Jérémie D. A. Pelletier

Catalytic oxidation of light alkanes (C1-C4) by heteropoly compounds.

(C1−C4) by Heteropoly Compounds Miao Sun,*,† Jizhe Zhang,‡ Piotr Putaj, Valerie Caps, Fred́eŕic Lefebvre, Jeremie Pelletier, and Jean-Marie Basset* †Research and Development Center, Saudi Aramco Oil Company, Dhahran 31311, Saudi Arabia ‡Division of Chemical and Life Sciences and Engineering, and KAUST Catalysis Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia Laboratoire de Chimie Organomet́allique de Surface, CPE Lyon, 43 Bd du 11 Novembre 1918, 69622 Villeurbanne, France

Synthesis of Cyclic Carbonates from Epoxides and CO2 under Mild Conditions Using a Simple, Highly Efficient Niobium-Based Catalyst

Carbon dioxide is increasingly regarded as a ubiquitous and nontoxic C1 feedstock for the preparation of bulk commodity chemicals, and thus, it can be considered as a promising future alternative to depleting carbon-based fossil fuel sources. In contrast, the steadily increasing concentration of CO2 in the atmosphere has already reached unsustainably high levels as a result of human activities. Hence, the search for low-energy, carbon-neutral processes to convert CO2 into useful chemicals is of paramount importance. In this context, the exothermic reaction of epoxides and CO2 to form cyclic carbonates is of particular interest in catalysis research. Propylene carbonate (PC, 2 a) and ethylene carbonate (EC, 2 b) find wide application in industry. 5] Recently, sophisticated catalysts were reported for the homogeneous-phase synthesis of organic carbonates at room temperature and at atmospheric pressure, including twoor single-component bimetallic aluminum–salen systems, iron and bismuth complexes, and m-oxotetranuclear zinc and cobalt clusters. Alternative catalytic tools formed by combining Lewis acidic metal halides with nucleophilic co-catalysts (i.e. , MoCl5/PPh3, [10] ZnCl2/NBu4I, [11] InBr3/PPh3 ) have so far shown only low to moderate levels of activity for the synthesis of PC under ambient conditions. Nevertheless, inorganic metal complexes are inexpensive, readily available, and can serve as useful benchmarks to examine the potential of a metal towards the development of more elaborate organometallic complexes and clusters with enhanced activities and stabilities. With the exception of chromium, there are only few reports on the application of metals of group 4–6 for the synthesis of cyclic carbonates. 10] We explored group 4–6 transition-metal complexes (halides and oxychlorides) in combination with a standard nucleophilic co-catalyst : N,N-dimethylaminopyridine (DMAP). Preliminary screening was carried out under mild conditions [50 8C, 5 bar (1 bar = 100 kPa)] . All reactions led to the formation of PC as the only product. The halides and oxychlorides of 4d transition metals, including ZrCl4, NbCl5, MoCl5, and MoOCl4, were the most active (Table 1). In particular, NbCl5/ DMAP formed a very active catalyst (Table 1, entries 6 and 7).

Cooperative Effect of Monopodal Silica-Supported Niobium Complex Pairs Enhancing Catalytic Cyclic Carbonate Production

Recent discoveries highlighted the activity and the intriguing mechanistic features of NbCl5 as a molecular catalyst for the cycloaddition of CO2 and epoxides under ambient conditions. This has inspired the preparation of novel silica-supported Nb species by reacting a molecular niobium precursor, [NbCl5·OEt2], with silica dehydroxylated at 700 °C (SiO(2-700)) or at 200 °C (SiO(2-200)) to generate diverse surface complexes. The product of the reaction between SiO(2-700) and [NbCl5·OEt2] was identified as a monopodal supported surface species, [≡SiONbCl4·OEt2] (1a). The reactions of SiO(2-200) with the niobium precursor, according to two different protocols, generated surface complexes 2a and 3a, presenting significant, but different, populations of the monopodal surface complex along with bipodal [(≡SiO)2NbCl3·OEt2]. (93)Nb solid-state NMR spectra of 1a-3a and (31)P solid-state NMR on their PMe3 derivatives 1b-3b led to the unambiguous assignment of 1a as a single-site monopodal Nb species, while 2a and 3a were found to present two distinct surface-supported components, with 2a being mostly monopodal [≡SiONbCl4·OEt2] and 3a being mostly bipodal [(≡SiO)2NbCl3·OEt2]. A double-quantum/single-quantum (31)P NMR correlation experiment carried out on 2b supported the existence of vicinal Nb centers on the silica surface for this species. 1a-3a were active heterogeneous catalysts for the synthesis of propylene carbonate from CO2 and propylene oxide under mild catalytic conditions; the performance of 2a was found to significantly surpass that of 1a and 3a. With the support of a systematic DFT study carried out on model silica surfaces, the observed differences in catalytic efficiency were correlated with an unprecedented cooperative effect between two neighboring Nb centers on the surface of 2a. This is in an excellent agreement with our previous discoveries regarding the mechanism of NbCl5-catalyzed cycloaddition in the homogeneous phase.

Highly integrated CO2 capture and conversion: direct synthesis of cyclic carbonates from industrial flue gas

Robust and selective catalytic systems based on early transition metal halides (Y, Sc, Zr) and organic nucleophiles were found able to quantitatively capture CO2 from diluted streams via formation of hemicarbonate species and to convert it to cyclic organic carbonates under ambient conditions. This observation was exploited in the direct and selective chemical fixation of flue gas CO2 collected from an industrial exhaust, affording high degrees of CO2 capture and conversion.

Cycloadditions to Epoxides Catalyzed by Group III–V Transition-Metal Complexes

Complexes of group III–V transition metals are gaining increasing importance as Lewis acid catalysts for the cycloaddition of dipolarophiles to epoxides. This review examines the latest reports, including homogeneous and heterogeneous applications. The pivotal step for the cycloaddition reactions is the ring opening of the epoxide following activation by the Lewis acid. Two modes of cleavage (CC versus CO) have been identified depending primarily on the substitution pattern of the epoxide, with lesser influence observed from the Lewis acid employed. The widely studied cycloaddition of CO2 to epoxides to afford cyclic carbonates (CO bond cleavage) has been scrutinized in terms of catalytic efficiency and reaction mechanism, showing that unsophisticated complexes of group III–V transition metals are excellent molecular catalysts. These metals have been incorporated, as well, in highly performing, recyclable heterogeneous catalysts. Cycloadditions to epoxides with other dipolarophiles (alkynes, imines, indoles) have been conducted with scandium triflate with remarkable performances (CC bond cleavage).

Dynamics of the NbCl5-Catalyzed Cycloaddition of Propylene Oxide and CO2: Assessing the Dual Role of the Nucleophilic Co-Catalysts

A mechanistic study on the synthesis of propylene carbonate (PC) from CO2 and propylene oxide (PO) catalyzed by NbCl5 and organic nucleophiles such as 4-dimethylaminopyridine (DMAP) or tetra-n-butylammonium bromide (NBu4 Br) is reported. A combination of in situ spectroscopic techniques and kinetic studies has been used to provide detailed insight into the reaction mechanism, the formation of intermediates, and interactions between the reaction partners. The results of DFT calculations support the experimental observations and allow us to propose a mechanism for this reaction.

Nucleophile-directed selectivity towards linear carbonates in the niobium pentaethoxide-catalysed cycloaddition of CO2 and propylene oxide

Homoleptic Nb-complexes combined with selected organic nucleophiles generate very active catalytic systems for the cycloaddition of propylene oxide and CO2 under ambient conditions. An unprecedented reaction pathway towards an acyclic organic carbonate is observed when extending the study to [Nb(OEt)5] in combination with 4-dimethylamino-pyridine (DMAP) or tetra-n-butylammonium bromide (TBAB). Mechanistic insights of the reaction are provided based on experimental and spectroscopic evidences.

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.

Design and Application of a Hybrid Material Featuring Well-Defined, Tuneable Grafting Sites for Supported Catalysis.

A new material based on amorphous silica, and featuring a well‐defined phenolic functionality was prepared in two simple steps by using commercially available, cheap reagents. Silica was first reacted with aluminum isobutyl etherate to yield an aluminum isobutyl site [(≡SiO)2Al(iBu)(Et2O)], which then selectively reacted with a hydroquinone spacer to yield [(≡SiO)2(AlOC6X4OH)(Et2O)] (X=H or F). This support material was then used to tether the organometallic tungstenocarbyne complex [W(≡CtBu)(CH2tBu)3] to yield the surface species [(≡SiO)2(AlOC6X4O‐W(≡CtBu)(CH2tBu)2(Et2O)] (X=H or F). Both H‐ and F‐containing species were fully characterized, and their activities in the self‐metathesis reaction of propene to ethylene and 2‐butenes were found to be two and three times higher, respectively, than the activity of the corresponding tungstenocarbyne complex directly grafted onto silica.

Methane Reacts with Heteropolyacids Chemisorbed on Silica to Produce Acetic Acid under Soft Conditions

Selective functionalization of methane at moderate temperature is of crucial economic, environmental, and scientific importance. Here, we report that methane reacts with heteropolyacids (HPAs) chemisorbed on silica to produce acetic acid under soft conditions. Specially, when chemisorbed on silica, H(4)SiW(12)O(40), H(3)PW(12)O(40), H(4)SiMo(12)O(40), and H(3)PMo(12)O(40) activate the primary C-H bond of methane at room temperature and atmospheric pressure. With these systems, acetic acid is produced directly from methane, in a single step, in the absence of Pd and without adding CO. Extensive surface characterization by solid-state NMR spectroscopy, IR spectroscopy, cyclic voltammetry, and X-ray photoelectron spectroscopy suggests that C-H activation of methane is triggered by the protons in the HPA-silica interface with concerted reduction of the Keggin cage, leading to water formation and hydration of the interface. This is the simplest and mildest way reported to date to functionalize methane.