This work describes kinetic studies of the catalytic oxidation of thiols (RSHs) found in kerosene to disulphides using a magnetically separable iron oxide coated Mg-Al layered double hydroxide supported tetra-sulphonated cobalt phthalocyanine (CoPcS/LDH@Fe3O4) catalyst in an alkali-free environment. Using 1-octanethiol as a representative RSH, we investigated the effects of different experimental parameters on RSH oxidation kinetics, including catalyst concentration, temperature (30–60 °C), and initial thiol concentration ([RSH]0, 100–300 ppm). The catalyst concentration was varied so that the [RSH]0/[Co]tot molar ratio ranged from 45 to 180. Based on the results, we propose a mechanistic rate expression to explain the observed oxidation of RSH in the presence of the CoPcS/LDH@Fe3O4 catalyst. The proposed rate law resembles double substrate Michaelis-Menten kinetics, however, for commonly encountered industrial conditions, we were able to simplify it to a linear form. This rate law for RSH oxidation can be used to design industrial reactors for an alkali-free sweetening process.

1. Research article
Kinetics and feasibility studies of thiol oxidation using magnetically
separable Mg-Al layered double hydroxide supported cobalt
phthalocyanine catalyst
Deepak K. Chauhan, Pawan Kumar, Rahul Painuly, Sunil Kumar, Suman L. Jain ⁎, Sudip K. Ganguly ⁎
CSIR-Indian Institute of Petroleum, Dehradun 248005, India
a b s t r a c ta r t i c l e i n f o
Article history:
Received 4 December 2016
Received in revised form 5 April 2017
Accepted 5 April 2017
Available online xxxx
This work describes kinetic studies of the catalytic oxidation of thiols (RSHs) found in kerosene to disulphides
using a magnetically separable iron oxide coated Mg-Al layered double hydroxide supported tetra-sulphonated
cobalt phthalocyanine (CoPcS/LDH@Fe3O4) catalyst in an alkali-free environment. Using 1-octanethiol as a rep-
resentative RSH, we investigated the effects of different experimental parameters on RSH oxidation kinetics, in-
cluding catalyst concentration, temperature (30–60 °C), and initial thiol concentration ([RSH]0
, 100–300 ppm).
The catalyst concentration was varied so that the [RSH]0
/[Co]tot molar ratio ranged from 45 to 180. Based on
the results, we propose a mechanistic rate expression to explain the observed oxidation of RSH in the presence
of the CoPcS/LDH@Fe3O4 catalyst. The proposed rate law resembles double substrate Michaelis-Menten kinetics,
however, for commonly encountered industrial conditions, we were able to simplify it to a linear form. This rate
law for RSH oxidation can be used to design industrial reactors for an alkali-free sweetening process.
© 2017 Published by Elsevier B.V.
1. Introduction
The presence of various thiols (RSHs) in petroleum products, such as
kerosene and aviation turbine fuel (ATF), is undesirable due to their cor-
rosive nature and foul odor [1]. Therefore, the conversion of RSHs into
an innocuous form is necessary before the fuel's end use. Phthalocya-
nines of metals, such as cobalt, iron, manganese, molybdenum, and va-
nadium, are well known to catalyze the oxidation of RSHs into
disulphides in alkaline media [1]. As a result they are commonly used
in fixed-bed reactors to catalytically oxidize the higher molecular
weight RSHs that are typically present in heavier petroleum products.
This reaction takes place when exposed to air in the presence of an alkali
medium and is known as fixed-bed sweetening. Today, cobalt phthalo-
cyanine (CoPc)-based catalysts are among the most popular commer-
cial catalysts available for such use [1]. The overall sweetening
reaction [1] may be represented by Eq. (1):
2RSH þ 0:5 O2 →
NaOH;catalyst
RSSR þ H2O ð1Þ
Fixed-bed sweetening is the most widely used process in refineries
worldwide for the removal of RSHs from kerosene and ATF. For this re-
action to take place, the phthalocyanine catalyst is typically impregnat-
ed on a suitable support (e.g., activated carbon). However, due to
stringent environmental regulations, the use of caustic materials for
fixed-bed sweetening is undesirable due to their hazardous nature
and the difficulty in disposing them [2]. Furthermore, the high solubility
of phthalocyanine catalysts in alkali media makes these materials less
attractive for RSH oxidation due to the continuous loss of the catalyst
during operations, leading to depleted activity.
One of the logical approaches to overcome these limitations is to im-
mobilize the homogenous metal phthalocyanine catalyst on a solid
basic support, which would not only make this process less caustic but
also easier to both recover and recycle the catalyst [3–7]. Researchers
have developed a few such catalyst systems consisting of solid basic ox-
ides, such as MgO [8] and Mg/Al-hydrotalcite grafted with tetra-
sulphonated cobalt phthalocyanine (CoPcS) [9] for alkali-free oxidation
of RSHs. In addition, there have been other reports in the literature of
catalytic systems designed for alkali-free sweetening using non-
aqueous media [10–13]. However, the use of toxic and volatile organic
solvents, such as dimethylformamide, makes the process environmen-
tally unfriendly and less cost-effective [10]. Besides these drawbacks,
supported catalysts usually involve conventional recovery techniques,
like filtration or centrifugation, which are typically inefficient at recov-
ering the catalyst.
Fuel Processing Technology 162 (2017) 135–146
⁎ Corresponding authors.
E-mail addresses: deepak_chauhan986@ymail.com (D.K. Chauhan),
choudhary.2486pawan@yahoo.in (P. Kumar), sunilkp@iip.res.in (S. Kumar),
suman@iip.res.in (S.L. Jain), sganguly.iip@gmail.com, sganguly@iip.res.in (S.K. Ganguly).
http://dx.doi.org/10.1016/j.fuproc.2017.04.003
0378-3820/© 2017 Published by Elsevier B.V.
Contents lists available at ScienceDirect
Fuel Processing Technology
journal homepage: www.elsevier.com/locate/fuproc

2. Recently, researchers have developed nano-sized catalytic supports
that can be magnetically separated, demonstrating intriguing potential
for the immobilization of homogeneous metal catalysts with improved
recovery [14–16]. In this design, the magnetic properties of the Fe3O4
catalytic core, which has been coated with a Mg-Al layered double hy-
droxide (LDH), not only allows these materials to be easily separated,
but also acts as a stabilizer that prevents the aggregation of the nanopar-
ticles. The LDH coating provides the catalyst with the –OH groups that
create the basic environment for the RSH oxidation reaction to occur.
Considering the advantages of magnetically-separable catalytic sup-
ports and in continuation of our on-going efforts to develop eco-friendly
technologies, we report the kinetics of RSH oxidation using a novel mag-
netically separable catalyst composed of an Fe3O4 nanoparticle core
coated with Mg-Al LDH and grafted with the CoPcS complex (CoPcS/
LDH@Fe3O4) [17] to establish its application for fixed-bed sweetening
of heavy petroleum products in an alkali-free environment. Kerosene
contains high molecular weight RSHs in the range of C8–C12, though
the majority component is C8 RSH [18]. Accordingly we selected 1-
octanethiol as a representative molecule of the kinds of RSHs present
in kerosene. To investigate the feasibility of using CoPcS/LDH@Fe3O4
as a fixed-bed sweetening catalyst, we studied the effects of tempera-
ture (30–60 °C) and the initial 1-octanethiol concentration ([RSH]0
,
100–300 ppm) to represent typical industrial conditions for RSH oxida-
tion. In these studies, we also varied the molar ratio of [RSH]0
to the total
cobalt concentration [Co]tot of the catalyst ([RSH]0
/[Co]tot) from 45 to
180. We systematically analyzed the experimental data to estimate
the kinetic parameters and propose a viable kinetic model. This rate
law will be useful for designing future industrial scale reactors that
use CoPcS/LDH@Fe3O4 as a sweetening catalyst. Our stability experi-
ments showed that the catalyst could be recycled several times without
any significant loss in its catalytic activity. Due to the promising activity
and stability of this magnetically separable CoPcS/LDH@Fe3O4 catalyst,
we believe it has strong potential for future application in an environ-
mentally friendly alkali-free sweetening process for heavier petroleum
products.
2. Materials and methods
2.1. Materials
All chemicals used were purchased from Sigma-Aldrich, including
iron (II) chloride (FeCl2, 98%), iron (III) chloride (FeCl3, 97%), magne-
sium nitrate hexahydrate (Mg(NO3)2·6H2O, 99.99%), aluminium nitrate
nonahydrate (Al(NO3)3·9H2O, 99.99%), ethanol (C2H5OH, 99.99%), so-
dium carbonate (Na2CO3), sodium hydroxide (NaOH, 97%), hydrochlo-
ric acid (HCl, 37%), 1-octanethiol (1-C8H17SH, 98.5%), 1-dodecane (1-
C12H26, 99%), and chlorosulphonic acid (ClHSO3, 99%). Cobalt phthalo-
cyanine (CoPc, 98%) was purchased from Lona Industries, Mumbai,
India. All chemical reagents were used without further purification.
We also used deionized water and instrument grade air (Sigma Gases,
New Delhi, India) throughout the experimental studies.
We synthesized CoPcS by treating CoPc with chlorosulphonic acid
according to a previously reported procedure [18]. In brief, the CoPcS
complex was obtained by the chlorosulphonation reaction of CoPc
using chlorosulphonic acid, which was carried out at 130–135 °C for
4 h. The resulting product was subsequently used for the synthesis of
CoPcS/LDH@Fe3O4.
2.2. Experimental procedure for the synthesis of the magnetically separable
CoPcS/LDH@Fe3O4 catalyst
Magnetic nanoparticles of Fe3O4 were synthesized by following our
previously reported procedure [17]. In brief, a solution containing
FeCl3 and FeCl2 (2:1 molar ratio) in 25 mL of deionized water containing
1 mL of HCl (Normality = 12.1) was prepared. This solution was then
added in a dropwise manner to a 1.5 M NaOH solution to precipitate
Fe3O4. In a subsequent step, thus obtained Fe3O4 nanoparticles were
coated with LDH using a co-precipitation method. In a typical synthesis,
the LDH was precipitated on Fe3O4 using a solution of Mg(NO3)2·6H2O
and Al(NO3)2·9H2O (3:1 molar ratio or 2:1 weight ratio) in a water/
ethanol (1:1) mixture containing Fe3O4 nanoparticles (0.11 wt%)
while maintaining a pH of 10 by the simultaneous addition of Na2CO3
and NaOH solutions (1:3 molar ratio). Thus obtained gel was kept at 60
°C for 24 h to facilitate the growth of LDH platelets on the Fe3O4 magnetic
nanoparticles (Fe3O4@LDH). The synthesized Fe3O4@LDH material was
used as a support to graft the CoPcS complex units according to
Scheme 1. This process involved the addition of 0.5 g of CoPcS and 3 g
of LDH@Fe3O4 to a 50 mL ethanol/water (1:1) mixture, which was stirred
for 24 h at 80 °C. The resulting product was washed with ethanol, water,
and then dried at 60 °C for 24 h to obtain the CoPcS/LDH@Fe3O4 catalyst.
The finally obtained supported catalyst was characterized using field
emission scanning electron microscopy (FE-SEM, Quanta 200F, FEI) at
an acceleration voltage of 15 kV. Energy dispersive X-ray (EDX) spectros-
copy was performed at an acceleration voltage of 20 kV. High resolution
transmission electron microscopy (HR-TEM, FEI-Tecnai G2, Tecnai) was
performed at an acceleration voltage of 200 kV. Finally, X-ray diffraction
(XRD, Bruker D8 Advanced diffractometer) was done at 40 kV and 40 Ma
with Cu Kα radiation (λ = 1.5418 nm) and a surface pore analyser
(ASAP2010, Micrometrics) using liquid nitrogen at 77 K.
2.3. Experimental set-up for the kinetics studies
The experimental setup for the kinetics studies is shown in Fig. 1a.
Briefly, these experiments were carried out in a 100 mL three-necked
and jacketed glass reactor. The central neck was used for uniform air
sparging through the perforated glass distributor, which dipped down
into the reaction mixture. The design of the air sparger is shown in
Fig. 1b. We continuously purged the reactor with air at 2.2 normal liters
per minute (NLPM) to ensure a greater amount was present than what
was required for a stoichiometric RSH oxidation reaction (Eq. (1)) to
occur. The second neck of the glass reactor was used for sampling the re-
action at different time intervals so that we could estimate the % RSH
conversion as the reaction progressed, according to the following equa-
tion:
%RSH ¼ RSH½ Š0
− RSH½ Š
 100= RSH½ Š0
ð2Þ
The third neck was kept open to allow the continuous passage of air
during operations.
We doped 1-octanethiol in 1-dodecane to represent synthetic kero-
sene, varying [RSH]0
in the range of 100–300 ppm. A calculated amount
of the CoPcS/LDH@Fe3O4 catalyst was added to maintain the desired
[RSH]0
/[Co]tot molar ratio, which we controlled from 45 to 180. The
glass reactor jacket was connected to a circulating water bath to main-
tain a uniform temperature inside the reactor. The contents of the reac-
tor were uniformly mixed using a magnetic stirrer to ensure the
temperature was uniform throughout the reaction mixture, which is es-
sential for kinetics studies. The experiments were carried out at differ-
ent temperatures, ranging from 30 to 60 °C. We estimated the RSH
concentration using the UOP163–89 method with a Metrohm 888
Titrando potentiometric titrator and determined the cobalt content of
the catalyst using inductively coupled plasma-atomic emission spec-
troscopy (ICP-AES, DRE, PS-3000UV, Leeman Labs Inc.). For comparison,
we also used a homogeneous unsupported CoPcS catalyst as a control
for RSH oxidation studies in an alkali-free environment.
3. Results and discussion
3.1. Characterization of CoPcS/LDH@Fe3O4
The heterogeneous catalyst was synthesized using a two-step
procedure as shown in Scheme 1. Units of the CoPcS complex were
attached to the surface of the magnetically separable LDH@Fe3O4
136 D.K. Chauhan et al. / Fuel Processing Technology 162 (2017) 135–146

3. support, presumably via the ionic interaction between the -SO3
−
ions
of the CoPcS and the –OH groups on the surface of the LDH@Fe3O4
material. We subsequently determined the morphology and the ele-
mental surface distribution of the resulting CoPcS/LDH@Fe3O4 cata-
lyst using electron microscopy and surface characterization
techniques.
Electron microscopy imaging revealed that the diameter of the Fe3O4
nanoparticles was approximately 20 nm (Fig. 2a,d,g). After coating the
magnetic Fe3O4 nanoparticles with LDH, the resulting material featured
a rough, ridge-like surface due to the formation of LDH platelets (Fig.
2b,e,h). The subsequent grafting of CoPcS on the LDH@Fe3O4 support re-
veals no visible changes in the material's morphology, which is probably
due to the lower loading of Co (as compared to Mg–Al loadings) on the
LDH@Fe3O4 support (Fig. 2c,f,i). EDX confirmed the presence of Co on
the surface of the CoPcS/LDH@Fe3O4 catalyst (Fig. 3 (a)–(c)). The aver-
age Co content of three different areas measured on the surface was
found to be around 1.9 wt%. ICP-AES analysis determined the total Co
content to be around 1.4 wt%. Furthermore, we used ICP-AES to find
the wt% of Mg, Al, and Fe in the catalyst, which was 10.45, 5.13, and
2.37 wt%, respectively. These values well matched the weight ratio of
Mg to Al (2:1) of the Mg and Al precursors (Mg (NO3)2·6H2O and Al
(NO3)2·9H2O) used for the LDH synthesis [17]. The composition analy-
sis of the CoPcS/LDH@Fe3O4 material as obtained using EDX and ICP-
AES is reported in Table 1.
We used the surface pore analyser to determine the Brauner–
Emmett–Teller (BET) average total surface area of the LDH@Fe3O4 and
CoPcS/LDH@Fe3O4 samples. The average total surface area was deter-
mined to be ~90 m2
/g for LDH@Fe3O4 and ~80 m2
/g for CoPcS/LDH@
Fe3O4. Similarly, measured the average pore volume to be ~0.29 cm3
/g
for LDH@Fe3O4 and ~0.20 cm3
/g for CoPcS/LDH@Fe3O4·The XRD pat-
tern of the CoPcS/LDH@Fe3O4 material was identical to the precursor
LDH@Fe3O4 substrate, demonstrating that the presence of CoPcS did
not lead to changes in the phase of the magnetic LDH@Fe3O4 compound
(Fig. 4). This further suggested that CoPcS treatment was limited to the
surface, with negligible intercalation between the LDH layers. The re-
duction in total surface area and pore volume after immobilization of
CoPcS units also confirms this hypothesis. In combination, the SEM,
EDX, and XRD results further support the hypothesized attachment of
CoPcS units on the surface of the LDH@Fe3O4 support.
Scheme 1. Synthesis of the CoPcS/LDH@Fe3O4 catalyst.
Fig. 1. (a) Experimental laboratory glass reactor set-up for the kinetic studies, including the (b) design of the air sparger.
137D.K. Chauhan et al. / Fuel Processing Technology 162 (2017) 135–146

4. 3.2. Mechanism kinetics of RSH oxidation
A number of researchers have reported the kinetics of catalytic RSH
oxidation using different types of RSHs, including C2H5SH (ethanethiol),
i-C3H7SH (isopropanethiol), n-C4H9SH (1-butanethiol), i-C4H9SH (2-
methyl-2-propanethiol), and 1-C8H17SH (1-octanethiol) [19–29]. The
majority of these experimental studies reported first order reaction ki-
netics for catalytic RSH oxidation with respect to the RSH concentration
[19,20,22,28–30]. Researchers have also proposed a radical anion mech-
anism of RSH oxidation [16,22]. The RS• radical is generated through
electron transfer from the thiolate anion (RS−
) to the cobalt ions pres-
ent on the catalyst. These RS• radicals subsequently dimerize to gener-
ate the corresponding disulphides. The suggested mechanistic
pathway [25,26,31] depicting oxidation of thiols to the corresponding
disulphides is shown in Eqs. (3)–(7).
Ionization of thiols
RSH þ OH−
→
λ1
RS þ H2O ð3Þ
Generation of thiol radicals
CoII
PcS þ RS−
⇄
λ2
λ−2
RS−
—CoII
PcS ð4Þ
RS−
—CoII
PcS þ O2 ⇄
λ3
λ−3
RS−
—CoIII
PcS—O2
−
ð5Þ
RS−
—CoIII
PcS—O2
−
→
λ4
CoII
PcS þ RSg
O2
−
ð6Þ
Generation of disulphides
2RSg
→
λ5
RSSR ð7Þ
Base regeneration
O2
−
þ 1=2H2O →
λ6
OH−
þ 3=4O2 ð8Þ
Based on the suggested/assumed mechanistic pathway, we obtained
a rate law similar to a double substrate Michaelis-Menten rate expres-
sion: [25,26]
−rRSH ¼
λ2λ3λ4 cat½ Štot RSH½ Š O2½ Š
λ−2 λ4 þ λ−3ð Þ þ λ3λ4 O2½ Š þ λ2 λ4 þ λ−3ð Þ RSH½ Š þ λ2λ3 RSH½ Š O2½ Š½ Š
ð9Þ
in which λi s represents forward/backward reaction rate constants.
Our catalyst, being a solid basic oxide, is rich in –OH groups on its
surface, which provide the necessary alkaline conditions for RSH
Fig. 2. SEM images of (a) Fe3O4, (b) LDH@Fe3O4, (c) CoPcS/LDH@Fe3O4, and HR-TEM images of (d) (g) Fe3O4, (e) (h) LDH@Fe3O4, and (f) (i) CoPcS/LDH@Fe3O4.
138 D.K. Chauhan et al. / Fuel Processing Technology 162 (2017) 135–146

5. oxidation. Therefore, we performed our oxidation kinetics studies in
aqueous media without any additional alkali. Furthermore, as evi-
denced during the characterization studies presented in Section 3.1,
the Co(II) ions appear to be located on the surface of the CoPcS/LDH@
Fe3O4 catalyst, which features significant surface area. The Co(II) ions
are therefore in direct contact with the reaction medium and available
for participating in the RSH oxidation reaction. If we assume a similar
radical anion reaction mechanism takes place, wherein abstraction of
H+
from the thiols is mediated by OH−
groups present in Mg-Al
LDH, the resulting anions (RS−
) may be assumed to be responsible for
Fig. 3. EDX patterns of the CoPcS/LDH@Fe3O4 catalyst at three distinct locations (a–c) on the sample surface.
139D.K. Chauhan et al. / Fuel Processing Technology 162 (2017) 135–146

6. production of the free radicals (RS•) in the presence of CoPcS/O2, which
subsequently dimerize to form disulphides. Therefore, we can custom-
ize the rate law for the CoPcS/LDH@Fe3O4 catalytic system to rewrite
Eq. (9) as:
−rRSH ¼
α1 Co½ Štot RSH½ Š O2½ Š
α2 þ α3 O2½ Š þ α4 RSH½ Š þ α5 RSH½ Š O2½ Š½ Š
ð10Þ
in which αi s represents modified forward/backward reaction rate
constants.
Excess air than what is required stoichiometrically for RSH oxidation
(Eq. (1)) is usually present within industrial-scale reactors. Under these
conditions, we can simplify the rate expression to a single substrate
Michaelis-Menten equation: [14,16,17]
−rRSH ¼
αI
1 Co½ Štot RSH½ Š
αM þ RSH½ Š½ Š
ð11Þ
in which α1
I
is the lumped rate constant.
In commonly encountered industrial conditions, the [RSH]0
in kero-
sene is usually 300 ppm or less, which is lower in magnitude than the
Michaelis-Menten constant (αM). Under such circumstances, the rate
expression in Eq. (11) may be written as a first order rate law:
−rRSH ¼ αcat RSH½ Š ¼ αeff Co½ Štot RSH½ Š ¼ αef f 0 Co½ Štote− E
R:T RSH½ Š ð12Þ
in which αeff ¼ αeff 0e− E
R:T.
Our laboratory reactor design must therefore allow an ample supply
of air/oxygen to the bulk solution so that we may reproduce near indus-
trial conditions that enable us to achieve kinetics that follow a linear
rate law.
3.3. Laboratory reactor design, hydrodynamics, and mass transfer
limitations
Catalytic RSH oxidation in liquefied petroleum gas sweetening is a
mass transfer limited reaction [23]. In order to estimate the kinetics of
this reaction, it is necessary to control the hydrodynamics/operational
parameters of our lab-scale reactor, such as the rotational speed of the
magnetic stirrer (N), to achieve maximum oxygen transport rates that
overcome any mass transfer limitations. It is therefore essential to es-
tablish hydrodynamic conditions that ensure a maximum supply of
air/oxygen to the bulk liquid in order to satisfy the pre-requirements
of rate Eq. (12). Based on this rationale, we determined conditions for
the agitated and sparged semi-batch glass reactor used in this study
that would satisfy both the design and operational requirements for
conducting the kinetic studies.
Since, the liquid phase of the reaction contains a light dispersion of
CoPcS/LDH@Fe3O4 catalyst, it is necessary to overcome mass transfer
limitations from both internal and external perspectives of mass trans-
fer resistance with respect to the nanoparticles (i.e., within the pores
and on the external surface) [32,33]. SEM, TEM, and other surface char-
acterization of the catalyst showed that the surface area to volume ratio
of the nano-material was high due to the very small diameter (~20 nm).
There was also reasonable evidence via EDX and XRD to assume that
CoPcS was attached to the surface of the CoPcS/LDH@Fe3O4 nanoparti-
cles. Therefore most of the cobalt ions are presumably in direct contact
with the reaction medium and readily available for participating in the
RSH oxidation reaction. It is logical to conclude that there is negligible
resistance offered to the approaching RSH reactant molecules due to
the pores, since most of the oxidation reaction may be assumed to be
happening on the external surface of the nanoparticles given the loca-
tion of the CoPcS groups. Therefore in this case, we can assume that
the mass transfer rates are controlled by external mass transfer resis-
tance. We can represent the external mass transport by KLS, which is
the mass transfer coefficient [32]. In brief, KLS represents the overall
rate of oxygen transport from the air to the bulk reaction medium in
the laboratory glass reactor. For a given system and reactor geometry,
KLS is dependent on N and the superficial gas velocity (vg), as shown
in Eq. (13):
KLS∝ Nβ1
vg
β2 ð13Þ
in which exponent β1 falls in the range of 1.2 to 3.02, and β2 is in the
range of 0 to 0.29 [32].
It is evident from the magnitude of the exponents that N determines
KLS to a greater degree than vg. In order to maximize KLS, or in other
words to minimize the mass transfer resistance, we need to clearly un-
derstand the relationship between the hydrodynamic regimes and the
hydrodynamic parameter, N. The literature reports four different hydro-
dynamic regimes [32] in an agitated and sparged reactor system, which
are dependent on N. For a given vg and at low values of N, the air within
this sparged and agitated system escapes axially to the liquid surface,
which is called the by-pass regime. Further increasing N moves the sys-
tem into the loading regime, in which gas is dispersed inefficiently. Next
is the transition regime, which has limited gas re-circulation. Finally,
further increase in N results in the total gas recirculation regime (N =
Nc). Any further increase in N does not lead to significant improvement
in the % RSH conversion. Warmoeskerken and Smith [34] report that in
the total recirculation regime, the bulk liquid within a reactor system is
uniformly aerated. Experiments conducted just above Nc are expected
to be free of mass transfer limitations and therefore operate in the kinet-
ic regime.
Accordingly, we determined the value of Nc in our reactor system by
measuring the % RSH conversion of the thiol oxidation reaction at a fixed
time (15 min) as a function of N at a given air flow rate of 2.2 NLPM. We
varied N from 0 to 800 rpm using an [RSH]0
of 300 ppm and a [RSH]0
/
[Co]tot of 90 at 30 °C. The % RSH conversion for different values of N is
Table 1
Metal analysis of CoPcS/LDH@Fe3O4 catalyst.
Parameters Units SEM-EDX ICP-AES
Set 1 Set 2 Set 3 Avg
Co wt% 1.81 1.66 2.30 1.92 1.40
Mg wt% 10.28 8.73 8.38 9.13 10.45
Al wt% 3.52 3.27 4.27 3.68 5.13
Fe wt% 0.94 0.66 0.63 0.74 2.37
Fig. 4. XRD patterns of the (a) Fe3O4, (b) LDH@Fe3O4, and (c) CoPcS/LDH@Fe3O4 samples.
140 D.K. Chauhan et al. / Fuel Processing Technology 162 (2017) 135–146

7. reported in Fig. 5. It is evident from these results that the % RSH conver-
sion increased with N up to 600 rpm. No significant variation was
observed in the % RSH conversion as N was further increased from 600
to 800 rpm (between 68.4% to 73.3%, with an average of 70.8%),
indicating the system had entered the total recirculation regime. We
therefore concluded that the value of Nc for our system was 600 rpm.
Thus, we conducted all the kinetics experiments at 625 rpm (i.e., just
above Nc).
3.4. RSH oxidation using unsupported CoPcS catalyst and LDH@Fe3O4
scaffold
For the control experiments, we studied the RSH oxidation perfor-
mance of the unsupported CoPcS catalyst (i.e., without the LDH@Fe3O4
scaffold), as well as the reactivity of the LDH@Fe3O4 scaffold without
CoPcS in an alkali-free environment. The purpose of these control ex-
periments was to determine the efficacy of the catalyst without the
basic OH groups of the LDH support, as well as the effects of the
LDH@Fe3O4 scaffold without the CoPcS. We doped 300 ppm 1-
octanethiol in 1-dodecane to represent the kerosene and added the un-
supported CoPcS catalyst to this medium such that [RSH]0
/[Co]tot = 180.
We conducted the experiment at 30 °C, an airflow rate of 2.2 NLPM, and
N of 625 rpm to ensure that mass transfer limitations were overcome, as
described in Section 3.3. Samples of the reaction mixture were collected
during the progress of the reaction at 8 h, 15 h, 20 h, and 25 h of reaction
time. Analysis indicated that the RSH content (UOP 163-89 method)
ranged between 255 and 260 ppm for all reaction times studied. In
other words, we observed no significant change in the amount of RSH
content between 8 h and 25 h of reaction. Therefore, we confirmed
that RSH oxidation using the conventional CoPcS catalyst was not effec-
tive in an alkali free environment, indicating the importance of the LDH
support.
Similarly, we doped 335 ppm 1-octanethiol in 1-dodecane to repre-
sent kerosene and added 0.02 g of the LDH@Fe3O4 scaffold (without
CoPcS) to this medium, which was comparable to the catalyst weight
at [RSH]0
/[Co]tot = 90. We conducted the experiment at 30 °C, an air-
flow rate of 2.2 NLPM, and N of 625 rpm to ensure that mass transfer
limitations were overcome, as described in Section 3.3. Samples of the
reaction mixture were collected during the progress of the reaction at
3 h, 6 h, 8 h, and 10 h of reaction time. Analysis indicated that the RSH
content (UOP 163-89 method) ranged between 300 and 310 ppm for
all reaction times studied. In other words, we observed no significant
change in the amount of RSH content between 3 h and 10 h of reaction
in this case as well. Therefore, we confirmed that RSH oxidation using
just the LDH@Fe3O4 scaffold was not an effective catalyst in an alkali-
free environment, indicating the importance of the CoPcS.
3.5. Estimating the kinetic parameters
3.5.1. RSH mole balance
RSH mole balance was carried out for the semi-batch reactor (Fig.
1a) based on Eq. (12). On integrating the mole balance over the reaction
time (t), the expression obtained is shown in Eqs. (14) and (15). 1-
Octanethiol was doped in 1-dodecane to represent the synthetic kero-
sene feed. All parametric experimental studies were conducted at N of
625 rpm with an air flow rate of 2.2 NLPM to ensure mass transfer lim-
itations were overcome (see Section 3.3). We evaluated the CoPcS/
LDH@Fe3O4 performance by studying the effects of experimental pa-
rameters like temperature, initial RSH concentration, and CoPcS/LDH@
Fe3O4 catalyst concentration on the effective (αeff) and lumped (αcat)
rate constants, specifically measuring the RSH content in the reactor at
these different experimental conditions. We analyzed this data using
Eqs. (14) and (15) to estimate αeff and αcat, which is discussed in subse-
quent Sections 3.5.2 to 3.5.4.
ln
RSH½ Š0
RSH½ Š
¼ αeff Co½ Štott ð14Þ
Alternatively, the equation can be written as:
ln
RSH½ Š0
RSH½ Š
¼ αcatt ð15Þ
In terms of fractional RSH conversion (X) Eq. (15) can be expressed
as:
ln
1
1−X½ Š
¼ αcatt ð16Þ
where:
X ¼ RSH½ Š0
− RSH½ Š
= RSH½ Š0
ð17Þ
3.5.2. Effect of temperature
In order to estimate the effective activation energy (E) of the catalyst
thiol oxidation reaction, we studied how αcat changed at three different
temperatures, including 30o
, 40o
, and 60 °C, which represent typical in-
dustrial conditions. We also maintained hydrodynamic conditions es-
sential to establishing a kinetic regime throughout these experiments
(see Section 3.3). We used an [RSH]0
of 300 ppm as well as [RSH]0
/
[Co]tot = 90, which allowed αcat and αeff to be dependent on tempera-
ture alone, as evident from Eqs. (12), (14), and (15). The regression pro-
files of αcat with respect to reaction time at 30°, 40°, and 60 °C are shown
in Fig. 6, enabling us to obtain a slope for αcat of 0.12 h−1
, 0.14 h−1
, and
0.18 h−1
, respectively. The regression model had a reasonably good fit
Fig. 5. Determination of the critical agitation speed, Nc, for identification of the kinetic
regime.
Fig. 6. Effect of temperature on the kinetics of RSH oxidation.
141D.K. Chauhan et al. / Fuel Processing Technology 162 (2017) 135–146

8. with R2
values greater than 0.96 for all the cases studied. The corre-
sponding values of αeff were computed using Eqs. (12) and (14) at a
constant [Co]tot. We estimated E using the Arrhenius equation, shown
in Eq. (16). Our regression analysis estimated the value of E for the cat-
alytic thiol oxidation reaction to be 10.143 kJ/mol, which compared well
to previous reports in the literature for similar RSHs (16.29 kJ/mol and
12 kJ/mol) [25,29]. The regression model for Eq. (18) showed a reason-
ably good fit, featuring an R2
value of nearly 1 (Fig. 7).
lnαeff ¼ −
E
RT
þ lnαeff0 ð18Þ
3.5.3. Effect of initial RSH concentration
We also studied the effect of [RSH]0
on the kinetics of RSH oxidation.
We looked at three concentrations of [RSH]0
(100, 200, and 300 ppm) at
30 °C and a [RSH]0
/[Co]tot of 90. We also maintained the necessary hy-
drodynamic conditions to establish a kinetic regime during the experi-
ments (see Section 3.3). In Fig. 8, the regression analysis composite
determined an αcat of 0.12 h−1
for all the [RSH]0
concentrations
employed. The regression model showed a reasonably good fit with
an R2
value of nearly 1. The composite αcat was found to be comparable
with the value obtained for [RSH]0
= 300 ppm at a temperature of 30 °C
reported earlier in Fig. 6 in Section 3.5.2. Hence, we can conclude that
the magnitude of αcat is independent of [RSH]0
. This behaviour can be
explained by the alternate form of Eq. (15), as shown in Eq. (16),
which is independent of [RSH]0
and expressed in terms of X defined in
Eq. (17), supporting the first order kinetics of the catalytic RSH oxida-
tion reaction. To confirm this, the conversion data was also collected
at the initial reaction times for three [RSH]0
so that an averaged αcat
value could be obtained. It can be seen that a similar trend was observed
at a higher catalyst concentration of [RSH]0
/[Co]tot = 9 as shown in Fig.
9. The resulting αcat value obtained (1.18 h−1
) at this higher catalyst
concentration was almost 10 times the αcat (0.12 h−1
) reported earlier
for [RSH]0
/[Co]tot = 90, which can be explained by Eqs. (14)–(15). The
regression model in this case also showed a reasonably good fit, with
an R2
value of 0.96.
3.5.4. Effect of CoPcS/LDH@Fe3O4 catalyst concentration
We evaluated the performance of the CoPcS/LDH@Fe3O4 catalyst at
30 °C for three different [RSH]0
/[Co]tot molar ratios (45, 90, 180) to
study the effect of catalyst concentration on the RSH oxidation kinetics
using a feed containing 300 ppm of 1-octanethiol in 1-dodecane. We
maintained the necessary hydrodynamic conditions to establish a kinet-
ic regime during the experiments (see Section 3.3).
We used regression analysis to obtain the best fitting equations of
the resulting RSH concentration profiles as a function of reaction time,
which allowed us to estimate the αcat parameter for the three different
[RSH]0
/[Co]tot ratios at 30 °C (Fig. 10). The regression models showed a
reasonably good fit with R2
values greater than 0.97 for all the molar ra-
tios studied (Table 2). We observed that for [RSH]0
/[Co]tot = 90, the
slope of the regressed line was 0.12 h−1
, whereas for double the
molar ratio, [RSH]0
/[Co]tot = 180, the slope was half as large at
0.06 h−1
. A similar phenomenon was observed for the experimental
case of [RSH]0
/[Co]tot = 45 (0.24 h−1
) compared with [RSH]0
/[Co]tot
= 90 (0.12 h−1
). This trend can be explained with the help of Eqs.
(14) and (15), which show that αcat at a constant temperature depends
on the [Co]tot concentration in the reaction medium. We therefore plot-
ted the values of αcat as a function of [Co]tot, as shown in Fig. 11, which
confirms the assumption of the linear dependence of αcat on [Co]tot as
described in Eqs. (12), (14), and (15) at a constant temperature. The re-
gression model in Fig. 10 showed a reasonably good fit with an R2
value
of nearly 1.
Furthermore, we also observed that for similar % RSH conversion
levels when the [RSH]0
/[Co]tot molar ratio was reduced from 180 to
90, the total reaction time (tr) was reduced by half (from ~40 h to
~20 h), indicating the effect of double the [Co]tot concentration. A simi-
lar phenomenon was observed when [RSH]0
/[Co]tot was reduced from
90 to 45, in which tr decreased from ~20 h to ~10 h. This linear trend
Fig. 7. Arrhenius plot for the estimation of effective activation energy, E, for the catalytic thiol oxidation reaction.
Fig. 8. Effect of [RSH]0
on the kinetics of RSH oxidation. Experimental conditions: [RSH]0
= 100–300 ppm, Air flow rate = 2.2 NLPM, N = 625 rpm, T = 30 °C, [RSH]0
/[Co]tot = 90.
142 D.K. Chauhan et al. / Fuel Processing Technology 162 (2017) 135–146

9. in decreasing reaction time with increasing catalyst concentration may
be explained using Eqs. (12) and (16), which on re-arrangement into
Eq. (19) demonstrate that for the same % RSH conversion levels at a con-
stant temperature, a reduction in [Co]tot by half causes tr to double:
tr ¼
C1
αeff Co½ Štot
¼
C2
Co½ Štot
ð19Þ
in which C1 and C2 are constants. Furthermore, this was also confirmed
by the trend in Fig. 12, which shows the linear dependence of tr on 1/
[Co]tot. Our combined data analysis of the RSH oxidation kinetics con-
firms the CoPcS/LDH@Fe3O4 catalyst follows first order kinetics for ex-
perimental parameters that reflect actual industrial conditions.
3.6. Rate law
We conducted these kinetic studies using representative [RSH]0
levels in the range of 100–300 ppm and at temperatures between 30
and 60 °C to represent actual industrial conditions. All the experiments
were conducted at an air flow rate of 2.2 NLPM and N of 625 rpm to en-
sure that prevailing hydrodynamics helped overcome mass transfer
limitations so that we could determine the intrinsic kinetics of the
reaction. Based on the kinetic studies conducted as described in
Section 3.5 and our subsequent data analysis for estimating the kinetic
parameters, we were thus able to simplify our proposed rate law to
the following linear form for commonly encountered industrial condi-
tions:
−rRSH ¼ 87:44 Co½ Štote−10143
R:T RSH½ Š ð20Þ
To validate the rate law, we predicted the total reaction time (tr
pre
)
for previous % RSH conversion experiments using these estimated ki-
netic parameters and compared those predictions with the actual ex-
perimental reaction time results (tr). Table 3 reports the details of the
experimental conditions used to decrease the [RSH]0
levels to a final
[RSH] level in the range of 25–40 ppm, along with the corresponding ex-
perimental tr. In Fig. 13, we constructed a parity plot of the predicted
versus actual experimental reaction times and calculated the extent of
fit in terms of root mean square deviation, which was determined to
be 0.2%. We can see that most of the parity points lie on the y = x
line, showing a reasonably good fit and thus validating the rate model.
3.7. Stability of CoPcS/LDH@Fe3O4
To study the long-term stability of CoPcS/LDH@Fe3O4, we performed
a series of catalyst recycling experiments. After completion of the reac-
tion, the heterogeneous catalyst could be readily be recovered using an
external magnet. Then the recovered material was washed with ethanol
and water, and dried for 24 h at 60 °C for reuse. The recovered catalyst
was recycled 7 times, each time using a fresh feed of 1-octanethiol
doped in 1-dodecane for the thiol oxidation reaction. The experiments
were conducted using an [RSH]0
of 200 ppm at 60 °C, which is the
Fig. 9. Estimation of αcat at the first few minutes of RSH oxidation. Experimental conditions: [RSH]0
= 103–355 ppm, air flowrate = 2.2 NLPM, N = 625 rpm, T = 30 °C, [RSH]0
/[Co]tot = 9.
Fig. 10. Effect of the CoPcS/LDH@Fe3O4 catalyst concentration on the kinetics of RSH
oxidation. Experimental conditions: [RSH]0
= 300 ppm, air flowrate = 2.2 NLPM, N =
625 rpm, T = 30 °C, [RSH]0
/[Co]tot = 45–180.
Table 2
Effect of CoPcS/LDH@Fe3O4 catalyst concentration on αcat.a
Parameters Units Set 1 Set 2 Set 3
[RSH]0
/[Co]tot Molar ratio 45 90 180
[RSH]0
ppm 300 300 300
[Co]tot mmol L−1
0.016 0.008 0.004
αcat h−1
0.24 0.12 0.06
1/[Co]tot mmol−1
L 6.25 12.5 25
tr h 10 19 41
αeff mmol−1
L h−1
1.56 1.56 1.56
T °C 30 30 30
a
Experimental conditions: air flow rate = 2.2 NLPM, N = 625 rpm.
143D.K. Chauhan et al. / Fuel Processing Technology 162 (2017) 135–146

10. highest possible operating temperature consistent with industrial con-
ditions. We estimated the RSH content (UOP 163-89 method) at the
end of 9 h in all 7 runs in order to compare the oxidation reaction
under identical experimental conditions (Fig. 14 (a)). Our findings
show that the % RSH conversion showed negligible change for each
run conducted with the recycled CoPcS/LDH@Fe3O4 catalyst, which re-
flects the material's stability. Furthermore, we found that the cobalt
content of the recovered catalyst after 7 cycles as estimated by ICP-
AES was the same (1.4%) as the fresh catalyst, which further confirmed
that the CoPcS/LDH@Fe3O4 material demonstrates long-term stability.
Moreover, the morphology of the material after 7 cycles was also stud-
ied, as shown in Fig. 14(b) and observed to be quite similar to the fresh
CoPcS/LDH@Fe3O4 catalyst.
Furthermore, we compared the synthesized heterogeneous catalyst
and protocol with previously reported methodologies for the oxidation
of thiols under alkali-free conditions [2,3,10,35–37]. Menini et al. [10] re-
ported an efficient Co-Fe magnetic composite for the liquid phase aerobic
oxidation of thiols into disulphides using dimethylformamide as a reac-
tion medium. However, the highly toxic and volatile nature of organic
solvents as well as the tedious procedures for removing/separating
them from the products makes this method of less practical relevance.
Similarly, a number of solid-based materials, such as Mg-containing ox-
ides [2,3,35] and surface modified carbons [36] have been proposed as
supports for cobalt pthalocyanine catalysts for this reaction. However,
these solid basic oxides possess limited stability due to their rapid de-ac-
tivation, which limits their use for large scale applications. Gao et al. [37]
reported silica-supported cobalt (II) tetrasulphophthalocyanine as a cat-
alyst for the oxidation of thiols present in gasoline. However, the process
involves very high temperatures (150–300 °C), which makes it highly
energy intensive and less cost effective. In contrast, herein we report
highly stable magnetically separable CoPcS/LDH@Fe3O4 catalyst for the
oxidation of thiols to disulphides under comparatively mild reaction con-
ditions. In addition, the magnetic core of the catalyst allows it to be easily
recovered with an external magnet for reuse. The CoPcS/LDH@Fe3O4 cat-
alyst exhibits consistent activity for several cycles without showing any
detectable leaching or morphological changes during the reaction. In
combination, these attributes suggest this heterogeneous material
could serve as a highly effective and practical catalyst for the oxidation
of thiols under alkali-free conditions.
4. Conclusions
We developed the CoPcS/LDH@Fe3O4 catalyst for an alkali-free
fixed-bed sweetening process for heavier petroleum products. The ki-
netics of RSH oxidation using this heterogeneous catalyst were investi-
gated in order to assess this material's feasibility for potential scale-up
and further process development. We conducted the kinetics studies
with an [RSH]0
concentration of 1-octanethiol ranging from 100 to
300 ppm and a temperature in the range of 30–60 °C, which is quite
similar to industrial conditions. The catalyst concentration was varied
such that the [RSH]0
/[Co]tot molar ratio ranged from 45 to 180. We pro-
posed a mechanistic rate law to explain the observed catalytic RSH oxi-
dation reaction in the presence of CoPcS/LDH@Fe3O4. The proposed rate
law resembles double substrate Michaelis-Menten kinetics, however,
for commonly encountered industrial conditions, we simplified it to a
linear form that fits the kinetic data reasonably well. Based on experi-
mental data, we were able to successfully estimate the kinetics param-
eters for the rate law.
Fig. 11. Dependence of αcat on the [Co]tot concentration of the catalyst.
Fig. 12. Effect of the CoPcS/LDH@Fe3O4 catalyst concentration on the total reaction time, tr.
Table 3
Summary of experimental conditions and tr data.a
S.No [RSH]0
[RSH]0
/[Co]tot T tr
ppm Molar ratio °C H
1 100 90 30 10.50
2 100 90 40 7.50
3 100 90 60 6.00
4 200 90 30 13.00
5 200 90 40 10.50
6 200 90 60 9.00
7 300 45 30 10.00
8 300 90 30 19.00
9 300 180 30 41.00
10 300 90 40 16.50
11 300 90 60 13.00
a
Experimental conditions: air flow rate = 2.2 NLPM, N = 625 rpm.
Fig. 13. Parity plot for the comparison of the experimental (tr) and predicted (tr
pred
) total
reaction time of the RSH oxidation reactions. Experimental Conditions: [RSH]0
= 100–
300 ppm, air flowrate = 2.2 NLPM, N = 625 rpm, T = 30–60 °C, [RSH]0
/[Co]tot = 45–180.
144 D.K. Chauhan et al. / Fuel Processing Technology 162 (2017) 135–146

11. The salient conclusions of the kinetics studies are as follows:
(i) A magnetically separable Fe3O4 coated Mg-Al layered double hy-
droxide that supports a tetra-sulphonated cobalt phthalocyanine
catalyst (CoPcS/LDH@Fe3O4) was found to be effective for heavi-
er RSH oxidation in an alkali-free environment.
(ii) The activity of the CoPcS/LDH@Fe3O4 catalyst was maintained
even after being reused 7 times for thiol oxidation, which dem-
onstrates its stability.
(iii) Due to its magnetic behaviour, the CoPcS/LDH@Fe3O4 material
could be easily separated and recycled.
(iv) Based on the kinetic studies and for commonly encountered in-
dustrial conditions, we simplified the proposed rate law to the
following linear form, which can explain the RSH oxidation re-
sults with reasonably good accuracy.
−rRSH ¼ 87:44 Co½ Štote−10143
R:T RSH½ Š
Acknowledgements
The authors are thankful to Director, Indian Institute of Petroleum,
Dr. Anjan Ray for granting permission to publish these results. Further-
more, we kindly acknowledge CSIR (New Delhi) for funding via the CSC-
0117, 12th Five Year Projects. Dr. S. Bojja, CSIR-Indian Institute of Chem-
ical Technology is kindly acknowledged for providing HR-TEM analysis
of the samples. We also thank the analytical department of the CSIR-In-
dian Institute of Petroleum for providing support in analysis of the
samples.
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