Butenes formation

The remarkable NEMCA behavior of the isomerization reaction is shown in Fig. 9.31. At potentials negative with respect to the open circuit potential ( 0.38V) the rates of cis- and tram-2-butene formation start to increase dramatically. At a cell voltage of 0.16 to 0.10V the observed maximum p values are 38 and 46, respectively. The absolute A values are approximately equal to 28, as computed from the ratio Ar/(I/F) (with I/F presenting the rate of proton supply to the catalyst). The system thus exhibits a strong non-faradaic electrophilic behavior. 

The addition of a spillover proton to an adsorbed alkene to yield a secondary carbonium ion followed by abstraction of a proton from the C3 carbon would yield both isomers of 2-butene. The estimated faradaic efficiencies show that each electromigrated proton causes up to 28 molecules of butene to undergo isomerization. This catalytic step is for intermediate potentials much faster than the consumption of the proton by the electrochemical reduction of butene to butane. However, the reduction of butene to butane becomes significant at lower potentials, i.e., less than 0.1V, with a concomitant inhibition of the isomerization process, as manifest in Fig. 9.31 by the appearance of the maxima of the cis- and tram-butene formation rates. 

Extrapolation of the rate data in Fig. 24 to zero conversion shows that the initial ratio of butene-1 to frans-butene formation is about unity. Thus, butene-1 is not an intermediate in the cis-trans isomerization and direct cis-trans isomerization occurs. Similar results are found for the heterogeneous base catalyzed isomerization over sodium on alumina (17). 

The intermetallic compounds CePd3 and ZrPd3 exhibited higher selectivity for butene formation than Pd. On Pd the hydrogen and butadiene are adsorbed on similar sites, whereas on the intermetallic compounds different sites may be involved in these adsorption processes44. 

Practical Applications. IFP s Alphabutol process is used to dimerize ethylene selectively to 1-butene.43,85 The significance of this technology is the use of 1-butene as a comonomer in the polymerization of ethylene to produce linear low-density polyethylene (see Section 13.2.6). Under the reaction conditions applied in industry (50-60°C, 22-27 atm), the selectivity of 1-butene formation is higher than 90% at the conversion of 80-85%. Since no metal hydride is involved in this system, isomerization does not take place and only a small amount of higher-molecular-weight terminal alkenes is formed. 

For instance, the activation energy for butene formation from n-butyl alcohol is 140 lOkJmol-1 on HZSM-5 and only 95 lOkJmol-1 on AAS. At 378 K, 94% ether plus 6% butene are formed over HZSM-5, whereas 43% ether and 57% butene are formed over AAS. Bearing in mind that butyl alcohol molecules, as well as those of intermediates and products of their dehydration, have dimensions closely similar to the diameter of the zeolite channels, we infer that a liquid-like packing of butyl alcohol molecules and other reaction participants occurs in the channels (as schematized in Fig. 5). We opine that some specific ordering of the adsorbed species in the catalyst channels may be induced by hydrogen bonding and hydrophobic interactions between them. 

Transient kinetic phenomena of another type were observed in the so-called purging experiments, whereby we switched from feeding the flow reactor with a helium-butyl alcohol mixture to one with pure helium and then back to the previous helium-butyl alcohol. A typical response of a catalyst to such purging is given in Fig. 6, referring to the dehydration of sec- and isobutyl alcohols over HZSM-5. For sec-butyl alcohol, the rate of butene formation initially increases by a factor of about 10 upon purging and then drops to zero. Return (8k) to the... 

That such conversions are indeed reversible under steady state conditions follows from the fact that the steady-state rates of butene formation over both HZSM-5 and AAS catalysts remain the same when n-butyl alcohol is substituted for di-n-butyl ether in the flow that feeds the microreactor (8j). 

Thermolysis of these complexes at 9°C produced ethylene, cyclobutane, and butenes. The ratio of the gaseous products was found to be a function of the coordination number of the complex, or intermediate. Thus three coordinate complexes favoured butene formation, while four coordinate complexes favoured reductive elimination to form cyclobutane, and five coordinate complexes produced ethylene as shown in Scheme 25.83... 

A recent report by Mayr of slow polymerization of PhC=CH by (PMe3)2Cl2(PhC=CPh)W=CHPh fulfills expectations based on the classic Chauvin mechanism for olefin metathesis (78). The presence of a carbene and a vacant coordination site are prerequisites for metallocyclo-butene formation with free alkyne. Mayr has both the carbene and the alkyne initially present in the catalyst, but there is no evidence for direct involvement of the cis alkyne in the actual polymerization mechanism. 

With such strong base catalysts, carbanions can be formed even from simple olefins. Evidence for carbanionic intermediates is found in butene isomerization. Starting from 1-butene, the rate of czs-2-butene formation is twenty times higher than the rate of frans-2-butene formation (42) ... 

It should also be mentioned that Allen and Pitts (1) studied the CH3-radical-sensitized decomposition of trans-crotonaldehvde to show that the 2-butene formation observed in the earlier photolysis studies is due to methyl-radical displacement of the formyl group. 

An electronic effect was also used to explain the difference in 1,3-butadiene hydrogenation selectivity observed over various types of nickel catalysts such as Ni(B), Raney nickel, nickel powder from the decomposition of nickel formate, Ni(P), and Ni(S). As discussed in Chapter 12, chemical shifts in XPS binding energies (Aq) for the various nickel species were compared with that of the decomposed nickel catalyst to determine the extent of 1-butene formation as related to the electron density on the metal. The higher the electron density, the more 1-butene formation was favored. 

It is reasonable to assume that butene formation starts with a palladium hydride. This inserts ethylene twice (the first insertion is probably reversible) and then terminates by / -hydride elimination to regenerate the palladium hydride and form butene. Thus, butene formation shows that olefin insertion (in a Pd-alkyl bond) and y6-elimination are intrinsically rapid reactions. However, the copolymer produced in the same experiment shows neither double olefin insertion errors nor the unsaturated end-groups indicative of -elimination. 

Tetrahydrothiophen was also detected during thiophen hds over a commercial C0-M0/AI2O3 catalyst (523-586 K, 1 atm). The rate of disappearance of tetrahydrothiophen was twice that of thiophen, but the rate of butene formation from each compound was about the same. Therefore, the greater rate of disappearance of tetrahydrothiophen was attributed to dehydrogenation to thiophen [reverse step (/)] rather than to more facile desulphurization. 

In the experiment shown in Fig. 47, it is A = 74000 and p = 26, that is, the rate of C2H4 oxidation increases by a factor of 25 and the increase in the rate of O consumption is 74000 times larger than the rate, I/2F, of O2- supply to the catalyst. In the experiment shown in Fig. 48, the maximum p values for the production of cis-2-butene, trans-2-butene, and butene are of the order of 50 and the corresponding maximum A values are of the order of 40 for cis-2-butene formation, 10 for trans-2-butene formation, and less than 1 for butene formation. Thus, each proton supplied to the Pd catalyst can cause the isomerization of up to 40 1-butene molecules to cis-2-butene and up to 10 1-butene molecules to trans-2-butene, whereas the hydrogenation of 1-butene to butane is electrocatalytic, that is, Faradaic. 

When the catalyst was exposed to i-butanol at 500°C, the coke formation was relatively fast. 1.5 wt% coke was deposited during 10 minutes. The formation of olefins from methanol was lower for this precoked sample. This is probably due to n-butene formation, originating from i-butene isomerization at 500°C. n-Butene probably forms inactive coke in the pores or at the pore openings, which results in significant deactivation. 

The selectivity of palladium and gold for alkene oxidation to aldehydes 28,29,170) was attributed initially to adsorption strength. However, electrooxidation in the presence of palladium ions indicates possible homogeneous alkene insertion, similar to the Wacker process 304). Homogeneous reaction is also involved in redox oxidations of hydrocarbons. In this case, the nature of the metal ions is expected to control selectivity. Indeed, toluene yields 20% benzaldehyde in electrolytes containing Ce salts, while oxidation proceeds to benzoic acid with Cr redox catalysts 311). In addition, the concentration of redox catalysts appears to affect yields in nonelectrochemical oxidation of ethylene large amounts of palladium chloride promote butene formation at the expense of acetaldehyde 312). Finally, the role of the electrolyte and solvent should not be ignored. For instance, electrooxidation of ethylene on carbon, in aqueous solution of acetic acid yields acetaldehyde 313) in the... 

The product distributions will be considered under three headings (1) the selectivity for olefin formation, (2) the transjcis ratio in the 2-butene, and (3) the stereoselectivity for 1-butene formation. 

Oxidation of butane has been performed on rare earth oxides at 400°C to 500°C [41]. Ceria had the highest activity, the better selectivity in CO2 production and the lowest one in butenes formation. A nice correlation was found between CO2 yield and the rare earth fourth ionisation potential I4 the lower the I4 value the higher the activity. According to the authors the value is in connection with the rate limiting step. Similar results were previously observed with butane oxidation also at 550°C, [42]. All these findings were summarised in a short review published by Pomonis [43]. 

Catalysts based on transition metal molybdates, typically bismuth, cobalt and nickel molybdates [2-6], have received recent attention. Of the transition metal molybdates, those based on nickel, and in particular the stoichiometric NiMo04, have attracted the greatest interest. NiMo04 presents two polymorphic phases at atmospheric pressure a low temperature a phase, and a high temperature P phase [2,7]. Both phases are monoclinic with space group dim. These phases differ primarily in the coordination of molybdenum which is distorted octahedral in the a phase and distorted tetrahedral in the P phase. The P phase has been shown to be almost twice more selective in propene formation than the a phase for comparable conversion at the same temp>erature [2]. A similar effect has been noted for oxidative dehydrogenation of butane, with the P phase being approximately three times more selective in butene formation than the a phase [8]. The reason for the difference in selectivities is unknown, but the properties of the phases are known to be dependent on the precursors from which they are derived. Typically, nickel molybdates are prepared by calcination of precipitated precursors. 

Of course, this high activation energy means that dehydrogenation reactions are less favoured than the side -decomposition reactions (Dean, 1985 Weissman and Benson, 1984). This is why butene formation is limited to less than 2-3 wt%. 

For example, some proposed mechanisms have the chain making the vinyl end first, by abstraction of a vinylic H from ethylene that is later added back to yield the terminating methyl group [319]. However, this idea seems to be at odds with many of the chain transfer responses, and has been reported to be inconsistent with some features of the kinetics, including H2 response and 1-butene formation [329]. 

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