(175a) Kinetics, Mechanism, and Structure Requirements for Selective Terminal Oxidation of Linear Alkanes on Mn-Modified Microporous Catalysts

Moden, B., University of California, Berkeley
Zhan, B., University of California, Berkeley
Dakka, J., ExxonMobil Research and Engineering Company
Santiesteban, J. G., ExxonMobil Research and Engineering Company
Iglesia, E., University of California at Berkeley

Selective oxidation of terminal C-H bonds in linear alkanes (RH) to oxygenates [e.g. alcohols (ROH), aldehydes/ketones (R(-H)=O) and acids] is a useful route to intermediates for fine chemicals, polymers and pharmaceuticals, but remains a significant challenge. We report here a systematic study of the oxidation of n-hexane by O2 on Mn-exchanged zeolites (Mn-ZSM-5, Mn-ZSM-57, Mn-ZSM-58 and Mn-MOR) and Mn-containing aluminophosphate materials (MnAPO-5 and MnAPO-18) with different channel structures to explore the effects of spatial constraints on rate and regioselectivity. n-Hexane oxidation proceeds with hexyl hydroperoxides (ROOH) as reactive intermediates on all materials. Formation rates for ROH and R(-H)=O were first order in ROOH concentration, as previously observed for cyclohexane oxidation on MnAPO-5. This suggests that decomposition of ROOH on redox-active Mn sites is the kinetically-relevant step to form Mn-bound reactive intermediates such as ROO, RO, and R, which further react with n-hexane forming primary products and also regenerate ROOH intermediates in heterogeneous chain transfer steps. Pseudo-first-order rate constants for ROOH decomposition to form ROH, R(-H)=O, and acids were 2.5, 1.4, 0.41, 0.31 and 0.38 mol (mol Mn-h)-1 (mM ROOH)-1 on Mn-ZSM-5, Mn-ZSM-57, Mn-MOR, MnAPO-5, and MnAPO-18 catalysts, respectively. In contrast, product formation rates on Mn-ZSM-58 gave a stronger than first order ROOH dependence as in the case of non-catalytic reactions, suggesting that small windows (0.36 nm) in Mn-ZSM-58 restrict entry of reactants. These results suggest that n-hexane oxidation on these microporous materials is strongly influenced by the environment of the active Mn sites. The terminal selectivity of O-attachment during n-hexane oxidation was also influenced by spatial constraints within microporous channels. Zeolites with 10-membered rings gave higher terminal selectivity (24% on Mn-ZSM-5; 14% on Mn-ZSM-57) than the 8- and 12-membered ring structures (8-10%), which showed selectivities similar to those for non-catalytic reactions (8%) and in agreement with linear free energy relationships based on the relative energies of the various C-H bonds in n-hexane. Addition of 10-ring and 12-ring H-form zeolites (e.g. H-ZSM-5, H-ZSM-57, and H-MOR) to scavenge ROOH intermediates and preventing unselective non-catalytic pathways indicates that the main role of confined Mn sites is to decompose ROOH and catalyze heterogeneous chain transfer processes. Density functional theory simulations showed that ZSM-5 channel intersections tend to orient n-hexane molecules so as to favor contacting active Mn-species with terminal C-H bonds over secondary C-H bonds in n-hexane.