Claude Gueguen

Hydration of n-butenes using zeolite catalysts. Influence of the aluminium content on activity

The hydration of n-butene over various protonic zeolites has been investigated in a batch reactor. At 523 K and 65 atm, and with a water/olefin ratio of 4.4, the alcohol yields per gram of catalyst followed the pattern mordenite > ZSM-5 > offretite ≅ omega ≅ Y. In particular, mordenites with intermediate Al contents (7 < SiAl < 9) enabled 75% of the thermodynamic equilibrium to be reached with 100% selectivity. Overdealumination of mordenite as well as more severe experimental conditions led to the formation of dimer by-products. Zeolites exhibiting a hydrophobic character had a very good activity but hydrophobicity does not appear to play a determining role in the process. It is found that the activities per protonic site are correlated with the aluminium content of the catalyst regardless of its framework structure. It is proposed that, as in aqueous acid media, the hydration activity of zeolites is a direct and single function of their protonic acid strength.

Activation of zeolite Ω: I. Physicochemical characterization of calcined and self-steamed samples

Abstract 29Si and27Al magic angle spinning NMR spectroscopy, X-ray diffraction, porosity measurements, chemical analysis and infrared spectroscopy of the framework and hydroxyl regions were used to study the changes in properties of zeolite Ω which occurred upon thermal activation. Calcination in the presence of dry air at 500°C and self-steaming at 600°C resulted in 25% and 50% of framework dealumination respectively. The dislodged species contained aluminium in tetrahedral and octahedral configurations. The former were characterized by an27Al NMR signal at 30 ppm and an IR band at 3575 cmt-1. Most of these species could be removed by acid leaching with dilute nitric acid. Activated zeolite Ω possessed acid sites which were stronger than those of Y type zeolites; some of them were not accessible to ammonia. In the parent material, aluminium was incorporated preferentially in the six-membered rings of the gmelinite cages. Dealumination affected selectively the sites located in the four-membered rings.

Sorption properties of zeolite omega

Abstract The sorption properties of synthetic zeolite omega have been studied using nitrogen, water, and hydrocarbons as sorbates. Samples were prepared from two parent materials appearing as hexagonal single crystals (0.7 × 2μm) or spherulitic aggregates of small particles (0.03 × 0.2 μm). The sorption characteristics of the hexagonal single crystals are fully consistent with the proposed framework structure. After exchange of the sodium and tetramethylammonium ions by protons their intracrystalline nitrogen and water pore volume corresponds to the calculated void space of the main channel, i.e., 0.09 ml g −1 . Eighty-five and seventy percent of this volume are readily available to n -hexane and 1,3,5-trimethylbenzene, respectively. The polycrystalline spherulitic form of zeolite omega contains mesopores in addition to the micropores. The mesopores are able to sorb 1,3,5-triisopropylbenzene and may contribute to one-half of the sorption capacity determined at high relative pressure. The influence of the activation procedures on the porosity of zeolite omega is examined and it is suggested that the deactivation characteristics of the samples in the cracking of isooctane are related to diffusion kinetics.

Linear prediction in the singular case and the stability of eigen models

The paper gives some insight on the use of Levinsons recursion on non positive Toeplitz matrices. The critical case, where a principal minor is zero, leads to define a new lossless lattice structure with zeros on the unit circle, also useful in the standard case. The stability of the AR models built on the eigen vectors of a Toeplitz is also examined, most of the corresponding zeros are shown to lie on the unit circle. The results apply to noise cancellation, spectral line analysis and computation of eigenvectors of correlation matrices.

Thermal Stability and Acidity of Al3+ Cross Linked Smectites

The preparation of a novel type of wide pore zeolites has been proposed by exchanging dioctaedral smectites by aluminium macrocations. The thermal stability of these solids has been investigated by X-ray diffraction and measurements of surface areas and pore size distributions. The evolution of texture with the calcination temperature corresponds to the sintering of a fraction of the micropores. This evolution is similar for several clays of different origins and is correlated with the degree of cross linking. By optimization of this parameter a residual surface area of 130 m2g-1 is obtained after calcination in air at 800°C. The resulting solid has a strong acidity, mainly of the Lewis type. Pyridine retention after evacuation at 500°C shows that the acidic strength is higher than that of Y zeolites.

Hydrothermal modifications of matrix-embedded Y-faujasite and offretite zeolites

Abstract The crystallinity of hydrothermally treated rare-earth-exchange Y-zeolite (REY) and ammonium-exchanged offretite samples has been determined using infrared and X-ray diffraction measurements. Both techniques indicated that the offretite sample was much more resistant to steam deactivation than the REY sample. Incorporating the zeolite into an amorphous silica alumina matrix results in an increased resistance of both zeolites to hydrothermal aging. IR results suggest that steaming of the matrix/zeolite catalysts induces the partial dealumination of the zeolitic component of the catalyst. A possible explanation for the stabilizing effect of the matrix on the zeolite component is proposed.

27Al MAS n.m.r. evidence for the reversible transformation of the coordination of aluminum in dealuminated mazzite

Abstract The influence of acetylacetone treatment on the aluminum coordination in zeolite mazzite has been studied by 27Al MAS n.m.r. and FTi.r. spectroscopy. Three steam-dealuminated samples, one before and two after acid leaching, with nearly identical Si Al framework ratios, have been examined. In the case of the nonleached sample, a progressive increase of the 27Al resonance of octahedrally coordinated aluminum was seen with increasing contact time with acetylacetone due to the transformation of n.m.r. invisible species into visible ones by complexation. The addition of acetylacetone on the acid-leached samples produces a decrease in peak intensity of the aluminum atoms located in the four-member rings of the framework and an increase of the intensity of the signal due to the octahedrally coordinated atoms. The reversibility of these intensity changes by washing leads to the assumption that it is caused by an in situ complexation and transformation of the coordination of the aluminum T-atoms by the acetylacetone molecules. I.r. measurements of adsorbed acetylacetone support this proposal.

Factorial linear modelling, algorithms and applications

The paper emphasizes the importance of normalization of parameters in identification of linear models as now commonly applied to digital signal processing. Classical LPC, Pisarenko, Prony methods are unified and compared. The factorial approach plays a central role when additive noise is considered. The computational requirement is the determination of eigen vectors of correlation and covariance matrices. Various algorithms are then given including sequential estimation procedures in the covariance case. The methods are compared on close sinewaves merged in noise in terms of resolution, windowing, signal-to-noise ratio.

The factorial linear modelling: A Karhumen-Loeve approach to speech analysis

This paper introduces a modified linear prediction method based on the Karhumen-Loeve expansion of the correlation matrix of the speech samples. This result is obtained via a new normalization of the parameters. It is shown that, due to some important properties of Toeplitz matrices, the poles of the A. R. model lie on the unit circle. Consequently, only the formant frequencies are computed and the result can be interpreted as a special discrete Fourier transform. Application to speech analysis is developped with a comparison to the usual linear prediction and ceptrum methods.

Exact likelihood for stationary vector autoregressive moving average process

Abstract Autoregressive moving average (ARMA) models are useful approximants to the kinds of random processes commonly encountered in discrete-time signal processing applications. Such models may be used to compress data in low bit-rate information transmission, to improve frequency resolution in spectrum analysis, and to forecast in economic, meteorological, and other time series. In this paper we discuss several aspects of the maximum likelihood theory of parameter identification in ARMA models. We highlight the role of ‘innovations’ representations in exact likelihood theory and show how internal model structure may be used to speed up calculation of likelihood in either fast Kalman predictor or fast lattice implementations.