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TiO2 Catalysts: A FT-IR Study
Guczi, L et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier science Publishers B.V. All rights resewed
OXIDATION OF PROPENE ON ALKALINE METAL-DOPED MoOfliO, CATALYSTS: A m-IRSTUDY
C. Martin, I. Martin, C. Menduabal and V. Rives Dpto. de Quimica Inorganica, Universidad de Salamanca, Facultad de Farmacia, Salamanca, Spain
Abstract The reactivity of MoOfli02 systems (pure or Na- or K-doped) with propene has been studied by FT-IR spectroscopy. Reactivity decreases in the presence of the alkaline cations, and thus reactive adsorption (leading to acrolein formation) is only observed in the doped catalysts above room temperature. This behaviour has been related to the decrease in surface acidity. 1. INTRODUCTION
Molybdena-based catalysts is a set of materials widely used nowadays for partial oxidation of olefins [ 1,2]. The most of the papen existing in the literature on this subject demonstratethe relationship existing between the nature of the active phase (stnrcture, dispersion, etc.) and the activity and/or selectivity of the catalyst. In the present paper, a FT-IR study has been carried out on the adsorption and oxidation of propene on titania-supported molybdena, doped with alkaline cations (Na and K), in order to correlate the reactivity of these systems and their surface acidity 2. EXPERIMENTAL Catalysts were obtained by impregnation of the support (titania P-25 from Degussa, Germany, a.50 mVg) with aqueous solutions of ammonium heptamolybdate (Merck, p.a.), then calcined in oxygen at 770K for 3 h. In some cases, calcinationwas carried out at 1 100K, in order to analyze the effect of molybdena melting on the properties of the solids. The alkaline dopants were incorporated (1 or 3% in weight) to the support, before incorporation of molybdenum, by impregnation with aqueous solutions of KNO3 or NaOH in a rotiry vacuum evaporator. The amounts of molybdena correspond to one or two monolayers of MoO3. Samples are named m M(number of monolayers of Mo03)Alkaline(percentage).The sample calcined at 1 lOOK is named M lT, and does not contain any alkaline dopant. Adsorption of propene (from Sociedad Castellana del Oxigeno, 99.95%), acroleine and acetone (Carlo Erba, p.a.) were followed by FT-IRspectroscopy in special cells with CaF2 windows, after outgassing the sample in situ at 670K for 2 h.
1988 Physicochemicalcharacterization of the catalysts had been canied out by X-ray diffraction, Exafs and Visible-Ultravioletiffuse reflectance spectroscopies and specific surface area determination [3], as well as by FT-IRmonitoring of pyridine adsorption [41. According to the results obtained, molybdenum species exist mainly as [Moos]. Alkalinefree samples contain MoO3, its dispersion increasing as the Mo content decreases and in sample M1T. Polymolybdate species are detected in the presence of alkaline cations, its dispersion degree being higher for the low alkaline-loaded samples; for sample M2K1 both molybdena and polymolybdatesexist. Pyridine adsorption indicate the presence of both Bronsted and Lewis surface acid sites for the alkaline-free samples. As the percentage of alkaline doping cation increases, the number of Bronsted acid sites decreases, being completely cancelled in those samples containing 3% of alkaline cations; in these systems, the Lewis acid sites are weaker than in the absence of alkaline dopants. However, for sample M2K1 the amount of Brdnsted sites is similar to that existing in the alkaline-free samples, due to the presence of MoO3 species (as detected by Xray diffraction). 3. RESULTS A N D DISCUSSION
Adsorption of propene was carried out at room temperature (r.t.),373 or 573 K, and the samples were then outgassed at r.t.. The spectrum recorded after adsorption and outgassing at r.t. (Fig. 1) on the bare SUpDort, indicates the presence of molecularly adsorbed propene, with bands at 1625, 1615 (uC-C), 1452, 1429, 1414 and 1373 cm-1 (bCH3), due to olefinx-bonded to exposed surface cations (Ti4+),and no perturbation of the bands due to uOH modes is observed. At 373K the bands are recorded at 2963,2927 (UGH),1625, 1609, 1463, 1448, 1386, 1164 and 1097 cm-1, due to the mentioned x-bonded species and to isopropoxy species (uC-0, 1097 cm-I), formed upon reaction of propene with surface hydropxyl groups at this temperature. At 573K the intensities of these bands decreases, and new bands develop with maxima at 1567 and 1445 cml; these bands can be ascribed to ua(CO0) and us(C00) modes of carboxylate species, but, contrary to the results reported by Graham et al. [S], formation of acetone is not observed. (i.e., with one or two monolayen of Mo03, but with no doping), For samples Ml and the spectra recorded upon propene adsorption show, in addition to the bands above described for x bonded catiodoleh complex (The most characteristicbeing that at 1620 cm-1,u(C-C), a medium intensity bandat 1678 cm-1 (Fig. I), similar to that recorded upon adsorption of acetone on these samples, and has been ascribed to acetone coordinated to surface Lewis acid sites through the oxygen atom (uC-0 for gaseous acetone is recorded at 1734 cm-1 [a]). The shape and position of the other bands (1465, 1390, 1378, 1326 and 1270 cm-1, due to uC-C and BCH3) indicate that they are originated by materials formed upon propene polymerisation [7,8], a reaction easily taking place on these systems due to their high acidity (both Br6nsted and Lewis sites). Upon increasing of the adsorption temperature, the intensity of the acetonerelated band increases, while those due to x bonded olefin species decreases; acrolein nor carboxylate species are detected.
-
-
1989
For the sample calcined at 1100 K 0 ,
no reactive adsorption is observed at room
temperature, and only the n-bonded olefin bands at 1623 and 1615 cm-1 (uC=C) are recorded. As the temperature is increased, surface coordinated acetone is detected (uC-0 at 1678 cm-1). The lack of formation of polymerised species can be due to the lower surface acidity of this sample, as shown by pyridine adsorption [41. The & and 5-doDed samples (MlKl, MlNal) show similar spectra upon propene adsorption. At room temperature several bands are recorded in the 1640-1620cm-1 range, due to the uC=C mode of n - bonded complexes, where the olefin molecule is coordinated to different surface cations: the main absorption at 1639 cm-1 has been ascribed [7] to a n-bonded K+/olefin complex. Other bands at 1457, 1442, 1383 and 1314 cm-1 are due to NCH3). At 373K, Fig. 1, the intensities of these bands decrease, while a new one arises at 1690 cml; this band, together with those at 1618, 1426, 1365, 1279 and 1165 cm-1, have been also recorded upon adsorption of acrolein on these samples, and are due to the u(C=O), u(C=C), 6(C-H) and u(C-C) modes of acrolein weakly adsorbed on cationic sites (uC=O for gaseous acrolein is recorded at 1724 cm-l[91).
Ti02
J
q2K1
Samples with a larger alkaline content, M1K3 and MlNa3, are not reactive at r.t., but at 373 and 573K a very complex spectrum is recorded, with several bands between 1682 and 1659 cm- 1, probably due to different carbonylcontaining species adsorbed on the surface. (where X-ray difFinally, for sample fraction indicated the simultaneouspresence of polymolybdatesand molybdena),again both acetone bonded to Lewis sites (uC=O at 1678 cm-1) and Ir-bonded catiodolefm species (uC=C at 1635-1610cml) are detected at r.t., but nopolymeric species, Fig. 1. At 373K several bands
1
1800
I
I
1500
cm-l
1200
Figure 1.-Spectra recorded upon adsorption of propene on the samples indicated at the given temperatures.
1990
that can be originated by carbonyl-containingspecies (acrolein, acetone, etc.) coordinated to Lewis sites, are recorded at 1695, 1675, 1659 and 1651 cm-1.
4. CONCLUSIONS
The different behaviour shown by these samples upon propene adsorption should be related to their different surface acidity. The results obtained agree with the mechanism proposed by several authors [10,11] for olefins oxidation on these systems. According to these authors, Bronsted sites (Mo6'-OH) are responsible for propene hydrationdehydrogenation to acetone, while coordinatively unsaturated molybdenyl species are responsible for the allylic oxidation (acrolein formation). So, on our samples, acetone is only detected in Bronsted sites rich-samples (alkaline-free samples and M2KI), and acrolein in samples where these sites do not exist or in a small amount. This larger selectivity to acrolein upon alkaline doping is also in agreement with previous results with V205niO2 systems doped with sodium [ 121. So, it should be concluded that addition of alkaline species (K+ or Na+)decreases the activity of MoOfliO2 systems (no reactive adsorption is observed at r.t.), although increases the selectivity to acrolein.
5. ACKNOWLEDGMENTS Authors thank finantial support from CAICYT (MAT88-556)and Consejeria de Culturn de la Junta de Castilla y Leon. I.M. acknowledgesa grant from DGICyT.
6. REFERENCES R. Grabowski, S. Sloczynski, K.Dyrek and M. Labanowska, Appl. Catal., 32 (1987) 103. 2 T. Ono, Y. Nakagawa, H. Miyata and Y. Kubokawa, Bull. Chem. SOC.Japan, 57 (1984) 1205. 3 A. Muiioz-Paez, P. Malet, C. Martin and V. Rives, J. Catal., in press. 4 C. Martin, I. Martin, C. Mendizabal and V. Rives, Proc. 7th. Int. Symp. Heterogeneous Catal., Bourgas, Bulgaria (Eds. L. Petrov, A. Andreev and G . Kadinov), Vol. 1, p. 163 (1991). 5 J. Graham, R. Rudham and C. H. Rochester, J. Chem. Soc. Faraday Trans 1,80 (1984) 895. 6 G.Delle Piane and J. Overend, Spectrochim. Acta, 22 (1966) 593. 7 G.Busca, G.Porcile and V. Lorenzelli, J.Mol. Struct., 141 (1986) 395. 8 G.Ramis, G.Busca and V. Lorenzelli, Appl. Catal.,32 (1987) 305. 9 R. K.H a m s , Spectrochim. Acta, 20 (1964) 1129. 10 V. Shchez-Escribano, G.Busca, V. Lorenzelli, J. Phys. Chem., 94 (1990) 8939. 11 J. fiber, Proc. 8th Intern. Congr. Catalysis, West Berlin (1984) Vol. 1, 88. 12 C. Martin and V. Rives, J. Molecular Catal., 48 (1988) 381. 1
OXIDATION OF PROPENE ON ALKALINE METAL-DOPED MoOfliO, CATALYSTS: A m-IRSTUDY
C. Martin, I. Martin, C. Menduabal and V. Rives Dpto. de Quimica Inorganica, Universidad de Salamanca, Facultad de Farmacia, Salamanca, Spain
Abstract The reactivity of MoOfli02 systems (pure or Na- or K-doped) with propene has been studied by FT-IR spectroscopy. Reactivity decreases in the presence of the alkaline cations, and thus reactive adsorption (leading to acrolein formation) is only observed in the doped catalysts above room temperature. This behaviour has been related to the decrease in surface acidity. 1. INTRODUCTION
Molybdena-based catalysts is a set of materials widely used nowadays for partial oxidation of olefins [ 1,2]. The most of the papen existing in the literature on this subject demonstratethe relationship existing between the nature of the active phase (stnrcture, dispersion, etc.) and the activity and/or selectivity of the catalyst. In the present paper, a FT-IR study has been carried out on the adsorption and oxidation of propene on titania-supported molybdena, doped with alkaline cations (Na and K), in order to correlate the reactivity of these systems and their surface acidity 2. EXPERIMENTAL Catalysts were obtained by impregnation of the support (titania P-25 from Degussa, Germany, a.50 mVg) with aqueous solutions of ammonium heptamolybdate (Merck, p.a.), then calcined in oxygen at 770K for 3 h. In some cases, calcinationwas carried out at 1 100K, in order to analyze the effect of molybdena melting on the properties of the solids. The alkaline dopants were incorporated (1 or 3% in weight) to the support, before incorporation of molybdenum, by impregnation with aqueous solutions of KNO3 or NaOH in a rotiry vacuum evaporator. The amounts of molybdena correspond to one or two monolayers of MoO3. Samples are named m M(number of monolayers of Mo03)Alkaline(percentage).The sample calcined at 1 lOOK is named M lT, and does not contain any alkaline dopant. Adsorption of propene (from Sociedad Castellana del Oxigeno, 99.95%), acroleine and acetone (Carlo Erba, p.a.) were followed by FT-IRspectroscopy in special cells with CaF2 windows, after outgassing the sample in situ at 670K for 2 h.
1988 Physicochemicalcharacterization of the catalysts had been canied out by X-ray diffraction, Exafs and Visible-Ultravioletiffuse reflectance spectroscopies and specific surface area determination [3], as well as by FT-IRmonitoring of pyridine adsorption [41. According to the results obtained, molybdenum species exist mainly as [Moos]. Alkalinefree samples contain MoO3, its dispersion increasing as the Mo content decreases and in sample M1T. Polymolybdate species are detected in the presence of alkaline cations, its dispersion degree being higher for the low alkaline-loaded samples; for sample M2K1 both molybdena and polymolybdatesexist. Pyridine adsorption indicate the presence of both Bronsted and Lewis surface acid sites for the alkaline-free samples. As the percentage of alkaline doping cation increases, the number of Bronsted acid sites decreases, being completely cancelled in those samples containing 3% of alkaline cations; in these systems, the Lewis acid sites are weaker than in the absence of alkaline dopants. However, for sample M2K1 the amount of Brdnsted sites is similar to that existing in the alkaline-free samples, due to the presence of MoO3 species (as detected by Xray diffraction). 3. RESULTS A N D DISCUSSION
Adsorption of propene was carried out at room temperature (r.t.),373 or 573 K, and the samples were then outgassed at r.t.. The spectrum recorded after adsorption and outgassing at r.t. (Fig. 1) on the bare SUpDort, indicates the presence of molecularly adsorbed propene, with bands at 1625, 1615 (uC-C), 1452, 1429, 1414 and 1373 cm-1 (bCH3), due to olefinx-bonded to exposed surface cations (Ti4+),and no perturbation of the bands due to uOH modes is observed. At 373K the bands are recorded at 2963,2927 (UGH),1625, 1609, 1463, 1448, 1386, 1164 and 1097 cm-1, due to the mentioned x-bonded species and to isopropoxy species (uC-0, 1097 cm-I), formed upon reaction of propene with surface hydropxyl groups at this temperature. At 573K the intensities of these bands decreases, and new bands develop with maxima at 1567 and 1445 cml; these bands can be ascribed to ua(CO0) and us(C00) modes of carboxylate species, but, contrary to the results reported by Graham et al. [S], formation of acetone is not observed. (i.e., with one or two monolayen of Mo03, but with no doping), For samples Ml and the spectra recorded upon propene adsorption show, in addition to the bands above described for x bonded catiodoleh complex (The most characteristicbeing that at 1620 cm-1,u(C-C), a medium intensity bandat 1678 cm-1 (Fig. I), similar to that recorded upon adsorption of acetone on these samples, and has been ascribed to acetone coordinated to surface Lewis acid sites through the oxygen atom (uC-0 for gaseous acetone is recorded at 1734 cm-1 [a]). The shape and position of the other bands (1465, 1390, 1378, 1326 and 1270 cm-1, due to uC-C and BCH3) indicate that they are originated by materials formed upon propene polymerisation [7,8], a reaction easily taking place on these systems due to their high acidity (both Br6nsted and Lewis sites). Upon increasing of the adsorption temperature, the intensity of the acetonerelated band increases, while those due to x bonded olefin species decreases; acrolein nor carboxylate species are detected.
-
-
1989
For the sample calcined at 1100 K 0 ,
no reactive adsorption is observed at room
temperature, and only the n-bonded olefin bands at 1623 and 1615 cm-1 (uC=C) are recorded. As the temperature is increased, surface coordinated acetone is detected (uC-0 at 1678 cm-1). The lack of formation of polymerised species can be due to the lower surface acidity of this sample, as shown by pyridine adsorption [41. The & and 5-doDed samples (MlKl, MlNal) show similar spectra upon propene adsorption. At room temperature several bands are recorded in the 1640-1620cm-1 range, due to the uC=C mode of n - bonded complexes, where the olefin molecule is coordinated to different surface cations: the main absorption at 1639 cm-1 has been ascribed [7] to a n-bonded K+/olefin complex. Other bands at 1457, 1442, 1383 and 1314 cm-1 are due to NCH3). At 373K, Fig. 1, the intensities of these bands decrease, while a new one arises at 1690 cml; this band, together with those at 1618, 1426, 1365, 1279 and 1165 cm-1, have been also recorded upon adsorption of acrolein on these samples, and are due to the u(C=O), u(C=C), 6(C-H) and u(C-C) modes of acrolein weakly adsorbed on cationic sites (uC=O for gaseous acrolein is recorded at 1724 cm-l[91).
Ti02
J
q2K1
Samples with a larger alkaline content, M1K3 and MlNa3, are not reactive at r.t., but at 373 and 573K a very complex spectrum is recorded, with several bands between 1682 and 1659 cm- 1, probably due to different carbonylcontaining species adsorbed on the surface. (where X-ray difFinally, for sample fraction indicated the simultaneouspresence of polymolybdatesand molybdena),again both acetone bonded to Lewis sites (uC=O at 1678 cm-1) and Ir-bonded catiodolefm species (uC=C at 1635-1610cml) are detected at r.t., but nopolymeric species, Fig. 1. At 373K several bands
1
1800
I
I
1500
cm-l
1200
Figure 1.-Spectra recorded upon adsorption of propene on the samples indicated at the given temperatures.
1990
that can be originated by carbonyl-containingspecies (acrolein, acetone, etc.) coordinated to Lewis sites, are recorded at 1695, 1675, 1659 and 1651 cm-1.
4. CONCLUSIONS
The different behaviour shown by these samples upon propene adsorption should be related to their different surface acidity. The results obtained agree with the mechanism proposed by several authors [10,11] for olefins oxidation on these systems. According to these authors, Bronsted sites (Mo6'-OH) are responsible for propene hydrationdehydrogenation to acetone, while coordinatively unsaturated molybdenyl species are responsible for the allylic oxidation (acrolein formation). So, on our samples, acetone is only detected in Bronsted sites rich-samples (alkaline-free samples and M2KI), and acrolein in samples where these sites do not exist or in a small amount. This larger selectivity to acrolein upon alkaline doping is also in agreement with previous results with V205niO2 systems doped with sodium [ 121. So, it should be concluded that addition of alkaline species (K+ or Na+)decreases the activity of MoOfliO2 systems (no reactive adsorption is observed at r.t.), although increases the selectivity to acrolein.
5. ACKNOWLEDGMENTS Authors thank finantial support from CAICYT (MAT88-556)and Consejeria de Culturn de la Junta de Castilla y Leon. I.M. acknowledgesa grant from DGICyT.
6. REFERENCES R. Grabowski, S. Sloczynski, K.Dyrek and M. Labanowska, Appl. Catal., 32 (1987) 103. 2 T. Ono, Y. Nakagawa, H. Miyata and Y. Kubokawa, Bull. Chem. SOC.Japan, 57 (1984) 1205. 3 A. Muiioz-Paez, P. Malet, C. Martin and V. Rives, J. Catal., in press. 4 C. Martin, I. Martin, C. Mendizabal and V. Rives, Proc. 7th. Int. Symp. Heterogeneous Catal., Bourgas, Bulgaria (Eds. L. Petrov, A. Andreev and G . Kadinov), Vol. 1, p. 163 (1991). 5 J. Graham, R. Rudham and C. H. Rochester, J. Chem. Soc. Faraday Trans 1,80 (1984) 895. 6 G.Delle Piane and J. Overend, Spectrochim. Acta, 22 (1966) 593. 7 G.Busca, G.Porcile and V. Lorenzelli, J.Mol. Struct., 141 (1986) 395. 8 G.Ramis, G.Busca and V. Lorenzelli, Appl. Catal.,32 (1987) 305. 9 R. K.H a m s , Spectrochim. Acta, 20 (1964) 1129. 10 V. Shchez-Escribano, G.Busca, V. Lorenzelli, J. Phys. Chem., 94 (1990) 8939. 11 J. fiber, Proc. 8th Intern. Congr. Catalysis, West Berlin (1984) Vol. 1, 88. 12 C. Martin and V. Rives, J. Molecular Catal., 48 (1988) 381. 1