Green Chemistry approaches for the Indian fine chemicals industry

The issue of wastes cuts across several segments of the chemical industry and all the more so in the chemistry-intensive fine chemicals industry. Due the sequential processes employed in multi-step synthesis of several organic compounds, this industry tends to have high waste generation, especially as liquid effluents. As with all wastes, it is far better to avoid its generation than to handle them in end-of-pipe effluent treatment plants. A plethora of waste minimisation strategies are available for the industry to deploy, and these are codified in the practices and principles of Green Chemistry (GC).

The focus of GC is to reduce, if not entirely eliminate, the use and generation of hazardous substances in the design and manufacture of chemical products. Strategies to enable this include use of catalytic, instead of stoichiometric amounts of reagents; use of non-toxic reagents & solvents; improving the atom efficiency of the reactions; and use of renewable resources wherever possible. Designing for degradation to benign products is also an integral part of GC approaches.

The environmental impact of the chemical industry

The environmental impacts of the chemical industry come from its products and processes. Several processes – some would say most – generate waste in one form or other – to AIR, water or land. The quantum generated vary, depending on the kind of chemicals made, but in areas as pharmaceuticals or agrochemicals this could be even a few hundred times the quantity of the desired product. This ratio – famously called the E-factor (the ‘E’ standing for environment) – is now widely recognised as a simplistic barometer of how ‘green’ or environment-friendly a process is.

The chemical industry’s approach to tackling the problem of wastes has changed significantly, especially in the last few decades. From ignoring the problem, the industry has evolved to integrate waste treatment strategies into overall manufacturing schemes. Most reputed chemical companies today monitor their release of wastes, and ensure compliance as per the laws of the land or, at times, mandates of customers. They also have clearly defined, and often well publicised, targets for improvement, and, when all else fails, contain the damage wastes may cause by keeping them secure (as in a well-built landfill).

While this still continues to be the model for much of the industry, it does have obvious limitations. Society is clearly expecting more from industry and is less tolerant of its lapses. At the receiving end, ecological systems are becoming over-burdened; landfills, for example, are filling out and permissions to build more are hard to come by.

Importance of fine chemicals

The importance of GC for fine chemicals hence cannot be overemphasised. This is an industry in which India has core strengths – adequate human resources (e.g. trained chemists); low capital needs; less dependence on access to cheap hydrocarbon feedstock; amongst others. But the industry does have a high environmental burden. The ‘E-factor’ of fine chemicals production typically ranges between 5-50, depending on the product and the process, which is far higher than the 1-5 level seen for bulk chemicals typically produced in dedicated plants at higher volumes.

At the same time, end-of-pipe treatment efforts to bring effluents to prescribed standards are expensive – accounting, at times, for as much as a quarter of production costs, and only surpassed by the cost of raw materials needed for the process.

Improving the atom economy

One of the easiest metrics to measure ‘greenness’ of a process is its atom economy, i.e. the atoms of the reagents that end up in the final product. While 100% atom economy is ideal (implying all the atoms in the reagents used end up in the desired product, with no waste generated), the more practical approach is to maximise it by choice of the manufacturing route. There is usually a sound economic rationale for doing so as well – after all, every molecule that does not end up in the desired product turns up in co-products, by-products or in waste streams.

Some of the widely practiced reactions in the fine chemicals industry are notoriously low in their atom economy and these have been the focus of significant research. Take Friedel-Crafts acylation, which typically used Aluminium trichloride as catalyst. All of the aluminium lands up as aluminium hydroxide (which can be recovered, albeit at a cost), while the chlorine end-ups as hydrochloric acid, which is hard to dispose into water bodies, except by neutralisation and dilution. The atom economies of such a process can be as low as 30%. Heterogeneous catalysts, such as zeolites, can be used alternatively, with near doubling of atom economy and the added advantage of catalyst recovery and reuse.

A change in the feedstock used can also be a way of improving atom economy. Take the manufacture of maleic anhydride (MAN), which can be done using either benzene or n-butane as raw material. While the former yields an atom economy of just 44%, the latter has 65% (with the rest only benign water). From the standpoint of carbon alone, the butane-based MAN process has an atom economy of 100%! To no surprise, new MAN plants are almost always based on butane, not benzene.

Some chemical transformations such as rearrangements, additions, Diels-Alder condensations, are inherently atom economical. Others such as substitutions, eliminations, Wittig and Grignard are inherently not so. The Wittig reaction, for example, is widely used for making vitamins and other drugs, but it has poor atom economy due to the production of triphenyl phosphine oxide (TPPO). Recycle strategies can convert it to the starting material, triphenyl phosphine (TPP) – a practical approach given the high cost of TPP – but it does add to costs.

The use of solid acid & base catalysts is now widespread in the bulk chemicals industry, with more than 125 industrial processes now using them for a variety of condensations, alkylations, etherifications etc. Nearly 180 different catalysts are commercially deployed, with about 75 from the class of zeolites. Products made in large scale using these catalysts include cumene (for phenol & acetone), linear alkyl benzene (detergent raw material), and cyclohexanol (for nylon 6).

While opting for a reaction with a high atom economy makes eminent sense, it is also important to consider other reaction parameters such as yield, reaction conditions, number of steps, ease of separation, nature of by-products and solvents, as well. If reactions with poor atom economy need to be used, one can try and minimise its effects by, for example, careful choice of catalysts, in-process reuse or recycle, and by ensuring that by-products are benign.

Solvent selection

Another area where a significant body of work is available is in the choice of organic solvents, widely used as reaction media. Concerns over emissions of volatile organic compounds (VOCs) are determining solvent choices not just in reactions, but also in the finished formulations (such as paints & coatings). For fine chemical synthesis alternatives to solvents include water, supercritical fluids (e.g., carbon dioxide), ionic liquids and even solvent-free synthesis. Solvent selection guides are now widely available, and identify preferred, usable and undesirable options for chemists to consider.

Relevance to India

The relevance of GC to India is unquestionable and its wide adoption will be central not just for sustainable growth, but even the industry’s survival at a time when regulators, governments and society are sceptical and suspicious of the industry.

There are many examples of successes in GC – especially from the US and Western Europe – and much to be learnt from them. But GC is more than just hazard, risk and pollution reduction. It is a philosophy that can enable the chemical industry transition to a more sustainable path of growth. India’s chemical industry must embrace it far more than it now does!