Nano‐web surface‐mounted metal‐organic framework films with tunable amounts of acid sites as tailored catalysts

Metal‐Organic Frameworks (MOFs) are a promising class of materials for many applications, due to their high chemical tunability and superb porosity. By growing MOFs as (thin‐) films, additional properties and potential applications become available. Here we report on copper (II) 1,3,5‐benzenetricarboxylate (Cu‐BTC) MOF thin‐films, synthesized via spin‐coating, resulting in “nano‐webs” – fiber‐like structures. These surface‐mounted MOFs were studied using photo‐induced force microscopy and time‐of‐flight secondary ion mass spectrometry. We revealed the optimal concentration of precursors, that resulted in chemically homogeneous, pure nano‐webs. Furthermore, we tuned the morphology and (un)coordinated Cu‐sites in the web by varying the rotation speed of the spin‐coating process. XRD, showed that rotation speeds ≥2000 rpm (with precursors in a water/ethanol solution) generate the catena‐triaqua‐µ‐(1,3,5‐benzenetricarboxylate)‐copper(II), or Cu(BTC)(H 2 O)­ 3 coordination polymer. XPS highlights the strong decrease in number of (defective) Cu + ‐sites, as the nano‐webs mainly consist of coordinated Cu 2+ Lewis acid sites (LAS) and organic linker‐linker, e.g. hydrogen bonding, interactions. Finally, we illustrate the Lewis‐acidic character of the Cu‐sites by testing the films in the isomerization of α ‐pinene oxide. The higher number of LAS (≥3000 rpm), result in higher campholenic aldehyde selectivity reaching up to 87.7%