An investigation of the roles of surface aluminum and acid sites in the zeolite MCM-22

Ammonia adsorption studies reveal that the observed Lewis acidity in the zeolite MCM-22 is derived from at least two types of framework aluminum sites (Al(F)), that is, octahedral Al(F) and three-coordinate Al(F). Comparative ammonia or trimethylphosphine (TMP) adsorption experiments with MCM-22 confirm that octahedral Al species gives rise to the signal at delta(ISO) approximate to 0 in the (27)Al NMR spectrum; this is a superposition of two NMR signals from the different Al species on the water-re constructed zeolite surface. A sharp resonance assigned to framework Al reversibly transforms on ammonia adsorption to delta(ISO) (27)Al approximate to 55 from tetrahedral Al(F), while the broad peak is assigned to nonframework aluminium which results from hydrothermal treatment. This study also demonstrates the effectiveness of (27)Al magic angle spinning (MAS) and multiple quantum (MQ) MAS NMR spectroscopy as a technique for the study of zeolite reactions.

Effect of pore ring number of zeolites on catalytic performance of Mo/Zeolite in methane aromatization under non-oxygen condition

The catalytic behavior of Mo-based zeolite catalysts with different pore structure and size, particularly with 8 membered ring ( M R), 10 M R, coexisted 10 and 12 M R, and 12 M R, was studied in methane aromatization under the conditions of SV=1500 ml/(g.h), p=0.1 MPa and T = 973 K. It was found that the catalytic performance is correlated with the pore structure of the zeolite supports. The zeolites that possess 10 MR or 10 and 12 MR pore structure with a pore diameter equal to or slightly larger than the dynamic diameter of benzene molecule, such as ZSM-5, ZSM-11, ZRP-1 and MCM-22, are fine supports. Among the tested zeolite supports, MCM-22 exhibits the highest activity and selectivity for benzene. A methane conversion of 10.5% with benzene selectivity of 80% was achieved over Mo/MCM-22 catalyst. The Mo/ERS-7 catalyst with 8 MR (0.45 nm) does not show any activity in methane dehydro-aromatization, while Mo/JQX-1 and Mo/SBA-15 catalysts with 12 MR pore exhibit little activity in the reaction. It can be concluded that the zeolites with 10 MR pore or coexisted 10 and 12 MR, having pore size equal to or slightly larger than the dynamic diameter of benzene molecule, are fine supports for methane activation and aromatization.

NMR study on the acidity of TS-1 zeolite

The acidic properties of TS-1 and Silicalite-I zeolites have been investigated by the solid-state MAS NMR technique capable of in situ sample pretreatment. As shown by a combination of the P-31 MAS NMR and H-1 MAS NMR techniques with trimethylphosphine, not only Bronsted acid sites but also Lewis acid sites exist in the TS-1 zeolites. Moreover, TS-1 zeolite is more acidic compared with Silicalite-1. The H-1, Si-29 MAS NMR spectra and the resonance related to Bronsted acid species in the P-31 MAS NMR spectra demonstrate clearly that the presence of titanium in the framework results in the formation of a new hydroxy group, titanols, which is more acidic than silanols of Silicalite-1. The P-31 MAS NMR measurements also illustrate convincingly the existence of at least two different Lewis acid species on the TS-1 zeolites. The conversion of propylene oxide into methoxypropanol catalyzed by TS-1 or Silicalite-I zeolite in methanol solution as a test reaction has also been described. With the increase of titanium in zeolite, TS-1 appears to have a higher activity during the reaction of propylene oxide to methoxypropanol.

Methane dehydro-aromatization over a Mo/phosphoric rare earth-containing penta-sil type zeolite in the absence of oxygen

The dehydro-aromatization of methane over a Mo-modified penta-sil type high-silica zeolite containing phosphoric and rare earth oxide (abbreviated as Mo/HZRP-1) was investigated. As a modification of HZSM-5, HZRP-1 is also a good support for the preparation of Mo-based zeolite catalysts, and is active for methane dehydro-aromatization. Mo/HZRP-1 catalysts are more active at high Mo loadings compared with Mo/HZSM-5 catalysts. Al-27 MAS NMR spectra of Mo/HZRP-1 reveal that there are two kinds of framework Al in HZRP-1. It is suggested that only the tetrahedral coordinated Al atoms in the form of Al-O-Si species in the zeolite, in the proton forms, are responsible for the formation of aromatics.

Oriented growth of MCM-22 zeolite films

MCM-22 zeolite films have been grown with layers vertical to the substrate under hydrothermal synthesis conditions, and characterized by X-ray diffraction and scanning electron microscopy.

Growth of ultrafine zeolite Y crystals on metakaolin microspheres

Ultrafine zeolite Y crystals (ca. 100-200 nm) have been successfully grown on metakaolin microspheres (< 100 mu m) for which good hydrothermal stability was observed; products were characterized by powder X-ray diffraction, scanning electronic microscopy and transmission electronic microscopy.

Synthesis and gas permeation properties of an NaA zeolite membrane

A high quality NaA zeolite membrane, which shows a H-2/n-C4H10 permselectivity of 106, has been synthesized on a seeded alpha-Al2O3 support by a multistage synthesis method.

Fast formation of NaA zeolite membrane in the microwave field

NaA zeolite membrane was successfully synthesized on the porous alpha-Al2O3 support by microwave heating. The synthesis of NaA zeolite membrane in the microwave field only needs 15 min and the synthesis time is 10 times shorter than that by conventional heating. SEM characterization indicates that the zeolite crystals in the NaA zeolite membrane synthesized by microwave heating are uniform in size; the membrane thickness is about 4 mu m and is thinner than that of the NaA zeolite membrane synthesized by conventional heating. Gas permeation studies indicate that the permeances of the NaA zeolite membrane synthesized by microwave heating are 3-4 times higher than those of the NaA zeolite membrane synthesized by conventional heating, while their permselectivities are comparable.

Synthesis of a high-permeance NaA zeolite membrane by microwave heating

Zeolite membranes with high permeance and separation factors are highly desirable for practical applications. Although, in the past, very good separation factors have been obtained, it has proved difficult to achieve a high permeance. Ken a comparative study of microwave versus conventional heating in the hydrothermal synthesis of NaA zeolite membranes is made. It is demonstrated that membranes prepared by microwave heating have not only a higher permeance but also a considerably shorter synthesis time. These observations are rationalized by examining the mechanism of membrane formation.