- Email: [email protected]
Are there any probe molecules suitable for measurement of acid—base pair sites?
164
trends in analytical chemistry, vol. 73, no. 4, 1994
USA. He has more than 17years of experience in agricultural, environmental, engineering sciences and related disciplines. Dr. Prasad earned his Ph.D. from Rutgers University His areas of expertise and interest focus on quality assurance and quality control in environmental contamination
studies, environmental chemical partition analysis, transport, transformation and effects of chemicals, risk and endangerment assessments as an overall input to comprehensive environmental restoration, and management studies on regulated chemicals and hazardous waste.
Are there any probe molecules suitable for measurement of acid-base pair sites? Kozo Tanabe Osaka, Japan The increasing importance of acid-base bifunctional catalysis is described. In order to design and develop the optimum bifunctional catalysts for particular reactions, the development of probe molecules which are suitable for the characterization of acidbase pair sites on solid surfaces is necessary.
Introduction It is known that the cooperation of a weak acid site with a weak base site on a solid surface, where the acid-base pair site is suitably oriented for a reacting molecule, can be surprisingly effective for giving high catalytic activity, selectivity, and long catalyst life [l-3]. Recently, several successful industrial processes have been achieved by using acid-base bifunctional catalysts. However, it is still difficult to design an optimum acid-base bifunctional catalyst for a desired reaction, because there are only a few probe molecules to characterize acid-base pair sites. In this article, the present situation of the problem is introduced briefly, to stimulate a search for suggestions and comments on possible probe molecules.
Acid-base
bifunctional
catalysis
To understand acid-base bifunctional catalysis and to demonstrate its intriguing features and im0 1994 Elsevier Science B.V. All rights reserved
portance, I shall first discuss homogeneous catalysis in solution. The most remarkable example is seen in the mutarotation of tetramethylglucose catalyzed by 2-hydroxypyridine [4]. The acid and base strengths of 2-hydroxypyridine are l/100 of the acid strength of phenol and l/10 000 of the base strength of pyridine. Nevertheless, the catalytic activity of 2-hydroxypyridine is 7000 times higher than that of a mixture of phenol and pyridine. Phenol alone, or pyridine alone, show no activity. The surprisingly high activity of 2-hydroxypyridine is considered to be caused by the concerted acid-base bifunctional catalysis, and the stereospecific orientation of the acidic and basic groups of the glucose to the basic and acidic groups of the catalyst, as shown in Fig. 1 [4]. In this case, the acidic groups (-OH) and the basic group (-N=) of the catalyst act as a proton donor and a proton acceptor, respectively, and an electron shift can occur in the direction indicated by the arrows. The activity difference between 2-hydroxypyridine and a mixture of phenol and pyridine is actually 102~104x7~103 = 7.109, if the difference in acid and base strengths is taken into consideration. This kind of catalysis has been termed acidbase concerted (synchronous, or push-pull) bifunctional catalysis, which for brevity is expressed as acid-base bifunctional catalysis in this article. In heterogeneous acid-base bifunctional catalysis, Zr02, Th02, MgO, Ti02-ZrO2 and modified Zr02 have been reported to exhibit pronounced catalytic activity and selectivity for particular reactions, and some reasonable evidence has been given [l-3]. Several examples are explained below. 0165.9936/94/$07.00
165
trends in analytical chemistry, vol. 13, no. 4, 1994
tetramethyl glucose
2-hydroxy pyridine
Fig. 1. Mutarotation of tetramethylglucose
a-Olefins
(part of structure) catalyzed by 2-hydroxypyridine.
from 2-a/coho/s
The selectivity for the formation of 1-butene in the dehydration of 2-butanol is 27% over Al2O3, but 90% over ZrO2 or ThO2. The high selectivity of Zr02 or Th02 can be attributed to acid-base bifunctional catalysis, as shown in poisoning experiments with n-butylamine (for the acid site, Z#+) and carbon dioxide (for the basic site, 02-). In the concerted mechanism shown below, no carbenium ion is formed, with the result that there is less formation of the more stable isomers, 2butene.
-YH-YH2 PH iP+
-CH=CH, I;I &-
2%’
+ H,O
02-
Allyllic a/coho/s from epoxides
The rearrangement of 2-carene oxide to the corresponding allyllic alcohol (Fig. 2) occurs over Ti02--Zt-02 (molar ratio = 1) with 100% selectivity. However, the selectivity over the highly acidic SiOz-A1203 is 0%, with many other products being formed. The rearrangement over Ti02-ZrO2, which possesses both acidic sites (Ti4 + and Zti +) and basic sites (02-) is considered to proceed by an acid-base bifunctional mechanism, as shown in Fig. 2. Hydrogenation
K and MgO pretreated at 1373 K. From butadiene, cis-2-butene is formed selectively over MgO. The hydrogenation is considered to proceed via an anionic intermediate having the stable cis form, which is formed from adsorbed butadiene and hydride ion. The characteristic nature of MgO pretreated at 1373 K is to provide a pair with an acid site (Mg2 + ) and a basic site (02-) suitably oriented so as to split a hydrogen molecule into a proton (H + ) and a hydride ion (H-) as shown in Fig. 3, although no hydrogenation occurs on MgO pretreated at 873 K. The unique nature of MgO is that hydrogen ions (H + and H-) attack only C-l and C-4 of butadiene, keeping the molecular identity of a hydrogen such that the exchange reaction between H2 and 2H2 does not take place. On the other hand, with Zr02 the reaction products from the hydrogenation of butadiene with hydrogen and with cyclohexadiene as hydrogen sources are trans-2-butene (80%) and 1-butene (64%), respectively. It is interesting to compare this with the results using MgO, where the main product is cis-2-butene (77%). Although neither the acidity nor the basicity of ZrO2 change much with a change of evacuation temperature, ZrOz evacuated at 873 K shows
of olefins and diolefins
The hydrogenation of olefins and diolefins with hydrogen takes place over ZrO2 pretreated at 873
Fig. 2. Rearrangement of 2-carene oxide over Ti02ZrOe.
166
trends in analytical chemistry
vol. 73, no. 4, 1994
ably oriented so as to split heterolytically H + and H- as is the case with MgO. ' 2+
02-
Mg2+
Mg
;2H-
\I
\
Hf
I
E'ig2+ ;2-
Fig. 3. Heterolytic splitting of hydrogen molecule on MgO.
for the hydrogenation of 1,3butadiene with hydrogen, whereas ZrOz evacuated at 1073 K gives maximum activity for the same hydrogenation with cyclohexadiene, as seen in Fig. 4. Since it is known that the lattice constant of Zr02 changes considerably with a change of evacuation temperature, the appearance of two kinds of maximum activities is considered to be caused mainly by differences in the distance between an acid site (Zr4 +) and a basic site (02-), suggesting the importance of the orientation of the acid-base pair sites for acid-base bifunctional catalysis. An IR study revealed that, for the activation of hydrogen, ZrO2 provides acid-base pair sites suitmaximum
activity
47
>r .5
I
3
.->
5 * 2 aJ .-> 5 lz 1
673 773 873 973 1633 1173 Pretreatment
H:! into
temperature
IK
Fig. 4. Catalytic activities of ZrO2 pretreated at different temperatures. o = hydrogenation of butadiene with H2; 0 = Hz-D2 exchange; A = isomerization of 1-butene; A = hydrogenation of butadiene with cyclohexadiene.
Industrial applications Synthesis of vinylcyclohexane hexyle thanol
from I-cyclo-
This is an application of the selective dehydration of 2-butanol catalyzed by Zr02, as mentioned above. The dehydration of 1-cyclohexylethanol was found to proceed over Zr02 treated with NaOH. The conversion and the selectivity for vinylcyclohexane are 80 and 90%, respectively, almost no deactivation of catalyst being observed during 3000 h [5,6]. Since polyvinylcyclohexane is a useful compound which improves the transparency of polyethylene at a concentration of several ppm, the dehydration process has been industrialized recently by Sumitomo Chemical Co. The acid-base bifunctional catalysis by ZrO2 is attributed to the fit between the distance of an acid site (Z#+) and a basic site (02-) of Zr02-NaOH calcined at 673 K with that (2.64 A) between a basic group (C-OH) and an acidic group (C-H) of 1-cyclohexylethanol. Synthesis of ethylene imine (aziridine) from ethanolamine
Ethylene imine derivatives are commercially important chemicals which are used in pharmaceuticals and in coatings for paper and textiles. The parent compound has been produced by the intramolecular dehydration of ethanolamine in the liquid phase, using sulfuric acid and sodium hydroxide, but the process has some problems such as formation of large amounts of sodium sulfate and low productivity. Recently, the vapor process of ethylene imine production has been developed by using new heterogeneous acid-base bifunctional catalysts, such as Cs-Ba-P-0-Si [7-91, with 2000 manufactured by Nippon tons/year being Shokubai Co. since 1991. The conversion of ethanolamine and the selectivity for ethylene imine are 86 and 8 1 %, respectively. The almost neutral catalyst, whose acid and base strengths are weaker than Ho = + 4.8 and H= 9.4, respectively, does not produce undesirable byproducts such as acetaldehyde and piperidine, and is highly active and selective as a result of the
167
trends in analytical chemistry, vol. 13, no. 4, 1994
cooperative action of weakly weakly basic sites. Synthesis ofaromatic aldehydes sponding carboxylic acids
acidic sites with
fromthe
corre-
Aromatic aldehydes are important intermediates in the production of organic fine chemicals such as pharmaceuticals, agrochemicals, and perfumes. These aldehydes have been produced mainly by a halogenation method. However, the method has some disadvantages such as poor yield, the formation of undesirable byproducts, and environmental effects. A novel process for synthesizing aromatic aldehydes by the direct hydrogenation of the corresponding carboxylic acids using ZrO2-Cr203 as a catalyst has been developed recently [ 10,111. The Mitsubishi Kasei Corporation has successfully commercialized the production of p-tert. -butylbenzaldehyde, m-phenoxybenzaldehyde and pmethylbenzaldehyde by this process, 2000 tons/year of the aldehydes being manufactured since 1988. The conversion of benzoic acid and the selectivity for benzaldehyde are 98 and 96%, respectively, over Zr02-Cr203 (atomic ratio of Cr/Zr = 0.05). The addition of Cr is considered to reduce the acid strength of ZrO;! and hence to enhance the activity and prolong the catalyst life.
Probe molecules Phenol is one of the probe molecules that is suitable for the measurement of acid-base pair sites. Phenol is known to adsorb on almost neutral ZrO2, acidic Si02-A1203, and basic MgO. The maximum temperatures of desorption of phenol are 780, 630, and 360 K, over ZrO2, MgO, and Si02-A1203, respectively. Thus phenol adsorption is strongest on Zr02, owing to the bifunctional nature of the latter, which is shown in Fig. 5. This gives evidence for a characteristic acid-base bifunctional catalysis by Zr-02, of which several examples were described earlier. Various carboxylic acids, which have both acidic (C-OH) and basic (C=O) groups, can be used as probe molecules, provided they do not decompose on the active catalyst surface. Another limit to their use is the adsorption and desorption temperature. 2-Hydroxypyridine has been men-
Fig. 5. Adsorbed state of phenol on Zr02.
tioned already, and should be a suitable probe molecule, although no one has applied it to solid catalysts.
Conclusion Fig. 6 shows an oversimplified model for acidbase bifunctional catalysis. If an acidic site (M) and a basic site (0) on a catalyst surface are in close contact with a basic group (B) and an acidic group (A) of a reacting molecule, as seen at the top of Fig. 6, a concerted acid-base bifunctional catalysis would be highly efficient. If the same catalyst is used for a molecule in which the distance of B-A is longer, M (or 0) could be in close contact with B (or A), but 0 (or M) would be far from A (or B), B-A M
B M
0
B
A
A M
o
B
A
M
0
o
Fig. 6. Model for acid-base bifunctional catalysis over metal oxide: M = metal cation; 0 = oxygen anion; B-A = basic and acidic group of reacting molecule.
168 as seen on the left and right of Fig. 6, and the efficiency of bifunctional catalysis would become low. For such a molecule, we have to select a catalyst whose distance M-O is greater, as seen at the bottom of Fig. 6. Thus, it is important for bifunctional catalysis that the distance between an acidic site and a basic site should be suitable for a particular molecule. In this sense, the sizes and structures of the acidic cation and basic anion should be taken into account. Crystalline materials, whose lattice constants (M-O distance) can be determined by X-ray diffraction, permit easy prediction of whether they should be suitable as bifunctional catalysts for particular reacting molecules. However, since most acid-base catalysts are more or less amorphous, it is difficult to determine the M-O distances accurately, even by EXAFS at present. The use of probe molecules suitable for the estimation of acid-base pair sites is therefore necessary for designing and developing efficient bifunctional catalysts. Thus, the introduction of probe molecules which have different acid and base strengths, and also different distances between their acidic and basic groups, is desired.
References 1 K. Tanabe, in K. Tanabe, H. Hattori, T. Yamaguchi and T. Tanaka (Editors), Acid-Base Catalysis, Ko-
TrAC Contributions Articles for this journal are generally commissioned. Prospective authors who have not been invited to write should first approach one of the Contributing Editors, or the Staff Editor in Amsterdam (see below), with a brief outline of the proposed article including a few references. Authors should note that all manuscripts are subject to peer review, and commissioning does not auto- matically guarantee publication. Short items of news, etc. and letters may be sent without prior arrangement to: Mr. D.C. Coleman, Staff Editor TrAC, P.O. Box 330, 1000 AH Amsterdam, Netherlands, Tel.: (+31 20) 5862784; Fax: (+31 20) 5862304.
trends in analytical chemistry, vol. 13, no. 4, 1994
dansha, Tokyo, VCH, Weinheim, 1989, p. 5 13. K. Tanabe, in M.J. Phillips and M. Teman (Editors), Proc. Int. Congr: Catal., 9th, 5 (1988) 85. K. Tanabe, M. Misono, Y. Ono and H. Hattori, New Solid Acids and Bases, Kodansha, Tokyo, Elsevier, Amsterdam, 1989. C.G. Swain and J.F. Brown, J. Am. Chem. Sot., 74 (1952) 2534,2538. K. Takahashi, T. Hibi, Y. Higashio and M. Araki, Shokubai, 35 (1993) 12. Sumitomo Chem. Co., Ltd., Jpn. Kokai Tokkyo Koho, 61-130240 (1986). M. Ueshima, Y. Shimasaki, Y. Hino and H. Tsuneki, in K. Tanabe (Editors), Acid-Base Catalysis, Kodansha, Tokyo; VCH, Weinheim, 1989, p. 41 M. Ueshima, Y. Shimasaki, K. Ariyoshi, H. Yano and H. Tsuneki, Proc. Int. Congl: Catal., IOth, (1992) 2447. Nippon Shokubai Co. Ltd., Jpn. Kokai Tokkyo Koho, 62- 149337 (1987). 10 Mitsubishi Kasei Corporation (T. Maki and T. Yokoyama), U.S. Put., 4 613 700 (1986). 11 T. Yokoyama, T. Setoyama, N. Fujita, M. Nakajima and T. Maki, Appl. Catal., 88 (1992) 149.
Kozo Tanabe is R & D consultant of Nippon Shokubai Co. Ltd., Osaka. He is Professor Emeritus of Hokkaido University and Associa te Editor of Applied Catalysis. His mailing address (home) is 14- 71 Sonomachi, Oasa, Ebetsu-shi, Hokkaido 069, Japan.
I
Manuscripts on disk TrAC encourages submission of machinereadable manuscripts. The general instructions for style, arrangement and reference format as given in the Instructions to Authors should be followed. Illustrations and tables should be provided in the usual manner. Authors may provide files from all wordprocessing systems. The preferred storage medium is a 5”4 or 3’” inch disk in MS-DOS compatible format. A hardcopy version should accompany the disk. For more details contact the Editorial Office of TrAC: Tel. (+31 20) 5862784 or Fax (+31 20) 5862304.
trends in analytical chemistry, vol. 73, no. 4, 1994
USA. He has more than 17years of experience in agricultural, environmental, engineering sciences and related disciplines. Dr. Prasad earned his Ph.D. from Rutgers University His areas of expertise and interest focus on quality assurance and quality control in environmental contamination
studies, environmental chemical partition analysis, transport, transformation and effects of chemicals, risk and endangerment assessments as an overall input to comprehensive environmental restoration, and management studies on regulated chemicals and hazardous waste.
Are there any probe molecules suitable for measurement of acid-base pair sites? Kozo Tanabe Osaka, Japan The increasing importance of acid-base bifunctional catalysis is described. In order to design and develop the optimum bifunctional catalysts for particular reactions, the development of probe molecules which are suitable for the characterization of acidbase pair sites on solid surfaces is necessary.
Introduction It is known that the cooperation of a weak acid site with a weak base site on a solid surface, where the acid-base pair site is suitably oriented for a reacting molecule, can be surprisingly effective for giving high catalytic activity, selectivity, and long catalyst life [l-3]. Recently, several successful industrial processes have been achieved by using acid-base bifunctional catalysts. However, it is still difficult to design an optimum acid-base bifunctional catalyst for a desired reaction, because there are only a few probe molecules to characterize acid-base pair sites. In this article, the present situation of the problem is introduced briefly, to stimulate a search for suggestions and comments on possible probe molecules.
Acid-base
bifunctional
catalysis
To understand acid-base bifunctional catalysis and to demonstrate its intriguing features and im0 1994 Elsevier Science B.V. All rights reserved
portance, I shall first discuss homogeneous catalysis in solution. The most remarkable example is seen in the mutarotation of tetramethylglucose catalyzed by 2-hydroxypyridine [4]. The acid and base strengths of 2-hydroxypyridine are l/100 of the acid strength of phenol and l/10 000 of the base strength of pyridine. Nevertheless, the catalytic activity of 2-hydroxypyridine is 7000 times higher than that of a mixture of phenol and pyridine. Phenol alone, or pyridine alone, show no activity. The surprisingly high activity of 2-hydroxypyridine is considered to be caused by the concerted acid-base bifunctional catalysis, and the stereospecific orientation of the acidic and basic groups of the glucose to the basic and acidic groups of the catalyst, as shown in Fig. 1 [4]. In this case, the acidic groups (-OH) and the basic group (-N=) of the catalyst act as a proton donor and a proton acceptor, respectively, and an electron shift can occur in the direction indicated by the arrows. The activity difference between 2-hydroxypyridine and a mixture of phenol and pyridine is actually 102~104x7~103 = 7.109, if the difference in acid and base strengths is taken into consideration. This kind of catalysis has been termed acidbase concerted (synchronous, or push-pull) bifunctional catalysis, which for brevity is expressed as acid-base bifunctional catalysis in this article. In heterogeneous acid-base bifunctional catalysis, Zr02, Th02, MgO, Ti02-ZrO2 and modified Zr02 have been reported to exhibit pronounced catalytic activity and selectivity for particular reactions, and some reasonable evidence has been given [l-3]. Several examples are explained below. 0165.9936/94/$07.00
165
trends in analytical chemistry, vol. 13, no. 4, 1994
tetramethyl glucose
2-hydroxy pyridine
Fig. 1. Mutarotation of tetramethylglucose
a-Olefins
(part of structure) catalyzed by 2-hydroxypyridine.
from 2-a/coho/s
The selectivity for the formation of 1-butene in the dehydration of 2-butanol is 27% over Al2O3, but 90% over ZrO2 or ThO2. The high selectivity of Zr02 or Th02 can be attributed to acid-base bifunctional catalysis, as shown in poisoning experiments with n-butylamine (for the acid site, Z#+) and carbon dioxide (for the basic site, 02-). In the concerted mechanism shown below, no carbenium ion is formed, with the result that there is less formation of the more stable isomers, 2butene.
-YH-YH2 PH iP+
-CH=CH, I;I &-
2%’
+ H,O
02-
Allyllic a/coho/s from epoxides
The rearrangement of 2-carene oxide to the corresponding allyllic alcohol (Fig. 2) occurs over Ti02--Zt-02 (molar ratio = 1) with 100% selectivity. However, the selectivity over the highly acidic SiOz-A1203 is 0%, with many other products being formed. The rearrangement over Ti02-ZrO2, which possesses both acidic sites (Ti4 + and Zti +) and basic sites (02-) is considered to proceed by an acid-base bifunctional mechanism, as shown in Fig. 2. Hydrogenation
K and MgO pretreated at 1373 K. From butadiene, cis-2-butene is formed selectively over MgO. The hydrogenation is considered to proceed via an anionic intermediate having the stable cis form, which is formed from adsorbed butadiene and hydride ion. The characteristic nature of MgO pretreated at 1373 K is to provide a pair with an acid site (Mg2 + ) and a basic site (02-) suitably oriented so as to split a hydrogen molecule into a proton (H + ) and a hydride ion (H-) as shown in Fig. 3, although no hydrogenation occurs on MgO pretreated at 873 K. The unique nature of MgO is that hydrogen ions (H + and H-) attack only C-l and C-4 of butadiene, keeping the molecular identity of a hydrogen such that the exchange reaction between H2 and 2H2 does not take place. On the other hand, with Zr02 the reaction products from the hydrogenation of butadiene with hydrogen and with cyclohexadiene as hydrogen sources are trans-2-butene (80%) and 1-butene (64%), respectively. It is interesting to compare this with the results using MgO, where the main product is cis-2-butene (77%). Although neither the acidity nor the basicity of ZrO2 change much with a change of evacuation temperature, ZrOz evacuated at 873 K shows
of olefins and diolefins
The hydrogenation of olefins and diolefins with hydrogen takes place over ZrO2 pretreated at 873
Fig. 2. Rearrangement of 2-carene oxide over Ti02ZrOe.
166
trends in analytical chemistry
vol. 73, no. 4, 1994
ably oriented so as to split heterolytically H + and H- as is the case with MgO. ' 2+
02-
Mg2+
Mg
;2H-
\I
\
Hf
I
E'ig2+ ;2-
Fig. 3. Heterolytic splitting of hydrogen molecule on MgO.
for the hydrogenation of 1,3butadiene with hydrogen, whereas ZrOz evacuated at 1073 K gives maximum activity for the same hydrogenation with cyclohexadiene, as seen in Fig. 4. Since it is known that the lattice constant of Zr02 changes considerably with a change of evacuation temperature, the appearance of two kinds of maximum activities is considered to be caused mainly by differences in the distance between an acid site (Zr4 +) and a basic site (02-), suggesting the importance of the orientation of the acid-base pair sites for acid-base bifunctional catalysis. An IR study revealed that, for the activation of hydrogen, ZrO2 provides acid-base pair sites suitmaximum
activity
47
>r .5
I
3
.->
5 * 2 aJ .-> 5 lz 1
673 773 873 973 1633 1173 Pretreatment
H:! into
temperature
IK
Fig. 4. Catalytic activities of ZrO2 pretreated at different temperatures. o = hydrogenation of butadiene with H2; 0 = Hz-D2 exchange; A = isomerization of 1-butene; A = hydrogenation of butadiene with cyclohexadiene.
Industrial applications Synthesis of vinylcyclohexane hexyle thanol
from I-cyclo-
This is an application of the selective dehydration of 2-butanol catalyzed by Zr02, as mentioned above. The dehydration of 1-cyclohexylethanol was found to proceed over Zr02 treated with NaOH. The conversion and the selectivity for vinylcyclohexane are 80 and 90%, respectively, almost no deactivation of catalyst being observed during 3000 h [5,6]. Since polyvinylcyclohexane is a useful compound which improves the transparency of polyethylene at a concentration of several ppm, the dehydration process has been industrialized recently by Sumitomo Chemical Co. The acid-base bifunctional catalysis by ZrO2 is attributed to the fit between the distance of an acid site (Z#+) and a basic site (02-) of Zr02-NaOH calcined at 673 K with that (2.64 A) between a basic group (C-OH) and an acidic group (C-H) of 1-cyclohexylethanol. Synthesis of ethylene imine (aziridine) from ethanolamine
Ethylene imine derivatives are commercially important chemicals which are used in pharmaceuticals and in coatings for paper and textiles. The parent compound has been produced by the intramolecular dehydration of ethanolamine in the liquid phase, using sulfuric acid and sodium hydroxide, but the process has some problems such as formation of large amounts of sodium sulfate and low productivity. Recently, the vapor process of ethylene imine production has been developed by using new heterogeneous acid-base bifunctional catalysts, such as Cs-Ba-P-0-Si [7-91, with 2000 manufactured by Nippon tons/year being Shokubai Co. since 1991. The conversion of ethanolamine and the selectivity for ethylene imine are 86 and 8 1 %, respectively. The almost neutral catalyst, whose acid and base strengths are weaker than Ho = + 4.8 and H= 9.4, respectively, does not produce undesirable byproducts such as acetaldehyde and piperidine, and is highly active and selective as a result of the
167
trends in analytical chemistry, vol. 13, no. 4, 1994
cooperative action of weakly weakly basic sites. Synthesis ofaromatic aldehydes sponding carboxylic acids
acidic sites with
fromthe
corre-
Aromatic aldehydes are important intermediates in the production of organic fine chemicals such as pharmaceuticals, agrochemicals, and perfumes. These aldehydes have been produced mainly by a halogenation method. However, the method has some disadvantages such as poor yield, the formation of undesirable byproducts, and environmental effects. A novel process for synthesizing aromatic aldehydes by the direct hydrogenation of the corresponding carboxylic acids using ZrO2-Cr203 as a catalyst has been developed recently [ 10,111. The Mitsubishi Kasei Corporation has successfully commercialized the production of p-tert. -butylbenzaldehyde, m-phenoxybenzaldehyde and pmethylbenzaldehyde by this process, 2000 tons/year of the aldehydes being manufactured since 1988. The conversion of benzoic acid and the selectivity for benzaldehyde are 98 and 96%, respectively, over Zr02-Cr203 (atomic ratio of Cr/Zr = 0.05). The addition of Cr is considered to reduce the acid strength of ZrO;! and hence to enhance the activity and prolong the catalyst life.
Probe molecules Phenol is one of the probe molecules that is suitable for the measurement of acid-base pair sites. Phenol is known to adsorb on almost neutral ZrO2, acidic Si02-A1203, and basic MgO. The maximum temperatures of desorption of phenol are 780, 630, and 360 K, over ZrO2, MgO, and Si02-A1203, respectively. Thus phenol adsorption is strongest on Zr02, owing to the bifunctional nature of the latter, which is shown in Fig. 5. This gives evidence for a characteristic acid-base bifunctional catalysis by Zr-02, of which several examples were described earlier. Various carboxylic acids, which have both acidic (C-OH) and basic (C=O) groups, can be used as probe molecules, provided they do not decompose on the active catalyst surface. Another limit to their use is the adsorption and desorption temperature. 2-Hydroxypyridine has been men-
Fig. 5. Adsorbed state of phenol on Zr02.
tioned already, and should be a suitable probe molecule, although no one has applied it to solid catalysts.
Conclusion Fig. 6 shows an oversimplified model for acidbase bifunctional catalysis. If an acidic site (M) and a basic site (0) on a catalyst surface are in close contact with a basic group (B) and an acidic group (A) of a reacting molecule, as seen at the top of Fig. 6, a concerted acid-base bifunctional catalysis would be highly efficient. If the same catalyst is used for a molecule in which the distance of B-A is longer, M (or 0) could be in close contact with B (or A), but 0 (or M) would be far from A (or B), B-A M
B M
0
B
A
A M
o
B
A
M
0
o
Fig. 6. Model for acid-base bifunctional catalysis over metal oxide: M = metal cation; 0 = oxygen anion; B-A = basic and acidic group of reacting molecule.
168 as seen on the left and right of Fig. 6, and the efficiency of bifunctional catalysis would become low. For such a molecule, we have to select a catalyst whose distance M-O is greater, as seen at the bottom of Fig. 6. Thus, it is important for bifunctional catalysis that the distance between an acidic site and a basic site should be suitable for a particular molecule. In this sense, the sizes and structures of the acidic cation and basic anion should be taken into account. Crystalline materials, whose lattice constants (M-O distance) can be determined by X-ray diffraction, permit easy prediction of whether they should be suitable as bifunctional catalysts for particular reacting molecules. However, since most acid-base catalysts are more or less amorphous, it is difficult to determine the M-O distances accurately, even by EXAFS at present. The use of probe molecules suitable for the estimation of acid-base pair sites is therefore necessary for designing and developing efficient bifunctional catalysts. Thus, the introduction of probe molecules which have different acid and base strengths, and also different distances between their acidic and basic groups, is desired.
References 1 K. Tanabe, in K. Tanabe, H. Hattori, T. Yamaguchi and T. Tanaka (Editors), Acid-Base Catalysis, Ko-
TrAC Contributions Articles for this journal are generally commissioned. Prospective authors who have not been invited to write should first approach one of the Contributing Editors, or the Staff Editor in Amsterdam (see below), with a brief outline of the proposed article including a few references. Authors should note that all manuscripts are subject to peer review, and commissioning does not auto- matically guarantee publication. Short items of news, etc. and letters may be sent without prior arrangement to: Mr. D.C. Coleman, Staff Editor TrAC, P.O. Box 330, 1000 AH Amsterdam, Netherlands, Tel.: (+31 20) 5862784; Fax: (+31 20) 5862304.
trends in analytical chemistry, vol. 13, no. 4, 1994
dansha, Tokyo, VCH, Weinheim, 1989, p. 5 13. K. Tanabe, in M.J. Phillips and M. Teman (Editors), Proc. Int. Congr: Catal., 9th, 5 (1988) 85. K. Tanabe, M. Misono, Y. Ono and H. Hattori, New Solid Acids and Bases, Kodansha, Tokyo, Elsevier, Amsterdam, 1989. C.G. Swain and J.F. Brown, J. Am. Chem. Sot., 74 (1952) 2534,2538. K. Takahashi, T. Hibi, Y. Higashio and M. Araki, Shokubai, 35 (1993) 12. Sumitomo Chem. Co., Ltd., Jpn. Kokai Tokkyo Koho, 61-130240 (1986). M. Ueshima, Y. Shimasaki, Y. Hino and H. Tsuneki, in K. Tanabe (Editors), Acid-Base Catalysis, Kodansha, Tokyo; VCH, Weinheim, 1989, p. 41 M. Ueshima, Y. Shimasaki, K. Ariyoshi, H. Yano and H. Tsuneki, Proc. Int. Congl: Catal., IOth, (1992) 2447. Nippon Shokubai Co. Ltd., Jpn. Kokai Tokkyo Koho, 62- 149337 (1987). 10 Mitsubishi Kasei Corporation (T. Maki and T. Yokoyama), U.S. Put., 4 613 700 (1986). 11 T. Yokoyama, T. Setoyama, N. Fujita, M. Nakajima and T. Maki, Appl. Catal., 88 (1992) 149.
Kozo Tanabe is R & D consultant of Nippon Shokubai Co. Ltd., Osaka. He is Professor Emeritus of Hokkaido University and Associa te Editor of Applied Catalysis. His mailing address (home) is 14- 71 Sonomachi, Oasa, Ebetsu-shi, Hokkaido 069, Japan.
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