The Low‐Temperature Oxidation of Propane by using H₂O₂ and Fe/ZSM‐5 Catalysts: Insights into the Active Site and Enhancement of Catalytic Turnover Frequencies

Abstract

Fe‐containing ZSM‐5 catalysts are reported to be efficient catalysts for the partial oxidation of propane to oxygenated products at reaction temperatures as low as 50 °C in an aqueous phase reaction when using the green oxidant H2O2. It was previously proposed that extra framework Fe species at the exchange sites of the zeolite are responsible for activation of both the alkane and hydrogen peroxide. Through a systematic study of the influence of framework topology and exchange properties, it is now shown that this high catalytic activity is specific to the MFI‐type Brønsted acidic zeolite ZSM‐5. Furthermore, through a simple aqueous acid washing treatment, leaching of approximately 77 % of iron present within a Fe/ZSM‐5 catalyst only caused the relative propane conversion to decrease by 17 %; implying that most of the initially loaded Fe does not actually contribute to the catalytic activity. This small change in conversion after ‘excess’ Fe removal, amounts to a three‐fold increase in turnover frequency (TOF) (Fe) from 66 h−1 to 232 h−1 compared with the parent Fe/ZSM‐5 catalyst. By comparing these samples, it is shown by NH3 temperature‐programmed desorption, 27Al magic angle spinning NMR spectroscopy, X‐ray photoelectron spectroscopy and TEM analysis that surface iron oxide species are effectively spectators in the oxidation of propane with H2O2. This provides further insight as to the location and true nature of the catalytically active Fe species.

Recent Advances in Iron Catalyzed Oxidation Reactions of Organic Compounds

“…The number of Fe sites on the zeolite had an impact on the selectivity. Fe‐containing zeolites were employed by Hutchings in the partial oxidation of propane utilizing H 2 O 2 as the oxidant to obtain a mixture of oxidation products . Davis showed that Fe atoms dispersed throughout a nitrogen‐containing carbon matrix catalyze the oxidation of benzyl alcohol and 5‐hydroxymethylfurfural by O 2 in the aqueous phase …”

The Role of Copper Speciation in the Low Temperature Oxidative Upgrading of Short Chain Alkanes over Cu/ZSM‐5 Catalysts

“…From am echanistic perspective it is interesting to note that whilst 2.5 %C u/ Silicalite-1 showedc omparable propane conversion to 2.5% Cu/ZSM-5 (23) (c propane = 0.36 %), no propene was observed in the GC-FID (sensitivet o< 1ppm), with selectivity favouring IPA( 38.4 %s electivity) and the rate of H 2 O 2 conversion was an order of magnitude greater.A sw ith ethane, the catalytic oxidation of propane under these reaction conditions wasp reviously shown to be at least partially dependent upon oxygenbased radicals. [32,71] The catalysis and characterisation studies reported herein show strong trends between Cu/ZSM-5 catalysedo xidative upgradingo fC 1 -C 3 alkanes in water using hydrogen peroxide as oxidant. It is clear that Cu speciation plays ak ey role in achieving high primary product selectivity,b ei tt om ethanol, ethene or propene.…”

“…From a mechanistic perspective it is interesting to note that whilst 2.5 % Cu/Silicalite‐1 showed comparable propane conversion to 2.5 % Cu/ZSM‐5 (23) (χ propane=0.36 %), no propene was observed in the GC‐FID (sensitive to <1 ppm), with selectivity favouring IPA (38.4 % selectivity) and the rate of H 2 O 2 conversion was an order of magnitude greater. As with ethane, the catalytic oxidation of propane under these reaction conditions was previously shown to be at least partially dependent upon oxygen‐based radicals …”

Investigating the Influence of Fe Speciation on N2O Decomposition Over Fe–ZSM-5 Catalysts

“…During the deposition of Fe on zeolites it is possible to form various types of Fe species such as framework Fe 3+ (formed during isomorphous substitution), isolated Fe 3+ or Fe 2+ anchored to the zeolite framework by either Si–O–Fe or Al–O–Fe bridges, di-nuclear Fe–O–Fe species either in the framework or in the channels, oligomeric Fe oxo-species, and both small nanoparticles and bulk FeOx particles [26, 59–61]. Determination of the active Fe species for N 2 O decomposition remains a challenge; thus far nano-particulate iron [26, 62] and extra-framework Fe have been suggested to catalyse the decomposition of N 2 O.…”