Rolf Fricke

Acidity and active sites of Al-MCM-41

Abstract The influence of the aluminum contents of MCM-41 (Si/Al ratio varied between 2.7 and 69) on the coordination of Al, on the acidity, and on the catalytic properties is studied by 27 Al MAS NMR, temperature programmed desorption of ammonia, and the conversion of acetone. Based on TPDA results, an assignment of the peaks of desorption of ammonia is proposed. With low Al contents, the concentration of strong Bronsted acid sites, which are attributed to tetrahedral aluminum, increases with growing Al amounts. At higher Al contents, however, the number of strong acid sites decreases again. Besides usual strong Bronsted sites, TPDA reveals the existence of weak Bronsted and Lewis sites and of Lewis sites of a high acidic strength. Separation between the two types of sites of weak acidity is incomplete. In the temperature programmed decomposition of NH 4 -exchanged MCM-41 samples, strong Lewis acid sites temporarily re-adsorb ammonia generated by the thermal decomposition of the NH 4 + ions located at the Bronsted sites. Ammonia re-adsorption at Lewis sites results in a shift of the maximum of the TPDA peak to a higher temperature. Calcination of samples leads to the formation of strong Lewis sites at the expense of Bronsted sites. To evaluate the acidity of Al-MCM-41, recording of decomposition profiles has been extended to Ga- and Fe-MCM-41. Catalytic activity in the conversion of acetone reaches its maximum with the sample based on Al-MCM-41 with the molar Si/Al ratio of 6.85. Highly favored formation of isobutene points to a comparably high acidic strength of the active sites.

Physico-chemical and catalytic properties of Al-, Ga- and Fe-substituted mesoporous materials related to MCM-41

Al-, Ga-, and Fe-substituted mesoporous materials related to MCM-41 were hydrothermally synthesized. The samples were characterized by XRD, SEM, MIR, TA, 29Si and 27Al MAS NMR, ESR, and IR spectroscopy of adsorbed pyridine. The results reveal the substitution of Si in tetrahedral positions in the pore walls by Al, Ga, and Fe. In as-synthesized Al-MCM-41, aluminum is tetrahedrally coordinated only. Calcination causes the formation of octahedrally coordinated aluminum at the expense of framework aluminum. 27Al satellite transition spectra could be recorded from which the isotropic chemical shift δiso and the structure-sensitive mean quadrupole coupling constant cq were determined. These results show that the tetrahedrally coordinated aluminum atoms in the pore walls of mesoporous molecular sieves and in the framework positions of zeolites are closely related. From the δiso values, also the mean intertetrahedral T-O-T angles ϑ at the framework Al were estimated. For the as-synthesized sample, ϑ amounts to 159° and for the H-form to 142°, i.e. Al is highly ‘stressed’ in the as-synthesized MCM-41. This stress is considerably reduced after calcination which is accompanied by strong dealumination. The acidity was studied by temperature-programmed FTIR spectroscopy of chemisorbed pyridine. Both Bronsted and Lewis sites of different strength are found in all substituted samples. Bronsted acidity conclusively confirms the isomorphous substitution of Si by Al, Ga, and Fe in the MCM-41 framework. The samples show a remarkable catalytic activity in the acetone conversion which decreases in the order Al>Ga>Fe. This is in line with the decrease of the strength and of the number of acid sites in the above order. Changes of the acidity also influence the selectivity behaviour. Thus, formation of isobutene is favoured by increased acidity.

ACIDITY OF SUBSTITUTED MCM-41-TYPE MESOPOROUS SILICATES PROBED BY AMMONIA

The acid properties of Al-, Ga- and Fe-substituted mesoporous silicates have been investigated using ammonia as a probe molecule. Substitution leads to the formation of Bronsted and Lewis acid sites of different type and strength. The IR spectra of ammonia-loaded samples show bending vibration bands near 1450 cm -1 (ammonium ions at Bronsted sites) and at ca. 1620 and 1300 cm -1 (ammonia located at weak and strong Lewis sites, respectively). The vibration bands of bridging Bronsted acid hydroxy groups appear at 3605 (Al) and 3591 cm -1 (Ga). Initial heats of ammonia chemisorption decrease in the order: 185 (Al), 162 (Ga) and 144 (Fe) kJ mol -1 . The degree of isomorphous substitution of framework silicon by trivalent atoms decreases in the order AlGa>Fe. The ratio of strong to medium-strong Bronsted sites falls in the same order. The strength of medium-to-strong Bronsted sites of Al-substituted mesoporous silicate is similar to the strength of corresponding sites in HY. Desorption of ammonia from Lewis sites indicates the existence of a broad acid-site distribution ranging in strength from weak to strong. In contrast, Fe-substituted material contains only Lewis sites of weak acid strength. The results indicate that isomorphous substitution is a useful tool to tailor the acidic properties of mesoporous molecular sieves.

Low-temperature conversion of NOx to N2 by zeolite-fixed ammonium ions

Abstract The selective catalytic reduction (SCR) of NOx in oxygen excess is reported to proceed with good efficiency by NH4+ ions fixed to zeolites at temperatures as low as 373 K. The reaction starts with almost complete reduction of NOx to N2 under consumption of NH4+ ions thereby converting the zeolite into its H+ form. The consumed NH4+ ions can easily be replenished at reaction temperature by admission of gaseous ammonia. A key step of the reaction sequence is the oxidation of NO to NO2 by O2 which is catalyzed by the zeolite. Both properties of the zeolite, namely, its storage capacity for NH3 (in the form of NH4+ ions) and its oxidation capability for NO allow the abatement of NOx in exhaust gases at low temperatures and in the presence of oxygen without permanent addition of a reductant. This process is named catalytic low temperature conversion (CLTC) of NOx.

Acidity and catalytic properties of MeAPO-5 molecular sieves

Abstract Molecular sieves of AlPO4-5 type containing ions of various metals isomorphously incorporated into the framework have been characterized by temperature programmed desorption of ammonia, infrared spectroscopy, and catalytic effectiveness in the conversion of methanol. The incorporation of metal ions (Mn, Co, Ni, Mg, Zn, Zr, and silicon) gives rise to the formation of bridged hydroxyl groups (Bronsted sites) of different acidic strength. Simultaneously, Lewis sites of low as well as of high acidity are generated. Catalysis of C C bonding reactions is connected to the presence of acidic sites of higher strength. Best catalytic results with respect to the formation of lower olefins are achieved with NiAPO-5 varieties, which contain a sufficiently high density of Bronsted sites of moderate strength and comparatively small portions of strong acid Lewis sites.

The effect of AI, Fe, and in substitution in the MFI silicate structure on the aromatic hydrocarbon transformation: SiOHM site strength

The conversion of toluene in toluene disproportionation and its alkylation with ethylene over metallosilicates (AI, Fe, In) of the MFI structure has been found to be proportional to the concentration of the individual framework bridging SiOHM groups, and when related to one SiOHM site, it increases in the sequence of the isomorphous substitution of Si for In < Fe <AI. This clearly reflects the increasing acid strength of the SiOHM group from In, Fe, to AI. As the alkylation of toluene with isopropanol is controlled by the desorption/transport rate of the bulky propyltoluenes from the molecular sieve channels, no correlation between the molecular sieve acidity and conversion- selectivity for this reaction has been found. All metallosilicates (AI, Fe, In) exhibit para-shape selectivity; however, no definite conclusion on the effect of the acidity on the para-shape selectivity can be drawn. Although the AI-silicates contain SiOHM sites with the highest acid strength, the deactivation of Fe and In-silicates by coking is greater compared to AI analogs, likely due to the higher relative amount of extraframework metal species.

Synthesis and properties of the silicoaluminophosphate molecular sieve SAPO-31

The silicoaluminophosphate molecular sieve SAPO-31 has been synthesized and characterized by XRD, 27Al, 29Si, and 31P MASNMR, IR spectroscopy, electron microscopy, DTA-TGA, chemical analysis, and the catalytic conversion of methanol to light olefins. The results from XRD and NMR investigations show that hydration causes structural changes of the framework.

On the confinement of liquid crystals in molecular sieves: dielectric measurements

Abstract The molecular dynamics of 4-octyl-4′-cyanobiphenyl and 4-pentyl-4′-cyanobiphenyl, confined to the mesopores with diameters less than 25 A of the molecular sieve AlMCM-41, was investigated over a wide temperature range using broadband dielectric spectroscopy. As expected, the phase transitions of the bulk liquid crystals cannot be detected for the confined molecules. A new, slow relaxational process occurs over the whole temperature range due to the motions of the molecules confined in the pores; the temperature dependence of the characteristic frequency obeys a Vogel–Fulcher–Tamman (VFT) law associated with a glassy state. The orientational order within these small pores is mostly dominated by the local environment.

Microwave synthesis of MnAPOMnAPSO-44 and MnAPO-5-stability of Mn2+ ions on framework positions

Abstract Aluminophosphate molecular sieves of structure 44 and structure 5 with incorporation of manganese (up to 11 mass% MnO) were prepared by microwave synthesis. The MnAPO samples were characterized in comparison to the corresponding SAPO samples by X-ray diffraction, light microscopy, infrared spectroscopy, thermal analysis, ESR spectra, nitrogen adsorption and calorimetric measurements of the heat of NH3 adsorption. Manganese replaces aluminium on tetrahedrally coordinated framework positions. The ESR measurements reveal a cluster-like arrangement of Mn2+ ions (Mn2+OPOMn2+). The as-synthesized samples are colourless. After calcination in air under static conditions a blue/deep blue colour is observed which is associated with the full adsorption capacity. The adsorption of polar molecules, as water and ammonia, causes marked structure destruction, and the colour changes from blue to pink or to grey shades. POH groups and Lewis acid sites are detected by infrared spectroscopy.

Investigation of hydroxyl groups in crystalline silicoaluminophosphate SAPO-34 by diffuse reflectance infrared spectroscopy

Hydroxyl groups in crystalline silicoaluminophosphate SAPO-34 have been investigated by diffuse reflectance IR spectroscopy. By carefully comparing the intensities of stretching vibrations with the appropriate combination and overtone bands the two types of bridged Si—OH—Al group were assigned to the fundamental stretching vibrations at 3625 and 3600 cm–1. Both types of group exhibited approximately the same acidic properties but differed in their localization. The bridged groups with ν= 3600 cm–1 were assumed to be localized in the hexagonal prisms, forming an H-bond with adjacent oxygen anions of the framework. In-plane and out-of-plane bending vibrations were observed for the first time in SAPOs.