IOP Conference Series: Materials Science and Engineering OPEN ACCESS Samarium Ion Exchanged Montmorillonite for High Temperature Cumene Cracking Reaction To cite this article: N N Binitha et al 2011 IOP Conf. Ser.: Mater. Sci. Eng. 17 012018 View the article online for updates and enhancements. You may also like Analysis of Heat Exchange Performance of Heat Exchange Tubes of Evaporative Heat Exchanger Based on Fluent Changting Li, Fan Bai and Fanghuai Gou - Mathematical model of process of production of phenol and acetone from cumene hydroperoxide I Z Baynazarov, Y S Lavrenteva, I V Akhmetov et al. - Effect of Methylated Polybenzimidazole As an Electrolyte for Anion-Exchange Membrane Fuel Cells Ziyi Han, Tsuyohiko Fujigaya and Naotoshi Nakashima - This content was downloaded from IP address 116.123.109.65 on 21/10/2021 at 17:59
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IOP Conference Series: Materials Science and Engineering
OPEN ACCESS
Samarium Ion Exchanged Montmorillonite for HighTemperature Cumene Cracking ReactionTo cite this article: N N Binitha et al 2011 IOP Conf. Ser.: Mater. Sci. Eng. 17 012018
View the article online for updates and enhancements.
You may also likeAnalysis of Heat Exchange Performanceof Heat Exchange Tubes of EvaporativeHeat Exchanger Based on FluentChangting Li, Fan Bai and Fanghuai Gou
-
Mathematical model of process ofproduction of phenol and acetone fromcumene hydroperoxideI Z Baynazarov, Y S Lavrenteva, I VAkhmetov et al.
-
Effect of Methylated Polybenzimidazole Asan Electrolyte for Anion-ExchangeMembrane Fuel CellsZiyi Han, Tsuyohiko Fujigaya and NaotoshiNakashima
-
This content was downloaded from IP address 116.123.109.65 on 21/10/2021 at 17:59
Samarium Ion Exchanged Montmorillonite for High Temperature Cumene Cracking Reaction
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]
Abstract – Montmorillonite clay is cation exchanged with samarium and its catalytic influence in cumene cracking reaction is investigated. Effect of exchange with sodium ions on further exchange with samarium ions is also noted. Acidity measurements are done using Temperature Programmed Desorption (TPD) of ammonia. The retention of basic structure is proved from FTIR spectra and XRD patterns. Elemental analysis result shows that samarium exchange has occurred, which is responsible for the higher catalytic activity. Surface area and pore volume remains more or less unaffected upon exchange. Thermogravimetric analysis indicates the enhanced thermal stability on exchanging. Cumene cracking reaction is carried out at atmospheric pressure in a fixed bed glass reactor at 673 K. The predominance of Brønsted acidity is confirmed from high selectivity to benzene.
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
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
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
Na K 150.56 0.22
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.
Fig.1 shows the XRD patterns of the as – synthesized catalysts. Due to the poorly crystalline nature of
the clay, XRD peaks are not sharp. The d001 peak of parent montmorillonite is at a 2θ value of 8.9 degrees corresponding to a spacing of 9.8Å. It is clear that exchanging did not make considerable changes to the layer distance. The slight shift towards left and broadening are due to exchanged cations and the corresponding small increase of d spacing. Eventhough Sm3+ has appreciable size, the percentage exchanged is very less as evident from the elemental analysis data and the shifting also is not to a measurable extent.
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.
Fig. 2 FTIR of the parent as well as exchanged systems
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
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
3.2 Catalytic tests Cumene cracking reaction gives a picture of the catalytic activity of different systems. Cracking results benzene whereas dehydrogenation leads to α -methyl styrene. A comparison of the relative amounts of these two products over the different systems provide some information about the relative effectiveness of the cracking and dehydrogenation sites i.e. Brønsted and Lewis acid sites on the catalysts respectively [9-11]. The formation of other products, like ethyl benzene, toluene and styrene is minimal. Dealkylation and side chain cracking results in the formation of ethyl benzene and toluene while dehydrogenation of ethyl benzene gives styrene [12]. The percentage conversion and selectivity to the products are greatly influenced by the contact time and reaction temperature. Inorder to study the effect of contact time cumene cracking reactions are performed under different weight hourly space velocities (WHSV). The reactions are done at different temperatures also. The catalyst SmI is selected as the representative one to find out the optimized reaction conditions.
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.
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,
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- AES analysis. References: [1] A. Nasreen, Montmorrillonite. Synlett, 2001. 8: p. 1341-1342. [2] P. Laszlo, Preparative chemistry using supported reagents. Academic Press, San Diego, 1987: p.
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