- Email: [email protected]
Deactivation of Ni supported on alumina-titania: Modelling of coke deposition in the phenylacetylene hydrogenation
CatalystDeactivation 1999 B. Delmonand G.F.Froment(Editors) 9 1999ElsevierScienceB.V.All rightsreserved.
439
Deactivation of Ni Supported on Alumina-Titania: Modelling of Coke Deposition in the Phenylacetylene Hydrogenation Gustavo Prrez l, Ju~in Carlos Rodriguez 2, Antonio Monzrn 2, Tomr.s Viveros I* lArea de Ingenieria Quimica, Depto. Ingenieria de Procesos e Hidrhulica, UAM-Iztapalapa, Apdo. Postal 55-534, Mrxico, D.F., 09340, Mrxico. 2Departamemo de Ingenieria Quimica y Tecnologias del Medio Ambiente, Facultad de Ciencias, Universidad de Zaragoza, Zaragoza 50009, Spain.
Abstract
The deactivation by coke of Ni supported on alumina-titania was studied in the hydrogenation of phenylacetylene. The method of preparation of the support, precipitation or sol-gel, had an important effect on the activity, selectivity and deactivation rate. The process was modelled using a consecutive reaction scheme and assuming the parallel formation of coke with phenylacetylene as the precursor.
I. INTRODUCTION Alumina is a catalytic support widely used. The acidic nature of this material favors coke deposition and it is possible to modify the acid-base properties of A1203 by Ti promotion. The preparation of mixed oxides by precipitation and sol-gel methods allows the synthesis of materials with differences in textural, structural and surface properties. This is the case of A1203-TiO2 mixed oxides, as has been reported [1,2]. The deactivation of nickel catalysts by coke deposition and sintering was reported to be dependent on the type of support, the preparation procedure and the reduction temperature; demonstrating also that the presence of Ti in the catalyst produced higher selectivities of the intermediate product in the hydrogenation of acetylene [3]. The deactivation by coke has been studied through models representing a variety of mechanisms, catalysts and reactions [4,5]. In general these mechanisms represent the main reaction occurring in parallel with the deactivating reaction. In this work a model for the deactivation by coke of Ni/AI203-TiO2 catalysts in the hydrogenation of phenylacetylene is presented. The model is based on the occurrence of a consecutive reaction and the parallel deactivation scheme. The data were obtained in an atmospheric system and the effect of the method of preparation and the composition of the supports were studied.
* To whom correspondence should be addressed.
440 2. EXPERIMENTAL 2.1 Catalyst preparation and characterization Two sets of catalysts were prepared, based on supports synthesized by sol-gel or precipitation. Sol-gel supports were obtained by the hydrolysis of the alkoxides (AI trisecbutoxide, and Ti isopropoxide), whereas the precipitation supports were prepared by the hydrolysis of the corresponding chlorides. Two AI/Ti atomic ratios were prepared, AI/Ti = 10 & 25. The nickel catalysts were obtained by the wet impregnation of the supports, previously calcined at 600~ The samples were calcined at 400~ and reduced at 500~ for 12 h. The crystallographic analysis was made by X-ray diffraction (Siemens) and the textural analysis was made by nitrogen adsorption (Quantachrom). Ni metallic areas were determined by hydrogen chemisorption (Micromeritics). 2.2 Reaction tests The reaction was carried out at 300~ at atmospheric pressure in a glass reactor. The gas feed was hydrogen satured with phenylacetylene with a partial pressure of 4.8 Torr. The exit gases were analyzed by gas chromatography. 3. DEACTIVATION MODEL The modelling of the deactivation processs was performed considering a consecutive reaction scheme, with styrene as intermediate product; the surface hydrogenation as the reaction control step and the formation of coke with phenylacetylene as the coke precursor [2,31"
(1)
A- ki ; B - - - - ~ C A -~ P $
(2)
Where A, B and C are phenylacetylene, styrene and ethylbenzene. P is the coke deposited on the catalytic surface. The activities at and a2 of the consecutive reactions are given by the equations [2]: aI =
a2 =
T, Pj
-I l
r, Pj
=
L
(4' =
L
For a flow reactor with a x spacetime, the rates of reaction and rates of deactivation are given by:
(5) dPA = kl PAa I dT
(6)
dPB = k 1PAal - k 2 PBa2 dV da I dt
kotpAadl
(7)
441
da 2 _ ko2 PAad2z (8) dt The model estimates the consecutive reaction constants (kl, k2) and the deactivation constants (kDl, kD2).
4. RESULTS AND DISCUSSION The results show that the deactivation by coke deposition on Ni catalysts in the phenylacetylene hydrogenation shows differences in the activity and selectivity as a function of the method of preparation and composition of the support. Figures 1 and 2 illustrate the changes in the activity and the products distribution, both experimental and calculated by the model, for two catalysts using a support with the same method of synthesis and different composition. It was found that as the titania content was increased the deactivation rate decreased and a better selectivity to the intermediate product (styrene) was obtained. The same behaviour was observed for the catalysts prepared using sol-gel supports. Spillover and strong metal-support interaction effects are thought to have an important role in this type of catalysts, modifying the deactivation performance. 0.008
0.008
0.006
0.006
o
.o 0.004
~ 0.004
0.002
~ 0.002
0
0
~
0
2
4 6 8 Time (hours) 9 Aexp = Bexp a Cexp e Acal = Bcal ~ Coal
Figure 1. Distribution of products, experimental and calculated. Ni soported on AI/Ti = 10 precipitated
0
.
.
.
.
.
.
.
!
2
4 6 Time (hours) 9 Aexp m Bexp a Cexp e ~Acal
=
Bcal
~.". Coal
Figure 2. Distribution of products, experimental and calculated. Ni soported on AI/Ti=25 precipitated
The model was fitted to the experimental results using a least squares method. The results obtained are given in Table 1. In this case besides the surface area and the metal content of the catalysts, the values ofkl, k2, kD1, kD2, dl and d2 are also given. According to the kDi and kD2 values obtained, a lower deactivation is observed for catalysts supported on mixed oxides. These results are noticeable due to the fact that alumina-titania mixed oxide supports have shown to be more acidic than alumina [ 1, 2]. This contradictory behaviour is indicative of the change in the interaction of the support with the active phase by Ti promotion.
442 0.008
0.008
0.006
.o
.o
o
o
~ o.004
0.006
lunn
0.004
0
0
0.002
0.002
0
2
4
6
0
Time (hours) 9 Aexp [] Bexp 9 Cexp e Acal m Bcal ~ Ccal Figure 3. Distribution of products, experimental and calculated. Ni soported on AI/Ti=25 sol-gel.
2
4
6
8
Time (hours) 9 Aexp [] Bexp 9 Cexp e Acal =~ Bcal ~ Coal Figure 4. Distribution of products, experimental and calculated. Ni soported on A1/Ti=I 0 sol-gel.
Table 1. Catalysts propertie s and model parameters. Sample a
Ni SBET XRD klx 10.3 k2x 10.4 kvt ko2 dl d2 wt% m2/g Phases mol.s"1g! mol's"~g,~ atm~s~ atmIs ! ....... Ni/A1203P .....15.1 247 rI 0.510 0.978 0.259 0.158 2.76 3.88 Ni/AITi25P 12.6 260 rl 0.269 0.357 0.223 0.113 2.40 3.77 Ni/AITil0P 14.5 268 rl 0.425 0.252 0.076 0.033 1.56 2.28 Ni/A1Ti25S 14.6 426 ~, 0.478 0.677 0.271 0.163 1.82 2.36 Ni/A1Til0S 17.7.... 368 ~/ 0.544 0.785 0.171 0.120 1.90 2.28 aThe letter P or S after the sample name denotes the method of preparation: precipitation (P) or sol-gel (S). ACKNOWLEDGMENTS The authors thank the financial support of Conacyt (Mexico) and ICI (Spain). REFERENCES 1. J.A. Montoya, D. Chadwick, J.M. Dominguez, I. Schifter, J. Navarrete, K. Zheng, T. Viveros, J. Sol-gel Sci. & Tech., 2 (1994) 431 2. T. Viveros, A. Zarate, M.A. Lopez, J.A. Montoya, R. Ruiz, M. Portilla, Stud. Surf. Sci. & Catal., 91 (1995) 807. 3. J.C. Rodriguez, T. Viveros, A. Monz6n, Stud. Surf. Sci. & Catal., 111 (1997) 609. 4. J. Corella, J. M. Asfia, Ind. Eng. Chem. Process Des. Dev. 21(1982)55 5. J.C. Rodriguez, J.A. Pefia, A. Monzon, R. Hughes, K. Li, Chem. Eng. J., 58 (1995) 7.
439
Deactivation of Ni Supported on Alumina-Titania: Modelling of Coke Deposition in the Phenylacetylene Hydrogenation Gustavo Prrez l, Ju~in Carlos Rodriguez 2, Antonio Monzrn 2, Tomr.s Viveros I* lArea de Ingenieria Quimica, Depto. Ingenieria de Procesos e Hidrhulica, UAM-Iztapalapa, Apdo. Postal 55-534, Mrxico, D.F., 09340, Mrxico. 2Departamemo de Ingenieria Quimica y Tecnologias del Medio Ambiente, Facultad de Ciencias, Universidad de Zaragoza, Zaragoza 50009, Spain.
Abstract
The deactivation by coke of Ni supported on alumina-titania was studied in the hydrogenation of phenylacetylene. The method of preparation of the support, precipitation or sol-gel, had an important effect on the activity, selectivity and deactivation rate. The process was modelled using a consecutive reaction scheme and assuming the parallel formation of coke with phenylacetylene as the precursor.
I. INTRODUCTION Alumina is a catalytic support widely used. The acidic nature of this material favors coke deposition and it is possible to modify the acid-base properties of A1203 by Ti promotion. The preparation of mixed oxides by precipitation and sol-gel methods allows the synthesis of materials with differences in textural, structural and surface properties. This is the case of A1203-TiO2 mixed oxides, as has been reported [1,2]. The deactivation of nickel catalysts by coke deposition and sintering was reported to be dependent on the type of support, the preparation procedure and the reduction temperature; demonstrating also that the presence of Ti in the catalyst produced higher selectivities of the intermediate product in the hydrogenation of acetylene [3]. The deactivation by coke has been studied through models representing a variety of mechanisms, catalysts and reactions [4,5]. In general these mechanisms represent the main reaction occurring in parallel with the deactivating reaction. In this work a model for the deactivation by coke of Ni/AI203-TiO2 catalysts in the hydrogenation of phenylacetylene is presented. The model is based on the occurrence of a consecutive reaction and the parallel deactivation scheme. The data were obtained in an atmospheric system and the effect of the method of preparation and the composition of the supports were studied.
* To whom correspondence should be addressed.
440 2. EXPERIMENTAL 2.1 Catalyst preparation and characterization Two sets of catalysts were prepared, based on supports synthesized by sol-gel or precipitation. Sol-gel supports were obtained by the hydrolysis of the alkoxides (AI trisecbutoxide, and Ti isopropoxide), whereas the precipitation supports were prepared by the hydrolysis of the corresponding chlorides. Two AI/Ti atomic ratios were prepared, AI/Ti = 10 & 25. The nickel catalysts were obtained by the wet impregnation of the supports, previously calcined at 600~ The samples were calcined at 400~ and reduced at 500~ for 12 h. The crystallographic analysis was made by X-ray diffraction (Siemens) and the textural analysis was made by nitrogen adsorption (Quantachrom). Ni metallic areas were determined by hydrogen chemisorption (Micromeritics). 2.2 Reaction tests The reaction was carried out at 300~ at atmospheric pressure in a glass reactor. The gas feed was hydrogen satured with phenylacetylene with a partial pressure of 4.8 Torr. The exit gases were analyzed by gas chromatography. 3. DEACTIVATION MODEL The modelling of the deactivation processs was performed considering a consecutive reaction scheme, with styrene as intermediate product; the surface hydrogenation as the reaction control step and the formation of coke with phenylacetylene as the coke precursor [2,31"
(1)
A- ki ; B - - - - ~ C A -~ P $
(2)
Where A, B and C are phenylacetylene, styrene and ethylbenzene. P is the coke deposited on the catalytic surface. The activities at and a2 of the consecutive reactions are given by the equations [2]: aI =
a2 =
T, Pj
-I l
r, Pj
=
L
(4' =
L
For a flow reactor with a x spacetime, the rates of reaction and rates of deactivation are given by:
(5) dPA = kl PAa I dT
(6)
dPB = k 1PAal - k 2 PBa2 dV da I dt
kotpAadl
(7)
441
da 2 _ ko2 PAad2z (8) dt The model estimates the consecutive reaction constants (kl, k2) and the deactivation constants (kDl, kD2).
4. RESULTS AND DISCUSSION The results show that the deactivation by coke deposition on Ni catalysts in the phenylacetylene hydrogenation shows differences in the activity and selectivity as a function of the method of preparation and composition of the support. Figures 1 and 2 illustrate the changes in the activity and the products distribution, both experimental and calculated by the model, for two catalysts using a support with the same method of synthesis and different composition. It was found that as the titania content was increased the deactivation rate decreased and a better selectivity to the intermediate product (styrene) was obtained. The same behaviour was observed for the catalysts prepared using sol-gel supports. Spillover and strong metal-support interaction effects are thought to have an important role in this type of catalysts, modifying the deactivation performance. 0.008
0.008
0.006
0.006
o
.o 0.004
~ 0.004
0.002
~ 0.002
0
0
~
0
2
4 6 8 Time (hours) 9 Aexp = Bexp a Cexp e Acal = Bcal ~ Coal
Figure 1. Distribution of products, experimental and calculated. Ni soported on AI/Ti = 10 precipitated
0
.
.
.
.
.
.
.
!
2
4 6 Time (hours) 9 Aexp m Bexp a Cexp e ~Acal
=
Bcal
~.". Coal
Figure 2. Distribution of products, experimental and calculated. Ni soported on AI/Ti=25 precipitated
The model was fitted to the experimental results using a least squares method. The results obtained are given in Table 1. In this case besides the surface area and the metal content of the catalysts, the values ofkl, k2, kD1, kD2, dl and d2 are also given. According to the kDi and kD2 values obtained, a lower deactivation is observed for catalysts supported on mixed oxides. These results are noticeable due to the fact that alumina-titania mixed oxide supports have shown to be more acidic than alumina [ 1, 2]. This contradictory behaviour is indicative of the change in the interaction of the support with the active phase by Ti promotion.
442 0.008
0.008
0.006
.o
.o
o
o
~ o.004
0.006
lunn
0.004
0
0
0.002
0.002
0
2
4
6
0
Time (hours) 9 Aexp [] Bexp 9 Cexp e Acal m Bcal ~ Ccal Figure 3. Distribution of products, experimental and calculated. Ni soported on AI/Ti=25 sol-gel.
2
4
6
8
Time (hours) 9 Aexp [] Bexp 9 Cexp e Acal =~ Bcal ~ Coal Figure 4. Distribution of products, experimental and calculated. Ni soported on A1/Ti=I 0 sol-gel.
Table 1. Catalysts propertie s and model parameters. Sample a
Ni SBET XRD klx 10.3 k2x 10.4 kvt ko2 dl d2 wt% m2/g Phases mol.s"1g! mol's"~g,~ atm~s~ atmIs ! ....... Ni/A1203P .....15.1 247 rI 0.510 0.978 0.259 0.158 2.76 3.88 Ni/AITi25P 12.6 260 rl 0.269 0.357 0.223 0.113 2.40 3.77 Ni/AITil0P 14.5 268 rl 0.425 0.252 0.076 0.033 1.56 2.28 Ni/A1Ti25S 14.6 426 ~, 0.478 0.677 0.271 0.163 1.82 2.36 Ni/A1Til0S 17.7.... 368 ~/ 0.544 0.785 0.171 0.120 1.90 2.28 aThe letter P or S after the sample name denotes the method of preparation: precipitation (P) or sol-gel (S). ACKNOWLEDGMENTS The authors thank the financial support of Conacyt (Mexico) and ICI (Spain). REFERENCES 1. J.A. Montoya, D. Chadwick, J.M. Dominguez, I. Schifter, J. Navarrete, K. Zheng, T. Viveros, J. Sol-gel Sci. & Tech., 2 (1994) 431 2. T. Viveros, A. Zarate, M.A. Lopez, J.A. Montoya, R. Ruiz, M. Portilla, Stud. Surf. Sci. & Catal., 91 (1995) 807. 3. J.C. Rodriguez, T. Viveros, A. Monz6n, Stud. Surf. Sci. & Catal., 111 (1997) 609. 4. J. Corella, J. M. Asfia, Ind. Eng. Chem. Process Des. Dev. 21(1982)55 5. J.C. Rodriguez, J.A. Pefia, A. Monzon, R. Hughes, K. Li, Chem. Eng. J., 58 (1995) 7.