Clarification of the long standing question of the gas phase acidity of zeolites with respect to classical strong acids and superacids
A recent collaboration work between University of Sofia, Bulgaria and Laboratoire Catalyse & Spectrochimie, CNRS, Caen, France reported key findings, related to the acidity of zeolite materials, which is their key functionality for applications in the petrochemical industry. In particular, the paper addressed the long standing question of the acidity of the zeolite acid sites with respect to the well-known acidic compounds as nitric acid, sulfuric acid, and superacids. The deprotonation enthalpies of the the hydroxyl groups in zeolites were evaluated using available experimental values for deprotonation enthalpy of small acid molecules in the gas phase and the corresponding values for zeolite hydroxyls calculated with density functional method. Based on the obtained data, the authors were able to position properly Brønsted acid sites (bridging hydroxyl groups) and silanol groups in zeolites into the general scale of gas phase acidity. Thus, the simulated deprotonation enthalpies imply that the Brønsted acid sites in zeolites fall in the range of superacids while silanols, depending on their surroundings, cover wide range from strong acids to superacids. Comparison of the acidity values with the infrared and NMR spectra of the same hydroxyl group, however, showed that the acidity of the hydroxyls cannot be estimated based on the spectral characteristics. Both for silanols and bridging hydroxyl the spectral characteristics accounts for the formation and strength of hydrogen bonds, while the hydrogen bonding is not the main factor determining the variations in their acidity.
The reported new results and their interpretation will be very useful for understanding various aspects related to catalytic and sorption properties of zeolites and similar silica or aluminosilicate materials, which are among the most important industrial catalysts, supports, or sorbents. Thus, analogous approach may be applied to other silica or aluminosilicate materials. The proper evaluation of their acidity with respect to traditional acidic compounds will help to substitute the latter acids, which are environmentally harmful and dangerous for the health, with environmentally friendly solid acid catalysts.
The paper is published in Microporous and Mesoporous Materials and is free of charge with gold open access provided by Bulgarian Ministry of Education and Science: