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Chemistry has an important role to play in the achieving of a sustainable civilization on Earth. A proof is given by the recent history of the US Environmental Protection Agency (US EPA), founded in 1970. Initially, US EPA adopted (as Europe) a command and control policy in the execution of environmental regulations while a shift in paradigm started in the 1980s when pollution prevention become the priority instead of end-of-pipeline control. Consequently, this approach requires to completely design ex novo the existing and necessary chemical processes making immediately evident the strategic role of synthetic chemists. With the Pollution Prevention Act, approved by the American Congress in 1990, US officially pointed the attention on the ‘‘millions of tons of pollution’’ and the related cost of ‘‘tens of billions of dollars per year’’ [1]. It is evident how the common interest of Government, industries and research institutions to cooperate is basically directed to solve obvious environmental issues but also to reach common economic interests. Such approach will be of great benefit to the entire society allowing to keep the production in highly advanced regions (such Europe and US) and saving job positions.
The”2011 Roadmap for the Chemical Sciences” recently disclosed by the European Association for Chemical and Molecular Sciences is in fully agreement with the Green Chemistry approach. The challenge for chemists is to continue the search for new methods to maintain access to key products currently synthesized using costly procedures. An advanced region such as Europe must actively operate in this field and encourage those industries that contribute to the wealth of the society by creating new and appealing job positions.
Following the Pollution Act a growing attention has been dedicated to the so-called Green Chemistry. The “12 principles” were published by the Green Chemistry Institute in US [2] and they represent a sort of guidelines for realizing efficient modern chemical processes. They can be summarized as follow: 1. prevent waste; 2. atom economy; 3. design safer chemicals; 4. less hazardous chemical synthesis; 5. safer or no solvents; 6. energy efficiency; 7. use renewable feedstock; 8. avoid protecting groups and derivatives; 9. catalysis; 10. design chemicals considering their degradation; 11. real-time analysis for pollution prevention; 12. inherently safer chemistry for accident prevention.
Considering that great attention must be paid to waste production, the efficiency of a chemical process cannot be correctly measured by just the yield and selectivity parameters. To evaluate the environmental impact of a synthetic procedure other metrics should be used. Among these, one of the simplest and very effective is the Environmental factor (E-factor) introduced by Sheldon [3]. This simple value is the ratio between the Kg of waste produced per Kg of desired product and gives the immediate idea of how an elegant and complex chemistry may results in a highly environmentally costly process. E factor, atom economy [4] and the “12 Principles” have emerged as key driving factors for the definition of sustainable chemical processes.
“Catalysis” is of key relevance to synthetic organic chemists. Catalytic methods offer numerous benefits related to sustainability, including lowered energetic reaction requirements; the use of catalytic rather than stoichiometric amounts of materials; increased selectivities; and, in many cases, the ability to use less-toxic reagents [5]. A truly crucial area concerns heterogeneous catalysis that embodies many requirements of sustainable processing by allowing the easy separation and recovery of a catalyst from the reaction products.
In addition, synthetic transformations should be preferentially performed under temperature and pressure conditions that do not require costly control equipments (minimization of energy consumption). The combination of catalysis and continuous-flow processes is the most effective solution to this issue.
Generally, continuous flow processes are performed in mini or microreactors, are more efficient than standard batch protocols and offer much higher throughput per unit volume and per unit time [6]. Most importantly, for a long time the chemical industry has relied on the continuous production of chemicals, commonly for commodities and less much for fine chemicals and pharmaceuticals. As no flow equipment on the laboratory scale was available until very recently, the chemistry developed in the laboratory was based on batch processes. This means that research was often disconnected from process chemistry which led to many problems and extra optimizations or even total redesign of the initial synthetic strategy. Anyway in our opinion, a significant step must be undertaken towards a green production. Intensified processes should be defined based on continuous-flow reactors able to drastically reduce waste and to operate in safer media taking green chemistry principles in flow.
In fact, much attention should be paid to the reaction medium. The Green Chemistry principle number five points out that in view of realizing an environmentally-efficient process, the use of solvent should be made unnecessary or minimized. This is the most crucial aspect of a chemical process, both at a laboratory small-scale or in large production. Generally, the largest contribution to waste is due to the solvents used for running the reaction, isolate and purify the products [7]. Several of the solvents normally used, especially in fine chemical syntheses, are volatile and their dispersion in the environment is almost unavoidable (one example above all is given by dichloromethane, the 70% of which is inevitably dispersed). Attention is directed towards safer alternatives, supercritical fluids (mainly CO2), ionic liquids, and water are the most representatively investigated options. Anyway a careful evaluation of the actual beneficial effects of such media on the greenness of the process should be always considered before considering them “green”. A very efficient option is the use of NO SOLVENT at all (solvent-free conditions, SolFC).
In the Laboratory of Green Synthetic Organic Chemistry (Green SOC) at the University of Perugia, for many year we have been trying to contribute to the development of novel synthetic processes efficient from both chemical and environmental point of views [8]. We have mainly directed our attention towards the identification of chemical processes that could be realized using water as reaction medium or under solvent-free conditions (SolFC).
We have exploited the peculiar properties of water, and more specifically the careful control of pH, to realize efficient catalytic synthetic processes always defining the recovery and reuse of both water and catalyst [8].
Recently, we have directed our attention towards heterogeneous catalytic systems in order to realize large scale processes operating in automated continuous-flow conditions. Generally, a significant loss of catalytic efficiency is observed when a catalyst is immobilized over a solid support and often this is related to its swelling [9]. Such process is strongly dependant on the reaction medium employed and dramatically affects the activity of the solid catalyst, influencing the access of the reactants at the catalytic sites and also the pressure needed to “push” the reactants mixture through the solid matrix hampering the realization of continuous-flow reactors [10].
According to our experience, we have identified several crucial issues in the use of an immobilized catalyst under SolFC. Commercially available solid supports (organic or inorganic) have been developed to be used in the presence of a reaction medium and designed to deal with the related swelling processes. Mechanical stirring of the reaction mixture often causes the complete crunching of the solid catalyst hampering its recovery and reuse. In addition, an organic solvent is generally necessary to isolate the products and recover the catalyst.
We have proposed as an alternative solution to these problems, the combined use of specifically designed catalytic systems and continuous-flow reactors able to operate under SolFC or in the presence of a dispersion medium such as water. Accordingly, novel solid supports should be designed in order to be able to operate effectively under these flow alternative conditions and consequently eliminate the problems related to the swelling and the internal pressure. This approach should allow a) the highest intimacy between the reactants and their optimal access to the catalytic sites, b) to avoid mechanical stirring and high pressures and b) the recovery of the final products using the minimal amount of organic solvents and therefore minimizing the production of waste.
[1] Pollution Prevention Act of 1990. US Government Printing Office, Washington,1995, p. 617.[2] P. T. Anastas, J. C. Warner, Green chemistry: theory and practice, Oxford University Press Inc., New York.[3] R. A. Sheldon, Chem. Commun. 2008, 3352–3365. [4] B. M. Trost, Science, 1991, 254, 1471. [5] R. A. Sheldon, I. Arends, U. Hanefeld, Green Chemistry and Catalysis, Wiley-VCH, Weinheim, 2007. [6] a) M. Irfan, T. N. Glasnov, C. O. Kappe ChemSusChem 2011, 4, 300; b) J. Wegner, S. Ceylan, A. Kirschning Chem. Commun. 2011, 47, 4583. [7] R. H. Henderson et al., Green Chem. 2011, 13, 854. [8] For recent examples see: L. Vaccaro et al. a) Org. Lett. 2011, 13, 2150; b) Eur. J.Org. Chem. 2011, 2587. [9] M. Benaglia, A. Puglisi, F. Cozzi, Chem. Rev. 2003, 103, 3401. [10] A. Kirschning et al. Angew. Chem.,Int. Ed., 2001, 40, 3995. [11] for representative examples see L. Vaccaro et al., a) Chem. Commun., 2004, 2587; b) Eur. J. Org. Chem. 2006, 1231; c) J. Org. Chem. 2006, 72, 9536; d) Eur. J. Org. Chem. 2008, 3928.
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