Stability of mesocellular foam supported copper catalysts for methanol synthesis

Abstract

Mesocellular foam is a well-defined mesoporous silica comprising large cages (˜20 nm), connected by small windows (2–10 nm). Here, we used mesocellular foam as a support material in the preparation of Cu based model catalyst for methanol synthesis at high temperature and pressure. To this end, we synthesized two types of mesocellular foam, with the same cage size, but a different window size of 8 and 2.3 nm, and compared them to the use of a silica gel support. Cu particles were deposited by impregnation with a copper nitrate precursor solution and decomposition in either N2 or H2-containing gas stream. We followed the phase evolution in situ, and identified a method using direct reduction in H2 to deposit 3 nm Cu particles on all different supports. The catalysts prepared on the mesocellular foam displayed a much higher stability than the silica gel supported catalysts. The impact of the window size of the MCF was small, but significant: the catalysts supported on the small window mesocellular foam were more stable on the long term. However, they also had a lower activity due to embedment of the Cu particles in the windows, as revealed with electron tomography. A higher metal loading on the small window mesocellular foam resulted in enhanced coalescence of the Cu particles, which was attributed to smaller interparticle distances. Increasing the loading on this support even further increased the size and polydispersity of the Cu nanoparticles, leading to a higher deactivation rate due to Ostwald ripening. Hence, the loading and size polydispersity are important parameters in determining the catalyst stability. We also found that the nature of the SiO2 support has more impact on the catalyst stability than the restricting pore sizes, probably due to the fact that Ostwald ripening dominates particle growth on the longer time scales.