Sailing the seven 'C's: 2,2,3-Trimethylbutane (triptane) selectively forms from dimethyl ether at low temperatures on acid zeolites. Selective methylation at less-substituted carbons, relative rates of methylation to hydrogen transfer as a function of chain size, slow skeletal isomerization, and beta-scission cracking of triptyl chains and their precursors are intrinsic properties of carbenium ions and account for the remarkable triptane selectivities within C(7) .
The
reaction of toluene methylation was investigated with four
acidic zeolites of different pore geometries: the medium pore zeolites
H-ZSM5 and H-ZSM11 as well as the large pore zeolites H-MOR and H-BEA.
The methylation, methanol consumption, light hydrocarbon formation,
and disproportionation rates for the reaction of toluene, p-xylene, and 1,2,4-trimethylbenezene with methanol were
determined. The products of toluene methylation (e.g., xylenes and
trimethylbenzenes) were readily methylated further in both medium
and large pore zeolites. A considerably higher fraction of methanol
was used to form light hydrocarbons with the medium pore zeolites
than with large pore zeolites. This was related to the fact that the
dealkylation of light hydrocarbons from highly methylated aromatics
became more favorable relative to methylation at an earlier stage,
that is, after fewer methyl groups were added to the aromatic ring.
Increasing the effective residence time of bulky aromatic molecules
with medium pore zeolites, modified either by coating the surface
with tetraethyl orthosilicate or by increasing the intracrystal pore
length, converted a larger fraction of methanol to light hydrocarbons
via methylation and subsequent dealkylation of light hydrocarbons.
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 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 Brønsted acids.
An increase in p-xylene selectivity was observed without losing the catalytic activity over novel mesoporous nano-sized ZSM5 crystals covered with an external SiO2 overlayer created by deposition of tetraethyl orthosilicate.
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