CHAPTER 1: General Introduction to the Field of Environmental Catalysis: Green Catalysis with Supported Gold and Gold Bimetallic Nanoparticles

The production of chemicals and energy is crucial to the way of life we have grown accustomed to over the recent decades of sustained growth. In some sense we take the availability of new materials and plentiful energy for granted and when there are disruptions to supplies this provides a major shock to the system. However, we now recognise that the resources available within the world are finite and consequently the desire to use sustainable resources other than fossil carbon reserves is now gaining momentum. Catalysis can and does play a major role with respect to developing the means by which bio‐renewable feedstocks can be utilised. However, catalysis can play a wider role in environmental chemistry. In this respect green chemistry is now becoming a major driving force helping to shape the way in which chemical processes should be configured. The 12 principles of green chemistry embody the key tenets of what should be aimed for in new environmentally friendly processes:

• Prevent waste: Design chemical syntheses to prevent waste, negating end‐of‐pipe clean up.

• Design safer chemicals and products: Prepare effective chemical products with little or no toxicity.

• Design less hazardous chemical syntheses: Develop syntheses to use and generate substances with little or no toxicity to humans and the environment.

• Use renewable feedstocks: Utilise raw materials and feedstocks that are renewable rather than depleting.

• Use catalysts, not stoichiometric reagents: Minimise waste by using catalytic reactions.

• Avoid chemical derivatives: Avoid using blocking or protecting groups or any temporary modifications if possible, as these lead to waste.

• Maximise atom economy: Design syntheses so that the final product contains the maximum proportion of the starting materials.

• Eliminate solvents or use safer solvents and reaction conditions: Avoid using solvents, separation agents, or other auxiliary chemicals. If these chemicals are strictly necessary, then use environmentally friendly chemicals.

• Increase energy efficiency: Run chemical reactions at minimal temperature and pressure whenever possible as this is consistent with energy utilisation and recovery.

• Design chemicals and products to degrade after use: Where the applications permit such an approach, design chemical products that will break down into innocuous substances after use so that they do not accumulate in the environment.

• Analyse in real time to prevent pollution: Include in‐process real‐time monitoring and control during syntheses to minimise or eliminate the formation of by‐products.

• Minimise the potential for accidents: Design chemical processes to minimize the potential for chemical accidents and releases to the environment.

It is clear that environmental catalysis should seek to maximise atom and energy efficiency and should eliminate the use of solvent and encompass renewable feedstocks. In this introductory chapter we will give examples that encompass these two key features using supported gold and gold bimetallic nanoparticles. A key feature that is observed for gold catalysis is the high specificity that is found in many catalytic reactions, and this is of key importance for environmental catalysis.

It is generally recognised that oil supplies are both finite and will eventually become increasingly scarce. The geographical disposition of the major oil supplies has made the price of oil a political variable for the majority of recent years. However, it is also generally understood that there exist plentiful supplies of natural gas, particularly with the discovery of larges deposits of shale gas. Despite this, there exist political drivers to diminish the dependency on materials production based solely on fossil fuels. In this respect the use of sustainable feedstocks is providing a driving force to discover new and improved chemical processes using these materials. However this drive to use sustainable feedstocks is not wholly embraced at present, as there are still widespread supplies of natural gas and coal. Indeed, several new Fischer–Tropsch plants based on natural gas have been commissioned, partly as a route to sulfur‐free diesel and gasoline, and similar processes based on coal‐derived syngas are expected to be commissioned in economies where coal is plentiful, e.g. India and China. The consumption of fossil fuels using existing pathways is therefore likely to continue for some time. Although the situation is complicated, the time is still right to consider the alternatives because in the longer term we can anticipate that the use of sustainable feedstocks will become politically expedient if viable catalytic chemistries can be developed.

More than 80% of the world's energy consumption and production of chemicals originates from fossil resources (i.e. oil, gas and coal). The finite nature of these fossil resources coupled with concerns regarding global warming have spurred a drive to develop new technologies for the generation of energy, chemicals and a materials supply from renewable resources. In this respect, it has recently been considered that biomass could become a major source for the production of energy and chemicals. Biomass derived from plants is generated from carbon dioxide and water using sunlight as the energy source while producing oxygen as an important by‐product. The total current biomass production on the planet is estimated to be about 170 billion tonnes and consists of roughly 75% carbohydrates (sugars), 20% lignins and 5% of other substances in minor amounts in forms such as oils, fats, proteins, terpenes, alkaloids, terpenoids and waxes. Based on this biomass production, only 3.5% is presently being used for human needs, where the major part of this amount is used for human food (around 62%), 33% for energy use, paper and construction needs and the rest (about 5%) is used for technical (i.e. non‐food) raw materials such as clothing, detergents and chemicals. The remaining 96.5% of biomass production is utilised within natural ecosystems.^1–3 

At the current time, the available biomass, and, therefore, the renewable raw materials, are almost entirely provided by agriculture and forestry. Important renewable raw materials such as carbohydrates can be supplied by the sugar, starch and wood processing sectors. Specifically, plant raw materials, such as sugar beet, sugar cane, wheat, corn, potatoes and rice can be used for the production of sucrose and starch, whereas cellulose and hemi‐cellulose can be derived from the processing of wood. These basic products (starch, cellulose and hemi‐cellulose) can in principle be further converted into a very broad range of products, by employing physical and chemical processes. These processes are based on partial or complete acid or enzymatic hydrolysis for breaking down the polysaccharides into sugar monomers. Starch and cellulose are polysaccharides having a glucose monomer unit and α‐1,4 or β‐1,4 glycoside linkages respectively; therefore hydrolysis will break them down into glucose, whereas sucrose hydrolysis will lead to the formation of fructose and glucose.⁴  Hemi‐cellulose contains five different sugars, of which two are 5‐carbon sugars (xylose and arabinose) and the other three are 6‐carbon sugars (galactose, glucose and mannose). In addition, the most abundant building block of hemi‐cellulose is xylan (a xylose polymer), which consists of xylose monomer units linked at the 1‐ and 4‐positions. Therefore, hydrolysis of hemi‐cellulose will provide a variety of 5‐carbon and 6‐carbon monomer sugars. In addition, other sugars, such as maltose and lactose, can be produced from biomass (barley and whey, respectively); their hydrolysis will produce glucose from the former and glucose and galactose from the latter. There is therefore a very wide range of sugars that can be made available from the utilisation of biomass. Many of these can be generated from biomass resources that cannot be utilised as foodstuffs and can be harvested on land not suitable for crop generation, and it is these starting materials that need to be the focus of future research attention.

As noted previously, throughout the world there are huge resources of bio‐renewable feedstocks, e.g. starch, cellulose, vegetable oils, and attention has been turning to considering whether these materials can be utilised more effectively. Until now the production of liquid fuels from biomass has been based on the use of processes such as acid hydrolysis of biomass for sugar production, thermochemical liquefaction and/or pyrolysis for bio‐oils production, and gasification of biomass to produce syngas (CO+H₂).⁴  Further treatment of the sugars produced, employing a fermentation process, leads to the production of ethanol, whereas the use of a dehydration process makes it possible to produce aromatic hydrocarbons.^5,6  In the case of liquefaction and pyrolysis, further treatment of these products by refining allows one to generate liquid fuels.^7,8  Finally, the Fischer–Tropsch process can be used for the synthesis of alkanes from syngas, whereas methanol production is also possible from syngas.⁹ 

An alternative strategy is the use of carbohydrates as a liquid fuel in fuel cell systems and this idea has recently attracted significant attention. For example, it has been demonstrated that carbohydrates can be used for the production of electricity in a fuel cell which comprises a vanadium flow battery.¹⁰ 

At the current time, the primary technology for the generation of liquid fuels using renewable biomass resources is based on the fermentation of carbohydrates for the production of ethanol (bio‐ethanol). However, this process is still not economically viable, owing to the high cost of the processing route. Consequently, research is now focused on the use of more plentiful and inexpensive sugars, such as ligno‐cellulose, as this will enable a reduction in the cost. Production of bio‐oils by liquefaction or pyrolysis, based on thermochemical treatment of biomass, is an inherently simpler process, but it produces a wide range of products (e.g. aromatic compounds, CO, CH₄, H₂, tars, chars, alcohols, aldehydes, ketones, esters and acids). Therefore, improvements in selectivity are considered essential for this latter approach. Research is currently being conducted with the goal of upgrading bio‐oils to more valuable products.⁹  Finally, the gasification of biomass can produce syngas (CO and H₂); however, this process requires volatilisation of water, thus decreasing the overall energy efficiency.¹¹  It can be concluded that the majority of these methods have complex processing requirements and the efficiency of liquid fuel synthesis remains low.

There has been much attention to the concept of a bio‐refinery that is capable of producing either syngas or a new raft of platform chemicals (Figure 1.1). Biomass from any source can indeed, in principle, be gasified, but this will incur an energy penalty, as noted earlier. The production of H₂ and CO by vapour‐phase and aqueous‐phase reforming of biomass‐derived oxygenated hydrocarbons has recently been described.⁴  Vapour‐phase reforming of biomass‐derived oxygenated hydrocarbons having high volatility (such as methanol, glycerol, ethylene and propylene glycol) is advantageous over the use of alkanes derived from petrochemical feedstocks. However, for less volatile biomass‐derived oxygenates such as glucose and sorbitol, it is advantageous to use the aqueous phase reforming process to avoid using excessively high temperatures.

Routes using bio‐renewable feedstocks that generate syngas can be used to interface with the need for the existing platform chemicals to be sustained. For the last 70 years the petrochemical and energy industries have focused their process technology on oil and hence huge processes have become entrenched for making key platform chemicals, e.g. ethene, propene, benzene, xylenes, from which many finished products are derived (Figure 1.2). There is therefore a desire to maintain this infrastructure and technology; after all it works very well. The initial new technologies based on methanol conversion, which sought to replace oil‐derived approaches to chemicals and fuels,^12,13  were also aimed at providing these platform chemicals, as can also be expected for the new coal‐based Fischer–Tropsch processes. Hence there is a major driving force to convert everything to syngas and start from there. While technologically sound, such an approach loses all of the in‐built complexity and functionality of bio‐derived molecules. It has been the hallmark of the chemical industry for the last century to strip every initial feedstock down to, or almost to, its elemental components, thereby incurring massive energy penalties and potential loss of selectivity. For example, the synthesis of ammonia involves a catalytic pathway on an iron‐based catalyst whereby the nitrogen–nitrogen triple bond is initially broken and then the atomic nitrogen hydrogenated. This should be contrasted with natural processes in which the bio‐synthetic pathway to ammonia sequentially hydrogenates molecular nitrogen, requiring the final fission of a nitrogen–nitrogen single bond, and this represents a much lower energy pathway.¹⁴  It would be unfortunate if such a similar approach is now taken and the routes developed for the use of bio‐renewable feedstocks are based on synthesis gas chemistry, because this would completely disregard all the chemical complexity intrinsic to the biological molecule.

An alternative to gasification and pyrolysis of biomass is fermentation. Using fermentation (see Figure 1.1) a broad range of carbohydrates are formed, principally sugars, as well as ethanol. The ethanol can be used as bio‐renewable gasoline. A major feedstock produced from fermentation of biomass is glucose because, after all, cellulose is essentially polymeric glucose (see Figure 1.3). Glucose represents a highly functionalised molecule with a number of stereogenic centres. It can be used to prepare a range of useful products, which find applications in pharmaceuticals and foodstuffs (Figure 1.4).

Alternative bio‐renewable materials are triglycerides, which are the major components of vegetable oils and animal fats. For many years, these materials have been used as the basis for soap manufacture by saponification with aqueous sodium hydroxide. This generates glycerol as a bio‐product which represents a new, highly functionalised, bio‐renewable feedstock. The quantities of glycerol made from saponification are readily utilised in the manufacture of refined glycerol for use in pharmaceuticals. However, more recently, additional interest has focused on consuming glycerol, because triglycerides can be reacted with methanol to produce biodiesel (Figure 1.5). Each mole of triglyceride reacts with 3 moles of methanol producing 3 moles of biodiesel and 1 mole of glycerol. Hence, roughly, 10 weight % of the product is glycerol.

This process is likely to transform the availability of glycerol as bio‐derived resource. At present the global production of glycerol is >1 million tonnes, and this is growing at a rate of ca. 10% per annum¹⁵  as the quest to introduce biodiesel continues. There are two drivers underpinning this surge in biodiesel production and hence glycerol production. First, within Europe there is the European Directive 2003/30 which required that 5.75% of all transport fuels be derived from renewable sources by 2010. Secondly, the Kyoto Agreement requires that emissions of CO₂ are 8% below 1990 levels by 2012, for those countries that are signatories to this agreement. There is indeed much political consensus on the need to decrease greenhouse gas emissions dramatically, and the reduction in CO₂ is a major goal to which the use of bio‐renewable feedstocks can contribute significantly. To achieve this, it is essential that bio‐renewable sources of energy are used effectively, since, for example, it is known that greenhouse gas emissions are ca. 50% less for biodiesel when compared with fossil fuel‐derived diesel on a per kilometre travelled basis. However, these drivers also produce a significant humanitarian tension because many of the bio‐renewable resources can be, and indeed are, used as foodstuffs. The competition between the need for energy and producing foodstuffs is an important issue that has yet to be resolved. Crucially though, it should be recognised that cellulose and many triglycerides are not used as precursors to foodstuffs, and it is here we should logically focus our attention.

Glycerol is a highly functionalised molecule; of course, it too can be gasified and thereby can be used as a feedstock to generate existing platform chemicals. However, it can be more effectively utilised to make a broad range of potential chemicals (Figure 1.6). For example, glyceric acid can be produced from glycerol and this topic will be discussed subsequently. Hence it is apparent that there is considerable scope for the use of bio‐renewable feedstocks for the generation of a new range of platform chemicals.

Selective oxidation using gold catalysts continues to make significant progress. Early observations concerning CO oxidation at low temperatures^16,17  and ethyne hydrochlorination^18–20  demonstrated that supported gold nanoparticles could be the catalysts of choice for these two reactions. Since then, gold catalysts have been found to be highly effective for alkene epoxidation^21–24  and alcohol oxidation.²⁶  In addition, such catalysts are highly selective for the hydrogenation of a range of alkenes^27–30  and for the hydrogenation of α,β‐unsaturated aldehydes to the corresponding unsaturated alcohols, e.g. acrolein^31,32  and crotonaldehyde.^33–35 

That gold could be a selective catalyst for alcohol oxidation was first clearly demonstrated by Rossi, Prati and co‐workers,^36–38  in their seminal studies. They showed that supported gold nanoparticles can be very effective catalysts for the oxidation of alcohols, including diols to the corresponding acid. For example 1,2 propane diol can be formed from lactic acid by oxidation of the primary alcohol group rather than the secondary alcohol group, which might have been expected to be the more reactive entity. The presence of a base (typically NaOH) was found to be essential for the observation of activity, and consequently sodium salts of the acids were formed as products. The base was considered to be essential for the first hydrogen abstraction, and this represents a significant difference between the supported gold catalysts and their counterpart Pd and Pt catalysts that are effective in acidic as well as basic conditions.

Christensen and co‐workers^39,40  have made a number of significant advances in the direct oxidation of primary alcohols using supported gold nanocrystals, in which they concentrated on decreasing the amount of base present in this reaction. They have shown that gold can catalyse the oxidation of aqueous solutions of ethanol to give acetic acid in high yields.³⁹  This provides a potential new route to a key commodity chemical that is based on a bio‐renewable feedstock using a substantially green technology approach. Recently, this group have also shown that methyl esters of a broad range of primary alcohols can be produced using a similar approach.⁴⁰ 

One of the most significant advances in the field of alcohol oxidation has been the observation of Corma and co‐workers^26,41  that an Au/CeO₂ catalyst is active for (i) the selective oxidation of alcohols to aldehydes and ketones, and (ii) the oxidation of aldehydes to acids. Most significantly, they showed that the oxidation could occur in the absence of base under very mild reaction conditions, without the addition of solvent and, consequently, the highly desirable aldehyde rather than the acid was selectively formed as the product. The ability to omit both the base and the solvent from this reaction is highly significant, since this permits the use of green reaction conditions for these oxidation reactions. The results were shown to be comparable to, or even higher than, the highest activities that had been previously observed with supported Pd catalysts.⁴²  Subsequently, Enache et al.²⁵  showed that addition of Pd to Au significantly enhanced the activity of the supported gold nanoparticles for alcohol oxidation under these mild reaction conditions using AuPd alloy nanoparticles supported on TiO₂ that comprised core–shell structures (Figure 1.7).

Oxidation of glycerol can lead to the formation of a number of valuable oxygenated compounds, such as dihydroxyacetone, hydroxypyruvic acid, glyceric acid, glycolic acid, oxalic acid and tartronic acid, and hence the control of product selectivity becomes crucial. To date these products have a limited market because they are produced mainly using costly and non‐green stoichiometric processes (e.g. using potassium permanganate or chromic acid as oxidants) or low productivity fermentation processes.^43,44  However, in the last 10 years, the selective oxidation of glycerol has been extensively studied, mainly over supported noble metal nanoparticles such as Pd, Pt and Au using molecular oxygen. Kimura and co‐workers demonstrated that, by using acidic conditions, secondary alcoholic groups could be oxidised mainly to dihydroxyacetone,⁴⁵  but also hydroxypyruvic acid,⁴⁶  using Pt and Pd catalysts. Changing the pH of the solution and moving forward to basic conditions, the primary alcoholic groups were preferentially oxidised and glyceric acid was obtained,⁴⁷  and the use of palladium‐supported catalysts led to increased selectivity in the formation of glyceric acid.^47,48  Furthermore, using Bi as a promoter on a Pt/C catalyst gave the highest selectivity to dihydroxyacetone, indicating a change in the direction of the reaction pathway towards to the secondary alcoholic group.^47,49 

The use of Au/carbon catalysts was extended by Carrettin et al.^50–53  for the oxidation of glycerol to glycerate with 100% selectivity, using dioxygen as the oxidant under relatively mild conditions, and resulted in yields approaching 60% as long as base was present. Under comparable conditions, supported Pd/C and Pt/C always generated other C₃ and C₂ products in addition to glyceric acid and, in particular, also gave some C₁ by‐products, including large amounts of carbon oxides. With the Au/carbon catalyst, the selectivity to glyceric acid and the glycerol conversion were very dependent upon the glycerol∶NaOH ratio (Table 1.1).

In general, with high concentrations of NaOH, exceptionally high selectivities to glyceric acid could be observed. However, decreasing the concentration of glycerol, and increasing the mass of the catalyst and the concentration of oxygen, led to the formation of tartronic acid via consecutive oxidation of glyceric acid. Interestingly, this latter product is stable with these catalysts. It is apparent that, with careful control of the reaction conditions, 100% selectivity to glyceric acid can be obtained with 1 wt% Au/activated C or 1% Au/graphite. The activity is dependent on the gold loading. For catalysts containing 0.25 or 0.5 wt% Au supported on graphite, lower glycerol conversions were observed (18% and 26% respectively as compared to 54% for the higher 1 wt% Au loading on graphite under the same conditions) and lower selectivities to glyceric acid were also observed. This was found to be consistent with the earlier studies on diol oxidation by Prati and co‐workers^36–38  which also showed that the conversion is very dependent on the concentration of Au in the catalyst. Characterisation of the selective 1 wt% Au/C catalysts demonstrated that they comprised Au particles as small as 5 nm and as large as 50 nm in diameter (Figure 1.8).

The majority, however, were about 25 nm in size and were multiply twinned in character. It should be noted that many researchers consider such large nanoparticles to be inactive and so it is possible that the activity is associated with a low population of smaller nanoparticles. Decreasing the loading to 0.5 or 0.25 wt% did not appreciably change the particle size distribution; however, the particle number density per unit area was observed to decrease proportionately, which could be correlated to the decrease in glycerol conversion and selectivity to glyceric acid. Subsequently, cyclic voltammetry was used to study Au catalysts supported on graphite,⁵³  since in this case the support is conducting and this very incisive characterisation technique could be used effectively under in situ conditions. The conversion has been found to correlate with specific features in the cyclic voltamogram, particularly those associated with electro‐oxidation of surface intermediates.

A common feature of the preceding studies is the use of a base to help activate the glycerol, and indeed for many years it was not possible to design gold catalysts that could activate glycerol under base‐free conditions. However, Prati and co‐workers⁵⁴  have recently demonstrated the oxidation of glycerol under base‐free conditions using a gold–platinum catalyst supported on carbon. Brett et al. then extended this approach by using supported bimetallic Au–Pd and Au–Pt nanoparticles supported on MgO.⁵⁵  Our hypothesis was that the use of a solid base as the support material could potentially aid the activation of glycerol and thereby facilitate the use of much milder reaction conditions. Initially catalysts were evaluated at 60 °C (Table 1.2) with supported Au–Pt nanoparticles (1∶3 mole fraction, 1% by mass total metal loading; Figure 1.9).

The Au–Pt catalyst with 1∶3 mol fraction retained significant activity when the temperature was decreased to 40 °C. By extending the reaction time to 24 h and further decreasing the reaction temperature to ambient (23 °C), high conversion was retained with a simultaneous increase in the C₃ product selectivity (>90% by mol.). Furthermore, no formation of formic acid, an undesired C₁ by‐product, was detected at the lower temperature. In contrast, an Au–Pd catalyst prepared with a 1∶3 mol fraction and similar metal loading demonstrated significantly lower activity under these conditions, and the selectivity to glyceric acid was lower than that displayed by the corresponding Au–Pt catalyst, despite the lower conversion observed. These results suggested that, at similar conversion levels, the Au–Pd bimetallic catalysts were significantly less selective to the desired C₃ products, and data at iso‐conversion showed this to be the case. A concern was that MgO might be acting as a sacrificial base during the reaction and hence the possible leaching of Mg²⁺ during reaction was investigated but found to be negligible. Indeed, the Mg²⁺ concentration was 2–3 orders of magnitude lower than that of the products observed. Furthermore, it was shown that the addition of this minimal level of base did not affect conversion significantly. These results conclusively demonstrated the potential of enhancing performance and effectiveness through appropriate catalyst design.

A central aspect of environmental catalysis is the need to minimise or eliminate solvents. In previous studies we have shown that alcohols can be oxidised to aldehydes in very high yields under solvent‐free conditions using supported gold–palladium nanoparticles.²⁵  Recently, we have extended this concept to demonstrate that the primary carbon–hydrogen bonds in toluene can be selectively oxidised using molecular oxygen under solvent‐free conditions.⁵⁶  We considered that the supported gold–palladium nanoparticles operate by producing a reactive hydroperoxy intermediate and, because these intermediates are known to be involved in the enzymatic oxidation of primary carbon–hydrogen bonds,⁵⁷  it was reasoned that Au–Pd nanoparticles could be active for the oxidation of toluene. Rossi and Prati first showed the applicability of colloidal methods for the preparation of gold catalysts and their use in liquid phase oxidation reactions,⁵⁸  and subsequently we have intensively studied this methodology and found that a sol immobilisation method produces more active Au–Pd catalysts for a range of selective oxidation reactions, providing us with an effective means of control over particle size, morphology and composition (Figure 1.10).^59–61  The Au–Pd alloyed nanoparticles prepared using sol immobilisation are indeed very active for the oxidation of toluene with molecular oxygen and unexpectedly selective to benzyl benzoate under solvent‐free conditions (Figure 1.11).⁵⁶ 

To show general applicability, the selective oxidation of xylenes was also studied, and the catalyst formed the aldehyde, acid and esters, with the relative amounts being dependent on conversion levels. The reaction profile was also investigated using a lower substrate∶metal molar ratio, and conversion continued to increase steadily, fully depleting the toluene after 110 h, while the selectivity to benzyl benzoate progressively increased (Figure 1.12). A key feature of this catalytic reaction was the high selectivity to benzyl benzoate that was observed, which was created via the formation and oxidation of a hemiacetal (see Figure 1.11). Structural characterisation and reactivity studies confirmed that any sintering or structural modification of these highly active catalysts is minimal, and that the catalysts are stable and re‐usable.

Supported gold and gold‐based bimetallic nanoparticles can play an important role in the field of environmental catalysis. Two examples of areas of importance in environmental catalysis where gold catalysts have already made an impact have been described. Bio‐renewable feedstocks can provide a valuable new route to the generation of platform chemicals on which the subsequent generation of finished products can be based. Although these bio‐feedstock materials can be gasified and pyrolysed to produce syngas and oil‐like materials, which can be processed by existing technology, we have argued that retention of the chemical complexity of the bio‐derived molecules provides the basis for new catalytic pathways to be explored. The oxidation of glycerol to glycerate using supported gold catalysts clearly demonstrates that this can be achieved under mild reaction conditions using oxygen. In addition, mono‐ and bimetallic Au nanoparticles can be used under solvent‐free conditions, which is another key consideration for the development of benign environmental catalysis. In the future, we can expect catalyst design to focus on improving atom and energy efficiency, and of course the quest for ever more efficient catalysts will continue.