Gas–Solid Reaction of Carbon Dioxide with Alanates

Abstract

An empirical study of the gas–solid reaction of carbon dioxide (CO2) with alanates is presented. This investigation was triggered by reports of hazards related to the reaction of lithium aluminum hydride with carbon dioxide, together with the recent emergence of alanates as potential hydrogen storage materials. Furthermore, the reduction of CO2 by hydrides is an alternative to the conventional CO2 reduction employing hydrogen gas in combination with a catalyst. Experimentally this work was carried out by studying the decomposition of alanate samples in a carbon dioxide atmosphere with thermogravimetric and ex situ IR spectroscopic techniques. It is shown that alanates react with CO2 at atmospheric pressure in two distinct temperature regions, yielding methane, hydrogen gas, and metal oxides as the major products. The experimental findings allowed us to postulate a mechanism for the complex metal hydride reduction of CO2, involving alane as a highly reactive intermediate. It is suggested that nucleophilic attack of alane hydride ions onto the carbonyl carbon of CO2 leads to sequential formation of aluminum formate and methoxy species which get converted into methane and metal oxides as the final products.