The Development of Metal-Carbon Catalysts for Oxidative Desulfurization of Diesel Fractions

Metal-carbon materials M/CNTs (M = Ce, Сu, Mo) were synthesized by incipient wetness impregnation and their physicochemical characteristics were studied using various methods (inductively coupled plasma optical emission spectrometry, thermal analysis coupled with mass spectrometry, low-temperature nitrogen adsorption, X-ray diffraction and structural analysis, scanning electron microscopy, and Raman spectroscopy). It was found that M/CNTs (M = Ce, Сu, Mo) are the mesoporous materials consisting of carbon nanotubes with deposited СeO2, Сu2O/Cu or МоО3/MoO2 particles, respectively. The dispersion of supported species and their deposition uniformity improve in the series Сu < Се < Мо. The type of metal was shown to affect thermal stability as well as the textural and structural properties of the samples. The thermal stability of materials increases in the series Ce < Cu ≈ Mo, which is caused by different redox properties of the metals and also by the composition of products of the metal precursor decomposition. It is promising to use the developed materials as the catalysts for deep purification of diesel fraction components from sulfur compounds. Article info Received: 16 November 2019 Received in revised form: 28 January 2020 Accepted: 10 March 2010


Introduction
The necessity to provide ecologically safe and sustainable development of society at a growing consumption of energy resources implies a decrease in discharge of harmful wastes to the environment. However, technogenic emission of SO 2 to the atmosphere still remains high, reaching 3.7 million tons a year in Russia [1]. Contamination of the atmosphere by sulfur dioxide raises the acidity of rain, thus deteriorating natural reforestation, and leads to corrosion of steel constructions and development of bronchopulmonary diseases. Contamination of the atmosphere with sulfur oxides is caused mostly by fuel combustion. Sulfur compounds are present in all commercial fuels, and restrictions on their content become more and more severe. In particular, for diesel fuel corresponding to the requirements of Euro-3, Euro-4 and Euro-5, the sulfur content should not exceed 350, 50 and 10 mg/kg, respectively [2]. In this connection, the development of technologies for reducing the concentration of sulfur compounds in hydrocarbon feedstock is an urgent problem.
At present, the main industrial method for desulfurization of components of diesel fractions is their catalytic hydrodesulfurization [3,4]. Economical advisability of this method decreases with stiffening of environmental requirements to the fuel quality, which makes it necessary to develop new approaches to the purification of hydrocarbon feedstock. Oxidative desulfurization (ODS) is among promising alternative methods for the removal of sulfur compounds and has considerable advantages over hydrodesulfurization [5][6][7][8][9][10][11][12][13][14][15]. The ODS process can be performed at lower temperatures and pressures and does not require the use of hydrogen, which is an expensive reagent. Such inexpensive reagents as air oxygen, hydrogen peroxide or organic peroxides can be used as the oxidants.
The  [20]). A combination of high adsorption capacity to the substrate and ability to activate the oxidant is a necessary condition for creating the catalytic systems efficient in ODS. It was shown [19] that carbon nanomaterials (CNMs) can serve as the promising materials for ODS. The oxidation of sulfur-containing compounds was studied in the presence of carbon nanotubes (CNTs) [26], reduced graphene oxide (rGO) [27], and Au/CNT composites [25].
In order to develop the efficient catalysts for ODS of motor fuels, metal-carbon materials M/CNTs (M = Ce, Сu, Mo) were synthesized in this work, and their properties were investigated using various physicochemical methods.

Experimental
Metal-carbon materials M/CNTs (M = Ce, Сu, Mo) were synthesized by incipient wetness impregnation. To this end, Taunit carbon nanotubes (NanoTechCenter Ltd., Tambov, Russia) [28,29] were impregnated with an aqueous solution of the corresponding metal salt (Ce( with a specified concentration. After that, the samples were dried in a drying oven in an air medium at 80 °C during 6 h and then calcined in a flow reactor in an argon flow at 600 °C for 2 h. The elemental composition of the samples was determined by inductively coupled plasma optical emission spectrometry on an atomic emission spectrometer iCAP 6500 Duo LA (ThermoFisher-Scientific, USA). Thermal analysis (differential thermal analysis (DTA), thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG)) of the samples was carried out on an STA 449 F3 Jupiter® (NETZSCH Geratebau GmbH, Germany) thermoanalyzer over the temperature range of 25-1000 °С at a heating rate 5 °C/min in an N 2 atmosphere.
Textural characteristics of the materials (specific surface area S BET , pore volume V pore and average pore diameter D pore ) were examined on an automated volumetric setup ASAP 2400 (Micromeritics, USA) by measuring and processing the low-temperature nitrogen adsorption isotherms taken at 77 K.
X-ray diffraction and structural analysis (XRD and XSA) of the samples was carried out on a powder X-ray diffractometer Bruker D8 ADVANCE A25 (Bruker, Germany) (CuKα radiation, λ = 1.5406 Å, Ni filter with secondary radiation) at room temperature using polycrystal method. XRD analysis was made using the diffraction patterns obtained by scanning over the angular region 2θ = 10° -90° with a step 0.02° and accumulation time 2 s. Diffraction peaks were identified by means of ICDD and PDF2 powder diffraction databases. Sizes of the coherent scattering regions (CSR) were calculated using the Debye -Scherrer formula: where h, k, l are the Miller indices; D hkl -the CSR size along the direction perpendicular to (hkl) planes; λ -the X-ray wavelength (λ = 1.5406 Å); β hkl -the true physical broadening of the line of the tested sample; cosθ hkl -the cosine of the X-ray diffraction angle.
Raman spectra were recorded in the spectral shift range of 100-4000 cm -1 on a Raman spectrometer Renishaw Invia (Renishaw plc., UK) with excitation by an argon laser with the wavelength 514.5 nm, diffraction lattice 1800 l/mm and L50× objective. Laser power on a sample did not exceed 0.175 mW, and the accumulation time of useful signal was 120 s.
Scanning electron microscopy (SEM) images were recorded on a JEOL JSM-6390 LA (JEOL, Japan) electron microscope with an X-ray energy dispersive detector JED 2300. Thermal analysis revealed [30] that heating of the dried samples in an inert atmosphere leads to decomposition of salts -precursors of metals and to a partial degradation of the carbon matrix. The temperature at which M/CNT starts to degrade (T d ) depends on the nature of a metal precursor compound and increases in the series Ce < Сu ≈ Мо (see Table). Different thermal stability of M/CNTs Eurasian Chemico-Technological Journal 22 (2020) 81-88 may be caused by features of the interaction between precursor compound and functional groups of CNTs as well as by the composition of the gas medium formed upon decomposition of inorganic salt. In particular, the decomposition of cerium nitrate hexahydrate is accompanied by the formation of nitrogen oxide and oxygen [31], which serve as oxidants of the carbon matrix:

Result and discussion
The decomposition of copper sulfate is a multistep process leading to a release of SO 2 and О 2 [32,33]: In addition, the reduction of copper cations (Cu 2+ → Cu + → Cu o ) in the presence of a carbon matrix cannot be ruled out.
The decomposition of ammonium paramolybdate tetrahydrate is accompanied by the formation of some intermediate compounds and a release of ammonia to the gas medium [34,35]: According to the low-temperature nitrogen adsorption, the adsorption isotherms of type IV with the hysteresis loop of H3 type at p/p 0 above 0.45 are observed for M/CNTs, which testify to the presence of mesopores. The specific surface area of the samples is lower as compared to the initial support (see Table). This may be caused by the blocking of thin pores by the active component particles. This effect becomes more pronounced in the series Ce < Cu < Mo. The pore volume of the samples weakly depends on the features of metal and is close to V ∑ of the support. The average pore diameter varies in the range of 7.4-8.6 nm.
The XRD study revealed that M/CNTs are the heterophase samples containing phases of the support (the graphitic carbon phase and the phase of metallic nickel and mixed nickel-magnesium oxide) and active component (see Table and Fig. 1). It should be noted that modification of CNTs with metals virtually does not change the position and half-width of the (002) diffraction maximum (at 2θ ≈ 26°) from (00l) planes, which are typical of graphitic carbon. The average lattice parameter c is equal to ~ 6.81 Å, and the size along the direction perpendicular to graphite layers, ~ 6 nm. The presence of mixed nickel-magnesium oxide and the metallic nickel phase is caused by the synthesis procedure of CNTs used as the support [29].
Stabilization forms of the active component in the carbon matrix were established. In the case of Ce/CNTs, the active component is represented by highly dispersed cerium dioxide particles. The CSR size of the СеО 2 phase is ~ 8 nm. For Сu/CNMs, the formation of the copper(I) oxide phase and metallic Cu o particles with the CSR size of 20 and 40 nm, respectively, is observed. For Mo/CNM samples, the active component is stabilized as oxides of molybdenum VI and IV, the CSR size of which can hardly be measured because their diffraction maxima cannot be resolved.
The Raman spectroscopy study revealed that the introduction of active component into the carbon matrix of support and subsequent thermal treatment change the ordering of the carbon framework, which is indicated by changes in the Raman spectra (Fig. 2a) and values of the I D /I G ratio (Table) used for estimation of structural defectness.
In particular, the decrease in I D /I G for M/CNTs (M = Ce and Мо) in comparison with unmodified CNTs may testify to both a decrease in the fraction of amorphous component of the carbon framework and a decrease in the number of defects [36]. The degree of change in the state of carbon matrix depends on the composition of the active component (Table). One can see that I D /I G decreases in the following series of metals: Cu > Ce > Mo. It was shown (Fig. 2b) that the active component forms surface oxide structures (CuO, Cu 2 O, МоО x ) [37,38], which is consistent with the XRD data. The SEM study demonstrated that modification of the CNT material with metal salts followed by thermal treatment in an inert atmosphere at 600 °C produces virtually no changes in its morphology (Fig. 3). According to EDX mapping, the distribu-tion of active component depends on the type of metal (Fig. 4). It is seen that the uniformity of metal distribution on the surface of support increases in the series Сu < Се < Мо, which agrees well with the data obtained by structural methods.

Conclusion
A series of metal-carbon materials M/CNTs was synthesized by incipient wetness impregnation with the variation of the type of metal M = Ce, Сu, Mo used for modification of the carbon matrix. Various physicochemical methods were employed to perform a comparative analysis of textural, structural and morphological properties of the samples. It was shown that thermal stability of the samples increases in the series Се < Сu ≈ Мо, which is caused by different redox properties of the metals and also by the composition of products of the metal precursor decomposition. It was found that the active component in M/CNTs, M = Ce, Сu, Mo, is stabilized on the carbon support surface as СeO 2 , Сu 2 O/Cu or МоО 3 /МоО 2 respectively, and its dispersion and deposition uniformity improve in the series Сu < Се < Мо. The synthesized materials are promising for kinetic studies in catalytic ODS of components of diеsel fraction from sulfur compound.