Summary of chemistry doctoral thesis: Study on synthesis, characteristics, and adsorption properties of toxic organic substances in the water environment of mesoporous carbon materials

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The thesis has found a new method to increase the pore size of mesoporous carbon by filling the liquid glass into the pore of the template (silica SBA-15) before impregnating the carbon presource to limit the penetration of carbon sealed the pore system of SBA-15. Stability of mesoporous carbon is increases due to silicon are partially retained in materials. This technique opens the way of synthesis of mesoporous carbon as an adsorbent with a desired pore size and stability.

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Nội dung Text: Summary of chemistry doctoral thesis: Study on synthesis, characteristics, and adsorption properties of toxic organic substances in the water environment of mesoporous carbon materials

MINISTRY OF EDUCATION VIETNAM ACADEMY OF AND TRAINING SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY- ---------------------------- NGUYEN THI HONG HOA STUDY ON SYNTHESIS, CHARACTERISTICS AND ADSORPTION PROPERTIES OF TOXIC ORGANIC SUBSTANCES IN THE WATER ENVIRONMENT OF MESOPOROUS CARBON MATERIALS Major: Theoretical Chemistry and Physical Chemistry Code : 62.44.01.19 SUMMARY OF CHEMISTRY DOCTORAL THESIS Ha Noi – 2019
The work was completed at: Graduate Universty of Science and Technology - Vietnam Academy of Science and Technology. Science supervisor 1: Assoc.Prof.Dr. Dang Tuyet Phuong Science supervisor 2: Dr. Tran Thi Kim Hoa Reviewer 1: … Reviewer 2: … Reviewer 3: …. The thesis will be defended in front of doctoral thesis, held at the Graduate University Science and Technology - Vietnam Academy of Science and Technology at ... o’clock, on day ... month ... year 2019. Thesis can be found at: - Library of the Graduate University Science and Technology - National Library of Vietnam
1 INTRODUCTION 1. The necessity of the thesis Mesoporous carbon materials have an ordered structure, uniform pore size. They often were synthesized by two methods: soft-templating and hard-templating. With the soft-templating method, materials have been prepared via self-assembly by using soft-templating (surfactant). The obtained materials have less orderly structure. The pore size of the material is difficult to control and the template is difficult to remove. With the hard-templating method, MCM-48, SBA-15, etc. are used as the templates. The materials have highly order structure, uniform and easily controlled pore size. Therefore, hard-templating method is used more widely. However, the pore size of materials is smaller than that of the hard-templates because obtained materials are inverse copies of the templates. The thickness of the wall and the pore size are limited by size and shape form of hard-templates. So far, the pore size of mesoporous carbon materials are synthesized by hard-templating method only reach the maximum of ~ 5.5 nm. The increasing in pore size is not feasible because it is limited by the size of the templates, resulting in framework collapse and pore breakage due to decrease stability. Hence, it is necessary to find new methods to synthesize mesoporous carbon materials with larger sizes, higher stability. Mesoporous carbon materials are said to be a good adsorbent of organic substances in water environment. However, these materials are not stability. The structure of the materials is easily broken during the reuse process and it is difficult to recover. So, the regeneration and reuse of mesoporous carbon materials are very difficult. Because of, if heat is used to remove completely adsorbed,
2 it is necessary to perform high temperature causing to burn mesoporous carbon materials. Also, the solvents are used to remove the adsorbed, resulting less-economical effect and secondary pollution. Therefore, the research to find the effective and feasible methods for regeneration and reuse of mesoporous carbon materials is necessary. From the above reasons, the thesis topic “Study on synthesis, characteristics, and adsorption properties of toxic organic substances in the water environment of mesoporous carbon materials” was studied. 2. The purpose of the thesis Study on control the process of synthesizing mesoporous carbon materials with an ordered structure, large pore size, high stability. They are as an effective adsorbent for toxic organic substances with different molecular sizes in water environment. Synthesis of mesoporous carbon materials with desired order structure, large pore size, high durability for effective adsorption of different molecular size toxic organic substances in water environment. 3. Scientific and practical significance of the thesis The thesis has found a new method to increase the pore size of mesoporous carbon by filling the liquid glass into the pore of the template (silica SBA-15) before impregnating the carbon presource to limit the penetration of carbon sealed the pore system of SBA-15. Stability of mesoporous carbon is increases due to silicon are partially retained in materials. This technique opens the way of synthesis of mesoporous carbon as an adsorbent with a desired pore size and stability.
3 Doping iron into the framework of mesoporous carbon materials creates catalysts to decompose adsorbed, release the adsorption sites, regeneration and reuse of mesoporous carbon, extend the scope of application of materials for treatment of toxic organic substances in water. 4. New findings of the thesis 1. For the first time, a new technique is used to control the pore size of the mesoporous carbon materials which is synthesized by hard – temlating method by filling the liquid glass into the pore of SBA-15 before impregnating the carbon source to prevent penetration carbon to seal the pore system of SBA-15. This technique opens new direction for mesoporous carbon synthesis technologies as the adsorbent with the desired pore size. 2. Retaining a silicon part in synthesiszed material to increase the stability of the mesoporous carbon material. 3. Using atom-planting method to put iron into framework of the mesoporous carbon material do not change the structure of the materials. Iron exists on the surface of materials in the highly dispersed Fe2O3 and FeO forms, favorable for adsorption and decomposition of methylene blue, enhance the ability of regeneration, reuse and do not cause secondary pollution. 5. The structure of the thesis The thesis consists of 140 pages with 83 figures, 31 tables. The thesis includes the following sections: Introduction (2 pages); Chapter 1: Overview (44 pages); Chapter 2: Research methods and experiment (16 pages); Chapter 3: Results and discussion (59 pages); Conclusions(2 pages); Novel scientific contributions of the thesis; List of publications; References and appendices.
4 CHAPTER 1. OVERVIEW Chapter 1 includes a general introduction of synthesis methods, application of mesoporous carbon materials and metal containing mesoporous carbon. Mesoporous carbon materials are synthesized by two methods: soft-templating and hard-templating. Metal containing mesoporous carbon materials are synthesized by two methods: impregnation and atom-planting. In this chapter, adsorption properties, application and adsorption mechanism of mesoporous carbon materials in the field of adsorption were introduced. CHAPTER 2. RESEARCH METHODS AND EXPERIMENT 2.1. Chemistry - F127 (Sigma-Aldrich); Phenol (China); Focmaldehit (China); SBA- 15, MCF (Synthesis from liquid glass - Department of Surface Chemistry - Institute of Chemistry - Vietnam Academy of Science and Technology); Refined sugar (Vietnam); Liquid glass (Vietnam). 2.2. Synthesis of materials 2.2.1. Synthesis of mesoporous carbon - Soft–templating method: Template F127; pH = 1, 2, 3; Temperature: 80 oC, 100 oC, 120 oC. - Hard-templating method: Templates of SBA-15 or MCF; Number of impregnations: 1, 2, 3; Figure 2.3. Process of synthesizing mesoporous carbon The CMQTBC(TTL) pattern is synthesized using a hard-templating method, but the liquid glass is filled into the pore of SBA-15 before impregnating the carbon source.
5 Table 2.2. Samples of mesoporous carbon 1 2 3 Materials Method pH Templating T (oC) N1 N2 CMQTBM1 1 100 - F127 - CMQTBM2; CMQTBM100 2 100 - F127 - Soft- CMQTBM3 3 100 - F127 - templating CMQTBM80 2 80 - F127 - CMQTBM120 2 120 - F127 - CMQTBC1(SBA-15) - - 1 SBA-15 0 CMQTBC2(SBA-15);CMQTBC(SBA-15) - - 2 SBA-15 0 CMQTBC3(SBA-15) - - 3 SBA-15 0 Hard- CMQTBC(MCF) - - 2 MCF 0 templating CMQTBC(TTL) - - 2 SBA-15 4 Fe-t-CMQTBC(TTL) (Impregnation) - - 2 SBA-15 4 Fe-b-CMQTBC(TTL) (Atom-planting) - - 2 SBA-15 4 1 2 3 Temperature; Number of impregnation; Number of g Na2SiO3
6 2.2.2. Synthesis of iron containing mesoporous carbon - Synthesis of Fe-t-CMQTBC(TTL) by impregnating iron nitrate 0.2 M (6% mass of Fe). - Synthesis of Fe-b-CMQTBC(TTL) by the atom-planting method. 2.3. Characterizations - Characterization techniques: XRD, SEM, TEM, BET, EDX, TA, FTIR, XPS. 2.4. Determination of the isoelectric point of mesoporous carbon 2.5. Determination of adsorption properties Langmuir, Freundlich adsorption isotherm models The pseudo-fisrt-order and pseudo-second-order adsorption kinetic models 2.6. Method of evaluating the ability to reuse materials Recover the material after adsorption and wash with water and ethanol + methanol (methanol and ethanol 1: 2 ratio, V = 60 ml) stir for 2 hours at 60 ° C. Then, the material is used to adsorb MB CHAPTER 3. RESULTS AND DISCUSSION 3.1. Synthesis of mesoporous carbon 3.1.1. Soft-templating method *) Effect of temperature 80 oC, 100 oC, 120 oC: Figure 3.1; 3.2. XRD patterns (A) and nitrogen adsorption- desorption isotherms (B) of mesoporous carbon are synthesized at different temperatures
7 Temperature increase → Brown motion increases → Self- assembly of surfactants increase → The length of the hydrophobic chain increases → pore size increases. Temperature high (over 100 o C) → evaporate water, flocculate surfactants → pore size decreases. So, the optimal synthetic temperature is 100 oC. *) Effect of pH = 1, 2, 3: Figure 3.5; 3.6. XRD patterns (A) and Nitrogen adsorption- desorption isotherms (B) of CMQTBM1, CMQTBM2 and CMQTBM3 The zero charge point of silicon is 2, if pH = 2, mesoporous carbon materials are formed according to the correct mechanism S0H+X− I (S: F127, X− Cl−; I: Si) Thus, conditions of suitable syntheting of materials are at 100 °C and pH = 2, the obtained materials have a mesoporous structure with pore size of 5.4 nm, surface area BET of 1693 m2/g . 3.1.2. Hard-templating method 3.1.2.1. Templating: using two templating with the same hexagonal structure, but the pore size of MCF is larger than that of SBA-15. Figure 3.9; 3.10. XRD patterns of SBA-15; CMQTBC(SBA-15) (A) and MCF; CMQTBC(MCF) (B)
8 XRD pattern shows that the structure of CMQTBC(SBA-15) and CMQTBC(MCF) is similar to that of SBA-15 and MCF. Figure 3.11. TEM images of CMQTBC(SBA-15) and CMQTBC(MCF) The structure of CMQTBC(SBA-15) and CMQTBC(MCF) samples have a hexagonal structure and uniform pore size (Figure 3.11 and 3.12). The pore size of CMQTBC(MCF) is larger than that of CMQTBC(SBA-15) because of the pore size of MCF is larger than that of SBA-15. Figure 3.12. Nitrogen adsorption- desorption isotherms CMQTBC(SBA- 15) and CMQTBC(MCF) Figure 3.12 shows that both CMQTBC(SBA-15) and CMQTBC(MCF) belong to type IV isotherm with a hysteresis. The pore sizes of CMQTBC(SBA-15) and CMQTBC(MCF) are in the range of respectively 4.2 nm; 5.6 nm. Figure 3.13. TGA patterns of CMQTBC(SBA-15) (A) and CMQTBC(MCF) (B)
9 TGA data show that CMQTBC(SBA-15) (complete o combustion temperature of 595 C) has higher thermal stability than CMQTBC(MCF) (552 oC) does. Therefore, SBA-15 is selected as templating to synthesize mesoporous carbon materials. 3.1.2.2. Amount (number of impregnation) of carbon source Figure 3.14. XRD patterns of SBA-15, CMQTBC1(SBA-15), CMQTBC2(SBA-15) and CMQTBC3(SBA-15) Figure 3.14 shows that all three materials CMQTBC1(SBA-15), CMQTBC2(SBA-15) and CMQTBC3(SBA-15) with the respective impregnated sample l, 2 and 3 times the carbon precursor have characteristics of mesoporous materials which are similar to those of SBA-15 material. Figure 3.15. Nitrogen adsorption-desorption isotherms (A) and pore size distributions (B) of SBA-15, CMQTBC1(SBA-15), CMQTBC2(SBA-15) and CMQTBC3(SBA- 15) Figure 3.15A shows that all four samples SBA-15, CMQTBC1(SBA-15), CMQTBC2(SBA-15) and CMQTBC3(SBA- 15) belong to type IV isotherm with a hysteresis which are typical for
10 mesoporous materials. Figure 3.15B shows that the pore distribution of CMQTBC2(SBA-15) is the narrowest with the pore size concentrated mainly in the 4-5 nm range. Thus, the most number of impregnated carbon precursor is 2. 3.1.2.3. Controlling pore size We use silicon from liquid glass to fill the pore of SBA-15 before impregnating the carbon precursor and prevent carbon from penetrating into the pore. Then the silicon is removed by HF and the obtained mesoporous carbon materials have the pore system larger than that of the initial SBA-15 (Figure 3.19). Figure 3.19. Simulate the synthesis process of CMQTBC(TTL) Figure 3.16. XRD pattern of CMQTBC(TTL) Figure 3.16 shows that CMQTBC(TTL) has a peak at very low scanning angle (below 0.5o), outside the detection threshold of the meter. Due to the small angle θ, a large distance d can be predicted, leading to large pore size. Figure 3.17. Nitrogen adsorption-desorption isotherms (A) and pore size distributions (B) of SBA-15, CMQTBC(SBA-15) and CMQTBC(TTL)
11 Figure 3.18. TEM images of CMQTBC(SBA-15) (A) and CMQTBC(TTL) (B). Figure 3.17 and 3.18 show that CMQTBC (TTL) has a large pore size (10.4 nm), fairly uniform pore. This result is consistent with the XRD analysis data. The synthesis process consists of stages (Figure 3.19): Stage 1: mixing liquid glass and SBA-15 templating obtain SBA-15(TTL) with pore of SBA-15 filled by liquid glass. Stage 2: impregnating carbon precursor onto SBA-15(TTL) and carbonization obtained C- SiO2 material. Stage 3: C-SiO2 is washed with HF 10% for the first time to obtain C3 material. Stage 4: C3 is washed with HF 10% for the second time to obtain CMQTBC(TTL) material. Stage 5: washing CMQTBC(TTL) with HF 10% 3 times obtain C5 material. Table 3.6. The characteristic parameter for porous properties of SBA-15(TTL), C-SiO2, C3 (washing HF 1st), CMQTBC(TTL) (washing HF 2nd) và C5 (washing HF 3rd). Materials SBET (m2/g) Vpore (cm3/g) D (nm) SBA-15 493 0,941 7,6 SBA-15(TTL) 1,4 0,008 27,7 C-SiO2 47,4 0,087 8,7 C3 221 0,486 11,0 CMQTBC(TTL) 772 1,698 10,4 C5 1276 4,304 15,0 From table 3.6 shows CMQTBC(TTL), is washed 2 times by HF, has a surface area SBET (772 m2/g) and a porosity Vpore (1,698 cm3/g) higher than material is no washing or washing 1 time. After the 3rd washing (C5 sample), Si is completely removed, the surface
12 area SBET and porosity Vpore increase to 1276 cm3/g and 4.304 cm3/g respectively because silicon was further removed, causing the expanding of pore, increasing in the average pore volume but the structure is less stable and the signs of structural collapse occur (Figure 3.21). Figure 3.21. SEM images of CMQTBC(TTL) and C5 Figure 3.23. TGA pattern of Figure 3.24. XPS spectra of CMQTBC(TTL) CMQTBC(TTL) Figure 3.23 shows that CMQTBC(TTL) (complete o combustion temperature of 605 C) has higher thermal stability than CMQTBC(SBA-15) (559 oC). XPS spectra (Figure 3.24) show the peaks at the energy level of 103 eV; 285 eV; 530 eV which are assigned to the presence of Si2p; C1s, O1s in CMQTBC(TTL) materials. Thus, with the technique of using pore-filled liquid glass SBA-15, synthesized MC material with large pore size (10.4 nm), surface area (772 m2/g) and high pore volume (1.603 cm3/g). Sumary: For soft-templating method: conditions of suitable synthetic materials are at 100 oC, pH = 2. The obtained materials have a
13 mesoporous structure with a pore size of 5.4 nm, porous characteristics, and BET surface area of 1693 m2/g. The order of materials is not high. For hard-templating method: - Suitable conditions for synthesizing materials: SBA-15 is template and number of impregnation is 2; - It is possible to change the pore size of materials by using different templates with different pore sizes such as SBA-15 and MCF. - filling the liquid glass into the pore of SBA-15 before impregnating the carbon source to prevent penetration carbon to seal the pore system of SBA-15 resulting the adsorbent with the desired pore size.is a new technique that has never been reported in the literature. - Stability of the mesoporous carbon materials increases due to retaining a part of silicon in the material. 3.2. Synthesis of iron containing mesoporous carbon Figure 3.25. XRD patterns of Fe-t-CMQTBC(TTL) and Fe-b- CMQTBC(TTL)( small corners) Figure 3.26. XRD patterns of Fe-t-CMQTBC(TTL) and Fe-b- CMQTBC(TTL) (large corners)
14 XRD patterns (Figure 3.25) show that the structure of Fe-t- CMQTBC(TTL) and Fe-b-CMQTBC(TTL) is similar to that of CMQTBC(TTL) (Figure 3.16), demonstrating that doping iron into the material does not affect the structure of the material. Figure 3.26 shows that Fe-t-CMQTBC(TTL) material does not have characteristic peak for iron on the material, may be small iron content below the detection threshold of XRD method or exists amorphous form. Fe-b-CMQTBC(TTL) has peaks with a value of 2θ in accordance with the standard data for the structure of Fe 2O3. This shows that with the atomic implant method, iron exists the form of oxide on mesoporous carbon. TEM images (Figure 3.27) show that the doping Fe does not change the structure of mesoporous carbon material and highly disperses iron. Figure 3.28. FTIR spectra of CMQTBC(TTL), Fe-t- CMQTBC(TTL) and Fe-t- CMQTBC(TTL) FTIR spectra (Figure 3.28) show the existence of –OH, C–H, -C=C, -C=O, and -C–O groups in structure of CMQTBC(TTL), Fe-t- CMQTBC(TTL) and Fe-b-CMQTBC(TTL). With iron containing samples (Fe-t-CMQTBC(TTL) and Fe-b-CMQTBC(TTL)) have additional peaks at 457.13 và 435.91 cm-1 assigned to peak of the link Fe–O.
15 Figure 3.29. Nitrogen adsorption- desorption isotherms of Fe-t- CMQTBC(TTL) and Fe-b- CMQTBC(TTL) Figure 3.29 shows that Fe-t-CMQTBC(TTL) và Fe-b- CMQTBC(TTL) materials have the same structure with the surface areas of 749 m2/g và 542 m2/g respectively, lower than that of CMQTBC(TTL) (772 m2/g), consistent with XRD data. The pore size of Fe-t-CMQTBC(TTL) is smaller than that of Fe-b- CMQTB(CTTL) which may be due to the pore partially covered the by iron oxide. Figure 3.30 shows the existence of element Fe in Fe-t- CMQTBC(TTL) and Fe-b-CMQTBC(TTL) with the percentage of 4,63% and 6,20%, respectively. XPS spectra (Figure 3.31) show the occurrence of peaks at the energy level 103 eV; 285 eV; 530 eV; 711 eV assigned to the presence of Si2p; C1s, O1s and Fe2p in Fe-t-CMQTBC(TTL) and Fe-b-CMQTBC(TTL). The peaks of Fe-t-CMQTBC(TTL) has peaks at 710.5 eV and 724 eV corresponding to Fe2O3 Fe2p3/2 and Fe2p1/2 structures, is not only the same Fe-b-CMQTBC(TTL) but also two peaks with a small intensity at 720 eV and 714 eV. This may be due to the process formation of CO at high temperatures (400-500 oC) which reduced Fe3+ to lower valence iron such as Fe2+.
16 Figure 3.31. XPS spectra of Fe-t-CMQTBC(TTL) and Fe-b- CMQTBC(TTL) a: Total spectra, b: Fe2p In addition, on the Fe-b-CMQTBC(TTL) spectra C1s (not shown here), there is also the appearance of the peak with power level at 291 eV. This is because the process of introducing iron at high temperatures has resulted in the process of breaking carbon creating many π-π * bonds of the material. Sumary: The addition of iron by the atom-planting method is superior to the impregnation method: iron oxide particle is highly dispersed on the CMQTBC(TTL) material. There is the formation of a new iron state Fe2+ and the π-π bond in the structure of Fe-b-CMQTBC(TTL) material is increased, the pore size is almost unchanged. 3.3. Evaluation of adsorption capacity of mesoporous carbons 3.3.1. Affecting factors Survey of factors: different adsorbents (MB and RhB), initial concentrations and pH showed that the adsorption capacity of MB, RhB on CMQTBC(SBA-15) is nearly the same, due to the surface area and the pore size of CMQTBC(SBA-15) is larger than the size of MB and RhB. MB adsorption capacity on CMQTBC(SBA-15), CMQTBC(TTL) increases when the initial MB concentration in the solution increases and it is possible to use mesoporous carbon adsorption MB in pH = 7 suitable to actual conditions, because the
17 isoelectric point of CMQTBC(SBA-15) and CMQTBC(TTL) has values of 5.5 and 5.7 respectively. 3.3.2. Study of the adsorption isotherm Table 3.10, 3.11. Langmuir, Freundlich adsorption isotherm parameters describe MB adsorption process on CMQTBC(SBA-15),CMQTBC (TTL). Model Material CMQTBC(SBA-15) CMQTBC(TTL) qm (mg/g) 398,41 476,19 KL (L/mg) 1,4022 0,4375 Langmuir 2 R 0,9992 0,9999 RL 0,00038 – 0,00077 0,00455 – 0,02235 R2 0,6394 0,7863 Freundlich n 9,4697 6,7935 KF (mg/g) 286,00 233,41 MB adsorption on CMQTBC(SBA-15), CMQTBC(TTL) materials obeys to the Langmuir isothermal model. The maximum MB adsorption capacity (qm, mg/g) on CMQTBC(TTL) is 476.19 mg/g, greater than on CMQTBC(SBA-15) of 398.41 mg/g, due to the large pore size of CMQTBC(TTL) makes the MB molecule easy to adsorb, in addition to the presence of surface functional groups as well as π-π bonds during MB adsorption on these materials and electrostatic interaction between the surface of material and adsorbent. 3.3.3. Study adsorption kinetics Table 3.13, 3.14 show that the pseudo-second-order kinetic equation is fitted to the adsorption process MB on CMQTBC(SBA- 15), CMQTBC(TTL).
18 Table 3.13, 3.14. Kinetic parameters for pseudo-fisrt-order and pseudo-second-order kinetic equations of adsorption process on CMQTBC(SBA-15) and CMQTBC(TTL) Co k1 (1/ q1e, cal qe, exp R22 k2 q2e,cal v0 R12 (mg/L) min) (mg/g) (mg/g) (g/(mg.min)) (mg/g) (mg/(g.min)) CMQTBC(SBA-15) 100 0,2869 0,0232 0,55 166,57 1 0,0450 166,67 1250 150 0,8871 0,0329 12,10 249,84 1 0,0047 250,00 294 200 0,9841 0,0183 43,56 332,58 1 0,0011 333,33 122 CMQTBC(TTL) 100 0,3128 0,0097 0,37 197,58 1 0,2601 196,08 10000 150 0,6622 0,0082 10,15 291,77 1 0,0050 294,12 433 200 0,4842 0,0043 9,06 386,54 1 0,0097 384,62 1435 2 2 R1 , R2 , k1, k2, q1e,cal, q2e,cal are correlation coefficient, the rate constant, adsorption capacity calculated according to pseudo-first-order and pseudo-second-order kinetic equations, respectively; v0: start adsorption rate.