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Black plate (270,1) hydrocarbon pool view of the MTO process since it does not demand any highly activated concerted steps169. Some details of the H-D exchange reactions of propene over D- ZSM-5, its boron analogue D-(B)ZSM-5 and D-SAPO-34 have been reported. Given the huge excess of exchangeable deuterium atoms on the catalyst in relation to the number of hydrogen atoms in the propene molecules loaded onto it, complete exchange would be expected to yield C3D6 as the exclusive equilibrium product of the reaction. However, a distribution of exchanged propene isoto- pomers was measured by mass spectrometry: 6% D4, 72% D5 and 22% D6 propene. By increasing the size of the propene pulse, the deuterium distribution was substantially but reproducibly different: 28% D4, 44% D5 and 29% D6 propene. The products were found to include not only propene but additional substances arising from cracking and oligomerisation, i.e. butenes, pentenes and hexenes. A mechanism is suggested to account for these changes170. In a separate study, the H-D exchange reaction of 2,2-dimethyl- propane (neopentane) over D2O-exchanged zeolites (MOR, FAU, BEA, MFI) in a batch recirculation reactor was studied by means of gas chromatographyymass spectrometry. It was found that H-D exchange proceeds without any side reactions at short contact times, in the temperature range 473–573K. The activation energies of the HyD exchange between neopentane and zeolites are the same for all zeolites which may indicate a common carbonium ion type transi- tion state, in analogy with the exchange between methane and liquid superacids. Accordingly, the deuterium exchange rates in neopen- tane were tentatively related to the acidity of the solids. However the order of activity MOR4MFI4BEA4FAU seems to be related to the size of the pores, which may suggest that a confinement effect in the zeolite cavities is critical171. However, as we have already discussed, a genuine carbocation mechanism appears dubious. Ivanova et al. have written a short review on the impact of in situ 13C and 1H MAS NMR on the elucidation of the mechanisms by which alkylaromatic compounds are transformed over zeolite catalysts. Conversions of deuterated benzene and cumene deriva- tives on protonic forms of zeolites are used to account for H-D exchange both in the aromatic ring and in the alkyl chain. The nature of 13C label transfers during the course of the rearrangements of specifically labeled benzene, cumene, n-propylbenzene and ethylbenzene are evaluated to provide deeper insight in the mechan- isms of transalkylation, disproportionation, skeletal and nonskeletal rearrangements of alkyl chain, fragmentation and dealkyla- 270 Christopher J. RhodesPDF Image | Properties and applications of zeolites
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