Zeolites Offretite Research Articles

Pressure-mediated crystal-fluid interaction in the zeolite offretite

The high-pressure behaviour and the crystal-fluid interaction of a natural offretite have been investigated by in situ single-crystal synchrotron X-ray diffraction, using a diamond anvil cell. Five different P-transmitting fluids have been employed: the non penetrating daphne oil 7575 and the potentially penetrating methanol:ethanol:H2O ​= ​16:3:1 mixture, ethanol:water ​= ​1:1 mixture, distilled H2O and liquid Ne. The daphne oil 7575 experiment allowed to describe the intrinsic compressional behaviour of offretite, without any pressure-induced crystal-fluid interaction, with an isothermal bulk modulus KV0 ​= ​59(2) GPa (βV0 ​= ​0.0170(9) GPa−1) for the range Pamb-1.83(5) GPa. All the experiments in potentially penetrating P-transmitting fluids showed the P-induced sorption of extra molecules within the large 12 ​mRs channel of offretite, obtained on the basis of X-ray structure refinements. For the aqueous mixtures, the magnitude of the adsorption appears to be governed by the H2O content of the P-transmitting fluid. Ne atoms are also able to penetrate into the 12 ​mRs channel in response to the applied pressure, interacting with the extraframework population via weak Van der Waals attraction. A comparative analysis of the P-mediated crystal-fluid interaction phenomena between offretite and the structurally similar erionite is provided.

Solid solutions of offretite, direnzoite, and ferrierite

The three-dimensional geometric analysis carried out in this study has made it possible to clearly outline the homogeneity regions of solid solutions rich in magnesium and potassium oxides. Common elements in the structures of the zeolites (offretite, direnzoite, and ferrierite-Mg) have been identified and the factors responsible for the distinctions between their structures have been analyzed.

The synthesis of offretite single crystals in the system containing pyrocatechol or F −

Zeolite offretite (OFF), especially large single crystal, is widely used in many fields. But there has been seldom report about the synthesis of large single crystals of offretite. Recently, offretite single crystals have been successfully prepared in the systems containing pyrocatechol and F −. Compared with other methods, zeolite offretite synthesized in these two systems has much better morphology and larger size. Pyrocatechol and F − are effectively used as complex for Si and Al species. Slowly and continuously, the species could be hydrolized to the active species, which favors to build the preliminary structure unite of Si–O–Si or Si–O–Al via condensation reaction. The influences of pyrocatechol, F − and other factors, which affect the morphology of offretite crystals, such as aluminum source, temperature, the ratio of SiO 2 to Al 2O 3, have been studied, and the SEM photographs have been shown.

Cation Siting and Dynamical Properties of Zeolite Offretite from First-Principles Molecular Dynamics

We have used density functional theory, both within the local density (LDA) and generalized gradient (GGA) approximations, to study the structure, energetics, and vibrational properties of zeolite offretite in the presence of different monovalent cations (H+, Na+, K+, and Cu+). We find that the spatial locations of the most favorable cation-binding sites are similar for the different cations, being related to the minima of the electrostatic potential. However, the relative stability of the sites does depend on the nature of the counterion, as well as on the Al/Si ratio and on the mutual interactions between cations. At low Al/Si ratios, the preferred site for H+ is in the channel, where it is accessible for reaction with incoming molecules. For both Na+ and Cu+, the most stable site is within the 6-fold ring of the gmelinite cage, but for Na+, two other sites are present within a few tenths of a kilocalorie/mole from the lowest site (small site selectivity). For K+, two sites, one inside the cancrinite ca...

A reexamination of the crystal structure of erionite

The crystal structure of a natural erionite from Nizhnyaya Tunguska, Siberia (schematic formula K 2 Ca 3.5 Al 9 Si 27 O 72 · 32H 2 O) has been refined to a residual wR of 0.044. The erionite framework can be described by a sequence AABAAC … of 6-membered rings of tetrahedra. The (Si, Al) distribution (24% of Al in T1 and 31% of Al in T2) is consistent with the strong disorder found in tetrahedral sites for all zeolites describable by a sequence of 6-rings. Erionite, as in the closely related zeolite offretite, shows a slight enrichment of Al in the single 6-ring. K is at the center of the cancrinite cage and coordinates 6 framework oxygen atoms. Ca atoms alternately occupy three sites, quite near each other, on the triad axis parallel to c , at the center of the erionite cage. Their coordination number varies between 6 and 9. The marked differences in T—O distances found between hydrated and dehydrated erionite are explained in terms of interactions between extraframework ions and framework oxygens.

First-principles molecular dynamics investigations of the stability of zeolite offretite under various Si4+/(Al3+, H+) substitutions

A local density functional study of Si4+/(A13+, H+)-substituted offretites is presented. Proton siting and dynamical properties are investigated within the First-Principles Molecular Dynamics method, using a periodically repeated unit cell. Results for monoaluminated offretites,e.g., with one Al per unit cell, show that the proton is located inside the channel of the zeolite, where it is accessible to incoming molecules for reaction. Calculated vibrational spectra of the framework, extracted from a dynamical simulation, reproduce experimental data well. The determined OH stretching frequencies show a rather weak dependence on the H+ position. A comparison of these frequencies with those of offretites containing three Al per unit cell does not indicate a significative change.

A reexamination of the crystal structure of the zeolite offretite

The crystal structure of a natural offretite from Poia Creek, Adamello, Italy (schematic formula KCaMgAl 5 Si 13 O 36 :17 H 2 O) has been refined to a residual wR of 0.038. The offretite framework can be described by a sequence AABAAB … of 6-membered rings of tetrahedra. The (Si,Al) distribution (29% of AI in T1 and 37% of AI in T2) is consistent with the disorder found in tetrahedral sites for ail zeolites describable by sequences of 6-rings. K is at the center of the cancrinite cage and is coordinated to 6 framework oxygen atoms. Mg, placed at the center of the gmelinite cage, is coordinated to 6 water molecules; 4 of these are disordered on 9 possible positions relative to two sites with multiplicity 6 and 3, respectively. Ca atoms alternately occupy two sites near each other, at the center of the wide channel parallel to parameter c, and are octahedrally coordinated to the same water molecules. The strong differences in the T−O distances found between hydrated and dehydrated offretite are here explained in terms of interactions between extraframe-work ions and framework oxygens.

Partitioning of aluminum atoms in crystallographically non-equivalent tetrahedral sites of the zeolite offretite by 29Si MAS NMR

For the zeolite offretite, a formula is proposed which includes the framework Si/Al ratio ( R), the partitioning ratio of Al over two crystallographically non-equivalent tetrahedral sites ( r) and intensities of the observed peaks in the 29Si MAS NMR spectrum. By this formula, the framework Si/Al ratio of offretite can be estimated from the 29Si MAS NMR spectrum. Combined with chemical analysis of the Si/Al ratio, Al partitioning in two kinds of T sites can also be deduced. It is concluded that the T B sites are favored by Al atoms in parent offretites and Al atoms at T B sites can more easily be substituted isomorphously by Si when treated with (NH 4) 2SiF 6. The formula proposed here is based only on experiments and may be used to testify some statistical models of Al distributions in offretites.

Results of the crystallisation of zeolite molecular sieves under microgravity conditions during the EURECA-1 mission

The results of the physical and chemical characterisation of the crystalline zeolite phase obtained in the solution growth facility during the EURECA-1 mission are presented. Comparisons of crystal size and morphology of materials obtained in parallel ground experiments and typical optimised gel syntheses show significant, though not marked differences. Two crystalline phases were obtained, the zeolites offretite and sodalite, in both microgravity and flight experiments. Offretite crystals from the ground experiments exhibited near spherical morphology of dimensions less than 0.5 μm. Equivalent crystals obtained in the flight experiments had developed into thick rod-like crystals of dimensions approximately 2 × 0.6 μm. Both these results can be contrasted with the typical gel syntheses under the same conditions in which a pure offretite phase is obtained with spherical crystals under 0.5 μm in dimensions.

Chabazite-offretite epitaxial overgrowths in cornubianite from Passo Forcel Rosso, Adamello, Italy

The first occurrence of the zeolite offretite in Italy is described in a cornubianite rock from Passo Forcel Rosso, Adamello. The mineral occurs as milky-white fibrous overgrowths in structural epitaxy on the (0001) crystal faces of hyaline platy chabazite («herschelite habit»), associated with large chabazite rhombohedra and yellowish spherules of saponite. Crystal chemical characterization of the zeolitic phases indicates anomalously high Mg contents in the offretite as well as the chabazite overgrowths. The chemistry of the offretites is reviewed, showing that the Si/Al ratio and K content are relatively constant, whereas the Ca/Mg ratio varies considerably, there being very few Mg-rich samples. The paragenesis of the newly-formed minerals is interpreted in the light of the host-rock chemistry

Preparation of zeolites in space

The synthesis of the zeolite offretite under microgravity conditions is planned during the EURECA-1 mission in 1991. The main experimental goal is the preparation of large single crystals with a perfect structure. Zeolites are of special interest with respect to heterogenous catalysis and adsorbents in connection with several industrial processes.

Aluminium distribution in the zeolites offretite and erionite

In earlier interpretations of offretite and erionite 29Si n.m.r. spectra a random aluminium distribution is assumed; the best agreement between observed and calculated spectra is normally obtained with a lower Si Al ratio than expected from chemical analyses. In this work observed spectra from offretite and erionite with different Si Al ratios have been compared with calculated spectra assuming both a random and a non-random aluminium distribution. A non-random aluminium distribution with a preferred aluminium occupation on the T 2 sites, ( Si Al(T 1 site) in the range 0.43–0.55), results in an optimum fit between the observed and the calculated spectra with the Si Al ratios in agreement with that determined by other analytical methods. An aluminium displacement in this range is also to be expected from considerations of the cation filling in the structures.

Toluene conversion on the zeolites offretite, omega and ZSM-5

The conversion of toluene and the accompanying formation of coke has been studied on the H-forms of the zeolites offretite, omega and Z5M-5. A qualitative correlation between the initial activity, the acidity and coke formation could be established. The p-selectivity of the ZSM-5 sample remains constant over a long time on stream. The selectivity of omega, and especially that of offretite increases with decreasing activity. As an explanation for these effects an increase of the diffusion paths by a closure of the pore entrances for the ZSM-5 and the interior of the pores of the omega and the offretite is discussed together with transition state selectivity. The results are further compared with (a) the selectivities of a number of other zeolites and (b) the results of theoretical calculations by Wei 8,9.

Detailed interpretation of the 29Si and 27Al high-field MAS n.m.r. spectra of zeolites offretite and omega

The 29Si MAS n.m.r. spectra of highly dealuminated zeolites are shown to exhibit separate Si(OAl) resonances due to crystallographically inequivalent lattice sites. The chemical shift dispersion due to crystallographic inequivalence is of the same order of magnitude as that from the presence of aluminium atoms in the first coordination sphere and can result in the overlapping of resonances. Thus, the interpretation of the 29Si MAS n.m.r. spectra of zeolites of low Si Al ratios can be ambiguous. The correct peak assignments are presented for the zeolites offretite and omega and the problems involved in obtaining quantitatively reliable Si Al ratios from the spectra discussed. In the case of offretite it is not possible to completely describe the distribution of silicon and aluminium over its two non-equivalent crystallographic sites, although the Si Al ratios for a number of samples are shown to agree with a random distribution. For zeolite omega it is possible to completely describe the non random Si distribution and to predict the aluminium distribution, which is in excellent agreement with that obtained independently from the high field (14.1T) 27Al MAS n.m.r. spectrum. All data indicate that Si and Al distributions in omega are not random in nature.

Thermochemical and acidic properties of the zeolites offretite, omega and ZSM—5

The thermochemical and acidic properties of offretite, omega and ZSM—5 have been compared by thermal and X-ray analysis, infrared spectroscopy and temperatureprogrammed desorption of ammonia. It has been established that the temperature range of the decomposition of the organic cation depends on the zeolite structure, and is wider in the case of offretite. The decomposition of the NH 4-forms of the three types of zeolites proceeds at higher temperatures than in the case of NH 4-Y and also for the decomposition of the organic cation. The appearance of a high-temperature part in the t.p.d. curves of the samples is attributed to their stronger acidic properties after deammoniation. With respect to the strength of ammonia adsorption at 573 K, the decationized forms of the zeolites showed the sequence omega ∼ offretite>ZSM—5.

Crystal structure of natural zeolite offretite after carbon monoxide adsorption

Crystal structure of natural zeolite offretite after carbon monoxide adsorption

Crystal structure of natural zeolite offretite after carbon monoxide adsorption

Crystal structure of natural zeolite offretite after carbon monoxide adsorption