Abstract
Attempts to optimize heterogeneous catalysis often lack quantitative comparative analysis. The use of kinetic modelling leads to rate (k) and relative sorption equilibrium constants (K), which can be further rationalized using Quantitative Structure-Property Relationships (QSPR) based on Multiple Linear Regressions (MLR). Friedel-Crafts acylation using commercial and hierarchical BEA zeolites as heterogeneous catalysts, acetic anhydride as the acylating agent, and a set of seven substrates with different sizes and chemical functionalities were herein studied. Catalytic results were correlated with the physicochemical properties of substrates and catalysts. From this analysis, a robust set of equations was obtained allowing inferences about the dominant factors governing the processes. Not entirely surprising, the rate and sorption equilibrium constants were found to be explained in part by common factors but of opposite signs: higher and stronger adsorption forces increase reaction rates, but they also make the zeolite active sites less accessible to new reactant molecules. The most relevant parameters are related to the substrates’ molecular size, which can be associated with different reaction steps, namely accessibility to micropores, diffusion capacity, and polarizability of molecules. The relatively large set of substrates used here reinforces previous findings and brings further insights into the factors that hamper/speed up Friedel-Crafts reactions in heterogeneous media.
Highlights
Quantitative Structure-Property Relationships (QSPR) are currently applied to an ever-growing plethora of physicochemical processes, be it to predict new data, rationalize phenomena, or both.In this study, Multiple Linear Regression (MLR) is applied to a heterogenous catalytic reaction to study which changes in the substrates’ characteristics affect the kinetic and equilibrium properties of the system
One of the major limitations currently described in the literature is the presence of large molecules that cannot access the active sites located inside the microporous structure of zeolites, leading to low catalytic activities
To facilitate the discussion, and since several of them will be used in the QSPR analysis, the main properties of the catalysts are presented in
Summary
Quantitative Structure-Property Relationships (QSPR) are currently applied to an ever-growing plethora of physicochemical processes, be it to predict new data, rationalize phenomena, or both.In this study, Multiple Linear Regression (MLR) is applied to a heterogenous catalytic reaction to study which changes in the substrates’ characteristics (size, polarity, etc.) affect the kinetic and equilibrium properties of the system. FC homogeneous reactions suffer from serious drawbacks such as the need to work at high temperatures and the use of over stoichiometric amounts of catalysts that cannot be regenerated at the end of the production cycle The latter, in turn, leads to complicated end-product purifications, often resulting in toxic and corrosive disposal issues [1,2]. Solid acid materials, such as zeolites, are widely used as catalysts in refining and petrochemical processes, since they can be separated from the reaction media and reused. One of the major limitations currently described in the literature is the presence of large molecules that cannot access the active sites located inside the microporous structure of zeolites, leading to low catalytic activities
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