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.

We have previously shown in our previous investigations that the presence of oxygen is required for the benzene methylation with methane over zeolite catalysts in a high pressure batch reactor. Thus, a two-step mechanism involving the formation of methanol intermediate by methane partial oxidation followed by benzene methylation with methanol in the second-step was postulated. This paper now presents the results of further investigation that provide more evidence that the catalytic benzene methylation with methane does occur and that the reaction is limited by the amount of oxygen. Further support is therefore given to the two-step oxidative reaction mechanism. Suggestions for further work in this area are also given. (C) 2003 Elsevier B.V. All rights reserved.

The methanol to aromatics conversion is shown to contribute significantly to the benzene methylation with methanol at 400degreesC over acidic ZSM-5 catalysts but such contribution over all catalysts is negligible at 250degreesC. Benzene-only conversion shows only negligible contribution at both temperatures over the catalysts studied. However, it appears that the methanol-only conversion can be minimised, to enhance the benzene methylation with methanol (and indirectly the oxidative benzene methylation with methane), by using NaZSM-5 with highly reduced Bronsted acidity. (C) 2002 Elsevier Science B.V. All rights reserved.

The methylation of benzene with methane over ZSM-5 zeolite catalysts in a high pressure static reactor is shown to require oxygen as a reactant, implicating methanol as a key intermediate species. In the case of zeolite H-beta, methyl aromatics can be formed in the absence of oxygen, consistent with an earlier report that these products are formed from cracking of benzene over the acid zeolite.

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]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.

In the simultaneous irradiation procedure, sensitized W radiation is shown to be a complementary initiator to ionizing radiation for the grafting of monomers to polyolefins. Parameters influencing the yield of graft have been considered, including the role of sensitizer, solvent, and the type of monomer. These results indicate why difficulties have been experienced by previous authors in photografting monomers to polymers using analogous procedures. Grafting of styrene to cellulose acetate using ionizing radiation at low total doses is also reported. Difficulties associated with the poor grafting yield to gel beads as opposed to films are considered predominantly in terms of moisture content and swelling of the gel beads. The feasibility of using the polyolefins and cellulose acetate beads modified by the present grafting methods in reagent insolubilization reactions is discussed.

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.

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.

The effect of solvent structure, particularly the application of the low molecular weight aliphatic alcohols, on the radiation grafting of monomers to PVC with the use of cobalt-60 and the simultaneous procedure is discussed. The addition of sulfuric acid to a typical system such as styrene/methanol/PVC enhances both the grafting yield and the size of the Trommsdorff peak. The results are shown to be consistent with a mechanism involving charge-transfer complex formation between monomer and grafting site in PVC. The role of hydrogen atoms in these reactions, particularly in the presence of sulfuric acid, is emphasized, and the importance of the magnitude of G(H) on the grafting yields and the Trommsdorff effect is discussed. These principles, developed for radiation grafting styrene, are shown to be of value for the radiation copolymerization of p-nitrostyrene and vinyl phosphorus monomers to PVC. The use of these PVC copolymers for immobilizing enzymes such as trypsin and heterogenizing typical ...

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.

Additives which can increase the yield in the radiation grafting of monomers to polymers are useful. The use of polyfunctional monomers as additives (approx. = 1% v/v) are shown to enhance significantly the copolymerization yields of styrene in methanol to films of polyethylene and polypropylene under certain radiation conditions. The results obtained when the polyolefins are replaced by PVC as backbone polymer in these accelerated grafting reactions are reported. Divinylbenzene (DVB) and trimethylolpropane triacrylate (TMPTA) were used as representative polyfunctional monomers for the enhancement effect. When polypropylene was used as backbone polymer, the inclusion of DVB significantly enhanced the radiation grafting of styrene in methanol at all monomer concentrations studied above 35%. At certain monomer concentrations (50% , 60%), the yield of graft copolymer was almost doubled by the addition of DVB. 3 tables. (DP)

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.