Abstract
Heat treating Fe/ZSM-5 under hydrogen leads to high dispersion of Fe species and higher alcohol selectivity in the oxidation of alkanes, as compared to oxygen treated catalysts.
Highlights
The direct oxidation of lower alkanes to partially oxygenated products remains a major challenge in catalysis
We reported high intrinsic catalytic activity for ZSM-5 in this system and found that the active site can be attributed to trace impurities of iron, which exist as extra-framework di-iron-m-oxo-hydroxo complexes or oligomeric iron complexes.[24,25,26]
A summary of metal loadings and heat treatment conditions used are given in Table 1 and the preparation methodology is described in fuller detail in the Experimental section of the Electronic supplementary information (ESI).† ICP analysis on samples with nominal iron loadings of 2.5 wt%, 1.1 wt%, 0.4 wt% were found to have actual loadings of 2.4 wt%, 1.02 wt% and 0.41 wt% Fe respectively, which supports the assertion that our method allows the controlled deposition of pre-determined amounts of iron onto H-ZSM-5
Summary
The direct oxidation of lower alkanes to partially oxygenated products remains a major challenge in catalysis. Several strategies have emerged which generally fall into three categories namely: (i) direct high temperature - high pressure activation of alkanes in low yield - high selectivity processes;[1,2,3] (ii) oxidation in strong acidic media using noble metal catalysts to produce alcohol derivatives as esters;[4,5,6,7,8] and (iii) activation under moderate conditions using micro/mesoporous catalysts containing transition metal sites and N2O/O2 which operate without catalytic turnover.[9,10,11,12,13,14,15] To-date whilst many studies have focussed on the direct oxidation of lower alkanes to alcohols and aldehydes, none have proven commercially viable This has created a situation where energy intensive technologies (e.g. steam reforming of methane to syngas as an integral part of methanol production,[16] steam cracking of ethane to ethene followed by hydroxylation to ethanol17) or other technologies (e.g. the BP Cativa Process18) are in global operation. Our recent work has focussed on using ZSM5-based materials for the direct low temperature selective oxidation of methane to oxygenated products using hydrogen peroxide as the oxidant.[24,25,26] We reported high intrinsic catalytic activity for ZSM-5 in this system and found that the active site can be attributed to trace impurities of iron, which exist as extra-framework di-iron-m-oxo-hydroxo complexes or oligomeric iron complexes.[24,25,26] Cu was found to successfully mitigate the oxidation of methanol to formic acid and thereby binary Cu–Fe/ ZSM-5 catalysts are capable of giving very high selectivity to methanol.[24,26]
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