Improvements in selectivity and stability of Rh catalysts modified by SnBu4. dehydrogenation of isobutane to isobutene

RhSn/SiO2 bimetallic catalysts prepared via an organometallic route have proved to be very active and selective toward several hydrogenation reactions. In this work these catalysts were studied for the dehydrogenation of isobutane to isobutene. It was found that Rh/SiO2 monometallic catalysts had a null selectivity to isobutene, and this selectivity increased up to more than 90% after the addition of tin, using SnBu4 as precursor.


INTRODUCTION
Surface organometallic chemistry on metals can be a new route to generate supported bimetallic and organometallic materials, according to previous works [1][2][3]. For example, the selective reaction of Group 14 organometallic complexes with Group 8 metals supported on silica gave rise to a new generation of bimetallic materials with high activity and selectivity in heterogeneous catalysis. For instance, it has been shown that SnBu4 reacts with supported rhodium, ruthenium or nickel to give the corresponding bimetallic catalysts (RhSn, RuSn 0133-1736/98AJS$12.00. 9 Akad~miai Kiad6, Budapest. All fights reserved or NiSn), which are highly selective and active in the hydrogenation of ethyl acetate to ethanol [4]. This fact was interpreted as an isolation of the transition metal by tin neighbors [5], Bimetallic systems can be obtained as well as phases where several organic fragments are retained by using this preparation method [6]. For these organometallic catalysts the activities and selectivities are much higher than those of the corresponding bimetallic catalysts for selective hydrogenation of citral to geraniol [7]. The aim of this work is to describe the catalytic properties of bimetallic catalysts prepared by the reaction between Rh/SiOz and SnBu4, in the dehydrogenation of isobutane to isobutene. This olefin is an intermediate of interest in the production of non-pollutant naphtha (MTBE) or polymeric materials (polyisobutylene). In dehydrogenation reactions of paraffins to olefins, thermodynamic limitations force the reaction to be carried out under severe operating conditions (high temperatures, low H2 to paraffin ratios) resulting in catalyst deactivation mainly caused by coke formation. Therefore, stability, selectivity and regeneration ability are very important features to be improved in dehydrogenation catalysts.

EXPERIMENTAL
Rh was deposited on silica (Aerosil, Degussa, 380 mZg ~) by cationic exchange of amine complexes [RhCI(NH3)5] ~-at pH=10 [8], and the resultant materials were calcined in air at 623 K and then reduced in H2 at 623 K to obtain Rh/SiO2 monometallic catalysts. Bimetallic catalysts were prepared under H2 atmosphere by reaction of Rh/SiO2 with SnBu4 in n-CTHl6 between 298 to 363 K, according to: Tetra n-butyltin and n-butane (the only hydrocarbon detected) were monitored by gas chromatographic analysis. The catalyst was then washed with n-heptane and treated under hydrogen at 623 K. After this treatment all the butyl groups evolved as shown by Didillon et al. [6], obtaining a R_hSn bimetallic catalyst. The content of metals on the resulting materials was determined by atomic absorption. H2 adsorption was followed in a conventional volumetric equipment [8]. Temperature programmed reductions (TPR) of Rh203 and Rh203-SnO2 / SiO2 were conducted in a conventional dynamic equipment under flow of a mixture of At and H2 (95]5, 20 cm 3 rain q, 10 K minq). Transmission Electronic Microscopy (TEM) and Scanning Transmission Electronic Microscopy (STEM) experiments were made using JEOL 100 CX and Vacuum Generator HB5 electron microscopes, respectively.
Isobutane dehydrogenation was carried out in a dynamic reactor according to the following procedure: 2 h in H2 flow at 823 K, then the temperature was set to 723 K. The reaction started by feeding a mixture of H2 and isobutane to the reactor (66/34, flow rate 42 cm 3 min "1, mass 0.1 g). The latter temperature was kept for an initial period of 2 h and increased up to 773 K for another 2 h, then up to 823 K for further 2 h and finally decreased to the initial value (723 K) for two additional hours to complete eight hours of reaction and to determine the stability of the samples.

RESULTS AND DISCUSSION
Rh/SiO2 monometallic material showed high dispersion in the metallic phase. TEM observations indicated that the particle size distribution was centered at about 1.5 nm, in close agreement with the results of H2 volumetric adsorption (H/Rh=1.2, D --80%, considering H/Rh~=1.5 for an equilibrium pressure of 150 mbar at room temperature [8]).  (1) is carried out at 298 K, all the SnBu4 in solution reacts with Rh/SiO2 monometallic catalysts until a Sn/Rhs ratio of ca. 0.8 is reached. When Sn/ILhs ratios higher than 0.8 are required, the temperature must be increased [9]. Kinetic results of reaction (1) are shown in Fig. 1.
Previous papers [1,3,9] provided evidence on the interaction of Rh with Sn, and this was confirmed in this work by STEM and TPR. The STEM analyses always showed the peak of Sn (3.44 keV) associated with that of Rh (2.69 keV). Sn peak was never found alone on the support. TPR of the samples obtained on calcined Rh and RhSn/SiO2 catalysts, exhibit only one peak at about 370 K and 450 K for monometaUic and bimetallic catalysts, respectively. Hydrogen consumption corresponded approximately to the complete reduction of Rh203 and Rh203-SnO2 to Rh ~~ and to Rh(~ (~ respectively. The reaction test led to complete decomposition of isobutane to methane (hydrogenolysis reaction) and coke when using Rh/SiO2 catalyst. In contrast, the addition of Sn caused noticeable changes in selectivity toward isobutene, as is shown in Table 1. In this sense, the catalyst having Sn/Rh~= 0.5 produced, besides methane, a small amount ofisobutene together with much less coke. The catalyst became as selective to isobutene as it was to methane when increasing the amount of tin to Sn/Rhs = 0.8. The best results in selectivity to isobutene (>90%) were reached for a catalytic material with Sn/Rh,=l.4. Moreover, from coke analysis data taken at the end of the tests, a direct relationship was found between the inhibition of coke formation and the tin concentration used. On the other hand, skeletal isomerization was low on all the catalysts most possibly because of the weak acidity of silica. The above mentioned geometrical effect of Sn on Rh (isolation of Rh "ensembles") addressed to explain the high activities and selectivities in the reaction of ethyl acetate to ethanol [10] could also explain the inhibition of coke and methane formation (hydrogenolysis reactions are favored by Rh "ensembles").
Concerning the catalyst with Sn/Rhs ratio =1.4, the stability of these bimetallic catalysts was very high (Fig. 2) almost undoubtedly because of the lower coke formation rate. It was observed that, while the selectivity was not much affected by temperature (above 90% between 723 and 823 K), the activity increased considerably. For the working conditions covered here, the apparent activation energy was in the order of 80 kJ tool ~, in agreement with those obtained for a Ptbased commercial catalyst. Catalysts with Sn/Rhs ratios of 0.5 and 0.8 deactivated noticeably during the experimental runs (see behavior of the latter in  2). These results evidence the inhibition effect on coke formation produced by tin, but coke still affects the catalytic activity. A very interesting property of this kind of catalysts is their ability to regenerate when treated in a flow of air at 673 K for 8 h, aRer which coke is eliminated and the activity is restored to the values of fresh catalysts.
In this work the promotion effect of Sn on Rh was observed. Monometallic Rh catalysts are characterized by high hydrogenolysis activity and high rate of coke formation, and, after the addition of tin, switch to excellent selectivity (higher than 90%) and stability in the reaction of isobutane to isobutene. The reaction between transition metals and tin organometallic compounds is a very interesting way to produce catalysts with a controlled amount of promoters. For dehydrogenation reactions of paraffirts to olefins, tin could be used to promote other elements (apart fi'om Rh) such as Ni, Ru, Ir, etc., giving rise to a new family of dehydrogenation catalysts.