Ellen Schreier

Acidity of aluminophosphate structures. Part 2.—Incorporation of cobalt into CHA and AFI by microwave synthesis

Samples of CoAPO/CoAPSO-44 and CoAPO-5 have been prepared using cyclohexylamine as templating agent and a microwave power system for the crystallization. Cobalt substitutes for aluminium. The Co/Al ratios are in the range of 0.05 to 0.3 for the structure 44 and Co/Al = 0.06 for CoAPO-5. The crystallinity of the samples was studied by X-ray diffraction (XRD), IR spectroscopy, optical microscopy, thermal analysis and adsorption measurements and compared with the results for SAPO-44 and SAPO-5. The thermal stability of the samples decreases strongly as a function of the cobalt content.The Co2+ incorporation-generated acid sites were investigated by diffuse reflectance IR Fourier transform (DRIFT) spectroscopy in the OH-stretching vibration region and by the adsorption of probe molecules (NH3 and CO), calorimetric measurements of the differential molar heat of NH3 sorption and temperature-programmed desorption (TPD) of ammonia. From the results of calorimetry and TPD of ammonia it follows that each cobalt atom inserted into a tetrahedrally coordinated framework position creates one acid site. Concerning the nature of acid sites, an equilibrium between bridging hydroxy groups, paired centres of Lewis sites and –POH groups is proposed.The formation of Co3+ in framework positions could not be confirmed. Thermal analysis combined with mass spectrometry indicate the addition of molecular oxygen in calcined CoAPO-44.

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.

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.

Synthesis of zeolite beta. Part 1.—Using tetraethylammonium hydroxide/bromide with addition of chelates as templating agents

The synthesis of zeolite beta has been studied using the following templating agents: (i) tetraethylammonium hydroxide (TEA;)(ii) tetraethylammonium bromide–diethanolamine; and (iii) tetraethylammonium hydroxide–tetraethylammonium bromide–triethanolamine. TEA+ ions and TEAOH molecules only have a structure-directing effect. The ratio (Si + Al)/TEA+ in the gel was varied from 2 to 10, the Si/Al ratio from 12 to 1000. The temperature of crystallization was chosen in the range 368–443 K. The samples were characterized using X-ray diffraction, micrographs, chemical analysis, thermal analysis, IR spectroscopy and nitrogen adsorption. Zeolite beta of good crystallinity (micropore volume = 0.28 cm3 g–1) was obtained with Si/Al ratios from 14 to 72. The acid strength and number of acid sites were determined from the calorimetrically measured heat of NH3 sorption and from the TPD of NH3. The initial acid strengths of zeolite beta (up to 185 kJ mol–1) are higher than those of other high-silica zeolites.Polycrystalline agglomerates are formed instead of small single crystals on the addition of diethanolamine to the synthesis gel. The agglomerates are characterized by a high intercrystalline volume (> 0.47 cm3 g–1), which can be influenced in a definite manner by variation of the TEABr/diethanolamine ratio. The presence of triethanolamine initiates the transformation of zeolite beta into a SiO2 layer structure at 368 K.

Synthesis and characterization of VAPSO-44 and VAPSO-5

Aluminophosphate molecular sieves of structures 5 (AFI) and 44 (CHA) have been prepared with different vanadium contents. The crystallization was performed by means of a microwave power system. The crystallinity of the samples was checked by X-ray diffraction (XRD), thermal analysis and adsorption measurements, and compared with the data for the corresponding SAPO molecular sieves. The acidity was studied by calorimetric measurements of the differential molar heat of NH3 sorption and by temperature-programmed desorption (TPD) of ammonia.Electron paramagnetic resonance (EPR) spectra indicate the presence of isolated immobile VO2+ species in square-pyramidal symmetry. VO2 clusters were also found in samples with higher vanadium content. On calcination V4+ is partially oxidized to V2O5. Only a small amount of VO2+(< 1 atom%) may be stabilized on the inner surface of the pore system. The incorporation of V4+ into tetrahedrally coordinated framework positions can be excluded from the EPR results. It follows from IR measurements that VO2+ ions are fixed on the inner surface via a condensation process with hydroxy groups.

Acidity of aluminophosphate structures. Part 1.—Incorporation of sillicon into chabazite-like structure 44

SAPO-44 samples have been prepared with different silicon contents up to 34.3 mass% SiO2. Characterization of the samples by X-ray diffraction (XRD), thermal analysis, adsorption measurements, chemical analysis, microprobe analysis and IR spectroscopy reveals that the samples are highly crystalline, even those with the highest SiO2 content. The number of Bronsted acid sites determined by NH3 sorption [calorimetric and temperature-programmed desorption (TPD) measurements] is independent of the SiO2 content of the samples and amounts to about one acid centre per double-six-ring of the chabazite-like structure.Concerning the silicon ordering within the framework, it is proposed that aluminophosphate-based as well as aluminosilicate-based double-six-rings form the structure.

Improvement of catalytic properties of SAPO-31 molecular sieves by using an activated form of Si02

Abstract SAPO-31 molecular sieves the synthesis of which is based on different types of SiO2 were physicochemically characterized by X-ray diffraction, IR, temperature-programmed desorption of ammonia,29Si magic angle spinning nuclear magnetic resonance and catalytically investigated in the conversion of methanol to lower alkenes and in the isomerization of n-heptane. The use of ultradispersed SiO2 generated in a capacitively coupled high frequency plasma (CCP) allowed us to synthesize SAPO-31 molecular sieves of Bronsted acidities which are higher than with samples based on traditional silicon sources. Results of the conversion of methanol and of the isomerization of n-heptane show that the use of plasma-chemically generated silica in the synthesis of SAPO type molecular sieves can be a way to improve their catalytic properties.

On the trapping of SOx on CaO–Al2O3-based novel high capacity sorbents

Calcium-aluminum mixed oxide based materials doped with Na and Mn were explored as sulfur trapping materials. The materials showed a three times higher total storage capacity and a higher time on stream with complete SO2 removal compared to a second generation SOx trapping material which was mesoporous with calcium mainly present in oxidic form. Combining in situ XANES at the S K-edge and IR spectroscopy the key properties of the storage materials and the affiliated storage processes were identified. CaO-Al2O3 acts as the primary support and storage component, while Na+ cations adjust the base strength and enhances the storage capacity. Manganese cations provide the appropriate oxidation capacity in absence and presence of up to 10% water. The transport into the bulk phase, which is markedly influenced by a layer of sorbed water, is the rate-limiting step in presence of Mn cations. In the absence of manganese cations the oxidation step appears controlling the rate. The overall reaction network, identified by in situ IR spectroscopy and the 2D Correlation Analysis, is similar on all materials.

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.

Synthesis, characterization and catalytic activity of amorphous Al,SiOx gels from weakly acidic aqueous solutions

Al,SiOxgels have been prepared by reaction of different inorganic aluminium compounds with silicic acid. Characterization by chemical analysis,27Al MAS NMR spectroscopy and adsorption measurements shows that the gels obtained from a mixture (pH ca. 3) of basic aluminium chlorides and silicic acid in water are porous and contain tetrahedrally coordinated aluminium in the framework. IR spectroscopy and TPDA measurements confirm that Bronsted and Lewis acid sites are present in the gels. Finally, catalytic activity was found in hex-1-ene isomerization.