Industrial Chemicals via C1 Processes - American Chemical Society

Chapter 13

Ammoxidation of Methanol to Hydrogen Cyanide Binary Oxide Catalysts and Mechanistic Aspects J. R. Ebner, J. T. Gleaves, T. C. Kuechler, and T. P. Li

Downloaded by TUFTS UNIV on November 24, 2015 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0328.ch013

Monsanto Chemical Company, St. Louis, MO 63167 Hydrogen cyanide can be obtained in high yields by the ammoxidation of methanol using silica supported FeMo oxide or MnP oxide catalysts. The yields of HCN observed in small fluid bed reactors approach 90%.

These catalysts have been studied by XRD, vibrational spectroscopy, TPR and TAP (Temporal Analysis of Products - a pulsed microreactor embedded in a vacuum chamber that allows direct monitoring of reaction intermediates and the microscopic reaction kinetics). The FeMo oxide catalyst is composed of a single phase,

Fe2(MoO4)3. Two Mn2+ phases were found to be present

in the MnP oxide catalyst, the pyrophosphate, Mn2P2O7, and the orthophosphate, Mn3(PO4)2. Methanol, ammonia, and hydrogen reduced the FeMo oxide catalyst at

temperatures below 300°C, whereas the MnP oxide system was found to be unreactive up to 500°C. In the presence of ammonia and methanol under anaerobic conditions, the FeMo oxide catalyst produced HCN, and the MnP oxide produced methylamine. The methylamine was found to be oxidized selectively to HCN in the presence of molecular oxygen. The Mars van Krevelen redox mechanism is operational only for the FeMo oxide system, and we suggest a surface activated oxo species is formed with the MnP oxide system.

Hydrogen cyanide is an important building block chemical for the synthesis of a variety of industrially important chemicals, such as 2-hydroxy-4-methylthiobutyric acid, adiponitrile, nitrilotriacetic acid, lactic acid, and methyl methacrylate. The primary commercial routes to hydrogen cyanide are the reaction of methane and ammonia under aerobic (Andrussow Process) or anaerobic conditions (Degussa

Process), or the separation of hydrogen cyanide as a by-product of the ammoxidation of propylene

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Ammoxidation of Methanol to Hydrogen Cyanide

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Maximum attainable HCN yield versus composition of

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allowed continual dosing of the surface with oxygen while pulsing methanol and ammonia, and only in this case is HCN formed (Figure 6) . These experiments point to the importance of gas phase oxygen with the MnP oxide catalyst.

The reducibility of the catalyst systems was further examined using temperature programmed reduction with a 3* hydrogen/argon gas

mixture.

The TPR curves shown in Figure 7 illustrate the MnP oxide

catalyst is not readily reduced at reaction temperatures.

In

contrast, the FeMo oxide catalyst begins to reduce at 250°C, and the rate of reduction is fast at temperatures of methanol ammoxidation

activity (425°-475°C) .

The poor lability of lattice oxygen for the

MnP oxide catalyst provides additional evidence for a non-redox Downloaded by TUFTS UNIV on November 24, 2015 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0328.ch013

process. Temperature Programmed Desorption Studies.

Methanol

adsorption/desorption studies of ferric molybdate are reported in the literature (26).

The salient features of these studies parallel

our findings for the silica supported FeMo catalyst.

These are as

follows: First, there are two desorption peaks, one at 98°C

attributed to adsorbed methanol and one at 275°C attributed to the decomposition of methoxy groups to formaldehyde (Figure 8). Second, the skewing of the methanol desorption peak on the high temperature side is attributed to the recombination of the methoxy groups with

available H from surface hydroxy groups to form methanol. This has been unambiguously established in the previous study (26.) using deuterated methanol.

Third, because of the oxidation of methanol to

formaldehyde, there is a net weight loss due to reduction of the catalyst, and exposure to oxygen restores the initial state of the catalyst. The total amount of oxygen removed resulted in a calculated surface oxide concentration of 9 x 1013 oxide sites/cm2. Analogous methanol adsorption/desorption experiments conducted with the supported manganese pyrophosphate catalyst gave the

following results: DA single desorption peak centered at 100°C is observed (Figure 8).

2) Mass spectral analysis indicates the

desorption product is methanol. 3) The final weight of the sample is unchanged from the initial weight, consistent with absence of

oxidation products during the desorption cycle. These results further support our hypothesis that the MnP oxide catalyst does not possess labile surface oxide species for oxidation chemistry. Ammonia adsorption/desorption experiments were also conducted over both catalyst systems in a microbalance system not interfaced

with a mass spectrometer. The desorption curves obtained after saturating the surface with ammonia and equilibrating in helium are shown in Figure 9. The weight loss spectrum of the FeMo oxide catalyst sample shows a high temperature shoulder and the MnP oxide catalyst does not. FTIR studies of the FeMo catalyst were conducted under an ammonia atmosphere. The strongly bound ammonia species seen in Figure 10 is identified as ammonium ion by the

characteristic ammonium bending mode at 1450 cm"1 (17) .

The

formation of ammonium ions on the surface by reaction of ammonia with surface hydroxyls is consistent with our FTIR results on the

catalyst in N2 which show the presence of surface hydroxyis up to 400°C.

The surface ammonium species disappears completely from the

surface by 350°C, and we believe this species accounts for the high temperature shoulder observed in the TPD experiment.

We found no



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SYSTEM TEMPERATURE (deg C) Figure 8.

Temperature Programmed Desorption of methanol from

the ferric molybdate (dashed line) and the manganese

pyrophosphate (solid line) catalysts determined gravimetrically. In Industrial Chemicals via C1 Processes; Fahey, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


13. EBNER ET AL.


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Figure 9. Temperature Programmed Desorption of ammonia from the ferric molybdenum (dashed line) and the manganese pyrophosphate (solid line) catalysts determined gravimetrically.


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spectroscopic evidence for the surface imido (=NH) group generally proposed in ammoxidation mechanisms with molybdate catalysts (28.) . As with methanol desorption, a net weight loss was observed for the

FeMo catalyst after ammonia desorption.

This was caused by

oxidation of the ammonia substrate to nitrogen and consequent catalyst reduction. The relative number of oxygen atoms removed was ca. 20* less than with methanol surface reduction. Conclusions

Reaction Mechanism.

Our current proposal for the mechanism of

ammoxidation of methanol to HCN over the two catalyst systems discussed is shown in Figure 11. The experimental data presented Downloaded by TUFTS UNIV on November 24, 2015 | http://pubs.acs.org Publication Date: December 16, 1987 | doi: 10.1021/bk-1987-0328.ch013

strongly indicates the ferric molybdate catalyst operates by a Mars Van Krevelen redox mechanism in the formation of HCN.

This

mechanism is not applicable in the case of the manganese phosphate catalyst.

Two possibilities exist: (1) a Rideal mechanism in which

an activated suface methylamine or methanol species reacts with gas phase oxygen, or (2) a mechanism involving formation of a surface activated molecular oxygen species capable of selective oxidation. Although we do not have sufficient evidence to distinguish these two possibilities, we prefer the latter. It is quite plausible that the

surface Mn2* sites activate molecular oxygen to form a Mn or a dinuclear Mn

superoxo

dibridged peroxo species which could lead to

cleavage of the O2 bond to form 0" species. The 0~ species is

believed to be very selective for C-H bond cleavage (10.). Mn pyrophosphate is a well known low temperature oxidant

Industrial Chemicals via C1 Processes - American Chemical Society

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