Nanoscale Reactor Engineering: Hydrothermal Synthesis of Uniform Zeolite Particles in Massively Parallel Reaction Chambers

Abstract

Nanocasting is a powerful technique for molding materials with nanostructures that are dictated by a porous preform, such as a micro-, meso-, or macroporous solid.[1] The pores are filled with precursors for the desired product. After processing and removal of the preform, replica structures are obtained. Micromolding techniques in inverse opals or "lost wax templating" methods have been used to generate replica opal structures or monodisperse spherical particles that resemble the shape of pores in the inverse opal.[2] The concept of synthesizing nanoparticles in confined environments [3] has also been applied to block copolymer micelles, miniemulsions, polymer microgels, and similar "soft" molds. [4] Among the range of materials prepared by nanocasting, nanosized [5] and uniformly shaped [6] zeolite particles have been synthesized in the confinement of mesoporous carbon, which forms a matrix that is resistant to hydrothermal (HT) processing conditions and is easily removed by combustion. [7] In those studies, the role of the porous carbon matrix was mainly to physically confine the product within the pore space in order to impose the shapes and dimensions of the pores onto the product structure, irrespective of the gel composition.[5] Herein we demonstrate that three-dimensionally ordered macroporous (3DOM) carbon matrixes can act as massively parallel reaction chambers for high-yield HT syntheses of zeolite particles with uniform sizes (in this case the molecular sieve TPA-silicalite-1); the product morphology is controlled not only by the shape of the macropores but also by several parameters that can be designed into the nanoreactor structure or adjusted during HT processing. These include the charge of polyelectrolytes deposited on the surface of the porous host, the precursor concentration, the number of infiltration steps, the proximity of a pore to the monolith exterior, and the dimensions of windows that connect adjacent pores and define transport properties. Depending on these parameters, the products can be uniform solid spheres; geode-like, hollow zeolite spheres; or colloidal crystal arrays of such spheres. The sphere surface may be smooth or corrugated. Under specific conditions, needleshaped zeolite particles can also be manufactured. Such particles with controllable shapes and defined dimensions are of great interest for preparations of hierarchically structured zeolite catalysts. This nanoreactor engineering approach promises improved control over the morphology of materials prepared by confined syntheses, and the design principles should also be applicable to HT syntheses of other materials.3DOM carbon [8,9] is a suitable reactor for HT processing of small particles because of its unique architecture, its chemical stability under HT synthesis conditions, and because it can be prepared in monolithic form, which facilitates removal of any excess deposits on external surfaces. 3DOM C was synthesized by an established colloidal crystal templating method using a resorcinol formaldehyde precursor.[...

Growth Patterns and Shape Development of Zeolite Nanocrystals in Confined Syntheses

“…Spheres can be isolated by calcination and extended sonication. 26 For a detailed investigation of crystallographic features in these templated zeolite particles, the sample obtained after L 11 was analyzed by high-resolution TEM. Figure 3A shows a TEM image of a particle that had filled a whole macropore and therefore took on the spherical shape of the macropore.…”

“…In our previous work, which employed only precursor H, a large number of seeds was produced in the initial IHT step and grew into irregular particles and then polycrystalline spheres after multiple cycles. 26 In contrast, using a sequence of an initial H 1 step and subsequent L cycles made it possible to produce small, well-faceted zeolite crystals. Reaction conditions favored nucleation during cycle H 1 and produced irregular zeolite particles ca.…”

“…10-40 nm in length (average length 24.5 ( 5.4 nm) throughout most of the 3DOM carbon monolith ( Figure 4A), that is, slightly smaller than after an identical hydrothermal reaction without confinement (average diameter 67.7 ( 8.7 nm, Figure S2B). Outermost layers of the monolith containing more completely filled macropores 26 were removed by mechanical polishing before continuing with L n cycles. The new conditions favored crystal growth from the original nuclei.…”

Effective use of flexible low-dimensional colloidal particles and colloidal crystals for the control of hierarchically porous materials

“…Stein et al have deposited zeolites in 3DOM carbon to provide periodically arranged spherical zeolite particles, which have meso/macropores among the particles, enhancing the diffusivity of guest molecules. 47), 48) This strategy was also applicable for zeolite nanoparticles in three-dimensionally ordered mesoporous (3DOm) carbon. 49) Such hierarchical materials could be disassembled into individual particles to form a homogeneous suspension.…”

The Synthesis of Novel Hierarchical Zeolites and Their Performances in Cracking Large Molecules

“…As compared with conventional zeolites, these hierarchical zeolites could overcome the limit of the pore size, allow efficient and rapid mass transport and possess the capability of cracking bulky molecules [8]. Among the various templating and non-templating methods, the nanocasting way with ''hard template'', such as various kinds of carbon [9][10][11][12][13][14], aerogel [15][16][17][18] and other porous materials [19][20][21][22], are very popular since the porosity of the as-synthesized materials can be tailored by controlling the properties of original templates.…”