High-pressure CO conversion

Arc-discharge deposition Laser vaporization deposition High-pressure CO conversion (HiPCO)... 

High-pressure CO conversion (HiPCO) is a new method for the bulk production of SWCNTs under high-pressure, high-temperature flowing CO on catalytic clusters of Fe. Fe catalyst is formed in situ by thermal decomposition of iron pentacarbonyl (i.e., Fe(CO)j) which is delivered intact within a cold CO flow and then rapidly mixed with hot CO in the reaction zone. Upon heating, the FefCO) decomposes into atoms that condense into larger clusters. SWCNTs nucleate and grow on these particles in the gas phase via CO disproportionation CO-i-CO (catalyzed) CO -i-C(SWCNT) [14,15],... 

Lefebvre et al. (170) have conducted the high pressure CO + H2 reaction (30 atm, 503-523 K) over Rh-NaY catalysts. Whatever the rhodium precursors [e.g., Rh -NaY and Rh (CO)2-NaY], the reaction data were similar. This is in agreement with the fact that all the precursors were ultimately converted to Rh6(CO),6 under catalytic conditions. The external Rh crystals deposited on the zeolite surface exhibit significant activity for hydrocarbons, mainly methane, whereas the carbonyl clusters gave lower conversion to hydrocarbons with a small amount of oxygenates such as methanol and ethanol. 

High pressure hydrothermal conversion of Spirulina was studied with iron as catalyst (Matsui et al., 1997). It showed that the bio-oil yield increased linearly from 54.4 to 63.7 wt. % with increasing amount of Fe(CO)5-S from 0 to 1 mmol. The conversion and gas yield were nearly constant. In a similar study, brown macroalga Laminaria saccharina was hydrothermally liquefied to bio-crude in a batch reactor (Anastasakis and Ross, 2011). A maximum bio-crude yield of 19.3 wt. % was obtained with a biomass to water ratio of 1 10 at 350°C and 15 min of residence time. The solid residue contained large proportion of calcium and magnesium, whereas the liquid phase was rich in sugars, ammonium, potassium and sodium. 

Because the synthesis reactions are exothermic with a net decrease in molar volume, equiUbrium conversions of the carbon oxides to methanol by reactions 1 and 2 are favored by high pressure and low temperature, as shown for the indicated reformed natural gas composition in Figure 1. The mechanism of methanol synthesis on the copper—zinc—alumina catalyst was elucidated as recentiy as 1990 (7). For a pure H2—CO mixture, carbon monoxide is adsorbed on the copper surface where it is hydrogenated to methanol. When CO2 is added to the reacting mixture, the copper surface becomes partially covered by adsorbed oxygen by the reaction C02 CO + O (ads). This results in a change in mechanism where CO reacts with the adsorbed oxygen to form CO2, which becomes the primary source of carbon for methanol. 

At a pressure of 30 bar and with excess steam the fractional conversion of methane in the reformer is reasonably satisfactory. The high pressure of 30 bar will favour the removal of carbon dioxide, following the shift reaction CO + H2O CO2 + H2, and reduce the cost of compressing the purified hydrogen to a value, typically in the range 50-200 bar, required for ammonia synthesis. 

A typical example for a reaction with substantial contraction of volume is the synthesis of methanol from syngas. Formally, 1 mol CO and 2 mol of H2 react to form 1 mol of methanol. This means that at high degrees of conversion, the contraction in volume can be a factor of three. This has dramatic implications for the pressure as the gas volume drops, the total pressure also drops. As a surplus, the analytical evaluation of the reaction is also complicated owing to the change in volume as a function of the degree of conversion. 

As part of ongoing research into the behavior of (vinylcarbene)iron complexes,119120 Mitsudo and Watanabe found that the trifluoromethyl-substituted vinylcarbene 174 exhibited a reactivity different from that of both 166 and 169.107 Upon treatment of the complex 174 with triphenylphos-phine the vinylketene complex 175 is formed, a reaction identical to that seen in the series of vinylcarbene complexes 166 (R = H). However, when the vinylcarbene 174 is exposed to a high pressure of carbon monoxide, it is converted cleanly to the ferracyclopentenone 176. Remember that when the vinylcarbene complex 166 (R = H) was treated in the same manner, conversion stopped at the vinylketene complex 167 Even when exposed to a pressure of 80 atmospheres of CO(g), no further reaction was seen to occur. An electron donating ligand (L = PR3) is required for conversion to cyclopentenone structure 168. Conversely, when the more electron-rich vinylcarbene 169 is exposed to carbon monoxide in the same manner, the pyrone complex 172 is formed. 

Concerning the operation of catalysts under adiabatic conditions, Basini et al. [156] reported the results of methane partial oxidation runs in a pilot-scale reactor operating at high pressure and short contact times, showing stable activity (almost complete conversion of methane and over 90% selectivity to CO and H2) during more than 500 h on-stream. In addition, operability for 20 000 h bench-scale testing has been claimed recently by the same group [157]. 

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