Efficient Toluene Adsorption on Metal Salt-Activated Porous Carbons Derived from Low-Cost Biomass: A Discussion of Mechanism.

Porous carbons (PCS) derived from sodium lignin sulfonate were activated by four common metal salts. The samples exhibit distinct characteristics of irregular, sunflower-like, interconnected sheet, and tine block morphologies under the impact of NaCl, CaCl2, ZnCl2, and FeCl3, respectively (PCS-MCl x ). Surprisingly, the maximum and minimum specific surface areas are 1524 and 44 m2/g corresponding to PCS-ZnCl2 and PCS-NaCl. All of the samples have plentiful functional groups; herein, PCS-NaCl and PCS-FeCl3 are detected with the highest O and S contents (11.85, 1.08%), respectively, which signifies sufficient active sites for adsorption. These porous materials were applied in toluene adsorption from paraffin liquid and matched the Langmuir isotherm models well. Thus, the activation mechanism was discussed in detail. PCS-MCl x has a completely different pyrolysis behavior according to thermogravimetry/derivative thermogravimetry (TG/DTG) analysis. It is speculated that H[ZnCl2(OH)] would have an etching effect on the carbon structure of PCS-ZnCl2, and HCl or H2SO4, resulting from FeCl3 hydrolysis and a reduction reaction, would be corrosive to the sodium lignin sulfonate (SLS) surface. Each metal salt plays a different role in activation. The devised method for the synthesis of porous carbons is green and economical, which is suited to mass production.

Introduction

Aromatic hydrocarbons are widely found in petroleum products and their concentrations are extremely dependent on the method of purification. n class="Chemical">Mineral oil (MO), for instance, is a complex mixture containing saturated hydrocarbons (MOSHs) and aromatic hydrocarbons (MOAHs) mainly alkylated like toluene.[1,2] Paraffin liquid (white oil) is a highly refined mineral oil[3,4] and is divided into industry grade,[5] cosmetics grade,[6] and food or pharmaceutical grade.[2,7,8] Significantly, some of these applications require MO to have a low or naught content of aromatic hydrocarbons for which carcinogenicity has been verified.

However, MO often contains a high content of aromatic hydrocarbons (30–50%),[7] which is found even in food (10%).[9] The classical methodologies to acquire purified n class="Chemical">paraffin liquid are sulfonation or catalytic hydrogenation,[10−12] which have a multitude of problems such as inefficiency, heavy pollution, expensive equipment, and risky work environments. As a matter of fact, adsorption methods for MO purification have recently been considered with simple activated carbons[13,14] or resins[15] on account of convenience and economy. But more studies are required in this direction and exploring adsorbents with high adsorption capacity is still vital. In this regard, porous carbons are generally accepted due to their great value in adsorption. Among the assorted carbon materials, porous carbons derived from biomass are not only environmentally benign, cost-efficient, and especially abundant in nature but also have adjustable pore structures, remarkable chemical stability, and prominent specific surface area (SSA).[16−18] Generally speaking, biomass as precursors often accompanies special templates[19] or activation agents that include alkalis,[20,21] acids,[22] and metal salts.[23,24] The templates might be inorganic particles mixed with biomass precursors. After removing the templates by specific methods, the mixtures are made into a regular and controllable porous structure whose morphology copies the templates’ morphology in reverse. As far as activation methods go, lignocellulose in biomass easily reacts with activators leading to micropores at a temperature much lower than that of the template methods.

Lignin, one of the three main components in biomass (the other two being cellulose and n class="Chemical">hemicellulose) in the range of 15–40%, is a complex organic polymer in nature.[25] It is separated from cellulose by adding sulfurous acid or a sulfite salt in the paper industry to form sodium lignin sulfonate (SLS), which is a totally underutilized material.[26] As such, it has vast potential that can be tapped to develop value-added products. Cotoruelo et al.[27] via simple lignin carbonization obtained activated carbons for sodium dodecylbenzene sulfonate adsorption. Vapor activation was included by Fu et al. for removing methylene blue.[28] SLS derived microporous carbons with KOH activation were obtained by He and co-workers for the adsorption of tetracycline.[20] Based on this, halloysite as templates and KOH activation were improved to prepare hierarchical porous carbons by Xie et al.[21] Nevertheless, some of the synthetic methods are either too facile to obtain developed porosity or too complicated to operate. The applications of SLS still have not been discussed, and there have been no reports on SLS-based porous carbons for toluene purification. In this work, SLS was used as a carbon source with mild MCl (NaCl, CaCl2, ZnCl2, and FeCl3) as both a template and an activation agent to prepare porous carbons at a certain temperature for toluene adsorption. Four entirely different morphology samples were synthesized with oxygen content from 8.58 to 11.85% and they all had a good adsorption capacity on toluene, which fitted the Langmuir isotherm well. The activation process was recorded by thermogravimetry/derivative thermogravimetry(TG/DTG) analysis and surface functional groups were analyzed in detail. Also, the activation and adsorption mechanisms were discussed comprehensively.

Results and Discussion

Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) Analysis

Direct carbonization of SLS leads to poor pore channel development, which has been reported in previous work.[20,21] Four totally different morphologies of porous carbons were observed through SEM. The surface of PCS-NaCl appears ragged and uneven (Figure a), which has plenty of irregular particles, less than 1 μm in diameter, distributed on it. Only a few places with pores can be found on closer inspection (Figure b). Interestingly, abundant divided cake-like lumps with a diameter of 1–2 μm are present on the surface of PCS-ZnCl2, as shown in Figure S1a. They possess developed and hierarchical pore channel structures crossing each other shaped like the center of a sunflower (Figure c,d). The morphology of the sample activated by CaCl2 manifests an erect and interconnected sheet structure in three dimensions, which is cross-linked randomly as seen in Figure S1b. Closer observations suggest that the surface of the sheets is hierarchically spongy and has a regular porous structure with a diameter less than 40 nm (Figure e,f). As for SLS via FeCl3 activation, as shown in Figure g,h, the carbon materials generate a mass of homogeneous tine blocks. These massive blocks are stacked like rocks with a great number of slit-pores, which offer extra surface area and volume. All of the samples have modified pore structures to some extent, which provides enough room for adsorption.

Figure 1

SEM images of (a, b) PCS-NaCl, (c, d) PCS-ZnCl2, (e, f) PCS-CaCl2, and (g, h) PCS-FeCl3.

SEM images of (a, b) PCS-NaCl, (c, d) n class="Chemical">PCS-ZnCl2, (e, f) PCS-CaCl2, and (g, h) PCS-FeCl3.

The images of TEM can further depict the morphologies of four samples, as shown in Figure . The PCS-NaCl (Figure a) has many uneven particles, which are dispersed at random. Although the sunflower shape for PCS-ZnCl2 cannot be seen in Figure b, it roughly reveals a circular shadow. Figure c displays the cross-linked sheets clearly. One unanticipated finding was that many dark spots occur in the TEM images of PCS-FeCl3 (Figure d), which is attributed to γ-Fe2O3 particles and is consistent with previous reports.[29] In addition, the high-resolution TEM (HRTEM) images of all of the samples (Figure S2) depict the disordered or amorphous structure of PCS-MCl, which might be due to the low graphitization degree.[30]

Figure 2

TEM images of (a) PCS-NaCl, (b) PCS-ZnCl2, (c) PCS-CaCl2, and (d) PCS-FeCl3.

TEM images of (a) PCS-NaCl, (b) n class="Chemical">PCS-ZnCl2, (c) PCS-CaCl2, and (d) PCS-FeCl3.

2.2. X-ray Diffraction (XRD) and Raman Analysis

The XRD curves indicate the crystal structure features of the samples. As shown in Figure a, all patterns have two broad and weak diffraction peaks at 2θ = 24.7 and 43.6° related to the (002) and (100) lattice planes in the amorphous graphitic n class="Chemical">carbon structure, respectively,[31] which fits the TEM images well. The two weak-intensity peaks reveal that porous carbons have poor graphitization degree at a relatively low carbonization temperature.[32] It is worth mentioning that an extra peak of PCS-FeCl3 at 2θ = 35.4° is identified as the γ-Fe2O3 lattice,[29] and the formation mechanism is discussed below. Additionally, the defects and graphene features of PCS-MCl were investigated using the Raman spectra (Figure b). Two typical peaks center at 1347 and 1584 cm–1 according to the D band and G band, which represent defective or disordered carbon and sp2 hybridized graphite phase, respectively.[33] The intensity ratio of D and G bands (R = ID/IG) can mirror the extent of carbon defects and active sites.[32,34] The calculated R values of PCS-NaCl, PCS-ZnCl2, PCS-CaCl2, and PCS-FeCl3 are 0.74, 0.86, 0.68, and 0.72, respectively. It is noticeable that PCS-CaCl2 has higher graphitization than other samples, while the lower R value suggests fewer active sites, which is not conducive to the toluene adsorption process.

Figure 3

(a) XRD and (b) Raman spectra of PCS-NaCl, PCS-ZnCl2, PCS-CaCl2, and PCS-FeCl3.

(a) XRD and (b) Raman spectra of PCS-NaCl, n class="Chemical">PCS-ZnCl2, PCS-CaCl2, and PCS-FeCl3.

Fourier Transform Infrared (FTIR) and X-ray Photoelectron Spectroscopy (XPS) Analysis

The FTIR spectra of the four samples are displayed in Figure . It is clear that the broad peak at 3430 cm–1 should be attributed to the fundamental vibrations of O–H. The band appearing at 1621 cm–1 can be ascribed to C=O or C=C aromatic skeletal vibrations. The band at 1252 cm–1 is assigned to the C–O stretching vibration. Also, the peak at 1040 cm–1 is ascribed to −SO3H, which is consistent with previous results,[21] and the region of 750 cm–1 indicates C=C stretching (aromatic rings).[35]

Figure 4

FTIR spectra of PCS-NaCl, PCS-ZnCl2, PCS-CaCl2, and PCS-FeCl3.

FTIR spectra of PCS-NaCl, n class="Chemical">PCS-ZnCl2, PCS-CaCl2, and PCS-FeCl3.

To further investigate the nature of surface functional groups in detail, XPS analysis was carried out for all of the samples. As can be seen in Figure a, the broad scan XPS spectra show mainly C 1s and O 1s peaks, which contain negligible S content (Table ) as most of the sulphur escape in the form of sulfur dioxide gas. The deconvolution of C 1s peaks for the four n class="Chemical">PCS-MCl samples is shown in Figure b–e. The peaks at 284.66–284.78 eV are ascribed to C=C aromatic groups or C–C. The contribution at 285.29–285.4 eV is attributed to C–O groups. The peaks at 286.1–286.7 eV are ascribed to C=O groups, while the highest energy contribution (288.21–289.3 eV) is attributed to O–C=O bonding.[36,37] Similarly, the XPS spectra of PCS-FeCl3 show an extremely low Fe content corresponding to TEM and XRD as well as S content[21] (1.08%) in Figure S3. The discovered Fe 2p3/2 peak is in accord with γ-Fe2O3 (712.7 eV) coated with carbon[38] because PCS-FeCl3 has an acid-resistant ability. In addition, the high-resolution photoelectron spectra of O 1s signals are depicted in Figure a–d. It is striking for PCS-NaCl that the sample has a unique peak at 531.2 eV corresponding to O–C=O groups. Other peaks located at 531.12–531.90, 532.18–532.92, and 533.60–534.07 eV can be assigned to C=O, C–O, and −OH groups, respectively.[39] Accordingly, the SLS activated by NaCl has the highest O content of 11.85% (Table ). PCS-CaCl2 possesses the lowest O content (8.58%) and the highest C content (91.17%). The single most interesting observation to emerge from the data comparison is that FeCl3 activation of SLS has 2–4 times larger S content than others. Briefly, functional groups can have a positive effect on interaction with toluene.

Figure 5

(a) XPS survey scan and high-resolution spectra of the C 1s of (b) PCS-NaCl, (c) PCS-ZnCl2, (d) PCS-CaCl2, and (e) PCS-FeCl3.

Table 1

Chemical Composition of PCS-MCl from XPS

samples	C (atom %)	O (atom %)	S (atom %)
PCS-NaCl	87.83	11.85	0.32
PCS-ZnCl2	90.65	8.88	0.47
PCS-CaCl2	91.17	8.58	0.25
PCS-FeCl3	88.6	10.32	1.08

Figure 6

XPS high-resolution spectra of the O 1s of (a) PCS-NaCl, (b) PCS-ZnCl2, (c) PCS-CaCl2, and (d) PCS-FeCl3.

(a) XPS survey scan and high-resolution spectra of the C 1s of (b) PCS-NaCl, (c) n class="Chemical">PCS-ZnCl2, (d) PCS-CaCl2, and (e) PCS-FeCl3.

XPS high-resolution spectra of the O 1s of (a) PCS-NaCl, (b) n class="Chemical">PCS-ZnCl2, (c) PCS-CaCl2, and (d) PCS-FeCl3.

Thermal Characteristic Analysis

TG and n class="Chemical">DTG were utilized to obtain quantitative and qualitative data of SLS activated by different metal salts. There are distinct differences in the total weight loss from 18 to 85% in Figure a, which is closely related to activating agents. As for the PCS-NaCl sample, it is noticeable that the curve is steadily flat before 200 °C and then witnesses a slight decrease corresponding to a wide peak in DTG (Figure b) between 200 and 700 °C. This can be explained by the pyrolysis of SLS and the extra peak in the XPS because not all −COOH groups transform into CO2 and H2O. However, PCS-ZnCl2, PCS-CaCl2, and PCS-FeCl3 show a rapid weight loss before 200 °C, corresponding to the obvious peaks in Figure b, which is ascribed to the removal of adsorbed moisture. PCS-CaCl2 has a similar trend to PCS-NaCl except for an extra gentle descent after 550 °C with a 40% weight loss up to 700 °C. Both PCS-ZnCl2 and PCS-FeCl3 samples experience a marginal decrease between 200 and 400 °C, indicating the pyrolysis of SLS and carboxylic or hydroxyl groups with the release of gases H2O, CO2, and SO2.[40] PCS-FeCl3 maintains the tendency and loses 60% of weight at 600 °C and becomes steady in accord with the broad and weak peak in DTG. Nevertheless, the SLS activated by ZnCl2 follows a dramatic descent with the strong peak of DTG during 400–600 °C, which could be explained by the reaction and disintegration of carbon in SLS and the entry into stable aromatization. Although PCS-NaCl shows excellent thermal stability, NaCl brings about limited changes to SLS surface properties compared to the other three metal salts, which is reflected in the pore structure being consistent with the SEM characterization.

Figure 7

(a) TG and (b) DTG for all samples.

(a) TG and (b) n class="Chemical">DTG for all samples.

Pore Structure Analysis

To investigate the impact of PCS-MCl activation on the specific surface area (SSA) and pore structure, the n class="Chemical">N2 adsorption/desorption isotherms were obtained, as shown in Figure . The isotherms, except for PCS-NaCl, exhibit a sharp N2 uptake increase at a relative pressure of 0–0.1P/P0, which occurs in typical microporous materials due to micropore filling. All of the profiles feature a H4 hysteresis loop of IV (a) type curves at 0.4–1.0P/P0 relative pressure according to the IUPAC classification,[41] which is regarded as the characteristic of slit pores containing both micropores (<2 nm) and mesopores (2–50 nm). Meanwhile, broad and distinct hysteresis loops also suggest that some pores would exist in the form of hierarchical structures certified in the SEM image. The SSA of PCS-NaCl, PCS-ZnCl2, PCS-CaCl2, and PCS-FeCl3 is 44, 1524, 390, and 703 m2/g, respectively, showing a clear difference with the four activating agents (Table ). It is worth mentioning that in the table, the t-plot micropore area (Smicro) of PCS-NaCl comes to nought. Similarly, the Brunauer–Joyner–Halenda (BJH) adsorption cumulative surface area of PCS-ZnCl2 (Smeso) also takes the major proportion (72%) with the highest Smeso of 1092 m2/g, which is well fitted to the TG/DTG analysis because of full activation and carbonization in the heating process. However, the Smicro dominates the sample surface area with 260 and 422 m2/g for PCS-CaCl2 and PCS-FeCl3, respectively. The pore size distribution of the four adsorbents is, respectively, depicted in Figure b,d,f,h. PCS-NaCl has a dispersed aperture distribution but it actually obtained the lowest single point adsorption total pore volume (Vtotal) of 0.076 cm3/g (Table ). It is similar to the Smeso proportion (72%) of PCS-ZnCl2 and the BJH adsorption cumulative volume (Vmeso) is as large as 1.409 cm3/g compared to the Vtotal of 1.616 cm3/g. Both PCS-CaCl2 and PCS-FeCl3 record sharp peaks below 2 nm in Figure f,h, which indicates that they yield a relatively small pore size focused at the micropore. Moreover, there is a significant difference in the Horvath–Kawazoe (HK) micropore volume (Vmicro) of PCS-CaCl2 because it takes half of Vtotal with 0.15 cm3/g andVmeso occupies the majority as for the other samples. It is apparent from Table that PCS-ZnCl2 and PCS-FeCl3 would take advantage of adsorption with the Vtotal of 1.616 and 1.011 cm3/g, respectively.

Figure 8

N2 adsorption/desorption isotherms and pore size distribution of (a, b) PCS-NaCl, (c, d) PCS-ZnCl2, (e, f) PCS-CaCl2, and (g, h) PCS-FeCl3.

Table 2

Specific Surface Area and Pore Parameter of PCS-MCl

samples	SSA (m2/g)	Smicro (m2/g)	Smeso (m2/g)	proportion of Smeso (%)	Vtotal (cm3/g)	Vmicro (cm3/g)	Vmeso (cm3/g)
PCS-NaCl	44	0	37	100	0.076	0.015	0.074
PCS-ZnCl2	1524	93	1092	72	1.616	0.557	1.409
PCS-CaCl2	390	260	83	21	0.299	0.150	0.173
PCS-FeCl3	703	422	223	32	1.011	0.274	0.818

N2 adsorption/desorption isotherms and pore size distribution of (a, b) n class="Chemical">PCS-NaCl, (c, d) PCS-ZnCl2, (e, f) PCS-CaCl2, and (g, h) PCS-FeCl3.

Adsorption Isotherm

To match the toluene adsorption equilibrium on n class="Chemical">PCS-MCl, two well-known theoretical models, that is, the Langmuir and Freundlich isotherm models, were considered. Langmuir assumed that the surface properties of the adsorbent were homogeneous. His work suggested that atoms or molecules on the surface of an adsorbent had an outward force that could trap adsorbate molecules. There is no force interaction between adsorbates, and the range of this force was equivalent to the diameter of the adsorbate molecules; thus, only monolayer adsorption could occur on the surface of the adsorbent.[42] Freundlich hypothesized heterogeneous surfaces, finite adsorption sites, and changeable potential energy interaction for adsorbents. He surmised that the adsorption ability is enhanced with the concentration of adsorbates, which indicates that an infinite adsorption capacity might be acquired in theory.[43] The two theoretical models can be represented, respectively, aswhere Qm (mg/g) indicates the maximum toluene adsorption capacity; KL and KF are the Langmuir and Freundlich adsorption constants associated with the adsorbent’s adsorption ability, respectively; and 1/n is a nonlinear factor normally between 0 and 1, suggesting an efficient adsorption process.

The nonlinear fitting results and the parameters of Langmuir and Freundlich adsorption models are given in Figure and Table S1. All of the regression coefficients (R2) are larger than 0.91 for Langmuir models, which fits better than Freundlich models as a whole (>0.85). The value of 1/n certifies that the adsorption is available and the calculated Qm is greater than 2300 n class="Chemical">mg/g for the four samples. The order of theoretical Qm is PCS-ZnCl2 > PCS-FeCl3 > PCS-NaCl > PCS-CaCl2. Contrary to expectations, the maximum toluene adsorption capacity does not find a sharp distinction between PCS-NaCl and PCS-ZnCl2, which exhibit the lowest and highest SSA and total pore volume, respectively. It can be explained that the adsorption ability is related not only to the size of the area or inner space but also closely to surface properties including functional groups and heteroatom contents (O, S), which contribute to PCS-NaCl adsorption of toluene. As a matter of fact, the positive effects of suitable surface oxygen contents on toluene adsorption have been proved by previous studies.[44]

Figure 9

Adsorption isotherm plots (25 °C) and Langmuir and Freundlich fitting curves for PCS-MCl.

Activation Mechanism

NaCl is one of the most common n class="Chemical">metal salts used as template agents for porous carbon synthesis. Many reports have proved that it facilitated nanosheet networks of the carbon framework either two-dimensional (2D) or three-dimensional (3D).[45−47] However, it seems that the current study does not support the former result. The reason for this is that the calcination temperature of 500 °C of this study is far lower than those of the previous studies of 750–1100 °C. NaCl, with a melting point of 801 °C, is a typical ionic crystal with a stable regular octahedral structure in which the smaller sodium ions (102 pm) fill the gap between the chloride ion arrangements and each ion (Na+/Cl–) is surrounded by six other ions (Cl–/Na+). Therefore, NaCl crystals replace Na+ as the templates at the current temperature, while the SLS molecules coat the NaCl particles, which contributes to the ragged and uneven surface of PCS-NaCl. Meanwhile, the carbonization process of SLS is impeded as shown by TG/DTG analysis. As a result, limited pores are generated with 44 m2/g specific surface area and 0.076 cm3/g total volume.

Although ZnCl2 has been applied as both a template and an activation agent far and wide,[17,24] its activation mechanism has been little explored. In the first place, free n class="Chemical">Zn2+ (74 pm) should play a role in activation for pore formation as a consequence of the ionic bond fracture at the low ZnCl2 melting point of 283–293 °C, which makes full contact with and generates micropores on the SLS surface in the molten state. Additionally, ZnCl2 is strongly water adsorbing and might react with water to form a complex acid (ZnCl2 + H2O → H[ZnCl2(OH)]), which makes it corrosive to SLS and produces an etching effect, thereby generating a pore structure. On the other hand, ZnCl2 has been verified not to volatilize in spite of the temperature being higher than the boiling point (732 °C), which may possibly be explanted by transforming into ZnO with a melting point of 1975 °C (ZnCl2 + −OH → ZnO + HCl). Apparently, it could conceivably be speculated that the Zn2+/ZnO acts as the template to further facilitate the development of micropores and mesopores (SSA = 1524 m2/g, Vtotal = 1.616 cm3/g) in the internal structure because the complex acid accelerates the activation from the surface to the inner layer. Generally speaking, ZnCl2 is considered as a dehydrating agent, which influences the pyrolysis reaction and cuts down the production of tar so as to help carbon framework aromatization, manifesting in the dramatic descent of the thermogravimetric curve (Figure ). In detail, H and O atoms in the SLS are removed as H2O for dehydration instead of forming hydrogen and oxygen functional groups on the surface so that PCS-ZnCl2 obtains a high C content of 90.65%.

Porous carbons based on biomass by CaCl2 activation have almost the same properties (C%: 91.7%, O%: 7.38%) and the erect and interconnected sheet structure in three dimensions as investigated by others.[48,49] It is known to us all that CaCl2 is a good adsorbent that can easily adsorb a great deal of H2O, CO2, SO2 and O2, which leads to volume expansion. Thus, the cross-linked sheet structure is formed with a large hole (Figure S1b) under the extrusion force after high-temperature calcination and washing off impurities. CaCl2 plays an extraordinary part in hard templates and expansion agents. Simultaneously, hydrated calcium chloride would release the adsorbed H2O, CO2, and O2 as the temperature increases, which occurs in the following common chemical reactionsThese carbon elimination reactions fit well to the TG curve of PCS-CaCl2 with a slight descent from 150 to 700 °C and generate massive micropores and mesopores. There was another speculation in the research of Li and his co-workers[49]that implied that CaCl2 infiltrated into the SLS facilitating pyrolysis or carbonization process and dissolved lignin providing a porous structure.[49]

As ZnCl2 and n class="Chemical">CaCl2, FeCl3 has strong water adsorption and is one of the most acidic salts in aqueous solutions due to the intense hydrolysis reaction of Fe3+ expressed asSO2 as a reducing agent released by SLS, with increasing temperature, will react with FeCl3Combining eqs and 8, the acid can affect the surface of SLS and induce a pore channel for its corrosivity. Hence, it can be deduced that a part of S atoms fixes on PCS-FeCl3 through transforming into H2SO4 at high temperatures, which is measured by XPS with a S content of 1.08% far more than in other samples. Obviously, Fe(OH)3 is also not stable in the calcination process and proceed carbon elimination reaction[29,38] except for eqs –6It can be reasonable to surmise that the remaining Fe2O3 is coated with amorphous carbon because 5 wt % HCl does not get rid of it, fitting well to the preceding characterization results. Moreover, FeCl3 possesses a low melting and boiling point (306 and 315 °C, respectively) and might evaporate before it can activate SLS, which explains the poorer pore properties than those of PCS-ZnCl2.

Adsorption Mechanism

There are many possibilities that occur in the adsorption process, which render the mechanism complicated. To put it simply, they might include the following three steps: first, the toluene molecules diffuse from the liquid n class="Chemical">paraffin solution to the adsorbent surface; then, it is followed by intraparticle diffusion within the channel of PC-MCl; and finally, toluene molecules are adsorbed on active sites or continue dispersing in micropores. Polanyi put forward the adsorption potential theory for the first time in 1916.[50] He believed that there was an adsorption field around the adsorbent surface where the adsorbates were captured while diffusing to this range. The adsorption potential existed at every point in this particular adsorption space.

Theoretically, the larger the SSA or pore volume provided by the adsorbent, the greater the adsorption capacity of adsorbents for toluene because high-energy pore channels are generally filled with the small n class="Chemical">toluene molecules at the beginning (pore-filling effect).[51] From the data, we can infer that the mesopores play a vital role in adsorption. However, it is obvious that SSA and Vtotal are not decisive from the results of isothermal adsorption. Raman characterization provides some support for the adsorption capacity, in which the order of R values is PCS-ZnCl2 > PCS-NaCl > PCS-FeCl3 > PCS-CaCl2. The higher R value accords with the lower graphitization degree of porous carbon; nevertheless, it induces more defects and active sites.[32] There are similarities of our study between the explanations demonstrated by Ji et al.[52] They described the reasons as adsorption affinity and thought that the adsorption affinity of porous carbons is larger than graphite for the small molecule compound (benzene and toluene) adsorption.

Besides, sufficient functional groups have been detected via FTIR and XPS, which may support the hypothesis of the electron donor–acceptor (EDA) mechanism.[52] The aromatic rings of toluene would act as acceptors with powerful electron-withdrawing ability for the unique π structure, and the n class="Chemical">carbonyl oxygen, −SO3H, or aromatic carbon on the PCS-MCl surface can be regarded as electron donors. Therefore, the adsorption process is associated with the number of donors and would be hindered as long as every donor is exhausted. In terms of O atom content, the orders is PCS-NaCl > PCS-FeCl3 > PCS-ZnCl2 > PCS-CaCl2, and PCS-FeCl3 has much more S atoms than others. In this sense, PCS-NaCl makes up for its lack of specific surface area and pore volume. To sum up, PCS-CaCl2 performs poorly in the above aspects, which contributes to its relatively low adsorption capacity.

Conclusions

In this work, SLS-based n class="Chemical">porous carbons with metal salt activation exhibit quite a few differences in morphologies, pore parameters, and thermal properties. PCS-MCl shows prominent toluene adsorption of more than 2300 mg/g fitting well to Langmuir isotherm models. For the activation mechanism, our preliminary conclusion is that ZnCl2 and FeCl3 further chemically react on the surface of biomass, resulting in a larger specific surface area and total pore volume, which endows them with better adsorbability on account of the pore-filling effect. However, NaCl activated SLS covers the shortage of SSA and Vtotal by abundant active sites; in other words, the highest O content of 11.85% as electron donors establishes interactions with toluene molecules to augment the adsorption affinity. PCS-CaCl2 possesses the lowest O and S content and a very poor pore parameter, leading to a relatively bad adsorption ability. The low-cost carbon resources and mild activation agents make it possible to prepare efficient adsorbents for industrial applications. It is believed that the unique porous structure in this study could also be developed for other fields.

Experimental Section

Materials

Sodium n class="Chemical">chloride (NaCl, AR), anhydrous calcium chloride (CaCl2, AR), zinc chloride (ZnCl2, AR), anhydrous ferric chloride (FeCl3, AR), paraffin liquid (CAS: 8012-95-1, AR), 2,2,4-trimethylpentane (C8H18, AR), and hydrochloric acid (HCl, AR, 37 wt %) were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China) and sodium lignin sulfonate (SLS, AR, 96%) was bought from Aladdin Reagent Co. (Shanghai, China).

Synthesis of Samples

Initially, moderate SLS was mixed homogeneously with n class="Chemical">NaCl, CaCl2, ZnCl2, or FeCl3 in the mass ratio of 1:4, respectively, and the mixture was ground to powder. Then, the mixture was directly carbonized at 500 °C for 2 h in a tube furnace with a heating rate of 5 °C/min under a nitrogen atmosphere. After cooling to room temperature, the obtained product was poured into 5 wt % HCl with magnetic stirring for 12 h. Finally, the solution was filtered and then washed with distilled water until it became neutral. The dried samples were marked as PCS-MCl (M is Na, Zn, Ca, and Fe, respectively).

Adsorption Experiment

The adsorption isotherm experiment was performed at 25 °C with 20 mg of n class="Chemical">PCS-MCl. The adsorbents were poured into 100 mL flasks with a series of initial toluene concentrations of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 g/L in paraffin solution (50 mL). Static adsorption was conducted in a shaker (SHZ-82A, China) with a speed lower than 130 rpm for more than 24 h to reach equilibrium. After centrifugation to obtain the supernatant, the toluene concentrations were analyzed by a UV–visible spectrophotometer (UV-2600 Shimadzu, Japan) in the range from 200 to 800 nm with 2,2,4-trimethylpentane dilution. The adsorption capacity at equilibrium (Qe, mg/g) was calculated aswhere C0 and Ce (g/L) are the initial and equilibrium concentrations of toluene, m (mg) is the mass of adsorbents, and V (mL) is the volume of the solution.

Characterization

Scanning electron microscopy (SEM, Hitachi SU8010, Japan) and high-resolution transmission electron microscopy (TEM, Tecnai G2 20S-TWIN) were utilized to observe the morphologies of the samples. X-ray photoelectron spectroscopy (XPS) was conducted on a ThermoScientific Multilab 2000 spectrometer. X-ray diffraction (XRD) patterns were obtained on a D8 Advance diffractometer (Bruker AXS, Germany). Fourier Transform infrared (FTIR) spectra were collected on a Nicolet Nexus 470 (ThermoScientific). Thermal decomposition (TG) of the samples was studies by a n class="Chemical">TG 209 F3 Tarsus (Netzsch, Germany). Nitrogen adsorption/desorption measurements were performed on a V-Sorb 2800P with a degassing condition of 300 °C for 4 h. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method and the pore volume was estimated by the Barrett–Joyner–Halenda (BJH) and Horvath–Kawazoe (HK) method. The pore size distribution was analyzed by nonlocal density functional theory (NLDFT).