Chemistry

2.1.1. Nanocarbons:

Nanocarbons materials (NCs) represents a massive nanomaterial with different nanoscale dimension [20]. Nanocarbons refer to a broad range of carbon materials with nanoscale dimension, structures, and textures [1]. Nanocarbons known for their physiochemical properties and can be synthesis via bottom up (i.e. templated deposition) or top down (i.e. structural controlled carbonization of precursor). Generally, nanocarbons have phenomenal properties either the internal and inherent properties which are related to the surface chemistry and electronic structure or the external one that is the morphology and specific surface area. To improve the chemical stabilities, reusability and increase the activity can be obtained via the decorating of the nanocarbon either with metal nanoparticle MNPs or metal oxide MO. NCs have been used in different applications such as energy storage, photocatalysis, nanomedicine and industrial catalysis (). These carbon nanostructures (CNSs) can be defines as the allotropes of carbon with a graphitic structure of sp2 graphitized carbon atoms. They vary in shapes and sizes. The morphological diversity of these graphitic structure has led to a strong drive to discover more chemical and physical properties [21]. Due to this, there has been growth of interest in materials science and engineering communities. In the last twenty years, CNSs that would include graphene, fullerene and carbon nanotubes (CNTs) were researched ().

The catalytic composites of the low dimensional nanocarbons such as CNT, graphene, ordered mesoporous carbon (OMC) and carbon nanofibers can be representive; see Figure 2.2. We focus on the mechanistic aspects of catalytic composites of several representative low-dimensional nanocarbons used for catalytic composites, including CNT, graphene, OMC, and carbon nanofibers (CNFs)

Figure 2.1. Scheme represents catalytic nanocarbon composites.

2.1.2. Catalytic nanocarbon composites

Carbon nanotube:

Carbon nanotubes were originally discovered by Iijima in 1991(). CNTs have cylindrical structure with a nanoscale diameter and consist of rolled up graphene sheets. Their characteristic one-dimensioned morphology gives rise to unique thermal, electric and mechanic properties (). This structure of walls has the same honeycomb lattice structure of sp2 hybridized carbon as graphene. Moreover, CNT have many advanced structure and properties that are fascinating and are discussed in a wide range of different research topics. The superiority of the electronic can be optical, mechanical and even thermal. On the basis of tube layers, CNTs can be divided into distinct categories: SWNTs, single-walled carbon nanotubes, and MWNTs, multi-walled carbon nanotubes, see Figure 2.3 [25-38]. CNTs employed to different catalytic reactions such as Fischer-Tropsch, hydrogenation reaction, hydrogen evolution reaction and electrocatalysis. These phenomenon properties of CNTs cause an improvement on the synergistic performance as they enable the CNT with highly conductive surface for stronger interfacial interaction between the CNT and the catalyst and improve catalyst dispersion, etc. These CNTs can be stabilize electronically the intrinsic transition state of semiconducting nanoparticles, influence activation energy and extend the absorption wavelength of the catalyst. The catalyst can be immobilized on CNT via different methods such as impegration followed by reduction/oxidation, evaporation deposition and electrochemical deposition [39].

Figure2.2. CNTs structures: single-walled carbon nanotube (SWNT) and multi-walled carbon nanotube (MWNT) [40].

Graphene:

Graphene is known for being single atom their Sp2 hybridized carbon atoms, where the graphene in the form of a hexagonal ring structure with the length of 0.124 nm. Graphite is known by 3-dimensional (3D) layered crystal lattice structure which consist of 2-dimensional (2D) graphene sheets as shown in Figure 2.4 [41]. These graphite sheets are stacked together due to van der waals forces with a calculated distance equal to 0.335 nm for each other. Graphene well known also as the main material for the different kinds of graphitic forms including carbon nanotube CNT and fullerenes, see Figure 2.5 [42].

Figure 2.3. Basis of all graphitic forms. Graphene is a 2D building material for other dimensionalities of carbon materials.

Figure 2.4 Layered structure of graphite showing the Sp2 hybridized carbon

atoms tightly bonded in hexagonal rings.

Graphitic Nanofibres (GNF)

Graphitic nanofibers (GNFs) are a CNS with a unique morphology. They are known for their cylindrical structure. The internal surface has step edges with the size of 3-4 nm which can work as a support position for guest species. The fibres are of stack orientation and are made of graphite planes. Step edges provide large inner hollow cavity (50 – 70 nm) that permit for the transfer of different molecules which have the GNFs unique and interesting for heterogeneous catalytic reactions, see Figure2.6 [43, 44].

Figure 2.6 GNF and their unique step-edges [45].

Inorganic Aerogels

Inorganic aerogels are one of the most exciting materials of the 21st century, based on their remarkable properties such as extremely high porosity and low densities, excessive dialectical strengths and large active surface areas [46, 47]. Due to these properties, inorganic aerogels have been used in numerous applications such as aerospace, energy generation and storage, biomedical devices and implants, sensors and coating [48]. The first inorganic aerogel was reported by Kistler who introduced silica-based aerogels. Since then, aerogels have been produced from many other inorganic materials, such as metal oxides, chalcogenides, and carbons. More recently, aerogels have also been fabricated from carbon nanostructures such as carbon nanotubes and graphene, see Figure2.7 [49, 50].

Figure2.7 Types and evolution timeline of aerogels [51].

Aerogels can be very generally classified into two categories: single-component aerogels and aerogel composites as shown in Figure2.8 [52]. This project will introduce a new class of single-component aerogels, specifically GNF aerogels, and study their synthesis and properties. Subsequently the synthesis of new aerogel composites will be explored, namely metal-NP/GNF aerogels, followed by the study of important structure-function relationships in context of heterogeneous catalysis.

In general, aerogel can be prepared using sol gel method, either using supercritical fluid drying or freeze drying

3 Introduction

Over the last decade significant advancement in noble metal nanoparticles supported carbon-based materials for applications in catalysis has been made. This is a result of important progress in synthesis, characterization and materials leading to better understanding of the fundamental properties of these materials allowing transfer into industrial applications.

3.1 Supported MNPs for heterogeneous catalysis

Noble metal nanoparticles (MNPs) such as Ru, Rh, Pd, Ag, Au, and Pt are known to exhibit exciting chemical and optical properties [51-53]. As MNPs have low coordinate metal atoms on their surface area and a colossal surface to volume ratio they typically display high catalytic performance. However, since they are intrinsically metastable, they tend to leach or irreversibly aggregate atoms of metal under standard chemical process conditions. Therefore, MNPs/CNTs nanohybrids have an interesting application in numerous sectors, see Figure3.1. They have limited practical application in catalysis reactions, as process conditions result in rapid loss of intrinsic catalytic performance due to aggregation or agglomeration. Support materials, however, can be utilized to stabilise them, for example nanocarbon materials [54].

Figure3.1 Brief summary of preparation and application of noble metal NPs/CNTs nanohybrids. (Reproduced from Wu et al. [54] with permission from Elsevier Science.)

Graphene have phenomenal properties due to its thermal, mechanical stability and its extreme mobility with an ideal atomic lactic. Graphene is also ideal for supported system in heterogeneous catalysis reactions due to the synergy between its sp2 hybridised surface and organic molecules [55].

Active catalysts have been reported in which graphene-based nanomaterials act as stabilising supports, reducing the size the particulates and simultaneously enhancing both the stability and solubility [56-57]. Wang, P. et al. [58] reported enhanced catalytic activity in the Heck carbon-carbon coupling reaction using palladium supported graphene. The material was prepared by first intercalation of M+ ions between layers of a graphene oxide support, prepared using the Hummer’s method [59]. Chemical reduction was then used for Pd NPs in the graphene oxide using H2O and hydrazine hydrate. The catalytic activity of the resultant materials was investigated, and the performance compared to that of various supported palladium-based catalysts in the coupling reaction of iodo-benzene and styrene, see Table3.1.

Table3.1 Performance of numerous supported Pd based catalysts in coupling reaction of iodo-benzene and styrene.

a The temperature was obtained by microwave irradiation.

In a separate study B. Cornelio, et al. [65] investigated how the preparation method as described by Zhi et al. [66] and the structure of a range of catalytic nanoreactors, based on nanoparticles located in hollow encapsulated graphitized nanofibers, effected the catalytic performance. In this study, the optimum catalytic material was observed to have small (2-5 nm) palladium nanoparticles located at the graphitic step-edges within the GNFs, see Table3.2, scheme 3.1 and scheme 3.2. The materials exhibited impressive catalytic properties in Suzuki Miyaura cross coupling reactions. The results reveal that the addition of the oxidative agents to the Pd NPs in the Suzuki Miyaura reaction happens at the step-edge of the nanofibers with the confinement at the nanoscale playing a sensitive role in the mechanism and kinetics of the reactions. Hence, the extent of the confinement imposed by the carbon nanoreactors can be controlled through careful selection of the aryl iodide reactant. To explore this the Suzuki Miyaura reaction was performed with variety of confinement in the cross-coupling reaction, see Table3 and scheme 3.1 and scheme 3.2.

Schem3.1 The palladium catalysed reactions of aryl iodides 1 and phenylboronic acids 2 leading to the products of Suzuki Miyaura cross coupling 3, dehalogenation 4 and Ullmann coupling 5.

Scheme3.2 The competitive Suzuki Miyaura reactions of (a) two aromatic boronic acids with a common aryl iodide and (b) two aryl iodides with a common aromatic boronic acid.

Table3.2 Comparison of the selectivity and activity of catalytic nanoreactors in the Suzuki Miyaura cross coupling of 1-iodo-4-nitrobenzene 1a and phenylboronic acid 2aa.

a Standard conditions: 1-iodo-4-nitrobenzene 1a (0.056 mmol), phenylboronic acid 2a (0.075 mmol), sodium acetate (0.13 mmol), catalyst (2 mol%), methanol (5 mL), 700C, 24 h.

b Conversion and selectivity for 4-nitrobiphenyl (3aa), nitrobenzene (4) and 4,4̀-dinitrobiphenyl (5a), the products of cross-coupling, dehalogenation and Ullmann coupling respectively, were determined by 1H NMR spectroscopy of the crude mixture.

Table3.3 Comparison of the activity and selectivity of PdNP@GNF and PdNP/GNF catalysts in the Suzuki Miyaura cross-coupling reaction of aryl iodides 1a–e and phenylboronic acids 2a–ea.

a Standard conditions: aryl iodides 1a–e (0.041–0.056 mmoll) boronic acids 2a–e (0.053–0.075 mmol), sodium acetate (0.094–0.13 mmol), catalyst (2 mol%), methanol (5 mL), 70 ͦ C, 24 h. b Conversion and selectivity for biphenyls 3aa–eb, nitrobenzene 4 and 4,4̀- and 3,3̀-dinitrobiphenyls 5a and 5b, respectively, the products of cross-coupling, dehalogenation and llmann coupling respectively were determined by 1HNMR spectroscopy of the crude mixture.

Table3.4 Catalytic activity of Pd and bimetallic Pd–Ni NPs in the Suzuki Miyaura reaction using phenylboronic acid in water[a] showing better results with MWNT supported Pd NPs coated with PVP.

[a] Reaction conditions: 4-bromoanisole (0.5 mmol), phenylboronic acid (0.75 mmol), catalyst (0.1 mol %Pd), K2CO3 (1.5 mmol) in H2O (0.3 mL).[b] Determined by GC analysis based on 4-bromoanisole consumption. [c] In 0.5 mL of H2O.

Table3.5 Reaction data for the Suzuki Miyaura reaction with phenylboronic acid in water[a] catalysed by various Pd and Ni catalyst showing optimization of the reaction conditions by using the bimetallic Pd30Ni70/MWNTs as a precatalyst.

[a] Reaction conditions: 4-bromoanisole (0.5 mmol), phenylboronic acid (0.75 mmol), catalyst, base (1.5 mmol) in H2O (0.3 mL). [b] Determined by GC analysis based on 4-bromoanisole consumption. [c] PEG300/H2O (1:5) in 0.5 mL volume was used as a solvent. [d] 1.2 equiv. of phenylboronic acid were used. [e] TBAB (0.5 mmol).

Table3.6 Reaction conditions studies for the Hiyama reaction using Pd NPs/MWNTs

in water[a] showing variation of the yield due the amount of the Ph Si(OMe)3.

[a] Reaction conditions: 4-iodoanisole (0.5 mmol), trimethoxyphenylsilane, catalyst

(0.1 mol% Pd),50% aqueousNaOH (2.5 mmol). b] Determined by GC analysis based on

4-iodoanisole consumption.

Table3.7 Reaction conditions studies for the Sonogashira Hagihara reaction in water[a] showing variation of the yield due the amount of the phenyl acetylene.

[a] Reaction conditions: 4-iodoanisole (0.5 mmol), phenyl acetylene (0.6 mmol), catalyst (0.1 mol% Pd), pyrrolidine (1 mmol). [b] Determined by GC analysis based on 4-iodoanisole consumption. [c] PEG300/H2O (1:1) in 0.5 mL volume was used as a solvent. [d] 1.5 equiv. of phenyl acetylene were used. [e] 2 equiv. of phenyl acetylene were used.

The results show palladium particles were recorded to yield maximum activity and show stability, with high recyclability while retaining high activity from cycle to cycle. Further, a microscopic investigation of the nanoscale morphology discovered an increase in NPs size after catalysis. However, the extent of growth is highly dependent on the type of nanocarbon support. In summation, the overall performance found the process to be functional in term of catalytic properties and stable in several cross-coupling reactions.

Aygün M. et al, [68] investigated the catalytic performance of Pt and Pd NPs encapsulated in magnetically fabricated GNF and tested the reduction of nitrobenzene with both materials that exhibited high activity and selectivity, particularly PdNPs@GNF. Magnetic nanoreactor allows the confinement of the activity and recyclability. where, see Table3.8. The reduction of nitrobenzene has been done in the presence of catalysts that are of high glass pressure and molecular H2. It has to be quantified by 1H NMR. PdNP@GNF becomes the lower catalytic activity that cannot be compared to PdNPs@GNF-2 which is considered to be in large NPs size. As a result, this brings a surface area to the Pd NPs in PdNPs@GNF-1. On the other hand, there are no spotted reactions on the smaller PtNPs in PtNPs@GNF-2, this was a different case in the larger PtNPs@GNF-1, in this case the observation confirmed that it was likely to be active. The lack of reactivity was due to the fact that small Pt NPs in PtNPs@GNF-2 needed more investigation, as a result we thought out that the small nanoparticles would probably be amorphous to some point and so they had a poorly defined site for catalysis. On the other hand, they could be having residual dibenzylideneacetone (dba) ligands blocking the surface.

Table3.8 Reaction data for the reduction of nitrobenzene using PdNP@ GNF and PtNP@GNF catalytic nanoreactors using a high pressure H2 glass reactor showing the activity and selectivity of nitrobenzene due the NPs size.

Reaction conditions: Nitrobenzene (0.78 mmol); ethanol (0.5 mL); catalyst (0.00047 mmol of metal); H2 (8 bar); room temperature. All reactions were performed in duplicate and nitrobenzene conversion was determined by 1H NMR with an error of ±2%. a) GNF were annealed at 450 °C for 1 h prior to use; b) PtNPs@GNF-2 was annealed under H2 flow for 5 h at 150 °C prior to the reaction to get rid of any impurities on the surface of Pt which can cause deactivation of the catalyst.

Finally, Liu S. et al, [69] reported that single-wall carbon nanotubes (SWNTs) provide support to palladium catalysts that have been prepared through various methods. The influence in these preparatory methods on the structure and performances of the catalysts can be seen in Table3.9. Where (-1, -2,-3,-4,-5 and -6) represent these methods; Methanol pregnation method, water pregnation method, two-step water pregnation method, ion exchange method, methanol reduction method and KBH4 reduction method. Furthermore, the effect of the reduction temperatures has been studied in the hydrogenation of nitrocyclohexane to cyclohexanone oxime. Pd/SWNTs-2 prepared through water pregnation and reduced through hydrogen that showed the best result at 723 K and gives selective cyclohexanone oxime formation with nitro cyclohexanone conversion of 96 % in mild conditions of 0.3 MPA and 323 K, see Table3.10.

Table3.9 Nitro cyclohexane hydrogenation catalyzed by different catalystsa showing the effect of different preparation methods on the selectivity.

a Reaction conditions: nitro cyclohexane 0.6 g; temperature 323 K; time 6 h; pressure 0.3 MPa; ethylenediamine 5 mL; NCH ¼ nitro cyclohexane; CHO ¼ cyclohexanone oxime; CHA ¼ cyclohexylamine.

Table3.10 Effect of reduction temperatures with catalysts on nitrocyclohexane Hydrogenation a.

a Reaction conditions: nitrocyclohexane 0.6 g; temperature 323 K; time 6 h; pressure 0.3 MPa ethylenediamine 5 mL; NCH ¼ nitrocyclohexane; CHO ¼ cyclohexanone oxime; CHA ¼ cyclohexylamine.

There are few reported examples of routs to decorate nanocarbon aerogels with metal NPs to introduce the metal nanoparticles nanocarbon with aerogel. Here, I will review some examples regarding the graphene/CNT aerogel decorated pallidum and their performance in catalytic reactions.

Zhong W. et al, [70] reported a three-dimensional Pd/graphen aerogel hybrid that was used successfully. The preparation process of the Pd/graphene was in a systematic way. Graphene oxide was produced through the use of Hammer and Offeman’s procedure [71]. Pd NPs were formulated as reported in previous works [72]. The results showed that this type of 3D Pd/ graphene aerogel hybrid has a very excellent catalytic activity in the ammonia borane hydrolysis releasing the hydrogen with a frequency turnover of 9.70 molH2molmetal-1min-1 with an activation energy value equal to 30.82 kj/mol, see Table3.11. This is considered to be higher than that of 2D Pd/RGO and the original Pd nanoparticles. Moreover, the defects in the graphene aerogel are as a result of electron transfer of Pd nanoparticles to the graphene aerogel via metal support interactions. This is due to the benefits arising from the ammonia borane hydrolysis reaction. Besides, the pore rich 3D Pd/ graphene aerogel has provided a very big surface area for the ammonia borane hydrolysis. This is important for reutilization. The use of PTHF (Polytetrahydrofuran) makes Pd/graphene aerogel to have more structural pores. This offers graphene aerogels large specific areas and structures that are considered durable and help to recycle a catalyst.

Table3.11 H2 generation from hydrolysis of AB catalyzed by Pd-based catalysts and graphene-based composites that affect of metal-support interaction between graphene and Pd NPs in the hydrolysis of AB.

In another study, Huang Y. et al, [86] reported a 3D amine terminated ionic liquid functionalized graphene through a covalent bond. There is successful fabrication of composite aerogel through a green reduction that induces self-assembly method. While ensuring there is no combination of particles on the graphene surface, Pd NPs are spread on graphene surface equally. The edifice of 3D IL rGO/Pd is composed of a structure that is permeable and with an extensive surface area which is strong. It provides a favourable environment for Suzuki cross coupling catalyst reactions.

Also, the catalyst can be recycled for more than 10 times without losing its effectiveness. Some the characteristics of 3D IL rGO/Pd catalyst that makes it necessary include; ability to be easily separated, recyclable, cost effective and being eco-friendly. Conclusively, the potential of 3D IL rGO/Pd composite aerogels in industrial catalysis cannot be exhausted, see Table3.12.

Table3.12 Optimization of the reaction conditions for Suzuki cross-coupling reactions of iodobenzene and phenylboronic acid catalyzed by catalyst-4a shown the effect of the solvents which mixed on the Suzuki cross coupling reactions.

a Reaction conditions: iodobenzene (1 mmol), phenylboronic acid (1.5 mmol),base (2 mmol), catalyst-4 (Pd 5 wt%, 0.5 mol%), and solvent (25 mL) at 80 _C, 15 min under air, b GC yield, c The polar aprotic Solvent. d The polar protic solvent.

However, to date research has focussed on CNT aerogels. To our knowledge, there has been no research that has investigated GNF aerogel yet, either in their pure form or functionalised with nanoparticle for catalytic reactions. Thus, the development of a synthetic route for GNF aerogels and their decoration with catalyst nanoparticles is a very topical issue that my PhD project will provide an insight on.

Summary:

Until now researcher’s efforts have been focused on the using MNPs and bimetallic NPs which are supported with CNTs either SWNT or MWNT using different methods. There are disadvantages in using these supports which can be notice in the efficiency of the catalytic properties. While using the GNF aerogel will increase the efficiency due the unique step edges that improve the NPs movement during the applied reactions. Since GNF aerogel with its high porosity, well-controlled pores and its high surface area () can offer the best quality for the NPs to be deposited on/in the nanostructure material. The material has not been explored or used in the catalytic application. My project will discuss the affect of GNF aerogel; and their different routes, that can improve the catalytic products and their applications. Furthermore, the GNF aerogel/rGNF areogel - either route A or route B- will be an excellent assistant in the process of the reactions regardless the where .

1931: First invention of Silica Aerogel

1970s: SiO

2

,

Metal Oxide

Aerogels

1990s: Organic/Polymer and Carbon Aerogels 2010s: CNT /Graphene Aerogels