Pd(/Fe3O4)-on-ZIFs: nanoparticle deposition on (nano-)MOFs from ionic liquids

Abstract

Well-defined spherical Pd-NPs (∼6–12 nm size, 4–17% wt content) were efficiently deposited on nano- or micro- (∼100–2500 nm) crystals of zeolite imidazolate frameworks (ZIFs) from different ionic liquids (ILs, typically 1-butyl-3-methylimidazolium bistriflimidate, [BMIm]NTf₂) at 180–230 °C under microwave (MW) heating for 1–10 min. The firm, nearly exclusive on-surface deposition, proven by systematic analysis of TEM micrographs, is achieved via size-exclusion of the IL-solvated PdCl₂ precursor. The one-pot high-quality NP deposition from partially stabilizing IL media leads to particularly well-defined metal nanoparticles-on-metal–organic frameworks, MNP-on-MOF(NP)s. They are contraposed to MNP@MOFs or non-specified MNP/MOFs, and are viewed as an extension of MNPs from ionic liquids, securing MNP-agglomeration protection after removal of the IL and reversible redispersion, and offering the functionality of the porous support. Pd-on-ZIF-8-NP (R = Me substituent of the azole ligand) demonstrated the highest stability and morphological quality, while the ZIF-90 analogues with the same sod topology (R = COOH, CHO, CH₂CH₂NH₂) or the triazolate MAF-66 (R = NH₂) with dia topology showed lesser stability (particularly for R = COOH and CHO). Ferromagnetic Pd/Fe₃O₄- and Fe₃O₄/Pd-on-ZIF-8-NP, ZIF-90-NH₂(-NP), and MAF-66 composites with nanoparticulated Fe₃O₄ (5–13 nm size, 5–22%_wt content) demonstrate the feasibility of sequential NP deposition of varied nature. The Pd-on-ZIF composites demonstrated high catalytic activity in the reduction of 4-nitrophenol to 4-aminophenol by NaBH₄ in water (TOF 0.38–4.10 vs. 0.088 s⁻¹ for 10%_wt Pd/C; amino-functionalization and the presence of Fe₃O₄ are a major and a minor factor, respectively). The reduction of the same substrate with H₂ in apolar media was much slower for the composite catalysts compared to Pd/C (10%_wt) due to possible poisoning by the residual IL.