Applications of Ionic Liquids in the Oil Industry: Towards A Sustainable Industry

PREFACE

Ionic liquids (ILs) are ionic organic compounds, which unlike inorganic ionic compounds (salts), present, in general, very low melting points (below 100°C by and large) and in other cases, they are liquid at ambient temperature with negligible vapor pressure (non-volatile like common organic solvents), slight corrosive nature, low flammability and high chemical stability. These and other properties have positioned such compounds as environmentally friendly and drawn the researchers’ attention, exploring a number of applications in different chemistry fields like that of the oil industry.

In the present compendium, some of the works published by the Mexican Petroleum Institute (In Spanish: Instituto Mexicano del Petróleo, IMP) are reviewed. The IMP is a public institution that has been devoted to carry out research projects aimed at providing solutions to the Mexican Oil Industry since 1965, the year when it was founded, and the synthesis and field application of ILs for dealing with the technical challenges faced by such national industry represent some of the current works developed at this research center. General aspects and recent bibliography of different topics are reviewed; in addition, IMP contributions by means of scientific papers and granted patents on ILs synthesized and used to solve technological problems found in the Mexican oil reservoirs are discussed. In this context, the removal of solid, liquid, and gaseous pollutants and the breaking of emulsions that are formed naturally between crude oil and water, which increases the oil viscosity and makes the transport of heavy and extra heavy crude oil difficult, are drawbacks that can be attacked by employing chemical compounds to control water in mature fields. ILs can be used to inhibit corrosion (corrosion inhibitors, CIs), as inhibitors of the formation of methane hydrates in deepwater wells, and as catalysts to obtain alkylate gasoline by the reaction between isobutane and butene.

This book is addressed to passionate organic chemistry researchers interested in the wide universe of ILs and more specifically to experts in research works focused on the synthesis and use of chemical compounds to support and help the Oil Industry be safer and more sustainable.

CONSENT FOR PUBLICATION

Not applicable.

CONFLICT OF INTEREST

The authors declare no conflict of interest, financial or otherwise.

ACKNOWLEDGEMENTS

The authors are grateful for the facilities granted by IMP for the writing of this chapter under project Y.62011 and thank CONACyT for financial support through project CF19-191973.

Rafael Martínez Palou

&

Natalya V. Likhanova

Dirección de Investigación en Transformación de Hidrocarburos

Instituto Mexicano del Petróleo. Eje Central Lázaro Cárdenas Norte 152, 07730

Mexico City

Mexico

Structure, Properties and Applications of Ionic Liquids

Rafael Martínez-Palou, Natalya V. Likhanova

Abstract

ILs have attracted the attention of researchers in recent decades. The number of applications in which these unusual compounds show good performance has grown dramatically in the last century. This chapter presents an overview of ionic liquids, their structure, properties and general applications that have made them one of the families of chemicals to which most research efforts have been devoted.

Keywords: Applications, Catalysis, Dissolution, Electrochemistry, Energy storage, Extraction, Ionic liquids, Properties, Polymerization reactions, Solvent, Synthesis, Separations, Synthesis of nanomaterials.

INTRODUCTION

Over the last two decades, ILs have strongly drawn the attention of the scientific community due to their interesting physical properties [1, 2] and applications as solvents with exceptional properties in organic synthesis [3-6], catalysis [7-10], biocatalysis [11-13], liquid-liquid separations [14], extraction [15-19], dissolu-tion, [20-23] synthesis of nanomaterials [24], polymerization reactions [25, 26], electrochemistry [27, 28], and energy storage [29].

ILs are ionic compounds in which at least the cation is of organic type and have the particularity of being liquid compounds at ambient temperature or close to it (< 100°C), which makes them different from other ionic compounds or molten salts that display very high melting points (> 800°C). Fig. (1.1) shows the general structure of the most common cations present in ILs.

Fig. (1.1))

Some typical IL cations. R, R’, R, R’ represent alkyl, benzylic or alkyl-functionalized chains.

The cations can be of the heterocyclic type, derived from imidazole (1), pyridine (2) or quinoline (3) or aliphatic compounds such as quaternary compounds derived from amines (4), phosphorous (5) or sulfur (6) compounds. ILs have the special feature of displaying a heteroatom (nitrogen, phosphorous or sulfur) with a positive charge or electron deficiency, which in the case of aromatic derivatives, are delocalized through the ring.

In the case of anions, they can be inorganic or organic and their type affects significantly the physicochemical properties of the ILs [30].

Some of the most common ions found in ILs are as follows: Cl-, Br-, [BF4]-, [PF6]-, [SbF6]-, [AlCl4]-, [FeCl4]-, [AuCl4]-, [InCl4]-, [NO3]-, [NO2]-, [SO4]-, [SCN]-, [AcO]-, [N(OTf)2]-, [CF3CO2]-, [CF3SO3]-, [PhCOO]-, [C(CN)2]-, [RSO4]- and [OTs]-.

The possible combinations between cations with different chain types (R) and anions allow the generation of more than 2 million of ILs with diverse physical and chemical properties [31]; some of the characteristics that make ILs so attractive in different chemical areas are the following:

Negligible vapor pressure. For this reason, ILs are considered as environmentally friendly solvents and exceptional substituents of common organic solvents, which in most cases are volatile, toxic and handled in high volumes in industrial processes.

Not flammable. This property makes them safe to be handled.

Excellent catalyst properties. The catalytic properties of these compounds are exceptional and the number of examples featuring processes where ILs have worked as catalysts is increasing exponentially in the scientific literature.

High ionic conductivity. The structure of both the cation and anion considerably We agree with the proposed change the ionic conductivity of ILs, which in general is very high.

Wide electrochemical potential window. Thanks to this feature, numberless applications in electrochemical processes are possible.

Broad thermal stability interval. For the same organic cation, the thermal stability can vary within a more or less wide interval; for this reason, these ions can be employed in processes that take place at relatively high temperatures (between 200 and 400 °C).

Variable dissolving properties. Wide range variability of the properties to dissolve organic compounds or to be dissolved in common organic solvents. The

structure of both the cation and anion affects considerably the solvent properties of ILs.

Easily recyclable. ILs can be purified and reused for various cycles for many applications without altering significantly their properties or activity. The regeneration process is generally carried out by washing with conventional organic solvents and subsequent vacuum drying.

Synthesis of ILs

In general, ILs are synthesized by means of nucleophilic substitution reactions through which an alkyl halide reacts with a heteroatom in a heterocyclic or aliphatic compound, where the free electron pair from such heteroatom is involved in the formation of a new heteroatom-carbon bond, thus generating electron deficiency in the heteroatom in question.

The classical synthesis methodology of ILs occurs through the alkylation of a heteroatom with short-chain alkyl halides; for this reason, in general, at the first synthesis stage, the ILs present a halide as anion. At the second stage, the anion can be exchanged or modified through either a metathesis or acid-base reaction.

The cation in the ILs can be symmetric or asymmetric. In the case of the symmetric ILs, the reaction includes a previous stage at which the heteroatom-hydrogen bond is broken through the treatment of the heterocycles with a strong base (sodium hydride, NaH, in most cases).

The synthesis requires heating conditions under reflux with or without the presence of a solvent. The reaction time will depend mainly on the reactivity of the alkyl halides, and according to them, the reaction time can be from 24 to 72 h.

Since ILs are not volatile (practically negligible vapor pressure), purification cannot be carried out by distillation and, in general, it is performed through washings employing organic solvents capable of eliminating soluble impurities without dissolving the IL.

The preparation methods of ILs by conventional heating require many reflux hours in organic solvents, however, in the last years, synthesis methodologies using microwaves, with which both alkylation and metathesis reactions are accelerated dramatically, have been described. The microwave synthesis methodology of ILs became very popular due to the high product yields in a few reaction minutes [32]. Likewise, ILs have been a very useful auxiliary tool for microwave organic synthesis [31, 33].

Varma et al. described for the first time the microwave synthesis of ILs of the imidazole and bis-imidazolium type by means of the reaction between 1-methylimidazole and alkyl halides or dihalides in an open system without using a solvent. The ILs were produced in less than 2 min with yields above 70% [34]. These researchers also published an efficient methodology for the synthesis of these compounds using ultrasound as an alternative energy source [35].

Fig. (1.2) shows a simplified reaction diagram for the synthesis of ILs, both symmetric and asymmetric, employing conventional heating (∆), ultrasound [)))] or microwaves (MW) as nonconventional heating sources. The schematic representation displays the synthesis from imidazole, which is one of the most used starting materials, but in general, the synthesis procedure is valid for most ILs described in the scientific literature containing the cations 2-6 described in the general structures [36-39].

Fig. (1.2))

General synthesis methodology of ILs from imidazole.

As it can be observed in Fig. (1.2), the synthesis time of the ILs is reduced considerably by employing a microwave piece of equipment. According to the aforementioned, it would be recommendable that such a piece be at hand for the synthesis of the ILs that are intended to be evaluated.

Once the ILs are synthesized and purified, they are submitted to a structural characterization process in order to obtain unequivocal information of the synthesized compound and its purity by means of techniques such as nuclear magnetic resonance (NMR), infrared spectroscopy and mass spectrometry.

Technology Development Using ILs

Since the 1980s, the interest in the use of ILs as solvents in chemical processes has increased notably and since then, a large number of applications have emerged. Many of these research works have focused on the employment of ILs to create biphasic systems for alkylation and acetylation reactions [40].

The ILs can be classified as design solvents, for by varying the characteristics of the implied ions, millions of different combinations can be synthesized; which is an immense amount in comparison with the less than 300 most used organic solvents in the chemical industry. Due to the excellent hybrid properties of ILs, which stem from their organic-inorganic nature (thinking of a cleaner chemical industry), the most varied uses of ILs in different application areas have been suggested and studied, as described in Table 1.1.

Table 1.1 Some applications of the ILs.

Large-scale Applications in the Oil Industry

As for the applications of ILs in the Oil Industry, some recent developments have been described; for example, the French Petroleum Institute patented the use of ILs as solvents for alkylation, polymerization, and catalysts for the Diels-Alder reaction [41].

The company BP Chemicals tested the use of pyridinium or imidazole chloride in combination with an alkyl aluminum halide (RnAlX3-n) as IL for the polymerization of butane [42, 43]. On the other hand, Akzo Nobel described a process for the formation of alkylbenzenes employing ILs [44, 45]. Exxon patented a process using ILs for extracting aromatic compounds from a hydrocarbon mixture, where triethylammonium dihydroxybenzoate salts were employed [46].

One of the applications that have had the largest scaling of a productive process employing ILs is the technology for gasoline production by alkylation through the reaction between isobutane and butenes. This technology has been widely studied to replace the conventional acid catalysts (H2SO4 and HF) by ILs [47, 48]. These ILs contain transition metals in their anion and have been used as catalysts of the alkylation reaction between light olefins and hydrocarbons, typically between 2-butene and isobutane to obtain trimethylpentane. The company PetroChina developed an alkylation process based on ILs known as Ionikylation. Currently, the company has an alkylation unit based on this technology that produces 150,000 ton/year [49-51]. As for Honeywell UOP, it licensed the technology known as Isoalky™, which was developed by Chevron, at the test stage in Salt Lake City based on a chloroaluminate-type catalyst [52].

Another advantage associated with the use of ILs as solvents in chemical reactions is that they require milder temperature conditions than when conventional solvents are employed, which implies as a consequence, the reduction of energy and environmental costs [53-55]. For example, the reactions, where ILs are used as catalysts, occur at lower temperatures and with higher yields. The Friedel-Crafts reaction, which plays a key role in the oil cracking process, is carried out with conventional solvents at 80 ° C, takes 8 h and has a yield of 80%; in contrast, the same reaction using ILs is performed at 0 °C, occurs in 30 seconds with a yield of 98% and a product that is purer and homogeneous [56-60].

At present, ILs have widened their applications in processes at a large scale [61] and many of them are commercially available [62]. Recently, some current and future applications have been reviewed [63, 64].

CONCLUDING REMARKS

As we have seen in the present chapters, ILs have such interesting, varied, and unique properties, besides being such a wide and diverse family of compounds that make them a focus of attention for their application in different scientific fields, such as organic synthesis, catalysis, biocatalysts, separations, extraction, dissolution, synthesis of nanomaterials, polymerization reactions, electrochemistry, and energy storage. As you can see by reading this book, the application of ionic liquids has been very extensive in a wide variety of applications in the Oil Industry, from facilitating the processes of primary and improved extraction of crude oil from the deep sea to the addition of these products in the crude transport and refining processes. Some of these applications are still in the early stages of research and certain challenges remain to be resolved, while other results have already been tested on an industrial scale.

REFERENCES