The Effect of a Nitrogen-Based Ionic Liquid as a Coating over 1Pd9Ag/Al2O3 for the Selective Hydrogenation of 1,7-Octadiene vs 1-Octene

Abstract

The effect of tetrabutylammonium nitrate ([N4444][NO3]) was studied as a surface coating over 1Pd9Ag/Al2O3 and applied in the selective hydrogenation of 1,7-octadiene in a mixture with 1-octene. Weight loadings up to a surface of three monolayers (MLs) were investigated and a further comparison coating with 1-ethylimidazole ([EIM]) was carried out to assess anionic effects in the Solid Catalysts with an Ionic Liquid Layer (SCILLs). Catalysts were characterised by H2-chemisorption, TGA-DSC, BET measurements, XPS, and HR-TEM. Catalytic studies showed that the uncoated and EIM-coated (10 wt%) catalysts gave nearly a 100% conversion of 1,7-octadiene and 1-octene with a selectivity mainly towards octane. Coating with [N4444][NO3] at 1 ML significantly decreased the 1-octene conversion by almost 50%, as well as the selectivity to octane (38%) at close to a 100% diene conversion. However, no net gain in 1-octene in the output stream was noted. At 2 ML IL/EIM coverage, a further decline in 1-octene conversion and octane selectivity was found at a diene conversion of 75%. The selectivity to 1-octene steadily increased from over the bare catalyst (52%) to the EIM-coated (62%) catalyst and SCILL (75%). At 3 ML IL coverage, the diene conversion (35%) was significantly reduced due to mass transfer limitations of hydrogen through the thick IL layer. Characterisation of the used catalysts by TG and BET analyses confirmed a leaching of up to 14% of the ionic liquid in the SCILLs coated at 1 ML and 2 ML, with an increase in surface area noted. Furthermore, smaller particle sizes of the used catalysts showed that the metal–support interaction was re-established. These results confirm a mild ligand coordination between the nitrogen in the IL anion and Pd and Ag where the ionic liquid remained physisorbed over the surface of the catalyst. In addition, component miscibility tests revealed partial solubility of the diene in the ionic liquid, indicating the presence of solvent effects also.