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The Oxidation of Isobutane to Methacrylic Acid: An Alternative Technology for MMA Production

“…Key points that limit the industrialization of the process were recently illustrated by researchers from Sumitomo. 38 Since the selectivity to methacrylic acid plus methacrolein typically decreases with temperature as the conversion increases, this implies that the rate of production of useful products increases only until the higher conversion compensates for the fall of selectivity. As a consequence of this, the maximum productivity value is reached at a specified temperature.…”

“…For instance, when a selectivity of 45% is reached at 22% isobutane conversion, with a residence time of 5.4 s, a temperature of 3708C, and a feed containing 25% isobutane, 25% oxygen, and 15% steam, a productivity equal to 0.72 mmol/h/g cat is obtained, which is one order of magnitude lower than the one needed to make the process industrially viable. 38 However, the productivity is limited by the oxygen conversion, the maximum concentration of which is dictated by the flammability limits (see Figure 14.1), and by temperature, since the POM decomposes above 3808C.…”

“…Possible solutions to overcome this problem are: 38 (1) decrease the residence time; the decrease of conversion is more than compensated by an increase of selectivity (due to the lower extent of methacrylic acid combustion), and in overall the productivity increases; (2) increase the total pressure, while simultaneously increasing both the oxygen and the isobutane partial pressure, as well as the total gas flow (so as to keep a constant contact time in the reactor). A higher pressure also implies smaller reactor volume, and hence lower investment costs.…”

“…Under these circumstances, productivity as high as 6.4 mmol/h/g cat was reached, which is acceptable for industrial production. 38 The additional heat required for the recirculation of unconverted isobutane and for increased pressure would be equalized by the higher heat generated by the reaction.…”

“…30,31 Methacrylic acid undergoes consecutive reactions of combustion to carbon oxides and acetic acid, but only under conditions of high isobutane conversion. 38,39 Isobutene is believed to be an intermediate of isobutane transformation to methacrylic acid, but it can be isolated as a reaction product only for very low alkane conversion. 39 From a kinetic point of view, the formation of methacrolein and methacrylic acid occurs through two parallel reactions.…”