Chemical Modification Effect on the Sorption Capacities of Natural Clinoptilolite By Haldun Kurama, Andreas Zimmer, and Wladimir Reschetilowski* Dedicated to Professor Dr. Helmut Papp on the occasion of his 60th birthday This work determines the effect of chemical modification on the sorption capacity of natural Turkish zeolite, clinoptilolite, for its potential application as a sorbent. Pore size distribution and surface area are critical for assessing the suitability of the zeolite for sorbent application. Because natural clinoptilolite has small pore sizes and low surface area compared to synthetic zeolites, modification studies have been performed to improve the sorption capacity. The conversion of natural clinoptilolite to the hydrogen form has been carried out by two different ion exchange procedures, namely ammonium exchange followed by calcination and direct treatment with HCl. The natural and modified clinoptilolite samples were characterized by XRD, 27 Al MAS NMR and BET methods. 1 Introduction Natural zeolites are crystalline aluminosilicates of alkaline and alkaline-earth metals. They possess many desirable ion exchange, molecular sieving and catalytic properties, which make them valuable minerals [1]. Synthetic molecular sieves are being used extensively as adsorbents for the purification of gaseous waste in commercial applications. But certain natural zeolites such as mordenite and clinoptilolite have also made headway in these areas because of the ready availability, low prices and high Si/Al ratio. They have been used extensively to remove water and carbon dioxide from natural/waste gas streams. Another important application of clinoptilolites involves the removal of corrosive H 2 S from methane gas produced during the anaerobic digestion of sludge in municipal sewage treatment plants [2]. The availability of large volumes of internal space is one of the most desirable characteristics of zeolites for sorbent applications. Window dimensions generally depend on the structure of the zeolite, but the ªfine tuningº can be achieved by variation of the cation type which is located at preferred sites within the framework of the zeolite. Larger suitable volumes are associated with the presence of small cations including protons [1,3]. In addition, the composition of the zeolite framework (Si/Al ratio) has a significant effect on the sorptive properties of the adsorbent. There are two main types of modification procedure, which can be used to form proton exchanged clinoptilolite (H-CLN); ammonium exchange followed by calcination or direct ion exchange with dilute acid solution. The numerous applications of these methods for zeolite X, Y, ZSM-5 etc. have been reported [4±6]. Direct hydrogen ion exchange occurs readily in a zeolite to a limited extent (10±15 wt. %) with water washing. Achieve- ment of higher exchange requires use of a solution with a greater proton concentration, i.e. dilute acid. Since most zeolites are acid sensitive, their applications are limited. But silica rich zeolites such as mordenite, clinoptilolite and erionite can be directly treated with suitable reagents (e.g. acids, chelating agents, salts) to increase the sorption capacity of sorbent. In zeolites containing a high percentage silica, clinoptilolite was of special interest to early research. Selective acid treatment with strong mineral acid was first reported by Barrer and Makki [7]. They pointed out that the sorption capacity of clinoptilolite could be increased up to mild acid concentration. Since then several researchers have shown different modifications to clinoptilolites which deter- mine their catalytic applicability [8,9]. However, much of this work was not focused on the key areas which give most information about the structural and textural changes occurring during modification. The aim of this study is to investigate the effect of modification of natural clinoptilolite on the structural and textural parameters namely structure stability, total and framework Si/Al ratio, pore size and pore volume. The two most important modification methods, deammonation of the ammonium form and direct acid treatment are described in detail. 2 Experimental 2.1 Materials In this study a representative sample of clinoptilolite rich tuffs of Bigadic (Turkey) was used. Physical and chemical analyses revealed that tuffs mainly consist of clinoptilolite (87 %), quartz (1 %) and K-feldspar (11 %) and can be classified as a Ca-rich clinoptilolite (Ca-CLN). Clinoptilolite with an average particle size of 1 cm was crushed and graded with US standard mesh sieves. The 0.355 mm±2 mm particle Chem. Eng. Technol. 25 (2002) 3, Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2002 0930-7516/02/0303-0301 $ 17.50+.50/0 301 ± [*] Asst. Prof. Dr. H. Kurama, Mining Engineering Department, Osmangazi University, 26030 Eskisehir, Turkey; Dr. A. Zimmer, Prof. Dr. W. Reschetilowski, Institut für Technische Chemie der TU Dresden, D-01062 Dresden, Germany (e-mail: wladimir.reschetilowski@chemie. tu-dresden.de) 0930-7516/02/0303-0301 $ 17.50+.50/0 Full Paper