Industrial Chemistry
Open Access

Photo catalysis; Hydrothermal; Benzyl alcohol; Benzyl aldehyde; UV-visible irradiation

Introduction

The conversion of alcohols into carbonyl compounds through selective oxidation is a crucial process in organic chemistry. However, this transformation has traditionally been carried out using hazardous and toxic stoichiometric oxidants under harsh conditions such as high temperatures and pressures as reported by relevant literatures. Recently, ultra-violet/visible light-driven organic photochemical synthesis has been a versatile research interest area as researchers covering the academia and Industry due to the unique features of green and sustainable chemistry. The Visible region is useful to perform many green organic photochemical reactions under catalytic conditions due to its sustainable and abundant nature [1]. Various research areas have been prompted by the characteristic property of photo catalyst in achieving the aim of green chemistry in the modern world today [2]. This photo catalyst which are formulated and synthesized using different mediums are evident to many plausible reactions and synthesis ranging from Water Treatment, Degradation of Organic waste, green photo catalytic oxy- functionalization of organic compounds and selective organic transformations under environmentally friendly conditions [3]. ZnO nanoparticle has extensive applications in system and processes of biosensors, gaseous sensors, solar cells, ceramics, nano generators, photo detectors, catalyzers, ultraviolet (UV) adsorbents in cosmetic and veneer of antivirus, pigments, optical materials, photocatalyst, electrical and optical processes [4]. Aldehyde compounds are indispensable substances for drugs, fragrances and organic synthesis in the chemical industry, and they are generally produced through reduction of corresponding carboxylic acids or oxidation of corresponding alcohols [5]. The production of aldehydes is conducted in environmentally harmful organic solvents with the use of stoichiometric oxygen donors such as KMnO₄ and CrO₃ [6]. Since these reagents are toxic and expensive and produce a large amount of waste, the process should be replaced with a more environmentally friendly method. Photo catalysis is a promising technique for environmental remediation due to its ability to degrade organic pollutants under mild conditions. Several Reports has employed several matrices inclusive of ZnO, TiO₂ as photo catalyst due to its high chemical stability, low toxicity, and low cost [7,8].

This research works deals with selective oxidation reaction, which is important from the viewpoint of green and sustainable chemistry in controlling the release of toxic waste. Photo catalytic oxidation as one of the advanced oxidation processes (AOPs) due to strong decomposition characteristic of hydroxyl radicals in removal of organic pollutant has attracted much attention [9]. In view of this, the electron gap generation by the catalyst helps in the oxidation of organic reaction, thereby causing conversion. Various catalysts can be used in photo catalytic process. The primary aim of this study is to examine the potential of synthesized zinc oxide and Zinc doped Titanium oxide as a photo catalyst for converting benzyl alcohol into benzyl aldehyde.

Methodology

Preparation of ZnO nanoparticle

1.3g Zinc Acetate was dissolved in 50 mL distilled water. Separately, 1g of NaOH was dissolved in 50 mL Ethanol. A magnetic Stirrer was used to yield homogeneity of the solution. Then, NaOH_(aq) was added drop wise to the Zinc Acetate Solution and allowed to stir for 1hr. The final mixture was transferred to an autoclave and placed in an Oven at 95^oC for 2hrs. The mixture was then filtered and rinsed with distilled water, then placed in an oven to dry at 60^oC. The obtained powder was then annealed at 400^oC to improve the crystal structure. The whole process was repeated as required using 0.65g of Zinc Acetate which was weighed into a 250mL beaker. The Powder was then characterized using XRD.

Preparation of ZnO doped TiO₂ nanoparticle

Hydrothermally Synthesized Zinc Oxide was mixed with Commercial TiO₂ powder at 0.1 – 1% of ZnO. A magnetic Stirrer was used to yield homogeneity of the solution. Then, NaOH_(aq) was added drop wise to the Solution and allowed to stir for 1hr. The final mixture was transferred to an autoclave and placed in an Oven at 95^oC for 2hrs.

The mixture was then filtered and rinsed with distilled water, then placed in an oven to dry at 60^oC. The obtained powder was then annealed at 400^oC.

Alcohol conversion

Photo catalytic conversion using synthesized Zn-TiO₂

1g of the Synthesized catalyst (ZnO/TiO₂) was placed in a 50mL Conical Flask containing 100mL of Distilled water as solvent and 20micro Liter of Benzyl alcohol and placed in the UV Chamber, for 5 mins, 10 mins, 15 mins, 20 mins, 25 mins and 30 mins. At each run, the process was repeated using 0.5g, 1g, 1.5g and 2g of the photo catalyst. The Result of each process per time was transferred in a clean amber reagent bottle for GC-FID analysis of the resulting product.

Photo catalytic conversion using synthesized ZnO

0.051g of the Synthesized catalyst (ZnO) was placed in a 50mL Conical Flask containing 100mL of Distilled water as solvent and 20micro Liter of Benzyl alcohol and placed in the UV Chamber, for 5min, 10mins, 15mins, 20mins, 25mins and 30mins. At each run, the process was repeated using 0.5g, 1g, 1.5g and 2g of the photo catalyst. The Result of each process per time was transferred in a clean amber reagent bottle for GC-FID analysis of the resulting product.

Photo catalytic activity of the synthesized particles in oxidation of benzyl alcohol to benzyl aldehyde

The Photo catalytic activity of the Synthesized ZnO and Zn-doped Titanium Oxide were studied by the use of Gas Chromatography-Flame Ionization Detector, using a plot of Concentration (mMol/L) vs Time (min). The purpose of the experiment, which is to investigate the photo catalytic oxidation of benzyl alcohol to benzyl aldehyde using ZnO and Zn-TiO₂ nanoparticles as catalysts.

The photo catalytic conversion of BA (Benzyl alcohol) using ZnO and Zn-TiO₂ respectively with respect to Catalyst Loading at 0.5g, 1.0g, 1.5g and 2g. Catalyst loading at 0.5g of ZnO for BA conversion was observed at high concentration and BA was converted on a steady state with initial time 0 – 4 mins, after which a considerable decline was observed in the plot with increase time, whereas at 2.0 ZnO catalyst loading, the conversion of BA occurred rapidly and further conversion was completed with increased time. This implies that Catalyst loading of ZnO at 2.0g shows high or preferential BA photo catalytic conversion.

Catalyst loading of Zn-TiO₂ at 0.5g for BA conversion was not efficient as the conversion experienced less conversion rate as seen in the plot. Whereas Catalyst loading of Zn-TiO₂ at 2.0g for BA conversion was sharp and occurred in a steady state with increase in time. Thus for Zn-TiO₂ catalyst loading at 0.5g the conversion is low whereas for catalyst loading at 2.0g, the BA Conversion rate is efficient.

Photo catalytic activity of the synthesized particles in oxidation of benzyl alcohol to benzyl aldehyde

The Photo catalytic activity of the Synthesized ZnO and Zn-doped Titanium Oxide were studied by the use of Gas Chromatography-Flame Ionization Detector, using a plot of Concentration (mMol/L) vs Time (min).

The purpose of the experiment, which is to investigate the photo catalytic oxidation of benzyl alcohol to benzyl aldehyde using ZnO and Zn-TiO₂ nanoparticles as catalysts.

The photo catalytic conversion of BA (Benzyl alcohol) using ZnO and Zn-TiO₂ respectively with respect to Catalyst Loading at 0.5g, 1.0g, 1.5g and 2g. Catalyst loading at 0.5g of ZnO for BA conversion was observed at high concentration and BA was converted on a steady state with initial time 0 – 4 mins, after which a considerable decline was observed in the plot with increase time, whereas at 2.0 ZnO catalyst loading, the conversion of BA occurred rapidly and further conversion was completed with increased time. This implies that Catalyst loading of ZnO at 2.0g shows high or preferential BA photo catalytic conversion. Catalyst loading of Zn-TiO₂ at 0.5g for BA conversion was not efficient as the conversion experienced less conversion rate as seen. Whereas Catalyst loading of Zn-TiO₂ at 2.0g for BA conversion was sharp and occurred in a steady state with increase in time. Thus for Zn-TiO₂ catalyst loading at 0.5g the conversion is low where as for catalyst loading at 2.0g, the BA Conversion rate is efficient.

Conclusion

Although TiO₂ has been thoroughly investigated as an effective photo catalyst, ZnO has been considered a suitable alternative since it is less expensive and possesses a similar band gap (∼3.3 eV). Zinc doped Titanium oxide has been reported to produce better oxidation in terms of percentage conversion and selectivity of BA due to the higher efficiency of photo excitation under Uv-visible light. Furthermore, ZnO and Zn-TiO₂ synthesized using Hydrothermal Method provides a suitable and cost effective method of synthesis, and the resultant material synthesized was crystalline with Wurzite structure as confirmed by XRD analysis. To further improve the photo activity, there is a need to increase the photo catalyst surface area, for optimal mass transfer and efficient dye degradation, without additional equipment cost in catalyst separation from water.

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