Chromium-free Cu@Mg/γ-Al2O3 – an active catalyst for selective hydrogenation of furfural to furfuryl alcohol

Development of a chromium (Cr)-free hydrogenation catalyst is very important to replace the existing hazardous Cr based catalyst used in the furfural hydrogenation to furfuryl alcohol. Herein, we report synthesis of well-dispersed copper nanoparticles supported on hydrothermally stable magnesium doped alumina (Cu@Mg/γ-Al2O3) for selective hydrogenation of furfural to furfuryl alcohol. The prepared catalyst was characterized by X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES), Powder X-ray Diffraction (PXRD), Surface Area Analysis (SAA), High Resolution-Transmission Electron Microscopy (HR-TEM), Temperature Programmed Reduction/Desorption (TPR/TPD) and Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) to understand textural properties of the catalyst. The prepared catalyst was found to be highly active and selective with 99% conversion of furfural and 94% selectivity for furfuryl alcohol under solvent free conditions at 443.15 K and 2 MPa of hydrogen pressure. It was also observed that the Cu@Mg/γ-Al2O3 catalyst is reusable (up to six runs) while maintaining its high activity and selectivity (≥94%) in the hydrogenation of furfural to furfuryl alcohol.


Experimental 1) General Information
All reagents were purchased from commercial suppliers and used without further purification.
All hydrogenation experiments were carried out under hydrogen (purity: 99.95%). Column chromatography was carried out with Merck silica gel 60-120 mesh and the products were visualized by GC detection. 1 H NMR and 13 C NMR (Bruker (Germany) Avance III) spectra were recorded in CDCl 3 . Chemical shifts (δ) are reported in ppm using TMS as an internal standard, and spin -spin coupling constants (J) are given in Hz. 1 H NMR and 13  and FWHM of the products, which were not obtained as pure species, were adjusted until the best fit was obtained. Symmetric Gaussian shapes were used in all cases. Auger electron spectroscopic (AES) analysis is conducted, at a base pressure of 10 -10 Torr, within the K.E.

Catalyst Characterization.
3a. BET surface area, Pore Volume, Pore Size and Particle Size:  collapsed partly during the calcination process, which could also decrease the specific surface area. All catalysts have similar surface area and pore volume as they have same active component loading. They all have an average pore size of 6.0 nm, which is favor to the diffusion of reactant during the hydrogenation process. In addition, large surface area can provide much more active sites for hydrogenation, as the reaction mainly occurred on the surface of the catalysts. The overall increase in surface area in Cu@Mg/γ-Al 2 O 3 than Cu/γ-Al 2 O 3 can be attributed to an increase in specific surface area of the active metal with an addition of Mg as promoter.

3c. Temperature Programmed Desorption (TPD)
The basic property of the catalysts were measured by temperature programmed desorption of CO 2 equipped with TCD detector. 100 mg of the catalysts were placed in a quartz reactor. The sample was pretreated at 400 o C in He of 60 mL/min for 180 min, and cooled down to room temperature. Then the sample was exposed in CO 2 (30 mL/min) using a six-way valve for 120 min and then flushed with He (40 mL/min) flow to remove all the physically adsorbed CO 2 .
Once the physically adsorbed CO 2 was purged off, the CO 2 -TPD experiments were started. The             Furfural (5.2 mol%), Cu@Mg/γ-Al 2 O 3 (0.2 g), were taken in a 600-mL, round-PARR reactor equipped with an overhead stirrer and stirred for an appropriate time. The progress of the reaction was monitored by TLC and on completion of the reaction, the reaction mixture was centrifuged to separate the catalyst, the solid residue was washed with EtOAc (1 X 10 mL) to make the catalyst free from organic matter, the reaction mixture was diluted with water (20 mL), and then extracted with EtOAc (3 X 10 mL). The combined organic layers were washed with brine (10 mL) and dried over anhydrous Na 2 SO 4 . The solvent was evaporated under reduced pressure to yield the crude product. It was then purified by flash chromatography over silica gel (60-120 mesh) column using hexane/ethyl acetate (80:20) v/v as an eluent to afford the pure product. The products were characterized by 1 H NMR, 13 C NMR and mass spectrometric analysis. ethyl acetate to make the catalyst free from organic matter, and finally with acetone, dried, and used in the next cycle. Almost consistent activity was noticed even after the fifth cycle.

Reaction Kinetics for the Hydrogenation Reaction
Copper content (0.2 mol%) of the fresh and used (after 5 th cycle) catalyst was found to be almost the same (by ICP-AES). To check the heterogeneity of the catalyst, the hydrogenation of furfural was terminated at 16% conversion (after 30 min) and the catalyst was separated by simple centrifugation. The reaction was continued for an additional 4.5 hours and the conversion remained almost unchanged. Moreover, the filtrate was tested for copper by ICP-AES after the 6 th cycle and no copper leaching was found for the five consecutive cycles. These studies clearly demonstrate that no leaching of copper from the catalyst was taken place during the reaction. .   Figure 21. 13 C NMR spectra of furfuryl alcohol