RU2388535C1 - Catalyst, method of its preparation and method of methane oxidation - Google Patents

Abstract

FIELD: process engineering. ^ SUBSTANCE: invention relates to complete methane oxidation catalysts and can be used in industries using diesel fuel. Invention covers complete methane oxidation catalysts based on strontium hexaferrites of the following composition: SrMnxFe12-xO19, where x=0, 1, 2, 6. Proposed method comprises settling catalyst components with the help of NH4HCO3 solution at constant pH equal to (7.1 to 8.0) and temperature not lower than 70C with subsequent stages of filtration, rinsing, drying and roasting. Proposed method comprises also the stage of heat treatment at 800 to 1000 C and is realised in the presence of above described catalysts. ^ EFFECT: high degree of methane conversion at relatively low temperatures. ^ 6 cl, 2 tbl, 14 ex

Description

The invention relates to catalysts for the complete oxidation of methane and a process for their preparation in order to protect the environment from pollution caused by untreated emissions from various enterprises characterized by high activity and thermal stability.

To date, the catalytic method of neutralizing various gas emissions from industries containing carbon monoxide, flue gases, hydrocarbons, including methane, and other components has proven itself well.

In recent years, the processes of catalytic oxidation of hydrocarbons, which have found industrial applications in the form of chemical heaters, catalytic combustion chambers, catalytic afterburners of exhaust gases and volatile organic compounds, etc., have developed greatly. [R. Kikuchi, Y. Tanaka, K. Sasaki, K. Eguchi, High temperature catalytic combustion of methane and propane over hexaaluminate catalysts: NO _x emission characteristics. Catal. Today 23 (2003) 223-231], which are carried out in the presence of heat-resistant catalysts.

Among various hydrocarbons, methane oxidation is receiving increased attention as the cleanest way to produce energy without any harmful emissions of CO, NO _x and unburned hydrocarbons [J.Wang, Zh.Tian, J.Xu, Yu.Xu, Zh.Xu, L. Lin, Preparation of Mn substituted La-hexaaluminate catalysts by using supercritical drying. Catal. Today 23 (2003) 213-222]. The inert nature of methane requires relatively high temperatures for its oxidation. In addition, active catalysts are also required for the afterburning of gas emissions containing small concentrations of CH ₄ , since the methane oxidation reaction is highly exothermic [VVAlegre, MAP da Silva, M. Schmal Catalytic combustion of methane over palladium alumina modified by niobia. Catal. Commun. 7 (2006) 314-322] and proceeds according to the equation:

CH ₄ + 2O ₂ = CO ₂ + 2H ₂ O; ΔH ₂₉₈ = -802.7 kJ / mol.

Therefore, one of the main problems in the development of a catalytic process for the oxidation of methane is the search for suitable catalysts with high activity and high thermal stability (800-1000 ° C).

It is known [P.Artizzu-Duart, JMMillet, N. Guilhaume, E. Garbowski, M. Primet, Catalytic combustion of methane on substituted barium hexaaluminates. Catal. Today 59 (2000) 163-177}, that systems based on Mn (Fe) -substituted hexaaluminates, characterized by high thermal stability due to their unique layered structure [G. Groppi, C. Cristiani, P. Forzatti, Preparation characterization and catalytic activity of pure and substituted La-hexaaluminate systems for high temperature catalytic combustion. Appl. Catal. B.35 (2001) 137-148; US 7442669, B01J 23/00, C01F 7/02, 10.28.2008], are widely used in the oxidation of methane. Among the monosubstituted BaMAl ₁₁ O ₁₉ hexaaluminates (M = Mn, Fe, Cr, Co, Ni), according to [L., Lietti, C. Cristiani, G. Groppi, P. Forzatti, Preparation, characterization and reactivity of Me-hexaaluminate (Me = Mn, Co, Fe, Ni, Cr) catalysts in the catalytic combustion of NH ₃ -containing gasified biomasses. Catal. Today 59 (2000) 191-204}, the most active are Fe- and Mn-containing samples. The methane conversion on these catalysts begins at a temperature of 550-600 ° C, and the temperature at which 50% conversion is achieved is 700 ° C. The reactivity of Co-, Cr- and Ni-substituted hexaaluminates is somewhat lower. Since Mn- and Fe-containing hexaaluminates are characterized by higher activity in the methane oxidation reaction, more research is devoted to them. A comparison of temperatures corresponding to 50% methane conversion on various hexaaluminates showed (Table 1) that BaMn ₃ Al ₉ O ₁₉ , for which T ₅₀ is 560 ° C, is more active. Thus, the catalysts based on substituted hexaaluminates allow the oxidation of methane, however, the temperature corresponding to 50% conversion varies between 560-776 ° C.

To reduce the indicated methane conversion temperatures, systems based on alkaline-earth hexaferrites MFe ₁₂ O ₁₉ (M = Ca, Sr, Ba) may be of interest.

Closest to the claimed technical essence is a catalyst for the oxidation of methane based on barium hexaferrite, described in [G. Groppi, C. Cristiani, P. Forzatti, BaFe _x Al _12-x O ₁₉ system for high temperature catalytic combustion: physico-chemical characterization and catalytic activity. J. Catal. 168 (1997) 95-103].

Table 1 The activity of hexaaluminates in the oxidation of methane. Hexaaluminates T _50% , ° C Literature BaAl ₁₂ O ₁₉ 810 G. Groppi, M. Bellotto, C. Cristiani, P. Forzatti, P. L. Villa, Preparation and characterization of hexaaluminate-based materials for catalytic combustion. Appl. Catal. A 104 (1993) 101-108. BaMn _0.5 Al _11.5 O ₁₉ 700 BaMn ₁ Al ₁₁ O ₁₉ 660 BaMn ₂ Al ₁₀ O ₁₉ 642 BaAl ₁₂ O ₁₉ 770 P. Artizzu-Duart, J. M. Millet, N. Guilhaume, E. Garbowski, M. Prime Catalytic combustion of methane on substituted barium hexaaluminates. Catal. Today 59 (2000) 163-177. BaMnAl ₁₁ O ₁₉ 610 BaMn ₂ Al ₁₀ O ₁₉ 590 BaMn ₃ Al ₉ O ₁₉ 560 BaFeAl ₁₁ O ₁₉ 635 BaFe ₂ Al ₁₀ O ₁₉ 605 BaFe ₃ Al ₉ O ₁₉ 625 LaFeAl ₁₁ O ₁₉ 702 X. Ren, J. Zheng, Y.Song, P. Liu, Catalytic properties of Fe and Mn modified lanthanum hexaaluminates for catalytic combustion of methane. Catal. Commun. 9 (2008) 807-810. LaFeMn _0.5 Al _10.5 O ₁₉ 624 LaFeMn ₁ Al ₁₀ O ₁₉ 621 LaFeMn ₂ Al ₉ O ₁₉ 662 LaFeMn ₃ Al ₈ O ₁₉ 776

The catalyst is obtained by precipitation of nitric salts of barium and iron with an aqueous solution of (NH ₄ ) ₂ CO ₃ at pH 7.5 and a temperature of 60 ° C. The precipitates obtained after filtration and washing were dried at 110 ° C. The dried samples were calcined at 500, 700, 900, 1000 ° C. One of the characteristics of the catalyst is the specific surface area: for a catalyst calcined at 700 ° C, it is 7.5 m ² / g, and for a catalyst calcined at 900 ° C, it is 5.0 m ² / g.

The activity of the catalysts was determined in the light-off mode and was characterized by the temperature (T ₅₀ and T ₉₀ ) at which the methane conversion was 50 and 90%, respectively. The reaction mixture containing: CH ₄ - 1 vol.%, The rest is air, was passed at a pressure of 1 atm. And a space velocity of 48000 h ^-1 through a catalyst bed, the volume of which was 0.5 cm ³ , at this is the ratio of V _cat. / V _inert = 2 (quartz was used as an inert diluent). On a BaFe ₁₂ O ₁₉ catalyst calcined at 700 ° C, T ₅₀ = 533 °, T ₉₀ = 600 ° C. An increase in the calcination temperature to 900 ° C led to a decrease in activity: T ₅₀ = 575 °, T ₉₀ = 649 ° C.

A disadvantage of the known catalyst is the relatively low activity in the oxidation of methane, possibly due to the relatively low specific surface area. In addition, since water is formed during the methane oxidation reaction, it can affect the catalytic properties of the catalyst, however, there is no evidence of such an effect. Of particular interest are the results of testing catalysts in gas mixtures less concentrated with respect to methane, since the concentration of methane in the emissions of various enterprises does not exceed 0.1–0.5 vol%, and, as noted above, more active reactions are required for the oxidation of methane at such concentrations catalysts.

The invention solves the problem of obtaining an active catalyst for the oxidation of methane in order to reduce temperatures corresponding to the corresponding degrees of methane conversion.

The problem is solved using a catalyst based on strontium hexaferrite composition: SrMn _x Fe _12-x O ₁₉ , where: x = 0, 1, 2, 6.

The problem is also solved by the method of preparation of the catalyst, which is prepared by precipitation of a mixed solution of salts of strontium and iron and manganese, preferably nitric acid, taken in the desired ratio, with an aqueous solution of NH ₄ HCO ₃ at a pH of 7.1 ÷ 8.0 and a temperature of at least 70 ° C at vigorous stirring, followed by maintaining the suspension for a certain time under the specified conditions. After that, the precipitate is filtered off and washed with distilled water, dried in air, then at 110-120 ° C for 12-14 hours, after which it is calcined at 700 ÷ 900 ° C.

The calcined catalyst is subjected to additional heat treatment - thermal "aging" at a temperature of 800-1000 ° C in a mixture containing not more than 10 vol.% H ₂ O, the rest is nitrogen, for 14-16 hours

The problem is also solved by the method of methane oxidation, the concentration of which in the reaction mixture is not higher than 0.1 vol.%, In the presence of the catalyst described above.

Distinctive features of the proposed catalyst and method for its preparation are:

1. The composition of the catalyst based on strontium hexaferrite - SrMn _x Fe _12-x O ₁₉ , where: x = 0, 1, 2, 6.

2. The method of preparation of the catalyst, including the precipitation of its components with an aqueous solution of NH ₄ HCO ₃ and a temperature of at least 70 ° C, providing a catalyst with a larger specific surface area: the catalyst calcined at 700 ° C is characterized by a specific surface of 30 ÷ 60 m ² / t

4. A method of producing a catalyst, including the stage of thermal "aging" at 800-1000 ° C in a gas mixture containing not more than 10 vol.% H ₂ O in nitrogen, for 14-16 hours

5. The method of oxidation of methane in the presence of the catalyst described above, the concentration of methane in the reaction mixture is not more than 0.1 vol.%.

The essence of the invention is illustrated in Figures 1, 2, 3 and the following examples showing the change in activity (temperatures at which the corresponding degrees of methane conversion are reached) depending on the composition of the catalyst, its preparation method and thermal “aging”, at a methane concentration in the reaction mixture of 0 , 1 vol.%.

The main characteristics of the catalysts and temperatures corresponding to the corresponding methane conversion values are shown in table 2.

Figure 1 presents the diffraction patterns of the Sr-Fe-O sample, differing in the processing temperature.

Figure 2 presents the theoretical diffraction pattern of strontium hexaferrite.

Figure 3 presents the IR spectra of the Sr-Fe-O sample, calcined at different temperatures.

According to the XRD data (FIG. 1), the samples calcined at 700 ° (Example 1) contain mainly SrCO ₃ and Fe ₂ O ₃ phases; in samples promoted with manganese, the Mn ₂ O ₃ phase is also present. Raising the treatment temperature to 900 ° C (example 2) promotes crystallization of the hexaferrite phase - SrFe ₁₂ O ₁₉ (Figure 1) and a decrease in the proportion of excess phases: α-Fe ₂ O ₃ , Mn ₂ O ₃ , while the strontium carbonate phase completely disappears .

Comparison of the theoretical [K. Kimura, M. Ohgaki, K. Tanaka, H. Morikawa, F. Marumo // J. Solid State Chem. 87 (1990) 186] and experimental diffraction patterns (Figs. 1 and 2) show that calcination of the samples at 900 ° C leads to the formation of the strontium hexaferrite phase — SrFe ₁₂ O ₁₉ . For the predominant hexaferrite phase, the parameters are: a = 5.88, c = 22.99 Å, which practically coincides with the tabular data: a = 5.886, c = 23.03 (ICDD, PDF-2, [00-33-1340]).

To identify the phase composition of the samples calcined at 700 ° C (example 1), the method of IR spectroscopy was used in the frequency range 250-2000 cm ^-1 . According to the results obtained (Figure 3), in the IR spectrum of the sample calcined at 900 ° C (example 2), items 288, 304, 359, 396, 438, 548, 591 and 779 cm ^-1 are observed, related to the SrFe ₁₂ O ₁₉ phase [Wen-Yu Zhao, Ping Wei, Hai-Bin Cheng, Xin-Feng Tang, and Qing-Jie Zhang, J. Am. Ceram. Soc. 90 (7) (2007) 2095; J. Jiang, L. AI, L. Li, J. Phys. Chem. In 113 (2009) 1376], and low-intensity pp. 323, 377, 458 and 799 cm ^-1 , characteristic of α-Fe ₂ O ₃ [E.N. Yurchenko, G.N. Kustova, S.S. Batsanov. Vibrational spectra of inorganic compounds. Novosibirsk, Nauka, 1981 p. 52; Onari S, Ari T., Kudo K. Phys. Rev. B 16 (1977) 1717]. Thus, the results obtained are consistent with the results of phase analysis, according to which the sample contains SrFe ₁₂ O ₁₉ and α-Fe ₂ O ₃ phases.

Testing of the catalysts in the methane oxidation reaction is carried out under conditions of a thermoprogrammed increase in the reaction temperature (light-off test) from 100 to 625 ° C with a heating rate of 10 ° C / min. The reaction mixture containing, vol.%: CH ₄ - 0.1, O ₂ - 20, Ne - 0.5, He - balance, is passed at a speed of 30 l / h through a catalyst bed, the volume of which is 0.6 cm ³ , fractional composition 0 , 25-0.50 mm at a space velocity of 50,000 h ^-1 . Analysis of the reaction gas mixture is carried out using a quadrupole mass spectrometer (Stanford Research Systems, QMS 200 Gas Analyzer).

Examples 1-7 illustrate the composition and method of producing catalysts based on strontium hexaferrite, while the methane oxidation reaction is carried out in the light-off mode from 100 to 625 ° C with a heating rate of 10 ° C / min.

Examples 8-14 illustrate the thermal "aging" of hexaferrite-based catalysts at 800-1000 ° C in a reaction mixture containing not more than 10 vol.% H ₂ O, the rest is nitrogen, for 14 ÷ 16 hours

Example 1

600 ml of distilled water are poured into a reactor placed in a thermostat, a pH meter is installed and heating of the reactor and stirrer are turned on; when the temperature reaches 70 ° C, a mixed solution of strontium and iron nitrate salts containing 4.9 g SrO and 45.1 g Fe ₂ O _{3 is} metered into the reactor at a rate of 30 ml / min, while adding 395 ml of NH ₄ HCO ₃ solution to maintain the pH of the precipitation, equal to 7.5. The resulting suspension was maintained under these conditions for 2 hours, after which it was filtered and washed with distilled water. The resulting precipitate was dried in air, then at 110 ° C for 12-14 hours, after which it was calcined at 700 ° C for 4 hours. The resulting catalyst composition SrFe ₁₂ O ₁₉ , corresponding to strontium hexaferrite.

Example 2

Similar to example 1, the difference is that the resulting catalyst is calcined at 900 ° C for 4 hours. The resulting catalyst composition SrFe ₁₂ O ₁₉ corresponding to strontium hexaferrite.

Example 3

Similar to example 1, the difference lies in the fact that a mixed solution of nitric acid salts of strontium, manganese and iron containing 4.9 g SrO, 3.7 g Mn ₂ O ₃ and 41.4 g Fe ₂ O _{3 is} dosed into the reactor. The resulting catalyst composition SrMnFe ₁₀ O ₁₉ corresponding to strontium hexaferrite substituted with manganese.

Example 4

Similar to example 1, the difference is that a mixed solution of nitric acid salts of strontium, manganese and iron containing 4.9 g of SrO, 7.5 g of Mn ₂ O ₃ and 37.6 g of Fe ₂ O _{3 is} dosed into the reactor. The resulting catalyst composition SrMn ₂ Fe ₁₀ O ₁₉ corresponding to strontium hexaferrite substituted with manganese.

Example 5

Similar to example 1, the difference is that a mixed solution of nitric acid salts of strontium, manganese and iron containing 4.9 g of SrO, 22.5 g of Mn ₂ O ₃ and 22.6 g of Fe ₂ O _{3 is} dosed into the reactor. The resulting catalyst composition SrMn ₆ Fe ₆ O ₁₉ corresponding to strontium hexaferrite substituted with manganese.

Example 6

Similar to example 1, the difference is that a mixed solution of nitric acid salts of strontium, manganese, iron and aluminum containing 5.2 g of SrO, 23.7 g of Mn ₂ O ₃ , 16.0 g of Fe ₂ O ₃ and 5.1 g of Al ₂ O _{3 is} dosed into the reactor . The resulting catalyst composition SrMn ₆ Fe ₄ Al ₂ O ₁₉ corresponding to strontium hexaferrite substituted with manganese and aluminum.

Example 7

Similar to example 1, the difference is that a mixed solution of nitric acid salts of strontium, manganese and aluminum containing 6.7 g of SrO, 10.3 g of Mn ₂ O ₃ and 33.0 g of Al ₂ O _{3 is} dosed into the reactor. The resulting catalyst for comparing the composition of SrMn ₂ Al ₁₀ O ₁₉ corresponding to strontium hexaaluminate substituted with manganese.

Example 8

Similar to example 1, the difference is that the calcined catalyst composition SrFe ₁₂ O ₁₉ was aged at 800 ° C in a reaction mixture containing 10 vol.% H ₂ O, the rest was nitrogen, for 16 hours

Example 9

Similar to example 8, the difference is that the calcined SrMnFe ₁₁ O ₁₉ catalyst corresponding to strontium hexaferrite substituted with manganese was aged.

Example 10

Similar to example 9, the difference is that thermal aging was carried out at 1000 ° C in a reaction mixture containing 10 vol.% H ₂ O, the rest was nitrogen, for 14 hours

Example 11

Similar to example 8, the difference is that the catalyst composition SrMn ₂ Fe ₁₀ O ₁₉ , corresponding to strontium hexaferrite substituted with manganese, was aged.

Example 12

Similar to example 8, the difference is that the catalyst composition SrMn ₆ Fe ₆ O ₁₉ , corresponding to strontium hexaferrite substituted with manganese, was aged.

Example 13

Similar to example 8, the difference is that the catalyst composition SrMn ₆ Fe ₄ Al ₂ O ₁₉ , corresponding to strontium hexaferrite substituted with manganese, was aged.

Example 14

Similar to example 8, the difference is that the SrMn ₂ Al ₁₀ O ₁₉ composition comparison catalyst corresponding to strontium hexaaluminate substituted with manganese was aged.

The methane oxidation rates for all examples are shown in table 2.

As can be seen from the above examples and the tables, the proposed catalysts allow us to solve the problem of efficient methane oxidation at a concentration of 0.1% by volume in the reaction mixture: the use of strontium hexaferrites both unsubstituted and substituted with manganese can reduce the temperatures corresponding to the corresponding degrees of methane conversion, approximately 100 ° C compared with strontium hexaaluminate substituted with manganese. An increase in the calcination temperature of strontium hexaferrite to 900 ° C leads to a decrease in activity in the oxidation of methane, however, it remains higher than on the reference catalyst calcined at 700 ° C. Even greater differences in activity are observed on the proposed catalysts and comparison catalysts after their thermal aging at 800-1000 ° C in a mixture containing water vapor for 14-16 hours

Claims (6)

1. Methane oxidation catalyst containing oxides of metals of groups II, III and VIII of the Periodic system, characterized in that it is a composition based on strontium hexaferrite composition: SrMn _x Fe _12-x O ₁₉ , where x = 0, 1, 2, 6. 2. The method of preparation of the catalyst for the oxidation of methane containing oxides of metals of groups II, III and VIII of the Periodic system, by precipitation with subsequent stages of filtration, washing, drying and calcination, characterized in that it is prepared by precipitation of a mixed solution of salts of Sr, Mn and Fe at constant values pH 7.1-8.0 and temperatures not lower than 70 ° С with an aqueous solution of NH ₄ HCO, and a composition based on hexaferrite of the composition: SrM _x Fe _12-x O ₁₉ , where x = 0, 1, 2, 6, is obtained. 3. The method according to claim 2, characterized in that the calcination is carried out at a temperature of 700-900 ° C. 4. The method according to claim 2, characterized in that the calcined catalyst is subjected to additional heat treatment at 800-1000 ° C. 5. The method of oxidation of methane in the presence of a catalyst, characterized in that it is carried out in the presence of a catalyst according to claim 1 or prepared according to claims 2-4. 6. The method according to claim 5, characterized in that the concentration of methane in the reaction mixture is not higher than 0.1 vol.%.

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