MODELING THE HETEROGENEOUS CATALYTIC RECOVERY PROCESSES OF ALDEHYDES AND KETONES

In the catalysis and green chemistry, there is a need for environmentally friendly technologies in order to obtain substances using the heterogeneous catalysts. In the synthesis of esters out of a homogeneous catalyst, it is often necessary to use additional solvent, and to recover the catalyst through its deactivation. These problems significantly complicate the process of receiving organic compounds. In contrast to a homogeneous catalyst, a heterogeneous catalyst can be used repeatedly, and it is much easier to separate this catalyst from the products of reaction (employing the mechanical methods). Promising catalysts of several processes of fine organic synthesis are zeolites, which have a dual structure of porosity – containing micropores and mesopores. Therefore, the development of new catalysts and exploring catalytic properties is a relevant task. The application of computational technologies and software packages allows calculating, with accuracy acceptable for practice, the required parameters of the examined processes, in particular velocity constants of recovery reactions of cyclohexanone and anise aldehyde. The aforementioned defines the relevance of the chosen research subject aimed at solving an important applied problem – mathematical modeling of heterogeneous catalytic recovery processes of aldehydes and ketones by the mechanism of Meerwein-Ponndorf-Verley (MPV). Thus, the implementation of organic synthesis in the presence of a heterogeneous catalyst is an important scientific task [1].


Introduction
In the catalysis and green chemistry, there is a need for environmentally friendly technologies in order to obtain substances using the heterogeneous catalysts. In the synthesis of esters out of a homogeneous catalyst, it is often necessary to use additional solvent, and to recover the catalyst through its deactivation. These problems significantly complicate the process of receiving organic compounds. In contrast to a homogeneous catalyst, a heterogeneous catalyst can be used repeatedly, and it is much easier to separate this catalyst from the products of reaction (employing the mechanical methods).
Promising catalysts of several processes of fine organic synthesis are zeolites, which have a dual structure of porosity -containing micropores and mesopores. Therefore, the development of new catalysts and exploring catalytic properties is a relevant task.
The application of computational technologies and software packages allows calculating, with accuracy acceptable for practice, the required parameters of the examined processes, in particular velocity constants of recovery reactions of cyclohexanone and anise aldehyde.
The aforementioned defines the relevance of the chosen research subject aimed at solving an important applied problem -mathematical modeling of heterogeneous catalytic recovery processes of aldehydes and ketones by the mechanism of Meerwein-Ponndorf-Verley (MPV). Thus, the implementation of organic synthesis in the pres-ence of a heterogeneous catalyst is an important scientific task [1].

Literature review and problem statement
Selective recovery of carbonyl compounds to alcohols in the presence of alcoholates is commonly called the restoration of ketones by the MPV mechanism.
At present, the mechanisms of reactions of ketone and aldehyde recovery by MPV are extensively described in the scientific literature. Some authors [2] described special features of these processes. But no papers have been found up to now, which would have included the results of mathematical modeling and parametric identification of kinetic parameters of mathematical models.
Zeolite is a crystalline form of oxides of metals, which have considerable potential to be applied as heterogeneous catalysts in many chemical reactions [3]. In addition, due to the unique microporous structure, they display different selectivity. A few examples of zeolites are known that are employed in the reaction which proceeds by MPV. For example, in articles [4][5][6], the reaction occurs in a gas phase on zeolites. There are as well publications about variants of using the zeolite systems for asymmetric aldol process of aldehyde recovery with subsequent esterification [7].
Article [8] presents results of research into selectivity of the recovery reaction of 4-tetra-butyl cyclohexanone to cis-4-tetra-butyl cyclohexanol on the zeolite catalysts of structural group BEA. This reaction has a commercial application -cis-isomers are widely used in the perfume industry.
A theoretical possibility of heterogeneous catalytic process for obtaining the alcohol by the MPV mechanism is shown in [9,10]. Zeolites, mesoporous and micro-mesoporous silicates and aluminum oxide materials may prove to be promising catalysts for this process. The given work describes the synthesis of alcohol on zeolites.
As far as the mathematical modeling of the recovery processes of aldehydes and ketones in the presence of heterogeneous catalysts is concerned, the scientific literature (in particular, the above mentioned sources) lacks the description of models of such systems. There are only theoretical provisions and features of modeling the heterogeneous catalytic processes. Thus, [14] describes basic assumptions put forward when compiling a mathematical model for the aforementioned processes, and considers the patterns that are applied to construct mathematical models of heterogeneous catalytic processes. At present, there are no developed algorithms for solving the inverse problem of chemical kinetics (calculation of velocity constants) for the recovery process of aldehydes and ketones by MPV. Therefore, with regard to the above status of research into the chosen subject, it is expedient to develop an adequate mathematical model of the heterogeneous catalytic recovery process of aldehydes and ketones. The mathematical model to be developed might be used for solving the inverse problems of chemical kinetics.

The aim and tasks of research
The aim of present work is to calculate the rate constants of the examined reactions in the presence of heterogeneous catalysts based on the obtained experimental data.
To accomplish the set aim, the following tasks were formulated: -to conduct experimental study of the recovery reaction of cyclohexanone to cyclohexanol in the presence of various zeolite catalysts; -to conduct experimental study of the recovery reaction of anise aldehyde to anise alcohol with its subsequent esterification; -to construct a mathematical model of the heterogeneous catalytic recovery processes of aldehydes and ketones; -to process the obtained experimental data and to calculate the rate constants of the examined reactions, using the developed mathematical model in the automated mathematical software package MathCad 15.0 (USA); to analyze the received results of calculations and to choose the most efficient catalyst.

Materials and methods for examining the heterogeneous catalytic recovery processes of aldehydes and ketones
At the Institute of Physical Chemistry of the Ukrainian NAN (Kyiv, Ukraine), the zeolite systems of structural group beta (β), which incorporated ions of metals, were investigated: Sn and Al. The main difference between the given zeolites from those already known is the dual structure of porosity (they have micro-and meso-pores).

1. The examined materials and equipment used in the experiment
Studies were carried out by using the extra wide porous zeolites of the type BEA (beta or β) in the process. These are complex oxides based on magnesium and aluminum prepared by the decomposition of hydrotalcite (Mg 6 Al 2 (OH) 16 CO 3 ·4H 2 O). The crystal lattice of such zeolites is incorporated with ions of tin (Sn) whose specific surface is about 100-120 mm 2 /g. The content of built-in metal is around 2 %. To activate the catalyst, it needs to be calcinated for 2 hours at a temperature of 600 ºC in an autoclave.
The experiment is carried out in the following way. Starting reagents and a catalyst (powdered) are loaded in a laboratory reactor. The reactor is equipped with the magnetic agitator with heating "WiseStir® MSH-20D DAIHAN" (South Korea). Intensity of stirring is 100 rotations of the agitator per minute [15].
The stirring reactor is warmed to 95 ºC in advance. The installation contains a rotation control unit of the agitator (this experiment is carried out at 100 revolutions per minute).
As a stirrer we used a "tablet", which consists of a magnet covered with inert teflon coating [16]. It can be used several times. A needle for selecting the samples is connected to the reactor through a special plug. After the installation is enabled, the samples are selected for examination in certain range of time (15 min, 30 min, 1 h, 2 h, 4 h, 6 h, 10 h, etc.).
When the reaction is completed, the catalyst is removed by centrifugation. Zeolite heterogeneous catalyst can be reused.
Water or condensate formed in the course of reaction as a result of esterification of alcohol (applicable only for the recovery reactions of anise aldehyde) is discharged to the refrigerator.
In order to analyze selected samples, we use the automated gas chromatograph of the 4th generation "Kristallux-4000M" (Russia) with a proportional-integral differential detector and a capillary column employing mesitylene as internal standard. Analysis of a sample takes about 15 minutes (depending on the size and complexity of the examined molecule). Next is the preparation for taking the next sample (lasting for 3 minutes on average).
The chromatograph "Kristallux-4000M" processes chromatographic information in automated mode using the compatible software NetChrom (Russia). The software application receives and processes data on the sample. It also has the implemented functions of automatic thermostat temperature control, consumption of the gas-carrier and auxiliary gases, automatic control of the flame combustion in the process. Measuring the signals from detectors is conducted by using the 24-bit analog-to-digital converter (ADC) "E-24 L" (Russia) [17].

1. The recovery reaction of anise aldehyde to anise alcohol with subsequent esterification
We studied experimentally the recovery process of anise aldehyde and subsequent esterification of the resulting anise alcohol by the MPV mechanism. As a result of the experiment, we obtained dependences of the concentrations of these substances on the time (Table 1). SnAl zeolites were employed as the catalysts. We processed mathematically results of the experiment and obtained a mathematic model of this process. The examined reaction is: where A is the anise aldehyde; B is the anise alcohol; C is the 4-methoxybenzyl-1-methylpropyl ester. Table 1 Concentrations of the reaction components*  Table 1 gives preliminary processed experimental results Data given in Table 1 were used to calculate the rate constants in the MathCad 15.0 programming environment.

2. The recovery reaction of cyclohexanone to cyclohexanol
In the course of the experiment we obtained the values of conversion degrees (Table 2) where A is cyclohexanone, B is cyclohexanol. Table 2 were recalculated to concentrations (in kmol/m 3 ) and used to calculate the rate constants in the MathCad 15.0 programming environment.  Table 2 gives preliminary processed experimental results

1. Mathematical modeling of the recovery process of anise aldehyde
The assumptions that were accepted for modeling the kinetics of recovery process of anise aldehyde: -a layer of catalyst is the quasi-homogeneous medium; -since 2-butanol is fed to the reactor in excess, then the change in its concentration can be considered negligible and it can be neglected; -a layer of catalyst is isothermal; -the displacement of substance occurs under an ideal agitation mode; -the transfer of substance along the axial direction is not considered.
Mathematical model of the process with regard to the assumptions takes the following form: Initial conditions: -C A (0)=0.234 kmol/m 3 is the starting concentration of component А in reaction (1); -C B (0)=C C (0)=0 kmol/m 3 is the starting concentration of components B and С in reaction (1).

2. Mathematical modeling of the recovery process of cyclohexanone
The assumptions that were accepted in the construction of a mathematical model of the recovery process of cyclohexanol [18]:

1. Rate constants of the recovery reaction of cyclohexanone
By using the calculated values of cyclohexanone and cyclohexanol concentrations, given in Table 3, in the MathCad 15.0 programming environment we calculated the rate constants of reaction (2) for three zeolite catalysts. We employed the listing given in [19] for the calculation.
Fragment of the code in the MathCad 15.0 programming environment to calculate the rate constants is shown in Fig. 6 [20].
Using the algorithm shown in Fig. 6, we calculated the rate constants for all examined catalysts. The obtained results of calculation are given in Table 4.
Analysis of the received values of constants is given in chapter 9. Table 3 Results of calculation of the reaction components concentrations  Table 4 Calculated values of the reaction rate constants

2. Rate constants of the recovery reaction of anise aldehyde
Using the values of concentrations of anise aldehyde, anise alcohol and the resulting ester, given in Table 1, in the MathCad 15.0 programming environment we calculated the rate constants of reaction (1) for the zeolite catalyst. We employed the listing given in [19] for the calculation.
Fragment of the code in the MathCad 15.0 programming environment to calculate the rate constants is shown in Fig. 7 [20].
Using the algorithm shown in Fig. 7, we calculated the rate constants for all stages of the examined reaction. The obtained results of calculation are given in Table 5. Analysis of the received values of constants is given in chapter 9.  Table 1 revealed that the maximum concentration of anise alcohol, which is achieved in the process of converting the anise aldehyde (component B in reaction (1)) by the MPV mechanism, is 0.00653 kmol/m 3 . Such a value of the concentration is achieved within 1 hour of conducting the experiment. Then the concentration of component B in reaction (1) starts to decrease. This is explained by the fact that in the reactor at this time there starts an esterification reaction of the obtained anise alcohol. The received values of the component concentrations could be also used for the simulation of chemical reactors with different modes of operation -agitation and extrusion.
On analyzing the obtained values of the rate constants of the recovery reaction of anise aldehyde (Table 5), we can state the following. The second stage of the reaction (formation of ester) proceeds much faster than the first stage. Thus, the rate constant of the first stage is 0.0323 s -1 , and that of the second is 0.4738 s -1 .
As far as the recovery reaction (2) of cyclohexanone to cyclohexanol is concerned, an analysis of the obtained data (Table 3, Fig. 4, 5) revealed the following. The maximum concentration of the product of reaction (cyclohexanol) is achieved when using the zeolite catalyst Sn-MgAl(CO 3 ). The maximum concentration of cyclohexanol is 0.2676 kmol/m 3 . Such value is achieved within 10 hours of the reaction