(519i) Experimental Studies of 4.6-Dimethyldibenzothiohene Adsorption on Metal-Exchanged Mesoporous Y Zeolites

Experimental Studies of
4.6-Dimethyldibenzothiohene Adsorption on Metal-exchanged Mesoporous Y Zeolites

Kevin X. Lee, George
Tsilomelekis 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

According to the latest transportation
fuel regulations, the maximum allowable concentration of sulfur in diesel is 15
ppmw.¹ This new limit corresponds to a 97%
reduction of sulfur since 2005. Owing to the strict administrative standard, a
majority of industries and research institutions face considerable challenges
as further reduction and removal of sulfur from diesel fuels require more
energy-intensive conditions and active catalysts. Alternatively, adsorptive
desulfurization using metal-exchanged mesoporous Y zeolites has shown to be
very effective in removing refractory sulfur compounds from transportation
fuels.² The introduction of mesoporosity
improves mass transfer and minimize diffusion limitations of bulky sulfur
compounds. The incorporation of metals such as Ce or Cu via the ion-exchanged
method has shown to increase the adsorptive active sites, consequently
improving the capacity and selectivity of sulfur compounds. The adsorption of
thiophenic rings on metal cations can proceed either via the
π-complexation or a direct S-M σ bond.^3,4 The objective of this work is to extend
our studies toward the desulfurization of a much larger thiophenic compound
such as the 4,6-dimethyldibenzothiophene (DMDBT) using our novel catalyst. To
the best of our knowledge, DMDBT removal by metal-exchanged mesoporous Y
zeolite has yet to be reported, which is expected due to the bulky and unreactive
nature of such molecule. This paper will highlight the remarkable
desulfurization performance of DMDBT using metal-exchanged mesoporous Y
zeolites, supported by fundamental IR spectroscopic experiments.

Metal-exchanged mesoporous Y zeolites
were first prepared by introducing mesoporosity via a top-down
surfactant-assistant technique.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
crystallinity and porosity, as well as the presence of metal
cations/nanoparticles were imaged using scanning transmission electron
microscopy (TEM). 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 the adsorption mechanism of thiophenic compounds on various modified
zeolites. The metal loadings on the zeolite were quantified using ICP.

Breakthrough experiments were
conducted in a fixed-bed column to study the dynamic adsorption of model fuel (DMDBT
in octane). The desulfurized effluent was collected periodically until
saturation had been reached and analyzed using a gas-chromatograph-sulfur
chemiluminescence detector (GC-SCD). Figure 1 shows the breakthrough curves of
DMDBT adsorbed on parent and modified Y zeolites. The parent Y shows an
extremely early breakthrough of DMDBT, suggesting diffusion limitations toward
the internal active sites. This is confirmed when the use of mesoporous Y
zeolite (ie. SAY) extended the breakthrough point from 0 to 150 mL/g of
sulfur-free fuel produced. Metal-exchanged mesoporous Y zeolite (ie. CuSAY) has
shown to improve the removal of DMDBT by about 20 mL/g.   The best
desulfurization sorbent was reported by bimetallic mesoporous Y zeolite as
nearly 200 mL/g of DMDBT-free fuel was produced. These results suggest that in
combination with the mesopores, the synergistic effects of Ce and Cu metals
significantly improve the capacity and selectivity of DMDBT.

Figure 1: Breakthrough Curves of DMDBT
on various Y-derived zeolites.

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, DMDBT solids were added into the cell and allowed to
adsorb at 200 °C. Temperature programmed desorption (TPD) studies were carried
out to investigate the sites at which DMDBT is adsorbed. Figure 2 shows the DMDBT
adsorption spectra on the parent Y zeolite. The OH region on the left shows
that DMDBT adsorbs on the acidic hydroxyl sites (3630 cm^-1 and 3550
cm^-1) more preferentially than the silanol sites (3741 cm^-1).
TPD profile shows that the hydroxyls begin to restore at 350 °C. On the CC
region, peaks at 1640, 1570, 1480, 1445, 1413, and 1375 cm^-1 are
apparent. The peaks at 1570 cm^-1 and 1480 cm^-1 are a
consequence of a shift to higher wavenumbers, implying an increase in electron
density. The positive shift is typical when new σ-bonds are formed,
suggesting a direct interaction (ie. H-bonding) of the proton in the zeolite with
the sulfur in the DMDBT ring. Meanwhile, the peaks located at 1445 cm^-1,
1413 cm^-1 and 1375 cm^-1 represent a shift to lower
wavenumbers upon adsorption of DMDBT on the parent Y. This negative shift
implies that the proton can also interact with the DMDBT ring, resulting in a
loss in electron density.  The same IR study can be conducted on the other
modified Y zeolites to fully understand the adsorption mechanism, as well as
the internal active sites at a fundamental level.

Figure 2: O-H
and C-C vibration regions of DMDBT adsorption parent Y zeolite.

References

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