Promoted Hexagonal Tungsten Bronzes as Selective Catalysts in the Aerobic Transformation of Alcohols: Glycerol and Methanol

Structure–Reactivity Correlations in Vanadium‐Containing Catalysts for One‐Pot Glycerol Oxidehydration to Acrylic Acid

“…However, the diffraction peaks of these two samples are slightly broader than those of hexagonal tungsten/vanadium oxide,W V ( Figure 1, trace c). As previously demonstrated by high-resolutiont ransmission electronm icroscopy (HRTEM) with selected area electron diffraction (SAED), as well as morphological and spectroscopic analyses, [18,28,29] there is remarkable evidencet hat provest hat the majority of vanadium in VO-WO x is incorporated as extra-framework species.O nly if another transition element (e.g.,V ,N b, Mo, Ti)e nters the hexagonal tungsten oxide framework is the HTB structure stable at temperatures significantly higher than those bearable by the materialb uilt up by only Wa toms (WO x ). Ar ole might also be played by the lower heat-treatmentt emperatureu sed for WO x and VO-WO x relative to that used for WV (i.e.,4 50 vs. 600 8C);i ndeed, if heateda tt emperatures > 450 8C, the former two structures evolve into monoclinic forms (i.e.,W O 3 and V-doped WO 3 ).…”

“…[30] However, if an additional amount of vanadium is added to the VCoAPOc atalyst by wet impregnation( sample VO x /VCoAPO;F igure 1, pattern f), extra-framework vanadium speciesa re formed as V 2 O 5 ,w hich are responsible for the additional diffraction peaks observed for the latter sample. [28] Hence, although the majority of vanadium occupies extraframework positions, as previously discussed, it can be inferred that am inor portion of it might replacet he in-framework W atoms, mostl ikely during heat treatment through as olid-solid reaction. [27] Raman spectroscopy was used to improvet he understanding on the structure of the various catalytic systemsp repared, particularly to shed light on the nature of the vanadium speciesi nt he HTB-like materials.…”

“…For both CoAPO and VCoAPO catalysts,n oc hanges are observed in the diffraction patterns, which confirms the insertion of cobalt and vanadium in the oxide framework as tetrahedral units. [14,28] In the VO-WO x sample, this band is very weak but is not completely absent;m oreover,t he main bands located at ñ = 694 and 819 cm À1 are shifted to lower frequencies relative to the main bands of the pristine WO x sample, as in the case of substituted-HTBs. The XRD pattern of the VPP catalyst ( Figure 1, pattern g) shows the presence of the characteristicp hase (VO) 2 P 2 O 7 ;t he lack of additional peaks confirms that no other VÀPÀOp hases constitute the bulk phase of the catalyst.…”

Vanadium Supported on Alumina and/or Zirconia Catalysts for the Selective Transformation of Ethane and Methanol

“…50 to 250 mg of catalyst (diluted with 0.5 g CSi) and a feed consisted of a methanol/oxygen/nitrogen mixture with a molar ratio of 6/13/81 (total flow of 100 mL min −1 ) were used. The analysis of reactants and products was carried out by means of gas-chromatography, using two different chromatographic columns [47]: (i) RTU-bond (30 m, 0.53 i.d. ); and (ii) molecular sieve 5A (3 m length).…”

“…According to their results, modifiers can cover the support acidic surface sites, without modification of the surface vanadia redox product yield. So, the use of methanol aerobic transformation could be of interest to study (in reaction conditions) the characteristics of the surface of catalysts in term of activity, but also selectivity when considering the acid/redox ratio in catalysts, since the presence of acid sites (and the DME formation in these catalysts) seems to be in a parallel behavior [47].…”