The ligand coordination approach for improving the stability of low-mercury catalyst in the hydrochlorination of acetylene☆

Xiaolong Xu,Haihua He,Jia Zhao,Bailin Wang,Shanchuan Gu,Xiaonian Li*

Institute of Industrial Catalysis of Zhejiang University of Technology,State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology,Hangzhou 310032,China

1.Introduction

Vinyl chloride monomer(VCM)is mainly used for the production of polyvinyl chloride(PVC),which is one of the five general resins and widely applied in the fields of industry,agriculture,and building materials[1].Generally,VCM is industrially synthesized by the dehydrochlorination of dichloroethane or the hydrochlorination of acetylene[2,3].The hydrochlorination of acetylene has received much attention in countries with large reserves of coal[4].For instance,the manufacture of PVCviathe hydrochlorination of acetylene accounts for about 70%of the total PVC production capacity in China[5].

In the industrial process of the hydrochlorination of acetylene,acetylene(C2H2)reacts with anhydrous hydrogen chloride(HCl)to obtain VCM under the catalysis of mercuric chloride(HgCl2)supported on activated carbon at optimum temperature range of 170–180 °C[6].However,HgCl2(B.P.=302°C)volatilizes easily from the surface ofactivated carbon underreaction conditions[7].Meanwhile,HgCl2ishighly toxic.Loss of HgCl2not only leads to an irreversible deactivation of the catalyst but also threatens to the environment and human health.Therefore,it is urgent to explore new types of environment-friendly non-mercury catalysts to replace mercury catalyst from the view of sustainable development of PVC industry.Based on the correlation of hydrochlorination activity of metal chlorides supported on activated carbon with the standard reduction potential[8],scholars focuses their attention on Au3+[9–13],Pd2+[14–16],Ru3+[17,18],and Ptδ+[19,20],etal.Nevertheless,these non-mercury catalysts cannotsimultaneously satisfy the requirements of high activity and favorable stability for industrialcatalyst.Thus activated carbon supported mercuric chloride(HgCl2/AC)has still been used as the catalyst for the hydrochlorination of acetylene by most of PVC enterprises.Consequently,aiming at the problems of the instability of HgCl2/AC,it's necessary to make improvement in the stability of mercury catalyst.Herein,how to inhibit the sublimation of the mercury species is the key to improving the stability of mercury catalyst.Xin Huang employed carbon-mixing expanded multilayered vermiculite(EML-VMT-C)as the high-mercury catalyst support and the as-obtained catalyst showed good stability for acetylene hydrochlorination[21].But vermiculite has low specific surface area[22],not in favor of the dispersion of active components.Furthermore,carbon deposition in the form of polymerization of acetylene would happen on non-carbon supports in the hydrochlorination of acetylene[23].Besides,some kinds of metal chlorides such as CdCl2[24],CsCl[25],BiCl3,CeCl3and KCl[26],were used as additives to improve the stability of mercury catalyst and reduce the amount of HgCl2effectively.However,itis worth noting thatloss ofHgCl2stillexists in the operating cycle of low mercury catalyst significantly[27].Hence,to improve the stability of mercury catalyst for the hydrochlorination of acetylene presently should focus on designing more stable active component.

The catalyst stability also puzzles the development of Au-based catalyst in the hydrochloriantion of acetylene.It has come to our notice that,in order to improve the stability of Au-based catalyst for acetylene hydrochlorination,Hutchings attempted to search or synthesize some gold complexes as the active precursor from the view of stability constant[28].Using Au(S2O3)32?with higher stability constant than AuCl?4as the active gold precursor could suppress the reduction of oxidized gold species effectively.Thus,we consider that whether this method to stabilize the gold species is valid for stabilizing mercury catalyst for acetylene hydrochlorination.Hg2+can react with halogen anions(Cl?,Br?,I?),forming a series of coordination ions(HgX24?,X=Cl,Br,I)with different stability constants and the stability of the coordination ions is enhanced in the orderofCl,Br,and I[29].In this paper,we have prepared some low-mercury catalysts(4%HgCl2loading),using potassium halides(KCl and KI)as additives.The results showed that the low-mercury catalyst HgCl2-4KI/AC performed better stability than HgCl2-4KCl/AC and HgCl2/AC under the acetylene hydrochlorination reaction conditions.The order of sublimation rates of HgCl2from the catalysts is:HgCl2-4KI/AC＜HgCl2-4KCl/AC＜HgCl2/AC,indicating that the HgCl2-4KI/AC catalyst using K2HgI4with larger stability constant as the main active component had better thermal stability.

2.Experimental

2.1.Pretreatment of activated carbon support

A commercial activated carbon(Norit ROX 0.8)was used as the support.Activated carbon was firstly washed with concentrated nitric solution(HNO3,65%–68%)at room temperature for 1 h to remove the contained impurities,then filtered and washed with deionized water to neutral and dried at 110°C for 12 h in the air.

2.2.Catalyst preparation

Taking into account the depletion and limited use of mercury resources,low-mercury(4 wt%HgCl2loading)catalyst was chose as the research object.The incipient wetness impregnation method was employed to prepare the catalysts.Firstly,HgCl2(0.2 g,GR,≥99.9%,Xiya Reagent)and KX(0.2197 g KCl,or 0.4891 g KI,n(HgCl2):n(KX)=1:4,X=Cl,I)were dissolved in deionized water(9 ml).Then,the mixed solution was added dropwise to the pretreated activated carbon(5 g)with stirring.After placed overnight,the catalyst was dried at 110°C for 12 h in the air.The as-prepared catalysts were labeled as HgCl2-4KCl/AC and HgCl2-4KI/AC,respectively.The same procedure was applied to prepare the pure HgCl2/AC catalyst.

2.3.Sublimation rate measurements of HgCl2

The sublimation rate measurements of HgCl2were carried outas described in the chemical industry standard of the People's Republic of China HG/T 4192-2011[30].The fresh catalyst(0.6 g)was set into a tubular reactor(Ф10 mm × 400 mm)and heated up to 250 °C.Nitrogen with a flow rate of 6 ml·min?1was fed into the reactor for 3 h before the catalyst was taken out to be analyzed.The content of HgCl2in the catalyst was analyzed by titration with C5H10NS2Na.After ground,screened with 80 mesh sieve and dried to constant weight,the sample(0.5 g)was put into a 250 ml flask with a re flux condenser.10 wt%NaCl solution(10 ml)and aqua regia(10 ml)were added before the flask was heated to boiling for 15 min.The mixture was filtrated and the obtained filtrate was titrated with standard C5H10NS2Na solution.Considering the easy sublimation of mercuric chloride and the high temperature during drying process,the real contents of HgCl2in fresh catalysts were determined by the same titration method above.

2.4.Catalyst characterization

X-ray diffraction(XRD)patterns of the catalysts were conducted on a PANalytical X'Pert diffractometer using a Cu Kα radiation(λ=0.1541 nm)in a scanning range of2θ=10°–80°thatwas operated at the voltage of 40 kV and the current of 40 mA.The morphology and components of the catalysts was characterized by using a transmission electronic microscope(TEM,Philips-FEI Tecnai G2 F30 S-Twin)equipped with high-angle annular dark- field(HAADF)detector and EDX spectroscopy.

2.5.Catalytic performance evaluation

Catalytic performance was carried out in a fixed bed reactor(i.d.10 mm).The temperature of the reactor was regulated by an AI-808P temperature controller(Xiamen Yudian Automation Technology Co.,Ltd.).Firstly,nitrogen gas(6 ml·min?1)was fed into the reactor containing catalyst(0.82 g)to eliminate water and air in the reaction system.After the reactor was heated up to 220°C,switch nitrogen gas to HCl gas(6 ml·min?1),which was controlled by a calibrated mass flow controller(Beijing Sevenstar Electronics Co.,Ltd.),passing through the catalyst bed and activating the catalyst for 0.5 h.Then,pretreated by saturated K2Cr2O7solution and concentrated sulfuric acid successively to eliminate the contained H2Sand H3Pimpurities,C2H2(5 ml·min?1)wasfed through anothercalibrated mass flowcontrollerand mixed with HClbefore passing through the reactor.Here,the gas hourly space velocity(GHSV)of C2H2is 180 h?1.The exit gas mixture was passed through an absorption bottle containing saturated NaOH solution to absorb excess HCl.The compositions of the product were analyzed using an online gas chromatography(GC 9790,Zhejiang Fuli Analytical Instruments Co.,Ltd.)equipped with flame ionization detection(FID).Chromatographic separation and identification of the compositions was determined by using a Porapak N packed column(6 ft.×1/800 stainless steel).The conversion of C2H2and the selectivity to VCM were de fined by the following equations.

In the above equations,FA0,FA,andFVCMrepresent the volume fraction of acetylene in the raw gas,the volume fraction of remaining acetylene in the product mixture gas,and the volume fraction of VCM in the product mixture gas,respectively.

3.Results and Discussion

3.1.Characterization

3.1.1.Crystal phase analysis

The X-ray diffraction patternsof the above three catalysts are shown in Fig.1.Itcan be seen thatthere are no obvious detectable re flections of Hg species,such as HgCl2,K2HgCl4,or K2HgI4,in the patterns.Itindicates that the active components are highly dispersed on the surface of activated carbon.According to Xie's findings[31],HgCl2,K2HgCl4and K2HgI4may disperse as a monolayer on the surface of the support because these substances are all salts.The extra KCl is generated when HgCl2reactswith excess KX.There were no diffraction peaks ofKCl,suggesting that the extra KCl is also highly dispersed.

3.1.2.Morphology of HgCl2-4KI/AC

In our previous study,when CsClwas used as an additive to be added into the low-mercury catalyst,the elements of Hg and Cl were homogeneously distributed on the surface of activated carbon[25].Therefore,we speculate that the distribution of Hg and Cl on the HgCl2-4KCl/AC catalyst is the same as that on Hg-Cs/AC catalyst.Fig.2 shows the mapping photographs of several major elements on HgCl2-4KI/AC.The mapping characterization displays that the elements of Hg and I are in homogeneous distribution on the surface of the support as well as K and Cl,and no obvious particles exist on the surface of activated carbon.It indicates that there are some interactions between HgCl2and excess KI,hinting the generation of K2HgI4,which is the active component of the HgCl2-4KI/AC catalyst.

3.2.Catalytic properties

3.2.1.The effect of KX on the performance of HgCl2/AC

In order to investigate the catalytic stability,the three catalysts were evaluated for 50 h under fixed acetylene hydrochlorination reaction conditions at 220°C,an acetylene gas hourly velocity(C2H2)of 180 h?1,and a fed volume ratioV(HCl)/V(C2H2)of1.2.The catalytic performances of the three catalysts were shown in Fig.3.The selectivity to VCM was all above 99.5%over the three catalysts.Although HgCl2/AC showed the highest hydrochlorination activity with C2H2conversion of 85.12%,it deactivated rapidly with an average deactivation rate of 0.49%·h?1.The deactivation rates of HgCl2-4KCl/AC and HgCl2-4KI/AC were 0.30%·h?1and 0.10%·h?1,respectively,smaller than that of HgCl2/AC.HgCl2-4KCl/AC and HgCl2-4KI/AC exhibited better catalytic stability than HgCl2/AC.

Fig.1.X-ray diffraction patterns of HgCl2/AC,HgCl2-4KCl/AC,and HgCl2-4KI/AC.

Fig.3.The catalytic performances of three low-mercury catalysts in acetylene hydrochlorination.(6 ml·min?1 HCl,5 ml·min?1 C2H2,0.82 g Cat.,T=220 °C).

3.2.2.The effect of variation of the active species on the performance of HgCl2/AC

Fig.2.HAADF image together with the EDS mapping of the HgCl2-4KI/AC catalyst.

Because ofthe formation of the excess KCl in HgCl2-4KX/AC,in order to investigate the intrinsic stability of the active component K2HgX4in the catalyst,we prepared some catalysts using pure K2HgX4(X=Cl,I)as precursor without excess KCl.The K2HgCl4/AC catalyst was prepared using 0.2 g HgCl2and 0.1099 g KCl(n(HgCl2):n(KCl)=1:2)as precursor.The K2HgI4/AC catalyst was prepared as follows:Firstly,the red deposit HgI2was obtained by reacting 0.2 g HgCl2with 0.2446 g KI(n(HgCl2):n(KI)=1:2),and washed with deionized water to eliminate K+and Cl?.Then,the K2HgI4/AC catalyst was got by using the as-prepared HgI2and 0.2446 g KI(n(HgI2):n(KI)=1:2)as precursor.The K2HgCl4-K2HgI4/AC catalyst was prepared by step impregnating method.The catalytic performances of the three catalysts were shown in Fig.4.We could see from Fig.4 that the three catalysts showed similar highest activity,about 78%-80%of acetylene conversion at the C2H2GHSV of 180 h?1and 220 °C.With the active component varying from K2HgCl4to K2HgI4,the stability of the catalyst was improved.The catalytic performances stated clearly that K2HgI4had better stability than K2HgCl4under acetylene hydrochlorination reaction conditions.

Fig.4.The catalytic performances of K2HgX4/AC in acetylene hydrochlorination.(6 ml·min?1 HCl,5 ml·min?1 C2H2,0.82 g Cat.,T=220 °C).

The reason why HgCl2-4KI/AC showed the bestcatalytic stability can be stated as below.Hg2+can react with halogen anions(Cl?,Br?,I?),forming a series of coordination ions(HgX42?,X=Cl,Br,I)with different stability constants.The chemical equations for the reactions of Hg2+and halogen ions to form a variety of coordination ions and corresponding stability constants ofthe coordination ions are listed in Table 1[29].As can be seen from Table 1,the active components of HgCl2/AC,HgCl2-4KCl/AC,and HgCl2-4KI/AC may be HgCl2,K2HgCl4and K2HgI4,respectively.The stability constant of HgI42?is 7.2×1029,far greater than that of HgCl42?,1.6×1015.According to the Principle of Hard and Soft Acids and Bases(HSAB)[32,33],hard acids prefer to bind to hard bases and soft acids prefer to bind to soft bases.Soft acids form stable complexes with bases that are highly polarizable.Hg2+is a soft acid with high polarizability.I?is a soft acid and Cl?is a hard acid.The large radius and loose electron cloud of I?results in high polarization degree of Hg--I bond and the nature of Hg--I bond is close to covalent bond.Cl?has small ionic radius,difficult to form stable covalent bond.The high polarizability of Hg2+makes the Hg--Cl bond difficult to form stable ionic bond.Therefore,the stability of HgI42?is better than that of HgCl42?.Likewise,the order of the thermal stabilities of HgCl2/AC,HgCl2-4KCl/AC and HgCl2-4KI/AC can be judged based on the stability constants.

Table 1The stability constants ofcoordination ions formed by reacting Hg2+with Cl?,Br?,and I?,respectively

3.3.The average sublimation rate of HgCl2

The orderofthe sublimation ratesofHgCl2for HgCl2/AC,HgCl2-4KCl/AC and HgCl2-4KI/AC can be predicted based on the stability constants in Table 1.The specific sublimation rates of HgCl2for the three catalysts are also compared in Table 2.As we can see from Table 2,the sublimation ofHgCl2for HgCl2/AC is the fastestwith the average HgCl2sublimation rate of 47.78 × 10?3mg·g?1·min?1and the sublimation of HgCl2for HgCl2-4KI/AC is slower than HgCl2-4KCl/AC at 250°C.The average HgCl2sublimation rate of HgCl2-4KCl/AC is 20 × 10?3mg·g?1·min?1,about 5/12 of that of HgCl2/AC.The average HgCl2sublimation rate of HgCl2-4KI/AC is 2.22 × 10?3mg·g?1·min?1,about 1/22 of that of HgCl2/AC and 1/9 of that of HgCl2-4KCl/AC.Loss percentage of HgCl2from the HgCl2-4KI/AC catalyst is 1.02%,less than 3%[30],which satis fies the requirement for industrial catalyst.The data in Table 2 reveal that the addition of KI further promotes the inhibition of sublimation of HgCl2from the low-mercury catalyst compared with the addition of KCl.The order of the average HgCl2sublimation rate is opposite to the order of stability constant of active components.

Table 2Comparison of the average HgCl2 sublimation rates of HgCl2/AC,HgCl2-4KCl/AC,and HgCl2-4KI/AC

4.Conclusions

A ligand coordination approach was employed to enhance the stability of low-mercury catalyst for the hydrochlorination of acetylene.According to the Principle of Hard and Soft Acids and Bases(HSAB),a more stable low-mercury catalyst(4%HgCl2loading),HgCl2-4KI/AC,was successfully prepared by using HgCl2and KIas precursors.The active component of the HgCl2-4KI/AC catalyst was K2HgI4,highly dispersed on the surface of activated carbon.The HgCl2-4KI/AC catalyst showed better catalytic stability than HgCl2/AC and HgCl2-4KCl/AC for the hydrochloriantion of acetylene under fixed acetylene hydrochlorination reaction conditions at 220°C,an acetylene gas hourly velocity(C2H2)of 180 h?1,and a fed volume ratioV(HCl)/V(C2H2)of 1.2.The comparison of the HgCl2sublimation rates for three catalysts indicated that the HgCl2-4KI/AC catalyst had best thermal stability than HgCl2/AC and HgCl2-4KCl/AC.This approach points out the direction to designing more stable mercury catalyst for the hydrochlorination of acetylene.

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