reactivity of the adsorbed methane forms in a C-C bond formation has been found at the joint conversion with n-pentane and with to aromatic hydrocarbons ...
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ScienceDirect Procedia Engineering 113 (2015) 79 – 83
International Conference on Oil and Gas Engineering, OGE-2015
Reactive capacity study of methane adsorbed in aluminic-platinum catalyst Ostanina N.V.a,b*, Golinsky D.V.a, Kryukova M.A.b, Pashkov V.V.a, Udras I.E.a, Beliy A.S.a,b a
Institute of Hydrocarbons processing SB RAS, 54, Neftezavodskaya St., Omsk 644040, Russian Federation b Omsk StateTechnical University, 11, Mira Pr., Omsk 644050, Russian Federation
Abstract The paper deals with the influence of the methane/platinum on the adsorption properties of aluminic-platinum catalyst. Methane dehydrogenation degree on the Pt/Al2O3 surface has been calculated for the entire range of the studied СН4/Pt ratios. A high reactivity of the adsorbed methane forms in a C-C bond formation has been found at the joint conversion with n-pentane and with to aromatic hydrocarbons formation. ©2015 2015Published The Authors. Published by Elsevier Ltd. © by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Omsk State Technical University. Peer-review under responsibility of the Omsk State Technical University Keywords: methane; n-alkane; aluminic-platinum catalyst; adsorption; binding site
1. Introduction Processing of natural (NG) and associated petroleum (APG) gas is a crucial task which remains unsolved since the 70s of the last century. In Russia, there is the annual 650-660 bln m3/year of natural gas and 69-72 bln. m3/year of associated gas production. Currently, natural gas is mainly used as fuel. Chemical industry consumes only 2.55.0% of the produced gas. The level of associated petroleum gas flame combustion is the highest in Russia (the amount of flame combustion in 2012 was 17.1 bln m3) [1], this leads to extremely negative economic, environmental and social consequences. Thus, the problem of the use of PG and APG is extremely relevant. The main component of natural and associated petroleum gas (up to 70 vol.%) is methane. Stable C-H bond in the СН4 molecule (dissociation energy of 435.8 kJ/mol) makes its effective use difficult on an industrial scale. A common method of methane activation is an appending of the oxygen-containing molecules into reactive atmosphere. In
* Corresponding author. Tel.: +7-962-053-5904. E-mail address: ostanina_n@mail.ru
1877-7058 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Omsk State Technical University
doi:10.1016/j.proeng.2015.07.295
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industrial scale СН4 oxidizing processes are implemented to obtain synthetic natural gas, further used to obtain oxychemicals and synthetic crude oil. One-step reaction of oxidative condensation into methane to С2–hydrocarbons has been extensively studied. [2] In the early 90-ies of XX century another way to activate СН 4 by conducting the reaction in two stages was described. In the first stage at temperatures of 573-723 K methane was chemisorbed with a С–Н bond breaking and a hydrogen molecule formation. Between proximate insufficient СНx (X≤3) units a C-C bond can be formed. To enter the gas phase the С2+–hydrocarbons precursors were at a lower temperature in a Н2 flow. [3] To date the question of methane adsorption with its subsequent activation and converting into С 2-hydrocarbons has been widely studied for catalysts with high metal content (3-10 wt. %) [4-6], mostly the VIII group. Another area of methane conversion is non-oxidizing conversion in the presence of high-silica zeolite catalysts [7-8]. The process is carried out in one stage at temperatures of 823-1023 K, and at atmospheric pressure. Currently, the greatest success has been achieved by Russian researchers from the Institute of Petroleum Chemistry. [9] However, extremely rapid deactivation of zeolite catalysts remains a serious problem. In this study we have studied the influence of the СН4/Pt ratio on the methane adsorption value in the presence of Pt/Al2O3 and reactive capability of the adsorbed СНх-particles in co-converting reaction with n-pentane. 2. Experimental 2.1 Catalyst preparation For the catalyst preparation the primary carrier γ-Al2O3 (CJSC "Industrial Catalysts" (Ryazan)) was modified by 3 wt. % ethanedioic acid water solution. Platinum in an amount of 1 wt. % from H2PtCl6 solution in acids competitors mixture – 1H chlorohydric acid (1 wt.%) and acetic acid 1H (1.5 wt%) – was applied on the sample dried at T = 413 K and calcined in dry air flow (T = 773 K). The dried (T = 393 K) catalyst was calcined in dry air flow at T = 773 K, and then disoxidated in a hydrogen atmosphere. 2.2 Catalyst testing The methane adsorption value was studied in the temperature range of 293-823 K. Methane was fed at a ratio of CH4/Pt - 1/1 – 16/1 mol/mol in the perfect-mixing reactor. The uploaded catalyst was pre disoxidated in a dry hydrogen flow at T = 773 K for 1 hour. Thereafter, to remove weakly bounded hydrogen from the system the catalyst was blown with argon for 2 hours. Then the reactor temperature was lowered to room temperature in the Ar flow, and a methane/argon mixture was added. Argon was used as an internal standard. After studying the adsorption properties the n-pentane was supplied to methane into the reactive atmosphere at T = 823 K in C5H12/СН4 = 1/1 ratio mol/mol. 3. Results and discussion It has been found that the beginning of СН4 adsorption is not dependent on the methane/platinum ratio and begins at 748 K (0.1 - 0.4 mol/mol Pt). With the temperature increasing up to 823 K, the methane adsorption value at СН4/Pt =1/1 mol/mol and СН 4/Pt =3/1 mol/mol and equals 0.2 and 0.7 mol СН4/ mol Pt, respectively (Fig. 1). The ratios range of СН4/Pt 8/1-16/1 mol/mol adsorption value reaches a maximum and equals 1,0-1,2 mol/mol Pt. In the reaction system in a gaseous phase at 823 K, the presence of hydrogen is observed, so when СН 4/Pt =1/1 mol/mol is 0.3 mol Н2/mol Pt. The ratios range of 3/1-16/1 mol СН4/mol Pt this value reaches 1.4-1.7 mol Н2/mol Pt (Fig.1).
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Fig. 1. The amount of adsorbed methane and released hydrogen at 823 K for1% Pt /Al 2O3 catalyst 1% Pt / Al2O3.
The obtained value dependence of methane adsorption on the СН4/Pt ratio corresponds to the Langmuir model, wherein at first adsorption increases proportionally to the gas concentration, then, as the metal sites on the surface are filled, the growth slows down and finally at high concentrations the excess adsorption is terminated, as the surface coating becomes close to the monolayer [10]. Based on the dependence the H/C ratio of the forming surface СНх-particles has been calculated (Fig. 2). It is clearly shown that with the increasing of СН4/Pt mol/mol ratio the dehydrogenation depth of methane decreases. When СН4/Pt =1/1 mol/mol and СН4/Pt =3/1 mol/mol the H/C ratio is 0.0 and 0.3 at./at., respectively, indicating the complete methane decomposition into carbon and hydrogen. The ratios range of 8/1-16/1 mol СН4/ mol Pt H/C varies from 1.1 to 1.4 at./at., indicating partial methane decomposition into C and Н 2 on the catalyst surface, and it makes it possible to assume that these particles can be active in co-converting with different hydrocarbon groups.
Fig.2. H/C ratio of the adsorbed hydrocarbon units.
In the n-pentane conversion in the absence of preadsorbed methane the conversion is 84.1%. There has been a significant cracking chemical yield (methane, ethane, propane) (52.1 mol%.). It is noted that С6+ hydrocarbons are
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virtually absent (0.1% mol.). The hydrogen yield is 31.7 mol%. that indicates the active heterolytic decomposition of n-pentane molecules (Table. 1). Table 1. The yield of hydrocarbons from n-pentane at various methane/platinum ratios. Raw materials
mol/mol Pt
methane
-
1
10
n-pentane
1
1
1
hydrocarbons
%mol.
hydrogen
31,7
5,9
20,3
methane
51,7
41,2
34,2
ethane
0,2
1,7
2,0
propane
0,2
1,4
1,6
isopentane
0,2
0,8
0,5
n-pentane
15,9
45,4
32,3
benzene
0,1
3,7
8,5
toluene
0,0
0,0
0,6
Sum
100,0
100,0
100,0
To study the reactive capability of the СНх hydrocarbon units adsorbed on the catalyst surface depending on the СН4/Pt ratio at a temperature of 823 K n-pentane was supplied into the reactor (Fig. 3). The results indicate that the degree of n-pentane conversion is higher at a high methane/platinum (8/1-16/1 mol/mol) ratio and is 62-67,7%, while when СН4/Pt =1/1 mol/mol and СН4/Pt =3/1 mol/mol the conversion н-С5Н12 is 54,6-57,6 %. Decreasing of the conversion degree at low СН4/Pt ratios is likely due to a partial carbonization of the active centers. The content of cracked products decreases with increasing methane/platinum ratio from 44.3 to 34.6 mol%. Total maximum yield of aromatic hydrocarbons (benzene, toluene) has been recorded at СН 4/Pt = 10 mol/mol (9.1 mol%.), the minimum corresponds СН4/Pt = 1/1 mol/mol and equals 3.7% mol. Reducing of the arenes yields at low СН4/Pt ratios can be explained by less reactive СНх-particles capabilities on the catalyst surface. The yield of hydrogen changes from 5.9 to 20.3 mol%. in the ratios range of 1-16 mole СН4/mol Pt. .
Fig.3. Product yields from n-pentane at 823 K at different СН4/Pt ratios.
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4. Conclusion The conducted researches have revealed that the methane adsorption on aluminic-platinum catalyst depends on the methane/platinum ratio. Quantitative estimation of СН4 dehydrogenation degree on Pt/Al2O3 surface at different СН4/Pt ratios indicate more complete methane decomposition into carbon and hydrogen at СН4/Pt 1-3 mol/mol. It is found that in the absence of СНх hydrocarbon units on the aluminic-platinum catalyst the n-pentane is poorly converted into aromatic hydrocarbons (0.1 mol%.). However, the addition of even minimal methane amounts (СН4/Pt = 1/1 mol/mol) into the reactive atmosphere leads to a significant increase in the aromatic hydrocarbons yield, which shows a high reactivity of the СНх-particles in the reaction with n-pentane. References [1] P.A. Kirjushin, A.Ju. Knizhnikov, K.V. Kochi, T.A. Puzanova, S.A. Uvarov, Poputnyj neftjanoj gaz v Rossii: Szhigat' nel'zja, pererabatyvat'. Vsemirnyj fond dikoj prirody (WWF), Moskva, 2013. [2] M.С. Alvarez-Galvan, N. Mota, M. Ojeda, S. Rojas, R.M. Navarro, J.L.G. Fierro, Direct methane conversion routes to chemicals and fuels. Catalysis Today. 171 (2011) 15-23. [3] T. Koerts, R.A. van Santen, Hydrocarbon formation from methane by a low-temperature two-step reaction sequence. Journal of Catalysis. 138 (1992) 101-104. [4] H. Amariglio, J. Saint-Justb, A. Amariglio, Homologation of methane under non-oxidative conditions, Fuel Proc. Tech. 42 (1995) 291-323. [5] R.L. Martins, M.A.S. Baldanza, M.M.V.M. Souza, M. Schmal, The effect of support on methane activation over Pt catalysts in the presence of MoO3. Appl. Catal. A. 318 (2007) 207-212. [6] S.F. Moya, R.L. Martins, M. Schmal, Monodispersed and nanostructrured Ni/SiO 2 catalyst and its activity for non oxidative methane activation. Appl. Catal. A. 396 (2011) 159-169. [7] M. Shuqi, G. Xiaoguang, Z. Lingxiao, S. Susannah, B. Xinhe, Recent progress in methane dehydroaromatization: from laboratory curiosities to promising technology. Journal of Energy Chemistry 22 (2013) 1–20. [8] L. Wang, M. Tao, G.X. Xie, Degydrogenation and aromatization of methane under nox-oxidizing conditions. Catalysis Letters, 21 (1996) 3541. [9] L.L. Korobicyna, N.V. Arbuzov, A.V. Vosmerikov, Prevrashhenie metana v aromaticheskie uglevodorody na metallsoderzhashhih ceolitah. Neftepererabotka i neftehimija. 8 (2013) 18-21. [10] A.P. Karnauhov, Adsorbcija. Tekstura dispersnyh i poristyh materialov. Nauka. Sib. Predprijatie RAN, Novosibirsk, 1999.
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