Lignin liquefaction

Based on experience obtained by hydrogenating coal during World War II, the Noguchi Institute of Japan began to study lignin liquefaction. By 1952 they had discovered a superior catalyst which converted a substantial portion of the lignin into relatively few monophenols and substantially suppressed additional hydrogenation of the phenols. This catalyst was improved some years later and, based on these two catalysts,... 

This method was used to construct a large set Np > 1000 of lignin oligomers whose average conformed to the PDF of Figures 1, 2, and 3. Monte Carlo simulation of the reaction of each allowed mathematical description of lignin liquefaction. 

The simulation of lignin liquefaction combined a stochastic interpretation of depolymerization kinetics with models for catalyst deactivation and polymer diffusion. The stochastic model was based on discrete mathematics, which allowed the transformations of a system between its discrete states to be chronicled by comparing random numbers to transition probabilities. The transition probability was dependent on both the time interval of reaction and a global reaction rate constant. McDermott s ( analysis of the random reaction trajectory of the linear polymer shown in Figure 6 permits illustration. 

Application of these ideas to lignin liquefaction required definition of allowable lignin states and the probabilities associated with transitions from one state to another. The random construction just considered provided the initial t = 0) states. The model compound reaction pathways provided the set of allowable states for t > 0, and the model compound kinetics and selectivities provided the transition probabilities. 

With models for catalyst decay and effectiveness now in hand, the simulation of lignin liquefaction could be achieved given the initial lignin structure (as described earlier) and model compound reaction pathways and kinetics, both thermal and catalytic. Construction of a random polymer, as outlined earlier, began the simulation. This structural information combined with the simulated process conditions to allow calculation of the reaction rate constants, selectivities and associated transition probabilities. The largest rate constant then specified the upper limit of the reaction time step size. 

Figure 10. Predicted Tar Fraction Product Yields of Kraft Lignin Liquefaction at 380°C.

Train, P.M. Stochastic and Chemical Modelling of Lignin Liquefaction. Ph.D. Thesis, University of Delaware, 1987. 

Supercritical fluid solvents have been tested for reactive extractions of liquid and gaseous fuels from heavy oils, coal, oil shale, and biomass. In some cases the solvent participates in the reactions, as in the hydrolysis of coal and heavy oils with water. Related applications include conversion of cellulose to glucose in water, dehgnincation of wood with ammonia, and liquefaction of lignin in water. 

Various solvents are being investigated to dissolve lignocellulosic materials. Some approaches focus on the selective depolymerization and extraction of lignin and hemicellulose as pre-treatment to produce clean cellulose fibers for subsequent fermentation or for pulping. Other approaches attempt to dissolve the whole lignocellulose with or without depolymerization. The liquefaction processes that are carried out at high temperature (>300 °C), and produce a complex oil mixture, are discussed above with the pyrolysis processes. 

Despite the economic picture, the Noguchi hydrogenation process remains the best process for liquifying lignin that has yet appeared. We were routinely able to obtain over a 55% yield of distillable products, based on the net organic content of the starting material, and in several cases the yields were over 65%. The drawback to high liquefaction is that more... 

Lin L., Nakagame S., Yao Y., Yoshioka M., Shiraishi N. Liquefaction mechanism of /3-0-4 lignin model compound in the presence of phenol under acid catalysis. Part 2. Reaction behavious and pathways. Holzforschung 55 625-630 (2001). 

The second method for liquefaction makes use of solvolysis during the process [8,11]. By using conditions which allow phenolysis of part of the lignin, especially in the presence of an appropriate catalyst, the liquefaction of chemically modified wood into phenols could be accomplished under milder conditions (at 80 C for 30-150 min). Allylated wood, methylated wood, ethylated wood, hydroxyethylated wood, acetylated wood, and others have been found to dissolve in polyhydric alcohols such as 1,6-hexanediol, 1,4-butanediol, 1,2-ethanediol, 1,2,3-propanetriol (glycerin), and bisphenol A using the liquefaction conditions just described. Each of them caused partial alcoholysis of lignin macromolecules [4]. 

Biomass is composed of various components such as cellulose, hemicellulose, lignin, extractives and mineral water. The composition of biomass plays a definitive role in altering the product distribution and their properties [2-3J. As is shown in earlier publications [4-S] different biomass, on pyrolysis, give different product yield with different product properties. In order to choose a biomass for a particular process (carbonisation, liquefaction, gasification or adsorbent char) knowledge on the product distribution and properties for various biomass are essential. 

The limitations of the use of lignin in polyols for urethanes are determined by the difficulties encountered during the liquefaction step of its preparation. This step generally reduces the lignin content of the polyol to between 10 and 30%. 

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