Shaped zeolite nanocrystals and larger zeolite particles with three-dimensionally ordered mesoporous (3DOm) features hold exciting technological implications for manufacturing thin, oriented molecular sieve films and realizing new selective, molecularly accessible and robust catalysts. A recognized means for controlled synthesis of such nanoparticulate and imprinted materials revolves around templating approaches, yet identification of an appropriately versatile template has remained elusive. Because of their highly interconnected pore space, ordered mesoporous carbon replicas serve as conceptually attractive materials for carrying out confined synthesis of zeolite crystals. Here, we demonstrate how a wide range of crystal morphologies can be realized through such confined growth within 3DOm carbon, synthesized by replication of colloidal crystals composed of size-tunable (about 10-40 nm) silica nanoparticles. Confined crystal growth within these templates leads to size-tunable, uniformly shaped silicalite-1 nanocrystals as well as 3DOm-imprinted single-crystal zeolite particles. In addition, novel crystal morphologies, consisting of faceted crystal outgrowths from primary crystalline particles have been discovered, providing new insight into constricted crystal growth mechanisms underlying confined synthesis.

No abstract

Multipodal nanoparticles (NPs) with controlled tethers are promising principal building blocks, useful for constructing more complex materials, much like atoms are connected into more complex molecules. Here we report colloidal sphere templating as a viable means to create tetrapodal NPs with site-specific tethers. Amorphous sol-gel materials were molded by the template into shaped NPs that mimic tetravalent atoms but on the length scale of colloids. Synthetic methods were developed to modify only the tips of the tetrapods with a range of possible functional groups to generate anisotropic NPs capable of directional bonding to other NPs. We also illustrate that sets of tethered "colloidal atoms" can assemble themselves into "colloidal molecules" with precise placement of the modifying colloids. The templating and tethering approaches to these anisotropic colloidal building blocks and the assembly methods are applicable to many compositions regardless of crystal structure, therefore lending themselves to the fabrication of complex assemblies, analogous to those found in the molecular regime.

Zeolite nanocrystals were prepared from three-dimensionally ordered mesoporous-imprinted (3DOm-i) silicalite-1 by a fragmentation method involving sonication and dissolution within a certain pH range. 3DOm-i silicalite-1 with spherical elements with diameters ranging from 10 to 40 nm and a wide range of crystal sizes (100-200 nm, 500-600 nm, and 1-2 μm) was used as the starting material. The highest yield (57%) of isolated nanocrystals was obtained for 3DOm-i silicalite-1 with a crystal size of 100-200 nm and a spherical element diameter of 40 nm. The smallest nanocrystals obtained, albeit in very low yields, had a 10 nm diameter. Preparation of stable silicalite-1 nanocrystal suspensions fragmented from 20 and 40 nm 3DOm-i silicalite-1 was demonstrated. Cryogenic transmission electron microscopy showed that the isolated zeolite nanocrystals can be used as seeds for the epitaxial growth of silicalite-1. An application of these findings was demonstrated: silicalite-1 nanocrystal suspensions were used to deposit seed layers on porous α-alumina disks, which were converted to continuous thin (300-400 nm) films by secondary growth that exhibited both high permeances and separation factors (3.5 × 10(-7) mol m(-2) s(-1) Pa(-1) and 94-120, respectively, at 150 °C) for p- and o-xylene.

The effects of confinement on the morphological development of the zeolite silicalite-1 were studied during hydrothermal synthesis in three-dimensionally ordered macroporous (3DOM) carbon monoliths. By scheduling multiple infiltration/hydrothermal reaction (IHT) steps using precursor solutions with high (H) or low nutrient content (L) in specific sequences, it was possible to obtain various zeolite morphologies of interest for technological applications. The special morphologies are also functions of shaping and templating effects by the 3DOM carbon reactor and functions of limited mass transport in the confined reaction environment. IHT steps employing high nutrient concentrations favor nucleation, whereas those using low nutrient concentrations provide growth-dominant conditions. Observed product morphologies include polycrystalline sphere arrays for the sequence HHH..., single crystal domains spanning dozens of macropores for the sequence LLL..., and faceted silicalite-1 crystallites with dimensions less than 100 nm with the sequence HLLL.... Most of these crystallites have dimensions less than 100 nm and would be suitable building blocks for seeded zeolite membrane growth. Finally, the sequence LLL...H introduces a secondary population of particles with smaller size, so that the size distribution of zeolite crystallites in the combined population may be tuned, for example, to optimize packing of particles. Hence, by choosing the appropriate infiltration program, it is possible to control grain sizes in polycrystalline particles (spheres and opaline arrays of spheres), which alters the concentration of grain boundaries in the particles and is expected to influence transport properties through the zeolite.

Magnesiothermic reduction of various types of silica/carbon (SiO 2 /C) composites has been frequently used to synthesize silicon/carbon (Si/C) composites and silicon carbide (SiC) materials, which are of great interest in the research areas of lithium-ion batteries (LIBs) and nonmetal oxide ceramics, respectively. Up to now, however, it has not been comprehensively understood how totally different crystal phases of Si or SiC can result from the compositionally identical parent materials (SiO 2 /C) via magnesiothermic reduction. In this article, we propose a formation mechanism of Si and SiC by magnesiothermic reduction of SiO 2 /C; SiC is formed at the interface between SiO 2 and carbon when silicon intermediates, mainly in situ-formed Mg 2 Si, encounter carbon through diffusion. Otherwise, Si is formed, which is supported by an ex situ reaction between Mg 2 Si and carbon nanosphere that results in SiC. In addition, the resultant crystalline phase ratio between Si and SiC can be controlled by manipulating the synthesis parameters such as the contact areas between silica and carbon of parent materials, reaction temperatures, heating rates, and amount of the reactant mixtures used. The reasons for the dependence on these synthesis parameters could be attributed to the modulated chance of an encounter between silicon intermediates and carbon, which determines the destination of silicon intermediates, namely, either thermodynamically preferred SiC or kinetic product of Si as a final product. Such a finding was applied to design and synthesize the hollow mesoporous shell (ca. 3−4 nm pore) SiC, which is particularly of interest as a catalyst support under harsh environments.

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.[...

Highly enhanced CO2 and H2 adsorption properties were achieved with a series of phenolic resin-based carbon spheres (resorcinol–formaldehyde carbon (RFC) and phenol–formaldehyde carbon (PFC)) by carbonization of RF and PF polymer (RFP and PFP) spheres synthesized via a sol–gel reaction and subsequent activation with hot CO2 or NH3 treatment. Monodisperse and size-tunable (100–600 nm) RFC and PFC spheres had intrinsic nitrogen contents (ca. 1.5 wt %), which are attributed to the synthesis conditions that utilized NH3 as a basic catalyst as well as nitrogen precursor. A series of CO2-activated and N-doped RFC and PFC spheres showed almost perfect correlation (R 2 = 0.99) between CO2 adsorption capacities and accumulated pore volumes of fine micropores (ultramicropore <1 nm) obtained using the nonlocal density functional theory (NLDFT) model. Interestingly, NH3 activation served not only as an effective method for heteroatom doping (i.e., nitrogen) into the carbon framework but also as an excellent activation process to fine-tune the surface area and pore size distribution (PSD). Increased nitrogen doping levels up to ca. 2.8 wt % for NH3-activated RFC spheres showed superior CO2 adsorption capacities of 4.54 (1 bar) and 7.14 mmol g–1 (1 bar) at 298 and 273 K, respectively. Compared to CO2-activated RFC spheres with similar ultramicropore volume presenting CO2 uptakes of 4.41 (1 bar) and 6.86 mmol g–1 (1 bar) at 298 and 273 K, respectively, NH3-activated nitrogen-enriched RFC was found to have elevated chemisorption ability. Moreover, prolonged activation of RFC and PFC spheres provided ultrahigh surface areas, one of which reached 4079 m2g–1 with an unprecedented superb H2 uptake capacity of 3.26 wt % at 77 K (1 bar), representing one of the best H2 storage media among carbonaceous materials and metal–organic frameworks (MOFs).