Catalytic oxidation of glycerol to high-value chemical dihydroxyacetone over Pt-Bi/C catalyst

Wenbin Hu, Purdue University

Abstract

With increasing biodiesel production, large amounts of glycerol are obtained as a byproduct, in a 1:10 ratio with biodiesel, which has resulted in an oversupply of glycerol on the market (current price: ∼1 ¢/lb). To maximize the economics of biodiesel production, it is essential to devise new methods to utilize the byproduct glycerol. Towards this end, one of the promising approaches is catalytic glycerol oxidation, leading to a combination of more than 10 products. The most important of these oxygenates is dihydroxyacetone (DHA), a specialty chemical (valued at ∼$20/lb) mainly used as a sunless tanning agent in cosmetics industry, a chemical intermediate in organic synthesis, a dietary supplement in nutritional industry, and a potential food additive to mitigate greenhouse gas emissions in dairy industry. Note that DHA is formed by oxidation of the secondary hydroxyl group in glycerol. Glycerol selective oxidation to DHA was investigated systematically in a semi-batch reactor over Pt-Bi/C catalyst. Catalysts with different metal loadings, supports and preparation methods were synthesized, characterized and tested. The sequential impregnation of Pt and then Bi, followed by NaBH 4 reduction, was the optimum synthesis method for maximum DHA yield and high glycerol oxidation turnover frequency (TOF). The optimum catalyst composition was determined to be 3 wt% Pt-0.6 wt% Bi, supported on Norit Darco 20-40 mesh activated carbon (BET surface area 600 m2/g, average pore size 4 nm, average pore volume 0.53 cm3/g). To investigate the nature of Pt-Bi/C bimetallic catalysis, a series of experiments were performed, and various characterization techniques, such as physisorption/chemisorption, ICP-OES, TEM-EDS, XRD and XPS, were applied. The measurements show that Pt0 and Bi3+ were the major states on the catalyst surface, and they did not alter under the reaction conditions described above, since no chemical shift was observed on the XPS spectra. The Bi/Pt ratio on the surface is higher than that in bulk, due to the sequential impregnation of Pt and then Bi. Further, the dissolution of Pt/Bi was very small, both <0.2%, indicating that metals were stable in the reaction media. From TEM micrographs, the Pt/Bi metal particles were about 4.5 nm in size. XRD measurements of both support and catalyst indicate that the metal particles were in amorphous state. The role of promoter Bi in Pt-Bi catalyst for selective oxidation of glycerol to DHA was investigated and a hypothesis was proposed. Due to the strong adsorption of terminal primary OH group on Bi particles (but with no reaction), the middle secondary OH group is more easily adsorbed and oxidized by Pt sites, leading to the rapid formation of DHA. This hypothesis was verified by reaction experiments conducted with Pt only, Bi only and Pt-Bi catalysts, separately. No reaction occurred with Bi only catalyst, while higher glycerol oxidation TOF and selectivity to DHA were observed with Pt-Bi catalyst compared to Pt only catalyst. The kinetics of the complete glycerol oxidation network over the optimum Pt-Bi/C catalyst was investigated systematically in this study. Based on a detailed reaction mechanism including adsorption/desorption and reaction steps on the catalyst surface, a kinetic model was developed. To overcome the difficulty in finding the global optimum in the model, the full complex network was decomposed into five progressively larger sub-networks with different intermediates as initial reactants. The corresponding reactions starting with these initial reactants were conducted and the collected data were used to fit the kinetic model for each sub-network. The experimental and calculated results were in good agreement and the kinetic parameters for each step were obtained. These results can be used to design and develop new appropriate catalysts with high stability, activity and selectivity for DHA. In fact, this approach is a powerful tool for the microkinetic study of complex reaction networks. Based on the concentration of each component and rate expression of each reaction step, a simplified network was identified and studied. The parameters thus obtained can be used to develop reactor models which could be used to determine the optimum operating reaction conditions for maximizing the yield of desired product DHA. In addition to the catalyst, the reaction conditions in a semi-batch reactor were also optimized by conducting experiments in the range of temperature 30-90 °C, oxygen pressure 0-180 psig, and initial pH 2-12. The optimum reaction conditions were identified as 80 °C, 30 psig, and initial pH=2. Under these conditions, a maximum DHA yield of 48% was obtained at 80% glycerol conversion, which is the highest yield obtained to-date from a semi-batch reactor. The optimum operating conditions in a trickle-bed reactor, on the other hand, was determined to be 70 °C, outlet oxygen pressure 3.4 bar, glycerol solution LHSF 1.5 h-1, feed pH 2.3 and feed glycerol concentration 1M. With these optimum conditions, a maximum DHA yield of 45.8% was obtained. Some future work following this thesis is also recommended: use of a membrane reactor, flow reactor modeling and exploration of gas-expanded-liquid solvents. Compared to conventional trickle-bed reactor, use of a membrane reactor changes two parameters: longer glycerol residence time and lower local oxygen pressure, which are good for glycerol conversion and partial oxidation of glycerol to intermediate DHA. Using the rate expressions obtained from the kinetic study in Chapter 4, along with the transport processes, a trickle-bed reactor could be developed. The experimental results in the trickle-bed reactor presented in Chapter 6 can be employed to validate the model. The validated model can then be used for process design and optimization. In this thesis work, water was the only solvent used to dissolve glycerol, which is green and does not introduce other components. The drawback, however, is the low solubility of oxygen in water. The use of other novel solvents such as supercritical solvents, gas-expanded liquids (GXL), and ionic liquids may change the thermodynamic and physical properties of the solution, thus improve the glycerol conversion and DHA yield. The research conducted in this thesis utilized both experimental and modeling approaches. The results provide insights into the process of catalytic conversion of glycerol to DHA and have the potential for commercialization. Effective utilization of glycerol will increase the profitability of biodiesel production, a promising alternative and sustainable energy source.

Degree

Ph.D.

Advisors

Varma, Purdue University.

Subject Area

Chemical engineering

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