Relating Structural and Microstructural Evolution to the Reactivity of

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Relating Structural and Microstructural Evolution to the Reactivity of Cellulose and Lignin during Alkaline Thermal Treatment with Ca(OH)2 for Sustainable Energy Production Integrated with CO2 Capture Hassnain Asgar, Sohaib Mohammed, Ivan Kuzmenko, and Greeshma Gadikota ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06584 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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ACS Sustainable Chemistry & Engineering

Relating Structural and Microstructural Evolution to the Reactivity of Cellulose and Lignin during Alkaline Thermal Treatment with Ca(OH)2 for Sustainable Energy Production Integrated with CO2 Capture Hassnain Asgar,1 Sohaib Mohammed,1 Ivan Kuzmenko2, and Greeshma Gadikota1,*

*

Corresponding Author. Phone: +1 608-262-0365. E-mail: [email protected]

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1Department

of Civil and Environmental Engineering and Grainger Institute for Engineering, 1415 Engineering Drive, 2228 Engineering Hall, University of Wisconsin – Madison, Madison, WI 53706 2X-Ray

Science Division, Advanced Photon Source, Building 433A, Argonne National Laboratory, 9700 Cass Avenue, Lemont, IL 60439


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Abstract The transition towards a low carbon economy necessitates the implementation of a wide range of negative carbon technologies including bioenergy integrated with CO2 capture and storage. Advancing large-scale or modular technologies for bioenergy with carbon capture and storage necessitates a fundamental understanding of chemo-morphological coupling in these material systems. In this context, integrated chemical pathways such as alkaline thermal treatment (ATT) of biomass which involve the production of bio-H2 while converting and storing CO2 as solid carbonates provides promising potential for integrating bioenergy with carbon capture and storage. In this study, we elucidate the structural and morphological changes when biomass feedstocks such as cellulose and lignin are reacted with calcium hydroxide to capture, convert and store CO2 as calcium carbonate while producing energy carrier such as H2. These structural and morphological changes were monitored using synchrotron based in-operando multiscale X-ray scattering measurements. Enhanced CO2 capture at temperatures above 375 °C was evident from the significant growth of the calcium carbonate phase at these conditions. Increase in the surface area and porosity of cellulose was noted compared to lignin due to the relatively fast decomposition of cellulose. Pore-solid interfaces in Ca(OH)2 + cellulose system became smoother in the temperature range of 500-700 °C compared to Ca(OH)2 + lignin system, where the interfaces became rougher. Enhanced roughness of the pore-solid interfaces in the presence of lignin is attributed to the simultaneous slow decomposition of lignin and formation of calcium carbonate. Keywords: biomass, cellulose, lignin, X-ray scattering, carbon capture, calcium carbonate, alkaline thermal treatment



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Introduction One of the grand challenges in sustainable energy production is the need to develop energy generating processes with in-built environmental controls. In this context, the increasing need to deploy negative emission technologies to rapidly transition towards a low carbon economy [1] necessitates the advancement of fundamental science associated with bioenergy production with CO2 capture and storage (BECCS) technologies. While several studies assessed the large-scale potential of integrating conventional carbon capture and storage technologies (CCS) with bioenergy production ([2]–[5]), few studies explored alternative chemical pathways which involve simultaneously upcycling biomass conversion to produce clean fuels such as H2 while converting and storing CO2 as solid carbonates ([6]–[9]). Conventional approaches of recovering energy from biomass have included pyrolysis in the temperature range of 375 - 525 °C which yield liquid oils, solid charcoal, and gaseous compounds ([10]–[13]). H2 gas can be produced from the pyrolysis of biomass in a controlled environment as shown by the following reaction: CxHyOz + heat → H2 + CO + CH4 + hydrocarbons

(1)

Methane and hydrocarbons produced via the reaction shown above can be reformed by steam to yield additional H2 via the water-gas shift reaction shown below [14]. Hydrocarbons + CH4 + H2O → CO + H2

(2)

CO + H2O → CO2 + H2

(3)

While energy production from biomass pyrolysis is considered CO2 neutral since this CO2 has origins in the atmosphere before it is converted to biomass via photosynthesis [15], capturing the produced CO2 adds the carbon-negative component to bioenergy production. The potential to capture CO2 emitted from burning coal using calcium oxide (CaO) and calcium hydroxide 4 ACS Paragon Plus Environment

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(Ca(OH)2) as sorbents were reported by Curran and co-workers [16] and Wang and co-workers [17], respectively. The conversion and storage of CO2 in the form of calcium and magnesium carbonates has been investigated in the context of carbon mineralization [18]–[27]. Further, the integration of calcium carbonate chemistry in the context of calcium or chemical looping combustion has been reported ([28], [29]). However, the approach of integrating alkaline sorbents including calcium, magnesium and sodium hydroxide for CO2 capture and storage with bioenergy production, also known as Alkaline Thermal Treatment (ATT) ([7]–[9]) is a highly promising route for integrating sustainable energy production with integrated CO2 capture. The general reaction pathway associated with the ATT of biomass is shown by the following reaction: CxHyOz + Ca(OH)2 + H2O → H2 + CaCO3

(4)

However, it is not certain if the reacting sorbent is CaO or Ca(OH)2 since the following sequence of reactions can occur in the presence of CO2 with increasing temperature to produce calcium carbonate (CaCO3) [30]. Ca(OH)2 → CaO + H2O

(5)

CaO + CO2 → CaCO3

(6)

Ca(OH)2 + CO2 → CaCO3 + H2O

(7)

Different studies have reported the gaseous products formed during the pyrolysis of biomass. The major products of cellulose and lignin pyrolysis are H2, CO, CH4, and CO2 with some minor quantities of C2H4 and C2H6, as reported by Yang and co-workers [10]. Higher H2 yield from lignin pyrolysis (20.84 mmol/g) was noted compared to cellulose (5.48 mmol/g) while the CO2 emissions from both cellulose and lignin were 6.58 and 7.81 mmol/g, respectively. Conversion of biomass to H2 rich fuel gas integrated with CO2 capture by CaO was reported by 5 ACS Paragon Plus Environment


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Guoxin et al. [6]. High yields of H2 production with significant suppression of CO2 during the ATT of cellulosic biomass was reported by Stonor et al. [7]. Group I and II hydroxides were shown to facilitate the ATT process by sequestering CO2 to produce solid carbonates [7]. Additionally, a 10% Ni/ZrO2 catalyst was reported to increase the overall yield of H2 gas produced. For example, improvements in H2 yield from 0.4% (without catalyst) to 16.1% (with catalyst) and 1.2% (without catalyst) to 31.4% (with catalyst) were noted on reacting Mg(OH)2 and Ca(OH)2 with cellulose, respectively. The recent studies on the ATT of cellulose have demonstrated the promise of using this process to produce higher yields of H2. However, it was mentioned earlier that the pyrolysis of lignin produces higher yield of H2 as compared to cellulose. Therefore, with the increasing interest in sustainably utilizing lignin-bearing biomass feedstocks for bioenergy production ([31]– [34]), the effectiveness of the ATT process for lignin valorization needs to be evaluated. Additionally, the nature of products evolving during the pyrolysis of different biomass is well known [10], [11] but there is a limited understanding of the temperatures that correspond to CO2 capture and the formation of carbonates. The evolution of gaseous products greatly depends upon the biomass feed and the temperature. For example, in a packed bed pyrolysis analysis, it was found that H2 was released at temperatures higher than 400 °C and its release was enhanced by increasing the temperature up to 600 °C or higher [10]. The temperature ranges that correspond to the high production of H2 in cellulose and lignin are 700 – 800 °C and 600 – 700 °C, respectively [10]. In the case of cellulose and lignin, significant quantities of CO2 were released at 400 – 600 °C [10]. Therefore, it is essential to evaluate the temperature transitions that correspond to the uptake of CO2 to produce carbonate phases for optimizing and enhancing the efficiency of the ATT process.


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Therefore, the precise tuning of the temperature conditions for the ATT process necessitate addressing the following research questions: (1) What are the detailed phase transitions that occur during the ATT of cellulose and lignin? (2) What are the corresponding morphological changes that occur during these phase transitions? (3) How do these phase and morphological transitions vary with lignin and cellulose? To address these questions, we utilized multi-scale in-operando Xray scattering measurements to elucidate the changes in the reactant and product phases with the corresponding textural changes in the materials. The Wide Angle X-Ray Scattering (WAXS) measurements provide detailed insights into the evolution of the solid reactant and product phases, while the Ultra Small/Small Angle X-Ray Scattering (USAXS/SAXS) measurements show the evolution of textural changes with reaction progress ([35]–[42]) (Figure 1). The cross-scale evolution in the structure and morphology of the solid products during bioenergy production integrated with carbon capture, utilization and storage starting from cellulose and lignin reacted with Ca(OH)2 can be effectively probed using in-operando USAXS/SAXS/WAXS measurements. Further, thermogravimetric analyses (TGA) were performed to relate weight changes to the temperature transitions. The evolution in the surface area and porosity were determined using BET analyses. These analyses were combined to develop a comprehensive understanding of chemomorphological coupling during alkaline thermal treatment process for sustainable bioenergy production integrated with carbon capture and storage. Materials and Methods Lignin and cellulose powders were purchased from Sigma Aldrich, whereas calcium hydroxide (Ca(OH)2) was provided by HiMedia Laboratories Pvt. Ltd. All the powders were used in the study as received.



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The in-operando USAXS/SAXS/WAXS measurements were performed to determine the cross-scale evolution in the structure and morphology of the solid products during bioenergy production integrated with carbon capture, utilization, and storage. These measurements were performed at Sector 9-ID at Advanced Photon Source (APS) in Argonne National Laboratory (ANL), Argonne, IL. The biomass (lignin and cellulose) and Ca(OH)2 powders were mixed in 50 wt.% ratio for the measurements. The temperature ramp rate was set at 3.3 °C/min and the samples were reacted at temperatures up to 750 °C. A schematic of the experimental setup is shown in Figure 1. The USAXS/SAXS instruments use multiple dispersive reflections from perfect crystals to reach a region of scattering vector q, where q = (4π/λ)sinθ, λ is the wavelength of the X-ray used, and 2θ is the scattering angle. These instruments are based on the original Bonse-Hart doublecrystal configuration [43]. The total X-ray flux, wavelength, and corresponding energy during the measurements were 10-13 photon s−1, 0.59 Å, and 21.0 keV, respectively. Silver behenate and the NIST standard reference material, SRM 640d (Si) were used to perform the calibrations. Finally, the Irena [44] and Nika [45] software packages embedded in IgorPro software (Wavemetrics, Lake Oswego, OR) were used to reduce and analyze the data. The variations in the mass with temperature were recorded via thermogravimetric (TG) measurements, performed up to 750 ºC with a ramp rate of 2.5 ºC/min in an N2 environment (purged at 25 mL/min) using a Thermogravimetric Analyzer (TGA, TA Instruments, Q550, New Castle, DE). TGA was also coupled with an online gas chromatography instrument (Micro GC Fusion® Gas Analyzer, Inficon, Bad Ragaz, Switzerland) to monitor the evolution of CO2 gas during the ATT process. To evaluate changes in the chemical bonding of the reactants, infrared (IR) spectra were acquired in an Attenuated Total Reflection (ATR) mode using a Fourier 8 ACS Paragon Plus Environment

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Transform Infrared - Attenuated Total Reflection spectrometer (FTIR-ATR, NicoletTM iSTM 10, Waltham, MA). The spectra were collected in the range of 4000 – 650 cm-1, both before and after the TGA measurements. The changes in the porosity and specific surface area of the powders because of the reaction of biomass and Ca(OH)2 were determined using the Brunauer–Emmett– Teller technique (BET, Quantachrome NOVAtouch® Analyzer, Boynton Beach, FL). These measurements coupled with the X-ray scattering data provided detailed insights into the structural and morphological evolution of the reactants and products over the course of the reaction. Results and Discussion Characterization of powders The surface area of cellulose, lignin, and Ca(OH)2 were found to be 1.02, 3.93, and 3.55 m2/g, respectively. Detailed infrared spectroscopy analyses were performed to evaluate the chemical composition of the reactants. The infrared spectra of cellulose, lignin and Ca(OH)2 are presented in Figure 2. The typical functional groups are labeled in Figure 2 and listed in Table 1. In the spectrum of Ca(OH)2, the sharp peak at 3639 cm-1 corresponds to the O-H stretching in Ca(OH)2 crystals [46]. Additionally, two peaks at 1420.29 and 874.44 cm-1 were observed corresponding to the out-of-plane bending of CO32- in CaCO3, which implies slight carbonation of Ca(OH)2 in the air ([46]–[48]). The typical functional groups in cellulose were identified as O-H stretching, C-H symmetrical stretch, CH2 rocking vibrations, and C-O-C symmetrical stretching groups at 3330.83, 2893.91, 1314.57, and 1159.99 cm-1, respectively. A comparison of the spectra of cellulose and lignin showed different oxygen-containing functional groups ([10], [49], [50]). For example, aromatic vibrations, Guaiacyl (G) units, and methoxyl (-O-CH3) groups were noted in lignin in the range of 1593.05 – 814.48 cm-1, which were characteristically absent in cellulose.



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Phase evolution in the biomass feedstocks and calcium hydroxide due to integrated CO2 capture To evaluate the phase transitions in the alkaline sorbent, which is Ca(OH)2 in this case, the changes in the wide-angle X-ray scattering patterns were carefully evaluated as a function of temperature. The changes in the characteristic reflections of Ca(OH)2 from (101) and (110) planes corresponding to d = 2.617 and 1.790 Å, respectively, when Ca(OH)2 is reacted with cellulose and lignin are presented in Figure 3 and Figure 4, respectively. Significant changes in the peak intensities of Ca(OH)2 were noted at reaction temperatures above 350 °C (Figure 3 (a-2 and b-2) and Figure 4 (a-2 and b-2)). The decomposition of Ca(OH)2 was noted at temperatures between 300 - 400 °C, with complete decomposition achieved at 500 °C in both cases with cellulose and lignin ([30], [51], [52]). The increase in the d-spacing of (110) and (101) planes of calcite with temperature is associated with the thermal expansion of the solids on heating. The phase transitions of Ca(OH)2 corresponded to the thermogravimetric analyses representing the weight loss at temperatures above 300 °C (Figure 5), associated with the dehydroxylation of Ca(OH)2. These data suggest that calcium initially present in the hydroxide state is either converted to calcium oxide or calcium carbonate. To probe the fate of calcium, the evolution of characteristic peaks of CaCO3 and CaO was determined. Since the overarching objective of the ATT process is to enhance the capture of CO2 using alkaline sorbents such as CaO while producing H2 (as shown in Equation 4), it is important to evaluate the temperature corresponding to the onset and growth of CaCO3. The characteristic reflections from (104) and (113) planes of calcite (CaCO3) during the reaction of cellulose and lignin with Ca(OH)2 corresponding to the d-spacing of 3.022 and 2.273 Å are presented in Figure 6 and Figure 7, respectively. The emergence of calcite phases is evident at 300 °C with a slow growth till 400 °C, as indicated by a small increase in the integrated peak intensities (Figure 6 10 ACS Paragon Plus Environment

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(a-2, and b-2) and Figure 7 (a-2, and b-2)). At temperatures above 400 °C, the peak intensities for calcite increase significantly. These data suggest that between the temperature range of 300 400 °C, Ca(OH)2 initially reacts with CO2 to form CaCO3. In the presence of water vapor, reaction 4 which represents the evolution of H2 through the ATT process may be dominant. However, at temperatures higher than 400 °C, the formation of CaCO3 is governed by the carbonation of CaO according to Equation (6). To test these hypotheses, the changes in the characteristic peaks of CaO were determined. The changes in the characteristic CaO reflections at (200) and (202) planes, and (111) plane of Ca(OH)2 are presented in Figure 8 and Figure 9. Tracking the peak intensities for CaO (200) and (202) planes provided detailed insights into the changes in the structure of CaO. In cellulose and lignin, the integrated intensities of the characteristic CaO peaks increase around 450 °C and continued to increase until 500 °C, which corresponds to the temperature of complete dehydroxylation of Ca(OH)2 (Figure 8 (a-2) and Figure 9 (a-2)). Further, the integrated intensities corresponding to the CaO peaks continued to decrease until 700 °C, which corresponded to the consumption of CaO to produce CaCO3. The subsequent increase in the peak intensity of CaO on heating above 700 °C corresponded to the decomposition of CaCO3 to produce CaO [53] (Figure 8 (a-2, b-2) and Figure 9 (a-2, b-2)). The detailed insights from the in-operando phase changes in the calcium-bearing species were intended to complement the previous studies reporting the gas-phase compositions of the ATT processes. For example, the gas phases compositions comprising H2, CO, CH4, and CO2 resulting from the pyrolysis of cellulose, hemicellulose, and lignin varied in the temperature range of 150 – 900 °C [10]. Higher yields of CO were noted from cellulose pyrolysis as opposed to H2 and CH4 from lignin [10]. ATT of cellulose with Group I and II hydroxides yielded similar gaseous 11 ACS Paragon Plus Environment


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products ([8], [9], [7]). Substantial suppression of gaseous CO2 production owing to its capture by alkali or alkaline metal conversion to the carbonate products resulted in higher H2 yields. To describe the changes in the solid-state chemistry that correspond to the studies focused on gasphase analyses ([7]–[10]), the evolution of various phases of calcium was related to the temperatures corresponding to CO2 production during biomass pyrolysis (Figure 10(a-1) and (a2)). The major product during the pyrolysis for both cellulose and lignin is CO2 and is readily available in the temperature range of 300 – 600 °C. The onset of CaCO3 formation and dehydroxylation of calcium hydroxide was noted at 300 °C. At temperatures in the range of 500700 °C, the conversion of CaO to CaCO3 was noted. Increase in temperature above 700 °C resulted in the calcination of CaCO3 to produce CaO. To confirm this, the evolution of CO2 being released during the pyrolysis of biomass and the reaction with Ca(OH)2 was monitored. The results of gas chromatography for cellulose and lignin with and without Ca(OH)2 are presented in Figure 10 (a2) and (b-2), respectively. The vol.% of the released CO2 was normalized with the initial mass of biomass used in all cases. For cellulose, maximum CO2 released was in the temperature range of 300 – 450 °C, followed by the fractional release up to 750 °C. However, in case of lignin, relatively large amount of CO2 was released over the broad temperature range of 300 – 700 °C. During reaction with Ca(OH)2, the amount of CO2 released from cellulose was decreased from 7 × 10-3 vol.%/mg of cellulose to 4.7 × 10-3 vol.%/mg of cellulose. Additionally, a peak for CO2 released right after 600 °C emerged, corresponding to the calcination of CaCO3 at higher temperatures (> 600 °C). Whereas, for lignin, during ATT almost all of the CO2 released was captured during the ATT process. Moreover, similar to the case of cellulose + Ca(OH)2, CO2 peak from the calcination of CaCO3 was noted. In case of lignin + Ca(OH)2, however, the CO2 peak at higher temperatures


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(> 600 °C) also contains some of the CO2 released from the secondary pyrolysis of lignin due to the degradation of complex aromatic groups [54], [55]. To understand the temperature transitions in greater detail, thermogravimetric analyses were performed (Figure 5). The onset of degradation temperatures and % residual weight is presented in Table S1. Additionally, the results from the gas chromatography experiments were superimposed with the results of TGA to relate the CO2 evolution with the change in weight % and are presented in Figure S1. Pyrolysis of cellulose resulted in the most significant weight loss with only 8% of retained weight after 750 °C, with the highest conversion of cellulose to CO2 at temperatures ~300 °C (Figure S1(a-1)). This was attributed to the fast pyrolysis and ease of degradation of cellulosic constituents [10]. Lignin exhibited a different trend compared to cellulose. The decomposition of lignin to CO2 was relatively slower owing to its complex structure and occurred over a wider temperature range of 200 – 750 °C (Figure S1 (b-1)). The initial decomposition temperature (onset) for lignin was observed at 268 °C, followed by secondary decompositions at 360 °C and 685 °C. Weight loss in the range 250 – 500 °C is attributed to the decomposition of carbohydrate components in lignin evolving as volatile gases such as CO2 and CH4. The weight loss at temperatures greater than 500 °C is attributed to the decomposition of the aromatic skeleton in lignin ([54], [55]). Moreover, the weight retained by lignin was 39.5 wt.% compared to 8 wt.% in the case of cellulose. ATT of cellulose and lignin with Ca(OH)2 was also studied using thermogravimetric analyses (Figures 5(a) and (b)). The onset of Ca(OH)2 decomposition occurred at 374 °C, which corresponded to the dehydroxylation of Ca(OH)2 to produce CaO and H2O [30]. Postdehydroxylation of Ca(OH)2, the residual weight % was ~77%. Further, different trends in the decomposition behavior of Ca(OH)2 + cellulose and Ca(OH)2 + lignin mixtures were observed. In 13 ACS Paragon Plus Environment


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the Ca(OH)2 and cellulose system, two separate decomposition regimes were noted: the first onset occurred ~316 °C which corresponded to the decomposition of cellulose to produce CO2 (Figure S1) and the second ~382 °C associated with the dehydroxylation of Ca(OH)2. In the Ca(OH)2 and lignin system, decomposition temperatures of both Ca(OH)2 and lignin overlapped with an onset ~328 °C. The lower mass of lignin and calcium hydroxide mixture compared to cellulose and calcium hydroxide suggests that the solid decomposition of lignin in calcium hydroxide is higher compared to that of cellulose and calcium hydroxide. To further understand the changes in the composition and structures of powders during the reaction, the infrared spectra were recorded before and after performing thermogravimetric analyses (Figure S2). The spectra of cellulose and lignin, after thermal decomposition, clearly indicate the removal of oxygen-containing functional groups. In the case of Ca(OH)2 and cellulose, almost all the oxygen-containing functional groups were decomposed, and obvious carbonate peaks were observed at 1420.08 and 896.20 cm-1. Similarly, the carbonate peaks in Ca(OH)2 and lignin material systems were present at 1410.04 and 865.66 cm-1. Additionally, in both cases, OH stretching vibrational bands were observed ~3640 cm-1 which was associated with the physical adsorption of H2O by CaO when exposed to air. Increased surface area and porosity of the postpyrolysis products of a mixture of biomass and calcium oxide suggests an enhanced affinity for water ([56]–[58]) (Table 2). Morphological changes in the biomass feedstocks and calcium hydroxide due to integrated CO2 capture With increasing interest in utilizing the residues post energy recovery, understanding the morphologies of these materials is essential. Based on the phase changes reported in the previous section, significant changes in the morphologies of cellulose or lignin as they are reacted with 14 ACS Paragon Plus Environment

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Ca(OH)2 were expected. The changes in the porosity, surface area, and the pore-solid interface were probed in this study. The cumulative pore volume (cc/g) and pore volume distribution (cc/nm/g) trends as a function of pore diameter (nm) for the samples under study are presented in Figure 11. The quantitative information about the surface area (m2/g) and cumulative pore volume (cc/g) are also reported in Table 2. As a result of alkaline thermal treatment, the cumulative pore volume of Ca(OH)2 + cellulose and Ca(OH)2 + lignin systems increased from 0.006 to 0.13 cc/g and 0.005 to 0.01 cc/g, respectively. Corresponding increases in the surface areas of Ca(OH)2 + cellulose and Ca(OH)2 + lignin systems from 4.22 to 98.67 m2/g and 2.70 to 11.74 m2/g were noted. The magnitude of the morphological changes with cellulose bearing systems was higher compared to lignin due to the greater ease of decomposing cellulose as opposed to lignin. To understand the origin of the changes in the surface areas and the porosities, pure Ca(OH)2, cellulose, and lignin were treated at similar experimental conditions as the mixtures of Ca(OH)2 and biomass (e.g., cellulose and lignin). The surface areas and the cumulative porosities of these materials post-reaction were significantly higher compared to the materials pre-reaction. This suggests that the increase in the surface area and porosities arising from the decomposition of the solid reactants dominates the formation of denser phases such as CaCO3. Furthermore, the changes in the basal planes and the pore-solid interface were effectively captured by USAXS and SAXS measurements, as shown in Figure 12. In the SAXS regime for both cases, the characteristic reflections from the (001) basal plane of Ca(OH)2 were observed at q = 1.30 Å−1, corresponding to the d-spacing of 4.83 Å [59]. The peak shifts of the (001) plane to smaller q values (larger d-spacing) with an increase in temperature is attributed to thermal expansion. In case of Ca(OH)2 + cellulose, the last (001) reflection appeared with a very low 15 ACS Paragon Plus Environment


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intensity at 500 °C. While, for Ca(OH)2 + lignin, the (001) peak diminished before 500 °C with a very low-intensity peak appearing at 475 °C. These findings were in agreement with the changes in the other phases of calcium hydroxide as shown in Figures 3 and 4. In addition to providing insights into the changes in the basal spacing of Ca(OH)2, detailed insights into the changes in the pore-solid interface from the Porod slope were obtained using USAXS/SAXS data ([35]–[38], [60]–[62]). The Porod slope provides information about the ‘fractal dimensions’ of the scattering objects. In this work, the Porod slope was calculated for the q values in the range 0.02 Å−1 – 0.2 Å−1 and the fractality of local structure was investigated. The Porod slope was calculated using the following equations: I(Q) =

A Qn

(8)

+B

log10[I(Q) ― B] = log10A ― nlog10Q

(9)

In the above equations, I(Q) is the scattering intensity and n is the slope. The values of Porod slopes provide useful insights into the morphology of scattering objects. Porod slopes between 3 and 4 represent scattering from rough interfaces with a fractal dimension, D, where n = 6 – D represents a surface fractal. Porod slopes of 4 represent scattering from smooth surfaces, while a Porod slope of 1 represents scattering from rigid rods. Porod slope between 2 and 3 is indicative of scattering from branched systems or networks also known as mass fractals. The Porod slopes of Ca(OH)2 + cellulose increased from 3.2 to 3.5 as the reaction proceeded from 100 °C to 400 °C, which suggested a smoothening of the pore-solid interface. (Figure 12 (a-2)). Further heating to 450 °C and 475 °C which corresponds to the dehydroxylation of Ca(OH)2 and rapid formation of CaCO3 enhanced the roughness of the pore-solid interface. Progressive growth of CaCO3 enabled the formation of smoother surfaces in Ca(OH)2 + cellulose 16 ACS Paragon Plus Environment

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system. In the case of lignin, however, the pore-solid interfaces became increasingly rougher with higher conversion to CaCO3 in the temperature range of 500-750 °C. Insights into the differences in the evolution of the pore-solid interface in the case of cellulose and lignin with increasing carbon mineralization behavior were obtained from thermogravimetric analyses (Figures 5 (a-1) and 5 (a-2)). For example, cellulose decomposition occurs quickly and significant changes in the weight do not occur at temperatures above 300 °C. In the case of lignin, however, decomposition occurs over a wider temperature range (Figure 5 (b-2)) and simultaneous CaCO3 formation and lignin decomposition were noted. The formation of well-defined CaCO3 phases is the dominant contributing factor for producing less rough pore-solid interfaces in cellulose. The simultaneous decomposition of lignin and formation of CaCO3 contributes to the formation of rougher poresolid interfaces as the reaction temperature increases above 500 °C. These morphological insights are useful when considering the reuse of the products post-pyrolysis. Conclusion In this study, we elucidate the evolution of the structural and morphological features of Ca(OH)2 during bioenergy production from cellulose and lignin when integrated with CO2 capture. Significant onset of calcium carbonate growth was only noted at temperatures above 375 °C suggesting that CO2 capture was enhanced at these conditions. Enhanced formation of CaCO3 phases corresponded to the decomposition of Ca(OH)2 to produce CaO. Although lignin decomposed slowly compared to cellulose, the onset of carbon mineralization and the temperature ranges for CO2 capture did not change. However, higher amount of CO2 was captured during ATT of lignin as compared to the ATT of cellulose. Increase in the surface area and porosity of cellulose system as opposed to the lignin system was attributed to the relatively fast decomposition of cellulose. Pore-solid interfaces in Ca(OH)2 + cellulose system became smoother in the temperature 17 ACS Paragon Plus Environment


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range of 500-700 °C compared to Ca(OH)2 + lignin system, where the interfaces became rougher. The roughness in the Ca(OH)2 + lignin system was attributed to the simultaneous decomposition of lignin and formation of CaCO3. These structural and morphological insights are useful in advancing our fundamental understanding of alkaline thermal treatment of biomass. Supporting Information Changes in the compositions of CO2 in cellulose without (a-1) and with (a-2) calcium hydroxide, and in lignin without (b-1) and with (b-2) calcium hydroxide using thermogravimetric analysis coupled with online micro gas chromatography measurements Changes in the characteristic functional groups in (a) cellulose and (b) lignin systems before and after thermogravimetric analyses from Attenuated Total Reflectance - Fourier transform infrared (ATR-FTIR) spectra Identification of the onset temperature for significant weight change and the residual weight % based on thermogravimetric analyses Acknowledgments The authors would like to acknowledge the experimental support provided by Dr. Jan Ilavsky at the USAXS facility, X-ray Science Division, Argonne National Laboratory. The use of the Advanced Photon Source, an Office of the Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne national Laboratory, is supported by the U.S. DOE under Contract DE-AC02-06CH11357. Wisconsin Alumni Research Foundation and the College of Engineering at the University of Wisconsin, Madison provided the financial support for this research.


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Table 1. Functional groups present on cellulose, lignin and Ca(OH)2 powders Sample

Wavenumber (cm-1)

Functional groups

Ca(OH)2

3639

O-H stretching mode

1420.29, 874.44

C-O from CO3-

3330.83

O-H stretching mode

2893.91

C-H symmetrical stretch

1427.23

HCH & OCH in-plane bending vibration

1314.57

CH2 rocking vibration

1201.73

C-O-C symmetrical stretching & O-H plane deformation

1159.99

C-O-C symmetrical stretching

1052.83, 1028.07

C-C, C-OH, C-H ring & side groups vibrations

661.71

C-OH out-of-plane bending

3249.16

O-H stretching from phenolic/aliphatic groups

2934.98

C-H stretching in aromatic methoxyl

1593.05. 1511.27, 1424.24

Aromatic skeleton vibrations

1450.66

C-H deformation & aromatic ring vibrations

1265.01

C=O stretch in Guaiacyl (G) unit

1123.71

C-H in-plane deformation

853.82, 814.48

C-H out of plane vibration in G-unit

Cellulose

Lignin


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Table 2. Surface area of samples before and after TGA Sample

Surface Area (m2/g)

Cumulative Pore Volume (cc/g)

Before TGA

After TGA

Before TGA

After TGA

Ca(OH)2

3.55

31.60

0.007

0.046

Cellulose + Ca(OH)2

4.22

98.67

0.006

0.126

Cellulose

1.02

55.19

0.002

0.065

Lignin + Ca(OH)2

2.70

11.74

0.005

0.014

Lignin

3.93

5.87

0.008

0.008



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Captions for Figures Figure 1. Schematic of the experimental setup for determining the structural and morphological features during the alkaline thermal treatment of biomass using in-operando USAXS/SAXS/WAXS measurements. Figure 2. Identification of the functional groups in cellulose, lignin and calcium hydroxide using Fourier Transform-Infrared spectroscopy measurements. Figure 3. Changes in the characteristic Ca(OH)2 peaks where (a-1) and (b-1) represent hkl reflections of (101) (q = 2.40 Å−1, d = 2.617 Å) and (110) (q = 3.51 Å−1, d = 1.790 Å), respectively during reaction with cellulose. The integrated peak intensities normalized with respect to the maximum values are shown in Figures (a-2) and (b-2). Figure 4. Changes in the characteristic Ca(OH)2 peaks where (a-1) and (b-1) represent hkl reflections of (101) (q = 2.40 Å−1, d = 2.617 Å) and (110) (q = 3.51 Å−1, d = 1.790 Å), respectively during reaction with lignin.The integrated peak intensities normalized with respect to the maximum values are shown in Figures (a-2) and (b-2). Figure 5. Changes in the weight of cellulose (a-1), and lignin (b-1) with and without calcium hydroxide. The corresponding first derivative of weight changes is shown in (a-2) and (b-2) for comparison. Figure 6. Changes in the characteristic CaCO3 peaks where (a-1) and (b-1) represent hkl reflections of (104) (q = 2.06 Å−1, d = 3.05 Å) and (113) (q = 2.76 Å−1, d = 2.27 Å), respectively during reaction with cellulose. The integrated peak intensities normalized with respect to the maximum values are shown in Figures (a-2) and (b-2). Figure 7. Changes in the characteristic CaCO3 peaks where (a-1) and (b-1) represent hkl reflections of (104) (q = 2.06 Å−1, d = 3.05 Å) and (113) (q = 2.76 Å−1, d = 2.27 Å), respectively during reaction with lignin. The integrated peak intensities normalized with respect to the maximum values are shown in Figures (a-2) and (b-2). Figure 8. Changes in the characteristic CaO peaks where (a-1) and (b-1) represent hkl reflections of (202) (q = 3.687 Å−1, d = 1.704 Å) and (200) (q = 2.607 Å−1, d = 2.41 Å), respectively during the reaction with cellulose. The hkl reflection for Ca(OH)2 at (111) (q = 3.727 Å−1, d = 1.685 Å) is shown in (a-1). The integrated peak intensities normalized with respect to the maximum values are shown in Figures (a-2) and (b-2). Figure 9. Changes in the characteristic CaO peaks where (a-1) and (b-1) represent hkl reflections of (202) (q = 3.687 Å−1, d = 1.704 Å) and (200) (q = 2.607 Å−1, d = 2.41 Å), respectively during the reaction with lignin. The hkl reflection for Ca(OH)2 at (111) (q = 3.727 Å−1, d = 1.685 Å) is shown in (a-1). The integrated peak intensities normalized with respect to the maximum values are shown in Figures (a-2) and (b-2). Figure 10. Comparison of the evolution of Ca(OH)2, CaO and CaCO3 phases in cellulose (a-1) and lignin (b-1). Changes in the release of CO2 from cellulose (a-2) and lignin (b-2) with and without Ca(OH)2. The shaded region indicates the presence of CO2 during the reaction of cellulose and lignin with calcium hydroxide ([7]–[10]). 26 ACS Paragon Plus Environment

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Figure 11. Changes in the cumulative pore volume and pore volume distributions in cellulose ((a1), (a-2) and lignin ((b-1), (b-2)), respectively. Figure 12. Changes in the combined slit-smeared UXAS/SAXS data for cases with Ca(OH)2 and cellulose (a-1) and Ca(OH)2 and lignin (b-1). The reflections from Ca(OH)2 (001) plane corresponding to q = 1.30 Å−1 and d = 4.83 Å are evident in (a-1) and (b-1). Figures (a-2) and (b-2) represent the changes in the Porod slopes for q in the range of 0.02 Å−1 – 0.2 Å−1 for Ca(OH)2 and cellulose and Ca(OH)2 and lignin, respectively.



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Figure 1. Schematic of the experimental setup for determining the structural and morphological features during the alkaline thermal treatment of biomass using in-operando USAXS/SAXS/WAXS measurements.


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Figure 2. Identification of the functional groups in cellulose, lignin and calcium hydroxide using Fourier Transform-Infrared spectroscopy measurements.



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Figure 3. Changes in the characteristic Ca(OH)2 peaks where (a-1) and (b-1) represent hkl reflections of (101) (q = 2.40 Å−1, d = 2.617 Å) and (110) (q = 3.51 Å−1, d = 1.790 Å), respectively during reaction with cellulose. The integrated peak intensities normalized with respect to the maximum values are shown in Figures (a-2) and (b-2).


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Figure 4. Changes in the characteristic Ca(OH)2 peaks where (a-1) and (b-1) represent hkl reflections of (101) (q = 2.40 Å−1, d = 2.617 Å) and (110) (q = 3.51 Å−1, d = 1.790 Å), respectively during reaction with lignin.The integrated peak intensities normalized with respect to the maximum values are shown in Figures (a-2) and (b-2).



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Figure 5. Changes in the weight of cellulose (a-1), and lignin (b-1) with and without calcium hydroxide. The corresponding first derivative of weight changes is shown in (a-2) and (b-2) for comparison.


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Figure 6. Changes in the characteristic CaCO3 peaks where (a-1) and (b-1) represent hkl reflections of (104) (q = 2.06 Å−1, d = 3.05 Å) and (113) (q = 2.76 Å−1, d = 2.27 Å), respectively during reaction with cellulose. The integrated peak intensities normalized with respect to the maximum values are shown in Figures (a-2) and (b-2).



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Figure 7. Changes in the characteristic CaCO3 peaks where (a-1) and (b-1) represent hkl reflections of (104) (q = 2.06 Å−1, d = 3.05 Å) and (113) (q = 2.76 Å−1, d = 2.27 Å), respectively during reaction with lignin. The integrated peak intensities normalized with respect to the maximum values are shown in Figures (a-2) and (b-2).


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Figure 8. Changes in the characteristic CaO peaks where (a-1) and (b-1) represent hkl reflections of (202) (q = 3.687 Å−1, d = 1.704 Å) and (200) (q = 2.607 Å−1, d = 2.41 Å), respectively during the reaction with cellulose. The hkl reflection for Ca(OH)2 at (111) (q = 3.727 Å−1, d = 1.685 Å) is shown in (a-1). The integrated peak intensities normalized with respect to the maximum values are shown in Figures (a-2) and (b-2).



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Figure 9. Changes in the characteristic CaO peaks where (a-1) and (b-1) represent hkl reflections of (202) (q = 3.687 Å−1, d = 1.704 Å) and (200) (q = 2.607 Å−1, d = 2.41 Å), respectively during the reaction with lignin. The hkl reflection for Ca(OH)2 at (111) (q = 3.727 Å−1, d = 1.685 Å) is shown in (a-1). The integrated peak intensities normalized with respect to the maximum values are shown in Figures (a-2) and (b-2).


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Figure 10. Comparison of the evolution of Ca(OH)2, CaO and CaCO3 phases in cellulose (a-1) and lignin (b-1). Changes in the release of CO2 from cellulose (a-2) and lignin (b-2) with and without Ca(OH)2. The shaded region indicates the presence of CO2 during the reaction of cellulose and lignin with calcium hydroxide ([7]–[10]).



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Figure 11. Changes in the cumulative pore volume and pore volume distributions in cellulose ((a1), (a-2) and lignin ((b-1), (b-2)), respectively.


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Figure 12. Changes in the combined slit-smeared UXAS/SAXS data for cases with Ca(OH)2 and cellulose (a-1) and Ca(OH)2 and lignin (b-1). The reflections from Ca(OH)2 (001) plane corresponding to q = 1.30 Å−1 and d = 4.83 Å are evident in (a-1) and (b-1). Figures (a-2) and (b-2) represent the changes in the Porod slopes for q in the range of 0.02 Å−1 – 0.2 Å−1 for Ca(OH)2 and cellulose and Ca(OH)2 and lignin, respectively.



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Synopsis Understanding chemo-morphological coupling in material systems for bioenergy with carbon capture and storage advances sustainable negative emissions technologies.


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ACS Paragon Plus Environment

Relating Structural and Microstructural Evolution to the Reactivity of

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