Partial Oxidation of C2-C4 Alkanes

Partial Oxidation of C2-C4 Alkanes 24.3.1. Partial oxidation of n-butane 

Maleic anhydride is currently used in the production of unsaturated polyesters and butanediol. World consumption over the last few years has exceeded 1.3 x 10 metric tons per year. Currently, ca. 70% of MA is produced from -butane partial oxidation. In this way, partial oxidation of -butane to MA is the only 

Replacement of the latter by -butane considerably reduces the number of byproducts, particularly in comparison to the heavy by-products obtained (i.e. phthahc anhydride and benzoquinone) from the benzene process. Moreover, the -butane process avoids the toxicity associated with the employment of benzene as the raw material (carcinogen) and becomes important in saving raw material costs (around 64%). It is also important to emphasize that the -butane process drastically decreases the formation of carbon oxides (atomic economy). Thus, whereas benzene has six carbon atoms, MA has four, and as a result, two CO2 molecules for each MA molecule are obtained. Accordingly, under optimal conditions, 100 kg of benzene generate 129 kg of maleic anhydride and 113 kg of CO2, whereas 100 kg of -butane generate 170 kg of maleic anhydride. 

Currently, the industrial single-step process from n-butane is carried out with an active phase based on vanadyl pyrophosphate, (VO)2P207, commonly named as a VPO catalyst. Yields to MA of around 60% obtained from alkane are lower than those from benzene (near 73%), but important cost savings on the final product (higher than 40%) are achieved through the n-butane process.  

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. 

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PARTIAL OXIDATION OF C2-C4 ALKANES INTO OXYGENATES BY ACTIVE OXYGEN GENERATED ELECTROCHEMICALLY ON GOLD THROUGH YTTRIA-STABILIZED ZIRCONIA... 

Figure 24.5. Selectivity to the main partial oxidation products achieved during the partial oxidation of C2-C4 alkanes over MoVTeNbO (a) and VPO (b) catalysts. AA=Acrylic acid MA = maleic anhydride MTA = methacrolein. From Ref. 170.

Recently we found that highly redueed H3PMol2O40 which was formed by the heat-treatment of pyridinium salt can catalyze the propane oxidation to acrylic acid and acetic acid selectively [24, 25]. After activation in N2 flow at 420°C for 2hr, the catalyst of HsPMo 12040 (Py) shows reduced state of molybdenum and a new stable structure in which pyridine remains as the linkage of the secondary structure. The activated H3PMoi2O40 (Py) also gives catalytic activities in the partial oxidations of ethane and isobutane to acetic acid and methacrylic acid respectively. In this paper, we will report the oxidation results of C2-C4 alkanes and discuss the roles of reduced state and aetivation of molecular oxygen over this catalyst. 

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