PHENOMENOLOGICAL ASPECTS OF HOMOGENEOUS CATALYSIS - THE CASE OF EQUILIBRIUM-CONTROLLED PRECURSOR CONVERSION

The homogeneous catalytic hydroformylation of cyclohexene was performed, starting with Rh4(CO)12 as catalyst precursor in n-hexane as solvent under the reaction conditions T = 283303 K, P(H2) = 1.0-4.0 MPa, P(CO) = 4.0-8.0 MPa, [Rh4(CO)12]0 = (5-21) x 10(-6) mole fraction, and [C6H10]0 = 0.05-0.18 mole fraction. The time-dependent concentrations of the precursor Rh4(CO)12, the intermediate C6H11CORh(CO)4, and the organic product C6H11CHO were measured under isobaric and isothermal conditions using in situ high-pressure infrared spectroscopy. The metal carbonyl Rh2(CO)8 Was also detected during the hydroformylation experiments. Complete conversion of the precursor Rh4(CO)12 to the intermediate C6H11CORh(CO)4 Was not observed during the 18-h experiments. After approximately 30 min of reaction, an apparent pseudo steady state was achieved between the precursor Rh4(CO)12 and the intermediate C6H11CORh(CO)4. Regression of the experimental concentration data gave the relationship [C6H11CORh(CO)4]ss = (K(eq)0.25k2/k1obs)[Rh4(CO)12]ss0.3[CO]1.1[H-2]-0.3[C6H10]0.9, with the apparent temperature dependence DELTA(ss)H(x) = -4.7 +/- 0.3 kJ/mol (-1.1 +/- 0.1 kcal/mol) and DELTA(ss)S(x) = -27.6 +/- 0.7 J/(mol K) (-6.6 +/- 0.2 cal/(mol K)). The results are consistent with (I) an equilibrium between the precursor Rh4(CO)12, H-2, and an unobservable intermediate {HRh(CO)3} and (II) the existence of a pseudo steady state between {HRh(CO)3} and the observed intermediate C6H11CORh(CO)4. The present catalytic system represents a case of equilibrium-controlled precursor conversion. The rate of hydroformylation was found to be proportional to the pseudo-steady-state concentrations Of C6H11CORh(CO)4 in each experiment. Regression of the experimental rate data provided the expression d[CrH11CHO]/dt = k1obs [C6H11CORh(CO)4]ss1[CO]-1.1[H-2]1 [C6H10]0.1, with the apparent activation parameters DELTA(double dagger)H(x) = 84.6 +/- 5.0 kJ/mol (20.3 +/- 1.2 kcal/mol) and DELTA(double dagger S(x)) = -8.7 +/- 17J/(mol K)(2.1 +/- 4.1 cal/(mol K)). The reaction orders and activation parameters are consistent with (I) an equilibrium between the intermediate C6H11CORh(CO)4, CO, and a coordinatively unsaturated species {C6H11CORh(CO)3} and (II) a rate-limiting step involving the oxidative addition of molecular hydrogen to {C6H11CORh(CO)3} to yield C6H11CHO and the highly reactive metal carbonyl hydride {HRh(CO)3}.