Computational Study of Brønsted Acidity of Mordenite. Effect of the Electric Field on the Infrared OH Stretching Frequencies

Abstract

Electric fields have been claimed to influence intrinsic acidity on zeolites, and in this work, we try to establish a relation between the electric field and Brønsted acidity. In this study, we use experimental data from commercial samples of mordenite (Si/Al ≈ 4−8) and we make periodic first-principles calculations of the infrared OH stretching bands with periodic density functional methods on several reliable models of a mordenite unit cell with a composition of Si40Al8O96H8 (Si/Al = 5). Experiments show a high-frequency band at 3610 cm−1 and a low-frequency band at 3585 cm−1, and the computational results give frequencies in the range of 3600−3510 cm−1, slightly lower due to the PAW-GGA methodology employed. A rationalization of the IR frequencies found has been attempted on the basis of physicochemical properties of the microporous material. No correlation has been found with geometrical parameters, such as SiO(H)Al angles, or chemical parameters, such as the number of Al neighbors. No clear relation was found between the cage size where the Brønsted site vibrates and the OH frequency, in disagreement with some common interpretations, this suggesting that a complex mixture of short- and long-range factors is at play. We, however, found that a parameter related to the electric field at the proton site correlates with the OH frequencies over a large sample of centers of different O types, SiO(H)Al angles, and number of Al neighbors.