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Ustynyuk, L. Yu.; Ustynyuk, Yu. A.; Laikov, Dimitri N.; Лунин, В. В.

doi:10.1023/a:1015076629218

“…In the particular case of amorphous silica, the periodic models are constructed from an ideal polymorph of SiO 2 , in particular a-quartz, 20 b-cristobalite [20][21][22][23][24] and edingtonite. [25][26][27][28] Transition metal based catalysts on silica surfaces have been modeled using finite clusters [29][30][31][32][33][34][35][36][37][38][39][40][41][42] and CPMD calculations have been carried out for a silica supported zirconium hydride. 22 In this work, we have carried out DFT periodic calculations of the experimental organometallic silica grafted complexes including the real set of ligands.…”

Section: Introductionmentioning

confidence: 99%

Structure, spectroscopic and electronic properties of a well defined silica supported olefin metathesis catalyst, [(SiO)Re(CR)(CHR)(CH₂R)], through DFT periodic calculations: silica is just a large siloxy ligand

Solans‐Monfort

¹

,

Filhol

²

,

Copéret

³

et al. 2006

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DFT plane-wave periodic calculations using the VASP code have been carried out to model the silica supported olefin metathesis catalyst, [(RSiO)Re(RCR)(QCHR)(CH 2 R)]. The structure, spectroscopic and electronic properties of this highly active catalyst have been compared with those of non-efficient molecular analogues, [(X 3 SiO)Re(RCR)(QCHR)(CH 2 R)] (X 3 SiO is triphenylsiloxy or polyoligomeric silsesquioxane (POSS)). The silica surface was modelled using cristobalite and edingtonite ideal polymorph surfaces, and the organometallic fragment has been represented with the experimental (R = tBu) and simplified (R = Me) ligands, [(RSiO)Re(RCR)(QCHR)(CH 2 R)]. The calculated structures, alkylidene J C-H coupling constants and n C-H stretching frequencies agree with experimental data. The syn and anti isomers of the Re complexes are close in energy, the former being always more stable. A secondary ReÁ Á ÁO interaction experimentally detected by EXAFS is found to have no stabilizing influence, but is possible because of the facile distortion of (RSiO)Re(RCR)(QCHR)(CH 2 R). More importantly, the geometry and electronic structure of the Re fragment is essentially the same for the triphenylsiloxy, the POSS and the silica surface, which shows that the siloxy group of the first coordination sphere of Re determines the metal properties. The silica surface is thus electronically equivalent to the other siloxy groups, and should be viewed as a large bulky ligand.

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“…We therefore propose that the active species is a zirconium dihydride complex and that the mechanism of formation of the products involves the following elementary steps (Scheme 2): s-bond metathesis (C À H activation and hydrogenolysis); [11,21,22] b-H and b-alkyl transfer [11,21] associated with intramolecular hydride transfer; [23] olefin insertion in the remaining alkyl chain leading to higher olefins (through b-H transfer) and alkanes (hydrogenolysis). [11,21,22] First, zirconium hydrides supported on silica and silicaalumina, respectively, are two different surface species, [( SiO) 3 ZrH] and [( SiO) 2 ZrH 2 ] (Supporting Information, Figure S5), [24] and C À H activation is faster on the more electrophilic dihydride species. [17] Second, the higher selectivity of isobutene compared to that of methyl-branched pentenes is not consistent with a simple insertion of adsorbed propene on Zr-alkyl species because the concentration of Zrpropyl has to be greater than that of Zr-Me species (Scheme 3).…”

mentioning

confidence: 99%

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

Thieuleux

¹

,

Maraval

²

,

Veyre

³

et al. 2007

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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...

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“…In the case of the zirconium catalysts, this mechanism cannot explain the product distribution obtained. We therefore propose that the active species is a zirconium dihydride complex and that the mechanism of formation of the products involves the following elementary steps (Scheme 2): σ‐bond metathesis (CH activation and hydrogenolysis);11, 21, 22 β‐H and β‐alkyl transfer11, 21 associated with intramolecular hydride transfer;23 olefin insertion in the remaining alkyl chain leading to higher olefins (through β‐H transfer) and alkanes (hydrogenolysis) 11. 21, 22…”

Section: Methodsmentioning

confidence: 99%

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

Thieuleux

¹

,

Maraval

²

,

Veyre

³

et al. 2007

View full text Add to dashboard Cite

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...

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Cited by 24 publications

References 6 publications

Structure, spectroscopic and electronic properties of a well defined silica supported olefin metathesis catalyst, [(SiO)Re(CR)(CHR)(CH₂R)], through DFT periodic calculations: silica is just a large siloxy ligand

Structure, spectroscopic and electronic properties of a well defined silica supported olefin metathesis catalyst, [(SiO)Re(CR)(CHR)(CH₂R)], through DFT periodic calculations: silica is just a large siloxy ligand

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

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

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Untitled

Cited by 24 publications

References 6 publications

Structure, spectroscopic and electronic properties of a well defined silica supported olefin metathesis catalyst, [(SiO)Re(CR)(CHR)(CH2R)], through DFT periodic calculations: silica is just a large siloxy ligand

Structure, spectroscopic and electronic properties of a well defined silica supported olefin metathesis catalyst, [(SiO)Re(CR)(CHR)(CH2R)], through DFT periodic calculations: silica is just a large siloxy ligand

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

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

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Structure, spectroscopic and electronic properties of a well defined silica supported olefin metathesis catalyst, [(SiO)Re(CR)(CHR)(CH₂R)], through DFT periodic calculations: silica is just a large siloxy ligand

Structure, spectroscopic and electronic properties of a well defined silica supported olefin metathesis catalyst, [(SiO)Re(CR)(CHR)(CH₂R)], through DFT periodic calculations: silica is just a large siloxy ligand