Reusable heterogeneous catalysts of α‐amino acid racemization, 2. Factors affecting the activity of heterogeneous catalysts based on strongly basic anion exchangers in OH− form and 4‐hydroxy‐3‐formylbenzenesulfonic acid (5‐sulfosalicylaldehyde)

Abstract

AbstractThe kinetics of (S)‐Ala racemization on heterogeneous catalysts based on eight strongly basic anion exchangers modified with 4‐hydroxy‐3‐formylbenzenesulfonic acid (5‐sulfosalicylaldehyde) (SSA) in the presence of Cu(II) ions at 25 °C was studied. The “internal capacity” value f = mp/(V0 — Vp) was introduced to characterize the polymers, (mp : number of quaternary ammonium groups of the polymer sample, V0 : total volume of the swollen polymer, Vp : volume of the inert polymeric matrix). The correlation of the observed first‐order rate constants of (S)‐Ala racemization with the f values of the anion exchangers was established. According to this correlation the selected anion exchangers could be divided into two types with respect to the chemical nature of the groups attached to the quaternary nitrogen atom. The chemical nature, physical structure and the degree of cross‐linking of the polymeric matrix do not affect the correlation. Anion exchangers of the first type have at least one ß‐hydroxyalkyl substituent at the Anion exchangers of the second type have (CH3)3 cationic groups. Catalysts prepared with the first type anion exchangers have greater activity as compared with catalysts based on the second type anion exchangers with the same f values. The increase in activity of heterogeneous catalysts can be partially attributed to the 10 to 68 fold increase in OH− concentration in the internal phase of the catalysts as compared to the model homogeneous system, which contains the same amounts of the components as the heterogeneous one. Moreover, an additional acceleration is observed in the heterogeneous racemization as compared to the homogeneous one, ca. 5 fold for the second type anion exchangers and ca. 14 fold for the first type anion exchangers.