In papers [1] [2] kinetic regularities of vapor phase catalytic ammoxidation of both a substrate-4-phenyl-o-xy- lene and an intermediate 4-phenyl-o-tolunitrile were studied. A following scheme of conversions, constituting a total process of 4-phenyl-o-xylene ammoxidation on V-Sb-Bi-Zr/γ-Al₂O₃-oxide catalyst (Figure 1), was proposed in contributions [1] [3] [4] . The old analog wearing a patent character [5] exists in this field; however, the kinetic regularities of 4-phenyl-o-xylene ammoxidation on V₂O₅-CrO₃/γ-Al₂O₃ catalyst have not been studied

and a mechanism of products formation on the basis of kinetic data has not been discussed by authors. In paper [6] the main routes of catalytic oxidative conversions of some methyl derivatives of biphenyl in the gas phase, including 3,4-dimethylbiphenyl (4-phenyl-o-xylene) ammoxidation, were studied. In the patent [5] it is shown that for 10% of V₂O₅ - 5.6% CrO₃ /g-Al₂O₃ catalyst at a temperature of 693K and a molar ratio of 4-phenyl-o-xy- lene: NH₃:O₂, equal to 1:39.71:6 turns out 4-phenylphthalonitrile with the 70% yield and conversion of the substrate in this case is 99%. Study of vapor oxidation and ammoxidation of 3,4-di-and 3,3', 4,4'-tetramethyldi- phenyls enables obtaining of the corresponding acid anhydrides, imides, nitriles of biphenylcarboxylic acids in a yield of 59 - 69 mol% [6] . The ammoxidation on many oxide catalysts is the exothermic catalytic reaction of o-xylene accompanied by the formation of major products, nitrites and imides. Therefore, an important object of research in the ammoxidation of xylene, and 4-brom- and 4-phenyl-o-xylene is the solution of the selectivity of the corresponding dinitrile and imide [7] . As a result of own study, it has been found that on the V-Sb-Bi-Zr/g- Al₂O₃-oxide catalyst, at high conversion of the starting 4-phenyl-o-xylene in single process [8] [9] , ammoxidation gives 80 mol% of 4-pfenylftalonitrile. Due to increased demands on the purity of the 4-pfenylftalonitrile while using and problem of the separation of hard separable impurities in the crystal 4-phenylftalimide, the variant of the technological design process with an average conversion and recycling unreacted 4-phenyl-o-xy- lene and intermediate 4-phenyl-o-tolunitrile is offered. In contrast to the single process, conducting of process with recirculation reduces quantity of the formed by-products, the share of deep oxidation and increases selectivity on a 4- phenyl phthalonitrile to 97.72% [8] .

Thus, the problem of the separation of hard separable crystal mixture of main products has been solved in the case of obtaining dinitrile from the substrate with the average conversion on the V-Sb-Bi-Zr/g-Al₂O₃-oxide catalyst, with the recirculation at higher concentrations of ammonia, and in the synthesis of imide with complete conversion of the substrate in a single process on V-Sb-Bi-Zr/g-Al₂O₃-oxide catalyst, using low concentrations of ammonia and water in the initial reaction mixture [9] [10] . As known, in general, and especially in cases where basic products are formed by competing pathways, to direct the reaction towards the important target substance, it is necessary to know its mechanism.

Major products of ammoxidation of aromatic compounds, having methyl groups at ortho-position, are both dinitrile and imide [10] - [12] , which are used in production of phthalocyanine pigments and dyes, photosensitizers of near IR spectrum for photodynamic cancer therapy [8] [9] , laser dyes, thermally stable polymers, medications, chemical means of plant protection and other fields of organic synthesis.

Kinetic measurements of 4-phenyl-o-xylene conversion and chromatographic separation of catalyzate components, and a quantitative calculation of their content were carried out in accordance with the earlier developed methods [1] . Kinetic measurements were performed in a setup with a vibrating fluidized-bed, gradientless flow reactor of 20-cm³ capacity made from 12Kh18N10T steel (Figure 2). Feed system of the reagents. Dosage of gaseous components of the initial reaction mixture, containing oxygen, ammonia and nitrogen (as gas-diluent), feed from cylinders, was being carried out by means of stopcocks {1} and a fine adjustment valve {5}. Gases cleaning and drying were being carried out in the gas columns {3}. Measurement of gases consumption was being performed by rheometers {4} with calibrated capillaries, and pressure in the system was being maintained constant by manostats {2}. 4-Phenyl-o-tolunitrile in liquid state by means of dispenser of syringe type {9} and gaseous reagents through the mixer {6} were being fed into the evaporator {7}, heated by oven {8}. The reaction assembly included a reactor {10} equipped with a thermowell, a grid (the same brand as the reactor), pre-

venting entrainment of catalyst dust, as well as a vibrator {14} with the oscillation frequency of 50 Hz and an adjustable amplitude. Temperature in the reaction zone was being measured using a sliding thermocouple and recorded by electronic potentiometer KSP-4M. Temperature of block furnace {11} and the reactor were being controlled by a potentiometer KVP I-503 with accuracy to ±2˚. The furnace with a reactor was being placed in a thermostat {12}. Temperature in the thermostat was being maintained at 570 - 580 K by electrical spirals {13, 15} sufficient to prevent condensation of the reaction products. A system for collecting the reaction products included a heated switch {16}, with solvent trap of reaction products {17, 18}, the three-way cock {19}, a trap filled with sulfuric acid solution for quantitative recovery of unreacted ammonia {20}, a desiccant {21}, a gas sampler {22}, foam flow meter of flow rate {23}.The described scheme provides a material balance for all substances-reaction participants.

The products absorbed by 1,4-dioxane were determined on a Chrome-5 chromatograph equipped with an FID and a column of 1.2 m length. The column packing was Chromaton N-AW (0.2 - 0.25 mm) coated with a mixed stationary phase of Apiezon L (21%) and PEG 40000 (0.5%) or with Polisorb-1 (0.25 - 0.5 mm) alone. Carbon dioxide was determined on LKhM-8MD chromatograph with triethylene glycol butyrate supported on INZ-600 (calcined diatomaceous earth) as the stationary phase. Separation of O₂ and N₂ was carried out on the same chromatoraph using a parallel column packed with NaX.

In products of the reaction of 4-phenyl-o-xylene (I) ammoxidation on V-Sb-Bi-Zr/γ-Al₂O₃-oxide catalyst 4- phenyl-o-tolunitrile (II), 4-phenylphthalonitrile (III), 4-phenylpthalimide (IV), 4-phenylbenzonitrile (V), carbon dioxide, unreacted I, oxygen and the diluent gas-nitrogen were determined by gas chromatography. The reaction gases were successively passed through a 1,4-dioxane-filled trap for absorption of nitriles, and IV and sulfuric acid-filled trap for absorption of ammonia. The concentration of ammonia at the reactor outlet was determined by titration of unreacted sulfuric acid from the second trap. The carrier-gas (nitrogen) flow rate was 80 mL/min. The sample injection temperature was 353 K, and the temperature programming rate was 20 K/min.

In previous papers [2] [3] it has been shown, that at 4-phenyl-o-xylene ammoxidation on the oxide catalyst V-Sb-Bi-Zr/γ-Al₂O₃ due to the electronic factor of a phenyl group para-methyl groups are activated first, then meta-methyl groups are activated second with respect to substituent in an aromatic nucleus. By the way, the phenyl group is related to substituent of the third kind [13] , which may show both electron-donating and electron-withdrawing properties and, taking into account lack of a significant role of conjugation, i.e. electron-do- nating property of the phenyl group, due to the fact that resonance constants of phenyl and methyl groups, located at the para-position, are not significantly different [14] [15] , therefore should be noted that the phenyl group in 4-phenyl-o-xylene molecule reveals the electron-withdrawing character. As the phenyl group in 4- phenyl-o-xylene molecule does not affect on activation of meta-methyl group [16] , then on the basis of the electron-withdrawing property of phenyl group and a positive inductive (+J) effect of para-methyl group it can be said, that both influence on activation of para-methyl group in regard to phenyl substituent. Thus, as a result of activation first intermediate 4-phenyl-o-tolunitrile and then 4-phenylphthalonitrile are formed [1] [2] .

For unsubstituted o-xylene it does not matter which of two methyl groups is activated first in an aromatic nucleus. Therefore, as a result of activation of one from two methyl groups only o-tolunitrile is formed, which in its turn, due to activation of the second methyl group under the conditions of oxidative ammonolysis [17] [18] forms dinitrile. On the other hand, study of ammoxidation processes of 4-substituted o-xylene [1] [2] [7] [19] - [23] on the modified V-Sb-Bi/γ-Al₂O₃ catalysts, including V-Sb-Bi-Zr/γ-Al₂O₃-oxide contact, shows, that introduction of the phenyl group, or bromine atom into the aromatic nucleus of o-xylene negatively affects on its reactivity, where the phenyl substituent exerts the less negative influence [15] . In modern organic chemistry summative influence of substituents on the reaction center can be subdivided into induction and resonance (conjugation), i.e. the electronic and steric constituents, from which upon adsorption of 4-phenyl-o-xylene by the methyl group the steric factor [15] and upon activation (chemisorption) electronic factor [3] play a noticeable role.

During the primary activation of alkyl aromatic compounds with the metal oxide catalyst under the ammoxidation conditions lateral C-H bonds, being in α-position to an aromatic nucleus, are broken [24] . Activation mechanism under the action of primary center-of nucleophilic oxygen ion is shown in Figure 3.

As seen, efficiency of heterolytic dissociation of the C-H bond in the methyl group is ensured, first of all, by interaction of nucleophilic oxygen (О²⁻) with the protonated hydrogen. The heterolytic breakage of C-H bonds occurs also at activation of the second methyl group of 4-phenyl-o-xylene, since 4-phenilphthalonitrile is formed on V-Sb-Bi-Zr/γ-Al₂O₃-oxide catalyst mainly from 4-phenyl-o-tolunitrile. Thus, the primary interaction of 4- phenyl-o-xylene with the contact surface has an acid-base character, i.e. stage 1 (Figure 3) proceeds without changing an oxidation state of the metal Mⁿ⁺ and is accompanied by separation of a proton from the substrate molecule (CH-acid) on nucleophilic catalyst center forming a surface compound of anionic type [3] [24] . For the oxidative reactions, mechanism of which comprises the acid-base stages, use of notions about the combined influence of redox and acid-base characteristics of the surface on the catalytic properties of substances is more productive [25] . Ammoxidation processes of toluene and its homologs, the primary interaction of which with the oxide catalyst can show the acid-base character [24] should be related to these reactions.

The purpose of the present article is to discuss a formation mechanism of the products of 4-phenyl-o-xylene ammoxidation, consistent with the observed kinetic regularities.

It is important to add, that as distinct from easiness of study of ammoxidation mechanism of olefinic and paraffinic hydrocarbons series [26] [27] , methodological difficulties, caused by comparatively high boiling and sublimation temperatures of initial substances and reaction products, highly detain conducting of detailed researches of ammoxidation mechanism of alkyl aromatic compounds [24] . The mechanism even for the simplest and sufficiently well-studied processes of ammoxidation remains still unexplained. An even greater difficulty is a study of the mechanism, as a result of which not only one but also two nitrile groups are formed, and imide group as well.

Earlier discussed by us, the formation mechanism of the products of o-xylene and p-xylene ammoxidation, consistent with the observed kinetic regularities, showed that in both cases dependence of the consumption rate of substrates allowed supposing their dissociative adsorption.

4-Phenyl-o-tolunitrile (II), 4-phenylphthalonitrile (III), 4-phenylphthalimide (IV), 4-phenylbenzonitrile (V) and CO₂ are formed at vapor phase ammoxidation of 4-phenyl-o-xylene (I) in accordance with the Figure 1. Analysis of the experimental data showed, that formation and consumption rates of the basic components of the reaction of 4-phenyl-o-xylene ammoxidation on V-Sb-Bi-Zr/γ-Al₂O₃-oxide catalyst in a range of the partial pressures of oxygen and ammonia (whose measuring unit is kPa), exceeding their minimum values, are described by the Equations (1)-(7).

Here к_i is a constant of the route rate (Figure 1).

Taking into account the observed dependence of the formation of the reaction products on the ratio of O₂ and NH₃ concentrations at values, lesser of some value, denoted as [1] , the rates of conversion of 4-phenyl-o-xylene into 4-phenyl-o-tolunitrile and 4-phenyl-o-tolunitrile into 4-phenylphthalonitrile are expressed by the Equations (8) and (9), respectively.

In the indicated range of ammonia concentrations the routes 7 and 8 are added according to the Figure 1, the rates of which are described by the Equations (10) and (11).

Here b is a share of 4-phenyl-o-xylene, which at small concentrations of ammonia is converted to 4-phe- nylphthalimide; b is a constant reflecting ratio of the adsorption equilibrium constants of O₂ and NH₃.

In accordance with Figure 1, W_sum is independent on NH₃ concentration and obeys the half-order equation on 4-phenyl-o-xylene. Such dependence allows assuming a dissociative adsorption of 4-phenyl-o-xylene at centers, which at is completely covered with oxygen. At the same time the adsorbed 4-phenyl-o-xylene fragment with probability к₉/к₁ + к₉ along the route 9 is oxidized to CO₂ and H₂O, with probability к₁/к₁ + к₉ leads to the formation of 4-phenyl-o-tolunitrile, and at small values of also produces 4-phenylphthalimide, CO₂ and H₂O. Correlation of the formation rates of 4-phenyl-o-tolunitrile on the route 1 (W₁) and other products of the reaction on the routes 7 and 8 (W₇ + W₈) depends on the ratio of. This together with independence of the conversion rate of 4-phenyl-o-xylene on gives a reason to suggest, that NH₃ is adsorbed on other centers, than 4-phenyl-o-xylene, besides in this adsorption it competes with oxygen, completely displacing it at the high values of. Obviously, these centers (Z) have less adsorption heat of oxygen than the adsorption centers of 4-phenyl-o-xylene (Y).

Taking into consideration the foregoing, sequence of the steps leading to adsorption of 4-phenyl-o-xylene and its complete oxidation on the route 9 can be represented in the following way:

1)

2)

3)

4¢)

5)

6¢)

Here, step 1) is the equilibrium stage, the stages with dashes are instant [28] , all other stages are slow, and C₆H₅ = Ph.

Equilibriumly adsorbed on centers ZO₂ and NH₃ completely cover their.

7)

8)

Hence:

Then O₂ and NH₃ covering can be calculated as follows.

Here, K_i is an equilibrium constant of the reaction stage.

If YO₂CHPhC₆H₃CH₃ interacts with ZNH₃, then 4-phenyl-o-tolunitrile is formed, in the case of interaction with ZO₂ either 4-phenylphthalimide or CO₂ and H₂O are formed on the routes 7 and 8 (Figure 1).

9¢)

10¢)

11¢)

12¢)

13¢)

14¢)

Covering of Y centers by the fragments of organic origin is little.

If consumption rate of YOCHPhC₆H₃CH₃ in the stages 2) and 3) is approximately equals to consumption rate of YOH₂ in the stage 5), then their concentrations are also equal with each other, i.e. [YOCHPhC₆H₃CH₃] » [YOH₂].

Then, from the equilibrium stage 1) and condition of full covering of Y centers by oxygen ([YO] ≈ 1) we obtain the expressions (16) and (17).

The consumption rate of 4-phenyl-o-xylene on the route 9, taking into account defining of this direction slow stage 3) at [YO] ≈ 1, is determined by the Equation (18), and on the routes 1, 7 and 8, taking into account defining of the direction slow stage 2) at [YO] ≈ 1 by the Equation (19).

Here, k_i is a rate constant of the reaction stage.

The values W₁ and [W₇ + W₈], taking into account coverings of NH₃ and O₂, which determine the routes 1 and [7 + 8] are found by the Equations (21) and (22).

In turn, the rates ratio of the routes 7 and 8 is defined by relation of the constants of the stages 11¢) and 13¢).

On the Y centers, analogously to adsorption of 4-phenyl-o-xylene, 4-phenyl-o-tolunitrile is dissociatively adsorbed.

15)

16)

17¢)

Since the formation rates of YOCHPhC₆H₃CN and YOH₂ at the stage 15) are the same, then [YOCHPhC₆H₃CN] = [YOH₂].

Therefore, from the equilibrium of the stage 15) and the conditions of full coverage of the Y centers by oxygen ([YO] ≈ 1) follows

hence

Then, the consumption rate of 4-phenyl-o-tolunitrile on the route 2, taking into account defining of this direction slow stage 16) at [YO] ≈ 1, is determined by the Equation (28).

Taking into consideration NH₃ covering, which determines the route 2, the reaction rate is determined by the Equation (29).

A character of kinetic dependencies of hydrolysis of 4-phenylphthalonitrile and reverse reaction of 4-phe- nylphthalimide ammonolysis on water and ammonia concentrations shows, that these reactions occur on the catalytic centers of different nature. Ammonia on them competes with water, and ammonia covering is much more than covering by adsorbed water and close to 1.

18)

19)

20)

21)

Here, the stages 18), 19) and 21) are equilibrium stages; stage 20) is reversible, but not the equilibrium.

Covering with adsorbed water is calculated by the Equation (31).

Then, the reaction rate on the route 3 in the forward direction, taking into account defining of the direction stage 20), is determined by the Equation (32).

The equilibrium constant of the stage 21), taking into account [X(NH₃)] » 1 is equal to:

Hence, covering with the intermediate compound:

Then, the reaction rate on the route 4 of the reverse direction of 4-phenylphthalimide ammonolysis, taking into account defining stage 20), is equal to:

Consequently,

At equilibrium,

Then,

and equilibrium constant of the hydrolysis reaction of 4-phenylphthalonitrile may be expressed in the following way (40).

If to judge on the observed kinetic regularities [1] [2] , then formation of 4-phenylbenzonitrile both from 4- phenylphthalimide and from 4-phenyl-o-tolunitrile occurs on other centers than ammoxidation. Existence of such centers on the surface of the multicomponent catalyst is quite possible. Therefore, a character of the kinetic dependences of 4-phenylphthalimide decarboxylation shows, that this reaction takes place on of catalytic centers (Ω), covered with imide.

22)

23)

24¢)

The consumption rate of 4-phenylphthalimide on the route 5, taking into account defining of the direction slow stage 23) at covering with imide, closed to 1,

Consequently,

Formation of 4-phenylbenzonitrile at the oxidative destruction (oxidative demethylation) of 4-phenyl-o-tolu- nitrile is impeded by 4-phenyl-o-xylene, and therefore, 4-phenyl-o-xylene and 4-phenyl-o-tolunitrile equilibriumly adsorbing on Λ centers cover them.

25)

26)

Then coverings of 4-phenyl-o-xylene and 4-phenyl-o-tolunitrile are determined by the equations (43, 44).

Sequence of the stages of 4-phenyl-o-tolunitrile oxidative destruction on the route 6 can be represented as follows.

27)

28)

29¢)

30¢)

31¢)

32)

From the foregoing follows, that

Taking into consideration this, and because of the relatively low yields of 4-phenylbenzonitrile even on the routes 5 and 6 (Figure 1) may assume, that Λ centers covering with the fragments of inorganic origin are very small (46).

From the stage 26) for [Λ(CH₃PhC₆H₃CN)], taking into account sum of the centers numbers, equal to unity, we obtain the following expressions (47) and (48).

Then, the consumption rate of Λ(CH₃PhC₆H₃CN)-adsorbed 4-phenyl-o-tolunitrile at PhC₆H₄CN formation on the slow stage 28) is determined by the equation (49).

Furthermore,

Hence,

Here a is a constant reflecting ratio of the constants of adsorption equilibrium of 4-phenyl-o-xylene and 4- phenyl-o-tolunitrile.

1) Ensuing from the proposed stage schemes of 4-phenyl-o-xylene ammoxidation mechanism, kinetic equations are in qualitative agreement with experimental data.

2) In deriving of the equations, physical meaning of a number of constants appearing in them is revealed.

3) The mechanism of formation of the main products as in the competing ways and its mutual transformation in each other are discussed.

4) The mechanism shows that the adsorbed 4-phenyl-o-xylol fragment as the general activated complex in interaction with ZNH₃ forms 4-phenyl-o-tolunitrile, which in its turn transforms in 4-pfenylftalonitrile and in the case of interaction with ZO₂ forms 4-pfenylftalimide.

5) Conducted research provides an opportunity to create an effective single technological process of obtaining 4-pfenylftalonitrile, by the ammoxidation reaction of 4-phenyl-o-xylene.

Used in this work software package OptimMe was developed by the support of Science Foundation of “SOCAR” under the grant project ET-27 (15/10/2014) at the Institute of Catalysis and Inorganic Chemistry named after Acad. M. F. Nagiyev.

Gulu A.Bagirzade,Dilgam B.Tagiyev,Manaf R.Manafov, (2015) Mechanism of the Products Formation in the Vapor Phase Ammoxidation Reaction of 4-Phenyl-o-Xylene. Advances in Chemical Engineering and Science,05,430-440. doi: 10.4236/aces.2015.54044