Eric G. Derouane

Infrared and temperature-programmed desorption study of the acidic properties of ZSM-5-type zeolites

Infrared spectroscopy and temperature-programmed desorption have been used to investigate the acidic properties of ZSM-5-type zeolites. Active acidic sites, in concentration related to the aluminum content and most probably located at the channel intersections, are characterized by an ir band at 3600 cm−1 and a desorption peak near 773 K (γ state). Weaker acidic sites are characterized by an ir band at 3720–3740 cm−1 and a desorption peak at about 500 K (β state). They probably correspond to terminal silanol groups on the external surface of the zeolite or possibly nonzeolitic impurities. Lewis acidic sites are also characterized by the γ-state desorption peak, while Na adsorption centers for ammonia give rise to a β-state peak. Pyridine desorption helps dehydroxylation of the zeolite.

Surface curvature effects in physisorption and catalysis by microporous solids and molecular sieves

Abstract Molecular sieves, including zeolites, distinguish themselves from other sorbents and catalysts by the curvature of the surface (internal pores and cages, and external “pockets”) which they offer to incoming molecules on their way to catalytically active sites. Elaborating on the recently proposed general concept of “nesting,” this paper attempts to quantify one of its aspects, namely the role of surface curvature when the size of the host structure and that of the guest molecule become comparable. Topics and examples are selected from the literature on physisorption and/or catalysis by zeolites. A simple van der Waals model for the interaction energy and the sticking force of a molecule lodging in a pore is used to rationalize semiquantitatively a number of well-accepted observations, e.g., (i) the role of zeolites as molecular traps; (ii) the origin of the surface barrier postulated to reconcile the large divergence between intracrystalline (self-diffusion) and macroscopically measured (nonequilibrium) diffusion coefficients; (iii) the rapid diffusion of molecules in tight-fitting zeolite pores; (iv) the “window effect” observed for the diffusion of C 3 C 14 linear chain paraffins in erionite; (v) the relationship among apparent acid strength, cracking activity, and molecular “nesting”; and (vi) the dependence of the “constraint index” on temperature. In particular, two new concepts are introduced: the floating molecule which acquires supermobility when its dimension(s) matches closely that of the surrounding channel and the serpentine or creeping motion of the molecule along the channel walls.

Infrared, Microcalorimetric, and Electron Spin Resonance Investigations of the Acidic Properties of the H-ZSM-5 Zeolite

The infrared spectra of the H-ZSM-5 zeolite calcined at temperatures up to 1173 K, and the corresponding electron spin resonance and microcalorimetric data are discussed. Two types of hydroxyl groups are characterized by absorption bands at 3720 and 3605 cm −1 , a small shoulder being present at 3665 cm −1 . Infrared spectra were also recorded after pyridine adsorption, showing the presence of Bronsted acid sites and Lewis acid sites. After calcination at increasing temperatures, dehydroxylation of the zeolite is observed: above 675 K, the number of Bronsted acid sites decreases, while that of strong Lewis acid sites increases; however, a small dealumination occurs as shown by chemical analysis measurements and XPS data. Moreover, dehydroxylation enhances the constraint character of this zeolite, as observed by pyridine inability to titrate the total Lewis sites. Electron spin resonance studies of trapped hydrogen atoms, of adsorbed NO, and of adsorbed benzene radical cations formed on H-ZSM-5 at various calcination temperatures are discussed in terms of the number and strength of the acid sites. It is suggested that the acid sites which are present are very similar to those of H-mordenite although slightly stronger. A microcalorimetric study of ammonia adsorption confirms the very strong acidic character of the acid sites and shows their dependence in strength and heterogeneity upon calcination temperature.

Elucidation of the mechanism of conversion of methanol and ethanol to hydrocarbons on a new type of synthetic zeolite

13C nuclear magnetic resonance and vapor-phase chromatography have been used to investigate the conversions of methanol and ethanol to hydrocarbons on a synthetic zeolite of the type H-ZSM-5 as described by Mobil. Methanol is first dehydrated to dimethyl ether and ethylene. Then the reaction proceeds by two competitive paths: first, successive dehydration-methanolation steps to give branched aliphatics, and, second, polycondensation reactions leading to linear aliphatic and aromatic compounds. The basic mechanism is essentially the same for ethanol, with the major difference being that ethylene can also be formed by direct dehydration of ethanol. At variance to earlier proposals, a mechanism involving carbenium ions is proposed which accounts well for the high yield in branched hydrocarbons and the observation of methyl ethyl ether which is detected in the methanol conversion products.

Synthesis and characterization of ZSM-5 type zeolites I. physico-chemical properties of precursors and intermediates

Abstract Several techniques have been used in combination to clarify the mechanisms by which crystallization of ZSM-5 type zeolites takes place from Na2O-(TPA)2-A12O3-SiO2-H2O synthesis mixtures. Two different mechanisms are observed depending on the source of silica and the Si/Al, Al/Na and (Si + A1)/TPA ratios in the reaction mixture. The first is a liquid phase ion transportation process in which few nuclei are formed and large crystallites are obtained. The second is a solid hydrogel phase transformation process in which numerous nuclei are formed, leading to polycrystalline aggregates. Classical sigmoid nucleation-growth crystallization kinetics characterize both types of syntheses with a higher rate of crystallization when solid hydrogel transformation takes place. A non-homogeneous distribution of aluminium may be observed in the zeolite crystallites due either to nucleation from nearly pure silicate species or because of crystallization of the latter as an isostructural outer shell. It is shown that the presence of ZSM-5 zeolite in intermediate phases may be evidenced by thermal analysis, in particular when a large number of microcrystallites which cannot be detected by X-ray diffraction is present. The clathrating-templating effect of TPA species is recognised. The latter are also incorporated in the framework as exchange cations upon heating at 500–600°C.

Reaction pathways for the conversion of methanol and olefins on H-ZSM-5 zeolite

The conversions of methanol, dimethyl ether, ethylene, propene, 1-butene, and 3,3-dimethyl-1-butene by reaction on various H-ZSM-5 catalysts at 370–380 °C demonstrate the importance of a carbenium ion mechanism in the formation of various aliphatic (C1C6) and aromatic (C6C10) hydrocarbons. C2C4 olefin reactions occur in a way very similar to the classical conjunct polymerization of olefins. The initial step in the formation of aromatics is a “concerted” cycloaddition of an olefin and a carbenium ion which is favored by the unique structural properties of the zeolite. From correlations between the SiAl ratio and yields in aromatics and C4 aliphatics, it is proposed that strong acid sites are responsible for the dehydrocyclization of C6+ olefins into aromatics. Some evidence is also presented for alkylation reactions. Some of the unique properties of H-ZSM-5 are apparently due to the combination of strongly acidic and molecular sieving properties.

A novel effect of shape selectivity: Molecular traffic control in zeolite ZSM-5

An analysis of new and available data on the adsorption of n-pentane, n-hexane, 3-methylpentane, p-xylene, toluene, and isopentane in ZMS-5 zeolites suggested that the reactant molecules enter the ZSM-5 catalyst essentially by its sinusoidal channels and that the isoaliphatic and aromatic products diffuse preferentially in the linear and elliptical channels. Conversion occurs at the channel intersection. These observations account for the absence of major counterdiffusion effects in ZSM-5 zeolites.

Synthesis and characterization of zsm-5 type zeolites: III. A critical evaluation of the role of alkali and ammonium cations

Abstract Various techniques were used to investigate the role of alkali and ammonium cations in governing nucleation and growth processes of (M I )ZSM-5 zeolites formed within the (Na 2 O, M 2 O)-(Pr 4 N) 2 O-A1 2 O 3 -SiO 2 -H 2 O synthesis mixtures (M I = Li, Na, NH 4 , K, Rb, Cs). Morphology, size, chemical composition and homogeneity of the(M I ) ZSM-5 crystallites depend on the competitive interaction between Pr 4 N + or alkali cationic species and aluminosilicate polymeric anions at the early stages of the nucleation process. The latter, in turn, is strongly affected by intrinsic properties of the alkali cations such as their size, their structure-forming or structure-breaking role towards water and their salting-out power. Structure-breaking cations such as K + , Rb + or Cs + favour the formation of large (15–25 μm) single crystals or twins. In the presence of structure-forming cations (Li + ,Na + ) a rapid nucleation yields Si-rich crystallites homogeneously distributed within the 5–15 μm range. Those are coated with numerous small (1 μm) Al-richer crystallites formed by a secondary nucleation process from the Si-deficient gel. In an cases, combined PIGE (bulk) and EDX (outer shell) analyses of Si and Al reveal that Al is homogeneously distributed within the individual crystallites and that Si/Al ratio increases with the particle size. As a result, K, Rb and Cs ZSM-5 zeolites appear homogeneous in composition while Li and Na polycrystalline aggregates show an apparent Al-enriched outer rim. In presence of NH 4 + ions, large single crystals of ZSM-5 having an Al-deficient core and an Al-rich outer shell, as well as small Si-rich crystallites stemming from a delayed nucleation process, are formed. This particular role of NH 4 + is explained in terms of its preferential interaction with aluminate rather than with silicate anions, during the nucleation stage. Our various findings suggest that both solution ion transportation and gel phase transformations (surface nuc1eation) mechanisms can govern simultaneously the nucleation and growth of ZSM-5 zeolites.

Zeolite microporous solids : synthesis, structure, and reactivity

I. Synthesis.- Some thermodynamic and kinetic effects related to zeolite crystallization.- Organic and inorganic agents in the synthesis of molecular sieves.- Non-conventional crystalline microporous solids.- Molecular engineering of lamellar solids. I. Principles derived from the pillaring of smectite clays.- Molecular engineering of layered structures II. Synthetic approaches to some new pillared derivatives.- II. Characterization.- General overview of the characterization of zeolites.- Sorption of single gases and their binary mixtures in zeolites.- Frequency-response measurements of diffusion of sorbates in zeolites.- Diffraction Studies of zeolites.- Spectroscopic investigations of zeolite properties.- The impact of NMR spectroscopy in molecular sieve characterization I. Low Si/Al ratio materials.- The impact of NMR spectroscopy in molecular sieve characterization II. Investigations of highly siliceous systems.- III. Modification, Reactivity, and Catalytic Activity.- Modification of zeolites and new routes to ion-exchange.- Zeolite framework substitution reliable characterization methods.- Evaluation and tailoring of acid-base properties of zeolites Part 1.- Evaluation and tailoring of acid-base properties of zeolites Part 2.- Catalysis by exchanged cations and zeolite framework sites.- Zeolites in oil refining and petrochemistry.- Composition of the carbonaceous compounds responsible for zeolite deactivation. Modes of formation.- Deactivation of zeolites by coking. Prevention of deactivation and regeneration.- IV. Novel Developments and Vistas.- Molecular sieves with pore openings consisting of more than 12-T atoms.- Chemical bonding in zeolites.- Some aspects of molecular shape-selective catalysis with hydrocarbons in zeolites.- New applications of nonclassical molecular sieve catalysts.- Enzyme mimicking with zeolites.- Microporous materials in organic synthesis.- Concluding Remarks.- List of Participants.

Position and configuration of the guest organic molecules within the framework of the ZSM-5 and ZSM-11 zeolites

Abstract Thermogravimetry (t.g.) and high resolution n.m.r. spectroscopy (HRNMR), using cross-polarization (CP) and magic angle spinning (MAS), enable the characterization of the configuration and conformation of tetrapropylammonium (TPA) and tetrabutylammonium (TBA) or tetrabutylphosphonium (TBP) species within the ZSM-5 and ZSM-11 frameworks respectively. It is observed by thermal analysis that 3.3 to 3.8 TPA entities are normally present per unit cell of the ZSM-5 precursor while for ZSM-11 the corresponding value is 2.6 for TBA and TBP. HRNMR shows a splitting of the terminal methyl groups resonance for all organic species. Differences in chemical shifts are accounted for by different environments, referring to the known structures of the zeolites, and locating these cations at their channel intersections. Specific interactions are suggested, which explain in addition the variations which occur in the other n.m.r. parameters (linewidths and relative intensities). Whilst each channel intersection in zeolite ZSM-5 tends to be occupied by one TPA entity during synthesis, the largest intersections in the ZSM-11 framework (two per unit cell) are preferentially occupied by the bigger TBA and TBP cations.