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 ex
change sites within
channels in 8-membered (ZSM-58), 10-membered (ZSM-5 and ZSM-57), and 12-membered
ring (MOR)
channels by sublimation of MnI
2. 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 whi
ch 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 intra
channel 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, whi
ch increased terminal selectivities above the values predicted from the relative bond energies of methyl and methylene CH
bonds in
n-hexane. Mn cations within 10-
ring channels gave higher terminal selectivities (Mn-ZSM-5: 24 % ,
kprim/ksec=0.42; Mn-ZSM-57: 14 % ,
kprim/ksec=0.22) than those within 8-membered or 12-membered
rings (Mn-MOR, Mn-ZSM-57: 8–10 % ,
kprim/ksec=0.12–0.14), because of restricted access in ZSM-58 and unconstrained transition states for CH 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, whi
ch 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.