APPROACH TO A DESCRIPTIVE MODEL OF CHARGE REDUCTION IN VERMICULITE BY HYDROTHERMAL TREATMENT A NA M. C AMPOS { ,S ONIA M ORENO, AND R AFAEL MOLINA* Estado So ´lido y Cata ´lisis Ambiental (ESCA), Departamento de Quı´mica, Facultad de Ciencias, Universidad Nacional de Colombia, AK 30 No. 45-03, Bogota ´, Colombia Abstract—Vermiculites have the potential to serve as effective catalysts if pillared with Al, but their high charge presents an obstacle to the pillaring process. The purpose of this study was to submit natural vermiculite to thermal treatments in the presence of water vapor in order to effect a reduction in the global negative charge and thereby to enhance its susceptibility to pillaring. The process of charge reduction in vermiculite under the conditions selected involved the extraction of 25% of IV Al accompanied by the extraction of structural Mg and charge-compensating cations (Ca 2+ , Na + , and K + ). The results indicate a reduction of 35% in the global negative charge in vermiculite by the end of the treatment. Some of the VI Al content was not removed during acid washing, and probably remained in the solid in structural positions in the octahedral sheet. Key Words—Hydrothermal Treatment, Layer Charge, Pillared Clay, Vermiculite, Water Vapor. INTRODUCTION The large negative charge generated by the elevated isomorphic substitutions of silica by Al in the tetrahedral sheet in vermiculite is an obstacle to the conventional cationic exchange process, which is the first step to obtaining a pillared clay. The limited number of attempts to pillar vermiculite is a result of this problem (d’Espinose de la Calleire and Fripiat, 1991; Michot et al., 1994; Campos et al., 2005). Recent development of efficient methods to achieve charge reduction have allowed for the pillaring of vermiculite, e.g. acid treatment (del Rey-Perez-Caballero and Poncelet 2000; Chmielarz et al., 2008), liquid-phase ultrasonic treat- ment in the presence of H 2 O 2 (Jime ´nez de Haro et al., 2005; Wiewio ´ra et al., 2003), and most recently the application of a hydrothermal treatment (HTT) (Cristiano et al., 2005). The immediate consequence of submitting vermicu- lites to a HTT process is an efficient reduction in the negative charge, which makes possible a cationic exchange process and subsequently the pillaring process, as observed by Cristiano et al. (2005) and by Campos et al. (2008). In order to study the HTT process, systematic study of the main parameters involved is essential to determine their effect on the structural and catalytic properties of the pillared mineral. The aim of the study was, therefore, to study the application of HTT to natural vermiculite, emphasizing the effects of both the relative humidity and the temperature. The particle size of the untreated material and water-vapor-flow effects on the HTT process were reported by Campos et al. (2008). EXPERIMENTAL The initial material, labeled V, is a natural Colombian vermiculite (from the Santa Marta region) which has been fully characterized (Campos et al., 2005; Cristiano et al., 2005; Campos, 2007). The structural formula for the mineral is [(Si 3.04 Al 0.92 Ti 0.04 )(Al 0.11 Fe 3+ 0.35 Fe 2+ 0.07 Mg 2.41 Mn 0.003 ) O 10 (OH) 2 ]Ca 0.21 K 0.05 Na 0.10 (Campos, 2007). The V was submitted to HTT under different conditions selected from a previous study (Campos et al., 2008) in order to assess the effect of both the relative humidity and temperature on the negative charge density and the capacity to generate pillaring structures. To assess the pillaring process, the solids were pillared with Al. Evaluation of the thermal effects was achieved mainly through use of X-ray diffraction (XRD), and of the catalytic performance by heptane hydro-conversion. Some solids were selected for detailed characteriza- tion by means of cation exchange capacity (CEC), X-ray fluorescence (XRF), N 2 adsorption/desorption isotherms, Al nuclear magnetic resonance 27 Al-NMR, X-ray photoelectron spectroscopy (XPS), and diffuse reflec- tance infrared spectroscopy (DRIFT). Hydrothermal treatment In the HTT process the material was loaded into a fixed-bed reactor (15 g of clay) and submitted to the desired temperature at a heating speed of 5ºC min À1 . Once the temperature had been reached, a N 2 flow with water vapor, generated through a saturator filled with distilled water at a temperature required to obtain the desired relative humidity in nitrogen, was circulated for * E-mail address of corresponding author: ramolinag@unal.edu.co { Present address: Departamento de Ciencias Ba ´sicas, Universidad Jorge Tadeo Lozano, Bogota ´, Colombia DOI: 10.1346/CCMN.2010.0580110 Clays and Clay Minerals, Vol. 58, No. 1, 97–109, 2010.