Challenges, prospects and comprehensive evolution of zeolite-based materials for the catalytic conversion of glycerol: A review

Abstract

The energetic demands of modern society urge a transition that relies on alternative and sustainable sources. Among the available possibilities focused on mitigating the use of fossil fuels, the biodiesel industry stands out. However, the excess of glycerol generated as a coproduct still raises debate regarding how it could be better used. A well-established approach is the use of the platform molecule, i.e., glycerol, in the presence of heterogeneous catalysts to obtain added value products. Zeolites are well-known for their versatility in numerous applications, such as in the oil industry. Besides, different types of aluminosilicates are being studied in the catalytic conversion of glycerol to acrolein, acetol, acrylic acid, allyl alcohol, solketal, etc. This review addresses the general properties, fundaments, synergetic aspects, theoretical modeling, resistance, and coke formation, as well as the zeolites limitations that pose obstacles for those reactions. This highlights the importance of developing zeolite materials with specific acid sites, synchronizing their amount and strength with the pore interconnectivity so that reagents diffusion within the zeolitic channels can be maximized, leading to a decrease in the obstruction of active sites and pores caused by coke deposition. A number of modifications, including hierarchization, isomorphic substitution, acidity tuning, and additional phases (SMSI effect), have been reported as alternatives for improving the performance of glycerol conversion and the resistance to deactivation. Several developments involving reactional mechanisms, coke deposition, and catalysts applied to glycerol conversion have been the subject of studies centered on process optimization, which is translated into the development of solids more resistant to deactivation. Among the zeolites with the best catalytic performance, the following stand out: BEA, MCM-22, MFI, ITQ-2, SAPO-34, and ZSM-5. Some complex technical aspects still need to be better understood so that the scalability of the catalytic conversion of glycerol becomes economically feasible, thereby arousing the interest of both the public and private sectors.