Solid-state 27Al MAS Research Articles

OSDA-free hydrothermal conversion of kenyaite into zeolite beta in the presence of seed crystals

Zeolite beta with high crystallinity has been successfully synthesized by using kenyaite conversion method with seed crystals under hydrothermal treatment at 125 °C for 36 h. The OSDA-free synthesis of zeolite beta could be obtained from the mixture composition of 0.96 NaOH: SiO2: 0.0167 Al2O3: 180 H2O by adding 25 wt % seeds. To study the crystallization behaviours and the medium-range structure changes during the conversion process, the prepared zeolite beta was characterized by powder X-ray diffraction, field-emission scanning electron microscope, Fourier transform infrared spectroscopy, energy dispersive spectroscopy, solid-state 27Al MAS and 29Si MAS NMR spectroscopy. The growth of zeolite beta happened on the surface of kenyaite. The five-membered rings and six-membered rings in kenyaite were partly preserved and the four-membered rings were formed by the connection of five-membered rings during the crystallization process. Meanwhile, the effects of reaction time, the initial mixture composition, the quantity of beta seed crystals and temperature in the synthesis were investigated. The as-synthesized zeolite beta, with relatively high thermal stability, exhibits good catalytic performance in the cyclohexanol dehydration reaction system.

Kreiranje mezopora u zeolitu ZSM-5 alkalnim tretmanom - uticaj koncentracije NaOH, temperature i trajanja tretmana

The work presented in this paper was dedicated to the investigation of the influence of the alkaline treatment parameters on the formation of mesopores in zeolite ZSM-5, SiO2/Al2O3=50. Concentration of NaOH, temperature and tretment duration were varied. The obtained samples were characterized using low temperature nitrogen adsorption, X ray diffraction and solid state 27Al MAS NMR spectroscopy. Formation of mesopores was found to be a process sensitive to changes of treatment parameters, which can lead to creation of hierarchical zeolites with different mesopore surfaces and distributions of pore size diameters. Also, treatment duration was found to have a lesser influence on the process, compared to other two investigated parameters. Mesopore introduction resulted in partial crystallinity reduction, in all modified samples. No significant buildup of extraframework Al was detected.

Solid-state MAS NMR investigations for pentavalent cation-replaced pollucite compounds with a negative thermal expansion coefficient

Pollucite compounds with an aluminosilicate structure, V-replaced pollucite, Cs0.7V0.1Al0.8Si2.1O6 (CVAS), P-replaced pollucite, Cs0.7P0.1Al0.8Si2.1O6 (CPAS), and Nb-replaced pollucite, Cs0.7Nb0.1Al0.8Si2.1O6 (CNbAS), were synthesized by a multistep heat treatment of 550–1200 °C. CVAS exhibited the superior thermal expansion with thermal expansion coefficients of 0.9 × 10−6 °C−1 (30–500 °C), −0.9 × 10−6 °C−1 (30–150 °C), and −0.4 × 10−6 °C−1 (30–100 °C). Solid-state 29Si MAS NMR spectroscopy clarified that the number of Si–O–Si bonds of tetrahedra in the framework of CVAS was larger than those of CPAS and CNbAS. The chemical shift for peaks of the 29Si MAS NMR spectrum of CVAS was larger than those of CPAS and CNbAS, suggesting that the bond angle of tetrahedra was enlarged by the V replacement. Solid-state 27Al MAS NMR spectroscopy implied that CVAS had a small amount of sixfold coordinated Al species, which resulted in an increase in the number of Si–O–Si bonds of tetrahedra of CVAS.

Synthesis of Mesoporous γ-alumina Assisted by Room Temperature Ionic Liquid

Boehmite Sols were used as aluminum precursors for preparing mesoporous alumina with crystalline framework walls in the presence of 1-butyl-3-methyl imidazolium tetrafluoroborate([BMIM][BF4]).The structure of the γ-Al2O3 was examined by N2 physical adsorption,X-ray powder diffraction(XRD),transmission electron microscope(TEM),and solid state 27Al MAS NMR spectra.The results show that alumina with the three-dimensional interconnected scaffold-like channels displays very rich porosities with large mesopores,and the addi-tion of [BMIM][BF4] could improve the pore properties of alumina significantly.

Surface alumina species on modified titanium dioxide: A solid-state 27Al MAS and 3QMAS NMR investigation of catalyst supports

27Al MAS and 3QMAS NMR have been used to study Al 2O 3/TiO 2 catalyst supports synthesized via excess-solution impregnation and surface sol–gel methods. Temperature and alumina loading level strongly affect chemical states of aluminum oxide species observed. Surface cations, Al ( H 2 O ) 6 3 + , a surface alumina monolayer, and disordered transitional aluminas (multilayers) and α-alumina, coexist on the TiO 2 surface. Chemical shift and quadrupole coupling constants are reported for the major species identified in 3QMAS experiments. Gold particle catalysts prepared from supports calcined at 500 °C have optimum catalytic activity in CO oxidation, and smallest gold particle size for supports, which show maximum monolayer type octahedral alumina on the titania surface.

Solid-State NMR Studies Of Ultramarine Pigments Discoloration

AbstractUltramarines are a family of pigments widely used as colorants in artists' paints, coatings, plastics, cosmetics, and various industrial materials. They are aluminosilicates characterized by a sodalite cage framework which enclarthrates paramagnetic (S3-., S2-.) and diamagnetic (S4or S3Cl) chromophores responsible for the color of these pigments. Solid-state27Al MAS NMR studies showed that the fading mechanisms in ultramarine pigments, both in acidic and alkaline environments, is initiated via de-alumination leading to framework destruction which in turn triggers the release of the chromophores. This results in color loss and in the emergence of extra-framework aluminum.29Si MAS studies provide new insight into these processes and suggest that acidic attack leads to the formation of Q1-Q3silicates and possibly to the emergence of Si(3Al), Si(2Al), Si(1Al) and Si(0Al) fragments indicative of the formation of secondary pores through which the guest chomophores can leave the cage. These findings are important for the design of proper conservation treatments and preservation procedures for artwork containing ultramarine pigments.

Effects of silica reactivity on the nature and formation of Al-substituted 1·1 nm tobermorite

The effects of the reactivity of the silica source on the formation and morphology of Al-substituted 1·1 nm tobermorite in a hydrothermally treated CaO-Al2O3-SiO2-H2O system were studied. Colloidal silica and fine quartz were used as the silica source and gibbsite, containing only octahedrally coordinated Al, was the aluminous source used in this investigation. From a combination of solid-state 27Al MAS NMR, XRD, DTA-TGA and SEM, unequivocal evidence can be found that, with increasing silica reactivity, tetrahedrally coordinated Al in Al-substituted 1·1 nm tobermorite prevails over octahedrally coordinated Al species. Reaction sequences in this system as a function of the reactivity of the silica source are presented.

Ultrasound as a tool to synthesize nano-sized silica-alumina catalysts with controlled mesoporous distribution by a novel sol-gel process

A novel sol–gel process is used to synthesize mesoporous silica–alumina catalysts with controlled pore size distribution without using templates or pore-regulating agents. The nitrogen sorption analysis shows that the synthesized materials have high specific surface area in the range 587–692 m2/g and similar mesoporous distribution within 2–11 nm. Ultrasound is applied to prepare precursor silica–alumina sol with narrow particle diameter distribution. By calculation, it is found that ultrasonic treatment is able to provide 22.94 kJ/mol energy, which is just in accordance with the weak bond's bonding energy. Based on the calculation result, it is proposed that the role of ultrasonic treatment followed by acid peptization is to provide energy to break down the weak bonds. After gelation, the regular and close packing of similar-sized sol particles is capable of formation of controlled-sized interstices which are the precursors of mesopores. Solid-state 27Al MAS NMR and TEM are used to characterize the samples' structure and particle morphology. They show that most of the aluminum is located in the tetrahedral position in the present materials. According to TEM results, it is shown that the synthesized materials have spherical particles with size of about 15 nm.

Observation of six-coordinate aluminum in andalusite by solid-state 27Al MAS NMR

Observation of six-coordinate aluminum in andalusite by solid-state 27Al MAS NMR