Silica-supported tantalum clusters: catalysts for conversion of methane with n-butane to give ethane, propane, and pentanes

Abstract

Si0 2 -supported tantalum clusters were prepared by adsorption of the precursor Ta(CH 2 Ph) 5 (Ph is phenyl) on the support followed by treatments in H 2 at 523, 623, and 723 K. The resultant clusters, had approximate average diameters of 0.3, 0.8, and 2 nm, as determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. The samples were tested as catalysts for conversion of methane with n-butane in a once-through flow reactor operated at atmospheric pressure and 523 K, and EXAFS spectroscopy was used to characterize the used catalysts. The results show that (a) the catalysts are active for the conversion of methane with n-butane to give ethane, propane, and pentanes; (b) catalytic activity decreased to nearly zero over a time on stream of 22 h; (c) the catalyst incorporating the smallest clusters exhibited the highest initial activity and that incorporating the largest clusters exhibited the lowest activity; (d) each used catalyst contained clusters of approximately the same nuclearity as the respective fresh catalyst, but with Ta-Ta bond lengths approximately 0.17 Å longer than those found in the fresh catalysts. The data are consistent with catalysis by the supported clusters, and the product distributions are consistent with disproportionation of n-butane accompanied by the reaction of methane with propane to give other alkanes.

Chemistry of Tantalum Clusters in Solution and on SiO2 Supports: Analogies and Contrasts

“…The reaction intermediates have not been determined. An analysis of the EXAFS data characterizing the deactivated catalysts indicated an increase in the Ta−Ta bond length in the clusters from 2.93 to 3.10 Å during operation in a flow reactor, without a substantial change in the average cluster nuclearity (Table ) . These results demonstrate that the gas-phase reaction environment affects the cluster structure.…”

“…The aforementioned catalysts consisting of tantalum clusters supported on fumed silica, formed by surface-mediated synthesis from Ta(CH 2 Ph) 5 , are active for the disproportionation of alkanes to give lower- and higher-molecular-weight alkanes . The reverse of this reaction was demonstrated approximately by the conversion of methane and n -butane to give ethane, propane, and pentane, among other (minor) products . The reaction intermediates have not been determined.…”

Trinuclear tantalum clusters grafted to hydroxylated silica surfaces: A density-functional embedded-cluster study

“…Active cen ters of these catalysts are usually formed by the reac tion of a metallic salt and an organometalic compound with surface OH groups and low coordinated oxygen atoms on the surface of the support [1,3,4]. Repre sentative examples of such systems are TM species ultra dispersed as cations on high area silica surfaces [5], also in the form of hydrides [6][7][8][9][10][11], small clusters [12,13], and nanoparticles [14]. Very often these spe cies are anchored to the surface to form oxo and oxo hydroxo complexes of early transition metals [4,[15][16][17][18][19][20][21].…”

Synthesis and spectroscopic properties of luminescent tantalum(v)-β-diketonate complexes and their use as optical sensors and the preparation of nanostructured Ta2O5

“…[3][4][5] In recent work, the preparation of tantalum complexes involves mostly metal alkoxides as precursors for obtaining alkoxidederived tantalum complexes. 6 The importance of synthesizing Ta(V) and Nb(V) β-diketonate complexes lies mainly in their use as precursors of nanoor microstructured oxides with applications in diverse fields such as microelectronics to fabricate semiconductor components, 7 production of various types of reaction catalysts, 8,9 and preparation of materials with special physicochemical characteristics like planar waveguides. [10][11][12] In addition, the use of metal complexes as precursors to produce thin films via metal organic chemical vapor deposition (MOCVD) has raised researchers' interest in obtaining these complexes and understanding their properties.…”