Elementary steps in acetone condensation reactions catalyzed by aluminosilicates with diverse void structures

Abstract

The unselective nature and ubiquitous deactivation of Brønsted acids in aldol condensations have precluded their practical use and unequivocal mechanistic assessments. H 2 and a Pt function are used here to confer stability by scavenging unsaturated intermediates to form stable products, thus allowing kinetic, spectroscopic, and isotopic assessments of elementary steps and their kinetic relevance, confirmed by density functional theory (DFT), for acetone condensation on microporous and mesoporous aluminosilicates (FER, TON, MFI, BEA, FAU, MCM-41). The selective titration of protons with 2,6-di-tert-butyl pyridine during catalysis shows that condensations occur exclusively on protons; the number of titrants required to suppress reactivity measures accessible sites and allows reactivity to be rigorously reported as turnover rates. Infrared spectra show that H-bonded acetone is present at saturation coverages during condensation catalysis. Taken together with rates that depend linearly on acetone pressure and with the absence of H/D kinetic isotope effects, these data indicate that condensation turnovers are mediated by the kinetically-relevant formation of a C-C bond via reactions of Hbonded acetone with another acetone molecule, a conclusion confirmed by DFT-derived free energies along the reaction coordinate. Measured rate constants reflect free energy differences between this transition state and its relevant precursors, a H-bonded and a gaseous acetone. These rate constants (per H + ) depend sensitively on size and shape of the confining voids among aluminosilicates of similar acid strength but diverse framework structure. Confinement effects are mediated by van der Waals contacts and are accurately described by energies derived from Lennard-Jones potentials of DFT-derived transition state structures; these energies are ensembleaveraged over all accessible configurations and T-site locations in each aluminosilicate framework. These energy descriptors replace incomplete metrics based solely on the sizes of voids and transition states, which fail to capture differences in reactivity among different confining frameworks.

Effects of Charge, Size, and Shape of Transition States, Bound Intermediates, and Confining Voids in Reactions of Alkenes on Solid Acids

“…Periodic plane‐wave density functional theory methods (Vienna ab initio Simulation Package; (VASP)) were used to determine optimized structures and energies for all stable intermediate and transition states with projector‐augmented wave (PAW5) pseudopotentials and their cut‐off energy of 396 eV (taken from O‐atoms value) ,. Exchange and correlation energies were calculated within the generalized gradient approximation using revised Perdew−Burke−Ernzerhof (RPBE) functionals with dispersive energies and forces calculated by DFT‐D3 algorithms during each energy minimization step,, which have been used previously in these systems to benchmark against experiment ,. A 1×1×1 Monkhorst−Pack k‐point mesh was used to sample the first Brillouin zone .…”

Identifying the roles of acid–base sites in formation pathways of tolualdehydes from acetaldehyde over MgO-based catalysts

“…As an electron acceptor, the Lewis acid site draws the negative charge and stabilizes the enolate intermediate, which is feasible because the conjugation of the molecule is preserved. 77 We hypothesize that this electron flow towards the carbonyl-O coordinated to the Lewis acid shifts the negative charge density in the carbon chain originally localized on the γ-C towards the α-C, thereby making the α-enolate a more represented nucleophile. This allows route 2B to contribute more appreciably.…”

A New Computational Interface for Catalysis

“…Lowfrequency (≤ 100 cm −1 ) modes have documented inaccuracies and are often inappropriate for harmonic oscillator entropy estimates; thus, frequency cutoffs can be specified, beneath which a representative value (e.g., 60 cm −1 ) can be used instead as done previously for zeolite catalyzed dehydration and condensation reactions. 13,14 Visualizing structures and reactions can be conveniently done through the webpage, but CCI also has tools to create high resolution stylized images for visualizing. These high-quality image tools utilize the POV-Ray package 15 to render a scene with raytracing.…”