Ping He

Kok-Giap Haw

Shichen Yan

Lingxue Tang

Qianrong Fang

Shilun Qiu

Valentin Valtchev
email: valentin.valtchev@ensicaen.fr

Carbon beads with well-defined pore structure derived from ionexchange resin beads

Carbon beads were prepared by using two types, a microporous and a macroporous, of cation exchange resin beads as precursors. The beads were subjected to potassium exchange prior to the carbonization reaction at high temperature. The physicochemical properties of the obtained carbon beads were thoroughly studied to evaluate the impact of all key parameters: i) the type of cation exchange resin; ii) the concentration of K + in the resin beads, and iii) the carbonization temperature. The beads prepared from macroporous resin showed narrow pore size distribution, higher specific surface area and retained a spherical shape. These beads showed impressive CO2 adsorption capacity at ambient temperature and variable pressure and excellent performance in the decontamination of water from medical pollutant. The method of preparation can be easily scaled up, and thus prepared carbon beads can be used for other adsorption applications where shaped bodies with high specific surface area and large pore volume are needed.

2

Introduction

The environmental deterioration, mainly due to global warming and waste discharge, is the highest priority issue for the modern society. [1][2][3] Global warming is namely due to the CO 2 release in the atmosphere. On the other side, the discharge of wastewater from the agriculture and medical industries pollute the surface and underground waters with antibiotics and other harmful chemicals which threaten animals, plants and human health through direct or indirect exposure. One of the most common antibiotic present in environmental water bodies is Oxytetracycline. 4 To overcome these environmental crises, constant efforts are targeted in the development of effective and sustainable methods to sequestrate the chemical contaminants and CO 2 worldwide. Among them, the selective adsorption has been proven as an efficient and versatile method to purify gaseous and liquid phases. Through the years, a large variety of porous materials, offering large specific surface area and particular chemical selectivity, have been studied as adsorbents. Zeolites, 5-7 ordered mesoporous silica, 8, 9 porous carbonaceous adsorbents 10, 11 and metal-organic frameworks (MOFs) 12, 13 are amongst the most widely investigated ones.

Porous carbonaceous adsorbents, due to their unique properties such as very high specific surface area, high hydrophobicity, variable pore structure, and availability, showed great potential for long-term industrial application. [14][15][16][17][18][19] However, carbonaceous adsorbent like activated carbon in fine powder form could lead to operational concerns like secondary pollution 20 and high pressure drop 21 which limits its application. Therefore, carbon materials with controlled macroscopic shape, for instance, beads, are highly desired since they can meet the requirements of most industrial applications. [22][23][24][25][26][27] Activated carbon beads can find different applications, for instance, can be used for CO 2 uptake in swing adsorption experiments.

28, 29 Tagliavini et al. studied the mechanisms of steroid micropollutants removal with polymerbased spherical activated carbon; 2 and Zeng et al. conducted water evaporation and oil sorption on hydrophobic hollow carbon beads by phase inversion method. 30, 31 Wickramaratne et al. reported the importance of micropore size in activated carbon for the efficient CO 2 capturing under ambient conditions. 32 Various kind of functionalized activated carbon beads with the goal to target specific application have been reported in the literature. [33][34][35][36] Preparation methods such as steam, 37 CO 2 , 28, 38 ZnCl 2 39 and KOH activations 32, 40 have been intensively studied and optimized to prepare carbon materials with high surface areas and appropriate pore structure for selective removal of target contaminant substances. However, the complicated post-synthesis modification procedures are time and cost consuming which limit the large-scale production of carbon spheres commercially. Consequently, much attention was paid to simplify the preparation of activated carbon. Singh et al. reported a single-step activation by mixing of an activating agent (solid KOH and ZnCl 2 ) with biomass. The prepared activated biocarbon featured high BET surface area and decent microporosity, but lack of welldefined morphology and particle size.

41, 42 At present, there is no well-established synthetic strategy to prepare carbon beads with desired particle size and uniform pore size distribution. Therefore, a novel and efficient strategy to fabricate efficiently porous carbon spheres is highly desirable. An avenue that could be explored is the use of shaped polymer beads as a

The objective of the present study is the preparation of carbon beads with a well-defined spherical shape and microporous structure which can be potentially used as an adsorbent for efficient CO 2 and medical pollutants capturing. We have selected millimeter-sized cation exchange resin bead as starting materials, justified by its proper particle size and morphology and also the possibility to select initial beads with different pore sizes, either micro-or macroporous. The pore structure of cation exchange resin beads offers the advantage to distribute the activator uniformly, in our case potassium, and thus to ensure a very uniform carbonization reaction. Both microporous gel-type (GCB) and macroporous (MSC) resin beads were subjected to ion exchange with KCl solutions of different concentration and carbonized at 700 to 1000 °C in order to optimize the pore structure of the resultant carbon beads. Carbon spheres with defined shape and size, high specific surface area and uniform micro-and mesopores network were obtained. These macroscopic carbon spheres showed excellent CO 2 adsorption capacity and great ability of medical pollutant removal from aqueous solutions.

Experimental Materials

All reagents were obtained from commercial sources and used without further purification. Gel-type microporous cation exchange resin beads (Dowex® 50WX2, H-form) and macroporous cation exchange resin beads (Dowex® MSC, Hform) were purchased from Aldrich Corp. KCl (99%), HCl (37%) and oxytetracycline dihydrate (OTC, 99%) were purchased from Shanghai Chemical Corp. (China). A Milli-Q integral ultrapure water system was employed to produce deionized (DI) water, which was used in all experiments.

Potassium exchange of resin beads

2.0 g raw resin beads were added into 80.0 mL KCl aqueous solution and heated under stirring for 6 h at 60 °C, and the exchange process were repeated three times with fresh Kcontaining solution. The concentrations of the KCl solutions were 0.1 M, 0.3 M, 0.5 M and 1.0 M, respectively. Then, the Kexchanged resin beads were separated by suction filtration, rinsed with water and dried overnight in an oven at 60 °C. Carbonization 0.1 g K-exchanged resin beads were placed on a ceramic boat and transferred into a horizontal quartz tube reactor. Porous carbon beads were prepared by one-pot carbonization and activation at 700, 800, 900 and 1000 °C for 2 h at a ramp rate of 3°C min -1 under an ultrahigh pure N 2 (99.999 %) with a flow rate of 50 mL min -1 . The obtained carbon beads were thoroughly washed with 1 M HCl in an ultrasonic bath to completely remove the residues followed by washing with DI water several times till the pH value reaches 7. The porous carbon beads were dried overnight in an oven at 60 °C. Porous carbon beads prepared from microporous gel-type Dowex® 50WX2 and macroporous Dowex® MSC resin beads were labelled as GCB-m-t and MSC-m-t respectively, where m is the concentration of KCl solution (in mol L -1 , M) and t is the carbonization temperature in °C. For comparison purpose, two control samples were prepared from Dowex® 50WX2 and Dowex® MSC resin beads at 900°C without preliminary KCl exchange and labelled as GCB-0-900 and MSC-0-900, respectively.

Characterization

Powder X-ray diffraction (PXRD) analysis was performed on a PANalytical B.V. Empyrean powder diffractometer at 40 kV and 40 mA using Cu Kα radiation (λ = 1.5418 Å) over the range of 2θ = 10.0 -80.0°. Confocal Raman spectra were measured using a RENISHAW invia Raman Microscope equipped with an argon-ion laser (514.5 nm). A JEOL JSM-6510 scanning electron microscope was used to study the morphology of the samples. Transmission electron microscopy (TEM) analysis was carried out on a JEM-2100 microscope. Before PXRD, TEM and Raman analysis, the microspheres were ground into powder in an agate mortar. CO 2 adsorption was performed on a Quantachrome Autosorb-IQ analyzer with ultra-high-purity grade CO 2 (99.999 %) at 273 K and 298 K. High-pressure CO 2 adsorption was performed on a home-built HPSA-2 instrument made by Shanghai University. Nitrogen adsorption analysis was performed on a Micromeritics ASAP 2460 volumetric adsorption analyzers with ultra-high-purity grade N 2 (99.998 %) at 77 K. Prior to the analysis, the samples were outgassed at 300 °C for 12 h. The specific surface area of the carbon beads was calculated using Brunauer-Emmett-Teller (BET) method within the relative pressure (P/P 0 ) range of 0.05-0.25. 43 The total pore volume (V total ) was calculated from the amount of nitrogen adsorbed at P/P 0 of ca. 0.99. The micropore volume was determined by the t-plot method. The pore size distributions were calculated using density functional theory (DFT) and Barrett-Joyner-Halenda theory (BJH) method, respectively.

44, 45 A SHIMADZU UV-2450 spectrophotometer was used to analyze the concentration of OTC during adsorption tests.

Results and discussion

Structure and morphology analysis

Cation exchange resin beads with gel-type microporous (GCB) and macroporous (MSC) structures were chosen as precursors. The building units in these two types of resin are similar (Scheme S1), however their structural organization is different. MSC resin exhibits large pores and highly accessible surface, which facilitates the removal of charged species from different solvents. In the case of the GCB resin, since it has no discrete pores, only solute ions could diffuse through the gel network and interact with the exchange sites. It is expected that Please do not adjust margins Please do not adjust margins different structural organization would lead to different diffusion paths during the carbonization process and thus carbon beads with different pore structure could be obtained.

In both ion exchange resins, sulfonic functional groups (Scheme S1) are present as cation exchange sites. Carbonization of polymer precursors using KOH activation has been extensively studied in the past. 24, 46, 47 In the present study we implemented KCl as the activation agent. The activation efficiency of KCl is similar to the conventionally used KOH. On the other side, the KCl offers the advantage to be moderate, neutral and thus environmentally benign. During the ion exchange, the proton of the sulfonic acid functional groups was exchanged to K + . After the exchange, the potassium is uniformly distributed in the resin network as can be seen in Fig. S1. The potassium content is almost equal to the sulfur content which confirms the homogeneous distribution of potassium ions (Fig. S2a). In the following carbonization step, K + cations play the role of activator which leads to oxidation of carbon and decomposes the hydrocarbons into H 2 , CO, CO 2 and H 2 O. The generate gas diffuse within the carbon matrix and contribute to porosity development. 41, 48 The chemical and thermal activation happens simultaneously maximizing the efficiency of the reaction and the development of porosity. During the conversion of the ion exchange into carbon beads, a part of the organic skeleton is lost. Nevertheless, a substantial part of the carbon is preserved since our measurements showed conversion between 30 and 50 %, as the lower yield was in the case of GCB-0.3-1000 (30 %) and the higher yield in the case of MSC-0.1-1000 (51%). Fig. 1a shows the typical XRD patterns of the activated carbon beads obtained under different conditions. In general, the XRD patterns exhibit two broad peaks at about 24.0° and 44.0°, characteristic of non-graphitic carbon, which is in agreement with the Raman data (Fig. 1b and1c). Raman spectra showed two well defined peaks at 1360 and 1585 cm -1 for all studied samples (Fig. 1b, 1c and Fig. S3). These bands correlate well with the disordered (D-band) and ordered (G-band) crystal structures of graphite, respectively. 49 The intensity ratio (I D /I G ) of these bands is almost similar (around 0.96-1.06) in all samples and reflect the low degree of graphitization for carbon beads. The Raman study also revealed that the level of graphitization does not correlate with the concentration of KCl (Fig. 1b) and the carbonization temperature (Fig. 1c).

In general, the carbon beads retained the spherical shape of the precursor, but the level of integrity of obtained carbon beads and the surface porosity strongly depend on the type of cation exchange resin employed and the K + content. Fig. S4 shows that the carbon beads obtained from GCB resin are broken with a rough surface and large macropores, as the central part of the beads is often missing. This result points out the violence of the reaction in the volume of the gel type resin beads, where no well define pore are present. The high pressure generated in the volume of the beads results in their disintegration, as shell parts with hemi-spherical morphology remain. The core part is missing, which is probably due to the disintegration into very small pieces. No substantial difference was observed between the beads activated at different temperatures (Fig. S4a-d). It should be noted that K + obviously promote the carbonization reaction. This result reveals that the gel-type resin is not an appropriate precursor for preparation of carbon beads since the absence of pores obstructs the release of generated gases (H 2 , H 2 O steam, CO and CO 2 ) during carbonization reaction. Fig. 2 shows representative SEM micrographs of carbon beads obtained from MSC resin beads after 0.1 M KCl exchange and carbonization at different temperatures. Different to the ones derived from GCB resin beads, these carbon beads exhibited well-defined spherical morphology (Fig. 2a,c,e,g). Small craters can be observed on the surface, which is due to the gas release during carbonization (Fig. 2b,d,f,h). The retained morphology of the carbon beads is obviously due to the presence of large well-defined pores which ensure the efficient release of generated gases. The presence of these pores in the carbon beads is also essential. for the envisaged sorption applications. 17, 32 The reference sample retained the perfect spherical shape and smooth external surface of initial ion exchange beads (Fig. 2i). As mentioned, the K + plays essential role in the carbonization reaction, which is further demonstrated by the results obtained after potassium exchange with solutions of higher concentration

The carbon beads obtained after an exchange with 0.3 M, 0.5 M and 1.0 M KCl showed larger surface pores (Fig. 3). The beads obtained after an exchange with 0.5M and 1.0 M KCl are often elongated, thus the morphology changes from spherical to elliptical. Again, the pressure generated in the volume of the beads is the reason for these morphological changes. Although the macroporous beads contain large pores, a considerable amount of gas generated by significant amount of potassiumthat cannot be evacuated immediately and lead to the described morphological changes. This result shows the possibility to tune the morphology and the porosity (vide supra) of the carbon beads by the potassium content. Elemental analysis of the MSC-0.3-900 sample was performed by using EDS analyses operated on SEM instrument. As shown in Fig. 4 and Fig. S2b, carbon, and oxygen are the major elements in these porous carbons, indicating the complete removal of the inorganic parts (K) after washing with HCl. Morover, a certain amount of sulfur uniformly distributed in the beads remains in the carbon network. These sulfur functionalities may act as binding sites for adsorption as reported in the literature. [50][51][52][53] The high resolution TEM image of MSC-0.3-900 (Fig. 5) reveals the amorphous structure of the carbon comprising 2-4 nm long graphitic layers.

Porosity analysis

The N 2 adsorption-desorption isotherms of the obtained activated carbon beads are presented in Fig. 6. All samples show rapid uptake at low relative pressure, typical of microporous type material, followed by horizontal adsorption/desorption branches. The isotherms are a combination of type I and IV according to the IUPAC classification. The hysteresis loop is characteristic of slit-like textural pores.

The data calculated from the analysis of the isotherms are summarized in Table 1. The obtained data reveals the impact of all variables, type of resin, the concentration of KCl solution and carbonization temperature, on the characteristics of carbon beads. Without the preliminary exchange of potassium, carbon beads with very low specific surface area and pore volume are obtained. The importance of KCl on the carbonization reaction was discussed already. However, the N 2 adsorption data provide additional information about the impact of KCl concentration during the ion exchange. In general, an increase in the concentration of KCl leads to carbon beads with higher specific surface area and pore volume. The best results are obtained from 0.3 M KCl Please do not adjust margins Please do not adjust margins exchange resin beads with both gel-type and macroporous configuration. The increase of carbonization temperature from 700 to 900 °C led to the increase in BET surface area, total and micropore pore volume of the carbon beads (Table 1). surface area. The optimum conditions for the preparation of the activated carbon beads was ion exchange with 0.3 M KCl solution and carbonization at 900 °C. Among all the samples, GCB-0.3-900 and MSC-0.3-900 showed the highest BET surface area (909 m 2 g -1 and 1398 m 2 g -1 ), microporous volume (0.41 cm 3 g -1 and 0.59 cm 3 g -1

), and total pore volume (0.49 cm 3 g -1 and 0.69 cm 3 g -1 ), respectively. The DFT pore size distribution analysis showed micropore width with sizes majorly below 1.5 nm with a maxium at about 0.8 nm (Fig. S5). Small mesopores (2-4 nm) with narrow pore size distribution were detected in the carbon beads derived from the GCB resin, while almost no mesopores were detected in the carbon beads derived from macroporous resin beads (Fig. 7 and Fig. S6).

CO 2 adsorption

CO 2 adsorption experiments were performed on the best, in terms of specific surface area and micropore volume, carbon samples. The CO 2 adsorption capacities of the selected carbon beads were investigated at 273 and 298 K under atmospheric pressure (1 bar) as shown in Fig. 8a,b. MSC-0.3-900, the sample with the highest surface area and pore volume, displayed the highest CO 2 uptake of 6.7 and 4.7 mmol g -1 at 273 K and 298 K, respectively. Micropores with size between 0.8 nm and 1.3 nm are preferable for CO 2 capture, owing to the stronger interactions between CO 2 molecules and pore walls.

23, 54-59 In the case of carbon obtained from ion exchange resin we anticipate an additional beneficial effect due to the presence of acidic groups generated by the remaining sulfur. 51, 53 The analysis of the adsorption results shows a correlation between the CO 2 capacity (Fig. 8a,b) and the micropore volume of carbon beads (Table 1). The best result was obtained with MSC resin beads pre-treated with 0.3 M KCl and carbonized at 900°C (Fig. 8a,b). It should be noted that the CO 2 adsorption capacity of MSC-derived carbons is substantially higher than their counterparts obtained from GCB resin (Fig. 8a,b). Thus, the MSC-derived carbon beads with well-defined micropores proved to be an excellent adsorbent for CO 2 capture. Please do not adjust margins Please do not adjust margins Adsorbents that can efficiently adsorb CO 2 at high pressure might be appropriate for high-density gas storage application. 60 Therefore, we studied the CO 2 sorption capacity at high pressure condition ranging from 1 bar to 26 bars at 298 K (Fig. 9). Ion exchange resin derived carbon beads shows impressive CO 2 uptake at high pressure (26 bar), as the highest uptake at 298 K (14.7 mmol g -1

) of MSC-0.3-900 is comparable with that of recently published carbon materials obtained by KOH activation.

41, 42, 60, 61 GCB-0.3-900 derived from the geltype resin beads showed lower CO 2 storage (Fig. 8), which is in accordance with its inferior pore structure (Table 1). The uptake mechanism at high pressure is more reliant to the available surface area and pore volume.

56, 61-63 Hence, the welldefined pore structure of carbon beads, with a remarkable proportion of micropores as well as the high surface area is more appropriate for CO 2 capture and storage, no matter of the conditions employed.

The isosteric heat of adsorption (Q st ) is an important parameter providing information for the thermal performance of an adsorbent (Table 2). For the series of carbon beads subjected to CO 2 adsorption (GCB-0.3-900, MSC-0.3-900, MSC-0.5-900, MSC-1.0-900), the Q st was calculated using the Clausius-Clapeyron relation at 273 K and 298 K (Fig. S7). The value of Q st decreases with the adsorbed amount, which is characteristic of adsorbent-adsorbate systems based on weak Van der Waals interactions. The obtained result also reveals the heterogeneous nature of the adsorbing surface. The initial Q st of the carbon beads ranges from 28.0 kJ mol -1 to 30 kJ mol -1 , which is within the characteristic range of activated carbon materials. 61 The difference between these two values is mainly due to the different strength of interaction between specific contact surfaces in the micropores and CO 2 molecule.

Oxytetracycline adsorption from aqueous solutions

We studied the OTC adsorption ability of the activated carbon beads from aqueous solutions with concentrations ranging from 20 to 800 mg L -1

. Each experiment was performed three times to verify the reproducibility of the results. Fig. 10 shows the OTC adsorption isotherms for the series GCB-0.3-900, MSC-0.3-900, MSC-0.5-900 and MSC-1.0-900 beads. The adsorption capacities increase with the incremental in OTC concentration. We interpret this result as a proof of the high capacity of the carbon beads combined with the good accessibility to the pore volume which enables fast transport and rapid uptake. All activated carbon beads exhibited remarkable adsorption capacity. For instance, the adsorption capacities with a solution of 800 mg L -1 were 320.1, 406.6, 382.0 and 386.0 mg g -1 for GCB-0.3-900, MSC-0.3-900, MSC-0.5-900 and MSC-1.0-900 beads, respectively. Among them, GCB-0.3-900 activated carbon beads showed the lowest uptake, attributed to its lower specific surface area and micropore volume. These values were comparable to the maximum adsorption capacity calculated from the Langmuir model (Table 3). It is worth noting that the differences between the carbon beads obtained from MSC-derived resins (MSC-0.3-900, MSC-0.5-900 and MSC-1.0-900) are not substantial. In the case of OTC adsorption, the available specific surface area is the key factor controlling the OTC adsorption, in contrast to the CO 2 adsorption where the type of pores is a crucial issue. It is noteworthy that sulfur resided in the pore structure increases the surface chemical polarity, thus, favoring adsorption of organic polar pollutants. In order to shed more light on the relationship between the adsorption capacity and the equilibrium concentration in water, the OTC adsorption isotherms were further analyzed by Langmuir and Freundlich models, following the method reported previously. 64 The linear regressions of the fitting results are shown in Fig. S8 and the calculated parameters of these models are listed in Table 3. Among these models, the Langmuir model (R 2 > 0.97) is the best-fitted one. The collected data points out that the adsorption of OTC molecules on activated carbon beads takes place as monolayer adsorption and there is a homogeneous interaction between the surfaces of the activated carbon beads and the OTC. Table 3 Langmuir and Freundlich isotherm constants for the OTC adsorption on activated carbon beads at 25 ℃

Conclusions

Activated carbon beads were prepared by employing two types, gel-type microporous (GCB) and macroporous (MSC), cation exchange resin beads as precursors. The precursor beads were first exchanged to potassium which serves as an activator of the carbonization reaction in the following high temperature step. The potassium efficiently generates pores upon carbonization and the final carbon structure strongly depends on the concentration of potassium and the type of resin beads used. After the activation, the K + can be completely removed by acid washing. The best results in term of high specific surface area, large micropore volume with narrow pore size distributions and retained spherical morphology were obtained with MSC resin beads exchanged with 0.3 M KCl solution and carbonized at 900 °C. The obtained activated carbon beads exhibit excellent CO 2 adsorption capacities at both ambient and elevated pressures. The excellent CO 2 adsorption ability is attributed to the appropriate type of micropores (ca. 1 nm) and large available sorption surface. The obtained results are amongst the best published in the open literature. The carbon beads also showed the excellent capability of medical pollutant sequestration from aqueous solutions. The application of shaped activated carbon beads produced according to the present method can certainly be extended to other adsorption applications where shaped bodies with high specific surface area, sulfur-containing functional surface and large pore volume are needed. This simple and efficient method for producing shaped carbon materials with excellent adsorption capabilities can be easily scaled up. This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins

Fig. 1 Fig. 2

 12 Fig. 1 (a) PXRD patterns of GCB-0.3-900, GCB-0.5-900, MSC-0.3-900 and MSC-0.5-900 activated carbon beads. Raman spectra of: (b) GCB-0.3-900, MSC-0.3-900, MSC-0.5-900, and MSC-1.0-900 activated carbon beads, and (c) MSC-0.3-900 and MSC-0.3-1000 activated carbon beads.

Fig. 3

 3 Fig. 3 Low and high magnification SEM micrographs of carbon beads obtained from MSC resin after potassium exchange with solut ions of different concentration: (a, b) MSC-0.3-900, (c, d) MSC-0.5-900 and (e, f) MSC-1.0-900.

Fig. 4

 4 Fig. 4 EDS mapping MSC-0.3-900 carbon beads: (a) general view and (b) cross-section view.

Fig. 5

 5 Fig. 5 High resolution TEM micrograph of the MSC-0.3-900 carbon beads.

Fig. 6

 6 Fig. 6 Nitrogen adsorption-desorption isotherms of GCB (a, b) and MSC (c, d) activated carbon beads obtained after exchanged with KCl of different concentration and carbonized at different temperatures.

Fig. 7 Fig. 8

 78 Fig. 7 BJH pore size distributions of (a) GCB and (b) MSC activated carbon beads.

Fig. 9

 9 Fig. 9 CO2 adsorption isotherms of activated carbon beads at 298 K and variable pressures.

Fig. 10

 10 Fig. 10 Adsorption behavior of activated carbon beads in decontamination of OTC from aqueous solutions with different concentrations (adsorption time: 12 h, adsorption temperature: 25 °C; C0 = 20-800 mg L -1 ;Dose of adsorbents: 1 g L -1 ).

17

 17

, 18

 18

65

 65

Table 1

 1 Surface and porous characteristics of the GCB and MSC activated carbon beads

	Sample	S BET (m	2 g -1 ) V total (cm	3 g -1 ) V micro (cm	3 g -1 )
	GCB-0-900	36	0.06	0.02
	GCB-0.1-700	693	0.35	0.31
	GCB-0.1-800	759	0.39	0.33
	GCB-0.1-900	901	0.46	0.38
	GCB-0.1-1000 832	0.41	0.33
	GCB-0.3-900	909	0.49	0.41
	GCB-0.5-900	856	0.43	0.37
	GCB-1.0-900	720	0.38	0.31
	MSC-0-900	133	0.07	0.06
	MSC-0.1-700	803	0.40	0.38
	MSC-0.1-800	852	0.46	0.39
	MSC-0.1-900	921	0.45	0.43
	MSC-0.1-1000 918	0.45	0.42
	MSC-0.3-900	1398	0.69	0.59
	MSC-0.5-900	1229	0.63	0.56
	MSC-1.0-900	1220	0.62	0.52

Table 2

 2 Comparison of CO2 adsorption capacities and isosteric heat of adsorption (Qst) for a series of adsorbents reported in the literature and the best material obtained in the present study

	Materials		S BET	Q st		Pressure	Temperature	Capacity	Reference
					(m	2 g -1 )	(kJ mol	-1 )	(bar)	(K)	(mmol g -1 )
	ITQ-6	a			580.4	22-26		1	293	1.2	64
	SBA-15			882	21-26		1	293	1	64
	KBM-700	a	1241	-		1	273	4.5	65
	PR3-700	b	826	33.3		1	273	4.6	1
	PAF-450	c		1191	27.8		1	273	4.5	66
	KLB2	d			1122	31		1	273	6.3	41
	MSC-0.3-900	1398	28		1	273	6.7	Present work

a Nitrogen-doped porous carbons; b Activated carbon beads; c Porous aromatic framework; d Activated bio-carbons Please do not adjust margins Please do not adjust margins

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 9

Acknowledgements

Q. F. and V. V. thank the National Natural Science Foundation of China (21571079, 21621001, 21390394, 21571076 and 21571078) for their financial support. V.V. and Q.F. acknowledge the support from the Thousand Talents Program (China).

Conflicts of interest

There are no conflicts to declare.

Langmuir Freundlich

Adsorbents qmax (mg g -1 ) KL (L mg -1 ) RL 2 KF (mg g -1 )

(L mg -1 )


Einar Knudsen
email: einar.knudsen@toulouse-inp.fr

Julien Perchoux

Thierry Mazoyer

José Imas

Mengkoung Veng

Francis Jayat

Clément Tronche

Thierry Bosch

Experimental Demonstration of the Impact of the Fringe Shape in Sub-Lambda / 2 Sensing With Optical Feedback Interferometry

It is established in the Optical Feedback Interferometry (OFI) theory that the shape of the interferometric fringe has an impact on the detector's response to very small displacement measurements. In this paper we validate -for the first time based on experimental results -this statement by comparing experiments to an established model implementation. Through these experiments we show that the amplitude of the signals induced by sub-lambda/2 optical path variations is linearly dependent on the slope of the underlying fringe. Thus, careful control of the phase allows us to maximize the detection amplitude of very small displacements by positioning the phase where the fringe slope is the steepest. These results are directly applicable to established OFI applications that measure sub-lambda/2 optical path variations, such as OFI vibrometers or acoustic imaging though the acousto-optic effect.

INTRODUCTION

Optical Feedback Interferometry (OFI) is a compact and versatile non-contact sensing scheme [START_REF] Donati | Developing self-mixing interferometry for instrumentation and measurements[END_REF][START_REF] Taimre | Laser feedback interferometry: a tutorial on the self-mixing effect for coherent sensing[END_REF] with numerous areas of application. It is used in displacement sensing [START_REF] Bes | Displacement measurements using a self-mixing laser diode under moderate feedback[END_REF][START_REF] Milesi | Measurement of local chest wall displacement by a custom self-mixing laser interferometer[END_REF][START_REF] Arriaga | Real-Time Algorithm for Versatile Displacement Sensors Based on Self-Mixing Interferometry[END_REF] including vibrometry [START_REF] Giuliani | Self-mixing laser diode vibrometer[END_REF][START_REF] Khan | Adaptive cancellation of parasitic vibrations affecting a self-mixing interferometric laser sensor[END_REF][START_REF] Donati | Self-Mixing Vibrometer has picometer sensitivity by exploiting the FM Channel[END_REF], absolute distance measurements [START_REF] Norgia | Absolute Distance Measurement With Improved Accuracy Using Laser Diode Self-Mixing Interferometry in a Closed Loop[END_REF][START_REF] Rovati | On the feasibility of absolute distance measurement by using optical-feedback into a superluminescent diode cavity[END_REF][START_REF] Veng | Fringe Disappearance in Self-Mixing Interferometry Laser Sensors: Model and Application to the Absolute Distance Measurement Scheme[END_REF], velocimetry [START_REF] Plantier | Real-time parametric estimation of velocity using optical feedback interferometry[END_REF][START_REF] Magnani | Real-Time Self-Mixing Interferometer for Long Distances[END_REF] and flowmetry [START_REF] Ozdemir | A Comparative Study for the Assessment on Blood Flow Measurement Using Self-Mixing Laser Speckle Interferometer[END_REF][START_REF] Norgia | Self-mixing laser Doppler: a model for extracorporeal blood flow measurement[END_REF][START_REF] Atashkhooei | Optical Feedback Flowmetry: Impact of Particle Concentration on the Signal Processing Method[END_REF], to mention a few. OFI is compact, robust, self-aligned and above all, it's need for very few optical elements compared to classical interferometric devices makes it a low cost sensing scheme. Recent advances [START_REF] Contreras | Edge filter enhanced self-mixing interferometry[END_REF][START_REF] Donati | Self-mixing interferometer with a laser diode: Unveiling the fm channel and its advantages respect to the am channel[END_REF][START_REF] Norgia | Exploiting the fm-signal in a laser-diode smi by means of a mach-zehnder filter[END_REF] have made the technology capable of measuring displacements in the range of 120 pm which is a major improvement of the OFI sensing scheme. This approach requires additional optical elements, and the increase of system complexity makes this approach cumbersome as it approaches the number of elements used in commercial interferometers today.

Nevertheless, the OFI sensing scheme gains importance in various sensing domains such as vibrometry or the very promising acoustic imaging through the acousto-optical effect. Bertling et al [START_REF] Bertling | Imaging of acoustic fields using optical feedback interferometry[END_REF] and Urgiles et al [START_REF] Ortiz | Visualization of an acoustic stationary wave by optical feedback interferometry[END_REF] created visual representations of sound waves in air using OFI. Their method is based on the sensing of the acousto-optic effect, where acoustic pressure variations induce modulations of the refractive index of air. These minute modulations create small changes in the optical path between the laser and the target. The variations are far below a half laser wavelength and at the edge of the limitations of the technology in it's classical compact form.

OFI operates the following way: Coherent light from the laser is backscattered from a target and returns back into the laser cavity. The returning light is coherently mixed with the lasing field. The mixing induces perturbations in the laser wave phase and amplitude, the latest which can be measured by the laser's in-package photodiode. An analysis of the signal provides information about the target it is pointing on. In particular, if the target is moving, interferometric fringes corresponding to a displacement of half the wavelength λ will be observed. By simply counting fringes we can record the target's movement along the beam axis.

Ameliorations were made to the OFI sensing scheme by Giuliani et al [START_REF] Giuliani | Self-mixing laser diode vibrometer[END_REF] to improve the lower limits of detection in vibrometry applications. They devised a non-contact vibrometer where the system was designed to maintain the laser phase at an operating point through an electronic feedback loop, a method called phase locking. In Giuliani's approach the operating point is set to the half fringe, as it allows a larger measurement dynamic. Measurements of down to 0.1 nm are reported. The advantage of this system was that there would be no drift in signal amplitude for a given measurement due to changes in optical conditions.

As explained by Taimre et al [START_REF] Taimre | Laser feedback interferometry: a tutorial on the self-mixing effect for coherent sensing[END_REF], for sub-λ/2 displacements, the target's movement is directly reproduced in the interferometric signal in an analog fashion. Lukashkin et al [START_REF] Lukashkin | A self-mixing laser-diode interferometer for measuring basilar membrane vibrations without opening the cochlea[END_REF], inspired by the approach in [START_REF] Giuliani | Self-mixing laser diode vibrometer[END_REF], regulated the mean distance between the laser and an oscillating target in order to fix an operating point around which the system would operate. This served to reduce distortions of the signal due to the fringe shape.

The shape of the fringe has an impact on measurements of sub-λ/2 optical path variations. A deeper knowledge of these effects would be beneficial for the further development of systems such as Giuliani's. The detection capacities of these types of OFI systems could be ameliorated by locking the phase at a point where the fringe slope is steeper than on the half-fringe point. This would increase the signal gain amplitude for a given excitation.

Experiments coupled with a comprehensive model has not yet been performed to explicitly show the impact of the fringe shape on the phase-to-power conversion for small displacements.

In this paper we present a dedicated, repeatable and robust experiment showing the impact of the fringe shape on small signals. We demonstrate experimentally that the signal amplitude is linearly coupled to the slope of the fringe, a result not yet explicitly published in the OFI literature. Furthermore we model an OFI system measuring sub-λ/2 optical path variations using Kliese's algorithm [START_REF] Kliese | Solving self-mixing equations for arbitrary feedback levels: a concise algorithm[END_REF]. Finally, we show the algorithm's performance in simulating sub-λ/2 optical path variations through accurately simulating the experimental results.

THEORY: OFI MODEL AND GOVERNING EQUATIONS

One of the most frequently used models for the OFI sensing scheme was made by Lang et Kobayashi [START_REF] Lang | External optical feedback effects on semiconductor injection laser properties[END_REF] using modified laser rate equations. The model was simplified by Petermann [START_REF] Petermann | Semiconductor Lasers with Optical Feedback[END_REF], supposing quasi static conditions, and later written into a concise algorithm in [START_REF] Kliese | Solving self-mixing equations for arbitrary feedback levels: a concise algorithm[END_REF]. The algorithm uses the Three Mirror Model [START_REF] Taimre | Laser feedback interferometry: a tutorial on the self-mixing effect for coherent sensing[END_REF][START_REF] Veng | Fringe Disappearance in Self-Mixing Interferometry Laser Sensors: Model and Application to the Absolute Distance Measurement Scheme[END_REF][START_REF] Petermann | Semiconductor Lasers with Optical Feedback[END_REF] as a base. This model is an extension of the two mirror laser model in which a phase condition is satisfied: the optical field in the laser has the same phase after one round trip [START_REF] Taimre | Laser feedback interferometry: a tutorial on the self-mixing effect for coherent sensing[END_REF][START_REF] Kliese | Solving self-mixing equations for arbitrary feedback levels: a concise algorithm[END_REF]. In the Three Mirror Model, the laser is forced to change its emission frequency ν 0 under feedback in order to satisfy the phase condition. The phase of the light returning into the laser is calculated through the excess phase equation [START_REF] Taimre | Laser feedback interferometry: a tutorial on the self-mixing effect for coherent sensing[END_REF][START_REF] Veng | Fringe Disappearance in Self-Mixing Interferometry Laser Sensors: Model and Application to the Absolute Distance Measurement Scheme[END_REF][START_REF] Kliese | Solving self-mixing equations for arbitrary feedback levels: a concise algorithm[END_REF].

φ F -φ 0 + C sin(φ F + arctan α) = 0 (1)
φ F = 2πντ ext is the phase response which is the external round- trip phase at the perturbed frequency. φ 0 = 2πν 0 τ ext is the phase stimulus which is the external round trip phase of the free running laser [START_REF] Taimre | Laser feedback interferometry: a tutorial on the self-mixing effect for coherent sensing[END_REF]. ν 0 is the laser's running emission frequency, ν is the perturbed laser frequency and α is the linewidth enhancement factor [START_REF] Henry | Theory of the linewidth of semiconductor lasers[END_REF]. The feedback parameter C describes the coupling strengh between the laser and the external cavity [START_REF] Taimre | Laser feedback interferometry: a tutorial on the self-mixing effect for coherent sensing[END_REF][START_REF] Acket | The influence of feedback intensity on longitudinal mode properties and optical noise in index-guided semiconductor lasers[END_REF][START_REF] Taimre | On the nature of Acket's characteristic parameter C in semiconductor lasers[END_REF], and can be written as

C = τ ext τ in R ext R 2 (1 -R 2 ) 1 + α 2 . ( 2
)
Here τ in is the laser cavity round-trip time for a photon, τ ext the external cavity round-trip time. R 2 is the reflectivity of the front facet of the laser, R ext is the ratio of the reinjected over the emitted power and is the fraction of reflected light coupled coherently back into the laser [START_REF] Kliese | Solving self-mixing equations for arbitrary feedback levels: a concise algorithm[END_REF][START_REF] Acket | The influence of feedback intensity on longitudinal mode properties and optical noise in index-guided semiconductor lasers[END_REF].

In our model we use the solver in [START_REF] Kliese | Solving self-mixing equations for arbitrary feedback levels: a concise algorithm[END_REF] to compute the excess phase equation eq. ( 1), and thus calculate the laser power eq. ( 3) as the laser beam strikes a moving target. The laser power can be expressed as

P F = P 0 [1 + m cos(φ F )], (3)
where P F is the laser power under feedback, P 0 is the laser power without feedback and m is a modulation index. To calculate the phase φ F in eq. ( 1) the solver discriminate two regimes of C. The first regime is for weak feedback, C ≤ 1.0, where the right hand side of eq. ( 1) is monotonic and a unique solution can be found. The second regime is for moderate or high feedback C ≥ 1.0, and several solutions for eq. ( 1) exist. The m parameter in eq.( 3) depends on the reflectivity R ext , and influences the amplitude of the fringes. For small displacements (a few half wavelengths) it can be considered as constant. However, for displacements with important amplitudes, m can vary significantly because of variations of the amount of light being coupled back in the cavity. Such variations may be due to Speckle effect or defocusing for example amongst others [START_REF] Zabit | Self-Mixing Laser Sensor for Large Displacements: Signal Recovery in the Presence of Speckle[END_REF].

In our experiments the optical conditions are set to remain unchanged and in simulations we choose to keep this term constant.

EXPERIMENT AND SIMULATION A. Experiment

A distributed feedback laser diode (ML720J11S) that emits a transverse and longitudinal single mode at 1310 nm is mounted on an Arroyo Instruments laser mount, including a temperature control driven by a ThorLabs TED 200C. The laser beam is focalized 0.3 meters from the laser source by a ThorLabs C230TMD-C lens. The target is the metal surface of the PI P-753.2CD piezo transducer, driven by a E-665.CR LVPZT-Amplifier. The setup is schematized in Fig. 1. The input voltage for the piezo controller is set to 2.24 V pp which generate 8 interferometric fringes. These fringes are visualized in the first plot in Fig. 2. The oscillation frequency is 8 Hz.

By modulating the piezo transducer at two separate frequencies simultaneously, we are able to produce the effects of the fringe's shape in the OFI signal at small displacement amplitudes. This is achieved by adding a small 7 kHz oscillation with an amplitude of 1 V pp to the 8 Hz main oscillation. The small oscillation corresponds to a movement of approx. 110 nm peak to peak. The second plot in Fig. 2 shows the smaller oscillation superposed on of the fringe. By using the piezo amplifier's manual offset, we can also move the mean position the piezo manually in order to set the desired operating point on the fringe.

B. Numerical simulations

Using Kliese's algorithm [START_REF] Kliese | Solving self-mixing equations for arbitrary feedback levels: a concise algorithm[END_REF] we simulate the OFI system's power response. The feedback parameter eq. ( 2) is empirically set to C = 2.5 and α = 3 for an external cavity length of L ext = nλ laser /2 ≈ 0.3 m so that the simulated fringes resemble as much as possible the measured ones.

In the simulations we modify the position of the operating point by modifying L ext . The operating point is best explained as the point on the fringe around which the power response oscillates. We simulate three scenarios where the operating point is Fringe with small oscillations on top. The underlying fringe has been approximated by a 3 rd degree polynomial. at three different positions, and for each operating point position we simulate a target displacement of amplitude 110 nm and frequency 7 kHz. Figures 3 and4 shows the output power variation as the target moves back and forth around each operating point.

DISCUSSION

The simulations presented in Fig. 3 show that the slope β of the fringe has an effect on the phase-to-power conversion gain, and thus the amplitude of the detected signal. The slope is calculated by considering the change in laser power per change in external cavity length. It can also be approximated by change in laser power per change in time.

β = d cos(φ F ) dφ 0 ≈ d cos(φ F ) dt (4)
The phase φ 0 = τ ext • 2πc/λ laser expresses the distance between the laser and the target through τ ext . c is the speed of light and t is time. Since the target moves at constant velocity over the range of the fringe, we can approximate β by deriving with respect to time dt instead. Simply put, the change in laser power per unit distance is equal to the change of laser power per unit time for a linearly moving target. As we'll see later, this approximation enables us to analyze the fringe slope in the time-domain signals acquired in experiments. By setting the operating point between positions A and B in Fig. 3 the gain is higher than between positions C and D by a factor 1.47. Furthermore one can estimate the fringe slopes over the parts that produced the interferometric signals by drawing a straight line from position A to B and then from C to D. The slope between positions A and B is calculated to β = 0.3 whereas the slope from C to D is β = 0.2. In applications such as the one in [START_REF] Giuliani | Self-mixing laser diode vibrometer[END_REF], locking the operation point to the beginning of the fringe would thus be more beneficial when measuring very small changes in the optical path. By further moving the operating point we can completely distort the output signals as shown in Fig. 4. Positions E and F have been placed on top of the fringe, right before the discontinuity. The simulation of the oscillation between these two positions confirm comments from [START_REF] Lukashkin | A self-mixing laser-diode interferometer for measuring basilar membrane vibrations without opening the cochlea[END_REF], where the authors stated that placing the operating point on the top of the fringe would distort their signal.

To further validate the performance of Kliese's algorithm [START_REF] Kliese | Solving self-mixing equations for arbitrary feedback levels: a concise algorithm[END_REF] in the sub-λ/2 domain we have compared the acquired signal to a simulation, as shown in Fig. 5. In the simulation we replicated the piezo's real movement while setting other experimental parameters such as L ext and C (described in section 3.A) equal to those in the experiment. Figure . 5 is a zoom on fringe number 3 from the left in Fig. 2. For both the simulated and acquired signal, the fringe's slow amplitude change is removed by a 1 st order Butterworth high-pass filter with a cutoff frequency of 6 kHz, thus leaving only the 7 kHz component.

Figure 5 offers a good visualization of how well Kliese's algorithm [START_REF] Kliese | Solving self-mixing equations for arbitrary feedback levels: a concise algorithm[END_REF] works for sub-λ/2 optical path variations as there is clear agreement between the experimental and simulated result. We observe how the amplitude of the 7 kHz oscillation diminishes towards the top of the fringe (towards the right) in both the experiment and in the simulation, followed by a distortion of the signal before the fringe discontinuity.

In order to demonstrate that the signal amplitude of a subλ/2 oscillation is linearly dependent on the slope of the underlying fringe we study the last plot Fig. 2. We have extracted the third fringe in the middle plot, and we have approximated the underlying fringe by a 3 rd order polynomial. The x-axis is converted to meters in order to reflect the target displacement of a half laser wavelength of 655 nm, which generate one fringe. We study the slope of the fringe, like in the simulations in Figures 3 and4, and compare it to the amplitude of the 7 kHz oscillation.

The linear relationship is found by comparing the peak-topeak amplitude variation ∆V over the full fringe, and the variation of slope β of the fringe. The peak to peak amplitude ∆V is extracted from the bottom plot in

β norm = β < β > (5)
and

V norm = ∆V < ∆V > (6)
Fig. 6. Evolution of the slope of the fringe and the evolution of the amplitude of the oscillations. Top: Based on signal from experiment (top plot in Fig. 5). Bottom: Based on simulated signal (bottom plot in Fig. 5).

are shown in the first plot Fig. 6. The second plot in the figure shows the result for the simulated signal. The recorded and the simulated signals provide similar results: The signal amplitude decreases at the same rate as the slope of the underlying fringe.

Thus we have demonstrated that the relationship between the amplitude of a sub-λ/2 signal and the fringe's slope is linear, a result that -to the best of our knowledge -has not been explicitly reported in literature so far. These results may have have a great impact on measurements of nanometric scale amplitude displacement or using the acousto-optic effect [START_REF] Zipser | 5E-4 Refracto-Vibrometry for Visualizing Ultrasound in Gases, Fluids and Condensed Matter[END_REF][START_REF] Zipser | Refracto-vibrometry for visualizing ultrasound in small-sized channels, cavities and objects[END_REF]. By adjusting the operating point on the fringe we believe we can increase the detectors sensitivity for very small displacements well below the laser's half-wavelength. This would also have an impact on acoustic detection using the OFI detection scheme as we would be able to detect lower acoustic powers than before.

Since the linear relationship between the slope of the fringe and the signal amplitude is demonstrated, an interesting solution would be to set the operating point where this slope is at the highest, that is to say in the fringe discontinuity. However, with large coupling coefficient C, maintaining such an operating point is difficult while controlling it with an open loop injection current command. Nevertheless, in order to demonstrate the validity of this hypothesis we have composed an experiment in which we degraded the strength of the feedback coupling by defocusing the laser beam. We estimated C ∼ 0.8 using [START_REF] Kliese | Solving self-mixing equations for arbitrary feedback levels: a concise algorithm[END_REF].

We then used the piezo amplifier's manual actuator to shift the mean position of the piezo, and hence the fringe operating point. The piezo was then modulated at 100 Hz at 3 mV peak to peak which corresponds to an amplitude of 8 nm peak to peak, which is the smallest stable oscillation we can produce with our system. First, the operating point was set approximately to the middle of the fringe. Then, the operating point was set in the discontinuity. The first plot in Fig. 7 is an acquisition of the fringes obtained with the coupling factor C ∼ 0.8. Estimations of the operating point positions for the following acquisitions are marked in black and red. The second plot shows the time-domain acquisition of the laser power variations while the operating point is within the black zone. The third plot shows the time-domain While the C increases it implies towards a steeper and steeper intra-fringe zone. However, a steep slope will limit the dynamic range of the measurement. If a displacement is larger than the intra-fringe zone allows, the operating point will jump to a new stable mode. Further studies of the laser dynamics for higher feedback levels will clarify the limitations and benefits of such a system. This result opens up for new investigations in the sub-λ/2 OFI sensing domain, with several exciting questions to investigate such as; The dynamic range dependency to the feedback conditions; The upper limitation of C, and; The determination of an optimal intra-fringe condition that could be achieved while modifying the feedback coupling. One could also imagine a system similar to the one employed in [START_REF] Giuliani | Self-mixing laser diode vibrometer[END_REF] to lock the phase onto the fringe jump, and thus exploit the higher phase-to-power gain factor. Further research shall clarify these questions and further advance the OFI sensing scheme.

CONCLUSION

In this paper we have shown experimentally and by simulation that the amplitude of a sub-λ/2 measurement is linearly dependent on the slope of the underlying fringe. This result has to our knowledge not yet been explicitly presented in the literature. We have validated experimentally the performances of Kliese's algorithm [START_REF] Kliese | Solving self-mixing equations for arbitrary feedback levels: a concise algorithm[END_REF] in the sub-λ/2 measurement domain by replicating our experiment in simulation and obtaining similar results.

As confirmed by experiments, a simulation of an optical path variation around several positions on the fringe resulted in different output signal amplitudes. The simulation show that a steeper fringe offers a higher gain in the phase to power conversion. A simulation of the piezo's real displacement from the experiment offers similar results to those recorded from the experiment.

Finally we have shown experimentally that placing the operating point in the phase jump, when C < 1, we can increase the signal amplitude for a small displacement. Exploring this very interesting piste shall provide further understanding of the OFI sensing scheme and its limits. A device such as the one presented in [START_REF] Giuliani | Self-mixing laser diode vibrometer[END_REF] could benefit from this study, by locking the phase at a more suitable position than the middle of the fringe.

Fig. 1 .

 1 Fig. 1. Schema of experimental setup (not to scale). Pizeo modulated at two separate frequencies simultaneously.

Fig. 2 .

 2 Fig. 2. Experimentally acquired, laser responses measured in Volts. Top: Fringes generated by moving target modulated at 8 Hz with amplitude 5.02 µm zero to peak. Middle: Fringes generated by moving target modulated at 8 Hz and at 7 kHz with amplitudes 5.02 µm and 110 nm respectively. Bottom: Zoom on fringe number 3 from the left in the middle plot.Fringe with small oscillations on top. The underlying fringe has been approximated by a 3 rd degree polynomial.

Fig. 3 .

 3 Fig. 3. Simulation of laser power response (arb. units) as function of operating point. Top figure: Black dotted line: Phase development cos(φ F ), Purple line: corresponding fringes, Blue and red lines: part of fringe traveled. Bottom figure: Blue line: Power response between positions A and B, Red line: Power response between positions C and D.

Fig. 4 .

 4 Fig. 4. Simulation of laser power response (arb. units) as function of operating point. Top figure: Black dotted line: Phase development cos(φ F ), Purple line: corresponding fringes, green line: part of fringe traveled. The top of the fringe (cos(φ F ) = 1) is marked with a red spot. Bottom figure: Power response between positions E and F.

Fig. 5 .

 5 Fig. 5. Comparison of measured interferometric signal (top) and simulation (bottom). The fringes have been removed using a high-pass filter, leaving only the 7 kHz oscillation in the signal. The amplitude of the 7 kHz oscillation is greater at the beginning of the fringe than at the end. The simulation result is remarkably true to the experiment.

  Fig 2 using a peak detection algorithm. The fringe slope is computed as the first derivative of the power variation depicted in Fig 2 (bottom). The amplitude and the fringe slope are normalized with regard to their average value over the full fringe (< β > and < ∆V >).

Fig. 7 .

 7 Fig. 7. Top: Shape of fringes used in experiment. Comparison between operating point placed mid-fringe (Middle), and operating point in phase jump (Bottom). A barely detectable displacement signal gains amplitude by clever positioning. Piezo movement: 8 nm pp . Marked in red and black in top plot: Approximate operating point zones for the two plots.

ACKNOWLEDGMENTS

This work has been performed with the LabCom CapIRO -a joint laboratory with the Toulouse INP, The LAAS -CNRS and ACOEM. Grants and funding provided by the Agence Nationale de la Recherche (ANR) and the Association Nationale de la Recherche et de la Technologie (ANRT).

DISCLOSURES

The authors declare no conflicts of interest.


Lingxue Tang

Kok-Giap Haw

Ping He

Qianrong Fang

Shilun Qiu

Valentin Valtchev
email: valentin.valtchev@ensicaen.fr

Synthesis of zeolite SSZ-24 using a catalytic amount of SSZ-13 seeds

SSZ-24 proved to be a zeolite with poor synthesis reproducibility and slow crystallization rate. The present study deals with the preparation of SSZ-24 by a seed-assisted approach. SSZ-13 and SSZ-24 zeolites are employed as seeds and the impact of seeds content on the formation of SSZ-24 studied. Both types of zeolite accelerate the zeolite nucleation, shorten crystallization time, and lead to the formation of pure and highly crystalline SSZ-24. However, the use of SSZ-13 seeds is more efficient in terms of rapid crystallization and crystallinity level. Pure and highly crystalline SSZ-24 is readily obtained even though the amount of SSZ-13 seeds has been decreases to a "catalytic level", i.e. 0.3 wt.%. The higher seeding power of SSZ-13 seeds is attributed to the presence of framework Al, which stabilizes structural elements that promote the SSZ-24 nucleation.

Introduction

Zeolites are extensively used in different areas of the chemical industry, such as shape selective catalysis, gas adsorption and separation, and ion exchange. [1][2][3][4][5][6] Crystalline structure and chemical compositions determine the properties and practical applications of a zeolite. However, the use of a particular zeolite does not depend solely on its properties. The production conditions in terms of operational simplicity, short production time and reactants employed are often decisive for the large-scale use of a particular zeolite. Hence, the optimization of synthesis parameters that lead to a more efficient and low-cost zeolite synthesis deserves particular attention. It is almost 60 years since the first seeded zeolite synthesis was reported. 7 In the 1960s, Kerr investigated the factors affecting the formation of zeolites A, B, and X using 10-15 wt.% seeds. 7,8 Later, Mirskii proposed a kinetic model of zeolite formation in the presence of seeds. 9 Kacirek employed seeds to achieve the separation of nucleation and growth, the amount of the added faujasite seeds nuclei with respect to the whole amount of the crystalline products was 2.5 to 3.0 wt. %. 10 Seeds are commonly used when zeolite synthesis reproducibility is poor since the seeded synthesis is less sensitive to the reactants and crystallization conditions used. Further, the seed-assisted approach is used to reduce the cost and enable the development of environmentally friendly production processes of relevant zeolites. 11,12 Important issues as the reduction of the synthesis time, elimination of impurities, controlling the particle size are also positively influenced by seeding. [13][14][15][16][17][18][19][20] Seeding is also commonly employed in the preparation of zeolite membranes and 3D structures. 21 The seed content in zeolite synthesis vary between 5 and 15 wt.% approximately, and the use of 1 to 3 wt.% is scracely reported. SSZ-24 is a high-silica, large-pore molecular sieve isostructural with AlPO-5 (AFI) aluminophoshpate. [START_REF] Ch | Atlas of Zeolite Framework Types[END_REF] SSZ-24 was first synthesized by Zones et al. using 1trimethylammonioadamantane as structure directing agent (SDA). [START_REF] Zones | [END_REF] The crystallization of SSZ-24 is substantially longer in respect to the most of high silica zeolites. Typically, the synthesis of SSZ-24 requires more than 10 days at 150 °C using N, N, N-trimethyl-1-adamantanamine cation as an organic structure-directing agent. [START_REF] Zones | [END_REF][START_REF] Van Nordstr | [END_REF][25][26] Over past decades, many efforts have been made to optimize the crystallization of zeolite SSZ-24. Zones et al. synthesized boron-containing SSZ-24 by using a calcined form of boronsubstituted zeolite beta ([B]-BEA) as primary silica sources. 27 Lobo and Davis reported the synthesis of [B]-SSZ-24 using N(16)-methylsparteinium (MeSPA) as SDA and sodium borate as the source of boron. 28 In the later case, the calcined [B]-BEA did not work as precursors for [B]-SSZ-24 synthesis. Maekawa et al. studied the phase transformation of [Al]-BEA to [Al]-SSZ-24 by using MeSPA as OSDA. 29,30 Valtchev et al. reported a synthesis of zeolite SSZ-24 with 5 wt.% seeds. 31 Peng et al. reported a fast synthesis of zeolite SSZ-24 at 210 o C in 60 min by fast heating in a stainlesssteel tubular reactor with 10 wt.% SSZ-24 seeds. 32 However, such fast synthesis This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins method involved special equipment and a series of complicated and time consuming pre-treatment steps before the hydrothermal treatment, which limits its practical application. For instance, 2 days stirring at room temperature was required for homogenization of the gel followed by an aging step at 95 °C for 3 days and then synthesis in an oil bath at 210 °C for 60 min.

Here we report the use of catalytic amounts of seeds for rapid synthesis of zeolite SSZ-24. Two types of seeds, zeolite SSZ-24 and SSZ-13, were employed. The structure similarity, i.e. the abundant presence of 6-rings, in the AFI-(SSZ-24) and CHA-(SSZ-13) type structures is the reason to employ SSZ-13 and compare its performance in respect to the SSZ-24 seeds. The ultimate goal of this study is to reduce both the mass of seeds used and the crystallization time, which would bring substantial practical advantages. To the best of our knowledge, no other synthesis of SSZ-24 under autogenous pressure hydrothermal conditions reported to date is shorter than 1 day and required only a catalytic amount of seeds.

Experimental

Materials

All chemicals and materials used are from commercially available sources and employed without further purification. Potassium hydroxide (KOH 98%, Aladdin Chemistry), Lithium hydroxide (LiOH 98%, Sigma-Aldrich), Sodium hydroxide (NaOH 98%, Aladdin Chemistry), Cesium hydroxide (CsOH 50 wt.% in water, Adamas-beta), N,N,N-trimethyl-1-adamantammonium hydroxide (TMAD 25%, Xikai Industry), Deionized water, Aluminum metal powder (99.5%, Aladdin Chemistry), Colloidal silica (Ludox HS-40, 40 wt.% SiO2, Sigma-Aldrich) and fumed silica (SiO2, Sigma-Aldrich). The zeolite Beta with Si/Al=15, which was used as a sillliiica-alumina source in the synthesis of SSZ-13, was purchased from Changchun Third Party Pharmaceutical Technology. A home-made SSZ-24 and SSZ-13 were used as seeds without prior calcination.

Conventional hydrothermal synthesis of SSZ-24

The reference SSZ-24 sample, denoted CS (conventional synthesis), was obtained from a gel with composition 0.1TMAD: 0.15KOH: 1SiO2: 44H2O. In a typical synthesis, 1.5 g fumed silica, 0.21 g KOH, and 1.51 g TMAD were mixed with 18.9 g H2O under stirring. [START_REF] Van Nordstr | [END_REF] The resulting gel was charged into a Teflon-lined stainless-steel autoclave and hydrothermally treated at 150 °C for 22 days. The final product was collected by vacuum filtration and washed with deionized water until a near neutral pH and dried overnight at 60 °C. The sample was calcined at 550 °C (at 60 °C•h -1 ramp) in air flow for 6 h. Synthesis of SSZ-13 and SSZ-24 seeds SSZ-13 was prepared according to reference 33 and used for seeding. The SSZ-13 seeds used in this study were prepared from a gel with molar composition 1SiO2:0.05Al2O3:0.18K2O:0.31TMAD:30H2O, where 1.7 g zeolite beta and the TMAD solution were mixed (solid/liquid weight ratio = 0.14) under stirring for 2 h at room temperature. The autoclave was heated at 150 °C for 24 h. After the crystallization step, the solid product was collected by vacuum filtration and washed thoroughly with deionized water until a near neutral pH and then dried overnight at 60 °C. The SSZ-24 seeds used in this study were prepared from a gel with molar composition 0.06TMAD: 0.1NaOH: SiO2: 21.1H20, where 0.3 wt.% of SSZ-13 seeds prepared according to the procedure described above were added. In a typical synthesis, 4.4 g colloidal silica (Ludox HS-40), 0.1 g sodium hydroxide and 0.005 g SSZ-13 seeds were mixed with 1.6 g TMAD solution under stirring, the autoclave was heated at 100 °C for 1 h and at 180 °C for 22 h. The final product was collected by vacuum filtration and washed with deionized water until a near neutral pH and dried overnight at 60 °C. Both SSZ-24 and SSZ-13 seeds were used without prior calcination.

Synthesis of zeolite SSZ-24 using SSZ-24 seeds

The synthesis of SSZ-24 was studied in the presence of KOH and NaOH. KOH system: Synthesis gel was prepared according to the following composition 0.1TMAD: 0.15KOH: 1SiO2: 44H2O and 5 wt.% of SSZ-24 seeds. The seed content (in wt.%) is calculated on the SiO2 basis in the starting gel. In a typical synthesis, 1.5 g fumed silica, 0.21 g KOH, and 1.51 g TMAD were mixed with 18.9 g H2O under stirring. 0.075 g SSZ-24 seeds were added into the above gel. The synthesis was performed in a Teflonlined stainless-steel autoclave and hydrothermally treated at 150 °C for 24 h and 72 h. The final product was collected by vacuum filtration and washed with deionized water until a near neutral pH and dried overnight at 60 °C. NaOH system: Synthesis gel was prepared according to the following composition 0.06TMAD: 0.10NaOH: xSiO2: 21.1H20, when x is 1.76 mole, the mass of seeds added was 0.0, 0.3, 1.2, 3.0 (wt.%); when x is 1.20 mole, the mass of seeds added was 4.2 (wt.%). The seed content (in wt.%) is calculated on the SiO2 basis in the starting gel. In a typical synthesis, 4.4 g colloidal silica (Ludox HS-40) and 0.1 g NaOH was mixed with 1.6 g TMAD solution under stirring. 0.07 g SSZ-24 seeds were added into the above gel. The gel was then mixed vigorously for 10 min. The resulting reaction mixture was charged into a Teflonlined, stainless-steel autoclaves. Before the hydrothermal treatment, the mixture was aged at 100 °C for 1 h. Then autoclaves were heated statically under autogenous pressure for 22 h at 180 °C. The final product was collected by vacuum filtration and washed with deionized water until a near neutral pH and dried overnight at 60 o C. The sample was calcined at 550 °C (at 60 °C•h -1 ramp) in air flow for 6 h and denoted as S24-Y, where S24 represents synthesis in the presence of SSZ-24 seeds, and Y is the numerical order of a series of experiments at variable syntheses conditions, as shown in Table 1.

Synthesis of zeolite SSZ-24 using SSZ-13 seeds

Similar to the previous part, the synthesis of SSZ-24 was studied in the presence of KOH and NaOH.

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Please do not adjust margins KOH system: Synthesis gel with composition 0.1TMAD: 0.15KOH: 1SiO2: 44H2O and 5 wt.% of SSZ-13 seeds was prepared. The seed content (in wt.%) is calculated on the SiO2 basis in the starting gel. In a typical synthesis, 1.5 g fumed silica, 0.21 g KOH, and 1.51 g TMAD were mixed with 18.9 g H2O under stirring. 0.075 g SSZ-13 seeds were added into the above gel. The synthesis was performed in a Teflon-lined stainless-steel autoclave and hydrothermally treated at 150 °C for 24 h and 72 h. The final product was collected by vacuum filtration and washed with deionized water until a near neutral pH and dried overnight at 60 °C. NaOH system: Synthesis gel was prepared using the following composition xTMAD: yNaOH: SiO2: zH20, where x= 0 -0.08, y= 0 -0.3, z= 21.1 -28.7 (moles); and the seed content was varied between 0.0 and 9.0 wt.% (Table 2). The seed content (in wt.%) is calculated on the SiO2 basis in the starting gel. In a typical synthesis, 4.4 g colloidal silica (Ludox HS-40) and 0.1 g NaOH was mixed with 1.6 g TMAD solution under stirring. The 0.005 g seeds were added into the gel. The gel was then mixed vigorously for 10 min and charged into a Teflon-lined stainlesssteel autoclave. Before the high temperature hydrothermal treatment, the mixture was aged at 100 °C for 1 h. Then autoclaves were heated statically under autogenous pressure for 22 h at 180 °C. The final product was collected by vacuum filtration and washed with deionized water until a near pH and dried overnight at 60 °C. The sample was denoted as S13-X, where S13 represents synthesis in the presence of SSZ-13 seeds, and X is the numerical order of a series of experiments at variable syntheses conditions, as shown in Table 2. Influence of the alkaline metal (Li, Na, K, Cs) on the synthesis of SSZ-24: Synthesis gel was prepared according to the following composition 0.06TMAD: 0.1MOH: SiO2: 21.1H20, M = Li, Na, K, Cs, and 0.3 wt.% of SSZ-13 seeds. The gel was then mixed vigorously for 10 min and then charged into Teflon-lined stainless-steel autoclaves. Before the high temperature hydrothermal treatment, the mixture was aged at 100 °C for 1 h. Then autoclaves were heated statically under autogenous pressure for 22 h at 180 °C. The final product was collected by vacuum filtration and washed with deionized water until a near neutral pH and dried overnight at 60 °C.

Crystal growth kinetics of SSZ-24:

The relative crystallinity of the samples prepared at different period of crystallization were determined by XRD analysis using the peak area at 7.42° 2θ in relative to the reference SSZ-24 sample (CS). For more accurate analysis, the XRD patterns of the calcined samples were used.

The yield of SSZ-24 was calculated by the following equation:

yield (%) =

Characterization

Powder X-ray diffraction (PXRD) analysis was performed on a PANalytical B.V. Empyrean powder diffractometer at 40 kV and 40 mA using Cu Kα radiation (λ = 1.5418 Å) over the range of 2θ = 4.0 -40.0°. Thermogravimetric analysis (TGA) was performed using a SHIMADZU DTG-60 thermal analyser system from 30 to 650 °C under an air atmosphere at a heating rate of 10 °C min -1 . A JEOL JSM6700 scanning electron microscope was used to study the morphology of the samples. Elemental analysis of the sample was measured by Energy Dispersive X-Ray Fluorescence Spectrometer (EDX) operated at JEM-2100F. Nitrogen and carbon dioxide physisorption analyses were performed with a Quantachrome Autosorb-IQ analyzer with ultra-high-purity nitrogen gas (99.999% purity) at 77 K and CO2 gas (99.999% purity) at 273 K and 298 K. The calcined samples were analyzed after degassing at 300 °C. The specific surface areas (SBET) were obtained using the Brunauer-Emmett-Teller (BET) equation. The microporous volume (Vmic, cm 3 g -1 ) and external surface area (Sext, m 2 g -1 ) were obtained from the t-plot based on the Harkins-Jura equation.

Results and discussion

Conventional hydrothermal synthesis of SSZ-24

Our preliminary study of SSZ-24 crystallization was based on the gel composition reported in reference 24. A highly crystalline SSZ-24 was obtained after 22 days crystallization at 150°C (Fig. S1). The adition of 5 wt.% SSZ-24 seeds to this system did not show the expected positive effect in term of substantially imporved crystallization rate. After 24 h crystallization amorphous and unidentified phase was found in the solid, while after 72 h traces of SSZ-24 with a large amount of amorphous material was observed (Fig. S2). In contrast, the addition of 5 wt.% SSZ-13 seeds to the system resulted in crystalline product after 72 h of synthesis (Fig. S3). However, in a mixture of SSZ-24, SSZ-13 and SSZ-31 was synthesized. Thus, further optimization of the initial gel was required in order to obtainpure SSZ-24.

Synthesis of zeolite SSZ-24 using SSZ-24 seeds

We have studied the performance of SSZ-24 seeds in the formation of pure and highly crystalline AFI-type zeolite in a NaOH system. Table 1 shows the synthesis conditions and the obtained products. The synthesis containing below 1.2 wt.% seeds did not yield a crystalline product (Run S24-1, S24-2). At higher seeds loading, i.e. 3.0 wt.%, SSZ-24 and crystoballite type SiO2 were formed (Run S24-4). A pure SSZ-24 was obtained after the increase of seeds content to 4.2 wt.% while decreasing the SiO2 content in the gel to 1.20 mole (Run S24-5). XRD pattern (Fig. 1 left) and SEM micrograph (Fig. 1 right) of S24-5 showed that a pure zeolite SSZ-24 was obtained and the crystals were relatively uniform in size. However, a careful SEM inspection indicated that some impurities were present in the sample. The amount of these impurities is below 3 wt.% since no other crystalline phase was detected in the XRD pattern of the solid. It should also be mentioned that the preliminary aging at 100 o C is indispensable to obtain pure SSZ-24. Without preliminary aging the formation of SSZ-31 as an impurity is observed (Fig. S4). This journal is © The Royal Society of Chemistry 20xx

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Synthesis of zeolite SSZ-24 using SSZ-13 seeds

Structure similarities between SSZ-24 and SSZ-13, i.e. the abundant presence of 6 member rings units, inspire us to try the use of SSZ-13 seeds in the synthesis of SSZ-24. All important factors controlling SSZ-24 crystallization were studied. First, we have investigated the impact of sodium on the formation of SSZ-24 in NaOH system in the presence of 0.6 wt. % SSZ-13 seeds. The experimental conditions and the results are summarized in Table 2 (Run S13-1 to S13-8) and the XRD patterns are presented in Fig. 2a. When NaOH/SiO2 ≥ 0.26, mixed phases of zeolite SSZ-13, SSZ-24, SSZ-31 were formed (Run S13-1, S13-2). Pure well crystalline SSZ-24 was obtained at 0.04 ≤ NaOH/SiO2 ≤ 0.13 (Run S13-5, S13-6, S13-7). It is worth noting that NaOH is crucial in SSZ-24 synthesis since without it (NaOH/SiO2 = 0) AFI-type material was not formed (Run The impact of seed content was studied in experiments S13-9 to S13-15 (Table 2). The XRD patterns are presented Fig. 2b. Pure SSZ-24 phase was constantly obtained even at seed content as low as 0.3 wt.% (Run S13-11). Noteworthy, at seed content of 0.2 wt.% still SSZ-24 can be observed in the product, but the amount of crystalline material was very low. Also, the crystallization rate decreased substantially. On the other hand, the use of 0.3 wt.% seeds (Run S13-11), resulted in SSZ-24 materials with 90% crystallinity. No crystallization was observed in the non-seeded system (Run S13-9). In the introduction section, it was pointed out that the zeolite synthesis required ≥ 3-5 wt.% seeds. The reduction of the seed content below 3 wt.% is rarely reported. The present study showed that upon optimization of the synthesis conditions, the mass of seeds could be reduced below 1 wt.% in respect to the silica content in the gel. Such a low seed content we consider as a "catalytic" since it is negligible with respect to

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The water content had a pronounced affect on the seeded SSZ-24 synthesis (Table 2, S13-11, S13-16, S13-17). For instance, as water content was increased to H2O/SiO2=28.7, the crystallinity of SSZ-24 decreased to 43% (Run S13-16), while the crystallinity reached 90% (Run S13-17) when the water content decreased to H2O/SiO2 = 21.1 (Fig. 2c).

The effect of TMAD on the synthesis of SSZ-24 was studied in the series S13-17 to S13-21 synthesis (Table 2). When TMAD/SiO2 mole ratio ≥ 0.06 (Run S13-17 and S13-18), a pure SSZ-24 was obtained. SSZ-31 was present as an impurity when TMAD/SiO2 < 0.06 (Run S13-19 and S13-20) and only amorphous was found in the TMAD-free system (Run S13-21).

The crystal growth kinetics of SSZ-24 in SSZ-13 seeded system was followed by SEM (Fig. 3). After 12 h large amorphous particles and crystals, the later are not well shaped, can be seen. The crystals are much better shaped after 18 h, but still some amorphous can be observed in the solid product. No traces of amorphous material are seen in the product after 22 h crystallization. The extension of the crystallization time to 48 h resulted in larger crystals, which is a consequence of the Ostwald repining.

In general, the use of SSZ-13 seeds in the crystallization of SSZ-24 is much more efficient with respect to the genuine SSZ-24 seeds. Often the crystal size and morphological influence the seed performance. However, the employed SSZ-13 and SSZ-14, where farely similar in morphology. Therefore, the seeding power of SSZ-13 is likely to be related to their composition. Hence we consider that the Al-containing SSZ-13 is more stable under conditions employed. More precisely, the framework aluminum is providing a stable configuration which retains the structure elements that promote the formation of SSZ-24. On the contrary, the all-silica SSZ-24 was readily dissolved in basic medium and lost its seeding power. In order to gain more insight into the phase transformation during the crystallization process, we have studied the crystal growth kinetics of SSZ-24 at 180 °C (Fig. 4). The relative crystallinity of the calcined samples was determined by the peak area at 7.42° 2θ under XRD patterns (Fig. S5) in relative to the reference SSZ-24 sample (CS). The reaction rate of using SSZ-13 (Run S13-18) as seeds is higher and a complete crystallization was obtained after 22 h, faster than the one using SSZ-24 (Run S24-5) as seeds (28 h). Consider the conventional synthesis of SSZ-24 required 22 days. [START_REF] Van Nordstr | [END_REF] The employment of seeds, as shown in the present study, improves substantially the crystallization rate of SSZ-24.

Generally, zeolites are synthesized in the presence of alkali metal cations, which influence not only the crystallization rate and framework Si/Al ratio but also the crystal size and morphology. 34,35 Since alkali metal cation plays an essential role in the zeolite synthesis, the effect of Li + , Na + , K + , Cs + on the formation of SSZ-24 were studied. Sodium is the most widely used alkali cation in zeolite synthesis. It promotes the formation of a number of zeolites and contributing to a rapid crystal growth rate. 37 Potassium is also largely used in zeolite Please do not adjust margins Please do not adjust margins synthesis with more pronounced structure specificity. In contrast, Li + and Cs + hardly promote the crystallization of zeolite structures. 38 A previous study showed that pure SSZ-24 was obtained from the K-TMAD system, while the replacement of K + with Na + would lead to bi-phases product. 36 However, when as-synthesized SSZ-13 seeds were used, the Nacontaining system yeilded highly crystalline SSZ-24 within 24 h. The crystallinity of K-SSZ-24 was lower with respect to Na-SSZ-24 (Fig. 5) and the increase pf K + content hindered the formation of SSZ-24 (Fig. S6a). The extension of the reaction time of K + -containinh system to 2 days resulted in highly crystalline SSZ-24, however zeolite SSZ-31 was formed as a side product (Fig. S6b). In contract, SSZ-24 was constantly obtained in Na + system even after 3 days of synthesis, revealing the essential role of the alkaline cation in SSZ-24 synthesis. In summary, SSZ-24 was successfully obtained in the presence of Na + and K + , while to Li + yielded an unknown phase and in the case of Cs + no crystalline product was found (Fig. 5).

For the sake of comparison the characteristics of the two best SSZ-24 samples obtained with SSZ-13 (S13-18) and SSZ-24 (S24-5) seeds were compared. Both samples showed X-ray crystallinity higher than 90%. Thermogravimetrical (TG) analysis showed similar wight losses, 12.6 wt.% and 12.9 wt.% for S13-18 and S24-5, respectively (Figure S7). The amount of TMAD (300 -650 °C) in the two samples is approximately 10 wt. %. The wight loss in the low temperature range (30 -250°C) attributed to the physisorb water release is a litle lower for S13-18 (1.9 wt.%) in respct to S24-5 (2.4 wt.%). Nitrogen adsorption-desorption analysis of S13-18 and S24-5 samples measured at 77 K showed Type I isotherms (Fig. 6a) with a sharp uptake at a low pressure of P/P0 < 0.05, which is characteristic of microporous materials. The Brunauer-Emmett-Teller (BET) equation was applied over the 0.05 < P/P0 < 0.25 range of the isotherms. The micropore volume (0.13 cm 3 g -1 ) of S13-18 are substantially higher in respect to S24-5 (0.10 cm 3 g -1 ). We attribute the higher micropore volume to thehigher crystallinity of SSZ-13 seeded sample (Table 3). S13-18 showed also a substantially higher CO2 adsorption than S24-5 (Fig. 6b). According to the elemental analysis (Table 3), the Si/Al ratio of S13-18 is 196 while S24-5 is 299. Both zeolites are almost all-silica materials, and thus impact of the framework composition on the adsporption properties is not expected.

Conclusions

In summary, we have developed a seed-assisted synthesis method for rapid crystallization of highly crystalline SSZ-24. The synthesis time was substantially shortened from 22 days in conventional synthesis to 22 h in our seeded system. This result was obtained with a catalytic amount (0.3 wt.%) of seeds, which make the synthesis particularly attractive for practical use. The reduction of the crystallization time and seed content was achieved without compromising the crystallinity (> 90 %) of the product. Thus the seed-assisted synthesis, with appropriate aging and selection of alkaline metal cations, directs the rapid crystallization of SSZ-24 and prevents the formation of undesired phase. This study illustrates that the seeded approach can properly address the crystallization of the most challenging zeolite.

Fig. 2

 2 Fig. 2 XRD patterns of SSZ-24 synthesized at different content of NaOH (a), SSZ-13 seeds (b), Water (c) and TMAD (d).

Fig. 1

 1 Fig. 1 XRD pattern (left) and SEM micrograph (right) of SSZ-24 obtained by using SSZ-24 as seed.

Fig. 3

 3 Fig. 3 SEM micrographs of of the series of samples S13-18 after 1, 3, 6, 12, 18, 22 and 48 h of hydrothermal synthesis.

Fig. 5

 5 Fig. 5 XRD patterns of SSZ-24 synthesized with different alkaline metal cations at 180 °C for 22 h.

Fig. 4

 4 Fig.4 Crystal growth kinetics of SSZ-24 synthesized with SSZ-13 (S13-18) and SSZ-24 (S24-5) seeds.

5

 5 This journal is © The Royal Society of Chemistry 20xx

Fig. 6

 6 Fig. 6 N2 physisorption isotherms (a) and CO2 adsorption isotherms (b) of S13-18 and S24-5 samples at 77 K and 273 K respectively.

Table 1

 1 Synthesis conditions and products of seeded SSZ-24 synthesis at 180 °C for 22 h.

			Synthesis conditions		Product(s)
	Run	SiO2 (mole)	Seeds (wt.%)	NaOH/ SiO2	TMAD/ SiO2	Crystalline phase(s)
	S24-1	1.76	0.0	0.10	0.06	Amorphous
	S24-2	1.76	0.3	0.10	0.06	Amorphous
	S24-3	1.76	1.2	0.10	0.06	SSZ-24, SiO2, SSZ-31
	S24-4	1.76	3.0	0.10	0.06	SSZ-24, SiO2
	S24-5	1.20	4.2	0.10	0.06	SSZ-24

Table 2

 2 Conditions of SSZ-24 synthesis with SSZ-13 seed at 180 °C for 22 h and the corresponding products.

			Synthesis conditions (mole ratio)		Product (s)
	Run	Seed (wt.%) NaOH/SiO2 TMAD/ SiO2	H2O/ SiO2	Phase (s)	Crystallinity (%)
	S13-1	0.6	0.30	0.08	24.9	SSZ-13, SSZ-24, SSZ-31	-
	S13-2	0.6	0.26	0.08	24.9	SSZ-13, SSZ-24, SSZ-31	-
	S13-3	0.6	0.22	0.08	24.9	SSZ-24, SSZ-31	-
	S13-4	0.6	0.17	0.08	24.9	SSZ-24, SSZ-31	-
	S13-5	0.6	0.13	0.08	24.9	SSZ-24	92
	S13-6	0.6	0.09	0.08	24.9	SSZ-24	88
	S13-7	0.6	0.04	0.08	24.9	SSZ-24	28
	S13-8	0.6	0.00	0.08	24.9	Amorphous	-
	S13-9	0.0	0.09	0.08	24.9	Layered silicate	-
	S13-10	0.2	0.09	0.08	24.9	SSZ-24	17
	S13-11	0.3	0.09	0.08	24.9	SSZ-24	82
	S13-12	0.6	0.09	0.08	24.9	SSZ-24	88
	S13-13	1.2	0.09	0.08	24.9	SSZ-24	89
	S13-14	6.0	0.09	0.08	24.9	SSZ-24	92
	S13-15	9.0	0.09	0.08	24.9	SSZ-24	92
	S13-16	0.3	0.09	0.08	28.7	SSZ-24	43
	S13-17	0.3	0.09	0.08	21.1	SSZ-24	90
	S13-18	0.3	0.09	0.06	21.1	SSZ-24	91
	S13-19	0.3	0.09	0.04	21.1	SSZ-24, SSZ-31	-
	S13-20	0.3	0.09	0.02	21.1	SSZ-24, SSZ-31	-
	S13-21	0.3	0.09	0.00	21.1	Amorphous	-

Table 3

 3 Comparison of physicochemical properties of S13-18 and S24-5 samples.

	Samples Seed (wt.%) Yield/ % SBET/m 2	g -1 Vmic/cm 3	g -1	Si/Al
	S24-5	4.2	61	407	0.10		299
	S13-18	0.3	80	413	0.13		196

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5

Acknowledgements

Q.F. and V.V. thank the National Natural Science Foundation of China (21571079, 21621001, 21390394, 21571076 and 21571078) for the financial support. Q.F and V.V. acknowledge the support from the Thousand Talents program (China).

Notes and references

Conflicts of interest

There are no conflicts to declare.