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Dedicated to Professor Rüdiger Lange on the occasion of his 65th birthdayThe continuous hydrogenation of a mixture of L-arabinose and D-galactose over a Ru/C catalyst was investigated in a miniaturized packed-bed reactor. The reaction is one important step of the transformation of the naturally occurring hemicellulose arabinogalactan into valuable sugar alcohols. Process intensification was accomplished by reducing the reactor dimensions to a few millimeters; thus leading to better mass and heat transfer performance. The effects of temperature, pressure, and liquid flow rate on the yield and by-product formation are discussed. Based on a kinetic model derived from batch experiments, a model of the continuous reactor was developed and used for scale-up purposes.

Microbiological photosynthesis is a promising tool for producing hydrogen in an ecologically friendly and economically efficient way. Certain microorganisms (e.g. algae and bacteria) can produce hydrogen using hydrogenase and/or nitrogenase enzymes. However, their natural capacity to produce hydrogen is relatively low. Thus, there is a need to optimize their core photosynthetic processes as well as their cultivation, for more efficient hydrogen production. This review aims to provide a holistic overview of the recent technological and research developments relating to photobiological hydrogen production and downstream processing. First we cover photobiological hydrogen synthesis within cells and the enzymes that catalyze the hydrogen production. This is followed by strategies for enhancing bacterial hydrogen production by genetic engineering, technological development, and innovation in bioreactor design. The remaining sections focus on hydrogen as a product, that is, quantification via (in-process) gas analysis, recent developments in gas separation technology. Finally, a discussion of the sociological (market) barriers to future hydrogen usage is provided as well as an overview of methods for life cycle assessment that can be used to calculate the environmental consequences of hydrogen production

The conversion of xylan to xylitol is commonly realized via the two separate process steps hydrolysis and hydrogenation (or fermentation). Recent research activities aim at developing one-pot processes to facilitate this conversion to save resources and operation time. To avoid hazardous chemicals like liquid acids for the initial xylan hydrolysis, enzymes are a suitable green alternative. A series of experiments are discussed wherein process conditions for chemical conversion were adjusted to more enzyme-friendly reaction conditions to overcome xylose-induced product inhibition by direct hydrogenation of sugar to xylitol. A novel combination of enzymes and precious metal catalyst (Ru/C) in an one-pot process is demonstrated. Based on these results, limiting factors and potentially advantageous process parameters as well as catalyst combinations are discussed.

The hydrolytic hydrogenation of xylan to xylitol by a one-pot process was studied in detail in a batch reactor. The reaction was catalyzed by a combination of diluted sulfuric acid and precious metal Ru on carbon powder. Process parameters were varied between 120–150°C, while maintaining constant hydrogen pressure at 20 bar and an acid concentration equivalent to pH 2. The xylan solution consisted of 1 wt% beechwood powder (Carl Roth, >90%) in deionized water. Sulfuric acid was added to the solution until pH two was reached, then the 0.3 wt% catalyst powder (5% Ru on Act. C) was added and the solution was put into the batch reactor. The first approach of kinetic modeling began with conventional first-order kinetics and compared this to a more complex model based on Langmuir–Hinshelwood kinetics. The xylan and xylitol data reached a good fit. However, the modeling results also showed that the rate-limiting step of xylose-formation was still not represented in a satisfactory manner. Therefore, the model was adapted and developed further. The advanced model finally showed a good fit with the intermediate product xylose and the target product xylitol. The overall modeling methods and results are presented and discussed.