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State-of-the-art synthesis of SAPO-34 zeolites catalysts for methanol-to-olefin conversion

September 21st, 2017
State-of-the-art synthesis of SAPO-34 zeolites catalysts for methanol-to-olefin conversion
Schematic representation of MTO conversion over the SAPO-34 catalyst. Credit: ©Science China Press

Methanol-to-olefin (MTO) conversion is one of the most successful non-petrochemical routes for producing light olefins. Silicoaluminophosphate zeolite SAPO-34 is one of the best industrial catalysts for MTO conversion, but suffers from rapid deactivation. To improve the MTO performance, considerable synthetic efforts are devoted to decrease the catalyst size or fabricate hierarchical catalysts. Scientists from China summarize the state-of-the-art synthetic strategies for nanosized and hierarchical SAPO-34 catalysts, and outline future development directions.

Light olefins, such as ethylene and propylene, have been widely used as important raw materials for chemicals. However, the shortage of oil resources forces researchers to develop an alternative technology for preparation of light olefins that is independent of oil resources. The methanol-to-olefin (MTO) process has proven to be a successful non-petrochemical route for the production of light olefins from abundant non-oil resources such as natural gas, coal and even biomass using methanol as the intermediate. Thus, the MTO reaction can act as a bridge between the non-petroleum chemical industry and modern petrochemical industry. In 2010, the first 600,000 ton/year MTO unit was brought online in China, a significant milestone for the conversion of coal to light olefins.

Because of the excellent shape selectivity, appropriate acidity, and superior thermal and hydrothermal stability, crystalline zeolites with ordered microporous in molecular dimensions have been widely used as the most important solid heterogeneous catalysts in a number of industrial processes. Silicoaluminophosphate zeolite SAPO-34 with CHA framework structure has proven to be the most ideal catalyst for MTO conversion to produce ethylene and propylene. SAPO-34 zeolite possesses a large cha cage (0.94 nm in diameter) and small 8-ring pore (0.38 nm) opening as well as moderate acidity, which can induce a very high selectivity of ethylene and propylene (>80%) in MTO reactions with complete conversion of methanol. In general, the reaction temperature of MTO conversion is in the range of 350~500ºC. The schematic representation of MTO conversion over the SAPO-34 catalyst is shown in Figure 1.

Based on the proposed hydrocarbon pool mechanism, the polymethylbenzenium ions are formed during the reaction, which act as the important reaction intermediates for olefin production. However, these polymethylbenzenium ions can further turn into bulk organic species as coke deposition accommodated in the large cavities connected by narrow channels, thus covering the active sites of catalysts leading to the rapid deactivation during methanol conversion. This is indeed the main problem associated with the SAPO-34 catalysts. To overcome the inherent diffusion limitations and retard coke deposition, various synthetic strategies have been developed in recent years, and considerable efforts are focused on the reduction of crystal sizes of the catalysts or the introduction of secondary larger pores into the zeolite crystals to form hierarchical structures. The nanosized and hierarchical SAPO-34 catalysts demonstrate significant advantages in the enhancement of mass transfer and decrease of coke formation rate as compared with their traditional microporous counterparts with larger crystal sizes.

In a new review published in the Beijing-based National Science Review, scientists at the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University in Changchun, China, and at SINOPEC Corporation, Science & Technology Development Department in Beijing, China, summarize recently advanced synthesis strategies for SAPO-34 zeolite catalysts in MTO Conversion. Co-authors Qiming Sun, Zaiku Xie and Jihong Yu mainly focus on representing the state-of-the-art synthetic strategies for preparing nanosized and hierarchical SAPO-34 catalysts with excellent MTO performance and the industrialization of SAPO-34 catalysts for the MTO reaction. The authors also discuss some current limitations as well as future prospects for the synthesis of SAPO-34 catalysts.

These authors consider that the development of MTO industrialization process in recent decade greatly promotes the continuous progress in the synthesis of the SAPO-34 catalysts. Particularly, some efficient synthetic methods have been developed for the preparation of nanosized and hierarchical SAPO-34 catalysts with excellent MTO conversion. Meanwhile, the authors also point out that more facile, cost-effective and environmentally-benign routes to synthesize nanosized and/or hierarchical SAPO-34 catalysts with enhanced catalytic performance are still highly desired for the large-scale industrial application. In the perspective of the review, the authors further put forward that precise controls of crystal morphology and intracrystalline hierarchically porous structure as well as distribution/acid strength of catalytic active sites are important issues for fabricating the high-efficient SAPO-34 catalysts and modulating the selectivity of ethylene and propylene in MTO reactions. This review will shed some light on the synthesis of SAPO-34 catalysts, and provide impetus for developing more efficient synthetic strategies for the SAPO-34 catalysts to meet the increasing industrial demands.

More information:
DOI: 10.1093/nsr/nwx0103

Provided by Science China Press

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