ARTICLE pubs.acs.org/IECR
Bubbling Reactor Technology for Rapid Synthesis of Uniform, Small MFI-Type Zeolite Crystals Wei Liu,*,† Yuxiang Rao,† Haiying Wan,† Abhijeet Karkamkar,† Jun Liu,† and Li Qiong Wang‡ † ‡
Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, United States Chemistry Department, Brown University, Providence, Rhode Island 02912, United States ABSTRACT: MFI-type zeolite is an important family of materials used in today’s industries as catalysts and adsorbents. Preparation of this type of zeolite material as uniform and pure crystals with sizes from tens to hundreds of nanometers are not only desired for current catalytic and adsorption processes for enhanced reaction kinetics and/or selectivity, but also much needed for some new applications, such as CO2 capture adsorbents and composite materials. However, it has been a major challenge in the field of zeolite synthesis to prepare small crystals of MFI-type zeolite over a range of Si/Al ratios with very high throughput. In this work, a gasbubbling flow reactor was used to conduct hydrothermal growth of the zeolite crystals with controllable Si/Al ratios and crystal sizes. Distinctive, uniform ZSM-5 crystals were successfully synthesized within 2 h of reaction time, which is exceptionally short compared to the conventional synthesis process. The crystals were small enough to form a stable milk-like suspension in water. The Si/Al ratio was controlled by adjusting the growth solution composition and reaction conditions over a range from about 9 to infinity. Characterization by SEM/EDS, XRD, TEM, N2 adsorption/desorption, and NMR spectroscopy confirmed the ZSM-5 crystal structures and revealed the presence of mesoporosity in the resulting crystals.
1. INTRODUCTION MFI-type zeolite materials, such as ZSM-5, are used in several key catalytic processes in the refining and petrochemical industries because of their exceptional stability and shape-selective attributes compared to other materials.1,2 They also comprise a useful class of adsorbent materials that can distinguish individual molecules based on slight differences in shape, size, and/or molecular weight. In addition to current industrial applications, interest in small or nanosized ZSM-5 crystals has continued to evolve for new applications, such as novel catalysts/adsorbents for NOx reduction,3 CO2 capture,4 zeolite membranes,5 and highly proton-conducting materials for fuel cells and batteries.6,7 Characteristic pore structures of the MFI-type zeolite framework are represented by two arrays of intersecting channels, elliptical 10-membered rings of straight channels that are 5.3 5.6 Å [100] size and near-circular 10-membered rings of zigzag channels that are 5.1 5.5 Å [010].8,9 The Si/Al ratio in the framework can be varied over a wide range from about 9, the lower limit for the ZSM-5 family, to infinity for silicalite.1 Thus, rich catalysis and adsorption properties can be obtained by adjusting the Si/Al ratio in the lattice framework and exchanging ions in the as-prepared crystals with other cations. The molecular transport inside the zeolite channel often proceeds through a configurationdiffusion mechanism such that the diffusivity could be orders of magnitude smaller than that of free-molecule diffusion. Thus, the pore diffusion rate inside ZSM-5 crystal can become a rate-limiting step for fast reaction/adsorption kinetics. It is known that the diffusion rate (volume-based) has a secondorder inverse dependence on the crystal size. Thus, reducing crystal sizes is highly desired for catalytic reaction or adsorption applications in which pore diffusion becomes a rate-limiting step. Along with reducing the crystal size, maintaining a uniform crystal size distribution is necessary to obtain good selectivity. r 2011 American Chemical Society
Because of its industrial significance, a great amount of effort has been devoted to prepare ZSM-5 with small and uniform crystals. An exhaustive literature review on ZSM-5 synthesis was not attempted for this work. Instead, a few approaches reported in the literature are introduced here for comparison purposes. Preparation of nanozeolite by modifying and improving conventional solgel and autoclave zeolite synthesis procedures often results in agglomerates of ZSM-5 crystals over a wide range of particle sizes.10,11 Synthesis routes starting with “clear” precursor solutions have been studied for the preparation of nanosized ZSM5 crystals,1219 as compared to gelled mixtures in the conventional zeolite synthesis. According to this approach, the solution is typically heated for a few days to obtain a white suspension of small zeolite crystals, and the amount of the resulting solid powder can be low (
