3.2 Effects of Na+ or K+ on the HCN formation pathways
The presence of metal ions will not change the possible pathways to form HCN from pyridine, but change the geometries of transition states and intermediates. The detailed pathways are compared below, and the letters “n” and “k” are used to represent the presence of Na+ and K+, respectively.
3.2.1 HCN formation from pathway-a
Energy profiles of the HCN formation with/without catalysis via path-a are depicted in Fig. 1. As discussed above, the decomposition of pyridine starts from the hydrogen transfer from C1 to N, disturbing the stability of the N-heterocycle. The transition states a-1t, a-n-1t, and a-k-1t are involved in this step for the non-catalytic, Na+- and K+-catalyzed conditions, with the activation energies of 357.7, 349.6, and 354.4 kJ/mol. Afterward, the six-membered intermediate a-1m isomerizes into the five-membered ring a-2m through the C1-N bond cleavage and the C1-C6 bond generation simultaneously. The presence of Na+ and K+ increases the activation energy from 462.2 kJ/mol to 468.1 and 470.2 kJ/mol, respectively. Thus, Na+ and K+ inhibit the isomerization reaction of the pyridine ring, and the impact of K+ is greater than Na+.
Subsequently, the five-membered ring of intermediate a-2m is cleaved via a concerted ring-opening reaction, where the C = N double bond turns into a triple bond. This step requires 511.3 kJ/mol of activation energy without catalysis. As for Na+- and K+-catalyzed pyridine pyrolysis, the ring-opening needs to overcome the activation energy of 460.4 and 467.0 kJ/mol, respectively. It indicates that Na+ and K+ can significantly reduce the activation energy (~ 50 kJ/mol) of the concerted ring-opening reaction of pyridine derivatives. Finally, the C4-C5 bond of intermediate a-3m cleaves via the hydrogen transfer from C4 to C5, producing a-4m (vinylacetylene) and HCN. The final step requires to overcome an activation energy of 561.2 kJ/mol under the non-catalytic condition. The addition of Na+ and K+ lowers the activation energy by 84.7 and 36.4 kJ/mol, respectively, showing that Na+ and K+ promote the breakage of the C-C bond to form HCN. Moreover, Na+ has a greater impact than K+ in this step. Notably, the final step is the rate-determining step regardless of the presence of alkali metal ions in path-a, and the overall activation energies are 561.2, 476.5, and 524.8 kJ/mol for the non-catalytic, Na+- and K+-catalyzed conditions, respectively.
In summary, alkali metal ions (Na+ and K+) can significantly promote the concerted ring-opening and final hydrogen transfer in path-a. What the two reactions have in common is both of them involve the C-C bond rupture. On the contrary, the isomerization reaction and the initial hydrogen transfer of path-a are less affected by the two alkali metal ions. Na+ and K+ can lower the overall activation energy to promote the HCN generation via path-a, with Na+ having a greater catalytic effect than K+.
3.2.2 HCN formation from pathway-b
The effect of alkali metal ions on the pyrolysis of pyridine in path-b is shown in Fig. 2. The initial hydrogen transfer from the C1 position to the C2 position is special, in comparison with those in other pathways. The N-heterocycle is cleaved by the C1-C2 bond cleavage, accompanied by the hydrogen transfer. As stated above, this reaction is facilitated in the presence of alkali metal ions. The activation energies are 413.9, 344.5, and 342.0 kJ/mol for the non-catalytic, Na+ and K+-catalyzed conditions. Then, intermediate b-1m undergoes isomerization by forming the C1-C5 bond and breaking the C5-N bond at the same time. The activation energy is 371.5 kJ/mol under the non-catalytic condition. During catalytic pyridine pyrolysis, the activation energies of the transition states in the presence of Na+ and K+ are 395.0 kJ/mol and 389.9 kJ/mol, which are 23.5 kJ/mol and 5.1 kJ/mol higher than that without alkali metal ions, respectively. This phenomenon shows that Na+ and K+ inhibit the isomerization reaction of the pyridine ring, and Na+ has a slightly stronger effect than K+. Finally, the newly formed C1-C5 bond cleaves through a hydrogen transfer reaction to produce b-3m (vinylacetylene) and HCN. Without catalysis, the activation energy is 561.2 kJ/mol, and it is the rate-determining step of path-c. In the presence of Na+ and K+, the activation energy decreases by 84.7 and 89.4 kJ/mol, respectively. Even so, this reaction is still the rate-determining step under the Na+- and K+-catalyzed conditions.
To sum up, the two alkali metal ions promote the formation of HCN by significantly reducing the overall activation energy in path-b. The overall catalytic ability of the two alkali metal ions is close, with K+ having a slight advantage over Na+. As expected, the two alkali metal ions have manifest promotion to the breakage of C-C bonds (step 1 and step 3).
3.2.3 HCN formation from pathway-c
Figure 3 shows the energy profiles of path-c under the non-catalytic and alkali metal ions-catalyzed conditions. The hydrogen transfer from C2 to C1 initiates the decomposition of pyridine in path-c. Under the non-catalytic condition, transition state c-1t is involved with an activation energy of 389.6 kJ/mol. Under the catalysis of Na+ and K+, transition states c-n-1t and c-k-1t are involved, with the activation energy increasing by 21.4 and 15.1 kJ/mol, respectively. Hence, both Na+ and K+ can hinder internal hydrogen transfer (C2 to C1), and the effect of Na+ is slightly greater than that of K+. Following the initial hydrogen transfer, the H at C1 then migrates to the N atom (485.1 kJ/mol) under the non-catalytic condition. Similarly, the activation energies of this step are also increased under the alkali metal ions, which are 535.2 and 527.6 kJ/mol for Na+ and K+, respectively.
Different from path-a, the intermediate c-2m undergoes the ring-opening reaction to break the C1-N bond, without involving a five-membered intermediate. The activation energy slightly decreases from 359.0 kJ/mol to 355.6 (Na+) and 354.2 kJ/mol (K+). Subsequently, the N-H bond rotates into a proper position for the final hydrogen transfer from the N atom to the C4 position. Finally, the concerted cleavage of the C4-C5 bond leads to the production of HCN and b-5m (vinylacetylene). The presence of Na+ and K+ greatly decreased the activation energy of the final hydrogen transfer (687.4 kJ/mol vs 503.0 (Na+) and 516.6 (K+) kJ/mol). In this step, Na+ has a greater promotion effect than K+.
According to the above results, the rupture of C-C bonds in the ring-opening and final hydrogen transfer reactions are facilitated by the alkali metal ions, which is similar to path-a. Differently, they have little effects on other reactions that do not involve the rupture of C-C bonds. In addition, the two alkali metal ions also alter the rate-determining step of path-c. The final step determines the reaction rate without catalysis, whereas the second step of hydrogen transfer (C1 to N) is the rate-determining step during the Na+/K+-catalyzed pyridine pyrolysis. In total, Na+ and K+ significantly promote the concerted decomposition during pyridine pyrolysis to form HCN, and Na+ shows a greater catalytic effect than K+.
3.2.4 HCN formation from pathway-d
Figure 4 gives the catalytic effects of Na+ and K+ on path-d. As mentioned above, the initial hydrogen transfer from C3 to C2 is hindered in the absence of the alkali metal ions. The activation energy increases from 406.6 kJ/mol to 423.0 (Na+) and 419.5 (K+) kJ/mol. Afterward, the C1-N bond breaks along with the hydrogen transfer from C5 to the N atom. It's hardly surprising that the activation energy is greatly reduced by the catalysis of Na+/K+ (660.5 kJ/mol vs 433.3 (Na+) and 431.6 (K+) kJ/mol). The H atom bound to the N atom then migrates to the C4 position to form the C ≡ N triple bond. It is notable that the activation energy is also reduced greatly with the catalysis of the two alkali metal ions. This reaction does not involve the breakage of a C-C bond but a transformation from the double bond to the triple bond. The non-catalytic activation energy is 587.1 kJ/mol. When Na+ is included, the activation energy (454.6 kJ/mol) is lowered by 132.5 kJ/mol, while the activation energy (461.8 kJ/mol) with K+ is 125.3 kJ/mol lower. Similarly, HCN and d-4m (vinylacetylene) are also formed by a final breakage of the C4-C5 bond via hydrogen transfer, and the activation energy is 620.9 kJ/mol. The activation energies with Na+ catalysis (540.4 kJ/mol) and K+ catalysis (550.6 kJ/mol) are 80.5 and 70.3 kJ/mol lower than that without catalysis. Thus, Na+ and K+ significantly promote the final C-C bond cleavage to form HCN, and the promotion effect of Na+ is stronger than K+.
In the presence of alkali metal ions, not only the scission of the C-C bond but also the formation of the triple bond from the double bond is facilitated by the catalysis of Na+ and K+. Additionally, the final step reaction determines the reaction rate of path-d, which is different from the non-catalytic condition where the second step of hydrogen transfer is the rate-determining step. All in all, both Na+ and K+ alter the rate-determining step of path-d, reduce the activation energy, and promote the formation of HCN, with Na+ having a greater influence than K+.
3.2.5 HCN formation from pathway-e
Figure 5 illustrates the energy profiles of path-e under the non-catalytic, Na+- and K+-catalyzed conditions. First, the H atom at the C3 position of pyridine transfers to the C2 position to form e-1m via an internal hydrogen transfer reaction. The transition state is e-1t in the absence of alkali metal ions, which has an activation energy of 389.4 kJ/mol. Both Na+ and K+ inhibit the internal hydrogen transfer reaction. The activation energy with Na+ catalysis is 423.4 kJ/mol, which is 34.0 kJ/mol higher than that without an alkali metal ion. K+ has a slightly weaker influence than Na+ by increasing the activation to 418.0 kJ/mol. Then, the intermediate e-1m undergoes a concerted ring-opening reaction to break the C2-C3 bond via the transition state e-2t (481.5 kJ/mol). As for the Na+/K+-catalyzed pyrolysis of pyridine, the activation energy is lowered to 399.6 kJ/mol and 402.5 kJ/mol, respectively. Different from the other four pathways, the breakage of the C5-N bond results in the formation of the C ≡ N triple bond, producing HCN and e-3m. The final step determines the reaction rate of path-e without catalysis, and the overall activation energy is 625.3 kJ/mol. The presence of Na+ and K+ significantly decreases the activation free energy of this reaction by 163.7 and 157.9 kJ/mol, respectively. Even so, the final C-N bond scission is still the rate-determining step under the Na+- and K+-catalyzed conditions, with the overall activation energy of 461.6 and 467.4 kJ/mol, respectively.
The above calculation results indicate that alkali metal ions significantly promote the C-C bond scission and the formation of the C ≡ N triple bond of path-e. Na+ and K+ lower the overall activation energies for pyridine pyrolysis, further promoting the formation of HCN. Overall, Na+ has a slightly greater catalytic effect than K+.