PNNL

Abstract

The ability to tailor alkene product distributions derived from ethanol conversion is of notable significance to the generation of renewable middle distillates with reduced energy input and capital costs. Here, we have developed the series of bimetallic Cu- and rare earth-containing (RE) Beta zeolite catalysts that yield high C3+ alkene selectivity from ethanol upgrading (>80% selectivity at ?100% conversion, 623K) by combining Cu sites for dehydrogenation/ hydrogenation reactions with a series of rare earth metals (e.g., Y, La, Gd, Ce, Yb) that catalyze aldol condensation and C4+ alcohol dehydration reactions. All RE-containing catalysts possess a uniform molar density of Lewis acid RE sites, while the strength of acidity follows the sequence of Yb12/Beta >Y7/Beta > Gd12/Beta > Ce10/Beta > La12/Beta as measured by IR pyridine adsorption and NH3 TPD technique. The formation rates of butene isomers and longer chain alkenes (C4/C5+) are linearly correlated with the strength of Lewis acidic RE identity. Rate measurements indicate that the RE selection plays the vital role in altering the rate of the key competitive reactions within the ethanol-to-alkenes reaction network, namely C4 alcohol dehydration and C-C chain growth, which dictate alkene product distributions. The catalyst with the strongest Lewis acid center, Cu-Yb12/Beta, exhibits the highest selectivity to butene due to either minimal successional aldol condensation reactions or increased butanol dehydration reactivity relative to the other Cu-RE/Beta sample. Cu-La12/Beta possesses the weakest Lewis acid strength of these samples and catalyzes the formation of longer carbon chain alkenes (C5+) due to favoring further aldol condensation reactions from prolonged C4 intermediates over dehydration reactions. These findings indicate a feasible and promising method for tailoring alkene product distributions from ethanol conversion over Cu-RE/Beta catalysts by tuning the RE metal identity and the associated kinetics of key intermediate reactions that control carbon chain growth. This offers a catalyst design strategy for the complex cascading reaction network such as ethanol-to-alkenes that allows for controlling individual and overall alkene yields and selectivities.