TEXTURE AND PHYSICO-CHEMICAL CHARACTERISTICS OF THE VINYL ACETATE SYNTHESIS CATALYST

ABSTRACT

Vinyl acetate is the most important monomer in the plastics industry and a raw material for the production of polyvinyl acetate, polyvinyl alcohol and polyvinyl acetals. Polyvinyl acetate is a polymer with high adhesive properties and is widely used in the production of adhesives, water soluble latex paints and fabric finishing. The catalyst was prepared using the following conditions 5-25% solutions of zinc acetate and cadmium acetate and 1-3% solutions of zirconyl nitrate were absorbed by a microspherical nanoporous support (expanded clay) at 60°C by a circulating absorption method. The time of salt uptake was varied between 60-90 min. The studies showed that the synthesised nanocrystals consist of agglomerates whose average size increases with increasing firing temperature.

АННОТАЦИЯ

Винилацетат является важнейшим мономером в промышленности пластмасс и сырьем для производства поливинилацетата, поливинилового спирта и поливинилацеталей. Поливинилацетат – полимер с высокими адгезионными свойствами, широко используемый в производстве клеев, водорастворимых латексных красок, отделке тканей. Катализатор готовили в следующих условиях: 5-25% растворы ацетата цинка и ацетата кадмия и 1-3% растворы нитрата цирконила абсорбировали микросферическим нанопористым носителем (керамзитом) при 60°С методом циркуляционной абсорбции. Время заглатывания солей меняли в пределах 60-90 минут. Исследования показали, что синтезированные нанокристаллы состоят из агломератов, средний размер которых увеличивается с увеличением температуры обжига.

Keywords: acetylene, acetic acid, vinyl acetate, catalyst, sol-gel technology, material balance, flow chart.

Ключевые слова: ацетилен, уксусная кислота, винилацетат, катализатор, технология “золь-гель”,материалний баланс, технологическая схема.

Introduction

Among the oxygen-containing compounds produced in the petrochemical and basic organic synthesis industries in the world, complex vinyl esters occupy a leading position. The most important among them is vinyl acetate. Vinyl acetate is the most important monomer in the plastics industry and a raw material for the production of polyvinyl acetate, polyvinyl alcohol and polyvinyl acetals. Polyvinyl acetate is a polymer with high adhesion properties, widely used in the production of adhesives, water-soluble latex paints, and fabric finishing. At the same time, polyvinyl acetate, polyvinyl alcohol are important in medicine, agriculture, in the creation of synthetic rubbers, artificial fibres, biologically active substances, modified polymer compounds and other materials with unique properties [1-4].

Currently, the production of vinyl acetate in developed countries is carried out in two ways:

1. oxidation-based ethylene esterification (Moiseyev reaction). The process is based on the reaction between ethylene, acetic acid and oxygen in the presence of a catalyst [5] :

C ₂ H ₄ + CH ₃ COOH + O ₂ → CH ₂ = CHOCOCH ₃ + H ₂ O

2. Based on the catalytic vapour phase reaction between acetylene and acetic acid [6-12]:

C ₂ H ₂ + CH ₃ COOH → CH ₂ = CHOCOCH ₃

The first method is widely used because the cost of ethylene is cheaper than acetylene. Nowadays, cheap sources of acetylene are found as a by-product of new production processes. Therefore, the production of vinyl acetate from acetylene remains promising.

Acetylene, ethylene or acetaldehyde are used as raw materials for the production of vinyl acetate. The competitiveness of one or another method is largely determined by the availability and cost of the starting reagents.

The Pd-Au bimetallic catalytic system is used in the ethylene method and has a good performance in industrial production [13, 14]. Miyazawa et al. [15, 16] synthesised several highly active metal-oxide catalysts, but they have obvious defects in terms of stability. In carrier modification, Bong and co-workers [17, 18] modified activated carbon with hydrogen peroxide, nitric acid and acetic acid, and the adsorption capacity and adsorption rate were increased using zinc acetate catalyst . In terms of engineering modelling and theoretical calculations, many researchers have done a lot of work on the reaction mechanism, active centre, carrier modification and side reactions of VA synthesis.

Today, the global annual demand for vinyl acetate is 8 million tonnes. For this purpose it is necessary to carry out research work on creation of the system of optimisation of technology of preparation of vinyl acetate and its derivatives with participation of local catalysts. Therefore, it is urgent to carry out research and development work on development and improvement of the technology of vinyl acetate production with the participation of local catalysts.

Experimental part

To produce vinyl acetate from acetylene and acetic acid, solutions of zinc acetate, cadmium acetate and zirconium nitrate were adsorbed by microspherical nanoporous expanded claystone with a size of 200-500 μm. The total pore volume of the catalyst is 0.3-0.41 cm³ /g and the specific surface area is 50-170 m²/g.

Ceramsite was used as catalyst carrier, its surface area is 690-720 m² /g, total pore volume is 0.87-0.92 cm³ /g, micropore volume is 0.24-0.25 cm³/g. Pellet diameter is 200-500 μm. Acetylation reaction of acetylene takes place at 170-220 0 C, the volume flow rate of acetylene 0.54-0.84 l/cm 3 (cat)∙ h, the mass flow rate of acetic acid 0.3-0.5 g/cm³ (cat)∙ h was carried out according to the conditions.

The catalyst was prepared under the following conditions: 5-25% solutions of zinc acetate and cadmium acetate and 1-3% solutions of zirconyl nitrate were absorbed by microspherical nanoporous carrier (expanded clay) at 60°C by circulating absorption method. The time of salt ingestion was varied between 60-90 minutes. Absorption was carried out in the ratio of substance-carrier (ceramsite):solution 1:3-1:8.5. After the decomposition process, the catalyst was dried at room temperature for 24 hours and then in a desiccator at 100-130°C (with a temperature increase of 10°C every 1 hour). The amount of zinc acetate in the catalyst was 11-30%. 9 cm 3 was loaded into a flowable quartz reactor and the system was purged with a stream of nitrogen at a rate of 15 l/h for 10 minutes. Synthesis of vinyl acetate was carried out at 180°C and normal atmospheric pressure.

Under the above conditions the catalyst service life was 2000 hours.

The reaction products were analysed by chromatographic method: Tsvet-100 chromatograph with flame ionisation detector; Glass column 2x0.04m, fixed phase 15% Lestosil in chromium (d=0.250-0.315mm), column thermostat temperature 100^oC, evaporator temperature 120^oC, carrier gas (nitrogen) flow rate 30 ml/min. Qualitative analysis was carried out based on comparison of "witnesses" and catch sizes, quantitative analysis and internal normalisation method. Data on the textural characteristics of the samples were obtained by low temperature liquid nitrogen adsorption at 77.35 K on an ASAP 2010 M instrument. Samples analysed from completion 4 hours ago dried at 120⁰C for and 6 hours burned at 550⁰C in time Comparison of the surface area in the BET method was full of determination. Pore total volume maximum at saturation adsorbed nitrogen amount according to was calculated. The pore size distribution was determined by the BJH (Barrett-Joyner-Halendra) method. 

Experimental results and their discussion

Studies obtained by N₂ adsorption-desorption method show that ZnO powder heated at 500⁰C is mesoporous with a pore size of 7-9 nm (presented in Figure 1-2).

Figure 1. Scanning electron micrograph of ZnO powders heated to 500°C from zinc acetate

700°C the specific surface decreases from 8.6 to 3.3 m² /g due to particle adhesion (specification). As a result, a new porous structure is formed, the size of mesopores is 10-50 nm. Macropores are also formed.

Figure 2. Scanning electron micrograph of ZnO obtained without the use of structuring agent (a), ethylenediamine (b), hexamethylenediamine (c) and citric acid

The gel-like nanoscale zinc oxide samples were investigated by transmission electron microscopy (TEM). Fig. 3. shows the micrograph taken.

Figure 3. Histogram of nanoparticle size distribution on the surface of zinc oxide plates and illumination of nanoparticles of different sizes electron microscopy

Analysis of micrographs shows that the structure of zinc oxide gel consists of a set of plates of different length (from 0.4 to 20 μm, width from 100 to 250 nm, thickness from 5 to 15 nm).

To investigate the effect of annealing temperature of the nanoscale zinc oxide samples on their spatial composition and structure, the synthesised samples were dried to 125° C and annealed from 175° C to 750° C. All the samples were then subjected to X-ray phase analysis. The results of the X-ray phase analysis are shown in Figure 4 below.

As can be seen from the diffractogram presented in Figure 4, characteristic peaks of zinc oxide are observed in the ZnO samples annealed at different temperatures. In addition, the intensity of the peaks increases with increasing calcination temperature, which is explained by the aggregation of particles and disruption of the gel structure by thermally induced sticking of particles.

Figure 4. Results of XRD analysis of samples annealed from 125 °C to 750 °C

We then investigated the IR spectrum by burning the sample from 125° C to 750° C. Analysis of the IR spectrum of the zinc oxide sample dried at 125° C shows that the sample contains a hydroxyl group corresponding to an absorption region of 1020-1067 cm^-1 , and an acetate group corresponding to an absorption region of 725, 1332, 1400, 1550, 2880-2970 cm^-1 has become known. Also chemically and physically adsorbed water (677, 877, 918, 1550, 3145-3425 cm ^-1 ), amino group (677, 877, 1550 cm -1 ) and Zn(H₂O)₂ 2+ and Zn(NH₃)₂ 2 2+ groups are also available .

Conclusion

The studies have shown that the synthesised nanocrystals consist of agglomerates whose average size increases with increasing firing temperature.

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