Selektive Umwandlung von Biomasse durch Dehydrierung und Reduktion (2017)

In this work, the transformation cascade of lignocellulosic biomass into levulinic acid (LA) and its further reduction into a potential fuel, gamma-valerolactone (GVL), was investigated. This investigation was performed regarding: a) catalysts preparation and characterization and b) transformation optimization. Being the main carbohydrate fraction of lignocellulosic biomass, glucose was selected as the starting material. The first transformation consisted in the acid catalyzed dehydration of glucose to LA. Here, the usage of sulfuric acid as a molecular Bronsted acid and of several sulfonated materials as solid acid catalysts was investigated and assessed. Using sulfuric acid, up to 70% yield of LA could be obtained under optimized reaction conditions in a highly reproducible manner. In all experiments, the formation of solid by-products (humins) was observed. Recently, some authors [1] reported on the formation of particularly high yields of LA over sulfonated graphene oxide (GO) as solid acid catalyst using low catalyst loadings. As a result of the advantages of solid catalysts over molecular ones, we investigated and assessed the usefulness and effectiveness of sulfonated GO in the above mentioned transformation. Sulfonation of GO was attempted by applying “one step” procedures using two different sulfonation reagents, namely ClSO3H and SO3, and by applying a “two-step” procedure consisting of a sequential reduction and sulfonation protocol of GO. Sulfonated graphene oxide derivatives were applied for the dehydration of glucose, but also several other sulfonated materials were tested. Mainly, all solid catalysts exhibited low yields of LA resulting likely from the deactivation of active sites by deposition of humins. While few samples of the graphene oxide derivatives resulted in the formation of high yields of LA, these experiments could not be reproduced using other catalysts batches under identical reaction conditions. Thus, the robustness of this system was found highly questionable, particularly when compared to sulfuric acid.The further hydrogenation of LA into GVL was investigated using molecular hydrogen over Pt and Pd nanocatalysts supported on SBA-15. The catalysts were synthesized by Chemical Fluid Deposition (CFD) using scCO2 as the deposition solvent [2]. The catalysts exhibited a homogeneous narrow particles size distribution mainly inside the pores of SBA-15. 3%Pt1-Pd3 (metals loading = 3 wt%, n (Pt) : n(Pd) = 1 : 3) supported on SBA-15 (pore size 6 nm) exhibited nearly quantitative yields of GVL under optimized reaction conditions in a highly reproducible manner. The catalysts were found stable and reusable over at least four catalytic runs.In total, the transformation chain of glucose as a biomass fraction into the platform chemical LA and its further transformation into the potential fuel additive GVL was investigated and assessed. The transformation of LA to GVL could be done in a very efficient manner whereby the usage of bimetallic Pt and Pd nanocatalysts were crucial for the high activity. However, the first step in the transformation cascade, i.e. the acid catalyzed dehydration of glucose to LA was found more challenging, particularly when solid catalysts should be used. The continuous by-production of solid humins is expected to be a severe problem for larger plants. In this direction, further research efforts appear to be necessary. 1. Upare, P.P., et al., Chemical conversion of biomass-derived hexose sugars to levulinic acid over sulfonic acid-functionalized graphene oxide catalysts. Green Chem., 2013. 15(10): p. 2935-2943. 2. Qiao, Y., et al., Preparation of SBA-15 supported Pt/Pd bimetallic catalysts using supercritical fluid reactive deposition: how do solvent effects during material synthesis affect catalytic properties? Green Chemistry, 2017. 19: p. 977-986.