R. Van Grieken

Anomalous crystallization mechanism in the synthesis of nanocrystalline ZSM-5

Abstract Nanocrystalline ZSM-5 (crystal size in the range 10–100 nm) has been synthesized in only 24 h by hydrothermal crystallization of clear supersaturated homogeneous synthesis mixtures. The influence of the aluminum source, aging time, pH, water content and presence of alkaline cations on the properties of the final nanocrystalline zeolite have been studied. Samples with different degrees of crystallinity have been prepared and characterized by X-ray fluorescence, X-ray diffraction, solid state MAS-NMR, transmission electron microscopy and nitrogen adsorption (BET) in order to investigate the crystallization mechanism. The process involves the formation of an initial amorphous solid prior to the zeolite crystallization, which is gradually transformed into nanocrystalline ZSM-5 through solid–solid transformations. Additionally, the conventional formation of ZSM-5 from crystalline nuclei generated in the remaining liquid solution is observed after 48 h.

Preparation of TS-1 by wetness impregnation of amorphous SiO2—TiO2 solids: influence of the synthesis variables

Abstract TS-1 has been synthesized by wetness impregnation of amorphous SiO 2 TiO 2 solids with TPAOH solutions and subsequent crystallization under autogenous pressure, the influence of the different variables involved being studied. The SiO 2 TiO 2 cogel used as raw material has been prepared following a two step (acid-base) sol-gel process which leads to the formation of Si O Ti bonds previously to the zeolite crystallization. The titanium content of TS-1 can be controlled through the variation of the raw SiO 2 TiO 2 composition, although a limit of the Ti/(Ti + Si) × 100 molar ratio around 2.3% in titanium effectively incorporated to the zeolite framework has been observed, even when starting from cogels with low Si/Ti ratios. The size and morphology of the crystals depend on the synthesis variables, small crystallites with 0.2 μm of diameter being obtained at high TPAOH concentration. The kinetics of TS-1 crystallization by this method is very fast in spite of taking place in static conditions. Thus, pure TS-1 can be achieved at 170°C in just 6 h of synthesis. All TS-1 samples were tested as catalysts for n-hexane oxyfunctionalization, high activities and selectivities being obtained with most of them.

Crystallization mechanism of all-silica zeolite beta in fluoride medium

Abstract The crystallization mechanism of pure silica zeolite beta in a fluoride medium has been investigated through the characterization of samples obtained at different synthesis times. The crystallization takes place in the presence of tetraethyl ammonium and fluoride ions at neutral pH, following a non-conventional mechanism based on the reorganization of an amorphous gel phase. The solid yield, referenced to the silica weight, remains almost constant and close to 100% during the whole crystallization process. Likewise, the amount of TEA + and F − ions present in the solid is not significantly changed as its crystallinity varies from 0% to 100%. The amorphous material initially observed is formed by a gel phase consisting of non-isolated primary units around 10–30 nm size distributed throughout the gel phase. The first crystals detected in the system are very large, with sizes around 7 μm. These crystals growth directly from the amorphous solid phase through a process of aggregation and densification of the primary units. The crystals finally observed in highly crystalline samples have sizes around 14 μm. This crystallization mechanism based on solid–solid transformations is probably favored by the low solubility of the silica species in the fluoride medium under neutral pH.

Evidence of solid-solid transformations during the TS-1 crystallization from amorphous wetness impregnated SiO2TiO2 xerogels

Abstract The crystallization mechanism of TS-1 has been investigated when this Ti-containing material is obtained from amorphous SiO2TiO2 solids impregnated with TPAOH solutions followed by thermal treatment under autogenous pressure. Samples with different degrees of crystallinity have been prepared and characterized by elemental analysis, X-ray diffraction (XRD), infrared (IR) and diffuse reflectance ultraviolet visible (DR UV-Vis) spectroscopies, scanning and transmission electron microscopy (SEM and TEM), n-hexane adsorption and catalytic tests of n-hexane oxyfunctionalization. In contrast to the classical mechanism proposed to describe the zeolite synthesis through a mediated solution process, in the system studied here the TS-1 crystallization is mainly governed by solid-solid transformations, and a true crystal growth step is not observed. The major role of the solution is to provide the TPA+ species necessary to complete the solid crystallization and to favour the migration of the secondary particles. During the first stages of the synthesis, a small amount of the raw material is dissolved yielding soluble Ti-rich species ( Si Ti = 5 ), which are no longer involved in the crystallization. Simultaneously, the starting polymeric SiO2TiO2 cogel is converted into a particulate amorphous material formed by a tight packing of primary particles with sizes around 50 nm. In a subsequent step, the latter are aggregated leading to secondary particles that become independent of the cogel as they reach a critical diameter of 0.5–0.7 μm. Once individualized, the secondary particles undergo a zeolitization process, being gradually transformed into TS-1 crystals of approximately the same size. In addition, the coordination of Ti sites become tetrahedral as the crystallization progresses due to the higher connectivity between Ti and Si atoms existing in the TS-1 network compared to the raw amorphous xerogel.

Synthesis of TS-1 by wetness impregnation of amorphous SiO2TiO2 solids prepared by the sol-gel method

Abstract TS-1 has been synthesized following a method based on the wetness impregnation with a template solution of amorphous SiO2TiO2 solids followed by crystallization under autogenous pressure. The preparation of the raw SiO2TiO2 solids has been carried out by different two-step sol-gel processes in order to achieve the formation of SiOTi bonds prior to the zeolite synthesis. The properties of the TS-1 samples obtained strongly depend on the method used to prepare the starting amorphous cogel. All TS-1 samples showed high catalytic activity for n-hexane oxyfunctionalization with H2O2. X-Ray diffraction (XRD), infrared (IR) and catalytic measurements evidence the Ti incorporation into the zeolite framework. Diffuse reflectance ultraviolet-visible (DR UV-VIS) spectroscopy shows that the Ti atoms are occupying tetrahedral positions in the TS-1 lattice, whereas bulk anatase and extraframework Ti species are not detected. Thermogravimetric analysis (TGA) and n-hexane adsorption measurements confirm the high crystallinity of the material obtained by this procedure. The size of the crystallites ranges between 0.4 and 5 μm depending on the method used to prepare the raw SiO2TiO2 cogel. When compared to the conventional procedures of TS-1 preparation by hydrothermal crystallization of a liquid gel, the method studied here is simpler, requires a lower reaction volume and proceeds with shorter synthesis time. In addition, TS-1 prepared by wetness impregnation of SiO2TiO2 cogels exhibits properties similar to those of the material obtained following the recipe of the original TS-1 synthesis patent.

Adsorption, acid and catalytic changes induced in ZSM-5 by coking with different hydrocarbons

Abstract Controlled coking of ZSM-5 has been carried out using mesitylene, toluene and isobutene as coke precursors. The coked samples have been characterized by thermogravimetry in air. X-ray diffraction, IR, adsorption-diffusion experiments, ammonia temperature-programmed desorption and catalytic measurements. It has been found that the amount, nature and location of the carbonaceous residues strongly depend on the hydrocarbon used as coke precursor. Coke formed from mesitylene is deposited on the external zeolite surface whereas coking with isobutene leads to deposits located within the internal pore structure and mainly of paraffinic nature. Coke from toluene is polyaromatic, being deposited both on the external and the internal zeolite surface. The catalytic properties of the coked ZSM-5 samples have been tested in toluene disproportionation. Coke formed from isobutene causes a decrease of toluene conversion almost without changing p -selectivity which has been assigned to the relative enhancement of the role of the external zeolite surface as the internal volume is filled by coke. On the other hand, p -selectivity is increased by coking with mesitylene because of the shape-selectivity enhancement produced by the deposition of the coke species on the external surface of the ZSM-5 zeolite. Coke from toluene shows an intermediate effect on the catalytic properties leading to a p -selectivity/toluene conversion relationship similar to that of the uncoked ZSM-5.

TS-2 synthesis from wetness-impregnated SiO2-TiO2 xerogels

TS-2 has been synthesized from SiO 2 -TiO 2 xerogels by wetness impregnation with TBAOH solutions and subsequent autoclaving at 170°C. The preparation of the raw materials through sol-gel methods leads to a stabilization of the Ti atoms by dispersion in isolated positions of the amorphous SiO 2 network, which prevents the formation of undesired TiO 2 phases during both gel and zeolite synthesis. Additional advantages of this method compared to the conventional TS-2 preparations are the use of lower amounts of TBAOH and smaller reactor volumes as well as the higher zeolite yields that are obtained. Characterization of the samples obtained by i.r., DR u.v.-vis, and n-hexane oxyfunctionalization measurements confirms that the Ti species are located in tetrahedral positions of the MEL framework, whereas extraframework Ti is not detected, t.g., n-hexane and N 2 adsorption, and TEM measurements reveal the presence of a relatively high external surface and a secondary mesoporosity in these samples. This mesoporosity is originated by the assembly of small crystallites (0.03 μm width) to yield zeolite macroparticles (1 μm). TS-2 prepared by wetness impregnation exhibits catalytic properties for the oxyfunctionalization of n -hexane with H 2 O 2 similar to those of the conventional TS-2. The TS-2 crystallization under these conditions takes place through a nonconventional mechanism governed mainly by solid-solid transformations, leading to a reordering of the amorphous starting solid into the TS-2 crystalline structure.

Crystallization of TS-1 and TS-2 zeolites with contribution of solid–solid transformations

The synthesis of TS-1 and TS-2 zeolites from SiO2–TiO2 xerogels wetness impregnated with solutions of the structure-directing agent proceeds mainly through solid–solid transformations which allow the raw amorphous network to be reordered into crystalline frameworks.

Bifunctional properties of Al-TS-1 synthesized by wetness impregnation of amorphous Al2O3-TiO2-SiO2 solids prepared by the sol-gel method

Ti-containing ZSM-5 (Al-TS-1) has been synthesized by wetness impregnation of amorphous Al2O3-TiO2-SiO2 solids with TPAOH solutions and subsequent crystallization under autogenous pressure at 170°C. The cogel containing aluminum, titanium and silicon oxides used as raw material has been prepared following a two-step (acid-base) sol-gel process which leads to the formation of Si-O-Ti and Si-O-Al bonds previously to the zeolite crystallization. X-ray diffraction (XRD) confirms the high crystallinity of TS-1 and Al-TS-1 obtained by this procedure whereas thermogravimetric analysis (TGA) of TPAOH-containing samples andnC6 adsorption measurements show the purity of the different samples. Fourier transformed infrared spectroscopy (FTIR) evidences the incorporation of Ti and NH3 temperature programmed desorption confirms the acidic properties of Al-TS-1. Diffuse reflectance ultraviolet-visible spectroscopy (DR UV-VIS) shows that Ti atoms occupy tetrahedral positions in the Al-TS-1 and TS-1 lat-tices whereas bulk anastase and/or extraframework Ti species are not detected.29Si and27Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy indicate the presence of Al occupying tetrahedral positions in the Al-TS-1 framework. Therefore, the simultaneous incorporation of Ti and Al provides the Al-TS-1 samples with bifunctional properties showing high catalytic activity forn-hexane oxyfunctionalization with H2O2 and for alcohols etherification reactions. When it is compared to the conventional procedures of Al-TS-1 preparation by hydrothermal crystallization of a liquid gel, the method studied here is simpler, requires a lower reaction volume and proceeds with shorter synthesis time. In addition Al-TS-1 prepared through this alternative method exhibits better catalytic properties than the material synthesized following a recipe based on hydrothermal crystallization of a liquid gel.

Selective production of methanol from syngas over LaTi1−xCuxO3 mixed oxides

Carbon monoxide hydrogenation was studied over partially substituted copper-containing LaTi1−xCuxO3 oxides and on copper supported on La2O3. The unsubstituted (x = 0) oxide was weakly active for CO hydrogenation, whereas all the other oxides were more active and exhibited high selectivity to methanol. Particularly, for substitutions x = 0.5–0.6, where the perovskite structure was observed, CO conversions close to 22% and selectivity close to 80% were found. Cu/La2O3 was, however, less active and yielded CO2 and hydrocarbons as the major products. Using X-ray photoelectron spectroscopy it was determined that under reaction conditions copper exists as reduced species. The L3VV X-ray induced Auger transition at 1849.2 eV observed for a representative LaMn0.5Cu0.5O3 catalyst prereduced and used in CO hydrogenation at 573 K, suggests that Cu+ species dominates in spent catalyst. These Cu+ species are believed to be stable under reaction conditions in the perovskite structure, only a slight Cu enrichment occurs on the topmost layer of catalysts.