Giuseppe Silvestri

Electrochemical oxidation of organics in water: Role of operative parameters in the absence and in the presence of NaCl

The electrochemical oxidation of organics in water was investigated theoretically and experimentally to determine the role of several operative parameters on the performances of the process in the presence and in the absence of sodium chloride. Theoretical considerations were used to design the experimental investigation and were confirmed by the results of the electrochemical oxidation of oxalic acid (OA) at boron doped diamond (BDD) or IrO(2)-Ta(2)O(5) (DSA-O(2)) anodes in a continuous batch recirculation reaction system equipped with a parallel plate undivided electrochemical cell. Polarization curves and chronoamperometric measurements indicated that, in the presence of chlorides, the anodic oxidation of OA is partially replaced by an indirect oxidation process. This result was confirmed by electrolyses experiments that show that, in the presence of suitable amount of chlorides, oxidation of OA takes place mainly by a homogeneous process. Interestingly, a very different influence of the nature of the anodic material, the flow rate and the current density on the performances of the process arises in the absence and in the presence of chlorides so that optimization of the two processes requires very different operative conditions. In the absence of chlorides, high current efficiency (CE) is obtained at BDD when most part of the process is under charge transfer controlled kinetics, i.e. when low current densities and high flow rates are imposed. On the other hand, in the presence of NaCl, higher CE are generally obtained at DSA anode when high current densities and low flow rates are imposed, i.e. when a high concentration of chemical oxidants is obtained as a result of the chloride oxidation. The effect of other operative parameters such as the OA concentration and the pH were further investigated.

Electrochemical reduction and carboxylation of halobenzophenones

Abstract The electrochemical reduction of a series of halogenated benzophenones XC6H4COC6H4Y (1) was studied in aprotic solvents, in the absence and presence of CO2, by cyclic voltammetry (CV) and controlled-potential electrolysis. The mechanism of electroreduction in dimethylformamide (DMF) has been investigated in some detail. Most of the compounds undergo reductive carbonhalogen bond cleavage in the time window of CV. The radical anions derived from difluorobenzophenones (X=Y=4-F; X=2-F, Y=4-F) as well as from the 3-chloro derivative are quite stable with a cleavage rate constant (kc) of the order of 10−2 s−1 or less. With 4-chloro-, 2-chloro-, 4,4′-dichloro- and 2,4′-dichorobenzophenones, kc of 1 − has values in the range 4–254 s−1 in DMF. In the presence of CO2 all radical anions react quite rapidly with CO2, carboxylation of 1 − being much faster than dehalogenation. Controlled-potential electrolyses were carried out in DMF, MeCN and N-methyl-2-pyrrolidinone (NMP) in an undivided cell using a compact graphite cathode and an aluminium sacrificial anode. The principal product under these conditions was always the α-hydroxyacid XC6H4C(OH)(CO2H)C6H4Y. In NMP under 1 atm of CO2, acid yields around 90% were obtained except in the case of 2-chloro- and 2,4′-dichlorobenzophenone, where acid yields of 58 and 68% were obtained, respectively.

Electrochemical metallization at the liquid–liquid interfaces of non-miscible electrolytic solutions

A direct current applied across the liquid-liquid interphase of a heterogeneous, unreactive Cu2+–[V(CO)6]– redox system, causes deposition of a copper layer at the interface.

Electrochemical carboxylation of aldehydes and ketones with sacrificial aluminum anodes.

Abstract The electrocarboxylation of aldehydes to the corresponding α-hydroxyacids was described as impossible with conventional electrochemical systems. It is reported here that it is possible to realize it in diaphragmless cells making of sacrificial aluminum anodes. The method can be used also for the electrocarboxylation of ketones with good yields.

Electrochemical carboxylation of benzal chloride

Abstract The clectrocarboxylation of benzal chloride to -chlorophenylacetic and phenylmalonic acids is realized in diaphragmless cells with aluminium sacrificial anodes. Yields respectively up to 50% and 30% can be obtained. Phenylacetic acid is always present among the products.

Electrochemical carboxylation of organic substrates. Synthesis of carboxylic derivatives of acenaphthylene

The electrochemical carboxylation of acenaphthylene totrans-acenaphthene-1,2-dicarboxylic acid and acenaphthene-1-carboxylic acid has been investigated. The yields of the synthesis are influenced by the cathode material by the current density and, in the case of acenaphthene-1-carboxylic acid, by the pressure of carbon dioxide. In preparative experiments, performed in a ‘gas lift’ electrochemical cell, up to 100 g oftrans-acenaphthene-1,2-dicarboxylic acid were obtained at cathodic current densities of 40 mA cm−2 and with current yields greater than or equal to 80%.

The electrochemistry of carbon monoxide reductive cyclotetramerization to squarate anion

Abstract The electrochemical reduction, of carbon monoxide to C4O2−4 squarate anion is investigated. Particular attention is devoted to the influence that solvents, electrolytes, electrodes and carbon monoxide pressure have on the yields. The main feature appears to be that strictly controlled conditions are not necessary for cyclotetramerization. In dmf — Bu4NBr the following yields vs reacted CO and circulated charge were obtained with different cathodes: Pt(49%, 34%); Au(48%, 29%); Al(47%, 38%); stainless steel (36%, 33%); graphite (39%, 31%). A distinct influence of the supporting electrolyte (inorganic and tetraalkylammonium halides) was however observed.

Catalytic systems based on transition metals for the carbonylation of methanol to dimethylcarbonate

Abstract The results of an investigation on catalytic systems arising from the electrochemical activation of salts and complexes of several transition metals for the carbonylation of methanol to dimethylcarbonate (DMC) are reported. Metals were tested as inorganic salts or complexes with ligands such as 2,2′-bipyridine (bipy), 1,6-bis (2-oxyphenyl)-2,5-diaza-1,5-hexadiene (salen), 2,4-pentanedionate (acac), triphenylphosphine (TPP) in methanol at room temperature saturated with carbon monoxide at atmospheric pressure. Best faradic yields vs DMC were observed as follows: CuCl(bipy): 84.8%; PdCl 2 (bipy): 64.0%; CoCl 2 : 26.0%; RhCl 3 : 25.0%; AgBF 4 (bipy): 10.2%; AuCl 3 : 9.4%; Cr(acac) 3 : 6.7%; PtCl 2 (bipy): 2.1%; NiCl 2 : 1.8%. The influence of the ligand on faradic yields has been also investigated.

Electrochemical activation of transition metal complexes for the carbonylation of methanol to dimethylcarbonate

Abstract The preliminary results on the electrochemical activation of catalytic systems based on copper and cobalt complexes for the carbonylation of methanol to dimethylcarbonate at room temperature and atmospheric pressure are reported. Faradic yields up to 70%, with very high selectivity towards dimethylcarbonate, were observed with CuCl(2,2′-bipyridine). Positive results were also obtained, although with poorer catalytic activity, with other complexes such as CuCl(tetramethylethylenediamine), [Co(1,6-bis(2-oxyphenyl)-2, 5-diaza-1,5-hexadiene)]0.5 H2O and Co(2,4-pentanedionate)2.

Electrochemical reduction of vanadium salts under carbon monoxide. Synthesis of tetrabutylammonium hexacarbonylvanadate(–L)

Vanadium(III) complexes can be carbonylated by electrochemical reduction in anhydrous solvents saturated with CO at low and medium pressures. With V(acac)3 as substrate, tetrabutylammonium hexacarbonylvanadate(–L) is obtained in pyridine, with conversion yields of up to 82%. A simplified process to obtain hexacarbonylvanadium is described.