NEW ADSORBENTS AND MATERIAL MODIFICATIONS- AN APPROACH
FOR ULTRACLEAN FUELS
Laxmi Gayatri Sorokhaibam, Vinay M. Bhandari and Vivek V. Ranade
CSIR-National Chemical Laboratory, Pune, India
e-mail: ls.gayatri@ncl.res.in; vm.bhandari@ncl.res.in; vv.ranade@ncl.res.in
Abstract
Experimental studies on sulfur removal by adsorption method on new forms of adsorbents
along with their transition metal incorporated forms are presented. These adsorbents have
been specially/selectively modified for removal of sulfur from liquid fuels like diesel and
gasoline. The characterisation results revealed the presence of elements like Al, Si, O and
successful incorporation of metals like Ni and Cu. The performance of these modified
materials on adsorption desulfurization was investigated mainly using benzothiophene and
mixtures comprising of thiophene, benzothiophene and dibenzothiophene as an approach
to mimic commercial liquid fuel. The adsorption data are modelled by different isotherm
equations and mechanism of interaction of the adsorbent surface with the sulfur moieties
to provide fundamental understanding on selectivity and role of modification are also
analysed. The present work shows that research on tailoring, fabrication, modification of
adsorbent materials is a promising approach in an effort to reduce the cost of
desulfurization by conventional hydrodesulfurization method when used in combination.
Keywords: Desulfurization, Adsorption, Fuel, Separation, Pollution
Introduction
Ultraclean fuels have created a worldwide heightened interest due to a) growing
environmental concern of SOx emission from liquid fuels like gasoline and diesel, b)
demand for lower and lower sulfur contents as required by different government
regulations and, c) the need for zero sulfur fuels in fuel cell applications to avoid poisoning
of the catalyst. The nature and quantities of different sulfur compounds in different types of
fuel such as diesel, gasoline or jet fuel are substantially different and therefore warrant
specific solutions to each type of fuel as far as sulfur removal is concerned to make it ultra
clean. As a result, generalized solutions are not available as yet and there is an urgent need
for devising more suitable, techno-economically feasible solutions on deep desulfurization,
especially in terms of increased capacities for sulfur removal and for the removal of
refractory compounds. Common sulfur compounds in transportation fuels include
mercaptans, sulphides, disulfides; and refractory groups like thiophene (T), benzothiophene
(BT), dibenzothiophene (DBT) and 4, 6-dimethyldibenzothiophene which are quite difficult
to be removed. The conventional method of sulfur removal from liquid fuels is
Hydrodesulfurization (HDS), which is considered to be highly cost intensive operation,
requiring high temperature (320-3800C) and high pressure of (3-7 Mpa) over Ni/Co
catalyst1. Currently, the emission standards as fixed by USEPA are 30 and 15 ppm for
gasoline and diesel2 respectively. In European countries, quality of diesel fuels as specified
by the EN 590 standard3, the permissible limits for sulfur content in fuels is 10 ppm. As
already stated, though HDS process can reduce the sulfur content below 10 ppmw in
diesel4, it is very capital intensive and difficult for higher alkyl/cyclic refractory sulfur
compounds. Thus in order to meet the demand for ultra-clean fuels for environmental
protection, newer approaches and modification of the existing technologies is required for
removal of these refractory sulfur compounds. In this context among the various emerging
technologies, research in adsorptive desulfurization using new class of adsorbents and
modification of the existing materials to increase the efficiency in sulfur removal capacity is
taking into new heights. The development of effective adsorbent which can selectively
remove sulfur compounds from real liquid fuels is a major challenge as commercial diesel
and gasoline comprises of a number of aromatic, cyclic organic compounds apart from
these refractory sulfur compounds. The main objective of the present study is to evaluate/
develop/ design suitable adsorbents with high capacity and selectivity to reduce the sulfur
levels to ultra-low levels and obtain more insight into sulfur removal behaviour on
adsorbent surfaces.
Adsorptive Deep Desulfurization For Ultra-clean Fuels
The reported adsorbents for desulfurization include activated carbon5,6, metal doped
zeolites7,8, silica, alumina and so on. The nature of the adsorbent, presence of specific
functional groups, active sites for sulfur moieties to bind, porosity-meso/microporous,
surface area etc play different roles in the overall adsorbent development. These elements
provide ample scope for developing new adsorbents, tailoring/ modification of the existing
materials with greater removal capacity. The incorporation of transition metal ions into
zeolite/carbon framework9,10 has attracted considerable interest in modification of the
existing materials. In this work both modified carbon based adsorbents, Shirasagi GH2x
4/6, SRCx 4/6 have been evaluated for their sulfur removal- specially for refractory sulfur
compounds and further modifications using Ni/Cu loaded forms for understanding surface
interactions using synthetic model fuel. These materials are hereafter referred as GH2x,
SRCx, Ni-GH2x, Ni-SRCx, Cu-GH2x and Cu-SRCx for convenience. The literature reports
have underlined need for research to provide detailed insights into the mechanism of
selective adsorption over different adsorbents. Some studies have attempted to provide
sulfur removal mechanism in the case of modified zeolites and other adsorbents by
proposing π-complexation11, hydrophobic interaction, van der Waals interaction, H-bonding
and/ or acid base interaction mechanism. The weak nature of bonding in these types of
mechanisms, however, is not reflected in the ease of regeneration and is not thoroughly
substantiated. It is expected that since adsorption method is relatively less cost intensive
and feasible option for industrial application, it can be intensified with current industrial
method of HDS to provide viable solution in reducing sulfur emission and meeting the
present norms and regulations. In the present work we have carried out elaborate study on
single component adsorption of thiophenic model fuels namely, benzothiophene in octane
and multicomponent adsorption of mixture of three common organosulfur compounds, viz.,
thiophene, benzothiophene and dibenzothiophene in order to provide fundamental
understanding, the role of adsorbent modification and ways of improving adsorption
efficiency. The sulfur specific adsorbents were then evaluated using synthetic model diesel
comprising thiophene, benzothiophene and dibenzothiophene.
Experimental
Materials And Analysis
Modified carbon based adsorbents, Shirasagi GH2x 4/6, SRCx 4/6 and metal
impregnated forms (Ni/Cu) were studied. The precursors, GH2x 4/6 and SRCx 4/6 were
obtained from Japan Envirochemicals Ltd. GH2x is pelletized form of coconut shell based
formulated for adsorption of sulfur compounds while SRCx is impregnated form of modified
carbon effective for adsorption of sulfur compound. All chemicals, octane, thiophene,
benzothiophene, di-benzothiophene, NiCl2.6H2O and Cu (NO3)2.3H2O used were of high
purity obtained from Sigma-Aldrich. Model fuels were prepared in octane solvent having
known predetermined initial concentrations and batch equilibrium experiments using
predetermined amount of adsorbents were carried out for equilibration time of 6 h.
Adsorption experiments were carried out normally at 300C and also using different
temperatures for investigating effect of temperature. The adsorbents were activated at a
temperature of 2000C for 12 h prior to use. Total sulfur analyser, TN-TS 3000,
Thermoelectron Corporation, Netherlands was used for investigating the residual sulfur
concentration after adsorption. Synthetic fuel is prepared by mixing equal proportions (100
mg.L-1) of thiophene, benzothiophene and dibenzothiophene and 10 mL of the aliquot was
used for batch run using GH2x and SRCx. For analysis of synthetic fuel, Gas
Chromatograph (Agilent GC 7980) with FPD was used.
Preparation Of Metal Incorporated Adsorbents
A series of nickel chloride and copper nitrate salt solutions of different
concentrations range 0.1-0.5 M in 0.01 M HCl were prepared. After the initial screening, we
have selected 0.5 M, as considerable metal impregnation resulted at this particular
concentration as confirmed by EDX data. Further higher concentrations were not attempted
as it may also result in blocking of active sites. 3 g each of GH2x and SRCx were treated
with 25 mL acidic salt solutions and stirred continuously at 120 rpm maintained at a
temperature of 60° C in an orbital shaker for 6 h and thereafter filter and washed to
remove excess acid until the pH of the mother liquor is almost neutral. They are referred as
Ni-GH2x, Ni-SRCx, Cu-GH2x and Cu-SRCx depending on the nature of the precursor and
salt solution we have selected for the purpose of modification. Finally, they are dried at
room temperature and activated at 200 °C for 16 h prior to batch run. The main objective
of this modification is to test whether there is enhancement in adsorption performance over
these already modified forms of activated carbon.
Characterization Methods
FTIR results were investigated for GH2x and SRCx. In the present work, spectrum
for Ni-GH2x using KBr pellet method on Cary 600 FT-IR (Agilent) spectrophotometer is
reported. XRD measurements were carried out by using Pan Analytical Diffractometer in
wide angle range. Scanning electron microscopic (SEM) studies were done by Leo- Leica,
Stereoscan 440, Cambridge, U.K. The BET surface areas of the samples were determined
by N2 adsorption isotherm at 77 K using Quantachrome Autosorb Automated gas adsorption
system. Energy Dispersive analysis of X-ray (EDX), Bruker, Quanrax-200, Berlin Germany
was performed using EDX associated with SEM.
Results and Discussion
Adsorbent Characterization
Characterizations by various technologies were conducted for selectivity analysis and
mechanistic understanding over these modified adsorbents. XRD patterns for GH2x and
SRCx exhibited well resolved peaks at 2θ value of 24 degree and 43 degree, characteristic
of carbonaceous adsorbent matrix. Figure 1 shows the FTIR spectrum of Ni-GH2x. Weak
broad band at 1184.06 cm-1 is the stretching vibration of C-N bond while the medium band
at 1498.40 cm-1 indicates aromatic C=C stretching. Depending on the nature of
modification and the ratio of Al/Si, the peak position for Si-O-Si (Al) which generally present
around 1000 cm-1 is shifted below and observed as a broad peak around 971 cm -1. The
presence of certain functional groups belonging to C-O stretching, N-H bending and OH
group have been indicated in our earlier studies. SEM images of Ni and Cu modified GH2x
are presented in Figure 2. It shows disordered structure with pores of non-uniform sizes.
Aggregates of metal deposition can be seen from the gray and white patches appearing in
contrast. Similar observations can be made from the SEM images of Ni-SRCx and Cu-SRCx,
though Cu depositions were found to be higher compared to Ni forms.
The EDX report indicated the presence of elements like C, O, Si and Al common to
all the four adsorbents while specifically incorporated Ni and Cu are present in Ni modified
and Cu modified forms. Among these metal impregnated forms, Ni forms have nearly 78 %
carbon while the carbon content is relatively lower for Cu forms. Other important
information that could be noted from the data include Al/Si ratio which is comparatively
higher in SRCx forms. Al/Si in Ni-SRCx is 4.55, 0.4 for Ni-GH2x while the ratio remains
almost same in Cu modified forms. Apart from EDX, proximate analysis method of ash
content was measured for Ni modified forms where the % ash content for Ni-SRCx is 1.77
and 1.23 for Ni-GH2x complementing the EDX data for higher inorganic content in Ni-SRCx.
All adsorbents, GH2x and SRCx have high surface area and metal impregnated
forms namely, Ni-GH2x has slightly lower surface area of 1077 m2. g-1 and relatively lower
pore volume of 0.711 cm3.g-1 with average pore diameter of 0.264 nm. These values for
the starting material, GH2x and SRCx are 1329 m2.g-1 and 1098m2.g-1 respectively. The
lower surface area in the Ni modified forms may be due to blocking of the pores.
Figure1. FTIR spectra of Ni-GH2x
A
B
Figure 2. SEM images of A) Ni-GH2x and B) Cu- GH2x
Adsorption studies
Adsorptive desulfurization performances of the different modified adsorbents,
including the adsorptive capacity and selectivity, were examined by using model fuels
containing known sulfur compounds in batch adsorption system.
Figure 3. Adsorption isotherms for benzothiophene on modified adsorbents
As seen from the adsorption isotherms (Figure 3), the order of desulfurization using
benzothiophene as model fuel follows SRCx> GH2x> Ni-SRCx> Ni-GH2x. Adsorption
modelling with Langmuir and Freundlich isotherm showed Freundlich isotherm giving better
fit with very high R2value for benzothiophene model fuel using GH2x, Ni-GH2x and Ni-SRCx
while the Langmuir isotherm fitted well for SRCx. The surface oxygen functional groups
might play an important role in adsorptive desulfurization over carbon based adsorbents.
FTIR studies of GH2x and SRCx also show the presence of surface oxygen groups. The
physical characteristics of the adsorbents like BET surface area, pore volume, pore size
distribution also has crucial contribution in adsorption process. Ni modified forms of GH2x
and SRCx did not show any effective improvement in the removal capacity. On the other
hand, the adsorption behaviour here showed lower adsorption at low concentrations while
capacity at higher concentrations was similar to that without Ni-modification. Similar trends
were indicated in case of Cu-GH2x and Cu-SRCx. It is possible that Ni- or Cu modification
does contribute in the sulfur removal capacity however at the cost of existing active sites
thereby giving no significant improvement in the overall capacity. This probably also
explains the fact that overall capacity remains largely unaffected in spite of possible
blocking of the pores as is evident in decreased surface area of Ni modified adsorbents.
The lower capacity at low sulfur concentrations for Ni- and Cu-modified adsorbents may be
partly attributed the increased reversibility that represents much weaker adsorption at low
concentrations. Since GH2x and SRCx have been specially tailored for removal of sulfur
compounds, further modification either in Ni or Cu form therefore appears to be ineffective
in enhancing its adsorption property or selectivity. In this regard, further research is
necessary.
Figure 4. Temperature effect of benzothiophene adsorption over A) GH2x and B)
SRCx
The study on effect of temperature indicated slight changes on adsorption
behaviour/capacity (Figure 4) in the range 30 to 50 C, clearly indicating role of
impregnation or modification of surface. Although for physical adsorption mechanism, the
temperature is expected to have significant impact, the chemical nature of interactions in
these modified adsorbents have probably dampened the temperature effect and hence not
much difference or adverse effect is seen the case of both GH2x and SRCx, though reduced
capacities at higher temperatures in adsorption is logical.
as:
The selectivity effect on the mixture was investigated in detail using the definition
%S x 
qe ( x )
qe ( x )  qe ( y )  qe ( z )
100
(1)
where, %S x is the % selectivity of the x component in the mixture consisting of x, y, z
components.
Figure 5. Selectivity for sulfur compounds in synthetic fuel
It is evident from Figure 5 that both the modified adsorbents have good capacity for sulfur
removal for refractory compounds such as benzothiophene and dibenzothiophene. Further,
the difference in the adsorption of different sulfur compounds is not significant, especially
at low sulfur concentrations. At higher total sulfur concentration and lower adsorbent dose
(0.1 g), the effect of selectivity over GH2x and SRCx adsorbent is higher for
dibenzothiophene, followed by benzothiophene and thiophene while the effect is less
pronounced with the increased adsorbent dose or lower total sulfur concentration at
equilibrium. The nature of modification and selectivity towards the refractory sulfur
compounds clearly underlines the utility of such adsorbents in obtaining ultra-clean fuels. It
also underlines the importance of understanding surface interactions that dictate the high
capacity for such adsorbents over other conventional adsorbents such as activated carbons.
The results of deep desulfurization, both in terms of higher capacity and surface
modifications indicate promising advantage for commercial application.
Conclusion
The adsorption performance of modified carbons and metal loaded adsorbents was
studied to obtain fundamental understanding of adsorption mechanism and selectivity. The
adsorption results showed that SRCx exhibited higher adsorption capacity followed by
GH2x, Ni- or Cu- modified GH2x and SRCx respectively for single component adsorption.
Adsorption using synthetic fuel comprising three refractory sulfur compounds showed
selectivity order of dibenzothiophene > benzothiophene > thiophene for GH2x and also for
SRCx. It was observed that metal modification did not significantly improve the adsorptive
capacity probably due to increased reversibility and possible blocking of active sites already
present in modified surfaces. With high capacities and selectivities for refractory sulfur
compounds, combination of adsorptive desulfurization with hydrodesulfurization process
may prove to be beneficial for achieving ultra-clean fuels.
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