The Design of a Shape Selective Platinum-carbon Catalyst with High Effectiveness
Open Access
- Author:
- Peer Lachegurabi, Maryam
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 04, 2013
- Committee Members:
- Professpr Henry C Foley, Dissertation Advisor/Co-Advisor
Professpr Henry C Foley, Committee Chair/Co-Chair
Darrell Velegol, Committee Member
Andrew Zydney, Committee Member
Chunshan Song, Committee Member - Keywords:
- nanoporous carbon
heterogeneous catalysis
Emulsion polymerization
Platinum nanoparticles
liquid phase hydrogenation - Abstract:
- Polyfurfuryl alcohol forms an amorphous non-graphitizing carbon upon pyrolysis at temperatures higher than 600 °C. This carbon is inherently microporous with the average pore size of 0.5 nm. The mean pore size and total porosity of PFA-derived carbon can be shifted and enhanced using different techniques either during polymerization (adding pore forming agents) or after carbonization (oxidation using CO2). The narrow pore size distribution which is a unique characteristic of this carbon, provides selective adsorptive properties and hence size and shape selectivity. Mass transport of bulky and non-planar molecules is significantly hindered within the slit-like ultra micropores of PFA-derived carbon. Because of having large surface area and high thermal and chemical stability, PFA-derived carbon can be utilized in a wide variety of applications such as gas separation, catalysis and energy storage. Heterogeneous catalysis is one of the most promising areas in which carbon offers several advantages. High hydrothermal stability and inert surface chemistry of PFA-derived carbon grant its long active life under harsh reaction conditions such as extremely high or low pH. On the other hand, shape selective catalysts with much higher sintering resistance compared to conventional impregnated catalysts, could be synthesized by embedding active metal nanoparticles within the carbon microstructure. These catalysts selectively convert those molecules that are able to diffuse into the pores, or promote the reaction in specific pathways that lead to the formation of molecules which are able to diffuse out. Due to the clean surface of the carbon pores, undesired side reactions, which are the main reason for decreased selectivity on zeolite-based catalysts, are avoided. However, engineering the textural and morphological properties of PFA-derived carbon (catalyst support) is the main challenge on the way to commercialization of these types of shape selective catalysts. In this study, based on the well-known fundamentals of heterogeneous catalysis, practical approaches are developed and utilized in order to design the optimum catalyst for liquid phase hydrogenation reactions. An emulsion polymerization approach using pluronic F-127 as the structure-directing agent and furfuryl alcohol as the monomer was utilized to prepare poly(furfuryl alcohol) spheres. Upon pyrolysis at 800 °C, the spheres transform to microporous carbon. By controlling the polymerization conditions, the diameter of the spheres, hence the diffusion length could be varied. Carbon spheres with the average size of a few microns all the way down to 50 nm were synthesized. In next step, platinum nanoparticles were embedded within the microstructure of the carbon spheres through emulsion polymerization of furfuryl alcohol in the presence of pre-formed Pt nanoparticles. The synthesized embedded catalyst was evaluated in liquid phase hydrogenation of linear and branched alkenes and showed high selectivity towards hydrogenation of smaller linear alkenes compared to larger and more branched ones. To improve the embedded catalyst activity, diffusion length, total porosity and average pore size were varied by varying the emulsion polymerization parameters, using pore forming agents and selective oxidation of carbon in CO2 stream. Catalyst activity was improved by more than one order of magnitude by optimizing the morphological and textural properties of the carbon support. The experimental catalysis data were analyzed and modeled with a transient reaction-diffusion equation within the ultramicropores, in order to obtain kinetic, adsorption and diffusion parameters. The results showed that those reactions that are happening within the confined spaces of the pores are significantly different from the reactions occurring on the outer surface of a supported conventional catalyst. The force field of the pore walls and the steric hindrance imposed by micropores, stabilize the molecules and increase the adsorption equilibrium constants and reaction rate constants. As the molecule size and bulkiness increases, the effect of confinement becomes more pronounced. It was shown that intrinsic forward reaction rate constant for 2-methyl-1-pentene hydrogenation within the micropores is almost one order of magnitude larger than it is for the same reaction over the supported catalyst. In order to tailor carbon-based catalysts for reactions that involve bulkier molecules, such as poly-aromatic hydrocarbons or biomass-derived chemicals, the presence of mesoporosity is essential to lower the mass transfer limitation imposed by the microporous nature of the carbons. In this study, a simple one step method that can be used to synthesize a carbon with a bimodal pore size distribution is presented. Simultaneous polymerization of furfuryl alcohol and phloroglucinol-formaldehyde in the presence of a structure directing agent (Pluronic F-127) was carried out and the resultant polymer was pyrolyzed to yield the bimodal carbon. The mean micro and mesopore size in this carbon could be varied by varying the surfactant and acid concentrations and monomer composition.