Ammoxidation Of Toluene Research Articles

Vanadium–phosphorous oxide supported on mesoporous SBA-15 catalysts for ammoxidation of toluene to benzonitrile

Reaction scheme of ammoxidation of toluene over VPO/SBA-15.

Unravelling the role of Ag[sbnd]Cr interfacial synergistic effect in Ag/Cr2O3 nanostructured catalyst for the ammoxidation of toluene via low temperature activation of Csp3-H bond

An environmentally benign and cost-effective synthesis of benzonitrile is reported by direct oxidative cyanation of aromatic Csp3-H bonds of toluene over Ag(I)/Cr2O3 nanostructured catalyst. The catalyst was prepared by a hydrothermal route using cationic surfactant cetyltrimethylammonium bromide and was characterized by powder XRD, XPS, SEM, TEM, TGA, TPD, TPR, UV-visible, FTIR, XANES, and ICP-AES methods. Conjugal effect of Lewis plus Brønsted acidity coupled with AgCr interfacial synergistic interaction appears as detrimental for its high catalytic activity and selectivity (98.4%) for benzonitrile. In situ DRIFTS analysis suggested that the catalytic system proceeded via a reaction pathway explained by Mars-van Krevelen mechanism.

Structural Properties of Gas-Phase Molybdenum Oxide Clusters [Mo4O13]2\u2212, [HMo4O13]\u2212, and [CH3Mo4O13]\u2212 Studied by Collision-Induced Dissociation

Molybdenum oxide-based catalysts are widely used for the ammoxidation of toluene, methanation of CO, or hydrodeoxygenation. As a first step towards a gas-phase model system, we investigate here structural properties of mass-selected [Mo4O13]2−, [HMo4O13]−, and [CH3Mo4O13]− by a combination of collision-induced dissociation (CID) experiments and quantum chemical calculations. According to calculations, the common structural motif is an eight-membered ring composed of four MoO2 units and four O atoms. The 13th O atom is located above the center of the ring and connects two to four Mo centers. For [Mo4O13]2− and [HMo4O13]−, dissociation requires opening or rearrangement of the ring structure, which is quite facile for the doubly charged [Mo4O13]2−, but energetically more demanding for [HMo4O13]−. In the latter case, the hydrogen atom is found to stay preferentially with the negatively charged fragments [HMo2O7]− or [HMoO4]−. The doubly charged species [Mo4O13]2− loses one MoO3 unit at low energies while Coulomb explosion into the complementary fragments [Mo2O6]− and [Mo2O7]− dominates at elevated collision energies. [CH3Mo4O13]− affords rearrangements of the methyl group with low barriers, preferentially eliminating formaldehyde, while the ring structure remains intact. [CH3Mo4O13]− also reacts efficiently with water, leading to methanol or formaldehyde elimination.

A critical perspective on the design and development of metal oxide catalysts for selective propylene ammoxidation and oxidation

The objective of this article is to use the example of the development of metal oxide catalysts for the selective ammoxidation and oxidation of propylene to illustrate successful catalyst design strategies for achieving enhanced yields of acrylonitrile and acrolein, respectively. The processes of catalytic selective ammoxidation and oxidation of propylene are not the only, albeit they are among the most industrially significant, examples of the commercial application of metal oxide selective oxidation catalysts. Other important commercial catalytic processes that use highly developed metal oxide catalysts systems include butane oxidation to maleic anhydride and alkyl aromatic ammoxidation of toluene, xylenes, and picolines (alkyl pyridines) to produce valuable aromatic nitriles including nicotinonitrile which is the precursor for nicotinic acid (niacin) an important B-complex vitamin. Using the example of industrial propylene selective (amm)oxidation, this article surveys the available patents, scientific and technical journals to chronical and analyze the development and advancement of the complex metal oxides from discovery to current state-of-the-art. Recognizing that commercial practices are proprietary, this analysis uses no known company sensitive or confidential information is included in these descriptions. From this analysis a comprehensive catalyst design strategy unfolds that is based on the enabling science of solid state chemistry and the results of the characterization tools it provides. This provides a viable platform to unlock the next level of metal oxide catalyst development with ultra-high selectivity for hydrocarbon oxidation.

NH3-efficient ammoxidation of toluene by hydrothermally synthesized layered tungsten-vanadium complex metal oxides

Hydrothermally synthesized WVO layered metal oxides (WVO) are studied for the vapor phase ammoxidation of toluene to benzonitrile (PhCN). Under similar conversion levels at 400°C, WVO shows higher selectivity (based on toluene) to PhCN and lower selectivity to COx than conventional V-based catalysts (V2O5 and VOx/TiO2). Under the conditions of high contact time, WVO shows 99.7% conversion of toluene and 93.5% selectivity to PhCN. Another important feature of WVO is high NH3-utilization efficiency in ammoxidation, which originates from the lower activity of WVO for NH3 oxidation than that of V2O5. In situ infrared (IR) study shows that toluene is oxidized by the surface oxygen species of W83V17 to yield benzaldehyde which undergoes the reaction with adsorbed NH3 to give benzonitrile. Model reaction studies with WVO suggest that the rate of NH3 conversion to PhCN in the benzaldehyde+NH3+O2 reaction is 3 times higher than the rate of NH3 oxidation to N2 in the NH3+O2 reaction. It is shown that the high NH3-efficiency of WVO is caused by the preferential reaction of NH3 in PhCHO+NH3+O2 over NH3+O2 reaction.

Characterization and catalytic properties of molybdenum oxide catalysts supported on ZrO2–γ-Al2O3 for ammoxidation of toluene

Molybdenum oxide catalysts with MoO3 loadings ranging from 6.6 to 25 wt% supported on ZrO2–γ-Al2O3 (1 : 1 wt%) mixed oxide were prepared by a wet impregnation method.

Spin–spin exchange in vanadium-containing catalysts studied by in situ-EPR: a sensitive monitor for disorder-related activity

In unsupported V-containing catalysts, V4+ sites are frequently present in amorphous oxidic clusters and/or crystalline paramagnetic bulk phases in which they are coupled by effective spin–spin exchange interactions. This work presents two EPR procedures for evaluating these interactions and relating their strength to the degree of disorder and catalytic performance, namely (1) evaluating permanent structural disorder by calculating exchange energies and integrals and (2) monitoring transient electronic disorder caused by catalyst–reactant interaction in working catalysts using the ratio of the fourth and the square of the second moment of the EPR signal. Selected application examples comprise VPO catalysts such as (i) (VO)2P2O7, in which a certain degree of structural disorder revealed to be essential for high catalytic performance in the selective oxidation of n-butane to maleic anhydride, (ii) (NH4)2(VO)3(P2O7)2 in which transient perturbation of spin–spin exchange sensitively reflects the strength of reactant adsorption during ammoxidation of toluene and (iii) (NH4)2VOP2O7 in which an amorphous VO2+-containing phase could be identified as active component besides inactive crystalline phases. Based on operando-EPR measurements, reasons for activity differences in MoVTeNb oxide catalysts could be suggested.

Characterization and reactivity of Al 2O 3–ZrO 2 supported vanadium oxide catalysts

Vanadium oxide catalysts with V 2O 5 loadings ranging from 2.5 to 20 wt.% supported on Al 2O 3–ZrO 2 (1:1 wt.%) mixed oxide have been prepared by wet impregnation method. The calcined samples were characterized by X-ray diffraction (XRD), BET surface area, pulse oxygen chemisorption, electron spin resonance (ESR), temperature-programmed reduction (TPR) of H 2, X-ray photoelectron spectroscopy (XPS), and UV–vis diffuse reflectance spectroscopy (UVDRS). The catalytic properties have been evaluated for vapor phase ammoxidation of toluene to benzonitrile. Dispersion of vanadia was determined by the pulse oxygen chemisorption method at 643 K. Vanadia is found to be present in a highly dispersed state at lower loadings and dispersion decreases steadily with increase of vanadia loading. The ESR spectra obtained under ambient conditions shows the presence of V 4+ in tetrahedral symmetry. The TPR results show a single reduction peak corresponding to V 5+→V 3+. XPS results reveal that vanadium is present in a fully oxidized state (+5) in all the samples. The intensity ratio V 2p 3/2/Al 2p 3/2 and V 2p 3/2/Zr 3d 5/2 is found to increase with increase in vanadia loading up to 12.5 wt.% and levels off at higher vanadia loadings. The UV–vis diffuse reflectance spectra indicate the existence of isolated and clusterized tetrahedral (Td) V 5+ species in all catalysts. Ammoxidation activity increases with vanadia loading up to 12.5 wt.%, which corresponds to monolayer coverage and remains constant at higher vanadia loadings. The catalytic activity in ammoxidation of toluene to benzonitrile has been correlated to the oxygen chemisorption sites.

Dispersion and reactivity of V 2O 5 catalysts supported on Al 2O 3–ZrO 2

Vanadium oxide catalysts with V 2O 5 loadings ranging from 2.5 to 20 wt% supported on Al 2O 3–ZrO 2 (1:1 wt%) mixed oxide have been prepared by wet impregnation method. The calcined samples were characterized by X-ray diffraction, BET surface area, pulse oxygen chemisorption, and temperature programmed desorption (TPD) of NH 3. Dispersion of vanadia was determined by the pulse oxygen chemisorption method at 643 K. At lower loadings, vanadia is found to be present in a highly dispersed state and dispersion decreases steadily with increase in vanadia loadings. X-ray diffraction results also suggest that vanadium oxide exists in a highly dispersed amorphous state at lower loadings and in a crystalline V 2O 5 phase at higher vanadia loadings. The effect of vanadia loading has been studied on the ammoxidation of toluene to benzonitrile over V 2O 5/Al 2O 3–ZrO 2 catalysts. The catalytic activity for the ammoxidation of toluene increases with vanadia loading up to 12.5 wt%, which corresponds to monolayer coverage and levels off at higher vanadia loadings. The correlation of catalytic activity with oxygen chemisorption sites and ammonia TPD measurements was discussed.

Structure and catalytic properties of molybdenum oxide catalysts supported on zirconia

MoO 3/ZrO 2 catalysts with different MoO 3 loadings (2–12 wt%) were prepared by the wet impregnation method. These catalysts were characterized by various techniques, such as X-ray diffraction (XRD), temperature-programmed reduction (TPR), laser Raman spectroscopy (LRS), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption of NH 3 and the catalytic properties were evaluated for vapor-phase ammoxidation of toluene to benzonitrile. XRD patterns show the presence of crystalline MoO 3 peaks above 6.6 wt% MoO 3, which corresponds to the theoretical monolayer loading of MoO 3 on the zirconia used in the present study. The TPR suggests that reduction of the catalysts occurs in two stages and indicates that the reducibility of the catalysts increases with increase in MoO 3 loading up to 6.6 wt%. The acidity of the catalysts was also found to increase up to 6.6 wt% of molybdena loading and it does not increase much at higher loadings. Raman results show that the surface molybdate species are present in low-loading samples, while crystalline MoO 3 bands are observed from 9 wt% of MoO 3 and above loadings. XPS spectra showed that molybdenum was present at Mo 6− on all fresh samples. The Mo/Zr atomic ratio shows that the dispersion of molybdena is high below 6.6 wt% MoO 3 and dispersion decreases at higher molybdena loadings. The catalytic activity of the catalysts during ammoxidation of toluene was found to increase with loading up to 6.6 wt% and did not change appreciably beyond this loading.

Studies on catalytic functionality of V2O5/Nb2O5 catalysts

Abstract A series of V 2 O 5 /Nb 2 O 5 catalysts with V 2 O 5 loadings ranging from 2 to 12 wt.% of V 2 O 5 were prepared by wet impregnation method. These catalyst samples were characterized by X-ray diffraction (XRD), oxygen chemisorption, electron spin resonance (EPR), temperature-programmed desorption (TPD) of ammonia, and scanning electron microscopy (SEM). The catalytic properties of these catalysts were tested for vapor phase ammoxidation of toluene to benzonitrile and compared with more traditional supports, such as Al 2 O 3 , SiO 2 and TiO 2 . Dispersion of vanadia was determined by the static oxygen chemisorption method at 640 K on the samples prereduced at the same temperature. At low vanadia loadings ( 2 O 5 phase at higher loadings of V 2 O 5 . The EPR spectra obtained under ambient conditions for the catalysts show the presence of V 4+ ions in a distorted tetrahedral symmetry. SEM results suggest that at low vanadia loadings vanadia is well dispersed and at higher loadings dense clusters are formed. The results of temperature programmed desorption of ammonia indicate that the acidity increases with vanadia loading up to 6 wt.% V 2 O 5 and decreases at higher vanadia loadings. The ammoxidation activity also increases with vanadia loading up to 6 wt.%, which corresponds to monolayer coverage and remains constant at higher vanadia loadings. The catalytic properties during ammoxidation of toluene are correlated to the oxygen chemisorption sites and acidic sites.

Toluene ammoxidation on α-Fe 2O 3-based catalysts

The catalytic behaviour of α-Fe 2O 3-based catalysts in the toluene ammoxidation reaction was investigated in a continuous fixed-bed microreactor at 673 K, under atmospheric pressure. Catalysts were prepared and characterized by chemical analysis, N 2 adsorption/desorption at 77 K, X-ray diffraction (XRD), temperature-programmed reduction (TPR) and adsorption microcalorimetry of ammonia. Differences in the reducibility of the surface sites were found between the pure Fe 2O 3 and the Na silicate containing catalysts. Doping by addition of V 2O 5, Sb 2O 5 or Cr 2O 3 did not lead to a significant modification of the reduction behaviour. On the contrary, the acidic properties of iron oxide were significantly affected by the presence of both Na silicate and V, Sb or Cr metal oxides. The addition of these minor components resulted in the improvement of the catalytic performance of the pure α-Fe 2O 3.

Structure and Reactivity of Molybdenum Oxide Catalysts Supported on La2O3-Stabilized Tetragonal ZrO2

A series of molybdenum oxide catalysts with Mo loadings in the range 2.5−12.5 wt % Mo supported on La2O3-stabilized ZrO2 was prepared by impregnation. X-ray diffraction results of the catalysts showed the diffraction patterns of crystalline MoO3 from 10 wt % Mo. Dispersion of molybdena was determined by the oxygen chemisorption method, and it was found to decrease with an increase in Mo loading due to formation of MoO3 crystallites at higher Mo loadings. Temperature-programmed reduction results suggest that the reducibility is found to decrease with Mo loading as a result of the decrease in dispersion. The acidity of the catalysts was found to increase with Mo loading, and the activity of the catalysts also found to increase with the increase in Mo loading up to 7.5 wt % Mo loading. Laser Raman spectra of the catalysts show the Raman bands due to MoO3 from 7.5 wt % Mo at 819 and 997 cm-1. At lower Mo loadings, highly dispersed species were observed. The catalytic properties were evaluated during the ammoxidation of toluene to benzonitrile and were related to the oxygen chemisorption sites on the support surface.

Looking on Heterogeneous Catalytic Systems from Different Perspectives: Multitechnique Approaches as a New Challenge for In Situ Studies

Adapting physico-chemical characterization techniques for in situ studies of real heterogeneous catalysts during conditioning, reaction, and deactivation processes is a research topic of growing importance. Up to now, in most studies only one single in situ technique is applied. However, new trends are focused on using a portfolio of several in situ methods to obtain comprehensive information on structure-reactivity relationships. This review discusses such application examples with the aim to point out the particular benefits that arise from multitechnique approaches for understanding processes like formation of active sites, interaction of those sites with reactant molecules, and catalyst deactivation. Except for Fourier-transform infrared spectroscopy and quasi-in situ X-ray photoelectron spectroscopy, the examples deal mainly with the combined use of bulk techniques such as electron paramagnetic resonance, ultraviolet/visible diffuse reflection spectroscopy, laser-Raman spectroscopy, temperature-programmed spectroscopy, X-ray diffraction, and X-ray absorption spectroscopy. However, although not being surface-sensitive per se, these techniques provide results relevant for the catalyzed reaction. Illustrative case studies comprise the investigation of (i) vanadia-based catalysts during selective oxidation and ammoxidation of toluene and during methanol synthesis, (ii) supported chromium oxides during octane aromatization, (iii) zeolite crystallization and catalysis, and (iv) a Cu/ZnO catalyst during methanol synthesis.

Defined vanadium phosphorus oxides and their use as highly effective catalysts in ammoxidation of methyl aromatics

The heterogeneous catalytic ammoxidation of methyl aromatics and methyl hetero aromatics is a preferred method for the synthesis of aromatic and hetero aromatic nitriles. The resulting nitriles are valuable intermediates for the production of dyestuffs, pesticides, pharmaceuticals and other chemical products. Usually, the ammoxidation is carried out using V-containing oxides (e.g. V/Ti, V/Sn, V/Mo) promoted by further transition metals as catalysts. However, we have shown recently that defined vanadium phosphates (VPO) revealed an excellent ammoxidation performance. The ammoxidation of toluene was used as model reaction. An intense characterisation of some VPO catalysts under pre-treatment and working conditions by the use of various in situ-methods has thrown more light in formation and stability of the used VPO solids as well as the effect of different reactants (ammonia, toluene) on VPO surface and bulk properties.

Dispersion and reactivity of Mo/Nb2O5 catalysts in the ammoxidation of toluene to benzonitrile

A series of Mo/Nb2O5 catalysts were prepared with Mo loadings ranging from 2.5–15 wt% of Mo and characterized by X-ray diffraction (XRD), pulse oxygen chemisorption, temperature-programmed desoprtion of ammonia. Dispersion of molybdena was determined by oxygen chemisorption at 623 K in a dynamic method. At low Mo loadings molybdenum oxide is found to be present in a highly dispersed state. XRD results show the presence of crystalline MoO3 at higher Mo loadings (from 10 wt%). The dispersion of molybdena was found to decrease with increased Mo loading due to formation of MoO3. The catalytic properties were evaluated for the vapour phase ammoxidation of toluene to benzonitrile and are related to different characterization results.

Vapour phase ammoxidation of toluene over vanadium oxide supported on Nb2O5–TiO2

Vanadium oxide catalysts with V2O5 loadings ranging 1 to 9% (w/w) supported on 1∶1 wt% Nb2O5–TiO2 mixed oxide have been prepared by the wet impregnation method. The calcined samples were characterized by X-ray diffraction (XRD), pulse oxygen chemisorption and temperature programmed desorption (TPD) of NH3. Dispersion of vanadia was determined by oxygen chemisorption at 643 K in a dynamic method. At low V2O5 loadings, vanadia is found to be present in a highly dispersed state. X-Ray diffraction results also suggest that vanadium oxide exists in a highly dispersed state at lower loadings and in a crystalline V2O5 phase at higher vanadia loadings (5 wt% and above). The dispersion of vanadia was found to decrease with V2O5 loading due to the formation of V2O5 crystallites. The catalytic properties were evaluated for the vapor phase ammoxidation of toluene to benzonitrile and correlated with characterization results.

Dispersion and reactivity of molybdenum oxide catalysts supported on titania

A series of MoO 3 catalysts with Mo loadings ranging from 2 to 10 w/w% supported on TiO 2 were prepared and investigated by X-ray diffraction (XRD), temperature programmed reduction (TPR) and oxygen chemisorption measurements. Dispersion of molybdena was determined by the oxygen chemisorption at 623 K and by static method on the samples prereduced at the same temperature. At low Mo loadings, i.e. below 6% Mo, molybdenum oxide was found to present in a highly dispersed amorphous state. TPR profiles of MoO 3/TiO 2 samples suggest that the reduction of MoO 3 to Mo proceeds in two stages and the reducibility of MoO 3 increases with Mo loading in the catalysts. The catalytic properties were evaluated for the vapor-phase ammoxidation of toluene to benzonitrile and are related to the oxygen chemisorption sites.

Redox Properties of VPO Ammoxidation Catalysts

The purpose of this study is to explore correlations between the molarity of V x IV/V O y proportions of (NH 4 ) 2 (V IV O) 3 (P 2 O 7 ) 2 catalysts, the vanadium valence states, and the reducibility of the vanadium on the one hand and the catalytic activity of the solids on the other. Pure (NH 4 ) 2 (V IV O) 3 (P 2 O 7 ) 2 and two ((NH 4 ) 2 (V IV O) 3 (P 2 O 7 ) 2 +V x IV/V O y ) solids (precursor V/P=1 and 1.5) are used as catalysts; the specimens are characterized by chemical analyses, different X-ray techniques (energy dispersive analysis of X-rays (EDX) and X-ray photoelectron spectroscopy (XPS)), temperature-programmed reduction (TPR), and catalytic measurements using the ammoxidation of toluene to benzonitrile as a model reaction.

Solid‐state reactions of vanadium(v) phosphates in the presence of ammonia

The structural transformation of vanadyl(V) phosphate dihydrate (V V OPO 4 ·2H 2 O, V/P = 1) and a Ga-containing vanadyl(V) phosphate dihydrate ([Ga(H 2 O)] x (V V O) 1–x PO 4 ·2H 2 O, V/P = 1 – x) in the presence of ammonia have been investigated. V V OPO 4 ·2H 2 O was transformed in the presence of an NH 3 –air–water vapour flow at temperatures of ca. 670 K mainly into distorted (NH 4 ) 2 (V IV O) 3 (P 2 O 7 ) 2 (V/P = 0.75). Additionally, the generation of crystalline V 2 O 5 (up to 10%) was observed, mainly representing the remainder of the vanadium of the precursor compound. [Ga(H 2 O)] x (V V O) 1–x PO 4 ·2H 2 O was synthesised by the replacement of a number of the (V V O) 3+ groups of the parent V V OPO 4 ·2H 2 O by [Ga(H 2 O)] 3+ . A similar solid-state transformation was observed when this material was treated under the same gas flow but, besides crystalline V 2 O 5 , a significant proportion of GaPO 4 was also formed. The heterogeneous catalytic ammoxidation of toluene to benzonitrile was applied as a test reaction in the temperature range 570–625 K for the evaluation of catalytic performance. The V V OPO 4 ·2H 2 O derived catalyst revealed an improved catalytic activity in comparison to similar catalysts obtained by the transformation of V(IV)-containing precursor compounds. It seems very likely that this is due to the existence of a proportion of crystalline V 2 O 5 . The catalytic activity of the Ga-containing material is much lower, but is still in the range of the V(IV)-derived catalysts. Characterisation of the parent samples and the generated products (after equilibration as well as catalytic runs) carried out by means of XRD, XPS and FTIR spectroscopy.