B. Marler

Influence of the sorbate type on the XRD peak intensities of loaded MCM-41

Abstract Nine organic sorbates of different scattering power were adsorbed into the mesopores of calcined, high quality B-MCM-41. The intensity of the observed X-ray diffraction peaks of the loaded samples results from the difference in the scattering power (or ‘scattering contrast’) between the two building blocks (amorphous silicate wall and amorphous sorbate) of the MCM-41 structure. In general, the intensity of the X-ray peaks decreases with decreasing ‘scattering contrast’ and is zero when the scattering power of the silicate wall and the pore filling material are similar (bromoform loaded B-MCM-41). After the recalcination of the loaded B-MCM-41 materials all X-ray diffraction intensities are recovered to about the same level as observed for the calcined sample which was never used for an adsorption experiment. It is concluded that the general structure of the MCM-41 material with its regular arrangement of the channels is maintained during all the calcination and adsorption processes. This is confirmed by transmission electron micrographs of B-MCM-41 which had been calcined, loaded with bromoform and calcined again.

PREFER: a new layered (alumino) silicate precursor of FER-type zeolite

Abstract A new layered microporous (alumino) silicate showing (100) ferrierite layers and called PREFER was synthesised from fluoride-containing aqueous media in the presence of a novel bulky template, the 4-amino-2,2,6,6-tetramethyl-piperidine. Calcination at about 500°C of this ‘as-made’ product leads to FER-type zeolite by a transformation occurring through the intermediate formation of poorly organised phases. The whole set of results obtained by several techniques such as variable-temperature XRD, chemical and thermal analyses and 29Si MAS NMR spectroscopy are consistent with a 2D→3D transformation process. Indeed, during the calcination, the (100) ferrierite layers progressively link together through condensation reactions between the Q3-type silicon atoms initially present.

Hydrous layer silicates as precursors for zeolites obtained through topotactic condensation: a review

In the search for new synthesis routes of zeolites, the topotactic condensation of hydrous layer silicates shows promising results in generating novel zeolite materials with distinct framework types which might have new, interesting properties as, e.g ., molecular sieves or form-selective catalysts. In order to manipulate and optimise the condensation process detailed knowledge of the crystal structures is essential. The layer silicates considered here are of a special type and can be designated as high-silica hydrous layer silicates , HLSs. The structures consist of a tetrahedral layer of interconnected [SiO4]-units containing equal numbers of terminal silanol/siloxy groups on either side of the layer and of an inter-layer region where cations of low charge density (predominantly organic cations) and water molecules are located. A topotactic condensation of the layers performed at temperatures of around 500°C with simultaneous expulsion of the inter-layer constituents is able to form fully condensed, uninterrupted framework silicates. The topotactic conversion has so far been described rarely in comparison to the classical hydrothermal synthesis of zeolites. Nevertheless, several hydrous layer silicates with different layer topologies were successfully converted into zeolites of different framework types using this synthesis route: CAS (type material: EU-20, EU-20b), CDO (type material: CDS-1), FER (type material: siliceous ferrierite), MWW (type material: MCM-22), NSI (type material: NU-6(2)), RRO (type material: RUB-41), RWR (type material: RUB-24), SOD (type material: guest-free silica sodalite). Thereby, four new zeolite framework types were obtained which have not been synthesized, so far, by direct hydrothermal synthesis (CDO, NSI, RRO, RWR). This review gives an overview on the hydrous layer silicates being structurally characterized in detail, on the condensation process and on some properties of the resulting zeolite materials.

Silica-ZSM-22: synthesis and single crystal structure refinement

Using hydrothermal techniques the aluminium-free end member of ZSM-22 (silica-ZSM-22) has been synthesized in the presence of any one of the amines — ethylenediamine, diethylamine, triethylenetetramine or 1-aminobutane. Using a single crystal with diethylamine as the guest molecule the crystal structure has been refined to Rw = 0.056. Silica-ZSM-22, 24SiO2·(C2H5)2NH, crystallizes in space group Cmc21 with ao = 13.859(3) A, bo = 17.420(4) A and co = 5.038(2) A. Its tetrahedral framework is isotypic with that of zeolites ZSM-22 and theta-1 whose structures have been derived from X-ray powder data. The SiO2 framework of silica-ZSM-22 contains five-, six- and ten-membered rings of [SiO4]-tetrahedra. Its one-dimensional channel system is made up by ten-membered rings running parallel to [001]. They are elliptical in cross section with free diameters of 4.7 and 5.5 A. The framework topology is closely related to those of ZSM-23 and ZSM-48. The formation of different porous tectosilicate frameworks appears to be dependent on the synthesis temperature and the nature of guest substances added.

Synthesis and crystal structure of the new borosilicate zeolite RUB-13

Abstract RUB-13 is a borosilicate zeolite which represents a new porous structure type. The material was synthesized in the system SiO 2 B 2 O 3 ethylenediamine-1,2,2,6,6-pentamethylpiperidinewater at 160°C. Single crystal structure analysis ( R =6.0%) revealed that RUB-13 has two-dimensional pore system with intersecting 8-membered ring channels. The void at the intersection is a large [4 6 5 8 6 4 8 4 ] cage with a free volume of ca. 550 a 3 . 13 C Cross-polarization magic-angle spinning nuclear magnetic resonance (CP-MAS NMR) spectroscopy proves that protonated pentamethylpiperidine (PMP) molecules occupy the cages and compensate the negative charge of the borosilicate framework. The unit cell composition is [Si 30.4 B 1.6 O 64 ]·1.6 PMP. All crystals are contact twins with a twin plane (001) and show signs of one-dimensional disorder.

Classification of tectosilicates and systematic nomenclature of clathrate type tectosilicates: a proposal

Abstract A classification for tectosilicates is proposed on the basis of the chemical nature of the T atoms in [TO4] tetrahedra, framework density and the nature of voids in the framework. In addition, a nomenclature for clathrate type tectosilicates is proposed which is based on polyhedral building units from which the tetrahedral frameworks can be generated by linkage through common corners, edges, faces and/or additional oxygen atoms. The proposed nomenclature gives information on the framework topology, symmetry and the distribution of the guest molecules in various cages of the framework. This nomenclature may be applicable to other clathrate type compounds with tetrahedral frameworks.

Synthetic tourmaline (olenite) with excess boron replacing silicon in the tetrahedral site: I. Synthesis conditions, chemical and spectroscopic evidence

One selected composition within the system Na 2 O-Al 2 O 3 -B 2 O 3 -SiO 2 -H 2 O (NABSH) was studied with the aim of synthesizing the new tourmaline end member olenite with the theoretical formula NaAl 3 Al 6 [Si 6 O 18 ] (BO 3 )O 3 (OH). The starting material consisted of a gel with the anhydrous composition 0.625Na 2 O.4.5Al 2 O 3 .6SiO 2 , but with 100% excess B 2 O 3 added over that of the above formula, and a surplus of water to aid crystallization. Run conditions ranged from 4 to 50 kbar, 400-900°C, but a tourmaline phase could only be obtained at or above 10 kbar, with yields between 80 and 100% when ignoring amorphous quench products from the coexisting fluids. The synthetic olenites exhibit much smaller cell parameters than those reported for natural olenites, and indeed the smallest ones ever measured for any tourmaline phase. Analytical data on an olenite prepared at 25 kbar, 600°C, show excess boron and water relative to the theoretical formula, coupled with deficiencies in Si, Al, and Na. Spectroscopic investigations (MAS NMR, EELS, IR) prove – directly or indirectly – that boron occurs not only in trigonal coordination, but is also located in the tetrahedral ring site. Thus, a provisional structural formula is derived as (Na 0.65 □ 0.35 ) (Al 2.72 □ 0.28 ) (Al 5.42 Si 0.58 ) [Si 3.73 B 2.27 O 18 ] (BO 3 ) 3 (OH) 3.87 O 0.13 . In this synthetic olenite the OH-content is close to the maximum of 4.0 p.f.u.; boron replaces tetrahedral Si according to BHSi −1 , with this substitution cancelling the proton deficiency of the theoretical olenite formula. Because the sum (B+Si) exceeds 9.0, some Si seems to replace octahedral Al. Nevertheless, octahedral vacancies remain. There are indications that the above tourmaline composition is not unique for the system studied, but that a range of olenite solid solutions exists as a function of starting material and run conditions, possibly extending to the ideal olenite formula. Excess-boron tourmalines are probably confined to very Al-rich (or M 3+ -rich) compositions which – for stoichiometric reasons – should have proton deficiencies, but these may be compensated by the BHSi −1 substitution.

New structures—new insights: Progress in structure analysis of nanoporous materials

In the recent past structure determination of microporous materials has experienced considerable developments in methodology. The FOCUS method: high resolution powder diffraction data used for direct method structure solution in combination with crystal chemistry based modelling. The models are retrieved from electron density maps calculated in direct method runs, energy minimized and checked through for realistic angles and distances values. The SUM-TF method: diffraction patterns at moderate resolution analysed with direct methods using a modified tangent formula which includes Patterson information for the structure solving. In this way the atomic resolution criterion for direct methods is bypassed. This overview gives a summary of the structures successfully solved using these new techniques.

Studies on clathrasils VIII. Nonasils-[4158], 88SiO2 · 8M8 · 8M9 · 4M20: Synthesis, thermal properties, and crystal structure

Nonasils-[4158], 88SiO2·8M8·8M9·4M20, have been synthesized with 2-methylpyrrolidine, hexamethyleneimine, 2-(aminomethyl)-tetrahydrofuran, 1,2-diaminocyclohexane, 2-methylpiperidine, 2-methylpiperazine, 1-aminobutane, 2-aminobutane, and 2-aminopentane as guest molecules, M20. The samples have been prepared from aqueous silicate solutions which were sealed in silica tubes and heated at about 200°C for several weeks. These clathrasils crystallize in space groupFmmm. For the nonasil with 2-aminopentane as the guest molecule and the unit cell dimensionsao=22.232(6) Å,b0=15.058(4) Å, andco=13.627(4) Å, the structure has been refined using 550 non-equivalent single crystal reflexions to a reliability factorRw=0.125. The 3-dimensional 4-connected silica host framework has three types of cage-like voids, [5464], [4158], and [58612], the latter housing the structure-controlling guest molecules, M20. The non-spherical shape of the guest molecules is the most important factor for the formation of nonasils-[4158]. On heating nonasils-[4158] up to 950°C the organic guest species are driven out and the pure silica form of nonasil is obtained.

Synthetic tourmaline (olenite) with excess boron replacing silicon in the tetrahedral site II. Structure analysis

Two excess-boron olenite samples which had been synthesized from a reaction mixture of 0.625 Na 2 O · 4.5 Al 2 O 3 · 6.0 SiO 2 · 3.0 B 2 O 3 + excess H 2 O at 600°C / 25 kbar (sample 1) and 650°C / 20 kbar (sample 2) were structurally analyzed by a Rietveld refinement. The investigated tourmalines possess space group symmetry R3m with lattice parameters of a = 15.5996(8) A, c = 7.0224(6) A for sample 1 and a = 15.6329(8) A, c = 7.0365(6) A for sample 2. 29 Si MAS NMR spectroscopy showed no octahedral Si, so that the octahedral Y and Z sites are exclusively occupied by Al 3+ ions. The average T-O distances are 1.573 A and 1.590 A for samples 1 and 2, respectively, indicating that B 3+ ions (B-O = 1.470 A) partly replace Si 4+ ions (Si-O = 1.620 A) at the T-position. The refined occupancy factors give a chemical composition of Na 0.8 Al 2.9 Al 6 [Si 3.8 B 2.2 O 18 ](B 3 O 9 )(OH,O) 4 for sample 1 and Na 0.7 Al 2.9 Al 6 [Si 4.2 B 1.8 O 18 ](B 3 O 9 )(OH,O) 4 for sample 2, approximately confirming an earlier chemical analysis of sample 1. The difference in charge which is generated by the partial replacement of Si 4+ ions by B 3+ ions is compensated by protons leading to OH-contents near 4. It is interesting to note that sample 1 synthesized at a higher pressure and lower temperature contains a larger amount of tetrahedral boron.