JOURNAL OF POLYMER SCIENCE: Polymer Chemistry Edition VOL. 12. 1781-1785 (1974)

Polymeric Nitrile-Palladium Chloride Complexes and Their Reaction with Olefins

M. KRAUS and D. TOMANOVA, Institute of Chemical Process Fundamentals, Czechoslovak Academy of Sciences, 165 02

Prague-Suchdol, Czechoslovakia

Synopsis Polymers containing cyano groups connected to the polymeric chain by means of a t

least one methylene group adsorb palladium dichloride reversibly from its slightly acidic solutions and form surface complexes of the probable structure (-CH&N)$-PdC12. These complexes can adsorb gaseous olefins; ethylene is consumed continuously, but adsorption of propylene and 1-butene reaches saturation. Additional ethylene can be added to the propylene or 1-butene adsorbate. Hydrolysis of the surface olefinic com- plexes yields aldehydes, the ethylene complex giving acetaldehyde and butanal, the mixed ethylene-1-butene complex giving acetaldehyde, butanal and hexanal.

INTRODUCTION

In connection with the development of metal complex-on-polymer hetero- geneous ~a ta lys t s l -~ the reaction of palladium dichloride with macroreticu- lar highly crosslinked polymers carrying cyano groups was studied. De- pending on the structure of the polymer, complexes corresponding to the well-known bis(alky1 or arylnitrile)dichloropalladium5 were obtained. On contact with ethylene not specially dried, the originally yellow to brown complexes changed their color to black. This indicated rapid reduction to metallic palladium which was then confirmed by analysis. With dry ethyl- ene no apparent color change was observed, and slow sorption of more than one molecule of the olefin per atom of the metal occurred. These observa- tions and the results of the study of the interaction of lower olefins with polymeric nitrile-palladium dichloride complexes are summarized in this paper. The vacuum-line technique was used in most experiments because it clearly indicates the stoichiometry of the formation of surface olefin-metal complexes.

EXPERIMENTAL

Polymers with Nitrile Groups

In most experiments a styrene-divinylbenzene copolymer carrying -CH&N substituents6 was used (polymer A). A commercial chloro- methylated macroreticular copolymer (80% styrene, 20% divinylbenzene)

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1782 KRAUS AND TOMANOVA

in bead form (diameter 0.5-1.5 mm), containing 16.7% C1, with a specific surface area of 120 m2/g and the most frequent pore diameter of 600 was reacted with sodium cyanide6 in dimethyl sulfoxide. The copolymer (100 g), NaCN (50 g), and dimethyl sulfoxide (400 ml) were heated on a boiling water bath for 6 hr. After washing with water and drying, the beads con- tained 2.6y0 C1 and 5.7Y0 N and their surface area was 142 m2/g.

Some other polymers were prepared from monomers already containing cyano groups. The polymerization of 2-cyanoethyl methacrylate cross- linked with ethylene dimethacrylate may serve as an example. A mixture of 8 ml of 2-cyanoethyl methacrylate, 2 ml of ethylene dimethacrylate, 5 ml of butyl acetate, and 0.1 g of benxoyl peroxide was heated in a test tube at 70°C for 3 hr. The resulting white rod of the copolymer was extracted with methanol in a Soxhlet apparatus for 6 hr, dried in vacuo, and ground in a mortar to a suitable grain size. The surface area of the macroreticular copolymer was 23 m2/g.

Palladium Complexes

A suspension of polymer beads or granules was suspended in water and placed in a chromatographic column. An excess of palladium dichloride solution containing 0.01 g Pd/ml was slowly soaked into the column and then displaced by distilled water. Nonadsorbed palladium salt was washed out and the impregnated polymer dried and analyzed for palladium.

Sorption of Olefins

A small flask with approximately 1 g of the complex from polymer A containing 2.65y0 Pd was sealed to a vacuum line and evacuated to torr a t 80°C for 8 hr. Dry olefin wm then admitted up to a pressure of 300 torr. The adsorbed amount was refilled when the pressure fell to approxi- mately 290 torr. The summarized readings gave the curves shown in Figure 1. In the case of propylene and l-butene, the remaining gaseous olefin was rapidly pumped off and ethylene introduced to the flask, again to a pressure of 300 torr, and the readings were continued.

Isolation of Aldehydes from Mixed Adsorbates

The same vacuum apparatus as above was used with 26.4 g of the com- plex. First, 23 ml (STP)/g of l-butene was adsorbed during 32 days and then additional 19.6 ml (STP)/g of ethylene during 50 days, both at 250 torr. The resulting gray-brown beads were boiled with 3 X 100 ml of wet diethyl ether; the beads changed in color to black. The ether solution was concentrated by distillation. The residue was analyzed by GLC (with polyethylene glycol as the stationary phase) and by TLC after conversion of the carbonyl compounds to 2,4-dinitrophenylhydrazones. Positions of the peaks and spots were compared with those of standard samples. Acet- aldehyde, butanal, and hexanal were identified. The ratio of C4:Cs alde- hydes was approximately 1 : 1.

NITRILE-PALLADIUM CHLORIDE COMPLEXES 1783

0 xx) 200 300 h

Fig. 1. Adsorption of ethylene, propene, and 1-butene on polymer A with 2.65% Pd at room temperature and 300 torr.

Isolation of Aldehydes from Ethylene Adsorption

A column containing 67 g of polymer A impregnated with 3.1% Pd was flushed with dry nitrogen for 3 days at room temperature. The column was weighed and the nitrogen stream was replaced by a stream of dry ethylene. Ih 24 hr the weight of the column increased by 0.8 g, i.e., 1.7 molecules of ethylene were adsorbed per atom of palladium. The beads were treated with wet diethyl ether and the products analyzed as above. Acetaldehyde and butanal were found to be formed in approximately equal amounts. The mixed dinitrophenylhydrazones weighed 1.7 g.

RESULTS AND DISCUSSION

The first observation of sorption of palladium dichloride on the cyano group containing polymers was made with poly(Zcyanoethy1 methacrylate) crosslinked with ethylene dimethacrylate (polymer B, see Experimental). A survey was then made of various polymers with the purpose of finding any structural dependence for complex formation. A series of polymers con- taining cyano groups was prepared either by chemical modification of com- mercial polymers (e.g., polymer A) or by polymerization of monomers con- taining cyano groups. All samples were highly crosslinked and macro- reticular, i.e., they possessed an internal pore system even in the dry state. Either divinylbenzene or ethylene dimethacrylate was used as crosslinking agent; for simplification, only the monomer unit carrying the cyano group will be mentioned in the following text. Sorption of palladium dichloride was observed on polycyanomethylstyrene, poly(cyanomethy1 methacry-

1784 KRAUS AND TOMANOVA

late), poly(Bcyanoethy1 methacrylate), and poly(ally1 cyanide). No com- plex of PdCh was formed with polyacrylonitrile, poly(3-cyanophenyl methacrylate), and polyfumarodinitrile, that is, with substances in which the cyano group is attached directly to the polymeric chain or phenyl group. On the contrary, the first series of the polymers contains the cyano group separated from the rigid structural unit by at least one methylene group. This gives flexibility to the cyano group which is probably neces- sary for formation of a surface complex of the structure (-CN)2PdC12, which is analogous to the complexes formed from low molecular weight nitriles.6 A surface complex with only one cyano group coordinated to palladium would not be so structure-sensitive.

Under anhydrous conditions, these palladium complexes reacted slowly with olefins, the rate of absorption of gaseous olefins being dependent on the pressure and structure of the absorbate. The pressure dependence was established by the experiments with ethylene at 300 torr (see Fig. 1)) at atmospheric pressure, and at 20 atm in an autoclave. In the last case, the absorption slowed down appreciably after few hours, at atmospheric pres- sure after one day, and at low pressure it proceeded at a very slow rate. The structure dependence is evident from Figure 1. The initial rate in- creases with the molecular weight of the olefin, and there is a marked dif- ference in the total amounts absorbed. The reaction with ethylene is slow but still proceeds even after 500 hr at 300 torr. Its stoichiometry exceeds the ratio of two molecules of ethylene per palladium atom. More rapid absorptions of propene and of 1-butene soon attained equilibrium, cor- responding to less than one molecule of olefin adsorbed per palladium atom. Additional ethylene could be added to preabsorbed propene and 1-butene, again at a slow rate.

These observations can be interpreted in accord with reported reactions of palladium complexes with 01efins.~*’ v 8 Bis(ary1nitrile)dichloropalladium dissolved in benzene and in contact with ethylene yields a precipitate, bis- (ethvlene)dichloro-p-dichlorodipalladium.6 This complex in which each ethylene molecule is coordinated to one palladium atom can also be prepared by direct action of ethylene on palladium dichloride in chloroform? Further treatment of its solution with ethylene gives, depending on conditions, butenes as the products of a catalytic reaction”8 or bis(ethy1ene)dichloro- palladium7, an unstable monomer with two ethylene molecules coordinated to one palladium atom. The latter compound is believed to be an inter- mediate in the catalytic dimeriaation? In contrast, butenes are not dimer- ized and form only bis(butene)dichl~rodipalladium.~ In our case, the sur- face nitrile complex is dispersed in the pores of the polymer and, with the exception of very narrow pores, easily attacked by olefin molecules. There- fore more than one molecule of ethylene per palladium atom can be ab- sorbed, a8 in solution. The lower stoichiometry for 1-butene is in agree- ment with that reported7 for soluble complexes. Propene resembles butene in our case, and this is a difference, because in solution it is dimerized by palladium dichloride? like ethylene.

NITRILE-PALLADIUM CHLORIDE COMPLEXES 1785

Some insight into the structure of our olefinic complexes was given by their decomposition by water. Acetaldehyde was formed as expected, but the ethylene-containing polymer also gave the C4 aldehyde (butanal). This is the evidence of the coordination of two ethylene molecules to one palla- dium atom which were dimerized either spontaneously or on attack by water. Similarly, the polymer with ethylene added to preabsorbed l-bu- tene yielded C4 and CS aldehydes. This indicates the formation of a mixed (ethylene + butene) complex on the palladium atoms. Such a complex was also assumed to be an intermediate in the catalytic dimerization.? No ketone was detected among the products of all our hydrolysis experiments, and this probably supports the idea of simultaneous decomposition and dimerization.

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Received March 11, 1974 Revised April 19, 1974