Reduction and metathesis activity of MoO3/Al2O3 catalysts II. The activation of MoO3/Al2O3 catalysts

Molecular Structure–Reactivity Relationships for Olefin Metathesis by Al2O3-Supported Surface MoOx Sites

“…The nature of the activated site(s) during propylene metathesis, however, cannot be determined from such indirect measurements, since the surface MoO x oxidation states are dynamic and depend on the metathesis reaction conditions. In spite of the limitations of this earlier study, these results qualitatively agree with the current in situ spectroscopic findings during propylene metathesis that the surface Mo sites on alumina are primarily present as Mo 6+ sites with a minor amount of Mo 4+ sites during propylene metathesis. , …”

“…The structure and oxidation state(s) of activated surface MoO x sites on Al 2 O 3 during olefin metathesis have previously been addressed by a few studies, but the catalysts were activated with H 2 or CO rather than olefins. ,, In two separate studies, Gruenert et al , examined the alumina-supported molybdena catalysts (0.5–13% MoO x /Al 2 O 3 prepared via impregnation of a solution of MoO 3 in NH 4 OH (pH ∼8)) for propylene metathesis activity by initial activation of Al 2 O 3 -supported MoO x with H 2 or thermal treatment in Ar and employed XPS to determine the resulting Mo oxidation states . The resulting H 2 -activated catalysts were subsequently examined for propylene metathesis at 200 °C using a conventional flow reactor.…”

“…In contrast to the detailed molecular information about oxidized, dehydrated supported MoO x /Al 2 O 3 catalysts from in situ characterization studies, little is known concerning the structures of the activated MoO x surface sites during olefin metathesis, although the reactive intermediates for olefin metathesis by homogeneous catalysts are well-known to be metal carbene and metallacyclobutane . In the earliest characterization studies of activated MoO x surface sites on Al 2 O 3 , the MoO x had been exposed to propylene metathesis reaction conditions and subsequently exposed to ambient conditions and examined with ex situ XPS and EPR spectroscopy. , These studies detected both Mo 6+ and Mo 4+ oxidation states, and it was suggested that other Mo oxidation states are not active for olefin metathesis. From ex situ solid-state 27 Al NMR of mesoporous Al 2 O 3 -supported MoO x catalysts, it was proposed that surface MoO x sites anchored to AlO 6 sites are the most active sites.…”

Room-Temperature Metathesis of Ethylene with 2-Butene to Propene Over MoOx-Based Catalysts: Mixed Oxides as Perspective Support Materials

“…Apparently, Brønsted acidic sites in close vicinity to MoO x species are relevant for high propene production at temperatures above 100 °C. The activity can also be enhanced after thermal catalyst pretreatment in inert gas [15], propene, ethylene, methane [16], methanol [13] or after photoreductive treatment in CO or H 2 . [17] The influence of co-catalysts such as CaO, Al 2 O 3 or SiO 2 -Al 2 O 3 on the metathesis of ethylene and 2-butene to propene over MoO x /Al x Si y O z was investigated by Goelden et al [8].…”

Simple and Efficient Way of Molybdenum Oxide-Based Catalyst Activation for Olefins Metathesis by Methane Pretreatment

“…As proposed here, the simple pretreatment procedure of the x MoO 3 /SBA-1 sample with methane gives catalysts with 1 order of magnitude higher catalytic performance, in comparison with the best MoO 3 -based catalysts activated in pure inert gas (550–650 °C). , Thermal activation of MoO 3 /Al 2 O 3 systems in inert gas at extremely high temperatures (>750 °C) brought comparable apparent TOF values, but the reaction temperature was also much higher (200 °C) . Moreover, such high activation temperatures lead to significant loss of activity in the case of catalysts with higher Mo loadings, because of MoO 3 sublimation and γ-Al 2 O 3 degradation.…”