Article pubs.acs.org/EF
Hydro-Pyrolysis of Biomass and Online Catalytic Vapor Upgrading with Ni-ZSM‑5 and Ni-MCM-41 F. Melligan, M. H. B. Hayes, W. Kwapinski, and J. J. Leahy* Carbolea Research Group, Department of Chemical and Environmental Sciences, University of Limerick, Ireland ABSTRACT: A catalyst reactor coupled with analytical pyrolysis gas chromatography/mass spectroscopy (Py-GC/MS) was used to carry out online analysis of the product vapors from the fast pyrolysis of Miscanthus x giganteus, Scots pine, and mahogany. Pyrolysis was carried out in both an inert atmosphere of He gas and in a highly reducing atmosphere of H2. Significant changes in the vapor compositions were achieved with the use of H2 as the carrier gas. The most notable of these were the increases in the hydrocarbon compositions. The roles of ZSM-5, Ni/ZSM-5, MCM-41, and of Ni/MCM-41 catalysts on the compositions of the pyrolysis vapors were investigated. Lower amounts of the higher molecular weight phenolic compounds and larger amounts of the lighter phenols were observed in the presence of Ni supported on ZSM-5 and MCM-41. This effect was more evident for the 10% than for the 2.5% Ni loadings. Overall, the results both from the use of H2 as the carrier gas and from all the catalysts demonstrates significant improvements in the composition of the vapors. However, this resulted in the lowering of the quantities of condensable products.
1.1. INTRODUCTION Bio-oil, an important product from biomass pyrolysis, can be regarded in its crude state as a low grade fuel. Many pyrolysis process parameters, such as temperature, pressure, heating rate, reactor configuration, biomass type, and particle size have been extensively studied1−3 and summarized.4−6 The majority of studies have used inert gas, usually N2 at ambient pressure. Biooil from this process possesses many undesirable properties, such as high quantities of carboxylic acids, water, and oxygenated molecules; furthermore, the H/C ratio is low. Thus, there is a very limited demand for the product. To meet the requirements for a fuel-oil, bio-oil produced by this method must undergo extensive and, in most cases, expensive upgrading processes. To achieve a better quality fuel, the process used must remove oxygen, convert carboxylic acids and other reactive species to more benign products, and also add hydrogen to the bio-oil. To date, the methods that have provided the most useful products are hydrodeoxygenation of the bio-oil and catalytic processing of the pyrolysis vapors. Neither method is without major problems. Bridgwater et al.7 has reported that it is possible to obtain hydrocarbon yields of up to 58% by weight of liquid bio-oil through hydrodeoxygenation. However, this process can be expensive and slow. Elliott et al.8 have shown that single stage hydrotreating is an inappropriate method, due to the formation of large quantities of both coke and tar, causing rapid catalyst deactivation and reactor clogging. To combat these problems, a multi stage process was developed, involving initially the stabilization of the bio-oil, followed by a more aggressive hydrotreatment.8 Similar issues, such as catalytic deactivation have been encountered with catalytic upgrading of pyrolysis vapors.9 Also, biomass is hydrogen deficient,10 and further hydrogen depletion takes place during catalytic upgrading of the vapors because oxygen is usually removed as H2O. When supplementary hydrogen is present during the pyrolysis process, very reactive H• radicals are generated. These react readily, adding hydrogen to biomass fragments, while simultaneously © 2012 American Chemical Society
removing oxygen and capping free radicals, thereby increasing hydrocarbon production and yielding an improved product. However, until recently, there have been limited reports on the use of ‘active’ gases such as H2 for pyrolysis. Thangalazhy et al.11 studied the production of hydrocarbon fuels from biomass pyrolysis using ZSM-5 as a catalyst under both He and H2 environment. Zhang et al.12 carried out a more detailed study, where N2, CO, CH4, and H2 were used as gases for the pyrolysis of corncobs. Other studies involving active carrier gases were carried out by Minkova et al.13 and Jindarom et al.14 All studies concluded that the type of carrier gas plays an important role in both the product distribution and composition. The higher heating value (HHV) of the bio-oil obtained under N2 was 17.8 MJ/kg, while that from H2 was 24.4 MJ/kg.15 Although use of atmospheric pressure pyrolysis with H2 results in an increase in HHV, some of the inherent problems associated with crude bio-oil still remain, such as thermal instability and immiscibility with crude-oil-based fuels, requiring further catalytic treatment.15 To date, zeolite and modified zeolite catalysts have received the majority of attention as materials for the catalytic upgrading of pyrolysis vapors. Several researchers have investigated the use of the zeolite based ZSM-5 and modified ZSM-5 catalysts for producing improved products.16−19 Pyrolysis vapors when passed over an acidic zeolite catalyst can become deoxygenated through simultaneous dehydration and decarboxylation reactions. An extensive study of zeolite catalysts, particularly modified ZSM-5, was carried out by French et al.20 They investigated 40 different catalysts and observed that the highest hydrocarbon yield was obtained from the Ni/ZSM-5 catalysts. The incorporation of transition metals, such as nickel, was Received: July 25, 2012 Revised: September 17, 2012 Published: September 18, 2012 6080
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However, as Azeez et al.30 have shown, there is close agreement between the composition of the volatile pyrolysis products from Py-GC/MS and bench-scale pyrolysis. Their data suggest that Py-GC/MS is a reliable technique for the qualitative determination of the pyrolysis products and for the study of reaction mechanisms. Py-GC/MS does not allow the products to be collected. Thus, it is not possible to obtain a mass balance. The GC/MS technique provides the chromatographic peak area of each compound which can be considered proportional to its quantity. However, due to the complexity of biomass pyrolysis, the accuracy of quantitative determination is questionable and was therefore not attempted in this study. Other investigations of biomass, using similar instrumentation, also do not carry out quantitative analysis.23,24
shown to increase the yields of aromatics; furthermore, metal impregnation will improve the hydrothermal stability of the ZSM-5.21 There are, however, drawbacks to the use of ZSM-5 for this process, even though its use can give rise to significant improvements in the bio-oil properties. Problems arise because of the rapid deactivation of the catalyst; caused by severe coking on the catalyst surface.9 Also the catalyst leads to a decrease in the yield of the organic fraction.22 In attempting to overcome these problems, mesoporous catalysts, with pore sizes greater than those of traditional zeolites, have been studied. These have the potential to crack large molecules, to be less prone to deactivation, and to give higher yields of organic liquid.23 MCM41, an extensively researched mesoporous material, with larger pore sizes, compared to ZSM-5,24 might be expected to be beneficial for the reforming of high molecular weight molecules from biomass pyrolysis.25 Larger pore size, combined with transition metals, has been shown to decrease the levels of ethanoic acid in pyrolysis vapors from analytical pyrolysis.24 The addition to the feedstock of MCM-41, prior to pyrolysis, can also decrease the content of unwanted carboxylic acids and carbonyl functionalities, such as those of carbaldehydes and ketones (which give undesirable properties to the bio-oil), while increasing the quantity of desirable phenols and hydrocarbons.26 Phenols and hydrocarbons are useful as fuels and have potential as platform chemicals. The upgrading method investigated in this study can be regarded as a mild hydrodeoxygenation comprising of hydrogenation, cracking, and decarboxylation reactions. The primary aim of the process is to improve the stability of the bio-oil by promoting the production of hydrocarbons and less reactive compounds. The pyrolysis reaction was carried out in an atmosphere of hydrogen, followed by catalytic treatment in a separate reactor. The catalysts investigated in this study were ZSM-5, Ni/ZSM-5, MCM-41, and Ni/MCM-41 with 2.5% and 10% Ni loading. The second section of the paper compares pyrolysis and hydropyrolysis of three different types of biomass, Miscanthus x giganteus (grass), Pinus sylvestris (Scots pine, softwood), and Swietenia mahagoni (mahogany, hardwood). This investigation will determine if the catalytic hydropyrolysis process will promote improvements, such as deoxygenation and hydrogenation in the pyrolysis vapors irrespective of the lignocellulosic composition of the biomass. 1.2. Py-GC/MS for Catalyst Screening. Useful fuels and valuable platform chemicals can be obtained from biomass pyrolysis.27,28 Py-GC/MS is a valuable tool for determining how reaction conditions and the use of catalysts can affect the quantities of individual compounds that are produced from the pyrolysis of biomass and thus its usefulness as a fuel. Park et al.29 have characterized compounds of bio-oils as either desirable or undesirable products. The so-called “desirable compounds” include phenolics, hydrocarbons, and alcohols, which have good fuel properties, and the undesirable compounds are carboxylic acids, aldehydes, ketones, and heavier oxygenates. High levels of aldehydes and ketones result in unstable bio-oils, and heavily oxygenated compounds increase the viscosity of the bio-oil. The pyrolysis reactor used for this study allows variations in process parameters, such as reaction time and temperature, heating rate, as well as both carrier gas flow and composition. Py-GC/MS carries out discontinuous pyrolysis; therefore, the composition of the product stream varies throughout time.
2. MATERIALS AND METHODS 2.1. Catalyst Preparation. MCM-41 and ZSM-5 were obtained from Alfa Aesar. All catalysts used in this study were prepared by wet impregnation using nickel nitrate (Alfa Aesar). Various amounts of nickel nitrate hexahydrate, depending on the quantity of nickel required on the catalyst, were added to water. This nickel solution was then added to the support, thoroughly mixed, and then dried at 105 °C. This was followed by calcination in air; samples were heated at 10 °C/min up to 450 °C with a hold time of 2 h. 2.2. Catalyst Characterization. XRD analyses were carried out on a Philips X′Pert Pro X-ray diffractometer with nickel filtered Cu Kα radiation (λ = 1.542 Å) between the X-ray source angles of 10° and 70°. Prior to analysis, all samples were ground using a mortar and pestle and then made into self-supporting disks. TEM (JEOL JEM2100F) analysis was used for the measurement of the metal particle size on the surface of the support. Prior to TEM analysis, all catalysts were reduced for 2 h at 400 °C under a constant flow of hydrogen. The specific surface area measurements and the pore size distributions of the catalytic materials were measured on a Micrometrics ASAP 2010 system. Nitrogen was used as the adsorptive and the adsorption process was carried out at −196 °C. Typically, the sample (100 mg) was degassed under vacuum at 200 °C for 12 h. The BET surface area was determined from the N2 adsorption−desorption isotherms. The pore volume and distribution were calculated from the adsorption isotherm using the Barrett−Joyner−Halenda theory. A measure of Brönsted acidity was made using NaCl (aq) as an ion-exchange agent. For this analysis, 0.05 g of catalyst was added to 40 mL of an aqueous solution of 0.1 M NaCl and was then allowed to equilibrate for 24 h. The resulting suspension was then titrated by the dropwise addition of standardized 0.1 M NaOH (aq). 2.3. Biomass Characterization. The moisture content of the feedstock was measured by placing the sample in an oven at 105 °C and weighing until a constant value was reached. Weight percentage of C, H, and N were measured using an Elementar Vario el Cube, according to ASTM d4442 (sulfanilamide was used as a standard). Oxygen was determined by difference. The HHV was measured using an oxygen bomb calorimeter 6200 no. 442 m (Parr Company), according to ASTM d240 (benzoic acid (99.5% Sigma Aldrich) was used as a standard). Volatile material associated with the biomass was determined from weight loss on heating for 7 min at 900 °C, according to CEN/TS 15148:2005. Cellulose, hemicellulose, and lignin contents were measured according to ASTM E1758-01. Thermogravimetric analysis was carried out using a TA Instruments SDT Q600. Analysis was carried out with samples of approximately 10 mg, with a nitrogen flow rate of 100 mL/min. The heating rate was 10 °C/min, with a final temperature of 800 °C. 2.4. Py-GC/MS. Pyrolysis was carried out using a CDS Analytical 5200 pyroprobe. This instrument contains two separate reactors: the first is for pyrolysis; the second is a catalyst reactor. For all experiments, 1.5 mg (±0.01 mg) of Miscanthus x giganteus (M) was placed in a quartz tube, and a plug of quartz wool was positioned above and below the feedstock. Due to the strict sample preparation process, the corresponding peak areas of the individual compounds 6081
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can be compared, indicating the changes of their yields. Moreover, the peak area percentage of the compounds can be compared to show the changing of the relative amount of each compound in the pyrolysis vapors. Catalyst (5 mg) was placed in the catalyst reactor. Due to the large amount of catalyst compared to biomass, a high gas hourly space velocity (GHSV) was used to allow a clearer distinction to be made between the activities of the catalysts. The GHSV was estimated to be about 240 000 h−1. Prior to all reactions, the catalysts were reduced at 400 °C under a flow of 40 cm3/min of H2 for 2 h. Pyrolysis was also carried out under a flow of either H2 or He. The reactor heating rate was approximately 20 °C/ms (20 000 °C/s) and was maintained at 600 °C for 20 s. Because of the small biomass sample size, the temperature lag was minimal. After pyrolysis, the vapors passed through the catalyst reactor held at 300 °C. All product vapors were then trapped on a Tenax adsorption column (held at 30 °C). Following vapor entrapment, the adsorption column was rapidly heated to 280 °C in order to desorb the vapors. These were passed directly to the GC/MS (Agilent GC 7890A and Agilent MS 5975C). The injector temperature was kept at 280 °C. The chromatographic separation was carried out using a RTX-5MS capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness). Helium (CP grade) was used as the carrier gas with a constant flow rate of 15.28 cm3/min and a 1:50 split ratio. The oven temperature was programmed from 30 (10 min hold time) to 280 °C at a heating rate of 5 °C/min and a final hold time of 10 min. The temperature of the GC/MS interface was held at 250 °C. The mass spectrometer was operated in EI mode at 70 eV. The mass spectra were obtained from m/z 40 to 400 with the scan rate of 500 amu/s. Identification of chromatographic peaks was determined according to the NIST MS library, and in cases where low probabilities were indicated by the library, identification was made starting from the top mass. Experiments were carried out at least three times for each catalyst in order to confirm the reproducibility of the procedures. To confirm the proposed reaction mechanisms, 2,3-dihydrobenzofuran (98%) and methoxy-vinylphenol (98%) were exposed to the same hydropyrolysis conditions as the biomass, before being passed into the GC/MS for analysis.
therefore mildly acidic, with a measured acidity of 9.76 mmol H+/g (Brönsted acid sites). Incorporation of Ni lowered the number of Brönsted acid sites present on the catalyst, due to the occlusion of acidic sites by the Ni particles. This was also observed by Iliopoulou et al.31 MCM-41 is a pure silica catalyst and therefore does not contain any acid sites. 3.2. XRD Analysis. Figure 1 shows the XRD spectra for the four catalysts containing nickel, which were used during the
Figure 1. XRD spectra (from top to bottom) for 10% and 2.5% Ni on ZSM-5 and 10% and 2.5% Ni on MCM-41 catalysts.
study. The peaks observed at 2θ values of 37.5, 43.4, and 62.9 in the XRD patterns (Figure 1) of the calcined materials can be attributed to NiO. These peaks were assigned to NiO (101), NiO (012), and NiO (110), respectively.32 These peaks were much smaller for both catalysts containing 2.5% Ni, indicating that the Ni is present as much smaller crystal sizes. Several other peaks were identified on the ZSM-5 catalysts, which can be attributed to the alumina within the ZSM-5 matrix; however, as the MCM-41 is a pure silica material, no other peaks were present on it spectra. The metal crystal size was estimated using the Scherrer equation. The estimated crystal sizes (NiO 012) for 10% Ni on ZSM-5 and MCM-41 were 18.4 and 6.6 nm, respectively. Small crystal size indicates that the particles are more evenly dispersed throughout the support, which will increase the number of metal sites, thus increasing catalysts activity. The peak area was too small to accurately calculate the crystal size for the catalysts with 2.5% Ni. The decreases in the peak size indicate that the metal was present primarily within the support matrix and as particles 95%) of the methoxy-ethenylphenol 6086
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Figure 8. Percentage peak areas for benzene derivatives from the pyrolysis of Miscanthus.
Scheme 2. Example of a Possible Reaction for the Formation of Benzene, Toluene, and Ethylbenzene
Scheme 3. Possible Reaction Mechanism for the Conversion of 2,3-Dihydro-benzofuran
remained unchanged; however, with the introduction of hydrogen, the double bond on the ethenyl group was easily hydrogenated to form methoxy-ethylphenol. Over 50% of the starting material was hydrogenated to methoxy-ethylphenol; small amounts of aromatic hydrocarbons were also observed, and these amounts increased significantly with the introduction of the catalysts, and in the case of the 10%Ni on ZSM-5 catalyst, about 6%, 3.5%, and 3.1% of ethylbenzene, toluene, and benzene, respectively, were formed. 3.6.4. Effect of Catalysts on Cellulose Derivatives. Figure 9 shows the catalytic effect on the yields of 1,4:3,6-dianhydro-αD-glucopyranose and furfural. Degradation of cellulose, a glucose ß-(1→4) linked homopolysaccharide, occurs mainly via
two competing reaction pathways. The first, a ring scission mechanism will result in the formation of large amounts of volatiles such as CO2, methanol, and ethanoic acid. In the second mechanism, the depolymerization of the cellulose in the degradation pathways would give rise to various types of anhydro oligosaccharides, monomeric anhydrosugars, furans, cyclopentanones, pyrans, and 1,4:3,6-dianhydro-α-D-glucopyranose. The percentage of 1,4:3,6-dianhydro-α-D-glucopyranose was observed to fall significantly after the treatment with H2 and in the presence of the catalysts, in the case of the ZSM-5 treatment complete elimination was achieved (Figure 9). This can be attributed to the additional acid sites on the ZSM-5 compared to MCM-41, suggesting that the acid sites on the support are very important for the degradation process. However, the effect of the catalysts is quite minimal when compared to the effect of the addition of hydrogen to the system. With 6087
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Figure 9. Effects of the catalysts on 1,4:3,6-dianhydro-α-D-glucopyranose and furfural from the pyrolysis of Miscanthus.
Figure 10. Effects of the catalysts on the production of 2,3-dihydrobenzofuran from the pyrolysis of Miscanthus.
the introduction of hydrogen, the level decreases from approximately 1.8% to below 0.25%; when the catalyst was introduced in the inert atmosphere there was no significant change. Furans are also valuable products with uses as organic solvents and as potential sources of fuel additives. Previous work has shown that furan compounds are produced in low quantities during the fast pyrolysis of biomass.40 According to Lu et al.,40 furans result from dehydration reactions of carbohydrates and these reactions can be promoted with catalysts containing either acid or metal sites. Paine et al.41 proposed several possible reaction mechanisms for the formation of furfural from the pyrolysis of glucose. A high yield of furfural can be obtained from xylose (up to 90%), whereas yields from glucose are much lower.42 Other compounds, detected in much smaller amounts, which can be related to the degradation of carbohydrates, include furan, methylfurfural, 5-hydroxylmethylfurfural, and levoglucosan. Figure 10 shows the impact of H2 and the selected catalysts on the yields of 2,3-dihydro-benzofuran. Fast pyrolysis of Miscanthus produced furans as major products, the most abundant of which was 2,3-dihydro-benzofuran (about 10%) (Figure 10). 2,3-Dihydro-benzofuran, is a product of both cellulose and lignin pyrolysis. In the presence of H2, the amount of 2,3-dihydro-benzofuran was lower, and the addition of the catalysts resulted in a further decrease in the relative peak area. A possible reaction mechanism for the transformation is shown in Scheme 3. The reaction shown in Scheme 3 is promoted in the presence of H2 and is further promoted by metal sites on the catalysts. ZSM-5, which has acid sites (unlike the MCM41), shows higher conversion, indicating that acid sites play an important role on the promotion of this reaction. All results presented in section 3 signify the importance of finding a balance between metal content and acidity of the catalysts. Metal sites are important for the depolymerization of lignin, indicated by the increase in the level of monomeric phenols in the presence of 10%Ni/ZSM-5 and 10%Ni/MCM-41. However, for the promotion of deoxygenation reactions,
particularly dehydration reactions, catalyst acidity plays a more important role than metal sites. The combination of both metal and acid sites in the 10%Ni/ZSM-5 catalysts wass shown to be a well balanced catalyst for the upgrading of the pyrolysis vapors, so it was chosen as the catalyst for the next stage of this investigation. The proposed reaction mechanism shown in Scheme 3 was confirmed by carrying out pyrolysis and hydropyrolysis on pure 2,3-dihydro-benzofuran. Conventional pyrolysis had very little impact on the 2,3-dihydro-benzofuran; with the incorporation of hydrogen into the reaction, the primiary decomposition product of 2,3-dihydro-benzofuran was ethylphenol (∼15%), with small amounts of aromatic hydrocarbons also observed (less than 1% in all cases). The overall conversion of 2,3dihydro-benzofuran increased to approximately 50% with the inclusion of the 10%Ni on ZSM-5 catalyst. Again, the most common decomposition product was ethylphenol. 4.1. Comparison of Pyrolysis and Hydropyrolysis of Hardwood, Softwood, and Energy Grass. Figure 11 shows the effect of H2 and catalysis (10% Ni on ZSM-5) on the total peak area obtained for the three feedstocks. The largest total peak area was obtained for pyrolysis of the wood materials. However, it should be noted that while the woods produced a larger total peak area, a larger portion of this is made up of high molecular weight compounds that could not be accurately identified by MS. For all three feedstocks, the peak area trends were comparable. The peak areas decreased during hydro-pyrolysis because dehydration reactions were promoted and larger amounts of noncondensable gases were formed. A further decrease in the total peak area was observed when the catalyst was included in the system. The catalyst used was the 10% Ni/ ZSM-5, described in section 3.1. Table 3 shows the peak area percentages of the compounds identified in the highest abundance. All compounds shown have a molecular weight less than 170 and can therefore be considered as low to medium molecular weight compounds. 6088
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4.2. Effect of Biomass Type on Carboxylic Acid Formation. During the pyrolysis of cellulose, two competing reactions occur. The cellulose can undergo either depolymerization or ring scission. Both of these reactions will result in the formation of very different compounds. The depolymerization process forms mainly anhydro oligosaccharides, monomeric anhydrosugars, furans, cyclopentanones, pyrans, and other related derivatives. The ring scission mainly produces hydroxyacetaldehyde, acetol, linear carbonyls, linear alcohols, esters, and other products.43,44 Very low levels of organic acids were produced for both of the wood biomasses, with the calculated peak areas less than half of that obtained for the pyrolysis of Miscanthus under similar reaction conditions. Hydropyrolysis caused acid increases of almost 300% for the softwood and about 250% in the case of hardwood. These increases correspond to a large decrease in the formation of monomeric anhydrosugars, particularly 1,4-anhydro-α-D-glucopyranose. This result may suggest that, during the conventional pyrolysis, cellulose primarily undergoes the depolymerization reaction pathway; however, with the use of H2, the reaction mechanism is altered and ring scission becomes more prominent. However, to obtain a definite conclusion, further work is required. 4.3. Effect of Biomass Type on Phenols and Aromatic Hydrocarbons. The yields of phenols produced from pyrolysis and hydropyrolysis are shown in Table 3. Trends varied between the woods and the Miscanthus. With addition
Figure 11. Total peak areas for the products of pyrolysis and of hydrogenolysis of Scots pine (softwood), mahogany (hardwood), and Miscanthus x giganteus (grass).
Table 3. Percentage Peak Areas for the Major Compounds Found in Pyrolysis Vapors of Scots Pine (SP), Mahogany (MH), and Miscanthus x giganteus (M) He SP
He MH
He M
H2 SP
H2MH
H2M
H2 cat SP
H2 cat MH
H2 cat M
acetic acid
3.42
4.54
11.84
9.87
10.13
10.01
2.6
1.57
4.01
phenol 4 methyl phenol 2-methyl phenol 4-ethyl phenol 4-ethyl-3-methyl phenol 2-methoxy phenol 2-methoxy-4-methyl phenol 4ethyl-2-methoxy phenol total low MW phenols (MW < 155)
0.79 0.3 0.93 0.79 0.69 1.79 2.46 1.36 9.11
3.17 1.45 1.5 1.26 0.31 2.1 1.42 11.21
0.58 0.6 0.62 0.85 0 6.58 1.25 0.86 11.34
1.02 0.85 1.63 2.96 1.91 3.78 4.87 1 18.02
1.66 0.48 1.45 1.21 1.5 5 3.03 2.02 16.35
1.14 0.66 0.21 9.03 1.67 0.1 0.27 0.77 13.85
1.91 1.35 2.26 0.6 1.53 1.18 3.14 1.22 13.19
1.67 1.63 2.7 0.37 0.76 1.71 3.79 3.5 16.13
3.83 1.46 2.04 12.86 3.88 2.59 0.65 3.8 31.11
2-methoxy-4-vinyl phenol 2-methoxy-4-(1-propenyl) phenol 2-methoxy-4-(2-propenyl) phenol total high MW phenols (MW >155)
2.92 1.49 1.37 5.78
7.73 0.92 2.85 11.5
3.72 1.18 2.85 7.75
2.63 0.47 1.15 4.25
6.47 0.06 3.05 9.58
0.54
2.29 1.5 3.79
0.23 2.76 0.75 3.74
4.16 1.48 0.42 6.06
benzene toluene ethylbenzene butyl benzene Propyl benzene 1,3-Dimethyl-benzene 1-ethyl-2-methyl-benzene 1,3,5-trimethyl-benzene total aromatic hydrocarbons
0.12 0.97 0.09
1.44 2.32 0.56
0.74 3.11 1.09
2.92 5.88 0.57
0.97
0.2 0.87 0.14 0.64 0.27 0.01 0.12
4.32
2.27
5.83
9.37
2.29 9.97 1.82 1.33 2.36 1.99 0.53 1.58 21.86
11.97 12.78 1.41 0.22 1 2.62
2.15
4.31 0.26 5.71 0.48 2.25 2.32 2.87 0.39 18.58
1.68 31.67
2.75 9.36 4.82 0.24 2.77 0.94 0.65 0.2 21.73
furfural di anhydro glucopyranose glucopyranose total carbohydrate derivatives
0.38 1.29 7.85 9.52
1.74 0.89 7.23 9.86
0.99 1.92 1.02 3.93
1.22 3.71 1.29 6.22
2.82 2.05 1.54 6.41
0.99 0.22 0.22 1.43
2.66 0 0 2.66
3.83 0 0 3.83
1.41 0 0 1.41
0.89
6089
0.74 1.28
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Energy & Fuels
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of the hydrogen, the percentage of light phenols (and the total level of light phenols) increased for all three feedstocks. The largest increase was observed for Scots pine, possibly due to a lower hydrothermal resistance of the lignin (this was confirmed experimentally by thermal analysis). A large increase in light phenols was observed for Miscanthus with the addition of the catalyst; an increase was not observed for the woods. However, the introduction of a catalyst resulted in a much larger increase in aromatic hydrocarbons, such as benzene, toluene, etc., for both the hard and the soft wood. The increases in aromatic hydrocarbons with the use of Ni on ZSM-5 were 16%, 25.8% and 3.1%, for Scots pine, mahogany, and Miscanthus, respectively. This is an important modification in the pyrolysis vapors with respect to the usefulness of the bio-oil as a fuel. The reaction mechanism responsible for this improvement in the pyrolysis vapors is shown in Schemes 2 and 3 and explained in section 3.6.2. This result confirms that, irrespective of the biomass composition, the level of aromatic hydrocarbons can be increased with the use of both hydrogen and Ni supported on an acidic zeolite. The reason for this was explained previously in section 3.6.2 and in Schemes 1, 2, and 3.
5. CONCLUSIONS Major improvements in the compositions of pyrolysis vapors can be achieved by using H2 as carrier gas and also Ni/ZSM-5 and Ni/MCM-41 catalysts. An increase in monomeric phenol was observed in all cases, resulting in a lower average molecular weight of the bio-oil. This was more evident for the catalysts with larger Ni loadings. The acid sites on the catalysts increased dehydration, decarboxylation, and cracking reactions, thereby increasing the yields of the aromatic hydrocarbons. Results indicate that hydro-pyrolysis along with the Ni catalysts promotes the formation of a higher grade bio-oil from lignocellulosic biomass.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge financial support from the Science Foundation Ireland under Grant No. 06/CP/E007. The authors wish to thank colleague Professor James Burdon (University of Limerick and also School of Chemistry, University of Birmingham) for his helpful suggestions and comments.
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dx.doi.org/10.1021/ef301244h | Energy Fuels 2012, 26, 6080−6090
