J.L. Guth

Synthesis, characterization, and catalytic properties of silica-rich faujasite-type zeolite (FAU) and its hexagonal analog (EMT) prepared by using crown-ethers as templates

High silica faujasite (FAU) and its hexagonal analog (EMT) were synthesized using 15-crown-5 and 18-crown-6, respectively, as templating agents. By mixing these two crown-ethers in the starting hydrogel, a material with a crystal morphology resulting from an overgrowth of both cubic and hexagonal structures was obtained. These three zeolites with a SiAl molar ratio close to 4 were ion-exchanged and transformed into their protonated forms. The samples were characterized by chemical analysis, 29Si, 27Al, and 23Na solid-state MAS n.m.r., and it was shown that the sodium removal was more complete in the hexagonal and overgrown samples. The acidity (determined by STD of ammonia) and the catalytic activity (evaluated by n-heptane cracking) are closely related to the residual amount of Na+, and they proved to be higher for the two latter samples.

Synthesis and characterization of cloverite: a novel gallophosphate molecular sieve with three-dimensional 20-membered ring channels

Abstract Cloverite, a novel cubic microporous gallophosphate, with three-dimensional channels circumscribed by 20-membered rings in the shape of a four-leafed clover, was synthesized in the presence of fluoride ions and quinuclidine. It is stable to at least 500°C in a dry atmosphere and shows high adsorption capacity for n -hexane, xylene, and mesitylene. By 13 C n.m.r., it was found that quinuclidine is protonated, thus compensating the negative charge of fluoride located in the double four-rings. The 31 P and 71 Ga MAS n.m.r. results are in agreement with the crystal structure analysis.

Influence of synthesis conditions and postsynthesis treatments on the nature and quantity of structural defects in highly siliceous MFI zeolites: A high-resolution solid-state 29Si n.m.r. study

The formation and annealing of various kinds of defects in the framework of highly siliceous MFI-type zeolites, as evidenced by high-resolution solid-state 29 Si n.m.r., can be explained by considering the systematic influence of synthesis parameters and postsynthesis treatments. High pH values and high concentrations of alkali metal and fluoride ions prevent a complete polycondensation of the silicate species and generate materials that contain significant levels of nonbonding point defects. By contrast, a high crystallization temperature results in the ready hydrolysis of the terminal Si-O − groups, leading to structures containing fewer defects. A large number of T vacancies (hydroxyl nests) are created in zeolites crystallized at low temperatures in the presence of high Pr 4 N + concentations and in the absence of F − or Na + ions. Such conditions favor the formation of double-5-ring silicate anions. The hypothesis, that such species may further condense, conducting to frameworks with an important number of empty T-sites, is proposed. On the basis of these findings, ideal synthesis conditions can be proposed to produce (Si) MFI zeolites that contain a minimum number of defects. Various postsynthesis treatments, namely, different calcination conditions followed, or preceded, by selected ionic (akali cation) exchanges, result in a partial or a nearly total healing of these defects in the final materials.

N.m.r. and i.r. study of B and BAl substitution in zeolites of the MFI-structure type obtained in non-alkaline fluoride medium

Pure boron and boron—aluminium MFI-type zeolites have been obtained by heating non-alkaline reaction mixtures containing F− instead of OH− anions and tetrapropylammonium bromide as a template. High resolution solid state magic angle spinning (MAS) 29Si-, 27Al- and 11B-n.m.r., and i.r. spectroscopy have indicated the incorporation of the trivalent elements in the zeolite framework. BO4 and AlO4 groups were shown to be present in the as-synthesized samples. For the dehydrated calcined samples, it is suggested that boron is in a trigonal environment still in the framework.

19F MAS n.m.r. studies of crystalline microporous solids synthesized in the fluoride medium

Abstract Highly siliceous TON-, MTT-, and MFI-type zeolites, alumino-, gallo-, titano-, and germanosilicates of MFI-type structure, and aluminophosphates and silicoaluminophosphates of AFI-, AEL-, FAU-, and CHA-type structure, containing various alkylammonium species R + , and synthesized in fluoride medium, were studied by 19 F MAS n.m.r. The as-synthesized samples, with a fluorine weight content up to 1.7% were characterized by a single main peak attributed to fluorine anions balancing the charge of the organic cation. The variations of chemical shifts with the nature of the templates were very small. The values ranged from −64 to −75 ppm in silicates and from −119 to −123 ppm in aluminophosphates. After calcination in dry gas, the remaining fluorine (0.1-0.3%) is bonded to Si (peak close to −155 ppm) or to T III elements (peak close to −140 ppm). When the samples were heated in moist atmosphere, no trace of F was found.

Characterization of siliceous MFI-type zeolites containing tetra-, tri-, and dipropylammonium fluoride species

Abstract Highly siliceous MFI-type zeolites containing tetra-, tri-, and dipropylammonium fluoride species, synthesized in the presence of fluoride ions, were characterized by powder X-ray diffraction t.g., d.s.c., and high-resolution solid-state n.m.r. of 13 C and 29 Si. The occupation of the channels by the different organic species could be elucidated by considering the refined unit cell parameters. In the case of, particularly, the dipropylammonium fluoride-containing sample the organic molecule is found to be located preferentially in the zigzag channels, in agreement with the results of the other techniques.

Fast crystallization of alumino- and silicoaluminophosphates with a VFI framework topology

Abstract A fast crystallization method for VPI-5-like products is reported. The method allows, for the first time, the full crystallization in less than 1 h of pure aluminophosphates and silicoaluminophosphates. This method requires a high rate of temperature increase of the hydrogel. Products are characterized by X-ray diffraction patterns, scanning electron microscopy, electronic microprobe analysis, 27Al MAS n.m.r., 29Si MAS n.m.r., and 31P MAS and CPMAS n.m.r.. N.m.r. spectra are unique and this is due to the high crystallinity of the products. These spectra are compared to the literature and are briefly commented on.

Identification of the volatile products resulting from the thermal decomposition of tetra-, tri-, di-, and mono-n-propylammonium cations occluded in MFI-type zeolites

The volatile thermal decomposition products of Pr4N+, Pr3NH+, P2NH2+, and PrNH3+ (Pr = n-propyl), occurring as templates in MFI-type zeolites prepared in various media (presence of F− or OH− anions and TIII substituents for Si), were analyzed by using differential thermal analysis (d.t.a.)-coupled mass spectrometry. In a second type of experiment, the products released during the d.t.a. were separated in a preliminary stage, using gas chromatography. The data from these combined techniques prompted a main decomposition scheme involving a Hofmann elimination, followed by β-eliminations. In addition to propene and its oligomers, a variety of other molecules are evolved. Their nature and proportions depend on the type of PrxNHy+ template, on the kind of anion (OH− or F−) accompanying the organic cation in the ion pairs, and on the acidity of the zeolite, which is related to the substitution of Si by a TIII element. The thermal decomposition products of a common template Pr4N+ were compared when T = Al, B, Fe, and Ga. A marked difference was observed when the in situ conversion of propene to aromatic or aliphatic hydrocarbons takes place. The extent of the catalytic side reactions in the thermal decomposition increases in the following sequence: mono-

Temperature-induced monoclinic/orthorhombic transition in germanium MFI-type zeolites

An X-ray diffractometer fitted with a variable temperature chamber was used to measure the temperature (Tt) of the phase transition (monoclinic to orthorhombic) occurring in an MFI-type zeolite in which GeIV was partially substituted for SiIV. The presence of a tetravalent element MIV (GeIV) induces an increase of Tt, in contrast to a trivalent element MIII (AlIII, FeIII, BIII, …) that is known to induce a decrease of Tt in MFI-type frameworks. A good correlation was obtained between Tt and the framework GeIV content in the zeolite.

EXAFS study of germanium-rich MFI-type zeolites

Two MFI-type zeolite samples containing 8.38 and 16.2 Ge atoms per 96 TO 2 (T = Si, Ge) were studied by Ge-K-edge X-ray absorption spectroscopy in the as-synthesized state with the (C 3 H 7 ) 4 N + template occluded and in the calcined form. XANES and EXAFS analysis proves the fourfold coordination of Ge in the samples and, hence, its incorporation into the framework. The Ge-O bond distance which is very close to that in quartz-type GeO 2 for the as-synthesized samples, 1.75 ± 0.02 A and 1.74 ± 0.02 A for samples 1 and 2, respectively, is shortened to 1.71 ± 0.02 A and 1.72 ± 0.02 A after calcination. The structural disorder around germanium is found to be lowered after such a treatment