Andreas Philippou

Structural investigation of ETS-4

The experimental findings of this work suggest that ETS-4 is structure related to zorite but is not the synthetic counterpart of this mineral, as proposed previously. 29 Si MAS n.m.r. results are not entirely in agreement with the zorite structure. These spectroscopic results can be explained by the presence of a second intergrown phase that would not easily be observed by techniques such as scanning electron microscopy. X-ray diffraction patterns of zorite and ETS-4 are, however, very similar. Two approaches have been taken to speculate on the structure of this material. First, ETS-4 is viewed as an intergrowth system of zorite with nenadkevichite. The second approach derives mainly from the work on ETS-10 where a number of defects were observed in the HREM images of this material. Combining both approaches seems to be a potential way of explaining the structure of ETS-4.

The strong basicity of the microporous titanosilicate ETS-10

Base-catalysed reactions in general are of great importance. In zeolite chemistry, alkali-metal-exchanged zeolites, such as zeolite X, display very strong basicity coupled with high catalytic activity. However, in order to achieve this strong basicity, considerable post-synthesis modification is necessary. In this report, it is shown that strong basicity is exhibited by the microporous titanosilicate ETS-10 in the as-prepared form which is superior to the presently known suite of zeolite catalysts. This opens up a new class of material for heterogeneous base catalysis.

Textural, structural and acid properties of a catalytically active mesoporous aluminosilicate MCM-41

As-prepared, calcined and catalytically tested MCM-41 materials have been comprehensively characterised by N2 adsorption/desorption at 77 K, 1H, 27Al and 29Si MAS NMR as well as by temperature-programmed ammonia desorption (TPAD) with in situ FTIR. 29Si MAS NMR spectra of MCM-41 closely resemble those of amorphous silica, suggesting a broad range of Si—O—Si bond angles in this material. The obtained results from 1H MAS NMR and a combination of FTIR and TPAD investigations indicate that the washed, template-free MCM-41 and its protonic form behave like a solid-state acid with a broad acid strength distribution. With regard to the presence of bridging hydroxy groups in the investigated MCM-41 materials, the findings of these two methods are not consistent. Therefore, it is assumed that the broad absorption between 3650 and 3400 cm–1 in the FTIR spectra is connected with hydrogen-bonded vicinal silanol pairs. A signal in the 27Al MAS NMR spectra at 40 ppm is identified as a separate resonance, assigned preferentially to an isolated extranetwork tetrahedral aluminium species, whereby the assignment to a pentahedrally coordinated network aluminium species is also considered. When calcined, these species are reinserted into network positions. Subsequent catalytic testing leads to partial recovery of the former resonance. The results of the catalytic testing imply that the weak Bronsted acid sites interact with neighbouring coordinatively unsaturated aluminium species and that synergistically stronger acid sites form.

Dehydration of t‐butanol over basic ETS‐10, ETAS‐10 and AM‐6 catalysts

The t‐butanol dehydration over basic ETS‐10, ETAS‐10 and AM‐6 has been investigated. All these as‐synthesised materials catalyse this reaction in a very similar fashion yielding remarkably high selectivities for isobutene, which are independent of both reaction conditions and conversion levels. The catalyst decay was more profound at lower reaction temperatures (ca. 200°C), whereas no decay was observed at higher temperatures (ca. 300°C). The basic sites of these materials are purported to be responsible for the excellent catalytic performance.

Isomorphous substitution in the microporous titanosilicate ETS-10

Abstract Silicon has been isomorphously substituted by aluminium and gallium in the microporous titanosilicate ETS-10 to produce ETAS-10 and ETGS-10 respectively. For aluminium substitution a series of samples have been prepared with Al/Ti ratios ranging from 0.10 to 0.48. Both aluminium and gallium substitute exclusively in tetrahedral silicon sites in a manner which avoids TiOAl and TiOGa linkages. Based upon detailed analysis of solid-state NMR spectra, models for the ordering of aluminium in ETAS-10 are discussed.

A comparison of direct synthesis and vapour phase alumination of MCM-41

Mesoporous AlMCM-41 materials were prepared by both direct synthesis and the reaction of siliceous MCM-41 with aluminium chloride vapour. The products were characterised using XRD, N2 sorption, EDAX and both MAS NMR and FTIR spectroscopy, and acid catalytic activity was evaluated using the conversion of 1,3,5-triisopropylbenzene (1,3,5-TIPB) as a test reaction. Acid site concentrations (Bronsted and Lewis) were estimated from sorption of pyridine, using FTIR. The concentrations of acid sites were correlated with the fraction of aluminium in the solids and comparisons were made with results published previously for similar mesoporous materials. Both the compositions of the gels used in the synthesis of Al-mesoporous materials and the procedures used to introduce aluminium into mesoporous silica strongly influenced the above correlation. In all cases, the ratio of total acid site concentration to aluminium concentration was less than unity. Consideration of the present catalytic results for conversion of 1,3,5-TIPB along with results published previously for conversion of cumene, demonstrates that acid catalytic activity is better correlated with Bronsted rather than with Lewis site concentration.

Characterisation studies on the new microporous aluminium-containing ETS-10 molecular sieve used for processing larger molecules

Abstract This study reports on the physico-chemical characterisation of an aluminium-containing microporous titanosilicate ETS-10 material in its protonic form in connection with catalytic testing. 27Al MAS NMR spectra reveal the successful incorporation of aluminium into the lattice structure. XRD patterns indicate that the investigated ETAS-10 material in its protonic form is relatively stable thermally, an indispensable requirement for the application as an industrial catalyst material. Nevertheless, for all samples, except parent ETAS-10, XRD and 29Si MAS NMR measurements indicate that an increasing amount of amorphous silica is present. Acidity investigations with combined FT-IR spectroscopy and temperature-programmed ammonia desorption and with 1H MAS NMR revealed that dehydroxylation of bridging hydroxyl groups, which are assumed to be of the type Ti-(OH)-Si, occur between 350 and 550°C. An increase in both the number and strength of catalytically active bridging hydroxyl groups seems to be limited by a conventional protonic ion-exchange. The acid properties corroborate excellently with the catalytic activity and selectivity in n-hexadecane and 1,3,5-triisopropylbenzene cracking. The results show that electron acceptor sites, which may originate from titaniumoxide impurities or defect sites especially in titanium octahedra, could be involved in the catalytic reactions forming radical precursors.

Dehydration of Alcohols by Microporous Niobium Silicate AM-11

The dehydration of ethanol, 1-propanol and 1-butanol has been studied at atmospheric pressure using the novel microporous niobium silicate AM-11 (Aveiro–Manchester structure number 11) in the temperature range 150–300 °C. The selectivity for alkenes increases with temperature for all three alcohols and reaches 100% for 100% conversion at 250–300 °C. At lower reaction temperatures (∼200 °C), ethers are also formed and their selectivity decreases markedly with the alcohol size.

Synthesis and characterisation of chromium-substituted ETS-10

A series of microporous chromium-containing ETS-10 (ETCrS-10) with different Cr/Ti molar ratios have been synthesised and characterised by energy dispersive absorption of X-rays (EDAX), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption measurements, diffuse-reflectance ultraviolet spectroscopy (DR-UV), electron paramagnetic resonance (EPR) and 29Si magic-angle-spinning nuclear magnetic resonance (MAS NMR). The diffractograms of the as-synthesised ETCrS-10 materials indicate that the ETS-10 structure has remained intact upon Cr substitution. DR-UV and EPR reveal that the as-prepared ETCrS-10 samples contain Cr3+ in octahedral coordination. The resonances in the 29Si MAS NMR spectra of ETCrS-10 are considerably broader than those of ETS-10 and such a broadening may relate to isomorphous substitution of Cr for Ti in the ETS-10 framework. The acid–base properties of ETCrS-10 materials are responsible for the high dehydrogenation activity in the conversion of isopropanol to acetone and dehydration activity in the conversion of tert-butanol to isobutene. Ethanol oxidation over ETCrS-10 yields acetaldehyde as a main product.

Investigation of surface methoxy groups on SAPO-34 A combined magic-angle turning NMR experimental approach with theoretical studies

The formation of stable surface methoxy groups in various zeolite-catalysed reaction processes has been widely accepted as an important intermediate stage and the understanding of such an intermediacy is of great significance. 13C magic-angle-turning (MAT) NMR experiments, in conjunction with theoretical calculations of 13C shielding tensors, are employed to investigate the chemical and electronic nature of methoxy groups on H-SAPO-34.