L- and D-Proline Adsorption by Chiral Ordered Mesoporous Silica Clara Casado, † Joaquín Casta ́ n, †,‡ Ismael Gracia, †,‡ Miriam Yus, †,‡ A ́ lvaro Mayoral, § Víctor Sebastia ́ n, † Pilar Ló pez-Ram-de-Viu, ‡,∥ Santiago Uriel, ‡ and Joaquín Coronas* ,† † Department of Chemical and Environmental Engineering and Instituto de Nanociencia de Aragó n (INA), ‡ Department of Organic Chemistry, and § Laboratorio de Microscopías Avanzadas, INA, Universidad de Zaragoza, 50018 Zaragoza, Spain ∥ Instituto de Síntesis Química y Homoge ́ nea (ISQCH), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain * S Supporting Information ABSTRACT: Chiral ordered mesoporous silica (COMS) was synthesized in the presence of amino acid proline by combining tetraethyl orthosilicate and quaternized aminosilane silica sources. The as-prepared materials were activated by calcination or microwave chemical extraction to remove the organic templates. The powder X-ray diffraction and N 2 adsorption characterization revealed in COMS the structural and textural features of MCM-41-type silica. The chirality of the material was disclosed by mixed and separate L- and D-proline adsorption on the COMS prepared with L-proline (L-Pro-COMS) and D- proline (D-Pro-COMS). It was found that the maximum L-proline and D-proline adsorption capacities on L-Pro-COMS were ca. 2.3 and 0.6 mmol/g, respectively, while the adsorption of D-proline was higher than that of L-proline on D-Pro-COMS. Finally, both activation routes yielded enantioselective silicas able to separate proline racemate. ■ INTRODUCTION α-Amino acids, an important class of organic compounds, containing an amino and a carboxyl group in the same carbon atom (the α-carbon), are critical to life because they constitute the building blocks of proteins and biopolymers carrying out the most diverse functions in organisms. The physiological importance of α-amino acids ensures a sustained interest in their chemistry and properties, particularly in the pharmaceut- ical exploration for new drugs or products with biological applications. Amino acids are commonly used in food technology, drug synthesis, and cosmetics. Continued research on α-amino acids has also led to their use in diverse areas such as the biodegradable plastics industry, 1 drug delivery systems, 2 or stereoselective laboratory synthesis. 3 There are several routes for the production of amino acids, all involving the use of separation techniques to recover and purify them. Besides chromatographic and electrophoresis methods, 4,5 amino acids are commonly separated by organic ion exchange resins. 6 In addition to these more classical procedures, adsorption from solution into molecular sieves is becoming increasingly widespread in separation and purifica- tion processes. In particular, zeolite β, 7,8 ZSM-5, 7,9 zeolite Y, 10 and also ordered mesoporous materials SBA-15 11 and MCM- 41 12 have been used to adsorb various amino acids from water solutions. Amino acids of different polarities have been separated by adjusting the pH of the solution appropriately. 7 The adsorption of amino acids on zeolites is in general dominated by electrostatic interactions, 10 although hydro- phobic interactions involving nonpolar side groups 7,9 and steric 8 interactions complete the overall molecule−adsorbent picture. In addition to providing information on the separation or purification of amino acids, 7 the study of the adsorption of these molecules can give insights into the adsorption of proteins or enzymes on solid materials. 13 Moreover, the adsorption of amino acids on minerals is an important step in the concentration of these molecules. Several papers have dealt with this topic 14,15 since it is thought to have played a role in the origin of life. 16 All the proteinogenic α-amino acids except glycine are chiral molecules, all of them possessing the L-configuration. This Received: February 29, 2012 Published: April 4, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 6638 dx.doi.org/10.1021/la300864n | Langmuir 2012, 28, 6638−6644