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Publication Title | Green chemical processing in the teaching laboratory: a convenient liquid CO2 extraction of natural products

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Green chemical processing in the teaching laboratory: a convenient liquid CO2 extraction of natural products†

Lallie C. McKenzie,a John E. Thompson,b Randy Sullivanc and James E. Hutchison*a aDepartment of Chemistry and the Material Science Institute, University of Oregon, OR 97403, USA. aE-mail: hutch@uoregon.edu

bDepartment of Chemistry, Lane Community College, OR 97405, USA

cDepartment of Chemistry, University of Oregon, OR 97403, USA

A unique liquid CO2 extraction laboratory developed for a greener organic teaching lab curriculum provides an effective, inexpensive, and convenient procedure for teaching natural products extraction concepts and techniques using modern green extraction technology. The procedure is appropriate for the teaching lab, does not require any special equipment, and allows the students to see the phase change and extraction as they occur. Students learn extraction and spectroscopic analysis skills, are exposed to a dramatic visual example of phase change, and are introduced to commercially successful green chemical processing with CO2.

Introduction

Although there have been many advances in green chemistry in the industrial and research fields, integration of these concepts into the teaching environment is still in its infancy. This may be due to the limited availability of educational materials that illustrate the methods, techniques, and principles of green chemistry. To address this problem, we developed a new green organic laboratory curriculum to teach fundamental chemical concepts and techniques along with the tools and strategies of green chemistry.2–6

Integration of these goals into the laboratory curriculum required the development of a broad collection of experiments that work well in the laboratory, improve the safety of the laboratory environment, and modernize the curriculum through the introduction of state-of-the-art methods. A green organic chemistry laboratory textbook2 and several manuscripts describe the criteria and process for greening experiments.3,4,6

Here we describe a new laboratory exercise that uses liquid CO2 to extract D- limonene from citrus rind. This safe and convenient procedure successfully addresses the diverse goals of green chemistry experiment development by simultaneously teaching practical techniques, fundamental chemical concepts, and green chemistry applications. In this experiment, the commonly taught concepts of natural product extraction and phase transitions are demonstrated through

a novel procedure employing liquid CO2 as a green solvent. The experiment introduces modern green chemical approaches through discussions of industrial use of liquid CO2 as a green replacement solvent (e.g., dry cleaning) and supercritical fluid extraction (SFE) as an example of a successful commercial green process (e.g., decaffeination of coffee). Replacement of traditional natural product extraction experiments through incorporation of this green chemistry research and technology promotes an understanding of the current practice of green chemical methods. This exciting, convenient, and straightforward procedure offers an opportunity to teach core organic chemistry concepts and skills in the context of applicable green chemistry.

In this report, a short review of the background and current industrial uses of CO2 and the relevance of this technology to the teaching laboratory are described. The new laboratory procedure is summarized in the Experimental section.7 In the Results and discussion section, both the chemical and green lessons of this process are discussed and extensions are proposed.

Supercritical and liquid CO2

At pressures above ambient, carbon dioxide can exist in forms usable as a solvent (i.e. as a liquid or supercritical fluid). As shown in the phase diagram in Fig. 1, CO2 is a liquid under relatively mild temperatures and pressures, in the ranges of 256.6 to 31.0 °C and 5.2 to 73.8 bar. Supercritical carbon dioxide (scCO2) is produced at temperatures higher than the critical temperature (31.0 °C) and between the critical pressure (73.8 bar) and extremely high pressures (approximately 104 bar). Supercritical fluids have no distinct liquid or vapor phase but retain

properties of each. ScCO2 is especially beneficial when used as a solvent in selective extraction processes. The gas-like properties, such as very low surface tension and viscosity, allow the solvent to penetrate into the substrate, while the liquid-like properties solubilize compounds and remove them from the substrate. Small changes in pressure or temperature alter the bulk density of the fluid leading to increased or decreased solubility of various compounds. In this way, the use of supercritical fluids can allow for control of separations of materials. Through manipulation of temperature and pressure conditions within accessible ranges, both the phase and properties of CO2 can be easily controlled.

During the past two decades, technical advances have been made in the industrial use of supercritical and liquid carbon dioxide in place of organic solvents.8–10 CO2 is useful as a green alternative solvent because it provides environmental and safety advantages: it is nonflammable, relatively nontoxic, readily available, and environmentally benign. Processing with CO2 also poses minimal hazard in the event of unintentional release or residual solvent in the product. Although CO2 is a greenhouse gas, when used as a solvent it is captured and employed, not generated, resulting in no net environmental harm. Additionally, a closed loop system can be used to compress the gas in order to use the supercritical fluid or liquid in processing, depressurize the solvent for separation of dissolved compounds, and recompress the CO2 to begin the cycle again. CO2 extraction processes can also be run at relatively constant pressure when liquid–liquid extraction against water is used for product recovery. These loop systems allow for easy recovery and recycling of the pure solvent.

†Electronic supplementary information (ESI) available: experimental procedure (including instructors’ notes, student handout, and demonstration procedure) and movie clip of extraction process. See http://www.rsc.org/ suppdata/gc/b4/b405810k/

This journal is © The Royal Society of Chemistry 2004

FOCUS

Green Chem., 2004, 6, 355–358

355

GC www.rsc.org/greenchem

DOI: 10.1039/b405810k

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