Furfural production from xylose using sulfonic ion-exchange resins (Amberlyst) and simultaneous stripping with nitrogen

Bioresource Technology 102 (2011) 7478–7485

Contents lists available at ScienceDirect

Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Furfural production from xylose using sulfonic ion-exchange resins (Amberlyst) and simultaneous stripping with nitrogen I. Agirrezabal-Telleria ⇑, A. Larreategui, J. Requies, M.B. Güemez, P.L. Arias Department of Chemical and Environmental Engineering, Engineering School of the University of the Basque Country, Alameda Urquijo s/n, Bilbao 48013, Spain

a r t i c l e

i n f o

Article history: Received 31 March 2011 Received in revised form 6 May 2011 Accepted 8 May 2011 Available online 14 May 2011 Keywords: Furfural Process optimization Xylose dehydration Amberlyst Stripping

a b s t r a c t The aim of this work deals with the development of new approaches to the production of furfural from xylose. It combines relatively cheap heterogeneous catalysts (Amberlyst 70) with simultaneous furfural stripping using nitrogen under semi-batch conditions. Nitrogen, compared to steam, does not dilute the vapor phase stream when condensed. This system allowed stripping 65% of the furfural converted from xylose and almost 100% of selectivity in the condensate. Moreover, high initial xylose loadings led to the formation of two water–furfural phases, which could reduce further purification costs. Constant liquid–vapor equilibrium along stripping could be maintained for different xylose loadings. The modeling of the experimental data was carried out in order to obtain a liquid–vapor mass-transfer coefficient. This value could be used for future studies under steady-state continuous conditions in similar reactionsystems. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Furfural (FUR) is a versatile key derivative produced from pentosan-rich biomass (Mamman et al., 2008; Mansilla et al., 1998). It is considered a selective solvent for organic compounds and serves as a building-block for its hydrogenation to furfuryl alcohol, for components of P-series fuels or liquid alkanes (Chheda et al., 2007; Weingarten et al., 2010). It is produced from the hemicellulosic fraction of the biomass. This fraction is produced through hydrolysis processes (Chareonlimkun et al., 2010; Matsumoto et al., 2011; Wang et al., 2011). During the initial stage of hemicellulose hydrolysis, the xylans generate pentose carbohydrates, which are further cyclodehydrated to furfural. Commercially, FUR is produced using sulfuric acid as homogeneous catalyst. Moreover, high steam to product ratio is used in order to strip the FUR and to avoid its further degradation (Mamman et al., 2008). However, these conditions show several drawbacks: high vapor product dilution leading to expensive purification stages, safety issues and environmental problems due to toxic waste effluents. For these reasons, the improvement of appropriate chemical technology remains of great interest for the growth of furan-based industry. Optimization studies of the current processes aim to reduce furfural degradation reactions operating at 230 °C (SUPRATHERM process), reducing yield–loss reactions (Zeitsch, 2000). In the SUPRAYIELD process, higher product purity is obtained using adia⇑ Corresponding author. Tel.: +34 61 7912 295; fax: +34 94 6014 179. E-mail address: [email protected] (I. Agirrezabal-Telleria). 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.05.015

batic flash distillation, leading to yields of up to 65% in the FUR vapor phase (Arnold and Buzzard, 2003). Nevertheless, 70% of the total industrial FUR production is still based on the fixed-bed reactor process using steam as stripping agent and reaching an overall yield of 50%. Recently, alternative liquid solvents instead of water have been researched, such as dimethylsulfoxide (Dias et al., 2005b), ionicliquids (Lima et al., 2009; Zhao et al., 2007) or supercritical CO2 (Kim and Lee, 2001). All of them have shown efficient solvent capacity using carbohydrates as model compounds. Biphasic systems using water and butanol, methyl isobutyl ketone or toluene (Román-Leshkov et al., 2006; Zhang et al., in press) as phase modifiers have been also applied to the dehydration of xylose (Dias et al., 2005a,b, 2006b; Lima et al., 2008; Moreau et al., 1998). Additional solvents show high capacity as isolating ‘‘chambers’’, where secondary reactions are minimized and as a result selectivity to FUR is improved. However, they present significant drawbacks: high furfural–solvent separation costs and their use would require separate hydrolysis and dehydration steps (Mamman et al., 2008). Another important field of research deals with the development of novel catalysts. Recent studies report hydrothermally stable heterogeneous catalysts, with strong acid capacity and shape/ adsorption selectivity for the dehydration of xylose as model compound. Zeolitic structures range from ZSM-5 in its protonated form (O’Neill et al., 2009) or layered zeolites with high active site accessibility (Lima et al., 2008) to H-mordenite/faujasite-HY type arrangements (90% selectivity at 30% XX) (Moreau et al., 1998). Other studies achieved a maximum yield of 55% using crystalline

7479

I. Agirrezabal-Telleria et al. / Bioresource Technology 102 (2011) 7478–7485

Nomenclature [X]0 [X]R Xx A70 A70L w [FUR]0 [FUR]R [FUR]T RI

initial xylose loading (%) xylose concentration in the reactor (g L1) xylose conversion (%) amberlyst 70 A70 loading fraction with respect to [X]0 (%) A70 mass load in the reactor (g) Initial FUR loading in the reactor (g L1) FUR concentration in the water-phase in the reactor (g L1) FUR concentration in the toluene phase (g L1) ratio of [FUR]T:[FUR]R in biphasic systems

layered exfoliated transition metal oxides in the H+ form (Dias et al., 2006b). Sulfonated MCM-41 achieved selectivities around 85–95% in water–toluene biphasic systems (Dias et al., 2005a) and micro–mesoporous materials of niobium-doped silicates showed good reusability (Dias et al., 2006a). However, the final FUR yield in water-phase systems is still limited to 50% due to secondary reactions. The objective of this work is to test, in semi-batch conditions, the use of nitrogen as stripping agent combined with relatively cheap acid heterogeneous catalysts. Nitrogen, compared to steam, shows several advantages as stripping agent for water–furfural mixtures: it is easily separable from the condensed vapor-phase stream, facilitating its reusability, and could strip a vapor stream (when condensed) richer in FUR than when steam is used, reducing further purification costs. Xylose will be used as model compound to simplify the reaction mechanism and its dehydration will be catalyzed by commercially available pure Brønsted acid sulfonic resin (Amberlyst 70). This catalyst features a low surface area (1 m2/g) but it shows enough acid capacity in order to study the reaction pathway and stripping efficiency. Kinetic data will be also studied in aqueous-phase and biphasic systems using toluene. Finally, the experimental data will be modeled to estimate the kinetic and stripping parameters. 2. Methods 2.1. Materials and reagents Furfural (99%) and D-xylose (minimum 99%) were purchased from Sigma–Aldrich. Amberlyst 70 was kindly obtained from Dow Chemical. Acetonitrile (HPLC-UV grade) was purchased from Panreac and NaOH (50 wt.% purity) was purchased from J.T. Baker.

[FUR]C FURY KS VR VC

dV R dt

k k0

FUR concentration in the condensate for nitrogen-stripping (g L1) FUR yield (%) stripping constant for liquid–vapor mass transfer (L min1) liquid volume in the reactor (L) total volume of stripped condensate (L) volume change of liquid in the reactor due to stripped vapor (L min1) kinetic constants in the absence of catalysts kinetic constants with respect to A70 mass load

mass-flow-controlled nitrogen at room temperature was bubbled into the liquid bottom at 150 mL min1 and 8 bar absolute pressure. This gas flow stripped the water–furfural vapor stream. The vapor flow was later fed to a condenser (cooled by Peltier effect at 10 °C) and the condensate was continuously weighted (precision 0.01 g). Each test was finished when the reactor volume was nearly zero. Liquid samples were taken at specific time intervals using a manual needle valve. Automatic control valves were used to regulate the reactor-pressure and liquid level in the condenser. For kinetic batch reaction studies (at constant volume), monophasic (water) and biphasic systems of water/toluene at 1:1 v/v were also tested.

2.3. Product analysis Reagents and products in the reactor and in the condensate were quantified using the HPLC module ICS-3000 from DIONEX coupled to an AS40 Autosampler. Xylose was quantified using a CarboPac PA20 3  150 mm column, at 30 °C and 0.5 mL min1, using 8 mM of NaOH as mobile phase. Detection was performed using an electrochemical cell, with integrated amperometry and Standard Carbohydrate Quad method. FUR was quantified using a Kinetex C18-XB 150  4.6 mm column from Phenomenex at 40 °C and coupled to a UV-2070 Plus detector from JASCO at 280 nm wavelength. The mobile phase was pumped at 1 mL min1 and consisted of 0.01 M H2SO4 and 10% v/v acetonitrile aqueous solution. Conversion of xylose (XX) and FUR product yield (FURY) were calculated using the equations below:

X X ð%Þ ¼ 1  ðg of xylose at time tS Þ=ðinitial g of xyloseÞ FURY ð%Þ ¼ðg of FUR at time t S Þ= 

2.2. Kinetic and stripping testing The kinetic and stripping tests were performed in a 2 L stirred reactor (Autoclave Engineers), with controlled electric-heating temperature (reactor unit designed by PID Eng&Tech). Moreover, high temperature–pressure ZrO2 pH electrodes were installed in the liquid phase. In a typical experiment operating at 175 °C and 8 bar, the reactor was first loaded with the desired amount of Amberlyst 70 (as received) and heated up to 190 °C with deionized water as solvent (75% of total initial volume). The rest 25% of total volume was fed from a nitrogen pressurized vessel, containing fixed compositions of xylose. This set-up allowed the xylose solution to be held at room temperature until the desired temperature was reached in the reactor and minimize initial xylose degradation. This procedure facilitated to end with a mixture quiet close to the desired constant reaction temperature (175 °C). For the semi-batch stripping tests,

ðg of potential FUR fromreacted xylose Þ

⁄

Potential furfural production (g) is 64% with respect to initial xylose mass load (g). Secondary product detection by GC–MS was performed using a HP 19091S-105 column from Agilent, 1 mL min1 of helium and an injection volume of 1 lL. The Thermogravimetric Analysis (TGA) of fresh and used Amberlyst 70 catalysts were performed in the TGA/ SDTA 851e module (Mettler-Toledo), measuring the weight change during oxidation at a heating ramp of 10 °C min1 and final temperature of 550 °C. Catalyst acid capacity was measured using the Cation Salt Splitting Capacity method (MTM 0230) provided by Dow Chemical. Firstly, sulfonic acid sites were activated with 1 N HCl. Later the protonated groups were substituted by Na+ (with

7480

I. Agirrezabal-Telleria et al. / Bioresource Technology 102 (2011) 7478–7485

1 N NaNO3) and leached in the HNO3 form. In order to know the acid capacity, the filtrate was titrated with 0.1 N NaOH.

Table 1 XX and FURY for aqueous and biphasic systems in the absence of catalyst and A70 as catalyst. [X] (%)

3. Results and discussion 3.1. Batch kinetic studies 3.1.1. In the absence of catalyst Kinetic studies of xylose dehydration were performed for [X]0 of 1, 3, 5 and 7 wt.%. Non-catalyzed xylose conversion rate to FUR has been reported as quasi instantaneous (60 s) at temperatures of 250 °C (Antal et al., 1991). However, reaction kinetics are better evaluated at lower conversion rates (relatively low temperatures). In this work, xylose and FURY evolution for non-catalyzed systems were studied at 175 °C (Fig. 1). The experimental tests showed no significant [X]0 influence, obtaining FURY of 65% and XX of 80% after 6 h of reaction (see Table 1). The start-up of xylose dehydration is catalyzed by the H3O+ ions present in water at the given temperature and later by decomposition products, such as acids. At low xylose conversion rates, slow side-reaction rates allowed high FURY, but long reaction periods promoted the yield–loss reactions listed below (Zeitsch, 2000): Lignocellulosic biomass hydrolysis (k0): Xylose cyclodehydration (k1): Side-reactions (k2):

Furfural resinification (k3):

1 3 5 7

In the absence of catalyst

Amberlyst 70 (60% A70L)

Waterphase

Water phase

Water/toluene biphasic

Water/toluene biphasic

XX

FURY

XX

FURY

XX

FURY

XX

FURY

80 79 83 82

69 63 59 59

76 77 81 80

73 75 74 75

83 93 96 97

38 9 5 2

81 90 94 94

54 37 46 44

Reaction conditions: batchwise at 175 °C and biphasic systems 1:1 v/v water/ toluene. XX: conversion of xylose after 6 h of reaction for non-catalyzed systems and 4 h of reaction for A70 catalyzed.

Biomass ? Xylose Xylose ? Furfural Xylose (interm.) + Furfural ? Condensation byproducts Furfural ? Furan resins

A similar kinetic study was performed using 1:1 v/v biphasic phase of water/toluene at 175 °C. The effect of FUR extraction is direct on the final yield, and [FUR]T showed an increasing trend even at long reaction periods (Fig. 2). Final FURY of 75% was obtained in the whole xylose concentration range (Table 1). FUR resinification effects in toluene (in the absence of catalyst) were checked as negligible (4% of degradation at 175 °C after 10 h of reaction). Compared to monophasic systems, XX was lower due to minimized condensation side-reactions: when FUR is extracted from the water-phase, reactions of xylose degradation with FUR are reduced, thus XX was also reduced. Moreover, mass-transfer rate of

Fig. 1. Experimental evolution of FURY (bullets) and model (line) at different [X]0 for non-catalyzed reaction systems at 175 °C.

Fig. 2. Evolution of [FUR]R and [FUR]T (g L1) in the reactor at different [X]0 for noncatalyzed reaction systems at 175 °C using biphasic water/toluene (1:1 v/v).

FUR through the interphase was observed to be proportional to [FUR]R and [FUR]T, since at different [X]0 an average value of 2.6 ± 0.2 was achieved for RI (obtained from Fig. 2). Toluene and FUR are practically miscible in the whole concentration range, so as observed in the literature (Mamman et al., 2008), toluene improved remarkably the FURY. 3.1.2. Catalyzed by A70 Xylose dehydration catalyzed by A70 sulfonic ion-exchange resin was also tested at 175 °C under batch conditions. The presence of A70 accelerated the initial XX due to its high Brønsted acid capacity. In the first instance, A70L of 60% was chosen. At intermediate [X]0, FUR resinification and condensation reactions were enhanced, ending up with a final FURY of 5%. However, low [X]0 (1 wt.%) decreased side-reaction effects and higher yields could be achieved. Contrary to the observed using non-catalyzed systems, XX was faster at increasing [X]0 and practically completed after 4 h of reaction. This can be explained by enhanced xylose decomposition activity at higher [X]0. FURY evolution in the reactor showed a maximum peak (Fig. 3a), where net FUR production/degradation was zero (XX of 26% and 70% of FURY). At this point, FURY using A70 catalyst is similar to the reported for other catalysts operating at similar temperatures, but it would have limited industrial applications because of such low XX. Indeed, FURY dropped considerably at long reaction times, leading to the formation of insoluble by-products in the liquid phase and an oligomer layer on the A70 surface.

7481

I. Agirrezabal-Telleria et al. / Bioresource Technology 102 (2011) 7478–7485

Fig. 3a. Experimental evolution of FURY (bullets) and model (line) at different [X]0 catalyzed by A70L of 60% at 175 °C.

Studies on FUR resinification have been performed under the influence of homogeneous catalysts (Zeitsch, 2000). In order to evaluate the effect of A70’s acid sites on FUR resinification, xylose dehydration conditions were applied to [FUR]0 of 25 g L1. These resinification effects followed an Arrhenius type reaction. Logarithmic of [FUR]R dropped linearly during time (almost 95% FUR decomposed after 6 h at 175 °C), leading to the formation of furan resins. This is one of the reasons for obtaining such low FURY using A70. The A70L effect was also studied for [X]0 3 wt.%. A70L of 40% and 60% showed similar FUR formation rates. As shown in Fig. 3b, the acid capacity of A70 sites remains high over the testing period, decreasing final [FUR]R. However, A70L of 20% might be optimum for 4 h of reaction time to obtain a final FURY of 33%. Similar yield maxima as at varying [X]0 (Fig. 3a) were obtained at XX of 28%. According to the GC–MS analysis, the main by-products in the water-phase were glycolaldehyde (a 2-carbon monosaccharide), dihydroxyacetone and decomposition products such as acetic and formic acid. FUR was also converted to 4-oxo-5-methoxy-2-penten-5-olide or reacted to form higher molecular weight coke deposits. Moreover, other decomposition trace sugars were also detected using integrated amperometry:lyxose (open chain isomer from xylose) or other C5 reagent impurities such as arabinose.

TGA analysis of A70 under an O2 atmosphere showed significant weight loss peaks. The sulfonic acid groups of fresh catalysts were converted to SO2 at 291 °C and no further significant weight loss was observed. However, used A70 samples showed two prominent peaks: one for acid site decomposition at 291 °C and the other one at 400 °C for coke gasification present on A70’s surface. According to the MTM 0230 method, the fresh A70 showed an acid capacity of 2.55 eq kg1. Catalyst stability after the reaction was checked using the MTM0230 method and it reused testing again with A70L of 60% and [X]0 of 3 wt.% at 175 °C. As observed in Table 2, catalyst acid capacity decreased considerably and thus xylose conversion in the reaction. Final FURY remained also low (12%). The addition of toluene as co-solvent considerably increased FURY as compared to water-phase systems, proving the high extracting capacity of toluene. FURY values were increased significantly at high [X]0 (Table 1), but these were still below non-catalyzed systems. This effect might be reasoned by two simultaneous effects: higher FUR formation rate than its extraction to the toluene phase and resinification effects taking also place in the toluene phase when FUR droplets contact the catalyst surface due to vigorous stirring. This last effect has not been published in other studies and it was checked for A70 in this study. Using only toluene as solvent, 20% of FUR was resinified at the same testing conditions used when water was the solvent (60% of A70L and [FUR]0 25 g L1). This effect should always be included in further kinetic biphasic tests if co-solvent liquid-phase contacts catalyst particles. As well as in non-catalyzed systems, lower XX rates than in water-phase systems were observed. 3.1.3. Modeling of the kinetic data A mathematical model was developed in order to obtain further insights during the kinetics of xylose dehydration and secondary reactions. MATLAB (version 7.5) was used as calculating software. The experimental data were fitted to the differential equations model using different Ordinary Differential Equation (ODE) solver types. The modeling equations of the kinetics in water-phase systems were based on the simplified reactions described in Section 3.1.1:

d½X R  ¼ k1  ½X R   k2 ½X R   ½FURR  dt

ð1Þ

d½FURR ¼ k1  ½XR  k2  ½XR  ½FURR  k3  ½FURR : dt

ð2Þ

Furfural resinification effects in the absence of catalyst at 175 °C showed a k3 value of 3.0E-04 min1 (obtained from the linear regression of [FUR]R degradation after 10 h of reaction) and it was used as a fix parameter in the model equations. The other k1 and k2 constants were iterated until reaching the tolerance range specified as sum of least-squares. As observed in Fig. 1, the model results and the experimental data were in good agreement, obtaining an average Standard Deviation of 13%. The ODE solver must be adjusted depending on the stiffness of the raw data: [X]0 at 1 wt.% required stiffer solvers (ODE23) than the data modeled at higher [X]0 (ODE45). Table 3 summarizes the kinetic constants with respect to [X]0. As a result, there was no evidence of significant variations among non-catalyzed xylose loading. Other research studies (Jing Table 2 Effect of fresh and used A70 catalyst on the acid capacity and the XX at 175 °C for [X]0 of 3 wt.% and A70L of 60% in water-phase.

Fig. 3b. Experimental evolution of FURY (bullets) and model (line) at 175 °C for [X]0 3 wt.% at different A70L.

A70

Acid capacity (eq. kg1)

Final XX (%)

Fresh Used

2.55 1.27

93 48

7482

I. Agirrezabal-Telleria et al. / Bioresource Technology 102 (2011) 7478–7485

Table 3 Kinetic constants for xylose dehydration in the absence of catalyst and using A70. [X]0 (%)

In the absence of catalyst Water-phase

3 5 7

Amberlyst 70 (60% A70L) Water/toluene biphasic

Water-phase

Water/ toluene biphasic

103k1

103k2

103k1

103k2

k4

103  k3

103  k2

103  k1

103  k2

103  k4

3.10 2.76 2.92

0.22 0.14 0.11

4.20 2.51 3.72

0.17 0.26 0.41

8.80 1.61 5.72

0.36 0.32 0.36

0.026 0.013 0.014

0.30 0.47

3.01 2.70

4.50 2.60

0

0

0

0

0

Reaction conditions: batchwise at 175 °C and biphasic systems 1:1 v/v water/toluene. 0 k3: 3.0E-04 min1, k3 : 4.5E-04 min1 g1 A70 . 0 0 0 1 1 1 Units ) k1 and k4: min1, k2: L min1 g1, k1 and k4 : min1 g1 g gA70 . A70 , k2 : L min

Table 4 Influence of A70L on the water-phase kinetic parameters at 175 °C for [X]0 = 3 wt.%. 0

A70L (%)

104  k1 (min1 g1)

105  k2 L min1 g1 g1 A70

20 40 60

3.6 + 0.04 5.1 + 0.01 7.5 + 0.50

2.48 + 0.05 3.50 + 0.25 2.12 + 0.60

0

and Lu, 2007) reported a k1 value of the same order of magnitude (2.2E-03 min1), extrapolated to 175 °C under high pressure conditions. The same model was applied to the studies performed with 60% of A70L. In this case, the constants (k0 ) were represented with respect to A70L and catalyst mass load (w) was also introduced into the equations. Under these conditions, a fixed FUR resinification 0 1 constant k3 was obtained: 4.5E-04 min g1 A70 (Degradation of [FUR]R at 175 °C after 6 h of reaction, as explained in Section 3.1.2). As an example, at [X]0 of 1 wt.%, FUR resinification rate was 13 times faster than in the absence of A70 and condensation reactions were also enhanced 9 times. This resulted in lower final FURY for A70 catalyzed systems. Contrary to [X]0 effects, different A70L for 3 wt.% [X]0 showed a non-proportional relationship. As observed in Section 3.1.2, the test using A70L of 20% showed higher FURY than the ones obtained using bigger loadings. According to the kinetic parameters presented in 0 Table 4, xylose dehydration reactions (k1 ) were also enhanced at the lowest A70L. This effect could be reasoned by the excess of coke formation on the A70 surface at high A70L, whereas catalyst deactivation was slower for the lowest A70L. This effect was not evidenced operating in homogeneous conditions (Zeitsch, 2000), where xylose dehydration showed order one with respect to sulfuric acid concentration. The kinetic modeling of the biphasic systems in the absence of catalyst (not shown here) were modeled inserting an additional k4 constant into Eq. (2) (see Table 3). This constant represented the diffusion of FUR through the water–toluene interphase. According to the literature (Weingarten et al., 2010), diffusion occurs in both directions and furfural mass-transfer is proportional (order one) to FUR concentration gradients. The relation between [FUR]T and [FUR]R (RI: 2.6, obtained from Fig. 2) was also introduced in the model equations:

d½FURR ¼ k1  ½XR  k2  ½XR  ½FURR  k3  ½FURR  k3 dt k4  ½FURT  ½FURR þ RI

ð3Þ

3.2. Semi-continuous stripping of furfural 3.2.1. In the absence of catalyst Preliminary furfural stripping tests were performed in the absence of catalysts. During FUR stripping tests, xylose was

initially converted to FUR, but it was not stripped until liquid saturation was achieved. For all the temperatures tested, after system stabilization, the first condensate droplets were obtained. At the lowest studied temperature (150 °C), reaction volume-reduction was not compensated by xylose conversion rate, leading to high final [X]R and thus accelerated side-reactions. However, the absence of a catalyst was compensated at higher temperature (200 °C), leading to FURY up to 65% (Table 5). Higher [X]0 (10 wt.%) at 200 °C using nitrogen as stripping agent led to the formation of two visible water–furfural phases. Indeed, increased FUR efficiency at 200 °C could be supported by the ‘‘entropy effect’’ (Zeitsch, 2000): the formation of oligomers decreases the entropy and the Gibbs free energy becomes less negative. When reaction temperature is increased, this leads to DG  0 (at ceiling temperature TC). At T > TC, polymerization becomes negligible and fragmentation occurs, increasing the final FURY. 3.2.2. Catalyzed by A70 Based on the preliminary xylose conversion rates measured under non-catalyzed conditions, further studies were focused on the use of A70 as catalyst at intermediate temperatures (175 °C). As observed in Fig. 4a, [X]R dropped continuously in spite of volume-reduction in the reactor. On the other hand, [FUR]R evolution showed a maximum concentration peak in the first 50 min of reaction and fell when [X]R reached 50% of its final value. [FUR]C followed a similar trend, obtaining stripped vapor flows richer in FUR (when condensed) than in the liquid phase. This is one of the advantages of water–furfural equilibrium. As shown in Table 5, final [FUR]Y via stripping was also proportional to [X]0. Even if FUR was continuously removed from the reacting medium, byproduct formation was unavoidable and even more enhanced at high [X]0. Coke deposits on A70’s surface deactivated the acid site capacity 40% by the end of the tests. Anyway, the efficiency of FUR production was considerably improved compared to batch A70 catalyzed conditions. As observed in Fig. 4b, the low final FUR yield can be attributed to the slow stripping flow compared to FUR formation rate (theoretically). When high amounts of catalyst were added into the reacting medium, initial FUR formation was so fast that the system was not able to strip it at a comparable rate. During the first 100 min, 80% of total FUR degradation occurred. However, when the system stabilized, FUR formation and stripping rates were equaled and the system allowed more efficient stripping conditions. Only 20% of potential FUR was degraded after this point. In order to evaluate the stripping efficiency, a water–furfural mixture was nitrogen-stripped at 175 °C in the absence of any other reagent. This procedure allowed to recover 95% of initial FUR and a linear relationship of 8.6:1 was obtained for [FUR]C:[FUR]R. As depicted in Fig. 4c, the studied systems followed a similar trend. This means that mixtures containing xylose, FUR and higher molecular-weight compounds did not alter significantly the liquid–vapor equilibrium. Compared to the current FUR production

7483

I. Agirrezabal-Telleria et al. / Bioresource Technology 102 (2011) 7478–7485 Table 5 XX and FURY using nitrogen as stripping agent for different [X]0. [X]0(%)

In the absence of catalyst T = 150 °C

1 3 5 7 10*

A70L 60%, T = 175 °C T = 175 °C

T = 200 °C

No stripping

Nitrogen stripping

XX

FURY

XX

FURY

XX

FURY

XX

FURY

XX

FURY

50 56 59 57

16 17 21 18

67 67 66 68

49 47 39 37

98 99 99 99 99

60 65 68 62 65

83 93 96 97

38 5 2

99 99 99 99 99

70 52 45 31 25

Reaction conditions: semi-continuous stripping using nitrogen at 150 mL min1 and set vapor pressure. XX: conversion of xylose after 200 min of reaction time. * Phase separation occurs at the initial stage of stripping.

Fig. 4a. Experimental (bullets) and model (line) evolution of xylose and FUR for [X]0 of 3 wt.%, 60% A70L at 175 °C and stripping of FUR using 150 mL min1 of nitrogen at 8 bar absolute pressure.

Fig. 4b. Evolution of theoretical FUR formation rate and experimental FUR stripping rate for [X]0 of 3 wt.% and 5 wt.% at 175 °C catalyzed by A70L of 60% using 150 mL min1 of nitrogen and 8 bar absolute pressure.

processes (5–6% FUR in the vapor-phase for 1–2% [FUR]R), nitrogen-stripping increased this separation factor: 7–8% in [FUR]C for [FUR]R of 1–2%. According to the GC–MS analyse of the condensate stream, no side-reaction sub-products were detected. This means that nitrogen-stripped FUR selectivity is 100% and that this stripping method

Fig. 4c. Ratio between [FUR]R and [FUR]C using nitrogen at 150 mL min1 as stripping agent at 175 °C and 8 bar absolute pressure. Ratios: 1% [FUR]0 = 8.6, 3% [X]0 = 7.9, 5% [X]0 = 7.1, 7% [X]0 = 7.2.

reduces considerably further purification costs for the production of high quality FUR. As mentioned in Section 1, the alternative to stripping could be the use of selective solvents since they allow reaching high FURY. Their first drawback is associated to the separation between furfural and solvent. Besides that, FUR selectivity values range around 80–90%, which also complicates further purification stages. Moreover, the procedure of installing a nitrogen-stripping unit in the current steam-stripping process is economically more feasible than the use of co-solvents in high-pressure reactors. When [X]0 was increased up to 10 wt.%, fast xylose conversion rates led to high initial [FUR]R. At this point, [FUR]C reached the solubility limit in water (8.3% at room temperature) and two visible phases were obtained: one phase rich in water and a second phase rich in FUR (Fig. 5). After 60% of XX, [FUR]R and [FUR]C followed the same trend as observed for other [X]0. Thus, if high xylose loading is maintained in the reactor under continuous feeding conditions, simultaneous water–furfural phase separation would considerably reduce further separation costs. This effect, combined with high FUR selectivity in the condensate stream, could considerably improve the FUR production process efficiency. Stripping-optimization studies were performed using 3 wt.% [X]0, because it is low enough for an easy reactor-operation and allows higher analytical precision than lower concentrations. As mentioned in section 3.1.3, A70L effect was experimentally and theoretically proved as optimum at 20%. This effect was also studied for stripping using A70L of 20% and 40% at 175 °C (3 wt.% [X]0). As a result, initial FUR degradation was reduced, and final FURY increased up to 61% and 66%, respectively. Kinetically, 20% A70L

7484

I. Agirrezabal-Telleria et al. / Bioresource Technology 102 (2011) 7478–7485

Fig. 5. Experimental evolution of xylose and FUR for [X]0 of 10 wt.%, 60% A70L at 175 °C and stripping of FUR using 150 mL min1 of nitrogen at 8 bar absolute pressure.

was proved as optimum, however, under FUR stripping conditions, higher FURY are obtained at A70L of 40%. The effect of the reaction temperature was also studied under nitrogen-stripping conditions. Low temperature simultaneous stripping at 150 °C at higher A70L (80%) improved FURY to 65% for 3 wt.% [X]0. On the other hand, 10% A70L at 200 °C yielded 70% of FUR. In this sense, reaction temperature and pressure conditions should be evaluated for future applications. Temperatures around 200 °C should operate at 15 bar of total pressure, which increases final manufacturing costs. On the other hand, the combination of more sophisticated catalysts with temperatures ranging from 150 (5 bar) to 175 °C (8 bar) would be a good solution to the current FUR production problem. The ratio of used N2: produced FUR showed an average value of 15:1 (weight ratio) for different [X]0, which is similar to the 16:1 steam-FUR ratio used in batch QUAKER OATS process but lower than the 30:1 steam-FUR ratio in the ROSENLEW process (Zeitsch, 2000). However, used nitrogen could be recompressed and recycled back to the stripping unit without too expensive additional costs. Moreover, cooling requirements for the vapor phase are much lower than in the current processes: using steam as stripping agent, initial steam input is required to be cooled, whereas using nitrogen-stripping just FUR stream has to be condensed. 3.2.3. Modeling of furfural stripping data The experimental data from nitrogen-stripping tests using A70 were modeled using the same solver types as in Section 3.1.3. The kinetic constant values obtained in Table 2 were introduced as data into the model equations. Reactor-volume variation was established according to the experimental data values and the masstransfer constant (KS) was optimized following the equations listed below:

d½XR 0 0 ¼ w  k1  ½XR  wk2  ½XR  ½FURR dt

ð4Þ

d½FURR 0 0 ¼  w  k1  ½XR  w  k2  ½XR  dt 0

½FURR  w  k3  ½FURR  K S  d½FURC ½FURR ½FURV   ¼ KS  dt VC VC

dV C dt

½FURR ½FURR    VR VR

dV R dt

As observed in Fig. 4a, the model results and the experimental data were in good agreement. In order to simplify the modeling equations, the experimental data were also shortened only to steady state stripping conditions (after 55% XX). Using this short model, a KS average value of 3.9E-02 ± 0.005 L min1 was obtained for intermediate [X]0 (3–7 wt.%). However, for low [X]0 (1 wt.%), the model did not converge similarly. This might be attributed to analytical errors at such low concentrations. On the other hand, modeling results at higher [X]0 (10 wt.%) were not considered due to the two water–furfural phases in the condensate stream, because this phase separation was not included in the model. Taking into account the kinetic data presented, more suitable catalysts than ion-exchange sulfonic resins have been published for the xylose dehydration in water-phase systems (Chareonlimkun et al., 2010; Dias et al., 2005b; Lima et al., 2010a,b; Moreau et al., 1998). However, compared to more sophisticated catalysts, the combination of A70 with nitrogen-stripping considerably improves the final possible FURY and product selectivity. For this reason, the application of the studied reaction/separation system combined with more adequate catalytic structures would optimize the current FUR production processes. These results could be similarly applied to pentosan-rich biomass feedstocks. It is true that by-product formation using biomass is higher, so stripping conditions should be adjusted to keep high FUR product selectivity. On the other hand, the use of co-solvents for the production of FUR from hemicellulosic biomass is a less promising alternative because it would require two separate hydrolysis/dehydration steps and further complex distillation units. The work presented here could be applied to the design of continuous steady-state conditions, where a fixed pentosan conversion is set in the reactor outlet stream with respect to an optimum FUR stripping efficiency. According to the stripping tests performed at 175 °C, an average XX of 55% in the reactor would produce FUR concentration maxima in the condensate stream. This way, non-reacted pentosan could be recycled back to the feeding point and the nitrogen re-used. 4. Conclusions Nitrogen-stripping of FUR, combined with sulfonic ion-exchange resins, shows several improvements to the current FUR production process. FUR selectivity in the condensate of almost 100% and high product yield were obtained. Moreover, high reactor xylose loadings led to the formation of two water–furfural phases. It is industrially feasible to use nitrogen in the current stripping processes. Besides this, nitrogen could be recycled back to the stripping unit without high additional costs. Energy requirements (cooling and purification) are reduced compared to steam-stripping or biphasic systems. Finally, the kinetic and stripping modeled data are valuable for future studies under continuous steady-state conditions. Acknowledgements This work was supported by funds from the Ministerio de Ciencia e Innovación ENE2009-12743-C04-04 and from the Gobierno Vasco (Programa de Formación de Personal Investigador del Departamento de Educación, Universidades e Investigación). The authors also gratefully acknowledge Dow Chemical for kindly supplying the Amberlyst 70 catalyst.

ð5Þ Appendix A. Supplementary data

ð6Þ

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biortech.2011.05.015.

I. Agirrezabal-Telleria et al. / Bioresource Technology 102 (2011) 7478–7485

References Antal, M.J., Leesomboon, T., Mok, W.S., Richards, G.N., 1991. Kinetic-studies of the reactions of ketoses and aldoses in water at high-temperature. Mechanism of formation of 2-furaldehyde from D-xylose. Carbohydr. Res. 217, 71–85. Arnold, D.R., Buzzard, J.L., 2003. A novel process for furfural production. In: Proc. South African Chem. Eng. Cong. September, 3-5. Chareonlimkun, A., Champreda, V., Shotipruk, A., Laosiripojana, N., 2010. Catalytic conversion of sugarcane bagasse, rice husk and corncob in the presence of TiO2, ZrO2 and mixed-oxide TiO2 – ZrO2 under hot compressed water (HCW) condition. Bioresour. Technol. 101, 4179–4186. Chheda, J.N., Huber, G.W., Dumesic, J.A., 2007. Liquid-phase catalytic processing of biomass-derived oxygenated hydrocarbons to fuels and chemicals. Angew. Chem. Int. Ed. 46, 7164–7183. Dias, A.S., Pillinger, M., Valente, A.A., 2005a. Dehydration of xylose into furfural over micro–mesoporous sulfonic acid catalysts. J. Catal. 229, 414–423. Dias, A.S., Pillinger, M., Valente, A.A., 2005b. Liquid phase dehydration of D-xylose in the presence of Keggin-type heteropolyacids. Appl. Catal., A. 285, 126–131. Dias, A.S., Lima, S., Brandao, P., Pillinger, M., Rocha, J., Valente, A.A., 2006a. Liquidphase dehydration of D-xylose over microporous and mesoporous niobium silicates. Catal. Lett. 108, 179–186. Dias, A.S., Lima, S., Carriazo, D., Rives, V., Pillinger, M., Valente, A.A., 2006b. Exfoliated titanate, niobate and titanoniobate nanosheets as solid acid catalysts for the liquid-phase dehydration of D-xylose into furfural. J. Catal. 244, 230–237. Jing, Q., Lu, X.Y., 2007. Kinetics of non-catalyzed decomposition of D-xylose in high temperature liquid water. Chin. J. Chem. Eng. 15, 666–669. Kim, Y.C., Lee, H.S., 2001. Selective synthesis of furfural from xylose with supercritical carbon dioxide and solid acid catalyst. J. Ind. Eng. Chem. 7, 424–429. Lima, S., Pillinger, M., Valente, A.A., 2008. Dehydration of D-xylose into furfural catalysed by solid acids derived from the layered zeolite Nu-6(1). Catal. Commun. 9, 2144–2148. Lima, S., Neves, P., Antunes, M.M., Pillinger, M., Ignatyev, N., Valente, A.A., 2009. Conversion of mono/di/polysaccharides into furan compounds using 1-alkyl-3methylimidazolium ionic liquids. Appl. Cat. A Gen. 363, 93–99. Lima, S., Fernandes, A., Antunes, M.M., Pillinger, M., Ribeiro, F., Valente, A.A., 2010a. Dehydration of xylose into furfural in the presence of crystalline microporous silicoaluminophosphates. Catal. Lett. 135, 41–47.

7485

Lima, S., Antunes, M.M., Fernandes, A., Pillinger, M., Ribeiro, M.F., Valente, A.A., 2010b. Catalytic cyclodehydration of xylose to furfural in the presence of zeolite H-Beta and a micro/mesoporous Beta/TUD-1 composite material. Appl. Catal., A. 388, 141–148. Mamman, A.S., Lee, J.M., Kim, Y.C., Hwang, I.T., Park, N.J., Hwang, Y.K., et al., 2008. Furfural: hemicellulose/xylose-derived biochemical. Biofuels Bioprod. Biorefin. 2, 438–454. Mansilla, H.D., Baeza, J., Urzua, S., Maturana, G., Villasenor, J., Duran, N., 1998. Acidcatalysed hydrolysis of rice hull: Evaluation of furfural production. Bioresour. Technol. 66, 189–193. Matsumoto, K., Kobayashi, H., Ikeda, K., Komanoya, T., Fukuoka, A., Taguchi, S., 2011. Chemo-microbial conversion of cellulose into polyhydroxybutyrate through ruthenium-catalyzed hydrolysis of cellulose into glucose. Bioresour. Technol. 102, 3564–3567. Moreau, C., Durand, R., Peyron, D., Duhamet, J., Rivalier, P., 1998. Selective preparation of furfural from xylose over microporous solid acid catalysts. Ind. Crop. Prod. 7, 95–99. O’Neill, R., Ahmad, M.N., Vanoye, L., Aiouache, F., 2009. Kinetics of aqueous phase dehydration of xylose into furfural catalyzed by ZSM-5 zeolite. Ind. Eng. Chem. Res. 48, 4300–4306. Román-Leshkov, Y., Chheda, J.N., Dumesic, J.A., 2006. Phase Modifiers Promote Efficient Production of Hydroxymethylfurfural from Fructose. Science 312, 1933–1937. Wang, P., Yu, H., Zhan, S., Wang, S., 2011. Catalytic hydrolysis of lignocellulosic biomass into 5-hydroxymethylfurfural in ionic liquid. Bioresour. Technol. 102, 4179–4183. Weingarten, R., Cho, J., Conner, W.C.J., Huber, G.W., 2010. Kinetics of furfural production by dehydration of xylose in a biphasic reactor with microwave heating. Green Chem. 12, 1423–1429. Zeitsch, K.J., 2000. The Chemistry and Technology of Furfural and Its Many By-Products, 1st ed., Sugar Series, Vol. 13, Elsevier, The Netherlands, pp. 13 1. Zhang, J., Zhuang, J., Lin, L., Liu, S., Zhang, Z., in press. Conversion of D-xylose into furfural with mesoporous molecular sieve MCM-41 as catalyst and butanol as the extraction phase. Biomass Bioenergy. Zhao, H., Holladay, J.E., Brown, H., Zhang, Z.C., 2007. Metal chlorides in ionic liquid solvents convert sugars to 5-hydroxymethylfurfural. Science 316, 1597–1600.