(453g) Diffusion and Adsorptive Desulfurization Studies on Bifunctional Zeolites

Diffusion and Adsorptive
Desulfurization Studies on Bifunctional Zeolites

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

Zeolite Y has shown to be an effective
sorbent material for the adsorptive removal of sulfur from transportation
fuels.¹ In their natural state, Y zeolites are microporous and
non-selective. To meet the mandated sulfur standards for gasoline and diesel,
several functionalities need to be added to the zeolites to improve the sulfur
uptake. Firstly, the introduction of mesoporosity enables the adsorption of
refractory sulfur compounds, which could not be achieved by conventional
hydrodesulfurization without excessive energy. The bigger pores improve mass
transfer and minimize diffusion limitations. Furthermore, adsorption sites for
sulfur can be increased through the addition of metals by ion-exchanging the
zeolite with active metals such as Ce or Ce. As a result, the adsorption
process becomes more selective to thiophenic molecules via the interaction of a
π-complexation or a direct S-M σ bond.^2,3 The
incorporation of metal ions increases the selectivity for sulfur, especially in
commercial fuels. The objective of this work is to fundamentally understand the
adsorption mechanism of various sulfur compounds on the zeolite active sites.
To achieve this goal, IR spectroscopy experiments were performed using model
thiophenic compounds, and parent Y and metal-modified Y zeolites.

Bifunctional zeolites were first
prepared by introducing mesoporosity via two top-down methods: (1) desilication
(DS) and (2) surfactant-assistant technique.^4,5 The mesoporous
zeolite was subjected to an ion-exchange procedure to replace the protons with
either Cu or Ce metals. The modified zeolites were characterized using various
techniques. The pore size distribution and crystal structure were identified
using N₂ adsorption/desorption isotherm and x-ray diffraction (XRD),
respectively. Surface acidity and metal distributions were determined using
pyridine and CO adsorptions by diffuse reflectance infrared spectroscopy
(DRIFT-FTIR), respectively. DRIFTS-FTIR experiments were also used to study
diffusion limitations and adsorption mechanism of thiophenic compounds on
various modified zeolites. The metal loadings on the zeolite were quantified
using ICP. Isosteric heat of adsorption calculations were conducted via a batch
method to study the strength of interaction between each sulfur component and
the sorbent material.⁶ The adsorption pattern was assumed to follow
the Langmuir isotherm.

Breakthrough experiments were
conducted in a fixed-bed column to study the dynamic adsorption of model fuel (thiophene
(TP), benzothiophene (BT) and dibenzothiophene (DBT) in octane). The desulfurized
effluent was collected periodically until saturation had been reached and
analyzed using a gas-chromatograph-sulfur chemiluminescence detector (GC-SCD). Results
indicated that the best materials for the removal of TP and BT were the metal-exchanged
zeolites (CeY and CuY). Larger pores, however, were required for the adsorption
of sulfur compounds with a larger kinetic diameter, such as DBT. Metal-exchanged
mesoporous Y (ie. CuSAY) has shown to be the most effective sorbent with an
increase in DBT uptake by about 75 mg/L compared to the parent Y, as
demonstrated in Figure 1(a). These results suggest that the adsorption of DBT
is perturbed by diffusion limitations, which can be overcome by bifunctional
zeolites. Figure 1(b) shows that in a mixture of model fuel containing all
three thiophenic compounds, the CeSAY material proved to be the best sorbent,
suggesting a preference to adsorb refractory sulfur compounds with a larger
kinetic diameter.

Figure 1:
Breakthrough Curves of (a) DBT and (b) model fuel on various Y-derived
zeolites.

In all cases, the tendency for
zeolites to adsorb sulfur compounds increases in the order of TP < BT <
DBT. The isosteric heat of adsorption calculations revealed that DH_ads
was the highest for BT adsorbed on CuY. This is likely due to the strong π-stacking
of the BT ring atop the Cu metal. The CuSAY, despite assisting in overcoming
diffusion challenges, showed the lowest DH_ads. This shows that
the isosteric heat of adsorption is only a descriptor of the interaction
strength.

The adsorption mechanism of sulfur on
Y zeolites can be revealed by FTIR studies. About 25 mg of zeolite was weighed
into the DRIFT cell and activated in-situ with either H₂ or N₂
for 1 hour. Upon activation, benzothiophene vapor was introduced into the cell
via a heated line until saturation was achieved. Temperature programmed desorption
(TPD) studies were carried out to investigate the sites at which BT is
adsorbed. Figure 2 shows the BT adsorption spectra on CuY and parent Y
zeolites. The OH region on the left shows that a higher temperature is required
to completely desorb the BT ring from CuY compared to parent Y. This is also
apparent in the 2000-1000 cm^-1 region where most of the C-C and C-H
vibration modes have disappeared from the parent Y spectra by 360 °C, but the
same peaks are still visible on CuY. As suggested by the breakthrough curves
and heats of adsorption, Cu-exchanged zeolites can form π-complexations
with sulfur, shifting the C=C band vibration of 1640 cm^-1 to lower
frequencies. As a result, more energy is needed to remove the strongly held BT
molecule from CuY.

Figure 2: O-H
vibration region of BT adsorption on (a) CuY and (c) Y and C-C vibration region
of BT adsorption on (b) CuY and (d) Y.

Breakthrough studies revealed that
CuSAY has the highest capacity for refractory sulfur compounds such as DBT due
to strong π-complexation interaction between the sorbate and the sorbent.
In a model fuel containing various thiophenic compounds, CeSAY proved to be the
best sorbent in selectively removing DBT via the direct σ-bond
interaction. Both isosteric heat of adsorption calculations and FTIR studies
suggested that higher energy is required to break the bonds between CuY and BT.
Collectively, our findings suggests that determining  the optimum balance
between the pore structure and metals in a bifunctional zeolite is essential
for maximizing capacity and selectivity for sulfur compounds.

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