Five-coordinate Aluminum Research Articles

Preparation of one-part geopolymers using coal gasification slag: Effect of alkali fusion product additive and liquid/solid ratio

In this work, one-part geopolymers were synthesized through the alkali fusion and component additive method (AFCAM), utilizing coal gasification slag as the primary raw material. The investigation delved into the influence of varying fusion product contents and liquid/solid (L/S) ratios on the geopolymer properties. The experimental results demonstrated a non-linear correlation between the increase in fusion product content and the compressive strength of geopolymers, revealing an initial ascent followed by a subsequent decline. Additionally, a lower L/S ratio of 0.3 was observed to enhance compressive strength compared to the L/S ratio of 0.4. Notably, the optimal combination of an L/S ratio of 0.3 and a fusion product percentage of 30 % resulted in a remarkable compressive strength of 25.2 MPa. The comprehensive strength analysis suggested that an adequate amount of fusion product enhances the geopolymerization reaction, evidenced by the transformation of aluminum coordination states. Specifically, the conversion of five-coordinated and six-coordinated aluminum in slag to four-coordinated aluminum in geopolymers was observed. Concurrently, increased silicon and aluminum reactivity led to the formation of a robust Q4(4Al) network in the geopolymer gel. The impact of L/S ratios was attributed to the accelerated dissolution and hydrolysis of aluminosilicates at an elevated L/S ratio of 0.4, hindering polycondensation. These outcomes underscore the pivotal role of fusion product content and L/S ratio in governing the strength development of one-part geopolymers.

Mapping the structural trends in zinc aluminosilicate glasses.

The structure of zinc aluminosilicate glasses with the composition (ZnO)x(Al2O3)y(SiO2)1-x-y, where 0 ≤ x < 1, 0 ≤ y < 1, and x + y < 1, was investigated over a wide composition range by combining neutron and high-energy x-ray diffraction with 27Al magic angle spinning nuclear magnetic resonance spectroscopy. The results were interpreted using an analytical model for the composition-dependent structure in which the zinc ions do not act as network formers. Four-coordinated aluminum atoms were found to be in the majority for all the investigated glasses, with five-coordinated aluminum atoms as the main minority species. Mean Al-O bond distances of 1.764(5) and 1.855(5) Å were obtained for the four- and five-coordinated aluminum atoms, respectively. The coordination environment of zinc was not observed to be invariant. Instead, it is dependent on whether zinc plays a predominantly network-modifying or charge-compensating role and, therefore, varies systematically with the glass composition. The Zn-O coordination number and bond distance were found to be 4.36(9) and 2.00(1) Å, respectively, for the network-modifying role vs 5.96(10) and 2.08(1) Å, respectively, for the charge-compensating role. The more open coordination environment of the charge-compensator is related to an enhanced probability of zinc finding bridging oxygen atoms as nearest-neighbors, reflecting a change in the connectivity of the glass network comprising four-coordinated silicon and aluminum atoms as the alumina content is increased.

Synthesis, spectroscopy, and crystallography of mononuclear, five-coordinate aluminum complexes that act as cyclic ester polymerization initiators

Three ketoimines with pendant quinolyl moieties and a series of five-coordinate ketoiminate aluminum complexes were prepared and characterized spectroscopically and crystallographically. The synthesis of the ketoimines completed a series of ten regioisomers for which the product ratio varied between the product resulting from quinoline addition adjacent to the alkyl/aryl group or adjacent to the trifluoromethyl substituent. Quantitation of the ketoimine regioisomer ratios with 19F NMR demonstrated the role of steric encumbrance in the nucleophilic attack mechanistic step of the Schiff base condensation reaction and was further supported by linear free energy relationships with the Charton steric parameter and DFT calculations. Eight five-coordinate aluminum complexes were prepared in 57–92% yield from ketoimines and tris(2,6-dimethylphenoxide) aluminum dimer and characterized with 1H, 13C, 19F, and 27Al nuclear magnetic resonance spectroscopy, UV–visible absorbance spectroscopy, elemental analysis, and single-crystal X-ray crystallography. These mononuclear complexes differ from previously reported aluminum ion-pair complexes due to the symmetric cleavage of tris-dimethylphenoxide aluminum dimer by THF and were shown to have distorted trigonal bipyramidal coordination geometries (τ5 = 0.73–0.89). In a preliminary study, the aluminum complexes were demonstrated to be effective initiators for ring-opening polymerization of ε-caprolactone (εCL) to poly-caprolactone (PCL) and l-lactide (LA) to poly-lactic acid (PLA), reaching 77–99% conversion in 2 h for εCL and 10 h for LA at 100 °C.

Facile and cost-effective synthesis of acidity-enhanced amorphous silica-alumina for high-performance isomerization

To achieve facile cost-effective synthesis of acidity-enhanced amorphous silica-alumina (ASA), a water-based sol-gel process is developed. Effectively, this method not only enhances acid sites density, but also lowers the cost by using aluminum nitrate nonahydrate instead of expensive organic aluminum precursor. The resulting material has a uniform mesopore distribution (~3.8 ​nm) and a large surface area (455 ​m2/g), with very few micropores. And the Brønsted-site density (200 ​°C) is 1.5 times higher than that of commercialized ASA Siral 70, which is ascribed to the abundant four- and five-coordinated aluminum species. The effectiveness of this as-synthesized ASA has been demonstrated for n-hexadecane hydroisomerization, affording a multi-branched isomer yield of 43.1%. In addition, good performance can be expected in other bulky-molecule-involved cracking and alkylation processes.

MIL-53 (Al) derived single-atom Rh catalyst for the selective hydrogenation of m-chloronitrobenzene into m-chloroaniline

The catalytic hydrogenation of halonitroarenes to haloanilines is a green and sustainable process for the production of key nitrogen-containing intermediates in fine chemical industry. Chemoselective hydrogenation poses a significant challenge, which requires the rational design of the catalysts with proper hydrogenation ability for nitro group and simultaneously preventing dehalogenation of halogen group. Herein, a highly effective Rh@Al2O3@C single-atom catalyst (SAC) was developed for the hydrogenation of m-chloronitrobenzene (m-CNB) to m-chloroaniline (m-CAN), through an in-situ grafting of metal during the assembly of MIL-53 (Al), followed by confined pyrolysis. Extensive characterizations reveal an exquisite structure of the Rh@Al2O3@C, containing atomically dispersed Rh sites onto Al2O3 confined by the amorphous carbon. The five-coordinated aluminum (AlV) species are essential for achieving the atomic dispersion of Rh atoms, providing the unsaturated coordinative sites for metal. Compared to the benchmark Rh/γ-Al2O3 and Rh/C nanocatalysts, the Rh@Al2O3@C SAC affords an excellent turnover frequency of 2317 molm-CNB·molRh–1·h–1, the highest value to date in heterogeneous catalyst systems for the hydrogenation of m-CNB at 313 K and 20 bar H2, together with a sustained selectivity to m-CAN (~98%) during five consecutive runs. The superior catalytic performance of the Rh@Al2O3@C is attributed to a proper modulation of electronic structure of hydrogenation metal by forming SAC, together with an enhanced accessibility of acid function sites.

Structural analysis of five-coordinate aluminium(salen) complexes and its relationship to their catalytic activity.

The crystal structure of [Al(tBu-salen)]2O·HCl shows major changes compared to that of [Al(tBu-salen)]2O. The additional proton is localized on the bridging oxygen atom, making the aluminium atoms more electron deficient. As a result, a water molecule coordinates to one of the aluminium atoms, which becomes six-coordinate. This pushes the salen ligand associated with the six-coordinate aluminium ion closer to the other salen ligand and results in the geometry around the five-coordinate aluminium atom becoming more trigonal bipyramidal. These results experimentally mirror the predications of DFT calculations on the interaction of [Al(tBu-salen)]2O and related complexes with carbon dioxide. Variable temperature NMR studies of protonated [Al(tBu-salen)]2O complexes revealed that the structures were dynamic and could be explained on the basis of an intramolecular rearrangement in which the non-salen substituent of a five-coordinate aluminium(tBu-salen) unit migrates from one face of a square based pyramidal structure to the other via the formation of structures with trigonal bipyramidal geometries. Protonated [Al(tBu-salen)]2O complexes were shown to have enhanced Lewis acidity relative to [Al(tBu-salen)]2O, coordinating to water, dioxane and 1,2-epoxyhexane. Coordinated epoxyhexane was activated towards ring-opening, to give various species which remained coordinated to the aluminium centers. The protonated [Al(tBu-salen)]2O complexes catalysed the synthesis of cyclic carbonates from epoxides and carbon dioxide both in the presence and absence of tetrabutylammonium bromide as a nucleophilic cocatalyst. The catalytic activity was principally determined by the nature of the nucleophilic species within the catalyst structure rather than by changes to the Lewis acidity of the metal centers.

New mixed-ligand organoaluminum heteroarylalkenolates

We have successfully synthesized and structurally characterized eight novel aluminum compounds with N,O-β-heteroarylalkenolate ligands (L) containing a CF3 group. The reactions of 3,3,3-trifluoro(dimethyl-1,3-oxazol-2-yl)propen-2-ol (H-DMOTFP), 3,3,3-trifluoro(pyridin-2-yl)propen-2-ol (H-PyTFP), and 3,3,3-trifluoro(1,3-benzthiazol-2-yl)propen-2-ol (H-BTTFP) with chloroalanes R2AlCl and RAlCl2 (R = Et, iBu) in dry aprotic solvents provided complexes displaying several types of molecular structures. Effects of ligand, solvent, and crystallization conditions were investigated. The most common and stable products were chloride complexes [AlCl(L)2] with five-coordinated aluminum centers represented by [AlCl(DMOTFP)2] (1) and [AlCl(PyTFP)2] (2). A six-coordinated aluminum complex with a coordinated THF was obtained as [AlCl(THF)(PyTFP)2] (2⋅THF). Complexes with preserved alkyl groups [AlR(L)2] were isolated as [AlEt(BTTFP)2] (3) and [AliBu(DMOTFP)2] (4). An intermolecular rearrangement led to an ionic complex [Al(PyTFP)2(THF)2][iBuAlCl3] (5). In the reactions of H-BTTFP, the complex 3 with two ligands, as well as molecules [AlRCl(L)] with only one ligand attached were characterized as [AlEtCl(BTTFP)] (6) and [AliBuCl(BTTFP)] (7). The thermal properties of obtained complexes were investigated by TG/DSC and HT/XRD analyses.

Synthesis and characterization of amorphous silica-alumina with enhanced acidity and its application in hydro-isomerization/cracking

Amorphous silica-aluminas with enhanced Brønsted acidity have been successfully synthesized via a novel acetic acid based sol-gel process. The resulting amorphous silica-aluminas possess an irregular spherical morphology, homogeneously element dispersion, and a high external surface area in the range of 374–594 m2/g with little contribution from micropores. By simply adjusting the molar Si/Al ratio in the synthesis precursor solution from 0.5 to 2.0, the total Brønsted acid site density firstly increases and then slightly decreases, consistent with the trend of summarized molar content of four and five-coordinated aluminum species. Among them, the amorphous silica-alumina with initial Si/Al ratio of 1.5 exhibits the highest Brønsted acid site density, and its supported Pt catalyst performs the highest activity resembling that of HBeta-based catalyst in the hydroisomerization of n-hexadecane. Meanwhile, a superior catalytic selectivity to multibranched isomers was received over the amorphous silica-alumina without strong Brønsted acid sites. Moreover, a remarkable stability (>2000 h) was also achieved in the hydrocracking of Fischer-Tropsch wax under industrially relevant reaction conditions. This sol-gel synthesis process provides a facile method to prepare amorphous silica-aluminas with outstanding texture and acid properties for catalytic refining in petrochemical fields.

HAlCl2 and H2AlCl as Precursors for the Preparation of Compounds with Four- and Five-Coordinate Aluminum.

Carbenes are known as donor molecules to form with chloroalane adducts, which enhances their aerobic and thermal stabilities. In contrast, the insertion products (cAACH)AlCl2(cAAC) (1) and (cAACH)AlHCl(THF) (2; THF = tetrahydrofuran) have been formed in the reaction of a cyclic alkyl(amino)carbene (cAAC:) with HAlCl2, and H2AlCl, respectively. PhC(NtBu)2Li as the precursor for the reaction with HAlCl2 in a molar ratio of 2:1 can easily form compound [PhC(NtBu)2]2AlH (3) with five-coordinate aluminum. The new products have been studied by spectroscopic methods and single-crystal X-ray diffraction.

Four- and five-coordinate aluminum complexes supported by N,O-bidentate β-pyrazylenolate ligands: synthesis, structure and application in ROP of ε-caprolactone and lactide

In this paper we report a series of Al(iii) complexes supported by N,O-bidentate β-pyrazyl functionalized enolate ligands HL1-HL5 (L = (6-Me-2,5-C4H2N2)-CH[double bond, length as m-dash]C(R)-O-), (R = tBu, Ph, p-tolyl, p-OMePh, o-tolyl) and their exploitation for the ring-opening polymerization of ε-caprolactone (ε-CL) and rac-lactide (rac-LA). The structures of the form LAlMe2 (1a-5a) or L2AlMe (1b-4b) were isolated depending on the ligand or stoichiometry of complexation. The investigation of these complexes toward the ROP of both ε-CL and rac-LA under the same conditions showed that dimethyl-aluminum complexes LAlMe2 (1a-5a) gave a higher activity than monomethyl-aluminum complexes L2AlMe (1b-4b). Meanwhile, monomethyl-aluminum complexes 1b-4b promoted the ring-opening polymerization with a better control over molecular weights and polydispersities than 1a-5a. Moreover, all of the LAlMe2 and L2AlMe produced PLA with strong isotactic bias (Pm up to 0.77) and narrow distributions. Generally, a β-pyrazyl enolate system exhibited significantly higher catalytic activity for the ROP of ε-CL and rac-LA than the analogous β-quinolyl enolate systems reported recently. The results are discussed in terms of electronic and steric properties affected by the substituents on the ligands.

Nitric Oxide Adsorption in MIL-100(Al) MOF Studied by Solid-State NMR

Adsorption of nitric oxide (NO) in the metal–organic framework (MOF) MIL-100(Al) is studied by solid-state NMR. Owing to a modified synthesis, no extraframework benzenetricarboxylate is present on the cost of a small amount of extraframework Al(OH)3 as evident from 27Al, 1H, as well as heteronuclear correlation spectra. Five-coordinated aluminum sites represent about 50% of the aluminum in a dehydrated sample, which remain open for adsorption. With increasing NO loading, a decrease of five-coordinated aluminum with a subsequent increase of six-coordinated aluminum site intensity is found. Additionally, 1H spin–lattice relaxation time T1 is decreasing with an increasing amount of NO, which also supports the NO interaction with the MOF because of the paramagnetism of NO. Fourier-transform infrared spectroscopy data further hint at Al–NO interactions.

Phenoxyaluminum(salophen) Scaffolds: Synthesis, Electrochemical Properties, and Self‐Assembly at Surfaces of Multifunctional Systems

Salophens and Salens are Schiff bases generated through the condensation of two equivalents of salicylaldehyde with either 1,2-phenylenediamines or aliphatic diamines, respectively. Both ligands have been extensively exploited as key building blocks in coordination chemistry and catalysis. In particular, their metal complexes have been widely used for various catalytical transformations with high yield and selectivity. Through the modification of the phenol unit it is possible to tune the steric hindrance and electronic properties of Salophen and Salen. The introduction of long aliphatic chains in salicylaldehydes can be used to promote their self-assembly into ordered supramolecular structures on solid surfaces. Herein, we report a novel method towards the facile synthesis of robust and air-stable [Al(Salophen)] derivatives capable of undergoing spontaneous self-assembly at the graphite/solution interface forming highly-ordered nanopatterns. The new synthetic approach relies on the use of [MeAlIII (Salophen)] as a building unit to introduce, via a simple acid/base reaction with functionalized acidic phenol derivatives, selected frameworks integrating multiple functions for efficient surface decoration. STM imaging at the solid/liquid interface made it possible to monitor the formation of ordered supramolecular structures. In addition, the redox properties of the Salophen derivatives functionalized with ferrocene units in solution and on surface were unraveled by cyclic voltammetry. The use of a five-coordinate aluminum alkyl Salophen precursor enables the tailoring of new Salophen molecules capable of undergoing controlled self-assembly on HOPG, and thereby it can be exploited to introduce multiple functionalities with subnanometer precision at surfaces, ultimately forming ordered functional patterns.

Biodegradable PHB from rac-β-Butyrolactone: Highly Controlled ROP Mediated by a Pentacoordinated Aluminum Complex

The diphenoxyimine five-coordinated aluminum complex [AlMeL2] (1), (L = N-(2,6-diisopropylphenyl)phenoxyimine), has been synthesized and fully characterized. The mononuclear structure observed in the solid state by X-ray diffraction analysis is maintained in solution, as shown by diffusion-ordered NMR spectroscopy (DOSY). Complex 1 is an active ROP catalyst able to provide well-controlled living ring-opening polymerization of rac-BBL at 100 °C, with PDI = 1.03, in the presence of BnOH to give the biodegrable polyester poly(hydroxybutyrate) (PHB). The suggested active species of this catalytic process, the aluminum complex [Al(OBn)L2] (2), has been synthesized and fully characterized. DFT calculations have been carried out to study the influence of the catalytic active species pocket over the polymer molecular weight control. Moreover, 1 is also active in the copolymerization of rac-β-butyrolactone and l-lactide to provide random copolymers.

Al and Sr environment in tectosilicate glasses and melts: Viscosity, Raman and NMR investigation

Structure and properties of SrO-Al2O3-SiO2 glasses and melts were investigated along the tectosilicate join (SrO/Al2O3=1), varying the amount of silica. The structure of the glasses was studied by means of various spectroscopic techniques: Raman, 27Al NMR and XAS at the Sr K-edge. Raman spectroscopy revealed that the fraction of high-membered tetrahedral rings diminishes upon substitution of SiO2 by SrAl2O4 favouring the formation of low-membered rings. Sr K-edge XANES shows that the strontium coordination number is around 9, based on the spectrum similarity with the one obtained for crystalline strontianite. 27Al NMR spectroscopy indicates the presence of four- and five-coordinated aluminium the latter being found in small quantities (<5%), i.e. smaller than for analogous Mg- and Ca-based aluminosilicate glasses. A minimum in Tg is found when the AlO5 content is maximum, both in Sr and Ca aluminosilicates. This fact indicates the importance of minor species such as five-fold aluminium in activating viscous flow, similarly to what has been proposed for five-fold silicon by Farnan and Stebbins (1994). Increase of Tg at low silica content was correlated to a decrease in AlO5 content as well as to a decrease of a number of different structural units and, as a consequence, an ordering of the system.

Archimedean (4,8)-tessellation of haeckelite ultrathin nanosheets composed of boron and aluminum-group V binary materials.

A compendium of unique haeckelite boron and aluminum-group V binary materials have been assessed for their fundamental thermodynamic and ground state electronic properties within density functional theory. We explore their thermodynamic stability relative to new bulk haeckelite crystal structures and find a number of stable polymorphs of planar and buckled ultrathin nanosheets. The bulk boron and aluminum haeckelite crystals display semiconducting and metallic behavior. From the dispersion curves, we predict the formation of both indirect and direct bandgap crystals. We also discover the existence of a five-coordinate aluminum antimonide crystal hitherto never before observed. Moreover, it is found that a number of the Archimedean four and eight membered ring tessellation planar nanosheets could form should synthesis be attempted. It is predicted that these nanosheets can attain two configurations - planar and buckled. From this work we find that combinations of elements such as boron and nitrogen or phosphorus, and aluminum and nitrogen will likely become true single-atom thick nanosheets. These materials show intrinsic indirect bandgap character, which spans the ultraviolet, visible, and infrared spectrum. In the boron series of these materials, the planar structures show double extrema in the bandstructures with van Hove singularities in the projected density of states at the Fermi energy suggesting strong light-matter interactions. The aluminum series we observe strong charge transfer and larger indirect bandgap nanosheets. This study serves as a starting point for a new class of inorganic bulk and ultrathin film materials, which can have many varied applications in nanotechnology.

Addition of dimethylaluminium chloride to N,N′-Disubstituted carbodiimides

Reactivity of trimethylaluminium, dimethylaluminium chloride and methylaluminium dichloride with N,N′-diisopropyl-, N,N′-dicyclohexyl- or N,N′-bis(diisopropylphenyl) carbodiimide (Dipp-NCN-Dipp) via the nucleophilic addition mechanism was studied. Six aluminium(III) complexes containing amidinato ligands were prepared and isolated. All reactions as well as the purity and structural confirmation of products were monitored using 1H and 13C NMR techniques. Reactions of prepared [Dipp-N-C(Me)–N-Dipp]AlMeCl with potassium mirror or pivalonitrile were also investigated. Molecular structures of five aluminium(III) mono- or bisamidinates and two dinuclear oxo-aluminium(III) amidinato by-products displaying four- or five-coordinated aluminium atom were determined by X-ray diffraction techniques.

Highly stable methylaluminum dimer complex with chiral tridentate ligand

The dimeric aluminum complex containing fully deprotonated 4-{[(1S,2R)-2-hydroxyindan-1-yl]amino}-pent-3-en-2-one was synthesized and characterized. X-ray analysis revealed that the two five-coordinate aluminum centers possess ideal tetragonal pyramid geometry and two methyl groups are located in cis position. It is found to be stable in the presence of isopropanol at room temperature. The compound was used as a catalyst for the ring opening polymerization of lactide accompanying lactide epimerization.

Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions

Despite their wide use in the oil refining process, little is known about the distribution of aluminium and silicon atoms in amorphous aluminosilicates (ASAs). In this paper, we report the synthesis of both Al/SiO2 and Si/Al2O3 ASAs by grafting aluminium and silicon alkoxides on silica respectively alumina under various conditions. For both supports, we evidence the central roles of the precursor molecule size and reactivity in the grafting yield and the deposit structure. Unless hydrolysis of the alkoxy groups by water and/or thermal decomposition occurs, deposition is saturated at a monolayer of precursor molecules on the support oxide surface. Additional species can be deposited by repeating the grafting process provided that hydroxyl groups of the top layer are recovered after calcination. 27Al NMR indicates the presence of five-coordinated aluminium species on Al/SiO2 materials prepared by two successive grafting steps. Transmission electron microscopy (TEM) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) confirm the controlled deposition of species by selecting the appropriate synthesis conditions. Homogeneous and regular deposition is favoured in anhydrous condition and at low temperature, while water addition during synthesis leads to inhomogeneous deposits. In case of Al/SiO2 grafting in aqueous conditions, alumina nanoparticles form. The accurate knowledge of the surface structure of these ASAs opens the way to a better understanding of the origin of their Brönsted acidity.

Interaction between composition and temperature effects on non-bridging oxygen and high-coordinated aluminum in calcium aluminosilicate glasses

The configurational changes that occur with temperature, and how these vary with composition, affect the thermodynamic and transport properties of aluminosilicate melts but are not well understood. We present here high-resolution 27 Al and 17 O NMR data on several calcium aluminosilicate glasses along two silica isopleths crossing the metaluminous (SiO 2 -CaAl 2 O 4 ) join, prepared with varying quench rates and thus with fictive temperatures spanning approximately 100 K. In all compositions, five-coordinated aluminum increases with increasing fictive temperature. For glasses along the 60 mol% SiO 2 isopleth, NBO also increases with increasing fictive temperature; this is less obvious for the glasses along the 30 mol% SiO 2 isopleth. The data suggest the mixing of bridging oxygen contributes to the magnitude of the observed temperature effect on NBO and continue to suggest the existence of multiple mechanisms that generate NBO and V Al.

Stereoselective Ring-Opening Polymerization of rac-Lactides Catalyzed by Aluminum Hemi-Salen Complexes

A series of unreported aluminum complexes supported by asymmetrically O,N,N,O-quadridentate hemi-salen ligands were synthesized by binaphthyl-imine derivatives. These complexes were characterized and used as catalysts in rac-lactide or l-lactide polymerization. The X-ray diffraction analysis showed that molecular structures of (S)-3 and (rac)-4 were mononuclear coordination compounds with five-coordinated aluminum atoms in the solid state. Using 2-propanol as cocatalyst, complex (S)-6 revealed the highest activity among these aluminum coordination compounds toward the ring-opening polymerization of l-lactide, and complex (S)-2 displayed the highest stereospecificity for the ring-opening polymerization of rac-lactide, affording partially isotactic polylactide with a Pm of 0.64. The polymerization kinetics using (S)-6 as a catalyst were investigated at great lengths. The kinetics of the polymerization consequences proved that the polymerization was first-order in monomer as well as catalyst. There was a lin...