Effect of phosphate ions on the surface chemistry and microstructure of amorphous alumina

Abstract

High-surface-area amorphous AI,O,-AIPO, samples (P/AI ratio : 04.17) have been prepared by reductive decomposition of aluminium nitrate in the presence of phosphoric acid. The materiais obtained are amorphous up to 1100-1200 K and present " zeolitic-type " monomodal microporosity (r = 10-15 A). They are weakly acidic.The Lewis acidity changes from being weak (associated with octahedrally coordinated A13+ ions) to being strong by increasing P content due to the formation of tetrahedral A13+ ions induced by phosphate species. Brsnsted sites are also present; these are associated with phosphate species. The catalytic activity in methanol dehydration increases with P content up to P/AI = 10/90. P inhibits both the crystallization of the y-alumina phase and the yto a-alumina phase transition.Amorphous aluminas constitute poorly known materials that are potentially useful in the field of heterogeneous catalys ~s . ' -~ In a previous paper4 we have characterized highsurface-area amorphous alumina prepared by an unconventional procedure. We found that the surface acidity of this material as well as its catalytic activity in methanol dehydration is lower than that of y-type crystalline alumina obtained by its crystallization. We attributed this effect to the preferential octahedral coordination of A13 + ions on the amorphous material, the acidity and catalytic activity of transition aluminas being assigned to tetrahedrally coordinated A13+.4 This conclusion found support in the results of Nortier et aL5 who found predominantly octahedral ions in another amorphous alumina sample by 27Al MAS NMR measurements.Alumina-based amorphous materials such as silica-alumina6 and alumina-boria7 are acidic materials of high technological interest in catalysis. In the first case A1 is thought to be tetrahedrally coordinated6 while in aluminaboria both octahedral and tetrahedral A1 ions have been found by NMR.7 Amorphous aluminium phosphate AlPO, and A1,0,-A1P04 mixed catalysts have been the object of increasing interest in recent years because they display relevant surface acidity and catalytic activities in several acidcatalysed reactions.8-' , A1 is reported to be tetrahedrally

Synthesis of zirconia-based solid acid nanoparticles for fuel cell application

“…It has been reported that S-ZrO 2 exhibited a Hammett acid strength H 0 of -16.03, whereas for 100% sulphuric acid it is only -11.99 and shows higher strength (Yadav & Nair, 1999). The modification with diammonium hydrogen phosphate acid also increases the specific surface areas to 37 m 2 /g, which is high in comparison with unmodified ZrO 2 , as has also been observed by other researchers (Abbattista et al, 1990). Figure 3 shows that the nanoparticles obtained from the ageing method had an enhanced specific surface area, which is necessary for the application of zirconia nanoparticles as catalyst support in fuel cells.…”

Gelification process to prepare phosphate modified alumina: Study on structure and surface properties

“…Phosphates have also been claimed to act as a support stabilizer [18][19][20][21][22][23][24][25]. Gishti et al [18] evidenced a stable effect of phosphate by studying the phase transitions of alumina in the range 673-1323 K. Abbattista et al [19] found phosphate ions hindered the transformation of amorphous alumina to crystallized transition alumina as well as the transformation of ␥-alumina to ␣-alumina.…”

“…Reductive decomposition of aluminium nitrate in the presence of phosphoric acid could also obtain highsurface-area P-modified alumina samples [19]. Mekhemer et al [22] area by impregnation, and it was attributed to the small particles after doping with phosphate ions.…”

“…Gishti et al [18] evidenced a stable effect of phosphate by studying the phase transitions of alumina in the range 673-1323 K. Abbattista et al [19] found phosphate ions hindered the transformation of amorphous alumina to crystallized transition alumina as well as the transformation of ␥-alumina to ␣-alumina. But the surface area decreased with the increasing content of phosphate in alumina, even less than the blank sample after calcinated at 1073 K. Morterra et al [20] also proved no surface area stabilization effect from phosphates in the case of the transition phase (spinel) aluminas.…”

“…Modification effect of phosphate on the surface acidity of alumina had been studied a lot [19][20][21][22]26]. Phosphates play a key role in stabilizing the amounts of the strong Lewis acid site [20].…”