Duncan Akporiaye

Combinatorial Approach to the Hydrothermal Synthesis of Zeolites

One hundred or more solid-state syntheses can be conducted in parallel and employed for the combinatorial hydrothermal syntheses of zeolites by using a novel multiautoclave design. The operation of the multiautoclave was ascertained by the reinvestigation of the complete Na2 O-Al2 O3 -SiO2 ternary system in a single experiment. In the picture on the right, the shaded areas on the left show the crystallization fields of the different phases obtained.

MCM-41: a model system for adsorption studies on mesoporous materials

Purely siliceous MCM-41 materials with different pore sizes were synthesised. The obtained nitrogen isotherms of these materials varied from a “type I-like” isotherm (small pore size) over a reversible type IV isotherm (intermediate pore size) to a typical type IV isotherm, which shows pore condensation with hysteresis. Similar types of transition states and hysteresis curves are observed by 1H nuclear magnetic resonance (NMR) by monitoring the proton signal intensity of the mobile water molecules confined in the pores versus temperature. The consistency observed between the two methods makes 1H NMR a valuable additional technique for characterisation of mesoporous materials.

Synthesis of a mesoporous MCM-41 material with high levels of tetrahedral aluminium

A Crystalline mesoporous member of the MCM-41 family has been synthesised with a Si : Al ratio as low as 8.5 : 1, without observing octahedral aluminium.

Structural and chemical influences on the MTO reaction: a comparison of chabazite and SAPO-34 as MTO catalysts

A comparison of isostructural H-SAPO-34 and H-chabazite in high space-velocity experiments of the methanol to olefins (MTO) reaction has been performed. The initial selectivity was shown to be similar for both catalysts, and also to be independent of acid site density. However, the deactivation rate was critically dependent on the acid site density. Thus, low acid-site density samples exhibited longer lifetimes at low space velocity conditions than their high acid-site density analogues.

Modified zeolites as catalysts in the Friedel-Crafts acylation

Abstract Modified zeolites were found to be active catalysts in the Friedel-Crafts acylation of anisole by acetyl chloride and acetic anhydride. The effect of two different modifications of the zeolites were tested; lanthanum-exchange and varying the Si Al ratio. For the rare-earth modified zeolites, the activity was found to be dependent on the lanthanum content, and the yield increased with the level of lanthanum even up to 93% exchange. Dealuminated Y-zeolites were also found to be very active, and an almost linear increase in the yield with decreasing aluminium fraction was found. This has been attributed to the increased hydrophobicity of dealuminated zeolites. Zeolite Beta was also found to be very active in the reaction.

High-resolution electron microscopy and X-ray diffraction studies of MCM-48

Abstract A cubic member of the M41S family with a three-dimensional pore system, denoted as MCM-48, was synthesised. By combining X-ray powder diffraction (XRD) with high-resolution electron microscopy (HREM), a cubic symmetry with an Ia 3 d space group was determined for the as-synthesised and the calcined MCM-48 material. Upon calcination the unit cell decreased from around 10.09 nm to about 8.1 nm. The overall pore structure was found to be quite unaffected by local variations in the structure and the decrease in the unit cell size upon calcination.

Synthesis, characterization and catalytic testing of SAPO-18, MgAPO-18, and ZnAPO-18 in the MTO reaction

SAPO-18, MgAPO-18 and ZnAPO-18 have been successfully synthesised using TEAOH as organic additive to the synthesis gel. SAPO-18 has catalytic properties comparable to SAPO-34, whereas the MeAPO-18s had much higher selectivities to C3=–C4= and shorter catalytic lifetime than SAPO-18.

Solid-state n.m.r. and XRD study of the thermal stability of VPI-5: Assignment of 31P and 27AI MAS n.m.r. spectra

Abstract A detailed study of VPI-5 has been carried out using XRD and in situ variable temperature (VT) 27AI and 31P MAS n.m.r. On the basis of a recent structural study of VPI-5 and the results from this study, two alternative assignments of the MAS n.m.r. spectra are possible. The initial assignment relies on the intensity ratio of the two signals in the dehydrated 31P spectra; however, it is shown not to be wholly consistent with the general trends expected for the phosphorous chemical shift with respect to the known structure and octahedral aluminum environment. On the basis of these incosistancies, the parallel study of the AIPO 4 -21 AIPO 4 -25 and previous MAS n.m.r. studies of AIPO4s, a second alternative assignment is proposed.

Combinatorial chemistry – The emperor's new clothes?

Considering the increasing pressures and constraints on research and development towards focussed innovation, the prospects of new fast and cost-effective approaches addressing the many challenges faced in more rapidly developing next generation products are expected to be very significant. The application of the new methodologies of combinatorial chemistry to materials discovery and development, though facing special challenges, has now been demonstrated in the hydrothermal synthesis of a series of aluminophosphate materials. Using this approach an extensive study of complex multi-component systems was easily achieved, demonstrating the highly reproducible experimental conditions that allows efficient screening of phase composition based on X-ray diffraction analysis.

Self-diffusion coefficient of water confined in mesoporous MCM-41 materials determined by 1H nuclear magnetic resonance spin-echo measurements

Abstract 1H Nuclear magnetic resonance (NMR) spin-echo measurements at 268 K have been performed on water-saturated mesoporous MCM-41 materials. The pores have cylindrical geometry with diameters in the range 18–40 A and length of approximately 1 mm. Using appropriate model calculations the self-diffusion coefficient (D) of water within the pore was estimated from the Carr-Purcell-Meiboom-Gill (CPMG) spin-echo envelope curves. D was found to be in the range 0.17·10−6 to 2.3·10−6 cm2/s and could be expressed by an equation of the form D−1∝D∞−1+α·lc−2 where D∞ is the self-diffusion coefficient of bulk water (lc = ∞) and α = (2.68 ± 0.14)·10−5s. The low values for the diffusion coefficients—compared to the diffusion coefficient of bulk water — are related to the strong interaction of the water molecules with the pore surface. Moreover, the spin-spin relaxation rate ( 1 T 2 ) was related to lc by application of the Bloembergen-Purcell-Pound (BPP) model.