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Znd. Eng. Chem. Res. 1992,31, 1610-1614
Solid-Liquid Reactions Catalyzed by Alumina and Ion Exchange Resin: Reactions of Benzyl Chloride/p -Chlorobenzyl Chloride with Solid Sodium Sulfide Narayan C. Pradhan and Man Mohan Sharma* Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India
Basic aluminum oxide and Amberlyst A27 (Cl- form) ion exchange resin, in their original form, were used to catalyze the solid-liquid reactions of benzyl chloridelp-chlorobenzyl chloride with sodium sulfide. At a 5 % loading of the catalysts, enhancement factors of 14 and 25 were obtained for the benzyl chloride reaction with alumina and Amberlyst A27, respectively. Alumina was also used as a cocatalyst with tetrabutylammonium bromide (TBAB)as a homogeneous-phase-transfer catalyst. Enhancement factors as high as 1.9 X lo3 and 4.9 X lo2 were obtained with benzyl chloride and p-chlorobenzyl chloride, respectively, as organic substrates at a 0.4% loading of TBAB and 5% loading of alumina. Ultrasound was also found to intensify the reaction.
Introduction Phase-tramfer catalysis (PTC) is very widely used in the intensification of heterogeneous reactions,and the inherent advantages of using this strategy are well-recognized. Catalyst recovery in the case of soluble phase-transfer catalysts is usually difficult-hence, they are not commonly reused. This poses problems as quaternary ammonium compounds are harmful in liquid effluents and are toxic to certain aquatic species. In triphase catalysis (TC), the problem associated with the loss of the catalyst can possibly be eliminated by anchoring the catalyst to some polymeric backbone which forms a third insoluble phase easily separable at the end of the reaction. This is, however, at the cost of reduced catalytic activity. In spite of a number of research papers and monographs, TC has apparently not been industrially exploited, which may be attributed to the limited life of the catalyst. The use of inorganic solid supports for organic synthesis has been a growing concept in the last decade. It has been found that the impregnation of porous solid supports with inorganic reagents, which are insoluble in organic solvents, can be, in some cases, as efficient in intensifying the reaction as the use of phase-transfer catalysts such as onium salts and crown compounds. The effectiveness of supported reagents is ascribed to a combination of several factors, such as (1)an increase of the effective surface area of the reagent owing to high dispersion on the support, (2) an activation of the reagent by the interaction between the support surface and the reagent, (3) a decrease in the activation energy of reactions due to preadsorption of substrates in close proximity, and (4) synergistic effect of acidic and basic sites of the support on substrates. It is of interest whether inorganic support materials as such, in their original form, can facilitate solid-liquid two-phase reactions and afford a more convenient alternative to soluble phase-transfer catalysts. The reactions of benzyl chloride and p-chlorobenzyl chloride with sodium sulfide are commercially important, as the products, namely, dibenzyl sulfide and bis(pchlorobenzyl) sulfide, find many applications as additives for high pressure lubricants, antiwear additives for motor oils, and stabilizers for photographic emulsions and in refining and recovery of precious metals and in different anticorrosive formulations. Pradhan and Sharma (1990) have made a detailed study of these reaction systems with ~
*Towhom all correspondence should be addressed.
soluble phase-transfer catalysts like tetrabutylammonium bromide (TBAB). It was thought desirable to study the same reaction systems in the presence of unimpregnated inorganic solids like alumina, and anion exchange resins to explore their "catalytic" activity in enhancing the rates, since in this case, additionally, the problem of catalyst recovery will be obviated.
Literature Review A considerable amount of literature is available on the activation of the inorganic reagents used in organic synthesis by impregnation on some solid supports like alumina, silica, etc. (Laszlo, 1987; Bram et al., 1980). A few reactions, however, have been reportedly studied in the presence of those solid support materials as such without impregnation with the inorganic reagent (Liu and Tong, 1978; Quici and Regen, 1979; Dalton and Regen, 1979; Ballini and Petrini, 1986; Veselovsky et al., 1988). The reaction rates in the case of unimpregnated supports are reported to be slower than those obtained with impregnated supports-however, the former strategy is inherently simpler, and reagent losses can be minimized. Liu and Tong (1978) have prepared disulfides, RSSR (R = Bu, MeCHEt, cyclohexyl, Ph, PhCH2)from RSH in benzene, in the presence of basic alumina, at room temperature, and yields of 8946% were reported. In the case of Me,CSH, only 10% disulfide was obtained due to steric hindrance. Quici and Regen (1979) have reported the use of free neutral alumina as a triphase catalyst for the halogen exchange reaction of 1-bromooctane. Thus, when neutral alumina and sodium iodide are suspended in a solution of 1-bromooctanein toluene and heated to 90 "C for 24 h with vigorous stirring, 95 % conversion to 1-iodooctane is achieved, compared to no conversion after 40 h in the absence of alumina. Czech et al. (1980) have prepared symmetrical organic sulfides by the alkylation of reagents impregnated on alumina support. Thus, the reaction of n-octyl bromide in toluene with sodium sulfide supported on alumina afforded dioctyl sulfide in 97% yield. Unsymmetrical sulfides have also been prepared by the reaction of organic halides with thiols adsorbed on alumina. However, significantly higher yields have been obtained when NaOH impregnated on alumina was used. Regen et al. (1981) have described the reaction of solid KCN with 1-bromooctane in toluene in the presence of alumina and have proposed that alumina could act as a triphase catalyst. They have also carried out alkylation
0888-5885/92/2631-1610$03.00/00 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 1611 of acetate ion in the presence of alumina acting as a triphase catalyst. In the cyanide displacement reaction on l-bromooctane in the presence of free alumina, Ando et al. (1984) have found that the reaction is further facilitated in the presence of a minute amount of water. This has been further confirmed by Sukata (1985) who has proved the inertness of dispersed KCN on alumina in the absence of even trace amounts of water. Ando et al. (1984) have also shown that the combined use of alumina, a minute quantity of water, and ultrasonic irradiation greatly accelerates nucleophilic substitution by the cyanide ion. Ogawa et al. (1985) have performed selective esterification of dicarboxylic acids by the use of monocarboxylate chemisorption on alumina. This selective esterification is known to be very difficult by conventional methods. It has been suggested that the dicarboxylic acids are adsorbed on the alumina surface as monocarboxylate anions. The unadsorbed carboxyl group held on a position remote from the alumina surface is then selectively esterified. In this way, terephthalic acid and isophthalic acid have been selectively esterified to the corresponding monomethyl esters by the reaction with dimmethane. Selective monomethyl esterification of phthalic acid on alumina was not successful, probably as a consequence of the close proximity of the two carboxyl groups and of the forced orientation of the second group when one is adsorbed. Polystyrene bound ammonium and phosphonium ions have been successfully used in the oxidation of alcohols with periodate ion (Schneider et al., 1982; Hodge et al., 1984). In a mechanistic investigation, it was found that the reaction rate can be drastically improved through proper adjustment of the volume ratio of the two liquid phases (Telford et al., 1986). Thus, the use of very hydrophilic commercial ion exchange resins as triphase catalysts should be generally feasible if the presence of excess water is prevented. This has been substantiated by Balakrishnan and Ford (1983) who found these resins to be very active triphase catalysts in the presence of a strong desiccant, such as 50% aqueous sodium hydroxide. Recently, nucleophilic substitution, base catalyzed alkylation, and oxidation of a secondary alcohol have been carried out in a nonpolar solvent with inorganic solid reagents using polymer-bound quaternary ammonium or phosphonium Catalysts (Arrad and Sasson, 1989). Some commercial ion exchange resins, which are known to be inactive in liquid-liquid systems are found to have a similar or even superior activity in comparison with the usual triphase catalysts. From the above discussion, it is evident that solids like alumina and silica are potential support materials for inorganic reagents for a number of reactions. These are also good triphase catalysts for some reactions. However, although there is some information in the literature on the use of alumina impregnated with sodium sulfide to prepare dialkyl sulfide, the information on the use of free alumina and ion exchange resins, in their original form, to catalyze the reaction of organic halides with sodium sulfide is lacking. Moreover, it has been reported that alumina can act as a good cocatalyst with tetrakis(tripheny1phosphine)palladium(0) (TTPP) as the catalyst for the reaction of iodobenzene with sodium cyanide. A 4-fold increase in yield was reported in the presence of alumina as cocatalyst compared to that obtained with TTPP as the sole catalyst (Dalton and Regen, 1979). It was, therefore, thought desirable to study the effect of free alumina and Amberlyst A27 anion exchange resin,
+
as well as the effect of a mixed catalyst system (TBAB alumina), on the reaction of benzyl chloride/p-chlorobenzyl chloride with solid sodium sulfide. The effect of ultrasound (US) on the reaction of benzyl chloride with solid sodium sulfide was also investigated.
Experimental Section Materials. Benzyl chloride (98.5% 1, sodium sulfide (extra pure), and basic aluminum oxide (chromatography grade) were obtained from Loba Chemie Pvt. Ltd.; toluene (99.5%) was obtained from S.D.Fine Chemicals Pvt. Ltd. and Amberlyst A27 ion exchange resin was obtained from Tulsi Fine Chemical Industries Pvt. Ltd. p-Chlorobenzyl chloride (>98%) and tetrabutylammonium bromide were Fluka grade. Apparatus and Procedure. The reactions of benzyl chloride and p-chlorobenzyl chloride with solid sodium sulfide were carried out in a 9.2-cm4.d. mechanically agitated glass reactor provided with a stainless steel cooling coil and a glass thermowell. A 2.5-cm-diameter six bladed glass disc turbine impeller located at a height of 1.5 cm from the bottom of the reactor was used for stirring the reaction mixture. Water was used as coolant through the cooling coils to control the increasing temperature of the reaction mixture due to the exothermic reaction between the organic substrate and sulfide. The reactor was kept in a constant temperature bath whose temperature could be controlled within *0.5 "C. Toluene was used exclusively as the solvent. In a typical run with tetrabutylammonium bromide (TBAB) as catalyst and alumina as cocatalyst, the organic mol/cm3 of benzyl chloride phase consisting of 1.25 X (or p-chlorobenzyl chloride) and 1.25 X mol/cm3 of catalyst TBAB in solvent toluene was kept in the reactor at a predetermined temperature. Then solid sulfide (Na& 3.5H20) of a particular particle size was added in one lot, followed by the addition of alumina. The average particle size of the solid sulfide was determined by screen analysis, and the particles in a particular size range were almost of the same shape. The reaction mixture was then agitated at a particular speed of agitation. Samples from the organic phase were withdrawn at regular intervals after the agitation was stopped and the solid particles settled. Method of Analysis. All the samples from the organic phase were analyzed by gas-liquid chromatography (GLC) using a 2 m X 3 mm stainless steel column packed with 10% OV-17 on chromosorb W (80/100). A Perkin-Elmer Model 8500 gas chromatograph interfaced with a GP 100 graphics printer was used with nitrogen as the carrier gas (initial oven temperature 150 "C, final oven temperature 300 "C, ramp rate 20 "C/min, carrier flow rate 15 cm3/min, injector and flame ionization detector (FID) temperature 300 "C). The rate of reaction of organic reactant was calculated as an average rate at a fixed conversion level. The experiments were repeated 3-5 times, and a maximum variation in rate of
