Hyper crosslinked polymer supported NHC stabilized gold nanoparticles with excellent catalytic performance in flow processes

Abstract

Highly active and selective heterogeneous catalysis driven by metallic nanoparticles relies on a high degree of stabilization of such nanomaterials facilitated by strong surface ligands or deposition on solid supports. In order to tackle these challenges, N-heterocyclic carbene stabilized gold nanoparticles (NHC@AuNPs) emerged as promising heterogeneous catalysts. Despite the high degree of stabilization obtained by NHCs as surface ligands, NHC@AuNPs still need to be loaded on support structures to obtain easily recyclable and reliable heterogeneous catalysts. Therefore, the combination of properties obtained by NHCs and support structures as NHC bearing "functional supports" for the stabilization of AuNPs is desirable. Here, we report the synthesis of hyper-crosslinked polymers containing benzimidazolium as NHC precursors to stabilize AuNPs. Following the successful synthesis of hyper-crosslinked polymers (HCP), a two-step procedure was developed to obtain HCP·NHC@AuNPs. Detailed characterization not only revealed the successful NHC formation but also proved that the NHC functions as a stabilizer to the AuNPs in the porous polymer network. Finally, HCP·NHC@AuNPs were evaluated in the catalytic decomposition of 4-nitrophenol. In batch reactions, a conversion of greater than 99% could be achieved in as little as 90 s. To further evaluate the catalytic capability of HCP·NHC@AuNP, the catalytic decomposition of 4-nitrophenol was also performed in a flow setup. Here the catalyst not only showed excellent catalytic conversion but also exceptional recyclability while maintaining the catalytic performance.

Fig. 1. (A) left to right: general structure benzimidazole based NHC, NHC@AuNP on solid support and AuNP supported in NHC containing support structure. (B) Structure and schematic of HCP/HCP·NHC structures.

Scheme 1. Synthesis of HCP·NHC@AuNP. P1 polymerization: BIMZ, benzene, DMM, FeCl₃ in 1,2-dichloroethane at 80 °C for 24 h. P2 complexation: KHMDS, toluene, 1 h, RT followed by [Au(DMS)Cl], DCM, 16 h, RT. P3 reduction: NaBH₄, DCM, 16 h, RT.

Fig. 2. (A and B) High resolution N 1s and Au 4f scans of HCP·NHC–Au(i) and HCP·NHC@AuNP respectively; (C) comparison of N 1s and Au 4f scan and corresponding Δ^1/2via envelopes of HCP·BIMZ (gray), HCP·NHC–Au(i) (black) and HCP·NHC@AuNP (red); (D) TEM micrograph of HCP·NHC@AuNP.

Fig. 3. (A) UV-vis absorption spectra of the catalytic reduction of 4-NP to 4-AP at different reaction times using conditions CAT 1; (B) kinetics and corresponding rate constants the reduction reaction of 4-NP⁻ to 4-AP with different HCP·NHC@AuNP loadings (CAT 1–3); (C) conversion of 4-NP⁻ over 10 consecutive cycles in the flow reactor (flow 0.25 mL min⁻¹); FCAT 1; (D) schematic of the flow reactor setup; (E) high resolution XPS of N 1s and (F) Au 4f scans after 10 flow-through cycles.