Catalytic performance of vanadyl pyrophosphate in the partial oxidation of toluene to benzaldehyde

Vanadyl pyrophosphate catalysts were generated by dehydrating VOHPO₄·(1/2)H₂O at different temperatures and duration of the dehydration procedure. The as-synthesised materials were characterised by X-ray diffractometry, FTIR spectroscopy and temperature-programmed reduction. The prolongation of the formation period at higher temperatures led to improved crystallinity and lower BET surface areas of the vanadyl pyrophosphate specimens and, additionally, a significantly impeded reducibility of the vanadyl sites was observed. The catalytic performance of the samples was tested in the partial oxidation of toluene to benzaldehyde. The obtained results revealed an increasing benzaldehyde selectivity with improved catalyst crystallinity. In situ FTIR and ESR spectroscopy were used to throw more light on the interaction of the toluene–air feed with the surface of the catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evolution of the active surface of the vanadyl pyrophosphate catalysts

Bulk crystallinity of vanadyl(IV) pyrophosphate catalysts forn-butane partial oxidation increased up to 23 days on stream as determined by XRD and Raman spectroscopy, while selectivity reached steady state after 8–10 days. Electron microscopy detected a 15 Å amorphous layer terminating the (200) planes of (VO)₂P₂O₇ in fresh catalysts that was not observed in the equilibrated catalysts. It is suggested that ordering of (200) planes at the surface of (VO)₂P₂O₇ is responsible for selective oxidation.     

Role of surface properties of the vanadyl pyrophosphate in the formation of maleic and phthalic anhydrides by n-pentane oxidation

The mechanism of synthesis of maleic and phthalic anhydrides by the gas phase oxidation of n-pentane over VPO-based catalysts was studied. The oxidation of the paraffin was carried out under conditions which may lead to the isolation of possible reaction intermediates: very low residence time and high n-pentane/oxygen ratio. Under these conditions a number of by-products were identified, which were attributed to the oxidation of intermediate pentene and pentadiene, as well as other products with more than 5 C atoms, among which were alkylaromatics. Alkylaromatics also were found to be formed in the anaerobic transformation of pentadiene. A mechanism is proposed, which takes into account the oxidehydrogenation of the paraffin to pentene and pentadiene, followed by two parallel pathways: (i) dimerization, dehydrocyclization and aromatization of the diolefin, followed by oxidation of the alkylaromatic to phthalic anhydride, and (ii) multi-step oxidation of pentadiene to maleic anhydride.     

Co-Mo催化剂上甲苯气相选择氧化制苯甲醛的研究

制备一系列的负载型Co-Mo-O/Al2O3-USY催化剂,并用于甲苯的气相选择氧化制苯甲醛。结果表明:在空速6000h-1,空气/甲苯摩尔比100,MoO3负载量为10.00%(质量分数),在480℃反应,得到甲苯转化率43.76%,苯甲醛选择性25.97%,苯甲醛收率为11.37%的结果。并用X射线衍射、X射线光电子能谱等测试手段,对催化剂进行结构表征。     

The dilution of vanadyl pyrophosphate, catalyst for n-butane oxidation to maleic anhydride, with aluminum phosphate: unexpected reactivity due to the contribution of the diluting agent

This paper describes the preparation, characterization and reactivity in n-butane oxidation of catalysts made of vanadyl pyrophosphate (VPP) diluted in aluminum phosphate. Catalysts were prepared by synthesizing at the same time the VPP precursor and AlPO₄, in order to develop mixed systems, suitable for the conglomeration into fluidizable particles by spray-drying. Al phosphate was chosen due to its presumed chemical inertness towards the active phase and towards n-butane. No evidence was obtained for the formation of mixed V/Al phosphates; an homogeneous reciprocal dispersion of the two compounds developed. However, Al phosphate showed an unexpected activity in n-butane oxidation; specifically, in the temperature range comprised between 340 and 400 °C, the hydrocarbon was converted to formaldehyde and carbon monoxide by radical-like reactions. The activity of Al phosphate became negligible above 400 °C, in the temperature range where instead VPP was active in hydrocarbon oxidation and selective to maleic anhydride. Moreover, Al phosphate was also responsible for the consecutive oxidative degradation of maleic anhydride, at high temperature.     

Anisotropic oxidation of crystallites of vanadyl pyrophosphate

Anisotropic oxidation of crystallites of vanadyl pyrophosphate ((VO)₂P₂O₇) has been demonstrated by Raman spectroscopy with samples having different microstructures. Oxidation of these samples by O₂ produced X₁ phase,- and-VOPO₄ phases. The relative peak intensity of the X₁ phase to the other phases correlated well with the ratio of the (100) plane to the side planes (surface area-basis). This correlation showed that the (100) plane was oxidized to X₁ phase and the side planes to- and-VOPO₄. For example, thin plate-like (VO)₂P₂O₇, of which the (100) fraction is 98%, was oxidized almost exclusively to X₁ phase. But when it was fractured into small plates to increase the side planes and then oxidized,- and-VOPO₄ were detected in addition to the X₁ phase. These results are consistent with our previous conclusion that the (100) plane of (VO)₂P₂O₇ is selective, but side planes are non-selective for catalytic oxidation ofn-butane.     

Spatially resolved infrared spectroscopy: a novel technique for in situ study of spatial surface coverage during CO oxidation on supported catalysts

A spatially resolved infrared (IR) imaging technique to monitor the linear adsorbed CO coverage on supported catalyst surface combining an IR bandpass filter and an IR thermography camera has been developed. Images acquired during the CO adsorption/desorption and ignition indicate that the technique provides an excellent method to image the change of surface coverage with a spatial resolution. It is expected that the combination of infrared thermography with spatially resolved imaging of surface coverage will provide a deeper insight in the dynamics of spatio-temporal patterns on heterogeneous catalysts.     

Selective oxidation of methane to formaldehyde on supported molybdate catalysts

A series of silica-supported molybdena catalysts with variable molybdenum content has been prepared and tested in the selective oxidation of methane to formaldehyde. The nature of the supported oxide phases has been studied by Fourier transform IR of chemisorbed NO molecule, X-ray photoelectron spectroscopy, UV-Visible reflectance and X-ray diffraction techniques. The results show that molybdenum oxide is highly dispersed on the silica at low molybdenum concentrations where two-dimensional polymolybdates are developed. Three-dimensional MoO₃ crystals grow in the region of high molybdenum concentrations. The analysis of the combined data suggests that there is a close relationship between methane conversion and formaldehyde selectivity and the presence of highly dispersed polymolybdate structures on the silica surface.     

Deactivation kinetics of V/Ti-oxide in toluene partial oxidation

Deactivation kinetics of a V/Ti-oxide catalyst was studied in partial oxidation of toluene to benzaldehyde (BA) and benzoic acid (BAc) at 523–573 K. The catalyst consisted of 0.37 monolayer of VO_x species and after oxidative pre-treatment contained isolated monomeric and polymeric metavanadate-like vanadia species under dehydrated conditions as was shown by FT Raman spectroscopy. Under the reaction conditions via in situ DRIFTS fast formation of adsorbed carboxylate and benzoate species was observed accompanied by disappearance of the band of the monomeric species (2038 cm⁻¹) (polymeric species were not controlled). Slow accumulation of maleic anhydride, coupling products and/or BAc on the surface caused deactivation of the catalyst during the reaction. Temperature-programmed oxidation (TPO) after the reaction showed formation of high amounts of CO, CO₂ and water. Rate constants for the steps of the toluene oxidation were derived via mathematical modelling of reaction kinetics at low conversion and constant oxygen/toluene ratio of 20:1. The model allows predicting deactivation dynamics, steady-state rates and selectivity. The highest rate constant was found for the transformation of BA into BAc explaining a low BA yield in the reaction.     

Benign oxidants and single-site solid catalysts for the solvent-free selective oxidation of toluene

Two types of single-site heterogeneous catalysts have been designed so as to facilitate either the side-chain oxidation or ring-hydroxylation of toluene in O₂ (solvent-free) or by employing aqueous (H₂O₂) or organic (cumene hydroperoxide) hydroperoxides in high yield. The use of H₂O₂ and cumene hydroperoxide in particular, facilitates the ring-hydroxylation of toluene when zeolite-encapsulated metal complexes, such as perhalogenated or tetra-nitro-substituted phthalocyanines, are used as catalysts. Nanoporous, redox molecular sieves, display a higher tendency for the side-chain oxidation of toluene with air as an oxidant, with benzoic acid as the predominant product.     

Relationship between the formation of surface species and catalyst deactivation during the gas-phase photocatalytic oxidation of toluene

Various partial oxidation products were identified on the surface of TiO₂ and an 8% SiO₂–TiO₂ binary catalyst used for the photocatalytic oxidation of gas-phase toluene. Using in situ FTIR spectroscopy, benzaldehyde and benzoic acid were identified on the surface of the deactivated photocatalysts. Additional GC/MS analysis of methanol-extracted surface species confirmed the presence of benzaldehyde and benzoic acid and detected small concentrations of benzyl alcohol. Apparently, benzaldehyde is the main partial oxidation product that is further oxidized to benzoic acid. Benzoic acid is strongly adsorbed on the surface of the catalyst. There seems to be a correlation between the accumulation of benzoic acid on the surface and catalyst deactivation. The presence of gas-phase water in the reactive mixture seems to retard the formation of benzoic acid.

The SiO₂–TiO₂ photocatalyst is more active and appears to deactivate slower than TiO₂. This binary oxide is photocatalytically active even in the absence of gas-phase oxygen. It also seems to have a higher toluene adsorption capacity than TiO₂. The acidity of the different oxides was examined using FTIR spectroscopy of adsorbed pyridine. The results indicate that no pure metal oxide displays Brønsted acidity but when SiO₂ is cofumed with TiO₂, Brønsted acidity of intermediate strength is generated. The generation of new surface sites may be responsible for the increased activity. The mechanism of this promotion effect is not clearly understood and further studies are required to elucidate it.     

Selective oxidation of toluene ando-xylene on MgF2-supported monolayer vanadium oxide catalyst

Information on the reactions of selective oxidation of toluene ando-xylene on monolayer V₂O₅/MgF₂ catalyst was obtained using IR spectroscopy. Also adsorption of aldehyde derivatives of these reagents, i.e. benzaldehyde ando-tolualdehyde, was investigated in order to identify intermediate species, which were formed during the oxidation process. The measurements were carried out at different temperatures, under conditions similar to that of the catalytic reaction. On the surface of the catalyst many intermediate species, as aldehyde-like, keton-like and benzoate species, were detected. On the basis of the data obtained the mechanism of these reactions has been proposed.     

Partial oxidation of toluene to benzaldehyde and benzoic acid over model vanadia/titania catalysts: role of vanadia species

Pure and K-doped vanadia/titania prepared by different methods have been studied in order to elucidate the role of vanadia species (monomeric, polymeric, bulk) in catalytic toluene partial oxidation. The ratio of different vanadia species was controlled by treating the catalysts in diluted HNO₃, which removes bulk vanadia and polymeric vanadia species, but not the monomeric ones, as was shown by FT-Raman spectroscopy and TPR in H₂. Monolayer vanadia species (monomeric and polymeric) are responsible for the catalytic activity and selectivity to benzaldehyde and benzoic acid independently on the catalyst preparation method. Bulk V₂O₅ and TiO₂ are considerably less active. Therefore, an increase of the vanadium concentration in the samples above the monolayer coverage results in a decrease of the specific rate in toluene oxidation due to the partial blockage of active monolayer species by bulk crystalline V₂O₅. Potassium diminishes the catalyst acidity resulting in a decrease of the total rate of toluene oxidation and suppression of deactivation. Deactivation due to coking is probably related to the Brønsted acid sites associated with the bridging oxygen in the polymeric species and bulk V₂O₅. Doping by K diminishes the amount of active monolayer vanadia leading to the formation of non-active K-doped monomeric vanadia species and KVO₃.     

In situ surface Raman spectroscopy studies on the interaction between oxygen and ethanol on electrolytic silver catalyst

In situ Raman spectroscopy is employed to investigate the oxidation of ethanol on the electrolytic silver catalyst under catalytic conditions. Over the temperature range of 300–873 K, the configuration of the surface intermediates is detected. The ethoxide species, acetate species, adsorbed acetaldehyde and surface hydroxide exist on the silver surface. The mechanism for the oxidation of ethanol on the silver surface under industrial conditions is discussed and compared with that obtained in ultrahigh vacuum systems.     

In situ IR,Raman, and UV-Vis DRS spectroscopy of supported vanadium oxide catalysts during methanol oxidation

The application of in situ Raman, IR, and UV-Vis DRS spectroscopies during steady-state methanol oxidation has demonstrated that the molecular structures of surface vanadium oxide species supported on metal oxides are very sensitive to the coordination and H-bonding effects of adsorbed methoxy surface species. Specifically, a decrease in the intensity of spectral bands associated with the fully oxidized surface (V⁵⁺) vanadia active phase occurred in all three studied spectroscopies during methanol oxidation. The terminal V = O (∼1030 cm⁻¹) and bridging V–O–V (∼900–940 cm⁻¹) vibrational bands also shifted toward lower frequency, while the in situ UV-Vis DRS spectra exhibited shifts in the surface V⁵⁺ LMCT band (>25,000 cm⁻¹) to higher edge energies. The magnitude of these distortions correlates with the concentration of adsorbed methoxy intermediates and is most severe at lower temperatures and higher methanol partial pressures, where the surface methoxy concentrations are greatest. Conversely, spectral changes caused by actual reductions in surface vanadia (V⁵⁺) species to reduced phases (V³⁺/V⁴⁺) would have been more severe at higher temperatures. Moreover, the catalyst (vanadia/silica) exhibiting the greatest shift in UV-Vis DRS edge energy did not exhibit any bands from reduced V³⁺/V⁴⁺ phases in the d–d transition region (10,000–30,000 cm⁻¹), even though d–d transitions were detected in vanadia/alumina and vanadia/zirconia catalysts. Therefore, V⁵⁺ spectral signals are generally not representative of the percent vanadia reduction during the methanol oxidation redox cycle, although estimates made from the high temperature, low methoxy surface coverage IR spectra suggest that the catalyst surfaces remain mostly oxidized during steady-state methanol oxidation (15–25% vanadia reduction). Finally, adsorbed surface methoxy intermediate species were easily detected with in situ IR spectroscopy during methanol oxidation in the C–H stretching region (2800–3000 cm⁻¹) for all studied catalysts, the vibrations occurring at different frequencies depending on the specific metal oxide upon which they chemisorb. However, methoxy bands were only found in a few cases using in situ Raman spectroscopy due to the sensitivity of the Raman scattering cross-sections to the specific substrate onto which the surface methoxy species are adsorbed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Probing the electronic structure of an active catalyst surface under high-pressure reaction conditions: the oxidation of methanol over copper

The active phase of a bulk metallic copper catalyst is investigated by surface sensitive X-ray absorption spectroscopy at the oxygen K-edge and the Cu L-edges in the total electron yield mode under practical steady state flow-through conditions. The active catalyst surface contains oxygen atoms revealing significant spectral differences compared to those of known copper oxides. The partial oxidation of methanol to formaldehyde is correlated to the abundance of this copper suboxide. These oxygen atoms probe defects of the copper lattice, which represent catalytically active sites. The suboxide is undetectable under UHV conditions. The total oxidation of methanol is catalysed by a conventional copper(I) oxide species and the abundance of carbon dioxide in the gas phase is increasing with decreasing integrated intensity of the oxide species. This revised version was published online in July 2006 with corrections to the Cover Date.     

微反应器内甲苯气固催化氧化反应动力学

微通道反应器具有优良的传热、传质性能，能有效避免催化剂床层内热点的形成，为研究强放热反应动力学提供有利条件。开展了微反应器内的V2O5/TiO2催化剂上的甲苯气相选择氧化动力学研究，在简化反应网络的基础上建立了动力学模型，并给出动力学参数。该模型能较好地反映和预测较宽的反应条件范围内的甲苯气固相催化氧化反应转化率及产物分布，为优化操作条件提供依据。     

A new approach to study the gas-phase oxidation of toluene: probing active sites in vanadia-based catalysts under working conditions

Differently prepared vanadyl pyrophosphate and potassium-doped vanadia catalysts have been studied during the selective oxidation of toluene and p-methoxy toluene to the corresponding benzaldehydes using a portfolio of various in situ-methods (EPR, UV–VIS–DRS, XRD and XPS). The poor catalytic performance of (VO)₂P₂O₇ is mainly due to strong product adsorption which is favoured by surface OH-groups formed under reaction conditions. This process can be partially suppressed by adding competitive adsorbates to the feed. In K-V₂O₅ catalysts, a potassium vanadate phase is formed on the surface in which about 25% of the vanadium ions are tetravalent. This leads to lower acidity and oxidation potential of the surface and, thus, to improved benzaldehyde selectivities.     

Spectroelectrochemical study of the oxidation of diaminophenols on platinum electrodes in acidic medium

The electrochemical oxidation of 2,3- 2,4- and 2,5-diaminophenol, on platinum electrode, in acid medium was studied using cyclic voltammetry, in situ UV-vis and in situ FTIR spectroscopies. The spectroscopic data indicates that 2,4-diaminophenol suffers hydrolysis giving the formation of 2-amino-hydroquinone/2-amino-p-benzoquinone in solution. The oxidation mechanism of 2,5-diaminophenol is similar to the p-phenylenediamine giving 2-hydroxy-p-benzoquinoneimine and its hydrolysis product, 2-hydroxy-p-benzoquinone. The electrochemical results suggest that 2,3-diaminophenol generates a non-electroactive polymeric material on the electrode surface.     

A study of carbon deposition on catalysts during the partial oxidation of methane to synthesis gas

The deposition of carbon on catalysts during the partial oxidation of methane to synthesis gas has been investigated and it has been found that the relative rate of carbon deposition follows the order Ni>Pd>Rh>Ir. Methane decomposition was found to be the principal route for carbon formation over a supported nickel catalyst, and electron micrographs showed that both whisker and encapsulate forms of carbon are present on the catalyst. Negligible carbon deposition occurred on iridium catalysts, even after 200 h.