Continuous-flow chemistry is defined as a chemical process carried out in continuous-flowing streams. Recently, flow-chemistry was included among one of the tools to achieve some of the United Nations Sustainable Development Goals (SDGs), whose target is to reach a more sustainable and better future by 2030 (Gomollón-Bel, 2019, Ten Chemical Innovations That Will Change Our World: IUPAC identifies emerging technologies in Chemistry with potential to make our planet more sustainable). In particular, flow chemistry, where the experiments are carried out in continuous-flow systems rather than in batch conditions, is particularly critical to accomplish SDG12: responsible consumption and production.
In addition, IUPAC has recently outlined the “top ten chemical innovations that will change our world” which could have “the potential to make our planet sustainable”. Among these revolutionary technologies, IUPAC had also included flow chemistry. Indeed, continuous-flow reactors can offer numerous advantages counting the reduction of consumption by using small volumes of solvents and small quantities of chemical reagents.(Akwi, 2018, Continuous flow chemistry: where are we now? Recent applications`, challenges and limitations). Moreover, flow chemistry processes can drastically reduce the risk of handling dangerous chemicals and increase reaction selectivity and yields, both avoiding harm and limiting the environmental impact.
As stated by Vaccaro et al., flow chemistry can ideally cooperate with green chemistry towards sustainability and can deliver new routes to make processes a more sustainable practice. These innovative technologies can build a bridge between the academic and the industrial world. Indeed, in terms of production, continuous-flow procedures are more effective than batch processes and offer much higher reaction throughput per unit of time and volume, providing more safe, scalable, efficient and reproducible performances.
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In fact the flow continuous-flow reactors have already been used in the chemical industries for over a hundred years. During this century, the progresses in flow chemistry have been remarkable and have led to significant upgrades in terms of safety, reduction of energy consumption and access to a broader variety of reaction conditions. Practically, as regards a continuous flow process, the starting materials are continuously introduced into the reactor where the reaction takes place in contact with the active catalytic materials at a well-defined temperature, pressure and flow rate; subsequently, the products of the process leave the flow reactor.
Flow chemistry offers numerous merits in chemical synthesis that cannot be easily reduced to green metrics. The advantages are enumerated and listed below.
- Reduction of reaction times
Continuous-flow processes permit to shorten the process reaction times compared to traditional batch processes. This technology allows multistep synthesis which are very efficient in avoiding the consumption of time needed to separate and isolate the reaction intermediates.
- Waste minimization
In general, the largest contribution in the generation of waste in a chemical process is due to the solvents used for carrying on the reaction and for the purification and isolation of the products. Continuous-flow reactors are effective tools for the minimization of wastes, reaching a highest environmental and chemical efficiency. In fact, in these protocols starting materials can be add in equimolar quantities and solvents can be used in the minimum amount. Furthermore, the use of a heterogeneous catalyst helps to reduce waste since it is simply recoverable and reusable.
