Thermocatalytic depolymerization of lignin and hydrodeoxygenation of lignin-derived monomers

Raikwar, Deepak and Shee, Debaprasad and Majumdar, Saptarshi (2019) Thermocatalytic depolymerization of lignin and hydrodeoxygenation of lignin-derived monomers. In: AIChE Annual Meeting, 8-15 November 2019, New York.

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Abstract

In recent years, lignin, the third fraction of lignocellulosic biomass, has evolved as one of the most prominent renewable resources for value-added aromatic chemicals. Lignin constitutes around 10-30% of the total lignocellulosic biomass. It is made up of aromatic monomers (coniferyl, sinapyl, and p-coumaryl), which brands it as the most abundant renewable source of aromatic chemicals. The removal of aromatic entities from lignin is a big challenge to date, mainly due to its heterogeneous nature and tendency of the polymer to repolymerize a more condensed structure. While working in a similar area in the last few years, we have narrowed down the specific needs and targets in this area of interest. The early and recent advancement has always given a strong focus on the development and screening of efficient catalysts for lignin depolymerization. Although the catalyst is a crucial component for the lignin depolymerization to monomeric products, significant attention is also required on the catalyst-to-lignin interactions and collision possibility of inter-unit linkages with the catalyst. An appropriate solvent might regulate the collision possibility of inter-unit linkages with active site of the catalyst. The use of a suitable solvent can also reduce the formation of char and restrain the repolymerization reactions. Therefore, Hansen solubility parameters (HSP) were utilized for selecting a suitable solvent for the lignin polymer. A mixture of water and THF was observed to have higher thermodynamic compatibility for lignin and guaiacol with a RED ~ 1. The beneficial effect of water and THF mixture was also observed in the depolymerization reactions of Kraft lignin using HZSM as a catalyst and NaOH as co-catalyst. A higher lignin conversion of 77% and 79% selectivity towards guaiacols with negligle char could be attained at the optimal reaction conditions from statistical analysis - Response Surface Methodology (RSM). The water-THF ratio was also found as the major independent variable influencing the conversion and selectivity of guaiacols. The higher selectivity of guaiacols directly from Kraft lignin gave us the motivation to perform the hydrodeoxygenation studies on the lignin-derived monomer model compounds. This upgradation step is necessary for the production of deoxygenated compounds for their utilization as a fuel, fuel additives, or chemical intermediates. Towards this direction, different studies based on inexpensive metal (s) catalysts, particularly Ni-Mo/ZrO2 and Ni-Co/Al2O3, were carried out on hydrodeoxygenation of lignin model compound “guaiacol”. For Ni-Mo/ZrO2, the presence of the optimum proportion of Ni/Mo4+ at the equimolar ratio of Ni and Mo is responsible for the higher HDO activity of the catalyst. The activation Caryl-O bonds are supposed to occur on oxophilic Mo4+ sites. Whereas, the hydride transfer from neighboring metallic Ni sites further results in hydrogenolysis of the Caryl-O bond giving phenol as the major product with 75% selectivity and complete guaiacol conversion. For the Ni-Co/ Al2O3 catalyst, a total metal loading of 9.2 mmolg-1cat and Ni/Co mole ratio of 1:2 indicated a higher proportion of NiCo2O4 spinel structure in the oxide catalyst. The spinel structure subsequently transformed to Ni-Co alloy after reduction in H2 at 573 K. The Ni-Co alloyed domains provide a different adsorption site than Ni and Co specific for direct methoxylation of guaiacol resulting in higher selectivity for benzene (35.1%) at complete guaiacol conversion. Research Interest My research interest focuses on the development of new catalysts and processes for the production of value-added fuels/chemicals from lignocellulosic biomass. A particular emphasis is also on the physicochemical characterization of biomass feedstock and catalyst with the help of various spectroscopic and non-spectroscopic characterization techniques (BET, powder-XRD, H2-TPR, UV–vis, FTIR, pyridine-FTIR, and NMR). The characterization information is used to improve the efficiency and selectivity of the catalyst and exploration of the reaction pathways. Recent examples of research topics include the catalytic depolymerization of lignin and the upgrading of lignin-derived monomers. Teaching Interest I am interested in teaching heterogeneous catalysis and developing a course on advanced instrumentation techniques in chemical engineering, which focus on the underlying working principles of various spectroscopic, analytical and separation techniques for undergraduate and graduate students. My research background in heterogeneous catalysis and bio-polymers has given me enough experience in different analytical methods in reaction engineering. I can teach handling and usage of equipment such as Gas Chromatography, X-ray Diffraction, Fourier Transform Infrared Spectroscopy, UV-vis, Raman Spectroscopy, Chemisorption Analyser, Surface Area and porosity system, Gel permeation chromatography, Zeta potential, and particle size analyzer, Scanning electron Microscopy. I also worked as a teaching assistant in various theoretical and laboratory courses for undergraduate students at the Indian Institute of Technology, Hyderabad. During my Ph.D. tenure, I also got the opportunity of mentoring M.Tech students in our research group.

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