Ping He

Jiehua Ding

Zhengxing Qin

Lingxue Tang

Kok-Giap Haw

Yiying Zhang

Qianrong Fang

Shilun Qiu

Valentin Valtchev
email: valentin.valtchev@ensicaen.fr

Binder-free preparation of ZSM-5@silica beads and their use for organic pollutants removal

Keywords: Zeolite@silica beads, binder-free, water decontamination, aniline

Zeolite is widely recognised as effective adsorbent for the removal of organic pollutants from aquous sources. However, its intrinsic powder form makes this application restricted by the arduous filtration-recovery process and decreased activity due to aggregation. Herein, we report an organic structure directing agent (OSDA) free and binder-free preparation of ZSM-5@silica beads for organic pollutants sequestration exemplified by the aniline removal. All silica beads were prepared by employing ion exchange resin beads as a macro template and further used to prepare ZSM-5@silica composite. First, anion exchange resin beads were charged with silica by ion exchange. Second, the anion exchange resin was eliminated by high temperature combustion leaving behind all-silica replica. The all-silica replica was partially transformed into ZSM-5 by hydrothermal treatment. ZSM-5 was synthesized from the organic template-free system, thus avoiding high temperature calcination of the composite. The effect of the crystallization temperature (120, 150, and 170 °C) on the formation of ZSM-5 was studied. The physicochemical properties of obtained beads were determined using different characterization methods such as powder X-ray diffraction (XRD), scanning-electron microscopy (SEM), N2 adsorption-desorption isotherms, and thermogravimetry (TG). The ZSM-5@silica composite demonstrated remarkable capacity outpacing most silica-based adsorbents and modified zeolitic materials, re-usability and retained spherical shape. The binder-free zeolite shaping technology offers a green and straightforward perspective of shaping zeolite-based adsorbents for the removal of organic pollutants from waste water.

Introduction

Toxic organic pollutants existing in water sources and soil are harmful to organisms even at trace concentrations. [START_REF] Li | Silicalite-1 zeolite membrane: Synthesis by seed method and application in organics removal[END_REF][2][START_REF] Jiang | Highsilica zeolites for adsorption of organic micro-pollutants in water treatment: A review[END_REF] Aniline from different origins, like petrochemical, dye and pharmaceutical industries, is one of the most common hazardous pollutants detected in nature. Thus, developing a highly efficient method to decontaminate aniline is urgently demand. [START_REF] Han | In-situ synthesis of SiO 2 @MOF composites for high-efficiency removal of aniline from aqueous solution[END_REF] In general, the sorption is the most versatile and promising method to treat intricate polluted water because of its high efficiency and the fact that it does not generate harmful byproducts.

3, 5

Various types of porous adsorbents have been employed

to sequestrate toxic aniline from waste water, [START_REF] Koyuncu | Removal of aniline from aqueous solution by activated kaolinite: Kinetic, equilibrium and thermodynamic studies[END_REF][START_REF] Ahmadi | Adsorptive removal of phenol and aniline by modified bentonite: adsorption isotherm and kinetics study[END_REF][START_REF] Albayati | Shape-Selective Adsorption of Substituted Aniline Pollutants from Wastewater[END_REF][START_REF] Hu | Application of graphitic carbon nitride for the removal of Pb(II) and aniline from aqueous solutions[END_REF] like actived carbon, metal organic frameworks (MOFs), functionalized porous silica, and zeolites materials. Considering costeffective, available and regenerative features, zeolite is considered as an excellent sorbent for wastewater treatment. Zeolites offer orderly distributed and uniformly sized micropores, which determine selective adsorption of a target molecule. Moreover, its excellent thermal stability can withstand oxidative treatment even thermal regeneration of the adsorbent. [START_REF] Tran | Surfactant modified zeolite as amphiphilic and dual-electronic adsorbent for removal of cationic and oxyanionic metal ions and organic compounds[END_REF][START_REF] Shaw | Surface Engineered Zeolite: An Active Interface for Rapid Adsorption and Degradation of Toxic Contaminants in Water[END_REF][START_REF] Mahmoodi | Zeolite nanoparticle as a superior adsorbent with high capacity: Synthesis, surface modification and pollutant adsorption ability from wastewater[END_REF] Nano-adsorbents of different nature and high specific surface area demonstrate good potential in the field of sewage treatment. [START_REF] Mahmoodi | Zeolite nanoparticle as a superior adsorbent with high capacity: Synthesis, surface modification and pollutant adsorption ability from wastewater[END_REF][START_REF] Kundu | Hierarchical Porous Carbon Nanospheres for Efficient Removal of Toxic Organic Water Contaminants of Phenol and Methylene Blue[END_REF][START_REF] Mcguinness | Nanotechnology Solutions for Global Water Challenges[END_REF] However, its practicability is significantly restricted by its arduous filtration-recovery process and decreased activity due to aggregation. 15-18 Zeolites in shaped forms (granules, extrudates, pellets, beads), which combine convenient handling with efficient adsorption and facile separation, are highly desirable for decontamination of wastewater. [START_REF] Wang | One-Pot Preparation of NaA Zeolite Microspheres for Highly Selective and Continuous Removal of Sr(II) from Aqueous Solution[END_REF][START_REF] Putra | Entrapment of micro-sized zeolites in porous hydrogels: Strategy to overcome drawbacks of zeolite particles and beads for adsorption of ammonium ions[END_REF][START_REF] Charkhi | Effect of Bentonite Binder on Adsorption and Cation Exchange Properties of Granulated Nano NaY Zeolite[END_REF] Typically, a binder is used to construct a solid shaped body, requires several preparation steps, thus results in a more expensive product. 16, 22 The binder also decreases the adsorption capacity of zeolite body. However, the preparation of such bodies inevitably involves a special treatment, like surfactant modification, [START_REF] Silva | Cationic polymer surface treatment for zeolite washcoating deposited over cordierite foam[END_REF] seeds coating, [START_REF] Fakin | Highly crystalline binder-free ZSM-5 granules preparation[END_REF] pseudomorphic transformation, [START_REF] Schumann | Investigation on the pore structure of binderless zeolite 13× shapes[END_REF][START_REF] Yue | Drygel synthesis of shaped binderless zeolites composed of nanosized ZSM-5[END_REF][START_REF] Moukahhal | Hierarchical ZSM-5 beads composed of zeolite nanosheets obtained by pseudomorphic transformation[END_REF] and others, 28-29 This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins which gives more synthesis steps and needs skilled technique.

In addition, most of them need expensive organic structure directing agent (OSDA), thus high temperature calcination is needed for removal of OSDA. The high temperature combustion is energy intense and environmental unfriendly step,which makes the product not cost-effective for water decontamination. [START_REF] Ng | Capturing ultrasmall EMT zeolite from template-free systems[END_REF][START_REF] Ren | Controllable and SDA-free synthesis of sub-micrometer sized zeolite ZSM-5. Part 1: Influence of alkalinity on the structural, particulate and chemical properties of the products[END_REF][START_REF] Ren | Controllable and SDA-free synthesis of sub-micrometer sized zeolite ZSM-5. Part 2: Influence of sodium ions and ageing of the reaction mixture on the chemical composition, crystallinity and particulate properties of the products[END_REF][START_REF] Wu | Sustainable synthesis of zeolites without addition of both organotemplates and solvents[END_REF][START_REF] Oleksiak | Organic-Free Synthesis of a Highly Siliceous Faujasite Zeolite with Spatially Biased Q(4) (nAl) Si Speciation[END_REF] Therefore, avoiding the use of OSDA in the preparation of shaped zeolite bodies is highly recommended. To the best of our knowledge, the preparation of binder-free micrometer sized ZSM-5 beads in the absence of OSDA and seeds is not reported to date.

The objective of the present work is the preparation of OSDA-and binder-free ZSM-5@silica beads. The analysis of their physicochemical properties and the ability to adsorb organic pollutants is also an objective of the study. Aniline is employed as a target pollutant in order to evaluate the decontamination ability of the composite sorbents.

Experimental Materials:

All reagents were obtained from commercial sources and used without further purification. Gel-type microporous anion exchange resin beads (Amberjet® 4200, chloride-form), Sodium silicate (water glass) (26.5 wt% SiO 2 , 10.6 wt% Na 2 O and 62.9 wt% H 2 O) and Ludox HS-40 (40 wt% SiO 2 ) were purchased from Sigma-Aldrich. Sodium hydroxide (NaOH, 98 %) and Sodium aluminate (NaAlO 2 , 98 %) were purchased from Sigma-Aldrich. A Milli-Q integral ultrapure water system was employed to produce deionized (DI) water, which was used in all experiments.

Synthesis of SiO 2 beads:

Microporous anionic exchange resin beads (Amberjet® 4200, chloride-form) was employed as shaping templates for the preparation of silica beads. [START_REF] Tosheva | Amorphous very high surface area silica macrostructures[END_REF] In a typical experiment, 2.0 g raw resin beads were mixed with 10 mL sodium silicate solutions and 90 mL deionized water. The ion exchange reaction was carried out in a silicon oil bath preheated to 100 °C for 6 h under reflux. After the reaction, the silicate-exchanged resin beads were separated from the liquid by suction filtration, subjected to ultrasonic treatment in deionized water to remove loosely attached silica species. After drying at 60 °C overnight, the resin-silicate composite was obtained. The resin beads were removed by calcination. The heating ramp was 1°C / min till 550 °C; then the beads were kept at 550 °C of 6 h.

Synthesis of ZSM-5@silica beads:

All-silica beads were subjected to hydrothermal treatment in a ZSM-5 yielding system free of OSDA. The precursor gel with composition of H 2 O: SiO 2 : Na 2 O: Al 2 O 3 = 30: 1: 0.11: 0.02 was prepared by dissolving Ludox HS-40, NaOH, NaAlO 2 and DI water. After vigorous stirring overnight, the gel was transferred to an autoclave containing SiO 2 beads. The ratio of gel: beads was 10: 0.5 in mass %. The synthesis was performed in a rotating oven (20 rpm) at 120, 150, 170 °C for 20 h. The ZSM-5@silica beads were separated from the product formed in the bulk and ultrasonicated in DI water. The procedure was repeated several times until the pH value reached 7. The composite beads were dried at 60 °C overnight. The obtained ZSM-5@silica beads were labelled as a function of synthesis temperature ZSB-120, ZSB-150, ZSB-170, respectively.

Characterization:

Powder X-ray diffraction (PXRD) analysis was performed on a PANalytical B.V. Empyrean powder diffractometer at 40kV and 40 mA using Cu Ka radiation (λ= 1.5418 Å) in the range of 2θ=4.0-40.0˚. A JEOL JSM-6510 scanning electron microscope equipped with EDX was used to study morphology and elemental analysis of the samples. Before PXRD analyses, the beads were ground into powder in an agate mortar. Thermogravimetric analysis (TGA) was performed on a SHIMADZU DTG-60 thermal analyzer system. Nitrogen adsorption analysis was performed on a Micromeritics ASAP 2460 volumetric adsorption analyzers with ultra-high-purity grade N 2 (99.999 %) at 77 K. Prior to the analysis, the samples were degassed at 300 °C for 12 h. The specific surface area of the beads was calculated using Brunauer-Emmett-Teller (BET) method within the relative pressure (P/P 0 ) range of 0.05-0.25.

The pore size distribution and total pore volume (V total ) were determined from the adsorption branch using the density function theory (DFT). A SHIMADZU UV-2450 spectrophotometer was used to analyze the concentration of aniline during adsorption tests.

Aniline decontamination test:

In a batch experiment, 10 mg beads were placed in a glass bottle containing 10 mL aniline solution with concentrations from 5 to 300 mg L -1

, then shaken at room temperature with speed of 200 rpm for 12 h. The residual concentration of aniline was determined using a UV spectrophotometer at the corresponding maximum absorption wavelength (286 nm). The adsorbed amount was calculated from the equation q e =V (C 0 -C e )/m, where q e is the equilibrium amount adsorbed (mg g -1

), V is the volume of the liquid phase (L), C 0 and C e are the initial and the equilibrium concentration of aniline solution (mg L -1

), respectively, and m is the mass of the adsorbent (g). All adsorption experiments were repeated three times to ensure an adequate reproducibility.

Results and discussion

Structure and morphology analysis

Anionic exchange resin beads with gel-type microporous structures were chosen as a hard template, and silicate species originating from a sodium silicate solution were introduced within the resin structure by ion exchange. After resin combustion, SiO 2 beads with a shape and size similar to that of resin beads were obtained. The obtained SiO 2 beads were subjected to hydrothermal treatment with ZSM-5 yielding gel, which did not contain an OSDA. Figure 1 presents the XRD patterns of the initial SiO 2 beads and ZSB-120, ZSB-150 and ZSB-170 composites. The SiO 2 beads are amorphous, while the Please do not adjust margins Please do not adjust margins hydrothermally treated ones contain ZSM-5. The ZSM-5 amount depends on the crystallization temperature. Thus, a small amount of crystalline phase is detected in the ZSB-120 sample. The ZSM-5 peaks are more intense in ZSB-150. As for ZSB-170, the crystallinity is higher compared with the ZSB-150 sample. The mother liquor from ZSB-120 and ZSB-150 synthesis system does not contain crystalline material (Figure S1), which indicates that the crystallization of ZSM-5 on the silica beads outpaces the formation of the zeolite in the bulk solution. The only exception is ZSB-170, where ZSM-5 formed in the bulk liquor due to the faster crystallization kinetics at this temperature. The broad peak, with a maximum at about 2θ≈22°indicates the presence of amorphous SiO 2 in all samples, i.e., the silica beads were not fully converted into the zeolite. In summary, the ZSM-5 growth is strongly influenced by the synthesis temperature in the OSDA-free system. At low temperature (120 °C), there is only a trace of ZSM-5 on the surface of silica beads. The raise of crystallization temperature to 150 °C resulted in more abundant formation of ZSM-5 on the beads, but no crystalline phase was found in the bulk solution. Further increasing of the synthesis temperature (170 °C) resulted in ZSM-5 formation in both, the silica beads and the bulk solution. It means silica beads plays an important role in accelerating the crystallization process in this OSDA-free system. Please do not adjust margins Please do not adjust margins Representative optical photographs and SEM micrographs of the obtained SiO 2 and ZSB-120, 150 and 170 beads are shown in Figure S2 and Figure 2. The SiO 2 and ZSBs beads all preserve their spherical morphology. The surface of silica beads is smooth (Figure 2a) built up of uniform particles (Figure 2e,i). Isolated ZSM-5 crystallites in size 100-200 nm can be found at the surface of the ZSB-120 sample (Figure 2b, f, j). Well-intergrown and continuous zeolite ZSM-5 shell is formed in the case of ZSB-150 (Figure 2c,g). It is difficult to distinguish the individual crystals due to their small size. The thickness of the shell is about 1 µm (Figure 2k). Also, wellintergrown but discontinues ZSM-5 shell was observed in the case of ZSB-170 (Figure 2d, h), the top layer is built of wellshaped crystallites with thickness in 3-5 um (Figure 2h,l). Moreover, ZSM-5 crystals collected from the bulk solution and denoted BS-170 show the characteristic of ZSM-5 coffin morphology (Figure S3). This phenomenon can be interpreted that the nucleus at 170 °C with beads is affected by its mother solution, resulting in the discontinuous morphology on the layer. Zeolite nucleation is a heterogeneous process where foreign surfaces play an important role, the surface of silica beads was dissolved and converted to intergrown ZSM-5 layer, forming core-shell zeolite composites. [START_REF] Smaihi | Investigation of the Crystallization Stages of LTA-Type Zeolite by Complementary Characterization Techniques[END_REF] Precisely, higher concentration of silicon species on the beads prompting the zeolite formation precedes that in the bulk mother liquor. Chemical composition of the formed beads was studied by EDX analysis (Table S1). Only silicon and a small amount of sodium were detected in the initial silica beads. The Si/Al ratio of ZSB-120 is relatively high (ca. 20), since the result is influenced by all silica beads laying below the thin ZSM-5 shell. The Si/Al ratio in the ZSB-150 and ZSB-170 is about 15, which is reasonable for template-free synthesized ZSM-5. The nitrogen adsorption-desorption isotherms of the beads are presented in Figure 3, and the DFT pore size distributions in Figure S4. All the isotherms showed a steep uptake at low pressure region, typical of a microporous material, which conforms to type I isotherm according to the IUPAC classification. The hysteresis loop at high relative pressure is a consequence of the presence of large mesopores. The isotherm of SiO 2 beads is different; the uptake at low relative pressure is small and followed by the inclination of the isotherm revealing the presence of mesopore with different sizes. The surface and porous characteristics calculated from the analysis of the isotherms are summarized in Table 1. All ZSB samples show a micropore volume higher than the SiO 2 beads. The micropore volume of the ZSM-5@silica beads obtained at 150 and 170 °C is higher which correlates with the higher zeolite content (Table 1). We attribute this result to the partial transformation of silica beads into zeolite and the generation of additional micro porosity in the silica matrx.

37-38

The XRD diffraction data of ZSB-150 and ZSM-170 (Figure 1) are in agreement with this conclusion.

Thermal analysis was employed to evaluate the impact of hydrothermal treatment on the all-silica and ZSB beads (Figure S5). The most significant weight loss is between 25 and 450 °C observed for all beads, which is due to the water release and dehydroxylation of the silica and zeolite framework. ZSB-120 shows slightly higher weight loss in respect of the SiO 2 beads. The release amount is again higher in ZSB-150 and ZSB-170 samples, which is a function of the zeolite content in the beads. Thus, the TG analysis confirm the results from the XRD and N 2 adsorption analyses.

Aniline adsorption from aqueous solution

The all-silica and zeolite@silica beads were tested in the decontamination of aqueous aniline-containing solutions. Absorption spectra with various concentrations of aniline were recorded to prepare the standard curves (Figure S6). The molecular size of aniline is 0.59 nm, which is close to the opening pore of the ZSM-5. There is the so-called close-fit theory, zeolite shows distinct adsorption affinity to molecules with close pore size. [START_REF] De Ridder | Zeolites for nitrosamine and pharmaceutical removal from demineralised and surface water: Mechanisms and efficacy[END_REF][START_REF] Rossner | Removal of emerging contaminants of concern by alternative adsorbents[END_REF][START_REF] Guo | Adsorption of Low-Molecular-Weight Amines in Aqueous Solutions to Zeolites: An Approach to Impeding Low-Molecular-Weight Amines from Regenerating N-Nitrosamines[END_REF] Moreover, acid-base interactions between the amino-group of aniline and Brønsted acid sites in Please do not adjust margins Please do not adjust margins zeolite@silica beads also contribute to the adsorption performance. [START_REF] Jiang | Preparation of a novel manganese oxide-modified diatomite and its aniline removal mechanism from solution[END_REF] Thus the size-dependent interaction and acidbase interactions between the targeted pollutant and the zeolite-containing beads is expected to provide a highly selective and efficient removal property for aniline.

The sorption kinetics of aniline over the beads as a function of contact time was studied (Figure 4a). The uptake was fast in the first 20 min, owing to the abundantly available adsorption sites. Then, the adsorption rate gradually slowed down and reached equilibrium after 90 min. The series of ZSB beads showed significantly enhanced sorption capacity toward aniline compared with SiO 2 beads. This result was related to the high selectivity of the zeolite to the aniline. The ZSB-150 beads demonstrated the highest aniline removal capacity, although this sample contains less zeolite than ZSB-170. We attribute this result to the good balance between zeolite content and accessibility in this composite. In other words, as explained by the diffusion-adsorption model, the formation of a large amount of intergrown ZSM-5 in ZSB-170 probably has a negative impact on the aniline diffusion and decreases the adsorption capacity. 23, 43-44 Therefore, the subsequent adsorption experiments were performed with ZSB-150 and allsilica beads.

The obtained data were used to build mathematical models using pseudo-first-order and pseudo-second-order equations: ln (q e -q t ) = ln q e -k 1 t and t/q t = 1/k 2 q e 2 + t/q e , where q t (mg g -1 ) and q e (mg g -1

) are the adsorption capacities at any time and equilibrium, respectively. The k 1 (min -1 ) and k 2 (g mg -1 min -1 ) represent the constants of the pseudo-firstorder and pseudo-second-order rates, respectively. The simulated data are shown in Figure S7 and the corresponding parameters calculated from the models are listed in Table 2. All values of correlation factor (R 2 ) obtained from the plots of pseudo-second-order kinetics are greater (R 2 >0.98), indicating that the adsorption kinetics of aniline on the studied adsorbent fits well with this model. It also showed that the calculated q e values are well-accorded with the actual experimental values, and the adsorption towards aniline almost reached equilibrium in 2 h. Similar results were reported for the aniline adsorption on different adsorbents.

6, 45

The equation q t = k id t 1/2 + C, was used to study the diffusion in the adsorbents. 46-47 where q t (mg g -1 ) is the adsorption capacities at any time, k id is the intra-particle diffusion rate constant (mg g -1 min

-1/2
), and C is the thickness of boundary layer. The multi-linear plot reveals that the adsorption process containing several stages. Indeed, the data presented in Figure 4b shows that the aniline adsorption occurred in three stages revealed by the three segment slopes in the scatter plot. The first linear portion, showing the fastest adsorption rate, is due to the transfer of the aniline molecules from the aqueous phase to the external surface of beads. The second slower step is related of the aniline diffusion in the intraparticle space, and the final linear portion is the result of diffusion through the micropore systems of ZSM-5 and the reaction with existing adsorption sites. This interpretation is in line with previously published data of adsorption of organic pollutants on porous materials. 48 The sorption of aniline on SiO 2 and ZSB-150 beads as a function of the initial concentration was studied. As can be seen in Figure 5a, the adsorption capacities increase with the increase in aniline concentration, which is attributed to the incremental driving force caused by the initial concentration of adsorbate. The plato in the isotherm appears when the available adsorption sites are exhausted, i.e., the concentration of adsorbate exceeds the adsorption capacity of the adsorbent. The ZSB-150 beads showed a much remarkable uptake, and its adsorption isotherm reached a plato more delayed than SiO 2 beads, which is attributed to a large number of active sites within zeolite micropores. In order to get deeper insights in the mechanism of aniline sequestration, the adsorption curves were fitted with the Langmuir (1), Freundlich (2) and Temkin models (3), [START_REF] He | MOF-cation exchange resin composites and their use for water decontamination[END_REF] according to the equations: C e /q e =1/K L q max + C e /q max (1) ln q e =ln K F + 1/n ln C e ( 2)

q e =RT/b ln K T + RT/b ln C e (3)
where C e is the residual concentration at equilibrium in solution (

); q e is the amount adsorbed at equilibrium (mg g -1

); q max is the maximum adsorption capacity at equilibrium (mg g -1 ); and K L , K F and K T are the corresponding constants, respectively. Analyses on the aniline adsorption isotherms by these three models are shown in Figure S8 and the fitted parameters summarized in Table 3. The R 2 value of the Langmuir model shows the best fit for the aniline sorption on the ZSB-150 sample (with R 2 of 0.9887), and the calculated maximum adsorption capacity is close to the experimental value. This result suggests that the adsorption of aniline on ZSB-150 could be modeled by monolayer adsorption. It reveals a highly uniform distribution of ZSM-5 in the SiO 2 matrix. The fitted models for the aniline adsorption on SiO 2 and ZSB-150 beads could be interpreted on the basis of acid-base interactions and adsorption in micropore space. As mentioned, the silanol of SiO 2 and ZSB-150 can form specific acid-base interactions with an aniline molecule, while the additional confinement effect in zeolite micropores greatly enhances the adsorption performance.

The effect of pH on the aniline uptake is shown in Figure 5b. The aniline uptake is higher under acidic conditions with a maximum at pH∼3. A gradual decrease of the aniline adsorption in the neutral and basic region is observed for ZSB-150 as the lowest value is 36 %. In the case of SiO 2 beads, a dramatic decrease of the adsorbed amount takes place at neutral pH, which reaches the lowest value (ca. 1 %) at pH = 12. The fact that aniline adsorption is significantly dependent on pH, is explained by the aniline state at different pH conditions.

45, 50 Aniline appears as aniline cation (Ph-NH 3 + ) under acid condition, the sizes of ionized aniline are smaller than that in neutral conditions. So, the shape-selective adsorption of ZSM-5 towards aniline gets enhanced. [START_REF] Guo | Adsorption of Low-Molecular-Weight Amines in Aqueous Solutions to Zeolites: An Approach to Impeding Low-Molecular-Weight Amines from Regenerating N-Nitrosamines[END_REF] When pH gets more alkaline, acid-base interactions become ineffective due to the dissociation of silanols into SiO -. It is worth noting that, compared with the SiO 2 beads, the adsorption capacity of ZSB-150 is less impacted by the pH due to the additional confinement effect in zeolite micropores. Thus, the zeolite@silica composites show promising performance in the sequestration of aniline from an aqueous source in the neutral to a slightly acidic environment.

Sodium chloride, as a widespread inorganic salt, largely present in the waste water. Therefore, the impact of NaCl concentration on the aniline adsorption was studied. The concentrations of NaCl was ranged from 0 to 20000 mg L -1 at fixed aniline concentration (C 0 = 20 mg L -1

). As shown in Figure 6a, the efficiency of aniline removal remained almost constant at different NaCl concentrations.

The reusability is a crucial factor for the industrial perspective of an adsorbent. Recycling experiments were carried out under the initial concentration aniline of 20 mg L -1 . The used beads were regenerated by shaking 10 mg beads in 10 ml ethanol solution, for 24 h and vacuum-dried at 60 °C. The consecutive adsorption-desorption cycles were repeated five times, and the results for each adsorption process are presented in Figure 6b. The ZSB-150 beads maintain 90 % adsorption capacity after 5 cycles. The integrity of the ZSB-150 adsorbent was checked after five adsorption-desorption cycles. As shown in Figure S9, the used ZSB-150 beads did not lose crystallinity and retained the spherical shape. Thus, the recyclability results show that ZSM-5@silica beads can withstand multiple uses in water purification.

The capacities toward aniline of most often used and efficient sorbents are presented in Table 4 and compared with the performance of ZSB-150. Composite ZSM-5@silica beads show remarkable capacity outpacing all silica-based adsorbents and modified zeolitic material. We attribute this to the result of the specific shaping technique that allows the preparation of binder-free sorbent of high adsorption efficiency. The high adsorption ability can also be ascribed to the well-defined microporous structure of zeolite with strong adsorption sites. Overall, the composite beads prepared by This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins present work is a potential candidate to uptake aniline from aqueous media.

Conclusions

All-silica beads of size 300-400 µm were prepared by using anion exchange resin as a macro template, followed by the combusting of the resin structure. The obtained all-silica beads were subjected to hydrothermal treatment in a ZSM-5 yielding OSDA-free system. The resultant ZSM-5@silica composite retains the macro-morphological features of the ion exchange resin beads. The zeolite content and the secondary mesopore system of the composite beads were controlled by varying the synthesis temperature.

The obtained composite ZSM-5@silica beads were used for sequestration of aniline from aquous solutions. The composite obtained at 150 °C showed the highest aniline uptake. The results of the adsorption model showed that the adsorption processes fit well with the pseudo-second-order model and Langmuir isotherm model. This result was attributed to the substantial conversion of silica into ZSM-5 and retained micromeso pore system, which favors the diffusion of the aniline.

It should be noted that the synthetic method developed in the present study offers a simple and green perspective of the preparation of zeolite-based adsorbents. The proposed method is expected to be applied for the preparation of other types zeolite-based composite materials.

Figure 1 .

 1 Figure 1. Powder XRD patterns of the SiO2 and the ZSM-5@silica beads.

Figure 2 .

 2 Figure 2. General view (a-d), external surface (e-h) and cross section (i-l) SEM micrographs of SiO2 (a, e, i), ZSB-120 (b, f, j), ZSB-150 (c, g, k), and ZSB-170 (d, h, l) beads.

31 - 32 Figure 3 .

 31323 Figure 3. Nitrogen sorption isotherms of SiO2, ZSB-120, ZSB-150, ZSB-170 beads. Note: adsorption -full symbols; desorption -empty symbols.

Figure 4 .

 4 Figure 4. The adsorbed amount of aniline(a) and Intra-particle diffusion plots of aniline (b) on the obtained SiO2, ZSB-120, ZSB-150, ZSB-170 beads (adsorption temperature: 25 °C, Co: 20 mg L -1 , adsorbent: 1 g L -1).

Figure 5 .

 5 Figure 5. Aniline adsorption capacity(a) and Impact of pH on the aniline adsorption (b) of SiO2 and ZSB-150 beads (adsorption time: 12 h, adsorption temperature: 25 °C, Co: 20 mg L -1 , adsorbent:1 g L -1).

Figure 6 .

 6 Figure 6. Effect of NaCl concentration (a) and Recyclability (b) on the aniline adsorption of ZSB-150 beads (adsorption time: 12 h, adsorption temperature: 25 °C, Co: 20 mg L -1 , adsorbent:1 g L -1).

Table 1 .

 1 Surface and porous characteristics of the obtained beads

	Sample	S BET (m	2 g -1 ) V total (cm	3 g -1 ) V micro (cm	3 g -1 )
	SiO 2 beads	289	0.16	0.06
	ZSB-120	287	0.14	0.10
	ZSB-150	361	0.21	0.13
	ZSB-170	345	0.32	0.11

Table 2 .

 2 Kinetic parameters of aniline adsorption on different beads at 25°C from solution with 20 mg L -1 concentration

		Pseudo-first-order kinetic model		Pseudo-second-order kinetic model
	Adsorbents	qe,	k1,	R 2	qe,	k2,	R 2
		mg g -1	min -1		mg g -1	g mg -1 min -1
	SiO2	15.4464	0.0139	0.9762	14.614	0.0053	0.9888
	ZSB-120	39.2285	0.0139	0.9952	38.461	0.0012	0.9842
	ZSB-150	85.8940	0.0191	0.9833	86.957	0.0009	0.9977
	ZSB-170	61.0261	0.0214	0.9328	64.103	0.0012	0.9992

6 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins

Table 3 .

 3 Adsorption isotherm model parameters derived from the Langmuir, Freundlich and Temkin model

		Langmuir					Freundlich				Temkin
	Adsorbents	q max		K L		R L	2	K F (mg g	-1 )	n	R F	2	K T	b	R T 2
		(mg g	-1 )	(L mg	-1 )			(L mg	-1 ) 1/n				(J g -1 )	(J mol	-1 )
	SiO 2	370.37	0.0012	0.3669	0.473		1.04	0.9923	0.1222	138.85	0.8177
	ZSB-150	120.48	0.0626	0.9887	12.367		2.14	0.9704	2.0678	140.90	0.9352

Table 4 .

 4 Comparison of the adsorption capacity of different sorbents used for aniline sequestration.

	Adsorbent	Adsorption capacity (mg g -1 )	Ref.
	Modified diatomite	43	42
	MCM-41	17	51
	Cu-Beta	110	52
	Siliceous Zeolitic Material	49	53
	Ca-Zeolitic Composites	34	54
	graphitic carbon nitride	66	9
	Electrospun PIM-1 fiber	161	55
	Poly acrylamide/SiO2	52	56
	Mesoporous silica nano-	18	48
	spheres
	ZSB-150	115	Present
			work

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 | 5

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7

Acknowledgements

Q. F. and V. V. thank the National Natural Science Foundation of China (21571079, 21621001, 21390394, 21571076 and 21571078) for their financial support. This study was partially supported by the Sino-French International Associated Laboratory "Zeolites".

Conflicts of interest

There are no conflicts to declare.

ARTICLE


Guoju Yang

Ji Han

Yujun Huang

Xiaoxin Chen
email: chenxiaoxin@jlu.edu.cn

Valentin Valtchev
email: valentin.valtchev@ensicaen.fr

Busting the efficiency of SAPO-34 catalysts for the methanol-to-olefin conversion by post-synthesis methods

Keywords: Zeolites, SAPO-34, Post-synthesis treatment, Hierarchical, MTO

As an effective non-petroleum based process for producing light olefins, the methanol-to-olefin (MTO) route has become an indispensable alternative to the industrial production of light olefins. The silicoaluminophosphate SAPO-34 zeolite (CHA-type structure) has proven to be an efficient industrial catalyst for the production of ethylene and propylene by the MTO reaction. However, the inherent structure and related diffusion limitations of SAPO-34 limit the mass transport and thus cause rapid deactivation of the catalyst. Fabrication of hierarchical SAPO-34 zeolite is one of the most effective strategies to address the intrinsic diffusion limitation. As simple, inexpensive, and efficient approach, the post-synthetic route has attracted considerable attention and widely used to introduce secondary meso-/macropores into the microporous SAPO-34 material. Significant effort has been dedicated to the development of post-synthesis strategies to prepare hierarchical SAPO-34 zeolite, thereby enhancing its catalytic performance in the MTO process. This mini-review addresses the post-synthesis preparation of hierarchical SAPO-34 catalysts and their MTO performance. Furthermore, some current problems and prospects of the post-synthesis route to hierarchical SAPO-34 catalysts are also revised. We expect this mini-review to inspire the more efficient preparation of hierarchical SAPO-34 catalysts for the MTO process.

Introduction

Light olefins, such as ethylene, propylene, are essential raw materials in the chemical industry of various polymers. [START_REF] Stocker | Methanol-to-hydrocarbons: Catalytic materials and their behavior[END_REF][START_REF] Yang | Recent progress in methanol-to-olefins (MTO) catalysts[END_REF] Traditionally, light olefins are petrochemical derivatives produced by naphtha cracking. The development of alternative non-oil route for light olefins production is imperative to overcome the expected oil shortage worldwide and meet the increasingly high demand for light olefins. The methanol-to-olefin (MTO) reaction, as a linkage between coal/natural gas industry and chemical production, has provided a successful non-petroleum route to produce light olefins from non-oil resources.

Consequently, intense research efforts have been dedicated to the understanding of MTO reaction from both the academia and industry. The MTO reaction was first reported by Mobil Oil Corporation in 1977. [START_REF] Chang | The conversion of methanol and other O-compounds to hydrocarbons over zeolite catalysts[END_REF] Through the years, significant progress was achieved in terms of the reaction mechanism, catalyst's synthesis, and process development in the MTO. [START_REF] Yang | Recent progress in methanol-to-olefins (MTO) catalysts[END_REF][START_REF] Tian | Methanol to olefins (MTO): From fundamentals to commercialization[END_REF] Design and optimization of an efficient catalyst is critical for any industrial process.

Microporous crystalline zeolites have been proved to be the most efficient catalysts for the MTO reaction, due to their regular microporous architectures, unique shape selectivity, appropriate acidity, and high thermal and hydrothermal stability. [START_REF] Tian | Methanol to olefins (MTO): From fundamentals to commercialization[END_REF][START_REF] Yu | Rational approaches toward the design and synthesis of zeolitic inorganic open-framework materials[END_REF][START_REF] Wang | Needs and trends in rational synthesis of zeolitic materials[END_REF][START_REF] Li | New stories of zeolite structures: Their descriptions, determinations, predictions, and evaluations[END_REF][START_REF] Weckhuysen | Recent advances in zeolite chemistry and catalysis[END_REF][START_REF] Li | Applications of zeolites in sustainable chemistry[END_REF][START_REF] Sun | The state-of-the-art synthetic strategies for SAPO-34 zeolite catalysts in methanol-to-olefin conversion[END_REF][START_REF] Yang | The Nature of Hydrogen Adsorption on Platinum in the Aqueous Phase[END_REF][START_REF] Yang | An amino acid-assisted approach to fabricate nanosized hierarchical TS-1 zeolites for efficient oxidative desulfurization[END_REF] Amongst them, the SAPO-34 (CHA-type) silicoaluminophosphate that possesses three-dimensional channels with 8-ring pore openings (0.38 nm × 0.38nm) and moderate acidity is particularly appropriate for the MTO reaction. [START_REF] Lok | Silicoaluminophosphate molecular sieves: another new class of microporous crystalline inorganic solids[END_REF] However, the SAPO-34 catalysts generally suffer from severe transport limitations leading to fast deactivation by the coke deposition. [START_REF] Chen | A methanol to olefins review: Diffusion, coke formation and deactivation SAPO type catalysts[END_REF][START_REF] Gao | Insight into the deactivation mode of methanol-to-olefins conversion over SAPO-34: Coke, diffusion, and acidic site accessibility[END_REF] Therefore, various synthetic strategies have been tested to minimize the intrapore diffusion limitation in SAPO-34 and improve their MTO performance. [START_REF] Bai | Creating hierarchical pores in zeolite catalysts[END_REF][START_REF] Pérez-Ramírez | Hierarchical zeolites: enhanced utilisation of microporous crystals in catalysis by advances in materials design[END_REF][START_REF] Valtchev | Tailored crystalline microporous materials by post-synthesis modification[END_REF] One of the most widely employed approaches is to introduce secondary meso-/macro-pores into the intrinsic microporous structure of SAPO-34, i.e., the fabrication of a hierarchical porous structure. In this review, we present recent progress in the post-synthesis approaches to hierarchical SAPO-34 zeolites. Besides the post-synthetic methods, the analysis addresses the current challenges and outline the perspectives. The ultimate goal of this mini-review is to highlight the most effective post-synthesis approaches for yielding hierarchical SAPO-34 catalysts with excellent catalytic performance in the MTO reaction.

Synthetic strategies employed in the preparation of hierarchical SAPO-34 zeolite

Materials and Methods

The synthetic routes operated in the development of hierarchical SAPO-34 zeolites can be classified into two groups, i.e., bottom-up and top-down approaches.

The bottom-up approach refers to the use of sacrificial meso-porogens (soft or hard template) besides the organic structure-directing agent (OSDA) in the starting zeolite synthesis system. [START_REF] Schmidt | Carbon templated SAPO-34 with improved adsorption kinetics and catalytic performance in the MTO-reaction[END_REF][START_REF] Varzaneh | Carbon nanotube templated synthesis of metal containing hierarchical SAPO-34 catalysts: Impact of the preparation method and metal avidities in the MTO reaction[END_REF][START_REF] Liu | Preparation of macroporous SAPO-34 microspheres by a spray drying method using polystyrene spheres as hard template[END_REF][START_REF] Sun | Organosilane surfactant-directed synthesis of hierarchical porous SAPO-34 catalysts with excellent MTO performance[END_REF][START_REF] Wang | Polyethyleneimine templated synthesis of hierarchical SAPO-34 zeolites with uniform mesopores[END_REF][START_REF] Egeblad | Templating mesoporous zeolites[END_REF] Although it is a flexible and efficient way to prepare hierarchical SAPO-34 there are substantial drawbacks that make this approach not very attractive from a practical viewpoint. First, the high-cost of meso-porogens should be considered in large-scale production. Second, the elimination of the meso-porogen usually includes high-temperature combustion, which is an energy-intensive process. However, the combustion of large organic species leads to the generation of substantial heat and water, which causes the partial collapse of zeolite framework or the generation of extra-framework aluminum species. Furthermore, defective sites might be generated during the formation of hierarchical structures via meso-porogens, which deteriorates catalyst's hydrothermal/mechanical stability. These problems are certainly challenging to be surmounted in large-scale operations.

Compared with the bottom-up method, the alternative top-down route, which includes a post-synthetic chemical etching of zeolite framework, is easier to be implemented and does not require high-temperature treatment. Different chemical etching procedures were developed to convert conventional aluminosilicate zeolites in hierarchical materials, such as vapor steaming and various mineral acid and base caustic leaching. [START_REF] Pérez-Ramírez | Hierarchical zeolites: enhanced utilisation of microporous crystals in catalysis by advances in materials design[END_REF][START_REF] Valtchev | Tailored crystalline microporous materials by post-synthesis modification[END_REF][START_REF] Jiao | Creation of Al-enriched mesoporous ZSM-5 nanoboxes with high catalytic activity: Converting tetrahedral extra-framework Al into framework sites by post treatment[END_REF][START_REF] Jiao | Inter-connected and open pore hierarchical TS-1 with controlled framework titanium for catalytic cyclohexene epoxidation[END_REF][START_REF] Zhang | Using ultrasound to improve the sequential post-synthesis modification method for making mesoporous Y zeolites[END_REF][START_REF] Van Donk | Generation, characterization, and impact of mesopores in zeolite catalysts[END_REF][START_REF] Valtchev | Hierarchical zeolites[END_REF][START_REF] Qin | Mesoporous zeolites by fluoride etching[END_REF][START_REF] Thibault-Starzyk | Quantification of enhanced acid site accessibility in hierarchical zeolites -The accessibility index[END_REF][START_REF] Verboekend | Design of hierarchical zeolite catalysts by desilication[END_REF] However, the generation of a hierarchical SAPO-34 material by post-synthesis treatment etching is relatively tricky owing to the alternation of Al and P in the framework, the latter being easily destabilized by chemical etching. Therefore, it was highly desirable to develop applicable post-synthetic approaches specific for SAPO-34, which generate hierarchical material without destabilizing the framework and substantially changing the framework composition. Fig. 1 illustrates the impact of the chemical etching on the fabrication of hierarchical SAPO-34 zeolites. In the following section, the representative post-synthesis approaches are summarized and discussed in detail.

Post-synthesis methods applied in the preparation of hierarchical SAPO-34

The conventional post-synthesis etching is a demetalizing process, which usually involves chemically selective extracting a framework cation, thereby changing the zeolite framework composition and acidity. Since the framework of SAPOs is generally less stable than the aluminosilicate zeolites, it is difficult to tailor SAPO-34 zeolites using traditional acid/base etching. The lower chemical stability of SAPO-34 is due to the fact that the silicon amount is limited, and the aluminum and phosphorous are alternate in the CHA-type framework. Thus, the selective chemical extraction destabilizes the structure (Fig. 2). Therefore, only an etching that allows unbiased extraction of framework elements would allow the efficient generation of hierarchical SAPO-34 with retained physicochemical properties.

Acid etching

Various mineral or organic acids have been employed to introduce secondary pores in SAPO-34. Ren et al. reported that SAPO-34 crystals subjected to nitric acid etching exhibit hierarchical structures with butterfly-shaped porous patterns on the faces, as shown in Fig.

3(A)

. [START_REF] Ren | Enhanced MTO performance over acid treated hierarchical SAPO-34[END_REF] The secondary pores in the obtained hierarchical SAPO-34 comprise meso-and macropores. The oxalic acid was also applied to etch SAPO-34 zeolites. As can be seen in Fig. 3(A), the crystal faces of SAPO-34 after post-etching by oxalic acid exhibit a similar secondary pore system but less obvious butterfly-shaped porous patterns in comparison with those of nitric acid etching SAPO-34. The BET specific surface area of the oxalic acid treated SAPO-34 increased sharply from 665 m 2 /g of parent sample to 876 m 2 /g. Compared with the parent counterpart, the obtained hierarchical SAPO-34 shows longer catalyst lifetime and higher selectivity to light olefins in the MTO process.

Qiao et al. prepared hollow SAPO-34 hierarchical crystals with a shell thickness of about 30 nm by hydrochloric acid etching (Fig. 3(B)). [START_REF] Qiao | Creation of hollow SAPO-34 single crystals via alkaline or acid etching[END_REF] The HCl acid treatment leads to preferential extraction of Al and P resulting in the formation of Si island in the crystal shell.

Silicon gradient in the parent crystals plays a vital role in the generation of a hollow structure.

A solvent-free process was developed to prepare hierarchical SAPO-34 by using solid oxalic acid. This melting-assisted route is beneficial from a water-saving viewpoint. [START_REF] Liu | Melting-assisted solvent-free synthesis of hierarchical SAPO-34 with enhanced methanol to olefins (MTO) performance[END_REF] After post-treatment with solid oxalic acid, there are butterfly patterned abundant meso-/macro-pores penetrating in the SAPO-34 crystals (Fig. 3(C)). The resultant hierarchical SAPO-34 zeolite shows a micro-, meso-, and macro-porous composite pore structure, high BET surface area, as well as a higher concentration of weak acid sites, thus improving the catalytic performance in MTO reactions.

. Besides introducing a hierarchical porous structure in SAPO-34 crystals, it is important to tune the content and distribution of silicon, thereby adjusting the acidity of the material. Shen and co-workers prepare the hierarchical SAPO-34 zeolites with slit-shaped mesopores via citric acid etching. [START_REF] Jin | Selective desilication, mesopores formation, and MTO reaction enhancement via citric acid treatment of zeolite SAPO-34[END_REF] Thus generated mesopores start from the crystal surfaces and end at the center of the crystals. The citric acid leaching leads to a substantial increase in the concentrations Lewis and Brønsted acid sites on the external surface of SAPO-34. Still, it decreases the total amount of acid sites. The resultant hierarchical SAPO-34 material exhibits enhanced catalytic performance in the MTO reaction with about 2.5 times extended catalytic lifetime and a nearly 8% improvement of selectivity for ethylene and propylene.

Base etching

The NaOH aqueous solution is commonly used as an etchant in the preparation of hierarchical aluminosilicate zeolites. [START_REF] Verboekend | Design of hierarchical zeolite catalysts by desilication[END_REF][START_REF] Abello | Mesoporous ZSM-5 zeolite catalysts prepared by desilication with organic hydroxides and comparison with NaOH leaching[END_REF] However, due to the limited chemical stability, the SAPO structure is very sensitive to NaOH etching. Compared with the strongly alkaline inorganic base, the organic base with moderate strength, are expected to be more appropriate in terms of gentle treatment and thus to preserve the crystallinity of SAPO-34. [START_REF] Verboekend | Hierarchical silicoaluminophosphates by postsynthetic modification: Influence of topology, composition, and silicon distribution[END_REF] Liu and co-workers prepared hollow SAPO-34 crystals with a shell thickness of 50 nm through post-etching with tetraethylammonium hydroxide (TEAOH), tetrapropylammonium hydroxide (TPAOH), tetramethylammonium hydroxide (TMAOH) and diethylamine (DEA), as shown in Fig. 4(A). They use relatively high etching temperature (180 °C). According to these authors, the abundant and interconnected Si-O-Al domains that are highly stable in the SAPO framework are of great help to endure the base leaching. Owing to the higher Si content in the rim of the crystals hollow structures were formed. [START_REF] Verboekend | Hierarchical silicoaluminophosphates by postsynthetic modification: Influence of topology, composition, and silicon distribution[END_REF] These very open SAPO-34 materials demonstrated the enhanced catalytic performance in the MTO reaction.

Zeng et al. performed the post-treatment of SAPO-34 zeolites in a lower concentrated TEAOH etching solution and using lower etching temperature (90 °C). [START_REF] Liu | Coke suppression in MTO over hierarchical SAPO-34 zeolites[END_REF] Different from the hollow structures reported by Liu et al., [START_REF] Qiao | Creation of hollow SAPO-34 single crystals via alkaline or acid etching[END_REF] the obtained hierarchical SAPO-34 zeolites exhibit symmetric meso-/macropores dissolution with triangle pore patterns (Fig. 4(B)).

The different etching patterns might be a consequence of different templates used by the In order to develop an efficient post-synthetic treatment method to hierarchical SAPO-34 with minimum mass loss, trimethylamine (TEA) aqueous solution was used as an etchant. [START_REF] Pan | Efficient post-synthesis of hierarchical SAPO-34 zeolites via organic amine etching under hydrothermal conditions and their enhanced MTO performance[END_REF] The particularity of this treatment is that the etching is coupled with a recrystallization process, as dissolution and recrystallization occur simultaneously. Thus a high yield of hierarchical SAPO-34 zeolites (ca. 85%) with larger secondary pores after post-etching in TEA solution was obtained. The type and the level of dissolution are dependent on the treatment conditions (temperature, time, etc.), as shown in Fig. 4(C). Such an approach ensures less mass loss while retains the chemical composition. Meanwhile, there is an increase in the external surface area and the mesopore volume during the introduction of secondary porosity via such post-etching in TEA solution. Consequently, the resultant hierarchical SAPO-34 catalysts show improved catalytic activity in the MTO reaction due to the remarkably enhanced transport properties coupled with modified acidity.

Fluoride etching post-synthesis

Hydrofluoric acid (HF) is commonly used to dissolve silicon-containing materials.

Valtchev and co-workers have developed fluorine-based methods for the preparation of hierarchical zeolite materials by post-synthesis etching. [START_REF] Qin | Mesoporous zeolites by fluoride etching[END_REF][START_REF] Valtchev | High energy ion irradiation-induced ordered macropores in zeolite crystals[END_REF] The particularity of the method is the unbiased dissolution of framework Si and Al, thus retaining the chemical composition of the material. This is possible by employing the bi-fluoride anion as an etching agent, achieved by mixing HF and NH 4 F. [START_REF] Qin | Chemical equilibrium controlled etching of MFI-type zeolite and its influence on zeolite structure, acidity, and catalytic activity[END_REF] Later, the same group reported the unbiased chemical etching based on the sole use of NH 4 F. [START_REF] Qin | The mosaic structure of zeolite crystals[END_REF][START_REF] Qin | Opening the cages of Faujasite-type zeolite[END_REF] The fluoride post-etching method is very flexible and controllable for regulating both the density and size of the The unbiased chemical etching of zeolite framework is particularly appropriate for the etching SAPO-type materials. In 2016, Chen et al. applied the fluoride post-synthesis route to fabricate hierarchical SAPO-34 zeolites. [START_REF] Chen | The preparation of hierarchical SAPO-34 crystals via post-synthesis fluoride etching[END_REF] By controlling the ratio between HF and NH 4 F in the etching solution and using ultrasonic radiation, hierarchical SAPO-34 was prepared. Noteworthy, the preferential dissolution of highly defected interfaces between crystalline domains was observed, thus only the most crystalline part of SAPO-34 remain after the etching (Fig. 5(A)). The number of Brønsted acid sites of the hierarchical materials decreases substantially during the fluoride treatment. Compared to the parent sample, the obtained hierarchical SAPO-34 exhibits longer catalytic lifetime and higher selectivity of low carbon olefins in the MTO reaction. The improved catalytic performance is attributed to the remarkably enhanced mass transportation in CHA structure and preserved highly crystalline domains, as well as the more appropriate moderate acid strengths (Table S1).

Chen et al. have also investigated the effect of fluoride etching on SAPO-34 crystals templated by morpholine, triethylamine, and tetraethylammonium. [START_REF] Chen | A top-down approach to hierarchical SAPO-34 zeolites with improved selectivity of olefin[END_REF] The impact of the OSDA on the formation of a hierarchical SAPO-34 was studied in detail. The authors reported that the dissolution process strongly depended on the organic templates employed in the synthesis of SAPO-34 (Fig. 5(B)). The hierarchical SAPO-34 catalysts exhibited higher selectivity to C 3 H 6 than the parent SAPO-34 in the MTO reaction. It was also found that the post-synthesis fluoride etching not only generates a secondary large pore system in SAPO-34, but also extracts the silicon preferentially at highly defected zones between the crystalline domains. Thus, both the large pore generation and the decrease of Si content are beneficial for improving the catalytic performance of SAPO-34.

Alternative etching agents that could provide hierarchical SAPO-34 material are reported (Table 1). In general, a hierarchical SAPO-34 can be obtained with any enchantable to dissolve the CHA-framework.

Conclusion and outlook

The practical importance of the MTO process continuously boosts the development and optimization of SAPO-34-based catalysts. The fabrication of hierarchical structures in the microporous SAPO-34 zeolite by the facile post-synthesis method can improve mass-transfer resistance, open the access to the active-sites, modify the acidity and retard coke deposition. Therefore, it has been one of the most promising routes to prepare hierarchical SAPO-34 zeolite to facilitate its catalytic performance in MTO reactions.

However, the post-synthesis method modifications are often at the expense of the framework stability, crystallinity, and acidity. Moreover, it is important to gain precise regulation of hierarchical characteristics and active sites distribution for the construction of an efficient SAPO-34 catalysts. The efficient chemical etching coupled with the in-situ recrystallization process is expected to address stability and crystallinity issues.

The post-synthesis chemical etching also leads to substantial losses of crystalline material that could go over 50 wt.%. Furthermore, the use of an etching agent raises the synthetic cost, and in general, it is not environmentally benign. Therefore, it is highly desirable to optimize the current post-synthesis methods further. The development of alternative, economic, green, and efficient post-synthesis strategies to meet the increasing demand for hierarchical SAPO-34 is also highly desired. Hence, the solvent-free method presents considerable interest and an attractive alternative for the fabrication of hierarchical SAPO-34 catalysts.

This mini-review summarizes the most important issues related to the preparation of hierarchical SAPO-34 catalysts by post-synthetic treatment. Addressing these issues will open the boulevard to a more rational design of highly performing materials, and guide the future direction in this field.

Fig. 1 .

 1 Fig. 1. Conventional acid/base etching and its impact on the composition of hierarchical SAPO-34 zeolite.

Fig. 2 .

 2 Fig. 2. Summary of the post-synthesis etching approaches used in the preparation of hierarchical SAPO-34.

Fig. 3 .

 3 Fig. 3. SAPO-34 zeolites fabricated by acid etching. (A) SEM images of SAPO-34 crystals treated with nitric (S-1) and oxalic (S-2) acid. Reprinted from ref. 29 (Copyright 2017, Elsevier). (B) SEM (left) and TEM (right) images of SAPO-34 leached with HCl. Reprinted from ref. 30 (Copyright 2016, Royal Society of Chemistry). (C) SEM images of SAPO-34 (a and b) before and (d and e) after solid oxalic acid treatment, (c) schematic drawing of the growth of SAPO-34, and (f) schematic illustration of the morphology of treated SAPO-34. Reprinted from ref. 31 (Copyright 2018, Royal Society of Chemistry).

[ 38 ]Fig. 4 .

 384 Fig. 4. (A) SEM images of SAPO-34 samples after treatment with TEAOH and TEM images of SAPO-34 samples treated with TPAOH, TEAOH, and TMAOH. Reprinted from ref. 30 (Copyright 2016, Royal Society of Chemistry). (B) SEM images of SAPO-34 crystals (a) before and (b) after TEAOH etching; (c) TEM images of (c) a side face with touching triangles and (d) a side face without triangle dissolution. Insets: Schematics illustration of (a) parent SAPO-34 and (b) hierarchical SAPO-34 crystals, (c) and (d) illustration of the dissolution observed by TEM, and (d, left, down) selected area electron diffraction (SAED) patterns. Reprinted from ref. 36 (Copyright 2016, Royal Society of Chemistry). (C) SEM images of the hierarchical SAPO-34 samples etched at (a) 180 °C for 24 h and (b) 180 °C for 36 h. Reprinted from ref. 37 (Copyright 2019, Royal Society of Chemistry).

  two groups (triethylamine by Zeng et al., and TEAOH and TMEDA by Liu et al.), which might cause the different distribution of the framework atoms. No matter of different etching patterns, the catalytic lifetime of the obtained hierarchical zeolites was significantly prolonged, and the selectivity of light olefins enhanced.

Fig. 5 .

 5 Fig. 5. (A) Representative low magnification SEM micrographs of the parent SAPO-34 and the hierarchical material obtained by 30 min ultrasonic treatment in fluoride medium. Reprinted from ref. 47 (Copyright 2016, Royal Society of Chemistry). (B) SEM images of the parent and fluoride medium post-treated samples: SAPO-34-Mor: (a) parent SAPO-34, and treated for (b) 30 and (c) 60 min; SAPO-34-TEA: (d) parent SAPO-34, and treated for (e) 30 and (f) 60 min; SAPO-34-TEAOH: (g) parent SAPO-34, and (h) treated for 15 min. Reprinted from ref. 48 (Copyright 2016, Elsevier).

a

  Catalyst weight = 3 g (20-40 mesh), T = 723 K, WHSV = 1 h -1 . b Catalyst weight = 3 g (20-40 mesh), T = 673 K, WHSV = 1 h -1 . c Catalyst weight = 1 g (40-60 mesh), T = 673 K, WHSV = 1 h -1 .

Table 1 .

 1 Summary of various leaching agents employed for the post-treatment of SAPO-34 zeolites.

Table 1 . Continued

 1

	Leaching agent	Microporous template	Structure/morphology	Yield (%)	S BET (m 2 /g)	V meso (cm 3 /g)	Lifetime (min)	Ref.
	HNO 3	TEA	Butterfly-shaped porous patterns on the	70	671	0.01	a 360	[33]
	Oxalic acid		four side faces	72	876	0.04	390 a
	HCl	TEAOH + TMEDA	A hollow structure with cougher surface and wall thickness of about 30 nm	25	705	0.26		[34]
	Solid oxalic acid	TEA	A butterfly patterned hierarchical pore system and high relative crystallinity		673	0.03	580 b	[35]
			A novel slit shape starts from the crystal
	Citric acid	TEA + TEAOH	surfaces then ends at the center of the		415	0.08	720 c	[36]
			crystal
	HCl			81		0.01
			Strongly amorphous
	H 4 EDTA			78		0
	Na 2 H 2 EDTA		Preserving the crystallinity during treatment 84		0.01
	NaOH	Supplied by ACS Material	Decreasing the crystallinity during	73		0.02		[38]
	NaOH + TPABr		treatment	74		0.02
	TPAOH		Strongly amorphous	79		0
	DEA		Preserving the crystallinity during treatment 85		0.01

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant 21971082) and the Jilin Province Science and Technology Development Plan (Grant 20190201229JC and 20200201096JC). G.Y thanks the China Postdoctoral Science Foundation (2018M640280 and 2019T120235) for supporting this work. And this work acknowledges the National 111 Project (B17020). X.C. and V.V. acknowledge the collaboration in the framework of joint Sino-French international laboratory "Zeolites".