Carbon - Supported Platinum Catlyst for N-Hexane dehydrogenation

Journal of Chemistry, Vol. 41, No. 1, P. 110 - 114, 2003<br /> <br /> <br /> CARBON - SUPPORTED PLATINUM CATALYST FOR<br /> N-HEXANE DEHYDROGENATION<br /> Received 01-7-2002<br /> Nguyen Thi Dung, Tran Manh Tri<br /> Institute of Chemical Technology, Hochiminh City<br /> <br /> <br /> Summary<br /> Activated carbon (AC) from coconut shells is used as support for platinum catalysts. A<br /> series of catalysts based on platinum and Pt doped Sn were prepared by impregnation on<br /> different supports like AC and SiO2. It was shown that the nature of the support has a significant<br /> influence on the activity and selectivity to alkenes of the catalysts in the dehydrogenation of n-<br /> hexane to alkenes. The higher activity and selectivity of the carbon supported platinum catalyst<br /> can be explained by higher dispersion of platinum particles of the catalysts. The addition of Sn<br /> led higher stability and activity and that may be related to the formation of a PtSn alloy on the<br /> surface and the modification of interaction between the alkenes and the metal sites, making it<br /> weaker and keeping the metallic surface more free of cokes by migration of that to the carbon<br /> surface.<br /> <br /> <br /> I - Introduction presence of Li but is not dependent on the<br /> concentration. Castrol [7] found that bimetallic<br /> Platinum supported catalysts are widely Pt-Sn catalyst doped by alkali metals has a<br /> used in many industrial processes including better yield in alkenes and lower selectivity<br /> hydrogenation, dehydrogenation, and reforming towards gases and aromatics compared to<br /> of n-alkanes. Its chemical stability in both monometallic platinum. Bimetallic Pt-Ge and<br /> oxidizing and reducing conditions makes this Pt-Pb catalysts show a similar performance<br /> metal an ideal catalyst in many applications. compared to Pt-Sn. However, their selectivity to<br /> Catalytic dehydrogenation of alkanes is of alkenes is lower. Yang et al. [8] indicated that<br /> increasing importance due to the growing the oxidation states of tin and the possibility of<br /> demand for linear alkenes as raw materials for alloy formation between platinum and tin<br /> the production of biodegradable detergents. seems to be depended on the method of<br /> preparation, the nature of supports and the<br /> Bimetallic Pt-Sn/Al2O3 catalysts have been loading of the catalytic component.<br /> extensively studied by many research groups [1<br /> - 4] due to their importance in industrial Recent studies have showed that, the nature<br /> petrochemical processes. Many of these studies of the support is one of the important<br /> have focused on hydrocarbon conversion such characteristics of great influence on activity and<br /> as isomerization, aromatization, dehyrocyc- selectivity of the catalyst .<br /> lization and dehydrogenation of light alkanes. Activated carbon as a catalyst support is of<br /> Timofeeva et al. [6] studied Pt/Al2O3 and Li- increasing interest today due to its unique<br /> doped Pt/Al2O3 catalysts and showed that the properties like stability in both acidic and basic<br /> selectivity to monoalkenes is dependent on the media and thermal resistance but only a few<br /> <br /> 110<br /> publications are devoted to the investigation of impregnating solution under stirring at room<br /> the selective dehydrogenation of n-alkanes to temperature for 2 h. Following impregnation,<br /> alkenes over carbon supported platinum the catalysts were filtered, dried overnight at<br /> catalysts. 110oC and then calcined at 450oC in flowing<br /> The aim of the following study is the nitrogen for 2 h. Subsequently Sn was added by<br /> preparation and characterization of platinum impregnation from a solution of SnCl2 (0,04<br /> catalysts supported on activated carbon mol/l) following the same procedure.<br /> obtained from coconut shells of Vietnam in The composition of the catalysts used in the<br /> comparison with supported Pt/SiO2 catalysts in catalytic tests are given in table 2 (For selected<br /> the selective dehydrogenation of n-hexane to catalysts the metal dispersions were determined<br /> alkenes as a model reaction by transmission electron microscopy) carried<br /> out with a Philips EM-420 Instrument operated<br /> II - Experiment at 120 kV.<br /> <br /> 1. Preparation and characterization of the Table 2: Composition of the catalysts used<br /> AC support<br /> Pt Sn Pt/Sn<br /> The active carbon was prepared by Support weight, weight, (atomic<br /> carbonization and activation of coconut shells % % ratio)<br /> [10] according to the following procedure. The<br /> coconut shells were impregnated with a AC 0.52 0 1:0<br /> solution of phosphoric acid dried at 115oC for 2 0.52 0.71 1 : 0.43<br /> h and then carbonized by heating up to 850oC<br /> SiO2 0.55 0 1:0<br /> for 3 h in a furnace under flowing nitrogen. The<br /> carbon obtained was activated under CO2 at 0.55 0.71 1 : 0.43<br /> 850oC for 4 h, then washed with deionized<br /> water and dried at 115oC overnight. 3. Catalytic test<br /> The surface area and the porosity of the Dehydrogenation of n-hexane was<br /> support were determined from adsorption investigated in a micro-flow reactor at<br /> isotherms of nitrogen which were measured on atmospheric pressure. A feed of hydrogen<br /> a Micrometric ASAP-200-V20. saturated with n-hexane vapor was generated by<br /> SiO2 (Degussa Aerosil-type silica) has been bubbling hydrogen (30 ml/min) through a<br /> selected as a support for comparison purposes. thermostated saturator at 15oC. All catalysts<br /> The results are summarized in table 1. used were reduced in-situ prior to the reaction<br /> at 500oC in flowing hydrogen (20 ml/min) for 4<br /> Table 1: The textural properties of the activated h. After that the reactor was brought to reaction<br /> carbon (AC) and the SiO2 temperature (500oC) and then contacted with<br /> Surface area Pore volume, the feed consisting of hydrogen and n-hexane<br /> Supports with molar ratio of 6.6 and a n-hexane partial<br /> BET, m2/g cm3/g<br /> pressure of 100 Pa. Product analysis was<br /> AC 1026 0.29 performed on line by gas-chromatography<br /> SiO2 230 0.90 (Fisons Instrument 8130-00) equipped with a<br /> FID detector and a capillary column DB-5 from<br /> 2. Preparation of catalysts W&J (30 m, 0.32 mm). Catalytic measurements<br /> were carried out at 500oC.<br /> The platinum-supported catalysts (0.5wt%<br /> Pt) were prepared by impregnation of the The specific rate (R) was calculated as<br /> supports with a solution of H2PtCl6 (0.02 mol/l). mole of n-hexane transformed per second per<br /> The support was contacted with the gram of Pt: mole/sec.g (Pt).<br /> 111<br />  Conversion (X) was defined as mole of n- 1. The effect of the support<br /> hexane converted per mole of n-hexane in the Blank tests were carried out with the<br /> feed (%mole) and the selectivity (S) as moles of supports AC and SiO2 which showed no<br /> n-hexenes obtained per moles of total n-hexane activity.<br /> converted (%mole). The results of the catalytic activity<br /> evaluated at 500oC for the two Pt (0.5%)<br /> III - Results and discussion containing catalysts are summarized in table 3.<br /> <br /> Table 3: Catalytic activity and selectivity of supported platinum catalysts<br /> in the dehydrogenation of n-hexane at 500oC<br /> Phexane = 1.33.104 Pa, mcat.= 0.5 g of catalysts; H2/nC6 = 6.6<br /> Pt(0,52wt%)/AC Pt(0,55wt%)/SiO2<br /> On stream,<br /> min Conversion, Selectivity, Conversion, Selectivity,<br /> %mole %mole %mole %mole<br /> 1 12.4 80.5 8.5 65.7<br /> 30 10.5 79.8 6.2 62.3<br /> 60 9.8 78.2 5.1 60.8<br /> 90 7.7 78.9 4.2 61.2<br /> 120 6.9 78.1 3.4 61.8<br /> 150 6.8 77.6 2.8 60.9<br /> 180 6.7 78.5 2.1 61.5<br /> 210 6.8 77.8 1.9 60.8<br /> 240 6.6 78.6 1.8 60.1<br /> <br /> From the results, it can be seen that the explained by the accumulation of carbonaceous<br /> change of the support led to significant changes species that leads to the loss of platinum<br /> in the dehydrogenation activity of the platinum surface area. For the carbon supported catalyst<br /> catalyst. The carbon supported Pt catalyst the better stability according to Llorca [11]<br /> shows a higher activity and selectivity could be caused by the easy adsorption of<br /> compared with the SiO2 supported catalysts. alkenes from the metallic sites and by a less<br /> This is in agreement with results reported by favorable polymerization type reaction under<br /> Guerrenro-Ruiz [13] who showed that the the same reaction conditions. On the other<br /> activity of the carbon supported catalyst was hand, the observed results may also be related<br /> higher compared to the silica supported catalyst to the dispersion of metal particles. From the<br /> (conversion 6.3% and 0.9% respectively). TEM results shown that the Pt/C catalyst has a<br /> The results for both catalysts Pt/AC and Pt/ very homogeneous particle size distribution. No<br /> SiO2 showed the deactivation of the catalysts. Pt particles could be seen above the resolution<br /> The conversion and selectivity decreased with limit of the microscope i.e. They are below 1<br /> time on stream. The carbon supported catalyst nm (Fig. 2). This is in agreement with results<br /> remained more stable after 2 h reaction time. In published by Meriaudeau et al. [5] who found<br /> our case the selectivity to alkenes of Pt/SiO2 very small particles (about 1 nm) of Pt<br /> was higher than that indicated in [11] 5% and supported on NaY. In the case of the Pt/SiO2<br /> 35%, respectively). The difference may be sample, large particle sizes between 5 nm and<br /> caused by different reaction conditions, 30 nm were observed (Fig. 3).<br /> methods of preparation and comparison and According to Cortright et al. [12] the<br /> composition of the catalyst. The deactivation of dissociative adsorption of n-hexane is the rate<br /> the silica-supported platinum catalyst can be limiting step that controls the dehydrogenation<br /> 112<br />  40<br /> <br /> 35<br /> <br /> 30 Pt-Sn/AC<br /> Conversion (%mol)<br /> <br /> <br /> <br /> <br /> 25<br /> <br /> 20<br /> <br /> 15 Pt-Sn/SiO2<br /> Pt/AC<br /> 10<br /> Pt/SiO2<br /> 5<br /> <br /> 0<br /> 0 30 60 90 120<br /> time(min) 150 180 210 240<br /> <br /> Fig. 1: Conversion of n-hexane to hexene on different catalysts<br /> <br /> <br /> <br /> <br /> Fig. 2: TEM of Pt/C catalyst Fig. 3: TEM of Pt/SiO2 catalyst<br /> <br /> reaction. The adsorption and dissociation of n- monometallic catalysts for both catalyst and the<br /> hexane should take place favorably on the stability was improved. In the case of the<br /> small particles of Pt. This phenomenon can be supported carbon catalyst the activity slightly<br /> explained by the electron deficiency of small decreased from 33.5% to 29.5% while for the<br /> Pt particles that leads to strong electronic silica supported catalyst a considerable loss in<br /> affinity to the electron of the hydrocarbon. On activity from 25.5% to 10.2% after 4 h was<br /> large particles of Pt the adsorption and observed. The selectivity increased for both<br /> dissociation n-hexane is more difficultly catalysts Pt/AC and PtSiO2 from 80.5% to<br /> because of the weak electron affinity [14]. 97.2% and from 65.7 to 80.5%, respectively.<br /> The selectivity remained nearly stable for 4 h<br /> 2 The effect of the Sn time on stream.<br /> To examine the effect of tin on the catalytic Both bimetallic Pt-Sn catalysts demons-<br /> properties, a series of experiments were carried trated more stability than monometallic<br /> out and the data showed that the activity and catalyst, but for carbon supported that<br /> the selectivity to alkenes increased when tin characteristic was better. After 4 h, activity of<br /> was added to both Pt catalysts (Fig. 1). The Pt-Sn/C decreased 0.8 times while for Pt-<br /> activity increased three times compared to the Sn/SiO2 that diminual 2.5 times.<br /> 113<br />  It was shown repeatedly that the addition Acknowledgement: We are grateful to the<br /> of Sn led to an increase of the catalytic activity VOLSKWAGEN- STIFTUNG (.AZ I /75855) for<br /> and the selectivity of the catalyst. Some studies financial support. We thank Prof. Dr. Nils<br /> indicated that alloy is formed on silica with Jaeger, IAPC of Bremen University for helpful<br /> different modifications depending on the discussion and TEM measurement.<br /> content of tin. According to results reported by<br /> Yang et al. [8] a PtSn alloy was obtained on the References<br /> support Al2O3 high interaction between Sn and<br /> Al2O3 was performed, while on the carbon 1. R. Srinivasan, B. H Davis. Platinum Met.<br /> support the particle of Pt and Sn were separate Rev. 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