Oxide clusters as source of the third oxygen atom for the formation of carbonates in alkaline earth dehydrated zeolites

Theoretical identification of carbonate geometry in zeolites from IR spectra

“…None, one or two Al atoms are present in both 8R and 6R + 4R models as in our previous studies [9][10][11][12][13]. The initial chemical compositions of the cluster models, without carbonate species, with two Al atoms are Me n Al 2 Si 6 O 8 H 16 and Me n Al 2 Si 6 O 9 H 14 for 8R and 6R + 4R, respectively, Me being the alkali (n = 2) or AE (n = 1) cation.…”

“…Some of us already discussed the aspects related to the formation of bi-dentate carbonate groups in alkaline earth zeolites (AEZ) [9][10][11][12][13]. From calculations at the cluster and periodic levels, it was possible to propose a direct way for a barrierless formation of carbonate groups in AEZ via the interaction between CO 2 and stable MeO X Me species (X = 1 or 3 and Me = Mg, Ca, Sr) [9][10][11][12][13].…”

“…From calculations at the cluster and periodic levels, it was possible to propose a direct way for a barrierless formation of carbonate groups in AEZ via the interaction between CO 2 and stable MeO X Me species (X = 1 or 3 and Me = Mg, Ca, Sr) [9][10][11][12][13]. Even if bi-dentate carbonates dominate in the IR spectra for some zeolites like NaA, they seem to possess a lower stability versus symmetric carbonates.…”

Combined influence of alkaline earth metals and CO2 on performance of hierarchical zeolite Beta in n-hexane isomerization

“…In light of the observed mismatch between acidity and conversion Operating parameters Column 1 2 3 4 5 6 7 8 9 10 Temperature, °C 350 350 350 340 350 375 350 350 350 % CO 2 in H 2 0 0 15 15 15 15 15 0 15 15 Performance parameters Hexane conversion, % 53 33 80 33 42 60 67 47 37 51 Isomer selectivity, % 56 80 73 99 98 96 96 93 66 82 Isomer yield, % 30 26 58 33 41 58 64 44 24 42 values observed in the present and reported works may be lack of determination of proportions on alkaline earth metals, their oxides and their carbonates. Furthermore, as mentioned earlier, in the presence of CO 2 , formation of carbonates of these metals is facile [13]. These carbonates impart additional acid sites to the zeolite support with MgCO 3 having the highest acidity.…”

“…In line of the above studies, Sawa et al also reported significant improvement in catalyst life after dealumination of and upon exchanging zeolites mordenite with Ba [12]. In relevance to migration of Ca while regenerating the catalyst, Larin et al mention that carbonates can form without much energetic barrier and oxides of Mg, Ca, Sr, and Ba will convert to corresponding carbonates [13]. Summarily, one concludes that upon exchanging the zeolites with alkaline earth metals, acid sites (number, strength, and type) get altered, mostly to the benefit of the reaction at hand.…”