Separation of Grubbs-based catalysts with nanofiltration
Van der Gryp, Percy
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This thesis describes the use of organic solvent nanofiltration (OSN) in the field of metathesis for separating homogeneous Grubbs-type catalysts from their post-reaction mixtures for the model metathesis reaction of 1-octene to 7-tetradecene and ethene. The main contributions and objectives of this study were in demonstrating: (i) the successful separation and re-use of different Grubbs-type catalysts from their post-reaction mixtures, and (ii) the successful synthesis of a newly developed catalyst, Gr2Ph, that demonstrated a longer catalytic lifetime for re-usability. The study was twofold in firstly describing the catalytic performances of different Grubbs-type catalysts for the model reaction and secondly in characterizing and describing the separation performances of the 1-octene metathesis system with OSN. In terms of catalyst performances: The catalytic performance of different Grubbs-type precatalysts (Gr1, Gr2, HGr1, HGr2 and the newly developed Gr2Ph) was studied for the model reaction by varying operating parameters, such as reaction temperature (30 to 100 °C), catalyst load (1-octene/Ru molar ratio between 1:5000 and 1:14000) and reaction environment (reaction in the presence of various organic solvents). Quantities such as product distribution, selectivity, yield, catalyst lifetime and activity were used in comparing and evaluating the efficiency of these precatalysts with each other. It was found that all three precatalysts HGM, HGr2 and Gr2Ph showed both metathesis and isomerization activity for the model reaction that was strongly temperature-dependent. Precatalysts HGr2 and Gr2Ph showed significant secondary metathesis activity while precatalyst HGM did not. It was found that the optimal reaction temperature for precatalyst HGM was 30 °C, for precatalyst HGr2 50 °C and for precatalyst Gr2Ph 80 °C. The addition of different solvents to the reaction environment had an overall negative effect towards the formation of the primary metathesis products (PMPs) of 7-tetradecene and ethylene. In this study it was postulated and demonstrated with molecular modelling, that the metathesis reaction of 1-octene with the different Grubbs-type precatalysts (HGM, HGr2 and Gr2Ph) could accurately be described by a type of release-return dissociative mechanism. It was further found that the reaction kinetics of the model reaction with the three precatalysts (HGM, HGr2 and Gr2Ph) could fairly accurately be described by a set of three inter-dependent elementary reaction rate-equations. In terms of separation performances: Five different Grubbs-type precatalysts (Gr1, Gr2, HGr1, HGr2 and Gr2Ph) and the commercially available STARMEM™ series of OSN membranes were used in this study. Parameters such as feed concentration, feed pressure, membrane pretreat-solvent and catalyst load were varied in a dead-end setup. Quantities such as the permeation rate (flux), catalyst rejection, solvent separation (selectivity), degree of swelling and contact angles were measured. It was found that the STARMEM™ 228 membrane successfully separated the different Grubbs-type catalysts from their post-reaction mixtures to below 9 ppm with catalysts rejections greater than 99%. Relative moderate fluxes were obtained that ranged from 0.2 to 15 kg.m-².h-¹. It was shown that 7-tetradecene preferentially absorbed in the STARMEM™ 228 membrane. A solvent non-separating system was observed for binary mixtures of 1-octene, 1-tetradecene and 7-tetradecene. It was found that the predominant parameters that influenced the transport of the 1-octene metathesis system through the ST-228 membrane were solvent properties (such as viscosity) » membrane-solvent interaction properties (such as sorption) > solvent-solvent structural properties (such as molar volume or effective molecular volume). The experimental permeation results for the binary mixtures of 1-octene and 7-tetradecene through the STARMEM™ 228 membrane were described by using pore-flow models, solution-diffusion models and a newly developed model that incorporates structural solvent-solvent interaction. It was found that the newly developed model best described the experimental results. A coupled reaction-separation process was applied that demonstrated the successful reusability of the in-house synthesized catalyst, Gr2Ph. The turnover number was increased from 1400 for a single pass reaction to 5500 for the overall consecutively coupled reaction-separation steps of four cycles. Catalysts Gr1, Gr2, HGM and HGr2 did not show any catalytic activity after the first separation cycle due to extremely short catalytic lifetimes of less than ten hours compared to catalyst Gr2Ph's three days. The short catalytic lifetimes of the classical precatalysts such as Gr1, Gr2, HGr1 and HGr2 in the field of alkene metathesis were solved with the synthesizing concept of modifying and binding the dissociating ligand and anionic ligand with bidentate 0,N-chelated Schiff base ligand on the second generation Grubbs-precatalyst.
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