Metal borohydrides synthesis and their usability in hydrogen recycle

Environmental friendly systems are considered attractive to meet the increasing energy demand. In this field hydrogen is thought to be the best candidate for beyond application.Hydrogen production technologies are well developed unlike its storing opportunities. Hydrogen can be stored; in liquid form, under high pressure, in activated carbon or in chemical structure. Small size of hydrogen molecule and thermodynamic limitations bring out chemical structures in advantageous position. Metal borohydrides are the most investigated compounds of this field.In this study, LiBH4 and NaBH4 were tried to produce. In order to synthesis of LiBH4 various mol ratio of Li and B prepared under Ar atmosphere and operated in ball milling. LiBH4 was obtained after minimum 50 bar hydrogen pressure application. On the other hand, NaBH4 was synthesized with trimethyl borate and NaH. Trimethyl borate production conditions also constitute a part of this report. It was observed that LiBH4 yield is higher when the B/Li ratio equals 0.214. Besides, 250 oC temperature and 6 times excess using of trimethyl borate according to the reaction stoechiometry was found as the best operating conditions for NaBH4 production. Pridin, tetrahydrofuran, dimethyl ether of diethylene glycol and ethylene diamine were used to purify the raw products.Dehydrogenations of the synthesized metal borohydrides were carried out in the presence of NaOH stabilizer. One gram of produced lithium borohydride sample released 1423 mL of hydrogen with Ni catalyst while NiO caused 1972.94 ml of hydrogen gas desorption, equaling to 90.25 % of the theoretical yield of commercial LiBH4. Furthermore 225 % of hydrogen in NaBH4 could be desorbed by CoCl2 catalyst. Indicating that, hydrogen of water can be obtained by either NiO or CoCl2 catalysts.CoCl2 was embedded on different support materials and ?-Al2O3 was determined as the most stable material in alkaline solution. Also, variation of CoCl2 amounts on support materials show that it is possible to control the dehydrogenation time from 500 to 3000 minutes.Reducing of NaBH4 production price is essential to increase its marketing. Thus, side product NaBO2 was operated in ball milling with MgH2 under Ar atmosphere to establish NaBH4 recycle. Experimental results showed that 10 hours operation using 30 % excess MgH2 is enough to obtain NaBH4 products whose purities are mostly over 70 %.