MCM-41 as selective separator of chlorophyll-a from β-carotene and chlorophyll-a mixture

Abstract

In this work, microporous (zeolite A, X and HS) and mesoporous (MCM-41, F-MCM-41 and MCM-48) silicate materials were used as adsorbents to separate the chlorophyll-a (Chl-a) from the mixture of Chl-a and β-carotene (β-Car). The adsorbent capacities were determined to be MCM-41 ∼ MCM-48 > F-MCM-41 > zeolite X > zeolite A > zeolite HS. It was found that the mesoporous MCM-41 and MCM-48 selectively adsorb the Chl-a from β-Car. The adsorbed Chl-a on MCM-41 can be extracted by polar MeOH solvent. No degradation of MCM-41 was observed during the course of separation process. Powder X-ray diffraction (XRD) analysis and N₂ adsorption–desorption isotherm experiments showed that the well ordered hexagonal structure of MCM-41 is present and surface area, pore volume and pore diameter have been decreased after adsorption.

Introduction

M41S is the designation of a new type of mesoporous structure such as MCM-41 [1], [2], MCM-48 [3] and MCM-50 [4], with a hexagonal array of cylindrical pores, cubic ordered pores and lamellar structure, respectively. The main characteristics of these materials are large surface areas and very narrow pore size distributions [5], [6], [7], [8], [9]. In fact, they are good hosts for inorganic and organomethalic materials [10], [11]. The encapsulation of large molecules either organic or inorganic (dyes, pigments, etc.) in these hosts leads to composite materials with novel optical properties. Therefore, they can be used for optical data storage, optical sensing and photocatalysts [12], [13].

Chl-a and β-Car play an important role in photosynthetic systems as photoprotecting agents and light harvesting antenna pigments [12], [14]. Chlorophyll is the principal photoreceptor in photosynthesis. This cyclic tetrapyrole, like the heme groups of globins and cytochromes is derived biosynthetically from protoporphyrins. Since the various Chl’s are highly conjugated molecules, they strongly absorb visible light. In general, energy transfer from the antenna system to a reaction center occurs in 10⁻¹⁰ s with efficiency of more than 90%. The high efficiency arises from the appropriate spacing and relative orientations of chlorophyll molecules. Chlorophylls mostly function to gather light and act as light harvesting antennas. These antennas then pass the energy of an absorbed photon, by excitation transfer from molecules to molecules until the excitation reaches a photosynthetic reaction center [15], [16]. However, natural chlorophylls extracted from living leaves have been seldom used as photocatalysts because of their instability. In the layered thylakoid membrane of chloroplast in intact leaves, chlorophyll molecules bind to proteins to form chlorophyll–protein conjugates in which the chlorophyll interaction play an important role in stabilization and physiological functions of living plant lives. Itoh and coworkers have reported that Chl-a adsorbed to silicate layers of smectite showed photostable and photocatalyzed properties, which demonstrated the importance of chlorophyll-support interactions [12], [17]. Therefore, we thought that MCM-41 with polar Si–OH groups might be a good host for storing of chlorophyll.

Chl-a has been extracted by different methods such as HPLC [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], liquid chromatography [27], liquid–liquid extraction [30], and sonification [22], [28], [29]. Separation of the accompanied β-carotene from Chl-a is carried out by saponification method [28], [29].

In this work, we have studied the adsorption of Chl-a from the mixture of Chl-a and β-Car obtained from spinach extraction using microporous and mesoporous materials. Moreover, we thought that the successfulness of this procedure may provide an efficient method of Chl-a storage in solid adsorbants.