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Alcoholysis of ester and epoxide catalyzed by solid bases
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
3507
Alcoholysis of ester and epoxide catalyzed by solid bases Hideshi Hattori, Masaomi Shima, and Hajime Kabashima Center for Advanced Research of Energy Technology, Hokkaido University Sapporo 060-8628, Japan Reactions of alcohols with ethyl acetate (transesterification) and propylene oxide were investigated by use of a variety of solid base catalysts to elucidate the activity determining factors of the catalyst in relation with the type of alcohol. Solid base catalysts examined were alkaline earth oxides, hydroxides, and carbonates, alumina supported KF and KOH, rare earth oxide, zirconium oxide, etc. Reaction rate of the alcoholysis of ethyl acetate varies with the combination of type of alcohol and basic strength of solid base catalyst. Over strongly basic catalysts such as CaO, SrO, BaO, 2-propanol reacted much faster than methanol. On the other hand, over weakly basic catalysts such as alkaline earth hydroxides, methanol reacted faster than 2-propanol. 2-Methyl-2-propanol reacted only over strongly basic catalysts, and much slower than methanol and 2-propanol. Alcoholysis with propylene oxide was catalyzed only by strongly basic catalysts such as alkaline earth oxides, and KF/alumina, alkaline earth hydroxides scarcely showed activity. )'-Alumina, however, showed a high activity, though the selectivity of products was different from those for alkaline earth oxides. Reactivities of alcohols with propylene oxide were in the order, methanol > ethanol > 2-propanol > 2-methyl2-propanol, regardless of the type of catalyst. One of the characteristic features observed for both alcoholyses is that the catalysts are tolerant to air exposure, which is caused by strong adsorptivity of alcohol competitive to that of carbon dioxide and water. I. INTRODUCTION Alcoholyses of ester and epoxide are important reactions for fine chemicals synthesis in chemical, food, pharmaceutical, and cosmetic industries[I-4]. These reactions are catalyzed by both acidic and basic catalysts. So far, the catalysts employed for the alcoholyses are mostly homogeneous catalysts, and heterogeneous catalysts have not been extensively studied. Application of solid base catalysts to a variety of organic syntheses has been recently expanded[5]. In the present study, solid base catalysts efficient for alcoholyses of ester and epoxide are searched for, and the activity determining factors are elucidated in relation with the type of alcohol. The reactions are as follows.
transesterification of ethyl acetate CH3COOCH2CH3 + ROH ~
CH3COOR + CH3CH2OH
3508
alcoholysis ofpropylene oxide CH3-CH-CH2 + ROH
\/
=- CH3-CH-CH2-O-R + CH3-CH-CH2-OH
O
i
i
OH
OR
2. EXPERIMENTAL 2.1. Catalysts Hydroxides and carbonates of Ca, Sr, and Ba were purchased from Kanto Chemicals Ind. Mg(OH)2 was purchased from E. Merck, and MgCO3 from Wako Pure Chemicals Ind. MgO, CaO, SrO, and BaO were prepared from Mg(OH)2, Ca(OH)2, SrCO3, and BaCO3, respectively, by thermal decomposition at elevated temperature under a vacuum. La203 was prepared from La(OH)3 which was prepared from am aqueous solution of La(NO3)3 by hydrolysis with an aqueous ammonia, followed by washing with water and drying at 373K. ZrO2 and ZnO were prepared from Zr(OH)4 and Zn(OH)2, respectively, by thermal decomposition at elevated temperatures under a vacuum. KF/alumina was purchased from Fluka Chemicals, and its KF content was determined to be 8.2 mmol/g by XRF. Alumina used for the catalyst and the support for KOH/alumina was supplied from the Catalysis Society of Japan (JRC-ALO4). KOK/alumina was prepared by impregnation of the alumina with an aqueous solution of KOH, the content of KOH was 3.6 mmol/g. 2.2. Reaction procedures Alcohols used in the present study were methanol, ethanol, 2-propanol, and 2-methyl2-propanol. Ethylacetate and propylene oxide were used as ester and epoxide, respectively. All the reactants were passed through 4A molecular sieves to remove water and carbon dioxide exclusively from the reactants. An H-shaped glass batch reactor was employed to carry out the reaction. Two branches of the reactor were separated by a breakable seal. A sample of the catalyst was placed in one branch, outgassed at an elevated temperature for 2 h, and sealed. A mixture of purified ethyl acetate (5 mmol) and alcohol (5 mmol) was stored in the other branch until it was introduced 2arough the breakable seal by distillation into the branch containing the catalyst thermost~tLed at liquid nitrogen temperature. Reaction was started by rapid melting the reactant mixture at a reaction temperature followed by stirring. In the reaction of propylene oxide with alcohol, a mixture containing 4 mmol propylene oxide and 12 mmol alcohol was allowed to react. The products were analyzed by GC with a DB-1 capillary column, MS, and GC-MS. 3. RESULTS AND DISCUSSION 3.1. Transesterufication of ethyl acetate with alcohols For transesterification of ethylacetate with methanol, high activities were observed for MgO, CaO, SrO, BaO, La203, KF/alumina, and KOH/alumina. Sr(OH)28H20 and Ba(OH)28H:O also exhibited high activities, but Mg(OH)2 and Ca(OH)2 scarcely showed activity. Alkaline earth carbonates, ZrO2, ZnO, and 7-alumina were inactive.
3509 50 For transesterification of I ethyl acetate with 2-propanol, O MgO MgO, CaO, SrO, BaO, [0 CaO 40 [A SrO KF/alumina, and [m BaO KOH/alumina exhibited high OH)2" 8H20 I ~ Sr(Ol =o 30 activities, but La203, OH)2 "8H20 | i~ i D Ba(O! Sr(OH)28H20, and [~ I v La203 La20 Ba(OH)28H20 showed only a [ I ~ KF/alumina KF/al ~o 20 C KOH )H/alumina .......... .3 low activity. Alkaline earth carbonates, ZrO2, ZnO, and ~,-alumina were inactive. For transesterification of ethyl acetate with 2-methylmethanol 2-propanol 2-propanol, alkaline earth Type of alcohol oxides, KF/alumina, and Fig. 1 Reactivity of alcohol for transesterification. KOH/alumina showed Cat. weight, 100rag for MgO, CaO, SrO, BaO, La203, activities which were one KF/alumina, and KOH/alumina; 248mg for Sr(OH)2" order of magnitude smaller than those with methanol on 8H20; 206mg for Ba(OH)2"8H20; the same catalysts. The Ethyl acetate, 5 retool; Alcohol, 5 mmol; other catalysts showed no Reaction temperature, 273 K; Reaction time, 30 min activity. In Fig. 1 are shown the transesterification conversions plotted against the type of alcohols (methanol and 2-propanol) for different catalysts. The catalysts can be classified into two groups. The conversion is higher for methanol than for 2-propanol for one group, and the opposite tendency for the other one. The former group includes BaO, SrO, and CaO. The latter group includes MgO, Sr(OH)28H20, Ba(OH)28H20, La203, KF/alumna, and KOH/alumina. The basic strengths of alkaline earth oxides are in the order; BaO > SrO > CaO > MgO. The alkaline earth oxides are much stronger than the corresponding hydroxides in basicity. It seems that 2-propanol reacts with ethyl acetate faster than metanol over strong solid base catalysts, and that methanol reacts faster over relatively weak solid base catalysts. The same tendency of the reactivity of methanol and 2-propanol was observed for cyanoethylation of alcohols with acrylonitrile[6]. Methanol undergoes cyanoethylation faster than 2-propanol over Mg(OH)2 and Ca(OH)2, while the opposite was observed for MgO, CaO, SrO, and BaO. It was interpreted that the abstraction of an H + from alcohol to form alkoxide anion is slow step for a weak solid base catalyst, and the reaction of the alkoxide with acrylonitrile is a slow step for a strong solid base catalyst. Transesterification and cyanoethylation have a common mechanistic feature that the first step is the abstraction of an H + from alcohol by basic site to form alkoxide anion. The mechanism for transesterification is illustrated in Scheme 1. One of the possible interpretations for the difference in the reactivity of methanol and 2-propanol between a strong solid base catalyst and a weak solid base catalyst is as follows. Over a weak solid base catalyst, the abstraction of an H + from alcohol is slow step. Methanol is easier to release an H + than 2-propanol, and therefore, methanol reacts faster than 2-propanol. On the other hand, over a strong solid base catalyst, the o~
3510 abstraction of an H § from Step 1-A alcohol is easy even for 2R-OH R propanol. The following step that the alkoxide anion attacks the adsorbed ethyl . 7/ o // --//,",///PT/ acetate becomes slow step. Step 1-B (a) The other possible CH3-CH2-O-C-CH3 9 CH3-CHz-O-C-CH3 II I interpretation is as follows. O O For all catalysts, the slow step is the attack of alkoxide //,.,/, ~ //,./, anion to the adsorbed ethyl Step 2 CH3 (b) GIla acetate. Over a weak solid Ie I CH3-CH2-O-C R CH3-CHz-O-C-O-R base catalyst, the surface concentration of methoxide is higher than that of 2% ,M // ,/~.7/,o,~, //.~, //,.~/,o// propoxide because the acidity (b) (a) (b) (a) of methanol is higher than Step 3 that of 2-propanol, which ^ CHa results in a higher rate of the CH3-CH2-O~!IC-O-R CH3-C-O-R ~-O O '-~1 He CH3-CH2-O~ + II attack of methoxide to the H~ O adsorbed ethyl acetate. / / , " / //,",/~PT"/ --//,"/, //,",/~PT"/ Over a strong solid base (b) (a) (b) (a) catalyst, the surface Step 4 concentrations of methoxide CH3-CH2-O O He CH3-CH2-OH and 2-propoxide anions are not different because a strong ,// . 7 / 6,, / _-//.//~O,// basic site can abstract an H § from a weak acid like 2Scheme 1 Reaction route oftransesterification of propanol. The stability of 2ethyl acetate with alcohol. propoxide anion is less than that of methoxide anion, which results in the high reactivity of the anion toward the adsorbed ethyl acetate. It is not certain as to whether the low reactivity of 2-methyl-2-propanol as compared to methanol and 2-propanol arises from the difficulty of the abstraction of an H § or the steric hindrance of the alkoxide anion caused by bulky tertiary butoxide for attacking the adsorbed ethyl acetate. +
+
. .
.
.
.
_
.
.
.
.
-
_ -
+
3.2. Alcoholysis of propylene oxide A plausible reaction mechanism for alcoholysis of propylene oxide is shown in Scheme 2. In Fig. 2 are shown the conversions of propyrene oxide plotted against the type of alcohol for different catalysts. Reactivities of alcohol are in the order, methanol > ethanol > 2-propanol for all catalysts except for SrO. With SrO, 2-propanol reacted slightly faster than ethanol. 2-Methyl-2-propanol did not react with propylene oxide
3511 over any catalysts examined. The order of the reactivities is consistent with the order of acidities of alcohols. There seem to be two possible interpretation for the reactivities of alcohol with propylene oxide like those described above for the interpretation for the reactivities of alcohols with ethyl acetate over a weak solid base catalyst. As for the catalysts not shown in Fig. 2, Sr(OH)2, Ba(OH)2, and La203 exhibited considerable activities only for methanolysis, but scarcely showed the activities for alcoholysis with other alcohols. It is to be noted that alumina exhibited the highest conversion for the alcoholysis with methanol and ethanol, though alumina showed the activity between BaO and CaO for the alcoholysis with 2-propanol. The feature that distinguishes alumina from other catalysts is that the selectivity for the 2alkoxy- 1-propanol was relatively high. The difference in the selectivity suggests that the reaction mechanisms are different for alumina and the other catalysts. However, details are not certain at present.
Step 1-A R-OH
R
;o
+
/ / , M / / , ? ,/~ ~
,/, y //.6. / /
Step 1-B CH3-CH-CH2
(a)
\O1
CH3-CH-C~2 'o'
+
Step 2
//,, ~ "/
'
-- //,, ~ //
H2
(b)
CH3_CH_~e Ri \O / ~-_.__ OG)
(b) Step 3 Hz I
H2i CH3-CH-C-O-R ;|
He
(a)
CH3-CH-C-O-R ;G)
(b)
He
(a)
CH3-CH-CH2-O-R OH I
He
--
l/M~ (b)
_He
-
4-
/ / , u , / / p/ / (a)
_~
Mp ,/ / M/ / o ~, (b) (a)
Scheme 2 Reaction route for alcoholysis of propylene oxide.
10o-------~
I 80
..
I
~
9~
~
1
O MgO 9 CaO 9 SrO
El2,,
~ ~
9 BaO KF/alumina | KOH/alumina | alumina J
= 60 = o 40
"~ | |
9
20 o
,
~~-
methanol
ethanol
,~
2-propanol
Type of alcohol Fig. 2 Reactivity of alcohol for alcoholysis of propylene oxide. Cat. weight, 100 rag; Propylene oxide, 4 mmol; Alcohol, 12 mmol; Reaction temperature, 323 K; Reaction time, 120 rain
3512 3.3. Tolerance of catalyst to air-exposure One of the characteristic features of alcoholysis over solid base catalysts is that the catalysts are tolerant to air exposure. Solid base catalysts are very sensitive to carbon dioxide and water, and easily poisoned by exposure to air for most of the reactions. However, for alcoholysis, solid base catalysts retain their activity even after exposure to air though a short induction period is observed. To elucidate the tolerance of the catalyst to air exposure, co-adsorption of methanol and carbon dioxide on MgO was studied by IR and TPD. In Fig. 3 are shown IR spectra of adsorbed methanol ~~ CH30~ I 1.0 on two kinds of MgO bidentate CO2 l I surfaces. One MgO was 9 ~ A oAA c) , , ~A un,dentate CO2J pretreated at 1073K in a 0.8 vacuum, and the other was ,~ exposed to carbon dioxide at 0.6 298K after pretreatment at 1073K. Methanol is able to be adsorbed to form 0.4 methoxide on the MgO < surface which preadsorbed 0.2 carbon dioxide. Adsorption of methanol on the carbon dioxide-exposed surface of 0 3000 2000 1200 4000 MgO was also observed by Wave numbers / cm-~ TPD. Strong adsorption of Fig. 3 IR spectra of methanol adsorbed on MgO. alcohols in the presence of a) evacuation at 1073 K for 120 min carbon dioxide suggests b) CO2 adsorption at 1 Torr for 30 min potential use of solid base c) CO2 adsorption at 1 Torr for 30 min followed catalysts in practical by methanol adsorption at 1 Torr for 30 min processes in which alcohols are involved.
I
REFERENCES
1. Y. Ono, Catal. Today, 35 (1997) 15. 2. Z. Fu and Y. Ono, J. Mol. Catal., 118 (1997) 293. 3. J. B. Lauridsen, J. Am. Oil Chem. Soc., 53 (1976) 293. 4. E. Jungerman, Cosm. Sci. Tech. Serv., 11 (1991) 97. 5. H. Hattori, Chem. Rev. 95 (1995) 537. 6. H. Kabashima and H. Hattori, Catal. Today, 44 (1998) 277.
3507
Alcoholysis of ester and epoxide catalyzed by solid bases Hideshi Hattori, Masaomi Shima, and Hajime Kabashima Center for Advanced Research of Energy Technology, Hokkaido University Sapporo 060-8628, Japan Reactions of alcohols with ethyl acetate (transesterification) and propylene oxide were investigated by use of a variety of solid base catalysts to elucidate the activity determining factors of the catalyst in relation with the type of alcohol. Solid base catalysts examined were alkaline earth oxides, hydroxides, and carbonates, alumina supported KF and KOH, rare earth oxide, zirconium oxide, etc. Reaction rate of the alcoholysis of ethyl acetate varies with the combination of type of alcohol and basic strength of solid base catalyst. Over strongly basic catalysts such as CaO, SrO, BaO, 2-propanol reacted much faster than methanol. On the other hand, over weakly basic catalysts such as alkaline earth hydroxides, methanol reacted faster than 2-propanol. 2-Methyl-2-propanol reacted only over strongly basic catalysts, and much slower than methanol and 2-propanol. Alcoholysis with propylene oxide was catalyzed only by strongly basic catalysts such as alkaline earth oxides, and KF/alumina, alkaline earth hydroxides scarcely showed activity. )'-Alumina, however, showed a high activity, though the selectivity of products was different from those for alkaline earth oxides. Reactivities of alcohols with propylene oxide were in the order, methanol > ethanol > 2-propanol > 2-methyl2-propanol, regardless of the type of catalyst. One of the characteristic features observed for both alcoholyses is that the catalysts are tolerant to air exposure, which is caused by strong adsorptivity of alcohol competitive to that of carbon dioxide and water. I. INTRODUCTION Alcoholyses of ester and epoxide are important reactions for fine chemicals synthesis in chemical, food, pharmaceutical, and cosmetic industries[I-4]. These reactions are catalyzed by both acidic and basic catalysts. So far, the catalysts employed for the alcoholyses are mostly homogeneous catalysts, and heterogeneous catalysts have not been extensively studied. Application of solid base catalysts to a variety of organic syntheses has been recently expanded[5]. In the present study, solid base catalysts efficient for alcoholyses of ester and epoxide are searched for, and the activity determining factors are elucidated in relation with the type of alcohol. The reactions are as follows.
transesterification of ethyl acetate CH3COOCH2CH3 + ROH ~
CH3COOR + CH3CH2OH
3508
alcoholysis ofpropylene oxide CH3-CH-CH2 + ROH
\/
=- CH3-CH-CH2-O-R + CH3-CH-CH2-OH
O
i
i
OH
OR
2. EXPERIMENTAL 2.1. Catalysts Hydroxides and carbonates of Ca, Sr, and Ba were purchased from Kanto Chemicals Ind. Mg(OH)2 was purchased from E. Merck, and MgCO3 from Wako Pure Chemicals Ind. MgO, CaO, SrO, and BaO were prepared from Mg(OH)2, Ca(OH)2, SrCO3, and BaCO3, respectively, by thermal decomposition at elevated temperature under a vacuum. La203 was prepared from La(OH)3 which was prepared from am aqueous solution of La(NO3)3 by hydrolysis with an aqueous ammonia, followed by washing with water and drying at 373K. ZrO2 and ZnO were prepared from Zr(OH)4 and Zn(OH)2, respectively, by thermal decomposition at elevated temperatures under a vacuum. KF/alumina was purchased from Fluka Chemicals, and its KF content was determined to be 8.2 mmol/g by XRF. Alumina used for the catalyst and the support for KOH/alumina was supplied from the Catalysis Society of Japan (JRC-ALO4). KOK/alumina was prepared by impregnation of the alumina with an aqueous solution of KOH, the content of KOH was 3.6 mmol/g. 2.2. Reaction procedures Alcohols used in the present study were methanol, ethanol, 2-propanol, and 2-methyl2-propanol. Ethylacetate and propylene oxide were used as ester and epoxide, respectively. All the reactants were passed through 4A molecular sieves to remove water and carbon dioxide exclusively from the reactants. An H-shaped glass batch reactor was employed to carry out the reaction. Two branches of the reactor were separated by a breakable seal. A sample of the catalyst was placed in one branch, outgassed at an elevated temperature for 2 h, and sealed. A mixture of purified ethyl acetate (5 mmol) and alcohol (5 mmol) was stored in the other branch until it was introduced 2arough the breakable seal by distillation into the branch containing the catalyst thermost~tLed at liquid nitrogen temperature. Reaction was started by rapid melting the reactant mixture at a reaction temperature followed by stirring. In the reaction of propylene oxide with alcohol, a mixture containing 4 mmol propylene oxide and 12 mmol alcohol was allowed to react. The products were analyzed by GC with a DB-1 capillary column, MS, and GC-MS. 3. RESULTS AND DISCUSSION 3.1. Transesterufication of ethyl acetate with alcohols For transesterification of ethylacetate with methanol, high activities were observed for MgO, CaO, SrO, BaO, La203, KF/alumina, and KOH/alumina. Sr(OH)28H20 and Ba(OH)28H:O also exhibited high activities, but Mg(OH)2 and Ca(OH)2 scarcely showed activity. Alkaline earth carbonates, ZrO2, ZnO, and 7-alumina were inactive.
3509 50 For transesterification of I ethyl acetate with 2-propanol, O MgO MgO, CaO, SrO, BaO, [0 CaO 40 [A SrO KF/alumina, and [m BaO KOH/alumina exhibited high OH)2" 8H20 I ~ Sr(Ol =o 30 activities, but La203, OH)2 "8H20 | i~ i D Ba(O! Sr(OH)28H20, and [~ I v La203 La20 Ba(OH)28H20 showed only a [ I ~ KF/alumina KF/al ~o 20 C KOH )H/alumina .......... .3 low activity. Alkaline earth carbonates, ZrO2, ZnO, and ~,-alumina were inactive. For transesterification of ethyl acetate with 2-methylmethanol 2-propanol 2-propanol, alkaline earth Type of alcohol oxides, KF/alumina, and Fig. 1 Reactivity of alcohol for transesterification. KOH/alumina showed Cat. weight, 100rag for MgO, CaO, SrO, BaO, La203, activities which were one KF/alumina, and KOH/alumina; 248mg for Sr(OH)2" order of magnitude smaller than those with methanol on 8H20; 206mg for Ba(OH)2"8H20; the same catalysts. The Ethyl acetate, 5 retool; Alcohol, 5 mmol; other catalysts showed no Reaction temperature, 273 K; Reaction time, 30 min activity. In Fig. 1 are shown the transesterification conversions plotted against the type of alcohols (methanol and 2-propanol) for different catalysts. The catalysts can be classified into two groups. The conversion is higher for methanol than for 2-propanol for one group, and the opposite tendency for the other one. The former group includes BaO, SrO, and CaO. The latter group includes MgO, Sr(OH)28H20, Ba(OH)28H20, La203, KF/alumna, and KOH/alumina. The basic strengths of alkaline earth oxides are in the order; BaO > SrO > CaO > MgO. The alkaline earth oxides are much stronger than the corresponding hydroxides in basicity. It seems that 2-propanol reacts with ethyl acetate faster than metanol over strong solid base catalysts, and that methanol reacts faster over relatively weak solid base catalysts. The same tendency of the reactivity of methanol and 2-propanol was observed for cyanoethylation of alcohols with acrylonitrile[6]. Methanol undergoes cyanoethylation faster than 2-propanol over Mg(OH)2 and Ca(OH)2, while the opposite was observed for MgO, CaO, SrO, and BaO. It was interpreted that the abstraction of an H + from alcohol to form alkoxide anion is slow step for a weak solid base catalyst, and the reaction of the alkoxide with acrylonitrile is a slow step for a strong solid base catalyst. Transesterification and cyanoethylation have a common mechanistic feature that the first step is the abstraction of an H + from alcohol by basic site to form alkoxide anion. The mechanism for transesterification is illustrated in Scheme 1. One of the possible interpretations for the difference in the reactivity of methanol and 2-propanol between a strong solid base catalyst and a weak solid base catalyst is as follows. Over a weak solid base catalyst, the abstraction of an H + from alcohol is slow step. Methanol is easier to release an H + than 2-propanol, and therefore, methanol reacts faster than 2-propanol. On the other hand, over a strong solid base catalyst, the o~
3510 abstraction of an H § from Step 1-A alcohol is easy even for 2R-OH R propanol. The following step that the alkoxide anion attacks the adsorbed ethyl . 7/ o // --//,",///PT/ acetate becomes slow step. Step 1-B (a) The other possible CH3-CH2-O-C-CH3 9 CH3-CHz-O-C-CH3 II I interpretation is as follows. O O For all catalysts, the slow step is the attack of alkoxide //,.,/, ~ //,./, anion to the adsorbed ethyl Step 2 CH3 (b) GIla acetate. Over a weak solid Ie I CH3-CH2-O-C R CH3-CHz-O-C-O-R base catalyst, the surface concentration of methoxide is higher than that of 2% ,M // ,/~.7/,o,~, //.~, //,.~/,o// propoxide because the acidity (b) (a) (b) (a) of methanol is higher than Step 3 that of 2-propanol, which ^ CHa results in a higher rate of the CH3-CH2-O~!IC-O-R CH3-C-O-R ~-O O '-~1 He CH3-CH2-O~ + II attack of methoxide to the H~ O adsorbed ethyl acetate. / / , " / //,",/~PT"/ --//,"/, //,",/~PT"/ Over a strong solid base (b) (a) (b) (a) catalyst, the surface Step 4 concentrations of methoxide CH3-CH2-O O He CH3-CH2-OH and 2-propoxide anions are not different because a strong ,// . 7 / 6,, / _-//.//~O,// basic site can abstract an H § from a weak acid like 2Scheme 1 Reaction route oftransesterification of propanol. The stability of 2ethyl acetate with alcohol. propoxide anion is less than that of methoxide anion, which results in the high reactivity of the anion toward the adsorbed ethyl acetate. It is not certain as to whether the low reactivity of 2-methyl-2-propanol as compared to methanol and 2-propanol arises from the difficulty of the abstraction of an H § or the steric hindrance of the alkoxide anion caused by bulky tertiary butoxide for attacking the adsorbed ethyl acetate. +
+
. .
.
.
.
_
.
.
.
.
-
_ -
+
3.2. Alcoholysis of propylene oxide A plausible reaction mechanism for alcoholysis of propylene oxide is shown in Scheme 2. In Fig. 2 are shown the conversions of propyrene oxide plotted against the type of alcohol for different catalysts. Reactivities of alcohol are in the order, methanol > ethanol > 2-propanol for all catalysts except for SrO. With SrO, 2-propanol reacted slightly faster than ethanol. 2-Methyl-2-propanol did not react with propylene oxide
3511 over any catalysts examined. The order of the reactivities is consistent with the order of acidities of alcohols. There seem to be two possible interpretation for the reactivities of alcohol with propylene oxide like those described above for the interpretation for the reactivities of alcohols with ethyl acetate over a weak solid base catalyst. As for the catalysts not shown in Fig. 2, Sr(OH)2, Ba(OH)2, and La203 exhibited considerable activities only for methanolysis, but scarcely showed the activities for alcoholysis with other alcohols. It is to be noted that alumina exhibited the highest conversion for the alcoholysis with methanol and ethanol, though alumina showed the activity between BaO and CaO for the alcoholysis with 2-propanol. The feature that distinguishes alumina from other catalysts is that the selectivity for the 2alkoxy- 1-propanol was relatively high. The difference in the selectivity suggests that the reaction mechanisms are different for alumina and the other catalysts. However, details are not certain at present.
Step 1-A R-OH
R
;o
+
/ / , M / / , ? ,/~ ~
,/, y //.6. / /
Step 1-B CH3-CH-CH2
(a)
\O1
CH3-CH-C~2 'o'
+
Step 2
//,, ~ "/
'
-- //,, ~ //
H2
(b)
CH3_CH_~e Ri \O / ~-_.__ OG)
(b) Step 3 Hz I
H2i CH3-CH-C-O-R ;|
He
(a)
CH3-CH-C-O-R ;G)
(b)
He
(a)
CH3-CH-CH2-O-R OH I
He
--
l/M~ (b)
_He
-
4-
/ / , u , / / p/ / (a)
_~
Mp ,/ / M/ / o ~, (b) (a)
Scheme 2 Reaction route for alcoholysis of propylene oxide.
10o-------~
I 80
..
I
~
9~
~
1
O MgO 9 CaO 9 SrO
El2,,
~ ~
9 BaO KF/alumina | KOH/alumina | alumina J
= 60 = o 40
"~ | |
9
20 o
,
~~-
methanol
ethanol
,~
2-propanol
Type of alcohol Fig. 2 Reactivity of alcohol for alcoholysis of propylene oxide. Cat. weight, 100 rag; Propylene oxide, 4 mmol; Alcohol, 12 mmol; Reaction temperature, 323 K; Reaction time, 120 rain
3512 3.3. Tolerance of catalyst to air-exposure One of the characteristic features of alcoholysis over solid base catalysts is that the catalysts are tolerant to air exposure. Solid base catalysts are very sensitive to carbon dioxide and water, and easily poisoned by exposure to air for most of the reactions. However, for alcoholysis, solid base catalysts retain their activity even after exposure to air though a short induction period is observed. To elucidate the tolerance of the catalyst to air exposure, co-adsorption of methanol and carbon dioxide on MgO was studied by IR and TPD. In Fig. 3 are shown IR spectra of adsorbed methanol ~~ CH30~ I 1.0 on two kinds of MgO bidentate CO2 l I surfaces. One MgO was 9 ~ A oAA c) , , ~A un,dentate CO2J pretreated at 1073K in a 0.8 vacuum, and the other was ,~ exposed to carbon dioxide at 0.6 298K after pretreatment at 1073K. Methanol is able to be adsorbed to form 0.4 methoxide on the MgO < surface which preadsorbed 0.2 carbon dioxide. Adsorption of methanol on the carbon dioxide-exposed surface of 0 3000 2000 1200 4000 MgO was also observed by Wave numbers / cm-~ TPD. Strong adsorption of Fig. 3 IR spectra of methanol adsorbed on MgO. alcohols in the presence of a) evacuation at 1073 K for 120 min carbon dioxide suggests b) CO2 adsorption at 1 Torr for 30 min potential use of solid base c) CO2 adsorption at 1 Torr for 30 min followed catalysts in practical by methanol adsorption at 1 Torr for 30 min processes in which alcohols are involved.
I
REFERENCES
1. Y. Ono, Catal. Today, 35 (1997) 15. 2. Z. Fu and Y. Ono, J. Mol. Catal., 118 (1997) 293. 3. J. B. Lauridsen, J. Am. Oil Chem. Soc., 53 (1976) 293. 4. E. Jungerman, Cosm. Sci. Tech. Serv., 11 (1991) 97. 5. H. Hattori, Chem. Rev. 95 (1995) 537. 6. H. Kabashima and H. Hattori, Catal. Today, 44 (1998) 277.