Jacek Klinowski

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.

Aluminosilicates and senile plaque formation in Alzheimer's disease.

Aluminium and silicon were found to be colocalised in the central region of senile plaque cores in studies with energy dispersive X-ray microanalysis. The distribution of these elements was similar in cores isolated from the cerebral cortex of patients with senile dementia of the Alzheimer type and in cores studied in situ from tissue sections from the cerebral cortex of presenile and senile patients with Alzheimers disease, and elderly, mentally normal patients. High-resolution solid-state nuclear magnetic resonance techniques showed aluminium and silicon to be present as aluminosilicates. The presence of aluminosilicates at the centre of senile plaque cores contrasts with the distribution of other inorganic constituents and suggests that they may be involved in the initiation or early stages of senile plaque formation.

Taboo Search: An Approach to the Multiple Minima Problem

Described here is a method, based on Glovers taboo search for discrete functions, of solving the multiple minima problem for continuous functions. As demonstrated by model calculations, the algorithm avoids entrapment in local minima and continues the search to give a near-optimal final solution. Unlike other methods of global optimization, this procedure is generally applicable, easy to implement, derivative-free, and conceptually simple.

Systematic enumeration of crystalline networks

The systematic enumeration of all possible networks of atoms ininorganic structures is of considerable interest. Of particular importance are the 4-connected networks (those in which each atom is connected to exactly four neighbours), which are relevant to a wide range of systems — crystalline elements, hydrates, covalently bonded crystals, silicates and many synthetic compounds. Systematic enumeration is especially desirable in the study of zeolites and related materials, of which there are now 121 recognized structural types, with several new types being identified every year. But as the number of possible 4-connected three-dimensional networks is infinite, and as there exists no systematic procedure for their derivation, the prediction of new structural types has hitherto relied on empirical methods (see, for example, refs 2–4). Here we report a partial solution to this problem, basedon recent advances in mathematical tiling theory. We establish that there are exactly 9, 117 and 926 topological types of, respectively, 4-connected uninodal, binodal and trinodal networks, derived from simple tilings based on tetrahedra. (Here nodality refers to the number of topologically distinct vertices from which the network is composed.) We also show that there are at least 145 more distinct uninodal networks based on a more complex tiling unit. Of the total number of networks that we have derived, only two contain neither three- nor four-membered rings, and most of the binodal and trinodal networks are new.

The evaluation of non-equivalent tetrahedral sites from 29Si NMR chemical shifts in zeolites and related aluminosilicates

Abstract 29 Si MAS NMR spectra of zeolites are much simplified following dealumination and the resulting spectra yield a quantitative correlation between chemical shift and tetrahedral angle. This correlation is useful in the structural elucidation of zeolites, silicas and other aluminosilicates.

Acidity and catalytic activity of the mesoporous aluminosilicate molecular sieve MCM-41

The acidity and catalytic properties of aluminosilicate mesoporous molecular sieves with the MCM-41 structure and bulk Si/Al ratios in the 10–60 range have been investigated. The incorporation of 4-coordinate aluminium into the structure of MCM-41 generates both BrØnsted and Lewis acid sites in amounts increasing with the degree of incorporation. However, the BrØnsted/Lewis acid population ratio is independent of the content of aluminium. The number and strength of acid sites generated are comparable to those of a pillared acid-activated clay and lower than in zeolite H-Y with Si/Al=3.65. Aluminosilicate MCM-41 is a moderate catalyst for the conversion of cumene which proceeds predominantly via catalytic cracking to propene and benzene. The sample of MCM-41 with the highest content of framework aluminium (Si/Al=10) has the largest number of BrØnsted acid sites and exhibits highest catalytic activity.

29Si and 27Al magic-angle-spinning NMR studies of the thermal transformation of kaolinite

Thermal transformations of kaolinite of different degree of crystallinity have been monitored by 27Al and 29Si high-resolution NMR with magic-angle spinning (MAS NMR), X-ray diffraction, Fourier transform infrared, atomic absorption spectrophotometry and thermogravimetric analysis. NMR shows differences in the dehydroxylation process of kaolinites with different degree of crystallinity and reveals the presence of short-range order in metakaolinite. 29Si NMR spectra acquired with a 30 s recycle delay of poorly and highly crystalline samples heated at 480 and 500° C, respectively, contain three distinct signals; we discuss their assignment in the light of experiments involving leaching of the samples with aqueous KOH. Ca. 40% of Si sites retain their original Q3 symmetry just above the onset of dehydroxylation and the Q4 environment is present showing that a small amount of amorphous silica has already segregated. The spectrum of samples treated at 1000° C contains a signal at -110ppm (from Q4 silicons) and a faint resonance, from mullite, at ca. -87 ppm. 29Si NMR also shows that cristobalite germs are already present at 950–1000° C. The 27Al MAS NMR spectra of metakaolinite reveal the presence of 4-, 5-and 6-coordinated Al. Changes in the three Al populations as a function of temperature have been monitored quantitatively. Below 800° C, 4-and 5-coordinated Al appears at the expense of 6-coordinated Al, but above 800° C the amount of 6-coordinated Al increases again. We suggest a dehydroxylation scheme which accounts for the presence of 4-and 5 coordinated Al. Above 900–950° C the latter signal is no longer present in the 27Al NMR spectra and new 4-and 6-coordinated Al species (mullite and γ-alumina) appear. We propose new ideas for the structure of metakaolinite.

Adsorption complexes of methanol on zeolite ZSM-5

Infrared spectroscopy, 1H MAS NMR and thermogravimetric analysis have been used to study the chemistry of methanol adsorbed on zeolites H-ZSM-5, Na-ZSM-5 and K-ZSM-5. On H-ZSM-5, methanol forms the CH3OH+2 methoxonium ion at low coverages. At higher coverages, large protonated clusters of strongly hydrogen-bonded molecules associated with the Bronsted-acid sites are present. In zeolites Na-ZSM-5 and K-ZSM-5 methanol is coordinatively adsorbed on the cation and forms weakly hydrogen-bonded clusters at higher equilibrium pressures.

Solid-state NMR studies of the geometry of brønsted acid sites in zeolitic catalysts

Abstract 1 H, 27 Al and 29 Si solid-state NMR was used to determine the geometry of the Bronsted acid site in zeolites. The mean distance between acidic protons in bridging hydroxyl groups and the nearest framework Al atom is 2.38±0.04 A in zeolite H–Y and 2.48±0.04 A in H-ZSM-5. The quadrupole frequency of framework Al is 1.02±0.05 and 3.1±0.8 MHz in dehydrated Na-Y and H-Y, respectively.

Selective and non-selective NMR excitation of quadrupolar nuclei in the solid state

Abstract Spectral intensities of quadrupolar nuclei and the radiofrequency (rf) pulse lengths for selective and non-selective excitation of any single-quantum NMR transition in the solid state are calculated. When nuclei of the same element occupy sites which experience different electric field gradients, the spectra are quantitatively reliable only when the length of the rf pulse is much smaller than π/2ω rf √ I ( I +1)− m ( m +1). By using hydrated samples and short rf pulses, quantitative determination of aluminium by 27 Al NMR in thermally treated (ultrastable) zeolite Y has been achieved. Quadrupole nutation NMR reveals novel information about the various aluminium species present.