J.C. Jansen

On the location and disorder of the tetrapropylammonium (TPA) ion in zeolite ZSM-5 with improved framework accuracy

i11.96/~do.04024 . ½NC12H28OH + nH20 (+0.04Na+?), Mr = 822.65 + (n x 18), orthorhombic, Pnma (assumed), a = 20.022 (2), b = 19.899 (2), c = 13.383 (1) /~, V=5332(4)A 3, Z=8, Dx=2.06 (including TPAOH), Dx = 1.80 gcm -3 (for the anhy- drous framework), h(Mo Ka) =0.71069 A, /z = 6.91 cm -1, F(000) = 3343.7, T = 293 K, R = 0.042 for 4523 observed reflections with l>2.0cr(I). The framework topology agrees with earlier descriptions. Straight channels and sinusoidal channels run parallel to (010) and (100), respectively. The sym- metry of the framework atoms is too close to Pnma to permit refinement in Pn21a. In the straight channel an approximate mirror plane through N and the chan- nel axis, perpendicular to the crystallographic mirror plane, exists. The tetrapropylammonium ion lies at the intersection of the straight and sinusoidal chan- nels in two different orientations. The two orienta- tions, populated in a ratio of 3:2, are nearly related by the approximate mirror plane. Contrary to literature data the propyl-N-propyl fragments point- ing into the sinusoidal and straight channel, respec- tively, both have CNCC torsion angles around 60 °. No evidence is obtained for the tetrapropylam- monium ion breaking the rn symmetry and extra disorder around rn is assumed. The mean C-C and C-N distances and mean CCC, CCN and CNC angles, averaged over both ions, are 1.55 and 1.57 A and 109, 113 and 109 °, respectively. Contact distances smaller than 4.0/~, between terminal C atoms of adjacent template ions, are in the range 3.56 (7)- 3.75 (2) ~.

An Efficient Polymer Molecular Sieve for Membrane Gas Separations

Bicyclic Bridge to Improvement Polymers of intrinsic microporosity are a recently developed class of contorted rigid glassy ladderlike polymers having very high free volume (open internal spaces). The intrinsic porosity of these materials has made them of interest for ultrahigh permeability gas separation membranes. However, while the polymers show good gas permeability, they have only moderate gas selectivity. Carta et al. (p. 303; see the Perspective by Guiver and Lee) hypothesized that if they could replace the dioxin-like rings in their polymers with stiffer bridged bicyclic rings, they could improve the membrane properties of the polymer. By exploiting reactions connected to the formation of Trögers base to form the multiple covalent bonds needed to make the bicyclic rings, the resulting polymers showed significantly improved selectivity and permeability. Intrinsically porous polymers made using reactions associated with Tröger’s base manifested enhanced membrane properties. [Also see Perspective by Guiver and Lee] Microporous polymers of extreme rigidity are required for gas-separation membranes that combine high permeability with selectivity. We report a shape-persistent ladder polymer consisting of benzene rings fused together by inflexible bridged bicyclic units. The polymer’s contorted shape ensures both microporosity—with an internal surface area greater than 1000 square meters per gram—and solubility so that it is readily cast from solution into robust films. These films demonstrate exceptional performance as molecular sieves with high gas permeabilities and good selectivities for smaller gas molecules, such as hydrogen and oxygen, over larger molecules, such as nitrogen and methane. Hence, this polymer has excellent potential for making membranes suitable for large-scale gas separations of commercial and environmental relevance.

The monoclinic framework structure of zeolite H-ZSM-5. Comparison with the orthorhombic framework of as-synthesized ZSM-5

The monoclinic framework structure of zeolite H-ZSM-5 (Si 95.68 Al 0.32 H 0.32 O 192 ) has been determined by single crystal X-ray diffraction. The space group is P2 1 /n.1.1. The unit-cell, of dimensions a = 20.107(2), b = 19.879(2), c = 13.369(1)A and α = 90.67(1)°, has 24 independent T sites and Z = 4. The orthorhombic/monoclinic symmetry change, ascribed earlier to a shift of neighboring (010) pentasil layers along c , is realized by a complicated displacement of the framework atoms. The SiOSi angles in the bridges connecting the T 12 -building units into (100) and (010) layers are hardly affected. The transition affects the large SiOSi angles within the T 12 unit most prominently. The corresponding Si O distances are lengthened. The monoclinic framework seems the less strained one. The maximal pore size in the straight channel (∼ 5.8A) is hardly affected by the symmetry change. The sinusoidal cross-sectional area changes from nearly circular (5.3 × 5.6A) in the orthorhombic structure to elliptical in the monoclinic structure (MONO) with major axes of 5.9 and 5.8Ain the two O 10 -rings defining the pore window. In this way the accessibility of the sinusoidal channel in MONO, e.g., for diffusion of p-xylene, is enlarged substantially.

Microwave preparation of zeolite Y and ZSM-5

Abstract Agglomerated uniformly sized, zeolite Y crystallites were prepared in a microwave oven in 10 min, whereas 10–50 h are required by conventional heating techniques depending upon the lattice Si/AI ratio. Relatively high Si/AI ratios, up to 5, could be obtained from hydrogels containing low aluminum contents without crystallization of undesired phases. ZSM-5 could also be synthesized in 30 min at 140°C using this technique.

A Spirobifluorene‐Based Polymer of Intrinsic Microporosity with Improved Performance for Gas Separation

A highly gas-permeable polymer with enhanced selectivities is prepared using spirobifluorene as the main structural unit. The greater rigidity of this polymer of intrinsic microporosity (PIM-SBF) facilitates gas permeability data that lie above the 2008 Robeson upper bound, which is the universal performance indicator for polymer gas separation membranes.

Overgrowth of mesoporous MCM-41 on faujasite

Abstract Zeolite faujasite overgrown with a thin layer of mesoporous MCM-41 has been prepared. Transmission electron microscopy shows that a large part of the zeolite surface is covered with an MCM-41 layer of 5 to 20 nm thickness. The composite MCM-41/FAU is formed by successive synthesis of FAU and MCM-41. Also MCM-41 overgrowth on pre-added FAU crystals is investigated. The overgrowth of MCM-41 on FAU is stimulated by an increase in the external cation exchange capacity of FAU. This new biporous molecular sieve shows interesting catalytic applications. Cracking of vacuum gasoil by MCM-41 coated USY shows a higher conversion of heavy products compared to USY.

Nanoporous Organic Polymer/Cage Composite Membranes

Organic?organic composite membranes are prepared by in?situ crystallization of cage molecules in a polymer of intrinsic microporosity. This allows a direct one-step route to mixed-matrix membranes, starting with a homogeneous molecular solution. Extremely high gas permeabilities are achieved, even after ageing for more than a year, coupled with good selectivity for applications such as CO2 recovery.

Triptycene Induced Enhancement of Membrane Gas Selectivity for Microporous Tröger's Base Polymers

A highly gas permeable polymer with exceptional size selectivity is prepared by fusing triptycene units together via a poly-merization reaction involving Trögers base formation. The extreme rigidity of this polymer of intrinsic microporosity (PIM-Trip-TB) facilitates gas permeability data that lie well above the benchmark 2008 Robeson upper bounds for the important O2 /N2 and H2 /N2 gas pairs.

Growth model of oriented crystals of zeolite Si-ZSM-5

A model for the growth of Si-ZSM-5 (silicalite-1) crystals from zeolite synthesis mixtures is proposed. Nucleation and crystal growth occur at the interface of gel particles and liquid, the only place where both the silicon source and template are present in abundance. In the extension of this model to growth on a support surface, a precursor gel layer plays an important role as an anchoring site for the template molecules. Orientation results from the preferential growth of the ac-plane of the crystallites in the plane of the substrate surface. The presence of alkali ions at this surface may adversely affect zeolite nucleation.

In situ synthesis of binderless ZSM-5 zeolitic coatings on ceramic foam supports

In situ synthesis of binderless ZSM-5 zeolite coatings on ceramic foam supports was studied. The effects were investigated of the rotation of the autoclaves during synthesis, the reaction mixtures SiO2/Al2O3 and dilution ratios, the reaction mixture-to-support surface area ratio, the synthesis temperature and time as well as the composition of the support material (α-Al2O3, SiC/Al2O3 and ZrO2/Y2O3 foam). X-ray diffraction patterns showed highly crystalline ZSM-5 structures superposed on the ceramic foam carrier pattern. The deposition of ZSM-5 was quantified by measuring mass increase and BET surface of the samples. Uniform and continuous ZSM-5 coatings with overall specific surface areas of up to 200 m2/g were obtained. SEM micrographs showed coffin-like, partly intergrown crystals of 5–10 μm that appeared to be attached firmly to the support.