Effect of iron and chromium incorporation and surface sulphonation on the structural properties of sba-15

SBA-15, in addition to one dimensional hexagonal structure, uniform pore dimension and distribution, also contains micropores in the structure. Although having the high surface area, mechanical and hydrothermal stability, due to the neutral surface combination, it exhibits low catalytic activity. Catalytic activities of SBA-15 can be improved with some modifications done during and/or after the synthesis. In this study, to increase the catalytic activity, the active metals to be incorporated to the structure had been selected through the metals efficient in the low carbon-chain organic compounds reaction applications as iron and chromium. These metals ensures the formation of different type of catalytic active sites on the structure because of the presence of high number of valence electrons in their last orbits. Sulphonation process which is used as the other modification method, increases the catalytic activity by rising the number of surface functional groups. The metal loading to SBA-15 supported catalysts sythesised in the scope of the study, were done hyrothermally by the molar percent of nmetal/nsilicon as 3, 5 and 7%. The surface sulfonation was carried out by changing the synthesis acid and use of the sulfate containing salts of the active metals with single and multiple modifications. XRD (X Ray Diffraction) patterns indicated that the alteration of synthesis acid HCl with H2SO4 did not destroy the sequence of regular pore structure of the support material, however, the increase of pore wall thickness from 4.93 nm to 5.61 nm and again a decrease in the pore wall thickness was observed by metal loading. Due to the used low molar ratios, the presence of the metal active species in the structure was not identified in the X ray diffraction patterns. Adsorption isotherms of all synthesized samples exhibit Type 4 and hysteresis openings occurred during desorption Type 1 behaviour. This situation supported that no significant deformation in the pore shape and distribution of the support structure were occured by single and multiple modifications. Synthesis acid alteration increased the micro pore volume and surface area, but caused decrease of meso pore volume and meso pore sizes. Additional metal loading increased the values of total, micro and meso pore volumes and surface areas and it caused little decreases in the sizes of meso pore. It was determined that, the %5-Cr-H2SO4-SBA-15 sample with 5% chrominium loading having the highest surface area with 1130 m2/g, had 1.43 cm3/g pore volume, 11% micro porosity and 6.5 nm pore diameter. In FT-IR (Fourier Transform Infrared Spectroscopy) characterization, it was determined that the sulphate, sulphite and metal groups were localized to the surface structure. Changing the synthesis acid, increased the acidity and this increase rised a little by the active metal loading. By XPS (X-ray photoelectron spectroscopy) method applied to 5% metal loaded samples, it was seen that the presence of iron active species with +2, +3 and +4 valancies and the chromium active species with +2 and +3 valancies and bonding of them to the structure as in the forms of metal oxide and metal sulfur. In TEM/EDX (Transmission Electron Microscopy-Energy-Dispersive X-Ray Spectroscopy) characterization, it was detected that, the modifications did not destroy the regular pore distribution and the hexogonal array of SBA-15 incorpoarion of metal active species by much more loading success by bimetalic combination.