Penilaian Karya Ilmiah C-11

Recent experimental work has shown that variations in the confinement of <i>n</i>-butane at Brønsted acid sites due to changes in zeolite framework structure strongly affect the apparent and intrinsic enthalpy and entropy of activation for cracking and dehydrogenation. Quantum chemical calculations have provided good estimates of the intrinsic enthalpies and entropies of activation extracted from experimental rate data for MFI, but extending these calculations to less confining zeolites has proven challenging, particularly for activation entropies. Herein, we report our efforts to develop a theoretical model for the cracking and dehydrogenation of <i>n</i>-butane occurring in a series of zeolites containing 10-ring channels and differing in cavity size (TON, FER, -SVR, MFI, MEL, STF, and MWW). We combine a QM/MM approach to calculate intrinsic and apparent activation parameters, with thermal corrections to the apparent barriers obtained from configurational-bias Monte Carlo simulations, to account for configurational contributions due to global motions of the transition state. We obtain good agreement between theory and experiment for all activation parameters for central cracking in all zeolites. For terminal cracking and dehydrogenation, good agreement between theory and experiment is found only at the highest confinements. Experimental activation parameters, especially those for dehydrogenation, tend to increase with decreasing confinement. This trend is not captured by the theoretical calculations, such that deviations between theory and experiment increase as confinement decreases. We propose that, because transition states for dehydrogenation are later than those for cracking, relative movements between the fragments produced in the reaction become increasingly important in the less confining zeolites

Insights into the Kinetics of Cracking and Dehydrogenation Reactions of Light Alkanes in H‑MFI

Monomolecular reactions of alkanes in H-MFI were investigated by means of a dispersion-corrected density functional, ωB97X-D, combined with a hybrid quantum mechanics/molecular mechanics (QM/MM) method applied to a cluster model of the zeolite. The cluster contains 437 tetrahedral (T) atoms, within which a T5 region containing the acid site along with the representative alkane is treated quantum mechanically. The influence of active site location on reaction energetics was examined by studying cracking and dehydrogenation reactions of <i>n</i>-butane at two regions in H-MFI–T12, where the proton is at the intersection of straight and sinusoidal channels, and T10, where the proton is within the sinusoidal channel. Two transition states were observed for cracking: one where the proton attacks the C–C bond and another where it attacks a C atom. Dehydrogenation proceeds via a concerted mechanism, where the transition state indicates simultaneous H₂ formation and proton migration to the framework. Intrinsic activation energies can be determined accurately with this method, although heats of adsorption were found to be higher in magnitude relative to experiments, which is most likely mainly caused by the MM dispersion parameters for the zeolite framework atoms. Intrinsic activation energies calculated for reactions at the T10 site are higher than those at T12 owing to differences in interaction of the substrate with the acid site as well as with the zeolite framework, demonstrating that Brønsted acid sites in H-MFI are not equivalent for these reactions. Apparent activation energies, determined from calculated intrinsic activation energies and experimentally measured heats of adsorption taken from the literature, are in excellent agreement with experimental results

SBH10: A Benchmark Database of Barrier Heights on Transition Metal Surfaces

While the performance of density functional approximations (DFAs) for gas phase reaction energetics has been extensively benchmarked, their reliability for activation barriers on surfaces is not fully understood. The primary reason for this is the absence of well-defined, chemically accurate benchmark databases for chemistry on surfaces. We present a database of 10 surface barrier heights for dissociation of small molecules, SBH10, based on carefully chosen references from molecular beam scattering, laser assisted associative desorption, and thermal experiments. Our benchmarking study compares the performance of a dispersion-corrected generalized gradient approximation (GGA-vdW), BEEF-vdW, a meta-GGA, MS2, and a screened hybrid functional, HSE06. In stark contrast to gas phase reactions for which GGAs systematically underestimate barrier heights and hybrids tend to be most accurate, the BEEF-vdW functional determines barriers accurately to within 0.14 eV of experiments, while MS2 and HSE06 underestimate barrier heights on surfaces. Higher accuracy of BEEF-vdW stems from the fact that the functional is trained on chemisorption systems, and transition states for dissociation on surfaces closely resemble the final, chemisorbed states. Therefore, a functional that can describe chemisorption accurately can also reliably predict barrier heights on surfaces