A team at RWTH Aachen University has developed a novel route for the efficient production of BTX (aromatic hydrocarbons such as benzene, toluene, ethyl benzene and three xylene isomers)—which play an essential role in the petrochemical and fine chemical industries—from bio-derived isobutyraldehyde.
In a paper in the RSC journal Green Chemistry, they report that their process, which involves the aromatization of isobutyraldehyde over zeolite catalysts in a continuous fixed bed reactor, produced value-added aromatic compounds with a yield of 93%. Benzene, toluene and xylenes are the major compounds formed with 79% yield and a productivity of 65 mmol gcat−1 h−1.
Traditionally, the so-called BTX aromatics are obtained mainly by catalytic cracking and catalytic reforming processes of petroleum. Renewable and sustainable alternatives for accessing Bio-BTX are, however, rare.
One potential source is considered to be lignin, a major component of lignocellulose which is expected to be a large source of aromatic building blocks. The catalytic conversion and depolymerization of lignin are extremely difficult to perform, mainly due to the robustness and complexity of its structure resulting in challenging processes with respect to selectivity. The conversion of biomass compounds to aromatics by thermal decomposition in the presence of zeolite catalysts is another bio-based approach to obtain aromatics. Catalytic fast pyrolysis allows the direct conversion of solid biomass into liquid fuels (aromatics) in a single reactor with aromatic yields of about 30%. Its disadvantages include fast coking, low selectivity and high energy costs.
… Although there are several published studies on the conversion of alcohols and ketones to aromatic hydrocarbons, reports on aldehydes are scarce. In this regard, isobutyraldehyde (IBA) is a promising starting material because of its low boiling point (63 °C), high vapor pressure (66 mmHg at 4.4 °C), and sustainable production options. A biomass based synthesis from Escherichia coli yields industrially relevant titres using in situ product removal. Moreover, subsequent purification is relatively easy as it can be readily stripped from microbial cultures.
Direct photosynthetic recycling of carbon dioxide to isobutyraldehyde using genetically engineered Synechococcus elangatus PCC7942 is another reported approach, which enables the direct bioconversion of CO2 into IBA without the need for biomass deconstruction. On the industrial scale, IBA can be readily converted to various hydrocarbons currently derived from fossil sources (i.e., isobutanol, isobutyric acid, neopentyl glycol, and esters) using existing approaches. Direct tranformation of IBA into aromatic hydrocarbons has, to date and to the best of our knowledge, not yet been mentioned or reported.—Deischter et al.
Zeolite structure and acidity proved to be important in the efficiency of the process, with HZSM-5 performing better than other tested types.
The team tested catalyst stability and the performance of HZSM-5 in repetitive catalytic cycles with in situ catalyst regeneration through calcination. Therein, the catalyst was used for three consecutive runs, of 10 h time on stream each, showing only a minor decline in the BTX yield.
Jeff Deischter, Kai Schute, Dario S. Neves, Brigitta E. Ebert, Lars M. Blank and Regina Palkovits (2019) “Aromatisation of bio-derivable isobutyraldehyde over HZSM-5 zeolite catalysts” Green Chemistry doi: 10.1039/C9GC00483A