Quantifying Site Heterogeneity in Microporous Aluminosilicates and Implications for Catalysis

Abstract

Zeolites and related microporous materials are key acid catalysts for many crucial transformations in both the gas and liquid phases for processes such as hydrocarbon refining, isomerization, and biomass upgrading. However, their catalytic behavior becomes complex under harsh hydrothermal conditions due to the formation of nonframework sites, which can significantly impact reaction rates and selectivity, complicating reproducibility and research evaluations. Therefore, in this work, we set out to establish characterization and titration protocols to identify and quantify site heterogeneity (i.e., differentiate between framework, partially hydrolyzed, and extraframework sites) of steamed microporous aluminosilicates, in contrast to solely using Brønsted and Lewis designations. For this purpose, we employ commercial MFI aluminosilicates (ZSM-5) of differing site heterogeneity and Si/Al ratios to quantify their site distribution through a combination of temperature-programmed desorption and FTIR protocols while contextualizing their effect on propane cracking rate constants. From the conclusions obtained, the present work provides a nuanced titration strategy on how to quantitatively determine the site heterogeneity of aluminosilicates and Al content without catalyst modification and with considerations for physisorbed species, base type, and size. We also reinforce literature observations of how water can induce changes in Al coordination even at ambient conditions, especially with increasing Al content, before catalysis, which adds variability in rate measurements. These observations and approaches should be extendable to other acidic zeolites and present ways to determine the site heterogeneity of materials in their dried state, in an accessible manner, that can serve as a starting point to evaluate structure–performance relationships.