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Selective partial oxidation of CH4 (SPOM) to formaldehyde (HCHO)

Formaldehyde (FICHO) is an important industrial chemical that is a precursor for the production of several resins used in the automobile and textile industries, in addition to its uses in disinfectants and biocides. Initially, CH, was converted to HCHO by a three-step process, through CH4 to syngas, syngas to methanol, followed by air oxidation of methanol over a silver or iron molybdate catalyst to HCHO. The importance of HCHO as an industrial precursor led to research on direct oxidation of CH4 to HCHO with investigations of various oxidants, catalyst formulations, and operating conditions. One-step HCHO production from methane has been the most commonly studied over supported catalytic metal oxides, which include SiO,. MgO, Mo03, V,Os, WOr and ZnO. Their catalytic activity and performance as support materials were studied (Hall et al.. 1995).

Catalytic metal oxides utilize lattice oxygen, but have the added participation of the catalytic surface, which promotes formation of the desired products by incorporation of lattice oxygen, while suppressing further oxidation. This is similar to the CLOCM process. The main difference is that, in the CLOCM scheme, the catalytic metal oxides are re-oxidized in a separate reactor by air, whereas here, the catalytic metal oxides are simultaneously regenerated in the same reactor using molecular oxygen. On the other hand, according to the reaction mechanism of the SPOM to HCHO with catalytic metal oxides, the process could also be operated in the chemical looping mode, with cyclic reduction and regeneration of the catalytic metal oxides, and an effective reactor design. Thus, researches on the properties of some popular catalytic metal oxides that lead to high HCHO selectivity and the mechanisms are introduced here.

Recently, Mo03 and V,05-based catalysts are the most widely studied as they are considered to be the most effective for the selective partial oxidation of methane to HCHO (Hall et al., 1995). They har e been extensively studied for the effect of their different physicochemical properties, such as their structure, dispersion of the metal oxide species, density of active sites. Their reducibility on the selective oxidation of CH4SPOM to HCHO were detailed investigated.

Faraldos et al. (Faraldos et al., 1996) investigated SiO, supported V,0. and MoO, and compared their activity towards SPOM to HCHO. It was found that SiO, supported V,05 exhibits a higher CH4 conversion as compared to supported MoO,. To achieve the same methane conversion, V,Os-based catalysts require a lower onset reaction temperature as compared to Mo03. which is also an indication of the higher reactivity ofV,0$. Similar observations were reported from other studies (Spencer and Pereira, 1989; Pannaliana et al., 1991; Miceli et al., 1993). It was reported that with SiO, supported MoO}, CO, is produced ел ей at low CH4 conversions, and the CO, selectivity remained constant over the entire range of CH4 conversions studied. SiO, supported V,0;, on the other hand, formed no CO, at low CH4 conversions, with increasing CO, selectivity when the CH4 conversion is increased. This suggests that, for SiO, supported MoO, catalyst, CO, is the primary product, whereas for SiO, supported V,Os catalyst, it is formed by further oxidation of CO.

As for the HCHO selectivity of the two catalysts with similar metal oxide surface concentrations, Mo03 catalyst always exhibits a higher HCHO selectivity than V,05 under all methane conversion conditions. Faraldos et al.’s study suggested that there is no real correlation between HCHO selectivity and the reducibility of the two supported catalytic metal oxides, which is in agr eement with other similar studies (Miguel A. Baiiares et al., 1994).

It has been reported that SiO, supported Mo03 does not adsorb oxygen but some degree of adsorption occurs on SiO, supported V,Os (Miguel A. Baiiares et al.. 1994; Haber, 1979; Kartheuser et al., 1993). Thus, in the case of vanadium oxide, there is a higher interaction between the gaseous oxygen and the active sites. Isotopic studies har e shown that selective oxidation of CH4 to HCHO involves the incorporation of lattice oxygen from the catalytic metal oxides (Baiiares et al., 1993; Kartheuser et al., 1993; Korarme et al., 1994; Mauti and Mims, 1993). The vacancies created on the metal oxide surface are filled by gaseous oxygen. In the case of SiO, supported V,Os, in addition to the lattice oxygen, surface- adsorbed oxygen species are also present, which makes the vanadium oxide more reactive and promotes further oxidation of HCHO to carbon oxides, resulting in a lov'er HCHO selectivity. Thus, while HCHO is the primary product for both SiO, supported MoO, and V,05, the absence of adsorbed oxygen in molybdates suppresses the further oxidation of HCHO to CO and CO,, leading to higher selectivity.

The catalytic conversion of a hydrocarbon molecule to a desired chemical is often highly sensitive to its structure. Specifically, selectivity of the product is dependent on the distribution of the ciystal planes (Andersson and Hansen. 1988). Catalysts that exhibit this kind of behavior fall under the category of catalytic anisotropy (Volta et al., 1979), where the difference between ciystal planes depends on the ratio of various active sites on each plane. The selectivity of a catalytic metal oxide can be improved by selecting suitable bond strengths and active sites. Establishing the relation between product selectivity and ciystal planes/active sites is the first step in exploring the potential of catalytic metal oxides. MoO, exhibits such catalytic anisotropy.

Smith and Ozkan (Smith and Ozkan. 1993a) investigated the effect of structural specificity of unsupported MoO, on its reactivity and selectivity by preparing MoO, samples using temperature programmed techniques to preferentially expose different ciystal planes. The selectivity and production rates of methane oxidation on Mo03-C and Mo03-R. which were synthesized using two different techniques to expose different ciystal planes of Mo03, were investigated. It was revealed that Mo03-C had a significantly higher selectivity and production rate toward HCHO than Mo03-R. The much higher concentration of (100) side planes for Mo03-C was considered to contribute to the higher selectivity of Mo03-C towards HCHO. While the (010) basal planes which tend to form carbon oxide products are in larger concentrations in MoO.-R. The higher HCHO production rate for Mo03-C also confirmed the importance of the (100) side plane in HCHO formation. Laser Raman spectroscopy, used to identify the type of Mo and О sites in the two different Mo03 structures, suggests that Mo03-C has more exposed terminal Mo=0 sites. Hence, the Mo=0 sites are concluded to be the main surface species that promotes partial oxidation of CH,. On the other hand, the bridged Mo-O-Mo sites, which har e a higher density in MoO.-R surface, promote the formation of complete oxidation products. Consequently, oxidation of HCHO over the two different Mo03 catalytic metal oxides revealed that MoO.-R is more active towards complete oxidation.

The reducibility of both types of site was tested with temperature programmed reduction (TPR) experiments by subjecting both MoO,-C and Mo03-R to H,. The experimental results suggested that MoO,-R is more readily reducible than Mo03-C, meaning that the Mo-O-Mo sites are easier to reduce than the Mo=0 sites. The differences in reducibility of these sites are reflected in the selectivity towards HCHO. Metal oxides that are more difficult to reduce har e a higher energy barrier for the formation of complete and partial oxidation products (COs) and are more selective towards formation of HCHO. The difference, with respect to selectivity, between these two sites reveals the preferred structure of MoO, for selective oxidation of methane and provides insight when designing a MoO, catalytic metal oxide.

With the preferred planes and sites for partial oxidation of CH4 identified, isotopic labeling experiments using 160, and lsO, as reactive gases on both Mo03-C and MoO.-R samples revealed the reaction pathway (Smith and Ozkan, 1993b). The isotopic experiments investigated the interaction between oxygen and the metal oxide surface in the absence of CH4. as well as the source of О atoms in the products during CH4 oxidation. The experiment results suggested that Mo03-R is more susceptible to exchange of О atoms than Mo03-C. It was found that Mo03-R contributed ~ 50% more О in the gas phase than the contribution of Mo03-C and at more than twice the rate. The high rate of desorption and exchange of lattice oxygen in the metal oxide surface, combined with the comparatively easier reducibility, allow for the surface Mo-O-Mo site to be more active to donate lattice oxygen than the surface Mo=0 site. Such high activity promotes the formation of complete oxidation products with methane as the reactant.

Isotopic oxygen switching experiments with CH4 were conducted in order to determine the source of oxygen in the products. Normalized CO, isotope concentrations suggested that oxygen from MoO,-R depletes faster than that from Mo03-C. Isotopes of H,0 also showed similar trends. HCHO isotope concentrations suggested that the reaction path is highly dependent on the oxygen atoms from the metal oxide and not the gaseous O,. However, the gaseous oxygen plays a role in replenishing the oxygen atoms on the surface.

The reduction and oxidation mechanism on the catalytic metal oxide surface is dependent on the type of the terminal metal-oxygen bond. Laser Raman spectroscopy on the reduced and re-oxidized MoO. sample revealed that the bridged site with Mo-O-Mo termination is more readily oxidized by gaseous oxygen. Mo=0 site replenishment, on the other hand, is preferred via oxygen diffusion through the catalytic metal oxide lattice. Since HCHO is mainly formed from О, it is concluded that the Mo=0

site is key to the selectivity of HCHO. The differences between the re-oxidation mechanisms and the relative population of each bond on MoO ,-C and MoO ,-R distinguish the HCHO selectivity.

In practice, the structural advantage of Mo03 can be used to promote the activity of silica. Arena et al. (Arena et ah, 1997) extensively studied the conversion of CH4 to HCHO on SiO, supported MoO, by varying the amount of MoO, on silica and evaluating then interaction with CH4. The effect of MoO, loading has been studied using H, and CH4 TPR and high temperature oxygen chemisorption (HTOC) experiments. H,-TPR profiles of catalytic metal oxides of MoO , on precipitated silica with concentrations of 2 «7%, 4 wt% and 7 wt% were compared with pure precipitated silica and bulk MoO,. For all MoO, on precipitated silica catalytic metal oxides, the onset temperature and peak locations are lower than pure precipitated silica and bulk MoO}. This suggests a positive interaction that results in different phases of Mo03 on silica (Arena and Pannaliaua. 1996).

Methane reactivity with each catalytic metal oxide was studied using CH4-TPR experiments. The rate of oxygen consumption was calculated by performing an oxygen balance through the total summation of oxygen atoms in product molecules such as HCHO, CO, CO,, and H,0. CH4-TPR further confirmed the findings of the H,-TPR studies. 7 wt% Mo03 on precipitated silica exhibited the highest lattice oxygen consumption rate, while 2 wt% Mo03 on precipitated silica had the lowest among all the Mo03 on precipitated silica metal oxides. Pure precipitated silica exhibited low reactivity toward CH4 and only increased slightly with temperature. HCHO and carbon are the only products formed in the absence of oxygen. For Mo03 on precipitated silica catalytic metal oxides, reactivity with methane is slow, at 650 °C, but increases drastically at higher temperatures. The CH4 conversion rate is inversely proportional to Mo loading. Oxygenated product formation also increased at beyond 650 °C. At the same time, high HCHO selectivity is only observed at low temperatures.

 
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