水作为绿色化学中的溶剂的力量
8 12月 2023
人们对他们所做的每件事对环境的影响越来越关注，企业、政府和个人越来越意识到可持续性的必要性，因此这在他们的行动中得到了体现。化学家也不例外，他们在走向可持续未来的过程中发挥着关键作用，运用他们的专业知识在众多行业中寻找创新的绿色解决方案。然而，化学家本身通过广泛使用有机溶剂作为反应介质，产生了大量的废物。
下面我们将探讨有机溶剂造成的废物问题，以及水如何成为可持续的替代品。
有机溶剂的陷阱
有机溶剂通常对安全和健康都有害；它们往往易燃（例如丙酮）并具有皮肤刺激特性，但也可能具有致癌性（例如苯）、有生殖危害（例如2-乙氧基乙醇），或可能是神经毒素（例如正己烷）。这些特性意味着它们永远不应被排放到环境中，而应该被收集并焚烧，在这一点上它们又对温室气体排放有所贡献。这些化学物质通常也来源于化石燃料储备：全球用于无数目的的有限资源——这本身就不是“可持续”的。
Water - A Green Alternative
The search for substitute media in which to carry out these reactions, including the use of supercritical CO₂, ionic liquids and biomass-derived organics, is progressing, but each has limitations. Another option that has garnered the attention of many chemists over recent years is actually water - considered “nature’s solvent”¹, water is abundant, non-toxic, non-flammable and cheap. The physical properties of water, and how they change with temperature and pressure, allow many more reactions than are first expected to take place within this polar solvent. Water has a large dielectric constant and high polarity, which leads to the hydrophobic effect - the clustering of non-polar molecules or functionalities to reduce the polar/non-polar interfacial area. This can concentrate pockets of non-polar reactants, influencing their reaction rate, chemo- and regioselectivity². Reactions that take place in conditions like this are termed ‘on-water’, and the most well-documented example is the acceleration of the Diels-Alder reaction with water as solvent, as shown by Rideout and Breslow, 1980³. The ability to modulate the hydrophobic effect of water with the addition of different salts in salting-in and salting-out was further shown in this work. Volatile organic compounds (VOCs) have traditionally been used as solvent media for reactions such as this.
Altering the characteristics of water
The heating and pressurisation of water to subcritical or supercritical states provides further options in the use of water as a solvent, though brings safety concerns and energetic requirements that may limit the desirability and application of these conditions. These changes in state drastically change the properties of water, to have a low dielectric constant, low polarity and increased dissociation. Supercritical water is therefore a highly suitable solvent for non-polar compounds, allowing their reaction throughout the bulk reaction mixture. Further, the increased dissociation of H+ and OH- ions provide both acidic and basic catalysis, without needing to alter pH.
Surfactant Assisted ‘In-water’ Reactions
Another promising option for reaction using water as a solvent is the use of surfactants to provide better interaction between polar and nonpolar molecules in ‘in water’ reactions⁴. The Lipshutz group at UC Santa Barbara focus on improving green chemistry in synthetic organic chemistry, and one of their focuses are “designer surfactants” that can, when present in water in very limited quantities, provide nanometer micelles or so-called “nanoreactors” for non-polar reactions to take place. Lipshutz and his group have been able to undertake many big-name organic reactions using water as the solvent, including Suzuki-Miyaura, Sonogashira, Mizoroki-Heck and Negishi couplings⁽⁵⁾ and Lipshutz states: “Micellar catalysis is becoming rich with a growing toolbox of technologies that enable just about any reaction to be run in water.”⁽⁶⁾
The limited quantities of designer surfactants needed has the above-described benefit of “higher local substrate concentrations and hence, faster reaction rates” often also reducing the catalyst loadings and energy required for reactions⁴. Relatively mild conditions are actually often necessitated by the need to avoid surfactants reaching their cloud-point, where the properties of surfactant solutions change. Further, the work-up required for isolation and purification of reaction products can be majorly simplified by using in-water reaction conditions, with work up including ““in flask” extraction of the product from the aqueous reaction mixture using a single, minimal amount of a recyclable organic solvent, or by simply decanting or filtering to obtain the solid product”⁴. Other benefits of in-water reaction conditions include the ability to perform sequential reaction steps in one pot without work-up and particularly to merge sequential reactions using chemocatalysis and biocatalysis (using enzymes that may be deactivated by organic solvents)⁽⁷⁾.
A promising outlook for Water as a Green Solvent
Water appears to hold immense promise as a green solvent for the chemical industry and the environment. It is inevitable that changes will be made to current chemical practices, whether due to the reducing supply or more difficult sourcing of organic solvents or for economic as well as environmental reasons. Prioritising the development of water-based technologies would bring major benefits.
ELGA LabWater is established in the supply of high purity water for application in chemistry and related practices, with ELGA purifiers reliably and consistently providing high purity water that can be used with confidence to support the aforementioned ecologically driven changes in traditional Organic Chemistry. The environmental focus of the parent Veolia group makes ELGA the sustainable choice for high purity water in the laboratory.
Reference:
- Sachdeva H, Khaturia S. A mini-review on organic synthesis in water. MOJ Biorg Org Chem. 2017;1(7):239‒243. DOI: 10.15406/mojboc.2017.01.0004
- Zhou F, Hearne Z, Li C-J. Water – the greenest solvent overall, Current Opinion in Green and Sustainable Chemistry. 2019, 18, 118-123 DOI: 10.1016/j.cogsc.2019.05.004
- Rideout D C, Breslow R. Hydrophobic acceleration of Diels-Alder reactions. J. Am. Chem. Soc. 1980, 102, 26, 7816 - 7817 DOI: 10.1021/ja00546a048
- Cortes-Clerget M, Yu J, Kincaid J R A, Walde P, Gallou F, Lipshutz B. Water as the reaction medium in organic chemistry: from our worst enemy to our best friend. Chem. Sci. 2021, 12, 4237 DOI: 10.1039/d0sc06000c
- Lipshutz B H, Caravez J C, Iyer K S. Nanoparticle-catalyzed green synthetic chemistry… In water. Current Opinion in Green and Sustainable Chemistry. 2022, 38, DOI:10.1016/j.gogsc.2022.100686
- Lipshutz B. Chemistry in Water to the Rescue, 2023 (https://lipshutz.chem.ucsb.edu/news/announcement/406) [7] Gröger H, Gallou F, Lipshutz B H. Where Chemocatalysis Meets Biocatalysis: In Water. Chem. Rev. 2023, 123, 9, 5262-5296 DOI: 10.1021./acs.chemrev.2c00416
Dr Bethany Campbell
After completing a BA and MSc in Natural Sciences at the University of Cambridge, Bethany was awarded a PhD in Chemical and Process Engineering from the University of Surrey. During her PhD, Bethany investigated the treatment of process water from the hydrothermal carbonisation of biomass - a process for the valorisation of waste to produce hydrochar (a fossil coal alternative). Bethany is now the R&D Wastewater Specialist at ELGA, where she works to develop technologies and products for treatment of wastewaters.
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