Modification of the Textural Properties of Chitosan to Obtain Biochars for CO2-Capture Processes

Three chitosans with different morphologies have been used (commercial chitosan powder, chitosan in film form and chitosan in globular form synthesized by the freeze-dried method) for the synthesis of biochars. The pyrolytic treatment has revealed that the biochar synthesized from the chitosan formed by the freeze-dried method reaches the highest CO2-adsorption capacity (4.11 mmol/g at 0 °C and a pressure of 1 bar) due to this adsorbent is highly microporous. Moreover, this biochar is more resistant to the pyrolytic treatment in comparison to the biochars obtained from the commercial chitosan and chitosan in the form of film. CO2-adsorption studies at different temperatures have also shown that the adsorption capacity diminishes as the adsorption temperature increases, thus suggesting that the adsorption takes place by a physical process.


Introduction
In recent decades, the increase in world population and resource consumption has led to an increase in anthropogenic CO 2 emissions. The increase in the concentration of CO 2 in the atmosphere has caused serious damage from an environmental point of view and the presence of this atmospheric-CO 2 limits the release of heat from the planet, causing a phenomenon called the greenhouse effect. This phenomenon produces an increase in temperature on the earth's surface with serious environmental consequences. Among the main consequences of the greenhouse effect, the melting of glaciers, flooding of islands and coastal cities, migration of species, desertification of fertile areas, and impact on agriculture and livestock can be highlighted. In addition, it also affects human health and causes food shortages, the spread of diseases and pandemics. Considering these devastating effects of global warming, governments have become aware of how to minimize CO 2 emissions. Nowadays, dependence on fossil fuels is very high, so achieving zero-CO 2 emission processes is very complex. As an alternative to fossil fuels, renewable energies have been developing in recent decades, although they are not mature enough to provide the demands of the planet actually. Other alternatives are focused on the design of more efficient processes where CO 2 emissions can be minimized.
Currently, there is no sufficiently mature technology to mitigate CO 2 emissions into the atmosphere, so a set of complementary measures is required to reduce CO 2 emissions. One of the most advanced technologies are CO 2 capture and storage (CCS) and CO 2 capture and utilization (CCU) [1]. One of the most expensive steps in both processes is CO 2 capture, so it is necessary to search for and develop low-cost adsorbents to make these processes competitive and sustainable [2][3][4]. Among the low-cost solvents studied in CO 2 capture processes, the high adsorption values of alkaline earth oxides can be highlighted, although For the synthesis of the biochars obtained from chitosan, three approaches were carried out. The first type of chitosan used for pyrolysis was a commercial chitosan used directly. The second type of chitosan was obtained from the dissolution of the commercial chitosan in acid medium. In this synthesis, 2 g of the commercial chitosan were suspended in 200 mL of H 2 O. Then, 1 mL of acetic acid was added to dissolve chitosan, obtaining a gel. In these conditions, the -NH 2 group of chitosan was transformed into -NH 3 + , promoting its dissolution in aqueous medium.
Finally, the gel was transferred and dried at 50 • C overnight to evaporate the H 2 O of the solution, obtaining a chitosan in the form of film. The third type of chitosan followed the same procedure as the second. Commercial chitosan was suspended in H 2 O and dissolved in the same amount of acetic acid. Then, the obtained gel was freeze-dried to remove the H 2 O of the solution, obtaining chitosan with a globular form. The structures of the obtained chitosans are compiled in Figure 1, while the scheme of the chitosan solubility is included in Figure S1 of the Supplementary Materials.

Synthesis of the Adsorbents
For the synthesis of the biochars obtained from chitosan, three approaches were carried out. The first type of chitosan used for pyrolysis was a commercial chitosan used directly. The second type of chitosan was obtained from the dissolution of the commercial chitosan in acid medium. In this synthesis, 2 g of the commercial chitosan were suspended in 200 mL of H2O. Then, 1 mL of acetic acid was added to dissolve chitosan, obtaining a gel. In these conditions, the -NH2 group of chitosan was transformed into -NH3 + , promoting its dissolution in aqueous medium.
Finally, the gel was transferred and dried at 50 °C overnight to evaporate the H2O of the solution, obtaining a chitosan in the form of film. The third type of chitosan followed the same procedure as the second. Commercial chitosan was suspended in H2O and dissolved in the same amount of acetic acid. Then, the obtained gel was freeze-dried to remove the H2O of the solution, obtaining chitosan with a globular form. The structures of the obtained chitosans are compiled in Figure 1, while the scheme of the chitosan solubility is included in Figure S1 of the Supplementary Materials. In the next step, chitosans with different morphologies were pyrolized in N2-medium at different temperatures to obtain the corresponding biochars.
The synthesized biochars were labeled as Chi-X-Y, where X is the morphology of the starting chitosan (P, powder for the raw chitosan; F, film for the lamellar chitosan; G, globular for freeze-dried chitosan). The term Y represents the temperature of the pyrolytic treatment.

Characterization of the Adsorbents
The crystallinity of the biochars was studied by X-ray diffraction (XRD). The diffractograms were collected in an Empyrean PANalytical diffractometer (Malvern, United Kingdom) (Cu Kα1,2) equipped with a PIXcel 1D detector in a Bragg Brentano configuration between 2θ of 10° and 80° using a step size of 0.026° and a counting time of 24 s/step. The diffractometers were carried out with a voltage of 40 V and a current of 30 A at room temperature.
The morphology of the chitosans and the biochars obtained after the pyrolytic treatment at 900 °C was studied using a scanning electron microscope (SEM) with JEOL SM- In the next step, chitosans with different morphologies were pyrolized in N 2 -medium at different temperatures to obtain the corresponding biochars.
The synthesized biochars were labeled as Chi-X-Y, where X is the morphology of the starting chitosan (P, powder for the raw chitosan; F, film for the lamellar chitosan; G, globular for freeze-dried chitosan). The term Y represents the temperature of the pyrolytic treatment.

Characterization of the Adsorbents
The crystallinity of the biochars was studied by X-ray diffraction (XRD). The diffractograms were collected in an Empyrean PANalytical diffractometer (Malvern, UK) (Cu Kα 1,2 ) equipped with a PIXcel 1D detector in a Bragg Brentano configuration between 2θ of 10 • and 80 • using a step size of 0.026 • and a counting time of 24 s/step. The diffractometers were carried out with a voltage of 40 V and a current of 30 A at room temperature.
The morphology of the chitosans and the biochars obtained after the pyrolytic treatment at 900 • C was studied using a scanning electron microscope (SEM) with JEOL SM-6490 LV equipment (Tokyo, Japan). The samples for SEM observation were gold-sputtered to avoid charging of the surface. 13 C-NMR spectra were collected at room temperature using AVANCEIII HD 600 (Bruker AXS) equipment (Billerica, MA, USA) using a triple resonance CP-MAS probe of 3.2 mm at a spinning rate of 15 kHz. The magnetic field is 14.1 T, corresponding to a 13 C resonance frequency of 150.91 MHz. The spectra were recorded with 7-s delay with 1 H decoupling ( 13 C Hpdec with tppm15 decoupling sequence for C) and summing up 5000 scans. The 13 C chemical shifts are referenced to adamantane.
Raman experiments were performed in a micro-Raman JASCO NRS-5100 spectrometer (Easton, PA, USA). This equipment combines a dispersive Raman spectrometer and a confocal microscope, equipped with DSF (Dual Spatial Filter) to diminish optical aberrations and to improve the resolution up to 0.4 cm −1 . The calibration of the different laser lines was made with respect to silicon, while the detection was achieved by means of a high-resolution CCD (Charge Coupled Device). All the experiments were carried out using a Nd:YAG laser with a wavelength of 532 nm, a power of 2.4-4.6 mW, an acquisition time of 10 s with 10 accumulations and an objective of 100X.
Thermal analysis was carried out on a TG-DTG thermobalance (Mettler Toledo) (Columbus, OH, USA) with a continuous heating rate of 5 • C/min in N 2 -flow 50 mL/min from 30 to 900 • C.
The chemical composition of the biochars on the surface was performed by X-ray photoelectron spectra. The experiments were carried out using a Physical Electronic PHI 5700 spectrometer (Physical Electronics) (Chanhassen, MN, USA), which is equipped with an Electronics 80-365B multichannel hemispherical electron analyzer and a Mg Kα X-ray excitation source (300 W, 15 kV, hv = 1253.6 eV). High-resolution spectra were recorded by a concentric hemispherical analyzer in a constant energy mode of 29.35 eV, using an analysis area with a diameter of 720 µm. The pressure in the analysis chamber was kept below 5 × 10 −6 Pa. Binding energies were determined to an accuracy of ±0.1 eV, using the adventitious carbon C 1s signal, located at 284.8 eV, as reference. A PHI ACCESS ESCA-F V6 software was used for data acquisition and analysis. A Shirley-type background was subtracted from the signals. The recorded spectra were always analyzed with Gauss-Lorentz curves.
The N-content of the biochars obtained after the pyrolytic treatment of chitosan with different morphologies was determined by elemental chemical analysis using a CHNS 932 analyzer supplied by LECO (Geleen, The Netherlands) through the combustion of the samples at 1100 • C in pure O 2 .
The textural properties of the biochars were determined from CO 2 adsorption isotherms at 0 • C using a Micromeritics 2420 apparatus (Norcross, GA, USA). Prior the adsorption experiments, the samples were outgased at 120 • C overnight. After the adsorption isotherms, both surface area and micropore volume were determined using the Dubinin-Astakhov equation [29].

CO 2 Adsorption Isotherms
To evaluate the potential of the synthesized biochars in CO 2 adsorption processes, the samples were outgased at 120 • C overnight, and then the adsorbents were studied at 0, 25 and 45 • C, between 0 and 760 mm of Hg in a Micromeritics 2420 equipment (Norcross, GA, USA).

Adsorption Models
The empirical Toth equation [30] was employed to describe the adsorption equilibrium. The advantage of this model is that, at low pressures, the Toth equation (Equation (1)) reduces to Henrýs law. The Toth equation can be written as follows: The limiting adsorbed concentration q m , the affinity parameter b and the heterogeneity factor t were varied with the isotherm temperature according to the empiric Equations (2)-(4), respectively.
To estimate the accuracy of each fit, the Average Relative Error (ARE) (Equations (5)), was used: where N T is the total number of the data points, and q i,est and q i,exp are the estimated and experimental amounts of CO 2 adsorbed, respectively.

Characterization of the Adsorbents
The crystallinity of the biochars synthesized from chitosan was studied by XRD ( Figure 2). With the exception of the sample synthesized from the commercial chitosan (Chi-P-900), all diffractograms display two broad diffraction peaks located at about 2θ of 25 and 44 • , which are assigned to the presence of a graphitic carbon with low crystallinity (PDF N: 41-1487) [31]. In the case of the Chi-P-900, narrow but weak diffraction peaks can also be observed, which are ascribed to carbonates and/or hydrogen carbonates (PDF N: 00-025-0626) of K or Na (PDF N: 01-074-1203). These alkaline species must be ascribed to the purification treatment of the crustacean shells as well as to the deacetylation process of chitin to form chitosan [32]. tions (2), (3) and (4), respectively.
To estimate the accuracy of each fit, the Average Relative Error (ARE) (Equations (5)), was used: where NT is the total number of the data points, and qi,est and qi,exp are the estimated and experimental amounts of CO2 adsorbed, respectively.

Characterization of the Adsorbents
The crystallinity of the biochars synthesized from chitosan was studied by XRD (Figure 2). With the exception of the sample synthesized from the commercial chitosan (Chi-P-900), all diffractograms display two broad diffraction peaks located at about 2θ of 25 and 44°, which are assigned to the presence of a graphitic carbon with low crystallinity (PDF N: 41-1487) [31]. In the case of the Chi-P-900, narrow but weak diffraction peaks can also be observed, which are ascribed to carbonates and/or hydrogen carbonates (PDF N: 00-025-0626) of K or Na (PDF N: 01-074-1203). These alkaline species must be ascribed to the purification treatment of the crustacean shells as well as to the deacetylation process of chitin to form chitosan [32]. The morphology of the biochars obtained from the pyrolysis of chitosan with different treatments was determined by SEM ( Figure 3). From these images, how all starting chitosans, prior to the pyrolysis, display different morphologies can be observed ( Figure 3A-C).
In the case of commercial chitosan without treatment (Chi-P), the image with lower magnification ( Figure S2, Supplementary Materials) shows an anisotropic structure where the dimensions with the width and length of the particles range between 50 and 150 μm. The increase in the magnification of the Chi-P sample ( Figure 3A) reveals that these particles are formed by a set of layers stacked on each other, because of the electrostatic The morphology of the biochars obtained from the pyrolysis of chitosan with different treatments was determined by SEM ( Figure 3). From these images, how all starting chitosans, prior to the pyrolysis, display different morphologies can be observed ( Figure 3A-C).
In the case of commercial chitosan without treatment (Chi-P), the image with lower magnification ( Figure S2, Supplementary Materials) shows an anisotropic structure where the dimensions with the width and length of the particles range between 50 and 150 µm. The increase in the magnification of the Chi-P sample ( Figure 3A) reveals that these particles are formed by a set of layers stacked on each other, because of the electrostatic interactions with adjacent sheets. The treatment of chitosan in an acidic medium to solubilize it and its deposition in a crystallizer until the solution evaporates results in a layered chitosan without imperfections ( Figure 3B). When this dissolved chitosan is subjected to a freezedrying process instead of being dried in a crystallized process, how the morphology clearly changes can be seen ( Figure 3C). This image shows how the globular structure displays many cavities due to the loss of H 2 O in the lyophilization process. causing a partial shrinkage and collapse. Thus, the pyrolyzed commercial chitosan shows a partial shrinkage of the particles, whose size is below 100 μm ( Figures 3D and S2, Supplementary Materials). In the same way, the structure of the chitosan in the form of a film also collapses after the pyrolytic treatment at 900 °C ( Figure 3E), obtaining lamellar but highly fractured structures. In the case of chitosan obtained by the freeze-drying method, pyrolysis at 900 °C also causes a collapse of the structure ( Figure 3F), diminishing the dimensions of the cavities. The biochars obtained from the pyrolysis of chitosan were also studied by Raman spectroscopy (Figure 4). In the spectra of the three synthesized materials, two defined bands can be observed, which are located at 1340 and 1590 cm −1 . These bands are denoted in the literature as D (disorder) band and G (graphite) band, respectively [17,33]. These authors pointed out that the D band is related to the A1g vibration mode, which is like an in-plane breathing vibration type; however, this vibration mode is inactive in a large single crystal-like highly oriented pyrolytic graphite [33]. Regarding the G band, this signal is ascribed to the Raman active E2g in-plane vibration mode [33].
The biochars obtained from chitosan pyrolysis were also studied by Raman spectroscopy ( Figure 4). In the three synthesized materials, two defined bands can be observed, which are located at 1340 and 1590 cm −1 . These bands are denoted in the literature as the D band (disorder) and G band (graphite), respectively [17,33]. These authors pointed out that the D band is related to the A1g vibrational mode, which is like an in-plane breathing type of vibration; however, this vibrational mode is inactive in a large single crystal such as highly oriented pyrolytic graphite [33]. With respect to the G band, this signal is attributed to the Raman active E2g in-plane vibration mode [33]. After pyrolytic treatment of each sample at 900 • C, a change in morphology can be observed in comparison to their respective starting materials due to the thermal treatment causing a partial shrinkage and collapse. Thus, the pyrolyzed commercial chitosan shows a partial shrinkage of the particles, whose size is below 100 µm ( Figure 3D and Figure S2, Supplementary Materials). In the same way, the structure of the chitosan in the form of a film also collapses after the pyrolytic treatment at 900 • C ( Figure 3E), obtaining lamellar but highly fractured structures. In the case of chitosan obtained by the freeze-drying method, pyrolysis at 900 • C also causes a collapse of the structure ( Figure 3F), diminishing the dimensions of the cavities.
The biochars obtained from the pyrolysis of chitosan were also studied by Raman spectroscopy (Figure 4). In the spectra of the three synthesized materials, two defined bands can be observed, which are located at 1340 and 1590 cm −1 . These bands are denoted in the literature as D (disorder) band and G (graphite) band, respectively [17,33]. These authors pointed out that the D band is related to the A 1g vibration mode, which is like an in-plane breathing vibration type; however, this vibration mode is inactive in a large single crystal-like highly oriented pyrolytic graphite [33]. Regarding the G band, this signal is ascribed to the Raman active E 2g in-plane vibration mode [33].
The biochars obtained from chitosan pyrolysis were also studied by Raman spectroscopy ( Figure 4). In the three synthesized materials, two defined bands can be observed, which are located at 1340 and 1590 cm −1 . These bands are denoted in the literature as the D band (disorder) and G band (graphite), respectively [17,33]. These authors pointed out that the D band is related to the A 1g vibrational mode, which is like an in-plane breathing type of vibration; however, this vibrational mode is inactive in a large single crystal such as highly oriented pyrolytic graphite [33]. With respect to the G band, this signal is attributed to the Raman active E 2g in-plane vibration mode [33].
Considering these premises, it is possible to know the type of carbonaceous material obtained after the pyrolytic treatment. Thus, it has been reported in the literature that an increase in temperature in the pyrolytic treatment causes a progressive decrease in the D band [17,34]. As this band is inactive considering that a graphitic structure is formed, it is expected that the increase in temperature leads to graphitic materials although these materials must be amorphous, or the crystallinity must be very low, as suggested by XRD ( Figure 2).  Considering these premises, it is possible to know the type of carbonaceous material obtained after the pyrolytic treatment. Thus, it has been reported in the literature that an increase in temperature in the pyrolytic treatment causes a progressive decrease in the D band [17,34]. As this band is inactive considering that a graphitic structure is formed, it is expected that the increase in temperature leads to graphitic materials although these materials must be amorphous, or the crystallinity must be very low, as suggested by XRD ( Figure 2).
From the Raman data, reported in Figure 4, it can be observed how the maximum values of the D and G bands are similar. These intensities agree to those obtained for other biochars obtained from cellulose at 900 °C [17] so the type of biopolymer (polysaccharide) and its morphology do not seem to play a relevant role in the formation of the graphitic adsorbent, but rather the temperature is the key parameter for the formation of the adsorbent.
The synthesized biochars were also characterized by 13 C NMR ( Figure 5). Three welldefined regions can be observed, mainly in the case of biochar Chi-F-900. The main signal, whose maximum is located around 125 ppm, is assigned to the presence of sp 2 hybridization, thus confirming the formation of a graphitic structure [17,35]. In the same way, two less intense signals can also be observed, which are located between 0-50 and 190-240 ppm [17]. These signals are assigned to the presence of aliphatic and carbonyl groups, respectively. From the Raman data, reported in Figure 4, it can be observed how the maximum values of the D and G bands are similar. These intensities agree to those obtained for other biochars obtained from cellulose at 900 • C [17] so the type of biopolymer (polysaccharide) and its morphology do not seem to play a relevant role in the formation of the graphitic adsorbent, but rather the temperature is the key parameter for the formation of the adsorbent.
The synthesized biochars were also characterized by 13 C NMR ( Figure 5). Three well-defined regions can be observed, mainly in the case of biochar Chi-F-900. The main signal, whose maximum is located around 125 ppm, is assigned to the presence of sp 2 hybridization, thus confirming the formation of a graphitic structure [17,35]. In the same way, two less intense signals can also be observed, which are located between 0-50 and 190-240 ppm [17]. These signals are assigned to the presence of aliphatic and carbonyl groups, respectively. The analysis of the textural properties was determined from CO2 adsorption-desorption isotherms at 0 °C. Many studies have analyzed the textural properties using N2 as a molecular probe. However, this molecule is not suitable for poorly accessible pores since it is difficult to reach equilibrium conditions.  The analysis of the textural properties was determined from CO 2 adsorption-desorption isotherms at 0 • C. Many studies have analyzed the textural properties using N 2 as a molecular probe. However, this molecule is not suitable for poorly accessible pores since it is difficult to reach equilibrium conditions. Thus, the CO 2 adsorption isotherms at 0 • C ( Figure 6) shows how the pyrolysis of commercial chitosan (Chi-P) increases the CO 2 adsorption when the temperature of the pyrolytic treatment increases, obtaining the maximum value when the treatment takes place at 700 • C. At higher temperatures, it is striking how the adsorption capacity becomes practically negligible, probably due to the collapse of the carbonaceous structure, giving rise to a material with a very low microporosity. In the case of biochars derived from the pyrolysis of chitosan films (Chi-F), the trend of CO 2 adsorption is like commercial chitosan, i.e., CO 2 adsorption increases with the temperature of pyrolysis, obtaining the maximum adsorption value at 700 • C, and at higher temperatures, the adsorption capacity decreases, although to a lesser extent than for commercial chitosan. Finally, the chitosan treated by the freeze-drying method (Chi-G) shows an increase in CO 2 adsorption as the pyrolytic treatment increases. The analysis of the textural properties was determined from CO2 adsorption-desorption isotherms at 0 °C. Many studies have analyzed the textural properties using N2 as a molecular probe. However, this molecule is not suitable for poorly accessible pores since it is difficult to reach equilibrium conditions. Thus, the CO2 adsorption isotherms at 0 °C ( Figure 6) shows how the pyrolysis of commercial chitosan (Chi-P) increases the CO2 adsorption when the temperature of the pyrolytic treatment increases, obtaining the maximum value when the treatment takes place at 700 °C. At higher temperatures, it is striking how the adsorption capacity becomes practically negligible, probably due to the collapse of the carbonaceous structure, giving rise to a material with a very low microporosity. In the case of biochars derived from the pyrolysis of chitosan films (Chi-F), the trend of CO2 adsorption is like commercial chitosan, i.e., CO2 adsorption increases with the temperature of pyrolysis, obtaining the maximum adsorption value at 700 °C, and at higher temperatures, the adsorption capacity decreases, although to a lesser extent than for commercial chitosan. Finally, the chitosan treated by the freeze-drying method (Chi-G) shows an increase in CO2 adsorption as the pyrolytic treatment increases.
Therefore, these data evidence that, although all the materials display a similar chemical composition (amorphous graphite), the adsorption capacity of the different biochars depends on both the staring chitosan and the pyrolytic temperature employed. Therefore, these data evidence that, although all the materials display a similar chemical composition (amorphous graphite), the adsorption capacity of the different biochars depends on both the staring chitosan and the pyrolytic temperature employed.
The determination of the microtextural properties was determined from the CO 2 adsorption isotherms at 0 • C, using the Dubinin-Astakhov equation (Table 1) [29]. From these data, it can be observed that the higher microporosity is attained for those biochars synthesized from chitosan modified by the freeze-drying method, reaching a maximum micropore volume of 0.263 cm 3 /g and a surface area of 657 m 2 /g for the sample pyrolyzed at 900 • C (Chi-G-900). Commercial chitosan (Chi-P) displays higher porosity than chitosan in the form of film (Chi-F). In both cases, the maximum microporosity is obtained at 700 • C (0.158 cm 3 /g for Chi-P-700 and 0.148 cm 3 /g for Chi-F-700). At higher temperatures, the structure of commercial and film chitosan collapse, although this collapse is more pronounced in the case of commercial chitosan and is related to the loss of microporosity in the resultant biochar. In any case, the textural properties of the solids obtained are higher than the other solids reported in the literature [36][37][38][39]. Table 1. Textural properties of biochars determined from CO 2 adsorption isotherms, as determined using the Dubinin-Astakhov equation [29].

Sample
Micropore Volume (cm 3  Considering that the textural properties of the pyrolyzed chitosan depend on the starting morphology (Figures 3 and 6), the next characterization study was the analysis of the thermogravimetric loss using an inert gas such as N 2 (Figure 7). Figure 7 shows how the profiles for the synthesis of Chi-P and Chi-F adsorbents are very similar between them. In both cases, the physisorbed-H 2 O is removed until 250 • C, although the content is different since the Chi-G-900 sample loses about 13% of the H 2 O, while commercial chitosan (Chi-P-900 sample) hardly losses 6% of its starting weight. From 250 • C, a pronounced loss in the mass of chitosan because of the pyrolysis takes place until 350 • C. Then, at higher temperatures, the mass loss is more progressive, remaining a mass of 28% for Chi-P and 24% for Chi-G at 900 • C, respectively. In the case of Chi-F, the TG analysis shows a profile that differs from those shown for the Chi-P and Chi-G samples. Thus, the Chi-F sample seems to be stable up to 150 • C. Then, the physisorbed-H 2 O is released between 150 and 290 • C. From this temperature, the TG analysis shows a drastic mass loss between 320 and 400 • C. Then, the Chi-F sample hardly losses mass at higher temperatures. It should be noted that the adsorbents synthesized from chitosan films (Chi-F) are those that have a greater mass loss, as shown in the TG data, where the remaining mass at 900 • C is less than 10% compared to its initial weight. From these data, it can be inferred that, despite all the materials having the same chemical composition, the morphology seems to have an important role in the pyrolytic treatment and in the textural properties obtained where the biochar synthesized from chitosan in the form of film is the material that loses the most mass, breaking down its structure, as was observed from Figure 3.  The surface chemical composition, as well as the chemical states of the elements on the surface, were determined by XPS ( Table 2 and Figure 8). The analysis of the biochars pyrolyzed at 900 °C mainly shows the presence of C, N and O. The analysis of the C 1s core level spectra of the biochars ( Figure 8A) displays four contributions. In all cases, a main band located at 284.8 eV can be observed, which is assigned to the adventitious carbon or C-C bonds. In addition, the second contribution located about 286.0 eV is attributed to the C-N or C-O groups, thus confirming that O and N-species are present in the carbonaceous structure in the form of amorphous graphite. At higher binding energies, approx- The surface chemical composition, as well as the chemical states of the elements on the surface, were determined by XPS ( Table 2 and Figure 8). The analysis of the biochars pyrolyzed at 900 • C mainly shows the presence of C, N and O. The analysis of the C 1s core level spectra of the biochars ( Figure 8A) displays four contributions. In all cases, a main band located at 284.8 eV can be observed, which is assigned to the adventitious carbon or C-C bonds. In addition, the second contribution located about 286.0 eV is attributed to the C-N or C-O groups, thus confirming that O and N-species are present in the carbonaceous structure in the form of amorphous graphite. At higher binding energies, approximately 287.0 eV, other small contributions assigned to C=O can be observed, while the band located at higher binding energy in this region (289.0 eV) is attributed to the presence of the O-C=O groups [40,41].  The surface chemical composition, as well as the chemical states of the elements on the surface, were determined by XPS (Table 2 and Figure 8). The analysis of the biochars pyrolyzed at 900 °C mainly shows the presence of C, N and O. The analysis of the C 1s core level spectra of the biochars ( Figure 8A) displays four contributions. In all cases, a main band located at 284.8 eV can be observed, which is assigned to the adventitious carbon or C-C bonds. In addition, the second contribution located about 286.0 eV is attributed to the C-N or C-O groups, thus confirming that O and N-species are present in the carbonaceous structure in the form of amorphous graphite. At higher binding energies, approximately 287.0 eV, other small contributions assigned to C=O can be observed, while the band located at higher binding energy in this region (289.0 eV) is attributed to the presence of the O-C=O groups [40,41].
In the analysis of the O 1s region ( Figure 8B), two regions can be observed in all cases. The contribution located at lower binding energy, about 531 eV, is assigned to the presence of remaining -C=O groups, while the contribution located at higher binding energy value (533 eV) is assigned to -C-OH groups.
Regarding the N 1s core level spectra of the biochars ( Figure 8C), all spectra display two contributions. The binding energy located at a lower value (398.5 eV) is assigned to pyridinic-N, while the contribution at around 400.6-400.9 eV is assigned to pyrrolic-N [42], confirming that N-species are embedded within the graphitic structure. The surface atomic concentration of the biochar was also studied ( Table 2). As was expected, the main element in all cases is carbon with an atomic concentration between 80%-90%. The oxygen content is between 6%-12%, while the nitrogen content is between In the analysis of the O 1s region ( Figure 8B), two regions can be observed in all cases. The contribution located at lower binding energy, about 531 eV, is assigned to the presence of remaining -C=O groups, while the contribution located at higher binding energy value (533 eV) is assigned to -C-OH groups. Regarding the N 1s core level spectra of the biochars ( Figure 8C), all spectra display two contributions. The binding energy located at a lower value (398.5 eV) is assigned to pyridinic-N, while the contribution at around 400.6-400.9 eV is assigned to pyrrolic-N [42], confirming that N-species are embedded within the graphitic structure.
The surface atomic concentration of the biochar was also studied ( Table 2). As was expected, the main element in all cases is carbon with an atomic concentration between 80-90%. The oxygen content is between 6-12%, while the nitrogen content is between 3-5%. These values are below the values obtained by CHN where the N-content is 6.4% for Chi-P-900, 7.5% for Chi-F-900 and 5.4% for Chi-G-900, respectively, so a portion of the N-species must not be exposed on the surface of the adsorbents. From XPS data, it can be concluded that the pyrolytic treatment does not completely remove oxygen and nitrogen species. In fact, it seems that nitrogen is incorporated within the structure of amorphous graphite.

Adsorption Studies
Once the biochars synthesized from a pyrolytic treatment of chitosans with different textural properties were characterized, these materials were tested as potential adsorbents in CO 2 capture processes.
In the first study, biochars synthesized at different temperatures were studied (Figure 9). The biochars synthesized from commercial chitosan (Chi-P) ( Figure 9A) show how CO 2 adsorption is very poor for the adsorbent synthesized at 500 • C (Chi-P-500) since the adsorption capacity is 1.37 mmol/g at a pressure of 1 bar and 25 • C. The increase in temperature in the pyrolytic treatment causes an improvement in microporosity, as indicated by studies of textural properties (Table 1). This means that the biochar can retain a greater amount of CO 2 in its microporous structure, achieving a capture of CO 2 of 2.01 mmol/g at 25 • C and a pressure of 1 bar for Chi-P-700 sample. An increase in the pyrolysis temperature to 900 • C promotes the microporosity of biochars by physical activation. However, the textural properties (Table 1) and the adsorption capacity decrease drastically, reaching a CO 2 adsorption capacity of only 0.97 mmol/g at 25 • C and 1 bar of pressure. This decrease must be ascribed to the fact that the increase in porosity at 900 • C must cause a partial collapse of the porous structure, thus impoverishing its textural properties.
Polymers 2022, 14, x FOR PEER REVIEW 12 of 17 since, unlike adsorbents obtained from commercial chitosan or in the form of film, the treatment at the highest temperature improves the adsorption capacity, reaching the highest CO2 capture with a value of 3.64 mmol/g. These data suggest that the chitosan synthesized from the dissolution and its freeze-drying leads to a material with higher porosity and higher thermal stability, which is more resistant to a collapse in its structure at high temperature. However, it should be noted that these values are above those obtained by biochar from agricultural residues, where a CO2 adsorption of 2.63 mmol/g is obtained for a biochar obtained at 900 °C under similar synthetic and analytic conditions [43]. From the CO2 adsorption isotherms obtained at different temperatures of pyrolysis (Figure 9), it can be concluded that the CO2 adsorption capacity is directly related to the microporosity of the biochar, as shown in the linearity between micropore volume vs. CO2 adsorption capacity (Figure 10). When chitosan is dissolved in an acid medium, it is deposited to obtain a film that is finally pyrolyzed (Figure 9B), the CO 2 adsorption capacity worsens compared to biochar obtained from commercial chitosan ( Figure 9A), probably due to the formation of the film takes place at a stacking of chitosan sheets that can limit the physical activation in the pyrolytic treatment. Thus, the sample pyrolyzed at 500 • C (Chi-F-500) reaches a CO 2 adsorption capacity of 1.07 mmol/g at 25 • C and 1 bar of pressure. The increased pyrolytic treatment also improves the porosity and 1.82 mmol/g at 25 • C and 1 bar of pressure. As was observed for commercial chitosan pyrolyzed at 900 • C, the pyrolysis at the highest temperature worsens the adsorption capacity (1.31 mmol/g), although this decrease is less pronounced than that observed in commercial chitosan, so that chitosan in the form of film seems to be more stable and resistant to pyrolytic treatment, as was observed from the analysis of the textural properties ( Table 2).
The study of the CO 2 adsorption capacity for the adsorbents synthesized from modified chitosan by freeze-drying ( Figure 9C) shows the highest adsorption in the range of the studied pyrolysis temperature. These values must be ascribed to the prior treatment of the chitosan since its dissolution, and then, the freeze-drying method favors the removal of H 2 O, generating a chitosan structure with a high porosity, as was observed in Figure 3C. Thus, the pyrolytic treatment at 500 • C achieves an adsorption capacity of 2.18 mmol/g. The adsorption capacity improves at 700 • C, attaining a value of 2.53 mmol/g. However, the most striking result is obtained when this chitosan is pyrolyzed at 900 • C since, unlike adsorbents obtained from commercial chitosan or in the form of film, the treatment at the highest temperature improves the adsorption capacity, reaching the highest CO 2 capture with a value of 3.64 mmol/g. These data suggest that the chitosan synthesized from the dissolution and its freeze-drying leads to a material with higher porosity and higher thermal stability, which is more resistant to a collapse in its structure at high temperature. However, it should be noted that these values are above those obtained by biochar from agricultural residues, where a CO 2 adsorption of 2.63 mmol/g is obtained for a biochar obtained at 900 • C under similar synthetic and analytic conditions [43].
From the CO 2 adsorption isotherms obtained at different temperatures of pyrolysis (Figure 9), it can be concluded that the CO 2 adsorption capacity is directly related to the microporosity of the biochar, as shown in the linearity between micropore volume vs. CO 2 adsorption capacity ( Figure 10). From the CO2 adsorption isotherms obtained at different temperatures of pyrolysis (Figure 9), it can be concluded that the CO2 adsorption capacity is directly related to the microporosity of the biochar, as shown in the linearity between micropore volume vs. CO2 adsorption capacity ( Figure 10). Figure 10. Representation of the CO2 adsorption capacity at 1 bar and 25 °C vs. micropore volume, determined from the CO2 adsorption isotherm at 0 °C. Figure 10. Representation of the CO 2 adsorption capacity at 1 bar and 25 • C vs. micropore volume, determined from the CO 2 adsorption isotherm at 0 • C.
As the Chi-G-900 adsorbent reached the highest adsorption capacity, those adsorbents synthesized by the pyrolytic treatment at 900 • C were fitted to the Toth model [30] (Table 3). be observed that the higher q mi,0 is obtained for the Chi-G-900 sample, which reaches a value of 18.23 mol/kg. These values are well above those obtained for the other biochars synthesized from chitosans with other textural properties due to the greater microporosity of this adsorbent. The greater microporosity of the Chi-G-900 sample supposes a stronger interaction and affinity between the adsorbent and the CO 2 molecules, as indicated by the b 0,i parameter as well as a higher adsorption heat value, as shown in the Q value. However, the heterogeneity of all adsorbents is very similar among them. These values are far from the unity where all the centers would be equally accessible and available. The t 0,i values are in the range of 0.25-0.28. This implies that there must be active sites that are more likely to capture CO 2 . These sites must be the narrowest pores where the CO 2 molecules can be captured, or they could also be the pyrrolic and pyridinic groups where there could be a chemical interaction with the CO 2 molecules.
Once the CO 2 adsorption capacity of the biochars at different pyrolysis temperatures have been studied, the next step has is to study the adsorption capacity at different temperatures. For this purpose, adsorbents pyrolyzed at the highest temperature (900 • C) were chosen to carry out this study because the Chi-G-900 sample reached the highest CO 2 adsorption values ( Figure 9). In all cases, it can be observed how an increase in the adsorption temperature worsens the CO 2 capture capacity [44]. In this sense, it has been reported in the literature that those adsorbents where the adsorption of CO 2 takes place physically, i.e., the CO 2 molecules, are retained in highly microporous materials due to the high quadrupole moment of the CO 2 molecules [45]. Thus, the adsorption study at 45 • C, the temperature at which flue gases are frequently emitted, shows a clear decrease in the CO 2 adsorption capacity. In the case of commercial chitosan pyrolyzed at 900 • C ( Figure 11A), adsorption at 45 • C only attains a CO 2 adsorption of 0.74 mmol/g at 1 bar of pressure. This value is also low for chitosan in the form of film and subsequently pyrolyzed at 900 • C ( Figure 11B), reaching a CO 2 adsorption of 0.99 mmol/a at 45 • C and 1 bar of pressure. Finally, the chitosan treated with the freeze-drying method and pyrolyzed at 900 • C ( Figure 11C) achieves the highest CO 2 adsorption at this temperature with an adsorption capacity of 1.94 mmol/g at 1 bar of pressure. Once the adsorption isotherms at different adsorption temperatures were studied, these isotherms were fitted according to the Toth equation [30]. The low ARE values suggest that the isotherms were well-adjusted to the Toth adsorption model.
The adsorption parameters reported in Table 4 show how the adsorbent with the highest CO2 uptake values throughout the studied temperature range (Chi-G-900) is also the adsorbent with the higher affinity for the adsorbate, as indicated by the b0,I values. This stronger affinity is ascribed to the narrower pore diameter, which allows for the retention of higher CO2-levels (Table 1). However, the other adsorbents synthesized from chitosan with different textural properties (Chi-P-900 and Chi-F-900) display lower affinity values due to their poorer textural properties. However, it should also be noted that the affinity parameter values decrease when the adsorption temperature increases. From these data, it can Once the adsorption isotherms at different adsorption temperatures were studied, these isotherms were fitted according to the Toth equation [30]. The low ARE values suggest that the isotherms were well-adjusted to the Toth adsorption model.
The adsorption parameters reported in Table 4 show how the adsorbent with the highest CO 2 uptake values throughout the studied temperature range (Chi-G-900) is also the adsorbent with the higher affinity for the adsorbate, as indicated by the b 0,I values. This stronger affinity is ascribed to the narrower pore diameter, which allows for the retention of higher CO 2 -levels (Table 1). However, the other adsorbents synthesized from chitosan with different textural properties (Chi-P-900 and Chi-F-900) display lower affinity values due to their poorer textural properties. However, it should also be noted that the affinity parameter values decrease when the adsorption temperature increases. From these data, it can be inferred that the adsorption must take place from a physical process, since this type of adsorption is favored at a low temperature and adsorbent with a narrow pore [13,24,25]. Table 4. Toth model parameters for the CO 2 adsorption of the biochars obtained after the pyrolytic treatment of chitosans with different textural properties at 900 • C (CO 2 adsorption: 0-45 • C). The presence of N-species in the form of pyrrolic and pyridinic groups could promote the chemical adsorption of CO 2 molecules. With this chemical adsorption, increasing the adsorption temperature should improve CO 2 uptake values; however, these results have not been observed in the present study. Thus, it is expected that the CO 2 adsorption using biochars obtained from chitosans with different textural properties as adsorbents must be through physical adsorption, as suggested by the narrow pore diameter (Table 1), while the chemical adsorption of CO 2 must be negligible.

Conclusions
A commercial chitosan has been modified to give rise to films and globular structures. The pyrolysis of these materials between 500 and 900 • C has generated biochars with different textural properties between them. Thus, it can be observed that the commercial chitosan and the chitosan in the form of a film collapse their structure as the pyrolysis temperature increases. However, chitosan modified by the freeze-drying method seems to be more resistant to pyrolytic treatment, improving its textural properties, even at 900 • C. Under these conditions, the biochar obtained is an amorphous graphite with N-pyridinic and N-pyrrolic groups coming from the -NH 2 group at the C2 position of the chitosan.
The study of the CO 2 -adsorption capacity shows how the Chi-G-900 sample is the best adsorbent due the high and narrow microporosity, which allows it to host a high proportion of CO 2 molecules in its framework, obtaining the highest value of 4.11 mmol/g at 0 • C and 1 bar of pressure. In the case of the biochars obtained from the commercial chitosan and the chitosan modified by the freeze-drying method, the CO 2 -adsorption capacity worsens when the pyrolytic treatment takes place at 900 • C, confirming that the framework of both biochars must collapse under this temperature. Finally, the study of the CO 2 -adsorption capacity for Chi-P-900, Chi-F-900 and Chi-G-900 shows how the increase in the adsorption temperature causes a decrease in the CO 2 -adsorption values, suggesting that, despite the presence of N-species, the CO 2 -adsorption must take place from physical interactions.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/polym14235240/s1, Figure S1: Scheme of the solubilization of chitosan under acid pH, Figure