Study of the interaction between components in hybrid CuZnAl/HZSM-5 catalysts and its impact in the syngas-to-DME reaction

Hybrid CuZnAl(CZA)/HZSM-5 catalysts were prepared by three mixing methods in order to analyze the possible existence of interactions between components and their impact in the STD process, namely: (a) grinding of powders prior to pelletizing (grinding method), (b) slurrying the two solids in water followed by drying and pelletizing (slurry method), and (c) physical mixture of pre-pelletized components. The materials were characterized by ICP-OES, XRD, N2 physisorption, H2-TPR, 27Al MAS NMR, FTIR-pyridine, and EPR spectroscopy. Detrimental interactions producing a drastic reduction in the amount of available zeolitic Brønsted acid sites were observed for the hybrids prepared by slurry and grinding methods. Such interactions involved, on one hand, the partial blockage of micropores by CZA particles and, on the other hand, an inter-cationic exchange of Cu2+ (and possibly also Zn2+) species. The presence of isolated Cu2+ cations occupying exchange positions in the HZSM-5 zeolite was unambiguously evidenced by EPR spectroscopy. The decrease in Brønsted acidity significantly reduced the activity of the zeolite for dehydrating methanol leading to a much lower efficiency of the corresponding hybrids for DME synthesis at typical STD conditions as compared to the catalyst obtained by simple mixture of pre-pelletized components.