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Some prospects and priorities for future research on vanadyl pyrophosphate
147
Catalysis Today, 16 (1993) 147-153 Elsevier Science Publishers B.V., Amsterdam
Some Prospects and Priorities for F’uture Research on Vanadyl Pyrophosphate G. Centi Dept. o~r~~t~
Chemistry and Mc~teric~ls, VkRisorgimento
4.40136 Bologna,
klfg
The objective of this issue on vanadyl pyrophosphate was to stimulate analysis and discussion of recent advances on this topic in order to enunciate some general conclusions and develop priorities for research on this catalytic system. The various contributions were mainly centered around topics regarding the nature of the active sites, key reaction steps in alkane oxidation, modeling of the reaction mechanism and interrelationships between catalytic behavior and structural/surface modifications of vanadyl pyrophosphate. In my opinion, these topics are the central aspects of research on this catalytic system which can really provide the basis for the further development of this catalyst and of its industrial app~caffons, but in addition they also are the more important topics for general scientific advancement in the field of heterogeneous catalysis. Indeed, identification of the elementary steps in the reaction mechanism and the nature and architecture of the active sites on vanadyl pyrophosphate that allow the complex oxidation of an alkane to anhydrides (n-pentane to PA is a 34 electron oxldationl to be carried out selectively make possible rational design and tuning of VP0 performances. At the same time, iden~ca~on of the unique structural and reactivity surface characteristics of VP0 and the reasons behind its peculiar catalytic ability in the selective oxidation of C4 and Cs alkanes open new possibilities and stimulate new ideas valid for the entire field of heterogeneous catalysis. Indeed, the survey of the research on VP0 catalysts presented in this issue provides some very interesting ideas and suggestions on the problems involved which are of more general interest and which, if further confirmed and investigated, can lead to a substantial jump in the understanding of heterogeneous mechanisms of reaction and in the development of new catalytic applications. On the other hand, a second, perhaps ambitious scope of the “Forum on Vanadyl Pyrophosphate” was also to evidence aspects regarding VP0 that are less relevant, in order to focalize research efforts on the more important and innovative directions. This difficult task of the identiftcation of the future “more innovative” lines of research on VP0 is reserved to the reader and I hope that the original approach used in this issue will favor this task. For my part, I should like to discuss some personal reflections that may not be of general acceptance. Different suggestions can also be found in the two commentary papers by B.K. Hodnett and G.J. 0920.5861/93/$6.00
0 1993 Elsevier Science Publishers B.V. All rights reserved.
148
Hutchings. A useful premise is that in the entire issue little attention was devoted to the preparation of VP0 catalysts. This was not determined by a lack of scientific interest on this aspect, but rather by the fact that since the structure-surface-reactivity relationship was the central theme of this issue, less attention was devoted to the aspects of the preparation of VP0 that allow various structural or surface characteristics to be obtained. Another useful premise is that although there are various interpretations and points of view when reading the data presented in this issue, a more close examination shows that the common background is the necessity for further understanding of the catalytic chemistry of VPO. For example, from an industrial perspective the more critical problem may be the necessity to improve the activity and selectivity, the productivity or the resistance to deactivation. The doping of vanadyl pyrophosphate or a suitable modification in the preparation method thus appears to be the only relevant aspect. However, the analysis of the literature data addressed, in particular, in the commentary paper of G.J. Hutching [11reveals that doping mainly affects the microstructure of VP0 with a change in the specific surface area of some crystalline planes or introduction of defects (21. In particular, increasing the exposure of the (020) plane of vanadyl pyrophosphate increases the activity of the catalyst and therefore the problem seems to be to find a preparation method or suitable doping in order to reach the limiting ideal case of a bidimensional catalyst with only the (020) plane. However, the structural model of vanadyl pyrophosphate and its surface gives a different picture showing the considerable influence on the possible analysis of the doping effect as well as the choice of suitable dopants or modifiers. Indeed, the work of Ebner and Thompson [3] suggests, even if it does not definitively prove, that the surface chemistry of vanadyl pyrophosphate is dominated by the presence of pendant pyrophosphate groups providing steric isolation of active vanadium centers on the bottom of the clefts or cavities created by the pyrophosphate groups. In addition, probably these phosphate groups cooperate in the reaction mechanism, for example in H-abstraction, and can suitably orient the intermediate product of reaction. It has been shown that doping of P-OH groups with K inhibits the 0 insertion ability of the catalyst [4]. The dimensions of these surface cavities depend on the orientation of the vanadyl columns within the structure, ie. the disorder present in the structure of the vanadyl pyrophosphate (3.51. However, doping with relatively large ions that occupy definite sites at the surface and the formation during reaction of non- mobile organic adspecies are also additional ways to modify in a similar fashion the surface topology and “architecture” of vanadyl pyrophosphate. In addition, these outgrowing surface groups may cooperate in the reaction mechanism. For example, it has been suggested that organic adspecies cooperate in the H-abstraction mechanism in furan oxidation on vanadyl pyrophosphate 161.Therefore, the nature and dimension of these surface cavities not only influence the activity of the vanadium centres on the bottom of these cavities (number of accessible sites) and the selectivity (site isolation of the active sites, inhibition of the surface mobility of adspecies, cooperative effects in the
149
surface mechanism, stabilization of the reaction intermediates). but also the reaction pathways. The phthalic to maleic anhydride ratio from n-pentane would certainly be affected by the nature, form and dimensions of these surface cavities. This concept of a trfcSmertsionalactive site on the vanadyl pyrophosphate surface and its role in the mechanism of selective synthesis of anhydrides from C4 and Cs alkanes has a relevant effect on the analysis of catalytic data and therefore also on the study of doping effects. Profound reflection is necessary on these topics which address several open questions very relevant also for possible applications. Might it be possible to modify the surface topology by a controlled anchoring of organic or inorganic molecules? Will it be possible to realize a multifunctional tridimensional active site in this way7 Will it be possible to design a surface topology for the active site that can orient the intermediate in order to obtain uncommon reaction products or to stop the reaction at a desired level? For example, the synthesis of furan or dihydrofuran from n-butane is industrially very interesting. These are suggested reaction intermediates from butane to maleic anhydride, but their formation is usually negligible. On the contrary, furan can be synthesized from butadiene with relatively good yields 171.The concept of surface topology and tridimensionaliiy of the active sites on vanadyl pyrophosphate, however, opens new prospects for the possibility of stopping the reaction from butane to furan. A similar discussion is also valid regarding the problem of enhancing the selectivity to phthalic anhydride from n-pentane rather than maleic anhydride 181.Also in this case a possible approach is to control the surface topology of accessible vanadium sites through selected doping or secondary modifications. A better understanding of this concept of surface topology and its relationship to the catalytic behavior can therefore lead to new industrial applications of VPO, but is also of more general interest for the entire field of heterogeneous catalysis or materials science. For example, better and more selective sensors can be realized by controlling the surface topology of the accessible active sites. The concept of tridimensional active sites on vanadyl pyrophosphate which mimic an enzymatic system and of controlled surface topology due to the presence of disorder in the structure of VP0 are thus very interesting hypotheses emerging from the study of this catalytic system that can open new prospects and which therefore must be more thoroughly studied and verified. For example, the use of AFM (atomic force microscopy) on real catalysts can provide valuable information regarding these questions and will allow more detailed analyses of the relationship between surface topology, catalytic behavior and influence of the preparation or pretreatment method, including also the surface modifications occurring during reaction and deactivation phenomena. It also emerges that the theoretical modeling of the reaction mechanism (two interesting examples, using a crystalchemical approach or extended Hiickel calculations, are presented in this issue [9,101) must be revised to include the role of the possible tridimensionality of the active sites suggested on the basis of the previous considerations. A completely open question is furthermore the possibility of realization of ad-hoc surface topologies and architectures of the active sites by secondary modification
150
of the active surface of vanadyl pyrophosphate through a speciiic anchoring of organic or inorganic groups to Bronsted P-OH groups. Another interesting aspect deriving from the recent results on VP0 catalysts is the considerable iniluence of the strongly held adspecies on the catalytic reactivity. Usually, due to the presence of oxygen, the SUrfaCe COnCentdiOn of these species is neglected, but on vanadyl pyrophosphate the presence of strong Lewis and Bronsted acidity 1111(probably necessary for the alkane activation] and limited oxidation capacity (the overoxidation to CO, of intermediates that are various orders of magnitude more reactive than the starting alkane must be controlled] enhances their formation. As a consequence, the surface reactivity is affected considerably: the presence of these strongly held adspecies reduces the number of active sites, but also changes their turnover frequency I121. Probably, as mentioned before, the surface topology during reaction is also altered. The problem of the identification of the real active surface and the role of adspecies (their reactivity, surface mobility and influence on the nature and architecture of the active sites] still requires further study. Present data suggest the importance of these aspects on the catalytic behavior of VPO, but cannot be considered clear proof of their role and nature. However, it should be stressed that on VP0 their importance is probably more relevant than in other catalytic reactions. It is also interesting to observe that the role of adspecies on the change of surface reactivity, their surface mobility and interaction with other surface species shifts the perspective from a single molecule - single active site analysis of the reaction mechanism to a general view of the active surface which must include important aspects such as the competition from some adspecies on the adsorption of others, the role of the surface concentration of adspecies on the catalytic mechanism, the problem of the surface mobility of adspecies. A question emerging from the analysis of the behavior of VPO, but of more general interest for heterogeneous catalysis, is the role of the relative surface lifetime of adspecies and reaction intermediates. TAP transient response studies suggest that when the organic reaction intermediate and oxygen are not present simultaneously on the active site, other surface pathways of transformation become significant with a decrease in selectivity 1121.The picture of the model reaction mechanism emerging from the extended Hfickel calculations is in agreement with this concept [lo]. In order to have the selective formation of a product in a complex multistep - multisite mechanism it is necessary to have a calibrated reactivity for all sites participating in the mechanism of reaction and constituting the multinuclear cluster-type active site. For example, if the specific reactivity of sites for the stage of 0 insertion on the adsorbed hydrocarbon decreases, but the reactivity of the sites able to abstract H atoms does not change, the intermediate can be transformed according to a different route with a change in the selectivity. The same occurs when the nature of the hydrocarbon is changed. because more reactive sites may be present in the molecule which alter the delicate equilibrium of relative reactivity between the various sites, thus favoring a different pathway. This is the basic explanation, in my opinion, to understand why phthalic anhydride (Cs] forms from n-pentane (Cs). but not from
151
n-butane (C4) on vanadyl pyrophosphate, even though the mechanism of transformation of butane and per&me must be reasonably similar (81.However, ifwe assume the formation of a diene from both alkanes (butadiene and pentadiene, respectively), in the latter diene more reactive allylic H atoms are present which are not present in butadiene and further abstraction leads to the formation of products following a different evolution than that for butadiene. These problems thus evidence the necessity of modeling the reaction mechanism from a dynamic point of view that includes concepts such as the relative surface lifetime of adspecies, the relative turnover frequency of the active sites for the various elementary steps, and the mobility of adspecies. This is an evolution of “traditional” and “static” concepts of active sites that do not include the interrelationship with the other adspecies and the specific turnover frequency of the various components of a multinuclear active sites. In addition to these general questions, it is useful also to point out some other more specific topics on VP0 that require further inves~ga~on. An important aspect is the analysis of the changes occurring in the VP0 catalyst during reaction. and in particular, the influence of the hydrocarbon and oxygen concentration, the influence of the water vapor pressure, and the role of possible gas phase additives. Aspects such as the possible surface partial amorphization, the formation of mixed valence surface compounds containing V(II1) [ 131, the possibilities of in-siht regeneration of the active surface by gas-phase doping are also very relevant, in particular for a better understanding of the phenomena of deactivation of VP0 catalysts. There are also several structural aspects of the chemistry of VP0 catalysts that must be analyzed more closely, and in particular the relationship between defect state, structure, morphology and surface topology. However, it also should be mentioned that little attention has been devoted to analysis of the catalytic behavior and characteristics of some compounds with several analogies to vanadyl pyrophosphate and that can provide useful indications. For example, a novel mixedvalence nonstoichiometric vanadium pyrophosphate has been recently synthesized (141 and the possibility of intercalation of transition metals into hydrated vanadyl orthophosphate (precursor phase for vanadyl p~ophosphate) has been shown f 151. The necessity for better characterization of the surface nature of vanadyl pyrophosphate and of the evolution during reaction has been previously mentioned, Probably, the new available AFM microscope that can also operate using non-conducting samples such as VP0 catalysts will open new possibilities, but a wide integration of surface techniques is also necessary to have more reliable data. The central aspect is probably the analysis of the local structure of phosphate groups on the surface. The application of computational techniques for the analysis of the reaction mechanism and dynamics of surface transformations as well as for the modeling of the surface structure is also a promising field. In particular, it will be necessary to include in these theoretical models all nonagon deriving from the ch~cte~~on of the bulk and surface properties of vanadyl pyrophosphate and from the analysis
152
of the reaction mechanism. The use of labeled compounds or catalyst samples in order to derive more precise information on the reaction mechanism has been applied in the past for the study of VI?0 catalysts, but must be extended and combined more extensively with spectroscopic characterizations [for example, with FI’-IR and Raman spectroscopy) for a better analysis of the mechanism. However, since some of the conclusions are controversial, analyses of the changes of the spectroscopic characteristics or response using labeled molecules in a series of homogeneous VP0 samples. but with different catalytic behavior are recommended. For example, it is interesting to correlate changes in the catalytic behavior and surface properties (determined using spectroscopic or reactivity methods) during the evolution of the catalyst after contact with the flow of alkane/oxygen or after selective gas-phase doping or poisoning These tests can provide more clear indications on the reaction mechanism in comparison to detailed and sophisticated analyses, but on a single catalyst. Finally. I will mention that there are various examples, but not exhaustive for all possibilities, of the application ofVP0 catalysts for other selective oxidation reaction or acid-catalyzed reaction such as condensation. Probably, new applications can also be found. In conclusion, vanadyl pyrophosphate can be considered ca mell characterized ~~e~l for the combina~on of various surface and structural ph~icochemic~ methods used in its characterization, and for the large number of papers published on this topic. However, only in the recent years have some key hypotheses been proposed that can explain the reasons for its peculiar and unique catalytic behavior in alkane oxidation. A considerable research effort is still necessary to obtain a better understanding of this catalytic system and identificaff on of the nature of the active sites for each elementary step in the reaction mechanism, but preliminary indications suggest that this study may have a profound impact on the entire field of heterogeneous catalysis and in general on microstructural engineering of surfaces.
REFERENCES Ill G.J. Hutching. Cat& Today. this issue. I21A. Satsuma, A. Hattori. Y. Murakami, D. Ye, A. Hattori, Cati Today. this issue. I31J.R. Ebner, M.R Thompson, CcW. Today. this issue. I41G. Centi. G. GoIinelli, G. Busca, J P&s. Cherry.94 (1990) 6813. 151M.R Thompson. JR. Ebner. in New DenerOpments in Selective Oxiciatronby HeterogeneousCatiysfs. P. Ruiz and B. Delmon Eds., Elsevfer Science Pub.: Amsterdam 1992, p. 353. i61 G. Busca, G. Centf, J. Am Chem SOL 111 (1989) 46. I71G. Centi. F. Trifirb, J, i4olec. Cc&d.. 35 (1986) 255. I81G. Centi. J.T. Gleaves, G. Golinelli, F. TriQrB.in New Lkuef~pments in Selective Omi;lationby HeterogeneousCatalysis, P. Ruiz and B. Deknon Eds., Elslsevier Pub.: Amsterdam 1992, p. 231. 191E. Bordes, C&aI. m. this issue. 101B. Schist& K.A. Jorgensen, cu.&I.Tw. this issue. 111G. Busca, G. Centi, F. Trlflro, V. Lorenzelli.J. P&s. Chem. 90 (1986) 1337.
153
(12) J.T. Gleaves. G. Centi, CataL Today, this issue. (13) J.W. Johnson, D.. Johnston, H.E. King, T.R Halbert, J.F. Brody, D.P. Goshorn. Znorg. Chem. 27 (1988) 1646. (14) K.H. Lii. C.S. Lee, Znorg. Chem. 29 (1990) 3298. (15) A Datta. AS. Saple, RY. Kelkar, J. Chern Sot. Chem. Comm., (1991) 645.
Catalysis Today, 16 (1993) 147-153 Elsevier Science Publishers B.V., Amsterdam
Some Prospects and Priorities for F’uture Research on Vanadyl Pyrophosphate G. Centi Dept. o~r~~t~
Chemistry and Mc~teric~ls, VkRisorgimento
4.40136 Bologna,
klfg
The objective of this issue on vanadyl pyrophosphate was to stimulate analysis and discussion of recent advances on this topic in order to enunciate some general conclusions and develop priorities for research on this catalytic system. The various contributions were mainly centered around topics regarding the nature of the active sites, key reaction steps in alkane oxidation, modeling of the reaction mechanism and interrelationships between catalytic behavior and structural/surface modifications of vanadyl pyrophosphate. In my opinion, these topics are the central aspects of research on this catalytic system which can really provide the basis for the further development of this catalyst and of its industrial app~caffons, but in addition they also are the more important topics for general scientific advancement in the field of heterogeneous catalysis. Indeed, identification of the elementary steps in the reaction mechanism and the nature and architecture of the active sites on vanadyl pyrophosphate that allow the complex oxidation of an alkane to anhydrides (n-pentane to PA is a 34 electron oxldationl to be carried out selectively make possible rational design and tuning of VP0 performances. At the same time, iden~ca~on of the unique structural and reactivity surface characteristics of VP0 and the reasons behind its peculiar catalytic ability in the selective oxidation of C4 and Cs alkanes open new possibilities and stimulate new ideas valid for the entire field of heterogeneous catalysis. Indeed, the survey of the research on VP0 catalysts presented in this issue provides some very interesting ideas and suggestions on the problems involved which are of more general interest and which, if further confirmed and investigated, can lead to a substantial jump in the understanding of heterogeneous mechanisms of reaction and in the development of new catalytic applications. On the other hand, a second, perhaps ambitious scope of the “Forum on Vanadyl Pyrophosphate” was also to evidence aspects regarding VP0 that are less relevant, in order to focalize research efforts on the more important and innovative directions. This difficult task of the identiftcation of the future “more innovative” lines of research on VP0 is reserved to the reader and I hope that the original approach used in this issue will favor this task. For my part, I should like to discuss some personal reflections that may not be of general acceptance. Different suggestions can also be found in the two commentary papers by B.K. Hodnett and G.J. 0920.5861/93/$6.00
0 1993 Elsevier Science Publishers B.V. All rights reserved.
148
Hutchings. A useful premise is that in the entire issue little attention was devoted to the preparation of VP0 catalysts. This was not determined by a lack of scientific interest on this aspect, but rather by the fact that since the structure-surface-reactivity relationship was the central theme of this issue, less attention was devoted to the aspects of the preparation of VP0 that allow various structural or surface characteristics to be obtained. Another useful premise is that although there are various interpretations and points of view when reading the data presented in this issue, a more close examination shows that the common background is the necessity for further understanding of the catalytic chemistry of VPO. For example, from an industrial perspective the more critical problem may be the necessity to improve the activity and selectivity, the productivity or the resistance to deactivation. The doping of vanadyl pyrophosphate or a suitable modification in the preparation method thus appears to be the only relevant aspect. However, the analysis of the literature data addressed, in particular, in the commentary paper of G.J. Hutching [11reveals that doping mainly affects the microstructure of VP0 with a change in the specific surface area of some crystalline planes or introduction of defects (21. In particular, increasing the exposure of the (020) plane of vanadyl pyrophosphate increases the activity of the catalyst and therefore the problem seems to be to find a preparation method or suitable doping in order to reach the limiting ideal case of a bidimensional catalyst with only the (020) plane. However, the structural model of vanadyl pyrophosphate and its surface gives a different picture showing the considerable influence on the possible analysis of the doping effect as well as the choice of suitable dopants or modifiers. Indeed, the work of Ebner and Thompson [3] suggests, even if it does not definitively prove, that the surface chemistry of vanadyl pyrophosphate is dominated by the presence of pendant pyrophosphate groups providing steric isolation of active vanadium centers on the bottom of the clefts or cavities created by the pyrophosphate groups. In addition, probably these phosphate groups cooperate in the reaction mechanism, for example in H-abstraction, and can suitably orient the intermediate product of reaction. It has been shown that doping of P-OH groups with K inhibits the 0 insertion ability of the catalyst [4]. The dimensions of these surface cavities depend on the orientation of the vanadyl columns within the structure, ie. the disorder present in the structure of the vanadyl pyrophosphate (3.51. However, doping with relatively large ions that occupy definite sites at the surface and the formation during reaction of non- mobile organic adspecies are also additional ways to modify in a similar fashion the surface topology and “architecture” of vanadyl pyrophosphate. In addition, these outgrowing surface groups may cooperate in the reaction mechanism. For example, it has been suggested that organic adspecies cooperate in the H-abstraction mechanism in furan oxidation on vanadyl pyrophosphate 161.Therefore, the nature and dimension of these surface cavities not only influence the activity of the vanadium centres on the bottom of these cavities (number of accessible sites) and the selectivity (site isolation of the active sites, inhibition of the surface mobility of adspecies, cooperative effects in the
149
surface mechanism, stabilization of the reaction intermediates). but also the reaction pathways. The phthalic to maleic anhydride ratio from n-pentane would certainly be affected by the nature, form and dimensions of these surface cavities. This concept of a trfcSmertsionalactive site on the vanadyl pyrophosphate surface and its role in the mechanism of selective synthesis of anhydrides from C4 and Cs alkanes has a relevant effect on the analysis of catalytic data and therefore also on the study of doping effects. Profound reflection is necessary on these topics which address several open questions very relevant also for possible applications. Might it be possible to modify the surface topology by a controlled anchoring of organic or inorganic molecules? Will it be possible to realize a multifunctional tridimensional active site in this way7 Will it be possible to design a surface topology for the active site that can orient the intermediate in order to obtain uncommon reaction products or to stop the reaction at a desired level? For example, the synthesis of furan or dihydrofuran from n-butane is industrially very interesting. These are suggested reaction intermediates from butane to maleic anhydride, but their formation is usually negligible. On the contrary, furan can be synthesized from butadiene with relatively good yields 171.The concept of surface topology and tridimensionaliiy of the active sites on vanadyl pyrophosphate, however, opens new prospects for the possibility of stopping the reaction from butane to furan. A similar discussion is also valid regarding the problem of enhancing the selectivity to phthalic anhydride from n-pentane rather than maleic anhydride 181.Also in this case a possible approach is to control the surface topology of accessible vanadium sites through selected doping or secondary modifications. A better understanding of this concept of surface topology and its relationship to the catalytic behavior can therefore lead to new industrial applications of VPO, but is also of more general interest for the entire field of heterogeneous catalysis or materials science. For example, better and more selective sensors can be realized by controlling the surface topology of the accessible active sites. The concept of tridimensional active sites on vanadyl pyrophosphate which mimic an enzymatic system and of controlled surface topology due to the presence of disorder in the structure of VP0 are thus very interesting hypotheses emerging from the study of this catalytic system that can open new prospects and which therefore must be more thoroughly studied and verified. For example, the use of AFM (atomic force microscopy) on real catalysts can provide valuable information regarding these questions and will allow more detailed analyses of the relationship between surface topology, catalytic behavior and influence of the preparation or pretreatment method, including also the surface modifications occurring during reaction and deactivation phenomena. It also emerges that the theoretical modeling of the reaction mechanism (two interesting examples, using a crystalchemical approach or extended Hiickel calculations, are presented in this issue [9,101) must be revised to include the role of the possible tridimensionality of the active sites suggested on the basis of the previous considerations. A completely open question is furthermore the possibility of realization of ad-hoc surface topologies and architectures of the active sites by secondary modification
150
of the active surface of vanadyl pyrophosphate through a speciiic anchoring of organic or inorganic groups to Bronsted P-OH groups. Another interesting aspect deriving from the recent results on VP0 catalysts is the considerable iniluence of the strongly held adspecies on the catalytic reactivity. Usually, due to the presence of oxygen, the SUrfaCe COnCentdiOn of these species is neglected, but on vanadyl pyrophosphate the presence of strong Lewis and Bronsted acidity 1111(probably necessary for the alkane activation] and limited oxidation capacity (the overoxidation to CO, of intermediates that are various orders of magnitude more reactive than the starting alkane must be controlled] enhances their formation. As a consequence, the surface reactivity is affected considerably: the presence of these strongly held adspecies reduces the number of active sites, but also changes their turnover frequency I121. Probably, as mentioned before, the surface topology during reaction is also altered. The problem of the identification of the real active surface and the role of adspecies (their reactivity, surface mobility and influence on the nature and architecture of the active sites] still requires further study. Present data suggest the importance of these aspects on the catalytic behavior of VPO, but cannot be considered clear proof of their role and nature. However, it should be stressed that on VP0 their importance is probably more relevant than in other catalytic reactions. It is also interesting to observe that the role of adspecies on the change of surface reactivity, their surface mobility and interaction with other surface species shifts the perspective from a single molecule - single active site analysis of the reaction mechanism to a general view of the active surface which must include important aspects such as the competition from some adspecies on the adsorption of others, the role of the surface concentration of adspecies on the catalytic mechanism, the problem of the surface mobility of adspecies. A question emerging from the analysis of the behavior of VPO, but of more general interest for heterogeneous catalysis, is the role of the relative surface lifetime of adspecies and reaction intermediates. TAP transient response studies suggest that when the organic reaction intermediate and oxygen are not present simultaneously on the active site, other surface pathways of transformation become significant with a decrease in selectivity 1121.The picture of the model reaction mechanism emerging from the extended Hfickel calculations is in agreement with this concept [lo]. In order to have the selective formation of a product in a complex multistep - multisite mechanism it is necessary to have a calibrated reactivity for all sites participating in the mechanism of reaction and constituting the multinuclear cluster-type active site. For example, if the specific reactivity of sites for the stage of 0 insertion on the adsorbed hydrocarbon decreases, but the reactivity of the sites able to abstract H atoms does not change, the intermediate can be transformed according to a different route with a change in the selectivity. The same occurs when the nature of the hydrocarbon is changed. because more reactive sites may be present in the molecule which alter the delicate equilibrium of relative reactivity between the various sites, thus favoring a different pathway. This is the basic explanation, in my opinion, to understand why phthalic anhydride (Cs] forms from n-pentane (Cs). but not from
151
n-butane (C4) on vanadyl pyrophosphate, even though the mechanism of transformation of butane and per&me must be reasonably similar (81.However, ifwe assume the formation of a diene from both alkanes (butadiene and pentadiene, respectively), in the latter diene more reactive allylic H atoms are present which are not present in butadiene and further abstraction leads to the formation of products following a different evolution than that for butadiene. These problems thus evidence the necessity of modeling the reaction mechanism from a dynamic point of view that includes concepts such as the relative surface lifetime of adspecies, the relative turnover frequency of the active sites for the various elementary steps, and the mobility of adspecies. This is an evolution of “traditional” and “static” concepts of active sites that do not include the interrelationship with the other adspecies and the specific turnover frequency of the various components of a multinuclear active sites. In addition to these general questions, it is useful also to point out some other more specific topics on VP0 that require further inves~ga~on. An important aspect is the analysis of the changes occurring in the VP0 catalyst during reaction. and in particular, the influence of the hydrocarbon and oxygen concentration, the influence of the water vapor pressure, and the role of possible gas phase additives. Aspects such as the possible surface partial amorphization, the formation of mixed valence surface compounds containing V(II1) [ 131, the possibilities of in-siht regeneration of the active surface by gas-phase doping are also very relevant, in particular for a better understanding of the phenomena of deactivation of VP0 catalysts. There are also several structural aspects of the chemistry of VP0 catalysts that must be analyzed more closely, and in particular the relationship between defect state, structure, morphology and surface topology. However, it also should be mentioned that little attention has been devoted to analysis of the catalytic behavior and characteristics of some compounds with several analogies to vanadyl pyrophosphate and that can provide useful indications. For example, a novel mixedvalence nonstoichiometric vanadium pyrophosphate has been recently synthesized (141 and the possibility of intercalation of transition metals into hydrated vanadyl orthophosphate (precursor phase for vanadyl p~ophosphate) has been shown f 151. The necessity for better characterization of the surface nature of vanadyl pyrophosphate and of the evolution during reaction has been previously mentioned, Probably, the new available AFM microscope that can also operate using non-conducting samples such as VP0 catalysts will open new possibilities, but a wide integration of surface techniques is also necessary to have more reliable data. The central aspect is probably the analysis of the local structure of phosphate groups on the surface. The application of computational techniques for the analysis of the reaction mechanism and dynamics of surface transformations as well as for the modeling of the surface structure is also a promising field. In particular, it will be necessary to include in these theoretical models all nonagon deriving from the ch~cte~~on of the bulk and surface properties of vanadyl pyrophosphate and from the analysis
152
of the reaction mechanism. The use of labeled compounds or catalyst samples in order to derive more precise information on the reaction mechanism has been applied in the past for the study of VI?0 catalysts, but must be extended and combined more extensively with spectroscopic characterizations [for example, with FI’-IR and Raman spectroscopy) for a better analysis of the mechanism. However, since some of the conclusions are controversial, analyses of the changes of the spectroscopic characteristics or response using labeled molecules in a series of homogeneous VP0 samples. but with different catalytic behavior are recommended. For example, it is interesting to correlate changes in the catalytic behavior and surface properties (determined using spectroscopic or reactivity methods) during the evolution of the catalyst after contact with the flow of alkane/oxygen or after selective gas-phase doping or poisoning These tests can provide more clear indications on the reaction mechanism in comparison to detailed and sophisticated analyses, but on a single catalyst. Finally. I will mention that there are various examples, but not exhaustive for all possibilities, of the application ofVP0 catalysts for other selective oxidation reaction or acid-catalyzed reaction such as condensation. Probably, new applications can also be found. In conclusion, vanadyl pyrophosphate can be considered ca mell characterized ~~e~l for the combina~on of various surface and structural ph~icochemic~ methods used in its characterization, and for the large number of papers published on this topic. However, only in the recent years have some key hypotheses been proposed that can explain the reasons for its peculiar and unique catalytic behavior in alkane oxidation. A considerable research effort is still necessary to obtain a better understanding of this catalytic system and identificaff on of the nature of the active sites for each elementary step in the reaction mechanism, but preliminary indications suggest that this study may have a profound impact on the entire field of heterogeneous catalysis and in general on microstructural engineering of surfaces.
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