(4ef) Application and Mechanism Understanding Of NANO-Structured Catalysts In Biofuel Production

APPLICATION AND MECHANISM UNDERSTANDING OF NANO-STRUCTURED CATALYSTS IN BIOFUEL PRODUCTION

Cun Wen, University of South Carolina, Columbia, SC

Department of Energy has set the goal of biofuel production to 36 billion gallons by 2022, while the present production rated at 1.1 billion gallons at 2011. The capacity needs more than 3200% increase in total or 40% increase in annual for the next 10 years. One of the limiting factors in biofuel production is the low throughput of the batch reaction mode. Though it has been long known that heterogeneous processes can accelerate the biofuel production, the actual progress is hindered by poor understanding of reaction mechanisms and requires sophisticated material design and synthesis. The reaction mechanism research is intricate with numerous potential reaction pathways, similar to the possible routes to overcome Himalayas. Our strategy to tackle this issue is to combine the mechanism research with kinetics studies, which serves as loyal contour-mapper and tells us the most feasible way. This strategy has been proven to be effective.^1-10 Herein, I bring in nano-structured heterogeneous catalysts with our strength in advanced nanomaterial synthesis and fundamental reaction-mechanism studies to expedite the biofuel production.

To upgrade biomass to biofuels, the aromatic rings and the C-O bonds need to be broken down and C-C/C-H bonds need to be build up, on which stage heterogeneous catalysts can show off. Firstly, 2-methyfuran acid catalysis reaction is chosen as a model reaction to exemplify our idea of heterogenerizing biofuel production. The 2-methylfuran is a mode molecular representing biomass feed stocks, which usually need acid catalysts for deoxygenating because of oxygen as heteroatom and need chain growth for generating biofuel because of low carbon numbers (<C6). Usually concentrated sulfuric acid is used as acid catalyst to serve the purpose, but it is unfavorable for large scale industrial production. Our results show that the zeolite catalyst can replace the concentrated sulfuric acid with even better production selectivity in terms of cetane number. Furthermore, the cetane number of the product can be increased by decrease the acid strength of the zeolites. To understand the relationship between acid strength and product cetane number, the reaction mechanism on the zeolite catalyst is probed by in situtechniques, such as FTIR. The reaction mechanism study reveals that high acid strength increases the adsorption energy, and thus prolongs the resident time of the intermediates on the zeolite. Because longer resident time facilitates the C-C chain growth, the branched products with poor cetane number are favored on zeolites with high acid strength.

Secondly, the products from 2-methfuran acid catalysis reaction need further hydrogenation to upgrade to final biodiesel, and Co is reported to be active for the hydrogenation reaction, whose activity can be further promoted by Cu doping. Thus, bimetallic core-shell nanoparticle Cu@Co with alloy of CoCu on the shell (for high activity) and Cu as the core (for low catalyst cost) would be optimum for this reaction. Traditionally, the core-shell bimetallic nanoparticles are synthesized in two steps. In the first step, the core is formed with one metal, and then in the second step the core is coated with second metal as the shell. Thus, the two-step method not only is poor in economic, but also yields mono-metal instead of alloy on the surface, while alloy is needed for high activity in this hydrogenation reaction. Herein, I demonstrated a one-pot methodology for mono-dispersed Cu@Co Core-shell nanoparticle synthesis with alloy of CoCu on the shell as catalyst for hydrogenation reaction. Furthermore, I can not only synthesis the nanoparticle in one-step, but also keep a tight control over the size distribution (relative standard deviation 7.9%).¹¹

Thirdly, biomass upgrading to biofuel is not limited to the acid and hydrogenation reaction, and other process can also serve the purpose, such as partial oxidation. The partial oxidation has been proved to be an effective way to produce biofuels from biomass aligned with Fischer-Tropsch synthesis. However, the long-standing problem in partial oxidation is the hot spot formation in the catalyst bed, which can lead to the catalyst deactivation or even explosion. It has been known that the hot spot is formed by the side reaction that is the total oxidation taking place beside the partial oxidation. The key factors guiding the reaction path through total or partial oxidation is unknown, which hinders the catalyst development. To address this issue, I looked into the mechanism of partial oxidation production of syngas (CO+H₂). Our research has revealed that the lattice oxygen reducibility of the catalyst is the key factors governing the reaction mechanism. With high lattice oxygen reducibility, total oxidation rather than the partial oxidation is favorite. By tuning the lattice oxygen reducibility of Er₂O₃with Mn doping, I successfully shift the reaction mechanism from total oxidation to partial oxidation, and eliminated the hot-spot formation.

Read Me More:

1.       Wen C, Liu Y, Guo Y, Wang YQ, Lu GZ. Strategy to eliminate catalyst hot-spots in the partial oxidation of methane: enhancing its activity for direct hydrogen production by reducing the reactivity of lattice oxygen. Chem Commun. 2010;46(6):880-882.

2.       Wen C, Zhu, Y.; Ye, Y.; Zhang, S.; Cheng, F.; Liu, Y.; Wang, P.; Tao, F., Water–Gas Shift Reaction on Metal Nanoclusters Encapsulated in Mesoporous Ceria Studied with Ambient-Pressure X-ray Photoelectron Spectroscopy. ACS Nano. 2012, 6, 9305-9313.     

3.       Wen C, Liu Y, Guo Y, Wang Y, Lu G. Synthesis of the rare earth compound nanosheets induced by lamellar liquid crystal. Solid State Sciences. 2009;11(11):1985-1991.

4.       Wen C, Liu Y, Tao F. Integration of surface science, nanoscience, and catalysis. Pure Appl Chem. Jan 2011;83(1):243-252.

5.       Liu Y, Wen C, Guo Y, Lu GZ, Wang YQ. Modulated CO Oxidation Activity of M-Doped Ceria (M = Cu, Ti, Zr, and Tb): Role of the Pauling Electronegativity of M. J Phys Chem C. 2010;114(21):9889-9897.

6.       Liu Y, Wen C, Guo Y, Lu GZ, Wang YQ. Effects of surface area and oxygen vacancies on ceria in CO oxidation: Differences and relationships. J Mol Catal a-Chem. Feb 2010;316(1-2):59-64.

7.       Liu Y, Wen C, Guo Y, et al. Mechanism of CO Disproportionation on Reduced Ceria. ChemCatChem. 2010;2(3):336-341.

8.       Media highlights: Pichon A. Catalysis: Lower reactivity means more hydrogen. 2010; http://www.nature.com/nchina/2010/100120/full/nchina.2010.7.html.

9.       Media highlights: Saxton C. Enhancing catalytic activity. 2010; http://www.rsc.org/Publishing/ChemTech/Volume/2010/02/enhancing_catalytic.asp.

10.       Media highlights: Chinese New Year. 2010; http://www.rsc.org/Publishing/Journals/cc/News/2010/Chinese_New_Year.asp.

11.     Wen C, Hattrick-Simpers J, Lauterbach J, Inventors. One-step synthesis of monodisperse transition metal core-shell nanoparticles with solid solution shells, US provisional patent: 61/686,288, 2012.