Mechano-chemical reactions in LiBH 4 + VCl n ( n = 2 and 3) mixtures

Abstract

Mechano-chemical routes to the metathesis reaction 4LiBH 4 + VCl n → Li 4− n V(BH 4) 4 + nLiCl ( n = 2 and 3) are explored at room and liquid nitrogen temperatures. The produced samples consist of crystalline LiCl and unreacted LiBH 4. No other crystalline phases are observed directly by powder X-ray diffraction. The highest reactivity was observed when using VCl 3. Two possible reaction paths are proposed in order to explain this result: (a) 2LiBH 4 + VCl 2 → 2LiCl + VB 2 + 4H 2, with Δ H = −188 kJ/mol and (b) 3LiBH 4 + VCl 3 → 3LiCl + 1/2B 2H 6 + VB 2 + 9/2H 2, with Δ H = −258 kJ/mol. The calculated enthalpy values show that reactions with VCl 3 have stronger thermodynamical driving forces than with VCl 2, corroborating the experimental results. Still, the different experimental methods lead to the same final products, with longer milling times being necessary to remove the precursor traces in the case of VCl 2 and cryomilling. The results shed some light on the efficiency of mechano-chemical methods for the synthesis of mixed metal borohydrides based on LiBH 4 and V-chlorides.