The beneficial effect of cobalt on VPO catalysts

The addition of Co to VPO formulations improves the yield of n-butane to maleic anhydride. In this work, different modes of impregnation and two different organic cobalt salts were used. The equilibrated catalysts were characterized using XRD, ³¹P SEM NMR, FT-IR and acetonitrile adsorption to evaluate Lewis acidity.

The best catalyst was obtained using Co acetyl acetonate for impregnation of the VOHPO₄·0.5H₂O precursor. This catalyst after equilibration had an optimum concentration of very strong Lewis acid sites, very low concentration of isolated V(V) centers, and no V(V) phases.     

Relationship between structural/surface characteristics and reactivity in n-butane oxidation to maleic anhydride: The role of V species

V/P/O-based catalysts were prepared by thermal treatments of VOHPO₄0·5H₂O precursors prepared with the organic procedure. Different methods for precursor dehydration led to compounds which were characterized by the prevailing presence of crystalline (VO)₂P₂O₇, but which also contained either V⁵⁺ species or V³⁺ species. The catalytic performance of these compounds in n-butane oxidation under almost-equilibrated conditions was compared. It was found that the presence of either V³⁺ or V⁵⁺ enhances the specific activity in n-butane oxidation, while the selectivity to maleic anhydride at low n-butane conversion (30%) remains substantially unaffected. A fully equilibrated, well-crystallized (VO)₂P₂O₇ was reduced with H₂. The reduced compound was more active than the fully equilibrated vanadyl pyrophosphate, while exhibiting comparable selectivity to maleic anhydride.     

Influence of Bi–Fe additive on properties of vanadium phosphate catalysts for n-butane oxidation to maleic anhydride

The physico-chemical and catalytic properties of three ways of modified catalysts were studied, i.e. (i) the addition of both Bi and Fe (nitrate form) during the refluxing VOPO₄·2H₂O with isobutanol (Catalyst A), (ii) the simultaneous addition of BiFe oxide powder in the course of the synthesis of precursor VOHPO₄·0.5H₂O (Catalyst B) and (iii) the mechanochemical treatment of precursor VOHPO₄·0.5H₂O and BiFe oxide in ethanol (Catalyst C). It was found that surface area of the modified catalysts has increased except Catalyst B. The reactivity of the oxygen species linked to V⁵⁺ and V⁴⁺ was studied by using H₂-TPR, which also affected the catalytic performance of the catalyst. The conversion of n-butane decreases with an increment of oxygen species associated with V⁵⁺.     

Preparation of catalyst precursors for selective oxidation of n-butane by exfoliation–reduction of VOPO4·2H2O in primary alcohol

A new method through intercalation and exfoliation of VOPO₄·2H₂O crystallites in primary alcohol (1-propanol or 1-butanol), followed by reduction with the alcohol, have been investigated for the preparation of catalyst precursor. Lamellar compounds, consisting of V⁴⁺, P⁵⁺ and alkyl group with thin film-like morphology, were formed and was characterized by means of XRD, IR, TG/DTA, and elemental analysis. The chemical formula of the precursor obtained by exfoliation–reduction in 1-butanol was shown to be VO{(n-C₄H₉)_0.16H_0.84}PO₄·0.8H₂O. On the other hand, a direct reduction of VOPO₄·2H₂O in the alcohol gave a mixed phase shown by (VOHPO₄·0.5H₂O)_0.3(VO{(n-C₄H₉)_0.3H_0.7}PO₄·3H₂O)_0.7 comprising plate-like microcrystallites. These precursors transformed to (VO)₂P₂O₇ phase during an activation process at 703 K in the presence of a mixture of n-butane 1.5% and O₂ 17% in He balance. The obtained (VO)₂P₂O₇ through the exfoliation–reduction was well crystallized and consisted of thin flaky crystallites. It was found that (VO)₂P₂O₇ thus prepared through the exfoliation–reduction was highly active and selective for oxidation of n-butane.     

Dynamic processes on vanadium phosphorous oxides for selective alkane oxidation

The use of complementary physicochemical tools (XRD, Raman spectroscopy, XPS, ³¹P NMR, and electron microscopy techniques), sometimes used in in situ conditions has allowed to evidence the dynamic processes occurring during the oxidation of light alkanes on the vanadium phosphorus oxide (VPO) system. The transformations of the VPO system in the course of the oxidation of n-butane to maleic anhydride and of the oxidation of propane to acrylic acid are contrasted in connection with the evolution of the catalytic performances.     

Controlling factors in the selective conversion of n-butane over promoted VPO catalysts at low temperature

Unpromoted and 1.2, 2.3 and 4.3 molar % Co:V cobalt-promoted vanadium-phosphorous-oxide (VPO) catalysts were synthesized via an organic route. The catalyst precursors were calcined and then conditioned in a reactor, forming the active vanadyl pyrophosphate (VO)₂P₂O₇ phase, which was confirmed via X-ray diffraction studies (XRD). The effect of co-promotion on the yield of maleic anhydride (MA) from n-butane oxidation was examined at different temperatures and gas hourly space velocities (GHSV). 2.3% Co:V was the optimum promoter loading for a high yield towards MA. Higher GHSV's proved to enhance the selectivity towards MA.

The catalysts were tested over a 200 h period, generally taking some 24 h to reach steady-state performance. The best performing catalyst yielded 45% MA at 275 °C and a GHSV of 7200 ml g⁻¹ h⁻¹, with an n-butane conversion of 73%, whilst all previously reported VPO catalysts produced far lower MA yields at this temperature.     

Phase transformations in mesostructured vanadium–phosphorus-oxides

Mesostructured lamellar, hexagonal and cubic vanadium–phosphorus-oxide (VPO) phases were prepared employing cationic, anionic and alkylamine surfactants under mild conditions and low pH. The obtained mesophases displayed desirable vanadium oxidation states (+3.8 to +4.3) and P/V molar ratios 1.0 for the partial oxidation of n-butane to maleic anhydride. As-synthesized mesostructured VPO underwent phase transformations to various mesostructured and dense VPO phases depending on the post-synthesis treatment. The phase transformations of mesostructured VPO during Soxhlet extraction and thermal treatment in N₂ have been observed for the first time. These transformations were explained by the changes in the surfactant packing parameter, g. Calcination in air produced more disordered mesostructures and dense VPO phases such as γ-VOPO₄ and (VO)₂P₂O₇.     

Effect of different calcination environments on the vanadium phosphate catalysts for selective oxidation of propane and n-butane

Vanadium phosphate catalysts were synthesized via VOPO₄·2H₂O and were calcined in two different hydrocarbon reaction environments, i.e. n-butane/air and propane/air. Both catalysts are denoted VPDB and VPDP, respectively. Both catalysts exhibited a good crystalline with characteristic peaks of pyrophosphate phase. However, the peaks for VPDP are shown to be more prominent than those of VPDB. BET surface area showed that VPDB gave higher surface area (23 m² g⁻¹) compared to VPDP (18 m² g⁻¹). The average V valence state for VPDP is 4.08 and the higher V valence state for VPDB is 4.26 due to higher amount of V^V for VPDB. Furthermore 14.2% of V^III was found for VPDP but none for VPDB. SEM micrographs clearly revealed that the morphologies of both catalysts composed of plate-like crystallite that was arranged into the characteristic of rosette cluster. However, the catalyst calcined in n-butane/air environment (VPDB) resulted in an increment of the amount of plate-like crystal formed in the rosette rosebud agglomerates. TPR in H₂ profiles of both catalysts gave two reduction peaks corresponding to two kinetically different oxygen species which were associated with V^V and V^IV phases, respectively. VPDB removed larger amount of active oxygen species linked to V^IV phase which eventually caused a higher conversion rate in the selective oxidation of n-butane and propane to maleic anhydride and acrylic acid, respectively.     

Study of Mn incorporation into SAPO framework: Synthesis, characterization and catalysis in chloromethane conversion to light olefins

MnAPSO-34 molecular sieve has been synthesized with triethylamine as the template, characterized with XRD, XRF, ³¹P, ²⁷Al and ²⁹Si NMR and FT-IR techniques and compared with SAPO-34. The template decomposition and removal have been investigated with TG–DTG–DSC coupled with mass spectrometer. Mn incorporation generates a negligible difference on the chemical shift in ³¹P and ²⁷Al MAS NMR, while an effect on the intensity of resonance peaks is revealed. ²⁹Si MAS NMR study has demonstrated that Mn incorporation favors the Si island formation, which may give rise to the stronger acidic sites. The thermal analysis (TG–DSC) on template removal in a diluted oxygen atmosphere, leading to the formation of CO₂, NO and H₂O, showed, besides a low temperature endothermic weight loss due to the desorption of water, two weight losses (200–400 and 400–600 °C) for SAPO-34 and MnAPSO-34, suggesting two different chemical location environments of template molecules in these two molecular sieves. The quantity of template removed at higher temperature range is much higher in MnAPSO-34, indicating stronger template–framework interaction and stronger acidity after calcination. The acid difference caused by Mn incorporation has also been evidenced by ammonia adsorption evaluated by FT-IR. Chloromethane transformation was carried out over MnAPSO-34 and SAPO-34 and the catalytic performance showed that both molecular sieves are very active and selective catalyst for light olefins production. MnAPSO-34 demonstrated higher activity and light olefins selectivity.     

In situ Raman spectroscopy studies of bulk and surface metal oxide phases during oxidation reactions

Bulk V-P-O and model supported vanadia catalysts were investigated with in situ Raman spectroscopy during n-butane oxidation to maleic anhydride in order to determine the fundamental molecular structure-reactivity/selectivity insights that can be obtained from such experiments. The in situ Raman studies of the bulk V-P-O catalysts provided information about the bulk crystalline phases, the hemihydrate precursor and its transformation to vanadyl pyrophosphate. However, the Raman experiments could not provide any molecular structural information about the amorphous and surface phases also present in this bulk metal oxide catalyst because of the strong Raman scattering from the crystalline phases. In contrast, in situ Raman studies of the model supported vanadia catalysts, where the active phase is present as a two-dimensional surface metal oxide overlayer, provided new insights into this important hydrocarbon oxidation reaction. In addition, the surface properties of the supported vanadia catalysts could be molecularly engineered to probe the role of various functionalities upon the structure-reactivity/selectivity relationship of n-butane oxidation to maleic anhydride. These fundamental studies revealed that the oxidation of n-butane required only one surface vanadia site and that the critical rate determining step involved the bridging V---O---P or V---O-support bonds. The selective oxidation of n-butane to maleic anhydride could occur over one surface vanadia site as well as multiple adjacent surface vanadia sites, but the reaction is more efficient with multiple sites. The n-butane oxidation TOF increased with the introduction of both surface Brönsted and Lewis acid sites, but only the surface Lewis acid sites increased the maleic anhydride selectivity.     

Alternative solution for strongly exothermal catalytic reactions: a new metal-structured catalyst carrier

In many processes of selective catalytic oxidation of hydrocarbons, e.g. benzene or n-butane to maleic anhydride, large amounts of heat are evolved and the catalytic bed has to be intensely cooled by the mixture of molten salts. At higher temperatures, the reaction rate and thus the amount of released heat increase causing local superheatings of the catalyst (hot spots) and hence its deactivation. In these studies, we report on a new metal-structured catalyst carrier with better heat and hydraulic characteristics.

The pressure drop and heat transfer characteristics of over 30 novel-structured metal carriers as well as the standard ceramic ones have been experimentally investigated and correlated in terms of the Fanning friction factor and Nusselt number vs. Reynolds number.

The best-structured carrier gave 16–18% larger heat transfer coefficients and 10–40% lower pressure drops in the operation range of the industrial reactors in comparison with the classic random ceramic supports. The utility of this structured carrier has been confirmed by reaction experiment of n-butane oxidation to maleic anhydride.     

The role of molybdenum in Mo-doped V–Mg–O catalysts during the oxidative dehydrogenation of n-butane

A detailed study on the influence of the addition of molybdenum ions on the catalytic behaviour of a selective vanadium–magnesium mixed oxide catalyst in the oxidation of n-butane has been performed. The catalysts have been prepared by impregnation of a calcined V–Mg–O mixed oxides (23.8 wt% of V₂O₅) with an aqueous solution of ammonium heptamolybdate, and then calcined, and further characterised by several physico-chemical techniques, i.e. S_BET, XRD, FTIR, FT-Raman, XPS, H₂-TPR. MgMoO₄, in addition to Mg₃V₂O₈ and MgO, have been detected in all the Mo-doped samples. The incorporation of molybdenum modifies not only the number of V⁵⁺-species on the catalyst surface and the reducibility of selective sites but also the catalytic performance of V–Mg–O catalysts. The incorporation of MoO₃ favours a selectivity and a yield to oxydehydrogenation products (especially butadiene) higher than undoped sample. In this way, the best catalyst was obtained with a Mo-loading of 17.3 wt% of MoO₃ and a bulk Mo/V atomic ratio of 0.6. From the comparison between the catalytic properties and the catalyst characterisation of undoped and Mo-doped V–Mg–O catalysts, the nature of selective sites in the oxidative dehydrogenation of n-butane is also discussed.     

Facile template-free synthesis of meso-macroporous titanium phosphate with hierarchical pore structure

Meso-macroporous titanium phosphate materials with hierarchical structure have been synthesized through a facile and template-free method using titanium n-butoxide (TBT) as titanium source. Both the as-synthesized titanium phosphate and the sample after calcination at 500 °C are amorphous, as proved by the XRD measurements. The co-existence of mesopores and macropores is identified by N₂ adsorption–desorption measurements, SEM and TEM methods. The formation of Ti–O–P bonds during the synthesis process was supported by FT-IR spectroscopy, and was further verified by ³¹P MAS NMR spectra and XPS measurements. The BET surface area of the products can be adjusted by addition of n-butanol.     

Water-soluble niobium peroxo complexes as precursors for the preparation of Nb-based oxide catalysts

In the frame of research aimed at developing new synthetic procedures of multimetallic Nb-based catalysts, peroxo complexes of niobium(V) of general formula A^I₃[Nb(O₂)₄] and A^I₃[Nb(O₂)_x(H_yL)]·nH₂O (A^I: NH₄⁺, CN₃H₆⁺ (gu); L: oxalate, tartrate, citrate) have been prepared and characterized on the basis of elemental and thermal analysis, FTIR and ¹³C-NMR spectra. The crystal structure of (gu)₃[Nb(O₂)₄] and (gu)₃[Nb(O₂)₂(C₂O₄)₂]·2H₂O have been determined. The application of the obtained Nb complexes as precursors for the preparation of silica-supported Nb–Mo–O catalysts has been demonstrated. Combining Nb peroxo-carboxylato compounds with analogous Mo(VI) compounds in a silica-impregnation method carried out in aqueous medium leads to the formation of the supported Nb₂Mo₃O₁₄ phase.     

The effect of preparation methods and activation strategies upon the catalytic behavior of the vanadium-phosphorus oxides

The Catalyst precursors for the oxidation of n-butane to maleic anhydride were prepared using vanadium pentoxide and orthophosphoric acid. The former was reduced using different agents (HC1 and benzyl alcohol) and solvents (water, isobutyl and Isopropyl alcohols). The precursors were activated applying different strategies which were selected and adapted from the literature. The changes occurring in the solid were followed using XRD, XPS and ³¹P MAS NMR techniques. The Catalytic performance was evaluated using a flow reactor system. The equilibrated Catalysts obtained after 100–400 hours on stream contained vanadyl pyrophosphate as the only crystalline phase (XRD). The XPS data showed significant phosphorus surface enrichment and the existence of only V(iv) on the surface. The solvent used and the activation strategy strongly influenced the Catalytic performance which could be traced through ³¹P NMR to the presence of small amounts of V(v) containing phases. The results are discussed in terms of the current literature.     

Structure–reactivity relationships in VOx/TiO2 catalysts for the oxyhydrative scission of 1-butene and n-butane to acetic acid as examined by in situ-spectroscopic methods and catalytic tests

Different VO_x/TiO₂ catalyst have been catalytically tested and studied by in situ-spectroscopic methods (FT-IR, UV/vis, EPR) in the oxyhydrative scission (OHS) of 1-butene and n-butane to acetic acid (AcOH). While 1-butene OHS follows the sequence butene → butoxide → ketone → AcOH/acetate with a multitude of side products also formed, n-butane OHS leads to AcOH, CO_x and H₂O only. Water vapour in the feed improves AcOH selectivity by blocking adsorption sites for acetate. The admixture of Sb₂O₃ was found to improve AcOH selectivity which is due to deeper V reduction under steady state conditions and lowering of surface acidity. VO_x/TiO₂ catalysts with sulfate-containing anatase are the most effective ones. Covalently bonded sulfate at the catalyst surface causes specific bonding of VO_x, stabilizes active V species and ensures their high dispersity.     

Water modification of PEG-derived VPO for the partial oxidation of propane

In this study, the PEG-derived VPO precursors were subject to water refluxing (90 °C, 8 h) and in situ activation in a steam-containing environment. For comparison, the VPO precursors without water refluxing were also activated in a similar manner. The IR measurement indicated that the majority of PEG in the precursor has been removed during water refluxing, and the VPO is essentially unenwrapped by PEG. The consequence is an increase of particle size and crystallinity of VPO as well as decreases in surface acidity and site density. The activation of catalysts in a stream-containing environment has an influence on the content of V⁵⁺ species and on the reduction behavior of the VPO catalysts. The VPO catalyst activated with 20% water vapor (by volume) in the feed shows the highest crystallinity. Compared with the non-PEG-derived VPO precursor, the PEG-derived one undergoes structure changes of higher severity during the water/steam treatments. The VPO catalyst generated from the PEG-derived precursor with water refluxing and activated with steam (20%) exhibited a propane conversion of 25% and a (AA + HAc) selectivity of 70%. The superior catalyst behavior can be interpreted in terms of the higher crystallinity of the (VO)₂P₂O₇ phase, the lower content of V⁵⁺ species, and the milder surface acidity as caused by the water/steam treatments.     

Ammoxidation of methylaromatics over vanadium phosphate catalysts II. Catalyst formation, active sites and reaction mechanism

The interactions of VOHPO₄· 0.5H₂O and (VO)₂P₂O₇ with the ammoxidation feed and the single components such as ammonia, oxygen, water and component mixtures were studied in detail using XRD and temperature-programmed reaction spectroscopy. The aim of this work was to improve the knowledge of the formation of the active phases or active sites of the catalysts from their precursors under the condition of the ammoxidation reaction. Similar catalytic properties of various applied VPO materials were discussed in terms of the presence of similar structure elements (domains of adjacent edge-sharing VO₆ octahedra-units and P-O-NH₄ groups).     

Selectivity and mechanism for skeletal isomerization of alkanes over typical solid acids and their Pt-promoted catalysts

Selectivities for skeletal isomerizations of n-butane and n-pentane catalyzed by typical solid acids such as Cs_2.5H_0.5PW₁₂O₄₀ (Cs2.5), SO₄²⁻/ZrO₂, WO₃/ZrO₂, and H-ZSM-5 and their Pt-promoted catalysts were compared. High selectivities for n-butane and low selectivity for n-pentane were observed over Cs2.5 and SO₄²⁻/ZrO₂, while H-ZSM-5 was much less selective, and WO₃/ZrO₂ was highly selective for both reactions. The Pt-promoted solid acids were usually selective for these reactions in the presence of H₂ except for Pt-H-ZSM-5 for n-butane isomerization. Both the acid strength and pore structure would be factors influencing the selectivity. Mechanism of skeletal isomerization of n-butane was investigated by using 1,4-¹³C₂-n-butane over Cs2.5 and Pt–Cs2.5. It was concluded that n-butane isomerization proceeded mainly via monomolecular pathway with intramolecular rearrangement on Pt–Cs2.5, while it occurred through bimolecular pathway with intermolecular rearrangement on Cs2.5. The higher selectivity on Pt–Cs2.5 would be brought about by the monomolecular mechanism. In the skeletal isomerization of cyclohexane, Pt–Cs2.5/SiO₂ was highly active and selective, while Pt–Cs2.5 was less selective. Control in the acid strength of Cs2.5 by the supporting would be responsible for the high selectivity.     

Mesoporous hybrid zirconium oxophenylphosphate synthesized in absence of any structure directing agent

A new organic–inorganic hybrid mesoporous zirconium oxophenylphosphate (ZPP-1) has been synthesized hydrothermally at 443 K by using phenylphosphonic acid (PPA) as phosphorus source in absence of any structure directing agent. Powder XRD, TEM, FE SEM, N₂ sorption, CHN and ICP-AES chemical analyses, ¹³C CP/MAS and ³¹P MAS NMR, UV–visible and FT IR spectroscopic tools and thermal analysis were employed to characterize this novel material. XRD, N₂ sorption and TEM image analysis suggested the existence of multi-lamellar structure of the pore wall with large micropores and mesopores having peak maximums of ca. 1.5 and 5.0–6.0 nm, respectively in this ZPP-1 material. Interestingly, this hybrid material showed very high thermal stability together with retention of nanostructure and phenyl group when heated upto 723 K. ZPP-1 showed fairly high H₂ adsorption capacity under atmospheric pressure at 77 K. Possible templating role played by the framework phenyl groups has been discussed.