Acidity Characterization of Amorphous Silica-Alumina

Surface characterization of amorphous silica-alumina (ASA) by COads IR, pyridine(ads) IR, alkylamine temperature-programmed desorption (TPD), Cs+ and Cu(EDA)(2)(2+) exchange, H-1 NMR, and m-xylene isomerization points to the presence of a broad range of Bronsted and Lewis acid sites. Careful interpretation of IR spectra of adsorbed CO or pyridine confirms the presence of a few very strong Bronsted acid sites (BAS), typically at concentrations lower than 10 mu mol/g. The general procedure for alkylamine TPD, which probes both Bronsted and Lewis acidity, is modified to increase the selectivity to strong Bronsted acid sites. Poisoning of the m-xylene isomerization reaction by a base is presented as a novel method to quantify strong BAS. The surface also contains a weaker form of BAS, in concentrations between 50 and 150 mu mol/g, which can be quantified by COads IR Cu(EDA)(2)(2+) exchange also probes these sites. The structure of these sites remains unclear, but they might arise from the interaction of silanol groups with strong Lewis acid Al3+ sites. The surface also contains nonacidic aluminol and silanol sites (200-400 mu mol/g) and two forms of Lewis acid sites: (i) a weaker form associated with segregated alumina domains containing five-coordinated Al, which make up the interface between these domains and the ASA phase and (ii) a stronger form, which are undercoordinated Al sites grafted onto the silica surface. The acid catalytic activity in bifunctional n-heptane hydroconversion correlates with the concentration of strong BAS. The influence of the support electronegativity on the neopentane hydrogenolysis activity of supported Pt catalysts is considerably larger than that of the support Bronsted acidity. It is argued that strong Lewis acid sites, which are present in ASA but not in gamma-alumina, are essential to transmit the Sanderson electronegativity of the oxide support to the active Pt phase.

Dry gel conversion of organosilane templated mesoporous silica: from amorphous to crystalline catalysts for benzene oxidation

Mesoporous silica grown using [3-(trimethoxysilyl)propyl]octadecyldimethylammonium chloride as the mesoporogen in the presence of Fe and Al is X-ray amorphous, but contains very small domains with features of MFI zeolite as evidenced by IR and Raman spectroscopy. When applied as a catalyst, this amorphous sample shows good performance in the selective oxidation of benzene using nitrous oxide. Addition of tetrapropylammonium as structure directing agent to the as-synthesized mesoporous silica and subsequent dry gel conversion results in the formation of hierarchical Fe/ZSM-5 zeolite. During dry gel conversion the wormhole mesostructure of the initial material is completely lost. A dominant feature of the texture after crystallization is the high interconnectivity of micropores and mesopores. Substantial redistribution of low-dispersed Fe takes place during dry gel conversion towards highly dispersed isolated Fe species outside the zeolite framework. The catalytic performance in the oxidation of benzene to phenol of these highly mesoporous zeolites is appreciably higher than that of the parent material.

From non-porous crystalline to amorphous microporous metal(IV) bisphosphonates

Porous layered hybrid materials have been prepared by the reaction of organo-bisphosphonate ligands, 4-(4'-phosphonophenoxy)phenylphosphonic, 4,4'-biphenylenbisphosphonic and phenylphosphonic acids, with metal(IV) cations (Zr and Sn). Crystalline Zr(IV) and Sn(IV) layered bisphosphonates were also prepared, which were non-porous. The amorphous M(IV) bisphosphonates showed variable compositions and textural properties ranging from mainly mesoporous to highly microporous solids with BET surface areas varying from 300 to 480 m(2) g(-1), micropore volumes ranging 0.10-0.20 cm(3)/g, and narrow porous size distributions for some materials. N-2 isotherms suggest that Sn(IV) derivatives show a comparatively higher micropore contribution than the Zr(IV) analogous at least for the ether-bisphosphonate hybrids. Sn(IV) bisphosphonates exhibit high microporosities without the need of using harmful DMSO as solvent. If ether-bisphosphonic acid is partially replaced by less expensive phenylphosphonic ligand, porous products are also obtained. P-31 and F-17 MAS NMR and XPS data revealed the presence of hydrogen-phosphonate groups and small (F-, Cl- and OH-) anions, which act as spacer ligands within the inorganic layers, in these hybrid materials. The complexity of the inorganic layers is higher for the Sn(IV) bisphosphonates likely due to the larger amount of small bridging anions including fluorides. It is suggested that the presence of these small inorganic ligands may be a key factor influencing both, the interaction of the inorganic layer with the bisphosphonate groups, which bridge the inorganic layers, and the generation of internal voids within a given inorganic layer. Preliminary studies of gases adsorption (H-2 and NO) have been carried out for selected Sn(IV) bisphosphonates. The H-2 adsorption capacity at 77 K and 1 bar was low, 0.26 wt%, but the NO adsorption capacity at similar to 1 bar and 298 K was relatively high, 4.2 wt%. Moreover, the hysteresis in the NO isotherms is indicative of partial strong irreversible adsorption of NO. (C) 2008 Elsevier Inc. All rights reserved.

The effect of photo-oxidation on the sticking and reactivity of Ag on amorphous GeS2

Photo-oxidation of amorphous GeS2 films illuminated by band-gap radiation drastically alters the growth mode and reactivity of subsequently deposited Ag. In the former case (monolayer/simultaneous multilayer growth) the Ag reacts with both Ge and S sites. In the latter case (Stranski-Krastanov growth) Ge sites are selectively oxidized and film growth proceeds by Ag nucleation at the unoxidized S sites. The behaviour is very different from that reported earlier for Zn deposition on GeS2, where photo-oxidation results in very large changes in metal sticking probability. XPS, XAES and EXAFS data provide the basis for understanding both this phenomenon and the very different photodiffusion behaviour of Zn and Ag in GeS2.

Synthesis of visible-light-activated yellow amorphous TiO2 photocatalyst

Visible-light-activated yellow amorphous TiO₂ (yam- TiO ₂) was synthesised by a simple and organic-free precipitation method. TiN, an alternative precursor for TiO₂ preparation, was dissolved in hydrogen peroxide under acidic condition (pH∼1) adjusted by nitric acid. The yellow precipitate was obtained after adjusting pH of the resultant red brown solution to 2 with NH₄OH. The BET surface area of this sample was 261 m²/g. The visible light photoactivity was evaluated on the basis of the photobleaching of methylene blue (MB) in an aqueous solution by using a 250 W metal halide bulb equipped with UV cutoff filter (λ>420 nm) under aerobic conditions. Yam- TiO₂ exhibits an interesting property of being both surface adsorbent and photoactive under visible light. It was assigned to the η²-peroxide, an active intermediate form of the addition of H₂O₂ into crystallined TiO₂ photocatalyst. It can be concluded that an active intermediate form of titanium peroxo species in photocatalytic process can be synthesised and used as a visible-light-driven photocatalyst

Thermodynamically stable amorphous drug dispersions in amorphous hydrophilic polymers engineered by hot melt extrusion.

In this study thermodynamically stable dispersions of amorphous quinine, a model BCS class 2 therapeutic agent, within an amorphous polymeric platform (HPC), termed a solid-in-solid dispersion, were produced using hot melt extrusion. Characterisation of the pre-extrudates and extrudates was performed using hyper-differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD) and Raman spectroscopy. Water uptake by the raw materials was determined using dynamic vapour sorption (DVS) analysis. Furthermore, the presence or absence of crystalline drug following storage at 25 °C/60% relative humidity and 40 °C/75% relative humidity in a sealed glass jar, and at 40 °C/75% relative humidity in an open glass jar for 3 months was determined using PXRD. Amorphous quinine was generated in situ during extrusion from both quinine base (5%, 10%, 20% w/w drug loading) and from quinine hydrochloride (5%, 10% w/w drug loading) and remained thermodynamically stable as a solid-in-solid dispersion within the HPC extrudates. When processed with HPC, quinine hydrochloride (20% w/w) was converted to amorphous quinine hydrochloride. Whilst stable for up to 3 months when stored under sealed conditions, this amorphous form was unstable, resulting in recrystallisation of the hydrochloride salt following storage for 1 month at 40 °C/75% relative humidity in an open glass jar. The behaviour of the amorphous quinine hydrochloride (20% w/w) HPC extrudate was related, at least in part, to the lower stability and the hygroscopic properties of this amorphous form.

An Investigation into the Role of Polymeric Carriers on Crystal Growth within Amorphous Solid Dispersion Systems

Using phase diagrams derived from Flory–Huggins theory, we defined the thermodynamic state of amorphous felodipine within three different polymeric carriers. Variation in the solubility and miscibility of felodipine within different polymeric materials (using F–H theory) has been identified and used to select the most suitable polymeric carriers for the production of amorphous drug–polymer solid dispersions. With this information, amorphous felodipine solid dispersions were manufactured using three different polymeric materials (HPMCAS-HF, Soluplus, and PVPK15) at predefined drug loadings, and the crystal growth rates of felodipine from these solid dispersions were investigated. Crystallization of amorphous felodipine was studied using Raman spectral imaging and polarized light microscopy. Using this data, we examined the correlation among several characteristics of solid dispersions to the crystal growth rate of felodipine. An exponential relationship was found to exist between drug loading and crystal growth rate. Moreover, crystal growth within all selected amorphous drug–polymer solid dispersion systems were viscosity dependent (η–ξ). The exponent, ξ, was estimated to be 1.36 at a temperature of 80 °C. Values of ξ exceeding 1 may indicate strong viscosity dependent crystal growth in the amorphous drug–polymer solid dispersion systems. We argue that the elevated exponent value (ξ > 1) is a result of drug–polymer mixing which leads to a less fragile amorphous drug–polymer solid dispersion system. All systems investigated displayed an upper critical solution temperature, and the solid–liquid boundary was always higher than the spinodal decomposition curve. Furthermore, for PVP–FD amorphous dispersions at drug loadings exceeding 0.6 volume ratio, the mechanism of phase separation within the metastable zone was found to be driven by nucleation and growth rather than liquid–liquid separation.