3.1. Thermal Analyses
Thermogravimetric analyses (TGA)—FTIR spectra demonstrated that there were many oxygen-containing (carboxylic, carbonyl, acyl, alkoxyl, ketones, esters and ethers, et al.), alkyl and aromatic functional groups in kraft lignin [
18]. These functional groups in kraft lignin were broken down or cracked with increasing of temperature during the thermal process.
Figure 1 shows TG and DTG curves of four iron salts promoted lignin precursors.
Figure 1a shows that solid residues as percentage of starting weights of iron-promoted lignin precursors after decomposition were 52.6%, 50.9%, 41.6% and 36.6% for iron salts FeCl
3, FeCl
2, FeSO
4 and Fe (NO
3)
3, respectively, which indicates that the catalytic graphitization activity of four iron catalysts on kraft lignin materials was in ascending order FeCl
3 < FeCl
2 < FeSO
4 < Fe (NO
3)
3.
The thermal decomposition process of iron-promoted lignin precursors can be divided into four stages (
Figure 1b). The first stage is characterized by a mass loss because of the evaporation of surface moisture and dehydration of combined moistures from iron-promoted lignin precursors. The second stage corresponded to the de-polymerization of kraft lignin and decomposition of iron species. During the de-polymerization process, the oxygen-containing groups in alkyl side chains of lignin basic units were catalytically decomposed. The mass decreased rapidly due to the breakage of large number of ether and C–C bonds connected on phenyl propane units, which generated small-molecule gases and macromolecular condensable volatiles. The maximum rates of these weight losses occurred at the temperatures of 323 °C, 300 °C, 375 °C and 237 °C for FeCl
2-, FeCl
3-, FeSO
4- and Fe (NO
3)
3-promoted lignin precursors, respectively. The third mass loss (
Figure 1b), corresponding to decomposition of kraft lignin char yielded after the completion of the second stage, indicated that the functional groups of kraft lignin continued to decompose as the temperature increased, which led to the aromatization of kraft lignin char matrix. The fourth mass loss stage was characterized with a further carbonization and graphitization process of chars in a wider temperature range up to 1000 °C, where the mass loss was mainly because of the decomposition of phenols, ether and C–H groups of kraft lignin chars, which released out CO and H
2 as main gases.
Temperature-programmed decomposition (TPD) analyses—Lignin is an across-linked type macromolecule and mostly formed via free radical coupling of three basic hydroxyphenylpropanoid monolignols: coumaryl, coniferyl and sinapyl alcohols [
19]. With the aid of catalysts, oxidative agents, or thermal treatment, lignin will break down at positions 1, 4, 5 and β [
18], accompanying the release of incondensable gases like H
2, CO
2, CO, CH
4, C
2H
6, C
2H
4, H
2S, trace amounts of gaseous organics (CH
3OH, C
6H
6OH) and water vapor or volatile products. In addition, gaseous species are produced from the thermal decomposition of iron salts such as HCl from Fe-Cl
2 and Fe-Cl
3, NO
2 and O
2 from Fe-N and SO
2 and SO
3 from Fe-S.
Figure 2 shows the evolution curves of release intensities of various gaseous species as a function of heating temperature recorded during the temperature-programmed decomposition of raw kraft lignin and iron-promoted lignin precursors. For raw kraft lignin, the releasing of H
2 began at 520 °C and reached its maximum value at 726 °C. For Fe-N, the releasing of H
2 started at 466 °C and reached its maximum value at 709 °C. H
2 released from Fe-S at 466 °C and reached its maximum value at 790 °C. Fe-Cl
3 samples began its H
2 release at 493 °C with a maximum value reached at 810 °C, while Fe-Cl
2 samples began its H
2 release at 537 °C with a maximum value reached at 840 °C. More hydrogen was supposed to be produced from the samples with iron catalysts and evolution temperature were supposed to be lower than raw kraft lignin samples. However, the evolution peaks were weaker and flatter and the hydrogen peaks for FeSO
4, FeCl
3 and FeCl
2 shifted to the higher temperatures. This was caused by the reverse water-gas shift reaction (RWGSR) (CO
2(g) + H
2(g) = CO(g) + H
2O(g)). Part of hydrogen from the thermal cracking reaction was consumed through RWGSR since more CO
2 was generated from the samples with iron catalysts and iron is very active for the RWGSR under the process conditions. From H
2 evolution trends, the catalytic activities of iron catalysts from different precursors were with the following descending order: Fe-N > Fe-S > Fe-Cl
3 > Fe-Cl
2. Methane was released from the decomposition of kraft lignin between 200 and 1000 °C under an argon atmosphere (
Figure 2b). Methane released below 500 °C was mainly caused by the fragmentation of kraft lignin side chains. Demethylation of the aromatic methoxy groups (–O–CH
3) also contributed to the methane formation in the low temperature range. Methane formation at the temperature above 500 °C was attributed to the breaking down of aromatic ring skeletons. Methane was observed in two temperature zones for Fe-N lignin samples, that is, the relative small and sharp peak at 250 °C and a wide and strong methane peak above 400 °C were observed. The methane evolution peak at a low temperature for Fe-N lignin sample tended to shift to the lower temperature compared to that of raw kraft lignin, that is, the first peak ranged from 455 °C to 240 °C and the high temperature methane evolution peak area was increased significantly. This might be because of the promotion effect of iron on lignin decomposition. The formation of methane significantly increased above 400 °C for Fe-N samples, possibly because of iron components catalytically cracking down aromatic ring skeletons in kraft lignin. Methane evolution profiles of FeSO
4, FeCl
2 and FeCl
3 samples were similar to that of raw lignin but the methane evolution peaks of FeCl
2 and FeCl
3 samples at the low temperature shifted to lower temperatures compared to Fe-N sample, the first peak ranged from 455 °C to 250 °C for FeCl
3 samples and 455 °C to 350 °C for FeCl
2 samples. These shifts in temperatures attributed to the catalytic decomposition activity of iron ions (Fe
3+ and Fe
2+) to methoxy groups (–O–CH
3). No significant difference in the temperature shift was observed for FeSO
4 samples.
Two CO evolution peaks were observed for raw kraft lignin samples under an argon atmosphere. The CO evolution peak at a low temperature was 418 °C, which was mainly contributed to the decomposition of carboxyl (C=O) groups and the CO evolution peak centered at 770 °C was attributed to the cracking down of carbonyl (C–O–C). The CO formation of Fe-N lignin samples had three temperature zones, that is, a sharp peak centered at 237 °C, a wide flat peak at 640 °C and a strong CO evolution peak at 900 °C (
Figure 2c). The CO evolution at 237 °C was contributed by the decarbonylation reaction of the C
3 side-chains of lignin, which was catalytically decomposed at low temperature by Fe
3+. The CO peak at 640 °C was because of the cracking down of carbonyl (C–O–C), while the CO peak at 900 °C was most likely because of the thermal cracking down of char residues. The CO evolution profiles of FeSO
4, FeCl
2 and FeCl
3 samples were similar to the ones of raw lignin but the CO evolution peak of FeCl
3 samples at the low temperature shifted to the lower temperature region, which was because of the influence of the catalytic activity of iron ions (Fe
3+) and the decarbonylation reaction. No significant difference in CO formation was observed for FeSO
4 and FeCl
2 samples.
CO
2 released during lignin decomposition because of the decomposition of carboxyl and ester groups of kraft lignin. Two significant peaks presented during the decomposition of kraft lignin under an argon atmosphere (
Figure 2d). Carboxyl (–COO–) was considered to be predominantly responsible for the peak at the low temperature of 407 °C. The CO
2 evolution at the high temperature of 642 °C was assigned to ester groups when the thermal process was under an inert atmosphere.
CO2 was also observed in three temperature zones for Fe-N lignin samples, that is, a sharp peak centered at 237 °C, a wide strong peak at 630 °C and a weak flat evolution peak at 870 °C. The formation of CO2 at 237 °C was contributed by decomposition of carboxyl (–COO–) and COOH groups in lignin which were catalytically decomposed by Fe3+ at a lower temperature. The CO2 peak at 630 °C was because of the cracking down of ester groups, while the CO2 peak at 900 °C was assigned to the thermal cracking down of char residues. The CO2 evolution profiles of FeSO4, FeCl2 and FeCl3 samples were similar to the one of raw lignin but the CO2 evolution peak of FeCl3 samples shifted to the lower temperature region because of the influence of the catalytic activity of iron ions (Fe3+) to the decarbonylation reaction.
The formation of phenols started with the dehydration of –OH groups in alkyl side chains of lignin basic units, followed by the cleavage of ether bonds between these units. The profile of phenols from the decomposition of kraft lignin under an argon atmosphere (
Figure 2e) showed a phenol peak at 473 °C. Phenol formation peaks shifted to the low temperature region for Fe-kraft lignin samples.
The plot of the evolution of a typical aromatic compound of benzene from the thermal decomposition of kraft lignin samples (
Figure 2f) showed that the evolution of benzene from the decomposition of kraft lignin under an inert atmosphere was detected over a wide temperature range from 528 to 938 °C, with a peak temperature at 697 °C. The formation of benzene in Fe-N samples was detected over two temperature zones: a weak flat peak in the temperature range from 220 to 380 °C and a strong wide peak in the temperature range from 510 to 1000 °C with the peak centered at 778 °C. Benzene was observed to be generated at a steady increasing level for the Fe-S sample when the heating temperature was above 500 °C. The evolution of benzene in Fe-Cl
3 samples was detected over 520 °C with a wide flat peak in the temperature range from 520 to 1000 °C. Benzene was the only gas observed released from Fe-Cl
2 samples at temperatures above 720 °C.
The evolution of methanol from raw kraft lignin samples heated under an argon flow was detected over a temperature range from 351 to 695 °C with a peak temperature at 494 °C (
Figure 2g). The evolution profiles of methanol from Fe-lignin samples were significantly different than those of raw kraft lignin in intensity and temperature range. Methanol evolution peaks of Fe-lignin samples all shifted to the low temperature region and also had weaker intensity. This could be because the effect of the catalytic activity of iron and to the decomposition of aromatic methoxy groups (CH
3O–) and aliphatic –CH
2OH group in lignin. Light hydrocarbons (mainly methane) were the preferred products other than methanol for iron-promoted samples (
Figure 2g).
When lignin was de-polymerized, sulfur can be present in many components of the gas phase, such as dimethylsulfide, carbonyl sulfide and hydrogen sulfide (H
2S). The H
2S evolution was monitored during the decomposition of kraft lignin samples under an argon atmosphere (
Figure 2h). H
2S formed in the temperature range from 210 to 646 °C, with a maximum value reached at 341 °C. H
2S formation profiles in Fe-lignin samples were different with these in raw kraft lignin samples in terms of intensity and temperature range. The H
2S evolution peaks of Fe-lignin samples were all in low intensity range. No H
2S gas was detected in Fe-N samples. The H
2S peak in Fe-S samples shifted to a high temperature zone of 350–650 °C with its peak temperature at 470 °C. H
2S released as a flat peak between 308 °C and 690 °C in Fe-Cl
3 samples, while a wide flat H
2S evolution peak in a temperature range of 268–880 °C was observed in Fe-Cl
2 samples. The sulfur content in kraft lignin samples seems to be absorbed completely (Fe-N) or partially (Fe-S, Fe-Cl
3 and Fe-Cl
2) by iron components during the thermal decomposition.
There were two formation zones of HCl released as a volatile from both FeCl
3-and FeCl
2-promoted kraft lignin samples. Fe-Cl
3 samples had a HCl evolution peak centered at 256.6 °C within a low temperature range from 205 to 597 °C. The thermal decomposition of hexahydrate iron(III) chloride produced iron(III) oxide, hydrogen chloride and water. These reactions occurred at a temperature above 250 °C.
HCl began to free from the samples at a high temperature of 623.3 °C due to the desorption of surface Cl atoms with the assistance of hydrogen atoms: H· + Cl· → HCl.
For the Fe-Cl
2 sample, HCl was formed at higher temperature compared to the Fe-Cl
3 sample, the HCl peak attributed to the hydrolysis of FeCl
2 was centered at 333 °C with a temperature range of 249–793 °C:
the formation of HCl started at a high temperature of 798.8 °C. The formation peaks were contributed by the desorption of surface chlorine with the assistance of hydrogen atoms: H· + Cl· → HCl.
Figure 2j shows the trends of NO
2 and O
2 during temperature-programmed decomposition of iron nitrate-promoted lignin samples. NO
2 and O
2 appeared between 160 and 350 °C with a peak temperature of 230 °C. This indicated that the thermal decomposition of iron(III) nitrate produced iron(III) oxide, nitric oxide and oxygen [
20]:
Figure 2k shows the trends of SO
2 and SO
3 during temperature-programmed decomposition of FeSO
4-promoted lignin samples. The anhydrous FeSO
4 released sulfur dioxide and white fumes of sulfur trioxide, which left a reddish-brown iron(III) oxide [
21]:
The decomposition of sulfate initiated around 450 °C, formed a sharp peak at 500 °C.
3.3. Solid Products Characterization
Elemental analyses—
Table 2 summarizes the results of C–H–N–S–Cl elemental analyses performed for raw kraft lignin and thermally treated Fe-lignin samples. The weight contents of C, H, N and S in raw kraft lignin samples were 65.2%, 6.1%, 0.1% and 0.8%, respectively and no Cl was detected. For thermally treated Fe-lignin samples, Fe-N samples had the highest carbon content of 98.5 wt%, followed by Fe-S of 97.3%, Fe-Cl
3 of 95.7% and Fe-Cl
2 of 95.0%; hydrogen contents were 0.4%, 0.6%, 0.8% and 0.1% for Fe-S, Fe-Cl
3, Fe-Cl
2 and Fe-N samples, respectively. No nitrogen was found in thermally treated Fe-lignin samples. Sulfur contents in thermally treated Fe-lignin samples were 0.7%, 0.4%, 0.2% and 1.2% for Fe-S, Fe-Cl
3, Fe-Cl
2 and Fe-N samples, respectively. The mass contents of chlorine detected in both Fe-Cl
2 and Fe-Cl
3 samples were 0.8% and 0.5%, respectively.
Surface area—
Table 3 summarizes BET surface areas measured on samples promoted with different iron compounds. The surface area value was in the range of 55–108 m
2/g and ordered as Fe-N > Fe-S > Fe-Cl
3 > Fe-Cl
2 from high to low, that is, the sample prepared from iron nitrate had the highest surface area among four sample groups evaluated.
XRD—
Figure 3 shows the XRD patterns of thermally treated Fe-lignin samples prepared using Fe-N, Fe-S, Fe-Cl
2 and Fe-Cl
3 compounds. The symbols of *, x and o represent the characteristic diffraction peaks of metallic α-Fe (JCPDS, No. 87-0722), γ-Fe (JCPDS, No. 89-3939) and Fe
3C (JCPDS, No. 89-2867), respectively. The diffraction peaks at 26.5° indicated the (002) plane of graphitic carbon. Both thermally treated Fe-lignin samples prepared using Fe-Cl
2 and Fe-Cl
3 exhibited sharp and narrow peaks of metallic iron (mainly α-Fe and γ-Fe). Thermally treated Fe-lignin samples prepared using Fe-S showed both metallic iron peaks (α-Fe and γ-Fe) and small and broad peaks of iron carbide (Fe
3C). Thermally treated Fe-lignin samples prepared using Fe-N showed weak and broad peaks of metallic iron (mainly γ-Fe) and iron carbide (Fe
3C), which indicated the presence of smaller Fe particles in Fe-N prepared samples compared to Fe-Cl
2 and Fe-Cl
3 prepared samples. The short-broad diffraction peaks observed in Fe-N prepared samples suggested good dispersion of iron species in the product. The sharper diffraction peaks of Fe-Cl
2 and Fe-Cl
3 prepared samples implied a growth in the crystallite size of metallic irons or iron carbide (Fe
3C).
The mean particle size was calculated for the most intense diffraction peaks of α-Fe, γ-Fe and Fe
3C using the Scherrer formula [
15]. Only α-Fe and γ-Fe were detected in Fe-Cl
2 and Fe-Cl
3 prepared samples, while α-Fe, γ-Fe and Fe
3C were observed in Fe-S and Fe-N prepared samples. The mean sizes of α-Fe nanoparticles were 79.6, 40.2, 59.1 and 18.5 nm for Fe-Cl
2, Fe-Cl
3, Fe-S and Fe-N prepared samples, respectively; while the γ-Fe grain sizes averaged 15.3, 30.2, 21.9 and 9.8 nm, respectively. The mean sizes of Fe
3C nanoparticles were 22.5 and 19.7 nm for Fe-S and Fe-N prepared samples, respectively.
Raman—
Figure 4 is the Raman spectra of thermally treated Fe-lignin samples prepared with four iron sources at 1000 °C, displaying a characteristic graphite G-band at 1580 cm
−1, D-band at 1350 cm
−1 and 2D-band at 2710 cm
−1, respectively. The Raman spectra were fitted by Lorentz function to obtain values of I
D, I
G, I
D+G and I
2D values were calculated through fitting Raman spectra with the Lorentz function. The intensity ratios of the D to G band (I
D/I
G) of Fe-Cl
2 and Fe-Cl
3 prepared samples were 1.56 and 1.57, respectively, which were higher than those prepared with Fe-Sand Fe-N of values 1.43 and 1.29, respectively. These results suggested that thermally treated Fe-lignin samples prepared from iron sources of Fe-Cl
2 and Fe-Cl
3 had higher disorder structures, while Fe-N prepared samples had a higher graphitic degree.
Scanning electron microscope (SEM)—The SEM images (
Figure 5) show typical morphologies observed on thermally treated Fe-lignin samples prepared with four iron sources of Fe-N, Fe-S, Fe-Cl
2 and Fe-Cl
3. The morphologies of Fe-Cl
2, Fe-Cl
3 and Fe-S prepared samples were significantly different from ones of Fe-N samples. The Fe-Cl
2, Fe-Cl
3 and Fe-S prepared samples had large pieces of solid grains. At the low magnification, the surfaces of Fe-Cl
2, FeCl
3 and Fe-O prepared samples were smooth and clean and with some gas channels (holes). At the high magnification, a layer of fine particle structures was observed over the surface of these samples. The morphologies of Fe-Cl
2, Fe-Cl
3 and Fe-S prepared samples looked more like raw lignin samples thermally treated at 1000 °C without any catalyst added.
The SEM images of Fe-N prepared samples showed a very fine powder structure, that is, small particles at the low magnification. At the high magnification, these particles were spherical nanoparticles with a relatively uniform particle size ranging between 5 and 10 nm. XRD results proved that these nanoparticles were composed of α-Fe, γ-Fe, iron carbide and graphene structures.
High resolution transmission electron microscopy (HRTEM)—
Figure 6 displays HRTEM images of thermally treated Fe-lignin samples. The dark spots in HRTEM images represented iron species dispersed in carbon-based materials. The nanoparticles in Fe-N prepared samples had core-shell structures with the diameter of core nanospheres in 3–5 nm range. The carbon shells exhibited ordered graphene plane structures. These Fe-core and graphene structure shelled nanoparticles homogenously embedded in the amorphous carbon framework (gray matrix). High-magnification TEM images indicated that these core-shell structures contained onion-like graphitic carbon nanostructures. The morphology of Fe-lignin samples prepared with iron chlorides and iron(II) sulfate was much different compared with Fe-N prepared samples. When iron chlorides and iron(II) sulfate were used as catalyst sources, serious agglomeration of iron particles occurred and the particle size reached as large as 50–100 nm in diameters. These iron nanoparticles unevenly distributed in the amorphous carbon matrix contained non-uniformly shaped solid iron crystallites. Most of iron-core nanoparticles in Fe-Cl
2, FeCl
3 and Fe-S prepared Fe-lignin samples were shelled with disordered amorphous carbon structures. This was presumably the seriously sintering of metallic iron particles because XRD results revealed that larger crystallite sizes of iron particles observed in Fe-Cl
2, Fe-Cl
3 and Fe-S prepared Fe-lignin samples than ones prepared with Fe-N. These results disclosed that uniformly dispersed smaller iron nanoparticles were beneficial for obtaining a highly performed iron catalyst towards the graphitization to kraft lignin, while agglomerated larger iron nanoparticles deteriorated the catalytic performance of irons as a catalyst. Variation in crystallite size of iron species with different iron catalysts was observed in the order of Fe-Cl
2 > Fe-Cl
3 > Fe-S > Fe-N.