4 Experimental
4.3 The Continuous Flow Rig
4.3.1 Apparatus
Figure 4.3.1.1 Schematic of Continuous Flow Reactor
As part of a collaboration with the Chemical Engineering School of University College London (UCL), a specially designed continuous-flow biphasic rig had been commissioned and built. This rig was transported to the University of St Andrews Chemistry Department where testing of continuous flow hydroformylation was carried out.
The continuous flow apparatus, Figure 4.3.1.1, comprised three feed bottles, two HPLC pumps (organic and fluorous phases), a Parr continuous stirred tank reactor (CSTR) (100 cm3): fitted with a thermocouple, stirrer (1700 rpm), pressure gauge, inlet for liquid and gas, a liquid outlet, a gas purge port, a cold finger and a bursting disc; a heat exchanger (later removed), capillaries for flow control, separator (connected to allow recycling of the fluorous phase) and a collection vessel. The reactor was filled by pumping the catalyst in the fluorous phase and the octene substrate through a Jasco PU-2080 HPLC pump and a Waters 515 HPLC pump
respectively. The feed from the two pumps mixed at a T-junction of tubing and passed through a heat exchanger before it reached the liquid inlet of the reactor. The reactor was pressurised using a Brooks pressure controller (5866 series) coupled to a Brooks Microprocessor control and read out unit (model 0154), this maintained the pressure within the reactor as gas was consumed in the reaction and fed gas directly from the cylinder. The contents of the reactor were stirred by a mechanical stirrer with a Parr motor (1700 rpm) and heated using a Parr heating mantle controlled by a Watlow Microprocessor-based temperature controller (series 987). The reactor was emptied through a dip tube, which sat inside the CSTR just above the stirrer, through the heat exchanger and through a Brooks mass flow controller, which metered the flow of liquid throughout the experiment. In order to control the flow of liquid from the CSTR a set of four capillaries of different length were introduced with two two- way valves to allow selection of an individual capillary, by increasing the length of the capillary the rate of flow was slowed down and the residence time in the CSTR was increased.
4.3.2 Continuous Flow Operation Procedure
The entire system was connected and left under argon overnight and each section purged separately with argon. The catalyst solution was prepared in a Schlenk tube containing the Rh precursor (0.1548 g, 0.6 mmol), which was taken into the glove-box to add the phosphine ligand (3.6492 g, 3.0 mmol). The perfluorinated solvent, PFMC (150 cm3), was added to the Schlenk tube by syringe and the contents were stirred until all solid had dissolved. During this stage it was possible to see the evolution of CO as the [Rh(acac)(CO)2] and phosphine reacted. This solution and the octene feedstock were transferred to their respective feed-bottles via cannula under an argon atmosphere. The organic substrate and the fluorous solvent from the initial feed
bottles were pumped into the CSTR, using the two HPLC pumps to deliver the liquids at constant and identical flow rates (to ensure a 50/50 mix within the reactor (75 cm3)). The pumping was stopped and the reactor was pressurised to 12 bar CO / H2 before it was heated to 70ºC with vigorous stirring (1000 rpm). Once at temperature, the pressure was increased to 15 bar through the pressure controller, maintaining the reactor pressure throughout. The homogeneous mixture in the reactor was left to react as a batch reaction for 2 hours before the gas inlet was closed and the reactor outlet was opened to the separator. Once liquid was observed in the separator, the outlet valve and the gas inlet valve were closed and the stirring stopped. The pumps were started, feeding the liquids into the reactor to compensate for the volume of liquid removed to the tubing and separator; this was calculated by returning the pressure of the reactor to 15 bar and recording the length of time to reach this along with the flow rate of the pumps. Again, the reactor was left under batch conditions for a further 2 hours. After 2 hrs the reactor outlet was opened and the pumps started to deliver catalyst solution and substrate to the reactor at a slow rate (0.6 cm3 min-1). Initially there was no product collection, as the separator had to fill to approximately 60 cm3 before the liquid reached the overflow tube. Once the product had started overflowing and the level of fluorous solvent had equilibrated, the recycling of the catalyst was started simply by turning to the second inlet on the fluorous pump, which was connected to the recycling tube on the underneath of the separator. The fluorous level was monitored throughout the experiment to ensure equal levels of both phases in the entire system and adjustment made to the flow rate when necessary. The system was run continuously with the organic phase being sampled every 30 min and analysed by GC-FID. Samples throughout the reaction were also monitored by ICPMS for Rh leaching to the organic phase.
4.3.3 Semi-Continuous Flow Operation of Rig
The equipment was set up as for a continuous flow reaction but without the capillaries and shorter connecting tubing into and out of the reactor. The reactor was filled with the required volume of each liquid by pumping the two liquids through the respective HPLC pumps. The reactor was then pressurised to 12 bar CO / H2 and heated with stirring to 70oC, with the reactor open to the CO / H2 cylinder. Once at reaction temperature the pressure was increased to 15 bar CO / H2 and the timer started. The stirrer was set to maximum (1700 rpm).
After the required time the reactor was closed, the separator gas vent was opened and the outlet from reactor to separator was carefully opened and closed once no more liquid was observed entering the separator. The reactor was then refilled with the required liquid and the process repeated. Samples of the organic phase were collected by syringe through the injection port in the separator and were analysed by GC-FID and the entire experiment was stopped once the overall conversion had dropped below 25 %.
4.3.3.1 Alterations to Semi-Continuous Flow Method
In an attempt to lengthen the catalyst life, during the semi-continuous flow reactions, the dip tube was shortened so that it only removed the organic phase from the reactor. The theory being that the catalyst solution would remain in the reactor whilst just collecting pure product phase after each batch reaction.
Initially this was carried out with the contents of the reactor kept at reaction temperature, although the heater was turned off during separation to avoid any overheating and eventual decomposition of the catalyst.
In the second experiment removing only the organic phase, the reactor was cooled to 30 oC before opening to the separator, in an attempt to reduce the decomposition of the catalytic species and the removal of the fluorous phase through the venting of the reactor.
With only the organic phase being removed to the separator, the entire contents of the separator were collected after each batch reaction. These samples were analysed by GC-FID for the conversion data and ICPMS analysis was carried out to obtain the amounts of Rh leaching to the organic phase.
4.4 Reference List
1. P. Bhattacharyya, D. Gudmunsen, E. G. Hope, R. D. Kemmitt, D. R. Paige and A. M. Stuart,J. Chem. Soc., Perkin Trans. 1, 1997, 3609.
2. P. W. N. M. van Leeuwen, Rhodium Catalyzed Hydroformylation, Kluwer Academic Publishers, The Netherlands, First edn., 2000.
3. W. Chen, L. Xu, Y. Hu, A. M. Banet Osuna and J. Xiao,Tetrahedron, 2002,58, 3889.