Surface Organometallic Chemistry of Titanium on Silica−Alumina and Catalytic Hydrogenolysis of Waxes at Low Temperature

A strategy for high ethylene polymerization performance using titanium single-site catalysts

The synthesis of heterogeneous Ti(IV)-based catalysts for ethylene polymerization following surface organometallic chemistry concepts is described. The unique feature of this catalyst arises from the silica support, KCC-1700. It has (i) a 3D fibrous morphology that is essential to improve the diffusion of the reactants, and (ii) an aluminum-bound hydroxyl group, [(Si-O-Si)(Si-O-)2Al-OH] 2, used as an anchoring site. The [(Si-O-Si)(Si-O-)(Al-O-)TiNp3] 3 catalyst was obtained by reacting 2 with a tetrakis-(neopentyl) titanium TiNp4. The structure of 3 was fully characterized by FT-IR, advanced solid-state NMR spectroscopy [1H, 13C], elemental and gas-phase analysis (ICP-OES and CHNS analysis), and XPS. The benefits of combining these morphological (3D structure) and electronic properties of the support (aluminum plus titanium) were evidenced in ethylene polymerization. The results show a remarkable enhancement in the catalytic performance with the formation of HDPE. Notably, the resulting HDPE displays a molecular weight of 3 200 000 g mol-1 associated with a polydispersity index (PD) of 2.3. Moreover, the effect of the mesostructure (2D vs. 3D) was demonstrated in the catalytic activity for ethylene polymerization.

Molecular Alumo- and Gallosilicate Hydrides Functionalized with Terminal M(NR2)3 and Bridging M(NR2)2 (M = Ti, Zr, Hf; R = Me, Et) Moieties

A general synthetic strategy for the systematic synthesis of group 4 MIV heterometallic complexes LMIII(H)(μ-O)Si(μ-O)(OtBu)2}nMIV(NR2)4-n (L = {[HC{C(Me)N(2,6-iPr2C6H3)}2; MIII = Al or Ga; n = 1 or 2; MIV = Ti, Zr, Hf; R = Me, Et), based on alumo- or gallosilicate hydride ligands bearing a Si-OH moiety, is presented. The challenging isolation of these metalloligands involved two strategies. On the one hand, the acid-base reaction of LAlH2 with (HO)2Si(OtBu)2 yielded LAlH(μ-O)Si(OH)(OtBu)2 (1), while on the other hand, the oxidative addition of (HO)2Si(OtBu)2 to LGa produced the gallium analog (2). These metalloligands successfully stabilized two hydrogen atoms with different acid-base properties (MIII-H and SiO-H) in the same molecule. Reactivity studies between 1 and 2 and group 4 amides MIV(NR2)4 (MIV = Ti, Zr, Hf; R = Me, Et) and tuning the reactions conditions and stoichiometry led to isolation and structural characterization of heterometallic complexes 3-11 with a 1:1 or 2:1 metalloligand/MIV ratio. Notably, some of these molecular heterometallic silicate complexes stabilize for the first time terminal (O3Si-O-)MIV(NR2)3 moieties known from single-site silica-grafted species. Furthermore, the aluminum-containing heterometallic complexes possess Al-H vibrational energies similar to those reported for modified alumina surfaces, which makes them potentially suitable models for the proposed MIV species grafted onto silica/alumina surfaces with hydride and dihydride architectures.

A High-Throughput Approach to Repurposing Olefin Polymerization Catalysts for Polymer Upcycling

Efficient and economical plastic waste upcycling relies on the development of catalysts capable of polymer degradation. A systematic high-throughput screening of twenty-eight polymerization catalyst precursors, belonging to the catalyst families of metallocenes, ansa-metallocenes, and hemi- and post-metallocenes, in cis-1,4-polybutadiene (PB) degradation reveals, for the first time, important structure-activity correlations. The upcycling conditions involve activation of the catalysts (at 0.18 % catalyst loading) with tri-iso-butyl aluminum at 50 °C in toluene. The data indicate the ability to degrade PB is a general reactivity profile of neutral group 4 metal hydrides. A simple quantitative-structure activity relationship (QSAR) model utilizing two descriptors for the distribution of steric bulk in the active pocket and one measuring the metal ion electrophilicity reveals the degradation ability improves with increased but not overbearing steric congestion and lower electrophilicity.

A strategy to convert propane to aromatics (BTX) using TiNp4 grafted at the periphery of ZSM-5 by surface organometallic chemistry

The direct conversion of propane into aromatics (BTX) using modified ZSM-5 was achieved with a strategy of "catalysis by design". In contrast to the classical mode of action of classical aromatization catalysts which are purely based on acidity, we have designed the catalyst associating two functions: One function (Ti-hydride) was selected to activate the C-H bond of propane by σ-bond metathesis to further obtain olefin by β-H elimination and the other function (Brønsted acid) being responsible for the oligomerization, cyclization, and aromatization. This bifunctional catalyst was obtained by selectively grafting a bulky organometallic complex of tetrakis(neopentyl)titanium (TiNp4) at the external surface (external silanol ([triple bond, length as m-dash]Si-OH) group) of [H-ZSM-5300] to obtain [Ti/ZSM-5] catalyst 1. This metal was chosen to activate the C-H bond of paraffin at the periphery of the ZSM-5 while maintaining the Brønsted acid properties of the internal [H-ZSM-5] for oligomerization, cyclization, and aromatization. Catalyst 2 [Ti-H/ZSM-5] was obtained after treatment under H2 at 550 °C of freshly prepared catalyst 1 ([Ti/ZSM-5]) and catalyst 1 was thoroughly characterized by ICP analysis, DRIFT, XRD, N2-physisorption, multinuclear solid-state NMR, XPS and HR-TEM analysis including STEM imaging. The conversion of propane to aromatics was studied in a dynamic flow reactor. With the pristine [H-ZSM-5300] catalyst, the conversion of propane is very low. However, with [Ti-H/ZSM-5] catalyst 2 under the same reaction conditions, the conversion of propane remains significant during 60 h of the reaction (ca. 22%). Furthermore, the [Ti-H/ZSM-5] catalyst shows a good and stable selectivity (55%) for aromatics (BTX) of time on stream. With 2, it was found that the Ti remains at the periphery of the [H-ZSM-5] even after reaction time.

Regioselectivity and Reaction Mechanism of Ru-Catalyzed Hydrogenolysis of Squalane and Model Alkanes

The dependence of the C-C hydrogenolysis activity on reaction parameters and the structure of the substrate alkanes was investigated for Ru/CeO₂ catalyst with very small (dispersion: H/Ru=0.89) Ru particles. The substrate concentration and reaction temperature did not have a significant effect on the selectivity pattern, except that methane production was promoted at high temperatures. However, the hydrogen pressure had a marked effect on the selectivity pattern. C_tertiary -C bond dissociation, terminal C_secondary -C_primary bond dissociation, and fragmentation to form excess methane had negative reaction order with respect to hydrogen partial pressure, whereas C_secondary -C_secondary bond dissociation had an approximately zero reaction order. Therefore, a high hydrogen pressure is essential for the regioselective hydrogenolysis of C_secondary -C_secondary bonds in squalane. Ru/SiO₂ catalyst with larger Ru particles showed similar changes in the product distribution during the change in hydrogen pressure. The reaction mechanism for each type of C-C bond dissociation is proposed based on reactivity trends and DFT calculations. The proposed intermediate species for the internal C_secondary -C_secondary dissociation, terminal C_secondary -C_primary dissociation, and C_tertiary -C dissociation is alkyls, alkylidynes, and alkenes, respectively.