Role of Lewis acid sites of ZSM-5 zeolite on gaseous ozone abatement

Abstract

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author-deposited version published in : http://oatao.univ-toulouse.fr/ Eprints ID : 9735 We report the role played by acidic surface sites of ZSM-5 on ozone removal. Ozone removal is mainly due to ozone decomposition on mild/strong Lewis acid sites. Adsorbed oxygen species appear on ZSM-5 surface because of ozone decomposition. Keywords:Acidic sites Chemical surface properties Lewis acid sites Ozone Synthetic zeolites ZSM-5 zeolite a b s t r a c t In this work, chemical interactions between ozone and zeolite surface active sites are studied in order to propose a process for gaseous ozone removal. Synthetic ZSM-5 zeolites with three different Si/Al 2 ratios and similar specific surface areas and microporous volumes were used in this study. Zeolite samples were characterised using Fourier Transform InfraRed spectroscopy (FTIR) and pyridine sorption IR studies in order to determine acidic site concentrations and strength. Ozone removal experiments were conducted in a quartz fixed-bed flow reactor, at 20°C and 101 kPa. Experiments using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) were conducted in order to identify adsorbed ozone and/or adsorbed oxygen species on zeolite surface. Pyridine IR measurements evidence two kinds of Lewis acid sites induced by extra-framework aluminium species and electronic aluminium defaults inside zeolite structure. Results obtained here evidence the important role of acidic surface sites of ZSM-5 zeolite on gaseous ozone removal. The total amount of removed ozone is found to be directly proportional to the total content of Lewis acid sites. DRIFTS experiments exhibit two bands around 800 and 1400 cm À1 that could correspond to adsorbed oxygen species linked to zeolite surface. DRIFTS experiments also exhibit a band around 1100 cm À1 that correspond to adsorbed ozone on the zeolite surface. Gaseous ozone removal using ZSM-5 zeolite could be largely attributed to ozone decomposition on Lewis acid sites and also to ozone adsorption on the surface of the zeolites.

Effect of Al content on number and location of hydroxyl acid species in zeolites: a DRIFTS quantitative protocol without the need for molar extinction coefficients

“…Increasing the SAR, and hence decreasing the total Al content, the number of Brønsted acid sites decreases. The observed trend is in good agreement with previous results 7, 30,31 and suggests that the experimental DRIFTS setup designed to probe the number of acid sites on the internal and external surface of H-ZSM-5 provides quantitative results.…”

“…Several IR studies 7,30,31 have found that the number of Brønsted acid sites in ZSM-5 increases linearly with increasing total Al content by studying the Brønsted peak in the C]C stretching region using pyridine. It is worth mentioning that the C-C and C-N vibrational modes of pyridine can be used as well to study protonic acidity.…”

Tuning of product selectivity in the conversion of ethanol to hydrocarbons over H-ZSM-5 based zeolite catalysts

“…1. All samples show three main region bands as usual [27,28]: the strong and sharp OH stretching band (νOH) at 3742 cm −1 is typically due to weakly acidic terminal silanol groups located on the external zeolite surface while the band at 3606 cm −1 is assigned to the "zeolitic" bridging hydroxyl Si-OH-Al groups on the inner surface and possessing a strong Brønsted acidity [29,30]. The presence of one or more bands in the region near 3660 cm −1 corresponds to OH groups on extraframework aluminium species, to which also a very weak component at 3770 cm −1 is associated.…”

Ozone-catalytic oxidation of gaseous benzene over MnO2/ZSM-5 at ambient temperature: Catalytic deactivation and its suppression

“…In addition, the rich Lewis acid sites of ZSM-5 can further enhance ozone decomposition [34]. ZSM-5 should be a good candidate for supporting MnO x .…”