Zeolites最新文献

The acidity of aluminum in a nominally 70:1 SiO₂/Al₂O₃, ratio ZSM-5 prepared by conventional means was characterized by TPAD, ²⁷Al and ²⁹Si n.m.r., XRD, and FTi.r. Discrepancies observed between the amount of tetrahedral “acidic” aluminum measured by TPAD and i.r. versus Al n.m.r. are due to the presence of Lewis aluminum. Furthermore, it is suggested that this Lewis aluminum may be located in the zeolite framework. These sites are not thought to be inherent to the ZSM-5 structure but are probably generated during calcination, resulting in partial hydrolysis of AlO bonds. A small percentage of octahedral aluminum was observed in hydrated HZSM-5, but was quantitatively converted to tetrahedral aluminum by treatment with ammonium nitrate solution at pH 8.

A new titanium precursor, TiF₄, has been used to synthesize titanium silicalite-1 (TS-1). In contrast to the commonly used alkoxides, TiF₄ is stable in air, water, and even basic TPAOH solutions, which offers considerable advantages as compared to conventional synthesis routes. In particular, TS-1 can be obtained with commercial TPAOH solutions, containing alkali cations, without formation of extraframework species. For samples synthesized with alkali-free TPAOH solutions, the maximum Ti incorporation that can be achieved corresponds to about 2.05 Ti/u.c., and TS-1 samples are similar to previously reported solids prepared with titanium alkoxides. In particular, they are very active in the ammoximation of cyclohexanone and the hydroxylation of phenol with hydrogen peroxide. When TS-1 is prepared with commercial TPAOH solutions, alkali cations can be removed by washing the solid with dilute acid, and the catalytic performance of the washed zeolite is similar to that of samples synthesized with alkali-free TPAOH.

Mesoporous MCM-41 materials have been synthesized and loaded with copper pyridine complexes by an ion-exchange procedure. Nitrogen adsorption and X-ray diffraction studies of the parent and loaded MCM-41 materials confirm the incorporation of the copper complexes into the mesopores of the MCM-41 structure. The incorporation status of the organometallic copper complexes is monitored by electron spin resonance and electron spin echo envelope modulation spectroscopy during the preparation procedure. Immobilized isolated copper pyridine complexes as well as agglomerates of complexes have been found in the ion-exchanged mesoporous material.

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.

The XPS method is employed to compare the behavior of [Pt(NH₃)₄]²⁺ ions in the surface layers of KX and KY zeolites with that in the zeolite bulk. FT i.r. and MS-t.p.d. methods are used to characterize the bulk properties. XPS and FT i.r. reveal the formation of different intermediate NH_x species during the decomposition of the ammine complex in a vacuum. They are interpreted as Pt-diammine, framework ammine, and immine complexes. The maximum evolution of ammonia occurs in the temperature interval ranging from 150°C to 250°C. Positive shifts of Pt: (4f) photoelectrons, observed for samples with a decomposed Pt-ammine complex, are explained by a decrease of the population of Pt 5d levels in encaged metallic clusters. The surface concentration of Pt decreases with increasing temperature due to the inward diffusion of Pt atoms.

Hydrothermal synthesis of zeolite beta with an aluminum content as high as 8 atoms per unit cell has been performed in sodium-tetraethylammonium systems. Low alkalinity is required to avoid the formation of denser phases. Sodium cations are needed for charge balance beyond 6 Al/unit cell. The concentration of lattice defects in the solids richest in aluminum provides information about the templating effect of tetraethylammonium in this system.