Amorphous solids belonging to the systems A1203-Me20(Me = Na,K) and A1203-B203 were prepared by nitrate decomposition, introducing boron in the form of boric acid. Crystalline metastable solids with pseudotetragonal symmetry were obtained from thermal treatment at 850" to 900°C for the compositions A16Mer0~9+o.5x) (x = 1; Me = Na, K) and Al(6-x,B,09 (1 5 x I 3). The resultant solids were stable only within a definite temperature range and transformed, with further treatment increases, into stable equilibrium phases. The structures of the metastable phases were examined by X-ray diffraction and Fourier transform infrared spectroscopy, and both analyses showed a mullite type of framework, inside of which the atomic coordinates were refined in the Pbam (no. 55) space group. The present results indicate that these silica-free mullite structures are stabilized by two different mechanisms: (1) interstitial occupation of bulky cations (Na+, K+) or (2) substitution of B for A1 in some of the tetrahedral positions. [
Alumina-boria samples with B : (B + Al) 0-50 atom% have been prepared by an unconventional procedure consisting of the reductive decomposition of the mixture of nitrates in the presence of glycerol X-ray diffraction (XRD)-amorphous samples having surface areas of 380-470 m2 g-' were obtained after calcination at 673 K.Their thermal stability has been studied by differential thermal analysis (DTA) and XRD analysis, showing that crystallization occurred in all cases above 1150 K. FTlR data show the predominance of octahedral A13+ and triangular B3+ both in the bulk and on the surface. Correspondingly, all samples show lower catalytic activity in methanol dehydration compared with y-alumina. It was concluded that boron tends to hinder crystallization of amorphous alumina, to increase its surface area and to further decrease it surface acidity.
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
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