(3cz) Design of Solid Acid Catalysts for Aqueous Phase Conversion of Lignocellulosic Biomass to Liquid Fuels and Fuel Precursors

A worldwide initiative has been set forth to decrease our dependence on petroleum-based fuels and develop new renewable sources of energy.  Particularly, the use of agricultural and forest residues and other lignocellulosic crops is a viable option compared to first generation biomass which demonstrate several drawbacks including competition with food crops and water.  Lignocellulosic biomass is a sustainable source of liquid fuels which generates significantly less greenhouse gas emissions than do fossil fuels and may even be greenhouse gas neutral pending efficient production methods.  The production of biofuels calls for the defunctionalization of highly oxygenated compounds which make up lignocellulosic biomass.  The transformation of lignocellulosic biomass in aqueous media offers an environmentally friendly and economical route to produce targeted chemicals.  The major challenge in the commercialization of lignocellulosic biofuels is the development of efficient technologies to achieve process sustainability and convert high volumes of biomass feedstock to displace crude oil as the primary source of fuels.

In part, further improvement of conversion and selectivity for aqueous phase processes are essential to overcome these challenges.  Acid catalysts play fundamental roles in many applications focused on biomass upgrading and promising results have already been obtained with homogeneous acid catalysts.  Nevertheless, due to their economic and environmental viability, it would be desirable to obtain solid acid catalysts that exhibit activities and selectivities at least comparable to their homogeneous counterparts.  As such, the effect of water on the catalytic behavior of solid acid catalysts remains questionable, as exposure of the catalyst to a polar solvent such as water can potentially alter the intrinsic nature of the acid surface due to solvation effects.  This reality in turn poses difficulties in determining the acidity of solid acids in aqueous media.

My research thus far in the field of aqueous phase biomass conversion has led me to significant findings pertaining to the chemistry and design of solid acid catalyst for aqueous phase conversion of carbohydrates.  This consists of an integrated approach which includes: (1) understanding the fundamental chemistry and reaction pathways through kinetic studies with homogeneous acid catalysts and (2) investigating the structural properties of solid acid catalysts and their correlation with activity and selectivity.  We developed a kinetic model for the dehydration of xylose to furfural in a biphasic reactor using a homogeneous catalyst.¹  Xylose serves as a model compound for the hemicellulose fraction of lignocellulosic biomass, and furfural is a valuable precursor for the production of liquid alkanes.  We then used our model to describe the optimal reaction conditions for furfural production.  Subsequently, we examined the role of Lewis and Brønsted sites of solid acid catalysts for the dehydration of xylose in aqueous media.²  Our results have allowed us to deduce the optimal design of solid acid catalysts for this class of reactions. 

In other studies we introduced a new process to produce levulinic acid from cellulose at high concentrations without the use of a homogeneous acid catalyst.³  Levulinic acid is a versatile renewable platform molecule which can be used as the basis for the production of various high-volume chemicals and fuels.  We also developed a kinetic model to predict the optimal reactor design and operating conditions for HMF (5-hydroxymethylfurfural) and levulinic acid production from glucose in a continuous reactor system.⁴

The objective of my research is to develop new catalytic materials and processes for the production of liquid fuels and fuel precursors from lignocellulosic biomass.  Development of efficient technologies targeted for aqueous phase applications will play a significant role in the development of future biorefineries.  This notion of a biorefinery, analogous to a petrochemical refinery, will consist of an integrated system of biomass conversion processes to produce transportation fuels, chemicals, heat and power.  In particular, future work will focus on designing hydrothermally stable solid catalysts for a variety of aqueous phase reactions.  Implementing advanced in-situ characterization techniques will be fundamental to elucidate the role of water on the intrinsic nature of the catalyst.  Additional factors such as kinetic modeling and reactors engineering will also play fundamental roles in developing sustainable processes for the production of fuels and chemicals from biomass-derived feedstock.

References 

1.            Weingarten, R.; Cho, J.; Conner Jr, W. C.; Huber, G. W., Kinetics of Furfural Production by Dehydration of Xylose in a Biphasic Reactor with Microwave Heating. Green Chemistry 2010, 12 (8), 1423-1429.

2.         Weingarten, R.; Tompsett, G. A.; Conner Jr, W. C.; Huber, G. W., Design of Solid Acid Catalysts for Aqueous Phase Dehydration of Carbohydrates: The Role of Lewis and Brønsted Acid Sites. Journal of Catalysis 2011, 279 (1), 174-182.

3.         Weingarten, R.; Conner Jr, W. C.; Huber, G. W., Production of levulinic acid from cellulose by hydrothermal decomposition combined with aqueous phase dehydration with a solid acid catalyst. Energy and Environmental Science 2012, DOI: 10.1039/c2ee21593d.

4.         Weingarten, R.; Cho, J.; Xing, R.; Conner Jr, W. C.; Huber, G. W., Kinetics and Reaction Engineering of Levulinic Acid Production from Aqueous Glucose Solutions. ChemSusChem 2012, DOI: 10.1002/cssc.201100717.