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

• Published in: Chemistry of materials 2008-07, Vol.20 (14), p.4576-4587
• Main Authors: Müller, Maike, Hermes, Stephan, Kähler, Kevin, van den Berg, Maurits W. E, Muhler, Martin, Fischer, Roland A
• Format: Article
• Language: English
• Subjects: Catalysis and Catalysts; Characterization of Materials; Composites (including Ceramic Composites, Polymer Composites, and Nanocomposites)
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm703339h

The loading of [Zn4O(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 = PMe3, CNtBu) and ZnEt2 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. 17O labeling studies using H2 17O (30%) revealed that neither the bdc linkers nor the central oxide ion of the Zn4O 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 m2·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 N2O/H2 treatment, resulting in a (Cu2O/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 17O-MAS NMR spectroscopy gave evidence for a largely intact MOF-5 matrix with imbedded ZnO nanoparticles