RESUMEN
This paper reports the results of an international interlaboratory study led by the National Institute of Standards and Technology (NIST) on the measurement of high-pressure surface excess carbon dioxide adsorption isotherms on NIST Reference Material RM 8852 (ammonium ZSM-5 zeolite), at 293.15 K (20 °C) from 1 kPa up to 4.5 MPa. Eleven laboratories participated in this exercise and, for the first time, high-pressure adsorption reference data are reported using a reference material. An empirical reference equation n e x = d ( 1 + exp [ - ln ( P ) + a / b ] ) c , [n ex -surface excess uptake (mmol/g), P-equilibrium pressure (MPa), a = -6.22, b = 1.97, c = 4.73, and d = 3.87] along with the 95% uncertainty interval (U k = 2 = 0.075 mmol/g) were determined for the reference isotherm using a Bayesian, Markov Chain Monte Carlo method. Together, this zeolitic reference material and the associated adsorption data provide a means for laboratories to test and validate high-pressure adsorption equipment and measurements. Recommendations are provided for measuring reliable high-pressure adsorption isotherms using this material, including activation procedures, data processing methods to determine surface excess uptake, and the appropriate equation of state to be used.
RESUMEN
The formation process of the MCM-41 precursors (silicate/surfactant complex) was investigated on the basis of the pH titration curves of Na(4)SiO(4) in the presence of [C(16)H(33)N(CH(3))(3)]Cl. Measurements of the pH titration curves were carried out using the computer-controlled gravimetric titrator constructed in our laboratory. The white precipitate (MCM-41 precursor) was abruptly formed at pH 11.1 (298 K) and at pH 9.9 (343 K). Formation of the MCM-41 precursor can be explained by coagulation of the rod-like micelle colloids whose surface is covered by the condensed silicate anions of (HSiO(3))(n)(n-). The porous texture of the MCM-41 samples whose precursors were synthesized under different conditions was analyzed on the basis of the adsorption isotherms of nitrogen at 77 K. It has been shown that the MCM-41 sample whose precursor was prepared at pH 9.9 and 343 K shows one sharp peak (r(p) = 1.65 nm) in the pore size distribution curve, but the MCM-41 samples whose precursors were prepared at pH 6.5-5.0 and 343 K give two peaks (r(p) = 1.66 nm and r(p) = 2.12-2.36 nm). The appearance of the second peak (r(p) = 2.12-2.36 nm) has been considered to be in connection with the destruction of the MCM-41 precursor into small fragments in acidic medium. Copyright 2000 Academic Press.
RESUMEN
Two kinds of the porous silicas with uniform and cylindrical pores were prepared from calcined Na4SiO4 and cationic surfactants ([C16H33N(CH3)3]Cl and [C18H37N(CH3)3]Br). In this paper, we call these porous silicas PS-C16 and PS-C18. PS-C16 was calcined in vacuo at 873-1273 K in order to prepare a series of porous silicas having different pore sizes. All nitrogen adsorption isotherms for a series of the porous silicas show no hysteresis in the adsorption and desorption branches. The nitrogen adsorption isotherms were analyzed by the t method, and the pore surface area (Sp), the pore volume (Vp), and the core volume (Vc) were determined. The pore radius (rp), the core radius (rc), and the adsorbed thickness on the pore wall (tpore) were computed from the measured values of Sp, Vp, and Vc. We can determine the relationship between the Polanyi adsorption potential and the core radius in the range of rc = 0.6-1.2 nm. On the basis of the present data, we propose a new method of calculating the pore size distribution in the range of rp = 1-2 nm.
