Heterogeneous catalysts in aqueous phase reforming environments: an investigation of material stability
Ravenelle, Ryan M.
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There are many problems associated with the use of fossil fuels to produce fuels and chemicals, and lignocellulosic biomass stands as a promising alternative fuel/chemical feedstock. Large scale processing of biomass will likely take place in high temperature liquid water due to the low vapor pressure and polar nature of carbohydrates. However, little is known about the material stability of these catalysts in high temperature aqueous phase environments. This dissertation aims to investigate the structural integrity of some common catalytic materials under typical biomass reforming conditions. There are 3 main objectives of this study: 1) identify potentially stable candidates from commonly used materials, 2) understand the mechanism(s) by which these catalysts degrade, 3) design/modify catalysts in an effort to increase their hydrothermal stability. The two main materials investigated in this work are zeolites (faujasite, ZSM-5) and γ-Al2O3 as these are commonly used as catalysts and catalyst supports. A number of physicochemical techniques were used to characterize the materials as a function of treatment time at conditions relevant for biomass reforming. For zeolites, the major findings are that ZSM-5 framework is highly stable whereas faujasite stability depends on the Si/Al ratio, where silicon rich materials are less stable. For γ-Al2O3 based catalysts, it was found that the alumina support hydrates and undergoes a phase transformation to form crystalline boehmite (AlOOH) with a subsequent loss in surface area and Lewis acid sites. When metal particles are present on the support, the phase change kinetics are slowed. The role of metal precursor on the stability of γ-Al2O3 supported catalysts was also explored, and it was found that the precursor used in catalyst synthesis changes the boehmite formation kinetics and also affects alumina support dissolution. The final thrust aims to stabilize a Pt/γ-Al2O3 catalyst by depositing silicon on the catalyst surface. The silicon modification is effective in protecting the catalyst from boehmite formation upon exposure to hot liquid water while also stabilizing metal particles against sintering. Additionally, an increase in turnover number for hydrogen production via aqueous phase reforming of sorbitol was observed.