Key Stages of the Formation of AlPO4-11 via Crystallization of Aluminophosphate Gel Preparedwith Boehmite
The one-dimensional channel pore system and the moderately strong acid centers inherent in aluminophosphate (AlPO4-11) and silicoaluminophosphate (SAPO-11) molecular sieves make them promising catalyst for hydroisomerization of higher n -paraffins. However, the mechanism of crystallization of these materials is not well understood as yet. XRD, 27Al and 31P MAS NMR, low- temperature adsorption-desorption of nitrogen, and SEM techniques were used for the first time for studying the stage crystallization of aluminophosphate AlPO4-11 for commercial boehmite based aluminium source. AlPO4-11 was shown to form via an intermediate phase based on layered crystalline aluminophosphate. It was established that highly crystalline and phase-pure AlPO4-11 was formed at 200 °C during 6 to 24 hours. When the crystallization at 200 °C lasted for more than two days, AlPO4-11 turned into non-porous cristobalite. The results obtained will be used for developing methods for deliberate control of the phase composition and crystallinity of industrially important silicoaluminophosphate sieves SAPO-11 with required properties to develop promising catalysts based thereon for large-scale processes of hydroisomerization of n -paraffins.