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Further investigations on Constraint Index testing of zeolites that contain cages |
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| Authors: | John R Carpenter Sheila Yeh Stacey I Zones Mark E Davis |
| Affiliation: | 1. Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA;2. Chevron Energy Technology Company, 100 Chevron Way, Richmond, CA 94802, USA;1. Department of Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, Hajdrihova 19, Ljubljana 1001, Slovenia;2. PetroSA Synthetic Fuels Innovation Centre, South African Institute for Advanced Materials Chemistry, University of the Western Cape, Cape Town, Private Bag X17, Bellville, 7535, South Africa;1. College of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China;2. Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China;1. A.V. Topchiev Institute of Petrochemical Synthesis RAS, Leninsky Prospect, 29, 119991, Moscow, Russia;2. Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1, bld. 3, 119991, Moscow, Russia;1. National Engineering Laboratory for Methanol to Ole?ns, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China;2. University of Chinese Academy of Sciences, Beijing 100049, China |
| Abstract: | Zeolites such as SSZ-25 and SSZ-35 exhibit Constraint Indexes (CI) lower than expected. Two hypotheses for the origin of the lower CI values are that partial cages on the external surface of the zeolites significantly contribute to the overall conversions and that the large cages within the crystalline structures provide less steric hinderence at the active site than would be expected from the size of the pores. The effect of external surface activity is observed by comparing the as-synthesized materials to those passivated via dealumination with ammonium hexafluorosilicate. The external surface dealumination is verified by XPS and by the reduction in reactivity toward isopropanol dehydration observed from materials still containing their SDAs. The external surface of calcined zeolites is shown to have little influence on the CI value. Cage accessibility is investigated by comparisons of structures having larger cages to those that do not and by following the time dependent behavior of the CI values for these materials. The presence of large cages in the zeolites is shown to lower the CI values below those expected from the pore sizes, and the time dependence of the CI value can provide insight into the existence of these larger cage structures. |
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