Synthesis of hierarchical SAPO-34 zeolites with tuned mesopore structure for methanol to olefins (MTO) reaction using polyethylene glycol as a soft template

Abstract

Hierarchically porous zeolites with different porosity levels have attracted much attention because of their well-defined pore system, improved mass transfer and, higher catalytic performance. In the present work, mesoporosity was introduced into silicoaluminophosphate (SAPO) zeolite by the hydrothermal method using tetraethylammonium hydroxide and morpholine as structural directing agents and polyethylene glycol (PEG) as a cost-effective mesopore soft template with a wide range of molecular weights (MWs) and molar ratios. The selected catalysts were characterized by techniques of XRD, BET, FESEM, and NH3-TPD and, the catalytic performance of all hierarchical porous SAPO-34 samples in the methanol to olefins (MTO) reaction was evaluated in a fixed bed reactor at 425\u2009°C and methanol weight hourly space velocity (WHSV) of 2.3\u2009h−1 using a 20% methanol in water as a feed stream. Furthermore, the influences of mesoporosity were studied carefully to improve the catalytic performance of samples. The results showed that the tuned mesopores formed within the catalyst structure has a direct relation with the molecular weight of PEG, and it truly affects the diffusion pathways and acidity of the prepared samples. It was found that the stability and catalytic performance of hierarchical SAPO-34 samples improved in MTO reaction. SAPO-34 synthesized by PEG4000 using a molar ratio of 0.01 demonstrated 10% improvement in total light olefins selectivity (90.6%) with the highest selectivity to propylene (42%) and total butylene (12.7%), whereas its selectivity to C1–C4 alkanes (3.9%) and heavy hydrocarbons (2.5%) are relatively lower compared to the parent and other modified catalysts.