Polyionic Nanoclays: Tailorable Hybrid Organic-Inorganic Catalytic Platforms

摘要

We unveil the synthesis and initial application of a hybrid inorganic-organic architecture comprising two-dimensional, few nanometer-thick magnesium phyllo(organo)silicate nanosheets displaying an ordered array of covalently linked imidazolium functionalities. Accessible via a simple, one-pot ambient-temperature synthesis using a custom organosiloxane, these so-called polyionic nanoclays (PINCs) represent an emergent class of supported ionic liquid phase and hold a number of attractive features, including: (i) an extremely high ionic density (e.g., 3.8 mmol g(-1) for the 1-methyl-3-propylimidazolium chloride PINC, roughly two-thirds the imidazolium cation density observed in the analogous free-flowing ionic liquid); (ii) the exchangeability of the associated counteranions to impart supplementary functionality or modulate solvent dispersibility; and (iii) the facility for tailoring the organic component, for example incorporating different (e.g., pyrrolidinium, pyridinium, phosphonium) or mixed onium moieties by employing the appropriate organosiloxane precursor(s). As a preliminary investigation of PINCs as nanocatalytic supports, we performed anion exchange with [AuCl4](-), followed by in situ reduction to generate small (<5 nm gold nanoparticles (AuNPs) on the clay lamellae. The resulting nanosized gold-modified PINCs dispersed in water exhibited an astonishing turnover frequency of 25 000 h(-1) for 4-nitrophenol reduction at room temperature, making this hybrid material the most active gold nanocatalyst reported for this model reaction. Most remarkably, AuNPs supported on the PINC showed a dramatically enhanced catalytic activity (4000% increase) compared to similar gold nanoparticles suspended in water (600 h(-1)), suggesting a major synergistic effect arising from the PINC support. Overall, the generality and flexibility of this route to lamellar hybrid nanoclays, coupled with the ability to accommodate a wide range of pendant organic ionic moieties suggests an auspicious future for PINCs in catalysis, ion exchange, energy storage, separations, (bio)sensing, imaging, and the construction of nanoscale assemblies.