Tuning the activity by controlling the wettability of MOF eggshell catalysts: A quantitative structure-activity study

In this work, we report a quantitative structure-activity relationship of functionalized ZIF-type materials (SIM, Substituted Imidazolate Material). Keeping the catalytic center unmodified, post-synthetic modification allows an efficient control of the hydrophilic-hydrophobic balance which drives the competitive adsorption of reactants and products. Hence, we attribute the increase in the reaction rate of Knoevenagel condensation (up to seven times) to the modification of the environment of catalytic sites through the creation of hydrophobic environment surrounding the catalytic sites. Surface tension properties are monitored by measuring the contact angle with a water droplet on a SIM thin film grown on a flat support. We show that the contact angle is directly correlated to the catalytic activity of Knoevenagel condensation. Hence, we propose that the contact angle of a catalyst thin layer with water can be a quantitative descriptor of direct relevance for catalytic processes which produces water and/or for catalysts that are poisoned with water. Finally, we measured the intrinsic reaction rate free of external and internal diffusion limitations by controlling the thickness of the SIM shell on SIM/alumina beads. Insights on the localization of the active sites (surface vs. bulk) are provided. (C) 2011 Elsevier Inc. All rights reserved.