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Experimental and computational evidence points to unimolecular transformation of terminal alkynes on the title Rh(I) metal fragments. Lack of isotopic scrambling in double-crossover experiments is inconsistent with a previously proposed bimolecular pathway. Focusing on a unimolecular manifold, alkyne binding to the metal forms the Rh(I) alkyne π-complex 2, which isomerizes to the Rh(III) hydrido-(alkynyl) species 4, ultimately leading to Rh(I) vinylidene product 5. In making alkyne-free precursors, use of heterocyclic ligand (i-Pr) 2 PIm′ (1b, Im′ ) 1-methyl-4-tert-butylimidazol-2-yl) led to species 8 with a labile P,N chelate, whereas a geometrically similar o-tolyl ligand suffered metalation at the methyl group and was unsuitable for alkyne transformation studies. Kinetic studies comparing 1b and (i-Pr) 2 PPh (1c) allowed determination of rate constants for the alkyne binding event and conversion of 2 to 5 (the latter, k 2-5 , being 9.6 times faster for 1b). Based on a scan of the two-dimensional reaction surface, combined density functional/molecular mechanics calculations predict that η 2 -(C,H) alkyne complex 3 is in a fast equilibrium with the lower energy hydrido(alkynyl) complex 4, and neither species is expected to be present at observable concentrations. Eyring model estimates of the rate constants from these computational data predict the available experimental values in this work to within a factor of 2 and the ratio of the rate constants k 2-5 for 1b and 1c to within 10%. The calculations also agree with the qualitative observation that reaction rates are faster for both ligands 1b and 1c than for (i-Pr) 3 P and predict that reactions using triphenylphosphine will be faster than those with (i-Pr) 3 P. NMR coupling constants, particularly 1 J CC values, were used to evaluate bonding and back-bonding in isotopomers of 2a-c and 5a-c derived from H 13 C 13 CH.

Coal pyrolysis is generally performed in atmospheres without oxygen and at temperatures below 650 °C for producing pyrolysis liquid and gas. To support the development of a new two-stage gasification process integrating a fluidized-bed pyrolyzer and a downdraft fixed-bed gasifier, this paper investigated the coal pyrolysis in atmospheres containing oxygen and steam and at temperatures up to 900 °C to understand the viable pyrolysis conditions in view of process adaptation. The examined pyrolysis characteristics include the product distribution, char gasification reactivity, and tar composition. The pyrolysis gas yield, especially the yields of H2 and CO, increased with elevating the temperature and mass ratio of steam/coal (S/C). Adding O2 to the reaction atmosphere promoted the formation of CO and CO2 but decreased that of H2. The inclusion of O2 and steam in the atmosphere resulted in chars with a larger surface area and more micropores. At 900 °C in N2 atmosphere or at 850 °C with oxygen in the atmosphere (e.g., at an excessive air ratio of 0.22), graphitization was observed in the produced char, which lowered the char gasification reactivity. Analyzing the produced tar via thermogravimetry coupled with Fourier transform infrared (TG−FTIR) spectroscopy clarified that the presence of steam significantly affected the tar composition by leading to more aliphatic hydrocarbons and lowering the contents of single-ring aromatics, phenols, and ketonic species. Thus, the pyrolysis gas product from a steam-containing reaction atmosphere would be easier to crack and be reformed in the downstream char gasifier.