Heterogenous catalysis of inorganic reactions

Abstract

Aluminosilicates, especially zeolite A, Surrey powder and alumina, are efficient and mild catalysts for ligand substitution reactions of inert Cr(" and Co(ffl) complexes. The reaction kinetics for the aquation of chromium(1H) chloride and chromium(llI) bromide hexahydrates, the anation reaction of Cr(H 20)631 by Cl- and the aquation of [Co(NH3)5C1]C12 with and without aluminosilicate catalysts were studied using ion exchange chromatography. Rate constants were determined for each reaction by application of a least squares modelling in Excel. Experimental observations agreed with literature data, that under the conditions employed and without catalysts, the aquation reactions of trans-[CrCl 2(H20)4]Ct2H20 and lrans-[Cr]3r2(H20)4JBr2H20 to [Cr(H2O)6] 3 , follow an I mechanism with a conjugate base pathway, indicating that a weakening of the Cr-X bond is crucial for accelerating the aquation reactions.
The aluminosilicate catalysed aquation studies of CrC! j6H2O and CrBr3'6H20 appear to indicate that accessible tetrahedral aluminium species are necessary for successful catalysis of the reactions and the greater the number of these sites the faster the aquation. The aluminosilicates act simultaneously as bi-functional catalysts, i.e. as proton acceptors (base catalysis) and as electron pair acceptors (Lewis acid catalysis).
Thus, the two possible activation mechanisms that agree with the experimental observations, appear to be: i) association of a halide ion with an accessible Lewis site (e.g. Al—OH2 and/or (AJ-0)2Cr 4), which is developed from weak base site Al-OH on the surface of the catalyst, enabling the halide to dissociate more readily; ii) base catalysis where the catalyst absorbs H allowing the Cr cation to lose fbrther protons from its aquation sphere, reducing the charge on the complex, weakening the Cr—Cl bond and thus making the halide more labile to substitution. This bi-functional catalysis was confirmed by the anation of Cr(H 2O)63 by Cl- and the aquation of [Co(NH3)5Cl]C12 catalysed by Zeolite A and Surrey Powder.
The base catalytic functionality can be classified into two types according to how they increase the pH of the solution environments. The first group including zeolite A and intact montmorillonite clays, has low pzc but high cation ion exchange capacity and so raises the pH to a relatively high degree by proton/metal cation exchange. The second group consists of oxides with high pzc such as y-alumina, which raise the pH of their surrounding solutions by reactions to form surface hydroxyls. For the Lewis acid catalysis function, the three possible anion absorption mechanisms on the clay edges that appear to occur under our experimental conditions (pH !9 4 and in Cr3 solution), are: i) formation of Al—OH2 at low pH, which can attract anions by acid—base ion pairing; ii) formation of AI—OCr2 or (Al-0)2Cr by metal cation binding reactions; and iii) the absorption of anions by exchange with the surface OH groups. The adsorbed Cr3 (e.g. [A]-0]2Cr4) on the edge of the catalysts appeared to be playing the more important role as the Lewis acid sites.
Surrey Powder and 5A molecular sieve were successful catalysts for the direct syntheses of potassium and sodium tris(oxalato)chromate(ll1), potassium and sodium tris(malonato)chromate(Ill) and tris(ethylenediamine)chromium(ffl) chloride from chromium chloride hexahydrate in water at room temperature. Although no attempts at optimising these yields were made, all yields of the catalytic syntheses were improved over the un-catalysed reactions.