Article pubs.acs.org/IECR
Synthesis and Characterization of Carbonaceous Materials from Saccharides (Glucose and Lactose) and Two Waste Biomasses by Hydrothermal Carbonization Kıvanç Aydıncak,† Tuğrul Yumak,† Ali Sınağ,*,† and Bekir Esen† †
Science Faculty, Department of Chemistry, Ankara University, 06100 Beşevler/Ankara, Turkey S Supporting Information *
ABSTRACT: Carbonaceous particles are synthesized under hydrothermal conditions using two waste biomasses (olive oil waste and hazel nutshell) and saccharides (glucose and lactose). A stainless steel autoclave with 75 mL of capacity was used to apply the hydrothermal carbonization (HTC) process to the starting materials stated above. In the experiments, 2.5 g of sample dispersed in 50 mL of deionized water is subjected to HTC at 180 °C for 4 h. H/C and O/C ratios for the chars were found to be more similar to the lignite than those of the starting materials. The heating values for the chars were found to be higher as compared to that of the feedstocks. FTIR investigations of the chars reveal that biochars of saccharides have different chemical structures compared with glucose and lactose, while biochars of waste biomasses are similar chemical nature with their starting materials. Hydroxymethylfurfural (HMF), phenol, acids, and aldehyde contents of aqueous phases were also determined. Solid-state 13C CP/MAS NMR analysis of the chars gave hints about the formation mechanism of sphere-structured biochars. Two different formation mechanisms for the biochars of waste biomasses and saccharides were proposed according to solid-state 13C CP/MAS NMR analysis results. The structure of biochar obtained from glucose and lactose involved furanic chains, while the biochars from olive oil waste and hazel nutshell have mainly aromatic structure.
1. INTRODUCTION Hydrothermal carbonization (HTC) is a thermochemical conversion technique which uses water as a reaction medium for conversion of saccharides to value-added carbonaceous materials. According to different experimental conditions and reaction mechanism, two HTC processes can be classified: low temperature HTC (up to 250 °C) and high temperature HTC (between 300 and 800 °C). Low temperature HTC is a mild condition, environmentally friendly one-step process, that is possible to obtain various carbonaceous materials by this process. Low temperature HTC is presumably close to natural coalification and that acting the hundred million years process in contrast with high speed and short time reactions.1 In the field of hydrothermal carbonization, Titirici et al. hydrothermally transformed hard plant tissue (oak leafs, pine cones) and soft plant tissue (orange peels, pine needles) complex biomasses in the presence of an acid catalyst under mild operation conditions, and they reported that hydrothermal carbons from biomasses chemically and spectroscopically are similar to peat or lignite.2 Heilmann and co-workers reported hydrothermal carbonization of different types of microalgae and obtained algal char. They investigated the energy input/output, and they did comparisons of algal char, lignocellulosic char, and coal about combustion behavior, ash content, and fuel quality.3 Cui et al. hydrothermally carbonized starch and rice grains as carbon precursers in the presence of free iron ions and iron oxide nanoparticles to obtain carbon related nanomaterials.4 Paraknowitsch et al. investigated oxidation of the HTC coal derived from glucose with a comparative study between lignite, and they are also interested in carbon fuel cell performance of those coals.5 HTC also enables the synthesis of micro- and nanocarbon spheres, possessing diameters in the nanomicroscales,6 without © 2012 American Chemical Society
using toxic organic solvents, initiators, and surfactants in an affordable pathway for mass production.7,8 These nano- and microspheres generally have inert and hydrophilic surfaces with carrying −OH and CO groups, enabling ligand immobilization on the surface.7,9 Mi et al. synthesized carbon microspheres in high yields and narrow-range distributions, possessing regular and perfect shape with diameters ranging from 1 to 2 μm as determined by scanning electron microscopy micrographs and X-ray diffraction (XRD) patterns.10 Shu-Hong Yu et al.11 reported bulk processing of metal/carbon nanostructures into nanocables, hollow tubes, and hollow spheres by using saccharides exposed to the HTC. R. Demir-Cakan et al.12 synthesized mesoporous carbonaceous structures containing an imidazole group by regular HTC of glucose in the presence of vinylimidazole and tested them as catalysts for Diels−Alder reactions, Knoevenagel condensations, and transesterifications. Among the potential saccharides that can be employed to produce carbonaceous materials through hydrothermal carbonization, glucose is the most promising material as it is by far the most abundant and inexpensive saccharide available. Most of the works recently published in this area have been mainly focused on the synthesis of carbonaceous products and hybrid carbon/ inorganic materials from glucose alone or glucose based materials. Surprisingly little attention has been paid to comparison of the detailed chemical and structural properties of synthesized carbonaceous materials (biochars) from various saccharides and waste biomasses. Received: January 26, 2012 Accepted: June 11, 2012 Published: June 11, 2012 9145
dx.doi.org/10.1021/ie301236h | Ind. Eng. Chem. Res. 2012, 51, 9145−9152
Industrial & Engineering Chemistry Research
Article
Figure 1. XRD pattern of the biochars.
In the present work we investigate the chemical and structural characteristics of the biochars obtained by hydrothermal carbonization of saccharides (glucose, lactose) and waste biomasses (hazel nutshell and olive oil waste). For this purpose we subjected the starting materials to hydrothermal carbonization, and the synthesized biochars were characterized by means of the different experimental techniques: transmission electron microscopy (TEM), X-ray diffraction spectroscopy (XRD), infrared spectroscopy, nitrogen physisorption (BET analysis), elemental C/H/O analysis, thermogravimetric analysis, and particle size distribution analysis. Phenol and hydroxymethyl furfural contents of the aqueous phases were also determined by HPLC to reveal the effect of these intermediates on the formation mechanism of sphere structured biochar. The higher heating values (HHV) of biochars were also calculated theoretically.
Figure 2. Van Krevelen diagramm of biochars and raw materials.
2. MATERIALS AND METHODS To explain the effect of different structures of the starting materials on the formation mechanism of biochars, glucose and lactose were selected as saccharides. Olive oil waste and hazel nutshell were selected as the waste biomasses. D(+)Glucose and β-lactose were obtained from Merck. Olive oil waste was provided from a local olive oil factory at Muğla from Turkey, and the hazelnut shell was supplied from the Black sea region in the northern part of Turkey. The reaction was carried out in a stainless steel autoclave with 75 mL of capacity located on the hot plate with a temperature control system. In a typical procedure, 2.5 g of sample was dispersed or dissolved in 50 mL of deionized water and subjected to HTC at 180 °C for 4 h. The solid product was recovered by vacuum filtration and washed with deionized water and absolute ethanol several times and finally dried at 110 °C for 4 h. The obtained biochars from glucose, lactose, olive oil waste, and hazel nutshell were denoted as G-C1, L-C2, R-C3, and N-C4, respectively. A series of experiments was conducted using different reaction times and temperatures during this study. In order to avoid presenting a large amount of data, the data collected under optimum conditions was only presented in Table S1. 2.1. Chemical Analysis. 2.1.1. XRD, FTIR, Elemental, and HPLC Analysis. XRD analysis of the starting materials and biochars were carried out on a Rigaku D/Max −2200 ULTIMAN X-ray diffractometer, using Cu K radiation = 1.5418. Elemental analysis of the starting materials and the biochars was performed
Figure 3. FTIR spectras of (a) glucose, (b) G-C1, (c) lactose, (d) L-C2, (e) olive oil waste, (f) R-C3, (g) hazel nutshell, and (h) N-C4.
with a LECO 932 CHNS elemental analyzer. FTIR analysis was performed by Thermo Scientific Nicolet iS10. HPLC analysis of the aqueous phases were carried out by Hitachi Elite LaChrom with Macherey Nagel Nucleodur C18 Gravity column (flow rate: 0.75 mL min−1) to explain the formation mechanism of carbon spheres in biochar. 9146
dx.doi.org/10.1021/ie301236h | Ind. Eng. Chem. Res. 2012, 51, 9145−9152
Industrial & Engineering Chemistry Research
Article
Figure 4. Solid-state 13C CP/MAS NMR spectras of biochars obtained from a) L-C2, G-C1 and b) R-C3, N-C4.
was increased from room temperature to 1200 K at 10 °C min−1 of heating rate. 2.4. Transmission Electron Microscope Analysis. TEM examinations were performed by an FEI Tecnai G2 (at 200 kV) transmission electron microscope to determine the particle size of biochars.
Table 1. BET Analysis of the Samples C source
BET surface area (m2 g−1)
lignite G-C1 L-C2 R-C3 N-C4
11
