Nanoporous Carbon Mediated Catalysis and Hydrogen Adsorption

Open Access
Author:
Holbrook, Billy-Paul Matthew
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 06, 2009
Committee Members:
  • Dr Henry C Foley, Dissertation Advisor
  • Henry C Foley, Committee Chair
  • Chunshan Song, Committee Chair
  • Ayusman Sen, Committee Member
  • David Lawrence Allara, Committee Member
Keywords:
  • catalysis
  • carbon
Abstract:
Polyfurfuryl alcohol forms a microporous, glassy carbon upon heating to >600°C under inert conditions. Polyfurfuryl alcohol (PFA)-derived carbons fall under the category of porous, non-graphitizing carbons and have a tunable porosity and surface area. These carbons have received significant attention due to their potential application in capacitors, fuel cell anodes and cathodes, as adsorbents and molecular sieves. Their narrow pore size distribution provides adsorptive selectively which is based on size and shape. This limits the extent which those molecules can enter and diffuse through their porous network. By incorporating metal nanoparticles into PFA prior to the pyrolysis, catalytically active metal sites can be made that are sheathed by a layer of microporous carbon. The formation of active sites embedded within the porous surface can lead to selective catalysis with reactants that are able to diffuse through the pores. Gas phase reactions had been conducted on such catalysts and a high selectivity was observed for planar, linear molecules. Yet the use of these catalysts had not been expanded into liquid phase reactions due to pore size restrictions. In order to explore the use of such shape selective catalysts for liquid phase hydrogenation reactions, a newly synthesized PFA-derived carbon was examined for use in liquid phase. This forms the crux of this thesis. A literature survey is provided in Chapter 1. It details the synthesis and formation of PFA-derived carbons, their use as molecular sieves, and applications for these carbons. There is also a review of the synthesis of boron-doped carbon to be used as a support for dispersing metal for enhancing hydrogen exchange processes including adsorption and desorption. The formation and characterization of selective PFA-derived carbon catalysts is presented in Chapter 2. To form such catalysts, platinum nanoparticles were synthesized using the monomer, furfuryl alcohol, as the reducing agent. Then the nanoparticle and monomer mixture was polymerized and cured to immobilize the nanoparticles within the polymeric solid prior to pyrolysis. By altering the solvent used in polymerization, mesoporosity was created, or not, while microporosity was retained. Furthermore, the catalytic activity and selectivity of these catalysts were extensively studied and analyzed in Chapter 3 and Chapter 4. Chapter 3 describes catalytic activity and selectivity of the experimental catalysts compared to a conventional catalyst for liquid phase hydrogenations. Wholly microporous catalysts were not active, but those which were both microporous and mesoporous catalysts were. The active catalysts were shown to be highly selective toward linear, planar molecules. Chapter 4 applies mathematical models to describe the rate limitations of the catalysts as well as the effect of porosity on activity and selectivity. In Chapter 5 and Chapter 6, a boron substituted carbon, BCx, is described which was formed by chemical vapor deposition. This could be made either as a non-porous solid or as a thin layer on a high surface area silica aerogel. Chapter 5 outlines the effects of synthesis parameters on the as-synthesized bulk material. The chemical environment of boron in the material was thoroughly characterized and was found to be trigonal planar. Chapter 6 explores the use of high area silica aerogel as a support onto which BCx may be deposited. Synthesis and characterization of this material was followed by experiments aimed at measuring its adsorptive properties for hydrogen. Chapter 7 concludes the work with suggestions on how to increase the activity of PFA-derived carbon catalysts while retaining selectivity. Some of the suggestions, provided with preliminary data, display how mesoporosity and concentration of available active sites are the major factor in activity, while microporosity is the contributing factor in selectivity. These three parameters are intertwined, and altering one correlates to changes in the others.