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Al2O3 catalysts of cluster type
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
171
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
I N T E R A C T I O N B E T W E E N CO2 C A T A L Y S T S OF C L U S T E R T Y P E
AND
PROPYLENE
ON
Ru-Co/A1203
G.D.Zakumbaeva, L.B.Shapovalova, I.A.Shlygina
Institute of Organic Catalysis & Electrochemistry, 142 Kunaev St., 480100 Almaty, Republic of Kazakhstan, Grant TA-MOU-CA 13 041 US Agency for International Development.
I. Introduction The availability of CO2 makes it highly attractive for synthesis of oxygen containing compounds. CO2 molecule is inert and its catalytic activation is necessary. Application of heterogeneous catalytic systems opens wide possibilities for carrying out of organic reactions with CO2. Recently the interaction between CO2 and hexene-1 in solvent on Ru-Co/AI203 catalysts has been studied [ 1].
2.Experimental In this work the interaction between CO2 and C 3 H 6 o n Ru-Co (1:1)/A1203 - catalysts of clusters type has been studied. The process was carried out in flow type reactor in the range of 423 - 623 K and pressure variation from 0,1 to 2,0 MPa. The catalyst was prepared by impregnation of 7 - A1203 with mixture of RuOHCI3 and Co(NO3)2.6H20 solutions, and reduction with H2 during 3 hours at 773K.. Then it was washed from C1 " and N O 3 " ions and dried in the air at 302-323K. The catalyst was then reduced directly in the reactor during 1 hour at 473-673K before the reaction of CO2 and propylene. The preliminary experiments have shown independence of catalyst's activity on the reduction temperature in the indicated range. The Tred =573K was chosen as a standard in further experiments. The mixture C O 2 : C 3 H 6 = 4:1 with small concentration of propane impurity (<1,5%) was used. The space velocity of reagents was 150-200h -1. To elucidate the mechanism of CO2 + propylene reaction the physicochemical and quantum methods have been used. IR-spectra of CO2 were registered by UR-75-Spectrometer at room temperature in the 1200 - 3500 cm -l range. Quantum-chemical calculations have been made on the basis of EH method modified by the core-core repulsion in Anderson's ASED MO approximation [2]. Minimum number
172 of metal atoms has been examined because the chemisorption was-considered as highly localized phenomenon. The total optimization of geometry of CO2 molecule on a few rigid metal atoms has been made to estimate CO2 and its fragments' state on the catalyst surface. The structure and the state of Ru-Co/A1203 were studied by electron microscopy, electron diffraction method and ESXA. Quantum-chemical calculations were carried out by EHT method [3].
3. Results and discussion. The interaction of CO2 with propylene was studied under the mild conditions (Tex=423K, Pex=0,2 MPa). The propylene conversion is 10,5%. An increase of pressure from 0,2 to 1,0 MPa leads to raising the propylene conversion to 25,1%. The conversion is not changed with further increase of pressure up to 1,4 MPa and then at higher pressure (P=2,0 MPa) it decreases. At 1,1 MPa pressure the propylene conversion grows from 25,1 to 49,4% with the increase of temperature from 423 to 523K. At higher temperature (Tex=623K) the conversion decreases more than 2 times (20,5%). The optimal conditions for formation of oxygen containing compounds were Tex=523K and Pex=l 1 atm. The conversion of propylene is 49,4% under these conditions and total yield of oxygen containing products is 70%. The reaction products were analyzed by chromatography and chromatomassspectrometry. The complex mixture of oxygen containing organic compounds and C~ - C6 hydrocarbons was formed. Oxygen containing products consist of Cl - Ca aldehydes (formaldehyde, propionic aldehyde, butiraldehyde), C2 - C4 acids (acetic, propionic, butyric), acetone, ethanol and the traces of C3 - C4 alcohols of normal structure. The formation of C l - C6 hydrocarbons shows that the main reaction is accompanied with side processes of propylene destruction and dimerization with participation of CHx species and hydrogen is formed at propylene decomposition. The carbon oxide were detected in some experiments due to the dissociation of CO2 => Oads + COads on the specific centers of catalyst surface at high temperatures. Complex composition of reaction products shows that interaction of CO2+propylene is not selective process and occurs in several directions simultaneously. The IR-spectroscopy study of CO2 chemisorption was carried out to determine the mechanism of CO2 +propylene reaction. In a case of CO2 chemisorbtion on Ru-Co/AI203 reduced in H2 at 773K and stirred in the air there was not registered adsorption bands typical for adsorbed CO and CO2 structures in IR-spectrum. Intensive adsorption bands of OH-group (3400cm-1), at the range of deformation oscillation of OH-group (1620cm l ) and adsorption bands at 1570-1360cm -1 were recorded. The intensity of this bands was not changed after its treatment in H2 at 373-673K. Appearance of the adsorption bands at 2350cm 1 (CO2 ads), 1950-2020cm-l(COads) and 29302920cm -l (CHads) in IR-spectrum were observed for catalyst freshly reduced at 373K. The intensity of these adsorption bands increases at catalyst Treo rise to 673K. The influence of temperature on CO2 adsorption has been studied by IR spectroscopy. It was found that in the range of 373-573K (Tred=573K) the position of adsorption bands CO2 ads is not changed significantly. But the intensity of adsorption bands at 2350-2310cm -1,
173 1950-2020cm q and 2930-2920cm l grows due to the increase of CO2 amount and at the same time the dissociative processes are strengthened, giving rise to COads and CHx ads. The opportunity of deeper dissociation of CO2 according to scheme: C02 ads => COads => Cads +Oads
and interaction of Cads with adsorbed hydrogen together with CHx- structure formation (29302920cm l ) takes place. Ruthenium in Ru-Co/Al203 is in Ru 0 and RuO2 forms ( R u 3 p 3 / 2 = 460,8 - 461,8 eV) and cobalt is in oxidized state with C o 2 p 3 / 2 - 777,8 - 780,0 eV. However separate spots of aCo 0 and 13-Co 0 have been found by electron microscopy. Also the fine dispersed particles have been observed, its amount was increased with the rise of Co concentration and got maximum when content of Co was 70%. These particles were X-Ray amorphous. Oxidized states of Co bonded with tiny particles of Ru were presented in these formations. The clusters formation is available .Particles of Ru 0 bonded with A1203 have been also found. Quantum chemical results of full optimization of mono and bimetallic cluster geometry containing 2-4 atoms of metal of different multiplicity has been published in the Ref.[3]. It has been shown that bond energy decreases in the series: Co-Co > Co-Ru > RuRu. In mixed Co-Ru clusters the Co-Co bond is more stable and Ru-Ru bond is weakened in comparison with monometallic particles. So in process of cluster growth the formation of CoCo bond is preferable and it should be proposed that content of Ru atoms on bimetallic cluster surface is higher than in its volume. These results are in good correlation with experimentally found particle size of catalyst: Ru - clusters of d>40A, bimetallic particles forming in plenty of Ru form 80 to 160A, and in plenty of Co - 320A. Therefore we suppose, that the X-Ray amorphous structures are Co-Ru clusters. This suggestion is in accordance with quantum chemical calculations. The quantum chemical calculations showed, that depending on cluster structure and initial orientation of CO2 molecule either associative or dissociative (a) (b) CO2 adsorption can take place (Fig.l) . The transfer of CO2 molecule parallel to Ru7 and Co7 monometallic clusters plane is accompanied with complete OC )O destruction of CO2 molecule according to equation Figl. Optimized structure of adsorbed CO2 on Ru7 (a) and Co7 (b). Horizontal orientation CO2 in start C02 ads => Cads -i- 20ads 9 point of optimization and symmetrical structure of fixed monometallic 7-atomic clusters. At vertical approach of ,,
'
,-...~
,
,,
,_..~,
ec.
adsorbed molecule to monometallic Ru- and Co- clusters' plane or clusters containing one atom of second metal (Co6Rul and Ru6Co 1) the associative vertical adsorption occurs.
174 It must be noted, that axis deflection of CO2 molecules from vertical position (up to value of angle between normal to surface and axis of molecule CO2 ads close to 90 ~) accompanies with increase of total energy of the system to -1 eV. The complete optimization of the CO2 geometry from initial position at <45 ~ to surface brings to vertical reorientation of molecule. As follows from these results the approaching of COzadsorbed molecule to plane part of the surface must cause its vertical (linear) orientation, moreover in this case the adsorption proceeds via oxygen atom (Fig.2a,2b). The complete optimization of geometry and position of molecule C02 ads over bimetallic clusters from location corresponding to minimum of potential energy on adsorption curve showed: if content of second metal was > 30% and there was not local symmetry in the sample the dissociative adsorption was observed according to the scheme:
(b)
(a)
~.
(c)
_
~Ru
~Co
OC
eO
Fig.2a Optimized structure of adsorbed C02 on Ru7 (a), Co7 (b), Co7(H3). Vertical orientation C02 in the start point of the optimization and symmetrical structure of fixed monometallic 7-atomic clusters.
(a)
(b)
4
9
eCo
OC
eO
Fig.2b. Optimized structure of adsorbed C02 on Ru6Co(H3) (a), Co6Ru (b). Vertical orientation C02 in the start point of optimization and symmetrical structure of fixed bimetallic 7-atomic clusters.
O
(a) ,0
(b)
C02 ads => COads %-Oads (Fig.3)
The presence of defects on surface and small sizes of Ru-Co clusters lead to dissociative adsorption. As follows from IR-spectroscopy data and investigations of interactions process between CO2 and propylene the dissociative adsorption of CO2 is caused by high temperatures.
~Ru
~Co
OC
eO
Fig3. Optimized structure of adsorbed C02 on Co4Ru3 (a), Co5Ru2(H3) (b). Vertical orientation CO2 in the start point of optimization and nonsymmetrical structure of fixed bimetallic 7-atomic clusters.
175 Quantum-chemical calculations showed that Co and Ru in cluster have different affinity to CO2, CO and 02 molecules. Ru is characterized by lower affinity to CO than Co, but more high affinity to oxygen. So it may be suggested that during the CO2 dissociation on the Co-Ru-clusters the preferable formation of new bonds Co-COads and Ru-Oaos takes place. In complex Ru-Oads metal has positive charge and could activate the olefin molecule - typical donor of electrons. To clarify the mechanism of propylene adsorption on Ru-Co clusters the quantumchemical calculation of interaction between it and Ru-Co, Ru-Ru, and Co-Co clusters were carried out. During the calculation it was assumed that carbon atoms of C-C bond are situated parallel to metal-metal bond. The distance at which the cluster and absorbable molecule begin to interact is characterized by the nature of active center. Full optimization of C3H6 molecule geometry confirms that propylene adsorbs associatively on Co-Co cluster and forms ~-type complex. In other cases the dissociate adsorption of propylene is occurred. The presence of Ru atom provides significant electron density transfer from olefin molecule orbitals to dorbitals of ruthenium in bimetallic Ru-Co- or monometallic Ru-Ru-clasters (independently on either the tertiary carbon atom is located on ruthenium or cobalt atom.). At the same time the olefin C-C bond loosens substantially down to their break. Thus, analysis of experimental data and quantum-chemical calculations shows that the direction of interaction of propylene and CO2 or fragments of dissociation is determined by the mechanism of CO2 and propylene adsorption on clusters of different composition.
References. 1. G.D.Zakumbaeva, L.B.Shapovalova, Japan-FSU. Catalysis seminar'94. October 31 November 2, 1994, Japan, Tsukuba(1994)28-34. 2. A.B.Anderson,S.J.Hong,J.L.Smialek, J.Phys.Chem.No 16,V.91 (1987)4245-4250. 3. G.D.Zakumbaeva, L.B.Shapovalova, I.G.Efremenko, A.V.Gabdrakipov, J.Neftechimia, No5 (1996)427-438. The research was carried out at support of grant TA-MOU-CA 13 041 US Agency for Intemational development.
171
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
I N T E R A C T I O N B E T W E E N CO2 C A T A L Y S T S OF C L U S T E R T Y P E
AND
PROPYLENE
ON
Ru-Co/A1203
G.D.Zakumbaeva, L.B.Shapovalova, I.A.Shlygina
Institute of Organic Catalysis & Electrochemistry, 142 Kunaev St., 480100 Almaty, Republic of Kazakhstan, Grant TA-MOU-CA 13 041 US Agency for International Development.
I. Introduction The availability of CO2 makes it highly attractive for synthesis of oxygen containing compounds. CO2 molecule is inert and its catalytic activation is necessary. Application of heterogeneous catalytic systems opens wide possibilities for carrying out of organic reactions with CO2. Recently the interaction between CO2 and hexene-1 in solvent on Ru-Co/AI203 catalysts has been studied [ 1].
2.Experimental In this work the interaction between CO2 and C 3 H 6 o n Ru-Co (1:1)/A1203 - catalysts of clusters type has been studied. The process was carried out in flow type reactor in the range of 423 - 623 K and pressure variation from 0,1 to 2,0 MPa. The catalyst was prepared by impregnation of 7 - A1203 with mixture of RuOHCI3 and Co(NO3)2.6H20 solutions, and reduction with H2 during 3 hours at 773K.. Then it was washed from C1 " and N O 3 " ions and dried in the air at 302-323K. The catalyst was then reduced directly in the reactor during 1 hour at 473-673K before the reaction of CO2 and propylene. The preliminary experiments have shown independence of catalyst's activity on the reduction temperature in the indicated range. The Tred =573K was chosen as a standard in further experiments. The mixture C O 2 : C 3 H 6 = 4:1 with small concentration of propane impurity (<1,5%) was used. The space velocity of reagents was 150-200h -1. To elucidate the mechanism of CO2 + propylene reaction the physicochemical and quantum methods have been used. IR-spectra of CO2 were registered by UR-75-Spectrometer at room temperature in the 1200 - 3500 cm -l range. Quantum-chemical calculations have been made on the basis of EH method modified by the core-core repulsion in Anderson's ASED MO approximation [2]. Minimum number
172 of metal atoms has been examined because the chemisorption was-considered as highly localized phenomenon. The total optimization of geometry of CO2 molecule on a few rigid metal atoms has been made to estimate CO2 and its fragments' state on the catalyst surface. The structure and the state of Ru-Co/A1203 were studied by electron microscopy, electron diffraction method and ESXA. Quantum-chemical calculations were carried out by EHT method [3].
3. Results and discussion. The interaction of CO2 with propylene was studied under the mild conditions (Tex=423K, Pex=0,2 MPa). The propylene conversion is 10,5%. An increase of pressure from 0,2 to 1,0 MPa leads to raising the propylene conversion to 25,1%. The conversion is not changed with further increase of pressure up to 1,4 MPa and then at higher pressure (P=2,0 MPa) it decreases. At 1,1 MPa pressure the propylene conversion grows from 25,1 to 49,4% with the increase of temperature from 423 to 523K. At higher temperature (Tex=623K) the conversion decreases more than 2 times (20,5%). The optimal conditions for formation of oxygen containing compounds were Tex=523K and Pex=l 1 atm. The conversion of propylene is 49,4% under these conditions and total yield of oxygen containing products is 70%. The reaction products were analyzed by chromatography and chromatomassspectrometry. The complex mixture of oxygen containing organic compounds and C~ - C6 hydrocarbons was formed. Oxygen containing products consist of Cl - Ca aldehydes (formaldehyde, propionic aldehyde, butiraldehyde), C2 - C4 acids (acetic, propionic, butyric), acetone, ethanol and the traces of C3 - C4 alcohols of normal structure. The formation of C l - C6 hydrocarbons shows that the main reaction is accompanied with side processes of propylene destruction and dimerization with participation of CHx species and hydrogen is formed at propylene decomposition. The carbon oxide were detected in some experiments due to the dissociation of CO2 => Oads + COads on the specific centers of catalyst surface at high temperatures. Complex composition of reaction products shows that interaction of CO2+propylene is not selective process and occurs in several directions simultaneously. The IR-spectroscopy study of CO2 chemisorption was carried out to determine the mechanism of CO2 +propylene reaction. In a case of CO2 chemisorbtion on Ru-Co/AI203 reduced in H2 at 773K and stirred in the air there was not registered adsorption bands typical for adsorbed CO and CO2 structures in IR-spectrum. Intensive adsorption bands of OH-group (3400cm-1), at the range of deformation oscillation of OH-group (1620cm l ) and adsorption bands at 1570-1360cm -1 were recorded. The intensity of this bands was not changed after its treatment in H2 at 373-673K. Appearance of the adsorption bands at 2350cm 1 (CO2 ads), 1950-2020cm-l(COads) and 29302920cm -l (CHads) in IR-spectrum were observed for catalyst freshly reduced at 373K. The intensity of these adsorption bands increases at catalyst Treo rise to 673K. The influence of temperature on CO2 adsorption has been studied by IR spectroscopy. It was found that in the range of 373-573K (Tred=573K) the position of adsorption bands CO2 ads is not changed significantly. But the intensity of adsorption bands at 2350-2310cm -1,
173 1950-2020cm q and 2930-2920cm l grows due to the increase of CO2 amount and at the same time the dissociative processes are strengthened, giving rise to COads and CHx ads. The opportunity of deeper dissociation of CO2 according to scheme: C02 ads => COads => Cads +Oads
and interaction of Cads with adsorbed hydrogen together with CHx- structure formation (29302920cm l ) takes place. Ruthenium in Ru-Co/Al203 is in Ru 0 and RuO2 forms ( R u 3 p 3 / 2 = 460,8 - 461,8 eV) and cobalt is in oxidized state with C o 2 p 3 / 2 - 777,8 - 780,0 eV. However separate spots of aCo 0 and 13-Co 0 have been found by electron microscopy. Also the fine dispersed particles have been observed, its amount was increased with the rise of Co concentration and got maximum when content of Co was 70%. These particles were X-Ray amorphous. Oxidized states of Co bonded with tiny particles of Ru were presented in these formations. The clusters formation is available .Particles of Ru 0 bonded with A1203 have been also found. Quantum chemical results of full optimization of mono and bimetallic cluster geometry containing 2-4 atoms of metal of different multiplicity has been published in the Ref.[3]. It has been shown that bond energy decreases in the series: Co-Co > Co-Ru > RuRu. In mixed Co-Ru clusters the Co-Co bond is more stable and Ru-Ru bond is weakened in comparison with monometallic particles. So in process of cluster growth the formation of CoCo bond is preferable and it should be proposed that content of Ru atoms on bimetallic cluster surface is higher than in its volume. These results are in good correlation with experimentally found particle size of catalyst: Ru - clusters of d>40A, bimetallic particles forming in plenty of Ru form 80 to 160A, and in plenty of Co - 320A. Therefore we suppose, that the X-Ray amorphous structures are Co-Ru clusters. This suggestion is in accordance with quantum chemical calculations. The quantum chemical calculations showed, that depending on cluster structure and initial orientation of CO2 molecule either associative or dissociative (a) (b) CO2 adsorption can take place (Fig.l) . The transfer of CO2 molecule parallel to Ru7 and Co7 monometallic clusters plane is accompanied with complete OC )O destruction of CO2 molecule according to equation Figl. Optimized structure of adsorbed CO2 on Ru7 (a) and Co7 (b). Horizontal orientation CO2 in start C02 ads => Cads -i- 20ads 9 point of optimization and symmetrical structure of fixed monometallic 7-atomic clusters. At vertical approach of ,,
'
,-...~
,
,,
,_..~,
ec.
adsorbed molecule to monometallic Ru- and Co- clusters' plane or clusters containing one atom of second metal (Co6Rul and Ru6Co 1) the associative vertical adsorption occurs.
174 It must be noted, that axis deflection of CO2 molecules from vertical position (up to value of angle between normal to surface and axis of molecule CO2 ads close to 90 ~) accompanies with increase of total energy of the system to -1 eV. The complete optimization of the CO2 geometry from initial position at <45 ~ to surface brings to vertical reorientation of molecule. As follows from these results the approaching of COzadsorbed molecule to plane part of the surface must cause its vertical (linear) orientation, moreover in this case the adsorption proceeds via oxygen atom (Fig.2a,2b). The complete optimization of geometry and position of molecule C02 ads over bimetallic clusters from location corresponding to minimum of potential energy on adsorption curve showed: if content of second metal was > 30% and there was not local symmetry in the sample the dissociative adsorption was observed according to the scheme:
(b)
(a)
~.
(c)
_
~Ru
~Co
OC
eO
Fig.2a Optimized structure of adsorbed C02 on Ru7 (a), Co7 (b), Co7(H3). Vertical orientation C02 in the start point of the optimization and symmetrical structure of fixed monometallic 7-atomic clusters.
(a)
(b)
4
9
eCo
OC
eO
Fig.2b. Optimized structure of adsorbed C02 on Ru6Co(H3) (a), Co6Ru (b). Vertical orientation C02 in the start point of optimization and symmetrical structure of fixed bimetallic 7-atomic clusters.
O
(a) ,0
(b)
C02 ads => COads %-Oads (Fig.3)
The presence of defects on surface and small sizes of Ru-Co clusters lead to dissociative adsorption. As follows from IR-spectroscopy data and investigations of interactions process between CO2 and propylene the dissociative adsorption of CO2 is caused by high temperatures.
~Ru
~Co
OC
eO
Fig3. Optimized structure of adsorbed C02 on Co4Ru3 (a), Co5Ru2(H3) (b). Vertical orientation CO2 in the start point of optimization and nonsymmetrical structure of fixed bimetallic 7-atomic clusters.
175 Quantum-chemical calculations showed that Co and Ru in cluster have different affinity to CO2, CO and 02 molecules. Ru is characterized by lower affinity to CO than Co, but more high affinity to oxygen. So it may be suggested that during the CO2 dissociation on the Co-Ru-clusters the preferable formation of new bonds Co-COads and Ru-Oaos takes place. In complex Ru-Oads metal has positive charge and could activate the olefin molecule - typical donor of electrons. To clarify the mechanism of propylene adsorption on Ru-Co clusters the quantumchemical calculation of interaction between it and Ru-Co, Ru-Ru, and Co-Co clusters were carried out. During the calculation it was assumed that carbon atoms of C-C bond are situated parallel to metal-metal bond. The distance at which the cluster and absorbable molecule begin to interact is characterized by the nature of active center. Full optimization of C3H6 molecule geometry confirms that propylene adsorbs associatively on Co-Co cluster and forms ~-type complex. In other cases the dissociate adsorption of propylene is occurred. The presence of Ru atom provides significant electron density transfer from olefin molecule orbitals to dorbitals of ruthenium in bimetallic Ru-Co- or monometallic Ru-Ru-clasters (independently on either the tertiary carbon atom is located on ruthenium or cobalt atom.). At the same time the olefin C-C bond loosens substantially down to their break. Thus, analysis of experimental data and quantum-chemical calculations shows that the direction of interaction of propylene and CO2 or fragments of dissociation is determined by the mechanism of CO2 and propylene adsorption on clusters of different composition.
References. 1. G.D.Zakumbaeva, L.B.Shapovalova, Japan-FSU. Catalysis seminar'94. October 31 November 2, 1994, Japan, Tsukuba(1994)28-34. 2. A.B.Anderson,S.J.Hong,J.L.Smialek, J.Phys.Chem.No 16,V.91 (1987)4245-4250. 3. G.D.Zakumbaeva, L.B.Shapovalova, I.G.Efremenko, A.V.Gabdrakipov, J.Neftechimia, No5 (1996)427-438. The research was carried out at support of grant TA-MOU-CA 13 041 US Agency for Intemational development.