Facile Solvent-Free Synthesis of Anhydrous Alkali Metal Dodecaborate M₂B₁₂H₁₂ (M = Li, Na, K)

Metal dodecaborate, widely regarded as an obstacle of the rehydrogenation of high-density hydrogen storage materials metal borohydrides M­(BH₄)_<i>n</i>, is generally synthesized using liquid-phase process followed by a careful dehydration process. In this study, we propose a new and facile solvent-free synthesis process of dodecaborates using B₁₀H₁₄ with a low melting point of 99.6 °C as a boron source. As a case study, our first challenge focused on the syntheses of anhydrous M₂B₁₂H₁₂ (M = Li, Na, and K) by heat treatment of starting materials (a) 2MH + 1.2B₁₀H₁₄ or (b) 2MBH₄ + B₁₀H₁₄ at 200–450 °C conditions, which have been proved to be successful for the first time by X-ray diffraction (XRD), Raman, and NMR analysis. Starting materials (b) 2MBH₄ + B₁₀H₁₄ shows better reactivity than that of (a) 2MH + 1.2B₁₀H₁₄, which demonstrates that synthesis of anhydrous M₂B₁₂H₁₂ by heat treatment of 2MBH₄ + B₁₀H₁₄ is a feasible solvent-free process

Heteroatom-Substituted Delaminated Zeolites as Solid Lewis Acid Catalysts

This manuscript represents a comparative study of Lewis acid catalysis using heteroatom-substituted delaminated zeolites, which are synthesized using an approach that obviates the need for surfactants and sonication during exfoliation. The comparison involves heteroatom substitution into silanol nests of delaminated zeolites consisting of DZ-1 and deboronated UCB-4. Diffuse reflectance ultraviolet (DR-UV) spectroscopy demonstrates framework heteroatom sites, and the Lewis acidity of these sites is confirmed using infrared spectroscopy of adsorbed pyridine. The enhanced catalytic accessibility of these Lewis acid sites is confirmed when performing Baeyer–Villiger oxidation of substituted 2-adamantanones with hydrogen peroxide as the oxidant. Comparison of delaminated Sn-DZ-1 with three-dimensional Sn-Beta for this reaction shows that the delaminated zeolite is more active for bulkier ketone substrates. The role of the two-dimensional crystalline framework of the delaminated zeolite on catalysis is highlighted by comparing delaminated zeolites Sn-DZ-1 with Sn-UCB-4. The former exhibits a significantly higher activity for Baeyer–Villiger oxidation, yet when comparing Ti-DZ-1 with Ti-UCB-4, it is the latter that exhibits a significantly higher activity for olefin epoxidation with organic hydrogen peroxide, whereas both delaminated zeolites are more robust and selective in epoxidation catalysis compared with amorphous Ti/SiO₂

Active Sites in Sn-Beta for Glucose Isomerization to Fructose and Epimerization to Mannose

Framework Lewis acidic tin sites in hydrophobic, pure-silica molecular sieves with the zeolite beta topology (Sn-Beta) have been reported previously to predominantly catalyze glucose−fructose isomerization via 1,2 intramolecular hydride shift in water and glucose–mannose epimerization via 1,2 intramolecular carbon shift in methanol. Here, we show that alkali-free Sn-Beta predominantly isomerizes glucose to fructose via 1,2 intramolecular hydride shift in both water and methanol. Increasing extents of postsynthetic Na⁺ exchange onto Sn-Beta, however, progressively shifts the reaction pathway toward glucose–mannose epimerization via 1,2 intramolecular carbon shift. Na⁺ remains exchanged onto silanol groups proximal to Sn centers during reaction in methanol solvent, leading to nearly exclusive selectivity toward epimerization. In contrast, decationation occurs with increasing reaction time in aqueous solvent and gradually shifts the reaction selectivity to isomerization at the expense of epimerization. Decationation and the concomitant selectivity changes are mitigated by the addition of NaCl to the aqueous reaction solution. Preadsorption of ammonia onto Sn-Beta leads to near complete suppression of infrared and ¹¹⁹Sn nuclear magnetic resonance spectroscopic signatures attributed to open Sn sites and of glucose−fructose isomerization pathways in water and methanol. These data provide evidence that Lewis acidic open Sn sites with either proximal silanol groups or Na-exchanged silanol groups are respectively the active sites for glucose–fructose isomerization and glucose–mannose epimerization

Active Sites in Sn-Beta for Glucose Isomerization to Fructose and Epimerization to Mannose

Framework Lewis acidic tin sites in hydrophobic, pure-silica molecular sieves with the zeolite beta topology (Sn-Beta) have been reported previously to predominantly catalyze glucose−fructose isomerization via 1,2 intramolecular hydride shift in water and glucose–mannose epimerization via 1,2 intramolecular carbon shift in methanol. Here, we show that alkali-free Sn-Beta predominantly isomerizes glucose to fructose via 1,2 intramolecular hydride shift in both water and methanol. Increasing extents of postsynthetic Na⁺ exchange onto Sn-Beta, however, progressively shifts the reaction pathway toward glucose–mannose epimerization via 1,2 intramolecular carbon shift. Na⁺ remains exchanged onto silanol groups proximal to Sn centers during reaction in methanol solvent, leading to nearly exclusive selectivity toward epimerization. In contrast, decationation occurs with increasing reaction time in aqueous solvent and gradually shifts the reaction selectivity to isomerization at the expense of epimerization. Decationation and the concomitant selectivity changes are mitigated by the addition of NaCl to the aqueous reaction solution. Preadsorption of ammonia onto Sn-Beta leads to near complete suppression of infrared and ¹¹⁹Sn nuclear magnetic resonance spectroscopic signatures attributed to open Sn sites and of glucose−fructose isomerization pathways in water and methanol. These data provide evidence that Lewis acidic open Sn sites with either proximal silanol groups or Na-exchanged silanol groups are respectively the active sites for glucose–fructose isomerization and glucose–mannose epimerization

Discovery of Fluidic LiBH₄ on Scaffold Surfaces and Its Application for Fast Co-confinement of LiBH₄–Ca(BH₄)₂ into Mesopores

Generation of fluidic LiBH₄ molecules, <i>f</i>-LiBH₄, was demonstrated by NMR spectroscopy of LiBH₄ bulk powder mixed with silica scaffold surface materials under minor heat treatment. In the presence of the fumed silica or mesoporous MCM-41 and SBA-15, LiBH₄ shows increased translational mobility at relatively low temperature (ca. 95 °C) and becomes liquid-like by evidence from ¹H–¹¹B <i>J</i>-coupling in ¹H and ¹¹B MAS NMR or substantial line narrowing of ⁷Li static NMR. This high diffusional mobility of LiBH₄ at the molecular level has never been seen for bulk LiBH₄, and the property is attributed to the interfacial interaction with the mesoporous scaffold surfaces. While <i>f</i>-LiBH₄ facilitates the confinement of LiBH₄ itself into various scaffold materials, LiBH₄ migrates along the SBA-15 surface to reach other metal borohydride particles, Ca­(BH₄)₂ in this case, and promotes the formation of similarly fluidic LiBH₄–Ca­(BH₄)₂ composite (LC solid solution) for coconfinement into mesopores. <i>In situ</i> variable temperature (VT) NMR spectroscopy detects the co-infiltration process of eutectic LiBH₄–Ca­(BH₄)₂ composite (LC) into mesopores of SBA-15. The infiltration rates measured for LiBH₄ bulk powder or LC composite showed dependence on pore sizes (MCM-41 vs SBA-15) and heat treatment conditions (static vs MAS)

Hydrogen Back-Pressure Effects on the Dehydrogenation Reactions of Ca(BH₄)₂

The dehydrogenation reactions of Ca­(BH₄)₂ are investigated under different isobaric conditions using in situ synchrotron radiation powder X-ray diffraction and nuclear magnetic resonance measurements. Ca­(BH₄)₂ dissociates in multiple steps, and several intermediate phases, such as an amorphous phase(s), CaB₂H_<i>x</i>, and CaB₁₂H₁₂, are observed during dehydrogenation. Among the intermediate phases, it is known that CaB₂H_<i>x</i> is fully reversible, while the more stable CaB₁₂H₁₂ with an icosahedral structure hinders reversible reactions. Here, we try to control the dehydrogenation reaction pathway of Ca­(BH₄)₂ by applying different hydrogen back-pressures. The decomposition reaction of Ca­(BH₄)₂ in the absence of a catalyst was found to be sensitive to the H₂ back-pressure. At <i>p</i>(H₂) = 1 bar, Ca­(BH₄)₂ decomposes via two competitive dehydrogenation reaction routes to form CaB₂H_<i>x</i> or CaB₁₂H₁₂. At <i>p</i>(H₂) = 10 bar, the overall dehydrogenation reaction remains unchanged. However, the formation of CaB₂H_<i>x</i> is reduced, and amorphous elemental boron is observed as a final dehydrogenation product. At <i>p</i>(H₂) = 20 bar, the elemental boron formation is significantly increased, and the formation of the CaB₂H_<i>x</i> phase is suppressed. Possible routes to form CaH₂ and elemental boron are discussed

Ultrasmall MgH₂ Nanoparticles Embedded in an Ordered Microporous Carbon Exhibiting Rapid Hydrogen Sorption Kinetics

MgH₂ nanoparticles with different average sizes have been prepared as ordered microporous carbon by tuning the Mg amount from 15 to 50 wt %. Ultrasmall particles with mean sizes of 1.3 and 3.0 nm have been obtained for 15 and 25 wt % Mg contents, respectively. The hydrogen desorption properties strongly depend on the nanoparticle size, as evidenced by different thermal analysis techniques. The onset temperature of hydrogen desorption for MgH₂ nanoparticles below 3 nm occurs at a temperature about 245 K lower than for microcrystalline material. Two distinct hydrogen desorption peaks are noticed for nanoparticles with mean sizes of 1.3 and 3.0 nm, as confirmed by TDS and HP-DSC. ¹H NMR investigations suggest the presence of two MgH₂ populations with enhanced hydrogen mobility, as compared to the microcrystalline hydride. The short hydrogen diffusion path and the enhanced hydrogen mobility may explain the increased desorption kinetics of these ultrasmall nanoparticles

In Situ NMR Study on the Interaction between LiBH₄–Ca(BH₄)₂ and Mesoporous Scaffolds

We discuss the use of nuclear magnetic resonance (NMR) spectroscopy to investigate the physical state of the eutectic composition of LiBH₄–Ca­(BH₄)₂ (LC) infiltrated into mesoporous scaffolds and the interface effect of various scaffolds. Eutectic melting and the melt infiltration of mixed borohydrides were observed through in situ NMR. In situ and ex situ NMR results for LC mixed with mesoporous scaffolds indicate that LiBH₄ and Ca­(BH₄)₂ exist as an amorphous mixture inside of the pores after infiltration. Surprisingly, the confinement of the eutectic LC mixture within the mesopores is initiated below the melting temperature, which indicates a certain interaction between the borohydrides and the mesoporous scaffolds. The confined borohydrides remain inside of the pores after cooling. These phenomena were not observed in microporous or nonporous materials, and this observation highlights the importance of the pore structure of the scaffolds. Such surface interactions may be associated with a faster dehydrogenation of the nanoconfined borohydrides

Hydrogenation of Magnesium Nickel Boride for Reversible Hydrogen Storage

We report that a ternary magnesium nickel boride (MgNi_2.5B₂) mixed with LiH and MgH₂ can be hydrogenated reversibly forming LiBH₄ and Mg₂NiH₄ at temperatures below 300 °C. The ternary boride was prepared by sintering a mechanically milled mixture of MgB₂ and Ni precursors at 975 °C under inert atmosphere. Hydrogenation of the ternary, milled with LiH and MgH₂, was performed under 100 to 160 bar H₂ at temperatures up to 350 °C. Analysis using X-ray diffraction, Fourier transform infrared, and ¹¹B magic angle spinning NMR confirmed that the ternary boride was hydrogenated forming borohydride anions. The reaction was reversible with hydrogenation kinetics that improved over three cycles. This work suggests that there may be other ternary or higher order boride phases useful for reversible hydrogen storage

Nonaqueous Fluoride/Chloride Anion-Promoted Delamination of Layered Zeolite Precursors: Synthesis and Characterization of UCB-2

The delamination of layered zeolite precursor PREFER is demonstrated under mild nonaqueous conditions using a mixture of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride in <i>N</i>,<i>N</i>-dimethylformamide (DMF) as solvent. The delamination proceeds through a swollen material intermediate which is characterized using powder X-ray diffraction (PXRD). Subsequent addition of concentrated HCl at room temperature leads to synthesis of UCB-2 via delamination of the swollen PREFER material and is characterized using PXRD, transmission electron microscopy (TEM), and argon gas physisorption, which shows lack of microporosity in UCB-2. ²⁹Si magic angle spinning (MAS) NMR spectroscopy indicates lack of amorphization during delamination, as indicated by the entire absence of Q² resonances, and ²⁷Al MAS NMR spectroscopy shows exclusively tetrahedral aluminum in the framework following delamination. The delamination process requires both chloride and fluoride anions and is sensitive to solvent, working well in DMF. Experiments aimed at synthesizing UCB-2 using aqueous conditions previously used for UCB-1 synthesis leads to partial swelling and lack of delamination upon acidification. A similar lack of delamination is observed upon attempting synthesis of UCB-1 under conditions used for UCB-2 synthesis. The delamination of PREFER is reversible between delaminated and swollen states in the following manner. Treatment of as-made UCB-2 with the same reagents as used here for the swelling of PREFER causes the delaminated UCB-2 material to revert back to swollen PREFER. This causes the delaminated UCB-2 material to revert back to swollen PREFER. Altogether, these results highlight delamination as the reverse of zeolite synthesis and demonstrate the crucial role of noncovalent self-assembly involving the zeolitic framework and cations/anions/structure-directing agent and solvent during the delamination process