Oxydehydrogenation of Ethane and Propane over Alkaline Metal Modified Fe-ZSM-5 Zeolites

Abstract

Iron modified zeolites of MFI structure, calcined at 873 and 1173 K, were doped with sodium cations in amounts corresponding to , , and to the total number of acidic centres, estimated from TPD of ammonia. Sodium modified Fe-ZSM-5(900) gradually lost their oxidative activity with an increase in the concentration of alkaline cations, while the activity of Na/Fe-ZSM-5(600) changed slightly and diminished considerably only after significant removal of acidic centres. The results of propane ODH reaction, performed at different contact times over alkaline free and alkaline modified Fe-ZSM-5 indicated, that oxydehydrogenation and cracking reactions on the samples calcined at 1173 K occurred as consecutive reactions, while in the presence of catalysts activated at 873 K, and characterised by strong acidic centres, both reactions occurred simultaneously.

Oxidative dehydrogenation of propane with N2O over Fe-ZSM-5 and Fe–SiO2: Influence of the iron species and acid sites

“…For the LT treatment, irrespective of the Si/Al ratio, three desorption peaks were observed for all the parent zeolites with their intensity decreasing with increasing Si/Al ratio. After the HT treatment a strong decrease of the peak at ~400 o C was observed for all zeolites which is consistent with a loss of Brønsted acid sites [10]. It is worth noting that the loss of acid sites has been thought to be due to partial dealumination [45]; however, from the XRD (not shown) and BET (Table 2) data little change following HT treatment is observed and, therefore, this is unlikely to be the cause in the present case.…”

“…As stated in the introduction, this can be explained by the fact that only medium and weak acidic sites are responsible for the C-H bond activation, involved in oxidative dehydrogenation pathway [18,19]. This is supported by the work of Held et al who reported that the reduction of acid sites by means of high temperature treatment and sodium introduction, results in a clear decrease in selectivity to cracking products and CO 2 , while selectivity towards propene increased [10].…”

“…The [ 48,50,[52][53][54], the creation of hydroxyl nests by dealumination [12,14], the formation of mono and bi/oligonuclear iron [24], the migration of iron ions located on the outer surface into the pore system [10] and the conversion of Fe 3+ to Fe 2+ [55,56].…”

“…Introduction ZSM-5 zeolites with low iron contents have shown remarkable activity and selectivity in the oxidative dehydrogenation (ODH) of propane with N 2 O [1][2][3][4][5][6][7][8][9][10] as well as in the hydroxylation of benzene [11][12][13][14][15] and other hydrocarbons [11,16,17]. The activity and selectivity in ODH of propane (ODHP) can be related to the structure of the ZSM- as the oxidant over Fe-ZSM-5 prepared by liquid phase ion exchange [9,10,17]. A decrease in Brønsted acid sites via high temperature treatment, sodium doping and hydrothermal treatment resulted in an increase in the propene selectivity, a slight change in the distribution of the cracking products as well as a decrease in the alkane conversion.…”

“…The pre-treatment conditions have also been reported to have a significant effect on the activity and selectivity for the ODH of propane with N 2 O. These conditions have included high temperature calcination and/or steaming [3,4,9,10,[21][22][23]30]. The high temperature treatment and steaming have been reported to increase the amount of extra-framework iron species in Fe-ZSM-5 prepared by hydrothermal method.…”