Selective oxidation catalysts obtained by immobilization of iron(III) porphyrins on thiosalicylic acid-modified Mg-Al layered double hydroxides.

Abstract Nitrate-intercalated Mg-Al layered double hydroxides (LDHs) were synthesized and exfoliated in formamide. Reaction of the single layer suspension with thiosalicylic acid under different conditions afforded two types of solids: LDHA 1 , in which the outer surface was modified with the anion thiosalicylate, and LDHA 2 , which contained the anion thiosalicylate intercalated between the LDH layers. LDHA 1 and LDHA 2 were used as supports to immobilize neutral (FeP 1 and FeP 2 ) and anionic (FeP 3 ) iron(III) porphyrins. For comparison purposes, the iron(III) porphyrins (FePs) were also immobilized on LDH intercalated with nitrate anions obtained by the co-precipitation method. Chemical modification of LDH facilitated immobilization of the FePs through interaction of the functionalizing groups in LDH with the peripheral substituents on the porphyrin ring. The resulting FePx-LDHAy solids were characterized by X-ray diffraction (powder) and UV–Vis and EPR spectroscopies and were investigated as catalysts in the oxidation of cyclooctene and cyclohexane. The immobilized neutral FePs and their homogeneous counterparts gave similar product yields in the oxidation of cyclooctene, suggesting that immobilization of the FePs on the thiosalicylate-modified LDHs only supported the catalyst species without interfering in the catalytic outcome. On the other hand, in the oxidation of cyclohexane, the thiosalicylate anions on the outer surface of LDHA 1 or intercalated between the LDHA 2 layers influenced the catalytic activity of FePx-LDHAy, leading to different efficiency and selectivity results. FeP 1 -LDHA 2 performed the best (29.6% alcohol yield) due to changes in the polarity of the surface of the support and the presence of FeP 1 . Interestingly, FeP 1 also performed better in solution as compared to the other FePs. Finally, it was possible to recycle FeP 1 -LDHA 2 at least three times.