An evaluation of the application of a TiO₂/ Cu(II)/solar simulated radiation system for selective oxidation of benzyl alcohol derivatives

Abstract

BACKGROUND: The conversion of hydroxybenzyl alcohols, nitro‐benzyl alcohol and methoxybenzyl alcohols into the corresponding aldehydes is attempted in aqueous solution by Cu(II) (which reduces to Cu(0)), at room temperature, under acidic and deaerated conditions, using TiO2 solar simulated light photocatalysis. RESULTS: Under the experimental conditions adopted, the yields of the corresponding hydroxyaldehydes, when Cu(II) was completely reduced to Cu(0), were 21.6% for 4‐hydroxybenzaldehyde, 11.5% for 3‐hydroxybenzaldehyde and 20.2% for 2‐hydroxybenzaldehyde. Higher conversions of 2‐methoxybenzyl alcohol and 4‐methoxybenzyl alcohol were recorded relative to unsubstituted benzyl alcohol. A selectivity of 5.2%, at 50% conversion of the substrate, was observed for the oxidation of 4‐nitrobenzyl alcohol to 4‐nitrobenzaldehyde. For high degrees of aromatic alcohols conversion, oxidation of the generated aldehydes to the corresponding benzoic acid derivatives is observed. CONCLUSION: The introduction of substituents into the aromatic alcohols structure changes the photocatalytic oxidation rate and product selectivities with respect to that previously observed for unsubstituted benzyl alcohol. In particular, the presence of both electron donating (hydroxy, methoxy groups) and electron withdrawing (nitrogroup) groups on the aromatic ring of the substrate causes a detrimental effect on the selectivity of the process with respect to the case of benzyl alcohol. © 2012 Society of Chemical Industry

Selectivity Enhancement in Heterogeneous Photocatalytic Transformations

“…270 Similar to the aforementioned facts, the presence of both electron donating (−OCH 3 , −OH) and electron withdrawing (−NO 2 ) groups on the aromatic substrate obviously decreases the photocatalytic selectivity. 270 For the oxidation of 4-(nitrophenyl) methyl sulfide on mpg-C 3 N 4 , the reaction is faster due to the strong interaction between −NO 2 and the surface active site. 258 The transformation of all the benzylic amines examined shows high selectivity to deliver the imine products.…”

“…When Cu­(II) is added into the reaction solution (TiO 2 /Cu­((II)/solar h ν ), the substituent groups in the aromatic rings also play important role in the conversion rate and selectivities . Similar to the aforementioned facts, the presence of both electron donating (−OCH 3 , −OH) and electron withdrawing (−NO 2 ) groups on the aromatic substrate obviously decreases the photocatalytic selectivity . For the oxidation of 4-(nitrophenyl) methyl sulfide on mpg-C 3 N 4 , the reaction is faster due to the strong interaction between −NO 2 and the surface active site .…”

“…Recent studies demonstrate that the use of Cu­(II) ions as electron acceptor instead of O 2 under deaerated conditions, lead to an increase of selectivity for aldehyde formation as exemplified in the selective conversion of 4-nitro-benzyl alcohol to 4-nitrobenzaldehyde; Cu­(II) is completely reduced to Cu(0) by photogenerated electrons after the reaction. , Cu­(II) could be recovered and reused, via an aerial reoxidation of Cu(0), produced during the photolytic run in the dark . The oxidation rate of BzA is greatly affected by the original Cu ions concentration, light source, and temperature.…”

Selective photooxidation of ortho-substituted benzyl alcohols and the catalytic role of ortho-methoxybenzaldehyde

“…Homogeneous photocatalysis has been also deeply investigated during the last decades for carrying out reactions such as water splitting, degradation of harmful contaminants and organic syntheses [14][15][16][17][18][19][20]. On the last field, however, a limited number of studies on the selective oxidation of alcohols has been done, although this photoreaction has been deeply investigated in heterogeneous systems containing TiO 2 -based catalysts [13,[21][22][23].…”

Photocatalyst Nanomaterials for Environmental Challenges and Opportunities

“…In principle, catalyzed reactions energized by natural sunlight can be used to clean water, remove toxic chemicals, and create fuel and synthon stock and can be achieved using titanium dioxide particle systems [1]. Photoactive properties of titanium dioxide particles such as super-hydrophilicity, self-cleaning, and anti-bacterial qualities, were enhanced by doping with novel metals (Pt, Pd, Au, Ag, Cu, or Ni) and carbon nanomaterials (carbon nanotubes, graphene) [2][3][4][5][6][7][8]. Further tailoring of TiO 2 nanoparticle systems is under development to be used for air and water pollution removal for industrial and defense purposes.…”