Role of SAPO-18 Acidity in Direct Syngas Conversion to Light Olefins

The metal oxide-zeolite (OX-ZEO) bifunctional catalyst concept has aroused wide interest recently for direct syngas conversion. However, the mechanism of product selectivity control is far from being understood. Herein, we studied the role of acidity on the product selectivity by taking SAPO-18 as an example and physically mixing it with ZnCrOx because its Si/Al ratio can be varied in a relatively wide range. The results demonstrate a strong dependence of the product selectivity on the Si/Al ratio. Light olefin selectivity reaches as high as 81.6% with CO conversion 35.5% at Si/Al ratio = 0.011. A higher Si/Al ratio gives a stronger acid strength and higher acid density, which leads to a reduced ratio of olefins/paraffins (O/P) and an increased C3/C2 ratio in the products. This could be attributed to the enhanced secondary reactions of ethylene under a H2-rich atmosphere of syngas conversion. Therefore, it is essential to control the acid strength and density of SAPO-18 in order to achieve selective synthesis of light olefins.