Nitrogen‐Doped Carbon‐Assisted One‐pot Tandem Reaction for Vinyl Chloride Production via Ethylene Oxychlorination

Abstract A bifunctional catalyst comprising CuCl2/Al2O3 and nitrogen‐doped carbon was developed for an efficient one‐pot ethylene oxychlorination process to produce vinyl chloride monomer (VCM) up to 76 % yield at 250 °C and under ambient pressure, which is higher than the conventional industrial two‐step process (≈50 %) in a single pass. In the second bed, active sites containing N‐functional groups on the metal‐free N‐doped carbon catalyzed both ethylene oxychlorination and ethylene dichloride (EDC) dehydrochlorination under the mild conditions. Benefitting from the bifunctionality of the N‐doped carbon, VCM formation was intensified by the surface Cl*‐looping of EDC dehydrochlorination and ethylene oxychlorination. Both reactions were enhanced by in situ consumption of surface Cl* by oxychlorination, in which Cl* was generated by EDC dehydrochlorination. This work offers a promising alternative pathway to VCM production via ethylene oxychlorination at mild conditions through a single pass reactor.

. Pore parameters of the N-doped mesopores carbon.                  Table S6. Catalytic results of the dual-bed method (0.5 g CeCu/Al2O3 at the top, 1.5 g N0.5 at the bottom).
The CuCl2/γ-Al2O3-based oxychlorination catalyst was prepared by the incipient wetness method as described in our previous report. [1] The precursors CuCl2 and CeCl3 solution were impregnated on the Al2O3 with the Cu loading of 5 wt%, and the molar ratio of 0.4 for Ce/Cu. After impregnation, the samples were put in the oven at room temperature for 10 h, followed by heating to 120 °C with the ramping rate of 2 °C/min and kept for 6 h. The obtained samples were sieved to 45−100 μm before use.
Nitrogen-doped carbon catalysts were synthesized by a hard template method, in which formaldehyde (F, 37 wt%) and phenol (P) were used as carbon precursors, melamine (M) as the nitrogen source, and SiO2 nanoparticles as the hard template. Firstly, F and P were added together into 100 ml 0.2 M NaOH solution with a molar ratio of 2. The mixture was stirred at 700 °C for 40 min, followed by adding F and M into the solution and stirred for another 30 min.
Then Ludox SM-30 sol (30% SiO2) was added into the mixture during stirring. The resulting solution was placed in an oil bath at 85 °C for 5 days to form an organic gel. The gel was then dried at room temperature for 2 days at 120 °C for 24 h. The obtained gel was then carbonized at 700 °C for 4 h in the Ar atmosphere at a ramping rate of 5 °C/min. The carbon/SiO2 composite was submerged in 3M KOH at 85 °C overnight to remove the silica. Finally, the obtaining samples were washed until the pH was 7 and then dried at 100 °C to dry. The last sample obtained was nitrogen-doped carbon. The molar ratio of M/P was tuned to get different N contents. Higher M/P ratio of higher nitrogen content. The resulting samples were named as Nx, where x is the molar ratio of M/P.

Catalyst characterization
The specific surface area, pore volume and pore size of the catalysts were measured on a TriStar 3020 instrument using N2 isotherms adsorption at 77 K, calculated with the respective BET and BJH methods. The samples were degassed at 120 °C overnight before the BET measurements.
XRD profiles were recorded on a Bruker D8 Advanced DaVinci X-ray diffractometer using Cu Kα1 and 0.15 nm wavelength.
The high-resolution of transmission electron microscopy (HR-TEM) was performed on a JEM-2100F Microscope operated at an accelerating voltage of 200 kV. The sample powders were well dispersed in ethanol under ultrasonic treatment before the measurement. Energy dispersive X-ray (EDX) analysis of the catalysts was performed using the same instrument.
The surface analysis and N (and O) species were performed by X-ray photoelectron spectroscopy (XPS) using a ThermoFisher K-alpha X-ray photoelectron spectrometer system, equipped with monochromator Al Kα radiation. The C1s peak at 284.8 eV was used as a calibration for the other peaks.
The Raman spectroscopy was performed on LabRAM HR800, using a Ne-Ne laser in the visible range (633 nm), with 1800 gr/mm and a hole of 200 m.

Catalytic performance evaluation
The catalytic performance evaluation was performed in a fixed-bed glass reactor. The catalyst was heated to the target reaction temperature with a rate of 10 °C/min in Ar. The reactant gases (C2H4: pure, AGA, 3.5, 20% O2 in Ar: AGA 5.0, 20% HCl in Ar: AGA 5.0, N2: pure, AGA 5.0) were fed into the reactor with calibrated mass flow controllers. All the reactions were performed at atmospheric pressure. The products were analyzed by an on-line Agilent 7890B GC, which was equipped with a thermal conductivity detector (TCD) and a flame ionization detector.
Chlorine-containing compounds were analyzed by FID, while CO2, CO, C2H4, and N2 were analyzed by TCD. Herein, N2 was used as an internal standard for analysis. Generally, the reaction was conducted under the condition of 250 °C and ambient pressure, with the ethylene flow rate of 4 ml/min and a total flow rate of 58 ml/min. The C2H4 conversion and product selectivity were calculated with the following equations, where Ci denotes the molar fraction of a product i. the vi means the stoichiometric parameter for the converting of C2H4 to the product i to the number of carbon atom numbers, for example, For the ethylene oxychlorination reaction on the CuCl2/Al2O3-based catalyst (0.2 g), before cofeeding the reactant gas, the catalyst was treated in HCl gas to get rid of the paratacamite (due to be stored in the air).
For the physical mixture of the catalysts, the CuCl2/Al2O3-based catalyst and N-doped carbon were physically mixed with the specific mass ratios (0.2 g and 0.8 g) before mounting into the reactor.
As for the dual bed reactor, the N-doped carbon catalyst was put at the bottom of the reactor, the CuCl2/Al2O3-based catalyst was put on the top layer of the reactor, the two layers were separated by the glass wool.
EDC cracking was also performed on the same setup with the fix-bed reactor. The catalyst (0.5 g) was first heated to the target reaction temperature, followed by introducing EDC (Sigma Aldrich, 99.8%) through bubbling by He as the carrier gas.
In the Deacon reaction HCl and O2 were introduced into the N-doped carbon (N0.5, 0.5 g) catalyst with a molar ratio of 4:1. The gas stream flowed to an adsorption bottle containing an aqueous KI solution (Sigma Aldrich, 0.1 mol/L, 100 ml) for iodometric titration. This test was used to verify the production of Cl2.
In the ethyl chloride oxidation reaction on the N-doped carbon catalyst, C2H5Cl (AGA, 1% in Ar) and O2 were introduced into the catalyst at 250 °C and 1 bar. Herein, it is noted that 1% C2H5Cl is the maximum limit that the gas supplier (AGA) can provide. The catalyst was heated in Ar to 250 °C with a ramping rate of 10 °C/min. Then, the C2H5Cl and O2 were introduced into the reactor, and the product was analyzed on the online GC, which is equipped with TCD and FID.

HCl temperature-programmed desorption (HCl-TPD)
HCl-TPD was performed on our home-made setups combined with an MS to record the signal of HCl. The samples (0.5 g) were firstly treated at 100 °C in Ar to purge out the moisture for 1 h and cool down to 30 °C. The samples were then saturated with 20% HCl/Ar for 1 h.
Subsequently, the samples were purged with Ar in 100 °C for 1h to remove out the physical adsorbed HCl with the flow rate of 30 ml/min. Once a stable baseline was obtained, chemisorbed HCl was desorbed by heating from 100 °C to 350 °C with a ramping rate of 10 °C/min with the effluent gas monitored by an online MS (Hiden Analytical HPR-20 R&D). The final temperature was kept for 30 min for a stable signal.

C2H4 temperature-programmed desorption (C2H4-TPD)
C2H4-TPD was performed similarly with the step discussed above. The samples (0.5 g) were firstly treated at 100 °C in Ar to purge out the moisture and cool down to 30 °C. The samples were then saturated with 20% C2H4/Ar for 1 h. Subsequently, the samples were purged with Ar in 100 °C to remove the physical adsorbed C2H4 with a flow rate of 50 ml/min. Once a stable baseline was obtained, chemisorbed C2H4 was desorbed by heating from 100 °C to 350 °C with a rate of 10 °C/min with the effluent gas monitored by an online MS (Hiden Analytical HPR-20 R&D). The final temperature was kept for 30 min until a stable signal was obtained.

Temperature programmed surface reaction (TPSR)
All the TPSR experiments were also performed on the same setup with an online MS (Hiden Analytical HPR-20 R&D) recording the gas effluent.

TGA
TGA was performed to evaluate the stability of the catalyst on Linseis STA PT1600. The catalysts (0.05 g) were treated in Ar for 30 min at 30 °C to remove the adsorbed moisture. Then the catalysts were treated in 6% O2 in Ar to 800 °C with the ramping rate of 5 °C/min, with an online MS recording the CO2 signal. The temperature was kept for at least 5 min until a stable baseline was obtained.

S2. Catalyst characterization and properties
The pore structures of the N-doped carbon were measured by N2 physisorption. The BET surface area, BJH pore size distribution, and pore parameters are summarized in Table 1. The samples exhibited type-IV isotherms, combined with the pore size, revealing the typical mesoporous structures. [2] Doping N into the carbon, surface area, pore volume, and pore size increased a little bit. These findings suggested that melamine has a positive effect on the pore structure of the mesoporous carbon. While, the samples after the catalytic reaction were also tested, as shown in the column of spent in Table 1. The surface area and pore volume were slightly decreased compared with the fresh samples. But the pore size was slightly increased in all the samples.
The N concentration was identified by EDX with multiarea for better verification, while the type and fraction of different N species were determined by XPS. It is commonly used in the N-doped materials in a wide range of literature (actually, most of the researches is using the XPS technique). In the XPS tests, each component was tested three times, the spectra were averaged for the analysis.  Figure S1. N2 adsorption−desorption isotherms of the carbon catalysts.  *Typical XRS N peak and deconvolution of the peak of N3 is presented in Figure S7.      Figure S11. Product selectivity vs. ethylene conversion at 250 °C and 1 bar with the feeding molar ratio of C2H4/O2/HCl= 2:1:2 on N0.5. Figure S12. C2H4/O2 TPSR of N0.5 after saturated adsorbing of HCl. Conditions: Wcat=0.5 g, 10 °C/min, Ftotal=100 ml/min, F C 2 H 4 = 10 ml/min, P total = 1 bar, C2H4/O2=2 diluted in Ar. Figure S13. C2H4/O2 TPSR of N3 after saturated adsorbing of HCl. Conditions: Wcat=0.5 g, catalyst,10 °C/min, Ftotal=100 ml/min, F C 2 H 4 = 10 ml/min, P total = 1 bar, C2H4/O2=2 diluted in Ar.        The formed Cl2 will react with KI with I2 formed, and the color of the solution will be changed.
No color changes have been recorded, suggested that no Deacon reaction occurred.
With increasing conversions, the selectivity of EDC decreased and VCM selectivity increased.
It suggests that EDC is an unstable primary product, while VCM is the primary product plus the secondary product from EDC dehydrochlorination. The selectivity of ethyl chloride (EC) decreased slightly with increasing conversion, suggesting the unstable primary product.

Proposed reaction mechanism of ethylene oxychlorination:
It has been well known that hydrocarbons and oxygen can be adsorbed on the carbon surfaces and it has been reviewed previously. [4] Ethylene TPD spectra in Figure S10 indicated ethylene can be adsorbed ( step R1) on the surface of both carbon and N doped carbon. N doping slightly enhanced the ethylene adsorption and the desorption occurred at a slightly low temperature, suggesting slightly weaker adsorption on N-doped carbon than the carbon surface. The π-π stacking interactions were formed between the ethylene and carbon surfaces. [4] Oxygen can also be adsorbed (R2) and dissociated (R3) on the carbon surfaces and defects. [4] HCl−TPD spectra in Figure 3 indicated that HCl adsorbed (R4) on N sites but not on the carbon surface.
The temperature-programmed surface reaction (TPSR) of ethylene on the surface with the pre-adsorbed HCl is shown in Figure S18, only the ethyl chloride was produced (R10, R11).
Both the HCl−TPD and ethylene TPSR reveals the associative adsorption of HCl (R4).
The TPSR of ethylene and oxygen on the surface with the pre-adsorbed HCl shows the formation of both the VCM and ethyl chloride in Figure S12 and S13. The adsorbed EDC seems to be directly dehydrochlorinated to VCM. Compared to ethylene TPSR, oxygen is necessary for the formation of EDC and VCM. Directly dissociation of HCl seems to be difficult, and oxygen assisted HCl dissociation (R5) seems to be essential for the activation of HCl. H in HCl reacts with OH* to form water (R7, R9).
The adsorbed Cl* preferably reacts with adsorbed ethylene to EDC (R6, R8) instead to be combined to form Cl2 (R12). The recombination of surface Cl* seems to be not favorable and no Deacon reaction was observed.
Ethyl chloride (C2H5Cl) is the byproduct of the reaction. VCM can in principle be produced by oxidative dehydrogenation of ethyl chloride. The reaction of ethyl chloride and oxygen was performed on the N-doped carbon. No VCM was detected and the main product is ethylene.
Oxidative dehydrochlorination instead of oxidative dehydrogenation occurred.