I n d . E n g . C h e m . Res. 1989, 28, 1289-1292
1289
Tawarah, K. M.; Hansen, R. S. Kinetics and Mechanism of Methanol Decomposition over Zinc Oxide. J. Catal. 1984, 87, 305.
Perrard, A.; Joly, J.; Germain, J. RBactivit6 de l'oxyg6ne labile de l'oxyde chromique Cr203avec les gaz r6ducteurs. 11. M6thanol. Bull. Sci. Chim. Fr. 1984, I , 208. Sherwin, M. B. Chemicals from Methanol. Hydrocarbon Process. 1981, 60 (March), 79.
Received for review October 20, 1988 Accepted May 25, 1989
Investigation of Phase-Transfer Reactions Catalyzed by Poly(ethy1ene glycol) Bound to Macroporous Polystyrene Supports Ying Shan,* Ru-hong Kang, and Wei Li Department of Chemistry, Hebei Teachers' University, Shijiazhuang, Hebei, People's Republic of China
Several macroporous polystyrene-resin-supported poly(ethy1ene glycols) and comparable microporous analogues were prepared. These polymer catalysts were used t o catalyze various organic reactions such as esterification, etherification, and alkylation. The results show that the macroporous polymers exhibit significantly greater activity than the comparable microporous analogues and that the catalytic activity of macroporous polymers was influenced greatly by the cross-link density, PEG chain length, ring substitution, dielectric constant of solvents, and recovery of catalysts. The establishment of the triphase catalysis has brought about further development of phase-transfer catalysis (PTC). The triphase catalysts are usually synthesized by polymer-supported ammonium salts, phosphonium salts, crowns, cryptands, and poly(ethy1ene glycols) (PEG). Mostly divinylbenzene (DVB) cross-linked styrene (STY) resins have been used as supports. A few inorganic supports have been used (Tundo, 1977; Sawicki, 1982). Resins are mostly low cross-linked (Regen, 1979). Though these catalysts are very active, their mechanical strength is low. In order to discuss the effect of catalytic activity on the resin structure, Gui et al. (1986) used double-layer reticulate polystyrene resin to support PEG, and Regen and Durgadas (1981) used macroporous resin to support quaternary salts. These changed the structure of the catalyst, and the strength of the catalyst became stronger. It was found that the catalytic activity of the macroporous resin to support quaternary salts is lower than that of the microporous resin. The organic phase diffuses into the pore of the macroporous resin, which is unfavorable to ion exchange (Tomoi et al., 1984). Eight macroporous resin-supported PEG catalysts were synthesized. The results are different from those of polymer-supported quaternary salts.
Experiment and Results Potassium acetate was melted and powdered; n-decane (internal standard) was of GC purity. Other reagents were of AR or CP purity. I. Synthesis of Catalysts (Regen and Durgadas, 1981; He et al., 1984; Qian, 1984). 1. Cross-Linked Macroporous Poly(styrene-co -(chloromethyl)styrene) and 2% Divinylbenzene. A solution of 1.0 g of gelatin and 0.6 mL of 0.1'70 methylene blue in 100 g of water was added to a 250-mL round-bottom flask fitted with a reflux condenser, a magnetic stirrer, and a water bath temperature controller. A solution of 13.4 g of styrene, 5.7 g of (chloromethyl)styrene, 0.9 g of 43.2% divinylbenzene, 6.0 g of isooctyl alcohol, and 0.2 g of BPO was added. The flask was purged with nitrogen for 30 min, and a nitrogen atmosphere was maintained throughout polymerization. Stirring was started at room temperature.
* Author
t o whom correspondence should be addressed.
0888-588518912628-1289$01.50/0
The size of the organic droplet suspended in water can be determined by sampling the mixture and examining it under a microscope. If smaller droplets are desired, the stirring speed can be increased. After the droplet size was established, the mixture was heated with stirring to 72 f 1 "C for 2 h and then to 95 "C for 4 h. Insoluble polymer was collected on sieves and washed thoroughly with water. Particles of 100-200 mesh were used. The other macroporous resins were prepared as follows: for 4 % , 6%, 8%, 1070 cross-linked polymers (cross-link density, styrene (g), (chloromethy1)styrene (g), divinylbenzene (g)), 470, 13.4, 5.7, 1.8; 6%, 12.7, 5.5, 2.7; 8%,12.7, 5.5, 3.6; 1070, 12.7, 5.5, 4.6. The reactants used the above amount specifications. The resins were prepared with the procedure described above. Preparation of Microporous Polymer. Microporous polymer was synthesized as before except that no isooctyl alcohol existed. 2. Preparation of Graft Copolymers. Poly(ethy1ene glycol) was added to a 20-cm X 3-cm tube equipped with a no-air stopper and Teflon-coated magnetic stirring bar. A solution of 0.5 g of NaH dispersed in 16 mL of freshly distilled diglyme was dropped in the tube for 2-5 h during vigorous stirring. After the solution was stirred under nitrogen a t room temperature for 1 h, 1.0 g of chloromethylated polystyrene (ring substitution 30%) was added. The tube was then placed in an oil bath maintained at 90 "C, and the mixture was stirred for a certain time under nitrogen. The resulting resin was collected by filtration, washed successively with 200 mL of 4:l THF/water and 200 mL of THF, extracted (Soxhlet) with T H F for 24 h, and dried under vacuum to give the catalyst. Chlorine analysis indicated complete replacement of chloride ion by the polyether (Table I). The IR peaks of every catalyst are as follows: 3500 cm-' (0-H); 3000 cm-I (Ar-H); 2900 cm-' (C-H); 1600, 1490 cm-' (Bz); 1450 cm-' (CH-CH); 1100 cm-l (C-0-C). 3. Titration of OH in Catalysts. Use the technique of Regen (1979) (see Table 11). 4. Determination of Catalysts' Porosity and Swelling. The samples were covered with EPON 815/812 and cut into slices. The porosity was detected by HITACHI-500 emission electron microscope. The data are in Table 111. 0 1989 American Chemical Society
1290 Ind. Eng. Chem. Res., Vol. 28, No. 9, 1989 T a b l e I. Classification a n d P r e u a r a t i o n of MacroDorous a n d Microuorous Resins" ring chloromethylated substitummol of supported catalyst polystyrene wt, g tion, 0'9 Cl/g of resin NaH wt, g PEG wt, g 1 1.73 0.5 16 Ma2-400 20 1 16 2.52 0.5 Ma4-400 30 1 2.52 0.5 20 Ma6-400 30 Ma8-400 1 30 2.52 0.5 20 1 2.52 20 Malo-400 30 0.5 1 0.4 9 30 2.52 ~a~-200 1 20 Ma6-600 30 0.5 2.52 0.5 0.5 25 2.52 ~a~-800 30 16 1.73 Mi2-400 1 20 0.5 Mi,-400 1 0.5 16 2.52 30 1 16 2.52 30 Mi6-400 0.5 1 0.5 20 2.52 30 Mi8-400 20 1 30 Mi 400 0.5 2.52 1 0.4 9 2.52 30 ~i~-200 20 Mi6-600 0.5 30 0.5 2.52 0.4 30 2.52 0.5 Mi6-800 30
time, day 5 7 9 10 11 8 11 14 6 8 9 10 12 8 12 15
catalysts wt, g 1.60 1.81 1.74 1.64 1.47 1.32 2.10 1.01
yield, % 96 94 90 85 76 93 87 71
1.67
87
"Ma and Mi are macroporous and microporous resins, respectively. The subscript is the cross-link density (%). The number is the PEG average molecular weight. Catalysts are 100-200 mesh. T a b l e 11. OH C o n t e n t i n C a t a l y s t s OH content, cat ring subst "O1/g before theor actual value value catalvst sumort. '70 Ma2-400 20 1.05 0.85 1.31 1.13 30 Ma4-400 1.30 1.03 Mae-400 30 1.29 0.80 30 Ma8-400 1.28 0.91 30 MaLo-400 1.78 1.66 ~a~-200 30 1.04 0.79 30 Mae-600 0.86 0.32 Ma,-800 30 1.30 0.89 Mi6-400 30
ring subst after supports, 90 14 26 24 19 21 28 22 11
21
T a b l e 111. R a n g e of P o r e s (Polvmer S u o u o r t i n a PEG 400) pore class cross-link density range of pore, macroporous 2 70 700-1500 4 Yn 500-1000 6 90 870 150-380 10% 50-130 2 Yo microporous 4% -0 6 0'7 -0 -0 8% -0 10%
A
The ability of the solvent to swell the catalysts decreased in the order chlorobenzene > toluene > cyclohexane, as shown in Figure 1. 11. Catalysis Reaction. 1. Esterification. A 6-mL culture tube equipped with a Teflon-coated magnetic stir bar was charged with chlorobenzene (4 mL), benzyl bromide (52 mg, 0.3 mmol), anhydrous potassium acetate (60 mg, 0.6 mmol), n-decane (internal standard; 15 mg, 0.1 mmol), and Mq-400 (70 mg, 0.04 mmol). The mixture was stirred at 130 "C for 10 h. The yield of the organic layer was analyzed by GC. 2. Etherification. A mixture of phenol (47 mg, 0.5 mmol), toluene (1.5 mL), n-bromobutane (137 mg, 1.0 mmol), Mq-400 (70 mg, 0.04 mmol), n-decane, and 2.5 M aqueous NaOH (1.5 mL) was placed in a 6-mL culture tube with a Teflon-coated magnetic stir bar. The mixture was stirred for 15 h at 90 "C. Analysis of organic layer indicated a yield of product. 3. Alkylation. A mixture of phenylacetonitrile (50 mg, 0.42 mmol), n-bromobutane (80 mg, 0.6 mmol), Ma,-400 (70 mg, 0.04 mmol), and 6% aqueous KOH (0.5 mL) was
0
1
2
3
4
5
+ a 7
6
3
Cross l i n k
1
0
dens.ty
F i g u r e 1. Volume swelling and cross-link density of catalysts. (-) 1-3 represent the swelling of macrocatalysts in chlorobenzene, toluene, and cyclohexane, respectively. (- - -) 4-6 Swelling of microcatalysts in chlorobenzene, toluene, and cyclohexane, respectively.
stirred in a 10-cm X 1-cm-i.d. test tube for 5 h at room temperature. After addition of 1 mL of toluene containing 8 mg of n-decane, analysis of the organic layer indicated a yield. The experimental conditions and results are shown in Table IV. 111. Effect of Catalytic Structure. 1. Effect of Cross-Link Density on the Reaction of Macroporous and Microporous Catalysts. Esterification Catalysis. We used five macroporous catalysts, Ma2-400, Ma4-400, Ma6-400,Ma8-400, Malo-400, and five microporous catalysts, Mi,-400, Mi4-400, Mi6-400, Mi8-400, and Milo-400, respectively, to catalyze the reaction of benzyl bromide and potassium acetate in order to study the effect of the cross-link density on catalytic activity. (Figure 2) 2. Effect of PEG Chain Length on Esterification. We used four macroporous catalysts, Me-200, Ma6-400, Mq-600, Mq-800, and four microporous catalysts, M&-200, Mi6-400, Mi6-600, Mi6-800, respectively, to catalyze the same reaction (the results are shown in Figure 3). 3. Effect of Ring Substitution. A 6-mL test tube equipped with a Teflon-coated magnetic stir bar was charged with Ma6-400 (80 mg, 0.046 mmol; it was 12%,
Ind. Eng. Chem. Res., Vol. 28, No. 9, 1989 1291 Table IV. Triphase Catalytic Esterification, Etherification, and Alkylation reaction catalyst reactant product esterification none CH&OOK, PhCHzBr CH,COOCH,Ph Mae,-400 CH&OOK, PhCHiBr CHiCOOCHiPh Mi6-400 CH,COOK, PhCH,Br CH3COOCHzPh CH?COO-n-Bu Mae-400 CH,COOK, n-BuI CH~COO-~-BU Mi,-400 CH,COOK, n-BuI etherification none PhOH, n-BuBr PhO-n-Bu PhO-n-Bu NaOH, n-BuBr PhO-n-Bu Mae,-400 PhOH, n-BuBr PhO-n-Bu Mi6-400 NaOH, n-BuBr alkylation none PhCH,CN, n-BuBr PhCH(n-Bu)CN KOH, n-BuBr PhCH(n-Bu)CN Mas-400 PhCH,CN, n-BuBr PhCH(n-Bu)CN Mi6-400 KOH, n-BuBr PhCH(n-Bu)CN
temp, “C 130 130 130 130 130 90 90 90 90 20 20 20 20
time, day 10 10 10 10 10 15 15 15 15 5 5 5 5
yield, % 0 71.3 64.3 11.0 8.2 91.4 75.6 88.0 62.2
1no
-fl
--
/
N
/
rO
I
n M
2
> 40
?n n 0
in
4
7
10
x RIW
Cross-link d m s i t y (‘0
Figure 2. Effect of catalysts’ cross-link density on esterification. Curves 1 and 2 represent Ma-400 and Mi-400 catalytic activities when the cross-link density was different.
0
a00
4130
mn
300
PEG avei A g e m o l e c u l a r weight
Figure 3. Catalytic activity on the different PEG chain lengths. Curves a and b represent respectively the catalytic activity of macroporous and microporous catalysts supported on different PEG chain lengths.
30%, 5070,respectively), anhydrous potassium acetate (60 mg, 0.6 mmol), and chlorobenzene (4 mL). The mixture was stirred at room temperature for 4 h. After the addition of benzyl bromide (50 mg, 0.3 mmol) and n-decane (15 mg, 0.1 mmol), the mixture was stirred at 130 “C for 10 h. Analysis of the organic layer indicated a yield of product (Figure 4). 4. Effect of Solvent. We used cyclohexane, toluene, chlorobenzene, benzyl ethyl ketone, and nitrobenzene as solvents and M+-400 and Mi6-400as catalysts to catalyze the esterification of potassium acetate and benzyl bromide. The effects of solvent on the catalysts’ activities are shown in Table V.
70
qn
5n
sutlstitiition (I)
Figure 4. Effect on ring substitution on catalytic activity. Table V. Effect of Solvent on the Catalysts’ Activity catalyst solvent ( E ) yield, % Mae-400 cyclohexane (2.02) 13.2 toluene (2.38) 16.5 chlorobenzene (5.62) 23.3 94.5 benzyl ethyl ketone (17.39) 100.0 nitrobenzene (34.82) Mi6-400 cyclohexane 9.2 13.5 toluene chlorobenzene 18.2 benzyl ethyl ketone 86.0 nitrobenzene 100.0 Table VI. Macroporous Catalysts’ Recovery catalyst no. of times ester yield av lapse rate Ma4-400 1 100.0 6.5 2 97.2 3 93.4 4 88.4 5 80.4 6 67.5 Mae-400 1 82.3 3.4 2 81.1 3 80.5 4 78.3 5 73.0 6 65.2 Malo-400 1 55.6 0.5 2 55.0 3 54.2 4 53.6 5 53.5 6 53.0
5. Recovery of Catalysts. The catalyzing reaction is the same as before. After use, the catalyst was filtered and washed with water and T H F (100 mL) and then used to catalyze the next reaction.
1292 Ind. Eng. Chem. Res., Vol. 28, No. 9, 1989
The recovery of the macroporous catalysts used for esterification is shown in Table VI.
Discussion 1. Macroporous polystyrene-resin-supported PEG catalysts and microporous catalysts both have obvious catalytic activity. Macroporous catalytic activity is higher than microporous, in the same conditions of cross-link density, PEG chain length, and reaction conditions. 2. Macroporous and microporous catalytic activities decreased as the cross-link density increased. In general, the macroporous catalytic activity is higher than the microporous one. Resin-supported PEG is not only on the surface of the spheroid but also in the interior of the spheroid; that is to say, in the catalyzing reaction, the PEG of both the exterior and interior of the spheroid can complex the cation. Mackenzie and Sherrington (1980) put forward a mechanism to explain the reaction. Ester yields, i.e., the reaction rate, depended on the rate of reactant diffusing into the resin pore (Tomoi and Warren, 1981). The larger the cross-link density and the smaller the resin pore, the more difficult the reactant diffused into the resin pore. The pore of the macroporous catalysts in the same conditions was larger than that of the comparable microporous analogue, so its interior diffusion was easier and the reaction rate was higher. Thus, the resin swelling had an effect on catalytic activity (Tomoi et al., 1984). 3. As the PEG average molecular weight increased from 200 to 600, the ester yield increased. When the PEG average molecular weight increased continuously, the ester yield began to fall, obviously. This is probably because the longer the PEG chain, the more severely the PEG chain is twined, resulting in the hindrance of the reactants‘ interdiffusion becoming more obvious and the internal diameter shorter. Thus, the catalyst became more active. It is clear that the longer the PEG chain, the higher the chain cyclization, and so the PEG active chain is less. It is the OH content of the supported catalyst that caused the effects on catalytic activity. 4. Whether ring substitution is higher or lower, it will affect the catalytic activity. Its selection should be appropriate; 30% ring substitution was chosen in our article. 5 . The solvent has influence on the catalytic activity too. In weak polarity aprotic solvents, such as cyclohexane, toluene, and chlorobenzene, potassium acetate existed in compact ion pair form. When potassium acetate collides and touches the PEG chain, PEG complexed K+ to form (@-PEGK+)OAc- relaxed ion pair (Yanagida et al., 1979). So the dissociation energy of CH3 COOK was reduced. It is favorable to CH3COO- nucleophilic attack. In strong polarity aprotic solvents, such as benzyl ethyl ketone and nitrobenzene, potassium acetate existed in free ion form. When PEG complexed K+, CH3COO- is in the “uncovered” form (Reichardt, 1978) it increases greatly its nucleophilic activity and the reaction rate is speeded up.
6. Catalyst M+-400 is of moderate activity and has an average lapse rate, so it is supposed to be the appropriate catalyst used in laboratory and commercial productions. Thus, great activity but low mechanical strength and great mechanical strength but low activity can be avoided. Macroporous resin-supported PEG phase-transfer catalysts showed higher activity than comparable microporous catalysts. Macroporous resins are better supports. Registry No. CH3C02H.K, 127-08-2; PhCH2Br, 100-39-0; n-BuI, 542-69-8; PhOH, 108-95-2; n-BuBr, 109-65-9; PhCH2CN, 140-29-4; CH3C02CHZPh, 140-11-4; C H ~ C O ~ B U123-86-4; -~, P h O B u - n , 1126-79-0; P h C H ( C N ) B u - n , 3508-98-3; (chloromethylstyrene) (diallylbenzene)(polyethyleneglycol)(styrene) (graft copolymer), 122019-31-2; toluene, 108-88-3; chlorobenzene, 10890-7; benzyl ethyl ketone, 1007-32-5; nitrobenzene, 98-95-3; isooctyl alcohol, 26952-21-6; cyclohexane, 110-82-7.
Literature Cited Gui, Yizhi.; Liu. Bing-sheng; Liu, Jin-bi; Zhong, Zhen-sheng; Lin, Han-zhi “Studies on Polymer-Supported Phase-transfer Catalysts: 1. Synthesis of Polystyrene-Supported Dibenzo-18-Crown-6 and Long Alkyl Chain ether of Polyethylene Glycol”. Org. Chem. 1986,5, 373-376. He, Bing-lin; Wang, Lin-fu; Chen, Wei-zhu T h e Amidophosphonation of Styrene-Divinylbenzene Copolymer Prepared by the Suspension Polymerization of Styrene and Divinylbenzene in the Presence of Alkanol as a Porogenic Agent. Polym. Commun. 1984,5, 339-345. Mackenzie, W. M.; Sherrington, D. C. Mechanism of Solid-Liquid Phase Transfer Catalysis by Polymer-Supported Linear Polyethers. Polymer 1980, 21, 791-797. Qian, T. B. Ion Exchange Resin and Application; Tianjin Science and Technology Press: Tianjin, 1984. Reaen, Steven, L. Triphase Catalysis. Angew. Chem., Int. Ed. Engl. i979, i8(6), 421-429. Reeen. ‘ , , Steven. L.: Dureadas. Bolikel Triahase Catalvsis Backbone Org. Chem. 1981, 46, Structure-Activity fielationships. 2511-2514. Regen, Steven L.; Jacques, J . Beese; Jerome, McLick Solid-Phase Cosolvents. Triphase Catalytic Hydrolysis of 1-Bromoadamantane. J . Am. Chem. Soc. 1979, 101, 116-120. Reichardt, C. Solvents Ejject in Organic Chemistry; Weiheim: New York, 1978. Sawicki, Rebert A. Phase Transfer Cataysts Polyethylene Glycols Immobilized Onto Metal Oxide Surfaces. Tetrahedron Lett. 1982, 23(22), 2249-2252. Tomoi, M.;Warren, T. Ford Mechanisms of Polymer-Supported Catalysis. 1. Reaction of 1-Bromo-octane with Aqueous Sodium Cyanide Catalyzed by Polystyrene-Bound Benzyltri-n-butyl. 103, 3821-3828. phosphonium Ion. J . Am. Chem. S O C1981, Tomoi, M.: Yasunori, Hosokawa; Hiroshi, Kakiuchi Phase-Transfer Reactions Catalyzed by Phosphonium Salts Bound to Macroporous Polystyrene Supports. J . Polym. Sci., Polym. Chem. E d . 1984, 22, 1243-1250. Tundo, Pietro Silica Gel as a Polymeric Support for PTC. J . Chem. Soc. Chem. Commun. 1977, 641-642. Yanagida, Shozo; Kazutomo, Takahashi; Mitsuo, Okahara SolidSolid-Liquid Three Phase Transfer Catalysis of Polymer-Bound Acyclic Poly(oxyethy1ene) Derivatives, Applications to Organic Synthesis. J . O g . Chem. 1979, 4 4 ( 7 ) , 1099-1103.
i.
Received for reuiew October 17, 1988 Accepted May 1, 1989
