Mario Bressan

One-pot synthesis of lignin-stabilised platinum and palladium nanoparticles and their catalytic behaviour in oxidation and reduction reactions

A one-pot green method to synthesise Pt and Pd nanoparticles is reported. Two natural aromatic polymers, lignin and fulvic acid, were used as both reducing and stabilising agents at moderate temperature (80 °C) in water and under aerobic conditions. Full characterisation was performed using TEM, UV-vis, XRD, 195Pt and 1H NMR, FT-IR and GC-MS techniques. In the TEM images, we observed spherical nanoparticles of diameters in the range of 16 nm to 20 nm, in the case of Pd, and smaller ones of not so well defined shapes for Pt. GC-MS of the organic fractions formed during the preparation of the nanoparticles showed defined amounts of vanillin, a well known degradation product of these polymers. This finding indicates that the active participation of lignins and fulvic acids in the metal reduction step. The catalytic activity of the nanoparticles was tested for the NaBH4 reduction of 4-nitrophenol and for the aerobic oxidation of alcohols, reactions that are always conducted under green conditions. Both Pt and Pd nanoparticles show good catalytic activity in the reduction reaction, while in the aerobic oxidation reaction only the Pt nanoparticles were effective.

Ruthenium-catalyzed oxygenation of saturated hydrocarbons by t-butylhydroperoxide

Abstract Saturated hydrocarbons such as adamantane, cyclooctane, cyclohexane, hexane and heptane are oxygenated by t-butylhydroperoxide (TBHP) or hypochlorite in the presence of the homogeneous catalysts K5[Ru(H2O)PW11O39] and cis-[Ru(H2O)2(dmso)4](BF4)2. With the latter a free-radical mechanism appears to dominate when TBHP is employed, thus accounting for the remarkably high rates of alkane conversions (up to ca. 8 turnovers per minute). Hypochlorite oxygenations proceed via oxo—metal species.

Oxidation of dibenzothiophene by hydrogen peroxide or monopersulfate and metal–sulfophthalocyanine catalysts: an easy access to biphenylsultone or 2-(2′-hydroxybiphenyl)sulfonate under mild conditions

A catalytic system consisting of metal–sulfophthalocyanines (MPcS) and monopersulfate or hydrogen peroxide as oxidants was effective in the dibenzothiophene oxidative desulfurization with various yields and selectivities. Oxidations were conducted at room temperature in acetonitrile–water mixed solvent. The dibenzothiophene oxidation involved the step by step formation of dibenzothiophene dioxide and biphenylsultone (dibenzo-1,2-oxathiine 2,2-dioxide), followed by hydrolysis to 2(2′-hydroxybiphenyl)sulfonate and finally catalytic desulfurization to 2-hydroxybiphenyl (2-phenylphenol) and sulfuric acid; all the intermediate compounds were identified. Moreover, catalytic over-oxidation of 2-hydroxybiphenyl, with ring fission and formation of various oxidation products, among them carbon dioxide, oxalic and benzoic acid, was also observed. Among the various MPcS catalysts examined (M = Fe, Co and Ru), the ruthenium derivative exhibited the best performances with persulfate and iron derivative with hydrogen peroxide; in both cases the slow step of the process consisted in the oxidation of dibenzothiophene dioxide to biphenylsultone.

Deoxydehydration of glycerol to allyl alcohol catalyzed by rhenium derivatives

The deoxydehydration (DODH) of glycerol is effectively catalyzed by rhenium derivatives, either in neat glycerol or in the presence of solvents (in particular alcohols), in air or under hydrogen bubbling. Methyltrioxorhenium (MTO) and ReO3 were the only rhenium catalysts tested that can selectively catalyze the DODH reaction at very low temperatures (140 °C). The presence of oxygen is not necessary, although under nitrogen the reaction requires higher temperatures to occur. On the other hand, the presence of hydrogen often noticeably increased the selectivity versus allyl alcohol formation, reaching the considerable value of 90% in the case of reaction conducted in 2,4-dimethyl-3-pentanol with ReO3. The DODH reaction always exhibits a definite induction time that, in the case of MTO, corresponds more or less to the time required for its demethylation. Metal catalysts in both high – likely rhenium(VI) – and low oxidation states are involved. Re-addition of fresh glycerol at the end of the reaction indicates the feasibility of the reuse of the catalysts.

Ruthenium-catalyzed oxidative dehalogenation of organics

Abstract Water-soluble ruthenium(II) complexes are effective catalysts for the deep oxidation of chlorinated organics in the presence of hydrogen peroxide or mono-persulfate at room temperature. Reactions are conducted either in nitromethane–water two phase or in water–acetonitrile mixtures or in water alone, in the presence of a surfactant agent (if the case) with the ruthenium(II) catalysts [Ru(H 2 O) 2 (dmso) 4 ](BF 4 ) 4 , [RuCl 2 (dmso) 4 ] or [RuPcS] (dmso=dimethylsulfoxide; PcS=tetra-sulfo-phthalocyaninate). The oxidation of various chlorinated organics (chloro, bromo-, iodo- and nitro-benzene, polychlorobenzenes, polychlorophenols) was followed by monitoring the nature and the relative amounts of the final products: chlorinated substrates are often converted into hydrochloric acid and carbon dioxide. Factors such as solvent and oxidant affect the reactions, the most favorable conditions being achieved in aqueous media. Substituted benzenes are oxidized via an initial electrophilic attack followed by a series of faster steps, whereas with polychlorophenols, which are more sensitive to oxidation than substituted benzenes, the reaction is also radical in character.

Catalytic aerobic oxidation of allylic alcohols to carbonyl compounds under mild conditions

A new catalytic aerobic oxidation of alcohols to aldehydes under green conditions was developed (room temperature and pressure, water solution, open vials). The water-soluble platinum(II) tetrasulfophthalocyanine (PtPcS) catalyst showed the best selectivity for carbonyl derivatives, and in particular for α,β-unsaturated alcohols; the reactions are slow.

Direct synthesis of adipic acid by mono-persulfate oxidation of cyclohexane, cyclohexanone or cyclohexanol catalyzed by water-soluble transition-metal complexes

A catalytic system consisting of water-soluble metal sulfophthalocyanines (MPcS) or various ruthenium complexes and mono-persulfate as the oxidant was effective in the oxidation of cyclohexanone, cyclohexanol and cyclohexane to adipic acid with different yields and selectivity. Oxidations were conducted at room temperature and under atmospheric pressure in aqueous media (or, in the case of cyclohexane, in a water–neat substrate double phase). The oxidation of cyclohexanol involved step-by-step formation of cyclohexanone, e-caprolactone and 6-hydroxyhexanoic acid, all of which have been identified in the reaction mixtures; in selected cases moderate over-oxidation of adipic acid to glutaric and succinic acid was also observed. Various MPcS catalysts were examined (M = Fe, Co, Ni, Cu and Ru), and the ruthenium derivative exhibited the best performances in terms of rate and selectivity. Mono-persulfate was found to be a more convenient oxidizing reagent than hydrogen peroxide; related patterns were observed when H2O2 was used, however extended dismutation of the oxidant limited the overall yields. Cyclohexane underwent slow oxidation when reacted with persulfate (water–substrate double phase) in the presence of the water-soluble metal catalysts; adipic acid was selectively produced (95%) in the presence of RuPcS catalyst with yields as high as 21% (48 h). The catalytic performance of simpler ruthenium derivatives, such as [RuCl2(DMSO)4] (RuDMS) and K5[Ru(H2O)P11O39] (RuPW), was also examined for comparison purposes. A kinetic scheme for cyclohexane oxidation is proposed.

Ruthenium sulfophthalocyanine catalyst for the oxidation of chlorinated olefins with hydrogen peroxide

Abstract In aqueous solution and at room temperature, various α-chloro-alkenes are effectively dechlorinated by hydrogen peroxide oxidation using a water-soluble ruthenium(II)-tetrasulfophthalocyanine catalyst, RuPcS. The molecular structure of RuPcS has been elucidated by ESI-mass spectroscopy. In the reaction conditions, and specifically in acidic media, the complex rapidly gives rise to a novel species, most likely catalytically active, whose nature is investigated.

Oxidation of C1–C4 alcohols by iron- and ruthenium-sulfophthalocyanine precatalysts with hydrogen peroxide or mono-persulfate in water

Abstract A catalytic system consisting of iron- or ruthenium-sulfophthalocyanine and hydrogen peroxide or mono-persulfate was effective in the oxidation of simple primary and secondary alcohols as well as of simple ketones. The oxidation reactions were conducted in aqueous media with turnover rates, defined as moles of product per mole of catalyst per minute, up to 5. Primary alcohols, including methanol, were selectively oxidized into the corresponding carboxylic acids. Secondary alcohols were transformed into the corresponding ketones, which were found to undergo further oxidation to esters via Baeyer–Villiger reaction, followed by hydrolysis or alternatively in the case of acetone via direct oxidation to acetic acid and CO 2 . Moreover, t -butyl alcohol was also found to be slowly oxidized into acetone and methanol. Analysis of the oxidation reaction of cyclobutanol indicated an ionic mechanism; no deuterium kinetic isotope effect was measured in the cases of methanol and ethanol. The mechanistic origin of the catalytic efficiency is also discussed.

Competitive catalytic epoxidation and oxidative cleavage of stilbene by ruthenium complexes

Abstract The catalytic oxidation of stilbenes, cis - and trans -, by iodosobenzene in the presence of the diphosphino-complexes of ruthenium(II) [RuCI(LL) 2 ]PF 6 (LL = 1,3-bis(diphenylphosphino) propane, DPP; 1-diphenylphosphino-2-(2′pyridyl)ethane, PPY) gives rise to stilbene epoxides, cis - and trans -, and to benzaldehyde with distinctly different kinetic pathways. The reaction is first-order to the catalyst fpr the epoxidation and second-order for the oxidative cleavage, whereas the rate dependence upon substrate concentration indicates reversible formation of a common catalyst-substrate intermediate.