Advances and Environmental Aspects on the Synthesis of Hierarchical Zeolites Revisited: a state-of-the-art description

Hierarchical zeolites are versatile materials used as adsorbents, ion exchangers, and highly selective catalysts in reactions such as fluid catalytic cracking, methanol from hydrocarbons, and benzene alkylation. Two routes can be applied to synthesize hierarchical zeolites: top-down and bottom-up. Both methods have been successfully used to obtain zeolites with innovative properties that improve pore structure. However, most procedures for obtaining hierarchical zeolites are not ecologically correct due to the requirement of complex molecules, high cost, high thermal demand, and more significant environmental impact. For this reason, green synthesis has emerged as a more ecological strategy using sustainable starting materials, green mesoporogen, solvent-free synthesis, mesoporogen-free synthesis (template-free), recycling and reuse, and less generation of aqueous waste. Therefore, this review aims to critically discuss the latest advances in hierarchical zeolite synthesis methods, with particular emphasis on eco-friendly techniques. Firstly, the latest advances on the obtention of hierarchical zeolites applying conventional approaches are presented. Also, eco-friendly syntheses are described and critically analyzed as an alternative to traditional methods. A green template with potential to generate additional porosity in zeolites is critically presented and explained. Finally, the central insights of the work and gaps in the literature are shown to direct new approaches in future studies about the eco-friendly synthesis of hierarchical zeolites.