Rare Metals 2008,(01),89-94
Catalytic synthesis of acetals and ketals with H3PW12O40/PAn
Philippe G.Merleb
Department of Chemistry and Biochemistry Concordia University,Montreal H4B 1R6,Canada,
作者简介：YANG Shuijin E-mail:yangshuijin@163.com;
收稿日期：28 September 2006
Catalytic synthesis of acetals and ketals with H3PW12O40/PAn
Abstract：
A new environmental friendly catalyst H3PW12O40/PAn was prepared and identified by means of FT-IR,XRD,and TG/DTA.The optimum conditions have been found;that is,the mass ratio of PAn to H3PW12O40 is 1:1.5,the volume of methanol is 20 mL,and the reflux reaction time is 3 h.The structural identity of Keggin units is preserved after the incorporation into polyaniline matrix.Catalytic activities of H3PW12O40/PAn in synthesizing 2-methyl-2-ethoxycarbonylmethyl-1,3-dioxolane,2,4-dimethyl-2-ethoxycarbonylmethyl-1,3-dioxolane,cyclohexanone ethylene ketal,cyclohexanone 1,2-propanediol ketal,butanone ethylene ketal,butanone 1,2-propanediol ketal,2-phenyl-1,3-dioxolane,4-methyl-2-phenyl-1,3-dioxolane,2-propyl-1,3-dioxolane,and 4-methyl-2-propyl-1,3-dioxolane were reported.It has been demon-strated that H3PW12O40/PAn is an excellent catalyst.Various factors concerned in these reactions were investigated.The optimum conditions are as follows:the molar ratio of aldehyde/ketone to glycol(r) is 1:1.5,the mass ratio of the catalyst used to the reactants is 0.6%,and the reaction time is 1.0 h.Under these conditions,the yield is as follows:2-methyl-2-ethoxycarbonylmethyl-1,3-dioxolane,69.0%;2,4-dimethyl-2-ethoxycarbonylmethyl-1,3-dioxolane,79.5%;cyclohexanone ethylene ketal,78.9%;cyclohexanone 1,2-propanediol ketal,85.3%;butanone ethylene ketal,56.9%;butanone 1,2-propanediol ketal,78.1%;2-phenyl-1,3-dioxolane,76.3%;4-methyl-2-phenyl-1,3-dioxolane,94.2%;2-propyl-1,3-dioxolane,70.7%;and 4-methyl-2-propyl-1,3-dioxolane,79.2%.
Keyword：
phosphotungstic acid;acetal;ketal;polyaniline;catalysis;
Received： 28 September 2006
1. Introduction
There has been considerable interest in synthesizing acetals and ketals as important perfume substance and industrial raw materials of organic synthesis[1].The concentrated sulfuric acid,which has been used as catalysts in these industrial processes for many years,exposed their defects,such as the erosion of plant unit and low yield of the products.The alternative catalysts,for example,Lewis acid[2],molecular sieve,Cd I2[3],and solid superacid[4],have been developed for environmental protection and techno-economical profit.Late published reports have reported that the732-type ion exchange resin[5],NH4Fe(SO4)2·12H2O[6],and HY-type molecular sieve[7]have excellent catalytic activity to 4-methyl-2-phenyl-1,3-dioxolane.Heteropolyacids(HPA)of Keggin-type have been extensively studied,mainly because of their interesting catalytic properties[8-12].It has been shown that the catalytic activity of acidic centers of heteropolyacids was two orders of magnitude larger than that of mineral acids or aluminosilicate.The reportshave pointed out that the rate of dehydration and etherification is directly in proportion to the total acidity of heteropolyacids.HPA and their salts show excellent catalytic activity to the esterification and have recently attracted much attention as catalysts for various industrial processes,because their acidic and redox properties can be controlled at atomic/molecular levels.Heteropolyacids dispersed on the supports were applied in the industrial preparation.Ti O2 and Si O2 were used most frequently as the supports.Unfortunately,heteropolyacids were weakly bonded with such a type of support and could easily be removed from it.On the other hand,it has been shown that heteropolyacids can be incorporated into conjugated polymers and strongly bonded(ionic bond)with the polymeric matrices.Significant research effort has been directed toward the entrapment of heteropolyanions in suitable polymeric matrices with the main goal of preparing a new type of polymer-supported catalyst for various applications in heterogeneous and electrocatalysis.The molecular dispersion of heteropolyanions in conjugated polymer matrices significantly increased theircatalytic activity and changed catalytic selectivity in comparison to unsupported crystalline heteropolyacids.One can expect that the incorporation of heteropolyacids into a matrix of high thermal stability may significantly increase the stability of such a catalytic system.
Polyaniline(noted PAn)is another example of a polymer host which can accommodate anions originating from12-phosphotungstic acid.The polyaniline was prepared by condensation-polymerization of aniline in aqueous HCl using(NH4)2S2O8 as the oxidant.The polyaniline is a wellknown polymer of high thermal stability.H3PW12O40/PAn can be achieved by doping the protonation of the polyaniline with H3PW12O40 in a two-step procedure.However,there is no report on the synthesis of acetals and ketals catalyzed by H3PW12O40/PAn.In this article,the synthesis of acetals and ketals catalyzed by H3PW12O40/PAn is studied with aldehyde/ketone and glycol as reactants,and the reaction conditions are examined.The study of the doping of polyaniline with Keggin-type phosphotungstic acid is presented.IR,TG/DTA,and X-ray diffraction techniques are used to characterize the nature of the catalyst.
2. Experimental
2.1. Materials and measurements
Ethyl acetoacetate,cyclohexanone,butanone,benzaldehyde,butyraldehyde,ethylene glycol,1,2-propanediol,cyclohexane,methanol,(NH4)2S2O8,and aniline were of analytical reagent grade.H3PW12O40 was synthesized according to Ref.[10]and examined by IR(infrared)spectra.The IR spectra were obtained using a Nicolet 5DX IR spectrometer(Nicolet,USA).The X-ray powder diffraction pattern of the powder sample was measured by an XRD-6000X-ray diffractometer(Rigaku,Japan)using Cu Kαradiation.The data were collected over the diffraction angle ranging from 5°to 65°in 2θwith a step scanning.The accelerating voltage and the applied current were 40 k V and 30 m A,respectively.TG and DTA analyst were carried out with a Pyris Diamond TG/DTA thermogravimeter(Perkin-elmer,USA)both in dynamic nitrogen atmosphere and at a heating rate of 10°C?min-1.1HNMR spectra were determined on a Mercury-VX 300 MHz spectrometer with tetramethylsilane(TMS)as internal standard in CDCl3.
2.2. Preparation of H3PW12O40/PAn
The synthesis of polyaniline was similar to Ref.[13]by condensation-polymerization of freshly distilled aniline in aqueous HCl using(NH4)2S2O8 as the oxidant.The color of solid changed to black-green.A method of the orthogonal experiments was designed,and m(PAn):m(H3PW12O40),volume of methanol,and reflux reaction time were chosenas the factors of the orthogonal experiments.The factors,levels,and results of the orthogonal experiments are shown in Tables 1 and 2.
Table 1.Factors and levels of L9(33)orthogonal experiments 下载原图
Note:A—m(PAn):m(H3PW12O40);B—volume of methanol,mL;C—reaction time,h.
Table 1.Factors and levels of L9(33)orthogonal experiments
Table 2.Results of L9(33)orthogonal experiments 下载原图
Note:A—m(PAn):m(H3PW12O40);B—volume of methanol,mL;C—reaction time,h.
Table 2.Results of L9(33)orthogonal experiments
The solution of H3PW12O40 was prepared by dissolving H3PW12O40 in methanol solution.Then solid PAn was stoichiometrically dropped into the above-mentioned solution under vigorous stirring,and the resulting solution was refluxed.Cyclohexanone 1,2-propanediol ketal was synthesized from cyclohexanone and 1,2-propanediol as probing reaction in the presence of H3PW12O40/PAn.The optimum conditions of probing reaction are as follows:the molar ratio of cyclohexanone to 1,2-propanediol is 1:1.4,the quantity of catalyst is equal to 1.6%of feed stocks,and the reaction time is 50 min.Under these conditions,the yield of cyclohexanone 1,2-propanediol ketal can reach over 83.9%.
Table 2 shows that the most important factor in the preparation of H3PW12O40/PAn is found to be the reflux re-action time.The other factors are in order of the mass ratio of PAn to H3PW12O40 and the volume of methanol.Analytical results indicate the optimum composing as A3B1C3.Anew environmental friendly catalyst,H3PW12O40/PAn was prepared.The optimum conditions have been found;that is,the mass ratio of PAn to H3PW12O40 is 1:1.5,the volume of methanol is 20 m L,and the reflux reaction time is 3 h.Finally,the catalysts were stored in a desiccator until use.H3PW12O40,PAn,and H3PW12O40/PAn were identified by means of FT-IR,XRD,and TG/DTA.
2.3. Synthesis of acetals and ketals
The reaction was carried out in a three-neck flask equipped with a stirrer,a reflux condenser,and a thermometer.A certain amount of ethyl acetoacetate,cyclohexanone,butanone,benzaldehyde,and butyraldehyde were added separately with alcohol(ethylene glycol or 1,2-propylene glycol)and the catalyst.The solution was heated to boil and refluxed until no water flowed off.The resultant solution was distilled and purified.
3. Results and discussion
3.1. Characterization of catalyst H3PW12O40/PAn
3.1.1. IR spectra
From the features observed in the infrared spectra of solid samples,the characteristic peaks of PAn were explored at1580,1488,1389,1303,1245,1148 cm-1,and H3PW12O40at 1078,985,891,767 cm-1.The infrared spectrum of H3PW12O40/PAn suggested the bands at 1558;1485;1302;1151;1079;982;888;and 800 cm-1 to be assigned to the stretching vibrations of quinoid ring(N=Q=N);benzene ring(N-B-N);QBQ(Q:quinoid ring,B:benzene ring),QBB,and BBB;Q=NH+B;P-Oa;W-Od;W-Ob-W;andW-Oc-W bond,respectively.The characteristic band of PAn is N=Q=N stretching band at 1580 cm-1.The IR spectra will change after PAn is doped with heteropoly acid:(1)1580 and 1488 cm-1 band shifts slightly to the lower frequency(by 22 and 3 cm-1,respectively);(2)a band at 1151cm-1,characteristic of the protonated PAn states,appears;(3)the relative intensity of 1389 cm-1 descends and that of 1302cm-1 increases;(4)the bands in the range 500-1100 cm-1corresponding the characteristic vibrations of the heteropoly acid appears.Thus,the molecule of heteropoly acid has gone into the molecule of PAn.The IR frequencies including the assignments of H3PW12O40,PAn,and H3PW12O40/PAn are listed in Table 3.
3.1.2. XRD spectra
X-ray powder diffraction analysis is used widely as a method to study the structure of heteropoly complexes.The wide diffusive peak at 18°-32°for PAn in the X-ray diffraction pattern verifies its amorphous nature.The characteristic diffraction peaks of H3PW12O40/PAn explored at the range of 7°-11°,18°-22°,26°-31°,and 34°-37°are similar to those of H3PW12O40,and the wide diffusive peak at 18°-32°for PAn in the X-ray diffraction pattern descends.Thus,it can be proposed that the structure of H3PW12O40/PAn would be similar to that of H3PW12O40 with Keggin structure,and the molecule of H3PW12O40 has gone into the molecule of PAn.
3.1.3. Thermal behavior studies
TG/DTA data of PAn,H3PW12O40,and H3PW12O40/PAn are shown in Table 4.
It can be seen that the thermal behavior of H3PW12O40/PAn is similar to that of PAn.Thus,it can be proposed that the molecule of H3PW12O40 has gone into the molecule of PAn.
Table 3.Assignments of IR spectra of H3PW12O40,PAn,and H3PW12O40/PAn 下载原图
Table 3.Assignments of IR spectra of H3PW12O40,PAn,and H3PW12O40/PAn
Table 4.TG/DTA data of H3PW12O40/PAn 下载原图
Table 4.TG/DTA data of H3PW12O40/PAn
3.2. Optimizing of the reaction condition
For example,4-methyl-2-phenyl-1,3-dioxolane was synthesized,the effects of operation parameters,such as the molar ratio of benzaldehyde to 1,2-propanediol,the dosage of catalyst,and the reaction time on the yield were measured and analyzed to optimize these operation conditions.
3.2.1. Effect of the molar ratio of benzaldehyde to1,2-propanediol on the yield
Table 5 shows the effect of the molar ratio of benzaldehyde to 1,2-propanediol on the yield.The molar ratio of benzaldehyde to 1,2-propanediol(r)will directly influence the yield.
Table 5.Effect of molar ratio of benzaldehyde to1,2-propanediol(r)on the yield 下载原图
Note:benzaldehyde,0.2 mol;mass ratio of catalyst to reactants,0.8%;cyclohexane,8 m L;reaction time,60 min.
Table 5.Effect of molar ratio of benzaldehyde to1,2-propanediol(r)on the yield
The yield increases with an increase in molar ratio of benzaldehyde to 1,2-propanediol.When the ratio is 1:1.5,the yield is the highest.Further increasing the molar ratio of benzaldehyde to 1,2-propanediol will cause a lower yield because of the lowering of temperature.Thus the optimum molar ratio of benzaldehyde to 1,2-propanediol is 1:1.5.
3.2.2. Effect of catalyst dosage on the yield
The effect of catalyst dosage on the yield is shown in Table 6.
Table 6.Effect of catalyst dosage on the yield 下载原图
Note:benzaldehyde,0.2 mol;n(benzaldehyde):n(1,2-propanediol)=1.0:1.5;cyclohexane,8 m L;reaction time,60 min.
Table 6.Effect of catalyst dosage on the yield
According to Table 6,the yield increases when the catalyst concentration increases.When the mass ratio of the catalyst to the reactants is 0.6%,the yield is 94.2%.The yield does not vary greatly when more catalyst is addedTherefore,the mass ratio of the catalyst to the reactants is0.6%for these experiments.
3.2.3. Effect of reaction time on the yield
Reaction time is also an important factor for the reaction Table 7 shows the effect of reaction time on the yield.
According to Table 7,the yield increases with an increase in time.After 60 min,the yield is the highest.Thus,60 min is chosen as the optimum reaction time.
Table 7.Effect of reaction time on the yield 下载原图
Note:benzaldehyde,0.2 mol;n(benzaldehyde):n(1,2-propanediol)=1.0:1.5;mass ratio of catalyst to reactants,0.6%;cyclohexane,8m L.
Table 7.Effect of reaction time on the yield
3.3. Comparison of the catalytic activities of catalysts
Table 8 gives the catalytic activities of 732-type ion exchange resin[5],NH4Fe(SO4)2·12H2O[6],HY-type molecular sieve[7],and H3PW12O40/PAn.All the catalysts show certain catalytic activity for the synthesis of 4-methyl-2-phenyl-1,3-dioxolane,but the catalytic activity of H3PW12O40/PAn is the highest among these four catalysts.
3.4. Analysis and identification of the product
A purified product was analyzed by IR and 1HNMRspectrum.The IR spectrum of 4-methyl-2-phenyl-1,3-dioxolane shows peaks at 1097,1067,1011,and 976 cm-1,which are in accordance with the standard IR spectrum of4-methyl-2-phenyl-1,3-dioxolane;1HNMR(300 MHz,CDCl3)spectrum,δH is as follows:1.38(d,3H,CH3),3.45-3.60(d,2H,CH2),4.15-4.40(m,1H,CH),5.68-5.78(s,1H,CH),and 7.3-7.5(m,5H,Ar-H);20Dn=1.5094(20Dn=1.5103[7]);the boiling point of 4-methyl-2-phenyl-1,3-dioxolane is 218-222°C.
4. Synthesis of acetals and ketals
The synthesis of 10 important acetals and ketals were carried out under the route as follows.The reactant of aldehyde/ketone was fixed in 0.2 mol with cyclohexane(8.0 m L)as water-carrying reagent.And the results of these synthesis reactions are summarized in Table 9.
Table 8.Comparison of catalytic activity of different catalysts 下载原图
Table 8.Comparison of catalytic activity of different catalysts
From the data listed in Table 9,H3PW12O40/PAn is highly active.When the molar ratio of aldehyde/ketone to glycol(r)is 1:1.5,the mass ratio of the catalyst used to the reactants is0.6%,and the reaction time is 1.0 h,the yield is as follows:2-methyl-2-ethoxycarbonylmethyl-1,3-dioxolane,69.0%;2,4-dimethyl-2-ethoxycarbonylmethyl-1,3-dioxolane,79.5%cyclohexanone ethylene ketal,78.9%;cyclohexanone 1,2-propanediol ketal,85.3%;butanone ethylene ketal,56.9%;butanone 1,2-propanediol ketal,78.1%;2-phenyl-1,3-dioxolane,76.3%;4-methyl-2-phenyl-1,3-dioxolane,94.2%;2-propyl-1,3-dioxolane,70.7%;4-methyl-2-propyl-1,3-dioxolane,79.2%.Moreover,the reaction temperature was controlled at the temperature of reflux of reactant solution and the amount of H3PW12O40/PAn is much lower than other catalyst used in the reaction.All helped in solving the conditional problem faced by the industrial processing.
When the temperature rose,it was observed that the solution of reagents became dark in color because a temperature above the optimum condition would decrease the activity of catalyst.More reaction time was needed to maintain the yield,but the activity began to decrease after 1 h.Comparing the two alcoholic reactants,the special steric hindrance and the density of electron atmosphere on oxygen atoms exerted the opposite effects in yield.The former kept oxygen atoms from approaching carbonium ions and the C-O bonding was difficult to form.But the latter decreased the density of electron atmosphere on oxygen atoms bonded with carbonium ions so as to alleviate the rejection of the electron atmosphere of two oxygen atoms.Comparing the two alcoholic reactants,the special steric hindrance had the disadvantageous effect and stood the leading role.For the rest,the density of electron atmosphere on oxygen atoms predominated.
Table 9.Synthetic results of 10 classes of acetals or ketals 下载原图
Note:the reaction conditions are n(aldehyde/ketone):n(glycol)=1:1.5;catalyst dosage,0.6%;and reaction time,1.0 h.
Table 9.Synthetic results of 10 classes of acetals or ketals
5. Conclusions
A new environmental friendly catalyst,H3PW12O40/PAn was prepared and identified by means of FT-IR,XRD,and TG/DTA.The optimum conditions have been found;that is,the mass ratio of PAn to H3PW12O40 is 1:1.5,the volume of methanol is 20 m L,and the reflux reaction time is 3 h.The structural identity of Keggin units is preserved after the incorporation into polyaniline matrix.Heteropolyanions are molecularly dispersed in the polymer.When the molar ratio of aldehyde/ketone to glycol is 1:1.5,the quantity of catalyst is equal to 0.6%of feed stocks,and the reaction time is 1.0 h the yields of acetals and ketals can reach 56.9%-94.2%.H3PW12O40/PAn is an excellent catalyst for synthesizingacetals and ketals.
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