دانلود رایگان مقاله سنتز سبز فوتوکاتالیستی ویتامین B3 توسط فوتوکاتالیست پلاتین بار TiO2

Abstract

Selective photocatalytic oxidation of 3-pyridinemethanol to 3-pyridinemethanal and vitamin B3 by using pristine and Pt loaded home prepared (HP) rutile and commercial TiO2 photocatalysts, under UV, UV–vis and visible irradiations in water, was performed in friendly environmental conditions. The photocatalysts were characterized by XRD, SEM-EDAX, BET, DRS, XPS and TGA techniques. The influence of pH on reactivity and total selectivity to 3-pyridinemethanal and vitamin B3 was studied. Under very acidic conditions (pH = 2) no or low activity (depending on photocatalyst) was observed, whereas by increasing the pH from 4 to 12 very high total selectivity was achieved. The Pt loading was beneficial for selectivity whereas the reactivity was positively affected only for crystalline HP sample. This last sample showed good activity under visible irradiation, exhibing an about 4 times higher conversion than the other samples. The influence of the position of the benzylic group in pyridine (2-pyridinemethanol and 4-pyridinemethanol) was also studied. The results showed that to synthesize vitamin B3 in green conditions the photocatalyst should be poorly crystalline or Pt loaded.

4. Conclusions

Selective photocatalytic oxidation of 2, 3 and 4- pyridinemethanol to corresponding aldehydes and acids has been performed in water and aerated conditions using Pt loaded HP and commercial TiO2 samples. TiO2 photocatalysts in different crystal phase, particle size, crystallinity, specific surface area, and amount of surface hydroxyl groups were used for the synthesis reaction and the obtained results are compared each other. Pt loading, suspension pH and light source effects on the reactivity and selectivity were investigated. The influence of the ortho and para position of the benzylic group in pyridine was also studied. The highest vitamin B3 selectivity (75%) for 50% conversion was found with HPRT at pH 12 and under UV irradiation. In a 24 h run carried out at the same conditions the vitamin B3 yield was ca. 90%. The obtained results show that (i) in order to synthesize vitamin B3 in green conditions, the photocatalyst should be poorly crystalline or Pt loaded; (ii) nitrogen atom in the aromatic ring increases the stability of pyridinemethanoic acids; (iii) the product stability in the experimental conditions plays an important role in the selectivity; (iv) Pt-HPRT-400 catalyst showed significant activity under visible irradiation and (v) pH is very important parameter for both activity and product selectivity.