PREPARATION OF CATALYSTS WITH HIGH ACTIVITY AND SELECTIVITY FOR THE PYROLYSIS OF MONOCHLOROMETHANE AND TESTING OF CATALYSTS

ABSTRACT

In the work, catalysts with high activity and selectivity for the pyrolysis of monochloromethane were prepared and the catalysts were subjected to preliminary tests. To implement this process, catalysts with the composition of 1% Na₄P₂O₇ + 1.0% B₂O₃/YuKS, 1% Na₄P₂O₇ + 1.0% B₂O₃/YuKS^Si, 1% Na₄P₂O₇ + 1.0% B₂O₃/YuKS^Fe and 1% Na₄P₂O₇ + 1.0% B₂O₃/YuKS^Mg were prepared. The process temperature was in the range of 420-440 ^oC, the volume flow rate was in the range of 1000-2000 h^-1, and the pressure was atmospheric. The tests of the prepared catalysts were carried out in the range of 30-240 min. The aim of the work is to prepare catalysts with high activity and selectivity for the pyrolysis of monochloromethane and to conduct preliminary tests of the catalysts.

АННОТАЦИЯ

В работе приготовлены катализаторы пиролиза монохлорметана с высокой активностью и селективностью, проведены их предварительные испытания. Для реализации данного процесса были приготовлены катализаторы состава 1% Na4P2O7 + 1,0% B2O3/ЮКС, 1% Na4P2O7 + 1,0% B2O3/ЮКСSi, 1% Na4P2O7 + 1,0% B2O3/ЮКСFe и 1% Na4P2O7 + 1,0% B2O3/ЮКСMg. Температура процесса находилась в диапазоне 420-440 °С, объемная скорость потока – в диапазоне 1000-2000 ч-1, давление – атмосферное. Испытания приготовленных катализаторов проводили в диапазоне 30-240 мин. Цель работы-является приготовление катализаторов с высокой активностью и селективностью пиролиза монохлорметана и проведение предварительных испытаний катализаторов.

Keywords: Catalyst, monochloromethane, ethylene, propylene, pyrolysis, activity, magnesium, chromium, iron.

Ключевые слова: Катализатор, монохлорметан, этилен, пропилен, пиролиз, активность, магний, хром, железо.

INTRODUCTION

Among the methods mentioned in the literature[1-5], the catalytic oxidative dimerization (oxycondensation) reaction of methane is a method that proceeds in one step and at normal atmospheric pressure, and this method has not yet been introduced into industry. However, the laws of the reaction, mechanism, kinetics and other parameters have been studied in detail[6,7]. To date, highly catalytically active mesoporous catalysts with high catalytic activity, selectivity and efficiency have been developed for this reaction[7-11]. Mesoporous catalysts with such high catalytic activity include catalysts composed of MeMnW/SiO₂ (Me = Li, Na, K) [12] and (MoO₃)x∙(ZnO)y∙(ZrO₂)z [13-15].

The catalytic pyrolysis of methyl chloride to produce alkenes is based on the chemical processing of methane (natural gas)[16]. In this process, methane is first oxychlorinated to produce methyl chloride, and then, in a second step, methyl chloride is catalytically pyrolyzed to produce lower alkenes.[17-19]

EXPERIMENTAL PART

Method of preparing a mesoporous catalyst with high catalytic activity. A mesoporous catalyst with high catalytic activity is prepared by soaking aqueous solutions of Na₄P₂O₇, Mg(NO₃)₂ and Na₂B₄O₇ in YuKS (high silica zeolite). After soaking for 12 hours, it is dried at 1500 for 1 hour and calcined in an air stream at 550-600 ^oC.

For the catalytic pyrolysis of monochloromethane molecules, modified mesoporous catalysts with high catalytic activity were synthesized with the following composition: YuKS; 1% Na₂B₄O₇/YuKS; 1% Na₄P₂O₇ +1.0% B₂O₃/YuKS; 1.0% Na₄P₂O₇ +1.0%B₂O₃ +1.0%MgO/YuKS. Citric acid was used as a template in the synthesis of these catalytic systems.

The physicochemical properties of mesoporous catalysts with high catalytic activity prepared from modified high-silica zeolites are presented in the table.

Table 1.

Physicochemical properties of high silica zeolites

The maximum concentration and strength of acid centers is shown by 1%Na₄P₂O₇+1.0%B₂O₃/YuKS. As is known, acid centers allow the formation of unsaturated hydrocarbons, that is, hydrocarbons containing double bonds.

EXPERIMENTAL RESULTS AND DISCUSSION

The effect of space velocity on the pyrolysis process of monochloromethane molecules was studied at 420-440 ^oC in the range of 1000-1500 h^-1. With a decrease in the volume of the mesoporous catalyst with high catalytic activity, the conversion of monochloromethane molecules increases. With an increase in the space velocity of the initial feedstock, the overall selectivity for the formation of C₂-C₃-unsaturated, that is, hydrocarbons containing double bonds, increases. After 120 hours of operation of the mesoporous catalyst with high catalytic activity, the values ​​​​of the above-mentioned quantities decrease.

(1)-420^oC; (2)-430^oC; (3)-440^oC. Mesoporous catalyst with high catalytic activity: 1.0% Na₄P₂O₇ + 1.0% B₂O₃ + 1.0% MgO/YuKS. V=1500 h^-1

Figure 1. Dependence of monochloromethane conversion on the operating time of a mesoporous catalyst with high catalytic activity at 420-440^oC

As a result of the studies, it was found that the initial conversion of monochloromethane molecules from 420 ^oC to 440 ^oC is 80-85%.

When mesoporous catalysts with high catalytic activity are operated for the first 120 minutes in the range from 420^oC to 440^oC, the conversion of monochloromethane molecules decreases from 85% to 65%. It was found that during the next 12 minutes, the conversion of monochloromethane decreases from 65% to 18%. With increasing temperature, the initial selectivity to ethylene increases slightly, while the selectivity to propylene remains almost unchanged.

(1)-420^oC; (2)-430^oC; (3)-440^oC. Mesoporous catalyst with high catalytic activity: 1.0% Na₄P₂O₇ + 1.0% B₂O₃ + 1.0% MgO/YuKS. V=1500 h^-1

Figure 2. Dependence of the selectivity of ethylene formation on the duration of operation of a mesoporous catalyst with high catalytic activity at 420-440^oC

With an increase in the duration of the experiment and an increase in temperature, the selectivity of the process to propylene decreases sharply.

The results of the experiments showed that when the volume velocity of monochloromethane molecules is 1200 h^-1, the conversion of monochloromethane molecules is 65%, and the overall selectivity of the formation of C₂-C₃ unsaturated hydrocarbons, i.e. hydrocarbons containing double bonds, is 77%.

(1)-420^oC; (2)-430^oC; (3)-440^oC. Mesoporous catalyst with high catalytic activity: 1.0% Na₄P₂O₇ + 1.0% B₂O₃ + 1.0% MgO/YuKS. V=1500 h^-1

Figure 3. Dependence of the selectivity of propylene formation at 420-440^oC on the duration of operation of the mesoporous catalyst with high catalytic activity

As a result of the experiments, it was also found that when the volume velocity of monochloromethane molecules is changed in the range of 1000-2000 h^-1, the conversion of monochloromethane molecules practically does not change, that is, it is around 75-80%. At the same time, with an increase in the duration of the experiment, the activity of the mesoporous catalyst with high catalytic activity decreases sharply (from 75-80% to 15-25%). In this case, the volume velocity changes in the range of 1000-2000 h^-1.

CONCLUSION

Thus, it is recommended to carry out the pyrolysis process of monochloromethane molecules at temperatures in the range of 420-440^oC and volume velocities of monochloromethane molecules in the range of 1000-1500 h^-1.

Under the selected conditions, the active operating period of the highly catalytically active mesoporous catalyst 1.0%Na₄P₂O₇+1.0%B₂O₃+ 1.0%MgO/YuKS is 180 hours, which ensures the pyrolysis process of monochloromethane molecules with a conversion of more than 65% of monochloromethane molecules, as well as an overall selectivity of 79% for unsaturated hydrocarbons containing C₂-C₃, that is, hydrocarbons containing double bonds.

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