According to previous reports (Zheng et al. 2016;Guo et al. 2019;Mettler et al. 2012) and the PY-GC/MS experimental results of current work, the main products' pathways (5-hydroxymethylfurfural, furfural, and benzene) in the cellulose catalytic pyrolysis reaction pathway were proposed. Hu et al. (Hu et al. 2018) suggested a complete reaction route for the non-catalytic route of preparing 5-hydroxymethyl furfural and furfural from the pyrolysis of glucose as a model compound. The routes with the most significant probability of occurrence are shown in Fig3. The reaction of reactant (R) to 5-hydroxymethylfurfural (P1) is path 1, and the reaction of reactant (R) to furfural (P2) is path 2-1. However, this article mainly studies the mechanisms of catalytic pyrolysis. To reduce the simulation calculation error, we choose a calculation method that takes the weak interaction force between molecules into the functional—M06-2X. Then, based on the 6-31g(d) level and the temperature, the non-catalytic Gibbs free energy without catalyst participating in the reaction was calculated. The transition states of the elementary reactions are listed in the supplementary information Table 2.
Path 1 (yellow) is a typical path where D-fructose (DF) participates. Glucose first undergoes a ring-opening reaction to form acyclic D-glucose (IM1) with the activation free energy of 8.9 kcal/mol. This reaction involves a coordinated transition state, TS1 (Table 2). IM1 is re-isomerized to DF (IM2), with the activation free energy of 72.8 kcal/mol. This reaction involves a coordinated transition state, TS2 (Table 2), in which the 2-hydroxy H was transferred to the 1-carbonyl group, and the other H in the C2 position is transferred to the C1 position. Later, DF formed β-D-fructofuranose (IM3) by forming a transition state TS3 in the form of a 2,5-acetal ring (Table 2). The quaternary transition state TS4 dehydrates IM3 at 2-OH+1-H (hydroxyl at C2 position and hydrogen at C1 position) to form intermediate IM4 (ΔEa = 89.9 kcal/mol), which is the decisive rate of the reaction path step. IM4 undergoes one-step of electro-ring dehydration to form IM5 directly. The reaction is a six-membered transition state TS5 (Table 2), where 3-OH and 1-H (OH) are dehydrated, and C2=C3 bonds and C1=O bonds are formed. The reaction is thermodynamically favorable (ΔG = −25.2 kcal/mol) and requires relatively low activation energy (ΔEa = 34 kcal/mol). Finally, IM5 dehydrates at the 4-OH + 5-H site to form 5-HMF (P1).
Path 2-1 is the most advantageous way for 3-deoxy-glucosone (3-DG) to participate in FF formation. As shown in Fig3, IM1 is first dehydrated at the 3-OH+2-H site to form the enol isomer IM6 of 3-DG (IM1-TS7, ΔEa = 63 kcal/mol). Then IM6 undergoes enol-ketone tautomerism through the quaternary transition state TS8 (IM6-TS8, ΔEa = 70.1 kcal/mol) into 3-DG (IM7). Both of these reactions are thermodynamically favorable (ΔG = −18.2 and -0.5 kcal/mol). Similar to DF, 3-DG forms a 2,5-acetal ring through hemiacetal reaction to form a five-membered intermediate IM8. The formation of IM8 involves the quaternary transition state TS9, the structure of which is shown in Table 2. In Path 2-1 (blue), IM8 is dehydrated between 4-OH and 6-H (OH) and formaldehyde to form an intermediate product IM9 with a high activation free energy of 84.3 kcal/mol. This is the decisive step of path 2-1. The reaction involves the six-membered transition state TS10, the structure of which is shown in Table 2. Finally, IM9 is dehydrated at the 2-OH + 3-H site to form FF (P2). The acyclic D-glucose produced by the ring-opening reaction of β-D-glucopyranose, and DF and 3-DG as important intermediates, are essential for developing both 5-HMF and FF.
After DF and 3-DG were formed in Path 1 and Path 2-1, respectively, the five-membered intermediate could quickly form 5-HMF and FF through a continuous dehydration reaction. Since the boiling point of 5-HMF is 114~116℃, but the experimental temperature is 550℃. Therefore, under rapid high-temperature pyrolysis, 5-HMF will be further degraded. Degradation products include furans (5-methylfurfural, furfural, 2-methylfuran, etc.) and small molecules (CO, CO2, H2O, formic acid, formaldehyde, acetic acid, etc.). Therefore, we consider the reaction of P1 to directly remove formaldehyde through transition state TS12 to obtain P2 as path 2-2 (red). This process of eliminating formaldehyde requires activation-free energy of 85.5 kcal/mol.
According to our experimental results, after adding the zeolite catalyst, new aromatic products (benzene, toluene, p-xylene, etc.) appeared in the product. Some researchers have proposed that furan compounds will undergo DA cycloaddition reaction with olefins, and furfural will undergo decarbonylation to produce carbon monoxide and furan (Cheng and Huber 2012). For this reason, we propose a conversion path from furfural (P2) to benzene (P3) on this basis, as shown in Fig3, path 3 (orange). Furfural first removes free carbon monoxide through the transition state TS13 to form the furan intermediate IM10. This is the crucial step of the path, which requires activation free energy of 88.2 kcal/mol. During the reaction, IM10 and short-chain ethylene undergo a DA reaction (IM10-TS14, ΔEa=70.1 kcal/mol) to obtain intermediate IM11. Under the reaction of water molecules, the C4-O ether bond of IM11 together with the C2=C3 double bond are disturbed. Then, a new C3=C4 double bond is generated, and a C1-C2-O ether bond is generated to obtain a ternary epoxy structure. Finally, a hydrogen bond is formed between the hydrogen of the water molecule and the epoxy, resulting in the IM12 complex. This reaction requires activation free energy of 74.2 kcal/mol. However, the IM12 intermediate has a shorter residence time and is quickly isomerized to IM13. IM13 continuously removes two molecules of water to obtain the product benzene (P3).
The energy consumed by the non-catalytic reaction path was generally at a high level. The easiest way to reduce the reaction energy barrier is to add a catalyst, and the experimental results also confirm the yield gap of typical products in the pyrolysis reaction after adding the catalyst (Table 3). Next, by calculating the reaction's energy after adding the catalyst, we will explain the changes in the product yield from a theoretical point of view.
Possible reaction pathways of typical products of cellulose catalytic pyrolysis
In the catalytic reaction system, the reactant and the catalyst interact. To express the reaction energy more accurately and reduce the calculation error, we put the reactant and the catalyst in the same system for simulation. Therefore, add the prefix C_ when naming each compound. When the compound is close to the acidic site of the catalyst, a complex is formed, where this adsorption process has certain adsorption energy. Since adsorption has little effect on product selectivity, this article ignores the effect of adsorption.
Qian et al. found that the 2-OH protonation on D-glucose resulted in the formation of 5-HMF and furfural through carbocation rearrangement (Qian et al. 2005). The protonation of 3-OH and 4-OH leads to two different five-membered ring degradation products in addition to 5-HMF, which may be the reason for other non-cellulose materials observed in experiments (Abatzoglou et al. 1986;Qian et al. 2005). However, the protonation of 1-OH and 6-OH does not cause any observable reactions. The Multiwfn software was used to predict and analyze the surface electrostatic potential of glucose molecules. As shown in Table 4, the 3-OH of R was the most active (-40.2 kcal/mol), followed by the 4-OH position. Also, the active-H of the catalyst was an acidic site. Considering all together, we choose the 4-OH site on R to react with the catalyst first. Therefore, a catalytic pyrolysis path from D-glucose to 5-HMF was designed, as shown in Fig4.
Table 4. Extreme value of the electrostatic potential on the surface of glucose molecule.
Number of surface minima: 7
|
#
|
a.u.
|
eV
|
kcal/mol
|
1(2-OH)
|
-0.03936667
|
-1.071222
|
-24.702979
|
2(4-OH)
|
-0.06223913
|
-1.693613
|
-39.055676
|
3*(3-OH)
|
-0.06403766
|
-1.742553
|
-40.18427
|
4(C1-O7-C5)
|
-0.03468574
|
-0.943847
|
-21.765646
|
5(1-OH)
|
-0.04740712
|
-1.290013
|
-29.74844
|
6(C6-H)
|
0.02659904
|
0.723797
|
16.691161
|
7(6-OH)
|
0.01467596
|
0.399353
|
9.209311
|
Table 5 lists the transition state geometry involved in the catalytic pyrolysis. In path 1'(pink), C_R generates C_IM1 through the dehydration of transition state C_TS1, with the activation free energy of 2.2 kcal/mol. In this elementary reaction, the active site active-H of the catalyst was close to the 4-OH on the ring leading to dehydration. At the same time, 5-H, is returned to the catalyst to form the six-member epoxy olefin intermediate IM1. The second step was the ring-opening reaction. Active-H induces the breaking of the ether bond on the ring, forming 5-OH of the linear intermediate IM2. Also, 1-OH provides an H atom to the catalyst to form 1-carbonyl group; IM1 generates intermediate IM2, with the activation energy of 1.9 kcal/mol. The third step is the hydrogen transfer reaction. In the presence of a catalyst, the carbonyl group is attacked by active hydrogen, 2-H is transferred to the C1 position, and 2-OH gives a hydrogen atom. The activation free energy from IM2 to C_TS3 is 4.7 kcal/mol. In the absence of a catalyst, the fourth step is an aldol condensation reaction, but it is easier to induce ring formation by active-H atoms. The straight-chain needs to be rotated and bent to form a ring, while the catalyst's porous structure promotes the straight-chain to bend; resulting in a shorter distance between the 2-carbonyl group and the 5-hydroxy group. When the space structure is suitable, active-H attacks the 2-carbonyl group, and the 5-OH breaks away from hydrogen and returns to the catalyst. This condensation reaction forms the furan ring precursor intermediate IM4 through the transition state C_TS4 (ΔEa = 6.7 kcal/mol). Subsequently, IM4 is dehydrated at 2-OH+active-H and 1-H (OH)+C and forms a C2=C3 bond. An 18.4 kcal/mol activation energy was consumed in the process of forming the enol intermediate IM5. Finally, IM5 can directly undergo cyclization dehydration tautomerization reaction to form P1 in one step. The reaction is the ten-membered transition state C_TS6 (Table 5), where 3-OH+active-H and 1-H (OH)+C are dehydrated and form C2=C3 and C1=O.
There are two paths for non-catalytic pyrolysis to generate P2. We choose the shortest path (path 2-2', green) to calculate catalytic pyrolysis’s energy; P1 removes the terminal free methylene group. The active H of the catalyst promotes the breaking of the C5-C6 single bond, the methylene group is then separated from the furan ring, and then the active hydrogen is added to the C4=C5 double bond. The hydroxyl group of -CH2OH removes a hydrogen atom to neutralize the catalyst to maintain charge neutrality. The transition state C_TS7 is shown in Table 5. This formaldehyde removal process requires activation free energy of 35.3 kcal/mol, which is 50.2 kcal/mol lower than that of non-catalysis.
In the process where furfural reacts with ethylene to convert to benzene (path 3', purple), the catalyst has the same effect, and the catalytic reaction is induced by active hydrogen. The free 1-carbonyl group of furfural removes CO under the catalysis of active hydrogen. First, active-H attacks the C1-C2 positions to promote single bond cleavage. Subsequently, the reactive H undergoes an electrophilic attack to produce the intermediate furan (IM6). At the same time, 1-H is returned to the catalyst and forms free CO. The second step Diels-Alder (DA) reaction, is the key step for forming benzene ring. Since the addition reaction is bimolecular, the transition state C_TS9 with a roof-shaped spatial geometry is formed during the addition process. A double bond forms, so active-H cannot participate in the reaction, but active H affects the electronic structure around the reactant and can also reduce the energy required for the reaction to a certain extent, which is consistent with the report by Patet et al. (Patet et al. 2017). The third step is that active-H promotes the breaking of the ether bond and the free water molecules in the reaction process, and the oxygen on the ring form a hydrogen bond, preparing for the water addition reaction in the fourth step. Finally, two molecules of water are continuously removed to produce benzene (P3). Table 6 lists the activation energy and reaction heat of the reaction process. Fig7 is the potential energy curve of the non-catalytic and catalytic pathways for forming a benzene ring. We found that in non-catalysis, the decisive step is the first step of CO removal reaction. In catalysis, the final step is the DA reaction, and except for the DA reaction, the energy barrier of other elementary reactions is greatly reduced. This is all due to the excellent catalytic effect of the zeolite catalyst. It also shows that the effect of zeolite catalyst on the DA reaction is not favorable. Since the zeolite catalyst could not significantly reduce the energy required for DA reaction. We will further explore high-efficiency catalysts to reduce the DA reaction's energy barrier in the future study.
Table 6. Kinetic and thermodynamic parameters of cellulose catalytic pyrolysis reaction pathway (kcal/mol).
|
Activation energy (ΔEa)
|
Reaction heat (ΔG)
|
Path 1’
|
|
|
R- ts1- im1
|
2.18
|
-8.82
|
im1- ts2- im2
|
1.86
|
-5.98
|
im2- ts3- im3
|
4.73
|
3.35
|
im3- ts4- im4
|
6.66
|
-5.74
|
im4- ts5- im5
|
18.35
|
-11.72
|
im5- ts6- P1
|
5.30
|
-25.57
|
Path 2’
|
|
|
P1- ts7- P2
|
35.28
|
15.44
|
Path 3’
|
|
|
P2- ts8- im6
|
2.17
|
-17.41
|
im6- ts9- im7
|
45.35
|
2.25
|
im7- ts10- im8
|
26.70
|
11.16
|
im8- ts11- im9
|
15.80
|
-12.66
|
im9- ts12- im10
|
27.01
|
-17.07
|
im10- ts13- P3
|
1.42
|
-26.24
|
Fig8 shows the potential energy curves of different catalytic pathways. In the catalytic reaction, each step of the reaction’s activation free energy is greatly reduced, which is very beneficial to the reaction in terms of kinetics. The reaction heat of the non-catalyzed reaction is in the supporting information Table S1, and the reaction heat of the catalyzed reaction is listed in Table 6. The thermodynamic energy of most elementary reactions is negative, representing an exothermic process, which is also thermodynamically beneficial. We also found that most of the transition states involved in non-catalytic reactions are four-membered ring transition states. Still, the eight-membered transition state dominates the transition states of catalytic reactions, and there is also the ten-membered transition state C_TS6. Also, C_TS6 has the lowest activation energy of transition state structure in all dehydration reactions, which is only 5.3 kcal/mol. If the transition state ring formed at the position where the chemical reaction occurs is larger, the activation energy required for the reaction is less. This is consistent with our previous research results (Wang et al. 2021;Cheng et al. 2021).
The speed-determining steps of the three catalytic pathways are C_TS5 (ΔEapath1'-L=18.4 kcal/mol), C_TS7 (ΔEapath2-2'-L=35.3 kcal/mol), and C_TS9 (ΔEapath3'-L=45.4 kcal/mol), indicating that 5-hydroxymethyl furfural is the easiest to produce from glucose, followed by furfural, and finally benzene. In the experiment, cellulose was first decomposed into single-molecule glucose, and then the glucose had 5-hydroxymethylfurfural, furfural, and benzene according to the difficulty of the reaction. Moreover, benzene can only be produced when the catalyst is present. In the non-catalytic pyrolysis experiment, the yield of furfural reached 25.34%, while after adding the catalyst, it decreased to 12.81%. However, the content of benzene increased from almost zero to 13.04%. Therefore, it can be inferred that furfural participates in the production of benzene, showing that the experimental and theoretical results are consistent.
To study the Diels-Alder cycloaddition reaction of furan and ethylene on the Brønsted-acidic molecular sieve, Patet et al. used electronic structure calculations and microscopic kinetic models, and the subsequent dehydration reaction of furan and ethylene(Patet et al. 2017). The electronic structure calculation showed that the acid strength of the Brønsted-acidic zeolite changes with the changes of Al, Ga, Fe, and B metal substituents. For these four modified Brønsted-acidic zeolites, the catalytic effect of the DA reaction is not apparent. However, for dehydration reactions, Brønsted-acidic follows the general principle of acid catalysis for dehydration reactions and plays an essential role in catalyzing these reactions. Our calculation results also showed that the activation energy of the DA reaction catalyzed by zeolite was 45.4 kcal/mol, which is only 9.4 kcal/mol lower than the uncatalyzed activation energy. However, the dehydration reaction's activation energy is reduced by about 40-70 kcal/mol. Therefore, we suggest that steric hindrance may make the DA cycloaddition reaction more complex, so the change in acid strength shows an extensive role in catalysis.