Article pubs.acs.org/EF
Characterization of Wood Chars Produced at Different Temperatures Using Advanced Solid-State 13C NMR Spectroscopic Techniques Xiaoyan Cao,† Joseph J. Pignatello,‡ Yuan Li,† Charisma Lattao,‡ Mark A. Chappell,§ Na Chen,† Lesley F. Miller,§ and Jingdong Mao*,† †
Department of Chemistry and Biochemistry, Old Dominion University, 4541 Hampton Boulevard, Norfolk, Virginia 23529, United States ‡ Department of Environmental Sciences, The Connecticut Agricultural Experiment Station, Post Office Box 1106, New Haven, Connecticut 06504, United States § Environmental Laboratory, United States Army Corps of Engineers, 3909 Halls Ferry Road, Vicksburg, Mississippi 39180, United States ABSTRACT: Temperature is one of the controlling factors determining the chemical structure of char. We employed advanced solid-state 13C NMR techniques to characterize maple wood and its chars produced under N2 at temperatures from 300 to 700 °C. Our results indicated that 300 °C char was primarily composed of residues of biopolymers such as lignin and cellulose. Carbohydrates are completely lost for char prepared at 350 °C. At 400 °C, the char lost most of the ligno-cellulosic features and consisted predominantly of aromatic structures. By 500 °C, sp3-hybridized carbon had all but disappeared. Protonated aromatic carbons increased up to 400 °C chars but then decreased. Aromatic C−O groups decreased, whereas nonprotonated aromatic carbons, especially bridgehead carbons, increased as temperature increased. The minimum aromatic cluster sizes estimated from spectral analysis increased from 8 carbons in 300 °C char, to 20, 18, 40, 64, and 76 carbons, respectively, in 350 °C, 400 °C, 500 °C, 600 °C, and 700 °C chars. 1H−13C long-range dipolar dephasing displayed the same increasing trend of aromatic cluster sizes of wood chars with increasing temperature. We show for the first time quantitative changes of different aromatic C forms and aromatic cluster size as a function of heat treatment temperature. a function of HTT.12−15 Most studies to date have employed the 13C cross-polarization magic angle spinning (CP/MAS) technique. The CP technique, however, does not provide reliable quantification of condensed aromatic carbons for two reasons: (i) CP of 13C nuclei remote from protons is inefficient, and (ii) chars contain a high concentration of stabilized free radicals, which decrease proton rotating-frame spin−lattice relaxation time (T1ρH) and reduce the efficiency of magnetization transfer. The 13C NMR direct polarization (DP) technique more accurately quantifies the structure of chars. The degree of aromatic ring condensation is an important feature of char structure. Fused ring size generally increases with thermal alteration, tending toward the graphite structure.16 Until recently, there had been no widely accepted method, however, for evaluating the degree of aromatic condensation in chars.15,17−19 In a wet chemical approach, chars are partially oxidized with nitric acid into single-ring aromatic molecules with multiple carboxylic acid groups (benzene polycarboxylic acids or BPCAs), and a qualitative assessment of aromatic cluster size can be inferred from the distribution of individual BPCA molecular markers.20−24 Basically, aromatic rings at the edges of the char structure tend to yield more of the lesser carboxylated BPCAs, whereas core aromatic rings in the char are more likely to form more of the greater carboxylated BPCAs.21,25−27 The condensation degree of aromatic structures has also been explored using NMR methodology. Attempts
1. INTRODUCTION Char is the carbonaceous material that remains after pyrolysis or incomplete burning of organic matter. As part of the black carbon continuum, chars are a ubiquitous component of soils and sediments, and their concentrations depend on the frequency of fires or the magnitude of intentional inputs of produced char. Wood chars have been manufactured for millennia. In addition to their traditional use as fuel, they have currently attracted attention as a means to improve soil fertility,1−3 stabilize contaminated soil,4,5 and offset greenhouse gas emissions,6 among other applications. The physical and chemical properties of chars closely depend on feedstock types and processing variables such as temperature, duration of heating, oxygen availability, moisture content, mineral content, and how efficiently tarry volatiles are swept away. Temperature is a critical parameter in char production. It is well recognized that with increasing heat treatment temperature (HTT) char yields and volatile carbon content decrease, while fixed carbon content and heating values increase.7−11 The HTT affects the chemical composition of char3,11 in ways important for their use. An understanding of the chemical structural variations of chars as a function of temperature is a prerequisite to successful tailoring of chars for specific purposes, and a comprehensive understanding of the natural charring process, which occurs under temperature gradient.12 An indispensable tool for characterizing chars is solid-state 13 C NMR spectroscopy. This technique, which reveals structural information at the molecular level nondestructively, has been used to determine structural changes of wood chars as © 2012 American Chemical Society
Received: June 1, 2012 Published: August 9, 2012 5983
dx.doi.org/10.1021/ef300947s | Energy Fuels 2012, 26, 5983−5991
Energy & Fuels
Article
2.2. NMR Spectroscopy. 13C NMR analyses were performed using a Bruker Avance III 300 spectrometer at 75 MHz (300 MHz 1H frequency). All experiments were run in a double-resonance probe head using 4-mm sample rotors. 2.2.1. Quantitative 13C DP/MAS and DP/MAS with Recoupled Dipolar Dephasing (DP/MAS/DD) NMR. Quantitative 13C DP/MAS NMR experiments were performed at a spinning speed of 13 kHz. The 90° 13C pulse length was 4 μs. Recycle delays, ranging from 10 to 40 s, were determined by the cross-polarization/spin−lattice relaxation time/total sideband suppression (CP/T1-TOSS) technique to ensure all carbon nuclei were relaxed by more than 95%.32 Nonprotonated carbons and mobile carbon fractions were quantified using a combination of DP/MAS technique and a recoupled dipolardephasing delay of 68 μs.33 The recycle delays for DP/MAS and DP/MAS with dipolar dephasing were 40 s, 30 s, 40 s, 30 s, 40 s, 30 s, and 10 s, respectively, for maple wood, 300 °C, 350 °C, 400 °C, 500 °C, 600 °C, and 700 °C chars. For those not fully relaxed, the 13C NMR spectra were corrected for incomplete relaxation by factors measured in CP/T1-TOSS.32 The number of scans for DP/MAS and DP/MAS/DD experiments for all samples was 1024. 2.2.2. 13C Cross-Polarization and Total Suppression of Sidebands (CP/TOSS) and 13C CP/TOSS Plus Dipolar Dephasing (CP/TOSS/DD). Semiquantitative compositional information was obtained with good sensitivity using a 13C CP/MAS NMR technique with MAS 5 kHz, CP time 1 ms, and 1H 90° pulse-length 4 μs. Four-pulse total suppression of sidebands (TOSS)34 was employed before detection, with the twopulse phase-modulated (TPPM) decoupling applied for optimum resolution. Subspectra for nonprotonated and mobile carbon groups were obtained by combining the 13C CP/TOSS sequence with a 40-μs dipolar dephasing. The number of scans for 13C CP/TOSS and 13C CP/TOSS/DD experiments was 6144 for all samples. 2.2.3. 13C Chemical-Shift-Anisotropy (CSA) Filter. Since O−C−O carbons (e.g., anomeric C in carbohydrate rings) and aromatic carbon resonances between 120 and 90 ppm are difficult to resolve, the aromatic carbon signals were selectively suppressed using a five-pulse 13 C CSA filter with a CSA-filter time of 47 μs.35 The number of scans was 6144 for all samples. 2.2.4. Spectral Editing of Immobile CH2 + CH. Spectra for immobile CH2 + CH groups were obtained using the following spectral-editing techniques. First, a 13C CP/TOSS spectrum was recorded using a short CP time (50 μs) to emphasize protonated carbons in immobile segments. Afterward, a second 13C CP/TOSS spectrum was recorded using a short CP (50 μs) coupled with a 40 μs dipolar dephasing. The difference spectrum of the two represents immobile CH2 and CH carbons, with a small CH3 contribution.36 The number of scans was 6144 for all samples. 2.2.5. 1H−13C Long-Range Recoupled Dipolar Dephasing Experiments. The size of fused aromatic rings was estimated from the recoupled 1H−13C dipolar dephasing.31 In short, two 1H 180° pulses per rotation period prevent MAS from averaging out weak CH dipolar couplings. To detect nonprotonated carbons with good relative efficiency, direct polarization/total sideband suppression (DP/TOSS) was used at a spinning rate of 7 kHz. The 13C 90° and 180°-pulse lengths were 4 and 8 μs, respectively. The number of scans was 640 for all char samples at each dephasing time.
based on the quantification of bridgehead carbon (aromatic carbons shared by two or more aromatic rings) by the simple CP/MAS or DP/MAS technique are plausible28 but most likely to fail because spectral separation of bridgehead C from other nonprotonated aromatic C cannot be achieved, as detailed in Brewer et al.18 and Mao et al.29 Smernik and McBeath et al.17,19,30 introduced a method that involves adding 13C-labeled benzene to char and observing the changes in 13C chemical shift experienced by benzene when sorbed to the char. The principle is that conjugated aromatic structures in chars in the presence of magnetic field induce diamagnetic ring currents, which cause the displacement of 13C chemical shift of 13Clabeled benzene sorbed to the char. The magnitude of this shift is shown to depend on the degree of aromatic condensation.17 Brewer et al.18 proposed a NMR-based protocol to estimate aromatic cluster size of chars involving long-range 1H−13C dipolar dephasing31 and spectral analysis. This dual NMR approach has proven consistent and reliable for this purpose. In the present study, we employed advanced solid-state 13C NMR techniques such as quantitative DP/MAS in combination with dipolar dephasing, and 13C chemical shift anisotropy filter to characterize the structures of maple wood and its chars produced at HTT from 300 to 700 °C. The size of fused aromatic rings was evaluated using the dual NMR methodology.18,29 Brewer et al.18 used the techniques to compare slow pyrolysis, fast pyrolysis, and gasification charring processes applied to different herbaceous feedstocks. We apply the techniques here to explore how structural changes of wood from a single source vary systematically with HTT during slow pyrolysis. We show the quantitative changes of different aromatic C forms (aromatic C−H, aromatic C−C, aromatic C−O, aromatic edge C, or bridgehead C) and aromatic cluster sizes, and provide detailed structures of wood chars with increasing HTT. The present study also aims to compare aromatic cluster sizes derived from dual NMR approach with the published results from ring current measurements.19
2. EXPERIMENTAL SECTION 2.1. Pyrolysis Procedure. Maple wood shavings were packed into a stainless steel reactor placed in the center zone of a temperatureprogrammable three-zone tube furnace (Lindburg Blue M Tube Furnace; Asheville, NC, U.S.A.). In a flow of N2 (1.5 L/min), the shavings were heated at 100 °C for 1 h to drive off moisture and then heated at the rate of 25 °C/min to the desired HTT and held for 2 h. The HTT was varied from 300 to 700 °C. The chars were stored for at least 2 weeks in air to complete chemisorption of oxygen and then gently pulverized to pass a 100 mesh screen. Elemental analysis (C, H, N, O, and ash) was performed by Galbraith Laboratories, Inc. (Knoxville, TN) and the results are given in Table 1.
Table 1. Elemental Analysis of Maple Wood and Chars Prepared under Different Thermal Conditions % by weight
3. RESULTS AND DISCUSSION 3.1. Chemical Structural Changes of Chars with Increasing HTT. Elemental analysis of the samples (Table 1) shows loss of oxygen and hydrogen and enrichment of carbon with increasing HTT. The 700 °C char has slightly lower carbon content than the 600 °C char. The H/C and O/C atomic ratios decrease with HTT and correlate strongly with one another (R2 = 0.99), most likely associated with dehydration and condensation reactions. The nitrogen contents are relatively low (
