Catalytic Properties of V–Ti–O Catalysts in the Oxidation and Ammoxidation of o-xylene, Acenaphthene and β-picoline

Catalytic properties of vanadium–titanium oxide catalysts in the oxidation and ammoxidation of o-xylene, acenaphthene and β-picoline have been studied. Methods of preparation of catalysts with varying amounts contents of V2O5 have been described. It has been shown that activity and selectivity in oxidation of organic compounds of different nature depends on the V2O5 content and the ratio of the oxides in the solid solution. Maximum selectivity in relation to phthalic and naphthalic anhydrides, naphthalimide and nitrile of nicotinic acid with the maximum conversion is observed with the catalysts having 4–6 wt.% of V2O5. Catalysts with other compositions showed lesser conversion of the initial substance. A catalyst, representing VO2–TiO2 solid solution, exhibits the least activity and selectivity among the tested samples for oxidation of o-xylene and ammoxidation of β-picoline. For oxidation and ammoxidation of acenaphthene, the least activity and selectivity are exhibited by a catalyst, containing alongside with the solid solution no more than 3% by weight of V2O5. Catalysts with an equal V2O5 amount, but differing one from the other by a composition of a solid solution, display different catalytic activity in o-xylene oxidation and β-picoline ammoxidation. Enrichment of solid solution with VO2 or a decrease in the amount of the latter in the content decrease activity and selectivity of a catalyst.


Introduction
Catalysts on the basis of vanadium pentoxide, modified by oxides of transition and other metal oxides, display high activity and selectivity for oxidation and ammoxidation of alkylbenzenes, polycyclic hydrocarbons and pyridine bases [1][2][3][4]. It is known [5], that in a series of composition of vanadium-titanium catalysts, two-phase system is formed, consisting of a VO 2 -TiO 2 solid solutions and V 2 O 5 . The amount of the latter is determined by the initial composition of the catalyst and preparation conditions employed. The literature data on the effect of the composition of V 2 O 5 -TiO 2 catalysts on their catalytic properties in oxidation and ammoxidation of acenaphthene and β-picoline show the main concern on the influence of the ratio of the initial oxides on observed activity and selectivity [3,4]. The absence of information of catalytic properties of VO 2 -TiO 2 solid solution in oxidation reactions [6] and the effect of varying V 2 O 5 content on different compositions of VO 2 -TiO 2 solid solution on oxidation of o-xylene, acenaphthene and β-picoline have motivated an interest to this investigation presented in this paper.

Experimental
Catalysts were prepared from reagent-grade V 2 O 5 and TiO 2 . With this purpose the mixtures of oxides of vanadium (V) and titanium (IV) in the molar ratio of 1:16 and 1:32 were thoroughly mixed, formed in tablets, calcined in a continuous air feed furnace zone, then cooled and ground in granules of the dimensions 3-5 mm. The temperature (500-1000°C) and time of calcination (2-10 h) were chosen experimentally, while analyzing them repeatedly for vanadium pentoxide content in the heat treated samples. The preparation methods employed allowed the variation of V 2 O 5 content, thus permitting to employ them in catalyst evaluation with definite V 2 O 5 content. The V 2 O 5 content in the catalyst solid samples was determined by IR-spectroscopy using a SPECORD spectrophotometer in the form of KBr discs [6].
An X-ray phase analysis of the catalysts was car- ried out on a diffractometer "DRON-0.5" with Co K α radiation. The activity was determined in a gradient-free reactor of an ideal shifting [7]. Experiments for the study of oxidation of the initial organic compounds were carried out in a flow-through reactor with a quartz reaction tube with the length of 130 mm and diameter of 20 mm and in a reactor made of stainless steel with the length of 1000 and diameters of 20 mm respectively. The reaction products were analyzed by chromatography with the use of a flameionizing detector (FID) and TCD.

Results and discussion
The variation of Catalytic activity of vanadiumtitanium catalysts V 2 O 5 ⋅16TiO 2 which differ in the stationary state by a vanadium pentoxide content, and therefore by the ratio of V 2 O 5 and VO 2 -TiO 2 , in oxidation of o-xylene is shown in Fig. 1. as a function of V 2 O 5 content. The catalyst, representing a solid solution VO 2 -TiO 2 possessed the least activity for this reaction. On a vanadium-titanium catalyst, containing apart from a solid solution 0.7-1.0 wt.% of V 2 O 5 , o-xylene oxidation rate sharply increased, and was raising smoothly with the further increase of V 2 O 5 amount in the catalyst, reaching a maximum value with the V 2 O 5 concentration of 4 wt.%, and then was slowly decreasing. The variation of selectivity for phthalic anhydride formation also showed a similar trend. Catalysts with the highest activity, containing 4-5 wt.% of V 2 O 5 possessed a maximum selectivity in o-xylene oxidation.
Similar behaviour was also observed on these catalysts for the oxidation of acenaphthene and the corresponding data are given in Table 1. Vanadiumtitanium catalysts, possessing 5.0-6.5 wt.% of V 2 O 5 and solid solution, displayed the higher activity and selectivity. The yield of naphthalic anhydride on these catalysts reached 76-78% with the full conversion of initial hydrocarbon. With the content of vanadium oxide (V) in the catalyst up to 1.5-3.0 wt.%, as a result of its reduction to VO 2 and enrichment of the VO 2 -TiO 2 solid solution by the latter, a decrease in the acenaphthene conversion was observed as also a sharp decrease in the yield of naphthalic anhydride and acenaphthelene.
Change of the concentration of vanadium oxide (V) in the catalyst V 2 O 5 ⋅16TiO 2 influenced its catalytic activity also in acenaphthene ammoxidation. At The feed of o-xylene -188 g, air -5000 l⋅l -1 cat.⋅h -1 , T -400°C, contact time -0.14 s. 1 -The rate of o-xylene oxidation, 2 -selectivity to phthalic anhydride.   Study of ammoxidation reaction of β-picoline under comparable conditions on different samples of vanadium-titanium catalyst, which contained an initial composition 12.4 wt.% of V 2 O 5 , but differed one from the other in the stationary state by vanadium (V) oxide content and its ratio with a VO 2 -TiO 2 solid solution, testified to the fact that activity and selectivity of the samples depended on V 2 O 5 amount in the catalyst (Fig. 2). On the VO 2 -TiO 2 solid solution, in absence of V 2 O 5 in the catalyst, conversion of 3-methylpyridine didn't exceed 65%, with 40% selectivity of the formation of nicotinic acid nitrile. The presence of V 2 O 5 and an increase in its concentration resulted in an increase in activity and selectivity, especially noticeable in case of V 2 O 5 content more than 4 wt.%. Maximum conversion of β-picoline (95%) and selectivity of nicotinic acid nitrile (93-95%) were observed on catalysts, containing VO 2 -TiO 2 solid solution and 5.6-6.0 wt.% of V 2 O 5 .
It is noteworthy, that vanadium-titanium catalysts are less sensitive to the changes of the reaction conditions employed within certain limits. Thus, for example, durable tests (300 h) of vanadium-titanium catalyst, containing 5.6 wt.% of V 2 O 5 ammoxidation of 3-methylpyridine with oxygen excess in the reaction mixture showed that the yield of nicotinic acid nitrile remained practically constant and constituted 91.5 mol.% with 96.2% conversion of β-picoline. The same catalyst with a decrease of oxygen excess due to an increase in the concentration of the initial pyridine base with the molar ratio of β-picoline, oxygen, ammonia and water, equal to 1:35:8:47, provided 3-methylpyridine conversion at the level of 97.7% with the yield of nicotinic acid nitrile, equal to 93-95% from the theoretical one. An analysis of the catalyst showed that in the process of a durable exploitation, V 2 O 5 content therein did not change significantly.
X-ray phase analysis of vanadium-titanium cata-lysts, working in the conditions of oxidation and ammoxidation of o-xylene, acenaphthene and β-picoline, testified to the identity of their compositions. The catalysts represented a two-phase system, containing V 2 O 5 and VO 2 -TiO 2 solid solution. Reduction of V 2 O 5 into VO 2 proceeded both in the process of catalyst preparation under the influence of temperature, and under the conditions of oxidation reaction. The formed VO 2 wasn't determined by X-ray phase analysis in V-Ti-O catalysts, since it was directly included in the catalyst composition, forming with TiO 2 (rutile) a continuous series of solid solution of VO 2 -TiO 2 . It was proved by the comparison of lattice parameters of vanadium catalysts and specially synthesized solid solutions and the agreement with the known literature data [9]. The presence of V 2 O 5 in the catalyst was indicated by the IR-spectral data, possessing an absorption band at 1020 cm -1 , which was characteristic for V=O.
From the data presented in this paper it is seen, that the amount of vanadium (V) oxide in the catalyst V 2 O 5 ⋅16TiO 2 plays a decisive role in the observed activity and selectivity. The composition of a solid solution is also of importance, since a change in V 2 O 5 content in the catalyst immediately entails a change therein of not only the ratio between V 2 O 5 and VO 2 -TiO 2 , but also the composition of a solid solution. Enrichment of VO 2 solid solution or a decrease in the amount of the latter in its composition influences  the catalytic activity of the catalyst. This was confirmed by the results of o-xylene oxidation and βpicoline ammoxidation on the catalysts V 2 O 5 ⋅16TiO 2 and V 2 O 5 ⋅32TiO 2 , which contained after the preparation an equal V 2 O 5 amount (4.2 wt.%), but differed by the composition of a solid solution. Upon the oxylene oxidation on the catalyst V 2 O 5 ⋅32TiO 2 , in which a solid solution contained more TiO 2 and less VO 2 than the catalyst V 2 O 5 ⋅16TiO 2 , conversion of the initial substrate and the yield of phthalic anhydride were lower by 10-15% in comparison with the catalyst V 2 O 5 ⋅16TiO 2 , with an equal V 2 O 5 content therein ( Table 2). Upon β-picoline ammoxidation on the catalyst V 2 O 5 ⋅32TiO 2 , alkylpyridine conversion and nicotinonitrile selectivity constituted 78 and 70% respectively, which was by 10% lower than that on the catalyst V 2 O 5 ⋅16TiO 2 with the same V 2 O 5 amount, but a different composition of a solid solution.

Conclusion
The results obtained show that a change of catalytic properties of vanadium-titanium-oxide catalysts for the oxidation of organic compounds, different by their nature, is connected with a change of V 2 O 5 content in the catalysts and the composition of a solid solution VO 2 -TiO 2 formed during the reaction. With an optimal V 2 O 5 content (4-6 wt.%) in a stationary catalyst and a corresponding composition of a solid solution, a high selectivity for oxidation and ammoxidation of o-xylene, acenaphthene and β-picoline into phthalic and naphthalic anhydrides, naphthalimide and nicotinonitrile is attained.