טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentHayek Naseem
SubjectOxidative Coupling of Methane to Ethylene over
Mn-Na2Wo4/Sio2 Catalyst
DepartmentDepartment of Chemical Engineering
Supervisor Professor Oz Gazit
Full Thesis textFull thesis text - English Version


Abstract

Oxidative coupling of methane (OCM) is one of the most attractive routes where methane, the main component of natural gas, can be converted to other valuable materials. In this reaction, CH4 and O2 mixture react in a catalyst bed at > 700 °C to produce ethane and ethylene (C2), which are vital building blocks in the chemical industry. Unfortunately, it is still industrially inapplicable due to the lack of active and stable catalyst that can overcome the over-oxidation of methane to COx.

The MnOx-Na2WO4/SiO2 has attracted much attention for possible practical applications due to its good catalytic performance. However, despite more than two decades of research, much is still ambiguous regarding the nature of the active site and the factors that make this catalyst very selective. Moreover, its catalytic performance needs to be further promoted to gain industrial feasibility.

This research was performed to study the structure of the MnOx-Na2WO4/SiO2 catalyst, to reveal its surface critical parameters, and to promote its catalytic performance by further additives. To do so, the research was conducted in three steps: I) studying the dispersion of MnOx on SiO2 support, II) studying the effect of MnOx-Na2WO4/SiO2 surface parameters on its OCM catalytic performance, and III) evaluating the effect of different additives on the catalytic performance of the MnOx-Na2WO4/SiO2 catalyst.

In the first part, Mn(NO3)2 and Mn(Ac)2 were impregnated on dealuminated β-zeolite and compared. Because of the dealumination process, defect sites in the zeolite lattice were formed. The results showed that when the Mn(Ac)2 precursor was used for impregnation, the obtained MnOx species after calcination were highly dispersed and incorporated inside the lattice of the zeolite, inside and outside the pores. However, when the Mn(NO3)2 precursor was used, only large particles of poorly dispersed MnO2 were formed on the external surface area of the zeolite.

In the second part, six 2%Mn-5%Na2WO4/SiO2 were synthesized having the same chemical composition, but different surface properties. The different properties were achieved by using both Mn(Ac)2 and Mn(NO3)2 precursors, and three different silica types. Following the impregnation and calcination, all silica types were transformed into α-cristobalite. The catalysts were evaluated in OCM and important surface parameters were revealed. MnOx dispersion, open-porous surface morphology, and small Na2WO4 crystallite size, all were shown to increase the catalytic activity of the MnOx-Na2WO4/SiO2 catalyst. For the first time, the inverse linear correlation was found between the Na2WO4 crystallite size and the CH4 conversion.

In the last part, different additives (Ti, Sn, Ce, Fe, Nb and Ge) were studied as promoters for the MnOx-Na2WO4/SiO2 catalyst. The results proved that only Fe, Nb, and Ge promoted catalytic performance. However, time on stream analysis showed that the promotion is only for the first few hours of the reaction, and all catalysts deactivate with time. The results led to a significantly important criterion in catalyst evaluation, implying that catalysts should be evaluated with time on stream under partial oxygen consumption to have justified conclusions. Moreover, different mechanisms of catalyst deactivation were studied.