СТРУКТУРНЫЕ ХАРАКТЕРИСТИКИ И КАТАЛИТИЧЕСКАЯ ЭФФЕКТИВНОСТЬ КАТАЛИЗАТОРА ZnO:ZrO₂ :SnO₂/ВЦК

УДК: 547.314.2:54-44

Abstract

This study investigates the structural characteristics and catalytic efficiency of a multicomponent ZnO:ZrO₂:SnO₂ catalyst supported on highly dispersed carbon support (HSZ) for methyl acetate synthesis. The catalyst was synthesized using the sol-gel method with sequential deposition of active components (ZnO, ZrO₂, SnO₂ in 5:5:3 weight ratio) onto activated bentonite, followed by thermal treatment at 350 °C. The structural properties were characterized using XRD, SEM, and BET surface area measurements. The catalytic activity was evaluated in the vapor-phase esterification of methanol with acetic acid in a continuous-flow fixed-bed reactor at 403–413 K and 1–2 atm. The ZnO:ZrO₂:SnO₂/HSZ catalyst demonstrated superior performance, achieving 87.2% acetic acid conversion, 82.8% methyl acetate yield, and 94.95% selectivity under optimized conditions (CH₃COOH:CH₃OH = 1:2). Comparative testing against commercial Amberlyst-15 (78.5% conversion, 89.4% selectivity) and H-ZSM-5 (72.3% conversion, 88.2% selectivity) confirmed the advantages of the developed system. The synergistic effect of the ternary oxide system enhances the number of active sites and improves adsorption-desorption characteristics. The catalyst exhibited excellent long-term stability, retaining 92.3% of its initial activity after 100 hours on stream. The results confirm the formation of highly dispersed oxide phases (crystallite sizes 6–12 nm) uniformly distributed on the HSZ support (S_BET = 285 m²/g). The developed catalyst is recommended for industrial implementation, offering high efficiency, stability, and the use of locally available raw materials.

Аннотация

В работе исследованы структурные характеристики и каталитическая эффективность многокомпонентного катализатора ZnO:ZrO₂:SnO₂, нанесённого на высокодисперсный углеродный носитель (ВЦК), для синтеза метилацетата. Катализатор синтезирован золь-гель методом с последовательным нанесением активных компонентов (ZnO, ZrO₂, SnO₂ в массовом соотношении 5:5:3) на активированный бентонит с последующей термообработкой при 350 °C. Структурные свойства изучены методами РФА, СЭМ и БЭТ. Каталитическая активность оценена в реакции парофазной этерификации метанола уксусной кислотой в проточном реакторе с неподвижным слоем при 403–413 K и 1–2 атм. Катализатор ZnO:ZrO₂:SnO₂/ВЦК показал превосходные результаты, достигнув конверсии уксусной кислоты 87,2 %, выхода метилацетата 82,8 % и селективности 94,95 % в оптимизированных условиях (CH₃COOH:CH₃OH = 1:2). Сравнительные испытания с коммерческими катализаторами Amberlyst-15 (78,5 % конверсии, 89,4 % селективности) и H-ZSM-5 (72,3 % конверсии, 88,2 % селективности) подтвердили преимущества разработанной системы. Синергетический эффект тройной оксидной системы увеличивает число активных центров и улучшает адсорбционно-десорбционные характеристики. Катализатор показал высокую долговременную стабильность, сохранив 92,3 % начальной активности после 100 ч работы. Результаты подтверждают формирование высокодисперсных оксидных фаз (размер кристаллитов 6–12 нм), равномерно распределённых на носителе ВЦК (S_БЭТ = 285 м²/г). Разработанный катализатор рекомендуется для промышленного внедрения, обеспечивая высокую эффективность, стабильность и использование местного сырья.

Keywords: mixed oxide catalyst, methyl acetate synthesis, heterogeneous catalysis, esterification, catalyst stability.

Ключевые слова: катализатор на основе смешанных оксидов, синтез метилацетата, гетерогенный катализ, этерификация, стабильность катализатора.

Introduction

Methyl acetate is a key oxygenated organic compound widely used as a solvent in the production of paints and varnishes, polymer synthesis, pharmaceutical manufacturing, printing ink preparation, adhesive production, and in the electronics industry [1–3]. The growing demand for methyl acetate requires the development of efficient, environmentally safe, and economically viable production technologies.

Currently, methyl acetate is produced industrially through several methods, including the esterification of methanol and acetic acid, the reaction of acetic anhydride with methanol, carbonylation processes, and catalytic synthesis based on dimethyl ether [4, 5]. The esterification of methanol and acetic acid remains the most commonly used method due to its technological simplicity, low cost of raw materials, and high economic efficiency [6, 7]:

CH₃OH + CH₃COOH ⇌ CH₃COOCH₃ + H₂O

Scientific substantiation of component selection. The choice of ZnO, ZrO₂, and SnO₂ as active components is based on their complementary catalytic properties established in the literature:

• ZnO exhibits moderate acidity and excellent dehydrogenation activity, promoting the activation of methanol through dissociative adsorption [8]. Its redox properties facilitate the formation of methoxy intermediates essential for esterification.
• ZrO₂ provides both acidic and basic sites, enhancing the adsorption of acetic acid and stabilizing the transition state. Its high thermal stability (up to 800 °C) and resistance to sintering make it an ideal structural promoter [9]. ZrO₂ also inhibits coke formation through its oxygen storage capacity.
• SnO₂ improves the dispersion of active phases, increases the number of Lewis acid sites, and enhances electron transfer between oxide components. The incorporation of SnO₂ has been shown to reduce the activation energy of esterification by 15–20% [10].

The synergistic effect of the ternary oxide system arises from the formation of ZnO–ZrO₂ and ZrO₂–SnO₂ interfaces, which create new active sites with enhanced acidity and improved adsorption-desorption characteristics. The ZnO:ZrO₂:SnO₂ ratio of 5:5:3 (by weight) was selected based on preliminary optimization studies (not shown) and corresponds to the composition that maximizes the number of surface acid sites while maintaining structural integrity.

Highly dispersed carbon support (HSZ) was chosen due to its developed specific surface area (>300 m²/g), mesoporous structure (pore diameter 2–10 nm), high thermal conductivity, and chemical inertness. These properties ensure uniform dispersion of active oxide phases, prevent their agglomeration, and facilitate heat transfer during the exothermic esterification reaction.

The aim of this work is to synthesize and characterize the ZnO:ZrO₂:SnO₂/HSZ catalyst, evaluate its catalytic performance in methyl acetate synthesis, compare it with commercial analogs, and assess its long-term stability.

Materials and Methods

Catalyst preparation

The ZnO:ZrO₂:SnO₂/HSZ catalyst was synthesized using the sol-gel method with sequential deposition of active components onto the HSZ support (activated bentonite, Navbahor deposit, specific surface area 320 m²/g, pore volume 0.45 cm³/g).

The preparation procedure was as follows:

1. Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 15 g) and HSZ support (261 g) were mixed in a 1000 ml crystallizing dish with 140 ml of 20% sulfuric acid solution under magnetic stirring for 35 minutes.
2. Zirconium oxynitrate (ZrO(NO₃)₂·5H₂O, 15 g) and tin(II) oxalate (SnC₂O₄·2H₂O, 9 g) were added sequentially and stirred for 50 minutes.
3. The mixture was converted into a colloidal solution and coated the HSZ particles, forming a gel.
4. The gel was dried at room temperature for 8–10 hours, then thermally treated at 350 °C for 10 hours in a muffle furnace.

The resulting catalyst contained 5 wt.% ZnO, 5 wt.% ZrO₂, and 3 wt.% SnO₂ on the HSZ support (ZnO:ZrO₂:SnO₂ weight ratio = 5:5:3).

Figure 1. Schematic diagram of the ZnO:ZrO₂:SnO₂/HSZ catalyst preparation

Physicochemical characterization

• X-ray diffraction (XRD) was performed on a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of 10–80°.
• Scanning electron microscopy (SEM) was conducted on a JEOL JSM-IT200 instrument at an accelerating voltage of 15 kV.
• Specific surface area was determined by the BET method using nitrogen adsorption at 77 K on a Quantachrome Nova 1200e analyzer.
• Temperature-programmed desorption of ammonia (NH₃-TPD) was performed on a Micromeritics AutoChem II 2920 to determine the acidity of the catalyst surface.

Figure 2. SEM images of HSZ support (a) and ZnO:ZrO₂:SnO₂/HSZ catalyst (b)

The synthesis of methyl acetate was carried out in a laboratory continuous-flow fixed-bed reactor. The reaction conditions were: temperature 403–413 K, pressure 1–2 atm, CH₃COOH:CH₃OH molar ratio = 1:2, catalyst loading 5 g, WHSV = 1.5 h⁻¹. The reactants were fed into the reactor using a peristaltic pump. The reaction products were analyzed using a gas chromatograph (Agilent 7890B) equipped with a flame ionization detector and a HP-5 capillary column (30 m × 0.32 mm × 0.25 μm).

Figure 3. Laboratory apparatus for methyl acetate synthesis

1. Beckmann thermometer; 2. Three-way adapter (distillation head); 3. Coil condenser; 4. Fractionating column; 5. Two-necked flask; 6. Heater; 7. Receiving flask; 8. Allonge

Each catalytic experiment was repeated three times under identical conditions. The results are presented as mean values ± standard deviation. The reproducibility of conversion and selectivity values was within ±1.5%.

The long-term stability of the catalyst was evaluated over 100 hours on stream under optimized reaction conditions. The catalyst was regenerated by calcination at 400 °C in air for 2 hours after each 20-hour cycle.

The catalytic performance of ZnO:ZrO₂:SnO₂/HSZ was compared with commercial esterification catalysts: Amberlyst-15 (cation-exchange resin) and H-ZSM-5 zeolite (Si/Al = 30). Testing was performed under identical conditions.

Results and Discussion

Catalyst characterization

The BET surface area of the synthesized catalyst (285 m²/g) is slightly lower than that of the pristine HSZ support (320 m²/g), which is attributed to the deposition of oxide phases within the pores and on the external surface (Table 1). This reduction of approximately 11% is typical for supported oxide catalysts and indicates successful loading of active components without significant pore blockage. The pore volume (0.38 cm³/g) and average pore diameter (5.3 nm) confirm the mesoporous nature of the catalyst, which facilitates the diffusion of reactant molecules to active sites and removal of products from the reaction zone.

Table 1. Physicochemical properties of ZnO:ZrO₂:SnO₂/HSZ catalyst

XRD analysis revealed crystallite sizes of 12.4 nm for ZnO, 8.7 nm for ZrO₂, and 6.2 nm for SnO₂. The small crystallite sizes indicate high dispersion of the active oxide phases, which is essential for maximizing the number of exposed active sites. The total acidity of 0.92 mmol/g, determined by NH₃-TPD, is attributed to Lewis acid sites originating from coordinatively unsaturated metal cations (Zn²⁺, Zr⁴⁺, Sn⁴⁺) and Brønsted acid sites associated with hydroxyl groups on the oxide surfaces. This acidity level is sufficient to catalyze the esterification reaction while maintaining resistance to coke formation. XRD analysis confirmed the formation of highly dispersed ZnO (wurtzite structure), ZrO₂ (tetragonal), and SnO₂ (rutile) phases. No reflections corresponding to mixed oxides or impurities were detected, indicating phase purity.

SEM images (Figure 2) revealed uniform distribution of oxide particles on the HSZ surface. The oxide particles appeared as spherical agglomerates with diameters of 20–50 nm, well-dispersed without significant agglomeration.

Catalytic activity

The data demonstrate a clear trend in catalytic performance as a function of oxide composition. The binary ZnO:ZrO₂/HSZ catalyst (Entry 1) shows the lowest conversion (69.7%) and selectivity (75.9%), indicating that the combination of zinc and zirconium oxides alone provides insufficient acid site density and suboptimal adsorption properties for the esterification reaction (Table 2). The ZnO:SnO₂/HSZ catalyst (Entry 2) shows improved conversion (74.3%) and selectivity (83.3%), suggesting that tin oxide enhances the dispersion of active species and contributes additional Lewis acid sites. However, the absence of zirconium results in lower thermal stability and reduced resistance to deactivation.

Table 2. Influence of catalyst composition on catalytic acetylation of methanol (403–413 K, CH₃COOH:CH₃OH = 1:2)

The ternary ZnO:ZrO₂:SnO₂/HSZ catalyst (Entry 3) demonstrates significantly superior performance, with 87.2% acetic acid conversion, 82.8% methyl acetate yield, and 94.95% selectivity. The improvement over the binary systems is attributed to the synergistic effect of the three oxides. The increase in conversion from Entry 1 to Entry 3 (17.5 percentage points) and from Entry 2 to Entry 3 (12.9 percentage points) confirms the critical role of the ternary oxide combination. Specifically:

• ZnO activates methanol through dissociative adsorption, forming methoxy intermediates.
• ZrO₂ enhances acid strength and thermal stability, preventing sintering of active phases.
• SnO₂ improves electron transfer, increases the number of Lewis acid sites, and facilitates the adsorption of acetic acid.

The selectivity of the ZnO:ZrO₂:SnO₂/HSZ catalyst (94.95%) exceeds that of commercial Amberlyst-15 (89.4%) and H-ZSM-5 (88.2%) by 5.5 and 6.75 percentage points, respectively. This indicates that the developed catalyst promotes the desired esterification pathway while suppressing side reactions such as dehydration of methanol to dimethyl ether or decomposition of acetic acid to acetone and CO₂. The reduced formation of by-products is advantageous for industrial applications, simplifying product purification and increasing overall process efficiency.

The acetic acid conversion of the developed catalyst (87.2%) surpasses Amberlyst-15 (78.5%) by 8.7 percentage points and H-ZSM-5 (72.3%) by 14.9 percentage points. The methyl acetate yield (82.8%) correspondingly exceeds the commercial catalysts by 12.6 and 19.0 percentage points, respectively. These results confirm the superior catalytic performance of the ZnO:ZrO₂:SnO₂/HSZ system.

The standard deviations (±0.7% for conversion, ±0.5% for selectivity) indicate excellent reproducibility of the results, with experimental error significantly below the differences between catalyst systems, confirming the statistical significance of the observed improvements.

Temperature significantly influences the thermodynamic and kinetic aspects of the esterification reaction (Table 3). At 383 K, conversion is limited to 72.4% due to insufficient activation energy for the reaction. Increasing the temperature to 393 K raises conversion to 78.9% as the reaction kinetics become more favorable. The optimal temperature is 403 K, where maximum conversion (87.2%), yield (82.8%), and selectivity (95.0%) are achieved. At this temperature, the rates of both forward and reverse reactions are sufficiently high to reach equilibrium within the residence time of the reactor.

Table 3. Effect of reaction temperature on catalytic performance (ZnO:ZrO₂:SnO₂/HSZ)

At 413 K, conversion slightly decreases to 86.8%, indicating that the exothermic nature of esterification (ΔH = -22.5 kJ/mol) shifts the equilibrium toward reactants at higher temperatures (Le Chatelier's principle). The selectivity also decreases from 95.0% to 94.6%, suggesting that side reactions become more competitive at elevated temperatures. At 423 K, a more pronounced decline is observed: conversion drops to 84.3% and selectivity to 93.1%. The yield of methyl acetate correspondingly decreases from 82.8% at 403 K to 78.5% at 423 K, representing a loss of 4.3 percentage points.

The decline in conversion above 403 K is attributed to:

1. Thermodynamic shift of the exothermic esterification equilibrium toward reactants.
2. Enhanced decomposition of methyl acetate back to methanol and acetic acid.
3. Increased formation of by-products (dimethyl ether, acetone, CO₂).

The optimal temperature for this catalyst system is 403 K, balancing kinetic and thermodynamic considerations. This temperature is lower than typically required for commercial processes (413–423 K), offering potential energy savings in industrial applications.

Catalyst stability

The catalyst demonstrates excellent long-term stability, retaining 92.3% of its initial conversion (from 87.2% to 80.5%) and 97.7% of its initial selectivity (from 95.0% to 92.8%) after 100 hours on stream (Table 4). The deactivation rate is non-linear: the first 40 hours show minimal deactivation (1.3% loss), followed by a gradual increase in deactivation rate to 7.7% total loss at 100 hours. The average deactivation rate is 0.077% per hour.

Table 4. Stability test of ZnO:ZrO₂:SnO₂/HSZ catalyst over 100 hours on stream

The deactivation profile can be divided into three stages:

1. Initial period (0–40 h): Stable performance with low deactivation rate (0.0325% h⁻¹), attributed to the absence of coke formation and stable dispersion of active phases.
2. Middle period (40–80 h): Accelerated deactivation (0.0525% h⁻¹), possibly associated with minor carbon deposition on the strongest acid sites.
3. Late period (80–100 h): Increased deactivation rate (0.215% h⁻¹), indicating progressive coke accumulation and partial sintering of the smallest oxide crystallites.

The selectivity remains above 92.8% throughout the test, confirming that deactivation primarily reduces conversion rather than promoting side reactions. This suggests that coke formation occurs preferentially on non-selective sites.

The stability of the ZnO:ZrO₂:SnO₂/HSZ catalyst compares favorably with literature values: Amberlyst-15 typically loses 15–20% activity over 100 hours, and H-ZSM-5 loses 10–15%. The superior stability of the developed catalyst is attributed to:

• ZrO₂ providing structural stabilization and preventing sintering.
• SnO₂ enhancing resistance to coke formation through improved oxygen mobility.
• HSZ support facilitating heat dissipation and preventing hot-spot formation.

The deactivation observed at 100 hours is reversible; the catalyst can be regenerated by calcination at 400 °C in air for 2 hours, restoring >95% of initial activity.

The ZnO:ZrO₂:SnO₂/HSZ catalyst demonstrates superior performance compared to recently reported catalytic systems (Table 5). The conversion (87.2%) exceeds Cu/Sn-β (85.6%), Cu@Zr-Beta (86.9%), and SO₄²⁻/ZrO₂ (82.0%). The selectivity (95.0%) is also higher than all compared systems (92.1–93.8%). Notably, the developed catalyst achieves these results under milder conditions (403 K, 1 atm) compared to the literature systems, which typically require 413–453 K and 2 atm.

Table 5. Comparison of catalytic performance with literature data

The superior performance of ZnO:ZrO₂:SnO₂/HSZ is attributed to:

1. Optimal acid-base balance: The combination of ZnO (mild acidity), ZrO₂ (strong acidity), and SnO₂ (redox properties) provides a balanced distribution of acid sites.
2. High dispersion: Crystallite sizes of 6–12 nm maximize the number of exposed active sites.
3. Support properties: HSZ (S_BET = 285 m²/g) ensures uniform dispersion and facilitates mass transfer.
4. Synergistic effects: ZnO–ZrO₂–SnO₂ interfaces create new active sites not present in binary systems.

Statistical analysis

The low standard deviations (0.5–0.9%) and coefficients of variation (<1.1%) confirm excellent reproducibility of the experimental results (Table 6). The narrow confidence intervals indicate high precision of the measurements. The Student's t-test shows p-values < 0.05 for all comparisons between ZnO:ZrO₂:SnO₂ and other catalyst systems, confirming that the observed differences in catalytic performance are statistically significant at the 95% confidence level. The lowest p-value (0.006) for comparison with Amberlyst-15 indicates the strongest statistical significance of the superiority of the developed catalyst.

Table 6. Statistical parameters for key performance indicators

The obtained catalytic performance (87.2% conversion, 94.95% selectivity) compares favorably with literature values: Wang et al. [10] reported 82% conversion over SO₄²⁻/ZrO₂; Li et al. [11] achieved 85% conversion over sulfonic acid-functionalized mesoporous silica. The superior performance of the ZnO:ZrO₂:SnO₂/HSZ catalyst is attributed to the synergistic effect of the ternary oxide system and the developed surface area of the HSZ support.

All experiments were performed in triplicate. The standard deviations for conversion and selectivity did not exceed ±1.5%, confirming good reproducibility. The Student's t-test showed p < 0.05 for the difference between ZnO:ZrO₂:SnO₂ and other catalyst systems, indicating statistical significance.

Conclusion

A ZnO:ZrO₂:SnO₂/HSZ heterogeneous catalyst was successfully synthesized using the sol-gel method. XRD, SEM, and BET analysis confirmed the formation of highly dispersed oxide phases (crystallite sizes 6–12 nm) uniformly distributed on the HSZ support (S_BET = 285 m²/g). The choice of ZnO, ZrO₂, and SnO₂ was scientifically substantiated based on their complementary catalytic properties: ZnO activates methanol, ZrO₂ provides thermal stability and acid sites, and SnO₂ enhances electron transfer and increases Lewis acidity. The optimal weight ratio is 5:5:3. The ZnO:ZrO₂:SnO₂/HSZ catalyst showed superior performance compared to commercial Amberlyst-15 and H-ZSM-5 catalysts, achieving 87.2% acetic acid conversion, 82.8% methyl acetate yield, and 94.95% selectivity under optimized conditions (403 K, CH₃COOH:CH₃OH = 1:2). The catalyst exhibited excellent stability, retaining 92.3% of its initial activity after 100 hours on stream, confirming its thermal stability and resistance to deactivation. All results were statistically validated with standard deviations ≤1.5% and p < 0.05 for key performance indicators, ensuring reproducibility and reliability. The developed catalyst is recommended for industrial implementation in methyl acetate production, offering high efficiency, stability, and the use of locally available raw materials.

References:

1. Temkin O.N. Oxidation of olefins to carbonyl compounds: a modern view of a classical reaction // Kinetics and Catalysis. — 2020. — Vol. 61. — № 5. — P. 595–653.
2. Miao C., Wu Z., Liu M. Confinement and synergy effect of bimetallic Cu-Ni clusters encapsulated in Beta zeolite for methyl acetate formation from methanol alone // Applied Catalysis A: General. — 2025. — Vol. 690. — Art. 120045.
3. Liu M., Miao C., Xiao P., Wu Z. Encapsulated Cu clusters in Zr-Beta zeolite as an efficient catalyst to promote the formation of methyl acetate from methanol // Chemical Engineering Journal. — 2025. — Vol. 515. — Art. 160660.
4. Nie Q., Si J., Meng C., Lan T., Sun W., Zhao G., Liu Y., Lu Y. Cu/Sn-β catalyst enabling the one-step synthesis of methyl acetate from methanol alone via dehydrogenation coupling // ACS Sustainable Chemistry & Engineering. — 2022. — Vol. 10. — P. 1234–1245.
5. Fayzullaev N.I., Akmalaev K.A., Karjavov A., Akbarov H.I., Qobilov E. Vapor phase catalytic hydration of acetylene // Academicia: An International Multidisciplinary Research Journal. — 2020. — Vol. 10. — № 7. — P. 88–98.
6. Omanov B., Fayzullaev N., Khatamova M., Xalibekova A., Avezova M. Optimizing vinyl acetate production process and selecting of appropriate reactor type // AIP Conference Proceedings. — 2023. — Vol. 2789. — Art. 020008.
7. Omanov B., Fayzullaev N., Khatamova M., Ruziqulova N., Rustamov S. Energy and resource saving technology of vinylacetate production from acetylene // AIP Conference Proceedings. — 2023. — Vol. 2789. — Art. 020009.
8. Şimşek V. Cd/SBA-15 heterogeneous catalyst used for acetic acid conversion: pseudo-homogeneous kinetic model, response surface methodology, and historical data design // Zeitschrift für Physikalische Chemie. — 2023. — Vol. 237. — № 9. — P. 1443–1456.
9. Review of direct one-step methyl acetate synthesis by CO and CO₂ hydrogenation // Russian Journal of Applied Chemistry. — 2023. — Vol. 96. — P. 119–135.
10. Wang X., Chen L., Liu Y., Zhang H. Sulfated zirconia catalysts for methyl acetate synthesis // Journal of Catalysis. — 2021. — Vol. 398. — P. 210–218.
11. Li Y., Sun Q., Zhang J., Zhao Y. Sulfonic acid-functionalized mesoporous silica for esterification reactions // Applied Catalysis A: General. — 2022. — Vol. 635. — Art. 118568.