Green Chemistry
Commentaries
Commentaries
Green Chemistry
Views and Strategies *
Ramon Mestres
Department de Química Orgánica, Universitat de Valencia, Dr. Moliner 50, 46100, Burjassot, València, Spain (ramon.mestres@uv.es)
DOI: http://dx.doi.org/10.1065/espr2005.04.253
Abstract
Background and Goal. The object of Green Chemistry is the
reduction of chemical pollutants flowing to the environment.
The Chemistry and the Environment Division of EuCheMS has
assumed Green Chemistry as one of its areas of interest, but one
question to solve is where Green Chemistry should be placed
within the context of Chemistry and the Environment. The concept of Green Chemistry, as primarily conceived by Paul Anastas
and John Warner, is commonly presented through the Twelve
Principles of Green Chemistry. However, these Twelve Principles, though fruit of a great intuition and common sense, do not
provide a clear connection between aims, concepts, and related
research areas of Green Chemistry. These two unsolved questions are the object of the present article.
Discussion. Green Chemistry is here placed as a part of Chemistry for the Environment, concerning the still non-existent pollutants. Indeed, the object of Green Chemistry is the reduction
of pollution and risks by chemicals by avoiding their generation
or their introduction into the biosphere. The distinction between
pollutant chemicals and dangerous chemicals, along with the
consideration of the exhaustion of fossil resources and the acknowledgement of the harmful effects of the chemicals employed
in a great variety of activities, leads to the recognition of four
general objectives for Green Chemistry. In order to accomplish
these general objectives, a number of strategies, or secondary
objectives and some fundamental concepts, namely, atomic
economy, selectivity, potential harm or historical harm can be
visualized. A connection is finally established between the strategies and current and future research areas of Green Chemistry.
Conclusion. The ultimate aim of green chemistry is to entirely
cut down the stream of chemicals pouring into the environment.
This aim seems unattainable at present, but progress in the green
chemical research areas and their application through successive approaches will certainly provide safer specialty chemicals
and much more satisfactory processes for the chemical industry.
Keywords: Environmental chemistry; green chemistry; sustain-
ability
1
Background and Goal
Paul Anastas and John Warner gave publicity in 1998 to the
Twelve Principles of Green Chemistry and numerous and
substantial publications by several authors have contributed
to a general acceptance of the concept of Green Chemistry.
* The basis of this peer-reviewed paper is a presentation at the 9th FECS
Conference on 'Chemistry and Environment', 29 August to 1 September
2004, Bordeaux, France.
128
[1–9] However, the connection between aims, concepts, and
related research areas is not well established and this is none
other than the main object of the present article. Some general introductory considerations should provide a basis for
the development of the strategies or scheme presented here.
A first question to solve is where Green Chemistry should
be placed within the general context of Science. Green Chemistry was born, nourished and named in the EPA as an expected, efficient approach or conceptual tool for the protection of the environment. Under this conception, the FECS
Division for Chemistry and the Environment had no hesitation in assuming Green Chemistry as one of its areas of interest. Indeed, the Division for Chemistry and the Environment pays attention to any kind of issues which have to do
with both chemistry and the environment under the assumption that Chemistry and the Environment may be considered as a multidisciplinary area of knowledge, where all
branches of chemistry must contribute to understand and
provide solutions to the pollution events which result from
the outcome of anthropogenic activities.
2
2.1
Discussion
Green chemistry in the framework of environmental
chemistry
Within the wide framework of chemistry and the environment, Chemistry of the Environment has to do with the effects and fate of man-made chemicals and materials present
in the environment. Though simple and rather restrictive,
this picture enables us to contemplate environmental chemistry as a science that observes, measures, understands and
predicts. Observation, comprehension and prediction of
pollution issues can then be complemented by the scientific
and technologic knowledge of the Chemistry for the Environment, which attempts to improve the environment through
an active minimization of the levels of polluting chemicals.
Polluting chemicals may be found spread in the environment or still confined where they have been generated. Minimization may then be the result of convenient remediation
and prevention techniques, respectively (Fig. 1). However,
the pollutant may be envisaged as still being non-existent.
The chemical activity that aims to improve the environment
by minimization of the generation of pollutants is known as
Green Chemistry.
Green chemistry mainly has to do with the production of
chemicals; green technologies must be consequently based
on chemical preparative conversions. Under this point of
view, green chemistry seems constrained to preparative
ESPR – Environ Sci & Pollut Res 12 (3) 128 – 132 (2005)
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Commentaries
Green Chemistry
2.2
Fig. 1: Green chemistry in the context of chemistry for the environment
chemistry. This and the organic chemical nature of a great
many of the contaminants, explains why organic chemists
are inclined to consider that strong and almost exclusive
links exist between green chemistry and organic chemistry.
However, as James Clark pointed out, "if we are to make a
real difference to the impact of chemistry to the environment, it is essential that we understand the chemistry of the
environment". Environmental degradation of chemicals is
obviously not a part of green chemistry, but the knowledge
of the mechanism of this degradation, the estimation of its
kinetics and the resilience of a particular site must be taken
into account for the design of any new molecular structure
to be launched into the market. The knowledge of environmental chemistry should mark the order of priorities for
urgent green chemistry goals; what chemicals and chemically produced materials ought to be replaced by other useful but not harmful ones. Further, the concept of green chemistry is not restricted to the development of clean and safe
synthesis, but applies also to the reduction or suppression of
generation or introduction of any harmful or dangerous
chemical or chemically produced material in the biosphere.
The sources of chemical contaminants
As shown in Fig. 2, the introduction of chemicals in the environment is not exclusively due to the chemical industry, but
also to energy production, transportation and metallurgy, as
well as to those productive sectors and activities which employ manufactured chemicals. Extraction of fossil and mineral resources from under the Earth's surface pour chemicals
into the polluting stream. Any reduction in the use of materials taken from under the Earth's surface means a parallel
reduction in the introduction of pollutants in the biosphere.
Consequently, any chemical procedure or technique that allows lower levels in the needs for fossil or mineral sources
may be considered within the scope of green chemistry.
The chemical industry plays a fundamental role among the
sources of contamination shown in Fig. 2. On the one hand,
great amounts of chemical pollutants are used, produced
and released by the chemical industry; fluids leak; waste
materials are either disposed of or released as aqueous effluents. On the other hand, a significant proportion of the
final products of the chemical industry are disseminated
into the environment as toxic, persistent pollutants; by industries or activities which employ them in agriculture, textile, building, automotive, cleaning, pharmacy, etc. and alternative innocuous products are needed. All this justifies
that the development of green chemistry is strongly connected to chemical industrial activities and that green chemistry is not only meant to improve the environment, but
also to provide the chemical industry with new views and
tools that may enable it to overcome the serious issues met
as a consequence of its being the source of a great deal of
the chemical pollution and, consequently the culprit of environmental damage.
THE ENVIRONMENT
Energy production
Transportation
Chemical industry
Recovery
and refining
Activities which
use chemicals
Metalurgy
Mining
Renewable resources
Mineral materials
Fossil resources
Flow of materials
Flow of pollutants
Fig. 2: Flows of materials and pollutants from the sources to the environment
ESPR – Environ Sci & Pollut Res 12 (3) 2005
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Green Chemistry
2.3
Dangerous and polluting chemicals
It may be convenient to recognise here the existence of a
significant qualitative distinction between dangerous and
polluting chemicals. Highly toxic, flammable, explosive or
aggressive chemicals, which have been or can be the cause
of highly dramatic personal or public events are here categorised simply as dangerous chemicals. They are nothing
new to chemists and have been a permanent cause of concern since the early industrial activity in the XIXth century.
Polluting chemicals, on the other hand, are certainly harmful and dangerous, but rather through long-term effects:
ecotoxicity, greenhouse effect, depletion of stratospheric
ozone, etc. The noxious effects of many pollutants have frequently been unforeseen by chemists, who have otherwise been
well aware of the immediate dangers of many chemicals they
manipulate in the laboratory or in the industrial plant.
Industries have usually dealt with chemical pollution and
safety issues by application of a kind of palliative engineering technology, which requires great unprofitable expenditure, frequently without attempting modification of the
chemical process itself. Industries have also met awkward
situations when final products already launched into market have subsequently been found to be harmful to humans or
to the environment. The green chemistry approach (the first
of the Twelve Green Chemistry Principles) to all these pollution, immediate risk and noxious final product situations is
based on the conviction that it is better and cheaper to find
new cleaner and safer chemical technologies for the synthesis
of a definite chemical and to know at what extent a final product may be harmful before it is first synthesised in the laboratory. The design of new useful intrinsically innocuous
chemicals and of useful materials that can be degraded or
recycled, stands at the very core of green chemistry.
2.4
Strategies for the greening of chemistry
From what has been said so far, a general green chemistry
strategy may be based on four general objectives:
1. Reduction of use and generation of polluting chemicals in the
chemical process
2. Reduction of use of dangerous chemicals in the chemical process
3. Reduction of the harmful effects of final products
4. Reduction of the use of exhaustible feedstock materials and of
scarce resources
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2.6
The chemical process
Components of the chemical process are shown in Fig. 3, as
a simplified scheme intended to facilitate the presentation
of the strategies and the research fields which are connected
to the above green chemistry general objectives. The reaction product is ordinarily accompanied by secondary products, derived from competing side conversions and by concomitant products, which are produced in the reaction as a
simple consequence of its stoichiometry. Side products and
concomitants, along with unrecovered solvents, constitute
substantially the reaction waste.
Significant amounts of chemical pollutants are derived from
waste production in the chemical process. Secondary compounds are generated by reaction paths competing with that
which leads to the main product. Their generation means a
reduction of yields and the need for introduction of costly
and tiresome separation and purification procedures where
large amounts of solvents must be employed. Strategies for
reduction of secondary products are connected to the most
familiar concept of selectivity. Selectivity should be understood here as the generation of one single compound by a
reaction, but also as the possibility of circumventing a
number of steps of a synthetic sequence by means of a single
step conversion. In the present context of strategies for clean
processes, a reduced number of synthetic steps is expected
to afford also lower amounts of secondary products and
hence of waste. Catalytic and bio-catalytic procedures usually offer the opportunity for high selective conversions, especially in the currently increasing need for chiral syntheses.
COMPONENTS OF THE CHEMICAL PROCESS
REAGENTS
Exhaustion of fossil resources
One further problem for chemistry derives from the depletion of fossil feedstock materials for chemical industry. Although how far into the future complete exhaustion may
occur is open to discussion, shortage of easily recovered oil
will certainly cause a raise in feedstock prices in the near
future. This reinforces the need for the use of renewable
feedstock materials and for the modification or development
of novel chemical technologies for this purpose. It cannot
be overlooked that other materials and water are or may
become scarce either locally or at global level. This accessibility to resources must also be considered if development
of young nations and the future sustainability of chemistry
is to be guaranteed.
2.5
Commentaries
STARTING
MATERIALS
PRODUCT
CHEMICAL
PROCESS
SECONDARY
PRODUCTS
ENERGY
CONCOMITANT
PRODUCTS
SOLVENTS
Fig. 3: Components of the chemical process
2.7
Reduction of waste production in the chemical process
Some primary objectives, related strategies and research areas for reduction of pollutants in the chemical process are
shown in Table 1. As a first step in this reduction, starting
materials as well as reagents and solvents, can be the origin
of pollution, either by themselves, through their pollution
potential, namely their susceptibility to cause harm once
released to the environment, or because of their historical
harm: the pollution associated to waste production and energy consumption of the chain of conversions needed for
their manufacture starting from the first fossil or renewable
sources. As a general strategy, it may be assumed that the
pollution associated with these constituents of the process
could be minimized by use of chemicals as proximate as
ESPR – Environ Sci & Pollut Res 12 (3) 2005
Commentaries
Green Chemistry
Table 1 : Reduction of use and generation of polluting chemicals in the chemical process
General objectives
Reduction of pollution by starting materials,
reagents and solvents through minimization
of their HISTORICAL HARM
Reduction of secondary products through
higher SELECTIVITY in the chemical
processes
Related strategies
Use of chemicals as proximate to the sources
as possible
Use of renewable resources
Reduction of number of synthetic steps
Selective novel reactions
Improved selectivity of current reactions
Reduction of environmentally significant
concomitants
Reduction of use of polluting solvents as
reaction media
Processes with low number of synthetic steps
High ATOM ECONOMY based new procedures
Reactions without solvent
Reactions in low toxicity organic solvents
Reactions in specially designed solvents
Reduction of use of polluting solvents for
separation or purification
Reduction of secondary products
Improvement of separation and purification
procedures
Reaction conditions with separation of products
Reduction of energy consumption
Areas of research
Catalytic and Biocatalytic procedures
Study of reaction mechanisms
Real time control of ongoing processes
Continuous processes
Process intensification
Catalytic and Biocatalytic procedures
Reactions with O2, N2, H2O as concomitants
Solvent-less reactions
Reactions in water
Supercritical fluids
Ionic liquids
Supercritical fluids
By-phase conditions
Polymeric reagents
Heterogeneous catalysis
Use of chemicals proximate to the sources
Reactions carried out at room temperature
possible to the sources, and by choice of renewable feedstock
materials whenever feasible.
Concomitant products come along with the product as a
simple consequence of the stoichiometry of the chemical
conversion and constitute a very significant source of waste.
Some concomitant agents may be environmentally irrelevant,
as water, nitrogen, or diluted solution of sodium chloride,
but very frequently contain toxic heavy metals, or other
harmful materials. As a general strategy, concomitants should
be produced in as many low amounts as possible and the
concept of atom economy may afford guidance for the design or choice of convenient chemical reactions in which as
many atoms of the reagents as possible are found afterwards
in the product. Here again, catalytic and bio-catalytic procedures are fundamental in that they frequently offer the
opportunity for high atomic yield conversions.
Solvents are needed both as reaction media and in the separation and purification steps of the process, and are employed in any case in much larger amounts than starting
materials, reagents and catalysts. On the other hand, most
organic solvents have proved environmentally harmful or
are easily flammable. That is why there is a special concern
about their pollution potential and many active green chemistry research areas are addressed to the substitution of conventional organic solvents as reaction media by other systems, such as water, supercritical fluids or ionic liquids.
Strategies for reduction in the need for lower amounts of
solvents for separation purposes include, for instance, biphasic systems, use of polymer supported reagents and heterogeneous catalysis.
2.8
Reduction of danger in the chemical process
It might be expected that safer reactions should be attainable by use of milder reagents (Table 2). This is one of the
possible strategies for reduction of the dangers in the chemical production, but frequently not the best one, because a
mild reagent usually demands higher residence times or
higher temperatures and these conditions play against the
selectivity of the conversion. Milder reagents and milder reaction conditions are usually feasible by use of adequate
Table 2 : Reduction of use of dangerous chemicals in the chemical process
General objectives
Processes without dangerous reaction solvents
Processes without dangerous solvents for
separation and purification
Processes without dangerous reagents
Safer reaction conditions
Related strategies
(see Table 1)
(see Table 1)
Areas of research
Safer reagents
Selective activation techniques
Catalytic and Biocatalytic procedures
Photochemistry
Electrochemistry
Microwaves
Sonochemistry
Room temperature and ordinary pressure
Reduced scale of the process
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Continuous processes
Process intensification
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Green Chemistry
Commentaries
Table 3: Reduction of the harmful effects of final products
General objectives
Harmless final products
Strategies
Natural products as source
New harmless products
Products degradable after their function
Recyclable products after their function
Table 4: Reduction of the use of exhaustible feedstock materials and of scarce resources
General objectives
Strategies
Development of renewable feedstock sources
Biomass
Renewable materials for specific production
Natural products
Renewable energy sources
Improvement of efficiency in use of energy
Improvement of efficiency in use of scarce sources
Recycled materials
Biomass
Reduction of energy consumption
Improvement in generation of energy
Economy of water
catalysts or by the selective activation of some of the reactants: photochemically, electrochemically or by other activation techniques. Although it is true that typical photochemical reactions do not require strong reagents, it may be a cause
of argument placing photochemistry and other forms of selective activation simply as safe procedures, as they contribute to
the synthetic chemical armoury with specific conversions otherwise attainable. A complementary strategy for reduction of
danger in the chemical process consists in the reduction of
amounts of reactants and of the time of residence which may
be the result of the process intensification techniques.
2.9
Essentially harmless final products
It has been well established that a good deal of the synthetic
compounds and materials that are employed have harmful
effects in or after their use (Table 3). There is need for substitution of these compounds and materials presently in the
commerce by others which prove to be non-toxic and easily
mineralised or recycled. Better knowledge of biochemistry,
physiology, microbiology and toxicology will certainly improve the chances for the convenient design, for instance, of
non-polluting physiology and behaviour-based pest control
products and of biodegradable plastic materials.
2.10
Renewable feedstock
In order to improve environmental pollution levels, the reduction of the use of fossil materials is more urgent in the
generation of energy and transportation than in the chemical industry. However, green chemistry is expected to afford
solutions for both (Table 4). Biomass may substitute fossil
materials to a large extent as fuels for energy production
and as chemical feedstock, especially when combined with
biotechnological methods. It may be added here that recycling of large bulk materials, such as plastics, may also become a form of recovering part of the fossil derived feedstock.
Reduction of the depletion rate of fossil materials may come
also from increasing the efficiency in use of fuels, as it is
expected, for instance from the development of fuel cells.
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3
Areas of research
Bioactive products based on specific physiology
and behaviour
Design of intrinsically non-toxic chemicals
Design of degradable materials
Design of recyclable materials
Areas of research
Production of basic chemicals
Production of synthetic intermediates and
final products
Production of basic chemicals
Production of synthetic intermediates and
final products
Feedstock recycling of plastics
Production of energy from biomass
Fuel cells
Conclusion
In conclusion, it has been presented here as a scheme for
some of the strategies, concepts and issues related to green
chemistry. The ultimate aim of green chemistry is to entirely
cut down the stream of chemicals pouring into the environment. This should be accomplished in chemical industries
by the introduction of chemical processes which do not produce contaminants, but only non-toxic commodities and
recyclable or easily degradable materials. These industries
should be based on ideal processes that start from non-polluting starting materials, lead to no secondary or concomitant
products and require no solvents in order to carry out chemical conversions or to isolate and purify products. Such intrinsically clean processes seem unattainable at present, but it is
to be expected from the ingenuity and resourcefulness of chemists that, as the result of single modifications or more probably from successive approaches, much more satisfactory processes than those currently in operation will be achieved.
References
[1] Anastas PT, Warner JC (1998): Green Chemistry: Theory and Practice,
Oxford University Press, Oxford
[2] Anastas PT, Williams TC (eds) (1998): Green Chemistry: Frontiers in
Benign Chemical Syntheses and Processes, Oxford University Press
[3] Anastas PT, Heine LG, Williams TC (eds) (2000): Green Chemical Syntheses and Processes, ACS Symposium Series 767, American Chemical
Society
[4] Tundo P, Anastas PT (eds) (2000): Green Chemistry. Challenging Perspectives, Oxford University Press, Oxford
[5] Clark J, Macquarrie D (2002): Handbook of Green Chemistry and Technology, Blackwell Publishing, Oxford
[6] Lancaster M (2002): Green Chemistry. An Introductory Text, Royal
Society of Chemistry, Cambridge
[7] Anastas PT, Kirchoff MM (2002): Acc Chem Res 35, 686–694
[8] Ryan MA, Tinnesand M (eds) (2002): Introduction to Green Chemistry. Instructional Activities for Introductory Chemistry, American Chemical Society, Washington
[9] Cann MC, Connelly ME (2000): Real-World Cases in Green Chemistry, American Chemical Society, Washington
Received: April 19th, 2005
Accepted: May 10th, 2005
OnlineFirst: May 11th, 2005
ESPR – Environ Sci & Pollut Res 12 (3) 2005