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Amino acid adsorption onto mesoporous silica molecular sieves

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Abstract

Mesoporous molecular sieves are promising as adsorbents for purification of biological molecules, such as amino acids, due to their tuneable mesopore sizes and high surface area. In this study, the adsorption of the basic amino acid, lysine, onto MCM-41, a siliceous mesoporous molecular sieve, has been investigated under a range of solution conditions. It was found to adsorb according to a Langmuir-type isotherm with a maximum capacity at pH 6 of 0.21 mmol/g. The extent of adsorption depends strongly on the pH and ionic strength of the adsorbate solution, due to a combination of ion exchange and electrostatic interactions governing the adsorption process.

Introduction

Amino acids are significant in the food industry as nutritional supplements and ingredients in parenteral solutions, as well as being used as building blocks for production of pharmaceutical and agrochemical compounds. These applications require amino acids to be supplied at high purities by processes such as ion exchange chromatography from natural or synthetic sources. Amino acids are also interesting as model adsorbates due to their molecular size and zwitterionic nature.

Mesoporous molecular sieves have been identified as promising adsorbents for biochemical molecules due to their tuneable pore sizes with narrow pore size distributions in the mesopore range (i.e., 2–50 nm) of relevance for molecules such as amino acids, peptides and proteins, as well as their high surface areas and large pore volumes [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. To date, relatively few studies have investigated this potential, particularly for amino acids, which are small enough to fit easily within the pores of a wide range of mesoporous molecular sieves [1], [11], [12]. MCM-41 is a mesoporous molecular sieve, developed by scientists at Mobil Corporation [13], [14], with a regular hexagonal array of mesopores, the size of which can be tailored between 2 and 10 nm by altering the synthesis conditions. It also possesses high surface area (>1000 m2/g) and can be synthesised as a silicate or aluminosilicate matrix. It has been identified as an excellent model adsorbent as well as having potential for use in industrial separation processes [1], [15].

In this study, the adsorption of the divalent basic amino acid, lysine, on siliceous MCM-41 was studied under a range of solution conditions to provide insight into the adsorption kinetics, capacity and mechanisms. Lysine is a significant product of current industrial fermentation followed by ion exchange purification processes [16].

Section snippets

Materials

Sodium silicate solution (27% SiO2 and 14% NaOH) and cetyltrimethylammonium bromide (CTAB) were obtained from Aldrich. dl-Lysine monohydrochloride was purchased from Wako Pure Chemical Industries. Analytical grade sulfuric acid, sodium chloride, hydrochloric acid and sodium hydroxide were also used. Distilled and de-ionized water used throughout the experiments was prepared using a Milli-Q system (Nihon Millipore Kogyo, Tokyo). All reagents were used as received.

Synthesis

Siliceous MCM-41 was prepared by

Results and discussion

Lysine (Fig. 3) was selected for this study because it is a small (MW = 146.2) basic amino acid (pI = 9.7). Thus, it is positively charged in solutions around neutral pH, whereas silica carries a net negative charge (pI  2.0) [16], [25].

Conclusion

The basic amino acid, lysine, adsorbs on siliceous MCM-41 following a Langmuir-type isotherm with a maximum capacity at pH 6 of 0.21 mmol/g. The extent of adsorption depends on the solution pH and ionic strength, with a combination of electrostatic interactions and ion exchange governing the uptake of lysine. MCM-41 shows promise as a high surface area adsorbent for biological molecules such as amino acids, although its stability in the presence of water requires improvement before industrial

Acknowledgements

A.O. gratefully acknowledges support of a Science and Technology Agency of Japan Fellowship and the National Institute for Research into Inorganic Materials, Japan. Authors from the University of Melbourne acknowledge access to infrastructure from the Particulate Fluids Processing Centre, a special research centre of the Australian Research Council (ARC).

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