A Tan­dem for Biomass

Bio­mass, which is use­ful as a sup­ple­ment or replace­ment for pet­ro­leum, is pro­cessed in biore­finer­ies and can be used as fuels or start­ing mater­i­als for the pro­duc­tion of chem­ic­als. How­ever, the high oxy­gen con­tent of many bio­mass com­pon­ents poses a prob­lem. In the journ­al Ange­wandte Chemie, research­ers from the Max Planck Insti­tute for Coal Research have now intro­duced a pro­cess based on a tan­dem reac­tion that could reduce the oxy­gen con­tent of both lignin and bio-oils under unusu­ally mild conditions. In prin­ciple, a biore­finery works like a pet­ro­leum refinery: a com­plex mix­ture of sub­stances is sep­ar­ated into indi­vidu­al com­pon­ents and par­tially refined — chem­ic­ally con­ver­ted into oth­er, more use­ful com­pounds. Lignin is one of the major com­pon­ents of bio­mass and is a byproduct pro­duced in large amounts by the paper and pulp industry. Even in the biore­finer­ies, no bet­ter applic­a­tion than its use as sol­id fuel has been found so far. Lignin is a het­ero­gen­eous group of phen­olic mac­ro­molec­ules. Phen­ols are aro­mat­ic six-membered rings of car­bon atoms with some num­ber of alco­hol (OH) func­tion­al groups. The depoly­mer­iz­a­tion of lignin to form low-boil­ing arenes – oxy­gen-free aro­mat­ics – instead of high-boil­ing phen­ols would rep­res­ent a great sim­pli­fic­a­tion of con­ven­tion­al refin­ing pro­cesses. Unfor­tu­nately, phen­olic OH groups are not so easy to cleave because the bond between the phen­olic oxy­gen atom and the aro­mat­ic ring is very strong. Pre­vi­ous meth­ods have been forced to use a detour involving deriv­at­iz­a­tion, in which an elec­tron-with­draw­ing group is attached to the oxy­gen atom to weak­en its bond to the car­bon atom. This allows the bond to be broken cata­lyt­ic­ally in the pres­ence of hydro­gen. This pro­cess is not prac­tic­al on a large scale because it also res­ults in large quant­it­ies of non­re­cyc­lable byproducts. Xingyu Wang and Roberto Rinaldi at the Max Planck Insti­tute for Coal Research in Mül­heim (Ruhr, Ger­many) have now intro­duced a new, one-step pro­cess for the depoly­mer­iz­a­tion of lignin with sim­ul­tan­eous, highly select­ive con­ver­sion of phen­ols to arenes. In con­trast to oth­er pro­cesses, this one works under mild con­di­tions and with no form­a­tion of deriv­at­ives. Their trick is the clev­er link­ing of three reac­tions into a reac­tion cas­cade that con­sists of an ini­ti­ation reac­tion and a tan­dem reac­tion. The cata­lysts are Raney nick­el and a zeolite. Because hydro­gen gas would dis­rupt this reac­tion sys­tem, hydro­gen must be intro­duced to the sys­tem in anoth­er way: 2‑propanol is first used to provide hydro­gen for the ini­ti­ation reac­tion. The cyc­lo­hexene formed in phase 1 of the tan­dem reac­tion then provides the hydro­gen for phase 2 of the tan­dem reac­tion. This new meth­od is a highly prom­ising start­ing point for the devel­op­ment of innov­at­ive indus­tri­al pro­cesses for refin­ing lignin and the phen­olic frac­tion of bio-oils under mild con­di­tions. The con­ver­sion of lignin into simple arenes offers a nov­el aven­ue for boost­ing ligno­cel­lu­losic biore­finery.

About the Author

Dr. Roberto Rinaldi is an inde­pend­ent group lead­er at the Max Planck Insti­tute for Coal Research, and has been work­ing in the area of bioen­ergy for over 6 years. In 2010, he received one of the most pres­ti­gi­ous awards in Europe: the Sofja Kova­levskaja Award, giv­en by the Alex­an­der von Hum­boldt Found­a­tion, to start his inde­pend­ent research group in Mül­heim in the field of cata­lyt­ic val­or­iz­a­tion of lignin.

Ori­gin­al publication

Author: Roberto Rinaldi, Max Planck Insti­tute for Coal Research, Mül­heim (Ruhr, Ger­many)
Title: A Route for Lignin and Bio-Oil Con­ver­sion: Dehyd­roxyla­tion of Phen­ols into Arenes by Cata­lyt­ic Tan­dem Reac­tions
Ange­wandte Chemie Inter­na­tion­al Edi­tion, Permalink to the art­icle: http://​dx​.doi​.org/​1​0​.​1​0​0​2​/​a​n​i​e​.​2​0​1​3​04776 Source: Ange­wandte Chemie