Efficient Depolymerization of Alkaline Lignin to Phenolic Compounds at Low Temperatures with Formic Acid over Inexpensive Fe–Zn/Al₂O₃ Catalyst

Abstract

In this study, an inexpensive catalyst (Fe–Zn/Al2O3) was investigated to efficiently convert alkaline lignin into phenolic monomers with formic acid at low temperatures. The catalyst was successfully prepared by a co-impregnation method and showed high selectivity for production of phenolic monomers. With the optimal reaction condition, i.e., a mass ratio of formic acid to lignin of 4:1 and a reaction temperature of 180 °C for 6 h, the highest yield of bio-oil of 28.31 wt % was obtained with formic acid as the in situ hydrogen source in the presence of Fe–Zn/Al2O3 catalyst. It was indicated from the composition analysis of bio-oil that the main phenolic monomers were 2-methoxy-phenol (2.86 wt %, based on alkaline lignin), vanillin (2.83 wt %, based on alkaline lignin), and apocynin (2.06 wt %, based on alkaline lignin). Compared with fresh catalyst, the spent one was also investigated for physical and chemical properties, and results confirmed by SEM (scanning electron microscopy) and TG/DTG (thermogravimetry/derivate thermogravimetry) analyses showed that solid products formed by lignin depolymerization were adsorbed on the surface of the Fe–Zn/Al2O3 catalyst. Finally, recyclability tests demonstrated that the catalytic performance of Fe–Zn/Al2O3 was still active and stable after five consecutive recycling runs.

Selective Catalytic Transfer Hydrogenation of Lignin to Alkyl Guaiacols Over NiMo/Al‐MCM‐41

“…Textural properties (surface area and pore volume) continuously decreased with each subsequent reaction, indicative of pore blockage by coke deposition from lignin depolymerization. This is supported by thermogravimetric analysis (Figure S4) of a spent catalyst (after five reaction cycles) under flowing air (ramp rate 10 °C min −1 ), which revealed a significant weight loss between 200–650 °C associated with the combustion of char residue on the catalyst surface [91] . The changes in textural properties of recycled catalysts were mirrored by a decrease in lignin conversion and bio‐oil yield, and an increase in char production, consistent with catalyst coking.…”

“…This is supported by thermogravimetric analysis (Figure S4) of a spent catalyst (after five reaction cycles) under flowing air (ramp rate 10 °C min À 1 ), which revealed a significant weight loss between 200-650 °C associated with the combustion of char residue on the catalyst surface. [91] The changes in textural properties of recycled catalysts were mirrored by a decrease in lignin conversion and bio-oil yield, and an increase in char production, consistent with catalyst coking. Nevertheless, NiMo/Al-MCM-41 still achieved > 50 % lignin conversion and around 44 % bio-oil yield after five cycles.…”

Highly Selective Conversion from Alkali Lignin to Phenolic Products

“…With an increase in the number of recycle, the catalytic performance of Ni-ZrO 2 /γ-Al 2 O 3 catalyst is weakened gradually, reflected by the decrease of bio-oil yield (from 40.11 to 15.42 wt %) and the increase of solid yield (from 20.27 to 53.65 wt %), which might be due to the deactivation of catalyst caused by oxidation from oxygen in the reactor or acidic environment from FA. Additionally, the masking of active sites caused by the solid products absorbed on the surface of catalyst can also suppress the catalytic activities to a certain extent . As the catalyst gradually deactivates, the reaction is primarily dominated by FA, which is prone to generate solid products and is also beneficial for the lignin conversion (see section ), which is reflected by the lowest yield of unreacted lignin in the last recycle.…”

“…Additionally, the masking of active sites caused by the solid products absorbed on the surface of catalyst can also suppress the catalytic activities to a certain extent. 29 As the catalyst gradually deactivates, the reaction is primarily dominated by FA, which is prone to generate solid products and is also beneficial for the lignin conversion (see section 3.2.2), which is reflected by the lowest yield of unreacted lignin in the last recycle.…”

Efficient catalytic liquefaction of organosolv lignin over transition metal supported on HZSM-5

“…This might be related to its low hydrogenation activity and certain oxidizing properties (Yu et al 2020). However, iron and cobalt have low activity as single metal catalysts, low liquefaction rate of lignin and more char, generally combined with other active metals to form bimetallic catalysts with higher catalytic activity (Kim et al 2015c;Zhai et al 2017;Dou et al 2020;Lu et al 2020;Mauriello et al 2020). The char of copper-based catalysts was less than that of Fe, Co, and Cr single-metal catalysts, probably because copper could reduce char formation (Barta et al 2014), but its hydrogenation activity was not as good as nickel-based catalysts.…”