NaOH and KOH for preparing activated carbons used in energy and environmental applications

Alkaline hydroxides, especially sodium and potassium hydroxides, are multi-million-ton per annum commodities and strong chemical bases that have large scale applications. Some of them are related with their consequent ability to degrade most materials, depending on the temperature used. As an example, these chemicals are involved in the manufacture of pulp and paper, textiles, biodiesels, soaps and detergents, acid gases removal (e.g., SO2) and others, as well as in many organic synthesis processes. Sodium and potassium hydroxides are strong and corrosive bases, but they are also very stable chemicals that can melt without decomposition, NaOH at 318ºC, and KOH at 360ºC. Hence, they can react with most materials, even with relatively inert ones such as carbon materials. Thus, at temperatures higher than 360ºC these melted hydroxides easily react with most types of carbon-containing raw materials (coals, lignocellulosic materials, pitches, etc.), as well as with most pure carbon materials (carbon fibers, carbon nanofibers and carbon nanotubes). This reaction occurs via a solid-liquid redox reaction in which both hydroxides (NaOH or KOH) are converted to the following main products: hydrogen, alkaline metals and alkaline carbonates, as a result of the carbon precursor oxidation. By controlling this reaction, and after a suitable washing process, good quality activated carbons (ACs), a classical type of porous materials, can be prepared. Such carbon activation by hydroxides, known since long time ago, continues to be under research due to the unique properties of the resulting activated carbons. They have promising high porosity developments and interesting pore size distributions. These two properties are important for new applications such as gas storage (e.g., natural gas or hydrogen), capture, storage and transport of carbon dioxide, electricity storage demands (EDLC-supercapacitors-) or pollution control. Because these applications require new and superior quality activated carbons, there is no doubt that among the different existing activating processes, the one based on the chemical reaction between the carbon precursor and the alkaline hydroxide (NaOH or KOH) gives the best activation results. The present article covers different aspects of the activation by hydroxides, including the characteristics of the resulting activated carbons and their performance in some environment-related applications. The following topics are discussed: i) variables of the preparation method, such as the nature of the hydroxide, the type of carbon precursor, the hydroxide/carbon precursor ratio, the mixing procedure of carbon precursor and hydroxide (impregnation of the precursor with a hydroxide solution or mixing both, hydroxide and carbon precursor, as solids), or the temperature and time of the reaction are discussed, analyzing their effect on the resulting porosity; ii) analysis of the main reactions occurring during the activation process, iii) comparative analysis of the porosity development obtained from different activation processes (e.g., CO2, steam, phosphoric acid and hydroxides activation); and iv) performance of the prepared activated carbon materials on a few applications, such as VOC removal, electricity and gas storages.

El papel del Banco de España en el hundimiento de las Cajas de Ahorro españolas

En la última década, la organización del sistema financiero español ha tenido una convulsión de un alcance inimaginable y puede decirse sin ningún género de dudas que el cambio ha sido histórico. De un sistema en el que el reparto de las instituciones financieras estaba representado por las Cajas de Ahorro y la banca en un 50% por cada grupo, se ha pasado a un escenario en el que las Cajas de Ahorro han desaparecido casi en su totalidad y la banca ha ocupado completamente todo el sistema financiero español. Difícilmente se puede explicar este cambio por factores exclusivamente atribuibles a unas u otras instituciones ya que, en buena lógica, una crisis económica afecta con distinta intensidad a cada una de las empresas y, por lo tanto, hubiese cabido esperar que el mantenimiento de algunas cajas de ahorros ante el embate de la crisis hubiese significado que quedasen algunas en funcionamiento. Pero ¿todas? También hubiera cabido esperar que la crisis hubiera afectado a más bancos, pero ¿tan pocos? ¿Tan bien estaban o sabían más que las demás entidades financieras? En esta comunicación vamos a tratar de analizar el comportamiento del Banco de España en este proceso y ver hasta qué punto sus decisiones o falta de decisiones han contribuido a profundizar en la crisis económica y en el hundimiento de todo el sector de cajas de ahorro españolas.