ARTICLES Synthesis and Structural Properties of Zeolytic Nanocrystals I. MFI Type Zeolites P. Morales-Pacheco, ² F. Alvarez-Ramirez, ² P. Del Angel, ² L. Bucio, ‡ and J. M. Domı ´nguez* ,² Instituto Mexicano del Petro ´ leo, Programa de Posgrado, Programa de Ingenierı ´a Molecular, 152 Eje Central L. Cardenas, 07730 Me ´ xico D.F., and Instituto de Fı ´sica, UNAM, Apdo. Postal 20-374, 01000 Me ´ xico D.F. ReceiVed: July 27, 2006; In Final Form: October 28, 2006 MFI type zeolite nanocrystals with a SiO 2 /Al 2 O 3 ratio of 120 were synthesized using soft-chemistry conditions, at 353 K, P ) 582 mmHg, from clear solutions of tetrapropylammonium hydroxide (TPAOH), fumed silica (CAB-O-SIL), and aluminum sulfate. The formation of crystallites with a low dimensionality (10-30 nm) was examined in function of the reaction time by X-ray diffraction (XRD), Fourier transform infrared (FTIR) (with KBr), 27 Al and 29 Si nuclear magnetic resonance (NMR) magic-angle spinning (MAS), and high-resolution transmission electron microscopy (HRTEM). Complementary information on the critical parameters of the low-dimension crystals was obtained by HRTEM and Rietveld’s method. The theoretical kinematical approach was used for calculation of the high-resolution electron microscopy images (Cerius 2 ). The physicochemical properties of the zeolitic nanocrystallites were compared with macroscopic MFI type crystals; the unit cell parameters were a ) 20.02 Å, b )19.89 Å, and c )13.38 Å, which means a slight contraction of about 0.25, 0.16, and 0.3% with respect to large MFI type crystals, respectively. HRTEM indicated the ZSM5 nanocrystals in its aluminosilicate matrix present a disklike shape, a Pnma symmetry, and pore dimensions in the (010) plane of about 0.56 nm. The mean crystallite sizes along [020] and [200] were 20 and 46 nm, respectively. The high surface charge associated with the nanocrystals influences the cell dimensions and crystal morphology, mainly. Introduction The synthetic zeolites are used widely as catalysts or adsorbents in petroleum refining, petrochemicals production, and fine chemistry. For example, FAU (Y), MFI (ZSM5), and LTA (A) are used in FCC, aromatics alkylation, and separation media, respectively. 1-5 About 133 zeolites have been reported so far, and this number tends to increase, as well as properties and applications, 6 which depend on the chemical composition (i.e., SiO 2 /Al 2 O 3 ratio), crystal size, crystal thickness, surface chem- istry, and so forth. 7-9 In this respect, the search of new properties of known zeolites continues to be a subject of scientific research, where a recent trend is the synthesis of zeolitic materials with a low dimensionality, 10 that is, 5-100 nm size range. Mintova and co-workers 11,12 reported the synthesis of colloidal crystals of FAU and LTA-type zeolites, with a mean crystal size between 40 and 80 nm, which were synthesized using 15-Crown-5 ether as a co-template. Similarly, Holmberg and co-workers 13,14 reported the synthesis of small FAU crystals of about 32-120 nm diameter, which were obtained using tetramethylammonium bromide (TMABr) as a co-template. Van Grieken et al. 15 reported ZSM-5 crystallites of about 10-100 nm diameter, which were obtained using clear supersaturated homogeneous mixtures and a crystallization time of 24 h in hydrothermal conditions. Li et al. 16 studied the nucleation and crystal growth kinetics of nanosized FAU crystallites from clear solutions, thus emphasizing the influence of the growth limiting nutrient (Na + ) on the crystallization process. Also, Verduijn and Schoeman and Sterte 17-19 reported FAU, LTA, and MFI crystallites of less than 100-nm diameter. However, the low dimensionality of the zeolitic crystallites poses some questions regarding the crystal stability, the effects of a high surface charge on the unit cell parameters or pore dimensions, and the ability of these materials to act as efficient molecular diffusion media, with respect to conventional zeolites, that is, crystallites of about 1-µm diameter. Thus, a better understanding of the influence of low crystal dimensionality on the molecular sieving and diffusion properties of these materials must be based on a detailed characterization of the physicochemical properties of the zeolitic nanocrys- tals. 20,21 In this respect, Thiele’s modulus () is a useful parameter that can be scaled down to the nanometric dimensions, that is,  ) R(k int /D) 1/2 , where R is the crystal thickness, k int is the intrinsic rate constant, and D is the intracrystalline diffusion coefficient. The diffusion efficiency defined as η ) tan()/ expresses the effect of crystal thickness on the molecular diffusion paths. 20,21 Thus, the use of crystallites with sizes down to nanometric dimensions might cause a significant decrease of the molecular diffusion paths as well as the increase of external crystal surface area (S) or the surface to volume ratio (S/V), with respect to conventional zeolites; assuming a cubic crystallite and a constant volume (V), the corresponding S/V ratio is equal to 120 µm -1 for a crystal thickness of about 0.05 * Author to whom correspondence should be addressed. E-mail: jmdoming@imp.mx. ² Instituto Mexicano del Petro ´leo. ‡ Instituto de Fı ´sica. 2368 J. Phys. Chem. C 2007, 111, 2368-2378 10.1021/jp064780v CCC: $37.00 © 2007 American Chemical Society Published on Web 01/20/2007