Silylation of titanium-containing amorphous silica catalyst: effect on the alkenes epoxidation with H2O2

The surface of a Ti/SiO2 catalyst was silylated using hexamethyldisilazane (HMDS) and tetramethyldisilazane (TMDS) as silylating reagents in vapor phase. The silylation of silanol (Si–OH) on the catalysts was confirmed by diffuse reflectance UV-Vis, DRIFT spectroscopy and solid-state 29Si MAS NMR techniques. Silylation with TMDS improves the catalytic performance of Ti/SiO2 catalyst more significantly than with HMDS and this is mainly due to the steric bulk effects of trimethylsilyl groups from HMDS as compared to dimethylsilyl groups from TMDS. The silylation degree with TMDS was found to be higher than with HMDS and no changes in the silylation degree with TMDS was observed for silylation times longer than 2 h. Despite the silylation route employed, the silylated surface is hydrophobic which enhances not only in hydrogen peroxide efficiency but also and in selectivity to epoxide in the epoxidation of alkenes with H2O2. These effects are more evident when the hydrogen peroxide concentration is higher and particularly for the catalyst modified with TMDS.Peer reviewe

Theory of finite temperature crossovers near quantum critical points close to, or above, their upper-critical dimension

A systematic method for the computation of finite temperature ($T$) crossover functions near quantum critical points close to, or above, their upper-critical dimension is devised. We describe the physics of the various regions in the $T$ and critical tuning parameter ($t$) plane. The quantum critical point is at $T=0$, $t=0$, and in many cases there is a line of finite temperature transitions at $T = T_c (t)$, $t < 0$ with $T_c (0) = 0$. For the relativistic, $n$-component $\phi^4$ continuum quantum field theory (which describes lattice quantum rotor ($n \geq 2$) and transverse field Ising ($n=1$) models) the upper critical dimension is $d=3$, and for $d<3$, $\epsilon=3-d$ is the control parameter over the entire phase diagram. In the region $|T - T_c (t)| \ll T_c (t)$, we obtain an $\epsilon$ expansion for coupling constants which then are input as arguments of known {\em classical, tricritical,} crossover functions. In the high $T$ region of the continuum theory, an expansion in integer powers of $\sqrt{\epsilon}$, modulo powers of $\ln \epsilon$, holds for all thermodynamic observables, static correlators, and dynamic properties at all Matsubara frequencies; for the imaginary part of correlators at real frequencies ($\omega$), the perturbative $\sqrt{\epsilon}$ expansion describes quantum relaxation at $\hbar \omega \sim k_B T$ or larger, but fails for $\hbar \omega \sim \sqrt{\epsilon} k_B T$ or smaller. An important principle, underlying the whole calculation, is the analyticity of all observables as functions of $t$ at $t=0$, for $T>0$; indeed, analytic continuation in $t$ is used to obtain results in a portion of the phase diagram. Our method also applies to a large class of other quantum critical points and their associated continuum quantum field theories.Comment: 36 pages, 4 eps figure

Western Star, 1911-08-30

The Western Star began publication on Newfoundland's west coast on 4 April 1900, appearing weekly with brief semiweekly periods up to 1952, when it became a daily. As of 17 April 2019 it continues as a free weekly community paper

Ellipticity dependence of atomic and molecular high harmonic generation

High harmonic generation is compared in the dependence on the ellipticity of the fundamental laser radiation for an atomic and a molecular system. In particular argon and nitrogen are compared employing molecular beams and intense (3 x 10(14) W/cm(2)) and ultrashort (80 fs) 800 nm laser pulses. It turns out that for all the harmonics under investigation (H5, H13 and H21) the harmonic yield decreases slower with the ellipticity for the molecule than for the atom. This indicates differences in atomic and molecular high harmonic generation

L'univers romanesque de Calixthe Beyala (pour une illustration des orientations actuelles du roman féminin d'Afrique noire francophone)

De son point de départ en 1969 avec "Rencontres essentielles" de Thérèse Kuoh-Moukoury, jusqu'en 1987 lorsque paraît "C'est le soleil qui m'a brûlée" de Calixthe Beyala, le champ romanesque féminin d'Afrique noire francophone se caractérise majoritairement par une très grande pudibonderie, un certain conformisme idéologique et bientôt esthétique. De fait, à travers les neuf romans étudiés, il nous est apparu que la romancière déconstruit la phallocratie, au profit de l'avènement de la "clitocratie". Il s'agit d'un modèle de société sous influence matriarcale, dont le siège est lové dans la capacité érectile du pénis en miniature que constitue le clitoris. Tout comme la phallocratie repose sur la turgescence pénienne, le phallus. Cependant, réduire l'univers romanesque béyalien au terreau libidineux, dont il est communément et d'emblée assimilable, serait passer outre une masse non négligeable de suggestions esthétiques fécondes. En l'espèce, des figures marginales telles les prostituées, les fous, les nymphomanes et les bâtards rejoignent le centre du texte béyalien at par là même, la société. L'humanité dysfonctionnelle comme grille d'analyse de soi, des autres et du monde, celui dit "pôle institutionnalisé", voilà un des enjeux critiques phares que révèle la présente étude. Il en va de même, de la prépondérance chromatique, une distribution coloriste s'opérant essentiellement entre le Rose/Violet, le Blanc, le Noir, le Bleu, le Vert. Mais par dessus tout, le Jaune (très fortement rattaché au soleil et à son incandescence) et le Rouge. Couleur du sang, couleur de la "terre-mère", couleur du pouvoir, qui a échappé à la femme et que les héroïnes de Calixthe Beyala reconquièrent. Celles-ci q'appellent Andela, Beyala, Biloa, Gono ou Ngono... patronymes souvent à forte charge symbolique, et déjà mystique, chez les Eton ; famille ethnique de la romancière et sous-groupe du grand groupe Béti-Boulou-Fang, dont nombre d'artifices, d'éléments des mythologies fondatrices sont revisitées. Veine créatrice orale, épique entre autres, traversant ainsi, personnages, lieux et structures narratives béyaliennes.From its beginning with "Rencontres essentielles" by Thérèse Kuoh-Moukoury in 1969 until 1987 with Calixthe Beyala's "C'est le soleil qui m'a brulée", black women's fiction in Franch-speaking Africa has been characterized by strong prudishness, and by a kind of ideological and (sooner) aesthetic conformism. Actually, the study of the nine novels reveals that the author decontructs phallocracy in favor of the advent of "clitocracy". It is a model of society under matriarchal influence and which relies upon the erectile capacity of the clitoris as a penis on a small scale. In the same way phallocracy relies on the turgescence of the phallus. Yet, reducing Beyalian fictional universe to its libidinous characteristics would overlook significant aesthetic suggestions. In fact characters such as prostitutes, lunatic people, nymphomaniac and bastards are central in Beyala's texts and thus play an important role in society. The crisis of humanity as a self analysis chart, as an analysis of the other and the world (considered as an institutionalised pole), constitutes one of the main interest of this work. The same for the chromatic predominance, the mixture of colours between pink/purple, white, black, blue, green. But yellow (linked to the sun and its incandescence) and red are more predominant. Red referring to blood, mother earth, and power (out of women's control and which Beyala's heroines reconquer). Their names are Andela, Beyala, Biloa, Gono and Ngono... patronymic that are very often charged with symbolic meaning among the Eton, the ethnic group of the novelist and subgroup of the Beti-Boulou-Fang whose tricks and mythologies are revisited. Oral creative vein, epic, going through the author's characters, settings, and narrative structures.PARIS12-Bib. électronique (940280011) / SudocSudocFranceF