Abstract
Introducing mesopores into the channels and cages of conventional micropores CHA (Chabazite) topological structure SAPO-34 molecular sieves can effectively improve mass transport, retard coke deposition rate and enhance the catalytic performance for methanol to olefins (MTO) reaction, especially lifetime and olefins selectivity. In order to overcome the intrinsic diffusion limitation, a novel CO2-based polyurea copolymer with affluent amine group, ether segment and carbonyl group has been firstly applied to the synthesis of SAPO-34 zeolite under hydrothermal conditions. The as-synthesized micro-mesoporosity SAPO-34 molecular sieve catalysts show heterogeneous size distribution mesopores and exhibit slightly decrease of BET surface area due to the formation of defects and voids. Meanwhile, the catalysts exhibit superior catalytic performance in the MTO reaction with more than twice prolonged catalytic lifespan and improvement of selectivity for light olefins compared with conventional microporous SAPO-34. The methodology provides a new way to synthesize and control the structure of SAPO-34 catalysts.
Highlights
Methanol to lower olefins reaction is a critical process for conversion of nonoil resources to chemicals through methanol, and light olefins especially ethylene and propylene have been widely applied in polymerization reaction and petrochemical industry [1–6]
The results demonstrate that the addition of polyurea as mesoporogen can decrease the crystallinity of sample PUa0 is 89.6% while the crystallinity for sample PUa0.08 and PUa0.10 are 79% and 78%, crystallinity of the SAPO-34 molecular sieves to a certain degree
F, i were ball-milled in water with a speed of 300 rpm for 2 h. These results indicate that the SAPO-34 crystals synthesized with polyurea as mesoporogen have hierarchically porous structure, implying that polyurea is a successful mesoporogen for synthesizing the SAPO-34 molecular sieves
Summary
Methanol to lower olefins reaction is a critical process for conversion of nonoil resources to chemicals through methanol, and light olefins especially ethylene and propylene have been widely applied in polymerization reaction and petrochemical industry [1–6]. The produced large coke species can block the channels and interdict the contact of raw material with active sites, leading to a rapid decline of both methanol conversion and olefins selectivity [4,22–24] In this respect, overcoming the mass transfer restriction and retarding the coke deposition rate are vital to the extension of the catalytic lifespan. The CO2 -based polyurea was synthesized by a facile one step reaction of CO2 with diamine [49] It contains affluent amine group, ether segment and carbonyl group that have strong affinity with raw materials in the starting gel of synthesizing SAPO-34 molecular sieves, makes it easier to be introduced into the channels and cages of the CHA framework to form defects and voids, i.e., mesopores/macropores in the zeolite. The catalysts manifest superior catalytic performance in the MTO reaction with more than twice prolonged catalytic lifespan and improvement of selectivity for light olefins compared with the conventional microporous SAPO-34
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