Swimming in a purer stream

Green Chemistry
January 4, 2002

Green chemistry is clean and environmentally friendly chemistry. From large-scale processes such as crude oil refining, bulk industrial chemical production and the paper industry to the more specialised fine chemicals and pharmaceuticals sector, clean synthesis is the objective. Green-chemistry research is the discovery and development of environmentally benign methods of effecting chemical transformations. The approach is much more than paying lip-service to the increasing and justifiable public concern for the environmental damage and potential health risk derived from chemicals released into the environment. Companies now carry out "cradle-to-grave" analysis of the components of any process to be brought on stream.

Green chemistry research and processes aim to reduce the impact on the environment to a minimum by eliminating hazardous wastes; reducing or eliminating the use of solvents; reducing emissions; and by using lower temperatures. These aims are not new, but the rise of environmental issues up the political agenda has given new impetus to the search for the least polluting processes and technologies. Economic cost remains an important issue but as the new technologies are improved and refined, they are becoming competitive with established processes.

Green Chemistry: Challenging Perspectives is the third volume in a series on green chemistry research and process development. It features a selected number of papers presented at the international green chemistry conference in Venice at the end of 1997. All chapters have informative introductions setting out the background and issues, with references to the primary literature as well as to reports from governmental and non-governmental organisations. The subject coverage is not comprehensive because there are so many strands of green-chemistry research.

Two chapters cover petroleum refining processes for the production of ultra-low sulphur diesel and cleaner burning petrol. The present method of sulphur removal is energy intensive and requires hydrogen that has to be generated separately. The benefits of the new technology of sulphur removal by micro-organisms at ambient temperatures are obvious. Upcoming United States and European Union legislation on cleaner burning petrol will reduce the octane rating. The remedy is to alter the composition of the petrol by introducing more branched-chain isomers of octane. This is done by joining together smaller molecules in a reaction catalysed by strong acids. These corrosive compounds pose major challenges in reactor design and engineering, and the disposal of spent acids. The chapter on the development of new solid catalysts with highly acidic surface groups to replace these older technologies is informative. These new processes are not yet implemented on a large scale because of cost, but catalyst improvements, legislation and perhaps favourably biased petrol duty regimes could tip the balance in the near future.

Wood-pulp bleaching is the key initial process in the paper industry. To make white paper, the complex biological polymer lignin has to be broken down by oxidation. When paper with a high lignin content is exposed to light, browning occurs. The most efficient methods of pulp bleaching use chlorine compounds as oxidising agents, but significant quantities of by-products, such as chlorinated phenols and dioxins, are formed. Even with effluent treatment, some release of these highly hazardous compounds into the environment is inevitable. An accessible chapter discusses the science and economics of chlorine-free bleaching processes using hydrogen peroxide and new catalysts.

A typical chemical synthesis reaction involves heating two or more reagents in an organic solvent, perhaps with a catalyst, commonly an acid or a base, to accelerate the reaction. The organic solvent is usually in large excess, and solvent recycling and disposal add environmental costs to the process. The properties and applications of one environmentally benign solvent, dimethyl carbonate, are presented in one chapter. Although it cannot be used for all chemical reactions, its properties serve as a benchmark. Other chapters deal with innovations in other aspects of a chemical reaction. Instead of using mineral acids or other inorganic compounds as catalysts, solids such as zeolites with surface acid groups can be used without the need for any solvent. Microwave heating is a much more selective means of increasing the rate of reactions than conventional methods, and no solvent is necessary for some reactions.

Oxidation, that is adding an oxygen atom or removing two hydrogen atoms, is a general reaction in synthesis. Once oxidised, most compounds become more chemically reactive, and one chapter describes developments in photochemical methods for the degradation of environmental contaminants using modified titanium dioxide as the catalyst. Chemical oxidising agents are highly reactive compounds that are often toxic, hazardous and some contain heavy metals. The search for molecular catalysts that can use hydrogen peroxide or atmospheric oxygen as the source of oxygen continues, but these processes are a long way away from being commercially viable. Living organisms do utilise these as the oxygen source by using enzymes as catalysts. The challenges and progress in the biotechnology of microbial oxidation reactions are reviewed in a short chapter, which gives a good summary but is not sufficient to do justice to this area. Indeed the whole area of bio-catalysis is under-represented in this volume, which is a pity because bio-transformations employ mild conditions, do not generate waste and the excellent selectivity of enzymes can simplify many processes.

Finally, one fascinating chapter describes the elegant method of trapping plant cells in a solid matrix and then coating the walls of these delicate cells with a thin layer of a silicone-type material. These protected cells are more robust and can be used to produce trace-plant metabolites of medicinal importance without having to fell tonnes of trees.

This book is of value to scientists and decision-makers in scientific research. The scientific journal Green Chemistry , published by the Royal Society of Chemistry, is thriving. The Clean Technology Unit of the Engineering and Physical Sciences Research Council has provided funding for green chemistry research in the United Kingdom for the past ten years. Green chemistry has moved from being a "bandwagon" into the mainstream.

Luet-Lok Wong is lecturer in chemistry, University of Oxford.

Green Chemistry: Challenging Perspectives

Author - Pietro Tundo and Paul Anastas
ISBN - 0 198 50455 1
Publisher - Oxford University Press
Price - £65.00
Pages - 269

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