Lewis Acid Zeolites for Biomass Conversion: Perspectives and Challenges on Reactivity, Synthesis, and Stability

Abstract

Zeolites containing Sn, Zr, Hf, Nb, or Ta heteroatoms are versatile catalysts for the activation and conversion of oxygenated molecules owing to the unique Lewis acid character of their tetrahedral metal sites. Through fluoride-mediated synthesis, hydrophobic Lewis acid zeolites can behave as water-tolerant catalysts, which has resulted in a recent surge of experimental and computational studies in the field of biomass conversion. However, many open questions still surround these materials, especially relating to the nature of their active sites. This lack of fundamental understanding is exemplified by the many dissonant results that have been described in recent literature reports. In this review, we use a molecular-based approach to provide insight into the relationship between the structure of the metal center and its reactivity toward different substrates, with the ultimate goal of providing a robust framework to understand the properties that have the strongest influence on catalytic performance for the conversion of oxygenates.

Time‐Resolved In Situ MAS NMR Monitoring of the Nucleation and Growth of Zeolite BEA Catalysts under Hydrothermal Conditions

“…Different TEA species have been identified and the mechanism of their interaction with other species has been established. These results are of prime importance for the further development of novel BEA‐based materials (Sn‐BEA, Zr‐BEA, Ti‐BEA), attracting broad attention during the last years, as well as for the other TEA‐templated zeolites and zeotypes of different structural types (LTA, MFI, AEI, AFI, CHA, MOR, and so on) …”

“…Finally, zeolite BEA is an excellent catalyst and sorbent for a broad range of industrial processes such as cumene synthesis, C2/C3 transalkylation, acylation, and depollution/decontamination processes (SCR of NOx, VOC adsorption). Besides that, it shows high potential for the transformation and valorization of bio‐sourced substrates …”

Identifying Sn Site Heterogeneities Prevalent Among Sn‐Beta Zeolites

“…The incorporation of Lewis acid sites ( e.g ., Sn(IV), Ti(IV), Zr(IV)) into purely siliceous zeolite frameworks creates heterogeneous catalysts that enable the efficient transformation of oxygenated substrates. [1] First demonstrated for the Baeyer–Villiger oxidation of cyclic ketones with hydrogen peroxide,[2] Sn(IV) centers incorporated into β ‐zeolite were shown to be capable of selectively activating CO groups in aqueous media . Sn‐ β has been used among others in transfer hydrogenations,[4] C–C bond formation and cleaving reactions,[5] isomerizations,[6] and epimerizations[7] of saccharides, etherifications, and the ring‐opening of epoxides .…”

Enhancing the Catalytic Activity in the Solid State: Metal–Organic Frameworks to the Rescue

“…Taking inspiration from the modification of the Lewis acidic sites in zeolites, 9 it will be intriguing to see the advantages that MOFs can offer in catalyst design. Given the predominant position zeolites hold in industrial catalytic processes, it is tantalizing to think of the potential roles that MOF-based materials may play with their equally well-defined active sites.…”