The use and function of ferrous iron catalyst for lignin deconstruction and intensification of sugar production from fast pyrolysis of biomass

Abstract

Fast pyrolysis of biomass has great potential for the large-scale production of carbon negative fuels. There exist a number of problems with the technology that currently prevent the adoption of fast pyrolysis for the production of cellulosic sugars. The traditional solution of acid pretreatment, while effective at producing sugars, introduces a different set of problems that make scale up difficult. This research investigates a new approach to producing sugars from biomass fast pyrolysis without the problems associated with traditional pretreatments. Sulfuric acid passivation of the alkali and alkaline earth metals (AAEM) content of biomass has for years been the favored method for producing high yields of anhydrosugars from lignocellulosic biomass. The consequence of this treatment is a removal of catalyst for the deconstruction of the lignin fraction of biomass. Without a catalyst present lignin undergoes a phase change at elevated temperatures and melts. This melted lignin sticks to other material inside pyrolyzers forming large char agglomerates capable of plugging reactor systems. The replacement of sulfuric acid with ferrous sulfate to accomplish passivation of biomass ash content achieves the dual goals of, increasing sugar yields for corn stover from 0.9 wt% to 11.8 wt%, and catalyzing lignin deconstruction, enabling throughput increases of weight hourly space velocity from 1 h-1 to 10 h-1 under autothermal operation. Taken together these represent an increase in volumetric sugar productivity of an order of magnitude from 157 g L-1 h-1 to 2041 g L-1 h-1. The AAEM content of biomass is detrimental to the production of sugars from fast pyrolysis of biomass. Traditional methods of acid pretreatment are effective at stopping the catalytic cracking of pyranose rings by AAEM however, they introduce the new problem of char agglomeration within continuous pyrolyzers. The new method of ferrous sulfate pretreatment is able to solve this problem for high ash corn stover but proved ineffective in stopping char agglomeration of low ash woody biomass. Overcoming this issue required the development of a novel combined ferrous sulfate and ferrous acetate pretreatment that proved effective on both hardwoods and softwoods. This use of ferrous sulfate increased sugar yields from 4.4 wt% to 15.5 wt% for red oak and 5.4 wt% to 19.0 wt% for southern yellow pine. While at the same time the introduction of ferrous acetate at levels of 6 wt% for red oak and 2 wt% for southern yellow pine was sufficient to prevent char agglomeration from occurring. The discrepancy in the ferrous acetate requirement was determined to be the result of the syringyl lignin content of the biomass and a correlation was developed to help in determining appropriate catalyst requirement for a multitude of different feedstocks. Naturally occurring AAEM play an important catalytic role in the deconstruction of lignin during pyrolysis. Ferrous iron has not previously been studied to determine its potential as a lignin deconstruction catalyst. A combination of modeling and experiments were performed to explore the catalytic activity of ferrous iron in comparison to AAEM. Density functional theory (DFT) calculations for model lignin dimers and pyrolysis experiments with extracted lignin drew the same conclusions. Ferrous iron has a stronger destabilization effect on the β-O-4 ether bond than either potassium or calcium. The strength of the AAEM cations was very similar with DFT determining Calcium was stronger while experimental results were inconclusive. Potassium’s lower catalytic strength was made up for by its higher turnover frequency than calcium or ferrous iron. Pyrolysis of softwood lignin had a lower activation energy (9.2 kcal/mol) than for hardwood lignin (15.3 kcal/mol). Of the catalysts tested only ferrous iron prevented the melting of lignin during pyrolysis due to its ability to initial lignin deconstruction at a temperature lower than lignin’s melting point.