Existence of Colloidal Primitive Building Units Exhibiting Memory Effects in Zeolite Growth Compositions V. Radha Rani, Ramsharan Singh, Pramatha Payra, and Prabir K. Dutta* Department of Chemistry, 120 West 18th AVenue, The Ohio State UniVersity, Columbus, Ohio 43210 ReceiVed: July 14, 2004; In Final Form: October 11, 2004 This study focuses on the clear, stable mother liquors obtained after completion of crystallization of nanocrystalline sodalite, zeolite A, and zeolite Y. Characterization of the freeze-dried mother liquors by high- resolution transmission electron microscopy showed regions of crystallinity for both zeolites A and Y, and ∼5 nm regions of crystallinity could be identified in zeolite Y. Upon adding the mother liquor to nutrient solutions of aluminates and silicates, it was found that the mother liquor could direct formation of the corresponding framework, thereby manifesting a memory effect. Dynamic light scattering studies showed that there were units present in the mother liquors that grew into crystals at a faster rate than the parent compositions used to generate the mother liquors. We propose that the mother liquors recovered after crystallization contain primary structural units having memory of the zeolite formed and that these units can assemble to form viable nuclei if provided with a source of nutrients. 1. Introduction The nucleation and growth of aluminosilicate zeolites and related microporous materials continues to be an area of active research. 1 Even with modern spectroscopic and analytical measurements, the complexity of the synthesis process has eluded a molecular level understanding. The early events that lead to nucleation and subsequent zeolite growth are not well understood, especially the molecular features that direct a particular composition to a specific framework. For systems that involve transformation of a gel to crystalline phase, the problem is even more difficult. Nevertheless, advances have been made in understanding the synthesis process as well as development of new framework structures. In particular, synthesis of zeolites from “clear” solutions, 2-4 i.e., from solutions that do not lead to a gel, has made it possible to monitor the crystallization process by various physical techniques. Examples include 29 Si MAS NMR, 6,7 dynamic light scattering, 5,8 ultra-small-angle X-ray scattering (USAXS), and wide-angle X-ray scattering (WAXS). 9,10 Such studies have detected nanometer-sized precursors in the synthesis solutions, and aggregation of these units to eventually form crystals has been proposed. Most detailed studies have been reported on the silicalite-1 system. 11-13 Schoeman first reported the presence of 2.8 nm particles in a silicalite composition based on cryotransmission electron microscopy, light scattering, and chemical methods. 3,13 A more precise structural model proposed formation of nanoslabs via combination of specific “trimer” silicate species and their subsequent aggregation to form crystals. 14 However, this mechanism has come under criticism since other NMR studies have not found the large, open- framework silicate species. 15,16 More recently, transmission electron microscopy studies have also provided insight into the controversy. 17,18 High-resolution electron microscopic studies have noticed the evolution of amorphous nanometer-sized amorphous particles of 40-80 nm to crystals of zeolite A and 25-35 nm particles to zeolite Y. 19,20 AFM studies of the (111) face of zeolite Y has revealed triangular terraces 1.5 nm in height, corresponding to the width of a faujasite sheet. 21 Terraces were proposed to nucleate and grow in a layer-by-layer fashion on clean surfaces and therefore the necessity of high levels of supersaturation. On zeolite A surface, step heights of 1.2 nm, which corresponds to one-half unit cell in the (100) direction, was observed, and the crystal growth was proposed to occur with much higher rates at kink sites. 21 The role of aged reactant compositions in shortening the induction period is well-known. 10 In addition, addition of aged aluminosilicate solutions prior to synthesis as well as seed crystals is known to speed up crystallization, primarily by acting as a nuclei source, thereby shortening the induction period. 1,8 The nature of zeolitic nuclei and how they are formed is of great interest. We address this issue by investigating the proper- ties of solutions that were recovered after zeolite crystallization was complete. Trying to study nucleation by examining solutions after crystal growth is complete may sound counterintuitive, but the motivation to approach the study in this manner is be- cause of the numerous studies that suggest the presence of nanometer-sized units throughout the crystallization pro- cess. 3,14,19 In this study, we focus on nanocrystalline sodalite, zeolite A, and zeolite Y synthesized from clear solutions. Typically, the yields of zeolites from such compositions are quite low. 2-4 Our interest is in the clear, stable mother liquors obtained after completion of the crystallization of the nanocrystalline zeolites. The question we pose is the following: do these mother liquors have a memory of the zeolites? To demonstrate the manifestation of the memory effect, we have examined if solutions recovered from different zeolite syntheses can direct nutrient solutions to the corresponding framework. The dynam- ics of crystal growth was carried out using dynamic light scattering studies. Characterization of the freeze-dried solutions recovered from the crystallization was carried out with high- resolution transmission electron microscopy. From these data, we have been able to draw conclusions about the zeolite nucleation process. * To whom correspondence should be addressed. E-mail: dutta.1@osu.edu. 20465 J. Phys. Chem. B 2004, 108, 20465-20470 10.1021/jp046878k CCC: $27.50 © 2004 American Chemical Society Published on Web 11/25/2004