Syntheses Of SAPO-34 Zeolites With Excellent MTO Performance

Light olefins, such as ethylene and propylene, have been widely used as starting materials of polyolefins and as important raw materials for various chemicals in the petrochemical industry. However, the shortage of oil resources forces people to develop the preparation technology of light olefins that does not rely on oil resources. Methanol-to-olefin(MTO) process has proven to be the most successful non-petrochemical route for the production of light olefins from the abundant resources of natural gas, coal, and biomass. Studies on MTO conversion have received extensive attention in the past decades.Because of the unique shape selectivity, microporous crystalline zeolites or molecular sieves with regular pore architectures have been widely applied in chemical industry as one of the most important solid catalysts. Up to now, a variety of zeolite materials have been tested as MTO catalysts, among which the aluminosilicate zeolite ZSM-5 with the zeotype MFI and the silicoaluminophosphate molecular sieve SAPO-34 with zeotype CHA are the most excellent catalysts for MTO reactions. Compared with the ZSM-5, the selectivity towards light olefins is greatly improved by the application of SAPO-34 with large cha cage and small 8-ring pore opening. In general, SAPO-34 gives a very high selectivity(>80%) to light olefins with 100% conversion of methanol. However, the main problem associated with the SAPO-34 catalyst is the rapid deactivation during methanol conversion because a large amount of organic species as retained materials accommodate in the large cavities connected by narrow channels.Previous studies demonstrated that nano-sized and hierarchical SAPO-34 zeolites can remarkably enhence the transformation of reactants and products inside the crystals, reduce the coking rates, and then prolong the catalytic lifetime and improve the selectivity of light olefins. In this doctoral thesis, we focused on the syntheses of nanosized and hierarchical SAPO-34 zeolites with excellent performance in MTO reaction. We developed some new strategies to synthesize the nanosized and hierarchical SAPO-34 zeolites with excellent MTO performance. In addition, 3D EDT analysis, solid NMR, GC-MS as well as DFT calculation had been used to systemically study the structure, acidity, and catalytic performance, which provide further insight into the understanding of the superior catalytic performance. The main research results of this dissertation are as follows:1. The nano sheet-like silicoaluminophosphate(SAPO) molecular sieves SAPO-34(CHA-type) and SAPO-18(AEI-type) with different silicon contents have been synthesized with a high product yield under conventional hydrothermal conditions by using tetraethylammonium hydroxide as the template. 3D EDT technique has been successfully applied to determine the CHA-type and AEI-type structures of synthesized nano crystals ab initio. Electron microscopy observations reveal the existence of intergrowth of CHA- and AEI-type frameworks due to different stacking manners of double 6-ring layers. The topological structure and the silicon contents of the as-synthesized nanosized SAPO catalysts have a pronounced effect on the catalytic performance in MTO reaction. With the decrease of silicon contents in SAPO-34, the catalyst lifetimes increase gradually, which is in accordance with the decrease of acidic concentration of the catalysts. Significantly, sample S3 with CHA-type as the major phase and the lowest silicon contents exhibits so far the longest catalyst lifetime(exceeding 1200 min) and the lowest coking rate in MTO reaction than other reported catalysts tested under the similar catalytic conditions. Moreover, sample S3 possesses the highest selectivity of ethylene and propylene reaching up to 83.3%. DFT calculations further explain the different catalytic performance between H-CHA and H-AEI, showing that H-CHA possesses higher selectivity towards light olefins than H-AEI. This work provides a useful guidance for improving the methanol conversion process by utilizing nano crystalline silicoaluminophosphate catalysts. Moreover, it demonstrates the power of 3D EDT technique combined with EDS analysis in a single nanocrystal in providing clear crystal structural features and compositional information of nanocrystals with impurities or defects. This is of significance for better understanding the catalytic performance of nanosized crystalline catalysts.2. The hierarchically porous silicoaluminophosphate SAPO-34 catalysts have been successfully synthesized using the organosilane surfactant as the mesopore director by direct hydrothermal crystallization. The hierarchically porous SAPO-34 crystals are obtained as cubic aggregates of nanocrystallites. Thanks to the hierarchical porosity, decreased acidity as well as reduced nano crystallinite size, the hierarchical porous SAPO-34 catalysts exhibit four-times prolonged catalytic lifetime and more than 10% improvement of light olefin(C2H4+C3H6) selectivity in MTO conversion compared with the conventional microporous SAPO-34 catalyst. This work demonstrates the effective organosilane surfactant-directed approach to prepare hierarchical porous silicoaluminophosphate molecular sieves with improved catalytic properties.3. The tri-level hierarchically porous SAPO-34 zeolite has been successfully synthesized using an Al-rich method coupled with the use of polymer under hydrothermal conditions. The as-prepared hierarchically porous SAPO-34 crystals possess the hollow cubic-like morphology with intracrystalline micro-meso-macropore structure. Thanks to the enhanced mass transport within the hierarchical porous system and decreased acidity, the hierarchical SAPO-34 catalysts exhibit superior MTO performance with about six-times prolonged catalytic lifetime and nearly 5% improvement of selectivity for light olefin(C2H4+C3H6) compared to the conventional microporous SAPO-34 catalyst. This work demonstrates a novel and simple synthetic route to prepare tri-level hierarchical zeolites with intracrystalline micro-meso-macroporosity showing superior catalytic activity, which will open new perspective for the application of tri-level hierarchically porous zeolites with unique textural properties.4. Nano-sized zeolite SAPO-34 catalysts with high crystallinity have been successfully synthesized in 10 minutes by fast heating the reaction gel in a stainless steel tubular reactor combining with the seed-assisted method. The fast heating transfer and seeding greatly accelerate the formation of SAPO-34 zeolites. Importantly, the nano-sized SAPO-34 zeolites synthesized in the minute time scale exhibit excellent performance in MTO reaction and comparable catalytic lifetime and selectivity of light olefins with the nano-sized catalyst synthesized by the conventional hydrothermal method for 12 h. This work opens new perspective for fast synthesis of SAPO-34 zeolites with excellent MTO performance, and this method could also be realized as an efficient mass-production route for the synthesis of nano-sized SAPO-34 zeolites using a continuous flow reactor that meets the demand of future large-scale industrial application.