(223e) Kinetics of Hydroformylation of Higher Olefins Using Rhodium-Phosphite Catalyst in a Thermomorphic Solvent System

KINETICS OF HYDROFORMYLATION OF 1-OCTENE USING RHODIUM-PHOSPHITE CATALYST IN A THERMOMORPHIC SOLVENT SYSTEM

Maizatul S. Shaharun1, Hilmi Mukhtar1, Subhas Bhatia2 and Binay K. Dutta1*

1*Chemical Engineering Department, Universiti Teknologi PETRONAS, Malaysia

2Chemical Engineering Department, Universiti Sains Malaysia

* Corresponding author, e-mail: binaydutta@petronas.com.my

ABSTRACT

The use of a liquid-liquid biphasic ‘thermomorphic' or temperature dependent multicomponent solvent (TMS) process, in which the catalyst remains as a residue in one of the liquid phases and the product goes preferentially to the other liquid phase, can be an enabling strategy of commercial application of hydroformylation processes with high selectivity, efficiency and ease of product separation and catalyst recovery1. This paper describes the kinetics of hydroformylation of 1-octene using a homogeneous catalyst composed of HRh(PPh3)3(CO) and P(OPh)3 in a TMS- system which is a mixture of propylene carbonate, dodecane and 1,4-dioxane. These components were selected in consideration of their polarities – 1,4-dioxane having a moderate polarity in between those of propylene carbonate and dodecane. The reaction scheme for hydroformylation of 1-octene is shown in Figure 1.

The reaction was carried out over a temperature range of 353-373K since the solvent mixture forms a homogeneous liquid phase and good conversion and selectivity could be achieved. A high pressure reactor (model: Parr 4843) provided with a stirrer and a temperature control system was used for the study. The solvent mixture (220 ml), the reactant (1-octene), and the constituents of the catalyst – a rhodium catalyst precursor [RhH(CO)(PPh3)3] (ABCR, Germany) and triphenyl phosphate [P(OPh)3] – were fed into a cylindrical PTFE cylindrical container in the reactor. The temperature was raised to a desired level with mixing when the catalyst was formed in situ. The gas mixture containing CO and H2 of a desired composition was fed to the reactor quickly upto a certain pressure that was maintained constant thereafter. The reactor was operated in the batch mode. Liquid samples (less than 1 ml sample volume) were with drawn from time to time and analyzed to follow the course of the reaction. The analysis of reactants and products was carried out by a gas chromatographic method using 5 % phenylmethyl siloxane capillary column.

The effects of the major process parameters – namely, the olefin (1-octene) concentration, catalyst concentration, total pressure and gas composition, and temperature were studied. The ranges of the parameters studied are given in Table 1. Since the reaction occurs in the liquid phase, dissolution and mass transfer of the gaseous reactants (CO and H2) could have an effect on the reaction rate. In order to study the intrinsic kinetics of the reaction, prevalence of the kinetic regime was ensured by eliminating diffusional limitation. The effect of the agitator speed was investigated for this purpose and the experimental runs were taken at an rpm 450 below which a small reduction of the reaction rate was observed. Further, the maximum volumetric rate of physical mass transfer was calculated by separate experiments on absorption of the gas mixture in the solvent by recording the rate of change of total pressure of an initially pressurized reactor with individual pure gases.

The suitable solvent composition (PC:dodecane:1,4-dioxane = 0.332:0.102:0.617) was determined by cloud point titration at different temperatures from 298 to 373K. The rate of conversion of the substrate, 1-octene, expectedly increased with increasing catalyst concentration but a loss of regioselectivity was observed at higher conversions. A total yield of 97% of the aldehyde with a normal/iso ratio of 8.4 was attained at a catalyst concentration of 0.68 mM. The reaction rate increased with temperature but the selectivity declined and isomerization of the substrate occurred. All these effects were carefully monitored.

An increase in the total pressure increased the rate of conversion of 1-octene but with a sacrifice of the selectivity. The gas composition also affected both conversion and selectivity. A higher H2:CO ratio increases the reaction rate but facilitates hydrogenation of the substrate to octane. The ligand [P(OPh)3] to catalyst [RhH(CO)(PPh3)3] ratio also exhibited substantial influence. The highest yield of total aldehyde was obtained for a L/Rh ratio of 12.

The reaction proceeds through catalytic addition of the H and formyl (CHO) groups across the double bond of the olefin to give aldehydes. The catalytic cycle in such a process is governed by the Heck and Breslow mechanism developed for the cobalt catalyzed oxo-reaction2. High conversions of 1-octene up to 97% after 2 h at 90oC and good selectivities to the n-aldehyde up to 89% could be achieved. After the reaction and cooling down the reaction mixture, a major fraction of the catalyst could be easily recovered by simple phase separation. The rhodium leaching to the product phase is very low (3 %).

The reaction rate was found to be first order with respect to catalyst concentration, 1-octene concentration and partial pressure of hydrogen. A catalytic reaction cycle that proceeds through the formation of a set of intermediates and release of the catalyst at the terminal step has been used to develop a mechanistic rate equation. In fact, three kinetic models were developed by considering alternative rate-determining steps for the proposed reaction cycles, and subjected to rigorous parameter estimation and model discrimination in order to identify the appropriate one. The following mechanistic rate equation has been proposed based on plausible mechanism steps, yielding an average absolute error of 4.5 % only.

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These results also suggest that a mechanism of reaction featuring oxidative addition of H2 to acylrhodium intermediate species as rate determining is fair to describe the hydroformylation of 1-octene using HRh(CO)(PPh3)3/P(OPh)3. The comparison of experimental and predicted values of the rate of reaction is shown in Fig. 2. The results also support previous findings in demonstrating that under the experimental conditions listed in Table 1, the acylrhodium complex is the resting state and oxidative addition of H2 is rate determining3. The activation energy was found to be 66.3 kJ/mol. The value of the activation energy is in the range reported by other authors for the hydroformylation of 1-octene with different Rh-complexes by homogeneous, biphasic and supported aqueous phase catalysis (SAPC): 66-75 kJ/mol 4-6 The developed kinetic rate expression differs significantly from those previously obtained for HRh(CO)(PPh)3 in organic media. The most significant observation is the lack of olefin inhibition, and the absence of a critical catalyst concentration. This may be due to the solvent effects, the phosphite ligand and the increased H2 and CO concentrations relative to conventional systems. These results illustrate the usefulness of the TMS-system as a mechanistic tool for reactions that involve H2 and CO.

The work done so far establishes the potential of the thermomorphic solvent system and the rhodium phosphite complex catalyst for the hydroformylation of 1-octene. Further work is currently in progress to determine the kinetics of hydroformylation of another substrate, which is 1-dodecene. The developed rate equation for 1-octene will be assessed to determine the validity and quality of the mechanistic model with respect to C12 substrate (1-dodecene).

Acknowledgement:

The financial support from The Ministry of Science, Technology and Innovation (MOSTI) through e-Science Fund (03-02-02-SF0019) is gratefully acknowledged.

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