MoCo catalyst

Catalysts. MoCo-124 Alumina was impregnated first with... 

MoCo-122 and MoCo-123 were obtained by impregnation as carried out for MoCo-124. Only 2 and 3 wt% CoO resp. were brought on the catalyst. Final calcination temperature 650 C. 

CoMo-124 Alumina was impregnated first with cobalt stepwise. The sample was dried at 120°C and calcined at 650 °C after each impregnation step. Afterwards the catalyst was impregnated with molybdenum. Final calcination temperature 650°C. The composition was the same as for MoCo-124. 

Visible Reflection Spectra. The final calcination temperature of MoCo-124 samples has been varied in order to study its influence on the coordination of the cobalt ions. The reflection spectra are shown in Figure 1. The spectra of MoCo-124, calcined at 400 and 500°C show a broad absorption band, covering the whole spectral region, with a weak superposition of the characteristic triplet of cobalt aluminate. This indicates that the cobalt ions are for the greater part still on the catalyst surface and not in the alumina lattice. The spectra of the MoCo-124 samples, calcined at 650-700 °C show a strong increase in intensity of the triplet band, while the broad absorption band has disappeared. This indicates the formation of a cobalt aluminate phase. 

Spectra of adsorbed pyridine have been recorded for the MoCo-124 catalysts, for which the final calcination temperature after the cobalt impregnation has been varied. It turns out that the 400 and 500°C calcined samples and the 650 and 700°C calcined samples show very similar spectra. Therefore we show only the spectra of the 400°C (low calcined) and the 650°C (high calcined) samples. Figure 4 shows spectra after desorption at 150 and 250°C. Few Brdnsted acid sites are observed in the low calcined MoCo-124 samples. The reflection spectra (Figure 1) indicate for these low calcined samples the presence of cobalt on the catalyst surface, because no cobalt aluminate phase could be detected. The high calcined samples do show the presence of Brdnsted acid sites the presence of a cobalt aluminate phase is concluded from the reflection spectra (Figure 1) for these samples. 

However, the molybdenum-alumina and the high calcined cobalt-molybdenum-alumina samples still show an important difference. The pyridine spectra of MoCo-124 indicate a second Lewis acid site, characterized by the 1612 cm-1 band. This band differs from the weak Lewis acid sites of the alumina support (1614 cm- ) because the position is significantly different. It also appears that the strength of the bond between pyridine and the catalyst is stronger, for the 1612 cm-1 band is still present after evacuation at 250°C, while the weak Lewis band (1614 cm-1) of the alumina has disappeared at this desorption temperature. Obviously the second Lewis band for the MoCo-124 catalyst is introduced by the interaction of cobalt with the surface molybdate layer. This interaction is... 

Such an effect might be expected when boehmite supported cobalt is being calcined, viz. during the phase transition AIO(OH) - y-Al203. Figure 7 shows spectra of pyridine, adsorbed on the sample CoMo-124 B, which has been prepared in this way. Spectra for MoCo-122, -123 and -124, containing 2, 3 and 4 wt% CoO resp. are shown for comparison. All these catalysts have had a final calcination of 650°C. Comparison of the spectra of CoMo-124 B and MoCo-124 indicates that the intensity of the 1612 cm l band, which is introduced by the interaction of the cobalt ions and the molybdate layer, is lower for CoMo-124 B than for MoCo-124. The spectrum for CoMo-124 B resembles that of CoMo-123, indicating that a part of the cobalt ions does not participate in this interaction. 

The spectra of adsorbed pyridine for MoCo-153 and MoNi-153 are compared in Figure 9. Two final calcination temperatures have been applied, 480 and 650°C. The spectra of the 480°C samples (Figure 9a and 9b) are nearly identical. The BrtSnsted acid bands are weak, while the 1612 cm l Lewis bands are strong. The intensity of the Brdnsted acid bands increases for both 650°C calcined samples (Figure 9c and 9d). The Lewis acid bands show a marked difference now. The 1612 band remains high in intensity for the MoCo-153 catalyst, but this Lewis band decreases appreciably in intensity for the MoNi-153 catalyst. 

Some bulk cobalt aluminate formation is expected to take place for the boehmite based catalyst, owing to a Hedvall effect (24).The spectrum of adsorbed pyridine on CoMo-124 B shows indeed a weaker 1612 cm l band, comparable with the intensity of this band for the MoCo-123 catalyst. This indicates that about 25 % of the cobalt ions has disappeared in the bulk of the alumina. (Figure lid). 

Catalysts. MoCo-124 Alumina was Impregnated first with molybdenum, then calcined at 650 C, followed by the cobalt impregnation. The final calcination was varied from 400 - 700 C. Composition 12 wt% M0O3 and 4 wt7o CoO. 

Molybdenum is largely used in steel industry. Its compounds are widely used in coloring agents, solid lubricants and as catalysts. Molybdenum is an essential trace element and a component of xanthine oxidase. This enzyme catalyzes the formation of urate. Molybdenum cofactor (Moco)-defi-ciency is a lethal autosomal recessive disease. Moco-deficiency in humans can cause neurological damage, seizures and various brain dysmorphisms. 

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