Theoretical aspects investigation of the structure of phosphormolybdenum heteropoly acid catalysts for the oxidation of methacrolein to methacrylic acid

The present study investigates of the structure of phosphormolybdenum heteropoly acid catalysts for the oxidation of methacrolein to methacrylic acid. As a result of the studies of the reaction of oxidative transformation of unsaturated C₃-C₄- aldehydes into the corresponding acids, the composition of samples with empirical formula was established: Cs_0,2-1 W_0,1-1 Cu_0,1-0,5 Cr_0,07-0,4 Zn_2-4 P₁ Mo₁₂ V_0,1-0,5 (NH₄)_1-4 O_x  

The selected sample for a detailed study of the structure is as follows:

Cs_0,7 W_0,2 Cu_0,1 Cr_0,4 Zn ₃ P₁ Mo₁₂ V_0,3 (NH₄)_3-4 O _45,5   The first sample was prepared from ammonium salts of P, Mo, and V, and the second was based on phosphormolybdenum heteropoly acid. The same evaporation, drying, and calcination conditions were followed in all cases. The calcined aluminum oxide was used as a reference substance.      As the results showed, regardless of the catalyst preparation method, increasing temperature up to 430°С affects its properties. These changes lead to irreversible activity loss and selectivity by 5-6 times.    Presented X-ray pictures of samples that have worked for 75 hours in methacrolein oxidation mode indicate that the properties of the catalysts are unchanged. Diffractometers show that the catalyst samples are not amorphous but crystalline. The paper gives IR spectra of the catalyst samples before and after they reached stable activity. Data for the catalysts modified with cesium compared with the original heteropoly acid are presented. It is indicated that the position and relative intensity of several characteristic reflexes on the X-ray diffraction pattern change. The low catalytic activity of the samples revealed is caused by the presence on the surface of ammonia formed during the synthesis of the catalyst heating removes ammonia. The catalyst activity increases more than threefold, and the selectivity does not change. Heating of freshly prepared catalyst removes crystallization water (~170 °C). A series of experiments were conducted to determine the catalyst's thermal stability limit. Increasing of temperature to 430 °С does not affect its properties. It should be noted that the catalyst decomposition temperature is higher than that of phosphormolybdenum acid (370°C, 50° higher). This is achieved by modifying it with various additives. Thanks to the conducted research, it is possible to change the catalyst structures to improve their properties.