A One-Step Facile Encapsulation of Zeolite Microcrystallites in Ordered Mesoporous Microspheres

Abstract

A facile aerosol-assisted technique was employed for the design of a new class of composite zeolite catalyst material with spherical morphology. This technique enables the one-step encapsulation of zeolite microcrystals into the matrix of spherical mesoporous silica particle supports such as MCM-41. By introducing presynthesized zeolite microcrystals into precursor solutions containing the templating surfactant and the silica precursor followed by aerosolization through nozzles larger than the microcrystals, it is possible to entrain the microcrystals in the aerosol droplet. Transport of the droplet through the heated zone of the furnace leads to hydrolysis and condensation of the silica precursor (tetraethyl orthosilicate, TEOS) in each droplet and the formation of spherical particles of MCM-41 containing embedded zeolite microcrystals. This bottle-around-a-ship procedure to make zeolite-MCM-41 composites is extremely effective and can be easily scaled up. Detailed characterization of these composite particles reveals that up to 75 wt % of ZSM-5 zeolite can be embedded in MCM-41 microspheres with no loss of fidelity in particle morphology. To verify access of the reactants to the zeolite, we impregnated the ZSM-5 with nickel (Ni@ZSM-5) prior to encapsulation in MCM-41, and have shown the feasibility of the system to the model reaction of the liquid phase catalytic reduction of 4-nitrophenol to 4-aminophenol. While the reaction proceeds efficiently, there are diffusional restrictions to the transport of 4-nitrophenol resulting in a composite catalyst effectiveness factor of 0.4. The encapsulation of zeolite crystals within a micrometer-sized mesoporous MCM-41 shell provides structural stability to the zeolites and could reduce the pressure drop across a fixed bed tubular reactor due to the increased particle size of the composite. Potential applications of such composite particles include the ability of the MCM-41 to act as sacrificial adsorbents for coke and catalyst poisons, thus extending the life of the active material.

Integrating Halloysite Nanostraws in Porous Catalyst Supports to Enhance Molecular Transport

“…The step-by-step approach to the implementation of the concept starts with the synthesis of MCM-41 containing 30 wt % of Ni@ZSM-5 (M30NZ) using the aerosol-assisted synthesis technique. 23 Figure 3 a shows the SEM image of Ni-impregnated ZSM-5 with primary particles of sizes of 50–100 nm that are aggregated together. 22 , 29 The TEM image of Ni@ZSM-5 in Figure 3 b shows 5–10 nm clusters of the metal atom on the surface of ZSM-5 with better visualization in the high-resolution TEM shown in Figure 3 c. The obtained Ni@ZSM-5 was added to the MCM-41 precursor solution containing TEOS and CTAB and allowed to stir vigorously to maintain suspension stability during aerosolization.…”

“…The cut-section TEM image in Figure 5 a shows the presence of zeolite microcrystals with dark spots of the electron-dense Ni indicating encapsulation of Ni@ZSM-5. We note that our earlier work 23 has expanded on this process with the EDS results on the cut section to quantify the elemental composition. Figure 5 b shows a cut-section SEM of M30NZ/30HNT, which was simply done by cutting the sample fixed in epoxy just once prior to mounting on the SEM holder (in contrast, for TEM imaging, the epoxy-fixed sample is cut twice to generate the thin 100 nm section).…”

“…Rather than simply including a metal salt into the precursor solution or introducing metal sites after catalyst preparation, we have included microcrystallites of ZSM-5 with Ni clusters on the external surface (Ni@ZSM-5), which are the active sites for the model reaction of the reduction of nitrophenol used in this work. 23 We do this for the reason that the zeolite microcrystallites are larger than the HNT lumen and therefore have to reside in the matrix of MCM-41. Alternate ways of introducing the metal will result in the metal being deposited in the lumen and will not lead to a direct comparison of the reaction with and without the straws that is necessary for this study, although perhaps inconsequential for application.…”

One-Step Hydropyrolysis and Hydrotreating Tandem Reactions of Miscanthus × giganteus Using Ni Impregnated ZSM-5/MCM-41 Composites

“…MCM-41 and ZSM-5/ MCM-41 composites were prepared in the lab using the aerosol method following the protocol that was published previously. 49 A schematic of the aerosol method can be found in Figure S1. Briefly, for the synthesis of spherical MCM-41 particles, 1.5 g of CTAB and 2 mL of 0.1 M HCl were added to 15 mL of ethanol in a vial, followed by the dropwise addition of 4.5 mL of TEOS under magnetic stirring.…”

A Promising Solution for Food Waste: Preparing Activated Carbons for Phenol Removal from Water Streams

“…In the first step, the feedstock is pyrolyzed under an inert atmosphere and turned into biochar. 20 − 24 The pyrolysis temperature can vary from 400 to 900 °C. 25 − 29 In the second step, the biochar derived from pyrolysis is activated at temperatures ranging from 500 to 900 °C in the presence of steam or carbon dioxide.…”