MECHANISTIC INVESTIGATION OF THE AUTOOXIDATION OF CUMENE CATALYZED BYTRANSITION-METAL SALTS SUPPORTED ON POLYMER

The autooxidations of cumene to cumene hydroperoxide (CHP) in the pres ence of various transition metal salts supported on Bio-Rex 70 which i s a macroreticular polyacrylate with carboxylate functional group, wer e investigated. The polymer supported catalyst is denoted as MS-BR-r i n which MS represents transition metal salt, BR represents the polymer support and r is the loading of metal salt in the unit of mmoles per gram of dry support. In a catalyst loading of 0.20 g per 10 mi of cume ne and initial O-2 pressure 103 kPa at 363 K, the catalyzed autooxidat ion rate follows the order: Mn(OAc)(2)-BR-0.6 > Co(OAc)(2)-BR-0.6 > Fe Cl2-BR-0.6 > Cu(OAc)(2)-BR-0.6 > Cr(NO3)(3)-BR-0.6 much greater than N i(OAc)(2)-BR-0.6. The selectivities to CHP are 97% for Cu(OAc)(2)-BR-0 .6 and Cr(NO3)(3)-BR-0.6; and 92% far Mn(OAc)(2)-BR-0.6, Co(OAc)(2)-BR -0.6 and FeCl2-BR-0.6. These data indicate that Cu(OAc)(2)-BR-0.6 is t he best catalysts among the catalysts investigated in this work. The m etal loading effect was investigated for Ca(OAc),-BR-r, r = 0.3, 0.6, 1.5, 2.0 and 2.5. In the catalyst loading of 0.20 g per 10 mi of cumen e and initial O-2 pressure 100 kPa at 363 K, the oxidation rate increa ses with r from 3.96 x 10(-5) M/s at r = 0.3 to 8.35 x 10(-5) M/s for r = 2.5. The selectivity to CHP decreases with increasing r from 93.8% for r = 0.3 to 88.1% for r = 2.5 at a conversion of 7%. When cumene a utooxidation catalyzed by Ca(OAc)(2)-BR-2.0 was investigated at temper atures in the range of 363 K to 323 K, we found that oxidation rate de creases with temperature. However, unexpectedly, the selectivity decre ases with temperature. This is interpreted by considering the competin g reactions between the formation of CHP which has a high activation e nergy and the catalyzed redox decomposition of CHP which has a low act ivation energy. When temperature decreases, the rate of formation of C HP decreases more than that of the decomposition of CHP. When the auto oxidation is catalyzed by a small amount of soluble copper(II) laurate or copper(II) stearate, the oxidation rate is faster and the selectiv ity to CHP is lower than that catalyzed by Cu(OAc)(2)-BR-0.6 under sim ilar reaction conditions. The carboxylate coordination environment on copper(II) reaction center is not sufficient for Cu(OAc)(2)-BR-0.6 to be an effective catalyst in cumene autooxidation. We propose that the role played by the polymer support is that the backbone of the polymer reduces the rate of the catalyzed redox decomposition of CHP by hinde ring the change of the coordination environments on the copper center during the redox decomposition reaction of CHP. (C) 1998 Elsevier Scie nce B.V. All rights reserved.