- Research article
- Open Access
The use of combination of zeolites to pursue integrated refined pyrolysis oil from kraft lignin
© Huang et al.; licensee Chemistry Central Ltd. 2014
Received: 11 November 2013
Accepted: 4 March 2014
Published: 14 March 2014
A mixture of Y and M type zeolites were applied to pyrolyze kraft softwood (SW) lignin with the objective of studying the combination effect of different types of zeolite on pyrolysis. The chemical structures of the subsequent pyrolysis oils were examined. Nuclear Magnetic Resonance (NMR) spectroscopy including 13C, 31P of phosphitylated bio-oils, Heteronuclear Single-Quantum Correlation (HSQC)-NMR, and Gel Permeation Chromatography (GPC) were used to characterize the pyrolysis oils. The yields of pyrolysis products (light oil, heavy oil and char) from the zeolites combination ‘Y + M’ catalyzed pyrolysis ranged between the pyrolysis oil yields from zeolite Y or M catalyzed pyrolysis. 31P NMR analysis of the phosphitylated bio-oils revealed that the mixture of ‘Y + M’ during pyrolysis could decrease the carboxyl groups by 84%, which is close to the effect of the M zeolite. The yields of hydroxyl groups and other functional groups in the ‘Y + M’ generated bio-oil was between the individual Y and M generated oils. The molecular weight of the pyrolysis oil using a zeolite mixture of ‘Y + M’ was similar to the individual zeolite Y assisted pyrolysis. These results show that the zeolite mixture of ‘Y + M’ manifests additive characteristics for pyrolysis.
Over the last century, worldwide energy consumption has increased rapidly, and the known petroleum resources are predicted to be consumed in less than fifty years at present rates of consumption [1, 2]. Lignocellulosic biofuels are a promising fuel platform since substantial amounts of plant/wood residual biomass are readily available, with no competition for food resources and with relatively low environmental impact . There is a vast potential supply of sustainable renewable biomass from forest and fallow lands throughout the world. Indeed, the United States alone has the ability to provide more than 1.3 billion dry tons annually to supply bio-refineries , which is enough to address approximately one-third of current demand for transportation fuels in an environmentally compatible manner .
Lignocellulosic biomass represents a renewable and carbon-neutral resource for the production of bio-fuels and bio-chemicals. Generally, lignocellulosic biomass contains around 35-50% of cellulose, which is a polymer of β-(1,4)-glucan with a degree of polymerization of ∼1000-15,000 ; 25-30% of hemicellulose, which is a short-chain branched and substituted linear polymer of sugars with a degree of polymerization of ~70-200; and another 15-30% of lignin which is a polymer derived from coniferyl, coumaryl, and sinapyl alcohols . Cellulose in lignocellulosic biomass is now generally recognized as a major renewable resource for fuels and chemicals. However, lignin has received much less attention than plant polysaccharides as a resource for biofuels and is generally regarded as a low value compound. For example, large quantities of kraft lignin are produced from the pulp and paper industry that has been mainly used as an energy source in combustion processes and less than 5% of the world’s supply has been used for other purposes . Recently, lignin separation technology has been developed to recover and utilize lignin as resources of biofuels and chemicals . The pyrolysis of lignin, including kraft lignin, is a promising approach to utilize this resource for fuels and aromatic chemicals [9, 10].
The chemical components of the bio-oils from lignin pyrolysis are very complex containing large amounts of aromatic structures (~40-70%), such as phenol, cresol and xylenol, substituted with methoxyl and aliphatic groups [11–14]. However, lignin pyrolysis oils cannot be used directly as fuel due to several unfavorable properties, such as high oxygen content, poor volatility, high acidity and viscosity, which significantly limit its usage [15, 16]. Therefore, upgrading lignin-derived pyrolysis oil is needed so as to facilitate the generation of green diesel and/or gasoline.
Many researchers have examined the use of additives, including zeolite and metal salts to upgrade the properties of bio-oil during the pyrolysis of lignin. Mullen et al.  suggested that H-ZSM-5 zeolite could improve the depolymerization of lignin, and CoO/MoO3 facilitated the production of aromatic hydrocarbons through a direct deoxygenation of methoxyphenol units. French et al.  found that transition metal substituted ZSM-5 zeolites could increase hydrocarbon yields during the pyrolysis of lignin. Zhao et al.  also suggested that zeolites could improve deoxygenation reactions during lignin pyrolysis. Different zeolites have different frameworks and pore sizes which influence pyrolysis product yields and structures. Our previous research studies have investigated the pyrolysis of kraft lignin with various zeolites, including Beta (BEA), Y (FAU), ZSM-5 (MFI), Mordenite (MOR) and Ferrierite (FER) zeolites . The results indicated that the FAU (Y) and BEA (B) zeolites could significantly improve the cleavage of methoxyl-aromatic linkages and ether bonds in the lignin and yield a pyrolysis oil that has a ‘gasoline’ range molecular weight. The MFI (Z), FER (F) and MOR (M) zeolites could more efficiently decompose the carboxyl groups in a bio-oil which reduces the acidity of pyrolysis oils and makes it more suitable for use as a biofuel after hydrotreatment. It’s well known that different types of zeolite could exhibit different catalytic behavior during pyrolysis; however, as far as we know, there are no detailed reports of employing combinations of different types of zeolite to pursue enhanced deoxygenation during pyrolysis.
Based on our previous studies, different zeolites have different effects on the pyrolysis of kraft lignin. In this study, the combination of Y and M type zeolites was applied to pyrolyze kraft lignin to study the additive effect of different types of zeolite on pyrolysis. The pyrolysis oil yield and chemical structures were compared with the individual zeolite Y or M assisted pyrolysis of kraft lignin. A blank pyrolysis test (pyrolysis of kraft lignin without zeolite) was also conducted for comparisons. The goal of this work was to convert kraft lignin to low molecular weight aromatics (molecular weight is ~100 g · mol−1) with low acidity through zeolite assisted pyrolysis, which could then be used as a precursor for bio-gasoline and/or bio-chemicals. To characterize the whole portions of various pyrolysis oils, we used 1D and 2D NMR and GPC to determine the structure of the pyrolysis oils. In addition, the thermal characteristics of the pyrolysis oil were analyzed by thermogravimetric analysis (TGA).
Materials and methods
All reagents used in this study were purchased from VWR International or Sigma-Aldrich (St. Louis, MO) and used as received. Lignin was isolated from a commercial USA softwood kraft pulping liquor. Zeolites (CBV 720 and CBV 21A) were purchased from Zeolyst, Inc.
Lignin separation and purification
Lignin samples used in this study were isolated from a commercial softwood kraft pulping liquor following published methods . In brief, the cooking liquor was filtered through filter paper, and the filtrate was treated with ethylenediaminetetraacetic acid disodium salt (EDTA-2Na+, 0.50 g/100.00 mL liquor) and stirred for 1 h. The liquor was adjusted to a pH value of 6.0 with 2.0 M H2SO4 and stirred vigorously for 1 h. The liquors were then further acidified to a pH of 3.0 and frozen at −20°C. After thawing, the precipitates were collected on a medium sintered glass funnel and washed three times with cold water by suspending the precipitates in the water and stirring vigorously at 0°C for 1 h. The precipitates were collected, air dried, and Soxhlet extracted with pentane for 24 h. The solid product was air dried and further dried under high vacuum at 45°C for 48 h. The resulting purified kraft lignin sample was stored at −5°C.
Preparation of pyrolysis sample
SiO 2 /Al 2 O 3 mole ratio, framework, code name used in this work and channel structure of each zeolite
SiO2/Al2O3 mole ratio
Pore size (Å)
7.4 × 7.4
6.5 × 7.0
Equipment and process of pyrolysis
Pyrolysis experiments were conducted in a quartz pyrolysis tube heated with a split-tube furnace . Typically, the pyrolysis sample (6.00 g) was placed in a quartz sample boat that was then positioned in the center of a pre-heated pyrolysis tube. A K-type thermal couple was immersed in the sample powder during the pyrolysis to measure the heating rate. The pyrolysis tube was flushed with nitrogen gas, and the flow rate was adjusted to a value of 500 mL min−1 and then inserted in the pre-heated (600°C) furnace. The outflow from pyrolysis was passed through two condensers, which were immersed in liquid N2. (Note: Liquid N2 was used for experimental convenience). Upon completion of pyrolysis (~10 minutes), the reaction tube was removed from the furnace and allowed to cool to room temperature under constant N2 flow. The condensers were then removed from liquid nitrogen. The pyrolysis char and oil were collected for subsequent chemical analysis. In general, the liquid products contained two immiscible phases referred to as heavy and light oil. The light oil was acquired by decantation. The heavy oil was recovered by washing the reactor with acetone followed by evaporation under reduced pressure. Char yields were determined gravimetrically, and gas formation was calculated by mass difference.
Characterization of pyrolysis oils by GPC
The weight average molecular weights (M w ) of the heavy oils were determined by Gel Permeation Chromatography analysis following literature methods . Prior to GPC analysis, the heavy oil samples were dissolved in THF (1 mg · mL−1) and filtered through a 0.45 μm syringe filter. The samples were injected into a Polymer Standards Service (PSS) Security 1200 system featuring Agilent High-Performance Liquid Chromatography (HPLC) vacuum degasser, isocratic pump, refractive index (RI) detector and UV detector (270 nm). Separation was achieved with four Waters Styragel columns (HR0.5, HR2, HR4, HR6) using tetrahydrofuran (THF) as the mobile phase (1.0 mL · min−1) with injection volumes of 25 μL. Data collection and processing were performed using PSS WinGPC Unity software. Molecular weights (M w ) were calibrated against a calibration curve. The calibration curve was created by fitting a third order polynomial equation to the retention volumes obtained from a series of narrow molecular weight distribution polystyrene standards (i.e., 7.21 × 103, 4.43 × 103, 1.39 × 103, 5.80 × 102 Da), dioctyl phthalate (M w = 390 g · mol−1), 2,29-dihydroxy-4,49-dimethoxyl-benzophenone (M w = 274 g · mol−1), 2-phenylhydroquinone (M w = 186 g · mol−1), phenol (M w = 94 g · mol−1) and acetone (M w = 58 g · mol−1). The curve fit had an R2 value of 0.998.
Characterization of pyrolysis oil by NMR
Quantitative 13C NMR
All NMR spectral data reported in this study were recorded with a Bruker Avance/DMX 400 MHz NMR spectrometer. Quantitative 13C NMR were acquired using 100.0 mg heavy oil dissolved in 450 μL dimethyl sulfoxide-d 6 (DMSO-d 6 ) employing an inverse gated decoupling pulse sequence, 90° pulse angle, a pulse delay of 20 s and 6000 scans at room temperature with a line-broadening (LB) of 5.0 Hz. To reduce the measurement time, 1 mg · mL−1 relaxation reagent chromium acetylacetonate was added into the solutions.
Quantitative31P NMR were acquired after in situ derivatization of the samples using 10.0 mg of heavy oil with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP) in a solution of (1.6:1 v/v) pyridine/CDCl3, chromium acetylacetonate (relaxation agent) and endo-N-hydroxy-5-norbornene-2, 3-dicarboximide (NHND, internal standard). 31P NMR spectra data was acquired using an inverse gated decoupling pulse sequence, 90° pulse angle, 25 s pulse delay, and 128 scans at room temperature with a LB of 4.0 Hz.
Characterization of Pyrolysis Oil by HSQC-NMR
The sample used for HSQC-NMR was the same as 13C NMR. HSQC-NMR were acquired employing a standard Bruker pulse sequence “hsqcetgpsi.2” with a 90° pulse, 0.11 s acquisition time, a 1.5 s pulse delay, a 1JC–H of 145 Hz, 48 scans and acquisition of 1024 data points (for 1H) and 256 increments (for 13C). The 1H and 13C pulse widths are p1 = 11.30 μs and p3 = 10.00 μs, respectively. The 1H and 13C spectral widths are 13.02 ppm and 220.00 ppm, respectively. The central solvent peak was used for chemical shift calibration. HSQC data processing and plots were carried out using MestReNova v7.1.0 software’s default processing template and automatic phase and baseline correction.
Thermogravimetric analysis (TGA) of pyrolysis oil
The thermogravimetric analysis (TGA) of the pyrolysis oil was performed on Q50 TGA (TA Instruments, USA). About 10 mg of sample for this analysis was used and heated from room temperature to 600°C at the rate of 10°C/min under the flow of N2 at the rate of 60 mL/min.
Triplicate tests were conducted for all the pyrolysis experiments, and a mean value was reported for the characterization measurement. The standard deviations were marked in all the quantitative analysis figures.
Results and discussion
Pyrolysis yields analysis
Quantitative 31P NMR analysis of pyrolysis oil
Chemical shifts and integration regions for pyrolysis oil derivatized with TMDP in a quantitative 31 P NMR
Integration region (ppm)
endo-N-hydroxy-5-norbornene-2,3-dicarboximide (NHND, internal standard)
151.0 - 152.8
150.0 - 145.5
C5 substituted guaiacyl phenolic OH
144.7 - 142.8
142.8 - 141.7
141.7 - 140.2
Guaiacyl phenolic OH
140.2 - 139.0
Catechol type OH
139.0 - 138.2
138.2 - 137.3
136.6 - 133.6
Quantitative 13C NMR analysis of pyrolysis oil
13 C NMR chemical shift assignment range of lignin pyrolysis oil based on the chemical shift database created in our previous work
Integration region (ppm)
Carbonyl or Carboxyl bond
215.0 – 166.5
Aromatic C-O bond
166.5 – 142.0
Aromatic C-C bond
142.0 – 125.0
Aromatic C-H bond
125.0 – 95.8
Aliphatic C-O bond
95.8 – 60.8
60.8 – 55.2
Aliphatic C-C bond
55.2 – 0.0
Methyl – Aromatic (CH3-Ar)
21.6 – 19.1
Methyl – Aromatic at ortho position of a hydroxyl or methoxyl group (CH3-Ar’)
16.1 – 15.4
HSQC-NMR analysis of pyrolysis oil
Molecular weight analysis of pyrolysis oil
TGA analysis of pyrolysis oil
The effect of zeolite on the lignin pyrolysis was also reflected on the bio-oil TGA profiles. For example, during the temperature range 80-300°C, the rate of weight loss was highest in Y zeolite catalyzed bio-oil, followed by the M zeolite catalyzed bio-oil and the non-catalyzed bio-oil (L) that has the lowest rate. The TGA weight loss rate of zeolite mixture ‘Y + M’ catalyzed bio-oil was between the bio-oils generated from Y and M assisted pyrolysis. These findings were in good agreement with the molecular weight analysis of the bio-oils: the bio-oils with lower molecular weight generally decomposed faster than the higher molecular weight samples in the TGA analysis.
The mixture of Y and M type zeolites was applied to pyrolyze kraft lignin, aiming to study the combination effect of different types of zeolite on pyrolysis. The product yields (light oil, heavy oil and char) from the zeolite mixture ‘Y + M’ catalyzed pyrolysis lie between the individual pyrolysis with zeolite Y and M. The NMR analysis revealed that the yields of hydroxyl groups and other functional groups in the bio-oils acquired from the zeolites mixture ‘Y + M’ catalyzed pyrolysis were between individual Y and M zeolites assisted pyrolysis. The molecular weight of zeolite mixture ‘Y + M’ catalyzed pyrolysis oil was similar to the individual zeolite Y assisted pyrolysis. These results show that the mixture of ‘Y + M’ manifests the dual characteristics, which combine the catalytic effects of Y and M, such as the cleavage of methoxyl-aromatic and ether bonds as well as the decompose of the carboxyl groups in a bio-oil. The significantly decreased molecular weight and acidity present a more suitable precursor for bio-gasoline and bio-chemicals.
The authors thank the US Department of Energy (DOE) for providing financial support (project: DE-EE0003144) for these studies.
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