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Selectivity engineering with polymer-supported reagents: oxidation of benzyl chloride to benzaldehyde
Reactive & Functional Polymers 32 (1997) 187- 194
REACTIVE & FUNCTIONAL POLYMERS
Selectivity engineering with polymer-supported reagents: oxidation of benzyl chloride to benzaldehyde G.D. Yadav *, B.V. Haldavanekar Chemical Engineering Division, University Department of Chemical Technology, University of Bombay, Matunga, Bombay Mumbai, 400 019, India Received 22 May 1996; revised version received 1 October 1996; accepted 10 October 1996
Abstract Polymer-supported oxidising agents are very useful in organicsynthesis,due to easy recovery and regener&on. The selective formation of benzaldehyde from benzyl chloride by using chromate salts supported on a polystyrerpe matrix crosslinked with divinylbenzene containing a quaternary functional group has been investigated in detail. The effects of various parameters on the rates of reaction have been studied. The mechanism and kinetics of this reaction have been established. There is a total selectivity for benzaldehyde and the reduced chromate species can be regenerated. The results are novel. Keywords: Polymer support; Oxidation; Chromates; Benzyl chloride; Benzaldehyde; Selectivity engineering
1. Introduction Polymer-supported catalysts and reagents are very useful in organic synthesis and they could be applied in conjunction with phase transfer catalysis (PTC). PTC is a pervasive synthetic tool in a variety of multiphase liquid-liquid (L-L), solid-liquid (S-L), liquid-liquid-solid (L-L-S) and liquid-liquid-liquid (L-L-L) reactions where mass-transfer effects are likely to come into play. Further, complex reactions involving several parallel and/or series steps in the multiphase PTC systems have not been dealt with adequately in published literature. Recently we studied the LL PTC and L-L-S (catalyst) oxidation of benzyl chloride with aqueous chromate salts leading to *Correspondingauthor. Fax: f91 22 4145614.
the formation of benzaldehyde via benzyli alcohol as an intermediate[l]. A kinetic model walsdeveloped and tested against experimental data1 One of the drawbacks of the chromate oxidatioq in L-L or L-L-S PTC system is the difficulty in recovery and reuse of the reduced chromate salts which are a source of pollution. It would be advaatageous to support the oxidising chromate species itself on a polymer support in this oxidation Whereby the reduced chromate salt bound to the IPolymer could be easily separated and regenerated,. Oxidation is an important class of reactions from both industrial and academic points of view. In recent years, a large number of okidising agents have been used by different researchers for a variety of industrial reactions. Supported oxidising agents have attracted attention of many scientists due to their various unique features
1381-5148/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII S1381-5148(96)00085-S
188
G.D. Yiv.
B.K HaMavanekm/Reactive
such as utmost simplicity of work-up where the catalyst or reagent can be separated from the reaction mixture by filtration, ease in the reaction set-up and operating conditions, improvement in the selectivity of the desired product, etc. Besides, the selectivity could be totally altered due to change in mechanism. 2. Literature survey Importance of polymer-supported reagents is well documented [2]. McKillop and Taylor [3] have employed clay-supported thallium trinitrate (TTN) for the oxidation of olefins and enols. Chiang et al. [4] have supported TIN on K-10 clay and successfully used the supported catalyst for the oxidation of alkyl aryl ketones. Montmorillonite-supported potassium permanganate was employed by Kim et al. [5] for the oxidation of alcohols under sonication to get yields up to 100%. Takeshi and Fumi [6] have studied the oxidation of secondary and benzylic alcohols with the help of metal nitrites supported on silica. Jidong [7] has successfully employed chromic acid on keiselghur for the oxidation of primary alcohols. Use of chromium trioxide on activated carbon is also reported for the oxidation of primary and secondary alcohols to obtain yields of aldehydes and ketones in the range of 64-95% [2]. Takashi et al. [8] prepared zeolite-supported inorganic nucleophilic reagents by impregnation with the aqueous solutions of the reagents. In recent years, polymers are widely used as supports for various cation- and anion-exchange resins. The use of polymers as support for oxidising agent can also be found in the literature. Polymer-supported oxidising agents can be broadly classified into two groups, namely (1) neutral species, and (2) ionic species. In the case of neutral species, the oxidising agent is covalently bonded to the resin. In most of the cases, the oxidising agent is added during the preparation of polymer. Various oxidising agents such as quinones, N-halogenated amines, amides, imides, sulphonium halides, sulphur and sele-
& Functional Polymers 32 (1997) X87-194
nium halides, etc. are generally supported on polymer by this technique [2]. In the case of ionic species, initially, the polymer containing free ions is prepared and then the oxidising agent exchanged with these free ions. Different types of oxidising agents such as metallic ions, iodate, periodate, perhalides, chromates and complex chromates, etc. are generally supported by using ion-exchange technique [g-14]. Polymer-supported tert-butyl-hypohalides were used by Sreekumar et al. [15] in the synthesis of some organic compounds. They have also prepared polystyrene crosslinked by divinylbenzene matrix with tert-butyl-hypochlorite function separated by trimethyl amine spacer [ 161. Chromium is generally supported in various forms such as chromates, dichromate, complex chromate, etc. Frechet et al. [9] has supported ClCr-0~ and (Cr20,2) on polyvinylpyridine resin. Brunelet and Gelbard [17] were successful in supporting CFsC02CrO; on polyvinylpyridine resin. Brunelet et al. [18] employed anion-exchange resin for supporting HCrO;, CFsC02CrO3, etc. Cardillo et al. [19] have analysed the oxidation of allylic halides by using chromic acid supported on anion-exchange resin. Narayanan and Balasubramanian[20] have prepared poly(4-vinylpyridinium bromochromate) and poly(4-vinylpyridinium fluorochromate) for the oxidation. Hussanein [21] has shown that chromium trioxide can be supported on poly (chloromethylated styrene) and can be effectively used for the oxidation of alcohols to carbonyl compounds. He has also reported that these oxidising agents are reusable. Although the polymer-supported reagents have been frequently used in synthesis of organic compounds, there is a dearth of information on kinetics of such reactions. Besides, there is no report on selective formation of benzaldehyde from benzyl chloride. The current investigations were therefore undertaken.
G.D. Yadav, B.V Haldavanekar/Reactive
3. Experimental 3.1. Chemicals and catalysts Benzyl chloride, toluene, chromic acid, methanol, and diethyl ether were obtained from M/s s.d. Fine Chemicals Pvt. Ltd. All the chemicals were of analytical grade. It was decided to use quatemary ammonium anion exchangeable resin as a support. The resin, Indion 810, was obtained from Ion-Exchange India Ltd. Indion is a polystyrene resin crosslinked with divinylbenzene with a quatemary functional group P -N+(CHs)sCl-. The particle size was in the range of 0.3-1.2 mm. The total exchange capacity was 2.5 1 meq per g dry resin.
0
3.1.1. Preparation of supported oxidising agent Various techniques have been employed for impregnating the oxidising agent on different supports. The most commonly used technique is the ‘wet impregnation’ method in which the oxidising agent is first dissolved in a suitable solvent and mixed with the solid support. The solvent is vaporised to get the oxidising agent impregnated on the solid support. In the ‘dry method’ the oxidising agent and the support are thoroughly ground together. This is commonly known as dry dispersion. In the current work, Indion 810 was washed with acetone and methanol to remove traces of impurities present on the surface of the catalyst. 7.5 g of chromic acid (00s) was dissolved in 25 cm3 of water and a slurry of resin was prepared by using this solution which was stirred for 2 h. The resin was filtered off and washed successively with water, methanol and diethyl ether until excess of chromic acid was removed from the surface. The resin was then dried under vacuum for 5 h at 60°C. The resin-supported reagent has a general structure P -N+-(CHs)sHCrQ,
0
and is henceforth referred to as 0P [Q+HCrOJ. The capacity of the resin was determined by stirring 0.5 g of the resin with 10 cm3 of 2 N aqueous potassium hydroxide for 12 h, filtering off and titrating iodometrically the obtained chro-
& Functional Polymers 32 (1997) 187-194
189
mate solution. The capacity was found to be 1.86 mmol of HCrOa/g of the dry resin. The resin so obtained did not lose activity on storing in air at room temperature. Thus, the exchange of Clions by HCrO; ions was 74.1%. 3. I .2. Experimental set-up and procedure The experiments were conducted in a 5-cm i.d. mechanically agitated contactor of 250 cm3 capacity, equipped with a 6 blade pitched turbine impeller and a reflux condenser. The assembly was kept in a thermostatic bath whose temperature would be maintained within f0.5”C. Initially, a known amount of polymersupported chromic acid was added to a measured quantity of toluene and stirred for 30 min at the desired reaction temperature to allow swelling of the resin beads. It was observed that this initial stirring helped in reducing the induction period during the course of reaction. The required amount of benzyl chloride was then added to the reaction mixture which had already attained the desired temperature. The moment of addition of benzyl chloride was1considered as the zero time. The samples of the organic phase were withdrawn at a definite interval of time and analysed by using gas chromatography study the progress of reaction (Perkin Elmer 8500 model, 5% OV-17 on Chromosorb WHP). Synthetic mixture were used for quantification. Typical experiments were conducted ~with 10 g of polymer-supported chromic acid (1.816mmol chromic acid/g resin), 30 cm3 of toluene and 1.57 g (0.0124 gmol) of benzyl chloride at l(MK for 7 h. The mole ratio of benzyl chloride to, HCrO, was 0.667. Typical concentration profiles of the reactants and products are given in Fig. 1 up to 7.5 h. 4. Results and discussion 4.1. Development of kinetic model The preliminary experiments were conducted with the polymer-supported oxidising agent, chromic acid with benzyl chloride in tokuene as
G.D. Yadav, B.K Haldavanekar/Reactive
& Functional Polymers 32 (1997) 187-194
0
6
12
16
24
time x lOi sec.
Fig. 1. Concentration profile of typical oxidation of benzyl chloride to benzaldehyde. Temperature = lOO”C, BnCl = 0.0124 gmol, resin = 10 g, mole ratio of BnCl to HCrO; = 0.667, speed of agitation = 1000 rev/min., toluene = 30 ml.
Fig. 2. Effect of speed of agitation on the conversions of benzyl chloride to benzaldehyde. BnCl = 0.0124 gmol, resin = 10 g, mole ratio of BnCl to HCrOT = 0.667, toluene = 30 ml.
a solvent. No phase transfer catalyst was used. It was found that the reaction mixture contained only benzaldehyde as a product. No benzyl alcohol, benzoic acid or dibenzyl ether was noticed. This suggests that the anhydrous chromic acid reacts as given below: The effective oxidising species in chromate salt oxidation in presence of PTC is Q+HCrO, [I]. Upon supporting this species on the polymer, it is o-P [Q+HCrO& which with Indion 810
was very small in the organic phase. Since no water was released, hydrolysis of BnCl to benzyl alcohol (BnOH) did not take place. At no time BnOH was detected and the selectivity of the reaction to benzaldehyde was 100%. A typical concentration profile, given in Fig. 2, shows that the rate of reaction of benzyl chloride is equal to the rate of formation of benzaldehyde (the two concentration curves appear mirror images). One of the reasons why excess of supported reagents is chosen is to drive the reaction to completion. The supported reagent gets depleted as the reaction proceeds, like a homogeneous reactant. It thus appears that the usual law of mass action should hold. The polymer-bound chromic acid is surrounded by the organic phase containing BnCl. This is a typical solid-liquid reaction in which the products of the reaction are PhCHO and HCl. The organic phase film surrounding the solid offers resistance to the transfer of BnCl into the pores and that of PhCHO out of the pores. A typical concentration profile for such type of reactions is provided by Kumbhar and Yadav, [22] and is shown in Fig. 1. Since toluene was used as a solvent, the rate
resin becomes 0P [N + -(CH&HCrOq]. reaction with benzyl chloride is as follows.
The
PhcH2ci+@- [Q+HCIOJ + [PhCH20CrO;Q+]-@ [PhCH20Cr0, -Q+] -@
0P
-[Q+HCrO,]
+ HCl
(la)
+
+ PhCHO
(lb)
The valency of Cr on right hand side of Eq. lb is Cr(IV) and it is a reduced species still bound to the polymer surface. The coproduct HCl gets desorbed out of the pores. Since the reaction was carried out at lOO”C,the solubility of HCl
G.D. Yadav, B.V Haldavanekar/Reactive
transport of BnCl from the bulk liquid phase to the exterior of the solid support, t-Awill be given by: rA = ~L~P{[&I
- [&II
& Functional Polymers 32 (1997) 187-194
191
Case (2) when M = 1, = kRt
(9)
(2)
Thus, a plot XA/(~ - XA) VS. t will give a slope
where [A,] and [A,] are the bulk and surface concentration of BnCl gmol/cm3, respectively, and up is the exterior surface of the polymer per unit volume of organic phase.
kR[A,]. Case (3), when M > 1, the reaction obeys a pseudo-first-order rate and hence
= rlk~~[AlPl
This theory was then tested against experimental data.
(3)
of polymerwhere [P] is the concentration supported reagent, expressed as gmol per g polymer and w, the loading of the agent, g polymer/cm3 of organic phase. It is interesting _ to note that [P] is a function of time. n represents the effectiveness factor of typical solid catalysed reactions, where q will be a function of 4, the Thiele modulus. In the absence of intraparticle diffusional resistance, Eq. 3 becomes rA = k~W[Al[pl
(4)
If [Po]w = M[A,] at time t = o, where M is the molar ratio of [HCrO,] to [BnCl], then Eq. 4 can be written as: -- d[Al = rA = k~[&l*(l dt
- XA)(M - XA)
(5)
i.e. dXA = k~[A,l(l dt where -
x
A
- XA>(M - XA)
= ([&I - [AlI
, the fractional conversion.
[&I
Integrating the above equation, for three different cases the following are obtained: Case (I) when M # 1, ln
CM - xA> MU - xA>
= kn[A,](M
- 1)t = k’t
(7)
Thus a plot of ln((M - x~)/M(l - XA)) VS. t will give a slope equal to [A,](M - l)kR and hence kR could be calculated, where k’ = kR[&](M
- 1)
(8)
-ln(l
- XA) = kR[P,]Wt
(10)
4.2. Efect of speed of agitation Fig. 2 demonstrates the effect of speed of agitation on the rate of consumption of benzyl chloride. When the speed of agitation was increased from 500 rev/min to 1000 rev/min, the conversion was found to increase. However, it was observed that the conversion practically remained constant after 750 rev/min. Thus, the external mass transfer resistance was eliminated at and above 750 rev/min and therefore, the speed of agitation was maintained at 1000 rev/min during further experiments. Typical values of solid-liquid mass transfer for agitated reactors have been provided Doraiswamy and Sharma [23]. At a speed~of 1000 ‘pm, the kSLvalue was obtained from thenlimiting value of Sharwood number of (ksLdr/@A) of 2 for the maximum particle size (dp) of 606 pm and diffusivity of BnCl in toluene (DA) of 3.1 x 10V5 cm*/s as 1.033 x lop3 cm/s. The external surface area of the resin particle per unit volume of organic phase is 37.95 cm-‘. This gives a vdhmetric mass transfer coefficient (ksLap) as 0.0391s-‘. The rate of mass transfer calculated from the$e values at the existing concentration is 1.5 x 1Clm6gmol cmd3 s-l, whereas the observed rate of reaction is 9.34 x lo-’ gmol cme3 s-t. Thus mass transfer has no influence on the rate of the reaction. 4.3. E#ect of particle size The intraparticle pore diffusion of BnCl (species A) from the outer surface to the internal surface where species P is located plays
G.D. Yadav. B.V HaldavanekarIReactive
192
+
300-350
0
125 -154 urn
& Functional Polymers 32 (1997) 187-194
urn
20
0 6
12
16 time x @s*c.
24
xl
Fig. 3. Effect of particle size of polymer support on the conversion of benzyl chloride to benzaldehyde. Temp. = lOO”C,BnCl = 0.0124 gmol, resin = 10 g, mole ratio of BnCl to HCrOi = 0.667.
an important role. It was therefore necessary to examine whether the pore diffusion could play any role in the present reaction or not. In order to elucidate this, the reaction was carried out with chromium-supported resins of different sizes under otherwise identical reaction conditions. The conversion of benzyl chloride for different particle sizes is given in Fig. 3. It is seen that the rate of reaction remains unaltered when the particle size is varied from 7.5 pm to 600 pm. The value of Thiele modulus was also calculated to find that the effectiveness factor q is nearly equal to 1. This proves that the reaction is not internal diffusion controlled. 4.4. Effect of concentrations of benzyl chloride and supported reagent
Effect of concentration of benzyl chloride was checked by varying the concentration of benzyl chloride from 2 x 10e4 to 1.24 x 10V3gmol/cm3 and the amount of polymer-supported chromic acid was kept constant (0.0186 gmol). It was observed that the conversion of benzyl chloride increased with a decrease in the concentration (Fig. 4). This due to the fact that by decreas-
0
6
12
timex lO?scc Fig. 4. Effect of initial concentration of benzyl chloride on its conversion to benzaldehyde. BnCl: o = 2 x 10m4 gmol/cm3; + = 3.95 x 1O-4 gmol/cm3; A = 6.23 x 10e4 gmol/cm3; 0 = 1.24 x 1O-3 gmol/cm3. HCrO;: 0.0186 gmol. [HCr04]/[BnCI]: 3.1; 1.5; 1; 0.5. Resin = 10 g, Temp. = lOO”C,speed of agitation = 1000 rev/min.
ing the concentration of benzyl chloride more of HCrO, are available for oxidation. The conversion of benzyl chloride was found to increase with decrease in the mmol of benzyl chloride per g of dry resin. This also suggests that the reaction is kinetically controlled. 4.5. Kinetics of reaction The kinetics of the reaction was established by using the various equations derived above for different values of M, the molar ratio of HCrO, to BnCl. Eqs. 7,9 and 10 were found to fit the data represented by curves c (for M = 1.5), b (for M = 1) and d (for M = 3.1), respectively, in Fig. 4. A typical second-order kinetic plot is given in Fig. 5. The values of second-order rate constants obtained from Eqs. 7,9 and 10 were 0.062, 0.045 and 0.052 cm3 g-’ s-l, respectively. The average value of the rate constant is thus 0.053 cm3 g-l s -l. An experiment was conducted to take the conversion of benzyl chloride to 95% with M = 3.1. The calculated time for this was 12.62 h whereas the experimental value after 13.50 h was 94.5% with an error of 8%, which is fairly satis-
G.D. Yadav, B.K Haldavanekar/Reactive
& Functional Polymers 32 (1997) 187-194
193
60-
60.
II 0
60%
x
BOY
0
lC&
40.
30.
20.
10.
0
d
30
0
time x@sec
Fig. 5. Kinetic plot. Temperature = lOO”C,BnCl = 0.0124 g, Resin = 10 g, speed of agitation = 1000 rev/mitt., toluene = 30 ml.
Fig. 6. Effect of temperature on conversion of benzyl chloride. Temperature = 100°C. BnCl = 0.0124 g. -6-
factory. The reaction was continued for 18 h to get a conversion of 98%. The isolated yield of benzaldehyde was 95% with 5% benzoic acid. Therefore for the measurement of kinetics the conversions were limited to only 30 to 50% for which benzaldehyde was the only product. Up to 90% conversion no benzoic acid was detected.
-s E
-9.
4.6. Efect of temperature The reaction was carried out at three different temperatures under otherwise similar reaction conditions to study the effect of temperature on the rate of reaction. It was observed that rate of reaction increased with an increase in the temperature (Fig. 6). The activation energy was estimated by determining values of k’at different temperatures and was found to be 16.21 kcal/gmol (Fig. 7). The value of activation energy also confirms the fact that the reaction is kinetically controlled. 4.7. Effect of repeated use of supported oxidising agent The resin was filtered after the reaction and washed with 0.1 M HCl and 0.2 M NaOH suc-
-11.
-122.6
2.7
2.6
2.9
l/l x 10:
K'
3
Fig. 7. Arrhenius plot: Ink vs. l/T to study the effect df temperature on rate constant. BnCl = 0.0124 gmole, resin 10 g, mole ratio of BnCl to HCrOi = 0.607, speed of agitation = 1000 rev/mitt., toluene = 30 ml.
cessively to remove Cr02 deposition on the resin. The resin was then regenerated by stirring with chromic acid, followed by washing with water, methanol, diethyl ether and drying in vacbum at 60°C for 5 h. These regenerated beads were used for the reaction under identical reaction conditions
G.D. Yadav. B.V Haldavanekur/Reactive
194
& Functional Polymers 32 (1997) 187-194
found to be 16.21 kcal/gmol. The reduced oxidising species can be regenerated. Acknowledgements GDY gratefully acknowledges the award of Herdillia Chemicals-UDCT Diamond Jubilee Distinguished Fellow which enabled him to write this paper. References
100
200
300
4
time, min.
Fig. 8. Reuse of polymer-supported reagent. First use: freshly prepared reagent. Second use: removal of reduced chromate salt and reloading. Third use: direct regeneration of reduced salt on the polymer support.
(Fig. 8). The conversion of benzyl chloride was found to decrease slightly with the repeated use. There is a likelihood of having less supported agent on the resin whose pore structure could be altered by some dehydration of HCrG, leading to the formation of CrG;? which remains inside blocking some channels. Therefore, during the third use the reduced chromate bound to the resin was directly treated with excess of chromic acid and the usual process of washing and drying as given in Section 3.1.1. was followed. There was an improvement in the reactivity of the supported oxidising reagent as shown in Fig. 8. This suggests that the chromate salts can be effectively used on polymer supports. 5. Conclusions Polymer-supported oxidising agent proved to be exclusively selective towards the oxidation of benzyl chloride, giving benzaldehyde as the only product. The results are novel. The system was analysed by a kinetic model which fits the data well. The reaction is overall second order, first order in each benzyl chloride and supported reagent. The activation energy was
Cl1 G.D. Yadav and B.V. Haldavanekar, J. Phys. Chem., 100 (1996) in press. PI P Laszlo, Preparative Chemistry Using Supported Reagents. Academic Press, Inc., New York, N.Y., 1987. [31 A. McKillop and E.C. Taylor, Endeavor, 35 (1976) 88; P Laszlo, Preparative Chemistry Using Supported Reagents. Academic Press, Inc., New York, N.Y., 1987. [41 C.S. Chiang, A. McKillop, EC. Taylor and J.F. White, J. Am. Chem. Sot., 98 (1976) 6750. PI S.N. Kim, D.H. Shin and B.H. Han, J. Korean Chem. Sot., 36(l) (1992) 165; cf., Chem. Abstr. 116 (1993) 127815~. [61 N. Takeshi and A. Fumi, Tetrahedron Lett., 29(48) (1988) 6265. 171 L. Jidong, Synth. Commun., 19(11-12) (1989) 1841. PI T. Takashi, 0. Harushisa and T. Hiroo, Nippon Kagaka Kaishi, 10 (1988) 1759; cf., Chem. Abstr. 110 (1989) 191884p. PI J.M.J. Frechet, M.J. Farral and L.J. Nuyens, J. Macromol. Sci., Chem., Al 1 (1977) 507. DOI S. Cacchi, L. Caglioti and E. Cemia, Synthesis, (1979) 74. [Ill A. Bongini, G. Cainelli, F. Contento and F. Manescalchi, Synthesis, (1980) 143. [W A. Akelah, M. Hassanein and I. Abidel-Galil, Eur. Polym. J., 20 (1984) 221. [13] C.R. Harrison and I? Hodge, J. Chem. Sot. Perkin I, (1982) 509. [14] Y.J. Zupan and B. Sket, J. Chem. Sot. Perkin Trans., (1982) 2059. [15] K. Sreekumar, V.N. Pillai and Rajsekharan, Polymer, 28 (1987) 1599. [16] K. Sreekumar, V.N. Pillai and Rajsekharan, J. Appl. Polym. Sci., 37(8) (1989) 2109. [17] T. Brunelet and G. Gelbard, Nouv. J. Chim., 7 (1983) 483. [ 181 T. Brunelet, C. Jouitteau and G. Gelbard, J. Org. Chem., 51 (1986) 4016. [19] G. Cardillo, M. Orena and S. Sandri, Tetrahedron Lett., (1976) 3985. [20] N. Narayanan and T.R. Balasubramanian, J. Chem. Res., Synop., 4 (1992) 132; Chem. Abstr. 116 (1993) 234732f. [21] M. Hussanein, Eur. Polym. J., 27(3) (1991) 217. [22] P.S. Kumbhar and G.D. Yadav, Chem. Engg. Sci., 44(11) (1989) 2535. [23] L.K. Doraiswamy and M.M. Sharma, Heterogeneous Reaction, Analysis Examples and Reactor Design, Vol. II. Whey Interscience, New York, N.Y., 1984.
REACTIVE & FUNCTIONAL POLYMERS
Selectivity engineering with polymer-supported reagents: oxidation of benzyl chloride to benzaldehyde G.D. Yadav *, B.V. Haldavanekar Chemical Engineering Division, University Department of Chemical Technology, University of Bombay, Matunga, Bombay Mumbai, 400 019, India Received 22 May 1996; revised version received 1 October 1996; accepted 10 October 1996
Abstract Polymer-supported oxidising agents are very useful in organicsynthesis,due to easy recovery and regener&on. The selective formation of benzaldehyde from benzyl chloride by using chromate salts supported on a polystyrerpe matrix crosslinked with divinylbenzene containing a quaternary functional group has been investigated in detail. The effects of various parameters on the rates of reaction have been studied. The mechanism and kinetics of this reaction have been established. There is a total selectivity for benzaldehyde and the reduced chromate species can be regenerated. The results are novel. Keywords: Polymer support; Oxidation; Chromates; Benzyl chloride; Benzaldehyde; Selectivity engineering
1. Introduction Polymer-supported catalysts and reagents are very useful in organic synthesis and they could be applied in conjunction with phase transfer catalysis (PTC). PTC is a pervasive synthetic tool in a variety of multiphase liquid-liquid (L-L), solid-liquid (S-L), liquid-liquid-solid (L-L-S) and liquid-liquid-liquid (L-L-L) reactions where mass-transfer effects are likely to come into play. Further, complex reactions involving several parallel and/or series steps in the multiphase PTC systems have not been dealt with adequately in published literature. Recently we studied the LL PTC and L-L-S (catalyst) oxidation of benzyl chloride with aqueous chromate salts leading to *Correspondingauthor. Fax: f91 22 4145614.
the formation of benzaldehyde via benzyli alcohol as an intermediate[l]. A kinetic model walsdeveloped and tested against experimental data1 One of the drawbacks of the chromate oxidatioq in L-L or L-L-S PTC system is the difficulty in recovery and reuse of the reduced chromate salts which are a source of pollution. It would be advaatageous to support the oxidising chromate species itself on a polymer support in this oxidation Whereby the reduced chromate salt bound to the IPolymer could be easily separated and regenerated,. Oxidation is an important class of reactions from both industrial and academic points of view. In recent years, a large number of okidising agents have been used by different researchers for a variety of industrial reactions. Supported oxidising agents have attracted attention of many scientists due to their various unique features
1381-5148/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII S1381-5148(96)00085-S
188
G.D. Yiv.
B.K HaMavanekm/Reactive
such as utmost simplicity of work-up where the catalyst or reagent can be separated from the reaction mixture by filtration, ease in the reaction set-up and operating conditions, improvement in the selectivity of the desired product, etc. Besides, the selectivity could be totally altered due to change in mechanism. 2. Literature survey Importance of polymer-supported reagents is well documented [2]. McKillop and Taylor [3] have employed clay-supported thallium trinitrate (TTN) for the oxidation of olefins and enols. Chiang et al. [4] have supported TIN on K-10 clay and successfully used the supported catalyst for the oxidation of alkyl aryl ketones. Montmorillonite-supported potassium permanganate was employed by Kim et al. [5] for the oxidation of alcohols under sonication to get yields up to 100%. Takeshi and Fumi [6] have studied the oxidation of secondary and benzylic alcohols with the help of metal nitrites supported on silica. Jidong [7] has successfully employed chromic acid on keiselghur for the oxidation of primary alcohols. Use of chromium trioxide on activated carbon is also reported for the oxidation of primary and secondary alcohols to obtain yields of aldehydes and ketones in the range of 64-95% [2]. Takashi et al. [8] prepared zeolite-supported inorganic nucleophilic reagents by impregnation with the aqueous solutions of the reagents. In recent years, polymers are widely used as supports for various cation- and anion-exchange resins. The use of polymers as support for oxidising agent can also be found in the literature. Polymer-supported oxidising agents can be broadly classified into two groups, namely (1) neutral species, and (2) ionic species. In the case of neutral species, the oxidising agent is covalently bonded to the resin. In most of the cases, the oxidising agent is added during the preparation of polymer. Various oxidising agents such as quinones, N-halogenated amines, amides, imides, sulphonium halides, sulphur and sele-
& Functional Polymers 32 (1997) X87-194
nium halides, etc. are generally supported on polymer by this technique [2]. In the case of ionic species, initially, the polymer containing free ions is prepared and then the oxidising agent exchanged with these free ions. Different types of oxidising agents such as metallic ions, iodate, periodate, perhalides, chromates and complex chromates, etc. are generally supported by using ion-exchange technique [g-14]. Polymer-supported tert-butyl-hypohalides were used by Sreekumar et al. [15] in the synthesis of some organic compounds. They have also prepared polystyrene crosslinked by divinylbenzene matrix with tert-butyl-hypochlorite function separated by trimethyl amine spacer [ 161. Chromium is generally supported in various forms such as chromates, dichromate, complex chromate, etc. Frechet et al. [9] has supported ClCr-0~ and (Cr20,2) on polyvinylpyridine resin. Brunelet and Gelbard [17] were successful in supporting CFsC02CrO; on polyvinylpyridine resin. Brunelet et al. [18] employed anion-exchange resin for supporting HCrO;, CFsC02CrO3, etc. Cardillo et al. [19] have analysed the oxidation of allylic halides by using chromic acid supported on anion-exchange resin. Narayanan and Balasubramanian[20] have prepared poly(4-vinylpyridinium bromochromate) and poly(4-vinylpyridinium fluorochromate) for the oxidation. Hussanein [21] has shown that chromium trioxide can be supported on poly (chloromethylated styrene) and can be effectively used for the oxidation of alcohols to carbonyl compounds. He has also reported that these oxidising agents are reusable. Although the polymer-supported reagents have been frequently used in synthesis of organic compounds, there is a dearth of information on kinetics of such reactions. Besides, there is no report on selective formation of benzaldehyde from benzyl chloride. The current investigations were therefore undertaken.
G.D. Yadav, B.V Haldavanekar/Reactive
3. Experimental 3.1. Chemicals and catalysts Benzyl chloride, toluene, chromic acid, methanol, and diethyl ether were obtained from M/s s.d. Fine Chemicals Pvt. Ltd. All the chemicals were of analytical grade. It was decided to use quatemary ammonium anion exchangeable resin as a support. The resin, Indion 810, was obtained from Ion-Exchange India Ltd. Indion is a polystyrene resin crosslinked with divinylbenzene with a quatemary functional group P -N+(CHs)sCl-. The particle size was in the range of 0.3-1.2 mm. The total exchange capacity was 2.5 1 meq per g dry resin.
0
3.1.1. Preparation of supported oxidising agent Various techniques have been employed for impregnating the oxidising agent on different supports. The most commonly used technique is the ‘wet impregnation’ method in which the oxidising agent is first dissolved in a suitable solvent and mixed with the solid support. The solvent is vaporised to get the oxidising agent impregnated on the solid support. In the ‘dry method’ the oxidising agent and the support are thoroughly ground together. This is commonly known as dry dispersion. In the current work, Indion 810 was washed with acetone and methanol to remove traces of impurities present on the surface of the catalyst. 7.5 g of chromic acid (00s) was dissolved in 25 cm3 of water and a slurry of resin was prepared by using this solution which was stirred for 2 h. The resin was filtered off and washed successively with water, methanol and diethyl ether until excess of chromic acid was removed from the surface. The resin was then dried under vacuum for 5 h at 60°C. The resin-supported reagent has a general structure P -N+-(CHs)sHCrQ,
0
and is henceforth referred to as 0P [Q+HCrOJ. The capacity of the resin was determined by stirring 0.5 g of the resin with 10 cm3 of 2 N aqueous potassium hydroxide for 12 h, filtering off and titrating iodometrically the obtained chro-
& Functional Polymers 32 (1997) 187-194
189
mate solution. The capacity was found to be 1.86 mmol of HCrOa/g of the dry resin. The resin so obtained did not lose activity on storing in air at room temperature. Thus, the exchange of Clions by HCrO; ions was 74.1%. 3. I .2. Experimental set-up and procedure The experiments were conducted in a 5-cm i.d. mechanically agitated contactor of 250 cm3 capacity, equipped with a 6 blade pitched turbine impeller and a reflux condenser. The assembly was kept in a thermostatic bath whose temperature would be maintained within f0.5”C. Initially, a known amount of polymersupported chromic acid was added to a measured quantity of toluene and stirred for 30 min at the desired reaction temperature to allow swelling of the resin beads. It was observed that this initial stirring helped in reducing the induction period during the course of reaction. The required amount of benzyl chloride was then added to the reaction mixture which had already attained the desired temperature. The moment of addition of benzyl chloride was1considered as the zero time. The samples of the organic phase were withdrawn at a definite interval of time and analysed by using gas chromatography study the progress of reaction (Perkin Elmer 8500 model, 5% OV-17 on Chromosorb WHP). Synthetic mixture were used for quantification. Typical experiments were conducted ~with 10 g of polymer-supported chromic acid (1.816mmol chromic acid/g resin), 30 cm3 of toluene and 1.57 g (0.0124 gmol) of benzyl chloride at l(MK for 7 h. The mole ratio of benzyl chloride to, HCrO, was 0.667. Typical concentration profiles of the reactants and products are given in Fig. 1 up to 7.5 h. 4. Results and discussion 4.1. Development of kinetic model The preliminary experiments were conducted with the polymer-supported oxidising agent, chromic acid with benzyl chloride in tokuene as
G.D. Yadav, B.K Haldavanekar/Reactive
& Functional Polymers 32 (1997) 187-194
0
6
12
16
24
time x lOi sec.
Fig. 1. Concentration profile of typical oxidation of benzyl chloride to benzaldehyde. Temperature = lOO”C, BnCl = 0.0124 gmol, resin = 10 g, mole ratio of BnCl to HCrO; = 0.667, speed of agitation = 1000 rev/min., toluene = 30 ml.
Fig. 2. Effect of speed of agitation on the conversions of benzyl chloride to benzaldehyde. BnCl = 0.0124 gmol, resin = 10 g, mole ratio of BnCl to HCrOT = 0.667, toluene = 30 ml.
a solvent. No phase transfer catalyst was used. It was found that the reaction mixture contained only benzaldehyde as a product. No benzyl alcohol, benzoic acid or dibenzyl ether was noticed. This suggests that the anhydrous chromic acid reacts as given below: The effective oxidising species in chromate salt oxidation in presence of PTC is Q+HCrO, [I]. Upon supporting this species on the polymer, it is o-P [Q+HCrO& which with Indion 810
was very small in the organic phase. Since no water was released, hydrolysis of BnCl to benzyl alcohol (BnOH) did not take place. At no time BnOH was detected and the selectivity of the reaction to benzaldehyde was 100%. A typical concentration profile, given in Fig. 2, shows that the rate of reaction of benzyl chloride is equal to the rate of formation of benzaldehyde (the two concentration curves appear mirror images). One of the reasons why excess of supported reagents is chosen is to drive the reaction to completion. The supported reagent gets depleted as the reaction proceeds, like a homogeneous reactant. It thus appears that the usual law of mass action should hold. The polymer-bound chromic acid is surrounded by the organic phase containing BnCl. This is a typical solid-liquid reaction in which the products of the reaction are PhCHO and HCl. The organic phase film surrounding the solid offers resistance to the transfer of BnCl into the pores and that of PhCHO out of the pores. A typical concentration profile for such type of reactions is provided by Kumbhar and Yadav, [22] and is shown in Fig. 1. Since toluene was used as a solvent, the rate
resin becomes 0P [N + -(CH&HCrOq]. reaction with benzyl chloride is as follows.
The
PhcH2ci+@- [Q+HCIOJ + [PhCH20CrO;Q+]-@ [PhCH20Cr0, -Q+] -@
0P
-[Q+HCrO,]
+ HCl
(la)
+
+ PhCHO
(lb)
The valency of Cr on right hand side of Eq. lb is Cr(IV) and it is a reduced species still bound to the polymer surface. The coproduct HCl gets desorbed out of the pores. Since the reaction was carried out at lOO”C,the solubility of HCl
G.D. Yadav, B.V Haldavanekar/Reactive
transport of BnCl from the bulk liquid phase to the exterior of the solid support, t-Awill be given by: rA = ~L~P{[&I
- [&II
& Functional Polymers 32 (1997) 187-194
191
Case (2) when M = 1, = kRt
(9)
(2)
Thus, a plot XA/(~ - XA) VS. t will give a slope
where [A,] and [A,] are the bulk and surface concentration of BnCl gmol/cm3, respectively, and up is the exterior surface of the polymer per unit volume of organic phase.
kR[A,]. Case (3), when M > 1, the reaction obeys a pseudo-first-order rate and hence
= rlk~~[AlPl
This theory was then tested against experimental data.
(3)
of polymerwhere [P] is the concentration supported reagent, expressed as gmol per g polymer and w, the loading of the agent, g polymer/cm3 of organic phase. It is interesting _ to note that [P] is a function of time. n represents the effectiveness factor of typical solid catalysed reactions, where q will be a function of 4, the Thiele modulus. In the absence of intraparticle diffusional resistance, Eq. 3 becomes rA = k~W[Al[pl
(4)
If [Po]w = M[A,] at time t = o, where M is the molar ratio of [HCrO,] to [BnCl], then Eq. 4 can be written as: -- d[Al = rA = k~[&l*(l dt
- XA)(M - XA)
(5)
i.e. dXA = k~[A,l(l dt where -
x
A
- XA>(M - XA)
= ([&I - [AlI
, the fractional conversion.
[&I
Integrating the above equation, for three different cases the following are obtained: Case (I) when M # 1, ln
CM - xA> MU - xA>
= kn[A,](M
- 1)t = k’t
(7)
Thus a plot of ln((M - x~)/M(l - XA)) VS. t will give a slope equal to [A,](M - l)kR and hence kR could be calculated, where k’ = kR[&](M
- 1)
(8)
-ln(l
- XA) = kR[P,]Wt
(10)
4.2. Efect of speed of agitation Fig. 2 demonstrates the effect of speed of agitation on the rate of consumption of benzyl chloride. When the speed of agitation was increased from 500 rev/min to 1000 rev/min, the conversion was found to increase. However, it was observed that the conversion practically remained constant after 750 rev/min. Thus, the external mass transfer resistance was eliminated at and above 750 rev/min and therefore, the speed of agitation was maintained at 1000 rev/min during further experiments. Typical values of solid-liquid mass transfer for agitated reactors have been provided Doraiswamy and Sharma [23]. At a speed~of 1000 ‘pm, the kSLvalue was obtained from thenlimiting value of Sharwood number of (ksLdr/@A) of 2 for the maximum particle size (dp) of 606 pm and diffusivity of BnCl in toluene (DA) of 3.1 x 10V5 cm*/s as 1.033 x lop3 cm/s. The external surface area of the resin particle per unit volume of organic phase is 37.95 cm-‘. This gives a vdhmetric mass transfer coefficient (ksLap) as 0.0391s-‘. The rate of mass transfer calculated from the$e values at the existing concentration is 1.5 x 1Clm6gmol cmd3 s-l, whereas the observed rate of reaction is 9.34 x lo-’ gmol cme3 s-t. Thus mass transfer has no influence on the rate of the reaction. 4.3. E#ect of particle size The intraparticle pore diffusion of BnCl (species A) from the outer surface to the internal surface where species P is located plays
G.D. Yadav. B.V HaldavanekarIReactive
192
+
300-350
0
125 -154 urn
& Functional Polymers 32 (1997) 187-194
urn
20
0 6
12
16 time x @s*c.
24
xl
Fig. 3. Effect of particle size of polymer support on the conversion of benzyl chloride to benzaldehyde. Temp. = lOO”C,BnCl = 0.0124 gmol, resin = 10 g, mole ratio of BnCl to HCrOi = 0.667.
an important role. It was therefore necessary to examine whether the pore diffusion could play any role in the present reaction or not. In order to elucidate this, the reaction was carried out with chromium-supported resins of different sizes under otherwise identical reaction conditions. The conversion of benzyl chloride for different particle sizes is given in Fig. 3. It is seen that the rate of reaction remains unaltered when the particle size is varied from 7.5 pm to 600 pm. The value of Thiele modulus was also calculated to find that the effectiveness factor q is nearly equal to 1. This proves that the reaction is not internal diffusion controlled. 4.4. Effect of concentrations of benzyl chloride and supported reagent
Effect of concentration of benzyl chloride was checked by varying the concentration of benzyl chloride from 2 x 10e4 to 1.24 x 10V3gmol/cm3 and the amount of polymer-supported chromic acid was kept constant (0.0186 gmol). It was observed that the conversion of benzyl chloride increased with a decrease in the concentration (Fig. 4). This due to the fact that by decreas-
0
6
12
timex lO?scc Fig. 4. Effect of initial concentration of benzyl chloride on its conversion to benzaldehyde. BnCl: o = 2 x 10m4 gmol/cm3; + = 3.95 x 1O-4 gmol/cm3; A = 6.23 x 10e4 gmol/cm3; 0 = 1.24 x 1O-3 gmol/cm3. HCrO;: 0.0186 gmol. [HCr04]/[BnCI]: 3.1; 1.5; 1; 0.5. Resin = 10 g, Temp. = lOO”C,speed of agitation = 1000 rev/min.
ing the concentration of benzyl chloride more of HCrO, are available for oxidation. The conversion of benzyl chloride was found to increase with decrease in the mmol of benzyl chloride per g of dry resin. This also suggests that the reaction is kinetically controlled. 4.5. Kinetics of reaction The kinetics of the reaction was established by using the various equations derived above for different values of M, the molar ratio of HCrO, to BnCl. Eqs. 7,9 and 10 were found to fit the data represented by curves c (for M = 1.5), b (for M = 1) and d (for M = 3.1), respectively, in Fig. 4. A typical second-order kinetic plot is given in Fig. 5. The values of second-order rate constants obtained from Eqs. 7,9 and 10 were 0.062, 0.045 and 0.052 cm3 g-’ s-l, respectively. The average value of the rate constant is thus 0.053 cm3 g-l s -l. An experiment was conducted to take the conversion of benzyl chloride to 95% with M = 3.1. The calculated time for this was 12.62 h whereas the experimental value after 13.50 h was 94.5% with an error of 8%, which is fairly satis-
G.D. Yadav, B.K Haldavanekar/Reactive
& Functional Polymers 32 (1997) 187-194
193
60-
60.
II 0
60%
x
BOY
0
lC&
40.
30.
20.
10.
0
d
30
0
time x@sec
Fig. 5. Kinetic plot. Temperature = lOO”C,BnCl = 0.0124 g, Resin = 10 g, speed of agitation = 1000 rev/mitt., toluene = 30 ml.
Fig. 6. Effect of temperature on conversion of benzyl chloride. Temperature = 100°C. BnCl = 0.0124 g. -6-
factory. The reaction was continued for 18 h to get a conversion of 98%. The isolated yield of benzaldehyde was 95% with 5% benzoic acid. Therefore for the measurement of kinetics the conversions were limited to only 30 to 50% for which benzaldehyde was the only product. Up to 90% conversion no benzoic acid was detected.
-s E
-9.
4.6. Efect of temperature The reaction was carried out at three different temperatures under otherwise similar reaction conditions to study the effect of temperature on the rate of reaction. It was observed that rate of reaction increased with an increase in the temperature (Fig. 6). The activation energy was estimated by determining values of k’at different temperatures and was found to be 16.21 kcal/gmol (Fig. 7). The value of activation energy also confirms the fact that the reaction is kinetically controlled. 4.7. Effect of repeated use of supported oxidising agent The resin was filtered after the reaction and washed with 0.1 M HCl and 0.2 M NaOH suc-
-11.
-122.6
2.7
2.6
2.9
l/l x 10:
K'
3
Fig. 7. Arrhenius plot: Ink vs. l/T to study the effect df temperature on rate constant. BnCl = 0.0124 gmole, resin 10 g, mole ratio of BnCl to HCrOi = 0.607, speed of agitation = 1000 rev/mitt., toluene = 30 ml.
cessively to remove Cr02 deposition on the resin. The resin was then regenerated by stirring with chromic acid, followed by washing with water, methanol, diethyl ether and drying in vacbum at 60°C for 5 h. These regenerated beads were used for the reaction under identical reaction conditions
G.D. Yadav. B.V Haldavanekur/Reactive
194
& Functional Polymers 32 (1997) 187-194
found to be 16.21 kcal/gmol. The reduced oxidising species can be regenerated. Acknowledgements GDY gratefully acknowledges the award of Herdillia Chemicals-UDCT Diamond Jubilee Distinguished Fellow which enabled him to write this paper. References
100
200
300
4
time, min.
Fig. 8. Reuse of polymer-supported reagent. First use: freshly prepared reagent. Second use: removal of reduced chromate salt and reloading. Third use: direct regeneration of reduced salt on the polymer support.
(Fig. 8). The conversion of benzyl chloride was found to decrease slightly with the repeated use. There is a likelihood of having less supported agent on the resin whose pore structure could be altered by some dehydration of HCrG, leading to the formation of CrG;? which remains inside blocking some channels. Therefore, during the third use the reduced chromate bound to the resin was directly treated with excess of chromic acid and the usual process of washing and drying as given in Section 3.1.1. was followed. There was an improvement in the reactivity of the supported oxidising reagent as shown in Fig. 8. This suggests that the chromate salts can be effectively used on polymer supports. 5. Conclusions Polymer-supported oxidising agent proved to be exclusively selective towards the oxidation of benzyl chloride, giving benzaldehyde as the only product. The results are novel. The system was analysed by a kinetic model which fits the data well. The reaction is overall second order, first order in each benzyl chloride and supported reagent. The activation energy was
Cl1 G.D. Yadav and B.V. Haldavanekar, J. Phys. Chem., 100 (1996) in press. PI P Laszlo, Preparative Chemistry Using Supported Reagents. Academic Press, Inc., New York, N.Y., 1987. [31 A. McKillop and E.C. Taylor, Endeavor, 35 (1976) 88; P Laszlo, Preparative Chemistry Using Supported Reagents. Academic Press, Inc., New York, N.Y., 1987. [41 C.S. Chiang, A. McKillop, EC. Taylor and J.F. White, J. Am. Chem. Sot., 98 (1976) 6750. PI S.N. Kim, D.H. Shin and B.H. Han, J. Korean Chem. Sot., 36(l) (1992) 165; cf., Chem. Abstr. 116 (1993) 127815~. [61 N. Takeshi and A. Fumi, Tetrahedron Lett., 29(48) (1988) 6265. 171 L. Jidong, Synth. Commun., 19(11-12) (1989) 1841. PI T. Takashi, 0. Harushisa and T. Hiroo, Nippon Kagaka Kaishi, 10 (1988) 1759; cf., Chem. Abstr. 110 (1989) 191884p. PI J.M.J. Frechet, M.J. Farral and L.J. Nuyens, J. Macromol. Sci., Chem., Al 1 (1977) 507. DOI S. Cacchi, L. Caglioti and E. Cemia, Synthesis, (1979) 74. [Ill A. Bongini, G. Cainelli, F. Contento and F. Manescalchi, Synthesis, (1980) 143. [W A. Akelah, M. Hassanein and I. Abidel-Galil, Eur. Polym. J., 20 (1984) 221. [13] C.R. Harrison and I? Hodge, J. Chem. Sot. Perkin I, (1982) 509. [14] Y.J. Zupan and B. Sket, J. Chem. Sot. Perkin Trans., (1982) 2059. [15] K. Sreekumar, V.N. Pillai and Rajsekharan, Polymer, 28 (1987) 1599. [16] K. Sreekumar, V.N. Pillai and Rajsekharan, J. Appl. Polym. Sci., 37(8) (1989) 2109. [17] T. Brunelet and G. Gelbard, Nouv. J. Chim., 7 (1983) 483. [ 181 T. Brunelet, C. Jouitteau and G. Gelbard, J. Org. Chem., 51 (1986) 4016. [19] G. Cardillo, M. Orena and S. Sandri, Tetrahedron Lett., (1976) 3985. [20] N. Narayanan and T.R. Balasubramanian, J. Chem. Res., Synop., 4 (1992) 132; Chem. Abstr. 116 (1993) 234732f. [21] M. Hussanein, Eur. Polym. J., 27(3) (1991) 217. [22] P.S. Kumbhar and G.D. Yadav, Chem. Engg. Sci., 44(11) (1989) 2535. [23] L.K. Doraiswamy and M.M. Sharma, Heterogeneous Reaction, Analysis Examples and Reactor Design, Vol. II. Whey Interscience, New York, N.Y., 1984.