期刊
CHEMISTRY-A EUROPEAN JOURNAL
卷 20, 期 51, 页码 16922-16932出版社
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201404924
关键词
hexane cracking; hydrothermal stability; phosphorus; X-ray tomography; zeolites
资金
- Natural Sciences and Engineering Research Council of Canada
- National Research Council Canada
- Canadian Institutes of Health Research
- Province of Saskatchewan
- Western Economic Diversification Canada
- University of Saskatchewan
The nature behind the promotional effect of phosphorus on the catalytic performance and hydrothermal stability of zeolite H-ZSM-5 has been studied using a combination of Al-27 and P-31 MAS NMR spectroscopy, soft X-ray absorption tomography and n-hexane catalytic cracking, complemented with NH3 temperature-programmed desorption and N-2 physisorption. Phosphated H-ZSM-5 retains more acid sites and catalytic cracking activity after steam treatment than its non-phosphated counterpart, while the selectivity towards propylene is improved. It was established that the stabilization effect is twofold. First, the local framework silico-aluminophosphate (SAPO) interfaces, which form after phosphatation, are not affected by steam and hold aluminum atoms fixed in the zeolite lattice, preserving the pore structure of zeolite H-ZSM-5. Second, the four-coordinate framework aluminum can be forced into a reversible sixfold coordination by phosphate. These species remain stationary in the framework under hydrothermal conditions as well. Removal of physically coordinated phosphate after steam-treatment leads to an increase in the number of strong acid sites and increased catalytic activity. We propose that the improved selectivity towards propylene during catalytic cracking can be attributed to local SAPO interfaces located at channel intersections, where they act as impediments in the formation of bulky carbenium ions and therefore suppress the bimolecular cracking mechanism.
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