Chi-Feng Cheng

Optimal parameters for the synthesis of the mesoporous molecular sieve [Si]-MCM-41

Highly crystalline MCM-41 with a very narrow pore-size distribution (FWHM=1.5 A), high surface area (1185 m2 g -1 ), large grain size and thick channel walls (ca. 17 A) was prepared in alkali-free media. The properties of the product depend on the source and concentration of the reactants, the gel aging time, the temperature and the duration of the synthesis. There is no induction period in the course of the synthesis and Ostwalds rule of successive transformations applies. The initially produced hexagonal phase is transformed into the lamellar phase and then into an amorphous phase. In the 150°C synthesis the most stable product is amorphous silica. The course of the synthesis is conveniently monitored by pH measurement. Gel aging, during which a spatial distribution of silicate polyanions and micellar cations is established, is essential for preparing high-quality MCM-41. Surfactants with the same cationic organic group but different counter-anions alter the course of the synthesis. The degree of polymerization of silica is also important. Highly basic gels favour the lamellar product; when the gel is weakly basic the quality of MCM-41 is lower as insufficient TMAOH is available to dissolve the silica. The result of excess silica is similar but even more pronounced. Purely lamellar products are made at low SiO 2 concentrations, when the gel is more strongly basic. The best quality MCM-41 is prepared from a gel of molar composition SiO 2 :0.19 TMAOH:0.27 CTABr:40 H 2 O (with CTABr/SiO 2 =0.27, similar to the ratio in the solid product) aged at 20°C for 24 h and synthesis lasting for 48 h.

Effect of structural aluminium on the mesoporous structure of MCM-41

X-ray diffraction (XRD), 27Al magic-angle-spinning (MAS) NMR, N2 adsorption measurements and transmission electron microscopy (TEM) show that the one-dimensional channels of the mesoporous aluminosilicate MCM-41 containing only structural (four-coordinate) aluminium are much shorter in the sample with an Si/Al ratio of 34.1 (ca. 160 A) than in purely siliceous MCM-41 (ca. 400 A). The quality of the XRD patterns rapidly deteriorates as the aluminium content of the solid increases. However, aluminosilicate MCM-41 still preserves a perfect mesoporous structure with an average pore diameter of ca. 30 A, a surface area of 990 m2 g–1 and a pore volume of 0.74 cm3 g–1. When the Na+ form of aluminosilicate MCM-41 is transformed into the Bronsted acidic H+ form, and some of the aluminium is removed from the structure, the uniform hexagonal mesopores partially collapse and macropores 200–2000 A in diameter are formed.

Distribution of tetrahedral and octahedral A1 sites in gamma alumina

Abstract Experimental and computational studies of γ-Al 2 O 3 show that 70±2% of Al ions occupy octahedral interstitial sites in the fcc oxygen structure, and the rest tetrahedral sites. The experimental data come from 27 Al MAS NMR and the theoretical results from ab initio quantum mechanical energy calculations for nine superlattice structures, analysed to extract various parameters which were then fed into a Monte Carlo simulation of the γ-Al 2 O 3 structure. The simulation throws new light on the reasons for the structural disorder.

Controlling the channel diameter of the mesoporous molecular sieve MCM-41

We describe a simple method of controlling the channel diameter of the mesoporous molecular sieve MCM-41 in the 26.1–36.5 A range and the wall thickness in the 13.4–26.8 A range while using the same gel mixture. This is achieved by varying the synthesis temperature in the 70–200°C range and/or reaction times in the 0.5–96 h range. The unit cell parameter, channel diameter, thickness of the channel wall, surface area, degree of polymerization and grain morphology were monitored by X-ray diffraction, N 2 adsorption, 29 Si magic-angle-spinning NMR and transmission electron microscopy. MCM-41 with wider and thicker-walled channels and higher degree of polymerization is prepared at higher temperatures and at longer reaction times. Thick-wall MCM-41 has higher thermal stability but lower surface area. The material with the thickest channel wall ever reported (26.8 A) can withstand calcination at nearly 1000°C with little structural damage. We suggest a mechanism for the increase of wall thickness and channel diameter. Fascinating morphological features involving sealed silicate ‘tubes’ and ‘vesicles’ up to 1200 A in diameter are observed.

Directing the pore dimensions in the mesoporous molecular sieve MCM-41

Abstract We describe a straightforward method of controlling the channel diameter of the mesoporous molecular sieve MCM-41 in the 26.1–36.5 A range and the wall thickness in the 13.4–26.8 A range while using the same gel mixture. The thickest-walled material can withstand calcination at 900°C with little structural damage. We have observed new morphological features involving sealed silicate ‘tubes’ and ‘vesicles’ up to 1200 A across, and suggest a mechanism for the increase of wall thickness and channel diameter.

Synthesis and characterization of the mesoporous galloaluminosilicate molecular sieve MCM-41

A range of mesoporous galloaluminosilicate molecular sieves with the MCM-41 structure and different compositions have been synthesized using gallium and aluminium nitrates as the source of Ga and Al and characterized using atomic absorption, powder X-ray diffraction (XRD) and 71Ga and 27Al magic-angle spinning (MAS) NMR. Ga, Al and Si are incorporated into the MCM-41 structure in proportions that clearly reflect the composition of the synthesis gel. NMR demonstrates that in all as-made and calcined samples gallium is four-coordinate and forms a part of the MCM-41 structure. However, 7% of Al is expelled from the structure of [120,1,3]-MCM-41 upon calcination.

Confirmation of an ANNNI-Like Model for Polytypism in SiC

The remarkable similarity between the observed SiC polytypes and the phases in the ANNNI model has suggested that the polytypes are equilibrium phases at high temperatures. Their origin lies in a competition between the nearest (J1) and next-nearest (J2) neighbour interactions between SiC double layers. We find, using norm-conserving pseudo-potentials, J1 = +0.0050 eV and J2 = -0.0034 eV per SiC pair. The sign of J2 and the value of the ratio J1/|J2| are such as to lead to polytypes, thus broadly confirming the correctness of an ANNNI-like mechanism. The origins of J1 and J2 are discussed.

Crystal morphology supports the liquid crystal formation mechanism for the mesoporous molecular sieve MCM-41

Abstract Transmission electron microscopy of purely siliceous and gallosilicate mesoporous molecular sieves MCM-41 shows that the neighbouring crystal edges meet at the angle of 120°, thus suggesting the most likely mechanism of crystal growth. The morphology of MCM-41 alone strongly supports the liquid crystal mechanism initiated by the silicate anions, and not the mechanism involving a layered intermediate.

Silicon carbide polytypes are equilibrium structures

The phonon free energy is calculated for several silicon carbide polytypes using a valence overlap shell model. The three most commonly observed polytypes, i.e. (2), (23) and (3), are shown to be thermodynamically stable, with (2) and (3) as the low- and high-temperature phases, respectively, and (23) in between. The long-ranged phonon effect splits the multiphase degeneracy between phases (2) and (3) to which SiC is extremely close at T=0 K, and its stabilises (23) with a significantly large stability region of 500 K. The calculations of the interatomic displacement-displacement correlations demonstrate how the long-ranged interatomic interactions arise from the phonons. The formation theory of longer-period polytypes is also discussed.

Synthesis and characterization of vanadosilicate mesoporousmolecularsieves MCM-41

A range of mesoporous vanadosilicate molecular sieves with the MCM-41 structure and the atomic ratio Si/V=10–160 have been synthesized using vanadyl sulfate as the source of vanadium. The products were characterized by XRD and 51 V and 29 Si MAS NMR spectroscopy. The increase of the unit-cell parameter (monitored by XRD) and the decrease of the Q 3 /Q 4 ratio (determined from the 29 Si spectra) with increasing vanadium content indicate the incorporation of vanadium into the MCM-41 structure. In as-synthesized [Si,V]-MCM-41, vanadium is in an octahedral environment and is located mainly on the surface of the channels. The amount of octahedral vanadium decreases and tetrahedral V is increasingly created as the calcination temperature increases.