Design of Supported Metal Catalysts for Aqueous-Phase Conversion of Biomass-Derived Oxygenates.

摘要

Increases in the cost of fossil fuels along with growing concerns for greenhouse gas emissions are prompting the search for renewable sources of liquid fuel and chemicals. Biomass has been considered as the only realistic and sustainable source of renewable organic carbon for the foreseeable future. Heterogeneous catalysis has played an important role in the development of efficient chemical processes to convert biomass to fuels and chemicals. In this respect, the design of inexpensive active and stable heterogeneous catalysts is important to develop new and improved processes for the conversion of biomass.;This dissertation focuses on aqueous-phase hydrodeoxygenation (APHDO) and aqueous-phase hydrogenation (APH) as model reactions to design improved supported metal catalysts for the conversion of biomass-derived feedstocks. Both APHDO and APH are crucial in converting biomass-derived compounds into liquid fuels and chemicals.;In this dissertation, the activity of a number of monometallic and bimetallic catalysts is compared for APH of carbonyl compounds which is an important reaction in APHDO. Bimetallic Pd-Fe is the most active catalyst among the tested catalysts for APH of C=O and C=C bonds. APHDO of sorbitol was performed with the bimetallic Pd-Fe supported on zirconium phosphate (Zr-P). Zr-P was chosen as the support due to its high Bronsted to Lewis acid ratio and stability in the aqueous phase. The Pd1Fe3 /Zr-P catalyst is up to 14 times more active than monometallic Pd/Zr-P and Pt/Zr-P catalysts. Moreover, the Pd1Fe3/Zr-P catalyst produces more C4-C6 products by promoting the conversion of sorbitan and isosorbide and more C1-C3 products by promoting C-C bond cleavage (dehydrogenation/retro-aldol condensation) of sorbitol.;Another critical issue of designing heterogeneous catalysts for aqueous-phase reactions is stability of the catalysts. A method for stabilizing base-metal particles of a Co/TiO2 catalyst is developed using atomic layer deposition (ALD) of TiO2 film onto the surface of the Co/TiO 2 catalyst. The ALD TiO2 coated Co/TiO2 catalyst was tested for APH reactions in a continuous flow reactor and characterized using chemisorption, surface area analysis, electron microscopy, X-ray diffraction, and small-angle X-ray scattering. Through these techniques, it is shown that the ALD TiO2 coating protects the cobalt particles against leaching and sintering under aqueous conditions.;High-temperature treatments of a Co/TiO2 catalyst cause migration of partially reduced TiO2 onto cobalt particles caused by strong metal-support interaction (SMSI) between cobalt and TiO2. The SMSI effect in the Co/TiO2 catalyst is elucidated using in situ Raman spectroscopy and electron microscopy. By the SMSI effect, cobalt particles of the Co/TiO2 catalyst are decorated by TiOx (x < 2) species. The TiOx decoration stabilizes the cobalt particles in a similar way to ALD TiO 2 overcoating. The SMSI effect also creates a bifunctional catalytic site in the Co/TiO2 which facilitates a furanyl ring-opening reaction. The high-temperature treated Co/TiO2 catalyst had > 95 % yield for APH of carbonyl compounds to their corresponding alcohols. The two methods for stabilizing cobalt catalysts introduced in this dissertation, ALD and SMSI, may enable the replacement of expensive novel-metal catalysts with inexpensive base-metal catalysts for aqueous-phase conversion of biomass-derived feedstocks.