NaBH 4 sodalites were obtained by two new modified methods of crystallization: (1) autothermal synthesis and (2) crystallization with crossover from gel to melt flow in NaOH flux. Syntheses results were presented according to XRD, SEM and FTIR. Besides important features of both synthesis procedures product properties like crystal size and morphology were investigated. Spherical agglomerates of microcrystalline sodalite of composition Na 7[AlSiO 4] 6BH 4(H 2O) 2 were already observed after 4 h without any external heating by the autothermal procedure. Sodalites of the same average composition but in form of agglomerated nanoparticles are crystallized after very short times (2 h 30’) by the crossover reaction from gel to melt flow. Hydrogen release by heating was further studied for two selected samples with comparable composition from each synthesis procedure. Total hydrogen release by hydrolysis reaction with the internal cage water was found during heating of the autothermal product in synthetic air up to 550°C. In contrast hydrogen release from the nanocrystalline sample of crossover synthesis was not completed when heated under the same conditions. These differences were discussed in terms of crystal size and an earlier loss of the internal water from the nanocrystals of the crossover synthesis
Sodium-tetrahydroborate (NaBH4) is a representative material for hydrogen storage and contains 10.6 wt% H2 [
Up to now synthesis of Na8[AlSiO4]6(BH4)2 is exclusively performed under hydrothermal conditions, a method, general used in zeolite chemistry. This method is characterized by the presence of an excess amount of water and the need of autoclaves at elevated temperature. The successful insertion of the hydrothermal method year in and year out in zeolite chemistry caused some negligence of enhancements of common synthesis procedures. Thus future research on modified methods and alternative less energy consuming cheap procedures is essential to obtain new materials with tailored properties even under insertion of hydrolysis sensitive reagents.
The present investigation takes up this demand and presents a case study of tailored synthesis of NaBH4-sodalites according to two new enhanced crystallization techniques. The autothermal synthesis (1), first described for hydrosodalites in [
1) The autothermal synthesis
Here the thermal energy for the whole synthesis period is produced chemically within the reaction batch itself. A tailored exothermic process with high degree of energy transfer of the reaction enthalpy has to be applied. The hydrolysis of Al (∆H = −277 kJ/mol H2; .∆S = 26.2 J/K und ∆G = −284 KJ [
A temperature decrease during the reaction period is compensated by the chemical energy of the inserted strong alkaline solution. In [
The autothermal process is tested for the first time in the present work for tailored syntheses in the tetrahydroborate sodalite system.
2) synthesis under conditions of crossover from aqueous gel-like solution to NaOH melt flow
The crossover reaction from aqueous gel solution to low temperature melt flow is demonstrated here as a further method for tailored synthesis of NaBH4- sodalite. The reaction mechanism based upon two distinctive steps, both ruled by controlled heating of a tailored educt mixture. In step one nucleation and early growth starts under the influence of water, released from the hydrated educts themselves during heating up. This early period is thus comparable with conditions of gel crystallization but under insertion of minimal amounts of water. In step two further crystal growth occurs under continuous shift of the conditions into a melt flux process at elevated temperature. Here the relatively low melt temperatures of 320˚C - 370˚C are sufficient, when a suitable flux component is added to the educts [
Zeolite 13-X was selected as Si-Al-source that simultaneously also acts as “water provider” for the first step of the reaction. Furthermore NaOH granulate is added. NaOH is not only responsible to reach alkaline conditions for sodalites but also acts as fluxing agent at elevated temperature (Tmax.) in the second step of the whole process.
For a statement of the mechanism of this procedure one can consider the whole reaction as a crossover synthesis in (aqueous) gel-like solution followed by crystal growth in a melt as a one-pot process. The present work demonstrates this new preparation technique as a case study of insertion of a modified method for tailored synthesis in the NaBH4-sodalite system for the first time.
-autothermal synthesis of NaBH4-sodalite
According to Equation (1) the NaOH concentration is a main synthesis parameter and four experiments with 4-, 8-, 12- and -16 M solutions were performed. Adequate portions of 6.4 - 25.6 g solid NaOH granulate (Merck 1.06467) were therefore filled into a 200 ml Teflon coated thermobeaker. 40 ml water and 5 ml sodium silicate solution (Merck 1.05621, consisting of 7.5% - 8.5% Na2O and 25.5% - 28.5% SiO2) were added. Additionally 1 g NaBH4 (Merck 806373) was admixed before 0.8 g aluminium grit (Riedel-de Haen St 615/8162) was carefully purred in. The thermobeaker was then closed by a cup, allowing the outlet of hydrogen produced during the rapidly starting strong exothermic reaction (see Equation (1)). A schematic view of the simple experimental arrangement is given in
The intermediate product NaAlO2 and the sodium silicate solution react forming a zeolite precursor sludge according to Breck [
As a result of the rapidly rising temperature within the strong exothermic reaction system this process is overlapped by NaBH4-sodalite crystallization under inclusion of BH4 anions according to Equation (3):
After a reaction period of 4 hours the products were washed (300 ml water) and dried over night on air to prevent any effects of higher temperature of drying in a cabinet dryer. The experimental conditions are summarized in
-synthesis under conditions of crossover from aqueous gel-like solution to NaOH melt flow
A dry powder mixture of 200 mg zeolite 13-X (Fluka 69856), 50 mg NaOH granulate (Merck 1.06467) and 100 mg of NaBH4 (Merck 806373) was pressed into a pellet under a hydraulic press at 50 kN for 5 minutes, before heated under open conditions. A heating program RT → Tmax = 320˚C - 370˚C → RT was revealed for syntheses with 1.5 h heating up till Tmax. and 1.0 h holding time at Tmax. After this reaction period the dense as synthesized sample pellets were grinded in a mortar, before washed (100 ml water) and dried over night on air. A schematic view of the method is given in
-Analytical methods
X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy
Autothermal synthesis (A) of NaBH4-sodalite1) | |||||||
---|---|---|---|---|---|---|---|
exp. No. | NaOH M | Na2SiO3 solution (ml) | template NaBH4 (g) | mass ratio H2O:Al | product | ||
phase analysis2) | amount (g) | cell-parameter a0 (Å) | |||||
A1 | 4 | 5 | 1.0 | 50 | amorphous | 3.8 | - |
A2 | 8 | 5 | 1.0 | 50 | NaBH4-Sod + 30% amorphous | 4.2 | 8.927(5) |
A3 | 12 | 5 | 1.0 | 50 | NaBH4-Sod + (amorphous) | 3.6 | 8.925(1) |
A4 | 16 | 5 | 1.0 | 50 | NaBH4-Sod | 3.6 | 8.9336(4) |
1)reaction period always 4 h; 2)( ): very few amount.
synthesis from gel-like aqueous solution to melt flow (SM)1) | ||||||
---|---|---|---|---|---|---|
exp. No. | educts | temperature of melt step (˚C)2) | product according XRD3) | SOD-cell Parameter a0 (Å) | ||
Zeolite 13-X | NaOH (mg) | |||||
hydrated (mg) | dehy-drated (mg) | |||||
SM1 | - | 200 | 50 | 330 | 13-X + amorphous | - |
SM2 | 200 | - | - | 330 | 13-X + (NaBH4 Sod) + amorphous | - |
SM3 | 200 | - | 50 | 330 | NaBH4 Sod + 11% amorphous | 8.935 (6) |
SM4 | 200 | - | 50 | 350 | NaBH4 Sod | 8.9212 (6) |
SM5 | 200 | - | 50 | 370 | NaBH4 Sod | 8.9229 (7) |
1): addition of 200 mg NaBH4; 2)holding time 1 h; 3)( ): very small amounts.
(FTIR), scanning electron microscopy (SEM) and thermogravimetry were used for analytical characterization of all products. The conversion of the reactants of the autothermal reactions was further controlled by weighting of the products on a Kern EMB 200-3 laboratory balance.
X-ray powder diffraction was performed on a a Philips PW-1800 diffracto- meter with Bragg-Brentano geometry and CuKα radiation in the 2 Theta range 5˚ - 85˚ (step width 0.03˚ and 1 sec measurement time per step, data evaluation with the WinXPow software (STOE)). Powder patterns of selected samples were analyzed by Rietveld refinements using the TOPAS 4.2 software (Bruker AXS). These patterns were measured in the 2 Theta range 2˚ - 80˚ at 0.02 s step width and 5 sec measurement time per step.
MIR-FTIR spectroscopy was carried out with a Bruker Vertex 80 v spectro- meter in the range of 400 to 4000 cm−1 (KBr wafer method: 1 - 2 mg sample/200 mg KBr).
Scanning electron microscopic (SEM) investigations were performed using a JEOL JSM-6390A SEM at an acceleration voltage of 30 kV.
The best products of each series were heated on a Setaram Setsys evolution 1750 thermoanalyzer up to 550˚C at a heating rate of 5˚C/min under atmosphere of synthetic air (80 Vol% N2, 20 Vol% O2; flow rate 20 ml/min). The thermal products were analyzed by XRD and FTIR to get information on hydrogen release and stabilities of the samples.
-autothermal synthesis of NaBH4-sodalite
The results of phase analysis according XRD and quantitative evaluation of the products (amounts in g) are summarized in
The X-ray powder patterns of NaBH4 sodalites from autothermal syntheses are given in
pattern only consists of a broad signal in the 20˚ - 40˚ 2 Theta region caused by amorphous aluminosilicate beside a very strong background contribution. In contrast tetrahydroborate sodalite in adequate quality is already found under the conditions of the 8M NaOH solution in sample A2. Amounts of 70% crystalline tetrahydroborate sodalite beside 30% amorphous parts were calculated accord- ing Rietveld analysis.
Only small amounts of the amorphous phase were observed in the product A3 of 12 M NaOH synthesis whereas the fully crystalline product was obtained in 16 M NaOH. Here a lattice parameter a0 = 8.9336(4)Å was refined for sample A4. This cell parameter is somewhat larger, compared with the value a0 = 8.9161(2)Å of NaBH4 sodalite microcrystals from common hydrothermal synthesis. The cell parameter of sodalite reacts very sensitive on the species like (OH∙H2O)−, H2O or the [BH4]− anions enclathrated within the framework cavities. About 50% cage fillings with water as in expanded hydrosodalite [
The FTIR spectra of autothermal synthesized NaBH4 sodalites are given in
similarities: the sodalite framework vibrations were clearly resolved in accor- dance with literature data [
Beside the BH4 anions even water within the samples can be detected according to typical bands in the FTIR spectra. The water molecules inside the sodalite cages of products A2, A3 and A4 and within the amorphous parts of sample A2 are responsible for the bands at 1650 cm−1 and 3100 cm−1 - 3700 cm−1 [
SEM images of the products of autothermal NaBH4 sodalite synthesis in 8 M, 12 M and 16 M NaOH are summarized in
16 M NaOH, temperature 120˚C, reaction time 24 h, as described in [
Small sodalite crystals of a size between 0.5 μm - 1.0 μm were observed in the product of autothermal synthesis with 8 M NaOH (
The product, obtained in 12 M NaOH consists of larger ball-like crystals with 2 - 4 μm diameters and a slightly rough surface. A few of these crystals are agglomerated to bigger aggregates (
The NaBH4 sodalites synthesized with 16 M NaOH again look somewhat different. Spherulitic crystals of 1 - 2 μm now exhibit an edged surface roughness. Crystals appear as ball-like aggregates formed by intergrowths of thin lamellar shaped microcrystals. Big aggregates of agglomerated crystals with the same morphology were also observed within this batch (
-NaBH4-sodalites by crossover synthesis from aqueous gel solution to NaOH melt flow
The results of phase analysis according XRD are summarized in
The X-ray powder patterns are summarized in
essential hints on the significance of water and NaOH for the successful run of the crossover synthesis from gel to melt.
All further experiments (SM 3 - 5) were performed with hydrated 13-X and NaOH granulate within the educt pellets (see experimental). The powder patterns of these three products all clearly indicate formation of sodalites, in product SM 3 together with 11% of amorphous material but in SM 4 and SM 5 as single phases. The lattice constants, added in
The FTIR spectra of the products are summarized in
Exclusively well resolved typical vibrations of the sodalite framework can be seen in all the three spectra of samples SM 3 - 5: the strong broad band of asym- metric T-O-T stretching vibrations (T = Si, Al) with intensity maximum at ≈1000 cm−1, the symmetric T-O-T vibration modes (triplet in the 660 cm?1 - 740 cm?1 region) and the two strong bending modes at around 460 cm−1 and 430 cm−1―all in accordance with literature [
Again the BH4 anions inside the toc units can be clearly distinguished by their characteristic vibration bands in the FTIR spectra too [
The spectra of sodalites SM 3 - 5 include further bands at 1650 cm−1 and 3100 cm−1 - 3700 cm−1, caused by water molecules within the products [
SEM images of the products are summarized in
Sample SM 4 contains big polyhedral aggregates, sometimes as sperulites some other like cubes of dimensions around 2 - 3 µm, but each aggregate again consists of a loosely aggregation of nanoparticles or tin platelet-like particles. The pores, cavities and channels formed thereby are favored for adsorption of “external water” from moisture when the sample is kept under open conditions.
In contrast sample SM 5 exhibits more spherulite-like crystals with an average size around 2 - 3 µm. Here again nanocrystals in form of very tin platelets are grown together to these denser particles as can be clearly seen at high magnifica-
tion (image down left in
-Hydrogen release properties of selected products of both synthesis procedures
The products A4 and SM 4 of both series were now further investigated by thermal analysis to observe the stability and reactivity of the enclathrated BH4 under the conditions of heating up to 550˚C in synthetic air atmosphere. Both samples were selected as best single phase and pure crystalline products nearly without amorphous byproducts, obtained in both different synthesis procedures. Regarding thermal reactions it is known from literature, that hydrogen release is possible by hydrolysis (starting at about 250˚C) or by oxidation reaction at elevated temperature according to Equations (4) and (5) [
As reaction (5) requires high temperatures T > 400˚C [
The mass loss according to TG (
products, mass loss and dehydratation characteristics according TG analysis | ||||||
---|---|---|---|---|---|---|
exp. No. | Phase analysis | cell-parameter (Å) | Mass loss (%) | degree of dehydration (%) | ||
at 200˚C | at 250˚C | At 300˚C | ||||
A4 | NaBO2-Sod + carnegieite | Sod: 9.034 (2) carnegieite: a0 = 10.262 (14) b0 = 14.454 (14) | −5.46 | 67 | 71 | 83 |
SM4 | NaBO2/NaBH4-Sod + carnegieite | Sod: 8.945 (5) | −5.48 | 85 | 92 | 95 |
dehydration characteristics of sample A4 differs as remarkable amounts of water were released at higher temperatures. Thus a larger amount of water is therefore available for hydrolysis of BH4 in sample A4 at these temperatures.
From
The FTIR investigation (
ing procedure RT → 550˚C → RT in synthetic air atmosphere. It can be seen that the agglomerated crystals of sample A4 are now more separated into single microcrystals of spherical habit and about 1 µm diameter. In contrast the nanoparticles of sample SM 4 still remain in its agglomerated state.
NaBH4 sodalites can be obtained by two new modified methods of crystallization: (1) autothermal synthesis and (2) crossover synthesis from aqueous gel to melt flow in a NaOH flux. Important parameters like crystal size, crystal morphology, composition and hydrogen release mechanism (hydrolysis and oxidation) can be influenced in dependence of the inserted method of synthesis.
Beside tailoring of the products the enhanced methods of crystallization exhibit many other important peculiarities:
The generation of the whole process energy by the enthalpy of the exothermic reaction system without any external heating has to be mentioned here for procedure (1). Method (2) demonstrates the successful insertion of zeolite water of the hydrated educt zeolite 13× as the only water source for synthesis. Short reaction times and the abandonment of autoclaves are further advantages of both methods.
Many further possibilities are offered by the new synthesis pathways, even in other material systems than sodalites. Future optimization of conditions of both reactions is essential to obtain other special types of materials. As an example therefore maybe crystal coarsening and sinter processes at elevated Tmax. and/or prolonged heating times under conditions of method (2) surely yield to dense self-supported membrane-like wafers of NaBH4 sodalite.
Generalizing the results of the present study one can state that two modified experimental methods are presented with future significance for tailor made synthesis even under insertion of sensitive compounds like NaBH4.
Buhl, J.-Ch. (2017) NaBH4 Sodalites, Synthesized by Modified Methods: (1) Autothermal Synthesis and (2) Crossover Reaction from Gel to Melt Flow. Advances in Chemical Engineering and Science, 7, 108-124. https://doi.org/10.4236/aces.2017.72009