The influence of the nature and textural properties of different supports on the thermal behavior of Keggin type heteropolyacids

• Published in: Journal of the Serbian Chemical Society 2006, Vol.71 (3), p.235-249
• Main Authors: Popa, Alexandru, Sasca, Viorel, Stefanescu, Mircea, Kis, Erne, Marinkovic-Neducin, Radmila
• Format: Article
• Language: English
• Subjects: heteropolyacids; keggin type; non-isothermal decomposition; silica; titania
• ISSN: 0352-5139; 1820-7421
• DOI: 10.2298/JSC0603235P

In order to obtain highly dispersed heteropolyacids (HPAs) species, H3PMo12O40 and H4PVMo11O40 were supported on various supports: silica (Aerosil - Degussa and Romsil types) and TiO2. The structure and thermal decomposition of supported and unsupported HPAs were followed by different techniques (TGA-DTA, FTIR, XRD, low temperature nitrogen adsorption, scanning electron microscopy). All the supported HPAs were prepared by impregnation using the incipient wetness technique with a 1:1 mixture of water-ethanol. Samples were prepared with different concentrations to examine the effect of loading on the thermal behavior of the supported acid catalysts. The thermal stability was evaluated with reference to the bulk solid acids and mechanical mixtures. After deposition on silica types supports, an important decrease in thermal stability was observed on the Romsil types and a small decrease on the Aerosil type. The stability of the heteropolyacids supported on titania increased due to an anion-support interaction, as the thermal decomposition proceeded in two steps. The structure of the HPAs was not totally destroyed at 450 ?C as some IR bands were still preserved. A relatively uniform distribution of HPAs on the support surface was observed for all compositions of the active phase. No separate crystallites of solid phase HPAs were found in the SEM images. Da bi se dobili visoko dispergovani uzorci heteropolikiselina (HPAs),H3PMO12O40 i H4PVMo11O40 naneti su na razlicite nosace: silicijum-dioksid (Aerosil-Degussa i Romsil tip) i TiO2 . Ispitivane su struktura i termicko razlaganje nanetih i nenanetih HPAs razlicitim tehnikama (TGA-DTA, FTIR, XRD, adsorpcija azota na niskim temperaturama, SEM). HPAs nanosene su impregnacijom metodom pocetnog vlazenja 1:1 smesom voda - etanol. Da bi se ispitao uticaj nanete kolicine na termicko ponasanje nanete kiseline kao katalizatora, uzorci su pripremani sa razlicitim koncentracijama. Termicka stabilnost je proveravana u odnosu na cvrste kiseline i mehanicke smese. Kod deponovanja na silicijum-dioksidne nosace primeceno je znacajno smanjenje termicke stabilnosti na Romsil, a malo smanjenje na Aerosil tipu. Stabilnost HPAs nanetih na TiO2 je povecana zbog interakcije anjon-nosac posto termicka degradacija tece u dva stupnja. Struktura HPAs nije potpuno razorena na temperaturi 450 ?C sudeci po tome sto su neke IR trake ocuvane. Relativno uniformna raspodela HPAs na povrsini nosaca primecena je za sve sastave aktivne faze. Na SEM s