CATALYST SELECTION AND TECHNOLOGY FOR OBTAINING DIMETHYL ETHER

АННОТАЦИЯ

В работе рассмотрен процесс моделирования производства метанола и диметилового эфира.Количество веществ на выходе из реактора анализировали хроматографическим методом.Синтез органических веществ на основе природного газа снижает долю его использования в качестве топлива и создает возможности для получения необходимых продуктов.Именно поэтому разработка способов синтеза различных веществ на основе природного газа, получение катализаторов на основе местного сырья для процесса относятся к числу актуальных задач.Исследован процесс получения диметилового эфира из СО и Н₂, синтеза метанола и его дегидратации в присутствии катализатора, состоящего из ZnO·Al₂O₃·ZrO₂·CuO·CaO/бентонит: γ-Al₂O₃в условиях их послойного послойная загрузка.

ABSTRACT

In the work, the process of modelling methanol and dimethyl ether production was considered. The number of substances at the exit from the reactor was analysed by chromatographic method. Synthesis of organic substances based on natural gas reduces the share of its use as fuel and creates opportunities for obtaining necessary products. That is why the development of ways to synthesize various substances based on natural gas, and obtaining catalysts based on local raw materials for the process are among the urgent problems. The process of obtaining dimethyl ether from CO and H₂, synthesis of methanol and its dehydration in the presence of a catalyst consisting of ZnO·Al₂O₃·ZrO₂·CuO· CaO/bentonite: γ-A1₂O₃was studied in terms of their layer-by-layer loading.

Ключевые слова: метанол, диметиловый эфир, температура, катализатор, конверсия.

Keywords: methanol, dimethyl ether, temperature, catalyst, conversion.

Introduction

Currently, dimethyl ether is considered an alternative source of energy to diesel fuel. The synthesis of dimethyl ether (DME) from synthesis gas is carried out by direct or indirect methods. For the direct synthesis of DME, two types of compositions are used such as methanol synthesis catalyst and CH₃OH dehydration catalyst storage compositions. Dimethyl ether-dimethyl sulphate is an important chemical raw material for the production of chemicals such as methyl acetate and ethylene, propylene, and is also environmentally safe for aerosol cans. According to the physical properties of dimethyl ether, liquefied oil is similar to gases and can be stored under the same conditions, and easily transported [1-4].

Worldwide research shows that various carbon-containing raw materials can be processed into valuable chemical products. One of the recycling ideas is to oxidize raw materials (natural gas, household waste, gas from oil refineries, etc.) into carbon dioxide and hydrogen [5-6].

Carbon dioxide and hydrogen can then be converted to oxygenates (DME or CH₃OH), which can be a synthetic source of hydrocarbons [7-8] or valuable chemical products [9]. The combination of high activity in methanol synthesis and its dehydration can be achieved not only by optimal selection of process parameters but also by proper selection of methanol synthesis and dehydration components [10-11].

Physicochemical and catalytic properties of Cr₂O₃*ZnO*Al₂O₃, aluminosilicate and CuO*ZnO*Al₂O₃catalysts for dehydrating alcohol to dimethyl ether were previously studied and described in works [12-13]. Synthesis, textural characteristics and application of catalysts based on UKTs are detailed in works [14-16].

Experimental part

In the work, since two processes-methanol syntheses and their dehydration occurred simultaneously in one reactor, CuO·ZnO·Al₂O₃·ZrO₂·CaO/bentonite: γ- A1₂O₃in the presence of a catalyst provided a) 1:1, b) 2:3, c)2:1:2 ratios, fold-by-loading methods. Operating conditions T=290 ℃, P=0.5 MPa, H₂/CO=2, volume velocity at a pressure of 1000 h^-1, conversion of CO to methanol and DME in a single pass through the catalyst bed "Crystal 5000" gas chromatograph, Determinations were made using a TriStar II (3020) automatic gas adsorption analyzer model BJH (Barett-Joyner-Halenda). The phase composition of the catalyst samples was studied by X-ray phase analysis (XRD) using a Shimadzu XRD-6000 diffractometer. The surface morphology of the samples was studied by scanning electron microscope based on IILMU VEGA electron microscope and INCA ENERGY 370 energy dispersive microanalysis. Diffractometry (XRD Pan Analytical) was used to study the phase composition of the catalysts, and scanning electron microscopy (SEM) was used to study the surface characteristics.

Results and discussion

When several compounds are applied to aluminium oxide, its surface properties change significantly, and a new catalyst with certain acid-base and oxidation-reduction properties determines the selectivity of methanol decomposition or conversion. A sample with a small copper content (~1 wt.%) slightly reduces the catalytic activity of the system in the CH₃OH dehydration reaction, but its general laws are preserved. Modification of the support surface with zinc oxide also reduces the catalytic activity in this reaction, and the more zinc oxide in the system, the lower the catalyst performance for dimethyl ether. This is because zinc oxide reduces the concentration of acid centres on the surface. If copper oxide is added to the initial carrier (bentonite) in sufficiently high concentrations (5-10 wt.%), a significant change in the catalytic properties of the system is observed.Other components appear in the reaction products: H₂, HCOOCH₃, CH₄, carbon dioxide and carbon dioxide. As the amount of copper in the catalyst increases, the relative efficiency of dimethyl ether and methyl formate increases at relatively low temperatures (150-240⁰C) (Fig. 1).

Figure 1. Dependence of the reaction temperature on the degree of methanol decomposition (2, 2`) and the relative efficiency of the CuO*ZnO*Al<sub>2</sub>O<sub>3</sub>*ZrO<sub>2</sub>*СаО/bentonite catalyst on dimethyl ether with different amounts of copper in the catalyst: 1, 2 - 4.1 wt.% Cu, 1`, 2` - 8.2 wt.% Cu

The change in the copper content of the catalyst has almost no effect on the specific indicators of carbon dioxide and carbon dioxide. Light gases such as hydrogen, carbon dioxide and carbon dioxide are formed as a result of methanol conversion and decomposition reactions:

                                            (1)

                                                   (2)

Water is a dehydration product of methanol. With an increase in temperature, the formation of dimethyl ether in the substance component of the catalyst decreases, and methanol decomposes into carbon monoxide and two hydrogen molecules (reaction 2).

At relatively low temperatures (170-235⁰C), HCOOCH₃ is formed on the catalyst (reaction 3), and at higher temperatures, the yield of dimethyl ether decreases sharply and methane appears in the products, which suggests the following reactions:

                                            (3)

                                             (4)

In this system, there is a high activity for methyl formate (Figure 2), and in such a system, in the low-temperature range (170-215 ⁰C), the performance of dimethyl ether is higher than that of pure aluminium oxide. This indicates that the methoxy groups located in the copper component are also involved in the formation of dimethyl ether. The formation of methyl formate is endothermic, that is, instead of increasing the equilibrium concentration of methyl formate with increasing temperature, it decomposes into CO and hydrogen at high temperatures (above 220⁰C):

                                             (5)

Experiments to determine the catalytic activity of samples with different catalyst loadings confirm that DME, CH₃OH and HCOOCH₃ are intermediates in the decomposition reactions of methanol to simpler substances. With a decrease in the sample size of the catalyst, its relative productivity for CH₃OCH₃ and HCOOCH₃ increases, which means that these products do not have time to decompose due to the short reaction time of the substances with the catalyst (Fig. 2).

Since hydrogen is a product of the dehydration of CH₃OH, it pushes both the equilibrium concentration of HCOOCH₃ and the rate of the chemical reaction away from equilibrium. The degree of this effect depends on the mechanism of the catalytic reaction, since the dehydration reaction is a rather complex multi-step process, hydrogen atoms are not gradually separated from the CH₃OH molecule, and further interaction of oxygen groups (formyl and methoxy) is observed.

Figure 2. Temperature dependence of the specific efficiency of CuO*ZnO*Al₂O₃*ZrO₂*СаО/bentonite catalyst in the decomposition reaction of methanol on dimethyl ether (1, 2, 3) and methyl formate (1`, 2`, 3`) at different loading of the catalyst: 1 - 0.01 g; 2 – 0.025 g; 3 – 0.05 g

Figure 3. Dependence of methanol conversion rate on reaction temperature in CuO*ZnO*Al₂O₃*ZrO₂*СаО/bentonite catalyst: 1 - initial mixture СН₃ОН/Ar, 1` - initial mixture СН₃ОН/(Ar+Н₂)

1, 2, 3 - initial mixture СН₃ОН /Ar;

1`, 2`, 3` - initial mixture СН₃ОН /(Ar+Н₂).

Figure 4. Dependence of the reaction temperature on the relative performance of the CuO-ZnO/Al₂O₃catalyst in the decomposition reaction of methanol on dimethyl ether (1; 1`), MF (2; 2`) and methane (3; 3`)

However, the amount of CH₃OCH₃ formed generally increases slightly at both low and high reaction temperatures, although the reasons for this increase vary between different temperature ranges. If this increases at low temperatures, it is due to the fact that the parallel pathway of methyl formate formation is suppressed and, accordingly, most of the CH₃O groups can pass to DME, then at high temperatures, a large concentration of hydrogen suppresses the decomposition reaction of dimethyl ether, since hydrogen is one of its products. This effect is positive in terms of the direct production of dimethyl ether from synthesis gas. Calculation of the Gibbs energy values of the direct reaction for the production of DME: Figure 5 prepared in the temperature range 300–650 K. 2CO+4H₂ ↔ CH₃OCH₃+H₂О

Figure 5. Calculated values of the Gibbs energy of the direct reaction to produce DME from CO and H₂.Under pressure of 0.5 (1), 1 (2), 1.5 (3), 2.5 (4) and 3.0 (5) MPa

A complex of physicochemical methods was used to study the properties of catalysts for obtaining DME before and after catalysisby RFA showed that the initial CuO*ZnO*Al₂O₃*ZrO₂*СаО/bentonite catalyst consisted mainly of copper and zinc oxides. After hydrogen reduction, the intensity of the copper oxide peak decreased and a peak corresponding to metallic copper appeared.As shown by the REM method, after catalytic tests, the CuO*ZnO*Al₂O₃*ZrO₂*СаО/bentonite surface coalesces with the formation of distinct dark-coloured peaks ranging from 1 to 40 μm. The surface of γ-A1₂O₃ is covered with craters with an average diameter of 2 μm and individual particles of about 1 μm.

Conclusion

The influence of various factors on the process of direct synthesis of CH₃OCH₃ from CO in one step was studied on the ZnO·Al₂O₃·ZrO₂·CuO·CaO/bentonite: γ-A1₂O₃catalyst and the increase in the volumetric rate decreased the conversion of carbon dioxide and the yield of dimethyl ether, carbon dioxide and hydrogen mol. It has been proven that the increase in the ratio leads to an increase in the yield of dimethyl ether and the conversion of soot gas.

As a result of studies on the effect of various factors on the production of target products, the process of obtaining DME from synthesis gas, pressure, temperature, and the mole ratio of hydrogen to H₂, because the reaction of direct synthesis of CH₃OCH₃ from synthesis gas prevails was found to lead to an increase in gas conversion.

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