Pd/C Catalyzed selective hydrogenation of nitrobenzene to cyclohexanone oxime in the presence of NH2OH·HCl: Influence of the operative variables and insights on the reaction mechanism
Graphical abstract
Consecutive, parallel reactions and equilibria affecting the selectivity in cyclohexanone oxime.
Introduction
Nitrobenzene and in general nitro-aromatic compounds reductions are really important and useful reactions for several industrial processes [1]. Béchamps-Brimmeyr reductions method with Fe-FeCl2 system is still used mainly for the formation of the Black Fe3O4 inorganic pigment as by-product of reduction of the nitrocompound [2]. Other stoichiometric reduction methods of nitrocompounds employ Zn or hydrazine for achieving high selectivity with sensitive substituents or particular synthesis two-step one pot synthesis [1,3].
Hydrogenation, however, is actually the most important and more used method of reduction in aromatic nitro-compounds [1,4]. Among them, nitrobenzene hydrogenation to aniline is one of the most important reaction, since the latter is used as an intermediate in many fields of application, for instance, in the production of isocyanates, rubber processing chemicals, dyes and pigments, agricultural chemicals and pharmaceuticals [1]. Industrial hydrogenation methods employ several type of catalysts and engineering solutions and each one depends on its specific use [5]. For instance, supported Pt, Ni, Pd are commonly employed as catalysts in gas/solid or liquid/solid multiphase reactors, as well as in continuous or in batch operation [6].
The reduction of the nitro-group involves several steps following the well-known Haber reaction path, where nitro-group is reduced step by step to the amine together with the formation of condensation products between nitrosobenzene and phenyl hydroxyl amine (see Scheme 1) [7].
Recently, for the catalytic hydrogenation mechanism, it has been suggested a similar path, but the formation of nitrosobenzene has been related to a surface equilibrium phenyl-hydroxylamine/nitrosobenzene rather than the formation of an actual reaction intermediate (see Scheme 1 Jackson route). Condensation of nitrosobenzene and phenyl hydroxyl amine occurs in the same way of the Haber scheme [8,9]. This has been clearly pointed out by the study of Jackson and co-workers on the kinetics of the nitrosobenzene hydrogenation, they demonstrated indeed a different reaction path for nitrosobenzene hydrogenation with respect to that of nitrobenzene [8].
Among the one-pot multi-steps processes the selective hydrogenation of nitrobenzene to 4-aminophenol is an elegant example of an industrial application, which combine a noble metal catalyzed reaction with a homogeneous acid catalysis [[9], [10], [11], [12], [13]]. Industrially, the reaction is carried out in continuous stirred tank reactor in a biphasic reaction medium, in the presence of aqueous sulfuric acid and a supported Pt catalyst. At first, nitrobenzene is hydrogenated to N-phenylhydroxylamine, and then the acid catalyses the Bamberger rearrangement of the intermediate [3,[9], [10], [11], [12], [13]]. From an environmental point of view, the major drawback of the process is the neutralization of the sulphuric acidic phase, which causes huge corrosion concern, with the consequent increased costs [3,[9], [10], [11], [12], [13]]. However, the use a single reactor system allows an increase of the sustainability of the process compared to other multistep ones [9].
Besides, Corma and Co-workers studied the selectivity of aromatic nitrocompounds reductions and they direct selectivity of Au/supported catalyst towards azobenzene. They recognized responsible for such a selectivity the stabilization of surface nitrosobenzene-Au species [14,15].
The complexity of the reaction scheme of nitrocompounds reduction allow to take into consideration (see Scheme 1) the possibility of designing catalyst or processes that gives, under particular experimental conditions, quite selectively a particular intermediate [9]. For instance, formation of azo-compounds can be achieved by combining the partial reduction of nitrobenzene condensation and dehydration in basic media [16]. Other interesting reduction methods for nitro-aromatic compounds are hydrogen transfer reactions [[17], [18], [19]]. For instance nitrobenzene can be easily reduced to azo, azoxy, anilines, carbamates and ureas by using CO as reductant and Pd(II) complexes as catalysts, the operating conditions will determine the selectivity to each product [17,18]. Under similar reaction conditions in the presence of mild oxidant, nitrobenzene is, at first glance, reduced to an intermediate, which gives aniline in protic solvent, while in aprotic one the main products is isocyanate [17,18]. The reaction gives almost selectively diphenyl urea in the presence of Pd supported catalysts in liquid biphasic conditions and aqueous NaOCHO as reductant [20].
The further reduction of the aromatic amines and of aniline in particular, is relatively less studied compared to the hydrogenation of the nitro-aromatic compounds [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]]. Hydrogenation of aniline to the cyclohexylamine, particularly, has been investigated from long time, and the best catalytic system is mostly Ru supported catalysts at quite high temperature and pressure (373−473 K, 1−10 MPa) [[22], [23], [24], [25], [26]]. Even though, less studied than that of nitrobenzene reduction, the mechanism of this reaction has been already proposed by Greenfield in the early sixty. In this paper, the author has assumed the formation of the imine (Scheme 2) as the key intermediate, which, due to its high reactivity, undergoes facile condensation reactions with all the nucleophiles present in solution, as showed in Scheme 2 [21]. The reactivity of such an intermediate drives the selectivity toward the formation of cyclohexyl amine, di-cyclohexyl amine, and, in the presence of water, to cyclohexanone and cyclohexanol [21,[28], [29], [30], [31]]. The highly reactive imine intermediate has been never isolated in the hydrogenation medium, however, the final products observed strongly supports the goodness of such a hypothesis [21,[28], [29], [30], [31]]. Greenfield suggested also that the imine specie should be only stabilized in the adsorbed state on catalyst surface, and a direct coupling deamination of CHA can occur only at temperature higher than 200 °C on Ni catalysts [21].
Cho and Park have observed a strong influence of the fluid dynamic behavior of the reactor on the selectivity of the aniline hydrogenation to cyclohexyl amine [31]. The authors suggested a non-stationary concentration of hydrogen in the reactor, which cause different rates of reactions in the various steps suggested in the Greenfield mechanism [31]. This mechanism suggests of using the reactivity of the imine intermediate towards nucleophiles; for instance, the substitution with water gives cyclohexanone, a fundamental intermediate, in the production of nylon-6 and in several fine chemistry synthesis [18,[31], [32], [33]]. In fact recently, appears some patents claiming a direct route from nitrobenzene to cyclohexanone oxime by hydrogenation and direct oximation [34,35]. In particular, Corma and coworkers have carried out the hydrogenation of nitrobenzene to cyclohexanone oxime in the presence of hydroxyl amine hydrochloride, in multiphase systems, catalyzed by Pd and Au supported catalysts [35,36]. These authors have claimed that the presence of the two type of catalyst allows high selectivity in the cyclohexanone oxime. The highest selectivity towards the oxime is achieved in the presence of a physical mixture of the two catalysts with respect to the single catalytic system Pd/C or Au/C, moreover, the latter leads to aniline only. Finally, the bimetallic Pd-Au/C catalyst is less selective than the physical mixture of the two catalysts, as well [36]. The mechanism, suggested by the Authors (see Scheme 3), is based on the hypothesis that cyclohexyl amine is in dehydrogenative equilibrium with the cyclohexyl imine, which undergoes to the nucleophilic attack of the aniline, thus forming N-cyclohexylideneaniline. In turn, the latter may undergo nucleophilic attack of the hydroxylamine, or the hydrogenation to phenyl cyclohexyl amine and finally to dicyclohexyl amine. From a mechanistic point of view, the Authors have suggested the involvement of the gold catalyst in the transformation the N-cyclohexylideneaniline, which determines the overall selectivity to the various products [36]. In fact, the nucleophilic attack of the NH2OH catalyzed by Au/C catalyst to the N-cyclohexylideneaniline lead to cyclohexanone oxime and finally its hydrolysis may give cyclohexanone if water is present [36]. Besides, Pd/C is responsible for the formation of the di-cyclohexyl amine for the sequential hydrogenation of N-cyclohexylideneaniline to N-cyclohexyl aniline, which is finally hydrogenated to the di-alkyl amine [36]. In any case, in agreement with the mechanism suggested by Greenfield, the formation of the imine intermediate is, also in this case, an important step, since its reactivity lead to the formation of the N-cyclohexylideneaniline recognized by these Authors as the actual intermediate, whose reactivity catalyzed by Au determines the high selectivity in the cyclohexanone oxime.
In this work, we have studied the selective hydrogenation of nitrobenzene to cyclohexanone oxime with different solvents, in the presence of various hydroxylammonium salts, and catalyzed by a commercial Pd/C catalyst. The research will focus on the influence of the operative variables (Temperature pressure and solvent type), of the amount and type of catalyst on the selectivity to the oxime and to the various byproducts. Besides, a particular attention will be reserved to the study of the likely intermediates found in traces amount in the reaction media as well as the parallel reaction of NH2OH·HCl reduction whose products favors the selectivity in the cyclohexanone oxime.
Section snippets
Materials
Nitrobenzene, aniline, cyclohexylamine, dicyclohexylamine and N-phenylcyclohexylamine (PCA) were all Aldrich products, their purity were checked by the usual methods (melting point, TLC, HPLC, GC and GC–MS) and employed without any purification, acetonitrile HPLC gradient grade was supplied by BDH, 1,4-dioxane, methanol, nitromethane, diethyl ether, dioxane, tetrahydrofuran, dimethylformamide and dimethyl sulfoxide are ACS reagent supplied by Aldrich. N-cyclohexylideneaniline (PCNA) was
Results and discussion
In this study, we will propose a first attempt to analyze a complex multistep multiphase reaction starting from the choice of the catalytic system, in order to verify what metals are able to hydrogenate nitrobenzene (NB) to cyclohexanone oxime (COX) in the presence of NH2OH salts, selectively. Actually, the reaction presents a recognized intermediate, which is aniline (AN), and some compounds, recognized in the reaction media, which are candidates of being intermediate or byproducts [3,36].
Conclusions
All results states the complexity of the reaction path and the large number of factors it is necessary to take into account for optimizing the yield in COX. The new insights for this process are the importance of the secondary reactions that is the reduction of NH2OH·HCl and all nucleophiles equilibria. In fact, its reduction gives NH4Cl and water, which, both, influences the selectivity towards COX. Aniline is the key intermediate and at first sight, its hydrogenation gives a lower yield in
Author statement
All authors contributed to investigation, LR an AV had the major role on funding acquisition, LR and AV had the major role in all other aspects regarding the manuscript.
Acknowledgement
University Ca’ Foscari of Venice, Adir 2018.
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