Tuning SAPO-34 with a tailor-designed zwitterionic amino acid for improved MTO performance

Silicoaluminophosphate SAPO-34 zeotypes have been hydrothermally synthesized by using a tailor-designed zwitterionic amino acid of 3-hydroxy-4-triethylaminobutyric acid (Et-LC). The unnatural zwitterionic amino acid possessing both triethyl ammonium cationic moiety and anionic carboxylate group promoted SAPO-34 crystallization, allowed to decrease acidity of the as-obtained SAPO-34 catalysts, and slightly generated mesopores without affecting the microporosity. While natural amino acids either induced polymorphism due to the amino or ammonium moiety incommensurate with the chabazite unit, or showed inability to create mesopores because of the non-deprotonated carboxylic group, thus failing to meet the stringent SAPO-34 synthetic requirements of neutral pH, high temperature and appropriate templates. The Et-LC mediated SAPO-34 crystals were demonstrated to be much more effective in catalyzing the methanol to olefin (MTO) reactions with lifetime improved to 533 min, the ethylene plus propylene selectivity preserved at 81%, and coke content decreased to 8%, in comparison to those of the conventional microporous SAPO-34 zeolite of 389 min, 81%, and 17.5%.