Evidence for the participation of lattice nitrogen from vanadium aluminum oxynitrides in propane ammoxidation
Introduction
The direct conversion of alkanes to high-value products for the chemical industry constitutes an arduous and stimulating scientific and technological challenge. Among the most significant industrial applications in this field is the production of acrylonitrile (ACN) through the propane ammoxidation process. This chemical intermediate is used extensively in the production of nitriles, acrylic fibers, resins, rubbers, and other specialized substances. However, until now, about 95% of worldwide ACN production, which is estimated to about 5 million metric tons per year, is obtained through the propene ammoxidation process [1].
The idea of replacing olefins with alkanes in the ammoxidation process dates back to the early 1970s. The advantages are essentially (i) the price of propane as a raw material is about 50–65% lower than that of propene (PE); (ii) the lower risk of a propene shortage due to its increasing consumption in polymerization processes; and (iii) the increasing worldwide demand for ACN, plastics, synthetic fibers, and other derived products.
However, two fundamental problems arise in the transformation of alkanes: (i) alkane activation is more difficult, requiring severe operating conditions and very active, selective, and stable catalysts; and (ii) reaction products are generally less stable than the reactants, such that they can be decomposed during the reaction, leading to the formation of undesirable oxygenated compounds, such as acrylamide, acrylic acid, carbon and nitrogen oxides, etc.
To surpass the first problem, many catalysts have been proposed. According to the literature, they can be classified into two main categories: (i) vanadium-antimonates, VSbxMyOz, where M is elements used as promoters, such as W, Te, Nb, Sn, Bi, Al, and Ti [2], [3]; and (ii) vanadium-molybdates, VMoxMyOz, where M is most often Bi or Te [4]. Other catalytic systems based on Ga–Sb oxides modified with W, Ni, or P and vanadyl pyrophosphates [5], [6] were studied, but they were not so promising for the ammoxidation of alkanes, especially of propane.
A promising new catalytic material consists of vanadium aluminum oxynitrides. VAlONs are obtained by partial substitution of nitrogen for oxygen in the structure of the V–Al mixed oxides during a nitridation process [7]. These materials are amorphous and behave as bifunctional catalysts. Their active sites are essentially made up of redox sites () and basic sites (, , N3−) with different strength distributions. It was found that the catalytic performances in propane ammoxidation of these new materials strongly depend on the V/Al ratio, coprecipitation pH, V concentration in the solution, and the nitridation protocol (temperature, nitridation mixture, time) [8], [9]. The VAlON system shows a propane conversion of 55% and an acrylonitrile selectivity of 60% in steady-state conditions for a V/Al of 0.25 prepared at pH 5.5 and a V concentration in solution of 0.020 mol/l.
Despite this promising behavior, the selectivity of VAlONs for acrylonitrile from propane is still lower than that obtained with propene. Therefore, further improvement of the catalyst formulation is necessary. To realize this objective, knowledge of the details of the reaction mechanism and the nature of the active sites can be very helpful. Until now many conflicting mechanisms have been proposed for ammoxidation catalysts, involving different intermediates. On VSbxMyOz catalysts, ACN formation is suggested to occur sequentially via propene and acrolein (ACO) as intermediates [10], [11], [12]. For the (VO)2P2O7 catalysts, Centi and Perathoner [13] proposed the formation of propylamine or acrylate species as intermediates. Direct propane-to-ACN transformation was observed with the use of vanadium aluminum oxynitride catalysts [8].
However, the most controversial aspect was the nature of the N-insertion species. Data from the literature indicate that different nitrogen species are active in ammoxidation, depending on the nature of the catalyst. groups are reported to be active in nitrogen insertion on Ga–Sb oxides [14], and , NH3ads, , and NH groups are suggested to be active on vanadyl pyrophosphate [5]. NH3ads and have been suggested by Zanthoff et al. [15] as the N species to be inserted, and amido- and imido-like species have been proposed by Centi and Perathoner [13]. Martin et al. [16] reported isotopic pulse experiments indicating the participation of ammonium ions in the N-insertion step during benzonitrile formation from toluene over alpha-(14NH4)2[(VO)3(P2O7)2].
The aim of this study is to investigate the role of ammonia and oxygen in the reaction mechanism of the ammoxidation of propane on VAlONs. Labeled ammonia, 15NH3, was used to distinguish between adsorbed ammonia and lattice nitrogen. The influence of the V/Al ratio and nitrogen content on catalytic performance was investigated. A transient technique at low pressure, temporal analysis of products (TAP) [17], was used to minimize the influence of homogeneous gas-phase reactions, which are well known to influence ammoxidation pathways [1].
Section snippets
Catalyst preparation
V–Al oxide precursors with two different V/Al ratios were prepared by coprecipitation of aluminum nitrate and ammonium meta-vanadate solutions at a pH of 5.5, constant vanadium concentration (0.062 M), and variable aluminum concentration (0.250 M for V/Al ratio =0.25, and 0.083 M for V/Al ratio =0.7). These two samples are referred to here as VAlO025 and VAlO07. The ammonium meta-vanadate salt was dissolved in hot water (333 K) with stirring, and then nitric acid was added progressively to
Results
The vanadium aluminum oxide precursors are totally amorphous according to XRD and have a BET surface area of 225 m2/g in the case of VAlO07 and 250 m2/g for VAlO025. After the nitridation process, the surface area of the solids decreased by 40%. The BET specific surface area, total nitrogen, and surface nitrogen content after nitridation are presented in Table 1.
Before each experiment, we pretreated non-nitridated catalysts by pulsing oxygen at reaction temperature to create a completely
Discussion
The VAlO025 precursor behaves as an active (propane conversion amounted to 44%) and relatively selective catalyst for the oxidative dehydrogenation of propane. It was reported that its activity is controlled by the redox capacity of V ions while the selectivity is achieved by site isolation (due to the presence of alumina) [18]. XPS measurements revealed a high V5+/(V4+ + V5+) ratio (0.83) and a low V/Al surface ratio of 0.14 [8], [18], which, indeed, explains the relatively high activity and
Conclusions
Lattice NHx species ( or 2), formed most probably by irreversible reaction of NH3 with the catalysts, are involved in the formation of ACN, whereas coordinated NH3 or M, formed by weak adsorption of ammonia on the surface, is involved in the nonselective pathway. Oxygen adsorbed species are involved in the nonselective reactions catalyzed by the surface. A double Mars–van Krevelen mechanism, involving both oxygen and nitrogen from the lattice, is proposed. ACN is formed consecutively via
Acknowledgments
This paper is dedicated to the memory of Prof. Paul Grange. This work was performed in the framework of a Concerted Research Action (GOA) financed by Ghent University, and of the Belgian Programme on Interuniversity Poles of Attraction initiated by the Belgian State, Prime Minister's Office, Science Policy Programming. Co-authors from the Catholic University of Louvain gratefully acknowledge the financial support of the Communauté Française de Belgique (Action de recherche concertée) and of the
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