Trap efficiency of exhaust gas pollutants in microporous sorbents under representative driving conditions

International audienceThe objective of this study is to develop a Neutral Air Quality Impact Vehicle (NAQIV) using a system able to reduce the tailpipe emissions of a gasoline powertrain engine. A possible configuration is a bypass in the exhaust, close to the tailpipe, that stores pollutants during the cold start phase when most emissions occur. For this purpose, adsorption efficiencies of commercial sieved powders Activated Carbon (AC), and zeolites (BEA, MFI) were screened at trap temperatures of 25, 50, and 150 °C using two different exhaust gas compositions. These compositions are obtained from two different phases of the World Harmonized Light-Duty Test Cycle (WLTC): the first 100 s after the engine start, and the first “5 km” after a cold start, the latter being representative of typical emissions of an automotive vehicle on an average trip in urban areas. The aim is also to provide sorption capacity not only in respect to the conventional pollutants (CO, NOx, CO2), and the most common NMHCs present in the exhaust gas (toluene, i-pentane, n-pentane, acetylene, ethylene, and methane), but also towards unregulated pollutants such as NH3, CH4 and N2O (to be introduced in Euro 7 regulations). The experimental results reveal a superior performance of AC NMHCs adsorption (and particularly toluene, n-pentane and i-pentane) which is not affected by the presence of other pollutants, but is negatively impacted by rising temperature and flow gas composition. Among zeolites, only Cu/Beta and Cu/ZSM-5 display moderate adsorption capacity of NH3, NMHCs and NO. In particular, the adsorption of ethylene and acetylene over Cu/ZSM-5 is strongly promoted at lower H2O concentrations. Finally, desorption profiles of each pollutant were generated through Temperature Programmed Desorption (TPD) experiments, unraveling sorbate-sorbent interactions

Selective and regular localization of accessible Pt nanoparticles inside the walls of an ordered silica: Application as a highly active and well-defined heterogeneous catalyst for propene and styrene hydrogenation reactions

+PDEWe describe here an original methodology related to the "build-the-bottle-around-the-ship" approach yielding a highly ordered silica matrix containing regularly distributed Pt nanoparticles (NPs) located inside the silica walls, Pt@{walls}SiO(2). The starting colloidal solution of crystalline Pt nanoparticles was obtained from Pt(dba)(2) (dba = dibenzylidene acetone) and 3-chloropropylsilane. The resulting nanoparticles (diameter: 2.0 +/- 0.4 nm determined by HRTEM) resulted hydrophilic. The NPs present in the THF colloidal solution were incorporated inside the walls of a highly ordered 2D hexagonal mesoporous silica matrix via sol-gel process using a templating route with tetraethylorthosilicate, TEOS, as the silica source, and block copolymer (EthyleneOxide)(20)(PropyleneOxide)(70)(EthyleneOxide)(20) (Pluronic P123) as the structure-directing agent. Low-temperature calcination of the crude material at 593 K led to the final solid Pt@{walls}SiO(2). Characterization by IR, HRTEM, BF-STEM and HAADF-STEM, SAXS, WAXS, XRD, XPS, H(2) chemisorption, etc. of Pt@{walls}SiO(2) confirmed the 2D hexagonal structuration and high mesoporosity (870 m(2)/g) of the material as well as the presence of stable 2-nm-sized crystalline Pt(0) NPs embedded inside the walls of the silica matrix. The material displayed no tendency to NPs sintering or leaching (Pt loading 0.3 wt.%) during its preparation. Pt@{walls}SiO(2) was found to be a stable, selective and highly active hydrogenation catalyst. The catalytic performances in propene hydrogenation were tested under chemical 'regime conditions in a tubular flow reactor (278 K, propene/H(2)/He = 20/16/1.09 cm(3)/min, P(tot) = 1 bar) and were found superior to those of an homologous solid containing Pt NPs along its pore channels Pt@{pores}SiO(2) and to those of a classical industrial catalysts Pt/Al(2)O(3), (TOF = 2.3 s(-1) vs. TOF = 0.90 and 0.92 s(-1), respectively, calculated per surface platinum atoms). Pt@{walls}SiO(2) also catalyzes fast and selective styrene hydrogenation. A material containing by design Pt NPs both in its walls and in its pores, Pt@{walls + pores}SiO(2), is also described. (C) 2011 Elsevier Inc. All rights reserved

Production of Propylene from 1-Butene on Highly Active “Bi-Functional Single Active Site” Catalyst: Tungsten Carbene-Hydride Supported on Alumina

1-Butene is transformed in a continuous flow reactor over tungsten hydrides precursor W–H/Al₂O₃, <b>1</b>, giving a promising yield into propylene at 150 °C and different pressures. Tungsten carbene-hydride single active site operates as a “bi-functional catalyst” through 1-butene isomerization on W-hydride and 1-butene/2-butenes cross-metathesis on W-carbene. This active moiety is generated in situ at the initiation steps by insertion of 1-butene on tungsten hydrides precursor W–H/Al₂O₃, <b>1</b> followed by α-H and β-H abstraction

Metallacyclobutane Substitution and Its Effect on Alkene Metathesis for Propylene Production over W–H/Al₂O₃: Case of Isobutene/2-Butene Cross-Metathesis

Cross metathesis between 2-butenes and isobutene yielding the valuable products propylene and 2-methyl-2-butene has been investigated at low pressure and temperature using <b>WH</b>_<b>3</b><b>/Al</b>_<b>2</b><b>O</b>_<b>3</b>, a highly active and selective catalyst. Two parallel catalytic cycles for this reaction have been proposed where the cycle involving the less sterically hindered tungstacyclobutane intermediates is most likely favored. Moreover, it has been found that the arrangement of substituents on the least thermodynamically favored tungstacyclobutane governs the conversion rate of the cross metathesis reaction for propylene production from butenes and/or ethylene

Structural Characterization of the EtOH-TiCl 4 -MgCl 2 Ziegler-Natta Precatalyst

The authors thank the PSMN, CINES, and IDRIS for the computational resources.International audienceThe Ziegler-Natta polymerization is one major example of application of catalysis in industry. Since the first discovery of Ziegler and Natta, several modifications of the catalyst have been developed, in order to improve its performance. Nowadays, a typical Ziegler-Natta catalyst for polyethylene synthesis consists of a precatalyst, composed of TiCl4 supported on MgCl2 in the presence of a Lewis base, activated by organoaluminum. The atomic-scale characterization of the Ziegler-Natta catalyst is crucial for further improvement of the catalyst. Here, the precatalyst TiCl4-MgCl2 with EtOH as internal. Lewis base is characterized combining solid-state NMR spectroscopy and periodic density functional theory calculations. From total energy and NMR spectra, eight surface species were proposed showing EtO- ligands on the Ti and EtOH/EtO- on the surface Mg species, These species lead to a complete interpretation of the NMR two-dimensional spectra. Hence a detailed molecular scale description of the precatalyst was obtained

Evolution of Structure and of Grafting Properties of γ-Alumina with Pretreatment Temperature

In this study, the nature of the hydroxyl groups present on γ-alumina, γ-Al₂O₃, pretreated at various temperatures has been reinvestigated by ¹H NMR spectroscopy. The peaks are assigned by comparison between experimental and simulated spectra, in agreement with previous IR studies. The lowest chemical shifts δ correspond to OH groups strongly bound to the most acidic Al atoms (Al_IV and Al_V). High chemical shifts δ are assigned to OH groups making hydrogen bonds. A large range of values is found depending on the strength of these bonds. The structure of the surface complexes obtained by grafting Hf(CH₂<i>t</i>Bu)₄, <b>1</b>, on γ-Al₂O₃ at various pretreatment temperatures <i>T </i>(350, 500, 700 °C), referred to as <b>1-</b>γ-Al₂O_3‑(T), and of their thermolysis products has been determined, by a combined experimental (mass balance, in situ IR,) and theoretical (DFT calculations) study. These results unambiguously prove the presence of two kinds of neopentyl–metal bonds, Hf–CH₂<i>t</i>Bu and Al–CH₂<i>t</i>Bu for <b>1</b>-γ-Al₂O_3‑(500) and <b>1</b>-γ-Al₂O_3‑(700), hence the existence of surface cationic low coordinated hafnium complexes. In contrast, for <b>1</b>-γ-Al₂O_3‑(350), only neutral species exist. Hence, temperature pretreatment has a key role for controlling the chemistry of the alumina surface (density of OH groups, presence of highly Lewis acidic Al), the grafting mode of the Hf precursor, and the formation of cationic low coordinated active centers

Structural Characterization of the EtOH–TiCl₄–MgCl₂ Ziegler–Natta Precatalyst

The Ziegler–Natta polymerization is one major example of application of catalysis in industry. Since the first discovery of Ziegler and Natta, several modifications of the catalyst have been developed, in order to improve its performance. Nowadays, a typical Ziegler–Natta catalyst for polyethylene synthesis consists of a precatalyst, composed of TiCl₄ supported on MgCl₂ in the presence of a Lewis base, activated by organoaluminum. The atomic-scale characterization of the Ziegler–Natta catalyst is crucial for further improvement of the catalyst. Here, the precatalyst TiCl₄–MgCl₂ with EtOH as internal Lewis base is characterized combining solid-state NMR spectroscopy and periodic density functional theory calculations. From total energy and NMR spectra, eight surface species were proposed showing EtO^– ligands on the Ti and EtOH/EtO^– on the surface Mg species. These species lead to a complete interpretation of the NMR two-dimensional spectra. Hence a detailed molecular scale description of the precatalyst was obtained