Formic acid decomposition dehydration

Formic acid decomposition has been studied on the (110), (001), and (100) surfaces of Ti02 [23-25,40-51]. The degree to which surface reducibility influences the reaction paths e.g., dehydrogenation vs. dehydration unimolecular reactions vs. bimolecular ones) will be explored in more detail... 

Formic acid can decompose either by dehydration, HCOOH — H2O + CO (AG° = —30.1 kJ/mol AH° = 10.5 kJ/mol) or by dehydrogenation, HCOOH H2 + CO2 (AG° = —58.6 kJ/mol AH° = —31.0 kJ/mol). The kinetics of these reactions have been extensively studied (19). In the gas phase metallic catalysts favor dehydrogenation, whereas oxide catalysts favor dehydration. Dehydration is the predominant mode of decomposition ia the Hquid phase, and is cataly2ed by strong acids. The mechanism is beheved to be as follows (19) ... 

Isothermal a—time curves were sigmoid [1024] for the anhydrous Ca and Ba salts and also for Sr formate, providing that nucleation during dehydration was prevented by refluxing in 100% formic acid. From the observed obedience to the Avrami—Erofe ev equation [eqn. (6), n = 4], the values of E calculated were 199, 228 and 270 kJ mole"1 for the Ca, Sr and Ba salts, respectively. The value for calcium formate is in good agreement with that obtained [292] for the decomposition of this solid dispersed in a pressed KBr disc. Under the latter conditions, concentrations of both reactant (HCOJ) and product (CO3") were determined by infrared measurements and their variation followed first-order kinetics. 

Thermolysis of D-fructose in acid solution provides 11 and 2-(2-hydrox-yacetyl)furan (44) as major products. Earlier work had established the presence of 44 in the product mixtures obtained after acid-catalyzed dehydrations of D-glucose and sucrose. Eleven other products were identified in the D-fructose reaction-mixture, including formic acid, acetic acid, 2-furaldehyde, levulinic acid, 2-acetyl-3-hydroxyfuran (isomaltol), and 4-hydroxy-2-(hydroxymethyl)-5-methyl-3(2//)-furanone (59). Acetic acid and formic acid can be formed by an acid-catalyzed decomposition of 2-acetyl-3-hydroxyfuran, whereas levulinic acid is a degradation prod-uct of 11. 2,3-Dihydro-3,5-dihydroxy-6-methyl-4//-pyran-4-one has also been isolated after acid treatment of D-fructose.The pyranone is a dehydration product of the pyranose form of l-deoxy-D-eo f o-2,3-hexodiulose. In aqueous acid seems to be the major reaction product of the pyranone. 

Trillo et al. (47,137) have reported compensation behavior in oxide-catalyzed decomposition of formic acid and the Arrhenius parameters for the same reactions on cobalt and nickel metals are close to the same line, Table V, K. Since the values of E for the dehydration of this reactant on titania and on chromia were not influenced by doping or sintering, it was concluded (47) that the rate-limiting step here was not controlled by the semiconducting properties of the oxide. In contrast, the compensation effect found for the dehydrogenation reaction was ascribed to a dependence of the Arrhenius parameters on the ease of transfer of the electrons to the solid. The possibility that the compensation behavior arises through changes in the mobility of surface intermediates is also mentioned (137). 

Krupay and Ross (272b), in a study of the decomposition of formic acid on manganese (II) oxide, demonstrate that manganese (II) formate is produced during reaction and discuss the probable role of this participant in the catalytic process. The reported Arrhenius parameters (log A, E) for the dehydration and dehydrogenation reactions were (28.7, 132) and (24.9, 87), respectively both points were close to the compensation line (Table V, K) characteristic of the breakdown of formic acid on oxides. 

Fig. 31. Catalytic activities of acidic Na or Cs salts of HjPW 204o as a function of Na or Cs content, (a) M = Na (O) dehydration of 2-propanol, (A) decomposition of formic acid, ( ) conversion of methanol, ( ) conversion of dimethyl ether, (b) M = Cs (O) dehydration of 2-propanol, ( ) conversion of dimethyl ether, (A) alkylation of 1,3,5-trimethylbenzene with cyclohexene. (From Refs. 46 and 128.)...

As shown in Fig. 44a, the catalytic activity of NaxH3- PW,2O40 for dehydration of 2-propanol, conversion of methanol, and decomposition of formic acid decreased monotonically with the Na content in the salts. The activities for these... 

If the catalyst to be studied has acidic sites, formic acid may decompose into carbon-monoxide and water according to a class A mechanism (dehydration). We may assume the mechanism of decomposition to be that of an acid catalyzed decarboxylation 63> ... 

Several kinetic studies of alcohol and formic acid-dehydration have been described, using cation exchange resins, and the results led to postulates about the possible structure of the adsorbed molecule and the mechanism of its decomposition U2-H4)... 

Formic acid is a popular molecule for probing the catalytic properties of metal oxides [23-28], The selectivity of its decomposition has frequently been used as a measure of the acid-base properties of oxides. This is a tempting generalization to make oxides that produce dehydration products (H2O and CO) are described as acidic oxides, while their basic counterparts produce dehydrogenation products (H2 + CO2). It has been shown that in many cases the product selectivity is better connected to the surface redox behavior of the oxide [29], Thus, more reducible surfaces produce higher yields of CO2, Consequently, particular attention has been paid in surface science studies to the interaction between adsorbed formate ions (the primary reaction intermediate) and surface metal cations, as well as to the participation of lattice oxygen anions in the surface reaction mechanism,... 

Like formic acid, methanol decomposition has also been used to probe the acid-base properties of metal oxides [70]. However, methoxide decomposition is dependent on surface structure in much the same way as formate decomposition. For example, methanol undergoes parallel dehydration and dehydrogenation reactions on the same crystal surface of zinc oxide [25]. Once again, product selectivity ratios may not necessarily serve as a diagnostic of acid-base properties alone. 

Even in the case of a less drastic final step (dehydration as opposed to reduction), Dalmai et al. found that irradiation infiuenced the catalytic properties of the product 182). Aluminum oxide produced from gibbsite, A1(0H)3, by heating at 290-340° was considerably more active in the decomposition of formic acid if the hydroxide had been irradiated in a reactor to 10 nvt or with about 6 X 10 i ev/gm of y-rays. The changes in catalytic activity were accompanied by somewhat complex effects of radiation on the rate of decomposition of the gibbsite 182a). Thus, irradiated samples decomposed more rapidly at low temperatures and less rapidly at high (above 210°) than unirradiated blanks, although the differences were relatively small. 

The decomposition of formic acid on metal catalysts to form carbon dioxide and hydrogen has been studied extensively by many investigators. The reaction is of interest in connection with selective catalysis, as formic acid also decomposes to carbon monoxide and water on some dehydrating catalysts such as alumina. 

It has been thought that the acid-base character is an intrinsic property of oxide substrates. The selectivity in the catalytic decomposition reaction of formic acid has been used to scale the acid-base property dehydration over acidic oxide and dehydrogenation over basic oxide, though this classification is over simplified vide infra. 

The catalytic dehydration reaction of formic acid on TiO2(110) is suggested to involve the unimolecular decomposition of formate ions (HCOO (a)) as rate-determining step. The formate-surface interaction activates the unimolecular decomposition of formate to preferentially yield CO(g) and OH (a). An acidic proton of a HCOOH molecule, which encounters the surface in a steady state, reacts with the resultant OH (a) to form H2O as shown in Scheme 1. 

As it appeared that the main reactions on different catalytic systems had many features in common, the decomposition of formic acid will in the following chapters be discussed along the lines given by the two main reaction paths. Thus the dehydrogenation both on metals and oxides will be treated in the first part and the second part will be devoted to the dehydration reaction. As the latter reaction is also largely catalyzed in the liquid phase by strong acid, a short discussion of this phenomenon has been included. 

Results of Investigations on the Catalytic Decomposition of Formic Acid on Oxides (—H, dehydrogenation -CO dehydration)... 

The fact that the two rates are in the same range indicates not only that the dehydration of formic acid on alumina proceeds via a surface formate, as already shown by Hirota, but also that the decomposition of these formate ions determines the rate of the process. This is compatible with zero-order kinetics. 

The main lines of the mechanism of the dehydration of formic acid in the liquid phase under the influence of acids are clear. As pointed out in the paragraph in question the decomposition of formic acid is completely analogous to that of alcohol and proceeds via proton addition, water elimination, and the decomposition of an (unstable) carbonium ion. 

It may thus be taken for granted that in the dehydration of formic acid on A1203 formates play an essential role. This mechanism is visualized in Fig. 27, branch II. The infrared measurements made by Scholten showed that the decomposition of the formate ion determines the overall reaction rate, step lie being the slowest step. 

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