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Selective Hydrogenation of Aromatic Aldehydes Using Precious Metal Catalysts on New High Surface-Area TiO2 Supports
M. Guisnet et al. (Editors),Heterogeneous Catalysis and Fine Chemicals 111 0 1993 Elsevier Science Publishers B.V. All rights reserved.
91
Selective Hydrogenation of Aromatic Aldehydes Using Precious Metal Catalysts on New High Surface-Area TiO, Supports
M. Bankmann, R. Brand, A. Freund and T. Tacke Degussa AG, Inorganic Chemical Products Division, Research and Development, P.O.Box 1345, D-6450 Hanau 1, Germany
Abstract New high surface-area TiO, extrudates, based on pyrogenic and precipitated TiO,, were used as supports for precious metal (Pd, Pt, Rh) catalysts. The precious metal catalysts were employed in the selective hydrogenation of aromatic aldehydes which have different substituents 1 (R = COOH, CH,, CI). Depending on the choice of catalyst and substrate, the selectivity of the hydrogenation reaction can be directed either toward a benzyl alcohol derivative 2 or toward a methyl-substituted compound 3. The benzyl alcohol is the reaction product obtained using catalysts which have weak or no acidic properties. If acidic catalysts are used, hydrogenolysis to the methyl-compound 3 dominates. Selectivity changes in the order from l a to l c . If an alcohol is used as solvent, side reactions, e.g., ether formation or aromatic ring saturation, can be observed. In the case of catalysts on precipitated TiO,, the benzyl ether is obtained at high selectivity in almost quantitative yield. The rate of hydrogenation is determined by the precious metal profile as well as by the electronic properties of the substituents which are effective in the following order: COOH > CI > CH,.
1. INTRODUCTION
Selectivity is one of the most important goals of the use of heterogeneous catalysis in the production of fine chemicals. Recently, we have reported on new high surface-area supports based on Degussa’s pyrogenic TiO, [l 1. These catalyst supports, impregnated with precious metals, have
92
been further investigated to demonstrate their unique physicochemical characteristics in catalytic reactions. In this study, aromatic aldehydes having different substituents were subjected to selective hydrogenation in liquid phase. The 4-carboxy(l a), 4-methyl-(l b), and 4-chlorobenzaldehyde (1c) were chosen as representative examples because of their significance and of the different electronic properties of their aromatic rings [2]. Based on the substrate and the reaction conditions, as many as five products can be expected (Fig.1).
8 X
X
Hz/Solvent Cat.
+
Q
CHO
X = COOH X eCH3 IXIXmCI
+
[2a-c~=~ R
E
+
+
CHzOR
OCHzCH3
0 X
X
CHI
[3e-cl X
others and
CH3
=H
Figure 1. Selective hydrogenation of aromatic aldehydes
2. EXPERIMENTAL Precious metal catalysts on TiO, extrudates having anatasdrutile ratio, which range from 75/25 to 0/100, were prepared by the pore-volume impregnation method. After being dried and calcined in air, they were reduced in a hydrogednitrogen atmosphere. In the same manner, a reference catalyst on formed precipitated TiO,, which consisted of pure anatase, was prepared. Varying acidic properties were obtained by changing the impregnation procedure and the phase composition of the titania support. The classification of catalysts in strong or weak acidity was done by pH
93
measurement of an aqueous 4 wt.-% catalyst suspension [3]. Catalysts classified as strong had a pH value of below 3.5, whereas the pH value of weakly acidic catalysts was in the range of 3.5 to 6. The same correlation was found for formed supports before the impregnation step.
As reported by Nakabayashi et.al. decreasing acidic properties of pure anatase were linked with decreasing BET-surface area due to increasing cristallite size of TiO, [4]. A similar correlation between acidity and BET surface area could also be observed in our experiments. However, it has to be emphasized that in our case the ratio of anatase to rutile is a subject to change. In general, weakly acidic catalysts were obtained by choosing supports, which have a high rutile content, and by using a minimal concentration of anions, such as halides or sulfate, during catalyst preparation [5]. In contrast, the use of precipitated titania which contained substantial amounts of sulfate introduced during its manufacture, gave rise to strong acidity in the final catalyst [6]. Typical characteristics of prepared catalysts are given in Table 1.
Table 1 Characteristic properties of PM/TiO, catalysts PMniO,
PM [wt Yo]
Phase Ratio AnataselRuth
Acidic pH [3] Properties
Precious Metal Profile
CO adsorption [ml CO/g Cat.]
Pd 1 Pd 1C Pd 2 Pd 4 Pd 5 Pd 6 Pd 7 Pt 1 Rh 2
0.5 0.5a) 1 .o 0.5 0.5 0.5 0.5 1.o 1 .o
75/25 10010 75/25 01100 75/25 75/25 01100 75/25 75/25
strong strong strong weak strong weak weak weak strong
uniform shell uniform uniform shell shell shell
0.43 0.09 0.50 0.33 0.32 0.31 0.22 0.38 1.71
2.9 3.1 2.8 3.8 3.1 4.4 5.2 3.5 2.9
shell
uniform
a) PrecipitatedTiO, is used.
All catalyst tests were performed at 150 "C and 10 bar of hydrogen partial pressure in a one-liter stainless steel autoclave which was equipped with a catalyst basket. In each run, 500 ml of a 1 wt. YO ethanol or aqueous solution of the substituted benzaldehydes la-c were hydrogenated in the presence of 2 g of catalyst. Sampling was done after 10, 30, 60, 120 and 240 minutes. In the case of la, the reaction mixture was analyzed by HPLC. GC was used to analyse the products of 1b and 1 c.
94
3. RESULTS AND DISCUSSION 3.1. Hydrogenation of 1a The benzyl alcohol 2a was obtained at a selectivity greater than 90 % as the main product of the hydrogenation in aqueous phase when each of the catalysts, with the exception of Pd 5, was used. The most favourable results were obtained using either catalyst Pt 1 or catalysts Pd 4 and Pd 6 which have the lowest acidity of all of the catalysts investigated. This can be attributed to the preparation of these catalysts without the introduction of interfering anions and/or to the pure rutile support.
The highest selectivity to the hydrogenolysis product 3a was achieved using catalyst Pd 5 which is, not unexpectedly, one of the most acidic catalysts in Table 2. In this case, the use of a catalyst support based mainly on anatase and of palladium chloride as the PM precursor is particularly advantageous.
Table 2 Test results using PMTTiO, catalysts Catalyst
Variable Parameter
Selectivity
Pd 1 Pd 2 Pd 5 Pd 6 Pt 1 Pd 4
Reaction time PM loading PM profile Reaction ti
’
at Conversiona) 2a
[“/.I
[“/.I
3a
98.5 99.4’) 97.1d) 84.ge) 99.gC) 98.2 99.3 97.3c)
96.9 93.8 94.7 88.1 44.7 96.8 97.6 98.0
1.7 4.8 5.0 11.8 55.1 2.6 2.0 1.4
Lovctivity PM 100 Yo rutile
[“/.I
Others [> 1 71 . 1.4 1.4
a) after 2 h except where otherwise indicated b) and PM profile c ) after 4 h d) after 1 h e) after 0.5 h
As far as the rate of hydrogenation is concerned, a high surface concentration and dispersion of the precious metal is preferred because of existing mass-transport limitations. This can be accomplished by two different means: deposition of the precious metal in an external shell of the support
95
or less efficiently by increasing the PM-loading of a uniformly impregnated catalyst. Catalyst samples Pd 1 and Pd 5 illustrate this correlation in the formation of methyl compound 3a. Similar observations were made regarding the intermediate product 2a.
3.2. Hydrogenation of 1b Unlike the case with l a , water had to be replaced by ethanol as solvent in order to form a homogeneous liquid phase which contains 4-methylbenzaldehyde 1b. As a result, the diethyl acetal is temporarily formed. Because of the reversibility of the reaction, it has not been taken into further account.
Table 3 Hydrogenation of 4-methylbenzaldehyde 1b Catalyst
Variable Parameter
Selectivity at Conversion after 4 h PA]
Pd 5 Pd 6 Pd 7 Rh
Low acidity 100 % rutilea) PM
78.9 95.3 62.9 78.8
2b
3b
Others [> 1 Yo]
16.2 64.4
76.8 74.2 33.2 76.1
4b 21.9 4b 7.7 4b 2.4 4b 15.5 5b 6.3
[“I/.
Pol
6C
Pd 1 C
Strong acidity
74.1
2.1
4b 99.0
a) and low acidity
Despite the change of solvent, the correlations between product selectivity and catalyst properties were the same as those observed before. In general, the conversion of l b after 4 hours is lower than that of l a and it is more difficult to achieve high selectivity in regard to benzyl alcohol 2b. The hydrogenolysis reaction predominates. Typical product compositions contain approximately 75 % of 3b and 25 YO of other products. One of the latter is ethyl ether 4b which arises from the acid-base catalyzed reaction between benzyl alcohol 2b and ethanol. The amount of the ether, which is formed, depends on the acidity of the catalyst. It is well worth noting the remarkably high selectivity of catalyst Pd 1C. This catalyst was prepared on precipitated titania which is considered to be far more acidic than fumed TiO, P25 and is also having different kinds of acid sites generated
96 by sulphate [6]. Other PM/TiO, catalysts, such as Rh 2 gave rise to additional reactions, e.g., aromatic ring saturation or decarbonylation to toluene 6c.
3.3. Hydrogenation of 1c In addition to the reaction pathways of the previously discussed substrates, 4-chlorobenzaldehyde 1c can be hydrodehalogenated. Indeed, the main product obtained using the Pd catalysts on fumed titania was toluene 6c which is the hydrogenolysis product of 4-chlorotoluene 3c. No benzyl alcohol 2c was formed. Apart from about 50-60 % of toluene, 4-chlorotoluene 3c, ether 4b and saturated aromatic ring product 5c are present in the range of between 5 and 30 YO. In this case, the different acidic properties of the Pd catalysts on fumed titania are of minor importance. The release of hydrogen chloride from the hydrodechlorination reaction compensates for the initial differences of the catalyst acidity.
Table 4 Hydrogenation of 4-chlorobenzaldehyde 1c Catalyst
Variable Parameter
Selectivity at Conversion after 4 h [“I./
Pd 5
2b
3b
[“A]
Pol
Others [> 1 Yo]
78.2
18.7
4c 15.0 5c 3.1 6C 62.2
30.1
4c 10.2 5c 4.8 6C 54.9
Pd 7
100 o/o rutile and low acidity
91.2
Pd 1C
strong acidity
84.7
4c 100
As could have been seen from an analysis of the product during the course of the reaction, only the hydrodechlorination of 3c took place. No other possible dehalogenated products, e.g., benzyl alcohol or benzyl ethyl ether, were detected. This might explain why, particularly in the case of catalyst Pd l C , the chlorosubstituted benzyl ether (4b) was the only product formed in high yield. Because of the kind and strength of acidity,
97 the intermediate benzyl alcohol (2c) was intercepted by ether formation thus preventing further reaction. Taking these results and the previous ones into account, a general reaction scheme for the selective hydrogenation of aromatic aldehydes is outlined in Figure 2.
X
- ca
mb
Y
A
+ 2 EtOH I +Hz0 I
- HzO
- 2 EtOH
CHO
*
OEt
EtO
X
- Ha0 + H z 0 I - EtOH
EtOH 1
I
CH2 OH
T
I
I
CH3
Figure 2. Reaction scheme aldehydes
CH20Et
CH3
for
the
CH3
selective
hydrogenation
of
aromatic
4. CONCLUSION
PM catalysts on new high surface-area TiO, supports were employed in the selective hydrogenation of aromatic aldehydes. Depending on the choice of catalyst and substrate, the selectivity of the hydrogenation can be
98
directed toward a given product as follows: Benzyl Alcohols 2
Benzyl Ethers 4
0
Low Acidity
- Preparation method - Support based mainly on rutile
0
Low PM Loading
0
Pd as Precious Metal (occasionally Pt)
0
Strong Acidity
- Preparation method
- Support based on pure anatase (precipitated)
Hydrogenolysis 3
0
Strong Acidity
- Support based mainly on anatase (fumed) - Reaction time
The rate of hydrogenation is determined by the precious metal profile and the electronic properties of the substituent in the 4-position of the aromatic aldehyde. Unlike the electron-donating methyl group, electronattracting groups, such as chlorine or carboxyl, increase the reaction rate. This is in agreement with similar observations made comparing 4-carboxybenzaldehyde with the unsubstituted derivative [7]. Based on the present and unpublished experimental results, the order is: COOH > CI > CH,.
5. REFERENCES
(11 M. Bankmann, R. Brand, B. H. Engler and J. Ohmer, Catalysis Today, 14 (1992) 225. [2] a) Amoco Corp., USP 4,812,594 (1 989) and USP 4,721,808 (1988). b) Teijin Ltd., JP 53 002 441 (1978). [3] Measurements were performed analogous to the ASTM method D 3830-80. [4] H. Nakabayashi, N. Kakuta and A. Veno, Bull. Chem. SOC.Jpn., 64 (1991) 2428. [5] J.A.R. van Veen, Z. f. Phys. Chem., 162 (1989) 215. 16) C. Morterra, J. Chem. SOC.Faraday Trans. I , 84 (1988) 1617. [7] L. Kh. Freidlin E. F. Litvin, R.N. Gurskii, G. K. Oparrina and R. V. Istratova, Izv. Akad. Nauk SSR. Ser. Khim., 1972, 1738.
91
Selective Hydrogenation of Aromatic Aldehydes Using Precious Metal Catalysts on New High Surface-Area TiO, Supports
M. Bankmann, R. Brand, A. Freund and T. Tacke Degussa AG, Inorganic Chemical Products Division, Research and Development, P.O.Box 1345, D-6450 Hanau 1, Germany
Abstract New high surface-area TiO, extrudates, based on pyrogenic and precipitated TiO,, were used as supports for precious metal (Pd, Pt, Rh) catalysts. The precious metal catalysts were employed in the selective hydrogenation of aromatic aldehydes which have different substituents 1 (R = COOH, CH,, CI). Depending on the choice of catalyst and substrate, the selectivity of the hydrogenation reaction can be directed either toward a benzyl alcohol derivative 2 or toward a methyl-substituted compound 3. The benzyl alcohol is the reaction product obtained using catalysts which have weak or no acidic properties. If acidic catalysts are used, hydrogenolysis to the methyl-compound 3 dominates. Selectivity changes in the order from l a to l c . If an alcohol is used as solvent, side reactions, e.g., ether formation or aromatic ring saturation, can be observed. In the case of catalysts on precipitated TiO,, the benzyl ether is obtained at high selectivity in almost quantitative yield. The rate of hydrogenation is determined by the precious metal profile as well as by the electronic properties of the substituents which are effective in the following order: COOH > CI > CH,.
1. INTRODUCTION
Selectivity is one of the most important goals of the use of heterogeneous catalysis in the production of fine chemicals. Recently, we have reported on new high surface-area supports based on Degussa’s pyrogenic TiO, [l 1. These catalyst supports, impregnated with precious metals, have
92
been further investigated to demonstrate their unique physicochemical characteristics in catalytic reactions. In this study, aromatic aldehydes having different substituents were subjected to selective hydrogenation in liquid phase. The 4-carboxy(l a), 4-methyl-(l b), and 4-chlorobenzaldehyde (1c) were chosen as representative examples because of their significance and of the different electronic properties of their aromatic rings [2]. Based on the substrate and the reaction conditions, as many as five products can be expected (Fig.1).
8 X
X
Hz/Solvent Cat.
+
Q
CHO
X = COOH X eCH3 IXIXmCI
+
[2a-c~=~ R
E
+
+
CHzOR
OCHzCH3
0 X
X
CHI
[3e-cl X
others and
CH3
=H
Figure 1. Selective hydrogenation of aromatic aldehydes
2. EXPERIMENTAL Precious metal catalysts on TiO, extrudates having anatasdrutile ratio, which range from 75/25 to 0/100, were prepared by the pore-volume impregnation method. After being dried and calcined in air, they were reduced in a hydrogednitrogen atmosphere. In the same manner, a reference catalyst on formed precipitated TiO,, which consisted of pure anatase, was prepared. Varying acidic properties were obtained by changing the impregnation procedure and the phase composition of the titania support. The classification of catalysts in strong or weak acidity was done by pH
93
measurement of an aqueous 4 wt.-% catalyst suspension [3]. Catalysts classified as strong had a pH value of below 3.5, whereas the pH value of weakly acidic catalysts was in the range of 3.5 to 6. The same correlation was found for formed supports before the impregnation step.
As reported by Nakabayashi et.al. decreasing acidic properties of pure anatase were linked with decreasing BET-surface area due to increasing cristallite size of TiO, [4]. A similar correlation between acidity and BET surface area could also be observed in our experiments. However, it has to be emphasized that in our case the ratio of anatase to rutile is a subject to change. In general, weakly acidic catalysts were obtained by choosing supports, which have a high rutile content, and by using a minimal concentration of anions, such as halides or sulfate, during catalyst preparation [5]. In contrast, the use of precipitated titania which contained substantial amounts of sulfate introduced during its manufacture, gave rise to strong acidity in the final catalyst [6]. Typical characteristics of prepared catalysts are given in Table 1.
Table 1 Characteristic properties of PM/TiO, catalysts PMniO,
PM [wt Yo]
Phase Ratio AnataselRuth
Acidic pH [3] Properties
Precious Metal Profile
CO adsorption [ml CO/g Cat.]
Pd 1 Pd 1C Pd 2 Pd 4 Pd 5 Pd 6 Pd 7 Pt 1 Rh 2
0.5 0.5a) 1 .o 0.5 0.5 0.5 0.5 1.o 1 .o
75/25 10010 75/25 01100 75/25 75/25 01100 75/25 75/25
strong strong strong weak strong weak weak weak strong
uniform shell uniform uniform shell shell shell
0.43 0.09 0.50 0.33 0.32 0.31 0.22 0.38 1.71
2.9 3.1 2.8 3.8 3.1 4.4 5.2 3.5 2.9
shell
uniform
a) PrecipitatedTiO, is used.
All catalyst tests were performed at 150 "C and 10 bar of hydrogen partial pressure in a one-liter stainless steel autoclave which was equipped with a catalyst basket. In each run, 500 ml of a 1 wt. YO ethanol or aqueous solution of the substituted benzaldehydes la-c were hydrogenated in the presence of 2 g of catalyst. Sampling was done after 10, 30, 60, 120 and 240 minutes. In the case of la, the reaction mixture was analyzed by HPLC. GC was used to analyse the products of 1b and 1 c.
94
3. RESULTS AND DISCUSSION 3.1. Hydrogenation of 1a The benzyl alcohol 2a was obtained at a selectivity greater than 90 % as the main product of the hydrogenation in aqueous phase when each of the catalysts, with the exception of Pd 5, was used. The most favourable results were obtained using either catalyst Pt 1 or catalysts Pd 4 and Pd 6 which have the lowest acidity of all of the catalysts investigated. This can be attributed to the preparation of these catalysts without the introduction of interfering anions and/or to the pure rutile support.
The highest selectivity to the hydrogenolysis product 3a was achieved using catalyst Pd 5 which is, not unexpectedly, one of the most acidic catalysts in Table 2. In this case, the use of a catalyst support based mainly on anatase and of palladium chloride as the PM precursor is particularly advantageous.
Table 2 Test results using PMTTiO, catalysts Catalyst
Variable Parameter
Selectivity
Pd 1 Pd 2 Pd 5 Pd 6 Pt 1 Pd 4
Reaction time PM loading PM profile Reaction ti
’
at Conversiona) 2a
[“/.I
[“/.I
3a
98.5 99.4’) 97.1d) 84.ge) 99.gC) 98.2 99.3 97.3c)
96.9 93.8 94.7 88.1 44.7 96.8 97.6 98.0
1.7 4.8 5.0 11.8 55.1 2.6 2.0 1.4
Lovctivity PM 100 Yo rutile
[“/.I
Others [> 1 71 . 1.4 1.4
a) after 2 h except where otherwise indicated b) and PM profile c ) after 4 h d) after 1 h e) after 0.5 h
As far as the rate of hydrogenation is concerned, a high surface concentration and dispersion of the precious metal is preferred because of existing mass-transport limitations. This can be accomplished by two different means: deposition of the precious metal in an external shell of the support
95
or less efficiently by increasing the PM-loading of a uniformly impregnated catalyst. Catalyst samples Pd 1 and Pd 5 illustrate this correlation in the formation of methyl compound 3a. Similar observations were made regarding the intermediate product 2a.
3.2. Hydrogenation of 1b Unlike the case with l a , water had to be replaced by ethanol as solvent in order to form a homogeneous liquid phase which contains 4-methylbenzaldehyde 1b. As a result, the diethyl acetal is temporarily formed. Because of the reversibility of the reaction, it has not been taken into further account.
Table 3 Hydrogenation of 4-methylbenzaldehyde 1b Catalyst
Variable Parameter
Selectivity at Conversion after 4 h PA]
Pd 5 Pd 6 Pd 7 Rh
Low acidity 100 % rutilea) PM
78.9 95.3 62.9 78.8
2b
3b
Others [> 1 Yo]
16.2 64.4
76.8 74.2 33.2 76.1
4b 21.9 4b 7.7 4b 2.4 4b 15.5 5b 6.3
[“I/.
Pol
6C
Pd 1 C
Strong acidity
74.1
2.1
4b 99.0
a) and low acidity
Despite the change of solvent, the correlations between product selectivity and catalyst properties were the same as those observed before. In general, the conversion of l b after 4 hours is lower than that of l a and it is more difficult to achieve high selectivity in regard to benzyl alcohol 2b. The hydrogenolysis reaction predominates. Typical product compositions contain approximately 75 % of 3b and 25 YO of other products. One of the latter is ethyl ether 4b which arises from the acid-base catalyzed reaction between benzyl alcohol 2b and ethanol. The amount of the ether, which is formed, depends on the acidity of the catalyst. It is well worth noting the remarkably high selectivity of catalyst Pd 1C. This catalyst was prepared on precipitated titania which is considered to be far more acidic than fumed TiO, P25 and is also having different kinds of acid sites generated
96 by sulphate [6]. Other PM/TiO, catalysts, such as Rh 2 gave rise to additional reactions, e.g., aromatic ring saturation or decarbonylation to toluene 6c.
3.3. Hydrogenation of 1c In addition to the reaction pathways of the previously discussed substrates, 4-chlorobenzaldehyde 1c can be hydrodehalogenated. Indeed, the main product obtained using the Pd catalysts on fumed titania was toluene 6c which is the hydrogenolysis product of 4-chlorotoluene 3c. No benzyl alcohol 2c was formed. Apart from about 50-60 % of toluene, 4-chlorotoluene 3c, ether 4b and saturated aromatic ring product 5c are present in the range of between 5 and 30 YO. In this case, the different acidic properties of the Pd catalysts on fumed titania are of minor importance. The release of hydrogen chloride from the hydrodechlorination reaction compensates for the initial differences of the catalyst acidity.
Table 4 Hydrogenation of 4-chlorobenzaldehyde 1c Catalyst
Variable Parameter
Selectivity at Conversion after 4 h [“I./
Pd 5
2b
3b
[“A]
Pol
Others [> 1 Yo]
78.2
18.7
4c 15.0 5c 3.1 6C 62.2
30.1
4c 10.2 5c 4.8 6C 54.9
Pd 7
100 o/o rutile and low acidity
91.2
Pd 1C
strong acidity
84.7
4c 100
As could have been seen from an analysis of the product during the course of the reaction, only the hydrodechlorination of 3c took place. No other possible dehalogenated products, e.g., benzyl alcohol or benzyl ethyl ether, were detected. This might explain why, particularly in the case of catalyst Pd l C , the chlorosubstituted benzyl ether (4b) was the only product formed in high yield. Because of the kind and strength of acidity,
97 the intermediate benzyl alcohol (2c) was intercepted by ether formation thus preventing further reaction. Taking these results and the previous ones into account, a general reaction scheme for the selective hydrogenation of aromatic aldehydes is outlined in Figure 2.
X
- ca
mb
Y
A
+ 2 EtOH I +Hz0 I
- HzO
- 2 EtOH
CHO
*
OEt
EtO
X
- Ha0 + H z 0 I - EtOH
EtOH 1
I
CH2 OH
T
I
I
CH3
Figure 2. Reaction scheme aldehydes
CH20Et
CH3
for
the
CH3
selective
hydrogenation
of
aromatic
4. CONCLUSION
PM catalysts on new high surface-area TiO, supports were employed in the selective hydrogenation of aromatic aldehydes. Depending on the choice of catalyst and substrate, the selectivity of the hydrogenation can be
98
directed toward a given product as follows: Benzyl Alcohols 2
Benzyl Ethers 4
0
Low Acidity
- Preparation method - Support based mainly on rutile
0
Low PM Loading
0
Pd as Precious Metal (occasionally Pt)
0
Strong Acidity
- Preparation method
- Support based on pure anatase (precipitated)
Hydrogenolysis 3
0
Strong Acidity
- Support based mainly on anatase (fumed) - Reaction time
The rate of hydrogenation is determined by the precious metal profile and the electronic properties of the substituent in the 4-position of the aromatic aldehyde. Unlike the electron-donating methyl group, electronattracting groups, such as chlorine or carboxyl, increase the reaction rate. This is in agreement with similar observations made comparing 4-carboxybenzaldehyde with the unsubstituted derivative [7]. Based on the present and unpublished experimental results, the order is: COOH > CI > CH,.
5. REFERENCES
(11 M. Bankmann, R. Brand, B. H. Engler and J. Ohmer, Catalysis Today, 14 (1992) 225. [2] a) Amoco Corp., USP 4,812,594 (1 989) and USP 4,721,808 (1988). b) Teijin Ltd., JP 53 002 441 (1978). [3] Measurements were performed analogous to the ASTM method D 3830-80. [4] H. Nakabayashi, N. Kakuta and A. Veno, Bull. Chem. SOC.Jpn., 64 (1991) 2428. [5] J.A.R. van Veen, Z. f. Phys. Chem., 162 (1989) 215. 16) C. Morterra, J. Chem. SOC.Faraday Trans. I , 84 (1988) 1617. [7] L. Kh. Freidlin E. F. Litvin, R.N. Gurskii, G. K. Oparrina and R. V. Istratova, Izv. Akad. Nauk SSR. Ser. Khim., 1972, 1738.