Materials Chemistry
High Toluene Dynamic Adsorption/Desorption Characteristics for Super-Microporous Silica Synthesized by Using Collagen Fibril as a Template
2020 Volume 61 Issue 5 Pages 980-984
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2020 Volume 61 Issue 5 Pages 980-984
The removal of volatile organic compounds (VOCs) has been an urgent task all over the world because they are the main substances for environmental problems. Removal by adsorption technique is the most convenient method. Until now, adsorption by activated carbon has been studied in many cases. However, activated carbon has a fire risk and requires a large amount of energy to regenerate the adsorbent. Therefore, siliceous materials have been expected as novel adsorbents instead of activated carbon. We have been able to synthesize porous silica having super-micropores of about 0.8 nm by using collagen fibril as a template. In the present study, the author evaluates toluene dynamic adsorption/desorption properties of this super-microporous silica (SMPS) as well as the characterization studies. It was found that SMPS showed very high dynamic adsorption/desorption properties in comparison with other siliceous materials such as mesoporous silica MCM-41, commercial microporous silica gel, and HY zeolite. SMPS synthesized by using collagen fibril as a template can be expected as a novel VOCs adsorbent.
Since volatile organic compounds (VOCs) are harmful to the human body and are the main causative substances such as photochemical smog and destruction of the ozone layer, emission of VOCs has been regulated in many countries.1–4) Adsorption method and catalytic combustion method and so on have been studied as VOCs removal methods. Among them, the adsorption method has been widely used because it has advantages such as simple system and low operation cost. Furthermore, useful VOCs can be recovered and reused by being desorbed from the adsorbents.5–7)
Activated carbon has been widely used as a VOCs adsorbent. This is because activated carbon has micropores with a pore diameter of 2 nm or less and has large specific surface area and large pore volume. It also easily interacts with organic substances due to the hydrophobic surfaces.8) However, activated carbon has disadvantages such as closed pore structure and fire risk. Furthermore, in order to desorb the VOCs from the activated carbon completely, it is necessary to perform water vapor treatment at high temperatures. That is, a large amount of energy is required to regenerate it.9) Therefore, novel adsorbents superior to the heat resistance and regeneration characteristics as compared with activated carbon have been desired. As representative candidates for the novel adsorbents, zeolite,2,3,10–12) silica gel,13,14) mesoporous silica,15–27) and super-microporous silica (SMPS)28) have been studied. Zeolite has micropores and indicates relatively high VOC dynamic adsorption properties, however, it is necessary to treat it at high temperatures for regeneration because the aromatic hydrogens of toluene interact with the zeolite framework oxygen atoms that are Lewis basic sites. Mesoporous silica and silica gel have mesopores larger than 2 nm and large pore volumes, so the saturated adsorption capacities of VOCs are very large. However, since the interaction between the pore wall and VOCs is weak, the dynamic adsorption characteristics of VOCs are low. Kosuge et al. have reported that SBA-15 with both mesopores and micropores showed high characteristics both in dynamic adsorption and desorption of VOCs.29) Watanabe et al. have reported that super-microporous silica prepared by a solvent-free synthesis method using a surfactant exhibited high toluene dynamic adsorption properties.28) However, desorption properties of toluene for SMPS have not been clarified.
On the other hand, we have demonstrated a novel synthesis method of SMPS by using collagen fibril as a template.30,31) Since collagen is a rod-shaped molecule with a diameter of 1.5 nm and a length of 300 nm, it can be used as a template for the synthesis of SMPS. In addition, it is possible to synthesize SMPS at low cost by using collagen fibril as a template, because such collagen can effectively utilize the waste discharged at the time of leather production. In fact, we successfully synthesized SMPS with a diameter of about 0.8 nm by using collagen fibril obtained by pulverizing leather waste.30)
In the present study, the toluene dynamic adsorption/desorption properties of SMPS synthesized by using collagen fibril as a template were compared with those of siliceous materials such as mesoporous silica MCM-41, commercial microporous silica gel and HY zeolite. In addition, the regeneration characteristic which is an important characteristic for the adsorbent in practical use was also investigated.
Figure 1(a) shows SEM image of the collagen fibril used in this study as a template. Collagen fibril is fibrous protein with the diameter of about 100 nm and is composed of regularly arranged collagen molecules with the diameter of 1.5 nm, the length of 300 nm shown in Fig. 1(b). In the present study, tetraethylorthosilicate (TEOS) was used as a silica source. A 1.0 g of collagen fibril was dispersed in a 120 mL of 4.2 vol% HCl aqueous solution and stirred for 30 min. Then, TEOS was added and stirred vigorously for 24 h. The products were filtered off, washed with distilled water many times and dried at room temperature overnight. The obtained collagen/silica composites were calcined at 873 K for 5 h in air for the removal of collagen fibril before characterization studies and adsorption/desorption measurements.
SEM image of collagen fibril used in this study (a) and a schematic diagram of the regular arrangement of collagen molecules in the formation of collagen fibril (b).
Mesoporous silica MCM-41, commercial microporous silica gel (Fuji Silysia Chemical Ltd.) and HY-zeolite (SiO2/Al2O3 = 5.1, Zeolyst International) were used as reference samples. HY-zeolite and commercial microporous silica gel were used without further treatment after purchase. MCM-41 was prepared as follows. Cetyltrimethylammonium bromide (CTAB) and TEOS were used as a template and a silica source, respectively. A 2.9 g of CTAB was dissolved in a 120 mL of 4.2 vol% HCl aqueous solution. 8.9 mL of TEOS was then added and stirred for about 10 min. Thereafter, the pH of the mixture was raised to 10 by adding ammonium hydroxide, and the mixture was stirred at room temperature for 3 h. The precipitate was filtered off, washed with distilled water many times and dried at 323 K overnight. The product obtained was calcined in air at 873 K for 5 h for the removal of CTAB.
2.2 CharacterizationPore characteristics of the samples were evaluated by nitrogen adsorption/desorption isotherm measurements (BELSORP-MaxII, MicrotracBEL Corp.). Before measurement, pretreatment was carried out at 573 K for 3 h under vacuum. The specific surface area was analyzed by BET method. Micropore and mesopore analyses were performed by t-plot method and BJH method, respectively. The morphologies of the samples were observed by using a field emission scanning electron microscope (FE-SEM; S-4800, Hitachi High Technologies Corp.). Pt was coated on the surface of the samples in order to prevent charge up before SEM observation.
2.3 Toluene dynamic adsorption/desorption measurementFigure 2 shows experimental setup of toluene dynamic adsorption/desorption. Dry air was used as a carrier gas. The toluene saturator was placed in an ice bath kept at 273 K and dry air containing about 450 ppm of toluene was produced by flowing dry air to the toluene saturator at a flow rate of 50 mL/min. The concentration of toluene was measured by the VOC monitor (VM-08-OS, O.S.P. Inc.).
Experimental setup for toluene dynamic adsorption experiments.
The pelletized samples were crushed and sieved between 20 and 40 meshes. These samples were filled into a glass tube with the inner diameter of 10 mm. The height of the adsorption layer was 20 mm. As a pretreatment of the samples for the adsorption experiment, the samples that had been dried 378 K overnight were treated at 423 K for 2 h by flowing dry air at a flow rate of 50 mL/min. The adsorption measurement was carried out at 303 K. When the amount of toluene adsorbed on the sample was saturated, only dry air was flowed, followed by the desorption experiment. The desorption measurement was carried out until the exit toluene concentration became zero. Thereafter, the sample was heated to 673 K at a rate of 3 K/min, and the amount of toluene that had remained in the sample was measured.
Figure 3 shows SEM image and nitrogen adsorption/desorption isotherm of porous silica synthesized by using collagen fibril as a template. As shown in Fig. 3(a), the morphology of the porous silica synthesized was the same as that of the collagen fibril used as a template. No other forms of silica were observed. When silica was also synthesized under the same conditions in the absence of collagen fibril, formation of silica could not be confirmed even after 72 h. It was suggested from these results that the polymerization reaction of silica proceeds only around collagen fibril. As shown in Fig. 3(b), nitrogen adsorption/desorption isotherm of the porous silica synthesized showed an IUPAC I type isotherm, indicating that the porous silica had only micropores. The results of specific surface area, pore volume and mean pore diameter analyzed from the nitrogen adsorption isotherms for SMPS and reference samples are summarized in Table 1. The specific surface area and pore volume of SMPS were small compared to those of MCM-41, but comparable to those of HY zeolite. That is, it was found that porous silica having super-micropores of 1 nm or less and a large specific surface area and pore volume could be obtained by using collagen fibril as a template. Since SMPS synthesized has a pore diameter similar to the dynamic molecular diameter of toluene, it is expected to have very high performance as an adsorbent for toluene.
SEM image (a) and N2 adsorption/desorption isotherm (b) for SMPS.
Figure 4 shows the breakthrough curves of toluene adsorption for each sample. The typical breakthrough curves give the evolution of the C/C0 ratio as a function of time, where C is the concentration of toluene at the outlet of the adsorption bed and C0 is the concentration of toluene at the inlet. The dynamic adsorption characteristics of toluene for each sample obtained from the breakthrough curve are summarized in Table 2. In the actual VOC adsorption processes, the operation finishes when the adsorption reaches the breakthrough point. Therefore, the performance of the adsorbent is evaluated by the time to reach the breakthrough point. Namely, the longer the breakthrough time is, the higher the dynamic adsorption property becomes. Further, the more rapid increase after the breakthrough means the less intraparticle mass transfer resistance. That is, the curve in which the time up to breakthrough is long and the curve rapidly increases after breakthrough is the ideal for the adsorbent.
Breakthrough curves for toluene of MCM-41, commercial silica gel, HY zeolite and SMPS.
As listed in Table 2, SMPS showed a long breakthrough time compared with other siliceous materials. For the other three adsorbents, the breakthrough time was the following sequence, HY zeolite > commercial microporous silica gel > MCM-41. The corresponding toluene adsorption amount of SMPS, commercial microporous silica gel, MCM-41, and HY zeolite was 128, 26, 11, and 64 mg/g, respectively. Taking into consideration of the porous properties of the adsorbents listed in Table 1, it was found that no relationship was observed between the surface areas, the pore volumes and the toluene dynamic adsorption properties, whereas the mean pore diameters of the adsorbents were related to the toluene dynamic adsorption properties: the smaller the mean pore diameters of the adsorbents were, the higher the toluene dynamic adsorption properties became. It is assumed that the interaction between the pore walls and toluene molecules is strong when the mean pore diameter of the adsorbent is similar to the dynamic molecular diameter of toluene (0.68 nm) such as SMPS and HY zeolite. Therefore, the dynamic adsorption properties of SMPS and HY zeolite will be higher than those of commercial microporous silica gel and MCM-41. Also, the toluene dynamic adsorption property of SMPS was higher than that of HY zeolite in spite of being similar mean pore diameters. This may be due to the differences of the pore structures between SMPS and HY zeolite. SMPS can possess one-dimensional pore system with long channel, whereas HY zeolite can possess three-dimensional pore system with the connection of the cage structures. Hence, the longer one-dimensional channels of SMPS will afford a larger the dynamic adsorption property than that for the three-dimensional cage structure of HY zeolite due to the greater time spent inside the longer channels by toluene.
The difference in the increase of C/C0 after breakthrough point was also observed for these siliceous materials. The slope of the increase of C/C0 was the following sequence, MCM-41 > SMPS > commercial microporous silica gel > HY zeolite. This may be due to the differences of the intraparticle mass transfer resistances for these adsorbents. In the case of porous materials, pore and surface diffusions of the adsorbates can affect the intraparticle mass transfer resistance. MCM-41, commercial microporous silica gel, and SMPS have only silanol groups on their inner surfaces. In other words, surface diffusions of toluene for three materials are seemingly similar, and the intraparticle mass transfer resistances for these materials can depend on the pore diffusions of toluene: the pore structures of these materials. MCM-41 and SMPS can possess one-dimensional pore structures because they are synthesized by using rod-shaped templates: surfactant and collagen molecules, whereas commercial microporous silica gels possess the pore structure with three-dimensional networks. Therefore, it is suggested that the intraparticle mass transfer resistances for MCM-41 and SMPS are smaller than that for commercial microporous silica gel. On the other hand, the surface diffusion of toluene for HY zeolite will be smaller, in other words, the intraparticle mass transfer resistance will be larger than those for the other three materials, because the aromatic hydrogen atoms of toluene strongly interact with the zeolite framework oxygen atoms that are Lewis basic sites.
Next, the desorption characteristics of toluene after adsorption were evaluated. As adsorbents are recycled and repeatedly used, the desorption properties of adsorbents, i.e., the regeneration property of adsorbents, are one of the most important ones. The column five in Table 2 showed the amount of toluene desorbed when dry air was flowed through the sample at 303 K after the amount of toluene adsorbed was saturated. MCM-41, commercial microporous silica gel and SMPS which were composed of only silica showed high desorption rates of over 90%. In particular, the adsorbed toluene in MCM-41 was almost desorbed by flowing dry air at 303 K. It was found that the smaller the mean pore diameter was, the lower the desorption rate of toluene became. It is considered that the smaller the pore diameter, the stronger the interaction between the pore wall and toluene molecules. On the other hand, low desorption rate in toluene was shown for HY zeolite by only flowing dry air at 303 K because of the strong interaction between the Lewis basic sites of zeolite frameworks and toluene molecules. The amount of toluene which did not desorb by flowing dry air at 303 K was measured by temperature programed desorption (TPD) measurement. Figure 5 shows the TPD spectra of toluene for each sample. Figure 5(b) shows a magnified TPD spectra for MCM-41, commercial microporous silica gel and SMPS. It was found that treatment at a high temperature over 573 K was necessary to completely eliminate the toluene remaining in HY zeolite, whereas the toluene remaining in MCM-41, commercial microporous silica gel and SMPS could completely eliminate and regenerate by treating at a relatively low temperature of about 423 K. It was suggested that porous silica, which has very high characteristics in both dynamic adsorption and desorption properties of toluene, could be synthesized by using collagen fibril as a template.
Temperature programed desorption profiles for toluene of (a) MCM-41, commercial silica gel, HY zeolite and SMPS and (b) the magnified profiles of MCM-41, commercial silica gel and SMPS.
Finally, the recycling characteristic of toluene adsorption for SMPS was evaluated. After the adsorption of toluene reached saturation, SMPS was regenerated by flowing dry air at 423 K for 2 h. As shown in Fig. 6, no change was observed for the toluene dynamic adsorption property of SMPS when the regeneration treatment of SMPS was repeated three times. That is, it was also cleared that SMPS possesses excellent regeneration property.
Recycling experiments for dynamic adsorption of toluene using SMPS.
SMPS was synthesized by using collagen fibril as a template. The toluene dynamic adsorption/desorption properties for SMPS were evaluated and the following results were obtained.
These results indicated that SMPS synthesized by using collagen fibril as a template was an ideal adsorbent that exhibited extremely high toluene dynamic adsorption/desorption properties.
