Butanal, from crotonaldehyde

This conjugated enone/diene difference is more definitively seen in the 184 kJ mol-1 decrease in enthalpy of formation on going from 1-pentene to butanal, in contrast to the 177 kJ mol-1 going from (E)-l,3-pentadiene to irans -crotonaldehyde. For further discussion, see J. F. Liebman and R. M. Pollack, in The Chemistry ofEnones (Eds. S. Patai and Z. Rappoport), Wiley, Chichester,... 

In a study of the conversion of w-butane to maleic anhydride, Bissot and Benson 157) noted that cis-2-butene reacted faster and more selectively than n-butane, in agreement with data of Table XIV. Using a cobalt molybdate catalyst at 450-475°, they obtained 92 % conversion of cis-2-C4Hg and 36 % selectivity to maleic anhydride, for a C X Sj 100 (or yield) value of 32%. From n-butane the best yield was 15%, but this could be raised to 25% by a two-stage operation. Rather similar results for oxidation of 2-butenes were reported by Skinner and Tieszen 158), who used a Mo/V/P = 9/3/1 catalyst at about 350° and obtained a yield of 28.3 %. Other products were acetaldehyde, formaldehyde, acrolein, crotonaldehyde, formic acid, and acetic acid. A water spray to quench products leaving the catalyst bed was used for best results. 

On the other hand, yields to MA similar to those from -butane can be reached from processes using butenes, but the number of by-products obtained is also higher than from the -butane process. Indeed, the formation of small amounts of furan, acetaldehyde, crotonaldehyde, or methyl-vinyl-ketone are observed from n-butene, which makes the process more expensive due to the need for a purification step. During the selective oxidation of -butane (a cheaper raw material) these products are not formed, but the main by-products are CO and CO2. Indeed, both parallel and consecutive reactions of total combustion must be taken into account in the process from n-butane since they are mainly responsible for the limited yield to MA. In this way, -butane conversion lower than 75-80% has been reported to be adequate in order to avoid further oxidation of MA, an important factor in the decrease of the process selectivity. 

Sampson and Shooter, from their study on vanadium oxide/phos-phorus pentoxide catalyzed oxidation of n-butane, suggest that there are two potential routes of MA one via crotonaldehyde and the other via butadiene. Furan has also been suggested as an intermediate in the sequence. 

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