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Highly efficient Ru@IL/AC to substitute mercuric catalyst for acetylene hydrochlorination Shanshan Shang, Wei Zhao, Yan Wang, Xiaoyan Li, Jinli Zhang, You Han, and Wei Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00057 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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Highly efficient Ru@IL/AC to substitute mercuric catalyst for acetylene hydrochlorination Shanshan Shang,† Wei Zhao,‡ Yan Wang,† Xiaoyan Li,† Jinli Zhang,† You Han*,† and Wei Li*,† †

School of Chemical Engineering & Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Chemical Engineering (Tianjin), Tianjin 300350, P. R. China.

‡

Department of Research and Development, Zhuhai Coslight Battery Co., Ltd, Zhuhai 519100, P. R. China.

*Corresponding author: Tel: +86-22-27404495, Fax: +86-22-2740-3389. E-mail address: [email protected] (W. Li), [email protected] (Y. Han)

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Abstract We synthesized a series of Ru@IL/AC catalysts using the incipient wetness impregnation technique associated with five kinds of ionic liquids, aiming to explore an efficient non-mercuric catalyst for acetylene hydrochlorination reaction. Over the optimal 1%Ru@15%TPPB/AC catalyst the acetylene conversion maintained 99.7% at 48 h (T=170 °C, GHSVC2H2=360 h-1 and VC2H2/VHCl=1.15). And with lower Ru loading (0.2%Ru@15%TPPB/AC), the acetylene hydrochlorination still kept at 99.3% within 400 h. Characterized by CO pulse chemisorption, TEM, XPS, TGA, etc., it is indicated that TPPB IL could effectively improve the dispersion of Ru species, suppress the reduction of active Ru species and inhibit the coke deposition during acetylene hydrochlorination reaction. The interactive mechanism between TPPB and the reactants and the product was investigated to disclose the effect of TPPB IL on the catalytic performance of Ru-based catalyst, in combination with DFT calculations. The enhanced activity and long-term stability of Ru@IL/AC suggest the promising industrial application as the non-mercuric catalyst for acetylene hydrochlorination.

Keywords: acetylene hydrochlorination; ionic liquids; catalytic mechanism; HCl adsorption; Ru@IL/AC catalyst.

1. Introduction Acetylene hydrochlorination is the important industrial process to manufacture vinyl chloride monomer (VCM) in coal-rich regions, which can be polymerized to produce polyvinyl chloride (PVC), the third largest general plastics widely used in the production of commodities including pipes and fittings, doors, windows, packaging 2


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sheets, and so on.1,

2

However, the traditional industrial process of acetylene

hydrochlorination utilized the carbon-supported HgCl2 as the catalyst, which is volatile and toxic, resulting in the severe environmental pollution.3, 4 Since 2013, more than 140 countries have signed "the Minamata Convention on Mercury" in order to inhibit the trade and application of mercury over the world in near future.5 Therefore, it is urgent to explore the environmentally benign acetylene hydrochlorination process based on novel efficient non-mercury catalysts for the sustainable development of PVC industry. In recent decades, many active non-mercuric metal chloride catalysts, involving AuCl3,6, 7 K2PdCl4/PdCl2,8, 9 K2PtCl4,10 RuCl3,11, 12 RhCl3,13 BiCl3,14 and CuCl2,15 have been studied extensively for acetylene hydrochlorination. Nevertheless, almost all of the non-mercuric metal chloride catalysts encountered deactivation problems. The deactivation mechanism was once explored by experiments and density functional theory (DFT) calculations.8,

16-18

Overall, there were two main

deactivation reasons for the supported metal chloride catalysts: 1) the reduction of active metal species, for instance, the reduction of Au3+ to Au0; 2) the coke deposition on the catalyst surface, which would cover the active sites during the catalytic reaction.19 During the acetylene hydrochlorination reaction, the metal chloride catalysts, like AuCl3, RuCl3 and K2PdCl4,8,

16, 18

usually had superior

adsorption and activation ability for C2H2. Then HCl was added to the activated C2H2 to form VCM, which was the rate-determining step of the whole catalytic process for acetylene hydrochlorination.6, 8, 10, 18, 20 If HCl could not interact with the

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activated C2H2 timely and sufficiently, the activated C2H2 molecules would easily polymerize to form coke deposition. The polymerization of intermediates or certain radical groups associated with activated C2H2 and the product VCM could be another two probable pathways to form coke deposition. In order to address the deactivation problems, many efforts have been focused on the synergistic effects between metal additives and active components,20-30 and the support modified method.31-39 As for the study of metal additives, Conte et al.30 studied the role of the second metallic component, including Pd2+, Pt4+, Rh3+, Ir3+ and Ru3+, on Au-based catalysts for acetylene hydrochlorination and found that the activity and stability of these bimetallic catalysts had no obvious improvement. Wei and his co-workers24 reported that Bi3+ could inhibit the partial reduction of Au3+ to metallic Au thus improving the catalyst stability. Our group has been engaging in the study of Ru-based catalysts for acetylene hydrochlorination in recent years owing to many merits of Ru, such as, low price, high activity, environmental benignity and so on. Similar to Au-based catalyst, adding metal additives was also an effective method to improve catalytic performance. In respect to bimetallic Ru-based catalyst, Ru−Cu/SAC, Ru−Co/SAC and Ru-K/SAC catalysts were prepared to enhance the catalytic activity of the Ru/SAC catalyst for the acetylene hydrochlorination reaction.11, 28 As for trimetallic Ru-based catalyst, we investigated the synergistic effects

of

Co(III)

and

Cu(II)

for

Ru-based

catalyst

during

acetylene

hydrochlorination and found the co-addition of Co(III) and Cu(II) could not only improve Ru dispersion but also inhibit the reduction of RuCl3 precursors.40 However,

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the stability of current Ru-based catalysts is still unsatisfactory due to the loss of Ru active species and other two reasons mentioned above during the reaction. Apart from the study of metal additives, non-metal modified method, such as N-doped,32-36 P-doped,37, 38 and S-doped,39 was also adopted to enhance the catalyst activity and stability for acetylene hydrochlorination reaction. Despite the significant improvements for these efforts, it is still a challenge to inhibit the coke deposition and the reduction of active metal species so as to enhance the activity as well as the long-term stability of non-mercuric catalysts for acetylene hydrochlorination. Supported ionic liquid catalysts (SILC), combining the advantages of ionic liquids (ILs) with heterogeneous support materials, have drawn wide attention recently.41 Preliminary studies have shown that SILC have been used in hydroformylation reaction,42, 43 hydrogenation reaction,44 Suzuki-Miyaura cross-coupling,45 etc., owing to the unique properties of IL involving the extremely low vapor pressure and the high dissolving/adsorption for organic or inorganic chemicals.46, 47 Very recently, Xing et al.48 reported that the anionic surfactant carboxylate ionic liquids (ASC-ILs) could improve the performance of metal catalysts such as Pd, Au and Pt for gas-liquid acetylene hydrochlorination reaction. Many reports showed that the solubility of HCl gas is very high in ILs,49-51 which is even higher than that in water. The high solubility of HCl in ILs is not only caused by the physical adsorption but also induced by the chemical adsorption of HCl,50, 51 indicating that ILs may have good activation effects for HCl gas. Taking into account the mechanism of coking deposition on non-mercuric catalysts, we are enlightened to explore novel IL-supported Ru-based

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catalysts for acetylene hydrochlorination utilizing the adsorption property of ILs for HCl to accelerate the electrophilic attack in the reaction between acetylene and hydrogen chloride. Herein we successfully prepared five kinds of Ru@IL/AC catalysts using incipient wetness impregnation method. Catalytic performance tests were conducted to demonstrate the superior activity and stability of Ru@IL/AC catalysts. The mechanism of ILs for the strong enhancement of catalytic activity and selectivity in the reaction of acetylene hydrochlorination was discussed with catalysts characterization and DFT calculations.

2. Experimental and theoretical method 2.1 Materials Coconut activated carbon (marked as AC, 20-40 mesh) was purchased from Fujian Sensen Activated Carbon Industry Science and Technology Co., Ltd. RuCl3 (purity 99%) was purchased from Tianjin Fengchuan Chemical Reagent Science and Technology Co., Ltd. Tetrabutylphosphonium bromide (TBPB), (Tri-n-butyl)-ntetradecylphosphonium chloride (TBTDPC), (Ethyl)triphenylphosphonium bromide (ETPPB), Tetraphenylphosphonium chloride (TPPC) and Tetraphenylphosphonium bromide (TPPB) were all purchased from ShangHai D&B Biological Science and Technology Co. Ltd., with the purity higher than 99%. The molecular structure of five ionic liquids can be seen in Figure S1. Ethanol (purity 99.7%) and sodium hydroxide (purity 96%) were purchased from Tianjin Guangfu Fine Chemical Research Institute. The HCl gas, C2H2 gas and VCM gas (purity all about 99.99%) were purchased from Tianjin Dongxiang special gas Co., Ltd. All the materials and reagents were used without further purification.

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2.2 Catalyst preparation The Ru@IL/AC catalysts were prepared using an incipient wetness impregnation technique with ethanol as a solvent. We take the preparation of 1%Ru@1%TPPB/AC for example. Firstly, the precursor RuCl3 was first dissolved in deionized water, and then was quantitatively added into a 25 mL ethanol solution containing 0.1 g TPPB. After that, the mixture was dropwise added into 10 g AC under vigorous stirring, followed by an ultrasonic bath (SCIENTZ, SB-100D) for 15 minutes. Subsequently, the sample was incubated at 60 °C for 12 h and dessicated at 120 °C for 12 h to finally obtain the catalyst, denoted as 1%Ru@1%TPPB/AC. In order to study the relationship between the catalytic activity and the amount of TPPB, we changed the mass of TPPB with 0.5 g, 1 g, 1.5 g and 2 g, and the catalysts were respectively named as 1%Ru@5%TPPB/AC, 1%Ru@10%TPPB/AC, 1%Ru@15% TPPB/AC, and 1%Ru@20%TPPB/AC. The texture properties performed by low-temperature N2 adsorption/desorption experiments in Table S1 and FTIR spectra in Figure S2 of 1%Ru@TPPB/AC catalysts proved that TPPB was supported on AC support successfully. And other four Ru@IL/AC catalysts were synthesized with the same method above, including

1%Ru@15%TBPB/AC,

1%Ru@15%TBTDPC/AC,

1%Ru@15%

ETPPB/AC and 1%Ru@15%TPPC/AC. As a control, the 1%Ru/AC catalyst was also prepared using the same method just without ionic liquid. The Ru loading of the catalyst was fixed at 1wt% unless mentioned. 2.3 Catalyst characterization Low-temperature N2 adsorption/desorption experiments were performed to test the texture parameters of the catalysts using Quantachrome Autosorb Automated Gas Sorption System. The catalysts were heated at 200 °C and outgassed for 4 h and

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measured using liquid nitrogen adsorption at -196 °C. Fourier transform infrared spectrometry (FTIR) of the catalysts was performed by Bruker Vertex70 FT-IR spectrophotometer with a DTGS detector. The resolution is 4 cm-1 and the samples were scanned for 32 times. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were used to determine the distribution and size of the Ru particles with Tecnai G2F20. The samples for characterization were dispersed in ethanol and deposited on an ultrathin carbon film which was coated on a 300-mesh Cu grid. Temperature-programmed desorption (TPD) experiments were measured by Quantachrome Instruments AMI-90 to investigate the adsorption capacity of the catalysts for reactants and products. The experiments were operated under the helium atmosphere over a temperature ramp of 50 °C to 800 °C at a heating rate of 10 °C min-1. The experiments of CO pulse chemisorption were also measured by Quantachrome Instruments AMI-90. Samples were pretreated under 10% H2/Ar atmosphere at 400 °C for 4 h followed by cooling to room temperature, and thereafter 250 µL pulse of 10% CO/He was introduced. The CO uptake profile was measured using a TCD detector. X-ray photoelectron spectra (XPS) were recorded on a Thermo ESCALAB 250Xi to determine the valence states of different Ru species on the catalysts. And C1s line (284.6 eV) from the support was adopted to calibrate the binding energies. Thermogravimetric analysis (TGA) of samples was carried out to detect coke deposition using METTLER TOLEDO TGA/DSC 2 instrument. The experiments were operated under air atmosphere at a flow rate of 80 mL min-1, and the temperature

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increased from 35 °C to 900 °C at a heating rate of 10 °C min-1. Inductively coupled plasma emission spectrometry (ICP) was used to measure the absolute content of Ru in catalysts with Varian 725-ES (USA). 2.4 Catalyst tests Catalyst tests were carried out in a fixed-bed microreactor (i.d. of 10 mm) for acetylene hydrochlorination. CKW-1100 temperature controller from the ChaoYang Automation Instrument Factory (Beijing, China) was used to regulate the reaction temperature. Appropriate amount of nitrogen was imported into the reactor to remove the air and water of 5 mL catalyst. Clean HCl gas was passed through the reactor to activate the catalyst at a flow rate of 30 mL min-1 under the temperature of 170 °C for 30 min. The composition and morphology of the activated catalyst can be seen in Figure S3, showing no much difference compared with the fresh catalyst. Subsequently, C2H2 (26 mL min-1) and HCl (29 mL min-1) were fed through filters with an acetylene gas hourly space velocity (GHSV) of 360 h-1. The reaction temperature was 170 °C. The effluent of the reactor was passed into NaOH solution, followed by the analysis using Beifen 3420A gas chromatograph. The acetylene conversion (XA) and the selectivity to VCM (SV) were calculated by the following equations: =

× 100%

= × 100%

(1) (2)

where , and represent the volume fraction of acetylene in the feed gas, the volume fraction of acetylene in the product gas and the volume fraction of vinyl chloride in product gas, respectively. 2.5 Simulation details All

DFT calculations were

carried

out using

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DMol3

numerical-based

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density-functional module implemented in Materials Studio package from Accelrys Inc. Geometry optimizations were performed using a spin-polarized revised Perdew– Burke–Ernzerhof (RPBE)52, 53 functional within the formulation of the generalized gradient approximation (GGA). The DNP basis set and the all electron core treatment were used in all of the calculations. The energy convergence criterion was set as 0.00005 Ha and the global cutoff was set to 3.8 Å. A smearing of 0.005 Ha to the orbital occupation was applied to achieve the self-consistent field convergence. Aiming to analyze the interaction between the reactant (hydrogen chloride or acetylene,) and the IL, the adsorption energy (Ead) was defined as follows: Ead=Ead-state-(Ereactant+EIL)

(3)

where Ead-state , Ereactant and EIL are the total energies of the IL with the reactant adsorption, the clean IL and the free HCl or C2H2, respectively.

3. Results and discussion 3.1 Catalytic performance of Ru@IL/AC catalysts Firstly, we chose TPPB as the representative of ILs to test the catalytic performance of 1%Ru@IL/AC catalysts in the reaction of acetylene hydrochlorination. The prepared catalysts, including AC, 1%Ru/AC, 15%TPPB/AC, 1%Ru@1%TPPB/AC, 1%Ru@5%TPPB/AC,

1%Ru@10%TPPB/AC,

1%Ru@15%TPPB/AC

and

1%Ru@20%TPPB/AC, have been assessed under the condition of 170 °C, the GHSV (C2H2) of 360 h-1, VHCl/VC2H2= 1.15. As shown in Figure 1a, the 1%Ru/AC catalyst without IL exhibits the acetylene of 36.1% after 48 h. While the 1%Ru@1%TPPB/AC catalyst with 1%TPPB, the acetylene conversion rises to 42.9% at 48 h. With the rising amount of TPPB, the acetylene conversion increases greatly. The catalysts of 1%Ru@5%TPPB/AC, 1%Ru@10%TPPB/AC, 1%Ru@15%TPPB/AC achieve the conversion of acetylene to 83.8%, 96.1% and 99.7% at 48 h, respectively. The

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acetylene conversion shows no discernable increase when the loading amount of TPPB is up to 20%, implying that 15% is the optimal loading amount of TPPB for 1%Ru/AC catalyst. In particular, over the pure IL catalyst, 15%TPPB/AC, the acetylene conversion is only 7.9% at 48 h. The summation of the acetylene conversion over the individual 1%Ru/AC and 15%TPPB/AC equals 44%, which is much lower than that over the 1%Ru@15%TPPB/AC (99.7%) catalyst. Therefore, the highest catalytic activity over 1%Ru@15%TPPB/AC is due to the synergistic effect between TPPB and Ru species. As shown in Figure 1b, the selectivity to VCM on these catalysts is all above 99.5%.

Figure 1. The conversion of acetylene (a) and selectivity to VCM (b) in acetylene hydrochlorination over 1%Ru@TPPB/AC catalysts. Reaction conditions: T=170 °C, GHSV(C2H2)=360 h-1 and VHCl/VC2H2=1.15.

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The other four kinds IL-supported Ru catalysts were also assessed for acetylene hydrochlorination reaction. Figure 2 displays the catalytic performance of 1%Ru@15%TBPB/AC, 1%Ru@15%TBTDPC/AC, 1%Ru@15%ETPPB/AC and 1%Ru@15%TPPC/AC. The initial acetylene conversion over 1%Ru@15%TBPB/AC, 1%Ru@15%TBTDPC/AC, 1%Ru@15%ETPPB/AC and 1%Ru@15%TPPC/AC achieves at 99.1%, 99.0%, 99.8% and 99.6%, respectively, which are much higher than that on the pure 1%Ru/AC. The results demonstrate that these ionic liquids indeed can enhance the performance of Ru-based catalyst for acetylene hydrochlorination.

After

48

h

reaction,

the

acetylene

conversion

over

1%Ru@15%TPPB/AC and 1%Ru@15%TPPC/AC maintain the same as the initial value,

whereas

over

1%Ru@15%TBPB/AC,

1%Ru@15%TBTDPC/AC

and

1%Ru@15%ETPPB/AC the acetylene conversion decreases to 97.4%, 97.7% and 98.9%, respectively, which may be caused by the low melting point of TBTDPC, TBPB and ETPPB compared with TPPB and TPPC.54-57

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Figure 2. The conversion of acetylene (a) and selectivity to VCM (b) in acetylene hydrochlorination over 1%Ru@15%IL/AC catalysts. Reaction conditions: T=170°C, GHSV(C2H2)=360 h-1 and VHCl/VC2H2=1.15. From the perspective of economy and industrial application, the lifetime testing of Ru@TPPB/AC catalyst with ultra-low Ru loading of 0.2 wt% and 0.1 wt% was carried out under the GHSV (C2H2) of 90 h-1. The loading amount of TPPB is 15 wt% and other conditions are the same as before (T=170 °C, VHCl/VC2H2=1.15). Figure 3 shows

the

conversion

of

acetylene

and

the

selectivity

to

VCM

on

0.2%Ru@15%TPPB/AC and 0.1%Ru@15%TPPB/AC catalysts for acetylene hydrochlorination. The acetylene conversion of 0.2%Ru@15%TPPB/AC achieves the maximum of 99.7% after the reaction time of 25 h and then decreases very slowly within 400 h to obtain a final acetylene conversion of 99.3%, and the acetylene conversion of 0.1%Ru@15%TPPB/AC decreases from 98.2% to 96.0% within 400 h.

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We calculated and compared the deactivation rate of 1%Ru@15%TPPB/AC, 0.2%Ru@15%TPPB/AC and 0.1%Ru@15%TPPB/AC catalysts for acetylene hydrochlorination, and found the deactivation rate of 0.2%Ru@15%TPPB/AC was 0.001% h-1, which was much lower than those over Ru-based catalysts reported in the previous literature (listed in Table S2). The different deactivation rate among different Ru@TPPB/AC catalysts is probably attributed to the variations of acetylene GHSV and Ru loading amount. Combining with Table S2 and literatures, it is reasonable to conclude that Ru@IL/AC catalyst shows the best catalyst stability among Ru-based catalysts for acetylene hydrochlorination even we decreased the Ru loading to 0.2 wt% or 0.1 wt%.The selectivity to VCM is close to 100% throughout the whole process. Such a long period of lifetime testing suggests that the Ru@TPPB/AC catalysts have both excellent activity and stability for acetylene hydrochlorination. Furthermore, we characterized the 0.1%Ru@15%TPPB/AC catalyst after the long-term test using TEM, TGA and ICP. TEM images (Figure S4) show that the average particles size of Ru on the fresh and used 0.1%Ru@15%TPPB/AC catalysts are 1.23 nm and 3.09 nm, respectively, indicating that no obvious aggregation occurs in the catalyst 0.1%Ru@15%TPPB/AC experienced 400 h reaction. The TGA result of 0.1%Ru@15%TPPB/AC catalyst after the long-term test (Figure S5) demonstrates that the coke deposition amount is 6.1%, illustrating that the 0.1%Ru@15%TPPB/AC catalyst possesses a strong ability to inhibit coke deposition. And the ICP characterization shows that the Ru loss ratio is 15% after 400 h reaction (Table S3).

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Figure 3. The conversion of acetylene (a) and selectivity to VCM (b) in acetylene hydrochlorination

over

0.2%Ru@15%TPPB/AC

and

0.1%Ru@15%TPPB/AC

catalysts. Reaction conditions: T=170 °C, GHSV(C2H2)=90 h-1 and VHCl/VC2H2=1.15.

Therefore, considering the catalytic activity, environmental benignity and low price of TPPB ionic liquid, we believe that the Ru@IL/AC catalyst will be a promising alternative for HgCl2/AC catalyst in the industrial production of vinyl chloride monomer. 3.2 TPPB enhances Ru dispersion CO pulse chemisorption was carried out to calculate the dispersion of fresh 1%Ru/AC and 1%Ru@15%TPPB/AC catalysts. The result is shown in Table 1. In order to eliminate the potential interaction between CO and TPPB, the CO

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chemisorption of fresh 15%TPPB/AC was also performed. The result shows that the CO uptake on the fresh 15%TPPB/AC is zero, indicating that the pure TPPB has no interaction with CO. The Ru dispersion of the fresh 1%Ru@15%TPPB/AC is 60.0%, which is much higher than that of fresh 1%Ru/AC (23.3%). The CO pulse chemisorption analysis demonstrates that the addition of TPPB improves the dispersion of Ru species. Table 1. Ru dispersion calculated according to CO uptake a. CO uptake

Ru dispersion

(µmol g-1)

(%)

Fresh 1%Ru/AC

23.1

23.3

Fresh 1%Ru@15%TPPB/AC

59.4

60.0

0

/

Sample

Fresh 15%TPPB/AC a

We assumed that the stoichiometric ration of CO and Ru was 1:1.

The TEM characterization of the fresh 1%Ru/AC and 1%Ru@15%TPPB/AC and those used partners further demonstrates that TPPB can improve the dispersion of Ru, shown in Figure 4. For the used 1%Ru/AC, there appear more particles dispersed on the support with an average size about 5.91 nm, much larger than that on the fresh 1%Ru/AC (3.05 nm), which suggests certain aggregation of Ru species during the reaction. While for the used 1%Ru@15%TPPB/AC, the small amount of dispersed particles shows an average size about 2.86 nm, a little higher than that on the fresh 1%Ru@15%TPPB/AC (1.91 nm). These results show that TPPB can not only improve the dispersion of Ru species in the catalysts, but also inhibit to some extent the aggregation of Ru species. In fact, many researchers have reported that the supported ionic liquid can stabilize catalytically active complexes or metal nanoparticles to sustain their small size, making them a high dispersion and inhibiting

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them agglomeration.44,

58, 59

This effect may be related to the surface electronic

stabilization of ionic liquid aggregates forming protective layers.45

Figure 4. TEM images of fresh 1%Ru/AC, fresh 1%Ru@15%TPPB/AC, used 1%Ru/AC, and used 1%Ru@15%TPPB/AC catalysts. In order to visually determine the distribution of Ru species and TPPB on catalyst, elemental mapping from STEM image displays a detailed element distribution of the fresh 1%Ru@15%TPPB/AC catalyst. As shown in Figure 5 (c–h), the mapping of C, O, P, Ru, Cl and Br elements reveal that all the elements have distributed uniformly on the catalyst. Especially, Figure 5 (i) presents the overlapping of Ru and P elements, which verifies the homogeneous dispersion of TPPB around Ru on the AC support.

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Figure 5. (a) STEM image, (b) elemental mapping image of the overlapping for all elements, (c-h) elemental mapping images of C (red), O (green), P (purple), Ru (yellow), Cl (blue), and Br (cyan), respectively, (i) elemental mapping image of the overlapping for Ru and P elements on the fresh 1%Ru@15%TPPB/AC catalyst. 3.3 TPPB stabilizes the valence states of Ru species XPS spectra were used to investigate the valence states of different Ru species and the element composition on catalysts. From the surface elemental analysis with XPS (Table S4), the 1%Ru/AC catalyst mainly consists of C, O, Cl and Ru elements, and the 1%Ru@15%TPPB/AC catalyst mainly consists of C, O , P, Cl, Br and Ru. After calculation, the surface molar ratio of P : Ru and Br : Ru are 1.7 and 1.4, respectively, while the theoretical molar ratio of TPPB : Ru is 3.6 ( 0.1 g Ru and 1.5 g TPPB on 10 g AC support), indicating that most of Ru species are more dispersed at the surface rather than the bulk of 1%Ru@15%TPPB/AC catalyst. Ru3p3/2 spectra are adopted to analyze the Ru species because of the overlapping between C1s and Ru3d signals.60 18


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Through the deconvolution of Ru 3p profiles in Figure 6, there are five Ru species located around 461.0 eV, 462.2 eV, 463.4 eV, 464.8 eV and 466.0 eV, corresponding to the species of Ru0, Ru/RuOy, RuCl3, RuO2 and RuOx,40,

61-64

but with a little

vibration in both fresh and used catalysts. The binding energies and relative contents are listed in Table 2. The fresh 1%Ru/AC catalyst consists of 32.76% RuCl3, 29.81% Ru/RuOy, 17.04% RuO2, 11.23% Ru0 and 9.16% RuOx. For the fresh 1%Ru@15%TPPB/AC, the content of RuO2 is 29.48%, which is much higher than that in the fresh 1%Ru/AC. The previous studies demonstrated that RuO2 was an important active ingredient in the reaction of acetylene hydrochlorination.28, 33, 64 The higher the content of RuO2, the higher the activity in acetylene hydrochlorination. Therefore, the addition of TPPB in 1%Ru/AC promotes the formation of more active RuO2 during the catalyst preparation process.

Figure 6. Ru 3p XPS spectra of fresh 1%Ru/AC, fresh 1%Ru@15%TPPB/AC, used 1%Ru/AC, and used 1%Ru@15%TPPB/AC catalysts.

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Table 2. The relative contents and binding energies of Ru species in the fresh and used catalysts. Relative contents (Area %) Samples

Ru (461.0eV)

Ru/RuOy

RuCl3

RuO2

RuOx

(462.2±0.1eV)

(463.5±0.3eV)

(464.8±0.3eV)

(466.0±0.3eV)

Fresh 1%Ru/AC

11.23

29.81

32.76

17.04

9.16

Fresh 1%Ru@15%TPPB/AC

11.21

13.86

33.71

29.48

11.74

Used 1%Ru/AC

22.15

35.38

26.52

12.80

3.15

Used 1%Ru@15%TPPB/AC

12.67

17.46

31.90

27.43

10.54

After 48 h reaction, the used 1%Ru/AC is comprised of 26.52% RuCl3, 12.80% RuO2, 3.15%RuOx, 35.38% Ru/RuOy and 22.15% Ru0. Compared with the fresh 1%Ru/AC catalyst, the contents of RuCl3, RuO2 and RuOx decrease, which may be another major reason for the decline of the catalytic activity. Additionally, the contents of Ru/RuOy and Ru0 increase due to the reduction of high valence states Ru species by C2H2. For the used 1%Ru@15%TPPB/AC catalyst, the contents of different Ru species do not change much. For example, the content of RuO2 only decreases from 29.48% to 27.43%. It demonstrates that the addition of TPPB can effectively inhibit the reduction of high valence states of Ru species, thus stabilizing active species during the reaction of acetylene hydrochlorination. 3.4 TPPB inhibits the coke deposition It is well known that coke deposition is one of the main reasons for catalyst deactivation.65 In order to evaluate the amount of coke deposition on the surface of the used catalyst, TGA was performed in our experiments. Figure 7 shows the TGA results of the 1%Ru/AC and 1%Ru@15%TPPB/AC catalysts. Both the fresh and used catalysts have a slight weight loss before 150°C, owing to the desorption of water

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existing on the catalysts. Subsequently, there is an obvious weight loss for the used catalysts in the temperature range of 150-400°C, which is attributed to the coke deposition on the surface of catalysts.66 At the temperature higher than 400°C, there is a sharp mass decrease attributed to the burning of TPPB and AC support.

Figure 7. TGA and DTG curves of the fresh and used catalysts of 1%Ru/AC (a) and 1%Ru@15%TPPB/AC (b). Table 3 lists the amounts of coke deposition on the used catalysts experienced 48 h reaction, calculated according to TG profiles using the precise method.25 The 1%Ru/AC catalyst has the coke deposition amount of 11.2%. For the 1%Ru@TPPB/AC catalysts, the coke deposition amount decreases with the increase loading amount of TPPB, with the lowest amount of 1.8% on the used 1%Ru@20%TPPB/AC catalyst. It illustrates that TPPB can efficiently inhibit the coke deposition on the Ru-based catalyst.

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Table 3. The amount of coke deposition on catalysts. Sample

Amount of coke deposition (%)

1%Ru/AC

11.2

1%Ru@1%TPPB/AC

9.4

1%Ru@5%TPPB/AC

6.3

1%Ru@10%TPPB/AC

2.2

1%Ru@15%TPPB/AC

2.0

1%Ru@20%TPPB/AC

1.8

3.4 TPPB interacts with the reactants and product The above catalytic performance results show that the TPPB IL can effectively and significantly enhance the activity and stability of Ru-based catalyst for acetylene hydrochlorination. The catalysts characterization of CO pulse chemisorption, TEM, XPS and TGA results further reveal that TPPB IL not only can improve the dispersion of Ru species and increase the amount of more active RuO2 during the catalyst preparation process, but also can stabilize the valence states of active Ru species and suppress the coke deposition during the reaction process. In order to disclose the interactive mechanism between TPPB and the reactants and the product VCM, we further carried out a series of characterization, testing and simulation analysis. Firstly, the TPD was performed to study the adsorption property of the reactants and the product on different catalysts. Figures 8a, 8b and 8c display the C2H2-, HCland VCM-TPD profiles of different samples. According to the He-TPD profile shown in Figure 8d, the two peaks above 400 °C are attributed to the decomposition of TPPB. The peaks below 400 °C represent the desorption property of C2H2, HCl and VCM on different catalyst. In general, the desorption temperature in the TPD profiles reflect the binding strength of the adsorbed species on the catalyst surface, and the peak area

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correlates with the amount of adsorbed species. It is clear that VCM desorption peak of 1%Ru@15%TPPB/AC catalyst moves to the lower temperature compared with that of 1%Ru/AC, whereas the HCl desorption peak position of 1%Ru@15%TPPB/AC is obviously higher than that of 1%Ru/AC, indicating that TPPB can enhance the adsorption of HCl but reduce the adsorption of VCM on the catalyst. Furthermore, the C2H2 and VCM desorption peak areas of 1%Ru@15%TPPB/AC are much smaller than that of 1%Ru/AC (listed in Table S5). The adsorption amount of C2H2 on 1%Ru@15%TPPB/AC catalyst is even reduced to a quarter of that on 1%Ru/AC catalyst. Song et al.67 also found that the adsorption capacity of the catalyst for C2H2 has a negative correlation with the catalyst stability. On the contrast, the HCl desorption peak area of 1%Ru@15%TPPB/AC is almost three times as large as that of 1%Ru/AC. The capacity of a catalyst to adsorb hydrogen chloride is associated with its activity and stability.27 Comparing the TPD results of 1%Ru@15%TPPB/AC with 15%TPPB/AC, we can clearly see that the increased HCl and decreased C2H2 and VCM adsorption capacity of 1%Ru@15%TPPB/AC can be ascribed to the addition of TPPB IL. Certain ILs show the properties of high adsorption capacity toward HCl gas. For instance, Xing and his co-workers48 reported that 1 mol [P4444][C17COO] IL could absorb approximately 2 mol HCl at 180 oC. We measured the adsorption capacity of pure TPPB IL toward individual HCl and C2H2 under the reaction temperature of 170 o

C (The test details are shown in Figure S6). The results indicate that 1 mol TPPB can

absorb 0.86 mol HCl but only absorb 0.03 mol C2H2. Therefore, the TPPB IL can significantly enhance the adsorption capacity of HCl but weaken that of C2H2 on Ru-based catalyst. Previous literature have illustrated that the acetylene conversion would increase and the deactivation rate would decrease under plenty of HCl for

23


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acetylene hydrochlorination reaction.16, 17 It is the enhanced adsorption capacity of HCl caused by TPPB IL that inhibits the coke deposition on 1%Ru@15%TPPB/AC catalyst, which is confirmed by the following experiments.

Figure 8. TPD profiles of the fresh catalysts. (a) C2H2-TPD, (b) HCl-TPD, (c) VCM-TPD and (d) He-TPD. We treated the 1%Ru/AC and 1%Ru@15%TPPB/AC catalyst with pure C2H2 or VCM gas under the gas hourly space velocity of 360 h-1 and 170 oC for 48 h (same as the reaction condition), and then measured the coke deposition amounts. As listed in Table 4, over the Ru-based catalyst with TPPB IL, the coke deposition amount under the C2H2 atmosphere decreases from 4.1% to 2.1% while under VCM atmosphere it decreases from 1.4% to 0.8%, illustrating that TPPB can inhibit greatly the formation of coke associated with C2H2 and facilitate the desorption of VCM, which is in accord with our TPD results. More importantly, under the real reaction condition (HCl and C2H2 coexist), the coke deposition amount on 1%Ru/AC after reaction is 11.2%,

24


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which is over two times of the sum coke amount (5.5%) on 1%Ru/AC after the treatment with individual C2H2 or VCM. It is suggested that more than half of the coke deposition on 1%Ru/AC are associated with the polymerization of reaction intermediate (C2H3· or C2H2Cl·) at the absence of Cl or H. While after the addition of TPPB IL, the coke deposition amount on 1%Ru@15%TPPB/AC is only 2.0%, much lower than that on 1%Ru/AC (11.2%) after reaction. Because HCl can be chemically adsorbed into IL,50, 51 IL may have the ability to weaken the H-Cl bond to activate HCl molecules. Besides, the amount of HCl around C2H2 is sufficient. The two advantages facilitate the HCl to react with the C2H2 activated by Ru species8, 16, 18 timely, and result in no excessive C2H2 to form coke. This efficiently reduces the difficulty of rate-determining step (adding HCl to activated C2H2) and inhibits the coke deposition caused by the reaction intermediate polymerization. As a result, the addition of IL improves significantly the catalytic performance of Ru-based catalyst for acetylene hydrochlorination. Table 4. The coke deposition amount on catalysts under different atmosphere including pure C2H2, pure VCM, and the reaction mixture. Coke deposition amount (%)

Sample C2H2

VCM

Reaction

1%Ru/AC

4.1

1.4

11.2

1%Ru@15%TPPB/AC

2.1

0.8

2.0

In order to reveal the mechanism of IL activated HCl, we conducted a series of DFT calculations. The Ead for C2H2 on ILs, RuCl3 and RuO2 ( shown in Table S6), are -0.27~-0.36eV, -0.47eV and -1.09eV,18 respectively. It shows that the C2H2 on Ru species is more stable than that in ILs, indicating that C2H2 is prior to be adsorbed and

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activated by Ru species. The Ead of C2H2 adsorbed on RuO2 is higher than that on RuCl3, which can further illustrate that RuO2 is the important active component in Ru-based catalyst. However, the Ead of HCl on ILs, RuCl3 and RuO2 are -0.59~-0.82eV, -0.11eV and -0.23eV,18 respectively. It can be seen that the Ead of HCl on ILs is higher than that on the Ru species, demonstrating that the HCl gas prefer to adsorb in ILs rather than on the Ru species. The stable structure of HCl adsorbed on TPPB (Figure 9a) and other ILs (Figure S7) shows the formation of a bond between the H atom of HCl and the Br- or Cl- of IL. Table 5 shows the bond properties, Mulliken charges and adsorption energies of HCl when it interacts with different ILs. In gas phase, the length of H-Cl bond is 1.29 Å, while it stretches to 1.36-1.42 Å after activated by ILs, resulting in an extension in H-Cl bond. Mulliken bond order analysis shows that the covalency of the H-Cl bond decreased while the iconicity increased under the electrostatic forces of ILs. Furthermore, the Mulliken charge analysis in Table 5 and Table 6 shows that the bond order of HCl is negative while both the cation and the anion of ILs loses electrons when HCl interacts with ILs, demonstrating that HCl gets electrons from ILs.

Figure 9. The stable structure of HCl adsorbed on TPPB (a), the structure of HOMO of TPPB (b) and LUMO of HCl (c). 26


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Table 5. The bond properties, Mulliken charge and adsorption energy of HCl adsorbed by ionic liquids. HCl adsorbed by ionic liquids HCl (g) TPPB

TPPC

ETPPB

TBPB

TBTDPC

Bond length(Å)

1.29

1.41

1.42

1.39

1.37

1.36

Bond order

1.001

0.685

0.688

0.723

0.762

0.793

Mulliken charge of HCl

0

-0.179

-0.195

-0.165

-0.141

-0.14

Ead of IL to HCl (eV)

--

-0.60

-0.59

-0.71

-0.72

-0.82

Table 6. The Mulliken charge change in ionic liquids. The charge of cation (e)

The charge of anion (e)

before adsorbing

after adsorbing

The amount of losing electron

Before adsorbing

After adsorbing

The amount of losing electron

TPPB

0.701

0.821

0.12

-0.701

-0.642

0.059

TPPC

0.733

0.862

0.129

-0.733

-0.667

0.066

ETPPB

0.73

0.849

0.119

-0.73

-0.684

0.046

TBPB

0.781

0.832

0.051

-0.781

-0.691

0.09

TBTDPC

0.79

0.854

0.064

-0.79

-0.714

0.076

The eigenvalue of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of HCl and ILs are shown in Table S7. The comparison shows that the electrons prefer to transfer from the HOMO of ILs to the LUMO of HCl. The overall theoretical analysis revealed that the IL activated the HCl through the nucleophilic activation process. Due to the interaction between the HOMO of ILs and the LUMO of HCl (shown in Figure 9b and Figure S8), the electrons transfer from IL to the HCl, causing that the H-Cl bond is stretched and its bonding strength weakened. The high electron-donating ability of ILs also allows the Ru@IL/AC catalyst to combine with more hydrogen chloride. 27


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3.6 Catalytic mechanism over Ru@IL/AC catalyst To illustrate the possible catalytic mechanism of supported Ru@IL/AC catalyst, we propose a mechanism scheme for acetylene hydrochlorination (Scheme 1). A thin layer of ionic liquid is dispersed on the solid porous support AC and Ru species are dissolved in the layer of IL. The thin ionic liquid layer can immobilize the active species of catalysts, which has been proved in the aspects of Ru dispersion and valence states of Ru species through catalyst characterization. When the reactants of C2H2 and HCl absorb onto the interface of ionic liquid, C2H2 is prior to be adsorbed on the Ru species and activated via electrophilic attack. Meanwhile, HCl has the priority to be activated by ILs through nucleophilic catalysis. The stretched bond length of HCl and C2H2 are labeled in the scheme, obtained from our DFT calculations and previous literature.18 Moreover, the adsorption amount of HCl on Ru@IL/AC catalyst is much larger than that of C2H2, which contributes to the timely addition of HCl to activated HCl to the activated C2H2, thus eliminating the rate-determining step and avoiding the reduction of active Ru species by C2H2. At the same time, the unique adsorption and activation properties of ILs can suppress the coke deposition caused by the polymerization of activated C2H2 and reaction intermediate. At the product desorption step, VCM can remove quickly from the interface of IL owing to the weak adsorption ability of IL for VCM, which is also favorable for the inhibition of coke deposition. As ILs have excellent performance during the steps of the catalyst preparation, the adsorption and activation of reactant HCl as well as the product desorption, thus effectively and significantly improving the catalytic performance of the catalyst for acetylene hydrochlorination.

28


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Scheme 1. Catalytic mechanism of acetylene hydrochlorination over Ru@IL/AC catalyst.

4. Conclusion In summary, we adopted incipient wetness impregnation technique to synthesize a series of Ru@IL/AC catalysts with five kinds of ionic liquids for acetylene hydrochlorination. Compared to 1%Ru/AC catalyst, all the 1%Ru@IL/AC catalysts showed

greatly

improved

catalytic

performance.

Especially,

the

1%Ru@15%TPPB/AC catalyst showed superior activity and stability with the acetylene conversion of 99.7% and the selectivity to VCM of approximately 100% under the reaction conditions of 170°C, the GHSV (C2H2) of 360 h-1 and VHCl/VC2H2= 1.15. No visible decline in activity was observed within 48 h. Through catalyst characterization results, including CO pulse chemisorption, TEM, XPS and TGA, we discovered that the addition of TPPB could efficiently inhibit the coke deposition on the catalyst surface, improve the dispersion of Ru species and suppress the reduction of active Ru species. Besides, experiments combined DFT results showed that IL could not only enhance the adsorption amount, but also possess the activation ability 29


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of HCl during acetylene hydrochlorination, which could effectively reduce the difficulty of rate-determining step and decrease the amount of coke deposition caused by the polymerization of activated C2H2, reaction intermediate and VCM. In addition, the 0.2%Ru@15%TPPB/AC and 0.1%Ru@15%TPPB/AC catalysts lifetime testing within 400 h demonstrated that the Ru@IL/AC catalyst would be a promising non-mercuric catalyst for acetylene hydrochlorination with high catalytic activity, environmental benignity and economic applicability.

Supporting Information More catalysts characterization results of FTIR, TEM, TGA, ICP, etc., catalytic performance of the reported Ru-based catalysts for acetylene hydrochlorination, and DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments This work was supported by the National Basic Research Program of China (2012CB720302), Natural Science Foundation of China (21576205), NSFC (21621004) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT_15R46).

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