In today’s world, advances in science and technology are responsible for the development of new components meant to improve human life. At the center of these developments is the new materials used to make several products used by man. Depending on the desired properties of an object, a given material is structured in a way that reflects suits its application (Hibbeler 2013, p. 14). There is a wide range of materials that are characterized by their unique properties. The current study focuses on porous materials. More specifically, the study examines the applicability of microporous materials in terms of Carbon Dioxide sorption.
The suitability of a particular type of material relies on the strength of its various attributes. However, Callister and Rethwisch (2011, p. 9) argue that the application of naturally occurring materials is very limited compared to that of inorganic or man-made materials. Consequently, there is a need to improve the properties of materials to enhance their use. The current study looks at grafting as a way of improving materials to increase their range of applications. To this end, the current paper examines how amine is grafted on a given microporous material. Information on other porous materials, such as mesoporous elements, is also discussed.
Background to the Problem
As outlined in the previous section, the study reported in this paper revolves around the improvement of the properties of a porous material to enhance its applicability. Amine grafting is one of the ways through which the properties of porous materials can be improved (Hibbeler 2013, p. 43). It is important to appreciate the importance of porous materials, as well as their synthesis and applications.
Elementary chemistry suggests that materials attain their structure as a result of the alignment of their constituent elements (Hibbeler 2013, p. 40). The constituent elements can either be homogeneous or heterogeneous in nature. Nugent et al. (2013, p. 80) are of the opinion that there are instances where the structure of materials is characterized by spaces or voids. Such materials are said to be porous. In most cases, these porous materials have a solid skeletal structure, but the voids are occupied by fluids (Nugent et al. 2013, p. 80).
When examining the porosity of a material, there are important factors one has to consider. The electrical conductivity and permeability of a given material are some of the properties that define its porosity. Eddaoudi et al. (2002, p. 469) examined the application of a particularly porous material in their study. In this study, Eddaoudi et al. (2002) suggested that the properties of such materials emanate from their constituent elements. Depending on the applicability of a given porous material, its properties can be improved by taking into consideration the aforementioned factors.
Hibbeler (2013, p. 40) points out that there are various naturally occurring elements that are characterized by porosity. Soils and rocks like zeolites are typical examples of porous materials that occur naturally on the surface of the earth. However, with the rising demand for materials, there is increased production of artificial elements that are also porous in nature. Callister and Rethwisch (2011, p. 65) cite cement as the most common form of artificial porous material.
As already indicated, there are different categories of porous materials. According to Callister and Rethwisch (2011), these materials are usually categorized according to the size of their pores. Hibbeler (2013, p. 65) is of the view that the various types of these materials include, among others, mesoporous, microporous, and nanoporous. Each of these substances is named after the metric size used to measure the diameter of their respective pores. Porous materials are used in filtration, sorption, and catalysis. The current study explains how a porous material can be made suitable for the adsorption of gases like CO2. To achieve this, a number of synthetic microporous materials are examined. They include metal-organic frameworks and covalent organic frameworks.
Metal-Organic Frameworks as Porous Materials
In their study, Lee, Kim and Ahn (2013, p. 1667) found that metal-organic frameworks are classic examples of porous materials. However, caution should be taken when studying these substances. The reason for this is that they are synthetically produced (Lee et al. 2013, p. 1667). In addition, the properties of these materials vary depending on, among others, their porosity. Metal-organic frameworks are emerging as the most preferred materials in the areas of energy storage, luminescence, CO2 adsorption, and magnetism. As such, they are important aspects of the current study. A detailed analysis of the structures of these materials is provided in the literature review section of this paper. An analysis of the substances suggests that they are hybrid in nature. Shekha et al. (2007, p. 15118) hold the opinion that metal-organic frameworks are formed from clusters of ions. According to Shekha et al. (2007, p. 15119), these clusters are associated with organic linkers.
Shekha et al. (2007, p. 15118) indicate that the application of these materials in human processes is relatively new. Consequently, there are very few examples of metal-organic frameworks in existence. The observation is associated with the limited availability of the ligands and ions that form the constituent elements of these substances. 1,4-benzene dicarboxylic acid and 1,3,5-benzene tricarboxylic acids are closely related to these organic frameworks. The two are some of the common ligands used in the formation of metal-organic frameworks. Shekha et al. (2007, p. 15118) point out that Zn3+,Cu2+, Cr3+, Al3+,Fe3+, and Zr3+ are the most common ion clusters in this structure.
Given that they are artificial materials, Lee et al. (2013, p. 1667) point out that organic metal frameworks are prepared through synthesis. Farha et al. (2010, p. 947) are of the view that the metal frameworks are prepared through a thermal process. Farha et al. (2010) cite hydrothermal synthesis as the most preferred preparation mode. In some instances, the preparation process is referred to as solvothermal synthesis. The change in name results from the choice of solvent used in the process.
Covalent Organic Frameworks
Callister and Rethwisch (2011, p. 173) reiterate the notion that porous materials are a result of their constituent building blocks. Covalent organic frameworks are another example of porous materials (El-Kaderi et al. 2007, p. 268). However, contrary to metal-organic frameworks, this particular material consists of atoms as its building blocks. As the name of these substances suggests, the structure of these frameworks is unique. It emanates from covalent bonds formed by the constituent atoms.
Uribe-Romo et al. (2009, p. 4570) cite Oxygen, Hydrogen, Boron, and Nitrogen as some of the elements found in covalent organic frameworks. It is important to note that atoms that form covalent bonds are usually strong. For instance, a grapheme is an example of a covalent organic framework with such bonds (Uribe-Romo et al. 2009, p. 4570). Again, a detailed analysis of covalent organic frameworks is provided in the literature review section. The literature reveals that materials of this nature are usually used in catalytic processes and gas storage.
Lu (2012, p. 157) is of the opinion that the need for the development of new compounds and materials has made synthesis an important chemical process. In the past, the conventional chemical procedures of shaking and mixing gave rise to products with distorted structures. As a result of these structures, the products failed to function as expected.
Covalent organic frameworks are some of the materials prepared using the synthesis process. According to Ulrich and Husing (2012, p. 165), the kind of process required in the preparation of these substances is referred to as reticular synthesis. Ulrich and Husing (2012, p. 165) point out that the objective of this particular kind of synthesis is to ensure that the resultant structures are rigid and properly formed. Such structures improve the applicability of these materials.
Applications of Porous Materials
The ever-dynamic technological sphere of life gives rise to processes and equipment with improved and advanced applications (Ishizaki, Komarneni & Nanko 1998, p. 215). Materials like the ones discussed in this study are increasingly becoming relevant. Porous materials are particularly important when it comes to areas like catalysis, gas storage, and adsorption of matter.
Knowledge of materials like metal-organic frameworks and covalent organic frameworks is becoming increasingly important in many areas of human life. Ishizaki et al. (1998, p. 218) cite the example of hydrogen transportation. The transportation process requires the gas to be stored in a stable condition. Covalent organic frameworks can help attain this ability. The current study will examine the applicability of a porous material when it comes to the adsorption of carbon dioxide. The process has emerged as an important industrial undertaking.
Purpose of the Study
A study is carried out with various objectives in mind. Balnaves and Caputi (2001, p. 12) argue that study objectives are put in place to ensure that the researcher does not stray from the subject matter. The background information section above has outlined the importance of porous materials in the developing world (Menczel & Prime 2009, p. 67). Ekwere (2012, p. 73) affirms that grafting is an ideal way of improving the various properties of porous materials.
In line with the subject matter of the current study, a number of objectives were formulated. The following are the objectives of this research:
- To understand and identify the various types of porous materials.
- To determine the different properties of porous materials.
- To analyze the application of porous materials.
- To determine the various ways through which the properties of porous materials can be enhanced.
- To understand how amines can be used as grafting compounds on microporous material.
- To understand how CO2 can be captured by the use of a porous material.
Significance of the Study
While introducing the background of the problem, it was determined that porous materials have multiple applications in today’s world. Rackley (2009, p. 165) suggests that one of the various applications of porous materials is sorption. To this end, the study of a grafted microporous material will be important in explaining CO2 adsorption from the atmosphere or from a system (Wilcox 2012, p. 82). The discussion revolving around the grafting of amines on microporous materials helps to increase knowledge on how to improve the properties of porous materials.
At present, there exists a literature gap in the capture of carbon dioxide from the atmosphere and other sources (Archer 2010, p. 100). The discussions provided in the current study will provide new information (and improve existing knowledge) in the area of grafting and carbon dioxide capture. The discussion will be geared towards addressing the different ways to minimize CO2 and its greenhouse effect on the environment. Mills (2011, p. 88) argues that research on the sorption property of porous materials is essential in understanding the capture and storage of various gases. The current study affirms this importance.
A research undertaking is expected to respond to a number of fundamental issues that help to address its subject matter (Patton 2001, p. 23). That is the reason why researchers formulate research questions before embarking on their study. The questions are usually related to the research objectives. The reason is that by answering the questions, the researchers would have effectively addressed the objectives of the study. The following are the questions that the current study revolves around:
- What is a porous material and what are some of its uses?
- What are the various types of porous materials available?
- What is the difference between a microporous material and a mesoporous material?
- What is a covalent organic framework?
- What is a metal-organic framework?
- What is grafting?
- How are amines adapted to the grafting process?
A study can either be qualitative or quantitative. It can also be a combination of the two. The current study is qualitative in nature. Gorard (2013, p. 67) thinks this kind of research design is common in scientific studies. According to Gorard (2013, p. 32), a qualitative research undertaking is usually formal and objective. Separately, Saunders, Lewis and Thornhill (2009, p. 16) and Isadore and Benz (1998, p. 45) find this kind of study to be systematic. The information used is gathered sequentially. Notwithstanding its qualitative nature, the study does not involve experimentation. On the contrary, a descriptive orientation is adopted (Isadore & Benz 1998, p. 47).
The current study relies on secondary sources of information. According to Patton (2001, p. 106), scientific studies that take a descriptive approach rely on secondary sources of information in most cases. The secondary materials relied on containing information on the result of various experiments that have been conducted in the past. To this effect, the instrumentation aspect of the current study will be based on the various publications touching on the subject matter (amine grafting of porous material).
Presently, there exists an array of information touching on porous materials. However, the current study is guided by the need to determine how the structural properties of these materials can be improved. Thus, the research undertaking reported in this paper squarely relies on literature in which grafting of amines is carried out on microscopic material. In so doing, the research paper adheres to the element of conciseness as suggested by Saunders et al. (2009, p. 80). The conciseness of the study is further supported by the fact that the information collected contains verifiable results from the experiments carried out in the various microscopic materials.
Qualitative studies, such as the current one, are expected to be undertaken in a systematic manner (Isadore & Benz 1998, p. 53). As a result of this, this dissertation is structured in a manner that amplifies the literature review section. The idea is to gather as much data on microporous materials as possible. A large volume of information will likely justify the importance of amine grafting. A detailed analysis of the methodology used in this study is provided in the research and methodology section.
Assumptions made in the Study
Balnaves and Caputi (2001) are of the opinion that it is not possible to address all the aspects of a given research field in one research undertaking. That is the reason why researchers make assumptions in their study. To this end, it is assumed that variables that are not manipulated or addressed in the study will remain constant. Further, it is assumed that if the variables change, such fluctuations will not affect the results of the study. Such assumptions are common in quantitative studies, especially those involving experiments. However, they can also be made in quantitative studies, like the one reported in this thesis.
As already indicated, the current study is descriptive. The researcher made several assumptions (Kothari 2004, p. 34). The assumptions made are intended to justify some of the findings arrived at in the study. In point form, the assumptions made are as follows:
- It is assumed that the procedures used to collect data in the secondary sources are correct.
- The researcher assumes that grafting is the best form of improving the sorption of a porous material.
- It is further believed that mesoporous and microporous materials can be prepared through grafting.
- Finally, an assumption is made to the effect that it is possible to capture CO2 from the atmosphere or in a system with the help of the sorption property of a porous material.
Limitations, Delimitations, and Scope of the Study
Conducting research is not a simple undertaking. Arthur et al. (2012, p. 60) agree with this idea when they argue that there are various factors in given a study that is beyond the control of the researcher. However, the delimitations help to justify the research. The following are the limitations associated with the current study:
- The study relies on secondary sources of information even though primary sources would increase its credibility.
- The research on porosity is limited to microporous and mesoporous materials. The limitation is even though there are other porous materials that can help understand CO2 capture.
- Whereas primary sources of data (such as experiments) are ideal, the grafting procedure is standard. In addition, the materials required for the experiment are costly.
- Porous materials like nanoporous elements are rarely used in CO2 capture.
As mentioned in the background section, the current research undertaking primarily focuses on porous materials (Hibbeler 2013, p. 65). The materials examined have pores of varying diameters, ranging from 2 nanometers to 50 nanometers. Johns (2002, p. 55) points out that it is not possible to mention all the details of a given subject matter. That is one of the reasons why the study focused only on microporous materials and on mesoporous materials to some extent. The idea is to discover how grafting can improve the sorption property of these materials as far as carbon dioxide capture is concerned.
Definition of Terms
The study makes use of a number of terms that require some form of preliminary explanation. The following are some of the terms:
According to Ferdinand, Johnston, and DeWolf (2005, p. 67), this is a chemical process through which a compound is attached to another for the purposes of improving the latter’s properties. It is an important process in polymer science. Grafting is mainly used in the development of synthetic materials.
Eberle (2000, p. 47) observes that certain materials of a crystalline nature have ‘voids’ in their skeletal structures. The nature of such structures is referred to as porosity. Materials of this kind have a ‘sponge-like’ behavior. The behavior makes them appropriate for, among others, the sorption of other molecules.
In material science, synthesis is one of the many chemical processes that are used in the formation of complex materials. Such complex materials include polymers. Ebewele (2000, p. 73) defines this process as a chemical reaction. It is where chemical species are combined to yield an even larger species. Grafting can be regarded as a form of a synthesis reaction.
There are certain reactions, like in the case of synthesis, where different reactants are derived from different chemical species. In such cases, one element acts as a central point where the other species is attached. According to Painter and Coleman (2008, p. 61), the atom or molecule that binds an adjoining compound is known as a ligand.
There are cases where elements with different chemical properties react to give rise to a bigger compound. The resultant materials are described as being heterogeneous (Painter & Coleman 2008, p. 53). An example of such compounds is carbon dioxide molecules. The molecules have two carbon atoms and a single oxygen atom.
According to Painter and Coleman (2008, p. 54), a homogenous material is one whose constituent species are of the same chemical nature. An example of such a material is graphite. The compound is made up of carbon atoms only.
The discussion outlined in this chapter acts as the preliminary arguments of the entire study. The chapter introduced the entire element of porosity and, in extension, porous materials. In the background section, the researcher introduced some elements of metal-organic and covalent organic frameworks. The two frameworks are examples of porous materials. The purpose of the study was also outlined in this chapter. In this section, grafting was highlighted as one of the mechanisms used in improving the properties of a porous material.
The next chapter involves a detailed literature review. An in-depth analysis of grafting is provided in the section. The literature review makes the case for grafting by discussing the properties of amines as suitable compounds for the process. The major objective of this study is to highlight the importance of microporous materials that are grafted with amines. The ability of such materials to store gases like CO2 is also discussed. In the literature review, the knowledge gaps are identified and addressed. The same allows the study to build on the said knowledge.
The Chemistry of Grafting
The present-day trends in science and technology require an abundance of materials for the production of different commodities. However, there is an increasing decline in the availability of natural products. As a result of this, manufacturers are forced to shift to synthetic materials. In the current industrial atmosphere, polymers appear to be the most preferred form of synthetic materials (Bhattacharya and Misra 2004, p. 767). The said polymers are usually modified to suit their required specifications in a given industry. Bhattacharya and Misra (2004, p. 767) cite curing, blending, and grafting as the methods through which the said polymers are modified.
Porous materials have become popular in the fields of air purification, gas storage, gas separation, and catalytic reactions (Ekwere 2012, p. 16). The materials are preferred in these industries owing to their sponge-like nature. They operate the same way a sponge sucks water from a container. The same technique is used when porous materials purify the air or extract certain gases from a particular system.
According to Ekwere (2012, p. 16), the industrial application of porous materials relies on their structural adaptations. For instance, when a porous material is intended to capture CO2, its pores must be able to accommodate the molecules of this gas. In addition, the said material is expected to withstand environmental factors, such as temperature and pressure.
During the preparation of a particular polyisoalkane, Hong, Pakula and Matyjaszewski (2001, p. 3392) modified their polymer of choice through grafting. In their study, Hong et al. (2001) describe grafting as the bonding of monomers. The bonding is made possible by the monomer’s covalent bonds, giving rise to a large chain of polymers. A diagrammatic representation of grafting is illustrated in figure 1 below:
Techniques of Grafting
A summary of grafting
Over the years, researchers have constantly improved the various techniques used in the grafting process. The notion is advanced by Bhattacharya and Misra (2004, p. 767). Bhattacharya and Misra cite the case of grafting through chemical means, as well as photochemical forms of grafting. Further, the two scholars illustrate that grafting can be achieved through radiation, an enzymatic process. There are also instances where the procedure is realized through a process referred to as plasma radiation-induced grafting. An understanding of this study is simplified by a brief discussion about each of the aforementioned techniques.
Grafting through chemical means
In order to realize grafting through this mechanism, Fried (2003, p. 253) argues that there are two paths that can be used. One can settle for the ionic or the free radical paths. Regardless of one’s choice, the results are the same. The process leads to the formation of a large. For instance, when one wants to take the free radical path, a redox reaction is applied. The application gives rise to the said free radicals as shown in the equations below:
Fe2+ + H2O2 →Fe3+ + OH– + OH–
Fe2+ + –O3S-OO-SO3– → Fe3+SO2-4 + SO–4
From the two equations above, it is evident that the iron ion-induced the decomposition of hydrogen peroxide, as well as that of potassium-persulphate. Consequently, the free radicals that emerge are OH– and SO–4. The free radicals are later attached to a larger compound that is electrophilic in nature (Garcia, Pinto & Soares 2002, p. 759). An illustration of grafting in the case of Bhattacharya and Misra (2004, p. 767) is realized when the free radicals attach themselves to a polymer as illustrated below:
SO–4 + Xpolymer –OH →HSO4 + Xpolymer –O
Similarly, when it comes to the ionic process of grafting, there is an initiator as is the case in the free-radical process. Bhattacharya, Rawlins, and Ray (2008, p. 142) illustrate various forms of grafting using this method. The compounds that act as initiators in this process are sodium naphthalene and a suitable organometallic compound. Bhattacharya et al. (2008, p. 142) highlight the utilization of the ionic process of grafting to accomplish a polymerization procedure.
With regards to the ionic process, it is important to note that grafting can be attained in two ways. There is the cationic mechanism, where positively charged ions are involved (Fried 2003, p. 331). The other mechanism is anionic grafting. In this case, negatively charged ions are involved. An example to distinguish between the two mechanisms is highlighted in the following equations (cited in Bhattacharya & Misra 2004, p. 779):
ACl + X3Al → A+X3AlCl–
A+ + Mpolymer →AM+ – M → GRAFTED CO-POLYMER
P-OH + NaOX → PO–Na+ + XOH
PO– + Mpolymer → POM—M →GRAFTED CO-POLYMER
Fried (2003, p. 367) suggests that the mechanism is similar to all compounds that have the capability to be grafted onto others. Such compounds as amines are treated the same way as free radicals or ions. The objective is to ensure that there is a larger compound resulting from the process. Examples of grafting of amines into porous materials are discussed in detail in the sections that follow.
Factors Contributing to Grafting
Like any other chemical process, grafting is affected by several contributing factors. In an experiment to illustrate grafting through the polymerization of a free radical, Garcia et al. (2002) cite the nature of the backbone monomer as a contributing factor. In addition, Fried (2003, p. 400) illustrates that factors like the initiator and temperature play a huge role in determining the success of grafting. Other factors include the solvent and the monomer used.
Bhattacharya and Misra (2004, p. 789) point out that grafting is largely a kinetic process. Consequently, temperature plays a significant role in the entire process. Bhattacharya and Misra (2004) argue that an increase in temperature affects the yield of a particular graft. The implication is that grafting is an endothermic process. Thus, the higher the temperature, the higher the volume of the grafted compound produced. The grafting of a compound like an amine into a compound that is microporous in nature will occur at high temperatures.
Overview of Amines
Lide (2005, p. 56) and Lawrence (2004, p. 67) describe amine as a functional organic group. The most outstanding feature of this functional group is the presence of a single nitrogen atom. The atom is characterized by a lone pair of electrons. An amine is structured in such a way that it can be split either as a free radical or as an ion. Such splitting allows it to be grafted into another compound. Figure 2 is an illustration of an amine:
The figure above illustrates amines in comparison to the common solvents used in a laboratory. According to Timberlake (2011, p. 344), these compounds are classified depending on the number of carbon atoms that have a direct bond with the nitrogen atom. The splitting of an amine to expose its functional group largely depends on its classification.
Properties of Amines
According to Bruckner (2001, p. 127), amines, like any other organic compound, are characterized by several features. The properties associated with these compounds allow them to take part in certain processes like grafting. Bruckner (2001, p. 127) suggests that the compound can be utilized in grafting owing to its hydrogen bonding properties. The said bonding structure implies that the compound has a high boiling point.
Das et al. (2013, p. 79) found out that the process, as already indicated above, is highly endothermic. As a result, the high boiling point associated with the amine compound makes it a suitable component for grafting. Another property that is characteristic of amines is their solubility in water. Das et al. (2013, p. 80) point out that solubility in a solvent like water is an important factor in the grafting process. A compound that has poor solubility in a solvent that is the initiator has little chance of completing the grafting process (Bruckner 2001, p. 129).
One of the most important properties of an amine is its chirality (Lee et al. 2013, p. 983). Depending on the class of amine, the resulting stereoisomer facilitates the grafting process. Lee et al. (2013, p. 983) point out that amines have an electronic property. The same comes in handy in the event that the ionic path is taken for the grafting process. According to Bruckner (2001, p. 130), the compounds exhibit this electronic nature owing to their basicity. The same help in the salvation of the compound.
Grafting of Amines
As aforementioned, grafting is a chemical modification carried out on certain materials to improve some of their desirable properties (Martinez-Hernandez, Velasco-Santos & Castano 2010, p. 14). In a study to determine the adsorption of CO2, Lee et al. (2013, p. 982) cite the usage of compounds like carbon nanotubes and other organic materials. Carbon nanotubes have certain properties that inhibit the compounds’ applicability in certain engineering processes. The shortcomings are what necessitate the need for grafting.
Lawrence (2004, p. 201) points out that carbon nanotubes have properties that make them most suitable to act as filler materials in certain polymer composites. Lawrence (2004) further illustrates that the efficiency of the said composite relies on the dispersibility of each of the carbon nanotubes. Unfortunately, carbon nanotubes form agglomerates in their sidewalls. The agglomerates inhibit the required dispersion. To this end, Lawrence (2004, p. 201) suggests that grafting would greatly enhance dispersion in the carbon nanotubes.
Ruelle et al. (2012, p. 296) hold the opinion that a functional group like an amine is best suited to grafting into the carbon nanotubes. Ruelle et al. (2012, p. 296) argue that the grafting process in such a situation is realized covalently. In addition, Ruelle and colleagues illustrate that grafting on the carbon nanotubes is attainable in two processes. For example, one can graft a functional group into the defects of the carbon nanotube’s lattices. Alternatively, the compound can be grafted into the compounds’ π-conjugated skeleton. In both cases, the desired dispersion is achieved through grafting
Ruelle et al. (2012, p. 296) are of the view that the choice of amines for grafting into carbon nanotubes gives the latter a low level of reactivity. The high reactive exhibited by amines make them the ideal compounds for this process. However, other compounds like carbenes and nitrenes play a similar or closely related role. As pointed out by Das et al. (2013, p. 80), amines are highly soluble in organic solvents. A compound with such a property is very suitable for grafting carbon nanotubes. The reason for this is that they are similarly highly soluble in organic solvents.
In their study, Ruelle et al. (2012, p. 296) describe the grafting process using amines. Their objective is realized with the help of the free radical path described earlier. The carbon nanotubes are subjected to a micro-plasma post-discharge treatment. According to Ruelle et al. (2012, p. 296), the said treatment is very important to the process. The process is significant given that it ensures maximum incorporation of the amines into the surface of the nanotubes. The researchers (Ruelle and colleagues) rely on primary amines in their research. The selectivity process ensures that the content of the said amines is high. The procedure involves the reduction of O2 contamination levels and increasing the concentration of hydrogen compounds. Such treatment makes sure that the process attains high levels of selectivity.
Grafting is termed as successful or unsuccessful depending on specified outcomes of the procedure. For example, it is regarded as successful in the event that the compound grafted into another remains stable during the life of the host compound (Lawrence 2004, p 190). In their study, Ruelle et al. (2012, p. 305) pay attention to the stability of the primary amines grafted into the carbon nanotubes. The success of their study relies on the stability of the amines. To this end, regardless of oxidation levels, the compounds are not converted to amides.
Ruelle et al. (2012, p. 305) illustrate how the aging of the amines occurs. Several assumptions are made to contextualize this aging process. For example, it is assumed that the density of the amines is reduced by up to 50% of its original value. The most probable cause of this change, as aforementioned, is oxidation. In the study conducted by Ruelle et al. (2012), oxidation is realized during the selectivity phase. In this phase, the nanotubes are subjected to plasma treatment. In such a scenario, the amines are converted to amides. The conversion reduces the stability of these compounds.
The said aging hurts the grafting process. It follows that whenever primary amines are required for grafting, one should ensure that they can withstand the aging process. Ruelle et al. (2012, p. 305) sought to determine the stability of their primary amines before grafting. They achieved this by subjecting the pellets of carbon nanotubes to the aforementioned treatment.
The results indicated that the primary amines selected overcame the aging due to the oxidation involved (Ruelle et al. 2012, p. 305). The implication here is that amines have the ability to improve the properties of porous materials. The said effect proves useful in understanding the numerous applications of microporous and mesoporous materials.
Effects of Amines on Adsorption in Microporous Materials
As already indicated, the permeability property of porous materials allows them to perform certain functions, such as the adsorption of gases. According to Lide (2005, p. 53), microporous elements are mostly used to capture and store various types of gases. The gases include hydrogen and carbon dioxide. The adsorption is largely dependent on the affinities of the materials’ pores to the respective gases. As a result of this, porous materials can improve their gas affinity by modifying their structures through grafting (Lawrence 2004, p. 200).
Lee et al. (2013, p. 981) conducted a study to determine the effects of grafting on a mesoporous material with regards to the adsorption of CO2. The mesoporous material in their experiment was SBA-15. Lee et al. (2013) prepared the material using ethanol extraction. The intention was to synthesize a more efficient CO2 adsorbent compound by grafting an amine into the said mesoporous substance.
From the experiment carried out by Lee et al. (2013, p. 985), it was determined that grafting of amines into SBA-15 increased the thickness of their walls. Due to this thickening, the material was able to withstand the effects of heat treatment. In their research, Lee et al. (2013) found out that the adsorption nature of porous materials is enhanced by the density of the amine used. It is also affected by other physical properties. Furthermore, the study revealed that the surface area of an amine contributes to its adsorption rate.
Fried (2003, p. 172) holds that most mesoporous materials used in modern times are artificial. As such, synthesis reactions inform their preparation. Lee et al. (2013, p. 981) rely on this knowledge in the preparation of their preferred porous material, SBA-15. In a similar study to determine the process of CO2 capture, Das et al. (2013, p. 76) illustrate that most materials lose their adsorptive abilities due to heat treatment. However, the ability of amines to withstand the said treatment is the main reason why adsorption is increased after the compound is grafted into a porous material.
The importance of grafting is illustrated by the ability of an amine grafted mesoporous material to withstand heat treatment (Lee et al. 2013, p. 998). Chemical procedures like carbon capture are increasingly becoming important in the energy sector. The extraction of CO2 from petroleum is an expensive affair. To address this issue, it is important to conduct more research on how to improve the grafting process on similar porous materials to increase their carbon dioxide capture capabilities
Grafting into Microporous Materials
In their research, Kishida et al. (2011, p. 1) rely on porous materials whose pores are less than 2nm in diameter. Kishida et al. (2011) use the materials to determine their adsorptive ability with regards to hydrogen phosphide. To this effect, any material with the said specifications can be regarded as a microporous compound. Microporous materials have a wide range of applications in laboratories and other industries. For instance, the materials can be used to filter out contaminants from gas mixtures.
When a microporous material is observed through a microscope, the expectation is that pores will be visible (Kishida et al. 2011, p. 1). In nature, several naturally occurring elements make up microporous material. One such example is the group of rocks referred to as zeolites. An illustration of a microporous structure is provided in figure 3 below:
The spaces illustrated in the Zeolite above represent the said pores. Nugent et al. (2013) argue that the pores are large enough to accommodate such chemical species as ions. Na2+ and Mg2+ are examples of cations that can fit into the pores. Due to the said pores, grafting can easily be carried on these microporous materials.
Galo, Sanchez, Lebeau, and Patarin (2002, p. 4095) observe that microporous materials can be synthesized to improve their properties. Grafting is one of the processes that can be applied in the synthesis of microporous materials like zeolites. The objective of grafting in such cases is to strengthen the walls of the pores. Galo et al. (2002, p. 4095) suggest that when a microporous material is to be used as an air filter, grafting of the same is advised.
Roswell and Yaghi (2004, p. 4) provide a working definition of a mesoporous. They describe it as a porous material whose pores have a diameter that is greater than 2nm but less than 50nm. Most silicas and aluminas are examples of mesoporous compounds. The applications of these materials range from ion exchange to catalysis, which is an important chemical process. In addition, mesoporous materials are used in such processes as sorption (Harlick & Sayari 2007, p. 446).
Harlick and Sayari (2007, p. 446) point out the large surface area that is characteristic of mesoporous substances increases their application in catalytic processes. One area where the materials are used involves obtaining hydrogen from water. The process is catalytic in nature. Currently, platinum is the preferred catalyst in the separation process (Hibbeler 2013, p. 28). The high cost of platinum implies that the hydrogen energy platform remains costly. Mesoporous catalysts would significantly reduce the cost and make the procedure a feasible endeavor.
Harlick and Sayari (2007, p. 446) demonstrate the application of mesoporous materials in sorption. Harlick and Sayari (2007) investigate how mesoporous silica grafted with an amine can be used to absorb CO2 from N2. In their mission, the two analyze MCM-41 as the mesoporous silica. They improve the said silica’s properties by coating it with 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysaline. The amine used in the coating phase is grafted into the mesoporous silica. The grafting strengthens the walls of the compound.
Introducing Metal-Organic Frameworks
There are instances where the porosity of a material is attained by a heterogeneous composition of its constituent compounds. In such cases, the material is said to have attained its porosity through a metal-organic framework (herein referred to as MOF). Batten et al. (2013, p. 1719) define this framework as a compound made up of metal ions. The ions are connected to organic molecules. According to Batten et al. (2013), MOF gives rise to porous structures.
The applicability of these materials ranges with the size of the pores in the MOFs. For instance, Batten et al. (2013, p. 1719) point out that MOFs can be used in the storage of such compounds like hydrogen and carbon dioxide. Batten et al. (2013) argue that the said application is attainable due to the stability of the MOF pores when it comes to the elimination of foreign molecules. Other applications of such materials are in the area of gas purification and catalysis.
Batten et al. (2013, p. 1719) point out that MOFs are characterized using x-ray diffraction. When diffraction is carried out on the material, the crystalline nature of the material is made evident. The sorption property of MOFs is realized through a process referred to as neutron scattering. If a metal-organic framework is unable to absorb certain gases, the implication is that its porous nature is impaired.
Sorption Property of a Metal-Organic Framework
Czaja, Trukhan, and Muller (2009, p. 1284) think that the applicability of metal-organic frameworks relies on their structure. According to Czaja et al. (2009), the structure of MOF is dependent on two components. The two are the linker and the cluster. The former is what is defined as the organic compound. On its part, the latter is the metallic ion. The structure of a MOF is determined by the type of clusters and ligands (plural for linker). Czaja et al. (2009, p. 1284) point out that the layout of MOF with regards to shape and size is influenced by the number of ligands coordinating with the metal ions.
According to Czaja et al. (2009, p. 1284), there is a need to come up with more materials to address the rising need for synthetic compounds. The heterogeneous nature of MOFs determines their application in areas like gas storage. In the opinion of Cheetham, Rao, and Feller (2006, p. 4780), MOFs are capable of storing gases separated from a given system. In such cases, a metal-organic framework is structured in a manner that is consistent with the size of the gas to be captured from the system.
Rosi et al. (2003, p. 1127) suggest that hydrogen is an important component when it comes to alternative energy. However, the exploitation of this gas is limited owing to the challenges associated with its transportation. The volatility of the substance inhibits its movement from one point to the other. Rosi et al. (2003, p. 1127) suggest that a metal-organic framework can be synthesized to address this challenge. The framework ensures that the pores can contain the hydrogen molecules in a sponge-like mechanism.
The sponge-like mechanism outlined above is the principle behind the storage of CO2. Debatin et al. (2012, p.10221) developed a metal-organic framework that was synthesized using an amine for the purposes of CO2 adsorption. The framework was developed from a mixture of gases. Debatin et al. (2012) explain the mechanism through which the gas molecules can be separated from a system or from the environment. The mechanism is vividly illustrated in figure 4 below:
Debatin et al. (2012, p. 10221) developed a series of metal-organic frameworks. They called them the isoreticular metal-organic frameworks (IFPs). The illustration in the figure above indicates that the said IFPs are synthesized using a functional group possessing amine qualities. Debatin et al. (2012) used 2-substituted imidazolate-4-amide-5-imitates as the functional group to illustrate the grafting procedure. The resultant IFP is a microporous material suitable for the adsorption of CO2.
The adsorption of CO2 in the study conducted by Debatin et al. (2012, p. 10225) is analyzed about the ability of the IFPs to retain methane (CH4). Results from their experiment suggest that CO2 adsorption was higher than the adsorption of CH4. The simulation was conducted at different temperatures and pressures. The results of this experiment have major implications on studies revolving around climate change and global warming. Debatin et al. (2012, p. 10226) suggest that MOFs are the best microporous materials to be used in CO2 adsorption.
Drisdell et al. (2013, p. 18183) argue that the ability of a microporous material to capture CO2 lies in its adsorption capabilities. The study by Nugent et al. (2013, p. 80) supports the hypothesis by Drisdell et al. (2013, p. 18183). In their research, Nugent et al. (2013) highlight the separation of CO2 from a system. The benefit of such a process is realized when carbon dioxide acts as an impurity in petroleum. The need to capture and isolate the impurities requires a material with a high affinity for CO2 adsorption.
According to Millward and Yaghi (2005, p. 17998), the adsorptive affinity of a microporous material is increased when it is grafted with a suitable functional group. Millward and Yaghi (2005) are of the opinion that amines increase the property to absorb CO2 in most metal-organic frameworks. The data obtained from their study indicates that amine, as a functional group, has the ability to increase CO2 affinity in the pore of a metal-organic framework.
Covalent Organic Frameworks
Overview of Covalent Organic Frameworks
In the previous chapter, it was determined that covalent organic frameworks (COF) are examples of porous materials. Feng, Ding, and Jiang (2012, p. 6010) suggest that materials of this nature are known to take both a one-dimensional and a multidimensional structural orientation. Feng et al. (2012) argue that the structural orientation of a covalent organic framework is based on the orientation of the ligands (see figure 5).
Figure 5 above illustrates how a covalent organic compound is prepared. A look at the structural layout of the compound suggests a lot of stability on its part. For example, the pores are regular in shape. The rigid structure of COFs is one of the reasons why Feng et al. (2012, p. 6010) support its use in the storage of gases.
Feng et al. (2012, p. 6010) carried out a study to analyze covalent organic frameworks. The uniqueness of their study saw them perform a synthesis reaction to give rise to a 3-dimensional COF (3D COF). Their experiment highlighted an interesting feature of COFs. They found that the covalent nature of these compounds allows them to form strong bonds. Consequently, they are able to withstand high temperatures (Hwang et al. 2008).
Another physical property discovered was in relation to the surface area of the 3D COFs. Feng et al. (2012, p. 6018) hold that covalent organic frameworks have a uniquely large surface area. According to Han, Hurukawa, Yaghi and Goddard (2008, p. 11580), the large surface area of this kind of porous material is important in catalysis. It follows that COFs are good catalysts and can be used in place of relatively expensive catalysts like platinum. However, Feng et al. (2012, p. 6018) indicate that the materials have a relatively low density.
In a study to determine the storage capabilities of COFs, Han et al. (2008, p. 11580) suggest that the strong bonds make them suitable for the storage of gaseous molecules like hydrogen. Similar sentiments are shared by Mendoza-Cortes, Pascal and Goddard (2011, p. 13857). Such an application is relevant in cases where materials essential for the capture and storage of gas molecules are concerned. Nonetheless, Mendoza-Cortes et al. (2011, p. 13856) suggest that the gas affinity of such molecules can be increased through grafting.
According to Mendoza-Cortes et al. (2011), grafting improves the functionality of various materials. Mendoza-Cortes et al. (2011) point out that there is another process of grafting, which they term doping. When the scholars introduced Na-, K-, and Li- as the doping agents, a unique effect was noted. They found that there was a tremendous increase in COF’s absorption of hydrogen.
A significant gap in the literature exists with regards to CO2 capture and COFs. Mendoza-Cortes et al. (2011, p. 13852) and Cote et al. (2005, p. 1166) are of the view that covalent organic frameworks are ideal materials for the capture of gases. The cost of producing these materials is relatively low. As such, they are viable alternatives for metal-organic frameworks that require grafting for stability (Sperling 2005).
Synthesis of Covalent Organic Frameworks
According to Mendoza-Cortes et al. (2011), COFs are similar to other artificial materials. As such, they can be prepared using a synthesis reaction. Fried (2003, p. 267) cites three types of synthesis reactions that are used in the preparation of these compounds. The first procedure is referred to as boron condensation. Mendoza-Cortes, Han, and Goddard (2012, p. 1621) cite this particular synthesis as the most popularly used.
The reaction takes place between 3 acids that are boron in nature. The objective of this particular synthesis is to ensure that the water molecules in the acids are completely eliminated. The resultant molecule is referred to as a boroxine ring (Fried 2003, p. 117). An example of boron condensation is provided by Cote et al. (2005, p. 1166). Cote et al. (2011) carried out a study to investigate the porosity of COFs. They hypothesized that the materials were crystalline in nature. In the study, phenyl diboronic acid and hexahydroxytriphenylene were used for the synthesis.
Lawrence (2004, p. 79) indicates that COFs can be synthesized using triazine materials. The porous materials produced through this synthesis have a high surface area. A majority of the COFs developed for catalytic processes are prepared using triazine-based trimerization. The best example of a covalent organic framework produced using this process is COF-1(Lawrence 2004, p. 79).
The discussion provided in this section was built on the introduction chapter. To this end, various aspects of the study mentioned in the introduction chapter were elaborated on in chapter two. The literature review determined that amines are key ingredients in the grafting process. Given that this study revolved around these materials, an analysis of their properties was provided in this section. In addition, issues touching on grafting were discussed. The chapter was based on information from existing literature touching on the subject matter. At the same time, the various knowledge gaps in the field were highlighted.
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