yong He

A new structural model for graphite oxide

Abstract Solid-state 13 C NMR spectra of graphite oxide (GO) and its derivatives prompt us to propose a new structural model. The spectra of GO treated with KI and the course of the thermal decomposition of GO reveal the presence of epoxide groups, responsible for the oxidating nature of the material. GO is built of aromatic “islands” of variable size which have not been oxidized, and are separated from each other by aliphatic 6-membered rings containing C–OH, epoxide groups and double bonds. The carbon grid is nearly flat; a small degree of warping is caused by the carbons attached to OH groups, which are in a slightly distorted tetrahedral configuration.

Alumination of the purely siliceous mesoporous molecular sieve MCM-41 and its hydrothermal conversion into zeolite Na-A

Purely siliceous mesoporous [Si]-MCM-41 has been aluminated using an aqueous solution of sodium aluminate, NaAlO2, to form [Si,Al]-MCM-41 with the framework Si/Al ratio as low as 1.9. 27Al and 29Si magic-angel-spinning (MAS) NMR show that all aluminium is incorporated in the framework. The well resolved X-ray diffraction (XRD) pattern of calcined [Si,Al]-MCM-41 indicates that the mesopore system is stable and that the channel diameter decreases with the increased degree of incorporation of Al. The intensity of lines corresponding to Si(nAl) building blocks in the 29Si MAS NMR spectra reflect changes in the local atomic arrangement upon alumination. Increased reaction temperature at a constant concentration of NaAlO2 enhances the efficiency of the alumination, but increased concentration at constant temperature disrupts the framework structure. Upon treatment with a concentrated (>1 mol 1–1) solution of NaAlO2 at moderate temperatures (100 ± 20 °C), [Si]-MCM-41 is transformed into crystalline zeolite Na-A.

X-ray photoelectron spectroscopic studies of kaolinite and montmorillonite

Abstract X-ray photoelectron spectroscopy (XPS or ESCA) spectra of kaolinite and montmorillonite, two layered silicates used in catalytic processes, are compared with the spectra of zeolites and related silicates, where there are direct correlations between shifts in ESCA core-level and valence band features and bond ionicity. ESCA results for kaolinite and montmorillonite are distinctly different from one another and from the results for zeolites with similar Si Al ratios. The nature of the ESCA shifting patterns is interpreted in terms of the differences in bonding. We anticipate that our arguments will soon be extended to other sheet silicates.

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.

Autoclave synthesis and thermal transformations of the aluminophosphate molecular sieve VPI-5: an in situ X-ray diffraction study

Abstract The synthesis of DPA-VPI-5 and TBA-VPI-5 and their phase transformation to AlPO 4 -8 have been studied by X-ray diffraction in situ. The synchrotron radiation-energy dispersive diffraction is shown to be able to monitor these two fast reactions in real time. The results indicate that nucleation may occur already during the aging of the synthesis gel. The different rates of transformtion of DPA-VPI-5 and TBA-VPI-5 show that the microstructure of the two materials is different.

ESCA AND SOLID-STATE NMR STUDIES OF ALLOPHANE

Abstract The surface/near-surface chemistry of allophane has been studied by X-ray photoelectron spectroscopy (ESCA) and the bulk material by 27Al and 29Si solid-state NMR and other techniques. The surface/near-surface Si/Al ratio of allophane is c. 1.0, similar to that for kaolinite, zeolite Na-A and sodalite. The core level binding energies for kaolinite and allophane are almost identical, but quite different from those for zeolite Na-A and sodalite, both framework aluminosilicates. The nature and size of these differences is consistent with the differences between the chemistry of sheet and framework silicates. The small variations in the Si(2p) spectra for kaolinite and allophane are discussed in terms of bonding of the tetrahedral units in the two materials.

31P solid-state NMR study of spin diffusion in the molecular sieve VPI-5 using variable-speed MAS

Abstract Hydrated aluminophosphate molecular sieve VPI-5 has been studied by 31 P solid-state NMR with variable-speed magic-angle spinning. Cross-polarization and two-dimensional spin-diffusion spectra were used to assign NMR signals to particular phosphorus sites in the structure in the P6 3 space group. The aim was to assess the value of the technique for the study of phosphate molecular sieves.

High resolution solid-state1H nuclear magnetic resonance spectra of organic template in hydrated aluminophosphate VPI-5

A spin-lock pulse sequence has been used in solid-state1H NMR to suppress the dominant water signal and to observe, under magic-angle spinning, very narrow signals of di-n-propylamine template in the hydrated molecular sieve VPI-5. Results suggest that the technique can be extended to the study of other molecular sieves.

NMR studies of the molecular sieve VPI-5 containing only “framework” water

Abstract The preparation of partially hydrated VPI-5, in which water molecules are strongly bonded to 1 3 of framework Al atoms but there is no intracrystalline water, can be readily monitored in situ by 2 H NMR when fully hydrated samples are heated inside an NMR rotor. 27 Al and 31 P MAS NMR spectra have also been examined. Partially and fully hydrated samples of VPI-5 undergo a similar symmetry change at 343-353 K, which indicates that the mobile water in the channels is not the major factor in the process.

23Na NMR Studies of Na-DNA in the solid state

23Na MAS, CP/MAS and quadrupole nutation NMR spectra and the 23Na spin-lattice relaxation times in Na-DNA with and without competing species (Mg2+, ethidium bromide and [Ru(phen)3]2+) reveal the presence of two sodium sites with different NMR parameters. While in the presence of Mg2+, sodium resides far from the surface of the DNA molecule, ethidium bromide and [Ru(phen)3]2+ displace sodium closer towards the surface.