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This content was downloaded from IP address 116.123.109.65 on
21/10/2021 at 17:59
N. N. Binitha1,2, P.P. Silija1, V. Suraj3, Z. Yaakob1, S. Sugunan4,
1Department of Chemical and Process Engineering, Faculty of
Engineering and Built Environment, National University of Malaysia,
43600 UKM Bangi, Selangor, Malaysia chool of Materials Engineering,
Universiti Malaysia Perlis, Taman Muhibah, 02600, Jejawi, Perlis,
Malaysia 2Department of Chemistry, Sree Neelakanta Government
Sanskrit College, Pattambi, Palakkad-679306, Kerala, India
3Department of Applied Chemistry, CUSAT, Cochin 22, Kerala, India
4National Institute of Technology, Calicut, Kerala, India Email:
[email protected]
1. Introduction The major current challenges before chemists are to
develop synthetic methods that are less polluting, i.e., to design
clean or ‘green’ chemical transformations. Clays are widely used
due to its ecofriendly nature, these are non-toxic, non-corrosive,
economical and recyclable, and thus are efficiently used for a
variety of organic reactions [1]. Clays provide easy separation and
regeneration [2]; in addition it shows good selectivity for a
particular product due to its well-defined porous structure. The
efficient use of clays is restricted due to the low thermal
stability, which makes it less useful for high temperature
reactions. Modifications are done to improve stability and
activity. The catalytic properties of montmorillonite clays can be
greatly manipulated by ion exchange to suit the needs of chemical
industry and synthetic organic
CAMAN IOP Publishing IOP Conf. Series: Materials Science and
Engineering 17 (2011) 012018
doi:10.1088/1757-899X/17/1/012018
Published under licence by IOP Publishing Ltd 1
chemists. Chemically modified clays are considered to be
potentially the most cost-effective nanomaterials. Usual cations
present in parent clays can be exchanged with hydrated transition
metal ions as well as rare earth cations. Since cations of
interlamellar space have no fixed sites in the lattice, when the
mineral is immersed in an electrolyte, exchange readily takes place
according to the principle of equivalence between the external and
internal cations [3], i.e., hydrated cations of the interlamellar
surface of the native mineral can be replaced with almost any
desired cation by utilizing simple ion exchange methods. It has
been realized that metal exchanged clay minerals possess catalytic
potentialities at their own right as an acid and redox catalyst
without any additional treatment [4]. Catalytic activity arises
from intrinsic activity of metal ions and surface acidity due to
cation exchange. Restricted environment of clay interlayer will
cause shape selectivity of the reaction products. In the present
study montmorillonite K10 is subjected to ion exchange with
samarium ion. The exchange is done with and without the
pre-exchange of sodium ions. The cation exchanged clays are
characterized by spectral, XRD and elemental analysis. Its physical
properties like surface area, pore volume and surface acidity are
also studied. Thermograms are taken to compare the thermal
stabilities of exchanged clays with that of parent clay. The extent
of cation exchange is determined from ICP-AES analysis. Cumene
cracking reaction is performed to know the catalytic activity and
acid site distribution. These types of catalytic cracking reactions
are of primary interest in petroleum processing due to the large
demand for aromatic petrochemicals of low molecular weight i.e.,
gasoline and paraffinic molecules for diesel fuel [5]. As bulk
crude oil is not generally rich in these molecular types, their
formation is very important. The reactions, which are required to
produce the desired hydrocarbon fractions, are brought about
through the use of acidic solids that activate the hydrocarbons
through the formation of carbocations. Now the porous clays have
recently received a great deal of attention for these types of
reactions. Cumene is a convenient model compound for such catalytic
studies, because it undergoes different reactions over different
types of active sites. The cracking of cumene, producing benzene
and propene, is generally attributed to the action of Brønsted acid
sites, following a carbonium ion mechanism and it is commonly used
as a model reaction for characterizing the presence of this type of
site on catalyst surface. The formation of α -methyl styrene during
cumene cracking, due to dehydrogenation, has been ascribed to Lewis
acidity [6,7]. Thus the present study allows the comparison of the
distribution of both Brønsted as well as Lewis acid sites of the
catalysts prepared. The use of clays for high temperature reactions
and the improvement in catalytic properties by cation exchange is
also understood. 2. Experimental 2.1. Materials Montmorillonite K10
(Fluka) is used as the parent clay; sodium nitrate (S.D.Fine-Chem
Ltd) and samarium nitrate (IRE Ltd, Udyogamandal) are used for ion
exchange. Cumene (Aldrich chemicals) is used as such for cracking
reactions. 2.2. Catalyst preparation and characterization
Montmorillonite K10 is added to sodium nitrate solution (0.5M) and
stirred vigorously for 24 hours at 343K, aged for one day,
filtered, washed free of nitrate and dried at 383K to obtain sodium
exchanged montmorillonite. For the exchange of samarium both parent
as well as sodium-exchanged clays are suspended in samarium nitrate
solution (0.5M) and the same method as described earlier is
followed; calcinations of the samples are done at 500°C in air. A
Micromeritics Gemini 2360 surface area analyzer is used to get the
BET surface area with N2 adsorption at liquid N2 temperature (77
K). The catalysts are heated at 673 K for 3hrs under a flow of
nitrogen before running the analysis. Elemental analysis of
prepared catalysts was done using inductively coupled plasma-atomic
emission spectrometer after the quantitative separation of silica.
Analysis is done using ‘GBC’ Plasmalab 8440M instrument. X-ray
diffraction pattern of the catalysts is obtained using a
CAMAN IOP Publishing IOP Conf. Series: Materials Science and
Engineering 17 (2011) 012018
doi:10.1088/1757-899X/17/1/012018
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Rigaku X-ray diffractometer with Ni filtered Cu-Kα radiation
(λ=1.5414Ao). FT-IR spectra are obtained by the KBr pellet method
on ABB BOMEM (MB Series) spectrometer. TG data of the samples were
obtained using a METTLER TOLEDO STARSYSTEM TGA Q50 thermal analysis
instrument in Nitrogen atmosphere from 298K-1073K at a heating rate
of 20°C/min. To find the acidity of the catalysts, Temperature
programmed desorption of ammonia has been carried out and the acid
strength distribution is obtained from the amount of ammonia
desorbed against temperature, in the range 373 K- 873 K. 2.3.
Reaction Cumene cracking reaction is carried out at atmospheric
pressure in a fixed bed, vertical, down- flow glass reactor placed
inside a double-zone furnace under a continuous flow of nitrogen.
Exactly 0.5 g of the sample was sandwiched between the layers of
inert silica beads. A thermocouple detector will monitor the
temperature of the catalyst. The reactant is fed into the reactor
by means of a syringe pump (Cole palmer R) at required WHSV and
temperature in the range 573K-723K. The products and the unreacted
reactant were condensed by means of a water condenser, collected
and subjected to Gas Chromatographic analysis using Chemito GC 1000
gas chromatograph with BP1 capillary column connected to a FID
detector. The catalyst was activated at 773 K for 2 hours before
subjecting to the reaction. 3. Results and discussion
3.1. Catalysts characterization Table 1 lists the catalysts, their
BET surface area and pore volume after calcination at 773K. The
surface area and pore volume of the different systems remains more
or less unchanged after modification. The slight changes are due to
difference in size of the exchanged cations. Introduction of bulky
exchanged cations leads to the pore volume reduction.
Table 1 Surface area & pore volume values Catalys
t Surface Area (BET) m2/g
Pore Volume x10-6 m3/g
K 148.00 0.24
SmD 148.33 0.23
SmI 142.16 0.22
K – Montmorillonite K10. NaK – Sodium ionexchanged montmorillonite
K10. SmD – Samarium ion exchanged montmorillonite prepared directly
from montmorillonite K10. SmI - Samarium ion exchanged
montmorillonite prepared from NaK. Elemental analysis data are
given in Table 2. Only very small percentage exchange occurs for
both the cases. It is seen that more exchange occurs for the system
that is prepared by direct exchange with the parent montmorillonite
K10. Thus in the present study it is proved that pre exchange with
sodium ions decreases exchange capacity of the montmorillonite K10.
The small increase to silica - alumina ratio after ion exchange is
due to the leaching of aluminium during the treatment with
corresponding nitrate solutions.
CAMAN IOP Publishing IOP Conf. Series: Materials Science and
Engineering 17 (2011) 012018
doi:10.1088/1757-899X/17/1/012018
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Table 2 Elemental Analysis results
Fig.1 shows the XRD patterns of the as – synthesized catalysts. Due
to the poorly crystalline nature of
Fig. 1 XRD patterns of the parent as well as exchanged
systems
Vibrational spectroscopic investigations yield useful information
about hydration characteristics, interlayer cations and moisture
content in the clays. FTIR spectra of parent montmorillonite K10
and the exchanged samples are shown in Fig.2 Si-O-Si stretching
bands can be seen in the region 1115-1005cm- 1. In the present case
the intense peak at 1050 cm –1 clearly indicates Si-O-Si stretching
vibrations of tetrahedral layer. Broad band near 3400 cm-1
indicates the –OH group. Furthermore, H-O-H hydrogen bonded water
causes a band near 3430cm-1, which further broadens the -OH
stretching band. The band near 1650 cm-1 corresponds to H-O-H
deformation mode. The bands at 793 cm-1 and 627 cm-1 correspond to
Si-O deformation perpendicular and parallel to optical axis
respectively [8]. From the FTIR of K10 and other samples it is
clear that the basic structure of clay is not at all altered during
cation exchange and is also clear from XRD profiles.
CAMAN IOP Publishing IOP Conf. Series: Materials Science and
Engineering 17 (2011) 012018
doi:10.1088/1757-899X/17/1/012018
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Fig. 3 Thermograms of parent montmorillonite and Sm3+ exchanged
systems TG curves of the parent clay and the samarium ion exchanged
systems are shown in fig.3 The sharp
dip near 373K for all the samples indicates a cumulative weight
loss corresponding to the dehydration of physisorbed water from
clay structure. The second major weight loss are at around
873K-1173K due to the dehydroxylation of the clay structure. On
exchanging with Samarium ions no additional weight loss occurs in
the TG curves, which is present for pillared species. Further
dehydroxylation is absent in the exchanged cases support that the
cationic species exist as such. Samarium ion exchanged system
prepared directly from K10 shows enhanced thermal stability as
evident from the TG curves.The TPD results of the prepared samples
are tabulatedin table 3. The ammonia desorption in the range of
373K – 473K are from the weak acid sites, that in the range 473K
–673K and in 673K – 873K are of medium acid sites and
CAMAN IOP Publishing IOP Conf. Series: Materials Science and
Engineering 17 (2011) 012018
doi:10.1088/1757-899X/17/1/012018
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strong acid sites respectively. Among the different systems SmD
shows maximum acidity. All systems show more acid sites in the weak
and medium regions. Thus from TPD of ammonia it is inferred that
maximum acid sites are present in the weak acid region.
Table 3 Acidity values from TPD measurements
Fig. 4 WHSV on SmI at a temperature of 673K
The change in conversion with WHSV is shown in fig.4 First the
percentage conversion increases and then decreases with increase in
WHSV. This may be due to the fact that for low WHSV of 3.456h-1
surface adsorption of the reactant may be stronger than desorption
and surface reaction. Further increase in flow rate decreases the
conversion due to the lesser contact time. Thus the WHSV of
5.184h-1 is adopted as the optimum value.
CAMAN IOP Publishing IOP Conf. Series: Materials Science and
Engineering 17 (2011) 012018
doi:10.1088/1757-899X/17/1/012018
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Fig. 5 Effect of reaction temperature on SmI at a WHSV of
5.184h-1
Fig.5 reports the cumene conversion at different temperatures. As
the temperature is increased from 573K to 673K conversion increased
first but further increase of the temperature to 723K decreased the
conversion. This may be due to the higher coke deposition at high
temperatures and thus 673K is selected as the optimum reaction
temperature.
Doing the reaction at the optimized reaction conditions compares
catalytic activities of the different systems. Eventhough cumene
cracking is a low conversion reaction; the present systems gave
good results. The conversion and the selectivity are tabulated in
Table 4.
Table 4 Catalyst Comparisons
WHSV 5.184h-1, Temperature 673K, Time on stream 2h, Amount of
catalyst 0.5 g In all the systems cracking of cumene to benzene is
the major reaction. More than 65% selectivity is obtained for
benzene. It is observed that as Sm content increases, the
conversion also increases showing that the inherent property of
this metal ion has an influence in the catalytic activity. This is
also proved from the acidity values. Continuous run of the reaction
is done for several hours to check the stability of the catalysts
and is found that the conversion and selectivity remains almost
constant, which makes the process more economical. 3.3. Activity –
Acidity relationships The catalytic activity could be
correlated in terms of either Lewis or Brønsted acid centers
contributing to the reaction mechanism [13]. Clay catalysts have
been shown to contain both Brønsted and Lewis acid sites, with the
Brønsted sites mainly associated with the interlamellar region and
the Lewis sites mainly associated with edge sites [14]. In addition
to test the activity, the cumene cracking reaction gives a picture
of the acid site distribution and also compares the total acidity
of the different systems. The main incentive to carry out cumene
conversion reaction is to correlate activity and selectivity with
acidity of the prepared catalysts. The ratio of benzene to α-methyl
styrene shown in table 4 is a measure of the acid site distribution
to Brønsted and Lewis sites. The enhanced Brønsted sites on the
acid activated parent montmorillonite K10 and its exchanged
analogues makes higher selectivity to benzene in all the cases.
Eventhough the K and NaK possess considerable acid sites as evident
from TPD measurements,
CAMAN IOP Publishing IOP Conf. Series: Materials Science and
Engineering 17 (2011) 012018
doi:10.1088/1757-899X/17/1/012018
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the activity is found to be comparatively low due to the low
thermal stability of these systems making them less suitable for
high temperature reactions. The samarium ion exchange increased the
acid sites and this is confirmed from the increased conversion. In
addition to the enhanced Brønsted acidity, the samarium ion
exchanged systems possess considerable Lewis acid sites and thus
decreasing the benzene to α- methyl styrene ratio. TPD measurements
show maximum acid sites in the weak and medium acidic region. 4.
Conclusions Cation exchange of montmorillonite K10 is done
effectively to improve the catalytic properties for high
temperature reactions and is proved from cumene cracking reactions.
The stability of the basic structure is established from FTIR and
XRD results. The optimum conditions for cumene cracking reactions
are selected and the prepared systems are found to be useful for
high temperature reactions. Cation exchange with samarium ions is
thus proved to be an efficient method to enhance the acidic as well
as catalytic properties of montmorillonite. The present systems are
good ecofriendly catalysts for a number of organic transformations.
5. Acknowledgement The authors also thank RSIC, Mumbai for the ICP-
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CAMAN IOP Publishing IOP Conf. Series: Materials Science and
Engineering 17 (2011) 012018
doi:10.1088/1757-899X/17/1/012018
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