Re-usability of zeolites and modified clays for alkylation of cyclohexanol a contrast study

Article abstract of DOI:10.1039/c5ra09692h

Organic sulfonic acid treatment of montmorillonite results in micro-pores on the surface of modified clays providing access to acid sites in the interlayer. Performance of modified clay catalysts for alkylation of para-cresol with cyclohexanol were compared with microporous zeolites. Phenoldisulfonic acid treated clay catalyst showed maximum activity comparable with that of beta-zeolite. Modified clay- and zeolite-catalysts on reuse for the alkylation exhibited different behaviour. Modified clays showed same activity as in the first cycle while zeolites showed significantly reduced catalytic activities on reuse. Differences in behaviour were further investigated by characterizing the reused samples with TGA, acidity and surface measurements. Used zeolite samples showed considerable variations in acidity, surface characteristics and TGA pattern, which was attributed to the formation of coke. Absence of coke in clays and deposition of coke in zeolites is attributed to difference in their pore architecture.

Full text of DOI:10.1039/c5ra09692h

View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: V. N.J, B. Y.S and
J. P. B.S, RSC Adv., 2015, DOI: 10.1039/C5RA09692H.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

Page 1 of 16
RSC Advances
View Article Online
DOI: 10.1039/C5RA09692H
Re-usability of zeolites and modified clays for alkylation of cyclohexanol a
contrast study
N.J Venkatesha, Y.S. Bhat*, B.S. Jai Prakash
Chemistry Research Centre, Bangalore Institute of Technology, K.R. Road,
Bangalore 560 004, India
*Corresponding author: Eꢀmailꢀbhatys@yahoo.com,
Tel.: +91 80 26615865; fax: +91 80 26426796.
Abstract:
Organic sulfonic acid treatment of montmorillonite results in microꢀpores on the surface of modified clays
providing access to acid sites in the interlayer. Performance of modified clay catalysts for alkylation of
para-cresol with cyclohexanol were compared with microporous zeolites. Phenoldisulfonic acid treated
clay catalyst showed maximum activity comparable with that of betaꢀzeolite. Modified clayꢀ and zeoliteꢀ
catalysts on reuse for the alkylation exhibited different behaviour. Modified clays showed same activity
as in the first cycle while zeolites showed significantly reduced catalytic activities on reuse. Differences
in behaviour were further investigated by characterizing the reused samples with TGA, acidity and
surface measurements. Used zeolite samples showed considerable variations in acidity, surface
characteristics and TGA pattern, which was attributed to the formation of coke. Absence of coke in clays
and deposition of coke in zeolites is attributed to difference in their pore architecture.
Keywords:
Zeolite, Modified clay, Alkylation, Organic sulfonic acids, Reusability
1. Introduction:
Zeolites are widely used as solid acid catalysts in the field of heterogeneous catalysis. They have
aluminosilicate frame work structure with three dimensional channels possessing acid sites having
1
ꢀ4
potential applications in industrial organic transformations
.
Clays are aluminosilicates having two dimensional layered structures with stacked frame work
arrangement. In clays, such as montmorillonite, negative charge on the layer arises because of
isomorphous replacement of structural Al with divalent Mg and Fe ions. Cations such as Na, Ca and Mg
1
, 3, 5, 6
occupy the clay interlayer and neutralize the negative charge
.
+
Montmorillonite upon acid treatment undergoes dealumination and forms H /Alꢀexchanged clay, an acid
catalyst. There are reports in the literature that treatment of montmorillonite with organic acids generates
1

RSC Advances
Page 2 of 16
View Article Online
DOI: 10.1039/C5RA09692H
pores on the surface caused by voids due to controlled dealumination in the octahedral layer. The
7
ꢀ15
generated pores give enhanced accessibility to the interlayer acid sites
.
Organic acids remove structural Al, essentially through the formation of soluble complexes, a part of
which are carried to the interlamellar region on washing with water. In the interlamellar region, they get
3+
13
hydrolysed to Al ions which replace the Na ions in the interlayer resulting in Alꢀclay . Organic acid
treatment thus results in a clay retaining most of its cation exchange capacity (CEC). In comparison,
mineral acids remove, in addition to Al, the structural Fe and Mg ions. This results in reduced cation
exchange capacity and lower acidity. Organic acids thus have an advantage of providing a clay retaining
8ꢀ14
most of its CEC and acidity.
Both zeolites and modified clays have been used as catalysts in several organic reactions like acylation,
1
6ꢀ26
Fries rearrangement, esterification, cracking and methoxylation
.
Alkylation of phenol is one of the most important aromatic reactions both in organic synthesis and
chemical manufacturing. The alkyl phenols are widely used as precursors for detergents, pesticides, and
additives for fuels, herbicides, polymers, phenolic resins, antioxidants, lubricants, fragrances,
20, 27ꢀ29
thermoplastic elastomers and paints
.
Recently researchers have been exploring the possibility of using environmentally benign catalysts for
3
–5, 30
alkylation
. In this context, alkylation of p-cresol has been studied using environmentally friendly
catalysts such as zeolites and clays, wherein the main acidity of the zeolites occur within their pores and
in the interlayer region for the clay minerals. The present study aims at comparing the performance and
reusability of organic acid modified clay with zeolite for alkylation of pꢀcresol (PC) with cyclohexanol
(
CH).
2
. Experimental:
.1 Materials method:
Montmorillonite clay sample used in the present study belonged to the Bhuj area, Gujarat, India and is
2
3
1
obtained from Ashapura Chemicals. The 2ꢁm fraction of the clay sample was further homogenised
with NaCl to get the NaꢀClay. Its composition was analysed by ICPꢀOES after microwave digestion
in hydrofluoric acid. It was found to be 42.86% SiO , 18.64% Al O , 9.05% Fe O , 2.58% MgO and
2
2
3
2
3
32ꢀ34
4
.12% Na O.
2
2

Page 3 of 16
RSC Advances
View Article Online
DOI: 10.1039/C5RA09692H
Methanesulfonic acid (MSA), pꢀtoluenesulfonic acid (pꢀTSA) were obtained from SD fine chemicals and
phenoldisulfonic acid (PDSA) from Loba chemicals, India. ZSMꢀ5, Betaꢀzeolite (HBEA) and Yꢀzeolite
(
YꢀZ) were obtained from Reliance industries, Vadodara, Gujarat.
.2 Catalyst preparation:
Clay samples were modified by treating with organic acids MSA, pꢀTSA and PDSA by following the
2
13, 29, 32
procedure reported in the literature.
10 g montmorillonite clay was dispersed in 1M 100 ml organic
acid and the mixture was subjected to microwave irradiation for 30 min. A maximum power of 800 W
0
was applied to reach 110 C in the initial one minute and the same temperature was maintained by
applying the required power ranging from 200 to 800 W for 29 min. Then the mixture was cooled to
ambient temperature and washed repeatedly with deionized water by centrifugation until the centrifugate
0
becomes neutral. The clay residue was dried at 120 C and finely ground. The different organic acid
treated modified clay samples were designated as MSAꢀclay, pꢀTSAꢀclay and PDSAꢀclay.
2.3 Characterization:
The modified clay and zeolite samples were characterized using various physicochemical techniques such
2
7
as, pyridine FTꢀIR, TPDꢀNH , TGA, powder XRD, BET surface area, AlꢀMAS NMR and TEM. The
3
samples after the alkylation reaction were recovered and designated as pꢀTSAR, MSAR, PDSAR for
clays and BEAR, ZSMꢀ5R for zeolites respectively. The recovered samples were also characterized by
the techniques mentioned which are described below.
Acidities of samples were determined by recording pyridine adsorbed FTꢀIR spectra. All the samples
0
were activated by degassing at 110 C for 2 hours, then cooled under vacuum and saturated with liquid
0
pyridine for 30 min. Samples were again heated to 115 C to remove physisorbed pyridine and the spectra
ꢀ
were recorded by KBr pellet technique using Shimadzu, IRꢀAffinityꢀ1 instrument with resolution of 4 cm
1
ꢀ1
and 40 scans in the wavenumber range of 1600ꢀ1400 cm . Brønsted and Lewis acidities were evaluated
3
2
from the peak areas and expressed as mmol/g as given in the literature .
The total acidity of modified clays was also evaluated using TPDꢀNH method. In a typical experiment,
3
0
0
.1 g of sample was taken in a Uꢀshaped quartz sample tube. The catalyst was heated to 150 C in He(30
3
3
cm /min) for 2 h. A mixture of NH (90%) and He (10%) was passed at the rate of 30 cm /min for 30 min
3
0
3
over the catalyst maintained at 60 C. After this the catalyst sample was flushed with He(30 cm /min) for
0
0
6
0 minutes. TPD measurements were carried out in the range 100–650 C with heating rate of 10 C/min.
3
0, 32
Ammonia concentration in the effluent was monitored with thermal conductivity detector
.
3

RSC Advances
Page 4 of 16
View Article Online
DOI: 10.1039/C5RA09692H
Surface area and pore characteristics of parent clay, zeolites and modified clays were characterized under
liquid nitrogen temperature by Quanta Chrome Novaꢀ1000 surface analyser instrument. Adsorptionꢀ
desorption isotherm measurements were carried out for the evolution of porosity and textural properties.
BET, BJH (Barrett–Joyner–Halenda) methods were used to evaluate surface area, pore diameter and pore
3
2
volume and de Boer tꢀmethod (Temkin) for the newly generated microꢀpore volume .
Structural integrity of the catalyst samples was checked by powder XRD. The data were recorded by step
0
0
0
scanning at 2θ = 0.020 per second from 3 to 80 on Shimadzu Maxima XRDꢀ7000 Xꢀray diffractometer
with graphite monochromatized Cu Kα radiation (λ = 1.5406 Å).
Thermogravimetric analysis were performed using a Mettler Toledo 851e stare 7.01 TGAꢀDTA system
0
ꢀ1
0
by applying ramping rate of 5 C min to the temperature 100 to 650 C under flowing air.
A morphological change of montmorillonite before and after treatment with PDSA was recorded using
TecnaiꢀG2 Transmission Electron Microscope (TEM) under 120KV under vacuum mode.
27
AlꢀMASNMR pattern was recorded with resolution of 25Hz and 512 numbers of scans using single
pulse JEOL ECXꢀ2 400 MHz magic angle spin nuclear magnetic resonance spectrometer having
frequency 104.261 MHz with a 4 mm rotor of spinning speed 10k. The Al(NO ) is used as an external
3
3
standard.
2
.4 Catalytic activity:
The Catalytic activity of all the catalyst samples was studied in microwave reactor, microwave lab station
STARTꢀS” from Milestone, Italy, having software which enables the online control of reaction mixture
“
temperature with the aid of an infrared sensor that monitors the temperature by regulation of microwave
power output. The maximum variable power up to 1200 W was applied by the microprocessor controlled
singleꢀmagnetron system.
Initially, the reaction was conducted varying the catalyst amount and mole ratio at different reaction
temperatures in the microwave reactor. Under the optimized conditions, ten mmol of PC and ten mmol
of CH were mixed with 0.5 g of the catalyst in a microwave reactor vessel with a magnetic stirring bar.
The vessel was kept in a microwave reactor and an initial power of 1200 W was applied for 1 minute to
achieve reaction mixture temperature of 120 °C. Same temperature was maintained for 29 minutes. After
cooling the reaction mixture, 10 mL of toluene was added and stirred for 10 min. Then the mixture of
reactants and products were extracted by filtration and analysed using Chemito GCꢀ1000 gas
chromatograph having FID with a TRꢀWAX capillary column of 30 m length, 0.32 mm thickness and
33
0
.55 ꢁm internal diameter .
4

Page 5 of 16
RSC Advances
View Article Online
DOI: 10.1039/C5RA09692H
Reaction products formed were confirmed by GCꢀMS method using, Shimadzu GCMSꢀQP2010S with a
Column: Rtxꢀ5 (30m length, 0.25mm thickness 0.25 ꢁm internal diameter). The chemical reaction with
the products formed is shown in schemeꢀ1.
Catalytic activity measurement of samples was performed three times on each sample and mean values
were used for calculating the standard deviation. Under the experimental conditions, intraꢀparticle and
interꢀphase heat and mass transfer were calculated using Mears, KorosꢀNowak and Weiszꢀ Prater criteria
34, 35
were found to be negligible
.
After the completion of reaction, some of the catalyst samples exhibited discoloration. All the used
catalyst samples were separated by filtration and once again characterized by the techniques already
described. This was to identify the changes that are brought about on the catalyst samples by the reaction.
The spent catalyst samples recovered, (pꢀTSAR, MSAR, PDSAR for clays and BEAR, ZSMꢀ5R for
zeolites respectively) were reused to test the repeatability.
OH
OH
O
OH
Solid acid catalyst
MW
+
+
+
+ H2
O
p
ꢀCresol Cyclohexanol
Cyclohexene
OꢀAlkylation CꢀAlkylation
Scheme 1: Alkylation reaction carried using solid acid catalysts samples.
3
. Results and discussion:
3.1 Acid treated clays catalytic activity:
Alkylation reaction was conducted under optimized conditions after preliminary experiments at different
0
temperatures (in the range of 60 to 180 C), with increase in reaction time (from 5 to 240 minutes) and
varying the catalyst amount (0.1 to 1 g). Mole ratio 1:2 of CH to PC was maintained. Typical results are
shown for PDSA catalyst in Fig. 1. The trends were almost same for other catalyst samples.
Fig1 (a) shows an initial steep rise in conversion of CH along with formation of cyclohexene. Apparently,
initially formed cyclohexene favours the formation of Oꢀalkylated product, OꢀCH. With increase in time,
cyclohexene is used up for Cꢀalkylation. A decrease in the OꢀCH is also noticed. This continues to form
the alkylated products till an equilibrium ratio is reached.
5

RSC Advances
Page 6 of 16
View Article Online
DOI: 10.1039/C5RA09692H
0
Fig. 1(b) shows the progress of reaction as a function of temperature. Beyond 160 C, almost complete
0
conversion of CH is observed. At 100 C, cyclohexene concentration reached a maximum value but the
0
alkylated products formed were in small amounts. Beyond 100 C, the cyclohexene formed was
consumed for the alkylation favouring Cꢀalkylated product. A simultaneous decrease in the OꢀCH
0
product is observed beyond 100 C.
Formation of different products with the increase in catalyst amount is shown in Fig 1 (c). Almost
complete conversion of CH was noticed beyond 0.9 g of catalyst. Formation of cyclohexene initially
increased and reached a maximum with the catalyst amount up to 0.4 g, beyond which it was used up
preferentially for the formation of Cꢀalkylated product. At this point, a slight decrease in OꢀCH product is
also seen.
1
00
100
80
60
40
20
0
100
80
60
40
20
0
Cyclohexanol conversion
C-CH
O-CH
Cyclohexene
Cyclohexanol conversion
C-CH
O-CH
Cyclohexene
8
6
4
2
0
0
0
0
0
(
a)
Cyclohexanol conversion
(c)
(
b)
C-CH
O-CH
Cyclohexene
0
20
40
60
80
100 120 140 160 180 200
60
80
100
120
140
160
180
0.0
0.2
0.4
0.6
0.8
1.0
0
Reaction time (min)
Temaperature ( C)
Catalyst amount (g)
Fig 1: Effect of reaction conditions on alkylation activity of PDSA clay catalyst
0
(
(
(
a): Reaction conditions; catalyst 0.5 g; temperature, 120 C, mole ratio, PC: CH, 2:1.
b): Reaction conditions; catalyst 0.5 g; Reaction time, 30 min, mole ratio, PC: CH, 2:1.
0
c): Reaction conditions: temperature, 120 C, reaction time; 30 min, mole ratio, PC: CH, 2:1.
1
00
100
80
60
40
20
0
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
Cyclohexanol conversion
CꢀCH
OꢀCH
Cyclohexanol conversion
CꢀCH
OꢀCH
Cyclohexanol conversion
CꢀCH
OꢀCH
Cyclohexene
Cyclohexene
Cyclohexene
0
20
40
60
80
100 120 140 160 180 200
60
80
100
120
140
160
180
0.0
0.2
0.4
0.6
0.8
1.0
o
time (min)
temperatute ( C)
Catalyst amount (g)
Fig 2: Effect of reaction conditions on alkylation activity of HBEA catalyst
0
(
(
a): Reaction conditions; catalyst 0.5 g; temperature, 120 C, mole ratio, PC: CH, 2:1.
b): Reaction conditions; catalyst 0.5 g; Reaction time, 30 min, mole ratio, PC: CH, 2:1.
6

Page 7 of 16
RSC Advances
View Article Online
DOI: 10.1039/C5RA09692H
0
(
c): Reaction conditions: temperature, 120 C, reaction time; 30 min, mole ratio, PC: CH, 2:1.
Optimization studies for zeolite samples were also studied. They are presented in Fig.2 the trends were
similar to those observed in the case of modified clays.
The results of alkylation carried out at optimum conditions using different catalysts are given in Table 1.
Untreated clay did not show any catalytic activity and is not included in the Table. The results in Table 1
show that, modified clays and HꢀBEA showed almost 100 % conversions of CH, while ZSMꢀ5 and YꢀZ
showed low conversions.
Table 1 Catalytic activity of different catalyst for alkylation of PC with CH
Catalyst samples p-TSA-clay MSA-clay PDSA-clay H-BEA ZSM-5 Y-Z
CH
96
100
100
100
41
39
Conversion (%)
Oꢀalkylation
Cꢀalkylation
Cyclohexene (%)
29
35
32
26
46
28
18
61
21
12
54
10
9
4
18
6
12
15
0
Reaction conditionsꢀ mole ratio 1:2 of CH to PC; time 60 min; catalyst 0.5 g; temperature, 120 C,
Alkylation is an acid catalysed reaction. In order to explain the improved catalytic activity of clay
samples upon modification for alkylation, further studies were made on the modified clays with regard to
changes in Brønsted and Lewis acidity and surface characteristics. There are reports in the literature that
organic acid treated clays develop pores on their surface and show pore characteristics similar to zeolites
30, 32,34
but retaining their layer structure
.
Development of pores on the clay surface is explained on the observation of dissolution and chelation of
structural Al (III) on treatment with organic acids. The amount of Al leached out upon acid treatment was
quantitatively estimated by ICPOES. The structural Al leached out was found to be in the order PDSA
2
7
(
1.14meq/g) >MSA (1.02 meq/g) >pꢀTSA (0.97 meq/g). A qualitative examination of
Al NMR
spectrum of the untreated montmorillonite sample (Fig. 4) showed the presence of octahedral Al. The
1
0, 11, 13, 23,
removal of Al ions causes voids on surface resulting in the increase in surface area and pore size
30
.
The morphology also changes after the treatment as shown by the TEM images (Fig.3).
7

RSC Advances
Page 8 of 16
View Article Online
DOI: 10.1039/C5RA09692H
Fig.3 TEM images of (a) Untreated clay, (b) PDSAꢀclay
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
-
100 -80 -60 -40 -20
0
20 40 60 80 100
δ (ppm)
27
Fig 4 Al MAS NMR of untreated montmorillonite clay
Table 2 shows surface area and porosity measurements as determined by BET method and Lewis and
Brønsted acidities by pyridine adsorption using FTIR for zeolite samples and the modified clays (spectra
provided as supplementary material). Activity of catalyst samples were expressed as average TOF which
gives the number of reactant molecules reacted per minute on a single active site of the catalyst.
8

Page 9 of 16
RSC Advances
View Article Online
DOI: 10.1039/C5RA09692H
Table 2 Surface and acidity characteristics of modified clay samples and zeolites
Clay
samples
S
APD
V
V
mt
B
L
B+L
Total
alkylated
product (%)
Average
TOF
BET
tot
2
o
3
3
(m /g)
(A )
(cm /g)
(cm /g)
(µmol/g)
(µmol/g) (µmol/g)
ꢀ
1
(
min )
Untreated
pꢀTSAꢀclay
MSAꢀclay
PDSAꢀclay
ZSMꢀ5
28
50.3
35.3
32.7
24.0
ꢀ
0.078
0.120
0.195
0.343
0.214
0.395
0.486
0.011
0.026
0.053
0.201
0.076
0.300
0.132
ꢀ
70
69
72
574
222
423
ꢀ
68
70
68
320
184
263
ꢀ
ꢀ
ꢀ
135
206
276
346
638
542
138
139
140
894
406
686
64
72
79
13
18
66
0.773
0.863
0.940
0.024
0.074
0.160
YꢀZ
HꢀBEA
ꢀ
ꢀ
SBET: BET Surface area (sd;±2.99), APD: average pore diameter (sd;±0.241), V : total pore volume
tot
(
sd;±0.002), V : microꢀpore volume by de Boer tꢀmethod (sd;±0.005), BꢀBrønsted (sd;±4.11) and Lꢀ
mt
Lewis acidity (sd;±3.51) as Pyridine FTꢀIR. B+L acidity (sd;±3.51) and total alkylated product
sd;±0.766)
sdꢀ standard deviation
(
*
Untreated clay sample (Naꢀclay) did not show any activity because of the absence of acid sites. Modified
clays demonstrated better activity for alkylation than the zeolites (Table 2). ZSMꢀ5 and YꢀZ showed
relatively low activities as shown by the total alkylated product (OꢀCH + CꢀCH) formed, while HꢀBEA
showed maximum activity (66 %), among zeolites, almost comparable to that of pꢀTSA modified clay.
The reason for low activity of ZSMꢀ5, though it had higher acidity, is because of the relatively narrow
pores compared to HBEA. The low activity of YꢀZ is attributed to its low acidity and cage structure.
All the three zeolites had higher acidity values compared to modified clay samples. In the case of zeolites,
acid sites are mostly present in the channels in contrast with modified clays where they are present in the
interlayer. Since untreated clay does not possess any acid sites, the pores which are generated in the
modified clays are able to provide enhanced access to the acid sites present in the interlayer. The
2
9ꢀ32
enhanced access to the sites is apparently responsible for increased catalytic activities
.
Correlation calculations were made between the total alkylated products formed and the catalyst
parameters such as surface area, microꢀpore volume, total pore volume, average pore diameter and acidity
with the yield of Cꢀalkylated product. Every data represent an average of three replicate samples. The
correlation calculations were carried out by standard statistical methods for small sets of data.
2
Correlation coefficients as expressed in r value are shown in Table 3. No correlations were noticed when
yield of the alkylated product for all the samples were considered. When clays were considered separately
from zeolites, better correlation was observed. Surface area, acidity and total pore volume, microꢀpore
volume showed high correlation for the modified clays while for zeolites all these parameters poorly
correlated with catalytic activity.
9

RSC Advances
Page 10 of 16
View Article Online
DOI: 10.1039/C5RA09692H
Table 3 Correlation coefficients
2
2
Parameters
r for MODIFIED CLAYS
r for ZEOLITES
2
SBET (m /g)
tot
0.999
0.950
0.835
0.942
0.391
0.001
0.998
0.076
0.656
0.037
ꢀ
0.000
0.000
0.000
3
V (cm /g)
3
V (cm /g)
mt
o
APD (A )
B (µmol/g)
L (µmol/g)
B+L (µmol/g)
TPDꢀNH3
*Product %
MSAꢀclay
72)
pꢀTSAꢀclay
(64)
PDSAꢀclay
(79)
ZSMꢀ5
(13)
HꢀBEA
(66)
YꢀZ
(18)
(
*Total alkylated product (OꢀCH + CꢀCH)
3.2 Reuse of the used catalysts:
In order to understand the difference in behaviour between two categories of materials with regard to
0
their reusability, the used catalyst samples, recovered from reaction mixture and dried at 80 C, were
reused in reaction with fresh reagents. The results of alkylation are shown in Table 4 for three more
recycles.
Table 4 comparison of catalytic activity of used catalyst samples
Catalyst
pꢀTSAꢀclay
MSAꢀclay
PDSAꢀclay
HꢀBEA
Recycle CH conversion
%)
Total
alkylation
62
Cyclohexene
(
(%)
30
28
31
26
28
27
21
22
22
10
8
st
1
92
90
91
98
98
96
100
100
100
68
52
43
31
26
22
34
30
24
nd
2
62
60
72
70
71
79
78
78
50
36
28
12
8
6
20
15
rd
3
1
st
nd
rd
2
3
1
st
nd
rd
2
3
1
st
nd
rd
2
3
1
7
st
14
10
10
11
8
nd
ZSMꢀ5
2
rd
3
1
st
nd
HꢀY
2
rd
3
12
4
0
Reaction conditions; mole ratio 1:2 of CH to PC; time 60 min; catalyst 0.5 g; temperature, 120 C
1
0

Page 11 of 16
RSC Advances
View Article Online
DOI: 10.1039/C5RA09692H
Zeolites showed considerable reduction in catalytic activity as noticed by the amount of alkylated product
formed. This is perhaps due to the formation of coke in case of zeolites. To confirm this, used samples in
st
the 1 recycle were subjected to TGA, acidity and surface measurements. Table 5 shows the results for
changes in acidity and surface measurements. The changes noticed are given in parenthesis. Fig 5A and
5B shows variations in pyridineꢀFTIR acidity and Fig 6A and 6B for changes in TGA plots. Coke
formation in case of zeolites is evident by the TGA patterns (Fig 6B). TGA analysis was carried out to
find out deposition of any carbonaceous compounds on the catalyst surface during the reaction. It was
noticed that there was no significant weight loss for the clay sample. On the other hand, zeolite sample
showed 20ꢀ30% weight loss after the reaction indicating formation of coke on the surface. Reduction in
acidity and surface area (Table 5) provide supporting evidence. Deposition of coke in the zeolite
channels may be cause for these reductions in the alkylation (Table 4). On the other hand, modified clays
showed no such changes observed in case of zeolites.
3.3 Channels as opposed to open pores in recycling:
Zeolites used in this study are microporous materials and are known to possess cages interconnected
through channels for Yꢀ Z and intersecting channels for ZSMꢀ5 and HBEA. Modified clays, however,
are known to possess microꢀ and mesoꢀ pores generated by dealumination on acid treatment. The pores
3
0, 32, 34
formed, in case of modified clays, are open pores on the surface
.
The different behaviour of two
materials for their reusability in the alkylation reaction is apparently due to the difference in their pore
architecture. This is further explained below.
The mechanism of alkylation involves the initial formation of cyclohexene from the dehydration of CH
(
Scheme 2). A part of cyclohexene formed is used up for the alkylation, another part may undergo
oligomerization leading to formation of coke and remaining cyclohexene will be present in the reaction
mixture. Out of these three, formation of coke is favoured in the channels of zeolites where most of the
active sites are located. Furthermore, for several cyclohexene molecules to find the acid sites and get
oligomerized to form coke is more probable inside the channels. This clearly explains the coke formation
in case of zeolites and also limitation in their reuse for alkylation.
1
1

RSC Advances
Page 12 of 16
View Article Online
DOI: 10.1039/C5RA09692H
Clay-
Clay-
+
H
H
H
+
H
+
O
OH
HO
O
O
+
H
CH
HO
O
H
OH
Scheme 2 Possible reaction mechanism for alkylation of PC with CH
In contrast with zeolites, the active sites on the modified clays, are located in the interlayer accessed
through the generated pores which are open on the surface. A part of cyclohexene formed on active sites
of the clay brings about the alkylation and the rest diffuse into the medium. The diffusing cyclohexene
molecules do not find active sites to undergo oligomerization for coke formation. Retardation of coke
formation in the case of modified clays and their unchanged catalytic activity for reuse can thus be
attributed to their open pores.
Table 5 Surface and acidity characteristics of reused clay samples and zeolites.
Clay
samples
pꢀTSAꢀclay
MSAꢀclay
PDSAꢀclay
ZSMꢀ5
S
V
V
B
L
B+L
BET
tot
mt
2
3
3
(m /g)
133(2)
202(4)
275(1)
264(82)
564(74)
384(158)
(cm /g)
(cm /g)
(µmol/g) (µmol/g) (µmol/g)
0.118(0.002) 0.024(0.002)
0.189(0.006) 0.049(0.004)
0.332(0.011) 0.198(0.003)
69(1)
67(2)
70(2)
66(2)
68((2)
67(1)
135(3)
135(4)
137(3)
0.153(0.061) 0.049(0.027) 268(306) 202(118) 470(424)
0.305(0.090) 0.234(0.066) 168(54) 108(76) 276(130)
0.368(0.118) 0.099(0.033) 203(220) 143(120) 346(340)
YꢀZ
HꢀBEA
SBET: BET Surface area, D : Average pore diameter, V : total pore volume, V : microꢀpore volume by
p
tot
mt
de Boer tꢀmethod, BꢀBrønsted and LꢀLewis acidity as Pyridine FTꢀIR. The value in ( ) indicates
characteristics (before –after reaction)
1
2

Page 13 of 16
RSC Advances
View Article Online
DOI: 10.1039/C5RA09692H
5
B
5
A
a
b
%
T
a
b
1
600 1580 1560 1540 1520 1500 1480 1460 1440 1420 1400
16001580 1560154015201500148014601440 14201400
ꢀ1
Wavenumber (cm )
ꢀ1
wavenumber (cm )
Fig 5A pyridine FTꢀIR spectra of PDSA clay (a) before and (b) after the reaction
Fig 5B pyridine FTꢀIR spectra of HBEA (a) after and (b) before the reaction
1
00
100
90
80
70
60
100
90
9
9
8
8
7
7
6
6
5
0
5
0
5
0
5
0
80
P DSA
pTSA
pTSAR
MSA
MSAR
P DSAR
70
60
0
100
200
300
400
500
600
0
100
200
300
400
500
600
0
100
200
300
400
500
600
o
o
o
temperature ( C)
temperature ( C)
temperature ( C)
Fig 6A TGA pattern of PDSA, MSA and pꢀTSA clay samples before and PDSAR, MSAR and pꢀTSAR
samples after reaction
1
00
100
90
ZSM-5
ZSM-5R
HBEA
HBEAR
90
80
70
60
80
70
60
0
100
200
300
400
500
600
700
0
100
200
300
400
500
600
o
o
temperature ( C)
tempertature ( C)
Fig 6B. TGA pattern of ZSMꢀ5 and HBEA zeolites before and ZSMꢀ5R and HBEAR zeolites after
reaction
1
3

RSC Advances
Page 14 of 16
View Article Online
DOI: 10.1039/C5RA09692H
4. Conclusions:
Clays modified with organic sulfonic acids such as PDSA and MSA were found to be more effective than
zeolites for alkylation of PC with CH. Reuse of used catalyst samples showed different behaviour for
zeolites and modified clays. Zeolites exhibited reduced activity on reuse which was attributed to the
formation of coke on the active sites in zeolite channels. Coke formation in the channels is more probable
due to the availability of acid sites favouring cyclohexene molecules to oligomerize. Modified clays did
not show any change in activity on reuse. Retardation of coke formation in the case of modified clays and
their unchanged catalytic activity for reuse is attributed to their open pores.
Acknowledgments:
The authors would like to thank the Principal and Members of Governing Council of Bangalore Institute
of Technology for the facilities provided and VTU for financial support. The authors also extend their
thanks to Dr. Arun Kumar and Dr. S. Ramesh for TEM, L.V Narendra, NMRC, IISc, Bengaluru for
MAS NMR, Dr. Nagabhushan, Tumkur University, for XRD, Prof. P.Vishnu Kamath, Bangalore
University and Ms. Manjula, Department of Physics, IISc for TGA measurements .
References:
1
2
3
4
S. Yariv, H. Cross, Organo-Clay Complexes and Interactions; Marcel Dekker: New York. 2002.
J. M. Adams, R.W. McCabe, Handbook of Clay science, 2001, volume 1, Elsevier limited.
U. Kurschner, V. Seefeld, B. Parlitz, W. Geflner, H. Lieske, React. Kinetic. Catal. Lett., 1998, 65, 17.
S. Kulprathipanja, Zeolites in Industrial Separation and Catalysis. WileyꢀVCH Verlag GmbH & Co.
KGaA, Weinheim, 2010.
5
6
7
8
9
1
C. B. Jurado, E. A. Valdes, J. Phys. Chem. A., 2009, 113, 8994.
P. Boulet, H.C Greenwell, S. Stackhouse, P.V Coveney, Theo. Chem., 2006, 762, 33.
M.P Hart, D.R Brown, J. Mol. Catal. A: Chem., 2004, 212, 315.
L. Cerveny, K. Mikulcova, J. Cejka, Appl. Catal. A. Gen., 2002, 223, 65.
M. Lenarda, L. Storaro, A. Talon, E. Moretti, P. Riello, J. Colloid. Interface Science., 2007, 311, 537.
0 A.K. Panda, B.G. Mishra, D.K. Mishra, R.K. Singh, Colloids. Surfaces A: Physicochem. Eng.
Aspects., 2010, 363, 98.
1
1
1
1 O.S. Ahmed, D.K. Dutta, J. Mol. Catal. A., 2005, 229, 227.
2 I. Tkac, P. Komadel, D. Muller, Clay mineral., 1994, 29, 11.
3 S. Ramesh, Y.S. Bhat, B.S. Jai Prakash, Clay. Miner., 2012, 47, 231.
14
L. Biesekia, F. Bertella, H. Treichel, F. G. Penha, S. B. C. Pergher, Materials Research., 2013, 16,
122.
1
1
4

Page 15 of 16
RSC Advances
View Article Online
DOI: 10.1039/C5RA09692H
1
1
1
1
1
2
2
2
2
2
2
2
5 Y. Liu, S.S. Kim, T.J. Pinnavaia, J. Catal., 2004, 225, 381.
6 S.R. Kirumakki, N. Nagaraju, K.V.R. Chary, S. Narayanan, J. Catal., 2004, 221, 549.
7 R. Anand, K. Gore, B. Rao, Catal. Lett., 2002, 81, 33.
8 R. Anand, T. Daniel, R.J. Lahoti, K.V. Srinivasan, B.S. Rao, Catal. Lett., 2002, 81, 241.
9 G.D. Yadav, P. Kumar, Appl. Catal. A., 2005, 286, 61.
0 N. Bhatt, A. Patel, Indian J. Chem. Sect. A., 2008, 47, 387.
1 G. Wine, E. Vanhaecke, S. Ivanova, R. Ziessel, C.P. Huu, Catal. Commun., 2009, 10, 477.
2 T. Stoylkova, P. Matthae, C. Bezouhanova, H. Lechert, Stud. Surf. Sci.Catal., 1998, 117, 429.
3 E. Kozhevnikova, Appl. Catal. A., 2003, 245, 69.
4 P. Adryan, R.Ismail,F. Roessner, Kinet. Catal., 2008, 49, 587.
5 M.L. Occelli, S.D. Landau, T.J. Pinnavaia, J. Catal., 1984, 260, 256.
6 D.R. Fernandes, A.S. Rocha, E.F. Mai, C.J.A. Mota, V.T.D. Silva, Appl. Catal. A: Gen., 2012, 425,
1
99.
2
2
2
3
3
3
3
3
3
7 G.D. Yadav, G.S. Pathre, J. Mol. Catal. A: Chem., 2006, 243, 77.
8 V. Krishnan, K. Ojha, N.C. Pradhan, Org. Process Res. Dev., 2002, 6, 132.
9 S. Ramesh, B.S. Jai Prakash, Y.S. Bhat, Appl. Catal. A: Gen., 2012, 413, 157.
0 N. J. Venkatesha, B. S. Jai Prakash, Y. S Bhat, Catal. Sci. Technol., 2015, 5, 1629.
1 B. T. Christensen, Advances in Soil Science., 1992, 20, 1.
2 N.J Venkatesha, Y.S Bhat, B.S Jaiprakash, Appl. Catal. A: Gen., 2015, 496, 51.
3 N. J. Venkatesha, B.M. Chandrashekara, B.S. JaiPrakash, Y.S. Bhat, J. Mol. Catal. A., 2014, 392, 181.
4 M.Cobo, A. Quintero, C.M. de Correa, Catal. Today., 2008, 133, 509.
5 S.T. Oyama, X. Zhang, J. Lu, Y. Gud, T. Fujitani, J. Catal., 2008, 257, 1.
1
5

RSC Advances
Page 16 of 16
View Article Online
DOI: 10.1039/C5RA09692H
Graphical abstract:
Highlights
ꢀ
ꢀ
Organic sulfonic acid treatment generates open pores on surface of clays and these
provide access to active sites in the interlayer
Modified clays retards formation coke in p-cresol alkylation, while zeolites exhibit
diminished activity due to coke formation caused by pore architecture.