SYNTHESIS OF ETHYL TERT-BUTYL ETHER OVER HSZ-CATIONITES: STRUCTURE–ACIDITY–ADSORPTION–ACTIVITY CORRELATION

ABSTRACT

This work presents the preparation, modification, and application of mesoporous zeolite cationites with high sorption and catalytic properties for the synthesis of ethyl tert-butyl ether (ETBE) from an ethanol–isobutene mixture. Decationisation of the Na-form was achieved by ion exchange with a 25% NH₄Cl solution, while modification of the H-form was carried out by impregnation with various salt and acid solutions, sequential binary modification, drying at 383 K, and calcination at 773 K. Mechanochemical activation with titanium(IV) oxide (293 K, 24 h) and the synthesis of titanoaluminosilicate gels with SiO₂/Al₂O₃ = 30 in the presence of hexamethylenediamine were also tested. The samples were characterised using IR spectroscopy and XRD: absorption bands typical of TO₄ tetrahedral vibrations at 1120, 820, and 470 cm⁻¹, as well as the structure-sensitive band at 560 cm⁻¹, were identified.

АННОТАЦИЯ

В данной работе представлены результаты получения, модификации и применения мезопористых цеолитных катионитов с высокими сорбционными и каталитическими свойствами для синтеза этил-трет-бутилового эфира (ЭТБЭ) из смеси этанола и изобутена. Декатионирование Na-формы проводилось путем ионного обмена с 25%-ным раствором NH₄Cl, тогда как модификация Н-формы осуществлялась пропиткой различными солевыми и кислотными растворами, последовательной бинарной модификацией, сушкой при 383 К и прокаливанием при 773 К. Также были испытаны методы механохимической активации с использованием оксида титана (IV) (293 К, 24 ч) и получения титаноалюмосиликатных гелей с отношением SiO₂/Al₂O₃ = 30 в присутствии гексаметилендиамина. Образцы охарактеризованы методами ИК-спектроскопии и рентгенофазового анализа: идентифицированы полосы поглощения, характерные для колебаний тетраэдров TO₄ (1120, 820 и 470 см⁻¹), а также структура-чувствительная полоса при 560 см⁻¹.

Keywords: mesoporous zeolite cationites; decationisation; modification; acid sites; pKa distribution; ethyl tert-butyl ether (ETBE); catalytic activity.

Ключевые слова: мезопористые цеолитные катиониты; декатионирование; модификация; кислотные центры; распределение pKa; этил-трет-бутиловый эфир (ЭТБЭ); каталитическая активность.

Introduction

Since the 1970s, methyl tert-butyl ether (MTBE) has been widely used as one of the most important oxygenated additives in motor gasoline [1–4]. However, the large-scale use of MTBE has also raised  several environmental concerns. Market analyses indicate that the global production of MTBE continues to grow and is projected to reach 22.9 billion USD by 2026 [5–8]. This trend is mainly attributed to its economic efficiency and its effectiveness in improving fuel quality.

Nevertheless, the extensive application of MTBE has placed the issue of its environmental safety on the agenda. Environmental monitoring data show that MTBE is among the most frequently detected volatile organic compounds in groundwater [9–12]. This is explained by its high solubility in water and its weak adsorption capacity on soil particles [13–15]. As a result, MTBE can migrate rapidly through groundwater and contaminate large areas.

The potential impact of MTBE on human health has also attracted serious concern. At high concentrations, it has been shown to cause irritation of the eyes and skin.

Materials and Methods

The decationisation of the Na-form of mesoporous zeolite cationites with high sorption and catalytic properties was carried out by treating 10 g of the material with 100 g of a 25% ammonium chloride solution. A granular fraction of 0.05–0.2 mm of the zeolite cationite was separated and subjected to further treatment.

Modified mesoporous zeolite cationite catalysts were prepared from the H-form of the material by impregnation with aqueous solutions of selected salts or acids. After impregnation, the samples were dried at 383 K for 8 h and subsequently calcined at 873 K for 16 h.

The IR spectra of the synthesised mesoporous zeolite cationites (Figure 1) were recorded on a UR-20 spectrophotometer in the range 450–2000 cm⁻¹, as this region corresponds to the fundamental absorption bands of AlO₄ and SiO₄ tetrahedral vibrations within the framework. For this purpose, 1–2 mg of the sample was mixed with 400 mg of KBr, pressed into a pellet using a special ring die, and placed in a sample holder before being inserted into the spectrophotometer. The recorded spectra displayed characteristic bands attributable to two types of vibrations.

Figure 1. Synthesised mesoporous zeolite cationite with high sorption and catalytic properties

The first type of vibrations in all mesoporous zeolite cationites with high sorption and catalytic properties were observed as the most intense absorption bands at 1120 cm⁻¹, 820 cm⁻¹, and 470 cm⁻¹. The strong absorption band at 1120 cm⁻¹ is attributed to the antisymmetric stretching vibrations of TO₄ tetrahedra. With an increasing proportion of framework-coordinated Al atoms, this absorption band is shifted towards the lower-frequency region. The absorption band at 460 cm⁻¹ corresponds to the bending vibrations of the TO₄ tetrahedra.

The second type of vibrations, which are highly sensitive to the nature of the linkages between tetrahedra, their topology, and the environment of secondary structural units in mesoporous zeolite cationites, is manifested by the band at 560 cm⁻¹. The presence of intense absorption bands across all spectra confirms that the studied samples correspond to mesoporous zeolite cationites. Moreover, the characteristic features in the regions 1300–900 cm⁻¹, 820 cm⁻¹, and 400–600 cm⁻¹ are consistent for all investigated samples, thereby allowing them to be attributed to the HSZ structural type. The reproducibility of the 560 cm⁻¹ band in all IR spectra further supports this classification.

Results and Discussion

The acidic properties of sulfonic cation exchangers are associated with the presence of functional groups [–SO₂–OH] within the styrene–divinylbenzene matrix of the resin. The distribution of adsorption centres by pKa, ranging from 0.8 to 6.4, was determined from the adsorption of colour indicators. Table 1 presents the data on the distribution of active sites on the resin surface in aqueous medium.

As can be seen, three main groups of sites are clearly distinguished in the spectra at pKa 1.1, 2.5, and 5.5, corresponding to the acidity functions H₀ = 2.6, 2.8, and 5.7. Among these, the sites with pKa 2.5 (H₀ = 2.8) are particularly strong and are characteristic of all the studied resins. Such a distribution of adsorption centres suggests that the investigated cation exchangers contain Brønsted acid sites of a similar nature but present in different proportions.

The Amberlyst 15Dry and Amberlyst 36Dry samples exhibit the lowest concentration of acid centres, at 1.65×10⁻⁷ and 3.2×10⁻⁷ mol/m², respectively. In the case of Amberlite IR-120 and Tulsion T-52H, which contain equal amounts of divinylbenzene but differ in the  number of pore-forming agents, the concentration of acid centres increases in proportion to the quantity of stabilising agent in the cation exchanger. A similar trend was also observed in the (–R–SO₃H–)ₙ/HSZ and Amberlyst 36Dry series.

Tulsion T-52H and Amberlyst 15Dry catalysts are both characterised by equal amounts of pore-forming agent; however, the content of divinylbenzene in Amberlyst 15Dry is three times higher than in Tulsion T-52H. Consequently, the overall concentration of acid sites in Amberlyst 15Dry decreases sharply, reaching only 1.65×10⁻⁷ mol/m², whereas for Tulsion T-52H it amounts to 29.63×10⁻⁷ mol/m².

Table 1.

Acidic and catalytic properties of sulfonic cation exchangers

When comparing the obtained acidic characteristics of the sulfonic cation exchangers with their structural properties, it can be noted that an increase in the amount of pore-forming agent leads both to a larger micropore volume and to a higher number of acid sites in the case of Amberlite IR-120 and Tulsion T-52H. For the Amberlite IR-120 and (–R–SO₃H–)ₙ/HSZ samples, it should also be emphasised that, as the degree of correlation between the sample structure and the number of acid sites increases, the micropore volume (Vₘ.ₚ) tends to decrease.

Figure 2. Ethanol adsorption isotherms on sulfonic cation exchangers at T = 293 K

For the studied resin samples, the adsorption heats calculated from the experimental data are consistent with the expected values. Figure 3 presents the dependence of the isosteric heats of adsorption (q) on the amount adsorbed for the investigated samples. For KU-2-8, Amberlite IR-120, and Tulsion T-52H, the initial adsorption heats at α = 0.5 mmol/g are 36.5, 16.6, and 40.7 kJ/mol, respectively. These values are somewhat lower than, but in the case of Tulsion T-52H nearly equal to, the heat of ethanol condensation, which is 38.5 kJ/mol. The Amberlyst 15Dry sample exhibits the highest adsorption heat (51 kJ/mol) in the low-pressure region, exceeding the condensation heat. In KU-2-8, the heat of ethanol adsorption, after slightly elevated initial values, rapidly decreases and then changes only marginally.

Figure 3. Dependence of isosteric heats of adsorption on the amount adsorbed

The kinetic characteristics obtained for sulfonic cation exchangers indicate that, although the equilibrium adsorption time varies among the resin samples, it consistently decreases with increasing temperature. Table 2 presents the limiting values of ethanol adsorption on the studied sulfonic cation exchangers at two adsorption temperatures and at a pressure of P/Pₛ = 0.02 (where adsorption is assumed to occur predominantly on individual adsorption centres under low-pressure conditions). The Amberlyst 15Dry sample exhibits the highest ethanol adsorption capacity, reaching 1.19 mmol/g, which is nearly twice as high as that of the other sulfonic cation exchangers, demonstrating superior performance.

Table 2.

Limiting values of ethanol adsorption on sulfonic cation exchangers at different experimental temperatures and at

The minimum ethanol adsorption capacities are characteristic of Amberlite IR-120 and Tulsion T-52H catalysts (0.10 and 0.05 mmol/g at T = 293 K, respectively). These catalysts also possess the lowest microporous structural parameters, indicating a direct correlation between the adsorption capacity of sulfonic cation exchangers for ethanol and their structural characteristics.

During the experiments, we studied the influence of temperature, process pressure, space velocity, and the ethanol/n-butene ratio on the conversion of the feedstock and the selectivity of ethyl tert-butyl ether (ETBE) formation. The temperature range of the experiments was determined by the thermal stability of the ion-exchange resins; therefore, the reactions were conducted at 333, 343, 353, 363, and 373 K.

The results showed that, for KU-2-8, (–R–SO₃H–)ₙ/HSZ, Amberlite IR-120, Tulsion T-52H, and Amberlyst 15Dry catalysts, as the reaction temperature increased from 333 K to 353 K at a pressure of 0.8 MPa, both the conversion of the butane–butylene fraction and the selectivity of ETBE formation increased. However, further heating to 373 K caused a slight decline in both conversion and selectivity.

It is evident that samples lacking large mesopores and characterised by the lowest concentrations of strong (H₀ = 2.8) and weak (H₀ = 4.8) acid sites, such as Amberlyst 15Dry, were inactive under the investigated conditions. The predominance of relatively small mesopores (d = 18–40 Å) hinders the diffusion of bulky butene molecules into the pores of sulfonic cation exchangers and restricts their access to active sites within the porous network. As a result, the conversion of i-butene is significantly lower compared with other catalysts.

At 0.8 MPa, Amberlite IR-120 (d_max = 17.50 Å, >100 Å), Tulsion T-52H (d_max = 25.65 Å, >170 Å), and KU-2-8 gel-structured samples (d_max = 14 Å, >100 Å) displayed different catalytic behaviours. Among them, Amberlite IR-120 achieved the highest selectivity for ETBE synthesis, reaching 98%. In small-pore samples where diffusion limitations occur, the reaction proceeds mainly on the external surface, with minimal participation of the internal surface, which significantly decreases the catalytic activity in ETBE formation.

Conclusion

The modification methods for HSZ-cationites, including ion exchange of the Na-form with 25% NH₄Cl solution, impregnation–drying (383 K, 6 h)–calcination (773–873 K), and mechanochemical activation, were confirmed to be effective. The addition of titanium oxide and the formation of SiO₂/Al₂O₃ = 30 titanoaluminosilicate gels contributed to the development of mesoporous structures.

IR spectroscopy revealed absorption bands characteristic of TO₄ tetrahedral vibrations at 1120, 820, and 470 cm⁻¹, as well as a structure-sensitive band at 560 cm⁻¹, confirming both the zeolitic nature of all samples and the influence of the Si/Al ratio in the framework.

In sulfonic cation exchangers, acidity was distributed within the pKa range of 0.8–6.4, with the highest concentration of centres observed at approximately pKa 2.5 (H₀ ≈ 2.8). The number and tunability of Brønsted acid sites were shown to exert a decisive influence on catalytic activity.

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