Carbon Nanomaterials in Hydrogenation Catalysis

Edward Furimsky

• Publisher: Royal Society of Chemistry
• Year: 2019
• Edition: 1
• eBook ISBN:
  9781788017800
• Topic: Ciencias físicas
• Subtopic: Química orgánica

Hydrogenation is a key reaction in both the food and petrochemical industries, where it is used to reduce carbon–carbon double bonds. Without a catalyst, hydrogenation reactions require extreme temperatures to occur, meaning catalysts are essential for the reaction to be industrially useful. During the past decade, the properties of many carbon nanomaterials that are relevant to hydrogenation catalysis have been described, including carbon nanotubes (CNTs), carbon nanofibers (CNFs), carbon nanohorns (CNHs), graphene oxide (GO), reduced graphene oxides (rGO) and fullerenes, that are relevant to hydrogenation catalysis, have been described. For many of these the production methods have advanced to the commercial stage. Numerous studies on the development of catalysts on carbon nano-supports have appeared in the scientific literature and these catalysts have shown remarkable activity and specificity.
Carbon Nanomaterials in Hydrogenation Catalysis is a valuable reference for researchers and chemical engineers working on improving hydrogenation processes and those interested in applications for carbon nanomaterials. Covering their production, modification and applications as a catalyst support this book provides an in-depth review of the current state-of-the art in using carbon nanomaterials for hydrogenation reactions.

CHAPTER 1

Introduction

The database available in the scientific literature indicates remarkable activity and selectivity of some catalysts supported on carbon nanomaterials such as carbon nanotubes (CNTs), carbon nanofibers (CNFs), carbon nanohorns (CNHs), graphene oxide (GO) and reduced graphene oxide (rGO) in various hydrogenation reactions. These catalysts outperformed catalysts supported on activated carbons (ACs) and carbon blacks (CBs) and also on traditional oxidic supports such as Al₂O₃, SiO₂, TiO₂, SiO₂–Al₂O₃ and zeolites. Among active metals, noble metals such as Pt, Pd, Ru and Rh have been used most extensively. To a lesser extent, transition metals such as Ni, Co, Fe, Cu, Mo and W have also been attracting attention. In an effort to improve catalyst performance, bimetallic catalysts consisting either of two noble metals or of one noble metal combined with a transition metal have been developed.

Carbon nanomaterials such as carbon nanotubes (CNTs), carbon nanofibers (CNFs), carbon nanohorns (CNHs) and fullerenes and graphene-derived solids such as graphene oxide (GO) and reduced graphene oxide (rGO) have been evaluated as potential supports for catalysts active during hydrogen transfer such as required for hydrogenation (HYD) and hydroprocessing (HPR) reactions.¹ HYD relates to the conditions employed during the HYD of various feeds and reactants for the production of fine chemicals and additives whereas HPR relates to the production of hydrocarbons such as those present in fuels and lubricants. There may be a significant difference between the operating conditions employed in these processes. For example, in HYD, the temperature rarely exceeds 150 °C, whereas in HPR, temperatures above 250 °C are necessary. Of course, owing to the complexity of feeds, an overlap of these temperature ranges may occur. Similar or the same catalysts have been used in these applications. Catalysts consisting of carbon nanosupports combined with noble metals (e.g. Pt, Pd, Ru, Rh and Ir), rare earth metals (e.g. Y, La and Ce) and transition metals (e.g. Ni, Co, Mo, W, Cu and Fe) have been the focus of attention.

Amorphous carbon materials such as carbon blacks (CBs) are included because the size of the particles may be in the nanorange. Activated carbons (ACs) are included whenever corresponding catalysts have been tested in comparison with catalysts supported on carbon nanomaterials. The applications of CBs and ACs in HPR catalysis, both as catalysts and as catalyst supports, were reviewed in detail more than a decade ago.² At that time, the database on similar applications of carbon nanomaterials was limited. Catalysts supported on oxidic supports are also included whenever they were tested in the same studies under similar conditions as those for catalysts supported on carbon nanosupports. This gives an opportunity to evaluate the rather unique performance of the latter catalysts in comparison with conventional catalysts.

There are other potential applications of carbon nanomaterials. In catalysis, carbon nanomaterials have been evaluated both as catalysts and supports for catalysts in various non-HYD applications. These studies were reviewed by Serp and co-workers.^3,4 A detailed account of hydrogen-related applications of CNTs (e.g. production via electrolysis, storage) was provided by Orinakova and Orinak.⁵ The potential of CNTs in several other energy-related applications was addressed in a review by Centi and Perathoner.⁶ However these aspects, and many others, are outside the scope of the present review.

Feeds and reactants varying widely in structure and origin have been included in studies over catalysts supported on carbon nanomaterials. As the most abundant source of carbon, the components of lignocellulosic biomass such as cellulose, hemicellulose and lignin have attracted considerable attention as the feed for catalytic depolymerization and also a source of products requiring catalytic upgrading to either fine chemicals or fuels.⁷ The conversions of cellulose and hemicellulose to glucose and fructose and to xylose, respectively, have been identified as important routes for the production of chemicals and fuels. Monomeric phenols and aromatics (e.g. guaiacol, eugenol and syringol) derived from lignin have been used as model compounds for HYD, HPR and hydrodeoxygenation (HDO).

Unique model compounds such as reactants containing α,β-conjugated CC and CO double bonds (e.g. cinnamaldehyde, citral and crotonaldehyde) have been studied. In these applications, catalysts supported on carbon nanosupports have been extensively investigated. This resulted from their selectivity for the HYD of CO bonds while leaving CC bonds intact. In this case, fine chemicals required for the production of pharmaceuticals, food additives, perfumery, etc., were the products of interest. Conventional petroleum feeds and nonconventional feeds and also model compounds derived from these feeds have been used to evaluate the performance of catalysts supported on carbon nanosupports.

HYD is an important route for the conversion of carbon oxides (CO and CO₂) to useful products. Fischer–Tropsch (FT) synthesis is a method for the conversion of CO that has been used commercially for decades. In fact, the concept based on the pre-reduction of CO₂ to CO via the Boudart reaction using carbon or during the reduction with H₂:

CO₂+C→2CO

CO₂+H₂→CO+H₂O

followed by the utilization of CO as the feed for FT synthesis:

nCO+mH₂→chemicals and fuels

has merit because all steps involved are commercially proven. Studies in which either carbon nanomaterials alone or catalysts supported on carbon nanosupports, using CO and CO₂ as the feeds, have been noted. Because of its uniqueness, this concept deserves separate treatment to enhance the awareness of its merit.

In spite of the limited information available, the potential of catalysts supported on carbon nanomaterials in various environmentally related applications should be noted. For example, chlorinated solid waste (e.g. polychlorinated biphenyls) could be readily converted to hydrocarbons under fairly mild HYD conditions.⁸ Nitrate contaminants could also be converted to benign products during HYD over these catalysts.^3,6 These few examples are introduced to indicate the suitability of catalysts supported on carbon nanomaterials for the conversion of hazardous wastes.

References

1. J. M. Planeix, N. Coustel, B. Coq, V. Brotons, P. S. Kumbhar, R. Dutartre, P. Geneste, P. Bernier and P. M. Ajayan, J. Am. Chem. Soc., 1994, 116, 7935.

2. E. Furimsky, Carbons and Carbon Supported Catalysts in Hydroprocessing, RSC Publishing, Cambridge, UK, 2008.

3. P. Serp, M. Corrias and P. Kalck, Appl. Catal., 2003, 253, 337–358.

4. P. Serp and J. S. Figueiredo, Carbon Materials for Catalysis, Wiley, Hoboken, New Jersey, 2009.

5. R. Orinakova and A. Orinak, Fuel, 2011, 90, 3123–3140.

6. G. Centi and S. Perathoner, ChemSusChem, 2011, 4, 913–925.

7. H. Li, A. Riisager, S. Saravanamurugan, A. Pandey, R. S. Sangwan, S. Yang and R. Luque, ACS Catal., 2018, 8, 148–187.

8. X. Guo, C. Yu, Z. Yin, S. Sun and C. T. Seto, ChemSusChem, 2018, 11, 1617–1620.

CHAPTER 2

Properties of Carbons

Structurally, highly ordered nanomaterials in their pristine form, e.g. graphene, carbon nanotubes, carbon nanofibers, carbon nanohorns and fullerenes, and also the corresponding modified materials, are the primary objective of this book. Amorphous carbons, such as carbon blacks and activated carbons, are briefly discussed because they have been included for comparison in many studies involving carbon nanomaterials. The focus is on the properties that are relevant for catalysis under a wide range of hydrogenation conditions and also on those properties which can influence catalyst preparation, performance and stability.

Structurally, highly ordered nanomaterials in their pristine form, i.e. graphene, CNTs, CNFs, CNHs and fullerenes, and the corresponding modified materials are the primary objective of this book. Amorphous carbons, such as CBs and ACs, are briefly discussed because they have been included for comparison in many studies involving carbon nanomaterials. The focus is on the properties that are relevant for catalysis under HYD and HPR conditions and on those properties that can influence catalyst preparation, performance and stability.

2.1 Carbon Nanomaterials

Figure 2.1 shows graphite, which consists of a layered/planar structure. Only three layers are shown to indicate the covalent bonding between carbons (spheres) and the flow of van der Waals forces depicted by vertical lines. In reality, graphite consists of multiple layers of platelets. In this case, the two-dimensional layers are called graphene. It is evident that the layers are arranged as a honeycomb lattice. The distance between the planes is ca. 0.35 nm. Because the layers are held together by weak van der Waals forces only, they can be easily separated by various methods. One layer, i.e. graphene, is also shown in Figure 2.1. In this case, basal planes account for most of the surface area while the contribution of the edge regions to the total surface area is very small. In the diagram of graphene, the armchair and zigzag configurations of the edge regions are evident. A difference between the activities of ...

Table of contents

• Cover
• Title Page
• Copyright Page
• Preface
• List of Acronyms
• Contents
• Chapter 1 Introduction
• Chapter 2 Properties of Carbons
• Chapter 3 Modifications of Carbon Nanomaterials
• Chapter 4 Stability of Carbon Nanosupports
• Chapter 5 Reactants and Feeds
• Chapter 6 Development of Catalysts Supported on Carbon Nanosupports
• Chapter 7 Catalysts Supported on Carbon Nanotubes
• Chapter 8 Catalysts Supported on Carbon Nanofibers and Carbon Nanohorns
• Chapter 9 Catalysts Supported on Graphenes
• Chapter 10 Catalysts Supported on Fullerenes
• Chapter 11 Selection of Carbon Supports
• Chapter 12 Future Perspectives
• Subject Index