Edited by: Maria Chiara Bignozzi, University of Bologna, Italy
Reviewed by: Francisco Carrasco-Marín, University of Granada, Spain; Alexander M. Puziy, National Academy of Sciences of Ukraine (NAN Ukraine), Ukraine
This article was submitted to Carbon-Based Materials, a section of the journal Frontiers in Materials
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This work analyzes the effect of co-solution of carbon precursor and activating agent on the textural and surface chemistry properties of highly nanoporous activated carbons obtained by chemical activation of Alcell lignin with phosphoric acid. The success of this methodology highlights the possibility of directly using the liquors produced in organosolv process (Alcell) to prepare activated carbons by chemical activation with phosphoric acid. Co-solutions of lignin and phosphoric acid were submitted to a two steps thermal treatment, which consisted of a first oxidative treatment in air at 200°C, followed by a thermal treatment in N2 at 400°C, where activation of the oxidized lignin with phosphoric acid took place. A lignin-derived activated carbon with very high apparent surface area (2550 m2/g) and pore volume (1.30 cm3/g) was obtained with an initial phosphoric acid to lignin mass ratio of 2. Up to now, this is one of the highest values of apparent surface area reported not only for activated carbons prepared from lignin, but even for porous carbons prepared by chemical activation of other lignocellulosic materials with phosphoric acid. The use of lignin and phosphoric acid co-solution provided larger and more homogeneous effective interactions between the carbon precursor and the activating agent, by the formation of phosphate esters in the lignin matrix, which seems to be a key factor in the subsequent treatments: promoting crosslinking reactions in the carbonaceous matrix during the oxidative treatment in air at 200°C and enhancing the development of a wide porosity during the following activation thermal treatment.
Within a biorefinery scheme, the conversion of the major components of lignocellulosic biomass (cellulose, hemicellulose, and lignin) into energy and chemicals is essential. However, lignin has been used, up to date, mainly, as a fuel to recover energy and chemical reactants in the pulp and paper industry and only a 2% of the produced lignin is currently commercialized (
Chemical activation with phosphoric acid is a well-known method to prepare activated carbons from different biomass precursors (
With regard to lignin, activated carbons with very high ABET values (around 3000 m2/g) by chemical activation of Kraft lignin with KOH has been reported (
Thus, the objective of this work was to analyze the effect of co-solution of carbon precursor and activating agent on the textural and surface chemistry properties of activated carbons obtained by chemical activation of Alcell lignin with phosphoric acid, in order to assess the possibility of directly using the liquors produced in the Organosolv process (Alcell), which would avoid the lignin precipitation and separation steps in this process. The influence of the use of a low temperature oxidative thermal treatment, previous to the activation process, on the surface chemistry and textural properties of the final lignin-derived carbons obtained was also studied.
Alcell lignin (supplied by Repap Technologies Inc., Co.) was used as raw material for the activated carbons preparation. Different procedures were used for the preparation of activated carbons from lignin. Alcell lignin was dissolved in ethanol to simulate the composition of dissolved lignin in the Organosolv liquors. Solid Alcell lignin was directly impregnated with aqueous H3PO4 85% (w/w) at room temperature. Co-solution of lignin and the activating agent, H3PO4 85% (w/w), was also prepared at room temperature and the ethanol/lignin/H3PO4 solution was stirred until evaporation of the ethanol (which could be recovered). Both the impregnated lignin and the solid derived from the co-solution of lignin and H3PO4 were dried for 24 h at 60°C in a vacuum dryer. The amounts of lignin and H3PO4 used in both cases were those to obtain a final H3PO4/lignin mass ratio of 2. The as-received lignin and that dissolved in ethanol were denoted by L and LE, respectively. H3PO4 impregnated lignin and the solid derived from the lignin/ethanol/H3PO4 solution once dried were denoted as L2P and LE2P, respectively. These two samples were subsequently submitted to a low temperature oxidative treatment at 200°C in air atmosphere for 1.5 h, followed by an activation treatment under inert atmosphere at 400°C, both inside a laboratory horizontal tubular furnace. The heating rate to reach the oxidative and the activation temperature was 3 and 10°C/min, respectively. Once the activation temperature was reached the samples were cooled inside the furnace under inert atmosphere and then washed with distilled water at 60°C until neutral pH. The resulting activated carbons were dried at 60°C.
L2P and LE2P submitted to the oxidative treatment in air at 200°C for 1.5 h were denoted by adding the letter S to the corresponding names (L2PS and LE2PS, respectively). The final activated carbons obtained at 400°C in inert atmosphere were denoted by adding 400 to the respective notations (L2PS400 and LE2PS400, respectively). In case of oxidative treatments at different temperatures, the activated carbons notation will include at the end of the name the temperature of the treatment. For the sake of comparison, some samples were directly activated at 400°C under air atmosphere without being subjected to the previous oxidative low temperature treatment. In these cases, the letter S was not included in their corresponding notations (L2P400 and LE2P400) and the letter A was added at the end of its corresponding name. In order to also evaluate the influence of the gas atmosphere during the low temperature treatment, a sample was treated under inert atmosphere at 200°C for 1.5 h and was denoted by adding the letter N to its corresponding notation.
The porosity of the samples was characterized by N2 adsorption–desorption and CO2 adsorption at −196 and 0°C, respectively, using a micromeritics instrument (ASAP 2020 model). The samples were previously outgassed for at least 8 h at 150°C. From the N2 isotherm, the apparent surface area (ABET) was determined by applying the BET equation (
Thermal treatments of different samples were carried out in a gravimetric thermobalance system, CI electronics. The thermobalance automatically measures the weight of the sample and the temperature as a function of time. Experiments were carried out in inert atmosphere (N2) and in air atmosphere, for a total flow rate of 150 cm3 (STP)/min, employing sample mass of approximately 10 mg. The sample temperature was increased at a heating rate of 10°C/min. Differential scanning calorimetry (DSC) experiments were also obtained by a thermal analyzer (Mettler Toledo equipment) coupled to a Mass Spectrometer (Pfeiffer Vacuum model ThermoStar TM GSD 320).
The FTIR spectra of lignin and lignin-derived samples were recorded in a Bruker Optics Vertex 70 FTIR spectrometer, in the 500–4000 cm–1 range, in KBr disks [ca. 1% (w/w)]. A KBr beam splitter and a Golden gate single reflection diamond ATR system detector were used. The spectra were collected for 2 min, with 4 cm–1 resolution.
Solid-state 13C and 31P-NMR spectra of lignin and lignin-derived samples were recorded, as well, with a NMR spectrometer (400 WB Plus model from Bruker) using the Cross Polarization Magic Angle Spinning (CPMAS) and high power decoupling (HPDEC) techniques and a 3.2 mm MAS triple-channel probe at a spinning frequency of 15 kH, with SPINAL-64 1H decoupling conditions.
X-ray photoelectron spectroscopy (XPS) analyses of the samples were carried out in a PHI 5700C model Physical Electronics apparatus, with MgKα radiation (1253.6 eV). For the analysis of the XPS peaks, the C1s peak position was set at 284.5 eV and used as reference to position the other peaks. For the deconvolution of the peaks, Gaussian–Lorentzian curves were used. The deconvolution of the phosphorus spectrum was carried out by using two doublet peaks with an area ratio of 0.5 and a separation between peaks of 0.84 eV for each phosphorus groups. TPD experiments were carried out in a customized quartz fixed-bed reactor placed inside an electrical furnace. CO and CO2, as output gases, were measured by a non-dispersive infrared (NDIR) gas analyzer, Siemens ULTRAMAT 22 (Siemens AG, Munich, Germany). Eighty mg of dried carbon sample was heated from room temperature to 930°C at a heating rate of 10°C/min under N2 flow (200 cm3/min).
Alcell lignin is a type of lignin derived from Organosolv process, a pulping technique that uses an organic solvent to solubilize lignin and hemicellulose. This type of lignin contains very small amounts of inorganic compounds (
Normalized CPMAS 13C NMR spectra
The samples show the characteristic vibrations for the guaiacyl unit (G ring and C = O stretch at around 1260 cm–1; CH in-plane deformation at 1150 cm–1; and C–H out-of plane vibrations in position 2, 5, and 6 of guaiacyl units at 830 and 915 cm–1). Syringyl (S) and guaiacyl (G) units are detected by aromatic skeleton vibrations at 1600 and 1325 cm–1 (S), 1515 and 1260 cm–1 (G), and aromatic in plane C-H vibrations at 1110 (S) and 1030 cm–1 (G). This last band is more intense in case of the LE sample (
Finally, both samples also contain a weak band at 1370–1375 cm–1 originating from phenolic OH and aliphatic C–H in methyl groups and a strong vibration at 1215–1220 cm–1 that can be associated with C–C, C–O, and/or C = O stretching. The aromatic C–H deformation at 1030 cm–1 appears as a complex vibration associated with the C–O, C–C stretching and C–OH bending in polysaccharides (
FTIR spectra of the different H3PO4-lignin derived samples.
The typical bands for guaiacyl unit are also significantly reduced, those at 1215–1220 cm–1 and at 1030 cm–1, associated to C–C, C–O, and/or C = O stretching and C–O, C–C stretching and C–OH bending in polysaccharides, respectively. However, the appearance of other bands probably related to phosphorus compounds is also observed. In this sense, the broad band at 965 cm–1 and the intense peak at 635 cm–1 could be initially associated to P-O-P and P-C bending of aromatic compounds, respectively, although the peak at 635 cm–1 can be also related to C ≡ CH bonds (
In order to analyze if phosphoric acid can act as an oxygen donor to the lignin matrix at this low temperature, sample L2P was treated at 200°C for 1.5 h, but under inert (N2) atmosphere (L2PS-N). The FTIR results for L2PS-N are also shown in
The results obtained from XPS analysis confirmed the presence of phosphorus groups on the surface of L2P, LE2P, L2PS, and LE2PS samples (see
XPS P2p spectra of the different H3PO4-lignin derived samples and deconvolution of LE2PS sample (C-O-P groups: continuous line, P2O5: dotted line and C-PO3: dashed line).
Mass surface concentration of the different H3PO4-lignin derived samples and the corresponding contribution of different phosphorus surface group obtained by XPS analyses.
Surface concentration | Phosphorus | |||||
Sample | by XPS |
surface groups |
||||
C (%) | O (%) | P (%) | P2O5 (%) | C-O-P (%) | C-PO3 (%) | |
L2P | 30.7 | 45.3 | 24 | 3.9 | 89.05 | 7.05 |
LE2P | 40.1 | 39 | 20.9 | 9.9 | 89.6 | 0.5 |
L2PS | 48.8 | 31.3 | 19.9 | 5.85 | 93.7 | 0.45 |
LE2PS | 42.9 | 36.5 | 20.6 | 9.24 | 89.59 | 1.17 |
Thermal decomposition of the different lignin-derived samples was studied by TG analyses.
Between 200 and 500°C a further decrease of mass is observed associated to the release of light compounds from lignin degradation. In this temperature range, crosslinking reactions dominate over bond cleavage and de-polimerization reactions. At 250°C the structures are considered to be small polyaromatic units connected mainly by phosphate and polyphosphate bridges, including polyethylene linkages. As the temperature increases, cyclization and condensation reactions lead to increases in aromaticity and size of the poliaromatic units, enabled by the scission of P-O-C bonds (
A change in the slope of the weight-loss curve can be also observed at temperatures higher than 500°C, probably associated to volatilization of carbon–oxygen complexes generated by the activation process, and to a lesser extent, to decomposition of phosphorous-compounds (as P2O5), produced by the reaction of phosphoric acid with the carbon matrix (
The profiles of L2P and LE2P are very similar, suggesting the low impact of using dissolved lignin in ethanol in the thermal decomposition reactions. However, the weight loss curves of the samples treated with air (L2PS and LE2PS) are above those of L2P and LE2P samples. If the difference between the weight loss for a specific temperature of the non-treated and treated samples is represented as a function of temperature (
Differential scanning calorimetry (DSC) scans of all the lignin-derived samples are shown in
A further experiment was carried out trying to simulate the oxidative treatment at 200°C. The corresponding DSC curves are shown in
Yield values of the lignin activated carbons.
Activated carbon | Oxidative treatment yield (%) | Activation yield (%) | Washing yield (%) | Total yield (%) |
L2P400 | – | 75.9 | 28.0 | 63.8 |
LE2P400 | – | 75.9 | 25.9 | 59.2 |
L2PS400 | 87.3 | 87.8 | 27.3 | 62.9 |
LE2PS400 | 90.7 | 81.6 | 25.2 | 63.3 |
N2 adsorption-desorption isotherms obtained at –196°C of the different activated carbons obtained at 400°C and with H3PO4 to lignin mass ratio of 2.
The textural properties obtained from the N2 and CO2 isotherms of the different activated carbons are summarized in
Textural properties of the different lignin derived activated carbons obtained from the N2 and CO2 isotherms.
N2 Isotherm |
CO2 Isotherm |
||||||
ABET (m2/g) | At (m2/g) | Vt (cm3/g) | Vp (cm3/g) | Vmes (cm3/g) | ADR (m2/g) | VDR (cm3/g) | |
L2P400 | 912 | 55 | 0.412 | 0.477 | 0.063 | 438 | 0.176 |
LE2P400 | 845 | 36 | 0.395 | 0.444 | 0.054 | 407 | 0.163 |
L2PS400 | 1937 | 131 | 0.892 | 1.044 | 0.146 | 673 | 0.270 |
LE2PS400 | 2551 | 143 | 1.158 | 1.313 | 0.152 | 859 | 0.344 |
Micropore size distribution of the different activated carbons obtained at 400°C and with H3PO4 to lignin mass ratio of 2.
The present results indicate that the use of co-solution of lignin with phosphoric acid in combination with an oxidative treatment at 200°C provides activated carbons with extremely high specific surface area and a large and wide microporosity. These significant differences in the porous texture seem to be the result of larger and more homogeneous interactions between lignin and the activating agent when they are co-dissolved that may increase the formation of effective phosphate esters, which are the responsible of the cross-linking reactions in the carbonaceous matrix during the oxidative stabilization process (
TPD CO profiles for the activated carbon prepared from dissolved and not-dissolved lignin are very similar, showing most of their CO release between 700 and 900°C (
Although to our best knowledge the co-solution of the carbon precursor and the activating agent has not been previously reported, some authors have already proposed the activation in two different steps.
In order to deep in the role of this oxidative treatment, further oxidative treatments were carried out at different temperatures.
Yield values of the lignin-derived activated carbons obtained at 400°C and H3PO4/lignin mass ratio of 2, treated at different temperatures.
Sample | Oxidative treatment yield (%) | Activation yield (%) | Washing yield (%) | Total yield (%) |
LE2PS400-100°C | 98.1 | 85.8 | 25.7 | 65.0 |
LE2PS400-200°C | 90.7 | 81.6 | 22.2 | 63.3 |
LE2PS400-300°C | 87.7 | 88.7 | 24.0 | 56.0 |
LE2P400-A | – | 80.1 | 27.2 | 64.9 |
LE2PS(N)400 | 88.8 | 87.3 | 26.2 | 60.9 |
N2 adsorption-desorption isotherms obtained at –196°C of the different activated carbons obtained at 400°C and H3PO4/lignin mass ratio of 2, treated at different temperatures.
Textural properties of the different lignin activated carbons obtained at 400°C and H3PO4/lignin mass ratio of 2, treated at different temperatures under air atmosphere, obtained from the N2 and CO2 isotherms.
Sample | N2 isotherm |
CO2 isotherm |
|||||
ABET (m2/g) | At (m2/g) | Vt(cm3/g) | Vp(cm3/g) | Vmes (cm3/g) | ADR(m2/g) | VDR(cm3/g) | |
LE2PS400-100°C | 877 | 61 | 0.386 | 0.459 | 0.066 | 420 | 0.168 |
LE2PS400-200°C | 2551 | 143 | 1.158 | 1.313 | 0.152 | 859 | 0.344 |
LE2PS400-300°C | 2128 | 181 | 1.036 | 1.237 | 0.175 | 570 | 0.228 |
LE2P400-A | 707 | 26 | 0.306 | 0.339 | 0.031 | 501 | 0.201 |
LE2PS(N)400 | 1263 | 111 | 0.575 | 0.706 | 0.127 | 483 | 0.194 |
The treatment in air at low temperature seems to enhance the formation of phosphate and polyphosphate bridges, which are responsible for connecting and crosslinking the lignin fragments. These phosphorus groups are inserted in the carbon matrix, separating the organic species. These groups generate an expansional process that, after removal of the acid, leaves the matrix in an expanded state, with a high pore development structure. In this case, the highest porosity development is observed when the oxidative treatment is performed at intermediate temperatures (200°C). The treatment at 100°C does not produce any enhancement in the porosity, and at 300°C some reduction begins to be noticed. These differences can be explained, according to the findings of
In addition, other experiment was carried out but treating the lignin/H3PO4 samples at 200°C under inert (N2) atmosphere (instead of air), followed by activation also in N2 at 400°C (LE2PS(N)400). The amount of N2 adsorbed volume over the entire range of relative pressure for the sample thus obtained was significantly higher than that observed for the LE2P400 sample, evidencing the presence of larger and wider microporosity. These results suggest that some reactions are being promoted at 200°C even under inert atmosphere, when sufficient time is provided. According to DSC results, it must be related to crosslinking reactions taking place during the decomposition process (
In order to provide evidences of a possible over-oxidation of the activated carbon, which avoids the porosity development, a further activation of the LE2P sample at 400°C but directly in air was carried out. The isotherm of the corresponding activated carbon, LE2P400-A, (shown in
Based on our results and taking into account the statements of all these authors, it seems that in our experimental conditions, the presence of oxygen at intermediate temperatures, where cross-linking reactions dominate, promote the porosity development due to the creation of more phosphate esters, but at higher temperatures, where the aromatization process preferentially takes place, an excess of oxygen was counter-productive, generating an over-oxidation that would inhibit the increase of the porosity.
This new methodology can be an interesting alternative for the valorization of Alcell lignin into highly porous activated carbons. The high apparent surface area obtained and the presence of surface phosphorus functional groups, which provides to the activated carbons with surface acid character (
This work analyzes the effect of co-solution of carbon precursor and activating agent on the textural and surface chemistry properties of highly nanoporous activated carbons obtained by chemical activation of Alcell lignin with phosphoric acid. The success of this methodology highlights the possibility of directly using the liquors produced in organosolv process (Alcell) to prepare activated carbons by chemical activation with phosphoric acid. Co-solutions of lignin and phosphoric acid were submitted to a two steps thermal treatment, which consisted of a first oxidative stabilization in air at 200°C, followed by a thermal treatment in N2 at 400°C, where activation of the oxidized lignin with phosphoric acid took place. A lignin-derived activated carbon with very high apparent surface area (2550 m2/g) and pore volume (1.30 cm3/g) was obtained with an initial phosphoric acid to lignin mass ratio of 2. Up to now, this is one of the highest values of apparent surface area reported not only for activated carbons prepared from lignin, but even for porous carbons prepared by chemical activation of other lignocellulosic materials with phosphoric acid. The use of lignin and phosphoric acid solutions provided more homogeneous and effective interactions between the carbon precursor and the activating agent, by the formation of phosphate esters in the lignin matrix, which seems to be a key factor in the subsequent treatments: promoting crosslinking reactions in the carbonaceous matrix during the oxidative treatment in air at 200°C and enhancing the development of a wide porosity during the followed activation thermal treatment.
All datasets generated for this study are included in the article/supplementary material.
IM and FG-M prepared the activated carbons. IM, FG-M, and JR performed the characterization of the samples and prepared the manuscript. JR, AB, JR-M, and TC developed the synthesis concept and planned the experiments. All authors discussed the result and commented on the manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.