Formation of Carboxylic Acids from Small Alkanes in Zeolite H-ZSM-5

Abstract

The activation of propane and isobutane in acidic zeolite H-ZSM-5 in the presence of both CO and H2O has been studied by in situ solid-state NMR and GC analysis. Evidence was provided for the conversion of propane to isobutyric acid at 373-473 K by cleavage of the C-C bond; methane and ethane are also produced. Isobutane is transformed into pivalic acid with simultaneous production of hydrogen. The low conversion (1-2%) at this temperature was rationalized by the existence of a small number of sites that are capable of generating carbenium ions which are trapped by CO at this temperature. A formate species was observed when CO and H2O were present on H-ZSM-5. This species disappeared in the presence of the alkane. At 573 K, the generation of large amounts of CO2 indicates a much higher conversion of the alkanes into carboxylic acids which, however, decompose under the reaction conditions.

Propane carbonylation on sulfated zirconia catalyst as studied by 13C MAS NMR and FTIR spectroscopy

“…The distribution of the observed reaction products, different from that formed from propane in the presence of solid Brønsted acid catalysts [11] and superacidic media [8,9,37], indicates that the activation of propane by SZ is performed primarily toward the secondary C-H bond of the alkane rather than its C-C bond. This can be realized by either the Lewis acid sites of SZ or by direct formylation by the formyl cation formed as equilibrated species from the formate.…”

“…Note that if propane with a 13 C-labeled methylene group is used for the reaction, the signals from the labeled CH 2 group of the initial C 3 H 8 , as well as from the carbons in the products where the label migrates in the course of the reaction, should be observed in the spectra. Two signals at 45.5 and 38.2 ppm become clearly visible in the 13 C CP/MAS NMR spectrum, besides the signal from the CH 2 group of unreacted [2-13 C]C 3 H 8 at 18.0 ppm [11], when the sample with coadsorbed [2-13 C] C 3 H 8 and unlabeled CO was heated at 100 • C (Fig. 1b).…”

“…1a) in the range of the spectrum typical for the carbonyl groups [18]. The signal at 232 ppm belongs to the carbonyl moiety of aldehyde [7], while the signal at 195 ppm points to the formation of a carboxylic acid [5,7,11].…”

“…Activation of propane in the presence of CO on H-ZSM-5 zeolite [11] proceeds in full analogy with the process in superacid media [8,9] via C-C bond cleavage to form methane and an ethyl cation. The ethyl cation abstracts a hydride ion from propane to form an isopropyl carbenium ion.…”

“…Propane carbonylation with CO has been reported to occur in liquid superacids [8,9], in the presence of strong Lewis acids (AlCl 3 ) [10] or acidic zeolite H-ZSM-5 [11], the reactions presuming the generation of intermediate carbenium ions and their trapping by CO. It has been established recently that SZ catalyzes propane conversion at 200-450 • C by a dimerization-isomerization-cracking process, i.e., via a classical carbenium-ion mechanism [12,13].…”