Zofia Piwowarska

Transformation of nitrogen structures in carbonization of model compounds determined by XPS

Abstract Nitrogen-containing chars were prepared by low-temperature carbonization (460 °C) of compounds containing pyrrolic, pyridinic, amino and cyano groups. The changes of precursor functionalities and transformation of nitrogen structures at 600 and 800 °C were determined by X-ray photoelectron spectroscopy. From all precursors, at low-temperature carbonization, three types of nitrogen structure were obtained: pyrrolic, pyridinic and quaternary, but from a precursor containing cyano groups this group was also observed. The concentration of pyrrolic and pyridinic structures decreases with increasing calcination temperature, whereas the amount of quaternary nitrogen increases. Quaternary nitrogen is the most stable form of the observed nitrogen structures, but an additional peak appeared at 403 eV for chars calcined at 800 °C.

Physicochemical and catalytic properties of ployaniline protonated with 12-molybdophosphoric acid

Using acid–base-type doping of polyaniline with H3PMo12O40, two series of catalyst samples were prepared. In series Sl the doping and polymerization were carried out simultaneously, leading to a uniform distribution of the dopant over the whole volume of the polymer. In series SII the doping was achieved by protonation of already formed polyemeraldine base with H3PMo12O40. Since this process must involve diffusion in the solid matrix, the surface becomes enriched in heteropolyanions (Keggin units) due to their limited diffusivity. The examination of physicochemical properties of the samples by XP, EPR and FTIR spectroscopies indicated that the Keggin units incorporated into the polymer matrix retain their identity and represent catalytically active centres. The insertion of the dopant species increases the electrical conductivity of the polymer by several orders of magnitude. The catalytic activity was tested in ethyl alcohol conversion and both the products of acid–base [C2H4 and (C2H5)2O] and redox (CH3CHO) type reactions were observed. Owing to the surface enrichment of the SII series of catalysts their catalytic activity was much higher than that of samples of the Sl series nominally doped to the same level. In both types of catalysts the selectivity to the redox reaction product (CH3CHO) was greatly enhanced in comparison with unsupported H3PMo12O40.

Catalytic conversion of ethyl alcohol on polyaniline protonated with 12-tungstosilicic acid

Abstract It has been demonstrated that protonation of polyemeraldine base with 12-tungstosilicic acid leads to the preparation of a new, polymer supported catalyst which shows significantly higher activity and altered selectivity in ethyl alcohol conversion as compared to crystalline 12-tungstosilicic acid studied in the same experimental conditions. The improved activity is associated with higher dispersion of Keggin units bonded to the polymer surface via protonation reaction which in turn leads to their improved thermal stability as shown by DTA, FTIR and other studies.

Free vs chitosan-immobilized urease: Microenvironmental effects on enzyme inhibitions

Chitosan gel membranes were prepared by a solvent-evaporation method and used as a support for covalent immobilization of jack bean urease. The effects of the local microenvironment created by both the electrostatic potential of the polycationic support and the enzyme reaction on the inhibition of urease by phosphate buffer were investigated as a function of pH and compared with other urease competitive inhibitions. It was found that the kinetic behaviour of chitosan-immobilized urease in the inhibition is resultant of structural, diffusion limitation-related and microenvironmental factors. More importantly, it was shown that this behaviour is microenvironment-dependent when either a pH-dependence of enzyme inhibition (phosphate and F−) or electrostatic inhibitor-support repulsion (Ni2 + ) prevailed in the system. Other inhibitors uncharged that are do not show pH-dependented inhibition (boric and acetohydroxamic acids), the inhibition only depends on factors other than microenvironmental. Knowledge of such microenvironmental effects is of practical and theoretical importance in designing applications of immobilized enzymes and in clarifying the mode of action of enzymes in biological membranes in their native milieu.

XPS study of polyaniline supported dodecatungstosilicic acid catalyst

Abstract The presence of hydrogenoheteropolyanions at the polyaniline surface supported H 4 SiW 12 O 40 catalyst playing most probably the role of catalytically active Bronsted acid centers in cumene cracking was confirmed by the XPS measurements. These centers are partially removed by heating at 573 K in the atmosphere of dry air but easily regenerated by the water vapor introduced into the reactor in the stream of helium carrier gas.

Polyaniline doped with heteropolyanions: Spectroscopic and catalytic properties

Abstract Acid-base properties of heteropolyacids make it possible to introduce them quite easily into a polyaniline matrix via classical acid-base doping. The present work shows that a new type of polymer-supported catalyst can be obtained in this way. It has been discovered that such catalysts are active in ethyl alcohol conversion. Of the two methods of synthesis studied, i.e. preparation of polyaniline and its protonation in two separate steps and the one-step procedure, the former results in more catalytically active products. This is presumably caused by the hindered diffusion of heteropolyanions between polymer chains in the former case. Thus, they stay mainly at the surface of the polymer matrix and are more easily accessible for the alcohol.

The properties of boralites studied by infrared spectroscopy

Because of its small dimensions boron is three-coordinated in boralites and is situated in one face of a tetrahedral hole. The i.r. band of the B—O asymmetric stretch in BO3 units is split into two maxima (at 1380 and 1405 cm–1), thus suggesting that two kinds of boron sites exist in boralites. Boron becomes four-coordinated following adsorption of the electron-donor molecules NH3, H2O and pyridine, but not of benzene. Three kinds of OH groups (at 3680, 3720 and 3460 cm–1) are formed following the subsitution of Na+ ions by protons. The 3720 cm–1 OH groups act as Bronsted-acid sites in the reaction with pyridine. The concentration of Bronsted sites determined by pyridine sorption is comparable with theoretical values. The low strength of the Bronsted-acid sites (illustrated by pyridine thermodesorption experiments) can be explained by the large separation and weak interaction between boron and the OH groups in the O3B⋯OH–SiO3 units. The electrostatic field of the Lewis-acid sites in boralites is stronger than that in zeolites.

X-ray photoelectron spectroscopy (XPS) as a useful tool to characterize polyaniline doped by 12-tungstosilicic, 12-tungstophosphoric and 12-molybdophosphoric acids

Abstract X-ray photoelectron spectra of polyaniline (PANI) doped by molybdophosphoric, tungstophosphoric and tungstosilicic acids were measured and discussed. Three different nitrogen species (imine, amine and protonated) were distinguished, and their proportions were determined. The N(1s) binding energies of the protonated species varied within a 400.5–402.5 eV range, while the imine and amine species showed constant values of the electron binding energies. Protonation of PANI was manifested both in N(1s) and O(1s) spectra. Even a small protonation by water molecules was recognized in X-ray photoelectron spectroscopy (XPS). Differences in acidic strength of the heteropolyacids were noticed. The thermal treatment (catalytic activation) converted the surface H2O into OH. Catalytic conversion of ethanol resulted in a strong deprotonation of all the samples.

Changes in the structure of ZSM-5 zeolites during boron and aluminium substitution

XRD and chemical analysis reveal that the orthorhombic unit cell of zeolite ZSM-5 structure undergoes linear contraction or expansion with increasing substitution level of B or Al, respectively. Evidence for a full substitution of B or Al in the framework to ca. three heteroatoms per unit cell is also provided by the 11B MASNMR spectrum and a linear decrease in the framework vibration frequency with the increasing Al per unit cell, as observed in the IR spectra. Only the boralite with an analytical content of 3.7 B atoms per unit cell contained ca. 24% of extraframework B. The similar electronegativities of B and Si explain the structural vibration frequency in the IR region, which is independent of the amount of B per unit cell.

Hierarchically structured ZSM-5 obtained by optimized mesotemplate-free method as active catalyst for methanol to DME conversion

In the presented studies, a new method for the synthesis of hierarchical porous materials with ZSM-5 zeolite properties was applied. The proposed method is based on the acidification of the zeolite seeds slurry using HCl solution, followed by hydrothermal treatment, enabling the aggregation of zeolite nanoseeds with the formation of the interparticle mesoporous structure. The influence of the duration of zeolite parent mixture aging before and after acidification on the resulting properties of the samples was investigated. The physicochemical properties of the obtained micro-mesoporous samples were analyzed using techniques such as N2-sorption measurements, X-ray diffraction, TG analysis, NH3-TPD and electron microscopy. In the second part of the studies, the influence of the modified zeolite sample parameters (such as porosity, acidity and crystallinity) on their catalytic activity for dimethyl ether (DME) synthesis from methanol was studied. DME is considered as a future clean alternative to diesel fuel and the development of methods for its synthesis is currently of high scientific interest. It was shown that modification of the porous structure and acidity of the zeolitic samples strongly influences their catalytic activity, selectivity and stability for the DME synthesis process. The micro-mesoporous samples, despite their significantly lower acidity, exhibited high catalytic activity (similar to conventional ZSM-5 zeolite) and enhanced selectivity towards DME, as well as higher stability in a long term catalytic test (higher resistance to the formation of coke deposits) in comparison to standard MFI-type zeolites.