Loading of MOF-5 with Cu and ZnO Nanoparticles by Gas-Phase Infiltration with Organometallic Precursors: Properties of Cu/ZnO@MOF-5 as Catalyst for Methanol Synthesis

Abstract

The loading of [Zn 4 O(bdc) 3 ] (MOF-5; bdc = 1,4-benzenedicarbocylate) with nanocrystalline Cu and ZnO species was achieved in a two-step process. First, the solvent-free gas-phase adsorption of the volatile precursors [CpCuL] (L = PMe 3, CN t Bu) and ZnEt 2 leads to the isolable inclusion compounds precursor@MOF-5. These intermediates were then converted into Cu@MOF-5 and ZnO@MOF-5 by hydrogenolysis or photoassisted thermolysis at 200−220 °C in the case of Cu and hydrolysis or dry oxidation at 25 °C followed by annealing 250 °C in the case of ZnO. 17 O labeling studies using H 2 17 O (30%) revealed that neither the bdc linkers nor the central oxide ion of the Zn 4 O unit exchange oxygen atoms/ions with the imbedded ZnO species. The obtained material Cu@MOF-5 (11 wt % Cu), exhibiting an equivalent Langmuir surface of 1100 m 2 ·g −1, was further characterized by powder X-ray diffraction (PXRD), X-ray absorption spectroscopy (XAS), and transmission electron microscopy (TEM). The Cu nanoparticles are homogeneously distributed over the MOF-5 microcrystals, occupying only about 1% of the cavities. Their size distribution appears to be polydisperse with a majority around 1 nm in size (by EXAFS) together with a minority of larger particles up to 3 nm (PXRD). Cu@MOF-5 was reversibly surface oxidized/reduced by N 2 O/H 2 treatment, resulting in a (Cu 2 O/Cu)@MOF-5 material as revealed by PXRD and XAS. Depending on the preparation conditions of the ZnO@MOF-5 materials a variation of the ZnO loading from 10 to 35 wt % was achieved. PXRD, TEM, UV−vis, and 17 O-MAS NMR spectroscopy gave evidence for a largely intact MOF-5 matrix with imbedded ZnO nanoparticles <4 nm being in the quantum size regime. Doubly-loaded (Cu/ZnO)@MOF-5 samples were prepared by gas-phase loading of ZnO@MOF-5 with [CpCuL] followed by thermally activated hydrogenolysis. The initial catalytic productivity in methanol synthesis from a CO/CO 2 /H 2 gas mixture at 1 atm and 220 °C peaked at about 60% of an industrial reference catalyst. This result is particular surprising because of the comparably low Cu loading (1.4 wt %) and small Cu specific surface area <1 m 2 ·g −1, thus suggesting a superior interfacial contact between the Cu and ZnO nanophases. However, the materials (Cu/ZnO)@MOF-5 were unstable under catalytic conditions over several hoours, the metal organic framework collapsed, and the final catalytic activities were poor.