- Email: [email protected]
The influence of metal-support interactions during liquid-phase hydrogenation of an α, β-unsaturated aldehyde over Pt
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
497
The Influence of Metal-Support Interactions During Liquid-Phase Hydrogenation of an ~, [3-Unsaturated Aldehyde over Pt Utpal K. Singh and M. Albert Vannice Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802 Liquid-phase hydrogenation of citral was investigated over SiO2- and TiO2- supported Pt catalysts in the range of 298 - 423 K, 7 - 41 atm H2 pressure, and 0.5 - 6.0 M citral in hexane. The initial rate of citral hydrogenation over Pt/SiO2 catalysts exhibits an activity minimum with respect to temperature accompanied by an increase in selectivity for hydrogenation of the C=O bond with increasing reaction temperature. Furthermore, the initial rate of citral disappearance is strongly influenced by metal particle size since the rate of citral disappearance at 373 K decreased 20-fold from the 5 nm SiO2-supported Pt crystallites to the 1 nm SiO2-supported Pt crystallites. This difference is suppressed at 298 K and only a five-fold decrease in the rate is observed during this change in Pt crystallite size. Pt/TiO2-LTR (low temperature reduced - 473 K) and Pt/SiO2 catalysts exhibited zero- and first-order dependencies on citral concentration and hydrogen pressure, respectively, at 373 K. In contrast, the Pt/TiO2-HTR (high temperature reduced - 773 K) catalyst exhibited negative first- and zero-order dependencies on citral concentration and hydrogen pressure, respectively. The TOF on the Pt/TiOz-HTR catalyst was more than an order of magnitude greater than that on Pt/SiO2 and Pt/TiO2-LTR. In addition, the Pt/TiO2-HTR catalyst exhibited a marked enhancement in selectivity towards hydrogenation of the C=O bond. 1. INTRODUCTION It has been stated that approximately 50-100 kg of by-product are produced per kg of product in the fine chemicals and pharmaceutical sectors of the chemical industry [1]. Therefore, in light of the increased environmental awareness, it is of interest to develop heterogeneous catalysts for synthesis of pharmaceuticals and fine chemicals In the present work we report the influence of metal-support interactions (MSI) during the selective liquidphase hydrogenation of citral (3,7-dimethyl-2,6-octadienal), which contains three unsaturated bonds including a conjugated system of C=C and C=O bonds and an isolated C=C bond. From a thermodynamic perspective, the isolated C=C bond is the most favorable to hydrogenate followed by the conjugated C=C bond and lastly the C=O bond [2]. However, kinetic control of the reaction can be induced to yield high selectivity for hydrogenation of the C=O bond alone. In the present paper we examine the influence of reaction parameters and support effects on selective hydrogenation of citral.
498 2. EXPERIMENTAL
The details of catalyst synthesis, characterization, and hydrogenation reaction procedures are described in detail elsewhere [3]. Briefly, the catalysts were prepared via incipient wetness or ion exchange using H2PtC16 or Pt(NH3)4CI2, respectively, as the precursor. SiO2 (Davison Grade 57 silica gel - 220 m2/g) and TiO2 (Degussa P25 - 47 m2/g) were dried and calcined at 773 K for four hours prior to catalyst synthesis followed by drying at 393 K overnight. The catalysts were characterized using H2 and CO chemisorption at 300 K to evaluate dispersion and average particle size. Pt/SiO2, Pt/TiO2-LTR (low temperature reduced), and Pt/TiO2-HTR (high temperature reduced) were reduced in situ at 673 K, 473 K, and 773 K, respectively. Nitrogen was bubbled through both hexane and citral prior to their addition into the reactor to remove trace quantities of oxygen from the liquid phase. The reaction progress was monitored by GC analysis of liquid samples periodically withdrawn into a N2 purged vessel as well as by the instantaneous rate of H2 uptake [3]. 3. Results and Discussion 3.1 Citral Hydrogenation over Pt/SiO2
The kinetic data obtained with Pt/SiO2 catalysts was previously shown to be free of transport limitations and poisoning effects as verified by the Madon-Boudart test [3, 4]. The influence of reaction temperature on rate and product distribution is displayed in Figures 1 and 2, respectively, for reaction over a 1.44% Pt/SiO2 catalyst at 20 atm hydrogen pressure and 1 M citral in hexane in the range of 298 - 423 K [3]. .......
-"" -~
~"
~ ,-
L
80 /
|
0.6 t[
9
.
i
i 0.4 + i
i
-- - 2 9 8 K 1
', '~ ~-
, o
'
i
20
:~" i /-~ 40 ~-~",~ "~ ~
"",.,.,ih,... i."
~ - ~ ~
0
,
.... 373K ~ 423 K i
~
7 40
60
Citral Conversion (%)
Figure 1:Temporal H2 uptake profiles for reaction at 298 K, 373 K, 423 K with 1 M citral in hexane at 20 atm H2 pressure.
20 t
.
.
.
.
.
I
....
-0 ....
,i
I o Conjugated C=C Bond (298 K);; o Conjugated C=C Bond (373 K)':'
~ 1 ~ - - - - '
O~ L
t
0
20
~ ---
40
~
---
60
I "~
80
100
Citral Conversion (%)
Figure 2" Selectivity for hydrogenation of conjugated C=O and C=C bonds at 298 K, and 373 K with 1 M citral in hexane at 20 atm H2 pressure.
It is apparent from Figure 1 that there is an activity minimum in the rate with respect to temperature. Furthermore, significant deactivation is observed at 298 K in contrast to the negligible deactivation exhibited at 373 K and 423 K. The product distribution is dramatically altered by reaction temperature as seen in Figure 2. The selectivity towards
499
hydrogenation of the conjugated C=C bond is greater than that for the C=O bond at 298 K. This trend is reversed at 373 K where hydrogenation of C=O bond is the primary reaction pathway. This unusual kinetic behavior was rationalized by utilizing a conventional Langmuir-Hinshelwood-type model for each of the hydrogenation steps along with a concurrent inhibiting decarbonylation reaction. The activity minimum has been explained based on the lower activation barrier for the decomposition reaction yielding chemisorbed CO as compared to that for CO desorption [3, 5, 6]. Such a trend was shown to have three consequences including: strong deactivation during reaction at low temperatures i.e., 298 K, an activity minimum at approximately 373 K, and negligible deactivation at 373 K and 423 K [3]. The complex kinetics observed during liquid-phase hydrogenation is also manifested in the apparent structure sensitivity of the reaction at 373 K with 20 atm H2 pressure and 1 M citral in hexane as shown in Table 1. The differences in the rate of citral hydrogenation among catalysts with different dispersion was suppressed at 298 K in contrast to the behavior at 373 K. Due to the strong deactivation behavior at 298 K, there is significant uncertainty present in reporting a single value for the rate at this temperature. Therefore, Figure 3 displays the temporal H2 uptake profile for reaction over Pt/SiO2 catalysts with varying metal dispersion at 298 K, 20 atm H2 pressure, and 1 M citral in hexane. It should be noted that in spite of the dramatic differences in the initial rate of citral hydrogenation for catalysts with different particle sizes, the selectivity vs. conversion behavior was similar for all the catalysts, within experimental uncertainty, as shown in Figure 4. Table 1 Effect of average Pt particle size, determined from H2 chemisorption at 300 K, on the initial rate of citral hydrogenation at 373 K, 20 atm H2 pressure, with 1 M citral in hexane. Dispersion H/Pt
Metal Particle Size
Initial TOF
Pt/SiO 2
(nm)
(s")
3.59% Pt
0.04
28
0.120
6.65% Pt
0.09
2.65% Pt
0.22
5.1
0.092
3.80% Pt (sintered)
0.37
3.1
0.040
1.44% Pt
0.41
2.8
0.017
0.49% Pt
0.45
2.5
0.012
3.80% Pt
0.66
1.7
0.010
1.1
0.005
Catalyst
0.77% Pt
0.065
500
0.8
..............................................................................................................
r~
0.6
o H / p t = 0.09
,., e~
9 H / Pt =
0.4
o H / Pt = 0.4
\',,,
~.
\o
~ H / Pt =0.7
0.2
~o
" H/Pt
i o~....'-~___~---~__ [-
0
0.2
-----I_
~---~_---~___^
0
= 1.0
- ....... o ~ - - - - o _ o ~ _ o
60
"-=---u--~--~~---o 120
..............
!
240
300
180
Time (min)
Figure 3" Influence of metal dispersion on the rate of H2 uptake during reaction at 298 K and 20 atm H2 with 1 M citral in hexane. 100 -;. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-=
80
-
60:
L
.<
,,j
o
40
9
,, H / P t = 0.41
o 121
"" '=
~
+ 20
0-
oH/Pt=0.22
1
oH/Pt=0.09
i t I
9 9 H/Pt=0.06
i 0
10
20
30
40
Citrai Conversion (%)
Figure 4" Influence of metal dispersion on selectivity towards hydrogenation of the C=O bond at 373 K 20 atm H2 with 1 M citral in hexane. Lercher and coworkers have previously alluded to a particle size effect during the liquid-phase hydrogenation of crotonaldehyde [7]. However, in contrast to their results, the selectivity towards hydrogenation of the C=O functionality in the present study is not affected significantly by particle size. Furthermore, the 20-fold enhancement in rate with 5 nm Pt particles compared to 1 nm Pt particles observed in the present study is significantly greater
501 than the four- to five-fold rate increase over a similar range of particle size reported by Lercher and coworkers for crotonaldehyde hydrogenation [7]. The apparent structure sensitivity observed during citral hydrogenation may be related to an inhibiting reaction proposed earlier to explain the observed activity minimum with respect to temperature [3]. It is well known that highly dispersed metal catalysts have a larger fraction of coordinatively unsaturated atoms which may facilitate reactions involving C-C bond scission. Furthermore, the results of Barteau and coworkers suggest that alcohol decomposition reactions over Pd surfaces is structure sensitive [8]. Therefore, the lower rate exhibited by the highly dispersed catalyst at 373 K may be due to a larger fraction of sites being poisoned by CO from the decarbonylation reaction. The suppression of an apparent particle size effect at 298 K is in general agreement with the results of Galvagno and coworkers which suggest that citral hydrogenation is structure insensitive during reaction at low temperatures over Ru catalysts [9]. The suppression of apparent structure sensitivity at 298 K is not entirely clear but may be attributable to a lower rate of the alcohol decomposition reaction as compared to the hydrogenation reaction, thus reducing the influence of the structure sensitive decomposition reaction on the overall hydrogenation reaction kinetics. 3.2 Citral Hydrogenation over Pt/TiO2 The kinetic results reported in the present study for Pt/TiO2 catalysts are free of transport limitations as verified by the Madon-Boudart test and the Weisz-Prater criterion. The behavior of Pt/SiO2 catalysts is similar to that for Pt/TiO2-LTR catalysts but is in sharp contrast to that for Pt/TiO2-HTR catalysts. Table 2 compares the TOF for citral disappearance and selectivity towards hydrogenation of the C=O bond at 373 K and 20 atm H2 with 1 M citral in hexane. A comparison of reaction orders with respect to citral and hydrogen for Pt/SiO2, Pt/TiOz-LTR and Pt/TiO2-HTR catalysts is also shown in Table 2 for reaction at 373 K, concentrations of 1 - 6 M citral in hexane, and hydrogen pressures from 7 41 atm. Dramatic differences are apparent in the kinetics with the Pt/TiO2-HTR catalyst compared to that with Pt/SiO2 and Pt/TiOz-LTR catalysts. The TOF for the Pt/TiOz-HTR catalyst, based H2 chemisorption on a HTR catalyst, is around two orders of magnitude higher than that for Pt/SiO2 (H/Pt=I.0) and Pt/TiOz-LTR catalysts. Even when the initial rate on the Pt/TiOz-HTR catalyst is normalized with respect to H2 chemisorption on the LTR catalyst, the rate is still three-times larger than that for the Pt/TiOz-LTR catalyst, indicating a greater activity per gram of catalyst, and it is 15-fold higher than SiO2-supported Pt.
In addition to the rate enhancement, the reaction orders exhibited by the Pt/TiO2-HTR catalyst are significantly different from those observed with the Pt/SiO2 and Pt/TiO2-LTR catalysts. Approximately zero-order kinetics are observed with respect to citral concentration with Pt/SiO2 and Pt/TiO2-LTR at 373 K, 20 atm H2 and 1.0 - 6.0 M citral in hexane. This is comrasted with the -0.86 order observed with Pt/TiO2-HTR. Furthermore, in contrast with the first-order dependency of rate on hydrogen pressure observed with Pt/SiO2 and Pt/TiO2LTR, Pt/TiO2-HTR exhibits a near zero-order dependence on H2. The Pt/TiO2-HTR catalyst also exhibits enhanced selectivity towards hydrogenation of the C=O functionality as compared to Pt/SiO2. For example, selectivity towards
502 hydrogenation of the C=O bond at 373 K and 20 atm H2, extrapolated to zero time, is 40%, 87%, and 93 % over the Pt/SiO2, Pt/TiOa-LTR, and Pt/TiO2-HTR catalysts, respectively. Table 2 Initial rate of citral disappearance and reaction orders with respect to citral and hydrogen over Pt/SiO2, Pt/TiO2-LTR, and Pt/TiO2-HTR at 373 K. Pt/SiO 2
Pt/TiO2-LTR
Pt/TiO2-HTR
TOF*
0.004
0.02
0.06 (1.0)**
S (C=O Bond)***
40
87
90
Reaction Order in Citral
0.02
-0.21
-0.86
Reaction Order in H2
0.91
1.1
0.16
* TOF normalized to H2chemisorption on LTR catalyst ** TOF normalized to Hz chemisorption on HTR catalyst *** Extrapolated to zero citral conversion
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
Sheldon, R. A., J. Mol. Catal. A." Chem 107 75 (1996) Patil, A. Banares, M. A., Lei, X., Fehlner, T. P., and Wolf, E., J. Catal. 159 458 (1996) Singh, U. K., and Vannice, M. A. J. Catal., Submitted for publication Madon, R. J. and Boudart, M., Ind. Eng. Chem. Fundam. 21 438 (1982) McCabe, R. W., and Schmidt, L. D., Surf Sci., 66 I 01 (1977) Davis, J. L., and Barteau, M., J. Am., Chem. Soc., 111 1782 (1989) Englisch, M., Jentys, A., and Lercher, J. A., J. Catal. 166 25 (1997) Shekhar, R., Barteau, M., Cat. Lett. 31 221 (1995) Mercadante, L. Neri, G., Milone, C., Donato, A., and Galvagno, S., J. Mol. Catal 105 93 (1996)
497
The Influence of Metal-Support Interactions During Liquid-Phase Hydrogenation of an ~, [3-Unsaturated Aldehyde over Pt Utpal K. Singh and M. Albert Vannice Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802 Liquid-phase hydrogenation of citral was investigated over SiO2- and TiO2- supported Pt catalysts in the range of 298 - 423 K, 7 - 41 atm H2 pressure, and 0.5 - 6.0 M citral in hexane. The initial rate of citral hydrogenation over Pt/SiO2 catalysts exhibits an activity minimum with respect to temperature accompanied by an increase in selectivity for hydrogenation of the C=O bond with increasing reaction temperature. Furthermore, the initial rate of citral disappearance is strongly influenced by metal particle size since the rate of citral disappearance at 373 K decreased 20-fold from the 5 nm SiO2-supported Pt crystallites to the 1 nm SiO2-supported Pt crystallites. This difference is suppressed at 298 K and only a five-fold decrease in the rate is observed during this change in Pt crystallite size. Pt/TiO2-LTR (low temperature reduced - 473 K) and Pt/SiO2 catalysts exhibited zero- and first-order dependencies on citral concentration and hydrogen pressure, respectively, at 373 K. In contrast, the Pt/TiO2-HTR (high temperature reduced - 773 K) catalyst exhibited negative first- and zero-order dependencies on citral concentration and hydrogen pressure, respectively. The TOF on the Pt/TiOz-HTR catalyst was more than an order of magnitude greater than that on Pt/SiO2 and Pt/TiO2-LTR. In addition, the Pt/TiO2-HTR catalyst exhibited a marked enhancement in selectivity towards hydrogenation of the C=O bond. 1. INTRODUCTION It has been stated that approximately 50-100 kg of by-product are produced per kg of product in the fine chemicals and pharmaceutical sectors of the chemical industry [1]. Therefore, in light of the increased environmental awareness, it is of interest to develop heterogeneous catalysts for synthesis of pharmaceuticals and fine chemicals In the present work we report the influence of metal-support interactions (MSI) during the selective liquidphase hydrogenation of citral (3,7-dimethyl-2,6-octadienal), which contains three unsaturated bonds including a conjugated system of C=C and C=O bonds and an isolated C=C bond. From a thermodynamic perspective, the isolated C=C bond is the most favorable to hydrogenate followed by the conjugated C=C bond and lastly the C=O bond [2]. However, kinetic control of the reaction can be induced to yield high selectivity for hydrogenation of the C=O bond alone. In the present paper we examine the influence of reaction parameters and support effects on selective hydrogenation of citral.
498 2. EXPERIMENTAL
The details of catalyst synthesis, characterization, and hydrogenation reaction procedures are described in detail elsewhere [3]. Briefly, the catalysts were prepared via incipient wetness or ion exchange using H2PtC16 or Pt(NH3)4CI2, respectively, as the precursor. SiO2 (Davison Grade 57 silica gel - 220 m2/g) and TiO2 (Degussa P25 - 47 m2/g) were dried and calcined at 773 K for four hours prior to catalyst synthesis followed by drying at 393 K overnight. The catalysts were characterized using H2 and CO chemisorption at 300 K to evaluate dispersion and average particle size. Pt/SiO2, Pt/TiO2-LTR (low temperature reduced), and Pt/TiO2-HTR (high temperature reduced) were reduced in situ at 673 K, 473 K, and 773 K, respectively. Nitrogen was bubbled through both hexane and citral prior to their addition into the reactor to remove trace quantities of oxygen from the liquid phase. The reaction progress was monitored by GC analysis of liquid samples periodically withdrawn into a N2 purged vessel as well as by the instantaneous rate of H2 uptake [3]. 3. Results and Discussion 3.1 Citral Hydrogenation over Pt/SiO2
The kinetic data obtained with Pt/SiO2 catalysts was previously shown to be free of transport limitations and poisoning effects as verified by the Madon-Boudart test [3, 4]. The influence of reaction temperature on rate and product distribution is displayed in Figures 1 and 2, respectively, for reaction over a 1.44% Pt/SiO2 catalyst at 20 atm hydrogen pressure and 1 M citral in hexane in the range of 298 - 423 K [3]. .......
-"" -~
~"
~ ,-
L
80 /
|
0.6 t[
9
.
i
i 0.4 + i
i
-- - 2 9 8 K 1
', '~ ~-
, o
'
i
20
:~" i /-~ 40 ~-~",~ "~ ~
"",.,.,ih,... i."
~ - ~ ~
0
,
.... 373K ~ 423 K i
~
7 40
60
Citral Conversion (%)
Figure 1:Temporal H2 uptake profiles for reaction at 298 K, 373 K, 423 K with 1 M citral in hexane at 20 atm H2 pressure.
20 t
.
.
.
.
.
I
....
-0 ....
,i
I o Conjugated C=C Bond (298 K);; o Conjugated C=C Bond (373 K)':'
~ 1 ~ - - - - '
O~ L
t
0
20
~ ---
40
~
---
60
I "~
80
100
Citral Conversion (%)
Figure 2" Selectivity for hydrogenation of conjugated C=O and C=C bonds at 298 K, and 373 K with 1 M citral in hexane at 20 atm H2 pressure.
It is apparent from Figure 1 that there is an activity minimum in the rate with respect to temperature. Furthermore, significant deactivation is observed at 298 K in contrast to the negligible deactivation exhibited at 373 K and 423 K. The product distribution is dramatically altered by reaction temperature as seen in Figure 2. The selectivity towards
499
hydrogenation of the conjugated C=C bond is greater than that for the C=O bond at 298 K. This trend is reversed at 373 K where hydrogenation of C=O bond is the primary reaction pathway. This unusual kinetic behavior was rationalized by utilizing a conventional Langmuir-Hinshelwood-type model for each of the hydrogenation steps along with a concurrent inhibiting decarbonylation reaction. The activity minimum has been explained based on the lower activation barrier for the decomposition reaction yielding chemisorbed CO as compared to that for CO desorption [3, 5, 6]. Such a trend was shown to have three consequences including: strong deactivation during reaction at low temperatures i.e., 298 K, an activity minimum at approximately 373 K, and negligible deactivation at 373 K and 423 K [3]. The complex kinetics observed during liquid-phase hydrogenation is also manifested in the apparent structure sensitivity of the reaction at 373 K with 20 atm H2 pressure and 1 M citral in hexane as shown in Table 1. The differences in the rate of citral hydrogenation among catalysts with different dispersion was suppressed at 298 K in contrast to the behavior at 373 K. Due to the strong deactivation behavior at 298 K, there is significant uncertainty present in reporting a single value for the rate at this temperature. Therefore, Figure 3 displays the temporal H2 uptake profile for reaction over Pt/SiO2 catalysts with varying metal dispersion at 298 K, 20 atm H2 pressure, and 1 M citral in hexane. It should be noted that in spite of the dramatic differences in the initial rate of citral hydrogenation for catalysts with different particle sizes, the selectivity vs. conversion behavior was similar for all the catalysts, within experimental uncertainty, as shown in Figure 4. Table 1 Effect of average Pt particle size, determined from H2 chemisorption at 300 K, on the initial rate of citral hydrogenation at 373 K, 20 atm H2 pressure, with 1 M citral in hexane. Dispersion H/Pt
Metal Particle Size
Initial TOF
Pt/SiO 2
(nm)
(s")
3.59% Pt
0.04
28
0.120
6.65% Pt
0.09
2.65% Pt
0.22
5.1
0.092
3.80% Pt (sintered)
0.37
3.1
0.040
1.44% Pt
0.41
2.8
0.017
0.49% Pt
0.45
2.5
0.012
3.80% Pt
0.66
1.7
0.010
1.1
0.005
Catalyst
0.77% Pt
0.065
500
0.8
..............................................................................................................
r~
0.6
o H / p t = 0.09
,., e~
9 H / Pt =
0.4
o H / Pt = 0.4
\',,,
~.
\o
~ H / Pt =0.7
0.2
~o
" H/Pt
i o~....'-~___~---~__ [-
0
0.2
-----I_
~---~_---~___^
0
= 1.0
- ....... o ~ - - - - o _ o ~ _ o
60
"-=---u--~--~~---o 120
..............
!
240
300
180
Time (min)
Figure 3" Influence of metal dispersion on the rate of H2 uptake during reaction at 298 K and 20 atm H2 with 1 M citral in hexane. 100 -;. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-=
80
-
60:
L
.<
,,j
o
40
9
,, H / P t = 0.41
o 121
"" '=
~
+ 20
0-
oH/Pt=0.22
1
oH/Pt=0.09
i t I
9 9 H/Pt=0.06
i 0
10
20
30
40
Citrai Conversion (%)
Figure 4" Influence of metal dispersion on selectivity towards hydrogenation of the C=O bond at 373 K 20 atm H2 with 1 M citral in hexane. Lercher and coworkers have previously alluded to a particle size effect during the liquid-phase hydrogenation of crotonaldehyde [7]. However, in contrast to their results, the selectivity towards hydrogenation of the C=O functionality in the present study is not affected significantly by particle size. Furthermore, the 20-fold enhancement in rate with 5 nm Pt particles compared to 1 nm Pt particles observed in the present study is significantly greater
501 than the four- to five-fold rate increase over a similar range of particle size reported by Lercher and coworkers for crotonaldehyde hydrogenation [7]. The apparent structure sensitivity observed during citral hydrogenation may be related to an inhibiting reaction proposed earlier to explain the observed activity minimum with respect to temperature [3]. It is well known that highly dispersed metal catalysts have a larger fraction of coordinatively unsaturated atoms which may facilitate reactions involving C-C bond scission. Furthermore, the results of Barteau and coworkers suggest that alcohol decomposition reactions over Pd surfaces is structure sensitive [8]. Therefore, the lower rate exhibited by the highly dispersed catalyst at 373 K may be due to a larger fraction of sites being poisoned by CO from the decarbonylation reaction. The suppression of an apparent particle size effect at 298 K is in general agreement with the results of Galvagno and coworkers which suggest that citral hydrogenation is structure insensitive during reaction at low temperatures over Ru catalysts [9]. The suppression of apparent structure sensitivity at 298 K is not entirely clear but may be attributable to a lower rate of the alcohol decomposition reaction as compared to the hydrogenation reaction, thus reducing the influence of the structure sensitive decomposition reaction on the overall hydrogenation reaction kinetics. 3.2 Citral Hydrogenation over Pt/TiO2 The kinetic results reported in the present study for Pt/TiO2 catalysts are free of transport limitations as verified by the Madon-Boudart test and the Weisz-Prater criterion. The behavior of Pt/SiO2 catalysts is similar to that for Pt/TiO2-LTR catalysts but is in sharp contrast to that for Pt/TiO2-HTR catalysts. Table 2 compares the TOF for citral disappearance and selectivity towards hydrogenation of the C=O bond at 373 K and 20 atm H2 with 1 M citral in hexane. A comparison of reaction orders with respect to citral and hydrogen for Pt/SiO2, Pt/TiOz-LTR and Pt/TiO2-HTR catalysts is also shown in Table 2 for reaction at 373 K, concentrations of 1 - 6 M citral in hexane, and hydrogen pressures from 7 41 atm. Dramatic differences are apparent in the kinetics with the Pt/TiO2-HTR catalyst compared to that with Pt/SiO2 and Pt/TiOz-LTR catalysts. The TOF for the Pt/TiOz-HTR catalyst, based H2 chemisorption on a HTR catalyst, is around two orders of magnitude higher than that for Pt/SiO2 (H/Pt=I.0) and Pt/TiOz-LTR catalysts. Even when the initial rate on the Pt/TiOz-HTR catalyst is normalized with respect to H2 chemisorption on the LTR catalyst, the rate is still three-times larger than that for the Pt/TiOz-LTR catalyst, indicating a greater activity per gram of catalyst, and it is 15-fold higher than SiO2-supported Pt.
In addition to the rate enhancement, the reaction orders exhibited by the Pt/TiO2-HTR catalyst are significantly different from those observed with the Pt/SiO2 and Pt/TiO2-LTR catalysts. Approximately zero-order kinetics are observed with respect to citral concentration with Pt/SiO2 and Pt/TiO2-LTR at 373 K, 20 atm H2 and 1.0 - 6.0 M citral in hexane. This is comrasted with the -0.86 order observed with Pt/TiO2-HTR. Furthermore, in contrast with the first-order dependency of rate on hydrogen pressure observed with Pt/SiO2 and Pt/TiO2LTR, Pt/TiO2-HTR exhibits a near zero-order dependence on H2. The Pt/TiO2-HTR catalyst also exhibits enhanced selectivity towards hydrogenation of the C=O functionality as compared to Pt/SiO2. For example, selectivity towards
502 hydrogenation of the C=O bond at 373 K and 20 atm H2, extrapolated to zero time, is 40%, 87%, and 93 % over the Pt/SiO2, Pt/TiOa-LTR, and Pt/TiO2-HTR catalysts, respectively. Table 2 Initial rate of citral disappearance and reaction orders with respect to citral and hydrogen over Pt/SiO2, Pt/TiO2-LTR, and Pt/TiO2-HTR at 373 K. Pt/SiO 2
Pt/TiO2-LTR
Pt/TiO2-HTR
TOF*
0.004
0.02
0.06 (1.0)**
S (C=O Bond)***
40
87
90
Reaction Order in Citral
0.02
-0.21
-0.86
Reaction Order in H2
0.91
1.1
0.16
* TOF normalized to H2chemisorption on LTR catalyst ** TOF normalized to Hz chemisorption on HTR catalyst *** Extrapolated to zero citral conversion
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
Sheldon, R. A., J. Mol. Catal. A." Chem 107 75 (1996) Patil, A. Banares, M. A., Lei, X., Fehlner, T. P., and Wolf, E., J. Catal. 159 458 (1996) Singh, U. K., and Vannice, M. A. J. Catal., Submitted for publication Madon, R. J. and Boudart, M., Ind. Eng. Chem. Fundam. 21 438 (1982) McCabe, R. W., and Schmidt, L. D., Surf Sci., 66 I 01 (1977) Davis, J. L., and Barteau, M., J. Am., Chem. Soc., 111 1782 (1989) Englisch, M., Jentys, A., and Lercher, J. A., J. Catal. 166 25 (1997) Shekhar, R., Barteau, M., Cat. Lett. 31 221 (1995) Mercadante, L. Neri, G., Milone, C., Donato, A., and Galvagno, S., J. Mol. Catal 105 93 (1996)