Alkylation—Heterogeneous

Abstract Alkylate or alkylation gasoline is produced by reacting isobutane with light olefins, generally the C 3 –C 4 cut from catalytic cracking (FCC) units, in the presence of a strong acid catalyst. The alkylate product is mostly composed of a mixture of high octane isoparaffins being almost free of sulfur, olefins, and aromatics, thus making it a valuable component of reformulated gasoline. Unfortunately, today's commercial alkylation processes rely on the use of noxious HF and H 2 SO 4 liquid acids as catalysts and this imposes serious limitations, especially in the case of HF‐catalyzed processes, to expanding the worldwide alkylation capacity to the levels one might ideally envisage. This prompted to an intensive research work in the past 30 years in both academy and industry looking for alternative environment friendly solid acid catalysts that overcome the limitations of the current alkylation technologies. Since then, different types of solid acids, among which zeolites have deserved particular attention, have been investigated leading to outstanding advances in both catalyst and process design as well as in fundamental knowledge (nature of active sites, mechanism, etc.) that allowed several solid acid alkylation technologies being, at present, offered for commercialization. Unfortunately, refiners are still reluctant to assume the economic risks associated with the installation of a new alkylation technology, even if competitive, unless tighter environmental regulations become operational. In this article, the latest achievements in the use of green solid acid catalysts as potential alkylation catalysts are reviewed. After a short introduction, the main features of the different solid acids so far studied, including supported Brønsted and Lewis acids, acid resins both supported and unsupported, zeolites, sulfated and tungstated transition metal oxides, and heteropolyacids and related materials, in the alkylation reaction are addressed. Although they cannot be considered as true solid acids, the potential of ionic liquids as alkylation catalysts is also discussed. Finally, a brief description of the different solid acid alkylation processes so far developed is included, with special emphasis on the catalyst and reactor designs.