Hendrik Kosslick

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.

Suzuki reaction over palladium-complex loaded MCM-41 catalysts

Palladium-complex loaded Al-MCM-41 catalysts have been used to heterogenize the homogeneous catalyzed Suzuki reaction. The Suzuki reaction is a convenient method for the C–C coupling of arylhalogenides with arylboronacids yielding biphenyles. The mesoporous catalyst supports were prepared by anchoring of alkylsilylsulfonic acid to as-prepared calcined Al-MCM-41. The PdCl2[Ph2P(CH2)2S(CH2)3SO3Na]2 complex was used as catalytically active precursor and directly loaded onto the support from solution. Materials were characterized by 29Si MAS NMR, 13C CP/MAS NMR, nitrogen adsorption, TG/DTA, CHNS analysis, XPS, and TEM. The Si/Al ratio of the Al-MCM-41 was 10.7. The pore size was ≈30 A. The catalytic tests were performed in a batch reactor at 78°C in a toluene/ethanol/water mixture with sodium carbonate as base and cetyltrimethylammoniumbromide as surfactant. Conversions and selectivities were determined by gas chromatography. For comparison also large pore SBA type mesoporous molecular sieves were included. The results indicate that Pd-complex/MCM-41 catalysts can be successfully used in the Suzuki C–C-coupling reaction. Catalysts can be recycled and re-used. The recycled catalysts show an enhanced activity.

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

The acid properties of Al-, Ga- and Fe-substituted mesoporous nsilicates have been investigated using ammonia as a probe molecule. nSubstitution leads to the formation of Bronsted and Lewis acid nsites of different type and strength. The IR spectra of ammonia-loaded nsamples show bending vibration bands near 1450 cm n -1 n n(ammonium ions at Bronsted sites) and at ca. 1620 and n1300 cm n -1 n (ammonia located at weak and strong Lewis sites, nrespectively). The vibration bands of bridging Bronsted acid nhydroxy groups appear at 3605 (Al) and 3591 cm n -1 n (Ga). nInitial heats of ammonia chemisorption decrease in the order: 185 n(Al), 162 (Ga) and 144 (Fe) kJ mol n -1 n. The degree of nisomorphous substitution of framework silicon by trivalent atoms ndecreases in the order AlGa>Fe. The ratio of strong to nmedium-strong Bronsted sites falls in the same order. The nstrength of medium-to-strong Bronsted sites of Al-substituted nmesoporous silicate is similar to the strength of corresponding sites nin HY. Desorption of ammonia from Lewis sites indicates the existence nof a broad acid-site distribution ranging in strength from weak to nstrong. In contrast, Fe-substituted material contains only Lewis sites nof weak acid strength. The results indicate that isomorphous nsubstitution is a useful tool to tailor the acidic properties of nmesoporous molecular sieves.

Room-temperature synthesis of zinc oxide nanoparticles in different media and their application in cyanide photodegradation

Cyanide is an extreme hazard and extensively found in the wastes of refinery, coke plant, and metal plating industries. A simple, fast, cost-effective, room-temperature wet chemical route, based on cyclohexylamine, for synthesizing zinc oxide nanoparticles in aqueous and enthanolic media was established and tested for the photodegradation of cyanide ions. Particles of polyhedra morphology were obtained for zinc oxide, prepared in ethanol (ZnOE), while spherical and some chunky particles were observed for zinc oxide, prepared in water (ZnOW). The morphology was crucial in enhancing the cyanide ion photocatalytic degradation efficiency of ZnOE by a factor of 1.5 in comparison to the efficiency of ZnOW at an equivalent concentration of 0.02 wt.% ZnO. Increasing the concentration wt.% of ZnOE from 0.01 to 0.09 led to an increase in the photocatalytic degradation efficiency from 85% to almost 100% after 180 min and a doubling of the first-order rate constant (k).

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.

Acidity of substituted mesoporous molecular sieve MCM-48

The nature, strength and relative concentration of acid sites of siliceous and Al-, Ga- and Fe-substituted MCM-48 have been studied by FTIR spectroscopy in the region of the νOH vibrations and δNH vibrations of adsorbed ammonia. The interior surface is strongly hydroxylated. Different types of hydroxy groups could be distinguished, vibrating at 3740 (terminal silanols), 3710 (geminal silanols) and 3500 cm−1 (associated silanols). The surface is strongly hydrophilic but silanols are not acidic. Substitution of silicon by Al, Ga and Fe yields Bronsted acid sites giving rise to an absorbance at 1450 cm−1 in the spectra of adsorbed ammonia. The thermal desorption shows that the acidic strength of Bronsted sites decreases in the order Al > Ga > Fe as does the thermal stability. The acidity of sites is weak to medium strong and Al-MCM-48, especially, contains some strong Bronsted acid sites. Calcination causes the formation of Lewis acidity at the expense of tetrahedrally coordinated MeIII. Two types of Lewis sites absorbing at 1610 (L1) and ca. 1300 cm−1 (L2) are found on Al- and Ga-MCM-48, whereas Fe-MCM-48 contains L1 sites only. Lewis acidity decreases in the order Ga ≈ AlFe. Al and Ga Lewis sites are very strong and exceed the acidity of Bronsted sites considerably. The catalytic properties in cumene cracking confirm the results of acidity investigations and show that strong Bronsted acid sites of Al-MCM-48 have an important impact on the activity.

Comparison of the acidic properties of ZSM-5 zeolites isomorphously substituted by Ga, In, B and Fe

Abstract The acidic properties of isomorphously substituted MFI-type zeolites prepared and pretreated in the same way were studied to obtain an exact gradation of the intrinsic acidity caused by the incorporation of Ga, Fe, In and B instead of Al into the framework. The nature, strength and concentration of the acidic sites were studied by temperature-programmed desorption of ammonia (TPDA) and Fourier transform infrared spectroscopy. The assignment of the two peaks observed in the TPDA plots of all zeolite samples required the additional registration of the decomposition plot of the ammonium form in the cases of In-ZSM-5 and B-ZSM-5. A temperature program with two ramps and an intermediate isothermal hold was used to separate the low-temperature peak from the high-temperature peak. In this way, the Bronsted sites could be determined more clearly on samples containing a high portion of non-framework species that inhibit the egress of ammonia from the pores. The nature and the strength of the acidic sites were spectroscopically characterized by recording the stretching vibration of the bridged OH groups of Bronsted sites and the spectra of pyridine adsorbed on Bronsted and Lewis sites. The intrinsic acidity of the Bronsted sites on In-ZSM-5 could be clearly classified to be intermediate between the acid strength of B- and Fe-ZSM-5. A relation could be revealed between the frequencies of the OH-stretching vibration and the temperature of the peak maxima of ammonia desorption from the Bronsted sites of the isomorphously substituted ZSM-5 samples.

The gallophosphate molecular sieve cloverite as a host for liquid crystals

The gallophosphate cloverite has been investigated because of its applicability to act as a host for larger molecules. 4-n-pentyl-4′-cyanobiphenyl (5CB) and 4-n-octyl-4′-cyanobiphenyl (8CB) liquid crystals were used as guest molecules. The results show that the adsorption of liquid crystals from the gas phase proceeds without destruction of the cloverite structure. The typical phase properties of the liquid crystals remain unchanged after adsorption on cloverite: however, they appear with lower intensity in the DSC curve. A new smectic phase seems to be formed inside the cloverite pore system. FTIR measurements show the interaction of 5CB with the Lewis and Bronsted acid sites of cloverite. Desorption of liquid crystals out of the pore system proceeds slowly under liberation of the POH groups of the lattice. In summary, the results show that the gallophosphate cloverite is a suitable host for large molecules.

Comparison of mesoporous silica and alumina supports for palladium-catalyzed carbon–carbon coupling reactions: unexpected high acceleration by supported cetyltrimethylammonium bromide

The coupling reaction of p-iodoanisole and phenylboronic acid occurs with high yields in aqueous medium in the presence of palladium complexes with water-soluble phosphine ligands on different mesoporous inorganic supports. Silica and alumina supports were similarly prepared and functionalized with sulfonate groups. The catalyst was immobilized by ion exchange. A novel approach to the amphiphilization of the supports by cetyltrimethylammonium bromide was applied to modify phase transfer steps favorably and to embed the catalyst in this bipolar surface layer. The reaction is accelerated and conversions with these embedded catalysts exceed those in homogeneous systems.