Syngas to light olefins over ZnAlO_x and high-silica CHA prepared by boron-assisted hydrothermal synthesis

Transformation of syngas into light olefins (STO) over bifunctional oxide and zeolite (OX-ZEO) composite catalysts requires rational design of zeolites with moderate to low acid density and strength. As a promising zeolite component in OX-ZEO towards olefins production, the synthesis of CHA with desirable acidity remains to be a challenging task. Herein, high-silica CHA zeolite with Si/Al ranges from to 20 to ∞ were prepared via a boron-assisted hydrothermal synthesis method. The influence of Si/Al ratio on the acidity and textural properties of boron-containing CHA (BAl-CHA) was systematically studied. The syngas to olefin performance over ZnAlO_x/BAl-CHA was explored. The roles of acidity, framework boron and reaction parameters in determining the olefin selectivity as well as catalyst stability were revealed. The 85.1% selectivity of light olefins was achieved over the BAl-CHA catalyst with Si/Al ratio over 300. The boron played a vital role in directing the formation of CHA structure (Si/Al > 100) and reduced the strong acid sites. The H₂/CO ratio, reaction temperature, total pressure and proximity of the two components also played crucial role in determining the light olefins selectivity. A DME/methanol involved aromatic-cycle is proposed to play a predominant role in the olefin production. The residual CO and H₂ are likely involved in the aromatic-cycle, and then affect the product distribution, with CO promoting the ethene selectivity while H₂ enhancing the alkane selectivity.