Facile redbrick clay as splendid catalyst for selective dehydration of alcohols

Article abstract of DOI:10.1007/s11164-018-3577-8

Abstract: Excellent catalytic activity of activated redbrick clay (ARB) for dehydration and highly selective conversion of cyclohexanol to cyclohexene and 1-phenylethanol to styrene is reported. The efficacy of the catalyst is compared with commercial catalysts, silica and α-Fe₂O₃, which performed poorly giving side products. Synergistic acidic properties and combinational Lewis and Bronsted acidic sites of ARB afforded superior performances relative to silica and α-Fe₂O₃ catalysts. Pyridine Fourier-transform-infrared spectra (FT-IR) and temperature-programmed desorption (NH₃-TPD) measurements showed the presence of combinational Lewis and Bronsted acidic sites and a weak to mild acidic texture, which could facilitate favorable interaction between the ARB clay catalyst and the reactants (cyclohexanol and 1-phenylethanol). The physicochemical characteristics of ARB, commercial silica and α-Fe₂O₃ catalysts were investigated using X-ray diffraction, scanning electron microscopy, X-ray fluorescence, NH₃-TPD, UV–visible diffuse reflectance spectroscopy, and pyridine FT-IR. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-018-3577-8

Research on Chemical Intermediates
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Facile redbrick clay as splendid catalyst for selective
dehydration of alcohols
Venkata Rao Madduluri1 • Naresh Kumar Katari
2,3 •
Nagaiah Peddinti
1 •
1
1
•
Venkataramana Kumar Velpula
•
David Raju Burri
1
3
Seetha Rama Rao Kamaraju
•
Sreekantha B. Jonnalagadda
Received: 8 May 2018 / Accepted: 25 August 2018
Ó Springer Nature B.V. 2018
Abstract
Excellent catalytic activity of activated redbrick clay (ARB) for dehydration and
highly selective conversion of cyclohexanol to cyclohexene and 1-phenylethanol to
styrene is reported. The efficacy of the catalyst is compared with commercial cat-
alysts, silica and a-Fe O , which performed poorly giving side products. Synergistic
2
3
acidic properties and combinational Lewis and Bronsted acidic sites of ARB
afforded superior performances relative to silica and a-Fe O catalysts. Pyridine
2
3
Fourier-transform-infrared spectra (FT-IR) and temperature-programmed desorp-
tion (NH -TPD) measurements showed the presence of combinational Lewis and
3
Bronsted acidic sites and a weak to mild acidic texture, which could facilitate
favorable interaction between the ARB clay catalyst and the reactants (cyclohexanol
and 1-phenylethanol). The physicochemical characteristics of ARB, commercial
silica and a-Fe O catalysts were investigated using X-ray diffraction, scanning
2
3
electron microscopy, X-ray fluorescence, NH -TPD, UV–visible diffuse reflectance
spectroscopy, and pyridine FT-IR.
3
&
&
&
Naresh Kumar Katari
Seetha Rama Rao Kamaraju
Sreekantha B. Jonnalagadda
Jonnalagaddas@ukzn.ac.za
Extended author information available on the last page of the article
123

V. R. Madduluri et al.
Graphical abstract
Si
1
1
2
0
8
6
4
2
0
After
purification
&
Activation
Fe
Al
Ca
K
si
fe
ca
al
k
Si
Fe
Ca
k
Al
Keywords Activated red brick (ARB) clay catalyst Á Cyclohexanol Á
-Phenylethanol Á Selective dehydration Á Styrene Á Cyclohexene
1
Introduction
The dehydration reactions involving alcohols provide valuable feedstocks, which
are of importance for various industrial applications [1–4]. The last few decades
have witnessed growing interest in the dehydration of alcohols to produce organic
intermediates including olefins, which are valuable substrates for several industrial
applications [5, 6]. Mori et al. [7] reported the conversion of 1,2-propanediol to
propanal using silica-supported silicotungstic acid as catalyst with good selectivity.
The present study focuses on the dehydration of 1-phenylethanol and cyclohexanol
into useful styrene and cyclohexene compounds, which are value-added commodi-
ties in numerous applications.
In recent years, styrene has been emerging as an active ingredient in several
products, including thermoplastics, fibers, polymers and resins. The direct synthesis
of styrene from ethylbenzene dehydrogenation has been widely used via traditional
processes [8]. About 15–20% of styrene has been produced by direct dehydration of
1
-phenylethanol which was obtained from a process called the styrene monomer [9].
1
23

Facile redbrick clay as splendid catalyst for selective…
In conventional processes, styrene is produced through steam-assisted ethylbenzene
oxidative dehydrogenation at high temperature [10]. Styrene is also produced with
ethylbenzene dehydrogenation at high temperature by using potassium-promoted
iron oxide catalysts under soft and mild oxidants [11, 12]. These processes have a
few disadvantages, such as high-energy requirement, high coke deposition and
active phase changes during the reaction. To reduce the high-energy requirements,
1
-phenylethanol dehydration was carried out in batch mode, but it faces limitations
with respect to a higher production of the styrene monomer [13]. The liquid phase
production of styrene is remarkably poor with considerable side products, in
comparison with vapor phase reactions, which give higher yields [9, 14, 15]. Thus,
the vapor phase production of styrene is preferred over liquid phase conditions.
Recently, several novel solid acidic oxide catalysts have been introduced for
improved conversion of 1-phenylethanol to the styrene monomer in vapor phase
conditions [16]. The dehydration of 1-phenylethanol in gas phase conditions over
TiO and Al O as catalysts at around 200–300 °C has also been reported [17, 18].
2
2 3
Metallic phosphates have also been tested as catalysts in the dehydration of several
alcohols [19]. In most cases, although, the conversion of 1-phenylethanol was high,
the catalysts were not efficient due to poor selectivity towards styrene. Lange et al.
[
20] have reported longer lifetimes of ZSM-5 catalysts beyond 150 h, with ([ 90%)
-phenylethanol conversion and ([ 90%) styrene selectivity, by improving the
1
diffusion characteristics of the catalyst particle.s Good results were have beenre-
ported with ZSM-5 from high-silica kaolinite as the catalyst for vapor phase
dehydration of 1-phenylalcohol [21]. These catalysts have still had a few
disadvantages, such as lengthy catalyst preparation procedures and expensive raw
materials.
Cyclohexene is an essential precursor in the synthesis of many valuable
chemicals, such as adipic acid, maleic acid, dicyclohexyladipate, and cyclohexene
oxide. Furthermore, cyclohexene is used as a solvent and is a starting material in
different organic reactions. Hence, there is growing interest in its synthesis and in
selective cyclohexanol dehydration over various solid oxide catalysts [22].
Zirconium phosphate and Ce-MCM-41 have been tested as catalysts for the
conversion of cyclohexanol to cyclohexene [23, 24]. Laha et al. [25] used cerium
incorporated in MCM-41 as the catalyst, while Akiya et al. [26] used high-
temperature water as an acid catalyst to convert cyclohexanol to cyclohexene.
Chromium-based materials as heterogeneous catalysts have been used in a wide
variety of organic conversions, such as oxidation of aromatic compounds,
dehydrogenation, epo-oxidation of olefins and allylic oxidation reactions [27–30].
Chromium-based catalysts have been tested for the transformation of alkyl
aromatics to aromatic ketones, but the use of chromium has adverse effects on
the atmosphere due to its toxic nature [31]. There is a great necessity to replace
toxic chromium-based materials with efficient, green and sustainable solid catalysts.
In view of this, we introduce here a nontoxic, reusable and inexpensive catalyst
material, activated redbrick clay (ARB), for the selective dehydration of
1
-phenylethanol to styrene and cyclohexanol to cyclohexene. The potential and
scope of brick clay materials in water purification and separation as adsorbents and
for different organic transformations such as catalysts has been accentuated in
123

V. R. Madduluri et al.
various reviews [32–34]. Cultrone et al. [35] systematically studied the unique
physiochemical properties of red brick clay with different characterization
techniques.
Redbrick clay is an inexpensive environmental friendly material. ARB clay is
easily procurable and its regeneration is facile. ARB clay material possesses active
silica and iron oxides particles, which act through synergistic or combinational
ways, contributing to its superior catalytic efficiency. In addition, an ARB clay
catalyst acts as a source of Bronsted and Lewis acidic sites along with a weak to
mild acidic nature for different organic molecule transformations into industrial
value-added chemicals. Earlier, we reported the selective production of tetrahydro-
furan over the ARB catalyst at 375 °C with good catalytic activity [36].
The literature shows very few studies on the use of solid oxide catalysts for the
dehydration and selective conversions of 1-phenylethanol to styrene and cyclohex-
anol to cyclohexene in the gas phase [17, 25]. In the present study, we report the
efficacy of ARB as a catalyst for the selective conversion of cyclohexanol to
cyclohexene and 1-phenylethanol to styrene. The kinetic parameters such as liquid
hourly space velocity (LHSV), gas hourly space velocity (GHSV), time on stream
(
TOS) and temperature on the product yield and conversions were investigated.
Experimental
Catalyst preparation
The ARB clay catalyst was prepared by simple grinding. Fresh red bricks were
collected, crushed to a uniform size, washed several times with deionized water,
separated into small-sized granules and dried at 120 °C for 24 h. The obtained
powder was designated as the ARB clay catalyst. Commercial silica and a-Fe O3
2
catalysts were purchased from SD-Fine, Chennai (AR grade, 99% purity).
Catalyst characterization
Powder XRD measurements were carried using a Rigaku diffractometer (ULTIMA-
IV, 40 kV, 30 mA) with monochromatic Cuka radiation. FT-IR spectra were
recorded on a spectrum GX spectrometer (Perkin Elmer, USA) in the scan range of
-
1
4
000–400 cm . SEM and EDX analyses were performed using JEOL FE-SEM-
610F to elucidate the morphology and metal composition of the catalyst. X-ray
7
fluorescence (XRF) spectra were carried out on a Bruker model SH Pioneer) with an
X-ray tube and Rh anode and operated at 4KW. Pyridine FT-IR investigation on the
pyridine-adsorbed ARB clay catalyst was recorded on a Spectrum-GX spectrometer
-
1
(
Perkin Elmer, Germany) in the scan range of 1600–1400 cm . UV–Vis DRS were
recorded using a UV Win Lab spectrometer (Perkin Elmer, Germany) with an
integrating sphere reflectance accessory in the wavelength range of 200–800 nm.
1
23

Facile redbrick clay as splendid catalyst for selective…
Catalytic activity evaluation on ARB clay catalyst
The catalytic activity was tested in a fixed bed downflow quartz reactor (18 mm
ID 9 300 mm long) at atmospheric pressure in the temperature range of
2
00–300 °C (for 1-phenylethanol) and 300–400 °C (for cyclohexanol) with N2
gas flow. A catalyst mass of 1 g was packed at the center of the reactor between two
quartz wool plugs. Above the catalyst bed, quartz glass chips were filled to act as a
preheating zone. The reactor was placed vertically inside an electrically heated
tubular furnace, the temperature being controlled by a temperature indicator–
programmer with a K-type thermocouple. The catalyst was activated by flowing N2
gas at 400 °C for 1 h, prior to the reaction. After the activation, the temperature was
adjusted to the desired reaction environment, and the reaction was started by
pumping (cyclohexanol or 1-phenylethanol) fed by a syringe pump (Braun,
Germany) along with N gas. The liquid effluent from the reactor outlet was
2
condensed in ice-cooled trap and was analyzed at regular intervals on a flame
ionization detector-equipped gas chromatograph (HP, USA), using a 30-m wax GB
capillary column.
Results and discussion
X-ray diffraction (XRD)
Figure 1 illustrates the XRD patterns of fresh ARB clay along with commercial
silica and a-Fe O catalysts. The XRD pattern of the ARB indicates the presence of
2
3
2 3
Fig. 1 XRD patterns of commercial (a) a-Fe O , (b) commercial silica, and (c) ARB clay catalysts
123

V. R. Madduluri et al.
elevated crystalline SiO in the quartz form, resulting in 2h values at 20.8°, 26.7°,
2
3
9.5°, and 50.2° and iron oxide in the hematite form with respect to a 2h value at
3.3°, while the 2h value at 27.9° was attributed to feldspar which emerged from the
3
combination of calcium, sodium or potassium with alumino silicates [35]. In this
study, we did not observe any iron silicate peaks which could be due to the silica
and iron oxide particles being well dispersed. In the XRD patterns for commercial
silica and a-Fe O catalysts, the latter displayed similar 2h values at 33.3° as the
2
3
ARB clay with mild intensity, which clearly indicates the amorphous nature of the
iron oxide particles. The commercial silica catalyst displayed a broad peak in the
range between 2h values of 20 and 30°, indicating the amorphous nature of the silica
particles.
FT-IR spectra
Figure 2 depicts the FT-IR spectra of fresh ARB clay along with commercial silica
-
1
and a-Fe O catalysts. The absorption bands present at 535 and 580 cm
2
3
correspond to the stretching and torsional vibrational modes of hematite Fe–O
bonds in ARB clay and commercial a-Fe O catalysts [37, 38]. Bands at 470 and
2
3
-
1
7
95 cm
indicate the Si–O–Si symmetric stretching vibrations. The bands at
-
around 1097 and 940 cm are ascribed to the asymmetric stretching vibration of
Si–O–Si and Si–OH bonds, corresponding to the silica network in the ARB clay and
1
-
1
commercial silica catalysts [39]. The 462 and 531 cm bands in commercial a-
Fe O are ascribed to oxygen coordinated with Fe metal ions (Fe–O stretching
vibrations), which is clearly endorsed by the literature [38].
2
3
2 3
Fig. 2 FT-IR analysis of fresh (a) ARB clay, (b) commercial silica, and (c) a-Fe O catalysts
1
23

Facile redbrick clay as splendid catalyst for selective…
XRF analysis
The XRF characterization of the fresh ARB clay catalyst obviously demonstrates
the precise weight percentage (wt%) of corresponding metals present in the catalyst
and is summarized in Table 1. ARB clay with a high composition of solid acidic
oxides such as silica (quartz) and iron oxide (hematite) possesses sufficient Lewis
and Bronsted acidic sites. ARB clay mostly possesses Si and Fe and other inorganic
metals, Al, Ca, Mg and K in wt%. The wt% of different metals correlated well with
the XRF and EDX analysis data presented in Table 1.
SEM–EDX
SEM images of the fresh ARB clay catalyst indicate the presence of macropores and
a sheet-like morphology (Fig. 3). The appearance of the porous matrix was due to
the development of hematite with an alignment of uniform red Fe-rich small crystals
on the surface of the ARB clay catalyst. The energy dispersive X-ray (EDX)
analysis of fresh and used ARB clay catalysts confirmed the major presence of the
Si, Fe, and Al elements and minor quantities of Ca, K and Mg, as listed in Table 1.
NH - temperature programmed desorption (TPD)
3
NH -TPD measurements provide the distribution and strength of acidic sites in the
3
catalysts. Figure 4 illustrates the TPD profiles of commercial silica, a-Fe O and
2
3
ARB clay catalysts. With the ARB clay catalyst, high-intensity desorption peak
around 180 °C and a shoulder peak at 275 °C with maximum NH consumption
3
correspond to the weak and medium acidic sites. Thus, the co-existence of both
weak and medium acidic sites in ARB clay has strongly enhanced its catalytic
efficiency.
The NH desorption peak around 100–250 °C in commercial silica reveals the
3
existence of weak acidic sites. In the case of a-Fe O , desorption peaks in the range
2
3
of 500–750 °C indicate the presence of strong acidic sites.
Table 1 Metals analysis (wt%)
of fresh and spent ARB clay
catalysts by various techniques
Metal
Weight % (XRF)
Fresh ARB
Weight % (EDX)
Fresh ARB
Fe
Si
5.34
12.51
5.19
1.25
2.86
0.15
5.28
12.36
5.23
1.44
2.3
Al
K
Ca
Mg
0.14
123

V. R. Madduluri et al.
Fig. 3 SEM-EDX patterns of the fresh ARB clay catalyst: a SEM and b EDX
Fig. 4 NH
3
-TPD patterns of (a) commercial silica, (b) ARB clay, and (c) a-Fe
2
O
3
catalysts
Pyridine FT-IR
Pyridine FT-IR elucidates the nature of acidic sites present in the catalyst. Figure 5
presents the pyridine FT-IR spectra of fresh ARB clay, commercial silica and a-
-
Fe O materials. The two obvious weak bands at 1450 and 1472 cm for the ARB
1
2
3
3
clay catalyst are ascribed to pyridine adsorbed onto hematite Fe Lewis acidic
-
species [40, 41], while 1492 cm pertains to pyridine adsorbed onto both Lewis
?
1
-
1
and Bronsted acidic clusters [42]. A strong band at 1540 cm is ascribed to
apparent Bronsted acidic [15] moieties (silanol), which are wider and intense. In
-
addition, the adsorption band at 1575–1590 cm is ascribed to Lewis acidic sites in
1
the ARB clay catalyst [43]. Here, commercial a-Fe O merely displayed Lewis
2
3
-
1
acidic sites in the range of 1575–1590 cm . Those were mainly responsible for
dehydration as well as the dehydrogenation products. As a result, a small fraction of
unwanted products (cyclohexanone and acetophenone) were obtained over the a-
Fe O catalyst. In contrast, the commercial silica catalyst exhibited mild and intense
2
3
1
23

Facile redbrick clay as splendid catalyst for selective…
2 3
Fig. 5 Pyridine FT-IR patterns of fresh (a) a-Fe O , (b) ARB clay, and (c) commercial silica catalysts
Lewis acidic sites [44, 45] compared to that of the ARB clay catalyst; consequently,
undesired cyclohexanol and 1-phenylethanol conversion with poor product selec-
tivity. It seems that Bronsted acidic sites are quantitatively higher compared to
Lewis acidic sites in the ARB clay catalyst. The combined effect of both Bronsted
and Lewis acidity mainly drives the selective dehydration reaction of cyclohexanol
and 1-phenylethanol towards better yields of cyclohexene and styrene, respectively.
A small fraction of Bronsted acidic sites are observed in the commercial silica
catalyst, due to smaller numbers of surface silanol groups, whereas the a-Fe O
2
3
catalyst exhibits Lewis acidic sites, which are responsible for the poor catalytic
efficiency resulting in mild cyclohexene and styrene selectivity.
UV–Vis DRS spectra
Figure 6 depicts the UV–Vis DRS of the fresh ARB clay catalyst. A strong
absorption band with a maximum of 250 nm was observed, which corresponds to
-
2
the ligand to metal charge transfer character that involves O and the isolated four
?
3
coordinated Fe . In addition, a broad absorption band in the range of 500–550 nm
represents the surplus Fe atoms in the hematite domain of the ARB clay catalyst
[
46].
123

V. R. Madduluri et al.
Fig. 6 UV–Vis DRS patterns of the fresh ARB clay catalyst
Activity study
Temperature influence on catalytic activity of ARB clay, commercial silica and
a-Fe O catalyst
2
3
The catalytic dehydration of 1-phenylethanol and cyclohexanol was conducted in
the temperature ranges of 200–300 °C and 300–400 °C, as depicted in Figs. 7 and 8.
In the reaction at 300 °C, the cyclohexanol conversion was low because the
Fig. 7 Temperature effect of 1-phenylethanol dehydration on the ARB clay catalyst. Reaction conditions:
-
1
-1
1
2
-phenylethanol feed flow; LHSV 0.37 h , N gas flow; GHSV752 h , temperature; 250 °C
1
23

Facile redbrick clay as splendid catalyst for selective…
Fig. 8 Temperature effect of cyclohexanol dehydration on the ARB clay catalyst. Reaction conditions:
-
1
-1
cyclohexanol feed flow; LHSV 0.37 h , N gas flow; GHSV764 h , temperature; 250 °C
2
dehydration reactions are temperature-dependent in nature and there might be
inferior diffusion of the reactants and products over the catalyst surface. At the
temperature of 375 °C, the cyclohexanol conversion reached 100% which can be
ascribed to the high-temperature requirement of cyclohexanol conversion. At all the
reaction temperatures, the selectivity towards cyclohexene was 100%, the cyclo-
hexanol conversion increased with temperature up to 375 °C, as shown in Fig. 8.
At the optimum reaction temperature of 375 °C, a complete highly selective
dehydration and conversion of cyclohexanol to cyclohexene is achieved, with no
byproducts [23, 24].
The 1-phenylethanol conversion requires
200–300 °C). With ARB clay as the catalyst, during the 1-phenylethanol
dehydration, the % conversion increased with the reaction temperature up to
50 °C, then remained unchanged. No significant changes in styrene selectivity
100%) were observed. The catalytic activity of the commercial silica and a-Fe O
a low to mild temperature
(
2
(
2
3
were examined in the dehydration of 1-phenylethanol and cyclohexanol under
similar reaction conditions. Silica exhibited 44% cyclohexanol conversion at 375 °C
with 89% cyclohexene and 11% cyclohexanone selectivity, whereas a-Fe O
2
3
catalyst showed 74% conversion with 82% cyclohexene and 18% cyclohexanone
selectivity. In addition, the silica catalyst achieved 52% 1-phenylethanol conversion
with * 91% styrene selectivity at 250 °C, while a-Fe O obtained 78% conversion
2
3
and 79% styrene and 21% acetophenone selectivity. At all the reaction temperatures
using the commercial silica and a-Fe O catalysts, we observed mild cyclohexanol
2
3
and 1-phenylethanol conversions and lower selectivity. Appropriate acidic textures
and combinations of Lewis and Bronsted acidic sites are responsible for the
remarkable conversion of cyclohexanol to cyclohexene, and 1-phenylethanol to
styrene over the ARB clay catalyst. The dehydration of cyclohexanol into
123

V. R. Madduluri et al.
cyclohexene is thermodynamically favored at higher temperatures, leading to
complete dehydration (high catalytic activity at 375 °C) via intramolecular
phenomenon [23]. In the present study, low acidic sites (shown in NH -TPD
3
analysis) are responsible for excellent styrene selectivity. Cyclohexene was
produced from mild acidic sites on the ARB clay catalyst and it was in good
agreement with the literature [9, 23, 24]. ARB clay possesses a macropore texture as
shown in SEM images (see Fig. 3), thus endorsing the beneficial interface for
reactants with different active acidic sites. Impressive yields of styrene and
cyclohexene in their optimum temperature conditions are due to the strong interface
of reactants with a suitable acidic texture of the ARB clay catalyst [47]. Moreover,
the Lewis and Bronsted acidic sites on the ARB clay catalyst and Lewis basicity of
reactant alcohols contribute to the efficient catalytic performance. Here, the ARB
clay catalyst could also facilitatean optimal residence time of reactants in
macropores with beneficial contact time and product diffusion properties. On the
other hand, dehydration intermediates in both reactions, such as (1-phenylethyl)ox-
onium ion and cyclohexyl carbocation stability seem to be absent, and consequently
not detected during GC–MS analysis. Hence, we assume no formation of
stable intermediate molecules during the cyclohexanol and 1-phenylethanol
dehydration reactions, and consequently superior yields of cyclohexene and styrene
without side reactions. The reactivity of cyclohexanol and 1-phenylethanol in the
present study mainly depends on Bronsted and Lewis acidic sites (see the pyridine
FT-IR pattern) and weak to mild acidic texture (NH -TPD analysis) of the ARB clay
3
catalyst along with a reaction temperature of 375 °C. Cyclohexanol and
1
-phenylethanol reactions using the Ce-KIT-6 catalyst reportedly gave many
byproducts, including dicyclohexyl ether and other products in different acidic
conditions [24]. While weak acidic properties are beneficial for styrene production,
the mild acidic texture favors the selective cyclohexene [24] formation on the ARB
clay catalyst. Cr-MCM-41 gave lower catalytic efficiency (with respect to 80%
cyclohexene and 70% styrene selectivity) compared to that of the ARB clay
catalyst, which exhibited 100% product selectivity. This was obviously due to
significant growth in carbonaceous deposits on the catalyst surface, along with
lower thermal and mechanical stability under reaction operation conditions.
Negligible carbon deposits (B 5% evidence from CHNS analysis after 12 h TOS)
were observed on the ARB clay catalyst. ARB clay as a green catalyst proves
superior for selective cyclohexanol and 1-phenylethanol conversions with 100%
efficiency.
Effect of N gas hourly space velocity (GHSV) on catalytic activity of ARB clay, silica
2
and a-Fe O catalysts
2
3
The GHSV of N2 showed significant influence on the cyclohexanol and
1
rates of 1104 h
-phenylethanol conversion over the ARB clay catalyst (Table 2). The N flow
2
-
1
-1
(cyclohexanol) and 1093 h
(1-phenylethanol) were not
promising due to little contact time of the reactants with the active metal surface
of the ARB clay, consequently giving (76%) cyclohexanol and (82%) 1-pheny-
-
lethanol conversions. In medium GHSV conditions, 764 h (cyclohexanol) and
1
1
23

Facile redbrick clay as splendid catalyst for selective…
Table 2 N GHSV effect on cyclohexanol and 1-phenylethanol conversion over the ARB clay catalyst
2
Cyclohexanol
1-Phenylethanol
GHSV
)
Conversion
(%)
Cyclohexene yield
(%)
GHSV Conversion
-1
Styrene yield
(%)
-
1
(
h
(h
)
(%)
4
7
1
23
64
64
100
76
64
100
76
411
75
100
82
75
100
82
752
104
1093
Reaction conditions: temperature; 375 °C (cyclohexanol) and 250 °C (1-phenylethanol), feed flow;
-
1
0
2
.37 h ; N gas flow; 15, 30 and 45 mL/min
-
52 h (1-phenylethanol), the diffusion nature of the products as well as the
1
7
residence time of the reactants were better in comparison with the mild GHSV
-
conditions. The N flow rates of 764 and 752 h were excellent for the 100%
1
2
conversion of cyclohexanol and 1-phenylethanol, respectively. This is possibly due
to the mutual interface between the reactants and the active sites on the ARB clay
catalyst. As a result, cyclohexene and styrene were produced with 100% selectivity.
The commercial silica catalyst showed 44% cyclohexanol and 52% 1-phenylethanol
-
1
conversion with optimal GHSV of 764 and 752 h N gas flow. The a-Fe O
2
2 3
catalyst displayed 74% cyclohexanol and 78% 1-phenylethanol conversion, without
1
-
considerable changes in product selectivity. The N flow GHSV of 423 and 411 h
2
afforded decreased conversions of 64% cyclohexanol and 75% 1-phenylethanol
over the ARB clay catalyst. It was due to the poor interface between the active
acidic sites on the ARB and the reactant molecules during the reaction. At all flow
rates, the N gas on the ARB clay catalysts attained cyclohexene and styrene as
2
major products; no other products were observed. Similar observations have been
made by researchers in other reactions [23, 24, 26]. Among these, the N GHSV
2
-
1
-1
flows tested, 764 h (1-phenylethanol), were
optimized with a good interface of the reactants with the ARB clay catalyst leads
(cyclohexanol) and 752 h
-
1
to 100% cyclohexene and styrene selectivity. In contrast, the N GHSV of 764 h
2
-
1
(
cyclohexanol) and 752 h
(1-phenylethanol) gave poor results with the the
commercial silica and a-Fe O catalysts. Obviously, this was due to significant
2
3
changes in the surface acidic texture of the commercial silica and a-Fe O catalysts,
2
3
compared to that of the ARB clay catalyst, an observation which is supported by
various characterization techniques.
Effect of cyclohexanol and 1-phenylethanol flow on catalytic activity of ARB clay,
commercial silica and a-Fe O catalysts
2
3
Table 3 summarizes the results of the onversion and selectivity of cyclohexanol to
cyclohexene and 1-phenylethanol to styrene, as a function of feed flow rate
-
1
(
oxides were employed as catalysts and the dehydration reaction was measured at
LHSV = 0.37, 0.75 and 1.13 h ). ARB clay, commercial silica and a-Fe O
2
3
-
1
-1
3
75 °C at N GHSV of 764 h (cyclohexanol) and 752 h (1-phenylethanol)
2
123

V. R. Madduluri et al.
Table 3 LHSV effect on
cyclohexanol and
Cyclohexanol
1-phenylethanol
1-phenylethanol conversion over
the ARB clay catalyst
-1
LHSV (h
-1
)
)
Conversion (%)
LHSV (h
Conversion (%)
0
0
1
.37
.75
.13
100
75
0.37
0.75
1.13
100
82
59
68
Reaction conditions: temperature; 375 °C (cyclohexanol) and 250 °C
-
1
(
1-phenylethanol), feed flow; LHSV 0.37, 0.75 and 1.13 h ; catalyst
weight; 1 g
Table 4 Comparison cyclohexanol conversion and selectivity on ARB clay, commercial silica and a-
Fe catalysts
2 3
O
Catalyst
Temperature Cyclohexanol
conversion (%)
Cyclohexene
selectivity (%)
Cyclohexanone
selectivity (%)
(
°C)
ARB
375
375
375
100
44
100
89
–
SiO
2
11
18
a-Fe
2
O
3
74
82
-1
-1
Reaction conditions: 375 °C, LHSV 0.37 h with respect to cyclohexanol and GHSV 764 h with
respect to nitrogen, 1 atm. pressure, 1 g catalyst
2 3
Table 5 Comparison data of 1-phenylethanol on the ARB clay, commercial silica and a-Fe O catalysts
Catalyst
Temperature
°C)
1-Phenylethanol
conversion (%)
Styrene
selectivity (%)
Acetophenone
selectivity (%)
(
ARB
250
250
250
100
52
* 100
* 91
79
–
SiO
2
9
a-Fe
2
O
3
78
21
-1
-1
Reaction conditions: 250 °C, LHSV 0.37 h with respect to 1-phenylethanol and GHSV 752 h with
respect to nitrogen, 1 atm. pressure, 1 g catalyst
-
Tables 4, 5). At the very low feed flow (LHSV = 0.37 h ) for both reactions,
1
(
interestingly, 100% conversion with 100% selectivity towards the product were
demonstrated and the results were sustained until 12 h TOS.
This was possibly due to the increased contact times over the active sites of the
ARB clay catalyst, which favor the selective dehydration of 100% cyclohexanol and
1
At medium LHSV of 0.75 h flow, the conversion slightly decreased with 100%
-phenylethanol into cyclohexene and styrene, respectively, with 100% selectivity.
1
-
-
1
selectivity. As the feed flow rate increased (LHSV = 0.75 and 1.13 h ), the %
conversion decreased, owing to the smaller contact time between the catalyst and
-
1
the reactants. In addition, with reactants at high LHSV, i.e. 0.75 and 1.13 h , a
smaller number of the catalyst active sites are available for maximum cyclohexanol
-
1
and 1-phenylethanol conversion. Hence, 0.37 h (LHSV) was determined as the
optimal flow of cyclohexanol and 1-phenylethanol in optimum catalytic efficiency.
1
23

Facile redbrick clay as splendid catalyst for selective…
-
1
Under a low LHSV rate, i.e., 0.37 h , the commercial silica and a-Fe O catalysts
2
3
displayed mild conversion of cyclohexanol to cyclohexene and 1-phenylethanol to
styrene with poor selectivity, due to drastic changes in the surface acidic properties
(in terms of Lewis and Bronsted, along with weak to mild acidic sites) of the
catalysts (Tables 4, 5).
Time-on-stream (TOS)
The ARB clay catalyst displayed an excellent catalytic performance in terms of
cyclohexanol conversion along with 100% cyclohexene selectivity compared with
the commercial silica and a-Fe O catalysts and other reports [23]. The TOS on
2
3
ARB clay catalyst was remarkable with superior cyclohexanol and 1-phenylethanol
conversions without producing any side products, because of the synergistic
influence of the silica and iron oxide particles. Here, we assume that the
cyclohexanol and 1-phenylethanol selective dehydrations proceed via the
intramolecular concerted mechanisms shown in Schemes 1 and 2, rather than
through step-wise product formation [23]. During the dehydration reaction, there
was no intermediate formation on the external catalyst surface, leading to unwanted
products, except for B 1% cyclohexanone and acetophenone obtained as minor
products. As a result, high selectiveityproducts, such as styrene and cyclohexene
(
100%) are observed during the course of the TOS until 12 h, as depicted in Figs. 9
and 10.
As per the TOS, there is no significant decline in the 1-phenylethanol conversion
until 12 h (shown in Fig. 9), which might be due to the facile diffusion of the
reactants and products on the silica, alumina and iron oxide species in the ARB clay
catalyst. The silica particles on the surface, which display homophobic properties
[48], retard the adsorption of surplus H O molecules on the surface of the ARB clay
2
catalyst.
Scheme 1 A plausible dehydration reaction mechanism of 1-phenylethanol on the ARB clay catalyst
123

V. R. Madduluri et al.
Scheme 2 A plausible dehydration reaction mechanism of cyclohexanol on the ARB clay catalyst
Fig. 9 TOS of 1-phenylethanol dehydration on the ARB clay catalyst. Reaction conditions: 250 °C,
-
1
-1
LHSV 0.37 h with respect to 1-phenylethanol and GHSV 752 h with respect to nitrogen, 1 atm.
pressure, 1 g catalyst
On the other hand, iron oxide particles in the ARB clay catalyst considerably
enhance its acidic nature, playing a beneficial role in better conversions. The results
of mild cyclohexanol and 1-phenylethanol conversion observed over the commer-
cial silica and a-Fe O catalysts are summarized in Tables 3 and 4.
2
3
As per the cyclohexanol and 1-phenylethanol conversions observed, mild and
strong acidic sites contribute to the activity in the commercial silica and a-Fe O
2
3
catalytic texture, rather than the synergistic weak to medium acidic properties in the
ARB clay catalyst (see NH -TPD pattern). In addition, the commercial silica and a-
3
Fe O catalysts might accumulate a stream of reactant molecules over the catalyst
2
3
surface due to the absence of synergistic Lewis and Bronsted acidic sites. As a
result, the commercial silica and a-Fe O catalysts did not facilitate an efficient
2
3
1
23

Facile redbrick clay as splendid catalyst for selective…
Fig. 10 TOS of cyclohexanol dehydration on the ARB clay catalyst. Reaction conditions: 375 °C, LHSV
-1
-1
.37 h with respect to cyclohexanol and GHSV 764 h with respect to nitrogen, 1 atm. pressure, 1 g
0
catalyst
coordination between the reactants and the catalyst active acidic sites, and
consequently there were mild conversions as well as product selectivity. The
acidity-enhanced silica, and a-Fe O particles in the ARB clay catalyst played a
2
3
dramatic role in an expanded catalytic efficiency via Lewis and Bronsted acidic
sites.
The ARB clay catalyst possessing the Lewis and Bronsted combinational
property offers beneficial catalytic activity with the activation of different acidic
-
1
sites at the reaction temperature. The Bronsted acidic (1520–1560 cm ) sites offer
strong affinity towards the reactants 1-phenylethanol and cyclohexanol, thus leading
to the optimal contact time of the reactants with the active surface of the ARB clay
catalyst, and resulting in excellent cyclohexene and styrene yields. The ARB clay
catalyst provides more Bronsted acidic sites in terms of sufficient silanol (Si–OH)
moieties over the catalyst surface (see FT-IR; Fig. 2). The ARB clay catalyst is
highly selective in the dehydration of 1-phenylethanol and cyclohexanol with
prolonged catalytic steadiness compared to the literature [16, 23, 24, 26].
The cyclohexanol reaction mainly requires a mild temperature and an optimum
acidic atmosphere as observed in previous and present studies [18, 26, 49]. During
the course of TOS (on the ARB clay catalyst), there is a marginal decline in
1
-phenylethanol conversion, which might be due to the additional acidic texture
provided by the aromatic group in the reactant molecules. The contact time of
-phenylethanol was high along with adequate acidic sites, leading to * 100%
1
styrene selectivity., while lower TOS efficiency with cyclohexanol dehydration (on
the ARB clay catalyst) compared with the dehydration of 1-phenylethanol was due
to the aliphatic nature of cyclohexanol molecules. The mild catalytic activity on the
commercial silica and iron oxide catalysts is not vital in higher alcohol conversion
123

V. R. Madduluri et al.
due to the formation of fractions of byproducts such as cyclohexanone and
acetophenone (Tables 4, 5). The combinational or synergistic properties (Lewis and
Bronsted acidic sites) of the ARB clay catalyst are responsible for the prolonged
-
catalytic dehydration. During the reaction, acidic oxide moieties (SiO ) obviously
?
abstract acidic or activated protons (H ) from the transient intermediate molecules,
such as cyclohexyl carbocation and (1-phenylethyl)oxonium ion, as shown in
reaction Schemes 1 and 2. However, the commercial silica and a-Fe O catalysts
2
3
merely facilitate the weak and strong acidic properties shown by the NH -TPD
3
pattern, thus leading to the mild cyclohexanol and 1-phenylethanol conversion, as
well as the selectivity towards cyclohexene and styrene, experienced on the
commercial silica and a-Fe O catalysts. However, the commercial silica catalyst
2
3
achieved the excellent selectivity of (89%) cyclohexene and (91%) styrene
monomer due to the medium acidic properties in the catalyst. Selectivities of
8
strong acidic environment, as investigated by the NH -TPD pattern. As per the mild
2% cyclohexene and 79% styrene are observed on the a-Fe O catalyst due to its
2
3
3
selectivity on the commercial a-Fe O and silica catalysts, we speculate that the
2
3
reactions might proceed via dehydration along with little bit of dehydrogenation
compared to that of the concerted dehydration manner on the ARB clay catalyst.
Therefore, small fractions of the reaction intermediates (cyclohexyloxalateion and
1
converted into considerable fractions of other products, as shown in Schemes 3 and
-phenylethyl)oxalate ions) on the commercial silica and a-Fe O catalysts become
2
3
4 (i.e., acetophenone and cyclohexanone as mentioned in Tables 4, 5). Hence, the
formation of cyclohexanone and acetophenone are obtained as minor byproducts
over the commercial a-Fe O and silica catalysts, which is also supported by
2
3
reported studies [50, 51].
Commercial silica gave moderate conversions with 1-phenylethanol (52%) and
cyclohexanol (44%), due to weak acidic sites, when compared to a-Fe O with
2
3
Scheme 3 A plausible dehydration reaction mechanism of 1-phenylethanol on the commercial silica and
iron oxide catalysts
1
23

Facile redbrick clay as splendid catalyst for selective…
Scheme 4 A plausible dehydration reaction mechanism of cyclohexanol on the commercial silica and
iron oxide catalysts
strong acidic sites, which showed better 1-phenylethanol (78%) and cyclohexanol
(
74%) conversions. However, there was a drastic decrease in cyclohexene (82%)
and styrene (* 79%) selectivity with cthe ommercial a-Fe O and silica catalysts.
2
3
As a result, the commercial silica and a-Fe O catalysts could not achieve good
2
3
1
mild thermal and chemical conditions. As per the NH -TPD pattern, the commercial
-phenylethanol and cyclohexanol conversions, because of the unsuitability of the
3
silica achieved better styrene and cyclohexene selectivity, whereas the a-Fe O
2
3
catalyst (strong acidic properties in NH -TPD and pyridine-FT-IR) did not attain
better selectivity, due to the formation of byproducts, as shown in Tables 4 and 5.
3
Conclusions
The ease of activation, eco-friendly and inexpensive nature of ARB as a catalyst
opens an opportunity for commercial exploitation of the proposed method for
selective dehydration of 1-phenylethanol, cyclohexanol and other alcohols. The
ARB clay exhibits 100% conversion of 1-phenylethanol and cyclohexanol, as well
as excellent selectivity towards the respective products. The catalytic efficiency of
the ARB clay is due to the synergistic nature of the SiO and Fe O particles.
2
2 3
Therefore, the ARB clay proved a sustainable catalyst in cyclohexanol and
-phenylethanol conversions, as it possesses weak to mild acidic properties.
1
Moreover, the hydrophobic properties with respect to the inhibition of the water-
absorbing nature, due to the excessive silica content in the ARB catalyst and the
synergistic nature of the silica and hematite species, is a major contribution toward
the impressive catalytic efficiency. Lewis and Bronsted acidic sites on the surface of
the ARB clay are also responsible in high thermal and mechanical stability
compared to that of the commercial silica and a-Fe O catalysts. From the NH -
2
3
3
TPD pattern, adequate acidic sites at low temperature (100–200 °C) and mild acidic
clusters at high temperature (200–400 °C) on the ARB may have contributed
positively to the efficacy in the 100% conversion and 100% selectivity of
cyclohexanol to cyclohexene and 1-phenylethanol to styrene.
123

V. R. Madduluri et al.
Acknowledgements MVR is grateful to the University Grants Commission and the Council of Scientific
and Industrial Research, New Delhi, India, for the bursary.
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Affiliations
Venkata Rao Madduluri1 • Naresh Kumar Katari
2,3 •
Nagaiah Peddinti
1 •
1
1
Venkataramana Kumar Velpula
•
David Raju Burri
•
1
3
Seetha Rama Rao Kamaraju
•
Sreekantha B. Jonnalagadda
1
Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology,
Hyderabad 500007, India
2
3
Department of Chemistry, GITAM School of Technology, GITAM Deemed to be University-
Hyderabad, HTP Campus, Rudraram, Medak, Telangana 502 329, India
School of Chemistry and Physics, University of KwaZulu-Natal, P Bag X 54001, Durban 4000,
South Africa
123