Version of Record: https://www.sciencedirect.com/science/article/pii/S0378382017301637
Manuscript_521d517099bd82fe641aacbf3c8506b2
Zeolites for adsorptive desulfurization from fuels: A review
Roghaye Dehghan, Mansoor Anbia*
Department of Chemistry, Research Laboratory of Nanoporous materials, Iran University of
Science and Technology, Farjam Street, Narmak, P.O. Box 16846-13114, Tehran, Iran
Abstract
Removal of aliphatic and aromatic sulfur compounds from fuel by adsorption on zeolites, has been
reviewed. Zeolites can be loaded with different metal ions such as Fe2+, K+, Ag+, Cu+, Ni2+, Zn2+, and Ce
4+
and Pd 2+ via ion-exchange or impregnation methods. Modified zeolites with these metal ions increase
their adsorption capacity and selectivity. Specially, Ce4+and Pd2+ shows a selectivity towards the sulfur
compounds in the presence of the other compounds such as aromatics, O-containing fuel additives and
nitrogen compounds. Three types of adsorptive desulfurization include reactive adsorption, selective
adsorption, and π-complexation have been used by zeolites for the removal of sulfur compounds. Thermal
and solvent regeneration of zeolites in adsorptive desulfurization process have been discussed in detail.
Adsorption methods of the sulfur compounds on the zeolite, the charge of metal cations, texture
properties of the zeolite, the numbers of active sites on the frameworks of zeolite, acid properties of
zeolites, SiO2/Al2O3 ratio and the pore size have a significant impact on the adsorption of sulfur
compounds. X and Y zeolites have been widely studied for adsorption of sulfur compounds, due to their
tuneable selectivity regarding polar molecules and pore size. Zeolites have been shown good sulfur
loading capacity, good regenerability and stable structure for removal of sulfur compounds.
Keywords: Adsorption; Zeolite; Desulfurization; Fuel; Review.
1. Introduction
*
Corresponding Author: anbia@iust.ac.ir Tel: 0098 21 77240516 ; Fax: 0098 21 77491204
1
© 2018 published by Elsevier. This manuscript is made available under the Elsevier user license
https://www.elsevier.com/open-access/userlicense/1.0/
In the recent years, removal of aliphatic and aromatic sulfur compounds from liquid fuels (gasoline, jet
fuel, diesel fuel), gaseous fuels (natural gas, reformates and syngas) and liquid petroleum gas (LPG) has
received considerable attention by both the scientific community and the petroleum refineries. Table 1
shows some of structure of aliphatic and aromatic sulfur compounds in fuel. Combustion of sulfur
compounds in fuels leads to SOx emissions, which causes important health problems and are precursors
of the acid rain. Furthermore, sulfur concludes catalysts deactivation in the downstream refining
processes, corrosion problems in pipelines, pumps, and refining equipments. Total sulfur in crude oil can
vary between 0.05 and 6wt. % depending on the type and source of crude oil[1].
Table 1
The sulfur concentrations in transport fuels such as gasoline and diesel according to the environmental
regulations should be lower than 10 ppm[2]. There are several approaches such as adsorptive
desulfurization[3], extractive desulfurization [4], bio-desulfurization (BDS)[5], hydrodesulfurization
(HDS), Oxidative desulfurization (ODS) and precipitative desulfurization to reduce sulfur content in
fuels. These technologies are outlined in Fig. 1. The full details of these technologies have been
investigated by Tawfik et al[6].
Fig. 1
Among these methods used for desulfurization, hydrodesulphurization (HDS) is efficient in the
removal of most aliphatic sulfur compounds such as mercaptanes, sulfides and disulfides from fuels.
Nevertheless, it is not efficient in the removal of aromatic refractory sulfur compounds such as thiophene
derivatives. H2S generated in some thiophene compounds reaction is a major inhibitor for HDS[7, 8]. In
addition, it requires high temperature, pressure and high dosage of catalyst before achieving the desired
objective which is uneconomical[6]. The adsorption process has become a promising approach in the
ultra-deep desulfurization, because it is easily available, environmentally friendly, economical method
and being able to decrease content of sulfur to less than 1ppm [9]. Current area of studies in adsorptive
desulfurization focuses on the development of a novel and adequate adsorbent which has high adsorptive
capacity, high selectivity and can be easily regenerated[3]. A wide variety of adsorbent such as carbonbased sorbents[10], metal adsorbents (reduced metals, metal oxides and metal sulfides),[11-15]silicabased sorbents[16-18], metal-organic frameworks (MOFs)[19, 20] and zeolites have been reported.
Advantages of the Carbon-based sorbents such as activated carbons are that, sources for preparation of
them are wide and the production cost is relatively cheaper for removal of sulfur compounds. However,
lack of low thermal and mechanical stability are disadvantages of these materials[21]. Ordered
mesoporous carbons (CMKx) are another group of Carbon-based sorbents which are typically
2
biocompatible and quite chemically stable under nonoxidizing conditions and exhibits mechanical
stability but the hydrophobic and inert nature of mesoporous carbons can be unfavorable for adsorptive
desulfurization applications[22]. One of drawbacks of metal adsorbents is that they are used with
expensive components (metal or support) in significant quantities. Further, regeneration of them for
subsequent runs is relatively more difficult and also adsorption capacity of them is lower than that of
zeolite based adsorbents[23, 24]. MOF adsorbents have good adsorption selectivity and capacity for
adsorptive desulfurization. In 2008, Matzger et al. reported adsorptive desulfurization by the MOFs
HKUST-1 (also known as Cu-BTC), UMCM-150, MOF-5, MOF-505, and MOF-177[25]. In their report,
the adsorption capacities were 0.38 mmol/g of BT using MOF-5 and 0.45 and 0.19 mmol/g of DBT and
DMDBT, respectively, using UMCM-150. These amounts were higher than those obtained previously by
Na-Y zeolite. This observation was due to the MOFs offer the higher surface area and pore volume than
zeolites adsorbents. However, high cost of organic precursors for preparation of these adsorbents and
thermal/hydrothermal instability makes them difficult to be used in industry. Recent progressions in
adsorptive desulfurization with metal-organic frameworks (MOFs) are thoroughly discussed by Ahmed
and Hwa[26]. Aiming at selection of adequate adsorbent which can
have a high selectivity, high
adsorption capacity, regenerability and safe operations, zeolites are effective for removal of sulfur
compounds[27]. There are more than 200 unique zeolite frameworks and naturally occurring zeolites
which have different shapes (channels, cages, cavities, etc.) and sizes[28]. Applications of zeolites are
numerous. Zeolites can be used not only as adsorbents, but also as catalyst, separation media, catalyst
support and animal feeding. These applications depends on the presence of suitable active centers, size,
and geometry of cavities inside the zeolite. Various outstanding reviews on different application of
zeolites such as removal of H2S[29], upgrading of bioethanol to fuels[30], benzene removal from
gasoline[31] pyrolysis of biomass[32] anaerobic digestion processes[33] removal of volatile organic
compounds [34], wastewater treatment[35] crop protection [36] and aquaculture industry [37], have been
published. However, there is no review report on adsorptive desulfurization of zeolites from fuels. The
aim of this review is to describe the recent progress in adsorptive desulfurization of fuel by using some of
natural zeolites such as Clinoptilolite and some of synthesic zeolites, including FAU(X and Y), LTA,
ZSM-5 and Beta with respect to increasing demand for producing sulfur free fuels. In the first part of the
study, the possible mechanisms of adsorptive desulfurization by zeolites have been reviewed. In the
second part of the study, the use of zeolites in adsorptive desulfurization and in the last part, thermal and
solvent regeneration of zeolites in adsorptive desulfurization process have been discussed in details.
2. Mechanisms of absorptive desulfurization
Investigation of the interaction between sulfur compounds and zeolites are necessary to guide the design
and preparation of suitable adsorbent for adsorptive desulfurization. Van der Waal’s forces, chemical
3
affinity and electrostatic attraction are responsible for adsorption. Adsorptive desulfurization is based on
physisorption or chemisorption process of organosulfur compounds on adsorbents[38]. There are various
forms of adsorptive desulfurization include reactive adsorption, polar adsorption, selective adsorption,
integrated adsorption and π-complexation. Among these forms of adsorptive desulfurization, πcomplexation and selective adsorption are widely used for adsorptive desulfurization by zeolites. In
addition, reactive adsorption by zeolites has been reported in few studies. Tawfik et al. studied full details
of these forms adsorptive desulfurization[3].
2.1. π-complexation mechanism
Zeolites loaded with different metal ions such as Fe2+, K+, Ag+, Cu+, Ni2+, and Zn2+ or Pd2+ cause πcomplexation (chemichal complexation) between metal ions and sulfur compounds. The metal ions form
σ-bonds with free s-orbitals and the d-orbitals will back-donate electron density to the antibonding πorbitals in the sulfur containing ring of thiophenes. Bonds formed due to this interaction, are stronger and
yet easily broken by alternating the temperature or pressure which enhances the capacity and selectivity to
sulfur compounds[10]. π-complexation yields better results compared to the normal vanderwaals
interactions occurring in adsorption studies[3]. It should be pointed out that although zeolites ion
exchanged with metal ions increase the selectivity and adsorption capacity of sulfur compounds, but the
loaded metal ions are washed away during the procedure of adsorption- desorption desulfurization.
Therefore, the stability of zeolites ion exchanged with metal ions is reduced.[39]. Many studies have been
done on the π-complexation by zeolites. In the following, the zeolites used in π-complexation form have
been mentioned.
2.2. Selective Adsorption
Selective adsorption form removes sulfur compounds from fuels, which constitute only less than 1%
of the fuels. For selective adsorption mechanism, adsorbents need to be effective, selective and
appropriate for sulfur removal. Nickel based sorbents and air regenerable metal oxide based sorbents
are the most common adsorbent materials used in this process[3]. Ce4+ ion, with the valence electronic
configuration 4f05d06s0, has high positive charge and polarizability. Sulfur compounds are adsorbed
over zeolites ion exchanged with Ce4+ by a direct S–adsorbent (S–M) interaction.
2.3. Reactive adsorption
Reactive adsorption form removes the sulfur compounds by chemical interaction between the fuel and
the sorbent material. The adsorbents are regenerated by disposing off the sulfur compounds in the form of
SO2, H2S, or elemental sulfur depending on the method employed. This process can be carried out at
ambient conditions, at high temperatures and with the aid of H2 at high temperatures. This process is
extensively used as the S Zorb process at high temperatures (340 -410°C) and low pressure of H2 (2-20
4
bars). Reactive adsorption desulfurization has the advantages of both the catalytic HDS and adsorptive
desulfurization and thus highly efficient for deep desulfurization.[3]. Nanoti et al.[40]reported an
investigation into the reactive adsorptive removal of thiophenic sulfur from naphtha range hydrocarbons
with a series of single and double metal ion exchanged zeolites at elevated temperatures. The unit consists
of an adsorption column of 19 mm internal diameter placed inside a three zone electric furnace Fig. 2.
Fig. 2
In the fixed bed vapor phase adsorption studies, the adsorption column was packed with 10 g of the
adsorbent. Adsorption experiments were done using zeolites ion exchanged with Cu(II), K(I),
Zn(II),Co(II), Ni(II), Mn(II) and Ce(VI). Both model mixture of 2-methylthiophene in n-hexane
containing sulfur concentration of 1000mg/L as well as a refinery hydrotreated naphtha containing180
mg/L were used as feed. After the adsorption cycle, desorption was taken through burning off the
adsorbed sulfur with mixture of air–nitrogen at 350 °C followed by a hydrogen activation phase at the
same temperature. The efficiency of desorption was tested through breakthrough curve measurement with
a model feed mixtures of 2-methyl thiophene in n-hexane (500 mg/L) through repeated cycles of
adsorption–desorption. During the adsorption cycle, liquid hydrocarbon feed was pumped into a preheater and then the vapors were mixed with hydrogen and nitrogen and flowed into the adsorption
column. Gas and liquid streams of the effluent through a condenser were separated. The sampling of the
desulfurized liquid product at constant time interval was down once the liquid level in the gas–liquid
separator reached a predetermined level corresponding to a hold up of 75 mL in the separator. Total sulfur
content were analyzed by X-ray/UV fluorescence and UV spectroscopy. Breakthrough measurements
were carried out on the single and double ion exchanged Y-zeolites at 45°C using feed mixtures of 500
mg/L thiophene in ethylene, to calculate the adsorption capacities of the adsorbents. Results showed that
the behavior of single and double ion-exchanged zeolites differed greatly with sulfur adsorption capacities
ranging from below 10–60 mg/g. Ion exchanged zeolites (Nine zeolite)were screened based on high
throughput combinatorial chemistry using a model FCC gasoline mixture and evaluated further in vapor
phase adsorption conditions in a fixed bed adsorption set up. The volume of feed treated per gram
adsorbent at effluent sulfur concentration of 150 mg/L was interpolated and the comparative data for the
nine adsorbents showed that four adsorbents, namely Cu–Ni–Y, Zn–Y, Cu–Mn–Y and Cu–Ce–Y had
high levels of 44–54 mL feed treated per gram adsorbent. Two of these adsorbents, namely Cu–Mn–Y
and Cu–Ni–Y that had high capacities for desulfurization from model feeds were selected for studies with
an actual refinery hydrotreated naphtha. Breakthrough data with this naphtha as feed were the same as for
5
model feed mixture except that the temperature was increased from 250°C to 350° C due to the higher
final boiling point (FBP) of the naphtha (220°C).
The sulfur levels were reduced with Cu–Mn–Y and Cu–Ni–Y from 180mg/L to<30 mg/L under
conditions similar to typical conventional naphtha hydrodesulfurization processes. The adsorbent Cu–
MnY could treat over 54mL naphtha feed per gram adsorbent before the effluent exceeds 30mg/L. A
comparison of the breakthrough data of these adsorbents for an effluent sulfur concentration of 30 mg/L
indicated that over 31–54 mL feed was treated per gram adsorbent. Ma et al.[41] reported desulfurization
from a real gasoline containing 210 mg/L sulfur, at elevated temperatures of 200 °C with Ni based
adsorbent. Results indicated that Cu–Mn–Y and Cu–Ni–Y can treat higher volumes of naphtha. Cu–Mn–
Y adsorbent capable of treating more than twice the volume of feed per gram adsorbent compared to Ma
et al[41]. Generally, results demonstrated that it is possible to lower the sulfur levels in a hydrotreated
naphtha from 180 mg/L to less than 30 mg/L by vapor phase adsorption on exchanged zeolite at 350 °C in
presence of small amounts of hydrogen.
3. use of zeolites in adsorptive desulfurization
3.1. Natural zeolites
Until now, about fifty natural zeolites are known. Commercially significant natural zeolites include
mordenite (MOR), chabazite (CHA), and clinoptilolite (HEU)[42]. Among them, Clinoptilolite is the
most studied of all zeolites and it is considered as adsorbent for deep desulfurization. In this regard,
Mahmoudi and Falamaki used Ni2+ion exchanged dealuminated clinoptilolite for removal of some
sulfur compounds such as thiophene (T), benzothiophene (BT), dibenzothiophene (DBT) and iso-propyl
mercaptan (IPM). Oxalic acid was used as dealumination reagent. Si/Al molar ratio of raw clinoptilolite
was 5.65. After dealumination and the ion-exchange process with Ni2+, deep desulfurization with zeolite
powder (raw, dealuminated or dealuminated/ion-exchanged or solely ion-exchanged) was performed.
The Powders were exposed at 300 °C in an electrical furnace for 2 h. Then cooling was performed at
100°C and immediately isolated in 50 cm3 glass bottles. The adsorption process was performed by
shaking at 20°C for 1.5 h. The adsorption capacity of the optimum zeolite (0.3 oxalic acid molarity and
dealumination treatment time of 120 min) for IMP, T, BT and DBT were 10.10, 6.33, 3.60 and 2.70 mg
s/g. The saturation capacity of IMP for NiX and NaX has been reported 12.3 and 10.2, respectively[43].
Considering this fact that, Clinoptilolite is a natural zeolite and it is not pure, these saturation capacity
are reasonable. Adsorptive properties of sulfur compounds as a function of Si/Al molar ratio by
considering the strong correlations between the crystallinity and Si/Al molar ratio and also, Ni content
6
and the Si/Al ratio were investigated. Maximum removal at a specific Si/Al molar ratio near 10.50, both
for BT and DBT were achieved. An outstanding point of their study was that for the parent raw zeolite
(Si/Al=5.65) and its Ni-exchanged form, the sulfur removal percentage was minimal (ca. 5.4%).
However, this value was increased strongly to 43% for DBT at the optimum Si/Al ratio of 10.40 after
dealumination. With dealumination the micropore volume, external surface and adsorption capacity
were increased due to the new internal and external surface area creation and formation of
mesoporosity. These results were achived for BT, with an initial adsorption of ca. 8% for the raw and
Ni-exchanged raw zeolite to 68% after dealumination (Si/Al ratio=10.40). In addition, effect of Ni ions
in the adsorption characteristics without dealumination treatment was investigated. Results showed that
the creation of π-complexation bonds between the sulfur atoms and the empty d-orbital of the Ni atoms
in the zeolite had a determining factor in increasing its sulfur adsorptive characteristics[44].
3.2. FAU zeolite
The framework for FAU type zeolite are built by linking sodalite cages through double six-rings. This
creates a large cavity in FAU called the “supercage” accessible by a three-dimensional 12 -ring pore
system. X and Y Zeolites are two major types of synthetic forms of the framework for FAU type zeolite.
Zeolite X refers to zeolites with Si/Al ratios between 1 and 1.5 and Zeolite Y refers to zeolites with Si/Al
ratios higher than 1.5. X and Y zeolites have been widely studied for adsorption desulfurization, due to
their tuneable selectivity regarding polar molecules[42].
3.2.1. X zeolite
3.2.1.1. X zeolite exchanged with Na+, K+ and Cs+
Sotelo et al.[45]prepared agglomerated FAU zeolites with different Si/Al molar ratios and modified these
zeolites with exchange cations Na+, K+ and Cs+ for fixed bed adsorption of benzothiophene (BT)
dissolved in cyclohexane. Powder FAU zeolites were agglomerated with sodium bentonite (5/1, w/w).
Agglomerating was performed before use in fixed bed experiments in order to reduce the pressure drop.
Equilibrium adsorption experiments were carried out with the mixture of the benzothiophene (BT)
dissolved in cyclohexane (5 g) with different weights of adsorbent at 298 K during 2 days. After
adsorption and desorption, BT concentration was measured by gas chromatography (GC). Dynamic
adsorption tests were carried out with packing 3 g of adsorbent (bed length ca. 0.3 m) in a stainless steel
pipe with an internal diameter of 4.9 mm and 0.5 m length at 298 K and constant flow rate of 5 mL/min.
The content of sulfur was 250 ppmw. Results of equilibrium adsorption experiments indicated that BT
adsorption isotherms onto agglomerated low silica NaX zeolite(A-NaLSX), agglomerated NaX zeolite
(A-NaX) and agglomerated NaY zeolite (A-Na-Y) were of type I isotherm. Experimental data was fitted
7
to the Langmuir adsorption isotherm. Also, the influence of the Si/Al ratio on adsorption capacity of ANaLSX (Si/Al molar ratio:1.07), A-NaX (Si/Al molar ratio:1.27)and A-Na-Y(Si/Al molar ratio:2.85) was
investigated. The adsorption capacity is usually related to the pore volume and BET surface areas of the
adsorbent. Therefore, A-NaLSX adsorbent with medium pore volume and surface area will expect to have
medium adsorption capacity. Nevertheless, results indicated that A-NaLSX zeolite, with a lower Si/Al
molar ratio, had the lowest maximum adsorption capacity (35.50 mg/g) compared to that of A-NaY and
A-NaX which were 44.1 and 48.3 mg/g respectively. This is due to the blocking of the pore windows with
strongly adsorbed BT that hinder the adsorption of the other BT into the inner pores, because of higher
polarity and basicity of A-NaLSX zeolite. In the following study, the influence of the exchanged cations
on the maximum adsorption capacity were investigated. Two different adsorption mechanisms onto these
adsorbents were hypostasized. The first mechanism is the interaction of the π electron cloud of the BT
molecule with the cations, which is higher for cations with high electronic affinity such as sodium. Also,
the interaction of the hydrogen atoms which are in a BT molecule with the basic oxygen atoms of the
zeolite framework, which is higher for adsorbents exchanged with caesium. In other words, as the cation
electronegativity is lower, its charge density and the interaction with the π electron cloud of he BT
molecule decrease, although the basic character of the adsorbent is enhanced, since the interaction of
basic oxygen with hydrogen atoms is stronger.
3.2.1.2. X zeolite exchanged with La and Cu
In 2012, Tong et al[46]studied the selectivity adsorption of thiophene alkylated derivatives over the Cu+13X and La3+-Cu+-13X
zeolites by static adsorption equilibrium.
Model Gasoline used in this
experiments were Thiophene (TP), 3-methylthiophene (3-MT), 2, 5-dimethylthiophene (2,5-DMT,), and
benzothiophene (BT) dissolved in n-hexane. Single and mixed binary components (TP/BT, 3-MT/BT, or
2, 5-DMT/BT) were used for investigation of adsorption performance. The initial sulfur compounds
concentration and volume were 1000 µg/g and 100 ml, respectively. Before experimental, an appropriate
amount of adsorbents were heated at 380 °C for 180 min under N2 gas and then cooling was performed.
Static adsorption equilibrium experimental were performed in 100 ml Erlenmeyer flasks with different
concentrations of sulfur compounds for 48 h reaction time. Residual sulfur concentration was determined
by gas chromatography at intermittent time. Results of adsorption isotherms of single component on Cu+13X and La3+-Cu+-13X at 25 °C showed that the adsorption isotherms were fitted by the Langmuir.
Amount of adsorption for BT, TP, 3-MT and DMT with Cu+-13X zeolite were 179.8, 146.2, 140.5 and
135.8 mg/g, respectively. In addition, these amounts for La3+-Cu+-13X zeolite were 197.3, 159.2, 153.5
and 151 mg/g, respectively. This amount of adsorption for La3+-Cu+-13X was higher than that of Cu+13X. According to the studies performed [47] the ϭ-bond and d-л the anti-bond are the main factor of the
thiophene and the benzothiophene adsorption, respectively. The d-л anti-bond is stronger than the ϭ-bond,
8
which causes the larger adsorption amount of benzothiophene as compared with the thiophene. The lower
sulfur adsorption capacity of DMT was due to the decreasing of steric volume by methyl groups. These
results were based on three factors: increased reactivity of adsorbents by loading La element, enhanced
adsorption ability, and reduced activation energy of adsorbents. Results of their studies on adsorption
isotherms of the binary components on adsorbents revealed that the total adsorption amount for the binary
components is smaller than single component due the competitive adsorption in the binary components.
Adsorption capacity for La3+-Cu+-13X zeolite was higher than Cu+-13X similar to that of the single
component. The synergistic effect of La3+and copper ions into molecular sieves can cause the high
adsorption capacity. In addition, insertion of La3+into zeolite causes the stretching of the framework of
zeolite, and changes the pore size, and creates more acidic centers for accommodating larger sulfur
compounds. In addition, Kinetic adsorption curves were investigated. The diffusion of thiophene sulfur
compounds inside zeolite involves external diffusion, internal diffusion, and internal surface diffusion.
As the model sample was stirred continuously during the experiment, the internal diffusion was
considered as control step. Results of kinetic adsorption showed that diffusion coefficient of
benzothiophene adsorption on the La3+-Cu+-13X increases greatly as compared with that on the Cu+13X. Its value is greatly influenced by the distribution, size and charge amount of cations in structure of
zeolites. The insertion of La cations greatly increases the amount of adsorption sites, and thus both of
thiophene and benzothiophene in model sample can be adsorbed effectively. The kinetic adsorption on
Cu+-13X at the initial stage showed, thiophene adsorption in binary components was faster than
benzothiophene. Later, the adsorption rate of benzothiophene was faster than thiophene. For La3+-Cu+13X adsorbent, total adsorption amount of benzothiophene was larger than thiophene.
Table 2 lists the adsorption capacity of sulfur compounds on X zeolites from fuel. These zeolites have
been demonstrated good adsorption capacity for sulfur compounds, which are demonstrated as promising
adsorbents for deep desulfurization of fuels. However, these zeolites have the lower sulfur adsorption
capacity of DMT due to the steric volume by methyl groups. Creation of mesopore in the structure of
these zeolites can be overcome to the diffusion limitation. Further researches should be focused on the
study of creation of mesopore in the structure of zeolites for desulfurization.
.
Table 2
3.2.2.
Y zeolites
3.2.2.1. Y zeolite exchanged with Ag+, Ag0, H+, Na+, and Ag2O clusters
9
Lee et al.[48]studied Na-Y and AgNa-Y zeolites for adsorptive removal of tetrahydrothiophene (THT)
and tert-butylmercaptan (TBM) in pipeline at ambient temperature and atmospheric pressure. The effect
of various adsorption sites (Ag+, Ag0, H+, Na+, and Ag2O clusters) and their contributions on THT and
TBM adsorption were investigated. Three CH4-balanced feeds containing (1) 100 ppm THT, (2) 100 ppm
TBM, and (3) mixture of 100 ppm THT and 100 ppm TBM were used. Adsorption experiments were
carried out at 303 K and atmospheric pressure using fixed bed glass reactors (i.d. = 9 mm). After
pretreating the adsorbents, desulfurization was carried out at flow rate 55 cm3/ min. The loading amount
of the AgNa-Y was adjusted (0.10–0.18 g) in order to give the gas hourly space velocity (GHSV) at 1.0 ×
104 h−1 (NTP). Residual sulfur concentration was analyzed using a gas chromatograph fitted with a flame
ionization detector (FID). Results of single component uptake on AgNa-Y showed the breakthrough THT
uptake was changed from 0.38 to 0.60 THT mol/molAl with altering Ag/Al mol ratio from 0 to 0.99.
Furthermore, the total THT uptake on AgNa-Y was 0.81–0.99 mol THT/mol Al which was close to Na-Y
(0.85 mol THT/mol Al). Whereas, the breakthrough TBM uptake for AgNa-Y and NaY were 0.34 and
0.04 mol TBM/mol Al, respectively. The total TBM uptake on AgNa-Y samples was 0.57–0.65 mol
TBM/mol Al which was similar to Na-Y (0.53 mol TBM/mol Al). The total THT and TBM uptake on
AgNa-Y did not depend on the Ag+-exchange level whereas it increased significant enhancements in the
breakthrough TBM uptake. These results were due to the condensation of THT and TBM in the zeolite
pores, which depend largely on the size and volatility of adsorbate molecules. Furthermore, results of
adsorption strength investigations of THT on the sites developed on AgNa-Y were in order of Ag+ > Na+
≅Ag0 > H+ > Ag2O. THT was adsorbed on Na+ sites with sufficiently high adsorption strength, thus Ag+
resulted no net increase in the breakthrough THT uptake. Differently, the adsorption strength of TBM on
Na+ sites was weak, whereas on Ag+ sites was strong. This was led to a marked increase in the
breakthrough TBM uptake with an increase in Ag+-exchange level. The adsorption strength of THT was
much higher than that of TBM regardless of the nature of the sites formed in AgNa-Y. This was resulted
in nearly 100% total adsorption uptake selectivity for THT over TBM, when these two sulfur species
coexist in the feed stream.
3.2.2.2. Y zeolite exchanged with Cu, Zn, Ag and the combined Cu2+-Zn2+, Zn2+-Ag+, Ni2+-Nd3+
Zhang
et
al.[49]
studied
adsorptive
removal
of
dibenzothiophene
(DBT)
and
4,6
dimethyldibenzothiophene (4,6-DMDBT) on the various ion-exchanged Na-Y zeolites (with single Cu2+,
Zn2+, Ag+ and the combined Cu2+-Zn2+, Zn2+-Ag+, Ni2+-Nd3+). After Pre-oxidation of sulfur compounds,
adsorptive removal of sulfur compounds were carried out using 1g zeolite adsorbent in the n-octane
solutions (50 ml) with certain concentration of sulfur compounds under continuous stirring at ambient to
80°C for reaction time 1–24 h. Results of reaction time showed that DBT can be removed in large
amount from 1500ppm in the feed to 300ppm in the product in the first 30min, and the sulfur reduction
10
reached 42mgS/g. The residual sulfur concentration was further decreased to ca.220ppm (reduction
S=45mgS/g) after 1h adsorption and keep almost unchanged with extended period of adsorption. Results
of adsorptive removal of DBT on various modified Y adsorbents are presented in Table 3.
Table 3
Result showed that the Ag-Y and Cu-Zn-Y adsorbents have higher adsorption abilities than the other
adsorbents. Adsorption ability is dependent on the sort of metal ions exchanged and the synergy between
the co-exchanged metal ions. In addition, the adsorption of different sulfur compounds on Ag-Y and CuZn-Y was investigated. Results are presented in table 4 and 5. The adsorption performances of various
sulfur compounds obtained on Cu-Zn-Y were almost identical to those obtained on Ag-Y.
Table 4
Table 5
Results revealed that the adsorptive removal of 4,6-DMDBT on Ag-Y is as effective as that of
DBT. This is due to the little steric effect of 4,6-dimethyl group which ceases the π-complexation
predominant interaction between 4,6-DMDBT and Ag+ site. Also, results showed that the benzene or
toluene in the both solution of DBT and 4,6-DMDBT cease competitive adsorption via π -complexing.
The order of desulfurization efficiency were direct adsorption > oxidation-adsorption > direct-oxidation.
The pre-oxidation modifies the structural configuration and the polarity of DBT and 4 ,6-DMDBT
molecules. Because of the presence of S=O bonds in the oxidized DBT and 4,6-DMDBT, oxygen atoms
directly were coordinated with the exchanged cations via ϭ interaction. Although Ag-Y showed the
promising performance for the removal of DBT and 4,6-DMDBT, but, it is not very stable for practicable
use.
In another study, Seyedeyn-Azad et al.[50] used Cu(II)-Y type Zeolite for removal of mercaptans
from two types of oil cuts, namly L-SRG and L-Naphtha, from Isfahan Refinery (Iran) in a batch process.
The effects of some parameters such as ion-exchange level (69.8, 80.2, 91.02 and 101.81), the mass of
zeolite (0.05, 0.1 and 0.15 g), and contact time (5, 10, 30 and 60 min) on the removal of mercaptan (or
RSH) were investigated at ambient temperature. The numbers of experiments were minimized by using
Taguchi method. In each run, a certain amount of oil cut was contacted with a certain amount of
adsorbent for a certain time. The amounts of mercaptans were measured by a potentiometer before and
after of contacting the samples with the oil cuts. Results of mercaptans removal from 400 ml of L-SRG as
a function of contact time showed that the percentage of removal by Cu–Y zeolite is increased from 25%
to 66% after increasing contact time from 5 to 60 min. The percentage of removal was enhanced to only
5% while the contact time was increased from 5 to 60 min For the Na–Y zeolite. Results of mercaptans
removal from 400 ml of L-Naphtha as a function of contact time showed that the percentage of removal
by Cu–Y zeolite was increased from 30.5% to 74% while the contact time was enhanced from 5 to 60
11
min. For Na–Y zeolite, the percentage of removal was only increased from 8 to 23% after increasing
contact time from 5 to 60 min. Results of optimization of parameters revealed that in 400 ml of L-SRG,
56% mercaptans were removed when copper ion-exchange level was 70%. 67% of mercaptans were
removed when 0.25 g of Cu(II)-Y zeolite was used and also, 66% of mercaptans were removed in 60 min.
These results for 400 ml of L-Naphtha with 70% ion-exchange level, 0.3 g adsorbent and 60 min time
were 80.2%, 65% and 74% removal, respectively. From the results, it is obvious that Cu(II)-Y zeolite was
more effective for the mercaptans removal compared to the parent Na–Y zeolite. Mercaptan reacts with
Cu cations as follows;
2Cu2+ + 2RSH→RSSR + 2Cu+ + 2H+
2Cu++ 2RSH→2RS−Cu + 2H+
2RS−Cu→RSR + Cu2S
RS−Cu complexes are stable at low reaction temperatures and remain in the zeolite. Decomposition of
these complexes to monosulfides and Cu2S is unlikely at low temperatures, but cannot be excluded
without the more detailed analysis of the reaction products [51, 52]
3.2.2.3. Y zeolite exchanged with Cu and Ce
Shan et al.[53]synthesized Cu-Ce bimetal ion-exchanged Y zeolites for selective adsorption of thiophene
(T), benzothiophene (BT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT). Adsorptive desulfurization
experiments were performed by a batch method under ambient conditions. The model fuel and the
adsorbent were mixed in a flask with stirring for 4 h. After adsorption, the liquid phase was separated by
filtration, and the sulfur content was determined using the DL-2B-EE microcoulometer. Adsorption
isotherms of the adsorbents were investigated. Results showed that the sulfur adsorption capacities over
CeIVY, CuIY, and CuCeY were 0.37, 0.40, and 0.42 mmol/g, respectively in a model fuel without toluene.
Results also revealed that the adsorption selectivity trend of CuCeY and CuIY for the three sulfur
compounds was in the order of T < BT < 4,6DMDBT. CuCeY exhibited a much higher 4,6DMDBT
adsorption capacity (at least 5-fold) than CeIVY. The Ce ions accelerate the conversion of Cu2+ to Cu+ and
enhance the concentration of Cu+ on the CuCeY surface and and Cu+ is needed for π-complexation
adsorption. Also, effect of toluene on adsorptive desulfurization was studied. Results indicated that
toluene has a less effect on thiophene adsorption over CuCeY than that over CuIY. By increasing the
toluene concentration to 20wt%, the sulfur adsorption capacity was decreased to 0.33, 0.09, and 0.35
mmol/g over CeIVY, CuIY, and CuCeY, respectively. This is due to the sulfur compounds were adsorbed
over Ce-exchanged Y zeolites via direct sulfur-adsorbent interaction rather than via π-complexation.
In another study, Song et al.[54]studied deep adsorptive desulfurization over CuIY, CeIVY and
CuICeIVY molecule sieve in a fixed-bed unit. 1-octane solution of thiophene and benzothiophene and a
12
certain amount of toluene or cyclohexene was used as Model gasoline. Composition of model gasoline are
shown in Table 6.
Table 6
For adsorption experiments in a fixed-bed unit, initially, the adsorbents were loaded inside the stainless
steel tube and heated in situ at 613 K in a flowing H2 gas. After activation of the adsorbent, cooling was
performed to room temperature under H2 gas. Then, a sulfur-free hydrocarbon was passed through a
column packed to eliminate any entrapped gas. The adsorbent was wetted for several minutes, and then
the feed was switched to the model gasoline. Effluent samples were analyzed with a flame photometric
detector-gas chromatograph. In all the cases, the adsorbent quantity was 1 g. A predetermined constant
flow rate 0.28 mL/min was used. Results of adsorptive desulfurization of the model gasoline showed that
CuIY and CuICeIVY had a strong capacity for sulfur removal in model gasoline M1 (100 mg/L sulfur
(CSulfur) of TP and BT). These adsorbents offered deep desulfurization of about 200 mL model gasoline
per gram of adsorbent for BT (sulfur concentration below 5mg/L). Moreover, CuICeIVY showed a higher
capacity for sulfur removal than the CuIY/CeIVY. For CuIY and CuICeIVY, the loadings at breakthrough
were about 0.605 wt.% and 0.496 wt.% for TP as well as 2.422 wt.% and 2.096 wt.% for BT, and
loadings at saturation were about 0.819 wt.% and 0.762 wt.% for TP as well as 3.261 wt.% and 3.072
wt.% for BT, respectively. In these studies, they reported that the color of the CuIY and CuICeIVY zeolite
changed from pale white to French gray and gray during the adsorbing process of model gasoline M1
respectively, while the color of the CeIVY zeolite adsorbing the model gasoline M1 changed from light
yellow to black. The differences in color likely showed differences in adsorption mechanism for sulfur
removal. Results of adsorption desulfurization with the model gasoline M5 on the CuICeIVY showed that
the order of the selectivity for the removal of sulfur compounds were BT>2,5-DTP>3-MTP>TP. In
addition, the effect of aromatic compounds on TP and BT adsorption over the CuIY, CuICeIVY and CeIVY
were studied. The results showed that the influence of toluene and cyclohexene on the CuICeIVY and
CeIVY adsorbents were not obvious. The drop of sulfur loadings were decreased in the order: CuIY >
CuICeIVY > CeIVY. Furthermore, CuICeIVY had a high selectivity for sulfur compounds. The
breakthrough results with model gasoline M3 contains pyridine showed that for all the adsorbents had a
sharp drop in sulfur removal capacity. The effect of aromatic compounds on TP and BT adsorption on the
metal ion-exchanged Y zeolites was in the order: pyridine > cyclohexene > toluene. Adsorption methods
π-complexation and direct coordination via S atoms with Ce4+ (S–M) interaction were used for
desulfurization.
3.2.2.4. Y zeolite exchanged with Ce and Ni
Wang et al.[55]studied selective adsorption of dibenzothiophene (DBT) over Ce/Ni-loaded Y zeolites.
Adsorptive desulfurization were carried out with 0.1 or 0.2 g ion-exchanged NaY zeolite (Si/Al= 5) from
13
20 ml transportation fuels containing 500 mg/L sulfur with 5 vol% of toluene at room temperature in a
stirred batch system. Before adsorptive experiments, kinetic studies were performed to determine the
equilibration time. Results showed that the adsorption processes are fast and most of the sulfur
compounds are adsorbed within 1h. For the model solution without toluene, the DBT uptake reaches 54%
after treatment for 30min and the adsorption equilibrium is achieved after treatment for 2h. Sulfur
adsorption efficiency for NiCeY was increased with increase in the Ce loading. It indicated that the Ce as
a cocation play a promoting role in DBT adsorption. Results of selective adsorption of the adsorbents
showed that NiCeY with adsorption capacity of 7.8 mg/g had higher adsorption selectivity than the NaY,
NiY and CeY with the adsorption capacity of 2.3, 5.4 and 6.6, respectively. Results of adsorption
mechanism showed that the two types of adsorption modes, namely π-complexation and selective
adsorption were observed in adsorption desulfurization. In another study[56], adsorptive desulfurization
was performed over the Ni(II)Y. Experiments were carried out with a commercial high speed diesel
pretreated by alumina beads having 50 ppm total sulfur at pressure 4 bar absolute. The feed flow rate was
at either 0.5 ml/min and 1 ml/min during initial experiments. The H2 flow rate was in the range of 1- 2
mL/min. Results indicated that 50 ppm sulfur can be brought down to below 5 ppm level.
3.2.2.5. Y zeolite exchanged with La
Shi et al.[57]used NaY and LaNaY zeolites to remove thiophene from model gasoline (MG) with and
without cyclohexene (CHE). The mechanisms of the effect of olefin on adsorption desulfurization were
investigated with FT-IR. The Si/Al ratio of LaNaY and NaY were 4.72 and 4.68, respectively. After
pretreatments of adsorbent, the adsorption experiment were carried out with 0.25 g dried adsorbents, 5 mL
of MG (MG1 to MG5), for 3 h at room temperature and atmospheric pressure. The detailed compositions
of the model fuels are summarized in Table 7.
Table 7
Results of their study on thiophene adsorption modes revealed that thiophene is adsorbed onto NaY by
π electron interaction and LaNaY adsorbed thiophene by direct interaction between the S atoms of and
La3+ ions. In addition, results of their study on CHE adsorption modes revealed that CHE is molecularly
adsorbed on NaY by π–OH complex. Some CHE molecules have been adsorbed molecularly on LaNaY
by alkyl–OH complexes which was due to the alkylation reactions on the Brönsted acid sites of
LaNaY[58]. Results of sulfur removal of MG with sulfur concentration of ca. 100 mg S/L (MG1 to MG5)
over NaY and LaNaY has been displayed in Fig. 3.
Fig. 3
Results showed that sulfur removal over NaY decreases continuously, but LaNaY exhibitd a volcanic
type curve with the adding of CHE. The tendency on NaY and LaNaY were due to the adsorption
14
methods of thiophene and CHE. Brönsted acid sites of LaNaY had an important role in removal of
thiophene from model gasoline containing olefin. The larger sizes and the higher electron densities on
sulfur atoms of alkylated thiophenes were main reasons for the improved sulfur removal from model
gasoline containing suitable amount of olefin.
3.2.2.6. Y zeolite exchanged with Ag and Ce
Lin et al.[59]investigated adsorption behavior of AgY and CeY zeolites for thiophenic removal. In
their study, Batch and fixed-bed adsorption experiments were performed for different purposes. The
adsorption isotherm of thiophene on adsorbent were obtained by batch adsorption experiments. The Si/Al
ratio of NaY, AgY and CeY zeolites were 2.2, 2.4 and 2.2, respectively. Batch adsorption experiments
were carried out by 0.3 g adsorbent for 5 h at 25 °C. Initial sulfur concentration level (thiophene/nheptane solution) was 180 mg/L. Fixed-bed adsorption experiments were performed in a vertical custom
quartz adsorber with an inside diameter of 10 mm and a column length of 130 mm. The adsorbent amount
and flow rate were 3.0 g, and 69.38 mL/h, respectively. The saturation capacity for NaY, AgY and CeY
zeolites were 0.39, 11.01 and 13.67 mg s/g, respectively. Adsorptive desulfurization of the NaY, AgY and
CeY zeolites were relied on the physical adsorption of zeolite pore channels, π-complexation and direct Sadsorbent (S–M) interaction, respectively. Desorption studies by solvent washing were investigated,
which is cited in regeneration section.
Song et al.[60]used the AgY, CeY and AgCeY bimetal ion-exchanged zeolites for adsorption of
benzothiophene and thiophene in a fixed-bed unit through different types of model gasoline containing
pyridine, cyclohexene and toluene. The compositions of different types of model gasoline are summarized
in Table 8.
Table 8
The experiments were carried out at a feed flow rate of 20 mL/h, temperature 50 °C and 1 g adsorbent.
The adsorption affinity was the following order: BT > TP. Results of fixed-bed breakthrough experiments
are summarized in Table 9.
Table 9
From the results, the capacity of sulfur removal with model gasoline without and by aromatics, nitrogen,
and olefins were in order ranking of AgY > AgCeY > CeY and AgCeY > CeY > AgY, respectively.
Similar to that of some previous reported works, TP and BT has been adsorbed on the Ce-exchanged Y
zeolite mainly by forming S-M bonds between Ce4+ and the sulfur atoms; so the effect of the aromatics,
nitrogen, and olefins on CeY and AgCeY is less than that on AgY. The order of adsorption affinity was
15
BT>TP. The effect of aromatics, nitrogen, and olefins for sulfur removal on zeolites were an order of
pyridine>cyclohexene>toluene. Desulfurization was performed by the mechanisms π-complexation and
direct S-adsorbent (S–M) interaction.
3.2.2.7. Y zeolite exchanged with Ni, Cu(I), Cu(II), Co and Ce
Yi et al.[61]investigated adsorption-desorption behavior and also mechanism of dimethyl disulfide in
liquid hydrocarbon streams on the Cu(I)–Y , Ni–Y, Co-Y, Cu(II)–Y, Na–Y and Ce–Y by the dynamic and
static experiments. The dynamic experiments were carried out at atmospheric pressure and LHSV (liquid
hourly space velocity) 10.0h−1. Also, the Static experiments were performed at ambient temperature for
12 h with 0.1 g adsorbent at 15 mL of sample containing dimethyl disulfide. Concentration of dimethyl
disulfide in liquid hydrocarbon streams was 45.77 ppm. The Si/Alratio of NaY adsorbent was 3.26.
Activation of the adsorbent was performed at 450°C to promote autoreduction of Cu2+ species to Cu+.
Results of desulfurization showed that the order of breakthrough adsorption capacity for dimethyl
disulfide were Cu(I)–Y > Ni–Y > Co-Y > Cu(II)–Y > Na–Y > Ce–Y. The Pyridine-FTIR analyses and
other techniques indicated that the maximum of the weak Lewis acid sites and maximum Brønsted acid
cite were observed on the Cu(I)–Y and Ce–Y, respectively. Results also revealed that Lewis acid was
contributed to the S–M (ϭ) bond between metal cation and sulfur of DMDS. In addition, Brønsted acid is
harmful to the S–M (ϭ) bond. Results of effect of calcination temperature on the performance
desulfurization indicated that the optimal calcination temperature of the modified Y zeolites was 450°C
and exhibited the sulfur adsorption capacity 157.4 mg s/g adsorbent.
3.2.2.8. hierarchical Y (meso-Y)
Only lower than o.5 % of the total content of active sites (sites placed over the external surface of the
zeolites, or close to the micropore openings) at zeolites are accessible for bulky molecules. Synthesis of
zeolites with hierarchical structures is one of the approaches to diminish of the steric limitations of bulky
molecules[28]. For this purpose, Tian et al.[62]studied adsorptive desulfurization performance of
hierarchical CeY (meso-CeY) in order to investigate the role of the pore structure of adsorbents in
adsorption of TP, 3-methylthiophene (3-MTP), and BTP dissolved into cyclohexane. The Si/Al ratio
parent NaY was 2.6. The sulfur contents of different MGs were ca. 100 mg/L and the sulfur content of the
FCC gasoline was initially 113.3 mg/L. Detailed compositions of the MGs are listed in Table 10.
Table 10
Prior to the desulfurization, the zeolites were dried at 393 K overnight. The adsorptive experiments were
carried out in a batch system at Room Temperature and atmospheric pressure. 0.250 g-dried adsorbents
16
were mixed rapidly with 5.00 mL of MG or 2.50 mL of FCC gasoline in flasks for 3 h. The desulfurized
MG was separated by filtration, and the sulfur content of MG before and after adsorption was analyzed by
microcoulometry. The microporous CeY was employed as reference to investigate the effect of porous
structure on the adsorptive desulfurization. Results indicated that the removal percentage of the TP and 3MTP over the meso-CeY were 97.9% and 96.2%, respectively, nearly the same as those over CeY (97.3%
and 95.8%). Removal of BTP with bigger molecular size was increased from 92.0% over CeY to 97.0%
over meso-CeY. These results indicated that the pore structure of adsorbent plays an important role in the
adsorptive desulfurization and the mesopores are beneficial to the adsorption of the thiophenic
compounds with larger molecular size. Also, the effect of competitors on the TP adsorption was
investigated. Results showed that the mesopores created play a helpful role in resisting the influence of
aromatics and olefins.
Increasing the content of cyclohexene in MG was led to a dramatic decrease in sulfur removal over CeY,
from 83.4% in MG8 to only 62.1% in MG11, while only a slight decline over meso-CeY, from 92.1% in
MG 8 to 76.5% in MG11. Results of desulfurization of FCC gasoline showed that sulfur removal on
meso-CeY and CeY were 36.6% and 21.9%, respectively. IR-spectra of TP, toluene and cyclohexene
adsorption were recorded on meso-CeY to clarify the key factor in the enhanced adsorptive
desulfurization. Results indicated that the IR peak of TP, toluene and cyclohexene on meso-CeY was
similar to that of CeY zeolite, indicating that the significantly decreased acid amount had little effect on
the adsorption modes of these competitors. The quantities of Brönsted and Lewis acid sites on the mesoCeY were only 144 and 13 μmol g−1, respectively, much less than 1099 and 129 μmol g−1 on the
microporous CeY due to the partial collapse of -Si-O-Al-O-Si framework in CeY microporous.
In another study, Sun et al.[39]used NaY zeolites modified with 0.09 M NaOH (NaY-0.09) aqueous
solution and transition-metal ions Ce and then investigated adsorption property of these zeolites(NaY,
NaY-0.09, and CeY) for adsorption of bensothophene from the simulated diesel oil. Modification with
NaOH solution is an effective method to adjust the acidity and pore structure of NaY zeolites Adsorption
experiments were carried out in a mixture of n-octane and benzothiophene (sulfur content, 275 mg/L)
with 1.0 g adsorbent and 6 mL of the simulated diesel oil. The adsorption experiments were performed at
303, 323, 343, 363, 383, and 403 K for 1 h under stirring by a static method. The sulfur content in the
feed and reaction mixture was measured using a ZWK-2001 sulfur and chlorine analyzer. Results of the
characterization showed that the surface area of parent NaY and NaY-0.09 adsorbents were 20 m2/g and
75 m2/g, respectively. The mesopore volumes of the parent NaY zeolite and NaY-0.09 adsorbents were
0.02 and 0.07 cm3/g, respectively. Results of characterization indicated that the both the adsorbents do not
have any Brønsted acidity on their surface. Moreover, introduction of mesopores in NaY, increased the
17
number of both the strong and weak Lewis acid sites and the mesopore surface area of the NaY zeolites
and these changes in the properties of the adsorbents cause some differences in their adsorption
desulfurization performance. Results of desulfurization of simulated and No. 0 diesel oils on NaY, NaY0.09, and CeY zeolites are summarized in Table11.
Table 11
Results showed that competitive adsorption occurs on the adsorbents. The selectivity of the
desulfurization for benzothiophene coexisted toluene over NaY-0.09 adsorbent was higher them that of
CeY. The NaY-0.09 zeolite had the sulfur removal and the adsorption capacity 99.9% and 1.65 mg/g,
respectively from the simulated diesel. This was the result of the increase in both the mesopore surface
area and the number of weak Lewis acid sites. The NH3-TPD results combined with the Pyridine-IR
results showed that the weak Lewis acid sites are responsible for the adsorption of benzothiophene. Also,
adsorptive desulfurization of the NaY-0.09, and CeY were compared. Results indicated that the
desulfurization property of the mesopores NaY zeolites were similar to that of the conventional CeY
adsorbents, while the loss of transition-metal ions do not need to be considered. Thus, the desulfurization
stability of the mesopores NaY zeolites was improved compared to CeY adsorbent. Therefore, the NaY0.09 zeolites can be used for application in the continuous fixed-bed adsorption desulfurization.
3.2.2.9. Y zeolites with different Si/Al ratio
Song et al.[63]investigated the effects of Si/Al ratio(3.0, 4.8, and 5.3) on adsorptive removal of
Thiophene and Benzothiophene over the ion-exchanged AgCeY zeolites for a model oil comprising
thiophene (TP), benzothiophene (BT), and certain amounts of toluene or cyclohexene. Total sulfur
concentration in model oil was of 200 mg/L (M1). In static adsorption, 0.2 g of the AgCeY zeolite was
added to 40 mL of model oil at 323 K for 1 h under magnetic stirring. In dynamic adsorption, a stainless
steel tube with an internal diameter of 10 mm and a length of 200 mm was used. The adsorbent amount,
flow rate and temperature were 1 g, 20 mL/h and 323 K, respectively. The breakthrough concentration
was 20 mg/L. Results of characterization showed that Ag+ and Ce4+ surface contents of AgCeY zeolite
with Si/Al ratio 5.3 (AgCeY-5.3 )were higher than those of AgCeY zeolites with Si/Al ratio 3.0, 4.8
(AgCeY-3.0 and AgCeY-4.8). The reason for this difference was due to the higher pore volume and
diameter of AgCeY-5.3 than those of AgCeY-3.0 and AgCeY-4.8. As the Si/Al ratio of AgCeY zeolites
increases, the number of lewis acid sites increases and the number of Brönsted sites decreases. Static
desulfurization experiments showed that adsorption capacity for sulfur removal on AgCeY-n were
decreased in the order AgCeY-5.3 > AgCeY-4.8 > AgCeY-3.0. AgCeY-n adsorbents with model oil
without competitive (M1) had BT removals higher that 93%. This removal amount was decreased in M2
(model oil comprising TP, BT and certain amounts of toluene) and M3 (model oil comprising TP, BT and
18
certain amounts of cyclohexene) in order of 83% and 79%, respectively. Dynamic desulfurization
experiments showed that breakthrough and saturation loading on AgCeY-n were decreased in the order of
AgCeY-5.3 > AgCeY-4.8 > AgCeY-3.0 and that the order selectivity of adsorption were followed the
order BT > TP. Results are listed in Table12.
Table 12
It is clear that a higher content of Lewis acid sites is beneficial for increasing the adsorption capacity in
adsorptive desulfurization. Therefore, these properties of AgCeY-5.3 zeolite was beneficial for increasing
the sulfur adsorption capacity, with a high adsorption selectivity to TP and BT in model oils containing
toluene or cyclohexene. The π-complexation and direct coordination (S−M) interactions were responsible
for the removal of TP and BT on the AgCeY-n zeolites.
3.2.2.10.
Y zeolite exchanged with H, Ni and K
There are two types of Brönsted (OH) and Lewis(≅Al) acid sites which are present in the structure of
zeolites. The Brönsted acid sites causes the catalytic reactions for sulfur compounds, which resulted in
pore blockage and the coverage of adsorption active centers [64-67]. According to this, Li et al.
synthesized NiY and KNiY adsorbents by incipient wetness impregnation to reduce the amount of
Brönsted acid sites and improve the desulfurization performance of these adsorbent. Ratio of Si/Al of the
HY zeolite was =5.2. Composition of gasoline model are listed in Table13.
Table 13
Desulfurization experiments were performed by 1 g of adsorbents at 360 °C under H2 for 5 h. Fuel was
pumped into the fixed-bed flow reactor with flow rate of 0.4 mL/min, and at ambient temperature and
pressure. Then, effluent samples were collected at regular intervals until breakthrough (breakthrough
concentration was 10 mg/L) was achieved, and sulfur content of the effluent was detected by TCS-2000S
ultraviolet fluorescence sulfur analyzer. The breakthrough volumes have been summarized in Table 14.
Table 14
This study showed that breakthrough volumes of HY, NiY, and KNiY adsorbents for M1, M2 and M3
model gasolines were HY< NiY< KNiY. Adsorption mechanisms were studied by in situ FT-IR in range
of 1600–1300 cm-1. Results showed that both strong and weak Brönsted acid sites were detected in these
adsorbents and these acid sites were decreased by introducing the K cation. Whereas, the weak lewis acid
sites were increased by introducing the K cation. As a conclusion, KNiY zeolite revealed the advantages
in desulfurization performance with 5 vol% olefins or 5 vol% aromatics involvement[68].
19
3.2.2.11.
Y zeolite exchanged with Ni and Pd
In another study, Han et al.[69]investigated adsorptive desulfurization by the NiY and NiPdY
adsorbents which were prepared by impregnation method. Adsorptive desulfurization for fuels were
carried out in a fixed-bed reactor (Fig. 4). The diameter of the fixed-bed flow reactor was 1 cm and the
length was 40 cm. Before each experiment, activation of the adsorbent was performed at 360°C under H2
for 5 h to reduce Ni2+ ions. Then, adsorbents were cooled to room temperature. Fuel was pumped into the
reactor with a flow rate 0.5 mL/min at room temperature and ambient pressure. The breakthrough
concentration was 10 mg/L. The sulfur content of the effluent was detected by a TCS-2000S Ultraviolet
fluorescence sulfur analyzer. Compositions of the model gasoline are listed in Table15.
Fig. 4
Table 15
The adsorption mechanisms of thiophene, olefin, and aromatics were investigated by in situ FT-IR. The
results showed that all of the compounds were adsorbed on HY through л-complexation, which cause to
have a strong competitive adsorption. For M-I only containing thiophene and cyclohexane, HY exerted a
good adsorption desulfurization performance. NiY zeolite had the adsorption desulfurization rate
improved with respect to that of HY from 57.0% to 65.7% in the olefin–thiophene system and from
21.8% to 73.1% in the aromatic–thiophene system. The breakthrough sulfur capacity of NiY was 0.68 mg
g-1 in olefin–thiophene system and 0.88 mg g-1 in aromatic–thiophene system. In addition, Pd was used for
reduction of the number of Bronsted acid sites. Results showed that not only the adsorptive
desulfurization rate was improved further from 65.7% to 85.8% in olefin–thiophene system and from
73.1% to 87.0% in aromatic–thiophene system, but breakthrough sulfur capacity was also enhanced from
0.68 to 1.64 mg/g in olefin–thiophene system and also from 0.88 to 2.01 mg/g in aromatic–thiophene
system. Pd has greater selectivity for thiophene on the NiPdY adsorbent despite the olefin or aromatics. A
summary of adsorptive desulfurization by Y zeolites are highlighted in Table16 including zeolite types,
adsorption capacity/efficiency values, reaction time, mechanism and experimental condition.
Table 16
3.3. LTA zeolite
20
LTA type zeolites are built by linking sodalite cages through double four-rings. LTA is commonly
synthesized with a Si/Al ratio of 1. A fully Na- exchanged LTA zeolite with a Si/Al ratio of 1 has 12
cations per alpha cavity (4A zeolite). Exchanging the Na(I) in a 4A zeolite with Ca(II) (5A) increases the
effective aperture size from about 4A to 5A. exchanging the sodium cations with potassium reduces the
aperture size to about 3A[42]. As can be seen in Fig. 5. The Sulfur compounds must be small enough to
enter the cavities via pores of the zeolite. Hence, 3A and 4A molecular sieves with low pore size cannot
be used in adsorptive desulfurization.
Fig.5
Xiang et al.[70]prepared high purity propane from liquefied petroleum gas (LPG) by the removal of
sulfur and butanes (i-butane and n-butane) over three commercial zeolites 13X, NaY and 5A. Adsorption
measurements were carried out in a fixed bed adsorption at atmospheric pressure, temperature of 303 K,
feed flow rate of 20 mL/min and bed length of 15 cm. After activation of adsorbents in air at 573 K for 4
h, desulfurization experiments were performed. Effluent sulfur content was sampled every 3 or 5 min.
The breakthrough concentration of sulfur was 2 ppm. Comparison of the adsorptive desulfurization in
LPG over zeolite 13X, NaY and 5A at 20 mL/min and 303 K showed that zeolite 13X and NaY presented
better desulfurization performance than zeolite 5A. It was due to the both zeolite 13X and NaY have
larger pore diameter than zeolite 5A for sulfur species to access and be adsorbed. Thus, zeolite 13X and
NaY can be employed in adsorbing sulfur compounds from LPG.
3.4. ZSM- 5 (MFI) zeolite
The MFI zeolites are built from five-rings and contains cavities interconnected by a straight ten - ring
channel systems and a zigzag ten-ring channel system. Si/Al ratios in MFI zeolites are from about 10 to
much highy. Since MFI zeolites are highly siliceous, the number of cations is small. However, since all
the cation sites are in the MFI channels, all changes to cation number and type can affect the adsorption
properties[42]. Adsorptive desulfurization with ZSM-5 zeolite have been studied by some researchers [65,
71]. Cristina and Lercher investigated adsorption and surface chemistry of Thiophene on Na-, K-, and HZSM-5 by IR Spectroscopy, mass spectroscopy and gravimetry and clarified the role of the strong
Bronsted acidity of H-ZSM5 in determining the complex adsorption behavior of thiophene. Results of
thiophene adsorption on SiO2 and Na- and K-ZSM-5, under the same experimental conditions, were
compared. On hydroxyl groups of SiO2, thiophene was reversibly adsorbed by hydrogen bonding at room
temperature. A strong coordinative type of bonding between the thiophene ring and the cations Na+ and
K+ occurred in the cation-exchanged ZSM-5 samples. The strength of the interaction increases in parallel
with the Lewis acid strength of the metal cation. Initial interaction (thiophene hydrogen bonds to the
21
SiOHAl groups) was followed by ring opening and oligomerization reactions promoted by the strong
Bronsted acidity of the H-ZSM-S zeolite. Temperature-programmed desorption of dimers and trimers of
thiophene was observed at temperatures higher than 550 K. In addition, parallel and consecutive reactions
including cracking, cyclization, alkylation, and condensation form alkylated aromatics and condensed
rings which are precursors for coke deposition in the pores and at the external surface of the H-ZSM-S
zeolite. These procedures are unfavorable for easy regeneration purposes. In another study, Chica et al.,
investigated adsorption and desorption of thiophene and the reactions of thiophene-derived adsorbed
species on H-ZSM-5, H-Beta, and H-Y with varying Si/Al ratios. Thiophene adsorption–desorption
measurements were measured at 363 K in a quartz cell using a 0.25 g packed-bed of zeolite. Activation of
zeolite was performed in flowing dry air at 773 K for 1 h. The adsorbent was cooled to 363 K in He/Ar.
Samples were exposed to thiophene in He/Ar flow at 363 K. After adsorption, weakly adsorbed thiophene
molecules were removed by He/Ar flow at 363 K for 0.25 h. Thiophene concentrations of the effluent
stream were measured continuously by mass spectrometric analysis. Results indicated that Thiophene
adsorption uptakes (per Al) were independent of Al content, and were 1.7, 2.2, and 2.9 on H-ZSM-5, HBeta, and H-Y, respectively, after removal of physisorbed thiophene. These data indicated that thiophene
oligomers are formed during adsorption and that their size depends on spatial constraints within zeolite
channels. Adsorption and oligomerization occured on Brønsted acid sites at 363 K. Thiophene/toluene
adsorption from their mixtures showed significant thiophene selectivity ratios (10.3, 7.9, and 6.4, for
HZSM-5, H-Beta, and H-Y zeolites), which exceed those expected from van der Waals interactions and
indicated that thiophene adsorption occurs concurrently with oligomerization on Brønsted acid sites and
forms toluene–thiophene reaction products. For decomposition of the thiophene oligomers high thermal
treatment used (534 K) to form molecular thiophene with all carriers. The oligomerization of thiophenic
compounds on ZSM-5 zeolites in desulfurization process indicating such materials are not suitable for
removing sulfur derivatives from hydrocarbons which is unfavorable for easy regeneration purposes.
3.5. Beta zeolite
The beta zeolite family are high-silica frameworks with the three-dimensional wide-pore system
comprising 12-membered ring channels and are synthesized with Si/Al ratios> 5. The wide Si/Al
ratios of these zeolites create the easy controlled active sites and high thermal stability beneficial for
the adsorption and regeneration, and thus it could be recognized as an attractive material for the use in
adsorptive desulfurization. Furthermore, the high degree of intergrowth can lead to a large number of
defects giving beta unique acid properties and these acid properties are also beneficial for
desulfurization[42].
22
3.5.1.
Beta zeolite exchanged with Cu(I) and Ag(I)
Gong et al.[72] studied deep desulfurization of gasoline using Cu(I)- and Ag(I)-beta zeolite through
fixed-bed adsorption technique. Model gasoline was thiophene (150 ppm) and benzothiophene (50 ppm)
in cyclohexane. The activation of Cu(I)-Beta and Ag(I)-Beta were carried out more than 18 h at the
temperature 450 °C in inert helium gas atmosphere. Dynamic adsorption experiments were performed
with 1 g of adsorbent, liquid space velocity 60 h−1, ambient temperature and pressure. The obtained
results from characterization revealed that the ratio of CuCl/beta or AgNO3/beta zeolite higher than 15
wt.%, jammed the pore channels of the adsorbents and block off the active sites resulting in reduction of
surface area and pore volume and this has an influence on the desulfurization performance. Results of
static adsorptive desulfurization in the model gasoline over Na(I), Cu(I)- and Ag(I)-beta zeolite, indicated
that Na-beta zeolite is very inefficient with only 20% sulfur removal, while Cu(I)- and Ag(I)-beta zeolite
is 92% and 87%, respectively. So, desulfurization in fixed adsorber was investigated on the Cu(I)- and
Ag(I)-beta zeolite. Results of Fixed-bed experiments showed the breakthrough capacity for Cu(I)-beta
and Ag(I)-beta of 0.239 and 0.237 mmol S/g, respectively. For 1 g Cu(I)- and Ag(I)-beta adsorbent, about
46 and 42 mL model gasoline were treated with the sulfur content reduced significantly from 200 ppmw
to below 1 ppmw, respectively. Adsorptive desulfurization experiments with actual FCC gasoline on the
Cu(I) beta and Ag(I)-beta zeolite were carried out. For 1 g Cu(I) beta and Ag(I)-beta zeolite, 30 ml and 22
mL actual FCC gasoline were treated with sulfur content reduced from 200 ppmw to below 1 ppmw. With
this amounts of actual FCC gasoline, the breakthrough capacity were 0.168 mmol S/g and 0.145 mmol
S/g, respectively. The decrease of sulfur removal capacity in actual FCC gasoline is an evidence of some
competitive adsorption from the olefins and aromatic compounds. The different acidic–basic properties or
framework charge distribution that can change the adsorption interaction of thiophene and unsaturated
compounds onto the Cu(I)- and Ag(I)-beta zeolite caused difference of the desulfurization capacity with
the actual FCC gasoline. Effect of Si/Al ratio on the adsorption capacity was investigated with the Si/Al
ratios of 8, 12, 25, 30, 60 and 80. Results showed that adsorption capacity of the adsorbents first increases
with the Si/Al ratio rise (10 ratio), and then decreases. The experimental results indicated that the acidity
and exchangeable cation number in zeolite decreases with the increase of the Si/ Al ratio but BET surface
area and pore volume increases. For zeolite with Si/Al ratios of 8, the lower sulfur adsorption capacity
was due to its lower BET surface area and pore volume.
3.5.2.
NaY/Beta composite zeolite exchanged with Ca and Ce
In another study, Fu et al.[73] prepared and modified NaY/Beta composite zeolite with Ca and Ce and
investigated the adsorption of organic thiophene by static adsorption experiments[73]. Y/Beta composite
23
has higher acid strength than Y, Beta and Y Beta physical mixture, and exhibited high activity and
selectivity[74]. Model gas oil was n-hexan/thiophene with sulfur content 200 ppm. The results showed
that the best desulfurization reaction conditions of CeY/Beta were found to be 1: 20, 363 K, 4 h and 94.9
% for adsorbent/oil ratio, temperature, reaction time and desulfurization rate, respectively. CaY/Beta were
found to be 1: 20, 363 K, 3 h and 42.1% for these parameters, respectively. The best desulfurization
reaction condition of Na Y/Beta were found to be 1: 20, 373 K, 4 h and 26.8%, respectively.
Adsorbent/oil ratio had the main influence factor with the results of orthogonal experimental and analysis
of variance. Table 17 presents the adsorptive desulfurization on Beta zeolites.
Table 17
4. Regeneration of zeolites
One of the factors can be effect on the efficiency of adsorptive desulfurization is regenerability of
adsorbent. This factor is important for adsorbent development. There are two common methods, namely
thermal regeneration (destructive regeneration), which is performed by, gases (Ar, Helium, N2 or
vacuum) at moderately high temperatures, and solvent regeneration (Non-Destructive regeneration).
4.1. Thermal regeneration
During the thermal regeneration process, some side products are formed which cause regeneration of
adsorbent are not easily. Especially at higher temperatures, these products will undergo further reactions
and will lead to coke formation. For example, mercaptans are oxidized to sulfides, disulfides and/or
polysulfides. These newly formed sulfur compounds are then adsorbed more strongly onto the synthetic
faujasite, due to their higher molecular weight, thereby reducing the adsorbent capacity of the faujasite.
Hawes et al.[75] reported novel desorption process for removal of sulfur compounds, including
mercaptans, sulfides, disulfides, thiophenes and thiophanes from liquid and gas feed streams with
synthetic 13X adsorbents. The process is characterized as a dedicated way to regenerate the adsorbent to
avoid decomposition and coking of the adsorbed sulfur compounds. Degradation of the mercaptans takes
place above 200° C. Degradation of the mercaptans can be reduced remarkably, if temperature increase is
done stepwise with a halt step at different levels. In one successful experiment, the temperature was
increased in steps of 10° C at a time and the temperature was left unchanged for 30 minutes before it was
increased by another 10° C. Using this approach, no exothermic reaction could be observed, indicating
that no degradation of the mercaptans took place. Thermal regeneration was used successfully for
regeneration of Ag-Y after sulfur removal at air-calcination at 450 °C for 6 h and a full regeneration of
used Ag-Y was achieved. Air-calcination of used Ag-Y at temperatures<350 °C causes incomplete
decomposition of the adsorbed sulfur species, resulting in declined adsorption performance[49].
24
Yi et al.[61]regenerated the Dimethyl disulfide absorbed on Cu(I)–Y zeolite in air at 450°C for 4 h and
then switched into N2 atmosphere for 4 h at the same temperature in the fixed bed. After three times of
cyclic regeneration, desulfurization property of the adsorbent was recovered to 66.7%. The Dimethyl
disulfide absorbed on Cu(I)–Y zeolite fully was burned at 450°C in air, nevertheless, cuprous ions were
oxidated to cupric ions. In the N2 regeneration, the sulfur onto the adsorbent was not removed completely
and the regeneration effect was unsatisfactory. Moreover, results of the Pyridine-FTIR spectra and XRD
of the regenerated Cu(I)–Y zeolites showed the Lewis acid sites and crystal structure were destroyed in
the regeneration process and so the desulfurization performance of Dimethyl disulfide onto the Cu(I)–Y
zeolite was affected.
Tian et al.[27]performed regeneration of used Ce/beta-40 at 450 °C, first under dried nitrogen flow for 2
h, and then in a muffle for 4 h. The XRD analysis and FT-IR of the regenerated Ce/beta-40 indicated that
the zeolite structure was not affected by the thermal treatment. The sulfur removal of the model fuel
containing DBT and toluene over fresh and regenerated Ce/ beta-40 indicated that the sulfur removal
decreases from 39.2 % over fresh Ce/beta-40 to 31.3% and 30.4% after the first and second regeneration,
respectively. That is, ca. 80% of the sulfur removal of the fresh adsorbent can be recovered by thermal
treatment, which is ascribed to the open 3D channels and better thermal stability of beta zeolite-based
adsorbent[27]. Bakhtiari et al.[76]performed the regeneration process for situation AgX-zeolite that had
been situated by sulfur compounds in air at 210 °C for 1 h. Regenerated AgX-zeolite adsorbent was used
five times.
Gong et al.[72]regenerated Cu(I) and Ag(I)-beta zeolite saturated with thiophene and benzothiophene
through fixed-bed adsorption technique under dried nitrogen flow at 623 K for over 6 h with a gas space
velocity of 2400 h−1. Dried nitrogen was used for the direct reduction of the Cu2+ to Cu+. The rate of
weight-loss of regenerated Cu(I)- and Ag(I)-beta zeolite were below 1 wt %. The results of XRD analysis
of the adsorbents showed that after 9 times regeneration the zeolite structure was not affected by the
thermal treatment. In addition, the adsorbed sulfur compounds were easily removed due to the open 3D
channels.
Sotelo et al.[45]investigated the influence of the Si/Al molar ratio and the exchanged cation on the
thermal regeneration of benzothiophene (BT) onto the agglomerated zeolites with FAU structure. TG
analyses were performed on the spent adsorbents after the dynamic adsorption experiments. TGA plots of
the recovered adsorbents after the dynamic adsorption experiments indicated that the low silica zeolites
due to its lower Si/Al molar ratio, presents both a higher amount of adsorption sites and highest
interaction with the BT molecule. Finally, investigation of the exchange cations on the thermal
25
regeneration of adsorbents showed that all adsorbents presented quite similar TG profiles and BT
desorption temperatures.
4.2. Solvent regeneration
For solvent elution method options, the solvents used undergo some type of interaction with the spent
sorbent, and these interactions have effect on the adsorption of the organosulfur compounds during the
second adsorption cycle. The solvent regeneration is applied to the adsorbate that is weakly bound to the
surface of the sorbents. On the other hand, this is used for the interactions with adsorption energy
typically 5100 kJ/mole. Regeneration by solvent is an environmentally kind method because it avoids the
emissions of SOx and H2S generated by thermal regeneration. In other cases where solvent regeneration of
the spent adsorbent was recommended a large solvent inventory along with suitable solvent recovery and
recycle system has to be considered which will add to the complexity of the overall process. solvent
regeneration is suitable for certain applications for those involving heat sensitive materials[77].
Lin et al.[59]used ethanol as desorption agent at certain temperature until the sulfur concentration in
the washing solvent became unchangeable. The outgoing solvent fractions were collected from the fixed
bed in order to estimate the amount of solvent required. The sulfur concentration in the initial fractions
were over 2000 mg/L. The results indicate that most of the sulfur compounds were recovered using 20 g
of the solvent per gram of adsorbent. Washing solvent desorption process is presented in Fig. 6.
Fig. 6
In another study, shan et al.[53]regenerated the saturated CuCeY with Thiophen in a static bath of
solvent containing a mixture of 30 wt % toluene and 70 wt % iso-octane for 4 h, followed by reactivation
in nitrogen at 450 °C for 4 h. Desorption was about 90% of the initial one. Also, helium atmosphere was
used for regeneration of the consumed adsorbent. Results showed that helium exhibited a similar
performance as that regenerated in nitrogen.
Sun et al.[39]investigated regeneration performance of the NaY, NaY-0.09 and CeY adsorbents in
toluene at an ambient temperature for 24 h. Results showed that NaY zeolite reviled has poor regeneration
properties compared with NaY-0.09 and CeY adsorbents. The initial sulfur removal for NaY zeolites was
7.4%. The sulfur removal was decreased sharply to around 0% after the first cycle of desulfurization on
NaY zeolites. Both NaY 0.09 and CeY adsorbents exhibit similar regeneration performance. After the
fourth cycle of desulfurization for NaY-0.09 and CeY adsorbents, the sulfur removal and adsorption
capacity of the simulated diesel oil were remained 99.9% and 1.65 mg/g, respectively. These values were
reduced when the two regenerated adsorbents were used in the fifth cycle of desulfurization. The
26
adsorption capacity after the fifth cycle was 1.49 and 1.47 mg/g for NaY-0.09 and CeY adsorbents,
respectively. When NaY-0.09 adsorbent was used in the adsorption desulfurization from No.0 diesel oil
(NaY-0.09 diesel), the initial desulfurization was 21.2%. The value was remained 18.5% after the fifth
cycle of desulfurization.
Shirani et al.[78]recycled magnetic NaY zeolite immobilized with 1-butyl-3-methylimidazolium
tetrachloroferrate ([bmim]Cl/ FeCl3) ionic liquid by a two-step process. At first, these adsorbent were reextracted by stirring with carbon tetrachloride to clean the immobilized ionic liquid. Then, adsorbent was
dried in vacuum. In the second step, these adsorbents were heated in n-hexane solvent at 50°C for 10 min
to remove the rest of dibenzothiophene, which had been stuck in the zeolite pores. The recycled sorbent
was used four times for sulfur removal. When the sorbent was used for more than four times the active
sites were occupied and polluted by dibenzothiophene. They were not completely removed from the
sorbent structure.
According to the performed studies, regeneration of zeolites is perrformed by air oxidation at 350450°C. Such large temperature will be considered a major disadvantage in the context of industrial
applicability of these processes. Moreover, the emissions of SOx and H2S generated are the disadvantages
of these processes. In other cases, solvent regeneration has to be considered a large solvent inventory
along with suitable solvent recovery and recycle system and also will add to the complexity of the overall
process.
5. Conclusion
Recently, zeolites have been developed for deep adsorptive desulfurization of fuel, which shows a high
selectivity, high adsorption capacity, regenerability and safe operations. Therefore, this review has
focused on the recent studies on adsorptive desulfurization of fuel by zeolites. Deep adsorptive
desulfurization are strongly dependent on adsorption methods of the sulfur compounds on the zeolites, the
charge of metal cations, texture properties of the zeolite, number of active sites on the framework of
zeolite, acid properties of the zeolites, Si/Al ratio and the pore size of the zeolites. X and Y zeolites have
been widely studied for adsorption of sulfur compounds, due to their tuneable selectivity regarding polar
molecules. Zeolites showed good sulfur loading capacity, good regenerability and stable structure for
removal of sulfur compounds. Nevertheless, some of zeolites such as LTA zeolites fail when using as
adsorbent for adsorption of sulfur compounds. This is due to the presence of pores with molecular size
typically below 1 nm, cease steric limitations to the diffusion of bulky molecules. Also, ZSM-5 zeolites
are not suitable for removing sulfur derivatives from hydrocarbons duo to oligomerization of thiophenic
compounds on these zeolites, which is unfavorable for easy regeneration purposes. Modification of
27
HZSM-5 with alkali or alkaline-earth metals cations can be decreased the number of strong Brönsted acid
sites and diminish oligomerization of thiophenic compounds on these zeolites. These adsorbents for
adsorptive desulfurization were rarely reported. Zeolites loaded with different metal ions such as Fe2+, K+,
Ag+, Cu+, Ni2+, and Zn2+ or Pd2+ conclude π-complexation between metal ions and the sulfur compounds
and have shown good adsorption performance for most of refractory sulfur compounds. However,
because of steric effect of the DBTs with one or two alkyl groups it is very difficult to remove these sulfur
compounds. One of the most successful strategies for improving accessibility is the case of hierarchical
zeolites. The presence of a bimodal pore size distribution, formed by both micropores and mesopores
improve mass transfer properties of micropores zeolites materials, which can be have advantageous for
interacting with bulky molecules. However, hierarchical zeolites have not been studied for adsorptive
desulfurization so much with respect to the limitation of zeolites in adsorption of bulky sulfur
compounds.
Acknowledgments
The authors are thankful to Research Council of Iran National Science Foundation(INSF) and to Iran
University of Science and Technology (Tehran) for financial support to this study.
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Table 1. Some of structures to related Aliphatic and aromatic sulfur compounds in crude oil.
Sulfur compounds
Structure
mercaptanes
RSH
sulfides
R2S
disulfides
RSSR
thiophenes
benzothiophenes
dibenzothiophenes
34
Table 2
Adsorptive desulfurization by X zeolite
Zeolite type
A-NaLSX,
A-NaX and
A-Na-Y
+
Cu -13X and
La3+/Cu+-13X
zeolites
Adsorption capacity / removal
efficiency
Sulfur compounds
benzothiophene (BT) dissolved
in cyclohexane(Model fuel)
A-NaLSX zeolite, A-NaX and A-NaY
had maximum adsorption capacity
35.50, 44.1 and 48.3 mg/g
respectively.
Thiophene (TP),
3-methylthiophene (3-MT),
2, 5-dimethylthiophene (2,5DMT) and benzothiophene
(BT) dissolved in n-hexane(
Model fuel)
Amount of adsorption for BT, TP, 3MT and DMT for Cu+-13X were
179.8, 146.2, 140.5 and 135.8 mg/g,
respectively. In addition, these
amounts for La3+/Cu+-13X were
197.3, 159.2, 153.5 and 151 mg/g,
respectively.
mechanism
Experimental condition
refrences
-
Dynamic adsorption tests were
carried out with packing 3 g of
adsorbent at 298 K and constant
flow rate of 5 mL/min. The content
of sulfur was 250 ppmw.
[45]
π-complexation
Static adsorption equilibriums were
performed in the initial sulfur
compounds concentration and
volume was 1000 µg/g and 100 ml,
respectively.
[46]
Table 3
Adsorptive removal of DBT on various modified Y adsorbents. Reprinted from[49], copyright (2008), With
permission from Elsevier.
Sample
Feed
Product
Reduction
Reduction
S (ppm)
S (ppm)
(%)
S (mg S/g)
Na-Yb
500
240
52
9.1
Na-Y
500
140
72
12.6
Ag-Y(I)
500
7
99
17.3
Zn-Y
500
75
85
14.9
Cu-Y
500
113
77
13.6
CuZn-Y(I)
500
10
98
17.2
ZnNd-Y
500
56
89
15.6
NiNd-Y
500
140
72
12.6
a
Adsorption was operated at 60 8C for 24 h. b Uncalcined sample.
Table 4
Adsorption of various sulfur compounds over Ag-Y(II). Reprinted from [49], copyright (2008), With permission
from Elsevier.
Feed S (ppm)
700 (400 DBT + 300 thiophene)
500 (4,6-DMDBT)
Product
Reduction
S (ppm)
(%)
Reduction
S (mg S/g)
22
97
23.8
17.4
3
>99
500 (DBT) + 2.5% toluene
119
76
13.4
500 (DBT) + 1% benzene
89
82
14.4
64
87
15.3
127
75
13.1
500 (4,6-DMDBT) + 1% benzene
500 (4,6-DMDBT) + 1%benzene, pre-oxidation
35
Table 5
Adsorption of different sulfur compounds over CuZn-Y(I). Reprinted from [49], copyright (2008), With
permission from Elsevier.
Feed S (ppm)
Product
Reduction
Reduction
S (ppm)
(%)
S (mg S/g)
700 (400 DBT + 300 thiophene)
36
95
500 (4,6-DMDBT)
<3
>99
17.5
500 (DBT) + 2.5% toluene
104
79
13.9
500 (DBT) + 1% benzene
134
73
12.8
16.4
500 (4,6-DMDBT) + 1% benzene
23.3
32
94
500 (DBT), pre-oxidation
308
38
6.7
500 (4,6-DMDBT), pre-oxidation
190
62
10.9
Table 6
Composition of model gasoline used. Reprinted from [54], copyright (2013), With permission from
Elsevier.
No.
Sulfur concentration(mg/L)
TP
3-MTP
2,5-DTP
BT
Toluene
(mg/L)
Pyridine
(mg/L)
Cyclohexene
(mg/L)
M1
100
-
-
100
-
-
-
M2
M3
100
100
-
-
100
100
500
-
500
-
M4
M5
100
100
100
100
100
100
-
-
500
-
Table 7
The compositions and proportions of model gasolines. Reprinted from [57], copyright (2013), With
permission from Elsevier.
Model gasoline
Sulfur content
composition
nCHE/ nThio
Vol,% of CHE
0
0
1
10
0.0315
0.315
100
3.15
500
10
15.76
3.15
(mg/L)
M1
94.5
M2
M3
97.5
99.0
M4
96.4
M5
M6
94.7
1038.9
Tiophene/cyclohexene
Tiophene+cyclohexene/cyclohexane
36
104.6
90.9
M7
M8
3-methylthiophene/ cyclohexane
2,5-dimethylthiophene/cyclohexane
0
0
0
0
Table 8
Composition of Model Gasoline Used. Reprinted from [60], Copyright (2014) American Chemical
Society.
No.
Sulfur concentration(mg/L)
TP
BT
M1
100
100
M2
100
100
M3
100
100
M4
100
100
Toluene
pyridine
cyclohexene
500
500
500
Table 9
Breakthrough and Saturation Loadings for TP and BT from Model Gasoline over AgY, CeY, and AgCeY
Adsorbents. Reprinted from [60], Copyright (2014) American Chemical Society.
Breakthrough Loading a (wt %)
TP
BT
Saturation loading (wt %)
TP
BT
M1
M2
M3
M4
0.822
0.243
0.090
0.192
1.550
0.525
0.098
0.417
0.873
0.254
0.098
0.205
M1
M2
M3
M4
0.424
0.406
0.165
0.395
1.044
0.989
0.217
0.943
0.447
0.411
0.176
0.404
feed
Sample AgY
1.577
0.543
0.099
0.444
Sample AgCeY
1.061
1.007
0.233
0.997
37
TP
Decline Loadings (%)
BT
TP
BT
70.438
89.051
76.606
66.129
93.677
73.129
70.905
89.003
76.518
65.586
93.722
71.833
4.245
61.085
6.769
5.268
79.215
9.646
8.054
60.626
9.642
5.090
78.040
6.051
M1
M2
M3
M4
a.
0.149
0.148
0.103
0.142
0.267
0.257
0.154
0.251
0.159
0.154
0.105
0.151
Sample CeY
0.275
0.265
0.158
0.255
0.671
30.872
4.698
3.745
42.322
6.067
3.145
33.962
4.843
3.636
42.554
7.200
Measured at a sulfur concentration in effluent of 20 mg/L for TP and BT
Table 10
The compositions of MG distillates. Reprinted from [62], copyright (2014), With permission from
Elsevier.
MG
Sulfur
content(mg/L)
Component
ncyclohexane/nTP
MG1
MG2
93.2
101.3
TP/cyclohexane
3-MTP/cyclohexane
MG3
MG4
96.8
99.5
BT/cyclohexane
TP+toluene/cyclohexane
MG5
MG6
98.5
101.9
MG7
MG8
92.5
97.7
MG9
MG10
MG11
TP+cyclohexene/cyclohexane
Cyclohexane
(vol. %)
1
0.032
96.5
100.4
10
100
0.320
3.200
93.5
500
15.800
nToluene/nTP
Toluene
(vol.%)
1
0.033
10
100
0.330
3.300
500
16.600
Table 11
Desulfurization of simulated and No. 0 diesel oils on NaY, NaY-0.09, and CeY zeolites. Reprinted from
[39], copyright (2015), With permission from Elsevier.
Adsorbent
Sulfur removal (%)
Simulated diesel oil
q (mg/g)
No. 0
Simulated diesel oil
No. 0
Without Toluene
Toluene
diesel oil
Without Toluene
Toluene
diesel oil
NaY
7.4
0.9
0.4
0.12
0.02
0.01
NaY-0.09
99.9
61.2
21.2
1.65
1.01
0.42
CeY
99.9
53
19.3
1.65
0.87
0.39
38
Table 12
Breakthrough and Saturation Loadings for TP and BT on AgCeY-n Zeolites. Reprinted from [63],
Copyright (2016) American Chemical Society.
Decrease in loading (%)
Breakthrough
AgCeY-3.0
AgCeY-4.8
AgCeY-5.3
Saturation loading
Breakthrough
Saturation
BT
TP
BT
TP
BT
0.45
1.06
-
-
-
-
0.97
0.41
0.99
4.88
5.83
8.89
6.61
0.38
0.93
0.40
0.95
7.32
9.71
11.11
10.37
M1
0.44
1.06
0.46
1.08
-
-
-
-
M2
0.42
1.00
0.44
1.02
4.55
5.66
4.35
5.56
M3
0.41
0.96
0.42
0.98
6.82
9.43
8.70
9.26
M1
0.45
1.07
0.48
1.09
-
-
-
-
M2
0.43
1.02
0.46
1.05
4.44
4.67
4.17
3.67
M3
0.42
0.97
0.44
0.99
6.67
9.35
8.33
9.17
loading (wt %)
( wt%)
TP
BT
TP
M1
0.41
1.03
M2
0.39
M3
feed
sample
a
a. Measured at a sulfur concentration in the effluent of 10% for TP and BT
Table 13
Composition of the model gasolines. Reprinted from [68], copyright (2016), With permission from
Elsevier.
No.
component
MG-1
MG-2
MG-3
Thiophene+cyclohexane
Thiophene+1-hexene/cyclohexane
Thiophene+toluene/cyclohexane
1-hexene or
Sulfur content
Toluene content (vol%) (mg/L)
52.3
45.6
48.5
5.0
5.0
39
Table 14
Breakthrough volumes of HY, NiY, and KNiY adsorbents for different model gasolines. Reprinted from
[68], copyright (2016), With permission from Elsevier.
Breakthrough volume (mg/g)
Model gasoline
HY
Ni
KNiY
MG-1
32
48
48
MG-2
4
8
16
MG-3
0
8
12
Table 15
Compositions of the model gasoline. Reprinted from [69], copyright (2016), With permission from Royal
Society of Chemistry
Model gasoline
Sulfur content( mg/L)
compositions
M-I
52.3
Thiophene+cyclohexane
M-II
51.7
5 vol% l-hexene and Thiophene+cyclohexane
M-III
49.8
5 vol% toluene and Thiophene+cyclohexane
Table 16
Adsorptive desulfurization by Y zeolites.
Zeolite
type
Y zeolite
exchanged
with Ag+,
Ag0, H+,
Na+, and
Ag2O
clusters
Y zeolite
exchanged
with Cu,
Zn and Ag
Sulfur compounds
Tetrahydrothiophene (THT)
and tert-butylmercaptan
(TBM) in pipeline natural
dibenzothiophene (DBT) and
4,6dimethyldibenzothiophene
(4,6-DMDBT)(model fuel)
Adsorption capacity or removal
efficiency
The breakthrough THT uptake changed
from 0.38 to 0.60 THT mol/molAl with
altering Ag/Al mol ratio from 0 to 0.99.
Total THT uptake on AgNa-Y was
0.81–0.99 mol THT/mol Al witch was
close to Na-Y (0.85 mol THT/mol Al).
the breakthrough TBM uptake for
AgNa-Y and NaY were 0.34 and 0.04
mol TBM/mol Al. the total TBM uptake
on AgNa-Y was 0.57–0.65 mol
TBM/mol Al which was similar to Na-Y
(0.53 mol TBM/mol Al).
Results have been listed in table 3, 4and
5.
40
Reaction time
-
DBT was
removed from
1500ppm in the
feed to 300ppm
in the product in
the first 30min,
and the sulfur
Experimental
condition
ref
π -complexation
Adsorption
experiments were
carried out at 303 K
and atmospheric
pressure using fixed
bed glass reactors at
flow rate 55 cm3/min.
The loading amount
of the AgNa-Y was
adjusted (0.10–0.18
g).
[48]
π -complexation
experimental tests
were carried out
using 1g adsorbent in
the n-octane solutions
(50 ml) with certain
concentration of
sulfur compounds
[49]
Mechanism
reduction
reaches
42mgS/g.
Cu-Ce
bimetal
ionexchanged
Y zeolites
thiophene (T),
benzothiophene (BT),
and 4,6dimethyldibenzothiophene
(4,6-DMDBT) (model fuel)
CuIY,
CeIVY and
CuICeIVY
model gasoline made up of 1octane solution of thiophene
and benzothiophene and a
certain amount of toluene or
cyclohexene(model fuel)
Ce/Niloaded Y
zeolites
dibenzothiophene (DBT)
(model fuel)
Cu(II)-Y
type
Zeolite
AgY and
CeY
zeolites
The sulfur adsorption capacities over
CeIVY, CuIY, and CuCeY were 0.37,
0.40, and 0.42 mmol/g, respectively.
With increasing the toluene
concentration to 20wt %, the sulfur
adsorption capacity was decreased to
0.33, 0.09, and 0.35 mmol/g.
For CuIY and CuICeIVY, the loadings
at breakthrough were about 0.605 wt.%
and 0.496 wt.% for TP as well as 2.422
wt.% and 2.096 wt.% for BT, and
loadings at saturation were about 0.819
wt.% and 0.762 wt.% for TP as well as
3.261 wt.% and 3.072 wt.% for BT,
respectively
Reaction time
was 4 h.
under continuous
stirring at RT to
80°C.
π-complexation
-
π-complexation
and S–M
interaction
NiCeY with adsorption capacity 7.8
mg/g had higher adsorption selectivity
than NaY, NiY and CeY with the
adsorption capacity of 2.3, 5.4 and 6.6,
respectively.
most of the
sulfur
compounds are
adsorbed within
1h
π-complexation
and S–M
interaction
mercaptans in two types of
oil cuts, L-SRG and LNaphtha
In 400 ml of L-SRG, 56% mercaptans
was removed when copper ion-exchange
level was 70%. 67% of mercaptans was
removed when 0.25 g of Cu(II)-Y
zeolite was used and also, 66% of
mercaptans was removed at 60 min.
These results for 400 ml of L-Naphtha
with 70% ion-exchange level, 0.3 g
adsorbent and 60 min time were 80.2%,
65% and 74% removal, respectively
Reaction time
was 1 h
Reactive
adsorption
thiophenic sulfur(model fuel)
The saturation capacity for NaY, AgY
and CeY zeolites were 0.39, 11.01 and
13.67 mg s/g, respectively.
Reaction time
was 5 h
π-complexation
and S–M
interaction
AgY, CeY
and
AgCeY
benzothiophene and
thiophene(model fuel)
Results of fixed-bed breakthrough
experiments have been summarized in
table 9
-
π-complexation
and S–M
interaction
Ni(II)Y
Sulfur compounds
50 ppm sulfur were brought down to
below 5 ppm level.
-
π-complexation
Cu(I)–Y ,
Ni–Y , CoY , Cu(II)–
Y , Na–Y
and Ce–Y
dimethyl disulfide
sulfur adsorption capacity of modified Y
zeolites was 157.4 mg s/g adsorbent
Reaction time
was 12 h.
π-complexation
and S–M
interaction
meso-CeY
TP, 3-methylthiophene (3-
Removal percentage of TP and 3-MTP
Reaction time
-
41
batch method under
ambient conditions
was used.
Fixed-bed
experimantes were
performed with
adsorbent quantity
was 1 g and
predetermined
constant flow rate of
0.28 mL/min.
Adsorption
desulfurization were
carried out with 0.1
or 0.2 g adsorbents
from 20 ml
transportation fuels
containing 500 mg/L
sulfur with 5 vol% of
toluene at room
temperature in a
stirred batch system.
The effects of ionexchange level (69.8,
80.2, 91.02 and
101.81), the mass of
zeolite(0.05, 0.1 and
0.15 g), and contact
time (5, 10, 30 and 60
min) on the removal
of mercaptan (RSH)
were investigated at
ambient temperature
in batch method.
Batch adsorption
experiments were
carried out by 0.3 g
adsorbent for 5 h at
25 °C. the adsorbent
amount and flow rate
were was 3.0 g, and
69.38 mL/h,
respectively in the
Fixed-bed adsorption
experiments.
The fixed bed
experiments were
carried out at a feed
flow rate of 20 mL/h,
temperature 50 °C
and 1 g adsorbent.
The feed flow rate
was at either 0.5
ml/min and 1 ml/min
during initial
experiments.
Dynamic experiments
were carried out at
atmospheric pressure
and LHSV 10.0h−1
and also, the Static
experiments at
ambient temperature
for 12 h , 0.1 g
adsorbent at 15 mL of
sample with.
Concentration 45.77
ppm.
Batch experiments
[53]
[54]
[55]
[50]
[59]
[60]
[56]
[61]
[62]
MTP), and BTP dissolved
into cyclohexane
was 3 h.
over meso-CeY were 97.9% and 96.2%,
respectively, nearly the same as those
over CeY (97.3% and 95.8%). Removal
of BTP with bigger molecular size was
increased from 92.0% over CeY to
97.0% over meso-CeY.
NaY
zeolites
modified
with 0.09
M NaOH
bensothophene from the
simulated diesel oil
Results of desulfurization of simulated
and No. 0 diesel oils on NaY, NaY-0.09,
and CeY zeolites have been summarized
in table11.
Reaction time
was 1 h.
-
IonExchanged
AgCeY
zeolites
model oil comprising
thiophene (TP),
benzothiophene (BT), and
certain amounts of toluene or
cyclohexene
Results have been listed in table12
Reaction time
was 1 h.
π-complexation
and S–M
interaction
NiY and
KNiY
thiophenic sulfur
The breakthrough volumes have been
summarized in table 14.
-
thiophenic sulfur
The breakthrough sulfur capacity of NiY
was 0.68 mg /g in olefin–thiophene
system and 0.88 mg /g in aromatic–
thiophene system. For NiPdY The
breakthrough sulfur capacity was 1.84
mg/ g in olefin–thiophene system and
2.01 mg /g in aromatic–thiophene
system
Physical
adsorption
were carried out at
R.T and atmospheric
pressure with 0.250 g
adsorbents with 5.00
mL of MG or 2.50
mL of FCC gasoline
in flasks for 3 h.
The sulfur contents of
MGs were ca. 100
mg/L and the sulfur
content of the FCC
gasoline was initially
113.3 mg/L.
experiments were
carried out in a
mixture of n-octane
and benzothiophene
(sulfur content, 275
mg/L) by a static
method with 1.0 g
adsorbent and 6 mL
diesel oil.
Total sulfur
concentration in
model oil was 200
mg/L.
In static adsorption,
0.2 g of AgCeY was
added to 40 mL of
model oil at 323 K
for 1 h under
magnetic stirring. In
dynamic adsorption,
a stainless steel tube
with an internal
diameter of 10 mm
and a length of 200
mm was used..
experiments was
down by 1 g of
adsorbents at 360°C
under H2 for 5 h.
Then, fuel was
pumped into the
fixed-bed flow
reactor with flow rate
of 0.4 mL/min, at
ambient temperature
[39]
[63]
[68]
and pressure.
NiY and
NiPdY
π-complexation
and S–M
interaction
-
fixed-bed
experiments were
performed with a
flow rate 0.5 mL min1
at room temperature
and ambient pressure.
[69]
Table 17
Adsorptive desulfurization by beta zeolites
Zeolite type
Sulfur compounds
Adsorption capacity or
removal efficiency
Reaction
time
mechanism
Cu(I)- and Ag(I)beta zeolite
thiophene
the breakthrough capacity for
Cu(I)-beta and Ag(I)-beta
were of 0.239 and 0.237
mmol S/g, respectively
-
лcomplexation
CaY/Beta and
CeY/Beta
thiophene
CeY/Beta were found to be
94.9 % removal and
CaY/Beta were found to be
42.1% removal.
Reaction
time was
3 h.
S–M
interaction
42
condition
Dynamic adsorption experiments
were performed with 1 g of
adsorbent, liquid space velocity 60
h−1, ambient temperature and
pressure.
Static adsorption experiments were
performed in the Model gas oil Nhexan/thiophene with sulfur content
200 ppm
[72]
[73]
Hydrodesulphurization(HDS)
Ring destructive pathway
Desulfurization
Extractive desulfurization
Sulfur specific pathway
Bio desulfurization
Polar adsorptive
Reactive adsorption
Adsorptive desulfurization
Selective adsorptive
Non-reactive adsorption
Integrated Adsorptive
Metal thiolates precipitation
π-complexation
Precipitate desulfurization
Sulfonium salts precipitation
Oxidative desulfurization(ODS)
Fig. 1. Technologies used for desulfurization[6].
43
Fig. 2. Diagram of gasoline desulfurization unit. Reprinted from [40], copyright (2011), With permission
from Elsevier.
Fig. 3. Sulfur removal(R%)of model gasoline over NaY (a) and LaNaY (b). Reprinted from [57],
copyright (2013), With permission from Elsevier.
44
Fig. 4. Process flow diagram of fixed bed adsorption desulfurization. Reprinted from [69], copyright
(2016), With permission from Royal Society of Chemistry
Fig. 5. LTA and X zeolite selection Chart for desulfurization. Reprinted from [79], copyright (2015),
With permission from Elsevier.
45
Fig.6. Washing solvent desorption process. Reprinted from [59], copyright (2011), With permission from
Elsevier.
46