Aromatization

Abstract: ConspectusThe substitution of expensive and potentially toxic noble-metal catalysts by cheap, abundant, environmentally benign, and less toxic metals is highly desirable and in line with green chemistry guidelines.We have recently discovered a new type of metal–ligand cooperation, which is based on the reversible dearomatization/aromatization of different heteroaromatic ligand cores caused by deprotonation/protonation of the ligand. More specifically, we have studied complexes of various transition metals (Ru, Fe, Co, Rh, Ir, Ni, Pd, Pt, and Re) bearing pyridine- and bipyridine-based PNP and PNN pincer ligands, which have slightly acidic methylene protons. In addition, we have discovered long-range metal–ligand cooperation in acridine-based pincer ligands, where the cooperation takes place at the electrophilic C-9 position of the acridine moiety leading to dearomatization of its middle ring. This type of metal–ligand cooperation was used for the activation of chemical bonds, including H–H, C–H (sp2 and sp...

Abstract: Recent developments in natural gas production technology have led to lower prices for methane and renewed interest in converting methane to higher value products Processes such as those based on syngas from methane reforming are being investigated Another option is methane aromatization, which produces benzene and hydrogen: 6CH4(g) → C6H6(g) + 9H2(g) ΔGor = +433 kJ mol−1 ΔHor = +531 kJ mol−1 Thermodynamic calculations for this reaction show that benzene formation is insignificant below ∼600 °C, and that the formation of solid carbon [C(s)] is thermodynamically favored at temperatures above ∼300 °C Benzene formation is insignificant at all temperatures up to 1000 °C when C(s) is included in the calculation of equilibrium composition Interestingly, the thermodynamic limitation on benzene formation can be minimized by the addition of alkanes/alkenes to the methane feed By far the most widely studied catalysts for this reaction are Mo/HZSM-5 and Mo/MCM-22 Benzene selectivities are generally between 60 and 80% at methane conversions of ∼10%, corresponding to net benzene yields of less than 10% Major byproducts include lower molecular weight hydrocarbons and higher molecular weight substituted aromatics However, carbon formation is inevitable, but the experimental findings show this can be kinetically limited by the use of H2 or oxidants in the feed, including CO2 or steam A number of reactor configurations involving regeneration of the carbon-containing catalyst have been developed with the goal of minimizing the cost of regeneration of the catalyst once deactivated by carbon deposition In this tutorial review we discuss the thermodynamics of this process, the catalysts used and the potential reactor configurations that can be applied

Abstract: Acylation of aromatic compounds alkylations, dealkylations, transalkylations, disproportionations halogenations nitrations miscellaneous substitution reactions addition reactions elimination reactions oxidation, dehydrogenation reactions aromatization hydrogenation, hydrogenolysis cyclization reactions Diels-Alder reactions isomerization dimerization, oxidative dimerization rearrangements condensation reactions thermal and hydrolytic decompositions reactions of carbonyl compounds reactions of carboxylic acids and derivatives amino acid and peptide formation under prebiotic conditions miscellaneous reactions.

Abstract: On MFI catalysts, low cost liquefied petroleum gas (LPG ) can be transformed into valuable aromatics (mainly C 6 -C 8 benzenics) and into hydrogen. Unfortunately methane and ethane are also produced in significant amounts. A reduction of the production of these unwanted compounds would render the aromatization process economically more attractive. The reaction pathways of propane aromatization were established on H-ZSM-5 pure or loaded with platinum or with gallium. On H-ZSM-5 the first step is the dehydrogenation and the cracking of the reactant through carbonium ion intermediates. The resulting alkenes (propene and ethylene) undergo rapid successive reactions via carbenium ion intermediates: oligomerization, cyclization, hydrogen transfer. The selectivity to aromatics is limited because of the formation of methane by propane cracking and of alkanes by hydrogen transfer. Platinum increases the rate of propane transformation significantly but a higher production of methane and ethane is found on PtH-ZSM-5 catalysts, owing to the hydrogenolysis of alkanes and of alkylaromatics and to the hydrogenation of ethylene on the platinum sites. Gallium improves both the rate and the selectivity of propane aromatization. The aromatization occurs, like on PtH-ZSM-5, through a bifunctional pathway, gallium catalyzing the dehydrogenation of the alkane reactant to alkenes and of naphthenic intermediates to aromatics, and the acid sites catalyzing the oligomerization of light alkenes and the cyclization of C 6 -C 8 alkenes. The better selectivity to aromatics is obtained for GaMFI catalysts with gallium species well dispersed within the zeolite and being very active for dehydrogenation (e.g. with gallosilicates or galloaluminosilicates steamed under mild conditions). At high temperature, in presence of hydrogen (hence during aromatization) GaMFI catalysts would undergo various modifications: reduction of gallium species, migration within the zeolite, reaction of these species with the protonic sites. The nature of the gallium active species and their role in the various steps are discussed.