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Selective hydrogenation is one of the largest class of catalytic reactions used by both the chemical process and specialty/fine chemistry industries


III.3 Title:  Selective hydrochlorination of acetylene to vinyl chloride using Au-containing catalysts 

Participants:  John R. Monnier, JR Regalbuto, and Donna A. Chen  

    

Motivation: Polyvinyl chloride (PVC) is the third highest volume plastic after polyethylene and polypropylene; global demand is expected to reach 80 billion lbs by 2016 [1]. Polyvinyl chloride is produced by polymerization of vinyl chloride monomer (VCM) which is manufactured either by the oxidative chlorination of ethylene to produce 1,2-dichloroethane which is thermally cracked to produce VCM or by the direct hydrochlorination of acetylene to produce VCM in very high selectivity in a single step.  The oxychlorination method generates the majority of VCM, although this route is highly corrosive, multi-step, non-selective, and energy intensive. Production of VCM from direct hydrochlorination of acetylene is simple and very selective (> 99%), although HgCl2/C, the catalyst currently used, undergoes reduction to Hg metal after a short period of time and is then volatilized into the environment [2]. It is estimated that 320 tons of Hg are lost annually in China during production off VCM [1].  In areas such as China where coal is plentiful, the formation of acetylene from coke (CaC2 + H2O reaction) is a well-developed technology. Thus, it is necessary to develop more environmentally-friendly catalysts for acetylene hydrochlorination. Carbon-supported Au catalysts have been found to be active and selective catalysts for this reaction; however, the high reduction potential for Au3+ to Auo also means that the rechlorination/regeneration of Au to the presumed, active AuClx site is slow and rate limiting [3,4].  This can give rise to agglomeration of Auo to form large metallic Au particles. The irreversible reduction of AuCl3 surface species to form Au atoms, and subsequent sintering of Au metallic particles is often given as the reason for the high rates of catalyst deactivation.

    

Recent results shown in Figure 1 from our laboratory [5,6] have shown that, regardless of HCl pretreatment conditions, all Au/C catalysts exhibit the same activities after 2 – 3 h of reaction time, suggesting that all Au particle sizes and surface compositions are similar.  More recent work summarized in Table 1 [7] has also shown that sintering of Au is greatly enhanced by HCl, which is, of course one of the essential reactants. Results in Table 1 indicate that exposure to HCl for periods as low as 15 minutes sinters Au particles from 3.5 nm to 18.8 nm (particle sizes determined by x-ray line broadening and application of Debye-Scherrer equation). In fact, sintering is virtually completed within the first several minutes of exposure to HCl. If the Au/C catalysts are exposed to only flowing He at the same conditions and time periods, no Au sintering is observed.  Conversely, exposure of 2.2 nm Pt particles to HCl as long as 480 minutes gives no detectable sintering. Thus, sintering of Au is caused by exposure to HCl gas streams; one possible Au-Cl species that may be formed at these conditions is the dimer Au2Cl6 [8,9], which has physical properties more susceptible to sintering. These results indicate that only if Au can be maintained in the active AuClx state during reaction and sintering can be prevented can direct hydrochlorination become commercially feasible. 

Hypothesis Bimetallic catalysts can provide structures more resistant to HCl-induced sintering and also potentially form a bimetallic surface that helps maintain the Au surface in an active and sinter-resistant AuClx state. Intelligent catalyst synthesis using advanced preparation methods will be used to prepare bimetallic particles composed of sinter-resistant shells of Au (with stable AuClx surface sites) on a core metal having a higher surface free energy than the surface free energy of the Au shell.  Surface free energy is defined as the excess energy at the surface of a material compared to the bulk and it is thermodynamically favorable to achieve the lowest possible surface free energy for a surface.  Metal-metal interactions can give greater resistance to particle sintering at conditions typically used for catalytic reactions by using metals with higher surface free energies (SFE) as core materials upon which a shell metal is selectively placed. Surface thermodynamics state that at equilibrium a system composed of solid components will rearrange to the lowest surface free energy; thus, the shell metal with lower SFE will remain on the surface of the core metal having higher SFE [10,11]. Within this hypothesis, a metal such as Pt (SFE = 2691 ergs/cm2 surface) will be deposited on carbon followed by the deposition of a Au shell (SFE = 1626 ergs/cm2 surface) to form a bimetallic surface in which the Au is thermodynamically constrained to remain on the stable Pt core and not agglomerate.

      The second component of the above hypothesis addresses the need to maintain the Au surface in an active AuClx state. The high reduction potential for Au3+ to Auo means that rechlorination and regeneration of Au to form the presumed, active AuClx site is slow and rate limiting [3,4].  One way to improve the kinetics for rechlorination is to form a bimetallic surface of Au with another metal M, where M is more reactive with adsorbed HCl to form an active M-Cl species that can shuttle the Cl to an adjacent Au site to form Au-Cl. Such M metals include Cu, Ag, and Pd. This essentially uses the Wacker-model (Cu-Pd system plus HCl and O2 for oxidation of C2H4 to form CH3CHO) [12] for combination of the active Au surface with a more active metal for activation of HCl that can supply Cl to the Au site. 

Objective: Synthesize and characterize more active and stable catalysts for hydrochlorination of acetylene. 

Research Plan:  Carbon-supported primary metal M (Cu, Pt, Pd, Ni) catalysts will be prepared using the strong electrostatic adsorption SEA) methodology [13] to generate small, supported metal particles; each of these metals has higher SFE than Au.  In SEA, an aqueous solution containing a charged metal precursor such as anionic platinum hexachloride, [PtCl6]2-, is contacted with an oppositely charged carbon surface.  The charge is induced on the carbon support surface by utilizing a solution with a pH different from the support’s point of zero charge; the pi bonds of the aromatic rings on the carbon surface can be protonated and positively charged using acidic pH impregnating solutions [14].  In this way, a monolayer or submonolayer of strongly adsorbed metal precursor will be adsorbed on the support.  These are converted to highly dispersed metal nanoparticles, typically less than 1.5 nm average particle size, by reduction in hydrogen at elevated temperature [14].

      Electroless deposition [15-19] will then be used to deposit controlled amounts of Au on the primary metal surface. In this instance electroless deposition will involve exposure of the supported, monometallic core metal catalyst to an aqueous solution containing a suitable reducing agent, such as hydrazine (N2H4), dimethylamine borane (DMAB), or sodium hypophosphite (NaH2PO2) and a reducible metal Au salt, such as tetrachloroauric acid (HAuCl4) or potassium dicyanoaurate [KAu(CN)2]. Catalytic activation of the reducing agent on the surface of the core metal forms an active reducing species where the Au salt is reduced to form Au metal deposited on the surface of the core metal.  Repeated cycles of activation of reducing agent followed by reduction of the Au salt increases the coverage of Au metal on the surface of the core metal to ultimately form a Au monolayer.  Because the freshly-deposited Au metal is also catalytic for activation of the reducing agent, deposition of Au on the Au surface can also occur after some time interval of this process to form controlled, multi-layers of Au on the core metal surface.  If it is desirable to form a bimetallic surface of Au with the base metal, the extent of Au deposition will be restricted to form only a partial monolayer of Au on the core metal.  For example if SEA is used to form small, uniform-sized particles of Cu (SFE = 1934 ergs/cm2 surface) on carbon, subsequent deposition of controlled amounts of Au could form a thermodynamically-stable Au surface that has adjacent Cu sites.  Such a surface might combine Au stability with a more active surface for acetylene hydrochlorination. The catalytic performance of bimetallic Au-containing catalysts will be studied using a dedicated, gas phase micro-reactor in the Monnier laboratories.

 Thus far we have assumed the active surface is an unspecified AuClx species; however, virtually nothing is known regarding the composition and reactivity patterns of the active site.  The Au-Cl surface will be studied using the UHV methods of Chen to identify the unidentified, active Au surface site.  Specifically, Au particles will be deposited (by evaporation) on TiO2 surfaces and then partially chlorided at elevated temperatures using dichloroethane (DCE) as the Cl source to simulate the Au-Cl surface site. Identification and better understanding of this site should help to synthesize more active and stable catalysts.  Chen [20,21 ] has extensively studied the structural and electronic properties of uniform Au particles vapor-deposited on TiO2(110) surfaces. An STM image taken form a recent study is shown in Figure 2. Gold particles such as these will be prepared at UHV conditions and then exposed to DCE using a custom one atmosphere recirculation loop reactor attached to the UHV chamber [22].  The chlorided Au particles will then be transferred back into the UHV chamber for analysis using the full suite of UHV attachments in the UHV chamber. 

First year milestone:  Several series of mono and Au-containing bimetallic catalysts will be prepared using strong electrostatic adsorption (core metal) and electroless deposition (Au shell) and evaluated for vinyl chloride formation from acetylene and hydrogen chloride in an automated flow reactor.  Characterization of supported catalysts by XRD, XPS, and STEM will be done.  Surface analysis of model Au surfaces that have been pre-chlorided with 1,2-DCE will identify possible Au-Cl surface sites.  

Cost:  $60,000 for full support of graduate student, materials for reactor modifications, Au salts for catalyst preparation, and electron microscopy charges.    
 
 
 
 
 
 
 
 
 
 
 
 

References 

[1].    Y. Saeki, T. Emura, Prog. Polym. Sci. 27 (2002) 2055–2131

[2]. J. Zhang, N. Liu, W. Li, B. Dai, “Progress on cleaner production of vinyl chloride

         monomers over non-mercury catalysts,” Front. Chem. Sci. Eng. 5 (2011) 514–520.

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[4].  M. Conte, C.J. Davies, D.J. Morgan, T.E. Davies, A.F. Carley, P. Johnston and G.J. Hutchings, “Modifications of the metal and support during the deactivation and regeneration of Au/CCatalysts for the hydrochlorination of acetylene,” Cat. Sci. Technol., 3 (2013) 128.

[5].  W. Wittanadecha, N. Laosiripojana, A. Ketcong, N. Ningnuek, P. Praserthdam, J.R Monnier, and S. Assabumrungrat, “Preparation of Au/C catalysts using microwave–assisted and ultrasonic-assisted methods for acetylene hydrochlorination,” Appl. Catal A: General, 475 (2014) 292.

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[7].  To be submitted.

[8].   Egon Wiberg, Nils Wiberg, and A. F. Holleman), Inorganic Chemistry (101 ed), Academic

         Press,1286–1287 (2001).

[9].   J. Zhang, Z. He, W. Li, and Y. Han, “Deactivation mechanism of AuCl3 catalyst in acetylene

         hydrochlorination reaction: a DFT study,” RSC Advances, 2 (2012) 4814.

[10].  S.H. Overbury, P.A. Bertrand, and G.A. Somorjai, “The surface composition of binary

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[11].  L.Z. Mezey and J. Giber, “The surface free energies of solid chemical elements:

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[12].  R. Jira, “Acetaldehyde from ethylene-a retrospective on the discovery of the Wacker

       process,” Angew. Chem. Int. Ed., 48 (2009) 9034.

[13]. J.R. Regalbuto, in “Synthesis of Solid Catalysts, Chapter 3: Electrostatic Adsorption,” K.de Jong, ed., Wiley-VCH Verlag, 2009.

[14]. X. Hao, S. Barnes, and J.R. Regalbuto, “A fundamental study of Pt adsorption onto carbon 1.  Adsorption equilibrium and particle synthesis,” J. Catal., 279 (2011) 48.

[15].  S.S. Dkokić, “Electroless deposition of metals and alloys,” Mod. Aspects Electrochem., 35,

         (2002) 51.  

[16].  M.T. Schaal, A.C. Pickerell, C.T. Williams, and J.R. Monnier, “Characterization and

        evaluation of Ag-Pt/SiO2 catalysts prepared by electroless deposition,”J. Catal.,254 (2008)

       131.

[17].  K.D. Beard, D. Borelli, A.M. Cramer, D. Blom, J.W. Van Zee, and J.R. Monnier,

       “Preparation and structural analysis of carbon-supported Co core/Pt shell electrocatalysts

        using electroless deposition methods,” ACS Nano, 3 (2009) 2841.

[18]. Rebelli,, A.A. Rodriguez, S. Ma, C.T. Williams, J.R. Monnier, “Preparation and

       characterization of silica-supported, Group IB-Pd bimetallic catalysts prepared by

        electroless deposition methods,” Catal. Today, 160 (2011) 170.

[19].  A.A. Rodriguez, C.T. Williams, and J.R. Monnier, “Selective liquid-phase oxidation of

         glycerol over Au-Pd/C bimetallic catalysts prepared by electroless deposition,” Appl.

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[20]   S.A. Tenney, B.A. Cagg, M.S. Levine, W. He, K. Manandhar, and D.A. Chen,

         “Enhanced activity for supported Au clusters: methanol oxidation on Au/TiO2(110),”

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[21]. J.B. Park, S.F. Conner, and D.A. Chen, “Bimetallic Pt-Au clusters on TiO2(110):

         growth, surface composition and metal-support interactions,” J. Phys. Chem. C,

      112 (2008) 5490.

[22].  S.A. Tenney, K. Xie, J.R. Monnier, A. Rodriguez, R.P. Galhenage, and D.A. Chen,

     “Novel recirculating loop reactor for studies on model catalysts: CO oxidation on

    Pt/TiO2(110),” Rev. Sci. Instrument., 84 (2013) 104101. 


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