Article pubs.acs.org/EF
Solid-State Nuclear Magnetic Resonance Characterization of Chars Obtained from Hydrothermal Carbonization of Corncob and Miscanthus Lucia Calucci,† Daniel P. Rasse,‡ and Claudia Forte*,† †
Istituto di Chimica dei Composti OrganoMetallici, Consiglio Nazionale delle Ricerche − CNR, Via Giuseppe Moruzzi 1, 56124 Pisa, Italy ‡ Bioforsk, Norwegian Institute for Agricultural and Environmental Research, NO-1432 Ås, Norway ABSTRACT: Corncob and Miscanthus feedstocks were hydrothermally carbonized at 230 °C for 6 h in an autoclave under autogenous pressure, obtaining fine brown powders with higher carbon and lower hydrogen and oxygen contents than the starting materials. The chemical structure of feedstocks and hydrochars was investigated by 13C cross-polarization magic angle spinning (CP-MAS) nuclear magnetic resonance (NMR) spectroscopy complemented by Fourier transform infrared (FTIR) spectroscopy and elemental analysis. In particular, a procedure including CP dynamics analysis and spectral deconvolution was applied to obtain quantitative information on the composition of the analyzed materials from 13C CP-MAS spectra. Miscanthus feedstock contained larger amounts of lignin and crystalline cellulose, which concurred to reduce cellulose exposition to hot water during the hydrothermal carbonization (HTC) process. As a consequence, a lower furan/aromatic ratio was found for Miscanthus versus corncob hydrochars.
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INTRODUCTION In recent years, hydrothermal carbonization (HTC) has attracted much interest as a powerful method for the conversion of biomass into highly valuable carbon materials in a relatively cheap and sustainable way.1−17 HTC is conducted by applying mild temperatures (180−250 °C) to biomass in a water suspension under self-generated pressures for several hours. The obtained carbon-rich solid (also called hydrochar), which forms through hydrolysis, dehydration, decarboxylation, condensation polymerization, and aromatization reactions, possesses a core−shell structure consisting of a hydrophobic aromatic nucleus and a hydrophilic shell containing a high concentration of reactive oxygen functional groups (i.e., hydroxyl/phenol, carbonyl, or carboxyl). Because of its intrinsic properties, such as high efficiency in carbon fixation under mild conditions, application to wet biomass, and abundant functional groups remaining on the product surface, HTC presents many advantages with respect to more traditional pyrolysis techniques employed to obtain carbonaceous materials from biomass. Indeed, HTC looks promising for the production of biochar for CO2 sequestration13,15,18 and improving soil properties19−22 and for waste management.12,17 Moreover, HTC can be easily coupled to post or in situ casting and activation procedures, thermal treatments, and functionalization reactions to obtain carbon materials with controlled physical (porosity, size, crystallinity, etc.), chemical (surface functional groups, aromaticity, etc.), and electronic properties suitable for applications in fundamental fields, such as water purification, gas storage, catalysis, electronics, and biomedicine.2,4,6−10,15 However, until now, only simple monosaccharides and oligosaccharides have been effectively employed for the production of functionalized carbon materials by HTC, while the exploitation of lignocellulosic biomass as readily accessible © 2012 American Chemical Society
and carbon-negative feedstock can at present be considered as the next challenging step in HTC application to materials science.15 In particular, glucose has been extensively studied, being one of the cheapest and most abundant carbohydrates, and the mechanism of its transformation by HTC has been described in detail in recent publications.3,23−25 This process includes the transformation of glucose in hydroxymethylfurfural (HMF) and then the polymerization and polycondensation of HMF into polyfuranic chains first and subsequently into aromatic networks, with the proportion of polyfuranic and condensed aromatics being dependent upon the HTC process conditions, namely temperature and time. The same mechanism has been found to govern the formation of char from oligosaccharides and simple carbohydrates.23,26,27 On the other hand, very recently, a fundamental difference in the formation mechanism of hydrochar has been highlighted in the case of cellulose, the main component of raw biomass.25,28 In fact, a limited amount of cellulose present at the interface with water is hydrolyzed to glucose and then transformed following the above-described mechanism, whereas the bulk of the cellulose is directly transformed into aromatic networks following a mechanism similar to that of classical pyrolysis, although the exact chemical pathways are not yet clear. General conclusions about HTC-induced transformations of raw biomass are even more difficult to draw because of the different cellulose, hemicellulose, and lignin ratios and the different behavior of these components during HTC.5,15,29−31 To the best of our knowledge, studies on the physicochemical properties of chars obtained by HTC of lignocellulosic biomass are still limited in number and, furthermore, are performed on different feedReceived: October 22, 2012 Revised: December 4, 2012 Published: December 12, 2012 303
dx.doi.org/10.1021/ef3017128 | Energy Fuels 2013, 27, 303−309
Energy & Fuels
Article
Table 1. Volatile Matter Content, Fixed Carbon, Ash Content, Elemental Composition, and H/C and O/C Atomic Ratios of Feedstocks and HTC Chars elemental composition (%) volatile matter (%) corncob Miscanthus HTC corncob HTC Miscanthus a
81.1 78.0 67.2 61.4
(±0.5) (±0.8) (±0.1) (±0.9)
fixed carbon (%) 17.5 13.5 31.3 34.3
(±0.5) (±0.8) (±0.3) (±0.3)
C 48.4 48.9 59.8 62.9
N
(±0.3) (±0.4) (±1.3) (±0.5)
0.38 0.18 0.38 0.25
(±0.01) (±0.01) (±0.11) (±0.01)
atomic ratio
H 6.5 6.3 5.9 5.7
(
