The current use of chemical elements in any application including catalysis should not restrict their use for future generations. By promoting holistic strategies for extraction, manufacture, utilisation and recovery of elements it will be possible to develop a sustainable global circular economy, whereby all elements are continually recirculated for use. More specifically, the importance of catalysis for the production of chemicals and materials is widely accepted, but current rates of recovery and reuse of elements for catalysis are typically low. It is hoped that the Earth-abundant elements would fulfil all the catalysis needs of the chemical industry but in practice this is not possible, many transformations are only economically feasible when a critical or scarce element is utilised as a catalyst. There is therefore a pressing need to consider more sustainable strategies for the improved recovery of catalytically relevant elements, with a view towards reuse. The sustainability of catalysts must consider aspects such as energy consumption for preparation and recovery, whilst also minimising the detrimental impact on the environment. In this chapter we highlight catalytic elements of concern and suggest strategies for their sustainable use.
1.1 Introduction
The chemical industry utilises the vast majority of the elements within the periodic table in the synthesis of chemicals, materials and fuels. An extensive range of elements (including metals) are used as catalysts in chemical processes. Many people have become familiar with the sustainability of carbon and the concept of being “carbon neutral” is now well established. As a society, we are currently not neutral in the use of all elements!1,2 As traditional supplies of elements are being depleted, resource deficits are being created. In fact, elements are not destroyed but are simply being spread throughout the technosphere in low concentration waste streams;3,4 thus generating a range of costly problems associated with the recapture of these unique elements. For an element to be truly sustainable, its use by this generation should not restrict its use by future generations.1 This concept has been termed “Elemental Sustainability” and aims to guarantee the sustainability of all elements. Considering the environmental, societal and economic effects of these elements and their use is a vital aspect of this philosophy.5,6 Elemental sustainability should not stop or prohibit the use of elements but should be used to promote holistic multidisciplinary strategies for extraction, manufacture, utilisation and recovery of elements.7 Such activities are vital in order to develop a sustainable circular economy.
Known tonnages of elements that can be economically and legally extracted using existing technologies are known as reserves.8 These reserves represent only a small proportion of the element's total abundance within the Earth's continental crust.8,9 Consumption patterns, materials efficiency and also the final application in which the elements are being used are in a constant state of change. The ability to quickly respond to such changes maintains balance in markets, but is highly challenging to achieve.1 Increased demand for these finite resources and limited reserves of some elements has led to concerns over the security of future accessibility and supply.10 Many elements used as catalysts (or even in ligands) are now considered as “critical” for businesses or national economies.8 The elements that are considered to be “critical” vary depending on the needs of the organisation and purpose of the assessment. These elements are typically those that have a high risk for supply restriction issues and would have a significant impact on a business or economy if this resource becomes limited.11,12,17
The factors that influence if elements are deemed as having a high associated risk of supply issues (or are deemed to be critical) can include but are not limited to;1
geopolitical issues
∘ trade restrictions;
∘ political factors;
∘ manipulation of markets;
∘ international monopolies in elements;
conflicts;
elements that are mined as a byproducts of other elements;
elements of low crustal abundance;
ease of accessibility;
restrictive investment.
The availability of elements within the Earth's crust is finite.9 These crustal abundances are by no means evenly distributed between the elements. Some elements such as aluminium, iron and silicon are available in many orders of magnitude higher quantities than others such as platinum, silver and selenium.9 Many mineral resources have a high potential geological abundance, however, the concentration of many elements within the ores is low when compared to Earth-abundant elements (industrial or base metals), such as iron.8 The cost of elements and also the negative impact on the surrounding environment can be dramatically influenced by the grade of the ores and the challenges associated with mining in geographically or politically hostile locations.1
Table 1.1 highlights those elements that are currently regarded as critical.1 This table also demonstrates that when considering the current known reserves and rates of use many of these resources will be consumed in less than 50 years. However, both the consumption and reserves of these finite elements are constantly changing in response to:
I. movements in markets;
II. discovery of new mineral deposits;
III. development of new applications;
IV. advances in extraction technologies;
V. rate of recycling;
VI. improvements in the efficiency of use, recovery and recycling.18
Elements of critical importance.a
| Element | Symbol | Estimated continental crustal abundance (in mg/kg)d | Resources remaining from traditional sourcesb (Years) | Main geographical location of known resource | Estimated recycling rates (%)c |
|---|---|---|---|---|---|
| Critical elements of global importance | |||||
| Rare Earths | / | 100+ | China | <1 | |
| Gallium | Ga | 1.9×101 | 5–50 | China | <1 |
| Indium | In | 2.5×101 | 5–50 | China | <1 |
| Platinum Group Metals | / | Pd (1.5×10−2) | 5–50 | South Africa | >50 |
| Pt (5.0×10−3) | (50–100 years for Pd) | ||||
| Tantalum | Ta | 2.0 | 50–100 | Brazil | <1 |
| Cobalt | Co | 2.5×101 | 50–100 | Congo | >50 |
| Niobium | Nb | 2.0×101 | 50–100 | Brazil | >50 |
| Antimony | Sb | 2.0×101 | 5–50 | China | 1–10 |
| Beryllium | Be | 2.8 | 100+ | USA | <1 |
| Lithium | Li | 2.0×101 | 100+ | Chile | <1 |
| Tellurium | Te | 1.0×103 | 50–100 | Peru | <1 |
| Germanium | Ge | 1.5 | 5–50 | China | <1 |
| Vanadium | V | 1.2×102 | 100+ | China | <1 |
| Tungsten | W | 1.25 | 5–50 | China | 10–25 |
| Molybdenum | Mo | 1.2 | 50–100 | China | 25–50 |
| Selenium | Se | 5.0×10−3 | 50–100 | Chile | <1 |
| Critical elements of international importance | |||||
| Hafnium | Hf | 3.0 | 5–50 | Australia | <1 |
| Nickel | Ni | 8.4×101 | 50–100 | Australia | >50 |
| Bismuth | Bi | 8.5×10−3 | 5–50 | China | <1 |
| Strontium | Sr | 3.7×102 | 5–50 | China | <1 |
| Barium | Ba | 4.25×102 | / | / | <1 |
| Magnesium | Mg | 2.33×104 | 100+ | Oceans | 25–50 |
| Manganese | Mn | 9.5×102 | 5–50 | South Africa | >50 |
| Titanium | Ti | 5.65×103 | 100+ | China | >50 |
| Critical elements of national importance | |||||
| Copper | Cu | 6.0×101 | 50–100 | Canada | >50 |
| Cadmium | Cd | 1.5×10−1 | 5–50 | India | 10–25 |
| Silver | Ag | 7.5×10−2 | 5–50 | Peru | >50 |
| Tin | Sn | 2.3 | 5–50 | China | >50 |
| Mercury | Hg | 8.5×10−2 | 50–100 | Mexico | 10–25 |
| Thorium | Th | 9.6 | / | / | / |
| Arsenic | As | 1.8 | 5–50 | China | <1 |
| Yttrium | Y | 3.3×101 | 100+ | China | <1 |
| Rubidium | Rb | 9.0×101 | / | / | <1 |
| Cesium | Ce | 6.65×101 | / | / | <1 |
| Zirconium | Zr | 1.65×102 | 50–100 | Australia | <1 |
| Chromium | Cr | 1.02×102 | 100+ | Kazakhstan | >50 |
| Rhenium | Re | 7.0×10−4 | 50–100 | Chile | >50 |
| Boron | B | 1.0×101 | 100+ | Turkey | <1 |
| Thallium | Tl | 8.5×10−1 | 50–100 | North America | <1 |
| Element | Symbol | Estimated continental crustal abundance (in mg/kg)d | Resources remaining from traditional sourcesb (Years) | Main geographical location of known resource | Estimated recycling rates (%)c |
|---|---|---|---|---|---|
| Critical elements of global importance | |||||
| Rare Earths | / | 100+ | China | <1 | |
| Gallium | Ga | 1.9×101 | 5–50 | China | <1 |
| Indium | In | 2.5×101 | 5–50 | China | <1 |
| Platinum Group Metals | / | Pd (1.5×10−2) | 5–50 | South Africa | >50 |
| Pt (5.0×10−3) | (50–100 years for Pd) | ||||
| Tantalum | Ta | 2.0 | 50–100 | Brazil | <1 |
| Cobalt | Co | 2.5×101 | 50–100 | Congo | >50 |
| Niobium | Nb | 2.0×101 | 50–100 | Brazil | >50 |
| Antimony | Sb | 2.0×101 | 5–50 | China | 1–10 |
| Beryllium | Be | 2.8 | 100+ | USA | <1 |
| Lithium | Li | 2.0×101 | 100+ | Chile | <1 |
| Tellurium | Te | 1.0×103 | 50–100 | Peru | <1 |
| Germanium | Ge | 1.5 | 5–50 | China | <1 |
| Vanadium | V | 1.2×102 | 100+ | China | <1 |
| Tungsten | W | 1.25 | 5–50 | China | 10–25 |
| Molybdenum | Mo | 1.2 | 50–100 | China | 25–50 |
| Selenium | Se | 5.0×10−3 | 50–100 | Chile | <1 |
| Critical elements of international importance | |||||
| Hafnium | Hf | 3.0 | 5–50 | Australia | <1 |
| Nickel | Ni | 8.4×101 | 50–100 | Australia | >50 |
| Bismuth | Bi | 8.5×10−3 | 5–50 | China | <1 |
| Strontium | Sr | 3.7×102 | 5–50 | China | <1 |
| Barium | Ba | 4.25×102 | / | / | <1 |
| Magnesium | Mg | 2.33×104 | 100+ | Oceans | 25–50 |
| Manganese | Mn | 9.5×102 | 5–50 | South Africa | >50 |
| Titanium | Ti | 5.65×103 | 100+ | China | >50 |
| Critical elements of national importance | |||||
| Copper | Cu | 6.0×101 | 50–100 | Canada | >50 |
| Cadmium | Cd | 1.5×10−1 | 5–50 | India | 10–25 |
| Silver | Ag | 7.5×10−2 | 5–50 | Peru | >50 |
| Tin | Sn | 2.3 | 5–50 | China | >50 |
| Mercury | Hg | 8.5×10−2 | 50–100 | Mexico | 10–25 |
| Thorium | Th | 9.6 | / | / | / |
| Arsenic | As | 1.8 | 5–50 | China | <1 |
| Yttrium | Y | 3.3×101 | 100+ | China | <1 |
| Rubidium | Rb | 9.0×101 | / | / | <1 |
| Cesium | Ce | 6.65×101 | / | / | <1 |
| Zirconium | Zr | 1.65×102 | 50–100 | Australia | <1 |
| Chromium | Cr | 1.02×102 | 100+ | Kazakhstan | >50 |
| Rhenium | Re | 7.0×10−4 | 50–100 | Chile | >50 |
| Boron | B | 1.0×101 | 100+ | Turkey | <1 |
| Thallium | Tl | 8.5×10−1 | 50–100 | North America | <1 |
/ data not available.
Based on the current known reserves and rate of use (Data adapted from ref. 10, 13, 14),
Data adapted from ref. 15, 16.
Adapted from ref. 9.
As such, care must be taken when using the rate of consumption versus known reserves as a metric for the criticality of elements (Table 1.1). Importantly, in the case of many elements recycling rates are still low and as such a holistic approach to elemental use as catalysts must be adopted including processing, manufacture, recycling and substitution.1,15,16
1.2 Perspectives on Sustainable Catalysis
For a catalyst to be regarded as sustainable, several routes are proposed (see Figure 1.1). The first utilises those elements with high crustal abundance and availability, known as Earth-abundant catalysts. Such catalysts offer several advantages including wide availability and low cost. The second route utilises those elements that are regarded as critical or rare. For these to be used as sustainable catalysts, holistic strategies must be implemented for manufacture, use and most importantly recovery of these catalysts. Finally, the use of organic catalysts can be sustainable if recycled, recovered and used for energy production wherever possible at the end of life. Ideally, organic-based catalysts should be produced from renewable feedstocks wherever possible. Sustainability of the catalyst should also consider how the catalyst is used and consider aspects such as energy consumption for its preparation and recovery, whilst also minimising detrimental impact on the environment (i.e. ensuring that no toxic materials are released into the environment through the use of the catalyst).
Routes to sustainable catalysis.
When adhering to the principles of Green Chemistry the ideal is that the catalyst is heterogeneous, i.e. that it is a separate phase to the reaction media, and this is typically achieved through the use of solid catalysts. The solid material of a heterogeneous catalyst may inherently contain catalytic sites, such as the acid form of zeolites or clays containing Lewis-acid ions within the interstitial gap. Alternatively, the solid may be a support for a catalytic species, such as nanoparticles of palladium (catalytic site) on carbon (support) or an aluminium salen complex (catalytic site) tethered to silica (support). In all these instances the use of heterogeneous catalysis is primarily there to facilitate separation at the end of the reaction, often by simple filtration, but can also be used beneficially to promote improved reaction selectivity or allow harsher reaction conditions to be used (i.e. higher temperature).
The chemical industry relies upon many of the elements within the periodic table to act as catalysts for the plethora of chemical transformations it uses, lists of catalysts used and catalysed reactions are far too large and wide-reaching to state here. Nevertheless the importance of catalysis for the production of chemicals and materials is well known and widely accepted. It should be noted that although we would hope that the Earth-abundant elements would fulfil all the needs of the chemical industry in practice this is not possible, many transformations are only economically feasible when a critical or scarce element is utilised as a catalyst.19 Indeed, many of the rare-earth elements and platinum-group metals show activities as catalysts many orders of magnitude higher than for cheaper, more abundant, equivalents, and overall processes save money when using a more expensive but highly active catalyst. One such example of this is the formation of acetic acid through the carbonylation of methanol. In the 1950s Walter Reppe and coworkers at BASF developed a process of methanol carbonylation based on relatively earth abundant cobalt (continental crustal abundance 2.5×101 mg/kg),20,21 though in the late 1960s this was replaced by a more-efficient process using the more-expensive and less-abundant rhodium (continental crustal abundance 1×10−3 mg/kg) catalyst of the Monsanto process.22 In the 1990s the Monsanto process was then itself superseded by the Cativa™ process, which was based on an iridium catalyst that demonstrated superior catalytic activity and selectivity and despite, iridium also having an exceptionally low crustal abundance (continental crustal abundance 1×10−3 mg/kg). For the last two decades the Cativa™ process has remained the dominant route to acetic acid production.23,24
As highlighted previously, great importance should be put on the use of heterogeneous catalysts, especially when critical or scarce elements are required. Indeed, the ease of recovery for heterogeneous catalysts would go a long way to improving the cyclic use of scarce elements, as some reuse of the critical or rare element would be ensured through the facile separation of the catalyst from the reaction media.25 Additionally, heterogeneous catalysts are ideally suited to applications in continuous reactions such as those in flow. Shown in Figure 1.2 are some examples of the reactions that typically require rare or critical metals. Many of the highlighted critical metals of global importance have found widespread applications in catalysis.19 The REEs, gallium, indium, scandium and yttrium are predominantly used as ions in complexes, and although the precious/PGM can be used in complexes they are more often utilised as nanoparticles. When using supported nanoparticles, appreciation should be given as to whether the catalyst are truly heterogeneous or instead function in a release and capture mechanism.26 A significant proportion of the uses of supported precious metal nanoparticles apply to hydrogenation reactions. Reduction of carbon–carbon double bonds is widespread, using a range of metals and supports for a plethora of substrates.27,28 Carbonyl-group reduction is also prevalent, with an example being terephthalic acid to 1,4-dimethanolcyclohexane for use in polyesters. The choice of metal and reactions conditions is vital for regioselectivity in hydrogenation reactions, mesityl oxide to methyl isobutyl ketone being one such example.29,30 Reductions of other functional groups also often utilise metal nanoparticles, examples include nitrobenzene to aniline,31 ammonia production from N2,32 or the conversion of imines to amines as part of reductive aminations.33 Precious-metal nanoparticles can also be used for dehydrogenations such as propane to propene, though the high temperatures required mean that inorganic supports, such as silica and alumina, are required to ensure thermal stability.34 Nanoparticles of PGMs on carbonaceous supports have been applied extensively to various C–C coupling reactions, including Heck, Suzuki, Sonogashira, Negishi, Kumda and Stille couplings. However, in the various coupling reactions an understanding of the true nature of the catalyst is often overlooked where sometimes the nanoparticles are simply a reserve of metal whose catalytic species is in fact monometallic.35 Metals captured in ionic form by ligands on the surface of a solid support can also find use as catalysts for some import chemical transformations. Grison et al. demonstrated that Friedel–Crafts alkylations and acylations can be catalysed effectively by various plant extracts that have hyperaccumulated Lewis-acid metals such as zinc, cadmium and nickel.36 Beyond those examples shown in Figure 1.2 precious and critical metals on solid supports have also been extensively applied in numerous oxidation reactions. Bulushev et al. and Prati et al. have focused on gold as a catalyst for industrial oxidation processes,37 Bulushev demonstrated the effective application of supported gold nanoparticles for carbon-monoxide oxidation. In this case, phenolic groups from activated carbon fibres were able to attach Au3+ ions, which were consecutively transformed into gold nanoparticles by reduction with hydrogen.37a Prati et al. successfully employed gold on carbon as a catalyst in diol oxidation. Gold ions were reduced by formaldehyde and by their own carbonaceous support.37b Zielinski et al. used nickel catalysts supported on activated carbon for hydrogen storage.38 Significant amounts of hydrogen were adsorbed, with hydrogen pressure, metal content and metal precursor key variables in the process. In a different study, Wang et al. used a nickel catalyst supported on activated carbon as catalyst in methane reforming.39
Examples of current chemical transformation catalysed by precious/critical metals supported on a solid material. For each general chemical transformation one specific example is also shown. Reproduced from ref. 47 with permission from The Royal Society of Chemistry.
Examples of current chemical transformation catalysed by precious/critical metals supported on a solid material. For each general chemical transformation one specific example is also shown. Reproduced from ref. 47 with permission from The Royal Society of Chemistry.
A variety of these transformations have been regarded as green (e.g. the atom economy of metathesis) and even applied in the valorisation of waste, a key move towards a more sustainable chemical industry.40 Endeavours towards the sustainable use of catalysts should go hand-in-hand with the sustainable use of all chemical feedstocks, and this primarily means moving away from nonrenewable fossil resources to produce the substrates, and instead use of renewable biomass or postconsumer waste. When considering the chemical transformation typically required to convert biomass through to valuable products, again it is clear that critical and rare elements will play a pivotal role.41 Figure 1.3 highlights some of these reactions that will be vital to a biobased chemical industry; all of the examples have been demonstrated at laboratory scale.47 Carbon dioxide produced during fermentation process could be converted to carbon monoxide via electrocatalytic reduction and is currently being investigated using supported gold nanoparticles.42 Reductions of alkenes and carbonyls are equally as important for biobased platform molecules as they are for the fossil derived chemicals shown in Figure 1.2. Of particular importance are reductions of the carbon–carbon double bonds in fatty esters, hydrogenations of sugars to polyols such as the conversion of glucose to sorbitol and reductions of various carboxylic acids to alcohols. Hydrogenolysis of polyols, predominately glycerol, is a promising route to 1,2-propanediol, 1,3-propanediol, 1,2-ethanediol and methanol and in some cases platinum or palladium on carbon have proven effective.43 Oxidations using these catalysts will likely also grow in importance with precious and critical metals often utilised for the oxidation of 5-(hydroxymethyl)furfural to 2,5-furandicarboxylic acid, a potential replacement for terephthalic acid in the synthesis of polyesters.44 Many biobased chemicals contain functional groups that can undergo various reactions catalysed by supported Lewis-acid complexes, these include Michael additions, aldol additions and condensations, carbonyl-ene reactions, Claisen condensations and Diels–Alder additions. Further research into metal capture and reuse could therefore be applied effectively to a variety of conversions of biobased chemicals to higher-value products and it is likely that these catalysed reactions will be vital in supporting the emerging biobased chemical industry. Other future areas of research for precious/critical metal capture and use include a thorough investigation into the effect of mixing metals, where in some instances synergistic effects between two or more metals can result in improved activity,45 and further studies into the use of flow reactors for combined capture and use.
Examples of potential future applications that could be catalysed by precious/critical metals supported on carbon. For each general chemical transformation one specific example is also shown. Reproduced from ref. 47 with permission from The Royal Society of Chemistry.
Examples of potential future applications that could be catalysed by precious/critical metals supported on carbon. For each general chemical transformation one specific example is also shown. Reproduced from ref. 47 with permission from The Royal Society of Chemistry.
Two green methods under development for the capture and use of elements as catalysts from waste streams includes phytoremediation and biosorption.46,47 Phytoextraction through hyperaccumulation in plants refers to the accumulation of elements at 100 times greater concentrations (normally toxic to plants) than typically observed for plants growing in the same location.48 Metal uptake and subsequent reuse of the plant materials as catalysts is of growing interest, however, the full potential of this process is yet to be realised. Living plants have been shown to recover palladium and produce catalytically active palladium nanoparticles.49 Such a process can reduce the number of production steps compared to traditional catalyst palladium on carbon. These heterogeneous plant catalysts have demonstrated high catalytic activity in Suzuki coupling reactions.49 Further development of this technology could demonstrate great potential for sustainable catalysis.50,51
Biosorption is a key technology for the benign recovery of diffuse elements from liquid effluents and hydrometallurgy processes.47 During the capture of metals via biosorption the reduction of the metal down to nanoparticles is commonly observed. One such example is the use of starch-derived carbonaceous mesoporous materials (Starbon®) for the selective adsorption and recovery of critical metals (Au3+, Pt2+ and Pd2+) from a mixture containing Earth-abundant elements (Ni2+, Cu2+ and Zn2+) with the consequent formation of metal nanoparticles.52 Of particular importance is whether the catalysts are truly heterogeneous or instead function in a release and capture mechanism.26 The direct synthesis of palladium catalysts using biosorption onto alginic acid and seaweed supports has also been successfully achieved and applied to carbon–carbon coupling reactions.53 Ultimately, it is envisaged that critical metals (or precious metals) captured on biosorbents will play a key role in biomass conversion to higher-value chemicals, therefore aiding in the progression towards a sustainable biobased chemical industry.47
It is vital that we seek to maximise the metals catalytic activity and recover 100% of elements from catalytic processes at both the end of reaction and end of life (the only exception may be carbon that can be burnt for energy production at end of life). Development and application of Earth-abundant catalysts for a wider range of catalytic applications is possible in the midterm. However, the long-term and ideal scenario would be that even critical elements can be used as sustainable catalysts if total recovery from anthropogenic cycles is guaranteed. The concept of elemental sustainability for catalysis is likely to become increasingly important in the future. Now is the time for producers and users alike to progress to circular economies and embrace sustainable catalysis.
Contained within this book are various chapters that review the possibilities for the sustainable use of catalysts in our chemical industry. Earth abundant metals are discussed in Sustainable Catalysis: With Non-endangered Metals, Parts 1 and 2, while the options for organocatalysis are discussed in Sustainable Catalysis: Without Metals or Other Endangered Elements, Parts 1 and 2. The future chemical industry cannot survive by the use of just one of the above catalyst classes, but will require the flexibility and versatility of both. An important aspect of sustainable catalysis that is also vital for the long-term security of elements is ensuring that we establish improved methods of catalyst recovery and reuse.
