Ba-exchanged Zeolite Research Articles

The role of residual Na + and Li + on the thermal transformation of Ba-exchanged zeolite A

This work intends to critically review the data concerning the variations of reactivity of Ba-exchanged zeolite A precursor with Na + and Li + content originally present in the zeolite framework. The main results were the following: a) obtaining a fully monoclinic celsian from Ba-exchanged zeolite A bearing Na + residual content ranging between 0.20 and 0.74 meq/g required a 5–22 h thermal treatment at 1300 °C; b) obtaining a fully monoclinic celsian from Ba-exchanged zeolite A bearing the not further reducible Na + residual content (about 0.20 meq/g) and Li + contents ranging from 0.11 to 0.37 meq/g required a 4–10 h thermal treatment at 1000–1100 °C. New insight into the mechanism through which Na + and Li + develop their mineralizing action on the thermal transformation of Ba-exchanged zeolite A was attained.

Parameters Expediting the Thermal Conversion of Ba-Exchanged Zeolite A to Monoclinic Celsian

Four samples of Ba-exchanged zeolite A, bearing small residual amounts of Na (0.27, 0.43, 0.58, and 0.74 meq/g), were thermally treated in the temperature range 200–1500∘C for times up to 28 hours. The same samples were pressed at 30 and 60 MPa to form cylindrical pellets which were thermally treated at 1300∘C for 5 hours. All materials were characterized by room temperature XRD. The sequence of thermal transformations that Ba-exchanged zeolite A undergoes (zeolite → amorphous phase → hexacelsian → monoclinic celsian) and the strong mineralizing action developed by Na are confirmed. Pressing the Ba-exchanged zeolite A powder-like samples to obtain cylindrical pellets is found to expedite the sluggish final phase transition hexacelsian → monoclinic celsian. The optimum residual Na content of Ba-exchanged zeolite A for transformation into monoclinic celsian is assessed to be between 0.27 and 0.43 meq/g.

Corrigendum to “Densification and crystallization of Ba-exchanged zeolite A powders” [Ceram. Int. 34 (2008) 543–549

Corrigendum to “Densification and crystallization of Ba-exchanged zeolite A powders” [Ceram. Int. 34 (2008) 543–549

The lithium- and sodium-enhanced transformation of Ba-exchanged zeolite LTA into celsian phase

The Li +- and Na +-doped hexacelsians (HC) synthesized from Ba-LTA synthetic zeolite as precursors were used for preparation of monoclinic celsians (MC). The doped pure MC and mixture HC/MC species were obtained by thermally induced polymorphous transformation at 1200 °C. Synthesized-doped MC have been characterized by X-ray powder diffraction and spectroscopic (infrared, Raman and 29Si magic angle spinning nuclear magnetic resonance) methods. The obtained results suggest that in all investigated samples Na + or Li + dopants were incorporated in MC crystal structures during thermal transformation of a zeolite. It has been shown that HC→MC transformation depends on molar fractions of alkali cations. These findings combined with used temperature/time conditions could help in optimization of MC synthesis route.

Densification and crystallization of Ba-exchanged zeolite A powders

The effect of thermal treatment, Na content and mineralizer ion on the sintering process of Ba-exchanged zeolite A on the zeolite → celsian thermal transformation are investigated. The powder samples containing different amounts of Na + and Li + were pressed at 30 MPa and thermally treated at temperatures from 1000 to 1400 °C for times up to 5 h and subsequently were characterized by room temperature X-ray diffraction and by scanning electron microscopy. Increasing the Na residual content in the Ba-zeolite A samples improves the sintering process, even if the highest Na content appears to inhibit the zeolite → celsian transformation, since a new crystalline phase appears at the highest temperature. Moreover, the porosity of all samples thermally treated is quite high. Finally the manufacture of pressed samples allowed lower temperatures and times to be used to obtain the transformation zeolite Ba-A → monoclinic celsian, which suggests it is a potential method to prepare celsian low temperature refractory materials. At last an ANOVA analysis was carried out to identify the independent parameters from a statistical point of view.

Microwave assisted hydrothermal conversion of Ba-exchanged zeolite A into metastable paracelsian

A microwave assisted crystallization of paracelsian, a metastable phase for which no synthetic occurrence is at present reported, was obtained. A sample of thoroughly Ba-exchanged zeolite A was prepared and reacted in alkaline media and hydrothermal conditions under the action of microwaves (frequency 2.45 GHz, irradiation power 1000 W). The alkaline environment was created by using aqueous solutions of LiOH and tetraethylammonium hydroxide. Treatment temperature, reaction time and weight solid/liquid ratio ranged between 150 and 230 °C, 40 and 120 min, 1/5 and 1/40, respectively. The pressure was autogeneous. The action of microwaves appears to strongly enhance the ability of hydrothermal systems to produce highly metastable phases, an important observation.

New Perspectives on Basic Zeolites as Catalysts and Catalyst Supports

AbstractFor Abstract see ChemInform Abstract in Full Text.

New perspectives on basic zeolites as catalysts and catalyst supports

Zeolites are aluminosilicate molecular sieves that have great potential as basic catalysts and catalyst supports. In particular, this paper discusses recent work and future directions with zeolite X containing exchanged alkali metal cations, occluded alkali metal oxides, and occluded alkali metals. Probe reactions such as alkylation of toluene with methanol, cyclo-addition of carbon dioxide to ethylene oxide, isomerization of 1-butene, and alkylation of toluene with ethylene are catalyzed by the basic X zeolites, depending on the preparation method. Basic zeolites are also excellent supports for nanometer-sized Ru metal clusters used to catalyze the ammonia synthesis reaction. In that case, occluded barium oxide effectively promotes the reaction on Ru-loaded Ba-exchanged zeolite X. These examples illustrate important principles that can be used to guide future design of shape-selective base catalysts.

FTIR study of the thermal transformation of barium-exchanged zeolite A to celsian

In this study room temperature X-ray diffraction patterns and FTIR spectra of zeolite A in its Na form, Ba-exchanged zeolite A, hexacelsian, monoclinic celsian and Ba-exchanged zeolite A samples thermally treated at temperatures ranging from 200 to 1000 °C are reported. The combined inspection of room temperature X-ray diffraction patterns and FTIR spectra reveals the following points. 1) The thermal collapse of the microporous zeolite structure goes to completion upon thermal treatment at 200–300 °C; in the resulting amorphous phase the presence of the secondary building units of the zeolite A structure may be detected. 2) Thermal treatment in the temperature range 200–400 °C results in a middle-range order which favours the crystallisation of small crystallites of monoclinic celsian at 500 °C. 3) The further evolution of the amorphous phase in the 500–800 °C temperature range of thermal treatment instead of promoting the crystalline growth of monoclinic celsian, creates a middle-range order favourable to the crystallisation of hexacelsian which occurs at 1000 °C.

New Insight into the Thermal Transformation of Barium-Exchanged Zeolite A to Celsian

The thermal transformation of a sample of Ba-exchanged zeolite A, containing a residual Na content of 0.20 meq/g, into monoclinic celsian is thoroughly investigated. The samples were thermally treated at temperatures ranging from 200 to 1500 °C for times up to 40 h and subsequently were characterized by room-temperature X-ray diffraction. The following are the main results of this investigation:(1) Above 500 °C, the presence of small crystalline aggregates of monoclinic celsian dispersed in the amorphous phase arising from the thermal collapse of Ba-exchanged zeolite A is recorded.(2) Above 900 °C, crystallization of hexacelsian occurs.(3) Above 1100 °C, hexacelsian converts into monoclinic celsian. The conversion is complete with thermal treatments at 1300, 1400, and 1500 °C within 22, 15, and 22 h, respectively.The presence of monoclinic celsian nuclei above 500 °C appears to favor the conversion of hexacelsian into monoclinic celsian at temperatures higher than 1100 °C. The residual Na content of Ba-exchanged zeolite A was found to strongly affect the temperatures and times at which the transformations of amorphous phase → hexacelsian and hexacelsian → monoclinic celsian occur. Actually a higher Na residual content results in lower temperatures and times, ceteris paribus.

Synthesis of Monoclinic Celsian from Ba-Exchanged Zeolite A.

Two kinds of zeolites with different crystal structures with the composition of BaO Al 2 O 3 2SiO 2 xH 2 O prepared by an ion-exchange technique were used as a precursor of celsian. The difference between the zeolite structures resulted in the difference in modifications of celsian obtained through a heating treatment. Ba-exchanged zeolite A became an amorphous material at 300°C with dehydration, and then generates a small amount of monocelsian in the amorphous material at 500°C. When successively at 950°C for 1 h, hexacelsian formed in addition to the previously produced monocelsian. A single phase of monocelsian was obtained finally by heating at 1500°C for 8 h. The specimen heated at 1700°C, however, became a single phase of hexacelsian, owing to monocelsian-to-hexacelsian phase transformation. The structure of Ba-exchanged Linde F zeolite collapsed at 300°C with dehydration, and only hexacelsian formed at temperatures above 980°C. The reason for the formation of monocelsian only from Ba-exchanged zeolite A was explained in terms of the similarity in framework structures of zeolite A and monocelsian.

Thermal transformation of Ba-exchanged A and X zeolites into monoclinic celsian

The thermal transformation of Ba- and (Ba+Li)-exchanged precursors of zeolites A and X into monoclinic celsian has been investigated by X-ray diffractometry, differential thermal analysis, and thermodilatometry. Upon the proper thermal treatment either Ba-exchanged zeolite A or X give rise to the following transformations: zeolite→amorphous phase→hexacelsian→monoclinic celsian. Zeolite A-based precursors are more reactive than zeolite X-based precursors allowing all the reported transformations to occur at far lower temperatures starting from Ba-exchanged zeolite A rather than from Ba-exchanged zeolite X. In particular, starting from zeolite X-based precursors, fully monoclinic celsian may be obtained after 24 h at 1550°C in the absence of Li and after 6 h at 1100°C in the presence of Li, whereas starting from zeolite A-based precursors it may be obtained after 6 h at 1100°C in the absence of Li and by slow heating (1°C/min) up to 900°C in the presence of Li.

A theoretical study of the stability of barium-exchanged Na—A type zeolites

From the geometry shown by X-ray crystallographic studies, potential energy functions suitable for energetic calculations in Ba-exchanged zeolite A, (Ba x 2 Na 12− x −A, 1 ⩽ x ⩽ 12), have been obtained by using the method of constraints. The influence of the water content and degree of Ba exchange on the stability of the framework has been investigated energetically using these potential functions. The stability of the framework decreases as the Ba content increases and as the water content decreases.