FAU Crystals Research Articles

Synthesis and single-crystal structure of sodium sulfide cationic cluster in the sodalite cavity of zeolite Y (FAU, Si/Al = 1.56)

The extraframework sodium sulfide cationic cluster, Na4S2+, has been introduced into zeolite Y (FAU, Si/Al = 1.56). A single crystal of |Na73(Na4S2+)1.0|[Si117Al75O384]-FAU was prepared by allowing aqueous 0.1 M Na2S solution to flow past a |Na75|[Si117Al75O384]-FAU crystal, and followed by vacuum dehydration at 723 K. Its structure was determined by single-crystal synchrotron X-ray diffraction techniques in the cubic space group Fd $$\overline{3}$$ m. The structure was refined using all intensities to the final error index R1/wR2 = 0.044/0.125. About 77 Na+ ions per unit cell are found at the crystallographically distinct positions, six. One Na+ ion per unit cell is at the center of double 6-ring. Two site-I′ positions are occupied by 26.5 and 4 Na+ ions, respectively, per unit cell. Thirty-two Na+ ions per unit cell are found at site II, in the supercage opposite single 6-rings. The remaining 4.5 and 9 Na+ ions per unit cell occupy two sites III′ near triple 4-rings in the supercage. Finally, one sulfide ion per unit cell, associated with Na+ ions, is found at the center of sodalite cavity. The 4 Na+ ions at site I′ coordinate to a sulfide ion at the center of sodalite cavity to give tetrahedral cationic cluster, Na4S2+. In |Na73(Na4S2+)1.0|[Si117Al75O384]-FAU, tetrahedral Na4S2+ ions center about 12.5% of the sodalite cavities.

A crystallographic study of Sr2+ and K+ ion-exchanged zeolite Y (FAU, Si/Al = 1.56) from binary solution with different mole ratio of Sr2+ and K+

To study the Sr2+-ion selectivity of zeolite Y (Si/Al = 1.56) in binary solution with different Sr2+ and K+ concentration during exchange, two single-crystals of fully dehydrated, Sr2+- and K+-exchanged zeolites Y were prepared by the flow method using a mixed ion-exchange solution whose Sr(NO3)2:KNO3 mol ratios were 1:1 (crystal 1) and 1:100 (crystal 2), respectively, with a total concentration of 0.05 M, followed by vacuum dehydration at 723 K. Their crystal structures were determined by single-crystal synchrotron X-ray diffraction techniques in the cubic space group $$ Fd\bar{3}m $$, respectively, and were refined to the final error indices R1/wR2 = 0.0429/0.1437 and 0.0356/0.1239 for crystals 1 and 2, respectively. In the structure of |Sr28K19|[Si117Al75O384]-FAU (crystal 1), 28 Sr2+ ions per unit cell occupy four different crystallographically distinct sites; 15, 2, 2, and 9 are at sites I, I′, II′, and II, respectively, whereas, the K+ ions occupy only one site: 19 are at site II. In the structure of |Sr17K41|[Si117Al75O384]-FAU (crystal 2), 17 Sr2+ ions per unit cell occupy three equipoints; 10.5, 2, and 4.5 are at sites I, I′, and II, respectively. The residual 41 K+ ions per unit cell are found at four different sites; 8, 24, 3, and 6 are at sites I′, II, III′a, and III′b, respectively. The degrees of ion exchange are 74.7 and 45.3% for crystals 1 and 2, respectively. This result shows that the degree of Sr2+ exchange decreased sharply by decreasing the initial Sr2+ concentration and increasing the initial K+ concentration in the given ion-exchange solution.

Combining Phase Separation with Pseudomorphic Transformation for the Control of the Pore Architecture of Functional Materials: A Review

Chemical phase separation and pseudomorphic transformation are very powerful tools for fine tuning the porous architecture of inorganic oxides. In this short review the basic principles of the preparation of centimetric bodies made of zeolites is introduced. The synthesis of monoliths with homogeneous distributions of interconnected macropores with skeleton made of SOD, LTA and FAU crystals is described. Their unique hierarchical porous texture featuring the structural micropores, intercrystalline mesopores and flow-through macropores leads to remarkable hydrodynamic behavior in separation, catalysis and ion exchange processes operated in continuous-flow mode. In the base-catalyzed carbon-carbon bond formation, productivities twice those achievable with fixed-beds of packed particles are observed. In the capture of strontium present in radioactive effluents their efficiency is three orders of magnitude that of traditional powder in batch.

Selection between LTA or FAU phases by manipulating the synthesis gel composition, crystallization time and temperature

Abstract LTA and FAU zeolites were synthesized with the addition of cetyltrimethylammonium bromide (CTAB) to assess the effects of Si/Al, Na/Al and H2O/Al ratios, time and temperature on the yield of each structure. The significance of each variable was determined by means of experimental designs, whose response variables were the areas of specific XRD peaks. CTAB did not act as a structure-directing agent, but rather as a growth modifier, increasing the growth rate of LTA and FAU crystals. At low temperature, LTA was afforded at low Si/Al ratio, while FAU crystallized at higher ratios, indicating a competition between the phases. Increasing temperature accelerated the kinetics of recrystallization of the less dense and metastable LTA and FAU phases in denser and more stable phases, such as sodalite and cancrinite. Decreasing the concentration of Na+ and water reduced the crystallinity of both structures, while increasing the concentration of Na+ favored LTA over FAU.

Hierarchical FAU/ZIF-8 Hybrid Materials as Highly Efficient Acid-Base Catalysts for Aldol Condensation.

The composite of hierarchical faujasite nanosheets and zeolitic imidazolate framework-8 (Hie-FAU-ZIF-8) has been successfully prepared via a stepwise deposition of ZIF-8 on modified zeolite surfaces. Compared to the direct deposition of metal organic frameworks (MOFs) on zeolite surfaces, ZIF-8 nanospheres were selectively attached to the external surfaces of the MOF ligand-grafted FAU crystals because of the enhancing interaction between the zeolite and MOF in the composite. In addition, the degree of surface functionalization can be greatly enhanced because of the presence of hierarchical structures. This behavior leads to an increase in the deposited MOF content, improving the hydrophobic properties of the zeolite surfaces. Interestingly, the designed hierarchical composite exhibits outstanding catalytic properties as an acid-base catalyst for the aldol condensation of 5-hydroxymethylfurfural with acetone. Compared to the isolated FAU and ZIF-8, a high yield of the product, 4-[5-(hydroxymethyl)furan-2-yl]but-3-en-2-one (67%), can be observed in the composite because of the synergistic effect between the Na+-stabilized zeolite framework and the imidazolate linkers bearing basic nitrogen functions. This opens up interesting perspectives for the development of new organic and inorganic hybrid materials as heterogeneous acid-base catalysts.

Selective synthesis of FAU-type zeolites

In the present work, parameters influencing the selectivity of the synthesis of FAU-zeolites from diatomite were studied. The final products after varying synthesis time were characterized by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and gas adsorption. It was found that high concentrations of NaCl could completely inhibit the formation of zeolite P, which otherwise usually forms as soon as maximum FAU crystallinity is reached. In the presence of NaCl, the FAU crystals were stable for extended time after completed crystallization of FAU before formation of sodalite. It was also found that addition of NaCl barely changed the crystallization kinetics of FAU zeolite and only reduced the final FAU particle size and SiO2/Al2O3 ratio slightly. Other salts containing either Na or Cl were also investigated. Our results suggest that there is a synergistic effect between Na+ and Cl−. This is attributed to the formation of (Na4Cl)3+ clusters that stabilize the sodalite cages. This new finding may be used to increase the selectivity of syntheses leading to FAU-zeolites and avoid the formation of undesirable by-products, especially if impure natural sources of aluminosilica are used.

Crystallographic studies of fully dehydrated partially Zn2+-exchanged zeolite Y (FAU, Si/Al = 1.56) depending on Zn2+ concentration of aqueous solution during exchange

Four single crystals of fully dehydrated and partially Zn2+-exchanged zeolites Y (Si/Al = 1.56) were prepared by the static ion-exchange method using a mixed ion-exchange solution in which Zn(NO3)2:NaCl mole ratios were 1:1 (crystal 1), 1:25 (crystal 2), 1:50 (crystal 3), and 1:100 (crystal 4), respectively, with a total concentration of 0.05 M, and followed by vacuum dehydration at 673 K. Their single-crystal structures were determined by single-crystal synchrotron X-ray diffraction techniques in the cubic space group Fd\(\bar {3}\)m and refined to the final error indices R1/wR2 = 0.0459/0.1454, 0.0449/0.1283, 0.0427/0.1284, and 0.0486/0.1680, respectively. Their unit-cell formulas are |Zn25Na25|[Si117Al75O384]-FAU (crystal 1), |Zn19.5Na36|[Si117Al75O384]-FAU (crystal 2), |Zn19.5Na36|[Si117Al75O384]-FAU (crystal 3), and |Zn7Na61|[Si117Al75O384]-FAU (crystal 4), respectively. The degree of Zn2+ exchange decreases from 67 to 19% as the initial concentration of Zn2+ decrease and the initial concentration of Na+ increases in given ion-exchange solutions.

Facile fabrication of NaX zeolite film on PSSF as a potential structured catalyst support

This article presents a facile method to synthesize NaX zeolite film on paper-like sintered stainless-steel fiber, which can be used as a potential support for structured catalysts. Unlike the secondary growth method, in which the nano-sized zeolite seeds should be synthesized and attached to the support using the appropriate procedure, the facile method is much easier to operate. It involves a simple pretreatment with 3-aminopropyltrimethoxysilane (APTMS) followed by in-situ hydrothermal synthesis. The synthesis conditions such as the molar composition of initial solution, pretreatment, aging duration, crystallization temperature, crystallization duration, synthesis times play pivotal roles in the growth of zeolite film. We systematically investigated these parameters. SEM and N2 adsorption/desorption were used for characterizations. It was found that film growth is extremely sensitive to synthesis conditions. The modification of molar composition resulted in the change in crystal size and the appearance of impurity phase (LTA, P-type zeolite and the amorphous); the pretreatment of APTMS led to even distribution of zeolite crystal on the surface; longer aging duration contributed to the growth of pure phase FAU crystal; lower crystallization temperature triggered incomplete crystallization while higher temperature gave rise to the crack of zeolite layer; both the shorter and longer crystallization duration caused impurity phase; increasing synthesis times thickened the zeolite film. Additionally, the dense P zeolite film was also prepared under various conditions.

Direct Hydrothermal Synthesis of High-silica SSZ-39 Zeolite with Small Particle Size

A high-silica SSZ-39 zeolite with an Si/Al molar ratio as high as 16 and with a particle size as low as 300 nm has been directly synthesized by tuning both alkali and water contents in the synthesis mixture, followed by adding relatively high-silica FAU crystals.

Behavior of cesium cation in zeolite Y (FAU, Si/Al = 1.56) and their single-crystal structures, |Cs75−xNax|[Si117Al75O384]-FAU (x = 35 and 54)

To study the tendency of Cs+ exchange into zeolite Y (Si/Al = 1.56) dependence on Cs+ and Na+ concentration of aqueous solution during exchange, two single-crystals of fully dehydrated, Cs+-and Na+-exchanged zeolites Y were prepared by the flow method using a mixed ion-exchange solution whose CsNO3:NaNO3 mol ratios were 1:1 (crystal 1) and 1:100 (crystal 2), respectively, with a total concentration of 0.1 M, followed by vacuum dehydration at 723 K. Their crystals were determined by single-crystal synchrotron X-ray diffraction techniques in the cubic space group Fd $$\overline{ 3}$$ m, respectively, and were refined to the final error indices R 1/wR 2 = 0.084/0.248 and 0.088/0.274 for crystals 1 and 2, respectively. In the structure of |Cs40Na35|[Si117Al75O384]-FAU (crystal 1), 40 Cs+ ions per unit cell occupy five different equipoints; 3, 3, 14, 9, and 11 are at sites I, II′, II, IIIa and IIIb, respectively, whereas, the remaining 35 Na+ ions occupy three different sites: 9, 11, and 15 are at sites I, I′, and II, respectively. In the structure of |Cs21Na54|[Si117Al75O384]-FAU (crystal 2), 21 Cs + ions per unit cell occupy three equipoints; 4, 6, and 11 are at sites II, IIIa, and IIIb, respectively. The residual 54 Na+ ions per unit cell are found at three different sites; 6, 20 and 28 are at sites I, I′, and II, respectively. The degrees of ion exchange are 53 and 28 % for crystals 1and 2, respectively. This result shows that the degree of Cs+ exchange decreased sharply by decreasing the initial Cs+ concentration and increasing the initial Na+ concentration in given ion-exchange solution.

Synthesis of Zeolite Nanomolecular Sieves of Different Si/Al Ratios

Nanosized zeolite molecular sieves of different Si/Al ratios have been prepared using microwave hydrothermal reactor (MHR) for their greater application in separation and catalytic science. The as‐synthesized molecular sieves belong to four different type zeolite families: MFI (infinite and high silica), FAU (moderate silica), LTA (low silica and high alumina), and AFI (alumina rich and silica‐free). The phase purity of molecular sieves has been assessed by X‐ray diffraction (XRD) analysis and morphological evaluation done by electron microscopy. Broad XRD peaks reveal that each zeolite molecular sieve sample is composed of nanocrystallites. Scanning electron microscopic images feature the notion that the incorporation of aluminum to MFI zeolite synthesis results in morphological change. The crystals of pure silica MFI zeolite (silicalite‐1) have hexagon lump/disk‐like shape, whereas MFI zeolite particles with Si/Al molar ratios 250 and 100 have distorted hexagonal lump/disk and pseudo spherical shapes, respectively. Furthermore, phase pure zeolite nanocrystals of octahedron (FAU), cubic (LTA), and rod (AFI) shape have been synthesized. The average sizes of MFI, FAU, LTA, and AFI zeolite crystals are 250, 150, 50, and 3000 nm, respectively. Although the length of AFI zeolite rods is in micron scale, the thickness and width are of a few nanometers.

Preparation of NaY zeolite membranes in fluoride media and their application in dehydration of bio-alcohols

Improved NaY zeolite membranes were prepared in the presence of ammonium fluoride. The crystallization and separation performance of fluoride-derived NaY zeolite membranes were studied in comparison with fluoride-free membranes. The addition of ammonium fluoride in gel influenced the crystallization and the morphologies of NaY zeolite and zeolite layers, and effectively suppressed the formation of the impurity phase of P-type zeolite in NaY zeolite layers. The fluoride-derived NaY zeolite membranes had thinner zeolite layers and displayed both higher fluxes and separation factors for the dehydration of bio-alcohols (ethanol and butanol) than the fluoride-free membranes. The separation factors increased with the increasing kinetic diameters of alcohols for the methanol, ethanol, i-propanol and n-butanol aqueous solutions. Synthesis reproducibility of fluoride-derived membranes and membrane stability during the dehydration process were also investigated. 29Si and 19F MAS NMR results confirmed that fluorine anions were not included in FAU crystals.

Three single-crystal structures of Cd2+-exchanged zeolite Y (FAU). Cd(OH)2 imbibition increases with increasing ion exchange temperature

Three single crystals of excessively Cd2+-exchanged zeolite Y were prepared by the exchange of Na-Y (|Na75|[Si117Al75O384]-FAU, Si/Al=1.56) with an aqueous stream 0.25M each in Cd(NO3)2 and Cd(C2H3O2)2 at 294K (crystals 1 and 2) and 353K (crystal 3), followed by vacuum dehydration at 1×10−6Torr and 423K (crystal 1) and 723K (crystals 2 and 3). Their crystal structures were determined by diffraction experiment using synchrotron X-radiation in the cubic space group Fd3¯m. They were refined to the final error indices R1/wR2=0.0502/0.1445, 0.0702/0.1960, and 0.0643/0.1747 for crystals 1, 2, and 3, respectively. The number of Cd2+, H2O, OH−, and O2− ions or molecules in the three crystal structures are about 43.5, 23, 12, 0; 43.5, 0, 0, 6; and 47.5, 0, 0, 10 per unit cell for crystals 1, 2, and 3, respectively. In all cases, over-exchange due to uptake of OH− occurred. Crystals 1 and 2 had imbibed about 6 Cd(OH)2 molecules per unit cell, perhaps as CdOH+ ions, crystal 3 ten, indicating this increase in imbibition as temperature increased from 294K to 353K. These decomposed upon vacuum dehydration at 723K to give, with additional Cd2+ ions, Cd8O48+ clusters (symmetry 4¯3m(Td)) in the sodalite cavities of crystals 2 and 3. The crystal compositions are |Cd8.5(CdOH)12(CdOH2)23|[Si117Al75O384]-FAU (crystal 1), |Cd31.5(Cd8O4)1.5|[Si117Al75O384]-FAU (crystal 2), and |Cd27.5(Cd8O4)2.5|[Si117Al75O384]-FAU (crystal 3), respectively.

Synthesis and single-crystal structures of fully dehydrated fully Sr2+-exchanged zeolite Y (FAU) and its benzene sorption complex, |Sr37.5|[Si117Al75O384]-FAU and |Sr37.5(C6H6)33(H2O)15|[Si117Al75O384]-FAU

Single crystals of zeolite Y, |Na75|[Si117Al75O384]-FAU, were Sr2+ ion exchanged and fully dehydrated to generate |Sr37.5|[Si117Al75O384]-FAU (crystal 1) and next, benzene adsorbed to generate |Sr37.5(C6H6)33(H2O)15|[Si117Al75O384]-FAU (crystal 2, partially dehydrated). Crystal 1 was prepared by ion exchange in a flowing stream of aqueous 0.05 M Sr(ClO4)2 for 3 days followed by dehydration at 673 K and 1 × 10−6 Torr for 2 days. To prepare the benzene sorption complex (crystal 2), another dehydrated |Sr37.5|[Si117Al75O384]-FAU crystal was exposed to 50 Torr of benzene for 3 days at 294 K followed by evacuation for 30 min. at this temperature and 5 × 10−5 Torr. Their structures were determined crystallographically at 100(1) K using synchrotron X-radiation in the cubic space group Fd $$ \bar{3} $$ m. They were refined to the final error indices R 1/wR 2 = 0.051/0.125 and 0.057/0.154 using 598 and 590 reflections with F o > 4σ(F o) for crystals 1 and 2, respectively. In crystal 1, about 37.5 Sr2+ ions per unit cell are found at an unusually large number of crystallographically distinct positions, six. Eight Sr2+ ions per unit cell are at the centers of the double 6-rings (D6Rs, site I). Five additional Sr2+ ions are near site I. The site-I’ positions (in the sodalite cavities opposite D6Rs) are occupied by 2.5 Sr2+ ions per unit cell. Two Sr2+ ions are located at site II’ in the sodalite cavity. The remaining twenty Sr2+ ions are found at two nonequivalent sites II (in the supercages) with occupancies of 9 and 11 ions per unit cell, respectively. Each of these Sr2+ ions coordinates to three framework oxygens. In this crystal, all sites are only sparsely occupied. In crystal 2, all Sr2+ ions are located at four crystallographic sites and 33 benzene molecules are found at two distinct sites within the supercages. One Sr2+ ion is at the center of the D6R. Fifteen Sr2+ ions are located at site I’. The remaining 21.5 Sr2+ ions are found at two nonequivalent sites II with occupancies of 19 and 2.5 ions per unit cell. Nineteen benzenes lie on threefold axes in the supercages, where they interact facially with the latter 19 site-II Sr2+ ions; occupancy = 19 molecules/32 sites. The remaining fourteen benzene molecules are found in 12-ring planes; occupancy = 14 molecules/16 sites. Each hydrogen of these 14 benzenes is ca. 2.9 A from six 12-ring oxygens.

Single-crystal structures of highly [formula omitted]-exchanged, fully deaminated, and fully Tl +-exchanged zeolite Y (FAU, Si/Al = 1.56), all fully dehydrated

Single crystals of zeolite Y, |Na 75|[Si 117Al 75O 384]-FAU, with diameters up to 0.30 mm were grown from gels; Si/Al = 1.56. They were ion exchanged and fully dehydrated to generate |(NH 4) 67H 5Na 3|[Si 117Al 75O 384]-FAU (crystal 1, partially deaminated), |H 72Na 3|[Si 117Al 75O 384]-FAU (crystal 2, fully deaminated at 473 K) and |Tl 75|[Si 117Al 75O 384]-FAU (crystal 3). Their structures were determined crystallographically at 294 K using synchrotron X-radiation in the cubic space group Fd 3 ¯ m . They were refined to the final error indices R 1/ wR 2 = 0.037/0.126, 0.055/0.190, and 0.044/0.121 using 884, 723, and 851 reflections with F o > 4 σ( F o) for crystals 1, 2, and 3, respectively. In crystal 1, 67 NH 4 + and 3 Na + ions per unit cell are found at four different crystallographic sites. Six NH 4 + ions were found at the centers of double 6-rings (site I; N···O = 2.888(3) Å). Filling site I′ are 32 NH 4 + ions (N···O = 2.774(5) Å); each is recessed 1.59 Å into the sodalite cavity from the plane of the three oxygens to which it bonds. Filling site II in the supercages are 29 NH 4 + and 3 Na + ions, each of which extends 1.69 Å and 1.10 Å, respectively, into the supercage from its 3-oxygen plane (N···O = 2.818(5) Å, Na–O = 2.512(19) Å). The remaining 5 of the 72 NH 4 + ions originally present per unit cell have apparently been deaminated by vacuum dehydration at 294 K. NH 4 + ions simultaneously occupy sites I and I′ as K + ions did in |K 71|[Si 121Al 71O 384]-FAU, |K 92|[Si 100Al 92O 384]-FAU, and |K 96|[Si 96Al 96O 384]-FAU; the internitrogen distance is 3.36 Å. In crystal 2, three Na + ions per unit cell are found at site I′ (Na–O = 2.207(13) Å); no other cations were found so the deamination of NH 4 + is complete at 473 K. In crystal 3, 30, 30, 9, 4, and 2 Tl + ions were found at sites I′, II, and three sites III′ per unit cell, respectively. The total, 75, agrees with the results of EDX analysis to establish the framework composition (the Si/Al ratio) of these crystals.

Why is it so extremely difficult to prepare shape-selective Al-rich zeolite membranes like LTA and FAU for gas separation?

Two possible reasons for the difficulty to prepare shape-selective Al-rich zeolite membranes for gas separation are discussed. One reason is the strongly negative electric surface charge of Al-rich zeolites like LTA or FAU in aqueous media as found by zeta potential (ZP) measurements of suspended zeolite powder. This negative surface charge is expected to prevent the negatively charged silica species to enter the slits between the growing crystallites in a zeolite membrane layer. This effect can be avoided by adsorption of positively charged species called intergrowth supporting substance (ISS) neutralizing the electric surface charges of the growing zeolite membrane surface. Another problem for Al-rich membranes are extreme mismatches in the expansion coefficients between the zeolite layer and the ceramic or metal support upon drying the as synthesized membranes. The as synthesized FAU or LTA membrane layers contain water. We have simulated a membrane activation in the permeation cell by drying zeolite powder and recording in situ the XRD spectra. We found huge expansion and shrinking effects during the in situ de-watering of LTA and FAU powder samples. This observation is correlated with the experimental finding continuous damage of model membranes upon repeated re-activation cycles in vacuum at 150 °C. The model membranes consisted of a few large LTA and FAU crystals embedded into epoxy resin. After each de-watering step, (i) the fluxes of CO 2, CH 4 and N 2 increased and (ii) the ideal selectivities decreased which indicates a continuous membrane damage. However, these semi-damaged LTA and FAU membranes can still separate water from alcohols but do not show any gas separation.

Crystal structures of the NO and NO 2 sorption complexes of fully dehydrated fully Mn 2+-exchanged zeolite X (FAU)

The crystal structures of the NO and NO 2 sorption complexes of fully dehydrated, fully Mn 2+-exchanged zeolite X, ∣Mn 46(16NO)∣[Si 100Al 92O 384]-FAU (crystal 1, a = 25.017(10) Å) and ∣Mn 46(28NO 2)∣[Si 100Al 92O 384]-FAU (crystal 2, a = 24.672(7) Å), have been determined by single-crystal X-ray diffraction techniques at 21(1) °C in the cubic space groups Fd 3 ¯ m and Fd 3 ¯ , respectively. Each crystal was prepared by ion exchange in a flowing stream of 0.05 M aqueous Mn(NO 3) 2 for 3 days, followed by dehydration at 500 °C and 2 × 10 −6 Torr for 2 days, and exposure to 100 Torr of NO or NO 2/N 2O 4 gas at 21(1) °C for 1.5 h. The structures were determined in these atmospheres and were refined using all data to the final error indices (based only on the 250 reflections for which F o > 4 σ( F o)) of R 1 = 0.059 and wR 2 = 0.206 for crystal 1; the corresponding values for crystal 2 are 347, 0.063, and 0.168. In crystal 1, Mn 2+ ions occupy four crystallographic sites: 15 are at the octahedral site I (at the centers of hexagonal prisms), 2 are at site I′, and the remaining 29 occupy two different sites II. Sixteen NO molecules lie in the supercage where each coordinates laterally to a Mn 2+ ion at one of the sites II (Mn–framework oxygens = 2.244(15), Mn–N = 2.31(3), Mn–O = 2.32(3) Å). The observed N–O bond length, 1.6(3) Å, does not differ significantly from that in NO gas, 1.15 Å. In crystal 2, the Mn 2+ ions also occupy four crystallographic sites: 14, 4, 20, and 8 are at sites I, I′, II, and III, respectively. Twenty-eight NO 2 molecules lie in the supercage; each coordinates either to a site-II or a site-III Mn 2+ ion (Mn–framework oxygens = 2.286(10)/2.41(6) Å, Mn–N = 2.27(7)/2.17(11) Å, N–O = 1.20(12), 1.10(10)/1.27(12), 1.21(13) Å, respectively). The latter four values agree with the N–O bond length in NO 2 gas, 1.193 Å.

Structural characterization of a new “core-shell” zeolite overgrowth system: faujasite on micro sized EMT-crystals

The nature of a new zeolite overgrowth has been characterized. Two overgrown samples were studied. Each sample was made up of the phases faujasite and EMT. By Rietveld refinement of the powder patterns the structure of both phases including the distribution of cations in the overgrowth was determined as well as the weight fractions of the phases in both samples. The growth of FAU crystals on the surface of EMT crystals is epitactic, as shown by HRSEM images. The epitaxial relation is [111] FAU//[001] EMT, (-110) FAU//(110) EMT. The true atomic interfacial model of the interface between faujasite and EMT is still subject of further studies.

Overgrowth of mesoporous MCM-41 on faujasite

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.