(600ca) A Fundamental Study of the Reaction and Diffusion of Poly-Aromatic Hydrocarbons in Hierarchical Pore Structure Zeolites

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.

References

[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

Acknowledgement

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