Using In Situ Raman Spectroscopy to Study Methanol Conversion over TiO2 Supported Oxomolybdate Catalysts

Conversion of methanol involves oxidation, condensation and dehydration reactions. These reactions provide an appropriate way to study both acidic and redox properties of a catalyst. The selective oxidation of methanol is a suitable way to achieve expensive products such as methylformate (MF), dimethoxymethane (DMM), or formaldehyde (Figure 1).

Figure 1. Oxidation of methanol

Raman spectroscopy

With the help of Raman spectroscopy, the structure of the active phase can be studied with suitable spatial resolution and a short collection time. Under certain favorable conditions, it is possible to observe the nature of poisons, adsorbates or intermediates. Space and time resolution make Raman spectroscopy suitable for in situ analysis of the catalyst under working conditions.

Experimental Framework

For the experiment, the catalysts were formulated by means of wetness impregnation of an ammonium heptamolybdate solution on an anatase provided by Sachtleben™ whose specific area is set to 50m²/g. The concentration of the impregnation ammonium heptamolybdate solution was altered so as to achieve a molybdenum loading matching to 5% _wt MoO₃/TiO₂.

Next, using the 531.95nm second harmonic line of a Nd:YAG laser, micro-Raman spectra were recorded in operating conditions under selected atmospheres and at different temperatures. Subsequently, a 50X microscope objective was utilized to focus the excitation beam and simultaneously collect the dispersed light.

The dispersed light was collected via a confocal hole (150µm) with a nitrogen cooled CCD. The in situ study was facilitated by means of Harrick‘s environmental spectroscopic chamber fitted with a novel planar dome including a pure silica window to allow Raman measurements in the UV-visible range. In the spectroscopic cell, the methanol was introduced using a He flow bubbling in a saturator fitted with a condenser, which was set to 11°C temperature.

Results and Discussion

In the environmental spectroscopic cell, the 5%_wt MoO₃/TiO₂ catalyst was activated by applying heat treatment for a period of 3 hours under a flow of pure oxygen at 350°C temperature.

This procedure improves activation and dehydration of the redox sites. In Figure 2, the blue line indicates the Raman spectrum of the activated material under pure oxygen flow sans making any contact with air following activation treatment.

Figure 2. In situ Raman spectra of the supported 5% MoO₃ catalyst upon MeOH/He flow

The effect of the reactive mixture on the active phase structure has been studied by in situ Raman spectroscopy carried out on the activated material. Figure 2 shows the Raman spectra recorded upon heating in pure MeOH/He flow.

Considerable changes in the Raman features of the active phase were observed following exposure to pure methanol carried by means of helium. With increasing temperature, the intensity of the line seen at 950cm^-1 disappears slowly.

This trend, which was seen in other catalytic systems, was related to effective reduction of Mo^{V I} to Mo^{V 4} and thus supports the conversion of methanol (15% at 240°C) to formaldehyde (yield: 10%) and MF (yield: 5%) seen for this catalyst in plug flow reactor (feed: MeOH/O₂).

When oxygen was introduced in the reactive mixture, it resulted in reverse spectral changes and hence it was possible to recover half the initial intensity of the Raman peak at 955cm^-1.

Raman spectroscopy can also provide a better understanding about the nature of adsorbates formed upon methanol flow. Therefore, the in situ Raman spectra shown in Figure 3 peaks at 2855 and 2957cm^-1, and those that are assigned to the symmetric and antisymmetric stretching modes of CH₃ are seen simultaneously with novel features at 1444 and 1577cm^-1 upon pure MeOH/He flow, as shown in Figure 3a.

Figure 3. In situ Raman spectra recorded at 240°C upon a) pure MeOH/He flow b) mix MeOH/O₂ c) pure O₂

However, when oxygen is introduced in the feed, these last bands are not seen while CH3 stretching vibrations can still be observed upon MeOH/O₂ flow. Nevertheless, assignment of the lines observed around 1500cm^-1 is not simple, but based on earlier analyses the latter can be attributed to COO vibration in formate adsorbed intermediates.

When oxygen was introduced in the reactive mixture (Figure 3b), it resulted in a total loss of the assumed formate vibration bands, and the residual line seen around 1660cm^-1 is a harmonic mode of anatase.

This trend can support a van Krevelen - type mechanism, which involves adsorption of methanol on the oxomolybdate phase and ensuing release of the oxidation reaction products.

The CH₃ stretching modes can still be seen, with a clear doubling which is yet to be explained fully. Following the flow of pure oxygen, all the adsorbates are removed from the surface of the catalyst.

Conclusion

Methanol conversion involves oxidation, dehydration and condensation reactions, which provide a relevant way to study the acidic and redox properties of catalysts.

By means of Raman spectroscopy, the structure of the active phase can be examined with a short collection time and suitable spatial resolution.

About Harrick Scientific Products, Inc.

Since its beginnings in 1969, Harrick Scientific has advanced the frontiers of optical spectroscopy through its innovations to transmission, internal reflection, external reflection, diffuse reflection, and emission spectroscopy. The president and founder of the corporation, Dr. N. J. Harrick, pioneered internal reflection spectroscopy and became the principal developer of this technique.

Harrick Scientific offers a large selection of standard and custom-built accessories for IR and UV-VIS spectrometers. Many of these attachments were originally forerunners in their field and their contemporary versions are considered industry standards. Harrick Scientific continues to introduce innovative new products. In addition to these state-of-the-art accessories, Harrick Scientific supplies a complete line of optical elements, including windows, ATR plates, prisms, and hemispheres.

This information has been sourced, reviewed and adapted from materials provided by Harrick Scientific Products, Inc.

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