Nitrogen and phosphorus co-doped activated carbon induces high density Cu+ active center for acetylene hydrochlorination

Fei Li, Xuemei Wang, Pengze Zhang, Qinqin Wang, Mingyuan Zhu,*, Bin Dai

1 College of Chemistry and Chemical Engineering, Yantai University, Yantai 264004, China

2 School of Chemistry and Chemical Engineering, Shihezi University, Shihezi 832000, China

Keywords:

ABSTRACT

1. Introduction

Polyvinyl chloride(PVC)is made by polymerizing vinyl chloride monomers (VCM) and is widely applied in industrial production and daily life. Due to China’s abundant coal resources, acetylene hydrochlorination using coal and limestone as raw materials has become the primary method for VCM synthesis.However, the catalyst synthesized by this method still contains toxic HgCl2,which is harmful to the environment [1].It is very significance to synthetic a green and efficient non-mercury catalyst for the continued growth PVC industry [2–4].

Previous research has investigated various metals in acetylene hydrochlorination reactions, such as Au [5–7], Ru [8–10], Pd [11],Cu[12–15],Co[16],and Bi[17].Au and Ru displayed excellent catalytic activity among these metals, but the high cost limited their industrial applications. Therefore, many efforts were devoted to exploring catalysts with a low content of Au and Ru. Li et al. [18]developed0.2%(mass)RucatalystwithN,N′-dimethylpropyleneurea (DMPU) as ligand; the conversion rate of acetylene was 81.3% after 200 h under the conditions of reaction temperature was 170 °C and gas hourly space velocity (GHSV) of C2H2was 180 h-1. Li et al. [19] reduced the loading of Au to 0.1%(mass)by using ligands,and the acetylene conversion rate reached 96%.However,the working life of these low content precious metals catalysts was far too short to meet industrial needs,which promotes the development and utilization of non-precious metals.

Cu-based catalyst is considered to be the most suitable nonnoble metal catalyst in acetylene hydrochlorination, but acetylene conversion of copper-based catalysts was much worse than that of Au and Ru catalysts.In order to improve the catalytic performance of Cu-based catalyst,Zhou et al.[20]prepared a catalyst with copper supported on carbon nanotubes doped with N for acetylene hydrochlorination. They found the catalyst activity was about ten times higher than the copper directly supported on the carbon nanotube.The electronic affinity of carbon nanotubes was adjusted by doping with N, which enhanced the anchoring of Cu and thus improved the activity of the catalyst. However, nitrogen was shown in the structure of pyrrole nitrogen, pyridine nitrogen,and graphite nitrogen on the carrier; these forms had different effects on the reaction.Dai et al.[21]found that increasing the relative content of pyrrole nitrogen can improve the acetylene conversion of Cu-based catalysts. The reason was that the interaction between pyrrole nitrogen and Cu enhanced the adsorbability of HCl and reduced the electron cloud density of Cu atoms. However, the electronic cloud density of Cu can also be reduced by ligand complexation. Li et al. [22] complexed CuCl2with nitromethyl pyrrolidone and loaded spherical activated carbon(SAC)to prepare the catalyst.At 180 h-1and 180°C,the acetylene conversion the catalyst is up to 94.2%. They found that the introduction of the N ligand reduced the adsorption strength of Cu to acetylene, inhibiting the catalyst’s carbon deposition and improving the activity. Consequently, the carrier was doped with a P element with low electron cloud density to adjust the electronic structure of the Cu-based catalyst. Li et al. [23] prepared Cu supported on phosphorus-doped SAC catalyzer with a different mol ratio of Cu and P (named x CuP/SAC). Under the conditions of 140 °C, GHSV (C2H2)=180 h-1and VHCl:VC2H2=1.1, the acetylene conversion of the 2.5 CuP/SAC catalyst reached 72.4%. The introduction of the P element in the carrier improved the dispersibility of Cu species through complexation reaction, and the particle size of Cu was also significantly reduced.

Therefore, Wang et al. [24] synthesized Cu-complexed hydroxyethyl phosphonic acid (HEDP)-supported AC catalyst and used it in acetylene hydrochlorination. At GHSV(C2H2) was 90 h-1and the reaction temperature of 180 °C. The vinyl chloride formation of Cu-1HEDP/AC catalyst was 83.4%.Adding the P ligand improved the dispersibility of Cu and repressed the reduction of Cu ions.Since the introduction of N or P improved the acetylene hydrochlorination activity of Cu-based catalysts, the simultaneous introduction of N and P ligands into Cu-based catalysts has been studied.Hu et al.[25]successfully prepared Cu supported on SAC modified by hexamethylphosphoramide (HMPA). At 180 °C and GHSV(C2H2) = 180 h-1, 15% Cu 10% HMPA/SAC catalyst Catalytic performance reached 87.25%.When HMPA ligand is added to 15%Cu/SAC catalyst, the dispersion of copper species is enhanced and carbon deposition is inhibited. Whether the Cu is supported on the N- or P-doped AC or the Cu is first complexed with a ligand and then loaded onto the AC, the same effect is observed in the acetylene hydrochlorination reaction. However, the cost of the heteroatom doped method is lower than that of the ligand complexation method. Therefore, the heteroatom doping method was further investigated to enhance the acetylene hydrochlorination performance of Cu-based catalysts.

This paper shares the successful preparation of a catalyst of Cu supported on AC with N and P co-doped using urea phosphate as the N and P source. Adding N and P improved the initial activity(95.59%) of the catalyst for acetylene hydrochlorination under the optimal reaction condition.In addition,the change of structure of Cu-based catalyst and catalytic performance for acetylene hydrochlorination was also described.

2. Experimental

2.1. Experimental materials

Cupric chloride dihydrate (CuCl2?2H2O, 99%), coconut activated carbon(AC,neutral,0.25–0.42 mm),C2H2gas(98%),HCl gas(99%),and urea phosphate (UP, 98%). All the materials and chemicals were used without processing.

2.2. Catalyst synthesis

2.2.1. The preparation of xNP/AC

N and P co-doped AC support was synthesized using UP as the N and P sources. First, Added a certain amount of AC and UP into a beaker with 30 ml deionized water and mixed for 24 h at 25 °C.Then it was dried for 24 h at 80 °C. Finally, the sample needs to be calcined in N2tube furnace for 3 hours(10°C?min-1).The pyrolysis temperature was 500 °C, 600 °C, 700 °C, 800 °C and 900 °C,respectively. The sample were recorded as xNP/AC (x = 5, 10, 20,30, and 40). For comparison, AC was also calcined at different temperatures.

2.2.2. The preparation of Cu-xNP/AC

The xNP/AC-supported Cu was synthesized using an impregnation technique. Firstly, immersed the xNP/AC in a CuCl2?2H2O solution with different masses, mixed for 24 h at 25 °C, and dehydrated for 24 h at 80 °C. The solutions containing various masses of CuCl2?2H2O were labeled as m = 1%, 3%, 5%, and 7%; the synthesised catalysts were labeled as mCu-xNP/AC. In addition, the AC supporting the same mass fraction of Cu was prepared as a control group.

2.3. Catalyst characterization

The Brunauer-Emmett-Teller (BET) textual properties of materials analysis was known by a Micromeritics ASAP 2020 instrument.X-ray diffraction (XRD) of 2θ from 10° to 80° in was performed by a Bruker D8 advance X-ray diffractometer. Transmission electron microscope (TEM) was conducted by a Titan G260-300 electron microscope. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Thermo Escalab 250Xi spectrometer with 150 W monochromatized Al-KαX-ray source (hv = 1486.6 eV).Inductively coupled plasma atomic emission spectroscopy (ICPOES; Thermo Scientific iCAP 6000 Series) was employed as measure the actual content of Cu and P elements in samples. C and N contents in supports was measured by a Elementar vario EL cube.ChemBET Pulsar TPR/TPD automated Chemisorption analyzer instrument (Quantachrome, USA) was employed in HCl temperature-programmed desorption (TPD) with a heating rate of 10 °C?min-1in Heatmosphere. Carbon deposited of samples was carried out on a thermogravimetric analysis (TGA, Discovery TGA 5500, TA Instruments, USA) under air atmosphere.

2.4. Catalyst activity evaluation

A fixed-bed microreactor (i.d. 10 mm) was applied to test the conversion of acetylene. 2 ml sample was purged in the reaction tube containing N2for 30 min to remove air, HCl (10.5 ml?min-1)was aerated into the reactor to activate the catalyst for 1 h; Then,reaction was heated from 180 °C to 240 °C. HCl and C2H2(VHCl: VC2H2= 1.15) were bubbled into the heated reaction tube,with a GHSV(C2H2) of 90 h-1. Finally, the gas reaction products were analyzed by gas chromatography (GC-2014C, Shimadzu,Japan) with a flame ionization detector (FID).

3. Results and Discussion

The pore structure parameters of AC,5%Cu/AC,and 5%Cu-xNP/AC catalysts are displayed in Table S1(in Supplementary Material).The specific surface areas of AC, 5%Cu/AC, 5%Cu-5NP/AC, 5%Cu-10NP/AC, 5%Cu-15NP/AC, 5%Cu-25NP/AC, 5%Cu-30NP/AC, and 5%Cu-40NP/AC are 1123.4 m2?g-1, 869.0 m2?g-1, 812.4 m2?g-1,769.2 m2?g-1, 734.1 m2?g-1, 588.1 m2?g-1, and 602.1 m2?g-1,respectively. With the increase of the doping amount of UP in the support, the specific surface area and hole capacity of the catalyst decrease [26,27]. The minimum is reached when the doping amount of UP is 30%; however, when the doping content of UP increases to 40%, the specific surface area and hole capacity of the catalyst are approximately the same as those of 30%. At this point, the doping amount of UP has reached saturation.

Additionally,the influence of different calcination temperatures on the conversion rate of acetylene was studied.The 30NP/AC catalyst calcinated at different temperatures,then 5%Cu was loaded to prepare a 5%Cu-30NP/AC catalyst,display in Fig. S1. The acetylene conversion increases, reaches a maximum, and then decreases as the calcination temperature rises; the conversion of acetylene reaches its maximum at the calcination temperature of 700 °C.The influence of catalysts with different Cu loadings on acetylene conversion display in Fig. 1(a). With the increase of Cu loadings,acetylene conversion gradually increases, reaching the maximum when Cu loadings are 5%and slightly decreasing when Cu loadings are 7%. Therefore, 5%Cu is determined as the optimum catalyst loading. Next, the influence of different UP doping amounts on the acetylene conversion rate was investigated. As shown in Fig.1(b),the acetylene conversion rate increases with the increase of urea phosphate doping amount; however, when the doping amount exceeds 30%, the acetylene conversion plateaus. It shows that the N and P doped by the catalyst reach the maximum,which is consistent with the specific surface area,and the hole capacity of the catalyst not decreasing. The acetylene conversion of 5%Cu-30NP/AC catalyst was 88.65% at the reaction temperature of 180 °C and GHSV(C2H2) of 90 h-1.

The acetylene hydrochlorination was tested at 180–240 °C and GHSV(C2H2) of 270 h-1to find the optimum reaction temperature of the 5%Cu-30NP/AC catalyst. The results are shown in Fig. 1(c).The acetylene conversion increases with the increase in reaction temperature, but when the reaction temperature exceeds 220 °C,the conversion rate increases slowly. Then, 5%Cu/AC and 5%Cu-30NP/AC catalysts were further reacted at GHSV(C2H2) of 90 h-1and 220 °C. In Fig. 1(d), the acetylene conversion of 5%Cu-30NP/AC catalyst decreased from 95.59% to 78.64% after the reaction time of 27 h,and the acetylene conversion rate of Cu/AC decreased from 76.53% to 59.80%. It is proved that the carrier doped with N and P can indeed improve the activity of the catalyst, but it does not change the stability.

In Fig. 2. XRD detection was carried out on 5%Cu/AC and 5%Cu-30NP/AC catalysts to analyze the aggregation state of Cu in the catalysts. There are new diffraction peaks at 16.1°, 32.2°,and 39.6° fresh and used of the 5%Cu/AC catalyst, which is the same as the CuCl2peak position (JCPID: 35–0690) [28]. The intensity of the characteristic peak of the 5%Cu/AC catalyst is greater after the reaction than before, showing that the catalyst has Cu aggregation both before and after the reaction. However,this Cu aggregation is more severe after the reaction and causes the inactivation of the 5%Cu/AC catalyst. For the 5%Cu-30NP/AC catalyst, the characteristic peak only appeared at 23° and 43.5°before and after the reaction [29]. These peaks are the same as that of the carrier AC, indicating that there was no agglomeration of Cu in the 5%Cu-30NP/AC catalyst and proving that doping N and P into the carrier AC could enhance the dispersion of Cu.This result was also confirmed by TEM.

Fig. 1. (a) Conversion of C2H2 over Cu-30NP/AC catalysts with different Cu loadings (Reaction conditions: T = 180 °C, GHSV (C2H2)=90 h-1, and VHCl :VC2H2 =1.15). (b)Conversion of C2H2 over 5%Cu/AC catalyst with different UP loadings(Reaction conditions:T=180°C,GHSV(C2H2)=90 h-1,and VHCl :VC2H2 =1.15).(c)acetylene conversion with 5%Cu-30NP/AC catalysts(Reaction conditions:temperature ranging from 180°C to 240°C,GHSV(C2H2)=270 h-1,VHCl :VC2H2 =1.15)(d)Test of optimum conditions of 5%Cu-30NP/AC and 5%Cu/AC catalysts (Reaction conditions: T=220 °C, GHSV (C2H2)=90 h-1, VHCl :VC2H2 =1.15).

Fig.2. XRD patterns of fresh and used 5%Cu/AC and 5%Cu-30NP/AC catalysts and AC was selected as contrast.

Fig.3(a)–(d)shows the TEM and HAADF-STEM picture of 5%Cu/AC and 5%Cu-30NP/AC catalysts, respectively. Fig. 3(a) and (c)show black particle aggregation in the 5%Cu/AC. In contrast, no aggregation was observed in the 5%Cu-30NP/AC (Fig. 3(b) and (d)),which means N and P co-doped AC can improve the dispersion of Cu and inhibit agglomeration in the catalyst. The elemental mapping diagrams of fresh 5%Cu/AC and 5%Cu-30NP/AC catalysts were tested to further prove this point.It can be seen from Fig.3(e)and(f)that the Cu constituent in the 5%Cu/AC catalyst is agglomerated,while the Cu constituent in the 5%Cu-30NP/AC catalyst is highly dispersed. It can be seen from Fig. 3(g) and (h) that the N and P in 5%Cu-30NP/AC catalyst are uniformly dispersed, which proves that the N and P elements are successfully doped [30].

The actual contents of Cu,P,and N are listed in Table 1.The contents of Cu and P in fresh 5%Cu/AC and 5%Cu-30NP/AC catalysts were analyzed by ICP-OES.The Cu content in the 5%Cu/AC catalyst was 4.42%(mass). The contents of Cu and P in the 5%Cu-30NP/AC catalyst were 4.27% (mass) and 4.11% (mass). Meanwhile, the N and carbon contents of the 5%Cu-30NP/AC catalyst were analyzed by an elemental analyzer.The N content of 5%Cu-30NP/AC catalyst is 0.80% (mass), and the carbon content is 76.26% (mass). It is directly proved that N and P are successfully doped on activated carbon. Moreover, the Cu content of the two catalysts is similar,but the performance of the two catalysts is different, which may be due to the interaction between N and P with Cu.

Fig.3. TEM images of fresh catalysts:(a)5%Cu/AC;(b)5%Cu-30NP/AC;TEM elemental mapping images of catalysts:(c)Representative HAADF-STEM images of fresh 5%Cu/AC catalysts;(d)Representative HAADF-STEM image of fresh 5%Cu-30NP/AC catalysts;(e)Element mapping image of Cu in 5%Cu/AC catalyst.;(f)-(i)Element mapping image of Cu, N, P, all elements in 5%Cu-30NP/AC catalyst.

Table 1 Elements contents in fresh catalysts

Fig. 4. XPS spectra and LMM Auger spectra of 5%Cu-xNP/AC catalyst catalyst.

Table 2 Surface Cu components of catalysts

XPS and Auger spectra were characterized to analyze the valence state of Cu in the catalysts modified by UP with different contents. The results are displayed in Fig. 4 and Table 2. Fig. 4(a)reveals the XPS spectrum of Cu in catalysts where the characteristic peak at 934.6 eV is Cu2+;the characteristic peak of Cu+and Cu0is at 932.2 eV[31–34].Fig.4(b)is the Cu XAES spectrum of the catalyst,showing three characteristic peaks:one peak at 918.6 eV corresponds to Cu0, another at 915.5 eV corresponds to Cu+, and the peak at 910.5 eV is the special valence of Cu [35,36]. Calculate the content of Cu0,Cu+and Cu2+by the area of the peak,and Table 2 shows the peak area size results. With the increase of UP content,the peak area of Cu2+reduced from 67.07%to 35.24%,the peak area of Cu+augmented from 29.38% to 63.61%, the peak area of Cu0reduced from 3.55% to 1.15%, and The ratio of Cu+/Cu2+increased from 0.43 to 1.81. Comparing the data in the chart shows that the loading of UP exceeds 20%,and the change of the relative contents of Cu2+,Cu+and Cu0tends to be equilibrium,which is consistent with the acetylene conversion rate of the catalyst. There is a strong chemical force between Cun+and the carrier because the carrier contains abundant electrons, which induces the transformation of Cu2+to Cu+, improving the activity of the catalyst [37].As can be seen from Fig. S3 and Table S2, the proportion of Cu+/Cu2+slightly increases after the reaction, because Cu2+in the catalyst is reduced to Cu+,and part of Cu+was reduced to Cu0,the content of Cu0increases, the catalyst activity decreases.

HCl-TPD curves of AC, 5%Cu/AC and 5%Cu-xNP/AC catalysts are shown in Fig. 5(a). It can be seen that the desorption peak of HCl appears at 200-270 °C, and the desorption area of the catalyst is in the order of 5%Cu-40NP/AC≈5%Cu-30NP/AC >5%Cu-20NP/AC > 5%Cu-10NP/AC > 5%Cu-5NP/AC > 5%Cu/AC > AC.The C2H2-TPD curve is displayed in Fig. 5(b). Compared with 5%Cu/AC catalyst, the desorption temperature of UP-doped catalyst is lower, which indicates that 5%Cu-xNP/AC catalyst reduces the adsorption capacity of C2H2. And slightly reduces the adsorption amount of C2H2. Comprehensively, the activated carbon modified by nitrogen and phosphorus can obviously enhance the chemical adsorption of HCl on mCu-xNP/AC catalyst[38],which is the reason for the improvement of catalyst activity.

4. Conclusions

Fig. 5. (a) HCl-TPD profiles of catalysts; (b) C2H2-TPD profiles of catalysts.

In this work, a new type of N—P Co-doped AC-supported Cubased catalyst (mCu-xNP/AC) has been successfully synthesized,the optimum loading of Cu is 5% and the optimum reaction temperature is 220 °C. Under the optimum reaction conditions,acetylene conversion of 5%Cu-30NP/AC (95.59%) is outstrip that of 5%Cu/AC (76.15%). The characterization of the catalyst shows that doping UP can reduce the aggregation of Cu on AC carrier,increase the content of Cu+, increase the chemical adsorption of HCl on the catalyst, and reduce the chemical adsorption of C2H2.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Taishan Scholars Program of Shandong Province (tsqn202103051), the Project of Scientific Research in Shihezi University (CXFZ202205).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2023.01.017.