M.A. Uguina

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.

Synthesis of ZSM-5 zeolites in the C2H5OHNa2OAl2O3SiO2H2O system

Abstract ZSM-5 type zeolites have been synthesized using ethanol as template. The effect of initial gel composition (in terms of molar rations H 2 O SiO 2 , SiO 2 Al 2 O 3 , OH − SiO 2 , and C 2 H 5 OH SiO 2 , with Na 2 O SiO 2 constant), temperature, stirring velocity, and seeding has been investigated. A semiempirical mathematical model for the kinetics of zeolite growth has been developed, reproducing the experimental data with deviations of less than 10%.

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.

Toluene disproportionation over ZSM-5 zeolite: Effects of crystal size, silicon-to-aluminum ratio, activation method and pelletization

Abstract The catalytic activity and selectivity of ZSM-5 zeolite in toluene disproportionation were studied as a function of crystal size, silicon-to-aluminum ratio, acidity, activation method and pelletization of zeolite. Although para -xylene formation was slightly favoured by increasing the crystal size, high para -selectivity was not achieved. At the lowest silicon-to-aluminum ratio, catalytic activity was enhanced with a drop on the selectivity to disproportionation, suggesting the presence of “super acid sites” related with extra lattice aluminum species. From IR spectroscopy measurements with adsorption/desorption of pyridine it was observed that during the activation step, a high H + concentration in the cation exchange increases the strength of Brensted sites, increasing the number of acid sites which are active in toluene disproportionation. Subsequent calcination completes zeolite activation, increasing the number of Bronsted acid sites active in that reaction. In the pelletization there is a cation exchange between Na + from the montmorillonite and H + from the zeolite when both are mixed and suspended in water. Nevertheless, sodium montmorillonite proved to be a suitable binder of ZSM-5 provided that the catalysts are reactivated after pelletization.

Carbon dioxide/methane separation by adsorption on sepiolite

In this work, the use of sepiolite for the removal of carbon dioxide from a carbon dioxide/methane mixture by a pressure swing adsorption (PSA) process has been researched. Adsorption equilibrium and kinetics have been measured in a fixed-bed, and the adsorption equilibrium parameters of carbon dioxide and methane on sepiolite have been obtained. A model based on the LDF approximation has been employed to simulate the fixed-bed kinetics, using the Langmuir equation to describe the adsorption equilibrium isotherm. The functioning of a PSA cycle for separating carbon dioxide/methane mixtures using sepiolite as adsorbent has also been studied. The experimental results were compared with the ones predicted by the model adapted to a PSA system. Methane with purity higher than 97% can be obtained from feeds containing carbon dioxide with concentrations ranging from 34% to 56% with the proposed PSA cycle. These results suggest that sepiolite is an adsorbent with good properties for its employment in a PSA cycle for carbon dioxide removal from landfill gases.

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.

Deactivation of toluene alkylation with methanol over magnesium-modified ZSM-5 Shape selectivity changes induced by coke formation

Abstract Coke deposition during toluene alkylation with methanol over a Mg-modified ZSM-5 catalyst leads to drastic changes in the zeolite shape selectivity with the time-on-stream. Besides a decrease in toluene and methanol conversions, a maximum in para selectivity is observed due to coke deposition. Nevertheless, the deactivated catalysts are less para-selective than the fresh ZSM-5 when they are compared at the same toluene conversion. Para selectivity of the primary product decreases due to coking below the value of 100% corresponding to the uncoked catalyst. At low times-on-stream, coke deposition takes place mainly by pore filling. At higher times-on-stream, the proportion of polyaromatic species in the coke deposits is increased, which leads to a partial blockage of the zeolite pore structure, lowering the length of the diffusional pathway. Therefore, the intracrystalline diffusional control on the internal xylene isomerization is less pronounced in the coked catalysts, which causes the lower para selectivity of the primary product observed in these samples.

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.

Synthesis of titanium silicalite-1 from an SiO2–TiO2 cogel using a wetness impregnation method

An alternative to the conventional methods of TS-1 synthesis is based on the wetness impregnation of an SiO2–TiO2 cogel, which leads to more effective Ti incorporation in a simpler way.