FORMATION OF CRYSTAL STRUCTURES DURING ACTIVATED CARBON PRODUCTION FROM TURKISH ELBISTAN LIGNITE

Transcription

1 FORMATION OF CRYSTAL STRUCTURES DURING ACTIVATED CARBON PRODUCTION FROM TURKISH ELBISTAN LIGNITE Billur Sakintuna 1, Sevil Çetinkaya 2, and Yuda Yürüm 1 1 Sabanci University Faculty of Engineering and Natural Sciences Tuzla, Istanbul 34956, Turkey 2 Hacettepe University Department of Chemistry Beytepe, Ankara 06532, Turkey

2 Introduction The physical and chemical characteristics of activated carbons make them interesting materials for use in many industrial processes. They are prepared by carbonization and activation of natural carbonaceous materials, coal is widely used as a precursor. The sorbent characteristics of activated carbons are related to their high surface area and pore volume and their particular pore size distribution.

3 Chemical and physical activation: Physical activation: thermal treatment in an inert environment and activation of the resulting char by steam or CO 2. Removal of mineral constituents resulted with better adsorption capacity (impurities were trapped on all ranges of carbon pores). Acid treatments of carbons are known to produce significant changes in carbon surface chemistry by removing of minerals

4 The turbostratic structure of carbon was suggested by Warren in Currently used X-ray diffraction techniques for the measurement of graphene sheet size and turbostratic crystallite thickness were developed by Warren in The most widely accepted model of the structure of turbostratic carbon the atoms are arranged in layers but stacked randomly instead of the order ABABA... Sequence of graphite and interlayer spacings also occur randomly. With increased carbonization temperature, decreased crystallite boundary area (from turbostratic crystallite growth) and increased graphene sheet. Warren BE. X-ray diffraction of carbon black. Phys Rev1934;2(9):551 5 Warren BE. X-ray diffraction in random layer lattices. Phys Rev 1941;59(9):693 8

5 Experimental Procedure Materials : Elbistan lignite was sieved to a particle size 100µm Washed with 1 h in 5N HCl and 1h in 22N HF Carbonization Activation : Raw, HCl and HCl-HF treated coal samples dried at 100 C an inert atm. Heated at 700, 800, 900 and 1000 C under N 2 flow (100 ml/min) : Carbonized samples activated under CO 2 flow (100 ml/min) Surface Analysis : ASAP2000 Accelerated Surface Area and Porosimetry (Relative pressure range : ) X-Ray Diffraction : Bruker axs Advance powder diffractometer, Siemens X- ray gun, Cu Kα, Bruker axs Diffrac PLUS software (0.3 step angle, 7s step time, 2θ = 5-80, generator: 40kV at 40mA)

6 Elbistan Lignite contains a high amount of mineral matter, which could effect the pore structure development and the adsorptive properties of the activated carbon. Proximate and Ultimate Analysis of Elbistan Lignite Proximate Analysis Volatiles Fixed Carbon Ash Moisture Ultimate Analysis C H N S O (by difference) %, dry %, dmmf

7 XRD of Raw Elbistan Lignite

8 Q C Q : Quartz, SiO 2, Will et al. (1988) C : Calcite, Ca(CO 3 ), Wartchow et.al, (1989) G: Gypsum, Ca(SO 4 ).2H 2 O, Pederson et.al, (1982) P: Pyrite, FeS 2, Anderson et.al, (1933) CL: Clay minerals, Ca-Mg-Al-Si-O, (1934) K: Kaolinite, Al 2 Si 2 O 5, Melka et.al., (1946) Ar : Aragonite, Ca(CO 3 ), Negro et.al., (1971) F: Feldspar, Grundy et.al. (1974) G G F CL Q CL C Cr Cr K F CL F Q G C G Cr G C C Q Ar P CL P Ar Q F Ar C Cr F Q P F C G C Q K Ar CL Cr : Cristobalite, SiO 2, Lacks et.al. (1993) C P C C Ar P K P CL Ar Ar Cr Cr Q Will et.al.j.appl.cryst. v.21, p.182, 1988 Melka et.al. Mineral. Mag., v.27, p.242, 1946 Wartchow et.al. Kristallogr. v.186, p.300, 1989 Negro et.al. Am. Mineral v.56, p768, 1971 Pederson et.al. Acta Crystollogr. v.38, p.1074, 1982 Grundy et.al., Am. Mineral v.59, p1319, 1974 Anderson et.al. Am.J.Sci. v.25, p.317, 1933 Lacks el.al, Phys. Rev. B. v.48, p.2889, 1993 Kristallogr. v.87, p.140, 1934 Karayiğit et.al., Int. J. Of Coal Geology, 47 (2001) 73-89

9 Carbonized Samples 900 C 800 C 700 C Raw Dominant components : Calcite and Quartz Carbonization : cleavage of bonds, loss of small molecules, bond reforming

10 Carbonized Samples Carbon Graphite 900 C 800 C 700 C Raw

11 Activated Samples 1000 C 900 C 800 C 700 C Raw The effect of carbonization and activation temperature on the inorganic components of the lignite is minimal.

12 BET Surface Area of Carbonized and Activated Samples BET BET Surface Areas (m2/g) raw raw Temperature ( C) ( C) Carbonized Activated Carbonization and activation caused to open closed or non-accessible pores andtowiden thepores.

13 XRD of raw samples activated and carbonized at 800 C Activated Carbonized Raw

14 XRD of raw samples activated and carbonized at 800 C Graphite Carbon Carbon Activated Carbonized Raw

15 XRD of raw samples activated and carbonized at 900 C Activated Carbonized Raw

16 XRD of Raw Elbistan Lignite, HCl and HCl-HF Treated HCl-HF HCl Raw The treatment with HCl caused removal of the main part of calcite, gypsum and cations in an exchangeable form associated with the inorganic structure, such as alkaline earths and alkalis, which are soluble in HCl solutions. Treatment with HCl-HF caused the removal of quartz and clay minerals. The presence of the small peak is possibly due to the a stable element which is formed by the reaction between calcium and fluoride ions.

17 700 C Carbonized, HCl and HCl-HF Treated HCl-HF HCl Raw This is also consistent with the results of Gao and Wu, HCl treatment could lead to the decomposition of the carboxyl and lactone surface groups while increasing the concentration of the highly stable compounds, phenolic and carbonyl groups. HF produces an acidic surface, a large amount of oxygen-containing complexes were removed from the carbon surface. Gao Z, Wu Y. React Kinet Catal Lett 1996;159:359, Zhu et.al., Carbon 38 (2000)

18 XRD of 800 C Carbonized, HCl and HCl-HF Treated Samples HCl-HF HCl Raw

19 XRD of 900 C Carbonized, HCl and HCl-HF Treated Samples HCl-HF HCl Raw

20 XRD of 700 C Activated, HCl and HCl-HF Treated Samples HCl-HF HCl Raw

21 XRD of 800 C Activated and HCl Treated Samples HCl Raw

22 XRD of 900 C Activated, HCl and HCl-HF Treated Samples HCl-HF HCl Raw

23 BET Surface Area of HCl and HCl-HF Treated Carbonized Samples BET BET Surface areas (m2/g) HCl HCl -- HF HF treated treated carbonized carbonized HCl HCl treated treated carbonized 1000 carbonized Carbonization Temperature ( C) ( C) The dominant effect of acid treatment is pore opening. Davini et.al., Carbon 39 (2001)

24 BET Surface Area of HCl and HCl-HF Treated Activated Samples BET Surface areas (m2/g) HCl HCl treated treated activated Activation Temperature ( C) HCl HCl -- HF HF treated treated activated As activation temperature increases both the sum of micropore and mesopore volume and the contribution of the micropores in the sum of pore volume was increased, indicating the development of microporous structure, due to the absence of inherent inorganic compounds that impede the development of pores. The dominant effect of acid treatment is pore opening.

25 Carbonized and HCl-HF Treated Samples An Ac An N N An An N An Ac An Ac An An:Anthracene, C 28 H 20 N: Naphthalene, C 10 H 8 Ac: Acenaphthene, C 12 H 10 N An An N An An N N N N 1000 C 900 C 800 C 700 C The diffraction line at 2θ = 24 represents (002) reflection of graphite. The diffraction line at 2θ = 44 arises from very broad, in plane (hk0) and mixed (hkl) diffraction lines. These are irregularly arranged in 3-D so that all X-ray diffraction peaks are very broad.

26 HCl-HF Treated and Carbonized Samples 700 C 1000 C

27 FWHM, L a and L c Calculations (002) (10) The position of the interplanar spacing d 002 obtained by direct application of Bragg s law. Mean crystalline dimensions obtained from Debye-Scherrer Equation. L c = 0.90λ / β cosθ 002 L a = 1.94λ / β cosθ 10 n = L c /d For the dimension of turbostratic crystallites perpendicular to the graphene sheets L c,(002) data is used and const. equals 0.9. For the dimension in graphene sheet planes L a, (100) data is used and const. equals β : Full Width Half Maxima, FWHM (in radians of theta) n : # of graphene sheets

28 La La (nm) Carbonization Temperature ( C) ( C) La La d 002 spacing is app FWHM FWHM (degrees (degrees of of theata) theata) FWHM (Degrees of of theata) HCl-HF treatments had the positive effect on the size of particles, L a of graphene sheets steadily increased with T carb. Growth of aromatic layers, Higher C content lower H content.

29 Lc Lc (nm) (nm) Carbonization Temperature ( C) ( C) Lc Lc L c values increased with T carb. FWHM FWHM (degrees (degrees of of theata) theata) Increasing in L a and L c indicating a certain increase in structural crystalline order. B.Feng et.al. Carbon 40 (2002) FWHM (Degrees of of theata)

30 Lc Lc (nm) (nm) Lc Lc (nm) (nm) C C C C C activated activated HCl- HCl C activated activated HCl- HCl- HF HF treated treated HF HF treated treated FWHM FWHM (Degrees (Degrees of of theata) theata) Carbonized Carbonized (Original) (Original) Activated Activated (Original) (Original) FWHM FWHM (Carbonized) (Carbonized) FWHM FWHM (Activated) (Activated) FWHM FWHM (degrees (degrees of of Theata) Theata) Lc Lc (nm) (nm) L c growth effect would cause a lower FWHM.

31 The rigid covalent structure of the disordered carbon cannot rearrange during decomposition to allow the uniform growth of turbostratic crystallites. Instead, some graphene sheets grow extensively, and other sheets become terminated and pinned by structural defects. The conversion of low-density disordered carbon into high-density graphene sheets causes the volumetric contraction observed during carbonization. Disordered carbon phase Growth of graphene sheets at high T Graphene sheets in turbostratic crystallites present at low T. Porosity Kercher and Nagle, Carbon 41(2003) 15-17

32 Average Average #of #of graphene graphene sheets sheets per per stack stack for for scherrer's scherrer's approach approach Carbonization Carbonization Temperature Temperature ( C) ( C) Carbon atoms in one layer interact with carbon atoms in adjacent layers by means of dispersion interactions, but which only become larger number of layers. N and BET surface area were almost inversely proportional. H.Darmstadt et.al. Carbon 38 (2000)

33 Conclusions A treatment technique involving three sequential stages (demineralization, carbonization and activation) was used for the production of carbonized and activated carbons from Elbistan Lignite. After carbonization and activation processes intensities of carbon crystallites detected in the XRD were increased. Treatment with acid solutions was proven effective in removing inorganic constituents from lignite. Upon carbonization, activation and acid treatment, BET surface areas of the samples were increased since inorganic compounds have been removed.

34 Conclusions After HCl-HF treatment, and carbonization, increasing T carb resulted in new crystallite structures HCl-HF treatments had the positive effect on the size of particles, L a and L c values of graphene sheets steadily increased with T. Average number of graphene sheets increased with T The increase of BET surface areas indicated that treatment with acid, was useful for increasing porosity which was advantageous for catalytic and absorptive purposes. Crystalline structures in acid treated samples formed with temperature increase.

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