Formaldehyde, methane conversion

In addition to these principal commercial uses of molybdenum catalysts, there is great research interest in molybdenum oxides, often supported on siHca, ie, MoO —Si02, as partial oxidation catalysts for such processes as methane-to-methanol or methane-to-formaldehyde (80). Both O2 and N2O have been used as oxidants, and photochemical activation of the MoO catalyst has been reported (81). The research is driven by the increased use of natural gas as a feedstock for Hquid fuels and chemicals (82). Various heteropolymolybdates (83), MoO.-containing ultrastable Y-zeoHtes (84), and certain mixed metal molybdates, eg, MnMoO Ee2(MoO)2, photoactivated CuMoO, and ZnMoO, have also been studied as partial oxidation catalysts for methane conversion to methanol or formaldehyde (80) and for the oxidation of C-4-hydrocarbons to maleic anhydride (85). Heteropolymolybdates have also been shown to effect ethylene (qv) conversion to acetaldehyde (qv) in a possible replacement for the Wacker process. 

Methane Conversion. The results for the conversion of methane on praseodymium oxide are shown in Figure 1 and Table I. The major products were carbon monoxide, carbon dioxide, ethylene, and ethane both in the presence and absence of TCM in the feedstream while small amounts of formaldehyde and C3 compounds were detected. Water and hydrogen were also produced. The catalyst produced low methane conversion (ca. 6%) and selectivity to C2+ compounds (ca. 30%) in the absence of TCM in the feedstream. On addition of TCM the conversion of methane after 0.5 h on-stream was increased by almost two-fold (11.9%) and increased still further to 17.2% after 6 h on-stream. The selectivity to C2+ also increased with time on-stream to 43.3% after 6 h on-stream. It is noteworthy that over the 6 h on-stream with TCM present the ratio increased from 1.0 to 2.1. No methyl chloride was... 

Figure 4. Influence of molybdenum content on methane conversion (circles) and formaldehyde selectivity (rhombus) at 873 K, a contact time of 3 sec and CH4 02 molar ratio of 11. Filled symbols for reactor 1 and open symbols for reactor 2.

Numerous works on the oxidation of methane to methanol and/or formaldehyde as well as on the oxidative dimerization of methane were reviewed by many authors [22-27]. First, high selectivity of methane oxidation by N20 was reported by Lunsford et al. [28-30], Over a supported Mo oxide [30], the total selectivity to methanol and formaldehyde at low methane conversions attained 100%, although this rapidly dropped as the conversion increased (Table 7.4). High selectivity for this reaction was obtained also with supported vanadium oxide [31]. 

Figure 4.10 shows temperature influence on the process results formaldehyde yield reaches its maximum (about 40%) with temperature raise to 520 °C and total methane conversion increase. Above 520 °C, CO and C02 are detected in reaction products. Their formation rates noticeably increase with temperature. The occurrence of these compounds in the system is explained by sequential formaldehyde transformation to intense degradation products in the high temperature range. After-oxidation of methanol synthesized in the system also contributes to formation of these products. 

Figure 4.10 Temperature dependencies of reaction product yields and selectivity at methane oxidation molar ratio CH4 25% H202 = 1 1, t= 1.2s (1 methanol 2 CO + C02 3 formaldehyde 4 selectivity by formaldehyde and 5 total methane conversion).

As follows from the above, at short contact times (below 2.9 s) the monooxygenase activity of the mimic remains low, whereas catalase activity is maximal (molecular oxygen yield exceeds 90 wt.%). Methanol yield and methane conversion increase with contact time up to r = 10 s and then stabilize at a level of 49-50 wt.% with —96% selectivity. Formaldehyde and formic acid are side products, giving total 2.7 wt.% no CO and C02 are detected in gaseous products. 

It now remains for us to consider the oxidation of monofunctional alcohols and molecules containing the -OH group remote from other functions. The conversion of methanol to formaldehyde (methanal) can be performed either by dehydrogenation (difficult, see Chapter 9) or by oxidative dehydrogenation according to the equation ... 

Other steps used in the model assume that the heterogeneous conversion of methane is limited to the gas-phase availability of oxygen, O2 adsorption is fast relative to the rate of methane conversion, and heat and mass transports are fast relative to the reaction rates. Calculations for the above model were conducted for a batch reactor using some kinetic parameters available for the oxidative coupling of methane over sodium-promoted CaO. The results of the computer simulation performed for methane dimerization at 800 °C can be found in Figure 7. It is seen that the major products of the reaction are ethane, ethylene, and CO. The formation of methanol and formaldehyde decreases as the contact time increases. 

Many other partial oxidations have been studied methane conversion to methanol and formaldehyde. 

Formaldehyde can also be produced in the body Drinking wood alcohol (methanol) causes blindness, respiratory failure, convulsions, and death. The liver enzyme alcohol dehydrogenase, whose function it is to detoxify alcohols, catalyzes the conversion of methanol to formaldehyde (methanal). The formalde-... 

A high oxygenate selectivity (75% to methanol and 5% to formaldehyde) and a low methane conversion (3.5%) were reported by Dowden and Walker using mixed metal oxides as catalysts. Among many oxides mentioned, the most preferable ones are oxides of V, Fe, and Zn mixed with MoOj. The best selectivity and conversion were obtained at 470 C, 54 atm, and 23,200 h space velocity. 

Nb- and Ti-containing silica-based mesoporous molecular sieves were used as catalysts for photocatalytic oxidation of methane. It is found that methanol was formed at 323 K and water pre-adsorbed samples exhibited higher catalytic activity than their pure metal oxides. By comparison with Nb-MCM-41, Ti-MCM-41 gave better methane conversion and methanol yield although traces of formaldehyde were observed over Nb-MCM-41 catalyst. Interestingly, OH radical was detected by the in-situ ESR spectroscopy using DMPO as trapper. 

A different approach was reported by the Wang group. They observed that, when comparing the catalytic behavior of transition metal oxides deposited over mesoporous materials, the Cu-containing catalyst was the most effective. After this preliminary result, they studied the influence of the catalyst preparation on catalytic behavior, reporting a formaldehyde selectivity of ccl 60-70% at methane conversion of 2% when working at 500-650°C. They proposed a relatively different mechanism with the stabilization of Cu"+ species during the catalytic tests. ... 

Methane reacts to methanol and CO at 450°C over Cr20s in batch reactors and reaction times up to 40 min. The reproducibility was limited due to the small reactor size (1.26 ml). At 10% methane conversion, methanol selectivity reached 40%. Compared to a gas-phase reaction, conversion was less but the yield was higher. Continuous partial oxidation of methane with catalysts Cr203/Al203 and Mn02/Ce02 at 400 75°C led to the formation of methanol, formic acid and other partial oxidation products. Metals Ag, Cu and Au/Ag as catalysts are also able to convert methane (375-500°C, 220-350 bar, residence time 0.5-60 s) into methanol and formaldehyde with 50-80% selectivity at conversion below 1%. ... 

In the past few years, there have been many active research programs around the world on the direct conversion of methane to methanol and/or formaldehyde, C2 hydrocarbons, and others. Methanol and formaldehyde can be produced by partial oxidation of methane under controlled conditions in a homogeneous or catafytic reaction process. Many catalysts, such as Mo-based oxides, aluminosilicates, promoted superacids, and silicoferrate, have been used for the reaction. Since the activation energy for the subsequent oxidation of methanol and formaldehyde to carbon oxides is usualfy smaller than that for partial oxidation, hi selectivities for methanol and formaldehyde have been demonstrated onfy at low methane conversions. Reaction conditions (e.g., 02 or N2Q to CH4 ratio, temperature, and resistance time) and surface area of supports play important roles in methanol and formaldehyde yield. In neral, low pressure favors the formation of formaldehyde. Hi pressure and low 02/methane ratios favor the formation of methanol The low yields achieved to data are a major obstacle to economical commercialization of this route. 

One of the most thorough comparisons of formaldehyde formation in the presence of oxide catalysts and in a void reactor was performed in [57]. Figure 2.5 displays the dependences of the methane conversion, selectivity of CH2O formation, and the yield of CH2O in an empty quartz reactor on the reaction temperature for a 1 5 methane—air mixture at a pressure of P = 5 atm and a reaction time of = 2.3 s. 

Methanol is also formed at atmospheric pressure, for example, in experiments [25] with CH4/O2 = 9 1, 4 1, and 2 1 mixtures at 456 °C in a static quartz reactor washed with nitric acid. The selectivity of methanol formation reached a maximum at the end of the highest-rate period of methane conversion, exceeding 20% for the CH4/O2 = 9 1 and 4 1 mixtures, after which it rapidly decreased, leading to a low integrated methanol yield. Formaldehyde was detected only in trace amounts (0.01—0.1%). 

On the other hand, a combination of high temperatures and relatively low pressures favours formaldehyde formation. As already noted, the homogeneous oxidation of methane at T = 625 °C, P = 5 atm, methane/air = 1 5, and a residence time of 2.3 s provided a formaldehyde yield of 3.5%, selectivity of its formation of Sch20 = 50%, and a methane conversion of 7% [57]. 

The same is true regarding no less sensational results of [154], where the DMTM process in the presence NO (0—2.92%) featured a selectivity of formation of Ci oxygenates of 40% at a very high methane conversion (40%, i.e., 16% yield) on a 205/8102 catalyst with a low surface area. The conditions used in this study ([N2] [CH4] [02] = 4 2 1, atmospheric pressure, and T = 530—750 °C) suggest that formaldehyde is mostly formed. 

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