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Article Cite This: ACS Omega 2019, 4, 12736−12744
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Alkali Earth Catalysts Based on Mesoporous MCM-41 and Al-SBA-15 for Sulfone Removal from Middle Distillates Eduard Karakhanov,*,† Argam Akopyan,† Oleg Golubev,† Alexander Anisimov,† Aleksandr Glotov,†,‡ Anna Vutolkina,† and Anton Maximov†,§ †
Chemistry Department, Lomonosov Moscow State University, GSP-1, 1-3 Leninskiye Gory, Moscow 119991, Russia Gubkin Russian State University of Oil and Gas (National Research University), Leninsky Prospekt 65, Moscow 119991, Russia § Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninsky Prospekt 29, Moscow 119991, Russia Downloaded via 5.101.220.152 on August 7, 2019 at 11:29:35 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Mg, Ca, and Ba catalysts supported on structured mesoporous silica oxides types MCM-41 and Al-SBA-15 were synthesized and investigated in sulfone cracking for sulfur removal from oxidized diesel fuel. Functional materials and catalysts were characterized by low-temperature nitrogen adsorption/desorption, transmission electron microscopy, and inductively coupled plasma atomic emission spectroscopy techniques. Catalytic tests were carried out in fixed-bed and batch reactors with a model compound dibenzothiophene sulfone and oxidized diesel fraction as a feed. MgO/MCM41 and MgO/Al-MCM-41 possess high activity in sulfone cracking. The sulfur content in the diesel fraction decreases from initial 450 up to 100 ppmw. Catalysts can be regenerated for reuse in several cycles and may be potentially scaled up for industrial applications
1. INTRODUCTION
An alternative approach for sulfur removal is oxidative desulfurization9−11 where petroleum sulfur compounds are oxidized by various agents such as sulfuric acid, nitric acid, potassium persulfate, nitrogen oxides, hypochlorites, peroxy acids, hydroperoxides, hydrogen peroxide, ozone, and molecular oxygen. The reactivity of oxidation increases with the higher electron density of the sulfur compounds. Thus, DBT is more readily oxidized than benzothiophene and thiophene reverses the reactivity of the HDS.12,13 As a result, sulfurcontaining organic compounds (sulfides, disulfides, mercaptans, thiophenes, and DBTs) are oxidized to the corresponding sulfoxides and sulfones.14−16 These compounds can be withdrawn by extraction, adsorption,17−21 or even membrane filtration22 methods. The process of extractive desulfurization is based on the better solubility of sulfur-containing components and aromatic hydrocarbons compared to nonaromatic ones in suitable polar solvents.23 Adsorption methods of fuel refining are based on the selective extraction of sulfur compounds by solid adsorbents.24 Using these methods, however, leads to the loss of refined diesel fuel, the more the higher sulfur content in the feedstock. Thus, where diesel fuel contains 10 000 ppm of sulfur (1%), after extraction of 95% of sulfur organic compounds, the loss of fuel is 4−6 wt %. To avoid the loss of products, it would be possible to replace the process of extracting sulfur- and oxygen-containing organic
Reducing the sulfur content in oil fractions is a permanent issue in the oil refining industry to meet the standards for motor fuels. Thus, the technical regulation of the Customs Union limits the sulfur content of class 5 diesel fuel to the level of 10 ppmw. Nowadays, the main industrial processes in oil refining, to reduce the sulfur content, are hydrotreating and hydrocracking. These technologies are referred to as “hydroprocesses”, i.e., processes where the main reaction is hydrogenolysis of sulfur-, nitrogen-, and oxygen-containing organic compounds in petroleum fractions. Hydrogenolysis results in hydrocarbons with hydrogen sulfide, ammonia, and water formation.1 Thus, mercaptans, sulfides, disulfides, and thiophenes are converted to paraffins or aromatic compounds with H2S release. Among the sulfur compounds, aliphatic ones (sulfides, disulfides, mercaptans, etc.) are the most facile to hydrogenate. Upon increasing the molecular weight of the fractions (and boiling temperatures, consequently), the rate of the hydrodesulfurization (HDS) decreases.2 Substituted thiophenes, benzothiophenes, and especially dibenzothiophenes (DBT) are sulfur compounds that are most difficult to remove from various oil fractions.3,4 The conversion of alkyl-substituted DBTs to hydrocarbons during middle distillates hydrotreating requires rather severe conditions due to steric hindrances of the sulfur atom (hydrogen pressure of 50 atm and higher and a temperature of 330−380 °C). The combination of factors above significantly increase the cost of processes.5−8 © 2019 American Chemical Society
Received: June 19, 2019 Accepted: July 17, 2019 Published: July 26, 2019 12736
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specific surface areas (500−630 m2/g) and pore volumes in the range of 0.5−1.2 cm3/g. The ordered porous structure of samples is shown in transmission electron microphotographs (Figure 1).
compounds (oxidation products) for their conversion into hydrocarbons. One of such oxidation products of sulfuric compounds are sulfones (organosulfur compounds having two oxygen atoms attached to sulfur). In this regard, the development of catalytic processes for selective removal of sulfur from the sulfone molecule is of current interest. It should be noted that there are a limited number of reports on this issue in the literature. It is known that the desulfonylation reaction (the destruction of sulfones) is carried out with the help of catalysts based on oxides and hydroxides of alkali and alkaline earth metals.24−27 The most active desulfonylation agents, quite attractive in cost as well, are oxides of alkaline earth metals, namely, MgO28,29 and CaO.30 Their catalytic activity can be increased by various carriers having a developed surface (to ensure an optimal distribution of active sites of the catalyst). One class of such supports is the mesoporous materials having a pore size from 2 to 50 nm.31 Such materials are characterized by an ordered structure and a narrow pore size distribution.32−35 Owing to these properties, branched sulfur compounds penetrate into the pores and it becomes possible to use materials in the field of heterogeneous catalysis,36−41 oxidative desulfurization,42−44 as well as in sorption processes.45,46 The silicate structure of mesoporous materials has no catalytic activity; therefore, for carrying out various processes, surface modification or introduction of active components into the pore volume is required. In the present work, catalytic desulfonylation was studied using a composition of basic oxides (MgO, CaO, and BaO) supported on mesoporous carriers of Al-SBA-15, Al-MCM-41, and MCM-41 types.
Figure 1. Microphotographs of CaO/MCM-41 (1) and MgO/MCM41 (2) samples.
It can be seen that the synthesized catalysts possess an ordered pore structure with a pore diameter of about 3 nm (MCM-41 supported). The formation of the ordered mesoporous structure is confirmed by low-temperature N2 adsorption/desorption data and low-angle X-ray diffraction (XRD) analysis (Figure 2).
2. RESULTS AND DISCUSSION 2.1. Catalyst Synthesis and Characterization. The samples containing MCM-41 and Al-MCM-41 carriers were prepared by impregnation thereof with metal salt solutions. MgO/Al-SBA-15 and CaO/Al-SBA-15 catalysts were prepared by direct synthesis (the mesoporous structure of aluminosilicates was doped with active components during formation thereof). After preparation, the composition of catalysts obtained was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Table 1).
Figure 2. X-ray diffraction pattern of the MgO/MCM-41 catalyst.
The catalyst with the MCM-41 support (for example) shows a strong peak at about 2.5° (2θ) due to (100) diffractions lines and weak peaks between 4 and 4.5° (2θ) and between 4.5 and 5.2° (2θ) due to higher-order (110) and (200) diffractions, respectively, indicating the formation of well-ordered mesoporous materials. All of these patterns are assigned to the hexagonal symmetry of MCM-41-type mesoporous silica.32 2.2. Reactions in the Autoclave. 2.2.1. Model Feed. Diesel fractions contain a large number of substituted condensed thiophenes.42 Typical representatives of such compounds are DBT and its derivatives. DBT molecules are resistant to hydrogenation due to steric hindrance, so this compound was chosen as a model compound in the oxidation reaction to sulfones. The thermal destruction of DBT sulfone (DBTS) at temperatures 300−450 °C was performed as a comparison experiment (Table 2). The composition of the products was determined by gas chromatography (GC). It was revealed that at temperatures up to 350 °C inclusive DBTS is stable under the given conditions. However, when it reaches 400 °C or more, DBTS starts to decompose. A sharp decrease in the amount of sulfone at 450 °C is due to the process of thermal cracking in a solvent with the evolution of hydrogen, as a result of which a reduction of DBTS to DBT takes place in a closed system.
Table 1. Quantitative Analysis of Oxide/Aluminosilicatea Catalysts quantity of oxides, wt % sample
SiO2
MgO
MgO/MCM-41 CaO/MCM-41 BaO/MCM-41 MgO/Al-MCM-41 CaO/Al-MCM-41 MgO/Al-SBA-15 CaO/Al-SBA-15
91.45 89.65 89.16 80.23 80.57 79.56 77.05
8.01
CaO
BaO
10.26 9.58 9.34 8.89 9.26 9.03
a
Si/Al molar ratio equals 10.
The textural characteristics of catalysts were investigated using low-temperature adsorption−desorption of nitrogen. The adsorption isotherms of samples correspond to type IV with a hysteresis loop, which is characteristic of mesoporous materials (Figure S1). All of the catalysts obtained have high 12737
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2.2.2. Real Feed. Experiments in the autoclave were also carried out on the thermal and catalytic decomposition of oxidized sulfur compounds, which are a part of the oxidized diesel fraction. Oxidation was conducted with a homogeneous catalytic system (formic acid and hydrogen peroxide). The oxidized diesel fraction was subjected to thermal decomposition at 300 °C and catalytic decomposition at 300 and 400 °C (Figure 4).
Table 2. Results of DBTS Destruction at Various Temperatures (300−450 °C) and a Reaction Time of 1−6 h DBTS conversion, % temperature, °C
1h
3h
6h
300 350 400 450
0 0 0 70
0 0 20 100
0 0 25 100
When studying the general trends of the catalytic destruction of DBTS, the tests were first carried out with unsupported calcium and magnesium oxides to compare with the published results.28,30 As shown by experiments, the degree of sulfone removal was 0% at 300 °C. This indicated the inapplicability of these oxides under above conditions and the necessity of using a mesoporous carrier. Thus, further studies on the catalytic degradation process were carried out using the CaO/MCM-41 catalyst under various conditions of temperature, reaction time, and catalyst/feedstock ratio. It was found that with the increase of temperature (Table 3) the degree of sulfone removal increased, which is partly due to the contribution of the thermal destruction process.
Figure 4. Sulfur content in the products of the sulfone decomposition at various temperature regimes (in real feedstock samples).
Table 3. Destruction of DBTS on CaO/MCM-41 at Different Temperatures (300−400 °C) and a Reaction Time of 3 h temperature, °C
DBTS removal degree, %
300 350 400
20 24 42
As can be seen in Figure 4, the sulfur content is not decreased in the case of thermal decomposition of fuel; however, with the addition of the catalyst, a sharp decrease in the amount of sulfur in the product is observed, which is directly related to the adsorption effect of the mesoporous carrier. The figure shows that without heating the system about 60% of the sulfur compounds are adsorbed inside the pores of the catalyst. When heated to 400 °C, the sulfur content decreases by 80%. 2.3. Reactions in the Flow System with the Fixed-Bed Catalyst. Due to high contribution of adsorption during reactions in an autoclave, catalytic tests of the catalysts for the destruction of sulfones on real feedstock were carried out in a flow system. The experiment was carried out at the operating temperature of the process in the absence of a catalyst as a blank test. These blank experiments were carried out at 300 and 400 °C. The oxidized diesel fraction was passed through a heated steel reactor in a circulating mode; after cooling, the reaction products were fed by a pump to the feed tank, from which the mixture was fed back to the reactor. The samples are taken every 30 min, and the sulfur concentration in the sample is measured. It is established that an increase in temperature from 300 to 400 °C does not significantly reduce the sulfur content of the product, but it accelerates the setting of steady-state desulfurization (Figure 5). Furthermore, the oxidized diesel fraction samples were desulfurized with the catalyst in a flow-through mode. Based on the sulfur content of each sample, a kinetic curve is constructed for each experiment. The typical kinetic curve is represented in Figure 6. As can be seen, sulfur concentration decreases steeply due to adsorption properties of the support. The amount of adsorbed sulfur is approximately 1.6 mg/g of catalyst, which is typical for mesoporous materials.47 Then, fresh fuel stream elutes the adsorbed substances and the decomposition of sulfones begins. The decomposition of sulfur compounds occurs in the first 60 min of the experiment and then proceeds to a certain final value, which is symbolized by the gradual emergence of the kinetic curve on the plateau.
Experiments on varying the duration of the sulfone destruction process were carried out at 300 and 400 °C. It was found that as the reaction time increases the amount of the removed sulfone increases at 400 °C (Figure 3).
Figure 3. DBTS Conversion in the Presence of the CaO/MCM-41 Catalyst.
However, when the reaction is carried out at a lower temperature, the degree of sulfone removal decreases with the increase of reaction time. This fact can be explained by desorption of unreacted sulfone from the pores of the carrier. The composition of the reaction mixture of catalytic degradation at 400 °C was investigated by GC with a mass spectrometric detector, and it was found biphenyl as the main product of the DBTS destruction (Figure S2). Moreover, DBT was observed as a byproduct in the mixture, which can be explained by the reduction of the sulfone with hydrogen liberated by partial cracking of hexadecane (a reaction solvent). 12738
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material type in the support on the sulfur removal activity, additional investigations were made. The acidity of the supports was measured using the thermoprogrammed ammonia desorption technique (Table 4). Table 4. Quantity of Ammonia Desorbed, μmol/g sample
medium acid sitesa
strong acid sitesb
total quantity of desorbed ammonia
Al-SBA-15 Al-MCM-41 MCM-41
36 82 25
142 33 53
178 115 78
Ammonia desorbed below 300 °C. bAmmonia desorbed above 300 °C. a
Figure 5. Desulfurization in the course of thermal destruction of feedstock in a flow reactor (blue line, 300 °C; violet line, 400 °C).
As can be seen from the table, the acidity of the samples increases with the addition of alumina into the mesoporous structure. That means that alkaline earth metals are more active in the desulfonylation reaction when supported on a silica-based mesoporous material rather than aluminosilicate one. Additional analysis was made to compare the basicity of catalysts with that of mesoporous supports. The basicity is characterized by chloroform adsorption using IR spectroscopy. The CHCl3 molecule is used as a probe that is adsorbed by basic sites of a catalyst. The position of the C−H stretching vibration is shifted to the low-frequency region relative to the chloroform band in the liquid phase (3033 cm−1) due to the interaction of hydrogen with the basic centers of the catalyst. Therefore, the shift of the band for the studied samples indicates the presence of the basic centers in them, and according to the degree of the shift, the strength of these centers is estimated.48−50 IR spectra of adsorbed chloroform on pure MCM-41 and CaO (MgO)/MCM-41 are represented in Figure 8.
Figure 6. Kinetic reaction curve in the presence of MgO/Al-SBA-15 at 400 °C; LHSV = 20 h−1.
When investigating the catalytic activity of the samples, AlSBA-15- and Al-MCM-41-supported catalysts were first compared. It revealed that at 400 °C the decomposition degree of sulfones was almost the same for both supports (60% for CaO and 69% for MgO). However, at 300 °C, Al-MCM41-supported catalysts were more active (3% more for CaO and 10% more for MgO). Then, MCM-41-supported catalysts were tested for comparison with Al-MCM-41 ones. The results of catalytic decomposition of sulfones in oxidized diesel fraction are summarized in Figure 7. As can be seen, optimal results on sulfur reduction in the fraction can be achieved by magnesium oxide as an active component. The sample containing BaO showed the worst conversion when compared to that containing CaO and MgO. As for the carrier, it can be seen that the one without Al shows better results. To study the dependence of mesoporous
Figure 8. IR spectra of adsorbed CHCl3 (peq = 2 Torr) on MCM-41 samples (spectra of the samples before adsorption are subtracted).
For the MCM-41 sample, only one band of 3031 cm−1 is fixed, the appearance of which is accompanied by a decrease in the band of 3743 cm−1 of free silanol groups and the appearance of the 3687 cm−1 band (perturbed silanol groups). The negative area of the spectra (3743 cm−1) corresponds to the reduced bands intensity after the adsorption of the CHCl3. The frequency of C−H vibrations, close to the vibration of
Figure 7. Sulfur removal from oxidized diesel in the presence of synthesized catalysts. 12739
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catalyst occurs at a lower degree than resinification of the feedstock. For regenerated MgO/MCM-41 and CaO/MCM-41 catalysts, the textural characteristics were analyzed by the method of low-temperature adsorption−desorption of nitrogen (Table 5).
chloroform in the liquid phase, indicates a weak interaction of chloroform with the surface of the support, probably through coordination with Si−OH groups. When chloroform is adsorbed on samples MgO/MCM-41 and CaO/MCM-41, low-frequency shoulders appear at 2994 and 2981 cm−1, respectively, which indicate the presence of basic centers on these samples.49 Obtained data correlates with the literature28−30 where basic materials (catalysts) are used for the desulfonylation reaction. The degree of sulfur removal in the sulfone destruction process is comparable to that in oxidative desulfurization followed by adsorption or extraction.51 One of the main reasons for insufficient sulfur removal is the incomplete oxidation of sulfur compounds. To prove this hypothesis, infrared spectroscopy was carried out. IR spectra of the diesel fraction before oxidation and after the sulfone destruction process were taken. The results (Figure 9) show that the SO2
Table 5. Textural Characteristics of Catalysts S (BET), m2/g Vpores, cm3/g MgO/MCM-41 before reaction MgO/MCM-41 after regeneration CaO/MCM-41 before reaction CaO/MCM-41 after regeneration
Dpores, Å
508 528
0.42 0.44
50 55
527 377
0.66 0.70
27 30
As can be seen from the data, the specific surface area of the sample MgO/MCM-41, along with the remaining characteristics, slightly increases. In contrast, the CaO/MCM-41 catalyst decreases the value of the specific surface area, but the pore diameter remains unchanged. An isotherm of type IV with a hysteresis loop is observed for all samples. Taking into account these data, the catalytic activity tests were carried out using the regenerated MgO/MCM-41 catalyst (Figure 10).
Figure 9. IR spectra of diesel fuel before oxidation (left) and after the reaction (right).
Figure 10. Degree of sulfur removal in the presence of fresh (A) and regenerated catalysts (B) in flow mode.
peaks (sulfones) are not observed in the product, which means all of the oxidized sulfur compounds are destroyed completely. The thiophene content is greatly reduced, but the residual content is still observed in the product of the reaction. This proves that the reaction of desulfonylation effectively converts all of the oxidized sulfur compounds to the corresponding sulfur-free hydrocarbons (in contrast to the adsorption or extraction technique). However, to achieve full removal of sulfur, special conditions are needed to oxidize the residual sulfur. 2.4. Catalysts and Feed Properties after the Reaction. The unloaded catalyst after the reaction was in the form of a powder of dark gray color, which indicated that the feedstock was partly resinified and coked during the reaction. The amount of coke was determined by the thermogravimetric analysis−differential scanning calorimetry method (Figure S3). The weight loss in the first stage (to 290 °C) was 8.7%. In the second stage, in the temperature range 290−640 °C, the loss was about 4.3% (the maximum weight loss rate was observed at the temperature of about 490 °C). On the basis of the indicated data, it can be concluded that coke formation on the
With the use of the regenerated catalyst, the conversion of sulfur compounds is reduced at 300 and 400 °C by 3 and 4.5%, respectively. The test on the catalyst activity was made additionally at 400 °C during four more recycles. After each recycle, the catalyst was regenerated in the air flow at 650 °C. The results of several recycles show that the catalyst activity is retained; however, it is slightly lower than that in the first run (Figure 11). The fractional composition of the hydrocarbon fraction was investigated by the simulated distillation method. The original diesel fraction before oxidation, the oxidized fraction that served as the feedstock for the reaction, and the product mixture after the reaction (at 300 and 400 °C) were studied (Table 6). As a result of the reaction at 400 °C, the yield of the diesel fraction does not decrease. However, at 300 °C, there is a slight decrease in the fraction (350 °C to EBP), which can be explained by partial resinification of heavier components of the fraction and deposition on the catalyst. 12740
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Catalysts based on Al-SBA-15 were synthesized by the direct synthesis method. In a typical synthesis, the mixture of (EtO)4Si (TEOS) and (sec-BuO)3Al was added to a dilute solution of HCl (pH = 1.5). Then, Pluronic P123 polymer was dissolved in a HCl solution (pH 1.5). Portion of Mg(OAc)2· 4H2O (Ca(NO3)2·4H2O) was then added to the Pluronic P123 solution. Then, a mixture of silicon and aluminum compounds in a dilute HCl solution was added to the solution of the template and the metal salt at 40 °C and was stirred for 3 h. The flask was sealed, placed in an oven, and held for 48 h at 95 °C. The resulting precipitate was evaporated on a rotary evaporator and air-dried. The sample was then dried for 5 h at 90 °C, for 2 h at 110 °C, heated in a muffle furnace at a rate of 1 °C/min to 600 °C, and calcined at this temperature in air for 4 h. Reagents quantities used for synthesis are given in Table S1. 4.2. Characterization. Adsorption/desorption isotherms of nitrogen were determined at 77 K using a Micromeritics Gemini VII 2390t equipment. Before measurements, the samples were degassed at 350 °C for 6 h. To calculate the surface area, the BET method was applied with adsorption data in the range of relative pressures (P/P0) of 0.04−0.20. The pore volume and the pore size distribution were determined from the adsorption branch of the isotherms using the Barrett−Joyner−Halenda model. The specific volume of the pores was determined based on the amount of adsorbed nitrogen at a relative pressure (P/P0) of 0.99. The transmission electron microscopy analysis was performed using a LEO 912 ABOMEGA apparatus, with magnification from 80 to 500 000× and image resolution of 0.2−0.34 nm. X-ray diffraction (XRD) analysis was carried out using a Rigaku Rotaflex RU-200 apparatus (Cu Kα radiation) in the 2θ range of 5−100°, with a goniometer (Rigaku D/Max-RC) at a rotation speed of 1° 2θ/min and a step of 0.04°. The content of silicon, aluminum, magnesium, calcium, and barium was measured by an iCAP-6500 Duo atomic emission analyzer from Thermo Fisher Scientific with a plasma source of radiation (ICP-AES). The analyzed sample was transformed into a soluble state; then, the solution was sprayed with a strong argon stream and subjected to a stream of atomization gas in a spark discharge. Radiation in the near ultraviolet and visible region (166−847 nm) emitted by the probe was compared with the emission of standards with a known content of the analyzed elements. The acidity of mesoporous materials was determined by the NH3-temperature programmed desorption (TPD) method with a USGA-101 instrument (UNISIT, Russia). The test sample of ∼0.1 g with a particle size of 0.5−0.25 mm was placed in a quartz reactor and treated in a flow of helium at 500 °C for 1 h with subsequent blowing by nitrogen. Saturation was performed in a flow of dried ammonia diluted
Figure 11. MgO/MCM-41 catalyst recycle tests at 400 °C; LHSV = 20 h−1.
3. CONCLUSIONS In this study, different catalysts for sulfone destruction were synthesized and characterized. The experiment on model feed showed that the mesoporous carrier was essential for sulfur removal from DBT sulfone and other sulfones contained in diesel fractions. The use of the mesoporous material also aids in reducing the active component content that is needed for efficient removal of oxidized sulfur compounds (compared to literature data24). A comparison between different catalyst supports revealed that mesoporous materials containing silica show better results in the desulfonylation reaction due to its low acidity. Thus, the use of sulfone destruction catalysts based on mesoporous materials makes it possible to achieve a reduction in the sulfur content in diesel fuel by 75%. The degree of sulfur removal is comparable to adsorption or extraction methods while saving valuable hydrocarbons in diesel fuel. The mesoporous structure of the catalyst is not destroyed, and the fractional composition of diesel fuel remains practically unchanged. 4. EXPERIMENTAL SECTION 4.1. Catalyst Preparation. Catalysts based on MCM-41 and Al-MCM-41 were prepared using the impregnation procedure with excess of water. Mesoporous materials MCM-41 and Al-MCM-41 were synthesized previously using standard techniques.52,53 Mg, Ca, and Ba oxides were incorporated using water-soluble salts: Mg(OAc)2·4H2O, Ca(NO3)2·4H2O, and Ba(OAc)2·3H2O, respectively. The calculation was carried out on the oxide content in the catalyst in amount of 10%. The mixture of catalyst and impregnation solution was stirred for 4 h without heating. Excess of water was vapored using a rotary evaporator. The solid residue was dried at 110 °C for 2 h and then calcined in a muffle furnace at 600 °C for 4 h. Table 6. Characteristics of Distillation Fractions sample diesel fraction before oxidation oxidized diesel fraction mixture of products (test using mixture of products (test using mixture of products (test using mixture of products (test using
Tb < 200 °C, %
200 °C < Tb < 350 °C, %
Tb > 350 °C, %
19 23 24 24 25.7 21
74 71 70.5 71.5 69 73.2
7 6 5.5 4.5 5.3 5.8
MgO/MCM-41 cat.; 300 °C) MgO/Al-MCM-41 cat.; 300 °C) MgO/Al-SBA-15 cat.; 300 °C) MgO/Al-SBA-15 cat.; 400 °C) 12741
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with nitrogen at the temperature of 60 °C for 15 min. Physically adsorbed ammonia was removed at 100 °C in a flow (30 mL/min) of dry helium for 1 h. For obtaining a TPD curve, the sample was cooled down to 50−60 °C and then the temperature was ramped (8 °C/min) up to 500 °C. The basicity of catalysts was measured by the chloroform chemisorption method. The sorption degree was detected by IR spectroscopy with a Nicolet Protégé 460 spectrometer (optical resolution, 4 cm−1; range, 4000−400 cm−1). Samples were activated in the IR cell at 400 °C (7.5 °C/min heating rate) and 10−5 Torr during 1 h. Chloroform adsorption was carried out at room temperature by addition of CHCl3 portions until the equilibrium pressure was 2 Torr. 4.3. Activity Measurement. Thermal and catalytic decomposition reactions were carried out in a stirred stainless steel autoclave (45 cm3) wherein 0.1 g of catalyst powder and 4 mL of model feed were placed. To prepare the model feed, DBT sulfone was dissolved in a solvent mixture consisting of 70 vol % benzene and 30 vol % hexadecane. The benzene/ hexadecane ratio was selected based on solubility of the amount of sulfone required to produce a mixture with the elemental sulfur content of 500 ppm. While using real feedstock (oxidized diesel fraction), the amounts of catalyst and feedstock were increased by 2 times. Reactions in the flow system were carried out in a laboratory catalytic unit with a fixed-bed catalyst at atmospheric pressure. The tests were carried out at 300 and 400 °C. Samples were taken after 15, 30, and 60 min of the feedstock time-on-stream and then hourly. The experiments were performed until the sulfur content in two subsequent samples differed less than 5 ppm. The reaction product composition of model mixtures and control of the purity of starting materials was analyzed by gas chromatography (GC) using “Kristall-2000M” with the flame ionization detector (glass capillary column, l = 30 m, d = 0.32 mm, liquid phase ZB-1, thermoprogrammed from 100 to 250 °C, and nitrogen as a carrier gas). Chromass spectrometry was performed with a LECO Pegasus 4D instrument for one-dimensional chromatography in electron impact mode with an ionizing radiation energy of 70 eV. Separation by GC was carried out on a silicone capillary column Rxi-5Sil MS (30 m, diameter 0.25 mm) thermoprogrammed from 50 °C (for 2 min) to 280 °C (for 5 min) with a rate of 20 °C/min. The scanned masses are 29−500 Da. Sulfur concentration in liquid products was determined using an energy-dispersive X-ray fluorescent sulfur analyzer by ASTM D4294.54 Fourier transform infrared (FTIR) spectra of diesel fractions were recorded on a Nicolet IR200 FTIR spectrometer in the range of 500−4000 cm−1. The fractional composition of the possible liquid products was obtained by simulated distillation by ASTM D288755 using a Chromos GC-1000 chromatograph. The coke content in the catalyst was determined gravimetrically using an SDT Q600 device from Thermal Analysis Instruments. The temperature ramp was 5 °C/min in the air flow up to 800 and 1000 °C.
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Reagents quantities for catalyst synthesis; adsorption isotherm of the MgO/Al-SBA-15 sample; chromatogram of the product mixture of DBTS destruction at 400 °C; and TGA curves of the spent MgO/MCM-41 catalyst (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Eduard Karakhanov: 0000-0003-4727-954X Oleg Golubev: 0000-0002-8558-3094 Alexander Anisimov: 0000-0001-9272-2913 Aleksandr Glotov: 0000-0002-2877-0395 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported by the Ministry of Education and Science of the Russian Federation, FTP Activity 1.3, Agreement on granting the subsidy No. 14.607.21.0173 of 26.09.2017. The unique identifier of applied scientific research is RFMEFI60717X0173. The authors thank the group of companies “Chromos” for providing equipment (Chromos GC-1000 gas chromatograph).
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b01819. 12742
DOI: 10.1021/acsomega.9b01819 ACS Omega 2019, 4, 12736−12744
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