Visible-Light-Induced Partial Oxidation of Cyclohexane by Cr/Ti/Si Ternary Mixed Oxides with Molecular Oxygen

Abstract

Cr/Ti/Si ternary mixed oxides (CrTiSi) were prepared by a hydrolysis of tetraethyl orthosilicate (TEOS), Cr(NO3)3, and titanium tetraisopropoxide (TTIP) followed by calcination. These oxides were used as catalysts for partial oxidation of cyclohexane with molecular oxygen under irradiation of visible light (λ > 400 nm). The CrTiSi catalysts produce cyclohexanol and cyclohexanone with high selectivity (>91%) and show much higher activity than the CrSi binary oxides. Among them, the catalyst with equimolar amounts of Cr and Ti shows the highest activity. Visible-light irradiation of the CrSi catalyst promotes a reduction of tetrahedrally coordinated Cr oxide species (T d 6+) and produces the excited state (T d 4+*), which behaves as an active site for oxidation. In contrast, the CrTiSi catalyst contains tetrahedrally coordinated Cr and Ti oxide species that are connected through the Cr–O–Ti bond. The photoformed T d 4+* species is strongly stabilized due to delocalization of excited electrons on the Cr–O–Ti species. This suppresses rapid deactivation of T d 4+* and results in enhanced photocatalytic activity.

Iron Aquo Complex as an Efficient and Selective Homogeneous Photocatalyst for Organic Synthetic Reactions

“…We discussed the mechanism for the photocatalytic cyclohexane oxidation on the [Fe III (H 2 O) 6 ] 3+ homogeneous photocatalyst. In the photocatalytic oxidation of cyclohexane on microporous zeolites or mesoporous silicas with isolated tetrahedrally coordinated transition metals linked in the silica matrices (so called single‐site photocatalysts), such as Cr 6+ ‐containing porous silicas, it was proposed that excitation of the Cr 6+ species produced the excited Cr via electron transfer from the terminal O 2− into the Cr 6+ species, and the resulting electrophilic O − attracted the H atoms of cyclohexane (Cy) to produce a complex of the excited Cr and Cy, which then reacted with molecular O 2 to form the partially oxidized products . For the present [Fe III (H 2 O) 6 ] 3+ , a similar mechanism should be proposed; excitation of the [Fe III (H 2 O) 6 ] 3+ produces the excited Fe 2+ via electron transfer from the coordinated O 2− (in H 2 O) into the Fe 3+ and the resulting electrophilic O − is attributable to formation of the intermediate complex like [Fe III (H 2 O⋅⋅⋅Cy)(H 2 O) 5 ] 3+ (Figure S1).…”

“…Remarkably, the Fe(III)@SBA heterogeneous photocatalyst showed the improved yield of up to 160 μmol (100 TON, cyclohexane conversion of 1.2 %) while maintaining nearly 100 % selectivity with the same amount of Fe(III) (Figure ). To the best of our knowledge, this activity of Fe(III)@SBA is by far the highest ever reported for heterogeneous photocatalytic partial cyclohexane oxidation with molecular O 2 under irradiation of visible and/or solar light . The enhancement of the photocatalytic activity of the Fe(III)@SBA was explained by the fact that SBA‐15 can effectively concentrate cyclohexane from the acetonitrile solution containing cyclohexanone and cyclohexanol (Figure ).…”

“…One such reaction is the partial oxidation of cyclohexane into cyclohexanol and cyclohexanone, which are important intermediates of adipic acid and ϵ‐caprolactam that are used in the manufacture of nylon‐66 and nylon‐6, respectively . Recently, effort has been directed to develop processes to convert cyclohexane to the partially oxidized products using heterogenous photocatalysts including TiO 2 . However, only low yield and selectivity have been frequently obtained so far.…”

“…[11] Recently, effort has been directed to develop processes to convert cyclohexane to the partially oxidized products using heterogenous photocatalysts including TiO 2 . [12][13][14][15][16][17][18][19] However, only low yield and selectivity have been frequently obtained so far. Thus, effective and selective partial oxidation of cyclohexane using homogeneous iron photocatalysis is significant and a formidable challenge.…”

Titanium Oxide-based Photocatalysts for Selective Organic Transformations

“…Sequential reaction of 1,2,4-trihydroxybenzene on the catalysts, therefore, scarcely occurs and results in high selectivity. These photocatalytic systems en- , V 34) , and Cr _ O _ Ti bimetallic species 35) are also effective for selective organic transformations. In particular, Cr oxide species are very effective for partial oxidation of cyclohexane.…”