Towards process intensification in homogeneous catalysis

Abstract

The central theme of this thesis is to enhance the recyclability of homogeneous catalysts of the type phosphine/phosphite/transition-metal-complexes as well as Brønsted acids to make processes based on such catalysts ever more feasible. The initial approach was to apply organic solvent nanofiltration (OSN) membranes in conjunction with molecular weight enlargement (MWE) of the catalyst/ligands. Specifically, polyhedral oligomeric silsesquioxane (POSS) was the MWE-unit of choice. It is important to have control over the ligands/catalyst’s stability under typical reaction conditions for extended periods of time in order to justify such modifications. The stability of phosphine ligands and Brønsted acids is relatively unproblematic and so POSS modifications of such ligands was directly attempted. The stability of phosphite ligands was known to be more difficult and so this was explored more deeply. Hence, in this thesis one chapter is dedicated to study the long-term stability of the phosphite ligand Biphephos when in solution, exposed to elevated temperature and under typical Rh-catalysed hydroformylation reaction conditions. It was concluded that the way a Biphephos/Rh solution is prepared has a major impact, the use of stabilising additives is important and that using an O2-free hydroformylation system is decisive in order to achieve long-term stability of such ligands. Two chapters of this thesis were dedicated to the POSS modification of phosphine ligands. One chapter presents a set of TPP-POSS ligands bearing 1, 2 or 3 POSS cages, highlighting that grafting a POSS cage so close to the phosphorus atom can have significant effects upon the ligand’s catalytic activity. The other chapter presents work on POSS modification of Xantphos-type ligands in the rear of their ligand backbone. The synthesis of NixantPOSS, a POSS modified version of Nixantphos, was realised in a one-pot reaction at ambient temperature giving the desired ligand at 94% isolated yield. When NixantPOSS was applied in the Rh-catalysed hydroformylation reaction of 1-octene it showed the same selectivity and only an 18% reduction in TOF as compared to the unmodified Nixantphos. NixantPOSS was rejected at 99% by a ceramic nanofiltration membrane during continuous hydroformylation. The last chapter of this thesis explores the application of sulfonated octaphenyl-POSS as a Brønsted acid catalyst in the ketalisation reaction of glycerol with butanone. The H-S-POSS catalysts were almost as active as PTSA and significantly more active than Amberlyst-36. The solubility of H-S-POSS was poor in all common laboratory solvents and so a recycling strategy based upon centrifugation and sedimentation was explored that turned out successful for H-S-POSS catalyst with a lower degree of sulfonation.