Raw material
Sugarcane bagasse was harvested from Dehong in Yunnan, China. Corncob was obtained from Baodi feed mill, Tianjin. Sugarcane bagasse and corncob were ground and sieved to the particle size range 0.11–0.18 mm and then dried in an oven at 105 °C until constant weights. Sulfuric acid was purchased from Chuandong Chemical Co. Ltd., Chongqing. The standard samples of glucose, xylose, levoglucosan, acetic acid, furfural and 5-hydroxymethylfurfural (5-HMF) were purchased from sigma (Shanghai).
Elemental analysis
Carbon (C), hydrogen (H) and nitrogen (N) contents were measured with an organic elemental analyzer (Vario EL cube, Hanau, Germany). The contents of potassium (K), sodium (Na), calcium (Ca) and magnesium (Mg) were determined by an inductively coupled plasma optical emission spectrometry (ICP-OES) (Optima 8000, PerkinElmer, USA). For ICP-OES analysis, oven-dried (at 105 °C) biomass samples (0.3 g) were weighted into a test tube. The biomass was digested for 10 h in the 4 mL mixed acids of concentrated HNO3 and HClO4 (3:1, v/v). Then the digested sample was diluted to 10 mL with deionized water. In ICP-OES analysis, nebulizer flow was 1.5 L/min. The flush time, delay time and wash time were 10, 40 and 40 s, respectively. Five standard solutions of each metal were prepared and analyzed to generate external calibration curves for quantitative determination.
Compositional analysis
The composition of carbohydrates was determined following the National Renewable Energy Laboratory (NREL) procedure [29]. Briefly, dried sample (0.3 g) was incubated with 3 mL of 72 % H2SO4 for 1 h at 30 °C with mixing. The mixture was diluted with 84 mL deionized water to a final acid concentration of 4 %. The solution was autoclaved for 1 h at 121 °C. The hydrolysate was filtered to separate the filtrate and solid residue. Calcium carbonate was used to neutralize the filtrate to pH 5–6. The sugars in the neutralized filtrate were analyzed by high performance liquid chromatography (HPLC, Waters 2695) with quantification referenced to standards, which were also autoclaved in 4 % H2SO4 to compensate for degradation. Glucose and xylose were separated by Aminex HPX-87P column (Bio-Rad, USA) at 80 °C with deionized water as mobile phase at a flow rate of 0.4 mL/min. Monosaccharides were detected by refractive index (RI) detector. The detector was operated at 50 °C. The samples were filtered through a 0.22 μm nylon filter before injection. The contents of glucan and xylan were determined from the concentration of the glucose and xylose, using an anhydro correction of a correction of 0.90 for glucose and 0.88 for xylose, respectively. Each sample was analyzed in triplicate.
Thermogravimetric analysis (TGA)
TGA experiments were performed with a thermogravimetric analyzer (TGAQ50, TA, USA). The samples (4–6 mg) were loaded into an alumina crucible, and then were heated from 50 to 105 °C at a rate of 20 °C/min and held at 105 °C for 10 min. Consequently, samples were heated to 750 °C at a rate of 20 °C/min. Nitrogen was used as carrier gas (20 mL/min).
Crystallinity measurement
Crystallinity of biomass before and after pretreatment was analyzed by X-ray diffraction (XRD) in X’Pert PROMPD X-ray diffract-meter (PANalytical V.B., Holland) employing Cu-Kα radiation. X-ray diffract-meter was set at 40 kV and 40 mA. Each sample (80 mg) was pressed into a lamellar container 20 mm in diameter and was scanned over diffraction angle (2θ°) of 5°–45° at a 0.01° per second of scanning rate by Cu radiation (λ = 1.54 Å). The percentage of crystalline material in the biomass was expressed as the crystallinity index (CrI), which was calculated by the equation following the procedure proposed by Segal [30]:
$$CrI = \frac{{ \, I {\text{002}}-I{\text{am}}}}{{ \, I 0 0 2 { }}} \, \times 1 0 0\;{\text{\% }}$$
(1)
where I
002 was the intensity of the peak in crystalline phase (2θ = 22°) and I
am was the intensity of the peak in amorphous phase (2θ = 16°).
Dilute acid hydrolysis
The dilute acid hydrolysis was performed in a 100 mL high pressure autoclave (HKY-3, Haian Petroleum Research Co. Ltd., Jiangsu, China). G0, G1, G2, G3, G4, G5 were used to denote as the un-pretreated (raw material), 0 % (hot water washing), 0.05, 0.5, 1 and 2 % dilute sulfuric acid pretreated sugarcane bagasse, respectively. C0, C1, C2, C3, C4, C5 were used to denote as the un-pretreated (raw material), 0 % (hot water washing), 0.05, 0.5, 1 and 2 % dilute sulfuric acid pretreated corncob, respectively. Sugarcane bagasse or corncob (3 g) was loaded in the 100 mL flasks containing 30 mL 0–2 wt % dilute sulfuric acid solution. The flasks were placed in a high pressure autoclave and reacted at 120 °C for 1 h. After hydrolysis, the solid phase and liquid phase were separated by filtration. Solid residue was washed with 300 mL distilled water to remove residual sulfuric acid, then freeze dried for 24 h (Boyikang Co., Ltd, Beijing). The pretreated biomass was then dried in an oven at 105 °C until constant weights. After drying, the pretreated feedstock was stored in sealed plastic containers for pyrolysis experiments. Concentration of glucose and xylose in dilute acid hydrolysate were determined by HPLC fitted with an Aminex HPX-87P column (Bio-Rad, USA) and RI detector. The concentration of acetic acid, furfural and 5-HMF in dilute acid hydrolysate were determined by HPLC fitted with an Aminex HPX-87H column (Bio-Rad, USA). The column and detector were operated at 60 and 50 °C, respectively. H2SO4 (5 mM) was utilized as mobile phase and the flow rate of the mobile phase was held constant at 0.6 mL/min. Acetic acid was analyzed by RI detector, while furfural and 5-HMF were analyzed by ultraviolet–visible (UV) detector at 280 nm. Compounds were identified and quantified by comparison to authentic standards using a five-point calibration curve. The hydrolysis yields of xylose and glucose were calculated as:
$${\text{Xylose yield (wt}} \%) = \frac{\text{ mass of xylose in the acid hydrolysate (g)}}{\text{ xylan mass of biomass (g) }} \, \times \;0.88\; \times 100 \; \%$$
(2)
$${\text{Glucose yield (wt}}\% ) = \frac{\text{ mass of glucose in the acid hydrolysate (g)}}{\text{ glucan mass of biomass (g) }} \, \times \;0.90\; \times 100\;\%$$
(3)
Fast pyrolysis of biomass
Fast pyrolysis was conducted on a CDS pyroprobe 5200 series (CDS Analytical, USA), which was connected to a gas chromatograph/mass spectrometer (GC/MS) system (Agilent 7890 gas chromatograph, Agilent 7975C mass spectrometer, Agilent Technologies). The pyrolyzer used a heated filament to heat a quartz tube containing the sample. Sample (200–400 µg) weighted by a microbalance with an accuracy of 1 µg (XP6152, METTLER TOLEDO, Germany) was pyrolyzed during each test. The pyrolysis temperature, residence time and heating rate were fixed at 500 °C, 20 s and 10 K ms−1, respectively. The helium carrier gas continuously passed through the interface at a flow rate of 20 mL/min to transport the pyrolysate from the quartz tube into 240 °C GC injection port. The interface line between the pyrolyzer and GC maintained at 300 °C to prevent condensation of vapors. The split ratio was 50:1. A HP-INNO wax capillary column (Agilent 19091 N-133, 30 m length, 0.25 mm ID, 0.25 µm film thickness) was utilized for the chromatographic separation of pyrolysis products. The GC oven temperature program: initial temperature was 50 °C, held for 2 min, heated to 90 °C at a rate of 10 °C/min, 4 °C/min to 129 °C, and then 8 °C/min to 230 °C with a dwell time of 29 min. Helium flow rate remained at 1 mL/min. The mass spectrometer was operated at 150 °C in an electron impact mode (70 eV) and the mass scanned from m/z 12 to 500. Compound identification was achieved by matching with NIST mass spectral data library. Compounds were quantified by comparison to authentic standards using a five-point calibration curve. All experiments were tested in triplicate and averaged for quality assurance to compensate experimental reproducibility. The yields of main pyrolysis products were calculated based on the dry weight of solid sample in pyrolysis experiment. The potential yield of levoglucosan was experimentally observed maximum yield of levoglucosan from pure cellulose (59 wt%) [22]. The effectiveness was defined as the actual yield of levoglucosan from acid pretreated biomass divided by the potential yield of levoglucosan from the glucan contained in that biomass sample. The compound yield and effectiveness were calculated as:
$${\text{Compound yield (wt}}\%) = \frac{\text{ mass of compound (g)}}{\text{ mass of biomass (g) }} \, \times 1 0 0 \, \%$$
(4)
$${\text{Effectiveness}}\, (\% )= \frac{\text{actual levoglucosan yield}}{\text{ potential yield }} \, \times 1 0 0\, \%$$
(5)