Homogeneously catalyzed hydroformylation in supercritical carbon dioxide : kinetics, thermodynamics, and membrane reactor technology for continuous operation

The increased awareness for environmental issues and concomitant environmentally conscious governmental policies has prompted the chemical process industry to implement "greener" production and synthesis methods. In particular, the reduction of the emission of harmful, often organic, substances, reduction of the production of waste, and increasing the energy efficiency are three important aspects in the development of environmentally benign chemical production processes. For the chemical and chemical engineering academic community this has given rise to a new direction, where the concept of "green chemistry" is being explored. Supercritical fluids have been established as promising substitutes to organic solvents. Carbon dioxide is of particular interest as an alternative solvent as it has a low toxicity, is non-flammable and has an accessible critical temperature and pressure. In addition, catalysis is an important tool for the optimization of atom efficiency of a chemical conversion, and therefore for the reduction of waste production. Additionally, catalysis allows for reactions to take place under milder conditions, which can also contribute to an increase in energy efficiency. In particular, soluble molecular organometallic catalysts allow chemical conversions with a higher rate and a better selectivity than their heterogeneous counterparts. The difficult separation of a homogeneous catalyst from reaction products, without deactivating the catalyst, is one of the main obstacles for their application on an industrial scale. Nanofiltration using a microporous ceramic membrane has the potential to be a solution to this problem. A large enough catalyst molecule will be retained while reaction products and solvent can permeate across the membrane. In the field of separation technology membranes have emerged as an energy efficient alternative to conventional separation methods, like distillation and extraction. Ceramic membranes are seen as one of the most promising candidates to purify process streams under demanding conditions. The main objectives of this thesis are the evaluation of the possible advantages of using supercritical carbon dioxide as a solvent as an alternative for organic solvents, and the investigation into the potential of membrane technology for the retention of homogeneous catalysts. The hydroformylation of 1-octene, which is an example of a homogeneously catalyzed reaction on an industrial scale, is considered as a model reaction. To perform the hydroformylation in supercritical carbon dioxide an experimental procedure has been developed, which allows for catalyst preparation under hydroformylation conditions and for carrying out the hydroformylation reaction with a well-defined starting point. It has been demonstrated that with this experimental procedure it is possible to obtain highly reproducible results. Furthermore, a relationship between the change in pressure and the change in reaction mixture composition as a function of time has been established. Using this experimental procedure the effect of total pressure, temperature, concentration of reactants, and concentration of catalyst precursors on the reaction rate, chemoselectivity, and the regioselectivity of the hydroformylation of 1-octene has been studied. The concentration of carbon dioxide had an effect on the regioselectivity of the reaction. Therefore, the same density of solvent has been used for each experiment rather than the more common approach of applying the same total pressure for each experiment. Based on the results obtained by the variation of the reaction parameters a kinetic model has been developed. An optimization method has been applied to find the model parameter values that best describe the experimental data. The observed kinetics for the catalyst based on rhodium(I) dicarbonyl acetylacetonate and tris(3,5-bis(trifluoromethyl)phenyl)phosphine shows resemblance to that observed for the hydroformylation where bulky phosphites have been used as the ligand. For this catalyst a high activity in the order of 5×103 to 12×103 mol1-octene molRh-1 h-1 has been observed at 70 °C. Organometallic complexes based on rhodium with phosphine ligands with a varying number of trifluoromethyl groups have been screened for their activity and selectivity for the hydroformylation of 1-octene. Furthermore, the effect of the type of solvent: carbon dioxide, hexane, and toluene has been included in this study. An increase in the number of trifluoromethyl substituents on the triphenylphosphine ligand results in an increase in 1-octene conversion rate and a decrease in the overall selectivity towards aldehydes. This behaviour is observed in all three solvents. For supercritical carbon dioxide or hexane, as the solvent, the outcome of the hydroformylation reaction in terms of activity and selectivity shows great similarity. By following the hydroformylation of 1-octene in time, it was observed that during batch operation rhodium catalysts with trifluoromethyl-substituted triarylphosphines showed a higher differential regioselectivity than based on the overall regioselectivity at the end of the reaction. For the hydroformylation in carbon dioxide this effect was most pronounced. Both the mode of operation, batch or semi batch, and the type of solvent had a significant influence on this phenomenon. The transport of a supercritical fluid across a microporous alumina supported titania membrane has been investigated. The dependence of the permeation of carbon dioxide across the titania membrane on the feed pressure is similar to what has been previously observed for microporous alumina supported silica membranes. At high feed pressure viscous flow appears to be the main mechanism of mass transport across the membrane. Furthermore, the titania membrane shows a reasonable stability over a period of operation of about at least six months in varying conditions. Finally, the first continuously operated experiment has been performed, in which hydroformylation of 1-octene and separation of the catalyst have been integrated using a membrane reactor. During a 27.5 h of operation of the membrane reactor, spread over four consecutive days, a maximum conversion of 17 % and a maximum regioselectivity of 5 in terms of n:iso ratio has been observed. The conversion and the n:iso ratio, which is the ratio between the linear and branched aldehyde product, decrease as a function of the number of permeated reactor volumes indicating a loss of catalyst. Permeation of free ligand and the catalytic species through the membrane appear to be the main reasons for the decrease in activity and selectivity. A good match between membrane retention characteristics and the size of the catalyst and its precursors is not found yet. However, a number of feasible improvements can be made to improve the retention of the catalyst. Using a membrane for retention of a homogeneous catalyst in combination with the application of carbon dioxide as a solvent for the continuous hydroformylation of 1-octene has great potential. Successful application of the envisioned membrane reactor process can have implications for other homogeneously catalyzed reactions of which asymmetric hydrogenation is a commercially relevant example. As a result of the experimental methods used in this thesis the potential benefits of using carbon dioxide as an environmentally benign alternative to organic solvents could be further extended

Придністровський конфлікт: чинники існування напруги

Стаття присвячена аналізу та систематизації чинників, що обумовлюють збереження статус-кво у процесі придністровського врегулювання на глобальному, регіональному та локальному рівні.The article is devoted to the analysis and systematization of factors leading to the preservation of the status-quo in the Transnistrian settlement process on global, regional and local level

A bulky phosphite modified rhodium catalyst for efficient hydroformylation of disubstituted alkenes and macromonomers in supercritical carbon dioxide

The hydroformylation of disubstituted alkenes and related macromonomers in supercritical CO2 is demonstrated. Higher turnover frequencies were observed for the 1,2-disubstituted alkenes than for the 1,1-disubstituted alkenes. The turnover frequency for poly(styrene) macromonomer hydroformylation compares well with that observed for cyclohexene. The turnover frequency observed for poly(methyl methacrylate) macromonomer hydroformylation is considerably lower than that observed for methyl methacrylate. Unprecedented turnover frequencies are observed in comparison with previous studies where CO2 has been applied as a solvent. This is achieved using rhodium modified with a readily available bulky phosphite ligand without the need of ligand modification to improve solubility in supercritical CO2

Facile and selective synthesis of aldehyde end-functionalized polymers using a combination of catalytic chain transfer and rhodium catalyzed hydroformylation

A novel synthetic pathway towards aldehyde end-functionalized polymers is presented from a combination of catalytic chain transfer polymerization (CCTP) and rhodium catalyzed hydroformylation in supercritical carbon dioxide. CCTP allows for the synthesis of well-defined macromonomers in terms of the average molecular weight and the terminal unit carrying the unsaturated bond. The rhodium catalyzed hydroformylation allows for a high selectivity towards aldehyde end-group functionalized polymers. The introduction of the synthetically versatile aldehyde end-group opens up a broad range of possible applications

Full kinetic description of 1-octene hydroformylation in a supercritical medium

The kinetics of the hydroformylation of 1-octene in a supercritical carbon dioxide medium, catalyzed by a tris(3,5-bis[trifluoromethyl]phenyl)phosphine-modified rhodium catalyst, have been investigated. The influence of the concentration of carbon dioxide, reactants, catalyst precursors, and the reaction temperature has been determined. A kinetic model was developed, which describes the concentration-time profiles of the reactants, the linear and branched aldehydes, and the internal alkenes. Using the kinetic model activation energies for hydroformylation of 1-octene to nonanal and 2-methyloctanal were determined. Throughout the concentration ranges studied an approximate first order dependence of the hydroformylation rate on the hydrogen and catalyst concentration was found which indicated that oxidative addition of hydrogen was the rate limiting step. The increase in reaction rate and regioselectivity with an increase in ligand concentration is a striking feature of the catalyst investigated here. At higher concentrations the reaction rate was found to have a strong negative order dependence on the carbon monoxide concentration. The reaction rate had a positive order in 1-octene at a concentration lower than 0.5 mol L-1 while saturation kinetics were observed at a higher concentration. The results were explained by invoking the contribution of both monophosphine and diphosphine rhodium species to the hydroformylation catalysis

Triphenylphosphine modified rhodium catalyst for hydroformylation in supercritical carbon dioxide

The kinetics of 1-octene hydroformylation catalyzed by triphenylphosphine modified rhodium in carbon dioxide have been explored at 90 °C and pressures up to 48 MPa. The apparent catalyst solubility was determined by evaluating the reaction rate for different rhodium amounts. The kinetics follow a first order in 1-octene, a negative order of -1.2 in carbon monoxide, an order of 0.25 in hydrogen, and a negative order of -0.2 in triphenylphosphine, which is to a great extent in agreement with studies on the hydroformylation of linear 1-alkenes in organic solvents. The observed apparent turnover frequencies range from 1900 to 7000 molaldehyde molRh-1 h-1. These turnover frequencies are of the same order of magnitude as observed for hydroformylations in organic solvents, indicating that rhodium modified with triphenylphosphine can be used with high efficiency in supercritical carbon dioxide rich mixtures

Application of non-fluorous phosphorus ligands in rhodium catalyzed hydroformylation in a supercritical carbon dioxide-rich medium

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Hydroformylation of 1-Octene in Supercritical Carbon Dioxide and Organic Solvents using Trifluoromethyl-Substituted Triphenylphosphine Ligands

Two different in situ prepared catalysts generated from Rh(CO)2acac and trifluoromethyl-substituted triphenylphosphine ligands have been evaluated for their activity and selectivity in the hydroformylation of 1-octene. The solvents used were supercritical carbon dioxide, hexane, toluene, and perfluoromethylcyclohexane. The highest value for the turnover frequency, 9820 mol1-octene molRh-1 h-1, has been obtained in supercritical carbon dioxide using ligand I, P[C6H3-3,5-(CF3)2]3. For both supercritical carbon dioxide and hexane employing ligand II, P(C6H4-3-CF3)3, a selectivity towards the linear aldehyde product, nonanal, and an n:iso ratio of 79.3¿% and 4.6–4.8 have been obtained, respectively. These values are significantly higher than those obtained with triphenylphosphine as ligand (nonanal: 74–76¿%, n:iso: 3.1–3.3). An increase in trifluoromethyl substitution on the triphenyl ligand results in an increase in the 1-octene conversion rate, an increase in the n:iso ratio and a decrease in the overall selectivity towards aldehydes. In terms of turn-over frequency and selectivity the three ligands give comparable results in supercritical carbon dioxide and hexane. This leads to the conclusion that the properties of supercritical carbon dioxide as a solvent for hydroformylation can be better compared with those of hexane rather than with those of toluene

Evaluation of pressure and correlation to reaction rates during homogeneously catalyzed hydroformylation in supercritical carbon dioxide

For the hydroformylation of 1-octene in supercritical carbon dioxide the relationship between the change in pressure and the change in reaction mixture composition as a function of time has been investigated. The activity and selectivity has been studied for the catalyst based on tris(3,5-bis(trifluoromethyl)phenyl)phosphine and rhodium(I) dicarbonyl acetylacetonate. The influence of the ligand to rhodium ratio on the hydroformylation has been used to demonstrate how the pressure can be correlated to the conversion and yield. The initial rate of reaction is in good agreement with the initial pressure change in the batch reactor. Up to an aldehyde yield of 80%, the pressure drop appears to be independent of the reaction rate and selectivity. The highest average reaction rate, , has been obtained for a ligand to rhodium ratio of 50 and an initial concentration of 1-octene of 0.5 mol L-1. Both the reaction rate and the selectivity increase when the ligand to rhodium ratio is increased. The Peng–Robinson equation of state has been used to describe the pressure as a function of the concentration of the reactants and products. The calculated pressure corresponds reasonably well with the observed reactor pressure. Following the progress of the reaction by monitoring the pressure is a good alternative to reaction mixture sampling, especially for fast reactions