Jj Janchen

Aluminum incorporation in MCM-41 mesoporous molecular sieves

Twelve MCM-41 samples with a SiAl ratio varying from 12 to 200 were prepared by using sodium aluminate in a tetramethylammonium hydroxide solution as aluminum source and an unexchanged CTMACl template solution. The thermogravimetric analysis (TGA) weight losses caused by thermal removal of the template cations, which are associated with the siloxy groups on the one hand and the aluminum sites on the other hand, correspond with these different Al contents. X-Ray diffraction (XRD) revealed that at SiAl = 5, in addition to MCM-41, a lamellar phase and a dense crystalline tridymite structure are formed. 27Al Magic-angle spinning nuclear magnetic resonance (MAS NMR) indicates that upon calcination the tetrahedral aluminum sites are altered. Fourier transform infrared (FTIR) analysis of proton-exchanged MCM-41 with acetonitrile as probe molecule showed a very weak Bronsted and a rather strong Lewis acidity. The latter may be the result of the distortion of the aluminum sites by thermal treatment.

CoAPO molecular sieve acidity investigated by adsorption calorimetry and IR spectroscopy

The acidic properties of cobalt-and silicon-substituted AlPO-5, –11 and –44 have been characterized by adsorption calorimetry and IR measurements. Adsorption calorimetric measurements indicate that the adsorption potential of the samples for acetonitrile is enhanced upon cobalt incorporation. The calorimetrically measured heats of adsorption indicate the formation of strong acid sites, due to this cobalt incorporation, as well as the presence of weaker acid sites, probably terminal P—OH groups. IR measurements of adsorbed ammonia and acetonitrile indicate that some Bronsted-bonded ammonia and acetonitrile are present, in combination with Lewis-bonded species. The Bronsted-bonded species presumably originate from interaction of the base with the weakly acidic P—OH groups. The Lewis-bonded species are due to the coordinative interaction of the lone pair of the base with framework cobalt ions.

FTIR spectroscopic and 'H MAS NMR studies of the influence of lattice chemistry and structure on Bronsted acidity in zeolites

The influence of the Si/Al ratio, of the nature of the T-atom and of the pore size on the acidic strength of Brønsted sites in zeolites has been investigated using changes of the vibrational properties of Brønsted OH(OD) groups and a shift change of Brønsted protons in nuclear magnetic resonance upon adsorption of weak bases. Deuterated acetonitrile and trichloro-acetonitrile have been chosen to probe the acidic strengths of ZSM-5, FeZSM-5, mordenite and zeolite Y, which are often used as catalysts. From the results of the FTIR and 1 H MAS NMR studies it can be concluded that the chemical composition of the lattice dominates the acidic strength of the Brønsted sites in zeolites. Differences in structure or pore size play a much smaller role.

Acid properties of ZSM-20-type zeolite

The acid properties of an HZSM-20 Zeolite have been characterized, in comparison to an Hy Zeolite, by IR spectroscopy, temperature-programmed desorption (TPD) of ammonia, time-resolved temperature-programmed FTIR of ammonia desorption and by microcalorimetry. Also, catalytic properties were tested for n-hexane conversion. These studies show that for HZSM-20 there is a shift of the bridging OH band to lower wavenumbers, an increase in the ammounia desorption temperature and a higher initial heat of ammonia chemisorption compared to HY. Thus, HZSM-20 contains medium–strong Bronsted sites, ranging in strength between zeilites HZSM-5 and HY. Obviously, the higher Si: Al ratio of HZSM-20 results in an enhanced number of strong acid sites with no second-neighbour aluminium atoms. This leads to a change in the distribution of Bronsted-acid sites. Superacid Bronsted sites could not be detected. Furthermore, the IR spectra of adsorbed pyridine and ammonia revealed the presence of at least two kinds of Lewis-acid sites. Some Lewis sites of HZSM-20 exhibit a weak acid strength and the strong lewis sites are weaker than those of HY. The different acidic properties of both zeolites are reflected in their catalytic activity and selectivity in n-hexane conversion.

Adsorption of linear alkanes in the alpha-Cages and gamma-Cages of H-ZK-5 and K-ZK-5

The adsorption properties of linear alkanes in H-ZK-5 and K-ZK-5 were studied at room temperature by means of sorption and 13C NMR measurements and by molecular simulations. Alkane molecules up to n-heptane are able to adsorb in the α- and γ-cages of ZK-5, but adsorption of longer alkane molecules is limited to the large α-cages as a result of size exclusion from the small γ-cages. These γ-cages are the preferred adsorption sites for propane and n-butane because of the more favorable heats of adsorption. In contrast, n-hexane and n-heptane adsorb preferentially in the large α-cages because of the larger entropy in these cages. For n-pentane, the γ-cages are the preferred adsorption sites in H-ZK-5, whereas in K-ZK-5 the α-cages are the preferred adsorption sites. This difference is caused by the presence of the potassium cations in the γ-cages of K-ZK-5 which constrains the adsorption in these cages. The difference in siting of molecules with different chain lengths is the result of a steep decrease of the entropy of the adsorbed molecules in the γ-cages with increasing chain length. The 13C NMR measurements and molecular simulations show that the molecules in the γ-cages are in general more coiled than those in the α-cages. Although the conformational equilibria of the molecules in the α-cages of H-ZK-5 and K-ZK-5 are comparable, the molecules in the γ-cages of K-ZK-5 are considerably more coiled than those in the γ-cages of H-ZK-5. The simulation parameters describing the zeolite-alkane interaction were tested by a comparison of the simulation results with the experimental findings.

Relationship between acid strength and framework aluminium content in dealuminated faujasites

Calorimetric measurements of the chemisorption heats of ammonia were used to investigate the acid strength of dealuminated Y-zeolites. Our measurements show that the number of very strong acid sites rises, passes through a pronounced maximum and then decreases with increasing aluminium content in the framework. The results are in good agreement with literature data and prove the validity of the concept of the aluminium topological density of zeolites given by Barthomeuf.

Adsorption studies on ordered mesoporous materials (MCM-41)

Ordered mesoporous materials, MCM-41, with varying Si/Al ratios have been synthesised and characterised by adsorption, XRD, NMR, and FTIR measurements. 27Al MAS NMR shows four-fold co-ordination of Al incorporated into the structure. The combined study by microcalorimetry and FTIR, using acetonitrile as probe shows a few strong Lewis sites and only weak Bronsted acidity. The physisorption properties were analysed by N2, n-hexane, benzene, acetonitrile, methanol and water adsorption using microcalorimetry and isotherm measurements. MCM-41 shows adsorption behaviour of a well-defined mesoporous molecular sieve. The heat of adsorption of a non-polar molecule is mainly determined by the pore diameter of the porous material. A characteristic feature of all applied adsorptives is capillary condensation. Whereas isotherms of water are completely irreversible and acetonitrile shows a small hysteresis, isotherms of the non-polar benzene, n-hexane, and nitrogen are completely reversible.

The IR transmission windows of hydrogen bonded complexes in zeolites: A new interpretation of IR data of acetonitrile and water adsorption on zeolitic Broensted sites

Abstract The ∼2800, ∼2400 and ∼1700 cm−1 infrared OH bands recently found upon adsorption of many basic molecules on Bronsted sites of zeolites, including acetonitrile and water, are suggested to be analogous to the (A,B,C) triplet of medium-strong molecular X-OH‥Y complexes in vapours, liquids and solids. IR studies of CD3CN and CCI3CN adsorptions on H-ZSM5, H-FeSil, H/D-ZSM5 and H/D-FeSil zeolites show these bands to be in a good agreement with the resonance theory of the (A,B,C) triplet, developed for molecular H-complexes. A new interpretation of IR data of water adsorption on zeolites, using this theory and results of ab initio calculations, suggests the water complex with the bridging OH group of zeolites to be “neutral”, the one water OH group interacting by hydrogen bonds both with the bridging OH group and with the nearest AlOSi oxygen and the other OH group being free.

Specific modification of the external surface of ZSM-5 zeolites by 12-tungstosilicic acid

A medium-pore zeolite ZSM-5 has been modified by impregnation with an aqueous solution of 12-tungstosilicic acid. Adsorption measurements, IR spectroscopy and catalytic probe reactions (ethylbenzene conversion and m-xylene isomerization) show that the Keggin unit [SiW12O40]4– cannot penetrate the ZSM-5 pore system but merely becomes deposited on the external zeolite surface. Thermal decomposition of the acid yields WO3 at high loading accompanied by a loss of acid sites in the channel system. This is not exclusively caused by further dealumination but has partially to be attributed to site blocking by WO3. Obviously, the decomposition of the acid involves mobile species of smaller size which are able to migrate into the interior of the zeolite.

Acidic properties of metal substituted aluminophosphates studied by adsorption calorimetry and IR spectroscopy

The heats of adsorption of acetonitrile on metal substituted AlPO 4 -5 and AlPO 4 -11 compared to the pure AlPOs indicate the formation of strong acidic sites. Besides these strong sites some weaker acidic centres, probably P-OH groups have been found. Accordingly, IR measurements on the loaded MeAPOs show some weakly Bransted bonded acetonitrile in combination with a strong Lewis complex. The Lewis bonded acetonitrile in the MeAPOs originate from coordinative interaction of the lone pair with the bivalent framework metal ions such as Mg 2+ , Mn 2+ , Co 2+ or Zn 2+ .