Micro-mesoporous Aluminosilicates Research Articles

Furfural Oxidation with Hydrogen Peroxide Over ZSM-5 Based Micro-Mesoporous Aluminosilicates

Micro-mesoporous aluminosilicates based on ZSM-5 zeolite, obtained by a dual template method, as well as in the presence of a dual-functional template (i.e. a Gemini-type surfactant), were tested in the oxidation of furfural with hydrogen peroxide. Even substantial changes in acidity and porosity of the catalysts result in minor variations of selectivity towards the desired products. Application of the synthesized zeolite-based materials in the oxidation of furfural with hydrogen peroxide leads to formation of 2(5H)-furanone (yield up to 28.5%) and succinic acid (up to 19.5%) as the main C4 reaction products. The kinetic model developed previously to treat the results for oxidation of furfural over sulfated zirconia was able to describe the data also for micro-mesoporous aluminosilicates.Graphical

Selective Hierarchical Aluminosilicates for Acetalization Reaction with Propylene Glycol

Hierarchical micro-mesoporous aluminosilicates nanoparticles were synthesized at different of Si/Al ratios and were directly used as a solid acid catalyst for acetalization reaction with propylene glycol. TEM and N2 adsorption analysis of the resulting aluminosilicates revealed the formation of the hierarchical structure occurs on the Si/Al ratio increases the formation of mesoporous within the structure of aluminosilicate. The aluminosilicates exhibit high selectivity towards acetalization reaction in comparison with the homogeneous PTSA. The presence of mesoporous structures is crucial for increased conversion and selectivity of the reaction which presumably due to the improved diffusion of substrate to reach acid sites.

Ethylene oligomerization over H- and Ni-form aluminosilicate composite with ZSM-5 and MCM-41 structure: Effect of acidity strength, nickel site and porosity

Abstract The micro-mesoporous aluminosilicates with composite structure of ZSM-5 phase encapsulated in MCM-41 structure were prepared. The corresponding H- and Ni-form composites were characterized and compared with the ZSM-5, MCM-41 to investigate the influence of acid strength and porosity on the catalytic performance of ethylene oligomerization. Their acidity increases with the increased ZSM-5 components incorporated in the MCM-41 structure. After loading Ni species the amount of weak acid site increases and that of strong acid site decreases. The interaction between the exchanged Ni-ion and the support is improved by the increased strength of acidity of zeolite. The catalytic character of the ZSM-5 and MCM-41 phase exhibited in the products distribution was studied under two typical reaction conditions for α-olefins and liquid fuel productions. The synergetic effect of Ni-ions and acid sites with different acid strength zeolite structure shows that the Ni-ion on MCM-41 is superior for ethylene conversion and determines the Anderson-Schultz-Flory products distribution at 523 K, and the strong acid site from microporous ZSM-5 is preponderant for ethylene conversion and dominates products distribution at 723 K. The nickel species are not only favorable to dimerization of α-olefins at 523 K, but also to dehydrogenation for aromatics generation at 723 K.

Isomerization of α-pinene oxide over ZSM-5 based micro-mesoporous materials

Few types of ZSM-5 based micro-mesoporous materials obtained via a dual template method, steam-assisted conversion and dual-functional templating were evaluated in α-pinene oxide isomerization. Complete conversion and the highest selectivity towards trans-carveol (ca. 40–43%) were achieved over X-ray amorphous micro-mesoporous aluminosilicates as well as mesoporous molecular sieves AlSi-SBA-15. In addition, X-ray amorphous samples containing the secondary building units of ZSM-5 zeolite demonstrated the highest rate of α-pinene oxide isomerization. The yield of the most desired product trans-carveol to a large extent depends on the accessibility of acid sites to the reagents molecules and acidity of the catalysts (strength and concentration of Brønsted and Lewis acid sites).

Influence of SBA-15 Zeolitization Conditions on Structural-Sorption and Acidic Properties of Micro-Mesoporous AluminosilicateS SBA-15/ZSM-5

Partly zeolitized micro-mesoporous aluminosilicates SBA-15/ZSM-5 were synthesized by conversion of dry gel of template-containing SBA-15 in the presence of tetrapropylammonium hydroxide. They have a hexagonally ordered mesostructure and developed surface and mesoporosity for which the concentration and strength of the Bronsted and Lewis acid sites are higher than in AlSi-SBA-15. The formation of Bronsted acid sites of moderate strength in the samples with a low degree of zeolitization is due to the presence of ZSM-5 precursors as well as crystallites in the obtained aluminosilicates.

Synthesis of micro-mesoporous aluminosilicates on the basis of ZSM-5 zeolite using dual-functional templates at presence of micellar and molecular templates

Micro-mesoporous aluminosilicates consisting of agglomerates of the ZSM-5 nanoparticles were obtained using dual-functional templates [C6H13–N+(CH3)2–C6H12–N+(CH3)2–C6H13](Br−)2 (C6–6–6Br2), [C8H17–N+(CH3)2–C6H12–N+(CH3)2–C8H17](Br−)2 (C8–6–8Br2). Aluminosilicates with randomly oriented flake-like particles built from ZSM-5 layers were obtained using [C16H33–N+(CH3)2–C6H12–N+(CH3)2–C6H13](Br−)2 (C16–6–6Br2). Use of С8–6–8Br2 and additive of cetyltrimethylammonium bromide CTAB (CTAB concentration is lower than the first critical micelle concentration, CMC1) leads to an increase of the total specific surface area, mesopore surface area and the mesopore size uniformity in the product, as well as the concentration of Brønsted acid sites accessible for bulk molecules. It is assumed that CTAB role is to limit the zeolite crystals growth. Addition of CTAB to С16–6–6Br2 (CTAB concentration is also lower than CMC1) promotes the formation of micro-mesoporous aluminosilicate possessing disordered lamellar mesostructure stable upon calcination which is characterized by a higher total specific surface area and better mesopores uniformity than ZSM-5 obtained in the presence of С16–6–6Br2. The additive of a molecular template tetrapropylammonium hydroxide (TPAOH) shifts the structure formation toward zeolite and leads to an increase in the ratio of concentrations of Brønsted and Lewis acid sites for the obtained aluminosilicates. The addition of small amounts of TPAOH into the reaction mixture (TPAOH/C16–6–6Br2 = 6.5·10−3) leads to formation of self-pillared aluminosilicate. In this material the nanoparticles act as “pillars” between the layers of ZSM-5 and thus contribute to retention of low-ordered lamellar mesostructure after calcination, leading to increase of the total specific surface area and improving mesopores uniformity.

Two-stage dual template alkali metal-free synthesis of micro-mesoporous aluminosilicates based on ZSM-5 zeolite precursors

Carrying out the dual template synthesis (in the presence of molecular and micellar templates) in the alkali metal-free reaction media (pH 11.7–12) at a relatively low-temperature hydrothermal treatment (100–120 °C, duration 3–5 days) allowed to obtain micro-mesoporous aluminosilicates combining the properties of zeolite ZSM-5 and mesoporous molecular sieve AlSi-MCM-41. X-ray amorphous sol-precursors of ZSM-5 zeolite obtained by preliminary hydrothermal treatment of alkali metal-free reaction mixture at 160 °C and containing secondary building units of zeolite were used as initial substances for synthesis of micro-mesoporous aluminosilicates. During the joint formation of the structure of zeolite and mesoporous molecular sieve in one reaction mixture and in contiguous conditions close to optimal for their individual formation, composites zeolite/mesoporous molecular sieve were formed. The structural and sorption characteristics and acidity of the obtained composites differ from mechanical mixture of two phases. X-ray amorphous micro-mesoporous aluminosilicate with secondary building units and precursors of ZSM-5 in mesostructure is characterized by higher concentration and strength of Bronsted and Lewis acid sites as well as a higher catalytic activity in the reaction of cracking of cumene compared to AlSi-MCM-41 of similar composition. This aluminosilicate also exhibits a higher total selectivity toward the major products of reaction, in comparison with ZSM-5.

Low-temperature and alkali-free dual template synthesis of micro-mesoporous aluminosilicates based on precursors of zeolite ZSM-5

The aim of this study was to determine the influence of the conditions of low-temperature sequential dual template synthesis of micro-mesoporous aluminosilicates based on precursors of ZSM-5 on their phase composition, structure, sorption properties, and acidity. The methods of X-ray powder diffraction and volumetric adsorption (N2, 77 K) analysis, dynamic light scattering (for characterization of sol-precursors), IR-spectroscopy, energy-dispersive X-ray spectroscopy, scanning and transmission electron microscopy, solid-state 29Si MAS NMR and 29Si{1H} cross-polarization MAS NMR spectroscopy, temperature-programmed desorption of ammonia, ad(de)sorption of pyridine and 2,6-di-tert-butylpyridine with IR-spectral control were used for characterization of the obtained micro-mesoporous aluminosilicates. It was shown that the convergence of compositions of the reaction mixtures and conditions of formation of zeolites and mesoporous molecular sieves, in particular, the use of alkali-free reaction medium and relatively low temperature of hydrothermal treatment (100 °C) allowed to obtain the X-ray amorphous and partially zeolitized micro-mesoporous aluminosilicates. Such aluminosilicates have appropriate structural and sorption properties and the acidity inherent to ZSM-5 and MCM-41, including the Bronsted and Lewis acid sites which are accessible for bulk molecules of organic substances. The possibility of purposeful regulation of characteristics of micro-mesoporous aluminosilicates depending on the conditions of their preparation was confirmed. Herewith the investigated aluminosilicates possess the sorption and acidic properties which are different from the mechanical mixture of two phases.

Low-temperature synthesis, structure, sorption properties and acidity of zeolite ZSM-5

High structure-directing activity and specificity of templating action of tetrapropylammonium cations TPA+ as well as high concentration of framework-forming substances in the initial reaction mixtures contribute to the formation of zeolite ZSM-5 at relatively low temperature (100 °C) in the alkali-free media. Zeolite ZSM-5 obtained at 100 °C has higher micropore and mesopore volumes, specific surface area, less uniform micropore size distribution in comparison with ZSM-5 synthesized at 170 °C in an alkaline medium in the presence of hydrated Na+ cations. Samples synthesized at 100 °C possess a lower concentration of acid sites and broader distribution of them in strength, compared with ZSM-5 obtained at 170 °C in an alkaline medium. ZSM-5 obtained at 100 °C exhibits a high catalytic activity in the reaction of cracking of cumene, as well as a higher selectivity to propylene and styrene, in comparison with isostructural analogue obtained at 170 °C in an alkaline medium. It is assumed that the investigated alkali-free reaction mixtures and the corresponding sol-precursors of zeolite ZSM-5 containing the elements of secondary building units can be used for obtaining of micro-mesoporous aluminosilicates in the conditions of low-temperature synthesis (100 °C) by dual-template method—in the presence of molecular and micellar templates.

Sol-gel synthesis of micro-mesoporous aluminosilicates using oligomeric esters of orthosilicic acid

A method for the sol-gel synthesis of micro- and mesoporous aluminosilicates (MAS) using commercially available raw materials (mixtures of oligomeric esters of the orthosilisic acid of ethyl silicate-40 and water-alcohol solutions of aluminum nitrate) is developed. The effect of pH has on the porous structures of aluminosilicates with different Si/Al ratios is studied. It is shown that varying the pH when studying hydrolysis allows the synthesis of both micro- and mesoporous aluminosilicates with relatively narrow distributions of pores, changing the specific surface area from 341 to 684 m2/g, the specific pore volume from 0.2 to 0.95 cm3/g, the pore diameter from 0.17 to 40 nm, and the total acidity on ammonia from 226 to 405 μmol/g. The acid properties of the samples are studied at different SiO2/Al2O3 ratios, and their catalytic activities are measured in the model dimerization of methylstyrene (80°C; 10 wt % MAS; initial concentration of α-methylstyrene in the chlorobenzene, 3.3 mol/L). It is established that the highest conversion of α-methylstyrene (99.8 mol %) is observed for samples with SiO2/Al2O3 ratios of 5 and 20, for which the maximum concentrations of acid sites are characteristic. Micro-mesoporous aluminosilicates are promising for making acid-base type catalytic changes in different organic substances.

Understanding and direct strategy to synthesize hydrothermally stable micro-mesoporous aluminosilicates with largely enhanced acidity

A general synthetic route of bimodal micro-mesoporous aluminosilicates with strong acidity and excellent hydrothermal stability has been developed. This method is built on the understanding that mesoporous aluminosilicates with high hydrothermal stability can be obtained via assembly of zeolite precursors in acidic conditions by copolymer templates, and the acidity can be improved by the introduction of Al species in neutral and alkaline systems. It is proposed that in order to obtain the mesoporous aluminosilicates with simultaneously strong acidity and excellent hydrothermal stability, an important requirement is that the paradox between stability and acidity must be harmonized. In this article, the crystallization of the resulting aluminosilicates was significantly improved by taking advantage of the Beta zeolite precursors in acidic media. When the mesostructure was basically formed, the pH of the reaction system was adjusted from strong acid to neutral, followed by a second hydrothermal crystallization, during which a large amount of Al could be introduced into the mesophases. XRF and Al NMR show that BPs possessed a high tetrahedrally coordinated Al loading. Py-IR results reveal that the total amount of Brønsted and Lewis acid sites increases 7.23 times. Such bimodal micro-mesoporous aluminosilicates demonstrated superior catalytic properties for heavy crude oil catalytic cracking, because of the increased hydrothermal stability and acidity. Our achievements have added new contributions to understanding the preparation of mesoporous aluminosilicates with strong acidity and excellent hydrothermal stability, which sheds a light on the practical application of mesoporous zeolites in petrochemical industry.

Preparation of hierarchical micro-mesoporous aluminosilicate composites by simple Y zeolite/MCM-48 silica assembly

• Hierarchical micro-mesoporous aluminosilicates were synthesized. • Y zeolite core/MCM-48 silica shell structures were obtained. • Y zeolite favors the formation of the mesostructure. • Porosity and structure can be varied by modifying the preparation variables. • Duration of the hydrothermal step has a great effect on the materials properties. A simple procedure to obtain hierarchical micro-mesoporous aluminosilicate composites was developed by growing MCM-48 silica over commercial Y zeolite. The obtained hierarchical composites have a microporous core and a mesoporous shell. The process consists in assembling dispersed Y zeolite with a mesoporous silica phase that is formed “in situ” by “soft-templating” with cetryltrimethylammonium bromide (CTAB) as surfactant. The Y zeolite/MCM-48 silica ratio and aging time were varied to study their effects on the final porosity and structure of the hierarchical composites. The pore textural and structural characteristics of the composites did not match those of the corresponding Y zeolite/MCM-48 silica physical mixtures. This implies that the synthesized composites integrate micropores and mesopores in the same bulk. The obtained composites exhibited micropore and mesopore volumes ranging between 0.15–0.31 and 0.30–0.51 cm 3 /g, respectively. X-ray diffraction and N 2 adsorption results revealed that the presence of zeolite in the reaction medium favors the formation of mesopores in the obtained materials, especially for short hydrothermal treatments. TEM results showed that the obtained adsorbents are constituted by an integrated micro-mesoporous bimodal system in which Y zeolite is surrounded by a thin cover of MCM-48 silica.