(349b) Mechanistic Insights Into the Production of Highly Branched Alkanes From Dimethyl Ether On Acidic Zeolites

The production of hydrocarbons that comprise gasoline, diesel, and jet fuel from non-petroleum feedstocks is becoming increasingly important. Methanol-to-hydrocarbon (MTH) processes have been studied and developed to produce gasoline range hydrocarbons from methanol on solid acid catalysts. It is advantageous to develop a catalytic system with milder conditions that leads to the selective production of hydrocarbons that are appropriate for transportation applications, because current MTH processes operate at temperatures above 573 K and exhibit low selectivities to specific hydrocarbon molecules, in general, and specifically to branched aliphatic molecules. Triptane (2,2,3-trimethylbutane), a desirable gasoline-range hydrocarbon and a valuable fuel additive (research octane number = 112), can be synthesized from methanol in homogeneous solutions using zinc halides and indium iodide at ~473 K with nearly 50% of the gasoline-range products (C₅-C₁₃) consisting of triptane. Recent work proposed that methanol homologation to triptane in ZnI₂ systems proceeds via carbocationic intermediates and that the remarkable selectivity to triptane reflects an increase in the rate of alkene methylation with degree of substitution in which 2,3-dimethyl-2-butene (a tetra-substituted alkene) methylates faster than 2-methyl-2-butene (a tri-substituted alkene), which methylates faster than isobutene (a di-substituted alkene). The rate of methylation of C₇ chains (triptene, a di-substituted alkene) is slower than its smaller alkene precursors, thus allowing them to be hydrogenated to triptane via competitive H-transfer reactions. High selectivity at C₇/triptane is observed as a consequence of higher rates of desorption as triptane via hydrogen transfer with alkenes, which contain weaker C-H bonds than alkanes, than of desorption of triptene via deprotonation. The loss of hydrogen from these alkenes results in the formation of increasingly hydrogen-deficient species and eventually aromatic species such as hexamethyl benzene (HMB). Methanol homologation to triptane with ZnI₂, although selective, presents commercial challenges because of difficulties in separation of homogeneous reaction mixtures and the presence of toxic, corrosive halide species within alkane products. Synthesizing triptane from methanol heterogeneously on halide-free solid acids, such as zeolites, is, therefore, desirable. We present here the conversion of dimethyl ether (DME) to triptane on acidic zeolites and elucidate the mechanism of this reaction in unprecedented detail. We also present the co-homologation of DME with low value, high volume alkanes as an attractive option for the upgrading of light alkanes to highly branched gasoline range hydrocarbons.

Reaction rate and selectivity data for the homologation of DME was measured on acidic zeolites of varying pore geometry. Beta-zeolite exhibited the best combination of reaction rate (740 µmol C [mol Al s]^-1) and selectivity to triptane (total carbon selectivity of 20%) as compared to FER (15 µmol C [mol Al s]^-1, <1% carbon selectivity to triptane), MFI (600 µmol C [mol Al s]^-1, 1% carbon selectivity to triptane), and FAU (800 µmol C [mol Al s]^-1, 3% carbon selectivity to triptane). These selective homologation reactions occur at modest temperatures (400-500 K) and DME pressures (60-250 kPa) with the preferential formation of isobutane (34% total carbon selectivity) in addition to triptane. Competitive reactions of ¹³C-labeled DME with unlabeled alkenes (propene, trans-2-butene, isobutene, 2-methyl-2-butene, 2,3-dimethyl-2-butene, and 2,3,3-trimethyl-1-butene) led to a rigorous assessment of the relative rates of methylation, hydrogen transfer, and isomerization and to an unprecedented level of detail about chain growth pathways during DME homologation. These data provided the basis for the remarkable specificity of the pathways to isobutane and triptane synthesis. We found that reactions leading to the formation of species with the most stable carbenium ions are the favored chain growth pathways in DME homologation, similar to the chemistry proposed for the previously mentioned reaction in homogenous solutions. C-C bond formation occurs between alkenes and DME-derived surface methylating species leading to the formation of an alkoxide surface intermediate. Chain propagation occurs via deprotonation of the alkoxide to form a gas phase alkene with subsequent methylation of the alkene. Chain termination occurs via aforementioned hydride transfer of the alkoxide and desorption as an alkane. Specifically, methylation occurs in a manner that leads to the formation sequence of n-butane, 2-methylbutane, 2,3-dimethylbutane, and 2,2,3-trimethylbutane as opposed to other isomers of these molecules. The relative rate of methylation to hydride transfer for the 2,2,3-trimethyl sec-butoxide intermediates is much smaller than for the C₃-C₆ precursors to triptane. Thus, chain growth is fast relative to termination until triptane formation where termination becomes favored. Methyl and hydride shifts occur readily along the backbone of growing chains, but the rate of the methyl shifts that lengthen or shorten chains is negligible, thus preserving the structures required for triptane synthesis. Methylation of 2,2,3-trimethylbutane leads to C₈ and C₉ molecules that undergo rapid β-scission to form isobutane (causing high selectivity to this molecule) and other smaller hydrocarbon species. These smaller species from β-scission events can be reincorporated into homologation pathways, especially in the presence of a hydride transfer co-catalyst, such as adamantane. Thus, DME homologation on H-BEA zeolite selectively produces triptane via kinetic pathways in which triptane is protected against further growth while its precursors are protected against skeletal rearrangements that lead to non-triptane products.

The elucidation of the DME homologation mechanism indicates that high-volume, low-value alkanes can be upgraded to higher value alkanes via co-homologation with DME, provided that a suitable hydride transfer co-catalyst facilitates the conversion of the alkane co-feed to alkenes. We have found that isobutane, 2-methylbutane (2MB) and 2,3-dimethylbutane (23DMB) can be converted to triptane via methylation with DME over H-BEA in presence of an adamantane co-catalyst. The co-processing of DME (39 kPa) and isobutane (39 kPa) at 473 K on H-BEA leads to an increase in reaction rates by a factor of 1.5 - 2.0 as compared to the homologation of DME (39 kPa) under the same conditions. The addition of adamantane to the isobutane/DME feed results in an additional increase in the rates of production of 2MB by a factor of 10, 23DMB by a factor of 6, and triptane by a factor of 2. The alkene to alkane molar ratios for C₅ and C₆ products decrease by orders of magnitude upon adamantane addition, indicating that adamantane serves as a dehydrogenation/hydrogenation co-catalyst. Competitive reactions between ¹³C-labeled DME and unlabeled alkanes (n-butane, isobutane, 2MB, and 23DMB) reveal details about chain growth pathways and the role of adamantane. The most abundant triptane isotopomers contain 3, 2, and 1 ¹³C atoms for isobutane, 2MB, and 23DMB co-feeds, respectively. Ratios of hydride transfer to methylation rates and triptane to HMB formation rates increase upon adamantane addition to the alkane/DME feeds while ratios of triptane to smaller homologation products decrease upon adamantane addition. The data from these isotopic labeling studies show that alkane formation occurs via methylation of alkenes derived from the alkane co-feed and that the same growth rules apply for the co-processing of DME and alkanes as for de novo synthesis of triptane from DME. Adamantane serves as a dehydrogenation/hydrogenation catalyst facilitating the conversion of the alkane co-feed to alkenes which serve as chain growth initiators. The alkane co-feed also provides the required amount of hydrogen for the synthesis of alkanes minimizing the rejection of carbon as unsaturated products (HMB) as observed in the case of DME homologation, and adamantane facilitate the transfer of hydrogen from the alkane co-feed to surface alkoxide intermediates. The presence of adamantane can lead to a decrease in triptane selectivity, however, because of premature termination of C₅-C₆ intermediates via hydride transfer.

We have shown that the homologation of DME on acidic zeolites leads to the selective formation of the high value fuel additive, triptane. Isotopic and kinetic studies reveal a homologation pathway involving the preferential methylation of growing chains leading to structures that cannot readily change backbone chain length by cracking or isomerization. Furthermore, these results show that selective C-C bond formation between DME and light alkane co-feeds is to an attractive option for the upgrading of low value, high volume alkanes. Adamantane serves as a hydride transfer co-catalyst that facilitates both the conversion of the alkane co-feed to alkenes that participate in homologation pathways as chain carriers and the transfer of hydrogen from the alkane co-feed to surface alkoxides resulting in alkane formation.