Browsing by Subject "ZSM-5"

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    The characterization and elimination of the external acidity of ZSM-5.
    (Elsevier, 1996) Weber, R; Fletcher, J; Möller, K; O'Connor, C
    The external acidity of ZSM-5 was modified either by coating the crystals with a silicalite shell or by chemical vapour deposition (CVD) of tetraethoxysilane (TEOS). Temperature programmed desorption (TPD) techniques were used to characterize the acidity changes arising from these modifications. Ammonia and pyridine were used as probes for the total acidity, and 4-methylquinoline was used to probe the external acidity of the modified and unmodified ZSM-5 samples. Si/Al ratios of the modified samples were also used to characterize the samples. Both methods of modification were able to make the external acid sites inert. Chemical vapour deposition was found to be a better method, because the external surface acidity was eliminated without significantly changing the catalyst in terms of total acidity. Optimum elimination of the external surface acidity by CVD was obtained for the sample with the higher deposition time (16 h) and temperature (400°C). Optimum elimination of the external surface acidity using the silicalite shell method was obtained using the detemplated NH4-form of the parent crystal as seed material. The channel structures of crystals modified by both processes remained accessible to ammonia and pyridine.
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    The chemical vapour and liquid deposition of tetraethoxysilane on the external surface of ZSM-5
    (Elsevier, 1998) Weber, R W; Möller, K P; Unger, M; O'Connor, C
    The external acidity of ZSM-5 was modified by chemical vapour deposition (CVD) and chemical liquid deposition (CLD) of tetraethoxysilane [Si(OC2 H5 ) 4 ] using a static vacuum system, a vapour phase flow system and liquid phase deposition. Temperature programmed desorption (TPD) techniques were used to characterise the acidity changes arising from these modifications. Pyridine was used as a probe for the total acidity and 4-methyl quinoline (MQ) was used to probe the external acidity. The adsorption capacities of the samples were measured using n-hexane, p-xylene, o-xylene and 1,2,4-trimethyl benzene. The extent of Si(OC2 H5 ) 4 deposition was strongly dependent on temperature in both vapour phase flow and static vacuum systems. Continuous Si(OC2 H5 ) 4 deposition was observed in the presence of H2 O at relatively high temperatures when decomposition products were removed from the sample. It is proposed that physisorbed species need to be removed by evacuation or calcination to re-expose active sites, thereby enabling complete inertisation of the external surface acidity to occur, and that a more uniform covering can be obtained when a gradual deposition process is used. Such a process may be achieved by the use of diluents or by preventing overexposure of Si(OC2 H5 ) 4 to the sample under conditions where continuous deposition may occur. It was shown that it is possible to reduce the relative external surface acidity by 97% without significant changes in acidity as measured by Py-TPD or adsorption capacity.
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    Effects of in-situ chemical vapour deposition using tetraethoxysilane on the catalytic and sorption properties of ZSM-5.
    (Elsevier, 1998) Röger, H; Krämer, M; Möller, K; O'Connor, C
    The external surface of HZSM-5 crystals was gradually modified by in-situ CVD of tetraethoxysilane. Using the particular procedures described in the paper, it was shown from probe reactions in combination with sorption studies that the external surface was deactivated and the pore-opening size was reduced while the intracrystalline pore space remained unmodified. The positive effect of the CVD treatment procedure on the shape selective properties of HZSM-5 was demonstrated by an enhancement in p-selectivity during toluene disproportionation. The paper discusses the relation between the effects of the CVD treatment procedure on catalytic properties, sorption properties and structural properties and conclusions are made on the use of the conversion of 1,2,4-trimethylbenzene, 1,3,5-triisopropylbenzene, toluene and n-hexane, respectively, to probe the effects of external surface modifications on properties of ZSM-5.