Vladimir L. Zholobenko

Structure of Micelles of a Nonionic Block Copolymer Determined by SANS and SAXS

The micellar state of Pluronic P123, which is a poly(ethylene oxide)-b-poly(propylene oxide)-b- poly(ethylene oxide) block polymer (EO(20)PO(70)EO(20)), has been investigated using SANS, SAXS, and differential scanning calorimetry under the conditions utilized in the synthesis of ordered mesoporous materials, such as SBA-15. The absolute intensity measurements, both with SANS and SAXS, have provided a detailed quantitative description of the P123 micelles in the framework of a simple core-shell spherical model. The model developed has been used to establish the structure of the copolymer micelles, including their size, shape, aggregation number and detailed composition, as well as the structural changes induced by varying reaction conditions. The effects of temperature, pH, acidic source and the addition of swelling agents (toluene and TMB) are reported and discussed.

Initial stages of SBA-15 synthesis: An overview☆

This work presents an overview of the data obtained for SBA-15 synthesis under the reaction conditions using synchrotron based small angle X-ray scattering and small angle neutron scattering. Three major stages in the synthesis of SBA-15 materials proceeding according to the cooperative self-assembly mechanism have been identified, and the structures of the intermediates species have been established. Our in situ time-resolved neutron scattering experiments demonstrate that only spherical micelles of the templating agent are present in the synthesis mixture during the first stage of the reaction. According to the neutron scattering and X-ray scattering data, in the second stage of the reaction the formation of hybrid organic-inorganic micelles is accompanied with the transformation from spherical to cylindrical micelles, which takes place before the precipitation of the ordered SBA-15 phase. During the third stage, these micelles aggregate into a two-dimensional hexagonal structure, confirming that the precipitation takes place as the result of self-assembly of the hybrid cylindrical micelles. As the synthesis proceeds, the voids between the cylinders are filled with the silicate species which undergo condensation reactions resulting in cross-linking and covalent bonding, leading to the formation of highly ordered SBA-15 mesostructure. This work demonstrates that valuable structural information can be obtained from X-ray and neutron scattering characterisation of complex systems containing periodic phases with d-spacing values up to 30 nm, and that both techniques are powerful means for in situ monitoring of the formation of nanostructured materials.

In situ FTIR study of the formation of MCM-41

The formation of both siliceous and aluminium-containing MCM-41 has been followed insitu using attenuated total reflectance Fourier-transform infrared (ATR FTIR) spectroscopy together with a range of complementary techniques. Key stages in the reaction are reflected by changes in the intensity of IR bands at 1030 and 1105 cm−1, which are assigned to the internal Si–O vibrations of Si(OSi)3 and Si(OSi)4 groupings (Q3 and Q4, respectively). Assignments were confirmed using solid state NMR. Under the conditions of these experiments, the reaction rate is limited by the relatively slow dissolution of the silica source. The resulting silicate oligomers then react more rapidly with surfactant cations in a co-operative assembly process to give the embryonic MCM-41 structure. Slower condensation reactions subsequently increase the product Q4/Q3 ratio as further crosslinking develops. From an analysis of both the CH3 head group and CH2 in-chain IR vibrations, the surfactant appears to be complexed or micellised at all times during the synthesis.

Synthesis of MCM-41 materials: an in situ FTIR study

Abstract A new approach has been developed to monitor the synthesis of MCM-41 mesoporous materials based on in situ ATR FTIR measurements. The transformation between Q4 and Q3 species, which is an essential step in the synthesis and structural development of MCM-41 materials, can be followed using this technique.

Identification of isolated Pt atoms in H-mordenite

The state and dispersion of Pt supported on mordenite have been studied using FTIR spectroscopy, CO Chemisorption, and H–D isotope exchange of cyclopentane. Under conditions for which other criteria indicate the presence of monatomic Pt0, a characteristic IR band of adsorbed CO is identified at 2123 cm–1. The position of this band does not change with CO coverage, indicating an interaction between CO and highly dispersed Pt. A significant blue shift of the band with respect to that of CO on multiatomic Pt clusters and the disappearance of this band upon neutralizing the acidic OH groups show that the Pt atoms are electron-deficient and interact with zeolite protons. These FTIR data thus confirm the presence of extremely small Pt clusters, presumably [Pt1—Hz]z+ adducts, in H-mordenite.

Brønsted acid sites in zeolites. FTIR study of molecular hydrogen as a probe for acidity testing

Hydrogen is used as a probe molecule for characterisation of Bronsted acidity in zeolites. Hydrogen adsorption is monitored volumetrically (physisorption of hydrogen) and by FTIR (OH⋯H2 complex formation), simultaneously. The physisorption of hydrogen is a function of the pore size of a zeolite whereas the OH⋯H2 interaction reflects the strength of the acid sites. Hydrogen complexes observed by FTIR are characterised by four parameters which depend upon acid strength: shifts in the position and the increase in absorbance of the hydroxy and the H2 bands. For the zeolites studied the behaviour of these parameters is consistent and shows the following order of acidity: SAPO-37 <H-Y <H-EMT <H-ZSM-5 <H-MOR. Hydrogen is also compared with carbon monoxide, another probe widely used for acidity determination.

New insights into the initial steps of the formation of SBA-15 materials: an in situ small angle neutron scattering investigation.

Time-resolved in situ SANS investigations have provided direct experimental evidence for the three initial steps in the formation of the SBA-15 mesoporous material: an induction period is followed by a shape transformation of the micelles from spherical to cylindrical ones followed by the precipitation of a two-dimensional hexagonal phase.

Acid sites in mesoporous materials: A DRIFTS study

Abstract A range of MCM-41 materials have been prepared and characterised employing diffuse reflectance FTIR spectroscopy (DRIFTS) to investigate fundamental and combination frequencies of the OH groups in amorphous and crystalline aluminosilicates. Comparative characterisation of mesoporous materials shows that SiOH groups in Al-MCM-41 are acidic, unlike those in Si-MCM-41. At the same time, no indication of the presence of bridging hydroxyls in any of the studied MCM-41 samples has been found. It appears that the ‘mild acidity’ of Al-MCM-41 materials is not due to the bridging Al(OH)Si groups, which are commonly found in zeolites, but it is associated with SiOH groups similar to those in amorphous aluminosilicates. Acidic properties of these OH groups are probably modified by the inductive effect of the Al present in the structure, which however does not affect their stretching frequency. A significant concentration of Bronsted acid sites is observed in the Al grafted sample obtained via a post-synthesis modification of Si-MCM-41 indicating that this may be a suitable way to generate acid sites compared with the aluminium introduction during the synthesis.

Brönsted acidity in US-Y zeolites

FTIR analysis of the hydroxyl region in a series of US-Y zeolites was used for semiquantification of the enhanced acidity, involving Bronsted hydroxyls in both super and β cages, which correlates with the catalytic activity in n-hexane cracking.

The Role of Steric Effects and Acidity in the Direct Synthesis of iso‐Paraffins from Syngas on Cobalt Zeolite Catalysts

This study focuses on the effects of the localization of Co species, zeolite structure, and acidity on the performance of Co bifunctional catalysts promoted with Pt for the direct synthesis of iso‐paraffins from syngas. ZSM‐5, MOR, and BEA were chosen as zeolites with different structures and pore diameters. The catalysts were prepared either by incipient wetness impregnation or by the mechanical mixing of the zeolite with a conventional silica‐supported Co catalyst. The increase in the pore size and open character of the zeolite structure from ZSM‐5 to BEA resulted in a higher fraction of Co located inside the pores of the catalysts prepared by impregnation. The catalytic performance was affected strongly by the zeolite acidity, pore structure, and Co distribution between the pores and the external surface. The selectivity to short‐chain iso‐paraffins is affected principally by the zeolite acidity, whereas the selectivity to long‐chain branched hydrocarbons mostly depends on steric effects.