versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.52 n.2 Concepción jun. 2007
J. Chil. Chem. S °C, 52, Nº 2 (2007), págs.: 1126-1129
POROSITY ENHANCEMENT IN SOME BAKED CLAY SAMPLES UNDER HYDROTHERMAL TREATMENTS
IMTIAZ AHMAD*Pa, M.SHAKIRULLAHPa, M.ISHAQPa, M.ARSALA KHANPb, HABIB UR REHMANa, MUHAMMAD OMERc AND HAMEED ULLAHPa
a :Institute of Chemical Sciences, University of Peshawar,25120, N.W.F.P, Pakistan
This paper demonstrates the chemically assisted enhancement in porosity of l °Cal clay samples activated at a temperature of 150 oC. Aluminosilicates enriched baked clay samples were used and hydro thermally treated with NaOH, KOH, Ba (OH)2 & ZrO. Pore size and surface area analyzer was used for the determination of surface area. Average pore width, pore volume and surface area were calculated. Treatment with NaOH and KOH has shown to be a very good activation pr °Cedure for high surface area development. Zirconium oxide has caused no pronounced effect on the development of surface area while barium hydroxide showed adverse effect and caused a decline in porosity.
Keywords: Porosity, clay, pillared clay; isotherm, DR method.
There is a great demand for multi scale porous materials for a variety of applications (1-6). To get improved overall reactions and adsorption/separation performances ass °Ciated with such materials, elimination of significant amount of blind or closed pores and improvement in the thermal stability to avoid collapse of pores is the f °Cus of modern research(7). Suitable adsorbents can be looked upon as sponges or mazes with large openings that lead successively to smaller and smaller channels with smaller and smaller openings. The capacity and, to some extent, the adsorptive characteristics of a given adsorbent depend on the emptiness within matrix and its surface area. The greater the surface area, the greater is the capacity; and the greater the pore volume, the greater is the efficiency. Chemical and thermal activation is a well-known method for preparing such adsorbents which has been the objectives of studies in the last few years. Selective leaching using an acid solution resulting in a maximum surface area generation in silica has been reported elsewhere (8). Some clays have been exfoliated by chemical and thermal treatment methods to obtain chemically inert adsorbent, fire-resistant, low-density materials with excellent thermal and acoustic insulation properties (9). Effect of acid treatment and alkali treatment on nanopore properties of selected minerals has been studied recently (10). Hydrothermal alteration in some clay samples has been reported elsewhere (11-14).
We report on the enhancement of porosity and changes in average pore width, micropore volume and micropore surface area as a consequence of hydrothermal treatment with sodium hydroxide, potassium hydroxide, barium hydroxide and zirconium oxide studied at mild temperature (150 oC)
2.1. Collection of Sample
The sample was collected from the local brick kiln according to the standard method of sample collection, crushed, and dried in an oven at 105 oC in order to remove the surface moisture.
2.2. Hydrothermal Treatment
One g of clay sample was treated separately with NaOH, KOH, Ba(OH)2 and ZrO (10%). 10ml of the respective solution was added to each sample and kept in an oven at 150°C for time duration of 24 hr. Each of the hydro thermally treated samples was washed with copious amount of de-ionized water, dried in a vacuum oven in order to remove all the surface and entrapped moisture. The prepared samples were kept in bottles for analysis of surface area.
2.3. Surface area analysis
For the determination of surface area, pore volume, and average pore width, the virgin as well as the hydro thermally treated samples were analyzed using pore size and surface area analyzer Model (Quanta Chrome Nova 2200e) under the conditions provided in Table-1 . The surface area, pore volume, average pore width were calculated using Dubinin Radushkevic Method (DR method). Average pore width was calculated using the formula:
3. RESULTS AND DISCUSSION
The nitrogen adsorptiondesorption isotherms of the virgin clay and variously hydro thermally treated clay samples obtained through the optimized procedure ( Table 2 ) have been provided in Figures 1-5 , which show a certain mesoporosity, as they exhibit a hysteresis loop type H3, according to the IUPAC classification, which can be associated with the presence of slit-shaped pores with parallel walls. On the other hand, the initial step in adsorption isotherms indicates the presence of a microporous structure. Different methods for analysis of isotherms were used in order to obtain more information on the porous properties of the clay under study. We have followed the Dubinin-Radushkevish (DR) equation (15). The DR plots are provided in Figures 6-10 . The pore size, pore volume, average pore width and micropore surface area were determined. The data has been assembled in Table 2 .
The surface area of virgin clay sample is 97.07m2g-1 by appling DR Equation. Upon treatment with NaOH, the surface area was increased up to 141.99 m2g-1. It is evident from the data that increase in surface area is quite significant in case of NaOH treatment. In case of KOH, the surface area was enhanced up to 131.11 m2g-1. This enhancement is quite significant and comparable with NaOH treatment. Treatment with ZrO caused a marginal increase in surface area (104.33 m2g-1). In case of Ba (OH)2, a decrease was observed in surface area when the hydrothermal treatment was performed at the same temperature and the porosity was decreased up to 91.28m2/g.
The micro pore volumes are 0.03 cm3g-1 in case of virgin clay, 0.05 cm3g-1 in case of of NaOH & KOH treated clay samples, 0.04 cm3g-1 in case of ZrO, and 0.03 cm3g-1 in case of Ba (OH)2 treated clay. It is evident from the data that the pore volume has been increased in case of NaOH & KOH treatments followed by ZrO. No change in pore volume was observed in case of Ba (OH)2 Average pore width calculated in case of virgin clay was 6.182 A0, in case of NaOH & KOH was 7.043 A0 & 7.627 A0, respectively. Incase of ZrO, the average pore width calculated was 7.668 A0 and in case of Ba (OH)2, was 7.574
The increase in surface area in case of NaOH & KOH corresponds to the surface roughness and generation of cracks due to the action of the chemicals used on clay dispersability and dissolution (16). Moreover, both sodium hydroxide and potassium hydroxide have the ability to swell the matrix which exert pressure on the pore walls and resulted in dilatations leading to emptiness in the matrix.
The marginal increase in surface area upon treatment with zirconium oxide corresponds to poor intercalation of zirconium oxide in to the natural cracks and emptiness present in the matrix. The reason is high rates of hydrolysis of ZrO which in turn results in the precipitation of agglomerated zirconia, thereby limiting its access to the pores of the clay. Zirconium oxide is itself a highly porous material and widely used as adsorbent in a variety of application (17-19). More over, zirconium oxide is often used because of high hydrothermal stability, and can have large interlayer spacing (20). However, it is not suggested to be a suitable agent for hydrothermal treatment of the clay under study.
The reduction in surface area in case of Ba (OH)2 is attributed to the gelation /thixotropic nature and reheology of Ba (OH) 2 (21) . Due to gelatinous behavior, the clay pores were blinded with colloids and the diffusion in to the matrix was limited.
Treatment with NaOH and KOH has shown to be a very good activation procedure for high surface area development in baked clay samples under study. Zirconium oxide has no pronounced effect on the development of surface area while barium hydroxide showed adverse effect and caused a decline in porosity.
The authors are grateful to Prof. Dr Mohammad Riaz, Director Centralized Resource Laboratory, Department of Physics, University of Peshawar for analysis part of this work
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