engleski

Influence of ammonia borane pretreatment in sodium amidoborane synthesis

As a hydrogen storage material, sodium amidoborane (NaNH2BH3, SAB) was for the first time mechanochemicaly synthesized in reaction between NaH and ammonia borane (NH3BH3, AB) [1, 2]. Although mechanochemical preparation of SAB is previously described in several publications, no one analyzed the optimal synthesis conditions. Namely, after few minutes of milling, newly formed SAB is subjected to elevated temperature due to the heating of milling jar, which directly cause its decomposition. In order to prolongate its milling time and thus analyze by ex-situ method we have mechanochemically modified one of the starting reactants – AB. The longer monitoring of the reaction can be conducted very well when AB particles are obtained by adequate preparation, i.e. by milling it for 4.5 min and after that mixed with NaH. In comparison with as-received AB flake-like crops, the prepared AB consists of larger fish scale-like agglomerates. It is observed that, in such a way, mutual reactivity of AB and NaH can be significantly promoted. We investigated a series of NH3BH3 + NaH ball-milling reactions that should result in SAB. Fixing the parameters of the milling jar and balls as well as influence of the milling conditions, we investigated influence of milling time and interventions during the milling process (i.e. degassing of milling jar), together with preparation of the starting chemicals. A detailed ex-situ XRD monitoring of the reaction mixture during mechanical milling shows that reaction accelerates after 5 min of milling. During the 50 min milling period, intensity of SAB lines increases with simultaneous drop of NaH and AB lines. However, a prolonged milling up to 100 min results in decomposition of SAB, probably due to the warming of the milling jar. Thus, the optimal milling time for these conditions is between 5 and 50 min. Intensity of characteristic SAB maxima in 100 min milled sample are significantly decreased in comparison to 30 and 50 min milled samples due to SAB decomposition to polymeric BN compounds. IR spectroscopy shows that SAB formation occurs in at least two steps, through production of the NaAB•AB adduct as a crucial intermediate whose reaction with starting species other than AB determines further reaction pathway toward the final product. SAB by itself suffers further rearrangement to long-chain product. The main message of this study is that the reaction conditions significantly influence the reactivity and must be very carefully controlled for successful synthesis of SAB. H2 and NH3 are released in significant quantities during milling, which is in agreement with the fact that the reaction occurs over more than one step. The starting 1:1 composition mixture after shorter milling times always result in adduct of the NaAB•AB composition with release of 1 equivalent of H2. The final product is then determined by reaction of this intermediate with some reagent, that remains in the mixture after completion of the first step. In the concrete case, the entire quantity of AB is consumed over the first step, while half of the starting quantity of NaH remains in the reaction mixture. It reacts with NaAB•AB, finally giving SAB. A prolonged milling, i.e. milling longer than 50 minutes, leads to decomposition of SAB into an amorphous product, a complex and undefined mixture of B-N-H containing oligomers and polymers. Also, a high influence of released gaseous products on the final product is observed. Deggasing of milling jar after some time spent in milling and continuing the milling after that shifts the reaction in direction of SAB (product) formation. This work was supported by the Croatian Science Foundation under the project ”New Materials for Energy Storage” in the total amount of 2M HRK. This work has been partially supported by SAFU, project KK.01.1.1.01.0001.

Amidoborane ; mechanochemistry ; ammonia borane ; hydrogen storage

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