Production of Isobutylene from Syngas – Isosynthesis Over Zirconia-Based Catalysts

This study presents the conversion of syngas (synthesis gas, CO + H₂) into isobutylene with monoclinic zirconia (m-ZrO₂), mixed monoclinic/tetragonal zirconia (m/t-ZrO₂), tetragonal zirconia (t-ZrO₂) and Ce/La-doped zirconia samples. The physical and chemical properties of the catalyst samples were characterised with nitrogen physisorption, NH₃ and CO₂ temperature-programmed desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Sample material crystal structures were analysed with powder X-ray diffraction (PXRD) analysis. The pure monoclinic ZrO₂ sample showed the best catalytic performance with 66% CO conversion and 44 mol-% C/hydrocarbons isobutylene selectivity at reaction conditions of 450 °C, 45 bar, GHSV = 2000 h⁻¹, and CO: H₂ = 1:1. While pure monoclinic ZrO₂ was a highly efficient catalyst for the isosynthesis reaction, pure tetragonal ZrO₂ produced only a minor amount of isobutylene (~ 1 mol-%C/hydrocarbons). Interestingly, Ce/La-doped zirconia had a cubic/tetragonal crystal structure, while showing selectivity and activity comparable to m-ZrO₂ (CO conversion 64% and isobutylene selectivity of 40 mol-% C/hydrocarbons). Although samples with a purely t-ZrO₂ crystal phase significantly reduced the desired isobutylene selectivity, catalysts with relatively high Ce/La loadings (17% and 5% of the total catalyst mass) in the cubic/tetragonal crystal structure achieved high isobutylene activity and selectivity, owing to the dopant-induced high base-to-acid site ratio and enhanced synthesis gas adsorption capacity. According to the results from density functional theory (DFT) calculations, the poor selectivity of the t-ZrO₂ phase towards isobutylene was due to the promoted intermediate methanation on the coordinatively saturated sites of t-ZrO₂. Compared to the t-ZrO₂ sample, m-ZrO₂ showed high isobutylene selectivity and activity related to high base to acid site ratio, high synthesis gas adsorption capacity, and kinetically favourable crystal structure to form C₂₊ hydrocarbons from C₁ intermediates. These results are important factors in the future work to prepare intrinsically active and isobutylene selective catalysts for the isosynthesis reaction.