Xueyi Zhang

Synthesis of self-pillared zeolite nanosheets by repetitive branching

Go with the Flow Effective absorption or filtration can be achieved by having a material with multiple levels of porosity, so that the main flow can occur in the larger channels, while smaller passageways can be used to sequester a secondary material. It can be difficult to make these materials because the pores need to be different sizes, but still fully connected to each other. Zhang et al. (p. 1684) show that a hierarchical zeolite can be made through a simple process using a single structure-directing agent that causes repetitive branching. This leads to a material with improved transport and catalytic properties. Single-step synthesis of pillared zeolite nanosheets is achieved with a common structure-directing agent. Hierarchical zeolites are a class of microporous catalysts and adsorbents that also contain mesopores, which allow for fast transport of bulky molecules and thereby enable improved performance in petrochemical and biomass processing. We used repetitive branching during one-step hydrothermal crystal growth to synthesize a new hierarchical zeolite made of orthogonally connected microporous nanosheets. The nanosheets are 2 nanometers thick and contain a network of 0.5-nanometer micropores. The house-of-cards arrangement of the nanosheets creates a permanent network of 2- to 7-nanometer mesopores, which, along with the high external surface area and reduced micropore diffusion length, account for higher reaction rates for bulky molecules relative to those of other mesoporous and conventional MFI zeolites.

Dispersible exfoliated zeolite nanosheets and their application as a selective membrane

Thin zeolite films prepared through a polymer exfoliation method were used as selective membranes. Thin zeolite films are attractive for a wide range of applications, including molecular sieve membranes, catalytic membrane reactors, permeation barriers, and low-dielectric-constant materials. Synthesis of thin zeolite films using high-aspect-ratio zeolite nanosheets is desirable because of the packing and processing advantages of the nanosheets over isotropic zeolite nanoparticles. Attempts to obtain a dispersed suspension of zeolite nanosheets via exfoliation of their lamellar precursors have been hampered because of their structure deterioration and morphological damage (fragmentation, curling, and aggregation). We demonstrated the synthesis and structure determination of highly crystalline nanosheets of zeolite frameworks MWW and MFI. The purity and morphological integrity of these nanosheets allow them to pack well on porous supports, facilitating the fabrication of molecular sieve membranes.

Hydrothermal synthesis of zeolites with three-dimensionally ordered mesoporous-imprinted structure.

Zeolites are microporous materials with pores and channels of molecular dimensions that find numerous applications in catalysis, separations, ion exchange, etc. However, whereas uniformity of micropore size is a most desirable and enabling characteristic for many of their uses, in certain cases, for example in reactions involving bulky molecules, it is a limitation. For this reason, synthesis of hierarchical zeolites with micro- and mesoporosity is of considerable interest as a way to control molecular traffic for improved catalytic and separation performance. Herein, we report a general synthesis route for the confined synthesis of zeolites within three-dimensionally ordered mesoporous carbon templates by conventional hydrothermal synthesis. Various zeolites, including BEA, LTA, FAU, and LTL, with three-dimensionally ordered mesoporous-imprinted structure have been synthesized by this approach. It is expected that these hierarchical zeolite materials will provide building blocks for thin-film and other syntheses and may provide a basis for quantitatively studying the mass-transfer limitation on the catalytic performance of zeolite catalysts.

Sub-40 nm zeolite suspensions via disassembly of three-dimensionally ordered mesoporous-imprinted silicalite-1.

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.

Oriented CoSAPO‐5 Membranes by Microwave‐Enhanced Growth on TiO2‐Coated Porous Alumina

We thank Dr. Christopher Lew for assistance with diffuse reflectance UV/Vis experiments. Support from the NSF (grant NSF-NIRT CMMI 0707610) and the Petroleum Institute of Abu Dhabi through the ADMIRE partnership (Abu Dhabi-Minnesota Institute for Research Excellence) is appreciated. M.P. acknowledges CSIC for a JAE doctoral fellowship, A.C. would like to thank CONSOLIDER Ingenio 2010-MULTICAT, and G.N.K. acknowledges a European Marie Curie International Reintegration Grant (FP7, grant agreement no. 210947). Portions of this work were carried out in the Nanofabrication Center, part of the National Nanotechnology Infrastructure Network (NNIN) which receives support from the NSF, and in the Characterization Facility on the campus of the University of Minnesota-Twin Cities which receives partial support from the NSF through the MRSEC program.

Hydrothermal Formation of the Head-to-Head Coalesced Szaibelyite MgBO2(OH) Nanowires

The significant effect of the feeding mode on the morphology and size distribution of the hydrothermal synthesized MgBO2(OH) is investigated, which indicates that, slow dropping rate (0.5 drop s−1) and small droplet size (0.02 mL d−1) of the dropwise added NaOH solution are favorable for promoting the one-dimensional (1D) preferential growth and thus enlarging the aspect ratio of the 1D MgBO2(OH) nanostructures. The joint effect of the low concentration of the reactants and feeding mode on the hydrothermal product results in the head-to-head coalesced MgBO2(OH) nanowires with a length of 0.5–9.0 μm, a diameter of 20–70 nm, and an aspect ratio of 20–300 in absence of any capping reagents/surfactants or seeds.

Influence of process parameters on hydrothermal formation of magnesium borate hydroxide nanowhiskers

Abstract Uniform magnesium borate hydroxide [MgBO2(OH)] nanowhiskers with a diameter of 20–60 nm and an aspect ratio of 10–70 were synthesised via a coprecipitation/hydrothermal reaction route, using NaOH, MgCl2 and H3BO3 as the reactants. The experimental results indicated that the control of the Mg/B/Na molar ratio at 2 : 3 : 4, the decrease in the NaOH concentration from 2˙0 to 0˙33 mol L–1, the increase in the hydrothermal temperature from 160 to 240°C and the prolongation of the hydrothermal reaction time from 2˙0 to 30˙0 h could promote the growth of the MgBO2(OH) nanowhiskers along the c axis, leading to the formation of the MgBO2(OH) nanowhiskers with high aspect ratio.

Ethylene Oligomerization to Select Oligomers on Ni‐ETS‐10

The oligomerization of short alkenes (ethylene and propylene) can be used for producing commodity chemicals. Various catalysts have been used for alkene oligomerization, among which ordered microporous catalysts are thermally and mechanically stable and are already established for large‐scale industrial applications. In this work, we demonstrate ethylene oligomerization reaction on a microporous titanosilicate ETS‐10 (Engelhard Titanosiliate‐10) exchanged with Ni2+ (Ni‐ETS‐10). We demonstrate a template‐free and fluoride‐free ETS‐10 synthesis method that does not produce impurities commonly seen in hydrothermal ETS‐10 synthesis. Ni‐ETS‐10 showed high C2 conversion rate, high selectivity to C4 and high stability comparing to other microporous catalysts investigated in this work for ethylene oligomerization reaction.

Structure Determination of Molecular Sieve Nanoparticles with Electron Microscopy and Powder X-Ray Diffraction

Molecular sieves are crystalline materials with ordered pores of molecular sizes. Zeolites and metalorganic frameworks (MOFs) are the most utilized molecular sieve materials. Zeolites are silicon dioxide with ordered micropores (0.5 nm – 2 nm), some of which have silicon substituted with metal atoms for added functionality. [1] MOFs are 3-D coordination networks composed of metal nodes and linkers, where the linkers create a 3-D scaffold that lead to ordered porosity. [2] These molecular cavities give molecular sieves the capability to recognize and manipulate molecules at microscale. For example, zeolite ZSM-5 is an important catalyst that can produce gasoline by catalytic cracking of heavy hydrocarbons, due to its special pore size and pore connections in the crystal structure. ZIF-8 and related materials are MOFs that can be used for CO2/CH4 separation, due to its chemical composition that results in selective adsorption and accelerated transport of CO2 in its molecular cavities. In addition to the crystal structure that decides pore size and topology, the morphology of the crystals is also of great importance. For example, 2-D molecular sieve particles are beneficial for gas separation due to the shortened diffusion path that leads to higher gas permeance; catalyst nanoparticles with sub-100 nm size are beneficial for fully exploiting the reactive surface area of nanoparticles for catalytic reactions.

Atomic Structure of Self-Pillared, Single-Unit-Cell Sn-MFI Zeolite Nanosheets

Two-dimensional MFI-type zeolites are single or near single-unit-cell thick microporous nanosheets with pore sizes of ~ 5 Å in diameter, formed by a network of interconnected silicon and oxygen atoms [1-3]. These nanosheets can be fabricated into ultra-thin membranes for the separation of isomer mixtures, such as p-xylene/o-xylene and n-butane/i-butane by allowing only the passage of isomer that fits within its micropores. In addition, application of these nanosheets can be further extended by incorporating heteroatoms, such as Sn atoms, into its framework as catalytically active sites for the isomerization of glucose to fructose, the reaction involved in production of high-fructose corn syrup. The yield of fructose during this reaction depends on the accessibility of catalytically active sites in the zeolite framework.