Role of Ga3+ promoter in the direct synthesis of iso-butanol via syngas over a K–ZnO/ZnCr2O4 catalyst

Abstract

The direct synthesis of iso-butanol is an important reaction in syngas (composed of CO and H₂) conversion. K–ZnO/ZnCr₂O₄ (K–ZnCr) is a commonly used catalyst. Here, Ga³⁺ is used as an effective promoter to boost the efficiency of the catalyst and retard the production of CO₂. X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet-visible diffuse reflection spectroscopy and electron microscopy were used to characterize the structural variations with different amounts of Ga³⁺, the results showed that the particle size of the catalyst decreases with the addition of Ga³⁺. The temperature-programmed desorption of NH₃ and CO₂, and diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTs) analysis of the CO adsorption revealed that the acidity and basicity were altered owing to the different forms of Ga³⁺ adoption. X-ray photoelectron spectroscopy and density functional theory (DFT) calculations revealed that the formation of Ga clusters that are coordinated on the exposed surfaces of ZnCr₂O₄, and undergo a tetra-coordinated Ga³⁺ exchange with one of the Zn in ZnCr₂O₄ (ZG) and ZnGa₂O₄, probably depends on the amount of Ga added. The structural evolution of the Ga³⁺ promoted K–ZnO/ZnCr₂O₄ catalysts can be described as follows: (i) the main forms are ZG and Ga coordinated ZnCr₂O₄, in which the amount of Ga³⁺ is below 1.10 wt%; and (ii) the Ga³⁺ containing compound is gradually changed from ZG to ZnGa₂O₄ and the amount of gallium clusters increased when the amount of Ga³⁺ was higher than 1.10 wt%. The catalytic performance evaluation results show that K–Ga_1.10ZnCr exhibits the highest space time yield and selectivity of alcohols, in which the three compounds play different roles in syngas conversion: ZG is the main active site that boosts the efficiency of the catalysts, owing to the intensified CO adsorption and decreased activation energy of CHO formation through CO hydrogenation; ZnGa₂O₄ only modifies the surface basicity and acidity on the catalyst, thereby impacting the carbon chain growth after the CO is adsorbed. The effects of Ga coordinated with ZnCr₂O₄ shows little impact on the CO adsorption owing to the weak electron donating effects of Ga.