Structural analysis of hydrothermal char and its models by density functional theory simulation of vibrational spectroscopy
Graphical abstract
Introduction
Biochars [1] are carbonaceous materials made from the thermal decomposition of biomass and waste-derived carbon [2]. Biochars are potentially useful for a variety of applications, including water purification [3], soil amendment [4], catalysis [5], and gas storage [6]. Several recent reviews [7], [8], [9] establish the potential usefulness of biochar materials. For example, Wei et al. [10] synthesized biochars from cellulose and activated them with a KOH treatment to prepare activated carbons with surface areas as great as 2967 m2 g−1 (nitrogen sorption). These activated carbons were used to make carbon electrodes with a specific capacitance of 236 F g−1 (100 F cm−3) at a sweep rate of 1 mV s−1. Similarly, Regmi et al. [11] converted switch grass into activated carbon by using a two-step process consisting of carbonization and KOH activation. The cadmium capacity of the resulting material was reported to be 34 mg g−1, compared to 1.5 mg g−1 for commercially available activated carbon (Calgon WPH).
Biochars can be divided into two categories, pyrolysis chars and hydrothermal chars (which we will refer to as “hydrochar”). Hydrochars are differentiated from pyrolysis biochars by the presence of a bulk water phase during their synthesis; hence, the synthesis process itself is termed “hydrothermal carbonization” or HTC [12]. Since many biorenewable feedstocks have high moisture content (for example, municipal waste [13] and many agricultural residues [14]), the energy efficiency of HTC can be greater than that of pyrolysis [15], a process which requires a dried feedstock [16]. Also, the physical and chemical composition of hydrochar is distinct from pyrolysis biochar [17], a factor which may be beneficial for specific applications [17]. For example, Liu et al. [18] compared the composition and performance of pinewood biochars synthesized under both hydrothermal and pyrolytic conditions. Interestingly, Liu et al. [18] reported that the pyrolysis chars possessed greater surface areas than the hydrochar (29 m2 g−1 for pyrochar to 21 m2 g−1 of hydrochar) as well as a significantly larger meso and macropore volume (0.89 cm3 g−1 for pyrochar and 0.05 cm3 g−1 for hydrochar). Despite having greater surface area, the copper adsorption capacity of the pyrolysis chars was less than that of the hydrochar (2.73 mg g−1 for pyrochar and 4.21 mg g−1 for hydrochar), a difference the authors attributed to the greater oxygen content of the hydrochar (35%) compared to the pyrolysis char (4%) [18].
The molecular composition of hydrochar remains the subject of debate. Fig. 1 shows 3 model hydrochar structures proposed by Sevilla et al. [19], Latham et al. [20], and Chuntanapum et al. [21], respectively. The models clearly have some consistent features, namely the presence of aromatic rings and oxygen functional groups. However, important differences are also apparent, including differences in the types of aromatic ring (arene vs. furan), presence or absence of cyclic alkanes, and the number of rings present in fused aromatic structures. An accepted molecular model for hydrochar would help guide technological efforts to utilize and tailor hydrochar for specific applications.
Some of the differences in hydrochar model structures are attributable to inherent differences in the different methods used for structural analysis. Hydrochar has been examined using methods that capture primarily elemental composition [5] (for instance, combustion and atomic spectroscopy), functional group content [22], [23], [24] (including NMR, C-XANES, and vibrational spectroscopy), or physical/textural properties (nitrogen/carbon dioxide sorption and electron microscopy). The hydrochar models shown in Fig. 1a [19] and 1c [21] were based primarily on IR and Raman spectroscopy; these techniques suggest that hydrochar is a highly defective graphitic structure with significant oxygen functional group substitution. Fig. 1b was based on C-XANES measurements of hydrochar, a method which indicates that the structure is composed primarily of individual furanic rings [20]. Similarly, Baccile et al. [25] used NMR analysis to propose furan rings as the main repeat unit in hydrochar. While differences in synthesis conditions (temperature, residence time, starting reactant) may play a role in the differences between model structures [26], different experimental techniques clearly provide conflicting information that leads to inference of different structures.
NMR and Raman spectra are two of the most common methods used for carbon material characterization. NMR analysis of hydrochars benefits from several inherent advantages, most notably that it allows nuclei-nuclei cross correlation analysis and spectral editing that can help clarify assignments [27]. On the other hand, Raman scattering has advantages of rapid sample preparation and scan times (minutes compared to hours for NMR) and is particularly sensitive to CC bonds [28]. In fact, Raman scattering has been useful for characterization of graphene and graphite structures [29], [30], and has recently been carefully studied as a tool for characterization of pyrolysis chars [31]. Unfortunately, the information that Raman spectroscopy can provide for analysis of hydrochars is limited largely due to their complex Raman spectra that apparently consists of many overlapping bands [32]. Careful study of hydrochar Raman spectra is required to increase the usefulness of this tool for hydrochar characterization and resolve apparent differences between model structures based on Raman spectroscopy from those determined by other methods.
Several authors have previously suggested methods to interpret the Raman spectra of disorganized carbon materials [28], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]. For example, Ferrari et al. [33], suggested that the Raman spectra of highly graphitic carbons could be characterized as consisting primarily of 2 distinctive bands, a “G band”, generally appearing at 1580 cm−1, and a “D band”, generally appearing at 1350 cm−1. Tuinstra et al. [35] established the origin of the G band to be defect-free graphene sheets, an assignment that remains generally accepted. The D band is typically attributed to the CC vibrations of defects in the graphene sheets [35], [36], [37]. Furthermore, Tuinstra et al. [35] originally attributed the D band to particle size or edge effects, suggesting that the symmetric A1G mode of the graphene lattice becomes active at the particle boundary. Ferrari [36] later refined the attribution of the D band in graphitic carbons, demonstrating that the intensity and width of the D band were greatest at the edges of a graphene sheet. As a further refinement, Castiglioni et al. [37] suggested that the D band originates from activation of phonons in sp2 domains around the edge of aromatic rings or defects in the graphitic lattice.
Hydrochar Raman spectra universally exhibit characteristic bands in the same wavenumber range as is typical of D and G bands [42], [43]. However, while the G and D band assignments for graphite are straightforward, the distinction between the basal plane and the edge is not well-defined for even the largest PAH sub-units thought to comprise hydrochar (see Fig. 1). Furthermore, while graphite Raman spectra consist of narrow G and D bands (<20 cm−1), the hydrochar G and D bands are broad (>50 cm−1). Likewise, Raman spectra of graphitic materials are typically dominated by a major G band, with only a minor D band, so that the G/D intensity ratio of graphitic materials is typically greater than 10. In contrast with graphite, the G/D intensity ratio of hydrochars is only about 2. Lacking a better alternative, many authors generally interpret observed hydrochar spectra as suggesting a defect-rich graphitic material, even though the graphitic content in hydrochar is generally accepted to be negligible [22], [44].
Based on their analysis of the Raman spectra of defect-rich carbonaceous materials (primarily soot), Sadezky et al. [39] and Beyssac et al. [28] suggested an approach that was more comprehensive than the traditional G/D band assignment. The new assignment [28] contains a G band at 1580 cm−1 and sub-divides the D band into 4 sub-bands, each associated with specific structural defects. This method has been applied by other research groups analyzing carbonaceous materials [40], but it was derived primarily for soot and other low oxygen content carbonaceous materials making unclear its extension to carbon materials with high oxygen content, including hydrochar.
More comprehensive Raman interpretations have been published, which build on the work of Sadezki et al. [40] and Beyssac et al. [41]. Li et al. [32] published an ambitious Raman analysis of Victorian brown coal that suggested that sub-features contained within the G/D vibration manifold might be used to identify molecular composition characteristics such as aliphatic side chains and average sizes of aromatic structures. Similarly, Smith et al. [31] have suggested a very detailed fit for pyrolysis char to explain the G/D region as a collection of vibrations related to different sizes of aromatic domains and various defects present in these domains, including seven and five membered rings. While the work of Li et al. [32] and Smith et al. [31] may provide a basis for understanding brown coal and pyrochar Raman spectra, as with earlier methods for interpreting Raman spectra these approaches were developed primarily for defect-rich graphitic carbon materials with low oxygen content and large PAH structures. Hydrochars are more structurally diverse than pyrochars and soot, as hydrochars have greater hydrogen-to-carbon and oxygen-to-carbon ratios than pyrochars [44]. Thus, a model developed for hydrochars must take into account a wider range of oxygen and hydrogen defect vibrations than either the Li et al. [32] or Smith et al. [31] models include.
The objective of this work was to develop a method for interpreting Raman spectra appropriate for hydrochars. Specifically, we have used computational simulations to predict the Raman spectra of model hydrochar compounds, including a series of PAHs, functionalized PAHs, and other model compounds proposed in the literature as potential subunits contained within hydrochar. To establish method accuracy, predicted Raman spectra were compared to measured spectra for a significant subset of the model compounds included in the study. The resulting data set was then analyzed to determine average band locations (and their ranges) and relative intensities of the primary PAH vibrations. The effect of the number of rings in the PAH structure and the effect of functionalization on band location and intensity were evaluated. The calculated average band locations constituted the basis of a new fitting algorithm for interpretation of hydrochar Raman spectra which was then used to interpret the Raman spectrum of a glucose-based hydrochar. IR and NMR spectroscopy were used to confirm general conclusions drawn from Raman spectroscopy. The results of this study reconcile Raman spectroscopy with IR and NMR analysis; provide a tool for interpreting hydrochar Raman spectra; and establish the utility and limits of Raman spectroscopy for characterizing hydrochar structure.
Section snippets
Density Functional Theory (DFT) parameters
All DFT calculations were performed using the Gaussian 09 software package [45] and the Webmo interface [46]. Geometry optimizations and frequency calculations were performed using Becke's three parameter Lee-Yang-Parr (B3LYP) hybrid functional [47] using the 6-31G basis set. The B3LYP/6-31G combination is commonly used for the simulation of organic molecules consisting of 10 or more heavy atoms [31], [48], [49]. For example, Zerbi et al. [37] used DFT to simulate vibrations of PAH molecules
Method accuracy
To establish the accuracy of the B3LYP-631G DFT theory method for predicting the locations and intensities of Raman bands of PAH molecules, we simulated the spectra of the compounds shown in Tables SI–1 and compared the simulated spectra to experimentally measured spectra, either obtained in our lab or reported in the literature [54], [55], [56], [57], [58]. Fig. SI-3 provides simulated and measured Raman spectra of pyrene, anthracene, and naphthalene as representative PAH molecules. The
Conclusions
The Raman spectra of model constituents of hydrochar were simulated using DFT, with a specific focus on arenes and furans. In nearly all cases, the most important aromatic vibrational bands were associated with G, D, and Kekulé modes. The positions and relative intensities of these bands varied with respect to the number of rings in the aromatic structure and when substituted by carbonyls. Other functional groups did not impact aromatic band positions or intensities and the effect of the
Acknowledgements
The U.S. National Science Foundation supported this work (1605916 and 1342320). The authors thanks Thomas Partington (WPI) for help constructing and maintaining the hydrothermal carbonization reactors.
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