389866 A Fundamental Study of the Reaction and Diffusion of Poly-Aromatic Hydrocarbons in Hierarchical Pore Structure Zeolites

Wednesday, November 19, 2014
Galleria Exhibit Hall (Hilton Atlanta)
David P. Gamliel, Shoucheng Du, George M. Bollas and Julia Valla, Department of Chemical & Biomolecular Engineering, University of Connecticut, Storrs, CT

A Fundamental Study of the Reaction and Diffusion of Poly-Aromatic Hydrocarbons in Hierarchical Pore Structure Zeolites

David P. Gamliel, Shoucheng Du, George M. Bollas, and Julia A. Valla

Department of Chemical & Biomolecular Engineering, University of Connecticut, Storrs, CT

Thermochemical conversion of biomass via gasification and pyrolysis has attracted significant scientific attention as a means for converting biomass and waste products to useful energy and chemical feedstocks. A major factor deterring large-scale commercialization of this technology is the presence of high molecular-weight poly-aromatic hydrocarbons (PAHs), also known as tars, in producer streams. Tars deactivate catalysts, cause blockages in transfer lines, and damage downstream units such as compressors, turbines, and fuel cells [1]. Additionally, PAHs are typically carcinogenic in nature. A process to convert PAHs into potentially useful products would be very desirable.

Zeolites are a promising catalyst for PAH transformation. They exhibit unique properties such as acidity, well-defined microporosity, and shape selectivity, which make them ideal for catalytic applications. Zeolites are used as the workhorse of the fluid catalytic cracking (FCC) unit, in which they are used to crack heavy hydrocarbons into lighter, more valuable products. However, low diffusion rates of large molecules through the highly microporous structure results in low conversion and coke formation. Bulky molecules may even be totally excluded from the pore system. The introduction of mesoporosity may be one way to reduce these diffusion limitations and increase access of bulky molecules to catalyst active sites.

Dou et al. [2] have studied the catalytic cracking of 1-methyl naphthalene in a fixed bed reactor co-fed with hydrogen. They found that Y zeolite and Ni/Mo catalyst were the best for cracking the tar, achieving greater than 95% conversion. Buchireddy et al. [3] studied the conversion of naphthalene over Ni impregnated Y zeolite in the presence of syngas. They found that the presence of the Ni increases the reforming capability of the catalyst and that activity increases with an increase in zeolite acidity.

The main focus of our work is the catalytic conversion of PAHs using microporous and mesoporous zeolites with and without impregnated metals. Mesoporous zeolites have been prepared by using two different methods: desilication and the surfactant assisted method [4]. Desilication is accomplished by introduction of random mesoporosity via alkaline treatment. Surfactant assisted desilication is performed by alkaline treatment in the presence of a surfactant, in this case CTAB (cetyltrimethylammonium bromide). Complete characterization of the prepared zeolite includes TEM (Figure 1), XRD, and N2 adsorption. We found that increasing base concentration increases the mesoporosity (measured by N2 adsorption), but drastically reduces crystal integrity. These findings are consistent with previously reported results [5-6].

Figure 1 TEM images of parent ZSM-5 (left) and mesoporous ZSM-5, prepared with 0.1 M base treatment

Catalytic experiments have been performed in a pyroprobe microreactor (CDS Analytical). Naphthalene is chosen as a representative tar compound. Experiments are performed at a variety of temperatures, residence times, and catalyst to reactant ratios.  For both the Y zeolite and ZSM-5 it was found that conversion of naphthalene is decreased when mesoporosity was introduced. However, selectivity to desirable compounds, those lighter than naphthalene, is increased when performing mesoporous zeolite was used. Figure 2 shows the selectivity of both Y zeolite and ZSM-5 to lighter compounds for both microporous and mesoporous catalyst.  This may indicate a decrease in diffusion limitations for the mesoporous catalyst.

Figure 2 Selectivity to compounds lighter than naphthalene

When the mesoporous and microporous catalysts are compared on an activity basis, as in Figure 3, the two catalysts are essentially equivalent.

Figure 3 Conversion of naphthalene to coke and liquids prepared on an activity basis

The pyridine adsorption FTIR technique is used in this study to determine the effect of introduced mesoporosity on the Brønsted and Lewis acidity of each zeolite, and their relation to the Si/Al ratio. Additionally, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) diffusion and adsorption studies with benzene and naphthalene have been performed to determine the mass transport and kinetic properties of each catalyst. Catalytic reactions have also been performed in reactive syngas, as opposed to inert nitrogen, to promote reforming reactions, and simulate a more realistic producer gas environment. Microporous and mesoporus zeolites impregnated with metals to further increase catalytic activity have also been studied and these results will be presented.


[1] S.D. Phillips, Ind. Eng. Chem. Res. 46(2007) 8887.

[2] B. Dou, J. Gao, X. Sha, S.W. Baek. Applied Thermal Engineeirng. 23 (2003) 2229-2239

[3] P.R. Buchireddy, R.M. Bricka, J. Rodriguez, W. Holmes. Energy Fuels. 24 (2010) 2707-2715.

[4] J. Garcia-Martinez, M. Johnson, J.A. Valla, K. Li, J.Y. Ying. Catal. Sci. Technol. 2 (2012) 987-994

[5] D. Verboekend, G. Vile, J.Perez-Ramirez. Advanced Functional Materials 22 (2012) 916-928

[6] K. Li, J.A. Valla, J. Garcia-Martinez. ChemCatChem.  5 (2013) 2-23


This work was generously funded by the National Science Foundation (Award CBET-1236738)

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