To add to Vidak Raičević's comment, in addition to Sc, Ln, Yb and Y triflates, bismuth triflate is another viable option (non-toxic).

you can also check this paper:

http://link.springer.com/article/10.1007%2Fs10562-012-0958-2#page-1

Zinc, iron, bismuth, copper compounds may be considered green

Zeolites might be the best way for the future when many transition metals become rare and prohibitively expensive. They are a proven technology in many applications. For elemental sustainability refer to this:

http://pubs.rsc.org/en/content/chapter/bk9781849736169-00001/978-1-84973-616-9#!divabstract

I was wondering about how people perceive catalysts in terms of greenness. The Kobayashi paper suggested by Vidak claims the Lewis acids are green without much evidence. Is indefinite recyclability enough, if achievable?

Would anyone be happy using a catalyst that a colleague had previously used and recovered? Could you be confident that the performance would exactly mirror fresh catalyst? Finally would anyone feel comfortable placing your recovered catalyst back on the shelf for someone else to use in their own reactions?

I wouldn't agree to any of these given current practice!

I don't think this is realistic at present, but when/if achieved it would be a promising indicator that efficient recycling had been achieved to a satisfactory standard. Recovery efficiency might be a different matter of course.

Yea

I have used PEG 400 as a Polar solvent and it is recovered and reusable for next reaction.

May be you used solid acid catalyst bearing Lewis acidic functionality, as Glucose supported sold acid bearing both types acidity, recyclable more than 7 time, high acidity.

A sustainability catalyst must be; non-toxic, heterogeneous, efficient, friendly with the environment, non costly, the process must be socially responsible.

James the attributes which you have mentioned like sustainability,recyclability,low toxicity,low adverse environmental impact the whole thing is a big complex web which the Scientific community has never focused some 15 years ago and even people thought were impossible to achieve( rather they felt there was never a need).

But the scenario has changed over the decade where many new concepts have taken shape like the concept of Green Chemistry itself (of course all these together would be called as Green Chemistry or sustainable chemistry). It is totally not a new concept,it was already there but the emphasis was not much 15-20 years ago.

There are many developments in these Green Concepts and this place would nt be sufficient enough to write it all. So I would mention one such thing which is worth mentioning, the concept of “Lewis Acid–Surfactant Combined Catalysts (LASCs) by Shu Kobayashi a dedicated green scientist. I would rather call this a breakthrough because they satisfy all the conditions you have mentioned in the case of Lewis acid catalysis like recoverability and reusability,you can use them in water,though that lanthanides are toxic not that notorious when compared to lead,Cadmium,Mercury.So far one is able to recycle them with no compromise on catalysis and conversion efficiently it is good choice,I would then say they are excellent sustainable catalysts.This is just the tip of the ice berg which I have discussed there are many such new findings and breakthroughs for example Carbocatalysis,Zeolites/Zeolite immobilized catalysts( SBA-15,all the ship in a bottle approaches,etc) Ionic liquid immobilized lewis acids,etc.

Hope I have given you the information which you require.If you need further help you can ask me and I would be very happy to reply to your queries and questions.

Good luck!

Zn/HY-Zeolite, should be prepared with minimal Zn content and calcined at T>600 C.

If you could double the yield of a process (45%to 90% for example) by replacing an aluminum catalyst with an indium catalyst would that be better than dealing with the lower efficiency offered by the more abundant (possibly 'greener') catalyst? Thoughts?

We used HPA supported on silica gel or HClO4 supported on SiO2 as catalyst. Please refer Chem. Commun. 2010, 1127; Tetrahedron 2011, 5621; Tetrahedron Lett. 2011, 5827.