Influence of Different Improved Hummers Method Modifications on the

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INFLUENCE OF DIFFERENT IMPROVED HUMMERS METHOD MODIFICATIONS ON THE CHARACTERISTICS OF GRAPHITE OXIDE IN ORDER TO MAKE A MORE EASILY SCALABLE METHOD Maria del Prado Lavin Lopez, Amaya Romero, Jesus Manuel Garrido, Luz Sanchez-Silva, and José Luis Valverde Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03533 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 2, 2016

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Industrial & Engineering Chemistry Research

INFLUENCE OF DIFFERENT IMPROVED HUMMERS ON

THE

METHOD

MODIFICATIONS

CHARACTERISTICS

OF

GRAPHITE OXIDE IN ORDER TO MAKE A MORE EASILY SCALABLE METHOD ‡M.P. Lavin-Lopez*, ‡A. Romero, ‡J. Garrido, ‡L. Sanchez-Silva, ‡J.L. Valverde Department of Chemical Engineering, University of Castilla-La Mancha, Avda. Camilo José Cela 12, Ciudad Real, Spain, 13071 KEYWORDS. Graphite oxide, Improved Hummers method, Optimization, Economic assessment.

ABSTRACT

Different samples of graphite oxide (GrO) were prepared by modification of the Improved Hummers Method reported in literature. Graphite was used as the raw material and KMnO4 and H2SO4 were used as chemical reagents. GrO samples were analyzed by Raman spectroscopy, SEM, TEM, elemental analysis, X-ray diffraction, N2 adsorption-desorption and thermogravimetric analysis. The results indicate that, after optimize the synthesis method, the oxidation time was reduced from 12 to 3 hours,

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coagulation step was removed, wash step was halved, use of H3PO4 during the oxidation step was no longer necessary, the production of graphite oxide per batch was increased and the amount of graphite that could be processed was increased five times. Therefore, synthesis costs were significantly lower than those related to the published Improved Hummers Method. An economic assessments for the industrial scale demonstrated that this process could be viable from the third year after its implementation.

Introduction: Graphene is a material that has attracted a great deal of attention in recent years due to its unique properties and numerous applications. This material can be described as a two-dimensional sheet of carbon atoms with a honeycomb structure

1, 2

. Graphene was

firstly obtained by Andre Geim and Kostya Novoselov using a mechanical exfoliation method called the ‘Scotch Tape® method’ in the form of small flakes with a thickness of several microns 3. Graphene can be synthesized by two different strategies, called bottom-up and topdown4. The bottom-up strategy comprises those methods in which a carbonaceous gas source is used to synthesize graphene. These methods include Epitaxial Growth on Silicon Carbide 5, 6 and Chemical Vapor Deposition 1, 3, 6. In contrast, top-down methods are based on an attack of graphite (powdered raw material) to break down its layers to form graphene sheets. These methods include Micromechanical Cleavage

7, 8

,

Exfoliation of Graphite Intercalated Compounds (GIC) 9, Solvent-Base Exfoliation, Unzipping Carbon Nanotubes

10, 11

and Exfoliation or Reduction of Graphene Oxide 12-

14

.


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Great emphasis is currently placed on the development of large-scale production methods for graphene synthesis to make graphene-based products. In this respect, graphite oxide (GrO) and its derivative, graphene oxide (expanded graphite oxide), have been identified as the most important graphene derivatives. The main properties of graphite oxide are as follows: it is an insulator material but its conductivity varies depending on its chemical and structural properties

15

; it is an amphiphilic material,

which allows the dispersion of GrO in both aqueous and organic solvents 16, 17; GrO has antibacterial properties

18

. All of these properties make graphite oxide a suitable

material for numerous applications such as supercapacitors radioactive waste from water

19

, the removal of

20

, the fabrication of transparent conductors

21

and in

biomedical applications 22, for example to inhibit tumor formation in multiple cell lines 23

.

Graphite oxide can be defined as a set of functionalized graphene sheets that are mainly composed of carbon, oxygen and hydrogen atoms. This material can be used as a precursor of graphene itself. The structure of graphite oxide is similar to that of graphite as they differ only in the oxygenated groups present in GrO, which give rise to a greater distance between the graphene layers 15. This material consists of a hexagonal network of sp2- and sp3-hybridized carbon atoms that bear hydroxyl and epoxide functional groups on the ‘basal’ plane and carboxyl and carbonyl groups at the edges 24. Graphite oxide can be synthesized by either the Brodie 25, Staudenmaier 26 or Hummers 14

methods or by a variation of the latter one, namely Modified Hummers Method or

Improved Hummers Method

27

. The main differences between the above mentioned

methods are summarized in Table 1, with particular emphasis on the nature of the oxidant, the toxicity and the main advantages or disadvantages of each approach. It can



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be observed that Improved Hummers Method is characterized by lower toxicity and several advantages in terms of the synthesized product. The main objective of the present work was to optimize the synthesis of graphite oxide by the Improved Hummers Method with the ultimate goal of reducing the reaction time and the quantities of chemical reagents required in the synthesis. As a result of this optimization, a considerably decrease of the production costs associated with the synthesis was observed. In addition, an economic assessment of the industrial scale of the process was study in order to know the viability of the graphite oxide synthesis.


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Table 1: Synthesis Methods for Graphite Oxide 12-14, 25-28. Brodie Method 12, 25 • Oxidants: KClO3, HNO3 • Toxicity: Yes • Disadvantages: • Weak acidity. • Soft dispersability in basic solutions. • Small size, limiting thickness and providing an imperfect structure. Staudenmaier Method 13, 26 • Oxidants: KClO3 (NaClO3), HNO3, H2SO4 • Toxicity: Yes • Disadvantages: • Time-consuming and dangerous method. • Addition of KClO3 generally takes longer than a week and CO2 is evolved, thus making necessary to remove an inert gas. • The risk of explosions is a constant danger. Hummers Method 27, 14 • Oxidants: KMnO4, H2SO4, NaNO3 • Toxicity: No (NOx is released) • Advantages: • Higher oxidation degree than that obtained in Brodie or Staudenmaier Methods. • Disadvantages: • It is still considered that the oxidation is incomplete. • Separation and purification processes are tedious process. • Highly time-consuming process. Modified Hummers Method 27, 28 • Oxidants: KMnO4, H2SO4, NaNO3, KMnO4, H2SO4 • Toxicity: No (NOx is released) • Advantages: • Improved level of oxidation and, therefore, product performance. • Disadvantages: • Separation and purification processes are tedious process. • Highly time-consuming process. Improved Hummers Method 27 • Oxidants: KMnO4, H2SO4, H3PO4 • Toxicity: No • Advantages: • Defects in the basal plane are reduced. • Larger amount of oxidized graphite is provided. • The degree of reduction provides an equivalent level of conductivity when compared to other methods. • Best process yield compared to Brodie, Staudenmaier and Hummers method. • Environmentally friendly, toxic gases are not generated during the preparation. • The product has a more organized structure compared to graphite oxide obtained by Brodie and Staudenmaier methods. • Disadvantages: • Separation and purification processes are tedious process. • Highly time-consuming process.



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Experimental: Materials: Graphite powder with a particle size

Influence of Different Improved Hummers Method Modifications on the

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