The Application of Green Solvents in Separation Processes

Francisco Pena-Pereira, Marek Tobiszewski

• Publisher: Elsevier
• Year: 2017
• ISBN:
  9780128054437
• Topic: Physical Sciences
• Subtopic: Environmental Science

The Application of Green Solvents in Separation Processes features a logical progression of a wide range of topics and methods, beginning with an overview of green solvents, covering everything from water and organic solvents, to ionic liquids, switchable solvents, eutectic mixtures, supercritical fluids, gas-expanded solvents, and more.

In addition, the book outlines green extraction techniques, such as green membrane extraction, ultrasound-assisted extraction, and surfactant-mediated extraction techniques. Green sampling and sample preparation techniques are then explored, followed by green analytical separations, including green gas and liquid capillary chromatography, counter current chromatography, supercritical fluid chromatography, capillary electrophoresis, and other electrical separations.

Applications of green chemistry techniques that are relevant for a broad range of scientific and technological areas are covered, including the benefits and challenges associated with their application.

• Provides insights into recent advances in greener extraction and separation processes
• Gives an understanding of alternatives to harmful solvents commonly used in extraction and separation processes, as well as advanced techniques for such processes
• Written by a multidisciplinary group of internationally recognized scientists

Section II

Green Solvents

Outline

Chapter 2 Water as the First Choice Green Solvent

Chapter 3 A Systematic Approach to Green Solvent Selection, Design, and Verification

Chapter 4 Bio-Based Molecular Solvents

Chapter 5 Supramolecular Solvents for Green Chemistry

Chapter 6 Ionic Liquids, Switchable Solvents, and Eutectic Mixtures

Chapter 7 Supercritical Fluids and Gas-Expanded Liquids

Chapter 2

Water as the First Choice Green Solvent

Kari Hartonen and Marja-Liisa Riekkola, University of Helsinki, Helsinki, Finland

Abstract

Water is the most environmentally friendly solvent to be considered for green separation processes. Thanks to its good and tunable physicochemical properties, its use for separation, and especially for extraction and chromatography, has been remarkably expanded. Although water has been most frequently used mixed with organic solvents in numerous applications carried out by many separation techniques at ambient or slightly elevated temperatures, the main goal of this chapter is to describe its exploitation alone in extractions and chromatographic separations at high temperatures. Material and method development together with the most recent applications are presented. Extraction applications using high-temperature water are mainly focused on studies related to biorefineries, bioactive compounds, energy technology, amino acids, and essential oils. Liquid chromatography with water at high temperature has been most commonly applied to analysis of pharmaceuticals, cosmetics, and food.

Keywords

Water; green solvent; high temperature; extraction; liquid chromatography; applications

2.1 Introduction

During the last decade, environmentally friendly (green) solvents have attracted continuing and increasing interest in clean chemical processes of biorefineries [1,2], chemical reactions/synthesis [3], energy technology [4,5], and chemical separations [6]. In-depth research on different green solvents, such as gas expanded liquids, ionic liquids, supercritical fluids, and water, has greatly complemented these studies [7]. Although cold or room temperature water has also many interesting applications, for example, in the extraction of bioactive compounds [8–10], this chapter will mainly focus on the use of water alone as a green solvent in different chemical separations at elevated temperatures. Namely water, mixed with organic solvents is also widely employed in many separation techniques and processes to enhance solvation of less polar analytes. Special attention here is given to most recent developments and applications.

2.1.1 Why to Use Water as a Solvent?

Molecular structure of water and its capability to form hydrogen bonds (and hydrogen-bonded network) result in unique physicochemical properties making water an interesting solvent and vital substance for life. Ionic and polar compounds are readily dissolved in water at ambient condition and less polar compounds at elevated temperatures. Water is also nontoxic, nonflammable, cheap, and widely available in pure form. In addition, it exists in liquid state at relatively high temperatures (0–100°C) at atmospheric pressure. Thanks to its good solvation properties, water plays an important role in the transport of many nutrients and other substances in the environment, enabling various biological processes. Due to its high heat capacity, it also stabilizes temperatures of organisms and in various regions of the earth. Based on all these facts, it is no wonder that water is the most natural choice for the solvent. However, aqueous waste generated need to be cleaned to guarantee the quantity and quality of our water resources also in the future.

2.1.2 Water—The Most Green Choice

Water is definitely the safest and the most environmentally friendly solvent that can be considered for use as solvent in different, widely studied, and developed green separation processes. It is favorably assessed in recent solvent selection guides [11,12]. In addition to aqueous separations, water is also popular medium and reagent in synthesis/reactions [3,13], oxidation of waste materials [14,15], and conversion of biomass into fuels and chemicals [4,5,16].

2.2 Solvent Properties of Water

Water can most likely solubilize more different kinds of compounds than any other solvent. Its solvent properties can be characterized by its dipole moment, relative permittivity ε_r, which is a measure of solute solvent interactions, and by its polarizability π*. For water at ambient conditions (25°C), all these three parameters have high values 1.85, 78.5, and 1.12, respectively [17]. In addition, for better solvent characterization, one should include hydrogen bond donating (α=1.2 at 25°C) and accepting (β=0.37 at 25°C) abilities included in solvatochromic Kamlet-Taft parameters [18]. The values listed above explain why water is such a good solvent for ionic and polar compounds. However, it should be noted that at elevated temperatures, it becomes less polar being an alternative to organic solvents.

As can be seen from Table 2.1 and Fig. 2.1, relative permittivity (dielectric constant), polarizability, and degree of hydrogen bonding in water are decreased when temperature of water is increased. At the same time, its viscosity and surface tension are decreased. These large changes result in remarkable changes in solvent properties of water that are very unique com...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of Contributors
• Section I: Introduction
• Section II: Green Solvents
• Section III: Green Extraction Techniques
• Section IV: Green Sampling and Sample Preparation Techniques
• Section V: Green Analytical Separations
• Index