Title : Effect the cluster nature (Rh and Pd) and the reaction conditions in the catalytic amination of phenols
Lignin-derived phenol is an attractive platform molecule to produce secondary amines through its reductive with primary amines on supported-transition metals. The performance of this reaction for different metals (Ni, Pd, Rh, Cu, and oxides) has been previously inspected, indicating that the active site plays a major role in the formation and disproportionation of intermediaries. However, the correlation between metal cluster’s nature with the of change in operation parameters (i.e., temperature, H2 pressure, reactant initial concentration) is still elusive to the science. Therefore, the effect of the operating conditions (353<T(K)<433, 1<pH2<2 bar, 0.1<C0<0.4 mol/L) and their kinetic implications for the catalytic amination of phenol with cyclohexylamine over Pd/C and Rh/C are studied here. The catalysts were analysed for BET, BHJ, and NH3-TPD in a 3FLEX apparatus, while their XRD patters were measured in a Bruker D4 diffractometer, and TEM images recorded in a JOEL JEM 1200 EXII microscope. The liquid phase reaction was carried out for 6 hours in 4 mL and 20 mL autoclave reactors equipped with a pressurization gas line. Chemical species were identified in gas chromatograph equipped with a mass spectrometer (Clarus 690, QS8), whilst the product`s quantification was carried out by GC-FID using nonane as internal standard. XRD results showed strong signals for Pd/C at 2θ = 40.0, 46.4, 68.2 and 82.1°, consistent with Pd0, while the Rh/C catalyst shows attenuated signs at 2θ = 41.2° and 47.8°, also showing zero valences. The results of dynamic tests and DelPlot analysis revealed that both catalysts carry out similar multi-step routes, consisting of a first step of hydrogenation of phenol to cyclohexanone (CyO), which is then condensed with cyclohexylamine (CyA) to form an intermediate imine, followed by disproportionation of the imine into secondary amines. In addition, a parallel self-condensation of CyA to produce Diciclohexylamine (DCyA) can take place via a hemi-aminal intermediate. The Rh/C and Pd/C yielded differences in product distribution owing to their ability to dehydrogenate. This was measured for model molecules (CyA and diphenylmethanimine) for which Pd0 sites demonstrated and enhanced dehydrogenation capacity as compared to Rh0 sites, leading to the formation of cyclohexylamine (CyPhA) preferentially on Pd/C. It was observed that selectivity to DCyA is favored until 415 K over oth catalysts (SDCyA = 98% (Rh) and SDCyA = 96% (Pd)) while at 433 K the formation of CyPhA is enhanced (SCyPhA = 16% (Rh) and SCyPhA = 58 % (Pd)), evidencing the higher dehydrogenation capacity of Pd/C. The increment in the initial concentration of CyA, and the H2 availability, favoured the formation of DCyA, presumably by their implications in the kinetically-relevant steps. A preliminary kinetic analysis, (based in a Power law approach) resulted in a activation energy of 62.3 kJ/mol on Pd/C and 70 kJ/mol on Rh/C, while the reaction order for phenol was 1 suggesting that it is not affecting the site balance and order > 2 for CyA suggesting its participation in more than one reaction step (consistent with the proposed reaction route).
Audience take away:
- The results of the characterization of Pd- and Rh-based catalysts and the correlation with their activity for the amination of phenol with cyclohexylamine.
- The results of this research are applicable to other catalyzed amination reactions of lignin-derived phenolics.
- This study provides information on the elemental reactions occurring at the catalyst active sites under different reaction conditions and will allow the development of more elaborated hypothesis on the structure-activity relationships.