Synthesis Of Nanosized SAPO-34 Molecular Sieve

SAPO-34 is molecular sieve with the framework topology of the natural zeolite chabazite with narrow diameter pores of 0.38 nm, moderate acidity and excellent thermal stability and hydrothermal stability, which is an excellent catalyst for many important reactions. For example, SAPO-34 was used to the reaction of methanol to olefins (MTO) as catalyst and gave high conversion of methanol and selectivity to olefins in the reaction. As SAPO-34 with small crystal size could improve the anti-coking ability of the catalyst and hence could prolong the lifetime of catalyst.In this study, to hydrothermally synthesize the SAPO-34 molecular sieve with nano particale size, pseudoboehmite, silica sol and phosphoric acid was separately used as the source of aluminium, silicon and the phosphorus, and tetraethylammonium hydroxide used as template. The effects of following synthetic conditions on the particale size of SAPO-34 synthesized were investigated by means of XRD and SEM techniques. The corresponding results are as follows:(1) For the synthesis of SAPO-34, the factors influencing the particale size were studied, including crystallization temperature and time, silica-alumina ratio, pH value, of the abtained aluminasilicate gel, and the TEAOH amount being added to the synthesis system. For obtaining nano-sized SAPO-34, the following conditions (optimized) should be used:Adding the precursor and TEAOH with silica-alumina ratio of 0.3 (molar ratio) and alumina-TEAOH ratio of 1 (molar ratio), which displayed a pH value of 5.2, aging the resulted material at room temperature for 24 h and then crystallized it at 200℃for 24 h. In these conditions, SAPO-34 with ca.1.5μm partical size could be obtained.(2) The enfluence of employing tween 20, emulsifier (OP-10), polyacrylamide, dextrose and carbon (BP-2000) as crystal growth inhibitor upon crystal size of SAPO-34 obtained in the synthesis was studied. It seems that all of these materials being added to the synthesis system have less effect on the partical size.(3) The effects of treating the aluminasilicate by microwave and ultrasonic on the crystal size of SAPO-34 were also studied. It was found that not only the crystallization time but also the crystal size of SAPO-34 could be successfully reduced by treating the aluminasilicate with microwave for 5 min and with ultrasonic for 40 min. In this way, crystallization time for synthesizing SAPO-34 molecular sieve was reduced to 6 h, and thus a nanosized SAPO-34 molecular sieve with average particle size of 100-200 nm was obtained at 200℃in the hydrothermal synthesis.