Ionothermal preparation of triclinic SAPO-34 and its catalytic performance in the MTO process

This work reports the ionothermal synthesis of SAPO-34 material, which is the per-excellence catalyst in the methanol-to-olefins (MTO) process. The direct addition of Si sources to the aluminophosphate and ionic liquid (1-ethyl-3-methyl imidazolium) mixture gave rise to the undesired AEL-structured materials. Therefore, some other strategies were necessary to apply. In particular, two unexplored strategies have been developed: (i) the addition of further heteroatom ions able to direct CHA materials, V ions being particularly efficient; and (ii) to carry out the synthesis in open systems rather than in autoclaves under autogenous pressure. Interestingly, the combination of both strategies led to V4+-free SAPO-34 samples, so any Bronsted acidity of the samples should be assigned to the incorporated Si atoms. These materials, far from being conventional SAPO-34, have triclinic CHA structure, which provides some structural singularities and have not been tested as catalysts in the MTO reaction yet. Despite their conversion level achieved was lower than that given by conventional SAPO-34, probably because of the scarce optimization of the physicochemical properties of the material, the selectivity towards the different olefins is interestingly different, favoring C4 olefins at the expense of ethylene and propylene.