Giorgio Perez

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.

PLS vs. zeolites as sorbents and catalysts 4. Effects of acidity and porosity on alkylation of benzene by primary alcohols

The alkylation of benzene by MeOH, EtOH, iso-PrOH and n-octanol has been investigated in H-ZSM-5, mordenite, USY and Theta-1 and the acidity/porosity relations obtained compared with those in an alumina-pillared montmorillonite (BP-PILC) and an alumina-pillared saponite (ATOS). Pseudo-first order alkylation rates follow the order: H-ZSM-5 > mordenite > USY > Theta-1 > ATOS > BP-PILC which for the zeolites is not that expected on the basis of acidity. Conversely, EtOH conversion in H-ZSM-5s with Si/Al ratios 5/35→5/400 do follow expected acidity trends and MeOH consumption during alkylation can be used to construct an acidity scale. It is concluded that the reaction occurs via electrophilic attack of intermediate matrix-attached oxonium ion at incoming benzene molecules and that pore size, rather than acidity, is the determining factor. Thus, the smaller channels of H-ZSM-5 are the most favourable because they allow close approach between oxonium ion and benzene, whereas both pore sizes in both PILCs are too large, which hinders efficient alkylation.

CATALYTIC PYROLYSIS-GAS CHROMATOGRAPHY

Abstract A new technique which allows the rapid catalytic testing of a small amount of solid (3–10 mg) has been developed and tested on standard ethylbenzene dealkylation and disproportion reactions in mid-pore zeolites. It consists of an adaptation of a commercial pyrolyser, to function as a fixed bed catalytic reactor, connected on line to a gas chromatograph. Test results with H+-ZSM-5 zeolite, as the fixed bed, obtained under different experimental conditions of flow rate and amount of catalyst, show that data can be evaluated and compared with standard methods, provided they are referred to the same space velocity. All other standard catalytic characterisation parameters (temperature, conversion, selectivity, including estimates of the difference between the activation energies of different catalysts) can also be extracted directly. The exception is TOS (time on stream) which can be measured indirectly after injecting an excess of volatile organic reactant (here, ethylbenzene). Since a small concentration of ethylbenzene with respect to the zeolite is required for each test, coking phenomena (which reduce the catalyst activity) cannot be observed.

Mesoporous M41S materials in capillary gas chromatography

M-41S materials anchored to glass capillary columns via sol–gel techniques can be advantageously utilised for gas chromatographic separations.

Pillared layered structuresvs. zeolites as sorbents andcatalysts

The pore structures of several alumina-pillared clays (PILCs) have been investigatedvia conversions of ethylbenzene (EB) to diethylbenzenes (DEB) and to styrene (Styr) under Lewis acid conditions. PILCs (including those containing Fe in the pillar) give higher EB→DEB conversion, with a comparable selectivity towards p-DEB to that of ZSM-5s (SiO 2 /AlO 2 =35, 235) and other mid-pore zeolites. Detailed comparisons of catalytic properties suggest the PILCs contain specific pore exit channels, or dimensions similar to those found in ZSM-5 zeolites, rather than a random supermesh structure. They differ from mid-pore zeolites in acting more as acidic reaction vessels (conversely the shape-selectivity characteristic of ZSM-5s depends on pore access–exit modes). This implies that the alumina pillar controls sorbate molecular orientation during this bimolecular reaction. Zeolites do not give EB→Styr conversion, and the PILCs themselves are active only at >450 °C, and then non-selectively. They become much more active and highly selective (over a wide temperature range: 100–600 °C) only after cation exchange. Beidellitic-PILCs are more active and selective than montmorillonite-PILCs and those containing mixed Fe 2+ Fe 3+ /Al 3+ pillars (e.g. FAZA, derived from a Greek bentonite) more than those with unsubstituted alumina pillars. Conversion yields for FAZA lie in the order: Ni 2+ >Co 2+ >Cu 2+ , with Ni 2+ -FAZA the most unfavoured as regards disproportionation (2.5% at 450 °C). Although Cu 2+ -FAZA gives low conversions of styrene (<12% at 500 °C) it shows the highest selectivity (100%). The high activity and selectivity for K + -FAZA suggest that KFeO 2 is formed at the alumina pillar, as suggested for commercial α-Fe 2 O 3 catalysts. Possible reaction mechanisms and active sites are proposed.

Conversions of resins and asphaltenes in porous catalysts

Abstract Resins and asphaltenes can be converted to alkenes and other products in gismondine-structured zeolites, pillared clays (PILCs) and large-pore zeotypes. Different product spectra: alkanes, alkenes, oxygenates, etc. are obtained with different catalysts. In particular, mesoporous zeotypes i.e. MCM(Fe)-41 containing framework iron gives a total bitumen conversion of >15%, opening routes to possibly useful products.

Decomposition of nitrous oxide on pillared clays

Alumina-pillared smectites have been found to abate nitrous oxide in the presence of methane. The results indicate that the yield of the reaction (N20 --> N2 + (1/2)O2) increases when pillared clays are exchanged with transition metals, single-pass conversion rates of >70% being attainable. In particular, when double exchanged (calcium and subsequently copper) alumina pillared montmorillonite/beidellite is used as a catalyst, de-N2O activity reaches a maximum, which is maintained even after 4 h of work at a space velocity of 5.5 h(-1). A mechanism for the reaction is suggested, which implies that N2O is first adsorbed by the catalyst and then decomposes through two different paths: catalyst oxidation and catalyst reduction. Such a redox process explains the kinetic data.

Radiation induced competitive chlorination

The γ-radiolysis of solutions of benzene and naphthalene in carbon tetrachloride has been studied to obtain information about chlorination reactions. The α position of naphthalene showed a reactivity 2.5 higher with respect to benzene, while the absence of 2-chloronaphthalene among the products confirmed the relatively high activation energy required for the attack in β position.