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Reactivity of Mixed Zn-Cr Oxide Towards Linear C4 Oxygenated Molecules by the Tpsr Method.*
C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactivity of Surfaces 0 1989 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
581
REACTIVITY OF MIXED Zn-Cr OXIDE TOWARDS LINEAR C4 OXYGENATED MOLECULES BY THE TPSR METHOD*.
L. LIETTI, E. TRONCONI, P. FORZATTI**, and I. PASQUON Dipartimento di Chimica Industriale ed Ingegneria Chimica "G. Natta" del Politecnico, Piazza Leonard0 da Vinci 32, 1-20133 Milano (Italy)
ABSTRACT The TPSR method is applied to assess the reactivity of the surface of a mixed Zn-Cr oxide towards linear C4 aldehyde, alcohol and acid molecules. The results indicate that molecularly adsorbed aldehyde, C4 alkoxide and C4 carboxylate species are formed at the surface together with the corresponding c8 intermediates originating from the aldol-like condensation of two aldehyde molecules. The functions of the surface of the mixed ZnCr oxide involved in the desorption upon decomposition or reaction of the above intermediates have been identified, including aldol-like condensation, hydrogenation, dehydrogenation, dehydration, decarboxylation and double bond isomerization. INTRODUCTION The Temperature Programmed Surface Reaction (TPSR) technique is usually referred as a TPD experiment in which the desorbed species are not produced by simple desorption of the initially adsorbed molecules but originate from surface catalyzed decomposition of the adsorbate or reaction between co-adsorbates. If suitable operating conditions are selected, intermediate species can in principle be desorbed and detected as soon as they form. Thus, valuable information on the reactivity of the surface and the underlying chemistry can be extracted from the desorption spectra of the products released upon decomposition or surface reaction of the adsorbates. Accordingly, the TPSR method focuses attention on the chemical functions of the catalyst and on the reactivity of the catalyst surface towards specific molecules.
* This is note no. XVI in the series "Synthesis of Alcohols from $$rbon Oxides and Hydrogen". Author to whom correspondence should be addressed.
582
In this paper we show the application of the TPSR technique to assess the reactivity of a mixed Zn-Cr oxide surface towards linear C4 aldehyde, acid and alcohol molecules. The present study is also of relevance for elucidating the mechanism of the alcohol chain growth over Zn-Cr mixed oxides based catalysts in the direct synthesis of methanol and higher alcohols from CO and H2. METHODS The TPSR apparatus and the related experimental procedures have been described in details elsewhere (ref. 1). For the present study, the analysis section has been modified including a 16 loops gas. sampling valve, so that several on-line G.C. analyses of the gas exiting the reactor can be performed during a single experiment. The G.C. analyses of the products were performed using a OV1 25m long capillary column (0.32 mm I.D.) with high phase thickness (3 pm) which allows to detect both light and heavy products. Typical heating program included 5 min at 3OoC, followed by heating up to 100°C (heating rate 5OC/min) and then up to 23OOC (heating rate 9°C/min). Profiles of the single desorbed species were obtained by interpolating the experimental points. A commercial Zn-Cr-0 catalyst with Zn/Cr atomic ratio = 3/1 and BET surface area = 120 m2g-l has been used in this study. XRD analysis indicated the presence of both ZnO and ZnCr204 phases. Chromatographic helium, further purified by molecular sieves, and Fluka puriss. p.a. reagents were used during all runs. RESULTS AND DISCUSSION The overall TPSR trace and the profiles of the main species desorbed upon n-butanal adsorption at 35OC on the mixed Zn-Cr oxide surface is shown in Figs. la and lb. Table 1 gives the relative amounts of the various products, as percentage of the G.C. area of the peak to the area of the overall TPSR trace, together with the corresponding peak temperature. Most of the observed products can be explained by assuming the presence of two classes of intermediate surface species represented in Figs. 2a and 2b (ref. 1). The molecularly adsorbed species A1 is formed by interaction of n-butanal with a Lewis acid site. The alkoxide species A2 and
583
100
200
300
400
+
Temperature ( ' C ) Fig. la. TPSR traces obtained after n-butanal adsorption at 35°C. a) Overall FID-TPSR trace; b) n-butanal; c) 1-butanol; d) propylene; e) butenes. the carboxylate ions A4 can be formed either by reduction with H2 adsorbed during catalyst's pretreatment and by oxidative dehydrogenation of the molecularly adsorbed C4 aldehyde respectively, or by a Cannizzaro-type disproportionation reaction of species A1. The molecularly adsorbed species A3, on the other hand, is formed by interaction of 1-butanol, once produced from n-butanal, with a Lewis acid site.
TABLE 1 Desorption products, peak temperatures and relative amounts following n-butanal adsorption at 35°C on Zn-Cr-0. desorption products n-butanal 2-ethyl-2-hexenal 1-butanol 4-heptanone c8 dienes C7 linear olefins C5 hydrocarbons methane + ethylene propylene butenes c8 ketones others
peak temperatures ("C)
relative amount( % )
79 97 90, 200
8 27 6
337 378 380 381 300, 400
11 2 2 19 5
100, 303 250
-
5 3
2 10
584
t
L
200
100
400
300
Temperature
b
('C)
Fig. lb. TPSR traces obtained after n-butanal adsorption at 35OC. a) Overall FID-TPSR trace; b) 2-ethyl-2-hexenal; c) c8 dienes; d) 4-heptanone; e) C7 linear olefins. n-BUTANAL
BUTE NE S
1 - BUTANOL
PROPYLENE
Fig. 2a. Intermediate surface species originating adsorption of n-butanal, and products desorbed therefrom.
upon
Since species A2, A3 and A4 can also originate upon adsorption of 1-butanol and n-butyric acid, the TPSR of these reagents was performed in order to provide complementary
585
information on the nature of the surfaces intermediates and on the mechanism by which the different desorption products are formed. Tables 2 and 3 report the relative amounts and the temperature of the maximum of the desorption peaks for each product during 1-butanol (ref. 2) and n-butyric acid TPSR from the mixed Zn-Cr oxide respectively. In the following, results of n-butanal TPSR are discussed and compared with those of 1-butanol and n-butyric acid TPSR.
-MFig. 2b. Intermediate surface species originating from the aldol-like condensation of two molecules, and products desorbed therefrom. The desorption of n-butanal in the low temperature region originates from species Al, which is reversible in nature, and is characterized by a temperature of the peak maximum which closely corresponds to the evaporation temperature of n-butanal. The 1-butanol desorption peak exhibits a broad shape with two maxima at about 90 and 2OO0C, respectively. These temperatures roughly correspond to those observed during TPSR of 1-butanol (Table 2) and are associated with the desorption of physisorbed 1-butanol (species A 3 ) and with the hydrolysis of the butoxy species A2 by surface OH groups, followed by desorption, respectively. Dehydration of species A2 at higher temperatures (TW=4000C) accounts for the evolution of butenes during n-butanal and 1butanol TPSR.
586
Propylene desorption is primarily responsible for the peak at 381°C in the overall TPSR trace of n-butanal and is accompanied by the evolution of large quantities of C02. Propylene is believed to originate from a decarboxylation reaction of species Ad:
A similar mechanism has been suggested to account for the decarboxylation of carboxylic acids over oxide surfaces (ref. 3). The strict correspondence observed in the temperature of the maximum of the propylene peak during n-butyric acid TPSR (TM=38loC in both Table 1 and Table 3) confirms that propylene desorption originates from decomposition of a carboxylic intermediate. On the other hand, propylene is detected at the same temperature also during 1-butanol TPSR (Table 2). This implies that species A4 is also formed at high temperatures upon adsorption of 1-butanol, in line with previous reports on the formation of carboxylate species after alcohol adsorption on various oxide surfaces (ref. 4).
TABLE 2 Desorption products, peak temperatures and relative following 1-butanol adsorption at 35OC on Zn-Cr-0. desorption products 1-butanol C5 hydrocarbons methane + ethylene propylene butenes others
peak temperatures ("C) 85, 185 365 380 381 390
-
amounts
relative amount( % 1 53 3 3 36 3 2
Species Bl, B2 and B3, on the other hand, result from the surface aldol-like condensation of two aldehyde molecules. In addition to the molecularly adsorbed species, the corresponding carboxylate and alkoxy species are likely to be present, and are produced through oxidation and reduction reactions or Cannizzaro type disproportionation. Condensation of a surface C4 carboxylate species with a neighbouring molecularly adsorbed n-butanal molecule, leading to species B2, is also possible (ref. 3).
587
TABLE 3 Desorption products and peak tem2eratures following n-butyric acid adsorption at 35OC on Zn-Cr-0
.
desorntion prodkts n-butyric acid 4-heptanone C5 hydrocarbons methane + ethylene propylene butenes
peak tempeiatures
( oc
100, 225 333 364 378 381 398
* Analytical problems in the 24O+31O0C temperature range prevented evaluation of the relative amounts of the single desorbed species. Formation of 2-ethyl-2-hexenalI in the low temperature region, is explained by dehydration of species Bl, followed by desorption. Dehydrogenation and decarboxylation of species B2 originates 4-heptanoneI whereas dehydration and decarboxylation leads to C7 linear olefins. Accordingly, the olefins/ketone ratio should be governed by the dehydration-dehydrogenation capabilities of the oxide surface. As a matter of fact, the evolution of C7 linear olefins occurs to a lesser extent, whereas the desorption of 4-heptanone is greatly increased during TPSR of n-butanal from the K-promoted Zn-Cr oxide (ref. 5). The desorption of 4-heptanone occurs in the same temperature range during n-butyric acid TPSR, but no C7 linear olefins have been detected in this case. The decarboxylative condensation of carboxylic acid over metal oxides is a reaction of wide applicability, and may proceed according to alternative mechanisms involving either the condensation of two acid molecules to form P-keto acids and a subsequent decarboxylation reaction or a decarboxylative alkyl anion transfer leading to the intermediate species C and D shown in Fig. 3, respectively (ref. 3). Keto-en01 tautomerism and decarboxylation of species C leads to 4-heptanoneI whereas keto-enol tautomerism is involved in the formation of 4-heptanone from species D. It is worth noting that neither species C nor species D can lead to olefin formation, in line with experimental results. The slightly different temperatures of the maximum of 4-heptanone desorption during TPSR of n-butyric
588
i
-M-
C
Fig. 3 . Intermediate surface condensation of carboxylic acids.
D
species
originating
from
acid and of n-butanal (333OC vs 303OC) may be due to the different reactivity of the intermediate species (C or D in Fig. 3 vs B2 in Fig. 2b). On the other hand, desorption of 4-heptanone in the low temperature region (TM = 100°Cl during n-butanal TPSR is likely to originate from decomposition of a different intermediate species, whose formation involves aldehydic intermediate molecules, since only traces of 4-heptanone have been detected in the low temperature region of n-butyric acid TPSR. Finally, species B3 is involved in c&3 dienes and c g ketones desorption. Evolution of Cf3 dienes is associated with dehydration on both Ca and Cb carbon atoms of species B3, whereas c&3 ketones are likely to result from dehydration on the Cb carbon atom of the same species, followed by double bond migration and keto-enol tautomerism. It is worth noting that the formation of cg ketones can also be associated with an aldol-like condensation reaction of two n-butanal molecules with oxygen retention reversal (ref. 6), as already suggested (ref. 1). The above results point to the reactivity of the Zn-Cr-0 surface in performing a series of characteristic chemical reactions, which are summarized below. (a) Aldol-like condensation. The capability of the Zn-Cr-0 system to promote aldolic condensation, is worth noting. Actually, the condensation products of n-butanal, 2-ethyl-2hexenal, is observed already in the low temperature region during n-butanal TPSR. Besides, compounds with more than four carbon atoms are predominant among the desorbed species. This function
589
is also effective for carboxylic molecules, as reflected by the formation of 4-heptanone during n-butyric acid TPSR. (b) Hydrogenation. The desorption of 1-butanol and butenes from prereduced Zn-Cr-0 surface is related to the presence of a C4 alkoxy species, which can originate either by hydrogenation of the molecularly adsorbed n-butanal or by a Cannizzaro-type disproportionation of a surface dioxybutylene intermediate. (c) Hydrolysis. This function is likely to account for 1butanol desorption in correspondence of the peak with T ~ = 2 0 0 ~ C . (d) Dehydrogenation. The formation of dienes, trienes and aromatics from the corresponding olefins, as well as the production of 4-heptanone and propylene by decarboxylation, requires a dehydrogenation function. (e) Decarboxylation. This is one of the most evident functions of the Zn-Cr-0 surface: intermediate surface species A4, B2 and C undergo decarboxylation in the high temperature region. (f) Dehydration. The formation of a number of desorbed products involves a dehydration step, including butenes and c8 dienes from the corresponding alkoxy species and C7 linear olefins from species B2. (9) Double bond isomerization. This function is revealed by the presence of isomers for both hydrocarbons and oxygenated unsaturated compounds (ref. 1). Also, keto-enol tautomerism, which can be seen as a particular form of double-bond migration, is involved in the formation of 4-heptanone and c8 ketones. ACKNOWLEDGEMENTS This work was supported by Progetto Finalizzato Energetica/2 and Minister0 Pubblica Istruzione (Roma). REFERENCES 1 2 3 4 5 6
L. Lietti, D. Botta, P. Forzatti, E. Mantica, E. Tronconi and I. Pasquon, J. Catal., 111 (1988) 360-373. L. Lietti, E. Tronconi and P. Forzatti, J. Mol. Cat., 44 (1988) 201-206. M. Jayamani and C. N. Pillai, J. Catal., 87 (1984) 93-97. D. Chadwick and P. J. R. O'Malley, J. Chem. SOC., Faraday Trans. 1, 83 (1987) 2227-2241. Unpublished results from our laboratories. K. Klier, Plenary Lecture, XI Simposio Iberoamericano de Catalisis, Guanajuato, Mexico (1988).
581
REACTIVITY OF MIXED Zn-Cr OXIDE TOWARDS LINEAR C4 OXYGENATED MOLECULES BY THE TPSR METHOD*.
L. LIETTI, E. TRONCONI, P. FORZATTI**, and I. PASQUON Dipartimento di Chimica Industriale ed Ingegneria Chimica "G. Natta" del Politecnico, Piazza Leonard0 da Vinci 32, 1-20133 Milano (Italy)
ABSTRACT The TPSR method is applied to assess the reactivity of the surface of a mixed Zn-Cr oxide towards linear C4 aldehyde, alcohol and acid molecules. The results indicate that molecularly adsorbed aldehyde, C4 alkoxide and C4 carboxylate species are formed at the surface together with the corresponding c8 intermediates originating from the aldol-like condensation of two aldehyde molecules. The functions of the surface of the mixed ZnCr oxide involved in the desorption upon decomposition or reaction of the above intermediates have been identified, including aldol-like condensation, hydrogenation, dehydrogenation, dehydration, decarboxylation and double bond isomerization. INTRODUCTION The Temperature Programmed Surface Reaction (TPSR) technique is usually referred as a TPD experiment in which the desorbed species are not produced by simple desorption of the initially adsorbed molecules but originate from surface catalyzed decomposition of the adsorbate or reaction between co-adsorbates. If suitable operating conditions are selected, intermediate species can in principle be desorbed and detected as soon as they form. Thus, valuable information on the reactivity of the surface and the underlying chemistry can be extracted from the desorption spectra of the products released upon decomposition or surface reaction of the adsorbates. Accordingly, the TPSR method focuses attention on the chemical functions of the catalyst and on the reactivity of the catalyst surface towards specific molecules.
* This is note no. XVI in the series "Synthesis of Alcohols from $$rbon Oxides and Hydrogen". Author to whom correspondence should be addressed.
582
In this paper we show the application of the TPSR technique to assess the reactivity of a mixed Zn-Cr oxide surface towards linear C4 aldehyde, acid and alcohol molecules. The present study is also of relevance for elucidating the mechanism of the alcohol chain growth over Zn-Cr mixed oxides based catalysts in the direct synthesis of methanol and higher alcohols from CO and H2. METHODS The TPSR apparatus and the related experimental procedures have been described in details elsewhere (ref. 1). For the present study, the analysis section has been modified including a 16 loops gas. sampling valve, so that several on-line G.C. analyses of the gas exiting the reactor can be performed during a single experiment. The G.C. analyses of the products were performed using a OV1 25m long capillary column (0.32 mm I.D.) with high phase thickness (3 pm) which allows to detect both light and heavy products. Typical heating program included 5 min at 3OoC, followed by heating up to 100°C (heating rate 5OC/min) and then up to 23OOC (heating rate 9°C/min). Profiles of the single desorbed species were obtained by interpolating the experimental points. A commercial Zn-Cr-0 catalyst with Zn/Cr atomic ratio = 3/1 and BET surface area = 120 m2g-l has been used in this study. XRD analysis indicated the presence of both ZnO and ZnCr204 phases. Chromatographic helium, further purified by molecular sieves, and Fluka puriss. p.a. reagents were used during all runs. RESULTS AND DISCUSSION The overall TPSR trace and the profiles of the main species desorbed upon n-butanal adsorption at 35OC on the mixed Zn-Cr oxide surface is shown in Figs. la and lb. Table 1 gives the relative amounts of the various products, as percentage of the G.C. area of the peak to the area of the overall TPSR trace, together with the corresponding peak temperature. Most of the observed products can be explained by assuming the presence of two classes of intermediate surface species represented in Figs. 2a and 2b (ref. 1). The molecularly adsorbed species A1 is formed by interaction of n-butanal with a Lewis acid site. The alkoxide species A2 and
583
100
200
300
400
+
Temperature ( ' C ) Fig. la. TPSR traces obtained after n-butanal adsorption at 35°C. a) Overall FID-TPSR trace; b) n-butanal; c) 1-butanol; d) propylene; e) butenes. the carboxylate ions A4 can be formed either by reduction with H2 adsorbed during catalyst's pretreatment and by oxidative dehydrogenation of the molecularly adsorbed C4 aldehyde respectively, or by a Cannizzaro-type disproportionation reaction of species A1. The molecularly adsorbed species A3, on the other hand, is formed by interaction of 1-butanol, once produced from n-butanal, with a Lewis acid site.
TABLE 1 Desorption products, peak temperatures and relative amounts following n-butanal adsorption at 35°C on Zn-Cr-0. desorption products n-butanal 2-ethyl-2-hexenal 1-butanol 4-heptanone c8 dienes C7 linear olefins C5 hydrocarbons methane + ethylene propylene butenes c8 ketones others
peak temperatures ("C)
relative amount( % )
79 97 90, 200
8 27 6
337 378 380 381 300, 400
11 2 2 19 5
100, 303 250
-
5 3
2 10
584
t
L
200
100
400
300
Temperature
b
('C)
Fig. lb. TPSR traces obtained after n-butanal adsorption at 35OC. a) Overall FID-TPSR trace; b) 2-ethyl-2-hexenal; c) c8 dienes; d) 4-heptanone; e) C7 linear olefins. n-BUTANAL
BUTE NE S
1 - BUTANOL
PROPYLENE
Fig. 2a. Intermediate surface species originating adsorption of n-butanal, and products desorbed therefrom.
upon
Since species A2, A3 and A4 can also originate upon adsorption of 1-butanol and n-butyric acid, the TPSR of these reagents was performed in order to provide complementary
585
information on the nature of the surfaces intermediates and on the mechanism by which the different desorption products are formed. Tables 2 and 3 report the relative amounts and the temperature of the maximum of the desorption peaks for each product during 1-butanol (ref. 2) and n-butyric acid TPSR from the mixed Zn-Cr oxide respectively. In the following, results of n-butanal TPSR are discussed and compared with those of 1-butanol and n-butyric acid TPSR.
-MFig. 2b. Intermediate surface species originating from the aldol-like condensation of two molecules, and products desorbed therefrom. The desorption of n-butanal in the low temperature region originates from species Al, which is reversible in nature, and is characterized by a temperature of the peak maximum which closely corresponds to the evaporation temperature of n-butanal. The 1-butanol desorption peak exhibits a broad shape with two maxima at about 90 and 2OO0C, respectively. These temperatures roughly correspond to those observed during TPSR of 1-butanol (Table 2) and are associated with the desorption of physisorbed 1-butanol (species A 3 ) and with the hydrolysis of the butoxy species A2 by surface OH groups, followed by desorption, respectively. Dehydration of species A2 at higher temperatures (TW=4000C) accounts for the evolution of butenes during n-butanal and 1butanol TPSR.
586
Propylene desorption is primarily responsible for the peak at 381°C in the overall TPSR trace of n-butanal and is accompanied by the evolution of large quantities of C02. Propylene is believed to originate from a decarboxylation reaction of species Ad:
A similar mechanism has been suggested to account for the decarboxylation of carboxylic acids over oxide surfaces (ref. 3). The strict correspondence observed in the temperature of the maximum of the propylene peak during n-butyric acid TPSR (TM=38loC in both Table 1 and Table 3) confirms that propylene desorption originates from decomposition of a carboxylic intermediate. On the other hand, propylene is detected at the same temperature also during 1-butanol TPSR (Table 2). This implies that species A4 is also formed at high temperatures upon adsorption of 1-butanol, in line with previous reports on the formation of carboxylate species after alcohol adsorption on various oxide surfaces (ref. 4).
TABLE 2 Desorption products, peak temperatures and relative following 1-butanol adsorption at 35OC on Zn-Cr-0. desorption products 1-butanol C5 hydrocarbons methane + ethylene propylene butenes others
peak temperatures ("C) 85, 185 365 380 381 390
-
amounts
relative amount( % 1 53 3 3 36 3 2
Species Bl, B2 and B3, on the other hand, result from the surface aldol-like condensation of two aldehyde molecules. In addition to the molecularly adsorbed species, the corresponding carboxylate and alkoxy species are likely to be present, and are produced through oxidation and reduction reactions or Cannizzaro type disproportionation. Condensation of a surface C4 carboxylate species with a neighbouring molecularly adsorbed n-butanal molecule, leading to species B2, is also possible (ref. 3).
587
TABLE 3 Desorption products and peak tem2eratures following n-butyric acid adsorption at 35OC on Zn-Cr-0
.
desorntion prodkts n-butyric acid 4-heptanone C5 hydrocarbons methane + ethylene propylene butenes
peak tempeiatures
( oc
100, 225 333 364 378 381 398
* Analytical problems in the 24O+31O0C temperature range prevented evaluation of the relative amounts of the single desorbed species. Formation of 2-ethyl-2-hexenalI in the low temperature region, is explained by dehydration of species Bl, followed by desorption. Dehydrogenation and decarboxylation of species B2 originates 4-heptanoneI whereas dehydration and decarboxylation leads to C7 linear olefins. Accordingly, the olefins/ketone ratio should be governed by the dehydration-dehydrogenation capabilities of the oxide surface. As a matter of fact, the evolution of C7 linear olefins occurs to a lesser extent, whereas the desorption of 4-heptanone is greatly increased during TPSR of n-butanal from the K-promoted Zn-Cr oxide (ref. 5). The desorption of 4-heptanone occurs in the same temperature range during n-butyric acid TPSR, but no C7 linear olefins have been detected in this case. The decarboxylative condensation of carboxylic acid over metal oxides is a reaction of wide applicability, and may proceed according to alternative mechanisms involving either the condensation of two acid molecules to form P-keto acids and a subsequent decarboxylation reaction or a decarboxylative alkyl anion transfer leading to the intermediate species C and D shown in Fig. 3, respectively (ref. 3). Keto-en01 tautomerism and decarboxylation of species C leads to 4-heptanoneI whereas keto-enol tautomerism is involved in the formation of 4-heptanone from species D. It is worth noting that neither species C nor species D can lead to olefin formation, in line with experimental results. The slightly different temperatures of the maximum of 4-heptanone desorption during TPSR of n-butyric
588
i
-M-
C
Fig. 3 . Intermediate surface condensation of carboxylic acids.
D
species
originating
from
acid and of n-butanal (333OC vs 303OC) may be due to the different reactivity of the intermediate species (C or D in Fig. 3 vs B2 in Fig. 2b). On the other hand, desorption of 4-heptanone in the low temperature region (TM = 100°Cl during n-butanal TPSR is likely to originate from decomposition of a different intermediate species, whose formation involves aldehydic intermediate molecules, since only traces of 4-heptanone have been detected in the low temperature region of n-butyric acid TPSR. Finally, species B3 is involved in c&3 dienes and c g ketones desorption. Evolution of Cf3 dienes is associated with dehydration on both Ca and Cb carbon atoms of species B3, whereas c&3 ketones are likely to result from dehydration on the Cb carbon atom of the same species, followed by double bond migration and keto-enol tautomerism. It is worth noting that the formation of cg ketones can also be associated with an aldol-like condensation reaction of two n-butanal molecules with oxygen retention reversal (ref. 6), as already suggested (ref. 1). The above results point to the reactivity of the Zn-Cr-0 surface in performing a series of characteristic chemical reactions, which are summarized below. (a) Aldol-like condensation. The capability of the Zn-Cr-0 system to promote aldolic condensation, is worth noting. Actually, the condensation products of n-butanal, 2-ethyl-2hexenal, is observed already in the low temperature region during n-butanal TPSR. Besides, compounds with more than four carbon atoms are predominant among the desorbed species. This function
589
is also effective for carboxylic molecules, as reflected by the formation of 4-heptanone during n-butyric acid TPSR. (b) Hydrogenation. The desorption of 1-butanol and butenes from prereduced Zn-Cr-0 surface is related to the presence of a C4 alkoxy species, which can originate either by hydrogenation of the molecularly adsorbed n-butanal or by a Cannizzaro-type disproportionation of a surface dioxybutylene intermediate. (c) Hydrolysis. This function is likely to account for 1butanol desorption in correspondence of the peak with T ~ = 2 0 0 ~ C . (d) Dehydrogenation. The formation of dienes, trienes and aromatics from the corresponding olefins, as well as the production of 4-heptanone and propylene by decarboxylation, requires a dehydrogenation function. (e) Decarboxylation. This is one of the most evident functions of the Zn-Cr-0 surface: intermediate surface species A4, B2 and C undergo decarboxylation in the high temperature region. (f) Dehydration. The formation of a number of desorbed products involves a dehydration step, including butenes and c8 dienes from the corresponding alkoxy species and C7 linear olefins from species B2. (9) Double bond isomerization. This function is revealed by the presence of isomers for both hydrocarbons and oxygenated unsaturated compounds (ref. 1). Also, keto-enol tautomerism, which can be seen as a particular form of double-bond migration, is involved in the formation of 4-heptanone and c8 ketones. ACKNOWLEDGEMENTS This work was supported by Progetto Finalizzato Energetica/2 and Minister0 Pubblica Istruzione (Roma). REFERENCES 1 2 3 4 5 6
L. Lietti, D. Botta, P. Forzatti, E. Mantica, E. Tronconi and I. Pasquon, J. Catal., 111 (1988) 360-373. L. Lietti, E. Tronconi and P. Forzatti, J. Mol. Cat., 44 (1988) 201-206. M. Jayamani and C. N. Pillai, J. Catal., 87 (1984) 93-97. D. Chadwick and P. J. R. O'Malley, J. Chem. SOC., Faraday Trans. 1, 83 (1987) 2227-2241. Unpublished results from our laboratories. K. Klier, Plenary Lecture, XI Simposio Iberoamericano de Catalisis, Guanajuato, Mexico (1988).