BEA-type Zeolite Research Articles

Synthesis and Crystal Structure of a Layered Silicate HUS-1 with a Halved Sodalite-Cage Topology

A new layered silicate, HUS-1, was synthesized by hydrothermal synthesis using decomposed FAU- and *BEA-type zeolites as nanosized silica parts. Structural analyses by X-ray powder diffractometry and solid-state magic-angle-spinning (MAS) NMR spectroscopy revealed that HUS-1 has a layered structure containing a silicate layer per unit cell along a stacking direction. Its framework topology is similar to that of SOD-type zeolites and consists of a halved sodalite cage, which includes four- and six-membered Si rings. Structure refinement by the Rietveld method showed that tetramethylammonium (TMA) ions used as a structure-directing agent (SDA) were incorporated into the interlayer. The four methyl groups of the TMA molecule were located orderly in a hemispherical cage in the silicate layer, which suggests restraint of molecular motion. The interlayer distance is estimated at about 0.15 nm, which is unusually short in comparison with that in other layered silicates (e.g., β-HLS or RUB-15) with similar framework topologies. The presence of hydrogen bonding between adjacent terminal O atoms was clearly revealed by the (1)H MAS NMR spectroscopy and by electron-density distribution obtained by the maximum entropy method.

On the influence of pore geometry and acidity on the activity of parent and modified zeolites in the synthesis of methylenedianiline

Methylenedianiline synthesis from formaldehyde and aniline is catalyzed by solid acid catalysts, such as zeolites, which offer strong acidity and high acid site density. Critical parameters for catalyst activity were explored using materials with MFI, BEA, MOR and FAU structure. While materials with MOR and MFI structure hardly show activity, BEA and FAU type zeolites are active and selective. The higher activity for the latter materials is related to their larger pore structure. Nevertheless, the reaction is limited by pore diffusion over parent BEA- and FAU-type zeolites. Mesoporous materials, such as dealuminated FAU, offer significantly higher activity as pore diffusion limitation can be overcome. Therefore, we synthesized and tested other mesoporous aluminosilicates such as desilicated BEA and the delaminated MWW materials ITQ-2 and MCM-36. It was shown that desilication increases the activity of parent BEA type zeolites for MDA formation, yet not to the same extend as dealumination in case of FAU. ITQ-2 shows promising results, but MCM-36 surpasses ITQ-2 in terms of activity by a factor of two displaying activities similar to the dealuminated FAU materials.

Selective production of isobutylene from acetone over alkali metal ion-exchanged BEA zeolites

Selective production of isobutylene from acetone was examined over BEA zeolites. Isobutylene production from acetone proceeds over solid-acid catalysts via a series of consecutive reactions, which include propylene and ethylene formation by cracking and coke formation. Because the isobutylene is an intermediate chemical in this process, low diffusion resistance of the isobutylene within the pore of the catalyst is needed to prevent its further conversion. To achieve this goal, we selected a BEA-type zeolite with 12-membered rings and a 3-dimensional pore structure. To obtain high yields of isobutylene, it is also important to inhibit undesired reactions of isobutylene and other intermediates that produce aromatics and coke. In this study, the acidity of the BEA zeolites was controlled using an ion-exchange method with alkali metals. The order of catalytic activity of the ion-exchanged BEA zeolites is as follows: H-BEA, Na-BEA, K-BEA, Rb-BEA, Cs-BEA, and is the same as the order of acidity of the zeolites. Though H-BEA zeolite showed a high acetone conversion at an initial reaction time, the activity of the catalyst drastically decreased due to coke formation. In contrast, the alkali metal ion-exchanged BEA zeolites exhibited high isobutylene selectivity at high acetone conversion conditions. Moreover, the amount of coke deposited on the catalyst decreased as compared to H-BEA zeolite. In K-BEA zeolite, the isobutylene yield reached 55%.

Influence of starting zeolite on synthesis of RUT type zeolite by interzeolite conversion method

In this study, hydrothermal conversions of ⁎BEA and FAU type zeolites using various structure-directing agents were carried out. Highly crystalline and pure RUT type zeolites were obtained from both zeolites in the presence of tetramethylammonium hydroxide. There were no major differences between the characteristics of the RUT type zeolites obtained from the two starting zeolites. However, the Si/Al ratio and the crystallization rate of the RUT type zeolites were strongly dependent on both the framework structure and the Si/Al ratio of the starting zeolite. That is, the crystallization rate of the RUT type zeolite from the *BEA type zeolite did not depend on the Si/Al ratio of the starting *BEA type zeolite, whereas the crystallization rate of the RUT type zeolite from the FAU type zeolite was dependent on the Si/Al ratio of the starting FAU type zeolite. This suggests that the chemical structure and the concentration of locally ordered aluminosilicate species produced by the decomposition/dissolution of the starting zeolite can be altered by changing the framework structure of the zeolite.

Plasma Synthesis of Highly Dispersed Metal Clusters Confined in Nanosized Zeolite

Metal-containing zeolites (and, more generally, porous materials) are very important for catalysis, electronics, and sensors, but their preparation is still a problem. Reproducibility and control of the properties of the metal cluster is difficult. Broad size distribution of the metal particles together with a low stability of the clusters is often observed. New methods have been reported, such as UV photolysis and g irradiation of hydrated metal complex ions incorporated within the microporous molecular sieves. Organic templates such as tetraalkylammonium cations can stabilize separate metal clusters and prevent agglomeration and also limit the metal loading in the material. However, such solids still need to be heat treated (calcined), which may result in a slow agglomeration of the clusters and a possible migration to the outer surface of the matrix, with pore blocking and low energy efficiency. Postsynthetic treatment by nonthermal plasma was recently applied to metal-loaded templated mesoporous materials (MCM and SBA-15). The plasma is believed to dissociate and ionize residual water molecules in the mesoporous matrix, creating hydrated electrons e ðaqÞ and radicals H· for reduction of the metal ions to clusters. The authors however still needed to use additional water and apply a final thermal treatment to remove residual impurities, resulting in partial sintering of the metal clusters. We report herein a one-step environmentally friendly approach for the preparation of active and highly dispersed metal clusters confined predominantly in the channels of nanosized BEA type zeolite (diameter of individual particles of 10 nm) by plasma treatment. This approach, which was discovered using operando FTIR in the plasma reactor, leads to a fast and simultaneous calcination of the nanosized molecular sieve (removal of the organic template) and reduction of the metal cations to clusters. Prior to calcination, the nanosized pure BEA and Pt-containing BEA (Pt/BEA, 0.75 wt% Pt) zeolites were directly subjected to Ar or O2 cold plasma treatments at low pressure (350 Pa). The changes in the samples under plasma treatment followed by operando FTIR spectroscopy are shown in Figure 1. The initial spectra of the pure BEA and Pt/BEA samples exhibit the methyl stretching modes at 3100–2800 cm 1 originating from the tetraalkylammonium (TEA) organic template (Figure 1). From the beginning of the plasma ignition, the intensity of the CH bands decreased due to the removal of the TEA template. In oxygen plasma, the IR feature of acidic Bronsted sites appeared at 3600 cm 1 (Figure 1b, c). This phenomenon is better pronounced in the Pt-containing BEA samples (Figure 1b). The degree of degradation of the organic template (D%) could be calculated by integrating the total absorbance, AHC, in the region of 3100–2800 cm 1 as a function of time according to the Equation (1):

Effect of Steam on the Steam-Assisted Crystallization of *BEA-Type Zeolite

Abstract We have studied the effect of steam in the steam-assisted crystallization (SAC) of *BEA-type zeolite. It was found that after an aluminosilicate dry gel with SiO2/Al2O3 = 100 was treated with steam in an autoclave at 130 °C and autogenous pressure, the thermal treatment of a resulting product at 130 °C in an autoclave without the addition of water gave *BEA nanoparticles with a high crystallinity. It was presumed that pseudo-*BEA structures as intermediates were formed during the SAC treatment, and the dehydration of silanols in the pseudo-*BEA solids during the thermal treatment in the presence of a minimum amount of water possibly promoted the transformation of the pseudo-*BEA phase into *BEA crystalline phase with a periodicity in long-range order.

Promising zeolite-type hydrocarbon trap catalyst by a knowledge-based combinatorial approach

Libraries consisting of more than 100 zeolite samples were prepared and examined for developing a promising HC trap catalyst. Parallel adsorptions of toluene onto the catalyst samples were conducted over a 10 × 10 array reactor under dry and wet conditions with or without a heating process three knowledge-based conditions for developing an automotive catalyst during the cold-start period. FAU and BEA type zeolites revealed a high performance of toluene adsorption under the dry condition. However, FAU type zeolite significantly decreased the amount of toluene adsorbed in the presence of water in the feed gas stream, mainly due to the hydrophobicity of the catalyst surface. Over Beta type zeolites, the toluene adsorbed was found to be considerably preserved, even after forced desorption temperature-ramping to the warm-up condition of an automotive engine. Li, K, or Ag ion-exchanged Beta zeolites seem to be particularly promising as an HC trap catalyst.

Elucidation of Medium-Range Structure in a Dry Gel-Forming *BEA-Type Zeolite

High-energy X-ray diffraction, 29Si nuclear magnetic resonance and Raman spectroscopy were applied to investigate medium-range structures in *BEA-type zeolite containing tetraethylammonium (TEA+) cation. In particular, the intermediates to form *BEA were focused the subject of focus. Double three-membered silicate rings (D3Rs) stabilized by TEA+ cation existed in the mother TEA+−silicate solution to prepare a dry gel that gave *BEA-type zeolite by the steam-assisted crystallization method. The D3R units transformed “4−2”-type secondary building units (SBUs) in the course of preparing the dry gel. Such “4−2”-type SBUs remained in the dry gel and played a significant role in the crystallization of *BEA-type zeolite. On the basis of these observations, we proposed that the presence of “4−2”-type SBUs in the dry gel is essential for the formation of *BEA-type zeolite in the steam-assisted crystallization, suggesting a plausible formation scheme of the periodic structure of *BEA-type zeolite in a long-range order via rearrangement of “4−2”-type SBUs.

Indication of Successful Crystallization of Dry Gel into *BEA-type Zeolite

Abstract Tetraethylammonium (TEA+)-containing silicate dry gels exhibiting a maximum in the reflection peak at 2θ = 28° in an XRD pattern using Cu Kα radiation are always transformed into *BEA-type zeolite with steam-assisted crystallization.

Control of the Morphology of All-Silica BEA-type Zeolite Synthesized in Basic Media

All-silica BEA-type (Si-BEA) zeolite readily crystallizes in fluoride quasi-neutral media, while its synthesis under basic conditions is difficult. To compare the morphological features of BEA-type materials synthesized in quasi-neutral and basic media, a two-step nucleation procedure, which allows controlled synthesis of Si-BEA under basic conditions, was developed. The procedure consists of (i) aging of the initial system at a close to neutral pH, and (ii) raising the pH and a secondary aging in basic media. This procedure was further employed to study the effect of the synthesis formulation, especially its sodium and potassium content, on the crystal morphology of Si-BEA. With an increase in the pH of the system, a gradual decrease of the size of the pyramidal (h0l) face coupled with an increase of the pinacoidal (00l) face was observed. Thus, the morphology of zeolite crystals can be varied from well-developed truncated bipyramids to platelike crystals, where the pinacoidal face is dominant. A consequ...

NO Oxidation Kinetics on Iron Zeolites: Influence of Framework Type and Iron Speciation

Zeolites having MFI, FER and *BEA topology were loaded with iron using solid state cation exchange method. The Fe:Al atomic ratio was 1:4. The zeolites were characterized using nitrogen adsorption, FTIR and DR UV–Vis–NIR spectroscopy. The catalytic activity in NO oxidation and the occurrence of NOx adsorption was determined in a fixed-bed mini reactor using gas mixtures containing oxygen and water in addition to NO and NO2 and temperatures of 200–350 °C. Under these reaction conditions, the NOx adsorption capacity of these iron zeolites was negligible. The kinetic data could be fitted with a LHHW rate expression assuming a surface reaction between adsorbed NO and adsorbed O2. The kinetic analysis revealed the occurrence of strong reaction inhibition by adsorbed NO2. FER and MFI zeolites were more active than *BEA type zeolite. MFI zeolite is most active but suffers most from NO2 inhibition of the reaction rate. FTIR and UV–Vis spectra suggest that isolated Fe3+ cations and binuclear Fe3+ complexes are active NO oxidation sites. Compared to the isolated Fe3+ species, the binuclear complexes abundantly present in the MFI zeolite seem to be most sensitive to poisoning by NO2.