Journal of Molecular Catalysis A: Chemical 335 (2011) 7181

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Journal of Molecular Catalysis A: Chemical

journa l homepage: www.e lsev ier .com

A study d P

Wei Zhan c

a State Key Lab .O. Boxb Graduate Unic Synfuels Chin

a r t i c l

Article history:Received 15 SeReceived in re10 NovemberAccepted 14 NAvailable onlin

Keywords:DecarbonylatiFurfuralFuranPd/Al2O3 catal2-Methylfuran

ral tostswconrbonK2Cat 2rmorurfuron thK-doas d

sidesby K doping is the main reason for suppressing the furfural hydrogenation.

2010 Elsevier B.V. All rights reserved.

1. Introdu

Transfortant sustainyears [1]. Fubiomass, apchemicalprtive for theOne importis a useful intetrahydroftion of furfuis usually pdling and re

Somemchromium oPd, Pt, Rh ualysts for deof the non-nwhichmay

CorresponCoal ChemistrChina. Tel.: +8

E-mail add

1381-1169/$ doi:10.1016/j.ction

mation of non-food biomass to chemicals is an impor-ability issue and has attractedmuch attention in recentrfural, as an important chemical readily accessible frompears to be the only unsaturated large-volume organicepared fromcarbohydrate resources and is akeyderiva-production of non-petroleum-derived chemicals [2].

ant chemical coming from furfural is furan [15], whichtermediate and employed primarily in the synthesis ofuran. Both vapor [3] and liquid phase [5] decarbonyla-ral can produce furan, but vapor phase decarbonylationreferred for its simple operation, easy separation, han-cycling of catalysts.etal oxides such as iron, zinc,manganese,molybdenum,xides, or their mixtures [68] and noblemetals such assed as supported form [9,10] are employed as the cat-carbonylation of furfural to furan. The main drawbackoble oxide catalysts is the drastic operating conditions,bringabouta signicantbreakdownof furanand the for-

ding author at: State Key Laboratory of Coal Conversion, Institute ofy, Chinese Academy of Sciences, P.O. Box 165, Taiyuan 030001, PR6 351 7117097; fax: +86 351 7560668.ress: zhuyulei@sxicc.ac.cn (Y. Zhu).

mation of heavy products resulting in deactivation of the catalysts[6,7]. In contrast to the non-noble catalyst systems, the supportednoble metal catalysts exhibit high conversion and yield at muchlower temperatures [9]. For the supported Pd catalysts, the mosteffective andwidelyused supports for thedecarbonylation reactionare active carbon and alumina [11]. Furthermore, some additives,such as nickel, rhodium, ruthenium, alkali and alkaline-earth met-als, are added to modify the properties of supports and interactwith the metal, among which alkali metals are the best promotersto the reaction [12].

It should be noted that the vapor-phase decarbonylation mustbe carried out in the presence of hydrogen as carrier gas, whichhas the function of prolonging the catalysts life [9]. However, thelife expectancy of the catalysts is still not long enough for indus-trial application and the reason may be the lower ratio of H2 tofurfural. Most of the references reported that the molar ratio ofH2 to furfural should not be higher than 2, and the best rangeis from 0.5 to 1 [9]. Srivastava and Guha [13] reported that themain reason for catalysts deactivation is the formation of coke, andexplained that the rate of deactivation is presumed to be controlledby the reaction between two adsorbed furfural to form coke pre-cursor followed by coking. Therefore, the ratio of H2 to furfuralis chosen as 20 in this study, which obviously prolongs the cata-lysts life and shows no sign of deactivation through hundreds ofhours. However, the high content of hydrogen existed in the reac-tant gases can produce signicant quantities of C O hydrogenation

see front matter 2010 Elsevier B.V. All rights reserved.molcata.2010.11.016of furfural decarbonylation on K-dope

ga,b, Yulei Zhua,c,, Shasha Niua,b, Yongwang Lia,

oratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Pversity of the Chinese Academy of Sciences, Beijing 100039, PR Chinaa Co. Ltd, Taiyuan 030001, PR China

e i n f o

ptember 2010vised form2010ovember 2010e 21 November 2010

on

ysts

a b s t r a c t

Vapor-phasedecarbonylationof furfubed reactor. A series of K-doped catalysalt precursors anddifferentpotassiumK not only promoted the furfural decaAmong the investigated catalysts, Pdachieved the furan yields up to 99.5%at high K content (K%>4wt.%). FurtheH2-TPD, H2-chemisorption, CO-FTIR, fdopant phase had a remarkable effectTPSR and furfural-FTIR exhibited thatthe decarbonylation reaction, which won K-doped and undoped samples. Be/ locate /molcata

d/Al2O3 catalysts

165, Taiyuan 030001, PR China

furanwasperformedonK-dopedPd/Al2O3 catalysts in axed-ere preparedby impregnationmethodwith various potassiumtent. Thecatalyst evaluation results revealed that thedopingofylation, but also suppressed the hydrogenation side reaction.O3/Al2O3 (K%=8wt.%) presented the best performance and60 C. The selectivity of 2-methylfuran would approach zeroe, the catalysts were characterized by BET, SEM, ICP, H2-TPR,al-TPSR, and furfural-FTIR experiments. It was shown that thee precursor Pd2+ and the reduced Pdmetal particles. Furfural-ped catalysts enhanced the furfural adsorption and promotedue to the different adsorption mode and intensity of furfural, the decrease of CO adsorption of furfural (2(C,O)-furfural)

72 W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 7181

O CHO

(1)

O-CO (2)

(

Fi

by-productas shown in(1), hydrogof furfuraldrofurfurylduring sepatetrahydrofseparatingtion. Amontetrahydrofwith furanever, 2-mettetrahydroffore, it shouhydrogenatbe employewhich hasT synthesispromoter c

To thebedecarbonyllow reactioK promoterreaction. Thyieldof furaMore specicursor; (b) tof K on the cpropertiesH2-TPR, COand furfuraand adsorp

2. Experim

2.1. Catalys

-Al2O3area =233mAl2O catalyFirstly, Pd(Nent temperat 120 C anreferred to

Five diffusing differas K precurthe Al2O3 fsubsequentthose of thX denotingdesignates

Different content of potassium was doped to the Al2O3 usingK2CO3 as K precursor and the loading range was from 0.75 to10wt.%. The preparation procedures of the catalysts were the same

K2CO3. These catalysts are labeled as PAK(x), while x isng thK/A

talys

surfeasuMicrfor 9moruan

catalop

Elmecata

comptemed

USA)eateC t

min.con

chemer (Merminorpteral hby dvacuptionNexten,ed.temed

duceempdsorr agaelineoutFTIRRTEXat thind

nlessor theduche syed.tem+2H2

O

O CH2OH OCH2OH

O CH3

4) +H2

(5)

+2H2

+H2-H2O

(3)

g. 1. Main reactions existed in furfural decarbonylation.

s. The main reactions occur in furfural decarbonylationFig. 1, including the decarbonylation of furfural to furanenation of furan to tetrahydrofuran (2), hydrogenationto 2-methylfuran (3), furfuryl alcohol (4) and tetrahy-alcohol (5). Accordingly, this complication may ariseration and product purication. As the nal product,uran can be obtained by furan hydrogenating withoutit from the gaseous products of furfural decarbonyla-g these products, furfuryl alcohol (Tboil = 171 C) andurfuryl alcohol (Tboil = 178 C) can be easily separated(Tboil = 32 C) and tetrahydrofuran (Tboil = 66 C). How-hylfuran (Tboil = 63.5 C) has a similar boiling point withuran,whichmakes it verydifcult for separation. There-ld be considered to reduce the by-products of furfuralion, especially 2-methylfuran. Potassium promoter cand to inhibit the hydrogenation function of catalyst,

been proved to be effective in the iron catalysts for F-[14]. So, it is expected that the addition of potassiuman suppress the hydrogenation of Pd/Al2O3.st of our knowledge, fewreports have studied thevaporation of furfural on high H2/furfural ratio (20) and then temperature range (180260 C). Besides, the effect ofon Pd catalysts is not systematically investigated in thiserefore, the objectives of this work are to achieve highnat lowtemperatureandstudy theeffect ofKpromoter.cally, thiswork is devoted: (a) to choose the best K pre-o study the effect of K content; (c) to investigate the roleatalytic behavior of Pd catalyst. The structure and redoxof PdK/Al2O3 catalysts are characterized by BET, ICP,-FTIR, H2 chemisorption, H2-TPD, SEM, furfural-TPSR,l-FTIR aimed at dening the interaction betweenmetalstion mode of furfural.

ental

t preparation

, obtained from the AluminumCo., Ltd, of China (surface2/g, 2040 mesh), was used as support. 1wt.% Pd/-st was prepared by incipient wetness impregnation.O3)2 solution was impregnated to the Al2O3 at ambi-

ature for 5h. After that, the sample was dried overnight

as PAdenotithe Pd

2.2. Ca

Thewerem2420 (350 C

TheSEM (Qof the

ICPPerkinof themetal

Theperform2920,were hor 8050ml/used to

H2analyzto detchemisfor sevlowedunder(adsorsured.hydrogobtain

Theperformrst resame tAfter awith Athebascarried

CO-ter (VEworksZnSe wof staiment fwere rdure, trecord

The

d then calcined at 450 C for 6h. The 1wt.% Pd/Al2O3 isas PA.erent 11wt.% for PdK/Al2O3 catalysts were preparedent potassium salts (K2CO3, KOH, KCl, KNO3, CH3COOK)sors. Firstly, potassium solutions were impregnated toor 0.5h before Pd(NO3)2 solutions were added in. Thedrying and calcination procedures were the same ase Pd/Al2O3. The catalysts are designed as PAX, withthe different potassium salts. For example, PAK2CO3the K in PdK/Al2O3 catalyst coming from K2CO3.

(furfural-TPto an on-linlysts were rat the samein order to dwas in situcarried gaspurged wit40 C untilments weree K content. The PAK(1%) means the content of K inl2O3 catalyst is 1wt.%.

ts characterization

ace area and textural propertied of the fresh catalystsredbynitrogenphysical adsorption at 77.4KwithASAPomeritics, USA). The catalyst samples were degassed ath prior to the measurement.phologies of as-prepared samples were investigated byta 400F). From this technique the surface morphologyysts can be obtained.tical emission spectroscopy (Optima 2100DV,r) was used to analyze the chemical compositionlysts. From this technique the exact composition ofonents can be obtained.perature-programmed reduction of H2 (H2-TPR) wasin a conventional apparatus (Micromeritics Auto Chem.with a thermal conductivity detector. The catalysts

d at a linear rate of 10 C/min from room temperatureo 500 C in a 5% H2/Ar mixture (mol/mol) owing atDuring the TPR, a liquid nitrogen/isopropanol trap wasdense the product water in the efuent.isorption was performed in a surface area and porosityicromeritics. ASAP 2020, USA). The method was usede the dispersion of Pd and the amount of hydrogenion. Prior tomeasurements, the catalystsweredegassedours, and then reduced inhydrogen at 350 C for 2h fol-egassing at the same temperature for 2h. After coolingum to the adsorption temperature, the total H2 uptakeat the Pd surface and adsorption in the bulk) was mea-, inert gas (He) was used to remove the physisorbedand then the amount of chemisorbed hydrogen was

perature-programmed desorption of H2 (H2-TPD) wasin the same apparatus as H2-TPR. The catalysts wered by H2 at 350 C for 2h, and then purged with Ar at theerature for 30min followed by cooling down to 50 C.ption of H2 at 50 C for 30min, the sample was purgedin in order to remove theweakly adsorbed species untilleveled off. Following this, theH2 desorptionwasbeingfrom 50 C to 800 C at a linear rate of 10 C/min.spectra were collected using an infrared spectrome-70, Bruker, Germany), equipped with KBr optics whiche liquid nitrogen temperature. The infrared cell withows was connected to a gas-feed system with a setsteel gas lines, which allowed the in situ measure-

e adsorption of CO. Before measurements, the catalystsed in situ at 350 C for 2h. After the reduction proce-stemwas cooled to 20 C and the CO-FTIR spectra were

perature-programmed surface reaction of furfuralSR) was performed in a self-made reactor connectede mass spectrometer (OmniStar TM). Firstly, the cata-educed by H2 at 350 C for 2h, and then purgedwith Hetemperature for 30min and cooled to 40 C overnightiminish the effect of H2. Next, the absorption of furfuralperformed at 40 C by a stream of furfural (diluted withHe) for 1.5h. After the absorption, the catalysts wereh He for removing the physically adsorbed furfural atthe values of m/e became constant. The TPSR experi-performed with a heating rate of 10 C/min in a ow

W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 7181 73

Table 1Surface area and pore structure of the catalysts with doping different K salts.

Catalyst Surface area(m2/g)

Pore volume(cm3/g)

Average poresize (nm)

-Al2O3 196.6 0.619 9.07PA 191.12 0.611 9.36PAK2CO3 184.34 0.605 9.73PAKOH 182.70 0.602 9.59PAKCl 178.48 0.601 9.73PAKNO3 176.42 0.605 9.92PAKAc 193.11 0.610 9.26

of He stream, and the desorbing products weremonitoredwith theonline MS.

Furfural-FTIR spectra were collected with a VERTEX70 spec-trometer cooled by liquid nitrogen. A vacuum pump was used topull the furfural vapor through the chamber for in situ FTIR mea-surements. Before the measurements, the catalysts were reducedin situ at 35was cooledThen, the tefurther evafurfural dec

2.3. Catalyt

The actithe Pd catalspheric pre2.5 g) wereat 200 C fointo the xeow lines ocondensatiotests were pthe liquid pThe reactanchromatogra capillary c

3. Results

3.1. Catalys

3.1.1. MorpThe BET

summarizeTable 1

well as an inin all the cathe deposithas no obvvariations i

as a negligible factor for the different catalytic activities of furfuraldecarbonylation, which will be shown in catalytic performance.

Table 2 displays the morphology and sample composition of Pdcatalysts with different K content. With the increase of K loading,the surface area and pore volume are decreased monotonous dueto pore blocking. The ICP results indicate that the content of Pd doesnot change too much upon different K content. However, there is amore rapid decline in K content at high K loading.

Fig. 2 exhibits that the outer surface of the catalysts with higherK content (such as PAK(8%)) is covered by elongated crystals otherthan the support that are not observable before doping K or lower Kcontent, which is constituted mainly by alkaline metal carbonatesas found by EDS analysis [15].

3.1.2. H2-TPRTPR experiments can be used to investigate the reducibility of

the Pd2+ species loaded on a given support. By analyzing the TPRof thhet

sor bTPRx)) amentraturwitht begcatal6]. Inraturformed duraturreasr disthes, wthe Prderonofeasirve oraturig. 3a25 Coratit 15h teaticaoes%). Ce int, and

Table 2Characterizati , chem

Catalyst (wt.%)

PAPAK (0.75%PAK (2%)PAK (4%)PAK (6%)PAK (8%)PAK (10%)

a The differeb Pd dispers0 C for 2h. After the reduction procedure, the systemto 30 C and the Furfural-FTIR spectra were recorded.mperature was increased from 30 C to 200 C so as toluate the strength of adsorption and the capability ofarbonylation.

ic test

vity tests for catalytic decarbonylation of furfural overysts were performed in a xed bed reactor under atmo-ssure. Prior to the reactions, the catalysts (2040 mesh,in situ reduced in a stream of pure H2 (150ml/min)r 12h. After reduction, furfural and H2 were introducedd-bed reactor through a saturator, and the downstreamf the saturator were maintained at 140 C to preventn of vaporized furfural. The decarbonylation activityerformed in the temperature range of 180260 C, androducts were obtained by a cold trap lled with water.ts and the products were analyzed using a GC-950 gas-aph (GC) equipped with ame ionization detector andolumn (OV-101).

ts characterization

hology and sample composition (BET, ICP, SEM)surface area, pore volume and average pore size ared in Tables 1 and 2.shows a decrease in surface area and pore volume, ascrease in average pore size comparedwith the supporttalysts, which may be due to pore blocking. However,of Pd and different K salt precursors on-Al2O3 supportious effects on the surface areas changes. That is, then the surface areas of these catalysts can be considered

resultsmine wprecur

The(PAK(experitempealyststhe rson PAtion [1tempein theoccurrtempethe incweak omay bePd sitecauses

In obilizatito be ation cutempewith Fing atincorpat abouthe higsystemundergPAK(8lyst, thextent

on of the catalysts with doping different K content: surface area and pore structure

Surface area(m2/g)

Pore volume(cm3/g)

Average poresize (nm)

Pd (wt.%) K191.12 0.611 9.36 0.86 0) 191.82 0.609 9.42 0.80 0.66

186.46 0.590 9.23 0.89 1.76179.77 0.563 9.03 0.89 3.67166.28 0.520 9.09 0.92 4.93146.62 0.497 9.02 0.87 6.64140.10 0.456 9.40 0.88 8.43

nce value of calculated value and true value determined by ICP.ion determined from H2-chemisorption.e undoped and doped samples, it is possible to deter-her dopants have interacted with the supported Pd2+

efore or during reduction [15].proles of PA and the whole set of K-doped samplesre shown in Fig. 3, inwhich (a) and (b) represent that thestarts from room temperature and far below ambiente (80 C), respectively. As displayed in Fig. 3a, the cat-low K content or without K can be easily reduced frominning, and a negative peak at about 72.5 C is appearedyst, which may be due to -palladium hydride desorp-hydrogen atmosphere, PdO is reduced easily at ambiente to Pdmetal, further interacts with hydrogen resultingation of PdHx species. This progress appears to havering the passage of reducing gas prior to the start of thee program, and then the PdHx will be decomposedwithe of temperature [17]. However, the desorption peak isappeared with the increase of K content and the reasonpresence of electropositive alkali atoms in the vicinityhich exhibits higher electron-donating properties anddH bond strength weaker.to avoid the partial reduction of Pd2+ species during sta-f thebaseline, a low-temperatureH2-TPRanalysis seemsble approach (Fig. 3b). As displayed in Fig. 3b, the reduc-f the PA catalyst shows no negative peak and the highe peak has a little shift to low temperature compared. The PA sample shows a single reduction peak start-and terminating at 75 C (the peak at 50 C). With theon of small amounts of K, an additional peak appears5 C. Further increasing the K content, the intensity ofmperature peak is progressive increased, and the peaklly shifts to lower temperature. For example, the peaka shift from the value 122 C in PAK(4%) to 107 C inontrarily, with the increase of K loading in the cata-ensity of low temperature peak is decreased to a greatthepeak simultaneous shifts tohigher temperature. The

ical composition, hydrogen chemisorption and metallic dispersion.

(K/wt.%)a H2 chemisorption(mmol/g)

Pd dispersionb (%)

0 0.00988 21.02330.09 0.00948 20.17530.24 0.01398 29.75780.33 0.01644 34.97391.07 0.01811 38.54751.36 0.01567 33.33991.57 0.02379 50.6189

74 W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 7181

, (b) P

results indiwith the inctemperaturcal interactlead to thecontent [15

3.1.3. H2-TPThe H2-

interacted wwhether threduced Pdchemisorptresults are s

It is obseples exhibiis due to hlites, wherespillover htemperatursites locatetactedwiththe positionthe high-teand have aK. The reasport,whichthe mediumintensity orcatalyst PAcorrespondof K. Howedecrease thadsorbed hchemisorpt

etaat Kouttion

CO-Fspe

of thFig. 2. SEM images of the surface of catalyst particles (10m): (a) PA

cate thatmore andmore K+ will be interactedwith Pd2+

rease of K content, which lead to change the reductione. The behavior up to K/Pd=4 reects a strong chemi-ion between K+ and Pd2+ species, whichmay eventuallyformation of a mixed compound (KPdO) even at low K].

D and H2 chemisorption

of Pd msion thcarriedinterac

3.1.4.FTIR

zation

TPR results exhibit that the dopant phase has alreadyith the precursor Pd2+ phase. One of the questions is

e interaction between K and Pd is still holding on themetal particles, so the experiments of H2-TPD and H2ion were carried out on the reduced Pd particles. Thehown in Fig. 4 and Table 2.rved from Fig. 4 that hydrogen desorbs from the sam-ting three peaks centered. The low-temperature peakydrogen chemisorption on the surface of Pd crystal-as the high-temperature feature can be attributed toydrogen associated with the support. The medium-e peak has been assigned to hydrogen adsorbed on thed at the periphery of Pd crystallites, which are con-the support [18]. K addition does not appreciably affectof the low-temperature peak (at ca. 85 C). However,

mperature peaks (at ca. 480 C) decrease in intensitylittle shift to higher temperature in the presence ofon is that K can induce the dehydroxylation of sup-is known to suppress the hydrogen spillover. Regarding-temperature peaks (at ca. 300 C), they decrease inoverlap with the high-temperature peaks except the(6%). This implies that the adsorption strength of the

ing sites toward hydrogen decreases in the presencever, it is interesting to see that these effects do note quantity of adsorbed hydrogen. On the contrary, theydrogen is increased as indicated by the results of H2-ion (Table 2),whichmaybedue to the higher dispersion

bindinide molecuvalence d-eof CO boneffective ch

The FTIRK-doped Pdtra of the cregion (20The interprcatalysts isrange 1850PdCOPdadsorptionattributed tor the twof[22], respeccontent cauthat the dop

As implispectra witmain phensity of adsPAK(8%), wbands I andto the PA c8% K loadinAK(2%), (c) PAK(4%), and (d) PAK(8%).

l particles. From the results, we may draw the conclu-has interacted with the reduced Pd. Then, CO-FTIR wasas a complement experiment to further investigate this.

TIRctroscopy of adsorbedCO is amethod for the characteri-e electronic states of supportedmetals. According to the

g mode reported by Davydov [19], the carbon monox-les adsorbing onmetal atoms or ions interact with theirlections and the frequency of the stretching vibrationd of the surface MCO complex depends directly on thearge of the adsorption.spectra of CO adsorbed at 20 C on Pd/Al2O3 and the/Al2O3 samples are displayed in Fig. 5ag. The spec-atalysts show three main peaks from high-frequency002100 cm1) to low-frequency (18002000 cm1).etation of CO absorption bands on supported palladiumwell dened in the literature [19,20]. The signals in the1980 and 20502100 cm1 are attributed to bridgeand linear PdCO species. Moreover, the two bridgedbands, from high-frequency to low-frequency, can beo the compressed and isolated bridged carbonyls [21]old bridged carbonyls on Pd(100) and Pd(111) facestively. As shown in Fig. 5, the progressive increase of Kses a gradualmodication of the CO spectra, suggestingant has interactedwith the reduced Pdmetal particles.ed by the CO-FTIR characterization, the changes in COh the increase of K loading can be summarized in threeomena. (1) With the increase of K content, the inten-orption band is increased, especially for the catalysthich shows the highest CO adsorption band. (2) TheII are progressive decrease of the intensitywith respectatalyst, and a simultaneous increase of band III. At thegs, the spectra are dominated by the last band, which is

W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 7181 75

5004003002001000

H2 C

onsu

mpti

on (

a.u.)

PA-K(2)

Temperature (C)

PA-K(10)

PA-K(8)

PA-K(6)

PA-K(4)

PA

PA-K(0.75)

a

-100

b PA

PA-K(0.75)

PA-K(2)

PA-K(4)

PA-K(6)

H2 C

onsu

mpti

on (

a.u.)

Fig. 3. H2-TPRand (b) start a

quite sharpby a generaundergoesin PAK(8%

800700600500400300200100

H2-D

esorp

tion (

a.u.)

Temperature(C)

PA-K(8%)

PA-K(10%)

PA-K(4%)

PA-K(2%)

PA-K(0.75%)

PA-K(6%)

PA

Fig. 4. H2-TPD proles on reduced catalysts.

downward shift of116 cm1 (from1940 cm1 in PA to 1824 cm1in PAK(8%), value taken at peak). The phenomenon shows that thedoping of K results in a red shift of the (CO) stretching frequencyof species adsorbed on these sites, which implies strengthening of

CO

Furfugram

2102200

PA-K(1

PA-K(8

PA-K(6

PA-K(4

PA-K(2

PA-K(0

PA

2102200

PA-(d)5004003002001000

PA-K(8)

PA-K(10)

the Pd

3.1.5.ProTemperature (C)

proles of the investigated catalysts: (a) start at ambient temperaturet 80 C.

and intense. (3) The change in intensity is accompaniedl downward shift of all the bands: for example, band Ia shift from the value 2083 cm1 in PA to 2063 cm1

) which is taken at peak, whereas band III undergoes a

lation prodoccurring dcatalysts. Tof furfural/purging thedesorbing pdifferent defural (C5H4(CO, 28 amu

Wavenumber (cm1)

1800190020000

0)

)

)

)

)

.75)

21002200

PA-K(0.75

2080

(b)

18001900200021002200

1940

1984

2083

(a) PA

18001900200021002200

1830

1942

2065

PA-K(6%)(e)

21002200

2063

PA-K(8%)(f)

1800190020000

1855

19532065

K(4%)

Fig. 5. FTIR spectra of CO dosed at 20 C on thbond with the increase of K content.

ral-TPSRmed thermodesorption of furfural and the decarbony-ucts was done to verify the stability of surface speciesuring desorption and decomposition on PA and PAKhese experiments were performed with the adsorptionHe ow on reduced catalysts at 40 C for 1.5h, thenphysisorption of furfural, after that, monitoring theroducts with the online MS. Masses characteristic ofsorption products are shown in Fig. 6, including fur-O2, 96 amu), furan (C4H4O, 68 amu), carbon monoxide) and hydrogen (H2, 2 amu).

180019002000

%)1979

18001900200021002200

PA-K(2%) 1878(c)1934

1959

2065

180019002000

1824

1948

18001900200021002200

1828

1943

PA-K(10%)

2071

(g)

e catalysts.

76 W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 7181

O

100

Furan

O

b(68 amu)

s)

100

Furfural

O C

O

H

a(96 amu)

sorpt

Fig. 6a sand the inteincrease ofbetween fuadsorb a cePAK and loof furfural o

Fig. 6b a(the productwo desorppeak at ca.PAK(4%, 8temperatursame temp152 C, whifuran can beIt is interestpeaks of futest showssupport (Alof CO on Al2Furthermorexhibit theother wordThe CO desothe peaks opeak (290

CO, whichan addition530 C, whi

Fig. 6d dlysts. In this

befoas m0

Inte

nsi

ty (

arb.u

nit

6005004003002001000

CO (28 amu)

c

PA

PA-K(4%)

PA-K(8%)

Al2O

3

Temperature (C)

Inte

nsi

ty (

arb.u

nit

s)

0

Inte

nsi

ty (

arb.u

nit

s)

d

6005004003002001000

PA

Al2O

3

PA-K(4%)

PA-K(8%)

Temperature (C)

Inte

nsi

ty (

arb.u

nit

s)

Fig. 6. Furfural-TPSR on the catalysts: (a) furfural desorption, (b) furan de

hows that furfural desorption takes place at ca. 67 C,nsity of furfural desorption peak is decreased with the

350 Cgen wK content, which may be due to the strong adsorptionrfural and Pd in the presence of K. Also, the support canrtain amount of furfural, with its intensity higher thanwer than PA catalyst, indicating that the adsorbabilityn Al2O3 is weak and the adsorption quantity is small.nd c expresses the desorption peaks of furan and COts of furfural decarbonylation), respectively. In Fig. 6b,tion peaks of furan are presented in PA catalyst: a small67 C and a large peak at ca. 195 C. In comparison, the%) catalysts also have two desorption peaks: the lowe peaks are decreased of the intensity, but keeping theerature as PA; the high temperature peaks are at ca.ch is 43 C lower than that of PA catalyst, implying thateasier desorbed on PAK catalysts than on PA catalyst.ing to see that the support also presents twodesorptionran at 66 C and 208 C. However, the result of blankthat the decarbonylation reaction cannot occur on the2O3). Fig. 6c also exhibits that the desorption quantityO3 is obviously less than other Pd-containing catalysts.e, therst twodesorptionpeaksof COon these catalystssimilar desorption temperature as furan desorption, ins, furan desorption is accompanied by desorption of CO.rption peaks on PA appear at 68, 214 and 290 C, whilen PAK are at 68, 158 and 290 C. The third desorptionC) may be due to the strong interaction between Pd andis decreased with the increase of K content. However,al desorption peak of CO on PAK (8%) is present at ca.ch may be due to the decomposition of polymer.isplays the H2 desorption from the investigated cata-characterization, the samples were purged with He at

H2 is desocatalyst. Inorption onformed inH2 can bespill to thegen is hardTherefore,gen atomsfurfural adlorptionpeaderived frorespectivel

3.1.6. FurfuTo bette

lation reactPAK(8%)wat different

Fig. 7a slyst. At 30

be attributesimilar to tincreasingthe intensichemisorbenew band aCO adsorptincrease thature prom600500400300200

PA

Al2O

3

PA-K(4%)

PA-K(8%)

Temperature (C)

600500400300200

PA

PA-K(4%)

PA-K(8%)

Al2O

3

Temperature (C)

H2 (2 amu)

ion, (c) CO desorption, and (d) H2 desorption.

re the absorption of furfural, so the desorbed hydro-ainly coming from furfural. As shown in the gure,

rbed more easily from PAK catalysts than from PAaddition, it is obvious that there are no sign of H2 des-Al2O3, which suggests that the desorbed hydrogen isthe presence of Pd. During the process of reduction,adsorbed dissociatively on the Pd surface and furthersupport. As revealed by H2-TPD, the spillover hydro-ly desorbed completely lower than 350 C (Fig. 4).the rst desorption peak may be due to the hydro-released to the surface by the decomposition of theayer recombined and desorbed, while the second des-kmaybedue to the combinationof twohydrogenatomsm dehydrogenation of furfural and spillover hydrogen,y.

ral-FTIRr understand the promoter effects on the decarbony-ion, we investigated the interaction of reduced PA andith furfural. The in situ FTIR spectra of furfural adsorbedtemperature on the samples are displayed in Fig. 7.hows the adsorbed acyl species of furfural on PA cata-C, two bands observed at 1888 cm1 and 1695 cm1 cand to physisorbed and chemisorbed furfural, which arehe adsorption of benzaldehyde on Pd/Al2O3 [23]. Withthe temperature, the physisorbed furfural is decreasedty and the bands shift to lower frequency, while thed furfural only decreases the intensity. In addition, appeared at 80 C (1957 cm1), which is assigned to theion, is progressive increased the intensity with furthere temperature. It indicates that the increase of temper-otes the decarbonylation of furfural.

W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 7181 77

140

18071676

1624

PA-K(8%)

PA-K(8%)

200 o

150 o

1888-1850

ba

100 o

80 oC

50 o

30 oC

PA1695

c

-1)

Fig. 7. In situ on PAK-doped and u

Fig. 7b excatalyst. Thfurfural, whtemperaturfural on thand 1624 cmmodes. Witis decreaseadsorbed futhe band atCO bands caature highe

As indicatra on the Kthree mainbands (bothare higher tfurfural is sshows a confrequency cundergoesin PAK(8%chemisorbeadsorptionis at 1624 ctemperaturtion mode oon the catal

3.2. Catalyt

Sampleexamine thtent on theplotted in Fas furfural cyield, 2-mesize that thand tetrahyminimize 2

Fig. 8 shabout the calyst revealowest fura

risond caan s, theomos thaeaseyielded thn thof Pt grats hal to 2methtalyst, thet onn froAK(pectereasee largby P1600180020001400160018002000

2021 1913

1957-1942 200 oC

150 oC

100 oC

80 oC

50 oC

30oC

Wavenumber (cm

Ab

sorb

ance

FTIR of furfural adsorbed on the catalysts at different temperature: (a) adsorbedndoped catalysts.

hibits the adsorbedacyl species of furfural onPAK(8%)e band at 1807 cm1 is attributed to the physisorbedich is also decreased the intensity with increasing thee. It is interesting to see that the chemisorbed fur-e K-doped catalyst shows two bands at 1676 cm11, suggesting the presence of two different adsorbedh increasing the temperature, the band at 1676 cm1

d the intensity, which is the same as the change ofrfural at 1695 cm1 on PA catalyst (Fig. 7A). However,1624 cm1 has no signicant change. Furthermore, then be seen at 1913 cm1 and 2021 cm1 at the temper-r than 100 C.ted in Fig. 7c, the differences of furfural in situ IR spec--doped and undoped samples can be summarized inphenomena. (1) The intensity of furfural adsorptionphysisorbed and chemisorbed furfural) on PAK(8%)

han that on PA, which indicates that the adsorption oftronger on K-doped samples. (2) The PAK(8%) catalystspicuous shift of all furfural adsorption bands to lowerompared with PA. For example, the chemisorbed banda shift from the value 1695 cm1 in PA to 1676 cm1

). (3) In contrast to PA, the PAK(8%) displays twod bands: the rst band is at 1676 cm1, having the samemode with band 1695 cm1 on PA; and the new band

1

compaK-dopethe furwordsand prexhibitall incrlowestpoisonfuran oorderThe lascatalysfurfuraand 2-best ca

Nexried oube seealyst Pthe exthe inchas thlowedm , which is gradually dominant with increasing thee. Accordingly, the doping of K can change the adsorp-f furfural on Pd catalysts, which has promoting effectytic performance.

ic performance

catalysts were tested for furfural decarbonylation toe effect of added different K sources and different K con-performance of Pd/Al2O3 (PA) catalyst. The results areigs. 8 and 9, which have four small graphs representedonversion, furan selectivity, furan and tetrahydrofuranthylfuran selectivity, respectively. Again, we empha-e focus of our research is to improve the yield of furandrofuran at low temperature (below 300 C), as well as-methylfuran selectivity.ows the results of conversions, selectivities and yieldsatalytic tests, and it can be clearly seen that PA cat-l a maximum for furfural conversion and in contrast,n selectivity and highest 2-methylfuran selectivity. In

with the furorbed hydris eventuallsite trendexhibits thelowest. Asof furan (99yields thanselectivity o4% (Fig. 9d)

4. Discussi

FurfuralreadilyhydmodynamicC O bond[24,25]. RebondoverC0 1400160018002000

PA-K(8%)

C

C

PA

PA

C

C

200 oC

200 oC

30 oC

30oC

catalyst, (b) adsorbed on PAK(8%) catalyst, and (c) comparison of

with PA catalyst, the conversions of furfural on thetalysts are all decreased to a certain extent, whereaselectivities are all increased (Fig. 8a and b). In otherdoping of K can suppress the hydrogenation of furfuralte the decarbonylation reaction on Pd catalyst. Fig. 8ct the total yield of furan and tetrahydrofuran does notwith K doping, for example, PAKCl catalyst shows the, whichmay be due to the residual presence of chlorinee active sites. The total yields of furan and tetrahydro-e K-doping catalysts with different K salts follow theAK2CO3 >PAKOH>PA>PAKNO3 >PAKAc>PAKCl.ph (Fig. 8d) shows that PAKCl, PAKAc and PAK2CO3ve excellent behavior on suppressing hydrogenation of-methylfuran. Therefore, considering the yield of furanylfuran selectivity, K2CO3 as K precursor can obtain thet performance in the decarbonylation reaction.experiments of the decarbonylation reactionwere car-

different content of K (from K2CO3), and the results canm Fig. 9. The rst graph (Fig. 9a) shows that the cat-6%) has the highest furfural conversion, which is notd result that the furfural conversion is decreased withof K content. H2-TPD (Fig. 4) shows that the PAK(6%)est amount of hydrogen desorption below 300 C fol-AK(4%), PAK(2%) and PAK(8%), which is coincident

fural conversion. It is revealed that the decrease of des-ogen would decrease the hydrogenation activity, whichydecreased the furfural conversion.However, theoppo-is observed for furan selectivity (Fig. 9b). PAK(8%)highest furan selectivity, while PAK(0.75%) shows theshown in Fig. 9c, PAK(8%) displays the highest yield.5%), while PAK(0.75%) and PAK(10%) exhibit lowerother catalysts above 220 C. It is also noticed that thef 2-methylfuran is zero when the loading of K is above.

on

is an unsaturated aldehyde of ve carbons, which isrogenatedunderhydrogenatmosphere.Generally, ther-studies favor hydrogenation of C C bond over the

for hydrogenation reaction of unsaturated aldehydesaction kinetics also prefer hydrogenation of the C CObond for smallmolecules,whereas steric constraints

78 W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 7181

80

90

100

%)

a

180

80

90

100b

)

180

Fig. 8. Catalyt ral: (aand tetrahydro sure, W

for larger mbond. Accomore difcsteric effectever, the hypossible, esreported thbond comption [26]. Inand positivis present bgenation onaccount theally, alkali mto increasethe role plathe typeof stralize thepoisoning bthe alkalimsition metaapproved bas oxides, hthat K stronof adsorbed

4.1. Charac

The cataPd catalysts2602402202001800

10

20

30

40

50

60

70

PA

PA-K 2CO3

PA-KOH

PA-KCl

PA-KNO 3

PA-KAc

Temperature (C)

Furf

ura

l co

nver

sion (

20

30

40

50

60

70

Fura

n s

elec

tivit

y (

%

2602402202001800

10

20

30

40

50

60

70

80

90

100c

PA

PA-K2CO

3

PA-KOH

PA-KCl

PA-KNO3

PA-KAcFura

n a

nd t

etra

hydro

fura

n y

ield

(%

)

0

2

4

6

8

10

12

14

16d

2-m

ethylf

ura

n s

elec

tivit

y (

%)

Temperature (C)

ic activity of PdK catalysts with doping different K salts in decarbonylation of furfufuran, and (d) selectivity of 2-methylfuran. Reaction conditions: atmospheric presolecules decrease the rates for hydrogenation of C Crdingly, the hydrogenation of C C bond in furfural isult than that of C O bond, which may be due to thesor thearomatic character of furfural. In ourwork, how-drogenation of furfural should be decreased asmuch aspecially for the C O bond hydrogenation. It has beenat Pd exhibits a low rate for hydrogenation the C Oared with other metals commonly used for hydrogena-addition, the catalytic performance shows that a cleare effect for the suppressing hydrogenation of furfuraly doping of K. The role of K in the suppressing hydro-the Pd catalysts can be better interpreted taking intointeraction between Pd particles and the dopant. Usu-etals are employed as catalytic promoters ormodiers

the activity and lifetime of heterogeneous catalysts, butyed by alkali metals is not unequivocal, depending onolid andcatalytic reaction. Firstly, alkali oxides canneu-surface acidic sites, thus enhancing their resistance toy acid. Furthermore, many studies [27,28] suggest thatetal atomsmodify the local electron density of the tran-l either directly or through the support, which can bey XPS. However, the alkalimetals are always introducedydroxides, carbonates and nitrates. Thus, it is explainedgly segregates at the surface, forming a submonolayerK and O having a stoichiometric ratio of unity.

ters and activity effect of K on Pd catalysts

lyst evaluation results exhibit that the doping of K oncan enhance the decarbonylation of furfural to furan

and simultaproducts, e

Accordincatalyst exin furfuralPAKCl (Figples with lowith the incsurface areathe conversfore, the cathan the spcrystallinepromoter pate [29]. In osome chembetween K

H2-TPRprole of Ptures. It alsattributed ttent is overto lower tethe decreas(do not inteWhentheKdisappeare2-methylfudecreased wzero at the260240220200

PA

PA-K 2CO3

PA-KOH

PA-KCl

PA-KNO3

PA-KAc

260240220200

PA

PA-K 2CO3

PA-KOH

PA-KCl

PA-KNO 3

PA-KAc

Temperature (C)

Temperature (C)

) conversion of furfural, (b) selectivity of furan, (c) total yield of furanHSV=0.77h1, H2/furfural = 20 (molar ratio).neously decrease the selectivity of hydrogenation by-specially 2-methylfuran.g to the BET results of PAX catalysts, the PAKAchibits the highest surface area but shows less activedecarbonylation compared with other catalysts except. 8a). A reversed trend is observed for PAKNO3 sam-wer surface area and higher catalytic activity. Besides,rease of K content (using K2CO3 as the K precursor), theand pore volume are decreasedmonotonous, whereasion of furfural does not follow the same order. There-talytic activity should depend on other factors, ratherecic surface area. Fig. 10 shows that large amounts ofalkali carbonate are deposited on the surface, but thehase is always an alkali compound other than carbon-therwords, the promoter phase is part of dispersedK inical state, whichmay be formed through the interactionand the activity species.shows that the addition of K changes the reductionA catalyst, favoring the reduction at higher tempera-o indicates that the high temperature reduction peak iso the interaction between K and Pd2+. When the K con-4%, the high temperature peak is dominated, and shiftsmperature with further increase of K content. Besides,e of low temperature peak proves that free Pd atomsract with K) are decreased with increasing K content.content ishigher than4%, the freePdatomsarealmostd, which presents the same trend as the selectivity ofran. In other words, the selectivity of 2-methylfuran isith the decrease of free Pd atoms and is approach toK content of 4wt.%. For the PAK(10%), the reduction

W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 7181 79

260240220200180

60

70

80

90

100

Furf

ura

l co

nver

sion (

%)

Temperature (C)

PA-K(0.75%)

PA-K(2)

PA-K(4%)

PA-K(6%)

PA-K(8%)

PA-K(10%)

a

180

20

30

40

50

60

70

80

90

100b

Fura

n s

elec

tivit

y (

%)

70

80

90

100c

rofu

ran y

ield

(%

)

180

3

4

5d

elec

tivit

y (

%)

Fig. 9. Catalytfuran and tetra

peak shiftsactivity is osites are blothe promotthe higher a

Themodpropertiesstudying hythat these agen dissocia

Fig. 10.260240220200180

20

30

40

50

60 PA-K(0.75%)

PA-K(2)

PA-K(4%)

PA-K(6%)

PA-K(8%)

PA-K(10%)

Fura

n a

nd t

etra

hyd

0

1

22-m

ethylf

ura

n s

Temperature (C)ic activity of PdK catalysts with doping different K content in decarbonylation of furfurhydrofuran, and (d) selectivity of 2-methylfuran. Reaction conditions: atmospheric pres

to high temperature compared with PAK(8%), and thebviously decreased. One reasonmight be that the activecked by carbonate, and another reason might be thating effect is suppressed by some poisoning function oflkali content [30].ication induced by alkali addition onmetal adsorptionhas been widely reported in the literature. Kiskinovadrogen adsorption on alkali modied nickel indicateddditives give rise to an important decrease in hydro-tion ability and saturation coverage [31]. Weatherbee

SEM images of the surface of PdK(8%)/Al2O3 particles (5m).

indicates thhydrogen aaud reporteexplained trise of hydrinduced byof the metaresults (Fig.hydrogen aother wordK-doping acthis effect seither by indecreasingof K contenCO on Pd psity is increbands. TheCO and theAlso, the PAbonylation,strong inter

4.2. Electro

As indicamay be forK+ and Pd2

With the in260240220200

Temperature (C)

PA-K(0.75%)

PA-K(2)

PA-K(4%)

PA-K(6%)

PA-K(8%)

PA-K(10%)

260240220200

PA-K(0.75%)

PA-K(2)

PA-K(4%)

PA-K(6%)

PA-K(8%)

PA-K(10%)

Temperature (C)al: (a) conversion of furfural, (b) selectivity of furan, (c) total yield ofsure, WHSV=0.77h1, H2/furfural = 20 (molar ratio).

at K produced an increase in the activation energy fordsorption on K promoted Fe/SiO2 catalyst [32]. Prali-d that K caused a decrease of the hydrogen adsorption,his by a decrease in metallic phase accessibility or by aogen bond stoichiometry, and claimed this occurrencean alkali sieving effect that limits the accessibilityl surface to a larger probe molecules [33]. The H2-TPD4) show that the doping of K can suppress the spillovernd H2 may interact with the alkali-modied support, ins, interact with hydroxyl groups on support. Therefore,ts to enhance the adsorption strength of hydrogen, andhould tend to reduce the rate of hydrogenation reaction,crease the hydrogenation-reaction activation energy,the concentration of free Ha [34]. Besides, increasingt induces different behavior towards the adsorption ofarticles and the CO-FTIR spectra show that the inten-ased and accompanied by a general red-shift of all thesample PAK(8%) shows the strongest interaction withmain CO absorption band is isolated bridged carbonyls.K(8%) exhibits the best performance on furfural decar-which may be owing to the adsorption mode and theaction between furfural and the catalyst.

nic effect of K on Pd catalysts

tedby the results ofH2-TPR, amixed compound (KPdO)med through the strong chemical interaction between+ species, which is more difcult to reduce than PdO.crease of K content (from 2% to 8%), the mixed com-

80 W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 7181

pound is more likely to be reduced and the reason might be thatincreasing of K content can share more electrons with Pd2+ duringthe process of reduction. Due to the higher electronegative of Pdatom, an electronic density transfer is readily from alkali metal toPd, which isthe Pd 3d bidramatic ensurface exteeffect alsothe CO banlysts, owingmetal to the

Electronthrough dirsupport proactions. Thechange frommetals and[38,39]. Figof styrene othe electronsupport. Frdensity sitewith addedgenation ovclaimed tha[39]. In ourelectron traand ultimaPd. This intfurfural becther hydrogin a decreas

4.3. Adsorp

The resufer electronCO. Duringnegative chsimultaneoAs a resultthat of PA cFTIR (Fig. 7)of furfural dcontent (FiPAK can padsorptionthe furan dtion modecombinatiotemperaturto bond in tface througwhich desostrongly boand oxygenthat decom[42]. Howevof aldehydewhich is tra

On thefollowing bfurfural decadsorptionational tem

O C

O

O C

H

O

Pd

(1) 1(O)

H

H

11. Ad

ratih th)-furobtationly, thhargducelt onl wirodue ofity of the rst furan desorption peaks is decreased accom-by increasing K content. Furthermore, the hydrogenationral always occurs through 2(C,O) conguration, that is the

ond hydrogenation [23], so the decrease of 2(C,O)-furfuralby K doping will inhibit the hydrogenation reaction. For PAt, the main decarbonylation occurs through 1(C) congu-(Fig. 11b), which is transformed from 2(C,O) congurationowever, for PAKcatalysts, the results of furfural-FTIR (Fig. 7)hat the adsorption mode of acyl species is different on K-and undoped catalysts, and furfural-TPSR indicates that the) conguration is difcult to form and unstable on PAK cat-Accordingly, the main decarbonylation of furfural on thed samplesmay occur through 1(C) conguration (Fig. 11c),is more stable than 1(C) conguration.

clusions

his work, the catalytic behaviors of K-doped Pd/Al2O3 cat-for furfural decarbonylation have been investigated in therature range of 180260 C and ambient pressure. Accordingresults, the PdK/Al2O3 catalyst doped with K2CO3 and withding of 8wt.% shows the best performance in the decarbony-reaction (the conversion of furfural is 100% and the yield ofs 99.5% at 260 C). Besides, the selectivity of 2-methylfuranproach zerowhen the loading of K is above 4wt.%. The effectproved by XPS with a shift towards negative values ofnding energies [35]. Also, alkali adsorbates also cause ahancement of the electronic polarizability of the metalnding it several angstroms into the vacuum [36]. This

produces a general shift towards lower frequencies ofds as indicated by the CO-FTIR spectra on these cata-to an enhanced transfer of electron density from the* molecular orbitals of CO.

ic effects of K promoter on Pd can take place eitherect KPd interaction or through a modication of thepertieswhich induces a change inmetalsupport inter-addition of K2CO3 can make the support propertiesacidic to basic [37] and the electron transfer betweensupports has been suggested by several researchers

oli and Largeniere studied the selective hydrogenationver Pd/Al2O3 catalysts and discovered the changes inic state of Pd via XPS when sodium was added to the

ee electrons on Pd were transferred to lower electrons on acidic Al2O3 and electron transfer was attenuatedsodium [38]. Figueras et al. studied benzene hydro-er Pd supported on several different acidic oxides andt the support inuences the electronic properties of Pdwork, more basic nature of K-doped Al2O3 favors annsfer to Pd, resulting in a higher Pd electron density,tely a reduced strength of hydrocarbon adsorption oneraction leads to an increase in furan selectivity fromause the furan desorbsmore readily before it can be fur-enated to tetrahydrofuran, and simultaneously resultse of the furfural hydrogenation by-products.

tion mode of furfural on PA and PA-K catalysts

lts of CO-FTIR indicate that the doping of K can trans-ic density to Pd and change the adsorption mode ofthe decarbonylation reaction, the surface of Pd displaysarge in the presence of K, while the adsorbed furfuralusly dehydrogenates and then forms carbonyl C+ ions., the adsorption of furfural is stronger on PAK thanatalyst, which can be proved by the results of furfural-and is in agreement with the results that the intensityesorption peaks are decreased with the increase of K

g. 6a). Also, the strong binding between furfural andromote the cleavage of CC, that is to say the strongfacilitates the furfural decarbonylation, as indicated byesorption (Fig. 6b). Barteau have studied the adsorp-of aldehydes on Pd(111) and Pd(110) surfaces by an of TPD and HREELS, mainly for adsorption in the lowe regime[40,41]. At lower temperatures aldehydes tendhe 1(O) conguration (that is, adsorb on themetal sur-h the oxygen lone pair electrons), a weakly held formrbs at low temperatures, typically ca. 200K. A morended 2(C,O) conguration (-bonded through carbonand at-lying) is identied above 200K, and it appearsposition or decarbonylation occurs through this stateer, Barteau proposed that the decarbonylation processs is carried out through the state of 1(C) on Pd(110),nsformed from 2(C,O) [43].basis of furfural-TPSR and furfural-FTIR studies, theond activation sequence is proposed to occur duringarbonylation on Pd catalysts (Fig. 11). Firstly, furfuralin the 1(O) conguration is not stable under our oper-perature and can be transformed easily to 2(C,O)

(2)

(3)

Fig.

conguthroug2(C,Ocan bedesorpSecondative celd indifcufurfurafuran pincreasintenspaniedof furfuC O bcausedcatalysration[43].Hshow tdoped2(C,Oalysts.K-dopewhich

5. Con

In talyststempeto theaK-loalationfuran iwill apO C

O

H

O

O C O

PdPd

2(C,O)

O C

H

O

PdPd

+ CO a

O C

H

O

PdPd

O C+

O

Pd

1(C)

O C+

O

Pd

O

+ CO b

O C

O

H O C+

O1'(C)

O C+

O

O

+ CO c

Pd-K

Pd-K

sorption mode of furfural on Pd/Al2O3 and PdK/Al2O3 catalysts.

on [40]. Then, a small part of furfural decarbonylatesis adsorption mode [42] (Fig. 11a) which is labeled asfural. Accordingly, the decarbonylation product furanined at ca. 67 C, which is observed as the rst furanpeak occurred both on PA and PAK catalysts (Fig. 6b).e lone-pair electrons of carbonyl oxygen displays neg-

e and the surface of Pd also exhibits a negative electricd by K, so the adsorption of carbonyl oxygen is morePAK than on PA. As a result, the amount of 2(C,O)-ll be decreased by K doping, which suggests that theced through2(C,O)-furfuralwill be decreasedwith theK content. As shown in Fig. 6b, it is obvious that the

W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 335 (2011) 7181 81

of K is systematically investigated through several characterizationmethods, which can be summarized as follows: (1) K doping has agreat inuence on Pd2+ precursor phase and the reduced Pd atom.TPR data indicates that there exists a strong interaction between Kand Pd2+, which inhibits the reduction of Pd2+ species. The resultsof CO-FTIR andH2 chemisorption also suggest that the strong inter-action still exists betweenK and the reduced Pd atoms. (2) K dopinghas caused the red-shift of COadsorption,which suggests thatmorebasic nature of K-doped Al2O3 results in an electron transfer to Pd,leading to a higher Pd electron density, and ultimately promotingthe decarbonylation of furfural. (3) The furfural-TPSR and furfural-FTIR results indicate that the doping of K can change the adsorptionmode of furfural on Pd catalysts. Finally, it is important to mentionthat the decrease of2(C,O) congurationmight be themain reasonfor suppressing hydrogenation of furfural on K-doped Pd catalysts.

Acknowledgement

The authors gratefully thank the nancial supports of NaturalScience Foundation of China (No. 20976185).

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A study of furfural decarbonylation on K-doped Pd/Al2O3 catalystsIntroductionExperimentalCatalyst preparationCatalysts characterizationCatalytic test

ResultsCatalysts characterizationMorphology and sample composition (BET, ICP, SEM)H2-TPRH2-TPD and H2 chemisorptionCO-FTIRFurfural-TPSRFurfural-FTIR

Catalytic performance

DiscussionCharacters and activity effect of K on Pd catalystsElectronic effect of K on Pd catalystsAdsorption mode of furfural on PA and PA-K catalysts

ConclusionsAcknowledgementReferences