Homologation of Propane Catalyzed by Oxide‐Supported Zirconium Dihydride and Dialkyl Complexes

Abstract

Zirconium hydrides supported on oxide materials catalyze several classical reactions, such as hydrogenation, reductive cyclization, cyclotrimerization, and olefin polymerization. [1][2][3][4][5] Notably, they also catalyze the hydrogenolysis of polyolefins and alkanes, except ethane, and this reaction corresponds to the reverse of the polymerization of olefins. [6][7][8][9] Under similar conditions, alkanes, including ethane, are transformed into their lower homologues in the presence of a catalytic amount of silica-supported tantalum hydride.[10] Cleavage of the carbon-carbon bond is different for both catalysts (Group 4 vs. Group 5): b-alkyl transfer for d 0 zirconium catalysts versus a-alkyl transfer for d 2 tantalum catalysts so that hydrogenolysis of a C À C bond in the skeleton of an alkane occurs by removal of one carbon atom at a time for tantalum catalysts and of at least two carbon atoms for zirconium catalysts, hence the difference in the selectivity of the final product (Scheme 1). [9,11,12] Silica-supported tantalum hydrides also catalyze the conversion of alkanes into their direct lower and higher homologues. This reaction, named alkane metathesis, [13] involves carbene and metallacyclobutane intermediates, [14,15] and it explains the selective formation of the directly lower and higher linear homologues of the starting alkane through the transfer of one carbon atom at a time, thereby preventing the formation of long-chain alkanes. We have exploited the specific properties of Zr, which is able to 1) activate C À H bonds of alkanes through s-bond metathesis, 2) carry out hydrogenolysis (the transfer of at least two carbon atoms through b-alkyl transfer), and 3) polymerize olefins by insertion. [16,17] Herein, we report on the transformation of propane into longer-chain higher homologues (up to C 10 ) at moderate temperatures.In an autoclave, zirconium hydride supported on silicaalumina was brought into contact with 500 equivalents of propane under supercritical conditions (43 bar, d = 510 Kg m À3 ). After 48 h at 200 8C, 12.6 % of the propane was converted into lower (39.1 %) and higher homologues (60.9 %). Notably, the major product was 2-methylpropane (20.7 %), and overall branched higher homologues up to C 10 are favored, and particularly the iso homologues, except for pentanes (see the Supporting Information, Table S1 for detailed selectivities). This selectivity is in contrast with that reported with tantalum hydride or tungsten hydride supported on oxide supports: [13,18] 1) linear higher homologues are the major products; 2) higher homologues are mainly butanes and pentanes along with trace amounts of hexanes.The reaction was studied in a flow reactor to identify primary products by investigating the effect of inverse space velocity (contact time) on product selectivity.First, using a constant flow of propane (600 kPa, 3 N mL) at 216 8C, we observed the formation of H 2 , methane, and ethane, which resulted from the C À H bond activation of propane, and its subsequent hydrogenolysis (Supportin...

Methane activation and exchange by titanium-carbon multiple bonds

“…4,8 It is now well established that transition metal alkylidene 9 and alkylidyne 10 complexes can activate aromatic and some aliphatic C-H bonds in an intermolecular manner.…”

“…t Bu, 10 so as to slow down the rate of decay of the precursor complex and expedite data collection at moderate pressures of CH 4 . When 1-d 1 was exposed to 310 psi of CH 4 pressure, KIE ¼ 2.3 at 31 C was observed ( Fig.…”

“…However, methane's low binding affinity for potential C-H activation and functionalization catalysts in addition to its low reactivity render this substrate untameable for these goals. One strategy towards employing this C 1 feedstock is alkane metathesis, [2][3][4][5] where long alkane skeletons are constructed by the combination of C-H bond breaking and C-C bond forming steps. Metal-carbon multiple bonds are thought to play a critical role as reactive intermediates in the aforementioned reactions promoted by solid-state supported Ta, Mo, and W hydrides, [2][3][4][5][6] and are certainly involved in recently discovered homogeneous tandem alkane dehydrogenation/olefin metathesis catalytic systems.…”

“…4, a conceivable pathway for observed hydrogen exchange in 4 is by way of the alkane sigmacomplex, A-CH 4 A and d Rh-C ¼ 2.38 A). These longer C-H bonds are in line with the thermodynamics which suggest that methane is very loosely bound in A-CH 4 .…”

“…4, a conceivable pathway for observed hydrogen exchange in 4 is by way of the alkane sigmacomplex, A-CH 4 A and d Rh-C ¼ 2.38 A). These longer C-H bonds are in line with the thermodynamics which suggest that methane is very loosely bound in A-CH 4 . The calculated enthalpy difference of 0.8 kcal mol À1 between the separated products and A-CH 4 indicates that the methane extrusion step is almost entirely entropy driven 24 and the upper limit of the corresponding barrier can be estimated to be 3-4 kcal mol…”