Science - USA (2020-03-20)

INSIGHTS | PERSPECTIVES

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of-the-art lignin conversion strategies have
produced a mixture of phenols, arenes,
and their oligomers ( 4 , 5 ) that cannot be
directly used as chemicals without further
expensive and energy-intensive separation
operations in downstream utilization.
In their first RCF step, hardwood re-
acted with added hydrogen (H 2 ) over a
metal catalyst (ruthenium on a carbon
support, Ru/C) to yield lignin oil contain-
ing phenolic monomers in near-theoretical
yields (~50 wt%) and a solid carbohydrate
pulp. This “lignin-first” strategy overcame
the condensation of the reactive interme-
diates and achieves near-complete deligni-
fication with little carbohydrate degrada-
tion. The monomers in the lignin oil
were easily extracted with a less than
sixfold added mass of n-hexane in a
cost-effective way.
Given the excellent yields of pheno-
lic monomers and the high extraction
efficiency, Liao et al. sought an effec-
tive catalytic process to convert the
lignin monomers into phenol, which
is the key step in demonstrating the
concept of lignin-to-phenol biorefin-
ery because the phenolic monomers
contain both methoxy groups and
other versatile substitute groups on
different positions of the benzene
ring ( 1 ). Removal of these functional
groups without destroying the ben-
zene ring and the phenolic hydroxy
represents a huge challenge in cataly-
sis, given their similar bond dissocia-
tion energy.
To reach this target, a stepwise
catalytic process was needed. First,
removal of methoxy groups was ex-
plored. To make the process economi-
cally viable, a solvent- and sulfur-free
continuous catalytic gas-phase hydro-
processing was performed over a non–
noble metal catalyst, nickel on a silica
support (Ni/SiO 2 ). Model compound
study indicates that the well-dispersed Ni/
SiO 2 catalyst showed remarkable versatil-
ity on different methoxylated alkylphenols
through either direct demethoxylation or
tandem demethylation-dehydroxylation.
Hydroprocessing of the lignin monomers
mixture provides high selectivity (75 to
85%) toward n-propylphenols and ethyl-
phenols at nearly complete conversion.
The major byproducts of methoxy cleav-
age were water and methane, and the re-
action proceeded without formation of
carbon monoxide or carbon dioxide. Water
was beneficial to maintain robust cata-
lytic activity in the next dealkylation step.
Methane, together with small molecules
(such as excess H 2 , ethylene, methanol, and
methyl acetate) generated in other steps,

could be incinerated to provide heating,

cooling, and electricity.

In the subsequent dealkylation step, the

researchers developed a hierarchical cata-

lyst based on ZSM-5, which they named

Z140-H, that had a balanced network of

micro- and mesopores for the dealkylation

of the crude alkylphenol condensates.

Near-quantitative transformation through

a carbenium mechanism led to a combined

yield for phenol and olefins of 82%. Zeolites

with smaller or larger pore sizes did not

work, because of either pore restriction or

a lack of the pore confinement needed for

shape-selective conversion. These findings

underline the importance of zeolite hierar-

chization in the dealkylation reaction and

the need for customized catalysts.

To make the technology sustainable and

economically feasible, full valorization and

utilization of all of the components in the

woody biomass are essential. After extrac-

tion of monophenols from lignin crude

oil, Liao et al. disclosed a new application

of the residue oligomers to replace fossil-

based p-nonylphenol by making high-

quality printing ink, which provides an

unexplored market potential for phenolic

oligomers, a major fraction in almost all

the lignin depolymerization processes. As

for the carbohydrate pulp generated in the

RCF step, it can be readily converted to

ethanol after a near-simultaneous sacchar-

ification-fermentation process.

A technoeconomic analysis indicates that

self-energy supply is enough to operate the

integrated biorefinery. For a factory with

capacity production of 100 kilotons of bio-

phenol per year, an internal rate of return of

23.33% and a payout time of ~4 years for a

plant with a lifetime of 20 years are antici-

pated. Moreover, life-cycle assessment sug-

gested a much lower carbon footprint for

phenol and propylene compared to their fos-

sil-based counterparts. Taking into account

the manufacturing investment, production

efficiency, and product price, as well as the

lower carbon footprint, this process shows

great potential for practical application.

The study by Liao et al. not only offers

an attractive approach toward the

production of phenol and propylene

from lignin, it also realizes full uti-

lization of all components in ligno-

cellulose. However, some key issues

must be addressed for large-scale

commercialization. For instance,

nonhardwood feedstocks such as

pinewood, cornstalk, and bagasse

should be studied to improve versa-

tility. In the RCF step, a large amount

of Ru/C catalyst (10% based on wood

weight) was used, and a non–noble

metal catalyst would be less costly

and more sustainable ( 6 ). Further,

unlike fuels, the value of chemicals

depends on purity, especially for phe-

nol and propylene as monomers for

the polymer industry. At present, how

to obtain these two products in suffi-

ciently high purity is not clear.

For biorefineries to play a greater

role in the production of liquid fuels

and chemicals, feedstock conversion

must be maximized. Robust catalysts

combined with high-efficiency pro-

cess engineering are still desirable.

In fundamental studies, some elegant

progress has been achieved recently,

including a solar energy–driven

lignin-first approach ( 7 ), a formaldehyde

stabilization strategy ( 8 ), and mild redox-

neutral depolymerization ( 9 ). j

REFERENCES AND NOTES

1. A. J. Ragauskas et al., Science 344 , 1246843 (2014).


2. Y. Liao et al., Science 367 , 1385 (2020).


3. C. Li, X. Zhao, A. Wang, G. W. Huber, T. Zhang, Chem. Rev.
   115 , 11559 (2015).


4. R. Rinaldi et al., Angew. Chem. Int. Ed. 55 , 8164 (2016).


5. W. Schutyser et al., Chem. Soc. Rev. 47 , 852 (2018).


6. H. Guo et al., ChemSusChem 9 , 3220 (2016).


7. X. J. Wu et al., Nat. Catal. 1 , 772 (2018).


8. L. Shuai et al., Science 354 , 329 (2016).


9. Y. X. Liu et al., ACS Catal. 9 , 4441 (2019).

ACKNOWLEDGMENTS

Support from the National Natural Science Foundation of

China (21690080, 21721004) is gratefully acknowledged.

10.1126/science.abb1463

OH

O

OH

OH

OH

Lignocellulosic biomass

Lignin-derived

monomers

Lignin-derived

oligomers

Carbohydrate

pulp

Valorization

Hydroprocessing

Dealkylation

Fermentation

Bioresin, varnish, ink Bioethanol

Catalytic funneling

O O

Reductive catalytic fractionation

Conquer and divide

Liao et al. processed hardwood pulp through reductive catalytic

fractionation to produce three feedstocks for further upgrading.

The diverse mix of lignin-derived monomers undergoes catalytic

funneling to produce mainly phenol and propylene.

1306 20 MARCH 2020 • VOL 367 ISSUE 6484