Catalyst Screening for the Production of Furfural from Corncob

Article pubs.acs.org/EF

Catalyst Screening for the Production of Furfural from Corncob Pyrolysis C. Branca,† C. Di Blasi,*,‡ and A. Galgano† †

Istituto di Ricerche sulla Combustione, C.N.R., P.le V. Tecchio, 80125 Napoli, Italy Dipartimento di Ingegneria Chimica, Università degli Studi di Napoli “Federico II”, P.le V. Tecchio, 80125 Napoli, Italy

‡

ABSTRACT: Acidic pyrolysis of corncobs is carried out at 800 K in a packed-bed reactor using several catalysts (H2SO4, H3PO4, H3BO3, (NH4)2SO4, ZnCl2, NiCl2, MgCl2, and Fe2(SO4)3). In all cases, the yields of char, water, and some condensable organic compounds (acetic acid, levoglucosenone, 1,4:3,6-dyanhydro-α-D-glucopyranose, and furfural) are significantly increased at the expense of the phenolic and carbohydrate products typical of uncatalyzed pyrolysis. The maximum yields of furfural (FF) are around 5−6% for the H2SO4, Fe2(SO4)3 ,and ZnCl2 treatments, with potential further increase, following secondary degradation of the main anhydrosugars, up to 8.5% (H2SO4, Fe2(SO4)3) or 6.5% (ZnCl2). Additional experiments made with a wide range of Fe2(SO4)3 concentrations, and a comparison with previous results obtained for H2SO4 and ZnCl2 treatments indicate that the best performances are shown by the first catalyst with potential FF yields up to 10%. Moreover, at low concentrations, it also highly promotes the formation of levoglucosan, 5-methylfurfural, 5-hydroxymethylfurfural, 4-hydroxy-5,6-dihydro-(2H)-pyran-2one, and 3-hydroxy-2-penteno-1,5-lactone, 1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one.

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hydrolysis of corncobs, have been estimated29 to amount at 22%, but depending on the severity conditions (residence time, temperature, acid concentration), actual yields are between about 10−15%. Acid hydrolysis presents the inconvenience that the catalysts are extremely corrosive and highly toxic and suffer from serious drawbacks concerning homogeneous catalytic processes, such as difficult separation and recycling of the mineral acid and product contamination.30 Moreover, the problem of waste disposal is always relevant. Pyrolysis, which permits the complete conversion of the substrate into valuable solid, gaseous, and liquid products, appears, in principle, very attractive for the production of FF. Indeed, the remaining fraction of the oil, once the compound of interest is recovered, and the other products are still useful for the usual applications, thus improving the economics and reducing the environmental impact of the process. However, only two catalysts (ZnCl2 and H2SO4) have been specifically investigated23−27 for the production of this chemical, and the results currently available are not comparable because of the different feedstocks and impregnation and reaction conditions. The results of this research group24,27 report maximum yields around 5−6% from corncobs, which, although testifying a 10fold increase with respect to noncatalytic pyrolysis, are still too low for commercial applications, indicating that additional research efforts are still needed in this direction. This study is a further investigation to optimize the catalytic pyrolysis of corncobs for FF production. The analysis has been carried out by considering the aqueous impregnation of the biomass with several strong and weak acidic substances (H2SO4, H3PO4, H3BO3, (NH4)2SO4, ZnCl2, NiCl2, MgCl2, and Fe 2 (SO 4 ) 3 ), with concentrations in the substrate

INTRODUCTION Among the biomass thermochemical conversion technologies, pyrolysis offers the advantage of producing a high energy density oil and char that can be exploited for the usual energetic applications and the recovery/production of high-value chemicals.1−3 The byproduct gas can be used to provide the heat needed by the endothermicity of the pyrolytic process. The reaction paths, and consequently the yields and composition of pyrolysis products, are strongly affected by indigenous or added inorganic matter. There have been several studies that consider the deliberate addition of inorganic substances to the biomass structure with the motivation of either to develop flame retardancy or to increase the yields of specialty chemicals. The mechanisms of inorganic matter catalysis of lignocellulosic fuel pyrolysis can be broadly classified as alkaline4−11 or acid.12−22 Both catalysis mechanisms appear to favor the yields of char and water at the expense of the total condensable organic products. Moreover, alkaline compounds, which promote the activity of dehydration, demethoxylation, decarboxylation, and charring reactions, in small quantities, lead to an enhancement in the production of some cyclopentenones (by factors of 4−6) and phenolics (by factors of 2−6).9,10 Acidic catalysts, which promote the activity of dehydration and depolymerization reactions, enhance the formation of levoglucosenone (LGone) and furan derivatives. In particular, significant attention has been paid to the conditions that maximize the production of furfural (FF) by means of ZnCl223−26 or H2SO4.27 Corncobs, whose chemical composition is rich in xylan and cellulose, are a feedstock especially suited for the production of this chemical.28 FF is currently produced by dehydrating pentoses, present in significant amounts in the hemicelluloses of some agricultural residues and hardwoods, after acid hydrolysis. Based on the chemical composition of the hemicellulose, potential yields of FF, that can be produced at commercial scale from acid © 2012 American Chemical Society

Received: December 30, 2011 Revised: February 17, 2012 Published: February 18, 2012 1520

dx.doi.org/10.1021/ef202038n | Energy Fuels 2012, 26, 1520−1530

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also independent from the amount of mass fed that, for the problem under study, is around 150 g. The forced nitrogen flow across the particle bed is adjusted to give a nominal (ambient) residence time of gases and vapors along the isothermal reactor zone of about 2 s. This figure is significantly reduced at the pyrolysis conditions, owing to the higher temperatures and the addition of quite high mass fluxes of volatile products. The direction of the hot forced flow is from the hot bottom zone toward the cold top zone of the reactor. Therefore, the activity of secondary reactions of the vapor phase products of pyrolysis is highly hindered, leading to optimal conditions for bio-oil production. Nitrogen and volatile pyrolysis products pass through a condensation train consisting of two water/ice cooled condensers (with a catch pot, where the largest fraction of liquids is collected and chemically characterized), two wet scrubbers, three cotton wool traps, and a silica gel bed (all connected in series). Gas sampling and analysis are made at selected times, allowing the exit volumetric flow rate and mass of each gaseous species to be determined. Gas analysis is carried out through a gas chromatography (GC) instrument (Perkin-Elmer Auto-System XL), equipped with a thermal conductivity detector (TCD) and a packed column (Supelco 60−80 Carboxen 1000, 15 ft), with helium as carrier gas. The liquid products are stored at a temperature of 277 K with no light exposure, and chemical analysis is typically completed within two days after the experiment. After filtration with microfilters (Millex-Gx of 0.45 μm), chemical analysis is performed by means of GC/MS instrument (Focus GC-DSQ, Thermo Electron) with a quadrupole detector and a DB-1701 capillary column (60 m × 0.25 mm i.d., 0.25 mm film thickness). Helium is used as carrier gas with a constant flow of 1.0 mL/min. The oven temperature is programmed from 318 K (4 min) to 508 K at a heating rate of 3 K/min and held at 508 K for 13 min. The injector and the GC/MS interface are kept at a constant temperature of 523 K and 513 K, respectively. A sample volume of 1 μL (2.5−25% pyrolysis liquid in acetone) is injected. The MS instrument is operated in electron ionization (EI) mode, and a m/z range from 30 to 300 is scanned. Standard mass spectra with 70 eV ionization energy are recorded. Qualitative analysis uses the total ion chromatography (TIC) results obtained from a full scan acquisition method. The identification of the peaks is based on computer matching of the mass spectra with the NIST library or on the retention times of known species injected in the chromatographic column. For the quantitative analysis, selected ion monitoring (SIM) chromatography results, obtained by monitoring only three masses corresponding to the three major fragments of each compound, are employed. Quantification is carried out by means of the internal standard method with fluoranthene as the internal standard. For each of the quantified compounds, calibration lines are prepared by injection of 4−10 standard solutions. For sample injection and compound quantification, the concentration range is determined by successive approximations, until it is within the range of calibration line. Finally, the water content of the pyrolysis liquid, collected from the condensation train, is determined by means of Karl Fischer titration (Crison Compact titrator) according to the standard test method ASTM E203-96. The organic fraction is computed as the difference between the measured total liquids and water so determined.

corresponding to the optimal conditions previously determined for H2SO4 and ZnCl2, respectively. Although these additives were already examined in biomass pyrolysis, a comparison is again not possible owing to the differences in the feedstocks and the pretreatment and reaction conditions. Besides, the information on the FF yields is generally lacking. As a result of this screening, it has been found that the addition of Fe2(SO4)3 to corncobs produces results that, in relation to the yields of FF and anydrosugars, are comparable or better than the maximum values obtained for H2SO4 and ZnCl2. A further set of experiments, made by varying its concentration in the substrate, shows that the yields of FF can be increased up to 5.4% and potentially (by considering the secondary conversion of the anydrosugar yields that are simultaneously formed) up to 10%, a figure higher than those obtainable with the H2SO4 or the ZnCl2 treatments.

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MATERIALS AND METHODS

The chemical composition of corncobs, produced in the region Campania (South Italy), consists of lignin (14%, Klason method), extractives (1.1% acetone extraction with a Soxhtec HT2 apparatus), ash (1%, calcination), and holocellulose (84%, as difference). The reported contents of cellulose and hemicellulose vary between 45− 31% and 37−41%, respectively, with hemicellulose made of xylan (31%), arabinan (3.8%), and acetyl groups (3.4%) (see the references reported in ref 28). Experiments are made with corncob particles approximately 5 mm thick, which present an intrinsic density of 0.330 g/cm3 and a density of the packed-bed of 0.15 g/cm3. Before impregnation, particles are washed in hot (333 K) twice-distilled water (1 L for 100 g) for 2 h, with stirring to remove, in part, alkali compounds, thus reducing their catalytic action on the conversion process. Washed and predried samples are referred to in the following as washed or additive-free. Impregnated samples are obtained by soaking the sample in aqueous solutions (100 g for 0.5 L) for 3 h, with stirring. The solution is prepared by adding deionized water to a proper amount of catalyst (H2SO4, H3PO4, H3BO3, (NH4)2SO4, ZnCl2, NiCl2, MgCl2, and Fe2(SO4)3) to obtain the desired concentration. After impregnation, wet particles increase their weight by about 200%. Theoretical evaluations based on the amount of impregnated aqueous solution and assuming that, after drying, the entire amount of catalyst in water is impregnated in the biomass sample, permits the calculation of the catalyst content. Drying of H2SO4, H3PO4, H3BO3, Fe2(SO4)3, and (NH4)2SO4 impregnated particles is performed by exposure to a forced air flow at 343 K for 1 h, followed by oven drying for 20 h at 313 K, and repeated exposure (before feeding) to forced air flow at 343 K for 15 min. For the other catalysts (ZnCl2, NiCl2, MgCl2) and the washed sample, the same procedure is applied, but the period of oven drying is at a temperature of 353 K. The pyrolysis reactor has already been used in previous work by this research group,9−11,19−22,24,27,28 but to facilitate the understanding of the conversion dynamics, the main features are again provided here. It is a cylindrical steel reactor. Nitrogen, fed through a jacket at the reactor top, is heated by an electrical furnace and distributed at the bottom by a perforated steel plate, which also supports the bed. Temperature profiles along the reactor axis are measured by seven thermocouples. The lower reactor zone (about 20 cm), at steady conditions, is isothermal at a temperature determined by the set point of the furnace. The temperature along the isothermal region, indicated in the following as heating temperature, is taken equal to 800 K during the tests. After nitrogen flushing, when steady profiles are established and the desired heating temperature is achieved, the sample consisting of biomass particles is suddenly dropped inside the hot isothermal part of the reactor. This feeding modality permits the attainment of sample heating rates and conversion times that are essentially determined by the particle properties and not by the reactor heating time. Results are

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CHEMICAL AND PHYSICAL CHANGES OF THE CATALYSTS The catalysts examined include some Brønsted acids such as H2SO4 (strong), H3PO4(intermediate), and H3BO3 (weak) ((NH4)2SO4 also belongs to this category and decomposes at the pyrolysis conditions to H2SO4) and Lewis acids such as ZnCl2, NiCl2, MgCl2, and Fe2(SO4)3 (Mg2+ and Fe3+ are hard Lewis acids, while Ni2+ and Zn2+ are borderline Lewis acids). At the reaction conditions, the catalysts also undergo important chemical and physical transformations that may include dissociation, possibly leading to different acidic properties, and/or decomposition reactions that, in some cases, are still the 1521


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K and 573 K (3.2−3.6 water molecules23). Above 773 K, the gradual evaporation takes place of the liquid-phase ZnCl2. Although it is difficult to quantify the exact amount of ZnCl2 retained by the solid-phase char for maximum reaction temperatures of 800 K, it can be reasonably thought that a large part of the initial mass is not vaporized. Here, it is assumed that the entire amount of catalyst is retained in the solid phase char. Dehydration of MgCl2 hexahydrate takes place according to several steps in the temperature range 390−793 K,34 with the hydrolysis reactions between MgCl2 and the crystallization water also leading to the formation of MgO (solid) and HCl, according to the global reaction:

subject of investigation and debate. An attempt is made here to briefly discuss some information on these aspects that can be helpful for understanding the action of the additives on the pyrolysis products and, more specifically, for quantifying how much of the catalyst mass is retained by the product char. The interactions between the two reactants (additive and substrate), which may give rise to the formation of new compounds, are assumed to be small and not taken into account. Following the analysis (and references) made in ref 19, the decomposition of H3PO4 (melting temperature of 315 K31) can be described according to the endothermic reactions a1 and a2: 432,482 K 1

H3PO4 (l) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→

2

H2O(g) +

1 H 4P2O7 (l) 2

> 482 K 1 1 H 4P2O7 (l) ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ H2O(g) + P2O5(s) 2 2

(a1)

390 − 793 K 1

MgCl 2· 6H2O(s) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→

2 11 + H2O(g) + HCl(g) 2

(a2)

Phosphoric acid is dehydrated, as in reaction a1, producing pyrophoric acid. Reaction a2 proceeds at a very slow rate with the formation of P2O5, which presents a melting temperature of 842 K and a boiling point of 864 K.31 For a temperature of 800 K, reactions a1 and a2 can be assumed to have reached completion, so that the stoichiometric amount of P2O5 formed is retained in the product char, corresponding to 72% of the initial catalyst mass. Information on the decomposition dynamics and products of (NH4)2SO4 is available (reactions b1−b3 from ref 22 and references there reported): (b1)

557 K

NH 4HSO4 (l) ⎯⎯⎯⎯⎯⎯→ NH3(g) + H2SO4 (l)

(b2)

656 K

H2SO4 (l) ⎯⎯⎯⎯⎯⎯→ H2O(g) + SO3(g)

(b3)

RESULTS A first set of pyrolysis experiments is performed to carry out a screening among the most used acid catalyst to evaluate their performances in relation to the production of FF from corncobs. However, as the results are significantly dependent on the catalyst content, it is important to select appropriate values for the comparison to be meaningful. Previous results of this research group regarding H2SO427 and ZnCl224 catalyzed pyrolysis of corncobs are used for this purpose (these two catalysts are assumed to represent strong and weak acids). More precisely, for a heating temperature of 800 K, concentrations of 2.1% H2SO4 and 4.8% ZnCl2 in the substrate are required to maximize the FF production. Thus, the former value is selected for the treatments with H3PO4. Also, the same H2SO4 concentration is reached once, in the course of the reaction, (NH4)2SO4 degrades, provided an initial concentration of 2.6%. On the other hand, similar to ZnCl2, a concentration of 4.8% is used for NiCl2, MgCl2, and Fe2(SO4)3, which was commercially available in the hydrated form, is assumed to contain 25% of water.36 Two concentrations (2.1 and 4.8%) are used for H3BO3. Results already available for H2SO4 and ZnCl2 are also included in the comparison. The measured pH values of the aqueous solution used for the impregnation, given the selected acid concentrations, are summarized in Table 1. As expected, the lowest pH is obtained for H2SO4, but H3PO4 and Fe2(SO4)3 also show rather low

393 − 573 K

533 − 543 K 1

HBO2 (s) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→

2

B2O3(s) +

1 H2O(g) 2

(d1)

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(NH4)2SO4 decomposes in two steps: In the first, ammonia is released according to reactions b1 and b2. The second step, which is the decomposition of H2SO4, proceeds through reaction b3 at 656 K, giving rise to the formation of H2O and SO3. Hence, both (NH4)2SO4 and H2SO4 do not give rise to solid-phase products. The decomposition dynamics and products of H3BO3 can be found in ref 20 (and references there reported). Upon heating, this compound loses water endothermically into two stages: H3BO3(s) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ HBO2 (s) + H2O(g)

1 MgO(s) 2

Therefore, for a reaction temperature of 800 K, it can be assumed that 50% of the initial additive mass is retained by the char as MgCl2 (melting point 985 K31) and 21% as MgO (melting point 3073 K31). Dehydration and decomposition of NiCl2 hydrate has been investigated.33 It is shown that in nitrogen atmosphere dehydration takes place according to several steps in the temperature range 433−553 K. Then, the weight loss curve shows a zone of nearly constant values up to about 873 K. Therefore, it can be assumed that the entire amount of NiCl2 (boiling point at 1243 K31) impregnated in the sample is retained by the char. Fe2(SO4)3 releases the crystallization water around 673 K and decomposes into Fe2O3(s) and SO3(g) at temperatures of 950−1100 K.35 Therefore, it can be again assumed that the entire amount of anhydrous Fe2(SO4)3 impregnated into the sample is retained by the product char.

557 K

(NH 4)2 SO4 (s) ⎯⎯⎯⎯⎯⎯→ NH3(g) + (NH4)HSO4 (l)

MgCl 2(s) +

(c1)

(c2)

where the softening, melting, and boiling temperatures of B2O3 are 598 K, 723 K, and 2133 K, respectively.31 Reaction c1 gives rise to the formation of HBO2, with further heating to waterfree B2O3. For the temperatures of interest in this study (up to 800 K), reactions c1 and c2 can be assumed to have reached completion, so that the formed amount of B2O3 retained by the char corresponds to 56% of the initial mass of H3BO3. Although considered in the anhydrous form, MgCl2, ZnCl2, and NiCl2 most likely acquire some crystallization water during the drying step, 23,32,33 which then participates in the modifications driven by thermal heating. Following the analysis (and references) reported in ref 21, ZnCl2 (melting temperature 556.7 K31) releases the crystallization water between 473 1522


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Table 1. Measured pH Values of the Aqueous Acidic Solutions Used for Sample Impregnation, Corresponding to Acid Concentration (in the Substrate) of 2.1 and 4.8%a catalyst Acid Concentration 2.1% H2SO4 H3BO3 H3PO4 (NH4)2SO4 Acid Concentration 4.8% MgCl2 NiCl2 ZnCl2 Fe2(SO4)3 H3BO3 a

pH 0.9 5.6 1.6 5.4 6.7 6.7 6.3 1.8 4.3

Figure 1. Profiles of the release rates of gas (sample mass basis) and bed temperature at a bed height of 10 cm versus time for the washed sample and the samples impregnated with various catalysts at an acid concentration of 2.1%.

Concentration of (NH4)2SO4 of 2.6%.

values; so, it is plausible that significant changes in the biomass structure already take place during the impregnation stage. The other catalysts show values near (NH4)2SO4, H3BO3, ZnCl2, or almost coincident (MgCl2, NiCl2) with the limit value of acidic solutions. The second set of experiments is made for variable concentrations (up to 5.8%) of Fe2(SO4)3 in the substrate, which, according to the screening, appears particularly promising for the production of FF. In all cases, the packedbed is instantaneously exposed to a heating temperature of 800 K. The total mass closure for the pyrolysis experiments is around 91−92% and the products are expressed as wt % of the sample fed to the reactor. For each condition, two experiments are made which show a good reproducibility (average variations on the measured variables between 0 and 3%). The chemical characterization of the liquid phase products is made on the basis of at least three chromatographic injections for each compound which are free from evident flaws of the measuring device and the operator. Catalyst Screening. To investigate the effects of the nature of the additive on the characteristics of corncob pyrolysis, an acid concentration of 2.1% has been considered for H2SO4, H3PO4, and (NH4)2SO4 and a concentration of 4.8% for ZnCl2, NiCl2, MgCl2 and Fe2(SO4)3. For H3BO3 both concentrations are examined. The qualitative dynamics of the process are, for main lines, not altered by the presence of catalysts. Examples of the quantitative modifications can be seen from Figures 1 and 2, which report the time curves of the temperature, at a bed height of 10 cm, and the gas release rate, −dYG/dt, defined as the time derivative of the produced gas mass fraction, for the two groups of catalysts with acid concentrations of 2.1 and 4.8%, respectively. An initial rapid decrease in temperature is followed by a slow increase toward the initial heating value. The first trend can be attributed to the feed preheating from ambient to the pyrolysis temperature, the global endothermic character of the process and the convective transport of heat outside the bed by the produced hot vapors and gases. Decomposition reactions become active at very short times, as testified by the prompt attainment of the maximum gas release rate, which is followed by a more or less wide tailing zone. Using the results of the extracted sample for comparison, it is evident that the presence of catalysts always causes a significant reduction in the peak of the gas release rate, which is also localized at slightly longer times. The reduction factors are

Figure 2. Profiles of the release rates of gas (sample mass basis) and bed temperature at a bed height of 10 cm versus time for the washed sample and the samples impregnated with various catalysts at an acid concentration of 4.8%.

approximately between 1.5 (H3PO4) and 1.7 (H2SO4) for the first group and between 1.4 (ZnCl2) and 1.8 (Fe2(SO4)3) for the second group. In general, the conversion times (defined as a the times when the production of 75% of the total gas has occurred) are slightly longer (182−240 s versus 169 s), especially for NiCl2 and H2SO4 (times of about 240 s). While qualitative variations in the shape of the curve are small for H3PO4, H3BO3, and (NH4)2SO4, a more evident tail appears for chlorides (especially NiCl2) and Fe2(SO4)3. The corresponding temperatures, limited to this zone, are also significantly lower than those of the washed sample. In can be thought that the slower temperature rise, associated with the slow release of crystallization water, also delays the devolatilization of the lignin fraction that mainly takes place in the tailing zone of the gas release rate curves. The differences in the contents of crystallization water and the corresponding dehydration temperatures may account for the variations in the temperature versus time curves (and, consequently, in the rate of volatile species release). For H2SO4, a wide zone of relatively high and constant gas release rates can be seen displaced at longer times. This was attributed27 to the formation of an azeotrope mixture of acid and water that is not removed during the drying step. Given the presence of spatial gradients across the bed, it is possible that, locally, corncob pyrolysis does not occur significantly as long as water evaporation and/or H2SO4 decomposition are under way. 1523


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expected, there is always a significant diminution in the yields of organic products with a concomitant increase in the production of char and water. These effects are increased by the concentration of the catalyst, as can be seen for the addition of H3BO3, and already found in previous investigations (for instance, refs 24 and 27). The lowest reduction in the production of condensable organic products is obtained for NiCl2 and H3BO3 at the low concentration (yield equal to 26.6%) and the highest for H3BO3 at the highest concentration (yield of 19%) and H3PO4 (yield equal to 20%) (versus 37.6% of the washed sample) with reduction factors of about 1.4−2. The yields of water are around 26−27% (NiCl2, MgCl2, H3PO4, and ZnCl2) or 28−29% ((NH4)2SO4, H2SO4, and Fe2(SO4)3) versus 20% of the washed sample (factors of increase 1.3−1.5). For the two H3BO3 concentrations, they vary from 23 to 30%. In accordance with the analysis already presented, these yields also include contributions that may derive from the intrinsic transformations of the catalysts. The highest yields of solid residue, on catalyst-free basis, are obtained for H3PO4 (31.7% versus 22% of the washed sample). Even if the correction is made for entire amount of the catalyst, to take into account that its interactions with the substrate are strong and give rise to new products,38 the char yields still remain the highest (31%). Quite high yields (catalyst-free basis, Figure 4) are also obtained for the treatments with H3BO3 (28.8 or 30%), and (NH4)2SO4, H2SO4, and ZnCl2 (yields around 27.5%). The yields of the gaseous species vary in a narrow range of about 10−12% (versus 13.5% of the washed sample). In particular, the addition of MgCl2 has the lowest influence in this regard. Figure 5 reports the yields of the main gaseous species (CO2, CO, and CH4) for the various catalysts and the washed sample.

Thus, corncob pyrolysis approximately takes place at the same temperature over a reaction zone of constant size for the entire duration of the conversion process, producing constant rates of gas release over a wide time interval. It is worth observing that the process of water evaporation is plausibly much more important in this regard than the endothermic decomposition of H2SO4 given that the gas release curve obtained for the (NH4)2SO4 treatment, which produces H2SO4 during its decomposition, does not present a flat zone displaced at longer times. On the whole, the variations induced by catalysts in the temperature versus time profiles in some cases are small (H3PO4, H3BO3, (NH4)2SO4, and ZnCl2). They often appear as slightly higher values, resulting from the increased exothermic formation rates of char.37 In other cases, the temperature values are lower than those measured for the washed sample presumably owing to the removal of the residual water from the azeotropic mixture (H2SO4) or the crystallization water (NiCl2, MgCl2, and Fe2(SO4)3). The endothermic decomposition reactions of the catalysts may also have a role, although this contribution is not expected to be large given the small concentrations examined. Figure 3 compares the yields of the lumped classes of products, char, organics, water, and gas, obtained for

Figure 3. Yields of the main classes of pyrolysis products (gas, organics, water, and char), expressed as percent of the initial sample mass, for the washed sample and the catalyst impregnated samples with acid concentration of 2.1% (A) and 4.8% (B).

impregnated and washed corncobs. Figure 4 reports the char yields, corrected for the contents of inorganic matter derived from the chemical and physical changes undergone by the catalysts in accordance with the description given above. As

Figure 5. Yields of the gaseous species, expressed as percent of the initial sample mass, for the washed sample and the catalyst impregnated samples with acid concentration of 2.1% (A) and 4.8% (B).

CO2 is still the most abundant species with yields of 6−8.6% (versus 9.3% of the washed sample). The yields of CO remain approximately the same or slightly decrease. Variations on the yields of CH4 are very small. The yields of the main components of the organic fraction (hydroxyacetaldehyde (HAA), acetic acid (AA), hydroxypropanone (HP), levoglucosan (LG), LGone, 1,4:3,6-dyanhydroα-D-glucopyranose (DGP), guaiacol (G), 4-methylguaiacol (4MG), 4-ethylguaiacol (4-EG), FF, 2-(5H)-furanone (2(5H)Fone), 5-hydroxymethylfurfural (5-HMF), 5-methylfurfural (5MF), phenol (PH), o-,m-,p-cresols (CR), 2-ethylphenol (2EPH), syringol (S), 4-methylphenol (4-MS)) are listed in Table 2. The yields of LG, LGone, and DGP are also shown in Figure 6. Furthermore, Figure 7 reports the actual and the potential

Figure 4. Yields of the char on catalyst-free basis, expressed as percent of the initial sample mass, for the washed sample and the catalyst impregnated samples with acid concentration of 2.1% (A) and 4.8% (B). 1524


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Table 2. Yields of Some Condensable Organic Compoundsa for the Washed Sample and Samples Impregnated with Acid Concentrations of 2.1 and 4.8% catalysts [2.1 wt %] HAA AA HP LG LGone DGP G 4-MG 4-EG FF 2(5H)Fone 5-HMF 5-MF PH CR 2-EPH S 4-MS

catalysts [4.8 wt %]

washed

H2SO4

H3BO3

H3PO4

(NH4)2SO4

MgCl2

NiCl2

ZnCl2

H3BO3

Fe2(SO4)3

1.20 3.96 2.72 0.65 0.00 0.06 0.18 0.08 0.15 0.60 0.26 0.07 0.07 0.16 0.15 0.11 0.22 0.07

0.07 4.08 0.11 0.08 4.28 0.52 0.06 0.07 0.05 5.11 0.01 0.01 0.05 0.09 0.14 0.00 0.02 0.02

0.31 4.74 0.22 0.34 1.13 0.07 0.06 0.03 0.05 1.59 0.05 0.19 0.22 0.11 0.07 0.07 0.03 0.02

0.05 2.71 0.04 0.04 4.82 0.16 0.05 0.02 0.03 1.96 0.02 0.04 0.12 0.10 0.05 0.04 0.04 0.01

0.11 3.70 0.15 0.71 3.93 0.82 0.09 0.07 0.08 2.11 0.05 0.05 0.09 0.11 0.10 0.07 0.00 0.00

0.10 4.67 0.23 0.10 0.30 0.04 0.05 0.02 0.04 3.24 0.05 0.15 0.11 0.10 0.11 0.06 0.02 0.01

0.15 7.70 0.18 0.14 1.53 0.07 0.06 0.03 0.04 4.33 0.06 0.15 0.15 0.14 0.07 0.08 0.03 0.01

0.03 4.96 0.06 0.05 0.91 0.09 0.05 0.01 0.02 5.79 0.04 0.06 0.05 0.07 0.02 0.01 0.01 0.00

0.25 4.93 0.08 0.07 0.35 0.03 0.02 0.00 0.02 1.28 0.03 0.06 0.17 0.08 0.07 0.05 0.02 0.01

0.09 5.58 0.10 0.13 4.56 0.72 0.05 0.05 0.05 4.62 0.03 0.03 0.15 0.11 0.08 0.06 0.03 0.02

a

Hydroxyacetaldehyde (HAA), acetic acid (AA), hydroxypropanone (HP), levoglucosan (LG), levoglucosenone (LGone), 1,4:3,6-dyanhydro-α-Dglucopyranose (DGP), guaiacol (G), 4-methylguaiacol (4-MG), 4-ethylguaiacol (4-EG), furfural (FF), 2-5H-furanone (2(5H)Fone), 5hydroxymethylfurfural (5-HMF), 5-methylfurfural (5-MF), phenol (PH), o-,m-,p-cresols (CR), 2-ethylphenol (2-EPH), syringol (S), 4-methylphenol (4-MS).

and hemicellulose and secondary degradation of some vaporphase products. Thus, it is meaningful to consider the combination of primary pyrolysis of biomass and secondary pyrolysis of the generated vapors, both catalyzed by acidic substances, for maximizing the production of FF.27 In this study, the potential yields of FF are computed by summing up the actual yields with those obtained from the complete conversion of the corresponding yields of anhydrosugars produced. It is assumed that conversion of LGone to FF takes place according to the mechanism proposed by Kawamoto et al.39 The first step is hydrolysis via ring-opening of the C1−O5 bond, justified by the significant presence of water vapor. Then, there is the elimination of C6 as formaldehyde before rearrangement into a five-member ring, which, upon elimination of H2O, produces FF. On a mass basis, from 1 g of LGone, 0.76 g of FF are formed. Moreover, it is also assumed that FF can be formed from LG and DGP, which, in a preliminary step, are dehydrated to LGone via the release of 2 and 1 mole of water, respectively.40 In this way, on a mass basis, from 1 g of LG or DGP, 0.59 g or 0.67 g of FF are formed. The composition of the organic products is highly affected by acidic additives. They catalyze the dehydration, depolymerization and cross-linking reactions of all the three main chemical components of the substrate, as testified by the increased production of water and char, according to mechanisms that are additive dependent.13,14,17,38,41,42 Their action already starts during the impregnation stage, probably causing an hydrolytic attack of the main components, whose entity is again dependent upon the nature and properties of the acid. As a result of the modifications in the structure and the reaction paths, some species, typically produced from noncatalytic pyrolysis, such as HAA, HP, and all the phenolic compounds are almost eliminated or strongly diminished. For instance, for HAA and HP yields, the reduction factors are in the range 4 (H3BO3 at the low concentration) to 35 (ZnCl2) and 11.5

Figure 6. Yields of levoglucosan (LG), 1,4:3,6-dyanhydro-α-Dglucopyranose (DGP) and levoglucosenone (LGone), expressed as percent of the initial sample mass, for the washed sample and the catalyst impregnated samples with acid concentration of 2.1% (A) and 4.8% (B).

Figure 7. Actual and potential yields of furfural (FF), expressed as percent of the initial sample mass, for the washed sample and the catalyst impregnated samples with acid concentration of 2.1% (A) and 4.8% (B).

yields of FF. The latter values are evaluated, taking into account that FF is a product of both primary decomposition of cellulose 1525


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FF is also formed from secondary degradation. It is wellknown that acidic substances, in particular Lewis acids, are especially effective for the conversion of anhydrosugars to FF.49 DGP, in this case, partially decomposes to LGone, FF, and 5MF.50 Therefore, it can act as a precursor for LGone and its subsequent decomposition products. It has been confirmed25 that LG and LGone are promptly converted to FF at 673 K in the presence of ZnCl2, whereas FF does not undergo any degradation. This result is important in the view of considering a two-step process for FF production, that is, primary pyrolysis of corncobs and secondary pyrolysis of the gas/vapor stream, both in the presence of acid catalysts. The results obtained here show that H2SO4, and Fe2(SO4)3 are particularly effective to catalyze the primary formation of FF, as the high yields of this product are also associated with high yields of DGP and LGone. On the other hand ZnCl2, which gives rise to high yields of FF but very small quantities of LGone and DGP, is a catalyst particularly effective for the secondary route of FF production. Thus, a combination of H2SO4 or Fe2(SO4)3 as catalysts for primary pyrolysis and ZnCl2 as a catalyst for secondary degradation can result particularly effective for augmenting the yields of FF. Finally, in some cases (MgCl2, NiCl2, and H3BO3 at the lowest concentration), a factor of increase around 2 in the yields of 5-HMF is observed, also resulting from an enhancement in the dehydration process of the component cellulose.23,43 Influence of the Fe2(SO4)3 Concentration. The examination of a certain number of acidic catalysts for FF production has indicated that those potentially more effective are H2SO4, ZnCl2, and Fe2(SO4)3. Previous investigations carried out by this research group24,27 have examined the effects of the concentrations of the first two additives and, as already pointed out, the optimal concentrations have been considered here. As no information was available, an additional set of experiments has been carried out about the effects on the products of corncob pyrolysis of Fe2(SO4)3 concentrations up to values of about 5.7%. In fact, the concentration of 4.8% examined in the first set of experiments may not be the optimal one for the purpose of this study. Some examples of the temperature versus time profiles (at a bed height of 10 cm) and the curves of the gas release rate (Figure 8) show that the conversion process becomes barely slower as the amount of catalyst in the substrate is increased.

(MgCl2) to 63 (H3PO4), respectively. Another significant product of noncatalytic pyrolysis, AA, is slightly reduced (H3PO4, factor of 1.5), left almost unchanged ((NH4)2SO4, H2SO4) or augmented (factors of 1.2 (MgCl2, H3BO3) to 2 (NiCl2)). Indeed, acidic catalysts impregnated in cellulose reduce the activity of fragmentation (versus depolymerization) reactions, with a consequent diminution in the yields of HAA, HP, AA, and other light compounds. The observed increase in the AA yields can be attributed to an enhancement in the cleavage of the acetyl group from the xylan structure.43,44 Also, the predominance of dehydration and condensation reactions of the ligninic fraction is responsible for the large diminution in the volatile products of this component. The reduction in the yields of LG is also always very large with factors of 1.9 (H3BO3) to 17 (H3PO4) with the exception of (NH4)2SO4, which causes a small increase. LGone, which is not present among the products of noncatalytic pyrolysis, becomes one of the dominant species with yields that vary between 0.3% (MgCl2) and 4.8% (H3PO4), with rather high yields obtained also for the treatments with Fe2(SO4)3 (4.6%), H2SO4 (4.3%), and (NH4)2SO4 (4%). With the exception of MgCl2 and H3BO3 at high concentration, the yields of DGP are also increased (factors of 1.1 (NiCl2) to 13 ((NH4)2SO4)), although they always remain quite low. For the most effective catalyst ((NH4)2SO4), they reach 0.8%, followed by 0.7% (Fe2(SO4)3) and 0.5% (H2SO4). It can be assumed that, in the presence of acidic additives, the chief chemical reactions of the cellulose structure are still dehydration (low temperature) and depolymerization (high temperature). However, depolymerization of the dehydrated cellulose mainly produces DGP and LGone instead of LG.27 As for the exception observed for (NH4)2SO4, it can be thought that the mild acidic character of this compound, on one hand, exerts during the impregnation stage a beneficial effect on the removal of indigenous matter content, which is detrimental for the formation of LG,45 and, on the other hand, the formation of a strong acid at a relatively high temperature (H2SO4 above 557 K) may preserve unaltered part of the substrate structure, thus increasing the formation rate of LG. All the catalysts examined contribute for augmenting the production of FF. The highest actual yields (5.8%) are observed for the treatment with ZnCl2, followed by those of H2SO4 (5%), Fe2(SO4)3) (4.6%), and NiCl2 (4.3%). However, the yields of FF are potentially the highest, reaching 8.7−8.6%, when H2SO4 or Fe2(SO4)3 are used, followed by values around 6.6−6% in the presence of ZnCl2, H3PO4, or (NH4)2SO4. One of the primary paths of FF formation is from the dehydration of pentoses (xylose and arabinose), which is catalyzed by acidic substances.46 These may also contribute in the preliminary depolymerization step of pentosans into pentoses, with the intensity of the action most likely proportional to the acidity level of the additive. Another primary path for FF formation is from the dehydration of the D-glucosyl residue in cellulose,47 again catalyzed by acids. ZnCl2 favors the breakdown of the glycosidic linkage of cellulose23 followed by hydrolysis of the cellulose, where the water from the hydrated ZnCl2 participates as a nucleophile to give glucose, which, upon dehydration, produces FF. A similar mechanism has also been proposed for MgCl2:48 hydrated MgCl2 promotes the solid-state hydrolysis of cellulose with Mg2+ acting as Lewis acid and hydrated water as a nucleophile. It can be postulated that a similar action is also exerted by the other Lewis acids.

Figure 8. Profiles of the release rates of gas (sample mass basis) and bed temperature at a bed height of 10 cm versus time for the washed sample and samples impregnated with various concentrations of Fe2(SO4)3. 1526


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Table 3. Yields of Some Condensable Organic Compoundsa for the Washed Sample and Samples Impregnated with Different Concentrations of Fe2(SO4)3

The delay is associated with a lower minimum temperature and mainly a much slower rise toward the external heating value, that becomes evident already at relative small catalyst loads. Also, the maximum gas release rate is soon highly reduced (by a factor of 2.9 for the range of concentrations examined) followed by an enlargement of the curve at longer times. The conversion time globally increases from 169 s (washed sample) to 248 s (Fe2(SO4)3 concentration of about 5.7%). Given that the enhancement in the formation rate of char also makes larger the exothermic contribution of the pyrolysis reactions, as already observed, it can be hypothesized that the delay in the temperature rise is due to the slow release of the crystallization water, which, in turn, also slows down the pyrolysis process. The yields of the main products classes are shown in Figure 9 on dependence of the catalyst concentration. The qualitative

Fe2(SO4)3 [wt %] HAA AA HP LG LGone DGP G 4-MG 4-EG FF 2(5H)Fone 5-HMF 5-MF PH CR 2-EPH S 4-MS

0

0.7

1.5

2.8

3.7

4.8

5.7

1.20 3.96 2.72 0.65 0.00 0.06 0.18 0.08 0.15 0.60 0.26 0.07 0.07 0.16 0.15 0.11 0.22 0.07

0.38 4.49 0.34 1.74 1.02 0.52 0.09 0.07 0.08 2.16 0.09 0.28 0.25 0.12 0.15 0.10 0.08 0.07

0.26 5.35 0.23 1.21 2.84 0.67 0.09 0.06 0.08 3.02 0.06 0.14 0.28 0.11 0.15 0.11 0.05 0.07

0.12 5.48 0.16 0.26 4.92 0.80 0.07 0.05 0.06 4.38 0.03 0.05 0.18 0.11 0.11 0.08 0.03 0.04

0.14 5.79 0.12 0.26 5.20 0.84 0.06 0.05 0.06 5.34 0.02 0.04 0.16 0.11 0.10 0.06 0.03 0.03

0.09 5.58 0.10 0.13 4.56 0.72 0.05 0.05 0.05 4.62 0.02 0.03 0.15 0.11 0.08 0.06 0.03 0.02

0.12 5.45 0.09 0.17 3.87 0.70 0.04 0.03 0.03 4.40 0.02 0.03 0.15 0.08 0.08 0.06 0.02 0.02

a

Hydroxyacetaldehyde (HAA), acetic acid (AA), hydroxypropanone (HP), levoglucosan (LG), levoglucosenone (LGone), 1,4:3,6-dyanhydro-α-D-glucopyranose (DGP), guaiacol (G), 4-methylguaiacol (4MG), 4-ethylguaiacol (4-EG), furfural (FF), 2-5H-furanone (2(5H)Fone), 5-hydroxymethylfurfural (5-HMF), 5-methylfurfural (5-MF), phenol (PH), o-,m-,p-cresols (CR), 2-ethylphenol (2-EPH), syringol (S), 4-methylphenol (4-MS).

Figure 9. Yields of the main classes of pyrolysis products (gas, organics, water, and char), expressed as percent of the initial sample mass, as functions of the impregnated Fe2(SO4)3, H2SO4, and ZnCl2 concentrations.

trends are similar to those obtained for the treatments with H2SO427 and ZnCl2.24 However, from the quantitative point of view, the yields of organics are higher with lower yields of water (comparable with those obtained in the case of ZnCl2) and solid residue (comparable with those obtained in the case of H2SO4). The gas yields are approximately the same, and the variations in the composition (not shown) are also small. The yields of the main compounds of the organic fraction are listed in Table 3. A comparison for FF (including the potential yields evaluated in accordance with the description given above), DGP, LGone, LG, and AA with the results already available for H2SO4 and ZnCl2, is made in Figures 10−12. Moreover, Figure 13 compares the relative peak area of 4hydroxy-5,6-dihydro-(2H)-pyran-2-one (HDP) (this compound is also indicated as 3-hydroxy-2-penteno-1,5-lactone) and 1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one (LAC), which are mainly formed from the five-carbon structures of hemicellulose44,51 and cellulose,52 respectively. Similar to the dependence of the yields of the total condensable organic fraction, the yields of some compounds, including HAA, HP, and phenolics, continuously decrease as the catalyst concentration is increased, as a result of a successive suppression of the volatile products in favor of dehydration and condensation reactions. However, the decay is less sharp than that observed for the other two catalysts, especially H2SO4. On the other hand, the Fe2(SO4)3 treatment also produces a significant increase in the formation rate of some species (FF, DGP, LGone) for concentrations that are displaced at relatively high values (about 3.5−4.5%) similarly to ZnCl2. Moreover, these high values are preserved for a relatively large range of catalyst

Figure 10. Actual and potential yields of furfural (FF), expressed as percent of the initial sample mass, as functions of the impregnated Fe2(SO4)3, H2SO4, and ZnCl2 concentrations.

concentrations, a feature that can permit at a real scale some fluctuations in the actual impregnated amount of additive without any significant effect on the desired product. In quantitative terms, the yields of these compounds are comparable with those obtained with the H2SO4 treatments: together with significant yields of FF, the production of LGone and DGP is also high. Indeed, the optimal conditions (Fe2(SO4)3 concentrations around 3.7%) correspond to a maximum in the actual yields of FF around 5.4% (versus 5 and 5.8% for H2SO4 and ZnCl2) but potential yields increase to about 10% (versus 8.7 and 6.6%). Therefore, as already observed, it can be thought that Fe2(SO4)3 mainly catalyzes primary decomposition reactions and shows better performances than the other two catalysts. Finally, at low concentrations (around 0.7%) Fe2(SO4)3 highly favors the production 1527


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formation of products potentially noxious should also be taken into consideration. It can be reasonably assumed that the failure in the perfect mass closure (around 91−92%), which has not been optimized for the laboratory scale reactor used in this study, is due to the well-known difficulties in the complete collection of the liquid products, mostly those derived from lignin decomposition in the form of aerosols.53 Therefore, in a two-step catalytic process, based on the application of H2SO4 or Fe2(SO4)3 as primary catalysts and ZnCl2 as secondary catalyst, the total organic fraction of liquid products would be around 30%, which is constituted by about 33% FF, 20% AA, and the remaining fraction by phenolic compounds and pyrolytic lignin.1 The high reduction in the number of chemical compounds of the liquid-phase product can highly simplify the FF extraction process, which can be based, for instance, on successive distillation.46 Apart from FF, AA is also an economically valuable product,54 while the other fraction can be exploited for the usual energetic applications or the production of biomaterials.1 Also, the gas produced (yields around 10%) can be exploited to partly provide the heat needed for the pyrolysis process. Some cleaning of the gas stream may be required, to remove for instance SO3, whereas the acidic aqueous solution can be continuously applied, upon adequate adjustments, for feedstock impregnation. It is desirable that future investigations carry out careful analyses about the properties and reactivity of chars produced from the catalytic pyrolysis of biomass. Indeed, char is a product quantitatively very important and may be associated with possible pollution problems, as it retains a large part of the catalysts and/or products of their transformations. Chars resulting from the low-temperature pyrolysis of Fe2(SO4)3 treated biomass are known to become more reactive in both combustion and gasification.55 Indeed, this has been indicated as a dual-function catalyst for the two stage utilization of biomass (pyrolysis followed by char conversion). Another possible use of this char is as raw material for carbon−iron sorbents.8 In this case, the iron present in the char, owing to its redox properties, permits the activation of the pore network without the further addition of activating agents. Char is also produced in significant amounts from the pyrolysis of corncobs catalyzed by H2SO4. Based on the decomposition reactions of this additive, it is likely that no significant residue is retained by the char. This hypothesis is confirmed by the scanning electron microscopy (SEM) pictures.27 Hence, it is possible that, in this case, alternative applications,2 in combination with the use as biofuel, can be sought. On the other hand, in the case of ZnCl2 addition to corncobs as primary catalyst, during successive combustion or gasification of char, problems may arise such as ash deposition, corrosion, and harmful emissions of gases and particulate matter.24 Instead, the use of ZnCl2 as a secondary catalyst for the gas/vapor streams completely avoids such drawbacks.

Figure 11. Yields of 1,4:3,6-dyanhydro-α-D-glucopyranose (DGP) and levoglucosenone (LGone), expressed as percent of the initial sample mass, as functions of the impregnated Fe2(SO4)3, H2SO4, and ZnCl2 concentrations.

Figure 12. Yields of levoglucosan (LG), acetic acid (AA) and 5hydroxymethylfurfural (5-HMF), expressed as percent of the initial sample mass, as functions of the impregnated Fe2(SO4)3, H2SO4, and ZnCl2 concentrations.

Figure 13. Ratio of the peak area (peak area of the compound/peak area of the internal standard) for 4-hydroxy-5,6-dihydro-(2H)-pyran-2one (HDP) and 1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one.

of LG, 5-HMF, 5-MF, and mainly HDP and LAC. In particular, it is worth noting the latter two compounds are produced in very small quantities by the other two catalysts (the LAC peak is practically zero for the H2SO4 treatment). Some Considerations on the Selection of the Optimal Catalyst. The main specific feature of biomass pyrolysis is the high number of products that, though drastically modified in the presence of catalysts, never results in a single compound. Thus, in the selection of the optimal catalyst for FF production from corncob pyrolysis, in addition to the yields of this compound, yields and properties of the other products and

■

CONCLUSIONS The effects of a number of catalysts (H2SO4, H3PO4, H3BO3, (NH4)2SO4, ZnCl2, NiCl2, MgCl2, and Fe2(SO4)3), at selected concentrations, have been evaluated on the product yields of corncob pyrolysis, in particular FF. Moreover, for Fe2(SO4)3, experiments at variable concentrations have been made and compared with results already available for H2SO4 and ZnCl2. The addition of catalysts to the substrate has been made by means of water impregnation followed by a drying stage. 1528


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The presence of acidic catalysts always make slower the conversion process although these effects are relatively small. They are especially evident in some cases and plausibly associated with the formation of an azetropic water/acid mixture (H2SO4) or the removal of significant amounts of crystallization water at relatively high temperature (NiCl2, MgCl2, and Fe2(SO4)3). In all cases, dehydration and depolymerization reactions are favored, which lead to a decrease in the total yields of condensable organic products to the advantage of char and water production with only small diminutions in the yields of CO2 and CO. The production of LGone, DGP, and FF is enhanced. The more effective catalysts for the production of FF are H2SO4 and Fe2(SO4)3, which act essentially on the primary decomposition reactions, and ZnCl2, which mainly favors the secondary degradation reactions of anhydrosugars. The actual maximum yields are around 5−6%; however, it could be advantageous to combine primary catalytic pyrolysis of corncobs with secondary catalytic degradation of the gas/ vapor stream produced. In this way, the maximum potential yields of FF, considering the complete conversion of anhydrosugars, increase up to about 10 and 9% using, as primary catalysts, Fe2(SO4)3 or H2SO4, and possibly ZnCl2, as secondary catalyst for the gas/vapor stream. These results are encouraging and may justify future efforts to actually evaluate the practical feasibility of FF production via catalytic pyrolysis. For the conditions/catalysts that maximize the production of FF, AA yields are also quite high (around 4−6%) although maximum values (around 8%) are obtained with the use of NiCl2. Another product quantitatively important is char obtained in yields around 25−28% (catalyst-free basis). However, when the production of char is of interest, improved yields (up to about 30%) are obtained with the use of H3PO4, which also maximizes the production of LGone (yields around 5%).

■

■

4-MG = 4-methylguaiacol 5-HMF = 5-hydroxy-methylfurfural 5-MF = 5-methylfurfural

REFERENCES

(1) Czernik, S.; Bridgwater, A. V. Overview of applications of biomass fast pyrolysis oil. Energy Fuels 2004, 18, 590−598. (2) Antal, M. J.; Mok, W. S. L.; Varhegyi, G.; Szekely, T. Review of methods for improving the yields of charcoal from biomass. Energy Fuels 1990, 4, 221−225. (3) Antal, M. J.; Gronli, M. G. The art, science and technology of charcoal production. Ind. Eng. Chem. Res. 2003, 42, 1619−1640. (4) Krieger-Brockett, B. Microwave pyrolysis of biomass. Res. Chem. Intermed. 1994, 20, 39−49. (5) Szabo, P.; Varhegyi, G.; Till, F.; Faix, O. Thermogravimetric/ mass spectrometric characterization of two energy crops, Arundo donax and Miscanthus sinensis. J. Anal. Appl. Pyrolysis 1996, 36, 179− 190. (6) Jakab, E.; Faix, O.; Till, F. Thermal decomposition of milled wood lignins studied by thermogravimetry/mass spectroscopy. J. Anal. Appl. Pyrol. 1997, 40−41, 171−186. (7) Nowakowski, D. J.; Jones, J. M.; Brydson, R. K.; Ross, A. B. Potassium catalysis in the pyrolysis behavior of short rotation willow coppice. Fuel 2007, 86, 2389−2402. (8) Dobele, G.; Urbanovich, I.; Zhurins, A.; Kampars, V.; Meier, D. Application of analytical pyrolysis for wood fire protection control. J. Anal. Appl. Pyrolysis 2007, 79, 47−51. (9) Di Blasi, C.; Galgano, A.; Branca, C. Effects of potassium hydroxide impregnation on wood pyrolysis. Energy Fuels 2009, 23, 1045−1054. (10) Di Blasi, C.; Galgano, A.; Branca, C. Influences of the chemical state of alkaline compounds and the nature of alkali metal on wood pyrolysis. Ind. Eng. Chem. Res. 2009, 48, 3359−3369. (11) Di Blasi, C.; Galgano, A.; Branca, C. Analysis of the physical and chemical mechanisms of potassium catalysis in the decomposition reactions of wood. Ind. Eng. Chem. Res. 2011, 50, 3864−3873. (12) Shafizadeh, F. Thermal conversion of cellulose materials to fuel and chemicals. In Wood and Agricultural ResiduesResearch on Use for Feed, Fuels, And Chemicals; Soltes, J, Ed.; Academic Press: New York, 1983; pp 415−438. (13) Shafizadeh, F. The chemistry of pyrolysis and combustion. In The Chemistry of Solid Wood, Advances in Chemistry Series 207; Rowell, R., Ed.; ACS: Washington, DC, 1984; pp 489−529. (14) Richards, G. N.; Zheng, G. Influence of metal ions and salts on products from pyrolysis of wood: applications to thermochemical processing of newsprint and biomass. J. Anal. Appl. Pyrolysis 1991, 21, 133−146. (15) Dobele, G.; Rossinskaja, G.; Telysheva, G.; Meier, D.; Faix, O. Cellulose dehydration and depolymerization reactions during pyrolysis in the presence of phosphoric acid. J. Anal. Appl. Pyrolysis 1999, 49, 307−317. (16) Dobele, G.; Meier, D.; Faix, O.; Radtke, S.; Rossinskaja, G.; Telysheva, G. Volatile products of catalytic flash pyrolysis of celluloses. J. Anal. Appl. Pyrolysis 2001, 58−59, 453−463. (17) Dobele, G.; Dizhbite, T.; Rossinskaja, G.; Telysheva, G.; Radtke, S.; Meier, D.; Faix, O. Pre-treatment of biomass with phosphoric acid prior to fast pyrolysis. A promising method for obtaining 1,6anhydrosaccarides in high yields. J. Anal. Appl. Pyrolysis 2003, 68−69, 197−211. (18) Dobele, G.; Rossinskaja, G.; Dizhbite, T.; Telysheva, G.; Meier, D.; Faix, O. Application of catalysts for obtaining 1,6-anydropolysaccarides from cellulose and wood by fast pyrolysis. J. Anal. Appl. Pyrolysis 2005, 74, 401−405. (19) Di Blasi, C.; Branca, C.; Galgano, A. Effects of diammonium phosphate on the yields and composition of products from fir wood pyrolysis. Ind. Eng. Chem. Res. 2007, 46, 430−438.

AUTHOR INFORMATION

Corresponding Author

*Tel: 39-081-7682232. Fax: 39-081-2391800. E-mail: diblasi@ unina.it. Notes

The authors declare no competing financial interest.

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ACRONYMS FOR CHEMICAL COMPOUNDS AA = acetic acid CR = o-,m-,p-cresol DGP = 1,4:3,6-dianhydro-α-D-glucopyranose FF = furfural FFA = furfuryl alcohol G = guaiacol HAA = hydroxyacetaldehyde HP = hydroxypropanone HDP = 4-hydroxy-5,6-dihydro-(2H)-pyran-2-one LAC = 1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one LG = levoglucosan LGone = levoglucosenone PH = phenol S = syringol 2-EPH = 2-ethylphenol 4-MS = 4-methylphenol 2(5H)Fone = 2−5H-furanone 4-EG = 4-ethylguaiacol 1529


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Catalyst Screening for the Production of Furfural from Corncob

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