Zeolites with uniform micropores, tunable acidities, and high thermal/hydrothermal stability are important heterogeneous acidic catalysts in the petrochemical and fine chemical industries^[1,2,3]. In recent years, as a successful alternative route of producing light olefins via a nonpetrochemical approach from the abundant resources of coal, natural gas, and even biomass, the zeolite- catalyzed methanol-to-olefins (MTO) reaction has gained considerable attention from academic and industrial communities^[4,5,6,7,8]. After extensive experimentation using various molecular sieves, it was demonstrated that small- pore 8-membered ring (8-MR) molecular sieves served as excellent catalysts in the MTO reaction (100% methanol conversion)^[9]. In particular, silicoaluminophosphate zeolite SAPO-34 with the CHA topological structure showed high selectivity to light olefins (over 90%) for this reaction because of the presence of 3D channels in its structure and moderate acid strength^{[10,11,12,13,14,15]}. However, the main problem associated with the SAPO-34 catalyst was the rapid deactivation during the MTO reaction due to the formation and trapping of heavy secondary products (coke) within the pores (channels and cages)^[16,17,18].

For the SAPO-34 catalyst, its deactivation in the MTO reaction is closely related to its acidity. It was reported that Brønsted acid sites played a critical role as active sites in this catalytic process^[19]. On one hand, the Brønsted acid sites facilitate the generation of cyclic carbenium ions and aromatics during the induction period, which then serve as hydrocarbon pool species on the working catalyst^[20]. On the other hand, coke compounds, i.e., large polycyclic aromatics, are formed at the Brønsted acid sites with further progress of the reaction, eventually causing catalyst deactivation^{[17-18, 21]}. Numerous studies have revealed that the coke rate increases with stronger acid sites and higher acid site density. Therefore, the SAPO-34 catalyst with mild acid strength and relatively low acidity density should be more appropriate for the MTO reaction. As such, various strategies have been developed for controlling the acid sites to obtain the highly active SAPO-34 catalyst with a long lifetime. These strategies include adjusting the silicon content^[22,23,24], metal heteroatom modification (by isomorphous substitution of heteroatoms, ion exchange, or impregnation)^{[25,26,27,28,29]}, silanation^[30,31], seed-assisted synthesis^[32], and recently reported low-temperature synthesis approach^[33]. Among these strategies, seed-assisted synthesis was probably the most intriguing one because the seeding approach has been proven to own a high capability of modulating the physico-chemical properties of zeolites by changing the crystallization kinetics.

Very recently, Xiao’s group reported the solvent-free synthesis of SAPO-34, which exhibited many advantages such as reduced waste production, increased zeolite yield, and extremely low autogenous pressure^[34,35]. However, the catalytic lifetime of SAPO-34 in the MTO reaction was only 200 min, which did not meet the strict industrial demand^[35]. In this study, a facile seed-assisted solvent-free method was developed to synthesize SAPO- 34 by using acid-etched seed crystals in order to modulate the physico-chemical properties of the zeolite.

1 Experimental

1.1 Chemicals and materials

Pseudoboehmite (Al₂O₃, 72wt%, Shandong Zibo Chemical Reagent Company), fumed silica (SiO₂, 100wt%, Aladdin Reagent), diethylamine (DEA, 99wt%, Tianjin Kaixin Chemical Industry Co., Ltd.), phosphoric acid (H₃PO₄, 85wt%, Yantai Shuangshuang Chemical Reagent Co., Ltd.), and distilled water were used in this study.

1.2 Activated seed crystals

The seed crystals were synthesized under hydrothermal conditions according to the literature^[36]. The activated seeds were prepared via acid etching with various concentrations of H₃PO₄ solution (0.1, 0.01, 0.001, and 0.0001 mol/L). The detailed procedure was as follows. First, 0.25 g of calcined seeds was slowly added to H₃PO₄ solution (50 mL) and stirred at room temperature for 30 min. After this mild etching process, the solid product was collected by centrifugation and thoroughly washed with distilled water followed by ethanol. Finally, the solid product was dried at 110 ℃ for 2 h.

1.3 Sedd-assisted synthesis of SAPO-34 catalysts

The parent SAPO-34 sample (SP34) was prepared under solvent-free conditions with the molar composition of 1.0Al₂O₃ : 0.6SiO₂ : 1.1DEA·H₃PO₄ according to the patent^[37]. The procedure for the seed-assisted synthesis of SAPO-34 catalysts was similar to that of SP34 except that specific seed crystals were introduced in the former syntheses. First, 0.85 g of pseudoboehmite, 0.216 g of fumed silica, 1.128 g of solid compounds of diethylamine phosphoric acid (DEA·H₃PO₄), 0.084 g of various seeds (20% of the total mass of Al₂O₃, P₂O₅, and SiO₂ in the raw materials) were mixed together and ground for 15 min. Then, the thoroughly mixed mixture was transferred to a 50 mL Teflon-lined stainless autoc- lave and crystallized at 200 ℃ for 48 h. Finally, the crystallized SAPO-34 samples were washed with distilled water and dried at 80 ℃ for 24 h. All samples were calcined at 550 ℃ for 5 h to remove the template. The resultant products were designated as SP34 (parent SAPO-34), SP34-RS (RS: Raw seeds), SP34-AS_X (AS: Acid-etched seeds, X: concentration of H₃PO₄ solution used to activate seed crystals, i.e., 0.1, 0.01, 0.001, 0.0001 mol/L).

1.4 Characterizations

The phase purity and crystallinity of samples were characterized through powder X-ray diffraction (XRD) patterns obtained from a D/Max-2400 Rigaku diffractometer with CuKα radiation operated at 40 kV and 150 mA. The crystallinities of SAPO-34 samples were calculated according to the intensity of peaks at 2θ=9.5°, 12.8°, 20.5°, and 30.5°^[36]. Scanning electron microscopy (SEM) images were recorded with the JSM-6701F instruments equipped with energy dispersed spectroscopy (EDS) to observe the morphology and crystal size of SAPO-34. Nitrogen adsorption/desorption measurements were carried out on a Micromeritics 2020 analyzer at -196 ℃ after the samples were degassed at 350 ℃ in vacuum. The Brunauer-Emmett-Teller (BET) surface area was calculated from the linear part of the BET plot by the IUPAC recommendation. A Micromeritics AutoChem II 2920 automated chemisorption analysis unit was used for conducting temperature programmed desorption of ammonia (NH₃-TPD) experiments to characterize the acidity of the samples.

1.5 Catalytic reaction

The MTO reaction was carried out in a quartz tubular fixed-bed steel reactor with a length of 30 cm and an inner diameter of 6 mm under atmospheric pressure. For each test, 0.3 g of catalyst was packed in the center of quartz wools. Before the catalytic reaction, the sample was pretreated in a N₂ flow of 30 mL/min at 500 ℃ for 1 h and then cooled to the reaction temperature. The reactant, methanol, was fed by passing the carrier gas (N₂, 15 mL/min) into the reactor at 400 ℃, which resulted in the weight hourly space velocity (WHSV) of 2.0 h^-1. The effluent products were detected by an online gas chromatograph GC7900 system (Tianmei, Shanghai, China) equipped with a flame ionization detector (FID) and Plot-Q column.

2 Results and discussion

XRD patterns of SP34, SP34-RS, and SP34-AS_X are displayed in Fig. 1. All samples exhibited typical diffraction peaks assigned to the CHA structure. However, a significant difference in crystallinity can be clearly observed. The crystallinities of SP34-RS and SP34-AS_X was evidently higher than that of SP34 (Fig. S1), demonstrating that the addition of seed crystals can effectively accelerate the nucleation and growth of SAPO-34 crystals.

Fig. 2 exhibits the SEM images of SP34, SP34-RS, and SP34-AS_X. Sample SP34 presents irregular aggregate morphology of approximately 8 μm, which consists of diverse micrometer- and nanometer-sized SAPO-34 crystals with a rough surface. When seed crystals were added, the obtained samples (SP34-RS and SP34-AS_X, X=0.1, 0.01. 0.001, 0.0001) were inter-grown with micrometer- and nanometer-sized smooth SAPO-34 crystals, especially for the SP34-AS_X samples. Accordingly, the number of isolated well-shaped crystals was less. The abundant intergrowth could be attributed to the presence of crystal seeds, which governed the growth of SAPO-34 crystals, while the low number of single isolated crystals can be explained by limited breeding^[38]. It was worth noting that the crystal sizes of samples with seeds were larger than those of SP34. The difference in crystal size could be ascribed to the crystallization of SAPO-34 only occurs on the surface of the seeds, thus the primary particle becomes larger than seeds, which is consistent with the results of Tang’s group^[39].

The physico-chemical properties of all samples are summarized in Table 1, N₂ adsorption/desorption isotherms and corresponding pore distribution curves are depicted in Fig. 3 and Fig. S2, respectively. As shown in Fig. 3, SP34 displayed a type IV isotherm, revealing the presence of abundant mesopores. The presence of a hysteresis loop in the relative pressure range of 0.4 ＜ p/p₀ ＜ 1 is typical for such stacking materials^[40]. The pore size distribution of SP34, with a maximum of around 4 nm and the texture data in Table 1 unambiguously illustrate that this sample is a material with more mesoporous and less microporous. In comparison, the samples prepared by seed-assisted synthesis (SP34-RS, SP34-AS_X) exhibited the type I isotherms characteristic of a purely microporous zeolite. In particular, it can be seen from Table 1 that the BET surface area and micropore volume of the SP34-AS_X samples, comparable to the texture parameter in the literature^[15,32], however, were much higher than those of SP34. For the SP34-AS_X samples, their enhanced microporosity may be attributed to the increased crystallinity, as determined by XRD. In contradiction with what we expected, the microporous surface of SP34-RS was not the largest, although its crystallinity was the highest among the six samples.

The acid site density and strength of the six samples were examined through NH₃-TPD measurements. As shown in Fig. 4, each of the NH₃-TPD profiles showed only one asymmetrical desorption peak centered at ~161 ℃. After suitable deconvolution (Fig. S3), the peak can be divided into two distinct desorption peaks, corresponding to weak acid sites and medium strong acid sites^[41,42]. The low-temperature peak was attributed to the desorption of physisorbed NH₃, and NH₃ adsorbed on lattice defects or terminal Si(OH) and Al(OH), whereas the high-temperature peak was assigned to the NH₃ desorption from the Brønsted acid sites. The acid site density values of SP34, SP34-RS, and SP34-AS_X (X=0.1, 0.01, 0.001, 0.0001) are listed in Table 1. For the SP34-RS and SP34-AS_X samples, both high- and low- temperature peaks were less intense than those of SP34, which suggested the decreasing trend of acid site density in these samples. It is well known that SAPO-34 catalysts with mild acid strength and relatively low acid site density are more appropriate for the MTO reaction^[5].

The catalytic activities of the six samples in the MTO reaction were evaluated in a fixed bed reactor at 400 ℃ with a methanol WHSV of 2.0 h^-1, and the results were plotted in Fig. 5 and Table 1. The duration time of methanol conversion over 99% was defined as the catalytic lifetime. All catalysts synthesized in the presence of activated seeds (SP34-AS_0.1, SP34-AS_0.01, SP34-AS_0.001, and SP34-AS_0.0001) exhibited remarkably prolonged catalytic lifetime (300-480 min) compared to SP34 (40 min) and SP34-RS catalysts with raw seeds (100 min).

Notably, there was a large difference in the catalytic lifetime of the samples prepared by seed-assisted synthesis although they have comparable crystal size, crystallinity, and BET surface area. The catalytic lifetime order is SP34-AS_0.1 (480 min) ＞ SP34-AS_0.001 (420 min) ＞ SP34-AS_0.0001 (330 min) ＞ SP34-AS_0.01 (300 min). This result indicated that the acid properties of SP34-AS_X might be the key factor affecting the catalytic lifetime. Particularly, SP34-AS_0.1 and SP34-AS_0.001 with appropriate strong acid site density exhibited longer lifetime than SP34-AS_0.01 and SP34-AS_0.0001. This is possibly because the increased strong-acid amount (SP34-AS_0.01 and SP34-AS_0.0001) in SAPO-34 often causes enhanced coke formation and hydride transfer, thus leading to faster catalyst deactivation^[43,44]. Therefore, the shortest lifetime of SP34 is well supported, because this catalyst possessed considerably higher strong acid site density than other catalysts. However, the catalyst with the lowest acid amount in strong acid sites, SP34-RS, did not have the longest lifetime, likely resulting from the insufficient number of acid sites, which may be rapidly covered by coke due to the diffusion limitation promoted by its low microporous surface area^[22]. Therefore, in order to achieve optimal performance for the SAPO-34 catalyst, it is essential to precisely control acidity^[13] as well as diffusion properties.

3 Conclusions

A facile seed-assisted solvent-free approach was developed to synthesize SAPO-34 with desirable properties. The resulting SAPO-34 samples with seed crystals exhibited considerably higher crystallinity, larger surface area, but lower strong acid site density, than the parent SAPO-34. Particularly, the SAPO-34 catalysts prepared from acid-etched seed crystals showed remarkably prolonged catalytic lifetime of up to 480 min in the MTO reaction. The approach developed herein may open new avenues for the cost-effective fabrication of high- performance MTO catalysts.

7 Supporting materials

Modulation of SAPO-34 Property with Activated Seeds and its Enhanced Lifetime in Methanol to Olefins Reaction

ZHANG Dongqiang, LU Huihui, SU Na, LI Guixian, JI Dong, ZHAO Xinhong

(School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, China)