SYNTHESIS AND CHARACTERIZATION OF Mo-V-Nb-Te-O M1 CATALYSTS
Date
2017-05
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Publisher
University of Delaware
Abstract
Mo-V-Nb-Te-O mixed metal oxides are the most efficient catalysts for new alkane-fed processes leading to some important C2 and C3 compounds in the chemical industry, such as ethylene, acrylic acid and acrylonitrile [1]. The purpose of this research is to study factors influencing the morphology of the orthorhombic M1 phase catalyst. Control of the crystallite dimensions of the desired catalyst phase, M1, requires control of composition and impurity phase content. The M1 phase was synthesized by mixing ammonium vanadate, ammonium heptamolybdate, telluric acid, and ammonium niobium oxalate in sequence, similar to a procedure used earlier by PhD student Xin Li [2,3]. The M1 phase was observed for calcination temperatures between 575°C and 625°C and for several starting compositions. The lower V/Mo ratios, longer calcination times, or close proximity to the limits of calcination temperature produced higher-purity crystalline M1 phase, as long as all conditions fall within reasonable ranges. Higher calcination temperatures resulted in increased crystal size along the c-axis. One of our goals is to obtain crystallites with small dimension along c-axis, but large dimensions perpendicular to c-axis, since the catalytically active surface is oriented perpendicular to c-axis. For this reason, reducing the c/ab_average ratio is a priority. Under the same element ratio, pH, and argon calcination time, 550°C argon calcination is found to be near optimal to approach the goal. For temperatures below 500°C, for the calcination temperature, the Mo-V-Nb-Te-O mixed metal oxides did not crystalize very well. The pentagonal ring building-blocks found in the M1 phase do not fully self-assemble if the calcination temperature is too low or if there is insufficient time for growth; they exist only as disordered clusters [4]. These disordered products are much less active than the well-ordered
orthorhombic M1 phase. Increasing the calcination time up to 96h for this sample did not result in a well-ordered orthorhombic M1 structure. Without adjusting the precursor solution pH, all samples prepared to date contain the desired M1 phase along with another phase, known as M2, as pseudo-hexagonal tungsten bronze-type (HTB-type) impurity. The M1 phase cannot be crystallized from precursor solutions with pH above about 3.5. For precursor solutions with pH < 2, the product becomes highly sintered after calcination and likely does not contain the M1 phase. The sample could not be ground into powder for XRD analysis, so the phase content has not yet been characterized. It appears that the M1 phase is only stable within the pH range 2.8-3.5.
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Keywords
CATALYSTS, Chemical Engineering