Decarbonisation Technology August 2025 Issue

with nitrogen (N 2 ) before being directed either to online GCs or the exhaust system. Two in-house configured online GCs enable detailed analysis of the complex reaction mixtures. Three thermal conductivity detectors (TCDs) are used for quantification of, for example, N2, Ar (internal standard), H2, and H2O. Moreover, three flame ionisation detectors (FIDs) provide detailed insight into carbon-based species: full paraffin/ olefin separation up to C4, oxygenate analysis by a Polyarc system, and carbon oxides (CO and CO 2) quantification (for example, during carbon burn-off) via a Jetanizer system featuring an overall method run time of approximately 25 minutes. (T, p, WHSV, H2 O-to-ethanol feed ratio) were investigated for both target reactions to obtain a robust foundation for process optimisation. Figure 1b shows the evolution of the target product yield as a function of ethanol conversion product: acetaldehyde) were tested at a lower temperature range (225-300°C) and higher weight hourly space velocity (WHSV) (5-10 h -1 ) compared to the Al2 O 3 -based catalysts (green; target product: ethylene). Initially, acetaldehyde formation increases with ethanol conversion, reaching a maximum in the conversion range of 30-50%, depending on the catalyst. At higher conversion levels, the acetaldehyde yield declines due to consecutive reactions leading to the formation of, for example, ethyl acetate and acetic acid in addition to other C 4 + oxygenates. Co-feeding H2 O decreases ethanol conversion under similar process conditions Results and discussion A broad range of process parameters for the respective catalyst systems. The Cu-based catalysts (blue; target while enhancing acetaldehyde selectivity. Al2 O 3 -based catalysts (green) required elevated temperatures (up to 450°C) and lower WHSV (3.3-4.5 h -1 ) to reach maximum ethylene yields (larger reactors which can hold more catalyst material were used for this reason). Notably, ethylene formation strongly increases in the conversion range of 70-90% and is more pronounced when H2 O is already present in the feed. Building on these findings, a full factorial experimental design (DoE) consisting of 18

Average selectivity acetaldehyde vs. Conversion (Cu catalysts) Ratio HO/EtOH = 0

Cu_1

Cu_2

Cu_3

Cu_4

50 0

Conversion (%)

Cu_1 (p=1 barg) Cu_2 (p=1 barg) Cu_4 (p=1 barg) Cu_3 (p=1 barg)

Cu_1 (p=4 barg) Cu_2 (p=4 barg) Cu_3 (p=4 barg) Cu_4 (p=4 barg)

Cu_1 (p=8 barg) Cu_2 (p=8 barg) Cu_3 (p=8 barg) Cu_4 (p=8 barg)

different parameter combinations for each of the four Cu-based catalysts revealed that all systems follow the same trends when plotting acetaldehyde selectivity as a function of ethanol conversion (see Figure 2 ). However, absolute selectivity towards acetaldehyde strongly depends on the catalytic system. In general, acetaldehyde selectivity decreases with increasing ethanol conversion. Based on the DoE results, it becomes clear that low pressures, low temperatures (low ethanol conversion), and high WHSV (short residence times) favour acetaldehyde formation. This underscores the fact that acetaldehyde is a primary reaction product ( Santacesaria, et al., 2012 ). Notably, the choice of catalyst system can strongly boost selectivity towards acetaldehyde (for example, ‘Cu_3’ with up to 88% selectivity vs. ‘Cu_1’ with 55% under identical process parameters). For this reason, selecting an appropriate catalyst and optimising the reaction parameters are crucial for the successful implementation of this process on an industrial scale. Figure 2 Results of the full factorial DoE for the Cu-based catalysts and pure ethanol feed. In total, 18 different combinations of reaction parameters were tested in only five days