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

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.


Materials and Methods
Commercially available reagents were used without further purification. Dry solvents were obtained from Acros/Fischer or Sigma Aldrich and stored over activated molecular sieves (3 Å). Ultra-pure water (18.2 MΩ.cm) was obtained by a Millipore Milli-q Advantage water purification system. X-ray photoelectron spectroscopy (XPS) was performed on a Nexsa Photoelectron Spectrometer (Thermo Fisher Scientific, UK). Samples were drop casted as suspensions in DCM onto silicon wafer to form thin films for analysis. Element specific high-resolution spectra for Carbon (C 1s 279-298 eV) Nitrogen (N 1s 392-410 eV) and Gold (Au 4f 80-96 eV) with step sizes of 0.1 eV and a pass energy of 50 eV were acquired. All measurements were performed using Al-Kα X-rays with a spot size of 400 µm. Obtained spectra were evaluated using the Advantage software package (v5.9929/build 06752) provided by Thermo Fisher Scientific.
Solid-state NMR (ssNMR) was carried out on a Bruker Avance NEO 500 wide bore system (Bruker BioSpin, Rheinstetten, Germany) using a 4 mm triple resonance magic angle spinning probe. Between 15 -25 mg of material was packed into a 4 mm zirconia CRAMPS rotor. The resonance frequency for 13 C NMR was 125.78 MHz, the MAS rotor spinning was set to 14 kHz. Cross polarisation was achieved by a ramped contact pulse with a contact time of 3 ms. During acquisition 1 H was high power decoupled using SPINAL with 64 phase permutations. The 1 H π/2 pulse was 2.5 µs, the relaxation delay was set to 4 s, and with roughly 2000 scans a sufficient signal to noise was achieved. The chemical shifts for 13C are reported in ppm and are referenced external to adamantane by setting the low field signal to 38.48 ppm.
UV-Vis measurements were conducted with an Agilent Technologies G1103A using open quartz cuvettes. Stirring was provided by an external magnetic stirrer.
Fourier-transform infrared spectroscopy (FT-IT) was performed on finely ground samples in the range 350-4000 cm -1 using a Tensor II FT-IR Spectrometer from Bruker.
Thermal analyses (TGA) were performed using a Discovery TGA (TA instruments, USA). Approximately 10 mg of each sample was heated from room temperature under air flow (100 mL·min -1 ) to 110 °C at a rate of 10 °C·min -1 and held at this temperature for 15 min. Following the isothermal step, samples were heated to 800 °C at a rate of 10 °C·min -1 .
Elemental analysis (EA) was performed using a Eurovector EA 3000 CHNS-O Elemental Analyser. Between 0.75 and 3.0 mg of each sample was weighed into tin vials (4×6 mm) for each individual run and each sample was ran at least in duplicate. Sample weighing is done using a micro balance (Sartorius, ME 5 OCE) for accuracy. The operating temperatures for the combustion and reduction were 1000 °C (1480 °C for O analysis) and 750 °C, respectively, with high purity helium (99.999+) used as a carrier gas.
Porous properties were determined using N2 sorption-desorption isotherms measured at -196 °C using a TriStar II from Micromeritics Instrument Corporation, controlled by the software TriStar II 3020 version 3.02. Samples were degassed for at least 4 h at 120 °C under N2 using a FlowPrep 060 from Micromeritics Instrument Corporation. Surface areas were calculated using the Brunauer-Emmett-Teller (BET) method on the adsorption branch between 0.05-0.2 P/P0. Total pore volumes were calculated from the volume of N2 adsorbed at P/P0 = 0.97 and micropore volumes were determined using the t-plot method for the relative pressure range between 0.15-0.4.
Powder X-ray diffraction (PXRD) measurements were performed on a Malvern Panalytical Empyrean instrument in reflectance mode configuration using Cu Kα1+2 radiation (wavelength of 1.5406 Å). Measurements were taken using a step size of 0.0263 °2θ, generator settings of 40 mA and 45 kV at a measurement temperature of 25 °C. The scan range was 5 to 50° with a scan step time of 59.4 s.
Transmission electron microscopy (TEM) was measured at the Electron Microscopy Facility at IST Austria using a Phillips Tecnai 12 (120kV) TEM equipped with a CMOS TVIPS TemCam-F216 camera. Prior to measurement, samples suspended in toluene were drop casted onto formvar carbon film on 200 mesh copper grids. TEM micrographs were analyzed using on Image J.

Synthesis of HCP•BIMZ
HCP•BIMZ was synthesized following a procedure reported previously. [1] Briefly, BIMZ (0.670 g, 2 mmol), benzene (0.468 g, 6 mmol), and dimethoxymethane (DMM) (1.360 g, 18 mmol) were added to 1,2-dichloroethane (8 mL). FeCl3 (2.920 g, 18 mmol) was quickly added, and the reaction vessel fitted with a reflux condenser. The reaction was then heated to 40 °C for 5 h to form a pre-network, before being heated at 80 °C overnight to drive the reaction to completion. The resulting polymer was then washed with methanol for 24 h via Soxhlet extraction before drying at 60 °C in vacuo, yielding HCP-BIMZ in quantitative yields.

XPS Spectra
All displayed spectra were deconvoluted using the Avantage software package (v5.9929/build 06752) provided by Thermo Fisher. Displayed baselines were generated by using the smart baseline feature of the software package. Raw data is displayed as gray symbol, generated envelope as red line and peak fits as colored areas.

Figure S7. TEM micrographs and statistical evaluation of size distribution of A) decomposed HCP•NHC-Au(I) and B) HCP•NHC@AuNP.
Statistical data based on n = 100 AuNPs.

C (CP/MAS) ssNMR Spectra of HCP•BIMZ, HCP•NHC-Au(I) and HCP•NHC@AuNP
13 C ssNMR allows the identification of characteristic peaks of HCP including aromatic R-CAr, H-CAr and crosslinking -CH2-moiety all associated to the HCP structure. [1][2] The C 2 position of the BIMZ and NHC structure (expected between~140 ppm and ~180 ppm, respectively) could not be detected due comparably low concentration in the HCP network. * are attributed to spinning sidebands.

Flow Catalysis
Experiments were carried out in Braun Injekt®-F 1 mL syringes with a compressed pad of HCP•NHC@AuNP between two layers of cotton. Flow was provided by a Landgraf Hill LA-30 syringe pump.

XPS Spectra and TEM Micrograph of Recovered HCP•NHC@AuNP from Flow Catalysis
All samples were obtained by careful removal of the HCP•NHC@AuNP pad from the syringe enclosure. The HCP•NHC@AuNP pad was rinsed with acetone to separate HCP•NHC@AuNP and cotton. HCP•NHC@AuNP was washed and collected by centrifugation.   . b Au content on based on published Au wt% of investigated catalyst systems. * System using identical reagent ratios.