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Ind. Eng. C h e m . Res. 1987,26, 1276-1280
Alkylation of Phenol and Pyrocatechol by Isobutyl Alcohol Using Superacid Catalysts Rajeev A. Rajadhyaksha* and Dilip D. C h a u d h a r i Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India Alkylation of phenol and pyrocatechol with isobutyl alcohol is investigated using a variety of acid catalysts including superacids. Triflic acid (trifluoromethanesulfonic acid, CF3S03H), which is the strongest known acid, showed the highest activity for the reaction. The perfluorinated resin sulfonic acid, Nafion-H, exhibited comparable activity; however, existence of an induction period was observed. Induction period was shown to be related to the rate of removal of water, formed due to the reaction. In contrast to the earlier reports, the catalytic activity of the “sulfate-treated zirconia” was found to be much lower than that of Nafion-H. Equivalent loadings of 98% sulfuric acid resulted in lower activity than Nafion-H and triflic acid and its activity decreased with progress of reaction, probably due to dilution of acid by water. The polystyrenesulfonic acid type cation-exchange resin showed considerable activity for the reaction. Interestingly, negligible conversion was obtained when an equal loading of p-toluenesulfonic acid was used. Alkylation of p-cresol by isobutyl alcohol under identical conditions was however unsuccessful. Alkylation of aromatic compounds by primary alcohols requires severe conditions since it proceeds through the formation of a primary carbonium ion. The conventional acid catalysts like zinc chloride, aluminium chloride, etc., are required to be employed in excessive amounts to carry out such reactions (Schriesheim, 1964). The present work has been undertaken with an objective to investigate whether such reactions can be carried out under milder conditions by employing superacids as catalysts. For this purpose, alkylation of phenolic compounds by isobutyl alcohol has been used as a model reaction. The choice of the system is essentially on account of the industrial relevance of the products (Varagnat, 1981; Reed, 1978). Use of isobutyl alcohol as a butylating agent can be of practical significance at locations where mixed butene stream, which is the normal source of isobutylene, is not readily available. Alkylation of phenolic compounds by isobutyl alcohol has not been studied extensively. A few studies using zinc chloride, phosphoric acid, 70% sulfuric acid, aluminium chloride, and CER (cation-exchange resin) have been reported (Schriesheim, 1964; Isagulyants, 1957). Most of the studies have been carried out using large quantities of the catalysts, and the effect of parameters such as catalyst concentration and reactant concentration on the progress of reaction has not been investigated systematically. In the present work, alkylation of phenol and pyrocatechol has been studied using a variety of superacid catalysts. The reactions have also been studied on some conventional acid catalysts for comparison. The superacid catalysts employed in the study include triflic acid (trifluoromethanesulfonic acid, CF,SO,H), perfluorinated resin sulfonic acid Nafion-H, and sulfate-treated zirconia. The choice of these catalysts was on account of their chemical stability in various reaction media and possibility of their repeated reuse. All the studies were carried out using catalytic quantities of the catalysts. Triflic acid is one of the strongest acids known. Its catalytic properties for a variety of reactions have been recently reviewed (Howell and McCown, 1977). Catalytic properties of Nafion-H have also been investigated extensively (Fung et al., 1981; Chaudhari, 1983). Being a solid superacid, it has advantages of the absence of corrosion problems, ease of separation from the reaction products, and reusability. A few studies on the catalytic properties of sulfate-treated zirconia have recently appeared (Arata and Hino, 1980). This catalyst showed activity comparable to Nafion-H for esterification reaction (Arata and Hino, 1981). Cracking 0888-5885/8712626-1276$01.50/0
and skeletal isomerization of hydrocarbons could be carried out on this catalyst at temperatures as low as 298 K (Arata and Hino, 1980). These observations indicate superacidic character of this material. In addition to the advantages offered by the resin catalysts, it can be cheaper and has better thermal stability. The reactions were also carried out using sulfuric acid, p-toluenesulfonic acid (PTSA), and cation-exchange resin (CER) catalyst for comparison. Experimental Section Materials. Phenol, pyrocatechol, isobutyl alcohol, zirconyl chloride, zirconium hydroxide of AR grade, and mixed xylenes of LR grade were used in the present study. The 4A molecular sieves were obtained from a firm of repute. Catalysts, The superacid catalysts Nafion-H (0.9 mequiv g-’ dry resin) (Hasegawa and Higashimura, 1980) and triflic acid were obtained from Du Pont and Fluka, respectively. The recently reported solid superacid, sulfate-treated zirconia, was prepared by treating zirconium hydroxide with 1 N sulfuric acid and subsequent drying and calcination at 650 “C for 3 h. Zr02-I was prepared by using zirconium hydroxide, obtained from hydrolysis of zirconyl chloride, while for the preparation of Zr02-II, zirconium hydroxide of AR grade was used. The details of the preparation procedure are reported elsewhere (Arata and Hino, 1980). Sulfuric acid (98%) and p-toluenesulfonic acid of AR grade were used. The CER (polystyrenesulfonic acid resin, Indion-CXC-125,4.8mequiv g-l dry resin) was procured from Ion-Exchange, India. Apparatus, Procedure, and Analysis. A schematic diagram of the experimental setup is given in Figure 1. The reactor consisted of a flat-bottom glass vessel of 6.0-cm i.d. and 325-mL volume. The reactor was equipped with glass baffles, Dean and Stark apparatus, and was stirred by a glass turbine stirrer at 800 rpm. The reactor was kept in a constant temperature bath. In a typical run, a mixture containing a required amount of phenol and isobutyl alcohol dissolved in xylene (as an entrainer) was brought to refluxing conditions (143-146 OC) under agitation, and the desired quantity of catalyst (which was dried at 110 “C for 1 h just before use) was added. Samples (0.5 mL) of the reaction mixture were withdrawn periodically. Samples were analyzed on Perkin-Elmer Sigma 3B gas chromatograph, connected to Sigma-15 data station using a flame ionization detector. The stationary phases em0 1987 American Chemical Society
Ind. Eng. Chem. Rea., Vol. 26, No. 7 , 1987 1277
a
m
I-
n
E
n
W
-8 I-H,O
k
a > z
W
8 a 0
z W
I
a. LL 0
-7
:: TIME ( H O U R S ) Figure 2. Effect of mole ratio on conversion: Nafion-H, lO%(w/w) phenol; initial concentration of phenol in xylene, 30% (w/w); temperature, 143-146 "C; mole ratio of reactants [(symbol) pheno1:iso4:1,(A)2:1, (0)1:1, (0) 1:2. butyl alcohol], (0)
W
I-
a W
15-
Figure 1. Experimental assembly: (1)reaction vessel, (2) stirrer, (3)sample point, (4)thermometer pocket, (5)baffles, (6) conetanttemperature bath, (7) Dean and Stark apparatus, (8) condenser, (9) CaClz guard tube.
ployed were 10% AT-1000 on W-HPCA for alkylation of phenol and Carbowax-2OM on Porapak-P for alkylation of pyrocatechol.
Results Alkylation of Phenol with Isobutyl Alcohol. In all the experiments,the product easentially consisted of PTEiP (p-tert-butylphenol), and negligible quantities of o-tertbutylphenol were observed. Alkylation Using Nafion-H as Catalyst. Figure 2 shows conversion of phenol at different reaction times for various feed compositions. The results clearly indicate the existence of an induction period. The reaction rates were found to be comparable when the initial molar ratios of phenol to isobutyl alcohol were 1 and 2. The rates were however found to be lower for molar ratios of 4 and 0.5. This may be attributed to the competitive adsorption of the two reactants on the catalyst. The induction period appears to increase with an increase in the concentration of isobutyl alcohol. It was therefore proposed that the induction period could be due to the presence of water, which is a reaction product. Increased concentration of isobutyl alcohol increases the solubility of water in the organic phase, thus decreasing the rate of removal of water. To verify the validity of the above, the experiment with phenol to isobutyl alcohol molar ratio of 2 was repeated, and the reflux condenser was packed with activated 4A molecular sieves. The progress of reaction, in the presence and in the absence of molecular sieves under otherwise
0
1
2 3 TIME ( H O U R S )
c
Figure 3. Effect of 4A molecular sieves on induction period Nafion-H, 10% (w/w) phenol; mole ratio of phenolisobutyl alcohol, 21; initial concentration of phenol in xylene, 30% (w/w); temperature, 143-146 "C; (0)with molecular sieves; ( 0 )without molecular sieves.
identical conditions, is compared in Figure 3. The results clearly indicate that removal of water significantly reduces the induction period, giving credence to the reasoning given above. To examine the reusability of the catalyst, the experiment of phenol with isobutyl alcohol at molar ratio of 2 was repeated 3 times with the same Nafion-H catalyst. The catalytic activity was found to be unchanged. An experiment was also conducted using phenol to isobutyl alcohol molar ratio of 2 but using half the concentration of phenol in xylene. The results are shown in Figure 4. A reaction carried out at 110 "C using toluene as entrainer resulted in negligible conversion. Alkylation Using Triflic Acid as Catalyst. Results obtained using triflic acid as catalyst are presented in Figure 5. No induction period is observed at 5% and 10% loading of the catalyst [(w/w) of phenol]. However, with 1% loading of triflic acid, there appears to be some induction period.
1278 Ind. Eng. Chem. Res., Vol. 26, No. 7 , 1987 "I
I
TIME (HOUSE 1 Figure 4. Effect of initial concentration of reactants: Nafion-H, 10% (w/w)phenol; mole ratio of pheno1:isobutyl alcohol, 2:l; temphenol, 30% (w/w)xylene; (A)phenol, perature, 143-146 'C; (0) 15% (w/w)xylene.
TIME (HOURS1 Figure 6. Alkylation of phenol on various acid catalysts: catalyst, 10% (w/w)phenol; mole ratio of phenolisobutyl alcohol, 2:l; initial temperature, 143-146 concentration of phenol in xylene, 30% (w/w); "C; ( 0 )Zr02-I,( X ) Zr02-II, (0) Nafion-H, (A)triflic acid, (0) CER, (A)98% HZSO,.
TIME (HOURS 1
Figure 5. Effect of triflic acid concentration [(w/w) of phenol] on conversion: mole ratio of pheno1:isobutyl alcohol, 2:l; initial concentration of phenol in xylene, 30% (w/w); temperature, 143-146 'C; (A)1 % , (0) 5%, (0) 10%.
The rate of reaction is approximately proportional to triflic acid concentration. Triflic acid is reported to form stable monohydrate with water (Howell and McCown, 1977). Absence of induction period would probably imply that triflic acid preferentially interacts with isobutyl alcohol in the presence of water. Alkylation Using Other Acid Catalysts. Results obtained using Zr02-I,Zr02-II,98% sulfuric acid, and CER
[Indion-CXC-125] are presented in Figure 6. All the experiments were carried out using identical catalyst loadings [lo70(w/w) phenol]. The results obtained using similar loadings of Nafion-H and triflic acid are also included in the figure, for the purpose of comparison. Zr02-I shows higher catalytic activity than Zr02-II, which is in agreement with the previous results on these catalysts (Arata and Hino, 1980). However, in contrast to the earlier observation, the activity of Zr02-I is considerably less than that of Nafion-H (Arata and Hino, 1981). The initial catalytic activity of 98% sulfuric acid is higher than that of Zr02-I. Activity of sulfuric acid, however, decreases with an increase in conversion and becomes negligible at about 20% conversion of phenol. This may be due to the strong affinity of sulfuric acid for water. Dilution of sulfuric acid with water will result in a decrease in acid strength which may be responsible for a decrease in catalytic activity. The CER shows significant catalytic activity for the reaction. The reaction temperature, however, is higher than the recommended upper limit of the temperature for such catalysts (Helfferich, 1962). Unlike in the case of Nafion-H, no induction period was observed. When the reaction was carried out using equal loadings (10% (w/w) phenol) of p-toluenesulfonic acid (PTSA), very small conversion was obtained (less than 4% conversion after 7 h). This was in spite of higher concentrations of sulfonic acid groups in comparison to the CER catalyst. Alkylation of Pyrocatechol with Isobutyl Alcohol. The experimental procedure for alkylation of pyrocatechol was essentially similar to that in the case of phenol. Figure 7 shows conversion of pyrocatechol to p-tert-butylpyrocatechol (TBP) using different acid catalysts including superacids. Triflic acid, which is the strongest known acid,
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n
m +
35
2
0 30 W l-
a
5
25
z
8 A
0
20
I
0
E
15
U
0
0
E
10
n LL
0
::
HO
5
0 0
1
2 3 4 TIME (HOURS)
5
6
7
Figure 7. Comparison of activity of various acid catalysts for alkylation of pyrocatechol: catalyst concentration, 10% (w/w) pyrocatechol [5% (w/w) in the case of triflic acid]; mole ratio of phenolisobutyl alcohol, 2:l; initial concentration of pyrocatechol in xylene, 30% (w/w); temperature, 143-146 "C; ( 0 )Zr0,-I, (0)NaCER, ( X ) 98% HzS04. fion-H, (A)triflic acid, (0)
displays the highest catalytic activity. In this case also an induction period was observed when Nafion-H was employed as catalyst. The activity of ZrOz-Iis considerably less than that of Nafion-H. The activity of 98% sulfuric acid is higher than that of Zr02-I and is observed to decrease with conversion. As indicated earlier, this is probably due to dilution of sulfuric acid by water. The CER also showed significant activity for the reaction. The attempts of alkylation of p-cresol by isobutyl alcohol under identical conditions with the above catalysts, however, were unsuccessful. This may be attributed to the steric hindrance of the hydroxyl group.
Discussion The trend of variation of activity with the catalyst was very similar for alkylation of phenol and pyrocatechol. A rational comparison of the efficacy of the various catalysts can be made only by comparing the activity per acid site. The number of protonic sites could be readily calculated for sulfuric acid, triflic acid, and PTSA from their molecular weights. For Nafion-H and CER, the number of acid sites per unit mass of catalyst (mequiv/g) are specified by the manufacturer. The acidity of ZrOz-I was measured by Benesi's method. The dried catalyst sample was titrated in anhydrous benzene with 0.1 M n-butylamine in benzene, using the Hammett indicators in the range of Ho = +1.5 to -9.3 (Benesi, 1957). The number of acid sites with Ho 1-9.3 were found to be 0.083 mequivfg. The turnover number for various catalysts could then be evaluated as turnover no. = (no. of mol of phenol)(initial rate)/ (Avogadro's no.)(no. of acid sites) The differences in the turnover number can be rationalized by comparing them with the strengths of the respective catalysts, which are commonly quantified by the Hammett acidity function, Ho.The Ho values for triflic acid, PTSA, and 98% sulfuric acid are widely reported and
Figure 8. Correlation between turnover number and Hammett acidity function, Ho:(- - - -) alkylation of phenol; (-) alkylation of pyrocatechol; ( 0 / O ) ZrOz-I, (A/A)Nafion-H, (0/.) triflic acid, ( V / V ) 98% HZSOk
are -13, -1.34, and -10.43, respectively (Howell and McCown, 1977; Albert and Serjeant, 1971; Gillespie and Peel, 1971). The Ho value of Nafion-H is not reported. Therefore, Ho value of perfluorobenzenesulfonic acid (-12.3) has been used as an approximation (Commeyras et al., 1976). The Ho value of Amberlyst-15 which is very similar to the CER employed in the present study has been reported to be -2.15 (Rys and Steinegger, 1979). The turnover numbers for alkylation of phenol and pyrocatechol are plotted against the Hammett acidity function of the respective catalysts in Figure 8. In the case of Nafion-H, activity just after the induction period is used. A reasonable correlation is observed for all the catalysts. The point for CER goes out of the figure, since it has a much lower value of Ho. The relevant figures for this catalyst are Ho= -2.15 and turnover numbers = 3.3 X lo-= and 2.1 X lovz2g-mol of reactant/[(mequiv or g-mol of catalyst).h] respectively in the cases of phenol and pyrocatechol. The point corresponding to zirconia catalyst lies much above the curve. Unlike other catalysts, zirconia is expected to have a wide distribution of strength of acid sites on the surface. The acid concentration measured by the Benesi's method refers to acid sites with H, 5 -9.3. The higher activity per acid site of this catalyst suggests that a large proportion of acid sites probably have Homuch lower than -9.3. CER shows considerably higher activity than PTSA. This has been reported in a few earlier studies (Gates et al., 1972; Ancillotti et al., 1977). It has been attributed to the fact that in the case of CER the sulfonic acid groups are concentrated in the small volume of polymer matrix which results in a very high effective acid concentration in the sorbed phase. In this highly acidic environment which exists in the polymer matrix, simultaneous interactions of reactant molecules with several acid sites become possible.
Conclusion The results indicate that alkylation of phenolic compounds by primary alcohols can be carried out using a catalytic amount of superacid catalysts. The superacid catalysts like triflic acid and Nafion-H were observed to be the most efficient. Complete conversion of the limiting reactant could be observed with triflic acid as catalyst. The sulfate-treated zirconia appeared less active when compared on the basis of equal loading. However, its activity
Ind. Eng. C h e m . Res. 1987, 26, 1280-1284
1280
was found to be comparable to the other superacid catalysts when the acid site concentration was taken into account. Sulfuric acid showed high initial activity; however, the activity declined almost to a zero value probably as a result of dilution by the water which is formed as the reaction product. Thus, the reason why sulfuric acid is required to be used in large excess is probably to reduce the effect of dilution by water. Cation-exchange resins do not appear suitable for these reactions, since the temperature which was found to be necessary to obtain a reasonable rate is above the recommended upper limit of temperature from the consideration of their structural stability.
Acknowledgment D.D.C. thanks the C.S.I.R., New Delhi, for the award of a Junior Research Fellowship. Registry No. Phenol, 108-95-2; pyrocatechol, 120-80-9; isobutyl alcohol, 78-83-1; triflic acid, 1493-13-6; Nafion-H, 63937-00-8; p-toluenesulfonic acid, 104-15-4; sulfuric acid, 7664-93-9; zirconium hydroxide, 14475-63-9; p-tert-butylphenol, 98-54-4; o-tert-butylphenol, 88-18-6; p-tert-butylpyrocatechol, 98-29-3.
Literature Cited Albert, A.; Serjeant, E. The Determination of Ionization Constants: A Laboratory Manual, 2nd ed.; Chapman and Hall: London, 1971; p 88.
Ancillotti, P.; Mauri, M.; Pescarollo, E. J . Catal. 1977, 46, 49-57. Arata, K.; Hino, M. J . Chem. Soc., Chem. Cc nmun. 1980, 18, 851-852. Arata, K.; Hino, M. Chem. Lett. 1981, 12, 1671. Renesi, H. A. J . Phys. Chem. 1957, 61, 970-973. Chaudhari, D. D. Chem. Ind. (London) 1983, 14. 568-569. Commeyras, A.; Grondin, J.; Sagnes, R. Bull. Soc. Chim. Fr. 1976, 11-22 ( l ) ,1779-1783. Fung, A. P.; Olah, G. A.; Malhotra, R. Synthesis 1981, 6, 474-476. Gates, B. C.; Heath, H. W., Jr.; Wisnouskas, .J. S. J . Catal. 1972,24, 320-3 2 7. Gillespie, R. J.; Peel, T. E. Advances in Physical Organic Chemistry; Gold, V., Ed.; Academic: London, 1971; Vol. 9, pp 1-24. Hasegawa, H.; Higashimura, T. Polym. J . (Tokyo) 1980, 12(6), 407-409. Helfferich, F. Ion Exchange; McGraw-Hill: New York, 1962; p 523. Howell, R. D.; McCown, J. D. Chem. Rev. 1977, 77, 69-92. Isagulyants, V. I. Gaz. Prom. im. I. M. Gubkina 1957,24,286;(‘hem. Abstr. 1960, 54, 13497i. Reed, H. W. B. K i r k - 0 t h ” Encyclopedia of Chemical Technalogy, 3rd ed.; Grayson, kl., Ed.; Wiley: New York, 1978; Vol. 2, p 87. Rys, P.; Steinegger, W. J. J . Am. Chem. SOC. 1979,101,4801-4806. Schriesheim, A. Friedel-Crafts and Related Reactions; Olah, G. A. Ed.; Interscience: London, 1964; Vol. IZ, Part 1, pp 477-595. Varagnat, J. Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Grayson, M., Ed.; Wiley: New York, 1981; Vol. 13. pp 63-64. Received for review February 6, 1985 Revised manuscript received July 20, 1986 Accepted March 16, 1987
A Deactivation Correlation for PtRe/Al,03 in n -Hexane Conversion Victor K. Shum,t Wolfgang M. H. Sachtler, and John B. Butt* Ipatieff Laboratory a n d Department of Chemical Engineering, Northwestern University, Evanston, Illinois 60201
A correlation of deactivation by coke formation in n-hexane conversion on PtRe/Al,O, is given following t h e suggestions of Levenspiel. Some data are also included for n-heptane and naphtha under similar conditions. Deactivation kinetics of these catalysts are not well correlated by established methods such as that of Voorhies, which yield little insight into the reaction pathways in any event. The present method provides information on apparent overall kinetics as well as deactivation parameters and would seem to provide a rapid method for screening of reaction/deactivation behavior a t one level beyond pure empiricism.
Background Some time ago, Levenspiel (1972) presented a simple method of analysis of catalyst deactivation bhsed on nth order pseudohomogeneous rate correlations. The approach has not been applied very widely in confrontation with experimental data, but at least in areas bearing on catalytic reforming, it would appear to provide better insight into possible reaction pathways involved (Franck and Martino, 1985) than more familiar empirical approaches (Voorhies, 1945; Mahoney, 1974), though not to the detail developed in other proposals (Froment and Bischoff, 1961; Wojciechowski, 1974; Corella and Asua, 1982) at the expense of more complex analysis. Our interest here is in the conversion of n-hexane (primarily), n-heptane, and naphtha on PtRe/Al,O, (Pt/Al,O, in the latter case) in a typical condition in which
* To whom correspondence should be addressed. ‘Present address: Amoco Oil Company, Research and Development Department. Naperville, 11, 60566. 0888-5885/87/2626-1280$01.50/0
hydrogen is present in significant excess. In such a case, a simultaneous-consecutive deactivation process with both reactant and product coke precursors may be appropriate. Since the net effects of all reactant molecules on coke formation are taken together, this represents a single “lump” in the reaction scheme, and for both reactant and product, we have the two lump scheme A--R
\\/
(1)
coke
Under such conditions, the main reaction is first order in the feed component and, at fixed total pressure, zero order in hydrogen. In effect, the hydrogen dependence is combined into the rate coefficient since hydrogen partial pressure is essentially constant. Thus, for separable deactivation kinetics (Szbpe and Levenspiel, 1970), -rA = kPAs
(1)
-ds/dt = kds
(21
62 1987 American Chemical Society
