Carbonyl ¹³C Shielding Tensors and Heats of Adsorption of Acetone Adsorbed in Silicalite and the 1:1 Stoichiometric Complex in H−ZSM-5

Abstract

The principal components of the carbonyl carbon chemical shift tensor of the hydrogen-bonded 1:1 stoichiometric acetone-H-ZSM-5 adsorption complex have been determined from an analysis of 13 C NMR spectra of static and magic angle sample spinning powder samples at 78 and 130 K, respectively. In a similar manner the principal elements of this tensor have been determined for physisorbed acetone in silicalite and the pure solid in order to separate changes due to hydrogen bonding of the acetone molecule in the zeolite complex from confinement effects defined as interactions of the adsorbed molecule with the siliceous cavity. The energetics associated with such changes have also been measured using microcalorimetry. The differential heats of adsorption of acetone adsorbed in H-ZSM-5 and silicalite over a wide range of surface coverage are reported. The results are compared with ab-initio calculations of the reaction of acetone with model zeolite structures to form a stoichiometric hydrogen-bonded cluster-molecule complex. At the Hartree-Fock level the agreement with respect to both energetics and isotropic shifts is good but only fair for the shielding anisotropy. The magnitudes of the chemical shifts due simply to confinement of the acetone molecule are of the same order of magnitude as those associated with hydrogen bonding.

Molecular Structure and Confining Environment of Sn Sites in Single-Site Chabazite Zeolites

“…1,2,[6][7][8] Their catalytic behavior also depends on their secondary environments, which can result from organic ligands, 1,2,[6][7][8] or from confinement of sites within an inorganic cavity that provides enthalpic and entropic stabilization of bound reactive intermediates through van der Waals and electrostatic interactions. [9][10][11][12] As a result, accurate descriptions of catalytic active sites require precise definitions of both the local structure and the secondary environments of the binding sites, in turn, requiring spectroscopic and kinetic probes sensitive to both environments. Zeolites belong to one of the most widely used and studied classes of heterogeneous catalysts 13,14 for which primary and secondary environments influence catalytic reactivity.…”

An Examination of Brønsted Acid Sites in H-[Fe]ZSM-5 for Olefin Oligomerization and Adsorption

“…Since one would expect Cs to poison the most active sites preferentially, this result, together with the linear increase in rates with Al content, implies that all Al sites in H-ZSM-5 must have essentially identical activities. In our own work, we have shown that stoichiometric adsorption complexes, one molecule per Brønsted site, can be formed for a wide range of compounds in H-ZSM-5, including alcohols [7][8][9], amines [10], nitriles [11], pyridines [12], thiols [13], ketones [14,15], CO [16], and diethyl ether [11]. In agreement with the concept of equivalent acid sites, the reactivities of molecules associated with the adsorption complexes are independent of site density [17].…”

An Infrared Spectroscopic Study of Acetone and Mesityl Oxide Adsorption on Acid Catalyst

“…Earlier studies have focused on the exploration of the Lewis acidity, while in recent studies, the center of interest has been the interaction with Brönsted sites. Besides IR spectroscopy, − the other physical tool that has been extensively used is the 13 C nuclear magnetic resonance. − The extent of the proton transfer from the Brönsted sites to the carbonyl is small for Ac and almost complete for MO (absorbed on HZSM-5) . Biaglow et al. , reported that at coverage below one molecule Ac/Brönsted sites, the hydrogen-bonded absorption complex is stable and Ac has a low mobility .…”