Hydrodechlorination of tetrachloroethene over Pd/Al2O3: influence of process conditions on catalyst performance and stability

Applied Catalysis B: Environmental 40 (2003) 119–130

Hydrodechlorination of tetrachloroethene over Pd/Al2O3: influenceof process conditions on catalyst performance and stability

Salvador Ordóñez, Herminio Sastre, Fernando V. Dı́ez∗Department of Chemical and Environmental Engineering, University of Oviedo, Julián Claverı́a s/n, 33006 Oviedo, Spain

Received 7 August 2001; received in revised form 12 June 2002; accepted 12 June 2002

Abstract

The influence of process parameters (temperature, pressure, hydrogen flow rate, and nature of solvent) on both activityand stability of a 0.5% Pd on alumina catalyst used for tetrachloroethene (TTCE) hydrodechlorination in an organic matrixwas studied. In the range of temperature studied (250–350◦C), higher temperatures lead to higher initial activity but fasterdeactivation. Increasing hydrogen flow rates, up to 0.8 L/min (STP), produce higher activity and stability of the catalyst,whereas pressure in the range 0.5–2 MPa has no significant effect. In all the cases, both hydrodechlorination and hydrogenationof the double bond take place, yielding ethane as the main product. Concerning to the solvent, there is no difference in theinitial catalytic activity for either toluene orn-decane, butn-decane leads to faster catalyst deactivation.

The effect of temperature and space time in TTCE conversion at the period of constant catalytic activity can be modelledby a kinetic model assuming first order for TTCE and zero-order for H2.

Finally, the performance of the Pd alumina-supported catalyst is compared with that of a Pd carbon-supported catalyst withthe same metal load, used in previous works. Although the carbon-supported catalyst yields higher initial conversion, thealumina-supported catalyst is more resistant to deactivation.© 2003 Elsevier Science B.V. All rights reserved.

Keywords:Tetrachloroethene; Hydrodechlorination; Pd catalyst; Catalyst deactivation

1. Introduction

Tetrachloroethene (TTCE, also called perchloroethy-lene or tetrachloroethylene) is a chlorinated compoundwidely used in industrial applications as solvent,metal surface cleaning agent, and especially in drycleaning of textiles. Although as most organochlo-rinated compounds, tetrachloroethene is harmful forthe environment and human health, it is very difficultto replace in these uses, because of its physical andchemical properties.

∗ Corresponding author. Tel.:+34-985-103437;fax: +34-985-103434.

As the result of the use of TTCE in dry cleaning oftextiles, solid or highly viscous liquid wastes contain-ing 40–90% TTCE (depending on the dry-cleaningtechnology), solids and grease are produced[1]. Inmost countries, these wastes are treated by inciner-ation. However, this technique has important draw-backs, the most important being the risk of formationof dioxins and other incomplete combustion highlytoxic by-products[2]. As the formation of these com-pounds is avoided if the wasted are treated underreductive conditions, several processes based on re-duction of the organochlorinated compounds havebeen developed, using different reductive agents suchas metals[3,4], hydrazine[5], or enzymatic systemsin the case of aqueous media[6]. However, hydrogen

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120 S. Ord́oñez et al. / Applied Catalysis B: Environmental 40 (2003) 119–130

is considered to be the most suitable reductive agentbecause it is widely available and relatively inexpen-sive, and the non-toxic character of the hydrogenationproducts (hydrocarbons and hydrogen chloride, thatcan be easily removed by absorption). On the otherhand, reduction with hydrogen requires either tem-peratures higher than 700◦C [7], or the presence ofa catalyst. In the presence of a catalyst, temperaturesneeded are much lower (200–350◦C, depending onthe catalyst). Concerning to the pressure, althoughcatalytic hydrodechlorination usually requires pres-sures above 0.1 MPa, when noble metal catalysts areused relatively mild conditions (0.5 MPa) are needed.Besides the environmental advantages (absence ofhighly toxic by-products), catalytic hydrodechlorina-tion can be an interesting alternative to incinerationfrom the economical point of view, as pointed out byKalnes et al.[8].

Most studies on catalytic hydrodechlorination of or-ganic compounds have been carried out in gas phase.Earlier works were carried out using hydrotreatmentcatalysts (sulfides of Mo, promoted by Ni or Co), be-cause of the chemical likeness between hydrodechlo-rination and hydrorefining of heteroatoms in theprocessing of oil products[8–14]. In these works, cat-alytic hydrodechlorination was shown to be a very effi-cient technique for the removal of chlorine from manycompounds in organic wastes, such as polychloroben-zenes, chlorophenols, and polychloro-olefins. Typi-cal reaction conditions are temperature in the range275–350◦C, and pressure in the range 7.5–10 MPa.

Hydrodechlorination over noble metal catalysts hasbeen studied from the 1970s[15,16]. Hydrodechlo-rination reactions catalysed by noble metals occur atlower temperature, and specially at lower pressure(0.1–0.5 MPa). Although, this reaction can be carriedout at room temperature and pressure in a multiphasesystem developed recently (using an heterogeneouscatalyst, aqueous solutions and phase transfer cata-lysts)[9], these systems are very complex and difficultto scale-up.

Most studies on catalytic hydrodechlorination havebeen carried out at very low concentration of theorganochlorinated compound, and working only withthe organochlorinated compound and hydrogen. How-ever, in industrial practice the chlorinated compoundis present at high concentration, and frequently dis-solved in an organic matrix, or is a solid or sludge

that must be dissolved before treatment. In particu-lar, TTCE containing dry-cleaning wastes must bedissolved in an organic solvent and filtered, in orderto remove solid particles, resulting a liquid organicstream, suitable for catalytic hydrogenation.

Concerning to the catalytic reaction, in the case ofTTCE, as for all polychloro-olefins, the double bondis also hydrogenated, yielding ethane as main organicproduct.

In a previous work, we made a preliminary studyon the activity of hydrotreatment, nickel reduced,and supported precious metal catalysts (Rh, Ru, Ptand Pd) for the hydrodechlorination of the most en-vironmentally relevant organochlorinated compounds(tetrachloroethene, trichloroethene, dichloromethane,chloroform, methyl-chloroform and carbon tetrachlo-ride). Experiments were carried out in discontinuousmicroreactors at 300◦C and 5 MPa in presence ofheptane as organic solvent[17]. Among the cata-lysts tested, Pd and Pt, supported both on alumina oractivated carbon, were found to be the most active.

An important aspect to be considered in the devel-opment of an industrial hydrodechlorination processis the catalyst deactivation, especially taking intoaccount that hydrodechlorination takes place in areductive environment (risk of coke formation) andthat hydrogen chloride is produced (risk of catalystpoisoning). In this way, our research group has stud-ied the deactivation of hydrotreatment and preciousmetal (carbon and alumina-supported Pd, Pt and Rh)catalysts in the deactivation of TTCE at fixed temper-ature (250◦C for noble metal catalysts and 350◦C forhydrotreatment catalysts)[18–20]. The main conclu-sions obtained were as follows. (1) At the same spacetime, hydrotreatment catalysts are less active and lessresistant to deactivation than noble metal catalysts.Among noble metals, Pd presents the best perfor-mance. (2) Hydrotreatment catalysts are stronglypoisoned by HCl, whereas in the case of noble metalcatalyst this effect is not so strong, being more impor-tant the deactivation caused by carbonaceous deposits.Considering these aspects, Pd supported on aluminacan be preliminarily selected as the most appropriatecatalyst for TTCE hydrodechlorination.

In order to develop an industrial TTCE hydro-dechlorination process, it is necessary to study morein depth the performance of this catalyst. For this pur-pose, in this work the influence of different process

S. Ord́oñez et al. / Applied Catalysis B: Environmental 40 (2003) 119–130 121

variables (operation temperature and pressure, H2/TTCE ratio, and organic matrix of the chlorinatedcompound) on the activity and stability of the cat-alyst is systematically studied. The mathematicalmodelling of the reaction kinetics and deactivationis very useful in order to evaluate the technical andeconomical feasibility of the process, and eventuallyto proceed to the design of a pilot or demonstrationplant. Results obtained allow also a better comparisonof alumina- and carbon-supported Pd catalysts. To thebest of our knowledge, there are no available studiesof this type in the literature.

2. Experimental

2.1. Materials

Experiments were carried out over a commercial Pdon �-alumina catalyst (0.5% Pd) supplied by Engel-hard, whose main textural parameters (determined bynitrogen adsorption) and metal dispersion (determinedby hydrogen pulse chemisorption[21]), are given inTable 1. The catalyst, available in extrusions, wasmilled and crushed, and the fraction between 100 and200�m was selected. The catalyst was introduced in-side the reactor diluted with inert corundum (Janssen,Belgium, maximum particle size 100�m).

TTCE, supplied by Panreac (99.9% purity), was dis-solved in toluene orn-decane (99.5% purity, Panreac).Hydrogen (>99.9995% purity) was supplied by AirLiquide.

Table 1Main characteristics of the catalysts used in this work

ESCAT-26 ESCAT-18

Composition (wt.%) 0.5% Pd,activated carbon

0.5% Pd,alumina

Surface area (m2/g) 1690a 103.4b

Pore volume (cm3/g) 0.160c, 0.401d 0.45c

Average pore diameter (nm) 1.5 16.5Metal dispersion

(%, H2 chemisorption)45.9 30.8

a Determined by the method of Langmuir.b Determined by the method of Brunauer, Emmet and Teller.c Mesopore volume, determined by the method of Barret,

Joyner and Halenda (desorption).d Micropore volume, determined by the method t-de Lippens.

2.2. Reaction studies

Reaction studies were carried out in a continuouspacked bed reactor. The reactor was a stainless-steeltube of 9 mm i.d. and 450 mm length, placed inside anelectrically heated furnace. Five thermocouples mea-sured the reactor wall external temperature at differentheights. The reaction zone was located in the centreof the bed, with a height of 30 mm approximately,the rest of the reactor being filled with corundum.Temperature in the reaction zone was measured by athermocouple inserted in the reactor, which providedthe control signal for a PID controller acting on theelectric furnace. The flow rate of hydrogen fed to thereactor (0.8 L/min (STP), except in the studies on theinfluence of H2/TTCE ratio and the kinetic studies)was controlled by a Brooks 5850TR/X-5879E masscontroller. The liquid feed (0.55 mol/l of TTCE dis-solved in toluene orn-decane) was impelled by aKontron LC T-414 metering pump. Liquid flow ratewas 7× 10−4 l/min, except in the case of kineticsexperiments, in which liquid flow was varied between3×10−4 and 3×10−3 l/min. The reactor effluent wascollected into a 1 l stainless-steel Teflon-coated cylin-der, acting as gas–liquid separator and reservoir forliquid reaction products. Pressure inside the reactorwas controlled by a Tescom back-pressure regulator,that vented gas reaction products. A two-valve systemallowed periodical withdrawn of liquid samples. Theamount of catalyst charged to the reactor was 0.5 g.Catalysts were reduced in situ before use by passing0.8 L/min (STP) hydrogen at 350◦C during 6 h.

Reactions were started after catalyst activation,maintaining the catalyst at reaction temperature in hy-drogen flow for 3 h, being then the liquid feed started.To avoid transient effects (especially in the kineticstudies), samples taken after changes in the liquid orgas flow rates, solvent, temperature, or pressure werediscarded.

2.3. Analysis and catalysts characterisation

Reaction feed and liquid products were analysedby gas chromatography in a Hewlett-Packard appara-tus equipped with a 60 m semipolar VOCOL column.Light alkanes formed in the reaction were analysedby injecting samples of the gas reaction product inthe same chromatograph using a Carbowax packed


column. Hydrogen chloride concentration was mea-sured by absorption of the gas in distilled water andtitration of the resulting solution with NaOH. Themain reaction products were ethane and hydrogenchloride. Trichloroethane was the only chlorinatedby-product detected. Neither ethylene nor other chlo-rinated compounds were found in the reaction prod-ucts in any case.

When the reaction was stopped, the catalyst wascooled in situ by flowing nitrogen, and then sepa-rated from the corundum by sieving and collected forsubsequent characterisation. Fresh and used catalystswere characterised by different techniques. Nitrogenadsorption measurements were performed in a Mi-cromeritics ASAP 2000 apparatus. Crystallographicchanges were studied by X-ray diffraction in a PhilipsPW1710 diffractometer, working with the K� lineof copper. Surface concentrations were calculated asthe average of measurements in 10 different points inthe catalyst surface. Carbonaceous deposits on usedcatalysts were evaluated by thermogravimetric (TG)studies carried out in a Mettler TA4000-TG50 ther-mobalance in a synthetic air oxidant atmosphere. Allthe thermograms were duplicated, the discrepanciesbeing lower than 2%.

3. Results and discussion

3.1. TTCE conversion

The occurrence of homogeneous reactions wasdiscarded, negligible conversions being obtained inexperiments carried out in absence of catalyst at tem-peratures up to 350◦C, maximum temperature used inthis work. All the experiments, except those devotedto the study of catalyst deactivation, were carried outin the period of constant catalytic activity.

The first series of experiments was devoted tostudy the effect of pressure, in the range 0.5–2 MPa,on TTCE conversion at 250 and 300◦C, for a spacetime of 1.3 gcatmin/mmol TTCE. Pressures higherthan 0.5 MPa were selected in order to minimise thelosses of reaction products in the gas phase. Resultsobtained show that total operation pressure does nothave an important effect on TTCE conversion at theconditions studied. This behaviour is consistent withthe kinetic model developed in theSection 3.5

Fig. 1. TTCE conversion when decane is used as solvent at (�)250◦C and (�) 300◦C compared with the performance in tolueneat the same temperatures (opened symbols). Other operation con-ditions: P = 0.5 MPa and 0.8 L/min (STP) of hydrogen.

The influence of the operation temperature on TTCEconversion in the constant activity period is discussedin detail in theSection 3.5.

Subsequent experiments were devoted to study theinfluence of the nature of the organic solvent. This is animportant issue considering that, as mentioned in theSection 1, TTCE-containing organic wastes often areaccompanied by organic compounds, or must be dis-solved before being processed. The effect of the pres-ence of an aromatic and an aliphatic solvent (tolueneand n-decane, respectively) at 0.5 MPa and differenttemperatures and space times is shown inFig. 1. Itcan be observed that TTCE conversion attained in thepresence of both solvents is similar.

Another important aspect to consider is the influ-ence of the H2/TTCE ratio. The effect of this variablewas studied in a series of experiments at 250◦C,0.5 MPa, constant liquid flow rate (7× 10−4 l/min)and hydrogen flow rates from 0.1 to 1 L/min (STP).Results are shown inFig. 2. Two different zones areobserved: for low hydrogen flow rates, TTCE con-version increases as hydrogen flow rate increases,whereas for hydrogen flow rates higher than 0.4 L/min(STP), hydrogen flow rate does not influence reactionconversion. The positive effect of higher hydrogenflow ratio in the first region could be due, both to theresulting increase in hydrogen partial pressure, whichcould affect the intrinsic reaction kinetics, or to theincrease in gas velocity, which would increase the gas


Fig. 2. Effect of H2 flow on (�) TTCE conversion and (�)selectivity at 250◦C, 0.5 MPa and 1.3 g min/mmol space time.

phase-catalyst particle mass transfer coefficient. Thecomparison between the rate of gas phase-catalyst par-ticle mass transfer (estimated according to the proce-dures proposed in the literature[22]) and the observedreaction rate indicates that gas phase-catalyst particlemass transfer effects are negligible, so it can be con-cluded that the observed increase in TTCE conversionis caused by the effect of the increase of hydrogenpartial pressure on the intrinsic reaction kinetics. Atsufficiently high hydrogen partial pressure, the satu-ration is attained, and conversion remains constant.

3.2. Catalyst stability

In this section, the influence of temperature, totalpressure, H2/TTCE ratio and solvent on catalyst sta-bility is discussed. The experimental conditions usedin the experiments are given inTable 2.

The change with time of TTCE conversion,recorded at 250, 300 and 350◦C, is depicted inFig. 3.It is observed that higher temperatures lead to higher

Table 2Experimental conditions for the different deactivation experiments

Studied parameter W/FA0

((g min)/mmol)Support Temperature

(◦C)Pressure(MPa)

H2/TTCE(molar)

Solvent

Temperature 1.3 �-Alumina 250, 300, 350 0.5 25/1 ToluenePressure 1.3 �-Alumina 300 0.5, 2 25/1 TolueneH2/TTCE mol ratio 1.3 �-Alumina 250 0.5 10/1, 25/1 TolueneSolvent 1.3 �-Alumina 300 0.5 25/1 Toluene,n-decaneCatalyst support 1.3 Carbon,�-alumina 250 0.5 25/1 Toluene

Fig. 3. Decrease of TTCE conversion with time-on-stream (TOS,h) at (�) 250◦C, (�) 300◦C and (�) 350◦C. See other reactionconditions inTable 2.

initial conversions, but the deactivation is also fasteras temperature increases. As a result, the highestTTCE conversion after time-on-stream 40 h is attainedat 250◦C. At this temperature, the catalyst remainedhighly active at the end of the experiment (110 h).

In order to quantify the carbonaceous deposits, sam-ples of catalysts after 110 h reaction time were charac-terised by thermogravimetric analysis. The results ofthese studies, expressed as weight losses between 200and 900◦C, together with BET surface areas and sur-face chlorine concentrations (measured by EDX) aresummarised inTable 3. There is not a good agreementbetween deactivation data and coke content of the de-activated catalyst samples. So, in spite of the observeddifferences on catalytic activity, the difference in cokecontent in the used catalysts is not very marked, andthe tendency of variation with temperature is unclear.On the other hand, the tendencies of variation of sur-face area and activity of the used catalyst are opposite,


Table 3Surface area, coke content and surface chlorine concentration forthe catalysts after 110 h on stream

Reactionconditions

Surface area(m2/g)a

Coke weight(%, w/w)b

Surface chlorine(%, w/w)c

250/0.5/0.8/A/T 84.4 4.25 2.09300/0.5/0.8/A/T 104.1 3.72 2.3350/0.5/0.8/A/T 137 4.17 2.26250/0.5/0.2/A/T 79.2 5.95 2.77300/2/0.8/A/T 102.8 4.31 2.29300/0.5/0.8/A/D 100.9 6.11 2.29250/0.5/0.8/C/T 1090 3.51 –

Reaction conditions: temperature (◦C)/pressure (MPa)/hydrogenflow rate (L/min (STP))/support (A: alumina, C: carbon)/solvent(T: toluene, D:n-decane).

a Determined according to the method of Brunauer, Emmet andTeller for alumina-supported catalysts and the method of Langmuirfor carbon-supported catalysts.

b Correspond to the weight losses between 200 and 900◦Cin a thermobalance and dynamic air atmosphere. Percentages arereferred to the catalyst after the desorption of the volatile com-pounds. More experimental details are given in reference[19].

c Determined by EDX, it is not possible to obtain reliable datafor organic supports.

the highest surface area corresponding to the most de-activated samples. XRD patterns do not reveal any rel-evant crystallographic change in the catalysts, as onlythe peak of�-alumina is observed in all the catalystsamples, Pd being undetected due to its low contentin the catalyst.

In earlier works, the deactivation of noble metal cat-alysts (Rh, Pd and Pt, both supported on�-alumina[19] or on activated carbon[20]) was studied at 250◦C.In these works, XPS and TG analysis showed that de-position of carbonaceous deposits was an importantfactor on catalyst deactivation. Results presented heresuggest that other phenomena such as crystallite sin-tering, or HCl poisoning could be also important atthe higher temperatures studied here. Although, chlo-rine concentrations measured by EDX are very sim-ilar in all the deactivated catalyst samples, it shouldbe noted that these data give little information on theamount of chlorine linked to the active phase, becausean important fraction of the chlorine measured proba-bly corresponds to the coke deposits, which have beenreported to present high chlorine concentrations[20].

The temperature dependence of the deactivation canbe modelled using the empirical deactivation model:

x = x0 exp(−kdt) (1)

Table 4Values of kd, x0 (Eq. (1)) and goodness of the fit (correlationcoefficient) for the deactivation rate of the catalyst at differenttemperatures, pressures and hydrogen flow rates

Reaction conditions kd (h−1) x0 r2

250/0.5/0.8/A/T 0.0007 0.685 0.871300/0.5/0.8/A/T 0.0044 0.807 0.969350/0.5/0.8/A/T 0.0067 0.863 0.981300/2/0.8/A/T 0.0023 0.786 0.902250/0.5/0.2/A/T 0.0054 0.624 0.933

For code of reaction conditions, seeTable 3.

where x is TTCE fractional conversion,x0 TTCEfractional conversion extrapolated tot = 0, kd is adeactivation constant, andt is reaction time. Similardeactivation models have been proposed in the liter-ature for the deactivation of metallic catalyst used inhydrodechlorination reactions[23]. The values ofkdandx0 at the three temperatures studied, calculated byfitting Eq. (1)to the experimental data, as well as thecorrelation coefficients, are shown inTable 4. The pro-posed model fits fairly well the experimental results at300 and 350◦C. The fit at 250◦C is worse, probablybecause of the low degree of deactivation at this tem-perature for the time-on-stream studied. Assuming anArrhenius dependence ofkd with absolute tempera-ture (T, in K) the following expression is obtained:

kd = 1130 exp

(−7427

T

)(2)

This expression leads to an activation energy forthe deactivation constant of 61.7 ± 10 kJ/mol. Thegoodness of the fit is shown inFig. 4.

An additional deactivation curve was obtained at250◦C for 0.2 L/min (STP) hydrogen flow rate, whichcorresponds to H2/TTCE molar ratio 10/1. Althoughfor this lower flow rate there is still a high excessof H2, a marked catalyst deactivation is observed(Table 4). This is in good agreement with data inTable 3, that shows a pronounced increase in coke con-tent and surface chlorine and decrease in surface areafor the deactivated catalyst, and with previous resultsthat indicated the importance of coking at low tem-peratures[19]. So, lower H2/TTCE ratios, and hencehigher partial pressure of organic compounds andlower partial pressure of hydrogen favour coke forma-tion [24]. Data inTable 3also indicates that, for thetwo experiments at 250◦C, the surface chlorine/coke


Fig. 4. Arrhenius plot for the evolution of the deactivation rateconstant with temperature.

content ratio remains approximately constant, whichsupports the quantitative importance of the chlorinecontained in the coke deposits. This agrees with XPSand TPO–MS studies cited in references[19,20].

Data inTable 4also allows to consider the effect oftotal pressure on catalyst stability, by comparing theresults obtained at 300◦C working at 0.5 and 2 MPa.Deactivation is slightly slower at higher total pressurefor reaction time above approximately 70 h. As shownin Table 3, the catalyst deactivated at 2 MPa totalpressure bears an amount of coke slightly higher (al-though probably not significant) than the deactivatedat 0.5 MPa. Total pressure has a two-fold effect on theamount and properties of coke formed. On one hand,TTCE and toluene partial pressures increase as totalpressure increases, hence favouring the formation ofcarbonaceous deposits. On the other hand, hydrogenpartial pressure increases as total pressure increases,with the opposite effect on the formation of coke. Ac-cording to this, it seems that these two effects are com-pensated in a great extension, resulting a very similarbehaviour of the catalyst at the two pressures studied.

The influence of the solvent in catalyst deactivationcan be observed inFig. 5, in which the deactiva-tion curves usingn-decane and toluene as solventsare shown. The deactivation model correspondingto Eq. (1) did not provide a good fitting to the dataobtained when decane is used as solvent. Experimen-tal results show that the catalyst deactivates fasterwhenn-decane is used as solvent, corresponding to a

Fig. 5. Decrease of TTCE conversion with time-on-stream (TOS,h) for TTCE dissolved in (�) decane and (�) toluene. See otherreaction conditions inTable 2.

noticeable increase in coke content (Table 3). Theseresults are in certain extent unexpected, as accord-ing to the literature, aromatic structures are currentlyinvolved in coke formation; on the other hand, theycould be explained by the higher molecular weightand boiling point ofn-decane, which would favourits deposition on the catalyst surface, and by theexistence ofn-decane radicalary cracking reactions,revealed by the appearance of lower molecular weighthydrocarbons (heptanes, hexanes, pentanes) in thereaction products whenn-decane is used as solvent.These radicals could be involved in coke formation.

3.3. Selectivity

In addition to TTCE conversion, it is also importantto consider the selectivity for total hydrodechlorina-tion, taking into account that the aim of the reactionis to transform organic chlorine into inorganic. Se-lectivity for total dechlorination, is defined as mol ofnon-chlorinated organic compounds formed per molof TTCE reacted.

Selectivities for the experiments corresponding tothe studies on the effect of solvent and operation pres-sure on the catalyst performance (Section 3.1), carriedout at space time 1.3 min g/mmol and 250–300◦C,were higher than 96% in all the cases. However, inthe experiments devoted to study the influence of


H2/TTCE ratio, selectivities were high and almostconstant for high hydrogen flow rate (>0.5 L/min(STP)), decreasing then as hydrogen flow rate de-creases, selectivity for 0.1 L/min (STP) being 83%(Fig. 2). This behaviour is similar to that observed forthe evolution of TTCE conversion (Fig. 2), althoughthere is a zone (hydrogen flow rates between 0.35 and0.55 L/min (STP)) where TTCE conversion remainsconstant whereas selectivity increases as hydrogenflow rate increases.

This behaviour could be explained considering thatthe reaction takes place through successive formalsteps, hydrodechlorination of TTCE yielding trichlo-roethene (the only intermediate product detected),followed by hydrodechlorination–hydrogenation oftrichloroethene to ethane:

Cl2C=CCl2 + H2 → Cl2C=CHCl + HCl (3)

Cl2C=CHCl + H2

→ H2C=CCl2(or HClC=CHCl) + HCl (4)

H2C=CCl2 + H2 → H2C=CHCl + HCl (5)

H2C=HCCl + H2 → H2C=CH2 + HCl (6)

H2C=CH2 + H2 → CH3–CH3 (7)

Reactions (c), (d) and (e) are very fast, so that the onlyintermediate detected is trichloroethene. Reaction (b),which determines TTCE selectivity to total dechlori-nation, is faster than reaction (a), which determinesTTCE conversion, and reaches saturation at higher hy-drogen partial pressure than reaction (a), that is whyselectivity is higher than TTCE than TTCE conversionin all cases.

Table 5Selectivity for total hydrodechlorination and solvent conversion in the deactivation experiments

Experiment Selectivity for ethane Toluene conversion

TOS = 1 h TOS= 110 h TOS= 1 h TOS= 110 h

250/0.5/0.8/A/T 0.959 0.939 0.476 0.003300/0.5/0.8/A/T 0.935 0.908 0.266 0.008350/0.5/0.8/A/T 0.867 0.846 0.194 0.005250/0.5/0.2/A/T 0.926 0.802 0.090 0.001300/2/0.8/A/T 0.999 0.998 0.651 0.026300/0.5/0.8/A/D 0.972 0.617 – –250/0.5/0.8/C/T 0.978 0.952 0.005 0.003

For code of reaction conditions, seeTable 3.

Fig. 6. Evolution of selectivity for total hydrodechlorination withTOS for the following experiments (temperature (◦C)/pressure(MPa)/hydrogen flow rate (L/min (STP))/solvent): (�) 250/0.5/0.8/toluene; (�) 250/0.5/0.2/toluene; (�) 300/0.5/0.8/toluene; (�)300/2/0.8/toluene; (�) 300/0.5/0.8/decane.

Changes of selectivities for total hydrodechlorina-tion with time-on-stream, corresponding to ageing ex-periments, are given inFig. 6 andTable 5. For hightime-on-stream, the trend in the range studied is higherselectivities for lower temperature and higher totalpressure and hydrogen/TTCE ratio. When toluene isused as solvent, very high selectivities are attained,even after 110 h, working at adequate operating condi-tions (i.e. 250◦C, 0.5 MPa, 0.8 L/min (STP) or 300◦C,2 MPa, 0.8 L/min (STP)). Selectivity decreases sharplywith time-on-stream whenn-decane is used as solvent.

3.4. Conversion of the solvent

Reaction of the solvent is undesired, as it con-sumes valuable hydrogen and hinders the reutilization


Fig. 7. Decrease of toluene conversion with TOS for the followingexperiments (temperature (◦C)/pressure (MPa)/hydrogen flow rate(L/min (STP))): (�) 250/0.5/0.8; (�) 250/0.5/0.2; (�) 300/0.5/0.8;(�) 300/2/0.8.

of the solvent. When toluene is used as solvent, theonly derived reaction product detected was methyl-cyclohexane (MCH). The formation of this productis important during the first reaction hours, but fallssharply with reaction time, and after 10 h on streamtoluene conversion is near zero in most experiments,as shown inFig. 7.

In this way, experiments reported in theSection 3.1,carried out in the constant activity period (time-on-stream higher than 10 h), gave similar results, withtoluene conversions lower than 2% in all the cases.Table 5shows toluene conversions for the “catalyststability experiments”. As expected, MCH is producedin higher amount as both total pressure and hydrogenflow (and hence hydrogen partial pressure) increase.Toluene conversions for 1 h on stream decrease astemperature increases, whereas after 110 h on stream,toluene conversions are very low and it is not possibleto observe any trend.

These results indicate that deactivation of the cat-alyst for toluene hydrogenation is much faster thanfor TTCE hydrodechlorination. This behaviour couldbe explained by the formation of surface palladiumchloride, which could be active for hydrodechlorina-tion, but not for the hydrogenation of toluene. So,Wu et al. described very high hydrodechlorinationconversions working with supported PdCl2 catalysts[25] although chlorine is often considered as a cat-alyst poison[23,26]. The inhibitory or poisoning ef-fect of the HCl released in the reaction could also

explain the observed faster deactivation of the cat-alyst for toluene conversion for higher temperature,as for short time-on-stream the reaction rate for hy-drodechlorination increases as temperature increases,and hence higher amounts of HCl are formed at highertemperature.

When n-decane was used as solvent many hydro-carbons (mainly C5, C6 and C7) were detected in thereaction product, resulting very complex gas chro-matographs, so that the reliable quantification of sol-vent conversion was not possible. However, it wasqualitatively observed that as the conversion of the sol-vent decreases with time, no reaction products fromthe solvent were detected at the end of the experiment.

3.5. Kinetic studies

Kinetic data for TTCE hydrodechlorination at dif-ferent space times and temperatures have been fittedto a first-order equation with an Arrhenius dependenceon temperature of the kinetic constant. Experimentaldata were obtained at 0.5 MPa, feeding to the reactora solution of 10 wt.% TTCE in toluene and using aconstant H2/TTCE ratio of 25/1. Different space timeswere obtained using different liquid and gas flow rates.

If diffusional and thermal effects are considerednegligible and integral ideal plug flow reactor is as-sumed, the following expression is obtained:

W

FA0

=∫ x

0

dx

−rA(8)

wherex is the conversion,−rA the rate of disappearingof TTCE (mmol TTCE/(gcatmin)) and (W/FA0) is thespace time for TTCE ((gcatmin)/mmol TTCE).

As shown below, in certain conditions first-orderkinetics is consistent with the most successful mod-els used in the literature to describe the kinetics ofhydrodechlorination reactions, based on Langmuir–Hinselwood mechanisms[10,27–29]. For noble metalcatalysts, analogous active sites for the adsorptionof hydrogen (dissociative in the case of Pd) and theorganochlorinated compound are generally consid-ered [26,28]. This mechanism has been proposedby different authors in order to explain mixture ef-fects in hydrodechlorination reactions[28] or tomodel the hydrodechlorination of chlorofluorocarbons[28].


With these assumptions, the kinetic expression isthe following:

−rA = kA√

KHpHKApA

(1 + √KHpH + KApA)2

(9)

wherepA and pH are the partial pressures of TTCEand H2, respectively,kA is the intrinsic kinetic con-stant andKH andKA are the adsorption constants ofhydrogen and TTCE, respectively. Considering thatpHis constant during all the experiments andKH is muchhigher thanKi [27,28], this expression is simplified toa first-order kinetics:

−rA = k′ApA (10)

wherek′A (mmol/min g MPa), is a function of the in-

trinsic kinetic constant and adsorption constants:

k′A = kAKA

√KHpH

(1 + √KHpH)2

(11)

The approximately zero-order dependence of the reac-tion rate with total pressure observed in theSection 3.1section is consistent withEq. (3), which predicts anorder in the range+0.5 to −0.5 for the variation ofthe reaction rate with total pressure.

According to Eqs. (1) and (4), ln(1/(1−x)) (be-ing x the TTCE fractional conversion), should in-crease linearly with (W/FA0). Fig. 8 shows that thepseudo-first-order kinetics assumption provides agood fit to the experimental results.

Fig. 8. Pseudo-first-order relationships for hydrodechlorination ofTTCE at (�) 200◦C, (�) 225◦C, (�) 250◦C and (�) 300◦C.

Fig. 9. Arrhenius plot for the evolution of the TTCE hydrodechlo-rination rate constant with temperature.

The equation obtained by fitting the experimen-tal data to first-order kinetics and Arrhenius depen-dence of the rate constant with temperature is (T,in K)

k′A = 4345.52 exp

(−3212.5

T

)(12)

that corresponds to an activation energy of 26.7 ±3.6 kJ/mol. The goodness of the proposed model (r2 =0.985) can be observed in the Arrhenius plot inFig. 9.

Concerning to values reported in the literature, Kimand Allen giveEA values of 50 kJ/mol for TTCE hy-drodechlorination, working with hydrotreatment cat-alysts[14]. This value is lower than the obtained inthis work, as expected considering the higher activityof the Pd catalyst[17]. As far as we know, no dataof EA for hydrodechlorination of polychloro-olefinsover noble metal catalysts are reported in the liter-ature, except the work of Weiss and Kreiger, whoreported an activation energy of 112 kJ/mol for thehydrodechlorination of different dichloroethylene iso-mers over Pt catalysts, but supposing zero-order ki-netics [15]. Values in the range 56–80 kJ/mol havebeen reported for other chlorinated compounds, suchas dichlorodifluoromethane or tetrachloromethane, as-suming first-order kinetics[30] and references citedtherein. In earlier works, it was observed that tetra-chloromethane is markedly more reactive than TTCE[17].


Fig. 10. Decrease of TTCE conversion with time-on-stream (TOS,h) for the (�) carbon-supported and (�) alumina-supported pal-ladium catalysts. See reaction conditions inTable 2.

3.6. Comparison of alumina and activated carbonas supports

In a previous work, we have studied the perfor-mance of a commercial 0.5% Pd on an activated carboncatalyst for the hydrodechlorination of TTCE, in con-ditions similar to that of this work (250◦C, 0.5 MPa,0.8 L/min (STP) and 1.3 g min/mmol space time)[20].The main characteristics of the carbon-supported cata-lyst, manufactured by Engelhard, are given inTable 1.In this section, the performances of the 0.5% Pd sup-ported on alumina and activated carbon catalysts forTTCE hydrodechlorination are compared. The natureof the catalyst support has great influence on the cat-alyst performance. For hydrodechlorination reactions,there are different opinions on the most adequate sup-port. So, some authors stated that the best supportsare those resistant to HCl, such as activated carbon orAlF3 [30,31], discouraging the use of other supportssuch as SiO2 or Al2O3, whereas others authors usedalumina-supported catalysts, with no mention of anydeactivation effect[32].

The deactivation curves for the carbon- and alu-mina-supported catalyst at the same operation condi-tions (seeTable 2) are shown inFig. 10, and somecharacteristics of the deactivated catalysts are inclu-ded in Table 3. It can be observed that initially thecarbon-supported catalyst is more active, but its de-activation is much faster than that of the alumina

supported, that deactivates very slowly. Accordingto these results, the alumina-supported catalyst ismore adequate to carry out the detoxification ofTTCE-containing wastes. Similar differences in thedeactivation of alumina- and carbon-supported cata-lyst were reported by Meyer et al. in the hydrodechlo-rination of 1,3-dichloropropene over a Pt catalyst[33].

The initial higher activity of the carbon-supportedcatalyst could be explained by the higher dispersionof Pd in this catalyst (as can be seen in the hydrogenchemisorption data quoted inTable 1). Another pos-sible cause of this higher activity is the presence ofspill-over phenomena, that are reported to be more im-portant in carbon-supported catalysts[34]. Spill-overcan cause an increase of concentration of reactantson the catalyst surface, and therein an increase inconversion.

The difference in the deactivation behaviour ofthe two catalysts could be explained consideringthe evolution of their surface area. While the usedalumina-supported catalyst maintains almost 82%of its initial surface area after 110 h on stream, thisvalue is only 64.5% for the carbon-supported, eventhough its coke content is lower. This can be causedby the microporous structure of activated carbon(smaller pore sizes), that are more easily blocked bycoke.

4. Conclusions

The effect of operation parameters on the perfor-mance of a Pd/Al2O3 catalyst for tetrachlorethylenehydrodechlorination has been studied. In the rangestudied, increasing temperatures lead to higher re-action rate but faster catalyst deactivation. The ef-fect of temperature in TTCE conversion is properlyquantified by a pseudo-first-order kinetic model withArrhenius dependence for the kinetic constant. To-tal pressure has little effect on conversion, whereashigher pressure led to slightly slower deactivation.Concerning to H2/TTCE ratio, higher values of thisparameter lead to both higher activity and higherresistance to deactivation. The initial conversion at-tained is the same when either toluene orn-decaneare used as solvents, but the catalyst deactivates fasterwhen decane is used as solvent.


When compared with a Pd/activated carbon, thealumina-supported Pd catalyst performs better, asthe alumina-supported maintains high activity muchlonger.

The final, and more important conclusion, is thatthe tested Pd/alumina catalyst appears to be appro-priate for industrial TTCE hydrodechlorination pro-cesses, because, when operated at adequate conditions,it provides high hydrodechlorination activity with highselectivity towards total hydrodechlorination, togetherwith high resistance to deactivation and little conver-sion of the solvent.

Acknowledgements

This work was financed by a research Grant ofthe Spanish Commission for Science and Technology(AMB97/850 and PPQ2000-0674). Engelhard, Italy,is acknowledged for its kind supply of catalysts.

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Documents

Hydrodechlorination of tetrachloroethene over Pd/Al2O3: influence of process conditions on catalyst performance and stability