Venezuelan natural alumosilicates as a feedstock in the synthesis of zeolite A F.E. Imbert, C. Moreno, and A. Montero Laboratorio de Cindticajy Catdlisis, Departamento de Biologia y Quimica, Ndcleo Universi- tario Rafael Rangel de Trujillo, ULA. Trujillo, Venezuela B. Fontal Laboratorio de Quimica Organometdlica, Departamento de Quimica, Facultad de Ciencias, Universidad de Los Andes, Mgrida, Venezuela j. Lujano Secci6n de Catdlisis Aplicada, Intevep S.A., Caracas, Venezuela Zeolite A has been synthesized from low-cost natural aluminosilicates. The samples were characterized by X-ray powder diffraction, chemical analysis, electron microscopy, FTi.r. spectroscopy, and nitrogen adsorption. Studies of calcination temperature effects and Na20/SiO2 and H20/Na20 ratios were performed. It was observed that mineral calcination at temperatures in the range of 500-850°C led to zeolite A formation, but when the calcination step was omitted, hydroxysodalite was formed. In samples calcined for 5 h at temperatures higher than 850°C, no zeolite was observed. A maximum in crystallinity was obtained for samples calcined in the range 700-750°C. The zeolite crystal mean size increased as the alkalinity increased. I.r. spectroscopy was used to characterize the products. An area band ratio at 570 and 470 cm -1 showed a linear correlation with % crystallinity determined by XRD. Keywords: Zeolite A; kaolin; calcination temperature effect; i.r. INTRODUCTION Zeolite A, due to its unique structural properties, is extensively used in adsorption and ion-exchange pro- cesses, such as drying gases and liquids, separation of normal from branched paraffins (isosieve process), and recently, as a detergent builder as a polyphos- phate substitute. 1-5 The use of synthetic zeolites as builders in detergent formulation has led to an in- crease on their production 5 from 1.46 x 105 metric tons in 1978 to 7.2 x 105 metric tons in 1990. Zeolites can be synthesized hydrothermally from aluminosili- care gels prepared by reacting a silicon salt or col- loidal silica with an aluminum salt in an alkaline medium, as well as from mineral clays.l-9 However, the synthesis of zeolite A from kaolin has been widely reported in the patent literature, 6 and very few papers are found in the current literature. 4,5'1°.I1 I.r. spectroscopy has been applied to characterize zeolite structures 2"12 and to investigate NaA zeolite crystallization mechanism.13 In this work, zeolite A was synthesized from Venezuelan kaolin Address reprint requests to Dr. Imbert at the Laboratorio de Cin6tica y Cat-~lisis, Departamento de Quimica, Facultad de Ciencias, Universidad de Los Andes, M~rida 5101A, Venezuela. Received 18 February 1993; revised 13 January 1994; accepted 20 January 1994 © 1994 Butterworth-Heinemann and the effects of calcination temperature, digestion time, and alkalinity on the zeolitization process were examined. The crystallization process was investi- gated by i.r. spectroscopy. EXPERIMENTAL Materials A Venezuelan kaolin (China Clay Guayana C.A.), white powder, 75 ~tm (200 mesh), sodium hydroxide pellets (Quimicas RB C.A.), and distilled water were used. Method of synthesis Zeolite A synthetic procedure comprises three steps: (a) sample calcination in the 500-1000°C range for 5 h; (b) sodium hydroxide solution digestion at room temperature for 0.5 to 24 h; and (c) crystalliza- tion at 80 and 98°C. The reaction mixtures were prepared by calcined kaolin suspension in a sodium hydroxide solution with the following oxide mole ratios SiO2/A1203 = 2.62, Na20/SiO2 = 0.36-1.04, and H20/Na~O = 29-43. The material obtained after crystallization was filtered, washed with distilled water up to pH 9, oven-dried at 120°C for 2 h, slightly milled, and weighed to determine the difference in respect to the original weight. The reaction yield was 374 ZEOLITES, 1994, Vol 14, June