Tongtong Zhang,Jia Zhao,Jiangtao Xu,Jinhui Xu,Xiaoxia Di,Xiaonian Li*

Industrial Catalysis Institute of Zhejiang University of Technology,Hangzhou 310014,China

1.Introduction

The production of vinyl chloride directly from acetylene via hydrochlorination has attracted great interests in China due to the rich coal reserves.However this progress is restricted by the volatility and toxicity of the mercury chloride catalyst adopted.Therefore,alternative catalysts with various metals have been investigated[1-8].Among these metal catalysts,AuCl3is proved to be highly active and selective to the desired products.But the commercialization of this catalyst is largely hindered by the high cost and low catalytic stability since AuCl3is readily reduced to Au0under reaction conditions[2,4,9-19].The replacement of AuCl3with a less expensive metal,or even a metal-free catalyst having a higher stability,is required for practical purposes[20-25].

Nitrogen-doped carbon and carbon nitride materials are known to act as promising metal-free catalysts for several reactions because the incorporation of nitrogen atoms in the carbon architecture provides the access to an even wider range of applications than pure carbon materials[26].Recently,some research efforts have been increasingly focused on the use of N-doped carbon materials as catalyst materials for the reaction of acetylene hydrochlorination.Dai et al.reported that a supported g-C3N4catalyst on activated carbon(AC)was active for the gas phase hydrochlorination of acetylene[21].Wei et al.evaluated the catalytic activity and stability of nitrogen-doped carbon nanotubes(N-CNTs)catalysts and found that N-CNTs were active for acetylene hydrochlorination reaction,possessing a good activity(TOF=2.3×10-3·s-1)and high selectivity(＞98%)[23].Very recently,Bao et al.reported that a nanocomposite of nitrogen-doped carbon derived from silicon carbide could also activate acetylene directly for hydrochlorination[25].By employing the pyrrolic materials,the acetylene conversion was twice that of SiC@N-C under the same conditions,butthey were less suitable for industrialpractice because of their powder nature.

To evaluate the potential of nitrogen-doped carbon materials as metal-free catalysts,it is desirable and interesting to first examine the in fluence of surface functional groups of carbon materials on the catalytic behaviors.As we all know,the nature of surface functional groups of the support is extremely importantbecause they can dramatically affect the catalytic performance of the carbon-based catalysts by varying their acid-base and/or hydrophilic characters on carbon surface[27,28].For instance,it has been shown that carbonyl/quinone groups were the active sites for the oxidative dehydrogenation ofethylbenzene to styrene,and a linear correlation between the activity of carbon catalysts and the concentration of active sites was established[29].Interestingly,it has been pointed out that the catalytic performance and even the reaction mechanism for the carbonaceous catalyst could be completely different,depending on different amounts and types of surface oxygenated groups[30,31].In the present work,the surface of a pristine AC material was modified by nitrogen doping and wet treatments with HNO3and the catalytic performance of these modified carbon materials was tested as catalysts for gas phase hydrochlorination of acetylene.The in fluence of the surface modifications on the reaction rate and the product selectivity was examined.

2.Experimental

2.1.Catalyst preparation

Acommercialactivated carbon NORIT ROX 0.8(pellets of0.8 mm diameter and 5 mm length)was selected as the starting materials for the preparation of catalysts.The activated carbon was first pretreated with HNO3(65 wt%)(room temperature for 12 h)to introduce surface oxygenated groups.The pretreated activated carbon was then filtered,rinsed by deionized water until neutral and eventually dried at 110°C for 12 h(AC-n).

The AC-n sample was used as the starting materialfor thermal treatment to modify surface oxygenated groups to obtain different oxygendoped AC-n.The pre-prepared AC-n was thermally treated in N2atmosphere under 400,600 and 900°C to obtain AC-n-N400,AC-n-N600 and AC-n-N900,respectively.The AC-n sample was again used as the starting material for nitrogen doping procedure.A mixture of AC-n(5 g),glacial acetic acid(3 ml)and distilled water(50 ml)was stirred for 30 min to obtain a carbon slurry.Urea(3 g),H2O2(10 ml,10%)and deionized water(30 ml)were added and the solution was stirred for 24 h at room temperature in the dark.Finally,the solution was filtered and calcined at 300,500 and 700°C under a nitrogen atmosphere for 1 h,to obtain AC-n-U300,AC-n-U500 and AC-n-U700,respectively.

2.2.Catalyst characterization

N2physisorption analysis was carried out in Micromeritics ASAP 2020 instrument at 77 K and the surface area was calculated by Brunauer-Emmett-Teller(BET)method.

X-ray diffraction(XRD)patterns were collected using a X'Pert PRO advanced X-ray diffractometer with CuKαirradiation (k =0.15406 nm)at 40 kV and 40 mA.

The Raman spectra were recorded using Micro-Raman spectrometer(HORIBA Jobin Yvon)equipped with a 632.81 nm Ar+ion laser.The integration time was 20 s for one measurement.

X-ray photoelectron spectroscopy(XPS)analysis was performed by Kratos AXIS Ultra DLD spectrometer with monochromatized aluminum X-ray source(1486.6 eV).The C1s peak at 284.8 eV was taken as references for correcting surface-charging effects.

Temperature programmed desorption(TPD)experiment was carried out in a tubular quartz reactor equipped with a mass spectrometry(QMS 200 Omnistar)for detecting the constitution of the off-gas.For C2H2or HCl-TPD,75 mg of sample was loaded in the reactor and the in-situ pre-adsorption of C2H2or HCl was achieved by using pure C2H2or HCl,respectively(at 180°C to pre-adsorption).The inlet gas was then switched to pure Ar with a flow rate of 30 ml·min-1for 0.5 h to sweep away physical adsorbed species.C2H2or HCl-TPD was then performed by heating the sample up to 450 °C or 400 °C at a ramp rate of 10 °C·min-1in the pure Ar atmosphere,respectively.To carry out CO2/CO-TPD,the sample with a weight of 75 mg also was loaded in the reactor which then was fed with pure Ar with a flow rate of 30 ml·min-1.The sample was then heated from 30 °C to 900 °C at a ramp rate of10 °C·min-1and the temperature was hold at900 °C for 1 h.

2.3.Catalytic test

Catalysts were tested for acetylene hydrochlorination in a fixed-bed glass microreactor(i.d.10 mm).Acetylene(2.8 ml·min-1,0.1 MPa)and hydrogen chloride(3.4 ml·min-1,0.1 MPa)were fed through a mixing vessel via calibrated mass flow controllers into a heated glass reactor containing catalyst(850 mg),with a total C2H2GHSV of 100 h-1.A reaction temperature of 180°C was chosen.The gas phase products were first passed through an absorption bottle containing NaOH solution and then analyzed on-line by GC equipped with a Porapak N packed column(6 ft×1/800 stainless steel)and a flame ionization detector(FID).

3.Results and Discussion

3.1.Textural and chemical properties of oxygen-and nitrogen-doped AC-n

Table 1 shows the surface area and chemicalcompositions ofpristine AC,AC-n,oxygen-and nitrogen-doped AC-n.As anticipated,pristine AC presents a low oxygen contenton surface.In order to investigate the effectofoxygen and nitrogen doping on the catalytic performance ofAC-n in hydrochlorination of acetylene,liquid phase oxidation using a concentrated HNO3solution was carried out to bring substantial oxygen containing groups on surface[32,33].As can be seen,HNO3oxidizing process did introduce a large amount of surface oxygenated groups and also a smallamountofnitrogen species which mostlikely presented in the form of-NO2[34].In addition,it caused a slight decrease of surface area,as a consequence of pore blocking by surface oxygenated groups[35].HNO3treated AC calcined in flowing N2under different temperatures showed a similar surface area with pristine AC,demonstrating the decomposing of surface oxygenated groups and relieving pore blocking upon thermal treatment,which also could be seen from the gradual decline of oxygen content with the increasing of temperature.Some works also showed that calcination treatment could recover the surface area of AC-n[32,36].AC-n-U300 had a relatively low surface area,which might be due to the low calcination temperature,which cannot embed nitrogen into carbon matrix as could be seen from the relatively low nitrogen doping content,and the blockage in pores by unreacted residual urea.AC-n-U500 and AC-n-U700 with higher calcination temperatures showed similar surface areas with pristine AC,demonstrating that the complete decomposing of urea during doping process.AC-n-U700 presented the lowest nitrogen content among the nitrogen-doped samples,which mightbe due to highercalcination temperature that would easier lead to the decomposition of nitrogen species[37,38].

Table 1 Surface area and chemical compositions of oxygen-and nitrogen-doped AC-n

3.2.The effect of oxygen doping on the performance of oxygen-doped AC-n

Fig.1 shows the conversions of acetylene over oxygen-doped AC-n.As can be seen from Fig.1,those catalysts subjected to the thermaltreatment in N2atmosphere under different temperatures present similar catalytic performances in terms of activity and stability.The conversion of acetylene less than 25%is observed in the gas phase for all the catalysts.

In order to investigate the effect of oxygen doping on the performance of AC-n,AC-n with different amounts and types of surface oxygenated groups should firstly be obtained.Fig.2 shows the TPD-MS profiles of oxygen-doped AC-n.As can be seen,four oxygen-doped AC-n showed completely different evolution curves,demonstrating the presence ofdifferentsurface functionalgroups.After liquid phase oxidation,AC-n presents the highest amount of surface oxygenated groups.After thermal treatments at 400 °C and 600 °C,part of the surface oxygenated groups,decomposes under these temperatures and were selectively eliminated,while the other surface oxygenated groups decomposing into COwith high decomposing temperature unexpectedly increased.AC-n-N900 after thermal treatment under 900°C presents negligible surface oxygenated groups.This result demonstrates that the surface oxygenated groups have negligible in fluence on the catalytic performances in hydrochlorination of acetylene by oxygen-doped AC-n.Temperature programmed desorption(TPD)is an effective technique providing direct comparison of the adsorption and activation of reactants on different catalysts.Fig.3 shows the C2H2-TPD profiles of oxygen-doped AC-n.Its desorption peaks at approximately 115°C.There is a negligible adsorption of acetylene on oxygen-doped AC-n(refer to Fig.7 for a comparison),which explains the reason for almost no acetylene conversion detected in the reaction.

3.3.The effect of nitrogen doping on performance of nitrogen-doped AC-n

Fig.3.C2H2-TPD profiles of oxygen-doped AC-n.

Fig.4(a)displays the results of acetylene hydrochlorination using the nitrogen-doped AC-n as catalysts under the reaction conditions of temperature 180°C and C2H2hourly space velocity 100 h-1.As can be seen,the three different nitrogen-doped AC-n catalysts presented apparently different catalytic performances.AC-n-U500 showed the highest acetylene conversion of 34%,while AC-n-U300 and AC-n-U700 showed relatively lower activities with the acetylene conversion of 20%and 27%,respectively.Additionally,the acetylene conversion of AC-n-U500 can be varied from 35 to 75%when the gas hourly space velocities(GHSVs)was decreased from 120 to 30 h-1(Fig.4(b)).The AC-n-U500 catalyst can also be operated at a higher temperature up to 240 °C(Fig.4(c)).For example,at240 °C and C2H2hourly space velocity 30 h-1,the conversion of acetylene reached 92%and the selectivity to acetylene chloride remained above 99%.It should be mentioned here that,as the nitrogen doping mechanism revealed by the work[37],the nitrogen precursor reacted with surface oxygenated groups could lead to different nitrogen species and these nitrogen species would transform gradually between each other.

As aforementioned,three nitrogen-doped AC-n presented obviously different catalytic performances(Fig.4(a)).Except for AC-n-U300 presenting a similar catalytic performance with oxygen-doped AC-n,which might be due to the fact that the calcination temperature of 300°C was too low to dope active nitrogen species into carbon matrix,all of the other nitrogen-doped AC-n showed evidently better catalytic performances than oxygen-doped AC-n.The distinct improvement of the catalytic performances of nitrogen-doped AC-n over oxygendoped AC-n could be attributed to the nitrogen species doped into carbon matrix.Moreover,AC-n-U500 with higher nitrogen content than AC-n-U300 and AC-n-U700(Table 1),also showed better catalytic performances than AC-n-U300 and AC-n-U700(Fig.4(a)).Herein,it might be concluded that doping into carbon matrix with nitrogen could improve the catalytic performance and the more nitrogen doping amount,the higher catalytic activity,which was consistent with the work reported by Dai et al.[21].

Fig.2.TPD-MS profiles of oxygen-doped AC-n.(a)CO2;(b)CO.

Fig.4.The conversions of acetylene hydrochlorination.(a)Different nitrogen-doped AC-n;(b)AC-n-U500 with differentGHSVs;(c)AC-n-U500 with different temperatures.((a)Reaction temperature 180 °C,catalyst mass 0.85 g,HCl/C2H2 mol ratio=1.2:1,C2H2 flow rate 2.8 ml·min-1,GHSV 100 h-1;(b)reaction temperature 180 °C,HCl/C2H2 mol ratio=1.2:1,GHSV ranging from 30 to 120 h-1;(c)reaction temperature ranging from 150 to 240°C,HCl/C2H2 mol ratio=1.2,GHSV 30 h-1).

XRD was performed to investigate the physical structure of the nitrogen-doped AC-n catalysts.Two obvious diffraction peaks in the pattern of the AC-n support appeared at 23.16 and 43.84°,corresponding to the(002)and(101)planes,respectively(Fig.5).As the annealing temperature increases,the(002)half peak width became narrow.The result suggests that the crystal form of the sample was gradually close to well-ordered graphite,indicating that nitrogen atoms have been successfully doped into the support[39].

Fig.5.XRD patterns of different samples.

Fig.6.Raman patterns of different samples.

Raman spectroscopy is a powerfultoolforcharacterizing carbon materials and detecting the doping effectofheteroatoms.Raman spectra of all samples exhibited two bands at around 1350 and 1580 cm-1corresponding to D(A1g mode)and G(E2g mode)bands,respectively(Fig.6).The increasing intensity of the nitrogen-doped AC-n in contrast to AC-n provided an evidence of the intensified graphitic nature of the N-doped carbon materials,which is in agreement with XRD results in Fig.5.In addition,it can be observed the D band slightly broadens with the increasing of calcination temperature.This effect could be indicative of the incorporation of N,specially pyridinic-N and quaternary-Nwhich would increase the structuralimperfection ofAC-n.

Fig.7 shows the C2H2and HCl-TPD profiles of nitrogen-doped AC-n.From Fig.7(a),after doping nitrogen into carbon matrix,there is a great enhancement of the ability to adsorb C2H2,which was quite consistent with the results of C2H2-TPD in the works[20,25].Wei et al.proposed that the stability of C2H2on nitrogen-doped carbon nanotubes(NCNTs)was attributed to the formation of covalent bonds between C2H2and N-CNTs,stemming from the interactions between the highest occupied molecular orbital(HOMO)of N-CNTs and the lowest unoccupied molecular orbital(LUMO)of C2H2[23].Nitrogen-doped AC-n,as shown in Fig.7(b),also showed an obvious improvementon the competence of adsorbing HCl.It has been revealed that,comparing with the AC-n without nitrogen doping,the desorption temperature of nitrogen-doped AC-n shifted to a higher value,indicating that the relatively strong bonding between HCl and nitrogen-doped AC-n[21,22].Therefore,itcould be concluded that the superior catalytic performance of nitrogen-doped AC-n than that of oxygen-doped AC-n was derived from the capability of adsorbing C2H2and the improvement of adsorbing HCl endowed by nitrogen doping.

3.4.Correlation between nitrogen species and the performance of nitrogen-doped AC-n

The doping ofnitrogen into carbon matrix gave rise to severaldifferent species,which may have differentrelationswith catalytic performance.In order to identify the role of nitrogen species in tuning catalytic performance and correlate the catalytic performance with the amounts of the nitrogen species,the XPS deconvolution of N1s was carried out to determine the amount and type of nitrogen species as displayed in Fig.8 and summarized in Table 2.The XPS N1s was deconvoluted into three differentnitrogen species,pyridinic-N with the binding energy about399.0 eV,pyrrolic-N with the binding energy about 400.0 eV and quaternary-N with the binding energy about 402.0 eV.

The correlation between the quantity of specific nitrogen species and catalytic performance was carried out and the results are shown in Fig.9.The catalytic performance of nitrogen-doped AC-n was improved with the increase of the amount of pyrrolic-N and quaternary-N species,which indicates that the catalytic activity was most likely linked with pyrrolic-N and quaternary-N species.Theoretically,the insertion of heteroatoms(for example,nitrogen)in the carbon lattice can convert the neutral carbon atoms into positively charged atoms(active sites).Indeed,as reported by Bao etal.[25]thatthe carbon atoms bonded with pyrrolic-N species were the active sites for acetylene hydrochlorination reaction via the formation of intermediate adsorbed C2H2complex species.By the theoretical simulations,the authors suggested that the pyrrolic-N-induced electronic state possessed a higher energy level and density,which bene fited the adsorption of acetylene.Besides,Wei et al.[23]revealed a good linearity between the quaternary-N species content and acetylene conversion when nitrogen-doped carbon nanotubes(N-CNTs)were employed as the catalyst for acetylene hydrochlorination reaction.DFT study showed that the nitrogen doping enhanced the formation of the covalent bond between C2H2and N-CNTs on these active sites,which promoted the hydrochlorination.According to the existing works and the correlating result obtained here,it is proposed that both pyrrolic-N and quaternary-N species can be active sites for catalyzing the hydrochlorination reaction.

3.5.Long-term stability and deactivation of AC-n-U500

The long-term stability is another critical measurement in the evaluation of non-metal hydrochlorination catalysts.In addition to the catalytic tests discussed in Fig.4,a long-term stability test consisting of approximately 200 h of operation,was performed on the very active AC-n-U500 catalyst.The catalytic performance of the catalyst during this experiment is shown in Fig.10.The C2H2conversions remained constant(～76%)over 200 h run under the upper limit of 50 h-1of industrial hydrochlorination space velocity except for its slight decrease at the initial stage.Evidently,the AC-n-U500 catalyst is stable under typical acetylene hydrochlorination reaction conditions.

C2H2and HCl-TPD and XPS analysis were carried out to investigate the adsorbing performance and surface chemistry of AC-n-U500 after reaction.The result of XPS analysis showed that the nitrogen content decreased from4.04%before reaction to 2.31%after reaction.The results of C2H2and HCl-TPD of reacted AC-n-U500 are shown in Fig.7.Compared with the fresh AC-n-U500,as anticipated,the reacted AC-n-U500 showed obviously lower adsorbing ability for both C2H2and HCl than the fresh one.Therefore,some possible reasons forthe deactivation are as follows:(1)the nitrogen species were decomposed during reaction,from the resultof XPS analysis that the nitrogen content decreased after reaction;(2)the active site was covered by carbon deposited during the reaction,leading to the decrease of the accessibility to active sites[21].As the XPS was a surface characterization technology,the covering of nitrogen species could also lead to the decrease of surface content of nitrogen.The reasons proposed here could result in the decrease of either total amount of active sites or the amount of valid active sites exposed to reactant,leading to the decrease of the adsorption ability of C2H2and HCl on nitrogen-doped AC-n and then the catalytic activity for the hydrochlorination of acetylene.

Fig.7.TPD profiles of nitrogen-doped AC-n.(a)C2H2-TPD;(b)HCl-TPD.

Fig.8.XPS deconvolution of N1s for nitrogen-doped AC-n.(a)AC-n;(b)AC-n-U300;(c)AC-n-U500;(d)AC-n-U700.

Table 2 Summary of binding energy and quantity of different nitrogen species obtained by the deconvolution of XPS N1s

Fig.9.Correlation between catalytic activity and nitrogen species for nitrogen-doped AC-n.

4.Conclusions

Fig.10.The stability test of AC-n-U500.(Reaction temperature 210°C,catalyst mass 2.5 g,HCl/C2H2 mol ratio=1.2:1,C2H2 flow rate 4.2 ml·min-1,GHSV 50 h-1).

In this work,AC-n was used as non-metal catalyst to catalyze hydrochlorination of acetylene directly.AC-n was subjected to different post-treatment process to obtain oxygen-doped and nitrogen-doped AC-n to investigate whether the surface functional groups have any influence on the catalytic performance.The result showed that the surface oxygenated groups introduced by liquid phase oxidizing process played a negligible role to affect catalytic performance,while the nitrogen species introduced by urea calcination did play a significant role and could be active sites for hydrochlorination of acetylene.The oxygen-doped AC-n failed to adsorb and activate C2H2,butafterbeing doped with nitrogen,the nitrogen-doped AC-n showed the ability to adsorb and activate C2H2and thus better catalytic performance.The correlation between nitrogen species and catalytic performance demonstrated that pyrrolic-N and quaternary-N might be the active sites.The nitrogen-doped AC-n suffered slight deactivation and the reasons for this deactivation were also investigated and proposed.The results obtained in this work do shed some light on the important role of surface functional groups of AC-n and will bene fit the rational design of carbon-based catalyst which is not just limited for hydrochlorination of acetylene.

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