Non-empirical quantum-chemical calculations on ZSM-5 zeolites. I. Brønsted acid sites

Abstract

This paper describes the study of the -OH group in ZSM-5 zeolite. The importance of zeolite ZSM-5 and the advantages in studying the OH groups by computer simulation techniques are highlighted. The methodology followed in the present study, which is a combination of classical and quantum-chemical calculations, is briefly described. The basis set SV3-21G has been chosen following trial calculations performed with different standard sets of basis functions. Results of geometrical optimizations carried out to determine the position of hydrogen in the ZSM-5 lattice are reported. The size of the point-charge (PC) cluster is varied and its effect on the calculated properties of the hydroxyl group is discussed. Calculations on a larger quantum-chemical (QC) cluster containing the hydroxyl group in both normalized and experimental structures reveal that geometry is an important factor for electronic properties. The procedure for generation of a suitable PC cluster to represent the rest of the lattice is described. Results of a calculation for a cluster model of 82 ions (PC cluster) in which a Si 2 O 7 (QC cluster) entity is explicitly treated by ab initio procedures, are discussed. The reported parameters include the O-H vibrational frequency, the proton binding energy and the aluminium substitution energy and we discuss their implications for the catalytic properties.

Consideration of the Aluminum Distribution in Zeolites in Theoretical and Experimental Catalysis Research

“…Ghorbanpour et al computed the stability of Al substitution at all 12 sites, finding T7 to be the most stable and T9 the least, although the span was less than 9 kcal/mol, suggesting that substitution at different T-sites is energetically similar . These computational findings are consistent with the experimental observations that Al siting in ZSM-5 depends on the synthesis conditions used and not by the thermodynamic stability of different T-sites. ,,, In addition to T12 and T7, some modeling studies have substituted Al at other T-sites, ,,,,− albeit less commonly. A recent computational study considered all 12 T-sites and used stability, accessibility, and acid strength as discerning criteria to conclude that T1 and T12 were the most favorable for Al substitution, while T2, T5, and T9 were also favorable.…”

“…Early computational studies of ZSM-5 largely focused on intrinsic structural properties of acidic zeolites, including comparing the stability of each T-site, and the stability of Al , (or Fe) substitution at various T-sites accounting for Lowenstein’s rule of avoiding next-nearest-neighbor Al substitution patterns. , Other studies focused on discriminating the different types of hydroxyl groups present (terminal SiOH and bridging Al–O­(H)-Si), their acid strength and how acid strength may be influenced by zeolite geometry, Al substitution, , and different types of internal silanol pairs . In addition, early interest focused on the adsorption behavior of various small molecules (benzene, toluene, butane, methanol, , p- xylene, CO, , H 2 O, NH 3 , ethylene, acetone, methyl ethyl ketone, diethyl ketone) and on elucidating catalytic mechanisms, such as the formation of carbenium ion intermediates from olefins .…”

“…In one of the first “zeolite” calculations, zero T-sites were included and the interactions of small molecules with a single hydroxyl group (sans tetrahedral atom) were calculated and compared with NMR experiments on zeolites . In what might be considered a precursor of the ONIOM method, some early studies embedded the ab initio QM region in a lattice of point charges modeling the zeolitic environment surrounding the active site. ,,, …”

“…The T12 site is the location most frequently chosen for Al substitution in studies of catalytic reactions in ZSM-5, ,− often attributed to its accessibility at the intersection of straight and sinusoidal channels. − As articulated by Ghorbanpour et al, however, two-thirds of the T-sites are also accessible at this intersection and only four T-sites are not at the channel intersection . Other studies have indeed chosen different T-sites for Al substitution for the same stated reason. ,, …”

Reliability of Small Cluster Models for Cu-Exchanged Zeolites

“…The water−ligand model was developed with the goal of providing a simple and versatile tool for exploring computationally various aspects of the catalytic chemistry of Cu-exchanged zeolites . While much more sophisticated methods have recently begun to be applied regularly to problems associated with Brønsted acid sites in zeolites, ,, one must recognize that (1) the problem of modeling exchanged transition ion sites is significantly more complicated, especially in the absence of definitive experimental information on the location and oxidation state of such ions, and (2) recent progress in applying large-cluster, , embedded-cluster, ,, and fully periodic 2 methods to Brønsted acid sites in zeolites represents the culmination of well over a decade of prior experience with relatively small clusters . In applying quantum-mechanical methods now to extraframework transition ions, our approach in ref and the present work has been to begin again with relatively small finite clusters in an attempt to extract as much qualitative understanding as possible before considering such additional complications as long-range electrostatic interactions, which are undoubtedly important quantitatively .…”