Adriana De Stefanis

Assembly and polymerisation of some aromatic amines in α-VOPO4·2H2O

Under soft conditions (dry EtOH, 25 °C), pyrazole, pyrazine and phenazine intercalate into α-VOPO4.2H2O as protonated, non-coordinated amines. Intermediates during pyrazine uptake include a metastable phase with d001= 13.3 A, ascribed to bilayer formation. Benzidine (bz) instead gives two bz+ charge transfer (CT) bronzes: VOPO4(bz)0.5·2H2O (dry EtOH) and VOPO4(bz)0.7·3.5H2O (95% EtOH) with interlayer distances lower (7.05 A and 6.65 A, respectively) than in α-VOPO42H2O itself. Interlayer ‘pocket’ orientation with layer shifting and turbostraticity to accommodate bz+ cations is suggested, which differs from the assembly of bz+ in smectites and V2O5. VOPO4(bz)0.5·2H2O shows ‘moving solid’ behaviour, probably due to changes in specific H-bonding between intercalate and sheet with water content. It is concluded that surface ruffling and specific H-bonding influence composite formation and assembly.Pyrrole (pyrr) almost completely polymerises on intercalation into α-VOPO4.2H2O, giving ‘nestled’ polymer composites with differing polypyrrole (Ppyrr) oxidation levels, whereas aniline uptake is governed by base protonation and proton induced reassembly. Both give differently loaded nanocomposites (with interlayer distances in the range 6.4–17.9 A) depending on amine delivery (vapour, liquid, solution) and intercalant state (powder dispersion, film). Pyrrole loadings are higher with dispersions and lower with films, but the opposite is the case for aniline (An). For example, neat An gives VOPO4(An)1.3·13.5H2O (interlayer distance 17.9 A), An-EtOH gives VOPO4(An)0.4(14.8 A), and gaseous An gives VOPO4(An)0.66·2.2H2O (6.65 A). As prepared, the Ppyrr shows mixed-polaron states, and both series give EPR spectra characteristic of highly anisotropic VIV-containing clusters and do not conduct. All become amorphous on calcination (160 °C), the An-containing composites polymerising to completion. Calcined VOPO4(Ppyrr)066·1.4H2O (from neat pyrrole) is a semiconductor, the EPR spectra (cationic amine and separated VIV ions) further supporting strong polymer-sheet electronic interactions with the formation of VIV island clusters. Assemblies of the two amines in VOPO4 are suggested.

PLS versus zeolites as sorbents and catalysts II. Terpene conversions in alumina-pillared clays and phosphates and medium pore zeolites

Abstract The reactions of α-pinene, limonene, and α-terpinene in several alumina-pillared clays (PILCs) and a layered α-tin phosphate analogue (Al-PILP) have been investigated under Lewis acid conditions and compared with the mid-pore zeolites USY, NH 4 + -ZSM-5 (with SiO 2 /Al 2 O 3 ratios = 35 and 235), and H + -mordenite. The bicyclic α-pinene gives the highest conversion, all catalysts giving > 50% camphene at 100°C. Total yields show that USY is the strongest acid, after which the acidity order is: BP-PILC = ZSM-5 (35) > FAZA > H + -mordenite, and the layered phosphate appears to be less acid than the PILCs. No fenchane carbocation-derived products are produced, indicating that all the solids promote formation of the norbornyl cation intermediate. Selectivities in the unsubstituted PILCs is comparable with those in the zeolites (e.g. both FAZA and USY show selectivity against limonene production in the α-pinene reaction). BP-PILC also shows appreciable activity for α-pinene at 25°C (as does USY) whereas Al-PILP is inactive. Specific carbocation precursors are deduced from the product distributions and a ‘carbocation cascade’ based on pore acidity provides a rationalisation of the results. However, site-selectivity effects do come into play in K + - and Ca 2+ -PILCs and it also appears that limonene occupies a specific site in USY. The reactions provide a means of generating terpene derived carbocations in the solid state.

PLS vs. zeolites as sorbents and catalysts. 5. Evidence for Brønsted/Lewis acid crossover and high acidity in conversions of C1–3 alcohols in some alumina-pillared smectite clays

Conversions of methanol, ethanol, and iso-propanol have been investigated in the activated alumina-pillared montmorillonite (BP-PILC), beidellite/montmorillonite (FAZA), and saponite (ATOS)-based PILCs obtained from standard [Al13O4(H2O)12(OH)24]7+ Keggin-ion. Product outcomes for MeOH differ from those on smectite clays themselves and on alumina (or transition metal ion oxides): all give rise to dimethylether, rather than methylformate or formic acid. Systematic study of conversion, yields and selectivity for cation-exchanged FAZA show that the large changes observed must be ascribed to both steric effects and selective blocking of proton-containing active sites. The latter are most important in Ni2+-exchanged FAZA-containing catalysts and are attributed to generation of highly acidic Lewis sites, e.g. hydrocarbons alone (in a distribution similar to the MTG process) are obtained on Ni2+-FAZA. Apart from ethene and acetaldehyde, EtOH conversion also gives diethylether, not obtained on smectites themselves, and produced via a bimolecular reaction. The total dehydration/dehydrogenation ratio varies in the order BP-PILC>FAZA>>AZA=ATOS, BP-PILC being the most acidic and most selective. Dehydration and dehydrogenation activity with temperature go through a crossover point. This is ascribed to “iso-acidity”, i.e. the iso-acidic point is where Lewis and Bronsted acids are equally strong. Trends in iso-acidity with metal-ion exchange in FAZA materials suggest that the Lewis acid sites are at the alumina pillar. Detailed study of reaction kinetics and contact times leads to the conclusion that saponite surfaces have higher dehydration activity than those in montmorillonite.

Pillared layered structures vs. zeolites as sorbents and catalysts. Part 1.—Hydrocarbon separations on two alumina-pillared clays and an α-tin phosphate analogue

The dynamic sorption of a series of hydrocarbons in an alumina-pillared montmorillonite (AZA), its mixed Fe3+–alumina-pillared analogue, (FAZA) and an alumina-pillared a-tin phosphate analogue (AI-PILP) has been investigated. Separations between binary mixtures of cyclic non-aromatic/aromatic, linear chain/cyclic non-aromatic hydrocarbons, among others, have been performed in the temperature range 200–350 °C. Aromatic chlorohydrocarbons can also be separated from one another and from their ternary mixtures with hydrocarbons. Although they are the most efficient in these separations (requiring lower temperatures), the AI-PILP samples show enhanced acid-catalysis characteristics at higher temperatures. In addition, naphthalene and biphenyl are sorbed by the montmorillonite-based PILCs, but separated by the AI-PILP, confirming that shape-selection behaviour is involved. The anomalous sorption of cyclohexane by the AI-PILP demonstrates that lateral oxide-pillar ordering differs between pillared smectite clays and phosphates.Catalytic cracking phenomena observed for n-heptane differ between AZA and FAZA, indicating that in-pillar Fe in FAZA is involved, and this is confirmed via cracking of 3-methylhexane. That adsorbate–wall (i.e. pillar) interactions are important, and differ from adsorbate interactions in medium-sized zeolites, is supported by the fact that AZA and FAZA act as weak Lewis-acid reaction ‘vessels’ in the isomerisation of p-xylene.

Variable assembly of imidazole into α-VOPO4·2H2O

α-VOPO4·2H2O reacts with Imidazole (Im) in anhydrous ethanol to give a layered composite with d00I, = 10.3 A, whereas in 95% ethanol it gives a phase with d00I, = 8.8 A; both have formula VOPO4(Im)1.0·nH2O (8.8 A phase, n= 0.2; 10.3 A phase, n= 0.6). The 8.8 A phase slowly changes to a third phase with d00I, = 9.63 A. Indirect evidence suggests that the 10.3 A phase contains Im coordinated to intralayer vanadium and oriented vertically to the planes, whereas in the 8.8 A phase the Im lies almost flat between the layers and is not coordinated, i.e., the materials may be considered ‘intercalation isomers’.

PLS versus zeolites as sorbents and catalysts. Part 6. Effect of pore-blocking of active sites in FAZA on conversions of C1-3 alcohols

Abstract The effect of exchanging transition metal ions in the alumina-pillared beidellite/montmorillonite FAZA on the conversion of methanol, ethanol, n -propanol and iso -propanol has been investigated. It is concluded that dehydration/dehydrogenation throughout the series of materials occurs preferentially at the oxidic pillar rather than at the aluminosilicate sheet. The basal structure of the smectite, i.e. whether protons lie inside (montmorillonitic) or outside (beidellitic) the aluminosilicate layers, little influences the reactions. By way of contrast, cation exchange (both single and double) greatly influences both product ratio outcomes and conversions, e.g. Ni-FAZA (FAZA=a Fe 2+ /Fe 3+ /alumina-pillared mixed beidellite/montmorillonite) has very acidic centres suppressing MeOH dehydration completely.

Ordering and manipulation of MoS2 platelets on differently charged micas by atomic force microscopy

Atomic force microscopy (AFM) has been used to investigate how dispersed MoS2 platelets (from colloidal suspensions) deposit onto three natural (Muscovite) mica (001) surfaces. Different platelet arrangements are observed, which are attributed to defects and charging effects of the mica (as well as the concentration of starting colloid). The provenance of the mica influences the self-organisation of the platelets into long tape-like assemblies (South Dakota mica) or individual flakes (Alps mica). Atomic scale imaging of the tapes reveals a distorted octahedral (Oh)-based local structure, different from the trigonal prismatic structure found in the 2H-polytype of annealed MoS2, in agreement with previous structural results on water-dispersed MoS2 platelets. The buckling and susceptibility to stripping of the tapes is ascribed to the presence of a water layer between the substrate mica and MoS2, and after stripping by the tip, the platelets ultimately form small clusters. The ordering of these clusters depends not only on the defect and charge structure of the mica, but also on complex hydration reactions between the H2O layer associated with the MoS2 and K + ions of the mica. Relatively symmetrical squares may be lifted out of the tapes, supporting the presence of weak bonding between tape and mica. Conversely, Alps and Bihar micas give rise to separate platelets, which are resistent to tip manipulation, which is attributed to hydrophobic interaction between MoS2 platelets and the mica surface.

PLS vs ZEOLITES AS SORBENTS AND CATALYSTS, PART 7. THE ROLE OF OXIDE PILLARS AND ALUMINOSILICATE SHEETS IN ALCOHOL DEHYDRATION ON PILCs

Reactions of primary aliphatic C2-C8 alcohols on alumina and iron-aluminium-oxides pillared montmorillonite and beidellite indicate that dehydration reaction occurs on Lewis sites of pillars, while clays are responsible of cracking.

Chirality of products in acid-catalysed rearrangement of α-pinene

Protonation of (1R)-(+)-a-pinene using gaseous H3+ ions led to a racemic mixture of camphene and limonene, to partial and no racemization, in acidic solutions and in solid acids, respectively.