Rh/ZrO2@C(MIL) catalytic activity and TEM images. CO2 conversion performance and structural systematic evaluation of novel catalysts derived from Zr-MOF metallated with Ru, Rh, Pd or In

A set of novel materials, denoted M/ZrO2@C(MIL) (M = Ru, Rh, Pd & In), were prepared by thermal trans-formation of MIL-140C containing 10% bipyridine linkers (MIL-140C-10), to provide sites for metal coordination within the framework. These materials were transformed into active catalysts for CO2 hydrogenation when heated in a gas mixture of H-2 and CO2 (3:1), at 500 degrees C. The thermal treatment provided high surface area catalysts with high stability and high Ru or Rh metal dispersion which were very effective for the hydrogenation of CO2 to CH4, giving a CH4 production of 3.0-3.7 mol/g Ru/h or 4.2-4.3 mol/g Rh/h (at 400 degrees C, 33 bar and WHSV 23 L/h/g cat.). PXRD, XPS and TEM indicated that the effective catalysts consisted of nanoparticles of Ru-0 (2-5 nm) or Rh-0 (6 nm) associated with larger ZrO2 nanoparticles (10-20 nm), which were dispersed upon carbonaceous ribbons. Interestingly, at 250-350 degrees C, Pd/ZrO2@C(MIL) yielded mainly CO rather than CH4, with some CH3OH. The CH4 and CO production were not stable at 400 degrees C. TEM results for this catalyst indicated Pd0 and ZrO2 nanoparticles (initially 20 nm and 10-20 nm diameter, respectively). The lower, unstable activity compared to Ru and Rh could have been due to the initially larger Pd particles and their tendency to grow in size with reaction time. In/ZrO2 has mainly been used to catalyse CH3OH production, but In/ZrO2@C(MIL) gave less CH3OH than In/monoclinic ZrO2 and was less selective. At 400 degrees C In/ZrO2@C(MIL) was a stable, reverse-water-gas-shift catalyst (producing 0.9 mol CO/g In/h at WHSV 20 L/g cat/h). The In was well dispersed in the ZrO2-C and of small particle size. The poor selectivity for methanol may have been due to the tetragonal phase of the ZrO2 and the low surface In concentration.

Automated summary

A set of novel materials were prepared and transformed into high-performance catalysts for CO2 hydrogenation, showing representative activity and selectivity. Ru/ZrO2@C(MIL) and Rh/ZrO2@C(MIL) exhibited excellent CO2 hydrogenation activity and stable CH4 production at high temperatures. Pd/ZrO2@C(MIL) mainly produced CO and CH3OH, but with lower and unstable activity. In/ZrO2@C(MIL) showed good reverse-water-gas-shift catalytic activity at high temperatures, but with poor selectivity for methanol.