Effect of Transition-Metal Cations on the Adsorption of H 2 S in Modified Pillared Clays Danh Nguyen-Thanh † and Teresa J. Bandosz* ,‡ Department of Chemistry, The City College of the City UniVersity of New York, and The Graduate School of the City UniVersity of New York, New York 10031 ReceiVed: October 31, 2002; In Final Form: March 26, 2003 Sodium montmorillonite was intercalated with the Keggin ion (hydroxyaluminum polycation) and calcined at 400 °C. Then, doping with Fe 3+ , Cu 2+ , or Zn 2+ was carried out. On the obtained samples, H 2 S breakthrough capacity tests were carried out under wet conditions. Doping with metals significantly improved the capacity of adsorbents, despite a noticeable decrease in microporosity. The sample doped with copper showed the best performance as a hydrogen sulfide adsorbent. The reason for this is likely the affinity of the metal to bind hydrogen sulfide, the degree of metal dispersion, and the accessibility of small pores. Introduction From their first syntheses during the late 1970s, 1-3 PILCs (pillared interlayered clays) have attracted a lot of attention and have been the subject of numerous studies. They are generated through the exchange of the small charge-compensating cations (usually alkali) of the clays, situated between the negatively charged clay layers, with metal polyoxycations. 4,5 Calcination at 400 °C or higher temperatures leads to the dehydration and dehydroxylation of hydroxycations and their conversion to metal oxide pillars separating the clay layers. 4,6,7 Hence, for example, pillaring a clay with Al implies first the intercalation of the Keggin ion [AlO 4 Al 12 (OH) 24 (H 2 O) 12 ] 7+ 8,9 and then the forma- tion of Al 2 O 3 pillars as a result of heat treatment. 9-12 Pillaring of clay with metal oxides results in the appearance of new and interesting properties. First, the presence of these large pillars instead of alkali cations in the clay interlayer space causes the development of micropores (pores with a width smaller than 20 Å). 6,13,14-23 Another valuable feature of PILCs is their acidity, whether it is of the Brønsted or of the Lewis kind. Even though clays originally have Brønsted acidic character, 8 introducing metal oxide pillars, especially aluminum, brings Lewis acid sites. 12 These properties explain why PILCs have been so extensively studied. They are potentially good materials for many industrial applications, mostly in cataly- sis 4,6,24 and gas adsorption. 7 But so far, most studies of PILC applications have been on their use as either catalysts or as catalyst supports. Meanwhile, studies using PILCs as gas adsorbents have been rather scarce. 25 Research on PILCs is currently going in new directions, one of them being the doping of PILCs with metallic cations. 12,26 This is done to introduce new acidic or oxido-reductive properties into the clay. In this study, we synthesize transition-metal-doped Al PILCs and test them as adsorbents of hydrogen sulfide. Earlier studies showed that various adsorbents (carbonaceous and mineral) modified with metal cations have improved capacities for H 2 S adsorption. 27-29 The metal cations used here for doping are Fe 3+ , Zn 2+ , and Cu 2+ . They were chosen by taking into account either oxidation-reduction properties (iron) or the affinity of metals to form stable sulfides. Experimental Section Materials. Sodium Montmorillonite (Na-M). The same original (unmodified) clay was used for the preparation of all of the samples. It is a Na-rich montmorillonite from Crook County, Wyoming purchased from The Source Clay Minerals Repository from the University of Missouri in Columbia. In all experiments, it was used as received without any purification. Al-Pillared Montmorillonite (Al-M). For Al pillaring, a 50 wt % solution of chlorhydrol from Reheis Inc. was used. It is a solution of aluminum chlorohydrate, which has a molecular formula of Al 2 Cl(OH) 5 ‚(2.5H 2 O). The chlorhydrol was diluted with distilled water to give a 0.2 M solution with respect to aluminum content. The montmorillonite clay sample (20 g) was dispersed to water and aged for 24 h, and then the intercalating solution was added dropwise, giving a final Al/clay ratio of 20 mmol/g. The obtained suspension was stirred for 24 h at room temperature. Excess pillaring agent was washed out with distilled water until its conductivity reached 30 μS. After drying, the intercalated montmorillonite was obtained. The samples were calcined at 400 °C for 4 h. This process led to the formation of pillared montmorillonite, which is referred to as Al-M. 1,2,13,26,30,31 Me-Doped Al-Pillared Montmorillonite (MeAl-M) with Me ) Fe 3+ , Zn 2+ , Cu 2+ . A 0.2 M NaCl solution (1 L) was prepared. Al-M (10 g) was dispersed into that solution with a Na/clay ratio of 20 mmol/g. The suspension was heated to 60 °C and stirred until all of the clay was well-dispersed. At that point, the pH of the solution was around 4.6. A dilute solution of 0.05 M NaOH was added dropwise to raise the pH of the suspension to 9. Once that pH was reached, small amounts of NaOH were added to maintain the pH at 9. After 24 h of stirring under these conditions, the suspension was washed by dialysis until the conductivity of the water was below 5 μS. Then the clay suspension was filtered and dried. A 20-g sample of treated Al-M was dispersed in 1 L of water, and 0.2 M solutions of either Fe 3+ , Zn 2+ , or Cu 2+ were added dropwise until the Me/ clay ratios were equal to 20 mmol/g. After the addition of the Me (iron, zinc, or copper) solution, the suspensions were stirred for 24 h. Then the clays were washed out using a dialysis membrane, filtered, dried, and calcined at 400 °C for 4 h. After * To whom correspondence should be addressed. E-mail: tbandosz@ ccny.cuny.edu. Tel: (212) 650-6017. Fax: (212) 650-6107. † Department of Chemistry. ‡ The Graduate School of the City University of New York. 5812 J. Phys. Chem. B 2003, 107, 5812-5817 10.1021/jp0223509 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/23/2003