Gaining Molecular Insights into Porous Niobium-based Catalysts for One-pot Biomass Upgrading

点击翻译按钮获取中文摘要

As a sustainable source of organic carbon, biomass is playing an increasingly important role in our energy landscape. Lignocellulose, as the main component of woody biomass, is composed of cellulose, hemicellulose, and lignin. The upgrading of renewable lignocellulosic biomass is particularly attractive to bridge future gaps in the supply of chemical fuels and feedstocks. However, due to the complexity of the molecular structure of lignocellulosic biomass, particularly for the lignin portion, and its notorious resistance to chemical transformation, energy-efficient and cost-effective production of liquid fuels and chemical feedstocks from lignocellulose remains a highly challenging task worldwide. Recently, a family of porous Nb-based catalysts (Ru, Pt or Pd loaded porous NbOPO4 or Nb2O5) have exhibited an outstanding performance for the conversion of lignocellulosic biomass (190 oC, 5 MPa H2, 20 h) and bulk lignin (250 oC, 0.5 MPa H2, 20 h) into alkanes and arenes, respectively, via one-pot reactions. These reactions enable the complete removal of oxygen from biomass to produce liquid hydrocarbons and avoid chemical pre-treatment to the raw biomass materials, thus leading to potential energy savings in the biomass refinery based upon these novel catalysts. However, to date, little information on the catalytic active site or mechanism is known for these systems and little effort has been devoted to investigating the structural changes of these catalysts upon cycling reactions, where decreased activity/selectivity was often seen. Gaining in-depth understanding on the reaction mechanism and catalysts stability is of fundamental importance for the development of improved catalytic systems. This proposal will systematically investigate the binding dynamics, activation and conversion of the substrate molecules on the surface of the catalysts by a combination of spectroscopic, crystallographic and computational approaches. In particular, inelastic neutron scattering, a very powerful but rarely used spectroscopic technique, will be applied extensively to gain molecular details on these catalytic upgrading reactions of renewable biomass for the production of liquid fuels and high value aromatic chemicals. More importantly, the stability and details on structural degradation of the catalysts will be studied in situ under flow conditions via time-resolved X-ray crystallography and a range of chemical analytic approaches. The essential goal of converting biomass (esp. for the cellulose and hemicellulose portion) into liquid hydrocarbon fuels is the complete removal of oxygen through the cleavage of C-O bonds during the one-pot reaction. This project will determine the most stable reaction intermediate/s on the surface of catalysts and the stepwise pathway for the rate-determining steps (esp. for the cleavage of C-O bonds) within the entire conversion. In this way, we will understand the unique feature of these porous Nb-based catalysts in cleaving the C-O bonds to achieve the complete removal of oxygen from the system. The success of this project will not only gain in-depth understanding of the catalytic mechanism for some highly important but challenging biomass upgrading reactions, but also afford key insights into the design of new catalysts with improved structural stability and catalytic activity. This proposal involves multiple collaborations with Central Facilities and will strengthen the links between neutron scattering and catalysis.