IR spectroscopic signs of surface intermediates of partial oxidation of propane by nitrogen monoxide on Fe-ZSM-5

Abstract

UDC 541.128.1:543.422 L. M. Alekseenko, G. M. Tel'biz, and S. N. OrlikWe have used IR spectroscopy to study adsorption of NO, propane, in the temperature range 20-350°C. We show that it is possible to form adsorbed nitrate ions, C-, N-, and O-containing intermediates leading to formation of partial oxidation products: oxygenates and nitrogen-containing compounds.Activation of light alkanes with the aim of obtaining valuable products remains an important direction in oxidative catalysis. In recent years, some successes achieved in direct oxidation of light alkanes have been connected with use of nitrogen oxide additives NO x as homogeneous catalysts [1, 2] and the specific effect of NO on the selectivity of formation of C 1 oxygenates in heterogeneous catalytic oxidation of methane on V 2 O 5 /SiO 2 [3].It was shown previously [4] that iron-containing samples based on the zeolite ZSM-5 and the mesoporous silica MCM-41 catalyze partial oxidation of C 3 -C 4 alkanes by nitrogen oxides (NO, N 2 O) with formation of oxygen-containing products. On the zeolite Fe-ZSM-5 (SiO 2 /Al 2 O 3 = 68), C 2 -C 4 oxygenates are formed (with the following selectivity: ethanol 5.5; butanol 2.0; propanol 0.8; propionic acid 0.4; acetone 0.2). In a study of the products of the indicated reaction by mass spectrometry, compounds were identified containing carbon, oxygen, hydrogen, and trivalent nitrogen atoms (CH 3 CN, C 2 H 5 NH 2 , HOCH 2 CH 2 NH 2 ): amines and nitriles.With the aim of determining possible surface intermediates and routes for their conversion in oxidation of propane by nitrogen monoxide, in this work we studied co-adsorption of NO and C 3 H 8 on a sample of the catalyst Fe-ZSM-5 by IR spectroscopy.The iron-containing form of the zeolite Fe-ZMS-5 was obtained by ion exchange with repeated treatment of the sodium form of the zeolite with an acidified (HCl) aqueous solution of iron nitrate at room temperature, followed by washing to remove NO 3 − ions. The extent of exchange of sodium cations by iron cations was 92%.In order to carry out the spectral studies, the samples of the catalyst, in the form of thin transparent tablets of density 2-3 mg/cm 2 , were placed in a specially designed cuvet connected to a vacuum apparatus. The cuvet design made it possible to carry out thermal vacuum treatment of the catalysts and admit the reagents without changing the position of the sample in the cuvet. Before making the measurements, the sample of the catalyst was subjected to thermal vacuum treatment at 450°C and a pressure of 10 -4 mm Hg for 3 h. Then the temperature was lowered down to 20°C. Adsorption of NO, propane, and their mixture was accomplished by admitting measured volumes of adsorbate at temperatures of 20, 100, 150, 250, 275, 350°C. The 0040-5760/09/4502-0131