Light olefins, such as ethylene and propylene, have been widely used as important raw materials for various chemicals in the chemical industry. However, the shortage of oil resources forces researchers to develop an alternative technology for preparation of light olefins that is independent of oil resources. Methanol-to-olefin (MTO) process has proven to be a successful non-petrochemical route for the production of light olefins from abundant nonoil resources, such as natural gas, coal, and even biomass using methanol as the intermediate. Thus, the MTO reaction can act as a bridge between nonpetroleum chemical industry and modern petrochemical industry. In the last 40 years, both of the fundamental research and industrial application of the MTO reaction have received great attention of many institutions and companies. In 2010, the first 600,000 ton/year MTO unit of world was built up and brought on stream successfully in China, which is regarded as 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.
See the article:
Qiming Sun, Zaiku Xie, and Jihong Yu
The State-of-the-Art Synthetic Strategies for SAPO-34 Zeolite Catalysts in Methanol-to-Olefin Conversion
Natl Sci Rev, 2017, doi: 10.1093/nsr/nwx0103
The National Science Review is the first comprehensive scholarly journal released in English in China that is aimed at linking the country's rapidly advancing community of scientists with the global frontiers of science and technology. The journal also aims to shine a worldwide spotlight on scientific research advances across China.