Synthesis, characterization, and function of Au nanoparticles within TS-1 zeotypes as catalysts for alkene epoxidation using O2/H2O reactants

Abstract

A synthetic procedure was developed for the encapsulation of Au nanoparticles within titanium silicalite-1 (TS-1). Encapsulation was achieved via crystallization of TS-1 synthesis gels containing ligated Au cations, resulting in TS-1 framework assembly around Au coordination complexes. X-Ray diffraction and micropore volume measurements indicated that TS-1 could be crystallized at much milder conditions than previously reported, allowing the assembly of TS-1 without concomitant decomposition of added Au complexes. Infrared (IR) and UV–vis spectroscopy showed that Ti was incorporated into the frameworks as tetrahedrally coordinated atoms. Changes to established crystallization procedures for TS-1 included lower temperature (393 K vs. 448 K) compensated by longer time (120 vs 48 h) and the use of seed crystals. Post-synthetic thermal treatments led to the formation of small Au nanoparticles (2.8–3.8 nm) visible in electron micrographs and free of organic debris, as indicated by IR spectroscopy. Such nanoparticles are protected from bulky titrants, and thus reside within TS-1 crystallites (>96% encapsulation). The Au particles were active for propene epoxidation with H2O/O2 even without promotion by alkali cations (e.g., Cs+), though alkali doping led to enhanced rates. These Au/TS-1 systems were less active for epoxidation than Au/TS-1 prepared by deposition–precipitation, likely because the latter contains a disproportionately active minority population of exceedingly small (<1 nm) Au clusters.