Insight Into The Structures And Isomerization Mechanisms Of C4 Intermediates Confined Inside Zeolite From Nmr Spectroscopy And Theoretical Calculation

Zeolites with adjustable acidity and well-defined porous structure play a major role in the large-scale processes of petrochemical industries,synthesis of fine chemicals and others like catalytic cracking,hydrocracking,olefin isomerization,etc.Similar to other acid-catalyed reactions,zeolitic catalysis often follow a carbocation route,therefore identification of the key carbocation intermediates and observation of their conversion can provide important mechanistic information of these catalytic reactions.The isomerization of C4 species in zeolite system has drawn much attention due to the critical role in multiple catalytic processes such as cracking of crude oil and reformulating gasolines.The key intermediates during the isomerization between branched and linear C4 species has long been discussed.Among various proposed intermediates,tert-butyl carbocation is believed to be the most promising answer and is under extensive investigations.The main work of this article is to detect the tert-butyl carbocation and monitor its conversion in ZSM-5 zeolite.(1)It has been a difficult task to observe short-chain carbocations because of the short lifetime and high reactivity.In this work,we adopt co-absorption of C4 reactant with base molecules(NH3)in ZSM-5 zeolite to capture the intermediate species.Combine theoretical calculations and two-dimensional 13C-13C MAS NMR experiments,we analyzed the structure of the captured molecule and confirmed it as tert-butyl carbocation.Besides,the 13C-27Al double-resonance solid-state NMR spectroscopy was carried out to determine the host/guest interaction between the carbocation and the zeolite framework.(2)The mechanism of conversion of C4 species has been a research hotspot.In this chapter,in-situ 13C CP/MAS NMR were carried out to study the conversion of 2-13C-tert-butanol on ZSM-5 zeolite and analyzed the 13C distribution in the products.The results indicated that carbon skeleton rearrangement caused the transformation from quaternary position into terminal position and scrambling of selectively labeled 13C.In order to figure out this phenomenon,a plausible reaction pathway has been derived involving a key intermediate methyl cyclopropane by the accurate DFT theoretical calculations.By ammonia absorption and excessive reactant absorption,we detected the olefinic intermediates(2-13C-2-butene,1-13C-isobutene and 2-13C-isobutene)which has been predicted in our mechanism.Therefore,the experimental results gave a solid proof to the proposed mechanism.Combine the theoretical and experimental methods,we observed a skeleton transformation of C4 species during the conversion and gave a reasonable explanation to this phenomenon.