Catalytic pyrolysis mechanism of lignin moieties driven by aldehyde, hydroxyl, methoxy, and allyl functionalization: the role of reactive quinone methide and ketene intermediates

The catalytic pyrolysis of guaiacol-based lignin monomers, vanillin, syringol, and eugenol over commercial HZSM-5 has been investigated using operando Photoelectron Photoion Coincidence (PEPICO) spectroscopy to unveil the reaction mechanism by detecting reactive intermediates, such as quinone methides and ketenes, and products. Vanillin shares the decomposition mechanism with guaiacol due to prompt and efficient decarbonylation, which allows us to control this reaction leading to a phenol selectivity increase by switching to a faujasite catalyst and decreasing the Si/Al ratio. Syringol first demethylates to 3-methoxycatechol, which mainly dehydroxylates to o- and m-guaiacol. Ketene formation channels over HZSM-5 are less important here than for guaiacol or vanillin, but product distribution remains similar. C3 addition to guaiacol yields eugenol, which shows a more complex product distribution upon catalytic pyrolysis. By analogies to monomers with simplified functionalization, namely allylbenzene, 4-allylcatechol, and 4-methylcatechol, the eugenol chemistry could be fully resolved. Previously postulated reactive semi-quinone intermediates are detected spectroscopically, and their involvement opens alternative pathways to condensation and phenol formation. Allyl groups, produced by dehydroxylation of the β-O-4 bond, may not only decompose via C1/C2/C3 loss, but also cyclize to indene and its derivatives over HZSM-5. This comparably high reactivity leads to an unselective branching of the chemistry and to a complex product distribution, which is difficult to control. Indenes and naphthalenes are also prototypical coke precursors efficiently deactivating the catalyst. We rely on these mechanistic insights to discuss strategies to fine-tune process conditions to increase the selectivities of desired products by enhancing either vanillin and guaiacol or supressing eugenol yields from native lignin.

Phenol selectivity increase during vanillin CFP over FAU The products of vanillin catalytic pyrolysis using HZSM-5 (red trace) are mainly small molecules, such as phenol (m/z 94), fulvene (m/z 78), benzene (m/z 78), and cyclopentadiene (m/z 66).When using HFAU40 (blue trace), more primary decomposition products are observed, such as 1,2-dihydroxylbenzaldehyde (m/z 138), guaiacol (m/z 124) and catechol (m/z 110) due to the low activity of the catalyst.Two factors are responsible for this observation: 1) HFAU40 has larger cage opening (7.4 Å) than HZSM-5 (5.5 Å).Thus, molecular diffusion is faster in HFAU40, leading to fewer secondary reactions; 2) HFAU40 has fewer active sites because of a higher Si/Al ratio (Figure S3b and e), which suppresses sequential reactions and thus secondary products.By using HFAU15 (green trace), with a higher BAS concentration (Figure S3b and e), the vanillin decarbonylation to guaiacol (m/z 124) is enhanced, which yields methyl phenol (m/z 108) and phenol (m/z 94) .The signals for small molecular products, such as benzene (m/z 78) and cyclopentadiene (m/z 66), from secondary chemistry are negligible.This aligns well with our previous findings during the guaiacol CFP experiment. 14Also upon vanillin CFP, catechol (m/z 110), as produced from demethylation of guaiacol, is dehydroxylated to phenol, which has the highest selectivity among the products.This can be explained by inhibition of the intramolecular dehydration to fulvenone, caused by the high density of adjacent active sites (Figure S3b, c), which isolate the neighboring OH groups in catechol.This favors the dihydroxylation to yield phenol.This experiment demonstrates that guaiacol shares the same chemistry with vanillin, and the strategies, developed to optimize the guaiacol decomposition, can also be applied to vanillin.

Figure S4
Photoion mass-selected threshold photoelectron spectra (ms-TPES) and photoionization spectra of syringol catalytic pyrolysis products, identified based on FC simulations combined with G4level ionization energy (IE) calculations or experimental spectra. 9,11,12,14S8).Further investigation of the ms-TPES of m/z 118 revealed that, similar to the blank experiments, isomerization of the parent allylbenzene is observed.Besides these abundant products, we identify toluene (m/z 92) and styrene (m/z 104) at 312 °C and 433 °C, respectively.They are produced via C2 and C1 loss at the allyl group, respectively.As the temperature increases, benzene (m/z 78, Figure S8) becomes the most abundant reaction product, while fulvene is not observed.Methylation is responsible for the species at m/z 130 and 128, corresponding to a mixture of naphthalene and two benzene substituted vinyl acetylenes (Figure S8).

Figure S10
Photoion mass-selected threshold photoelectron spectra (ms-TPES) or photoionization spectra of products upon 4-allylcatechol catalytic pyrolysis, identified based on FC simulations combined with G4 ionization energy (IE) calculations or reference spectra. 1,9,11,12,14re S11 The internal energy comparison of m/z 132 isomers.Hydroxylindenes have similar energies.The calculated ionization energy of 1H-inden-6-ol (red line) aligns with the vibrational peak at 7.65 eV in the m/z 132 ms-TPES (Figure S10), suggesting its presence.Although the 1H-inden-6-ol is has the lowest ionization energy and thus the easiest to detect, the other hydroxyindenes may also be produced during ring closure of the allyl substituent and subsequent dehydroxylation.In addition, the methylene unit in the 5-membred ring may undergo [1,5] sigmatropic rearrangement increasing the isomeric pool.In Figure S9, no pronounced peaks of these derivatives are observed, which is probably due to their low abundance during the reaction and overlapping signals.
Fulvenone derivatives lie at ca. 3.36 kJ/mol (0.035 eV) higher in energy compared than 1H-inden-6-ol.Although the simulated spectra of the isomers plotted in purple and blue fit with experimental m/z 132 ms-TPES (Figure S10), their existence cannot be fully confirmed due to absence of distinct features in the ms-TPES.p-Allylcyclohexadienone (orange line) lies ca.3.18 kJ/mol (0.03 eV) higher in energy than 1H-inden-6ol.In Figure S10, two vibrational peaks at ca. 8.7 and 8.9 eV agree with the FC simulation of pallylcyclohexadienone (orange curve), which illustrates that p-allylcyclohexadienone is possibly produced during 4-allylcatechol catalytic pyrolysis.Those were also suggested by Shen et al. in eugenol pyrolysis, but lack of a clear spectroscopic observation.

Figure
Figure S5 Time-of-flight mass spectra (ToF MS) upon syringol pyrolysis recorded at hν = 10.5 eV.Reaction conditions: < 0.1% syringol (m/z 154); H-ZSM-5; < 0.5 bar; 20 sccm Ar.At room temperature, syringol is present at m/z 154 accompanied by a minor peak at m/z 139, due to dissociative ionization.The peak at m/z 46 is attributed to residual ethanol from the reactor and sample container.The ethanol signal is depleted as time proceeds and becomes invisible at 433 °C.No products are observed by comparing the mass spectra at 433 °C and 25 °C.A tiny peak is seen at m/z 140 until the temperature reaches 580 °C and becomes pronounced at 624 °C, accompanied by a small peak at m/z 122.m/z 140 corresponds to the demethylation product of syringol, 3-methoxycatechol.Therefore, it is suggested that syringol does not decompose below 531 °C.

Figure S7
Figure S7 Allylbenzene pyrolysis.a) Temperature-dependent mass spectra collected at hv=10.5 eV.Reaction conditions: 1-2% allylbenzene (m/z 118); no catalyst; 0.11-0.16bar; 20 sccm Ar.No obvious product peaks are observed between 25-546 °C.b) ms-TPES of m/z 118 (black trace), at 546 °C, with FC simulations of indane (blue trace) and 1-phenylpropene (green trace) as well as allylbenzene ms-TPES (red trace) obtained at 25 °C.m/z 118 ms-TPES shows vibrational signals between 8.7 to 10.5 eV, which agree with the reference TPES of allylbenzene.Notably, the signals in the 8.0-8.7 eV region fit the 1-phenylpropene and indane FC simulations quite well, which indicates partial isomerization at high temperature.c) The ring closing mechanism of the allyl group is reflected by the identification of indene and its derivates during eugenol CFP, which is best be investigated by studying the chemistry of allylbenzene.Reaction conditions: 0.8% allylbenzene (m/z 118) in Ar; HZSM-5; 0.3 bar; 20 sccm Ar.Temperature-dependent mass spectra of allylbenzene without catalyst show no obvious change in the mass spectra (a), however the ms-TPES (b) confirms rearrangement to 1-phenylpropene and indane, via ring-closing reactions at 546 °C.With the addition of HZSM-5, the reaction is initiated already at 270 °C (c) and we assign propene (m/z 42), benzene (m/z 78) and indene (m/z 116) as products (ms-TPES in FigureS8).Further investigation of the ms-TPES of m/z 118 revealed that, similar to the blank experiments, isomerization of the parent allylbenzene is observed.Besides these abundant products, we identify toluene (m/z 92) and styrene (m/z 104) at 312 °C and 433 °C, respectively.They are produced via C2 and C1 loss at the allyl group, respectively.As the temperature increases, benzene (m/z 78, FigureS8) becomes the most abundant reaction product, while fulvene is not observed.Methylation is responsible for the species at m/z 130 and 128, corresponding to a mixture of naphthalene and two benzene substituted vinyl acetylenes (FigureS8).

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Figure S11The internal energy comparison of m/z 132 isomers.Hydroxylindenes have similar energies.The calculated ionization energy of 1H-inden-6-ol (red line) aligns with the vibrational peak at 7.65 eV in the m/z 132 ms-TPES (FigureS10), suggesting its presence.Although the 1H-inden-6-ol is has the lowest ionization energy and thus the easiest to detect, the other hydroxyindenes may also be produced during ring closure of the allyl substituent and subsequent dehydroxylation.In addition, the methylene unit in the 5-membred ring may undergo[1,5] sigmatropic rearrangement increasing the isomeric pool.In FigureS9, no pronounced peaks of these derivatives are observed, which is probably due to their low abundance during the reaction and overlapping signals.Fulvenone derivatives lie at ca. 3.36 kJ/mol (0.035 eV) higher in energy compared than 1H-inden-6-ol.Although the simulated spectra of the isomers plotted in purple and blue fit with experimental m/z 132 ms-TPES (FigureS10), their existence cannot be fully confirmed due to absence of distinct features in the ms-TPES.p-Allylcyclohexadienone (orange line) lies ca.3.18 kJ/mol (0.03 eV) higher in energy than 1H-inden-6ol.In FigureS10, two vibrational peaks at ca. 8.7 and 8.9 eV agree with the FC simulation of pallylcyclohexadienone (orange curve), which illustrates that p-allylcyclohexadienone is possibly produced during 4-allylcatechol catalytic pyrolysis.Those were also suggested by Shen et al. in eugenol pyrolysis, but lack of a clear spectroscopic observation.22