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Confining Noble Metal (Pd, Au, Pt) Nanoparticles in Surfactant Ionic Liquids: Active Non-Mercury Catalysts for Hydrochlorination of Acetylene Jingyi Hu, Qiwei Yang, Lifeng Yang, Zhiguo Zhang, Baogen Su, Zongbi Bao, Qilong Ren, Huabin Xing, and Sheng Dai ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01690 • Publication Date (Web): 01 Oct 2015 Downloaded from http://pubs.acs.org on October 3, 2015
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ACS Catalysis
Confining Noble Metal (Pd, Au, Pt) Nanoparticles in Surfactant Ionic Liquids: Active Non-Mercury Catalysts for Hydrochlorination of Acetylene Jingyi Hu,†Qiwei Yang,† Lifeng Yang,† Zhiguo Zhang,† Baogen Su,† Zongbi Bao,† Qilong Ren,† Huabin Xing,*† Sheng Dai‡§ †Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China. ‡Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States §Department of Chemistry, University of Tennessee, Knoxville, TN 37966, United States ABSTRACT: Metal catalysts often encounter the dilemma of rapid deactivation due to the reduction or particle aggregation/growth during the reaction. Here we reported an active and stable metal nanoparticles (NPs)/surfactant IL system for the catalytic hydrochlorination of acetylene. The NPs of Pd, Au, and Pt with a narrow size distribution and well-defined lattice fringes experienced in-situ generation in the reaction medium of anionic surfactant carboxylate ILs (ASC-ILs). Benefiting from the high reactivity of NPs and the self-assembly property of ASC-ILs, an effective redox cycle between Pd0 and PdII was established to reduce the deactivation of metal catalysts. The Pd NPs/surfactant IL systems showed excellent catalytic activity toward acetylene hydrochlorination. An acetylene conversion of 93% and a selectivity of 99.5% were achieved with no discernible deterioration over a reaction time of 55 hours. Furthermore, ASC-ILs were endowed with a unique property of the strong hydrogen-bond basicity, which was effective in absorbing and activating acetylene and HCl. This study manifests that metal NPs/surfactant IL systems are promising as substitutes for toxic mercury catalysts in the hydrochlorination of acetylene, and also is instructive for the stabilization of metal NPs.
KEYWORDS: nanoparticles, ionic liquid, acetylene, hydrochlorination, non-mercuric, palladium
INTRODUCTION Vinyl chloride monomer (VCM) is essential for synthesizing polyvinylchloride (PVC), one of the most widely used plastics around the world.1,2 The industrial VCM is generally produced either by the oxychlorination of ethylene obtained from petroleum or by the hydrochlorination of acetylene derived from coal. The coal-based acetylene hydrochlorination processes have remarkable economic advantage over ethylene route.3 Therefore, in the countries with a rich coal reserve, the acetylene hydrochlorination reaction catalyzed by mercuric chloride (HgCl2) is a dominant process to VCM.1-5 However, the hydrochlorination of acetylene is a highly exothermic reaction (∆H=-124.8 kJ·mol-1) and facile formation of hot spots is thus to occur, resulting in the loss of toxic Hg from activated carbon. This not only leads to the deactivation of catalysts but also causes severe environment pollution and ecology deterioration.1-5 Therefore, exploring an environmentally friendly catalytic system is imperative. The Hutchings group and other researchers have shown that non-mercuric metal chloride catalysts, such as AuIII, PtII, PdII, RhIII, RuIII, BiIII, and CuII, are active catalysts for acetylene hydrochlorination.6-14 Nevertheless metal chloride catalysts are often encountered with rapid deactivation problems because that some metallic cations are readily reduced to zero-valent metal by acetylene along with the reaction.7,8 The weak inter-
action between catalysts and substrates also results in loss of active components.10,14 To address the problem, extensive efforts have been centered on synergistic bimetallic catalysts in which a second metal component is introduced to improve the stability of metal chloride.3,15-20 Meanwhile, some examples also demonstrate that nitrogen or phosphorus-doped carbon supports are able to stabilize metal components.21-23 The enhanced basicity of N,P-doped carbon materials is believed to activate acidic acetylene and HCl molecules, and very recently, non-metallic catalysts including carbon nitride and pyrollic nitrogen-doped carbon are developed as new catalytic systems for hydrochlorination.1,24-26 Despite the significant improvements for this transformation, it remains a great challenge to develop active and stable catalytic system for acetylene hydrochlorination. Currently, increasing interest has fallen into metal nanoparticles (NPs) as new catalysts in a wide range of reactions because of their high surface-to-volume ratios and high surface energies.27-28 The reactivity of metal NPs can be tailored by controlling their size, shape, and crystal plane.29-32 Metal NPs with a large number of edge and corner atoms have been utilized as highly active and excellent catalysts.32 In general, stabilizers like polymers and surfactants should be added to generate highly dispersed metal NPs.33 Ionic liquids (ILs) have also been found as excellent media for stabilizing metal NPs.34-40 In addition, ILs have nearly negligible vapor pressures, wide liquid range, and large solubility of metal catalysts,
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which makes them be excellent media for various catalytic reactions.41-46 Herein, we for the first time reported a Pd NPs/surfactant IL system for acetylene hydrochlorination reaction. The Pd0 NPs with narrow size distribution and well-defined lattice fringes were synthesized in-situ with PdCl2 as precursor in anionic surfactant carboxylate ILs (ASC-ILs). Despite the zero-valent nature, the Pd NPs/surfactant IL systems showed excellent catalytic activity to acetylene hydrochlorination. We found that the highly active Pd0 NPs served as reservoirs that were easily oxidized to catalytically active PdII species by the reactant of HCl. Benefiting from the high reactivity of NPs and the unique self-assembly property of long-chain carboxylate ILs, an effective redox cycle between Pd0 and PdII was established to reduce the deactivation of metal catalysts. Till now, most of the researches on hydrochlorination of acetylene have focused on gas-solid reactions while heat spot and coking deactivation are easy to occur in these cases.16-18 Compared to gas-solid reaction, gas-liquid reaction can provide mild temperature control. Acetylene hydrochlorination using ILs as reaction media can not only provide environmentally friendly choices for gas-liquid systems than using volatile organic solvents but also have the potential of achieving better catalytic results due to the unique physicochemical properties of ILs.47-50 In this work, a series of ASC-ILs, tetrabutylphosphonium-based long-chain fatty acid ILs were prepared, which were featured with strong hydrogen-bond basicity, ranking the top level of all reported ILs and organic solvents, along with good lipophilicity. The strong basicity of ASC-ILs was expected to activate the reactants of acetylene and HCl. Therefore, in this article, we discussed the effect of physicochemical properties of ASC-ILs on catalytic activity. The effects of catalyst dosage, temperature, gas velocity, and reaction time on the catalytic performance of acetylene hydrochlorination were also investigated.
EXPERIMENTAL SECTION Materials. C2H2 (gas, 99.5%) and HCl (gas, 99.998%) were purchased from Jingong Special Gas Co., Ltd. The 40% aqueous solution of tetrabutylphosphonium hydroxide ([P4444]OH) was purchased from Tokyo Chemical Industry Co., Ltd. Octanoic acid (99%) was purchased from J&K Scientific Ltd. Lauric acid (98%), myristic acid (98%), palmitic acid (98%) and stearic acid (98%) were purchased from Aladdin Reagent Co., Ltd. 1-Butyl-3-methylimidazolium trifluoromethanesulfonate, ([Bmim][CF3SO3], 98.5%) was purchased from Green Chemistry and Catalysis, LICP, CAS. PdCl2 and HAuCl4·4H2O were purchased from Sinopharm Chemical Reagent Co., Ltd. PtCl2 was purchased from Aladdin Reagent Co., Ltd. Synthesis of butylphosphonium-based anionic surfactant carboxylate ILs (ASC-ILs). These surfactant ASC-ILs were prepared by the acid-base neutralization between different long chain fatty acids and tetrabutylphosphonium hydroxide in water as the similar procedures reported in the literature.51,52 The accurate concentration of [P4444]OH in aqueous solution was calibrated by HCl titration. Then equimolar [P4444]OH (aqueous solution) and fatty acid were added to stir well at 313.15 K for 24 h to prepare the aqueous solution of surfactant ILs following by high vacuum drying at 323.15 K for 48 h. The used long chain fatty acids were octanoic acid, lauric acid, myristic acid, palmitic acid and stearic acid, bearing a carbon number from eight to eighteen. Then a series of ASC-ILs
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could be synthesized, namely tetrabutylphosphonium octanoate ([P4444][C7COO]), tetrabutylphosphonium laureate ([P4444][C11COO]), tetrabutylphosphonium myristate ([P4444][C13COO]), tetrabutylphosphonium palmitate ([P4444][C15COO]) and tetrabutylphosphonium stearate ([P4444][C17COO]). Preparation of metal NPs/IL catalytic system. Nearly 22 mL ILs together with a certain amount of a catalyst precursor were mixed well at 120 oC for an hour to form a NPs/IL catalytic system. The catalyst precursors referred to PdCl2, PtCl2 and HAuCl4·4H2O. Catalytic experiment. The acetylene hydrochlorination reaction was carried out in a bubbling reactor with two coaxial glass tubes of different diameters. The outer tube has a diameter of 15 mm with a vertical length of nearly 400 mm and inner tube has a diameter of 12 mm. The inner tube is separated from the outer tube by 1.5 mm in order to achieve a long reaction path by using minimum amount of ILs.47 The bubble reactor was heated by a self-designed cylindrical oil bath with a heating length of 350 mm. Before reaction, the catalytic system should be activated by passing HCl gas for an hour at 120 o C and then increased to the specific reaction temperature. Initial experiments had been carried out to optimize the activation temperature at 90, 120 and 150 oC. The activation temperature exerted an impact on the initial catalytic activity and increase of activation temperature brought higher initial acetylene conversion while the equilibrium conversion of acetylene increased slightly by 0.8% and 1.5% when the activation temperature increased from 90 to 120 oC and 120 to 150 oC, respectively. In order to keep the same temperature as preparation of metal NPs/IL catalytic system, 120 oC was selected in this work. The dried feed gases, C2H2 (5 mL/min) and HCl (6 mL/min), controlled by mass flowmeters, were fed into the catalytic system, giving a gas hourly space velocity (GHSV) of 13.6 h-1 (acetylene based). The GHSV was similar to the reported gas-liquid acetylene hydrochlorination reaction47 while less than gas-solid reaction due to the dilute concentration of catalyst in IL. Finally, the outlet gases were analyzed online by gas chromatograph (GC) after eliminating acid and water. The experimental setup was displayed in Figure S1. The products were analyzed online by GC (Fuli, GC9790) with the use of GDX-301 column by flame ionization detector (FID). The temperatures for column, injector and detector were 80 oC, 90 oC and 150 oC, respectively. The performance of catalytic system was defined as acetylene conversion and vinyl chloride selectivity, which could be calculated as follows.15 Conversion =(φA0-φA)/φA0*100% (1) Selectivity =φVC/(1-φA)*100% (2) Where φA0 stood for volume fraction of acetylene in feed gas, φA stood for residual volume fraction of acetylene in product gas, φVC stood for volume fraction of vinyl chloride in product gas, and they were all determined by GC analysis. Characterization. High resolution transmission electron microscopic (HRTEM) (JEOL, JEM-2010F, 200 kV) was used to determine the distribution and size of the nanoparticles, and also the lattice fringes of NPs. Energy dispersive X-ray Detector (EDX) was used to identify the presence of metal elements. The samples for characterization were prepared by fully dispersing metal NPs/IL systems in ethanol and depositing 2 or 3 drops of the suspension on ultrathin carbon film coated on a 300-mesh Cu grid. X-ray photoelectron spectroscopy (XPS) (Thermo, ESCALAB 250Xi, Al Kα source) was used to ana-
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lyze the valence state of the elements. The single element spectra were recorded with a pass energy of 30 eV. The binding energies were calibrated by the C1s binding energy, taken as 284.6 eV. The concentration of Pd in the catalyst after reacting for 55 h was measured by Agilent AA 240 model atomic absorption spectrophotometer. The catalyst samples were dissolved in the diacid mixture (HC:HNO3 in 3:1 ratio) at 60 o C before measurement.
RESULTS AND DISCUSSION The modification of cations or anions to fine tune the supramolecular organization of ILs enabled some metal NPs to be prepared in IL media. From this idea, we synthesized a novel kind of tetrabutylphosphonium-based long-chain fatty acid ILs (ASC-ILs, molecular structures see Scheme S1) with surfactant properties. Nanoscale organizations were observed in these ASC-ILs due to the nano-segregation of alkyl tails of anion in ILs, and the degree of order of these nanostructured ASC-ILs was dependent on the length of the alkyl chain in the carboxylate anions.52 We expected that the unique selfassembly property of IL could contribute to enhanced dispersity/stability of NPs. The metal NPs could be obtained by directly mixing ASC-ILs with a catalyst precursor at the temperature of 120 oC for an hour. By adding precursor PdCl2 into [P4444][C17COO], the solution immediately turned black, indicative of in-situ Pd NPs formation, which was similar to those reported in literatures with other ILs as solvents.34,37,53 As evidence from previous work that acetate anions could act as the reducing agent,53 carboxylate anions of ASC ILs were likely to have the similar effect. The valence status of Pd was verified by X-ray photoelectron spectroscopy (XPS) (Figure S2). The two peaks at 339.5 eV and 334.3 eV were assigned to Pd 3d3/2 and 3d5/2 electrons of metallic Pd0, suggesting the reduction of PdII to Pd0. Transmission electron microscopic (TEM) images of Pd NPs/[P4444][C17COO] were presented in Figure 1. The visible
a)
b)
from high resolution transmission electron microscopic (HRTEM) image, the Pd NP clearly displayed highly organized lattice fringes corresponding to (111) crystal plane since the fringe spacing was estimated as 0.230 nm, which was composed of neatly arranged Pd atoms of high activity (Figure 1c). Among tetrabutylphosphonium-based ASC-ILs, by varying carbon chain length of anions, we prepared Pd nanoparticles in [P4444][C7COO] and [P4444][C13COO]. The TEM images showed that there experienced no obvious disparity in dispersion degree or particle size of Pd NPs when altering the anions of ASC-ILs (Figures S4a,5a) and Pd NPs in [P4444][C7COO] or [P4444][C13COO] could also display clear lattice fringes as in [P4444][C17COO] (Figures S4b,5b). Additionally, we found that the imidazolium-based IL [Bmim][CF3SO3] was also capable of forming Pd clusters. However, for [Bmim][CF3SO3], although the average size of Pd clusters was almost the same as those in ASC-ILs, the clusters had the tendency of small-scale aggregation (Figure S6a). Seen from HRTEM image, Pd NPs in [Bmim][CF3SO3] were a kind of laminar crystals that were much thinner than nanoparticles in ASC-ILs (Figure S6b). In some reports, ILs like [Bmim][PF6] or [C4py][Tf2N] could also afford Pd NPs, but the average size of NPs was usually about 5-10 nm and the dispersity of NPs was relatively poor.54-56 In consequence, the surfactant ASC-ILs we synthesized were more effective than traditional ILs in preparing metal NPs in terms of their size and shape. Pt NPs/[P4444][C17COO] and Au NPs/[P4444][C17COO] catalytic systems were also fabricated by simply blending [P4444][C17COO] with precursor PtCl2 and HAuCl4·4H2O at 120 oC, respectively. EDX identified the existence of Pt and Au element (Figures S7 and S8). Seen from Figure 2a, Pt nanoparticles could be distributed well in [P4444][C17COO] and the estimated size of Pt NPs was only about 1.5 nm, even smaller than that of Pd NPs. Figure 2b clearly exhibited that there existed highly ordered lattice fringes in a Pt nanoparticle. While in Au NPs/[P4444][C17COO], the particle size of Au was almost more than 20 nm that was much larger than Pd NPs or Pt NPs formed in the same IL (Figure 2c). Clear lattice fringes of an Au particle were as well exposed in
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Mean size: 3.2 nm
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Figure 1. TEM characterization of Pd NPs in [P4444][C17COO] IL. a) TEM image of Pd NPs for 50 nm scale, b) TEM image of Pd NPs for 10 nm scale, c) HRTEM image of a Pd NP, d) Particle size distribution of Pd NPs
clusters were confirmed as Pd element by Energy dispersive X-ray Detector (EDX) (Figure S3). Pd NPs were uniformly dispersed in IL with a narrow size distribution in the range of 2.4-4.4 nm and the mean size averaged from randomly selected 50 clusters was about 3.2 nm (Figures 1a,b,d). Observed
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Figure 2. TEM characterization of Pt and Au NPs in [P4444][C17COO] IL. a) TEM image of Pt NPs, b) HRTEM image of a Pt NP, c) TEM image of Au NPs, d) HRTEM image of a Au NP Table 1. Catalytic performance of acetylene hydrochlorination in different metal NPs/IL systems
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conversion
selectivity
[P4444][C7COO]
Pd
76.23%
99.46%
2
[P4444][C11COO]
Pd
79.69%
99.47%
3
[P4444][C13COO]
Pd
81.37%
99.47%
4
[P4444][C15COO]
Pd
87.50%
99.47%
5
[P4444][C17COO]
Pd
a
93.04%
99.46%
6
[P4444][C17COO]
Au
64.56%
99.24%
7
[P4444][C17COO]
Pt
74.25%
99.19%
8
[Bmim][CF3SO3]
Pd
69.91%
99.24%
a
The ratios of Pd(0) and Pd(II) after HCl activation is 28.02:71.98. Reaction conditions: 180 oC, HCl: 6 mL/min, C2H2: 5 mL/min, Catalyst precusor: 0.04 mol/L HRTEM images (Figure 2d). The catalytic performances of NPs/IL systems were tested in acetylene hydrochlorination reaction (Table 1). Results showed that Pd NPs in tetrabutylphosphonium-based ILs with different long chain fatty acids anions ([C7COO]-, [C11COO]-, [C13COO]-, [C15COO]-, [C17COO]-) gave pretty good conversion and selectivity. Entry 1-5 demonstrated a rising trend in acetylene conversion from 76.23% in [P4444][C7COO] to 93.04% in [P4444][C17COO] with the increasing carbon chain length of anions and all the ASC-ILs examined yielded high VCM selectivity of around 99.5%. Apart from Pd NPs, Au NPs and Pt NPs also afforded good results. The acetylene conversion of Au NPs/[P4444][C17COO] catalytic system was 64.56% (Entry 6) and that of Pt NPs/[P4444][C17COO] reached 74.25% (Entry 7). Both systems produced VCM with extremely high selectivity (>99%). While Au or Pd catalysts initially had a high activity for this transformation, quick deactivation was often observed in traditional catalytic systems within 3 hours, resulting in gradually decreasing activity.5,10 Our metal NPs/ASC-IL systems exhibited rather better stability with no obvious decrease in catalytic activity over 8 hours (Figures S9-S11). However, a gradual decrease of acetylene conversion was observed after reacting for just 2 hours with Pd NPs as catalyst and common IL of [Bmim][CF3SO3] as a solvent (Figure S12) and its acetylene conversion was about 69.91% (Table 1, Entry 8), much lower than that in Pd NPs/[P4444][C17COO]. In view of the catalytic activity and stability, the metal NPs in ASC-ILs were superior catalysts for acetylene hydrochlorination. To our knowledge, the oxidized metals like AuIII, PtII, and PdII are regarded as main catalytic active sites in acetylene hydrochlorination reaction.6 Therefore, the role of nanoparticles played in catalytic reaction is of significance to this transformation. In order to address this query, we conducted XPS analysis of Pd NPs/[P4444][C17COO] catalytic system after passing HCl and after reaction for 8 hours. In Figure 3a, the peaks centered at 335.1 eV and 340.4 eV belonging to Pd0 clusters while typical values around 337.1 eV and 342.1 eV corresponding to PdII species. According to the area integral, the fraction of PdII in total metal species was about 28.02%. This result showed that some of Pd0 atoms on the Pd nanoparticles had been oxidized to PdII after passing HCl, which meant that the highly active Pd0 acted as reservoirs to generate catalytically active PdII. We conducted a control experiment by heating the system for another hour without HCl treatment and XPS analysis manifested the only presence of Pd0 in the absence of PdII (Figure S13), indicating that HCl facilitated the
oxidation process. Hutchings also observed that HCl treatment facilitated the oxidation of Au.5 As far as we know, the oxidation of Pd NPs with HCl is rarely reported. After 8 hours of reaction, we also observed the presence of Pd0 and PdII (Figure 3b) and the ratio of PdII increased to 35.75%, suggesting that Pd0 could continuously be oxidized to PdII in the whole reaction process to maintain a relatively high level of PdII active component. The catalytic performances of NPs/IL systems without activating the catalyst by passing HCl gas were also investigated and the results showed that the initial activity was low and the conversion of acetylene less than 60% after reaction of 5 h (Figure S14). It is well known that the metal NPs tend to grow or aggregate during the catalysis processes which results in a significant decrease of acitivity.57-59 Interestingly, in [P4444][C17COO] system, the Pd nanoparticles after reacting for 8 hours could still evenly dispersed with clear crystal plane as depicted in HRTEM image (Figure 3c). Figure 3d exhibited a size distribution of nanoparticles ranging from 2.6 to 4.6 nm with a mean particle size estimated as 3.58 nm, only a slight change when compared to the fresh Pd clusters.
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5 nm
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Pd0 II Pd
338
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Binding Energy (eV)
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Mean size 3.58 nm
30 20 10 0 2.6
3.0 3.4 3.8 4.2 Particle Size (nm)
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Figure 3. a) Pd 3d XPS spectra of Pd NPs/[P4444][C17COO] after passing HCl, b) Pd 3d XPS spectra of Pd NPs/[P4444][C17COO] after reacting for 8 hours, c) HRTEM image of Pd NPs after reacting for 8 hours, d) Particle size distribution of Pd NPs after reacting for 8 hours
It was partially attributed to the stabilizing role of the surfactant ASC-ILs. On the other hand, the Pd NPs with narrow size distribution could go through in-situ generation in ASC-ILs. Therefore, in [P4444][C17COO], though PdII could be reduced to Pd0 in reaction process, the reduced Pd0 atoms could still reassemble into small sized Pd NPs with active crystal plane rather than larger aggregated particles that were less active. Hence, it was fairly probable that there existed a redox cycle between Pd0 and PdII in Pd nanoparticles/[P4444][C17COO] system for acetylene hydrochlorination as postulated in Figure 4, which was analogous to the mechanism of Heck reaction.37,60,61 As reservoirs, the highly active Pd0 atoms on the surface of Pd nanoparticles proceeded through facile oxidization by HCl to form catalytic active PdII. Along with PdII catalyzing the reaction of C2H2 and HCl to yield C2H3Cl, partial PdII would be reduced to Pd0 by C2H2. Benefiting from the unique property of surfactant ASC-ILs, the reduced Pd0 atoms could still experience in-situ formation of small-sized Pd NPs with active crystal facet. As illustrated in the amplified mi-
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crogram of a Pd nanoparticle composed of neatly organized Pd0 atoms, the ILs could form a protective layer around the Pd NPs, and those weakly coordinating ions would prevent the aggregation of NPs. In the next cycle, the Pd0 was ready to be re-oxidized to generate PdII, entering again into reaction path. In this way, the redox cycle between Pd0 and PdII involved in acetylene hydrochlorination was able to make sure continuous supply of active sites for prolonged catalysis. Hutchings had found that offline aqua regia reactivation could restore the activity of metal catalysts.62,63 In this work, we took advantages of the high activity of NPs and the unique surfactant property of ILs to realize an in-situ cycle between zero-valent metal and oxidized metal, which provided a facile approach instead of offline treatment to overcome the deactivation due to reduction of metal active component. As mentioned above, Au NPs/[P4444][C17COO] and Pt NPs/[P4444][C17COO] catalytic systems could also performed good activity and stability. Generally, Au had been regarded as the most active catalyst in acetylene hydrochlorination. However, to our great interest, it was not the case in the IL catalytic PdCl2 n
n
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0
II
Figure 4. The redox cycle between Pd and Pd involved in acetylene hydrochlorination reaction(gray sphere for C atom, white sphere for H atom and green sphere for Cl atom) with an amplified microgram of a Pd NP
system. Seen from Table 1, for the same [P4444][C17COO], Au NPs generated smaller acetylene conversion and VCM selectivity than Pd NPs. XPS result revealed the presence of both Au0 and AuIII after reaction but the ratio of AuIII was only 14.6% (Figure S15), which might be partially ascribed to the larger size of Au particles that hindered Au0 atoms to be efficiently oxidized. Although the ratio of PtII after reaction reached 43.9% thanks to their smaller size (Figure S16), the performance of Pt nanoparticles/[P4444][C17COO] was still inferior to Pd. It was believed that the performance was an integrated result affected by several factors and further researches to uncover the in-depth mechanism were still needed. The calculated turnover frequency (TOF) based on the concentration of active metal ions for Pd, Pt, Au nanoparticles were about 0.78 min-1, 1.44 min-1 and 0.52 min-1, respectively, which was higher than that of PtCl2 (0.13 min-1) and HAuCl4 (0.13 min-1) dissolved in IL of [Bmim][Cl].47 Therefore the activities of metal NPs have not been restrained by the long-chain carboxylate ILs. However, their TOF values were lower than the porous catalyst of Au-carbon (12 min-1) because of the relatively high viscosity and larger mass transfer resistance in ILs.21 The TEM images of Au and Pt nanoparticles/[P4444][C17COO] catalytic systems after reacting for 8 h were characterized to observe the changes of NPs. There existed two morphologies of Au particles
after reaction. One kind of morphology turned out to be irregular particles composed of smaller sized clusters ranging from 15 nm to 50 nm and some larger clusters even bigger than 200 nm (Figure S17a). Another morphological characteristic was evenly dispersed Au nanoparticles with a mean size of about 4 nm (Figure S17b). In comparison with fresh Au nanoparticles, the changes of Au nanoparticles revealed the occurrence of detachment and deposition of Au0 atoms on the surface of clusters, which also demonstrated a redox cycle between Au0 and AuIII in the reaction process. Figure S18 showed that Pt nanoparticles after reaction didn't appear to be much different from fresh nanoparticles, indicating the good stability of Pt-based catalyst in IL. As to the carrier of catalytic system, the physicochemical properties of ILs could also exert effect on the catalytic performance. In reported literatures, carbon materials which had good lipophilicity and relative weak polarity were considered to be the preferred supports,15-20 while silica-based support would produce some polymerization by-products due to their surface acidity.6 The introduce of basic sites on carbon materials was proved to activate HCl and acetylene effectively, contributing to enhanced activity and stability.21-23 Therefore, the physicochemical properties of ASC-ILs, hydrogen-bond basicity (β) and dipolarity/polarizabiliy (π*), were determined and results were listed in Table 2.51 ASC-ILs featured strong hydrogen-bond basicity and the β values of ASC-ILs reached Table.2 The hydrogen-bond basicity (β), dipolarity/polarizabiliy (π*) and onset temperature of decomposition (Tonset) of ASC-ILs ionic liquid
β
π*
Tonset（oC）
[P4444][C7COO]
1.55
0.86
301.3
[P4444][C11COO]
1.61
0.81
335.0
[P4444][C13COO]
1.61
0.80
337.0
[P4444][C15COO]
1.62
0.77
306.2
[P4444][C17COO]
1.61
0.76
301.1
almost 1.60, ranking the highest among all the reported ILs and organic solvents.51 The carboxyl with electron-donating effect of long carbon chain (RCOO-) mainly contributed to the strong hydrogen-bond basicity of ASC-ILs. Therefore, ASCILs were expected to exhibit good absorption capacity of HCl and C2H2. HCl gas absorption experiment showed that 1 mol [P4444][C17COO] could absorb approximately 2 mol HCl at reaction temperature, indicating that HCl could be well activated by IL. For the solubility evaluation of C2H2, with the test temperatures and pressures, the acetylene solubility could reach 0.1 mol/L~0.6 mol/L (Figure S19). In 298.15 K, the mole capacity of acetylene in [P4444][C17COO] reached about 0.46 bar-1 in molar fraction. ASC-ILs had been proved to be one of the best solvents to absorb C2H2 compared with other reported ILs and industrial organic solvents.64 On the other hand, ASC-ILs displayed good lipophilicity. The values of dipolarity/polarizabiliy (π*) of ILs were relatively lower between 0.76 and 0.86 compared to most of ILs (0.90~1.10), and π* values gradually dropped with the increasing carbon chain length of anions. Therefore, as shown in Table 1, the increase of conversion from [P4444][C7COO] to [P4444][C17COO] might be due to
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Figure 5. Catalytic activity in Pd NPs/[P4444][C17COO] for acetylene hydrochlorination. a) The effect of temperature (reaction conditions: HCl: 6 mL/min; C2H2:5 mL/min; PdCl2: 0.04 mol/L), b) The effect of velocity (reaction conditions: 180 oC; flowrate ratio of HCl:C2H2=1:2; PdCl2: 0.04 mol/L), c) The effect of catalyst dosage (reaction conditions: 180 oC; HCl: 6 mL/min; C2H2:5 mL/min), d) Long time stability test (reaction conditions: 180 oC; HCl: 6 mL/min; C2H2:5 mL/min; PdCl2: 0.04 mol/L)
the decrease in dipolarity/polarizabiliy of ILs since TEM results manifested Pd nanoparticles formed were almost the same. It was likely that the acetylene conversion probably formed an adverse relationship with π*. In addition, the onset decomposition temperatures (Tonset) of ASC-ILs determined by thermal gravimetric analysis (TGA) were also listed in Table 2.51 All these ILs showed very high Tonset of exceeding 300 oC, indicative of an excellent thermal stability to endure the high temperature of acetylene hydrochlorination reaction. The boiling points of conjugate acids of ASC-ILs’ anion, such as stearic acid (376.1 oC), were also very high, which effectively reduced the possibility of loss as volatile acids resulting from the possible anion exchange between ILs and HCl.65 To get better understanding of the catalytic system, we targeted [P4444][C17COO] as a preferable reaction medium for further investigation and the effect of temperature, velocity and catalyst dosage on catalytic performances were investigated. As illustrated in Figure 5a, with the rise of temperature from 120 oC to 200 oC, it made notable gains in acetylene conversion from 49.09% to 96.29%, whereas the VCM selectivity underwent a slight decline from 99.55% to 99.40%. It indicated that higher temperature was capable of boosting both main reaction of obtaining VCM and side reaction of producing dichloroethane. However, the relative content of VCM among product gases descended with increasing temperature. We also explored acetylene GHSV meanwhile keeping the flowrate ratio of HCl to C2H2 constantly at 1.2:1. Figure 5b showed that the acetylene conversion decreased from 95.32% in 8.2 h-1 (3 mL/min) to 84.56% in 30 h-1 (11 mL/min) while VCM selectivity nearly remained invariable. The fast flowrate might give rise to shortage of residence time, thus obstructing better reaction performance to be achieved. Therefore, designing a gas-
liquid reactor with better mass transfer effect was of great significance to improve the reaction efficiency. In addition, the molar concentration of catalyst precursor PdCl2 was taken into account from 0.005 mol/L to 0.05 mol/L. Under low molar concentration, acetylene conversion formed an increasing function of catalyst dosage. When the catalyst dosage reached 0.02 mol/L, the conversion became constant afterwards at around 93% and further addition of catalyst was hardly effective at improving the catalytic activity (Figure 5c). The stability of Pd nanoparticles/[P4444][C17COO] system was also surveyed. Figure 5d portrayed the catalytic performance over reaction time of 55 hours. In the first two hours, the acetylene conversion went up with time rapidly, and then the rising rate of conversion slowed down, and finally leveled off at 93.04%. The VCM selectivity always maintained a high level at nearly 99.5%. The system presented good stability as reaction time went on, showing no deterioration in both conversion and selectivity. Atomic absorption spectroscopy analysis showed that there still existed 74.6% Pd species (0.030 mol/L including Pd0 and PdII) after reacting for 55 h compared with the fresh Pd catalyst, which was much better than reported porous solid catalysts.10 As evident from the experimental results, the redox cycle between Pd0 and PdII might exert a significant effect on stability and study on longer time test is being carried out.
CONCLUSIONS In conclusion, we developed a novel metal NPs/surfactant IL system for acetylene hydrochlorination reaction. Pd, Au, and Pt NPs with a narrow size distribution and well-defined lattice fringes were prepared in surfactant ASC-ILs. Pd NPs were uniformly dispersed with a mean size of 3.2 nm and clear (111) crystal plane. The metal NPs/surfactant IL systems
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showed excellent catalytic activity to acetylene hydrochlorination. Our work demonstrated the highly active Pd0 clusters served as reservoirs that were easily oxidized to catalytically active PdII species by HCl. It was highly possible that the high reactivity of NPs and the surfactant property of ILs together made possible a redox cycle between Pd0 and PdII to provide sufficient active sites for prolonged catalysis. For Pd NPs/[P4444][C17COO] system, it achieved good acetylene conversion of surpassing 93% and VCM selectivity of about 99.5% and over the reaction time of 55 hours, the activity performance maintained invariable. Results showed Pd NPs could acquire better catalytic performance even than Au or Pt NPs in ASC-ILs. Furthermore, with the strong hydrogen-bond basicity of almost 1.60 and weak polarizability of 0.76~0.86, ASCILs were able to absorb approximately 2 molar equivalents HCl and obtain acetylene solubility of 0.1 mol/L~0.6 mol/L, which also gave rise to activation of reactants and improvement of activity. Metal NPs/surfactant IL catalytic systems are regarded as green substitutes for mercury catalysts for acetylene hydrochlorination reaction and also provide inspiration for design of efficient catalysts for activation of other alkynes. In addition, with the unique physicochemical property and excellent NPs stability, these systems are highly potential in extensive range of catalytic processes.
ASSOCIATED CONTENT Supporting Information The solubility experiment details, experimental setup, Pd XPS spectra, EDX confirmations of metal elements, TEM and HRTEM images of metal NPs, catalytic activity over reaction time and the solubility of acetylene in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. Fax: (+86)-571-87952375.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENTS The research was supported by the National Natural Science Foundation of China (21222601, 21476192, and 21436010), the Zhejiang Provincial Natural Science Foundation of China (LR13B060001), and SD was supported by the Office of Basic Energy Sciences, U.S. Department of Energy.
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