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Montmorillonite for selective hydroxyalkylation of p-cresol
Applied Clay Science 43 (2009) 113–117
Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Montmorillonite for selective hydroxyalkylation of p-cresol A.C. Garade, V.R. Mate, C.V. Rode ⁎ Chemical Engineering and Process Development Division, National Chemical Laboratory, Pune 411008, India
a r t i c l e
i n f o
Article history: Received 19 May 2008 Received in revised form 19 July 2008 Accepted 22 July 2008 Available online 26 July 2008 Keywords: Hydroxyalkylation Solid acids Ammonia TPD Montmorillonite Dihydroxydiarylmethane
a b s t r a c t Performances of montmorillonite titanium silicate (TS-1) and dodecatungstophosphoric acid (DTP) were compared for the hydroxyalkylation of p-cresol into dihydroxydiarylmethane (DAM). Ammonia TPD studies of various catalysts showed that an appropriate combination of both strong and weak acid sites of montmorillonite was mainly responsible rather than only the stronger acidity of bulk DTP for its highest catalytic activity for selective hydroxyalkylation of p-cresol to DAM. The selectivity to DAM could be enhanced by adjusting reaction conditions like mole ratio of p-cresol to formaldehyde, reaction temperature, catalyst concentration, solvent and reaction time. © 2008 Elsevier B.V. All rights reserved.
1. Introduction With increasing concern of the environmental problems in fine chemical industry, attempts are being made by the researchers to replace the conventional acid reagents by solid acid catalysts in various organic transformations (Corma, 1995; Singh et al., 2007). One such important reaction is the condensation of aldehyde or ketone with aromatic compounds commonly known as hydroxyalkylation for the synthesis of dihydroxydiarylmethane products, widely used in the chemical industry (Barthel et al., 2000; Angelis et al., 2004). Industrially hydroxyalkylation processes are carried out using strong mineral acids such as HCl, H2SO4 and H3PO4 (Okihama, 1996, 1997). Although, these reagents give acceptable yields of the desired product, these processes have some major drawbacks like difficulties in the separation and recovery of pure product from the reaction medium, handling of the reagents, problems due to corrosive nature of reagents and formation of inorganic wastes due to use of reagents in the stoichiometric quantities (Okihama, 1996). In order to overcome these drawbacks several solid catalysts such as zeolites, heteropolyacids, clay minerals, ion exchange resins and amorphous aluminosilicates have been proposed in the literature (Yadav and Kirthivasan, 1997; Alvaro et al., 1998, 2003; Jana et al., 2004, 2005; Udayakumar et al., 2006). Although, zeolites and sulphonic resins have been preferred as commercial solid acid catalysts, clay minerals and heteropolyacids are also an interesting class of solid acid catalysts having both Lewis and Brønsted acidity and therefore can be further explored for hydroxyalkylation reaction (Farhad, 1988; Perego et al., 2000; Bhure et al., 2007, 2008). In this paper, we report a com-
⁎ Corresponding author. Tel.: +91 2590 2349; fax: +91 20 2590 2620. E-mail address: [email protected] (C.V. Rode). 0169-1317/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2008.07.015
parative study of activity and selectivity of montmorillonite clay, titanium silicate (TS-1) and bulk dodecatungstophosphoric acid (DTP) as solid acid catalysts for the hydroxyalkylation reaction of p-cresol to 2, 2'-methylenebis (4-methylphenol), DAM (Scheme 1) which is used as an antioxidant in plastic and rubber industries (Huglin et al., 1972). Among these solid acids, montmorillonite clay was found to be the most active catalyst for selective hydroxyalkylation of p-cresol to DAM. Ammonia TPD studies of various catalysts showed that an appropriate combination of both strong and weak acid sites of montmorillonite was mainly responsible rather than only the stronger acidity of bulk DTP for its highest catalytic activity for selective hydroxyalkylation of p-cresol to DAM. Effects of various reaction parameters like mole ratio of p-cresol to formaldehyde, reaction temperature, catalyst concentration, solvent and reaction time on p-cresol conversion and DAM selectivity have also been investigated. 2. Experimental 2.1. Materials Montmorillonite was purchased from Sigma-Aldrich, Bangalore, India. TS-1 was prepared in NCL, Pune, India. p-Cresol, formaldehyde, toluene, isopropyl alcohol and DTP were purchased from Loba chemie, Mumbai, India. n-Decane was purchased from Sd fine chemicals, India and all solvents were used without further purification. 2.2. Physico-chemical characterization BET surface area measurements were carried out by N2 adsorption at 77 K (Quantachrome CHEMBET 3000). Ammonia temperature programmed desorption (TPD) was also done on Quantachrome CHEMBET
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A.C. Garade et al. / Applied Clay Science 43 (2009) 113–117
Scheme 1. Hydroxyalkylation of p-cresol to DAM.
3000 by: (i) pre-treating the samples from room temperature to 200 °C in a flow of nitrogen (ii) adsorption of ammonia at room temperature (iii) desorption of adsorbed ammonia with a heating rate 10 °C min− 1 starting from the adsorption temperature to 725 °C.
(curve c in Fig. 1). Among all these catalysts, montmorillonite sample has the highest total acid concentration.
2.3. Catalytic activity tests
Comparison of activities of montmorillonite clay with those of TS-1 and DTP for hydroxyalkylation of p-cresol are presented in Fig. 2. Montmorillonite clay and DTP showed almost similar conversion (32%) of p-cresol, while TS-1 showed the lowest activity (b4% conversion of p-cresol) inspite of its very high surface area (410 m2/g). Selectivity to DAM was maximum (N90%) for montmorillonite while it was much lower for both DTP and TS-1 (73% and 68% respectively). This indicates the nature and strength of acid sites play a dominant role in hydroxyalkylation of p-cresol with formaldehyde. The lowest activity (b4% conversion) of TS-1 could be attributed to its lowest acid strength as well as the presence of only Lewis acidic sites in the low temperature region at 100–200 °C (Mariscal et al., 2000; Capel-Sanchez et al., 2003). Montmorillonite showed marginally higher activity than that for DTP although its total acid strength was 1.75 times more than that of DTP, indicating that beyond a certain value, total acid strength does not affect the activity of the catalyst. However, selectivity to the desired DAM was maximum (N90%) in case of montmorillonite which can be due to the fact that density of low temperature acid sites was much higher (by about 50%) than that of high temperature acid sites. In case of DTP, both the acid sites were almost equal in concentration but the selectivity to DAM was lower (73%) while for TS-1, only low temperature acid sites were present which resulted in lowest selectivity to DAM. The decrease in selectivity of DAM for TS-1 was mainly because of predominant formation of carbinol. This clearly indicates that the presence of both types of acid sites as well as the total acid strength is crucial in determining the activity and selectivity performance for hydroxylation of p-cresol. Since, montmorillonite clay was found to be the best catalyst because of its mixed acidic sites (Stackhouse et al., 2001) for hydroxyalkylation of p-cresol, further study on the effect of
The hydroxyalkylation of p-cresol with formaldehyde was carried out in a magnetically stirred glass reactor (50 ml) fitted with a reflux condenser and an arrangement for temperature controller. In a typical hydroxyalkylation experiment, p-cresol (1 g), formaldehyde (0.15 g), toluene (2.5 ml) were taken and catalyst (0.15 g) without any pretreatment was added to the reaction mixture. The reaction mixture was then heated to a desired temperature and the reaction was continued for 2 h. p-cresol conversion and product selectivity were determined by HP6890 series GC System (Hewlett Packard) coupled with FID detector and capillary column (HP-1 capillary column, 30 m length X 0.32 mm i.d.). The products were identified by 1H NMR, 13CMR and by GC-MS. Recycling experiments were carried out at 353 K using p-cresol/ formaldehyde mole ratio 3:1 for 2 h with catalyst concentration (montmorillonite) 0.042 g/cm3 as follows. After the first hydroxyalkylation run, the used catalyst was filtered and washed with methanol followed by several times with deionized water. Then the catalyst was dried in an oven at 393 K for 8 h and reused for the subsequent run. The procedure was followed for two subsequent hydroxyalkylation experiments. 3. Results and discussion 3.1. Catalyst characterization Table 1 presents the specific BET surface area and NH3-TPD results of montmorillonite TS-1 and DTP. The surface areas of various catalysts were in the following order: TS-1 N montmorillonite clay N DTP. NH3-TPD profiles of montmorillonite clay, TS-1 and DTP are shown in Fig. 1 and the values of ammonia adsorbed are presented in Table 1. DTP showed two desorption peaks, one for the strongly chemisorbed ammonia at 650 °C (curve b, in Fig. 1) and another at 200 °C. The amount of desorbed ammonia for these two different acid sites was 0.70 and 0.66 mmol/g indicating almost same concentration of both the acid sites in case of DTP. Montmorillonite sample also exhibited two desorption peaks, one for the strongly chemisorbed ammonia in the range of 600–650 °C (curve a in Fig. 1) and another at 200 °C. However, the amount of ammonia desorbed at low temperature was more that 1.5 times than that at higher temperature. Interestingly, TS1 showed only one peak in the low temperature region at 100–200 °C
3.2. Catalyst screening
Table 1 Textural properties of the catalysts Sr. No. Catalysts
1 2 3
SBET (m2/g)
Montmorillonite clay 59.2 TS-1 410.6 DTP 8.3
TPD of NH3 Region I (LT-Peak)
Region II (HT-Peak)
Total acid site density (mmol/g)
1.48 1.18 0.66
0.91 – 0.70
2.39 1.18 1.36
Fig. 1. Ammonia TPD profile over montmorillonite clay, DTP and TS-1.
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Fig. 2. Catalyst screening for hydroxyalkylation reaction of p-cresol. Reaction conditions: p-cresol, 9.26 mmol; catalyst concentration, 0.042 g/cm3; temperature, 353 K; time, 2 h; mole ratio of p-cresol to formaldehyde, 5:1; solvent, toluene.
Fig. 4. Conversion and selectivity vs time. Reaction conditions: p-cresol, 9.26 mmol; catalyst concn, 0.042 g/cm3; temperature, 353 K; time, 2 h; mole ratio of p-cresol to formaldehyde, 3:1; solvent, toluene.
reaction parameters on activity and selectivity was carried using the same clay catalyst.
hydroxyalkylation of phenol with acetone for the bisphenol A synthesis (Nowinska and Kaleta, 2000).
3.3. Effect of mole ratio p-cresol/formaldehyde
3.4. Effect of reaction time
Mole ratio p-cresol/formaldehyde plays a significant role in hydroxyalkylation of p-cresol (Fig. 3). For a mole ratio b2, p-cresol conversion was almost negligible while, highest conversion of pcresol achieved was 53% as the mole ratio of p-cresol to formaldehyde increased from 2 to 3. However at this mole ratio, selectivity to DAM obtained was 77% while the other product formed was a trimer. The conversion of p-cresol decreased from 53 to 32% and selectivity to DAM increased from 77 to 90% when the mole ratio of p-cresol to formaldehyde increased from 3 to 5. This decreased conversion of p-cresol was obvious, since p-cresol was used in excessive amount as compared to formaldehyde and the reaction was almost ceased when all formaldehyde was consumed in hydroxyalkylation reaction, thus preventing further p-cresol conversion. The lower concentration of formaldehyde also retards the further reaction of a DAM with pcresol, hence selectivity to DAM is maximum for higher molar ratio of p-cresol to formaldehyde. Similar results were obtained for the
The effect of reaction time on the p-cresol conversion and DAM selectivity was studied for p-cresol to formaldehyde mole ratio of 3:1 (Fig. 4). p-cresol conversion increased from 12 to 53% with increasing reaction time from 0.5 to 2 h. The selectivity to DAM decreased from 90 to 77% and that of trimer increased from 10 to 23% with increasing reaction time from 0.5 to 2 h. This decreased selectivity of DAM was mainly because of the reaction of DAM formed initially with formaldehyde followed by with p-cresol.
Fig. 3. Effect of mole ratio p-cresol/formaldehyde on conversion and selectivity. Reaction conditions: p-cresol, 9.26 mmol; catalyst concn, 0.042 g/cm3; temperature, 353 K; time, 2 h; mole ratio of p-cresol to formaldehyde, 2:1-5:1; solvent, toluene.
Fig. 5. Effect of temperature on conversion and selectivity. Reaction conditions: p-cresol, 9.26 mmol; catalyst concn, 0.042 g/cm3; temperature, 313–373 K; time, 2 h; mole ratio of p-cresol to formaldehyde, 3:1; solvent, toluene.
3.5. Effect of temperature The effect of temperature on both conversion of p-cresol and selectivity to DAM was studied in a temperature range 313–373 K (Fig. 5). The conversion of p-cresol was b5% at 313 K and at 333 K which increased to 53% with increasing temperature up to 353 K and remained almost constant up to 373 K. The selectivity to DAM also increased from
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Fig. 6. Effect of catalyst concentration on conversion and selectivity. Reaction conditions: p-cresol, 9.26 mmol; catalyst concn, 0.014–0.042 g/cm3; temperature, 353 K; time, 2 h; mole ratio of p-cresol to formaldehyde, 3:1; solvent, toluene.
53 to 77% with increasing temperature from 333 to 353 K, then remained constant up to 373 K. There was considerable amount (36%) of carbinol (2-(hydroxymethyl)-4-methyl phenol) formed at 333 K, which was then converted into DAM when temperature was raised to 353 K resulting in enhanced selectivity to DAM. 3.6. Effect of catalyst concentration The effect of catalyst concentration on the conversion of p-cresol and product distribution was studied in the range of 0.014–0.042 g/cm3 at 353 K and at 3:1 mole ratio. The conversion of p-cresol increased with increasing in catalyst concentration (Fig. 6). The maximum conversion (53%) was observed with catalyst concentration of 0.042 g/cm3. At the lowest catalyst concentration of 0.014 g/cm3, selectivity to DAM achieved was 71% which marginally increased to 77%, when the catalyst concentration was increased to 0.042 g/cm3. The selectivity to the trimer [2, 2'-(2-hydroxy-5-methyl-1, 3-phenylene)bis(methylene) bis(4-methylphenol)] increased from 12 to 23% with increasing catalyst concentration from 0.014 to 0.042 g/cm3. The predominant formation (17%) of carbinol (2-(hydroxymethyl)-4-methyl phenol) was observed at lower catalyst concentration (0.014 g/cm3) which then decreased
Fig. 8. Catalyst recycling study. Reaction conditions: p-cresol, 9.26 mmol; catalyst concn, 0.042 g/cm3; temperature, 353 K; time, 2 h; mole ratio of p-cresol to formaldehyde, 3:1; solvent, toluene.
steeply due to its further reaction with p-cresol to form DAM, with increasing catalyst concentration up to 0.042 g/cm3. 3.7. Effect of solvents Fig. 7 shows the role of three solvents having different polarity viz decane, toluene and isopropyl alcohol. The p-cresol conversion was b5% for highly polar solvent such as isopropyl alcohol. This could be explained based on the competition between isopropyl alcohol and formaldehyde for active Brønsted acid sites of montmorillonite. Since nucleophilicity of the oxygen atom of isopropyl alcohol is higher than that of formaldehyde, causing better protonation of isopropyl alcohol than formaldehyde, resulted in lower conversion of p-cresol. With less polar solvents like n-decane and toluene, the conversion of p-cresol was much higher than that for polar solvent. Toluene showed the highest conversion of p-cresol (53%) and selectivity (77%) to DAM. 3.8. Catalyst recycling studies Recycling experiments were also carried out at 353 K. The catalyst was found to retain its activity even after the second recycle without affecting the conversion of p-cresol (53%) (Fig. 8). The selectivity to DAM (77%) was also almost the same for three experiments. 4. Conclusion
Fig. 7. Role of solvent in hydroxyalkylation of p-cresol. Reaction conditions: p-cresol, 9.26 mmol; catalyst concn, 0.042 g/cm3; temperature, 353 K; time, 2 h; mole ratio of p -cresol to formaldehyde, 3:1.
Among various solid acid catalysts studied for hydroxyalkylation of p-cresol, montmorillonite clay was found to be an effective catalyst. Inspite of the highest surface area TS-1 showed very poor activity (conversion b 4%) which could be attributed to its lowest acid strength as well as the presence of only Lewis acidic sites at 100–200 °C. Both montmorillonite and DTP showed almost comparable activity. Selectivity to the desired dimer (DAM) was maximum (N90%) in case of montmorillonite which can be due to the fact that density of low temperature acidic sites was much higher (by about 50%) than that of high temperature acidic sites. The conversion of p-cresol as well as the selectivity to DAM increased from 40 to 53% and 76 to 77% respectively with increasing mole ratio p-cresol/formaldehyde from 2 to 3 while, the conversion of p-cresol decreased from 53 to 32% and the selectivity to DAM increased from 77 to 90% when the mole ratio increased from 3 to 5. The predominant formation of another product, 2-(hydroxymethyl)-4-methyl phenol was observed at lower catalyst concentration (0.014 g/cm3), then decreased steeply due to its further reaction
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with p-cresol to form DAM, with increasing catalyst concentration up to 0.042 g/cm3. Acknowledgement One of the authors, ACG thanks, University Grant Commission (UGC) New Delhi, for the award of senior research fellowship to him. References Alvaro, M., Garcia, H., Sanjuan, A., Espla, M., 1998. Hydroxyalkylation of benzene derivatives by benzaldehyde in the presence of acid zeolites. Appl. Catal. A Gen. 175, 105–112. Alvaro, M., Das, D., Cano, M., Garcia, H., 2003. Friedel-craft hydroxyalkylation: reaction of anisole with paraformaldehyde catalyzed by zeolites in supercritical CO2. J. Catal. 219, 464–468. Angelis, A., Ingallina, P., Perego, C., 2004. Solid acid catalysts for industrial condensations of ketones and aldehydes with aromatics. Ind. Eng. Chem. Res. 43, 1169–1178. Barthel, N., Finniels, A., Moreau, C., Jacuot, R., Spagnol, M., 2000. Hydroxyalkylation of aromatic compounds over protonic zeolites. Top. Catal. 13, 269–274. Bhure, M.H., Rode, C.V., Chikate, R.C., Patwardhan, N., Patil, S., 2007. Phosphotungstic acid as an efficient solid catalyst for intramolecular rearrangement of benzyl phenyl ether to 2-benzyl phenol. Cat. Commun. 8, 139–144. Bhure, M.H., Kumar, I., Natu, A.D., Chikate, R.C., Rode, C.V., 2008. Phosphotungstic acid on silica with modified acid sites as a solid catalyst for selective cleavage of tertbutyldimethylsilyl ethers. Cat. Commun. 9, 1863–1868. Capel-Sanchez, M.C., Campos-Martin, J.M., Fierro, J.L.G., 2003. Impregnation treatments of TS-1 catalysts and their relevance in alkene epoxidation with hydrogen peroxide. Appl. Catal. A Gen. 246, 69–77. Corma, A., 1995. Inorganic solid acids and their use in acid catalyzed hydrocarbon reaction. Chem. Rev. 95, 559–614.
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Farhad, N., 1988. Bisphenol A and Alkylated Phenols, PEP Report No. 81. SRI International, Menlo Park, CA. Huglin, M., Knight, G., Wright, W., 1972. Studies on homologous series of oligomeric antioxidants derived from p-cresol/formaldehyde condensates. I. Synthesis and characterisation. Makromol. Chem. 152, 67–82. Jana, S., Kugita, T., Namba, S., 2004. Aluminium-grafted MCM-41 molecular sieve: an active catalyst for bisphenol F synthesis process. Appl. Catal. A Gen. 266, 245–250. Jana, S., Okamoto, T., Kugita, T., Namba, S., 2005. Selective synthesis of bisphenol F catalyzed by microporous H-beta zeolites. Appl. Catal. A Gen. 288, 80–85. Mariscal, R., Lopez-Granados, M., Fierro, J.L.G., Sotelo, J.L., Martos, C., Van Grieken, R., 2000. Morphology and surface properties of titania-silica hydrophobic xerogels. Langmuir 16, 9460–9467. Nowinska, K., Kaleta, W., 2000. Synthesis of Bisphenol-A over heteropoly compounds encapsulated into mesoporous molecular sieves. Appl. Catal. A Gen 203, 91–100. Okihama, M., Kunitake, J., 1996. Production of bisphenol F. Jpn. Kokai JP 08 268 943. Okihama, M., Kunitake, J., 1997. Production of bisphenol F. Jpn. Kokai JP 09 067 287. Perego, C., Angelis, A., Farias, O., Bosetti, A., 2000. Process for the production of diaminodiphenylmethane and its higher homologues. U. S. Patent 6 380 433. Singh, B., Patial, J., Sharma, P., Agarwal, S.G., Qazi, G.N., Maity, S., 2007. Influence of acidity of montmorillonite and modified montmorillonite clay minerals for the conversion of longifolene to isolongifolene. J. Mol. Catal. A Chem 266, 215–220. Stackhouse, S., Coveney, P.V., Sandre, E., 2001. Plane-wave density functional theoretic study of formation of clay-polymer nanocomosite materials by self-catalyzed in situ intercalative polymerization. J. Am. Chem. Soc. 123, 11764–11774. Udayakumar, S., Ajaikumar, S., Pandurangan, A., 2006. Electrophilic substitution reaction of phenols with aldehydes: Enhance the yield of bisphenols by HPA and supported HPA. Appl. Catal. A Gen. 302, 86–95. Yadav, G.D., Kirthivasan, N., 1997. Synthesis of bisphenol-A: comparison of efficacy of ion exchange resin catalysts vis-à-vis heteropolyacid supported on clay and kinetic modeling. Appl. Catal. A Gen. 154, 29–53.
Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Montmorillonite for selective hydroxyalkylation of p-cresol A.C. Garade, V.R. Mate, C.V. Rode ⁎ Chemical Engineering and Process Development Division, National Chemical Laboratory, Pune 411008, India
a r t i c l e
i n f o
Article history: Received 19 May 2008 Received in revised form 19 July 2008 Accepted 22 July 2008 Available online 26 July 2008 Keywords: Hydroxyalkylation Solid acids Ammonia TPD Montmorillonite Dihydroxydiarylmethane
a b s t r a c t Performances of montmorillonite titanium silicate (TS-1) and dodecatungstophosphoric acid (DTP) were compared for the hydroxyalkylation of p-cresol into dihydroxydiarylmethane (DAM). Ammonia TPD studies of various catalysts showed that an appropriate combination of both strong and weak acid sites of montmorillonite was mainly responsible rather than only the stronger acidity of bulk DTP for its highest catalytic activity for selective hydroxyalkylation of p-cresol to DAM. The selectivity to DAM could be enhanced by adjusting reaction conditions like mole ratio of p-cresol to formaldehyde, reaction temperature, catalyst concentration, solvent and reaction time. © 2008 Elsevier B.V. All rights reserved.
1. Introduction With increasing concern of the environmental problems in fine chemical industry, attempts are being made by the researchers to replace the conventional acid reagents by solid acid catalysts in various organic transformations (Corma, 1995; Singh et al., 2007). One such important reaction is the condensation of aldehyde or ketone with aromatic compounds commonly known as hydroxyalkylation for the synthesis of dihydroxydiarylmethane products, widely used in the chemical industry (Barthel et al., 2000; Angelis et al., 2004). Industrially hydroxyalkylation processes are carried out using strong mineral acids such as HCl, H2SO4 and H3PO4 (Okihama, 1996, 1997). Although, these reagents give acceptable yields of the desired product, these processes have some major drawbacks like difficulties in the separation and recovery of pure product from the reaction medium, handling of the reagents, problems due to corrosive nature of reagents and formation of inorganic wastes due to use of reagents in the stoichiometric quantities (Okihama, 1996). In order to overcome these drawbacks several solid catalysts such as zeolites, heteropolyacids, clay minerals, ion exchange resins and amorphous aluminosilicates have been proposed in the literature (Yadav and Kirthivasan, 1997; Alvaro et al., 1998, 2003; Jana et al., 2004, 2005; Udayakumar et al., 2006). Although, zeolites and sulphonic resins have been preferred as commercial solid acid catalysts, clay minerals and heteropolyacids are also an interesting class of solid acid catalysts having both Lewis and Brønsted acidity and therefore can be further explored for hydroxyalkylation reaction (Farhad, 1988; Perego et al., 2000; Bhure et al., 2007, 2008). In this paper, we report a com-
⁎ Corresponding author. Tel.: +91 2590 2349; fax: +91 20 2590 2620. E-mail address: [email protected] (C.V. Rode). 0169-1317/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2008.07.015
parative study of activity and selectivity of montmorillonite clay, titanium silicate (TS-1) and bulk dodecatungstophosphoric acid (DTP) as solid acid catalysts for the hydroxyalkylation reaction of p-cresol to 2, 2'-methylenebis (4-methylphenol), DAM (Scheme 1) which is used as an antioxidant in plastic and rubber industries (Huglin et al., 1972). Among these solid acids, montmorillonite clay was found to be the most active catalyst for selective hydroxyalkylation of p-cresol to DAM. Ammonia TPD studies of various catalysts showed that an appropriate combination of both strong and weak acid sites of montmorillonite was mainly responsible rather than only the stronger acidity of bulk DTP for its highest catalytic activity for selective hydroxyalkylation of p-cresol to DAM. Effects of various reaction parameters like mole ratio of p-cresol to formaldehyde, reaction temperature, catalyst concentration, solvent and reaction time on p-cresol conversion and DAM selectivity have also been investigated. 2. Experimental 2.1. Materials Montmorillonite was purchased from Sigma-Aldrich, Bangalore, India. TS-1 was prepared in NCL, Pune, India. p-Cresol, formaldehyde, toluene, isopropyl alcohol and DTP were purchased from Loba chemie, Mumbai, India. n-Decane was purchased from Sd fine chemicals, India and all solvents were used without further purification. 2.2. Physico-chemical characterization BET surface area measurements were carried out by N2 adsorption at 77 K (Quantachrome CHEMBET 3000). Ammonia temperature programmed desorption (TPD) was also done on Quantachrome CHEMBET
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Scheme 1. Hydroxyalkylation of p-cresol to DAM.
3000 by: (i) pre-treating the samples from room temperature to 200 °C in a flow of nitrogen (ii) adsorption of ammonia at room temperature (iii) desorption of adsorbed ammonia with a heating rate 10 °C min− 1 starting from the adsorption temperature to 725 °C.
(curve c in Fig. 1). Among all these catalysts, montmorillonite sample has the highest total acid concentration.
2.3. Catalytic activity tests
Comparison of activities of montmorillonite clay with those of TS-1 and DTP for hydroxyalkylation of p-cresol are presented in Fig. 2. Montmorillonite clay and DTP showed almost similar conversion (32%) of p-cresol, while TS-1 showed the lowest activity (b4% conversion of p-cresol) inspite of its very high surface area (410 m2/g). Selectivity to DAM was maximum (N90%) for montmorillonite while it was much lower for both DTP and TS-1 (73% and 68% respectively). This indicates the nature and strength of acid sites play a dominant role in hydroxyalkylation of p-cresol with formaldehyde. The lowest activity (b4% conversion) of TS-1 could be attributed to its lowest acid strength as well as the presence of only Lewis acidic sites in the low temperature region at 100–200 °C (Mariscal et al., 2000; Capel-Sanchez et al., 2003). Montmorillonite showed marginally higher activity than that for DTP although its total acid strength was 1.75 times more than that of DTP, indicating that beyond a certain value, total acid strength does not affect the activity of the catalyst. However, selectivity to the desired DAM was maximum (N90%) in case of montmorillonite which can be due to the fact that density of low temperature acid sites was much higher (by about 50%) than that of high temperature acid sites. In case of DTP, both the acid sites were almost equal in concentration but the selectivity to DAM was lower (73%) while for TS-1, only low temperature acid sites were present which resulted in lowest selectivity to DAM. The decrease in selectivity of DAM for TS-1 was mainly because of predominant formation of carbinol. This clearly indicates that the presence of both types of acid sites as well as the total acid strength is crucial in determining the activity and selectivity performance for hydroxylation of p-cresol. Since, montmorillonite clay was found to be the best catalyst because of its mixed acidic sites (Stackhouse et al., 2001) for hydroxyalkylation of p-cresol, further study on the effect of
The hydroxyalkylation of p-cresol with formaldehyde was carried out in a magnetically stirred glass reactor (50 ml) fitted with a reflux condenser and an arrangement for temperature controller. In a typical hydroxyalkylation experiment, p-cresol (1 g), formaldehyde (0.15 g), toluene (2.5 ml) were taken and catalyst (0.15 g) without any pretreatment was added to the reaction mixture. The reaction mixture was then heated to a desired temperature and the reaction was continued for 2 h. p-cresol conversion and product selectivity were determined by HP6890 series GC System (Hewlett Packard) coupled with FID detector and capillary column (HP-1 capillary column, 30 m length X 0.32 mm i.d.). The products were identified by 1H NMR, 13CMR and by GC-MS. Recycling experiments were carried out at 353 K using p-cresol/ formaldehyde mole ratio 3:1 for 2 h with catalyst concentration (montmorillonite) 0.042 g/cm3 as follows. After the first hydroxyalkylation run, the used catalyst was filtered and washed with methanol followed by several times with deionized water. Then the catalyst was dried in an oven at 393 K for 8 h and reused for the subsequent run. The procedure was followed for two subsequent hydroxyalkylation experiments. 3. Results and discussion 3.1. Catalyst characterization Table 1 presents the specific BET surface area and NH3-TPD results of montmorillonite TS-1 and DTP. The surface areas of various catalysts were in the following order: TS-1 N montmorillonite clay N DTP. NH3-TPD profiles of montmorillonite clay, TS-1 and DTP are shown in Fig. 1 and the values of ammonia adsorbed are presented in Table 1. DTP showed two desorption peaks, one for the strongly chemisorbed ammonia at 650 °C (curve b, in Fig. 1) and another at 200 °C. The amount of desorbed ammonia for these two different acid sites was 0.70 and 0.66 mmol/g indicating almost same concentration of both the acid sites in case of DTP. Montmorillonite sample also exhibited two desorption peaks, one for the strongly chemisorbed ammonia in the range of 600–650 °C (curve a in Fig. 1) and another at 200 °C. However, the amount of ammonia desorbed at low temperature was more that 1.5 times than that at higher temperature. Interestingly, TS1 showed only one peak in the low temperature region at 100–200 °C
3.2. Catalyst screening
Table 1 Textural properties of the catalysts Sr. No. Catalysts
1 2 3
SBET (m2/g)
Montmorillonite clay 59.2 TS-1 410.6 DTP 8.3
TPD of NH3 Region I (LT-Peak)
Region II (HT-Peak)
Total acid site density (mmol/g)
1.48 1.18 0.66
0.91 – 0.70
2.39 1.18 1.36
Fig. 1. Ammonia TPD profile over montmorillonite clay, DTP and TS-1.
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Fig. 2. Catalyst screening for hydroxyalkylation reaction of p-cresol. Reaction conditions: p-cresol, 9.26 mmol; catalyst concentration, 0.042 g/cm3; temperature, 353 K; time, 2 h; mole ratio of p-cresol to formaldehyde, 5:1; solvent, toluene.
Fig. 4. Conversion and selectivity vs time. Reaction conditions: p-cresol, 9.26 mmol; catalyst concn, 0.042 g/cm3; temperature, 353 K; time, 2 h; mole ratio of p-cresol to formaldehyde, 3:1; solvent, toluene.
reaction parameters on activity and selectivity was carried using the same clay catalyst.
hydroxyalkylation of phenol with acetone for the bisphenol A synthesis (Nowinska and Kaleta, 2000).
3.3. Effect of mole ratio p-cresol/formaldehyde
3.4. Effect of reaction time
Mole ratio p-cresol/formaldehyde plays a significant role in hydroxyalkylation of p-cresol (Fig. 3). For a mole ratio b2, p-cresol conversion was almost negligible while, highest conversion of pcresol achieved was 53% as the mole ratio of p-cresol to formaldehyde increased from 2 to 3. However at this mole ratio, selectivity to DAM obtained was 77% while the other product formed was a trimer. The conversion of p-cresol decreased from 53 to 32% and selectivity to DAM increased from 77 to 90% when the mole ratio of p-cresol to formaldehyde increased from 3 to 5. This decreased conversion of p-cresol was obvious, since p-cresol was used in excessive amount as compared to formaldehyde and the reaction was almost ceased when all formaldehyde was consumed in hydroxyalkylation reaction, thus preventing further p-cresol conversion. The lower concentration of formaldehyde also retards the further reaction of a DAM with pcresol, hence selectivity to DAM is maximum for higher molar ratio of p-cresol to formaldehyde. Similar results were obtained for the
The effect of reaction time on the p-cresol conversion and DAM selectivity was studied for p-cresol to formaldehyde mole ratio of 3:1 (Fig. 4). p-cresol conversion increased from 12 to 53% with increasing reaction time from 0.5 to 2 h. The selectivity to DAM decreased from 90 to 77% and that of trimer increased from 10 to 23% with increasing reaction time from 0.5 to 2 h. This decreased selectivity of DAM was mainly because of the reaction of DAM formed initially with formaldehyde followed by with p-cresol.
Fig. 3. Effect of mole ratio p-cresol/formaldehyde on conversion and selectivity. Reaction conditions: p-cresol, 9.26 mmol; catalyst concn, 0.042 g/cm3; temperature, 353 K; time, 2 h; mole ratio of p-cresol to formaldehyde, 2:1-5:1; solvent, toluene.
Fig. 5. Effect of temperature on conversion and selectivity. Reaction conditions: p-cresol, 9.26 mmol; catalyst concn, 0.042 g/cm3; temperature, 313–373 K; time, 2 h; mole ratio of p-cresol to formaldehyde, 3:1; solvent, toluene.
3.5. Effect of temperature The effect of temperature on both conversion of p-cresol and selectivity to DAM was studied in a temperature range 313–373 K (Fig. 5). The conversion of p-cresol was b5% at 313 K and at 333 K which increased to 53% with increasing temperature up to 353 K and remained almost constant up to 373 K. The selectivity to DAM also increased from
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Fig. 6. Effect of catalyst concentration on conversion and selectivity. Reaction conditions: p-cresol, 9.26 mmol; catalyst concn, 0.014–0.042 g/cm3; temperature, 353 K; time, 2 h; mole ratio of p-cresol to formaldehyde, 3:1; solvent, toluene.
53 to 77% with increasing temperature from 333 to 353 K, then remained constant up to 373 K. There was considerable amount (36%) of carbinol (2-(hydroxymethyl)-4-methyl phenol) formed at 333 K, which was then converted into DAM when temperature was raised to 353 K resulting in enhanced selectivity to DAM. 3.6. Effect of catalyst concentration The effect of catalyst concentration on the conversion of p-cresol and product distribution was studied in the range of 0.014–0.042 g/cm3 at 353 K and at 3:1 mole ratio. The conversion of p-cresol increased with increasing in catalyst concentration (Fig. 6). The maximum conversion (53%) was observed with catalyst concentration of 0.042 g/cm3. At the lowest catalyst concentration of 0.014 g/cm3, selectivity to DAM achieved was 71% which marginally increased to 77%, when the catalyst concentration was increased to 0.042 g/cm3. The selectivity to the trimer [2, 2'-(2-hydroxy-5-methyl-1, 3-phenylene)bis(methylene) bis(4-methylphenol)] increased from 12 to 23% with increasing catalyst concentration from 0.014 to 0.042 g/cm3. The predominant formation (17%) of carbinol (2-(hydroxymethyl)-4-methyl phenol) was observed at lower catalyst concentration (0.014 g/cm3) which then decreased
Fig. 8. Catalyst recycling study. Reaction conditions: p-cresol, 9.26 mmol; catalyst concn, 0.042 g/cm3; temperature, 353 K; time, 2 h; mole ratio of p-cresol to formaldehyde, 3:1; solvent, toluene.
steeply due to its further reaction with p-cresol to form DAM, with increasing catalyst concentration up to 0.042 g/cm3. 3.7. Effect of solvents Fig. 7 shows the role of three solvents having different polarity viz decane, toluene and isopropyl alcohol. The p-cresol conversion was b5% for highly polar solvent such as isopropyl alcohol. This could be explained based on the competition between isopropyl alcohol and formaldehyde for active Brønsted acid sites of montmorillonite. Since nucleophilicity of the oxygen atom of isopropyl alcohol is higher than that of formaldehyde, causing better protonation of isopropyl alcohol than formaldehyde, resulted in lower conversion of p-cresol. With less polar solvents like n-decane and toluene, the conversion of p-cresol was much higher than that for polar solvent. Toluene showed the highest conversion of p-cresol (53%) and selectivity (77%) to DAM. 3.8. Catalyst recycling studies Recycling experiments were also carried out at 353 K. The catalyst was found to retain its activity even after the second recycle without affecting the conversion of p-cresol (53%) (Fig. 8). The selectivity to DAM (77%) was also almost the same for three experiments. 4. Conclusion
Fig. 7. Role of solvent in hydroxyalkylation of p-cresol. Reaction conditions: p-cresol, 9.26 mmol; catalyst concn, 0.042 g/cm3; temperature, 353 K; time, 2 h; mole ratio of p -cresol to formaldehyde, 3:1.
Among various solid acid catalysts studied for hydroxyalkylation of p-cresol, montmorillonite clay was found to be an effective catalyst. Inspite of the highest surface area TS-1 showed very poor activity (conversion b 4%) which could be attributed to its lowest acid strength as well as the presence of only Lewis acidic sites at 100–200 °C. Both montmorillonite and DTP showed almost comparable activity. Selectivity to the desired dimer (DAM) was maximum (N90%) in case of montmorillonite which can be due to the fact that density of low temperature acidic sites was much higher (by about 50%) than that of high temperature acidic sites. The conversion of p-cresol as well as the selectivity to DAM increased from 40 to 53% and 76 to 77% respectively with increasing mole ratio p-cresol/formaldehyde from 2 to 3 while, the conversion of p-cresol decreased from 53 to 32% and the selectivity to DAM increased from 77 to 90% when the mole ratio increased from 3 to 5. The predominant formation of another product, 2-(hydroxymethyl)-4-methyl phenol was observed at lower catalyst concentration (0.014 g/cm3), then decreased steeply due to its further reaction
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