**3. Oxidation of BD by a heterolytic mechanism involving palladium catalysts**

Palladium catalysts are widely used in the liquid-phase heterolytic oxidation of olefins [38]. The most significant mechanisms for practice are acetoxylation of ethylene to vinyl acetate and Wacker oxidation of olefins converting ethylene to acetaldehyde and but-1-ene to methyl ethyl ketone. A mechanism of olefin oxygenation under the action of Pd(II) complexes established by Moiseev et al. and Henry et al. [39, 40] is now described in numerous publications (e.g., chapter by Reinhard Jira in book [24]). The mechanism includes the formation of Pd(II) complex with olefin and inner sphere transformations resulting in the reduction of Pd2+ to form carbonyl compound and Pd0 black. Assisted by Cu(II) chloride or other intermediate oxidant, reoxidation of Pd0 with oxygen closes the catalytic cycle, allowing the use of oxygen as a stoichiometric oxidant.

in a radical process. Telluric acid inhibits the radical process but does not operate as an oxidant

inactive in oxidation of BD (second row in **Table 2**). By contrast, the heterogeneous 5%Pd2%Te/C catalyst is able to provide nonradical oxidation, with the radical chain oxidation being inhibited by Te. As a result of inhibiting action of Te, the large amount of the catalyst and low concentration of BD appear unfavorable for the development of the chain process. The oxidation on the 5%Pd2%Te/C catalyst in aqueous dimethylacetamide (DMA) has been observed to give crotonaldehyde and methyl vinyl ketone as main products (third row in **Table 2**). Interestingly, crotonal-

of crotonaldehyde and methyl vinyl ketone are produced on the 5%Pd2%Te/C catalyst in the

Besides DMA, other polar solvents can be used in this oxidation. The presence of proton additive is required in the solvent (**Table 3**). No reaction has been observed in anhydrous

 **(mmol/L) Time (h) Products (mmol)**

**Furan Methyl vinyl ketone**

with H<sup>6</sup>

TeO<sup>6</sup>

, but nearly equal amounts

**Croton aldehyde1**

/N2 = 10/90, 40 atm) on 5% Pd 2%Te/C

solution is

O), T 90°C.

**Others**

105

to maintain the nonradical oxidation by Pd2+. As a result, the PdCl2

dehyde is a predominant product of heterolytic oxidation with PdCl2

**О (%) H2**

Crotonaldehyde can be partly subjected to further oxidation to crotonic acid.

**Table 3.** GC detected products from oxidation of BD (4.5 mmol) by oxygen (O2

**SO4**

DMA 1 17 — 4 0.2 1.4 1.2 Dioxane 0.5 — 5 6 0.5 1.0 0.7 Acetonitrile 1 17 — 5 <0.1 1.0 0.8 Acetonitrile 1 17 8 5 0.3 1.7 0.9 Acetonitrile 0.5 — 2 4 0.8 2.0 1.2 Acetonitrile 0.5 — — 3 0 0 0

for Pd0

**Catalyst (mg)**

PdCl2

PdCl2 120,

5% Pd 2% Te/C 2000

H6 TeO<sup>6</sup> 800 **BD (mmol)** **Time (h)**

**Table 2.** GC detected products from oxidation of BD by oxygen (O2

**Products (mmol)**

**Furan Acrolein Methyl vinyl** 

**ketone**

120 43 3 0.4 1.1 0.3 1.6 3.2 3.9 0.2

43 3 0.2 0.5 <0.1 0.4 <0.1 0.5 0.1

22 6 0.1 <0.1 0.8 0.6 — 0.2 0.3

**Crotonaldehyde**

Reactivity of a Simplest Conjugated Diolefin in Liquid-Phase Oxidation: Mechanisms and Products

**3-Butene-1,2-diol**

/N2 = 10/90, 60 atm) in DMA (30 mL, 3% H2

**2-Butene-1,4-diol, 4-hydroxybut-2-enal**

http://dx.doi.org/10.5772/intechopen.71259

same conditions.

**Solvent (g) Catalyst (g) Н<sup>2</sup>**

catalyst in a solvent (35 mL), T 100°C.

acetonitrile.

1

Analogous to light olefins, BD reacts under homogeneous conditions in an aqueous solution of PdCl2 catalyst and CuCl2 oxidant. The oxygenation is directed to one of the double bonds with the retention of the second double bond to produce crotonaldehyde [41, 42]. The oxidation conditions are identical to those applied for oxidation of ethylene to acetaldehyde and 1-butene to methyl ethyl ketone (Wacker-type oxidation), but the kinetics is different [43], in particular the order of reaction with respect to Cl− and H+ ions. Unlike the oxidation of ethylene and other olefins, the oxidation of BD is zero-order with respect to the hydrocarbon. The kinetic parameters of BD oxidation are determined by high reactivity of the conjugated π-bonds, in particular by a strong BD to Pd2+ bonding in the intermediate complex. Unlike propylene, the oxygenation of the BD double bond is directed at the terminal rather than inner carbon atom to form crotonaldehyde. This is probably due to the stabilizing effect of the second double bond. In the presence of Pd2+ ions and another strong oxidizing agents of P-Mo-V heteropolyacids, BD is converted to furan in the similar conditions [44]. It seems like crotonaldehyde was initially formed and then converted under oxidizing conditions to furan, as in a similar homogeneous system [45]. Oxygen is a final stoichiometric oxidant, but the strong intermediate oxidant (Cu2+ or heteropolyacid) is necessary for easy regeneration of the ionic palladium in the oxidation of BD and olefins, as well.

We have observed catalysis by PdCl2 when the radical chain oxidation of BD to diols, furan, and acrolein proceeds along with nonradical oxidation to form mainly crotonaldehyde together with small amounts of methyl vinyl ketone and furan (**Scheme 3**) (first row in **Table 2**). It is interesting that the system does not contain an oxidizing agent, except oxygen. There is no need of any intermediate oxidant since reoxidation of Pd0 to Pd2+ is provided by peroxide intermediates generated

**Scheme 3.** Nonradical reaction of BD on PdTe/C catalyst in polar solvents.


**3. Oxidation of BD by a heterolytic mechanism involving palladium** 

Palladium catalysts are widely used in the liquid-phase heterolytic oxidation of olefins [38]. The most significant mechanisms for practice are acetoxylation of ethylene to vinyl acetate and Wacker oxidation of olefins converting ethylene to acetaldehyde and but-1-ene to methyl ethyl ketone. A mechanism of olefin oxygenation under the action of Pd(II) complexes established by Moiseev et al. and Henry et al. [39, 40] is now described in numerous publications (e.g., chapter by Reinhard Jira in book [24]). The mechanism includes the formation of Pd(II) complex with olefin and inner sphere transformations resulting in the reduction of Pd2+ to

Analogous to light olefins, BD reacts under homogeneous conditions in an aqueous solution

with the retention of the second double bond to produce crotonaldehyde [41, 42]. The oxidation conditions are identical to those applied for oxidation of ethylene to acetaldehyde and 1-butene to methyl ethyl ketone (Wacker-type oxidation), but the kinetics is different [43],

ethylene and other olefins, the oxidation of BD is zero-order with respect to the hydrocarbon. The kinetic parameters of BD oxidation are determined by high reactivity of the conjugated π-bonds, in particular by a strong BD to Pd2+ bonding in the intermediate complex. Unlike propylene, the oxygenation of the BD double bond is directed at the terminal rather than inner carbon atom to form crotonaldehyde. This is probably due to the stabilizing effect of the second double bond. In the presence of Pd2+ ions and another strong oxidizing agents of P-Mo-V heteropolyacids, BD is converted to furan in the similar conditions [44]. It seems like crotonaldehyde was initially formed and then converted under oxidizing conditions to furan, as in a similar homogeneous system [45]. Oxygen is a final stoichiometric oxidant, but the strong intermediate oxidant (Cu2+ or heteropolyacid) is necessary for easy regeneration of the

acrolein proceeds along with nonradical oxidation to form mainly crotonaldehyde together with small amounts of methyl vinyl ketone and furan (**Scheme 3**) (first row in **Table 2**). It is interesting that the system does not contain an oxidizing agent, except oxygen. There is no need of any inter-

black. Assisted by Cu(II) chloride or other intermediate

when the radical chain oxidation of BD to diols, furan, and

to Pd2+ is provided by peroxide intermediates generated

ions. Unlike the oxidation of

with oxygen closes the catalytic cycle, allowing the use of oxygen

oxidant. The oxygenation is directed to one of the double bonds

and H+

**catalysts**

104 Alkenes

of PdCl2

form carbonyl compound and Pd0

catalyst and CuCl2

We have observed catalysis by PdCl2

mediate oxidant since reoxidation of Pd0

in particular the order of reaction with respect to Cl−

ionic palladium in the oxidation of BD and olefins, as well.

**Scheme 3.** Nonradical reaction of BD on PdTe/C catalyst in polar solvents.

oxidant, reoxidation of Pd0

as a stoichiometric oxidant.

**Table 2.** GC detected products from oxidation of BD by oxygen (O2 /N2 = 10/90, 60 atm) in DMA (30 mL, 3% H2 O), T 90°C.

in a radical process. Telluric acid inhibits the radical process but does not operate as an oxidant for Pd0 to maintain the nonradical oxidation by Pd2+. As a result, the PdCl2 with H<sup>6</sup> TeO<sup>6</sup> solution is inactive in oxidation of BD (second row in **Table 2**). By contrast, the heterogeneous 5%Pd2%Te/C catalyst is able to provide nonradical oxidation, with the radical chain oxidation being inhibited by Te. As a result of inhibiting action of Te, the large amount of the catalyst and low concentration of BD appear unfavorable for the development of the chain process. The oxidation on the 5%Pd2%Te/C catalyst in aqueous dimethylacetamide (DMA) has been observed to give crotonaldehyde and methyl vinyl ketone as main products (third row in **Table 2**). Interestingly, crotonaldehyde is a predominant product of heterolytic oxidation with PdCl2 , but nearly equal amounts of crotonaldehyde and methyl vinyl ketone are produced on the 5%Pd2%Te/C catalyst in the same conditions.

Besides DMA, other polar solvents can be used in this oxidation. The presence of proton additive is required in the solvent (**Table 3**). No reaction has been observed in anhydrous acetonitrile.


1 Crotonaldehyde can be partly subjected to further oxidation to crotonic acid.

**Table 3.** GC detected products from oxidation of BD (4.5 mmol) by oxygen (O2 /N2 = 10/90, 40 atm) on 5% Pd 2%Te/C catalyst in a solvent (35 mL), T 100°C.

**4. Heterolytic mechanism of 1,4-oxidative addition to BD**

BD selectively to 2-butene-1,4-diol diacetate (**Scheme 5**) [52, 53].

fins [57] proceed effectively, but BD reacts with low yield and selectivity.

(OAc−

**Scheme 5.** Oxidative 1,4-addition to BD.

Wacker-type oxidation of olefins and analogous Pd-catalyzed nonradical oxidation of BD produce usually carbonyl compounds, but special additives are required for obtaining dioxygenates. Nevertheless, the oxidative 1,2-addition to olefins is known to occur under the action of Pd2+ complex and oxoanion strong oxidants, such as periodate [48] or nitrate anions, in acetic acid solution to form glycol derivatives [49–51]. Mechanism of the oxidation is based on a nonradical inner sphere interaction of olefin with oxidant in Pd2+ complex. Similar interaction is probably realized in oxidation of BD in the presence of palladium as the catalyst of nonradical heterolytic olefin oxidation and Sb, Bi, Te, or Se promoters. Heterogeneous catalysts containing these active components have shown unique catalytic properties in oxidation of

Reactivity of a Simplest Conjugated Diolefin in Liquid-Phase Oxidation: Mechanisms and Products

http://dx.doi.org/10.5772/intechopen.71259

107

XPS analysis of the Pd and PdTe catalysts indicates that Te-oxide is able to increase positive charge on Pd surface [46], thus being an oxidation promoter for palladium. The ionic state of surface palladium is responsible for heterolytic oxidation. Acetic acid is used as a solvent for this reaction. The mechanism of formation of 2-butene-1,4-diol diacetate is proposed by Takehira et al. for PdTe catalyst (**Scheme 6**) [54], and fundamentally identical one is proposed for the RhTe catalyst [55]. The details in intermediate structures explain the preferential formation of trans-2-butene-1,4-diol in the case of Pd-containing catalyst and cis-isomer in the case of Rh.

Exceptionally high selectivity of BD to 2-butene-1,4-diol diacetate conversion is explained by a concert interaction of BD with surface Pd and with acetate anions. Adsorbed on Pd, BD forms π-allyl-type intermediate that undergoes acetoxylation on the terminal carbon atom. Resulting monoacetoxyl reacts with the second acetate to give 2-butene-1,4-diol diacetates and 3-butene-1,2-diol diacetate in amounts proportional to the reactivity of carbon atoms 1 and 2 (**Scheme 6**). In fact, only 2-butene-1,4-diol diacetates are produced. Analogous mechanisms are realized in homogeneous oxidation of various dienes in the presence of Pd complexes and p-benzoquinone oxidizing agent, instead of Te. Oxidation of diene alcohols [56] and substituted conjugated diole-

As noted earlier, Te-oxide is able to inhibit radical chain oxidation of BD, the selectivity of which is lower than the selectivity of the heterolytic process. Besides, Te operates as an inhibitor of radical polymerization of BD and oxidation products, thus preventing the formation of side high-boiling products. Acetic acid (possibly, other carboxylic acids) also contributes to the achievement of high selectivity in BD oxidation. Being not only solvent but also reagent

anions), it is involved in an intermediate interaction with olefin to form the surface Pd

**Scheme 4.** Tentative routes for nonradical oxidation of BD on PdTe/C catalyst.

According to XPS analysis, the 5%Pd2%Te/C catalyst contains both reduced Pd0 and ionic Pd2+, and two oxidation states of tellurium Te0 and Te4+ [46]. The Pd2+ to Pd0 ratio on the catalyst surface becomes larger with an increase in tellurium content that indicates an oxidizing influence of TeO2 . It can be expected that the oxidation state of the surface is enhanced under the reaction conditions. Nevertheless, dissolution of Pd and Te during reaction does not exceed 1% of the content of both components in the solid catalyst, the solution exhibiting no catalytic activity. Therefore, activity of the catalyst refers to the active components on the surface of carrier and is associated with their reversible redox transformations. Based on the known mechanisms of homogeneous oxidation of olefin, one can propose two possibilities for oxidation of BD by oxygen on the PdTe species, both assuming a nonradical heterolytic interaction. Perhaps the mechanism is in general similar to that postulated for the oxidation of BD and olefins in the presence of Pd2+ complexes, oxygen, and intermediate oxidant (**Scheme 4**, Route 1). It involves surface Pd2+ ions and TeO2 oxidant providing regeneration of Pd2+.

However, there is a difference in products composition. Crotonaldehyde and furan are produced in above-mentioned oxidations of BD with homogeneous Pd2+ catalysts [41, 42], whereas methyl vinyl ketone is the second product formed in our oxidation on the PdTe catalyst. To explain this difference, one can consider an oxidation of BD by hydrogen peroxide as an alternative or parallel reaction (Route 2 in **Scheme 4**). Hydrogen peroxide is generated from oxygen on Pd0 species. The high reactivity of olefins with respect to peroxide compounds is known [47]. It is known that hydrogen peroxide does not accumulate during reaction. But it is found in trace amounts in the reaction solution and can form a reactive peroxide compound of Te4+ on the surface of the catalyst. In both mechanisms proposed, Te serves as a carrier of molecular or peroxide oxygen, and the surface of Pd2+/Pd0 activates reagents due to the adsorption of O2 and BD. Thus, the PdTe/C catalyst opens the possibility of oxidation of BD by a nonradical heterolytic mechanism due to the combined effect of the two active components.
