Kenneth J.D. MacKenzie

Preparation of microporous silica from metakaolinite by selective leaching method

Microporous silica was prepared by selective leaching of metakaolinite, derived from the 1:1 layered clay mineral kaolinite (2SiO2Al2O32H2O) by firing at 600 °C for 24 h. Treatment with 20 mass% H2SO4 solution at 90 °C for 0.5–5 h with stirring, led to a decrease in the Al2O3 content and an increase in the specific surface area (SA) with increasing leaching time. The adsorption-desorption isotherms of N2 measured at 77 K show a type I curve, the average pore size, calculated by t-plot method, being about 0.6 nm and the maximum SA about 340 m2g−1. The 29Si MAS NMR spectrum of the 2 h leached sample showed peaks at −101 and − 110 ppm, which can be assigned to the layer structure unit (Q3) and the SiO4 tetrahedral framework structure unit (Q4), respectively. It. is suggested that the SiO4 tetrahedral structure of the microporous silica basically maintains the original kaolinite and is different from the general framework structure of silica gel.

NMR studies on rapidly solidified SiO2Al2O3 and SiO2Al2O3Na2O-glasses

Abstract 27Al and 29Si MAS NMR studies were performed on roller-quenched SiO2Al2O3-glasses with Al2O3 contents ranging from 10 to 60 mol% and on SiO2Al2O3Na2O glasses containing 10 mol% Al2O3 and 2.5 to 10 mol% Na2O. Pure aluminium silicate glasses show NMR peaks at 0, 30 and 60 ppm. The frequency distribution of the different Al-sites is not affected by the glass composition. In glasses of the system SiO2Al2O3Na2O the 30 ppm peak decreases to zero as the Na2O content increases. The 30 ppm peak is assigned to distorted triclustered AlO-tetrahedra, rather than to fivefold coordinated Al. Triclustering of tetrahedra may provide for charge neutrality in glasses with molar excess of Al2O3 over Na2O. As charge balance is increasingly achieved by addition of alkali ions, the tendency of tetrahedral triclustering is reduced, reflected by the disappearance of the 30 ppm peak in glasses containing ≥ 7.5 mol% Na2O.

Carbothermal synthesis of titanium nitride

Studies of the reactivity of six TiO2 samples (two rutiles and four anatases) and nine carbon samples towards the formation of TiN by reduction of TiO2 with carbon in a nitrogen atmosphere at 1150°C show that the reaction is influenced by the chemical and physical properties of both the TiO2 and the carbon. Although anatases and rutiles behave similarly, their reactivities are adversely affected by the presence of impurities such as those deliberately added as surface coatings in pigment-grade TiO2. There is some evidence that the reactivity of the TiO2 increases with increasing surface area. Carbons with higher ash contents appear to be more reactive. The reactivity of the carbons generally increases with increasing surface area, as measured by gas penetration methods (BET nitrogen adsorption and Blaine gas permeation).

Carbothermal formation of β′-sialon from kaolinite and halloysite studied by 29Si and 27Al solid state MAS NMR

Abstract27Al and 29Si magic-angle spinning(MAS) nuclear magnetic resonance(NMR)and complementary X-ray diffraction (XRD) studies of carbothermal formation of sialons from kaolinite and halloysite confirm that the reaction involves the initial formation of mullite (3Al2O3·2SiO2) and amorphous silica. In the presence of carbon, Si-N bonds are formed at ≈1200 °C, giving a continuum of silicon oxynitride compositions which become progressively more N-rich. These do not become sufficiently ordered to be detected by XRD until later in the reaction, when crystalline silicon oxynitride, possibly containing a little Al (O′-sialon) and x-phase sialon are formed, followed by β′-sialon. The O′- and x-phase sialons are transitory, but the β′-sialon persists throughout the reaction. Si-O bonds survive the destruction of the mullite and persist throughout the reaction, especially with kaolinite starting material. The 29Si MAS NMR results indicate that Si-C bonds are formed later in the reaction than previously suggested, the SiC phase behaving more like a secondary product than a transitory intermediate. Al-N bonds are not detectable by 27Al MAS NMR until very late in the reaction (after 8 h firing at 1400 °C), and coincide with the appearance of the secondary product AlN. The implications for the carbothermal reaction sequence in kaolinite and halloysite are discussed.

The thermal reactions of muscovite studied by high-resolution solid-state 29-Si and 27-AI NMR

Studies of two muscovites of different iron contents, using solid-state NMR with magic-angle-spinning (MAS) combined with X-ray powder diffraction, thermal analysis and57Fe Mössbauer spectroscopy, suggest that dehydroxylation occurs by a homogeneous rather than an inhomogeneous mechanism, forming a dehydroxylate in which the aluminium is predominantly 5-coordinate. On further decomposition at about 1100° C, the tetrahedral layer and interlayer K+ form a feldspar-like phase similar to leucite (KAISi2O6), the remainder forming a spinel, which, contrary to previous suggestions, appears to contain little silicon. Further heating induces the formation of mullite (AI6Si2OP13), and, in the higher-iron sample, corundum (α-Al2O3), in addition to the feldspar-like phase. The presence of the iron impurity enhances the recrystallization reactions and promotes the conversion of mullite to corundum, which eventually becomes the sole aluminous product in the high-iron sample. In samples fired to higher temperatures, only the tetrahedral aluminium resonance is detectable by27AI NMR, probably because most of the iron is located in either the mullite or corundum phases, in which it broadens the octahedral aluminium resonance beyond detection.

Structure and thermal transformations of imogolite studied by 29 Si and 27 Al high-resolution solid-state nuclear magnetic resonance

Solid-state nuclear magnetic resonance (NMR) spectroscopy, thermal analysis, and X-ray powder diffraction data on the tubular, hydrous aluminosilicate imogolite were found to be fully consistent with a previously proposed crystal structure consisting of a rolled-up, 6-coordinate Al-O(OH) sheet, bonded to isolated orthosilicate groups. The calculated 29Si chemical shift of this structure agreed with the observed shift within 3 ppm. Thermal dehydroxylation of the Al-O(OH) sheet produced predominantly NMR-transparent 5-coordinate Al, but a few 4- and 6-coordinate sites and some residual hydroxyl groups may also have formed, as shown by NMR spectroscopy. Changes in the 29Si NMR spectrum on dehydroxylation suggest a condensation of the orthosilicate groups, but steric considerations rule out bonding between adjacent silicons. To account for these observations, an alternative mechanism to orthosilicate condensation has been proposed, involving the fracture and unrolling of the tubes, followed by the condensation of fragments to form a layer structure. The layer structure has a calculated 29Si chemical shift of -95.6 ppm, in good agreement with the observed value of -93 ppm.

The role of iron in the formation of inorganic polymers (geopolymers) from volcanic ash: a 57Fe Mössbauer spectroscopy study

The behavior of the iron present in two volcanic ashes was investigated during geopolymer synthesis using sodium hydroxide as the sole alkali activator. XRD, SEM, and room-temperature 57Fe Mössbauer spectroscopy were used to monitor the behavior of the iron during the synthesis reaction. Geopolymers with very good compressive strengths were formed, especially with the finer ash, in which the iron is present in the crystalline minerals ferroan forsterite and augite. Mössbauer spectroscopy identified the ferrous sites in these minerals, plus a ferric site, probably located in an X-ray amorphous phase. The ferroan forsterite in the original ashes did not react with NaOH, but a substantial proportion of the augite reacted to form new ferric sites with parameters similar to distorted tetrahedral or 5-coordinated environments, suggesting the possible incorporation of ferric iron in the tetrahedral network of the geopolymer product. These results indicate that iron is not necessarily deleterious to geopolymer formation, as has sometimes been suggested.

Formation of mullite from mechanochemically activated oxides and hydroxides

Abstract The formation of mullite from different precursors prepared by a mechanochemical method were investigated. The structural changes caused by grinding various oxide/hydroxide mixtures were characterized by the 29 Si and 27 Al MAS-NMR, FTIR, DTA and XRD. Thermal treatment of precursors prepared by mechanochemical activation from gibbsite and silica gel led to the crystallization of mullite at ≈ 1200 °C via a spinel-phase, while precursors prepared from α-Al 2 O 3 and silica gel or gibbsite and fused silica formed mullite at about 1400 °C by solid state reaction between α-Al 2 O 3 and cristobalite. MAS-NMR spectroscopy shows that the crystallization of mullite at lower temperatures is associated with good homogeneity, as judged by the amount of Si present in the aluminosilicate resonances.

Structural evolution in gel-derived mullite precursors

Abstract The evolution of mullite from organo-metal gel precursors above 700 °C is found to be strongly influenced in both gel pieces and powdered samples by the thermal pretreatment at lower temperatures. Under the present conditions, the optimum preheating temperature was found to be 350 °C, at which temperature an anomalously high concentration was found of an Al species with a characteristic 27 Al magic-angle spinning NMR resonance at about 30 ppm. Such Al sites are often described as pentaco-ordinated, but an alternative assignment is considered. The optimum temperature for the formation of this Al site is also optimal for the catalytic formation of aromatic molecules from the residual organic fragments and/or solvent present. Mass spectrometry shows that under the present reaction conditions, these aromatics are thermally stable up to at least 900 °C in air, and the prolonged presence of their decomposition products (CO and water) could facilitate the transformation of the gel to crystalline mullite. The 29 Si NMR spectra indicate at least three different Si environments, including one which may arise from the formation of silicon oxycarbide glasses in these gels.

The structure and thermal transformations of allophanes studied by 29 Si and 27 Al high resolution solid-state NMR

Examination of two volcanic and two precipitated allophanes by solid-state NMR, thermal analysis and X-ray powder diffraction shows three of the samples to contain structural features similar to both tubular imogolite and defect layer-lattice aluminosilicates such as kaolinite. The fourth allophane, a precipitated sample from New Zealand, had no imogolite-like features and contained tetrahedral as well as octahedral aluminum. The imogolite-like units in allophane are less stable thermally than tubular imogolite. The NMR spectra and their changes on heating can be accounted for by a structural model in which a two-sheet, kaolinite-like structure containing defects (holes in the tetrahedral sheet) is curved into a sphere in which imogolite-like orthosilicate units are anchored into the octahedral sheet and fit into the tetrahedral defects. Computer simulation shows that the model is crystallographically sound, and accounts for all the known facts, including the spherical morphology, the solid-state NMR spectra and the thermal dehydroxylation behavior of all except the New Zealand allophane, which is of a different structural type.