Introduction

Light olefins, particularly ethylene and propylene, are among the most important primary feedstocks in the modern petrochemical industry and have a wide range of applications [1, 2]. These compounds play a key role in the production of polymeric materials, synthetic fibers, and various organic chemicals. Therefore, their efficient production remains a subject of extensive research [3].

Traditionally, olefins are produced via steam cracking, a process characterized by high energy consumption and strong dependence on liquid hydrocarbon feedstocks [3, 4]. As a result, increasing attention has been directed toward alternative technologies based on natural gas resources [4, 5].

The catalytic conversion of methyl chloride to light olefins (CTO process) has emerged as a promising alternative route. This process enables the direct transformation of an intermediate obtained via methane chlorination into olefins, thereby reducing process complexity and eliminating intermediate synthesis steps [5]. From a mechanistic perspective, the CTO process is analogous to the methanol-to-olefins (MTO) reaction, and therefore similar catalytic systems are employed [6, 7].

SAPO-34 molecular sieve catalysts exhibit high performance in these processes due to their CHA-type microporous structure. This structure creates a confined reaction environment that promotes the formation of hydrocarbon pool intermediates, which play a central role in olefin generation [8,9]. As a result, the formation of heavier hydrocarbons is suppressed, while selectivity toward light olefins is enhanced.

Furthermore, the efficiency of the process depends not only on catalyst properties but also on key operating parameters such as temperature, contact time, and feed flow rate [10, 11]. Careful optimization of these parameters is essential to achieve high conversion and selectivity.

Catalyst deactivation during operation, mainly caused by coke formation, represents a major challenge affecting process stability [12, 13]. Recent studies have shown that the use of naturally derived materials and composite catalyst systems can improve catalytic performance and resistance to deactivation [14, 15].

In this context, a comprehensive understanding of the structural properties of SAPO-34 catalysts, the reaction mechanism, and the interplay between operating parameters is essential for improving process efficiency and advancing CTO technology [11,16].

Experimental Section

The SAPO-34 molecular sieve catalyst was synthesized using a hydrothermal approach. For this purpose, aluminum, phosphorus, and silicon precursors were combined in appropriate proportions to form a uniform reaction mixture. An organic structure-directing agent was then introduced, and the system was transformed into a homogeneous gel. The resulting gel was transferred into a Teflon-lined autoclave and maintained under autogenous pressure at 180°C for 24 hours to allow crystallization to proceed.

Following the crystallization step, the solid product was separated by filtration, thoroughly washed several times with distilled water, and subsequently dried at moderate temperature. The dried material was then subjected to thermal treatment at approximately 550°C. During this stage, the organic template decomposed, leading to the development of the internal pore structure and activation of the catalyst.

The structural features of the synthesized material were examined by Fourier transform infrared (FTIR) spectroscopy. Spectra were recorded over a wide wavenumber range, and the observed absorption bands were interpreted in relation to the framework structure. Strong bands in the region of 1080–1100 cm⁻¹ were attributed to vibrations associated with oxygen bridges within the silicoaluminophosphate framework. Additional signals observed at lower frequencies, particularly around 600 cm⁻¹, confirmed the presence of a microporous structure. Broad absorption features detected in the higher wavenumber region were assigned to surface hydroxyl groups.

Catalytic performance was evaluated in a tubular fixed-bed reactor under controlled conditions. A defined amount of catalyst was loaded into the reactor and activated in situ under an inert atmosphere prior to the reaction. The feed containing methyl chloride was introduced together with a carrier gas, ensuring steady operating conditions throughout the experiment. Reaction temperature was varied over a wide range, while the feed rate was adjusted to achieve the desired WHSV values.

The gaseous reaction products were analyzed using gas chromatography equipped with a flame ionization detector (FID). Quantitative analysis was carried out based on prior calibration of the system. Conversion of the feed and product distribution were calculated from the composition of the outlet stream.

Since a gradual decline in catalytic activity was observed during operation, the regeneration behavior of the catalyst was also investigated. The spent catalyst was treated at elevated temperature in an oxygen-containing atmosphere, and subsequent tests demonstrated that the catalytic performance was largely restored.

To ensure reliability, each experimental condition was repeated multiple times, and the reported values represent averaged results. The variation between repeated measurements remained low, confirming the consistency and reproducibility of the data.

Results and Discussion

The effect of reaction temperature on methyl chloride conversion and light olefin selectivity is presented in Table 1. Temperature is one of the most important variables in the CTO process because it influences both the activation of methyl chloride and the rate of secondary transformations.

Table 1

Effect of temperature on catalytic performance

As the temperature increased from 420 to 445°C, methyl chloride conversion increased from 57 ± 2 % to 72 ± 3 %. This behavior can be explained by the improved activation of CH3Cl molecules on Brønsted acid sites of SAPO-34. In the same temperature region, selectivity toward light olefins also increased and reached 82 ± 2 mol %, indicating that the formation of the desired products was favored under these conditions.

A further increase in temperature did not improve the process. At 455°C the conversion slightly decreased, while at 470°C both conversion and selectivity dropped more noticeably. This decline is associated with the increasing contribution of hydrogen transfer, oligomerization and aromatization reactions. These side reactions consume active intermediates and accelerate the formation of heavier hydrocarbons and carbonaceous deposits. Therefore, the best catalytic performance was achieved near 445°C, where the balance between methyl chloride activation and secondary reactions was most favorable.

Fig. 1. Effect of temperature on methyl chloride conversion over SAPO-34 catalyst. Error bars represent standard deviation

The influence of weight hourly space velocity on catalytic performance is summarized in Table 2. WHSV controls the contact time between methyl chloride and the catalyst surface. Therefore, it directly affects both conversion and product selectivity.

Table 2

Effect of WHSV on catalytic performance

Gas chromatographic analysis showed that ethylene and propylene dominate the product distribution, with estimated yields of 20–25 % and 18–22 %, respectively.

At the lowest WHSV value, 600 h ^-1 , the conversion was relatively high because of the longer contact time. However, selectivity toward light olefins remained lower than under intermediate conditions. This indicates that prolonged residence time promotes not only the primary formation of olefins but also further transformations of these products.

When WHSV increased to 800 h ^-1 , selectivity reached its highest value of 82 ± 2 mol %. The shorter residence time limited the extent of secondary reactions and helped preserve the desired product fraction. At WHSV values higher than 900 h ^-1 , the conversion gradually decreased because the contact time became insufficient for complete conversion of methyl chloride. Thus, the most favorable WHSV range was 700–850 h ^-1, where conversion and selectivity were both maintained at acceptable levels.

Fig. 2. Effect of WHSV on light olefin selectivity over SAPO-34 catalyst. Error bars represent standard deviation

The trends shown in Figures 1 and 2 confirm that the catalytic system has a clear optimum rather than a simple monotonic response. In Figure 1, the conversion curve rises sharply up to 445°C and then decreases. This shape reflects the transition from a temperature-limited regime to a region where secondary reactions and catalyst deactivation become more important.

Figure 2 shows that the selectivity profile follows a bell-shaped dependence on WHSV. At low WHSV, the longer residence time favors secondary conversion of olefinic products. At very high WHSV, reactant-catalyst contact is insufficient. The intermediate WHSV region therefore provides the most suitable kinetic environment for light olefin formation.

The catalytic conversion of methyl chloride over SAPO-34 can be interpreted through the hydrocarbon pool mechanism. According to this model, the reaction is not limited to a direct transformation of CH3Cl into olefins. Instead, organic intermediate species are generated inside the microporous cages of the catalyst and participate in repeated methylation and cracking cycles.

Polymethylated aromatic species are usually considered the key active intermediates in this type of reaction network. Their formation explains the induction behavior often observed at the early stage of the process. Once these organic species are established inside the catalyst pores, they promote the selective formation of ethylene and propylene.

The FTIR results support the structural suitability of the synthesized SAPO-34 catalyst. The presence of framework vibration bands and surface hydroxyl groups indicates that the material contains the structural and acidic features required for catalytic activity. At the same time, excessive accumulation of hydrocarbon intermediates can lead to carbon deposition, which gradually decreases catalyst performance.

Catalyst deactivation is mainly related to the formation of carbonaceous deposits inside the microporous structure. These deposits reduce the accessibility of active acid sites and hinder the diffusion of reactants and products. The decrease in performance at higher temperatures is consistent with this interpretation, because elevated temperature accelerates secondary reactions and coke formation.

Regeneration experiments showed that this deactivation is largely reversible. Oxidative treatment removes a significant part of the carbon-containing deposits and restores catalytic activity. This behavior is important for practical application, since long-term use of the catalyst requires repeated reaction-regeneration cycles.

The obtained results suggest that SAPO-34-based CTO systems can be further considered for continuous operation. The identified temperature and WHSV ranges provide useful guidance for process optimization. In industrial practice, fluidized-bed reactor configurations may be advantageous because they allow continuous catalyst circulation, efficient heat transfer and in situ regeneration.

The combination of high olefin selectivity, controllable deactivation and effective regeneration makes SAPO-34 a promising catalyst for the conversion of methyl chloride into light olefins. These features are especially relevant for processes aimed at the chemical valorization of natural gas-derived feedstocks.

Conclusion

The obtained results demonstrate that the SAPO-34 catalyst provides selective performance in the conversion of methyl chloride to light olefins. The reaction outcome is strongly dependent on operating parameters, and a distinct optimum is observed within a relatively narrow range of temperature and space velocity.

At approximately 445°C and WHSV of 700–850 h⁻¹, the process reaches its most favorable regime, where methyl chloride conversion approaches 70–72 %, while the selectivity toward C₂-C₃ olefins remains at about 80–82 mol %. Under these conditions, the overall yield of light olefins is close to 55–60 %.

Analysis of the product composition shows that ethylene and propylene constitute the main fraction of the reaction products, with comparable contributions from both components. The limited formation of heavier hydrocarbons indicates that the catalyst effectively suppresses secondary growth reactions.

The observed trends are consistent with the hydrocarbon pool mechanism, where intermediate species confined within the SAPO-34 framework play a key role in olefin formation. At the same time, gradual loss of activity can be linked to carbon deposition, which partially restricts access to active sites. A significant fraction of the initial catalytic activity can be restored after oxidative treatment.

Taken together, these findings demonstrate clear potential for the application of SAPO-34 in processes aimed at producing light olefins from methyl chloride.

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