Search

The benzene methylation with methane over zeolite catalysts was previously shown in our laboratory to require the presence of oxygen. Thus, a two-step mechanism involving the intermediate formation of methanol by partial oxidation of methane followed by the methylation of benzene with methanol in the second step, was postulated. This paper now reports the results of the characterisation of the zeolite catalysts used for the oxidative benzene methylation reaction in order to provide some information about their composition, structure, properties and their behaviour before and after the reaction. The catalysts were characterised by X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray fluorescence (XRF), FT-IR and solid state NMR. XRD results indicate that the crystalline structures of all the ZSM-5 and H-beta catalysts remained unchanged after batch reaction of benzene with methane over the catalysts in agreement with the observation that the catalysts recovered from the reactor could be reused without loss of activity. Elemental analyses and FT-IR data show that as the level of metal ion exchange increases, the Brönsted acid concentration decreases but this metal ion exchange does not totally remove Brönsted acidity. FT-IR results further show that only a small amount of acid sites is actually necessary for a catalyst to be active since used catalysts containing highly reduced Brönsted acidity are found to be reusable without any loss of their activity. 29Si and 27Al magic angle spinning (MAS) NMR together with FT-IR spectra also show that all the active zeolites catalysts contain some extra-framework octahedral aluminium in addition to the normal tetrahedral framework aluminium. The presence of this extra-lattice aluminium does not, however, have any adverse effect on the crystallinity of the catalysts both before and after oxidative benzene methylation reaction. There appears also to be no significant dealumination of the zeolite catalysts during reaction since their catalytic performance was retained after use.

The experimental conditions which control the exchange from deuterium or tritium gas into the C-1 proton position of glucose in aqueous solution have been studied in detail, with a view to determining the factors which maximize exchange into glucose, but minimize exchange into the solvent water. The favoured conditions for producing glucose with close to 100% isotope labelling in the C-1 position, and with negligible formation of labelled byproducts, were a reaction time of 6 to 8 hours, a temperature of 60°C, and a pH of 7 or higher. Pd/BaSO 4 was the preferred catalyst and slow pre-addition of the deuterium or tritium gas to the catalyst bed was essential to maximize the subsequent isotope exchange into the substrate.

A range of organic substrates was successfully exchanged with tritium gas using NaBH4-reduced palladium oxide or chloride as catalyst. Exchange conditions involved a reaction temperature of 100 °C, a reaction time of 3 days, and products were analyzed by radio-gas chromatography and 3H NMR spectroscopy. Both aromatic and aliphatic compounds underwent exchange, 18 of the 22 substrates incorporated > 35% of the available tritium, and most products showed high radiochemical purity. 3H NMR analyses showed that both alkanes and the alkyl chains of alkylbenzenes were rapidly and evenly labelled. Some aromatic compounds showed general ring exchange while others showed high specificity. These results for bulk palladium catalyst are strikingly different from those obtained with bulk platinum and supported palladium catalysts. The results are discussed in terms of currently accepted mechanisms of heterogeneous metal catalyzed hydrogen isotope exchange reactions.

Platinum-loaded microporous aluminophosphate molecular sieves (Pt-AlPO4-5) were synthesized and studied as catalysts for promoting tritium isotope labelling by exchange between tritium gas and aromatic, heteroaromatic and aliphatic hydrocarbons. The presence of the Pt metal considerably enhanced the activation of the tritium gas. Detailed isotope orientation patterns, as determined by 3H nmr spectroscopy, showed that the AlPO4-5 component of the catalyst dominated the exchange reactions, but some influence of the Pt metal was apparent with some substrates.

A synthesis of [3H]R- and [3H]S-4-amino-3-phenylbutanoic acid ([3H]R- and [3H]S-β-phenylGABA) from the unsaturated intermediate by catalytic hydrogenation using tritium gas in the presence of palladium on charcoal is reported. The [3H]R- and [3H]S-diastereoisomers of pantolactone ester of β-phenylGABA were separated by HPLC.

A synthesis of [3H]E- and [3H]Z-4-aminobut-2-enoic acids from methyl 4-N-phthalimidobut-2-ynoate by catalytic hydrogenation using tritium gas in the presence of a homogeneous catalyst, tris(triphenylphosphine)rhodium(I) chloride, is reported. HPLC separation of the E- and Z-isomers, and the saturated analogue, 4-aminobutanoic acid (GABA), is also described.

Microporous AlPO4-5 catalysts exhibit catalytic activity for tritium isotope exchange and other reactions but the activity of the catalyst may vary widely from one batch of catalyst to another although all catalyst samples appear to show pure AlPO4-5 phase. This catalytic activity appears to correlate with the presence of octahedral aluminium in the catalyst samples as revealed by solid sate 27Al NMR spectroscopy and this is attributed to defect sites in the lattice. The exchange patterns may show high regional specificity. These patterns have some of the features of acid catalysis but are assumed to be modified by the constraints imposed by the micropore geometry.

A wide range of simple organic substrates has been successfully tritium labelled using reduced platinum oxide as catalyst. Exchange conditions involved reaction temperatures in the range 60–150°C, and reaction times of 3–7 days. The isotope sources for the reactions were tritium gas and tritiated water. Analysis of the labelled products was accomplished by radio‐glc and by H NMR spectroscopy. The specific activities ranged up to 1100 mCi/mL, and most of the labelled products showed high radiochemical purity. The regioselectivity of tritium incorporation, as obtained from H NMR studies, supports currently accepted theories for mechanisms of platinum‐catalyzed hydrogen isotope exchange. The results also show remarkable consistency for a heterogeneous system under widely varying reaction conditions.

Levamisole (LEV) is a widely used anti-parasitic agent. In order to characterise the biochemical pharmacology of LEV in parasitic nematodes, [3H]LEV and a more active analogue [3H]m-aminolevamisole (MAL) have been prepared. Labelling was accomplished by tritiated water exchange of MAL under acid conditions. Multiple site labelling was achieved in the positions ortho and para to the amino group of MAL. Tritiation of MAL HCl in [3H]2O was achieved at 103°C for 23 h. Crude [3H]MAL was diazotised (hydrochloric acid and sodium nitrite) and deaminated (hypophosphorous acid) to effect the synthesis of [3H]LEV. Products of both reactions were purified by preparative h.p.l.c. and characterised by h.p.l.c., t.l.c. and mass spectrometry. (Radiochemical yield was about 15% and purity >90%.) Specific activities of 39 Ci/mmol for [3H]MAL and 37 Ci/mmol for [3H]LEV were obtained.

The tritiation of a range of low molecular weight alkanes and alkenes has been investigated by self-radiation induced exchange with HTO at room temperature and 115°. By comparison with the Wilzbach T2 gas exposure procedure, the HTO method yields less byproducts and radiochemical purification of the parent compound is facilitated. With alkenes, saturation of the double bond is a competing process in the HTO procedure, however the significance of this side reaction decreases with increasing temperature of exchange. The HTO technique is shown to be suitable for tritiating a wide range of hydrocarbons to medium levels of specific activity.

Deuterated analogues of various phenolic steroids, catechols and indole derivatives were prepared in high chemical yield by heating the relevant compound in D2O at 190°C for 24 hours.

A method for the synthesis and purification of mebendazole specifically labelled with tritium in the 4′ position is described.

Boron tribromide has been assessed as a catalyst for rapid tritium labeling of organic compounds using tritiated water under mild conditions. Alkyl and halogen substituted aromatics labeled rapidly, the labeling being most pronounced in the ortho and para positions. Compounds containing electrophilic deactivating groups were found to be difficult to label. Very little byproduct formation was observed.

Adaptation du second detecteur d'ionisation de flamme d'un chromatographe gazeux a deux detecteurs, pour en faire une chambre d'ionisation

A practical synthesis of [14-3H]dibenz[a,j]acridine and [7-3H]dibenz[a,h]acridine is reported from the parent hydrocarbon by catalytic exchange with tritium gas in the presence of palladium on calcium carbonate. Analogous regiospecific labelling with deuterium was also affected. In addition [14-14C]dibenz[a,j]acridine was prepared from [14C]paraformaldehyde.

Benzimidazole carbamates (BZCs) act as inhibitors of the tubulin-microtubule equilibria in eukaryotic organisms. Recently drug resistance to this class of compounds in helminth parasites has been shown to be due to a reduced ability of resistant tubulin to bind BZCs. In order to quantitate the nature of the tubulin-BZC interaction a general method for the specific tritium labelling of BZCs has been developed. The BZCs: mebendazole, oxfendazole, parbendazole, oxibendazole, albendazole and fenbendazole were labelled by catalytic exchange using palladium on calcium carbonate in pure dioxane at 60°C under tritium gas. The position of label incorporation for tritiated albendazole was determined by tritium-NMR as the 4-position of benzimidazole nucleus. The yields for individual BZCs varied from 8 to 68% for a range of specific activity of 0.44 to 13.4 Ci/mmole.

Chemical shifts for tritons in chlorobenzene have been measured in the pure compound and in a dilute solution in cyclohexane using a 64 MHz NMR spectrometer. The order of shifts is o>m>p. Triton shifts in pure bromobenzene, fluorobenzene and toluene have also been measured and the influence of CCl4 solvent on the aromatic triton shifts in toluene explored. Complete resolution of the three aromatic triton positions is achieved in pure toluene, but the ortho and para shifts are not resolved in 25% solution in CCl4.

Chemical shifts for aromatic tritons have been varying by up to 0.1 ppm when measured in different laboratories. This is attributable to solvent effects. For truly identical solvents this error is shown to reduce to about 0.002 ppm (less than the digitization error). New chemical shift values for aromatic tritons in a series of alkylbenzenes are reported.