Operando Molecular Spectroscopy during Catalytic Biomass Pyrolysis

Operando Molecular Spectroscopy during Catalytic Biomass Pyrolysis

Christopher J. Keturakis,^1* Olga B. Lapina², Victor V. Terskikh³, and Israel E. Wachs¹

¹Operando Molecular Spectroscopy and Catalysis Laboratory, Chemical Engineering Department, Lehigh University, Bethlehem, PA 18015 USA

²Boreskov Institute of Catalysis, SB RAS, Prosp. Lavrentieva 5, 630090 Novosibirsk, Russia

³Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N6N5

*CJK209@lehigh.edu

Perpetually increasing petroleum prices and finite petroleum resources have become large societal concerns in recent decades, causing a search for new, renewable and energy-efficient sources of liquid hydrocarbon fuel. With biomass being the only sustainable source of carbon, advances in technology for biomass conversion are critical for the development of renewable fuel sources. Significant scientific understanding and improvement of catalysts for biomass pyrolysis can only come from a fundamental research approach to all aspects of biomass conversion.

We have developed an operando Raman/IR-MS spectroscopy system for catalytic biomass pyrolysis that simultaneously monitors catalyst structure, biomass structure, surface reaction intermediates, coking, char formation, and over 40 major gas phase pyrolysis products (determined by GC-MS). Model supported 1-10% Al₂O₃/SiO₂ catalysts were synthesized and characterized (in situ Raman, in situ IR, and high field (21.1T) ex situ dehydrated ²⁷Al NMR spectroscopy). The chemical nature (Brönsted or Lewis acid sites; BAS or LAS) of specific surface AlO_x acid sites was determined from in situ IR-CO adsorption and DFT calculations. These well-defined catalysts were employed during the catalytic pyrolysis of cellulose, hemicellulose (xylan), lignin, their monomers (glucose and xylose), and woody biomass to determine correlations between specific value-added chemicals, required acid site nature, and surface AlO_x active site coordination. Results were also compared to popular HZSM-5 catalysts (Zeolyst) with different Si/Al ratios (15 to 140).

Results of operando spectroscopy experiments during catalytic biomass pyrolysis revealed that there are several intermediates on the catalyst surface depending on the biomass source, including furans, conjugated alkenes, alkenes conjugated to aromatics, and small cyclic rings with carbonyl groups. Demethylation of small aromatics (such as toluene or xylene) was determined to be the pathway for benzene production while methylation is the route to larger methylated aromatics such as 1-methylnaphthalene. The catalytically assisted dehydration of xylose into furfural was observed to be a key step in furfural production. Aromatic polymerization into graphite-like coke was observed to be the major catalyst deactivation mechanism. The production of furfural, benzene, toluene, and xylene correlated with the presence of BAS associated with AlO₅ while large aromatics, such as naphthalene, correlated with AlO₆ LAS. Similar correlations were found for glucose and cellulose catalytic pyrolysis. In general, cellulose and xylan (polymers) pyrolysis trends indicate that greater acid site strength is needed than for glucose and xylose (monomers) pyrolysis. For lignin pyrolysis, strong AlO₄ BAS selectively cleaved the C₁-C_α propenyl ligand bond and enhanced demethoxylation.

This talk will explore the developed correlations for value-added chemicals from each major fraction of biomass (cellulose, hemicellulose, lignin and whole woody biomass). This is the first application of operando spectroscopy to biomass conversion and the method developed here paves the way for definitively establishing catalytic structure-activity relationships for catalytic biomass pyrolysis in future studies.