Vanadium-magnesium mixed oxides have been prepared by thermal decomposition of decavanadate-intercalated Mg, Al LDHs and by impregnation of MgO or without Al
2O
3 with a decavanadate solution, and ...they have tested for ODH of propane. The differences observed for the catalytic behaviour are mainly related to their acid properties, increased by Al
3+ decreasing the selectivity to propene.
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Vanadium-magnesium mixed oxides have been prepared by thermal decomposition of decavanadate-intercalated Mg, Al layered double hydroxides (LDHs), MgAlVO-LDH, and by impregnation of MgO (mixed, MgAlVO-I, or not, MgVO-I, with Al
2O
3) with an aqueous decavanadate solution. The activity of catalyst MgAlVO-LDH in oxidative dehydrogenation of propane is higher than that measured for the mixed oxides obtained by conventional impregnation. However, the largest normalised conversion measured over sample MgAlVO-I is assigned to the higher surface density of V centers, which are stabilised along the reaction runs. Normalised selectivity and conversion profiles show that propene is first formed and then oxidized to CO on catalyst MgAlVO-I, a process favoured by the high acidity of this solid. However, catalyst MgVO-I, with a medium strength surface acidity, is rapidly deactivated, forming CO
2, by coke deposition on exposed V sites. The low selectivity to propene of catalyst MgAlVO-LDH is related to its low surface density of V sites.
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▶ Co-feeding of H
2, CO or CO
2 (reaction products) affects catalytic performances. ▶ The observed results are not explained by thermodynamics but by kinetic changes. ▶ The ...modifications on catalytic performances induced by gas co-feeds are reversible. ▶ An optimal oxidation state of Rh could explain the high selectivity of syngas. ▶ H
2/CO ratio can be modulated by adding proper amounts of co-feeds during POM.
The influence of the addition of 1, 2 or 5
vol.% of CO, H
2 or CO
2 to the feed during the partial oxidation of methane (POM) was studied over Rh/Ti-modified support catalysts (Rh/Ti–SiO
2, Rh/Ti–Al
2O
3 and Rh/Ti–MgO). The changes observed in the conversion and syngas selectivity in the presence of gaseous co-feeds are due to changes in the kinetics of POM and may be mainly explained by modifications in the oxidation state of rhodium during the reaction. A higher reduction of Rh is observed when 5% of H
2 or CO is co-fed while Rh is maintained in a higher oxidation state in the presence of CO
2 co-feed. Reversibility tests show that the modifications induced by the gaseous promoters in the catalytic performances are reversible. Such changes can alter not only the kinetics of POM but also the kinetics of the other reactions involved (dry reforming and the reverse water-gas shift) during POM reaction. Results have implications in the expression of the reaction kinetics to be used in the modelling of POM reaction mechanism.
Bismuth molybdenum titanium oxides were prepared in a highly dispersed state by the sol–gel method and in a largely crystallized state by coprecipitation and impregnation. The catalysts contained 14 ...or 25
wt.% of bismuth molybdate. Their catalytic performance in propene oxidation to acrolein was studied. The increase in activity of Bi-Mo-Ti oxides in propene oxidation as compared to that of bulk bismuth molybdate can be tentatively related, in the case of the sol–gel samples, to the stabilization of small aggregates of Bi- and Mo-containing phases due to the beneficial presence of the titania matrix. The catalytic performance of the samples prepared by coprecipitation or impregnation is enhanced once a crystal size of bismuth molybdate close to 23
nm is reached, i.e., the crystal size should not be too small (samples prepared by the sol–gel method) nor too large (samples prepared by impregnation and coprecipitation), but an optimum size should exist to enhance the catalytic performance.
The performance of Pd supported on alumina or titania, prepared by impregnation or sol–gel, for catalytic combustion of methane is studied. The addition of the Pd precursor after gelification of ...alumina has a beneficial effect on the catalytic properties of the catalysts. The better activity of the alumina catalysts is related to the high dispersion of the palladium particles, the low crystallite size and the high specific surface area. The catalytic activity of titania-based catalysts was only slightly affected by the preparation procedure. The addition of CO
2 during the oxidation of methane promotes oxidation for the titania-based catalysts.
The surface acid properties of WO x supported on different oxides (alumina, silica, magnesia, titania, zirconia, and niobia), as well as the decomposition of butan-2-ol on these systems, have been ...studied by FT-IR spectroscopy. Incorporation of supported tungsten-containing species significantly increases the surface acid properties, and new surface acid sites develop, except for in the magnesia-supported system. The increase in surface acidity is responsible for the high selectivity to butene formation on these systems, except for in the magnesia-supported system, where a larger selectivity to methyl ethyl ketone is observed.
Synergetic effects in the oxidative dehydrogenation of propane have been studied over magnesium vanadate catalysts containing three different Mg/V ratios: 1/2, 2/2, and 3/2, denoted as MgV(1/2), ...MgV(2/2), and MgV(3/2). Four types of catalysts were analysed: (a) pure magnesium vanadate oxides, (b) mechanical mixtures of the pure magnesium vanadate oxides, (c) mechanical mixtures of the magnesium vanadate oxides with α-Sb2O4, and (d) impregnated MgV(2/2) and MgV(3/2) with Sb ions. Synergetic effects are observed in MgV(3/2) and in MgV(2/2) oxides when they are in presence of α-Sb2O4. In the mixtures of MgV(3/2) with α-Sb2O4, the principal effect is an increase in the selectivity with a corresponding decrease in propane conversion, whereas in the mixtures of MgV(2/2) with α-Sb2O4, there is a strong increase in propane conversion with a moderate increase in propene yield. Concerning the mixtures of MgV(3/2) and MgV(2/2), synergetic effects in the conversion, in the yield, and in the selectivity are observed. However, no synergetic effects in selectivity or conversion are exhibited by MgV(2/2) and MgV(3/2) when they are mixed with MgV(1/2). Highly dispersed (SbxOy), formed on impregnated MgV(2/2) and MgV(3/2), sinters and detaches from MgV(2/2) and MgV(3/2) surfaces. No formation of a new phase or contamination in the presence of α-Sb2O4takes place when MgV(3/2) is used. Thus, these catalysts contain two separate phases in contact. In MgV(2/2) + α-Sb2O4mechanical mixtures a new phase, MgSb2O6, (formed during the test) is also present in small quantities. The mixtures of MgV(3/2) with MgV(2/2) reveal neither formation of a new phase nor contamination. The synergetic effects in the selectivity exhibited by MgV(3/2) mixed with MgV(2/2) and with α-Sb2O4is explained by a remote control mechanism. MgSb2O6, formed in MgV(2/2)-containing catalysts, is responsible for the increase in complete oxidation and the strong decrease in selectivity.
MgVO oxide with
Mg
V
atomic ratios of
2
2
and
3
2
, were mixed mechanically with α-Sb
2O
4. Chemical changes during oxidative dehydrogenation of propane (ODP) and under calcination treatment were ...studied. In
Mg
V(
2
2
)
+ α-Sb
2O
4
, magnesium vanadate is contaminated during ODP and a new phase, MgSb
2O
6, was formed after only 1 day of calcination at 823 K. In
Mg
V(
3
2
)
+ α-Sb
2O
4
, no indication of surface contamination or new phase formation was detected during ODP or calcination. The catalytic performances of both mixtures are explained in the light of these results.
