Light Hydrocarbon Reactions on Solid Bronsted Acids: Confinement and Acid Strength Effects on Ion-Pair Transition States and Their Precursors

Light hydrocarbons are converted on solid Brønsted acids via oligomerization, isomerization, β-scission, hydride transfer and cyclization reactions; these reactions are mediated by ion-pair transition states (TS). The relative stability of these ion-pairs, consisting of organic cations derived from reactants and inorganic anions formed by deprotonation of the inorganic catalyst, relative to their relevant precursors determine the rate of each reaction and their prevalence over the others. The anion contributes to TS stability by (i) van der Waals interactions conferred by the geometry of its surroundings (confinement) and (ii) its willingness to accept the negative charge (acid strength) and to interact electrostatically with the organic cation. The cation, in contrast, determines TS stability through its ability to accept the proton and then interact with the anion. These confinement and electrostatic effects can be probed using reactions mediated by TS that differ in size, shape or charge to various extents from each other and from their relevant kinetic precursors; density function theory (DFT) methods that account for the pertinent dispersive interactions further provide detailed information that is inaccessible from experiments alone, such as how the cation and anion conform to each other.

Here, we report turnover rates (per proton) for C₂-C₄ alkene oligomerization reactions and for the incorporation of C₄-C₅ alkanes via hydride transfer on acids of different acid strength and confining environments (TON, MFI, BEA, MOR, FAU zeolites; amorphous silica-alumina (SiAl), Keggin polyoxometalate clusters (POM) clusters on SiO₂). C-C bond formation and hydride transfer rates show that kinetically-relevant steps involve reactions of an alkane or alkane to alkene-derived alkoxides present as saturation coverages, consistent with in-situ infrared spectra and DFT estimates of activation free energy barriers and of the stability of alkoxide intermediates. These kinetic data, obtained over a very broad range of reactant pressures, allow a systematic comparison between theory and experiment and also accurate estimates of alkoxide adsorption constants. In doing so, we provide a quantitative assessment of the effects of surface curvature and of alkene size and structure on alkoxide stabilities, which indicates bulkier alkoxides become sterically hindered within smaller, concave environments (TON) as compared to larger pore environments (MOR, FAU, SiAl, POM).

The rate constants for oligomerization and hydride transfer increase exponentially as the deprotonation energy (DPE) of the solid acid decreases and the stability of its conjugate anion concomitantly increases. These effects reflect DFT-derived transition states that differ in charge from their alkoxide precursors. Rates constants also generally increase with increasing TS size for both oligomerization and hydride transfer on each zeolite framework, because of the combined effects of the greater stability of the larger TS carbenium ions and their more effective contact with the void walls. DFT treatments show that zeolite frameworks distort locally, so as to enhance van der Waals contacts at the expense of a slight distortion of the framework lattice, which ultimately becomes too costly as the TS reach the size of the confining voids. For example, the lattice of TON (0.57 nm channel) locally moves closer to the TS for smaller TS (ethene dimerization; 0.46 nm diameter) and moves away for larger TS (isobutene dimerization; 0.58 nm diameter); the distortions become too costly, however, when the TS is much larger than the void (isobutane-C₆ hydride transfer; 0.81 nm diameter), consistent with a lower rate constant for TON than for larger pore environments. These energy compromises are also critical in alkoxide formation. DFT-calculated alkoxide energies indicate bulkier alkoxides distort the framework more in smaller, concave environments (TON) than larger pore ones (MOR, HPW).

This array of transition states and their precursors—formed from a range of reactants and catalysts—exploits the diversity in size, shape and charge in solid acid upgrading of alkenes/alkanes and provides unprecedented clarity of the complex relationship between organic moieties and inorganic catalysts when combined with state-of-the-art theoretical methods. By developing descriptors that relate reactivity and selectivity to transition state properties (intrinsic stability of the organic cation, electrostatic interaction with the anion (acid strength) and stabilization via confinement provided by the flexible inorganic framework), we aim to extrapolate these relationships to other transition states and acid catalysts than those studied here.

Financial support from BP through the ICC Program and National Science Foundation Graduate Research Fellowship Programs and computational resources from National Science Foundation’s Extreme Science and Engineering Discovery Environment (XSEDE; ACI- 1053575) are gratefully acknowledged.