Abstract
Evaluation of copper doped silica-alumina and γ-alumina catalysts for lignin decomposition was conducted using a suite of chemical analysis protocols that enabled a comprehensive characterization of the reaction product. X-ray diffraction analysis was used to verify the concentration of doped copper on catalyst supports. Then, batch experiments were performed to study the significance of catalyst support type, catalyst dopant concentration, lignin concentration, catalyst-to-lignin ratio, reactor stirring rate and reaction time. Aqueous products were extracted with dichloromethane and analyzed using a detailed gas chromatography-mass spectrophotometry analytical protocol, allowing for quantification of over 20 compounds. Solid residues were analyzed by thermogravimetric analysis and scanning electron microscopy. The highest yield of monomeric products from these screening experiments occurred with 5 wt% Cu on silica-alumina with a 1:1 w/w ratio of catalyst to lignin. A second set of experiments were conducted at these conditions to evaluate the effect of varying the reaction temperature between 300 and 350 oC. Lower reaction temperatures (300 oC) resulted in more unreacted lignin while higher temperatures (>350 oC) led to an increased formation of liquid phase products, but also increased char formation. While the total amount of liquid phase products increased, the combined yield of monomer phenolic products was only 5–7 wt% of the liquid extracted product and statistically independent of temperature and other operational parameters, although the yields of different chemicals varied with temperature. Unlike most pyrolytic processes, the concentration of gas phase products gradually decreased with increasing reaction temperature and became negligible at 400 oC, while the formation of coke increased with temperature. This seemingly contradictory result is likely due to increased product polymerization occurring at higher temperatures.
Highlights
Lignocellulosic feedstock is well known as a renewable source of biofuels
Characteristic peaks of copper showed up in both silica-alumina and γ-alumina catalyst supports, which verifies the success of the doping protocol
Copper was well-dispersed on the surface of the silica-alumina catalyst and its characteristic peak was identified in the EDS profile
Summary
Lignocellulosic feedstock is well known as a renewable source of biofuels. This resource is attractive because it does not compete directly with edible plant production. The ability to generate higher value fuel and chemical intermediates from this lignin would increase the economic attractiveness of lignocellulosic biofuel facilities. Lignin is a complex three-dimensional polymer, which is rich in aromatic phenolic units. Lignin isolation from plant biomass by most of the available industrial methods, e.g., obtaining Kraft lignin, was recently shown to replace ether phenolic unit links, the most abundant β-O-4 bonds, into much more recalcitrant C-C bonds [1,2,3,4,5]
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