Sustainable chlorine recycling via catalysed HCl oxidation: from fundamentals to implementation

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Cite this: Energy Environ. Sci., 2011, 4, 4786

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Sustainable chlorine recycling via catalysed HCl oxidation: fromfundamentals to implementation

Javier P�erez-Ram�ırez,*a Cecilia Mondelli,a Timm Schmidt,*b Oliver F.-K. Schl€uter,c Aurel Wolf,c

Leslaw Mleczko*c and Thorsten Dreierb

Received 21st July 2011, Accepted 17th August 2011

DOI: 10.1039/c1ee02190g

The heterogeneously catalysed oxidation of HCl to Cl2 comprises a sustainable route to recover

chlorine from HCl-containing streams in the chemical industry. Conceived by Henry Deacon in 1868,

this process has been rejuvenated in the last decade due to increased chlorine demand and the growing

excess of by-product HCl from chlorination processes. This reaction suffered from many sterile

attempts in the past two centuries to obtain sufficiently active and durable catalysts. Intense research

efforts have culminated in the recent industrial implementation of RuO2-based catalysts for HCl

oxidation. This paper reviews the new generation of technologies for chlorine recycling under the

umbrella of Catalysis Engineering, that is, tackling the microlevel (catalyst design), mesolevel (reactor

design), and macrolevel (process design). Key steps in the development are emphasised, including lab-

scale catalyst screening, advanced catalyst characterisation, mechanistic and kinetic studies over model

and real systems, strategies for large-scale catalyst production, mini-plant tests with a technical catalyst,

and reactor design. Future perspectives, challenges, and needs in the field of catalysed Cl2 production

are discussed. Scenarios motivating the choice between catalysed HCl oxidation and HCl electrolysis or

their integration for optimal chlorine recycling technology are put forward.

Introduction

Chlorine is a highly reactive element, which practically does not

exist as Cl2 in nature, but is found in combination with other

aInstitute for Chemical and Bioengineering, Department of Chemistry andApplied Biosciences, ETH Zurich, Wolfgang-Pauli-Strasse 10, HCI E 125,CH-8093 Zurich, Switzerland. E-mail: [email protected]; Fax: +41 44 6331405; Tel: +41 44 633 7120bBayer MaterialScience AG, PUR-PTI-PRI, Chempark B598, D-41538Dormagen, Germany. E-mail: [email protected] Technology Services GmbH, BTS-PT-RPT-REC, D-51368Leverkusen, Germany. E-mail: [email protected]

Broader context

The energy crisis and environment protection constitute major co

dialogue and discussion between politicians, industrialists, and s

development, enabling chemical reactions with substantial energy sa

inputs. A particularly prominent example highlighting the essential

processes in the chemical industry is the oxidation of by-product HC

the 19th century, the industrialisation of this process has suffered from

recent development of a catalytic route for chlorine recycling is desc

the process. Proper integration of knowledge from these three lev

practical experience in process scale-up, is essential in order to i

reviewed greens up the manufacture of polyurethanes and polycarb

4786 | Energy Environ. Sci., 2011, 4, 4786–4799

elements. More than 2000 naturally occurring chlorine-based

compounds have been identified, and many of them accomplish

a number of useful functions in a wide range of living organisms.

In addition, Cl2 is a key building block for the manufacturing of

important industrial chemicals and consumer products.1 Many

sectors like healthcare, agro-food, construction, electronics,

textiles, transport, cosmetics, and leisure activities depend on

chlorine chemistry. Nearly two million jobs in the European

industry are directly or indirectly related to Cl2. Fig. 1 shows

a breakdown of the applications of chlorine within Europe in

2009. About two thirds of chlorine usage was in engineering

materials, i.e., polymers, resins, and elastomers. Polyvinyl

ncerns for today’s society, becoming the subject of constant

cientists. Catalysis is a key operational tool for sustainable

ving, reduced waste generation, and increasing use of renewable

role of catalysis in introducing energy-efficient and eco-friendly

l to high-purity Cl2. Since its introduction in the second half of

many sterile attempts due to the lack of suitable catalysts. The

ribed by rationalising the design of the catalyst, the reactor, and

els, bridging fundamental understanding at the nanoscale and

mplement a superior technology. The HCl oxidation process

onates, two of the currently most versatile synthetic plastics.

This journal is ª The Royal Society of Chemistry 2011

http://dx.doi.org/10.1039/c1ee02190g






http://pubs.rsc.org/en/journals/journal/EE

http://pubs.rsc.org/en/journals/journal/EE?issueid=EE004012

Fig. 1 European chlorine applications in 2009. Total European chlorine production was ca. 9 Mton. Germany was the main producer with a quota of

43.5%. Data retrieved from ref. 1.

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chloride (PVC) manufacturing constituted the main application

of chlorine, closely followed by the preparation of isocyanates

and oxygenates. The world chlorine production capacity was 18

Mton in 1965 but, as a result of the burgeoning demand for

plastics, it reached around 65 Mton in 2010 (Fig. 2a).2 According

to Euro Chlor, the European federation representing the chlor-

alkali industry, the demand is expected to continue to increase at

an annual rate of 4.4% in the coming years. Undoubtedly,

a country’s chlorine production reflects the state of development

of its chemical industry.3

Electrochemical processes have dominated the scene of large-

scale Cl2 production for more than a century. Basically, Cl2 is

formed by passing an electric current through an aqueous NaCl

Prof: Javier P�erez-Ram�ırez and Dr: Cecilia Mondelli

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solution, generating NaOH and H2 as by-products. The

membrane process is the cleanest, most energy-effective, and

economic technology available today,2 representing almost half

of the installed production capacity in Europe (Fig. 2b). The

mercury process is still widely implemented. However, due to

environmental reasons and relatively high energy consumption,

this technology is progressively being phased out. The chlor-

alkali industry has indeed experienced continuous improvements

in safety, health, and environmental aspects. Nonetheless, it

should be underlined that electrochemical Cl2 production

remains one of the most energy-intensive and costly processes in

the chemical industry, with power supply accounting for around

50% of the production costs. This aspect becomes even more

P�erez-Ram�ırez (Benidorm, Spain, 1974) earned his PhD in

cal engineering (cum laude) at TUDelft, the Netherlands (2002).

en worked in Norsk Hydro and Yara International in Norway

–2005) and later as ICREA research professor and group leader in

(2005–2009). Since 2010, he is full professor and chair of Catalysis

eering at the Institute for Chemical and Bioengineering of the ETH

, Switzerland. He researches the science and engineering of

geneous catalysis to design sustainable processes. His activities on

st development for HCl oxidation began in 2006 through collabo-

with Bayer MaterialScience. Cecilia Mondelli (Como, Italy,

earned her PhD in chemistry at the University ofMilan in 2007 and

the P�erez-Ram�ırez group in 2010. She currently works as senior

st and is active in the area of catalytic chlorine recycling.

Schmidt (Stuttgart, Germany, 1977) earned his PhD degree in

cal engineering at Saarland University, Germany (2006). Since

he is an employee of Bayer MaterialScience (BMS), heading the

w Processes Isocyanates within Process Research Isocyanates. He

harge of the R&D in the field of catalytic HCl oxidation within

Thorsten Dreier (M€unster, Germany, 1972) has over 9 years of

ence in research and technology in polyurethanes. Since 2009, he is

of Process Research Isocyanates at the business unit polyurethanes

S. In this role, he leads the process research department for MDI

DI, including the Deacon HCl recycling process, at BMS. He

his PhD in chemistry from the University of M€unster, Germany.

Energy Environ. Sci., 2011, 4, 4786–4799 | 4787


Fig. 2 (a) Evolution of the worldwide chlorine production since 1965.

The dominance of electrolytic processes for large-scale Cl2 manufacture is

illustrated in (b), showing the progressive shift to membrane cells in the

last two decades.

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important in relation to the present energy crisis and raises

concerns about anthropogenic CO2 emissions contributing to the

global warming. Advantageously, it has been shown that the

energy demand of conventional membrane technology for NaCl

electrolysis can be reduced by up to 30% by replacing the

hydrogen-evolving cathodes with oxygen depolarised cathodes

(ODC), which are well known from fuel cells.3,4 The first indus-

trial scale demonstration unit with a capacity of 20 kton Cl2 per

year is currently being realised in Uerdingen, Germany.5

It is interesting to note that one-third of all Cl2-derived

products do not contain chlorine. Polyurethanes and poly-

carbonates are representative chlorine-free end materials

produced using chlorine chemistry. It can be generally stated that

ca. 50% of the Cl2 used ends up forming part of secondary

products, namely HCl and chloride salts, during the

manufacturing chain. Processes that valorise chlorine-containing

by-product streams by conversion into high-purity Cl2 are in

great demand. This paper reviews recently developed technolo-

gies for sustainable chlorine recycling, with emphasis on the

heterogeneously catalysed oxidation of HCl to Cl2. The term

‘sustainable’ embraces both environmental and energetic aspects,

since the catalytic process converts the HCl by-product into

valuable Cl2 with very low energy requirements.

Past and present of catalysed Cl2 production

Prior to describing the new advancements in catalysed HCl

oxidation to Cl2, it is interesting to survey key aspects of the

Dr: Oliver Schl€uter; Prof: Leslaw Mleczko and Dr: Aurel Wolf

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history of chlorine production (Fig. 3). A detailed account for

each of the milestones illustrated can be found elsewhere.2,6 As

mentioned above, NaCl electrolysis, first developed using dia-

phragm and mercury cells towards the last quarter of the 19th

century and later propelled by the introduction of membrane

cells, possesses long-standing supremacy in the chlorine

manufacturing technology.3 However, industrial Cl2 production

started as a catalytic process. Chlorine gas was first produced in

1774 by Carl Scheele by heating pyrolusite (MnO2) with HCl.7

Nevertheless, Humphry Davy is often credited as the discoverer

of the element in 1808.8 On the basis of Scheele’s experiments, Cl2started to be manufactured by oxidising an aqueous solution of

HCl with huge (stoichiometric) amounts of MnO2, forming

MnCl2. Walter Weldon improved this process in 1866, including

a step to regenerate the metal oxide in the presence of oxygen and

lime (Ca(OH)2).9 Still, about half of the chlorine introduced as

HCl was finally wasted as CaCl2. Around 1870, chemists Henry

Deacon and Ferdinand Hurter established the first catalytic

process for large-scale Cl2 production via the gas-phase oxidation

of hydrogen chloride on CuCl2/pumice.10–13 Deacon was one of

the founders of the chemical industry through the alkali

manufacturing business Gaskell, Deacon & Co. at Widnes,

England. The Deacon process was primarily conceived to curtail

HCl emissions from the Leblanc process for production of soda

ash (Na2CO3). The resulting diluted and impure chlorine streams

were used to manufacture valuable bleaching powder.14 In 1900,

the Solvay process replaced the Leblanc process, and Cl2production was taken over by the chlor-alkali industry. In fact,

although the electrochemical production of chlorine was shown

byWilliam Cruickshank as early as 1800,2 its impact remained of

little importance until the development of suitable generators by

Siemens and by Acheson and Castner in 1892.

The catalytic HCl oxidation continued attracting industrial

interest during the 20th century, following a parallel but less

successful pathway to electrolytic methods. During 1939–1944,

IG Farben introduced a gas-phase HCl oxidation process at the

pilot scale using molten sodium (or potassium) chloride and iron

chloride salts.15 This process presented a very low space-time

yield.16 In the 1960s, Shell established the Shell-Chlor process

using a CuCl2–KCl/SiO2 catalyst in a fluidised-bed reactor.17–19

The improved stability of the catalyst compared to the original

Deacon process was a result of the molten salt formed by CuCl2and KCl and the lower operating temperature in a fluidised bed

liver Schl€uter (Cologne, Germany, 1973) earned his PhD at the

niversity of Bochum (2004). He works for Bayer Technology

ervices (BTS) since 2005, researching on heterogeneous and

hotocatalysis. LeslawMleczko (Zabrze, Poland, 1954) earned his

hD at the Silesian Technical University and worked at the Ruhr-

niversity Bochum, Germany. There, he is professor of Industrial

hemistry since 2003. In 1996, he moved to the R&D organization

f Bayer AG. He currently heads the Competence Center Reaction

ngineering & Catalysis at BTS.Aurel Wolf (Kronstadt, Romania,

970) earned his PhD at the University of Essen, Germany. Since

000, he is an employee of BTS, currently working as senior scientist

n catalyst development for basic chemicals and polymer synthesis.

hey were the core team who started the catalyst development for

Cl oxidation at Bayer in 2004.



Fig. 3 Chronological development of Cl2 production, indicating

important milestones from its discovery in 1774 to the various

manufacturing technologies developed to date. The panel underneath

summarises ‘past and present’ commercial catalytic processes for HCl

oxidation. Catalyst composition, reactor type, operating temperature

range, and current status are indicated.

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compared with a fixed bed. The Shell process was apparently

realised in a facility with a capacity of 30 kton Cl2 per year, but

the operation was eventually shut down.20General disadvantages

of the use of copper-based catalysts have been the limited HCl

conversion and the fast catalyst deactivation due to (i) volatili-

sation of the active metal species in the form of chlorides (copper

chlorides evaporate at an appreciable rate at temperatures above

673 K) and (ii) severe corrosion issues in the plant caused by

unreacted HCl in the presence of H2O (making the choice of

materials of construction extremely problematic).21 Some of


these drawbacks could be improved by conducting the chlori-

nation (453–473 K) and the oxidation (613–673 K) steps over

supported CuCl2–KCl catalysts in a circulating dual fluidised-

bed reactor system, which can be operated to attain 100% HCl

conversion.22,23 This configuration reached the pilot scale but was

not commercially demonstrated. In this context, a two-step fixed-

bed process with alternating conditions and optimized dynamic

heat transfer from the exothermic chlorination to the endo-

thermic re-oxidation was also proposed.24 In the 1960s, Kellogg

introduced the Kel-Chlor process for Cl2 manufacturing, using

nitrogen oxides (NO and NO2) as the catalyst and sulfuric acid as

a circulating medium.25 From 1975 to 1988, DuPont operated

a full-scale plant based on this process recovering up to 600 ton

Cl2 per day, which was later discontinued due to a change in the

structure of the plant and material problems.2 In 1980, Mitsui

Chemicals established the MT-Chlor process using a Cr2O3/SiO2

catalyst in a fluidised-bed reactor.26,27 Under reaction conditions,

the catalyst operates without melting. That is, the reaction takes

place using only the oxidation–reduction reaction without going

through the chloride-oxide reaction cycle of copper-based cata-

lysts. As a consequence, the stability of the catalyst was greatly

improved compared with the Shell-Chlor process. Only one plant

of 60 kton Cl2 per year was erected in Japan, which is said to be

still operative.20 Wider application of Mitsui’s catalytic tech-

nology did not occur, probably due to the environmental

concerns associated with the use of a chromium-based material.

A major breakthrough in Cl2 production via HCl oxidation has

been achieved with the application of ruthenium-based catalysts

(Fig. 3, bottom): RuO2/TiO2-rutile by Sumitomo20,28 and RuO2/

SnO2-cassiterite by Bayer.29–32 Unlike previous industrial cata-

lysts, ruthenium-based materials exhibit a very high activity at

low temperatures and remarkable longevity, since ruthenium

oxide is very stable against (bulk) chlorination (vide infra).

Sumitomo licensed a plant with a capacity of 100 kton per year to

a Japanese chemical manufacturer in 2002, followed by three

additional plants worldwide.20 Bayer’s technology has been

successfully piloted and is ready for application in large-scale

chlorine recycling facilities. In broader terms, ruthenium-based

catalysts satisfy a long-standing industrial need to count on solid

alternatives to electrolysis to manufacture Cl2.

Chlorine recycling

Hydrochloric acid is produced as a by-product in many organic

processes, in particular those using phosgene as a carbonylating

agent. The most representative example comprises the produc-

tion of isocyanates, which are key precursors in the manufacture

of polyurethanes (PU). In the phosgenation step, 4 mol HCl are

produced per mol TDI (Fig. 4). An equivalent scheme applies to

other polyurethane precursors such as methylene diisocyanate

(MDI). Polycarbonate (PC) production is another prominent

source of HCl, formed in the phosgenation of bisphenol A. PU

and PC currently stand at the forefront of synthetic plastics

procuring various market segments with the required products.

The global demand for these products has increased steadily over

the past years (annual growth rate of 5% for PU and 6–7% for

PC), and prospects indicate an even more pronounced growth,

particularly in China. Therefore, the increased Cl2 demand to

produce phosgene is a fact.



Fig. 5 Integration of catalytic and electrolytic technologies for high-

purity Cl2 production from HCl. This configuration is attractive for large

capacity expansions of phosgene-based processes.

Fig. 6 Typical sequence of activities in a catalyst development program.

Fig. 4 Simplified flowsheet of toluene diisocyanate (TDI) production,

exemplifying the concept of ‘chlorine recycling’. TDI is a key precursor

for the manufacture of polyurethanes. In the phosgenation of toluene

diamine (TDA), 4 mol HCl are produced per mol TDI. Catalytic HCl

oxidation and/or ODC-based HCl electrolysis are attractive options for

Cl2 recovery.

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The by-product HCl can be marketed for its use as a raw

material in the production of 1,2-dichloroethane (EDC) by the

CuCl2-catalysed oxychlorination of ethylene. EDC is primarily

used to produce a vinyl chloride monomer (VCM, chloroethene).

However, the EDC/VCM demand for PVC production grows at

a slower pace than that for PU and PC. Therefore, the increasing

HCl excess cannot be absorbed by the PVC business. The option

of neutralising the excess HCl is unattractive for obvious reasons.

Therefore, if selling HCl is not feasible, an intelligent way of

valorising HCl is by means of recycling strategies.

Technologies to convert HCl to Cl2 have existed for decades,

but have been greatly refined in recent years. Particularly, two

routes can be effectively applied for producing high-purity

chlorine from by-product HCl: (i) catalytic oxidation and (ii)

electrolysis.20,33 The electrolysis of aqueous hydrochloric acid

developed by the former Hoechst, Bayer, and Uhde operates in

multiple plants around the world since its commercialisation in

the 1960s.2 An aqueous 22%HCl solution is converted by electric

current on graphite electrodes separated by a diaphragm into Cl2(anode) and H2 (cathode). The oxygen-depolarised cathode

(ODC) technology,3,34 jointly developed by Bayer andUhdeNora

in the 1990s, lowers the power consumption of the conventional

diaphragm electrolysis process by up to 30%. The voltage

required to overcome electrochemical polarisation is reduced by

replacing hydrogen-evolving cathodes proton recombination

with oxygen-depolarised cathodes. In the latter cell, an aqueous

solution of 14% HCl is fed to a dimensionally stable anode

(DSA), where the chloride ions are oxidised to gaseous Cl2.

Protons pass through an ion-exchange membrane and combine

with O2 and electrons to form water on the oxygen-depolarised

cathode. Bayer MaterialScience commercially demonstrated the

ODC-based HCl electrolysis by starting up two units at its

Brunsb€uttel site in Germany, each with a capacity of 10 kton Cl2per year. In 2008, a commercial plant with a production capacity

of 215 kton Cl2 per year was installed at Bayer MaterialScience,

Caojing site, China. The HCl gas-phase electrolysis process

developed by DuPont33,35 is described to have lower capital and

operating costs relative to the diaphragm electrolysis process and


a pilot unit (3-cell stack with 2 m2 active area) was started up in

1995.

Because of its unbeatably low power consumption,20 at a first

glance catalytic HCl oxidation outperforms HCl electrolysis by

far. This is certainly true in terms of operating costs. However,

the complex process cycle (vide infra), obligatory safety

measures, and generally expensive construction materials result

in high specific investment costs (per ton of chlorine) for new

small gas-phase catalytic HCl oxidation plants. The capital

investment can be compensated by maximising the single line

capacity of the whole process cycle, which is commonly referred

to as the economy of scale. Due to their modular design, the

investment costs of electrolysis units are less scale driven.

Consequently, optimised ODC-based HCl electrolysis is a suit-

able technology for small capacity expansions of phosgene-based

processes and to complement the catalytic route by converting

unreacted HCl from the equilibrium-limited Deacon reaction

(Fig. 5).

New generation of Deacon processes

In this section, key results related to the attainment of a catalytic

process for HCl oxidation to Cl2 are reviewed. The ruthenium-

based catalysts collected in the lower panels of Fig. 3 are in focus.

Catalyst development comprises a sequence of interactive tasks

that ultimately leads to the implementation of an optimised

formulation (Fig. 6).36,37 In the primary screening stage, realisa-

tion of concepts or ideas generated in response to given targets

leads to the lab-synthesis of catalysts, whose performance is

screened in miniaturised reactor systems. During secondary

screening, the catalysts are tested in a more quantitative manner



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and are characterised in more depth. Insights into the reaction

mechanism, kinetics, nature of the active sites, and long-term

stability are obtained. Both primary and secondary screening

activities are encircled at the microlevel. At the mesolevel,

promising catalyst formulations are scaled up. The synthesis

recipe is translated from the gram scale to the kilogram scale in

order to conduct pilot tests over catalyst bodies under relevant

process conditions. In addition to optimal catalyst geometries,

important considerations at the reactor level, such as mass and

heat-transfer phenomena and fluid-dynamics, come into play.

The final verdict for implementation of the catalyst is formed

after extensive pilot and plant testing and successful process

design, which comprises the macrolevel.

Catalyst screening

The gas-phase oxidation of hydrogen chloride (2HCl + 0.5O2 4

Cl2 + H2O) is an equilibrium-limited and exothermic reaction,

DH0 ¼�57 kJ per mol Cl2. The equilibrium conversion of HCl is

favoured at low temperature, high feed O2/HCl ratio, and high

total pressure. Catalysts with high activity at low temperatures

(<700 K) are pursued not only for thermodynamic reasons, but

also for stability purposes. At higher temperatures, volatilisation

of the active phase occurs in the form of metal chlorides. Oxides

and, less frequently, chlorides of a number of metals (Cu, Cr,

Mn, Ce, Fe, Ni, Ru, Au, Bi, etc.) have been claimed for the

Deacon reaction. Reviewing all catalytic systems would be

tedious. In our screening program, ca. 200 samples were evalu-

ated, including bulk and supported catalysts. The lab-scale

testing unit consists of (i) a feed section with mass flow

controllers, (ii) a continuous-flow fixed-bed quartz micro-reactor

(8 mm i.d.) heated in an oven, and (iii) an analysis section.38–40

The set-up is equivalent to any conventional continuous-flow

reactor system used in any gas-phase catalysed reaction. Teflon�was used for the tubing to prevent corrosion problems, which can

be particularly prominent downstream of the reactor due to the

presence of unreacted HCl and produced H2O. The most rele-

vant testing variables are the feed O2 : HCl ratio (0–10), the

temperature (373–723 K), and the space velocity (1000–

20 000 h�1), that is, the ratio of total gas flow to catalyst volume.

Particle fractions in the range of 0.1–0.5 mm were used. The

Deacon activity was assessed by two protocols: temperature-

programmed reaction (TPR) and isothermal (steady state)

testing. TPR is performed by ramping the temperature and

monitoring on-line the Cl2 production with a miniature fibre

optic spectrophotometer (absorbance at 330 nm). This dynamic

method is suitable for ranking catalytic activity on the basis of

the temperature at which Cl2 evolution begins.38 An example is

shown in Fig. 7a, which compares the Cl2 evolution profiles of

representative Ru- and Cu-based samples. The onset tempera-

ture of the ruthenium catalyst is 100 K lower. The superior HCl

oxidation performance of bulk RuO2 with respect to bulk metal

oxides was quantified by means of isothermal tests using iodo-

metric titration for Cl2 analysis (Fig. 7b).

Once the suitable active phase is identified, further develop-

ment in the formulation is needed to minimise metal loading and

maximise the catalyst specific activity, expressed as mol Cl2 per

hour and mol Ru. Accordingly, the ruthenium precursor was

deposited by incipient wetness impregnation on supports with


different chemical nature and textural properties. The choice of

the carrier for ruthenium is crucial to obtain a superior Deacon

catalyst. Sumitomo reported that RuO2/TiO2-rutile is at least 10

times more active than RuO2 on traditionally applied supports

and 50 times more active than Cr, Cu, Fe, Mn, or Ni-based

systems (Fig. 8a).20 Ru-based catalysts for the Deacon process

using SiO241 and a-Al2O3

42 supports have been claimed. Shell

introduced SiO2-supported RuCl3 as a Deacon catalyst.41

However, pilot trials and industrial application were not repor-

ted. Our screening program identified ruthenium-containing

samples as the most active Deacon catalysts (Fig. 7c).43 The key

feature shared by the Sumitomo and the Bayer catalysts is the use

of carriers for ruthenium oxide having a rutile-type structure (see

next section) with similar lattice parameters. The attainment of

active Deacon catalysts in relatively high-throughput lab-tests is

only the tip of the iceberg towards the identification of a promi-

sing system. The assessment of stability is critical to optimally

selecting the formulations to be scaled up at the mesolevel. Severe

deactivation was commonly observed in the first 100 h on stream

over the majority of the samples regarded as attractive on the

basis of the initial activity. An example is shown for CuO/Al2O3

in Fig. 7d and relates to extensive copper volatilisation. In

addition to metal loss, other undesirable situations are experi-

enced such as bed coagulation. This phenomenon causes an

excessive pressure drop and safety-wise requires the immediate

shutdown of the catalytic run. The absence of operational issues

in the long term is a prerequisite for large-scale implementation

of any catalytic system. For this purpose, formulations that were

stable for several hundred hours in lab-scale tests were further

considered. The stringent longevity requirements were fulfilled

by some ruthenium-containing catalysts (Fig. 7d). Accelerated

deactivation tests at relatively high temperatures revealed that

RuO2/SnO2 lost up to 75% of the initial activity after 50 h on

stream, while RuO2/SnO2–Al2O3 displayed stable performance

for several thousand hours (Fig. 8c). The stabilising effect of

alumina in the Bayer catalyst is fulfilled by silica in the Sumitomo

catalyst (Fig. 8d).20 As elaborated in the next section, these

additives act as sintering-prevention agents.

Catalyst characterisation

This section summarises detailed characterisation studies per-

formed on RuO2-based materials to rationalise the effect of the

nature of the oxide employed as a carrier, to elucidate the

characteristics of the deactivation phenomenon, and to ascertain

the role of alumina or silica as additives on minimising the latter.

In order to understand the origin of the differences in catalytic

behaviour in dependence on the support employed, catalysts

were studied by transmission electron microscopy (TEM).20,43,44

RuO2 on TiO2 is shown to adopt different morphologies

depending on the crystal structure of the support (Fig. 8a). RuO2

on TiO2-anatase appears in the form of well-distributed nano-

particles, while RuO2 on TiO2-rutile features a thin film coating

the carrier. As the RuO2 phase possesses the rutile structure, the

latter morphology can be generated via epitaxial growth only

onto TiO2-rutile due to lattice matching. High-temperature

treatments of the rutile-type carrier prior to impregnation of the

ruthenium precursor are often employed in order to favour the

formation of the rutile structure at the surface level. Investigation



Fig. 7 Lab-scale evaluation of HCl oxidation catalysts in a continuous-flow reactor at ambient pressure. (a) Catalyst screening by temperature-pro-

grammed reaction.38 (b) Isothermal testing over various bulk metal oxides.40 (c) Dependence of HCl conversion on temperature during steady-state tests.

(d) Long-term isothermal testing is essential to realistically assess the stability of Deacon catalysts. Typical Cu-based catalysts suffer from severe

deactivation due to volatilisation of the active phase under reaction conditions. Decolouration of the catalyst bed can be seen in the photographs of the

reactor before and after the test (inset). RuO2/SnO2–Al2O3 displays an extraordinary stability.43

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by X-ray absorption spectroscopy (XAS) indicated that the film

was composed of two-dimensional RuO2 crystallites of about

1 nm in size, while particles of a bigger size are formed when

using non-pre-treated TiO2. High-resolution TEM of RuO2/

SnO2-cassiterite indicated that the support particles are

uniformly coated by RuO2 nanostructures (Fig. 8b), similar to

the RuO2/TiO2-rutile case. Higher magnification of different

surface regions revealed that the RuO2 phase is present on SnO2

both as protruding nanoparticles, exhibiting the typical planes

associated with the rutile structure, and as a thin layer grown

with the same orientation as the support. On other standard

carriers, like SiO2 and Al2O3, RuO2 would be exhibited as

nanoparticles as in the case of TiO2-anatase.

The synthetic strategy of lattice matching produces highly

active catalysts, as the film-like RuO2 morphology maximises

metal dispersion. Additionally, the intimate contact of RuO2

with the support improves structural stability, producing durable

catalysts. Still, as mentioned in the previous section, the presence

of additives is required in view of their long-term use. The origin

of the deactivation behaviour is traced back to RuO2 sintering

for both the Bayer and Sumitomo catalysts. Added alumina and

silica play a key stabilising role. In the case of RuO2/SnO2-

cassiterite, dramatic agglomeration of the RuO2 phase into bulky

particles (average size ca. 20 nm), partially losing contact with

the support, was evidenced by HRTEM analysis upon equili-

bration (denoted by ‘e’ in Fig. 8c). In contrast, grains of RuO2/

SnO2 resembling those observed for the alumina-free fresh

catalyst and nano-sized alumina filling the interstitial spaces are

detected for RuO2/SnO2–Al2O3, with this morphology being

substantially unmodified after reaction for 1500 h. On the basis

of the knowledge on the mechanistic aspects of HCl oxidation

over RuO2 (next section), the sintering process was suggested to

be driven by the elimination of water by interaction of two

hydroxyl-ruthenium (Ru-OH) species formed during the reac-

tion. The presence of alumina in between neighbouring particles

provides a mean to address the inter-particle contact, which thus

seems to dominate the sintering scenario in its absence.43

In the case of the Sumitomo catalyst, characterisation by XAS

of the RuO2 crystallites size at increasing operating times

provides profiles for the growth along the a, b, and c axes

(Fig. 8d) indicating sintering and that this process namely has

a two-dimensional character. Accordingly, the RuO2 phase

agglomeration in RuO2/TiO2-rutile during the catalytic run is


described on the basis of crystallite migration, preferentially

occurring within the same support grain (intra-particle). The

addition of SiO2 of ca. 1 wt% to the catalyst formulation, referred

to as nano-sized sintering blocking material, aims at separating

adjacent nanoparticles on the same support grain. The inset of

Fig. 8d represents a schematic illustration of the expected

placement of SiO2 in between the individual RuO2 nano-

structures. The profiles obtained for the crystallite size versus

operating time for RuO2/SiO2/TiO2-rutile proved that recon-

struction along the a, b-axes is drastically reduced.20

Reaction mechanism and kinetics

The description of the microlevel is completed by surveying the

mechanistic and kinetic investigations of HCl oxidation on

RuO2-based materials. Studies have been performed extensively

and with a variety of techniques, including detailed analysis and

theoretical calculations of well-defined model RuO2 surfaces

under ultra-high vacuum (UHV) conditions, as well as of poly-

crystalline RuO2 samples at pressures more relevant to real

catalysis, thus bridging the material and pressure gaps.

The most evident feature that distinguishes RuO2 from other

metal oxides employed for HCl oxidation, such as CuO, is the

preservation of its bulk structure upon contact with the aggres-

sive Deacon mixture, as shown by X-ray diffraction analysis

(Fig. 9a).45 Excessive chlorination typically leads to metal loss by

volatilisation.39 Nevertheless, X-ray photoelectron spectroscopy

analysis indicates that chlorine is present at the surface level in

RuO2 after the reaction.45 The surface chlorination process was

described by Over et al.46,47 based on UHV studies on RuO2(110).

RuO2(110) is typically used as a model catalyst, since it is the

energetically most stable surface. The termination of RuO2(110)

is depicted as an alternation of rows of coordinatively unsatu-

rated Ru atoms (Rucus) and protruding rows of O atoms in the

bridging position (Obr) (Fig. 9b, stick-and-ball model). High-

resolution core-level shift (HRCLS) spectroscopy evidenced that,

upon exposure of this surface to HCl and subsequent annealing,

selective replacement of Obr atoms by bridging Cl atoms (Clbr)

occurs. The chlorination process appears to be self-limiting, as it

terminates when most of the Obr atoms are replaced. The thus

formed surface is indicated as RuO2�xClx(110).46 Ab initio ther-

modynamics,45 predicting that the initial state of RuO2(110) is

partially over-oxidised, while the surface after the reaction



Fig. 8 Characterisation studies of supported RuO2 catalysts. (a) Influence of the carrier on the catalytic activity.20 (b) Morphology of RuO2/SnO2

studied by TEM.43,44 (c) Studies of the deactivation mechanism of RuO2/SnO2 and of the stabilising effect of the Al2O3 binder.43 (d) Details of the

sintering process of RuO2/TiO2 and development of a durable catalyst using SiO2 as the stabiliser.20

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contains both oxygen and chlorine, supports the results of UHV

investigations. The self-limiting nature of chlorination offers

a molecular level explanation of the remarkable stability

observed for bulk RuO2 in the Deacon reaction.46 The chlori-

nation degree was quantitatively determined for a polycrystalline

RuO2 sample in the Temporal Analysis of Products (TAP)

reactor (Fig. 9c, individual HCl transient over time as the inset).38

The chlorine uptake of RuO2 results are equivalent to a molar

Cl/Rusurface ratio of 0.74, i.e. ca. 75% of the total surface Ru sites

(cus + bridge) are chlorinated.

HRCLS spectroscopy and temperature programmed desorp-

tion (TPD) experiments showed that the RuO2�xClx(110) surface

is able to catalyse the oxidation of HCl releasing Cl2 and H2O


when exposed to both HCl and O2 and then heated above 600 K

(Fig. 9d).46 RuO2�xClx, ruthenium oxychloride, is thus regarded

as the actual catalytically active phase. Furthermore, the Deacon

reaction is proposed to proceed according to a Langmuir–Hin-

shelwood scheme along the rows of Rucus.47 The proposed

catalytic cycle, illustrating the elementary steps and the relative

energy requirements, is schematically represented in Fig. 9b.

Density functional theory (DFT) analysis of the reaction on

RuO2(110) by L�opez et al.45 indicated that the reaction follows

a mechanism composed of five simplified steps, namely,

hydrogen abstraction from hydrogen chloride by adsorbed

atomic oxygen and adsorption of the chlorine atom, recombi-

nation of surface chlorine atoms and desorption as gas-phase



Fig. 9 Mechanistic studies of HCl oxidation over RuO2-based catalysts. In some cases, comparison with other metal oxides is established. (a) XRD

patterns of bulk RuO2 and CuO prior to and after the Deacon test.39,45 (b) Reaction scheme derived from surface science techniques and DFT simu-

lations on RuO2(110).45–47 (c) Chlorination profile of polycrystalline of RuO2 determined by controlled doses of HCl at 573 K in the TAP reactor.38 (d)

TPD after co-adsorption of O2 and HCl on a chlorinated RuO2�xClx(110) surface. Analogous experiment with no oxygen supply is also displayed.47 (e)

Dependence of the energy of Cl2 desorption on the coverage by X (Cl, O) species.45 (f) Cl2 production on polycrystalline RuO2 as a function of the time

delay between O2 and HCl pulses (pump-probe experiments) in the TAP reactor.38 (g) Volcano dependence between the calculated turnover frequency in

HCl oxidation over different surfaces and the energy of oxygen dissociation.49 (h) Relationship between the space time yield in ambient-pressure

continuous-flow tests over different bulk oxides and a characteristic parameter derived from transient experiments in the TAP reactor.40 (i) Relative

reaction rate in HCl oxidation as a function of the feed O2/HCl ratio and the resulting degree of surface chlorination in RuO2/SnO2–Al2O3, which was

determined under reaction conditions by PGAA.44 (j) Dependence of the reaction rate on the partial pressure of reactants and products on RuO2/SnO2–

Al2O3 from steady-state tests at 1 bar.44 (b), (d), and (h) Reprinted with permission from the American Chemical Society. (c), (e), and (f) Reprinted with

permission from Elsevier. (g) Reprinted with permission from Wiley-VCH Verlag.

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Cl2, recombination of surface hydroxyl groups, water desorp-

tion, and dissociative oxygen adsorption for surface regeneration

(Fig. 9b). According to temperature-programmed experiments

(Fig. 9d), chlorine evolution appeared more difficult than water


evolution and was observed to be totally dependent on the

presence of gas-phase O2. The energy values calculated for the

reaction steps supported that the recombination of chlorine

atoms to gas-phase Cl2 is the most demanding step, being



Fig. 10 Simplified variants for the preparation of a technical ruthenium-

based Deacon catalyst.

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favoured at higher oxygen and chlorine coverage (Fig. 9e).45,47

In situ studies of HCl oxidation over RuO2(110) and RuO2(100)

model surfaces showed that similar turnover frequencies were

attained for the two catalysts, thus concluding that the reaction is

structure insensitive.48 DFT calculations revealed that the

intrinsic activity of the (110) and (100) surfaces is actually

different. The experimental outcome was rationalised on the

basis of the ability of the (100) surface to reconstruct into the

more energetically stable (110) surface under reaction

conditions.44

Experiments conducted on bulk RuO2 in the TAP reactor

separating oxidation and chlorination confirm the tight depen-

dence of the net Cl2 production on both oxygen (Fig. 9f) and

chlorine coverage, thus concluding that the reaction mechanism

for HCl oxidation on a polycrystalline sample predominantly

follows a Langmuir–Hinshelwood scheme. A limited participa-

tion of lattice surface species is detected at very low coverage,

thus indicating that the impact of a 2D Mars-van Krevelen

contribution is actually minor.38 It is worth noting that these

mechanistic fingerprints are substantially preserved when sup-

porting RuO2 on a carrier.38,44 Still, DFT simulations indicated

that the activity of supported RuO2 is dependent on the number

of layers coating the carrier. When SnO2 is used as the support,

an epitaxially grown RuO2 monolayer is found to be less reactive

than pure RuO2 (110), while two layers result in considerably

more activity.44 Accordingly, a good strategy could be to create

a bilayer of ruthenium oxide in order to maximise catalyst

activity.

RuO2 has been compared with other metal oxides to gain

insights into the mechanistic aspects accounting for its superior

performance. A combined DFT and micro-kinetic model deve-

loped by Bligaard et al.49 for screening purposes, which consid-

ered chlorine recombination as well as molecular oxygen

dissociation as rate determining steps, identifies RuO2 as the best

performing catalyst (Fig. 9g). This result highlights the ease of re-

oxidation of this catalyst as another characteristic beneficial

feature. Still the model predicts that the optimal conditions have

not been reached, thus even better catalysts may be identified in

the future. According to this work, substitution of bridged

oxygen by chlorine slightly diminishes the catalytic performance

of the material. The formation of a chlorinated phase during the

reaction is, however, inevitable. Higher temperatures cause an

increase in activity (inset a), and chlorine production from

chlorinated RuO2 surfaces is found to take off at about 500 K

(inset b), in good agreement with the TPR data reviewed above

(Fig. 7a). Very recently, structure–mechanism relationships have

been established (Fig. 9h), categorising the Deacon activity by

means of mechanistic descriptors derived from TAP studies and

related to chlorine recombination, chlorine evolution, and cata-

lyst re-oxidation.40 According to this classification, the superior

performance of RuO2 is explained by the combination of the

three crucial features for an optimal catalyst: limited surface

chlorination favouring long-term stability, fast Cl2 evolution,

and relatively easy re-oxidation, allowing low-temperature

operation.

In view of a suitable reactor design, kinetic investigations have

been performed on catalysts of practical interest, i.e., supported

RuO2-based catalysts. Prompt gamma activation analysis

(PGAA) of RuO2/SnO2 during the reaction indicated that the


reaction rate decreases, when increasing the amount of chlorine

on the catalyst surface (by increasing the feed HCl : O2 ratio)

(Fig. 9i).44 Steady-state kinetic tests on RuO2/SnO2–Al2O3

showed that the dependences of the reaction rate on reactants

(HCl and O2) were positive, while products strongly inhibited the

reaction (Fig. 9j).

Development of a technical Deacon catalyst

Technical catalysts must not only faithfully reproduce the

performance of laboratory preparations but must also have the

required mechanical and chemical stability to ensure smooth

operation and a long lifetime in an industrial reactor. Catalyst

particles with a low mechanical strength tend to produce fines,

eventually causing plant shut down due to increased pressure

drop. Turning a promising candidate identified during initial

screening into an industrial catalyst implies (i) adaptation of

laboratory protocols for its multi-ton manufacture and (ii)

shaping of the catalyst powder into practical millimetre-sized

bodies. For the latter task, forming agents such as binders and

other additives are often required.

As detailed in previous sections, one of the most successful

catalysts for HCl oxidation comprises cassiterite as the carrier

and ruthenium oxide/oxychloride as the active phase. The

preparation of such a catalyst is detailed elsewhere.29,43 Among

the various procedural steps, two points of practical nature

should be stressed. First of all, the as-received SnO2 powder must

be pre-calcined above 973 K to ensure the formation of the

cassiterite structure at the surface level, thereby favouring the

epitaxial growth of RuO2 over the entire support surface.

Secondly, the calcination temperature of the as impregnated

catalyst should be relatively low (<523 K) to minimise sintering

of the active phase, thus achieving a higher dispersion.

Fig. 10 depicts two strategies to prepare a technical RuO2/

SnO2 catalyst, which differ in the order between the incorpora-

tion of the ruthenium precursor by dry impregnation and the

catalyst shaping. In the scale up of a supported catalyst, the

shaping could be done after impregnation of the support with

the active phase precursor (method 1). This variant is not suited

for ruthenium-based Deacon catalysts due to the extensive

sintering of RuO2 and/or the loss of volatile RuO4 occurring at

the high temperatures required for hardening of the pelletised

catalysts during the forming step. These drawbacks must be

avoided since an optimal utilisation requires maximised disper-

sion and no loss of the expensive ruthenium phase. Accordingly,

the aqueous ruthenium precursor is preferably impregnated on



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a SnO2 carrier that has been pre-formed as spheres or extrudates

with the aid of a suitable binder (method 2). This method is

efficient if the selected binder does not influence the impregnation

step, that is, if the deposition selectively occurs on the supporting

SnO2 and not on the (typically high-surface area) binder parti-

cles. The relative surface properties of the binder and support

phases play a crucial role in this respect. For example, greater

selective impregnation of the SnO2 carrier is observed on use of

an acidic Al2O3 binder. Similar results were obtained using SiO2

as the binder. However, a basic clay-like binder, such as kaolin,

unselectively captures a significant fraction of the impregnated

metal. The metal distribution using both binders was determined

by high-resolution transmission electron microscopy coupled to

energy-dispersive X-ray analysis. Kaolin-supported ruthenium

species are suboptimal for HCl oxidation, since the strong

interaction between RuO2 and SnO2 is crucial to guarantee the

remarkable activity and stability of the catalyst. The choice of the

appropriate binder is important (surface chemistry and pore

structure), but also the forming procedure and the catalyst

geometry should be adequately selected. Fig. 11 shows the

mechanical stability of various technical catalysts as a function of

different parameters such as type and content of binder, the SnO2

grade by different suppliers, and the catalyst geometry. The

mechanical stability increases with increasing binder content and

is higher for alumina in comparison to silica. Furthermore,

spherical granules are significantly mechanically more stable

than extrudates. The tin oxide grade of different suppliers plays

a minor role. Spherical granules (2 mm) of RuO2/SnO2/Al2O3

catalyst containing 2 wt% Ru and 10 wt% g-Al2O3 were selected

for the pilot studies shown in Fig. 7d. The Cl2 production over

this catalyst, evaluated in a three-zone adiabatic reactor, has

been stable for more than 12 000 hours on stream. As discussed

in the characterisation section, alumina does not only fulfil

a standard binder function, but also acts as a stabilising agent,

impeding the most highly detrimental inter-particle RuO2

agglomeration.

Reactor design

Due to the high exothermicity of HCl oxidation, temperature

control is the main challenge at the reactor scale. High temper-

atures promote catalyst deactivation due to the formation of

volatile metal (oxy)chlorides and active phase sintering. Heat

Fig. 11 Mechanical strength by attrition and side crush strength tests on

RuO2/SnO2 catalysts with different type and amount of binder, SnO2

supplier, shaping method, and particle geometry.


removal therefore is a crucial point. As the investment for heat

exchangers is very high under the highly corrosive Deacon

conditions, it is important to minimise the heat exchange area.

The alternative heat-exchange designs are dictated by the reactor

type. Generally, two reactor types have been extensively studied:

fixed and fluidised beds. An attempt to perform the reaction in

bubble columns using molten salt catalysts16 was hampered by

serious corrosion issues and the process was never scaled up.

The first industrial application was realised by Shell in a flui-

dised-bed reactor using a Cu-based catalyst operated in the

bubbling-bed regime.50 Reactor temperature was controlled by

an immersed heat exchanger. A similar concept was applied in

the MT-Chlor process by Mitsui over a Cr-based catalyst. The

principal advantage of a fluidised-bed reactor is the highly effi-

cient heat exchange between the bed and the immersed heat

exchangers. A very good mixing of solids results in isothermal

conditions. However, the catalyst must be mechanically stable.

RuO2/a-Al2O3 designed by BASF can be mentioned as an

example of the development of novel catalysts for fluidised-bed

applications.42 In a fluidised bed, attrition cannot be completely

eliminated, and therefore the reactor has to be equipped with

space and cyclones for fines separation, leading to increased

complexity and size. A good catalyst flowability in a fluidised bed

has been utilised in a two-stage reactor concept developed by the

University of Southern California.22,23 In this design, the overall

reaction is separated into two stages: chlorination (CuO + 2HCl

/ CuCl2 + H2O, operated at 423–493 K) and oxidation (CuCl2+ 1/2O2 / CuO + Cl2, operated at 573–673 K). The two-stage

reactor operated in the riser regime was piloted, but never

commercialised mainly due to corrosion problems encountered

in the complex solids handling system. However, the idea was

further developed by the Tsinghua University in China, who

applied a stationary fluidised-bed with internal solid circulation

in order to minimise mechanical complexity. This design was

subsequently scaled up and piloted.51

The development of robust ruthenium-based catalysts opened

the way for application of fixed-bed reactors. Sumitomo

patented a multi-tubular fixed-bed reactor (Fig. 12a). The

industrial RuO2/TiO2-rutile catalyst, which possesses a high

thermal conductivity, is placed in parallel tubes which are cooled

by a circulating heat transfer salt (HTS).20 Themain contributory

effort is set on the heat management and the temperature

control. Although the reaction is thermodynamically limited, hot

spots can be produced in the low-conversion ratio region of the

tubes.52 In order to overcome this problem, the reactor is filled

with a catalyst with staged activity. In the entrance zone,

a catalyst of reduced activity is applied, while in the zone where

conversion is already high, a catalyst of higher activity is used. In

addition, the cooling side is zoned by partitioning from the shell

side of the reactor. Using subtle reactor segmentation measures,

regarding catalyst activity and cooling control by the HTS, flat

temperature profiles can be obtained. Furthermore, despite the

occurrence of catalyst deactivation, it is possible to retain

constant conversion by selectively increasing the temperature in

the corresponding zones.52 This reactor design is elegant

although it is of high technical and operational complexity.

A fixed-bed reactor was also developed by Bayer, using the

alternative concept of a multi-stage adiabatic reactor (Fig. 12b).

The main advantage of this configuration is the simplicity of the



Fig. 13 Temperature profile in an adiabatic cascade of fixed-bed reac-

tors with intermediate heat exchanger and HCl dosing, as shown in

Fig. 12b. In this example,53 the overall HCl conversion was 82.4%.Fig. 12 Fixed-bed reactor concepts for large-scale HCl oxidation over

ruthenium-based catalysts. (a) Sumitomo’s multi-tubular reactor with

heat transfer salt (HTS) bath and (b) Bayer’s adiabatic reactor cascade

with intermediate heat exchangers.

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modular reactor design, which in turn, has important implica-

tions on the investment cost. Since an adiabatic reactor is easy to

scale up, the development time and costs can be reduced. Cata-

lyst loading and replacement are simple steps with this reactor in

comparison to a multi-tubular reactor. Separation of reaction

and heat removal steps permits the independent optimisation of

each process. The rise in temperature at each stage can be

controlled by appropriate distribution of the HCl feed (Fig. 13).

Since the heat exchange takes place at temperatures and gas

velocities that are significantly higher than those encountered in

a fixed bed, high rates of heat flow exchange are achievable. This,

in turn, allows for a reduction in volume of the expensive heat

exchangers. Furthermore, the heat can be directly removed with

water cooling, i.e., expensive molten salt loops are not necessary.

Both Sumitomo and Bayer reactor concepts are energetically

efficient as the reaction heat is recovered as steam by a heat

recovery boiler.

Fig. 14 Simplified flowsheet of the HCl oxidation work-up process. The

sequence of unit operations remains practically unchanged since the

1930s. However, module optimisation has been constantly undertaken.

Process design

The eldest complete Deacon process flowsheet was introduced by

IG Farben in the early decades of the 20th century, the so-called

Oppauer Deacon.15 The sequence of unit operations, shown in

Fig. 14, is still valid for modern plants. A number of patent

applications on the Deacon process have been filed in the past

two decades, pinpointing the coupling with other processes,

different module options, and/or module optimisation.54–56 An

account of the process steps is briefly summarised below.

HCl purification. Typically, residual amounts of solvents from

the isocyanate production (e.g., chlorobenzene and chloro-

toluene) and phosgene have to be separated from the gaseous

HCl feed by deep freezing, partial condensation,57 adsorption on

active carbon,58 or combinations thereof.59 Even extraction by

ionic liquids has been considered.60 There are also protocols

describing the removal of CO and low boiling sulfur compounds

in a guard bed by oxidation,61,62 or more elaborately by total HCl


condensation.63 CO favours hotspot formation in the Deacon

catalyst due to the 10-times higher exothermicity of CO oxida-

tion with respect to that of HCl, while sulfur compounds poison

the ruthenium catalysts.64

Reaction. To compensate for the consumption of oxygen and

for its loss in the gaseous purge, fresh O2 has to be supplied to the

reaction. Since a higher partial oxygen pressure and a higher

absolute pressure are beneficial to attain high equilibrium HCl

conversions, the reaction is typically carried out under (elevated)

oxygen excess and a gauge pressure of several bars in a fixed or

fluidised bed.65–67 The different reaction technologies and typical

modern ruthenium catalysts were described in previous sections.

HCl/H2O separation. In the first work-up step, unreacted HCl

and the water produced are removed from the process gas

producing a hydrochloric acid solution,68 which can be sold,

subjected to an azeotropic distillation to recover gaseous HCl,69

or converted into Cl2 in an HCl-electrolysis unit.34 There is no

technically applicable alternative to avoid hydrochloric acid by-

production in HCl oxidation.



Fig. 15 Evolution of the market price of ruthenium. Data retrieved from

ref. 82.

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Drying. Residual dew point water must be removed from the

process gas to avoid corrosion of down-stream construction

materials by established concentrated sulfuric acid technology.70

Cl2 separation. The simplest method to separate chlorine from

the process gas is by exploiting the boiling point differences,

applying high-pressure and/or low temperature liquefaction,71 or

distillation.72 Extraction by dichloromethane73 or gas perme-

ation74 has also been developed.

Recycle/off-gas. The use of pure oxygen, instead of air, favours

small apparatus volumes and enables the recycling of the process

gas after chlorine separation.75 A recycle-gas washer can serve as

a guard against poisoning of the ruthenium catalyst by sulfur

compounds from the drying step (SOx).76 The build-up of inert

compound (N2, CO2, Ar) has to be controlled by a gaseous

purge. Residual chlorine can be removed from the gaseous purge

by washing with a caustic soda solution77 or other oxidative

solutions.78 Control concepts for the whole loop,79 heat inte-

gration options80 and construction materials for all operation

steps81 are available.

Conclusions and outlook

The fast-growing demand for polyurethanes and polycarbonates

originates a concomitant generation of larger amounts of HCl as

by-product, due to the use of phosgene as a carbonylating agent

in the manufacturing chain of these chlorine-free plastics.

Although phosgene-free routes are being actively sought e.g., in

isocyanate production, phosgene-based technology is so far

unrivalled. These HCl-generating processes grow at a much

faster pace than HCl-consuming processes, e.g., 1,2-dichloro-

ethane production, and the increasing net excess requires

a sustainable solution. The heterogeneously catalysed oxidation

of HCl to Cl2 comprises an energy-efficient route for chlorine

recycling in the chemical industry. After many decades of limited

success, a new generation of industrially viable catalytic

processes are now available. Hurdles to overcome were associ-

ated with the development of sufficiently active and durable

catalysts, suitable reactors, and resistant construction materials.

RuO2 catalysts supported on rutile-type carriers (TiO2, SnO2)

used in fixed-bed reactors exhibit high activity and a long lifetime

in HCl oxidation. As such, they are expected to be the flagship of


the Deacon process for the industry in the next decade. The

combination of fundamental and applied research proved

essential for the successful development of this robust tech-

nology. Due to its very low power consumption, catalytic HCl

oxidation outperforms HCl electrolysis in terms of operating

costs. In contrast, optimised ODC-based HCl electrolysis

outperforms catalytic HCl oxidation in terms of specific invest-

ment costs per ton of chlorine, particularly, if small-to-medium

size capacities are considered. Consequently, both technologies

are highly complementary and contribute to sustainable chlorine

recovery in the chemical industry (Fig. 5).

The most important drawback of ruthenium is its high and

fluctuating market price (Fig. 15). The catalyst cost associated

with a world-scaleHCl oxidation plant equippedwith ruthenium-

based materials amounts to several million euros, clearly indi-

cating that the specific activity of ruthenium is the steering

parameter for further optimisation. Beyond that, progress in

recycling of the spent ruthenium catalyst is of eminent impor-

tance. An overall ruthenium recovery rate of up to 85% by

a sophisticated wet chemistry approach was recently reported83

amongst promising alternative approaches,84–86 targeted at

moderating the catalyst replacement costs. The development of an

industrially viable catalyst based on cheaper and more abundant

metals would certainly benefit the expansion of the gas phase HCl

oxidation technology. Some promising steps in this direction are

being taken, as shown by a recent example using cuprous dela-

fossite and other copper mixed oxides.87,88 Still, the significant

copper loss is still an issue. Academic studies aimed at the iden-

tification of new catalytic materials, as well as understanding how

Deacon catalysts function at a molecular level (ultimately leading

to structure–performance relationships), are encouraged. High-

throughput screening of novel formulations and operando studies

during reaction can also contribute to realisation of these

developments. In order to draw realistic conclusions about the

potential of a catalytic system, researchers should keep in mind

that lifetime is the primary factor. Accelerated deactivation tests

and stability studies must prevail over fast screening of non-

equilibrated catalysts, in order to save wasted efforts when

promising initial activities fall rapidly with time on stream.

Acknowledgements

JPR acknowledges co-workers in ICIQ Tarragona and ETH

Zurich for their valuable contribution to the Deacon project (A.

P. Amrute, Dr L. Dur�an Pach�on, Dr J. G�omez-Segura, R.P.

Mar�ın, Dr G. Novell-Leruth, and Dr N. L�opez) and the support

from Bayer MaterialScience. The authors appreciate the inter-

action with partners of the BMBF project, ref. 033R018A: Dr D.

Teschner, Prof. H. Over, Prof. R. Schom€acker, and Prof. W.F.

Maier. Dr J. Kintrup, Dr R. Weber, Dr D. Ogrin, and Dr S.

Mitchell are acknowledged for comments on the manuscript.

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Sustainable chlorine recycling via catalysed HCl oxidation: from fundamentals to implementation

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