Metal Fluorides, Metal Chlorides and Halogenated Metal Oxides As Lewis Acidic Heterogeneous Catalysts

Review Metal Fluorides, Metal Chlorides and Halogenated Metal Oxides as Lewis Acidic Heterogeneous Catalysts. Providing Some Context for Nanostructured Metal Fluorides

David Lennon and John M. Winﬁeld * School of Chemistry, University of Glasgow, G12 8QQ Glasgow, UK; [email protected] * Correspondence: John.Winﬁ[email protected]; Tel.: +44-141-563-7281

Academic Editor: Erhard Kemnitz Received: 1 December 2016; Accepted: 17 January 2017; Published: 28 January 2017

Abstract: Aspects of the chemistry of selected metal fluorides, which are pertinent to their real or potential use as Lewis acidic, heterogeneous catalysts, are reviewed. Particular attention is paid to β-aluminum trifluoride, aluminum chlorofluoride and aluminas γ and η, whose surfaces become partially fluorinated or chlorinated, through pre-treatment with halogenating reagents or during a catalytic reaction. In these cases, direct comparisons with nanostructured metal fluorides are possible. In the second part of the review, attention is directed to iron(III) and copper(II) metal chlorides, whose Lewis acidity and potential redox function have had important catalytic implications in large-scale chlorohydrocarbons chemistry. Recent work, which highlights the complexity of reactions that can occur in the presence of supported copper(II) chloride as an oxychlorination catalyst, is featured. Although direct comparisons with nanostructured fluorides are not currently possible, the work could be relevant to possible future catalytic developments in nanostructured materials.

Keywords: metal fluoride; metal fluoride; oxohalide; catalysis; lewis acid; oxychlorination; chlorofluorocarbon; chlorohydrocarbon

1. Introduction Close-packed, solid metal fluorides have, for the most part, relatively small surface areas, although in some cases the surface metal cations have significant Lewis acidity due to their highly electronegative fluoride anion nearest-neighbors. For heterogeneous catalysis, partially fluorinated oxide surfaces have often been preferred, in view of their significantly increased surface areas without great loss in Lewis acidity. The development of nanostructured metal fluorides, which has been rapid over the past 10–15 years [1], has resulted in renewed interest in the possible catalytic applications of these high-surface-area metal fluorides and it is timely to consider how they compare as catalysts with materials prepared by more conventional routes. Catalytic studies are described explicitly in other contributions to this themed collection; here, in the first part of the review, we describe some surface properties of what might be termed ‘competitor catalysts’. The emphasis is on various forms of aluminum fluoride and γ-alumina, which have been fluorinated using various fluorinating agents. Although fluorinated chromia is also an obvious comparator, its catalytic activity is not dealt with here, since we have very recently compiled a detailed account of this topic [2].

2. Aluminum Fluorides and Halogenated Aluminas Two of the important aluminum fluorides, which can be regarded as precursors to nanoscale metal fluorides, are β-aluminum trifluoride and aluminum chlorofluoride (ACF). The former contains

Molecules 2017, 22, 201; doi:10.3390/molecules22020201 www.mdpi.com/journal/molecules Molecules 2017, 22, 201 2 of 10

III a six-coordinate Al but, unlike α-AlF3, which is close-packed, has the more open hexagonal tungsten bronze (HTB) structure [3]. This phase behaves as a solid Lewis acid and has signiﬁcant catalytic Molecules 2017, 22, 201 2 of 10 behavior [4]; a model for the coordinatively unsaturated surface AlIII species has been deduced [5].

In contrast,coordinate ACF AlIII was but, unlike developed α-AlF3 from, which industrial is close-packed, research has the in fluorocarbonmore open hexagonal chemistry tungsten [6]. bronze Judged by its chemical(HTB) structure behavior, [3]. This its Lewisphase behaves acidity as is a comparable solid Lewis acid to thatand has of liquidsignificant antimony catalytic pentafluoride,behavior [4]; a benchmarka model infor thethe field.coordinatively Synthesized unsaturated by halogen surface exchange AlIII species between has been solid deduced aluminum [5]. In contrast, trichloride ACF and variouswas chlorofluorocarbons,developed from industrial hydrochlorofluorocarbons research in fluorocarbon chemistry or hexafluoropropene, [6]. Judged by its chemical the reactions behavior, do not its Lewis acidity is comparable to that of liquid antimony pentafluoride, a benchmark in the field. result in complete replacement of Cl by F; the stoichiometry of the solid product is AlClxF3−x, with Synthesized by halogen exchange between solid aluminum trichloride and various chlorofluorocarbons, x being in the range 0.05–0.3. Surface areas are in the range 100–150 m2·g−1. The structure of the hydrochlorofluorocarbons or hexafluoropropene, the reactions do not result in complete replacement amorphous solid has been deduced at the atomic level from a number of spectroscopic techniques [7]. of Cl by F; the stoichiometry of the solid product is AlClxF3−x, with x being in the range 0.05–0.3. Surface Chlorineareas isare distributed in the range throughout 100–150 m2·g the−1. The solid, structure rather of than the amorphous in a discrete solid AlCl has3 beenphase, deduced and is at possibly the III III bridgedatomic to threelevel from Al acenters. number There of spectroscopic appear to techniques be three different [7]. Chlorine types is of distributed six-coordinated throughout Al .the solid,The defect rather spinelthan in oxide a discreteγ-alumina AlCl3 phase, is an and ideal is basepossib materially bridged to investigateto three AlIII thecenters. effects There of fluorination appear uponto an be oxide three surface.different Thetypes surface of six-coordinated properties depend AlIII. on both the reagent employed and the conditions used, as illustratedThe defect spinel in Table oxide1. γ-alumina is an ideal base material to investigate the effects of fluorination upon an oxide surface. The surface properties depend on both the reagent employed and the conditions used, as illustrated in TableTable 1. 1. Some properties of fluorinated γ-alumina [8].

Table 1. Some propertiesBETArea of fluorinatedFluorine γ-alumina [8]. Fluorination Conditions a Other Properties (m2·g−1) Content (%) BET Area Fluorine Fluorination Conditions a Other Properties SF static conditions, nominally room 2 −1 4 (m80–90·g )b Contentca. (%) 22 c Some Brønsted sites also temperature, 2 h, procedure repeated ×2 SF4 static conditions, nominally room 80–90 b ca. 22 c Some Brønsted sites also temperature, 2 h, procedure repeated ×2 γ-alumina present; possibly an SF 20% in N , flow conditions, 523 K, 2 h 67 47.1 4 2 γ-aluminaamorphous present; phase possibly also SF4 20% in N2, flow conditions, 523 K, 2 h 67 47.1 CHF 20% in N , then 100%, flow an amorphousα- and β-AlF phasepresent; also 3 2 d 58.4 3 CHF3 20% in N2, then 100%, flow 34 α- and β-AlF3 present; conditions, 623 K, total time 5 h 34 d 58.4 possibly an amorphous phase conditions, 623 K, total time 5 h possibly an amorphous phase a In all cases samples were calcined before use; b Before fluorination 110 m2·g−1; c Calculated from a radiotracer a b 2 −1 c study; Ind Before all cases fluorination samples were 240 m calcined2·g−1. before use; Before fluorination 110 m ·g ; Calculated from a radiotracer study; d Before fluorination 240 m2·g−1.

TheseThese fluorinated fluorinated aluminas aluminas are are not not supported supported reagen reagents,ts, since since the the fluorine fluorine is incorporated is incorporated within within the surface,the surface, and and in somein some cases, cases, possibly possibly inin thethe bulkbulk to some some extent. extent. They They contrast contrast therefore therefore withwith materials,materials, such such as supportedas supported boron boron trifluoride trifluoride [[9],9], which have have been been used used widely widely as solid as solid Lewis Lewis acids. acids. TwoTwo of the of possiblethe possible surface surface species species that that have have been been suggestedsuggested are are shown shown in in Figure Figure 1. 1.

Figure 1. Possible surface species derived from BF3 and oxides. Redrawn from Reference [9]. Figure 1. Possible surface species derived from BF3 and oxides. Redrawn from Reference [9].

The uptake of F by γ-alumina is slow when CHF3 is used. The process is initiated at the surface ofThe oxide uptake particles, of F by subsequentlyγ-alumina being is slow incorporated when CHF 3intois used. sub-surface The process regions. is The initiated reaction at the between surface of oxidesulfur particles, tetrafluoride subsequently and γ-alumina being incorporated is essentially into a hydrolysis; sub-surface the regions. course Theof the reaction reaction between has been sulfur tetraﬂuoridefollowed using and γ the-alumina radiotracers is essentially fluorine-18 a hydrolysis;and sulfur-35. the The course hydrolysis of the involves reaction the has replacement been followed usingof thesurface radiotracers hydroxyls ﬂuorine-18 by F and the and partial sulfur-35. replacemen Thet hydrolysisof bridging Al-O-Al involves moieties the replacement by Al-F, with of the surface hydroxylsconcomitant by F and formation the partial of OSF replacement2 and SO2. These of bridging are not Al-O-Alstrongly moietiesadsorbed. by The Al-F, Lewis with acid the sites concomitant have disordered F/O environments rather than being fully fluorinated. When the SF4 treatment is performed formation of OSF2 and SO2. These are not strongly adsorbed. The Lewis acid sites have disordered

Molecules 2017, 22, 201 3 of 10

F/O environments rather than being fully fluorinated. When the SF4 treatment is performed under staticMolecules conditions, 2017, 22,nominally 201 at room temperature, the retention of the co-product, HF, appears3 of 10 to be complete. Consequently, γ-alumina fluorinated by this method possesses Brønsted acidity in addition to theunder expected static Lewisconditions, acidity. nominally at room temperature, the retention of the co-product, HF, appears toA be most complete. powerful Consequently, method to probeγ-alumina the Lewisfluorinated acidity by ofthis fluorinated method possesses surfaces Brønsted is FTIR acidity spectroscopy in usingaddition Lewis baseto the probe expected molecules Lewis acidity. such as pyridine (py) or carbon monoxide, and 2-methyl-substituted py as aA probe most wherepowerful Brønsted method to surface probe the hydroxyl Lewis acidit groupsy of fluorinated are present. surfaces Studies is FTIR using spectroscopy CO at low using Lewis base probe molecules such as pyridine (py) or carbon monoxide, and 2-methyl-substituted temperatures can be particularly powerful, since different types of surface sites can be detected and py as a probe where Brønsted surface hydroxyl groups are present. Studies using CO at low compared. The aluminum fluorides mentioned above and various oxofluorides of different structural temperatures can be particularly powerful, since different types of surface sites can be detected and typescompared. have been The studied aluminum in thisfluorides way mentioned and the results above comparedand various with oxofluorides nanoscale of different aluminum structural fluorides; accountstypesof have the been methodology studied in andthis someway and illustrative the results results compared are towith be nanoscale found in [aluminum10,11]. fluorides; accountsUseful data of the can methodology be obtained and in some addition, illustrative using results a radiolabeled are to be found probe in [10,11]. molecule to investigate a fluorinatedUseful surface. data Experimentscan be obtained using in addition, anhydrous using hydrogen a radiolabeled chloride probe and molecule its precursors, to investigate labeled a with chlorine-36,fluorinated are surface. described Experiments below. In us addition,ing anhydrous a radiotracer hydrogen can chloride provide and important its precursors, mechanistic labeled with details. chlorine-36,Two examples are described are provided below. by In the addition, behavior a ra ofdiotracer the trichlorotrifluoroethane can provide important mechanistic isomers at details.β-AlF3 and Two examples are provided by the behavior of the trichlorotrifluoroethane isomers at β-AlF3 and the fluorinated γ-alumina surfaces. Isomerization of CCl2FCClF2 to the thermodynamically preferred the fluorinated γ-alumina surfaces. Isomerization of CCl2FCClF2 to the thermodynamically preferred isomer CCl3CF3 does not involve any formation of Al-Cl surface bonds, as indicated by the labeling isomer CCl3CF3 does36 not involve any formation of Al-Cl surface bonds, as indicated by the labeling of CCl2FCClF2 with [ Cl]. This indicates that the isomerization proceeds by an intra- rather than by of CCl2FCClF2 with [36Cl]. This indicates that the isomerization proceeds by an intra- rather than by an an inter-molecular mechanism, as shown in Figure2a. Isomerization is followed by a dismutation inter-molecular mechanism, as shown in Figure 2a. Isomerization is followed by a dismutation reaction reaction giving a mixture of CCl FCF and CCl CClF , which, from the lack of the incorporation of giving a mixture of CCl2FCF3 and2 CCl33CClF2, which,3 from2 the lack of the incorporation of [36Cl] into the 36 [ Cl]solid into surface, the solid is believed surface, isto believedinvolve a tohalogen involve exchange a halogen between exchange two betweenadsorbed twoCCl3 adsorbedCF3 molecules, CCl3 CF3 molecules,as shown as in shown Figure in 2b Figure [12]. 2b [12].

Figure 2. Schematic representation of possible adsorbed states for (a) CCl2FCClF2 prior to isomerization to Figure 2. Schematic representation of possible adsorbed states for (a) CCl2FCClF2 prior to isomerization CCl3CF3; and (b) CCl3CF3 prior to its dismutation. Redrawn from Reference [12]. to CCl3CF3; and (b) CCl3CF3 prior to its dismutation. Redrawn from Reference [12]. These observations provide indirect evidence for the presence of strong Lewis acid surface sites on βThese-AlF3 and observations fluorinatedprovide γ-aluminas, indirect sufficiently evidence strong for to the allow presence C–F bonds, of strong unexpectedly, Lewis acid to behave surface as sites on βLewis-AlF3 andbases. fluorinated γ-aluminas, sufficiently strong to allow C–F bonds, unexpectedly, to behave as Lewis bases. 3. Anhydrous Hydrogen Chloride as a Surface Probe 3. Anhydrous Hydrogen Chloride as a Surface Probe It is not intuitively obvious that anhydrous HCl might be a useful probe molecule which can be usedIt is notto compare intuitively the surface obvious behavior that anhydrous of different HClfluorinated might surfaces, be a useful but this probe has molecule proven to whichbe the can be usedcase, to particularly compare thefor surfacecomparisons behavior among of nanoscal differente fluorinatedaluminum fluorides surfaces, an butd their this conventionally has proven to be the case,prepared particularly counterparts. for comparisons The genesis of among the approach nanoscale was a aluminumfortuitous observation fluorides andof reactions their conventionally that occur when 1,1,1-trichloroethane is exposed to an SF4-fluorinated (static conditions, see Table 1) γ-alumina at prepared counterparts. The genesis of the approach was a fortuitous observation of reactions that occur room temperature. The reactions involved are shown in Figure 3 [9]. when 1,1,1-trichloroethane is exposed to an SF -fluorinated (static conditions, see Table1) γ-alumina The surface-catalyzed reactions shown in4 Figure 3 are, respectively, a Lewis acid catalyzed at room temperature. The reactions involved are shown in Figure3[9]. dehydrochlorination of CH3CCl3 and an oligomerization, accompanied by a partial dehydrochlorination, ofThe adsorbed surface-catalyzed CH2=CCl2; both reactions have their shown equivalent in Figure reactions3 are, using respectively, the archetypal a Lewis Lewis acid acid, catalyzed solid dehydrochlorinationaluminum chloride. of CHThe3 presenceCCl3 and of an retained oligomerization, HF on the surface accompanied probably by accounts a partial for dehydrochlorination the fluorination ,

Molecules 2017, 22, 201 4 of 10

of adsorbed CH2=CCl2; both have their equivalent reactions using the archetypal Lewis acid, solid Molecules 2017, 22, 201 4 of 10 aluminum chloride. The presence of retained HF on the surface probably accounts for the fluorination step.step. This This can can be madebe made catalytic catalytic if if the the purple, purple, oligomericoligomeric supported supported layer layer is exposed is exposed to a tomixture a mixture of of HF andHF CHand3 CHCCl3CCl3. Although3. Although similar similar behavior behavior is observed when when BF BF3 is3 isused used to fluorinate to fluorinate γ-aluminaγ-alumina (cf. Figure(cf. Figure1), it 1), occurs it occurs to a to far a far smaller smaller extent, extent, reflecting, reflecting, no doubt, doubt, the the smaller smaller fluorine fluorine content content of the of the surfacesurface [9]. [9].

Figure 3. Behavior of CH3CCl3 on SF4-fluorinated γ-alumina. Redrawn from Reference [9]. Figure 3. Behavior of CH3CCl3 on SF4-ﬂuorinated γ-alumina. Redrawn from Reference [9].

Dehydrochlorination occurs also when t-butyl chloride is exposed to a fluorinated Lewis acidic Dehydrochlorinationsurface at room temperature, occurs although also when the chemistryt-butyl chloride is not as isdramatic exposed visually to a fluorinated as that derived Lewis from acidic surfaceCH at3CCl room3. It is temperature, a useful reaction, although however, the chemistrysince ButCl isis notreadily as dramaticlabeled with visually [36Cl] and as that the derivedresulting from t 36 CH3CClfate3 of. It the is H a36 usefulCl produced reaction, can however,be monitored since from Bu itsCl radioactivity is readily labeled [12]. with [ Cl] and the resulting fate of theThe H36 experimentalCl produced procedures can be monitored for quantifying from β its− activity radioactivity from the [12 long-lived]. chlorine-36 isotope Theby Geiger-Müller experimental counting procedures have been for quantifying well documentedβ− activity in the literature from the [10–12] long-lived and are chlorine-36 not described isotope by Geiger-Müllerhere. Although counting the behavior have observed been well when documented [36Cl]-ButCl is in exposed the literature to SF4-fluorinated [10–12] and γ-alumina, are not β described-AlF3 and aluminum chlorofluoride (ACF) is similar,36 there aret differences in the details [12]. In particular, here. Although the behavior observed when [ Cl]-Bu Cl is exposed to SF4-fluorinated γ-alumina, the outcomes with SF4-fluorinated γ-alumina appear to be dominated by oligomerization reactions β-AlF3 and aluminum chlorofluoride (ACF) is similar, there are differences in the details [12]. that cover the surface, whereas strongly and weakly surface-bound H36Cl are observed with ACF. In particular, the outcomes with SF -fluorinated γ-alumina appear to be dominated by oligomerization More controversially, a significant4 fraction of H36Cl appears to be retained in the bulk ACF, from reactions that cover the surface, whereas strongly and weakly surface-bound H36Cl are observed which it can be removed by prolonged pumping. The observations using β-AlF3 are similar; in this 36 with ACF.case, however, More controversially, HCl interacts awith significant residual fraction H2O located of H inCl the appears HTB tohexagonal be retained channels in the and bulk is ACF, fromdesorbed which it as can H2 beO·HCl. removed The behaviors by prolonged of ACF pumping. and β-AlF3 Thetowards observations H36Cl added using directlyβ-AlF replicate3 are similar;those in this case,described however, above. HCl interacts with residual H2O located in the HTB hexagonal channels and is 36 36 desorbedThe as H behavior2O·HCl. of The nanoscale behaviors aluminum(III) of ACF and fluoride,β-AlF HS-AlF3 towards3, towards H Cl H addedCl parallels directly that replicate of ACF; those describedweakly above. and strongly surface-bound H36Cl and material retained in the bulk are all observed [13]. Dehydrochlorination of ButCl is rapid, and HCl is observed immediately in the36 vapor; the analogous The behavior of nanoscale aluminum(III) fluoride, HS-AlF3, towards H Cl parallels that of ACF; reaction over β-AlF3 is far slower. Nanoscale magnesium fluoride, HS-MgF2, behaves in an identical weakly and strongly surface-bound H36Cl and material retained in the bulk are all observed [13]. way, indicating that unlike rutile MgF2, HS-MgF2 can behave as a Lewis acid. The composite material, Dehydrochlorination of ButCl is rapid, and HCl is observed immediately in the vapor; the analogous 15 mol % FeF3 in HS-MgF2, in its behavior towards [36Cl]-ButCl, appears to utilize Lewis acid sites β reactionbased over on Fe-AlFIII in 3additionis far slower. to those Nanoscale involving Mg magnesiumII [13]. fluoride, HS-MgF2, behaves in an identical way, indicatingAs is evident that unlike from the rutile summary MgF2, given HS-MgF above,2 can the behave level of as detail a Lewis available acid. about The compositethe interactions material, 36 t 15 molbetween % FeF HCl3 in HS-MgFand fluorinated2, in its surfaces behavior is limited, towards la [rgelyCl]-Bu by theCl, corrosive appears nature to utilize of the Lewis materials acid sites basedinvolved. on FeIII Morein addition detailed to descriptions those involving become Mg possibleII [13]. for the interactions between anhydrous HCl, Aswhich is evident of course from has its the own summary corrosion given problems, above, and the transitional level of detail aluminas available such as about γ- and the η-alumina. interactions betweenFTIR HClspectroscopy, and fluorinated utilizing surfaces temperature-programm is limited, largelyed techniques by the corrosive and, more nature recently, of theinelastic materials involved.neutron More scattering, detailed has descriptions been widely used. become Together possible with fora knowledge the interactions of the molecular between environments anhydrous HCl, of the various surface Lewis acid sites on the aluminas, it is possible to construct realistic hypotheses which of course has its own corrosion problems, and transitional aluminas such as γ- and η-alumina. for events that take place at a chlorinated surface [2,14]. FTIR spectroscopy, utilizing temperature-programmed techniques and, more recently, inelastic neutron An illustration of what can be achieved under favourable circumstances is the FTIR diffuse scattering,reflectance has beenspectra widely as a function used. Togetherof temperature with fo ar knowledgea saturated, chemisorbed of the molecular overlayer environments of HCl at the of the varioussurface surface of η-alumina, Lewis acid as shown siteson in Figure the aluminas, 4 [14]. it is possible to construct realistic hypotheses for events that take place at a chlorinated surface [2,14]. An illustration of what can be achieved under favourable circumstances is the FTIR diffuse reflectance spectra as a function of temperature for a saturated, chemisorbed overlayer of HCl at the surface of η-alumina, as shown in Figure4[14]. Molecules 2017, 22, 201 5 of 10

Molecules 2017, 22, 201 5 of 10

Figure 4. Diffuse reflectance spectra as a function of temperature of a saturated chemisorbed overlayer Figure 4. Diffuse reﬂectance spectra as a function of temperature of a saturated chemisorbed overlayer of HCl: (a) 4000–1200 cm—1 and (b) the free hydroxyl region; (i) Saturation spectrum dosed at 293 K. of HCl: (a) 4000–1200 cm−1 and (b) the free hydroxyl region; (i) Saturation spectrum dosed at 293 K. The sample was then progressively warmed to (ii) 423; (iii) 523; and (iv) 623 K. A flow of He was The samplecontinually was thenpassed progressively over the sample while warmed heating to to (ii progressively) 423; (iii) 523;higher and temperatures. (iv) 623 K.The A sample ﬂow was of He was continuallyFigureheld 4.at passed Diffusethe desorption over reflectance the temperature sample spectra while for as a15 function min heating and of thento temperature progressivelyallowed to ofcool a saturatedto higher room temperature, temperatures. chemisorbed where overlayer The the sample was heldof HCl:spectrum at ( thea) 4000–1200 was desorption acquired. cm—1 temperature All and spectra (b) the are free for background hydroxyl 15 min and region;subtracted, then (i) allowedSaturation where the to spectrum spectrum cool to roomdosed of the at temperature,clean 293 K. whereThe theactivated sample spectrum catalystwas wasthen has acquired.prog beenressively subtracted All warmed spectra from theto are ( iimeasured background) 423; (iii spectrum.) 523; subtracted, and Reproduced (iv) 623 where K. withA theflow permission spectrum of He was of the continuallyfrom Reference passed [14]. over the sample while heating to progressively higher temperatures. The sample was clean activated catalyst has been subtracted from the measured spectrum. Reproduced with permission held at the desorption temperature for 15 min and then allowed to cool to room temperature, where the from Reference [14]. spectrumFor a detailed was acquired. interpretation All spectra of the are spectra, background the re adersubtracted, is directed where to theReference spectrum [14]; of however, the clean in summary,activated therecatalyst is goodhas been evidence subtracted for weakly from the adsorbed measured molecular spectrum. HCl Reproduced and dissociatively with permission adsorbed Forspecies afrom detailed Referencein which interpretation HCl[14]. has interacted of the with spectra, Al-O specie the readers to form is Al-Cl directed and OH to groups, Reference or has [14 replaced]; however, in summary,a surface there Al-OH is good by evidenceAl-Cl with forthe formation weakly adsorbed of H2O. This molecular type of information HCl and is dissociatively very important adsorbedin producing realistic sequences of surface events. A very good example is the large-scale selective synthesis species inFor which a detailed HCl hasinterpretation interacted of with the spectra, Al-O species the reader to form is directed Al-Cl to and Reference OH groups, [14]; however, or has replaced in summary,of methyl there chloride is good from evidence methanol for and weakly HCl, using adsorbed an η-alumina molecular heterogeneous HCl and dissociatively catalyst [2]. The adsorbed full a surface Al-OH by Al-Cl with the formation of H2O. This type of information is very important speciessequence in which of events HCl involving has interacted HCl and with methanol Al-O specieat the surfaces to form is shown Al-Cl schematicallyand OH groups, in Figure or has 5 [15]. replaced in producing realistic sequences of surface events. A very good example is the large-scale selective a surface Al-OH by Al-Cl with the formation of H2O. This type of information is very important in η synthesisproducing of methyl realistic chloride sequences from of surface methanol events. and A very HCl, good using example an -alumina is the large-scale heterogeneous selective synthesis catalyst [2]. The fullof methyl sequence chloride of eventsfrom methanol involving and HCl HCl, and using methanol an η-alumina at the heterogeneous surface is shown catalyst schematically[2]. The full in Figuresequence5[15]. of events involving HCl and methanol at the surface is shown schematically in Figure 5 [15].

Figure 5. A schematic representation of the site-selective formation of methyl chloride over η-alumina. The red circles represent strong/medium-strong Lewis acid sites; the blue circles represent medium-weak Lewis acid sites; the red/blue shaded circles represent strong/medium-strong/medium-weak Lewis acid sites. The definition and form of these sites are described in D. T. Lundie et al. J. Phys. Chem. B 2005, 109, 11592–11601 [16]. Reproduced with permission from Reference [15].

FigureFigure 5. A 5. schematic A schematic representation representation of of the the site-selectivesite-selective formation formation of ofmethyl methyl chloride chloride over over η-alumina.η-alumina. The red circles represent strong/medium-strong Lewis acid sites; the blue circles represent medium-weak The red circles represent strong/medium-strong Lewis acid sites; the blue circles represent medium-weak Lewis acid sites; the red/blue shaded circles represent strong/medium-strong/medium-weak Lewis acid Lewis acid sites; the red/blue shaded circles represent strong/medium-strong/medium-weak Lewis sites. The definition and form of these sites are described in D. T. Lundie et al. J. Phys. Chem. B 2005, acid sites.109, 11592–11601 The deﬁnition [16]. Reproduced and form of with these permission sites are from described Reference in D.[15]. T. Lundie et al. J. Phys. Chem. B 2005, 109, 11592–11601 [16]. Reproduced with permission from Reference [15].

Molecules 2017, 22, 201 6 of 10

4. Iron(III) and Copper(II) Chlorides as Chlorination Catalysts: Redox and Lewis Acid Possibilities From a historical standpoint, iron(III) and copper(II) chlorides are two of the most important metal halides in heterogeneous catalysis, being used in a variety of situations, both in laboratory-based studies and in large-scale processes, where catalysis is an important factor. Anhydrous iron(III) chloride and its hexa-hydrate feature widely in contemporary organic syntheses, since, because of their Lewis acid properties, they can initiate or catalyze a variety of organic transformations [17–26]. Because of their redox properties arising from the 1e−, FeIII/FeII redox transformation, they are also the basis of a useful polymerization process [27–31], for example the polymerization of thiophene. Anhydrous FeCl3 has been widely reported as a Lewis acid catalyst for chlorination and hydrochlorination of a variety of hydrocarbons [32–34], particularly in large-scale process reactions such as the chlorination of ethene and the hydrochlorination of vinyl chloride [32,33]. Reaction pathways suggested for these types of reactions were traditionally based on polar intermediates but these are probably unrealistic based on current thinking. Reactions carried out in mild steel that involve Cl2 may be problematic, since surface chlorination can occur, leading to a chloride surface that behaves like solid FeCl3. As a result, the unwanted chlorination of hydrochlorocarbons can occur; this may be accompanied by oligomerization of unsaturated species [35–37]. This type of behavior in the 1,1,2,2-tetrachloroethane/Cl2 system was observed in a recent laboratory study of reactions carried out in stainless steel or Pyrex [36,37]. A rather different example of over-chlorination occurred in large-scale reactions of Cl2 with CH2=CH2 and led to unwanted 1,1,2-trichloroethane in the product, 1,2-dichloroethane. The over-chlorination is believed to be catalyzed by the presence of small quantities of FeCl3 formed adventitiously, either in solution or at the reactor wall. Considerable effort has been made to inhibit over-chlorination in large-scale processes; for example, by means of FeCl3 removal by complexation with various Lewis bases. However, fundamental studies targeted to the identification of molecular species present have apparently never been reported, and in the reports of inhibition by chloride ion–forming tetrachloroferrate(III) anions, differences exist as to the optimum stoichiometry to be used. Because anhydrous FeCl3 is extremely hygroscopic, its surface may be complexed by trace water, present even in purified CH2ClCH2Cl. Molecular species such as FeCl3(OH2), so formed, are soluble in a hydrochlorocarbon solvent and the resulting solid/solution distribution of chloroferrate(III) species is not simple [38]. The ability of copper(II) chloride, either as a component of a melt or supported on an oxide, to chlorinate a wide variety of saturated and unsaturated hydrocarbons has been known for many years [39,40]; the behavior of pumice-supported CuCl2 towards olefins suggests that CuCl2 is the chlorinating agent rather than adsorbed Cl* [40]. From a heterogeneous catalytic standpoint, one of the most important uses of CuCl2 is as the catalyst for the Deacon reaction, a process for the conversion of HCl to Cl2. There is renewed interest in this sequence of reactions, shown in Equations (1)–(4) below, in the context of the utilization of waste HCl. A scheme for the integration of processes producing and using HCl has the aim of “closing the chlorine cycle”; it includes the steps: chlorination, dehydrochlorination and oxychlorination [41].

II I 2Cu Cl2 → 2Cu Cl + Cl2 (1)

I II 2Cu Cl + 1/2O2 → Cu 2OCl2 (2) II II Cu 2OCl2 + 2HCl → 2Cu Cl2 + H2O (3) Overall 2HCl + 1/2O2 → H2O + Cl2 (4)

Since a Deacon, or oxychlorination, catalyst typically comprises, in addition to CuCl2, a support, generally a high-surface-area oxide, and a promoter, such as an ionic halide, for example KCl, Equations (1)–(3) are likely to be an approximation of the surface species present. Furthermore, in Molecules 2017, 22, 201 7 of 10 a processMolecules 2017 environment,, 22, 201 at the typical operating temperatures used, the species may well be7 liquid.of 10 One of the most detailed studies made to date under laboratory conditions involved CuCl2 supported on of the most detailed studies made to date under laboratory conditions involved CuCl2 supported on γ-alumina. The catalyst was used in the oxychlorination of ethene; the product, CH2ClCH2Cl, underwent γ-alumina. The catalyst was used in the oxychlorination of ethene; the product, CH2ClCH2Cl, underwent dehydrochlorination to CHCl=CH2, so the study was very relevant to PVC production [42–49]. dehydrochlorination to CHCl=CH2, so the study was very relevant to PVC production [42–49]. Physical Physical methods, particularly surface science–based together with reactor-based experiments, methods, particularly surface science–based together with reactor-based experiments, indicate that indicate that at least three CuII-containing phases are present and that further mixed CuII/Group I at least three CuII-containing phases are present and that further mixed CuII/Group I phases can be phasesformed can as be well. formed as well. Chlorination or oxychlorination of CH ClCH Cl in the presence of a CuCl /KCl catalyst Chlorination or oxychlorination of CH22ClCH2Cl in the presence of a CuCl2/KCl2 catalyst supportedsupported on on the the clay clay mineral, mineral, attapulgite, attapulgite, leadsleads to a complex mixture mixture of of C C2 2chlorohydrocarbonschlorohydrocarbons and and chlorinatedchlorinated oleﬁns; olefins; the the products products depend depend on on the the exact exact conditions conditions used used butbut carefulcareful controlcontrol cancan resultresult in CHCl=Clin CHCl=Cl2 or2 CCl or CCl2=CCl2=CCl2 being2 being the the predominant predominant product.product. The re reactionaction scheme scheme deduced deduced is isshown shown in in FigureFigure6[36 6 ,[36,37].37].

Figure 6. The dehydrochlorination (1, 4) and chlorination (2, 3) processes that connect CHCl2CHCl2 Figure 6. The dehydrochlorination (1, 4) and chlorination (2, 3) processes that connect CHCl2CHCl2 with CHCl=CCl2 and CCl2=CCl2. Possible oligomer formation (5, 6). Reproduced with permission with CHCl=CCl2 and CCl2=CCl2. Possible oligomer formation (5, 6). Reproduced with permission from Reference [36]. from Reference [36].

There is, inevitably, a trade-off to be made between high conversions and the loss of materials throughThere oligomerization, is, inevitably, a particularly trade-off to of be CHCl=CCl made between2. Product high distributions conversions are the and result the lossof competition of materials throughbetween oligomerization, chlorination, facilitated particularly by of the CHCl=CCl CuCl2 catalyst,2. Product and distributions dehydrochlorination. are the result The of competitionlatter for betweenCHCl2CHCl chlorination,2 is initiated facilitated by Cl*, whereas by the dehydrochlorination CuCl2 catalyst, and of dehydrochlorination. CHCl2CCl3 occurs at Lewis The sites latter on for CHClthe 2catalyst.CHCl2 is initiated by Cl*, whereas dehydrochlorination of CHCl2CCl3 occurs at Lewis sites on the catalyst.The CuII catalyst supports the Deacon reaction to convert the co-product, HCl, to the reactant II Cl2The; however, Cu catalyst the reaction supports is slow the compared Deacon reaction to the main to convert organo-chlorine the co-product, transformations. HCl, to the reactant Cl2; however,Phosgene the reaction is prepared is slow on compared a large scale to theusually main by organo-chlorine the reaction between transformations. carbon monoxide and Cl2 overPhosgene a carbon is catalyst. prepared An on oxychlorination a large scale usually route ha bys been the reactiondeveloped, between however, carbon on a monoxidelaboratory andscale Cl 2 overusing a carbon a catalyst catalyst. comprising An oxychlorination CuCl2/KCl supported route has on silica been gel developed, [50–52]. The however, challenge on ain laboratory this case is scaleto prepare OCCl2 continuously with good conversion and without hydrolysis. This has been achieved using a catalyst comprising CuCl2/KCl supported on silica gel [50–52]. The challenge in this case is to by employing a three-stage arrangement corresponding to Equations (1)–(4), viz. Equations (5)–(8). prepare OCCl2 continuously with good conversion and without hydrolysis. This has been achieved by employing a three-stage arrangement2CuCl corresponding2 + CO → 2CuCl to Equations + COCl2 (1)–(4), viz. Equations (5)–(8).(5)

2CuCl2CuCl2 + CO + 1/2O→ 2CuCl2 → Cu2 +OCl COCl2 2 (6) (5)

Molecules 2017, 22, 201 8 of 10

2CuCl + 1/2O2 → Cu2OCl2 (6)

Cu2OCl2 + 2HCl → 2CuCl2 + H2O (7) Overall: CO + 1/2O2 + 2HCl → COCl2 + H2O (8)

5. Conclusions The two parts of this short review have different objectives. In the ﬁrst part, we sought to show that selected metal ﬂuorides prepared in conventional and unconventional ways have many similarities in their Lewis acidity, using a variety of probe molecules, including the very weak Lewis base anhydrous HCl. In the second part, some aspects of FeCl3 and, particularly, CuCl2, chemistry demonstrated the complexity in their catalytic behavior. The challenge for those who are synthesizing nanoscale halides is to investigate whether there is a parallel chemistry to be uncovered.

Acknowledgments: The authors’ studies have benefitted enormously from talented students and post-doctoral fellows over many years. Particularly in the second part, we have been very fortunate to collaborate with colleagues based in Runcorn, UK. Our thanks go to all. Conflicts of Interest: The authors declare no conflict of interest.

References and Notes

1. Tressaud, A. Functionalised Inorganic Fluorides; Wiley: Chichester, UK, 2010. 2. Lennon, D.; Winﬁeld, J.M. Heterogeneous Catalysts Used for Large-Scale Syntheses of Selected Catalytic Applications; Parvulescu, V.I., Kemnitz, E., Eds.; Elsevier: Amsterdam, The Netherlands; Oxford, UK; Boston, MA, USA, 2016; Chapter 7; pp. 193–217. 3. Le Bail, A.; Jacaboni, C.; Leblanc, M.; De Pape, R.; Duroy, H.; Fourquet, J.L. Crystal Structure of the Metastable

form of Aluminium Trifluoride βAlF3 and the Gallium and Indium Homologues. J. Solid State Chem. 1988, 77, 96–101. 4. Hess, A.; Kemnitz, E. Characterisation of Catalytically Active Sites on Aluminium Oxides, Hydroxyfluorides and Fluorides in Correlation with their Catalytic Behaviour. J. Catal. 1994, 149, 449–457, and references therein. 5. Kemnitz, E.; Winfield, J.M. Fluoride Catalysts: Their Application to Heterogeneous Catalytic Fluorination and Related Processes. In Advanced Inorganic Fluorides: Synthesis, Characterization and Applications; Nakajima, T., Žemva, B., Tressaud, A., Eds.; Elsevier Science S.A.: Lausanne, Switzerland, 2000; Chapter 12; pp. 367–401. 6. Krespan, C.G.; Petrov, V.A. The Chemistry of Highly Fluorinated Carbocations. Chem. Rev. 1996, 96, 3269–3301, and references therein. 7. Krahl, T.; Kemnitz, E. The Very Strong Lewis Acids Aluminium Chlorofluoride (ACF) and Bromofluoride (ABF)—Synthesis, Structure and Lewis Acidity. J. Fluor. Chem. 2006, 127, 663–678, and references therein. 8. Kemnitz, E.; Skapin, T.; Winfield, J.M. Preparation of Fluorinated γ-Alumina. In Efficient Preparations of Fluorine Compounds, 1st ed.; Roesky, H.W., Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2013; Chapter 67; pp. 442–446. 9. Klapötke, T.M.; McMonagle, F.; Spence, R.R.; Winfield, J.M. γ-Alumina-Supported Boron Trifluoride: Catalysis, Radiotracer Studies and Computations. J. Fluor. Chem. 2006, 127, 1446–1453. 10. Vimont, A.; Daturi, M.; Winfield, J.M. Investigation of Surface Acidity using a Range of Probe Molecules. In Functionalised Inorganic Fluorides; Tressaud, A., Ed.; Wiley: Chichester, UK, 2010; Chapter 4; pp. 101–139. 11. Winfield, J.M. Investigating Acidity of Metal Fluoride Surfaces by Spectroscopic and Chemical Methods. J. Fluor. Chem. 2009, 130, 1069–1079. 12. Nickkho-Amiry, M.; Winfield, J.M. Investigation of Fluorinated Surfaces by means of Radio-Labelled Probe Molecules. J. Fluor. Chem. 2007, 128, 344–352. 13. Nickkho-Amiry, M.; Eltanany, G.; Wüttke, S.; Rüdiger, S.; Kemnitz, E.; Winfield, J.M. A Comparative Study of Surface Activity in the Amorphous, High Surface Area Solids, Aluminium Fluoride, Magnesium fluoride and Magnesium Fluoride containing Iron(III) or Aluminium(III) Fluoride. J. Fluor. Chem. 2008, 129, 366–375. Molecules 2017, 22, 201 9 of 10

14. McInroy, A.R.; Lundie, D.T.; Winfield, J.M.; Dudman, C.C.; Jones, P.; Parker, S.F.; Lennon, D. The Interaction of Alumina with HCl: An Infrared Spectroscopy, Temperature-Programmed Desorption and Inelastic Neutron Scattering Study. Catal. Today 2006, 114, 403–411. 15. McInroy, A.L.; Winfield, J.M.; Dudman, C.C.; Jones, P.; Lennon, D. The Development of a New Generation of Methyl Chloride Synthesis Catalysts. Faraday Discuss. 2016, 188, 467–499. 16. Lundie, D.T.; McInroy, A.R.; Marshall, R.; Winfield, J.M.; Jones, P.; Dudman, C.C.; Parker, S.F.; Mitchell, C.; Lennon, D. Improved Description of the Surface Acidity of η-Alumina. J. Phys. Chem. B 2005, 109, 11592–11601. 17. Zhang, X.; Fan, X.; Niu, H.; Wang, J. An Ionic Liquid As a Recyclable Medium for the Green Preparation of 0 α,α -Bis(substituted benzylidine)cycloalkanones Catalysed by FeCl3·6H2O. Green Chem. 2003, 5, 267–269. 18. Kabalka, G.W.; Yao, M.-L.; Borella, S.; Goins, L.K. Iron Trichloride Mediated Allylation of Lithium Alkoxides through an Unusual Carbon-Oxygen Bond Cleavage. Organometallics 2007, 26, 4112–4114. 19. Xa, W.-W.; Lin, M.; Huang, Y.; Chen, L.; Zhan, Z.-P. Iron Trichloride Catalysed One-Pot Synthesis of Substituted Oxazoles from Propargylic Acetates and Amides under Microwave Irradiation. Lett. Org. Chem. 2009, 6, 664–665. 20. Cantagrei, G.; De Canné-Carnavalet, B.; Meyer, C.; Cossy, J. IronTrichloride Promoted Cyclisation of O-Alkynylaryl Isocyanates. Synthesis of 3-(Chloromethylene)Oxindoles. Org. Lett. 2009, 11, 8584–8586. 21. Schröder, K.; Enthaler, S.; Bitterlich, B.; Schulz, T.; Spannenberg, A.; Tse, M.K.; Junge, K.; Beller, M. Design of and Mechanistic Studies on a Biomimetic Iron-Imidazole Catalyst System for Epoxidation of Olefins with Hydrogen Peroxide. Chem. Eur. J. 2009, 15, 5471–5481.

22. Buchwald, S.L.; Bolm, C. On the Role of Metal Contaminants in Catalyses with FeCl3. Angew. Chem. Int. Ed. 2009, 48, 5586–5587. 23. Schröder, K.; Junge, K.; Spannenberg, A.; Beller, M. Design of a Bio-Inspired Imidazole-Based Iron Catalyst for Epoxidation of Oleﬁns: Mechanistic Insights. Catal. Today 2010, 157, 364–370.

24. Su, L.; Lei, C.-Y.; Fan, W.-Y.; Liu, L.-X. FeCl3-Mediated Reaction of Alkynols with Iodine: An Efﬁcient and Convenient Route to Vinyl Iodides. Synth. Commun. 2011, 41, 1200–1207. 25. Blevins, D.W.; Yao, M.-L.; Yong, L. Iron Trichloride Promoted Hydrolysis of Potassium Organotriﬂuoroborates. Tetrahedron Lett. 2011, 52, 6534–6536. 26. Yeh, M.-C.P.; Fang, C.-W.; Lin, H.-H. Facile Synthesis of Azaspirocycles via Iron Trichloride Promoted Cyclisation/Chlorination of Cyclic 8-Aryl-5-aza-5-tosyl-2-en-7-yn-1-ols. Org. Lett. 2012, 14, 1830–1833. 27. Della Casa, C.; Bertinelli, F.; Costa Bizzarri, P.; Salatelli, E. Properties of a Hydroxydecyl-Functionalized Polythiophene Synthesized by the Iron Trichloride Route. Adv. Mater. 1995, 7, 1005–1007. 28. Tanaka, K.; Mihara, T.; Koide, N. Synthesis and Physical Properties of Regioregular Poly(3-Alkoxy-4- Methylthiophene)s. Polym. J. 2004, 36, 628–663. 29. Svoboda, J.; Bláha, M.; Sedláˇcek,J.; Vohlídal, H.; Balcar, H.; Mav-Golež, I.; Žigon, M. New Approaches to the Synthesis of Pure Conjugated Polymers. Acta Chim. Slov. 2006, 53, 407–416. 30. Cloutet, E.; Mumtaz, M.; Cramail, H. Synthesis of PEDOT Latexes by Dispersion Polymerization in Aqueous Media. Mater. Sci. Eng. C 2009, 29, 377–382. 31. Lanzi, M.; Paganin, L.; Errani, F. Synthesis, Characterization and Photovoltaic Properties of a New Thiophene-Based Double-Cable Polymer with Pendent Fullerene Group. Polymer 2012, 53, 2134–2145. 32. Reed, D.J. Chlorocarbons and Chlorohydrocarbons. In Kirk-Othmer Encyclopaedia of Chemical Technology, 4th ed.; Kroschwitz, J.I., Howe-Grant, M., Eds.; John Wiley & Sons: New York, NY, USA, 1993; Volume 5, pp. 1017–1028. 33. Weissermel, K.; Arpe, H.-J. Industrial Organic Chemistry, 4th ed.; Wiley-VCH: Weinheim, Germany, 2003; pp. 449–450.

34. Delaude, L.; Lazlo, P. Aromatic Chlorination of Toluene and of Anisole Using Clay-Supported FeCl3 and m-Chloroperbenzoic Acid. A Biomimetic Approach. Catal. Lett. 1990, 5, 35–44. 35. Kawaguchi, T.; Suzuki, Y.; Saito, R. Trichloroethylene and Perchloroethylene Coproduction by Ethylene Process. Ind. Eng. Chem. 1970, 62, 36–41. 36. Sutherland, I.W.; Hamilton, N.G.; Dudman, C.C.; Jones, P.; Lennon, D.; Winﬁeld, J.M. Chlorination and Dehydrochlorination Reactions Relevant to the Manufacture of Trichloroethene and Tetrachloroethene. Part 1. Reaction Pathways. Appl. Catal. A Gen. 2011, 399, 1–11. Molecules 2017, 22, 201 10 of 10

37. Sutherland, I.W.; Hamilton, N.G.; Dudman, C.C.; Jones, P.; Lennon, D.; Winﬁeld, J.M. Chlorination Reactions Relevant to the Manufacture of Trichloroethene and Tetrachloroethene. Part 2. Effects of Chlorine Supply. Appl. Catal. A Gen. 2014, 471, 149–156. 38. Taylor, D.S.C. Catalysis in the System 1,2-Dichloroethane, Iron(III) Chloride. Ph.D. Thesis, University of Glasgow, Glasgow, UK, 2002. 39. Fontana, C.M.; Gorin, E.; Kidder, G.A.; Meredith, C.S. Ternary System, Cuprous Chloride-Cupric Chloride-Potassium Chloride and its Equilibrium Chlorine Pressures. Ind. Eng. Chem. 1952, 44, 363–368. 40. Arganbright, R.P.; Yates, W.F. Chlorinations with Cupric Chloride. J. Org. Chem. 1962, 27, 1205–1208. 41. Cavani, F. Catalytic Selective Oxidation: The Forefront in the Challenge for a more Sustainable Chemical Industry. Catal. Today 2010, 157, 8–15. 42. Leofanti, G.; Padovan, M.; Garilli, M.; Carmello, D.; Zecchina, A.; Spoto, G.; Bordiga, S.; Turnes Palomino, G.; Lamberti, C. Alumina-Supported Copper Chloride 1. Characterization of freshly Prepared Catalyst. J. Catal. 2000, 189, 91–104. 43. Leofanti, G.; Padovan, M.; Garilli, M.; Carmello, D.; Marra, G.L.; Zecchina, A.; Spoto, G.; Bordiga, S.; Lamberti, C. Alumina-Supported Copper Chloride 2. Effect of Aging and Thermal Treatments. J. Catal. 2000, 189, 105–116. 44. Leofanti, G.; Marsella, A.; Cremaschi, B.; Garilli, M.; Zecchina, A.; Spoto, G.; Bordiga, S.; Fisicaro, P.; Berlier, G.; Prestipino, C.; et al. Alumina-Supported Copper Chloride 3. Effect of Exposure to Ethylene. J. Catal. 2001, 202, 279–295. 45. Leofanti, G.; Marsella, A.; Cremaschi, B.; Garilli, M.; Zecchina, A.; Spoto, G.; Bordiga, S.; Fisicaro, P.;

Prestipino, C.; Villain, F.; et al. Alumina-Supported Copper Chloride 4. Effect of Exposure to O2 and HCl. J. Catal. 2002, 205, 375–381. 46. Lamberti, C.; Prestipino, C.; Bonino, F.; Capello, L.; Bordiga, S.; Spoto, G.; Zecchina, A.; Diaz Moreno, S.; Cremaschi, B.; Garilli, M.; et al. The Chemistry of the Oxychlorination Catalyst: An In Situ, Time-Resolved XANES Study. Angew. Chem. Int. Ed. 2002, 41, 2341–2344. 47. Prestipino, C.; Bordiga, S.; Lamberti, C.; Vidotto, S.; Garilli, M.; Cremaschi, B.; Marsella, A.; Leofanti, G.; Fisicaro, P.; Spoto, G.; et al. Structural Determination of Copper Species on the Alumina-Supported Copper Chloride Catalyst: A Detailed EXAFS Study. J. Phys. Chem. B 2003, 107, 5022–5030. 48. Muddada, N.B.; Olsbye, U.; Caccialupi, L.; Cavani, F.; Leofanti, G.; Gianolio, D.; Bordiga, S.; Lamberti, C. Influence of Additives in Defining the Active Phase of the Ethylene Oxychlorination Catalyst. Phys. Chem. Chem. Phys. 2010, 12, 5605–5618. 49. Muddada, N.B.; Olsbye, U.; Leofanti, G.; Gianolio, D.; Bonino, F.; Bordiga, S.; Fuglerud, T.; Vidotto, S.; Marsella, A.; Lamberti, C. Quantification of Copper Phases, their Reducibility and Dispersion in

Doped-CuCl2/Al2O3 Catalysts for Ethylene Oxychlorination. Dalton Trans. 2010, 39, 8437–8449. 50. Zhang, T.; Troll, C.; Rieger, B.; Kintrup, J.; Schlüter, O.F.-K.; Weber, R. Reaction Kinetics of Oxychlorination of Carbon Monoxide to Phosgene based on Copper(II) Chloride. Appl. Catal. A Gen. 2009, 357, 51–57. 51. Zhang, T.; Troll, C.; Rieger, B.; Kintrup, J.; Schlüter, O.F.-K.; Weber, R. Composition Optimisation of Silica-Supported Copper(II) Chloride Substance for Phosgene Production. Appl. Catal. A Gen. 2009, 365, 20–27. 52. Zhang, T.; Troll, C.; Rieger, B.; Kintrup, J.; Schlüter, O.F.-K.; Weber, R. Oxychlorination of CO to Phosgene in a Three-Step Reaction Cycle and Corresponding Catalytic Mechanism. J. Catal. 2010, 270, 76–85.

© 2017 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).