(242c) A Novel Approach to Synthesis of Highly Selective Pt/Mg(Al)(Ga)O Catalysts for the Dehydrogenation of Alkanes to Alkenes and Hydrogen

The catalytic dehydrogenation of light alkanes is of industrial interest because it offers alternative to producing alkenes and hydrogen that in principle should be more selective than non-catalytic thermal dehydrogenation, which suffers from extensive alkane cracking and coke formation. While supported Pt is known to be an effective catalyst for alkane dehydrogenation, prior studies have shown that it suffers from a lack alkene selectivity and stability. The deficiencies can be offset to a degree by adding Sn, Zn, or Ga to Pt to produce a bimetallic catalyst. It has also been reported that the use of non acidic supports such as such as SiO2, MgO, activated carbon, K-LTL, MgAl2O4, and ZnAl2O4 enhances catalyst stability. Recently, it has been shown that calcined hydrotalcites, Mg(Al)O, can serve as an effective support for bimetallic PtSn catalysts because it exhibits high surface area (above 150 m2/g), basic properties, and high thermal stability. We report here an effective new approach for preparing supported bimetallic dehydrogenation catalysts by combining synthesis of the support and introduction of the Pt modifier into one step. The modifying elements were introduced into the hydrotalcite support synthesis, instead of after the synthesis of the support. This method not only creates a catalyst with high activity, selectivity close to 100%, and high stability, but also improves synthesis efficiency. For example, the introduction of Ga into the synthesis of Mg(Al)O, followed by calcinations of the resulting hydrotalcite produces a Mg(Al)(Ga)O mixed oxide. Pt is impregnated using organometallic precursors. Mg(Al)(Ga)O, with different Ga concentrations were synthesized, and then characterized by ICP, BET, XRD, and TGA-DSC, and the corresponding Pt/Mg(Al)(Ga)O catalyst was characterized further via EXAFS, NMR and TEM. The activity and selectivity of Pt/Mg(Al)(Ga)O for ethane dehydrogenation were found to be functions of the bulk Ga/Pt ratio. The highest dehydrogenation activity was obtained for Ga/Pt = 5.4, at which value the ethene selectivity was 100%. The catalyst shows no deactivation after 10 h on stream, and minimal coke formation compared to Pt/Mg(Al)O. The catalyst activity and its resistance to coke formation are increased by the presence of H2. Maximum ethene productivity was achieved for a feed H2/C2H6 ratio of 0.5. The high ethene selectivity is attributed to the formation of PtGa alloys, which were identified by EXAFS following catalyst reduction and use.