Mordenite Structure Research Articles

Influence of the Generation of Mesopores on the Hydroisomerization Activity and Selectivity of n-Hexane over Pt/Mordenite

The activity and selectivity of hydroisomerization of n-hexane over Pt/mordenite is strongly influenced by acid leaching. The activity increases and the selectivity changes to favor primary products. Leaching selectively modifies the mordenite structure, making more sites accessible for reaction and facilitating desorption of reaction products. It is argued that the activity of untreated mordenite is limited by mass transfer effects. These effects largely vanish after modification of the zeolite structure by generation of a 3-D micropore structure as well as mesopores. The alleviation of intracrystalline diffusion limitation is the major factor in activity enhancement after acid leaching of Pt/mordenite.

Crystal defects of mordenite structures

Crystal defects in mordenite materials were observed and characterized by high-resolution electron microscopy (HREM). These defects are identified as line, planar, and 3D defects, which may result in pore blockage of the mordenite channel structure and be detrimental to their chemical properties for catalytic applications. The planar defects are the most severe ones affecting the structural properties of mordenite because of their density and scale. The main emphasis of this investigation is focused on the crystallographic aspects of these defects.

Co2+ Ion Siting in Pentasil-Containing Zeolites. I. Co2+ Ion Sites and Their Occupation in Mordenite. A Vis−NIR Diffuse Reflectance Spectroscopy Study

VIS−NIR diffuse reflectance spectra of Co2+H−mordenites and co-cation (K, Cs, Ca, Ba)-containing Co−mordenites with Co/Al varying from 0.0015 to 0.35 were studied. Three different sites-coordinations of the Co2+ ions in dehydrated mordenite were found from interpretation of optical spectra, and the populations of these sites depending on Co-ion concentration were established. The assignment of characteristic absorption bands in the VIS spectrum to the Co2+ ions located at individual cationic sites was based on knowledge of siting of K, Cs, Ca, and Ba ions in mordenite structure. Co2+ ions are located in sites A and C of the mordenite pocket and in site E of the main channel. In site A, occupied preferentially in CoH−mordenite, the Co2+ ion is coordinated to six framework oxygens of the twisted eight-ring. The number of coordinated framework oxygens and detail geometry of the “boat-shape” site C is not known. This site is occupied only at high Co loading. Site E, consisted of four oxygens forming a rectang...

The role of matrix embedding on the properties of steamed mordenites

The influence of various hydrothermal treatments on the physicochemical properties of pure H-mordenites and matrix-embedded H-mordenites was investigated. It was found that the crystallinity and the chemical composition of pure mordenite were strongly dependent on the ageing conditions. Embedding of mordenite had no influence on the extent of dealumination and prevented the mordenite structure from collapsing. This protecting effect of the matrix on the zeolite structure was not observed for a physical mixture MOR/matrix, thus suggesting that intimate contact between the zeolite and the matrix is necessary for such an effect to take place. This intimate contact MOR/matrix was confirmed by the formation of new chemical bonds between hydroxyl groups on the outer surfaces of the zeolite crystals and those on the surface of the embedding matrix.

Adsorption and energetics of xenon in mordenite: A Monte Carlo simulation study

Grand canonical Monte Carlo (GCMC) simulations were used to calculate energy contour maps, isodensity surfaces and adsorption isotherms of xenon in purely siliceous mordenite at 300 K using previously published potential parameters. Then, in order to understand the effect of aluminum content, Al-containing mordenites were modeled by adding the appropriate negative charge to each of the oxygen atoms in the mordenite framework and by inserting Na+ ions at fixed extra-framework positions previously identified from x-ray crystallographic data. GCMC simulations were again performed using these structure models. Both main channel and sidepocket xenon adsorption were observed in all mordenite structures. The sidepockets were found to be only singly occupied whereas the main channel allowed multiple occupancy. Sidepocket xenon adsorption became more favorable when cations (and aluminum) were introduced. In the main channel, xenon adsorbed into localized sites that were distributed in an octahedral arrangement within each unit cell with two apices in front of opposing sidepockets. These sites remained at the same location over the entire range of loading (0–16 Xe/unit cell) and cation (and aluminum) content.

In-situ IR monitoring of the acidic and structural variations of mordenite caused by pressure and temperature

The effects of the surroundings (pressure and temperature) on the acidity and structure of mordenite zeolite have been investigated by monitoring the acidic bridging hydroxy groups, Si—OH—AI, by in-situ IR. Effects due to pressure have not been observed. However, severe effects due to temperature have been found. The frequencies of acidic bridging OH groups decreased and the acidity increased with increasing temperature. The weaker Bronsted acid sites could be turned into stronger Bronsted acid sites as the temperature was raised. In a series of mordenites, in the lower temperature region (300–473 K) the order of acidity depended on the conditions of treatment. In the higher temperature region (473–674 K), with increasing temperature the acidity of the sample decreased, reaching a minimum (Si : AI = 11.18) before increasing again (Si : AI > 11.18).The effect of temperature on the structure of mordenite has shown that the more sensitive the IR frequencies of the acid OH groups to temperature, the greater the degree of distortion of the zeolite rings and, correspondingly, the less thermostable the structure of the dealuminated mordenite. Moreover, SiOHAI fragments tend to locate in the positions at which the ∠ TOT angles are minimised.

Incorporation of vanadium into the mordenite structure by direct hydrothermal crystallization and secondary synthesis

The substitution of vanadium atoms into the zeolite framework structure could be applied to the large pore zeolites by means of modified treatments as well as direct hydrothermal synthesis. The incorporation of vanadium into the mordenite framework was demonstrated by XRD, FTIR, UV-Vis techniques and tested with catalytic oxidation reactions. These vanadium-containing zeolites have selective catalytic properties in the various oxidation reactions as compared with vanadia-impregnated catalysts.

Monte Carlo simulation of xenon adsorption with dealumination of an idealized mordenite structure

Grand canonical Monte Carlo simulations were used to study the effect of dealumination on adsorption of xenon in an idealized structural model of Na-mordenite. Simulations were carried out at Si/Al ratios of 5, 11, and ∞, chosen to represent prototype cation occupancies. A recent experimental study of xenon adsorption in mordenites with different Si/Al ratios found unexpected nonmonotonic trends. In disagreement with experiment, the isotherms of our idealized zeolite changed monotonically with the degree of dealumination. This trend supports recent observations that significant structural chages result from dealumination.

粉砕に伴う合成モルデナイトのメカノケミカル変化 陽イオン吸着特性変化について

The effect of dry grinding on the adsorption characteristics, e. g., distribution coefficient and saturated amount of adsorbed cations, of synthetic mordenite has been studied using a radioisotope of 137Cs+ as a tracer. The grinding operation for different periods of time, i. e., 15 to 360min, was carried out using a planetary ball mill.The grinding operation had no effect on the distribution coefficient onto the mordenite sample at the relatively lower concentration of Cs+ in the neutral and alkaline solutions, though the coefficient significantly decreased in the acidic solutions with an increase in grinding time. The saturated amount of Cs+ adsorbed on the mordenite sample in both neutral and acidic solutions decreased markedly during the early stage of grinding. However, for more than 30 minutes of grinding, the amount of absorbed Cs+ in the neutral solution increased slightly, while in the acidic solution it gradually decreased with grinding time. These observations may be interpreted from the formation of the hydroxyl group on the fractured surface, as well as from the considerable distortion of the channel structure of mordenite induced by grinding.

Characterization of dealuminated large-port mordenites

Four different types of large-port mordenite were studied. Three of these catalyst samples were dealuminated by treatment with nitric acid and were characterized by temperature-programmed desorption (TPD) of ammonia, infrared spectroscopy, cyclohexane adsorption, solid-state NMR, and X-ray diffraction (XRD). It was found that the extent of dealumination and the amount of octahedral aluminum remaining in the zeolite channels was strongly influenced by crystallite size. Extensive dealumination resulted in materials with surface areas greater than the parent materials, yet still retaining their characteristic mordenite structure as determined by XRD. Ammonia TPD was shown to uniquely determine the framework aluminum content, irrespective of the presence of extraframework aluminum. The previously disputed origin of the infrared absorption band at 955 cm −1 is assigned to new SiOSi bonds formed upon framework dealumination. A sequence of dealumination events is proposed.

Structure and acidity modification of mordenite through isomorphous Fe substitution

A series of Fe-modified mordenite were hydrothermally synthesized. The effects of substrate composition, aging time, reaction temperature, and reaction time upon the crystallization of the mordenite were investigated. Isomorphous substitution of Fe was confirmed by FT-IR and EPR analysis. Acid strength and acid sites distribution of each catalyst were measured by pyridine TPD, which showed that the number of strong acid sites in H-Fe-mordenite are smaller than in H-Al-mordenite. Structural characteristics and thermal stability were also examined using XRD, SEM and TG. Fe-substitution into the mordenite structure have resulted in substantial decrease in thermal stability. H-Fe-mordenite catalysts showed high selectivity towards p-xylene in toluene alkylation and in xylene isomerization reaction, compared with those obtained using H-Al-mordenite because of lower concentration of strong acid sites. In addition, these H-Fe-modified mordenite showed little hexane cracking activity. Deactivation in both toluene alkylation and xylene isomerization reaction and coke formation over H-Fe-mordenite in these reactions were negligible.

Change of pore-opening structure of mordenite upon dealumination by hydrochloric acid

The structure of dealuminated mordenites prepared at 340–358 K of hydrochloric acid was studied. 27Al MAS n.m.r. and XPS were measured to determine the species and depth profile of alumina, and these were corroborated with adsorption experiments using adsorbates of different sizes. Adsorption capacity of nitrogen was unaltered so that the structure of mordenite was retained, while that of 2,2,4-trimethylpentane with a larger kinetic diameter increased below 358 K of dealumination temperature, but then decreased above that. The H-mordenite (HM, Norton 100-H) contained a large amount of octahedral aluminum species (ca. 28%), and the nonframework aluminum was removed preferentially by the dealumination. Aluminum depth profile showed the homogeneous distribution on the inherent HM, but the enrichment on the samples prepared above 350 K. Based upon these findings, the following conclusions were drawn: (1) On the inherent HM, a small portion of nonframework aluminum near the pore opening disturbs the adsorption of large molecules: (2) The pore-opening size of the sample dealuminated at 358 K was large enough to adsorb 2,2,4-trimethylpentane: (3) However, upon the dealumination above 358 K, the pore opening was partly destroyed or blocked by nonframework alumina again to inhibit the adsorption of large molecules. Most of the aluminum impurity species seemed to be occluded as blocks or clusters in the mordenite. These variations of structure of the pore opening can be well related to those of the catalyst activity.

Alkylations of benzene, alkylbenzenes, and halobenzenes catalyzed by protonated mordenite pretreated with chlorofluorocarbons

The chlorofluorocarbon (CFC) treatment of protonated mordenite (HM) was carried out in a flow reactor under ordinary pressure, usually at 600°C for 10 min using CClF 3 as a treatment agent. The mordenite structure was almost completely retained during the CFC treatment. Evidence for dealumination and surface fluorination was observed by XPS examination. Decrease of surface acidity, as seen by NH 3-TPD, accompanied the treatment. The function of HM as a catalyst for alkylations of benzene and alkylated or halobenzenes with methanol was greatly enhanced by the CFC treatment. In particular, the activity maintenance was remarkably improved. Thus, in the alkylations at 300°C under the molar ratios of CH 3OH/Aromatic compound = 1 and W/ F = 81.2 (for toluene) or 97.5 (for 1,2,4-trimethylbenzene (TMB)) g h mol −1, the conversions at the initial stage of the run were increased from 27 to 65% (for toluene) and from 12 to 47% (for TMB) by CFC treatment of the HM catalyst. In addition, the conversions of benzene and TMB were well maintained at about 60 and 40%, respectively, throughout the running time of 3 h in the reactions catalyzed by the treated HM, while the conversions of benzene and TMB rapidly decreased and became almost zero within 90 min in the reaction catalyzed by untreated HM.

Crystal structure of synthetic mordenites

Crystal structures of synthetic sodium mordenites with different Si Al ratios have been studied by X-ray diffraction methods. Framework geometries of the synthetic mordenites are very similar to those of natural mordenites. Atomic positions and occupancies of extraframework cation and water molecules of synthetic mordenites with different Si Al ratios have been determined.

Aluminum siting in mordenite and dealumination mechanism

The dealumination of mordenite by acidification (HCl, HNO/sub 3/) and by SiCl/sub 4/ and steaming treatments is followed by high-resolution magic-angle-spinning solid-state /sup 29/Si and /sup 27/Al NMR spectroscopy. The combined use of these techniques leads to the determination of the silicon-aluminum ordering. It is demonstrated that aluminum atoms preferentially occupy tetrahedral positions in the four-membered rings of the mordenite structure. In addition, a mechanism of dealumination can be inferred, consisting in removing the aluminum atoms two by two from the four-membered rings. It is also possible to compute the number of SiOH groups left in the unit cell after the progressive removal of aluminum atoms: four groups per extracted Al atom are generated in the beginning of dealumination and this number gradually decreases to two, suggesting that a structural reorganization must necessarily occur for substantial dealumination. These silanol groups are also identified by cross-polarization MAS /sup 29/Si NMR spectroscopy.

The crystal structure refinement of a natural mordenite

AbstractThe crystal structure of natural mordenite from Elba (Italy) with chemical formula (Na1.49K2.80Mg0.04Ca2.05Sr0.05)(Al8.98Si39.13)O96· 29.07 H2O was refined in its hydrated form up to awRvalue of 5.2%. Systematic absences were consistent with theCmcmspace group but the asymmetry of the extraframework ions indicated as more probable the acentric subgroupCmc21. Consequently the atom O8 is no longer set on an inversion center and the T – O8 – T angle is not constrained to 180°. The Si/Al distribution is partially ordered, with an enrichment of Al in the tetrahedra of the four-membered ring. All Ca ions are localized in a eight-coordinated site; K ions alternate with water molecules in a six-coordinated site.

The Crystal Structure of Ion-exchanged Mordenite

Abstract The crystal structure of hydrated mordenite, yielded at Hokiyadake, Nagano City, Japan, Na1.4Ca2.9Al7.2Si40.8O96·nH2O, were refined to R=0.070 for 1526 observed reflections. Notable ordering of the aluminum positions of the aluminosilicate framework was observed.

Progressive ion exchange of lithium and cobalt(II) in sodium mordenite and its effect on the enthalpy of sorption of O 2 and N 2

The effects of progressive replacement of sodium ions in mordenite with lithium and cobalt(II) ions on the initial enthalpies of sorption of O 2 and N 2 have been studied. Zeolites, like mordenite, demonstrate higher enthalpies of sorption of N 2 than of O 2 and consequently are employed as selective sorbents in pressure swing adsorption (PSA) systems to separate O 2 and N 2 from air. Lithium exchange produced little effect on the enthalpy of sorption of N 2 up to an exchange level of 41% on an equivalent basis. As the level of lithium exchange increased from 41% to 49%, a sharp increase (from 5.7 to 6.2 kcal/mole, i.e., 23.8 to 25.9 kJ/mole) in the enthalpy of sorption of N 2 was exhibited. Cobalt(II) exchange produced a significant increase in the enthalpy of sorption of N 2 even at low levels of exchange. At an exchange level of 15% cobalt(II), the enthalpy of sorption of N 2 increased from 5.6 kcal/mole (23.4 kJ/mole) for sodium mordenite to 6.6 kcal/mole (27.6 kJ/mole). Cobalt(II) exchange also produced the same type of effect on the enthalpy of sorption of O 2 increasing that value from 4.3 to 4.9 kcal/mole (18.0 to 20.5 kJ/mole) at a 15% exchange level. However, lithium exchange little affected the enthalpy of sorption of O 2 regardless of the extent of exchange. The different effects of progressive lithium and cobalt(II) ion exchange on the enthalpies of sorption of O 2 and N 2 are explained in terms of the position the respective cations occupy in the outgassed mordenite structure.

Progressive ion exchange of lithium and cobalt(II) in sodium mordenite and its effect on the enthalpy of sorption of O2 and N2

The effects of progressive replacement of sodium ions in mordenite with lithium and cobalt(II) ions on the initial enthalpies of sorption of O2 and N2 have been studied. Zeolites, like mordenite, demonstrate higher enthalpies of sorption of N2 than of O2 and consequently are employed as selective sorbents in pressure swing adsorption (PSA) systems to separate O2 and N2 from air. Lithium exchange produced little effect on the enthalpy of sorption of N2 up to an exchange level of 41% on an equivalent basis. As the level of lithium exchange increased from 41% to 49%, a sharp increase (from 5.7 to 6.2 kcal/mole, i.e., 23.8 to 25.9 kJ/mole) in the enthalpy of sorption of N2 was exhibited. Cobalt(II) exchange produced a significant increase in the enthalpy of sorption of N2 even at low levels of exchange. At an exchange level of 15% cobalt(II), the enthalpy of sorption of N2 increased from 5.6 kcal/mole (23.4 kJ/mole) for sodium mordenite to 6.6 kcal/mole (27.6 kJ/mole). Cobalt(II) exchange also produced the same type of effect on the enthalpy of sorption of O2 increasing that value from 4.3 to 4.9 kcal/mole (18.0 to 20.5 kJ/mole) at a 15% exchange level. However, lithium exchange little affected the enthalpy of sorption of O2 regardless of the extent of exchange. The different effects of progressive lithium and cobalt(II) ion exchange on the enthalpies of sorption of O2 and N2 are explained in terms of the position the respective cations occupy in the outgassed mordenite structure.

Hydroisomerization of n-hexane on platinum zeolites : II. Comparison between the reaction mechanisms on platinum/Y-zeolite and on platinum/mordenite

The transformation of n-hexane was carried out under hydrogen pressure, on platinum/mordenite and platinum/Y-zeolite with platinum areas ranging from 0 to 10–15 m 2 g −1. For both series, as can be expected from the conventional bifunctional process, the isomerization activity is proportional to the platinum area for small platinum areas and almost independent of it for large platinum areas. However, the primary products of n-hexane and 3-methylpentane isomerization are notably different: the selectivity of platinum/Y-zeolite with a platinum area ≥0.5 m 2 g −1 is typical of a bifunctional reaction with, as limiting step, the skeletal isomerization of intermediate olefins; that of platinum/mordenite, even with a platinum area equal to 15 m 2 g −1, remains intermediate between those of platinum-free zeolites and large platinum area Y catalysts. These differences are clearly related to the higher activity of platinum/mordenite, to its lower hydrogenation activity as well as to diffusional limitations in the practically unidimensional porous structure of mordenite.