Aluminum Center Research Articles

Sulfate ion induced formation of unsaturated coordination of AlF3 nanocluster catalyst confined in carbon framework for catalytic dehydrofluorination

The gas-phase catalytic dehydrofluorination of hydrofluorocarbons (HFCs) to produce hydrofluoroolefins is an important strategy for the resource utilization of HFCs. AlF3 is the most suitable and efficient catalyst for dehydrofluorination of HFCs. However, sintering and preservation of coordinatively unsaturated aluminum centers pose significant challenges for AlF3 catalysts. Herein, we prepared carbon-confined AlF3 nanocluster catalysts with aluminum fumarate metal organic frameworks (AlFu-MOF) as the precursors, followed by carbonization and fluorination treatments. During catalyst preparation, anion ligand in the aluminum salt plays a major role in the formation of coordinatively unsaturated Al centers. Compared with Al(NO3)3·9H2O and AlCl3·6H2O, AlFu-MOF (named AlFu-S) prepared with Al2(SO4)3·18H2O exhibits a higher concentration of Al-OH species. Additionally, the residual SO42- ions can indeed retard the crystallization of AlF3 at elevated temperatures. Hence, the decomposition of Al-OH groups and the presence of residual SO42- ions in AlFu-S contribute together to the abundance of unsaturated coordinated Al centers in the finally carbon-confined AlF3 nanocluster catalyst (named AlFu-S-550-F). Also, the confinement effect of the carbon material formed by the carbonization of AlFu-S effectively prevent AlF3 nanocluster from sintering and phase transition, hence, in the dehydrofluorination of HFC-134a, AlFu-S-550-F demonstrates excellent stability compared to AlF3 catalysts prepared by precipitation methods. In summary, this study provides a strategy for the preservation of coordinatively unsaturated aluminum centers in AlF3 catalysts due to sintering and crystal structure transformation. Furthermore, the methodology proposed in this work is also applicable to the preparation of other coordinatively unsaturated metal catalysts.

Isomorphic Insertion of Ce(III)/Ce(IV) Centers into Layered Double Hydroxide as a Heterogeneous Multifunctional Catalyst for Efficient Meerwein-Ponndorf-Verley Reduction.

The development of highly active acid-base catalysts for transfer hydrogenations of biomass derived carbonyl compounds is a pressing challenge. Solid frustrated Lewis pairs (FLP) catalysis is possibly a solution, but the development of this concept is still at a very early stage. Herein, stable, phase-pure, crystalline hydrotalcite-like compounds were synthesized by incorporating cerium cations into layered double hydroxide (MgAlCe-LDH). Besides the insertion of well-isolated cerium centers surrounded by hydroxyl groups, the formation of hydroxyl vacancies near the aluminum centers, which were formed by the insertion of cerium centers into the layered double hydroxides (LDH) lattice, was also identified. Depending on the initial cerium concentration, LDHs with different Ce(III)/Ce(IV) ratios were produced, which had Lewis acidic and basic characters, respectively. However, the acid-base character of these LDHs was related to the actual Ce(III)/Ce(IV) molar ratios, resulting in significant differences in their catalytic performance. The as-prepared structures enabled varying degrees of transfer hydrogenation (Meerwein-Ponndorf-Verley MPV reduction) of biomass-derived carbonyl compounds to the corresponding alcohols without the collapse of the original lamellar structure of the LDH. The catalytic markers through the test reactions were changed as a function of the amount of Ce(III) centers, indicating the active role of Ce(III)-OH units. However, the cooperative interplay between the active sites of Ce(III)-containing specimens and the hydroxyl vacancies was necessary to maximize catalytic efficiency, pointing out that Ce-containing LDH is a potentially commercial solid FLP catalysts. Furthermore, the crucial role of the surface hydroxyl groups in the MPV reactions and the negative impact of the interlamellar water molecules on the catalytic activity of MgAlCe-LDH were demonstrated. These solid FLP-like catalysts exhibited excellent catalytic performance (cyclohexanol yield of 45%; furfuryl alcohol yield of 51%), which is competitive to the benchmark Sn- and Zr-containing zeolite catalysts, under mild reaction conditions, especially at low temperature (T = 65 °C).

Lewis Acidic Aluminosilicates: Synthesis, 27Al MQ/MAS NMR, and DFT-Calculated 27Al NMR Parameters.

Porous aluminosilicates are functional materials of paramount importance as Lewis acid catalysts in the synthetic industry, yet the participating aluminum species remain poorly studied. Herein, a series of model aluminosilicate networks containing [L-AlO3] (L = THF, Et3N, pyridine, triethylphosphine oxide (TEPO)) and [AlO4]- centers were prepared through nonhydrolytic sol-gel condensation reactions of the spherosilicate building block (Me3Sn)8Si8O20 with L-AlX3 (X = Cl, Me, Et) and [Me4N] [AlCl4] compounds in THF or toluene. The substoichiometric dosage of the Al precursors ensured complete condensation and uniform incorporation, with the bulky spherosilicate forcing a separation between neighboring aluminum centers. The materials were characterized by 1H, 13C, 27Al, 29Si, and 31P MAS NMR and FTIR spectroscopies, ICP-OES, gravimetry, and N2 adsorption porosimetry. The resulting aluminum centers were resolved by 27Al TQ/MAS NMR techniques and assigned based on their spectroscopic parameters obtained by peak fitting (δiso, CQ, η) and their correspondence to the values calculated on model structures by DFT methods. A clear correlation between the decrease in the symmetry of the Al centers and the increase of the observed CQ was established with values spanning from 4.4 MHz for distorted [AlO4]- to 15.1 MHz for [THF-AlO3]. Products containing exclusively [TEPO-AlO3] or [AlO4]- centers could be obtained (single-site materials). For L = THF, Et3N, and pyridine, the [AlO4]- centers were formed together with the expected [L-AlO3] species, and a viable mechanism for the unexpected emergence of [AlO4]- was proposed.

CO2 reduction using aluminum hydride: Generation of in-situ frustrated Lewis pairs and small molecule activation therein.

CO2 reduction is appealing for the long-term production of high-value fuels and chemicals. Herein, using density functional theory (DFT) based calculations, we study the CO2 reduction pathway to formic acid using aluminum hydride and phosphine derivatives. Our primary focus is on aluminum hydride derivatives, aimed at improving the efficiency of the CO2 reduction process. Substituents with σ-donating properties at the aluminum center are discovered to lower the activation barriers. We demonstrate how di-tert-butylphosphine oxide (LB-O)/di-tert-butylphosphine sulfide (LB-S)/di-tert-butylphosphanimine (LB-N) work together with aluminum hydride to facilitate CO2 reduction process and generate in-situ frustrated Lewis pairs (FLPs), such as FLP-O, FLP-S, and FLP-N. The activation strain model (ASM) analysis reveals the significance of strain energy in determining activation barriers. EDA-NOCV and PIO analyses elucidate the orbital interactions at the corresponding transition states. Furthermore, the study delves into the activation of various small molecules, such as dihydrogen, acetylene, ethylene, carbon dioxide, nitrous oxide, and acetonitrile, using those in-situ generated FLPs. The study highlights the low activation barriers and emphasizes the potential for small molecule activation in this context.

Aluminum complexes of a hemilabile pyrrolylaldiminate ligand

AbstractOverviewThis study describes the synthesis, characterization, and reactivity of aluminum complexes containing the amine‐functionalized pyrrolylaldiminate ligand N‐(2‐dimethylaminoethyl)‐5‐tert‐butyl‐pyrrol‐2‐ylaldiminate ([PyImN]−).SynthesisThe condensation reaction of N,N‐dimethylethylenediamine with 5‐tert‐butylpyrrole‐2‐carbaldehyde in methanol at room temperature produces H[PyImN]. Alkane elimination reactions of MeAlCl2 with H[PyImN] in toluene at 110°C and AlR3 (R = Me, Et) with H[PyImN] in toluene at −35°C afford [PyImN]AlCl2 and [PyImN]AlR2, respectively.CharacterizationSolution NMR studies reveal the coordination preferences of [PyImN]—whose hemilabile amine donor is bound to the aluminum center in [PyImN]AlCl2 but unbound in [PyImN]AlR2 (R = Me, Et).CatalysisThe catalytic activities of [PyImN]AlMe2 in controlled ring‐opening polymerization of ε‐caprolactone are discussed.

Pinacol Cross-Coupling Promoted by an Aluminyl Anion.

A simple sequential addition protocol for the reductive coupling of ketones and aldehydes by a potassium aluminyl grants access to unsymmetrical pinacolate derivatives. Isolation of an aluminium ketyl complex presents evidence for the accessibility of radical species. Product release from the aluminium centre was achieved using an iodosilane, forming the disilylated 1,2-diol and a neutral aluminium iodide, thereby demonstrating the steps required to generate a closed synthetic cycle for pinacol (cross) coupling at an aluminyl anion.

Quest for Active Species in Al/B-Catalyzed CO2 Hydrosilylation.

We demonstrate the catalytic role of aluminum and boron centers in aluminum borohydride [(2-Me2CH2C6H4)(C6H5)Al(μ-H)2B(C6H5)2] (6) during carbon dioxide (CO2) hydrosilylation. Preliminary investigations into CO2 reduction using [(2-Me2NCH2C6H4)(H)Al(μ-H)]2 (1) and [Ph3C][B(3,5-C6H3Cl2)4] (2) in the presence of Et3SiH and PhSiH3 resulted in CH2(OSiR3)2 and CH3OSiR3, which serve as formaldehyde and methanol surrogates, respectively. In pursuit of identifying the active catalytic species, three compounds, B(3,5-C6H3Cl2)3 (3), [(2-Me2NCH2C6H4)(3,5-C6H3Cl2)Al(μ-H)2B(3,5-C6H3Cl2)2] (4), and [(2-Me2NCH2C6H4)2Al(THF)][B(3,5-C6H3Cl2)4] (5), were isolated. Among compounds 2-5, the highest catalytic conversion was achieved by 4. Further, 4 and 6 were prepared in a straightforward method by treating 1 with 3 and BPh3, respectively. 6 was found to be in equilibrium with 1 and BPh3, thus making the catalytic process of 6 more efficient than that of 4. Computational investigations inferred that CO2 reduction occurs across the Al-H bond, while Si-H activation occurs through a concerted mechanism involving an in situ generated aluminum formate species and BPh3.

Mono- and Bimetallic Aluminum Complexes Supported by Thioether-Amide Ligands: Synthesis, Characterization, and Application in the Ring Opening Polymerization of l-Lactide

Mono- and bimetallic aluminum complexes bearing bidentate NS or tetradentate NSSN ligands have been synthesized via reaction of AlMe3 with the corresponding proligand and spectroscopically characterized. In these complexes the aluminum centers are surrounded by anionic nitrogen and neutral sulfur atoms that, together with the carbon atoms of the alkyl groups, produce tetrahedral coordination geometries. In the bimetallic complexes the two metal centers are linked by a thioether-bridge of different lengths. The solid state structure of the complexes featuring the thioethyl or thiopropyl bridge were determined by X-ray diffraction studies. Both mono- and bimetallic aluminum complexes display fluxional behaviors in solution. After proper activation, all complexes were able to promote the ring opening polymerization of l-lactide in toluene solution. The polymerizations were well controlled as testified by the linear increase of the molecular weights with the conversions and the narrow polydispersity indexes. The highest activity was observed for the bimetallic complex bearing the NSSN ligand with the shortest bridge (ethyl bridge) with a turnover frequency of 11.6 molLA·molAl–1·h–1 (toluene, 80 °C). The activity decreased as the distance between the two metal centers increased. Moreover, the activity of the bimetallic complex with the ethyl bridge was higher than those of the monometallic ones. These findings suggest that a cooperative interaction between two aluminum centers is operative in the bimetallic complex with the shortest bridge.

Undirected C-H Bond Activation in Aluminium Hydrido Enaminonates.

Two new aluminium hydrido complexes were synthesized by reacting AlH3 with the enaminone ligand N-(4,4,4-trifluorobut-1-en-3-on)-6,6,6-trifluoroethylamine (HTFB-TFEA) in different molar ratios to obtain mono- and di-hydrido-aluminium enaminonates. Both air and moisture sensitive compounds could be purified via sublimation under reduced pressure. The spectroscopic analysis and structural motif of the monohydrido compound [H-Al(TFB-TBA)2] (3) showed a monomeric 5-coordinated Al(III) centre bearing two chelating enaminone units and a terminal hydride ligand. However, the dihydrido compound exhibited a rapid C-H bond activation and C-C bond formation in the resulting compound [(Al-TFB-TBA)-HCH2] (4a), which was confirmed by single crystal structural data. The intramolecular hydride shift involving the migration of a hydride ligand from aluminium centre to the alkenyl carbon of the enaminone ligand was probed and verified by multi-nuclear spectral studies (1H,1H NOESY, 13C, 19F, and 27Al NMR).

Inducing Nucleophilic Reactivity at Beryllium with an Aluminyl Ligand.

The reactions of anionic aluminium or gallium nucleophiles {K[E(NON)]}2 (E = Al, 1; Ga, 2; NON = 4,5-bis(2,6-diisopropylanilido)-2,7-ditert-butyl-9,9-dimethylxanthene) with beryllocene (BeCp2) led to the displacement of one cyclopentadienyl ligand at beryllium and the formation of compounds containing Be-Al or Be-Ga bonds (NON)EBeCp (E = Al, 3; Ga, 4). The Be-Al bond in the beryllium-aluminyl complex [2.310(4) Å] is much shorter than that found in the small number of previous examples [2.368(2) to 2.432(6) Å], and quantum chemical calculations suggest the existence of a non-nuclear attractor (NNA) for the Be-Al interaction. This represents the first example of a NNA for a heteroatomic interaction in an isolated molecular complex. As a result of this unusual electronic structure and the similarity in the Pauling electronegativities of beryllium and aluminium, the charge at the beryllium center (+1.39) in 3 is calculated to be less positive than that of the aluminium center (+1.88). This calculated charge distribution suggests the possibility for nucleophilic behavior at beryllium and correlates with the observed reactivity of the beryllium-aluminyl complex with N,N'-diisopropylcarbodiimide─the electrophilic carbon center of the carbodiimide undergoes nucleophilic attack by beryllium, thereby yielding a beryllium-diaminocarbene complex.

Diverse Cooperative Reactivity at a Square Planar Aluminium Complex and Catalytic Reduction of CO2.

The use of a sterically demanding pincer ligand to prepare an unusual square planar aluminium complex is reported. Due to the constrained geometry imposed by the ligand scaffold, this four-coordinate aluminium centre remains Lewis acidic and reacts via differing metal-ligand cooperative pathways for activating ketones and CO2 . It is also a rare example of a single-component aluminium system for the catalytic reduction of CO2 to a methanol equivalent at room temperature.

Substitution at a Coordinatively Saturated Aluminum Center Stabilized by Imidazolidin-2-imine

In this article, the coordination chemistry of the imidazolidin-2-imine ligand (1) toward aluminum has been delineated. We commenced our investigation by preparing dimeric aluminum dihydride {μ-LAlH2}2 (2) from the reaction of 1 with AlH3. Substitution of the hydride with methyl (3), chloride (4), and bromide (7) was achieved from the reaction of 2 with BH3·SMe2, BHCl2·SMe2, and Br2/H2O, respectively. The reaction with BH3·THF afforded a 1 → BH3 adduct (5) with simultaneous elimination of AlH3. An unprecedented spirocyclic aluminate anion with imidazolidin-2-iminium cation (6) resulted from a reaction with PhB(OH)2. While sulfur undergoes insertion reaction into the Al–H bonds (8), the reactions with diselenides and ditellurides conveniently install unusual functionalities such as −SePh (10) and −TePh (11) onto alane.

Organoaluminum derived from Schiff bases: Synthesis, characterization and catalytic performance in hydroboration

In the process of the development of green chemistry, non-noble metal catalysts have become one of the research hotspots. The abundant aluminum has entered the research scope of chemists. Four organoaluminum derived from halogen-substituted Schiff bases were prepared: (2-(((2,6-difluorophenyl)imino)methyl)phenoxy)dimethylaluminum (C1), (2-(((3-chloro-4-fluorophenyl)imino)methyl)phenoxy)dimethylaluminum (C2), (2-(((2-chlorophenyl)imino)methyl)phenoxy)diethylaluminum (C3), (2-(((4-chlorophenyl)imino)methyl)phenoxy)diethylaluminum (C4). All organoaluminum compounds were characterized using 1H NMR, 13C NMR, 19F NMR spectroscopy, and X-ray single crystal diffraction. XRD proved that the aluminum centers of these organoaluminum compounds are all twisted six-membered ring structures. They were used for the hydroboration of aldehydes and ketones, showing excellent results. This provided an additional catalyst for the reduction of carbonyl compounds.

New chronological constraints for the lowermost stratigraphic unit of Atapuerca Gran Dolina (Burgos, N Spain)

We present the results of a combined Electron Spin Resonance (ESR) and Luminescence dating study for the lowermost stratigraphic unit (TD1) of the palaeoanthropological site of Atapuerca Gran Dolina, Spain. Ten samples collected through the Main Section were dated using either the Multiple Centre (MC) ESR approach or the single-grain thermally transferred optically stimulated luminescence (SG TT-OSL) technique, both of which were applied to quartz grains. The two methods yield consistent ages, providing a robust chronostratigraphic framework for the TD1 deposits and enabling improved correlation of the magnetostratigraphic succession (comprising three intervals of normal polarity; N1 to N3, from bottom to top) with the Geomagnetic Polarity Time Scale. Specifically, the results provide a late Early Pleistocene chronology for TD1, and clearly position the deposits in the post-Olduvai time range (<1.77 Ma). The final age estimates range from 1.30 ± 0.14 Ma (TT-OSL) close to the bottom of the section to a mean ESR age of 1.05 ± 0.12 Ma (1 s.d.) for three samples from the N3 interval towards the top. Consequently, the N3 interval may confidently be correlated to the Jaramillo Subchron, while we propose a correlation of N1 and N2 with the Cobb Mountain subchron and Punaruu excursion, respectively; although we cannot exclude that these two normal intervals may correspond to other, unknown, geomagnetic events given the existing uncertainties associated with the ESR and luminescence ages. From a methodological point of view, the ESR MC approach shows that the Titanium signal provides the most reliable burial ages for TD1. In contrast, the Aluminium centre yields systematically overestimated age estimates, as a result of incomplete resetting of the signal before sediment deposition. The SG TT-OSL ages presented here are amongst the oldest published so far, and highlight the potential of the high temperature SG TT-OSL 290 approach for obtaining finite ages exceeding 1 Ma at Atapuerca. Finally, these new dating results provide key insights into the rates and nature of sedimentary processes at Gran Dolina, modifying long-standing interpretations of the cave formation and infilling dynamics. An age of at least 1.4 Ma may be inferred for the base of the sedimentary infill at the Main Section, providing minimum age constraint for the formation of the cavity and, by extension, for the intermediate level of the Atapuerca karst. The opening of the cave, which pre-dates human occupation, most likely occurred close to or slightly after 0.9 Ma. This suggests that both TD4 and TD6 archaeological levels were deposited within a relatively short time range of <100 kyr. The so-called autochthonous fluvial deposits from TD1 unit, or at least those dated in the present study, were also clearly primarily derived from outside the karst system, and were not reworked from older deposits within the karst. They experienced sufficient daylight exposure and minimum transport times within the karst system to enable consistency between the ESR and TT-OSL burial ages and the independent magnetostratigraphic evidence. Our results highlight the need to reconsider the terminology traditionally employed to differentiate the two main phases of Gran Dolina's sedimentary infill. New numerical age results for stratigraphic unit TD1 of Atapuerca Gran Dolina, Spain The presence of Jaramillo magnetic Subchron (1.07-0.99 Ma) is confirmed The base of the sedimentary infill post-dates Oduvai Subchron (<1.77 Ma) Cave opening occurred ca. 0.9 Ma ago, suggesting that TD4-6 units deposited in < 100 kyr TD1 fluvial deposits do not originate from within the karst

σ-Aromaticity in planar pentacoordinate aluminium and gallium clusters

Planar hypercoordinate structures are gaining immense attention due to the shift from common paradigm. Herein, our high level ab initio calculations predict that planar pentacoordinate aluminium and gallium centres in Cu5Al2+ and Cu5Ga2+ clusters are global minima in their singlet ground states. These clusters are thermodynamically and kinetically very stable. Detailed electronic structure analyses reveal the presence of σ-aromaticity which is the driving force for the stability of the planar form.

A β-ray irradiation resistant MOF-based trap for efficient capture of Th(IV) ion

Efficient capture of radionuclides from aqueous solution is highly essential for environment safety and resource recycling, but still faces large challenge. In this study, a stable aluminum-based metal–organic framework (MOF-303) with high-density ion traps was used to selectively capture thorium (Th) ion from aqueous solution. The features of aluminum center endow this material with excellent resistance from β-ray irradiation even with a high dose of 1000 kGy. Further, due to the special chelation interaction from the ion trap, MOF-303 exhibits a large adsorption capacity for Th(IV) ion (461.7 mg g−1) and excellent separation coefficients up to 97.6, 97.3 and 81.3 for Th(IV)/Pr(III), Th(IV)/Eu(III) and Th(IV)/Nd(III) respectively. The adsorption equilibrium was reached at ∼ 420 min. Besides, the regeneration of MOF-303 as well as the elution of Th(IV) ion can be achieved via an acid-wash process. Experimental and theoretical calculation results indicate Th(IV) ion can be tightly anchored in the ion trap surrounded by six active atoms, via the chemical bonding mode.

Chiral Bis(oxazoline) Ligand‐Supported Alkyl Aluminum Cations

AbstractAlkyl aluminum cations have shown high Lewis acidity and capability of catalyzing challenging organic transformations, however, the related chiral catalyst is scarcely documented. Herein, we report the generation of a series of weakly solvated or Lewis base‐stabilized alkyl aluminum cationic complexes bearing chiral bis(oxazoline) ligands. The increase of steric hindrances at the ligand backbone and the aluminum center were found to significantly increase the complex stability. In the Gutmann–Beckett experiment and fluoride ion affinity calculation, these complexes display Lewis acidity greater than B(C6F5)3 and the reported tetra‐coordinate aluminum cation. Preliminary catalytic studies revealed that the aluminum cation catalysts can effectively promote hydroboration of ketone and hydrosilylation of un‐activated olefin, despite only poor ee values were obtained.

Front Cover: Iodine‐Chemisorption, Interpenetration and Polycatenation: Cationic MOFs and CPs from Group 13 Metal Halides and Di‐Pyridyl‐Linkers (Chem. Eur. J. 23/2022)

Mopping up radioactive iodine: A cationic MOF with the abundant metal aluminium shows chemisorption of iodine from a gas atmosphere at room temperature that is suitable for the capture of the radioisotopes 129I and 131I. The capture is explicitly stable as triiodide ions are formed with iodide inside the MOF pores. High uptake of up to one equivalent of I2 per aluminium centre marks the material out as highly suitable for applications in capturing radioactive isotopes, for example, from toxic nuclear waste as required after an unwanted hazardous exposure to the environment. More information can be found in the Research Article by K. Müller-Buschbaum et al. (DOI: 10.1002/chem.202104171).

1-Chloroalumole

A representative of the stable alumoles with a chlorine substituent at the aluminum atom was synthesized by the coupling of a cyclobutadiene dianion derivative and aluminum trichloride. The structure of alumole was established by X-ray crystallography, which showed that alumole exists as a monomer. The aluminum atom is tetracoordinated with the fourth coordination site occupied by a thf molecule, which reduces the intrinsically high Lewis acidity of the aluminum center, thus stabilizing the alumole molecule.

Investigation of quartz electron spin resonance residual signals in the last glacial and early Holocene fluvial deposits from the Lower Rhine

Abstract. In this study, we examined the residual doses of the quartz electron spin resonance (ESR) signals from eight young fluvial sediments with known luminescence ages from the Lower Rhine terraces. The single aliquot regenerative (SAR) protocol was applied to obtain the residual doses for both the aluminium (Al) and titanium (Ti) impurity centres. We show that all of the fluvial samples carry a significant amount of residual dose with a mean value of 1270 ± 120 Gy for the Al centre (including the unbleachable signal component), 591 ± 53 Gy for the lithium-compensated Ti centre (Ti-Li), 170 ± 21 Gy for the hydrogen-compensated Ti centre (Ti-H) and 453 ± 42 Gy for the signal that originated from both the Ti-Li and Ti-H centres (termed Ti-mix). To test the accuracy of the ESR SAR protocol, a dose recovery test was conducted and this confirmed the validity of the Ti-Li and Ti-mix signal results. The Al centre shows a dose recovery ratio of 1.75 ± 0.18, whereas the Ti-H signal shows a ratio of 0.55 ± 0.17, suggesting that the rate of signal production per unit dose changed for these signals after the thermal annealing. Nevertheless, all fluvial sediments investigated in this study carry a significant residual dose. Our result suggests that more direct comparisons between luminescence and ESR equivalent doses should be carried out, and, if necessary, the subtraction of residual dose obtained from the difference is essential to obtain reliable ESR ages.