III.3 Title: Selective hydrochlorination of acetylene to vinyl chloride using Au-containing
catalysts
Participants: John R. Monnier, JR Regalbuto, and Donna A. Chen
Acetylene Conv (% )
Acetylene Conv (% )
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 1,2dichloroethane 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
prechloride, then react
100
Reaction after drying at 120C
100
in Figure 1 from our
Reduce, then prechloride, then react
80
reduce, then react
laboratory [5,6] have shown
90
60
that, regardless of HCl
80
40
pretreatment conditions, all
Au/C catalysts exhibit the
20
70
same activities after 2 – 3 h
0
60
0.0
0.5
1.0
1.5
2.0
2.5
3.0
of reaction time, suggesting
0
5
10
15
20
Run Time (Hours)
Run Time (Hours)
that all Au particle sizes and
1. Evaluation of 1.0 wt% Au/C. Reaction conditions: T = 180°C, GHSV = 1460 hr-1 and feed = HCl : C2H2 : He
surface compositions are Figure
= 1.1 : 1.0 : 1.0. Prechloride is pretreatment at 180°C for 1 hr in 33% HCl, balance He. Drying is pretreatment at
similar. More recent work 120°C for 1 hr in flowing He. Reduction is pretreatment at 180°C for 1 hr in 20% H2, balance He.
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/cm 2 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 TiO 2
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].
[2].
Y. Saeki, T. Emura, Prog. Polym. Sci. 27 (2002) 2055–2131
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.
[3]. M. Conte, A.F. Carley, C. Heirene, D.J. Willock, P. Johnston, A. A. Herzing, C. J. Kiely,
and G.J. Hutchings, Hydrochlorination of acetylene using a supported gold catalyst: A
study of the reaction mechanism,” J. Catal., 250 (2007) 231.
[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.
[6]. W. Wittanadecha, N. Laosiripojana, A. Ketcong, N. Ningnuek, P. Praserthdam, J.R.
Monnier and S. Assabumrungrat, “Development of Au/C catalysts by the microwaveassisted method for the selective hydrochlorination of acetylene,” Reac. Kinet. Mech. Cat.,
112 (2014) 189.
[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
systems. Prediction of surface phase diagrams of solid solutions,” Chem. Rev., 75 (1975)
547.
[11]. L.Z. Mezey and J. Giber, “The surface free energies of solid chemical elements:
calculations from internal free enthalpies of atomization,” Jap. J. Appl, Phys., 21 (1982)
1569.
[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.
Cat. A: General, 475 (2014) 161.
[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),”
Surf. Sci., 606 (2012) 1233.
[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.