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Synthesis and Catalytic Properties of Phosphate-Based Molecular Sieves Using Diazolium-Based Organic Structure-Directing Agents

Title
Synthesis and Catalytic Properties of Phosphate-Based Molecular Sieves Using Diazolium-Based Organic Structure-Directing Agents
Authors
최수련
Date Issued
2021
Publisher
포항공과대학교
Abstract
A cage-based small-pore (silico)aluminophosphate with CHA, and AEI structures were synthesized using various diazolium-based dications, where two diazole molecules are connected by alkyl chain, as organic structuredirecting agent (SDA). It was revealed that head group properties of diquaternary organic SDA have influence on phase selectivity of crystallization whereas the influence of the length of central alkyl chain connecting two diazole moieties is relatively small. However, varying chain length of diquaternary organic SDA can be one way to control acidic properties such as acid site density and distribution of crystallized catalysts. In addition, combined use of 1MP-C4 and 1MI-C4, which direct CHA and AEI, respectively, enable the crystallization of SAPO-34/18 molecular sieves where the intergrowth ratio of CHA and AEI structure are controlled by adjusting the 1MP-C4/1MI-C4 ratio in the synthesis gel. Investigation of catalytic properties of synthesized catalysts for methanol-toolefin (MTO) conversion allows us to find out that crystal size has dominant effect on catalytic lifetime than acidic properties of catalysts. In addition, it was shown that isolated acid sites are beneficial to enhance catalytic stability when crystal size and acidic properties are similar. On the other hand, comparison between physical mixture of SAPO-34 and SAPO-18 and SAPO-34/18 intergrowth catalysts enable us to observe that formation of intergrowth structure is beneficial to enhance catalytic lifetime. More interestingly, SAPO34/18 with high ratio of aei cages (˻50 %) show high propylene selectivity with propylene/ethylene ratio of 2.0, to our best knowledge, which is the highest value among SAPO molecular sieves with 8-membered ring windows. Considering catalytic stability together, we can conclude SAPO-34/18 with intergrowth ratio of 0.5 is the best catalyst for selectively producing propylene from methanol.
URI
http://postech.dcollection.net/common/orgView/200000369047
https://oasis.postech.ac.kr/handle/2014.oak/111968
Article Type
Thesis
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