Oxidation to Formaldehyde

The selective oxidation of CH3OH to HCHO represents the industrial production of one of the major chemical intermediates. Two different catalytic oxidation industrial processes are in use for methanol oxidation. One process employs unsupported silver catalysts and the other employs bulk iron-molybdate, Fe2(Mo04)3, catalysts. The silver catalyst process is used when limited amounts of formaldehyde are desired and the process can be quickly started and shut down. 

The methanol oxidation silver process employs an autothermal reactor since the exothemtic heat of the reaction maintains the catalyst bed temperature at 650-725°C. The catalyst bed consists of a thin layer of metallic Ag particles (99.99%) on a gauze substrate. The reaction is conducted with excess methanol. 

These Raman bands have recently been assigned with the assistance of density functional theory (DFT) calculations. The Raman bands are assigned as follows 910-960 cm to Ag-O-O-Ag vibration on a partially oxidised surface, 780-870 cm to the 0-0 vibration of an adsorbed atomic oxygen coordinated to a lattice atomic oxygen of a partially oxidised silver (referred to as a hybrid Ag-O-O-Ag), and 364 cm (not shown in Fig. 17.1) that is a surface atomic oxygen atom 

The methanol oxidation iron-molybdate process takes place in a shell-and-tube fixed-bed reactor where the catalysts are packed within the tubes and the circulating oil coolant maintains catalyst temperatures of 350-450°C, with the higher temperatures occurring in the hot spot region. The process is conducted at 100% methanol conversion with greater than 90% formaldehyde selectivity, which results in formaldehyde yields greater than 90%. The higher formaldehyde yield of the methanol oxidation iron-molybdate process makes it the preferred process for new plants. 

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Another method for the hydrogenoiysis of aryl bromides and iodides is to use MeONa[696], The removal of chlorine and bromine from benzene rings is possible with MeOH under basic conditions by use of dippp as a ligand[697]. The reduction is explained by the formation of the phenylpalladium methoxide 812, which undergoes elimination of /i-hydrogen to form benzene, and MeOH is oxidized to formaldehyde. Based on this mechanistic consideration, reaction of alcohols with aryl halides has another application. For example, cyclohex-anol (813) is oxidized smoothly to cyclohexanone with bromobenzene under basic conditions[698]. 

Methanol can be converted to a dye after oxidation to formaldehyde and subsequent reaction with chromatropic acid [148-25-4]. The dye formed can be deterruined photometrically. However, gc methods are more convenient. Ammonium formate [540-69-2] is converted thermally to formic acid and ammonia. The latter is trapped by formaldehyde, which makes it possible to titrate the residual acid by conventional methods. The water content can be determined by standard Kad Eischer titration. In order to determine iron, it has to be reduced to the iron(II) form and converted to its bipyridyl complex. This compound is red and can be determined photometrically. Contamination with iron and impurities with polymeric hydrocyanic acid are mainly responsible for the color number of the merchandized formamide (<20 APHA). Hydrocyanic acid is detected by converting it to a blue dye that is analyzed and deterruined photometrically. 

Oxidation of cumene to cumene hydroperoxide is usually achieved in three to four oxidizers in series, where the fractional conversion is about the same for each reactor. Fresh cumene and recycled cumene are fed to the first reactor. Air is bubbled in at the bottom of the reactor and leaves at the top of each reactor. The oxidizers are operated at low to moderate pressure. Due to the exothermic nature of the oxidation reaction, heat is generated and must be removed by external cooling. A portion of cumene reacts to form dimethylbenzyl alcohol and acetophenone. Methanol is formed in the acetophenone reaction and is further oxidized to formaldehyde and formic acid. A small amount of water is also formed by the various reactions. The selectivity of the oxidation reaction is a function of oxidation conditions temperature, conversion level, residence time, and oxygen partial pressure. Typical commercial yield of cumene hydroperoxide is about 95 mol % in the oxidizers. The reaction effluent is stripped off unreacted cumene which is then recycled as feedstock. Spent air from the oxidizers is treated to recover 99.99% of the cumene and other volatile organic compounds. 

Methyl violet [8004-87-3] Cl Basic Violet 1 (17), is made by the air oxidation of dimethyl aniline in the presence of salt, phenol, and a copper sulfate catalyst. Initially, some of the dimethyl aniline is oxidized to formaldehyde and /V-methyl aniline under those conditions. The formaldehyde then reacts with dimethyl aniline to produce N,N,]S7,1S7-tetramethyldiaminodiphenylmethane, which is oxidized to Michler s hydrol [119-58-4]. The hydrol condenses with... 

Fast catalytic reac tions that must be quenched rapidly are done in contac t with wire screens or thin layers of fine granules. Ammonia in a 10% concentration in air is oxidized by flowthrough a fine gauze catalyst made of 2 to 10% Rh in Pt, 10 to 30 layers, 0.075-mm (0.0030-in) diameter wire. Contact time is 0.0003 s at 750°C (1,382°F) and 7 atm (103 psi) followed by rapid quenching. Methanol is oxidized to formaldehyde in a thin layer of finely divided silver or a multilayer screen, with a contact time of 0.01 s at 450 to 600°C (842 to 1,112°F). 

Hyphomicrobium sp. strain EG is able to grow at the expense of dimethyl sulfide or dimethyl sulfoxide (DMSO) and prodnces methanethiol initially. This is then further oxidized to formaldehyde, sulfide, and Ft202 by an oxidase that has been purified (Suylen etal. 1987). 

Redox reactions with metal porphyrins (MPs) as photocatalysts. A spectacular example here is the reaction that couples upon illumination with the sunlight, methanol oxidation to formaldehyde with the formation of hydrogen peroxide in be nzene-methanol mixture (90 10)... 

The Holy Grail of catalysis has been to identify what Taylor described as the active site that is, that ensemble of atoms which is responsible for the surface reactions involved in catalytic turnover. With the advent of atomically resolving techniques such as scanning tunnelling microscopy it is now possible to identify reaction centres on planar surfaces. This gives a greater insight also into reaction kinetics and mechanisms in catalysis. In this paper two examples of such work are described, namely CO oxidation on a Rh(llO) crystal and methanol selective oxidation to formaldehyde on Cu(llO). 

Similarly, the m/z = 60 ion current signal was converted into the partial current for methanol oxidation to formic acid in a four-electron reaction (dash-dotted line in Fig. 13.3c for calibration, see Section 13.2). The resulting partial current of methanol oxidation to formic acid does not exceed about 10% of the methanol oxidation current. Obviously, the sum of both partial currents of methanol oxidation to CO2 and formic acid also does not reach the measured faradaic current. Their difference is plotted in Fig. 13.3c as a dotted line, after the PtO formation/reduction currents and pseudoca-pacitive contributions, as evident in the base CV of a Pt/Vulcan electrode (dotted line in Fig. 13.1a), were subtracted as well. Apparently, a signihcant fraction of the faradaic current is used for the formation of another methanol oxidation product, other than CO2 and formic acid. Since formaldehyde formation has been shown in methanol oxidation at ambient temperatures as well, parallel to CO2 and formic acid formation [Ota et al., 1984 Iwasita and Vielstich, 1986 Korzeniewski and ChUders, 1998 ChUders et al., 1999], we attribute this current difference to the partial current of methanol oxidation to formaldehyde. (Note that direct detection of formaldehyde by DBMS is not possible under these conditions, owing to its low volatility and interference with methanol-related mass peaks, as discussed previously [Jusys et al., 2003]). Assuming that formaldehyde is the only other methanol oxidation product in addition to CO2 and formic acid, we can quantitatively determine the partial currents of all three major products during methanol oxidation, which are otherwise not accessible. Similarly, subtraction of the partial current for formaldehyde oxidation to CO2 from the measured faradaic current for formaldehyde oxidation yields an additional current, which corresponds to the partial oxidation of formaldehyde to formic acid. The characteristics of the different Ci oxidation reactions are presented in more detail in the following sections. 

The formation of peroxides and formaldehyde in the high-purity polyoxyethylene surfactants in toiletries has been shown to lead to contact dermatitis [31], Peroxides in hydrogenated castor oil can cause autoxidation of miconazole [32], Oxidative decomposition of the polyoxyethylene chains occurs at elevated temperature, leading to the formation of ethylene glycol, which may then be oxidized to formaldehyde. When polyethylene glycol and poloxamer were used to prepare solid dispersions of bendroflumethiazide, a potent, lipophilic diuretic drug, the drug reacted with the formaldehyde to produce hydroflumethiazide [33],... 

Glycerol can be oxidized to formaldehyde by periodic acid and the formaldehyde measured spectrophotometrically at 570 nm after reaction with chromotropic acid, or fluorimetrically after the addition of diacetylacetone and ammonia. The chromotropic acid reagent consists of 8-dihydroxynaphthalene-3,6-disulphonic acid dissolved in 50% sulphuric acid. 

Dehydrogenation. The removal of one or more hydrogen atoms from a molecule by chemical means, as in the conversion of alcohols to aldehydes. For example, methanol (CH3OH) can be oxidized to formaldehyde (HCHO) plus H2. ... 

Terpenoid DBPs were investigated by Joll et al. [124] and Qi et al. [125]. The main ozonation product of 2-methylisobomeol was camphor, which was further oxidized to formaldehyde, acetaldehyde, propanal, buntanal, glyoxal, and methyl glyoxal [125]. Chlorination of p-carotene, retinol, p-ionone, and geranyl acetate resulted in the formation of THMs [124]. 

Methanol is expected to oxidize to formaldehyde, both during combustion and after emission to the atmosphere. As discussed in Chapter 6.H, OH reacts with methanol primarily at the methyl group ... 

Alkenes can be cleaved by ozone followed by an oxidative or reductive work-up to generate carbonyl compounds. The products obtained from an ozonolysis reaction depend on the reaction conditions. If ozonolysis is followed by the reductive work-up (Z11/H2O), the products obtained are aldehydes and/or ketones. Unsubstituted carbon atoms are oxidized to formaldehyde, mono-substituted carbon atoms to aldehydes, and di-substituted carbon atoms to ketones. 

Methanol can be absorbed through the skin or from the respiratory or gastrointestinal tract and is then distributed in body water. The primary mechanism of elimination of methanol in humans is by oxidation to formaldehyde, formic acid, and C02 (Figure 23-3). 

The present chapter will primarily focus on oxidation reactions over supported vanadia catalysts because of the widespread applications of these interesting catalytic materials.5 6,22 24 Although this article is limited to well-defined supported vanadia catalysts, the supported vanadia catalysts are model catalyst systems that are also representative of other supported metal oxide catalysts employed in oxidation reactions (e.g., Mo, Cr, Re, etc.).25 26 The key chemical probe reaction to be employed in this chapter will be methanol oxidation to formaldehyde, but other oxidation reactions will also be discussed (methane oxidation to formaldehyde, propane oxidation to propylene, butane oxidation to maleic anhydride, CO oxidation to C02, S02 oxidation to S03 and the selective catalytic reduction of NOx with NH3 to N2 and H20). This chapter will combine the molecular structural and reactivity information of well-defined supported vanadia catalysts in order to develop the molecular structure-reactivity relationships for these oxidation catalysts. The molecular structure-reactivity relationships represent the molecular ingredients required for the molecular engineering of supported metal oxide catalysts. 

The TOFs for methanol oxidation to formaldehyde (95-99% selectivity), butane oxidation to maleic anhydride and C0/C02 (30% maleic anhydride selectivity) and S02 oxidation to SO3 are independent of surface vanadia coverage. This observation suggests that these oxidation reactions do not depend on the surface concentration of bridging V-O-V bonds since the reaction TOFs do not correlate with the surface density of bridging V-O-V bonds. Furthermore, the constant TOFs with surface vanadia coverage suggest that only one surface vanadia site is required for the activation of these molecules during the oxidation reactions. 

Many substances can be partially oxidized by oxygen if selective catalysts are used. In such a way, oxygen can be introduced in hydrocarbons such as olefins and aromatics to synthesize aldehydes (e.g. acrolein and benzaldehyde) and acids (e.g. acrylic acid, phthalic acid anhydride). A selective oxidation can also result in a dehydrogenation (butene - butadiene) or a dealkylation (toluene -> benzene). Other molecules can also be selectively attacked by oxygen. Methanol is oxidized to formaldehyde and ammonia to nitrogen oxides. Olefins and aromatics can be oxidized with oxygen together with ammonia to nitriles (ammoxidation). 

Methanol may be oxidized to formaldehyde, and the color developed with formaldehyde and roseaniline or chromotropic acid as indicator is used for methanol determination. GLC has also been used (31), and Martin et al (32) found good agreement between the two procedures at higher methanol levels. 

Methanol is oxidized to formaldehyde and dimethyl formate. Under the experimental conditions, neither formic acid nor its esters were detected. The other primary alcohols are oxidized to give aldehydes and, in a subsequent reaction, carboxylic acids. Acetals and esters are also formed. Profiles of the reaction products in the oxidation of l-propanol are shown in Fig. 19. 

The classic C—C bond-forming processes of aldehydes and ketones are aldol reactions. In Scheme 8.7, an iron-catalyzed sequential methanol oxidation to formaldehyde and its aldol reaction with [i-oxo ester 24a is shown [30]. The oxidant is 30% aqueous H202. Curiously for an oxidation, the reaction has to be performed under an atmosphere of Ar in order to prevent a-hydroxylation of the [i-oxo ester [31], The role of benzaldehyde (4f) as substoichiometric additive is not completely clear. 

Figure 15. Chemical reaction dynamics of methanol oxidation to formaldehyde one of the C-H bonds of the methyl group becomes elongated (top left) and eventually breaks (top right). The adsorbed hydroxymethyl group stabilized by forming a hydrogen bonded complex to a water molecule (bottom left) and dissociates rapidly into adsorbed formaldehyde and a hydronium ion (bottom right) which further stabilized by undergoing structural diffusion steps to form Zundel ions H50+.123,125 Reprinted from Chemical Physics, Vol. 319, C. Hartnig and E. Spohr, The role of water in the initial steps of methanol oxidation on Pt(l 1 1), p. 108, Copyright (2005), with permission from Elsevier.

A -dealkylation is a common reaction in the metabolism of drugs, insecticides, and other xenobiotics. The drug ethylmorphine is a useful model compound for this reaction. In this case the methyl group is oxidized to formaldehyde, which can be readily detected by the Nash reaction. 

In a process proposed in a recent application by Texaco 474), methanol is anodically oxidized to formaldehyde, which is then reduced cathodically to glycol in the same cell. 

The results of experiments on the contact time effect on methane oxidation reaction (Figure 4.11) show that formaldehyde yield increases to some extent (39%) with the contact time. Maximal yield of formaldehyde is reached at r = 1.2 s, which is optimal for execution of the reaction at selected parameters. As the contact time exceeds 1.2 s, side products occur in the system CO, C02 and CH3OH concentration of the last compound reaches its maximum at r = 1.4 h. The above results show that methane is oxidized to formaldehyde... 

Let us note once again that comparison of the results on methanol oxidation with hydrogen peroxide with methane oxidation data under atmospheric pressure (refer to Table 4.3, Figures 4.10 and 4.11) indicates significant differences in these processes. Methane is oxidized to formaldehyde at a higher rate and higher selectivity than at methanol oxidation. Low methanol yields at methane oxidation compared with formaldehyde confirm parallel proceeding of formaldehyde and methanol synthesis from methane. 

It is shown that methane oxidation to formaldehyde may be implemented with great efficiency by a conjugated process in the presence of H202. As mentioned in Chapter 3, spontaneous reactions may also be induced for which conditions with advantageous kinetics cannot be selected. The low-effective reaction (5.21) unambiguously belongs to this class of processes. 

Chapter 4 presents experimental data on methane oxidation to formaldehyde with hydrogen peroxide. We shall not discuss them here again, but use them only for determinant D calculation. The calculated D value is 0.2 which, according to currently developed ideas, indicates the conjugated type of CH4 oxidation with hydrogen peroxide. 

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