Solvothermal conversion of technical lignins over NiMo catalysts

Abstract

Scope: Lignin, cellulose and hemicellulose are the main constituents of plants cell walls. Lignin is an aromatic rich compound, composed of phenolic building blocks. Depending on the method used for isolation of lignin from cellulose and hemicellulose, several types of technical lignin are available such as Kraft lignin, Sulfonate lignin (Lignosulfonate), Soda lignin and Organosolv lignin. Despite being a natural aromatic compound, lignin is mainly treated as a low value by-product, and is burned for the energy supply of pulp and paper industries. By extraction of higher value products from lignin, a renewable source of valuable chemicals would be available and the economic viabilities of the relevant industries will be promoted. Most of the recent publications have targeted one type of lignin such as Kraft lignin 1. However, it is desirable to introduce a method suitable for conversion of the range of available technical lignins. In this work, catalytic conversion of different types of lignin using an alumina supported NiMo catalyst (provided by Haldor Topsøe A/S) is conducted in ethanol at 310 ˚C with initial hydrogen pressure of 25 barg. The reaction time was set to 3 hours. Proton-Lignosulfonate (H-LS, provided by Borregaard A/S in form of Sodium-Lignosulfonate), Kraft lignin, Protobind 1000 and Organosolv lignin are among the selected lignin types. Non-catalytic conversion of each type of lignin was also performed at similar reaction conditions for comparison. The catalyst: lignin: solvent ratio of 1 g: 10 g: 100 ml was selected. The NiMo catalyst is a sulfur tolerant catalyst, which is originally present in the oxide form. The NiMo catalyst was presulfided before use to form NiMoS2. Presulfidation was conducted overnight by in-situ decomposition of 10 ml dimethyl disulfide and formation of H2S. After each standard test, the solid and liquid reaction products were separated by vacuum filtration. Furthermore, the light and heavy liquid products were separated by rotary evaporation at 35 ˚C and 5 mbar. While the light fraction was rich in ethanol, the remaining fraction was heavy oil, attributed as ‘bio-oil’. GC-MS-FID analysis was used for identification and quantification of the bio-oil and ethanol rich light fraction. The molecular weight of the oil fraction was determined by size exclusion chromatography (SEC). Elemental analysis (Eurovector EuroEA3000) was conducted for measuring the organic C, H, S, N and O contents of the oil and solid fractions. The gas phase in the autoclave was analyzed using a gas chromatograph.