Synthesis and characterization of porous carbons using natural zeolites and AIPO4-5 as templates and diffusion of volatile organic chemicals in these porous media
Sakintuna, Billur (2005) Synthesis and characterization of porous carbons using natural zeolites and AIPO4-5 as templates and diffusion of volatile organic chemicals in these porous media. [Thesis]
The novel approach of template carbonization method has been successfully used in this study for the production of porous carbons templated from a Turkish natural zeolite and a microwave-assisted AIPO4-5. The effect of different templating agents and different carbonization temperatures was studied. The carbon precursor used was furfuryl alcohol, FA. The structures of the carbons produced were compared using SEM, EDS, XRD, FT-IR, 13C CPMAS, 29Si CPMAS, 27A1 MAS NMR and 19F MAS NMR and some surface analysis methods. FA was polymerized and carbonized at 700ʻC, 800ʻC, 900ʻC and 1000ʻC in the channels of natural zeolite and AIPO4-5. The doo2 values, stacking heights of graphene sheets, Lc, lateral size, La, number of graphene sheets per stack of resultant carbons were calculated from the Debye-Scherer equation. BET surface areas of natural zeolite templated carbons were 397, 350, 405 and 367 m2/g at 700, 800, 900 and 1000ʻC, respectively. By using sol-gel method, 804 m2/g BET surface area was obtained. EDS analyses showed that 91-99% C was obtained at 700-1000ʻC. In 13C NMR, sp, sp2 and sp3 hybridized carbons were observed. Pore diameters were 11 nm, indicating the mesoporosity. Lc values and average number of graphene sheets per stack were increasing with carbonization temperature. During removal of the zeolite template by washing with HF, some organoaluminium fluoride flower-like structures were observed in the SEM images which consisted of mainly fluorine, carbon and aluminium. According to 19F MAS NMR spectra of these carbons, AI2F2, AIFC3, AIF2C2, and AIF3C groups were observed. In XRD pattern, peaks were belonging to aluminium hydroxide fluoride. The structural differences in the natural zeolite templated carbons obtained after HC1, NaOH, HCl-NaOH, and HF-NaOH washing solutions besides HF were investigated. HF washed porous carbon had the highest surface area with 397 m /g. The systematic study of the parameters of the microwave synthesis of AIPO4-5 was examined for the use of further templating purposes. Powdered AIPO4-5 crystals of high quality could be synthesized using a microwave heating technique. Usage of microwave heating drastically reduced the crystallization times. The crystal growth depended on the nitial gel composition. Perfect hexagonal AIPO4-5 products formed with ca. 5 (u)j,m length with characteristic morphologies of AIPO4-5 crystals. BET surface area of selected AIPO4-5 for the further steps was 107 m /g and average pore diameter was 1.7 nm. ET ur ace areas of AIPO4-5 templated carbons were measured as 149, 125, 122 and 108 m2/g at 700, 800, 900 and 1000ʻC, respectively. The surface areas of natural zeolite templated porous carbons were higher than those of AIPO4-5 templated porous carbons, due to higher accessibility to the pores in natural zeolite. Average pore diameters of AIPO4-5 templated porous carbons were 1.7 nm, indicating microporosity. Carbon contents of the porous carbons detected in EDS were 96-82% at 700-1000ʻC. As observed in the natural zeolite templated carbons, Lc values and average number of graphene sheets per stack were increasing with carbonization temperature. The doo2 values of the AIPO4-5 templated porous carbons synthesized were 0.358-0.363 nm at 700-1000ʻC, respectively. Diffusion of volatile organic chemicals in natural zeolites and natural templated porous carbons was investigated. Diffusion coefficients, mode of transport and activation energies of diffusion of methanol, ethanol, n-propanol, i-propanol and n-butanol into the porous structure of a Turkish natural zeolite and natural zeolite templated porous carbons were measured in the range of 24.0-28.0ʻC. As the molecular weight of the alcohols increased diffusion coefficients into natural zeolite and natural zeolite templated carbons decreased, activation energy for diffusion increased, and time necessary to reach equilibrium increased. The diffusion constants increased linearly with an increase in the temperature. The diffusion of alcohols into zeolite and porous carbons obeyed the anomalous transport mechanism. Diffusion rate constants slightly increased as the temperature was increased. The calculated coefficients of diffusion of volatile molecules in the porous carbons were lower than that of observed in the natural zeolite. It is interesting to compare the activation energies measured in natural zeolite and natural zeolite templated porous carbons. The activation energies for diffusion of methanol were 90.1 kJ/mol and 220.7 kJ/mol for natural zeolite and porous carbon, carbonized at 700ʻC, respectively. This is most likely due to the polarity of alcohols and stronger interaction with carbon surface.
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