Yajing Wu

Framework-Substituted Lanthanide MCM-22 Zeolite: Synthesis and Characterization

Incorporation of Ce and La into the framework of MCM-22 zeolite has been achieved by cohydrolysis and condensation of tetraethylorthosilicate and lanthanide salts in moderate/weak acidic media followed by a switch of synthesis gels to basic conditions for hydrothermal crystallization. The promotion effect of the framework Ce (La) when Ce(La)-MCM-22 serves as the catalyst support for hydroisomerization of n-heptane is demonstrated. Framework substitutions of lanthanides are evidenced by a set of mutually complementary characterizations, catalytic tests, and item-by-item comparisons with the impregnated and ion-exchanged counterparts. The novel synthesis strategy may give further outlines for the production of other types of heteroatomic zeolites.

Unseeded organotemplate-free hydrothermal synthesis of heteroatomic MFI zeolite poly-nanocrystallites

A facile one-pot method without organic templates and seeds is reported for the fabrication of framework-substituted heteroatomic zeolites for the first time. Such a breakthrough is achieved by the acidic co-hydrolysis of tetraethylorthosilicate (TEOS) with metal salts, followed by basic aging and hydrothermal crystallization. Fe(Mo, Ti, Co, Cr) MFI zeolites can be obtained with high crystallinity, thermal stability and tunable chemical composition, in which Fe3+ and Al3+ are proposed to act as co-structure-directing agents, inducing the growth of the zeolite crystal and their special poly-nanocrystalline morphology. Structural characterizations and Knoevenagel condensations were conducted to evidence the incorporation of Fe3+ into the MFI framework. The catalytic activity on the as-synthesized framework-substituted Fe-ZSM-5 is not only tens of times higher than that of Fe3+-exchanged or ferric oxide impregnated analogues, but also much higher than that of its as-calcined analogue, suggesting the unique ability of our method to inhibit the transfer of heteroatoms out of the framework. This easy, controllable and green new strategy may be extended to other heteroatomic MFI zeolites, and is potentially important for practical utilization.

Hydrothermal incorporation of Ce(La) ions into the framework of ZSM-5 by a multiple pH-adjusting co-hydrolysis

Ce and La incorporated ZSM-5 zeolitic materials with different metal loadings were synthesized via the multiple pH-adjusting procedures involving a co-hydrolysis of TEOS (tetraethyl-orthosilicate) with rare earth salts under a strong acidic condition, a further gelation under a weak acidic condition, and a final basic hydrothermal crystallization. Thus obtained Ce(La)-ZSM-5, as well as Ce or La ion-exchanged ZSM-5 samples, were characterized by XRD, SEM, TG, UV–vis, 29Si MAS NMR, FT-IR, N2-physisorption, and elemental analysis. The characterizations provide systematic and complementary evidences to demonstrate the incorporation of Ce(La) ions into the tetrahedral sites of ZSM-5 framework. The upper limits of the rare earth loadings in the synthesis gel and the final product are ca. 50 and 70 of the molar ratio of Si/Ce(La), respectively. It is observed that the synthesized Ce(La)-ZSM-5 zeolites possess an unusual morphology of microspheres that are assembled by numerous cubic prism-shaped nanocrystallines.

Direct hydrothermal synthesis and characterization of framework-substituted Co(Mn)-Beta zeolites

Isomorphously substituted Co(Mn)-Beta zeolites were directly synthesized through a static hydrothermal approach. The silica-metal precursor was prepared by co-hydrolysis of tetraethylorthosilicate with cobalt or manganese nitrates in acidic media, which was adjusted to the strong basic condition by adding the structural directing agent (tetraethylammonium hydroxide) and Al source (sodium aluminate) for the subsequent hydrothermal crystallization. The products were characterized by X-ray diffraction, inductively coupling plasma elemental analysis, Brunauer–Emmett–Teller surface area, nitrogen sorption isotherm, FT-IR, UV–vis, and SEM. The results show that Co or Mn ions are incorporated into the framework of zeolite Beta, and the thus obtained Co(Mn)-Beta zeolites are nano-structured zeolite crystallites. The acidic co-hydrolysis/considention is proposed to facilitate the formation of metal-bearing nucleus for growing Co(Mn)-Beta crystals.