Advances in Zeolite Catalytic Processes: Current Trends and Challenges in Processing Nonconventional Feedstocks

Dear Colleagues,

For several decades, a significant number of industrial processes involving separation operations and/or solid-catalyzed reactions have used zeolites as adsorbents as well as catalysts or catalytic supports. Many types of zeolites were developed and used for industrial applications, ranging from their early use in petroleum refineries to their more recent use in environmentally friendly, sustainable chemical processes.

Depending on the catalytic reaction involved, a specific zeolite catalyst can be selected. The three key characteristics of the zeolites—acidity, shape selectivity, and loading property—are responsible for their good performance as catalysts. Since it helps to create active and selective catalysts, strong acidity can be considered the most significant characteristic of zeolites. Nonetheless, weak acidity may be adequate for chemical reactions in some cases, while strong acidity is not always necessary. Different post-synthesis techniques (steaming, dealumination/desilication, ion exchange, metal loading) allow modifying the density, nature, and strength of acid sites as well as the porous structure. The shape of the micropores in zeolite framework, with a size similar to those of a large number of reactive molecules, is responsible for the shape selectivity. This property allows for the regulation of observed activity and selectivity. Moreover, the high specific surface area and acid/base tunable properties allow zeolites to be used as support for metals and metal oxides, ensuring high loads and metal dispersion in the pore system. Novel zeolite catalysts have been prepared combining these three characteristics, in order to reach a good catalytic performance and elongate the catalyst life.

The number of zeolites that are synthesized and studied increases notably year after year, as well as the opportunities to develop novel related (micro/mesoporous) materials with different potential applications. Nevertheless, the variety of zeolites that have been successfully used in commercial processes is limited. Many different zeolite species are not commercially available due to insufficient thermal and mechanical stabilities, as well as the high cost of zeolite synthesis. Among the zeolites that can be recognized as the most classically used in industrial and environmental protection processes, we can mention the following: Y (FAU), in the catalytic cracking of petroleum cuts; ZSM-5 (MFI), in isomerization, alkylation, and aromatization processes, NO reduction, and methanol to olefin conversions; mordenite (MOR), in trans-alkylation, isomerization, and disproportionation reactions; and β (BEA), in the alkylation of benzene and acylation with acetic anhydride.

As zeolites are extremely active in a wide range of catalytic processes due to their adjustable textural and acidic properties and are also thermally stable, they are very promising in processes that require the conversion of non-conventional feedstocks of different origins. Hierarchical zeolites are very efficient in catalyzing reactions involving large molecules, such as those obtained from biomass conversion processes, from the pre-cracking of bulky hydrocarbon compounds, and from the thermal pyrolysis of plastic waste, among others.

This Special Issue aims to address the current trends and challenges in the use of zeolites as catalysts in novel processes involving non-conventional feedstocks. Topics of interest may include (but are not restricted to) the application of zeolite catalysts to intensify the processing of non-conventional feedstocks (both fossil cuts and streams coming from biomass conversion or from the thermal pyrolysis of different wastes, among others). Strategies that enhance the catalytic properties, improve the catalytic performance (activity and selectivity), and the thermal stability and regenerability of catalysts, as well as increase their lifetimes, are welcome.

Dr. Juan Rafael Garcia
Dr. María Eugenia Sad
Dr. Clara Saux
Guest Editors

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• hierarchical zeolites
• zeolites in compound catalysts
• structured zeolitic catalysts
• novel zeotype materials
• stability, deactivation, and regeneration
• activity and selectivity improvement