Effects of the addition of an organic polymer on the hydrolysis of sodium tetrahydroborate in batch reactors

Abstract

An experimental study is presented both on the generation and storage of molecular hydrogen (H 2) by small additions of an organic polymer – carboxymethyl cellulose (CMC) – to sodium borohydride (NaBH 4) through the alkaline hydrolysis, in the presence of a powdered nickel–ruthenium based catalyst reused from 274 to 282 times. The experiments were performed at 45 °C in two batch reactors with internal volumes of 0.229 L and 0.369 L, made of stainless-steel with bottom conical shape, positioned vertically. The results showed that working at moderate pressures, up to 2.7 MPa, increases slightly the H 2 dissolution in the liquid phase, enhanced by the changing of the polarity of the remained solution inside the reactor: a value of 0.182 for dimensionless H 2 solubility in the liquid phase with 0.25 wt% CMC was found, at 45 °C, based on Henry’s law. As a consequence, sodium tetrahydroxoborate, NaB(OH) 4 by-product was produced in the presence of CMC additive, showing the absence of crystalline water in its crystal structure (NaB(OH) 4 presents structural water, with boron atoms linked to four hydroxyl groups). This new finding never reported to form at < 50 °C, has a positive impact in recyclability costs of NaBO 2 back to NaBH 4 due to the elimination of two energy consuming steps in the metaborate dehydration kinetics. In fact our system of compressed hydrogen, shows that both H 2 generation rates and yields and hydrogen storage capacities can be augmented, the latter to reach ≈ 6 wt%, by adding small amounts of an organic polymer (CMC) to the classic NaBH 4 hydrolysis, performed with stoichiometric amount of water. The eventual success of this new route will depend upon developing a advantageous method of converting borates into tetrahydroborate and also finding materials (chemicals) which enhance the solubility of H 2, promoting its reversible storage in the liquid phase. We therefore attempted to show that a less Polar Organic Polymeric Solution ( lPOPS) of NaBH 4 is a suitable hydrogen carrier for portable and/or niche applications.