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What are the important trends in catalysis for the future?
Applied Catalysis A: General 113 ( 1994) 193-198
What are the important trends in catalysis for the future? I. Pasquon Dipartimento di Chimica Industriale ed Ingegneria Chimica G. Natta de1 Politecnico, Milan, Italy
1. What are the important trends in catalysis for the future?
Such trends are supposed to concern industrial chemical processes as well as issues of environmental protection in a broad sense. The following topics can be included in the former group: C 1 chemistry: Methane activation (oxidative coupling to ethylene, direct 0) oxidation to methanol and/or formaldehyde) ; syntheses from CO and H2 (e.g. higher alcohols, C2 oxygenates). (ii) Alkane activation (e.g. dehydrogenation to alkenes, ammoxidation of propane, controlled oxidation and oxychlorination of ethane) . Highly selective syntheses in the field of fine chemistry. Regarding the (iii) topics of environmental protection; Catalysts for the abatement of pollutants from motor vehicles. (iv) More selective catalysts designed to reduce the emission of polluting efflu(v) ents from industrial processes. Catalysts for alternative syntheses from less harmful feedstocks (or inter(vi) mediates), particularly where the present technology involves the use of hazardous reactants for the environment or for the human beings. (vii) Catalysts for the desulphurisation of liquid hydrocarbon fractions. (viii) Catalytic methods aimed at the elimination of pollutants in the flue gases (e.g. DeNOx for gas turbines, incinerators, boilers and Diesel engines, combined DeNOx and DeSOx, hot desulphurisation) . Catalytic combustion for power applications, aimed at reducing the NOx (ix) emissions. Catalysts for reformulated gasolines. (x) For each topic you choose to focus on (ideally two or more) : (a) what is the objective and the potential impact? (b) What progress should be made? (c) How realistic is the objective? (d) Is it related to other fields of catalysis? (e) If so, how could progress in the other fields help the objective? 0926-860X/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO926-860X (94)00063-W
194
I. Pasquon /Applied Catalysis A: General 113 (1994) 193-I 98
Oxidative coupling of methane to ethylene (a) Obtain ethylene (and propene) from methane, with the following advantages: (i) replace the petroleum derivatives as feedstocks; (ii) provide a better utilization of methane; (iii) construct plants devoted to manufacturing one or two products only. (b) Increase selectivities and yields by the development of new catalytic systems and of new reactor types (e.g. membrane reactors). (c) Achieving the above goals is not straightforward. (d) Other related fields of catalysis are gas-phase homogeneous catalysis, the knowledge of reaction mechanisms and active sites, the development and the study of catalysts supported on membranes; the study of catalytic phenomena in the presence of microwaves could be helpful, too. (e) Identification of new catalytic systems, and optimal design of chemical reactors, also based on new concepts. Abatement of polluting emissions from motor vehicles (a,b) More durable catalysts, which are also less expensive, less sensitive to poisoning and to the operating conditions of the engines; the practical consequences would include a drastic reduction of the sources of air pollution. (c) The objectives do not seem unrealistic. (d,e) Understanding the reaction mechanisms, the deactivation phenomena, the catalytically active phases, in order to improve the existing catalysts and to develop new systems.
2. What are the scientific challenges in catalysis and how do we develop a strategy to tackle them? The main goal remains the understanding of the reaction mechanisms, of the intermediate species, of the catalytic sites and of the active phases in heterogeneous catalysis, as well as the understanding of the active complexes in homogeneous catalysis, for real working conditions. This could result in an optimal catalyst design, once also aspects concerning the optimal choice of the pore structure, of the spatial distribution of the active phase and of the geometric characteristics are taken into account. A strategy suitable for addressing these issues could involve coordinated researches based on different approaches, the development of instruments for a more thorough investigation of surface phenomena and of solid-state reactions in the case of heterogeneous catalysis, and of catalytic complexes in the case of homogeneous catalysis. Also the study of enzymatic processes and the understanding of the related mechanisms could be remarkably helpful for the
I. Pasquon /Applied
Catalysis A: General I13 (I 994) 193-I 98
195
progress of knowledge in the field of catalysis. The in depth study of phenomena associated with catalyst deactivation appears also to be particularly helpful.
3. Are new concepts of catalysis required? If so, what are they and how can they be addressed?
An important breakthrough in the field of catalysis could be originated by the development of processes based on the use of membranes. The study of enzymatic phenomena could result in the development of biomimetic catalysts suitable for the synthesis of fine chemicals, but also of commodities (e.g. methanol). An ambitious goal would be the chlorophyllian synthesis. The study of catalytic reactions in extreme conditions, such as coupling of methane or dehydrogenation of alkenes, would greatly benefit from additional knowledge on the interaction between homogeneous and heterogeneous catalysis, especially in terms of identification and definition of a systematic approach. Professor Haber of the Polish Academy of Sciences, Krakow focused on the role of oxygen, which is chemically the most important reagent as virtually all chemical products have been subjected to some form of catalytic oxidation. The tailoring or even the molecular design of a catalyst surface, as suggested by Cairns, and reaction control by a set of parallel and consecutive reactors are areas for research. While molecular modelling is already fashionable in zeolite chemistry, it is more difficult for oxides. However, software is being developed and it should soon be possible to make more use of computational chemistry in screening for potential new catalysts. One of the problems is that catalysis is one dynamic process, not a series of
Prof. I. Pasquon
196
I. Pasquon /Applied
Catalysis A: General 113 (1994) 193-198
steps as it is usually conceptualised, and the quantum chemistry is not fully understood yet. Discussion Bader: I should like to underscore that one should apply biotechnology not only for speciality or fine chemicals. I think biotechnology should be used to make commodity chemicals too. We have heard this morning that it is often very difficult to obtain a highly selective, active and long-living catalyst. But nature works by a different concept. Nature steadily produces an amount of catalytic activity and loses, by deactivation, some of the catalyst. By using living cells you might be able to at least minimize the problem of having highly selective, active and also long living catalysts. Roth: I have to express some concern about that view because I think the economic feasibility and attractiveness of using enzymes or living cells for the production of commodity chemicals still seems doubtful. In the few examples where people, in the euphoria of the early days of genetic engineering, have attempted to develop a process for the production of ethylene or ethylene derivatives, that approach failed to achieve acceptable economics. While on the one hand it has some very appealing features, I think that the economic realities are something that may weigh against it. Bader: Take the nitrolases that are used for making acrylamide from acrylonitrile as an example. What I mean is using living cells, because using inactivated cells of isolated enzymes is a more demanding approach. If you have living cells that can reproduce themselves, you will have a long-lasting, active catalyst. Delmon: Don’t you think that in general using whole cells and natural catalysts would specifically be reserved for relatively sophisticated reactions where you have a succession of steps to obtain the given product with a high selectivity, and for commodity chemicals the use of natural catalysts will remainrelatively exceptional? Another consideration is that sometimes the thermodynamics of the reaction are very unfavourable at low-temperature and, hence, require a considerable energy consumption, e.g. like ATP. So there is certainly a place for these reactions but probably more so for molecules with a complicated structure. Bader: I agree with you where redox enzymes are concerned, where co-factors etc. are needed, but with ligases they are doing additions or other coupling reactions. We have, of course, to learn how to develop this application of catalysis. We should not make the mistake of reserving enzyme catalysis only for high-valued products. Pusquon: There are some examples of high-volume products prepared by enzymatic action. By studying enzymatic activity we can prepare biomimetic catalysts that can be used, for instance, in the activation of methane. I don’t think it will be possible for the production of ethylene from methane. Enzymes and biotechnology are very important for fine chemicals. There are numerous examples. I do not think
I. Pasquon /Applied
Catalysis A: General 113 (1994) 193-198
197
that this is the solution for all problems but, in my opinion, the technique is not being used to its full extent. Bader: The advantage is that bioorganisms can be fed a lot of different materials. If your process is based on methane or some other feedstocks you will have to make those first. Perhaps bioorganisms can help us clean up a lot of organic waste. Huber: On the other hand there is one great danger, namely if you apply biotechnology to commodity chemicals produced in large quantities, you will have a tremendous problem with the environmental pollution caused by the bacteria or other organisms. If you have ever visited a brewery you will have seen what remains from the beer production. Buder: Perhaps the solution is in recycling. You can use this biomass to feed cattle. Haber: But it remains dangerous, and the danger is even greater when you are polluting the environment with living organisms and you can never predict how they will develop. This is why I completely agree with what Jim Roth said, that it is probably in specialty products where this will be applied. Cusumuno: If you apply it to specialty chemicals as you develop enzymology and whole cell technology, if it is applicable it will find its own application and you will further develop the catalyst for these applications. It depends very much on a case-by-case basis. One other thing about this waste that you were talking about is that it is biodegradable. Huber: Sure, but on the other hand it might be dangerous because it is alive. Bader: We should learn to use the high-value products we have in the cells, such as the peptides, etc. Pusquon: I thing we can not generalize. There are several different situations. With enzymes there will be perhaps better chances, they are for instance used for cheese production. Delmon: We do not know what quantities of waste are produced in the acrylamide process and if they used the actual microorganism or just the enzyme. I have heard that they now use the immobilized enzyme in order to minimize the waste I have another question: to what extent are membrane catalytic reactors worth considering. There are admittedly some doubts over the validity of the concept for several reactions. If the idea is to work on the equilibrium of the reaction by removing the product it might be all right, but not if you have to deal with a very delicate mixture and you wish to work on the verge of explosive limits. For example, people now very much doubt that there is a sound reason for applying membranes in methane oxidative coupling or methane functionalization. I should like to have your comment on that. Pusquon: I should mention two types of reagents: hydrogen and oxygen. In the first case we can take away hydrogen in order to achieve a better equilibrium. This is of interest for dehydrogenation reactions. But there are competitive processes in order to obtain the hydrogenated products. There is reductive oxidation, but this does not have a high selectivity whereas hydrogenation has a good selectivity. If
198
I.Pasquon /Applied
Catalysis A: General 113 (1994) 193-198
we can conduct the reaction at low temperature in order to take away the hydrogen, so we will need a catalyst working a low temperature, perhaps this process will be better. The other, more interesting case is that of the oxidation. The oxidation of paraffins in general is also not selective. The ratio of oxygen versus paraffin in different parts of the reactor with different temperatures is important in order to control selectivity. In this case it will be important to introduce oxygen at the right point, and that is where a membrane can help. But for instance in the case of methane oxidative coupling we do not have a membrane that can resist the high temperatures. Delmon: To my knowledge there are no convincing arguments that one could possibly obtain better results for the oxidative coupling of methane, except if we consider not the action of the membrane as such, but the restricted volume of its pores in which the coupling takes place. This may enhance the concentration of radicals and their probability of recombining but this is not the membrane effect which is at stake. I believe that we should think about the real potential of catalytic membranes, in order to establish clearly what the fields are where they could be useful. Maxwell: One of the obvious areas where you could potentially use them is paraffin dehydrogenation, where you are very often equilibrium limited by the partial pressure of hydrogen. We have done some simulation experiments at low hydrogen partial pressures to look at the potential benefits in terms of shifting equilibrium and what we found was that most of the catalyst systems that have been developed become very unstable in the absence of the partial pressure of hydrogen. So you solve one problem, but you create another. Pasquon: The other topic I want to consider is what I call tools. Several of the tools I discussed are new tools and it is difficult to say at this moment which of them will become important and find commercial application. But we should look at the possibilities. I think it would be a mistake to discard some of them on the basis of the knowledge we have at the moment. In the future there might be new solutions, new ideas. I repeat that not all those tools will find applications, but I emphasize that we should take these new tools into account, as for instance the new preparation methods for catalysts. This morning we heard a good example. I once worked with a company preparing electronic parts by using high vacuum, by metal deposition, etc. With this company we studied a new type of catalyst. The catalyst was not applied for a simple reason, yet this new type of catalyst was very interesting. This happened in 1969, but now we are again thinking of this type of catalyst and now we have the experience. In our work we are concerned with the future, and not just tomorrow, but the more distant future. We have to take all opportunities into account and look critically at them.
What are the important trends in catalysis for the future? I. Pasquon Dipartimento di Chimica Industriale ed Ingegneria Chimica G. Natta de1 Politecnico, Milan, Italy
1. What are the important trends in catalysis for the future?
Such trends are supposed to concern industrial chemical processes as well as issues of environmental protection in a broad sense. The following topics can be included in the former group: C 1 chemistry: Methane activation (oxidative coupling to ethylene, direct 0) oxidation to methanol and/or formaldehyde) ; syntheses from CO and H2 (e.g. higher alcohols, C2 oxygenates). (ii) Alkane activation (e.g. dehydrogenation to alkenes, ammoxidation of propane, controlled oxidation and oxychlorination of ethane) . Highly selective syntheses in the field of fine chemistry. Regarding the (iii) topics of environmental protection; Catalysts for the abatement of pollutants from motor vehicles. (iv) More selective catalysts designed to reduce the emission of polluting efflu(v) ents from industrial processes. Catalysts for alternative syntheses from less harmful feedstocks (or inter(vi) mediates), particularly where the present technology involves the use of hazardous reactants for the environment or for the human beings. (vii) Catalysts for the desulphurisation of liquid hydrocarbon fractions. (viii) Catalytic methods aimed at the elimination of pollutants in the flue gases (e.g. DeNOx for gas turbines, incinerators, boilers and Diesel engines, combined DeNOx and DeSOx, hot desulphurisation) . Catalytic combustion for power applications, aimed at reducing the NOx (ix) emissions. Catalysts for reformulated gasolines. (x) For each topic you choose to focus on (ideally two or more) : (a) what is the objective and the potential impact? (b) What progress should be made? (c) How realistic is the objective? (d) Is it related to other fields of catalysis? (e) If so, how could progress in the other fields help the objective? 0926-860X/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO926-860X (94)00063-W
194
I. Pasquon /Applied Catalysis A: General 113 (1994) 193-I 98
Oxidative coupling of methane to ethylene (a) Obtain ethylene (and propene) from methane, with the following advantages: (i) replace the petroleum derivatives as feedstocks; (ii) provide a better utilization of methane; (iii) construct plants devoted to manufacturing one or two products only. (b) Increase selectivities and yields by the development of new catalytic systems and of new reactor types (e.g. membrane reactors). (c) Achieving the above goals is not straightforward. (d) Other related fields of catalysis are gas-phase homogeneous catalysis, the knowledge of reaction mechanisms and active sites, the development and the study of catalysts supported on membranes; the study of catalytic phenomena in the presence of microwaves could be helpful, too. (e) Identification of new catalytic systems, and optimal design of chemical reactors, also based on new concepts. Abatement of polluting emissions from motor vehicles (a,b) More durable catalysts, which are also less expensive, less sensitive to poisoning and to the operating conditions of the engines; the practical consequences would include a drastic reduction of the sources of air pollution. (c) The objectives do not seem unrealistic. (d,e) Understanding the reaction mechanisms, the deactivation phenomena, the catalytically active phases, in order to improve the existing catalysts and to develop new systems.
2. What are the scientific challenges in catalysis and how do we develop a strategy to tackle them? The main goal remains the understanding of the reaction mechanisms, of the intermediate species, of the catalytic sites and of the active phases in heterogeneous catalysis, as well as the understanding of the active complexes in homogeneous catalysis, for real working conditions. This could result in an optimal catalyst design, once also aspects concerning the optimal choice of the pore structure, of the spatial distribution of the active phase and of the geometric characteristics are taken into account. A strategy suitable for addressing these issues could involve coordinated researches based on different approaches, the development of instruments for a more thorough investigation of surface phenomena and of solid-state reactions in the case of heterogeneous catalysis, and of catalytic complexes in the case of homogeneous catalysis. Also the study of enzymatic processes and the understanding of the related mechanisms could be remarkably helpful for the
I. Pasquon /Applied
Catalysis A: General I13 (I 994) 193-I 98
195
progress of knowledge in the field of catalysis. The in depth study of phenomena associated with catalyst deactivation appears also to be particularly helpful.
3. Are new concepts of catalysis required? If so, what are they and how can they be addressed?
An important breakthrough in the field of catalysis could be originated by the development of processes based on the use of membranes. The study of enzymatic phenomena could result in the development of biomimetic catalysts suitable for the synthesis of fine chemicals, but also of commodities (e.g. methanol). An ambitious goal would be the chlorophyllian synthesis. The study of catalytic reactions in extreme conditions, such as coupling of methane or dehydrogenation of alkenes, would greatly benefit from additional knowledge on the interaction between homogeneous and heterogeneous catalysis, especially in terms of identification and definition of a systematic approach. Professor Haber of the Polish Academy of Sciences, Krakow focused on the role of oxygen, which is chemically the most important reagent as virtually all chemical products have been subjected to some form of catalytic oxidation. The tailoring or even the molecular design of a catalyst surface, as suggested by Cairns, and reaction control by a set of parallel and consecutive reactors are areas for research. While molecular modelling is already fashionable in zeolite chemistry, it is more difficult for oxides. However, software is being developed and it should soon be possible to make more use of computational chemistry in screening for potential new catalysts. One of the problems is that catalysis is one dynamic process, not a series of
Prof. I. Pasquon
196
I. Pasquon /Applied
Catalysis A: General 113 (1994) 193-198
steps as it is usually conceptualised, and the quantum chemistry is not fully understood yet. Discussion Bader: I should like to underscore that one should apply biotechnology not only for speciality or fine chemicals. I think biotechnology should be used to make commodity chemicals too. We have heard this morning that it is often very difficult to obtain a highly selective, active and long-living catalyst. But nature works by a different concept. Nature steadily produces an amount of catalytic activity and loses, by deactivation, some of the catalyst. By using living cells you might be able to at least minimize the problem of having highly selective, active and also long living catalysts. Roth: I have to express some concern about that view because I think the economic feasibility and attractiveness of using enzymes or living cells for the production of commodity chemicals still seems doubtful. In the few examples where people, in the euphoria of the early days of genetic engineering, have attempted to develop a process for the production of ethylene or ethylene derivatives, that approach failed to achieve acceptable economics. While on the one hand it has some very appealing features, I think that the economic realities are something that may weigh against it. Bader: Take the nitrolases that are used for making acrylamide from acrylonitrile as an example. What I mean is using living cells, because using inactivated cells of isolated enzymes is a more demanding approach. If you have living cells that can reproduce themselves, you will have a long-lasting, active catalyst. Delmon: Don’t you think that in general using whole cells and natural catalysts would specifically be reserved for relatively sophisticated reactions where you have a succession of steps to obtain the given product with a high selectivity, and for commodity chemicals the use of natural catalysts will remainrelatively exceptional? Another consideration is that sometimes the thermodynamics of the reaction are very unfavourable at low-temperature and, hence, require a considerable energy consumption, e.g. like ATP. So there is certainly a place for these reactions but probably more so for molecules with a complicated structure. Bader: I agree with you where redox enzymes are concerned, where co-factors etc. are needed, but with ligases they are doing additions or other coupling reactions. We have, of course, to learn how to develop this application of catalysis. We should not make the mistake of reserving enzyme catalysis only for high-valued products. Pusquon: There are some examples of high-volume products prepared by enzymatic action. By studying enzymatic activity we can prepare biomimetic catalysts that can be used, for instance, in the activation of methane. I don’t think it will be possible for the production of ethylene from methane. Enzymes and biotechnology are very important for fine chemicals. There are numerous examples. I do not think
I. Pasquon /Applied
Catalysis A: General 113 (1994) 193-198
197
that this is the solution for all problems but, in my opinion, the technique is not being used to its full extent. Bader: The advantage is that bioorganisms can be fed a lot of different materials. If your process is based on methane or some other feedstocks you will have to make those first. Perhaps bioorganisms can help us clean up a lot of organic waste. Huber: On the other hand there is one great danger, namely if you apply biotechnology to commodity chemicals produced in large quantities, you will have a tremendous problem with the environmental pollution caused by the bacteria or other organisms. If you have ever visited a brewery you will have seen what remains from the beer production. Buder: Perhaps the solution is in recycling. You can use this biomass to feed cattle. Haber: But it remains dangerous, and the danger is even greater when you are polluting the environment with living organisms and you can never predict how they will develop. This is why I completely agree with what Jim Roth said, that it is probably in specialty products where this will be applied. Cusumuno: If you apply it to specialty chemicals as you develop enzymology and whole cell technology, if it is applicable it will find its own application and you will further develop the catalyst for these applications. It depends very much on a case-by-case basis. One other thing about this waste that you were talking about is that it is biodegradable. Huber: Sure, but on the other hand it might be dangerous because it is alive. Bader: We should learn to use the high-value products we have in the cells, such as the peptides, etc. Pusquon: I thing we can not generalize. There are several different situations. With enzymes there will be perhaps better chances, they are for instance used for cheese production. Delmon: We do not know what quantities of waste are produced in the acrylamide process and if they used the actual microorganism or just the enzyme. I have heard that they now use the immobilized enzyme in order to minimize the waste I have another question: to what extent are membrane catalytic reactors worth considering. There are admittedly some doubts over the validity of the concept for several reactions. If the idea is to work on the equilibrium of the reaction by removing the product it might be all right, but not if you have to deal with a very delicate mixture and you wish to work on the verge of explosive limits. For example, people now very much doubt that there is a sound reason for applying membranes in methane oxidative coupling or methane functionalization. I should like to have your comment on that. Pusquon: I should mention two types of reagents: hydrogen and oxygen. In the first case we can take away hydrogen in order to achieve a better equilibrium. This is of interest for dehydrogenation reactions. But there are competitive processes in order to obtain the hydrogenated products. There is reductive oxidation, but this does not have a high selectivity whereas hydrogenation has a good selectivity. If
198
I.Pasquon /Applied
Catalysis A: General 113 (1994) 193-198
we can conduct the reaction at low temperature in order to take away the hydrogen, so we will need a catalyst working a low temperature, perhaps this process will be better. The other, more interesting case is that of the oxidation. The oxidation of paraffins in general is also not selective. The ratio of oxygen versus paraffin in different parts of the reactor with different temperatures is important in order to control selectivity. In this case it will be important to introduce oxygen at the right point, and that is where a membrane can help. But for instance in the case of methane oxidative coupling we do not have a membrane that can resist the high temperatures. Delmon: To my knowledge there are no convincing arguments that one could possibly obtain better results for the oxidative coupling of methane, except if we consider not the action of the membrane as such, but the restricted volume of its pores in which the coupling takes place. This may enhance the concentration of radicals and their probability of recombining but this is not the membrane effect which is at stake. I believe that we should think about the real potential of catalytic membranes, in order to establish clearly what the fields are where they could be useful. Maxwell: One of the obvious areas where you could potentially use them is paraffin dehydrogenation, where you are very often equilibrium limited by the partial pressure of hydrogen. We have done some simulation experiments at low hydrogen partial pressures to look at the potential benefits in terms of shifting equilibrium and what we found was that most of the catalyst systems that have been developed become very unstable in the absence of the partial pressure of hydrogen. So you solve one problem, but you create another. Pasquon: The other topic I want to consider is what I call tools. Several of the tools I discussed are new tools and it is difficult to say at this moment which of them will become important and find commercial application. But we should look at the possibilities. I think it would be a mistake to discard some of them on the basis of the knowledge we have at the moment. In the future there might be new solutions, new ideas. I repeat that not all those tools will find applications, but I emphasize that we should take these new tools into account, as for instance the new preparation methods for catalysts. This morning we heard a good example. I once worked with a company preparing electronic parts by using high vacuum, by metal deposition, etc. With this company we studied a new type of catalyst. The catalyst was not applied for a simple reason, yet this new type of catalyst was very interesting. This happened in 1969, but now we are again thinking of this type of catalyst and now we have the experience. In our work we are concerned with the future, and not just tomorrow, but the more distant future. We have to take all opportunities into account and look critically at them.