For a variety of reasons, the title of this article is significantly different, and so is its content, from the lecture I (JMT) gave on “Some of Tomorrow’s Catalysts: Actual and Desired” at the KOPRC Forum in August, . In the relatively short interval that has elapsed since that Forum, many significant new developments have occurred in the fields circumscribed by the title of this article. Also, I have been the author or co-author of other related articles, which have appeared, or are shortly to appear, that address many of the questions raised by this broad field. Moreover, even before presenting my KOPRC talk at Oxford, I had reviewed in detail the many facets that arise in considering how humankind is going to cope with powering the planet in an environmentally responsible manner in the next 50 years and beyond—see, for example, Refs. [1–5]. See also the definitive review by Centi and co-workers [6] published in , and references therein. Hence, the situation pertaining to all of the topics relevant to those touched upon or implied by the title of this paper is rapidly changing. For example, I did not know until late October that there is now perceived to be enough natural gas available in the world to serve the needs of the planet for an estimated 230 years! So the frequently repeated mantra [7] that it is necessary to seek more renewable sources of energy is simply not true. For example, as recently as 9 January , it was announced [8] that Australia is increasing natural gas production by roughly 150 % over the next 4 years.
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In addition to these macro developments, progress in the evolution of new catalysts, especially for the processing of CO2, continue to be reported. Furthermore, the agreement reached in the UN meeting in Paris, December , calls for further action to be taken, not only to stabilize but to decrease the amount of anthropogenically produced CO2 in the Earth’s atmosphere if the target of keeping the temperature rise of the Earth to less than 2 °C by is to be met.
In predicting and pontificating about what catalysts are likely to be needed and used to fulfil the desiderata implied by the title of this paper, it is salutary to recall that numerous, well-intentioned predictions—often made by experts of unimpeachable credentials—tend to fall short of realisation. As in my Oxford lecture, I recall here that the Commission set up by President Roosevelt to advise him on scientific and technological developments in the foreseeable future, missed many seemingly obvious developments when they reported in . They made no mention of the future, expected use of antibiotics, despite the fact that Alexander Fleming had discovered penicillin in . They also did not find it pertinent to mention that, in future, devices such as the fax machine (invented in the early s) or the fuel cell (also first described and tested in the s) would ultimately be in popular use.
Given the fact that there is still an abundance (almost a superabundance) of fossil-based feedstocks, and also that there is public clamour for the operations of a civilized life to generate less CO2, all kinds of socio-political, as well as technological, initiatives are constantly being pursued.
What is absolutely necessary is that more efficient ways are needed of utilizing nonrenewable feedstocks. To give an interesting recent example, aircraft manufacturers are “rethinking the airplane for climate sake”. (This statement figured as the title of an article in the International New York Times in January ). In essence, the concept called distributed propulsion is one of several being studied by the NASA Armstrong Flight Research Center in California, to develop technologies that could lead to completely new and far less polluting aircraft designs. For example, future planes may be powered by batteries or hybrid gas-electric systems, and have lighter wings that can quickly change shape so as to handle better the stresses brought on by turbulent air.
It is well known that the French worker, Sabatier, over a Century ago, demonstrated how methane could be produced synthetically by passing a mixture of CO2 and H2 over a supported Ni catalyst:
$${\text{CO}}_{ 2} + {\text{ 4H}}_{ 2} \to {\text{ CH}}_{ 4} + {\text{ 2H}}_{ 2} {\text{O}}$$Previously, when it was felt that natural gas could be in short supply in various parts of the world, the Sabatier reaction could be used to produce synthetic natural gas (SNG), for transport and other purposes. Nowadays, it is not the thermally activated Sabatier reaction that is attracting the attention of those chemists devoted to the diminution of anthropogenic CO2 but its photocatalytic equivalent. At present, there is no proven, highly active and desirable photocatalyst that has been discovered to convert CO2 to CH4 in a sustained manner and with high efficiency. The quest for such a photocatalyst, discussed by workers such as Corma et al. [9] and Ozin et al. [10] is likely to last for a long time.
For several decades CO2 has been used as a feedstock to produce salicylic acid (a precursor in the manufacture of aspirin), and urea, the most compact form of soil fertiliser. But these industrial processes, although they utilize CO2 as a feedstock, are far from being carbon neutral. It is gratifying to note, however, that many of the polymeric products previously synthesized from nonrenewable sources can now be readily prepared in an entirely sustainable manner. The case of polyurethane is a good example. Langanke et al. [11] have shown how CO2 can be used as feedstocks in copolymerizations using epoxides—see Fig. 1. In fact, the Bayer material science (BMS) organisation now produces this industrial synthesis on the scale of 103 ton per annum.
What is glaringly obvious is that the amount of CO2 liberated into the atmosphere as a result of human activity (some 50 gigatonnes per annum) far exceeds the amount of CO2 currently used as a feedstock for the production of useful materials. The latter amounts, at present, languish in the megaton range per annum.
A constructive development is the use of CO2 (sunlight and water) as a feedstock for the growth of algae using genetically enhanced cyanobacterias to generate ethanol and O2—see Fig. 2. This work is carried out by the Algenol Biofuels Company, which, in its facility in Florida, utilizes CO2 gas liberated from an industrial plant. At present, this Company produces gallons of ethanol per acre per year, and they quote a solar energy conversion of 2–3 %. When their plant grows to occupy acres (at the seashore [12]), the Algenol Company will produce ca. 14 x 106 gallons of ethanol per annum. An important advantage possessed by producing ethanol from algae rather than corn is that no land is used up that could otherwise have been used to produce food. The algae bio-photoreactors all operate at the seashore.
Important as this development is, its merits need to be carefully stated:
it converts anthropogenically produced CO2 to generate ethanol;
this ethanol can be readily catalytically dehydrated to yield ethylene using a number of single-site heterogeneous catalysts, such as DAF-4, SAPO-34 and others, as described elsewhere [13] by one of us (JMT);
if the final use of the ethanol is as a fuel (blended into gasoline) or as a source of polyethylene, its ultimate fate is to generate more CO2, which, however, can then be used as feedstock in subsequent algal-based solar-driven production of more ethanol.
We shall return below to the important part microalgae are likely to play in future production of “green” hydrocarbons (i.e. sulphur-free) and diesel fuels. Other biochemically oriented methods of consuming CO2 as a feedstock are being explored by the German investigator, Erb [14]. Such work involves the creation of customized, CO2-fixing biochemical pathways to produce biomass and fine chemicals from atmospheric CO2.
In Erb’s work, Rubisco chemistry figures eminently: Rubisco is d-ribulose-1, 5-bisphosphate carboxylase/oxyglucose. This is a member of the carboxylase family of enzymes—see Thomas and Harris [5] for more details. Rubisco chemistry has also influenced the recent, ingenious work of Kanan et al. [15, 16]. These workers set out to emulate nature’s strategy for C–C bond formation, which is to deprotonate C–H bonds to form carbanions and then to trap these intermediates with CO2 to form C–CO2 moieties. They use a purely inorganic approach, which has the advantage of circumventing the need to select the appropriate, robust enzyme co-factor, and some other complications associated with the approach of the synthetic biologist.
What is commendable about what Kanan et al. [15] accomplish is their ability to synthesize ethylene glycol and ethanol using only CO2 and H2. Their work leads to the ready synthesis of polyethylene furandicarboxylate—also designated polyethylene furanoate, PEF—which is a viable substitute for polyethylene terephthalate (PET) [17], and is used extensively as a container material for portable water and mineral drinks.
The H2 required for the preparation of ethylene glycol—a powerful chemical building block for other materials—may be produced by either wind-powered or solar-powered water-splitting, using either wind or photovoltaic power for electrolysis. Kanan [15] estimates that by replacing the entire 15 M tonne year−1 PET market with PEF, 20–35 M tonne year−1 of CO2 would be saved from liberation to the environment.
It is relevant at this junction to note that Freund [18] and colleagues in Berlin have constructed a method of attaching a neutral CO2 molecule to a radical ion of CO2, thus forming a \(( {\text{CO}}_{ 2} )_{ 2}^{ - }\) species, which may then be transformed into an oxalate species, whereby a C–C bond is formed. These oxalate species may then be further catalytically transformed with water or ammonia into useful fuels and other products.
It has been estimated (by environmentalists and city planners) that about 40 % of all fossil fuels are utilized in heating homes and other buildings. As a consequence, revolutionary methods are now being invoked to seek alternative methods of achieving the minimum amount of nonrenewable feedstock for this purpose. It is, therefore, relevant to note that an apartment block in Hamburg (Germany) has been built that uses microalgae placed within the façade to generate heat and biomass. This has been done by engineers [19] who are engaged in the establishment of zero—or even surplus—energy buildings as part of the goal to achieve sustainable architecture.
The solar fuel network is an international community of scientists dedicated to the idea of establishing a situation in which solar energy is harnessed for the production of fuels that can ultimately replace those derived from nonrenewable sources. Bearing in mind the scale of the operations involved when all the fuels and other products of the catalytic cracking of petroleum are considered—see Figs. 3 and 4 for some relevant quantitative facts—this is indeed a tall order, as has been pointed out previously [20]. Faced with the magnitude of this task, some scientists declare that there is a long road still to be travelled before we see the onset of widespread use of solar fuels.
On the other hand, as pointed out by Jacobson et al. [21, 22], civil engineers in Stanford University and the Nobel prizewinning Paul Krugman, in a recent (February ) article in the International New York Times, “wind, sun and tide”, are already making a significant contribution to civilized life. In the main, these three sources of power are used to convert sunlight to an abundant generation of H2. It is also relevant to mention that numerous concentrated solar power plants of 200 and 300 MW are also capable of providing the electricity to electrolyze water for the generation of H2. Krugman’s article quoted a recent report by the investment firm Lazard that the cost of electricity generation using wind power fell 61 % from to , while the cost of solar power fell 82 % in the same period. This trend is continuing.
But whilst there are increasingly more viable methods of generating relatively cheap H2—which can sustain the hydrogen economy, and bolster the use of fuel cells (burning H2) for transport, hospitals, shopping malls and the like—the next goal is to achieve highly efficient photocatalytic conversion of CO2 to CH4. In other words, it is the photochemical Sabatier reaction that needs to be conquered. (It must not be forgotten that as much energy from the sun reaches the Earth’s surface in a few hours as the world currently consumes (largely as fossil fuel) in 1 year.)
To date, some significant progress has been made. For example, Sastre et al. [9] have made a worthwhile attempt to achieve complete photocatalytic reduction of CO2 to methane by H2 under solar light irradiation (Fig. 5). Ozin and co-workers [23, 24] have also made a promising start, but their catalyst (In2O3−x(OH)y) goes only as far as photocatalytically converting CO2 to CO (which is still a worthwhile goal). In a thorough review of the entire question of photocatalytically converting CO2 to CH4, Dhakshinamoorthy et al. [7], focusing on TiO2 as the photocatalyst, concluded that the average productivity of the less-TiO2-based photocatalyst is about 100 μ mol g−1 h−1 of catalyst with sunlight. They emphasized the need to increase this productivity by orders of magnitude. In the main, the solar fuels community has concentrated most of its efforts on improving the efficiency of the hydrogen evolution reaction (HER), by employing solar light to drive a photochemical cell typified by the set up: TiO2−Ti−pn + Si as photocathode. At a symposium organised by J. A. Barber for the Solar Fuels Network (London, July ), a protagonist in the field, Professor Harry Gray of Caltech, urged those working on the generation of solar fuels to concentrate more on using CO2 rather than H2O as the material for the production of useful fuels.
An attractive feature of the photocatalytic conversion of CO2 to CH4 is that if successful on a large scale, the practice of blending H2 with natural gas—which involves careful control of the relative amounts of H2 and natural gas—can be replaced by the much safer and utilitarian blending of sunlight-derived CH4 with the natural gas grid. This strategy, if and when successful, would ultimately stabilize the amount of CO2 in the Earth’s atmosphere (CO2 → CH4 → CO2 → CH4, etc.).
A few further remarks on TiO2 are appropriate here. The high oxidation potential of photogenerated holes makes bulk TiO2 a powerful oxidant for water and organic molecules, a fact that has given rise to many practical applications of solid state photocatalytic activity, such as self-cleaning windows, tiles and cements (by Anpo et al. [25, 26]), anti-bacterial paints, and the purification of water by the solar breakdown of dyestuffs and other organic pollutants.
Recently, one of the present authors (JMT) has been associated with a fundamental study [27] of a single-site Ti(IV)-based photocatalyst, a titanosilicate, known as JDF-L1 [28], with the formula: Na4Ti2Si8O224H2O. The essential features of the active centre in this open-structure solid, a TiO5 arrangement, square pyramid, are shown in Fig. 6, in which there is one Ti=O apical bond and four Ti–O bonds. 4D ultrafast electron microscopy showed [27] that upon photoexcitation, the Ti(IV)=O bond is transformed to a single bond Ti(III)–O− with a consequential dilation of the length of the double bond from 1.7 Å–2.5 Å. This occurs on the femtosecond time scale, and a schematic illustration of this process as well as the ensuing photochemical possibilities are shown in Fig. 7. This illustration depicts the kind of pathways that are followed in the photoreduction of CO2 or H2O by the JDF-L1 catalyst.
Very recently, Ozin and co-workers investigated the unique photoactive behaviour of pristine and defected indium oxide surfaces [24]. Their combined theoretical and experimental study provided fundamental insights into excited state properties, as well as an explanation for the experimentally observed enhanced activity of defected indium oxide surfaces for the gas-phase reverse water gas shift reaction, CO2 + H2 + hν → CO + H2O, in the light compared to the dark (Fig. 8). A thorough excited state study of pristine and defected forms of indium oxide (In2O3, In2O3−x, In2O3(OH)y and In2O3−x(OH)y) surfaces was carried out using time dependent density functional theory calculations. The results were supported experimentally by transient absorption spectroscopy and photoconductivity measurements. They found that the surface frustrated Lewis pairs created by a Lewis acidic coordinately unsaturated surface indium site proximal to an oxygen vacancy and a Lewis basic surface hydroxide site in In2O3−x(OH)y become more acidic and basic, and hence more active in the excited state compared to the ground state. They described how this provides a theoretical mechanism responsible for the enhanced activity and reduced activation energy of the photochemical reverse water gas shift reaction observed experimentally for In2O3−x(OH)y compared to the thermochemical reaction. To conclude, they emphasize that such fundamental insight into the role of photoexcited surface frustrated Lewis pairs for catalytic CO2 reduction could lead to improved photocatalysts for solar fuel production.
It is relevant to note that very recently a novel approach, spearheaded by Ozin and colleagues in Canada and involving collaborators from other countries, uses visible and near infrared radiation to effect photothermal (not photocatalytic) conversion of CO2 in the presence of H2 to CO (i.e. the reverse water gas shift reaction) [29]. The solids used by the workers consist of Pd nanocrystals supported on nanorods of Nb2O5. Conversion rates as high as 1.8 mmol g−1 h−1 were achieved. Careful experiments disclosed that the photothermal catalysis originates from intra-band and/or inter-band optical excitation and nonradiative relaxation of Pd nanocrystals rather than being driven by UV plasmon excitation of the Pd nanocrystals or by electron-holes pair generation upon absorption of UV photons in the Nb2O5.
One of us (JMT) has recently given an account of many of the ways in which the action subsumed by this sub-heading can be fulfilled; and the reader is referred to that article, Ref [5]. There are, however, many more factors to outline in addition to those contained in Ref [5]. Three particular issues are considered here: catalytic cracking; the generation of bio-oil from renewable feedstocks; and the emergence of a new class of heterogeneous catalysts, i.e. those that are composed of supported single atoms.
This is an extremely important applied catalytic activity in that a large majority of the materials used in civilized life are currently the products of the cracking of petroleum—see again Fig. 4. It is unlikely, in the short term, that the catalytic cracking of petroleum will cease because it is well-nigh indispensable throughout the world. What is, however, desirable is that the process can be made more efficient. The yield of desirable products must be significantly increased. Fortunately, significant progress has recently been made in this direction. Before describing it, however, it is relevant to recall the scale of the operation of fluidized catalytic cracking (FCC) of petroleum.
Figure 3 reminds us of the massive scale of the cracking process by considering one single 75,000 barrel-a-day operation. And Fig. 9 illustrates the nature of the catalytically active centre (an (LaOH)2+ entity) inside the cavities of a faujasitic zeolite. In industrial parlance, this is the La-Y ultra-stabilized Brønsted acid zeolite used on a massive scale—it is estimated that 500 K tonnes of the cracking catalyst are ‘consumed’ each year—to break down the hydrocarbons of petroleum into small, more useful products, such as light alkanes and alkenes, as well as octanes and other hydrocarbons of larger molecular weight suitable as the fuel for diesel engines.
That such enormous amounts of the zeolite Y (La3+-exchanged) cracking catalyst are consumed every year arises because the catalyst matrix [consisting of zeolite Y, kaolin (as a filler), and typically, aluminium chlorohydrate (as a binder)] are friable and lack attrition resistance. Another important fact pertaining to faujasitic zeolite cracking catalysts is that they have quite large—but not large enough (see below)—pores (ca. 7.4 Å diameter). Thanks primarily to the work of the Spanish investigator, Garcia-Martinez et al. [30, 31], mesostructural zeolite Y has been prepared and is already now being utilized industrially. Mesoporous zeolites [32] and other so-called hierarchically structured zeolites are synthesized using large micelles made up of controllable diameter that consist of a long-chain surfactant template—see Fig. 10. The essential difference between a conventional zeolite (like Zeolite Y) and a mesostructured one is shown in Fig. 11, which also illustrates the improved pore characteristics of the mesoporous variety. (The Rive Company in the US manufactures the variety of mesoporous zeolites designed by Garcia-Martinez et al.). Additional indications of the improved catalytic performance of the Rive-type of mesoporous cracking catalyst are shown in Fig. 12.
The overall message derived from the introduction of mesostructured zeolites—and additional messages are continued in the collection of articles contained in Ref [32]—is that by judicious design of new solid catalysts, more efficient ways can be found for processing nonrenewable feedstocks.
A selection of important manufacturing processes that can be improved by utilizing designed new solid catalysts are given in the review by Harris and Thomas [5] and also in Refs. 1, 2, 13. Both catalytic dehydration reactions and a host of other processes can be facilitated by nanoporous catalysts that contain one or more types of spatially well-isolated active centres epitomized by the bifunctional catalyst shown in Fig. 13.
(This section is based largely on Sect. 9.7.1 of the book by Thomas and Thomas [2]).
So-called bio-oil is produced by fast pyrolysis or liquefaction of lignocellulosic biomass. It is an aqueous highly functionalized, but essentially sulphur-free, mixture of light to medium hydrocarbons containing up to 30 % water. Because it is corrosive and possesses rather low ‘heating value’ (<19 M J kg−1), and some other disadvantageous features, it is unattractive for use without prior treatment. If it is subjected to hydrogenation and hydrodeoxygenation (HDO) with cobalt-doped MoS2 catalysts, high-grade transportation fuels may be extracted from it.
An alternative approach based on zeolites involves simultaneously catalysing several reactions, including dehydration, cracking, polymerization, deoxygenation and aromatization at temperatures between 350° and 450°. These conditions serve to convert the oxygenates into aromatic molecules and carbonaceous deposits, while the yield of alkanes does not exceed some 25 %. Very recently, Lercher [33, 34] has shown how widely applicable zeolitic catalyst HZSM-5 with a pore system containing a substantial fraction of Ni metal nanoparticles can quantitatively produce C5–C9 hydrocarbons from paraffins, napthenes and aromatic molecules in a cascade reaction by HDO of n-hexane-extracted crude bio-oil in the presence of substantial concentrations of water under mild reaction conditions (250° and 5 MPa H2).
The components of the n-hexane-extracted bio-oil include mainly C5–C6 substituted furans, ketones and aldehydes derived from the deconstruction of cellulose and hemicellulose, as well as C6–C9 substituted phenols derived from the deconstruction of lignin (Fig. 14a). HDO of such a mixture on Ni/HZSM-5 in a semi-batch reaction for 4 h succeeded in quantitative conversion into the corresponding C5–C9 hydrocarbons, as shown in Fig. 14b.
Many other studies reporting fast (or slow) pyrolysis of biomass have recently appeared. In one, emanating from the US Department of Agriculture [35], sustainable production of bioenergy and biochar from the straw of high-biomass soybean is described in the context of ‘on-farm’ biorefinery, where food and bioenergy can be sustainably produced. The biochar retains mineral constituents that are beneficial to plants and is deployed for soil remediation.
As mentioned above, microalgae constitute a very viable source of ‘green’ hydrocarbons. This arises because of their high cellular lipid content and unusually high rate of photosynthetic growth. Algae mass captures about 3–8 % of incident solar energy, in contrast to terrestrial plants, which do so at about 0.5 %. Some microalgae have higher than 60 % oil content by weight of dry biomass, and the average oil content attains approximately 20–50 %. Microalgae grow 12 times as fast and yield 30 times as much triglycerides per unit area compared to conventional oil-producing land plants such as sunflower and rape. Moreover, they do not require arable land—where they would compete with the production of food—and they can utilize waste water, sea water and industrial CO2 (flue gases) to grow as valuable biomass.
It is estimated that there are between 60,000 and 80,000 species of algae; and the number of products that may be derived from them are many and varied (especially from those that are genetically engineered): medicines, pharmaceuticals, foodstuffs, jet fuel, bioethanol, biodiesel and ingredients for several consumer products. In this brief review, we shall focus on but two main products: diesel-range alkanes and bioethanol.
Currently, three approaches are used for microalgae oil refining. The first involves transesterification of triglycerides and alcohol into fatty acid alkyl esters (FAAEs) and glycerol. The second employs conventional hydrotreating catalysts, such as sulfided NiMo and CoMo for upgrading. The third, which is the one we focus on here, relies on supported noble and base metal catalysts for decarboxylation and decarbonylation of carboxylic acids to alkanes at 300–300 °C, but these catalysts exhibited low activities and selectivities for C15–C18 alkanes when triglycerides were converted.
Recently, a more efficient method has been described by Peng et al. [36]. who used a novel Ni catalyst supported on an acidic zeolite, HBeta. This quantitatively converts crude microalgae oil under mild conditions (260 °C, 40 bar H2) to diesel-range alkanes as high-grade second-generation transportation biofuels. From measurements of product distributions for this transformation, these authors formulated the reaction mechanism given in Fig. 15. The pathway proceeds through an initial metal-catalysed hydrogenation of double bonds in the alkyl chain, followed by hydrogenolyses of the formed saturated triglyceride leading to fatty acid and propane. The subsequent hydrogenation of the carboxylic groups of fatty acid leads to the corresponding aldehyde (rate-determining step), followed by either decarbonylation or hydrogenation. Subsequent acid-catalysed dehydration and metal-catalysed hydrogenation lead to the long-chain n-alkane. Because of an abundance of acidic sites in the zeolite, hydroisomerization and hydrocracking of the alkanes also takes place; and CO may react with H2 to produce methane (by the classic methanation reaction).
Using catalytic hydrothermal technology, Maschmeyer and colleagues [37] have recently developed (and commercialized) a catalytic hydrothermal reactor which converts feedstocks such as sawdust into bio-crude oil of such good quality—it is essentially free of sulphur—that it can be readily blended with stocks of either diesel, kerosene or gasoline. The first commercial plant, now being completed in Canada, is intended to transform 200,000 tonnes of waste annually into the desired ‘green’ bio-oil. Figure 16 illustrates the overall conversion.
This development is of great significance in this environmentally conscious age, especially when one reflects on the fact that only some 18 % of a tree is used in paper production; it therefore means that converting the ‘tree waste’ into bio-oil is a significant step forward.
Great interest is now being given to the increasing variety of this category of catalysts. Short accounts have been given elsewhere—see Thomas and Harris [5], Yang et al. [38], Flytzani-Stephanopoulos [39], Vile et al. [40] and Thomas [41]. A spectacular example of a stable single atom Pd catalyst for selective hydrogenation has recently been reported by Vile et al. [40]. Unlike many other previous workers in this field, they use a structurally adept solid as the support for atomically dispersed Pd, namely, nanoporous carbon nitride, C3N4. The merit of their system (see Fig. 17) is that the individual atoms of Pd are so firmly anchored to the nanoporous walls of the C3N4 that they exhibit minimal tendency to migrate and coalesce to form nanoclusters. This catalyst was shown to be active in a three-phase hydrogenation of alkynes in flow mode, and both its selectivity and activity surpassed those of nanoparticulate Pd. Apart from C3N4, there is now great interest focused on another ‘indented’ (holey) nitrogenous graphene-like carbon of empirical formula C2N. Recent theoretical work done on this catalyst support for single atoms of members of the 3d transition metal series, using an augmented wave version of the density functional theory approach, indicates that Cr and Mn single atoms would catalyse the conversion (by an Eley–Rideal mechanism) of CO and O2 to yield CO2 [42].
In Fig. 18, we show high-resolution images of other few-atom catalytic systems, comprising Au nanoclusters or single atoms supported on TiO2 or activated carbon [43]. Each small spot here is the image of a single metal atom. Christopher and colleagues [44] have also studied isolated metals, in their case, supported on TiO2. They found significant differences in catalytic selectivity between individual atoms of Rh and nanoparticles of the same metal—see Fig. 19. In an elegant assessment of the energetics of single atoms and nanoparticle metal catalysts, namely of Cu on the (111) face of CeO2, Campbell and co-workers [45] were able to arrive at quantitative values of the chemical potential of this metal from the single atom extreme to the bulk state, as shown in Fig. 20. Such work is of great value in ascertaining the long-term thermal stability of single atom catalysts, which is a subject that is now of great relevance.
As a teenager in the s, Charles V. Johnson of Lake Geneva, Wis., was tinkering with his chemistry set when he discovered that earwax could serve as a catalyst for making pigments. Later on, as a zoology undergraduate student at the University of Wisconsin, Madison, Johnson took a daring chance in a chemistry lab: He applied earwax to a boiling chip and substituted it for a palladium catalyst in an organic synthesis experiment. It worked well to make trans-stilbene, although his professor didn’t seem impressed.
“That’s the thing that has bothered me most,” Johnson told C&EN in a interview. “My instructors didn’t think there was anything to it.” After graduating, Johnson worked as a chemical technician at Sigma-Aldrich until he retired. He occasionally toyed with using earwax as a catalyst over the years to, for example, polymerize a methacrylate-based material he bummed off his dentist.
Johnson often contemplated what the active catalyst might be in earwax, but he wasn’t able to do an analysis to find out. Most likely, it’s an amino acid or protein, he assumed. Amino acids such as proline are well-known organocatalysts. And catalytic proteins, known as enzymes, have been used since the dawn of civilization—though not knowingly until modern times—for food and beverage processing.
Chemists such as Johnson are “natural” experimentalists in every sense. And given that nature provides chemists everything they need to do experiments, it’s only natural that when it comes to looking for ways to improve chemical reactions—to make them greener and more sustainable—that researchers look for more natural, minimally processed ingredients.
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Earwax may not be in sufficient supply to serve as an industrial catalyst, but plenty of other inexpensive, naturally sourced materials are out there, whether they be enzymes derived from pig kidneys or millipede stomachs, metals sequestered by plants, minerals straight from the ground, or metals recovered from highway grit. Here, C&EN takes a look at the prospects of a few “natural catalysts” derived from animal, vegetable, and mineral resources.
Roger A. Sheldon of Delft University of Technology, an expert in developing enzymes and greener process chemistry for the fine chemicals and pharmaceutical industries, is quick to point out that “enzymes are nature’s sustainable catalysts.” They are derived from renewable resources and are biocompatible, biodegradable, essentially nonhazardous, and nontoxic, he explains. Plus, enzymatic reactions are often performed under mild conditions, avoid the need for scarce precious-metal catalysts, and proceed without waste-generating protection and deprotection steps common in conventional organic syntheses (Green Chem. , DOI: 10./c6gcc).
The use of enzymes produced by yeasts dates back thousands of years to bread and cheese making, beer brewing, and wine making, Sheldon says. The first commercial enzyme preparations were produced in the 19th century and included the use of dried calf stomachs in cheese manufacture and animal pancreatic extracts in laundry cleaning. Food and beverage processing together with animal feed and detergents accounts for more than 60% of the enzyme market today, he notes.
However, the development of modern biotechnology in the s brought about a shift from the traditional animal and plant sources, Sheldon says. Genetically modified microorganisms now provide cheaper and higher-purity enzymes that work more efficiently for industrial chemical production.
But can chemists come up with enzymes that are even more sustainable? Yan-Hong He and coworkers of Southwest University in Chongqing, China, think so. They have been developing an all-natural catalyst by grinding up earthworms. Like humans, earthworms rely on bacteria in their gut to oxidize and refashion plant and mineral matter to obtain their nutrients. And earthworms can be mass-produced anywhere in compost heaps, living off nothing more than garbage and crop wastes.
Among the enzymes that earthworms exude is a set of serine proteases called earthworm fibrinolytic enzymes. These enzymes are the active ingredients of medicinal extracts used in China for a range of applications, including treating a fever and improving cardiovascular health.
Because enzyme purification can be expensive and time-consuming, He and coworkers began looking for practical ways to use earthworm enzymes directly, without fussing over purifying them or trying to improve their performance through genetic modification. The team was inspired by prior reports of chemists using crude extracts from mushrooms, egg whites, and other sources as biocatalysts.
The researchers rinse common red worms from a worm farm and then puree them in a kitchen blender, He explains. Next, they centrifuge the puddinglike mixture and collect the supernatant liquid, which they dry and then grind into a powder.
The Southwest University chemists first tested the earthworm powder in a Mannich-Michael reaction to prepare isoquinuclidines, which are nitrogen-containing heterocyclic molecules that are useful chemical intermediates. The researchers compared the powder’s abilities with those of commercially available lumbrokinase, one of the earthworm fibrinolytic enzymes, finding that using 100 mg of earthworm powder—the same amount as the purified enzyme—yielded equal results.
They found that their enzyme concoction also works well in asymmetric aldol, Mannich, Henry, and Bignelli reactions (PLOS One , DOI: 10./journal.pone.; Tetrahedron , DOI: 10./j.tet..06.025). Continuing the work, the researchers have just completed the synthesis of the blood-thinning medication warfarin using their earthworm extract.
Natural catalysts provide multiple advantages for chemists, He says. “They are safe, accessible, economical, and environmentally benign.” He acknowledges that the use of the crude enzymes in organic synthesis is limited in terms of the range of reactions that can be catalyzed. “But our work provides a practical way to use sustainable catalysts from nature that could have a transformative effect on streamlining the practice of organic synthesis,” He says.
Yasuhisa Asano of Japan’s Toyama Prefectural University agrees. “Because industrial biotechnology requires more efficient and stable enzymes for sustainable development, we must continuously explore for potential enzyme sources,” Asano says. To that end, Asano’s team has found that an Asian millipede produces a hydroxynitrile lyase enzyme that is much faster at producing cyanohydrins than currently used industrial versions of the enzyme sourced from almonds are (Proc. Natl. Acad. Sci. USA , DOI: 10./pnas.).
Asano’s team initially collected kilograms of the millipedes to extract the enzyme. But the team studied the enzyme’s amino acid sequence and identified a gene for the enzyme that can be expressed in yeast to ease availability.
Besides working with enzymes from many-legged sources, Asano’s group has also developed a method to make α-aminonitriles and convert them into nonnatural α-amino acids by oxidative cyanation of primary amines using modified enzymes from pig kidneys or the western diamondback rattlesnake (Green Chem. , DOI: 10./c6gch).
“The discovery of millipede hydroxynitrile lyase and the porcine and snake oxidases can serve as a template both for the isolation of promising and efficient enzymes and the design of improved tailor-made enzymes by rational protein engineering,” Asano says.
As some researchers have examined metal life cycles—from how the elements are extracted from the ground to how they are recycled or disposed—they’ve started to think about alternative means of acquiring metals that don’t require traditional mining. Their work comes at a time when global ore quality is slipping from high-grade, low-bulk ore to low-grade, high-bulk ore, according to the International Council on Mining & Metals. For example, some economists estimate that we have extracted more commercially accessible metals from the earth than are left in the ground.
An international team of ecologists and metallurgists, including Antony van der Ent and James Vaughan of the University of Queensland and Marie-Odile Simonnot and Guillaume Echevarria of the University of Lorraine, don’t view that situation as a problem. They instead consider it an opportunity. The researchers have been promoting phytoextraction as a sustainable approach to sourcing metals (Environ. Sci. Technol. , DOI: 10./esu).
Phytoextraction, also known as phytoremediation or agromining, takes advantage of “hyperaccumulator” plants that have the ability to selectively concentrate high levels of metals from soil or water, van der Ent says. The approach has long been used as a means for cleaning up toxic metals such as arsenic and cadmium at abandoned industrial and mining sites and to prevent the metals from spreading farther in the environment.
Workers typically harvest crops grown on a site, dry and incinerate the biomass to reduce the volume of material, and then bury it in a landfill. But the plant material can also be considered a high-grade “bio-ore” that can be refined into pure metals, van der Ent explains. When you think about the process that way, as agromining, metals can be extracted from mineral-rich soils just about anywhere. Besides providing a source of metals, agromining can help local communities find a use for poor-quality land that isn’t amenable to traditional farming, as well as a source of income, he notes.
Chemists have already turned to using harvested metals as catalysts. Claude Grison and coworkers of France’s CNRS Laboratory of Bioinspired Chemistry & Ecological Innovations (ChimEco) reported in what is believed to be the first use of hyperaccumulator biomass in organic synthesis (Green Chem. Lett. Rev. , DOI: 10./..). The researchers used alpine pennycress grown on a zinc mine site to prepare Lucas’s reagent, a solution of zinc chloride in hydrochloric acid, which they used to make alkyl chlorides from aliphatic alcohols.
The ChimEco process requires a series of steps to isolate crude zinc chloride, including dissolving dried plant matter in acid and ion-exchange chromatography. Still, Grison notes, the process has advantages over purified salts normally used, which “are more expensive and less sustainable to produce.”
Grison and her coworkers continue to expand on the work, using an assortment of hyperaccumulator plants to selectively isolate zinc, nickel, copper, and manganese. They are developing a line of “ecocatalysts” based on the metal salts and have been testing them for possible commercial application in a range of reactions, such as manganese reductive amination of ketones and copper-catalyzed azide-alkyne click chemistry (Green Chem. , DOI: 10./c4gcb).
But can chemists come up with ways to use the plant material that are even more sustainable? As Grison and coworkers have shown, crudely isolated metals from plants can be used for some solution-phase reactions. This approach has a limitation, though: Homogeneous catalysis often depends on using a ligand to optimize the electronic and structural properties of a soluble metal complex. But taking a simpler approach and directly using incinerated plant ash as a catalyst is something researchers are starting to think could work.
“Definitely, the metal concentrations in hyperaccumulator plants are so high that specialist chemicals and catalysts can be obtained from them without refining,” van der Ent says. Nickel, which is widely used as a catalyst, can concentrate in hyperaccumulator plants up to about 4% in leaves, he notes, which translates to more than 25% nickel in the resulting ash. That amount compares with only about 2% nickel in low-grade ores.
Preliminary tests by van der Ent and his colleagues have shown that burning hyperaccumulator biomass with a restricted amount of air or reducing the ash with carbon monoxide results in a carbonized material like charcoal that contains nanometer-sized metallic particles. “These particles can be isolated by simple magnetic separation and presumably have excellent catalytic properties,” he says.
In one example, the researchers made carbon-supported metal nanoparticles containing 7% by weight of nickel by pyrolyzing the wood of a tree that grows in New Caledonia. Some 40 years ago, the latex of this tree was shown to contain 25% nickel, the highest known concentration of nickel in a plant. A single tree can contain up to 40 kg of nickel. The team is studying the nanoparticles as a potential hydrogenation catalyst.
The Queensland and Lorraine teams aren’t the first to make nanomaterials from plants partial to a diet of metals. Jiao Qu, Xing Yuan, and coworkers at Northeast Normal University have developed a process to use hyperaccumulator vegetation from a copper-zinc mine to make multiwalled carbon nanotubes, zinc oxide and copper-zinc oxide nanoparticles, and carbon nanotube-metal nanoparticle composites (Environ. Sci.: Nano. , DOI: 10./c6enk).
“The nanotubes extracted from hyperaccumulators have shown superior efficiency in electrocatalytic and photocatalytic degradation of organic pollutants in water,” Qu says. Qu believes the metal content is sufficient to use the materials as industrial catalysts as well.
Some researchers have in fact already shown that nanoparticles made from plants are good catalysts. For example, in a proof of concept, a team led by Neil C. Bruce, James H. Clark, and Elizabeth L. Rylott of the University of York grew plants using water spiked with palladium salts and then used the plants to produce catalytic palladium nanoparticles. They have used their heterogeneous plant catalysts in Suzuki-Miyaura coupling reactions to make diaryl compounds. The researchers are now testing the commercial viability of their approach in field trials using plants grown in soil containing mining waste (Environ. Sci. Technol. , DOI: 10./acs.est.6b).
Meanwhile, van der Ent and his colleagues continue to explore the possibilities of doing more with less. They’re running a trial in South Africa using a hyperaccumulator aster plant, Burkheya coddii, which is normally used to target nickel, to try to pull palladium and platinum from industrial waste. “The initial results are encouraging, with substantial accumulation of palladium in the plants,” van der Ent says. He points out that all it will take to build the case for the minerals industry to embrace the agromining idea is one large-scale demonstration that works through the operational challenges and provides “real-life evidence of profitability.”
According to some origin-of-life theories, minerals may have served as the first catalysts of biological chemistry on Earth. Exposed metal ores or minerals known as zeolites and clays might have helped simple molecules coalesce to form biochemical building blocks—amino acids, nucleobases, and sugars.
Chemists today still rely on the catalytic prowess of zeolites, which are porous hydrated aluminosilicates spiked with a variety of metals. For example, zeolite Y is a natural mineral used in catalytic cracking to turn crude oil into useful chemicals, and the synthetic zeolite ZSM-5 is used for hydrocarbon isomerization and alkylation reactions.
“The use of materials based on natural minerals is often the case in industrial catalysis,” says Graham J. Hutchings of Cardiff University, a leading expert in heterogeneous catalysis. In seeking to improve the minerals’ performance, researchers have designed a variety of pretreatment processes, an approach that parallels efforts to improve on enzymes via genetic engineering. For example, Hutchings and his colleagues recently developed a solution-processing method for the mineral malachite to prepare a new class of copper/zinc oxide materials, called zincian georgeite, that is more effective as a catalyst for methanol synthesis and for hydrogen production than currently used malachite catalysts (Chem. Sci. , DOI: 10./c6scb).
But can chemists come up with better uses of mineral resources to make catalysts that are more sustainable? For a growing number of researchers, the answer is yes, and the key is taking advantage of materials that are already out of the ground. Red mud, the noxious by-product of the Bayer process for extracting aluminum from bauxite ore, makes a good case study.
The majority of material processed in mining operations ultimately goes to waste. For every ton of alumina extracted from bauxite, more than a ton of red mud is produced; aluminum mining leaves behind some 120 million metric tons per year of the salty, highly alkaline, heavy-metal-laden material, according to the International Aluminum Institute. Some 4 billion metric tons of the material is lying about globally, much of it held in retention ponds.
Mining companies have long tried to find ways to recycle the environmentally problematic red mud. It is a classic problem in search of a solution. One approach is neutralizing red mud with seawater or treating it with CO2 or sulfur compounds. The modified materials have been tried as fill for mining and construction, as pigment and filler for bricks and cement, and as a sorbent for water treatment. Others have looked at extracting more aluminum from red mud, or obtaining other useful metals such as sodium, copper, and nickel. But so far there have been few safe and economical large-scale applications.
On a new front, some chemists are trying to go catalytic, focusing on iron oxide, the chief component of red mud. But given the purity and properties of red mud, researchers have found it typically is not an active enough catalyst to compete against existing commercial catalysts. That’s because the mineral composition, particle size, and surface properties are important in developing heterogeneous catalysts. With red mud, finding the right combination is a work in progress.
One early sign of success comes from Foster A. Agblevor of Utah State University’s USTAR Bioenergy Center and coworkers in conjunction with Pacific Northwest National Laboratory researchers. They have been testing red mud as a bulk catalyst to replace zeolites in a fluidized-bed reactor to pyrolyze biomass to make crude oil (Energy Fuels , DOI: 10./acs.energyfuels.6b). The team processes the biocrude oil using a traditional catalytic hydrotreating process to make a gasoline-type fuel and has tested it on a lawn mower or lawn trimmer. “We are able to run an engine on the fuel without difficulty,” Agblevor says.
The Utah State researchers have applied for a patent for their process. They are working with catalyst company Nexceris to scale up catalyst production and with Wildland Forestry & Environmental to harvest wood from pinyon-juniper range lands in the western U.S. to scale up biofuel production. The team is also expanding the scope of using red mud beyond biomass pyrolysis, Agblevor says. The researchers have applied the catalyst to coal gasification, he notes, as well as to a process for catalytic pyrolysis of waste tires for fuel production.
Despite raw red mud’s ultimate utility as a catalyst, its story points to other possibilities for recovering metals that have already been extracted and used. For example, industrial processing, the use of consumer goods and medicines, and even the wearing away of jewelry leads to measurable amounts of catalyst metals such as gold, silver, and platinum accumulating at wastewater treatment plants.
One of the more prolific sources of these metals, though, is catalytic converters. Automobiles in the U.S. have been using catalytic converters for 40 years. They do a good job on vehicle emissions by zapping pollutants such as unburned hydrocarbons, carbon monoxide, and nitrogen oxides and turning them into more benign products such as CO2, water, and nitrogen.
But as cars putt down the road, catalytic converters slowly disperse platinum, palladium, rhodium, and cerium into the environment. Researchers who have assessed the abundance of these dissipative metals think the concentrations are high enough in the environment, or will be over time, to make it worthwhile to recover them because of their high market values.
To assess the situation, environmental engineer Sebastien Rauch of Chalmers University of Technology and his colleagues measured platinum, palladium, and rhodium concentrations and fluxes in the environment using high-volume air particulate sampling. They have trapped micrometer-sized aerosol particles containing the metals in cities such as Boston and Mexico City and analyzed the filtered samples by inductively coupled plasma mass spectrometry. Concentration ratios of the metals and trace osmium isotope ratios allowed the team to peg catalytic converters as the source of the metals, rather than natural or industrial sources.
Whether one could develop natural catalysts from the errant catalytic converter metals will “depend on the process you want to catalyze and the size of the process,” Rauch says. And harvesting the metals from the air could certainly have some limitations. Consider platinum: A typical automobile exhaust catalyst contains 1 to 2 g of platinum in various combinations with other metals, and the researchers have measured platinum concentrations in urban air of only about 10 pg/m3.
“You can process about 1,500 m3 of air per day using a high-volume sampler,” Rauch says. “After one day, you would only collect about 15 ng. It is of course possible to sample for longer periods, but platinum amounts would remain relatively small.
“Street dust is potentially more interesting,” Rauch continues. Metal concentrations are often in the 100-ng/g (0.1-ppm) range but can be higher at specific locations, he says. “You would need to find a way to concentrate the recovered metals, such as sieving the dust to small particle sizes, so that you could also increase surface area for better contact with the chemicals in the reactions you want to catalyze.”
Testing out the idea, geoscientist Hazel M. Prichard of Cardiff University, who passed away on Jan. 1, became interested in hunting for places where metals tend to accumulate to determine if they are concentrated enough to be worth recovering. Prichard had the idea that recovering the metals could be as simple as scooping up samples from the street or roadsides, storm drains, and wastewater treatment plants. Prichard even investigated the collection bins in the bellies of street-sweeping machines.
Several years ago, Prichard and her colleagues collected samples in Sheffield, England, finding gold primarily from jewelry, and platinum, palladium, and rhodium from catalytic converters. The platinum, palladium, and rhodium combined made up as much as 1.5 ppm of street dust—just at the cusp of what’s needed for catalytic chemistry. They also found more than 3 ppm of gold, platinum, palladium, and rhodium concentrated in incinerated sewage sludge ash.
By comparison, Prichard estimated that the minimum concentration needed to economically extract platinum-group metals from ore deposits is 2–4 ppm. As Rauch points out, mechanical sieving could potentially help concentrate the metals for use as catalysts. As another option, Prichard and her team enlisted biochemist Lynne E. Macaskie of the University of Birmingham to help develop a fermentation process for metal-absorbing microbes to extract metals from the dust. Macaskie and her colleagues have tested palladium-containing bacterial biomass as a bioinorganic catalyst for cleaning up industrial waste and for hydrogenation reactions, finding that the material has potential for industrial applications, she says.
Although recovering metals could prove lucrative, Prichard was motivated by the fact that global supplies of precious metals are limited. Their ongoing use in catalytic converters and handheld electronics could put a crimp on future availability such that some technologies, such as zinc and palladium catalysis, might not be viable long term, and some new technologies, such as fuel-cell cars, won’t be worth pursuing.
“The story of seeking out alternative catalyst sources is an inspiration to the entire chemistry community and elevates the conversation about sustainability to a level at which we can all identify,” says Bruce H. Lipshutz of the University of California, Santa Barbara. Lipshutz and his group have taken a minimalist approach to organic synthesis, using the smallest possible amount of catalyst and organic solvent to see how green and efficient they can make everyday reactions. In one case, they showed that parts-per-million traces of palladium impurity in iron chloride is enough to catalyze cross-coupling reactions.
“Catalysis plays a huge role in minimizing the world’s consumption of precious resources, whether from the mining of metals or reliance on enzymes,” Lipshutz adds. “Claiming both types of catalysts from the earth just makes sense. Finding creative ways to recycle such gifts from nature are clearly important challenges we face, but they are problems we can solve.”
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