EP1608456A1

EP1608456A1 - Halocarbon destruction

Info

Publication number: EP1608456A1
Application number: EP03782616A
Authority: EP
Inventors: James Thomson
Original assignee: Ceimig Ltd
Current assignee: Ceimig Ltd
Priority date: 2002-12-11
Filing date: 2003-12-11
Publication date: 2005-12-28
Also published as: AU2003290252A1; WO2004052513A1; GB0228810D0

Abstract

This invention relates to the molecular fragmentation of halo-substituted hydrocarbons. In particular, this invention relates to the catalytic destruction of chlorofluorocarbons (CFCs) using a PdZn/ZrOX-Ϝ-Al2O3 catalyst.

Description

Halocarbon Destruction

Field of the Invention This invention relates to the molecular fragmentation of halo-substituted hydrocarbons. In particular, this invention relates to the catalytic destruction of chlorofluorocarbons (CFCs) .

Background of the Invention

CFCs are inert, non-toxic and non-flammable chemicals containing atoms of carbon, chlorine and fluorine. Before the discovery in 1974 of their detrimental impact on stratospheric ozone they were used extensively in the manufacture of aerosol sprays, blowing agents for foams and packing materials, as solvents and as refrigerants.

As a result of the Montreal Protocol in 1988 and further amendments which called for a complete phase-out of CFCs and their replacements, the production of CFCs was banned from 2000. Nevertheless, the current available replacements also have a very high global warming potential and therefore will also be phased out in the near future. Current CFC alternatives are listed below:

Despite their phase-out CFCs are still an environmental problem as there is a large stockpile of CFCs on industrial sites and also from old equipment such as fridges where CFCs are contained. In the UK alone, it is estimated that up to 3 million domestic refrigeration units are disposed of each year and a further half million commercial units are also replaced annually. Nine types of CFCs, 1 halon, 2 HCFCs and 11 HCFC blends are commonly contained in refrigeration equipment together with hydrocarbons and lubricating oils.

Following EC Regulation 2037/2000 which covers the recycling and discarding of fridges in the UK, the safe disposal of ozone depleting substances is required. Currently, this is done by granulating whole fridges in the sealed compartment of a cross-flow shredder and by grinding and heating the foam chunks to liberate remaining gases from the pores. Material fractions of steel, polystyrene, polyurethane, glass and metals are separated. The gases are collected and destroyed by high-temperature incineration. Incineration is the only destruction technique used in practice as it is the only method which can handle large amounts of CFCs and which is cheap. Several alternative approaches have been proposed such as UV or ultrasound treatment [1, 2] , aerosol mineralisation [3] , electrochemical reduction [4] , cement kiln and plasma [5] . However, none of these techniques is suitable for scaling up to dispose of large amounts of CFCs in an energy efficient, inexpensive way, and without creating toxic by-products.

Catalysis is regarded as having the greatest potential to safely dispose of CFCs. So far, two types of catalysts have been studied: supported Pd catalysts over which hydrogenolysis is performed [6, 7, 8, 9]; and hydrolysis catalysts such as metal oxides (Ti0₂; Zr0₂; 0₃) [10, 11, 12] , metal sulphates [13] and metal phosphates [14] .

Supported Pd

CFC + H₂ > HFA + HCl

Metal Oxide CFC + H₂ > C0₂ + HCl + HF

However, supported Pd catalysts quickly deactivate as they are easily attacked by hydrochloric acid generated in si tu resulting in a loss of both the metal and the support areas [6] .

One possibility to prevent the diffusion of chlorine and fluorine in the Pd bulk is by choosing a different support; although there is controversy in the literature on whether the initial aim is not cancelled out by other surface effects (e.g. acidic supports such as AlF₃ prevent the diffusion of halides into Pd [7] but are reported to enhance coking [15] or basic supports such as Zr0₂ may neutralise the HCl formed [16] but can lead to more rapid sintering of the Pd metal [17] ) . Another possibility to prevent the diffusion of halides into the bulk Pd is by adding a second metal

[18] . A synergistic relationship was found between

Pd°Zn^ι:ι:0 whereby a zinc-rich phase showed increased dispersion and also greater chemical stability of the Pd [19] .

In contrast, the hydrolysis catalysts show both greater stability towards deactivation and activity at lower operating temperatures, usually less than 500°C. 0₃/Ti0₂ is reported to achieve complete conversion of CFC- 12 at 265°C [11] , and sulphate promoted Ti₂-Zr0₂ at 280°C [20] . In these reactions, water prevents the fluorination of the Ti and metals and hence their loss by evaporation. Metal phosphates were previously used to obtain a greater 0₂ supply in the hydrolysis reaction. However, this system has only been tested with pure CFCs on a small scale. It would also not be suitable for CFCs containing impurities as found in refrigeration equipment .

It is an object of embodiments of the present invention to obviate or at least mitigate one or more of the aforementioned problems. It is a further object of the present invention to provide a method for the environmentally safe destruction of halo-substituted hydrocarbons such as CFCs.

Summary of the Invention According to a first aspect of the present invention there is provided a method of catalytically destroying halo-substituted hydrocarbons comprising the steps of first mixing a gas comprising a halo-substituted hydrocarbon with steam or hydrogen and steam to form a gas mixture and thereafter passing the gas mixture over a catalyst capable of destroying halo-substituted hydrocarbons wherein the temperature of the catalyst is about 100-800°C.

By destroying the halo-substituted hydrocarbon herein is meant converting the halo-substituted hydrocarbon into a more environmentally friendly form. On destroying the halo-substituted hydrocarbon, the chain length of the halo- substituted hydrocarbon may be reduced. The shorter chain molecules may, for example, be methane and ethane. Furthermore, on destroying the halo-substituted hydrocarbon the number of carbon-halo bonds such as carbon-chlorine, carbon-fluorine, carbon- bromine and carbon-iodine bonds are reduced.

The catalyst may be supported on an inert carrier. The inert carrier may be a ceramic. The ceramic may be alumina and in particular γ-Al₂0₃.

Typically, the catalyst comprises any of the following: palladium, rhodium, ruthenium, silver, gold, gallium, zinc and/or zirconia. The catalyst may, for example, be a palladium and zinc based catalyst and, in particular, may be a PdZn/ZrO_x based catalyst. The catalyst used may be PdZn/ZrO_x-γ-Al₂0₃. The ratio of Pd:Zn may range from 2:1 to 1:4. In particular, the ratio of Pd:Zn may be 1:2. x may range from 1 to 3.0 and preferably 1.5 to 2.0.

The catalyst used may also be in the form of an extrudate or in a monolith. The halo-substituted hydrocarbon may be a hydrochlorofluorocarbon, a chlorofluorocarbon (CFC) , a chlorocarbon or a fluorocarbon. The hydrocarbon may be in a saturated or unsaturated form.

The halo-substituted hydrocarbon may be selected, for example, from any of the following: bromocarbon; bromochlorocarbon; bromochloroiodocarbon; bromochlorofluorocarbon; bromochlorofluoroiodocarbon; bromofluorocarbon; bromofluroriodocarbon; bromoiodocarbon; chlorocarbon; chlorofluorocarbon; chlorofluoroiodocarbon; chloroiodocarbon; (per) fluorocarbon; fluoroiodocarbon; hydrobromocarbon; hydrobromochlorocarbon; hydrobromochlorofluorocarbon; hydrobromochlorofluoroiodocarbon; hydrobromochloroiodocarbon ; hydrobromofluorocarbon; hydrobromofluoroiodocarbon; hydrobromoiodocarbon ; hydrochlorocarbon; hydrochlorofluorocarbon ; hydrochlorofluoroiodocarbon; hydrochloroiodocarbon; hydrofluorocarbon; hydrofluoroiodocarbon; hydroiodocarbon; and iodocarbon, or mixtures thereof.

Preferably, the halo-substituted carbon is a chlorofluorocarbon (CFC) . Conveniently, the halo-substituted carbon may be selected from any of the following: CFC- 11; CFC- 12; CFC- 13; CFC-111; CFC-113; CFC-114; CFC-115; CFC 211 - 217 or mixtures thereof .

The halo-substituted hydrocarbon, for example CFC, may be destroyed via a reductively induced steam reaction wherein steam is passed over the catalyst along with N₂ and H₂. The reaction temperature may be held in the range of 110-800°C and is typically about 600°C. The halo-substituted hydrocarbon, for example

CFC, may also be destroyed via a hydrolysis reaction wherein the halo-substituted hydrocarbon is reacted with steam. For this reaction, carrier gas such as N₂ is used and bubbled both through the halo-substituted hydrocarbon and a water reservoir and then reacted over the catalyst.

The reaction may be carried out at high pressure such as

800 torr and at a temperature range of about 400-800°C.

Preferably, the reaction temperature may be about 500- 700°C or is typically 600°C.

In the reductively induced steam reaction, and the hydrolysis reaction, the halo-substituted hydrocarbons such as CFCs may be converted into C0₂, HCl, HF, HBr and HI. The C0₂, HCl and HF may be easily disposed of or reused in other processes.

According to a second aspect of the present invention there is provided use of a catalyst as used in the first aspect for use in the destruction of halo- substituted hydrocarbons. According to a third aspect of the present invention there is provided apparatus for catalytically destroying halo-substituted hydrocarbons comprising a catalyst as used in the first aspect.

According to a fourth aspect of the present invention there is provided a method of catalytically destroying halo-substituted hydrocarbons comprising the steps of first mixing a gas comprising a halo-substituted hydrocarbon with hydrogen to form a gas mixture and thereafter passing the gas mixture over a PdZn/ZrO_x-γ- Al0₃ catalyst wherein the temperature of the catalyst is about 400-800°C.

The ratio of Pd:Zn may range from 2:1 to 1:4 and may, in particular, be 1:2. The halo- substituted hydrocarbon may therefore be destroyed via a hydrogenolysis reaction wherein the halo- substituted hydrocarbon, for example CFC, is reacted with H₂. To perform this reaction N₂ (which is used as a carrier gas) and H₂ may be passed, for example, bubbled, through the halo- substituted hydrocarbon and reacted over the catalyst .

The reaction temperature may typically be about 600°C. The catalyst used may also be in the form of an extrudate or in a monolith.

The halo- substituted hydrocarbon may be a hydrochlorofluorocarbon, a chlorofluorocarbon (CFC) , a chlorocarbon or a fluorocarbon. The hydrocarbon may be in a saturated or unsaturated form.

The halo-substituted hydrocarbon may be selected, for example, from any of the following: bromocarbon ; bromochlorocarbon ; bromochloroiodocarbon; bromochlorofluorocarbon ; bromochlorofluoroiodocarbon ,^• bromofluorocarbon ; bromofluroriodocarbon ; bromoiodocarbon; chlorocarbon ; chlorofluorocarbon ; chlorofluoroiodocarbon; chloroiodocarbon ; (per) fluorocarbon; fluoroiodocarbon ; hydrobromocarbon ; hydrobromochlorocarbon ; hydrobromochlorofluorocarbon; hydrobromochlorofluoroiodocarbon; hydrobromochloroiodocarbon; hydrobromofluorocarbon; hydrobromofluoroiodocarbon; hydrobromoiodocarbon; hydrochlorocarbon; hydrochlorofluorocarbon; hydrochlorofluoroiodocarbon; hydrochloroiodocarbon; hydrofluorocarbon; hydrofluoroiodocarbon; hydroiodocarbon; and iodocarbon, or mixtures thereof. Preferably, the halo-substituted carbon is a chlorofluorocarbon (CFC) .

Conveniently, the halo-substituted carbon may be selected from any of the following: CFC- 11; CFC- 12; CFC- 113; CFC- 114 and CFC- 115, or mixtures thereof. In the hydrogenolysis reaction the halo-substituted hydrocarbons such as CFCs may be converted into C0₂, HCl and HF. The C0_2/ HCl and HF may be easily disposed of or reused in other processes.

According to a fifth aspect of the present invention there is provided use of a PdZn/ZrO_x-γ-Al₂0₃ catalyst for use in a hydrogenolysis reaction in the destruction of halo-substituted hydrocarbons.

According to a sixth aspect of the present invention there is provided apparatus for catalytically destroying halo-substituted hydrocarbons comprising a PdZn/ZrO_x-γ- Al₂0₃ catalyst . Brief Description of the Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a representation of apparatus used to carry out the catalytic destruction of CFCs according to the present invention;

Figure 2 is a representation of CFC- 113 peak areas on catalytic destruction at different temperatures and H₂ flow rates according to the present invention;

Figure 3 is a deactivation study using catalytic extrudate at 600°C according to the present invention;

Figure 4 is a representation of a catalytic deactivation study using catalytic monolith at 600°C according to the present invention;

Figure 5 is a chromatographic representation of gases formed from a catalytic extrudate at the start of the reductively induced steam reaction where the temperature of the water is 75°C;

Figure 6 is a chromatographic representation of gases formed from a catalytic extrudate at the halfway point of a reductively induced steam reaction wherein the temperature of the water is 75°C; Figure 7 is a chromatographic representation of gases formed from a catalytic extrudate at the halfway point of a reductively induced steam reaction wherein the temperature of the water is 95°C;

Figure 8 is chromatographic representation of gases formed from a catalytic extrudate at the end of a reductively induced steam reaction wherein the temperature of the water is 95°C; Figure 9 is a chromatographic representation of gases formed from a catalytic extrudate using a thermal conductivity detector of a reductively induced steam reaction wherein the temperature of the water is 95°C;

Figure 10 is chromatographic representation of gases formed from a catalytic extrudate at the start of a hydrogenolysis reaction;

Figure 11 is a chromatographic representation of gases formed from a catalytic extrudate at the halfway point of a hydrogenolysis reaction;

Figure 12 is a chromatographic representation of gases formed from a catalytic extrudate at the end of a hydrogenolysis reaction; Figure 13 is a chromatographic representation of gases formed from a catalytic monolith at the start of a reductively induced steam reaction wherein the temperature of the water is 95°C;

Figure 14 is a chromatographic representation of gases formed from a catalytic monolith at the halfway point of a reductively induced steam reaction wherein the temperature of the water is 95°C;

Figure 15 is a chromatographic representation of gases formed from a catalytic monolith at the end of a reductively induced steam reaction wherein the temperature of the water is 95°C;

Figure 16 is a chromatographic representation of gases formed from a catalytic monolith at the start of a hydrogenolysis reaction; Figure 17 is a chromatographic representation of gases formed from a catalytic monolith at the end of a hydrogenolysis reaction; Figure 18 is a chromatographic representation of gases formed from a catalytic monolith at the start of a steam reaction wherein the temperature of the water is 95°C;

Figure 19 is a chromatographic representation of gases formed from a catalytic monolith at the end of a steam reaction wherein the temperature of the water is 95°C; Figure 20 is an overlay of chromatographic representations of gases formed from a reductively induced steam reaction and a hydrogenolysis reaction using a catalytic extrudate wherein the temperature of the water is 95°C; Figure 21 is an overlay of chromatographic representations of gases formed from a reductively induced steam reaction, hydrogenolysis and a steam reaction using a catalytic monolith wherein the temperature of the water is 95°C; Figure 22 is a representation of hydrogenolysis of carbon tetrachloride;

Figure 23 is a representation of the HCL eluent during hydrogenolysis of carbon tetrachloride;

Figure 24 is a representation of the selectivity of CHC1₃ to hydrocarbons by hydrogenolysis;

Figure 25 is a representation of selectivity of CH₂C1₂ to hydrocarbons by hydrogenolysis;

Figure 26 is a representation of selectivity of CC1₄ to hydrocarbons by hydrolysis; Figure 27 is a representation of temperature dependence of HCl eluent during reaction of CC1₄ in the presence of H₂/steam; Figure 28 is a representation of the determination of light-off temperature for the conversion of CFC-113; and Figure 29 is a representation of the effect of reaction environment on conversion of CFC-113 at 600°C.

Detailed Description

Shown in Figure 1, there is a schematic representation of apparatus, generally designated 10, for the catalytic destruction of CFCs.

CFC containing gas is first of all fed through a CFC bubbler 12 which has an ice bath 13 and an H₂0 bubbler 14. A series of flow meters 16, 18, 20 may feed in 0₂, H₂, N₂, respectively. A pressure gauge 22 monitors the flow.

The CFC containing gas is then fed to a reactor 24 containing a catalyst 26. A furnace 28 is used to heat the reactor 24 and the catalyst 26 to about, for example,

600°C. Trap 30 is used to collect any excess aqueous acid formed in the reaction. Gases are passed out via vent 32.

A series of bubblers 34 (a to n) is then used prior to the gas being tested by gas chromatography and a thermal conductivity detector (i.e. GC/TCD) or gas chromatography and a flame ionisation detector (i.e. GC/FID) .

Catalyst Preparation

A series of batches of a Pd . Zn ( l : 2 ) /ZrO_x-γ -Al₂0₃ catalyst were prepared .

Batch A: 5wt% Pd. Z (1 : 2 ) /ZrO_x-γ-Al₂0₃ (extrudate)

15. Og of γ-Al₂0₃ extrudate (Degussa ^λC, 110m²g^"1) was impregnated with the metal precursors palladium (II) nitrate hydrate, Pd(N0₃)₂.H₂0 (1.635g; 7. lmmol ; Aldrich Chemical Co.), and zinc nitrate hexahydrate, Zn (N0₃) ₂.6H₂0 (4.183g; 14. lmmol ; Aldrich Chemical Co.), dissolved in 0.1wt% Pd.Zn(l:2) 100% ZrO_x sol (5ml). After the wet impregnation, Batch A was oven dried at 120°C overnight.

Batch B: 5wt% Pd.Zn(l :2) /ZrO_x-γ-Al₂0₃ (extrudate)

15.3g of γ-Al₂0₃ extrudate (Degussa 'C, 110m²g^"1) was impregnated with the metal precursors palladium (II) nitrate hydrate, Pd (N0₃) ₂.H₂0, (1.601g; 7.0mmol; Aldrich Chemical Co.) and zinc nitrate hexahydrate, Zn (N0₃) ₂.6H₂0, (4.188g; 14. lmmol ; Aldrich Chemical Co.) dissolved in 0.1wt% Pd.Zn(l:2) 100% Zr0_x sol (5ml). After the wet impregnation Batch B was oven dried at 120°C overnight.

Batch C: 5wt% Pd. Zn (1 : 2) /ZrO_x-γ-Al₂0₃ (monolith)

A 0.5 x 0.5 x 6 inch monolith section was coated twice with a 10wt% γAl₂0₃ sol based on the uptake of the γ-Al₂0₃ (0.4069g) , 0.0438g of palladium (II) nitrate hydrate, Pd(N0₃) ₂.H₂0, (0.2mmol; Aldrich Chemical Co.) and 0.1149g of zinc nitrate hexahydrate, Zn (N0₃) ₂.6H0, (0.4mmol; Aldrich Chemical Co.) were dissolved in 0.1wt% Pd.Zn(l:2) 100% ZrO_x sol (0.136ml) . 10ml of deionised water was added to facilitate a homogeneous distribution during the coating procedures. Each coating was followed by flowing helium (BOC) over the monolith section to clear the channels and subsequently baking at 650°C for a period of 7-15h. Batches A and B were placed in a dark bottle and stored in a desicator charged with dried silica until being used. Reduction of Catalyst

Batch A

Batch A was dried at 350°C under dinitrogen, N₂ (BICOFN; SScm nin^"1) for 20 min followed by calcinations in dioxygen, 0₂ (B0C;35cm³ min^"1) for 2h. The reactor temperature was then decreased to 50°C at a ramp rate of 10°C min^"1 under N₂ (10cm³ min^"1) overnight (18h) . Reduction of Batch B was performed with a 25% H₂/N₂ feedstream (100cm³ min^"1) . The reduction temperature was ramped to 350°C at 1°C min^"1 and held constant at 50°C intervals for lh each. Once 350°C was reached, the reverse temperature programme was performed. Finally, the reactor was heated up to 600°C at 5°C min^"1 under N₂ (10cm³min^"1) .

Batch B

Batch B was dried at 350°C under dinitrogen, N₂ (10cm ³ min^"1) for 14h followed by calcination in 0₂ (40cm³ min^"1) for 5h. The reactor was flushed with N₂ (45cm³ min^"1) for lh. The reactor temperature was then decreased to 50°C at a ramp rate of 10°C min^"1 under N₂ (10cm³ min^"1) overnight for 18h. Reduction of Batch B was performed with a 25% H₂/N₂ feedstream at 100cm³ min^"1. The reduction temperature was ramped to 350°C at 1°C min^"1 and held constant at 50°C intervals for lh each. Once 350°C was reached, the reverse temperature program was performed. Finally, the reactor was heated up to 600°C at 5°C min^"1 under N₂ (10cm³ min^"1) .

Batch C

Batch C was dried at 350°C under dinitrogen, N₂(35cm³ min^"1) for 20 min followed by calcinations in N₂ (80cm³ min^" ^x ) for lh. The reactor was flushed with N₂ (10cm³ min^"1) overnight and the reactor temperature was decreased to 50°C at a ramp rate of 1°C min^"1. Reduction of Batch C was performed with a 20% H₂/N₂ feedstream (100cm³ min^"1) . The reduction temperature was ramped to 350°C at 1°C min^"1 and held constant at 50°C intervals for lh each. Once 350°C was reached, the reverse temperature programme was performed. Finally, the reactor was heated up to 600°C at 1°C min^"1 under N₂ (10cm³ min^"1) .

Reactions

Hydrogenolysis Reaction

In the hydrologenolysis reaction, the CFC was reacted with dihydrogen, H₂. For this, N₂ (40cm³ min^"1) and H₂ (BOC; 25cm³ min^"1) were bubbled through the CFC bubbler 12 and reacted over the catalyst 26. The CFC bubbler 12 was submerged in an ice bath 13 and held at 0°C to give a time-averaged feed rate of 72.7±6.6μl min^"1. The reaction was carried out at a pressure of 800 torr and a reaction temperature of 600°C.

Reductively Induced Steam Reaction

The reductively induced steam reaction was performed by passing N₂ (15-55cm³ min^"1) , H₂ (10-50cm³ min^"1) , and CFC at 72.7±6.6μl min^"1 and steam over the catalyst 26. The

H₂0 bubbler 14 was initially held at 75°C and later at 95°C. The reactions were carried out at a pressure of 800 torr. The reaction temperature was held in the range of 110-600°C. Hydrolysis Reaction

In the hydrolysis reaction, the CFC was reacted with steam. For this, the carrier gas N (65cm³min^"1) was bubbled through both the CFC bubbler 12 (held at 0°C to give a time-averaged feed rate of 72.7±6.6μl min^"1) and the H₂0 bubbler 14 was held at 95°C and reacted over the catalyst 26. The reaction was carried out at a pressure of 800 torr and a reaction temperature of 600°C. The gas hourly spaced velocity (GHSV) was set to

780h^"1 for catalyst Batch A and to 28,700h^"1 for catalyst Batch C.

For all reactions, the change in weight of the CFC bubbler 12 was recorded and at the end of each experiment the reactor was left under a N₂ flow overnight at a rate of 10cm³ min^"1 and at a temperature range of 110-600°C as appropriate for the following experimental run.

Sampling and GC Settings The product distribution of the reactions was monitored by on-line gas chromatography (GC; Perkin-Elmer Autosystem XL) equipped with a flame ionisation detector (FID) and a thermal conductivity detector (TCD) . The system was given at least 15min to stabilise before the first sample of the eluent was taken. Subsequent aliquots were measured at 40-120min intervals.

The GC settings were set as shown in the table below.

The GC oven temperature was held at 170°C overnight to facilitate desorption of any residue left in the column.

Evolved hydrochloric acid (HCl) and hydrofluoric acid (HF) were trapped in bubblers 34 filled with sodium hydroxide (NaOH) and when possible titrated manually (burret reading ±0.05ml) with standardised HCl solution. Since in several reactions performed copious amounts of liquid, highly concentrated acid were generated an empty bubbler 30 serving as a trap was placed before the first NaOH bubbler 34. Evolved C0₂ was measured by GC/TCD.

Reactions were conducted as shown in the table below:

H = hydrogenolysis; rs = reductively induced steam reaction; s = steam reaction * = trap included; -/* = reaction carried out with and without trap FID = flame ionisation detector; TCD = thermal conductivity detector

Results

Catalyst Screening

Figure 2 is a representation of different reaction conditions for the reductively induced steam reaction of CFC-113.

From Figure 2 it can be seen that the reaction is thermally driven as higher reaction temperatures lead to a distinct reduction in the CFC peaks. For example, at 600°C the CFC-113 conversion is greater than 99%. This value is based on the peak area measured at 110°C where this temperature was chosen to obtain a reference value

with a constant gas hourly space velocity (GHSV) but avoiding condensation of the steam. Figure 2 also shows that the H₂ flow rate does not appear to influence the CFC peak area (i.e. the CFC concentration) to any great extent.

Deactivation Study of the Extrudate

Figure 3 is a representation of a study of the deactivation of catalyst Batch B. The percentage conversion of CFC-113 was measured as a function of time. It should be noted from Figure 3 that the scale of the percentage CFC-113 conversion ranges from 96-100%. From Figure 3 it can be seen that a conversion of greater than 99% was achieved for most of the time at 600°C and is independent of the reaction type. In the reductively induced steam reaction, the temperature rise of the water reservoir after 18h on stream improved that percentage conversion to greater than 99.9% once the system was stabilised. (The initial unsteady state at higher water temperature may be a result of the back pressure problems that occurred during the reactions) .

The CFC-113 conversion during the hydrogenolysis (27.7 - 46.4h on stream) was also consistently greater than 99.7% at 600°C.

The deactivation study finished after 43.8h on stream and CFC peaks were detected once again the reaction temperature was reduced to 500°C.

Deactivation Study of the Monolith

Figure 4 shows a deactivation study of the catalytic monolith Batch C which shows that the percentage of conversion of CFC-113 ranges from 80-100%. From Figure 3 it can be seen that at 600°C, in the H₂/steam reaction a conversion of greater than 99% was achieved. After

9.9h on stream the conversion stayed consistently above

99.9%.

In contrast, in the steam reaction (20.0 - 25.8h on stream) CFC-113 conversion varied from 96.7-100%. The optimum value of 100% coincides with an atypical product distribution from the steam reaction with peaks occurring

6.79min, 7.90min, 15.14min and most prominently at

20.33min. In the second H₂/steam reaction (26.8-32.3h on stream) the CFC conversion declined from 99.1-97.4% over time. The H₂ flow rate was held at 15cm³min^"1.

Finally, after 32.6h on stream the hydrogenolysis led to an initial increase in CFC conversion (99.9%) but declined rapidly after 35.9h on stream (88.1%) to reach

82.6% by 37.7h on stream.

Chromatographs

The chromatographs shown in Figures 5 to 8 show samples taken at the start, halfway and at the end of the reductively induced steam reaction carried out over the extrudate. Since the temperature of the water reservoirs was increased halfway through the experiment, two chromatographs are given for this stage during the experiment. It can be seen that the most prominent peak occurs at 6.8±0.2min followed by a peak at 2.48±0.04min. The effect of the increased water temperature can be noticed by the fact that the peak area of the by-products is smaller. The chromatograph shown in Figure 9 shows a distinct

C0₂ peak at 0.9min and a water peak at 6.5min as detected by TCD. The chromatographs shown in Figures 10 to 12 show that hydrogenolysis over the extrudate produces three major peaks at 1.7±0.2min, 2.43±0.07min and 6.6±0.2min. The peak at 2.43±0.07min is consistently the largest indicating a shift in the equilibrium position of the reaction intermediates in comparison to the reductively induced steam reaction.

The H₂/steam reaction over the monolith is shown in the chromatographs shown in Figures 13 to 15. As in the case of the extrudate, the largest peak occurs at 6.4±0.2min and the second largest at 2.43±0.02min. The chromatographs shown in Figures 16 and 17 show that the product distribution of hydrogenolysis over the monolith is initially similar to the one of the extrudate with peaks at 1.67min, 2.45min and 6.66min. (However, as the catalyst deactivated towards the end, the peak at 2.45min disappears) .

The chromatographs shown in Figures 18 and 19 show that product distribution of the steam reaction over the monolith initially has prominent peaks at 14.96min, 15.65min and 23.62min but also at 6.73min. Towards the end of the experimental run, the largest peak is observed at 6.59min whereas the other peaks diminished. The overlays shown in Figures 20 and 21 show a comparison of the reaction types over the extrudate or the monolith, respectively.

Turnover Frequency Since for most of the time conversion values greater than 99% were achieved, the time-averaged flow rate of CFC-113 was used to determine the turnover frequency (TOF) as shown below;

This means that, for example, in catalyst A every Pd atom works 413 molecules of CFC.

Discussion

From the catalyst screening it can be seen that the reaction is thermally driven and effective at 600°C. This is more than twice as high a temperature than that of Ti0₂ based model catalyst where a complete CFC- 112 conversion is reported at 265°C for W0₃/Ti0₂ [11] . However, the Pd. Zn(l :2) /ZrO_x-γ-Al₂0₃ catalyst used in the present invention is capable of burning off carbon as proven by the C0₂ detection from the TCD data shown in Figure 10. Both 0₃ and Ti0₂ are not active for burning hydrocarbons and there is no known chemistry for HFA or hydrocarbon conversion for these metal oxides.

From the deactivation studies of both the extrudate and the monolith it can be seen that the reactivity of the H₂/steam reaction is greater than that of the hydrogenolysis .

In addition, it was shown that the extrudate system is transferable to the monolith system which shows good performance despite significantly lower metal loading. The product distribution for the monolith system was found to be similar to that of the extrudate. In the hydrolysis of CFC-113 less by-products than in the H₂/steam reaction were generated. This indicates a further, higher activity towards the formation of C0₂, HCl and HF. However, a lower declining percentage conversion was observed which was only improved once dihydrogen was added to the reaction. Hence, hydrogen seems necessary to keep the metal in the reduced state which is equivalent to a fresher catalyst.

There are two important reaction intermediates observed in the chromatographs: at 2.44±0.8min and at 6.6±0.4min. The former is pronounced in during the hydrogenolysis whereas the latter becomes important as soon as steam is introduced.

The GHSVs shows that the extrudate system could have been run at 40 times higher flow rate and the turnover frequency data shows that the monolith catalyst has a good capacity to deal with a large amount of CFCs. The ability of Pd. Zn (1 : 2) /ZrO_x-γ-Al₂0₃ to catalytically destroy CFCs has therefore been shown. Firstly at 600°C, on average greater than 99% conversion was achieved using hydrogenolysis and a reductively induced steam reaction over a period 44h on stream. A monolith system has also shown a greater than 99% conversion for a H₂/steam reaction and also for the hydrogenolysis before the activity rapidly declined after 36h on stream. The hydrolysis showed a lower percentage conversion despite the product distribution indicating a greater activity towards C0₂, HCl and HF formation. As the percentage conversion increased as soon as dihydrogen was added to the reaction, this indicates that hydrogen is necessary in the steam reaction to keep the metal in a reduced state. In addition, it was shown by GC/TCD measurements that the Pd. Zn (1 : 2) /ZrO_x-γ-Al₂0₃ catalyst is capable of burning off carbons and hydrocarbons. Catalytic Performance

Hydrogenolysis of Carbon Tetrachloride and

Chi orohydrocarbons As shown in Figure 22, hydrogenolysis of carbon tetrachloride was performed in order to examine the performance of the PdZn(l : 2) /ZrO_x/γ-Al₂0₃ catalyst under stoichiometric ratios of CC1₄ and dihydrogen. The analysis confirms the formation of alkane compounds such as methane, ethane, ethene and propane. Figure 22 shows the normalised composition of the gas phase as a function of time. The breaks in the data points (e.g. 5h) indicate an overnight soak in dinitrogen at 600 °C. Integration of the hydrocarbon output indicates stability of the catalyst over the reaction period. The stability of the catalyst is also supported by the constant value obtained for the formation of hydrogen chloride as shown in Figure 23 where the mass balance for chloride ion is ca 45%. The formation of organics of C_>ι is consistent with radicalisation of the surface adsorbed moieties. The susceptibility of the halocarbon to undergo radical mechanisms may also be observed by bromoform and halons .

The observation of the formation of alkanes of C_>ι during a hydrogenolysis reaction is also found for the hydrogenolysis of methylene chloride and chloroform as shown in Figures 24 and 25, respectively.

For each of the Ci halocarbons, the major product species is methane indicating that the rate of formation of methane (i.e. methyl and hydrogen radical combination) is greater than the rate of methyl radical combination. Hydrolysis of Carbon Tetrachloride

The catalytic hydrolysis of CC1₄ by steam was performed and the data is shown in Figure 26. This data confirms a reduction by ca 50% in the partial pressure of hydrocarbons C_> (see Figures 22 and 26) . However, no reduction in the partial pressure of methane was achieved. As shown in Figure 26, the addition of steam to the feed lowers the rate of methyl recombination with respect to the hydrogenolysis reaction. The chlorine mass balance is shown in Figure 27. The analysis confirms that at the reaction temperature of 400 °C the recovered chlorine is ca 96%, an increase from 45% for the hydrogenolysis reaction (see Figure 23) . This increase in recovered chlorine confirms that during the reaction, the catalyst is in a "cleaner" form relative to that for the hydrogenolysis process.

H₂/steam Reaction of 1 , 1 . 2-Trichlorotrifluoroethane

The hydrolysis of 1 , 1, 2-trichlorotrifluoroethane was performed under reducing conditions as a function of reaction temperature. The results are shown in Figure 28. This reaction was carried out over PdZn (1 : 2) /ZrO_x/γ- Al₂0₃ granules .

The Conversion (ζ) of CFC-113 as • shown in Figure 28 was obtained indirectly as shown below: A conversion of 86.1% is obtained at 450 °C and

>99.9% at 600 °C. At temperatures below 250 °C the conversion is not real as CFC-113 adsorbs onto the catalyst surface; at 350 °C a negative percentage conversion indicates the desorption of the chlorofluorocarbon. This observation is consistent with a study over PdZn₂/CCA where no activity was observed below 280 °C and conversion started at 320 °C. TPR analysis confirmed activity of PdZn₂ at ca 350 °C. The effect of the reaction conditions for the decomposition of CFC-113 was tested over PdZn (1 : 2) /ZrO_x/γ- Al₂0₃ extrudate and monolith. The obtained results are shown in Figure 29. Over the extrudate conversions of >99.7% were achieved in the hydrogenolysis reaction and >99.9% in the presence of steam at steady state (initial instability was due to backpressure) . Over the monolith, steady state conversions were >99.9% for the H₂/steam reaction and >96.7% for the hydrolysis of CFC-113. The addition of dihydrogen could lift the conversion before deactivation occurred after 35.9h. FC/TCD analysis confirmed the decomposition of CFC-113 towards C0₂ in the steam reactions.

Discussion The catalyst shows during the hydrogenolysis of carbon tetrachloride good stability over a reaction period of 19h with a hydrocarbon output of methane and ethane as the main products (see Figure 22) . With time on line the catalyst is being chlorinated during the process as evidenced by the chlorine mass balance (see Figure 23). After 10.5h the partial pressure of methane clearly decreases as the partial pressure of ethane increases. The formation of C_>ι species is consistent with a radical mechanism of the adsorbed surface species as shown below:

Dissociative adsorption of reactants H₂(_g) → H₂(ads) → 2 H _(ads)

CCl₄(g) → CCl4(a s) → CCl3(_ads) + Cl (_ads) CCl₃(ads) → CCl₂(ads) + Cl (_ads) CCl₂(ads) → CCl(ads) + Cl (_ads) CCl(ads) → C(a s) + Cl(_ada)

Formation of Hydrochloric acid

H(ads) + Cl(ads) → HCl (_ads) → HCl <_g)

Formation of methane

C(ads) + H(ada) → CH(_ad_s) CH(_ads) + H(ads) → CH₂(ads) CH₂(ads) + H(_ad_s) → CH3 (_ad_s) CH3(_a s) + H(ads) → CH4 (_{a s}) → CH4 (_g)

Formation of C_>ι species

2 CH₂(ads) → C₂H4(ads) → C2H4 (_g) 2 CH3(_ads) → C₂H6(ads) → C₂H6 (g) C2H4(_{a s}) + 2 H(_ads) → C₂Hε (_ads) → C₂H6(g) C₂H4(ads) + CH₂(ads) → C3H6 (ads) ^→ C3H6 (g) C3H6(ads) + 2 H(ads) → C3Hβ(_ads) → CjKg (_g)

Formation of tetrachloroethene

2 CCl₂(ads) → C₂Cl4(ads) → C₂Cl₄(g) Dechlorination of Chlorohydrocarbons

CHCl3 (_ads) + H (ads) → CH₂Cl₂ (_ads) + Cl (_ads) → CH₂Cl₂ (g) CH₂Cl₂ (_ads) + H (ads) → CH3CI (_ads) + Cl (_ads) → CH3CI (_g) CH₃Cl (ads) + H (_a s) -» CH₄ (ads) + Cl (_ads) → CH4 _(g)

For the hydrogenolysis of carbon tetrachloride, experimental data excludes the possibility that methane is formed via dechlorination of reaction intermediates CHCI₃ and CH₂C1₂ as neither of them is observed in the gas phase. Hence, formation of methane and C_>x species is consistent with the above reactions. In these reactions, methyl and methylene radical recombination are competing reactions versus hydrogen radical combination. Hence, a high methane output indicates that the latter proceeds faster. Increased formation of ethane indicates the build-up of carbonaceous species onto the catalyst surface.

The formation of tetrachloroethene by dimerization of the CC1₂ group was not observed showing that .the recombination of fragmented chlorine atoms to be adsorbed carbon is not kinetically favoured.

In contrast, the product distribution of the hydrogenolysis of chloroform and methylene chloride indicates that the reaction proceeds via a dehalogenation mechanism which favours the recombination of methyl and methyl groups (as shown in Figures 24 and 25) . Methylene chloride is observed as a reaction intermediate in the hydrogenolysis of chloroform and propane is formed at the expense of methane. In the hydrogenolysis of methylene chloride, adsorbed hydrogen competes against the strong retention of chlorine onto the catalyst surface. As less hydrogen is available carbonaceous species build up as evidenced by the high propane output. Eventually, catalytically active sites are blocked yielding the reagent methylene chloride as the main product .

Overall, for the hydrogenolysis reactions the reactivity decreases in the order CC1₄>CHC1₃>CH₂C1₂ which is consistent with C-Cl bond energies of the respective chloromethanes . Decomposition of carbon tetrachloride in the presence of water vapour yields methane as main product while the formation of hydrocarbons C_>ι is suppressed (see Figure 26) . Methane output is maintained over 19h on line. Chlorine mass balance confirms that the retained chlorine is reduced to only 4% (see Figure 27) . Hence, the catalyst is in a "cleaner" form and maintains its stability as supported by XRD analysis. After ca 25,400 bed turnovers and a contact time of 0.043s, no metal chlorides were identified and the PdZn (1:2) formulation shows a minor indication for the presence of PdZn alloy.

The hydrolytic decomposition of 1,1,2- trichlorotrifluoroethane requires a higher reaction temperature and light off is observed at ca 600 °C (see Figure 28) . The decomposition towards carbon dioxide was confirmed by GC/TCD analysis and deactivation studies demonstrated the good stability of the catalyst (see Figure 29) .

This is consistent with XRD analysis which confirms catalyst stability. Phase change is greatly suppressed over all Pd/Zn formulations by reaction with a molar ratio of 1:3.7:1.1 for (CFC-113/H₂0/H₂) and residence time of ca 3s. In particular the PdZn (1:2) formulation maintains stability of the tetragonal zirconia phase. However, with a molar ratio of 1:5.8:1.5 and a shorter residence time of ca Is, the formation of metal chlorides is observed after a period of ca 8700 bed turnovers. The catalyst sample reacted at 450 °C shows that stability of t-ZrO_x is maintained while the BET surface area lies above the trendline. This is consistent with literature which shows that t-ZrO_x exhibits greater surface are than m- ZrO_x.

Conclusion

The hydrolytic decomposition of cholorocarbons, chlorohydrocarbons and chlorofluorocarbons on a zirconia supported palladium-zinc catalyst has been demonstrated. Catalyst stability was enhanced in the presence of water vapour as relative to the hydrogenolysis. Chlorine mass balance showed that in si tu dechlorination leads to a "fresher" state of the catalyst. It was shown that reactions of the chloro- (hydro) carbons proceed by radicalisation of surface adsorbed moieties. The presence of steam suppresses methyl recombination and hence favours formation of methane.

The light off temperature of 1,1,2- trichlorotrifluoroethane was established as 600 °C for a >99.9% conversion. Decomposition towards carbon dioxide was confirmed. The PdZn (1:2) formulation showed good activity with time on line and good stability of the catalyst . References

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Claims

1. A method of catalytically destroying halo- substituted hydrocarbons comprising the steps of first mixing a gas comprising a halo-substituted hydrocarbon with steam or hydrogen and steam to form a gas mixture and thereafter passing the gas mixture over a catalyst capable of destroying halo-substituted hydrocarbons wherein the temperature of the catalyst is about 100- 800°C.

2. A method according to claim 1 wherein the catalyst is supported on an inert carrier.

3. A method according to claim 2 wherein the inert carrier is a ceramic.

4. A method according to claim 3 wherein the ceramic is alumina.

5. A method according to claim 3 wherein the ceramic is γ-Al₂0₃.

6. A method according to any preceding claim wherein the catalyst comprises any combination of the following: palladium, rhodium, ruthenium, silver, gold, gallium, zinc and/or zirconia.

7. A method according to any preceding claim wherein the catalyst is a palladium and zinc based catalyst .

8. A method according to any preceding claim wherein the catalyst is a PdZn/ZrO_x based catalyst.

9. A method according to any preceding claim wherein the catalyst used is PdZn/ZrO_x-γ-Al₂0₃.

10. A method according to any of claims 6 to 9 wherein the ratio of Pd:Zn in the catalyst ranges from about 2:1 to 1:4.

11. A method according to any of claims 6 to 9 wherein ratio of Pd:Zn in the catalyst is about 1:2.

12. A method according to any of claims 9 to 11 wherein x ranges from about 1.0 to 3.0.

13. A method according to any of claims 9 to 11 wherein x ranges from about 1.5 to 2.0.

14. A method according to any preceding claims wherein the catalyst is in the form of an extrudate or in the form of a monolith.

15. A method according to any preceding claim wherein the halo-substituted hydrocarbon is a hydrochlorofluorocarbon, a chlorofluorocarbon (CFC) , a chlorocarbon or a fluorocarbon.

16. A method according to any preceding claim wherein the hydrocarbon is in a saturated or unsaturated form.

17. A method according to any preceding claim wherein the halo-substituted hydrocarbon is selected from any of the following: bromocarbon; bromochlorocarbon; bromochloroiodocarbon; bromochlorofluorocarbon; bromochlorofluoroiodocarbon; bromofluorocarbon; bromofluroriodocarbon; bromoiodocarbon; chlorocarbon; chlorofluorocarbon; chlorofluoroiodocarbon; chloroiodocarbon;

(per) fluorocarbon; fluoroiodocarbon; hydrobromocarbon; hydrobromochlorocarbon; hydrobromochlorofluorocarbon; hydrobromochlorofluoroiodocarbon; hydrobromochloroiodocarbon; hydrobromofluorocarbon; hydrobromofluoroiodocarbon; hydrobromoiodocarbon; hydrochlorocarbon; hydrochlorofluorocarbon; hydrochlorofluoroiodocarbon; hydrochloroiodocarbon; hydrofluorocarbon; hydrofluoroiodocarbon; hydroiodocarbon; and iodocarbon, or mixtures thereof.

18. A method according to any of claims 1 to 16 wherein the halo-substituted carbon is a chlorofluorocarbon (CFC) .

19. A method according to any preceding claim wherein the halo-substituted carbon is selected from any of the following: CFC- 11; CFC- 12; CFC- 13; CFC- 111; CFC- 113; CFC-114; CFC-115; CFC 211 - 217 or mixtures thereof.

20. A method according to any preceding claim wherein the halo-substituted hydrocarbon is destroyed via a reductively induced steam reaction wherein steam is passed over the catalyst along with N₂ and H₂.

21. A method according to claim 20 wherein the reaction temperature is held in the range of 110-800°C.

22. A method according to claim 20 wherein the reaction temperature is about 600°C.

23. A method according to any of claims 1 to 19 wherein the halo-substituted hydrocarbon is destroyed via a hydrolysis reaction wherein the halo-substituted hydrocarbon is reacted with steam.

24. A method according to claim 23 wherein a carrier gas such as N₂ is used and bubbled both through the halo-substituted hydrocarbon and a water reservoir and then reacted over the catalyst .

25. A method according to any of claims 23 and 24 wherein the reaction is carried out at high pressure such as 800 torr and at a temperature range of about 400-800°C.

26. A method according to any of claims 23 and 24 wherein the reaction temperature is about 600°C.

27. A method according to any of claims 20 to 26 wherein in the reductively induced steam reaction, and the hydrolysis reaction, the halo-substituted hydrocarbons such as CFCs are converted into C0₂, HCl and HF.

28. Use of a catalyst as used in any of claims 1 to

27 in the destruction of halo-substituted hydrocarbons.

29. Apparatus for catalytically destroying halo- substituted hydrocarbons comprising a catalyst as used in any of claims 1 to 27.

30. A method of catalytically destroying halo- substituted hydrocarbons comprising the steps of first mixing a gas comprising a halo-substituted hydrocarbon with hydrogen to form a gas mixture and thereafter passing the gas mixture over a PdZn/ZrO_x-γ-Al₂0₃ catalyst wherein the temperature of the catalyst is about 400- 800°C.

31. A method according to claim 30 wherein the ratio of Pd:Zn ranges from about 2:1 to 1:4.

32. A method according to claim 30 wherein the ratio of Pd:Zn is about 1:2.

33. A method according to any of claims 30 to 32 wherein the halo-substituted hydrocarbon is destroyed via a hydrogenolysis reaction wherein the halo-substituted hydrocarbon, for example CFC, is reacted with H₂.

34. A method according to claim 33 wherein N₂ which is used as a carrier gas and H₂ are passed, for example, bubbled, through the halo-substituted hydrocarbon and reacted over the catalyst .

35. A method according to any of claims 30 to 34 wherein the reaction temperature is about 600°C.

36. A method according to any of claims 30 to 35 wherein the catalyst used is in the form of an extrudate or in the form of a monolith.

37. A method according to any of claims 30 to 36 wherein the halo-substituted hydrocarbon is a hydrochlorofluorocarbon, a chlorofluorocarbon (CFC) , a chlorocarbon or a fluorocarbon.

38. A method according to any of claims 30 to 37 wherein the hydrocarbon is in a saturated or unsaturated form.

39. A method according to any of claims 30 to 38 wherein the halo-substituted hydrocarbon is selected, for example, from any of the following: bromocarbon; bromochlorocarbon; bromochloroiodocarbon; bromochlorofluorocarbon; bromochlorofluoroiodocarbon; bromofluorocarbon; bromofluroriodocarbon; bromoiodocarbon; chlorocarbon; chlorofluorocarbon; chlorofluoroiodocarbon; chloroiodocarbon;

40. A method according to any of claims 30 to 38 wherein the halo-substituted carbon is a chlorofluorocarbon (CFC) .

41. A method according to any of claims 30 to 40 wherein the halo-substituted carbon is selected from any of the following: CFC-11; CFC-12; CFC-113; CFC-114 and CFC-115, or mixtures thereof.

42. A method according to any of claims 33 to 41 wherein in the hydrogenolysis reaction the halo- substituted hydrocarbons such as CFCs are converted into C0₂, HCl and HF.

43. Use of a PdZn/ZrO_x-γ-Al₂0₃ catalyst in a hydrogenolysis reaction in the destruction of halo- substituted hydrocarbons.

44. Use of catalyst according to claim 43 in combination with steam and/or hydrogen.

45. Apparatus for catalytically destroying halo- substituted hydrocarbons comprising a PdZn/ZrO_x-γ-Al₂0₃ catalyst.

46. Apparatus according to claim 45 which comprises an inlet for steam and/or hydrogen.