Thermal treatments of precursors of molybdenum and vanadium oxides and the formed Mo x V y O z phases active in the oxydehydration of glycerol

Abstract

This paper presents an in situ study of the crystallographic phases formed during the thermal treatment of precursors of vanadium and molybdenum oxides, measured under synchrotron X-ray diffraction. The interest in the speciation of Mo x V y O z mixed oxides lies in the excellent catalytic performance of these materials for the selective conversion of glycerol to acrylic acid employing the oxydehydration reaction. The crystallographic structure of the active phases of Mo x V y O z directly influences on the nearby metal valence and, therefore, on the dynamic changes in metal oxidation states during the catalytic reaction. In the present study, the thermal treatment of a mixture of the precursors of Mo and V under oxidizing or inert atmospheres revealed the major formation of 61% of MoV 2 O 8 or 29% of Mo 4 V 6 O 25 , respectively, at a final temperature of 500 • C. The most active phase for acrylic acid formation was MoV 2 O 8 (3.5 times more active than the separate metal oxides), due to the instability of the phase with respect to framework oxygen at the reaction temperature. The cycle of reduction and oxidation of the vanadium in MoV 2 O 8 during the reaction caused pronounced dynamic formation of oxygen vacancies, resulting in 97% conversion of glycerol and 32% selectivity towards acrylic acid.

Surfactant-assisted synthesis of Mo–V mixed oxide catalysts for upgraded one-step conversion of glycerol to acrylic acid

“…8 Finally, these reduced forms of vanadium could be present in small quantities of amorphous phases, not detectable by XRD but, previously reported in the synthesis of vanadiummolybdenum based mixed oxides. 6,8,36 Another important fact noted is that the catalysts synthetized with surfactants showed higher quantities of O-Mo bonds compared to the reference sample, what can be associated with the higher quantities of MoO 3 crystalline phase formed in these samples. The quantity of surface O-H bonds decreased in CTAB-0.10 and SDS-0.10 samples, inuencing the total acidity of these catalysts (Table 1).…”

“…CTAB-0.10 and SDS-0.10 catalysts showed decreases of 2.3% (from 11.7 to 9.4%) and 0.2% (from 10. phase are being re-established as shown in eqn (8), and this cycle generates the equilibrium between MoV 2 O 8 /MoVO 5 phases observed in CTAB-0.10 and SDS-0.10 spent catalysts. The decrease in acrylic acid selectivity observed for reference sample, between the h and sixth hour of reaction could be associated with the migration of vanadium from MoV 2 O 8 crystalline phase to amorphous phase, but the presence of Mo 4.65 V 0.35 O 14 phase in the catalysts is contributing to the oxidation of acrolein during reaction, since it was already reported as active in the glycerol oxydehydration, 38 and is, probably, maintaining some activity in the reference catalyst.…”

“…Both mixtures were maintained under stirring for 10 min at 25 C and aer the complete dissolution, they were mixed and stirred for another 10 min at 25 C, resulting in a solution with a pH of 2.8 and a Mo/(Mo + V) mole ratio of 0.60. 6,8 The mole proportion of the reagents was 1 VOSO 4 : 0.2 (NH 4 ) 6 Mo 7 O 24 $4H 2 O : 300 H 2 O. Aerwards, sodium dodecyl sulfate (SDS) or cetyltrimethylammonium bromide (CTAB) were added to the solution and maintained under stirring for 40 min at 25 C. The mole ratio surfactant/(Mo + V) was 0.10. Later, the solution was transferred to a 50 mL Teon vessel contained in a stainless steel autoclave reactor, occupying approximately 70% of the vessel capacity.…”

“…Due to the increase in biodiesel production and, consequently, the coproduced glycerol, the search for new uses for it becomes indispensable. 1,2 Many studies have been reported for glycerol valorisation in the production of different substances using various processes like dehydration, [3][4][5] oxidative dehydration, [6][7][8] acetalization, 9 acetylation 10 and hydrogenolysis. 11 A high valueadded product that can be obtained from glycerol oxidative dehydration and that has attracted attention because of the increase in its market demand is acrylic acid.…”

“…15 Mixed oxides based on molybdenum and vanadium, Mo x V y O z , are prominent catalysts to perform the one-step conversion of glycerol due to the moderate acidity and well-known oxidative potential of vanadium in these structures, conducted by the Mars-van-Krevelen mechanism. 8,16 However, the main disadvantages of these catalysts are the textural properties, like low specic areas and porosity, 8,[17][18][19] and the instability of the mixed oxide phases in the reaction conditions. 20 The solid-state reaction and/or spreading of mixed oxide phases when applied in catalytic systems under high temperatures and oxidant atmospheres can lead to a restructuration of phases 20 to form single component oxides and/or migration of active species to amorphous phase.…”