Dear Colleagues,

Acid-catalyzed reactions are by far the most numerous and best-studied reaction types in which organic functional groups undergo an array of different transformations. These include various electrophilic aromatic substitutions, e.g. nitration, halogenations, and Friedel-crafts alkylation and acylation. In addition, numerous rearrangement reactions, such as Beckmann and Fries rearrangements, are also catalyzed by acids, as well as a variety of cyclization reactions such as Diels-Alder reactions and the synthesis of heterocyclic compounds.

Moreover, esterification and transesterification reactions which enhance the use of high acid value oil to be used as feedstock for synthesis of biodiesel, which is thought to be one suitable solution to meet the future demand of energy sources, are also examples of important reactions that are catalyzed by acids.

However, the waste drives from the use of mineral and Lewis acids which cannot be readily recovered and recycled are the main drawbacks of these conventional homogeneous acid-catalysts.

In contrast, heterogeneous acid catalysts have the advantages of easy recovery and reusability and are readily amenable to continuous processing. Moreover, these acids are less corrosive and safer in handling than the known homogeneous acids.

Various solid acid catalysts, such as resins, tungstated and sulfated metal oxides, such as zirconia, polyaniline sulfate, heteropolyacid, metal complexes, zeolites, acidic ionic liquid, and others have been explored as potential heterogeneous acid catalysts. Beside the activity of these catalysts—resulting in moderate to high conversions and yields, their deactivation, poisoning, and ability of leaching in the reaction medium—are important aspects.

The objective of this Special Issue of Molecules is to focus attention on the latest developments in heterogeneous acid catalysts and their applicability for different syntheses along with all other aspects.

Prof. Dr. Detlef W. Bahnemann
Dr. Amer Hakki
Guest Editors

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• solid acids
• acid catalysts
• cyclization
• heterogeneous catalysts
• biodiesel
• heterocyclic compounds
• rearrangement reactions