Regenerating spent ammonia-borane II

27 November 2012 - The hydrogen economy

In a previous episode Sutton et al. introduced a new method for the regeneration of ammonia borane (AB) using hydrazine. Bad idea! say Reller and Mertens because hydrazine is (1) a hazardous material and (2) therefore predominantly sold as the less hazardous hydrate. And the hydrate cannot be used in AB regeneration. In their recent publication (DOI) Reller and Mertens suggest an alternative scheme for AB regeneration featuring an entirely different intermediate as summarized below:

1 - Test reaction: conversion of ammonia borane to polymerized BHN at 95°C
2 - hydrochlorination with aluminum chloride / hydrogen chloride in carbon disulfide at 40 bar forming ammonium chloride and boron trichloride
3- Hydrodechlorination with hydrogen at 130°C / 60 bar and Ni3B as catalyst. Trietylamine is added as thermodynamic driver forming the Et3NBH3 adduct. Byproduct HCl can be recycled.
4 - Exchange reaction back to H3NBH3 starting with liquid ammonia then heated at 80°C for 12 hours.

An efficiency of 60% was reported.

Bottlenecks reported: small window of opportunity in step 4 as dehydrogenation sets in already at 90°C but eventually ammonia can replace triethylamine.
Lucky breaks reported: despite its abundance chlorine did not poison the nickel boride catalyst.