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Reaction of 2-propanol over heteropolyacids after pretreatment in air
Applied Catalysis, 77 (1991) 133-140 Elsevier Science Publishers B.V., Amsterdam
133
Reaction of 2-propanol over heteropolyacids after pretreatment in air Hideo Orita*, Takashi Hayakawa, Masao Shimixu and Katsuomi Takehira National Chemical Loboratory for Industry, Tsukuba, Zbaraki 305 (Japan), tel. (+ 81-298)544634, fax. (+ 81-298)551397 (Received 1 March 1991, revised manuscript received 14 May 1991)
Abstract The reaction of 2-propanol over various phosphorus-containing heteropolyacids was investigated as a function of pretreatment temperature and composition of the heteropolyacids. With catalysts pretreated at lOO-3OO”C,only propene was produced. However, after pretreatment at 45O”C,the selectivity changed in favor of acetone over some mixed molybdenum and tungsten heteropolyacids. From infrared (IR) and thermogravimetry-differential thermal analysis (TG-DTA) measurements, the improvement in acetone selectivity was ascribed to the destruction of Keggin units of the heteropolyacid. The maximum selectivity to acetone was observed for HSPMo12_-JWxOU) when x=6. Keyluords: acetone, dehydrogenation, heteropolyacid, 2-propanol.
INTRODUCTION
Recently, catalytic applications of heteropolyacids have attracted increased interests [ 1,2]. Heteropolyacids exhibit two important properties in acid-base and redox chemistry, and have been shown to be efficient in various chemical processes. Redox and acidic properties as well as catalytic performance can be modified by an appropriate change in the composition of the heteropolyacid [3-51. In the present work, the reaction of 2-propanol has been chosen as a test reaction in order to examine the properties of some heteropolyacids, because different reactions can proceed on acidic (dehydration) and redox (dehydrogenation) catalytic centers. Propene and acetone, which are the products of dehydration and dehydrogenation, respectively, are considered to be stable, undergoing no further secondary reaction, and thus can be used for a precise investigation of catalytic performance. The reaction of 2-propanol over a series of H,PMo12 _,W,O, catalysts was studied previously by Otake and Onoda [ 61, who detected only the products of dehydration and concluded that the heteropolyacids show higher dehydration activities than such commonly employed
0166-9634/91/$03.50
0 1991 Elsevier Science Publishers B.V. All rights reserved.
134
solid acid catalysts as silica-alumina. We have investigatedselectivityof the products in the 2-propanolreaction as a function of pretreatmenttemperature and composition of the heteropolyacid.Since the catalytic performance was observed in our study to change from “dehydration” to “dehydrogenation” following the pretreatmentat 450°C of H3PMo12_,W,0~ catalysts, we have also examined the thermal stability of some heteropolyacidsby IR and TGDTA methods. Results of these investigationsare reportedhere. EXPERIMENTAL
Reagents and reactor The H3PMoi2_,W,O, heteropolyacids(X= 3,6,9) werepreparedfrom mixed aqueous solutions of Na,MoO,, Na,WO, and Na,HPO, using the recommendedelementalcomposition accordingto the publishedprocedure [ 71. The mixed molybdenumand tungstencoordinatedheteropolyanionsthus prepared were probably mixturesof PMo,,_,W,O, (x=0-12) [8], where the x value only indicate an averagecomposition of molybdenumand tungsten.However, the IR bands of the Keggin structurein the heteropolyacidwere found to shift to a higherwavenumberwith increasingx. The metal salts of the heteropoly acids were also prepared according to the published procedure [ 71. The H3+xPMo,,_,V,0,~ series (x= 1,2,4) wereobtainedby courtesyof Japan New Metals.Otherchemicalswerepurchasedand used without furtherpurification. The reaction includingthe catalysts’pretreatmentwas carriedout in a conventional closed circulation system. The pretreatmentwas carried out in circulatingair at differenttemperaturesrangingfrom 100 to 600°C for l-2 h. A mixtureof 2-propanol and air [total initial pressure (PO is 86 Torr; the pressure ratio of 2-propanol to air is 2 : 5; 1 Torr = 133.3 Pa] in conjunction with 1 g of heteropolyacidwas used for the catalytic measurements.The reaction products were analyzedby gas chromatography. IR spectroscopy IR spectra were measuredbetween 4660-400 cm-’ by a Jasco FT-IR-7060 spectrometeras potassium bromide disks both before and after the pretreatment in the reactor.Some spectrawerealso recordedby the diffuse reflectance method with mercurycadmiumtelluride (MCT) detector. Thermd analysis TG-DTA were carriedout in air (100 ml/min) on a Mac Science TG-DTA 2000. The samples (ca. 40 mg in a platinumcell) were heated at 10oC/min up to 8oo”c.
135
X-ray difiaction (XRD) Powder XRD data were obtained with a Philips PW-1800 diffractometer, using Cu Kar radiation. RESULTS AND DISCUSSION
The typical results of the reaction of 2-propanol over variously pretreated heteropolyacid and its copper-salt are shown in Fig. 1. At pretreatment temperatures below 400” C, propene was the major product with acetone and diisopropyl ether also forming in minor amounts. The formation of propene from 2-propanol is considered to be the action of acidic sites in the heteropolyacid [6]. However, with catalysts pretreated at 45O”C, the selectivity of propene fell sharply and became lower than the selectivity to acetone. Acetone was then the major product. The selectivities to both acetone and propene decreased with catalysts pretreated above 500°C. The dependence of the consumption rate on pretreatment temperature was very different for the two series of cat-
pretreatment
temperature/ OC
Fig. 1. Reactivity of P-propanol as a function of pretreatment temperature in air: (a) over H,PMo,W,O,, reaction temperature, 75°C; (b) CU,(PMG,W,O,,,)~ T= 135°C. (0) Selectivity to propene, ( 0 ) selectivity to acetone, ( A ) consumption rate of 2-propanol. Conditions: P” = 80 Torr (the pressure ratio of 2-propanol to air is 2 : 5).
136 TABLE 1 Reaction of 2-propanol over various catalysts after pretreatment in air at 450°C P” = 80 Torr (the pressure ratio of 2-propanol to air is 2 : 5) Catalyst
Reaction temp. (“C)
Acetone selectivity (%)
Consumption rate (ml-STP h-l g-l)
H,PMo,W,O, Na3PMoeWBOlo KBPMosWsOlo Cu, (PMo,W&),
75 180 100 135 95 135 100 155 75 75 280 130 170 180 350
34 48 0.8 42 13 35 18 12 7.4 0.9 66 2.4 40 77 60
1.5 0.9 2.0 2.7 1.2 1.1 1.8 1.7 0.7 7.1 1.1 4.5 1.4 1.2 0.9
W(PMO~W~O~OL Mn6(PMo6W60dz W’Mdho
NasPMolzOlo H,SiMoIsOlo H3PW12040 NazMoO, Moos Bi2M03012 cue Na2W04
-
75
-50
3 2 0
-6 . 25
-0
200
600
400
temperature/
800
“C
Fig. 2. TG-DTA curve of H3PMosWsOm; heating rate, lO”C/min; air flow, 100 ml/min.
alysts. For H,PMo,W,O,, the consumption rate of 2-propanol decreased considerably after pretreatment at 300°C. On the other hand, the activity of Cu, ( PMo~W~O~)~ increased abruptly after pretreatment at 400” C and decreased to a low value after further treatment. The catalytic activities of various compounds after pretreatment at 450°C
137
-
0
I
400
200
600
BOO
temperature/ QC
Fig.
3. TG-DTA curve of KSPMo,W60m; heating rate, lO”C/min; air flow, 100 ml/min.
a
I c
\
00
d
!2 320 40.
0.""""""' 90 100
wavenumber/
cm
-1
110
120
130
140
150
1 0
reaction temperature/ %
Fig. 4. IR spectra of heteropolyacid (KRr disk method); (a) H,PMo6WsONas-prepared; (b) after pretreatment at 450” C and reaction of 2-propanol; (c) K,PMo,W,O, as-prepared; (d) after pretreatment at 450°C and reaction of 2-propanol. Fig. 5. Selectivity of the 2-propanol reaction over Cu~(PMo,W,O& as a function of reaction temperature; ( 0 ) acetone, (0 ) propene, ( n ) diieopropyl ether. PO= 80Torr (the pressure ratio of 2-propanol to air is 2:5).
138
Fig. 6. Effect of the composition of H,PMo~~_,W,O~~ upon selectivity for acetone; P” = 60 Torr (the pressure ratio of 2-propanol to air is 2 : 5). Fig. 7. Effect of the composition of H 3+xPMo12_-IVxOI0upon the selectivity for acetone; PO=80 Torr (the pressure ratio of 2-propanol to air is 2 : 5 ) .
are presentedin Table 1 (the reaction temperaturewas changed in order that the consumption rate of 2-propanol might be in the rangeof l-4 ml-STP h-’ g - ‘, i.e., the selectivitiesto acetone of variouscatalystsare comparedat almost the same range of consumption rate of 2-propanol). Among the heteropoly species investigated,Na3PMo6W6040showed the highest selectivity to acetone, althoughthe consumption rate of 2-propanol was reduced and a higher reaction temperaturewas required.The potassium salt showed the lowest selectivity,and propenewas the majorproduct even afterpretreatmentat 450°C. In order to investigatethe change of selectivity,thermal analysis (TG-DTA) and IR spectroscopy were applied, and typical resultsfor H3PMosWe040and K3PMo6W6040are shown in Figs. 2-4. For the acidic species, the exothermic peak appeared around 49O”C, associated with the irreversibledestruction of the Keggin units in the heteropolyacid.In contrast, the potassiumsalt did not show any distinct peak between 400 and 600” C. In the IR spectra before pretreatment (Fig. 4a and c), the bands assignedto the Keggin structure [9,10] are observed:1070cm-‘, P-O stretchingvibration;970 cm-‘, M-O (terminal) stretching;666 and 790 cm-‘, M-O-M stretching.The destructionof the Keggin units occurredafter the pretreatmentat 450” C for the acid, but not for the potassium salt (Fig. 4b and d). The destructionof the Keggin units was also observedfor all other heteropoly species shown in Table 1 except H,PW,,O,,. For H,S~MO~~O,pretreated at 45O”C, IR bands of MOO, appeared clearly, which is consistentwith the resultsof Rocchiccioli-Deltcheffet al. [ 111. From these results,it is consideredthat the active sites for the formation of acetone are produced after the destructionof the Keggin units. From XRD patternsof NaBPMoeWsOlo and Cu3( PMo~W~O~)~pretreatedat 456 “C, WO, was clearly identifiedbut Moos was not observed.Peaksdueto Moos wereobservedweakly
139
only for H3PMo12040 pretreated at 45O”C, suggesting that Moos produced through the decomposition of heteropolyacids was mainly amorphous to Xrays. However, the exact structure of the active site is unclear at the present stage since Fig. 4b is featureless. Rocchiccioli-Deltcheff et al. [ 111 reported that the methanol reaction over H4SiMo12040 which favors the formation of ether at low-pretreatment temperatures over acidic sites changes to favor the formation of formaldehyde by a redox mechanism as the pretreatment temperature exceeds 320 oC. They considered that the active site for the formation of formaldehyde is MOO, produced through decomposition of the heteropolyacid. Sodium molybdate and sodium tungstate showed high selectivity for acetone, but much higher reaction temperatures were required, indicating that a large amount of sodium (sodium/molybdenum or tungsten= 2) reduced the consumption rate of 2-propanol considerably. Molybdenum trioxide did not show a good selectivity to acetone, indicating a high acidic character. Bismuth molybdate exhibited almost the same properties as Na,PMo,W,O,. The conventional dehydrogenation catalyst of cupric oxide [ 121 showed the highest selectivity in the present work. But its activity declined gradually and the formation of carbon dioxide was increasingly evident (selectivity is equal to ca. 5% ). Preliminary results of the reaction of 1-propanol or ethanol over heteropolyacids showed that the dehydrogenation of primary alcohol also proceeds, especially, after the pretreatment at 45O”C, forming propionaldehyde or acetaldehyde. However, it required a somewhat higher reaction temperature than the reaction of 2-propanol (e.g., T= 155 oC for Cu, ( PMo~W~O~)~, selectivities to propionaldehyde and acetaldehyde of 58 and 66%, respectively). The reaction of 2-propanol over Cu3 ( PMo~W~O~)~ after pretreatment at 450” C was examined as a function of reaction temperature (Fig. 5). As the reaction temperature increased, selectivity for propene increased sharply and those for acetone and diisopropyl ether decreased at a reaction temperature higher than 135°C. Lower temperatures favored the formation of acetone, but the irreversible adsorption of 2-propanol occurred and the total mass balance of the reaction became less than 86%. Activation energies of product formation were also determined The activation energy calculated for propene formation (30 kcal/mol, 1 cal=4.18 J) was about twice that for acetone (17 kcal/mol) and diisopropyl ether (16 kcal/mol). The effect of the composition of the heteropolyacids upon selectivity for acetone was investigated and the results are shown in Fig. 6. Similar results with catalysts in which vanadium replaces tungsten, i.e., H,+,PMo,,_,V,O, are shown in Fig. 7. For the H3PMo12_-rWx040 series, selectivity for acetone was maximum at x=6. From IR and DTA measurements, the replacement of molybdenum with tungsten was found to stabilize the Keggin units. H,PMo~W~O~ and H3PW12040 showed exothermic peaks at 545’ C and at 591 and 606”C, respectively. The IR bands assigned to the Keggin structure were
140
observed even after pretreatment at 450°C. On the other hand, selectivity was
almost constant for H,+,PMo,,_,V,O,,. The Keggin units of all the H3 +rPMo12_-+Vx040series investigated were found to be destroyed by pretreatment at 45O”C, and the IR bands due to Moos were observed clearly for x = 2 and weakly for z= 1. When the Keggin structure of these mixed coordinated heteropolyacids was destroyed after pretreatment at 45O”C, mixed oxides of molybdenum and tungsten or vanadium were considered to be formed. From Figs. 6 and 7, it is concluded that mixed oxides of molybdenum and tungsten are much more active for the formation of acetone than those of molybdenum and vanadium. In the case of mixed molybdenum and tungsten coordination, the Moo3 phase was not observed from IR and XRD analyses, suggesting that amorphous Moos might be more selective for the formation of acetone. The effect of mixed coordination on homogeneous oxidation was studied by F’UNkawa et al. [ 131, who reported an increased yield of glutaraldehyde from cyclopentene on the mixed coordinated heteropolyacids of molybdenum and tungsten, but not on those of molybdenum and vanadium. They considered that the effect of mixed coordination is associated with the change in the redox potential of the polyanions. In the present study, it is necessary to destroy the Keggin structure in order to improve the selectivity to acetone, so the effect of mixed coordination is ascribed to the decomposition property of polyanions but not to the redox potential. CONCLUSION
The selectivity in the 2-propanol reaction over some mixed molybdenum and tungsten heteropolyacids is changed from dehydration to dehydrogenation after pretreatment in air at 450°C. The improvement in acetone selectivity is ascribed to the destruction of the Keggin structure by IR and DTA measurements. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
M. Miaono, Catal. Rev. Sci. Eng., 29 (1987) 269. I.V. Kozhevnikov end K.I. Matveev, Appl. Catal., 5 (1983) 135. K. Bruckman, J. Haber and E.M. Serwicka, Faraday Discuss. Chem. Sot., 87 (1989) 173. 0. Watzenberger, G. Emig and D.T. Lynch, J. Catal., 124 (1990) 247. A. Bieleneki, J. Pozmiczek and A. M&&a, React. Kinet. Catal. L&t., 41 (1990) 127. M. Otake and T. Onoda, Shokubai, 17 (1975) 13. T. Okuhara and M. Mieono, Shokubai, 23 ( 1981) 178. M.A. Schwegler, J.A. Peters and H. van Bekkum, J. Mol. Catal., 63 (1990) 343. C. Rocchiccioli-Deltcheff, R. Thouvenot and R. Franck, Spectrochim. Acta, 32A (1976) 587. L. Lyhamn, S.J. Cyvin, B.N. Cyvin and J. Bnmvoll, Z. Naturforsh., 31A (1976) 1589. C. Rocchiccioli-Deltcheff, M. Amirouche, G. Herve, M. Fournier, M. Che and J.-M. Tatibouet, J. Catal., 126 (1990) 591. J.C. Volta, P. Turlier and Y. Trambouze, J. Catal., 34 (1974) 329. H. Furukawa, T. Nakamura, H. Inagaki, E. Nishikawa, C. Imai and M. Mieono, Chem. Lett., (1988) 877.
133
Reaction of 2-propanol over heteropolyacids after pretreatment in air Hideo Orita*, Takashi Hayakawa, Masao Shimixu and Katsuomi Takehira National Chemical Loboratory for Industry, Tsukuba, Zbaraki 305 (Japan), tel. (+ 81-298)544634, fax. (+ 81-298)551397 (Received 1 March 1991, revised manuscript received 14 May 1991)
Abstract The reaction of 2-propanol over various phosphorus-containing heteropolyacids was investigated as a function of pretreatment temperature and composition of the heteropolyacids. With catalysts pretreated at lOO-3OO”C,only propene was produced. However, after pretreatment at 45O”C,the selectivity changed in favor of acetone over some mixed molybdenum and tungsten heteropolyacids. From infrared (IR) and thermogravimetry-differential thermal analysis (TG-DTA) measurements, the improvement in acetone selectivity was ascribed to the destruction of Keggin units of the heteropolyacid. The maximum selectivity to acetone was observed for HSPMo12_-JWxOU) when x=6. Keyluords: acetone, dehydrogenation, heteropolyacid, 2-propanol.
INTRODUCTION
Recently, catalytic applications of heteropolyacids have attracted increased interests [ 1,2]. Heteropolyacids exhibit two important properties in acid-base and redox chemistry, and have been shown to be efficient in various chemical processes. Redox and acidic properties as well as catalytic performance can be modified by an appropriate change in the composition of the heteropolyacid [3-51. In the present work, the reaction of 2-propanol has been chosen as a test reaction in order to examine the properties of some heteropolyacids, because different reactions can proceed on acidic (dehydration) and redox (dehydrogenation) catalytic centers. Propene and acetone, which are the products of dehydration and dehydrogenation, respectively, are considered to be stable, undergoing no further secondary reaction, and thus can be used for a precise investigation of catalytic performance. The reaction of 2-propanol over a series of H,PMo12 _,W,O, catalysts was studied previously by Otake and Onoda [ 61, who detected only the products of dehydration and concluded that the heteropolyacids show higher dehydration activities than such commonly employed
0166-9634/91/$03.50
0 1991 Elsevier Science Publishers B.V. All rights reserved.
134
solid acid catalysts as silica-alumina. We have investigatedselectivityof the products in the 2-propanolreaction as a function of pretreatmenttemperature and composition of the heteropolyacid.Since the catalytic performance was observed in our study to change from “dehydration” to “dehydrogenation” following the pretreatmentat 450°C of H3PMo12_,W,0~ catalysts, we have also examined the thermal stability of some heteropolyacidsby IR and TGDTA methods. Results of these investigationsare reportedhere. EXPERIMENTAL
Reagents and reactor The H3PMoi2_,W,O, heteropolyacids(X= 3,6,9) werepreparedfrom mixed aqueous solutions of Na,MoO,, Na,WO, and Na,HPO, using the recommendedelementalcomposition accordingto the publishedprocedure [ 71. The mixed molybdenumand tungstencoordinatedheteropolyanionsthus prepared were probably mixturesof PMo,,_,W,O, (x=0-12) [8], where the x value only indicate an averagecomposition of molybdenumand tungsten.However, the IR bands of the Keggin structurein the heteropolyacidwere found to shift to a higherwavenumberwith increasingx. The metal salts of the heteropoly acids were also prepared according to the published procedure [ 71. The H3+xPMo,,_,V,0,~ series (x= 1,2,4) wereobtainedby courtesyof Japan New Metals.Otherchemicalswerepurchasedand used without furtherpurification. The reaction includingthe catalysts’pretreatmentwas carriedout in a conventional closed circulation system. The pretreatmentwas carried out in circulatingair at differenttemperaturesrangingfrom 100 to 600°C for l-2 h. A mixtureof 2-propanol and air [total initial pressure (PO is 86 Torr; the pressure ratio of 2-propanol to air is 2 : 5; 1 Torr = 133.3 Pa] in conjunction with 1 g of heteropolyacidwas used for the catalytic measurements.The reaction products were analyzedby gas chromatography. IR spectroscopy IR spectra were measuredbetween 4660-400 cm-’ by a Jasco FT-IR-7060 spectrometeras potassium bromide disks both before and after the pretreatment in the reactor.Some spectrawerealso recordedby the diffuse reflectance method with mercurycadmiumtelluride (MCT) detector. Thermd analysis TG-DTA were carriedout in air (100 ml/min) on a Mac Science TG-DTA 2000. The samples (ca. 40 mg in a platinumcell) were heated at 10oC/min up to 8oo”c.
135
X-ray difiaction (XRD) Powder XRD data were obtained with a Philips PW-1800 diffractometer, using Cu Kar radiation. RESULTS AND DISCUSSION
The typical results of the reaction of 2-propanol over variously pretreated heteropolyacid and its copper-salt are shown in Fig. 1. At pretreatment temperatures below 400” C, propene was the major product with acetone and diisopropyl ether also forming in minor amounts. The formation of propene from 2-propanol is considered to be the action of acidic sites in the heteropolyacid [6]. However, with catalysts pretreated at 45O”C, the selectivity of propene fell sharply and became lower than the selectivity to acetone. Acetone was then the major product. The selectivities to both acetone and propene decreased with catalysts pretreated above 500°C. The dependence of the consumption rate on pretreatment temperature was very different for the two series of cat-
pretreatment
temperature/ OC
Fig. 1. Reactivity of P-propanol as a function of pretreatment temperature in air: (a) over H,PMo,W,O,, reaction temperature, 75°C; (b) CU,(PMG,W,O,,,)~ T= 135°C. (0) Selectivity to propene, ( 0 ) selectivity to acetone, ( A ) consumption rate of 2-propanol. Conditions: P” = 80 Torr (the pressure ratio of 2-propanol to air is 2 : 5).
136 TABLE 1 Reaction of 2-propanol over various catalysts after pretreatment in air at 450°C P” = 80 Torr (the pressure ratio of 2-propanol to air is 2 : 5) Catalyst
Reaction temp. (“C)
Acetone selectivity (%)
Consumption rate (ml-STP h-l g-l)
H,PMo,W,O, Na3PMoeWBOlo KBPMosWsOlo Cu, (PMo,W&),
75 180 100 135 95 135 100 155 75 75 280 130 170 180 350
34 48 0.8 42 13 35 18 12 7.4 0.9 66 2.4 40 77 60
1.5 0.9 2.0 2.7 1.2 1.1 1.8 1.7 0.7 7.1 1.1 4.5 1.4 1.2 0.9
W(PMO~W~O~OL Mn6(PMo6W60dz W’Mdho
NasPMolzOlo H,SiMoIsOlo H3PW12040 NazMoO, Moos Bi2M03012 cue Na2W04
-
75
-50
3 2 0
-6 . 25
-0
200
600
400
temperature/
800
“C
Fig. 2. TG-DTA curve of H3PMosWsOm; heating rate, lO”C/min; air flow, 100 ml/min.
alysts. For H,PMo,W,O,, the consumption rate of 2-propanol decreased considerably after pretreatment at 300°C. On the other hand, the activity of Cu, ( PMo~W~O~)~ increased abruptly after pretreatment at 400” C and decreased to a low value after further treatment. The catalytic activities of various compounds after pretreatment at 450°C
137
-
0
I
400
200
600
BOO
temperature/ QC
Fig.
3. TG-DTA curve of KSPMo,W60m; heating rate, lO”C/min; air flow, 100 ml/min.
a
I c
\
00
d
!2 320 40.
0.""""""' 90 100
wavenumber/
cm
-1
110
120
130
140
150
1 0
reaction temperature/ %
Fig. 4. IR spectra of heteropolyacid (KRr disk method); (a) H,PMo6WsONas-prepared; (b) after pretreatment at 450” C and reaction of 2-propanol; (c) K,PMo,W,O, as-prepared; (d) after pretreatment at 450°C and reaction of 2-propanol. Fig. 5. Selectivity of the 2-propanol reaction over Cu~(PMo,W,O& as a function of reaction temperature; ( 0 ) acetone, (0 ) propene, ( n ) diieopropyl ether. PO= 80Torr (the pressure ratio of 2-propanol to air is 2:5).
138
Fig. 6. Effect of the composition of H,PMo~~_,W,O~~ upon selectivity for acetone; P” = 60 Torr (the pressure ratio of 2-propanol to air is 2 : 5). Fig. 7. Effect of the composition of H 3+xPMo12_-IVxOI0upon the selectivity for acetone; PO=80 Torr (the pressure ratio of 2-propanol to air is 2 : 5 ) .
are presentedin Table 1 (the reaction temperaturewas changed in order that the consumption rate of 2-propanol might be in the rangeof l-4 ml-STP h-’ g - ‘, i.e., the selectivitiesto acetone of variouscatalystsare comparedat almost the same range of consumption rate of 2-propanol). Among the heteropoly species investigated,Na3PMo6W6040showed the highest selectivity to acetone, althoughthe consumption rate of 2-propanol was reduced and a higher reaction temperaturewas required.The potassium salt showed the lowest selectivity,and propenewas the majorproduct even afterpretreatmentat 450°C. In order to investigatethe change of selectivity,thermal analysis (TG-DTA) and IR spectroscopy were applied, and typical resultsfor H3PMosWe040and K3PMo6W6040are shown in Figs. 2-4. For the acidic species, the exothermic peak appeared around 49O”C, associated with the irreversibledestruction of the Keggin units in the heteropolyacid.In contrast, the potassiumsalt did not show any distinct peak between 400 and 600” C. In the IR spectra before pretreatment (Fig. 4a and c), the bands assignedto the Keggin structure [9,10] are observed:1070cm-‘, P-O stretchingvibration;970 cm-‘, M-O (terminal) stretching;666 and 790 cm-‘, M-O-M stretching.The destructionof the Keggin units occurredafter the pretreatmentat 450” C for the acid, but not for the potassium salt (Fig. 4b and d). The destructionof the Keggin units was also observedfor all other heteropoly species shown in Table 1 except H,PW,,O,,. For H,S~MO~~O,pretreated at 45O”C, IR bands of MOO, appeared clearly, which is consistentwith the resultsof Rocchiccioli-Deltcheffet al. [ 111. From these results,it is consideredthat the active sites for the formation of acetone are produced after the destructionof the Keggin units. From XRD patternsof NaBPMoeWsOlo and Cu3( PMo~W~O~)~pretreatedat 456 “C, WO, was clearly identifiedbut Moos was not observed.Peaksdueto Moos wereobservedweakly
139
only for H3PMo12040 pretreated at 45O”C, suggesting that Moos produced through the decomposition of heteropolyacids was mainly amorphous to Xrays. However, the exact structure of the active site is unclear at the present stage since Fig. 4b is featureless. Rocchiccioli-Deltcheff et al. [ 111 reported that the methanol reaction over H4SiMo12040 which favors the formation of ether at low-pretreatment temperatures over acidic sites changes to favor the formation of formaldehyde by a redox mechanism as the pretreatment temperature exceeds 320 oC. They considered that the active site for the formation of formaldehyde is MOO, produced through decomposition of the heteropolyacid. Sodium molybdate and sodium tungstate showed high selectivity for acetone, but much higher reaction temperatures were required, indicating that a large amount of sodium (sodium/molybdenum or tungsten= 2) reduced the consumption rate of 2-propanol considerably. Molybdenum trioxide did not show a good selectivity to acetone, indicating a high acidic character. Bismuth molybdate exhibited almost the same properties as Na,PMo,W,O,. The conventional dehydrogenation catalyst of cupric oxide [ 121 showed the highest selectivity in the present work. But its activity declined gradually and the formation of carbon dioxide was increasingly evident (selectivity is equal to ca. 5% ). Preliminary results of the reaction of 1-propanol or ethanol over heteropolyacids showed that the dehydrogenation of primary alcohol also proceeds, especially, after the pretreatment at 45O”C, forming propionaldehyde or acetaldehyde. However, it required a somewhat higher reaction temperature than the reaction of 2-propanol (e.g., T= 155 oC for Cu, ( PMo~W~O~)~, selectivities to propionaldehyde and acetaldehyde of 58 and 66%, respectively). The reaction of 2-propanol over Cu3 ( PMo~W~O~)~ after pretreatment at 450” C was examined as a function of reaction temperature (Fig. 5). As the reaction temperature increased, selectivity for propene increased sharply and those for acetone and diisopropyl ether decreased at a reaction temperature higher than 135°C. Lower temperatures favored the formation of acetone, but the irreversible adsorption of 2-propanol occurred and the total mass balance of the reaction became less than 86%. Activation energies of product formation were also determined The activation energy calculated for propene formation (30 kcal/mol, 1 cal=4.18 J) was about twice that for acetone (17 kcal/mol) and diisopropyl ether (16 kcal/mol). The effect of the composition of the heteropolyacids upon selectivity for acetone was investigated and the results are shown in Fig. 6. Similar results with catalysts in which vanadium replaces tungsten, i.e., H,+,PMo,,_,V,O, are shown in Fig. 7. For the H3PMo12_-rWx040 series, selectivity for acetone was maximum at x=6. From IR and DTA measurements, the replacement of molybdenum with tungsten was found to stabilize the Keggin units. H,PMo~W~O~ and H3PW12040 showed exothermic peaks at 545’ C and at 591 and 606”C, respectively. The IR bands assigned to the Keggin structure were
140
observed even after pretreatment at 450°C. On the other hand, selectivity was
almost constant for H,+,PMo,,_,V,O,,. The Keggin units of all the H3 +rPMo12_-+Vx040series investigated were found to be destroyed by pretreatment at 45O”C, and the IR bands due to Moos were observed clearly for x = 2 and weakly for z= 1. When the Keggin structure of these mixed coordinated heteropolyacids was destroyed after pretreatment at 45O”C, mixed oxides of molybdenum and tungsten or vanadium were considered to be formed. From Figs. 6 and 7, it is concluded that mixed oxides of molybdenum and tungsten are much more active for the formation of acetone than those of molybdenum and vanadium. In the case of mixed molybdenum and tungsten coordination, the Moo3 phase was not observed from IR and XRD analyses, suggesting that amorphous Moos might be more selective for the formation of acetone. The effect of mixed coordination on homogeneous oxidation was studied by F’UNkawa et al. [ 131, who reported an increased yield of glutaraldehyde from cyclopentene on the mixed coordinated heteropolyacids of molybdenum and tungsten, but not on those of molybdenum and vanadium. They considered that the effect of mixed coordination is associated with the change in the redox potential of the polyanions. In the present study, it is necessary to destroy the Keggin structure in order to improve the selectivity to acetone, so the effect of mixed coordination is ascribed to the decomposition property of polyanions but not to the redox potential. CONCLUSION
The selectivity in the 2-propanol reaction over some mixed molybdenum and tungsten heteropolyacids is changed from dehydration to dehydrogenation after pretreatment in air at 450°C. The improvement in acetone selectivity is ascribed to the destruction of the Keggin structure by IR and DTA measurements. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
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