Edited by: Arthur Jonas Ragauskas, University of Tennessee, Knoxville, United States
Reviewed by: Selhan Karagoz, Karabük University, Turkey; Ao Xia, Chongqing University, China
Specialty section: This article was submitted to Bioenergy and Biofuels, a section of the journal Frontiers in Energy Research
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This article reported a novel low-temperature process for aromatics production through lignin depolymerization catalyzed by 0.1 wt% Pd-zeolite Y catalyst prepared by a facile method. Under the same reactive condition, the as-prepared Pd-zeolite Y catalysts exhibited much higher catalytic efficiency than zeolite Y or commercial Pd/Al2O3–zeolite composites. The selectivity of the Pd-zeolite Y toward lignin depolymerization was also much higher than the other commercial zeolite-based catalysts. With the presence of hydrogen, aromatics were the predominant products with high yields of phenols and dimers (over 99%). The as-obtained aromatics can be promising feedstock for production of fuels or chemicals for various applications. Results revealed that the Pd-zeolite Y catalyst is a highly active catalyst for the cleavage of lignin interlinkages, especially the C–O–C bonds by hydrogenolysis.
Catalytic conversion of biomass lignin into value-added chemicals and fuels is a promising route to resolve the energy crisis (Jae et al.,
High catalytic efficiency of lignin depolymerization over low Pd-zeolite Y loading at mild temperature.
However, challenges remain for catalytic lignin depolymerization by zeolites because the current zeolite-based catalysis requires high temperature (250°C or above), which leads to faster catalyst deactivation by coking and dealumination (Jia et al.,
A group of catalysts with a combination of zeolite and noble metal nanoparticles drew increasing attention due to their excellent catalytic activity in lignin degradation. For example, lignin degradation and/or hydrodeoxygenation reaction were reportedly enhanced by a combination of Pt, Ru, and Pd nanoparticles supported on zeolites (Laskar et al.,
Combining the outstanding catalytic performance of zeolites and metals, metal-exchanged zeolites potentially have higher activity than non-exchanged zeolites (Hui et al.,
In this study, the Pd-zeolite Y catalysts were prepared
CBV 300 purchased from Zeolyst International was heated in air at 550°C for 5 h. Pd(NO3)2 (Aldrich) was dissolved in diluted aqueous HNO3 (0.01 M) solution. Then 1 g of zeolite (CBV300) was dispersed into 50 mL of water solution by stirring, followed by adding aqueous Pd(NO3)2 solution [containing 5 mg of Pd(NO3)2]. Stirring continued for 30 min. Then 20 mg of NH4NO3 was added, followed by stirring for another 2 h. The mixture was then centrifuged and thoroughly washed with distilled water. The precipitate was dried at room temperature before catalytic reaction tests. Commercial Pd/C (5 wt%) was purchased from Aldrich.
For a typical catalytic depolymerization experiment, 30 mL of deionized water was added into a 100 mL Parr reactor as solvent for the reaction, followed by adding 200 mg of lignin and 200 mg of Pd-zeolite catalysts. After sealing the vessel, H2 was used to flush out the air in the reactor at least four times, and then the reactor was pressurized to 3.5 MPa at room temperature. The reactor was heated to reach the reaction temperature (180–200°C) where the reaction time was started. The reaction was conducted for 3 h. Then the vessel was plunged into cold water to cool the reaction system. Ethyl acetate was used to extract products while the 0.5 wt% of toluene (in ethyl acetate) was added as an internal standard. Then a sample was acquired from the organic phase for GC–MS analysis. For GC–MS analysis, the response factor for each product was standardized with the use of efficient carbon number (Scanion and Willis,
After ethyl acetate extraction, the organic phase was analyzed by GC–MS. 1 µL of sample was injected with 0.6 mL/min of He (carrier gas) into a DB-5 (30 m length × 250 µm I.D. × 0.25 µm film thickness, J&W Scientific) capillary column installed in an Agilent Technologies 7890A GC that was set at splitless mode. The GC oven was set to maintain 45°C for 2 min, then heated to 200°C at the rate of 15°C/min then held at 200°C for another 1 min. After that, the oven was heated up to 300°C at the rate of 10°C/min and held for 5 min. Eluting compounds were determined using a MS (Agilent Technologies 5975C) inter XL EI/CI MSD with a triple axis detector and matched to NIST GC–MS libraries (Wang et al.,
Scanning electron microscopy (SEM) analysis was performed using a field emission SEM instrument (Hitachi S-4800), operating at an accelerating voltage of 10 kV. Transmission electron microscope analysis was performed using an FEI Tecnai G2 S-Twin instrument with a field emission gun operating at 200 kV. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements were performed on a Thermo Jarrell Ash Atom scan Advantage instrument.
The mass yield of each product and its selectivity from lignin depolymerization were calculated as follows:
Transmission electron microscopy (TEM) image in Figure
Transmission electron microscopy images of Pd-zeolite Y before
Energy dispersive spectrometer result on the surface of Pd-zeolite Y after H2 treatment under 200°C (inset: scanning electron microscopy image of Pd-zeolite Y).
The GC–MS results of the products derived from catalytic lignin conversion are concentrated in 18–28 min interval (Figure
GC–MS spectrum of the products from lignin depolymerization on Pd-zeolite Y at 200°C.
GC–MS analysis of the products from lignin depolymerization on the three catalysts: (a) Pd-zeolite, (b) zeolite, and (c) Pd2+ [Pd(NO3)2 solution] catalysts (reaction conditions: 200°C, 3.5 MPa H2, 3 h).
Furthermore, Pd-zeolite Y showed much higher activity than both zeolite Y and Pd when they were applied separately, suggesting strong synergistic effects of Pd and zeolite Y on lignin depolymerization. Under the same reaction conditions, as shown in Figure
The major products from lignin conversion.
Number | Name | Formula | |
---|---|---|---|
P1 | Phenol, 2-methoxy- | 1.25 | |
P2 | Phenol, 4-ethyl- | 6.6 | |
P3 | Phenol, 4-ethyl-2-methoxy- | 4.6 | |
P4 | Phenol, 2-methoxy-4-propyl- | 2.8 | |
P5 | Phenol, 2-methoxy-4-(1-propenyl)-,(E)- | – | |
P6 | 4-Propyl-1,1′-diphenyl | – | |
P7 | Phenol, 2,6-dimethoxy-4-(2-propenyl)- | 8.7 |
Zeolites have been widely used as solid acid catalysts for lignin depolymerization. In this work,
Based on the products in this work, potential reaction pathways to generate phenols and guaiacols are summarized in Figure
Proposed reaction pathways for the formation of the major products from cleavage of the β-O-4 lignin.
Besides the catalysts as mentioned in the previous mechanism, hydrogen also plays a pivotal role in lignin depolymerization. From previous reports, lignin depolymerization in aqueous phase through hydrolysis by a solid acid catalyst is the dominant route (Taarning et al.,
GC–MS spectrum of the products from lignin depolymerization on Pd-zeolite under the N2 atmosphere.
Percentage of lignin aromatic inter-unit linkages and the selectivity of the major products
Lignin inter-unit linkages (%) |
Major products selectivity (%) |
|||||||||
---|---|---|---|---|---|---|---|---|---|---|
β-O-4 | β-5 | β-β | β-1 | P1 | P2 | P3 | P4 | P5 | P6 | P7 |
43 | 44 | 9 | 4 | 2.9 | 19.9 | 28 | 10.3 | 5.4 | 17.8 | 15.7 |
Furthermore, compositions of lignin inter-unit linkages obtained from NMR analysis and the major product selectivity are shown in Table
GC–MS spectrum of the products from lignin depolymerization on Pd-zeolite Y and zeolite at 180°C.
Lignin depolymerization on the state-of-the-art commercial Pd/Al2O3–zeolite composites catalysts with equal amount of Pd and zeolite was tested to compare with the catalytic performance of Pd-zeolite Y. As shown in Figure
GC–MS spectrum of the products from lignin depolymerization on Pd-zeolite (a) and commercial Pd/Al2O3–zeolite composites catalysts [(b) 40 mg of Pd/Al2O3 + 200 mg of zeolite and (c) 4 mg of Pd/Al2O3 + 200 mg of zeolite] at 200°C.
Herein, we demonstrated that the Pd-zeolite Y catalysts prepared
YQ, HR, HW, MF, and BY conceptualized the work, designed experiment, analyzed data, and wrote the manuscript. YQ and HR conducted the experiment. All the authors have approved the manuscript and agreed with submission to
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Part of this work was conducted at the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility located at the Pacific Northwest National Laboratory (PNNL) and sponsored by the Department of Energy’s Office of Biological and Environmental Research (BER). The authors also thank Dr. Zheming Wang and Ms. Marie S. Swita for insightful discussions.