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 degrees C in the case of Cu and hydrolysis or dry oxidation at 25 degrees C followed by annealing 250 degrees 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), wag 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 mn 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 mn 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 degrees 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.