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

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.


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].

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 a six-coordinate Al III but, unlike α-AlF 3 , 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 behavior [4]; a model for the coordinatively unsaturated surface Al III species has been deduced [5]. In contrast, ACF was developed from industrial research in fluorocarbon chemistry [6]. Judged by its chemical behavior, its Lewis acidity is comparable to that of liquid antimony pentafluoride, a benchmark in the field. Synthesized by halogen exchange between solid aluminum trichloride and various chlorofluorocarbons, hydrochlorofluorocarbons or hexafluoropropene, the reactions do not result in complete replacement of Cl by F; the stoichiometry of the solid product is AlCl x F 3−x , with x being in the range 0.05-0.3. Surface areas are in the range 100-150 m 2 ·g −1 . The structure of the amorphous solid has been deduced at the atomic level from a number of spectroscopic techniques [7]. Chlorine is distributed throughout the solid, rather than in a discrete AlCl 3 phase, and is possibly bridged to three Al III centers. There appear to be three different types of six-coordinated Al III .
The defect spinel oxide 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 Table 1. These fluorinated aluminas are not supported reagents, since the fluorine is incorporated within the surface, and in some cases, possibly in the bulk to some extent. They contrast therefore with materials, such as supported boron trifluoride [9], which have been used widely as solid Lewis acids. Two of the possible surface species that have been suggested are shown in Figure 1. coordinate Al III 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 behavior [4]; a model for the coordinatively unsaturated surface Al III species has been deduced [5]. In contrast, ACF was developed from industrial research in fluorocarbon chemistry [6]. Judged by its chemical behavior, its Lewis acidity is comparable to that of liquid antimony pentafluoride, a benchmark in the field. Synthesized by halogen exchange between solid aluminum trichloride and various chlorofluorocarbons, hydrochlorofluorocarbons or hexafluoropropene, the reactions do not result in complete replacement of Cl by F; the stoichiometry of the solid product is AlClxF3−x, with x being in the range 0.05-0.3. Surface areas are in the range 100-150 m 2 ·g −1 . The structure of the amorphous solid has been deduced at the atomic level from a number of spectroscopic techniques [7]. Chlorine is distributed throughout the solid, rather than in a discrete AlCl3 phase, and is possibly bridged to three Al III centers. There appear to be three different types of six-coordinated Al III . The defect spinel oxide 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 Table 1. These fluorinated aluminas are not supported reagents, since the fluorine is incorporated within the surface, and in some cases, possibly in the bulk to some extent. They contrast therefore with materials, such as supported boron trifluoride [9], which have been used widely as solid Lewis acids. Two of the possible surface species that have been suggested are shown in Figure 1. The uptake of F by γ-alumina is slow when CHF3 is used. The process is initiated at the surface of oxide particles, subsequently being incorporated into sub-surface regions. The reaction between sulfur tetrafluoride and γ-alumina is essentially a hydrolysis; the course of the reaction has been followed using the radiotracers fluorine-18 and sulfur-35. The hydrolysis involves the replacement of surface hydroxyls by F and the partial replacement of bridging Al-O-Al moieties by Al-F, with the concomitant formation of OSF2 and SO2. These are not strongly adsorbed. The Lewis acid sites have disordered F/O environments rather than being fully fluorinated. When the SF4 treatment is performed The uptake of F by γ-alumina is slow when CHF 3 is used. The process is initiated at the surface of oxide particles, subsequently being incorporated into sub-surface regions. The reaction between sulfur tetrafluoride and γ-alumina is essentially a hydrolysis; the course of the reaction has been followed using the radiotracers fluorine-18 and sulfur-35. The hydrolysis involves the replacement of surface hydroxyls by F and the partial replacement of bridging Al-O-Al moieties by Al-F, with the concomitant formation of OSF 2 and SO 2 . These are not strongly adsorbed. The Lewis acid sites have disordered F/O environments rather than being fully fluorinated. When the SF 4 treatment is performed under static conditions, nominally at room temperature, the retention of the co-product, HF, appears to be complete. Consequently, γ-alumina fluorinated by this method possesses Brønsted acidity in addition to the expected Lewis acidity.
A most powerful method to probe the Lewis acidity of fluorinated surfaces is FTIR spectroscopy using Lewis base probe molecules such as pyridine (py) or carbon monoxide, and 2-methyl-substituted py as a probe where Brønsted surface hydroxyl groups are present. Studies using CO at low temperatures can be particularly powerful, since different types of surface sites can be detected and compared. The aluminum fluorides mentioned above and various oxofluorides of different structural types have been studied in this way and the results compared with nanoscale aluminum fluorides; accounts of the methodology and some illustrative results are to be found in [10,11].
Useful data can be obtained in addition, using a radiolabeled probe molecule to investigate a fluorinated surface. Experiments using anhydrous hydrogen chloride and its precursors, labeled with chlorine-36, are described below. In addition, a radiotracer can provide important mechanistic details.
Two examples are provided by the behavior of the trichlorotrifluoroethane isomers at β-AlF 3 and the fluorinated γ-alumina surfaces. Isomerization of CCl 2 FCClF 2 to the thermodynamically preferred isomer CCl 3 CF 3 does not involve any formation of Al-Cl surface bonds, as indicated by the labeling of CCl 2 FCClF 2 with [ 36 Cl]. This indicates that the isomerization proceeds by an intra-rather than by an inter-molecular mechanism, as shown in Figure 2a. Isomerization is followed by a dismutation reaction giving a mixture of CCl 2 FCF 3 and CCl 3 CClF 2 , which, from the lack of the incorporation of [ 36 Cl] into the solid surface, is believed to involve a halogen exchange between two adsorbed CCl 3 CF 3 molecules, as shown in Figure 2b [12]. under static conditions, nominally at room temperature, the retention of the co-product, HF, appears to be complete. Consequently, γ-alumina fluorinated by this method possesses Brønsted acidity in addition to the expected Lewis acidity. A most powerful method to probe the Lewis acidity of fluorinated surfaces is FTIR spectroscopy using Lewis base probe molecules such as pyridine (py) or carbon monoxide, and 2-methyl-substituted py as a probe where Brønsted surface hydroxyl groups are present. Studies using CO at low temperatures can be particularly powerful, since different types of surface sites can be detected and compared. The aluminum fluorides mentioned above and various oxofluorides of different structural types have been studied in this way and the results compared with nanoscale aluminum fluorides; accounts of the methodology and some illustrative results are to be found in [10,11].
Useful data can be obtained in addition, using a radiolabeled probe molecule to investigate a fluorinated surface. Experiments using anhydrous hydrogen chloride and its precursors, labeled with chlorine-36, are described below. In addition, a radiotracer can provide important mechanistic details.
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 isomer CCl3CF3 does not involve any formation of Al-Cl surface bonds, as indicated by the labeling of CCl2FCClF2 with [ 36 Cl]. This indicates that the isomerization proceeds by an intra-rather than by an inter-molecular mechanism, as shown in Figure 2a. Isomerization is followed by a dismutation reaction giving a mixture of CCl2FCF3 and CCl3CClF2, which, from the lack of the incorporation of [ 36 Cl] into the solid surface, is believed to involve a halogen exchange between two adsorbed CCl3CF3 molecules, as shown in Figure 2b [12]. These observations provide indirect evidence for the presence of strong Lewis acid surface sites on β-AlF3 and fluorinated γ-aluminas, sufficiently strong to allow C-F bonds, unexpectedly, to behave as Lewis bases.

Anhydrous Hydrogen Chloride as a Surface Probe
It is not intuitively obvious that anhydrous HCl might be a useful probe molecule which can be used to compare the surface behavior of different fluorinated surfaces, but this has proven to be the case, particularly for comparisons among nanoscale aluminum fluorides and their conventionally prepared counterparts. The genesis of the approach was a fortuitous observation of reactions that occur when 1,1,1-trichloroethane is exposed to an SF4-fluorinated (static conditions, see Table 1) γ-alumina at room temperature. The reactions involved are shown in Figure 3 [9].
The surface-catalyzed reactions shown in Figure 3 are, respectively, a Lewis acid catalyzed dehydrochlorination of CH3CCl3 and an oligomerization, accompanied by a partial dehydrochlorination, of adsorbed CH2=CCl2; both have their equivalent reactions using the archetypal Lewis acid, solid aluminum chloride. The presence of retained HF on the surface probably accounts for the fluorination These observations provide indirect evidence for the presence of strong Lewis acid surface sites on β-AlF 3 and fluorinated γ-aluminas, sufficiently strong to allow C-F bonds, unexpectedly, to behave as Lewis bases.

Anhydrous Hydrogen Chloride as a Surface Probe
It is not intuitively obvious that anhydrous HCl might be a useful probe molecule which can be used to compare the surface behavior of different fluorinated surfaces, but this has proven to be the case, particularly for comparisons among nanoscale aluminum fluorides and their conventionally prepared counterparts. The genesis of the approach was a fortuitous observation of reactions that occur when 1,1,1-trichloroethane is exposed to an SF 4 -fluorinated (static conditions, see Table 1) γ-alumina at room temperature. The reactions involved are shown in Figure 3 [9].
The surface-catalyzed reactions shown in Figure 3 are, respectively, a Lewis acid catalyzed dehydrochlorination of CH 3 CCl 3 and an oligomerization, accompanied by a partial dehydrochlorination, of adsorbed CH 2 =CCl 2 ; both have their equivalent reactions using the archetypal Lewis acid, solid aluminum chloride. The presence of retained HF on the surface probably accounts for the fluorination step. This can be made catalytic if the purple, oligomeric supported layer is exposed to a mixture of HF and CH 3 CCl 3 . Although similar behavior is observed when BF 3 is used to fluorinate γ-alumina (cf. Figure 1), it occurs to a far smaller extent, reflecting, no doubt, the smaller fluorine content of the surface [9]. step. This can be made catalytic if the purple, oligomeric supported layer is exposed to a mixture of HF and CH3CCl3. Although similar behavior is observed when BF3 is used to fluorinate γ-alumina (cf. Figure 1), it occurs to a far smaller extent, reflecting, no doubt, the smaller fluorine content of the surface [9]. Dehydrochlorination occurs also when t-butyl chloride is exposed to a fluorinated Lewis acidic surface at room temperature, although the chemistry is not as dramatic visually as that derived from CH3CCl3. It is a useful reaction, however, since Bu t Cl is readily labeled with [ 36 Cl] and the resulting fate of the H 36 Cl produced can be monitored from its radioactivity [12].
The experimental procedures for quantifying β − activity from the long-lived chlorine-36 isotope by Geiger-Müller counting have been well documented in the literature [10][11][12] and are not described here. Although the behavior observed when [ 36 Cl]-Bu t Cl is exposed to SF4-fluorinated γ-alumina, β-AlF3 and aluminum chlorofluoride (ACF) is similar, there are differences in the details [12]. In particular, the outcomes with SF4-fluorinated γ-alumina appear to be dominated by oligomerization reactions that cover the surface, whereas strongly and weakly surface-bound H 36 Cl are observed with ACF. More controversially, a significant fraction of H 36 Cl appears to be retained in the bulk ACF, from which it can be removed by prolonged pumping. The observations using β-AlF3 are similar; in this case, however, HCl interacts with residual H2O located in the HTB hexagonal channels and is desorbed as H2O·HCl. The behaviors of ACF and β-AlF3 towards H 36 Cl added directly replicate those described above.
The behavior of nanoscale aluminum(III) fluoride, HS-AlF3, towards H 36 Cl parallels that of ACF; weakly and strongly surface-bound H 36 Cl and material retained in the bulk are all observed [13]. Dehydrochlorination of Bu t Cl is rapid, and HCl is observed immediately in the vapor; the analogous reaction over β-AlF3 is far slower. Nanoscale magnesium fluoride, HS-MgF2, behaves in an identical way, indicating that unlike rutile MgF2, HS-MgF2 can behave as a Lewis acid. The composite material, 15 mol % FeF3 in HS-MgF2, in its behavior towards [ 36 Cl]-Bu t Cl, appears to utilize Lewis acid sites based on Fe III in addition to those involving Mg II [13].
As is evident from the summary given above, the level of detail available about the interactions between HCl and fluorinated surfaces is limited, largely by the corrosive nature of the materials involved. More detailed descriptions become possible for the interactions between anhydrous HCl, which of course has its own corrosion problems, and transitional aluminas such as γ-and η-alumina. FTIR spectroscopy, utilizing temperature-programmed techniques and, more recently, inelastic neutron scattering, has been widely used. Together with a knowledge of the molecular environments of the various surface Lewis acid sites on the aluminas, 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 Figure 4 [14]. Dehydrochlorination occurs also when t-butyl chloride is exposed to a fluorinated Lewis acidic surface at room temperature, although the chemistry is not as dramatic visually as that derived from CH 3 CCl 3 . It is a useful reaction, however, since Bu t Cl is readily labeled with [ 36 Cl] and the resulting fate of the H 36 Cl produced can be monitored from its radioactivity [12].
The experimental procedures for quantifying β − activity from the long-lived chlorine-36 isotope by Geiger-Müller counting have been well documented in the literature [10][11][12] and are not described here. Although the behavior observed when [ 36 Cl]-Bu t Cl is exposed to SF 4 -fluorinated γ-alumina, β-AlF 3 and aluminum chlorofluoride (ACF) is similar, there are differences in the details [12]. In particular, the outcomes with SF 4 -fluorinated γ-alumina appear to be dominated by oligomerization reactions that cover the surface, whereas strongly and weakly surface-bound H 36 Cl are observed with ACF. More controversially, a significant fraction of H 36 Cl appears to be retained in the bulk ACF, from which it can be removed by prolonged pumping. The observations using β-AlF 3 are similar; in this case, however, HCl interacts with residual H 2 O located in the HTB hexagonal channels and is desorbed as H 2 O·HCl. The behaviors of ACF and β-AlF 3 towards H 36 Cl added directly replicate those described above.
The behavior of nanoscale aluminum(III) fluoride, HS-AlF 3 , towards H 36 Cl parallels that of ACF; weakly and strongly surface-bound H 36 Cl and material retained in the bulk are all observed [13]. Dehydrochlorination of Bu t Cl is rapid, and HCl is observed immediately in the vapor; the analogous reaction over β-AlF 3 is far slower. Nanoscale magnesium fluoride, HS-MgF 2 , behaves in an identical way, indicating that unlike rutile MgF 2 , HS-MgF 2 can behave as a Lewis acid. The composite material, 15 mol % FeF 3 in HS-MgF 2 , in its behavior towards [ 36 Cl]-Bu t Cl, appears to utilize Lewis acid sites based on Fe III in addition to those involving Mg II [13].
As is evident from the summary given above, the level of detail available about the interactions between HCl and fluorinated surfaces is limited, largely by the corrosive nature of the materials involved. More detailed descriptions become possible for the interactions between anhydrous HCl, which of course has its own corrosion problems, and transitional aluminas such as γand η-alumina. FTIR spectroscopy, utilizing temperature-programmed techniques and, more recently, inelastic neutron scattering, has been widely used. Together with a knowledge of the molecular environments of the various surface Lewis acid sites on the aluminas, 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 Figure 4 [14]. The sample was then progressively warmed to (ii) 423; (iii) 523; and (iv) 623 K. A flow of He was continually passed over the sample while heating to progressively higher temperatures. The sample was held at the desorption temperature for 15 min and then allowed to cool to room temperature, where the spectrum was acquired. All spectra are background subtracted, where the spectrum of the clean activated catalyst has been subtracted from the measured spectrum. Reproduced with permission from Reference [14].
For a detailed interpretation of the spectra, the reader is directed to Reference [14]; however, in summary, there is good evidence for weakly adsorbed molecular HCl and dissociatively adsorbed species in which HCl has interacted with Al-O species to form Al-Cl and OH groups, or has replaced a surface Al-OH by Al-Cl with the formation of H2O. This type of information is very important in producing realistic sequences of surface events. A very good example is the large-scale selective synthesis of methyl chloride from methanol and HCl, using an η-alumina heterogeneous catalyst [2]. The full sequence of events involving HCl and methanol at the surface is shown schematically in Figure 5 [15].  The sample was then progressively warmed to (ii) 423; (iii) 523; and (iv) 623 K. A flow of He was continually passed over the sample while heating to progressively higher temperatures. The sample was held at the desorption temperature for 15 min and then allowed to cool to room temperature, where the spectrum was acquired. All spectra are background subtracted, where the spectrum of the clean activated catalyst has been subtracted from the measured spectrum. Reproduced with permission from Reference [14].
For a detailed interpretation of the spectra, the reader is directed to Reference [14]; however, in summary, there is good evidence for weakly adsorbed molecular HCl and dissociatively adsorbed species in which HCl has interacted with Al-O species to form Al-Cl and OH groups, or has replaced a surface Al-OH by Al-Cl with the formation of H 2 O. This type of information is very important in producing realistic sequences of surface events. A very good example is the large-scale selective synthesis of methyl chloride from methanol and HCl, using an η-alumina heterogeneous catalyst [2]. The full sequence of events involving HCl and methanol at the surface is shown schematically in Figure 5 [15]. The sample was then progressively warmed to (ii) 423; (iii) 523; and (iv) 623 K. A flow of He was continually passed over the sample while heating to progressively higher temperatures. The sample was held at the desorption temperature for 15 min and then allowed to cool to room temperature, where the spectrum was acquired. All spectra are background subtracted, where the spectrum of the clean activated catalyst has been subtracted from the measured spectrum. Reproduced with permission from Reference [14].
For a detailed interpretation of the spectra, the reader is directed to Reference [14]; however, in summary, there is good evidence for weakly adsorbed molecular HCl and dissociatively adsorbed species in which HCl has interacted with Al-O species to form Al-Cl and OH groups, or has replaced a surface Al-OH by Al-Cl with the formation of H2O. This type of information is very important in producing realistic sequences of surface events. A very good example is the large-scale selective synthesis of methyl chloride from methanol and HCl, using an η-alumina heterogeneous catalyst [2]. The full sequence of events involving HCl and methanol at the surface is shown schematically in Figure 5 [15].

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][18][19][20][21][22][23][24][25][26]. Because of their redox properties arising from the 1e − , Fe III /Fe II redox transformation, they are also the basis of a useful polymerization process [27][28][29][30][31], for example the polymerization of thiophene. Anhydrous FeCl 3 has been widely reported as a Lewis acid catalyst for chlorination and hydrochlorination of a variety of hydrocarbons [32][33][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 Cl 2 may be problematic, since surface chlorination can occur, leading to a chloride surface that behaves like solid FeCl 3 . As a result, the unwanted chlorination of hydrochlorocarbons can occur; this may be accompanied by oligomerization of unsaturated species [35][36][37]. This type of behavior in the 1,1,2,2-tetrachloroethane/Cl 2 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 Cl 2 with CH 2 =CH 2 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 FeCl 3 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 FeCl 3 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 FeCl 3 is extremely hygroscopic, its surface may be complexed by trace water, present even in purified CH 2 ClCH 2 Cl. Molecular species such as FeCl 3 (OH 2 ), 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 CuCl 2 towards olefins suggests that CuCl 2 is the chlorinating agent rather than adsorbed Cl* [40]. From a heterogeneous catalytic standpoint, one of the most important uses of CuCl 2 is as the catalyst for the Deacon reaction, a process for the conversion of HCl to Cl 2 . 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].
2Cu II Cl 2 → 2Cu I Cl + Cl 2 (1) Since a Deacon, or oxychlorination, catalyst typically comprises, in addition to CuCl 2 , 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 a process environment, at the typical operating temperatures used, the species may well be liquid. One of the most detailed studies made to date under laboratory conditions involved CuCl 2 supported on γ-alumina. The catalyst was used in the oxychlorination of ethene; the product, CH 2 ClCH 2 Cl, underwent dehydrochlorination to CHCl=CH 2 , so the study was very relevant to PVC production [42][43][44][45][46][47][48][49]. Physical methods, particularly surface science-based together with reactor-based experiments, indicate that at least three Cu II -containing phases are present and that further mixed Cu II /Group I phases can be formed as well.
Chlorination or oxychlorination of CH 2 ClCH 2 Cl in the presence of a CuCl 2 /KCl catalyst supported on the clay mineral, attapulgite, leads to a complex mixture of C 2 chlorohydrocarbons and chlorinated olefins; the products depend on the exact conditions used but careful control can result in CHCl=Cl 2 or CCl 2 =CCl 2 being the predominant product. The reaction scheme deduced is shown in Figure 6 [36,37]. 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 dehydrochlorination to CHCl=CH2, so the study was very relevant to PVC production [42][43][44][45][46][47][48][49]. Physical methods, particularly surface science-based together with reactor-based experiments, indicate that at least three Cu II -containing phases are present and that further mixed Cu II /Group I phases can be formed as well.
Chlorination or oxychlorination of CH2ClCH2Cl in the presence of a CuCl2/KCl catalyst supported on the clay mineral, attapulgite, leads to a complex mixture of C2 chlorohydrocarbons and chlorinated olefins; the products depend on the exact conditions used but careful control can result in CHCl=Cl2 or CCl2=CCl2 being the predominant product. The reaction scheme deduced is shown in Figure 6 [36,37].  (2,3) processes that connect CHCl2CHCl2 with CHCl=CCl2 and CCl2=CCl2. Possible oligomer formation (5,6). Reproduced with permission from Reference [36].
There is, inevitably, a trade-off to be made between high conversions and the loss of materials through oligomerization, particularly of CHCl=CCl2. Product distributions are the result of competition between chlorination, facilitated by the CuCl2 catalyst, and dehydrochlorination. The latter for CHCl2CHCl2 is initiated by Cl*, whereas dehydrochlorination of CHCl2CCl3 occurs at Lewis sites on the catalyst.
The Cu II catalyst supports the Deacon reaction to convert the co-product, HCl, to the reactant Cl2; however, the reaction is slow compared to the main organo-chlorine transformations.
Phosgene is prepared on a large scale usually by the reaction between carbon monoxide and Cl2 over a carbon catalyst. An oxychlorination route has been developed, however, on a laboratory scale using a catalyst comprising CuCl2/KCl supported on silica gel [50][51][52]. The challenge in this case is to prepare OCCl2 continuously with good conversion and without hydrolysis. This has been achieved by employing a three-stage arrangement corresponding to Equations (1)-(4), viz. Equations (5)- (8).
There is, inevitably, a trade-off to be made between high conversions and the loss of materials through oligomerization, particularly of CHCl=CCl 2. Product distributions are the result of competition between chlorination, facilitated by the CuCl 2 catalyst, and dehydrochlorination. The latter for CHCl 2 CHCl 2 is initiated by Cl*, whereas dehydrochlorination of CHCl 2 CCl 3 occurs at Lewis sites on the catalyst.
The Cu II catalyst supports the Deacon reaction to convert the co-product, HCl, to the reactant Cl 2 ; however, the reaction is slow compared to the main organo-chlorine transformations.

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 FeCl 3 and, particularly, CuCl 2 , 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.