Recently a colleague and I were debating the reality of a fuzzy concept referred to as “green chemistry”. Being in the specialty chemical business, as opposed to the commodity chemical business, our discussion was naturally biased to the medium batch reactor scale. This is no trivial distinction. Raw material consumption and side product streams from commodity continuous-process trains can be astronomical in comparison to batch reaction operations.
There was the suggestion that the whole concept was absurd and was just the latest incarnation of a tree hugging export from CA. I’m a little bit more circumspect about it, but one thing is obvious- economics will be the driver of any green process changeover.
I’m not an expert in the green chemistry field, but some of the concepts seem clear and highly desirable. The goal is to minimize the total chemical insult to the environment. To achieve this, a green process has to be as atom efficient as possible in the assembly of the target product, recycle solvents to the greatest extent possible, avoid all toxic metals (Pb, Hg, Cd, Cr(VI), etc., metals are forever), eliminate fugitive emissions, and probably 6 or 8 other things I can’t think of right now.
As far as I know, certain metals are already on the path to extinction as reagents in chemcial processing- mercury and lead are the obvious ones. The battery industry still uses some unfortunate metals, mostly because of the reality of the electromotive series. There are only so many electrode combinations that are feasble for commercial batteries.
One of the obvious approaches that will get us to a greener chemistry is the continued adoption of catalyzed processes. I’m not thinking about acid protons, I’m thinking of highly selective transition metal catalysts. Synthesis chemistry is about managing reactivity through the choice of appropriate functional groups and the sequence in which they appear. The chemist has to contend with the inverse relationship of reactivity and selectivity. Catalytic reactions form reactive intermediates from otherwise docile functional groups (olefins, boronic acids, or aryl halides, for example) and bring them together into close proximity in the coordination sphere of the metal. This is a kind of tuned reactivity management that reacts functional groups that, absent the catalyst, are relatively inert. All kinds of coupling reactions come to mind- the Suzuki coupling, etc.
Now, to be fair, to get a substrate suitably functionalized for a green transformation might require some brutish and not-so-green chemistry- preparation of specialty aryls, acetylenes, and olefins; borylation reactions; the chemistry needed to make these whizbang ligands for Pd; and, well you get the point. The final greenness has to be measured as the sum of all the green steps from some common level, if not the oil well in Kuwait itself.
Catalysis has the benefit of allowing the activation of relatively inert functionalities for subsequent transformations. Rather than making bulk quantities of highly reactive species, a catalyst can generate it in situ and do the deed straight away. That is certainly in the direction of green.
Lamentations on Chemistry 
I think this whole “Green” chemistry thing is just another buzz word that was thought up to ensure that crappy papers have a chance of getting published. I hate to be a bit cynical, but I have see more obvious transformations published in the past 3 years simply because it was run in water, and is hence “green”, than I can count.
Milo makes a good point. Just running a reaction in water (or an RTIL) does not necessarily constitute an innovation, but it still might be an improvement. The aq waste streams still have to be dealt with. Often the aq streams are sent to cement calcining units where any stray BTU’s are appreciated. But on the other hand, doing large scale chemistry in water may minimize the amount of flammable organics that have to be kept on site. A 10,000 gallon bullet tank full of THF is a serious thing to have on your site. To the extent that you can reduce the flammables loading per sq ft, the better off you’ll be in an accident.
We all focus on innovations and novelty. But sometimes all we want is some improvement that saves money and reduces waste and reactor operating hours. The solution to the problem is usually very mundane. Greenness is achieved by lowering the energy and material inputs to a process. It may also be achieved by increasing yields. The more mass you channel into product, the less that has to be hauled off aws waste. At least in principle.
Hi there,
I’m over at The Green Chemistry Technical Blog
Check it out – and if you fancy joining up for posts then let me know!
On the whole issue about “green chemistry” being simply a “Buzz word”, it is relatively easy to look narrowly at it, but if you dig a little deeper you’ll find that alot of green chemistry is “applied-chemistry”. Green Chemistry incorporates many different branches of thinking from outside the “traditional realms” of chemistry, those that I view as being rather institutionalised and really are lacking innovation – I’ve seen many many reactions that haven’t been optimised or changed for over 100 years, or give no consideration to environmental, economic or social consequences. Doing improvements through “Green Chemistry” encourages “lateral-thinking” – an aid to innovation.
To say that it allows “crappy papers” to be published is wrong and if you knew enough about the area, you would know where the better sources of information are. The relevant dedicated scientific journals are also peer-reviewed, just like any other journals! I’d also urge you to attend a relevant “green chemistry” conference – you will see that these are joined by very able scientists/economists and social scientists!
I’d urge you to look at some new EU regulations such as REACH and WEEE and others that will only simply encourage the adoption of Green Chemistry.
I’d also suggest you look at some of my previous posts on the subject – although they’re not exhaustive on the whole issues – they give some interesting pointers on the benefits of this system.
Indeed, I look forward to working with other specialised “synthetic organic chemists”, “inorganic chemists” and chemical engineers – since we all bring different things to the table! I’m very keen to learn the best practices from each!!
Best wishes on learning about “Green Chemistry” and how it really is here for the long term,
Mark