Nanocrystalline nickel prepared by glycol reduction method and characterized by XRD and magnetic measurements has been used as a catalyst for hydrogenation of styrene oxide to 2-phenylethanol. Effect of process variables such as particle size of the catalyst, temperature, and pressure have been optimized to achieve a maximum conversion of 98% of styrene oxide with 99% selectivity towards 2-phenylethanol. The structure of the transition state has been computed employing density functional theory and using Gaussian 09 suite. The enthalpy of reaction (
2-Phenylethanol (2-PEA) having smell of rose petals is a material of commerce and finds applications as fragrance chemical [
Conventional catalyst used for many hydrogenation reactions is Raney nickel (particle size of about 25 nm) which is prepared by dissolving Ni-Al alloy in NaOH and removing the sodium aluminate by repeated washing with water which is time consuming [
Liquid phase [
There are reports on the production of 2-phenylethanol by reduction of styrene oxide with hydrogen in slurry phase 5 using Pd supported on carbon [
In a three necked flask fitted with a thermometer and a water condenser a 2% solution of anhydrous nickel acetate in ethylene glycol was taken, pH adjusted with NaOH solution, and refluxed till whole of the nickel acetate was converted to Ni indicated by change of color from green to brown. Samples were prepared at pH 7, 11, and 13, and named as N7, N11, and N13. The anhydrous nickel acetate was prepared simply by drying laboratory grade nickel acetate in oven at 105°C for few hours till a constant weight was achieved. Small amount of the product was taken out for characterization. The catalyst was always kept covered with ethylene glycol and taken out only when needed for characterization or for application as a catalyst. X-ray diffraction “XRD” (
Catalytic performance of the prepared catalyst for reduction of styrene oxide was evaluated in a 2lt stainless steel pressure reactor supplied by Amar Engineering Mumbai. The reactor was fitted with a mechanical stirrer and pressure gauze. Heating was performed by circulating thermal fluid to the coil fitted internally to the reactor. In a typical rum 50 g of styrene oxide in 500 mL of methanol and 2.5 g of catalyst was charged to the reactor and heated to 110°C. The reactor was pressurized through a hydrogen gas cylinder to the desired pressure (40–100 kg·cm−2). The gas pressure decreases slowly with time and finally becomes constant at completion of the reaction. After completion of the reaction the reactor is opened, catalyst is filtered, solvent is distilled, and product mixture is analyzed over a Chemito GLC machine using SE-30 column and FID detector.
The XRD patterns of the catalyst prepared at different pH are reproduced in Figure
XRD pattern of nano Ni catalyst.
The pattern matches well with the reported pattern. The XRD pattern of the spent catalyst is shown in Figure
XRD pattern of spent catalyst.
Effect of pH on particle size of catalyst.
Magnetic measurements of Raney Ni as well as spent and fresh Ni nanoparticles prepared at pH 13 are depicted in Figure
(a) Raney nickel (b), N13 catalyst, and (c) N13 spent catalyst.
Effect of duration of reaction was studied to have an idea of completion of the reaction. Results are shown graphically in Figure
Effect of duration of reaction. B = conversion, C = 2-PEA yield, D = others, and E = 2-PEA selectivity.
Effect of catalyst loading was studied with the objective of finding optimum catalyst/styrene oxide ratio. The results are shown graphically in Figure
Effect of catalyst loading on the performance of the catalyst. B = conversion, C = 2-PEA yield, D = others, and E = 2-PEA selectivity.
Effect of temperature on the performance of the catalyst prepared at pH 13 was studied at pressure of 50 Kg·cm−2 and catalyst loading of 1%. The results are presented in Figure
Effect of temperature on performance of catalyst. B = conversion, C = 2-PEA yield, D = others, and E = 2-PEA selectivity.
Effect of pressure on the performance of the catalyst was studied in the range 0–50 kg·cm−2 and results are presented in Figure
Effect of pressure on catalyst performance. Temp. = 378 K, catalyst loading = 1%, duration = 4 hrs. B = conversion, C = PEA yield, D = Others, and E = Selectivity.
Most of mechanisms for hydrogenation of alkenes are based on the assumption that hydrogen is first atomized on the catalyst surface which is subsequently added to the double bond producing a saturated hydrocarbon. These descriptions are qualitative in general and there is no quantitative proof of the mechanism. In order to get a clue to the adsorption of hydrogen on the Ni surface and subsequent atomization we performed a density functional theory (DFT) calculation of the adsorption of hydrogen on Ni employing Gaussian 09 suite. We used Beck’s three parameter hybrid method with the Lee, Yang, and Parr (B3LYP) exchange correlation functional to perform these calculations. Geometries were optimized using standard 6-31 G
The optimized geometries of reactant model, product model, and transition state are shown in Figure
Structure of (a) reactant model, (b) product model, and (c) TS for hydrogenation of styrene oxide to 2-phenylethanol.
Potential energy diagram for hydrogenation of styrene oxide Over Ni catalyst.
The atomic charges (coulomb) in the reactant models are, respectively, Ni1 = 0.445, Ni2 = 0.332, O1 = −0.672, H1 = −0.082, H2 = −0.112, H3 = 0.126, H4 = 0.156, H5 = 0.122, H6 = 0.125, H7 = 0.124, H8 = 0.143, H9 = 0.154, H10 = 0.164, C1 = −0.127, C2 = −0.164, C3 = 0.144, C4 = −0.167, C5 = −0.124, C6 = −0.125, C7 = −0.484, and C8 = 0.021. Free nickel atom possesses no charge. Opposite charges on nickel atoms and negative charges on O1, H1, and H2 suggest electrostatic nature of adsorption. All carbon atoms accept C3 and C8 bears negative charge. This can also be ascribed to adsorption of SO. It is worth noting that in the reactant model, the epoxide bond is broken and new bonds are formed between O1 and C7 with Ni1 and Ni2. The adsorbed reactant model is therefore something like an intermediate. The charge distribution in styrene oxide (SO) is shown in Figure
Geometry of styrene oxide showing atomic charges (Coulomb).
Model of the catalytic cycle for hydrogenation of styrene oxide over Ni catalyst.
The authors declare that there is no conflict of interests regarding the publication of this paper.