Adsorption of Methane on the Pt/Al2O3 Catalyst. Studying of ...

Available online at www.sciencedirect.com

ScienceDirect Procedia Engineering 113 (2015) 13 – 18

International Conference on Oil and Gas Engineering, OGE-2015

Adsorption of methane on the Pt/Al2O3catalyst. Studying of reactionary activity of the adsorbed methane forms in reaction of joint transformation with N pentane Golinsky D.V.a* , Ostanina N.V.a,b, Ovcharenko A.I.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 State Technical University, 11, Mira Pr., Omsk 644050, Russian Federation

Abstract It is shown in the work that in the presence of the preadsorbed hydrogen forms on the catalyst 1%Pt/Al 2O3 the maximum size of adsorption of methane at 823 K makes 1,1 CH4/Pt of mol/mol. Existence on the surface of the preadsorbed oxygen leads to emergence in the reactionary environment 0,6 mol. CO, and the total of the turned methane makes 1,3 CH 4/Pt of mol/mol. On the catalyst surface (in lack of the preadsorbed forms of hydrogen and oxygen) the amount of the adsorbed methane decreases to 0,8 CH4/Pt mol/mol. Studying of reactionary ability of the adsorbed methane forms in reaction of joint transformation with N pentane shows quite high catalytic activity of alkanes in the formation of aromatic hydrocarbons. © 2015 2015Published The Authors. Published Elsevier Ltd.access article under the CC BY-NC-ND license © by Elsevier Ltd. by This is an open Peer-review under responsibility of the Omsk State Technical University. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Omsk State Technical University Keywords: methane; N pentane; aluminium platinum catalyst; adsorption; catalytic activity

1. Introduction According to the statistical report of bp group of companies 604, 8 billion m3 of natural gas was extracted in Russia in 2013 [1]. The main component of natural gas is methane which is considered as a potential source for chemical industry now. The main problem arising when developing processes of transformation of methane is the problem of its activation [2], because of high stability of a molecule of methane. Strong tetrahedral S-N tying with

* Corresponding author. Tel.:+7 -381-267-3334 E-mail address:[email protected]

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.2015.07.279

14

D.V. Golinsky et al. / Procedia Engineering 113 (2015) 13 – 18

energy of dissociation of 435, 8 kJ/mol [3] and absence of functional groups complicate its chemical interaction. Because of high stability of a molecule it was considered long time that effective transformation of methane is possibly only at temperatures more than 1300 K. So the simplest homologation of methane leading to the formation of ethane is characterized by high positive value of change of energy of Gibbs (+71kJ/mol) even at 1000 K [4] and therefore it is forbidden thermodynamically. The same reaction, but with the formation of ethylene is even less thermodynamically probable, as ΔG makes +79, 9 kJ/mol [5]. Thermodynamic restrictions can be overcome due to introduction to the reactionary environment of oxygen or the connections containing active oxygen (H 2O, CO2). So far the question of adsorption of methane with the subsequent its activation and transformation into C 2 hydrocarbons is quite widely considered for catalysts with the high content of metals (3-10%mass) [4], mainly the VIII groups on various oxidic and zeolitic catalyst [6-8]. Besides, catalytic activity of separate outlines of metals [3, 9-10], and also the possibility of activation of methane on the clusters of platinum and palladium applied on graphene and carbon nanotubes is studied [11]. The ability of methane to be adsorbed on the catalyst of 1% of Pt/Al 2O3 depending on conditions of its preliminary processing is studied in this work. Besides, catalytic activity of the adsorbed methane forms in reaction of joint transformation with N pentane is established. 2. Experimental 2.1. Catalyst preparation Studying of the adsorptive properties concerning methane was carried out in the range of temperatures of 293823 K, CH4/Pt ratio = by 10/1 mol/mol in the reactor of ideal mixture. The catalyst loaded into the reactor was restored previously in running of the drained hydrogen at T=773 K within 1 hour. After that for studying of adsorption of methane on the catalyst surface with the preadsorbed hydrogen (Pt-H/Al2O3), the temperature of the reactor was reduced to the room one. Then in the reactor mixture methane argon was supplied in the reactor. Argon was used as the internal standard. At research of the adsorptive properties of the catalyst with the preadsorbed oxygen (Pt-O/Al2O3) after restoration of the catalyst the temperature of the reactor was reduced to the room one with the replacement of hydrogen by oxygen and rise in temperature to 523 K, with endurance within 30 minutes. After decrease in temperature the mixture methane argon was supplied in the reactor. For studying of adsorption of methane on a "pure" surface of the catalyst (Pt/Al2O3) the restored catalyst at T=773 K was blown by argon within 2 hours. Then temperature of the reactor was reduced to the room in the running of argon and mixture methane argon was supplied. After studying of the adsorptive properties on reactionary operating environment N pentane from C5H12/CH4 ratio = 4/1 mol/mol was supplied to methane at T=823 K. 3. Results and discussion Research of adsorption of methane on the catalyst with the preadsorbed hydrogen showed that adsorption of methane begins at the temperature of 748 K (0,1 mol/mol of Pt) (fig. 1) with the growth of temperature to 823 K the amount of the adsorbed methane increases to 1,1 mol/mol of Pt. It is visible from the figure that in lack of the adsorbed methane at T=673 K in reactionary system the release of hydrogen of 1,4 mol/mol Pt is observed. Thus the further increase in temperature leads to the growth of amount of the emitted hydrogen to 5,2 mol/mol of Pt (T=823 K).


Fig.1. Amounts of the adsorbed methane and the emitted hydrogen for the catalyst (T=823 K). Pt-H/Al2O3.

For establishment of the reasons of observed hyper stoichiometric release of hydrogen the hydrogen desorption in the environment of argon from the surface of previously restored catalyst was carried out. It is followed from the obtained data (Fig. 2) that hydrogen starts desorbing with the catalyst surface at 673 K (1,4 mol/mol Pt). The increase in temperature to 823 K leads to growth of the content of hydrogen to 3,7 mol/mol of Pt.

Fig.2. Hydrogen desorption from the surface of the restored catalyst Pt-H/Al2O3.

Thus, it is possible to note that at restoration of the catalyst on its surface strongly connected forms of hydrogen are formed. In work [12] the authors also observed hyper stoichiometric release of hydrogen from platinum catalysts. The explanation of the observed effect was connected with possible "dissolution" of hydrogen in platinum microcrystals in the course of restoration of the catalyst. However this explanation was made only for the coarsely dispersed forms of platinum - monocrystals and black. In the case of the applied catalysts with high dispersion of platinum large amounts of the emitted hydrogen can be connected with the interhalogen compound of PtHx [13] which are formed at restoration. Taking into account the presented data on the amount of the emitted hydrogen from the surface of the restored PtH/Al2O3 catalyst it is possible to calculate true values of the hydrogen which is allocated at adsorption and dissociation of methane (Fig. 3a). So it is followed from the figure that from the beginning of adsorption of methane (T=748 K) in the gas phase hydrogen (0,1 mol/mol Pt) starts appearing, growth of temperature of reaction to 823 K leads to increase in amount of the emitted hydrogen to 1,5 mol/mol Pt.

15

16


Fig.3. Amounts of the adsorbed methane and the hydrogen which is allocated thus for the Pt-H/Al2O3 catalyst (a); the relation of N / C of the adsorbed hydrocarbonic fragments(b).

Besides, the executed calculation of structure of СНх of the hydrocarbonic fragments adsorbed on platinum shows that Н/C value makes 2,6 at T=773 K (3b). The further increase in temperature of adsorption (T=823 K) leads not only to growth of amount of the adsorbed methane, but also to deeper dehydrogenation of С-Н of communications in methane. It follows from research of adsorption of methane on the catalyst surface with the preadsorbed oxygen on platinum (Pt-O/Al2O3) (fig. 4a) that transformation of methane is observed from temperature 673 K (0, 2 mol/mol Pt). Thus, the content of hydrogen makes 0,2 mol/mol Pt. The increase in temperature of reaction to 773 K leads to the growth of amount of the turned methane to 0,7 mol/mol Pt, and the formed hydrogen to 1,3 mol/mol Pt. Besides, in the reactionary environment appears CO (0,1 mol / 1 mol Pt), formed as a result of oxidation of the adsorbed methane by the oxygen preadsorbed on platinum.

Fig.4. Amounts of the adsorbed methane, the escaped hydrogen and CO for the Pt-O/Al2O3 catalyst (a); the relation Н/С of the adsorbed hydrocarbonic fragments (b).

The increase in temperature of the reaction to 823 K leads to the growth of amount of the turned methane to 1,3 mol/mol Pt, to increase in concentration of hydrogen to 2,6 mol/mol Pt, and the contents of CO in the gas phase reaches 0,6 mol/mol Pt. Methane, unlike the previous case (adsorption on Pt-H/Al2O3), is exposed to dehydrogenation from 673 K (Fig. 4b). The further increase in temperature to 823 K leads to full removal of hydrogen with the formation of carbon on the catalyst surface.


Fig.5. Amounts of the adsorbed methane and the hydrogen which is escaped thus for the Pt/Al2O3 catalyst (a); the relation Н/С of the adsorbed hydrocarbonic fragments (b).

Adsorption of methane on the surface of the Pt/Al2O3 catalyst begins at 748 K (fig. 5a). Thus the amount of the turned methane makes 0,3 mol/mol Pt, and the emitted hydrogen of 0,1 mol/mol Pt. The increase in temperature of reaction to 823 K leads to the growth of amount of the adsorbed methane to 0,8 mol/mol Pt, and the emitted hydrogen to 1,0 mol/mol Pt. Thus, according to the results of research of adsorption of methane on the aluminum platinum catalyst with various conditions of preliminary processing it is possible to note that the maximum value of adsorption 1,3 mol/mol Pt is observed for the sample with the preadsorbed oxygen. Thus CO is formed in the reactionary environment, and with the growth of temperature of reaction to 823 K unoxidized methane as a result of deep dehydrogenation turns into carbon. Comparison of the adsorptive data for samples with a "pure" surface and the preadsorbed hydrogen at 823 K shows that the size of adsorption of CH4 on the Pt-H/Al2O3 catalyst makes 1,1 mol/mol Pt, against 0,8 mol/mol Pt for Pt/Al2O3 sample. Apparently, the preadsorbed hydrogen promotes adsorption of methane on disperse platinum and to increase in extent of dehydrogenation of methane. The explanation of this effect consists in ability of platinum to catalyze diffusion of hydrogen on the surface of Al 2O3 (spillover). Thanks to the process of a spillover of hydrogen probably there is a release of the active platinum center for adsorption of new molecules of methane. Data on calculation of stoichiometry of the formed СНх fragments from the adsorbed methane show that in the range of temperatures of 798-823 K the methane adsorbed on platinum with the preadsorbed hydrogen in slightly, but bigger degree, than the methane adsorbed on a "pure" surface of the catalyst is exposed to dehydrogenation. For research of reactionary ability of the Pt-H/Al2O3 and Pt/Al2O3 catalysts of hydrocarbonic fragments of СНх adsorbed on the surface at the temperature of 823 K the N pentane was supplied in the reactor. It follows from the received results that the depth of transformation of N pentane for the Pt-H/Al2O3 catalyst is equal to 90,6%. Thus the maintenance of cracking products (methane, ethane, propane, butane) at the total content of aromatic hydrocarbons (benzene, toluene) 6,0% mol makes 70,6% mol. At transformation of N pentane in lack of the preadsorbed methane its conversion decreases and makes 84,1%. Also decrease in yield of products of cracking reaction (65,9% mol) and aromatic hydrocarbons (3,3%мол.) is observed. In case of the catalyst with the adsorbed methane on Pt/Al2O3 the degree of transformation of N pentane is equal to 85,0%. The general content of cracking products makes 62, 1% mol., and the formed aromatic hydrocarbons 8,2%мол. Transformation of N pentane in lack of CH4 leads mainly to formation of gases (73,9% mol.) and to significantly smaller yield of aren (0,7% mol.) at conversion of N pentane of 87,0%. 4. Conclusion The researches conducted in the work allowed to establish that adsorption of methane on the aluminum platinum catalysts has dissociative character and is followed by release of hydrogen. Quantitative assessment of the degree of methane dehydrogenation at presence on the surface of the catalyst of hydrogen and oxygen are executed. The ability of the adsorbed СНх (х =1,3-1,5) of hydrocarbonic fragments at T=823 K to interact with N pentane with

17

18


formation of aromatic hydrocarbons (benzene, toluene) is found. In lack of СНх fragments of aluminum platinum the catalyst possesses such ability in much smaller degree. References [1] Official site of bp group of companies: www .bp.com [2] Frennet. Chemosorption and exchange with deuterium of methane on metals, Cat. Rev. Sci. Eng. 10 (1974) 37-68. [3] T. V. Cloudhary, D.W. Goodman. Methane activation on Ni and Ru model catalysts, J. Mol. Cat. A: Chemistry 163 (2000) 9-18. [4] T. Koerts, M.J.A.G. Deelen, R.A.van Santen. Hydrocarbon formation from methane by a low temperature two-step reaction sequence, J.Catal. 138 (1992) 101–114. [5] H. Amariglio, J. Saint-Justb, A. Amariglio. Homologation of methane under non-oxidative conditions, Fuel Proc. Tech. 42 (1995) 291-323. [6] 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. [7] S.F. Moya, R.L. Martins, M. Schmal. Monodispersed and nanostructrured Ni/SiO2 catalyst and its activity for non oxidative methane activation. Appl. Catal. A. 396 (2011) 159-169. [8] D. Zhou, D. Ma, Y. Wang, X. Liu, X. Bao. Study with density functional theory method on methane C–H bond activation on the MoO2/HZSM-5 active center. Chem. Phys. Let. 373 (2003) 46–51. [9] F. Abild-Pedersen, O. Lytken, J. Engbæk, G. Nielsen, I. Chorkendorơ J. K.Nørskov. Methane activation on Ni(111): Eơects of poison sand step defects. Surf. Sci. 590 (2005) 127–137. [10] I. Li, E. Croiset, L. Ricardez-Sandoval. Methane dissociation on Ni (100), Ni (111), and Ni (553): A comparative density functional theory study. J. of Mol. Catal. A: Chemical 365 (2012) 103–114 [11] J. Russell, P. Zapol, P. Král, L.A.Curtiss. Methane bond activation by Pt and Pd subnanometer clusters supported on grapheme and carbon nanotubes Chem. Phys. Let. 536 (2012) 9–13. [12] B. Lang, R.W. Youner, G.A. Somorjai. Studies of Chemisorbed Gases on Stepped Surfaces of Platinum. Surface science 30 (1972) 454-474. [13] V. V. Rozanov, J. Glend. A. V. Sklyarov. Studying of heterogeneous catalytic reactions by a thermal desorption method. Kinetics and catalysis 5. (1979) c. 1249-1255.