DFT insights into the oxygen-assisted selective oxidation of benzyl alcohol on manganese dioxide catalysts

Benzyl alcohol to benzaldehyde oxidation on MnOx clusters: Unraveling atomistic features

“…Previous mechanistic results reported by Gueci et al [32] are summarized in the catalytic cycle of Figure 1. This, in particular, shows the catalytic benzaldehyde oxidation mechanism already proposed after having examined all the possible ways of interaction of the considered molecular substrates with all the plausible adsorption sites present in the different catalytic fragments (Mn 4 O 7 , Mn 4 O 8 , Mn 4 O 9 ) involved [32].…”

“…Previous mechanistic results reported by Gueci et al [32] are summarized in the catalytic cycle of Figure 1. This, in particular, shows the catalytic benzaldehyde oxidation mechanism already proposed after having examined all the possible ways of interaction of the considered molecular substrates with all the plausible adsorption sites present in the different catalytic fragments (Mn 4 O 7 , Mn 4 O 8 , Mn 4 O 9 ) involved [32]. The catalytic cycle presents a starting Mn 4 O 8 catalytic model cluster -representative of the Mn(IV) sites characterizing the cluster-sized MnO x fragments present on the surface of real catalysts [6] -that, undergoing transient structural modifications, rules benzyl alcohol oxidation to benzaldehyde, following a complex reaction mechanism.…”

“…However, activity loss and the need of regeneration-rejuvenation procedures are generally reported also for such catalysts [6,[26][27][28][29][30][31]. The latter even shows deactivation phenomena related to the over-oxidation of benzyl alcohol to benzoic acid [32] but the intimate aspects concerning both the formation of benzoic acid and its role in poisoning the catalyst still deserve investigation. Recent synergistic approaches involving experimental investigations and quantum chemistry calculations, mostly based on Density Functional Theory (DFT), showed to be important in highlighting the fundamental aspects of catalytic reactions enabling the development of effective catalysts and processes [33].…”

“…In fact, the multifaceted panorama of the cluster structures and properties was investigated to design new catalysts whose characteristics could be tuned either changing the elemental nature, sizes and shapes of the clusters or dispersing them on supports having peculiarly addressed features [40][41][42][43][44][45][46]. This paper presents a systematic computational study, in the frame of the DFT paradigm, aimed at shedding light on the catalytic mechanisms, even including catalyst deactivation phenomena, which occur in the selective aerobic oxidation of benzyl alcohol to benzaldehyde on MnO x cluster-sized catalysts [32,47]. In particular, the role of possible over-oxidizing species, which could potentially lead to the formation of benzoic acid, is investigated.…”

“…Namely, the role of the oxygen species produced on (or displaced from the bulk to) the catalyst surface, of the O 2 (coming from the gas phase) and of the water molecules (formed during the reaction) was taken into consideration and analyzed. This was done in the hypothesis that benzoic acid or other oxidized intermediates could play a role in the deactivation of the catalyst [32,47]. The identification of these surface intermediates and especially of the paths determining their genesis could indeed suggest some corrections to be made into the process in order to improve the selectivity to benzaldehyde.…”