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Nanoporous Solid Acid Materials for Biomass Conversion into Value-Added Chemicals: Synthesis, Catalysis, and Chemistry

Growing environmental concerns associated with diminishing reserves of fossil fuels has led to accelerated research efforts towards the discovery of new catalytic processes for converting renewable lignocellulosic biomass into value-added chemicals. For this conversion, nanoporous solid acid materials have been widely used because of their excellent hydrothermal stability and molecular sieving capability. In the thesis, hierarchical Lewis acid zeolites with ordered mesoporosity and MFI topology (three dimensionally ordered mesoporous imprinted (3DOm-i) Sn-MFI) were successfully synthesized within the confined space of three dimensionally ordered mesoporous (3DOm) carbon by a seeded growth method. The obtained 3DOm-i Sn-MFI showed at least 3 times higher catalytic activities for the biomass-derived sugar isomerization than conventional Sn-MFI zeolites. This is because the mesopores in the hierarchical zeolites greatly enhance molecular transport. In addition, Lewis acid Sn-MFI combined with Pt metal nanoparticles (Pt/Sn-MFI) could oxidize glycerol to produce lactic acid (LA) under base-free conditions. Glycerol is a by-product in biodiesel synthesis. 80.5% selectivity of LA was achieved at 89.8% conversion of glycerol using a bifunctional Pt/Sn-MFI catalyst under base-free conditions. In the tandem reaction pathway, selective oxidation of glycerol to glyceraldehyde (GLA) and dihydroxyacetone (DHA) by using Pt catalysts was cascaded with Lewis acid catalyzed isomerization of GLA/DHA into LA. Moreover, morphology-tunable Lewis acid Sn-BEA with hydrophobicity was successfully synthesized by recrystallization of post-synthesized Sn-BEA (Sn-BEA-PS) using ammonium fluoride (NH4F) and tetraethylammonium bromide (TEABr). This recrystallization includes simultaneous procedures of dissolution-reassembly: i) the dissolution of Si-O bonds around silanol nests by fluoride ions, and ii) the reassembly of fragmented silica species into defect-free zeolite framework in the presence of TEA ions. The recrystallization also increased open Lewis acid Sn sites. These findings can explain why a 2.5 times higher rate of aqueous glucose isomerization was achieved on recrystallized Sn-BEA (Sn-BEA-RC), compared with Sn-BEA-PS. Moreover, in the isomerization of bulky lactose (C12 sugar) dissolved in MeOH, hierarchical Sn-BEA-RC showed a 3.2-fold higher activity than hydrothermally synthesized Sn-BEA (Sn-BEA-HF), due to the mesopores and enhanced organophobic character of the recrystallized catalyst. In the final part, renewable p-xylene synthesis was investigated. p-Xylene is a major commodity chemical used for the production of polyethylene terephthalate (PET) with applications in polyester fibers, films and bottles. Diels-Alder cycloaddition of 2,5-dimethylfuran (DMF) and ethylene with subsequent dehydration of the cycloadduct intermediate to produce p-xylene is an attractive reaction pathway for its production from biomass feedstocks. It was shown that phosphorous-containing zeolite BEA (P-BEA) is active, stable and selective for this reaction with an unprecedented p-xylene yield of 97%. It can selectively catalyze the dehydration reaction from the furan-ethylene cycloadduct to p-xylene, without performing side reactions which include alkylation and oligomerization. This acid catalyst establishes a commercially attractive process for renewable p-xylene production.
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