Dehydrochlorination of Intermediates in the Production of Vinyl Chloride over Lanthanum Oxide-Based Catalysts
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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.
KeywordsDehydrochlorination Heterogeneous catalysis Lanthanum oxide Chlorinated ethanes Gas phase IR
With an annual production of 45 million tons, chlorine is one of the most important chemicals for numerous commercial products . 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, 3, 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 .
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 . 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.
2.1 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 CoKα1,2 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
3.2 In Situ IR Studies on the Catalyst Activity and Selectivity
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 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.
This work was supported by the National Research School Combination Catalysis (NRSCC) and an NWO-CW VICI Grant.
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