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Catalytic Co‐Homologation of Alkanes and Dimethyl Ether and Promotion by Adamantane as a Hydride Transfer Co‐Catalyst
Author(s) -
Simonetti Dante A.,
Ahn John H.,
Iglesia Enrique
Publication year - 2011
Publication title -
chemcatchem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.497
H-Index - 106
eISSN - 1867-3899
pISSN - 1867-3880
DOI - 10.1002/cctc.201000383
Subject(s) - chemistry , hydride , adamantane , isobutane , catalysis , alkane , dimethyl ether , neopentane , photochemistry , alkene , isopentane , carbenium ion , organic chemistry , medicinal chemistry , hydrogen , molecule
We provide kinetic and isotopic evidence for the co‐homologation of linear and branched alkanes with dimethyl ether (DME) to form larger branched alkanes and isobutane on H‐BEA zeolites, and for the role of adamantane as a hydride transfer co‐catalyst that allows the activation of CH bonds in alkanes at low temperatures (<500 K) on Brønsted acid sites. Branched alkanes (isobutane, isopentane, and 2,3‐dimethylbutane) present in equimolar mixtures with DME form the corresponding alkenes via hydride transfer to bound alkoxides (formed in DME homologation steps) and subsequent deprotonation; these alkenes, derived from the added alkanes, are then methylated to lengthen their chain by using DME‐derived C 1 species, as shown by the isotopologues formed in reactions of 13 C‐DME with 12 C‐alkanes. Linear alkanes are much less reactive than branched alkanes, because of their stronger CH bonds and larger carbenium ion formation energies, which determines hydride‐transfer rates to a given acceptor molecule. Adamantane increased the hydride‐transfer rates to bound alkoxides from branched alkanes, and even from unreactive linear alkanes, while also increasing their extent of incorporation into DME homologation pathways; adamantane acts as a reversible hydrogen donor that mediates dehydrogenation of alkanes at low temperatures on acid sites. The co‐homologation of alkanes with DME avoids the need for carbon rejection in the form of arenes to satisfy the hydrogen balance in the DME conversion to alkanes, provides a robust strategy for increasing the chain length and extent of branching in light alkanes through the selective addition of C 1 species, and mitigates the formation of unsaturated by‐products ubiquitous in the homologation of DME or methanol on Brønsted acids.