Chapter three
3.2. Experimental
3.2.1 Catalyst synthesis
Materials: All purchased materials were used as supplied: colloidal silica Ludox 40 (Sigma-
Aldrich), sodium silicate (Sigma-Aldrich), sodium hydroxide (Merck), tetrapropylammonium bromide (TPA, Merck), iron(III) nitrate nonahydrate (Aldrich), perchloric acid 70% (Merck), sodium chloride (Associated Chemical Enterprises), hydrochloric acid (Associated Chemical Enterprises), hydroxylamine (Merck), ammonium hydroxide (Merck), Sc-zeolite (Süd Chemie), potassium hydroxide (Merck), acetonitrile (Sigma-Aldrich), n-octane (Fluka), hydrogen peroxide (Associated Chemical Enterprises), and hydrofluoric acid (Riedel-de Haën).
The catalyst material, Na-Fe-silicalite-1, was synthesised using the solid gel method [12] in the following procedure: 0.8 g of iron nitrate was dissolved in 100 mL of double distilled water, which was acidified by three drops of perchloric acid in a 500 mL Teflon beaker to get a clear solution, followed by further addition of 3.5 mL perchloric acid (solution 1). In a second 500 mL Teflon beaker, 11.5 mL of sodium silicate solution was added to 141 mL of deionized water (solution 2). Both solutions were kept in a refrigerator overnight. While still cold, solution 2 was added dropwise into solution 1 under vigorous stirring until pH 4.5 (using pH paper). Then 14.8 mL of colloidal silica was added into the remainder of solution 2 then this was added to solution 1. To the slurry that formed was added 2 g sodium chloride, 6.7 g TPA and 0.2 g Sc- zeolite seed. The pH of the resultant creamy slurry was 10.5. The slurry was transferred into a Teflon-lined autoclave and allowed to crystallize in a Parr reactor at 160 oC for 24 hours without agitation. After crystallization, the solids were filtered, washed with hot water and dried at 110 oC from which Na-Fe-silicalite-1(41) was obtained. The procedure was the same used in the synthesis of Na-Fe-silicalite-1(80) and Na-Fe-silicalite-1(128) except that different amounts of iron nitrate were used. Furthermore, the synthesis of ZSM-5 with Al and Fe added during the synthesis was also performed to get Na-Fe-ZSM-5. The similar procedure as mentioned above was followed except that solution 1 now contained appropriate amount of Al(NO3)3 and Fe(NO3)3. Masses of 0.8 and 0.4 g of Al(NO3)3 were used to yield Na-Fe-ZSM-5(66) and Na- Fe-ZSM-5(114) respectively while kept the mass of Fe(NO3)3 at 0.8 g for both synthesis. The catalysts were calcined under N2 and then air at 550 °C [13].
Furthermore, materials were modified in their Na form by the process of silanization to deactivate the external surface of zeolites. A mass of 2.5 g of parent Na-Fe-silicalite-1 or Na-Fe- ZSM-5 was stirred for 21 hours in 4 wt% tetraethoxysilane (TEOS) diluted in 96 mL hexane at
room temperature. After the sample was stirred for 24 hours, it was filtered, dried and calcined under air at 550 οC to yield Na-Fe-silicalite-1(41:Sil), Na-Fe-silicalite-5(80:Sil), Na-Fe-silicalite- 1(128:Sil), Na-Fe-ZSM-5(66:Sil) and Na-Fe-ZSM-5(114:Sil), respectively. Note: Sil = Silanized.
3.2.2 Characterisation
Fourier Transform-Infrared (FT-IR) spectroscopy data was obtained within the range of 4000-400 cm-1 using a Nicolet 400 D spectrophotometer. Powder X-Ray Diffraction (XRD) was performed using a Philips PW 1730/10 diffractometer, using Co Kα radiation, equipped with a long line focus operating with amperage of 20 mA and voltage of 40 kV. Data was collected in the range of 2 to 90° (2θ). Scanning Electron Microscope (SEM) data was obtained using a Philips XL30 ESEM at 20 kV operating at a low vacuum mode of 1 Torr. Transmission Electron Microscope (TEM) images were obtained from a JEOL JEM 1010. Inductively Coupled Plasma- Optical Emission Spectroscopy (ICP-OES) data was collected on a Perkin Elmer (Optima 5300 DV) instrument. Samples were digested in hydroflouric acid and analysed in triplicate. The X-Ray Fluorescence (XRF) Spectroscopy was performed on a PW 1404 X-Ray Fluorescence instrument equipped with a flow counter detector. Brunauer-Emmet-Teller (BET) surface area measurements were obtained using a Micromeritics Gemini instrument. The Hydrogen Temperature Programmed Reduction (H2-TPR) and Ammonia Temperature Programmed Desorption (NH3-TPD) were performed using a Micromeritics Autochem II chemisorption analyzer equipped with a TCD detector. The H2-TPR data was obtained between 40 and 950 °C (10 °C min-1) in a flow of 5% (v/v) at 15 mL min-1. The NH3-TPD analysis was collected in the temperature range 40 to 700 °C at a constant heating rate of 5 °C min-1 after preheating the sample to 500 °C for 2 hours under a flow of helium, then cooled to 40 °C. The nitrogen adsorption data was collected using a Mictromeritics Tristar instrument.
3.2.3 Catalyst testing
All reactions were performed under nitrogen. In a typical test run, a desired amount of acetonitrile was added a two-neck round bottom flask (100 mL) fitted with a stopper and a condensor. This was followed by the sequential addition of 7.01 mmol of n-octane, 3 mL H2O2
(30 wt%) and 0.2 g catalyst. The reactions were conducted in 13, 50, 60, 80 and 100 mL of acetonitrile for 8 hours at 80 °C using Na-Fe-silicalite-1(80). The products were analysed by a
Perkin Elmer Auto System GC equipped with Flame Ionisation Detector using a Pona 50 x 0.20 mm x 0.5 μm column. The residual H2O2 was determined by titrating 1 mL of the
reaction solution acidified with 6M H2SO4 with 0.1 M KMnO4. Chlorobenzene was used as an internal standand and the carbon balance was found to be +/-98% in all cases. Each reaction was repeated twice.
3.2.4 Oxidation of n-octane in the presence of cyclohexane
The mass of 0.1 g of catalyst, 1 mL octane, 0.5 mL cyclohexane, 0.2 mL of H2O2 (30 %) and chlorobenzene, as an internal standard, were added to a 5 mL round bottom flask. The reaction was conducted at room temperature and repeated at 80 ºC. The product analysis was performed as mentioned above.