Solid base catalysed 5-HMF oxidation to 2,5-FDCA over Au/hydrotalcites: fact or fiction?

Synergistic effects between alkali-free hydrotalcites and gold nanoparticles afford efficient heterogeneous catalysts for the cascade oxidation of 5-HMF to 2,5-FDCA.


Catalyst synthesis
Alkali-free hydrotalcites were synthesised according to our literature co-precipitation method. A mixture of 1 M Mg(NO 3 ) 2 .6H 2 O and Al(NO 3 ) 3 .9H 2 O in deionised water was first prepared in a 100 ml volumetric flask at a nominal Mg:Al molar ratio of 3:1. Separately, a 2 M aqueous solution of (NH 4 ) 2 CO 3 and NH 4 OH of pH 9 was prepared in a second volumetric flasks. The two solutions were simulatneously-fed via an Atlas syringe pump dropwise at room temperature into a 2 L Radleys Ready reactor, filled with 300 ml of de-ionised water, under 300 rpm stirring; the addition rate was set at 1 ml.min -1 and the pH maintained between 9.3-9.5 iva addition of concentrated NH 4 OH (35 wt.% NH 3 aqueous solution). The resulting white hydrogel was aged overnight at 65 °C, then washed with H 2 O until the pH was neutral, and finally dried in vacuo overnight at 100 °C and stored in a dessicator.
Au/HT catalysts were prepared subsequently prepared via deposition-precipitation with nominal gold loadings of 0

Catalyst characterisation
Nitrogen porosimetry was undertaken on a Quantachrome Nova 4000e porosimeter; samples were degassed at 120 °C for 2 h prior to analysis, with multi-point BET surface areas calculated over the relative pressure range 0.01-0.3. Powder XRD patterns were recorded on a Bruker D8 ADVANCE diffractometer with a Cu K α source between 2 θ = 10-80 ° and 0.02 ° step size. XPS was performed on a Kratos Axis HSi X-ray photoelectron spectrometer fitted with a charge neutraliser and magnetic focusing lens employing Al K α monochromated radiation (1486.7 eV); spectral fitting was performed using CasaXPS version 2.3.15, with spectra enery-corrected to the C 1s peak of adventitious carbon at 284.6 eV. Base site densities were measured via CO 2 pulse chemisorption and subsequent temperature programmed desorption on a Quantachrome ChemBET 3000 system; samples were outgassed at 120 °C under He (120 ml.min -1 ) for 1 h prior to CO 2 titration at 40 °C.
SEM analysis was conducted on an Oxford Instruments EVO SEM; samples were mounted on a carbon disc and coated using 90:10 Au:Pd. EDX analysis was undertaken via Oxford Instruments Inca software on uncoated samples. High-resolution TEM was performed on a Philips EM208 TEM with a tungsten filament at 80 kV. Samples were deposited from ethanolic solution on holey carbon copper grids. Elemental analysis was performed by ICP-MS (MEDAC UK).
Au L III K-edge (11.92 keV) XAS measurements were made on beamline B18 of the Diamond Light Source in fluorescence mode, using a Si(111) double crystal monochromator and 9-element Ge solid state detector. In situ thermal decomposition of the catalyst precursor was followed in a bespoke "Sankar" pellet furnace under flowing air (10 ml.min -1 ) between 25-500 °C. Operando measurements were made in a bespoke PTFE cell with Kapton windows on a catalyst reaction mixture (250 mg of 2 wt% Au/HT, 1 mmol HMF and 60 ml H 2 O) recirculated from an external oxygenated round bottom flask (10 ml.min -1 flowing O 2 at 1 bar) between room temperature and 90 °C; spectra were processed using Athena and Artemis software within the IFEFFIT software suite.

5-HMF oxidation
Selective oxidation of 5-HMF was conducted in a 3-neck round-bottomed flask using a Radleys Starfish reactor, employing 0.2 mmol HMF in 6 ml of deionised water, and either 25 or 50 mg of Au/HT catalyst under flowing O2 (10 ml.min -1 ) at 90 °C and 500 rpm stirring (sufficient to eliminate mass-transport effects). Additional 1 M NaOH solution (pH 14) was added to some experiments in order to regulate solution pH as described in the main manuscript.
Aliquots were sampled periodically and analysed on an Agilent Technologies 1200 Infinity HPLC equipped with UV-vis and R.I. detectors. A Zorbax Hilic plus HPLC column (4.6 mm x 100 mm x 3.5m) was employed, protected by a Rx-SIL guard column (4.6 mm x 12.5 mm), in conunction with a gradient method, developed in co-operation with Agilent Technologies, to achieve resolution of HMF ( = 282 nm), HMFCA ( = 260 nm), FFCA ( = 282 nm) and FDCA ( = 260 nm). The mobile phase was prepared with acetonitrile as eluent A, and eluent B an HPLC grade aqueous solution buffered by 50 mM CH 3 COONH 4 ; the pH was adjusted to 5.2 by adding the requiste amount of glacial CH 3 COOH. The gradient method is defined in Table S3: A typical chromatogram is shown in Fig. S9 highlighting the excellent resolution of all components. Figure S9. HPLC chromatogram of reaction mixture during 5-HMF oxidation over 2 wt% Au/HT.