chlorobenzene hydrodechlorination catalyst prepared via

ISSN 0023�1584, Kinetics and Catalysis, 2015, Vol. 56, No. 6, pp. 764–773. © Pleiades Publishing, Ltd., 2015.Original Russian Text © E.S. Lokteva, E.V. Golubina, M.V. Antonova, S.V. Klokov, K.I. Maslakov, A.V. Egorov, V.A. Likholobov, 2015, published in Kinetika i Kataliz, 2015,Vol. 56, No. 6, pp. 753–762.

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INTRODUCTION

Chlorinated organic compounds are among themost dangerous organic environmental contaminantsexerting a negative effect on nature and human health[1]. They are prone to bioaccumulation; migration insoil, water, and air; and accumulation in nutrientchains. Even small doses of these compounds caninduce various diseases, including cancer. In addition,these compounds possess exceptional chemical andbiological stability and toxicity [2]. Chlorine�contain�ing organic compounds form in various processes ofthe chemical industry as target products or by�prod�ucts. Mixtures of chlorinated benzenes and polychlo�rinated biphenyls were used earlier as electrotechnicalliquids, and, hence, the accumulated stocks of thesemixtures need qualified utilization.

There are different procedures for the utilization oforganochlorine compounds, including oxidative(combustion and catalytic oxidation), reductive(dehydrochlorination and hydrodechlorination),electrochemical, and some other methods of theirconversion to useful products; their landfilling wasalso reported [3]. The oxidation of chlorine�contain�ing compounds, which are incombustible in nature,requires large energy expenses and is almost alwaysaccompanied by the formation of more toxic dioxin�like compounds in the course of the process itself ordue to the interaction of waste gases with metalspresent in the corresponding equipment. An analysis

of trace amounts of dioxins is difficult and fairlyexpensive, so dioxins often remain unanalyzed. Thelandfilling of municipal or industrial wastes cannot berecommended as a safe method for waste manage�ment. At the same time, the reductive methods com�pletely exclude the formation of dioxins. In manycases, they make it possible to recover the hydrocarboncomponent of organochlorine waste and to reuse it. Inaddition, the reductive methods are more economi�cally profitable.

Hydrodechlorination is applicable to a wide rangeof organochlorine compounds and is one of the mostpromising and environmentally safe utilization meth�ods [4]. The active sites of efficient hydrodechlorina�tion catalysts contain such transition metals as Pd, Pt,or Ni [5]. The Pd/C catalysts are very stable [6] andexhibit a high efficiency due to the large specific sur�face area of active carbon, which favors the formationof small palladium particles. Along with active carbon[7], a number of other materials, including Al2O3 [8],TiO2 [9], SiO2 [10], and Sibunit [11, 12], are used assupports for the palladium catalysts.

Active carbon is thermally stable and retains a largespecific surface area even at relatively high tempera�tures [13]. However, nanosized Pd particles, which areactive in the catalytic reaction, are weakly bound tothe surface and, therefore, are unstable and can aggre�gate on heating. Active carbon mainly containsmicropores, owing to which it has a large specific sur�

Chlorobenzene Hydrodechlorination Catalyst Prepared via the Pyrolysis of Sawdust Impregnated with Palladium Nitrate

E. S. Loktevaa, b, *, E. V. Golubinaa, b, M. V. Antonovaa, S. V. Klokova, b, K. I. Maslakova, b, A. V. Egorova, and V. A. Likholobovb

a Moscow State University, Moscow, 119991 Russiab Institute of Hydrocarbons Processing, Siberian Branch, Russian Academy of Sciences, Omsk, 644040 Russia

*e�mail: [email protected] November 27, 2014

Abstract—(7% Pd)/C catalysts have been prepared by the pyrolysis of untreated sawdust and sawdust washedwith an acid to remove of Group I and II metal impurities, both impregnated with palladium nitrate. Studiesby transmission electron microscopy, X�ray photoelectron spectroscopy, and temperature�programmedreduction have demonstrated that the dominant palladium species in the catalysts is 2–5 nm Pd0 particles,there is no PdO on the surface, and the catalyst bulk contains small amounts of larger (10–20 nm) PdO par�ticles. The catalysts are active in chlorobenzene hydrodechlorination in a fixed�bed flow reactor and ensure100% conversion of the substrate into benzene in the temperature range from 250 to 350°C. At lower temper�atures (150–200°C), the catalyst containing calcium is the most active and the sample subjected to reductionafter pyrolysis shows the lowest activity.

Keywords: Palladium catalyst, carbon support, wood sawdust, pyrolysis, catalytic hydrodechlorination, chlo�robenzene

DOI: 10.1134/S0023158415060099

KINETICS AND CATALYSIS Vol. 56 No. 6 2015

CHLOROBENZENE HYDRODECHLORINATION CATALYST 765

face area. Active carbons are usually obtained fromsawdust or nutshell, including coconut shells [14].Their specific surface area SBET can reach 1500 m2/gand depends on the raw materials used and other fac�tors [15].

The activation of a carbon support is considered toensure a small particle size of the supported metal and,hence, a high efficiency of catalytic systems based onthese particles. Accordingly, the preparation of a car�bon support consists of several stages: initially carbonis impregnated with an active metal salt, and then it iscalcined to form metal oxide and is reduced to themetallic state. At the same time, CO and Н2 areevolved during the pyrolysis of wood in an inert gasflow (i.e., reductive conditions are created) and car�bon forms, which also exhibits reductive properties atelevated temperatures [16].

In this work, we attempted to decrease the numberof stages necessary for the formation of an efficientcatalyst. Supported reduced Pd/C catalysts were pre�pared in an inert atmosphere using the pyrolysis of thesawdust impregnated with a metal salt. Sawdust con�taining alkaline�earth metal impurities and sawdustfrom which the impurities were washed away were usedin the preparation of the catalyst. The catalytic prop�erties of the systems obtained were studied in thevapor�phase hydrodechlorination of chlorobenzene.

EXPERIMENTAL

Preparation of Catalysts

Birchwood sawdust (0.25–0.5 mm fraction) wasused as the initial material in the preparation of thesupport. A solution of Pd(NO3)2 (Pd content of464.6 g/L; Aurat, Russia) served as the source of pal�ladium. Part of the sawdust was used without pretreat�ment, and the other part was purified. For this pur�pose, the sawdust was first washed with a 5 M aqueoussolution of HNO3 under moderate heating. Then, atthe end of gas evolution, the sawdust was thoroughlywashed with distilled water to pH 7. The washed saw�dust was dried in a muffle furnace at 100°C for 6 h.

The purified sawdust was used in the preparation ofa series of Pd/C�1 catalysts. The sawdust was impreg�nated with a solution of Pd(NO3)2, dried in a mufflefurnace at 90°C for 6 h, and then subjected to pyrolysisin a quartz tubular reactor at 400°C for 4 h in an Aratmosphere (flow rate of 12 mL/min). A portion of thecatalyst was reduced with hydrogen at 280°C for 4 h.This sample was designated Pd/C�1�H2. The catalystPd/C�2 was prepared by the same method, but thesawdust was not washed with the acid. To determine anoptimum duration of the pyrolysis of the sawdustimpregnated with Pd(NO3)2, we prepared onemore sample, which was subjected to partial pyrolysis(PP) after impregnation. The PP duration was 2 hinstead of 4 h, and the sample obtained was designed

Pd/C�2�PP. According to the calculation that tookinto account the amount of the palladium salt used,the weight loss of the sawdust during pyrolysis, and thepossibility of the conversion of Pd to liquid pyrolysisproducts (under the chosen experimental conditions,the amount of liquid pyrolysis products was insignifi�cant), the Pd content of all catalysts was 7 wt %. Somesamples were examined by atomic absorption spec�troscopy on a Thermo Scientific iCE 3000 series AAspectrometer (Thermo Fisher Scientific, UnitedStates) with the Solaar software after palladium wasdissolved in nitric acid. The analysis results confirmedthat the Pd concentration was 7 ± 0.3 wt %.

The commercial catalyst 5% Pd/C (Fluka, SBET =700 m2/g, micropore volume of Vmicro = 0.171 cm3/g,total pore volume of Vpore = 0.52 cm3/g) and catalystscontaining 2 and 5% Pd on active carbon R4 (VebLaborchemie Apolda, SBET = 1090 m2/g, Vmicro =0.252 cm3/g, Vpore = 0.49 cm3/g), which were preparedusing the impregnation of the support with a solutionof palladium nitrate followed by drying at 120°С andreduction with hydrogen at 280°С for 3 h, were alsoused for comparison.

Physicochemical Characterization of the Catalysts

The specific surface area was measured by the low�temperature adsorption–desorption of nitrogen usingan ASAP 2000 system (Micromeritics, United States).

X�ray photoelectron spectra (XPS) were recordedon an Axis Ultra DLD spectrometer (Kratos Analyti�cal Limited, Great Britain) using an AlK

α1, 2 mono�chromatic radiation source. The pass energy of thespectrometer was 160 eV for survey spectra and 40 eVfor high�resolution spectra. The samples were depos�ited on a double�sided adhesive tape. A neutralizer wasused to compensate the charge of the samples. Thespectra were calibrated against the binding energy ofthe low�energy component of the C1s photoelectronspectrum, which was accepted to be 284.6 eV.

The samples were also studied by high�resolutiontransmission electron microscopy (HRTEM) on aJEM 2100F instrument (JEOL, Japan) equipped withan attachment for local energy dispersive analysis(EDX) at an accelerative voltage of 200 kV using brightfield and high angle annular dark field (HAADF)imaging methods. The phase composition of large par�ticles was determined by transmission electronmicroscopy (TEM) in the electron diffraction mode.The morphology of the samples was studied by scan�ning electron microscopy (SEM) on a JSM 6490 LVinstrument (JEOL). An EDX attachment was used forelemental analysis.

The morphology of palladium particles and theirsize distribution were determined by an analysis of1073 particles in the images of sample Pd/C�1,715 particles in the images of Pd/C�1�H2, and 453 par�ticles in the images of Pd/C�2. The average size of pal�ladium particles was estimated in terms of their num�

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ber average diameter dn, surface average diameterds (2), and volume average diameter d

v [17]:

dn = (1)

ds = (2)

(3)

where ni is the number of particles with the diame�ter di.

Temperature�programmed reduction in hydrogenwas carried out on a laboratory setup consisting of agas preparation system, a reactor with a tubular fur�nace, and a thermal�conductivity detector. A 5% H2 +95% Ar gas mixture was passed at a rate of 26 mL/min,and the rate of the linear heating of the sample (50 mg)was 12 deg/min. The concentration of hydrogen in theflowing gas was continuously monitored with the ther�mal�conductivity detector. The signals from the detec�tor were processed using the Ekokhrom programpackage.

Catalytic Hydrodechlorination of Chlorobenzene

The gas�phase hydrodechlorination of chloroben�zene was carried out in a tubular fixed�bed flow quartzreactor 10 mm in diameter. The catalyst (8 mg) wasplaced in the reactor between layers of a quartz filter(Whatman QM�A). The reactor was heated in ahydrogen flow (12 mL/min) to the preset temperature.Chlorobenzene (CB) was supplied to the reactor froma bubbler through which hydrogen was passed. TheН2 : CB molar ratio was 30 : 1. The reaction was car�ried out under isothermal conditions in the tempera�ture range from 100 to 350°С. After a constant conver�sion was reached, the supply of the CB + H2 mixturewas switched to hydrogen supply, and the temperaturewas increased to the next specified level.

Gaseous reaction products were analyzed by gas–liquid chromatography on an Agilent 6890N instru�ment (Agilent, United States) using a 30�m�long Sol�gel�Wax capillary column (SGE Analytical Science)and a flame�ionization detector. Samples were takenat the outlet of the reactor with a gas syringe and wereimmediately injected into the chromatograph. Thechlorine mass balance was determined by titration,passing the reaction products through a solution ofNaOH.

RESULTS AND DISCUSSION

SEM and Low�Temperature Nitrogen Adsorption Data

The SEM images of the initial sawdust and thePd/C�2 catalyst based on the sawdust are shown inFig. 1. The morphology of the catalyst after pyrolysis is

∑ ∑ ,i i i

i i

n d n

∑ ∑3 2,i i i i

i i

n d n d

=∑ ∑4 3,i i i i

i i

d n d n dv

similar to that of the sawdust used. This is also true forother samples (Pd/C�1 and Pd/C�1�H2), so theirSEM images are omitted.

The EDX data combined with the SEM datashowed that the sawdust that was not washed with theacid contained Ca (0.2 wt %) (Fig. 1f).

The specific surface area of the Pd/C�1 catalyst was6.3 m2/g, which is equal within one order of magni�tude to the corresponding parameters of biochar pre�pared by the pyrolysis of oak sawdust in an inert atmo�sphere (SBET = 2.8 and 7.7 m2/g after pyrolysis at 400and 500°С, respectively) [18]. Therefore, under theconditions described, the pyrolysis of the sawdustresults in the formation of carbon with a small specificsurface area. The adsorption–desorption isothermlooks like an unclosed loop, which is characteristic ofporous materials with low�strength pore walls that canbreak under analysis conditions. This prevented thereliable determination of the pore size of the samples.

TEM Data

The TEM images of the surfaces of the washedPd/C�1 catalyst and the same sample after its reduc�tion with hydrogen at 280°С (Pd/C�1�H2) are pre�sented in Figs. 2a–2e. The particle size distributions inthe three samples obtained (Pd/C�1, Pd/C�1�H2, andPd/C�2) are shown in Fig. 2f. Most of the palladiumparticles in the reduced catalyst Pd/C�1�H2 (Fig. 2d)are spherical and small. About 90% particles are 1 to2 nm in size, the size of 7% particles is smaller than1 nm, and larger particles are practically absent. Themean standard deviation is σm = 9.17 × 10–4 and,hence, the determined mean values of dispersion arereliable (dn = 1.72 nm, ds = 3.67 nm, dv = 8.57 nm).

As follows from Figs. 2a and 2b, the unreduced cat�alyst Pd/C�1 contains particles of two types and its sizedistribution is bimodal. This catalyst and its reducedanalogue Pd/C�1�H2 contain many (more than 75%)fine spherical palladium particles uniform in size,although the first peak of the bimodal distribution insample Pd/C�1 is somewhat broader (in this case, dn =2.3 nm, ds = 29.3 nm, and dv = 33.9 nm). The averageparticle size dn of the particles assigned to the first peakis 2–3 nm, whereas in the reduced sample this size is1–2 nm. Even such a small difference can have a sub�stantial effect on the catalytic properties of the samplesdue to the size effects that are observed for particlessmaller than 5–10 nm [19].

Up to 25% large particles with a size of 20–30 nmare also present in the unreduced catalyst Pd/C�1along with fine particles (Fig. 2b). According to theTEM data, all large particles are surrounded by a lightarea of different size and geometry. Their shape andsizes indicate that the presence of these light areas canbe due to channels in the structure of the carbon sup�port. The channels are formed due to the migration ofa large palladium�containing particle.



×100

20 µm

100 µm ×500 50 µm

×200 100 µm

20 µm

(а) (b)

(c) (d)

(e) Pd 10

2 3 4 5 6 7 8 9 10

90

180

270

Ca Kα Ca Kβ

C Kα

O Kα

Binding energy, keV

Content, arb. units

(f)

Fig. 1. SEM images of different regions on the (a)–(c) Pd/C�2 catalyst and (d) initial sawdust, (e) the distribution of Pd on thesurface (for image (c)), and (f) the energy dispersive spectrum of the initial sawdust.

The particle size distribution in Pd/C�2 is alsobimodal, as in Pd/C�1, but the proportion of largeparticles in this sample is lower (not more than 5%),and the distribution maximum of fine particles is

shifted toward higher values (at σm = 0.0047 dn =2.4 nm, ds = 3.06 nm, and dv = 3.5 nm).

The nature of large palladium�containing particlesin TEM was studied by electron diffraction (Fig. 2c).

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20 nm 50 nm

20 nm5 nm–1

5 nm

(а) (b)

(c) (d)

(e) 0 1 2 3 4 75 6 8 9 10 20 30

10

20

30

40

50

60

70

Fraction of particles, %

– Pd/C�1– Pd/C�1�H2

– Pd/C�2

(f)

Fig. 2. Bright field TEM images of (a) fine and (b) large particles in Pd/C�1, (c) electron diffraction on PdO particles, (d) theTEM image of sample Pd/C�1�H2, (e) the dark field TEM image (HAADF) of sample Pd/C�2, and (f) the particle size distribu�tion in the catalysts Pd/C�1, Pd/C�1�H2, and Pd/C�2.



It turned out that the large particles were palladium oxidePdO. The fine particles gave no diffraction pattern.

XPS Data

The Pd3d XPS spectra of the samples studied,including the reference sample Pd(NO3)2/sawdustprepared using the impregnation of the sawdust with apalladium nitrate solution but without pyrolysis, arepresented in Fig. 3.

The spectrum of the Pd(NO3)2/sawdust sample isdominated by the state with a Pd3d5/2 binding energy(BE) of 337.4 eV. This BE value is typical of oxidizedpalladium (Pd2+). At the same time, the spectrum ofthis sample contains no N1s line at BE ≈ 408 eV char�acteristic of nitrates, which would be observed if thesample surface contained Pd(NO3)2. The observed BE= 337.4 eV somewhat exceeds the values that are pre�sented in the literature for palladium oxide PdO(336.3–337.0 eV) [20–22]. The BE of the main com�ponent of the Pd3d spectrum can be attributed to theformation of palladium hydroxide Pd(OH)2, for whichBE(Pd3d5/2) has a similar value of 337.2 eV [23]. Alongwith the main state, a minor state shows itself in thePd3d spectrum of the Pd(NO3)2/sawdust sample atBE(Pd3d5/2) = 335.3 eV, which corrsponds to metallicpalladium Pd0 [21, 23]. The Pd3d XPS spectra of theother samples are very similar in shape and include themain peak at a Pd3d5/2 binding energy of 335.3 eV cor�responding to metallic palladium and a small shoulderat 337.4 eV corresponding to oxidized palladium Pd2+.

According to the XPS data, reduced palladiummainly exists on the surfaces of both reduced andunreduced catalysts obtained using the pyrolysis ofbirchwood sawdust impregnated with palladiumnitrate. Therefore, relatively large PdO particles foundby TEM of the unreduced Pd/C�1 sample are mainlyin the material bulk.

Thus, the pyrolysis of the sawdust impregnatedwith palladium nitrate gave predominantly fine parti�cles of reduced palladium, and it is these particles thatoccur on the surface. The simultaneous presence ofPdO and Pd0 particles in Pd/C�1 suggests that the firststage of pyrolysis results in the decomposition of pal�ladium hydroxide and/or nitrate to palladium oxide,which is then reduced in an inert atmosphere due tothe interaction with the support or under the action ofgases evolved upon pyrolysis. Thermogravimetricstudies combined with mass spectroscopy shows thatthe gases evolved in the pyrolysis of, for example, pinesawdust at 400°C can contain up to 20% Н2, up to 60%CO, up to 8% СН4, and up to 18% СО2; i.e., productswith reducing properties dominate [16]. The forma�tion of channels in the carbon support can be due tothe formation of nitrogen oxides upon the decomposi�tion of the metal precursor.

Figure 4 presents the C1s and O1s XPS spectra ofthe Pd(NO3)2/sawdust sample that was not subjectedto pyrolysis and the spectra of the samples subjected to

partial (Pd/C�2�PP) and complete (Pd/C�2) pyroly�sis. A comparison shows that the C1s and O1s spectrarecorded after partial pyrolysis are almost identical tothe spectra of the Pd(NO3)2/sawdust sample that wasnot subjected to pyrolysis. Therefore, the pyrolysis ofthe sawdust in this sample occurred to a very insignifi�cant extent, although the complete reduction of palla�dium took place. The C1s spectrum of Pd/C�2 indi�cates a substantial decrease in the contribution fromcarbon–oxygen bonds of different types: the peak atBE = 284.6 eV corresponding to C–C bonds becomesdominant. The intensity of the peaks in the O1s spectradecreases, indicating a decrease in the total oxygencontent of the sample. In addition, the spectral shapechanges due to the modification of the dominant typeof the C–O bond. All these data indicate the occur�rence of deep pyrolysis in Pd/C�2 mainly yielding acarbon material at least on the surface. It can beassumed that the formation of Pd0 favors the furtherpyrolysis of the sawdust in this direction. The insignif�icant yield of resins can be explained by the presenceof acid sites in the sawdust impregnated with palla�dium nitrate, since the impregnating solution hadpH <3.

332336344348 340 Binding energy, eV

Inte

nsi

ty

Pd0

5

4

3

2

1

Pd2+

Fig. 3. Pd3d XPS spectra of (1) Pd(NO3)2/sawdust, (2) Pd/C�1,(3) Pd/C�1�H2, (4) Pd/C�2�PP, and (5) Pd/C�2. The spectraare intensity�normalized.

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LOKTEVA et al.

Temperature�Programmed Reduction Data

The above assumption that PdO present in theunreduced sample occurs in the carbon material bulk,based on TEM and XPS data, is confirmed by the tem�perature�programmed reduction data for the unre�duced samples Pd/C�1 and Pd/C�2.

No hydrogen uptake peaks were detected whileheating the samples in a flowing 5% Н2 + 95% Ar mix�ture up to 150°С, although PdO is usually reduced atrelatively low temperatures (below 150°С) [24]. Thisfact confirms that Pd is predominantly in the reducedstate not only on the surface but also in the bulk ofboth samples. It is most likely that PdO particlesobserved by TEM occur in the carbon support matrixand are inaccessible to hydrogen. Above 150°С, abroad hydrogen uptake peak is observed, which is mostlikely due to the hydrogenation of the carbon material.Hydrogenation can facilitate pore opening to makePdO more accessible to the reductant. Therefore, noPdO particles are detected by TEM in the reducedPd/C�1�H2 sample. It is most likely that the reductionof PdO to Pd0 is accompanied by the dispersion of theparticles.

According to the results of physicochemical stud�ies, the material obtained using the pyrolysis of thesawdust impregnated with palladium nitrate is Pd/C.This material mainly contains Pd0 nanoparticles andshould be substantially efficient in reduction reac�tions.

Catalytic Activity Tests in Chlorobenzene Hydrodechlorination

The catalysts obtained in the work were tested inchlorobenzene hydrodechlorination in a fixed�bedflow reactor. Note that catalyst samples in the catalytictests were very small (only 8 mg), since the method ofchlorobenzene supply from the bubbler to the reactorlimited the supply rate range and use of large catalystsamples resulted in 100% chlorobenzene conversion.In all cases, benzene was the only reaction product.

The temperature dependences of the conversion ofchlorobenzene over Pd/C�1, Pd/C�1�H2, and Pd/C�2are shown in Fig. 5. Each data point was obtained dur�ing a prolonged (2–4 h) test under isothermal condi�tions, so that the values obtained are steady�state con�versions. In the temperature range from 250 to 350°C,all of the three catalysts ensure 100% chlorobenzene�to�benzene conversion. Differences are observed at100, 150, and 200°C. In this temperature range, theconversion of chlorobenzene decreases in the follow�ing order: Pd/C�2 > Pd/C�1 > Pd/C�1�H2. At 100°C,the conversion for all of the three catalysts does notexceed 40%. This is likely explained by the fact thatcondensed chlorobenzene (bp 130°C) can blockactive sites of the catalysts, thus decreasing theiractivity. The conversion of chlorobenzene to benzenereaches 100% already at 150°C on the unreduced sam�ple Pd/C�2 prepared suing sawdust untreated with theacid. Both catalysts obtained using the sawdust washed

280284292296 288Binding energy, eV

Inte

nsi

ty

3

2

1

C–C

C–O

O=C–O

(a)

528532540 536Binding energy, eV

Inte

nsi

ty

3

2

1

(b)

Fig. 4. (a) C1s and (b) O1s XPS spectra of (1) Pd(NO3)2/sawdust, (2) Pd/C�2�PP, and (3) Pd/C�2. The spectra are normalizedto the C1s peak areas.



with the acid are less efficient, and the conversion forthe reduced sample Pd/C�1�H2 is lower than for unre�duced Pd/C�1 (30 and 52% at 150°C, 48 and 85% at200°C, respectively).

The high activity of the Pd/C�2 catalyst is primarilydue to the presence of a calcium impurity in the initialsawdust. The Ca impurity was observed by EDX in theSEM experiment. Alkaline�earth metal compoundscan act as promoters: they can stabilize nanosized pal�ladium particles, may be a part of the active compo�nent [25], in particular, as the second component ofa bimetallic system [26, 27], and can prevent thedeactivation of the active site under the action ofHCl [28, 29].

The influence of iron, aluminum, silicon, calcium,and magnesium oxides in the Pd/Cact catalyst on theactivity of this system in the hydrodechlorination ofortho�dichlorobenzene was studied [30]. Active car�bon AG�2000 containing almost 12% ash impuritieswas the most efficient catalyst. There are literaturedata that heavy metals are adsorbed on birchwoodsawdust mainly due to their ion exchange to Ca2+ ionsand, to a substantially lesser extent, to Mg2+ ions [31].Therefore, the presence of these ions should favor thestabilization of Pd on the surface in the course of theimpregnation of the sawdust with the palladium salt. Itwas mentioned [31] that the acidity range 2 < pH < 4 isoptimal for the sorption of the palladium salt and it isapproximately in the same range that palladiumnitrate was deposited in the present work.

The lower activity of the reduced catalyst Pd/C�1�H2 compared to the unreduced sample can be due todifferent sizes of the Pd0 particles measured by TEM.A narrower distribution is observed in the reducedsample, and the average size of the Pd0 particle issmaller than that in sample Pd/C�1 (see the first peakof the bimodal distribution in Fig. 2). Therefore,Pd/C�1�H2 contains a large proportion of particlesthat are unstable under the reaction conditions,readily undergoing deactivation upon their interactionwith HCl. A substantial decrease in the proportion offine palladium particles (smaller than 3 nm) in the liq�uid�phase hydrodechlorination of chlorobenzeneunder the action of evolved HCl was reported in theliterature. The particles whose size is 3 nm or above aremore resistant to the action of the acid [30, 32]. Achange in the particle size of the active component canaffect both the stability and activity of the catalyst: thestructural sensitivity of hydrodechlorination was dis�covered [33, 34]. A coarsening of the palladium parti�cles and an increase in the conversion caused by thekinetic factor can occur with an increasing tempera�ture. As a result, at 250−350°С the catalysts Pd/C�1�H2and Pd/C�1 afford equally high conversions of chlo�robenzene (Fig. 5).

The catalytic systems studied contain a significantamount of palladium (7 wt %). This is substantiallyhigher than that in the palladium�containing catalystsused in practice (usually to 1 wt %), which is explained

by difficulties in a priori estimating the weight loss ofthe sawdust during pyrolysis in the presence of a cata�lytically active element. In addition, the high contentof palladium in the model samples facilitated physico�chemical studies.

The commercial catalyst 5%Pd/C (Fluka) andsamples of Pd/C with a palladium content of 2 and 5%obtained by palladium deposition on active carbonwith a large specific surface area were chosen as refer�ence catalysts. When testing these catalysts, the sam�ple weight was 50 mg, which more than 6 timesexceeds the weight of the samples prepared by thepyrolysis of the sawdust, and, hence, the results for thereference catalysts are omitted in Fig. 5. An importantdistinction of the three reference catalysts is the com�position of the products obtained on them: the hydro�genation of benzene to cyclohexane was observedalong with the hydrodechlorination of chlorobenzeneinto benzene. The conversion of chlorobenzene was50% on 2%Pd/C at 100°С, whereas at 150–300°С theconversion was 100% and cyclohexane was the onlyproduct. In the presence of 5%Pd/C, the conversionreached 51% already at 50°С and the conversion at100–150°С was 100%. The major product was ben�zene, but up to 20% cyclohexane was also formed. Forthe commercial catalyst 5%Pd/C at 50°С, the conver�sion of chlorobenzene was 69%, and only benzene wasformed. At 100°С, the complete conversion of chlo�robenzene occurred and the products contained 70%benzene and 30% cyclohexane, while the 100% con�version of chlorobenzene to cyclohexane was observedat 150°С.

Even this incomplete comparison shows that theprepared catalysts are fairly promising. In spite of theirlow specific surface area, the systems obtained usingthe pyrolysis of the sawdust impregnated with the pal�ladium salt ensure, at a very small catalyst weight, a

80

60

40

20

0350300200150100 250

100

Temperature, °С

Ch

loro

ben

zen

e co

nve

rsio

n,

mol

%

1

2

3

Fig. 5. Chlorobenzene conversion versus reaction temper�ature for the catalysts (1) Pd/C�1, (2) Pd/C�1�H2, and(3) Pd/C�2.

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LOKTEVA et al.

somewhat lower conversion of chlorobenzene at lowtemperatures (50–100°С), whereas at 150°С andhigher the conversion is comparable with those on thesamples containing active carbon with a highly devel�oped surface as the support and on the commercialcatalyst. A high benzene formation selectivity isattained on our catalysts. The differences can be due tothe narrower size distribution of Pd particles in thecatalysts obtained by the pyrolysis of the sawdust.Indeed, the size of palladium particles in the 5%Pd/Csample (Fluka) ranges from 5 to 30 nm, whereas thatin the 5%Pd/C sample ranges from 4 to 40 nm [35].

The results presented above show that the directdeposition of palladium on the sawdust followed bypyrolysis in an inert atmosphere makes it possible toobtain a catalytic system with a high activity in thehydrodechlorination of chlorobenzene. Undoubtedly,the “traditional” methods for the synthesis of Pd/C,for example, the deposition of the palladium salt onactive carbons or other commercial carbon supports,can produce cheap and very efficient catalytic systemsappropriate for the hydrodechlorination of chlori�nated organic compounds, including polychlorinatedpersistent organic pollutants [7, 11, 12, 36, 37]. Animportant feature of the systems obtained is that palla�dium is mainly in the metallic state due to its reductionduring pyrolysis and the narrow nanosized range ofmetallic palladium particles. The use of the proposedmethod excludes the necessity of the preliminarypreparation and activation of the carbon support andthermal reduction, which shortens the number ofpreparation stages of the catalyst. Among the draw�backs of the obtained systems are their low specificsurface area and a variable composition of the ashcomponents. The latter is also inherent, to someextent, in processes of obtaining active carbon fromnatural materials, which can be of variable ash compo�sitions as wood sawdust.

Further we are planning to use additions of othermetals for the optimization of the chemical composi�tion of the obtained catalytic systems. We also intendto improve the synthesis method for increasing thespecific surface area of the catalysts, to decrease thepercentage of the active component in them, and tocompare the systems obtained with the catalyst pre�pared by palladium deposition from nitrate on thepyrolyzed sawdust.

CONCLUSIONS

It was shown by TEM and XPS that the pyrolysis ofthe birchwood sawdust impregnated with a palladiumnitrate solution in an inert atmosphere at 400°C canprovide the Pd/C catalysts containing nanosized Pd0

particles with a size optimum for catalysis (2–4 nm)due to the reduction of the metal by the pyrolysis gasesand carbon of the support. The catalysts are highlyactive in the vapor�phase hydrodechlorination ofchlorobenzene in a fixed�bed flow reactor at 150–

350°C. The presence of Ca in the sawdust enhancesthe activity of the catalysts based on the sawdust at rel�atively low temperatures (150–200°C), which is likelydue to the shift of the maximum of the size distributionof Pd particles toward larger values (from 2 to 3 nm).An increase in the activity is also favored by the inter�action between Pd and Ca facilitating the immobiliza�tion of palladium particles on the support. In addition,calcium weakens the effect of HCl on the active com�ponent. The method described makes it possible toobtain an efficient palladium–carbon catalyst using asmaller number of stages than that used in usual meth�ods (for example, when palladium is deposited on acarbon support by traditional methods of impregna�tion and deposition).

ACKNOWLEDGMENTS

This work was supported by the Russian ScienceFoundation (agreement no. 14�33�00018) and Mos�cow State University Program of Development.

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Translated by E. Yablonskaya

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