Catalytic activity o palladium colloids in situ
a Inorganic Cemistry Department of State Institute of Technology, Moskovsky pr. 26, St-Petersburg, Russian Federation 198013
Catalytic properties of palladium colloid in situ have been studied in the hydrogenation and oxidation of benzilic alcohols. The kinetics of the homogeneous reaction has been investigated under static conditions. The concentration of catalyst, molecular hydrogen and oxygen into the reaction solution and initial concentration of alcohols have been varied. Physic-chemical properties of palladium catalyst have been studied using methods of TEM and UV-spectroscopy. Using experimental results and physic-chemical investigations, the mathematical models of the process and the reaction mechanism have been offered.
Author Keywords: Palladium colloid; Benzylic alcohols; Selective hydrogenation; Selective oxidation; Catalysis
During the last decades investigation of catalysts on the base of colloidal metals became one of the one most important problems in catalysis. It caused higher catalytic activity and selectivity of coloidal catalysts in compare with supported catalysts. We did not discover however publications dedicated to investigation of catalytic activity colloidal metal nanoparticles in absence supported or stabilizing ligands in literature. This investigation can give information about the key step of mechanisms reaction in the presence colloidal metal nanoparticles.
Recently we are established that solution formatted during reduction of [Pd(H2O)4](ClO4)2 (I) by a number of aliphatic allylic and benzylic alcohols have catalytic activity in oxidation by molecular oxygen and hydrogenation by molecular hydrogen of related alcohols and its disproportionation after palladium bulk recovery.
In particular during benzyl alcohol oxidation by complex (I) in inert atmosphere benzaldehyd and toluen was obtained. The both yields of a toluene and a benzaldehyde on Pd(II) reduced are equal and top the stoichiometry of eq. 1:
|PhCH2OH + (I) = PhCH(O) + Pd(0) + 2H+||(1)|
more than 2-5 time as functions of conditions the reaction proceeding.
After the complete reduction of complex (I) and separating of palladium black (Pdblack) as well as changing argon on dioxygen or hydrogen, the process leads to a benzaldehyde or a toluen corresponding, with high selectivity according general scheme 1
General route of catalytic transformations of benzylic alcohol in the Pd colloid in situ presence.
In this paper kinetics of above mentioned processes are investigated on the system with benzylic alcohol and the general scheme mechanism of those have been proposed. [ Ukraintsev V.B., Potechin V.V. // Zh. Obch. Khim.1997. V.67. N 10. P. 1606-1610. V.V. Potechin, V.A. Matsura, V.B. Ukraintsev // Zh. Obch. Khim. 2000, V.70., N. 12. P. 2058. V.V. Potechin, V.A. Matsura, V.B. Ukraintsev // Zh. Obch. Khim. 2000, V.70., N. 6. P. 886]
The experiments on investigation of palladium colloid in situ catalytic properties were carried out under static conditions. The reactions were carried out in a glass isothermal reactor installed in a shaker and connected to a gasometric burette [D.V. Sokolsky, Gidrirovanie v rastvorah, AN KazSSR, Alma-Ata, 1962, 487 pp.]. The process of hydrogenation or oxygenation was carried out under conditions providing the absence of diffusion limitations. Previously, it was found that stirring intensity less than 900 shakings/min provided diffusion limitations (the process rate was determined by the rate of reagents diffusion to the catalyst surface) [E. Sulman, V. Matveeva and T. Ankudinova. Kinet. Catal. 353 (1994), p. 385.]. Thus, by the process kinetics study it is necessary to stir the reaction mixture with the intensity more than 900 shakings/min. Then the process rate will be limited by the rate of the reaction itself. The rate of reactions was calculated as relation of gaseous uptake to the time of reaction on the stage of constant rate.
Analysis of reaction mixture was carried out by gas chromatography (GC) using chromatograph "Cvet-600" with FID and glass column 3 m/3 mm. The column was filled with phase "Apiezon L" (0.215–0.315 mm). Before analysis the column was conditioned with nitrogen under conditions: temperature: from 50 to 220°C, heating rate: 1°C/min, duration: 3–5 h. The temperature of the evaporator was 200°C, of the detector -180°C. Supporting gas was nitrogen. Outlet velocity of nitrogen was 80 ml/min and hydrogen - 35 ml/min. The analysis duration was approximately 10 min.
UV-spectra were recorded at room temperature at the spectrophotometer SP-26 in the interval of 186–1100 nm.
The homogeneous catalyst was prepared in Ar atmosphere and acidic media (HClO4 concentration was 0.2 mol/l) from complex 1 by reduction with benzylic alcohol. The reduction time was 20 min. After reduction have been done, the weakly gray or depend from concentration of 1 colorless solution of catalyst was obtained. The end of reduction was probed by the reaction with KI – formation of K2PdI4, accompanied by changes of color reaction solution was not obtained. The concentration of catalyst was taking equal to concentration of complex 1.
3. Results and discussion
Catalytic properties of the homogeneous catalyst created on the basis of complex 1 were investigated in the process of selective hydrogenation and oxidation according to the methods above described. During the course of catalyst formation the yellow color that result from 1 begun to decrease in intensity and solution eventually became colorless. The electron micrograph of Fig. 1 shows the presence of colloidal Pd with a broad distribution of particle size average diameter of about 5 nm. This results has good agreement with earlier reporter (Henglein, A.; Ershov, B. G.; Malow, M. J. Phys. Chem. Volume 99, Issue 38, 1995, P. 14129).
The effect of the catalyst concentration was studied at constant concentration of benzylic alcohol Co=0.072 mol/l, gas pressure 0.1MPa and solvent quantity of 10 ml by complex 1 concentration Cc varying from 5*10-6 to 1*10-3 mol/l.
Rate dependence in hydrogenation and oxygenation of benzylic alcohol from catalyst concentration
Table 1. The influence of the catalyst (complex 1) concentration on its activity in the process of homogeneous hydrogenation of benzylic alcohol (CROH 0.072, CHClO4 М mDHL: 1 g, th: 55°C, liquid phase volume: 10×10-6 m3, PH2 or O2: 0.1 MPa)
According to the results obtained, optimal rate is observed in the presence of 1*10-5 mol/l in hydrogenation and 1*10-4 mol/l of complex 1.
The results obtained are presented in Table 1 (mDHL is DHL quantity, th is the hydrogenation temperature).
Table 1. The influence of the reaction system components concentration of on the reactions rate in the process of homogeneous hydrogenation ((1): 1*10-5 mol/l) and oxidation ((1): 1*10-4) of benzyl alcohol (water, HClO4: 0.2 mol/l, th: 55°C, liquid phase volume: 10×10-6 m3, P= 0.1 MPa)
|СО2·104, mol/l||СH2 ·104, mol/l||CROH·102, mol/l||V·105, mol/(l·s)||K ·10-3, l2/(s·mol2)|
*The concentrations of hydrogen and oxygen in solution were calculated in depends from partial pressure according their solubility (18)
The reaction scheme was used for the experimental data description:
where k' is the rate constant, S1 – benzylic alcohol, S2 – H2 or O2 and Product – benzaldehyde.
According to this scheme the kinetic model were proposed using mathematical treatment and the kinetic data obtained. Substrate adsorption was not considered (the catalyst was homogeneous). The best results were obtained for the model described by the following equation:
where W' is the relative hydrogenation rate, k’ is the kinetic parameter of the reaction, xi=Vi/Vo, where Vi is the volume of gas (H2 or O2) up taking by the moment t.
The presence of induction period is probably related to additional processes leading to the reactive complex formation. The rate increasing in this period confirms the supposition. It is possible to suppose two-step reactive complex formation on the first step benzylic alcohol activation by catalyst with weakness of C-H-bond in α-position and at second – H2 or O2 interaction, according the presented scheme:
where Kat – is the colloidal particle of Pd catalyst
where A is the reactive complex "substrate–catalyst”. On the formation of A point the disproportionation of benzylic alcohol in inert atmosphere.
If the stage of the product formation is supposed to be the slowest one and the equilibrium of the processes (5-6) displaces to the reverse reaction, then the rate of hydrogenation and oxygenation is directly proportional to the initial substrate concentration.
Then, the rate of reaction can be described by equation () when the catalyst concentration is constant:
The complex A concentration if on conditions that [Kat]<<[S1]0 and [Kat]<<[S2]0 can be obtained from the following equation:
where W is the rate of process.
It would be important to note that the kinetic schemes are analogously both for hydrogenation and oxidation and that earlier the similarly kinetic equations were obtained by us for allilic alcohol hydrogenation and oxidation and for H2 and O2 interaction in the palladium colloid in situ presence that demonstrated the common key step for all mentioned reaction. Thus we are can propose of general scheme for all catalytic processes in the presence of palladium colloid in situ:
(1) The catalytic properties of the Pd in situ obtained from complex 1 were firstly investigated in selective hydrogenation and oxygenation of the benzylic alcohol.
(2) The kinetics of the process was studied, catalyzed by the homogeneous catalyst created on the basis of this complex. The kinetic model was proposed on the basis of the data obtained. The mechanism of the reaction was proposed.
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