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Investigation of Massive Catalyst based on Molybdenum Disulphide by Simultaneous Thermal Analysis and Mass Spectrometry Methods

The paper presents the results of experimental studies of massive sulfide catalysts by simultaneous thermal analysis and mass spectrometry. It is found that the STA/MS methods are quite informative for testing the catalyst systems based on MoS2 and are useful in identification of the reference features that could be used to predict their activity. It is also shown that the defect structure of molybdenum disulfide formed during mechanical activation is reflected on the DSC curves.


Introduction
The development of methods for the synthesis of domestic massive sulphide catalysts for hydrotreating of petroleum fractions is a strategic priority in the field of oil refining to produce high quality distillate fuels. Conventional western technologies allow producing bulk catalysts with a high activity and selectivity for all types of hydroprocessing procedures (hydrogenation, hydrodesulfurization, hydrodenitrogenation, hydrodemetallization, hydrodearomatization, hydroisomerization, hydrodewaxing, hydrocracking) [1]. These multi-stage and energy-intensive methods require the use of large amounts of aqueous solutions of salts-precursors as the active components, which is inevitably accompanied by the formation of substantial amounts of wastewater with toxic anions [2][3][4]. The use of mechanical activation method to prepare the hydrotreating catalyst systems has a number of advantages, as it allows reducing the number of stages and preparation time, and eliminating the use of aqueous solutions of reagents and organic solvents.
We propose a new promising approach to manufacturing 4-, 3-, 2-, and 1-component massive sulfide catalysts, which is implemented in a single-stage mixing of initial solid-phase reagents under mechanical activation (MA). In this case, MoS 2 is a dimensional inorganic precursor of the active component only and it has to be subjected to dispersion [5]. We have previously shown [6,7] that the catalysts of this series are capable of providing an ultra-low level of sulfur (about 10 ppm) in a model reaction of hydrogenolysis of dibenzothiophene (DBT) and in the course of hydrorefining the straightrun diesel fraction (S initial = 2.08%).
For 1-component catalysts based on MoS 2 , a unique experimental fact was discoveredtheir activity in the model reaction of dibenzothiophene hydrodesulfurization can be significantly increased in case of using the trace amounts of polar liquids (S res = 1-3 ppm) during MoS 2 MA-dispersion.
There is no answer to the question of the reason for such a high catalytic activity and diagnostics of such systems so far. It is well known that thermal analysis methods are based on detection of thermal effects which reflect the physical and chemical transformations occurring in the analyzed sample. For example, a simultaneous thermal analysis (STA) allows us to monitor the change in mass, oxidation, decomposition of the sample substance, temperature and enthalpy of the phase transition, the thermal effects of chemical reactions, and to study the kinetics of the reactions [8][9][10]. A combination of the simultaneous thermal analysis and mass spectrometry (MS-STA) allows us to obtain a lot of additional data on the composition of volatile products.
The aim of this study was to investigate massive sulfide catalysts based on MoS 2 via a simultaneous thermal analysis and mass spectrometry.

Experimental
A number of sulfide catalysts were prepared by the solid phase method. The commercial coarsegrained powder of molybdenum disulfide (the content of molybdenum disulfide is 99.72%) was used as a precursor of the active catalyst component. The powder was subjected to mechanical activation in a vertical vibratory mill in the presence of trace amounts of polar liquids (in this case, methanol) and in their absence.
The activity of the catalyst system was evaluated by the residual content of sulfur in the course of hydrogenolysis of a model dibenzothiophene compound which represents the most stable sulfur structures presented in the diesel fraction. The model reaction experiments were performed in an 'AUTOCLAVE ENGINEERS' autoclave with the reactor volume of 100 ml, at a pressure of 3.4 MPa, and a temperature of 340 °C (S initial = 500 ppm).
The samples were analyzed by the STA-MS method using a STA-449C ("Netzsch") apparatus combined with a quadrupole QMS 403C mass spectrometer. Heating was carried out in air (flow rate of 20 ml/min) to 650 ºC at a heating rate of 10 °C/min. The results of thermogravimetry (TG) and differential scanning calorimetry (DSC) were processed using the 'Proteus Analysis' program.

Discussion of Results
It was found out experimentally that in the presence of mono-component catalysts prepared by the solid phase method (Table 1), a complete hydrodesulfurization of dibenzothiophene was observed which had not been reported in the literature. For example, the coarse-grained molybdenum disulfide (catalyst system AK-0) used as a commercial reagent (DMI-7) exhibits certain catalytic activity resulting in a 20% conversion of DBT and a 100 ppm decrease in the residual sulfur content in the reaction mixture (from the initial 500 ppm). The sulfur content in hydrodesulphurizate was reduced by a factor of 5 (down to 100 µl) in the presence of mechanically activated MoS 2 . In the presence of trace amounts (100 µl) of a polar liquid (methanol) the MoS 2 monocatalyst exhibits its maximum activity in the model reaction of DBT hydrogenolysis (residual sulfur content 1-2 ppm; AK-60).  (Table 1), which is necessary to provide ultra-low sulfur content in the reaction products.
On the other hand, we have to answer the questions on the reason for the threshold effects in the activity of MoS 2 in the presence of trace amounts of CH 3 OH and the origin of active centers in the above catalysts. The method of simultaneous thermal analysis and mass spectrometry allowed us to obtain the following additional information on the AC-27, AC-60 and AC-74 catalysts before and after their participation in a model reaction. Figures 1 and 3 show typical TG, DTG, and DSC curves using the AC-74 catalyst system as an example. First of all, it should be noted that the mass loss occurs in the temperature region up to 170-280 C. The calculated content of methanol in the sample is 0.026% which could be hardly measured without special techniques. According to MS spectra, the ions with the following masses: m/e = 18(H 2 O + ), 12(C + ), 2(H 2 + ), 34(H 2 S + ), etc. have been identified in gas atmosphere in this temperature region, and an interpretation of their presence in the spectra is difficult. The mass loss of the AC-74 catalyst sample before the reaction at the temperature up to 300 C is 11.19% while after the reaction it is 8.66%. The mass changes in MoS 2 within the temperature range above 300 C are due to its oxidation.   According to thermal analysis for all investigated samples, the identical changes of the shape in DSC curves are observed for the catalysts before and after their use in the model reaction. For example (see Fig. 1, 3), the DSC curve for AC-74 catalyst before the reaction is a double-humped curve of heat emission with the temperature maxima of its major peaks at 415 C and 363 C (Table  2), while for the AC-60 and AC-27 catalysts, the DSC curves are polymodal. The first low temperature peak in the thermograms for these catalysts disappears after the reaction. One more specific feature of the formal external analysis of the curves should be noted: the peaks of heat emission in the temperature range 300-550 C (Fig. 1, 3) in the DSC curves repeat the shape of the mass spectral peaks in the MS curves (Fig. 2, 4). Moreover, these changes are accompanied by mass loss (DTG and TG curves in Fig. 1 and 3). The ratio m/e = 64 is identified as the molecular SO 2 + ion formed by oxidation of MoS 2 . According to the literature data, oxidation of a coarse-grained MoS 2 begins at about 400 C. For nanodisperse states, the initial oxidation temperature of the substrate can be much lower. It is well known from the literature [11,12] that the mechanical activation of the reaction mixture is accompanied by an increase in the number of point contacts, generation of defects, their constant regeneration, multiplication, and migration throughout the entire volume of the reaction mixture. Hence, the broadening of the MS peaks corresponds to a multiple set of different discrete states produced during mechanical activation of MoS 2 , which are due to varying degree of bonding of sulfur atoms with Mo and imperfection of its nanocrystallites. The shift of the MS peak temperature from 415 to 475 ºC in Fig. 4 might testify in favour of the initial activity of low-temperature catalytically active centers of MoS 2 and their functioning during the model reaction. In addition to that, there is a reasonable question about the effective lifetime of mono-component catalysts based on molybdenum disulphide in the reactions of DBT hydrodesulfurization in the absence of such centers.

Conclusion
Thus, for the first time we have shown a possibility of identifying structure defects formed during mechanical activation using the DSC curves. The experimental results obtained show that the STA/MS method is quite informative for testing the catalyst systems based on MoS 2 and is useful in identification of the reference features that could be used to predict their activity.