Don Seykens

A 27Al nuclear magnetic resonance study on the optimalization of the development of the Al13 polymer

Synthesis conditions strongly influence the yield of the tridecameric polymer Al13 ([AlO 4 Al 12 (OH) 24 (H 2 O) 12 ] 7+ ). An amount of 68% tridecamer was achieved by injection of alkali through a capillary tube into a 5 × 10 −2 M Al solution at a rate of 0.015 ml/s up to an OH/Al ratio of 2.2. Dropwise addition of alkali yielded significantly less tridecameric polymer. During progressive hydrolysis the monomeric Al NMR resonance moved from 0.1 to 0.9 ppm and the linewidth increased from 37 to 112 Hz. Simultaneously the resonance at 63.3 ppm due to tridecameric fourfold coordinated Al changed by 0.02 ppm. During aging the tridecamer rearranged to polymers undetectable by NMR, due to loss of the tetrahedral symmetry of the central Al, which was deduced from the decrease in intensity and the broadening of the 63.3 ppm resonance. The formation of tetrahedral Al(OH)4−, due to the inhomogeneous conditions at the point of base introduction, is essential for the synthesis of Al13. Aging over a period of 1 year caused a strong decrease in Al13 concentration, which showed that Al13 is a metastable polymer.

Thermal stability of basic aluminum sulfate

The tridecameric aluminum polymer [AlO4Al12(OH)24(H2O)12]7+ is prepared by forced hydrolysis of an Al(NO3)3 solution by NaOH up to an OH:Al mol ratio of 2.2. Upon addition of sulfate the tridecamer crystallizes into macroscopic crystallites of the basic aluminum sulfate Na0.1[Al13O4(OH)24(H2O)12](SO4)3.55, which is characterized structurally by means of X-ray diffraction, 27Al solid-state magic angle spinning NMR, IR and chemically by inductively coupled plasma atomic emission Spectroscopy. The basic aluminum sulfate has a monoclinic unit cell with a = 20.188±0.045 A, b = 11.489±0.026A, c = 24.980±0.056 A, and β= 102.957±0.022°. With TG analysis, DTA and heating stage X-ray diffraction the thermal decomposition is studied. The tridecamer persists as a stable unit in the sulfate structure to temperatures of 80°C. Approximately 9 mol H2O are adsorbed in excess per one mol basic aluminum sulfate; these are easily lost by heating to 80°C. From 80 to 360°C the tridecamer unit will gradually decompose losing its 12 water and 24 hydroxyl groups, to finally become X-ray amorphous. From 360 to 950°C, with a maximum between 880 and 950°C, SO3 is removed, leaving behind primary aluminum oxide.

Temperature influence on the Al13 complex in partially neutralized aluminum solutions: a 27Al nuclear magnetic resonance study*

Stepwise heating to 85°C in the NMR apparatus does not notably change the monomer and tridecamer (Al13) concentrations in a 0.2M Al(NO3)3 solution neutralized with 0.2M NaOH up to an OH/Al molar ratio of 2.4. Upon heating the fourfold coordinated 27Al NMR signal of Al13 at 63.3 ppm and the very broad sixfold coordinated 27Al NMR signal of Al13 at 12 ppm exhibit an increasing intensity and decreasing linewidth, due to diminishing ‘missing intensity’ and ‘quadrupole relaxation’, respectively. An analogous effect for a Na2CO3 neutralized 0.2M AlCl3 solution confirmed that the 12 ppm signal must be assigned to the sixfold coordinated Al of the Al13 complex. The surface ratio of fourfold coordinated Al to sixfold coordinated Al of the Al13 complex experimentally established is smaller than the theoretical 1 : 12 ratio. During heating, a more intensive exchange interaction between monomer and other Al-species is proposed without any effect on the actual concentrations. High symmetry in the Al13 complex is determined at elevated preparation temperatures from the decreasing linewidth of the 63.3 ppm resonance. Above 85°C the tridecamer transforms to other species, which cannot be observed with NMR.

Aluminum monomer line-broadening as evidence for the existence of [AlOH]2+ and [Al(OH)2]+ during forced hydrolysis: a 27Al nuclear magnetic resonance study☆

Abstract The 27 Al monomer resonance of 0.05M aluminum nitrate solutions gradually broadens from 7.82 to 89.40 Hz upon forced hydrolysis from an OH/Al molar ratio of 0.0 to 2.2. The broadening is due to increasing amounts of [AlOH] 2+ and [Al(OH) 2 ] + up to 2.24% and 0.11%, respectively. Additional measurements on the hydrolysis with ammonium hydroxide and the dissolution of aluminum sec-butoxide in dilute HCl give similar results. To calculate the linewidth of the monomer resonance, it is assumed that [AlOH] 2+ and [Al(OH) 2 ] + have identical linewidths as the isoelectronic compounds [AlF] 2+ and [AlF 2 ] + , respectively. The calculation is based on pK values of 4.99 for [AlOH] 2+ and 10.13 for [Al(OH) 2 ] + . The results of the theoretical calculations agree well with observed linewidth data.

Incipient polymerization of SiO2 in acid-catalyzed TMOS sol-gel systems with molar water/alkoxide ratio between 0.5 and 32

Abstract 29Si NMR spectroscopy is an adequate technique to record SiO2 polymerization in sol-gel systems. The influence of variation of the initial molar H2O/TMOS ratio, Rw, has been studied at a fixed reaction time in the early stages of the polymerization, since the ratio is a main factor in the development of linear, cyclic or network polymerization. Spectra of the systems with variation in hydrolysis degree facilitate the assignment of fine structures in the resonances of the SiO2 polymers. Improvements are suggested on previously reported assignments. The chemical shifts at −85.5 and −91.4 ppm are denoted to the (022) and (202)3c species, respectively. New designations are proposed for partly and total hydrolyzed cyclic trimer and tetramer building blocks: −83.3 ppm for (022)3c, −84.1 ppm for (112)3c, −90.4 ppm for (022)4c and −91.4 ppm for (112)4c. The linear (112) species exhibits a chemical shift at −92.3 ppm. The log-normal distributions of the involved Q0, Q1 and Q2 species depend on their degree of hydrolysis. Interpolation of functional group concentrations yields a maximum degree of condensation for the Rw ∼ 2.5. Below this Rw value, the condensation degree increases with increasing Rw. A decrease of the condensation degree with increasing Rw is noticed for Rw > 2.5. For the range 2 ≤ Rw ≤ 8, cyclic species are generated up to a maximum of 23.5% of the total silicon concentration for Rw = 2.

The tridecameric aluminum complex as an appropriate precursor for fibrous boehmite: A 27Al NMR study on the partial hydrolysis of aluminum sec-butoxide

The aging of the aluminum tridecamer in acidified aluminum sec-butoxide is studied with 27Al nuclear magnetic resonance. At 20°C the tridecamer content passes through a maximum of 19.7% after 133 h of aging to decline subsequently to a final content of approximately 12%. At 90°C the tridecamer disappears within 21 h of aging due to clustering. After 91 h colloidal gibbsite forms, while the pH drops continuously from 4.3 to 3.5. Simultaneously the monomer concentration increases to approximately 6.6%. The increase is possibly due to liberation of the central fourfold-coordinated Al from the tridecamer that rearranges to hexameric rings, and to dissolution of some gibbsite in the acid solution. The hexameric rings are considered to be the precursor for the crystallization of gibbsite and boehmite. Increase of the preparation temperature from 20°C to 80°C has no effect on the amount of tridecamer, although at 90°C the amount increases from approximately 9% to 14.1%. An increase of the preparation temperature leads to changes in the second-order quadrupole effects, which results in a line sharpening from 19.5 to 9.8 Hz for the central fourfold-coordinated Al signal of the tridecamer at 63.3 ppm.

The effects of concentration and hydrolysis on the oligomerization and polymerization of Al(III) as evident from the 27Al NMR chemical shifts and linewidths

Abstract The Al concentration and forced hydrolysis influence the polymerization of aqueous Al(III) and thus the 27 Al NMR spectra. An increase of the Al concentration results in an increase of the fraction present as monomer, in the formation of an oligomer of an OH/Al molar ratio of 2.4, and in the disappearance of the tridecamer above an Al concentration of 0.15M. Increasing the OH/Al molar ratio at a constant Al concentration leads to a linear drop of the fraction present as monomer over the entire range between 1 and 2.5 and a maximum in oligomer fraction between 1.5 and 2.5. At low Al concentrations, the fraction of tridecamer exhibits an optimum yield between OH/Al ratios of 2.2 and 2.4. At ratios higher than 2.4, NMR unobservable polymers are formed. The chemical shift and the linewidth of the monomer resonance are lowered by increasing spin-spin relaxation, T 2 , and a consequently decreasing quadrupole coupling constant upon increasing Al concentration. With increasing OH/Al molar ratio, [Al(H 2 O) 6 ] 3+ is mainly replaced by [Al(H 2 O) 5 (OH)] 2+ and [Al(H 2 O) 4 (OH) 2 ] + and subsequently the chemical shift and linewidth of the monomer resonance increase. The Al concentration or OH/Al molar ratio hardly affect the resonance of the central fourfold coordinated Al of the tridecamer, due to its very strong shielding.

DIFFERENCES BETWEEN LARVAL AND ADULT DROSOPHILA IN METABOLIC DEGRADATION OF ETHANOL

The alcohol dehydrogenase polymorphism in Drosophila melanogaster is a widely cited paradigm ofthe occurrence ofnatural selection (van Delden 1982). However, the fruit flys life stages seem to vary substantially in sensitivity to selective forces of alcohols (e.g., Middleton and Kaeser 1983; Heinstra et al. 1987; Hoffmann and McKechnie 1991). Drosophila uses mainly alcohol dehydrogenase (ADH, EC 1.1.1.1) to detoxify dietary ethanol (for a recent review see Geer et al. 1991). Alcohol dehydrogenase has a dual function in larvae; dehydrogenation of ethanol into acetaldehyde and subsequently of acetaldehyde into acetic acid (Heinstra et al. 1983, 1989). Recently two independent investigations suggested that the enzyme aldehyde dehydrogenase (ALDH, EC 1.2.1.3) is more important than ADH for the dehydrogenation of acetaldehyde in adults (Anderson and Barnett 1991; Leal and Barbancho 1992). This points to differences in alcohol elimination between these life stages. Ethanol-derived (end)products, like fatty acids, a,a-trehalose, alanine, lactate, glutamate, glutamine, and proline have been detected in lar-

Metabolic physiology of alcohol degradation and adaptation in Drosophila larvae as studied by means of carbon-13 nuclear magnetic resonance spectroscopy

Alcohol dehydrogenase-mediated degradation of [2-13C]ethanol was followed in third instar larvae of Drosophila by means of 13C NMR. The tricarboxylic acid (TCA) cycle intermediates, citrate-C(2),4 and succinate-C2,3; the amino acids, glutamate-C4,3,2, glutamine-C4,3,2, proline-C4, alanine-C2,3 and the carbon nuclei of the glucosyl units of the disaccharide, α,α-trehalose, were intensely labeled in perchloric acid extracts of whole larvae. A model of the intermediary metabolism of ethanol degradation in larvae was formulated from these observations. The C2 atom of ethanol enters the mitochondrial TCA cycle as C2-acetyl-CoA and is converted into the TCA cycle intermediates. The TCA cycle intermediate 2-oxoglutarate(-C4) apparently is readily converted into glutamate(-C4) and subsequently to glutamine(-C4) and proline(-C4). Dietary ethanol is also a substrate for trehalose synthesis. This may occur by an exchange of malate(-C2,3) between its mitochondrial and cytosolic pools. Part of the cytosolic malate(-C2,3) may be diverted into pyruvate then generating alanine(-C2,3) as another product. The other part may be converted into glucose and subsequently into α,α-trehalose by the gluconeogenic pathway. 13C natural abundance signals of stored fatty acids and glycerol were readily detectable in chloroform extracts of control larvae. De novo synthesis of fatty acids from labeled ethanol also occurred after a lag period. Our findings show the coordinated nature of metabolic pathways, and we point to its consequences in understanding the dynamics in evolutionary processes.

29Si NMR monitoring and kinetic modelling of an acid-catalyzed TMOS sol-gel system with molar H2O/Si = 8*

Simplified kinetics are used to describe the rates of hydrolysis and condensation in a Si(OCH3)4/CD3OD/H2O/HCl sol-gel system with H2O/Si = 8. The kinetics of the system can be expressed by three principal reactions: hydrolysis, esterification and water-producing condensation. The statistical model of Kay and Assink [J. Non-Cryst. Solids 104 (1988) 112] extended with a multiplicative factor, R, representing the increasing steric and inductive effects with progressing condensation [Brinker and Assink, J. Non-Cryst. Solids 111 (1989) 48], is additionally corrected for the esterification, which plays an important role in sol-gel systems with H2O/Si ⩾ 4. Numerically simulated concentration curves of species calculated with this model are compared with experimental data. In contradiction with the linear statistical model, good fittings are obtained with the extended Kay-Assink-Brinker (KABe) model when the experimental data are used without correction for intramolecular condensation. By simple reduction of the cyclic oligomers/species to their linear equivalents assuming (1) an additional or (2) a competitive intramolecular condensation, the model calculations do not fit. These results point to a more complex intramolecular condensation or to the failure of the model to simulate sol-gel processes attendent with intramolecular condensation.