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Role of Mo and Sb in oxide Catalysts for Selective Oxidation of Propylene
Guczi, L d al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1 9 2 , Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
ROLE OF Mo AND Sb IN OXIDE CATALYSTS FOR SELECTIVE OXIDATION OF PROPYLENE B. Zhofl, X. Cud and R T.Chuangb aNational Laboratory of Fundamental Research in Catalysts, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China bDepartment of Chemical Engineering, University of Alberta, Edmonton, T6G 2G6 Alberta, Canada
Abstract
The activity of Moo3 - Sb2O4 catalyst for propylene oxidation was found to relate with the acidic property of Moo3 and redox property of Sb2O4. Results also indicate that the adsorption of propylene occurs on the Lewis sites, and the subsequent abstraction of a-hydrogen, which is the rate-determining step, takes place on the Bronsted sites. Both sites are located on the surface of Moog. The presence of SbzO4 in the catalyst facilitates the dissociation of molecular oxygen into active species which can subsequently migrate onto the surface of Moog, resulting in the reoxidation of the reduced active sites. INTRODUCTlON
.Mo and Sb have been postulated as the most important catalytic elements for the amm)oxidation of propylene [l]. Many studies have focused on the relations ip between surface property and activity for catalysts containing Moo3 or SbzO4, but their role in these catalysts is still not clear. The objective of our study is to elucidate the effect of these two elements in the selective oxidation of propylene; more specifically, the influence of acidic property of Moo3 and redox property of Sb2O4 on different elemental step of propylene oxidation.
h
EXPERIMENTAL
The catalysts were prepared by dispersing MOO and Sb2O4 in n-pentane. The detailed procedure has been described elsewhere ?2] The catal sts were characterized by BET, XRD, SEM, AEM, XPS and ISS techniques whic were also reported in ref [2]. Pyridine adsorption was studied by infrared spectroscopy. The spectra were obtained on a Pekin E er 580B infrared spectrophotometer operating in the range of 800 to 4000 cm-? The results were used to identify the Lewis and Bronsted acid sites on the catalysts. TPD of ammonia was carried out in order to determine the amount of Lewis and Bronsted sites resonance. The reoxidation of reduced catalyst was studied by electron spin
K
1964 resonance (ESR). A previously reduced Moog. gfs mixed with Sb2O4 accordin to a procedure described in ref [2]. the d o in this mixture gave a ! signal. The mixture was then reoxidized by oxygen (1% 0 2 in 99% strong ER He t 403K. The ESR spectra were recorded over a period %f 8 hours. The Mc$$ concentration was expressed as the relative intensity ([Mo '1 rel.) [Mo5+] rel. = (I/I0)hH2. (l/m)(l/MoO3wt%) where I is intensity of the Mo5+ ignal,; lo is the intensity of the internal standard; AHis width of the si nal for Mo5'; m is mass of the sample ar&MoOg wt% is the weight percent of 003 in the sample. The decrease of [Mo ] rel. represents the reoxidation degree of the mixture. Catalytic activity was measured by on-line gas chromatography. Reaction products were acrolein, Cop and water.
d
RESULTS and DISCUSSION
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The structure of the Moo3 Sb2O4 catalyst was extensively characterized by the physico-chemical techniques, and the results were used to elucidate the reaction mechanism. Figure 1 presents the microanalysis data by AEM. A reat number of SbpO4 and MOO particles was analyzed and the results show t at each particle is pure, i.e. mutua contamination is not detected. The characterization results obtained from XRD, SEM, XPS and ISS 21 lead to a conclusion that (1) the catalyst is composed of pure Moo3 and Sb2 4 articles, (2) these particles are well dispersed and exist with an intimate contact etween each other. The IR spectra of a sample containing 50 wt% Moo3 before and after adsorption of pyridine are iven in Figure 2,, After the adsorption, the bands at 1640, 1617, 1580, 1542, 14 2 and 1451 cm' appear (compared Fig.2a with 2b). The bands at 1617, 1580, 1492 and 1451 cm; disappear after outgassing at 383K (Fi , 2c) and those at 1640 and 1542 cm' disappear only after evacuation at 503K Fig. qd). According to the literature [3,4], the bands at 1617, 1580, 1492 to pyridine chemisorbed on the Lewis acid sites; those at and 145 cm' are 1640 and 1542 cm' are due to pyridine chemisorbed on Bronsted acid sites. The bond between pyridine and Bronsted site is stronger than that with Lewis site because the desorption of pyridine from Bronsted site occurred at a temperature (503K) higher than that for the desorption from Lewis site (383K). Similar results were obtained for the temperature programmed desorption Two desorption peaks were of NH3 from the catalysts containing MOO observed, one at 383K which corresponds to d l 3 desorbed from Lewis sites and another at 498K for the desorption from Bronsted sites. The amount of NH3 desorbed from both sites was determined by comparing the peak area with the integral thermal conductivity signals obtained from known amounts of NH . Figure 3 presents the catalytic activity as a function of t e NH3 concentration on different sites. It shows linear relationship between the propylene conversion and the amount of NH3 desorbed from Lewis sites. Similarly, the acrolein yield is found to be directly proportional to the amount of NH3 desorbed from the Bronsted sites. It should be mentioned that pure Sb2O4 does not possess any acidic properties, 1.e. pyridine and ammonia are not adsorbed on the Sb20 surface. As a result, pure SbpO4 is not active for the propylene oxidation. 8 u t when
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Sb2O4 is mixed with Moog, a dramatic increase of catalytic activity can be observed. To understand the role of Sb2O4 in the catalyst, a series of experiments was carried out. A previous reduced molybdenum oxide, MOO^-^, was mixed with Sb 04 and then reoxidized by gaseous oxygen. The reoxidation was foll w e t b y ESR measurements. Figure 4 presents the relative intensity of the Mo9+ signal (g = 1.93, width: ca. 100 Gauss) as function of reoxidation ti?? Before reoxidation (time < o), all samples have practe$ly the same Mo concentration. As soon as the reoxidation begins, the Mo concentration in the mixture is alwaygdower p f n that in pure Moog. This indicates that the in the mixture occurs more rapidly than that in the reoxidation of Mo to Mo pure Moog. An increase of the Sb2O4 content in the mixtures progressively accelerates the reoxidation. It is generally accepted [I] that the selective oxidation of propylene consists of four elementary steps: (1) chemisorption of gropylene, (2) abstraction of a-hydrogen to generate allylic species, a rate- etermlning step, (3) insertion of lattice oxygen into the allylic intermediates and (4) the reoxidation of the active site back to its initial and active state once it has undergone a reduction cycle during the oxidation of propylene to acrolein. Step (4) is vital in maintaining the catalyst activity. Our results suggest that propylene, bein a weak base, can be chemisorbed on the Lewis acid sites and converte to either acrolein or C02. The abstract of a-hydrogen which is the rate-determinin step leading to the acrolein formation is certainly related to Bronsted sites. Bot sites are located on the surface of MOO The role of Sb2O4 is to dissociate the gaseous oxygen into active species whict can migrate, at the contact region between the two oxides, onto the surface of Moog, where they complete redox cycle by reoxidiring the reduced molybdenum oxide. This results in the regeneration of deactivated sites on the surface of Moo3 during the reaction, and thus maintains the catalyst in an active and selective state. The catalysts for (amm)oxidation of propylene used in the industrial applications usually contain Mo and/or Sb oxides. Our study gives a clue for understandingthe behaviors of these kind of catalysts.
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REFERENCES
1. 2. 3. 4.
R.K. Grasselli, G. Centi and R. Trifiro, Appl. Catal. 57 (1990) 147. B. Zhou, S. Ceckiewicz and B. Delman, J. Phys. Chem. 91 (1987) 5061. E.P. Parry, J. Catal. 2 (1963) 13. K. Tanabe, "Solid Acids and Bases", Academic Press, New York, N.Y. 1970.
1966 NH3 desorbed from Lewis sites (M mol/m2 cata)
fl0
2
I
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I
Energy I kev
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Figure 1 AEM spectraof (1) Sb204 partlcles (2) MoQ particles
I
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NH3 desorbed from Bronsted sites mol/rn* cata)
Figure 3 Relationship between (a) propylene conversion and N& desorbed from Lewis sites @) acrolein yield and N* desorbed from Bronsted sites.
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-a
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Figure 2 Infrared spectra of the sample Mom: (a) before pyridine containing -96 adsorption; @) after pyridine adsorption and outgassing at 3531(; (c) @) t outgassing at 383% (d) (c) t outgassing at 503K
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Time ( h l Figure 4 Concentrationof ~ o 5 in Mom Sb204 as a function of reoxidation time.
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ROLE OF Mo AND Sb IN OXIDE CATALYSTS FOR SELECTIVE OXIDATION OF PROPYLENE B. Zhofl, X. Cud and R T.Chuangb aNational Laboratory of Fundamental Research in Catalysts, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China bDepartment of Chemical Engineering, University of Alberta, Edmonton, T6G 2G6 Alberta, Canada
Abstract
The activity of Moo3 - Sb2O4 catalyst for propylene oxidation was found to relate with the acidic property of Moo3 and redox property of Sb2O4. Results also indicate that the adsorption of propylene occurs on the Lewis sites, and the subsequent abstraction of a-hydrogen, which is the rate-determining step, takes place on the Bronsted sites. Both sites are located on the surface of Moog. The presence of SbzO4 in the catalyst facilitates the dissociation of molecular oxygen into active species which can subsequently migrate onto the surface of Moog, resulting in the reoxidation of the reduced active sites. INTRODUCTlON
.Mo and Sb have been postulated as the most important catalytic elements for the amm)oxidation of propylene [l]. Many studies have focused on the relations ip between surface property and activity for catalysts containing Moo3 or SbzO4, but their role in these catalysts is still not clear. The objective of our study is to elucidate the effect of these two elements in the selective oxidation of propylene; more specifically, the influence of acidic property of Moo3 and redox property of Sb2O4 on different elemental step of propylene oxidation.
h
EXPERIMENTAL
The catalysts were prepared by dispersing MOO and Sb2O4 in n-pentane. The detailed procedure has been described elsewhere ?2] The catal sts were characterized by BET, XRD, SEM, AEM, XPS and ISS techniques whic were also reported in ref [2]. Pyridine adsorption was studied by infrared spectroscopy. The spectra were obtained on a Pekin E er 580B infrared spectrophotometer operating in the range of 800 to 4000 cm-? The results were used to identify the Lewis and Bronsted acid sites on the catalysts. TPD of ammonia was carried out in order to determine the amount of Lewis and Bronsted sites resonance. The reoxidation of reduced catalyst was studied by electron spin
K
1964 resonance (ESR). A previously reduced Moog. gfs mixed with Sb2O4 accordin to a procedure described in ref [2]. the d o in this mixture gave a ! signal. The mixture was then reoxidized by oxygen (1% 0 2 in 99% strong ER He t 403K. The ESR spectra were recorded over a period %f 8 hours. The Mc$$ concentration was expressed as the relative intensity ([Mo '1 rel.) [Mo5+] rel. = (I/I0)hH2. (l/m)(l/MoO3wt%) where I is intensity of the Mo5+ ignal,; lo is the intensity of the internal standard; AHis width of the si nal for Mo5'; m is mass of the sample ar&MoOg wt% is the weight percent of 003 in the sample. The decrease of [Mo ] rel. represents the reoxidation degree of the mixture. Catalytic activity was measured by on-line gas chromatography. Reaction products were acrolein, Cop and water.
d
RESULTS and DISCUSSION
-
The structure of the Moo3 Sb2O4 catalyst was extensively characterized by the physico-chemical techniques, and the results were used to elucidate the reaction mechanism. Figure 1 presents the microanalysis data by AEM. A reat number of SbpO4 and MOO particles was analyzed and the results show t at each particle is pure, i.e. mutua contamination is not detected. The characterization results obtained from XRD, SEM, XPS and ISS 21 lead to a conclusion that (1) the catalyst is composed of pure Moo3 and Sb2 4 articles, (2) these particles are well dispersed and exist with an intimate contact etween each other. The IR spectra of a sample containing 50 wt% Moo3 before and after adsorption of pyridine are iven in Figure 2,, After the adsorption, the bands at 1640, 1617, 1580, 1542, 14 2 and 1451 cm' appear (compared Fig.2a with 2b). The bands at 1617, 1580, 1492 and 1451 cm; disappear after outgassing at 383K (Fi , 2c) and those at 1640 and 1542 cm' disappear only after evacuation at 503K Fig. qd). According to the literature [3,4], the bands at 1617, 1580, 1492 to pyridine chemisorbed on the Lewis acid sites; those at and 145 cm' are 1640 and 1542 cm' are due to pyridine chemisorbed on Bronsted acid sites. The bond between pyridine and Bronsted site is stronger than that with Lewis site because the desorption of pyridine from Bronsted site occurred at a temperature (503K) higher than that for the desorption from Lewis site (383K). Similar results were obtained for the temperature programmed desorption Two desorption peaks were of NH3 from the catalysts containing MOO observed, one at 383K which corresponds to d l 3 desorbed from Lewis sites and another at 498K for the desorption from Bronsted sites. The amount of NH3 desorbed from both sites was determined by comparing the peak area with the integral thermal conductivity signals obtained from known amounts of NH . Figure 3 presents the catalytic activity as a function of t e NH3 concentration on different sites. It shows linear relationship between the propylene conversion and the amount of NH3 desorbed from Lewis sites. Similarly, the acrolein yield is found to be directly proportional to the amount of NH3 desorbed from the Bronsted sites. It should be mentioned that pure Sb2O4 does not possess any acidic properties, 1.e. pyridine and ammonia are not adsorbed on the Sb20 surface. As a result, pure SbpO4 is not active for the propylene oxidation. 8 u t when
p1
9
b
E
I
8
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1965
Sb2O4 is mixed with Moog, a dramatic increase of catalytic activity can be observed. To understand the role of Sb2O4 in the catalyst, a series of experiments was carried out. A previous reduced molybdenum oxide, MOO^-^, was mixed with Sb 04 and then reoxidized by gaseous oxygen. The reoxidation was foll w e t b y ESR measurements. Figure 4 presents the relative intensity of the Mo9+ signal (g = 1.93, width: ca. 100 Gauss) as function of reoxidation ti?? Before reoxidation (time < o), all samples have practe$ly the same Mo concentration. As soon as the reoxidation begins, the Mo concentration in the mixture is alwaygdower p f n that in pure Moog. This indicates that the in the mixture occurs more rapidly than that in the reoxidation of Mo to Mo pure Moog. An increase of the Sb2O4 content in the mixtures progressively accelerates the reoxidation. It is generally accepted [I] that the selective oxidation of propylene consists of four elementary steps: (1) chemisorption of gropylene, (2) abstraction of a-hydrogen to generate allylic species, a rate- etermlning step, (3) insertion of lattice oxygen into the allylic intermediates and (4) the reoxidation of the active site back to its initial and active state once it has undergone a reduction cycle during the oxidation of propylene to acrolein. Step (4) is vital in maintaining the catalyst activity. Our results suggest that propylene, bein a weak base, can be chemisorbed on the Lewis acid sites and converte to either acrolein or C02. The abstract of a-hydrogen which is the rate-determinin step leading to the acrolein formation is certainly related to Bronsted sites. Bot sites are located on the surface of MOO The role of Sb2O4 is to dissociate the gaseous oxygen into active species whict can migrate, at the contact region between the two oxides, onto the surface of Moog, where they complete redox cycle by reoxidiring the reduced molybdenum oxide. This results in the regeneration of deactivated sites on the surface of Moo3 during the reaction, and thus maintains the catalyst in an active and selective state. The catalysts for (amm)oxidation of propylene used in the industrial applications usually contain Mo and/or Sb oxides. Our study gives a clue for understandingthe behaviors of these kind of catalysts.
8
R
REFERENCES
1. 2. 3. 4.
R.K. Grasselli, G. Centi and R. Trifiro, Appl. Catal. 57 (1990) 147. B. Zhou, S. Ceckiewicz and B. Delman, J. Phys. Chem. 91 (1987) 5061. E.P. Parry, J. Catal. 2 (1963) 13. K. Tanabe, "Solid Acids and Bases", Academic Press, New York, N.Y. 1970.
1966 NH3 desorbed from Lewis sites (M mol/m2 cata)
fl0
2
I
I
I
Energy I kev
I
01 5
Figure 1 AEM spectraof (1) Sb204 partlcles (2) MoQ particles
I
1.5
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NH3 desorbed from Bronsted sites mol/rn* cata)
Figure 3 Relationship between (a) propylene conversion and N& desorbed from Lewis sites @) acrolein yield and N* desorbed from Bronsted sites.
150
L Y
*-
-r
100
-a
so
I
2000
1500 Wavenumber
1000
I C1-l 1
Figure 2 Infrared spectra of the sample Mom: (a) before pyridine containing -96 adsorption; @) after pyridine adsorption and outgassing at 3531(; (c) @) t outgassing at 383% (d) (c) t outgassing at 503K
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Time ( h l Figure 4 Concentrationof ~ o 5 in Mom Sb204 as a function of reoxidation time.
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