Alkane Oxidation Catalysis by Homogeneous and Heterogeneous Catalyst

Abstract

Abstract Cobalt-based complexes are widely used in industry and organic synthesis as catalysts for the oxidation of hydrocarbons. The Co/Mn/Br (known as "CAB system") catalyst system is effective for the oxidation of toluene. The Co/Mn/Br/Zr catalyst system is powerful for the oxidation of p-xylene, but not for the oxidation of toluene. [Co3O(OAc)5(OH)(py)3][PF6] (Co 3+ trimer 5) is more effective than [Co3O(OAc)6(py)3][PF6] (Co 3+ trimer 6) as a catalyst in the CAB catalyst system. Higher temperatures favour the oxidation of toluene. Zr 4+ does not enhance the oxidation of toluene. Zr 4+ could inhibit the oxidation of toluene in the combination of Co/Br/Zr, Co/Mn/Zr or Co/Zr. NHPI enhances the formation of benzyl alcohol, but the formation of other by-products is a problem for industrial processes. Complex(es) between cobalt, manganese and zirconium might be formed during the catalytic reaction. However, attempts at the preparation of complexes consisting of Co/Zr or Mn/Zr or Co3ZrP or Co8Zr4 clusters failed. The oxidation of cyclohexane to cyclohexanone and cyclohexanol is of great industrial significance. For the homogeneous catalysis at 50 o C and 3 bar N2 pressure, the activity order is: Mn(OAc)3 �2H2O > Mn12O12 cluster > Co 3+ trimer 6 > [Co3O(OAc)3(OH)2(py)5][PF6]2 (Co 3+ trimer 3) > Co 3+ trimer 5 > Co(OAc)2 �4H2O > [Co2(OAc)3(OH)2(py)4][PF6]-asym (Co dimerasym) > [Co2(OAc)3(OH)2(py)4][PF6]-sym (Co dimersym); whereas [Mn2CoO(OAc)6(py)3]�HOAc (Mn2Co complex) and zirconium(IV) acetate hydroxide showed almost no activity under these conditions. But at 120 o C and 3 bar N2 pressure, the activity order is changed to: Co dimerasym > Co(OAc)2 �4H2O > Co trimer 3 and Mn(OAc)3 �2H2O > Co 3+ trimer 6 > Mn2Co complex > Co 3+ trimer 5 > Co dimersym > Mn12O12 cluster. The molar ratio of the products was close to cyclohexanol/cyclohexanone=2/1. Mn(II) acetate and zirconium(IV) acetate hydroxide showed almost no activity under these conditions. Among those cobalt dimers and trimers, only the cobalt dimerasym survived after the stability tests, this means that [Co2(OAc)3(OH)2(py)4][PF6]-asym might be the active form for cobalt(II) acetate in the CAB system. Metal-substituted (silico)aluminophosphate-5 molecular sieves (MeAPO-5 and MeSAPO-5) are important heterogeneous catalysts for the oxidation of cyclohexane. The preparation of MeAPO-5 and MeSAPO-5 and their catalytic activities were studied. Pure MeAPO-5 and MeSAPO-5 are obtained and characterised. Four new pairs of bimetal-substituted MeAPO-5 and MeSAPO-5(CoZr, MnZr, CrZr and MnCo) were prepared successfully. Two novel trimetal-subtituted MeAPO-5 and MeSAPO-5 (MnCoZr) are reported here. Improved methods for the preparation of four monometal-substituted MeAPO-5 (Cr, Co, Mn and Zr) and for CoCe(S)APO-5 and CrCe(S)APO-5 are reported. Novel combinational mixing conditions for the formation of gel mixtures for Me(S)APO-5 syntheses have been developed. For the oxidation of cyclohexane by TBHP catalysed by MeAPO-5 and MeSAPO-5 materials, CrZrSAPO-5 is the only active MeSAPO-5 catalyst among those materials tested under conditions of refluxing in cyclohexane. Of the MeAPO-5 materials tested, whereas CrCeSAPO-5 has very little activity, CrZrAPO-5 and CrCeAPO-5 are very active catalysts under conditions of refluxing in cyclohexane. MnCoAPO-5, MnZrAPO-5 and CrAPO-5 are also active. When Cr is in the catalyst system, the product distribution is always cyclohexanone/cyclohexanol equals 2-3)/1, compared with 1/2 for other catalysts. For MeAPO-5, the activity at 150 o C and 10 bar N2 pressure is: CrZrAPO-5 > CrCeAPO-5 > CoZrAPO-5. For MeAPO-5 and MeSAPO-5, at 150 o C and 13 bar N2 pressure, the selectivity towards cyclohexanone is: CrZrAPO-5 > CrZrSAPO-5 > CrCeAPO-5 > CrAPO-5 > MnCoAPO-5 > MnZrAPO-5; and the selectivity towards cyclohexanol is: MnZrAPO-5 > CrZrAPO-5 > MnCoAPO-5 > CrZrSAPO-5 > CrCeAPO-5 > CrAPO-5. Overall the selectivity towards the oxidation of cyclohexane is: CrZrAPO-5 > CrZrSAPO-5 > CrCeAPO-5 > CrAPO-5 > MnCoAPO-5 > MnZrAPO-5. The amount of water in the system can affect the performance of CrCeAPO-5, but has almost no effect on CrZrAPO-5. Metal leaching is another concern in potential industrial applications of MeAPO-5 and MeSAPO-5 catalysts. The heterogeneous catalysts prepared in the present work showed very little metal leaching. This feature, coupled with the good selectivities and effectivities, makes them potentially very useful.