Formaldehyde from heterogeneous oxidation

However, formaldehyde can be rapidly destroyed through radical reactions or even decomposed by oxygen at very short residence times. This is one of the main reasons why formaldehyde selectivity is low at high conversions. Apart from this, it is difficult to compare the gas-phase and heterogenous reactions because there is a pressure gap and temperature gap, i. e., moderately high pressure and low temperatures in the purely gas-phase oxidation and high temperature and atmospheric pressure in the heterogenous oxidation. 

Oxidation of methane to formaldehyde. One of the first studies in this area was reportedly an experimental factory production of formalin in the United States from natural gas (Empire Refining Co., 1930) with a capacity of 70 million gallons (265 million litres) of a mixture of formaldehyde, methanol, and acetaldehyde. The description of the installation and the method, as well as the yields, has not been published. However, in contrast to the oxidation of propane and butane (associated gas), the processes of direct oxidation of methane have not received widespread in the United States. Two industrial processes for production of formaldehyde from methane were developed in Germany. To produce formaldehyde, methane was oxidized with molecular oxygen in the presence of 1—2% of nitrogen oxides or a heterogeneous catalyst (94% Cu, and 6% Sn). The oxidation of methane in the presence of platinum or palladium yielded mainly formic acid. In this case, the reaction proceeds at a very high rate, so it is impossible to isolate oxidation intermediates, formaldehyde, and methanol [174]. 

On the other hand, the oxidative coupling reaction of CH4 in the presence of Og, even when performed in membrane-type reactors, is mainly catalysed by metal oxide catalysts. Also oligomerisation, aromatisation and the partial oxidation to methanol or formaldehyde apply non-metallic heterogeneous catalysts (i.e. zeolites, supported metal oxides or heterogenized metalcarbon nanofibers or nanotubes from methane, these being catalysed by metal nanoparticles, but at the moment this is not considered as a Cl chemistry reaction. Again we direct the attention of the reader to some reviews on this type of process. ... 

In Table 1 the ancxlic reactions that have been studied so far in small cogenerative solid oxide fuel cells are listed. One simple and interesting rule which has emerged from these studies is that the selection of the anodic electrocatalyst for a selective electrocatalytic oxidation can be based on the heterogeneous catalytic literature for the corresponding selective catalytic oxidation. Thus, the selectivity of Pt and Pt-Rh alloy electrocatalysts for the anodic NH3 oxidation to NO turns out to be comparable (>95%) to the selectivity of Pt and Pt-Rh alloy catalysts for the corresponding commercial catalytic oxidation. The same applies for Ag, which turns out to be equally selective as an electrocatalyst for the anodic partial oxidation of methanol to formaldehyde, ... 

The set of reactions shown in Table 5.1 accounts for the formation of the main oxidation products methanol, formaldehyde, and water, but does not provide for their further transformation, since only the initial stage of the process is considered. Nevertheless, its analysis can explain the main qualitative features of the DMTM process, although its quantitative modeling requires much more complex, open-type models that would take into account the totality of homogeneous and heterogeneous elementary steps important in this range of conditions. This means that the model includes all the relevant elementary steps and, if required, it can be readily extended and that all the kinetic parameters are taken from independent databases. Thus, these parameters can and should be modified based only on the subsequent recommendations of these databases. 

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