Characterisation and deactivation studies of sulfided red mud used as catalyst for the hydrodechlorination of tetrachloroethylene

Applied Catalysis B: Environmental 29 (2001) 263–273

Characterisation and deactivation studies of sulfided red mud usedas catalyst for the hydrodechlorination of tetrachloroethylene

Salvador Ordóñez, Herminio Sastre, Fernando V. Dıez∗Department of Chemical and Environmental Engineering, University of Oviedo, 33071 Oviedo, Spain

Received 5 March 2000; received in revised form 3 July 2000; accepted 10 July 2000

Abstract

Sulfided red mud (a by-product in the production of alumina by the Bayer process) has been shown to be active as a catalystin hydrodechlorination reactions. In order to evaluate the feasibility of red mud in industrial processes, which would be mostinteresting as the cost of this material is much lower than that of commercial catalysts, its deactivation must be characterised.

In this study, the deactivation of sulfided red mud as a catalyst for the hydrodechlorination of tetrachloroethylene at 100 barand 350◦C was studied. The variation of conversion with reaction time was determined in the presence and absence of carbonsulfide in the feed, a notorious increase in the catalyst life being observed in the presence of carbon sulfide.

Fresh and used catalysts were characterised by nitrogen adsorption, X-ray diffraction, scanning electron microscopy andX-ray dispersion spectrometry. An increase in the specific surface and chlorine surface concentration of the catalyst and adecrease in sulphur surface concentration were observed, as well as crystallographic changes in iron species. © 2001 ElsevierScience B.V. All rights reserved.

Keywords:Red mud; Tetrachloroethylene; Catalytic hydrodechlorination; Catalyst deactivation; Iron sulfide catalysts

1. Introduction

Residues containing organochlorinated compoundsare a very important environmental problem due to thefact that organochlorinated compounds are very harm-ful for both humans (they are toxic and carcinogenic)and the environment (they are related to the destruc-tion of the ozone layer and the formation of photo-chemical smog) [1,2].

Tetrachloroethylene (TTCE) is the organochlori-nated compound that is produced and released intothe environment in the greatest amount, according tothe USA EPA [3]. TTCE is widely used in several

∗ Corresponding author. Tel.:+34-985-103-508;fax: +34-985-103-434.E-mail address:[email protected] (F.V. Dıez).

industries due to its non-flammability and high sol-vent power, mainly in textile dying and dry cleaning,metal degreasing, and solvent and paint manufacture.In contrast to other chlorinated and fluoro-chlorinatedcompounds, TTCE cannot be satisfactorily substi-tuted yet by less environmentally harmful prod-ucts. That is why the development of techniquesfor the economically and environmentally accept-able elimination of TTCE-containing effluents isneeded.

The conventional technique for the destruction ofTTCE residues is thermal incineration but this tech-nique has a number of disadvantages. First of all,chlorine acts as flame inhibitor, hence high combus-tion temperatures are needed, which increases the costof the process. Furthermore, very toxic partial oxi-dation by-products such as fosgene, dibenzofuranes

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264 S. Ordoñez et al. / Applied Catalysis B: Environmental 29 (2001) 263–273

and dibenzodioxines can be formed during thermalincineration [4].

Hydrodechlorination is an environmentally inter-esting alternative to thermal incineration [5]. Thisprocess consists in reacting the organochlorinatedcompound with hydrogen, the reaction products be-ing hydrogen chloride, which can be easily separatedby alkaline washing, and hydrocarbons, which can beburned. TTCE hydrodechlorination requires a catalystin order for it to take place at moderate temperatures,as do most organochlorinated compounds [6]. Severalprecious-metal (Pd, Pt and Rh) [7–9] and hydrotreat-ment catalysts (Ni–Mo) [9–11] have shown activityfor the hydrodechlorination of TTCE, but most arehighly sensitive to poisoning by hydrogen chlorideand are quickly deactivated, in addition to beinghighly sensitive to the organic impurities present inthe waste. This fact, together with the high price ofcommercial catalysts, supposes an important eco-nomical burden for the process. This burden wouldbe overcome if expensive commercial catalysts aresubstituted by low-priced disposable catalysts.

Red mud is a by-product in the manufacture ofalumina by the Bayer process, specifically the solidresidue of the caustic leaching of bauxite. Its main con-stituents are iron, titanium and aluminium oxides, witha significant content of silicon, calcium and sodiumoxides. Red mud is produced as a residue in greatamounts, and its storage is an important environmen-tal problem.

Red mud in sulfided form has shown catalyticactivity for the hydrogenation of pure organic com-pounds [12,13] and complex organic fractions [14].The active phase is pyrrhotite, a non-stoichiometricsulfide, thermodynamically stable at temperaturesabove 200◦C, nominally Fe7S8, with a regular NiAsstructure. This structure has ‘iron vacancies’ (formeddue to its non-stoichiometric character), which exhibitspatial order. Pyrrhotite has been shown to be catalyt-ically active in reactions involving the activation ofhydrogen, such as coal liquefaction [15] or tiophenehydrodesulfuration [16]. The other components arenot catalytically active, conferring stability, especiallytitanium oxide [17], and porosity.

Frimmel and Zdrazil [18] reported that iron sulfidesare catalytically active for the hydrodechlorination ofo-dichlorobenzene, although their catalytic activity islower than that of sulfides of other transition metals

such as Rh, Pd, and Mo. This is why sulfided red mudis a promising hydrodechlorination catalyst. In fact,in a previous study [19], sulfided red mud was foundto be active for the hydrodechlorination of TTCEat 20–350◦C and 20–100 bar, although its activity islower than that of commercial precious-metal and hy-droprocessing catalysts. TTCE conversion was foundto increase with temperature and pressure. With re-spect to the solvent present in the reaction (hexane,heptane, benzene or toluene), no influence of the sol-vent in TTCE conversion nor reaction products fromthe solvent were observed. The kinetics of the reactionwas studied at 350◦C and 100 bar, under conditionsfor which there were no mass transfer limitations, anda good fitting between the experimental results and aLangmuir–Hinselwood kinetic equation was obtained.

In spite of being of considerable industrial impor-tance, studies on catalyst deactivation in hydrodechlo-rination are scarce for precious metals [17,18] and formetal sulfide catalysts [19]. To the best of our knowl-edge, there are no available studies on deactivation ki-netics. Some authors state that the main deactivationmechanism is due to poisoning by HCl [20,21], whileothers suggest that the formation of carbonaceous de-posits plays an important role [22,23]. Determiningthe principal deactivation mechanism is very criticalin order to develop new, more resistant catalysts.

In the present study, the deactivation of sulfidedred mud as a catalyst for the hydrodechlorination ofTTCE at 100 bar and 350◦C was studied. The varia-tion of conversion with reaction time was determinedin the presence and absence of carbon sulfide in thefeed. Fresh and used catalysts were characterised bynitrogen adsorption, X-ray diffraction, scanning elec-tron microscopy and X-ray dispersion spectrometry.

2. Experimental

2.1. Materials

Red mud was supplied by the San Ciprián (Lugo,Spain) plant of the Spanish aluminium company,Inespal. Its main constituents (bulk) were analysed byatomic absorption, spectrometry and volumetric meth-ods after acid dissolution and alkaline fusion. Thesuperficial concentration of red mud (1mm depth of apolished and carbon-coated sample) was determined

S. Ordoñez et al. / Applied Catalysis B: Environmental 29 (2001) 263–273 265

Table 1Bulk and EDX superficial composition of red mud

Element Bulk composition(wt.%)

EDX composition (wt.%)

Oxidic form Sulfided form

Fe 19.07 21.7 36.7Ti 13.0 11.9 17.9Al 7.9 7.4 11.3Na 3.7 3.0 4.0Ca 5.1 4.9 7.6Si 4.7 3.6 6.2P – 0.7 0.8V – 0.3 0.3Cl – 0.3 1.0S – 0.0 14.5

by the X-ray microanalyzer of a JSM 6100 scanningelectron microscope. The compositions obtained forboth methods are given in Table 1; additional experi-mental details may be found in [24].

Red mud bulk composition, determined by classicalmethods, along with the surface composition, mea-sured by EDX, are given in Table 1. Textural char-acteristics of the red mud are given in Table 2. It isimportant to point out that the specific surface of thered mud is much higher that that of hematites (pureiron(III) oxide), which present surface areas in therange of 3–5 m2/g, depending on the method of prepa-ration.

The following mineralogical constituents wereidentified by X-ray diffraction (Fig. 1): rutile (RU),TiO2; hematite (HE), Fe2O3; goethite (GO) andlepidocrocite (LE), FeO(OH); iron hydroxide (IH),Fe(OH)3; halloysite (HA), Al2Si2O5(OH)4; andbayerite (BA), Al(OH)3. After sulfiding, pyrrhotite(PY), specifically pyrrhotite 4M, Fe7S8, was formed,while the content in crystalline iron oxides and hy-droxides decreased.

Hematites (a-Fe2O3) was obtained from a saturatedacid solution of FeCl3 (Merck) in water, followed by

Table 2Morphological parameters of red mud

Parameter Fresh redmud

Sulfided redmud

Specific surface BET (m2/g) 24.3 29.5Pore volume BJH (cm3/g) 0.086 0.090Average pore diameter BET (nm) 12.1 10.5

Fig. 1. XRD patterns for red mud (symbols are explained in thetext): (A) in oxidic form and (B) in sulfided form.

precipitation of Fe(OH)3 by the addition of aqueousammonia, filtering, washing with distilled water, dry-ing at 110◦C for 10 h, and calcination at 500◦C for2 h. XRD patterns of the product obtained show theformation of well crystallised hematites.

2.2. Reaction studies

Reaction studies were carried out in a continuousfixed bed reactor, consisting of a 45 mm long, 9 mminternal diameter stainless steel cylinder placed insidea tubular electric furnace, and equipped with five ther-mocouples at different reactor lengths. 2 g of red mud(75–100mm), previously dried at 110◦C and dilutedwith inert corundum, were placed in the mid-sectionof the reactor, the lower and upper sections being filledwith low area inert alumina (100–200mm).

The liquid feed consisted of 10 wt.% tetra-chloroethylene dissolved in hexane, pumped down-wards through the reactor by a liquid chromatographypump (Kontron LC). Hydrogen was fed co-currently,with the flow rate controlled by a mass-flow regula-tor. Reaction products were collected in a cylindricalreceiver connected to a back-pressure regulator thatregulated the system pressure by venting the excessgas. Liquid samples were taken by emptying thereceiver at selected time intervals. The equipmentwas provided with safety features such as redundanttemperature control and a rupture disk. More detailsabout the experimental set-up are given in [24,25].

Red mud was sulfided in situ before use by passing3 l/h of a mixture 10% H2S/90% H2 at 400◦C and at-mospheric pressure through the reactor for 4 h. In some


experiments, 1 wt.% carbon sulfide was added to theliquid feed to maintain the catalyst in the sulfide form.

The experimental conditions found to be optimal forthis reaction [19] were: hydrogen flow rate 0.8 l/min(at STP); temperature 350◦C and pressure 100 bar. Aspace time of 7.25 min g/mmol of TTCE was used inall the experiments. It has been theoretically demon-strated that, under these conditions, there are no masstransfer limitations [24].

Reaction products were analysed by gas chromatog-raphy in a Hewlett-Packard 5890 apparatus equippedwith an FID detector, using cycloheptane as the in-ternal standard and a 60 m× 0.53 mm internal diam-eter VOCOL fused-silica capillary column. The ovenwas maintained at 35◦C for an initial period of 15 minand then heated to 180◦C at 6◦C/min. Analyses werecarried out in split mode. Peak assignment was per-formed by gas chromatography–mass spectra (Finni-gan GCQ).

2.3. Catalyst characterisation

The catalyst pore size distribution and surface areawere measured by nitrogen adsorption at−196◦C witha Micromeritics ASAP 2000 surface analyser, consid-ering a value of 0.164 nm2 for the cross-section of thenitrogen molecule.

The catalyst morphology of gold-coated catalystsamples was observed by SEM with a JSM-6100 ap-paratus. The SEM apparatus is equipped with a LinkX-ray microanalyser that provides quantitative chem-ical analysis of the catalyst surface layer to a depthof about 1mm and maps of the distribution of certainelements. On these maps, the brightness of every pixelis proportional to the intensity of emission of the char-acteristic Ka line of each element and hence, to itsconcentration. For this analysis, catalyst samples mustbe polished and carbon-coated.

Powder X-ray diffraction patterns of the differ-ent catalyst samples were obtained with a D-5000Siemens diffractometer, using nickel-filtered Cu Kaas monochromatic X-ray radiation. The patterns wererecorded over a range of 2θ angles from 20 to 70◦ andcrystalline phases were identified using JCPDS files.

Temperature-programmed reduction was carriedout in a Micromeritics TPD/TPR 2900 apparatus witha TCD detector. Each sample, equivalent to 50 mg ofFe2O3, was heated from 50 to 1000◦C at 10◦C/min

in a stream of 10% H2/90% Ar with a flow rate of100 cm3/min. Pure CuO (Micromeritics) was used ascalibration standard. Temperature-programmed sul-fiding experiments were carried out in a home-madeapparatus, using a UV detector to evaluate the H2Sconcentration and a TCD for hydrogen concentration.The samples, also with a weight equivalent to 50 mgof Fe2O3, were first kept at 50◦C in a 10% H2S/H2flow (100 cm3/min) for 30 min, and then heated from50 to 1000◦C at 10◦C/min. A 6.4% MoO3 overg-alumina catalyst was used as calibration standard.More details about the procedure are given in [26].

3. Results and discussion

3.1. Reaction studies

The evolution of TTCE conversion with reactiontime was studied under the conditions mentioned inSection 2. TTCE and hydrogen react according to thefollowing reaction:

C2Cl4 + 5H2 → C2H6 + 4HCl

In all the experiments, the only reaction products de-tected were ethane and hydrogen chloride. TTCE con-version is defined as mole of TTCE reacted per molof TTCE fed to the reactor.

Experimental results are shown in Fig. 2. In theexperiment carried out without carbon sulfide in thefeed, a period of almost constant TTCE conversion of

Fig. 2. TTCE conversion vs. reaction time, for feed with 1% carbonsulfide added (r), and feed without carbon sulfide added (h).


about 10% is observed between 12 and 32 h reactiontime, after which time conversion decreases sharply;the catalyst being almost completely deactivated af-ter 42 h reaction time. When carbon sulfide is addedto the feed, TTCE conversion in the period of almostconstant activity increases to about 15%. During theperiod of 30–38 h reaction time, conversion decreasesslowly to 10%, and then sharply until complete deac-tivation at 48 h. This behaviour may be explained bythe presence of hydrogen sulfide in the reactor whencarbon sulfide is added to the feed, produced by thereaction between carbon sulfide and hydrogen. Hydro-gen sulfide helps to maintain the red mud iron contentin the catalytically active sulfide form, increasing bothcatalyst activity and active life. However some authorsclaim that this positive effect may be overcome bythe inhibition effect of hydrogen sulfide. In this sense,Hagh and Allen [27] indicate that the addition of CS2has a negative effect in hydrodechlorination reactionsover a sulfided NiMo catalyst. However, Murena et al.[28] found 0.3 wt.% as the optimal CS2 concentra-tion for hydrodechlorination of trichlorobenzene overa sulfided NiMo catalyst.

A sample of deactivated catalyst (experiment with1% carbon sulfide, after 60 h reaction time) waswashed in a Soxhlet apparatus consecutively withtoluene and THF, and reused as catalyst without previ-ous re-sulfiding, no appreciable recovery of catalyticactivity being observed. When sulfided red mud basedcatalysts were used in another hydrogenation reaction(anthracene oil hydrogenation), this treatment of thedeactivated catalysts led to a partial recovery in cata-lyst activity, which was explained by the cleaning ofthe carbonaceous deposits produced by the actions ofcatalyst washing [29]. This distinct behaviour wouldindicate that fouling by carbonaceous deposits doesnot play an important role in red mud deactivation inTTCE hydrodechlorination.

The leachate of the Soxhlet extraction was anal-ysed using GC–MS in order to found condensationmolecules, related to the formation of carbonaceousdeposits. No one of these molecules was found in theleachate.

3.2. Catalyst characterisation

Fresh unsulfided red mud and a bulk pure hematites,prepared according to the method specified previously

Fig. 3. TPR profiles for: (a) red mud and (b)a-Fe2O3.

and tested by XRD, were characterised by TPR andTPS. TPR profiles (Fig. 3) are very similar for red mudand hematites, three transitions being observed forboth materials at very close temperatures. Althoughthe form of the peaks is different, this difference couldbe supported on difussional effects. This is in goodagreement with the literature [30,31], in which a re-duction of Fe2O3 to elemental Fe in three steps, withFe3O4 and FeO as intermediate oxides, is postulated.The reaction scheme may be written as follows:

3Fe2O3 + H2 → 2Fe3O4 + H2O

Fe3O4 + H2 → 3FeO+ H2O

FeO+ H2 → Fe+ H2O

Thus, the global TPR reaction will be the following:

3Fe2O3 + 9H2 → 6Fe+ 9H2O


The observed molar ratio between hydrogen uptakeand Fe2O3 content in the solid samples for the TPRexperiment is very close to 3 for both catalysts (2.85for red mud and 2.95 fora-Fe2O3).

The stoichiometric values and the similarity of theTPR profiles of the two solids lead us to think that theother elements present in red mud have no marked ef-fect on its chemical properties with respect to reduc-tion. Some authors suggest that Ti could change thenature of the catalytically active phase, especially inthe case of molybdenum sulfides [17]. However, ac-cording to the TPR results, this effect is unlikely inour case.

The TPS results (uptake of H2 and H2S) for thesulfiding of both solids (red mud and hematites) areshown in Fig. 4. The fact that the sulfiding began witha consumption of H2S without consumption of H2(there is even a slight release of H2 at the lower tem-peratures) suggests that the reaction proceeds througha direct O–S exchange, which means that the Fe2O3 isnot first reduced to Fe3O4 or FeO. Indeed, the directreduction of Fe2O3 occurs at higher temperatures thanthe sulfiding (comparing TPR pattern with TPS pat-terns). If the Mössbauer studies of Ramselaar et al. onthe sulfiding of Fe2O3 [32] are considered, a mecha-nism for the sulfiding could be inferred. These authorsstudied the crystallographic structure of iron sulfides atreductive conditions and different temperatures, con-cluding that at low temperatures (200–300◦C), pyrite(FeS2) is the most stable sulfide, pyrrhotite (Fe7S8)being more stable at higher temperatures and iron(II)sulfide being the most stable phase at temperaturesabove 500–600◦C. Thus, the reactions taking placeduring the TPS experiment would be the following:

3.5Fe2O3 + 14H2S → 7FeS2 + 10.5H2O + 3.5H2

7FeS2 + 6H2 → Fe7S8 + 6H2S

Fe7S8 + H2 → 7FeS+ H2S

and the global TPS reaction would be

3.5Fe2O3 + 7H2S → 3.5H2 + 10.5H2O + 7FeS

The asymmetric aspect of the TPS profiles patterns(dents) may be explained by the overlapping of thethree reactions. Similar behaviour has been reportedin the literature for nickel and cobalt oxides [27,33],whereas for molybdenum and tungsten oxides, sulfid-

Fig. 4. TPS profiles for: (a) red mud and (b)a-Fe2O3. Solid linescorrespond to H2S uptake, and dashed lines to H2 uptake.

ing starts with a reducing step [26]. Although a mech-anism via oxi-sulfides could also be congruent withthe experimental results, this may be discarded takinginto account the Mössbauer studies mentioned above.

If the TPS patterns of red mud and pure iron oxideare compared, a displacement to higher temperaturesis observed for pure iron oxide. This behaviour couldbe caused both by the lower accessibility of iron oxidein hematites, taking into account their lower surfacearea, and by a promotional effect of Ca and Na presentin red mud in the sulfiding of iron oxide.

The uptake values agree with the proposed path-ways for a-Fe2O3 (2.1 mol H2S and 1.1 mol H2 per


mol Fe2O3). In the case of red mud, the value of H2Suptake (1.88 mol H2S per mol Fe2O3, discounted theuptake corresponding to Na and Ca, which also reactswith H2S) is congruent with the proposed pathway.However, the hydrogen uptake (1.63 mol of H2 permol Fe2O3) is much higher than the expected valueof 1. This additional hydrogen uptake could be causedby the reduction of titanium or silicon oxides, whichis favoured thermodynamically compared to the re-duction of aluminium oxide. These reactions, not ob-served in the TPR experiments, could take place atTPS conditions catalysed by hydrogen sulfide.

Ca, and specially Na, could modify the acidity ofred mud. TPD experiments with sulfided red mud sat-urated with ammonia or pyridine did not demonstratethe formation of acid sites in sulfided red mud, whileacid sites are found in other transition metal sulfidecatalysts [34] or even in red mud based catalysts fromwhich Na had been removed [24].

The nature of the deactivation process was studiedby characterising catalyst samples collected at differ-ent reaction times, for operation with carbon sulfideadded. The catalyst samples studied were: fresh un-sulfided, fresh sulfided, 2 and 4 h reaction time (cor-responding to the initial period of declining activity),12 h (corresponding to the constant activity period),and 60 h (catalyst completely deactivated). The cata-lyst samples were Soxleht-washed with toluene andTHF and then characterised by nitrogen adsorption,SEM-EDX and X-ray diffraction.

Fig. 5. Surface parameters (relative to fresh sulfided catalyst) andTTCE conversion (d) vs. time on stream. Surface parameters:surface area (e), mesopore volume (h), average pore diameter(n).

Fig. 6. EDX superficial concentration (% w/w) of Fe (e), Al (h)and Ti (n) and TTCE conversion (d) vs. time on stream.

The results of nitrogen adsorption are given inFig. 5, in which the evolution of surface area, porevolume and average pore diameter is depicted. Theaverage pore diameter decreases slightly as the cata-lyst deactivates, while pore volume does not follow aclear trend, and the specific area increases unexpect-edly with reaction time. These results indicate thatthe deactivation process is not caused by phenomenathat involve the loss of surface area.

The evolution of the superficial concentration ofthe most important red mud constituents, measuredby EDX, is given in Figs. 6 (metals) and 7 (sulfurand chlorine). The concentration of Al remains almostconstant while the concentration of Ti, and particu-larly Fe, decreases slightly. The concentration of S de-creases markedly, and it correlates well with the deac-tivation of the catalyst. The most important change in

Fig. 7. EDX superficial concentration (% w/w) of Cl (h) and S(e) and TTCE conversion (d) vs. time on stream.


Fig. 8. EDX superficial distribution maps of red mud: (A) fresh sulfided and (B) after 60 h reaction time.

superficial concentration is that of Cl, which increasesfrom nearly 0 in the fresh catalyst to close to 15% inthe deactivated catalyst.

EDX maps of superficial distribution of Fe, Al, Ti,Si, S and Cl for samples of both fresh catalyst and after60 h reaction time are in shown in Fig. 8. On these

maps, white corresponds to a high concentration of anelement, black to its absence, and greys to intermediateconcentrations. EDX maps clearly show the decreasein S and the increase in Cl superficial concentration. Inaddition, it can be observed that while the elements aredistributed quite homogeneously in the fresh catalyst,


Fig. 9. SEM photographs of red mud: (A) fresh sulfided; (B) after 4 h; (C) after 12 h and (D) after 60 h.

Al on the one hand, and Fe–S–Ti–Cl on the other, tendto concentrate in different zones in the deactivatedcatalyst.

Fig. 9 presents SEM photographs of the red mud af-ter different reaction times. During the first 12 h, someflat-surfaced large particles are formed, together withgranular zones. After 60 h reaction time, smaller parti-cles are formed, part of the sample remaining as darkflat-surfaced particles. Although it is not possible tocarry out a quantitative EDX analysis of the differentcatalyst zones, as this requires the sample to be pol-

ished, semi-quantitative information can be obtainedby pointing the electron beam at different zones andanalysing the resulting spectra. Fig. 10 shows the re-sults corresponding to two zones (granular, and com-pact, flat-surfaced) of the deactivated catalyst (60 h re-action time). In the flat-surfaced zone, the Fe contentis higher and the Al content lower than in the porouszone.

X-ray diffraction patterns of sulfided red mud atdifferent reaction times (0, 2, 4, 12 and 60 h) areshown in Fig. 11. Ti remains as rutile (TiO2) in all


Fig. 10. EDX spectra of two different zones of sulfided red mudafter 60 h reaction time: (A) porous zone and (B) compact zone.

the samples, whereas Fe, which in the fresh catalystsis mainly in pyrrhotite form, tends to form iron(II)sulfide (IS) as the reaction progress. Thus, acceptingthat pyrrhotite is the most active iron sulphide phase,its disappearance could be a cause of deactivation. It

Fig. 11. XRD profiles of sulfided red mud (symbols are explainedin the text): (A) fresh sulfided; (B) after 2 h; (C) after 4 h; (D)after 12 h and (E) after 60 h.

is important to consider the fact that pyrrhotite is re-ported to be the most stable form of iron sulfide underreaction conditions [35]. In like manner, Alvarez et al.,studying the deactivation of red mud catalysts in thehydrogenation of anthracene oil at the same pressureand temperature as in this work, reported that the onlycrystallographic phase found after 100 h reaction timewas pyrrhotite, [29]. This could indicate that the trans-formation of pyrrhotite to iron(II) sulfide is promotedby the presence of some compound (probably HCl).

In spite of the important increase in chlorine con-centration on the catalyst surface, the only chloridefound by XRD was sodium chloride (halite, HA),formed from the beginning of the reaction. Na-, Ca-or Al-containing crystalline phases were not found.However, in the fully deactivated sample there is anunidentified peak (2θ = 68◦), probably associatedwith complex phases.

According to these results, the catalyst deactivationmay be explained by the destruction of pyrrhotite, as-sumed to be the active phase, which could be partiallyinduced by the presence of hydrogen chloride.

4. Conclusions

Sulfided red mud is active as a catalyst for thehydrodechlorination of TTCE at 100 bar and 350◦C.


When carbon sulfide is not added to the feed, it re-mains active up to 32 h reaction time. When carbonsulfide is added to the feed, both TTCE conversionand the active life of the catalyst increase.

Characterisation of fresh and used catalysts showa slight increase in the catalyst specific surface andchlorine surface concentration and a decrease in sul-phur surface concentration, as well as crystallographicchanges in iron species. Sulfided red mud deactivationmay be explained by the destruction of pyrrhotite, as-sumed to be the active phase, that could be partiallyinduced by the presence of hydrogen chloride.

Acknowledgements

This work was financed by a research grant fromthe Spanish Commission for Science and Technology(AMB97-850).

The authors wish to thank Bas Vongelaar (Indus-trial Catalysis Section, Technical University of Delft,Netherlands), Mónica Fernández and Lara Suárez(University of Oviedo) for their co-operation in thiswork.

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Documents

Characterisation and deactivation studies of sulfided red mud used as catalyst for the hydrodechlorination of tetrachloroethylene