A simple and efficient approach to confine Cu/ZnO methanol synthesis catalysts in the ordered mesoporous SBA-15 silica

Abstract

The ammonia-driving deposition-precipitation (ADP) method has been applied with the purpose of confining active Cu/ZnO methanol synthesis phases inside the pores of the ordered mesoporous SBA-15 silica. Thus, a series of CuZnx/SBA-15 catalysts with total (Cu+Zn) metal loading of 35wt% and Cu/Zn mass ratios (x) of 0.5, 1, 2, 4, and 6, as well as a Zn-free Cu/SBA-15 sample, have been prepared. Additionally, a CuZn2/SBA-15 sample (Cu+Zn=35wt%, Cu/Zn=2) prepared by impregnation and a coprecipitated Cu-ZnO-Al2O3 (CZA) catalyst have been prepared as reference. The materials have been characterized by ICP-OES, N2 physisorption, XRD, in situ H2-XRD, TEM, H2-TPR, and N2O chemisorption, and their methanol synthesis activities determined, after in situ H2 reduction, under realistic conditions (533K, 4.0MPa, syngas: 66%/30%CO/4%CO2). Copper NPs in CuZnx/SBA-15 (ADP) catalysts with Cu/Zn mass ratios up to 2 were effectively confined within the SBA-15 pores (dCu<7nm) while at higher ratios part of Cu formed large particles sizing about 60–70nm on the external SBA-15 surface. Similar large Cu NPs were also found for the impregnated catalyst (Cu/Zn=2). A maximum in the methanol synthesis activity was attained for the catalyst prepared by ADP with Cu/Zn mass ratio of 2. This catalyst displayed a synthesis activity about 14 times higher than the impregnated sample with equal composition and nearly the same activity (per mass of Cu) than the reference CZA catalyst. No direct relationship between the methanol synthesis rate and Cu0 surface area was found for the studied catalysts. Instead, our results strongly suggested that the active sites are located at the Cu0-ZnOx interface, which can be maximized through the effective confinement of Cu0 NPs inside the SBA-15 channels using the simple ADP method.