Abstract
Organic solvent nanofiltration (OSN) has been widely applied to separate and recycle homogeneous catalysts, but the influence of ligand and solvent selection on the performance of OSN is not fully understood. Here we prepared four palladium (Pd) catalysts by combining palladium acetate with four ligands of different molecular weights. Morphological and functional properties of the Pd catalysts were characterized by TEM, FTIR, and NMR. OSN experiments were conducted in a lab-scale dead-end filtration rig. Two commercial OSN membranes, PuraMem S600 (PS600) and DuraMem 500 (D500), were used to separate the Pd catalysts from different organic solvents (toluene, isopropanol, butanol/water, and methanol) that are specified to be compatible with, respectively. For both membranes, the pure solvent permeance was positively related to the degree of membrane swelling induced by the solvent. The solvent permeance decreased significantly after the addition of a solute, as a result of membrane fouling and concentration polarization. For the PS600 membrane, the Pd rejection in any solvent was closely correlated to the molecular weight of the ligand, which agrees with the pore-flow model. For the D500 membrane, on the other hand, there was no conclusive link between the Pd rejection and the type of ligand. The one-way analysis of variance (ANOVA) confirmed that the separation processes in PS600 and D500 membranes were controlled by different transport models. The findings shed light on the selection of ligand and solvent in OSN in order to enhance the separation of homogeneous catalysts.
Highlights
Homogeneous catalysis by transition metal complexes offers many advantages over heterogeneous catalysis, such as high catalytic activity, high selectivity, and negligible mass transfer limitations (De Smet et al, 2001; Van Leeuwen, 2004)
This study aims to fill this knowledge gap by attaching a homogeneous Pd catalyst to four ligands with different molecular weights and geometries and investigating their separation in four solvents by two Organic solvent nanofiltration (OSN) membranes
As for the Pd(OAc)2 + dppp complex, the phosphorus-aryl bond at 1,590 cm−1 becomes more discrete and exhibits a larger peak when mixed with Pd(OAc)2. This can be explained as both overlapping with the more intense Pd stretching, and as a change in the local densities of the phosphorus bond, it can be determined that the Pd and ligand are interacting on a molecular scale (Pretsch et al, 2009)
Summary
Homogeneous catalysis by transition metal complexes offers many advantages over heterogeneous catalysis, such as high catalytic activity, high selectivity, and negligible mass transfer limitations (De Smet et al, 2001; Van Leeuwen, 2004). This study aims to fill this knowledge gap by attaching a homogeneous Pd catalyst to four ligands with different molecular weights and geometries and investigating their separation in four solvents by two OSN membranes. The data will help identify the suitable combination of ligand and solvent for use in order to achieve effective catalyst separation and recycle. It will shed some light on the transfer mechanisms of homogeneous catalyst through OSN membranes on a molecular level. Four ligands were chosen for use in this study, namely 1,3-Bis(diphenylphosphino)propane (dppp); 1,2-Bis(diphenylphosphino)benzene (dppBz); Tri(otolyl)phosphine (P(o-tol)3); and 2-Dicyclohexylphosphino2′,4′,6′-triisopropylbiphenyl (XPhos) Phosphorus (31P) NMR spectra were recorded by an Avance III 500 MHz NMR spectrometer (Bruker, UK) using the solvent suppression technique
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