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

Abstract

Solid acids with sites varying in acid strength and in confining environments are used here to examine, through kinetic experiments and theory, how size and shape of organic guests and inorganic hosts at ion-pair transition states (TS) influence reactivity. Reactions of C 2 ÀC 4 alkene mixtures serve as the illustrative example; they involve kinetically-relevant CÀC coupling between alkenes and bound alkoxides that saturate the surface on microporous (TON) and mesoporous (SiAl) aluminosilicates of similar acid strength and on stronger heteropolytungstic acids (HPW). Rate constants (per H + ) increase with increasing TS size (number of C-atoms) because of the combined effects of size/substitution on proton affinities of coupling products and on van der Waals (vdW) contacts between carbocations and voids. When the voids and organic moieties are similar in dimension, these contacts require framework distortions, leading to dispersive interactions that are compensated in part by enthalpic penalties associated with the distortions, which are quantified here with DFT-derived atomic displacements and distortion energies. DFT-derived proton affinities of the coupling products and vdW interaction energies between the TS and voids represent the most accurate and complete descriptors of reactivity, as the size and shape of TS structures vary in CÀC coupling steps and more generally in acid-catalyzed transformations, and can be used in designing voids for stabilizing specific host-guest pairs.[a] Dr.

Controlled Deposition of Silica on Titania-Silica to Alter the Active Site Surroundings on Epoxidation Catalysts

“…The entropy loss is smallest for Ti-β, which may be a result of the relatively large limonene fitting poorly into these zeolite pores, 42 but also can result from a large entropy gain by released acetonitrile solvent. In contrast, the low apparent activation enthalpies for 2cFO and 2cPO are reminiscent of the behavior seen in transition-state confinement.…”

Control of the Reaction Mechanism of Alkylaromatics Transalkylation by Means of Molecular Confinement Effects Associated to Zeolite Channel Architecture

“…[3][4][5][6] Thus, the zeolite topology, the size and dimensionality of the channel systems, the presence or absence of intersections and/or cavities, combined with the confinement effects can determine the selectivity to the desired products by controlling the extension of competing parallel or consecutive reactions. [7][8][9][10][11] The complexity of the alkylaromatics conversion processes has led to numerous studies regarding the possible reaction mechanisms within the microporous structure of medium, 12-25 large 14,15,19,[26][27][28][29][30][31][32] and multipore 33,34 zeolites. Focusing on the transalkylation of alkylbenzenes, it can either proceed by intermolecular alkyltransfer involving dealkylation-alkylation steps with surface alkoxy species as reaction intermediates, or through formation of bulkier diaryl intermediates 19 (see Scheme 1).…”

“…This is due to the large variety of well-defined microporous structures available, the flexibility regarding their chemical composition, their thermal and hydrothermal stability, and their environmentally benign properties. , Their main applications within the petrochemical field are related to upgrading aromatic fractions. In general terms, alkylaromatics can be converted by means of Brönsted acid catalyzed disproportionation/transalkylation, isomerization, and dealkylation-alkylation reactions, and although the reactivity of the alkylaromatic and the reaction conditions play a role on product selectivity, the extension of each of these competing processes mainly depends on the structural and physicochemical properties of the zeolite catalyst. − Thus, the zeolite topology, the size and dimensionality of the channel systems, the presence or absence of intersections and/or cavities, combined with the confinement effects, can determine the selectivity to the desired products by controlling the extension of competing parallel or consecutive reactions. − …”

Preise der North American Catalysis Society: J. Pérez‐Ramírez, E. Iglesia und A. Datye

“…Sie kombinieren kinetische und spektroskopische Verfahren mit Isotopenexperimenten und theoretischen Methoden und der Synthese anorganischer Feststoffe mit neuartigen Architekturen. Iglesia und Mitarbeiter verglichen in einer Angewandte‐Chemie ‐Zuschrift assoziative und dissoziative Routen für die Methanoldehydratisierung in Zeolithen, und sie beschrieben Ladungs‐, Größen‐ und Formeffekte auf Reaktionen von Alkenen an sauren Feststoffen in ChemCatChem …”