Labelled ethene

Data obtained from collision-induced dissociation experiments did not allow for a distinction of the isomeric metal-silene and-silylene species however, structure-specific ion-molecule reactions of the complexes with labeled ethene were used to clearly differentiate between the metal silene and the silylene. In this intriguing study, Jacobson and coworkers also bracketed the bond dissociation energies of the isomeric ions. 

The Hj/CO ratio in the synthesis gas was varied from 1 1 to 2 1 to 4 1. This resulted in 36, 72, and 99 percent conversion for CO and 95, 99.9, and 99.9 percent conversion of C-labeled ethene. The proportion of labeled ethene converted to methane increased from 2.7 to 5.1 to 8.8 percent with increasing H2/CO ratio. The olefinic portion of the hydrocarbon products decreased with increasing H2/CO ratio. For ethene, the amount of the labeled compound that was hydrogenated to labeled ethane was 66.6, 70.6, and 71.7 percent, respectively. The amount of in C3 products was 20 to 30 percent for the three experiments. This incorporation of label into C3. products may occur by three processes chain initiation, propagation, or termination. 

Another unique feature of C-labeled ethene incorporation into even carbon number products is illustrated in Figure 29. The n-paraffins (the data shown correspond to the total n-products since the hydrocarbon products were hydrogenated prior to analysis) have a higher activity than the odd carbon number compounds. This was taken to indicate that " C-Iabeled ethene undergoes oligomerization thus, the extent that the activity of an even carbon number product exceed that of the next carbon number compound is a relative measure of the amount of oligomerization. 

These results with C-labeled ethene are very informative however, one must note that the conversion of the labeled ethene is quite high. Thus, 95 percent or greater of the added ethene is converted to products. This provides great opportunity for a variety of secondary reactions to mask the initial reactions of ethene. 

Results were also obtained for the conversion of syngas containing C-labeled eth-ene or propene using a precipitated promoted iron catalyst. In addition, a fused iron catalyst was employed in a run with labeled ethene at 20 atm pressure. They found that the cracking reaction of ethene was of secondary importance with the iron catalyst, unlike the case with cobalt. The distribution of the synthesis products from C-ethene showed that about 50 percent of the transformation was to the C3 product the transformation to higher hydrocarbons decreased much quicker than for the cobalt normal pressure synthesis (Figure 33). With the addition of C-ethene the iso-paraffins had a lower activity than the normal paraffins this is consistent with the data for cobalt (Figure 34). 

If this is so, it follows that CO reacts to form propene by direct addition to a C-labeled ethene molecule that was added to the syngas. 

Percy and Walter converted a mixture of doubly labeled ethene ( CH2 CH2 2 mol percent), CO (24.5 mol percent) and H2 (73.5 mol percent). The propene in the effluent from runs with low (3 to 4 percent) conversions was trapped and analyzed with NMR to determine the distribution of C at each carbon position in the propene. It was possible to quantify the amount of each of the eight isotopomers that can result from various combinations of C and C that were present. Singly labeled propenes were formed from the doubly labeled ethene this required that some ethene dissociates to form C fragments. No evidence was obtained that would indicate that two Cj units, one from CO and one from a C2 unit, recombine to give C2H4 in the product. Thus, ethene is incorporated in part as C2 units and in part by dissociation to Cj units. Incorporation of single Cj units from ethene is equally probable at the three carbon positions of the propene that is eventually formed. The authors state that they see no contradiction between any of their data and the widely accepted carbide mechanism for FTS. 

C-Labeled ethene incorporation during synthesis with a Co catalyst at 195 °C produces hydrocarbons that contain a constant molar radioactivity (Figure 37). The added ethene underwent hydrogenation to ethane to the extent of 50 percent and 50 percent was converted to higher hydrocarbons. Eidus reports that comparing the published data indicates that ethene takes part in the synthesis with a cobalt catalyst to a greater extent than ethanol does under the same reaction conditions he proposed that the ethanol is... 

C-Labeled propene incorporated to produce hydrocarbons with a constant molar radioactivity just as was observed with labeled ethene. However, propene initiated the formation of products (one of 12 molecules) to a smaller extent than ethene (one of four to five molecules). 

A first example is C/H-labelled ethene which for H-labelling has the configuration-counting series ... 

Apart from labelled ethene,[ ° l FT synthesis was carried out in the presence of other labelled compounds such as ethanol,larger alkenesi and higher alcohols.Most results indicate that more than one mechanism is responsible for the distribution of radioactivity in the products. Figure illustrates that polymerisation of labelled ethene on Co produced more label in even C-number alkanes up to Cio- Almost constant radioactivities were observed in the >Ce products with added propanol. Smaller products showed more incorporated and the monomethyl-alkanes contained more radioactivity. The authors concluded that these alkenes participate both in chain initiation and chain propagation. Alcohols, in turn, initiated chain growth but did not participate in chain propagation on an industrial Fe catalyst.Neither ethene nor ethanol (or ethene formed by its dehydration) participated in the chain termination step. ... 

Table indicates that only a fraction of the C label from ethene was incorporated into chain growth products on Co and Fe catalysts. About the same amount of C (31%) was found in FT products formed with 1[ C]-1-propene, but only 18% when l-[ C]-l-hexadecene was applied.I About 50% of radioactive ethene gave methane, and 50% chain growth products on a Co catalyst, I the specific activity of higher products being practically constant between C4 and C32. Less than 10% of C from labelled ethene was incorporated into C10-C17 products, but the incorporation from C-ethanol was 60-80 times higher on a fused iron catalyst.The difference... 

Ethylidyne (8) has been recognised on Pt/SiOa at 300 K using the SEDOR NMR technique applied to heavily C-labelled ethene the C—C bond length was 149 pm. This seemed to occur on large platinum particles, where areas of (111) face are most likely it was also seen by SIMS on platinum black, but on small particles vinylidene (17) predominated. Similar SEDOR experiments with ethyne showed 75% vinylidene and 25% ethyne as 12A or 15. " Adsorbed benzene was shown to rotate freely at 300 K, and cyclopropane was adsorbed, but not strongly, i.e. without loss of hydrogen. ... 

---