As-prepared Zeolite Research Articles

<i>In Situ</i> Synthesis of Zeolite NaKL In Highly-Enriched Spinel Matrix

Zeolite NaKL was in-situ crystallized under static condition with kaolin microspheres calcined at 980 °C as silicon and aluminium sources. Influences of hydrothermal synthesis conditions such as the mole ratio of n(K2O)/n(Na2O), n(OH-)/n(SiO2) and crystallizaion time on crystallization of zeolite L were studied in detail. The crystal phase and morphology of the as-prepared zeolite samples were characterized by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The results showed that the as-synthesized samples owned characteristic peaks of zeolite L with high crystallinity and were covered uniformly by regular crystal of zeolite L on its kaolin microsphere surface , which maintained complete ball shape as before. The optimum conditions to get zeolite NaKL with high crystallinity by this method were found to be the mole ratio of n(K2O)/n(Na2O) =7:3 and n(OH-)/n(SiO2) =0.50 in the reaction mixture, an aging of the reactants at room temperature for 30 h and crystallization temperature 120 °C for 24 h.

A simple seeding method for MFI zeolite membrane synthesis on macroporous support by microwave heating

An effective seeding method is a mandatory step to have thin and defect-free zeolite membranes prepared by means of secondary growth method on macroporous tubular supports. The permeation properties of the as-prepared membranes depend strongly on the seeding method of zeolite crystals during which the whole support surface should be covered uniformly and sufficiently. In this work a novel and simple seeding method was designed and developed to prepare seed layer on low-cost macroporous Al 2O 3 support. This seeding technique includes three steps: (a) pre-modify the support surface by hot dip-coating in large seed (1.2 μm) suspension; (b) rub off the excess large seeds which stick on the support surface; (c) further modify the support by hot dip-coating in another seed (0.4 μm) suspension. After the composite seeding process, silicalite-1 membrane was synthesized by microwave heating (MH) at 423 K for 2 h using the synthesis solution of 1SiO 2:0.3TPAOH:80H 2O:4C 2H 5OH. Results showed the as-prepared zeolite membrane was dense and continuous with a thickness of ∼4 μm. Moreover, it exhibited high hydrogen permeance of 3.01 × 10 −6 mol m −2 s −1 Pa −1 and permselectivity of 66 for H 2/i-C 4H 10 at room temperature.

Following the evolution of iron from framework to extra-framework positions in isomorphously substituted [Fe,Al]MFI with 57Fe Mössbauer spectroscopy

Isomorphously substituted [Fe,Al]MFI zeolites enriched with 57Fe isotope were synthesized with varying iron (0.075–0.6 wt%) and aluminum (0–1.1 wt%) concentrations. The as-prepared zeolites underwent sequential post-treatment steps (i.e., calcination, transformation to H-form, and steam treatment), making them active in the one-step oxidation of benzene to phenol with N 2O as oxidant. Through 57Fe Mössbauer spectroscopy, and transmission electron microscopy (TEM) as a complementary technique, we followed the evolution of the physico-chemical states of iron during the different post-treatment steps. In general, the extent to which iron is removed from framework to extra-framework positions, during different post-treatments, is higher for [Fe,Al]MFI zeolites containing high iron and aluminum concentrations. For all steamed [Fe,Al]MFI zeolites with an aluminum content of ∼1.1 wt%, iron was predominantly present in the high-spin Fe 2+ state (ca. 90% based on spectral contribution). This extraordinarily high concentration of Fe 2+ species is significant, since the presence of Fe 2+ was correlated to the formation of α-sites, which were reported to be responsible for the direct oxidation of benzene to phenol [1]. For [Fe,Al]MFI with less aluminum (∼0.6 wt%), a mixture of Fe 2+ (at least 30%) and Fe 3+ ions was observed, whereas for the aluminum-free sample, only iron in the Fe 3+ state was obtained.

X-ray Absorption Fine Structure Analysis of the Local Environment of Fe in Fe/Al−MFI

An MFI zeolite framework containing iron and aluminum has been prepared and characterized by X-ray absorption spectroscopy. Extended X-ray absorption fine structure analysis indicates that in the as-prepared zeolite Fe is present in the framework but migrates to extraframework sites upon thermal activation at elevated temperature. In the absence of framework Al, Fe forms small particles of Fe3O4. When Al is included in the zeolite, extraframework Fe associates with framework Al in a manner identical to that observed in Fe-exchanged H-ZSM-5. Therefore, the presence of framework Al in the MFI structure plays a critical role in stabilizing atomically dispersed Fe.

Raman spectra of zeolites exchanged with uranyl(VI) cations—I. Zeolite Y

UO 2Y zeolites prepared by various methods have been characterized by Raman spectroscopy. The speciation of [UO 2] 2+ cations in as-prepared zeolites was found to be determined by hydrolytic reactions occurring at the zeolite—solution interface during ion-exchange, which resulted in the formation of hydroxy-bridged polynuclear complexes in all zeolites investigated. The extent of cation hydrolysis was found to be significantly reduced if the parent NaY zeolites were pretreated in mildly acidic media prior to ion-exchange, and a model for the chemical processes occurring during sample preparation is proposed.

1H mas NMR and IR studies of the acidic properties of realuminated zeolite Y

IR and1H MAS NMR confirm that extra-framework aluminium present in dealuminated (ultrastable) zeolite Y is reintroduced into the framework by treatment with strongly basic solutions at elevated temperatures. The realuminated sample contains twice as many Bronsted acid sites than the ultrastable precursor and, with an accuracy of 20%, the same number of acid sites as the parent as-prepared zeolite. However, not as many hydroxyl groups associated with framework Al in the product are acccesible to pyridine as in the parent sample.

Monitoring cation-site occupancy of nickel-exchanged zeolite Y catalysts by high-temperature in situ X-ray powder diffractometry

Procedures based on the Rietveld method of refinement of X-ray powder profiles have been used to determine the sites and occupancies of extra-framework cationic nickel, potassium and sodium in typical zeolitic catalysts at temperatures in the range 400–500 °C. The siting of Ni2+ and other ions at high temperature differs from that at room temperature previously reported. There are also significant differences in the location of cations and in the reducibility of Ni2+ which depend upon the nature of the treatment (e.g. steam treatment, treatment with an alkaline solution) to which the as-prepared zeolite is subjected prior to dehydration and/or reduction.

Solid-state NMR studies of framework defects in thermally treated zeolite Ca-A

29Si, 27Al and 1H magic-angle-spinning NMR and relaxation measurements on thermally treated zeolite Na 3.2Ca 4.4-A have been carried out in order to study framework defects responsible for, and surface barriers to, molecular diffusion. Various possible mechanisms for the formation of such defects are considered in detail. We conclude that no framework dealumination takes place and no acidic hydroxyl groups are present in the zeolite. Excess aluminium occluded in the as-prepared zeolite A is responsible for the production of aluminium hydroxide during hydrothermal treatment.