Ammoxidation Of Ethane Research Articles

Formation of acetonitrile and ethylene from activation of ethane over cobalt-exchanged aluminosilicates: Active sites and reaction pathways

Cobalt-exchanged ZSM-5 catalyzes the ammoxidation of ethane (2 C2H6 + 3 O2 + 2 NH3 → 2 CH3CN + 6 H2O), yet the active form of cobalt, the role of the Brønsted acidic zeolite, and the reaction pathways remain elusive. Comparisons of rates and selectivities of reactions at distinctive cobalt sites (Co2+ at Al pair sites (CoZ2), Co3O4, cobalt phyllosilicate, and cobalt aluminate) suggest sites that form from CoZ2 provide the greatest rates and selectivities for CH3CN and C2H4 formation. Significantly, we revealed a large fraction of CH3CN appears to form directly through a trimolecular reaction at cobalt ions without desorption of intermediates. In parallel, a more conventional sequential pathway dehydrogenates C2H6, aminates C2H4, and oxidatively dehydrogenates C2H5NH2 to produce CH3CN. Combined rate measurements with temperature programmed reactions and in situ infrared spectra demonstrate that O2-derived intermediates at cobalt ions initiate both reaction sequences by abstracting H-atom from C2H6.

Ethane Ammoxidation over Sn/H-Zeolite Catalysts: Toward the Factors Contributing to the Yield of Acetonitrile.

Different Sn/H-zeolite (β, MOR, SSZ-13, FER, and Y zeolite) catalysts are prepared with the improved impregnation method. The effects of reaction temperature and the composition of the reaction gas (ammonia, oxygen, and ethane) on the catalytic reaction are investigated. Adjusting the fraction of ammonia and/or ethane in the reaction gas can effectively strengthen the ethane dehydrogenation (ED) route and ethylamine dehydrogenation (EA) route and inhibit the ethylene peroxidation (EO) route, whereas the adjustment of oxygen cannot effectively promote acetonitrile formation because it cannot avoid enhancing the EO route. By comparing the acetonitrile yields on different Sn/H-zeolite catalysts at 600 °C, it is revealed that the ammonia pool effect, the residual Brönsted acid in the zeolite, and the Sn-Lewis acid synergistically catalyze ethane ammoxidation. Moreover, a higher L/B ratio of the Sn/H zeolite is beneficial to the improvement of acetonitrile yield. With a certain application potential, the Sn/H-FER-zeolite catalyst shows an ethane conversion of 35.2% and an acetonitrile yield of 22.9% at 600 °C; although a similar catalytic performance was observed on the best Co-zeolite catalyst in literature, the Sn/H-FER-zeolite catalyst is more selective to ethene and CO than the Co catalyst. In addition, the selectivity to CO2 is less than 2% of that on the Sn-zeolite catalyst. This may be attributed to the special 2D topology and pore/channel system of the FER zeolite, which guarantee an ideal synergistic effect of the ammonia pool, the residual Brönsted acid in the zeolite, and the Sn-Lewis acid for the Sn/H-FER-catalyzed ethane ammoxidation reaction.

Highly selective Sn/HZSM-5 catalyst for ethane ammoxidation to acetonitrile and ethylene

The ammoxidation of ethane over the conventional metal oxide and Co/zeolite catalysts faces the challenge of inadequate selectivity owing to the formation of CO2, N2, and NOx. Here we report that Sn modified ZSM-5 catalyst is highly selective for ethane ammoxidation. The C- and N-based acetonitrile selectivities are up to 80 % and 70 % over the Sn/HZSM-5, but they are below 60 % and 20 %, over the Co/HZSM-5. We suggested that the Sn (IV) cations are the critical active sites for acetonitrile formation, and the presence of nanosized SnOx particles favors CO2 formation. Transient kinetics analysis suggested that ethane ammoxidation over the Sn/HZSM-5 follows the same reaction pathways as that over the Co/HZSM-5. However, the surface abundancy of the reaction intermediates over these two catalysts is different, which could be the reason for the enhanced selectivity over the Sn (IV) cations in contrast to the Co (II) cations in the ZSM-5 zeolite.

Acetonitrile Synthesis via Ammoxidation: Mo/zeolites Catalysts Screening

Mo/zeolites catalysts were investigated in ethane and ethylene ammoxidation into acetonitrile. The catalysts were prepared either in solid‒solid or liquid‒solid interface after varying different parameters. The stabilization of Mo species upon the exchange is dependent on the hydrophilic/hydrophobic character of the zeolite and the type of Mo precursor. In fact, zeolites with low Si/Al molar ratios should be avoided due to their higher dehydration enthalpy values (Δdehyd.H). On the other hand, the use of MoOCl4, Mo(CO)6 and MoCl3 precursors and zeolites with high Si/Al ratios led to inefficient [Mo7O24]6‒ species and amorphous MoO3 which catalyzes the combustion reaction. Nevertheless, the use of MoCl5, MoO3 and MoO2(C5H7O2)2 led to promising activities. In catalysis, [MoO4]2‒ species are required to activate C2H6 into C2H4, while [MoxO3x+1]2‒ (x =1, 2) species catalyze the ammoniation of C2H4 and the ethylamine dehydrogenation into CH3CN. Interestingly, active catalysts could be obtained by humid impregnation and a simultaneous oxidative treatment. Such a treatment improves the dispersion state of crystalline MoO3, which activate ethane molecules. It is judicious to perform C2H6 oxidative dehydrogenation before ammoxidation since the interference between the different investigated parameters could be noted.

Ammoxidation of Ethane to Acetonitrile and Ethylene: Reaction Transient Analysis for the Co/HZSM-5 Catalyst.

Ethane ammoxidation to acetonitrile and ethylene over the Co/HZSM-5 catalysts was revisited based on both transient and steady-state performance evaluation to elucidate the structure/reactivity relationships. We suggested that the exchanged Co2+ cation encapsulated in the zeolite favors the formation of acetonitrile and ethylene, whereas nanosized cobalt oxide particles without close proximity with the HZSM-5 only favor CO2 formation. Excess Brønsted acid sites of the zeolites may act as a reservoir for NH3, which inhibits the CO2 formation through the NH3-mediated oxidative dehydrogenation mechanism. According to the transient kinetic analysis, the time constants τ from the back-transient decay for NH3 and CO2 are both 7.7 min, which decreased to 2.7 min for acetonitrile and further decreased to 3–4 s for ethane, ethylene, and O2. Assuming first-order reaction kinetics, the rate constants for the formation of acetonitrile and CO2 are 0.37 and 0.13 min–1, respectively, from their corresponding reactive intermediates.

Catalytic behaviour of molybdenum-based zeolitic materials prepared by organic-medium impregnation and sublimation methods

Molybdenum-exchanged ZSM-5 catalysts were tested in ethane ammoxidation into acetonitrile at 500 °C and at a very low contact time (0.08 s). The solids were prepared by sublimation, impregnation in CCl4 and solid-state ion exchange methods. The hydration state of the zeolite strongly affected the nature of MoCl5 and Mo(CO)6 decomposition products and, therefore, the concentration of stabilized Mo species in the final catalysts. In effect, using dehydrated ZSM-5 zeolite, the sublimation of MoCl5 led to the most active catalyst (TOF = 8.78 s−1) due to the presence, essentially, of [MoO4]2− (77%) and [Mo2O7]2− (10%) besides less-active crystalline MoO3 (12%) and traces of heptamers. However, the impregnation and the solid-state ion exchange of MoCl5 as well as the sublimation of Mo(CO)6 led to less-active catalysts owing to the presence of inefficient MoO3 oxide phase. In fact, moderate concentrations of crystalline MoO3 should coexist with [MoO4]2− species in order to activate C2H6 into C2H4 instead of enhancing the deep hydrocarbons’ oxidation.

Improvement of the conventional preparation methods in Co/BEA zeolites: Characterization and ethane ammoxidation

Ethane ammoxidation into acetonitrile (AN) was successfully catalyzed by Co/BEA solids issued from improved preparation methods. The catalyst issued from the impregnation of [Co(NH3)6]2+ complex into BEA zeolite is very active in ethane ammoxidation (48% of ethane conversion and 90% of selectivity towards AN at 450 °C). Over this solid, the combustion of hydrocarbon molecules into CO2 was inhibited since the inefficient oxidic species occupied hidden sites inside the zeolite host as demonstrated by the CO-TPR experiments. However, the exchange of [Co(OH2)6]2+ complex, accompanying the conventional impregnation and solid-state ion exchange, favoured the formation of Co3O4 oxide species, which restricted the ethane conversion into AN during ammoxidation. Nevertheless, the exchange of aqua complex in ammonia stream inhibited the formation of oxidic species and led to efficient ammoxidation catalysts. Meanwhile, ammonia favoured the formation of Co4N (detected by XRD, TPR and TPD) to the detriment of bare Co2+ ions. In this context, very low amounts (∼0.05 mmol g−1) of bare Co2+ ions, sited in the β-sites, are sufficient to catalyze ammoxidation at 450 °C. However, beside their amount, the stability of these ions is a crucial parameter to be considered and studied by FTIR upon CO adsorption at −196 °C.

Physicochemical and catalytic properties of over- and low-exchanged Mo‒ZSM-5 ammoxidation catalysts

A series of Mo/ZSM-5 catalysts prepared by solid-state ion exchange at different Mo/Al molar ratios were characterized and tested in ethane and ethylene ammoxidation into acetonitrile. It has been concluded that the low-exchanged solid (Mo/Al = 0.2) stabilized MoO3, [Mo2O7]2− and [Mo7O24]6− species. However, besides these species, the solids prepared at Mo/Al = 0.5 and 1.5 stabilized [MoO4]2−. Nevertheless, only MoO3 and [Mo2O7]2− species were stabilized at Mo/Al = 1. The study performed by diffuse reflectance spectroscopy allowed the determination of the molar fraction relative to each Mo specie and, therefore, the calculation of the turnover frequency values. The catalytic activities of the various solids have been classified by taking into consideration the inefficiency of Al2(MoO3)4 phase, which inhibits the diffusion of reactants molecules towards the active sites, and amorphous MoO3 which catalyzes the undesired hydrocarbons’ combustion. However, [MoO4]2‒ species are efficient in the oxidative dehydrogenation of C2H6 into C2H4, while dimeric species catalyze the ammoxidation.

Over– and low–exchanged Co/BEA catalysts: General characterization and catalytic behaviour in ethane ammoxidation

Ethane ammoxidation into acetonitrile was successfully catalyzed between 380 and 450°C over Co/BEA solids issued from solid–state ion exchange with different metal loads. During the preparation, in the presence of NH4+–Beta zeolite (Si/Al=12.5), CoCl2 precursor decomposes under helium stream without evaporation, leading to the stabilization of bare Co2+ at the exchange cationic sites as revealed by spectroscopic tools. However, at 4.13 and 5.63wt.% of Co (Co/Al molar ratio equal to 0.75 and 1, respectively), the corresponding solids stabilized, besides bare β–type Co2+ ions, Co oxo [Co(III)O]+ species, revealed by H2–TPR. These species exhibit highest catalytic activity on the basis of TOF calculation and play a key role in ethane ammoxidation into acetonitrile at low temperature (380°C). Nevertheless, the excess of cobalt is transformed into Co3O4 oxide (as revealed by XRD and TPR experiments) which catalyzes the hydrocarbons combustion into CO2.

Light hydrocarbons ammoxidation into acetonitrile over Mo–ZSM-5 catalysts: Effect of molybdenum precursor

The catalytic performances of Mo–ZSM-5 catalysts (6 wt% of Mo, Si/Al = 26), prepared by solid–state ion exchange from different molybdenum precursors, were evaluated in the ammoxidation of ethane and ethylene into acetonitrile in the temperature range of 425–500 °C. The catalysts were characterized by chemical analysis, N2–physisorption, XRD, FTIR, 27Al MAS NMR, DRIFT, UV/Vis DR, Raman, and XPS spectroscopies, NH3–TPD, and H2–TPR. Starting from MoCl5, Mo is stabilized in the dimeric form and only small crystallites of MoO3 were formed. However, the oxygenated precursors, i.e. MoO3, (NH4)6Mo7O24·4H2O, and MoO2(C5H7O2)2 are favorable for the agglomeration of amorphous MoO3. In the studied reaction, the required active sites are (MoO4)2–, (Mo2O7)2–, (Mo7O24)6–, and small crystallites of MoO3, while the undesired amorphous MoO3 inhibited the diffusion of reactants to the active sites and/or enhanced the hydrocarbon combustion. Upon the catalyst issued from MoCl5, UV/Vis DRS revealed the abundance of dimeric Mo at the detriment of mono and polymeric species. The former specie played a key role in the ammoxidation since the two Mo atoms in (Mo2O7)2– are spatially too separated and the steric hindrance between intermediate molecules is therefore limited.

Propane Versus Ethane Ammoxidation on Mixed Oxide Catalytic Systems: Influence of the Alkane Structure

Catalysts from three different catalytic systems, Ni–Nb–O, Mo–V–Nb–Te–O and Sb–V–O, have been prepared, characterized, and tested during both ethane and propane ammoxidation reactions, in order to obtain acetonitrile and acrylonitrile, respectively. The catalytic results show that Mo–V–Nb–Te–O and Sb–V–O catalyze propane ammoxidation but are inactive for ethane ammoxidation whereas Ni–Nb–O catalysts catalyze both, ethane and propane ammoxidation. The activity results, and the characterization of fresh and used catalysts along with some data from previous studies, indicate that the ammoxidation reaction mechanism that occurs in these catalytic systems is different. In the case of Mo–V–Nb–Te–O and Sb–V–O, two active sites appear to be involved. In the case of Ni–Nb–O catalysts, only one site seems to be involved, which underlines that the mechanism is different and take place via a different intermediate. These catalysts activate the methyl groups in ethane, on the contrary, neither ethane nor ethylene appear to adsorb on the Mo–V–Nb–Te–O and Sb–V–O active sites.

Ammoxidation of ethylene to acetonitrile over vanadium and molybdenum supported zeolite catalysts prepared by solid-state ion exchange

This work reports on the investigation of the influence of the parent zeolite topology (MFI, MOR and USY) on the ammoxidation of ethylene to acetonitrile over vanadium and molybdenum oxide supported zeolite catalysts. The physico-chemical properties were investigated by several characterization techniques such as XRD, N2-adsorption, 27Al MAS NMR, TEM, XPS, DR UV–vis, Raman and DRIFT spectroscopies and H2-TPR. From the catalytic results, V and Mo oxide species in USY and MOR zeolites led to less active catalysts when compared to MFI structure. These results suggest that the catalytic performances depend strongly on the zeolite structure and thus, the size of the formed metal oxide particles. The catalytic activity and selectivity are controlled by the porous structure and the chemical state of V and Mo species. The extent of dispersion and reducibility of supported M-Ox (MV or Mo) species are governed by the chemical identity of the support as detected by TPR analysis and optical absorption spectroscopy. The good catalytic performance of MFI-type zeolite might be related to the high dispersion of metal oxide species and to the internal pore space which permits an effective accessibility of the reactants.

Synergy between vanadium and molybdenum in bimetallic ZSM-5 supported catalysts for ethylene ammoxidation

Ammoxidation of ethylene to acetonitrile was studied on V/ZSM-5, Mo/ZSM-5 and V–Mo/ZSM-5 catalysts prepared by a solid-state ion exchange method.

In Silico Design of Highly Selective Mo-V-Te-Nb-O Mixed Metal Oxide Catalysts for Ammoxidation and Oxidative Dehydrogenation of Propane and Ethane

We used density functional theory quantum mechanics with periodic boundary conditions to determine the atomistic mechanism underlying catalytic activation of propane by the M1 phase of Mo-V-Nb-Te-O mixed metal oxides. We find that propane is activated by Te═O through our recently established reduction-coupled oxo activation mechanism. More importantly, we find that the C-H activation activity of Te═O is controlled by the distribution of nearby V atoms, leading to a range of activation barriers from 34 to 23 kcal/mol. On the basis of the new insight into this mechanism, we propose a synthesis strategy that we expect to form a much more selective single-phase Mo-V-Nb-Te-O catalyst.

Performance of NiO and Ni–Nb–O active phases during the ethane ammoxidation into acetonitrile

There is a need to develop catalysts for the direct ammoxidation of ethane to acetonitrile. This work reports a series of bulk Ni–Nb–O catalysts with increasing niobium content, analysing the structure and the effect on acidity and redox properties and how these relate to Ni–Nb interaction. Niobium doping affects NiO lattice leading to progressively smaller unit cell sizes. The Nb/Ni atomic ratio determines the formation of two Nb–Ni–O mixed phases identified by HRTEM, and which Raman bands are identified near 850 cm−1 and 800 cm−1 for a Nb-poor phase and near 800 cm−1 for a Nb-rich Ni–Nb–O one.

Cr–ZSM-5 catalysts for ethylene ammoxidation: Effects of precursor nature and Cr/Al molar ratio on the physicochemical and catalytic properties

Cr–ZSM-5 catalysts prepared by solid-state ion exchange with different Cr/Al molar ratios were characterized and tested in ethylene ammoxidation to acetonitrile in the temperature range 425–500°C. Starting from Cr acetate like precursor and lower Cr load (Cr/Al=0.5), small amount of chromate species, sited inside the zeolite matrix, exhibited lower catalytic activity in ammoxidation (12.5% of CH3CN yield). Increasing the Cr amount (Cr/Al=1) does not significantly improve the catalytic properties since agglomerates of amorphous Cr2O3 inhibited the accessibility of chromate species to the reactants. However, at higher Cr amount (Cr/Al=1.5), crystalline Cr2O3 particles reduced the diffusion of reactants to the internal active sites. Starting from Cr chloride like precursor, the corresponding solids are low-exchanged since CrCl3 evaporates. At Cr/Al molar ratios of 0.5 and 1, chromate species, sited in the exchange sites, exhibited interesting catalytic properties (18.5 and 25% of CH3CN yield at 500°C, respectively). Nevertheless, at higher metal load (Cr/Al=1.5), the excess of Cr transforms into crystalline Cr2O3 which inhibits the diffusion of reactants to the active sites and enhances the hydrocarbon oxidation mainly at low temperatures. As ethylene ammoxidation required ammonia activation over (poly)chromate sites, a possible pathway of such a step has been proposed on the basis of NH3-TPD results.

Niobia-Supported Nanoscaled Bulk-NiO Catalysts for the Ammoxidation of Ethane into Acetonitrile

Two series of supported nanoscaled NiO catalysts have been prepared using Nb2O5 supports with different surface areas. NiO nanoparticles interacting with Ni–O–Nb mixed phases are active and selective for ethane ammoxidation; both are required in order to have an active and selective catalyst. In this paper we report the dispersion of NiO nanoparticles on two Nb2O5 supports in order to generate an active phase for ethane ammoxidation. The use of a support stabilizes NiO nanoparticles, which otherwise would sinter. This work shows how the activity can be modulated by the size of the NiO nanoparticles and by the NiO/Ni–Nb–O ratio, which depends on the nickel coverage and support surface area. The catalysts obtained are very promising for ethane ammoxidation reaction.

Influence of the parent zeolite structure on chromium speciation and catalytic properties of Cr-zeolite catalysts in the ethylene ammoxidation

Cr-zeolites with MFI, BEA, MOR and FAU structures, prepared by solid-state ion exchange, were characterized and tested in C2H4 ammoxidation to acetonitrile in the temperature range 425–500°C. Based on characterization results, chromate and/or polychromate species, oxo-cations and small Cr2O3 oxide clusters played a key role in the ammoxidation of ethylene, while agglomerated Cr2O3 and bare Cr cations should be avoided. Cr ions sited in the sodalite and hexagonal cages of NH4+–Y are not accessible to the reactants while available catalytic sites are poorly active. However, the mesopores of the ultra stable Y zeolite (USY) favor the diffusion of reactants to the clustered Cr oxide. The corresponding catalyst is therefore active, but the presence of octahedral Al species is crucial to the ammoxidation. Cr ions in zeolites beta and mordenite led to less active catalysts when compared to ZSM-5. In zeolite beta, the micropores are small; therefore, pronounced interactions between ethylamine intermediate molecules could discourage the acetonitrile formation. In mordenite, agglomerates of Cr2O3 oxide inhibited the accessibility of active sites to the reactants and enhanced the hydrocarbon oxidation. The catalytic performance of Cr ions in ZSM-5 provided from a synergy of different parameters: structural, textural and the acid strength (Si/Al ratio).

Theoretical and Experimental Study of Light Hydrocarbon Ammoxidation and Oxidative Dehydrogenation on (110)-VSbO4 Surfaces

Density Functional Theory (DFT) calculations have been performed for ethane, ethylene, propane, and propylene adsorption on the rutile VSbO4 structure to uncover the reaction mechanism during both ODH and ammoxidation reactions. This study is complementary to a previous paper in which the adsorption of ammonia on this structure was deeply analyzed, and in addition, experimental activity results for both reactions are shown to complete the theoretical DFT calculations. These results show that ammonia, ethane, and ethylene compete for the same active sites, the former adsorbing strongly. This explains why this catalytic system is not active for ethane ammoxidation; the presence of ammonia blocks the other molecule adsorption sites and prevents ethane oxidative dehydrogenation to ethylene. Such competition does not occur with propane or propene since the adsorption of both ammonia and hydrocarbon is possible at different sites, explaining why this catalytic system is active for propane ODH reaction. During ammoxidation, molecularly dispersed VOx species are able to transform the propane molecule into propylene. Then, intermediate propylene can coadsorb along with ammonia on the trirutile VSbO4, inserting the nitrogen atom that forms acrylonitrile. Calculations show that the adsorptions studied are more favored when the rutile structure presents a cationic vacancy.

Ammoxidation of ethylene over low and over-exchanged Cr–ZSM-5 catalysts

Catalytic performances of Cr–ZSM-5 catalysts (5 wt.% of Cr, Si/Al = 26), prepared by solid-state reaction and aqueous exchange, from Cr nitrate and Cr acetate precursors, were evaluated in the selective ammoxidation of ethylene into acetonitrile in the temperature range 425–500 °C. Catalysts were characterized by chemical and thermal analysis, XRD, N 2 physisorption, 27Al MAS NMR, TEM, UV–vis DRS, Raman, DRIFTS and H 2-TPR. Characterization results shown that solid-state exchange was favorable for Cr 2O 3 formation, while exchanging chromium in aqueous phase led, essentially, to Cr(VI) species. Catalysts were actives and selectives in the studied reaction, and among them, those, prepared from aqueous exchange, exhibited the highest acetonitrile yields (23 ± 0.5%, at 500 °C). Improved catalytic properties can be correlated with the chromium species nature. In fact, mono/di-chromates and/or polychromate species, sited in the charge compensation positions, were definitively shown, as being, the active sites. Furthermore, during solid-state reaction, the agglomeration of Cr 2O 3 oxide should be avoided since these species inhibit the catalyst activity.