The organic nitroxyl radical, TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), finds a variety of industrial applications for chemical transformations. Because of economic and environmental concerns, the recovery and reuse of TEMPO with maintained high activity are of the utmost importance. In this Review, we summarize the most important advances made by the scientific community in TEMPO immobilization on various organic and inorganic support materials for recovery and reuse, and we discuss the activity and stability, as well as the procedures. Also summarized is the wide range of applications of TEMPO in both homogeneous and heterogeneous forms in chemical transformations, beginning from methodology tuning in synthetic chemistry to the use in polymer chemistry.

Thiazolium and imidazolium hybrid materials were prepared by radical reactions between a mercaptopropyl-modified SBA-15 mesoporous silica and bis-vinylthiazolium or bis-vinylimidazolium dibromide salts. These hybrid materials were characterized by several techniques and were employed in the etherification reaction of 1-phenylethanol. Solvent-free conditions at 160 8C under different gas phases (oxygen, air, nitrogen and argon) were used. The thiazolium-based material displayed excellent performances. Further studies were carried out using unsupported thiazolium salts, with or without a methyl group at the C-2 position of the thiazolium moiety. These studies allowed us to propose a reaction mechanism. The supported thiazolium-based material was successfully used in the etherification reaction of two other benzylic alcohols and also in seven consecutive cycles. This work represents the first use of thiazolium-based compounds as catalysts for the etherification reaction of alcohols.Scheme 1. Synthesis of SBA-15-Imi and SBA-15-Thia.

A novel fullerene [5:1]hexakisadduct bearing two 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) radicals and ten 1-propyl-3-methylimidazolium bromide moieties has been synthesized and characterized. Such an C 60 IL 10 TEMPO 2 hybrid has been successfully employed as a catalyst in the selective oxidation of a wide series of alcohols and is highly active at just 0.1 mol% loading. Moreover, it can be easily recovered by adsorption onto a multilayered covalently-linked SILP phase (mlc-SILP) through a "release and catch" approach and reused for up to 12 cycles without loss in efficiency. Interestingly, a catalytic synergistic effect of TEMPO and imidazolium bromide moieties combined in the same hybrid has been clearly shown.Keywords: alcohols; fullerene; ionic liquids; oxidation; TEMPO; 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)In the last decades TEMPO-like catalysts have emerged as powerful, metal-free and environmentally sustainable alternatives in the oxidation of primary and secondary alcohols to the corresponding carbonyl compounds.[1] Nevertheless, their high price prompted scientists examine the development of supported TEMPO catalysts in order to obtain recyclable materials. Hence, TEMPO has been covalently immobilized onto various supports such as silica, [2] magnetic nanoparticles, [3] soluble [4] and insoluble [5] organic polymers, and carbon nanotubes. [6] However, although several of these materials were catalytically active and easily recyclable, they often suffer from the inherent drawback of heterogeneous catalysis, namely the reduced mass transfer that makes reactions slower than in the case of their homogeneous counterparts. [7] In this regard, very recently a new approach called "release and catch" has been highlighted. [8] In this strategy, the catalytic system is non-covalently immobilized on the support, but the catalytic moiety is released in solution over the course of the reaction and it is recaptured at the end of the reaction. In such a manner, the benefits of homogeneous (high catalytic activity and reaction rates) and heterogeneous catalysis (easy separation and recycling) are combined. Interestingly, the "release and catch" approach has found application in organometallic-based catalysts, [9] organocatalysts [10] and metal-based catalysts. [11] In the past few years, we have been engaged in the use of silica-supported ionic liquid-like phase monolayers (SILLP) and multilayers (mlc-SILP) as reversible catalyst reservoirs. Both kinds of materials were excellently employed for organocatalyst recovery and recycling [10,12] as well as in the immobilization of Pd nanoparticles for C À C coupling processes in batch [13] and in flow. [14] On the other hand, we started exploring new recyclable TEMPO-based systems: 10 mol% of ionic liquidtagged TEMPO derivatives were easily recovered with the help of mlc-SILP and recycled in up to 13 consecutive cycles with no loss in activity, [12d] whilst a series of fullerene-TEMPO derivatives with two, four and twelve TEMPO moieties can be quickly ...

Bis(imidazolium)-tagged 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) catalysts were adsorbed on different supports such as silica gel, silica gel modified with highly cross-linked polymeric imidazolium networks, and highly cross-linked polymeric imidazolium networks entrapping magnetic particles. These systems\ud provided a convenient tool for the oxidation of both primary and secondary alcohols working as recyclable reservoirs for the bis(imidazolium)-tagged TEMPO catalysts. By using EPR spectroscopy it was demonstrated that the catalyst was released as the corresponding oxoammonium salt in the solution during the recycling step, thus promoting the oxidative process in a homogeneous fashion. After solvent removal, the catalyst was readsorbed on the support allowing an easy recovery and recycle of the catalytic material up to 13 consecutive cycles with no loss in activity. The bis(imidazolium)-tagged\ud TEMPO catalyst could be used in only 1 mol% both for the oxidation of benzylic and aliphatic alcohols. The catalytic material was highly recyclable if used on silica or imidazolium-modified\ud silica gel in 10 mol% loading. Loading could be scaled down to 1 mol% and the catalyst proved to be recyclable up to\ud 8 cycles only with imidazolium-modified silica gel. Such a “catalyst-sponge-like” system permits to combine the benefits of homogeneous and heterogeneous catalysi

Silica based catalysts containing organic framework with both amino and sulfonic or phosphonic functions were synthesized, characterized and tested in esterification reaction of aliphatic polyalcohols as glycerol. The aim of the research was to study the relations between the structure and the activity of the materials focusing on both, the effect of the amino group and the influence of aliphatic or aromatic chain on the acidic active sites. The effects of the sulfonic and phosphonic functions and the morphology of SiO 2 , amorphous or mesostructured Santa Barbara Amorphous (SBA-15), were also addressed. The materials were compared with some commercial catalysts, showing good performances in term of activity and stability. Fresh and spent specimens were analyzed by means of thermo-gravimetrical analysis (TGA), X-ray photoelectron spectroscopy (XPS), small angle X-ray scattering (SAXS), FT-Infrared spectroscopy (FT-IR) and acid capacity measurements. The best performances in terms of glycerol conversion and selectivity to acetins were achieved in presence of the sulfonic function linked to the amino-propyl SiO 2 , while the highest stability during three catalytic cycles was registered for the analogous SBA-15 sample.