Dehydrochlorination of Intermediates in the Production of Vinyl Chloride over Lanthanum Oxide-Based Catalysts

Abstract

Lanthanum oxide-based catalysts are active in the elimination of HCl from C₂H₅Cl, 1,2-C₂H₄Cl₂ and 1,1,2-C₂H₃Cl₃ leading to the formation of their respective chlorinated ethenes. An oxygen-rich catalytic surface may form CO, CO₂ and C₂HCl as side products, whereas with chlorine-rich catalytic surfaces a stable product distribution is achieved with 100% selectivity towards the formation of ethenes, such as the valuable C₂H₃Cl intermediate.

1 Introduction

With an annual production of 45 million tons, chlorine is one of the most important chemicals for numerous commercial products [1]. The reaction with chlorine activates hydrocarbons, making them suitable as building blocks for organic synthesis. Moreover, chlorinated hydrocarbons (CHCs) are used as organic solvents and are persistent, making them heat-resistant and relatively inert. On the other hand, the same properties which make these compounds useful in industry make them harmful when emitted into the environment. In the last decades, it was found that CHCs contribute to various environmental effects, such as acid rain formation, ozone layer depletion and the greenhouse effect [2–4]. Also, many CHCs are carcinogenic and toxic. Even though the better understanding of the effects of these substances has greatly reduced the use in commercial applications, they are still used and produced in large quantities in industry [5].

More than one third of all chlorine is used for the production of polyvinyl chloride (PVC), the most produced plastic in the world after polyethylene (PE) [1, 5]. C₂H₃Cl is the monomer of PVC and is industrially prepared from C₂H₄ and chlorine. C₂H₄ is chlorinated into 1,2-C₂H₄Cl₂ via oxychlorination and direct chlorination described by Reaction Equations (1) and (2), respectively. 1,2-C₂H₄Cl₂ is then thermally cracked into C₂H₃Cl as shown in Reaction Equation (3). The oxychlorination process is used to lower chlorine consumption by recycling HCl from the cracking of 1,2-C₂H₄Cl₂, though selectivity towards 1,2-C₂H₄Cl₂ is lower than for the direct chlorination process. Overall, the reactions are performed at relatively high selectivity (>98%), but the scale of the process result in the formation of large quantities of by-products [6]. Various chlorinated C₁ and C₂ are formed in side- reactions and are separated from 1,2-C₂H₄Cl₂ as the so-called light and heavy ends. Regulation on the production and emission [2–4] of CHCs has enforced efficient degradation of excess CHCs, which is done in most cases by incineration [7]. However, because of the high heat resistance of the CHCs and possible formation of furans and dioxins, a high temperature (>1,000 °C) is needed for this process, making it costly. In addition, incineration results in loss of feedstock.

$$ {\text{C}}_{{\text{2}}} {\text{H}}_{{\text{4}}} {\text{ + }}\raise0.5ex\hbox{$\scriptstyle {\text{1}}$} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{$\scriptstyle {\text{2}}$}{\text{ O}}_{{\text{2}}} {\text{ + 2 HCl}} \to {\text{1,2-C}}_{{\text{2}}} {\text{H}}_{{\text{4}}} {\text{Cl}}_{{\text{2}}} {\text{ + H}}_{{\text{2}}} {\text{O}} $$

(1)

$$ {\text{C}}_{{\text{2}}} {\text{H}}_{{\text{4}}} {\text{ + Cl}}_{{\text{2}}} \to {\text{1,2-C}}_{{\text{2}}} {\text{H}}_{{\text{4}}} {\text{Cl}}_{{\text{2}}} $$

(2)

$$ {\text{1,2-C}}_{{\text{2}}} {\text{H}}_{{\text{4}}} {\text{Cl}}_{{\text{2}}} \to {\text{C}}_{{\text{2}}} {\text{H}}_{{\text{3}}} {\text{Cl + HCl}} $$

(3)

Research efforts have been made on methods to efficiently convert CHCs at low temperature into non-hazardous or re-usable products. Catalytic (hydro)dechlorination can be categorized by three types of systems: noble metals, transition metal oxides and basic oxides. In the case of noble metals, poisoning of the catalyst by chlorine remains a difficult issue to overcome [8–13]. Even though transition metal oxides are very active for the dechlorination of CHCs, the chlorination of the oxide may result in the formation of toxic volatile transition metal oxychlorides [14–21]. The basic oxides have proven to be stable active materials for the conversion of CHCs [22–34]. In previous work, lanthanum oxide-based catalysts were found to have the highest destructive capacity for the catalytic destruction of chlorinated C₁ and have been studied in detail in our group [29–31, 33, 34]. However, activity towards both chlorinated C₁ and C₂ is important for conversion of the light ends mixture formed in C₂H₃Cl production.

1,1-C₂H₂Cl₂, which is the monomer for the production of polyvinylidene chloride (PVDC), is prepared from the dehydrochlorination of 1,1,2-C₂H₃Cl₃ using alkaline solutions [35]. Actually, in most cases 1,1,2-C₂H₃Cl₃ is used to produce 1,1-C₂H₂Cl₂, since sufficient quantities are formed during the production of C₂H₃Cl. Also, the cracking of 1,2-C₂H₄Cl₂ to C₂H₃Cl is a non-catalytic dehydrochlorination reaction. Not many heterogeneous catalysts are known for the dehydrochlorination of chlorinated ethanes, mainly because of low selectivity and chlorine poisoning. In fact, alumina is active for this reaction and has been studied because dehydrochlorination is an undesirable side-reaction in the oxychlorination of ethene into 1,2-C₂H₄Cl₂, which is catalyzed by CuCl₂/γ-Al₂O₃ [36, 37]. Therefore, catalytic dehydrochlorination of chlorinated ethanes is not only interesting from a waste conversion point of view as an active dehydrochlorination catalyst could be used in the preparation steps towards C₂H₃Cl as well. Here, we report for the first time on such new active dehydrochlorination catalyst based on lanthanum oxides, which leads to the selective formation of C₂H₃Cl when starting from 1,2-C₂H₄Cl₂.

2 Experimental

2.1 Materials and Characterization

Commercial samples of La₂O₃ (Acros Organics, 99.99%) were used without additional purification. LaOCl was synthesized by a precipitation process using LaCl₃.7H₂O (Acros Organics, 99.99%) as precursor and a NH₄OH (Merck, 25 wt% in water p.a.) solution. The obtained gel (La(OH)₂Cl) was filtered, washed and dried at 120 °C and heated at 550 °C in pure N₂ (Linde, ≥99.999%) for 6 h. The phase composition of La₂O₃ and LaOCl after the reaction as a function of time with 1,1,2-C₂H₃Cl₃ was determined using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). XRD measurements were performed at ambient conditions with a Bruker-AXS D8 diffractometer equipped with a Co_Kα1,2 source. The XPS spectra were acquired using a Perkin-Elmer (PHI) model 5500 spectrometer. All XPS spectra were obtained using samples in the form of pressed wafers.

2.2 Flow Gas Experiments

The activity experiments for the dehydrochlorination of C₂H₅Cl (Aldrich, ≥99.7%), 1,2-C₂H₄Cl₂ (Acros Organics, ≥99.8%) and 1,1,2-C₂H₃Cl₃ (Acros Organics, ≥98%) were performed in a tubular fixed-bed quartz reactor. The catalyst bed consisted of 0.5 g LaOCl or La₂O₃ pressed in a 200–500 μm sieve fraction, pretreated in 10 mL/min He (Linde, ≥99.996%) at 550 °C. The flow was regulated by Brooks 0–100 mL automatic mass flow controllers. To find the initial reaction temperature, the reaction was carried out from 50 to 400 °C using a 25 mL/min 3–4 vol% reactant/He flow. In the case of 1,2-C₂H₄Cl₂ and 1,1,2-C₂H₃Cl₃, the flow was generated by flowing He through a bubbler containing the liquid reactant. The C₂H₅Cl feed was generated from 2 mL/min C₂H₅Cl and 23 mL/min He. Once stabilized, the flow was led over the reactor bed, consisting of LaOCl or La₂O₃. The temperature of the reactor was raised from 50 to 400 °C in steps of 10 °C. The heating ramp was 3.3 °C/min, and after each step, the temperature was held constant for 5 min. The composition of the reactor effluent was analyzed by a Siemens Maxum Edition 2 gas chromatograph with a sampling time of 240 s. In the case of 1,1,2-C₂H₃Cl₃, the reactions were also performed at constant temperature of 400 °C over La₂O₃ and LaOCl. The composition of the reaction mixture was analyzed with time.

2.3 IR Experiments

During the flow gas experiments with 1,1,2-trichloethane over La₂O₃, several products were detected which could not be assigned by GC. To complement the GC data, the reaction of 1,1,2-trichloethane on La₂O₃ was monitored in situ by IR. A static vacuum quartz cell was employed and all IR spectra were recorded using a Perkin Elmer 2000 spectrometer with a resolution of 4 cm⁻¹. La₂O₃ (Acros Organics, 99.99%) was pressed into a self-supporting wafer (2 cm²), and activated in situ prior to the IR measurements in dynamic vacuum at 550 °C overnight. 1,1,2-C₂H₃Cl₃ (Acros Organics, 98%) was evaporated by injection via a septum into an evacuated flask, which was connected to the vacuum system. After pretreatment, 1,1,2-C₂H₃Cl₃ (30 mbar) was introduced into the cell, which was then closed. The wafer was positioned in a separate heated part of the cell, enabling the measurement of gas phase IR spectra. The temperature was raised from 100 to 400 °C in steps of 50 °C. After each step the temperature was held constant during which gas phase spectra were recorded.

3 Results and Discussion

3.1 Temperature Programmed Reaction Studies on the Catalyst Activity and Selectivity

The effluent composition of the temperature programmed dehydrochlorination reactions over LaOCl is shown in Fig. 1. In the case of C₂H₅Cl and 1,2-C₂H₄Cl₂, 100% selectivity is observed towards C₂H₄ and C₂H₃Cl, respectively (Reaction Equations (4) and (5)). 1,1,2-C₂H₃Cl₃ is converted into several products. A significantly lower initial reaction temperature is observed for chloroethane compared to 1,2-C₂H₄Cl₂ and 1,1,2-C₂H₃Cl₃. It has been established that the cleavage of the C–Cl bond typically precedes the removal of a H atom [38, 39]. Furthermore, it has been shown that the initial reaction rate for the dehydrochlorination of chlorinated ethanes increases with an increasing number of chlorine atoms [40]. It should be noted, however, that this trend was established based on mono-, di- and tri-substituted chloroethanes with all chlorine atoms on the same carbon atom. With more C–Cl bonds on the same carbon atom in a chlorinated ethane molecule, the C–Cl bonds become more polarized. As a result, the C–Cl bonds are more susceptible to cleavage. Instead, C–Cl bonds on both carbon atoms may stabilize the polarization of the C–Cl bonds, which explains the higher initial temperature of reaction of 1,2-C₂H₄Cl₂ and 1,1,2-C₂H₃Cl₃ with respect to C₂H₅Cl.

Fig. 1

Effluent composition for the dehydrochlorination of (a) C₂H₅Cl, (b) 1,2-C₂H₄Cl₂ and (c) 1,1,2- C₂H₃Cl₃ over LaOCl as a function of temperature. (GHSV = 2,000 h⁻¹, inlet concentration: [C₂H₅Cl] = 3.7 vol%, [1,2-C₂H₄Cl₂] = 3.4 vol% and [1,1,2-C₂H₃Cl₃] = 3.2 vol%, X = product unassigned based on GC)

The main product for the dehydrochlorination of 1,1,2-C₂H₃Cl₃ is 1,1-C₂H₂Cl₂ and in addition C₂H₃Cl and cis-1,2-C₂H₂Cl₂ are formed. The formation of C₂H₃Cl implies that Cl₂ elimination occurs. This reaction is, however, energetically highly unfavourable and is not expected to proceed. A possible explanation would be that a different product is co-eluting with C₂H₃Cl. Therefore, the product assigned by the GC as C₂H₃Cl has been labelled as X in Figs. 1 and 2. Chlorine and hydrogen atoms can be removed from the reactant molecule via two pathways of HCl elimination, shown in Reaction Scheme 1. The formation of 1,1-C₂H₂Cl₂ (Scheme 1, Reaction a) is favoured over the reaction towards 1,2-C₂H₂Cl₂ (Scheme 1, Reaction b). In addition, no formation of trans-1,2-C₂H₂Cl₂ (Scheme 1, Reaction c) was detected. At 400 °C, trace amounts of CO and CO₂ were detected. Blank experiments were also performed and no significant product formation was observed, which rules out non-catalytic gas phase reactions. Based on these results, it is concluded that LaOCl is an active catalyst for the dehydrochlorination of chlorinated ethanes.

$$ {\text{CH}}_{{\text{2}}} {\text{Cl{-}CH}}_{{\text{3}}} \to {\text{CH}}_{{\text{2}}} {\text{ = CH}}_{{\text{2}}} {\text{ + HCl}} $$

(4)

$$ {\text{CH}}_{{\text{2}}} {\text{Cl{-}CH}}_{{\text{2}}} {\text{Cl}} \to {\text{CH}}_{{\text{2}}} {\text{ = CHCl + HCl}} $$

(5)

The dehydrochlorination of chlorinated ethanes over heterogeneous catalysts is catalyzed by three types of active sites: acidic, basic or dual sites [41]. It has been shown that for the dehydrochlorination of 1,1,2-trichloroethane, the selectivity towards the 1,1- and 1,2-product are influenced by the acid-base properties of the catalyst [41]. Control of the chlorination degree of the catalyst is crucial to tune the acid-base properties and optimize the performance of the catalyst when converting mixtures of chlorinated C₁ and C₂. In previous work, it was shown that the acid-base properties of lanthanum oxide-based catalysts are a key factor for the activation of C–Cl and C–H bonds in chlorinated C₁ [33, 34]. Hence, the degree of chlorination may also be of influence on the selectivity of the dehydrochlorination of chlorinated C₂. Therefore, the reaction with 1,1,2-C₂H₃Cl₃ was repeated with La₂O₃, which contains weaker La³⁺ Lewis acid sites. The reactor effluent composition was analyzed throughout the experiment, as shown in Fig. 2. In addition to the products observed over LaOCl, trans-1,2-C₂H₂Cl₂ is detected. However, as the temperature increases the selectivity towards 1,1-C₂H₂Cl₂ increases, while it decreases towards the other products. Low concentrations of CO and CO₂ were detected as in the case of LaOCl. Also, small amounts of two products were detected which could not be assigned based on the GC results. Even though these products could not be identified, the retention times indicate that these compounds are CHCs. Therefore, the same response factor as for the other CHCs is assumed to be valid for these unknown products.

Fig. 2

Effluent composition for the dehydrochlorination of 1,1,2-C₂H₃Cl₃ over La₂O₃ as a function of temperature. (GHSV = 2,000 h⁻¹, inlet concentration: [1,1,2-C₂H₃Cl₃] = 3.2 vol%, X = product unassigned based on GC)

Scheme 1

Reaction pathways for the dehydrochlorination of 1,1,2-C₂H₃Cl₃ into (a) 1,1-C₂H₂Cl₂, (b) cis-1,2-C₂H₂Cl₂ and (c) trans-1,2-C₂H₂Cl₂

3.2 In Situ IR Studies on the Catalyst Activity and Selectivity

The flow-gas experiments indicate that La₂O₃ and LaOCl catalyze the dehydrochlorination of chlorinated ethanes and that the acid-base properties of the catalyst directly influence the selectivity. However, the assignment of the products is difficult with gas chromatography only. Therefore, the gas phase composition of the temperature programmed reaction on La₂O₃ in a vacuum cell was monitored using IR spectroscopy to complement the GC results. The recorded spectra are shown in Fig. 3. In the spectral regions 2,250–2,400 and 1,350–1,900 cm⁻¹, the strong bands of ambient CO₂ and H₂O, respectively, were removed by spectroscopic software. Figure 3a illustrates the increase and decrease of the bands in the gas phase spectra as a function of temperature during the experiment. In Fig. 3b, the spectrum of the reactant at room temperature (RT) is shown together with that of Spectrum 11 from Fig. 3a. Spectrum 11 was chosen for the assignment of the bands of the products, because all the bands which are observed during the experiment are present in this spectrum.

Fig. 3

(a) Gas phase IR spectra recorded during the dehydrochlorination of 1,1,2-C₂H₃Cl₃ over La₂O₃ as a function of temperature: (0) reactant at RT, (1) 5 min at 100 °C, (2) 5 min at 150 °C, (3) 5 min at 200 °C, (4–6) 5, 10 and 25 min at 250 °C, (7–9) 5, 10 and 25 min at 300 °C, (10–12) 5, 10 and 25 min at 350 °C, (13–15) 5, 10 and 75 min at 400 °C. (b) Spectrum (0) and (11) used for band assignment of products

The assignment of the bands based on reference spectra is shown in Table 1 [42]. The products that are formed throughout the experiment are 1,1-C₂H₂Cl₂, cis-1,2-C₂H₂Cl₂, trans-1,2-C₂H₂Cl₂, C₂HCl, CO and CO₂. Specific bands were chosen, which possesses high intensity and minimal overlap with other bands, to derive the relative ratios of the products as a function of temperature as shown in Fig. 3a. The spectra show that C₂H₂Cl₂ derivatives are formed simultaneously at 200 °C (Fig. 3a, spectrum 4).

Table 1 Assignment of the gas phase IR bands in spectrum recorded after 10 min of conversion of 1,1,2-trichloethane over La₂O₃ at 350 °C shown in Fig. 3b. (vs = strong, s = strong)

As the temperature increases (Fig. 3a, spectrum 5–9), the intensity of the chlorinated ethene bands increases with maximum intensity at 250 °C (Fig. 3a, spectrum 9). Further increase of the temperature results in a decrease in intensity of the bands assigned to cis/trans-1,2-C₂H₂Cl₂. The intensity of the 1,1-C₂H₂Cl₂ band, however, remains constant. The decrease in intensity of the bands of the 1,2-derivatives is accompanied by the formation of CO, CO₂ and C₂HCl (Fig. 3a, spectrum 10). The formation of C₂HCl indicates that a second dehydrochlorination reaction may occur resulting in the formation of a C≡C bond. At 350 °C, a band becomes visible at 730 cm⁻¹, for which a reference could not be found. Strong absorption at this position is characteristic for C–Cl stretch vibrations. When the temperature reaches 400 °C, the bands of C₂HCl and 1,1-C₂H₂Cl₂ also decrease and a strong increase in intensity of the CO₂ band is observed. The formation of CO and CO₂ is characteristic products for destructive adsorption of CHCs. After more than 1 h of reaction, only CO and CO₂ are detected. These results show that high temperatures should be avoided to prevent undesirable secondary reactions.

No bands indicative of C₂H₃Cl were observed in the spectra. The other products which were detected during the temperature programmed flow-gas experiment over La₂O₃ (Fig. 2), namely 1,1-C₂H₂Cl₂ and cis/trans-1,2-C₂H₂Cl₂, were also observed in the in situ IR experiment. Therefore, based on these spectra it is proposed that the product which co-elutes with C₂H₃Cl is C₂HCl. The dehydrochlorination of 1,1,2-C₂H₃Cl₃ proceeds at relatively low temperature. At higher temperature, two secondary reactions of the chlorinated ethenes are favorable; a second elimination of HCl or destructive adsorption. The former results in the formation of C₂HCl. The latter results in the breaking of the C–C bond and formation of CO and CO₂ via exchange of oxygen and chlorine atoms. The 1,2-C₂H₂Cl₂ derivatives are more susceptible to the secondary reactions than 1,1-C₂H₂Cl₂. It should be noted that no significant amounts of CO and CO₂ were detected during the flow-gas experiments. A possible reason for this may be the different nature of the experiments; the IR experiments are in a closed cell as opposed to the flow-gas experiment in which the reactant has a limited residence time.

3.3 Catalyst Behaviour

The temperature programmed flow-gas experiment over La₂O₃ suggests there is an induction period during which the selectivity changes. This is supported by the in situ IR experiments. Moreover, no bands indicative of HCl were found in the gas phase spectra. It is known that La₂O₃ and LaOCl can be chlorinated into a pure LaCl₃ phase using HCl. The HCl reacts with the basic oxygen sites according to Reaction Equation (6). During the dehydrochlorination of chlorinated ethanes, HCl may either be re-adsorbed or the H and Cl atom are abstracted directly by the La-O couple and remain on the surface. Either way, the catalytic surface will change which may affect the activity and selectivity. The stability of LaOCl and La₂O₃ materials for the dehydrochlorination of 1,1,2-C₂H₃Cl₃ was tested in a flow-gas experiment at 400 °C. The reactor effluent composition as a function of time is shown in Fig. 4. The experiment over La₂O₃ was stopped prematurely because the reactor became plugged. The experiments show that over La₂O₃, an induction period precedes a stable conversion and product distribution. The same products which were detected during the temperature programmed reactions are formed. The two unknown products, labelled x1 and x2, are formed at higher concentrations than during the temperature programmed experiments and are therefore included in Fig. 4. In the case of LaOCl, the induction period is not observed. After the induction period, the product distribution in the constant temperature experiments is similar for both LaOCl and La₂O₃. Although coke formation was observed, no significant loss of activity occurs in the experiments. LaOCl has proven to be a more stable catalyst with high selectivity towards chlorinated ethenes.

$$ {\text{La{-}O{-}La (s) + HCl (g)}} \to {\text{La{-}OH (s) + Cl{-}La (s)}} $$

(6)

Even though the GC is not calibrated for HCl and H₂O, these products are visible in the chromatogram at low retention times. The intensities of the peaks assigned to HCl and H₂O are shown in Fig. 5 as a function of time. The hydroxyl groups formed after HCl elimination can react into both HCl and H₂O. Initially, H₂O is eliminated from the oxygen-rich catalytic surface according to Reaction Equation (7). As the reaction proceeds, less hydroxyl groups are available and more chlorine is present on the surface. As a result, elimination of HCl from the surface becomes more favourable. The reaction time at which HCl formation becomes more dominant than H₂O formation, is also when the product distribution becomes stable. Therefore, it is proposed that a specific degree of chlorination of the catalyst material results in steady-state conversion of chlorinated ethanes. It should be noted that a small amount of H₂O is probably still formed together with HCl once the product distribution has stabilized, but due to overlap of the H₂O and HCl peak, the intensity of the H₂O peak is considered zero.

$$ 2{\text{La}}{-}{\text{OH (s)}} \to {\text{H}}_2{\text{O (g)}} + {\text{La}}{-}{\text{O}}{-}{\text{La (s)}}$$

(7)

If a specific chlorination degree is needed for steady-state conversion, the catalyst materials used in the constant temperature experiments should possess similar surface compositions. To verify this, the LaOCl and La₂O₃ catalyst were characterized before and after reaction with 1,1,2-trichloethane using XPS and XRD. The XRD results, as shown in Fig. 6, provide information on the bulk phase composition of the catalyst materials. Before reaction, the characteristic diffraction patterns of LaOCl and La₂O₃ are observed (Fig. 6a and c, respectively). The diffractogram of La₂O₃ after approximately 3 h of reaction at 400 °C is shown in Fig. 6b. The peaks observed after reaction show that during reaction the crystalline La₂O₃ phase has been converted into a pure crystalline LaOCl phase. Even though the peaks are much broader in the amorphous LaOCl catalyst material, the similarity is straightforward. The dehydrochlorination of 1,1,2-trichloethane at 400 °C over LaOCl was performed for ca. 6 h, which is over two times as long as in the case of La₂O₃. The pattern of the LaOCl phase is partially preserved after reaction. However, a second phase is present, which was assigned as LaCl₃ · 3H₂O. The peaks at 2θ = 17.3, 29.2, 38.5, 42.5 and 46.9 ° and their relative intensities are characteristic of LaCl₃ · 3H₂O. It is uncertain whether LaOCl is converted into LaCl₃ · 3H₂O directly or into LaCl₃, which becomes hydrated when it is exposed to air prior to the XRD measurement. Either way, the bulk phase of both La₂O₃ and LaOCl has become chlorinated during the constant temperature reactions. Chlorination of the bulk phase is also observed during the destructive adsorption and catalytic destruction of chlorinated C₁ [29–33]. This is caused by the solid-state diffusion of oxygen and chlorine atoms between the catalytic surface and the bulk. As the surface becomes chlorinated, the chlorine atoms diffuse into the bulk and surface oxygen is regenerated. It is therefore viable to assume that the same process occurs when the surface becomes chlorinated as a result of the dehydrochlorination reaction.

Fig. 4

Effluent composition for the dehydrochlorination of 1,1,2-C₂H₃Cl₃ over (a) La₂O₃ and (b) LaOCl at 400 °C as a function of time. (GHSV = 2,000 h⁻¹, inlet concentration: [1,1,2-C₂H₃Cl₃] = 3.2 vol%, x_n = product unassigned based on GC and IR)

Fig. 5

Intensity of H₂O and HCl peak in the chromatograms of for the dehydrochlorination of 1,1,2-C₂H₃Cl₃ over (a) La₂O₃ and (b) LaOCl at 400 °C as a function of time as shown in Fig. 4. (GHSV = 2,000 h⁻¹, inlet concentration: [1,1,2-C₂H₃Cl₃] = 3.2 vol%, x_n = product unassigned based on GC and IR)

Fig. 6

X-ray diffractograms of La₂O₃ and LaOCl before (a and c, respectively) and after (b and d, respectively) the dehydrochlorination of 1,1,2-C₂H₃Cl₃ at 400 °C, as shown in Fig. 4

The surface composition of LaOCl and La₂O₃ before and after reaction was determined with XPS. All spectra were normalized to the La3d band. Because lanthanum oxide-based materials strongly adsorb CO₂ and H₂O, the exposure to air may influence the characterization of O atoms. The intensity of the Cl2p band was therefore chosen as a measure of surface chlorination. Figure 7 shows the Cl2p band for LaOCl and La₂O₃ before and after reaction. The Cl2p band is obviously not observed in the spectrum of La₂O₃ before reaction (Fig. 7a). However, the La4p band is observed in the same region at 207 eV. Both these XPS bands are known to possess a shoulder at higher energy than the maximum, which is also observed here. The La4p band appears as a shoulder of the Cl2p band in the spectra of the materials containing chlorine. The Cl2p band positioned at 200 eV is observed in Fig. 7b–d. The spectra show that the surface chlorination of the materials after reaction is approximately the same and in both cases higher than before reaction. This result confirms the hypothesis that a specific degree of chlorination is required to reach steady-state conversion.

Fig. 7

X-ray photoelectron spectra of La₂O₃ and LaOCl before (a and c, respectively) and after (b and d, respectively) the dehydrochlorination of 1,1,2-C₂H₃Cl₃ at 400 °C, as shown in Fig. 4

4 Conclusion

Lanthanum oxide-based materials are active catalysts for the dehydrochlorination of chlorinated ethanes. The reaction scheme for the dehydrochlorination of chlorinated ethanes over La₂O₃-based catalyst materials is schematically shown in Scheme 2. A hydrogen and chlorine atom is abstracted from the chlorinated ethane, resulting in the formation of a hydroxyl group and a lattice chloride (Scheme 2, Reaction b). In the case of an O-rich surface, such as La₂O₃, the hydroxyl groups will react with other hydroxyl groups under formation of H₂O (Scheme 2, Reaction c). However, when a specific degree of chlorination of the catalytic surface is reached, the elimination of H₂O becomes less pronounced and HCl is formed according to Reaction a in Scheme 2. After the induction period characterized by the formation of H₂O, the product distribution and conversion become stable and the chlorination degree of the catalyst surface also remains constant.

Scheme 2

Catalytic cycle for the dehydrochlorination of chlorinated ethanes over lanthanum oxide-based materials: (a) desorption of HCl from the catalyst material, (b) H + Cl abstraction by catalytic surface and (c) desorption of H2O from the catalyst material

The in situ IR experiments have shown that at relatively high temperature, secondary reactions may occur, such as a second dehydrochlorination step resulting in the formation of an ethyne, or destructive adsorption leading to the formation of CO and CO₂. This indicates that the reaction temperature and chlorination degree are key factors to achieve optimal selectivity towards the formation of ethenes. Because the chlorination degree of the catalyst is of influence on both activity and selectivity for the conversion of chlorinated C₁ and C₂, it may be used to tune the catalytic properties of the La₂O₃-based catalyst. This control of activity and selectivity is crucial for the efficient conversion of chlorinated waste streams, such as the light ends in the production of C₂H₃Cl.