Krzysztof Bahranowski

THE EFFECT OF STRUCTURAL ORDER ON NANOTUBES DERIVED FROM KAOLIN-GROUP MINERALS

Kaolin-group clay minerals can be modified to form nanotubular and mesoporous structures with interesting catalytic properties, but knowledge of the best methods for preparing these structures is still incomplete. The objective of this study was to investigate intercalation/deintercalation as a method for the delamination and rolling of kaolinite layers in relation to structural order. To prepare nanotubular material, kaolinites of different crystallinities and halloysite (all from Polish deposits) were chosen. The experimental procedure consisted of four stages: (1) preparation of a dimethyl sulfoxide precursor intercalate; (2) interlayer grafting with 1,3-butanediol; (3) hexylamine intercalation; and (4) deintercalation of amine-intercalated minerals using toluene as the solvent. Structural perturbations and changes in the morphology of the minerals were examined by X-ray diffraction, Fourier transform infrared spectroscopy, differential scanning calorimetry, and transmission electron microscopy (TEM). The number of rolled kaolinite layers depended heavily on the efficiency of the intercalation steps. An increase in the structural disorder and extensive delamination of the minerals subjected to chemical treatment were recorded. Kaolinite particles which exhibited tubular morphology or showed rolling effects were observed using TEM. The nanotubes formed were ∼30 nm in diameter, with their length depending on the particle sizes of the minerals.

Oxidation of aromatic hydrocarbons with hydrogen peroxide over Zn,Cu,Al-layered double hydroxides

Abstract Layered double hydroxides (LDHs) with general formula [(Zn1−yCuy)1−xAlx(OH)2](NO3)x·nH2O (x≅0.33, 0.6 ≤y≤1.0, n≅0.5) have been synthesized and characterized by XRD, chemical analysis, and ESR. According to ESR, copper ions exist both as isolated species and as clusters, their relative contribution depending on the overall Cu content. The tendency to form clusters is observed even at a very low absolute Cu concentration. The materials are active in the low temperature liquid phase oxidation of toluene, o-, m-, and p-xylene with hydrogen peroxide. The catalytic reaction involves both the side chain oxidation and the hydroxylation/oxidation of the aromatic ring. The catalytically active centres are formed by isolated copper species. The clustered copper ions are inactive in the oxidation reaction but accelerate the non-productive decomposition of hydrogen peroxide. The catalytic performance is discussed in terms of competition between the process of H2O2 decomposition and its consumption in the oxidation reaction.

SURFACE AREA AND POROSITY OF NANOTUBES OBTAINED FROM KAOLIN MINERALS OF DIFFERENT STRUCTURAL ORDER

Mesoporous materials with pore diameters in the range 2–50 nm forming tubular or fibrous structures are of great interest due to their unique properties. Because they are commonly used as sorbents and catalyst carriers, knowledge of their surface area and porosity is critical. A modified intercalation/deintercalation method was used to increase the efficiency of nanotube formation from kaolin-group minerals which differ in terms of their degree of structural order. Unlike previous experiments, in the procedure adopted in the present study, methanol was used instead of 1,3-butanediol for grafting reactions and octadecylamine intercalation was also performed. The samples were examined using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The specific surface area and porosity of previously described and newly formed materials were investigated by N2 adsorption/desorption. Compared to results described earlier, the percent yield of nanotubes obtained in the present study was significantly greater only in the case of ‘Maria III’ kaolinite, which has high structural order. This increase was obtained mainly by the grafting reaction with methanol. Highly ordered stacking of kaolinite-methanol intercalates was noticed and, thus, the amine intercalation was more efficient. In particular, the use of long-chain octadecylamine significantly increased the nanotube yield. The grafting reaction with methanol procedure yielded fewer nanotubes, however, when applied to poorly ordered samples (‘Jaroszów’ kaolinite and ‘Dunino’ halloysite). In the case of the ‘Maria III’ kaolinite, the diameter of the rolled layers observed by TEM was ~30 nm and corresponded to average diameters of newly formed pores (DmN) determined using N2 adsorption/desorption, confirming that nanotubes contributed to an increase in surface area and total pore volume. In the case of ‘Jaroszów’ kaolinite and ‘Dunino’ halloysite mainly macropores (DmN > 100 nm) and mesopores (20 nm > DmN > 40 nm) were formed. The pores were attributed to interparticle and interaggregate spaces in the stacks of platy particles and to the small relative number of nanotubes.

Layered double hydroxide-derived vanadium catalysts for oxidative dehydrogenation of propane: Influence of interlayer-doping versus layer-doping

Abstract Mixed-oxide vanadium catalysts for oxidative dehydrogenation (ODH) of propane have been prepared by thermal decomposition of Mg, Al-layered double hydroxides (LDHs) containing vanadium either in the brucite layer or in the interlayer. The materials have been characterised by XRD, ICP-AES chemical analysis, XPS, BET and ESR. The catalytic performance of the samples depended on the manner of incorporation of the vanadium into the LDH structure. The sample obtained from interlayer-doped precursor was more active and more selective than mixed oxides obtained from layer-doped LDHs. The difference in the catalytic properties was attributed to the different magnesium vanadates nucleating in the calcined samples, the pyrovanadate formed from the interlayer-doped LDH giving better performance than ortho-vanadate crystallising from the layer-doped precursor. It has been suggested that one of the factors contributing to the difference in the behaviour of both types of catalysts might be the difference in the covalency of VO in-plane bonds around the reduced V centres.

ESR study of vanadium-doped alumina- and titania-pillared montmorillonites

Abstract Vanadium-doped alumina- and/or titania-pillared montmorillonites obtained by various preparative procedures have been investigated with the aid of ESR. The ESR signals of air-dried samples are characteristic of immobilized VO2+ ions, even after storage in a wet atmosphere. Comparison with model spectra shows that these species are bound to the pillars. Estimation of the V O bond strength from the experimentally determined ESR parameters shows that in the titania-pillared montmorillonite this bond is stronger than in alumina-pillared samples. In all cases the unpaired electron is strongly localized at the vanadium centre. A certain degree of delocalization into ligand orbitals is observed for the vanadyl ions present in the (V Ti)-PILC (pillared clay) sample obtained by calcination of the co-pillared precursor. A hypothesis is advanced that these species are responsible for the unique catalytic properties of this sample.

Insertion of electrochemically reduced Keggin anions into layered double hydroxides

A new procedure is described to enable the charge on a Keggin ion to be modified to improve incorporation into layered double hydroxides. The procedure, involving electrochemical reduction, has been tested using the 12-molybdophosphate anion. Chemical analysis and PXRD data confirm that from the otherwise hydrolytically unstable and low charge anions which are unsuitable for intercalation within the basic matrix of MgAl–LDH, a species is obtained in which the stoichiometry is preserved and which yields a pillared LDH structure of 14.8 A basal spacing.

Influence of doping with copper on the texture of pillared montmorillonite catalysts

Modification of the textural properties of Al-, Zr-, and Ti-pillared montmorillonites upon doping with copper has been investigated by nitrogen adsorption/desorption at 77 K. The doping methods included addition of copper during the post-pillaring treatment of uncalcined or calcined solids, and co-pillaring, i.e., the use of the pillaring agent containing copper ions. All investigated pillared samples are predominantly microporous, the dominant micropore radii of Al- and Zr-pillared samples (<3.50 A) being much lower than in the case of the Ti-pillared clay (5.25 A). The influence of the copper additive on the texture of pillared matrices depends on the nature of the pillars and on the method of doping, and affects both the micropore and the mesopore system. The results are interpreted in terms of the physicochemical properties of the samples as determined by the chemical, XRD and ESR analyses. The strong influence of post-pillaring doping on the texture of the Al-pillared clay, resulting in a decrease of microporosity, is assigned to the enhanced formation of clustered copper ions. An increase of the adsorption potential of the co-pillared samples is attributed mainly to a better structural order induced by the competitive exchange. It is suggested that the sensitivity of the mesopore system of the Ti-pillared clay to the repeated post-pillaring doping is due to crystallisation effects in the extra-framework titania.

Physico-chemical characterization and catalytic properties of copper-doped alumina-pillared montmorillonites

Cu-doped alumina-pillared montmorillonite samples have been prepared and characterized with X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), electron spin resonance (ESR) and inductive coupled plasma atomic emission spectroscopy (ICP AES) techniques. The results show that the catalysts are porous materials with copper species located in the interlayer, present either as isolated Cu2+ ions anchored at alumina pillars or as patches of amorphous CuO. Catalytic tests with hydroxylation of phenol show that the clay samples possess significant activity for dihydroxybenzene (DHB) formation, comparable with the reference TS-1 catalyst. Experiments with changing the substrate dosing indicate that adsorption and activation of phenol molecules is a necessary condition for the reaction to occur.

Synthesis and physicochemical properties of vanadium-doped zirconia-pillared montmorillonites in relation to oxidative dehydrogenation of propane

V-doped zirconia-pillared montmorillonites have been obtained by different methods and characterised with XRD, ESR, nitrogen adsorption and chemical analyses. The samples are microporous, their structure, texture and location of V species depending on the method of preparation. Catalytic results in the reaction of the oxidative dehydrogenation of propane to propene are discussed in terms of the physicochemical properties of the materials.

ESR study of the thermal decomposition of V-containing layered double hydroxides

Abstract Heat treatment induced transformations of the ESR signals present in the layered double hydroxides (LDHs) containing vanadium (introduced either as an interlayer decavanadate anion or as a layer-forming cation) are investigated, with particular stress on the changes in the content, electronic structure and symmetry of the vanadyl species created. The differences in the catalytic activity in the oxidative dehydrogenation of propane of the mixed oxide materials resulting from the decomposition of the investigated LDHs are related to the differences in the degree of in-plane π-covalent bonding observed for V(IV) centres present in these materials.