The conversion of lignin to alkylphenolic monomers using heterogeneous catalysis

McVeigh, Ashley (2016) The conversion of lignin to alkylphenolic monomers using heterogeneous catalysis. PhD thesis, University of Glasgow.

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Lignin is the most recalcitrant part of woody biomass yet is one of the few natural aromatic resources available in abundance. There is huge potential for this material to be used as a key feedstock in future applications however a conversion route to fine chemicals must first be established. In this thesis, a methodology to take raw wood sawdust and convert its lignin component to value added products whilst avoiding a pyrolysis route has been reported.
During biofuel processing, lignin is subjected to a number of pretreatment steps in order to separate it from the lignocellulosic material and it is during these steps that modification of the lignin can occur. The aim of this project was to selectively cleave the dominant β-O-4 linkage using heterogeneous catalysis to form functional aromatic monomers. It was found to be imperative that an appropriate pretreatment was firstly identified in order to avoid depletion of this dominant alkyl-aryl ether bond where, for example, the lignin isolated using an ammonia pretreatment from poplar wood gave an uncondensed structure in comparison to the condensed lignin produced using an organosolv pretreatment. We have shown that lignin can be effectively depolymerised to low molecular weight compounds when the starting material is rich in β-O-4 bonds and we were able to verify this finding using a Pt/Al2O3 catalyst under hydrodeoxygenation/hydrogenolysis conditions. Organosolv lignin from Poplar sawdust was found to have a low β-O-4 content (9.2 %) and subsequently gave limited conversion to fine chemicals (9.0 %) in comparison to the ammonia lignin with a high β-O-4 content (28.9 %) which gave much higher conversion to monomeric product (16.4 %). Further work using lignins isolated using different pretreatment methods supported this conclusion. Whilst GPC was used to give an overview of the initial cracking to breakdown products, GC-MS analysis enabled us to successfully identify 21 monomeric products.
Optimisation work using a Pt/Al2O3 catalyst with the ammonia lignin was carried out in order to minimise char production and maximize the yield of the fine chemical products to varying success. Experimental conditions were altered in terms of the solvent used and its composition, reaction temperatures, pressures, time and stirring rates, as well as changing the pH and active metal. It was found that the use of 100 % methanol under the standard conditions yielded the highest amount of monomeric product (43.5 %), whilst the use of 100 % water alternatively gave the highest char production (47.1 %). This knowledge obtained during the initial lignin conversion study was then used to tackle a sulfur-containing lignin known as Kraft lignin. This lignin is readily available as a by-product of the dominant pulping technique adopted by the paper and pulp industry but due to the nature of the technique used to isolate the lignin, the Kraft lignin often incorporates sulfur onto its structure. Such impurities could prove detrimental to the catalytic depolymerisation reaction via catalyst poisoning but it was found that the Kraft lignin behaved in a similar manner to the other condensed lignins under the standard conditions in terms of the total monomeric yield obtained. The Kraft lignin produced 9.2 % of alkylphenolic product in comparison to 7.4 % produced from the standard reaction with soda lignin. It was also noted that the optimised conditions found for the benchmark ammonia lignin did not depolymerise the Kraft lignin to the same extent where, for example, using 100 % methanol with the Kraft lignin hindered the reaction and gave an overall monomeric yield of 5.6 %. This highlighted the fact that whilst one set of conditions may be efficient for the conversion of one type of lignin, this may not be the case for another.
This work was followed by an isotopic labelling study to investigate the reaction mechanism which indicated an inverse kinetic isotope effect and that the depolymerisation reaction occurred in two steps. These two steps involved an initial cracking step of the entire lignin molecule to fragmented lignin pieces followed by conversion to monomeric product. Some post-catalyst analysis was also carried out using XRD, BET, TPO and Raman analysis in order to analyse the catalyst surface. The work showed that during the reaction, the lignin deposited onto the catalyst surface and formed a carbonaceous layer which was graphitic in nature. Some samples were found to be more ordered in nature but no obvious trend with respect to the type of lignin used was established.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Lignin, niomass conversion, heterogeneous catalysis, alkylphenollic
Subjects: Q Science > QD Chemistry
Colleges/Schools: College of Science and Engineering > School of Chemistry
Supervisor's Name: Jackson, Professor S. David
Date of Award: 2016
Unique ID: glathesis:2016-7136
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 29 Feb 2016 09:46
Last Modified: 10 Mar 2016 10:56

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