Methacrylic acid, isobutane oxidation

THE OXIDATION OF ISOBUTANE TO METHACRYLIC ACID AN ALTERNATIVE TECHNOLOGY FOR MMA PRODUCTION... 

Figure 14.2 shows the simplified flow sheet of the process, as reported in patents issued to Sumitomo. CO2 is maintained in the recycle loop to act as a ballast component the desired concentration of CO2 is obtained by combustion of CO, while excess CO2 is separated. Methacrolein is separated and recycled to the oxidation reactor. An overall recycle yield of 52% to methacrylic acid is reported, with a recycle conversion of 96% and a per-pass isobutane conversion of 10%. The heat of reaction produced, mainly deriving from the combustion reaction, is recovered as steam. 

Figure 14.2 Simplified flow sheet for isobutane oxidation to methacrylic acid, as stated by Sumitomo.

Figure 14.3 Different strategies for integration of isobutane (oxi)dehydrogenation to isobutene and isobutene oxidation to methacrolein and to methacrylic acid.

TABLE 14.2 Summary of Results Reported in the Scientific and Patent Literature for the Oxidation of Isobutane to Methacrolein and Methacrylic Acid Catalyzed by Keggin-Type POMs... 

The reaction network for isobutane selective oxidation catalyzed by POMs consists of parallel reactions for the formation of methacrolein, methacrylic acid, carbon monoxide, and carbon dioxide. Consecutive reactions occur on methacrolein, which is transformed to acetic acid, methacrylic acid, and carbon oxides. ° Methacrylic acid undergoes consecutive reactions of combustion to carbon oxides and acetic acid, but only under conditions of high isobutane conversion. 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. ... 

Figure 14.4 Mechanism of the oxidation of isobutane to methacrylic acid catalyzed by Keggin-type POMs.

After a steady catalytic behavior was reached, the catalyst was treated in air at 350°C, in order to reoxidize it. Thereafter, the reaction was run again under isobutane-rich conditions (Figure 14.5), in order to understand the role of the POM reduction level on catalytic performance. The reoxidized catalyst exhibited a selectivity to methacrylic acid that was initially around 20%, and approximately 20-30 hours were necessary to recover the original performance of the equilibrated, reduced catalyst. On the contrary, the activity of the catalyst was almost the same as before the oxidizing treatment. This confirms that a partially reduced POM is intrinsically more selective to methacrylic acid than a fully oxidized one, and that one reason for the progressive increase in selectivity to methacrylic acid that occurs during the equilibration period was the increase in the POM reduction level, as a consequence of the operation under isobutane-rich conditions. 

This chapter describes some aspects of the reactivity of POMs as catalysts for the selective oxidation of isobutane to methacrylic acid. If developed at the industrial level, this reaction could represent an alternative to the current production method via the ACH route. P/Mo Keggin-type POMs are active and selective catalysts for this reaction. 

The catalytic performance depends a great deal on the reaction conditions, and specifically on the isobutane-to-oxygen ratio in the feed. Usually isobutane-rich conditions are claimed to be more selective, and the reason for this is that under these conditions the operative POM is a partially reduced one, and a more reduced POM is intrinsically more selective than a fully oxidized one. High isobutane partial pressures help to improve the selectivity, avoiding further oxidation of methacrylic acid. 

Heteropoly catalysts have significant activities for the oxidation of isobutane into methacrolein and methacrylic acid. The yield increased up to 6% by vanadium substitution or salt formation, as follows. With Cs2.5Ni0.08H0.34+JrPV,Mo12 - O40, the highest conversion and selectivity were observed at x 1 (355). Increases in the reaction temperature to 613 K led to increased yields, up to 9.0%. A similar increase in the yield resulted from the substitution of As for P as a heteroatom or from the addition of various transition metals (106, 356). 

It is interesting to note that reduced heteropoly compounds show higher selectivity towards methacrylic acid than the non-reduced ones in the oxidation of isobutane. Mizuno and coworkers have also reported the oxidation of isobutane under oxygen-deficient conditions [67]. Ueda and coworkers have studied reduced 12-molybdophosphoric acid for the oxidation of propane [79]. This highly reduced... 

A second example concerns the use of alkanes instead of alkenes to both use alternative feedstocks and reduce the environmental impact. An interesting example is the oxidation of isobutane to methacrylic acid [331]. 

The aim of this review is to describe the reactivity of three catalytic systems whieh have been widely studied in recent years for the oxidative tran.sformation of light paraffins i) vanadyl pyrophosphate, which is the industrial catalyst for the oxidation of /i-butane, but has also been elaimed to be selective in the oxidation of n-pentane to maleie and phthalic anhydrides (18-22), ii) heteropolyeompounds, whieh are currently being studied for the oxidation of isobutane and propane to the corresponding unsaturated acids (methacrylic acid and acrylic acid) (5,23-29), and whose composition can be tuned to change the acidic and oxidizing properties and iii) rutile-based mixed oxides, which can act as the matrix to host various metal components, and whieh have been claimed as optimal eatalysts for the ammoxidation of propane to acrylonitrile (15,30-33). 

By optimizing the quantity and type of constituent elements of heteropolyanions and counter cations, fairly good yields were obtained for the oxidation of isobutane [13 -17]. Recently, it was found that acidic cesium salts of Keggin-type heteropolymolybdates can efficiently catalyze the oxidation of isobutane to methacrylic acid with molecular oxygen. The optimal contents of Cs and V were 2.5 and 1, respectively, and the addition of Ni enhanced the cataljd ic activity even further will be discussed below [13 -16]. 

It is interesting that the reduced heteropoly compounds showed higher selectivity to methacrylic acid for the oxidation of isobutane [10, 13,... 

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. 

The heteropolymolybdate catalysts, H3PMoi2O40, (NH4)3PMoi2O4 0 and H3PMol2O40(Py) were tested for the partial oxidation of isobutane to methacrylic acid. The... 

The effects of contact time on the isobutane oxidation over the H3PMol2O40(Py) catalyst are again shown in Figure 6. With the increase of the contact time, the conversion of isobutane increases greatly, and the selectivity of methacrolein decreases markedly to disappear. The change of the selectivity to methacrylic acid exhibits a mountain shape, and the selectivities to acetic acid and acrylic acid still increased slightly after the top of the mountain. It is, therefore, clear that methacrolein is the intermediate to form methacrylic acid. 

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