Support effects in rare earth element separation using diglycolamide-functionalized mesoporous silica

Due to the rapidly increasing energy demand and growing production of high technology devices, the development of new sequestration materials for rare earth elements (REEs) has become critical. Nowadays, REEs play a predominant role as supplies for the transition to cleaner energy and production of economically important modern devices, such as wind turbines (Pr, Nd, Sm, Dy), car catalysts (Ce) or hybrid vehicles (Dy, La, Nd). However, for all these applications, only a very pure and isolated form of element can be used. While several methods have been developed for REE extraction, such as liquid-liquid or liquid-solid extraction methods, the selective separation and purification of REEs still remain challenging. Industrially, the separation/purification process of REEs involves several liquid-liquid extraction (LLE) cycles. As a consequence, a large volume of solvents, time and labor are required. Moreover, LLE usually generates huge amounts of waste that is often environmentally harmful. Therefore, in our laboratories, we have recently focused on developing greener alternatives for the REE extraction process using solid extraction systems. In the present study, we use a tailored-made solid phase (SPE) extraction system, where appropriately modified mesoporous silica supports (i.e., SBA-15, SBA-16 and MCM-41) are used and compared as sorbents. As evidenced from our results, DGA-functionalized porous sorbents are characterized by a pronounced selectivity towards mid-size elements and high stability under the extraction conditions tested. Moreover, these sorbents show very fast REE uptake, in about 5 min. Furthermore, we focus our studies on elucidating the influence of the pore structure, pore size and pore connectivity of different silica materials on the static and dynamic extraction/purification of REEs.