Surface reactivity and morphology of vanadia-titania catalysts

Martı́n, Cristina; Rives, V.; Sánchez-Escribano, V.; Busca, Guido; Lorenzelli, V.; Ramis, Gianguido

doi:10.1016/0039-6028(91)91106-8

Cited by 18 publications

(9 citation statements)

References 16 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Notably, characteristic C–H stretching bands of olefinic methylene species , at ∼3010 and at ∼3090 cm –1 as well as CC stretching mode at 1610–1650 cm –1 are not detected, indicating the absence of CC bonds in strongly adsorbed species that persist evacuation at 323 K. Thus, strong adsorption involves a reaction, which leads to conversion of the olefinic CC bond in propene. The detected peak patterns agree well with the bands of isopropoxy species adsorbed on the surface of metal oxides. − In fact, adsorption of isopropanol on MoO x /SBA-15 results in a quite similar spectrum (Figure S4 (1) of the SI), confirming that isopropoxide is the major species formed by adsorption of propene at 323 K on MoO x /SBA-15. The transformation of propene into isopropoxide happens via protonation of the olefin by acidic surface OH groups.…”

Section: Resultssupporting

confidence: 72%

“…Moreover, new bands at 1668 and 1417 cm −1 are clearly visible, which can be assigned to the C O stretching mode of the carbonyl group and the deformation mode of methyl groups, respectively, of coordinated acetone. 38,40,42,43 Occurrence of these bands upon acetone adsorption (Figure S4 (3) of the SI) as well as the development of a similar carbonyl stretching band at 1673 cm −1 upon prolonged (18 h) isopropanol adsorption (Figure S4 (2) of the SI) corroborate the formation of acetone by oxidation of isopropoxide surface species. The modest intensity of the carbonyl band at 1668 cm −1 (blue line in Figure 5A) compared to the spectrum of acetone adsorption (Figure S4 (3) of the SI) suggests that only a fraction of the isopropoxide species undergo oxidation into acetone.…”

Section: Resultsmentioning

confidence: 62%

See 1 more Smart Citation

In Situ Generation of Active Sites in Olefin Metathesis

et al. 2012

View full text Add to dashboard Cite

The depth of our understanding in catalysis is governed by the information we have about the number of active sites and their molecular structure. The nature of an active center on the surface of a working heterogeneous catalyst is, however, extremely difficult to identify and precise quantification of active species is generally missing. In metathesis of propene over dispersed molybdenum oxide supported on silica, only 1.5% of all Mo atoms in the catalyst are captured to form the active centers. Here we combine infrared spectroscopy in operando with microcalorimetry and reactivity studies using isotopic labeling to monitor catalyst formation. We show that the active Mo(VI)-alkylidene moieties are generated in situ by surface reaction of grafted molybdenum oxide precursor species with the substrate molecule itself gaining insight into the pathways limiting the number of active centers on the surface of a heterogeneous catalyst. The active site formation involves sequential steps requiring multiple catalyst functions: protonation of propene to surface Mo(VI)-isopropoxide species driven by surface Brønsted acid sites, subsequent oxidation of isopropoxide to acetone in the adsorbed state owing to the red-ox capability of molybdenum leaving naked Mo(IV) sites after desorption of acetone, and oxidative addition of another propene molecule yielding finally the active Mo(VI)-alkylidene species. This view is quite different from the one-step mechanism, which has been accepted in the community for three decades, however, fully consistent with the empirically recognized importance of acidity, reducibility, and strict dehydration of the catalyst. The knowledge acquired in the present work has been successfully implemented for catalyst improvement. Simple heat treatment after the initial propene adsorption doubled the catalytic activity by accelerating the oxidation and desorption-capturing steps, demonstrating the merit of knowledge-based strategies in heterogeneous catalysis. Molecular structure of active Mo(VI)-alkylidene sites derived from surface molybdena is discussed in the context of similarity to the highly active Schrock-type homogeneous catalysts.

show abstract

Section: Resultssupporting

confidence: 72%

Section: Resultsmentioning

confidence: 62%

In Situ Generation of Active Sites in Olefin Metathesis

et al. 2012

View full text Add to dashboard Cite

show abstract

“…Alternatively, the bands at 3686 and 3647 cm –1 can be assigned to W–OH, as reported by Reiche et al in their studies of vanadia over rutilized titania . The weak bands between 2200 and 1700 cm –1 are due to overtones and combination bands . Upon ammonia introduction in the flow, we observe a perturbation of the band assigned to Ti–OH at 3720 cm –1 shifting toward lower frequencies, probably because of ammonia adsorption on a neighboring site.…”

Section: Resultssupporting

confidence: 82%

“…40 The weak bands between 2200 and 1700 cm −1 are due to overtones and combination bands. 41 Upon ammonia introduction in the flow, we observe a perturbation of the band assigned to Ti−OH at 3720 cm −1 shifting toward lower frequencies, probably because of ammonia adsorption on a neighboring site. In addition, the band near 3670 cm −1 undertakes a progressive decrease while a sharp band grows at ∼3680 cm −1 , rapidly reaching a steady state, accompanied by another peak at higher wavenumbers (∼3690 cm −1 ), which continue to increase.…”

Section: Resultsmentioning

confidence: 77%

Does Pelletizing Catalysts Influence the Efficiency Number of Activity Measurements? Spectrochemical Engineering Considerations for an Accurate Operando Study

et al. 2012

View full text Add to dashboard Cite

Porosity is a factor affecting catalyst efficiency in pelletized form. This implies that care should be taken with uncritically relating activity measurements from transmission operando FTIR to final catalyst performance. If the pelletizing pressure is excessive, a destruction of the pore structure of, for example, support oxides might take place, which in turn affects the pore size distribution and the porosity of the catalyst, leading to the observation of lower activity values due to decreased catalyst efficiency. This phenomenon can also apply to conventional activity measurements, in the cases that pelletizing and recrushing of samples are performed to obtain adequate particle size fractions for the catalytic bed. A case study of an operando investigation of a V2O5-WO3/TiO2-sepiolite catalyst is used as an example, and simple calculations of the influence of catalyst activity and internal pore diffusion properties are considered in this paper for the evaluation of catalyst performance in, for example, operando reactors. Thus, it is demonstrated that with a pelletizing pressure of <1–2 ton/cm2, the pore structure is only negligibly altered, and small deteriorations of estimated catalyst efficiencies are observed for first-order kinetic constants lower than 100 mL/gs. However, if the operando study deals with highly active catalysts, it is necessary to consider efficiency losses. A simple procedure for evaluating efficiencies based on pellet dimensions and solid phase characteristics is proposed. The Thiele modulus is directly proportional to the thickness of the pellet, and, thus, inversely related to the catalyst efficiency. As a rule of thumb, we found that for catalytic constants below 100 mL/gs, the maximum thickness of the pellet pressed at 2 tons/cm2 has to be as low as 80 μm to exhibit catalyst efficiencies above 90%. For catalysts with k′ = 10 mL/gs, the value is 260 μm. This strongly underlines the importance of taking internal diffusion limitations into account when working with highly active catalysts.

show abstract

“…Solid-state nuclear magnetic resonance (NMR) was performed on Bruker Advance III 600 MHz WB spectrometer. 27 Al MAS NMR spectra were recorded at a resonance frequency of 156.4 MHz with the recycle delay time was set to 1 s, and the number of scans was set at 2048 and spinning rate of 12 kHz. An aluminum nitrate solution (1 mol/L) was employed as an internal standard.…”

Section: Catalyst Characterizationmentioning

confidence: 99%

Enhancement of Propylene Metathesis Performance of MoO₃/Al₂O₃ with Promotion of Phosphoric Acid

Qin,

Yang,

Yin

et al. 2023

ChemCatChem

View full text Add to dashboard Cite

Catalytic olefin metathesis represents a vital reaction in the chemical industry, as it provides a feasible route for the interconversion of olefins. However, traditional supported catalysts usually suffer from low activity due to sluggish kinetics for the formation of catalytically active metal‐carbene species. This work tackles this issue by taking phosphoric acid as a promoter and constructing a Brønsted acidic site to cooperate with the MoOx sites, thus facilitating the formation of Mo‐carbene. It turns out that the turnover frequency of propene metathesis over the Mo−P/Al2O3 catalyst is 7.1 times higher than that over the unmodified Mo/Al2O3 catalyst. The combined characterization reveals the promotion of phosphorus results from the alignment of olefin toward adjacent MoOx and thus facilitates formation of active Mo‐carbene species.

show abstract

Surface reactivity and morphology of vanadia-titania catalysts

Cited by 18 publications

References 16 publications

In Situ Generation of Active Sites in Olefin Metathesis

In Situ Generation of Active Sites in Olefin Metathesis

Does Pelletizing Catalysts Influence the Efficiency Number of Activity Measurements? Spectrochemical Engineering Considerations for an Accurate Operando Study

Enhancement of Propylene Metathesis Performance of MoO₃/Al₂O₃ with Promotion of Phosphoric Acid

Contact Info

Product

Resources

About

Surface reactivity and morphology of vanadia-titania catalysts

Cited by 18 publications

References 16 publications

In Situ Generation of Active Sites in Olefin Metathesis

In Situ Generation of Active Sites in Olefin Metathesis

Does Pelletizing Catalysts Influence the Efficiency Number of Activity Measurements? Spectrochemical Engineering Considerations for an Accurate Operando Study

Enhancement of Propylene Metathesis Performance of MoO3/Al2O3 with Promotion of Phosphoric Acid

Contact Info

Product

Resources

About

Enhancement of Propylene Metathesis Performance of MoO₃/Al₂O₃ with Promotion of Phosphoric Acid