Iron oxide nanoparticles supported on zirconia were prepared by precipitation-deposition method and characterized by XRD, SEM, FT-IR, TGA/DTA, surface area and particle size analysis. Catalytic activities of the catalysts were tested in the gas-phase conversion of cyclohexanol in a fixed-bed flow type, Pyrex glass reactor, at 433 - 463 K. Major detected products were cyclohexanone, cyclohexene and benzene, depending on the used catalyst. The rate of reaction was significantly raised by the introduction of molecular oxygen in the feed gas, thereby suggesting the oxidation of cyclohexanol to cyclohexanone. Furthermore, the catalytic activity of iron oxide nanoparticles supported on zirconia treated with hydrogen at 553 K for 2 hours, was more selective and better than the unreduced iron oxide nanoparticles supported on zirconia, in the gas-phase oxidation of cyclohexanol to cyclohexanone. Experimental results showed that there was no leaching of metal, and that the catalyst was thus truly heterogeneous.
Since the early 1900’s, iron-based dispersed catalysts have been used for the liquification of coals. Continuous efforts have been made to reduce size of the particles with a simultaneous focus on preserving and enhancing the dispersion, in order to improve the quality and affordability of these iron-based catalysts. Several attempts have been made to prepare iron oxides and explore their catalytic activities in petrochemical industries [1-9]. Research has established the enhancing effect of promoters on the catalytic activity of iron-based catalysts, for example, molybdenum promotes the catalytic activity of sulfated hematite while tungsten has additive effect when used in combination with molybdenum. Similarly, nickelcobalt has synergetic effect while used with molybdenumetc. [10-12]. Recently, researchers have diverted attention towards nano-materials and their applications in the field of catalysis. In this scenario, iron oxide nanoparticle is a potent candidate to be investigated as a catalyst in various industrially important reactions, including the synthesis of NH3, water shift reaction, desulfurization of natural gas, dehydrogenation of ethyl benzene, oxidation of alcohol, and manufacture of butadiene [13-18].
In this work, we prepared iron oxide nanoparticles from iron nitrate with precipitating agents, i.e., ammonium hydroxide and ammonium acetate, without the aid of any surfactant. Iron oxide particles were found to be in the range of 8 - 10 nm as estimated from XRD of pure iron oxide nanoparticles. Following the same procedure, iron oxide nanoparticles supported on zirconia were prepared by adding deionized water and monoclinic zirconia to iron hydroxide prepared with ammonium hydroxide, or an excess of acetone and monoclinic zirconia to Fe2(CHOO)6 sol prepared with ammonium acetate. The prepared catalysts were characterized by various techniques, and tested for catalytic activities in the gas phase conversion of cyclohexanol to cyclohexanone, cyclohexene and benzene, keeping in view the fact that hematite and magnetite are semiconductors and can catalyze oxidation/reduction reactions [
Chemicals were of high purity grade and were used as such without further purification. The catalyst was prepared in the laboratory from its precursor compounds. Nitrogen, oxygen and hydrogen were supplied by BOC Pakistan limited.
The catalyst was prepared in two steps.
Monoclinic zirconia was used as a support for the catalyst which was prepared by ammonolysis of ZrOCl2∙8H2O (0.5 M aqueous solution) [
Fe2O3 nanoparticles were prepared by drop wise addition of 1) 0.5 M ammonium acetate solution to 0.1 M iron nitrate solution under vigorous stirring to obtain Fe2(CHOO)6 sol [
2) 0.5 M ammonium hydroxide solution to 0.1 M iron nitrate solution under vigorous stirring to obtain brownish precipitate. The precipitate was washed twice with triple distilled water and centrifuged. To the purified precipitates, deionized water and monoclinic zirconia were added and stirred, followed by centrifugation at 3000 rpm. Iron hydroxide/zirconia was then dried and calcined at 823 K which resulted in the nanoparticles of iron oxide supported on ZrO2.
The prepared catalyst was reduced at 553 K in tube furnace for 2 h. The nitrogen gas was continuously passed through the reactor until the desired temperature (i.e., 553 K) was achieved, and then a mixture of hydrogen and nitrogen (1:1) was passed for two hours at 40 mL/min, with a subsequent cooling in the nitrogen atmosphere.
Modern techniques such as XRD (X-ray differactrometer Rigaku D/Max-II, Cu tube, Japan), SEM (JSM 5910, JEOL, Japan), TGA/DTA (Diamond Series PerkinElmer, USA), U.K), FT-IR (Shimadzu prestige-21), Surface area and pore size analyzer (Quantachrome) were used for characterization of the catalyst.
100 mg of iron oxide nanoparticles supported on monoclinic ZrO2 catalysts were used for the gas phase oxidation of cyclohexanol to cyclohexanone in a fixed-bed flow type Pyrex glass reactor at 433 - 463 K in a tubular furnace attached to temperature controller. Cyclohexanol vapors were fed from saturators, using N2 as a carrier gas with a fixed flow rate of 40 mL/min. Reaction mixtures of 0.5 mL were injected at specified time intervals with six-port gas sampling valve to GC (PerkinElmer Clarus 580) with column (rtx@-Wax 30 m, 0.5 mm ID, 0.5 nm) and FID.
XRD of the catalysts, i.e., one prepared with ammonium acetate and the other prepared with ammonium hydroxide as a precipitating agent, was carried out. XRD pattern of the former catalyst presented with peaks at 2θ = 28.5 and 31.8 which point towards a retention of the monoclinic phase of zirconia. However, the monoclinic phase of zirconia was lost in the latter catalyst which could probably be due to the presence of water. Furthermore, peaks at 2θ = 35.4 and 45.3 were recorded for both samples prior to being reduced, which account for the crystallized nanoparticles of iron oxide (αFe2O3/γFe2O3). In case of reduced samples, a peak was recorded at 2θ = 69.2 as shown in
The average particle size was estimated (8 - 10 nm) by using the Debye-Scherrer equation from peak width broadening in the XRD data of pure iron oxide (
BET surface area of the catalysts was found to be in the range of 150 - 300 m2g−1. FT-IR spectra (
SEM images (
K, and for the preparation of magnetite nanoparticles on monoclinic zirconia by calcination at 823 K 4 hours followed by reduction at 553 K in hydrogen flow for 2 hours.
Oxidation/dehydrogenation was demonstrated by comparing the data obtained with and without oxygen under identical experimental conditions using magnetite nanoparticles supported on zirconia as a catalyst as shown in
The FT-IR spectra of the catalyst recovered from the reaction carried out in the oxygen flow using magnetite nanoparticles supported on zirconia showed intense peak for carbonyl group, thus showing that the major product of gas phase oxidation of cyclohexanol is cyclohexanone as shown in
The same reaction when carried out under identical condition using hematite nanoparticles supported on zirconia gives cyclohexene as a major product by dehydration of cyclohexanol as shown in FT-IR spectra (
Variation in the rate of reaction with temperature at con-
stant pressures ranging from 12 Torr to 33 Torr was observed, using magnetite nanoparticles supported on monoclinic zirconia. Activation energies were calculated using Equation (4). Activation energies at pressures 12, 15, 20, 26, 33Torr are 17.48, 18.2, 17.73, 19.27, 19.73 kJ·mol−1, respectively. The activation energies for all these pressures are in the range where reaction is diffusion controlled. The rate of a reaction is given by following equation.
According to Arrhenius equation
Putting the value of k from Equation (2) in Equation (1), we get
ln rate = (lnA + nlnP) (4)
Plot ln rate vs. 1/T will give slope = while intercept = lnA + nlnP as shown in
The catalytic activity of hematite and magnetite nanoparticles supported on monoclinic zirconia was investigated for the gas-phase reaction of cyclohexanol. It was found that the catalyst was more active when reduced and selective for cyclohexanol oxidation to cyclohexanone, but it showed activity and selectivity for dehydration (i.e., conversion of cyclohexanol to cyclohexene) when unreduced. Furthermore, the catalyst effectuated dehydration/ dehydrogenation of cyclohexanol to benzene at higher temperatures. The model reaction, i.e., the gas-phase oxidation of cyclohexanol to cyclohexanone using magnetite nanoparticles supported on monoclinic zirconia, in
flow reactor, was found to be in diffusion control regime. In addition, the iron oxide nanoparticles supported on monoclinic zirconia could be easily recovered and used several times without significant loss of catalytic activity.
The authors gratefully acknowledge financial support of Higher Education Commission of Pakistan (Project No: 20-1604/R&D/092198) and Pakistan Science Foundation (Project No.PSF/Res/F-UM/Chem-434).