SYNTHESIS AND CATALYTIC ACTIVITIES OF POLYMER ANCHORED METAL COMPLEXES

Abstract

The use of synthetic polymers as catalyst support has been a logical transition from inorganic support to the well-defined synthetic support. The earlier workers have used low cross-linked organic polymers as support for the catalysts. The rationale behind using lightly cross-linked polymers as support was due to the accessibility of catalytic sites to the reactants in low cross-linked polymer supports. The highly cross-linked polymer supports have shown low porosity, which generally reduces the quality of support to imbibe solvent to facilitate the accessibility of reactants to the active sites. Therefore, polymer supports with optimum degree of cross-linking have been found capable to enhance activity of catalysts in comparison to unsupported homogeneous analogues. In polymer support, the flexibility of chains is another positive factor, which contributes appreciably towards reactivity enhancement ofthe catalysts in comparison to rigid inorganic supports. In addition to reactivity enhancement of the catalysts, the supports also facilitate the separation ofcatalysts from the reactants and the products. The chloromethylated cross-linked polymer support has been used frequently by various workers but this polymer support has provided limited choice for loading ofcatalysts and to have control on its degree of swelling and porosity. Efforts have been made to solve these limitations by careful designing the support by copolymerizing vinyl derivatized ligands with styrene and divinyl benzene as cross-linking agent or by immobilizing the catalysts on a tailor made functionalized polymer support as reported in this dissertation. These methods of designing supports for catalysts have been found useful, hence both methods designing supports have been tried in this work. The metal complexes of five different Schiff bases have been immobilized on polymer support and were characterized for structure and catalytic activity in comparison to their unsupported analogues. The polymer supported metal complexes of N,N'-bis (3-allyl salicylidene)ethylenediamine Schiff base (N,N'-BSEDA) have been prepared by copolymerization of synthesized N,N'- BSEDA Schiff base with styrene and divinyl benzene and synthesized polymer beads were allowed to react with selected metals salt. The amount of N,N'-BSEDA Schiff base immobilized on polymer beads obtained at different amount of DVB ranging from 0.8 mmol to 2.0 mmol, has varied from 1.50 mmol to 2.26 mmol. The extent of complexation of cobalt (II), copper (II), iron (III), nickel (II) and zinc (II) ions on polymer immobilized N,N'-BSEDA Schiff base has also shown variations in beads prepared with different amount of DVB. The structure of metal complexes with N,N'- BSEDA Schiff base has been determined using infrared, ultraviolet and magnetic techniques and it has been observed that the structures of free and polymer supported metal complexes of N,N'-BSEDA Schiff base were same. The beads prepared with 1.50 mmol of DVB have shown maximum loading for cobalt (II) ions (1.70 mmol g"1 of beads).These beads have shown optimum degree ofswelling (9.09 %) and average pore volume of0.64 cm3 g'1. The catalytic activity ofmetal complexes ofN,N'-BSEDA Schiff base immobilized on these beads (Type III) was evaluated by determining the rate of decomposition of hydrogen peroxide. The polymer supported N,N'-BSEDA Schiff base complex of iron (III) ions has shown high turn over number (176.8xl0'20 molecules sec"1 mol'1) in comparison to unsupported N,N'-BSEDA Schiff base complex of iron (III) ions (88.90xl0-20 molecules sec-1 mol'1). The N,N'-BSEDA Schiff base complexes of cobalt (II) copper (II), nickel (II), and zinc(Il) metal ions have shown high catalytic activity on polymer support, which has clearly indicated that polymer support has played a positive role in increasing the activity of these catalysts.The energy of activation of polymer supported metal complexes ofN,N'-BSEDA Schiff base has been found to be higher than unsupported analogue. The kinetic parameters for decomposition of hydrogen peroxide have been determined as a function ofconcentration of catalyst and hydrogen peroxide and considering these parameters a rate expression was derived. Asimilar methodology has been applied to prepare polymer supported metal complexes of N,N-bis(3-allyl salicylidene)hydrazine Schiff base (N,N'-BSH) by copolymerization of allylchloride derivatized salicylidenehydrazine Schiff base in presence of styrene and DVB. The amount ofN,N'-BSH Schiff base anchored on cross-linked polymer beads and catalytic activity of metal immobilized beads have shown variations on varying the amount of DVB in reaction mixture. The concentration of DVB was varied from 0.8 mmol to 2.0 mmol. The degree of swelling, pore volume and the amount of anchored N,N'-BSH Schiff base was optimum in beads ( Type IV) prepared at 1.75 mmol of DVB. The structures of polymer supported N,N'-BSH Schiff base metal complexes and their free analogues were almost same as verified from spectral and magnetic data. However, the arrangements of these complexes on cross-linked polymer beads have shown variations on varying the amounts ofDVB in the reaction mixture. The catalytic activity of polymer supported metal complexes of N,N'-BSH Schiff base has been found highest in comparison to free metals complex of N,N'-BSH Schiff base despite of structural similarity between free and polymer supported metal complexes of N,N'-BSH Schiff base. The iron (III) complex of N,N'-BSH Schiff base has shown high turn over number (39.82 x 10' molecules sec'1 mol'1) in comparison to its unsupported analogue (6.32 x 10' molecules sec" mol' ).The energy of activation for decomposition of hydrogen in peroxide in presence ofpolymer supported iron (III) N,N'-BSH Schiff base complex was also low ( 37.43 kJ mol"') in comparison to unsupported iron (III) N,N'-BSH Schiff base complex (57.20 kJ mol'1). The kinetic parameters were used to derive rate expression for the dependence of reaction rate (Rp) with concentration of hydrogen peroxide and catalyst. The low catalytic activity for metal complexes of N,N'-BSH Schiff base has been assumed due to the presence of hydrazine in the Schiff base(N,N'-BSH) used to complex with metals ions. The metal complexes of a Schiff base containing aromatic diamine and salicylaldehyde were also synthesized to compare the effect of diamine structure on complexation with selected metal ions and to compare catalytic activity of metal complexes of this Schiff base with other Schiff base and also with its unsupported counterpart. To obtain polymer supported Schiff base (salphen), the salphen Schiff base was first derivatized with allylchloride, which produced N,N'-bis(3-allyl salicylidene)ophenylenediamine Schiff base( N,N'-BSPDA). The resultant N,N-BSPDA Schiff base monomer was subsequently copolymerized with styrene and divinyl benzene (DVB) in presence of azobisisobutyronitrile as initiator to obtain polymer anchored N,N'-BSPDA Schiff base. The extent of N,N'-BSPDA Schiff base on polymer beads has shown dependence on the concentration of DVB, which was varied from 0.8 mmol to 2.0 mmol. The polymer bead (Type III) obtained at 1.50 mmol of DVB were having highest amount of N,N'-bis (3-allyl salicylidene)o-phenylenediamine Schiff base (N,N'-BSEDA) (1.74 mmol g"1 beads).These beads (Type III) have shown maximum efficiency for loading (% EL) and complexation (% EC) for all selected metals ions. The beads prepared with different amount of DVB were characterized for their size and pore volume. The thermal stability of polymer anchored Schiff base (N,N'-BSEDA) was compared with metals IV anchored counterparts, which has clearly indicated that thermal stability of polymer anchored Schiff base has increased substantially on complexation with metal ions. The structures of free and polymer supported N,N'-BSPDA Schiffbase complexes of cobalt (II), copper (II), iron (III), nickel (II) and zinc(II) metal ions were analyzed by IR, UV and magnetic measurements, which indicated for structural similarity between free and polymer supported metal complexes of N,N'-BSPDA Schiff base. The catalytic activity of metals complex of N,N'-BSPDA Schiff base was evaluated by estimating the rate constant (k) for decomposition of hydrogen peroxide as well as by determining the turn over number. The value of rate constant for decomposition of hydrogen peroxide with iron (III) complex has been found to be high (24.75xl0"5 sec"') in comparison to its unsupported countered part (0.46x10'5 sec'1) but was low in comparison to iron (III) complex of N,N'-BSEDA Schiff base (15.30xl0"4 sec'1 and 8.0xl0'4 sec"1). The kinetic parameters that determined as a function of concentration of hydrogen peroxide, catalysts were used to derive a rate expression as derived with other Schiff base metal complexes. In addition to these three systems, two more Schiff base systems based on acetylacetone, ethylenediamine and o-phenylenediamine were studied. In these systems, the N,N'- bis(acetylacetone)ethylenediamine (N,N'-BSACEN) and N,N'-bis(acetylacetone)ophenylenediamine (N,N'-BSACPHEN) Schiff bases were immobilized on functionalized polymer beads prepared by copolymerization of styrene (48.97 mmol) and allylchloride (48.97 mmol) at different concentrations of divinyl benzene ranging from 0.8 mmol to 2.0 mmol in the reaction mixture. The amount of allylchloride in the beads was determined by estimating the content of chlorine by elemental analysis. The content of allylchloride in the beads has shown dependence on the amount of DVB taken in the reaction mixture. The content of allylchloride has shown variations from 7.40 mmol g"1 of beads to 5.40 mmol g"1 of beads.The extend of anchoring of N,N'-BSACEN Schiff base on polymer support was different from N,N'-BSACPHEN Schiff base due to structural difference in these Schiff bases. The structures of metal complexes of N,N'- BSACEN and N,N'-BSACPHEN Schiff bases both in free and polymer supported state were determined with IR, UV and magnetic measurements have provided complete similarity between free and polymer supported state of complexes of both Schiff bases. The extent of complexation of metal ions with N,N'-BSACEN Schiff base was higher than N,N'-BSACPHEN Schiff base, which was almost same as was observed between N,N'-BSEDA and N,N'-BSPDA Schiff bases. This difference was due to the introduction of phenylene group in place of ethylene group. The catalytic activity of metal complexes of N,N'-BSACEN Schiff base was higher in comparison to metal complexes of N,N'- BSACPHEN Schiff base. The kinetic data as a function of concentration of hydrogen peroxide and catalysts were estimated and used to derive rate expressions in presence of both Schiff bases and it was found that mechanism of decomposition of hydrogen peroxide has remained almost same except the rate of decomposition has shown variations with type of Schiff base and metal ions used. These investigations have clearly demonstrated that polymer support has played a significant role in activity modification of catalysts. The activity of catalysts has shown variations with the structure of Schiff base and also on with structures of beads used to immobilize these metal complexes. The immobilization of Schiff base using monomer derivatized legend has provided a solution to control the extent and distribution of Schiff base on polymer support in comparison to direct immobilization of Schiff base on polymer support of fixed composition.