Effect of phosphate ion on the textural and catalytic activity of titania-silica mixed oxide

Phosphate-modified TiO2-SiO2 mixed oxide catalysts have been prepared by va rying the method of preparation, source and concentration of phosphate ion. The prepared catalysts were compared for their catalytic activity/selectiv ity in nitration of toluene. The characterisation of the catalysts was perf ormed using X-ray powder diffraction (XRD), Fourier-transform infrared spec troscopy (FT-IR), thermal analysis (TG-DTA), nitrogen adsorption-desorption methods, surface acid strength measured by Hammett indicator method, surfa ce acid sites measured by amine titration method, and phosphate content mea sured by UV-VIS spectrophotometry. The XRD patterns revealed that phosphate ion stabilises the anatase phase up to 1173 K activation. FT-IR results sh ow that phosphate species strongly bound bidentately, and that both the int ernal weakly H-bonded hydroxyl groups and free hydroxyl groups are present on TiO2-SiO2 mixed oxide support. Surface area and surface acidity are foun d to increase with the increase in phosphate loading up to 7.5 wt.% and the reafter the values decrease drastically. However, average pore radius and t otal pore volume shows the reverse order. Phosphated samples prepared using H3PO4 as the source of phosphate ion exhibit higher acidity, and surface a rea but lower porosity than the samples prepared from (N-H-4)(3)PO4, though both the samples contain the same amount of phosphate (7.5 wt.%). Similar results were also observed when varying the method of preparation. TiO2-SiO 2 samples prepared at pH = 3 exhibit higher acidity and surface area but lo wer porosity than the samples prepared at pH = 7. The acid strength of 7.5P /TiO2-SiO2 (H) is found to be stronger than that of 100% concentrated H2SO4 . The material modified with phosphate ion was found to be an efficient and selective catalyst for solvent-free mono-nitration of toluene. Selectivity to the para-product is correlated with the porosity of the material. (C) 2 001 Elsevier Science B.V. All rights reserved.