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Changing lignin into beneficial chemical substances and fuels has attracted in depth consideration by chemists as a consequence of its aromatic-rich construction and huge pure abundance. The advanced molecular construction of lignin, nevertheless, results in a broad product distribution, which implies excessive prices for downstream processes. Understanding the response mechanism throughout catalytic lignin depolymerization will open up new avenues to rational catalyst design and course of optimization to manage the selectivity of this course of.
Our teams investigated many lignin mannequin compounds, starting from benzenediols to dimers, in (catalytic) pyrolysis utilizing operando photoelectron photoion coincidence (PEPICO) spectroscopy to acquire deep mechanistic insights in a bottom-up technique. [1-6] PEPICO has been established as a flexible operando software to detect and quantify reactive intermediates in heterogenous catalysis by a novel mixture of mass spectrometry and photoelectron spectroscopy.[7, 8] In accordance with the literature and our group’s work, a complete understanding of catalytic pyrolysis of e.g. guaiacol (Fig. 1), certainly one of three essential primary models within the lignin construction, was achieved.[1-3, 9, 10]
The mechanism of guaiacol 1 is initiated by a demethylation yielding catechol 2 and methyl and branches between a direct dehydroxylation response to phenol 3, and formation of fulvenone 4, a extremely reactive ketene intermediate, through dehydration. Formation of ketene 4 is accelerating the response charges, which additionally produces phenol 3 but additionally cyclopentadiene 6, fulvene 8 and different species, decreasing the selectivity of this course of.
For the reason that dehydroxylation 2 -> 3 is a Brønsted acid catalyzed response, rising the focus of those websites will probably enhance the phenol 3 selectivity by making it extra probably that catechol 2 will coordinate on two acid websites concurrently, thereby isolating the hydroxyl teams and suppressing fulvenone 4 formation through dehydration. This could enhance the selectivity in direction of dehydroxylation to phenol 3.
Fig. 1 | Response mechanism of guaiacol catalytic quick pyrolysis utilizing HZSM-5
We examined this speculation by decreasing the Si/Al ratio within the zeolite, which will increase the Brønsted acid web site density and utilized operando PEPICO spectroscopy as analytical software, enabling ketene detection.
Fig. 2 | Mass spectra of the catalytic quick pyrolysis merchandise of guaiacol over zeolite catalysts.
Fig. 3 | Temperature dependent mole fraction of the principle merchandise and intermediates in guaiacol CFP.
Mass spectra in Fig. 2 on the similar response circumstances present that the phenol selectivity is strongly elevated in HFAU(2.6) and HFAU(15), whereas the product distribution of HFAU(40) is much less selective. By quantification of intermediates and merchandise (Fig. 3), we discovered that HFAU(15 and a couple of.6) favors the phenol manufacturing, whereas fulvenone is strongly suppressed. To know how HFAU catalysts with totally different Si/Al ratios have an effect on the response mechanism, we traced the central intermediates fulvenone and fulvene through photoion mass-selected threshold photoelectron spectroscopy (ms-TPES), as proven in Fig. 4. Fulvenone and fulvene dominate within the HFAU(40) experiment, whereas they’re virtually absolutely suppressed in HFAU(2.6). This end result proves that HFAU(15) and HFAU(2.6) suppress the fulvenone manufacturing upon catalytic pyrolysis of guaiacol and is the important thing to extend the selectivity of the response.
Fig. 4 | Comparability of the experimental m/z 78 and 92 ms-TPES (squares) with reference spectra (coloured strains) of spectral carriers.
This may be understood as outlined in Fig. 5. Brønsted acid websites are the principle energetic websites in guaiacol catalytic pyrolysis. As HFAU(40) displays a excessive Si/Al ratio and thus low density of Brønsted acid websites, the preliminary product catechol is barely coordinated to a single Brønsted acid websites (Fig. 5 high). The 2 hydroxyl teams can work together with one another favoring the intramolecular dehydration of catechol and fulvenone formation. Because of the excessive reactivity of this elusive intermediate the response is hardly controllable, resulting in an unselective formation of many various merchandise. In distinction, the 2 hydroxyl teams in catechol are remoted on HFAU(15) and HFAU(2.6) as a result of bidentate bonding throughout the zeolite pore, which suppresses the fulvenone manufacturing, resulting in a dominant selectivity in direction of phenol. Along with detection of reactive intermediates and quantification of the merchandise, we carried out response pathway calculations in addition to 29Si MAS-NMR spectroscopy to confirm the response mechanism.
Fig. 5 |The impact of Brønsted acid web site density on the response mechanism of guaiacol catalytic pyrolysis
In conclusion, by linking reactive intermediate concentrations, selectivities and the conversion, it has been proven that the guaiacol catalytic pyrolysis may be optimized in a focused manner. This method is broadly relevant to many heterogeneous catalytic processes, starting from hydrogenation and syngas- or methanol-to-hydrocarbon reactions. Taking management of ketenes and their floor analogs might have extra advantages for the general selectivity, particularly for MTH reactions. Our operando PEPICO method can help rational catalyst design to manage product selectivities for focused course of optimization.
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[2] Pan, Z., et al. Isomer-dependent catalytic pyrolysis mechanism of the lignin mannequin compounds catechol, resorcinol and hydroquinone. Chemical science, 2021, 12(9), 3161-3169.
[3] Pan, Z., et al. Operando PEPICO unveils the catalytic quick pyrolysis mechanism of the three methoxyphenol isomers. Bodily Chemistry Chemical Physics, 2022, 24(36), 21786-21793.
[4] Wu, X., et al. Isomer-Dependent Selectivities within the Pyrolysis of Anisaldehyde. Power & Fuels, 2022, 36(13), 7200-7205.
[5] Gerlach, M., et al. Metamorphic meta isomer: carbon dioxide and ketenes are fashioned through retro-Diels–Alder reactions within the decomposition of meta-benzenediol. Bodily Chemistry Chemical Physics, 2019, 21(35), 19480-19487.
[6] Wu, X., et al. Unimolecular thermal decarbonylation of vanillin stifled by the bimolecular reactivity of methyl-loss intermediate. Journal of Analytical and Utilized Pyrolysis, 2022, 161, 105410.
[7] Hemberger, P., et al. Photoelectron photoion coincidence spectroscopy offers mechanistic insights in gasoline synthesis and conversion. Power & Fuels, 2021, 35(20), 16265-16302.
[8] Hemberger, P., et al. New analytical instruments for superior mechanistic research in catalysis: photoionization and photoelectron photoion coincidence spectroscopy. Catalysis Science & Expertise, 2020, 10(7), 1975-1990.
[9] Liu, P., et al. Exploring the response chemistry of biomass upgrading over HZSM-5 catalyst by way of mannequin compounds. Gasoline, 2022, 312, 122874.
[10] Jiang, X., et al. Catalytic conversion of guaiacol as a mannequin compound for fragrant hydrocarbon manufacturing. Biomass and Bioenergy. 2018, 111, 343–351.
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