Improving biomass processes via structure characterization and valorization of humins and process intensification
Date
2020
Authors
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Publisher
University of Delaware
Abstract
This thesis focuses on some of the grand challenges in biomass conversion to platform chemicals, and specifically, on humins and on converting batch processes to continuous processes. Humins are carbonaceous, polymeric byproducts formed during acid-catalyzed, hydrothermal processing of sugars to bio-based platform molecules, such as 5-hydroxymethylfurfural (HMF). Currently, humins are a low-value byproduct used mainly for combustion. Handling these solid particles increases maintenance costs. Minimization of humins formation and/or their valorization is essential for improving the process economics of the biorefineries. The lack of thorough understanding of humins’ structure hinders efforts towards their valorization. To bridge this gap, in the first part of the thesis, we developed methods to infer the microstructure of humins through solubility experiments. First, we conducted dissolution experiments in various solvents and correlated the solubility data to molecular properties of solvents to develop suitable descriptors. Next, Liquid Chromatography-Mass Spectrometry (LC-MS) was used to determine the species solubilized from humins in different solvents. Finally, multi-stage dissolution experiments were done to investigate the spatial homogeneity of humins as they get dissolved from the solid. Both Fourier Transform Infrared Spectroscopy (FT-IR) and LC-MS were used to characterize the solubilized and insoluble humin fractions obtained at each stage. Based on our results, an inhomogeneous structure, where macromolecular and molecular components are connected through weak forces, was proposed. ☐ Next, we use for the first time operando ultra-small angle X-ray scattering (USAXS) to investigate the evolution of size, volume fraction, and number concentration of humins formed from fructose and HMF in acidic solutions. The radius of gyration (Rg) of suspended humins particles grows linearly with respect to time accompanied by an increase in polydispersity (PD), before precipitation occurs. An apparent activation energy of humins growth was found to be 102 kJ/mol. By comparing the growth starting from fructose or HMF, we concluded that humins form mainly from HMF or its derivatives. The time evolution in the number of particles revealed two competing processes, namely formation of new particles characterized by inception of oligomers and humins nanoparticles < 20 nm as well as the aggregation of particles leading to potential precipitation. The results are briefly compared to those of growth of silica, a better-understood colloidal system. ☐ Armed with better understanding of the structure of humins, we then investigated the one-step valorization of humins in methanol using carbon supported catalysts. First, we screened four different noble metal catalysts. Aromatic hydrocarbons, phenols and esters were the main liquid products. Rh/C gave the best GC-detectable oil yield and was chosen for a subsequent set of experiments with varying reaction parameters. Up to 12 wt% light, GC-detectable oil yield was achieved at 75 % humins conversion. The light oil yield was shown to be a strong function of hydrogen pressure and temperature. High pressure and intermediate temperatures, time and catalyst loadings were found beneficial for the light oil yields. As temperature and time increase, the total oil yield decreases as a result of gasification. The product distribution shifts in favor of aromatics and phenols at high temperatures and long reaction times and of esters at short reaction times and high catalyst loadings. ☐ Lastly, we investigated continuous flow sugar chemistry. We leveraged a new group flow microchannel reactor with good heat and mass transfer for the effective conversion of glucose to HMF using CrCl3/HCl catalysts. Comparison with batch reactor data acquired at 140 °C demonstrated a two-fold increase in HMF productivity at otherwise identical reaction conditions. Then, we extended the study to 2-pentanol/water biphasic solvent mixtures to continuously extract HMF and improve its yield. We achieved the best HMF productivity of 33.6% at 51.4 % yield in 92 s under 200 °C. Compared to our single-phase reaction where 30.3% HMF yield is achieved in 120s, the 2-pentanol/water biphasic system shows a two-fold increase in productivity. Changing the 2-pentanol/water volumetric ratio from 1 to 3 did not lead to significant changes in glucose conversion or HMF yields possibly due to the process being kinetics rather than mass transfer limited. The maximum HMF yield of 43.0% was reached at 60s in the MIBK/water system, and the yields of other products, such as mannose and humins, also showed notable differences. Possible factors contributing to the different reactivity of different solvents are discussed.
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Keywords
Biomass processes, Structure characterization, Valorization, Humins, Process intensification