(717a) Investigation of Bifunctional Zeolites for the Adsorptive Desulfurization of Fuels

Investigation of
bifunctional zeolites for the adsorptive desulfurization of fuels.

Kevin X. Lee and Julia A.
Valla

Department of Chemical
& Biomolecular Engineering, University of Connecticut, 191 Auditorium Road,
Unit 3222, Storrs, CT 06269-3222, USA,

Phone: +1-860-486 4602, e-mail: ioulia.valla@.uconn.edu

Naturally occurring sulfur in
transportation fuels has been considered a threat to environment and health
safety. The combustion of sulfur produces toxic sulfur oxides (SO_x),
which contributes to acid rain and catalyst deactivation. Furthermore, the
slightest amount of sulfur can poison an entire fuel-cell system.  Due to this
global issue, many desulfurization methods have been developed to meet the stringent
emission standards of 10ppm by 2017.¹ A promising method to achieve
this goal is desulfurization by adsorption using zeolites because it is a cost
effective and environment friendly process.² Zeolites are microporous
aluminosilicates minerals that are widely used in catalysis, adsorption, and
other energy applications due to their unique pore structure and large surface
area. Unlike the energy intensive hydrodesulfurization method, zeolites can
effectively remove refractory sulfur compounds such as alkyl-benzothiophenes
and their derivatives at ambient conditions and without the consumption of
hydrogen.

Further modification to the microporous zeolite can be done either
by introducing mesoporosity and/or incorporating d-block metal cations.³
Metal-incorporated zeolites, in combination with the ordered and open pore
structures, were shown to be promising due to the sulfur-metal bond and the
π-complexation interactions.^4,5 The crystallinity of the
zeolite is also preserved to study the effect of acid sites and microposority on
desulfurization.⁶ Hierarchical zeolite Y and USY were prepared via
desilication and surfactant methods.⁷ The crystal structure and pore
size were identified using x-ray diffraction (XRD) and Brunauer-Emmett-Teller (BET),
respectively. Brönsted and Lewis acid sites were quantified using pyridine
adsorption by diffuse reflectance infrared spectroscopy (DRIFT-FTIR). DRIFTS-FTIR
experiments were also used to study diffusion limitations of thiophenic
compounds through the zeolites. Ce, Ni and Cu metals were introduced onto the
zeolites via either ion-exchange or wet incipient impregnation method. The
metal loadings on the zeolite were determined using ICP.

The adsorption of a model fuel (eg. benzothiophene in octane)
was studied via dynamic fixed-bed chromatography experiments. The sulfur
concentrations in the zeolite and in the effluent after reaching equilibrium
were evaluated using elemental analysis and the gas-chromatograph-sulfur chemiluminescence
detector (GC-SCD), respectively. Breakthrough curves of final benzothiophene
concentrations were generated and compared to evaluate the performance of each
type of zeolite. The most promising zeolite for sulfur removal appears to be the
Ni-impregnated on zeolite Y. Sigma bond connecting S in benzothiophene and Ni metal
yields a strong S-M interaction and longer sorbent lifetime. This convincing
result will allow for further investigation on other transition metals. Our
results also demonstrate that zeolite Y is a more effective sorbent than USY,
which suggests that the number of acid sites could play a more significant role
than the pore size for benzothiophene. Precise and careful modification of
zeolites is essential to optimize diffusivity, selectivity and adsorption rate.

The study has been extended to diesel fuels to examine the
feasibility of sulfur adsorption with zeolites in industrial applications. The
diesel fuel was spiked with one or several sulfur model compounds (eg.
thiophene, benzothiophene and dibenzothiophene) as well as other aromatic
hydrocarbons (eg. benzene, toluene, and pyridine). Dynamics experiments have
been conducted to assess the capacity and the lifetime of each zeolite. This
study explores wide applications of hierarchical pore structured zeolites as
promising adsorbents of thiophenic compounds by investigating adsorption
kinetics, selectivity, and capacity.

To simulate the adsorption behavior and predict sorbent
performances, a mathematical model was developed based with the following
assumptions: (1) isothermal conditions, (2) constant flow rate, (3) transport
of diluted species, (4) constant particle and bed porosities, (5) internal mass
transfer described by Linear Driving Force model, (6) adsorption based on
Langmuir isotherm.⁸ The overall differential-algebraic mass balance
equation of the bulk (liquid) phase is given by:

Where c is the concentration
of sulfur, D is the axial dispersion coefficient, u is the
interstitial velocity, ε is the bed porosity, q ia the
adsorbed sulfur concentration, k is the mass transfer coefficient, K_L
is the Langmuir constant, and q_m is the sorbent capacity. The
boundary conditions are given by:

The study on temperature effects allowed for thermodynamic
evaluation where the Gibbs free energy change is a function of the Langmuir
constant.

            The negative value of ΔG suggested that the adsorption of benzothiophene is a
spontaneous process.  The changes in entropy and enthalpy of the adsorption
process were evaluated by the following equation:

            The analysis of
thermodynamics indicated that the adsorption of benzothiophene is both
spontaneous and exothermic. Metal-modified zeolite Y has shown to be a
promising adsorbent for the desulfurization of fuels due to strong S-M or π-complexation interactions.
Microporosity of the zeolite also plays an important role in providing acid
sites for the sulfur compounds. For larger refractory thiophenic compounds
where diffusion limitations may be an issue, the introduction of mesoporosity could
enhance the adsorption of these molecules. The selectivity, capacity, lifetime,
and kinetics of the adsorbents are dictated by the type of zeolite. The overall
performance of each zeolite can be determined via the breakthrough curves. The
mathematical model has shown to be a good representation of the breakthrough
curves described by the non-linear Langmuir isotherm.

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