Catalytic oxidation of n-hexane on Mn-exchanged zeolites:: Turnover rates, regioselectivity, and spatial constraints

The effects of channel structure and spatial constraints on n-hexane oxidation rates and regioselectivity were examined on Mn cations within channels of acidic zeolites. Active Mn cations were placed at exchange sites within channels in 8-membered (ZSM-58), 10-membered (ZSM-5 and ZSM-57), and 12-membered ring (MOR) channels by sublimation of MnI2. Synthesis rates for hexanols (ROH), hexanal/hexanones (R(-H)=O), and acids were proportional to hexylhydroperoxide (ROOH) concentrations on all Mn-zeolite catalysts, except Mn-ZSM-58, on which products formed exclusively via noncatalytic autoxidation because of restricted access to Mn cations present within small channels (0.36 nm). Catalytic decomposition of ROOH intermediates occurs on intrachannel Mn cations and is the kinetically relevant step in alkane oxidation. ROOH decomposition rate constants were 2.5,1.4, and 0.41 mol (mol-Mn h)(-1) (mM-ROOH)(-1) on Mn-ZSM-5, Mn-ZSM-57, and Mn-MOR (403 K; 0.4 MPa O-2), respectively. Regioselectivity was influenced by the constrained environment around Mn cations, which increased terminal selectivities above the values predicted from the relative bond energies of methyl and methylene C-H bonds in n-hexane. Mn cations within 10-ring channels gave higher terminal selectivities (Mn-ZSM-5: 24%, k(pri)m/k(sec) = 0.42; Mn-ZSM-57: 14%, k(prim)/k(sec) = 0.22) than those within 8-membered or 12-membered rings (Mn-MOR, Mn-ZSM-57: 8-10%, k(pim)/k(sec) = 0.12-0.14), because of restricted access in ZSM-58 and unconstrained transition states for C-H bond activation in MOR. Terminal selectivities decreased with increasing alkane conversion, because unselective noncatalytic autoxidation pathways prevail as ROOH concentrations concurrently increase. ROOH intermediates can be scavenged from the extracrystalline liquid phase using H-zeolites with accessible protons, which inhibit unselective noncatalytic reactions and maintain higher terminal selectivities as conversion increases, albeit with a concomitant decrease in the rate of oxidation steps also involving ROOH intermediates. (c) 2006 Published by Elsevier Inc.