Preparation of A-type zeolite membranes on nonporous metal supports by using electrophoretic technique

A-type zeolite membranes were prepared on the nonporous metal supports by using electrophoretic technique. The as-synthesized membranes were characterized by XRD and SEM. The effect of the applied potential on the formation of the A-type zeolite membrane was investigated, and the formation mechanism of zeolite membrane in the electric field was discussed. The results showed that the negative charged zeolite particles could migrate to the anode metal surface homogenously and rapidly under the action of the applied electric field, consequently formed uniform and dense membranes in short time. The applied potential had great effect on the membrane formation, and more uniform and denser zeolite membranes were prepared on the nonporous metal supports with 1 V potential.

Synthesis and properties of A-type zeolite membranes by secondary growth method with vacuum seeding

A-type zeolite membranes were successfully synthesized on tubular alpha-Al2O3 supports by secondary growth method with vacuum seeding In the seeding process, a thin, uniform and continuous seeding layer was closely attached to the support surface by the pressure difference between the two sides of the support wall. The effects of seed particle size, suspension concentration, coating pressure difference and coating time on the membrane and its pervaporation properties were investigated. The as-synthesized membranes were characterized by XRD and SEM. The quality of the membranes was evaluated by the pervaporation dehydration of 95 wt. % isopropanol/water mixture at 343 K. High quality A-type zeolite membranes can be reproducibly prepared by the secondary growth method with vacuum seeding under the conditions: seed particle size of 500-1200 nm, suspension concentration of 4-8 g/l, coating pressure difference of 0.0100-0.0250 MPa and coating time of 45-180 s. (C) 2004 Elsevier B.V. All rights reserved.

A DFT study on isomorphously substituted MCM-22 zeolite

The density functional theory has been used to study the isomorphously substituted MCM-22 zeolite for the first time. The effect of the basis sets on the calculation results is discussed in details. Data of several index properties for characterizing the relative acidity of T-MCM-22 (T = B, Al, Ga, and Fe), including proton affinity, bond length and bond angle, OH stretching frequency, and charge on the acidic proton, show that the acidity of T-MCM-22 increases in the sequence of B-MCM-22 < Fe-MCM-22 < Ga-MCM-22 < Al-MCM-22. After making a correction, the calculated OH stretching frequencies for Al-MCM-22 and Fe-MCM-22 show a reasonable agreement with the experimental data. On the basis of an equilibrium structure of the B-MCM-22 zeolite, the effect of the B element in the synthesis of the Ti-MCM-22 is also discussed. The adding of the B element during the synthesis of the Ti-MCM-22 can decrease greatly the Ti substitution energy because of the forming of a structure quite similar to the terminal silanol group. The results can provide some constructively information for zeolite synthesis.

Solid-state MAS NMR studies on the hydrothermal stability of the zeolite catalysts for residual oil selective catalytic cracking

By using the solid-state MAS NMR technique, the hydrothermal stabilities (under 100% steam at 1073 K) of HZSM-5 zeolites modified by lanthanum and phosphorus have been studied. They are excellent zeolite catalysts for residual oil selective catalytic cracking (RSCC) processes. It was indicated that the introduction of phosphorus to the zeolite via impregnation with orthophosphoric acid led to dealumination as well as formation of different Al species, which were well distinguished by Al-27 3Q MAS NMR. Meanwhile, the hydrothermal stabilities of the zeolites (P/HZSM-5, La-P/HZSM-5) were enhanced even after the samples were treated under severe conditions for a prolonged time. It was found that the Si-O-Al bonds were broken under hydrothermal conditions, while at the same time the phosphorous compounds would occupy the silicon sites to form (SiO)(x)Al(OP)(4 - x) species. With increasing time, more silicon sites around the tetrahedral coordinated Al in the lattice can be replaced till the aluminum is completely expelled from the framework. The existence of lanthanum can partially restrict the breaking of the Si-O-Al bonds and the replacement of the silicon sites by phosphorus, thus preventing dealumination under hydrothermal conditions. This was also proved by P-31 MAS NMR spectra. (C) 2004 Elsevier Inc. All rights reserved.

Characterization of the acid sites in dealuminated nanosized HZSM-5 zeolite with the probe molecule trimethylphosphine

The acid sites in dealuminated HZSM-5 zeolite with crystal sizes down to the nanoscale were firstly characterized by the probe molecule trimethylphosphine (TMP). As evidenced by the combination of P-31 CP/MAS NMR, Al-27 MAS and H-1 --> Al-27 CP/MAS NMR measurements, the Bronsted acid sites of both microsized and nanosized HZSM-5 could be decreased upon the dealumination of zeolitic framework after hydrothermal treatment. At the same time, the appearance of Lewis acid sites was observed. The dealuminated nanosized HZSM-5 is easier to form Lewis acid sites than microsized HZSM-5, and the type of Lewis acid sites in nanosized HSM-5 is more than one. In addition, the origin of Lewis acid sites is mainly associated with the aluminum at ca. 30 ppm, in the Al-27 MAS NMR spectra, and only a part of which in the dealuminated HZSM-5 zeolite acts as Lewis acid sites. (C) 2002 Elsevier Science B.V. All rights reserved.