A thermo-responsive polymer such as poly(N-isopropylacrylamide) (PNIPAAM) was covalently grafted on the external surface of halloysite nanotubes (HNTs) by means of microwave irradiation. This nanomaterial was used as a support and stabilizer for palladium nanoparticles. The obtained HNT–PNIPAAM/PdNPs was characterized by means of TGA, SEM, EDS and TEM analyses. The palladium content of the catalyst was estimated to be 0.4 wt%. The stability of the catalytic material at different temperatures (below and above the PNIPAAM lower critical solution temperature) was tested in the Suzuki reaction under microwave irradiation. In addition, TEM analysis after five consecutive runs was performed. The catalyst showed a good catalytic activity toward the Suzuki cross-coupling reaction between phenylboronic acid and several aryl halides in aqueous media under microwave irradiation and low palladium loading (0.016 mol%, 8 μg of Pd). Turnover numbers (TONs) and frequencies (TOFs) up to 6250 and 37 500 h⁻¹, respectively, were reached. The catalyst was easily separated from the reaction mixture by centrifugation and reused for five consecutive cycles with a small drop in its catalytic activity.

The aim of this Perspective is to start a discussion about the real usefulness of more or less sophisticated catalytic systems for the Knoevenagel reaction with malononitrile, a reaction that can take place under mild conditions without the need of a catalyst. From a sustainable viewpoint the questions are: Is it useful to increase the rate of a reaction that already occurs under mild conditions? Is it useful to spend resources and time in designing, characterizing and realizing complex catalytic systems for such reaction? Does it make sense to carry out the reaction under conditions such as to have a slower reaction rate and therefore to find a catalyst that is able to increase it?