versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.50 n.1 Concepción mar. 2005
J. Chil. Chem. Soc., 50, N 1 (2005)
SELECTIVE HYDRODECHLORINATION OF 1,2-DICHLOROETHANE OVER Pd-Sn/SiO2 CATALYSTS
*F. ORELLANA(1), G. PECCHI(2) and P. REYES(2)
(1)Departamento de Química, Universidad del Bío-Bío, Concepción-CHILE, firstname.lastname@example.org
Pd/SiO2 and Pd-Sn/SiO2 catalysts prepared by sol-gel processing have been characterised and tested in the selective hydrodechlorination of 1,2-dichloroethane. The obtained solids were characterised by N2 adsorption at 77 K, H2 chemisorption, temperature programmed reduction, and X-ray diffraction. It was found that the Pd surface atoms decreases upon the Sn addition, without critical changes on the specific area and reducibility. The turnover frequency increases continuously with the reaction temperature and also as tin loading increases. The selectivity to ethylene is strongly enhanced as temperature increases up to reach almost a constant value of 90 %, in the temperature range 500-600 K. It is proposed that Pd-SnOx are the active sites for the selective hydrodechlorination of 1,2-dichloroethane.
Key words: Sol-gel, TEOS, hydrodechlorination, 1,2-dichloroethane, selectivity.
Chlorinated wastes generated by industrial processes are generally eliminated by thermal or catalytic incineration (1,2). Kalnes and James (3) have shown, however, that for wastes such as polychlorobiphenyl compounds and halogenated petrochemical by-products such as chlorinated alkanes, the hydrodechlorination and the reuse provide a more economical solution than incineration. The hydrodechlorination reactions involve the reaction between H2 and an organic molecule containing a C-Cl bond to form HCl and a C-H bond. These type of reactions are important steps in many syntheses. For example, hidrodechlorination is used in the manufacture of CF3CFH2 (a widely used refrigerant) from CF3CFCl2 (4) and in the manufacture of CF2=CFH (a monomer used in the production of Teflon-like polymers) from CF2ClCFCl2 (5). These reactions require selective cleavage of C-Cl bonds which in turn requires highly selective catalysts. Numerous examples of hydrodechlorination catalysts have been described in the literature (6-9). Noble metals are particularly attractive because of their high activities. The hydrogenolysis of C-Cl bond in chlorofluorcarbons is carried out primarily using a palladium catalysts because of the high selectivity toward the fully dechlorinated hydrofluorocarbon (10-12).
Another recent application of dechlorination in the presence of H2 lies in the conversion of a chlorinated by-product into a useful compound, for example, in the conversion of 1,2-dichloroethane into ethylene (13). The subsequent hydrogenation to alkanes is not desirable due to the lower value of these compounds. VIII-B group metals have displayed good performance in the hydrodechlorination, however this reaction would lead to ethane. Bozzelli et al. (13) have demonstrated the ability of bimetallic catalysts, composed of metals from Groups VIII and IB, to convert chlorinated alkanes into less-chlorinated or unchlorinated alkenes, due to the reduction in the hydrogenation capacity of these bimetallic systems. This new alternative is very attractive, because alkenes are raw materials for numerous of industrial reactions. These bimetallic catalysts display lower poisoning degree (8, 14) compared to monometallic catalysts, and the selectivity of the Pd/SiO2 catalyst may be improved by the addition of a second metal, by a dilution effect and/or alloy formation (8, 15).The activity and selectivity of hydrodechlorination catalysts can be affected by several parameters, among others the nature of the support and the metal component, metal particle size and reaction conditions (10). Heinrichs et al. (13) reported an increases in the activity and selectivity to ethylene of Pd-Ag/SiO2 catalysts during CClH2CClH2 hidrochlorination. On the other hand, the sol-gel method has been used by many authors in order to obtain supported mono- and bimetallic catalysts (16-18). Such method allows the obtaining of catalysts with high metal dispersion and having a homogeneous porosity and displaying also high resistance to sintering. This is due to the fact that metal crystals may remain strongly anchored to the support surface.
The aim of this work is to study the effect of Sn loading in Pd-Sn/SiO2 catalysts in the reaction of hydrodechlorination of 1,2-dichloroethane. Pd-Sn/SiO2 catalysts having a 0.5wt.% Pd and different Sn contents were prepared. Monometallic Pd/SiO2 catalyst was obtained by the sol-gel method, whereas the bimetallic Pd-Sn/SiO2 samples were obtained by impregnation of Sn precursor on the monometallic Pd/SiO2 catalyst. Catalysts were characterised by N2 adsorption at 77 K, H2 chemisorption, temperature programmed reduction (TPR), and X-ray diffraction. The hydrodechlorination of 1,2-dichloroethane was studied in a fixed bed reactor at different temperatures in the range 473 to 573 K.
Pd(0.5 wt.%)/SiO2 catalyst was obtained by the sol-gel technique. The silica precursor, tetraethylorthosilicate (TEOS), was gelated at 353 K in the presence of ethanol, water, Pd(CH3COOCH2COCH3) 2. A H2O/TEOS molar ratio equal to 10 and a pH of 3, was used. The pH was adjusted using CH3COOH solution. The obtained gel was dried at 383 K overnight. Sn(1wt.%)/SiO2 catalyst was prepared by impregnation of SnCl2 in a sol-gel silica obtained by a procedure similar to that described above. The bimetallic Pd-Sn catalysts were prepared by impregnation of the Pd/SiO2 catalyst with Sn chloride solution containing the required amount to get Pd/Sn atomic ratio of 0.75, 0.5 and 0.25, pH equal 3 was used. The Sn contents were 0.5, 1.0 and 2.4 wt.%. The solids were calcined in air at 673 K for 4 h, stored in stove. Prior there characterisation or catalytic evaluation the solid were reduced in situ under H2 flow for 4 h at 773 K.
Nitrogen adsorption isotherms at 77 K, in the 0.05-0.995 relative pressure range, were carried out in a Micromeritics Model Gemini 2370 apparatus. Hydrogen chemisorption measurements were carried out in a volumetric system at 373 K in a 0 to 40 mmHg pressure range, to prevent hydrides formation over palladium (19). Prior the chemisorption experiments, the samples were reduced in situ at 773 K for 2 h and then outgassed for 4 h at the same temperature. A stoichiometry of adsorption H/Pd=1 was used. Under these conditions Sn does not chemisorb H2 (20). Temperature programmed reduction experiments were carried out in a TPD/TPR 2900 Micromeritics system provided with a thermal conductivity detector. The reducing gas was a mixture of 5% H2/Ar (40cc min-1) and a heating rate of 10 Kmin-1 was used. X-ray diffractograms of the reduced solids were obtained in a Rigaku powder diffractometer using nickel filtered CuKa radiation. The diffractograms were taken in a continuous scanning method at a 2q range, between 3 and 70 °, at a 1°min-1 goniometer speed.
Catalytic evaluation in the hydrodechlorination of 1,2-dichloroethane was carried out in a fixed bed flow reactor at atmospheric pressure using 200 mg of catalysts and a space velocity of 2 h-1. The calcined samples were reduced in situ in flowing H2 (30 cc min-1) up to 673 K for 1 h. Then, the samples were cooled down to 473 K and the flowing H2 (30 cc min-1) was switched to a saturator containing 1,2-dichloroethane at 273 K and this mixture was fed to the reactor. The activity was evaluated at different temperatures (473, 523, 573, 623 and 673 K). The effluents of the reactor were analysed by an on-line GC. For this purpose a Heweltt Packard model 5890-A gas chromatograph equipped with a 6 ft capillary column HayeSep IIR(100/120) (Supelco).
RESULTS AND DISCUSSION
Figure 1 shows representative nitrogen adsorption on the studied catalysts. For the Sn/SiO2 catalyst the isotherm corresponds to Type-I in a BDDT classification (21). This is the expected behaviour considering that the support was prepared by the sol-gel method at acid pH (22), and the tin was then incorporated by impregnation. On the other hand, the isotherm obtained for Pd(0.5wt.%)/SiO2 catalyst, suggests the presence of mesopores. The mean pore diameter, obtained by the Gurvich´s rule, is 3.3 nm, in line with previous works, suggesting that the mesoporosity was developed during the decomposition of the organic precursors.
In the monometallic Sn(1wt.%)/SiO2 catalyst the shape of the adsorption isotherm is mainly type I, indicative of a microporous solid characteristic of those obtained by gelation in acid medium (23). For the Pd(0.5wt.%)/SiO2 and Pd(0.5wt.%)-Sn/SiO2 catalysts the shape of the isotherms indicate a significant proportion of both micropores and mesopores. This fact is explained taking into account that during the gelation of this solid, a non ionic metal precursor was used (Pd(acac)2). The above mentioned analysis can be confirmed by studying the t curves, statistical thickness (Fig.2). Linearity deviation observed for the Sn(1wt.%)/SiO2 and for the Pd-Sn bimetallic catalysts indicate an important contribution of micropores.
Table 1 summarises the specific area, pore volume and average pore diameter for all the studied catalysts. It can be seen a slight decreases in the specific area of the bimetallic catalysts upon the addition of Sn, as well as a drop in the pore volume. This can be attributed to partial blockage of porous structure during Sn impregnation, in agreement with results previously reported by da Silva et al.(23) for Pt-Sn/Carbon catalysts.
TPR experiments were carried out in the temperature range 195 to 773 K. The profiles of the calcined samples for both Pd monometallic and PdSn bimetallic catalysts are rather complex. This is of common occurrence in sol-gel generated catalysts; due to, in addition of the H2 consumption during reduction, the decomposition of remaining organic residue also contribute to the TCD signal (24). No H2 consumption occurs over 530 K for Pd monometallic catalyst, indicating that Pd reduction has been achieved. Similar results were reported by Rodriguez et al. (19) for Pd supported catalysts. The extent of reduction for Sn(1wt.%)/SiO2 catalyst suggests that Sn remain in oxide state on the surface (25). Chemisorption results of the studied catalysts "expressed as Pd surface atoms" and H/Pd ratio is displayed in Table 2. As expected the monometallic Sn(1wt.%)/SiO2 catalyst does not chemisorb H2 under these conditions, and the H/Pd ratio decreases as Sn content increases. This drops in the H/Pd ratio may be due to Pd surface coverage by SnOx species and/or Pd-Sn alloy formation. These type of sites have been reported for PtSn/SiO2 catalysts (26).
Figure 3 shows the XRD patterns of the studied Pd-Sn catalysts. It should be mentioned that the Sn/SiO2 catalyst does not shows any diffraction lines assigned to Sn or tin oxides because it remains highly dispersed on the support. Either as SnOx or SnO2 species. The diffraction pattern of Pd on SiO2 displays the main lines of Pd at 2q of 40.1 and 46.6 degrees corresponding to the planes (111) and (200) respectively. The addition of small amounts of tin does not change significantly the XRD patterns up to 2.4 wt.% of tin. In fact, for this compound the diffraction patterns show three main peaks in the 2q studied region, at 39.66, 41.00 and 44.60 degrees respectively. They do not coincide with the expected values for Pd, Sn or tin oxides, but they are the expected lines for an intermetallic Pd3Sn2 species.
Catalytic studies of the hydrodechlorination of 1,2-dichloroethane were carried out in the temperature range 473 to 573 K and the obtained products were ethylene, ethane and HCl. The activity of Sn monometallic catalyst was negligible during this reaction. The Pd/SiO2 catalyst exhibits the highest activity but showing an important deactivation during the catalytic evaluation. Fig. 4 a display the evolution of the conversion with time on stream at different temperatures for the monometallic catalyst. Eventhough the activity increases as temperature increases, the catalyst deactivate faster at higher temperature, reaching an almost steady state around 80 min. The deactivation is attributed to the presence of HCl close to the metallic sites (9). The selectivity to ethylene increases with the time leading to a constant value close to 60 minutes. Similar feature was observed at the different studied temperatures, showing an enhancement in the selectivity at higher temperatures. The catalytic behaviour of the Pd-Sn catalysts is different. The addition of Sn lead to a drastic decreases in the initial conversion level, however, almost no deactivation was observed. The drops in the initial conversion is attributed to a significant decreases in the amount of Pd surface atoms upon the addition of Sn as it was evidenced in Table 2. This change is a consequence of the formation of a bimetallic Pd-Sn phase and also by surface coverage of Pd crystals by SnOx species. Similar finding were reported previously by Reyes et al. for the hydrogenation of crotonaldehyde on Rh-Sn catalysts (27, 28). Table 2 compiles the initial conversion level, at 523 K, the selectivity to ethylene at similar conversion level (in the range 7-9%) for the studied catalysts. Additionally, the activity per site TOF has also been reported. It can be seen a drastic decreases in the initial conversion level comparing the Pd/SiO2 catalyst with the Pd-Sn counterparts. These later display similar values, in the range 7.7 to 10%. With regard to the selectivity to ethylene, the monometallic Pd exhibit a value of 49% whereas the bimetallic samples showed selectivities in the range 77 to 82%. An important enhancement in the TOF is observed in the bimetallic catalysts. The results suggest a modification in the nature of the active site, thus, Pd0 seems to be the responsible in the monometallic sample whereas Pd-SnOx are the active sites in the bimetallic sample. These results are in good agreement with the characterisation results. At higher temperatures, the Pd/SiO2 catalyst shows an important drops in the selectivity whereas the Pd-Sn/SiO2 samples show only slight changes in the selectivity to ethylene (close to 90%).
Pd-Sn/SiO2 catalysts prepared by the sol-gel method show high specific surface areas as well as essentially mesoporous structure. For Pd(0.5wt.%)/SiO2 monometallic catalyst the average pores size is close to 3.3 nm, having a significant fraction of micropores. H2 selective chemisorption showed that addition of Sn to Pd catalyst decreases the capacity of chemisorb H2. This effect increases with Sn content due to the surface coverage of Pd atoms by Sn oxidised and/or the formation of Pd-Sn alloys species. The X-ray diffraction studies showed the formation of metallic palladium particles in Pd/SiO2 and Pd-Sn/SiO2. The addition of Sn modified the metallic Pd surface, leading to Pd-Sn and Pd-SnOx species.
Catalytic activity in the hydrodechlorination of 1,2-dichloroethane showed significant changes with the reaction temperature and higher selectivity towards ethylene for bimetallic catalysts compared to the monometallic one. This behaviour can be explained on the basis of a lower hydrogenating capacity of the bimetallic samples. It is likely that the active sites consists in Pd clusters partially covered by SnOx entities (Pd-SnOx). This implies a creation of new type of active sites exhibiting higher activity compared to the monometallic Pd/SiO2 catalyst.
The authors thank Dirección de Investigación of the Universidad del Bío-Bío for the financial support and to Dr. Richard Gonzalez (Tulane University, New Orleans-U.S.A.).
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