Short Communication
Enhanced liquid phase catalytic hydrodechlorination of 2,4-dichlorophenol over mesoporous carbon supported Pd catalysts
Yun Shao,Zhaoyi Xu,Haiqin Wan ⁎,Yuqiu Wan,Huan Chen,Shourong Zheng ⁎,Dongqiang Zhu
State Key Laboratory of Pollution Control and Resource Reuse,School of the Environment,Nanjing University,Nanjing 210093,PR China
a b s t r a c t
a r t i c l e i n f o Article history:
Received 9March 2011
Received in revised form 6May 2011Accepted 9May 2011
Available online 17May 2011Keywords:
Catalytic hydrodechlorination of 2,4-dichlorophenol Supported Pd catalyst Activated carbon Mesoporo
us carbon
Supported Pd catalysts on ordered mesoporous carbon (OMC)and activated carbon (AC)were prepared and their catalytic behavior for the liquid phase catalytic hydrodechlorination (HDC)of 2,4-dichlorophenol was investigated.In comparison with Pd/AC,Pd particles of Pd/OMC were effectively con fined in the mesopores of OMC,resulting in high Pd dispersion and Pd 2+content.Accordingly,Pd/OMC exhibited higher catalytic activity than Pd/AC.Moreover,increasing catalyst reduction temperature lowered the catalytic activities and favored stepwise HDC of 2,4-dichlorophenol.
©2011Elsevier B.V.All rights reserved.
1.Introduction
Chlorinated phenols are important raw materials and intermediates in chemical industry.However,the wide use of chlorinated phenols has resulted in a large amount of chlorinated phenols-bearing wastewater,from which the effective removal of chlorinated phenols is legislatively obligatory due to their strong toxicity and low biodegradability [1].Liquid phase catalytic hydrodechlorination (HDC)over supported noble metal catalysts provides a nondestructive approach for the detoxi fica-tion of chlorinat
ed phenols and recovery of valuable chemicals [2].Supported Pd,Pt,and Rh catalysts are generally used in the liquid phase HDC,where supported Pd catalyst is the most active one [3].Additionally,activated carbon (AC)is usually used as catalyst support due to its large surface areas and high stability under acidic and alkaline conditions.However,AC primarily consists of irregular shaped micropore with a wide pore size distribution,incapable of hosting nano-scale metal particle.Such shortcomings can be avoided by using ordered mesoporous carbon (OMC)as the support.Because OMC is composed of ordered mesopore with a narrow pore distribution,nano-scale metal particle can be con fined in the mesopores [4].Hence,it is reasonable to hypothesize that OMC supported Pd catalyst may exhibit different catalytic behavior from AC supported Pd catalyst.However,few studies have been conducted on the catalytic behavior of OMC supported Pd catalyst for the catalytic HDC of chlorinated phenols thus far.
In this study,OMC and AC supported Pd catalysts were prepared and the liquid phase catalytic HDC of 2,4-dichlorophenol (2,4-DCP)was investigated.The results showed that Pd/OMC displayed superior catalytic activity to Pd/AC,highlighting the potential of using Pd/OMC as a more effective catalyst for the catalytic HDC of chlorinated phenol.2.Experimental 2.1.Catalyst preparation
Ordered mesoporous silica SBA-15was prepared using a triblock copolymer,EO 20PO 70EO 20(Pluro
nic P123,Aldrich),as the structure directing agent and tetraethoxysilane (TEOS,98%,Shanghai Chemical Co.)as the silica source [5,6].OMC was synthesized using calcined SBA-15as the hard template [6,7].Commercial AC (Filtrasorb-300,Calgon Carbon Co.)was included for comparison purpose.To avoid possible intraparticle diffusion,the supports were ground to pass through a 400-mesh sieve (b 37μm)prior to catalyst preparation [8].
Supported Pd catalysts were prepared using the incipient wetness impregnation method with H 2PdCl 4as the Pd precursor.The Pd loading amount was preset to be about 5wt.%,which was generally adopted for AC supported catalysts [8,9].The samples were dried at 80°C under vacuum for 4h,and then reduced in a H 2flow (40mL/min)at 200,300or 400°C for 2h.The resultant catalyst was referred to as Pd/OMC-X or Pd/AC-X respectively,where X is the reduction temperature.2.2.Catalyst characterization
X-ray diffraction (XRD)patterns of the samples were obtained from a Rigaku D/max-RA powder diffraction-meter.Transmission electron
Catalysis Communications 12(2011)1405–1409
⁎Corresponding authors.Tel.:+862589680373;fax:+862583707304.
E-mail addresses:wanhq@nju.edu (H.Wan),srzheng@nju.edu (S.
Zheng).1566-7367/$–see front matter ©2011Elsevier B.V.All rights reserved.doi:
10.1016/j.catcom.2011.05.007
Contents lists available at ScienceDirect
Catalysis Communications
j o u r n a l h o m e p a g e :w ww.e l s ev i e r.c o m /l o c a t e /c a tc o m
microscopy(TEM)images of the catalysts were obtained from a JEM-2100transmission electron microscope.Brunauer–Emmett–Teller (BET)specific surface areas of the supports and catalysts were measured using the nitrogen adsorption method on a Micromeritics ASAP2020 instrument.X-ray photoelectron spectroscopy(XPS)was performed on a Thermo ESCALAB250instrument equipped with a monochromatized Al Kαexcitation source(hν=1486.6eV).The C1s peak(284.6eV)was used for the calibration of binding energy values.The Pd contents of Pd/ OMC and Pd/AC were determined to be5.60and5.82wt.%,respectively, by X-rayfluorescence(ARL-9800).
2.3.Liquid phase catalytic HDC of2,4-DCP
Catalyst activity for the liquid phase catalytic HDC of2,4-DCP was evaluated using the batch reaction approach at20°C under atmospheric pressure of hydrogen.Catalysis tests were conducted in a500-mL of four-neckedflask reactor with a sample port,a pH-stat,H2inlet and a condenser.The reaction tempera
ture was stabilized at20±0.5°C with a water-bath(SDC-6,scientz Co.,China).Briefly,50mg of catalyst was added into the reactor containing200mL of3.5mmol L−12,4-DCP aqueous solution.In order to eliminate HCl poisoning,the pH of2,4-DCP solution was pre-adjusted to12.0[10].After purging with a N2flow (250mL min−1)under stirring(1200rpm)for30min,the N2flow was switched into a H2flow(250mL min−1)during the reaction process. Samples were taken at different time intervals and catalyst particles were removed byfiltration.Thefiltrate was neutralized using2.0mol L−1HCl and the concentration of reactant,intermediate and product in the filtrate was analyzed by a high performance liquid chromatography with an ultraviolet(UV)detector at270nm and a4.6×150mm HC-C18 column(Agilent).The mobile phase consisted of60%CH3CN and40% water(v:v).The results of two separate runs of the HDC of2,4-DCP showed high data reproducibility(see Fig.1S,supporting information).
3.Results and discussion
3.1.Catalyst characterization
The small-angle XRD patterns of the samples are compared in Fig.2S,supporting information.For OMC and Pd/OMC catalysts, diffraction peaks with2θat1.01°,1.76°and2.02°,indexed as(100), (110)and(200)
diffractions,were observed,reflecting the presence of typical hexagonal mesostructured phase[5,11].N2adsorption results are shown in Fig.3S,supporting information and resultant parameters are listed in Table1.For AC,N2adsorption amount mainly increased at low relative pressure(p/p0),characteristic of the microporous nature of AC.In contrast,typical capillary condensation was observed within a relative pressure range of0.40–0.65for OMC and Pd/OMC,reflecting the presence of mesopores.The pore size distribution profile of AC showed that AC was mainly composed of micropores dominated by pore sizes smaller than1.5nm,while OMC exhibited a narrow pore size distribution with the most probable pore diameter centered at4.2nm.Accordingly,the Pd/OMC catalysts had the most probable pore diameters around4.3nm,similar to that of OMC.
TEM images and histograms of Pd particle size distributions of the catalysts are compared in Fig.1.Ordered mesostructure and Pd particles were clearly identified in Pd/OMC.Moreover,much smaller Pd particles and narrower Pd particle distributions of Pd/OMC were observed as compared to Pd/AC(see Fig.1b),likely due to the effective confinement of Pd particles in OMC pores.For Pd/AC and Pd/OMC increasing reduction temperature led to broadened Pd particle size distribution toward larger particle size.The average Pd particle sizes of the catalysts were further quantified based on a surface area weighted diameter[10]:
s
=∑n i d3i=∑n i d2i:ð1Þ
Where n i is the number of counted Pd particles with diameter of d i and the total number of particles∑n i
ðÞis larger than150.
As shown in Table1,the average Pd particle sizes of Pd/OMC and Pd/AC increased with the reduction temperature,indicative of particle aggregation due to the high mobility of metal particle on carbona-ceous material surface[12].At given reduction temperature,the average Pd particle size of Pd/AC was larger than Pd/OMC,confirming the confinement effect of mesopore.
The surface atomic ratios of Pd/C and O/C based on XPS analysis are summarized in Table2.At similar reduction temperature,Pd/AC had higher surface atomic contents of Pd and O than Pd/OMC.Given smaller Pd particles,the lower surface Pd contents of Pd/OMC than Pd/ AC suggest that Pd particles are predominantly located in the mesopores of OMC,but on the external surface of AC.It is understandable that due to its microporous nature AC is incapable of hosting nano-scale Pd pa
rticles in the micropores.Alternatively, OMC consists of ordered mesopore with the most probable diameter of4.2nm,resulting in effective confinement of Pd particles in the mesopore.It should be emphasized that high surface oxygen containing functionality of carbonaceous support favors Pd dispersion [13].Given lower oxygen containing functionality of OMC the higher Pd dispersion of Pd/OMC than Pd/AC again confirms the confinement effect of OMC,wherein oxygen containing functionality of OMC is fully accessible,but oxygen containing functionality on external surface of AC is available for nano-scale Pd particles.
XPS spectra of the catalysts in the Pd3d5/2region are compiled in Fig.2.For Pd/OMC and Pd/AC,the binding energies of Pd3d5/2varied with catalyst reduction temperature,implying the coexistence of metallic and oxidized Pd.To quantify the contents of metallic and oxidized Pd,the XPS profiles were deconvoluted and thefitting parameters are listed in Table2.For Pd/OMC increasing reduction temperature lowered Pd2+contents.Similar trend but more marked decrease of Pd2+content with the reduction temperature was observed for Pd/AC.Upon H2reduction treatment,Pd2+is partially retained due to the stabilization of Pd2+by trace chlorine from
Table1
Properties of the supports and catalysts reduced at200,300and400°C.
Sample BET surface area
(m2g−1)Micropore volume
(cm3g−1)
Mesoporous volume
(cm3g−1)
Pore volume
(cm3g−1)
Pore diameter
(nm)
Average Pd particle size
(nm)
OMC9910.1  1.2  1.3  4.2–AC9810.40.10.5––Pd/AC-200–––––  3.8 Pd/AC-300–––––  4.7 Pd/AC-400–––––  5.4 Pd/OMC-2009300.1  1.1  1.2  4.4  2.4 Pd/OMC-3008640.1  1.0  1.1  4.3  2.6 Pd/OMC-4008660.1  1.0  1.1  4.3  3.5 1406Y.Shao et al./Catalysis Communications12(2011)1405–1409
palladium precursor and strong interaction between Pd 2+and oxygen containing groups from support surface [14,15].Despite of the lower oxygen content of OMC the oxygen containing functionality of OMC is
fully accessible.Moreover,stronger metal-support interaction can be expected for Pd/OMC due to its smaller Pd particle size [16],resulting in more effective stabilization of Pd 2+and higher Pd 2+content in Pd/OMC as compared to Pd/AC.
3.2.Liquid phase catalytic HDC of 2,4-DCP
Prior to catalyst activity evaluation,it is necessary to exclude the in fluence of mass-transfer limitation.Hence the initial catalytic activities of Pd/OMC-300and Pd/AC-300with varied catalyst dosages were compared,where the initial activity was calculated based on a first-order rate law at 2,4-DCP conversion lower than 25%.The results are presented in Fig.4S,supporting information.For both catalysts,initial HDC rates were proportional to the mass of catalyst,while the initial activities remained
nearly constant after catalyst mass
R e l a t i v e  a m o u n t  (%)
Particle size (nm)Pd/OMC-300Pd/OMC-400Pd/OMC-200Pd/AC-200
Pd/AC-300
Pd/AC-400
Particle size (nm)
R e l a t i v e  a m o u n t  (%)
b
a
Fig.1.(a)TEM images and (b)histograms of Pd particle size distributions of the catalysts.
Table 2
Binding energies of surface palladium species and surface atomic ratios.Sample
Binding energy (eV)Pd 2+
/
(Pd 2++Pd 0)
Pd/C (×10−3)O/C
(×10−2)
Pd 0
Pd 2+Pd/AC-200335.7337.50.457.969.05Pd/AC-300335.4337.30.317.078.05Pd/AC-400335.5337.10.22  5.937.19Pd/OMC-200335.6336.90.48  4.52  4.71Pd/OMC-300335.6336.80.46  4.40  4.66Pd/OMC-400
335.7
337.1
0.36
4.10
4.34
1407
Y.Shao et al./Catalysis Communications 12(2011)1405–1409
normalization,indicative of the absence of mass transfer limitation under the tested reaction conditions [8,17].
The liquid phase catalytic HDC of 2,4-DCP over the catalysts is compiled in Fig.3.Only 2-chlorophenol was identi fied as the partially dechlorinated product,and cyclohexanone from further hydrogena-tion of phenol was not detected.At given reduction temperature Pd/OMC exhibited higher catalytic activity than Pd/AC.For example,after reaction for 30min 2,4-DCP was removed by 75%for Pd/AC-200and by 92%for Pd/OMC-200.Accordingly,2-CP was completely reduced in 90min for Pd/OMC-200,but in 150min for Pd/AC-200.The higher catalytic activity of Pd/OMC is tentatively attributed to its higher Pd dispersion as compared to Pd/AC.Furthermore,increasing reduction temperature resulted in declined catalytic activities of Pd/OMC and Pd/AC;while more marked decrease was observed on Pd/AC.For example,increasing reduction temperature from 200to 400°C led to decreased 2,4-DCP removal from 99to 73%for Pd/OMC,but from 95to 16%for Pd/AC within 60min.Such marked difference cannot be explained in terms of Pd particle size because Pd/OMC displays similar Pd particle aggregation to Pd/AC upon H 2reduction treatment.Gomez-Sainero et al.[14]studied the catalytic HDC of CCl 4over Pd/
346344342340338336334332I n t e n s i t y
Binding Energy (eV)
Pd/OMC-200Pd/OMC-300Pd/OMC-400346344342340338336334332
I n t e n s i t y
Binding Energy (eV)
Pd/AC-200
Pd/AC-300
Pd/AC-400
Fig.2.XPS spectra of the catalysts.
C o n c e n t r a t i o n  (m m o l  L -1)
Time (min)C o n c e n t r a t i o n  (m m o l  L -1)
Time (min)
Fig.3.Liquid phase catalytic HDC of 2,4-DCP over the catalysts.(♦)2,4-DCP,(■)2-CP and (▲)phenol.Lines represent the fitting curves using Eqs.(2)–(4).
1408Y.Shao et al./Catalysis Communications 12(2011)1405–1409
AC and concluded that the coexistence of Pd 2+and Pd 0was essential for the effective HDC of CCl 4,wherein Pd 0acted as the active site for H 2activation and Pd 2+served as the active site for the activation of C \Cl bond via abstraction of nucleophilic chloride anion and formation of highly reactive +CCl 3ion.Hence,at similar reduction temperature the higher catalytic activity of Pd/OMC can be attributed to its higher Pd 2+content.Accordingly,more markedly decreased Pd 2+content and catalytic activity of Pd/AC was observed with catalyst reduction temperature.The results imply that the catalytic activity of the catalysts can be further optimized by adjusting Pd 2+/Pd 0ratio.Notably,the catalytic activities of the catalysts correlate to both Pd particle size and Pd 2+/Pd 0ratio.Undoubtedly,more research should be warranted in the future to determine their relative importance on catalyst activity.
The variation of catalyst structural properties may in fluence the reaction mechanism.The liquid phase catalytic HDC of 2,4-DCP could be implemented via stepwise and concerted pathways (Scheme 1)[8].
The rate constants could be further quanti fied by fitting the kinetic data using following equations [18]:C 2;4−DCP =C 0
2;4−DCP exp −k 1+k 3ðÞt ðÞ
ð2Þ
C 2−CP
=k 1C 02;4−DCP k 2−k 1−k 3
exp −k 1+k 3ðÞt ðÞ−exp −k 2t ðÞðÞð3Þ
C Phenol
=k 1k 2C o
2;4−DCP 2−k 1−k 3−113
exp −k 1+k 3ðÞt ðÞ−1ðÞ+12exp −k 2t ðÞ−1ðÞ
−
k 3C 0
2;4−DCP
13
exp −k 1+k 3ðÞt ðÞ−1ðÞ
ð4Þ
Where
C 02,4-DCP
is the initial 2,4-DCP concentration,C 2,4-DCP ,C 2-CP and C Phenol are the concentrations of 2,4-DCP,2-CP and phenol at reaction time t ,k 1,k 2and k 3are the rate constants of 2,4-DCP to 2-CP,2-CP to phenol,and 2,4-DCP to phenol,respectively.
The resultant fitting parameters are listed in Table 3.At similar reduction temperature Pd/OMC had higher rate constants (k 1,k 2and k 3)than Pd/AC,and the rate constants of Pd/OMC and Pd/AC decreased with catalyst reduction temperature,again con firming the positive relation-ship of catalytic activity with Pd 2+content.It is interesting to note that for Pd/OMC and Pd/AC lower catalyst reduction temperature results in higher k 3/k 1ratio.Such trend could be explained in terms of Pd 2+content,wherein high Pd 2+content favors simultaneous activation of ortho-and para-substituted Cl,g
iving rise to preferential HDC of 2,4-DCP via a concerted pathway.Additionally,at given reduction temperature higher k 3/k 1ratio was observed for Pd/AC than for Pd/OMC.This is probably because when compared with Pd/OMC Pd/AC has larger Pd particle,which provides more planar Pd surface and thus facilitates simultaneous access of Pd 2+sites by ortho-and para-substituted Cl of 2,4-DCP.
To verify possible catalyst deactivation,catalyst reuse was conducted and the results are compared in Fig.5S,supporting information.For Pd/OMC-300and Pd/AC-300,gradual deactivation was observed with catalyst recycle.XPS analysis indicated that Pd 2+/Pd 0ratios of the used catalysts remained identical to those of the fresh catalysts.Additionally,Pd content analysis indicated that after one cycle Pd content decreased by 13.0%for Pd/AC-300and 7.7%for Pd/OMC-300.In comparison with
Pd/AC-300,substantially slower deactivation was shown for Pd/OMC,likely due to the suppressed Pd leaching by the con finement effect.4.Conclusions
In the present study,the catalytic behaviors of Pd/OMC and Pd/AC for 2,4-DCP HDC were compared.For the supported catalysts,Pd particles are effectively con fined in the mesopores of OMC,but dominantly located on the external surface of AC,resulting in higher Pd dispersion and Pd 2+content of Pd/OMC than Pd/AC.Accordingly,at given catalyst reduction temperature Pd/OMC display
s higher catalytic activity than Pd/AC.For both catalysts,increasing catalyst reduction temperature lowers the catalytic activity and favors the stepwise HDC of 2,4-DCP.However,in comparison with Pd/AC the con finement effect of OMC stabilizes Pd 2+,leading to less marked decrease in the catalytic activity of Pd/OMC at elevated reduction temperature.Acknowledgements
The financial support from the Natural Science Foundation of China (no.20877039)and Program of New Century Excellent Talents in University (NECT-08-0277)are gratefully acknowledged.We are indebted to the Modern Analytical Center,Nanjing University for the material characterization.
Appendix A.Supplementary data
Supplementary data to this article can be found online at doi:10.1016/j.catcom.2011.05.007.References
[1]M.M.Häggblom,M.D.Rivera,L.Y.Young,Appl.Environ.Microbiol.59(1993)
ac reactor1162–1167.
[2]R.A.W.Johnstone,A.H.Wilby,Chem.Rev.85(1985)129–170.[3]  F.Murena,F.Gioia,Catal.Today 75(2002)57–61.
[4]  F.Letellier,J.Blanchard,K.Fajerwerg,C.Louis,M.Breysse,D.Guillaume,D.Uzio,
Catal.Lett.110(2006)115–124.
[5]  D.Y.Zhao,J.L.Feng,Q.S.Huo,N.Melosh,G.H.Fredrickson,B.F.Chmelka,G.D.
Stucky,Science 279(1998)548–552.
[6]J.S.Lee,S.H.Joo,R.Ryoo,J.Am.Chem.Soc.124(2002)1156–1157.[7]R.Ryoo,S.H.Joo,S.Jun,J.Phys.Chem.B 103(1999)7743–7746.[8]G.Yuan,M.A.Keane,Chem.Eng.Sci.58(2003)257–267.
[9]  C.H.Xia,Y.Liu,J.Xu,J.B.Yu,W.Qin,X.M.Liang,Catal.Commun.10(2009)
456–458.
[10]G.Yuan,M.A.Keane,Appl.Catal.B:Environ.52(2004)301–314.[11]  A.Sayari,Y.Yang,Chem.Mater.17(2005)6108–6113.
[12]Y.A.Ryndin,M.V.Stenin,A.I.Boronin,V.I.Bukhtiyarov,V.I.Zairovskii,Appl.Catal.
54(1989)277–288.
[13]H.Vu,F.Goncalves,R.Philippe,E.Lamouroux,M.Corrias,Y.Kihn,D.Plee,P.Kalck,
P.Serp,J.Catal.240(2006)18–22.
[14]L.M.Gomez-Sainero,X.L.Seoane,J.L.G.Fierro,A.Arcoya,J.Catal.209(2002)
279–288.
[15]  E.Diaz,S.Ordonez,R.F.Bueres,E.Asedegbega-Nieto,H.Sastre,Appl.Catal.B:
Environ.99(2010)181–190.
[16]  A.Y.Stakheev,L.M.Kustov,Appl.Catal.A:Gen.188(1999)3–35.
[17]O.M.Ilinitch,  F.P.Cuperus,L.V.Nosova,  E.N.Gribov,Catal.Today 56(2000)
137–145.
[18]  E.Diaz,J.A.Casas,A.F.Mohedano,L.Calvo,M.A.Gilarranz,J.J.Rodríguez,Ind.Eng.
Chem.Res.47(2008)3840–3846.
2,4-DCP
2-CP Phenol
1
2
Scheme 1.Reaction pathway of the liquid phase catalytic HDC of 2,4-DCP.
Table 3
Fitting parameters of the catalytic HDC of 2,4-DCP over the catalysts.Sample k 1
(mmol L −1min −1)k 2
(mmol L −1min −1)k 3
(mmol L −1min −1)k 3/k 1Pd/AC-2000.0250.0360.0230.92Pd/AC-3000.00900.00520.00570.63Pd/AC-4000.00120.00190.000220.18Pd/OMC-2000.0680.0500.0340.50Pd/OMC-3000.0400.0260.0170.43Pd/OMC-400
0.021
0.0065
0.0052
0.25
1409
Y.Shao et al./Catalysis Communications 12(2011)1405–1409

版权声明：本站内容均来自互联网，仅供演示用，请勿用于商业和其他非法用途。如果侵犯了您的权益请与我们联系QQ:729038198，我们将在24小时内删除。