The hierarchical structure can significantly improve the diffusion efficiency of the catalyst and regulate the product distribution. Therefore, the preparation of hierarchical SAPO-34 molecular sieve has been a hot research topic. With Cetyltrimethyl Ammonium Bromide (CTAB) and Diethylamine (DEA) as templates, a two-step crystallization process was employed to synthesize hierarchical SAPO-34 molecular sieves. We found that the aging process is vital for the formation of pure phase SAPO-34. It was investigated the relationship of crystallinity trend and mesoporous content with the crystallization time. The results showed that the prolongation of crystallization time was beneficial to enhance the crystallinity of the molecular sieve, but unfavourable to the retention of mesoporous structure. The formation process of hierarchical SAPO-34 molecular sieve involved agglomeration, disintegration, crystallization, re-agglomeration and growth. The hierarchical SAPO-34 molecular sieve with a satisfactory crystallinity and considerable mesoporous structure could be obtained after 36 hours of crystallization. Moreover, the sample had the most suitable acid strength as well as acid amount. The catalytic activity was investigated by catalytic dimethyl ether (DME) to olefin (DTO) reaction. It revealed that the conversion of DME and the selectivity to olefins over the hierarchical SAPO-34 molecular sieve were significantly enhanced with comparison to that over microporous SAPO-34 molecular sieve. The amount of coke deposition of the hierarchical SAPO-34 molecular sieve (14.2%) was lower than that over the microporous molecular sieve (16.5%). Meanwhile, the propylene selectivity of hierarchical SAPO-34 was higher than that of microporous SAPO-34 in the whole reaction. In a word, the hierarchical SAPO-34 molecular sieve synthesized in this study showed a longer catalytic life, higher coke deposition resistance and higher propylene selectivity.
Light olefins such as ethylene and propylene have become the backbone of the modern chemical industry. Light olefins are mainly produced by catalytic cracking of petroleum. Due to the decrease of petroleum resources, finding a broad route to prepare light olefins is very urgent. DTO reaction process is a common solution at present. SAPO-34 has attracted the attention of researchers because of its unique pore structure and suitable acid performance, etc. [
For the heterogeneous reaction, diffusion efficiency plays an important role in the reaction performance. The diffusion mode of reactants and products in the micropores adopts configuration diffusion [
Generally, there are two strategies to improve the diffusion efficiency of catalyst, one of which is to reduce the particle size of the catalyst. The other one is to fabricate the molecular sieves with hierarchical structures.
As for the former one, Li et al. [
As reported in the literature, the diffusion rates of gaseous products and reactants are closely dependent on the pore size. It means that the diffusion efficiency in mesoporous and macroporous materials is much higher than that in microporous materials [
Soft template method has been a common strategy for the preparation of hierarchical molecular sieves. It usually requires expensive organo-silicon, organo-aluminium and organo-phosphorus as raw materials. Its mesoporous templates usually are surfactants. Surfactants form micelles at relatively mild temperatures to produce mesoporous [
Due to these difference, the phenomena of non-crystallization of raw materials or the formation of various mixtures of crystals and amorphous materials are usually be found in the synthesis of hierarchical molecular sieves by soft template method [
The hierarchical SAPO-34 zeolites were synthesized by two-step hydrothermal process. The molar ratio of the precursors composition was: Al2O3:P2O5:SiO2:DEA:H2O:CTAB = 1:0.9:0.6:1.6:50:0.05. The typical synthesis procedure is as following: First, mix the pseudo-boehmite (Shandong Aluminum Company, 69.0 wt%) with distilled water (Qingdao Chengda Distilled Water Co., Ltd.) and stir until a homogeneous suspension is formed. Then, the phosphoric acid (Sinopharm Chemical Reagent Co., Ltd, 85.0 wt%) diluted with water was added dropwise into the pseudo-boehmite suspension under rapidly stirred, and kept on stirring for 40 minutes to obtain a mixed gel A. After that, the silica gel (Qingdao Ocean Chemical Co., Ltd., 25.0 wt%) was added into the mixed gel A and stirred for another 1 hour to obtain a mixed gel B. Then the gel B was dropped into the dissolve CTAB (Sinopharm Chemical Reagent Co., Ltd, 99 wt%) to obtain a mixed gel C. The gel C was aged at room temperature for 2 h with stirring, and then transferred into a 100 mL Teflon-lined stainless steel autoclave to pretreat at 100˚C for 12 hours (the sample was labeled S-F after calcination). After that, the template for the synthesis of SAPO-34DEA (Sinopharm Chemical Reagent Co., Ltd., 99 wt%) was added into the autoclave when it was cooled to room temperature. After stirred for 2 hours, the autoclave was re-covered and the mixed gel was crystallized at 180˚C for 12 hours, 24 hours, 36 hours and 48 hours, respectively. The products were separated and washed, and calcined at 550˚C for 5hours to obtain a series of hierarchical SAPO-34 molecular sieves, which were labeled as S-12, S-24, S-36 and S-48 samples.
In contrast, microporous SAPO-34 molecular sieve (labeled S-W) was synthesized by a conventional hydrothermal synthesis method according to the literature [
The crystal phase structure and crystallinity of the sample were characterized by X-ray diffractometer (DX-2700, China, Dandong Haoyuan Instrument Co., Ltd.). Test conditions: room temperature, Cu target, Kα source, tube voltage 40 kV, tube current 30 mA, scan range 2θ = 5˚ - 40˚. The morphologies of the samples were observed using a scanning electron microscope (JSM-6390LV, JEOL, Japan). The N2 adsorption-desorption isotherm, specific surface area, and pore volume of the samples were measured on a fully automatic specific surface area and porosity analyzer (ASAP 2460, Micron, USA). Prior to the measurement, the samples were degassed under vacuum at 300˚C for 4 hours. The coke amount for the used catalyst was analyzed by a thermogravimetric analyzer (TG209F3, Germany, NETZSCH). The coked catalyst was heated from 50˚C to 800˚C with a heating rate of 10˚C/min and air flow rate of 100 mL/min.
The acidity of the sample was measured by temperature?programmed desorption of ammonia (NH3-TPD) on a chemisorption analyzer (FINETEC FINSORB-3010). Typically, the sample was first pretreated at 200˚C for 30 min with a ramp rate of 20˚C/min in the helium flow of 20 mL/min. When the temperature was dropped to 50˚C, the 5% NH3/95% He was introduced to adsorbe on the catalyst for 40 min until saturation. After that, it was purged with helium at 20 mL/min for 1 h, followed by increasing temperature to 800˚C at a rate of 20˚C/min.
The catalytic performance of these samples for DTO reaction was measured with a fixed bed reactor. In brief, 2 g catalyst (20 - 40 mesh) was charged into the reactor (550 mm × 10 mm), and activated at 450˚C for 1 hour under a nitrogen (50 mL/min) atmosphere prior to reaction. Subsequently, DME gas with WHSV = 0.725 h-1 was introduced into the reactor for 6 h, when cooling to 400˚C. The reaction product was detected on-line by Shimadzu Corporation GC-2014C gas chromatography. After the reaction stopped, the catalyst was cooled to room temperature under nitrogen, and the used catalyst was subjected to coke deposition analysis by a thermos-gravimetric analyzer.
From
pores makes defects in crystals, which leads the diffraction peak of S-36 lower than that of S-48.
The hierarchical pores of S-36 were supported by CTAB micelles. These micelles could be destroyed by exposing to the high temperature for along time [
It can be inferred that the raw materials formed spherical aggregates under the
function of CTAB in the first step. These spherical aggregates contained CTAB micelles, and the micelles formed the hierarchical structure of spherical aggregates. Then the spherical agglomerates were destroyed, disintegrated and crystallized under the effect of DEA. From the XRD results, we can know it is difficult to obtain pure SAPO-34 without the aggregate in the first step. The aggregate generated in the first step will be disintegrated in the subsequent process, so the shape of the aggregate is not the key for this experiment. CTAB could improve the viscosity of crystalline system, which promoted the agglomeration and growth of minute crystals. Therefore, the large SAPO-34 molecular sieve with hierarchical structure was formed. The mesoporous structure assembled by CTAB was gradually destroyed and transformed into the microporous structure after a longer crystallization time. This change belongs to the variation of microstructure (supported by BET results) instead of the change of macro-morphology.
All the four samples prepared by adding CTAB show H4 hysteresis loops in type IV isotherm according to the IUPAC, which was obtained by the N2 adsorption-desorption characterization (
From the
Sample | BET Surface Area A/(m2・g−1) | Pore Volume V/(m3・g−1) | Average Pore Size d/nm | acid amount | ||||||
---|---|---|---|---|---|---|---|---|---|---|
Stotal | Smicr | Sext | Vtotal | Vmicr | Vext | Aw | As | At | ||
S-12 | 150 | 2 | 148 | 0.575 | 0.000 | 0.575 | 7.25 | 9040 | 796 | 9836 |
S-24 | 538 | 479 | 59 | 0.361 | 0.180 | 0.181 | 5.83 | 12,268 | 5285 | 17,553 |
S-36 | 551 | 467 | 84 | 0.255 | 0.187 | 0.068 | 3.18 | 15,597 | 8829 | 24,426 |
S-48 | 736 | 725 | 11 | 0.286 | 0.263 | 0.023 | 3.08 | 16,354 | 8220 | 24,574 |
S-W | 796 | 796 | 0 | 0.287 | 0.286 | 0.001 | 1.44 | 13,730 | 8500 | 22,230 |
Stotal: Total specific surface area, Smicr: Micropore specific surface area, Sext: Specific surface area outside the micropore, Vtotal: Total pore volume, Vmicr: Micropore volume, Vext: pore volume outside the micropore volume, Aw: Integral area of weak acid peak, As: Integral area of strong acid peak, At: total area.
The acidity of catalysts shows a vital impact on the selectivity of products and catalyst lifetime in the catalytic dimethyl ether (methanol) to olefins. The acid strength and acid amount of the present catalysts were determined by using NH3-TPD. The results are shown in
According to the crystallization rule of SAPO-34, most silicon made up the framework of molecular sieve by directly participating in the process of nucleation and crystal growth, while a small amount of silicon entered by substituting mechanism in the later stage of crystallization [
but also due to more silicon enters the crystal as the crystallization time increases [
The results of the respective catalysts in the DTO reaction are showed in
Coke deposition is considered to be the main reason for the deactivation of SAPO-34 molecular sieves [
According to the report in the literature [
Simple | S-W | S-24 | S-36 | S-48 |
---|---|---|---|---|
Time on stream/h | 6 | 6 | 6 | 6 |
DME conversion/% | 4.4 | 1.8 | 16.5 | 1.0 |
Coke/%, (g・g−1cat) | 16.5 | 4.4 | 14.2 | 9.5 |
Reaction conditions: T = 400˚C, WHSV = 0.725 h−1.
its strong acid content is reduced than S-36 (in
The product distribution of each sample is showed in
Sample | Selectivity/% | C3=/C2= | ||||||
---|---|---|---|---|---|---|---|---|
CH4 | C2H6 | C2H4 | C3H8 | C3H6 | C4+ | C2=+C3= | ||
S-24 | 30.58 | 3.62 | 39.43 | 4.65 | 18.84 | 2.89 | 58.27 | 0.48 |
S-36 | 1.95 | 1.35 | 51.29 | 2.71 | 35.35 | 7.35 | 86.64 | 0.67 |
S-48 | 15.54 | 4.28 | 45.57 | 4.98 | 23.59 | 6.05 | 69.16 | 0.52 |
S-W | 2.79 | 2.29 | 51.05 | 3.25 | 32.97 | 7.65 | 84.02 | 0.65 |
Reaction conditions: T = 400˚C, WHSV = 0.725 h-1, t = 4 h.
than the microporous SAPO-34. Its catalytic performance is superior to that of microporous SAPO-34 zeolite.
Hierarchical SAPO-34 zeolite was successfully synthesized from conventional raw materials and templates. The characterization results showing the crystallization process of SAPO-34 synthesized by this method were as follows: The raw materials formed relatively stable spherical agglomerates with hierarchical structure under the function of CTAB. Then the spherical agglomerates were broken and crystallized under the function of DEA. Then, the hierarchical SAPO-34 molecular sieves were formed by stacking growth under the combined function of the two. In this process, the unstable mesoporous structure will transform into microporous structure at high crystallization temperature with the prolongation of crystallization time. Therefore, it is necessary to control the crystallization time to obtain the ideal hierarchical structure. Furthermore, it was found that the hierarchical SAPO-34 molecular sieves with more hierarchical pores and moderate strength of strong acid centers were obtained at 36 hours of crystallization time. These properties were favorite to the DTO reaction showing in the low coke deposition, high propylene selectivity and long catalytic lifetime. This experiment will provide a new clue for the synthesis of hierarchical SAPO-34 molecular sieves, which is favorable to the application of SAPO-34 in industry.
This work was financially supported by the National Natural Science Foundation of China (No.21706140) and the Shandong Provincial Natural Science Foundation of China (ZR2017BB039).
The authors declare no conflicts of interest regarding the publication of this paper.
Li, G.M., Li, Z., Ren, X.Z., Zhang, Y., Chen, Z.W. and Yu, J.Q. (2019) A Two-Step Crystallization Route for Hierarchical SAPO-34 Molecular Sieves: Unique Structural Features and Catalytic Property for DTO. Materials Sciences and Applications, 10, 302-316. https://doi.org/10.4236/msa.2019.104023