A comparative study on basicity based on supported K-salt catalysts for isomerization of 1-methoxy-4- (2-propene-1-yl) benzene

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1 Indian Journal of Chemical Technology Vol.17, November 2010, pp A comparative study on basicity based on supported K-salt catalysts for isomerization of 1-methoxy-4- (2-propene-1-yl) benzene Baldev Singh a *, Jyoti Patial a, Parveen Sharma a, Suresh Chandra a, Sudip Maity b & N Lingaiah c a Indian Institute of Integrative Medicine (CSIR), Canal Road, Jammu, , India b Central Institute of Mining & Fuel Research(CSIR), Dhanbad , India c Indian Institute of Chemical Technology (CSIR), Hyderabad , India * drbaldevsingh@yahoo.co.in Received 21 April 2010; revised 5 October 2010 A comparative study on basicity of supported K-salt catalysts has been carried out for the preparation of trans-anethole [1-methoxy-4-(1-propen-1-yl) benzene]. It is observed that strong basic sites facilitated 99% conversion of methyl chavicol [1-methoxy-4-(2-propen-1-yl) benzene] with 89% selectivity of trans-anethole. Activation energy for isomerization of methyl chavicol was found to be in the range of kj/mol using 25% K 2 CO 3 loaded on alumina. The catalysts were characterized by CO 2 -TPD and BET surface area. Keywords: Methyl chavicol, Trans-anethole, Basicity, Isomerization Trans-anethole is a useful industrial chemical. Beside wide application in perfumery chemicals 1-3, transanethole also finds application in alcoholic beverage industries, food industry, synthesis of anisic aldehyde and synthetic intermediates for fragrance and flavor industries 4. The demand of trans-anethole in the world market increased rapidly during last few years due to its growing applications in various products. The production of trans-anethole from the natural source is not sufficient enough to fulfill the required demand. Industrial sources of trans-anethole have changed to address increasing demand. Therefore, the single step preparation of anethole (mixture of cis- & trans-isomers) from methyl chavicol via double bond isomerization has drawn a special attention from both industrial and academic perspective `The current technology uses homogeneous bases (NaOH/KOH) as catalyst 10 for the production of trans-anethole but it does have some serious drawbacks like catalyst cannot be reconverted and must be neutralized and separated from reaction products and effluent disposal problems. Additionally, the basic homogeneous process is very sensitive to the presence of water. In fact, solid base catalysts hold a distinct advantage over acid catalysts due to their ability to isomerize double bond without C C bond interruption. Solid base catalysts seem to be promising candidate for replacing a homogenous process by a heterogeneous catalytic process. In this work, loading of different sources of K + salts like K 2 CO 3 and KHCO 3 as well as KNO 3 on alumina are investigated in terms of total basicity and different basic sites which are co-related with activity for the isomerization of 1-methoxy-4-(2-propene-1-yl) benzene to trans-anethole. A detailed literature review reveals that, there has been no prior study in vapor phase in fixed bed continuous flow reactor. Experimental Procedure Materials All the chemicals like potassium carbonate, potassium bicarbonate, potassium nitrate and aluminum oxide were purchased from Qualigens Fine Chemicals, Bombay. Methyl chavicol (99%) and trans-anethole (98%) were procured from Hindustan Mint & Agro Products Pvt. Ltd. India. Catalyst preparation Incipient wetness impregnation method was used to prepare the series of potassium modified alumina catalysts. During the entire course of preparation of different catalysts, 25 and 30 g amount of potassium salts were dissolved in water and resulting solution was added drop wise under vigorous stirring on 100 g of support and temperature was maintained between C. The resulting mixture was kept overnight then evaporated to dryness. The dried mass was

2 SINGH et al.: STUDY ON BASICITY BASED ON K-SALT CATALYSTS FOR ISOMERIZATION 447 initially dried at 110 C for 8 h and then calcined at different temperatures from 350 to 650 C prior to the reaction and stored in vacuum desiccator, because their basic sites generated in situ in tubular reactor and CO 2 contamination from atmosphere can be avoided. Following catalysts were prepared using alumina oxide as support and designated as A, B, C, D, E and F A) Alumina oxide (purchased from Qualigens Fine Chemicals, Bombay, India) B) Alumina oxide modified with (25%) K 2 CO 3 and calcined at 450 C C) Alumina oxide modified with (25%) K 2 CO 3 and calcined at 550 C C-I) Alumina oxide modified with (30%) K 2 CO 3 and calcined at 550 C D) Alumina oxide modified with (25%) K 2 CO 3 and calcined at 650 C E) Alumina oxide modified with (25%) KHCO 3 and calcined at 550 C F) Alumina oxide modified with (25%) KNO 3 and calcined at 550 C Evalution of catalytic activity and product analysis Activity of the prepared catalysts for the vapor phase reaction was evaluated in fixed bed catalytic reactor, under atmospheric pressure at wide range of temperature from C under the flow of nitrogen gas. The arrangement for testing was employed with some modifications as described elsewhere 12, here instead of top feeding bottom feeding was employed. Analytical technique GC Analysis GC analysis with respect to trans-anethole conversion was carried out on Nucon 5765 gas chromatograph equipped with FID and AIMIL chromatography data processor. The separation was achieved using a FFAP, SE-30 fused-silica capillary column (20 m 0.25 mm i.d., 0.25 µm film thickness); column temperature, 90 C (2 min) to 220 C at 4 C/min; injection temperature, 240 C; detector temperature, 260 C; mode, split; carrier gas, helium at column flow rate of 1.05 ml/min (100 kpa). Retention indices (RI) of the sample components and authentic compounds were determined on the basis of homologous n-alkane hydrocarbons (n-nonane to n-nonadecane) under the same conditions. The quantitative composition was obtained by peak area normalization, and the response factor for each component was considered to be one. CO 2 TPD and surface area CO 2 -TPD and surface area were determined by CHEMBET-3000 (TPR/TPD/TPO) instrument, containing a quartz reactor (i.d. = 4 mm) and a T.C.D. detector. Prior to CO 2 adsorption, catalyst was pretreated in He gas at 300 C for 2 h to remove any adsorbed impurities. Subsequently, the sample cooled to room temperature in helium gas flow. The adsorption of CO 2 is carried out by passing a mixture of 10% CO 2 balanced He gas over the catalyst for one hour. Finally the system was heated from 80 to 1200 C at the rate of 10 C/min and the desorbed gas is monitored with a T.C.D. detector. All the flow rates were maintained at normal temperature and pressure (NTP). Results and Discussion An important step in isomerization reactions is the interconversion of π- and σ-bonded organometallic complexes 13. Earlier workers have shown that the double bond isomerization is highly influenced by the nature of the metals and ligands 14. In the present study, conversion and selectivity data show that basicity facilitate the catalytic activity and subsequently, different basic sites resulting from different K-salts impregnation show higher probability of exposure and react with molecules of methyl chavicol yielding higher conversion and selectivity of desired product. The K + ions replace the protons of isolated hydroxy groups of alumina to form Al-O - K + groups, which are considered to be the active species of the catalyst and likely related to the maximum activity. Table 1 shows the BET surface area of three different samples. Sample A shows surface area of m 2 /g, when modified with 25% potassium carbonate (sample C) surface area is decreased to m 2 /g, on further increasing the load on sample A (sample C-I) the surface area sharply decreased to m 2 /g. It seems that higher loading amount of potassium salt hinders the dispersion of K + and leads to agglomeration of bulk carbonate and as a result surface area reduces sharply. TPD profiles of CO 2 adsorbed on alumina, as well as modified with various potassium salts derived catalysts are shown in Fig. 1, quantified in

3 448 INDIAN J CHEM. TECHNOL., NOVEMBER 2010 Table 2 and different concentration of sites are presented in Table 3. The Fig. indicates that different basicity and basic sites are present on the surface. To facilitate the discussion, the basic sites have been divided into weak, medium and strong sites. The total basicity refers to the total amount of CO 2 desorbed. It is revealed from CO 2 TPD data that basicity changed with nature of potassium salt and calcination temperature. Table 2 shows that unmodified sample A exhibit different CO 2 desorption peaks with total basicity being 6.8 µmolg -1 but there is no major variations in basic sites. After modification of sample A with potassium carbonate and calcination at 450, 550 and 650 C (designated as sample B, C, and D), noticeable changes are observed in basic sites as well as in total basicity (Table 2). Sample B possesses total basicity of 17.0 µmolg -1 comprising concentration of strong basicity predominate i.e. 7.2 µmolg -1. When sample C calcined at 550 C, total basicity increased to 25.7 µmolg -1 and amount of medium and strong basicity is 2.8 and 22.6 µmolg -1 respectively. Whereas, after loading 30% K 2 CO 3 on alumina (sample C-I) total basicity decreased to 20.9 µmolg -1, this might be due to residual K 2 CO 3 on the surface support. However, sample D exhibits maximum total basicity of 26.7 µmolg -1, due to calcinations at higher temperature i.e. at 650 C. Samples E and F (modified with KHCO 3 and KNO 3 respectively), showed maximum total basicity of 19.0 µmolg -1 and 32.3 µmolg -1. Besides total basicity measurement, TPD curves show the distribution of strong, medium and weak basic sites. Sample A indicates (Table 3) greater number of strong basic sites up to 37.0%, with weak and moderate basic sites as 30.5% and 32.5% respectively. In sample B change in the distribution of strong, medium and weak basic sites is observed, with increase of total basicity up to 17.0 µmolg -1, under marked variation in concentration of basic sites Fig. 1 CO 2 -TPD of samples A-F Table 1 BET surface area of catalysts S. no. Catalyst BET surface area (m 2 g -1 ) 1 A C C-I Table 2 Distribution of basicity and conversion/selectivity S. no. Catalyst Basicity (µmol g -1 ) Conversion Selectivity Weak Moderate Strong Total trans cis 1 A B C C-I D E F

4 SINGH et al.: STUDY ON BASICITY BASED ON K-SALT CATALYSTS FOR ISOMERIZATION , 40.8 and 42.1% respectively. Samples C and D showed noticeable results with respect to total basicity but major concentrations are of strong basic sites with 85.2 and 57.2% respectively. The sample E possesses the total basicity of 19.0 µmolg -1, comprising 46.3% of medium and 53.0% strong basic sites and concentration of weak basic sites are very less compared to strong and medium basic sites. Interesting changes are evident in case of sample F; total strength of basicity is 32.3 µmolg -1, with greater number of strong basic sites up to 99.5% and weak basic sites around 0.5%. Thus, it may be inferred that the Al 2 O 3 modified either with K 2 CO 3, KHCO 3 or KNO 3, all have shown marked variation of total basicity and distribution in concentration of basicity. From Table 2, it is observed that sample C shows the maximum activity and yields of trans-and cis-isomers are 89 and 10% respectively. Table 3 indicates that the concentration of strong basic sites is 85.2% and amount of basicity is 22.6 µmolg -1. Noticeable results are observed when basicity is increased beyond 25.7 µmolg -1 i.e., 32.3 µmolg -1, which mainly dominated strong basicity in sample F, there is variation in the yield of trans- and cis-isomers viz 77.4 and 19.0% respectively. When basicity decreased to 17.0 µmolg -1 (sample B) comprising strong basicity 7.2 µmolg -1 the yield of trans-isomer sharply decreased to 43.0% and cis-isomer increased up to 40.0%. The results are very well correlated in term of total basicity and activity as seen from Table 2. When total basicity is slightly increased to 19.0 µmolg -1, comprising strong basicity 10.0 µmolg -1, the yield of trans isomer is increased to 63.7% and cis-isomer is decreased to 18.1% It appears that the strong basic sites, which provide reversible adsorption sites for reactant and products of catalytic reactions are very important in improving the yield. Medium basic sites also play a very important role. Table 2 showed that lower basicity gave better yield of trans-anethole. However, beyond 2.8 µmolg -1 conversion of methyl chavicol has no significant change; whereas yield of trans-anethole gradually decreases (Table 2). It also appears from Table 2 that if weak basic sites increased beyond 0.3 µmolg -1, there is decrease in the yield of trans-anethole. Thus, number and amount of the basic sites are very important to understand the catalytic phenomenon of the solid basic catalyst reaction. To synthesize an active, selective and stable catalyst for the conversion of methyl chavicol to trans-anethole, the amount and concentration of optimum weak, medium and strong basic sites is very essential. Kinetic analysis It was observed that the rate of reaction depends on the reaction temperature. At lower temperature, below the boiling point of methyl chavicol, conversion as well as selectivity of desired product was decreased. A rate equation for the first order irreversible reaction, when the concentrations are expressed in terms of conversion, in the integrated form is KP T W/F = -X 2ln (1 X) RT (1) W/F = g cat/g mole feed/h and X = mole converted/ g mole of feed. The reaction velocity constant (k) is related to the reaction temperature by Arrhenius equation, the constant of which are obtained by a plot of lnk against 1/T as shown in (Fig. 2). Activation energies for the isomerization of methyl chavicol under experimental conditions were found between Fig. 2 Arrhenius plot for the isomerization of methyl chavicol to trans-anethole Table 3 Distribution of base concentration (%) S. no Catalyst Basicity Weak Moderate Strong 1 A B C C-I D E F

5 450 INDIAN J CHEM. TECHNOL., NOVEMBER to kj/mol. Whereas, it was observed that 25% K 2 CO 3 loaded on alumina showed the higher conversion of methyl chavicol and for reasonably excellent selectivity of transanethole, activation energy for isomerization of methyl chavicol was found to be in the range of kj/mol. Conclusion The nature, number and strength and distribution of basic sites are very important for understanding the catalytic phenomenon for the conversion of methyl chavicol to trans-anethole reaction. Acknowledgements Authors wish to express their sincere gratitude to the Director IIIM, Jammu for his keen interest and permission to publish the paper. References 1 Gandilhon P, US Pat (1979). 2 Bauer K, Garbe D & Surberg H, Ullmann Encyclopedia of Industrial Chemistry, 6 th edn, (2002) 3 International Flavors & Fragrances Inc., (New York) Sharma S K, Srivastava V K & Jasra R V, J Molecular Catal A:Chem, 245 (2006) Wagner A P, Manuf Chem, 23 (1952) Reddy M R & Periasamy M, J Organomet Chem, 491 (1995) Martan M & Reichenbacher P H, US Pat (1977). 8 Lee H S & Lee G Y, Bull Korean Chem Soc, 26 (2005) Wu W & Verkade J G, Arkivoc, 9 (2004) Srivastava V K, Bajaj H C & Jasra R V, Catal Commun, 4 (2003) Bibb C H & Fla P, US Pat (1936). 12 Singh B, Roy S K, Sharma K P & Goswami T K, J Chem Technol Biotechnol, 71 (1988) Chaloner P A, Handbook of Coordination Catalysis in Organic Chemistry. (Butterworth, London), Sharma S K, Srivastava V K, Pandya P H & Jasra R V, Catal Commun, 6 (2005) 205.

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