Catalysis Communications 9 (2008) 2144–2148

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Catalysis Communications

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Dehydration of D-xylose into furfural catalysed by solid acids derived fromthe layered zeolite Nu-6(1)

Sérgio Lima, Martyn Pillinger, Anabela A. Valente *

Department of Chemistry, CICECO, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 February 2008Received in revised form 18 April 2008Accepted 23 April 2008Available online 29 April 2008

Keywords:XyloseFurfuralDehydrationAluminosilicateZeoliteDelaminationSolid acid

1566-7367/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.catcom.2008.04.016

* Corresponding author. Tel.: +351 234 378123; faxE-mail addresses: atav@ua.pt, avalente@dq.ua.pt (A

A delaminated zeolite with Si/Al = 29, obtained by swelling and ultrasonication of a laminar precursor ofNu-6(2), has been examined as a solid acid catalyst for the liquid phase cyclodehydration of xylose to fur-fural at 170 �C, using a water–toluene biphasic reactor system. The final material, with a specific surfacearea about seven times higher than that for proton-exchanged Nu-6(2), gave a reaction rate about twotimes higher than that for H-Nu-6(2), and could be recycled several times without loss of performanceor Al leaching. The furfural yield after 4 h reaction was 47%, compared with 34% in the presence of anH-mordenite sample with Si/Al � 6.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Furfural is an important chemical that is produced on an indus-trial scale from biomass rich in pentosan (e.g., sugar cane bagasse,oat hulls, corn cobs, etc.) [1]. The main outlet of furfural is as chem-ical feedstock for the production of furfuryl alcohol and for other 5-membered oxygen-containing heterocycles such as furan andtetrahydrofuran [1,2]. Most processes of furfural production em-ploy mineral acids as catalysts, especially sulfuric acid, which leadsto serious corrosion and safety problems, difficult catalyst separa-tion from the reaction products, and excessive waste disposal [2,3].For these reasons the production of furfural is one process wherethe demand for green chemistry and technology for sustainability[4,5] is stimulating the replacement of the ‘‘toxic liquid” acid cata-lysts by stable, recyclable, non-toxic solid acids [6]. Some recentprogress has been made using conventional microporous zeolites[7], modified mesoporous silicas [8–11] and exfoliated transitionmetal oxides [12] as catalysts for the dehydration of xylose tofurfural.

The use of conventional microporous zeolites as catalysts forxylose dehydration is compromised by the diffusion limitationsinherent in the relatively narrow pore dimensions. One solutionthat has been developed during the last decade involves the delam-ination of layered precursors of zeolites. The idea is to obtain, in

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: +351 234 370084..A. Valente).

the limit, single crystalline sheets of zeolitic nature with all catalyt-ically active sites being accessible to the reagent molecules. Cormaet al. prepared the materials ITQ-2, ITQ-6 and ITQ-18 by delaminat-ing the precursors of the aluminosilicates MCM-22, ferrierite andNu-6(2) [13–18]. The obtained materials share the acid propertiesand (hydro)thermal stability of zeolites, but possess enhanced spe-cific surface area and porosity as compared with the precursors,leading to improved diffusion and desorption of organic com-pounds, and lower rates of catalyst deactivation. To the best ofour knowledge, ITQ-18 has only been studied as a solid acid alter-native to HCl in the synthesis of an intermediate for the productionof polyurethanes [19].

In the present work, we report on the catalytic performance ofaluminosilicates prepared from the lamellar precursor Nu-6(1),namely alkali and proton-form Nu-6(2), and a material obtainedafter swelling/ultrasonication/calcination of Nu-6(1), in the li-quid-phase cyclodehydration of xylose to furfural, at 170 �C.

2. Experimental

2.1. Catalyst preparation

The lamellar precursor Nu-6(1) with a final Si/Al ratio of 38 (asdetermined by ICP–AES) was synthesised as described previously,using an initial Si/Al ratio (in the gel) of 30 [16]: 1.82 g of 4,4’-bipyridine (Aldrich, 98%) were dissolved in 10.08 g ethanol to givesolution A. 20.06 g sodium silicate (Merck, 8.02% Na2O, 24.92%

S. Lima et al. / Catalysis Communications 9 (2008) 2144–2148 2145

SiO2, 67.05% H2O) were diluted with 13.4 g of deionised water togive solution B. 0.62 g of aluminium sulfate (Merck, 51.34%Al2(SO4)3, 48.66% H2O) and 1.52 g of sulfuric acid (98%) were dis-solved in 22.8 g of deionised water to give solution C. Solution Bwas stirred into solution A followed by solution C. The mixturewas heated in a rotating stainless steel autoclave for 3 days at135 �C. The resultant product was filtered off and washed withca. 500 mL distilled water until a neutral filtrate was obtained.

Before drying, Nu-6(1) (1.5 g) was suspended in 100 mL dis-tilled water and 10 wt.% decyltrimethylammonium hydroxideadded until the pH reached 12. The mixture was stirred for 15 daysat room temperature. After 4 days, the mixture was sonicated in anultrasonication bath (50 W, 40 kHz) for 1 h, and the pH readjustedto 12. This procedure was repeated twice before filtering the resul-tant colloidal dispersion using a Glass MicroFibre Filter GF/A What-man membrane. The solid was washed with water (ca. 200 mL),dried overnight at 55 �C, and calcined under air at 580 �C for 7 h,giving the material denoted as del-Nu-6(1).

Nu-6(2) was prepared by calcining Nu-6(1) under air at 580 �Cfor 7 h [20]. The proton-exchanged Nu-6(2) (denoted as H-Nu-6(2)) was obtained by stirring the calcined material (1 g) in 2 MHCl (10 mL) at 90 �C for 1 h. Finally, the solid was filtered, washedwith distilled water until the pH of the filtrate was near neutraland the AgNO3 test showed the absence of chloride ions, driedovernight at 120 �C, and calcined under air at 450 �C for 6 h.

2.2. Catalyst characterisation

ICP–AES analyses for Na, Al and Si were carried out at the Cen-tral Laboratory for Analysis, University of Aveiro (by Soares andCarvalho). Powder XRD data were collected at room temperatureon a Philips X’pert diffractometer with a curved graphite mono-chromator (Cu-Ka radiation), in a Bragg–Brentano para-focusingoptics configuration (40 kV, 50 mA). Samples were step-scannedin 0.04� 2h steps with a counting time of 6 s per step. The BET spe-cific surface areas (SBET) and specific total pore volumes (Vp) wereestimated from N2 adsorption isotherms measured at �196 �C,using a gravimetric adsorption apparatus equipped with a CI elec-tronic MK2-M5 microbalance and an Edwards Barocel pressuresensor. The solids were outgassed at 350 �C before the measure-ments. SEM images were recorded using a Hitachi SU-70 UHRSchottky FE-SEM instrument. The 27Al MAS NMR spectra weremeasured at 104.26 MHz with a Bruker Avance 400 (9.4 T) spec-trometer. The spectra were acquired using a contact time of0.6 ls, a recycle delay of 0.8 s, and a spinning rate of 15 kHz. Chem-ical shifts are quoted in ppm from AlðH2OÞ3þ6 . Infrared spectra wererecorded on a Unican Mattson Mod 7000 FTIR spectrophotometerequipped with a Specac Golden Gate single reflection ATR system.

2.3. Catalytic experiments

Batch catalytic experiments were performed under nitrogen ina tubular glass micro-reactor equipped with a valve for gas purg-

Table 1Textural and catalytic properties of the aluminosilicate materials

Sample SBETa (m2 g�1) Vp

b (cm3 g�1) Si/Alc Na/Alc

Nu-6(2) 25 0.01 36 3.3H-Nu-6(2) 20 0.01 32 1.6del-Nu-6(1) 151 0.07 29 0.9

a BET specific surface area.b Pore volume calculated from N2 adsorption at p/p0 �0.97.c Molar ratio determined by ICP–AES.d Calculated at 1 h reaction.e Furfural selectivity at 90% xylose conversion.

ing. In a typical procedure, D-xylose (30 mg), powdered catalyst(20 mg) and a solvent mixture (W/T) comprising H2O (0.3 mL)and toluene (0.7 mL) were poured into the reactor. The reactionmixtures were heated with a thermostatically controlled oil bathand stirred magnetically at 600 ppm. The influence of the stirringrate on the initial reaction rate becomes negligible over 600 rpmand the desired reaction temperature was reached within the firstminute of reaction. Zero time was taken to be the instant the mi-cro-reactor was immersed in the oil bath. The products in theaqueous and organic phases were quantified using two HPLCequipments as described elsewhere [8–12]. The experimental errorin these values is estimated to be about 5%.

3. Results and discussion

3.1. Catalyst preparation and characterisation

The lamellar precursor Nu-6(1) with a bulk Si/Al ratio of 38 wasprepared using 4,40-bipyridine as a structure-directing agent. Calci-nation of Nu-6(1) gave Nu-6(2) with no significant change in theSi/Al ratio (Table 1). A proton-exchanged material, denoted as H-Nu-6(2), was prepared by treatment of Nu-6(2) with dilute HCl.The material referred to as del-Nu-6(1) was obtained after re-peated sonication of an aqueous suspension of Nu-6(1) interca-lated by decyltrimethylammonium ions [16,18]. A final Si/Al ratioof about 30 was found for H-Nu-6(2) and del-Nu-6(1), and theNa/Al ratio decreased in the order Nu-6(2) > H-Nu-6(2) > del-Nu-6(1) (Table 1).

The good agreement between the experimental and simulatedpowder XRD patterns for Nu-6(1) and Nu-6(2) confirms that thesynthesised materials have the desired structures (Fig. 1). Compar-ing the patterns for Nu-6(2) and del-Nu-6(1), it is evident that thelatter material contains structure units similar to those of Nu-6(2).Corma et al. noted a bigger difference between the patterns for Nu-6(2) and their delaminated material, called ITQ-18, with the pat-tern of ITQ-18 being of poorer quality and showing an additionallow angle peak at ca. 3� 2h uncharacteristic of Nu-6(2). As foundin a recent study by Zubowa et al. on the expansion and exfoliationof Nu-6(1) [18], the material del-Nu-6(1) did not exhibit the addi-tional low angle peak.

SEM images of Nu-6(2) and H-Nu-6(2) reveal large (ca. 8–10 lm) aggregates of thin flake-like crystallites (Fig. 2). Theimages of del-Nu-6(1) show a general breakdown of the aggre-gates, as well as some additional material with an amorphouscharacter.

A specific surface area of 151 m2 g�1 was determined for del-Nu-6(1), which is six to seven times higher than that measuredfor Nu-6(2) and H-Nu-6(2) (Table 1). Although this fractional incre-ment is in agreement with the ca. 7-fold increase described by Cor-ma et al. [16] and Zubowa et al. [18] in their studies of thetransformation of Nu-6(1) into ITQ-18, the absolute values foundin the present work are between one third and a quarter of thosereported by these workers. Nu-6(2) is a narrow pore zeolite with

Activityd ðmmol g�1cat h�1Þ Furfural selectivity at �90% conversione

6.2 241.7 503.8 53

Fig. 1. Powder XRD patterns of the as-synthesised layered precursor Nu-6(1) and ofthe aluminosilicate materials derived from it. The patterns Nu-6(1)-sim and Nu-6(2)-sim were simulated from the available crystallographic data [20].

Fig. 2. SEM images of (a) Nu-6(2), (b) H-Nu-6(2) and (c) del-Nu-6(1).

2146 S. Lima et al. / Catalysis Communications 9 (2008) 2144–2148

elliptical pore diameters of 3.2 � 4.3 Å and 2.4 � 4.8 Å [20]. In Refs.[16] and [18], the higher specific surface areas were mainly due toan increase in the external surface area, and it was proposed thatthe space between the ‘broken layers’ formed a ‘mesopore-like’volume. In the present work, the external specific surface area ofdel-Nu-6(1) was estimated to be 6 m2 g�1 using the t-plot method,indicating that essentially micropores were formed as a result ofthe intercalation and ultrasonication treatments.

The presence of framework Al in the Nu-6(2) and del-Nu-6(1)materials was confirmed by the 27Al MAS NMR spectra (notshown), which all contained a resonance at about 52 ppm for Alin tetrahedral coordination. An additional weak peak at about0 ppm for del-Nu-6(1) indicated the presence of some extraframe-work octahedral Al (IT/IO = 100/25). By comparison with similar re-sults for ITQ-2 [21], prepared via swelling and exfoliation of theprecursor to MCM-22, the extraframework Al species are probablyformed during the final calcination step.

The IR spectra of Nu-6(2) and del-Nu-6(1) showed bands at 545and 580 cm�1 characteristic of the zeolitic structure (electronicsupplementary material). In agreement with the spectrumreported by Zubowa et al. for ITQ-18 [18], the spectrum of del-Nu-6(1) showed an additional weak shoulder centred around940 cm�1, which can be ascribed to terminal �SiO� groups formedupon delamination [16,18].

From the above results we may infer that the swelling and sub-sequent ultrasonication of the precursor Nu-6(1) with Si/Al = 38 isessentially a fragmentation process resulting in exfoliated andother fragments, which upon calcination transform into Nu-6(2)structure units. Maheshwari et al. reached similar conclusions forthe swelling and exfoliation of the layered precursor of zeoliteMCM-22 [22]. According to Frontera et al. [21], the aluminium con-centration in the parent material is likely to influence the eventualoutcome of the whole process. In the present work the exfoliation

appears to have proceeded to a small degree compared with thatreported previously [16,18], resulting in a material with distincttextural properties. This may be related with the lower Si/Al ratioused [21].

3.2. Catalytic cyclodehydration of xylose into furfural

The H-Nu-6(2) and del-Nu-6(1) materials are active catalysts inthe conversion of xylose in aqueous phase at 170 �C, using toluene

Fig. 3. Kinetic profiles of xylose conversion (A) and dependence of furfural select-ivity on xylose conversion (B) in the presence of Nu-6(2) (s), H-Nu-6(2) (D) and del-Nu-6(1) (�), at 170 �C in W/T.

Scheme 1.

S. Lima et al. / Catalysis Communications 9 (2008) 2144–2148 2147

as a co-solvent for the extraction of furfural formed during thereaction. These operating conditions were chosen because theone-pot process of reaction of xylose or fructose plus separationof the product, furfural or hydroxymethylfurfural, leads to higheryields of the desired furan product than if only water is used as sol-vent [8,23]. Without catalyst, the reaction in the aqueous phasegave a maximum of 5% furfural yield during 6 h; throughout thediscussion the results have not been corrected for this non-cata-lytic contribution.

The reaction conditions were optimised by performing preli-minary experiments on the effects of temperature, substrate con-centration and amount of catalyst on the catalytic performanceof del-Nu-6(1) after 1 h reaction: (i) Increasing the reaction tem-perature from 140 to 170 �C increases the conversion and selectiv-ity by a factor of about 5 (with an increase in furfural yield from 1%to 16%); (ii) increasing the amount of xylose from 5 to 30 mg per20 mg catalyst increases the conversion by a factor of about 2.5(with an increase in furfural yield from 4% to 16%), but withamounts above 30 mg the conversion tends to level off; (iii)increasing the amount of catalyst from 5 mg to 20 mg per 30 mgxylose increases the conversion by a factor of about 2.5 (with an in-crease in furfural yield from 3% to 16%).

The kinetic profiles of xylose conversion at 170 �C and the con-version dependent curves of selectivity for Nu-6(2), H-Nu-6(2) anddel-Nu-6(1) are shown in Fig. 3. As noted in other studies with so-lid acid catalysts [8–10,12,24], selectivity tends to increase withxylose conversion, for conversions up to at least 60%. Mechanisticfactors are the likely cause of this behaviour. The cyclodehydrationof xylose follows a complex reaction mechanism with a series ofelementary steps involving the liberation of three water moleculesper xylose molecule converted into furfural [2,3]. The undesiredby-products result mainly from consecutive condensation reac-tions between furfural and intermediates of the xylose-to-furfuralconversion (Scheme 1) [2].

In the presence of (alkali-form) Nu-6(2) the reaction of xylose isnearly complete at 6 h, but furfural selectivity is quite low (<25%up to 97% conversion). Nu-6(2) probably possesses accessible basicsites, which may favour the aldolisation decomposition of the car-bohydrate to give oligomeric acid products [7,25]. The reaction ofxylose in the presence of H-Nu-6(2) is slower than that observedfor as-synthesised Nu-6(2), but the furfural selectivity is a factorof 2 higher at ca. 90% conversion (Table 1). A possible explanationfor this is that the exchange of sodium ions for protons (Table 1)enhanced the acidity of the material. When del-Nu-6(1) is usedas a catalyst the conversion rate is enhanced significantly com-pared with H-Nu-6(2), giving a maximum of 58% furfural selectiv-ity at 63% conversion (Fig. 3). After 4 h reaction, the furfural yieldsare 47% for del-Nu-6(1) and 20% for H-Nu-6(2). Furfural yieldsclose to 50% have also been achieved using the protonic form ofaluminium-containing MCM-41-type mesoporous silica (Si/Al = 12, SBET = 649 m2 g�1) as catalyst and the same reaction mix-ture composition (6 h, 160 �C, W/T) [11]. In studies with conven-tional microporous zeolites as catalysts for this reaction at 170 �Cin W/T, mordenite was the best zeolite in terms of furfural selectiv-ity [26]. We found that mordenite with Si/Al �5 gave 34% furfuralyield at 4 h, compared with 47% with del-Nu-6(1), under similarreaction conditions.

The superior catalytic performance of del-Nu-6(1) comparedwith H-Nu-6(2) can be ascribed to the considerably higher SBET,which would give a greater number of effective acid sites. A similarcorrelation was observed for the cyclodehydration of xylose in thepresence of layered titanate, niobates and titanoniobates, and thecorresponding exfoliated–aggregated materials with higher SBET

[12]. The most active catalyst was exfoliated H4Nb6O17

(SBET = 136 m2 g�1), which gave 54% selectivity at ca. 90% conver-sion, at 160 �C in W/T, using the same reaction mixture composi-

tion as that used in the present work. The hexaniobate catalystgave 9:1 mmol g�1

cat h�1, compared with 3:8 mmol g�1cat h�1 for del-

Nu-6(1).

Fig. 4. Xylose conversion and furfural yield obtained in recycling runs (run 1 – hash,run 2 – bricks, run 3 – diamonds) with del-Nu-6(1) (6 h runs, 170 �C), Al-MCM-41(6 h runs, 160 �C [11]) and exfoliated–aggregated eH4Nb6O17 (1 h runs, 160 �C [12])solid acid catalysts.

2148 S. Lima et al. / Catalysis Communications 9 (2008) 2144–2148

The catalytic stability of del-Nu-6(1) was investigated by usingthe solid in three consecutive 6 h runs. Prior to reuse, the catalystwas separated from the reaction mixture by centrifugation,washed thoroughly with methanol and acetone, and dried at100 �C overnight. In order to facilitate the removal of strongly ad-sorbed/entrapped coke, the recovered solid was calcined at 350 �Cunder air for 3 h. Furfural selectivity decreased marginally from thefirst to the third run, and xylose conversion decreased to a factor of0.95 (Fig. 4). The powder XRD pattern of the used catalyst is similarto that of the fresh catalyst (Fig. 1), and SBET decreased 16%. Forcomparison, Fig. 4 shows the catalytic results reported previouslyfor the mesoporous H-Al-MCM-41 [11] and exfoliated H4Nb6O17

[12] mentioned above, using the same reaction mixture composi-tion. Referring to the changes in xylose conversion and furfuralyield between runs, the catalytic performance of del-Nu-6(1) issteadier than that of H4Nb6O17, and similar to that of H-Al-MCM-41. ICP–AES indicated no measurable loss of Al from del-Nu-6(1)during the catalytic reactions, similar to that found for H-Al-MCM-41 [11].

4. Conclusions

In the present work, the layered aluminosilicate Nu-6(1) wassubjected to an exfoliation procedure and, after calcination, amaterial comprising Nu-6(2) structure units was obtained, butwith a specific surface area about seven times higher than thatmeasured for H-Nu-6(2) prepared by proton-exchange of calcinedNu-6(1). The material, designated as del-Nu-6(1), is a promisingalternative to conventional zeolites or mesoporous materials forthe production of furfural, using a biphasic reactor system for pro-cessing xylose monosaccharide. Compared with zeolite Nu-6(2) (inthe protonic form), del-Nu-6(1) is considerably more active (initialactivity 3:8 mmol g�1

cat h�1 vs. 1:7 mmol g�1cat h�1 for H-Nu-6(2)) and

somewhat more selective to furfural at 40–90% xylose conversion,giving a furfural yield of 47% after 4 h at 170 �C. The catalytic effi-ciency of del-Nu-6(1) is mainly attributed to the easier accessibil-ity of its active sites. Although materials such as exfoliatedniobates have been shown to be more active, del-Nu-6(1) is morestable in recycling runs (with no measurable Al leaching), and cat-alysts obtained by the delamination of aluminosilicates are likelyto be economically more attractive. The catalytic performancemay be further improved by optimising the Si/Al ratio and thedelamination procedure.

Acknowledgements

This work was partly funded by the FCT, POCTI and FEDER (Pro-ject POCTI/QUI/56112/2004). The authors wish to express theirgratitude to Prof. C.P. Neto for helpful discussions, Dr. Zhi Lin (CIC-ECO) for supplying a mordenite sample, Dr. F. Domingues (Depart-ment of Chemistry) for access to HPLC equipment, and M.F. Lucasfor assistance in the HPLC analyses. S.L. is grateful to the FCT fora post-doctoral Grant.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.catcom.2008.04.016.

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