Article Cite This: Ind. Eng. Chem. Res. 2017, 56, 15049−15057
pubs.acs.org/IECR
Hybrid Porous Molybdenum Disulfide Monolith for Liquid Removal of Dibenzothiophene Salama H. AlMarzooqi,† Marios S. Katsiotis,‡ and Saeed M. Alhassan*,§ †
The Abu Dhabi Oil Refining Company (TAKREER), Abu Dhabi, United Arab Emirates Titan Cement Company S.A., P.O. Box 18, 19200 Elefsina, Greece § Department of Chemical Engineering, The Petroleum Institute, Khalifa University of Science and Technology, P.O. Box 2533, Abu Dhabi, United Arab Emirates ‡
S Supporting Information *
ABSTRACT: Molybdenum disulfide (MoS2) has been historically used as a hydrodesulfurization (HDS) catalyst and industrial lubricant. Because of its 2D nature and unique properties, MoS2 is being considered for new applications in catalysis and electronics. In addition, there is great interest in designing new physical forms of MoS2 that will allow for improved implementation of its properties, such as a continuous porous monolithic form. In this work, we report a new synthesis method to fabricate continuous, centimeter-sized, open cell hybrid foam (monolith) that consists of molybdenum sulfide and carbon as its main constituents. The hybrid foam was characterized using XRD, electron microscopy (SEM and TEM), Raman, FTIR, and EELS spectroscopy. Furthermore, liquid adsorption of dibenzothiophene (DBT) in toluene solvent was used to test the foam affinity to adsorb organosulfur compounds. The monolith is of low density and exhibits high specific adsorption capacity compared to existing materials reported in the literature. a ready material for transistors applications.8 Research shows that the band gap of MoS2 can be increased by peeling the material structure to single layers.9 The most established applications of MoS2 include hydrodesulfurization and lubrication.10−13 Recent applications include electrochemical catalyst for hydrogen evolution reaction (HER),14−19 energy storage in lithium batteries,9,20,21 flexible transistors,8 photocatalysis,22 dye-sensitized solar cells fabrication (DSSC),23,24 and supercapacitors.25,26 Specifically for catalytic applications, the active sites in MoS2 are derived from unsaturated sulfur atoms on its surface. A general consensus exists that the edge sites are more catalytically active than the basal planes of MoS2 layers.11,27 Accordingly, significant efforts have been made to produce MoS2-based composites with a high ratio of exposed surfaces and edges by using nanoparticles, nanosheets, and open porous structures (3D materials) as guiding agents or support materials. A recent successful example reported the growth of MoSx on carbon nanotubes (CNTs), exhibiting unprecedented HER performance.28 The authors have reported the deposition of MoS2 nanosheets on CNTs and carbon nanofibers (CNFs) through thermal decomposition of its most common precursor,
1. INTRODUCTION The field of two-dimensional (2D) materials is growing rapidly, and it aims at providing new materials for several applications. Transition metal-dichalcogenides (TMDC) compounds are defined to be a group of materials with internal layered structure and weak interlayer interaction.1 The behavior of TMDC materials can be changed from metallic, semimetallic, or semiconductors based on the orientation and oxidation state of the metal atom.2 Molybdenum disulfide (MoS2) is one of the most stable metal-dichalcogenides with a current research interest as a 2D material.3−5 Molybdenum disulfide has three polytypes for its internal layered structure named as 1T-MoS2, 2H-MoS2, and 3R-MoS2. The most stable structure at normal conditions is the 2H-MoS2, which has the hexagonal unit cell, while the other structures are metastable.6,7 MoS2 is typically a black crystalline inorganic material and has a layered structure analogous to graphene. Its structure consists of a close packed layer of sulfur atoms stacked to create trigonal interstices with molybdenum held by strong bonds of S−Mo−S. The stack of layers are held together by weak van der Waals forces, which provide a useful characteristic for easy exfoliation and layer separation, though a careful selection of methods is required to achieve a mono layer of MoS2. Apart from composition, one main difference between MoS2 and graphene is its inherent semiconductor nature with a band gap of 1 eV (compared to 0 eV for graphene); this makes MoS2 © 2017 American Chemical Society
Received: Revised: Accepted: Published: 15049
August 10, 2017 November 1, 2017 November 21, 2017 November 21, 2017 DOI: 10.1021/acs.iecr.7b03313 Ind. Eng. Chem. Res. 2017, 56, 15049−15057
Article
Industrial & Engineering Chemistry Research Table 1. Physical Characteristics of MoS2 Hybrid Foam Specimens Sample no.
Density (g/cm3)
Surface area, As (m2/g)
Average pore diameter (Å)
Total pore volumea (cm3/g)
Porosity (%)b
1 2 3 Average
0.54 0.57 0.44 0.52 ± 0.07
29.18 28.80 25.79 27.92 ± 1.86
23.6 21.2 28.4 24.4 ± 3.7
0.034 0.028 0.037 0.033 ± 0.005
89 88 91 89 ± 1
Pores with radius smaller than 1.5 um. bPorosity defined as [1 − (density of foam/density of MoS2)] × 100. It is assumed that the majority of the sample is MoS2, which puts an upper bound to the above values.
a
ammonium tetrathiomolybdate (ATTM).29 Using the thermal decomposition method, MoS2 has been deposited on 3D structures, including mesoporous graphene,30 3D graphenebased network,31 and 3D MoS2-graphene hybrid structure.32 Additional examples include CNF foam33 and composite CNT−graphene foam;34 it should be noted that from the above-mentioned works concerning 3D MoS2-based materials only Liao et al. provide actual data on the pore volume of the synthesized foams.30 With the increasing worldwide environmental concern of sulfur content in fuels, the new studies aim to develop the HDS process to lower the sulfur content to achieve cleaner energy fuels. One of the model reactions of hydrodesulfurization of refinery feedstock is the HDS of DBT. The catalyst activity measurement is tested in a micro or batch reactor with a model sulfur compound (organosulfur) to estimate its activity in the HDS process. New research advances in sulfur removal from liquid fuels tend to develop new techniques to replace the conventional HDS process in less restricted conditions (lower temperature and pressure). Liquid adsorption using porous materials is a current developing method for desulfurization.35−37 Other new methods include oxidative desulfurization (ODS), biodesulfurization (BDS), and sulfur extraction (physical or chemical adsorption onto solid material) using solvents and ionic liquid.38 It must be noted that to the best of the authors’ knowledge, the published works concerning MoS2-based 3D structures available in the literature hold two facts in common: (i) MoS2 is in all cases grown or deposited on a 3D structure and not synthesized in situ. (ii) The cost of synthesis is high when considering industrial applications, mostly because of precursors required and the high value of “exotic” supports (CNTs, CNFs, graphene). In this context, the development of a porous hybrid material where MoS2 is in situ grown within the 3D structure in a cost-effective manner is missing from the literature. In this work, we present a new synthetic method to produce porous MoS2 material in continuous form. In our attempt, a hybrid material of carbon and MoS2 has been synthesized with emphasis on the importance and the future demand of MoS2 hybrid materials. The synthetic process involves a polymer solution of poly(vinyl alcohol) to induce high porosity, MoS2 powder as a starting material which is significantly cheaper than its precursor, a small quantity of ATTM which provided the link between MoS2 and the carbon substrate, and laponite. Laponite has been recently used to provide structural stability for poly(vinyl alcohol) aerogels39 while it exhibits excellent affinity for layered materials, being one itself.40 In the formulation presented herein, MoS2 constitutes ∼65% of the total foam mass, aiming toward our goal of synthesizing pure MoS2 foam. In addition to a foam structure and surface characterization, the MoS2 hybrid material has been tested for its affinity
to adsorb DBT from a toluene solvent in liquid adsorption and exhibited highly promising results.
2. EXPERIMENTAL SECTION 2.1. Materials. Poly(vinyl alcohol) (PVOH) 99+% hydrolyzed (Sigma-Aldrich), molybdenum(IV) sulfide (MoS2) powder
