Mostafa Taoufik

Metathesis of Alkanes and Related Reactions

The transformation of alkanes remains a difficult challenge because of the relative inertness of the C-H and C-C bonds. The rewards for asserting synthetic control over unfunctionalized, saturated hydrocarbons are considerable, however, because converting short alkanes into longer chain analogues is usually a value-adding process. Alkane metathesis is a novel catalytic and direct transformation of two molecules of a given alkane into its lower and higher homologues; moreover, the process proceeds at relatively low temperature (ambient conditions or higher). It was discovered through the use of a silica-supported tantalum hydride, ([triple bond]SiO)(2)TaH, a multifunctional catalyst with a single site of action. This reaction completes the story of the metathesis reactions discovered over the past 40 years: olefin metathesis, alkyne metathesis, and ene-yne cyclizations. In this Account, we examine the fundamental mechanistic aspects of alkane metathesis as well as the novel reactions that have been derived from its study. The silica-supported tantalum hydride catalyst was developed as the result of systematic and meticulous studies of the interaction between oxide supports and organometallic complexes, a field of study denoted surface organometallic chemistry (SOMC). A careful examination of this surface-supported tantalum hydride led to the later discovery of alumina-supported tungsten hydride, W(H)(3)/Al(2)O(3), which proved to be an even better catalyst for alkane metathesis. Supported tantalum and tungsten hydrides are highly unsaturated, electron-deficient species that are very reactive toward the C-H and C-C bonds of alkanes. They show a great versatility in various other reactions, such as cross-metathesis between methane and alkanes, cross-metathesis between toluene and ethane, or even methane nonoxidative coupling. Moreover, tungsten hydride exhibits a specific ability in the transformation of isobutane into 2,3-dimethylbutane as well as in the metathesis of olefins or the selective transformation of ethylene into propylene. Alkane metathesis represents a powerful tool for making progress in a variety of areas, perhaps most notably in the petroleum and petrochemical fields. Modern civilization is currently confronting a host of problems that relate to energy production and its effects on the environment, and judicious application of alkane metathesis to the processing of fuels such as crude oil and natural gas may well afford solutions to these difficulties.

“Hydro‐metathesis” of Olefins: A Catalytic Reaction Using a Bifunctional Single‐Site Tantalum Hydride Catalyst Supported on Fibrous Silica (KCC‐1) Nanospheres

We thank ESCPE-Lyon, the CNRS, and the KAUST for financial and logistic support and Anne Baudouin for NMR spectra acquisition.

Non-Oxidative Coupling Reaction of Methane to Ethane and Hydrogen Catalyzed by the Silica-Supported Tantalum Hydride: (≡SiO)2Ta−H

Silica-supported tantalum hydride, (SiO)2Ta-H (1), proves to be the first single-site catalyst for the direct non-oxidative coupling transformation of methane into ethane and hydrogen at moderate temperatures, with a high selectivity (>98%). The reaction likely involves the tantalum-methyl-methylidene species as a key intermediate, where the methyl ligand can migrate onto the tantalum-methylidene affording the tantalum-ethyl.

17O NMR gives unprecedented insights into the structure of supported catalysts and their interaction with the silica carrier.

Flame silica was surface-labeled with (17)O, through isotopic enrichment of both siloxanes and silanols. After heat treatment at 200 and 700 °C under vacuum, the resulting partially dehydroxylated silica materials were investigated by high-field solid-state (1)H and (17)O NMR. More specifically, MQ MAS and HMQC sequences were used to probe the (17)O local environment. In a further step, these (17)O-tagged supports were used for the preparation of supported catalysts by reaction with perhydrocarbyl transition metal derivatives (zirconium tetraalkyl, tantalum trisalkyl-alkylidene, and tungsten trisalkyl-alkylidyne complexes). Detailed (17)O 1D and 2D MQ and HMQC MAS NMR studies demonstrate that signals in the Si-OH, Si-O-Si, and Si-O-metal regions are highly sensitive to local structural modifications, thanks to (17)O wide chemical shift and quadrupolar constant ranges. Experimental results were supported by DFT calculations. From the selective surface labeling, unprecedented information on interactions between supported catalysts and their inorganic carrier has been extracted.

[(SiO)TaVCl2Me2]: A Well‐Defined Silica‐Supported Tantalum(V) Surface Complex as Catalyst Precursor for the Selective Cocatalyst‐Free Trimerization of Ethylene

Over the last 50 years, the production of linear a-olefins by ethylene oligomerization has gained increasing interest in industrial and academic research. Currently, numerous studies in this field have been reported and developed into industrial processes: titanium-based catalysts for the dimerization of ethylene (Alpha-butol, IFP); chromium-based catalysts for the trimerization of ethylene (Phillips Petroleum), and more recently chromium-bearing PNP ligand for ethylene tetramerization (SASOL). Along with these nowclassical systems, which require co-catalysts such as MAO, Sen also reported the use of tantalum pentachloride in combination with an alkylating agent, such as SnMe4, ZnMe2, AlMe3, or MeLi. In this case, the catalysts is assumed to be Ta species formed in situ by the reduction of TaMe2Cl3 in the presence of ethylene. In this context, Mashima and coworkers have shown that Ta active species can be alternatively formed by reduction of TaCl5 by 3,6-bis(trimethylsilyl)-1,4-cyclohexadiene derivatives. However the catalytic performances of these systems require the addition of cocatalysts. In several instances, site isolation on the oxide surface has been highly beneficial to the design of efficient catalysts, compared to inactive or rapidly deactivating molecular analogues, which is in particular due to the absence of bimolecular reactions between supported metal complexes. When considering the reaction intermediates, and in the view of developing well-defined silica supported species, we targeted the immobilization of Me3TaCl2 [9] onto an inorganic carrier, silica, by surface organometallic chemistry (SOMC) as a catalyst precursor. Herein, we report the synthesis and the surface characterization of the grafted organometallic species ( SiO)TaCl2Me2 2 in view of its application in ethylene oligomerization. Furthermore, mechanistic studies on this selective catalytic process have been successfully achieved thanks to the dynamic reactor used. They indicate three different pathways for the initiation process. SBA-15 was selected because of its ordered mesoporous network with large surface area (Supporting Information, Figures S1, S2). This porous silica was subjected to partial dehydroxylation under vacuum at 700 8C to afford SBA15(700), which features mostly isolated silanols, as indicated on the IR spectrum by the characteristic sharp peak at 3747 cm 1 (Supporting Information, Figure S3). SBA-15(700) was reacted with TaCl2Me3 1 (Supporting Information, Figure S4), and the resulting powder was characterized to determine the organometallic species on the surface prior to its catalytic application. Elemental analysis gave 14.8% Ta, 1.65% C, and 0.39 % H, with a ratio of Ta/C/Cl = 1:1.97:1.88 (theoretical: Ta/C/Cl = 1:2:2). H-MAS NMR spectrum of 2 unexpectedly displays two major signals at 1.27 ppm and 0.85 ppm with a broad peak at 1.90 ppm and a very weak signal at 0.03 ppm, which is probably due to methane or a trace amount of SiMe (see below for further comments, and the Supporting Information, Figure S5). The NMR signal at 1.9 ppm most likely corresponds to the small amount of unreacted silanols, in agreement with IR spectroscopy results. Two peaks appear at 1.27 and 0.85 ppm that would be consistent with two inequivalent methyl groups coming from one species or indicating the presence of two distinct species. Proton double (DQ)and triple (TQ)-quantum correlation spectra under 22 kHz MAS (Supporting Information, Figure S6) confirm that these two signals correspond to methyl groups, most likely from two different species in view of the absence of correlation of diagonal peak. Autocorrelation peaks are observed on the diagonal of the 2D DQ spectrum for all the protons (notably, this shows that unreacted silanols are in close proximity to each other and most likely located in micropores). A strong autocorrelation [*] Y. Chen, E. Callens, E. Abou-Hamad, J.-M. Basset KAUST Catalysis Center King Abdullah University of Science and Technology Thuwal 23955-6900 (Kingdom of Saudi Arabia) E-mail: jeanmarie.basset@kaust.edu.sa

On the track to silica-supported tungsten oxo metathesis catalysts: input from 17O solid-state NMR.

The grafting of an oxo chloro trisalkyl tungsten derivative on silica dehydroxylated at 700 °C was studied by several techniques that showed reaction via W-Cl cleavage, to afford a well-defined precatalyst for alkene metathesis. This was further confirmed by DFT calculations on the grafting process. (17)O labeling of the oxo moiety of a series of related molecular and supported tungsten oxo derivatives was achieved, and the corresponding (17)O MAS NMR spectra were recorded. Combined experimental and theoretical NMR studies yielded information on the local structure of the surface species. Assessment of the (17)O NMR parameters also confirmed the nature of the grafting pathway by ruling out other possible grafting schemes, thanks to highly characteristic anisotropic features arising from the quadrupolar and chemical shift interactions.

Heteronuclear NMR spectroscopy as a surface-selective technique: a unique look at the hydroxyl groups of γ-alumina.

The surface hydroxyl groups of γ-alumina dehydroxylated at 500 °C were studied by a combination of one- and two-dimensional homo- and heteronuclear (1)H and (27)Al NMR spectroscopy at high magnetic field. In particular, by harnessing (1)H-(27) Al dipolar interactions, a high selectivity was achieved in unveiling the topology of the alumina surface. The terminal versus bridging character of the hydroxyl groups observed in the (1)H magic-angle spinning (MAS) NMR spectrum was demonstrated thanks to (1)H-(27) Al RESPDOR (resonance-echo saturation-pulse double-resonance). In a further step the hydroxyl groups were assigned to their aluminium neighbours thanks to a {(1)H}-(27) Al dipolar heteronuclear multiple quantum correlation (D-HMQC), which was used to establish a first coordination map. Then, in combination with (1)H-(1) H double quantum (DQ) MAS, these elements helped to reveal intimate structural features of the surface hydroxyls. Finally, the nature of a peculiar reactive hydroxyl group was demonstrated following this methodology in the case of CO2 reactivity with alumina.

A Study of Transition-Metal Organometallic Complexes Combining 35Cl Solid-State NMR Spectroscopy and 35Cl NQR Spectroscopy and First-Principles DFT Calculations

A series of transition-metal organometallic complexes with commonly occurring metal-chlorine bonding motifs were characterized using (35)Cl solid-state NMR (SSNMR) spectroscopy, (35)Cl nuclear quadrupole resonance (NQR) spectroscopy, and first-principles density functional theory (DFT) calculations of NMR interaction tensors. Static (35)Cl ultra-wideline NMR spectra were acquired in a piecewise manner at standard (9.4 T) and high (21.1 T) magnetic field strengths using the WURST-QCPMG pulse sequence. The (35)Cl electric field gradient (EFG) and chemical shielding (CS) tensor parameters were readily extracted from analytical simulations of the spectra; in particular, the quadrupolar parameters are shown to be very sensitive to structural differences, and can easily differentiate between chlorine atoms in bridging and terminal bonding environments. (35)Cl NQR spectra were acquired for many of the complexes, which aided in resolving structurally similar, yet crystallographically distinct and magnetically inequivalent chlorine sites, and with the interpretation and assignment of (35)Cl SSNMR spectra. (35)Cl EFG tensors obtained from first-principles DFT calculations are consistently in good agreement with experiment, highlighting the importance of using a combined approach of theoretical and experimental methods for structural characterization. Finally, a preliminary example of a (35)Cl SSNMR spectrum of a transition-metal species (TiCl4) diluted and supported on non-porous silica is presented. The combination of (35)Cl SSNMR and (35)Cl NQR spectroscopy and DFT calculations is shown to be a promising and simple methodology for the characterization of all manner of chlorine-containing transition-metal complexes, in pure, impure bulk and supported forms.

Low temperature hydrogenolysis of waxes to diesel range gasoline and light alkanes: Comparison of catalytic properties of group 4, 5 and 6 metal hydrides supported on silica–alumina

A series of metal hydrides (M = Zr, Hf, Ta, W) supported on silica–alumina were studied for the first time in hydrogenolysis of light alkanes in a continuous flow reactor. It was found that there is a difference in the reaction mechanism between d0metal hydrides of group 4 and d0 ↔ d2metal hydrides of group 5 and group 6. Furthermore, the potential application of these catalysts has been demonstrated by the transformation of Fischer–Tropsch wax in a reactive distillation set-up into typical gasoline and diesel molecules in high selectivity (up to 86 wt%). Current results show that the group 4 metal hydrides have a promising yield toward liquid fuels.

Small changes have consequences: lessons from tetrabenzyltitanium and -zirconium surface organometallic chemistry.

Homoleptic benzyl derivatives of titanium and zirconium have been grafted onto silica that was dehydroxylated at 200 and 700 °C, thereby affording bi-grafted and mono-grafted single-site species, respectively, as shown by a combination of experimental techniques (IR, MAS NMR, EXAFS, and elemental analysis) and theoretical calculations. Marked differences between these compounds and their neopentyl analogues are discussed and rationalized by using DFT. These differences were assigned to the selectivity of the grafting process, which, depending on the structure of the molecular precursors, led to different outcomes in terms of the mono- versus bi-grafted species for the same surface concentration of silanol species. The benzylzirconium derivatives were active towards ethylene polymerization in the absence of an activator and the bi-grafted species displayed higher activity than their mono-grafted analogues. In contrast, the benzyltitanium and neopentylzirconium counterparts were not active under similar reaction conditions.