Acid strength and solvation effects on methylation, hydride transfer, and isomerization rates during catalytic homologation of C₁ species

Dimethyl ether (DME) homologation forms isobutane and triptane (2,2,3-trimethylbutane) with supra-equilibrium selectivities within C₄ and C₇ hydrocarbons on both mesoporous solid acids (SiO₂–Al₂O₃, H₃PW₁₂O₄₀/SiO₂) and the acid forms of various zeolites (BEA, FAU, MFI) via methylation and hydride transfer steps that favor isobutane and triptane formation because of the relative stabilities of ion-pairs at transition states for chains along the preferred growth path. The stabilities of ion-pair transition states increase as acid sites become stronger and energies for charge separation decrease and as van der Waals interactions within pores become stronger, which respectively lead to higher rates on H₃PW₁₂O₄₀/SiO₂ and aluminosilicate zeolites than on amorphous SiO₂–Al₂O₃. Solid acids with different strengths and abilities to solvate ion-pairs by confinement differ in selectivity because strength and solvation influence transition states for the hydride transfer, methylation, and isomerization steps to different extents. Stronger acid sites on H₃PW₁₂O₄₀/SiO₂ favor isomerization and hydride transfer over methylation; they lead to higher selectivities to n-butane and non-triptane C₇ isomers than the weaker acid sites on BEA, FAU, and mesoporous SiO₂–Al₂O₃. This preference for hydride transfer and isomerization on stronger acids reflects transition states with more diffuse cationic charge, which interact less effectively with conjugate anions than more localized cations at methylation transition states. The latter recover a larger fraction of the energy required to form the ion-pair, and their stabilities are less sensitive to acid strength than for diffuse cations. Large-pore zeolites (BEA, FAU) form triptane with higher selectivity than SiO₂–Al₂O₃ because confinement within large pores preferentially solvates the larger transition states for hydride transfer and methylation, which preserve the four-carbon backbone in triptane, over smaller transition states for alkoxide isomerization steps, which disrupt this backbone and cause growth beyond C₇ chains and subsequent facile β-scission to form isobutane. MFI forms isobutane and triptane with much lower selectivity than mesoporous acids or large-pore zeolites, because smaller pores restrict the formation of bimolecular methylation and hydride transfer transition states required for chain growth and termination steps to a greater extent than those for monomolecular alkoxide isomerization. These data and their mechanistic interpretations show that the selective formation of isobutane and triptane from C₁ precursors like DME is favored on all acids as a result of the relative stability of methylation, hydride transfer, and isomerization transition states, but to a lesser extent when small confining voids and stronger acid sites preferentially stabilize monomolecular isomerization transition states. The observed effects of acid strength and confinement on rates and selectivities reflect the more effective stabilization of all ion-pairs on stronger acids and within solvating environments, but a preference for transition states with more diffuse charge on stronger acids and for ion-pairs with the appropriate solvation within voids of molecular dimensions.