Themed issue: green materials

Green chemistry has become part of the language of chemists working in areas including organic synthesis, catalysis and process development. While its presence in materials chemistry may be less visible, its importance to the field is becoming increasingly evident. From a life-cycle perspective, we can see its importance at all of the key stages: in the use of renewable materials such as cellulose and starch both as the basis of new, greener materials and also as a source of future building block molecules; in the increasingly complex solid catalysts being designed to make chemical synthesis more selective and more efficient; and in the design of new generation material products to match the demanding requirements of an advanced but seeking-to-be-sustainable society—performance at minimal cost to the environment. This special issue of Journal of Materials Chemistry is timely: the depletion of traditional resources both petrochemical and mineral is a subject of increasing concern, while legislation impacting manufacturing, what can go into an article, and what happens to that article at the end of its use is growing exponentially. The articles included in this issue have been selected to reflect many of these new challenges and the opportunities they provide, in cleaner processes and in greener products.

One of the most important factors in green chemistry is the ability to assess the impact that the production, use and disposal of a chemical or material has on the environment. This allows us to improve existing processes in a rational manner, and also allows us to evaluate alternatives in terms of their environmental impact. One of the two feature articles (DOI: 10.1039/b911123a) in this issue discusses the concept of ecodesign in the context of sol-gel technology, with a particular emphasis on mesoporous silicas, to which there are a wide range of routes involving templates, catalysts and different solvent systems, as well as varying energy needs. The article provides important insights into the choice of processing conditions, and should help guide research towards further improvements in this important area.

The second feature article (DOI: 10.1039/b911224c) relates to the use of supercritical fluids in the production of nanocomposites, with a focus on two different applications—silver nanoparticles and semiconductor nanoparticles. Supercritical fluids have many advantages over conventional solvents including at least in some cases environmental benefits. In particular, carbon dioxide is one of only three solvents (along with water and (bio)ethanol) that can be used in the manufacture of a substance that can be classified as “natural” in the EU. Their solvent properties make them very well suited to producing nanocomposites.

Extraction technology is traditionally based on either liquid–liquid extraction or on solid–liquid sorption processes. Extractant impregnated resins combine the two, where a resin is used to encapsulate a liquid-phase extractant. This is described for the case of a series of ionic liquids entrapped within biopolymeric resins such as those derived from alginate. These materials are compared to the more traditional petroleum-derived resins. Applications in the extraction of several metal species are then discussed (DOI: 10.1039/b911318e).

There has been considerable interest in developing new materials from renewable resources, in particular widely available and inexpensive biopolymers such as starch, cellulose and chitin/chitosan. These materials are produced by nature on a scale of 10¹¹ to 10¹² tonnes per annum, and have a range of interesting functionality and structural features which make them very amenable to the synthesis of useful, multifunctional materials. Several papers describe applications of many of these materials:

The potential uses of xyloglucan, a polysaccharide derived from tamarind, is discussed (DOI: 10.1039/b911150f). This material has been investigated intensively over the last decade or so, and has potential in a variety of areas, including drug delivery.

Starch is one of the most versatile biopolymers, and has been used in a variety of applications, many of which rely on its complex nanostructure. Thermolysis of starch leads to interesting materials, including a liquid form of starch, which can be produced within a narrow temperature range. This issue has a contribution (DOI: 10.1039/b911342h; Fig. 1) which demonstrates that this “liquid starch” is a very good adhesive for direct bonding to aluminium, replacing non-renewable adhesives which are currently used, and obviating the need for pre-treatment of the aluminium surface with toxic and corrosive reagents.

Fig. 1 Figure from b911342h.

Chitin and chitosan are unusual polysaccharides as they contain a nitrogen in the sugar repeat unit (glucosamine) and therefore have somewhat different chemistry, which has led to many applications in catalysis and adsorbency. The ability of chitosan to form films and fibres readily has led to many medical applications. Chitin is found predominantly in the shells of crustaceans and insect exoskeletons, and is a major waste stream from the fishing industry. The thermal decomposition of prawn shells forms the theme of one contribution (DOI: 10.1039/b911528e) where the product is a nitrogen rich porous carbon. Such materials are not readily formed by other means, and the use of the natural mineral as a template for carbon formation is an elegant use of a natural nanocomposite material. A second paper discussing the uses of chitosan relates to the use of supercritical CO₂ to generate chitin whiskers, which can then be converted to chitosan without significant change in morphology (DOI: 10.1039/b910842d). Such porous chitosan structures are of potential use in tissue engineering.

Some of the intricacies of cellulose chemistry are discussed in an article about extractions and functionalisation to give a range of materials with industrial applications (DOI: 10.1039/b911610a). The importance of the effect of the source of cellulose on the material properties and the ability to control functionalisation to influence final properties are highlighted as key parameters for determining properties.

Small molecule-derived renewable materials are the focus of a paper on the polymerisation of furan monomers, readily available from the controlled decomposition of polysaccharides. This class of compounds has unique chemistry, which can be controlled to provide polymeric materials with fascinating properties (DOI: 10.1039/b909377j).

Materials with catalytic properties have a central importance in green chemistry, allowing processes to take place along efficient, low energy pathways with improved selectivity and hence easier separations and less waste. The clean synthesis of these materials is as important a consideration as their use, and several papers address these issues:

The use of supercritical CO₂ in the formation of cerium dioxide has been shown to provide a material which has better performance as a catalyst support than ceria produced by non-supercritical routes. Au-Pd nanoparticles produced on its surface have higher activity and a longer lifetime in the selective oxidation of alcohols than those produced conventionally (DOI: 10.1039/b911102f).

Heteropolyacid-silica composites have attracted interest for some time, and a contribution in this issue describes how direct synthesis of such materials can lead to acidic materials with a range of different geometries and morphologies. Application to the conversion of biomass-derived levulinic acid to diphenolic acid was demonstrated and allowed conclusions to be drawn about the relative importance of structural features (DOI: 10.1039/b910416j).

A different type of strongly acidic material based on sulfonic acid groups within the pores of SBA-15 is also discussed and shown to be a valuable catalyst for a three component coupling under solvent free conditions. The influence of surface chemistry and pore size is discussed (DOI: 10.1039/b911388f).

A second application of sulfonic acid-silica materials is discussed whereby the acid groups are entrapped on the internal surface of hollow nanospheres. These are used to protonate and immobilise chiral amines to give a highly enantioselective catalyst, capable of carrying out the asymmetric aldol reaction with very high enantioselectivity. The design of the silica structure is shown to have a significant effect on the outcome of the reaction, and hollow nanospheres are significantly more effective than normal mesoporous systems (DOI: 10.1039/b909002a).

Vanadium containing SBA-15 materials have also been developed for epoxidation reactions (DOI: 10.1039/b910891b) While these materials have many benefits, they require relatively long reaction times, not ideal from the point of view of energy use or of the reactivity of the epoxide products. Here, they are combined with microwave heating to provide a much more rapid and energy efficient process. Activity and selectivity are both improved under microwave conditions.

Heteropolyacids are used in a simple and clean synthesis of gold nanoparticles in aqueous solution at room temperature (DOI: 10.1039/b903599k; Fig. 2) The HPA plays dual roles as reductant and stabiliser of the nanoparticles, with conditions influencing the structure and morphology of the nanoparticles.

Fig. 2 Figure from b903599k.

An alternative and perhaps more sustainable approach to the synthesis of nanoparticles involves the use of tea derived polyphenols as reductant for the production of iron nanoparticles. No other species are required to reduce or stabilise the particles formed. The resultant nanoparticles are effective at decolorizing solutions of bromothymol blue, used as a model system for the oxidative decomposition of contaminants (DOI: 10.1039/b909148c).

Two titanium-silicate materials are described, which are of interest in catalytic cycles. The first (DOI: 10.1039/b911107g) is a composite material of Ti sites dispersed within a silica matrix along with an aromatic polymeric material. The combination of the mixed oxide catalyst and the hydrophobic nature of the material (derived from the polyaromatic) provide an excellent environment for catalysis to occur.

The second paper relates to the functionalisation of layered Ti-MWW to give a catalytically active lamellar material with expanded pore size (DOI: 10.1039/b910886f; Fig. 3). Control of material properties can be achieved by appropriate functionalisation which also stabilises the layers against condensation and pore closure. Catalytic results also reflect the more open and accessible structure of the material.

Fig. 3 Figure from b910886F.

A new acidic catalyst which combines the acidity of Ta-heteropolyacids with an appropriately modified silica support and its use in the synthesis of biodiesel is described in another article (DOI: 10.1039/b910694d) The walls of the support contain organic bridges to increase hydrophobicity and improve catalytic performance. The materials successfully transesterified vegetable oils containing significant amounts of free acids, an important benefit which would allow their use in converting waste vegetable oils into biodiesel.

By covering materials with applications ranging from adhesives to drug delivery, from nanoparticle stabilisation to composite formation, and from making chemicals to manufacturing fuels, we can see the diversity of potential for greener and more sustainable materials both in established industrial sectors and in emerging technologies. Life-cycle awareness of modern articles means that there is no hiding place for unreasonably hazardous, polluting or non-sustainable products and processes: new materials need green chemistry and green chemistry needs new materials.

We would like to express our gratitude to the scientists who have contributed articles to this special issue.

James H. Clark and Duncan J. Macquarrie, York, September 2009.

James H. Clark

Duncan J. Macquarrie

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