Doan Pham
email: doan.phamminh@mines-albi.fr

Ange Nzihou

Patrick Sharrock

Universite´ De

SODIUM DIHYDROGEN PHOSPHATE STARTING FROM SODIUM CHLORIDE AND ORTHOPHOSPHORIC ACID VIA CATION RESIN EXCHANGE

Keywords: Synthesis design, ion exchange, cations, sodium dihydrogen phosphate

Sodium phosphates are generally synthesized by neutralization of orthophosphoric acid with sodium hydroxide. In this paper, we communicate a new synthesis process, wherein sodium dihydrogen phosphate can be obtained from sodium chloride and orthophosphoric acid, using cation resin exchange at room operational conditions. This opens a new concept for the design of environmentally friendly processes in this inorganic synthesis field.

INTRODUCTION

Sodium phosphates include a family of several inorganic compounds. At least 15 solid sodium phosphates exist in the temperature range from 25 to 100 • C (Table 1). 1,[START_REF] Gilmour | Phosphoric Acid Purification, Uses, Technology, and Economics[END_REF] Many of them are commonly used as food additives, in pH control, and in detergent composition. [START_REF] Gilmour | Phosphoric Acid Purification, Uses, Technology, and Economics[END_REF][START_REF] Corbridge | Studies in Inorganic Chemistry 10. Phosphorus: An Outline of its Chemistry[END_REF] They can be also used in catalysis, 4 and as starting materials for the synthesis of other phosphates, such as sodium trimetaphosphate, [5][START_REF] Pham Minh | Phosphorus Sulfur Silicon[END_REF][START_REF] Pham Minh | Phosphorus Sulfur Silicon[END_REF][8] phosphate-based glasses and alloys, [9][10][11][12][13] apatitic calcium phosphates. [14][15][16][17] Sodium phosphates can be classically obtained from the neutralization of orthophosphoric acid with sodium hydroxide (and sometimes with sodium carbonate). [START_REF] Corbridge | Studies in Inorganic Chemistry 10. Phosphorus: An Outline of its Chemistry[END_REF] However, sodium hydroxide is industrially produced from the electrolysis of sodium chloride solution. [START_REF] Tilley | Understanding Solids: The Science of Materials[END_REF] Considering sodium chloride and orthophosphoric acid as starting materials, sodium phosphates are formed by multi-step synthesis process (Equations (1-2)) which release chlorine.

•H 2 O N a 2 HPO 4 •2H 2 O N a 3 HPO 4 •0.5H 2 O NaH 2 PO 4 •2H 2 O N a 2 HPO 4 •7H 2 O N a 3 HPO 4 •6H 2 O NaH 2 PO 4 •Na 2 HPO 4 Na 2 HPO 4 •8H 2 O N a 3 HPO 4 •8H 2 O NaH 2 PO 4 •H 3 PO 4 Na 2 HPO 4 •12H 2 O N a 3 HPO 4 •12H 2 O 2NaCl + 2H 2 O → 2NaOH + Cl 2 +H 2 (1) NaOH + H 3 PO 4 → NaH 2 PO 4 +H 2 O ( 2)
This study aimed to design a new process for getting sodium phosphates directly from sodium chloride and orthophosphoric acid as starting materials, in order to simplify and economize cost and energy of the existing process. This calls for the cationic exchange property of resin-based materials. A given cationic exchange resin contains generally a stable polymer matrix (R-) and functional groups such as sulfonic acid (-SO 3 -H). The proton (H + ) of these functional groups can be exchanged with other cations such as Na + and this process is generally reversible. Our synthesis process can be summarized by Equations (3-4). The first step (Equation ( 3)) is for fixing the sodium cations on the resin matrix and for releasing HCl as the co-product of the process. The second step (Equation ( 4)) is for regenerating the acidic resin matrix and for forming NaH 2 PO 4 as the desired product.

R -SO 3 -H + NaCl → R -SO 3 -Na + HCl (3) R -SO 3 -Na + H 3 PO 4 → R -SO 3 -H + NaH 2 PO 4 (4)

RESULTS AND DISCUSSION

Three liquid flow rates of 2, 10, and 20 mL min -1 of both NaCl and H 3 PO 4 solutions were investigated. Table 2 summarizes the results of sodium exchange yield, corresponding to Step 1. For sodium exchange, the quantity of sodium exchanged with the column varied between 17.4 and 20.5 mmol corresponding to the exchange yields of 51-60% (the exchange capacity of the column was 29.8 mEq.). This might be due to the quantity of sodium introduced into the column (34 mmol), which was only slightly higher than the capacity of the column. Contrary to what we expected, the decrease of the liquid flow rate (in the range 2-20 mL min -1 ) led to a slight decrease of the sodium exchange yield. The analysis of the chloride showed that the resin matrix practically had no interaction with Table 3 shows the results obtained for Step 2 at three different liquid flow rates. When orthophosphoric (34 mmol) passed through the column, all phosphate anions left the column, indicating that the resin had no affinity for this anion. For sodium cations previously fixed on the column at Step 1, they were only partially released from the column. As observed for Step 1, the liquid flow rate did not have a notable influence on the release of sodium. The aqueous solution recovered from Step 2 of this synthesis process contained 4.0-5.2 mmol of NaH 2 PO 4 and 28.8-30.0 mmol of H 3 PO 4 . This corresponded to the final yield of both steps 1 and 2 of the process of 11-15%. The difficulty of sodium release during the step 2 may be explained by the low dissociation constant of orthophosphoric acid (K a1 equal to 7.25 × 10 -3 ), compared to the strength of the sulfonate groups on the resin. At ambient temperature, the pH of the dilute aqueous solution of orthophosphoric acid of 100 mM was measured and equal to 1.6. At this pH, the species distribution of orthophosphoric acid was as following: 83.7%, 16 . 17 These first results show that it is possible to obtain monobasic sodium phosphate via cationic exchange process. The advantages of this process are: its operational simplicity; its moderate reaction conditions (room temperature and pressure); and the separation of two main products of the process (obtaining HCl in the step 1, and NaH 2 PO 4 in the step 2). However, the performance of the process needs to be improved. For this last, we think that the choice of resin plays an important role. The resin used in this work (R-SO 3 -H) contains strong acid groups. Sodium cations fixed by sulfonate groups are difficult to remove with orthophosphoric acid, having low acidity. A resin containing weaker acid groups (R-COO-H for example), which should easily participate in the cationic exchange with both NaCl and H 3 PO 4 solutions seems to be necessary for improving the performance of the process. This will be tried in future work.

CONCLUSIONS

For the first time, the synthesis of sodium phosphate from NaCl and H 3 PO 4 via cationic exchange process was reported. Using the resin containing sulfonic acid (-SO 3 -H) as functional groups, the final yield of the process reached 15%. The use of other resins having weaker acid groups seems to be a key factor for improving the performance of the process.

EXPERIMENTAL

NaCl powder (Fisher Scientific, >99%), HCl (Fisher Scientific, 36 wt.% in water), and orthophosphoric acid (85 wt.% in water, Merck) were used for the preparation of aqueous solutions containing 100 mM of NaCl, HCl, and H 3 PO 4 . Elemental analysis of liquid samples was performed by inductive coupled plasma atomic emission spectroscopy (ICP-AES, HORIBA Jobin Yvon Ultima 2). Phosphate anion (PO 4

3-

) and chloride anion (Cl -) of liquid samples was analyzed by ion chromatography (Dionex, conductivity detector, Dionex IonPac AS 19 column (4 × 250 mm)).

In the first step, we tested a commercial resin, which was purchased from Aldrich (Amberlite IR120). This resin is composed of styrene-divinylbenzene as resin matrix (R-) and sulfonic acid (-SO 3 -H) as functional groups. The cationic exchange capacity of this resin was 1.356 mEq per mL of resin, which was experimentally determined. Spherical resin particles of diameter of 297-1190 µm were used as received.

The cationic exchange process was carried out at ambient temperature and pressure, using a graduated glass column (internal diameter of 10 mm, height of 250 mm) (Fig. 1).

A porous glass disk (porosity of 101-160 µm) was set at the bottom of the column in order to keep the bed of resin particles in the column. The outlet of the column was connected with a peristaltic pump for the control of liquid flow rate. The procedure for a given test was the following:

• Step 0: The column was filled with resin particles for the apparent volume of 22 mL.

Then, it was completed with pure water in order to facilitate the contact of reactants with resin particles during exchange process. The cationic exchange capacity of the column was 29.8 mEq. The same resin sample was used for all the tests in this report. • Step 1: The resin in the column was in the protonated form (R-SO 3 -H). For the first synthesis step (Equation ( 3)), 340 mL of NaCl (100 mM) passed through the column at the liquid flow rate of 2-20 mL min -1 , followed by rinsing with 50 mL of water. • Step 2: The resin was now saturated with sodium cations (R-SO 3 -Na). For the second synthesis step (Equation ( 4)), 340 mL of H 3 PO 4 (100 mM) passed through the column at the same liquid flow rate as for the first synthesis step (2-20 mL). The column was also rinsed with 50 mL of water. • Step 3: To regenerate the column, 340 mL of HCl (100 mM) was pumped through the column at the flow rate of 2 mL min -1 . This quantity of HCl was largely sufficient for acidifying the quantity of resin present in the column. Then, the column was rinsed with 50 mL of water following the regeneration.

For all the steps 1 to 3, the total quantity of liquid phase from the outlet of the reactor (including rinsing water) was recovered. ICP-AES and ion chromatography were carried out for determining the content of sodium, chloride, and phosphate. This allowed calculating the mass balance of each component.

The final yield for the formation of NaH 2 PO 4 of both steps 1 and 2 of the process is calculated according to the following equation:

Y = Na released Na i x100
where Y is the final yield of both steps 1 and 2 of the process for the formation of NaH 2 PO 4 ; Na released is the value of "Na released by the column, mmol" in Table 3; Na i is the initial quantity of sodium chloride introduced into the column in Step 1 which is equal to the initial quantity of orthophosphoric acid introduced into the column in Step 2 (or 34 mmol).

Figure 1

 1 Figure 1 Illustration of the resin column used in this work.

Table 1

 1 List of solid crystalline sodium phosphates existing at the temperature range of 25 to 100 • C

	NaH 2 PO 4	Na 2 HPO 4	Na 3 HPO 4
	NaH 2 PO 4

Table 2

 2 Sodium exchange during the step 1 of the synthesis process at room temperature and pressure

	Liquid flow rate	Na introduced	Na captured by the	Cl -introduced	Cl -outlet
	(mL min -1 )	(mmol)	column (mmol)	(mmol)	(mmol)
	2	34	17.4	34	34
	10	34	18.5	34	33.9
	20	34	20.5	34	34

Table 3

 3 Sodium release during the step 2 of the synthesis process at room temperature and pressure

	Liquid flow rate	Na available in the	Na released by the	PO 4	3-introduced	PO 4	3-outlet
	(mL min -1 )	column (mmol)	column (mmol)		(mmol)	(mmol)
	2	17.4	4.6		34		33.8
	10	18.5	5.2		34		34
	20	20.5	4.0		34		34

chloride anions. The aqueous solution recovered from the outlet of the reactor during this step 1 contained 17.4-20.5 mmol of HCl and 16.6-13.5 mmol of NaCl.

  .3%, 0%, and 0% for H 3 PO 4 , H 2 PO 4

	HPO 4	2-, and PO 4	3-	, -

ACKNOWLEDGMENTS

The authors gratefully acknowledge colleagues at RAPSODEE Center for technical help.


Doan Pham

Jocelyn Ramaroson

Ange Nzihou

Patrick Sharrock

One-Step Synthesis of Sodium Trimetaphosphate (Na 3 P 3 O 9 ) from Sodium Chloride and Orthophosphoric Acid

One-step thermal synthesis of sodium trimetaphosphate (STM) from sodium chloride and 85 wt % orthophosphoric acid as starting materials was investigated. The reaction temperature and volume-mean diameter of sodium chloride influenced strongly the elimination of chloride and the selectivity in STM. STM of high quality (99%) was obtained at 600 °C from fine powder of sodium chloride. Hydrochloric acid was the only byproduct of the reaction.

INTRODUCTION

Polyphosphates, and particularly sodium trimetaphosphate (STM), could be used in different applications such as water treatment, metal surface treatment, detergent, food additives, etc. [START_REF] Papo | Sodium tripolyphosphate and polyphosphate as dispersing agents for kaolin suspensions: rheological characterization[END_REF][START_REF] Rashchi | Polyphophates: A review their chemistry and application with particular reference to mineral processing[END_REF][START_REF] Halliwell | Hydrolysis of triphosphate from detergents in a rural waste system[END_REF][START_REF] Ferrer | Synthesis and characterization of CdS nanocrystals stabilized in polyvinyl alcohol-sodium polyphosphate[END_REF][START_REF] Kutun | New elution agent, sodium trimetaphosphate, for the separation and determination of rare earths by anionexchange chromatography[END_REF] STM is one of the most popular cross-linkers used in food chemistry. [START_REF] Li | Physical properties and loading capacity of starch-based microparticles crosslinked with trisodium trimetaphosphate[END_REF][START_REF] Cury | Modeling a system of phosphated cross-linked high amylase for controlled drug release. Part 1: Synthesis and polymer characterization[END_REF][START_REF] Muhammad | Effect of pH on phosphorylation of sago starch[END_REF][START_REF] Lack | Highresolution nuclear magnetic resonance spectroscopy studies of polysaccharides crosslinked by sodium trimetaphosphate: A proposal for the reaction mechanism[END_REF] Papo et al. found that STM is a good dispersant for kaolin suspensions, which are important raw materials in advanced ceramic processing. [START_REF] Papo | Sodium tripolyphosphate and polyphosphate as dispersing agents for kaolin suspensions: rheological characterization[END_REF] In multistage flash seawater distillation, STM was found to be a good antiscale agent. [START_REF] Butt | Performance of a polyphosphatebased scale control additive in an MSF plant of Kuwait[END_REF] In addition, STM is the starting material for the synthesis of all other metal trimetaphosphates. [START_REF] Averbuch-Pouchot | Topics in Phosphate Chemistry[END_REF] STM is industrially synthesized following the process of Thilo and Grunze in which sodium dihydrogen phosphate (NaH 2 PO 4 ) was heated to 500 °C for 5 h. [START_REF] Thilo | Der Entwasserungsverlauf der Dihydrogenmonophosphate des Li, Na, K, und NH 4[END_REF][START_REF] Ondik | The structures of anhydrous sodium trimetaphosphate, Na 3 P 3 O 9 , and the monohydrate, Na 3 P 3 O 9 •H 2 O[END_REF][START_REF] Durif | The development of cyclophosphate crystal chemistry[END_REF] Solid product is then cooled down to room temperature and STM is obtained after a recrystallization step. However, NaH 2 PO 4 is a relatively expensive product because it is synthesized by the electrolysis of aqueous solution of sodium chloride to produce sodium hydroxide, following by neutralization of sodium hydroxide with orthophosphoric acid. The industrial production of STM can be summarized by following equations:

+ → + + 2NaCl 2H O 2NaOH Cl H 2 2 2 (1)
+ → + NaOH H PO NaH PO H O 3 4 2 4 2 (2)
→ + 3NaH PO Na P O 3H O 2 4 3 3 9 2 (3)
Therefore, from sodium chloride and orthophosphoric acid as starting reactants, STM is obtained after four steps: electrolysis, neutralization, thermal synthesis, and recrystallization, taking into account the first two steps for the synthesis of NaH 2 PO 4 . Gaseous H 2 and Cl 2 are two byproducts.

This work aims to investigate the one-step thermal synthesis of STM from sodium chloride and orthophosphoric acid as starting reactants. These reactants were chosen because of their good availability and low cost for an eventual real application.

MATERIALS AND METHODS

Sodium chloride powder with the volume-mean diameter (d NaCl ) of 375 μm (labeled SC1) from Acros Organics and 85 wt % orthophosphoric acid from Merck were used. Crushing this SC1 for 10 h in a rotary porcelain jar led to the formation of a finer powder with 78 μm d NaCl (labeled SC2). The particle size of sodium chloride powder was measured by laser scattering using dry dispersion technique in a Mastersizer 2000 (Malvern Instruments Ltd., Malvern, U.K.). The measurement was carried out at 3.5 bar of air pressure.

The reaction was carried out in a quartz reactor composed of a cylinder-shaped tube with gas input and output, a 65 mL crucible where the reaction will take place, and a stick for keeping the crucible fixed. The reactor was heated by a vertical furnace with a defined temperature program. For the reaction, sodium chloride (87.5 mmol) and orthophosphoric acid (87.5 mmol) were introduced into the crucible at room temperature. The reactor was heated to the reaction temperature and kept at this temperature for 120 min. It was then freely cooled down to room temperature. During the reaction, air circulation (120 L/ h) was applied. Output gas was trapped into a wash bottle containing water or an aqueous potassium hydroxide solution.

ICP-AES (inductive coupled plasma atomic emission spectroscopy) measurements were performed on a HORIBA Jobin Yvon Ultima 2 for the analysis of elementary phosphorus and sodium. Chloride (Cl -) and phosphorus-containing anions were analyzed using a Dionex ionic chromatography apparatus equipped with a conductivity detector and a Dionex IonPac AS 19 (4 × 250 mm) column.

TGA-DSC (simultaneous thermogravimetric analysis and differential scanning calorimetry) was carried out in a TA Instruments SDTQ600 analyzer. It simultaneously measures the heat flow and weight changes associated with transitions and reactions in materials. Infrared spectroscopy (IR) was performed with a Shimadzu FTIR 8400S spectrometer. XRD (X-ray diffraction) data was collected using a Phillips Panalytical X'pert Pro MPD diffractometer with a Cu Kα (1.543 Å) radiation source and a nickel film, which suppressed the Cu Kβ ray. Scanning electron microscopy (SEM) was carried out using a Philips XL30 ESEM apparatus (FEI Company), equipped with a energy dispersive X-ray (EDX) analysis module.

RESULTS AND DISCUSSION

3.1. Thermal Reaction. The reaction between sodium chloride powder and orthophosphoric acid occurred even at room temperature, observed by the formation of hydrochloric acid, and could be illustrated in eq 4.

+ → + + NaCl H PO Na H P O HCl H O a b c d 3 4 2 (4)
However, a high temperature was required for the formation of condensed phosphates. The analysis of the solid product (Na a H b P c O d ) by ionic chromatography showed the presence of different phosphorus-containing species: orthophosphate (PO 4 3-), pyrophosphate (P 2 O 7 4-), and trimetaphosphate (P 3 O 9

3-) anions. The selectivity of a given phosphorus anion "A" (S A ) in the solid product and chloride elimination rate (X Cl ) were calculated as follows:

= × S
quantity of phosphorus found in A form initial quantity of phosphorus 100

A = × X
quantity of eliminated chloride initial quantity of chloride 100

Cl Table 1 shows the analysis results of solid products formed in the reaction temperature range 300-600 °C, using two sodium chloride powders with two different volume-mean diameters of sodium chloride particles. Both the reaction temperature and the particle size of sodium chloride powder had important influences on the chloride elimination rate (X Cl ) and the selectivity in trimetaphosphate anion (S P 3 O 9 ). The higher the reaction temperature was, the better the chloride elimination rate and the selectivity in trimetaphosphate anion were. However, the reaction temperature was limited at 600 °C because a higher temperature could lead to the transformation of STM into Graham's salt product by melting at ca. 627 °C. [START_REF] Bastide | Etude de la cristallisation du trimetaphosphate de sodium vitreux. Premiere partie: aspect cristallochimique[END_REF] Low contents of PO 4 3-and P 2 O 7 4-anions were observed at 300 and 400 °C. This means that dehydration reactions of orthophosphate and pyrophosphate occurred already at these temperatures. At higher reaction temperatures, these anions disappeared to form P 3 O 9 3-anion. The formation of P 3 O 9

3anion increased continuously with the increase of the reaction temperature and reached up to 99% at 600 °C with fine sodium chloride powder. In parallel, the chloride elimination rate increased, too. At a given reaction temperature, the chloride elimination rate was higher with crushed sodium chloride powder (d NaCl = 78 μm) than with initial sodium chloride powder (d NaCl = 375 μm). The effect of the particle size of sodium chloride was minor at lower temperatures and became more important at higher temperatures. The best chloride elimination rate was 99%, obtained at 600 °C with SC2. The decrease of particle size of sodium chloride favored its contact with orthophosphoric acid and therefore favored the reaction. In fact, the product of the reaction, essentially STM at 500 and 600 °C, existed in solid phase and could isolate the sodium chloride core from orthophosphoric acid if the initial sodium chloride particles are too big. This phenomenon is particularly important because the reaction was carried out without stirring. In spite of a high temperature of 600 °C, some traces of chloride were until detected when fine sodium chloride (d NaCl = 78 μm) powder was used. This result can be explained by the presence of a small fraction (8%) of the particles in the range 200-316 μm) in this fine sodium chloride powder, as shown in Figure 1.

The analysis of gaseous products trapped in wash bottle showed that only chloride was present in HCl form. The total quantity of chloride in gaseous and solid products reached the initial quantity of chloride used in all cases.

3.2. Characterization of the Solid Products. Ionic chromatography analysis permitted the determination of chloride elimination rate and the distribution of phosphoruscontaining species in the solid products. However, supplementary characterizations are necessary for identifying its components. We present here the characterization results obtained with the solid product of 99% chloride elimination rate and 99% selectivity in trimetaphosphate anion that is labeled Syn-STM.

Figure 2 shows XRD patterns of Syn-STM and pure STM (commercial product from Aldrich). All peaks present in the Syn-STM pattern could be attributed to STM diffraction. No other crystalline phase was found. Synthesized STM was also very well crystallized at 600 °C with a free cooling under air atmosphere.

Figure 3 compares IR spectra of Syn-STM and pure STM. Both spectra were very similar to each other, characterized by vibrations of trimetaphosphate ring in the wavenumber range from 650 cm -1 to 1400 cm -1 . [START_REF] Sbai | Chemical preparation, crystal structure, thermal behavior and IR studies of barium thallium cyclotriphosphate dihydrate[END_REF][START_REF] Nahdi | Chemical preparation and thermal behavior of neodymium cyclotriphosphate pentahydrate NdP 3 O 9 •5H 2 O: A study by controlled rate thermal analysis (CRTA)[END_REF][START_REF] Jouini | Crystal structure, vibrational spectra, and optical properties of praseodymium cyclotriphosphate PrP 3 O 9 •3H 2 O[END_REF] IR results confirmed again that STM was the only compound present in Syn-STM.

Figure 4 presents ATG-DSC analysis of Syn-STM in the temperature range 25-750 °C. A mass reduction of 1.4% was recorded at 750 °C that must be due to the dehumidification and probably the elimination of the last traces of chloride remaining in Syn-STM. Two endothermic peaks were observed. The first peak at 546.4 °C could be due to the melting of an intermediate present at very low content in Syn-STM. The second peak at 626.6 °C could be attributed to the melting of STM. [START_REF] Bastide | Etude de la cristallisation du trimetaphosphate de sodium vitreux. Premiere partie: aspect cristallochimique[END_REF] As mentioned, the reaction temperature must not exceed this melting point to avoid the transformation of STM into Graham's salt. [START_REF] Bastide | Etude de la cristallisation du trimetaphosphate de sodium vitreux. Premiere partie: aspect cristallochimique[END_REF] SEM images of Syn-STM are shown in Figure 5. Solid product in homogeneous powder form was observed. EDX analysis was carried out on different points of sample and showed similar results. According to the results of ionic chromatography analysis, chlorine was detected at low content while phosphorus, sodium, and oxygen were found as the main elements. Note that the presence of carbon was due to carbonic gas in the air.

CONCLUSIONS

Sodium trimetaphosphate (STM) was successfully synthesized from sodium chloride and orthophosphoric acid by one-step thermal process (600 °C for 2 h). Using a fine sodium chloride powder of 78 μm volume-mean diameter, chloride could be nearly totally eliminated (99%) and STM of high quality (purity of 99%) was obtained. Hydrochloric acid was the only byproduct that could be easily recovered by condensation. In comparison with the competing industrial process, the present work seems to be more economical and ecological thanks to the overcoming of three steps (electrolysis, neutralization, and recrystallization).

Future work will focus on the synthesis using finer sodium chloride powders to increase the contact of sodium chloride particles with orthophosphoric acid and therefore reduce the temperature and/or time of the reaction. The synthesis of other alkali trimetaphosphates from LiCl and KCl and orthophosphoric acid should be investigated.

■ AUTHOR INFORMATION

Corresponding Author *Tel.: +33 563493258. Fax: +33 563493043. E-mail address: doan.phamminh@mines-albi.fr.

Figure 1 .

 1 Figure 1. Distribution of particle size of sodium chloride powder before (1) and after (2) crushing.

Figure 2 .

 2 Figure 2. XRD patterns of Syn-STM and pure STM.

Figure 3 .

 3 Figure 3. IR spectra of pure STM (1) and Syn-STM (2).

Figure 4 .

 4 Figure 4. ATG-DSC of Syn-STM in the temperature range 25-750 °C.

Figure 5 .

 5 Figure 5. SEM-EDX analysis of Syn-STM.

Table 1 .

 1 Elimination of Chloride and Distribution of Phosphorus-Containing Species in Solid Products

	T (°C)	d NaCl (μm)	X Cl (%)	S PO4 (%)	S P2O7 (%)	S P3O9 (%)
	300	375	76	5	5	51
	300	78	83	8	16	54
	400	375	89	1	7	74
	400	78	95	0	2	85
	500	375	90	0	5	80
	500	78	97	0	0	93
	600	375	98	0	0	90
	600	78	99	0	0	99

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

Dr. Nathalie Lyczko is gratefully acknowledged for her assistance in XRD analysis.


Bruna Rego De Vasconcelos

Doan Pham

Nathalie Lyczko

Thanh Son Phan

Patrick Sharrock

Ange Nzihou

Upgrading greenhouse gases (methane and carbon dioxide) into syngas using nickel-based catalysts

Keywords:

In this work, Mg-doped Al 2 O 3 supports were used for the preparation of Ni catalysts, which were then tested in dry reforming of methane reaction (DRM) for syngas production. The influence of MgO content (0-70 wt%) on the basicity of the catalysts as well as on the formation of a NiO-MgO solid solution was investigated and linked to the catalytic performance of the catalysts. The catalyst containing high amount of Mg (Ni/70MgAl) showed the best performance with negligible deactivation rate over 50 h of time on stream (TOS). The presence of strong basic sites in this catalyst was important to adsorb carbon dioxide (CO 2 ), to gasify the coke deposit and to increase the syngas production. Also, the presence of a NiO-MgO solid solution led to strong metal-support interaction, which limited the sintering of nickel particles. The quantification of the water formed during the reaction showed that its formation was crucial for the elimination of the coke deposits as well as to increase the syngas production.

Introduction

CO 2 reforming of methane, also known as dry reforming of methane, (Eq. ( 1)) has received great attention in the past few years since it converts CO 2 and CH 4 , the two main greenhouse gases, into syngas, which can be later used for the production of energy, fuels and added-value chemicals, such as dimethyl ether, formaldehyde, olefins, etc [START_REF] Newnham | Highly stable and active Ni-mesoporous alumina catalysts for dry reforming of methane[END_REF][START_REF] Hua | Novel layered solid oxide fuel cells with multiple-twinned Ni 0.8 Co 0.2 nanoparticles: the key to thermally independent CO2 utilization and power-chemical cogeneration[END_REF].

Dry reforming of methane (DRM): (1)

+ ↔ + °= °= - H G CH CO 2H 2CO ∆ 2
Despite the economic and environmental benefits of this process, there is not yet a DRM full scale plant operation up to date [START_REF] Liu | La-promoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF]. The main reasons are the high energy requirements of the reaction and the fast deactivation of the catalysts due to coke deposition and sintering of both support and active phase at high reaction temperatures.

So, much effort has been done to develop thermally stable catalysts with good performance for this process. Supported noble metal (Ru, Pd, Pt and Rh) catalysts proved to have good catalytically performance. However, their high cost and limited availability are significant drawbacks [START_REF] García-Diéguez | Characterization of alumina-supported Pt, Ni and PtNi alloy catalysts for the dry reforming of methane[END_REF][START_REF] Pakhare | A review of dry (CO2) reforming of methane over noble metal catalysts[END_REF][START_REF] Shang | Carbon dioxide reforming of methane to synthesis gas over Ni/Si3N4 catalysts[END_REF]. So, transition metals, especially Ni, have been widely studied due to their lower cost and higher accessibility compared to noble metals and proven activity in this process [START_REF] Jafarbegloo | Thermodynamic analysis of carbon dioxide reforming of methane and its practical relevance[END_REF]. Nevertheless, they were reported to be more prone to carbon deposition on the catalyst surface (Eqs. ( 2) and ( 3)) and present low resistance against the sintering of the active phase, which considerably reduce their stability [START_REF] Newnham | Highly stable and active Ni-mesoporous alumina catalysts for dry reforming of methane[END_REF][START_REF] Gangadharan | Evaluation of the economic and environmental impact of combining dry reforming with steam reforming of methane[END_REF][START_REF] Boukha | Methane dry reforming on Ni loaded hydroxyapatite and fluoroapatite[END_REF][START_REF] Guo | CO2 reforming of methane over Mn promoted Ni/Al2O3 catalyst treated by N2 glow discharge plasma[END_REF].

Boudouard reaction: (3)

↔ + °= - °= - + H G 2CO C CO ∆ 1
So, there has been many studies to improve the coke resistance of the Ni-based catalysts. One possible pathway is the addition of alkali promoters, which increase the basicity of the support and favors the CO 2 adsorption which will further oxidize the carbon deposit [START_REF] Bradford | CO2 reforming of CH4[END_REF][START_REF] Gao | Dry (CO2) reforming[END_REF][START_REF] Wang | Carbon dioxide reforming of methane to produce synthesis gas over metal-supported catalysts: state of the art[END_REF][START_REF] Ballarini | Platinum supported on alkaline and alkaline earth metal-doped alumina as catalysts for dry reforming and partial oxidation of methane[END_REF][START_REF] Sun | Effect of pore structure on Ni catalyst for CO2 reforming of CH4[END_REF]. According to Ballarini et al. [START_REF] Ballarini | Platinum supported on alkaline and alkaline earth metal-doped alumina as catalysts for dry reforming and partial oxidation of methane[END_REF], these promoters may act as texture, structure and activity promoters, enhancing the thermal stability of the alumina-based supports and preventing sintering of the support and of the active phase. They would also improve the metal dispersion and the ability of the catalyst to gasify the coke deposit. San Jose-Alonso et al. [START_REF] José-Alonso | K and Sr promoted Co alumina supported catalysts for the CO2 reforming of methane[END_REF] investigated the influence of potassium (K) over the performance of the Co/Al 2 O 3 catalyst. Promoted catalyst showed lower coke deposition compared to unpromoted catalyst. This was explained by a partial coverage of the active sites for the methane decomposition by K addition and its ability to catalyze the carbon gasification reaction.

Several works have demonstrated that the formation of a NiO-MgO solid solution could also hinder the deactivation by coke deposition. Bradford and Vannice [START_REF] Bradford | CO2 reforming of CH4[END_REF] showed that the formation of NiO-MgO solid solution could stabilize the Ni particles, which prevented carbon deposition over Ni/MgO catalyst in DRM reaction. Hou et al. [START_REF] Hou | Production of synthesis gas via methane reforming with CO2 on noble metals and small amount of noble-(Rh-) promoted Ni catalysts[END_REF] added that the combination between NiO and MgO results in a very stable solid solution with a basic character, which is efficient in inhibiting the coke deposit.

Addition of water to the feed has also been used to inhibit carbon formation via a carbon gasification reaction (Eq. ( 4)) [START_REF] Bradford | CO2 reforming of CH4[END_REF].

Carbon gasification:

+ ↔ + °= H C H O H CO ∆ 131 kJ/mol (s) 2 2 298 (4)
Rostrup-Nielsen [START_REF] Rostrup-Nielsen | Catalytic steam reforming[END_REF] showed that the coke could be gasified by different gases such as hydrogen, carbon dioxide and steam. However, steam would be the most effective one. Similar conclusions were obtained by Figueiredo et al. [START_REF] Figueiredo | Gasification of carbon deposits on nickel catalysts[END_REF]. They showed that the rate of coke gasification with hydrogen is considerably slower than with steam.

In the present work, Ni supported on Mg-doped Al 2 O 3 catalysts, with different amounts of MgO, were tested in the DRM reaction. The aim was to understand the influence of the catalysts basicity (linked to the amount of Mg) on the catalyst activity and stability, and the role of the water formed during the reaction. The novelty of this work is the successful combination of different methods for structural characterization of catalysts before and after catalytic test to understand the deactivation mechanism and the impact on the conversion of CO 2 and CH 4 into syngas. The water formed during the reaction could be experimentally quantified and linked to the catalytic performance of catalysts synthesized in this work. This contributes to the basic understanding of DRM process and to the design of an efficient catalyst for DRM.

Materials and methods

Catalyst preparation

Commercial γ-Al 2 O 3 and hydrotalcite-based supports (PURAL® MG) provided by Sasol (Germany) were employed as catalysts supports. In this work, two hydrotalcite-based supports with different MgO/Al 2 O 3 weight ratios were used: PURAL MG30 (30:70) and PURAL MG70 (70:30). The three supports (Al 2 O 3 , PuralMG30 and PuralMG70) were first sintered at 1200 °C for 5 h in order to stabilize them and avoid later catalyst deactivation by support sintering. Then, the catalysts were prepared by impregnating the sintered supports with an aqueous solution of Ni(NO 3 ) 2 •6H 2 O (Fisher Scientific) by incipient wetness impregnation method. The loading amount of Ni was 5.7 wt% for each sample. After the impregnation, the powder was dried at 105 °C. No further calcination was performed prior to the catalytic tests since the temperature employed during the reduction and reaction phases (700 °C) are enough to decompose the water and nitrates from the catalyst and these are the only species decomposed during a calcination of these catalysts (Fig. S1, supplementary material). The catalysts are referred as Ni/Al, Ni/30MgAl and Ni/70MgAl for Al 2 O 3 , PuralMG30 and PuralMG70 supports, respectively.

Catalytic reaction

The dry reforming of methane was carried out in a fixed-bed tubular reactor (8 mm i.d.). 300 mg of catalyst was diluted 2 times with nonporous alumina powder, which was then placed at the center of the reactor, between two layers of the same non-porous alumina. The pressure drop in the reactor was 1.6 bar. The reaction temperature was controlled by a thermocouple inserted into the reactor at the center of the catalytic bed. The catalyst was reduced in situ at 700 °C for 2 h under 4%H 2 /N 2 with a flow rate of 70 mL/min. After the reduction step, the reaction was carried out at 700 °C with a WHSV of 15,882 mL h -1 g cat -1 with a synthetic gas mixture containing 20% of CH 4 , 20% of CO 2 and 60% of N 2 (volume percentage) for 50 h of TOS (time on stream). A silica gel tube was placed at the outlet of the reactor as water trap for the quantification of water formed during the reaction. Gas products were analyzed by a µ-GC A3000 (Agilent) equipped with a thermal conductivity detector (TCD). A gas counter was also used for the measurement of the total gas flow rate at the reactor outlet. This is mandatory for correctly determining the selectivity and mass balance of the process.

The deactivation rate of the catalysts was calculated as follows:

where: X being equal to 26 or 50 h.

Catalyst characterization

X-ray diffraction (XRD) patterns of the powdered samples previously calcined at 700 °C under air for 3 h were obtained using a Phillips Panalytical X'Pert MPD diffractometer operating with Cu Kα radiation (λ = 1.543 Å) over a 2θ range of 10-150°and scan step size of 0.042°s -1 . Crystallite size was obtained using the Scherrer equation.

N 2 adsorption isotherms were recorded with a Micromeritics Tristar II 3020 in order to analyze the specific surface area and the porosity of the fresh catalysts. BET surface area was calculated from the nitrogen adsorption isotherms. Prior to the analyses, the catalysts samples were outgassed at 105 °C for 24 h at low pressure (< 100 mbar).

Basicity of the catalysts previously calcined at 700 °C under air for 3 h was investigated by carbon dioxide temperature-programmed desorption (CO 2 -TPD) using a Micromeritics AutoChem 2920 Analyzer equipped with a thermal conductivity detector (TCD). For each measurement, 100 mg of the sample was pretreated at 700 °C (10 °C/min) under helium flow for 1 h. The sample was then reduced under a flow of 25 mL/min of 5%H 2 /N 2 for 1 h and then it was cooled to 50 °C at 10 °C/ min under helium flow. The samples were then saturated with 5%CO 2 / N 2 (15 mL/min) for 30 min, before being flushed with helium (25 mL/ min) for 1 h in order to eliminate physisorbed CO 2 molecules. TPD was finally performed from 50 to 700 °C at a heating rate of 10 °C/min.

Transmission electron microscopy (TEM) images of the fresh catalysts calcined at 700 °C under air for 3 h and of the spent catalysts were recorded on a MET JEOL JEM-ARM200F-EDX/EELS. EDX mapping was also performed on the fresh calcined catalysts.

Thermogravimetric analyses (TGA) of spent catalysts were realized in order to qualitatively determine the different types of carbon deposit over the catalysts after the reaction. The analyses were carried out on a SDT Q600 apparatus from TA instruments. 30 mg of the spent catalyst was introduced into an alumina crucible. The sample was then heated under air flow (100 mL/min) up to 1000 °C at a heating rate of 1 °C/ min.

Results and discussion

Textural, structural and chemical features of the fresh catalysts

XRD patterns of fresh catalysts calcined at 700 °C under air for 3 h are presented in Fig. 1. For the Al support, a phase transition of γ-Al 2 O 3 to α-Al 2 O 3 was observed as expected. In fact, the α-Al 2 O 3 phase is usually formed during a high temperature calcination process [START_REF] Ballarini | Platinum supported on alkaline and alkaline earth metal-doped alumina as catalysts for dry reforming and partial oxidation of methane[END_REF]. The nickel crystallite size on the catalyst Ni/Al was estimated around at 19 nm.

XRD patterns of the 30MgAl support showed the only the presence of the MgAl 2 O 4 phase. The Ni/30MgAl catalyst showed the presence of the MgAl 2 O 4 phase and also another phase that could be either NiO or MgO or, most likely, a NiO-MgO solid solution due to the thermal treatment applied to the catalyst. In fact, it has been extensively reported in the literature that NiO and MgO oxides have a NaCl-like lattice structure with very similar lattice parameters. Feng et al. [START_REF] Feng | Calcination temperature effect on the adsorption and hydrogenated dissociation of CO2 over the NiO/MgO catalyst[END_REF] reported that for MgO, the 2θ values match at 62.6°(2 2 0), 75°(3 1 1) and 79°(2 2 2), while for NiO these values match at 63.2°, 75.8°and 79.81°. Moreover, Ni 2+ ionic radius is 0.070 nm while the Mg 2+ radius is 0.065 nm. These structural similarities explain why NiO and MgO can easily form a solid solution [START_REF] Feng | Calcination temperature effect on the adsorption and hydrogenated dissociation of CO2 over the NiO/MgO catalyst[END_REF][START_REF] Hou | Meso-porous Ni/Mg/Al catalysts for methane reforming with CO2[END_REF][START_REF] Hu | Binary MgO-based solid solution catalysts for methane conversion to syngas[END_REF][START_REF] Arena | Effect of calcination on the structure of Ni/MgO catalyst: an X-ray diffraction study[END_REF].

The 70MgAl support presented the MgAl 2 O 4 phase, along with MgO phase, with particle size of 12.8 nm (Table 1). The Ni/70MgAl catalyst still presented MgAl 2 O 4 and it also presented another phase that could be related to MgO or to a NiO-MgO solid solution. To confirm the presence of a NiO-MgO solid solution, the crystallite size and quantitative analysis were performed using Rietveld refinement. It is worth noting that MgO crystallite size in the initial 70MgAl support was of 12.8 nm (Table 1). On the other hand, the crystallite size of the possible MgO phase in Ni/70MgAl catalyst was of 16.9 nm. This result suggests the insertion of Ni 2+ ions into the initial MgO lattice leading to the formation of a NiO-MgO solid solution. Moreover, the 70MgAl support contained only 59.7 wt% of MgO. It was 66.2 wt% in the Ni/70MgAl catalyst considering that the major peak at 42.86°corresponded only to MgO (Table 1). In addition, no other crystalline phase of nickel oxide was detected in this catalyst. This confirms the formation of the NiO-MgO solid solution. Similar results were obtained by Feng et al. [START_REF] Feng | Calcination temperature effect on the adsorption and hydrogenated dissociation of CO2 over the NiO/MgO catalyst[END_REF]. Rietveld refinement fitted the measured XRD patterns satisfactorily as indicated by the low values of R Bragg parameter.

BET surface areas (S BET ) and pore volumes (V p ) measurements of the supports and catalysts revealed negligible S BET and V p , as expected since the supports were previously sintered for stabilization.

TEM images of the fresh catalysts are illustrated in Fig. 2. Ni/Al catalyst (Fig. 2a) presented well distinguished Ni-based particles of size around 10-50 nm. The formation of distinguished Ni-based particles (nickel oxide particles, as previously identified by XRD analysis) are confirmed by the EDX mapping presented in Fig. 3. Indeed, this figure highlighted the association between Ni and O. However, the Ni is not associated with the Al, highlighting the formation of well-defined nickel oxide particles. Ni-based particles on both Ni/30MgAl (Fig. 2b) and Ni/ 70MgAl catalysts (Fig. 2c) were not well-defined so their size could not ): • Ni/Al; ■ Ni/30MgAl; ▴ Ni/70MgAl. be estimated. Moreover, the EDX mapping results of these catalysts (Fig. 4 and Fig. 5, respectively) showed that Ni is always associated to all the other species, highlighting the formation of NiO-MgO solid solution as previously identified by the XRD results.

CO 2 -TPD experiments were conducted in order to evaluate the amount and the strength of basic sites presented in each catalyst. The objective was to show how the amount of MgO would influence the basicity of the supports and possibly help the coke removal. The amount of the basic sites is represented by the area under the CO 2 -TPD curves and the strength of the sites is related to the temperature of CO 2 desorption. It has been reported that high desorption temperatures (T > 300 °C) indicate the presence of strong basic sites, while temperatures between 200 and 300 °C and lower than 200 °C represent medium and weak basic sites, respectively [START_REF] Sengupta | Modifying alumina with CaO or MgO in supported Ni and Ni-Co catalysts and its effect on dry reforming of CH4[END_REF][START_REF] Siahvashi | Synthesis gas production via propane dry (CO2) reforming: influence of potassium promotion on bimetallic Mo-Ni/Al2O3[END_REF][START_REF] Han | Elevated temperature CO2 capture on nano-structured MgO-Al2O3 aerogel: effect of Mg/Al molar ratio[END_REF][START_REF] Navajas | Synthesis of biodiesel from the methanolysis of sunflower oil using PURAL Mg-Al hydrotalcites as catalyst precursors[END_REF]. Fig. 6 shows the CO 2 -TPD profiles of the catalysts samples after impregnation and calcination at 700 °C under air. The Ni/Al catalyst showed a small desorption peak at around 185 °C, indicating the presence of only small amounts of medium basic sites. This result was not surprising since alumina is known to be an acidic material [START_REF] Sengupta | Modifying alumina with CaO or MgO in supported Ni and Ni-Co catalysts and its effect on dry reforming of CH4[END_REF]. The Ni/30MgAl catalyst presented a desorption peak with the maximum at about 250 °C, also implying the presence of medium basic sites. However, the area under the CO 2 -TPD curve of this catalyst was larger than the one obtained with the Ni/Al catalyst, indicating that the total amount of basic sites was larger for the Ni/30MgAl catalyst, as expected due to the presence of Mg. This result is rather interesting confirming that the formation of a NiO-MgO solid solution did not eliminate the basic sites present initially on the 30MgAl support due to the presence of MgO. The Ni/ 70MgAl catalyst showed a very broad desorption peak ranging from 100 to almost 500 °C, indicating the presence of weak, medium and strong basic sites. Moreover, this catalyst presented the largest area under the curve, indicating a larger amount of basic sites. The difference between the two Mg-containing catalysts could be related to the initial MgO contents on the supports. Ballarini et al. [START_REF] Ballarini | Platinum supported on alkaline and alkaline earth metal-doped alumina as catalysts for dry reforming and partial oxidation of methane[END_REF] and Montanari et al. [START_REF] Montanari | Basic catalysis and catalysis assisted by basicity: FT-IR and TPD characterization of potassium-doped alumina[END_REF] reported that only high amounts of alkali and alkaline earth metals could produce significant amount of different basic sites for alumina-based support, which is the case of the 70MgAl support. Once again, the formation of a NiO-MgO solid solution did not eliminate the basic sites related to the presence of Mg. This is an important parameter since catalysts which are able to strongly adsorb CO 2 tend to present high catalytic stability in processes such as DRM. In fact, CO 2 can gasify coke deposit preventing the catalyst deactivation [START_REF] Bradford | CO2 reforming of CH4[END_REF].

DRM test

The catalytic performance of the prepared catalysts during DRM reaction at 700 °C is shown in Fig. 7. Also, the selectivity to syngas (desired product) and in water and solid carbon (undesired products) are shown in Fig. 8. The real amount of carbon deposited over the catalyst sample could not be measured due to the dilution applied to the catalyst that rendered the sample very heterogeneous. So, the solid carbon selectivity was calculated based on the carbon mass balance. Ni/70MgAl catalyst showed the best performance under the experimental conditions used. The conversion of methane and carbon dioxide was stable and kept around 80% over 50 h of time on stream (TOS). The selectivity to syngas was also very high (≈80-90%) and was kept constant during the test. In consequence, very low H 2 O and C (s) selectivity were observed. The low amounts of water and solid carbon indicate the occurrence of the carbon gasification reaction (Eq. ( 4)), where the carbon deposit reacts with the water formed to increase the syngas production. Moreover, the presence of strong basic sites in this catalyst favored CO 2 adsorption and consequently the solid carbon gasification (by adsorbed CO 2 ) through Boudouard reaction (Eq. ( 2)). This explains the low deactivation rate (only 0.004%/h at 50 h of TOS) of this catalyst (Table 2).

Finally, Ni/Al catalyst, which had negligible support basicity, showed high initial methane and carbon dioxide conversions (≈60%). However, they drastically decreased to ≈20-30% during the first 26 h of TOS before becoming relatively stable. Similarly, the initial syngas selectivity was very high (≈80%) and then it drastically decreased during the first 26 h of TOS. Consequently, high H 2 O (≈12%) and C (s) (≈30%) selectivity were observed after 26 h of TOS. This could be explained by the occurrence of side reactions such as reverse water-gas shift (

+ + °⇌= CO H CO H O ∆H 41.2kJ/mol 2 2 2 298
), that consumes H 2 and CO 2 to produce H 2 O and CO and also Boudouard (Eq. ( 2)) and methane cracking reactions (Eq. ( 3)), which are responsible for coke deposition on catalyst surface. This could explain the high deactivation rate (0.70%/h at 50 h of TOS) of this catalyst (Table 2).

Methane and carbon dioxide conversions were slightly higher with Ni/30MgAl than with Ni/Al catalyst. Ni/30MgAl catalyst still presented considerably high methane and carbon dioxide conversions (≈40-50%) after 50 h of TOS. Syngas selectivity obtained with this catalyst slightly decreased from 78 to 72% for H 2 , and 84 to 79% for CO during 50 h of TOS. However, its deactivation rate (0.40%/h at 50 h of TOS) was much smaller (Table 2) compared to pure alumina-based catalyst (0.7%/h at 50 h of TOS). Only trace amount of water was observed which suggests that MgO could promote the carbon gasification (Eq. ( 4)). However, high carbon deposition still took place with the selectivity in solid carbon around 20% after 50 h of TOS.

It is worth noticing that all three catalysts presented high deactivation rate during the first 26 h of TOS (Table 2), which indicates that the carbon deposition reactions occurred mainly during this time. Then, the process tended to equilibrium between the carbon deposition and the carbon gasification reactions.

Deactivation analysis

The main reasons for the catalyst deactivation are generally coke deposition, which covers the active sites, and the sintering of active phase. In order to better understand the deactivation of the prepared catalysts, TEM and TGA analyses of the spent catalysts were realized.

The morphology of the spent catalysts was observed by TEM with the emphasis on the size of the metal-based particles and on the coke deposit. The TEM images of the three spent catalysts are shown in Fig. 9.

The screening of different regions of Ni/Al spent catalyst (Fig. 9a andb) revealed the presence of significant amount of carbon deposit under core-shell structure, which covered the nickel-based particles. This type of carbon deposit leads to a complete coverage of the active sites, and consequently to the catalyst deactivation. Wang et al. [START_REF] Wang | The properties of individual carbon residuals and their influence on the deactivation of Ni-CaO-ZrO2 catalysts in CH4 dry reforming[END_REF] reported that the formation of the core-shell/graphite layers structures occurs when the carbon diffusion rate is low, leading to the retention of carbon on the surface of the Ni-based particles. The diffusion of the carbon species through the Ni-based particles also resulted in the formation of carbon nanotubes (CNTs) and carbon nanofibers (CNFs). Moreover, the size of Ni-based particles on the spent catalyst was around 50-60 nm while it was around 10-40 nm on the fresh catalyst (Fig. 2a). So the sintering of the active phase occurred during the reaction, which is also known to be responsible for the catalyst deactivation.

Spent Ni/30MgAl catalyst (Fig. 9c andd) also showed the presence of core-shell carbon. However, the carbon deposit occurred mostly under CNTs/CNFs form. Sintering of the nickel-based particles was also observed for this sample since the spent catalyst clearly showed nickelbased particles around 10-60 nm while they were not detectable on the fresh sample.

Ni/70MgAl catalyst (Fig. 9e andf) showed rarely core-shell carbon and CNTs/CNFs. Sintering of the nickel-based particles also occurred with the observation of small nickel-based particles (around 10-20 nm) which was not the case for the fresh sample.

The three samples showed the formation of whisker carbon (CNTs and CNFs). In some cases, the CNTs/CNFs embedded the active phase and thus contributed to the deactivation of the catalysts. However, it has been proved in the literature that this type of carbon has little influence on the catalysts deactivation. In fact, they mostly grow away from the catalyst surface and do not cover the active sites. Furthermore, in some cases, the active phase is located at the tip of the carbon structure and thus still accessible to the reactants [START_REF] Wang | The properties of individual carbon residuals and their influence on the deactivation of Ni-CaO-ZrO2 catalysts in CH4 dry reforming[END_REF][START_REF] Wei | Elucidation of the roles of Re in steam reforming of glycerol over Pt-Re/C catalysts[END_REF][START_REF] De Vasconcelos | Synthesis of carbon nanotubes/hydroxyapatite composites using catalytic methane cracking[END_REF][START_REF] Ay | Dry reforming of methane over CeO2 supported Ni, Co and Ni-Co catalysts[END_REF].

The comparison between TEM results and the deactivation rate (Table 2) of the catalysts revealed that besides the amount of carbon, the type of carbon deposited on the catalyst and the sintering of the active phase are crucial for the catalyst stability. Ni/Al catalyst, which showed the highest deactivation rate at 50 h of TOS (0.70%/h), strongly favored the formation of core-shell carbon and the sintering of the nickel-based particles. Conversely, the Ni/70MgAl catalyst, which presented the lowest deactivation rate at 50 h of TOS (0.004%/h), showed the lowest amount of core-shell carbon and the lowest degree of sintering of the active phase.

Thermogravimetric analysis (TGA) of the spent catalysts was performed in order to determinate the type of carbon deposited over the catalysts. As different types of carbon decomposes at different temperatures, the temperature of the DTG peaks (derivation of TGA curves) were used for the identification of the nature of carbon deposition. For each sample, the analysis was repeated three times and the deviation related to DTG peak position was less than 2%. In some cases, the deconvolution of DTG peaks was necessary in order to properly identify the type of carbon deposit. The deviation of deconvolution was less than 1.5%. The DTG curves of the samples are presented in Fig. 10.

Three main DTG peaks could be observed at 280, 450 and 550 °C. Based on TEM results presented above and literature data, these peaks could be assigned to the oxidation of amorphous carbon (C α ), CNTs/ CNFs (C β ) and core-shell/graphite carbon (C γ ), respectively [START_REF] Ay | Dry reforming of methane over CeO2 supported Ni, Co and Ni-Co catalysts[END_REF]. These results slightly differ from results reported in the literature. Wang et al. [START_REF] Wang | The properties of individual carbon residuals and their influence on the deactivation of Ni-CaO-ZrO2 catalysts in CH4 dry reforming[END_REF][START_REF] Wang | Coking and deactivation of a mesoporous Ni-CaO-ZrO2 catalyst in dry reforming of methane: a study under different feeding compositions[END_REF] showed that carbon whiskers (CNTs and CNFs) would be oxidized around 500 °C and the core-shell/graphite carbon would be oxidized only at temperatures above 600 °C. Zhang et al. [START_REF] Zhang | Carbon dioxide reforming of methane to synthesis gas over supported Ni catalysts[END_REF] concluded that the amorphous carbon (C α ), CNTs/CNFs (C β ) and graphite carbon (C γ ) would be oxidized in the temperature range of 150-220 °C, 530-600 °C and above 650 °C, respectively. However, these studies applied heating rates of 10-30 °C/min, while in this work the heating rate was 1 °C/min, which explains the low temperatures of DTG peaks.

Both DTG and TEM results are in agreement with the catalytic results (Table 2). Both Ni/Al and Ni/30MgAl catalysts, which had high deactivation rates, led to the formation of C β and C γ types of carbon, which are harder to remove causing the catalyst deactivation. On the other hand, Ni/70MgAl catalyst, which showed a very low deactivation rate, presented mostly C α and C β after DRM test. As stated previously, the C β carbon has little influence on the catalytic deactivation and C α is known as the active species for the syngas formation and do not contribute to the catalytic deactivation [START_REF] Wang | Facile route for synthesizing ordered mesoporous Ni-Ce-Al oxide materials and their catalytic performance for methane dry reforming to hydrogen and syngas[END_REF]. C γ was rarely observed in the spent Ni/70MgAl catalyst, explaining the lower deactivation rate presented by this catalyst.

Conclusions

Alumina-based supports containing various MgO amounts (0-70 wt %) were used for the preparation of supported nickel catalysts. The catalytic performance of these catalysts was investigated in dry reforming of methane reaction (DRM) for syngas production at 700 °C and 1.6 bar.

The basic sites on the catalysts related to the presence of Mg and the formation of NiO-MgO solid solutions revealed to be crucial to the performance of the catalysts. The Ni/Al catalyst with very low amount of basic sites quickly deactivated during the first 26 h of TOS due to the formation of C β (CNTs and CNFs) and C γ (core-shell carbon), and to the sintering of the nickel-based particles.

The Ni/30MgAl catalyst with medium basicity showed better catalytically performance than the Ni/Al catalyst with a higher selectivity to syngas. The difference between their performances were mainly related to the formation of a NiO-MgO solid solution, which might have prevented the sintering of the Ni-based particles in a large extent as well as to the capacity of the catalyst to remove the coke deposit in a certain extent due to the presence of medium basic sites.

The Ni/70MgAl catalyst presented better catalytically performance than two other catalysts showing a very low deactivation rate, mainly due to the presence of strong basic sites and to its ability to remove the C α and C β carbon deposits. The formation of the solid solution might also have helped preventing the sintering of the active phase in a large extent and the formation of core-shell carbon that completely deactivates the active phase.

Acknowledgment

The authors gratefully acknowledge Severine Patry for her technical help on the characterization of the catalysts.

Fig. 1 .

 1 Fig. 1. XRD patterns of the fresh catalysts.

Fig. 2 .Fig. 3 .

 23 Fig. 2. TEM of fresh catalysts: (a) Ni/Al, (b) Ni/30MgAl and (c) Ni/70MgAl; pink arrow: nickel particle. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4 .

 4 Fig. 4. EDX mapping of Ni/30MgAl catalyst.

Fig. 5 .Fig. 6 .

 56 Fig. 5. EDX mapping of Ni/70MgAl catalyst.

Fig. 7 .

 7 Fig. 7. CH 4 (a) and CO 2 (b) conversions as a function of TOS for the prepared catalysts (reaction conditions: 700 °C, 1.6 bar, 15,882 mL h -1 g cat -1

Fig. 8 .

 8 Fig. 8. Selectivity in (a-b) desired (H 2 and CO) and (c-d) undesired (H 2 O and C s ) products: • Ni/Al; ■ Ni/30MgAl; ▴ Ni/70MgAl.

Fig. 9 .

 9 Fig. 9. TEM images of spent catalysts after 50 h of TOS: (a) and (b) Ni/Al; (c) and (d) Ni/30MgAl; (e) and (f) Ni/70MgAl.

Fig. 10 .

 10 Fig. 10. TGA/DTG curves of the spent catalysts: (a) Ni/Al, (b) Ni/30MgAl, (c) Ni/70MgAl. -Blue line: original DTG curves; -red line: simulated DTG curves;dashed line: deconvoluted peaks. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of

Table 1

 1 XRD characterization of the 70MgAl support and Ni/70MgAl catalyst.

	Sample	MgO or solid solution XRD	Possible	MgO	R Bragg	***
			parameters		match	(wt%)
		2θ	Cell	d *
			a = b = c (Å)	(nm)
	70MgAl	42.88	4.214	12.8	MgO	59.7	1.81
	Ni/70MgAl	42.86	4.212	16.9	MgO or solid	66.2 ** 1.88
					solution
	* Crystallite size calculated using the Scherrer equation.
	** Considering that the peak at 42.86°correspond only to MgO.
	*** From Rietveld refinement.

Table 2

 2 Deactivation rates of the alumina-based catalysts.

	Catalysts	Deactivation rate (% h -1 )	Deactivation rate (% h -1 )
		at TOS = 26 h	at TOS = 50 h
	Ni/Al	1.09	0.70
	Ni/30MgAl	0.54	0.40
	Ni/70MgAl	0.01	0.004


Doan Pham
email: doan.phamminh@mines-albi.fr

Jocelyn Ramaroson

Ange Nzihou

Patrick Sharrock

Guy Depelsenaire

A NEW ROUTE FOR THE SYNTHESIS OF ALKALI POLYPHOSPHATE FROM ECONOMICAL STARTING MATERIALS: PREPARATION AND CHARACTERIZATION OF SODIUM CYCLOTRIPHOSPHATE

Keywords: Alkali polyphosphate, sodium cyclotriphosphate, thermal synthesis

Thermal synthesis of sodium cyclotriphosphate (SCTP) -Na 3 P 3 O 9 was investigated in the temperature range of 150 • C to 750 • C using sodium chloride (NaCl) and 85 wt% orthophosphoric acid (H 3 PO 4 ) as economical starting materials. Reaction temperature had a crucial impact on the chloride elimination rate and the formation of SCTP. The best result was obtained at 600 • C with 96% of elimination of the initial chloride as hydrochloric acid and 84% of selectivity in SCTP. At lower temperatures, residual chloride contents were high. At higher temperatures (650 • C and 750 • C), SCTP was melted and transformed into glassy products.

INTRODUCTION

Alkali polyphosphates, formed from the condensation of orthophosphates, are used in various fields such as water treatment, metal surface treatment, detergent, food additives, fuel cells, ceramic materials, etc. [1][2][3][4][5][6][7][8] For example, in multi-stages flash sea water distillation alkali polyphosphates were found to be good anti-scale agents. [9][10][11] At low concentration, alkali polyphosphates are used as inhibitors for soluble copper corrosion control to prevent the leaching of copper from potable water distribution system. 12,13 Steel corrosion in similar conditions of sea water was reduced by addition of small amounts of alkali polyphosphates in the presence of divalent cations, such as Ca, Mg, Zn, and dissolved oxygen thanks to the formation of thin film between polyphosphates and metal cations and thereby inhibits the electrochemical processes that lead to corrosion. [10][11][12][13][14] Sodium cyclotriphosphate (SCTP) is an important polyphosphate since it is used in different applications as mentioned above. In addition, SCTP is used as starting material for the preparation of all other metal cyclotriphosphates, such as K 3 P 3 O 9 , Ag 3 P 3 O 9 •H 2 O, Na 2 LiP 3 O 9 , Mn 3 (P 3 O 9 )2 • 10 H 2 O, Ca 3 (P 3 O 9 )2 • 10H 2 O, Ba 3 (P 3 O 9 )2 • 4H 2 O, Ln(Bi)Na 3 P 3 O 9 • 9H 2 O, Tl 3 P 3 O 9 , etc. [START_REF] Averbuch-Pouchot | Topics in Phosphate Chemistry[END_REF] Actually SCTP is industrially synthesized by a thermal route (500 • C, 5 h) using sodium dihydrogen phosphate (NaH 2 PO 4 ) as reactant which is a relatively expensive product. [START_REF] Durif | [END_REF][17][18] Here, we report a new route for the synthesis of SCTP starting from sodium chloride (NaCl) and orthophosphoric acid (H 3 PO 4 ). The results obtained are of interest for a potential industrial application from point of view of the availability and low cost of the starting materials used.

RESULTS AND DISCUSSION

Influence of the Reaction Temperature

When solid sodium chloride powder is brought in contact with 85 wt% H 3 PO 4 at a Na/P molar ratio of 1/1 a supersaturated solution of sodium chloride results because the quantity of water initially present in 85 wt% H 3 PO 4 is much smaller than that required to dissolve all the sodium chloride. Under this condition, we can predict that hydrochloric acid could be formed in solution because of the present of both H + and Cl -ions. Under thermal conditions, hydrochloric acid could evolve from the reaction mixture as gas. Simultaneously, water will evaporate also. So, the general reaction between sodium chloride and H 3 PO 4 can be written as follows: Table 1 shows the chloride elimination rate and the distribution of species in the solid products as a function of reaction temperature between 150 • C and 750 • C. We obtained The reaction temperature had an important influence on the chloride elimination rate which varied progressively from 9% to 99%. The elimination of chloride explained the appearance of the solid products. Between 150 • C and 300 • C, the chloride elimination rate changed from 9% to 73%. This means that high quantity of initial H 3 PO 4 remained after the reaction which caused the pasty consistency of the final product. No further characterization was performed for these pasty products. On the other hand, solid products were formed between 350 • C and 750 • C, thanks to the successive increase of chloride elimination rate, up to nearly total.

At 150 ) was found to be the main product, with selectivity (S P3O9 ) varying from 79% at 350 • C to 84% at 600 • C.

Analysis of the gaseous products trapped in a wash bottle showed that only chloride was detected in form of HCl. No trace of sodium or phosphorus was detected. The quantification of chloride in the gaseous phase and in the solid product permitted to regain the initial quantity of chloride introduced in the reactor.

Total phosphorus (P balance ) was calculated from the quantified phosphorus containing products (PO 4 ). At 150 • C and 200 • C, P balance reached the initial quantity of phosphorus introduced in the reactor (called also P initial :87.5 mmol). At higher temperature from 250 • C to 750 • C, P balance was smaller than P initial . This difference must be due to some intermediates which were not identified in this temperature range. In fact, besides the three identified phosphorus compounds mentioned above, ionic chromatography analysis showed also one or several other small peaks that could not be identified. As example, Figure 1 shows the ionic chromatography analysis of the solid product formed at 400 • C which shows two unknown peaks eluted after P 3 O 9 3-. Phosphorus deficit (the difference with P initial and P balance ) was small between 250 • C and 600 • C where the selectivity in P 3 O 9

3-was high. Exceptionally at 650 • C and 750 • C, great phosphorus deficits were observed. This phenomenon was certainly due to a transformation of P 3 O 9 3-into other products.

Characterization of the Solid Product

Simultaneous Thermogravimetry and Differential Scanning Calorimetry Analysis (TGA-DSC). Figure 2 shows TGA-DSC analysis of the solid product formed at 350 • C. The first weight loss of 3.5% (25 • C-400 • C) could be attributed to the dehydration of orthophosphate and pyrophosphate species. The second weight loss (400 • C-610 • C) could be due to the reaction between remaining chlorides (15% in this case) with phosphorus-containing species to form gaseous HCl (so the elimination of chloride). After this second weight loss, the product containing essentially P 3 O 9

3-(at 600 • C) The product formed at 600 • C was well crystallized. Most of the peaks could be attributed to SCTP and NaCl. This means that P 3 O 9 3-quantified in Table 1 exists only in SCTP form. Three peaks with low intensity (2 θ position: 21.9; 30.3; 39.1) were not identified and are probably due to the principal peaks of unknown intermediates, formed at low contents. Similar results were observed for the solid products formed in the temperature range 350 • C-600 • C. According to the results of the ionic chromatography analysis, a difference was found in the intensity of the peaks for NaCl (decrease from 350 • C to 600 • C) and SCTP (increase from 350 • C to 600 • C).

On the other hand, the product formed at 650 • C was amorphous. In fact, melting of the solid product at 628 • C, observed in Figure 2, resulted in the formation of a glassy product or Graham's salt-like products. 19 This latter is a mixture of different long-chain polyphosphates and was described as an amorphous product. The same result was obtained with the solid product formed at 750 • C. So, the reaction temperature must be limited to 600 • C to avoid the formation of amorphous products.

Scanning Electron Microscopy/Energy Dispersive X-Ray (SEM-EDX). The change of the reaction temperature from 350 • C to 750 • C led to a change of the chlorine elimination rate, degree of dehydration and composition of the products formed. Accordingly, some morphological changes were observed as showed in Figure 4. All SEM micrographs were at the same scale. The largest difference in these products was the homogeneity of particle appearance, which increased with increasing reaction temperature. At 350 • C (micrograph a), most particles were similar with a gray glossy surface covered partially by light crust. Besides, there were also some particles which had a totally glossy surface. At higher reaction temperatures, there were no more any totally glossy particles. The surface of the particles covered by light crust increased with increasing reaction temperature up to 600 • C. At 650 • C and 750 • C, a perfect homogeneity of particle surface was observed, explained by the melting of these products at 628 • C as demonstrated by TGA-DSC results.

EDX analysis was then performed on all solid products. For the solids which had non homogenous appearance (products formed from 350 • C to 600 • C), sodium and chlorine were detected as two major elements on glossy surfaces that must correspond to NaCl remaining after reaction. On the other hand, all EDX analyses of light crust's surface showed the presence of sodium, phosphorus, and oxygen as major elements. Chlorine was frequently present on the light crust's surface but its content decreased with increasing reaction temperature. So, the light crust corresponded to the solid product of the reaction. It grows and sticks directly on the surface of NaCl particles. As an example, Figure 5 presents EDX analysis on glossy and light crust surfaces of the same particle of the solid product formed at 600 • C. Note that the presence of C in EDX results was due to the CO 2 present in air during the analysis and the presence of Au was due to the metallization for SEM sample preparation.

The result of SEM-EDX analysis revealed also that when a particle of sodium chloride is totally wrapped in light crust (or product of the reaction), it could not be in contact with the H 3 PO 4 and the reaction slows up. So, the particle size of NaCl powder could be an important parameter of the reaction.

CONCLUSION

SCTP can be conveniently prepared in a one step thermal process by heating sodium chloride and H 3 PO 4 as economical starting materials. It can be claimed that this synthesis represents also a smaller ecological footprint because it avoids the use of a strong base in comparison with the competing industrial process. Hydrochloric acid is the by-product of the reaction and could be recovered by condensation.

Reaction temperature was found to be a crucial parameter in the formation of the desired product (SCTP). The most favorable reaction temperature was at 600 • C with 96% of chloride elimination rate and 84% of selectivity in SCTP. Lower temperatures led to important remains of chloride residues while higher temperatures led to the transformation of SCTP into a Graham's salt-like product after melting at 628 • C.

EXPERIMENTAL Reactor

A quartz reactor consisting of a cylinder-shaped tube with gas input and output, a 65 mL crucible where the reaction takes place and a stick for keeping the crucible fixed was used. The reactor was heated by a vertical furnace with defined temperature program.

Fine powder of sodium chloride from Acros Organics (volume-mean diameter of 375 µm) and H 3 PO 4 (85 wt% pure grade) from Merck were used as received without further modification. For the reaction, 5.1 g of sodium chloride and 10.1 g of 85 wt% H 3 PO 4 were introduced into the crucible at room temperature, corresponding to a 1/1 Na/P molar ratio. The reactor was then heated to the reaction temperature (between 150 • C and 750 • C) and kept at this temperature for 30 min. After this temperature plateau, the reactor was freely cooled down to room temperature. During the reaction, an air circulation at constant flow rate of 120 L/h was applied. Output gas was trapped into a wash bottle containing pure water or an aqueous potassium hydroxide solution.

The concentration of sodium and phosphorus in solution was measured by ICP-AES (inductive coupled plasma atomic emission spectroscopy) using a HORIBA Jobin Yvon Ultima 2. The concentration of different anions (Cl -, PO 4 ) in solution was analyzed using a Dionex ionic chromatography apparatus equipped with a Dionex P/N 061830 conductivity detector and a Dionex IonPac AS 19 (4 × 250 mm) column. The mobile phase was constituted by ultra-pure water eluted at a rate of 1 mL/min and the pH was regulated and kept constant at 13 by a Dionex ICS-3000 EG eluant generator. The anions listed above were identified by comparison of their retention times with those of authentic samples.

TGA-DSC analysis was carried out in a TA Instruments SDTQ600 analyzer. It measures simultaneously the heat flow and weight changes associated with transitions and reactions in materials over the temperature range from ambient temperature to 800 • C under air atmosphere (100 mL/min). About 10 mg of the sample were used for each measurement with a heating rate of 5 • C/min. XRD data for the solids was collected using a Phillips Panalytical X'pert Pro MPD diffractometer with a Cu Kα (1.543 Å) radiation source and a nickel film which suppressed the Cu Kβ ray. SEM-EDX measurement was performed on a Philips XL30 ESEM apparatus (FEI Company). Metallization of SEM-EDX sample was carried out with metallic gold in a SC 7640 Sputter Coater (Quorum Technologies).

(

  85 wt%) H 3 PO 4 (liquid) + NaCl (solid) T -→ Na a H b P c O d (solid) + HCl (gas) + H 2 O (gas)

Figure 1

 1 Figure 1 Example of ionic chromatography analysis of the solid product formed at 400 • C: the two peaks eluted after P 3 O 9 3-were not identified.

Figure 2

 2 Figure 2 ATG-DSC analysis of the solid product formed at 350 • C.

Figure 3

 3 Figure 3 XRD patterns of the solid products formed at 600 • C (a) and 650 • C (b).

Figure 4

 4 Figure 4 SEM micrographs of the solid products formed at 350 • C to 750 • C: (a) 350 • C, (b) 400 • C, (c) 450 • C, (d) 500 • C, (e) 550 • C, (f) 600 • C, (g) 650 • C, and (h) 750 • C.

Figure 5

 5 Figure 5 Example of EDX analysis on the glossy (a) and light crust (b) surfaces of the same particle from the product formed at 600 • C.

Table 1 Influence of the reaction temperature on the distribution of the species in the solid products. Reaction conditions: initial NaCl and H 3 PO 4 : 87.5 mmol, volume-mean diameter of NaCl powder: 375 µm, reaction plateau

 1 solid products in the temperature range 150 • C-300 • C or 350 • C-750 • C, respectively. The solid products were opaque between 350 • C and 600 • C and transparent between 650 • C and 750 • C. It seems that the reaction temperatures of 650 • C and 750 • C lead to a melt, indicated by the perfect homogeneity of the solid products.

	time: 30 min, air flow rate: 120 L/h
	T	C l -	X Cl	PO 4	3-	P 2 O 7	4-	P 3 O 9	3-	P balance	S P3O9
	( • C)	(mmol)	(%)	(mmol)	(mmol)	(mmol)	(mmol)	(%)	Observation
	150	80	9	88	trace	0		88	0	Pasty
	200	66	25	68	10		trace	88	3	Pasty
	250	43	51	40	17		1		77	3	Pasty
	300	23	73	27	7		4		53	15	Pasty
	350	10	85	4	2		23		77	79	Opaque
	400	10	88	1	1		23		72	79	Opaque
	450	10	89	2	1		23		73	79	Opaque
	500	9	90	trace	2		23		73	80	Opaque
	550	8	91	0	trace	23		69	80	Opaque
	600	3	96	0	trace	24		72	84	Opaque
	650	3	97	0	trace	2		6	6	Transparent
	750	1	99	0	trace	1		3	3	Transparent

  • C, the amount of orthophosphate (PO 4 With the increase of the reaction temperature, PO 4 3-was rapidly converted into intermediates and its disappearance was nearly total at 400 • C. P 2 O 7 4-was formed as the first intermediate with the highest amount of 17 mmol at 250 • C, corresponding to the transformation of 34 mmol of PO 4 3-(initial quantity of H 3 PO 4 used was 87.5 mmol). At higher reaction temperature, P 2 O 7 4-was then transformed into other intermediates. This transformation was nearly complete at 350 • C. Cyclotriphosphate (P 3 O 9

		3-) was nearly unchanged and only
	trace of pyrophosphate (P 2 O 7	4-) was detected. A dehydration of PO 4	3-occurred already at
	this temperature. 3-

This work was financed by the Solvay Group. The authors thank colleagues at RAPSODEE (Research Centre in Albi on Particulate Solids, Energy and the Environment) for technical help.


Doan Pham
email: doan.phamminh@mines-albi.fr

Ange Nzihou

Patrick Sharrock

Jocelyn Ramaroson

A NEW ROUTE FOR THE SYNTHESIS OF ALKALI POLYPHOSPHATE FROM ECONOMICAL STARTING MATERIALS: PART II-INFLUENCE OF REACTION CONDITIONS

Keywords: Sodium polyphosphate, sodium cyclotriphosphate, sodium chloride, orthophosphoric acid, thermal synthesis

Direct synthesis of sodium cyclotriphosphate, Na 3 P 3 O 9 (SCTP), from sodium chloride and orthophosphoric acid as economical starting materials was investigated. Reaction conditions including the heating rate, reaction plateau time, air flow rate, and Na/P molar ratio were found to be key parameters for the elimination of chloride and for the selectivity in SCTP. High yields in SCTP (94-99%) were obtained. The results are very interesting with respect to a potential industrial application for the synthesis of SCTP from point of view of the low cost of the starting materials.

INTRODUCTION

Condensed phosphates are phosphate anions having one or several P-O-P bonds, which are formed by hydrolysis and dehydration reactions of orthophosphate anion under thermal conditions. 1,[START_REF] Averbuch-Pouchot | Topics in Phosphate Chemistry[END_REF] Their formation and structure depend on several parameters of synthesis: reaction temperature, heating rate, reaction time, cooling rate of melts, phosphorus/metal molar ratio, source of orthophosphate, etc. [START_REF] Sidney | [END_REF]4 Sodium cyclotriphosphate, Na 3 P 3 O 9 (SCTP), is used in different applications like detergency, food chemistry, water treatment, etc. 1,[5][6][7][8] Moreover, SCTP is the starting material for the preparation of all other metal cyclotriphosphates such as K 3 P 3 O 9 , Ag 3 P 3 O 9 [START_REF] Averbuch-Pouchot | Topics in Phosphate Chemistry[END_REF] The actual competing industrial process for the synthesis of SCTP was developed by Thilo and Grunze in 1955. 10--12 Sodium dihydrogen orthophosphate is heated to about 550 • C for 5 h and cooled to room temperature. Pure SCTP is then obtained by a purification step using recrystallization technique. Considering the industrial synthesis of sodium dihydrogen orthophosphate from sodium hydroxide and orthophosphoric acid, we have the following scheme for SCTP industrial synthesis process: Thilo and Grunze process

Our previous work shows that SCTP could be obtained from the direct reaction of sodium chloride and orthophosphoric acid at reaction temperature lower than 628 • C. 13 The present study investigates the influence of different synthesis parameters on the conversion of the initial reactants and the selectivity in forming SCTP. In comparison with the Thilo and Grunze process, the proposed process is more economical and ecological as it avoids the electrolysis and neutralization steps.

RESULTS AND DISCUSSION

Influence of the Heating Rate

When the powder of sodium chloride is set in contact with 85 wt% orthophosphoric acid at high temperature, hydrochloric acid is formed and leaves the reaction mixture as vapor. The general reaction between sodium chloride and orthophosphoric acid can be described by the following equation: As for all thermal processes, the heating rate was first investigated at different reaction temperatures from 300 to 600 • C. The influence of the heating rate on the distribution of chloride and phosphorus-containing species in the solid products, as obtained from ionic chromatography analysis, is shown in Table 1.

No influence of the heating rate on the distribution of chloride, orthophosphate, and pyrophosphate was observed. However, the heating rate had some effects on the formation of cyclotriphosphate anion (P 3 O 9

3-) at 300 and at 400 • C. A slight influence of the heating rate was recognized at 300 • C with higher selectivity for P 3 O 9 3-at 2 and 5 • C/min than at 10 and 20 • C/min. In fact, the time to reach the reaction temperature of 300 • C at heating rates of 10 • C/min and 20 • C/min starting from ambient temperature was only 28 min and 14 min, respectively. These heating times could be too short for the transformation of initial orthophosphate species and intermediates into P 3 O 9 3-, which explains the low selectivity for P 3 O 9

3-(4%). On the other hand, the heating rates of 2 3-in the range of 76-79% was observed for the heating rate range 2-10 • C/min. A smaller selectivity of 60% for P 3 O 9 [START_REF] Sidney | [END_REF]-was observed at heating rate of 20 • C/min, probably due to the shorter time at this heating rate to reach the reaction temperature, as explained above for the reaction temperature of 300 • C.

At 500 • C, the results were similar for all heating rates with a constant selectivity for P 3 O 9 3-of 80%. Thus, the reaction at 600 • C was carried out only at the heating rate of 5 • C/min. At 600 • C, the elimination of chloride and the selectivity in P 3 O 9 3-were higher than those achieved at 300, 400, or 500 • C as observed in previous investigations. 13 In conclusion, the heating rate influenced mostly the selectivity for P 3 O 9 3-anion. At three investigated temperatures of 300, 400, and 500 • C, the heating rate of 5 • C/min was the most favorable for the formation of P 3 O 9 3-.

Influence of the Air Flow Rate

The air circulation permits to remove the vapor of water and hydrochloric acid during the reaction and, therefore, influences the performance of the process. Thus, the synthesis was carried out at 350 • C with an air flow rate varying from 30 to 240 L/h. The reaction temperature of 350 • C was chosen to better observe the effect of the air flow rate. The result of this study is presented in Table 2.

At the reaction temperature of 350 • C, no effect of the air flow rate on the elimination of chloride was observed. On the other hand, orthophosphate species and pyrophosphate decreased with increasing air flow rate. In fact, the increase of air flow rate leads to a more effective removal of the gaseous products and so the dehydration of orthophosphate species, pyrophosphate, and eventually other intermediates takes place more readily. At the same time, the selectivity for P 3 O 9

3-increases from 39 to 77% when the air flow rate is increased from 30 to 60 L/h. In the range 60-240 L/h, the selectivity for P 3 O 9 3-increases only slightly from 77 to 80%. This study suggests that the air flow rate should be in range 60-120 L/h. A higher air flow rate is not recommended because of the higher energy consumption in order to keep the reaction temperature unchanged.

Influence of the Reaction Plateau Time

The effect of the reaction plateau time on the elimination of chloride and the distribution of the phosphorus-containing species is shown in Table 3.

Despite the increase of the reaction plateau time up to 1440 min, the content of chloride in the final product was only slightly reduced. Alternatively, the reaction temperature of 300 • C was not sufficient for a total elimination of chloride during a long reaction time (24 h). On the other hand, the reaction plateau time influenced strongly the distribution of phosphorus-containing species and the selectivity for the P 3 O 9 3-anion. Orthophosphate species nearly disappear after 120 min of reaction time. Pyrophosphate, formed as the first intermediate at the beginning of the reaction also disappears after 120 min. The portion of P 3 O 9 [START_REF] Sidney | [END_REF]-anion strongly increases between 30 and 480 min of reaction plateau time. The highest selectivity for P 3 O 9 [START_REF] Sidney | [END_REF]-was found at 88% after allowing the reaction to proceed for 480 min. However, a longer reaction time (1440 min) resulted in a decrease of the selectivity for P 3 O 9 3-, which is probably due to the transformation of P 3 O 9 3-into other products. So the most favorable reaction plateau time was found to be 120 min for the elimination of chloride and for the best selectivity with respect to the P 3 O 9 3-anion. From the results obtained above, a synthesis was then carried out at the most favorable conditions found for the Na/P molar ratio of 1/1: reaction plateau time of 120 min, reaction temperature of 600 • C, heating rate of 5 • C/min, and an air flow rate of 120 L/h (Table 3). As expected, the elimination of chloride was nearly total (98%) and a high selectivity for P 3 O 9 3-of 94% was obtained. This was also the best result obtained with the Na/P molar ratio of 1/1; the solid thus obtained is labeled Syn1.

Influence of the Na/P Molar Ratio

This study was carried out at 300 and 400 • C with Na/P molar ratios of 1/2, 1/1, and 2/1 (Table 4). The result obtained at the molar ratio Na/P = 1/1, which was already shown in Table 1, is also included in Table 4 for comparison.

When an excess of orthophosphoric acid was used (Na/P = 1/2), the chloride elimination rate was nearly total even at 300 • C. Orthophosphate species (PO 4 3-) disappear completely and P 2 O 7 4-and P 3 O 9 3-anions were found only in small amounts. This means that other intermediates were formed, which were not detected by ionic chromatography. So the excess of orthophosphoric acid is not favorable for the formation of the P 3 O 9 3anion.

When an excess of sodium chloride was used (Na/P = 2/1), a great amount of chloride remained in the final product. At 300 • C, PO 4 3-and P 2 O 7 4-were still present and the selectivity for the P 3 O 9 3-anion reached only 29%. At 400 • C, PO 4 3-was nearly totally converted into P 3 O 9 [START_REF] Sidney | [END_REF]-anion (99% of selectivity) and a half of the initial amount of chloride was consumed. The solid product obtained with 99% of selectivity with respect to the P 3 O 9 3-anion is labeled Syn2. In conclusion, an equimolar Na/P ratio (Na/P = 1/1) or an excess of sodium chloride (Na/P > 1/1) is required to obtain a high selectivity for P 3 O 9 3-.

Mass Balance

For all experiments, the mass balance for elemental chlorine, sodium, and phosphorus was established. ICP-AES analysis showed that all initial sodium and phosphorus were found in the final solid product. At the same time, the analysis of the gaseous products trapped in the wash bottle showed the presence of only chlorine in form of hydrochloric acid, which was easily condensed from the gas output of the reactor. The total quantity of chlorine in the gaseous and solid products reached in all cases the initial quantity of chlorine in the NaCl used.

Characterization of the Solid Product

By varying the reaction conditions, high selectivity for the P 3 O 9 3-anion up to 99% was obtained, as showed by ionic chromatography analysis. However, additional characterization is necessary to identify the corresponding product.

Characterization by XRD. All solid products were analyzed by XRD technique. The results show that P 3 O 9 [START_REF] Sidney | [END_REF]-anion is present mainly as Na 3 P 3 O 9 (SCTP), but in some cases small amounts of Na 2 HP 3 O 9 could be detected. The compound Na 2 HP 3 O 9 was only observed when the selectivity for P 3 O 9 [START_REF] Sidney | [END_REF]-was high and the chloride elimination rate was low, resulting in a deficiency of sodium for the formation of SCTP. For example, in Table 3 within the reaction plateau time range from 120 to 480 min Na 2 HP 3 O 9 was present in the solid products because the quantity of sodium (in mmol) liberated from the elimination of chloride was much smaller than the quantity of orthophosphates (in mmol) transformed into P 3 O 9 3-anion. In addition, the reaction temperature of 300 • C might also not have been sufficient to remove the last hydrogen atom of P 3 O 9 [START_REF] Sidney | [END_REF]-ring and substitute it by sodium atom. [START_REF] Van Wazer | Phosphorus and Its Compounds[END_REF] Figure 1 shows the XRD patterns of the two solid products (Syn1 and Syn2) having the best selectivity for P 3 O 9

3-for the Na/P molar ratio of 1/1 or 2/1 in comparison with that Na/P molar ratio of 1/1 with 98% chloride elimination rate and 94% of selectivity for the P 3 O 9 3-anion (Table 3). (Syn2): XRD pattern of the solid product formed at 400 • C and a Na/P molar ratio of 2/1 with 99% of selectivity for P 3 O 9 [START_REF] Sidney | [END_REF]-anion (Table 4). ( * ): peak positions of the NaCl diffraction in 2θ range from 15 to 50 2 IR spectra of (STM): pure STM; (Syn1): solid product formed at 600 • C and a Na/P molar ratio of 1/1 with 98% chloride elimination rate and 94% of selectivity for the P 3 O 9

3-anion (Table 3); (Syn2): solid product formed at 400 • C and a Na/P molar ratio of 2/1 with 99% of selectivity for the P 3 O 9

3-anion (Table 4).

of pure SCTP. Both products, Syn1 and Syn2, did not contain Na 2 HP 3 O 9 as byproduct. XRD patterns of the product Syn1 and pure SCTP are very similar. NaCl was practically not detected in Syn1 according to the almost complete elimination of chlorine (98%). For the product Syn2, all peaks could be attributed to SCTP and NaCl. In fact, half of the NaCl remained in Syn2 because of the excess of NaCl in the synthesis using the Na/P molar ratio of 2/1. IR Analysis. Figure 2 compares the IR spectra of pure SCTP with the spectra of the two solid products Syn1 and Syn2.

Both solid products synthesized showed IR spectra similar to that of pure SCTP, and display characteristic bands in the range of 650-1400 cm -1 . 15--17 No other bands were observed in the range 1400-4000 cm -1 . Note that the remaining NaCl does not show an IR signal. These results confirm again that the SCTP synthesized is well crystallized.

CONCLUSIONS

A new route for the direct synthesis of SCTP from sodium chloride and orthophosphoric acid as economical starting materials was developed. The reaction temperature, heating rate, air flow rate, reaction plateau time, and Na/P molar ratio were found to be important parameters for the elimination of chloride and the selectivity with respect to SCTP. Complete elimination of chloride and high yield of SCTP could be obtained under the following conditions:

• Moderate heating rate of 5 • C/min and reaction temperature of 600 • C.

• Air flow rate of at least 60 L/h permitting an effective removal of the gaseous products.

• Reaction plateau time of about 120 min.

• Na/P molar ratio of 1/1 or 2/1.

Starting from a Na/P molar ratio of 1/1, the best result was obtained with 98% of chloride elimination and 94% of selectivity for SCTP (reaction conditions: temperature of 600 • C, heating rate of 5 • C/min, air flow rate of 120 L/h, and reaction plateau time of 120 min). Increase of the molar ratio Na/P to 2/1 results in a complete transformation of the initial orthophosphoric acid into SCTP at only 400 • C. A mixture of pure SCTP and NaCl was obtained. These results are technically and economically very interesting in comparison with the actual competing industrial process for the fabrication of SCTP. Hydrochloric acid was the only product in gas phase, which could be condensed and recovered in liquid form as pure chemical product.

Future investigations will focus on the purification of the solid product to obtain pure SCTP, for example, by recrystallization. The reactor engineering aspects of the process should be also considered, for example, a reactor equipped with a stirring system, for the improvement of the performance of the process.

EXPERIMENTAL

Fine powder of sodium chloride from Acros Organics (99.5%) with a volume-mean diameter of 375 µm and phosphoric acid (85 wt% pure grade) from Merck were used as starting reactants. Pure SCTP (99%) was purchased from Sigma-Aldrich. The experimental setup was described elsewhere. 13 For the reaction, the quartz reactor was loaded with sodium chloride powder and 85 wt% orthophosphoric acid at room temperature. The reactor was then heated to the reaction temperature and kept at this temperature between 5 min and 24 h. According to our previous work, all experiments were carried out at a temperature lower than 625 • C to avoid the transformation of SCTP in glass amorphous products. 13 After this plateau time, the reactor was freely cooled down to room temperature. During the reaction, an air circulation at constant flow rate was applied to remove the gaseous products formed. The gaseous products were trapped into a wash bottle containing an aqueous solution of potassium hydroxide.

The solid product was crushed into fine powder prior to all analyses and characterizations. ICP-AES (inductive coupled plasma atomic emission spectroscopy) analysis was carried out with a HORIBA Jobin Yvon Ultima 2 instrument for quantification of sodium and phosphorus. Chloride (Cl -), orthophosphate (PO 4 3-), pyrophosphate (P 2 O 7 4-), and cyclotriphosphate (P 3 O 9 3-) anions were analyzed with a Dionex ionic chromatography apparatus equipped with a Dionex IonPac AS 19 (4 × 250 mm) column, a Dionex P/N 061830 conductivity detector, and a Dionex ICS-3000 EG eluant generator. Ultra-pure water was used as mobile phase, eluted at a rate of 1 mL/min.

From the results of ionic chromatography analysis, the selectivity for the P 3 O 9 3anion (S P3O9 ) was calculated as follows: S P3O9 = 100 × n P P3O9 /n P i where n P P3O9 was the quantity (mole) of phosphorus converted in P 3 O 9 3-anion and n P i was the initial quantity (mole) of phosphorus introduced in the reactor.

IR (infra-red) spectra were measured with a Shimadzu FTIR 8400S spectrometer using a sensitive pyroelectric detector with an L-alanine-doped deuterated triglycine sulfate (DLATGS) element. Solid powder samples were directly measured in the wave number range 500-4000 cm -1 . XRD (X-ray diffraction) measurements were carried out using a Phillips Panalytical X'pert Pro MPD diffractometer with a Cu Kα (1.543 Å) radiation source.

(→

  85 wt%) H 3 PO 4 (liquid) + NaCl(solid) T Na a H b P c O d (solid) + HCl(gas) + H 2 O(gas) (1) where Na a H b P c O d is the solid product of the reaction.

Figure 1 (

 1 Figure1(SCTP): XRD pattern of pure SCTP. (Syn1): XRD pattern of the solid product formed at 600 • C and a Na/P molar ratio of 1/1 with 98% chloride elimination rate and 94% of selectivity for the P 3 O 93-anion (Table3). (Syn2): XRD pattern of the solid product formed at 400 • C and a Na/P molar ratio of 2/1 with 99% of selectivity for P 3 O 9[START_REF] Sidney | [END_REF]-anion (Table4). ( * ): peak positions of the NaCl diffraction in 2θ range from 15 to 50 • .

  •H 2 O, Mn 3 (P 3 O 9 ) 2 •10 H 2 O, Ca 3 (P 3 O 9 ) 2 •10 H 2 O, Ba 3 (P 3 O 9 ) 2 •4 H 2 O, Tl 3 P 3 O 9 , Rb 3 P 3 O 9 •H 2 O, Na 2 LiP 3 O 9 , Na 2 CsP 3 O 9 •2 H 2 O, Ln(Bi)Na 3 P 3 O 9 •9 H 2 O, etc.

Table 1

 1 Influence of the heating rate on the distribution of chloride and phosphorus species in the final solid product. S P3O9 : selectivity with respect to P 3 O 9 3-anion. Reaction conditions: initial quantity of NaCl and 85 wt% orthophosphoric acid: 87.5 mmol, reaction plateau time: 30 min, air flow rate: 120 L/h

	T ( • C)	Heating rate ( • C/min)	Cl -(mmol)	PO 3-4 (mmol)	P 2 O 4-7 (mmol)	P 3 O 3-9 (mmol)	S P3O9 (%)
	300	2	26	27	8	3	10
	300	5	23	27	7	4	15
	300	10	24	17	9	1	4
	300	20	23	18	9	1	4
	400	2	7	2	1	22	76
	400	5	10	1	1	23	79
	400	10	11	2	1	23	78
	400	20	12	3	2	17	60
	500	2	8	1	trace	23	79
	500	5	9	trace	2	23	80
	500	10	9	2	1	23	79
	500	20	9	1	1	23	79
	600	5	3	0	trace	24	84
	time of 140 min and 70 min, respectively, to reach 300 • C. So orthophosphate species and intermediates had more time to be converted into P 3 O 9 3-.
		At 400 • C, chloride elimination was more effective at 2 • C/min than at other heating
	rates. Similar selectivity for P 3 O 9

• C/min and 5 • C/min required a

Table 2

 2 Influence of the air flow rate on the distribution of chloride and phosphorus species in the final solid product. S P3O9 : selectivity for P 3 O 9 3-anion. Reaction conditions: temperature: 350 • C, initial NaCl and 85 wt% orthophosphoric acid: 87.5 mmol, reaction plateau time: 30 min

	Air flow rate (L/h)	Cl -(mmol)	PO 4	3-(mmol)	P 2 O 7	4-(mmol)	P 3 O 9	3-(mmol)	S P3O9 (%)
	30	10		12		3		11	39
	60	10		8		3		22	77
	120	10		4		2		23	79
	240	10		1		1		23	80

Table 3

 3 Influence of the reaction plateau time on the distribution of chloride and phosphorus species in the final solid product. S P3O9 : selectivity for P 3 O 9 3-anion. Reaction conditions: initial quantity of NaCl and 85 wt% orthophosphoric acid: 87.5 mmol, air flow rate: 120 L/h, reaction temperature: 300 • C

	Reaction plateau	Cl -	PO 4	3-	P 2 O 7	4-	P 3 O 9	3-	S P3O9
	time (min)	(mmol)	(mmol)	(mmol)	(mmol)	(%)
	5	2 3	28	8		4		15
	30	23	27	7		4		15
	120	21	4	2		23		79
	240	17	1	1		23		79
	480	15	1	trace	26		88
	1440	15	0	trace	19		65
	120 *	1	0	0		28		94

Table 4

 4 Influence of the Na/P molar ratio on the distribution of chloride and phosphorus species in the final solid product. S P3O9 : selectivity in P 3 O 9 3-anion. Reaction conditions: air flow rate: 120 L/h, reaction plateau time: 30 min, H 3 PO 4 (i) : initial quantity of orthophosphoric acid, NaCl(i) : initial quantity of NaCl

	H 3 PO 4	(i)	NaCl (i)	Na/P	T	Cl -	PO 4	3-	P 2 O 7	4-	P 3 O 9	3-	S P3O9
	(mmol)	(mmol)	molar ratio	( • C)	(mmol)	(mmol)	(mmol)	(mmol)	(%)
	87.5		175	1/2	300	1	12	11		2		7
					400	trace	3	1		2		6
	87.5		87.5	1/1	300	23	27	7		4		15
					400	10	1	1		23		79
	175		87.5	2/1	300	101	5	5		8		29
					400	86	1	trace	29		99

* Reaction temperature: 600 • C; air flow rate: 120 L/h, and reaction plateau time: 120 min.