Green photocatalytic synthesis of vitamin B3 by Pt loaded TiO2 photocatalysts

Authors dedicate this article to the retirement and to the career of Prof. Vincenzo Augugliaro (Palermo University, Italy).
https://doi.org/10.1016/j.apcatb.2016.09.063Get rights and content

Highlights

  • Photocatalytic green synthesis of vitamin B3 was investigated.

  • Significant amount of vitamin B3 was obtained in basic conditions.

  • Pt-loaded catalysts are very efficient for vitamin B3 synthesis in water.

  • Pt-HPRT-400 catalyst showed significant activity under visible irradiation.

  • Reactivity and product selectivity are very influenced by pH.

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.

Introduction

Heterogeneous photocatalysis by TiO2 fulfils the principles of Green Chemistry as the experiments are generally carried out in water at ambient temperature and under sunlight by using oxygen from air as the oxidant [1]. A semiconductor such as TiO2 is very suitable for green photocatalysis as it is very effective, cheap and stable to photocorrosion [2]. Using oxygen from air without doing any purification is very simple, economic and hazardless. Water is also very cheap, sustainable and un-flammable solvent [3]. By considering that fossil fuels will end near future, using sunlight as energy source becomes a very important challenge for mankind. In addition, working at room temperature and pressure decreases energy requirement and costs for reactor security.

Heterogeneous photocatalysis has been mainly employed to oxidise organic and inorganic pollutants in liquid or gaseous phases [2]. The oxidation reaction in water, due mainly to primary oxidant species as hydroxyl radicals, has been considered unselective and therefore synthesis reactions generally have been carried out in organic solvents [4]. Photocatalytic selective oxidation of benzyl alcohol derivatives [5], 5-(hydroxymethyl)-2-furaldehyde [6], amines [7], piperonyl alcohol [8], trans-ferulic acid [9], isoeugenol [10], and glycerol [11] or selective cyclization of aromatic acids [12] are examples of synthetic reactions performed in water.

It is well known that Pt loading of TiO2 photocatalysts increases the activity both under UV and visible irradiation [7], [12]; in fact Pt loading decreases the electron-hole recombination rate and therefore the reaction rate increases. By using metal loaded TiO2, photocatalytic reactions under visible or sunlight irradiation have been generally performed for degrading harmful compounds [13], some investigations having been also carried out to perform organic syntheses in organic solvents [14]. For instance photocatalytic oxidation of aniline to nitrosobenzene has been selectively (90%) performed by using Pt-loaded TiO2 in toluene and under visible irradiation [14c].

Vitamin B3 (pyridine-3-carboxylic acid), whose world production is around 35,000 tons per year [15], is generally used in the prevention and treatment of pellagra disease. Industrially vitamin B3 and other pyridine carboxylic acids are produced at high pressure by oxidation of picolinic isomers using nitric acid, permanganate or chromic acid and vanadia-titania-zirconia oxide supported catalysts [15]. Just three research papers were published on the photocatalytic selective oxidation of 3-pyridine-methanol [16], [17] and its derivatives [15] to their corresponding aldehydes and acids. These reactions were performed in water, under acidic conditions (pH 1–4) [15], [16], [17] and using commercial TiO2 [15], [17] and TiO2-graphen-like [16] composite photocatalysts. In addition, the reactions were performed under de-aerated conditions using cupric ions as electron acceptor in order to minimize the electron-hole recombination. pH and temperature influence on 3-pyridine-methanol oxidation was also investigated [15]. It was found that temperature effect was negligible for products yields while, by increasing pH from 1 to 4, both aldehyde and vitamin B3 yields and Cu2+ ions conversion decreased. In these works, high total selectivity values were obtained, being selectivities to aldehyde higher than those to acid. Selective production of pyridinecarboxaldehydes from methylpyridine isomers was also performed under de-aerated conditions in acetonitrile or acetonitrile-water solvents [18]. The obtained results show that the reactivity is strongly dependent on the photocatalyst properties.

In our recent work selective photocatalytic oxidation of 4-methoxybenzyl alcohol and 4-nitrobenzyl alcohol was performed in water under simulated solar light by using Pt, Au, Pd and Ag loaded Degussa P25 TiO2 catalysts [19]. The best activity and selectivity results were obtained with Pt-loaded (0.5%) TiO2 and the highest aldehyde selectivities were reached at low pH’s. Significant amount of 4-nitrobenzoic acid (ca. 50%) was obtained only from 4-nitrobenzyl alcohol at high pH values.

In this work, differently from the previously cited works [15], [16], [17], [19], Pt-loaded home prepared (HP) TiO2 photocatalysts were used for selective oxidation of 2, 3 and 4-pyridinemethanol to its corresponding aldehyde and acid in water. Oxygen from atmospheric air was used as oxidant; the influence of different light sources (UV, UV–vis and visible) on substrate degradation rates and products selectivities was also investigated for different pH’s (2–12). The optimal amount to be loaded on TiO2 was already investigated [19] and this amount (0.5%) has been used for the TiO2 samples of this work.

Section snippets

Preparation of HPRT

The precursor solution was obtained by adding 20 mL of TiCl4 to 1000 mL of water contained in a volumetric flask (2 L). At the end of the addition, the resulting solution was stirred for 2 min by a magnetic stirrer and then the flask was sealed and maintained at room temperature (ca. 298 K) for a total aging time of 6 days. After that time the precipitated powder was separated by decantation and dialysed several times with deionised water until a neutral pH was reached. Then, the sample was

Characterization

XRD patterns of P25 and HP TiO2 photocatalysts are given in Fig. 1. The peaks assignable to anatase are those at 25.6°, 38.08°, 48.1° and 54.6° [6]. Those referring to rutile are at 27.5°, 36.5°, 41°, 54.1° and 56.5° [6] and those referring to platinum are at 39.8°, 46.2° and 67.5° [21]. HPRT, HPRT-400 and Pt-HPRT-400 are in rutile phases and P25 and Pt-P25 are in anatase and rutile phases (A:80%, R:20%). Because Pt content of samples is very low, Pt peaks are not detected in the spectra of

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

Acknowledgements

Authors thank the Scientific Research Project Council of Afyon Kocatepe University (BAP project no:16.KARİYER.08) for financial support and Prof. Leonardo Palmisano (Palermo University, Italy) for his useful suggestions.

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