Heteropolyacid dehydration

Furfural is obtained industrially (200000 t a-1) by dehydration of pentoses produced from hemicelluloses. Furfurylic alcohol is obtained by selective hydrogenation of the C=0 bond of furfural, avoiding the hydrogenation of the furan ring. Liquid phase hydrogenation at 80 °C in ethanol on Raney nickel modified by heteropolyacid salts resulted in a 98% yield of furfuryl alcohol [31]. 

Scheme 3 Acrolein can be obtained by dehydration of glycerol. The reaction was reported many years ago using powdered KHSO4/K2SO4 as catalyst. Recently, the use of silica-supported heteropolyacids has also been described, notably with silicotungstic acid as catalyst.

Scheme 5 Xylose can be dehydrated to produce furfural. The reaction has been reported using several different catalysts including zeolites, sulfonic acid functionalized MCM-41 and immobilized heteropolyacids. The best selectivity towards furfural was achieved using zeolite H-mordenite, although at low conversion of xylose.Overall, the best yield of furfural was obtained using sulfonic acid functionalized MCM-41.

In catalytic dehydration of alcohols, pseudoliquid phase behavior (bulk type I reaction) of heteropolyacids has been demonstrated (Section VI). The high catalytic activity is associated with this behavior and the strong acidity. Unique pressure dependences of the catalytic activity and selectivity are found for H3PW1204o due to the pseudoliquid phase (Fig. 40). 

Varisli, D., Dogu, T. and Dogu, G. 2007. Ethylene and Diethyl-Ether Production by Dehydration Reaction of Ethanol Over Different Heteropolyacid Catalysts. Chem. Eng. Sci., 62, 5349-5352. 

Acid Form - Pseudoliquid Phase Behavior. Owing to a high affinity for polar molecules, large quantities of molecules such as alcohols and ether are absorbed within the bulk phase of crystalline heteropolyacids. The amounts of pyridine, methanol, and 2-propanol absorbed correspond to 50-100 times that which can be adsorbed on the surface, while nonpolar molecules like ethylene and benzene are adsorbed at the surface only. Catalytic reactions of polar molecules occiu both on the surface and in the bulk, so that the solid heteropolyacid behaves as a highly concentrated solution, called a pseudoliquid phase . The dehydration of alcohols, various conversions of methanol and dimethyl ether to hydrocarbons in gas-solid systems, and the alkylation of phenol and pinacol rearrangements can all occur in the pseudoliquid. The transient response using isotopically labeled 2-propanol provides evidence for the pseudoliquid phase behavior of H3PW12O40. This behavior influences the selectivity, for example, the aUcene/aUcane ratio, in the conversion of dimethyl ether. 

The behaviour of Dawson and Keggin-type heteropolycids has been studied over surface-type and bulk-type reactions. Both heteropolyacids have shown similar activity for all the reactions investigated, with the biggest differences found for t-BuOH dehydration and isobutene polymerisation. It is possible for t-BuOH dehydration that the pseudo-liquid phase is responsible for the higher peformance of the Dawson, as suggested by Misono for MTBE synthesis. However this does not explain why the Dawson, which is the most selective catalyst at low temperature for 1-BuOH and 2-BuOH dehydration, suddenly becomes the less selective for f-BuOH dehydration. 

Isopropanol was used as a probe molecule to characterize the acidity of heteropolyacid compounds because the product s distribution upon reaction depends on the nature of the surface active sites. Strong Brdnsted (H ) and Lewis acid sites catalyze the dehydration of isopropanol to propylene (di-isopropyl ether over weak Lewis acid sites), and redox/basic sites lead to the dehydrogenation of the alcohol to acetone. 

The chemisorption of isopropanol at 313 K leads to the coverage of the heteropolyacids with a stable monolayer of adsorbed isopropoxy species and avoids further surface reaction. These adsorbed intermediate-reactive alkoxy species further react and desorb as propylene (or other product depending on the nature of the site) upon controlled heating during the TPSR experiment. Therefore, the quantification of the desorbed product is proportional to the total number of acid sites active on isopropanol dehydration towards propylene. 

By inversion of the catalytic ethylene hydrolysis (Chapter 4.2) fuel-grade anhydrous bioethanol is rather easily dehydrated to ethylene at elevated temperature using, for example, a silicoaluminophosphate, HZSM-5 zeolite, or a heteropolyacid catalyst in a fixed bed or fluidized bed reactor (Figure 4A.25)." " ... 

For the best conversion of lactic acid into AA, Katryniok et al. (2010) used silica-supported heteropolyacids (HPAs) as catalysts at 275°C in a fixed-bed gas phase reactor. More improvement of lactic acid (91%) conversion was observed when using the highly acidic silicotungstic acid. The proposed mechanism for conversion was decarbonylation/dehydration. However, by using less acidic molybdenum-based HPAs, propanoic acid was the dominant product, pointing to a more pronoxmced decarboxylation pathway producing H2 (and CO2). The resulting H2 can be used in situ to reduce lactic acid to propanoic acid (Katryniok et al., 2010). 

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