Metastable LaOCl_x Phase Stabilization as an Effective Strategy for Controllable Chlorination of Ethane into 1,2-Dichloroethane

Abstract

LaOCl-mediated ethane chlorination into 1,2-dichloroethane offers a promising pathway for low-temperature, large-scale ethane upgrading. However, under Cl₂-rich conditions, LaOCl undergoes detrimental chlorination into lanthanum chloride (LaCl₃), accompanied by extensive surface hydroxylation. Such severe structural evolution limits the practical application of La-based catalysts under industrially relevant conditions. In this study, an alumina-stabilized La catalyst was prepared via a coprecipitation method. We demonstrated that strong La-O-Al interactions effectively resist structural degradation of La species under reaction conditions, enabling the modified catalyst to maintain exceptional stability under high Cl₂ concentrations. At a C₂H₆/Cl₂ ratio of 4:9, the optimized catalyst achieves an ethane conversion of 61%, with 1,2-dichloroethane selectivity sustained above 74% for 12 h without noticeable deactivation. In contrast, the bulk LaOCl counterpart suffers from rapid over-chlorination, shifting product dominance to trichloroethane within 10 h. Advanced spectroscopy characterization reveals that selectivity loss in LaOCl originates from phase collapse into LaCl₃, whereas Al₂O₃ stabilization preserves the metastable LaOCl_x phase in a highly dispersed state, ensuring selective C–Cl bond formation. These results highlight the critical role of stabilizing metastable oxychloride phases through robust metal oxide interactions, establishing a design framework for rare-earth catalysts in high-concentration chlorine environments.

1. Introduction

Over the past few decades, the direct conversion of ethane (C₂H₆) into value-added chemicals has emerged as a prominent research focus in heterogeneous catalysis [1,2,3,4,5], driven by its potential to revolutionize chemical feedstock supply through the utilization of abundant natural gas resources [6,7,8,9]. However, conventional C₂H₆ conversion processes are inherently constrained by thermodynamic limitations, with activation barriers persisting even at elevated temperatures approaching 700 K [10,11,12,13]. These harsh operational conditions typically induce undesirable side reactions, including coking and catalyst deactivation [14,15], while simultaneously incurring substantial energy requirements [16,17,18]. This fundamental challenge highlights the critical need for developing innovative catalytic systems that enable efficient ethane transformation under mild operating conditions.

Recently, Pérez-Ramírez and colleagues pioneered a catalytic process for the selective synthesis of 1,2-dichloroethane (1,2-C₂H₄Cl₂) via ethane chlorination under mild conditions [19]. This strategy unlocks new opportunities for large-scale ethane updating, as 1,2-C₂H₄Cl₂ serves as a pivotal precursor for polyvinyl chloride (PVC) [20,21,22,23], a commodity chemical with an annual global demand of 50 million tons that currently relies entirely on coal-and petroleum-derived feedstocks [24,25,26,27,28,29]. Compared to transition metals, non-redox rare-earth oxychlorides exhibit exceptional structural stability under harsh oxychlorination conditions [25,30,31,32], which inherently arise from the thermodynamically unfavorable chlorination under the temperatures required for C₂H₆ upgrading. This unique property positions rare-earth oxychlorides as preferred catalysts for C₂H₆ chlorination [33]. However, unsaturated metal cations with unpaired electrons on the oxychloride surface, which is critical for activating chlorine-containing intermediates [34,35], inevitably undergo structural degradation into inert chlorides under Cl₂-rich conditions [36,37,38]. For instance, the representative lanthanum oxychloride (LaOCl) catalyst, which exceeds 80% selectivity for 1,2-C₂H₄Cl₂, undergoes structural evolution during reaction, with the final catalyst structure primarily composed of LaCl₃ and only trace amounts of detectable LaOCl phases [37]. To mitigate selectivity loss from phase collapse, such catalysts must operate under Cl₂-lean conditions and maintain C₂H₆ conversion rates below 20%. This severely limits their industrial applications. Notably, since Pérez-Ramírez’s discovery in 2021 [19], no breakthroughs have been achieved in the field, highlighting the significant technical challenges that remain in converting C₂H₆ into target chlorinated hydrocarbons.

In this work, we demonstrate that robust La-O-Al interactions can effectively resist the segregation of La species in harsh Cl₂ environments. X-Ray Diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses reveal that at 8.8% La loading, the LaOCl_x phase is well stabilized by Al₂O₃. The highly dispersed LaOCl_x species, serving as active centers, can maintain the long-term controlled chlorination of C₂H₆. This optimized catalyst achieves 61% C₂H₆ conversion with >70% selectivity toward 1,2-C₂H₄Cl₂ at a C₂H₆/Cl₂ molar ratio of 4:9, maintaining stability for 12 h without deactivation. In contrast, we observed an obvious structural degradation of La catalytic centers in the bulk LaOCl counterparts. Intriguingly, this structural modulation directly governs the C₂H₆ chlorination pathway, shifting product selectivity from 1,2-C₂H₄Cl₂ to trichloroethane (C₂H₃Cl₃). This work elucidates the unique catalytic behavior of alumina-confined La-based catalysts, highlighting the potential of oxide-supported La single-atom catalysts for ethane chlorination.

2. Results and Discussion

The Al₂O₃-stabilized La-based catalyst was synthesized via coprecipitation of La and Al nitrate precursors using ammonia as the precipitating agent, followed by calcination at 900 °C. Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis confirmed a La loading of 8.8 wt% in the catalyst (denoted as LaAl₂O₃). Previous studies by Pérez-Ramírez et al. have identified LaOCl as the primary active phase for C₂H₆ chlorination [19]. Accordingly, we prepared La₂O₃ via the precipitation method and exposed it to the feed gas atmosphere (C₂H₆:Cl₂ = 4:9) for approximately 55 min to generate the LaOCl phase (Figure 1). For comparison, LaCl₃ was loaded onto Al₂O₃ via impregnation to simulate surface over-chlorination to the LaCl₃ phase, achieving a La loading of 9.0 wt% as determined by ICP-OES (labeled as LaCl₃/Al₂O₃). Systematic characterization and catalytic performance of LaOCl, LaAl₂O₃, and LaCl₃/Al₂O₃ were conducted to elucidate the structure-performance relationships of La-based catalysts in C₂H₆ chlorination.

XRD analysis was employed first to investigate the structural evolution of three La-based catalysts before and after C₂H₆ chlorination. As shown in Figure 1, the fresh LaOCl catalyst exhibited diffraction peaks matching the tetragonal LaOCl reference (ICCD 08-4330) [39]. After reaction, however, the LaOCl peaks significantly diminished, accompanied by the emergence of LaCl₃ (ICCD 02-3146) and LaCl₃·3H₂O phases (ICCD 07-5266), indicating irreversible chlorination and lattice collapse under Cl₂-rich conditions. In contrast, the LaAl₂O₃ catalyst demonstrated exceptional structural stability, with nearly identical XRD patterns before and after reaction. Its diffraction peak exclusively displayed Al₂O₃ peaks (ICCD 04-0877) with no detectable lanthanum oxide phases, suggesting uniform La incorporation into the alumina lattice. This hypothesis is further supported by a distinct shift of the Al₂O₃ (45°) peak to 46°—a consequence of lattice distortion induced by La doping [40,41]. Notably, the robust La-O-Al bonds formed during high-temperature calcination (900 °C) effectively stabilized La species, preventing LaCl₃ formation even after 12 h of exposure to reactive atmospheres at 260 °C. For LaCl₃/Al₂O₃, XRD patterns revealed Al₂O₃ (ICCD 04-0877) and LaCl₃·3H₂O (ICCD 07-5266) phases, signifying surface aggregation of LaCl₃ due to its high loading.

To further elucidate the structural evolution of LaOCl, LaAl₂O₃, and LaCl₃/Al₂O₃ catalysts under reactive atmospheres, we analyzed their surface elemental distributions using energy-dispersive X-ray spectroscopy mapping (Figure 2). For the spent LaOCl (Figure 2a–c), Cl exhibited localized enrichment with spatial overlap between La and Cl, confirming irreversible chlorination of LaOCl into LaCl₃. In contrast, LaAl₂O₃ (Figure 2d–f) showed negligible Cl signals and homogeneous distributions of La, Al, and O at the nanoscale, indicating that the robust La-O-Al interface suppressed Cl⁻ penetration and over-chlorination. The LaCl₃/Al₂O₃ catalyst (Figure 2g–i) displayed uniform distribution of Cl on Al₂O₃, with Cl enrichment zones colocalized with La aggregates, consistent with XRD observations. These results demonstrate the extreme sensitivity of LaOCl to HCl-induced structural collapse and LaCl₃ aggregation. Conversely, the La-O-Al interface in LaAl₂O₃ created a chlorine-resistant architecture that stabilizes LaOCl_x species, providing an ideal active surface for controlled ethane chlorination.

XPS was conducted to elucidate the evolution of surface electronic states in LaOCl, LaAl₂O₃, and LaCl₃/Al₂O₃ catalysts before and after reaction. As shown in Figure 3a–c, the Cl 2p spectra (193–203 eV) were deconvoluted into four characteristic peaks: spin-orbit splitting peaks of Cl⁻ species at 198.8 eV (Cl 2p_1/2) and 200.4 eV (Cl 2p_3/2), along with the La 4p_3/2 peak at 195.8 eV and its satellite peak at 197.5 eV [42,43]. Systematic analysis of binding energy shifts and peak area ratios revealed distinct chlorination mechanisms. For the spent LaOCl catalyst (Figure 3a), the Cl 2p peaks shifted to lower binding energies (Δ ≈ 0.2 eV), while the La 4p_3/2 and La 3d_5/2 peaks shifted to higher binding energies (Δ ≈ 0.6 eV; Figure 3d). This confirmed electron transfer from La to Cl. A significant increase in the Cl 2p/La 4p_3/2 peak area ratio further indicated surface over-chlorination. These observations suggest that continuous Cl coordination at La sites reduces the electron density of La centers, rationalizing the La 4p_3/2 upshift. In contrast, the LaAl₂O₃ catalyst exhibited exceptional stability (Figure 3b,e). Its Cl 2p, La 4p_3/2, and La 3d peaks showed minimal shifts (Δ ≤ 0.2 eV), and the Cl 2p/La 4p_3/2 peak area ratio remained unchanged after reaction. This demonstrates that robust La-O-Al bonds stabilize surface La species in a LaOCl_x configuration, preventing excessive chlorination under harsh conditions. Notably, LaCl₃/Al₂O₃ showed invariant peak positions and Cl 2p/La 4p_3/2 peak area ratios (Figure 3c,f), confirming structural integrity without significant surface reconstruction.

The distinct structural evolution behaviors of LaOCl, LaAl₂O₃, and LaCl₃/Al₂O₃ under reaction atmospheres result in differences in their catalytic activities. Through comparative analyses of product distributions and time-dependent evolution patterns in ethane chlorination over these three lanthanum-based catalysts, we elucidated the structure-dependent mechanism governing chlorination pathways. As illustrated in Figure 4a, LaOCl initially demonstrated superior catalytic activity with 70% ethane conversion, predominantly yielding 1,2-C₂H₄Cl₂. This observation aligns with the findings of Pérez-Ramírez regarding LaOCl’s low activation energy and high catalytic efficiency in ethane chlorination [19]. However, a prolonged reaction duration induced the structural degradation of LaOCl, leading to the progressive accumulation of over-chlorinated products. After 10 h of operation, C₂H₃Cl₃ became the dominant product (>50% selectivity), accompanied by a sharp conversion decline to 46%. The significant decline in ethane conversion rate was attributed to the over-chlorination of 1,2-C₂H₄Cl₂, which consumes chlorine from the reactants and reduces the concentration of gas-phase Cl radicals required to drive the ethane conversion. The over-chlorination of 1,2-C₂H₄Cl₂ may stem from the hydroxylation of LaOCl during surface chlorination. The hydrogen-bonding network formed by surface hydroxyl groups could enhance 1,2-C₂H₄Cl₂ adsorption beyond optimal levels. Notably, in their study of La_xEu_1-xOCl solid solutions, Weckhuysen et al. demonstrated that La³⁺ is readily chlorinated and acts as a chlorine buffer that can transfer chlorine to the active Eu³⁺ phase, thereby enhancing the methane oxychlorination activity [44]. This observation implies that chlorine atoms adsorbed on La³⁺ centers exhibit dynamic adsorption behavior. Under reactive atmospheres, the strong electrophilicity of La³⁺ centers may enable the adsorption of chlorine radicals beyond stoichiometric proportions, creating localized Cl radical enrichment. These chlorine reservoirs may serve as critical drivers in promoting the over-chlorination of 1,2-C₂H₄Cl₂.

Intriguingly, the LaAl₂O₃ catalyst demonstrated remarkable stability under the reaction conditions of 260 °C and a C₂H₆/Cl₂ ratio of 4:9. Throughout the 12-h reaction period, LaAl₂O₃ maintained consistent selectivity (~74%) for 1,2-C₂H₄Cl₂ synthesis, with only a marginal decline in ethane conversion from 64% to 61% (Figure 4b). This performance stability is attributed to the robust La-O-Al interfacial structure. The LaOCl_x species induced under reactive atmospheres efficiently catalyzed selective 1,2-C₂H₄Cl₂ synthesis, offering theoretical guidance for designing stable C₂H₆ chlorination catalysts. XRD and XPS characterizations confirmed the structural stability of Al₂O₃-supported LaCl₃ under reaction conditions, consistent with its stable catalytic performance: over a 10-h test, C₂H₆ conversion remained at 63% with 50% selectivity toward 1,2-C₂H₄Cl₂ (Figure 4c). This phenomenon suggests that highly dispersed LaCl₃ species stabilized by interfacial oxygen exhibit structural features like LaOCl_x, enabling selective 1,2-C₂H₄Cl₂ production. Notably, however, LaCl₃ aggregation into inert crystalline grains on Al₂O₃ reduced chlorination capacity, resulting in 40% selectivity toward C₂H₅Cl. The residual catalytic activity of LaCl₃/Al₂O₃ may originate from a minor fraction of highly dispersed LaCl₃ species. These dispersed species, under the modulation of interfacial oxygen, may exhibit LaOCl_x-like characteristics capable of stabilizing chlorine radicals. This inference can be confirmed by the study from Chen et al., which has shown that CuCl₂ undergoes dissociative adsorption on an γ-Al₂O₃ (110) surface, where only one chloride ion binds to copper, and the other binds to the Al₂O₃ surface [45]. These findings collectively highlight that the formation of surface LaOCl_x structures governs chlorination selectivity.

Through comprehensive analysis of temperature-dependent catalytic performance in ethane chlorination over LaAl₂O₃ systems, our study highlights the pivotal role of thermal effects in governing reaction pathway selection (Figure 4d). Notably, when reaction temperatures exceeded 300 °C, we observed the emergence of a competing dehydrochlorination pathway, producing ethylene (C₂H₄) and vinyl chloride (C₂H₃Cl). This phenomenon presents a significant challenge for developing efficient catalytic systems for selective chlorination of ethane to 1,2-C₂H₄Cl₂, as the formation of unsaturated byproducts not only reduces target product yield, but also accelerates catalyst poisoning through coking mechanisms. The findings emphasize the importance of precise thermal management in chlorination processes to optimize reaction kinetics while suppressing dehydrogenation pathways. It is crucial to maintain reaction temperatures below 300 °C and implement strategies for selective intermediate stabilization.

To investigate the adsorption behavior of La-based catalysts toward intermediate species in the chlorination reaction, we conducted temperature-programmed desorption (TPD) experiments with C₂H₅Cl and 1,2-C₂H₄Cl₂ on the spent LaOCl, LaAl₂O₃, and LaCl₃/Al₂O₃ catalysts (Figure 5). C₂H₅Cl-TPD profiles (Figure 5a) revealed negligible desorption peaks (50–400 °C) across all catalysts, confirming their extremely weak adsorption capacity for C₂H₅Cl. This suggests that the adsorption of C₂H₅Cl on the catalysts is not a critical factor influencing its further chlorination to 1,2-C₂H₄Cl₂. In the 1,2-C₂H₄Cl₂-TPD tests (Figure 5b), the LaOCl catalyst exhibited a pronounced desorption peak at 360 °C, far exceeding the desorption peaks of LaAl₂O₃ and LaCl₃/Al₂O₃ (Figure 5b). This indicates that surface chlorination of LaOCl during the reaction induces strong adsorption of 1,2-C₂H₄Cl₂, leading to its over-chlorination to form C₂H₃Cl₃. The absence of dichloroethane desorption peaks for LaAl₂O₃, and LaCl₃/Al₂O₃ implies that neither LaOCl_x nor LaCl₃ species promote strong adsorption of 1,2-C₂H₄Cl₂.

Since ethane-to-chloroethane conversion occurs via gas-phase chlorine radical pathway independent of catalysts [46,47], the chlorination kinetics of C₂H₅Cl were investigated to elucidate structural effects. Steady-state kinetic tests and Arrhenius analysis systematically compared LaAl₂O₃ and LaCl₃/Al₂O₃ in terms of light-off temperature, reaction orders, and apparent activation energy. As shown in Figure 6, LaAl₂O₃ exhibited superior low-temperature activity, with a 15 °C lower light-off temperature than LaCl₃/Al₂O₃, confirming its enhanced catalytic capability. Notably, over the LaAl₂O₃ catalyst, the primary product was 1,2-C₂H₄Cl₂, with byproducts mainly consisting of C₂H₃Cl₃ and C₂H₂Cl₄. Furthermore, no 1,1-C₂H₄Cl₂ isomers were detected in the products, indicating that the chlorination of C₂H₅Cl primarily occurred on the catalyst surface. Conversely, aggregated LaCl₃ clusters on Al₂O₃ hindered chlorination reaction, leaving unreacted C₂H₅Cl as the dominant byproduct. These performance disparities highlight that constructing La-O-Al interfaces optimizes active site dispersion, enhancing surface reactivity and selectivity.

Kinetic studies revealed near-first-order dependence on C₂H₅Cl for both catalysts (1.06 vs. 0.99, Figure 6b). Combined with the TPD characterization, it is evident that the reaction rate was not influenced by the strength of C₂H₅Cl adsorption. Worth noting is the significant difference in the reaction orders of Cl₂ between the two catalysts. As shown in Figure 6c, the reaction order for Cl₂ was 0.29 for LaAl₂O₃ and 0.69 for LaCl₃/Al₂O₃, indicating that LaAl₂O₃ promotes Cl₂ adsorption and activation. Combining the kinetic data with the catalytic performance, it can be conclusively inferred that the LaAl₂O₃ catalyst possessed more efficient chlorine adsorption sites, significantly accelerating the surface chlorination process. Notably, when the LaCl₃ species on the catalyst surface was overly aggregated, this structural advantage was compromised, leading to the active sites being covered and the efficiency of chlorine activation being reduced. As shown in Figure 6d, the apparent activation energy for LaAl₂O₃ was 54.1 kJ·mol⁻¹, significantly lower than the 85.7 kJ·mol⁻¹ observed for the LaCl₃/Al₂O₃ catalyst. This dynamic parameter difference was highly consistent with the catalytic performance evaluation and reaction order measurements.

The exceptional ethane chlorination performance of LaAl₂O₃ catalysts originates from their structural stability, prompting us to investigate the maximum La content that Al₂O₃ can stabilize to resist surface chlorination under reactive conditions. To address this, we engineered LaAl₂O₃ catalysts with elevated La loadings of 17.5% and 32.5%. XRD analysis (Figure 7a) revealed highly dispersed La species within the alumina lattice, as indicated by significant peak broadening at 46° and 67° corresponding to γ-Al₂O₃. Nevertheless, excessive La/Al ratios induced partial structural instability under chlorination atmospheres, resulting in LaCl₃ formation and a marked reduction in 1,2-C₂H₄Cl₂ selectivity (Figure 7b). These results unequivocally demonstrate that, while Al₂O₃ serves as an effective stabilization platform for La species, precise control over La loading is imperative to optimize both structural resilience and catalytic efficiency.

Based on the experimental analyses above, we present a mechanistic model for ethane chlorination (Figure 8). The catalyst-independent C₂H₆ conversion observed in Figure 4 suggests that the initial chlorination step to C₂H₅Cl is primarily mediated by gas-phase chlorine radicals. Weak adsorption of C₂H₅Cl, as evidenced by C₂H₅Cl-TPD profiles and its high reaction order across all catalysts, indicates that C₂H₅Cl adsorption/activation is not the rate-determining step (RDS) for its subsequent conversion to 1,2-C₂H₄Cl₂. The marked disparity in Cl₂ reaction orders between LaAl₂O₃ and LaCl₃/Al₂O₃ catalysts highlights that Cl₂ activation and the stabilization of chlorine radicals on the catalyst surface likely govern the RDS for C₂H₅Cl chlorination. In LaAl₂O₃, LaOCl_x centers stabilized by the La-O-Al framework act as electrophilic sites to efficiently anchor chlorine radicals. In contrast, LaCl₃, with its saturated Cl coordination, impedes chlorine radical access, thereby suppressing further chlorination. These findings underscore the critical importance of stabilizing La in a LaOCl_x configuration rather than a fully chlorinated state (LaCl₃), which mechanistically explains the experimentally observed high selectivity for C₂H₆-to-1,2-C₂H₄Cl₂ conversion on LaAl₂O₃ surfaces.

3. Experimental Section

3.1. Catalyst Preparation

Lanthanum nitrate hexahydrate (La(NO₃)₃·6H₂O), lanthanum chloride heptahydrate (LaCl₃·7H₂O), and aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O) were purchased from Macklin Chemical Reagent Factory. Ammonia (NH₃·H₂O) was purchased from Tianjin Damao Chemical Reagent Factory.

La₂O₃ was synthesized via the ammonia precipitation method. Initially, 5.6 g La(NO₃)₃·6H₂O was dissolved in 150 mL of deionized water. An alkaline solution was prepared by dissolving 20 mL NH₃·H₂O with a mass fraction of 25 vol% in 150 mL of deionized water. This alkaline ammonium hydroxide solution was then gradually added to the acidic La(NO₃)₃ solution while stirring continuously. The resulting mixture was placed on a magnetic stirrer and stirred for 8 h until the pH value of the solution exceeded 8. After that, the mixture was subjected to centrifugation to separate the solid phase, followed by drying and milling to obtain a fine powder. Finally, the powder was calcined at a high temperature of 800 °C for 3 h, yielding the La₂O₃ catalyst with a well-defined crystalline structure.

The synthesis of LaAl₂O₃ was carried out using the ammonia precipitation technique. To begin with, 0.52 g La(NO₃)₃·6H₂O and 11.38 g Al(NO₃)₃·9H₂O (17.5% LaAl₂O₃: 1.13 g La(NO₃)₃·6H₂O and 10.93 g Al (NO₃)₃·9H₂O, 32.5% LaAl₂O₃: 1.69 g La(NO₃)₃·6H₂O and 10.52 g Al (NO₃)₃·9H₂O). Meanwhile, an alkaline solution was prepared by dissolving 25 mL NH₃·H₂O with a mass fraction of 25 vol% in 150 mL of deionized water. This alkaline ammonium hydroxide solution was subsequently introduced into the acidic solution containing La(NO₃)₃ and Al(NO₃)₃ with continuous stirring. The resultant mixture was placed onto a magnetic stirrer and stirred for 8 h until the pH of the solution exceeded 8. Following this, the mixture underwent centrifugation to separate the solid phase, which was then dried and milled. Lastly, the milled material was calcined at a high temperature of 900 °C for 3 h, resulting in the formation of the LaAl₂O₃ catalyst. Prior to catalytic evaluation, LaAl₂O₃ was activated by treatment in a feed mixture containing HCl/N₂ = 10:90 at 260 °C for 6 h.

The alumina (Al₂O₃) carrier was prepared by the co-precipitation method. Initially, Al(NO₃)₃·9H₂O was dissolved in deionized water with continuous stirring until a clear solution was formed. Subsequently, NH₃·H₂O was slowly added while stirring until the solution pH reached around 8–9 (monitored using pH paper). At this stage, a white flocculent precipitate of aluminum hydroxide (Al(OH)₃) was observed. The solution was then allowed to stand for 1–2 h to age the precipitate and improve its crystallinity. After centrifugation, drying, and milling, the Al₂O₃ was obtained by calcination at 900 °C for 3 h. The LaCl₃/Al₂O₃ catalyst was synthesized via the incipient wetness impregnation method. Initially, 0.37 g LaCl₃·7H₂O was dissolved in water to prepare a solution. One gram of the previously prepared alumina was impregnated with a specific amount of the LaCl₃ solution, ensuring that the volume of the impregnation solution matched the volume of liquid absorbed by 1 g of alumina. The impregnated alumina was then placed in an oven at 155 °C and dried for 4 h to yield the LaCl₃/Al₂O₃ catalyst.

3.2. Characterization

High-resolution transmission electron microscopy (HRTEM) images and energy-dispersive X-ray spectroscopy (EDS) analyses were performed using a JEM2100F microscope (JEOL Ltd., Tokyo, Japan), which was operated at an accelerating voltage of 200 kV. The powder X-ray diffraction (XRD) patterns were obtained using a PW3040/60 X’Pert ProSuper diffractometer (PANalytical, Malvern, UK), which was equipped with a Cu Kα radiation source. The instrument was operated at a voltage of 40 kV and a current of 40 mA. X-ray photoelectron spectroscopy (XPS) was measured on a Thermofisher ESCALAB 250Xi instrument (Thermo Fisher Scientific, Waltham, MA, USA), which applied monochromatic Al Kα radiation (hυ = 1486.6 eV) as the X-ray source. The concentration of La in the samples was determined using an inductively coupled plasma optical emission spectrometer (ICP-OES). C₂H₅Cl-TPD and 1,2-C₂H₄Cl₂-TPD experiments were carried out using a Micromeritics AutoChem II 2920 chemisorption analyzer (Micromeritics, Norcross, GA, USA). Prior to the measurements, the sample was pretreated by drying in a helium (He) flow at 250 °C for 150 min. It was then cooled down to 40 °C and exposed to C₂H₅Cl (or 1,2-C₂H₄Cl₂) for 60 min to allow adsorption. Following this, the gas was switched to He for purging, and the temperature was increased at a rate of 10 °C per minute until it reached 400 °C. Throughout the process, a thermal conductivity detector was employed to monitor and record the signals.

3.3. Catalyst Evaluation

Ethane chlorination (EC) was executed at atmospheric pressure in a self-made continuous-flow fixed-bed reactor. The gases C₂H₆ (8% in N₂), C₂H₅Cl (9% in N₂), Cl₂ (18% in N₂), Ar (carrier gas), and N₂ (carrier gas) were fed into a mixing unit at controlled flow rates using digital mass-flow controllers (Bronkhorst^®, Veenendaa, The Netherlands). Catalytic performance was evaluated in a fixed-bed quartz reactor with an inner diameter of 8 mm. The inlet feed gas mixture consisted of C₂H₆/Cl₂/N₂ in a ratio of 4:9:87. The reaction was conducted at a temperature of 260 °C, with a total gas flow rate of 16.67 mL/min. A catalyst loading of 0.5 g was used, resulting in a weight hourly space velocity (WHSV) of 2000 mL·h⁻¹·g⁻¹. A gas chromatograph (GC) equipped with a FID detector (PANNA, Changzhou, China) was used for online analysis of the feed gas and the reaction products.

Kinetic studies of the catalysts were conducted in a fixed-bed quartz reactor with an inner diameter of 8 mm. A 0.2 g portion of catalyst was loaded into the reactor and pretreated in a feed gas mixture (C₂H₆/Cl₂/N₂ = 4:9:87) at 260 °C for 10 h under atmospheric pressure. Subsequently, the reaction was performed with the reactor bed temperature varying from 140 °C to 260 °C. The feed composition was adjusted to C₂H₅Cl:Cl₂ = 1–5:2–5 at temperatures between 200 and 230 °C. The catalyst loading remained at 0.2 g, corresponding to a weight hourly space velocity (WHSV) of 6000 mL·h⁻¹·g⁻¹. For each experimental condition, measurements were taken at 20-min intervals, and the average of at least three measurements was utilized to determine the concentrations.

C₂H₆ and C₂H₅Cl conversions were calculated from the following equation:

$${\mathrm{Con}}_{{\mathrm{C}}_2{\mathrm{H}}_6({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Cl})}={\displaystyle \frac{{\mathrm{C}}_2{\mathrm{H}}_6{({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Cl})}_{\mathrm{inlet}}-{\mathrm{C}}_2{\mathrm{H}}_6{({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Cl})}_{\mathrm{outlet}}}{{\mathrm{C}}_2{\mathrm{H}}_6{({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Cl})}_{\mathrm{inlet}}}}$$

The product selectivity X (x = C₂H₄, C₂H₅Cl, C₂H₃Cl, 1,2-C₂H₄Cl₂, 1,1-C₂H₄Cl₂, and 1,1,2-C₂H₃Cl₃ selectivity) was calculated according to the following equation,

$${\mathrm{Sel}}_{\mathrm{x}}={\displaystyle \frac{{\mathrm{X}}_{\mathrm{outlet}}}{{\mathrm{C}}_2{\mathrm{H}}_{6\left(\mathrm{inlet}\right)}-{\mathrm{C}}_2{\mathrm{H}}_{6\left(\mathrm{outlet}\right)}}}$$

where “inlet” and “outlet” represent chemicals in the inlet and outlet, separately.

The reaction order is typically determined experimentally using the rate expression

r = k⋅[A]^m[B]ⁿ

where r is the reaction rate, k is the rate constant, and m and n represent the partial reaction orders with respect to reactants A and B, respectively, measuring the initial reaction rate under varying initial concentrations of one reactant while keeping others constant. The reaction order for a specific reactant is the slope derived from the logarithmic relationship of

ln r = ln k + m⋅ln [A] + n⋅ln [B]

The activation energy (E_a) is calculated using the Arrhenius equation,

$$k=A\cdot e^{-E_a/\left(RT\right)}$$

(1)

where k is the rate constant, A is the pre-exponential factor, R is the gas constant (8.314 J·mol⁻¹·K⁻¹), and T is the temperature in Kelvin. By measuring rate constants (k) at multiple temperatures, Ea can be determined from the linearized form:

$$\mathrm{l}\mathrm{n}k=\mathrm{l}\mathrm{n}A-{\displaystyle \frac{E_a}{R}}\cdot {\displaystyle \frac{1}{T}}$$

A plot of lnk versus 1/T (Arrhenius plot) yields a straight line with the slope (−E_a/R), allowing E_a to be derived.

In the real experiment, both reaction orders and activation energies were calculated based on the average turnover frequencies (TOF) obtained at low conversion levels. Since the same reactor configuration was employed, the active sites on the catalyst surface remained consistent across experiments, and a normalization procedure was consequently applied. Critically, this normalization does not alter the slope of the LnTOF vs. LnPCl₂ relationship (i.e., the reaction order). The residence time used for calculating the average TOF was determined by the gas flow rate. Therefore, the data utilized for TOF calculations included the feed gas concentration, gas flow rate, and measured conversion. It is important to note that the gas flow rate was intentionally increased when necessary to ensure conversions remained below 20%.

4. Conclusions

This work systematically deciphers the structure-dependent selectivity and kinetics of La-based catalysts in ethane chlorination. LaOCl, due to its thermodynamic instability in a Cl₂-rich environment, undergoes irreversible chlorination (LaOCl → LaCl₃), enhancing its adsorption of 1,2-C₂H₄Cl₂ and triggering the over-chlorination of C₂H₆. By confining La within robust Al₂O₃ and with the assistance of La-Al-O bonds, La can be stabilized in a LaOCl_x structure, effectively promoting the selective generation of 1,2-C₂H₄Cl₂. Using the LaCl₃/Al₂O₃ model catalyst, we confirmed that aggregated LaCl₃ particles, due to their weakened chlorine adsorption activation capability, severely limit the further chlorination of C₂H₅Cl. Through further kinetic experiments, we validated the critical role of LaOCl_x sites in the selective chlorination of C₂H₆. Kinetic analysis confirmed that LaOCl_x sites stabilized by La-Al-O interfaces reduced chlorination barriers to 54.1 kJ·mol⁻¹, achieving high selectivity (74%) and long-term stability for 1,2-C₂H₄Cl₂. These findings elucidate the unique catalytic behavior of oxide-confined La-based systems, highlighting the potential of single-atom La catalysts in C₂H₆ chlorination and providing a theoretical framework for designing efficient, stable industrial chlorination catalysts.