Fuel cell grade hydrogen from fossil carbon reserves. Methanol or formic acid derivatives intermediates for hydrogen storage in CO2

The crucial aspect of the hydrogen economy, yet hypothetical, is the quality of hydrogen which, to feed a Proton Exchange Membrane Fuel Cell (PEMFC), must be Fuel Cell grade Hydrogen (FCH), that is CO free and undiluted by inert gases. The practical application of the fuel cell technology requires that FCH is obtained from H2O by means of a chemical energy largely available on the planet and that its storage is made in such a way that the release can occur on demand. It is well known that huge amounts of H2 together with CO and CO2 (syngas), are produced by reacting still abundant fossil carbon reserves with H2O and controlled amount of O2 (steam and oxidative reforming). However, FCH cannot be directly obtained from syngas, since economic methods for the complete removal of CO have not been found yet. Rather, hydrogen of syngas is employed to hydrogenate the carbon oxides to methanol. From the point of view of hydrogen extraction, the conversion of syngas into methanol looks attractive: the hydrogen is exceedingly concentrated to a 10% w/V into methanol which, furthermore, is a liquid easily transportable substance. In order to take advantage from these two favourable aspects, many attempts to obtain FCH from CH3OH, rather than directly from syngas, have been done. A variety of catalyst have been tested under various conditions but, invariably, hydrogen contaminated by CO has been obtained. We present here an alternative to exhaustively extract FCH from methanol through a three steps process involving i) methanol dehydrogenation to formaldehyde, ii) formaldehyde oxidation to formate, accompanied by FCH evolution and iii) formic acid decomposition to the easily removable CO2 and FCH. Since CO2, produced in the last step, can be hydrogenated to methanol, methanol itself can be regarded as an intermediate for hydrogen storage in CO2. A limit of such a strategy is the fact that hydrogen is lost in the water accompanying the CO2 hydrogenation to methanol, moreover, by this way, the FCH extraction is a laborious stepwise process. Such an idea of the hydrogen storage in CO2 is thirty years old but formic acid derivatives rather than methanol have been regarded as the possible intermediates. A more direct chemical loop for hydrogen storage in CO2 has been envisaged, but not implemented. It would require the production of formic acid derivatives under moderately high CO2/H2 pressure and the release back of H2 together with CO2 at atmospheric pressure. The first process producing substantial amounts of formic acid derivatives by employing CO2 rather than CO raw material is reported in the present thesis. By incorporating CO2 and H2 into neat NEt3, the amine has been quantitatively converted into HCOOH/NEt3 adducts which have been isolated as a pure and storable liquid. On the other hand these adducts are decomposed on demand by a proper catalyst to FCH, CO2 and NEt3. For the first time, both steps of a chemical loop for hydrogen storage in CO2 have been fulfilled.