Catalytic Co-Homologation of Alkanes and Dimethyl Ether and Promotion by Adamantane as a Hydride Transfer Co-Catalyst

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 Bronsted 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 C1 species, as shown by the isotopologues formed in reactions of 13C-DME with 12C-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 C1 species, and mitigates the formation of unsaturated by-products ubiquitous in the homologation of DME or methanol on Bronsted acids.