Zeolite catalysts for dehalogenation processes

Applied Catalysis A: General 271 (2004) 3–11

Zeolite catalysts for dehalogenation processes Russell F. Howe∗ Chemistry Department, University of Aberdeen, Aberdeen, Scotland AB24 3UE, UK Accepted 1 March 2004

Abstract A review is given of recent work in which zeolites have been used to catalyse the decomposition, oxidation or hydrodehalogenation of halogen containing molecules. The reactivity of CFCs depends strongly on the number of C–H and C–Cl bonds in the molecule. Reactions over FAU and MFI zeolites involve acid sites, and irreversible dealumination of the zeolites through the formation of extraframework AlF3 species. Halons (bromofluorocarbons) have been less widely studied, but both pyrolysis and hydrodehalogenation reactions occur over transition metal exchanged MFI zeolites. Both acidic and metal loaded zeolites have also been explored as catalysts for the oxidation and/or hydrodehalogenation of other organohalogen compounds of environmental concern. Features of zeolite chemistry common to these different applications are identified, and prospects for development of zeolite based dehalogenation processes for environmental protection discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Zeolite catalysts; Dehalogenation; Hydrodehalogenation; Halons

1. Introduction The importance of organohalogen compounds in the environment is well established. The Montreal Protocol relating to ozone depletion has led to bans on the production and use of chlorofluorocarbons (CFCs) and Halons, resulting in the accumulation of stockpiles of banned materials. The need for safe disposal of these, and/or the conversion of them to alternative materials which are environmentally acceptable has prompted increasing research into catalytic conversion of CFCs and Halons. Other organochlorine compounds are found in industrial waste streams, as volatile compounds, or in ground water contaminated with agricultural or industrial waste. Effective removal of such contaminants through separation and destruction or by direct catalytic conversion is an important objective. The conversion of organohalogen compounds to less environmentally hazardous materials involves one of three processes: direct decomposition, oxidation, and hydrodehalogenation. The direct thermal decomposition of organohalogens (pyrolysis) requires in most cases extremely high temperatures, involves gas phase radical chemistry, and

∗

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0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.05.031

is not at all selective. Combustion (oxidation) likewise is inefficient and may produce hazardous by-products [1]. The so-called plasma arc combustion process is environmentally more acceptable, but expensive [2]. Catalytic oxidation lowers the operating temperature for complete destruction of organohalogens, and has typically involved supported noble metal catalysts. Catalyst deactivation by HCl or Cl2 produced in the reaction is however a major problem. Hydrodehalogenation allows in principle the conversion of the organohalogen compound to more hydrogen rich and less environmentally harmful products, and would seem economically more desirable than complete destruction. Many research groups have investigated the hydrodechlorination of CFCs, producing HFCs and HCl as reaction products. The catalysts used are typically supported noble metals [3–11]. Supported palladium catalysts appear to be the most active and show the highest selectivity for retention of C–F bonds (the undesirable reaction is complete hydrogenation of the CFC to an alkane, forming HF). In all cases, however, catalyst deactivation continues to be a major problem, just as with catalytic oxidation. Zeolites as catalysts or catalyst supports offer a number of potential advantages over metal oxide or supported metal catalysts. The high internal surface area allows in principle high reactivity, and the cation exchange capacity of the zeolite permits facile introduction of acidic or transition metal

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catalytic function. Certain zeolites are particularly resistant to structural collapse, particularly mordenite (MOR), ZSM-5 (MFI) and zeolite beta (BEA). These features have recently attracted several groups to begin investigating zeolites as catalysts or catalyst supports for catalytic decomposition of organohalogens, for oxidation, and for hydrodehalogenation. This article will review the present state of this particular field, identify features of zeolite chemistry which are common to all three types of catalytic process and will discuss prospects for use of zeolite catalysts in dehalogenation processes in the future.

2. Interaction of CFCs with zeolites There have been numerous fundamental studies of the adsorption of various chloro- and fluorocarbons in zeolites. Adsorption isotherms and heats of adsorption have been measured for a number of systems, e.g. CCl2 F2 in cation exchanged zeolite Y samples [12], HFCs in zeolite X and Y [13], and C2 HCl3 in dealuminated Y and ZSM-5 [14]. Computational (force field simulation) studies of the location of adsorbed organohalogen molecules within the zeolite pores have been reported [15–24]. Of particular note is the conclusion reached by George et al. [24] that unidimensional channel zeolite topologies, such as mordenite, may be preferred over larger and more open pore structures such as FAU for efficient sorption of heavily halogenated molecules. A recent powder neutron diffraction study of CFCl3 in NaY has shown that the preferred location of the adsorbed CFC is in the 12-ring windows of the zeolite pore system, where electrostatic interactions with both the framework oxide ions and with extraframework sodium cautions are optimised [25]. Spectroscopic methods (FTIR, Raman and FTIR) allow the possibility of observing directly the interaction of adsorbed CFCs with zeolite adsorbents, and any subsequent reactions that may occur. Hannus, Konya and co-workers have reported extensively on studies by FTIR and solid state NMR of the interaction of CFCs with zeolites Y and ZSM-5 [26–33]. CF2 Cl2 (CFC-12) is adsorbed intact at room temperature in both NaY and HZSM-5. The 13 C NMR spectrum of the adsorbed molecule is shifted only slightly from that of the corresponding liquid [29], and infrared spectra show broadening of the OH stretching bands of the zeolite indicating a weak interaction of the CFC with both internal (acidic) OH groups and the silanol groups that terminate the external surfaces of the zeolites [26]. Similar conclusions were reached in a Raman study of CFC-12 in zeolite NaY [34]. The Raman spectrum of CFC-12 adsorbed in NaY at room temperature closely resembled that of the gas phase, and the bands of the adsorbed molecule could be removed by evacuation at room temperature. However, the Raman study did provide evidence of a stronger interaction between CFC-12 and zeolite NaX. In this case, a new band appeared in the Raman spectrum at 507 cm−1 which did not relate to the other bands in the

spectrum of adsorbed CFC-12. This band persisted on evacuation. A similar band was also formed on exposure of NaX and NaY to CCl4 and to Cl2 . Several possible assignments are suggested by the authors [34] for this band. One possibility is a molecular chlorine species adsorbed on cation sites (formed in the case of CFC-12 by decomposition of the CFC). Alternatively, the 507 cm−1 band may be due to terminal Si–O− defect sites formed by dealumination of the zeolite. The ability of chlorine containing molecules to promote dealumination of zeolites is well known [35,36]. The chemistry proposed for this process involves initial chlorination of the surface followed by elimination of AlCl3 . Whether or not this can happen on room temperature exposure of zeolite NaX to CFC-12 is less clear. The FTIR and NMR studies of Hannus et al. certainly provide evidence that CFC-12 reacts more aggressively with zeolites at elevated temperatures. The extent and nature of reaction depends on the zeolite type and cation exchange. For example, the 13 C NMR spectrum of CFC-12 adsorbed in NaY at room temperature is replaced on heating to 473 K by signals due to adsorbed phosgene (COCl2 ) and CO2 . At higher temperatures (600 K) the 29 Si NMR spectrum showed that the zeolite structure is significantly degraded (about 50% amorphous phase formed), the 27 Al NMR spectrum shows the formation of some octahedrally coordinated aluminium as well as a significant loss in total signal intensity, and the 23 Na NMR spectrum shows clearly the formation of NaCl, as well as possibly NaF [29]. None of these changes were observed when a basic NaY zeolite prepared by adding NaN3 to NaY was heated in the presence of CFC-12 [30]. From this observation it was concluded that reactivity in the zeolites is associated with acidic rather than basic sites, the parent NaY zeolite being assumed to contain some residual acidic hydroxyl sites. Consistent with this conclusion is the observation that zeolite HY is much more reactive than NaY towards CFC-12. In the case of HY, reaction with CFC-12 at 500 K causes almost complete collapse of the zeolite, as judged from the 29 Si NMR spectrum. The 27 Al NMR spectrum of HY treated with CFC-12 at 500 K shows the appearance of intense new features at about 0 ppm (octahedral aluminium) and around 30 ppm. This third signal has been attributed to 5 coordinated aluminium, or possibly a distorted tetrahedral aluminium [28]. Evidence for at least partial framework destruction in HY is also found in infrared spectra in the framework stretching region, which show the formation of a new band at 911 cm−1 assigned to defect sites [37]. Infrared spectra of HY in the hydroxyl stretching region show that the zeolite is irreversibly dehydroxylated upon heating in the presence of CFC-12. Infrared spectra of the gas phase show that the reaction products of CFC-12 decomposition over HY are HCl, CO2 , and COCl2 [30]. It is noteworthy that no fluorine containing products are detected in the gas phase. The zeolites MOR and MFI are known to be more resistant to structural damage than the FAU structure, and this

R.F. Howe / Applied Catalysis A: General 271 (2004) 3–11

is also true in their reaction with CFC-12 [37]. 13 C NMR spectra show that decomposition of CFC-12 adsorbed in HZSM-5 occurs through similar pathways to that in HY, involving COCl2 , CCl4 as intermediates, with adsorbed CO2 as the final carbon containing product. These molecules are also detected in the gas phase. The 29 Si NMR spectra of both HMOR and HMFI show relatively little perturbation after heating in CFC-12, while the corresponding 27 Al NMR spectra show the formation of small amounts of extraframework octahedral aluminium. 19 F NMR spectra showed that fluorine is retained in the zeolite as AlF3 (a 19 F signal at −163.4 ppm, relative to the −6.1 ppm signal for the initially adsorbed CFC-12). Hannus et al. have also compared the reactivity of different CFCs over the same zeolites. CFCs containing more than two fluorine atoms (CFC-13 and CFC-14) underwent no reaction with NaY, HY or CuY [32]. CFC-10 (CCl4 ) and CFC-11 (CCl3 F) reacted in a similar manner to CFC-12, forming phosgene, CCl4 and ultimately CO2 , and causing extensive structural damage to the zeolites. The phosgene intermediate is believed to be the key to the structural damage. Phosgene cannot be readily formed from CF4 or CF3 Cl, so that these molecules do not readily react. The hydrogen containing adduct HCFC-22 reacted more slowly and formed CO as a final product rather than CO2 . A different reaction pathway leading to CO is proposed. The reactions described above and observed spectroscopically are in the first place stoichiometric reactions of the CFCs with the zeolites, and do not constitute catalytic cycles. Nevertheless, they do provide considerable insight into how the CFC reactants interact with zeolite catalysts. Hannus et al. do comment that the presence of oxygen prolongs the lifetime of the phosgene intermediate species [37], which may influence the extent of lattice dealumination. Catalytic decomposition of CFCs over acid zeolites in the presence of water is reported by Tajima et al. [38]. The overall chemistry can be represented: CFCl3 + 2 H2 O → CO2 + 3 HCl + HF H-MOR was found to be the most active of the zeolites tested. The authors attribute this to the greater acidity of this zeolite. The reactivity of different CFCs over this catalyst varied widely. The reaction temperature needed to achieve 100% conversion increased in the order CCl4 < CFCl3 < CCl2 F2 < CF3 Cl. CF4 did not react up to 873 K, while CFC113 (C2 F3 Cl3 ) decomposed in a similar temperature range to CF3 Cl. These differences can be accounted for in terms of bond energy differences. The C–Cl bond energy increases from CCl4 (298 kJ mol−1 ) to CF3 Cl (360 kJ mol−1 ), which correlates well with reactivity, suggesting that C–Cl bond cleavage is the rate determining step in the reaction. Catalyst deactivation is observed with the HMOR catalyst after prolonged time on stream. Deactivation is associated with loss of Bronsted acid sites, as shown by FTIR and

5

XPS measurements. The extent of deactivation is strongly dependent on the concentration of water present in the reaction. When the water concentration is just sufficient for stoichiometric reaction, deactivation occurs relatively slowly. At higher water concentrations however, deactivation is more rapid, which can be attributed to the role of aqueous acid in dealuminating the zeolite framework.

3. Interaction of Halons with zeolites The catalytic chemistry of the Halons (bromofluorocarbons) has received much less attention than that of the corresponding CFCs. There is nevertheless an equally important need to develop efficient processes for the safe disposal of existing stockpiles of these ozone depleting chemicals or their conversion to less harmful alternatives. The most important application of Halons has been as fire suppressants, both in total flooding systems (typically Halon 1301,CF3 Br) and in hand held extinguishers (e.g. Halon1211, CF2 ClBr). The C–Br bond strength in Halons is significantly lower than the corresponding C–Cl bond strength in CFCs (average carbon–halogen single bond enthalpies are: C–F = 485 kJ mol−1 , C–Cl = 327 kJ mol−1 , C–Br = 285 kJ mol−1 and C–I = 228 kJ mol−1 [39]), suggesting that catalytic chemistry already proposed for CFCs may be applied also to Halons. The C–Br bond is cleaved homolytically when Halons are heated in the gas phase to elevated temperatures in the absence of a catalyst. The group of Dlugogorski and Kennedy has studied the homogeneous gas phase chemistry of Halons in some detail [40–43]. For example, Halon 1301 (CF3 Br) when heated above 873 K dissociates to CF3 radicals and Br atoms. The major carbon containing final products are C2 F6 and CF4 . The authors were able to model the observed conversions and selectivities to both major and minor products as a function of temperature and space velocity with a 12-step radical mechanism. C–Br bond cleavage is followed by attack of the Br atom on a second CF3 Br molecule: CF3 Br → CF3 • + Br • CF3 Br + Br • → CF3 • + Br 2 C2 F6 is formed by recombination of two CF3 • radicals, while CF4 results from attack of CF3 • on a further CF3 Br molecule: CF3 • + CF3 Br → CF4 + CF2 Br • Further subsequent reactions lead to the formation of minor products such as CBr2 F2 and ultimately to termination of the chain reaction. Given the relative stability of zeolites such as MFI in reactions with CFCs demonstrated by Hannus [30], Li et al. [44] undertook a study of the effects of MFI zeolite catalysts on the pyrolysis chemistry of Halon 1301. Catalysts were

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R.F. Howe / Applied Catalysis A: General 271 (2004) 3–11

mounted in a fixed bed microreactor using CF3 Br flow rates similar to those employed in the homogenous reaction. HZSM-5 catalysts give an initial high conversion of CF3 Br at temperatures 200 K lower than those required for onset of the homogeneous reaction. This initial conversion quickly falls however as a function of time on stream, and final steady state conversions are comparable with those found in the homogeneous reaction. Examination of used catalyst samples showed that the zeolite remained crystalline to X-ray powder diffraction. However, no Bronsted or Lewis acidity could be detected from infrared spectra of chemisorbed pyridine in samples of HZSM-5 exposed to CF3 Br at typical reaction temperatures (e.g. 873 K) for more than a few minutes. Surface analysis by XPS of such used catalysts revealed a high fluorine content and a depletion of the surface aluminium content. From these observations it was concluded that the severe deactivation of HZSM-5 in the pyrolysis of CF3 Br is due to the extensive framework dealumination that occurs and possible pore blockage by extraframework aluminium species. It is also clear that extensive C–F bond cleavage occurs over this catalyst. Catalyst deactivation was found to be much less severe when a Ni exchanged MFI zeolite was used for the pyrolysis of CF3 Br. In this case an initial very high conversion is observed for up to 1 h on stream, followed by deactivation to a steady state conversion still more than 10 times that observed in the absence of a catalyst under the same conditions. Interesting variations in product selectivity occur during time on stream. The major product during the first hour on stream is CF4 rather than C2 F6 . Subsequently, as the catalyst activity falls to its steady state value, C2 F6 becomes the major product. To try and understand why NiZSM-5 displays superior catalytic performance to HZSM-5 for CF3 Br pyrolysis, an extensive series of characterisation measurements were carried out on catalyst samples removed from the reactor at various times on stream [45]. The results of these experiments may be summarised as follows. • As in the case of HZSM-5, zeolite crystallinity is preserved throughout the reaction. • 27 Al and 29 Si NMR measurements show that framework dealumination occurs during the first few minutes on stream. The 27 Al spectra show an almost immediate loss of the characteristic signal at 59 ppm due to tetrahedral framework Al, indicating a lowering of symmetry around the framework sites, but the signal at 0 ppm due to octahedral extraframework aluminium appears only after several hours on stream. • XPS surface analysis shows a lowering of the surface aluminium content and the appearance of surface bound fluorine in used catalysts. The surface fluorine signal was reduced on argon ion sputtering, suggesting that the fluorine is largely associated with the external surface of the catalysts.

• FTIR spectra of the used catalysts show that these still contain Bronsted acid sites at levels comparable to those in the fresh zeolite. The spectrum of the pyridinium cation formed from pyridine chemisorbed on used catalysts differs from that on the fresh catalyst however, suggesting that the Bronsted acid sites differ. • Aluminium K-edge XANES spectra from used catalysts show the characteristic signature of AlF3 . • Nickel K-edge EXAFS data show rapid (within 5 min) conversion of the initial isolated ion exchanged cations to a nickel cluster species which is either an oxidic or more likely a fluoride species (EXAFS is unable to distinguish between O and F nearest neighbour ligands). Conclusions drawn from these studies were that although framework dealumination is still extremely rapid in NiZSM-5, the presence of Ni inhibits the rapid deactivation seen with HZSM-5. The nickel cluster species formed very quickly on initial exposure to CF3 Br may be responsible for the high initial selectivity to CF4 , although how this deactivates is not yet clear. The steady state formation of C2 F6 is presumed to be associated with extraframework aluminium species, probably AlF3 , which does not deactivate as quickly as in HZSM-5. Further experiments have been reported in which CF3 Br is reacted over zeolite catalysts in the presence of CH4 as a hydrogen source for hydrodehalogenation [46,47]. In the absence of a catalyst, the homogeneous reaction of CF3 Br with CH4 begins above 800 K and increases rapidly above 973 K. The products at lower temperature are almost exclusively HCF3 and CH3 Br, i.e. the reaction can be regarded as a simple hydrogen exchange between the two reactants. At higher temperatures, the selectivity to CH3 Br falls and a complex mix of other products is formed. The authors of this study show that the reaction can be understood in terms of gas phase radical chemistry. The initiation step is homogeneous cleavage of a C–Br bond (just as in the pyrolysis reaction), forming CF3 • and Br• . The CF3 • radicals then abstract a hydrogen atom from CH4 , and the resulting CH3 • radical reacts with a bromine atom. At higher temperatures, the CH3 Br reacts further. When the homogeneous reaction of CF3 Br is intercepted by MFI zeolite catalysts, the temperature required for the onset of reaction is lowered by some 150 K. Transition metal exchanged MFI catalysts (Ni, Cu, Mn) give, after an initial break in period during which CF3 Br conversion is complete, steady state conversions of up to 60% (873 K, GHSV 3500 h−1 ). Conversions at these levels are maintained for up to 20 h before gradual deactivation occurs. The methane conversion does not however equal that of CF3 Br, suggesting that the reaction path is more complex than that in the homogeneous reaction. This is also evident from the product distributions obtained. The CH3 Br selectivity consistently exceeds that for HCF3 (in contrast to the homogeneous reaction, where HCF3 has the highest yield). Other significant

R.F. Howe / Applied Catalysis A: General 271 (2004) 3–11

products detected in the catalysed reaction, even at 873 K, are CH2 Br2 , C2 F6 , C2 H4 and C2 F2 H2 . Over HZSM-5, the CF3 Br conversion quickly falls as a function of time on stream to a steady state value only slightly higher than that achieved without a catalyst under the same conditions. The selectivities to CH3 Br and HCF3 are more closely similar, and C2 F6 is the largest minor product. In this case, the chemistry appears to be dominated by the homogeneous gas phase reactions. As in the pyrolysis reaction, X-ray diffraction measurements show that zeolite crystallinity is retained throughout the reaction of CF3 Br with CH4 , and NMR measurements confirm that framework dealumination occurs in this case as well. Aluminium K-edge XANES measurements likewise show the formation of AlF3 species. There is however a significant difference between the Ni K-edge EXAFS data for used pyrolysis catalysts versus those used in the reaction with methane. In the latter case, the EXAFS shows that nickel remains monatomically dispersed. As noted above, EXAFS is unable to distinguish between O and F in the first coordination shell. Nevertheless, sintering of the nickel does not appear to occur in the presence of methane. The initial high CF3 Br conversions achieved with a fresh NiZSM-5 catalyst in the presence of methane could be completely restored by subjecting used catalysts to a hydrogen treatment at 623 K. Oxygen treatment, on the other hand, did not significantly alter the activity of partially deactivated catalysts. These observations suggest that coking, which would be expected to be reversed by an oxidative treatment, is not a major contributor to catalyst deactivation. The reversal of deactivation by a hydrogen treatment suggests instead that poisoning by halogen (probably fluorine, since bromine is not detected by XPS) is responsible for deactivation. Some evidence does emerge that long term irreversible deactivation may eventually occur with these catalysts. Traces of organosilicone oils have been detected by GC-MS in the product stream during reaction of CF3 Br with CH4 over NiZSM-5. These presumably arise from methylation of the zeolite during reaction. The resulting loss of silicon must eventually lead to lattice destruction, but this has not yet been quantified. The reaction mechanism of CF3 Br with CH4 over transition metal exchanged MFI zeolites is not yet understood. In particular, the extent to which, and in what manner the zeolite catalyst promotes a heterolytic cleavage of C–Br or C–F bonds is not clear. Nevertheless, the catalyst performance figures produced so far, and the ability of the catalyst to be regenerated in hydrogen, suggest that this hydrodehalogenation reaction with methane may be a viable process for conversion of CF3 Br to the useful feedstock HCF3 . HCF3 is a potential precursor for the Halon replacement material CF3 I. This material, known commercially as tri-iodide [48], has fire suppressant properties only slightly inferior to those of Halon 1301. Other important Halons whose catalytic chemistry has not yet been explored over zeolites or any other catalyst include

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1211 (CF2 ClBr) and 2402 (C2 F4 Br2 ). The homogeneous reaction of 1211 with CH4 has been shown recently to yield high conversions to the coupling product CF2 CH2 [42]. The possibility of enhancing this reaction over zeolite catalysts at lower temperatures has implications for both Halon conversion and the more fundamental issue of catalytic activation of methane.

4. Catalytic conversion of other halocarbons over zeolites The catalytic conversion of methyl chloride over zeolite catalysts has been investigated in detail by Lunsford and co-workers [49]. The motivation behind this work was the possible conversion of methane to light olefins via a methyl chloride intermediate [50]. The chemistry involved is quite analogous to the more familiar conversion of methane to light olefins via methanol (the so-called MTO process). Methyl chloride can be produced by oxychlorination of methane. The subsequent conversion to ethylene is written simply as: 2 CH3 Cl → C2 H4 + 2 HCl The optimum catalyst for this reaction was found to be a phosphorus modified magnesium exchanged MFI zeolite. The authors propose a mechanism in which magnesium cations activate methyl chloride to form HCl and a carbene intermediate. Surface bound methoxy groups are formed from reaction of methyl chloride with Bronsted acid sites (13 C NMR evidence was presented for the formation of such species). Reaction between the carbene intermediate and a surface methoxy group then forms ethylene. An MFI zeolite containing no magnesium has much lower activity for the same reaction, supporting the suggestion that the optimum catalyst is genuinely bifunctional, with the magnesium cations playing an important role. FTIR studies of chemisorbed pyridine showed that the phosphorus modification of MgZSM-5 reduces the strong Bronsted acidity of the zeolite, inhibiting the undesired further oligomerisation of ethylene to paraffins and aromatics. 27 Al NMR measurements showed that this loss of strong Bronsted acidity is due to partial framework dealumination. Notwithstanding the improved performance of the phosphorus modified catalysts, deactivation still occurred over 24 h on stream. Deactivated catalysts could however be completely regenerated by an oxidative treatment, and the catalyst lifetime improved in regenerated catalysts. This behaviour is very similar to that observed in methanol conversion over HMFI zeolites [51], where the improvement in catalyst lifetime is attributed to a further reduction in the number of strong acid sites which promote coke formation. 1,2-dichloroethane and trichloroethylene are common industrial air pollutants. The use of zeolite catalysts for the deep oxidation of these materials to CO2 , HCl and H2 O was

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R.F. Howe / Applied Catalysis A: General 271 (2004) 3–11

recently reported by Gonzalez-Velasco et al. [52]. The reaction pathways proposed for the two reactants were: C2 H4 Cl2 → C2 H3 Cl + HCl C2 H3 Cl +

5 2

followed by

O2 → 2CO2 + HCl

and C2 HCl3 + 2 O2 → 2 CO2 + HCl + Cl2 C2 HCl3 + Cl2 → C2 Cl4 + HCl

followed by

C2 Cl4 + 2 O2 → 2 CO2 + 2 Cl2 HZSM-5 was found to be a more active catalyst than HY for both reactions. Trichloroethylene requires a higher temperature for complete conversion than dichloroethane, which is consistent with earlier reports in the literature that highly chlorinated compounds are more resistant to complete oxidation [53]. Finnochio et al. have compared HZSM-5 with HY and a silica-alumina catalyst for the oxidative dechlorination of trichloroethylene, chloropropanes and dichloropropanes [54]. Dechlorination of 1- and 2-chloropropane occurred most effectively over the silica-alumina catalyst, producing propene and HCl with high selectivity: C3 H7 Cl → C3 H6 + HCl Dichloropropanes undergo initial conversion to monochloropropenes: C3 H6 Cl2 → C3 H5 Cl + HCl followed by complete combustion to CO2 , H2 O and HCl at much higher temperatures. The zeolite catalysts were found to be considerably inferior to silica-alumina for these reactions, becoming heavily coked during reaction. Silica-alumina is also reported to be the most effective catalyst for trichloroethylene oxidation, with no evidence of coke deactivation. The authors comment that zeolite HY may also be suitable for this reaction, but give no detailed results. The particular problem of dealing with halogenated hydrocarbons in contaminated ground-water using catalytic hydrodehalogenation has been recently addressed by Schuth et al. [55]. Palladium based catalysts can readily hydrogenate contaminants such as trichloroethylene and perchloroethylene even at room temperature, yielding ethane and HCl [56]. However, palladium catalysts are susceptible to poisoning by reduced sulphur species, which are often found in ground water. Schuth et al. have developed zeolite supported palladium catalysts which are resistant to such poisoning and shown that they can be effectively used in the treatment of real ground water samples. Palladium was supported in zeolites ZSM-5, zeolite Y with three different Si:Al ratios, and mesoporous MCM-41 with three different Si:Al ratios, and the hydrodechlorination of 1,2-dichlorobenzene in water investigated at room

temperature. In the absence of sulphite poisons, the activity for 1,2-dichlorobenzene conversion to benzene and HCl increased with increasing pore size of the zeolite. Pd-ZSM-5 catalysts showed very little activity, consistent with the steric exclusion of 1,2-dichlorobenzene from the zeolite pores. The highest activity was shown by Pd-MCM-41 catalysts, which have ca. 2.7 nm pore diameter. When sodium sulphite was added to the aqueous 1,2dichlorobenzene stream, all of the catalysts were severely poisoned except for palladium supported in zeolite Y having a Si:Al ratio of 200, i.e. a highly hydrophobic support. In this case, the conversion achieved in the presence of sulphite is only slightly lower than that found in the absence of sulphite. The authors attribute the remarkable poison resistance of the Pd-Y catalyst to the hydrophobicity of the support, arguing that while 1,2-dichlorobenzene can penetrate into the zeolite pores to be hydrogenated at palladium sites, aqueous sulphite cannot and so poisons only those few palladium sites that exist on the external surface of the zeolite. An authentic groundwater sample contaminated with trichloroethylene, dichloroethylene and vinyl chloride was then treated with hydrogen over the hydrophobic PdY catalyst. All of the contaminants were hydrodehalogenated to ethane and HCl, and the catalyst activity remained approximately constant for up to 10 days. This work clearly demonstrates the potential of engineering the pore size and hydrophobicity of zeolite supports to obtain hydrodehalogenation catalysts which may be used under realistic industrial waste treatment conditions. Wet gas phase oxidation of dichloromethane over NaX and NaY zeolites was described very recently by Dinard et al. [57]. There is an initial stoichiometric reaction between CH2 Cl2 , H2 O and the zeolite, written as: CH2 Cl2 + H2 O + NaOZ → HCHO + HCl + NaCl + HOZ The resulting modified zeolites then appear to show a steady state catalytic activity for the transformation of CH2 Cl2 to formaldehyde. NaX shows superior performance to NaY, attributed to the greater basicity of the zeolite framework. There is also evidence from infrared spectra of framework dealumination, presumably caused by HCl. The authors suggest that combination of such basic zeolites with an oxidising function associated with platinum should provide a viable route for the destruction of volatile organochlorine compounds, and they report that a Pt-NaX zeolite catalyst is able to completely transform dichloromethane into CO2 , HCl and H2 O above 330 ◦ C. A platinum loaded HBEA zeolite has been used by Creyghton et al. [58] for the gas phase hydrodehalogenation of chlorobenzene. The initial activity of the catalyst was similar to that of a Pt-alumina catayst with similar metal surface area under the same conditions. The major reaction products are benzene and cyclohexane (along with HCl). The authors suggest that benzene which is initially formed may be further hydrogenated to cyclohexane. Over the zeolite supported catalyst, some isomerisation of cyclohexane

R.F. Howe / Applied Catalysis A: General 271 (2004) 3–11

to methylcyclopentane is also observed. A much more significant difference between the two catalysts however is that the zeolite supported catalyst deactivates with time on stream, whereas the initial activity of the alumina supported catalyst continues indefinitely. Deactivation is attributed to the presence of Bronsted acid sites in the HBEA support. These acid sites promote the oligomerisation of cyclohexene intermediate species leading to coke. The acid sites also cause some isomerisation of cyclohexane. Partial replacement of Bronsted acid sites in the zeolite support by sodium ion exchange reduces the extent of deactivation, but completely suppresses cyclohexane isomerisation. The Pt-NaBEA catalyst still cannot match however the performance of Pt-alumina, suggesting that in this case there is no particular advantage to be gained by using a zeolite support. A similar conclusion can be drawn from the work of Shin and Keane [59], who compared the performance of nickel-silica and nickel-Y zeolite catalysts for the gas phase hydrodechlorination of pentachlorophenol. Over the nickel-silica catalyst a mix of dichloro-, monochloroand fully dechlorinated phenol products were obtained. For nickel supported on zeolite Y, on the other hand, hydrodechlorination was less complete, and the major reaction product was 2,4,5 trichlorophenol. The specific activity of the zeolite supported catalyst (per gram of nickel) was also significantly lower than that of the best silica supported catalyst. The authors of this study concluded that both zeolite and silica supported nickel catalysts are 100% selective in cleaving C–Cl bonds, leaving the aromatic ring and the hydroxyl group intact. The product distribution obtained, particularly the extent to which complete dechlorination occurs, is related to steric effects in the support. In the zeolite support, diffusion restrictions limit the extent to which complete dechlorination can occur. Both types of catalyst were found to deactivate; the more severe deactivation found in the zeolite supported catalyst was attributed to pore blockage, presumably by reaction products. An interesting use of zeolite catalysts in an area other than environmental remediation is the reported production of cyclohexene from the gas phase hydrodehalogenation of cyclohexyl chloride and cyclohexyl bromide [60]. This occurs over both silica and zeolite Y supported nickel catalysts. The conclusions reached in this study are very similar to those of the same research group in the study of pentachlorophenol dehalogenation over the same catalysts, namely that the silica supported catalysts are more active and deactivate less than the corresponding zeolite catalyst. Finally, a novel application of a zeolite catalyst to synthetic organic chemistry is reported by Adimurthy et al. [61]. These authors used an HBEA zeolite to catalyse debromination and deiodination of bromo- or iodo-phenols. This work was done in the context of using bromine or iodine as a protecting group in organic synthesis. It is then necessary to find an efficient method for removing the protecting group from

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the desired product. A solid acid zeolite such as HBEA offers potential advantages over conventional dehalogenation with corrosive hydrobromic acid. For example, quantitative dehalogenation of

was achieved by refluxing with HBEA zeolite in the presence of sodium sulphite as a halogen scavenger. The catalyst could be recovered and re-used reproducibly for removal of both bromine and iodine. Reaction did not occur under the same conditions however with the corresponding chloro compound, suggesting that C–Cl bonds could not be cleaved.

5. Zeolite chemistry: common features In considering the range of different reactions occurring over zeolites and zeolite supported catalysts summarised above, there are several common features in the chemistry which can be identified. Firstly, the use of zeolites as high surface area microporous supports for noble metal hydrodehalogenation or oxidation catalysts does not appear to offer any real advantage over conventional high surface area oxide or carbon supports, at least for high temperature gas phase reactions. The poisoning of the noble metal function by halogen is not inhibited by the zeolite support, and coke formation which occurs in all hydrocarbon reactions over zeolites containing acid sites may be an additional cause of deactivation. There are two notable exceptions to this generalisation. The use of a hydrophobic zeolite Y as a support for palladium catalysts in hydrodehalogenation of chlorocarbons in water is an innovative step forward. Under these low temperature reaction conditions coking is not an issue, and tuning of the hydrophobicity of the support apparently excludes aqueous sulphur containing poisons. The high silica zeolite support will also be more resistant to attack by aqueous HCl. The reported use of PtNaX zeolite for wet oxidation of dichloromethane involves very different chemistry. The initial stoichiometric reaction of dichloromethane with the zeolite apparently produces a partially dealuminated support which retains strong basic sites which can activate the dichloromethane, while the noble metal function catalyses the oxidation of formaldehyde intermediate to carbon dioxide and water. Further study of this system, particularly with regard to the extent of framework collapse of the zeolite and long term stability of the catalyst, will certainly be warranted. In all other cases so far studied, it is the interaction of the halocarbon with Bronsted acid sites in the zeolite which is

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the key to understanding the chemistry and to possibly developing improved catalyst performance. Bond strength considerations would suggest that it is the C–Cl or C–Br bonds that are most likely to react at Bronsted acid sites, through chemistry such as that proposed by Hannus et al. involving a phosgene intermediate. On the other hand, there is clear evidence also for the formation of aluminium and sodium fluoride species. The fluoriding of alumina by reaction with fluorine containing molecules at elevated temperatures is well known [5], and similar chemistry can be postulated to occur in zeolites. Indeed, the available evidence suggests it is AlF3 that is formed as the extraframework aluminium species rather than AlCl3 or AlBr3 . Thus C–F bonds are also being broken. As noted above, the best zeolites to use in high temperature gas phase reactions of halocarbons would therefore seem to be those whose structures most readily accommodate framework dealumination, i.e. MFI, MOR or possibly BEA. Zeolites containing both acid sites and a second cation (Mg2+ , Ni2+ , Mn2+ ) appear to offer some advantages both in terms of activity and catalyst lifetime. It is not yet clear however how the second cation acts to activate the halocarbon and to inhibit deactivation.

6. Challenges and opportunities A common feature in most of the zeolite catalysts so far studied is the occurrence of an initial break in period during which stoichiometric reactions are occurring between the halocarbon and the catalyst. For those systems in which a steady state activity is then achieved, the working catalyst may well be very different in composition and structure from the fresh zeolite. The development of improved catalyst performance calls for more detailed study of the changes occurring when zeolites or zeolite supported catalysts are exposed to typical halocarbon reactants. In situ spectroscopic studies of this chemistry are required. In terms of catalytic reactions, at least five areas of further development can be identified. • Low temperature wet oxidation appears to be the best route for disposal of low levels of gaseous halocarbon pollutants. Zeolite catalyst supports which resist coke formation and excessive framework dealumination provide a good means for dispersing the oxidation catalyst. Although these have been to date noble metals, transition metal oxides which may be more poison resistant can also be readily dispersed in zeolite supports, and warrant attention. • High temperature decomposition (pyrolysis) of CFCs or Halons is probably not an avenue to pursue, given the irreversible catalyst deactivation accompanying this chemistry. Further study of such reactions does neverthe-

less provide useful information about the ways in which halocarbons react with zeolites. • The use of methane as a hydrogen font for hydrodehalogenation reactions needs to be further explored. The initial work described above involving Halon 1301 and methane suggests that zeolite catalysts with stable and regenerable activity for these reactions are possible, and unpublished work from the Newcastle group [62] indicates that zeolites also catalyse the reaction of CFCs with methane. Gervasini et al. have very recently described the use of methane to assist in the catalytic oxidation of carbon tetrachloride over an alumina supported manganese oxide catalyst [63]. • The reported use of hydrophobic zeolite supported catalysts to effectively treat chlorocarbon contaminated waste water streams offers opportunities for further development. • The conversion of CFCs and Halons to more useful products calls for greater control of catalyst selectivity and lifetime than has so far been possible. Zeolite supports offer wide flexibility in this regard.

Acknowledgements The author’s collaboration with the University of Newcastle group (Professors Kennedy and Dlugogorski) has been funded by the Australian Research Council.

References [1] J.C. Dickerman, T.E. Emmel, G.E. Harris, M.E. Hammel, Technologies for CFC and Halon Destruction, EPA/600/7-89/011, U.S. Department of Commerce, 1989, p. 2. [2] G.M. Bickle, T. Suzuki, Y. Mitarai, Chem. Eng. Trans. I 70 (1992) 449. [3] B. Coq, J.M. Cognion, F. Figueras, D. Tournigant, J. Catal. 141 (1993) 21. [4] B. Coq, F. Figueras, S. Hub, D. Tournigant, J. Phys. Chem. 99 (1995) 11159. [5] E. Kemnitz, D.H. Menz, Prog. Solid State Chem. 26 (1998) 92. [6] W. Jusczyk, A. Malinowski, Z. Karpinski, Appl. Catal. A 166 (1998) 311. [7] R. Ohnishi, W.L. Wang, M. Ichikawa, Appl. Catal. A 113 (1994) 29. [8] Z. Karpinski, K. Early, J. D’Itri, J. Catal. 164 (1996) 378. [9] A. Malinowski, W. Jusczyk, Z. Karpinski, React. Kinet. Catal. Lett. 68 (1999) 53. [10] T. Mori, Y. Morikawa, Stud. Surface Sci. Catal. 130 (2000) 1985. [11] T. Mori, W. Ueda, Y. Morikawa, Catal. Lett. 38 (1996) 73. [12] S. Kobayashi, K. Mizuno, S. Kushiyama, R. Aizawa, Y. Koinuma, H. Ohuchi, Ind. Eng. Chem. Res. 30 (1991) 2340. [13] M.K. Crawford, K. Dobbs, R.J. Smalley, D.R. Lorbin, N. Maliszewsky, T.J. Udovic, R.R. Cavanagh, J.J. Rush, C.P. Grey, J. Phys. Chem. B 103 (1999) 431. [14] G. Weber, O. Bertrand, E. Froment, S. Bourg, F. Bouvier, D. Bissinger, M.H. Simonot-Grange, J. Chim. Phys. 93 (1996) 1412. [15] L. Abrams, J.C. Calabrese, D.R. Corbin, J.M. Newsam, S. Levine, C.M. Freeman, Stud. Surface Sci. Catal. 98 (1995) 248. [16] A.R. George, C.M. Freeman, C.R.A. Catlow, Zeolites 17 (1996) 466.

R.F. Howe / Applied Catalysis A: General 271 (2004) 3–11 [17] C.F. Mellot, A.M. Davidson, J. Eckert, A.K. Cheetham, J. Phys. Chem. B 102 (1998) 2531. [18] C.F. Mellot, A.K. Cheetham, S. Harris, R.J. Gorte, A.L. Myers, S. Savitz, J. Am. Chem. Soc. 120 (1998) 5788. [19] C.F. Mellot, A.K. Cheetham, S. Harris, S. Savitz, R.J. Gorte, A.L. Myers, Langmuir 14 (1998) 6728. [20] C.F. Mellot, A.K. Cheetham, J. Phys. Chem. B 103 (1999) 3864. [21] E. Jaramille, C.P. Grey, S. Auerbach, J. Phys. Chem. B 105 (2001) 12319. [22] H. Fuchs, A.K. Cheetham, J. Phys. Chem. B 105 (2001) 7375. [23] K.H. Lim, F. Jousse, S.M. Auerbach, C.P. Grey, J. Phys. Chem. B 105 (2001) 9918. [24] A.R. George, C.M. Freeman, C.R.A. Catlow, Zeolites 17 (1996) 466. [25] C. Mellot-Draznieks, J. Rodriguez-Carvajal, D.E. Cox, A.K. Cheetham, Phys. Chem. Chem. Phys. 5 (2003) 1882. [26] Z. Konya, I. Hannus, I. Kiricsi, Stud. Surface Sci. Catal. 105 (1997) 1509. [27] I. Hannus, Z. Konya, J.B. Nagy, I. Kiricsi, J. Molec. Struct. 410/411 (1997) 89. [28] I. Hannus, Z. Konya, J.B. Nagy, P. Lentz, I. Kiricsi, Appl. Catal. B 17 (1998) 157. [29] Z. Konya, I. Hannus, I. Kiricsi, P. Lentz, J.B. Nagy, Colloid. Surface. A 158 (1999) 35. [30] I. Hannus, Appl. Catal. A 189 (1999) 263. [31] I. Hannus, Z. Konya, T. Kollar, Y. Kiyozumi, F. Mizukami, P. Lentz, J.B. Nagy, I. Kiricsi, Stud. Surface Sci. Catal. 125 (1999) 245. [32] I. Hannus, Z. Konya, P. Lentz, J.B. Nagy, I. Kiricsi, J. Molec. Struct. 482/483 (1999) 359. [33] I. Hannus, Z. Konya, P. Lentz, J.B. Nagy, I. Kiricsi, J. Molec. Struct. 563/564 (2001) 167. [34] D.J. Wylie, R.P. Cooney, J.M. Seakins, G.J. Millar, Vib. Spec. 9 (1995) 245. [35] I. Hannus, I. Kiricsi, G.G. Tasi, P. Fejes, Appl. Catal. 66 (1990) L7. [36] H.K. Beyer, I.M. Belenkaya, Stud. Surface Sci. Catal. 5 (1980) 203. [37] I. Hannus, B. Imre, Z. Konya, J.B. Nagy, I. Kiricsi, React. Kinet. Catal. Lett. 74 (2001) 309. [38] M. Tajima, M. Niwa, Y. Fuji, Y. Koinuma, R. Aizawa, S. Kushiyama, S. Kobayashi, K. Mizuno, H. Ohuchi, Appl. Catal. B 9 (1996) 167. [39] G. Aylward, T. Findlay, S.I. Chemical Data, 4th ed., Wiley, Brisbane, 1999.

11

[40] K. Li, E. Kennedy, B. Moghtaderi, B. Dlugogorski, Environ. Sci. Tech. 34 (2000) 584. [41] K. Li, E. Kennedy, B. Dlugogorski, Chem. Eng. Sci. 55 (2000) 4067. [42] R. Tran, E. Kennedy, B. Dlugogorski, Ind. Eng. Chem. Res. 40 (2001) 3139. [43] E. Kennedy, K. Li, B. Moghtaderi, B. Dlugogorski, Chem. Eng. Comm. 176 (1999) 195. [44] K. Li, F. Oghanna, E. Kennedy, B. Dlugogorski, A. Fazeli, S. Thomson, R. Howe, Micro. Meso. Mater. 35/36 (2000) 219. [45] R. Howe, S. Thomson, Y. Yang, K. Li, E. Kennedy, B. Dlugogorski, J. Molec. Catal. A 181 (2002) 63. [46] K. Li, E. Kennedy, B. Dlugogorski, R. Howe, Chem. Commun. (1999) 709. [47] K. Li, E. Kennedy, B. Dlugogorski, R. Howe, Catal. Today 63 (2000) 655. [48] R.E. Tapscott, S.R. Skaggs, D. Diedorf, Am. Chem. Soc. Symp. Ser. 611 (1995) 151. [49] Y. Sun, S.M. Campbell, J.H. Lunsford, G.E. Lewis, D. Dahle, J. Catal. 143 (1993) 32. [50] P. Lersch, F. Bandemann, Appl. Catal. 75 (1991) 133. [51] D.M. Bibby, R.F. Howe, G.D. McLellan, Appl. Catal. 93 (1992) 1. [52] J.R. Gonzalez-Velasco, R. Lopez-Fanseca, A. Aranzabel, J.I. Guitierrez-Ortiz, P. Steltenpohl, Appl. Catal. B 24 (2000) 233. [53] P.S. Chintawar, H.L. Greene, J. Catal. 165 (1997) 12. [54] E. Finnochio, C. Pistarino, S. Dellepiane, B. Serra, S. Braggio, M. Baldi, G. Busca, Catal. Today 75 (2002) 263. [55] C. Schuth, S. Disser, F. Schuth, M. Reinhard, Appl. Catal. B 28 (2000) 147. [56] G. Lowry, M. Reinhard, Environ. Sci. Tech. 33 (1999) 1905. [57] L. Dinard, P. Magnoux, P. Aurault, M. Guisnet, J. Catal. 221 (2004) 662. [58] E.J. Creyghton, M. Burgers, J.C. Jansen, H. van Bekkum, Appl. Catal. A 128 (1995) 275. [59] E.J. Shin, M.A. Keane, Catal. Lett. 58 (1999) 141. [60] G. Tavourlaris, M.A. Keane, Appl. Catal. A 182 (1999) 309. [61] S. Adimurthy, G. Ramachandraiah, A. Bedakar, Tetrahed. Lett. 44 (2003) 6391. [62] E. Kennedy, B. Dlugogorski, Private communication. [63] A. Gervasini, C. Pirola, S. Zilo, V. Ragaini, Appl. Catal. B 47 (2004) 257.