Carbon monoliths performance

Figure 32 includes results illustrating the performance of lipase/car-bon monolith systems in an acylation reaction. For comparison, the free lipase and a commercial immobilized lipase (Novozyme) were also tested. As expected, in all cases the specific activity of immobilized lipase was foimd to be lower than that of the free enzyme. Such a difference is usually ascribed to conformational changes of the enzyme, steric effects, or denaturation. For the monolithic biocatalysts, the activity of the immobilized catalyst relative to that of the pure enzyme was found to be 30-35%, and for the Novozyme catalyst about 80% in the first rim. However, the Novozyme catalyst underwent significant deactivation, in contrast to the carbon monolith-supported catalysts. The deactivation of the Novozyme catalyst in consecutive runs is probably a consequence of the instability of the support matrix under reaction conditions (101,102). 

In this chapter two different catalytic systems with carbon monoliths as the support matrix are discussed in terms of preparation and performance. The main difference between these systems is the effect of the carbon monolith on selectivity. Finally, some practical considerations of the use of different carbon monoliths are discussed. 

In this example, lipase is immobilized on different carbon monoliths and applied in a transesterification reaction in toluene. The biocatalysts are compared in terms of carrier preparation, enzyme immobilization, and performance. A commercially available immobilized lipase is used as a comparison. A convenient tool to compare monolithic biocatalysts is the monolithic stirrer reactor (MSR), consisting of two monoliths that have the catalyst immobilized on the wall of their channels. These monoliths work as stirrer blades that can easily be removed from the reaction medium, thereby eliminating the need for a filtration step after reaction [37]. 

Catalytic tests with the lipase-monolithic catalysts were performed in a monolithic stirrer reactor consisting of a glass vessel equipped with a stirrer motor (V = 2.5 dm ). 1-Butanol and vinyl acetate concentrations were 0.6 M and 1 M, respectively. Activity tests with immobilized lipase Candida antarctica) were performed at varying stirrer rates and temperatures. Carbon monoliths (Westvaco integral carbon monoliths, with a loading of 30 wt% of microporous activated carbon, wall thickness 0.3 mm) were used as a reference material. 

Estevez, L., R. Dua, N. Bhandari, A. Ramanujapuram, R Wang, and E. R Giannelis. 2013. A facile approach for the synthesis of monolithic hierarchical porous carbons— High performance materials for amine based CO2 capture and supercapacitor electrode. Energy Environmental Science 6 1785-1790. 

Lang, J. W., X. B. Yan, W. W. Liu, R. T. Wang, and Q. J. Xue. 2012. Influence of nitric acid modification of ordered mesoporous carbon materials on their capacitive performances in different aqueous electrolytes. Journal of Power Sources 204 220-229. Wang, D. W., F. Li, L. C. Yin et al. 2012. Nitrogen-doped carbon monolith for alkaline supercapacitors and understanding nitrogen-induced redox transitions. Chemistry -A European Journal 18 5345-5351. 

Carbon constitutes the most abundant element of the different FC components. Setting aside the membrane, which is a polymer with a carbon backbone, all the other components, i.e. the CL, GDL and current collector plates (bipolar plates) are made almost entirely of graphitic carbon. The electrocatalyst support of the CL is commonly carbon black in the form of fine powder. GDLs are thin porous layers formed by carbon fibers interconnected as a web or fabric, while current collector plates are carbon monoliths with low bulk porosity. As explained above each of these components has a particular function within the fuel cell and in particular in the triple phase boundary. The structure and properties of the carbon in each of the different FC components will determine the whole performance of the cell. 

The synthesis of monolithic carbons generally relies on the means including sol-gel method and self-assembly approach [25, 26]. In recent years, much efforts have been devoted to create new types of carbon monoliths with enhanced functions, which are developing new polymerization systans (solvents and/or precursors), precise pore engineering toward mnltimodal porosities, and targeted surface/ bulk functionalization for a high performance in CO2 capture [27-30]. 

A sophisticated quantitative analysis of experimental data was performed by Voltz et al. (96). Their experiment was performed over commercially available platinum catalysts on pellets and monoliths, with temperatures and gaseous compositions simulating exhaust gases. They found that carbon monoxide, propylene, and nitric oxide all exhibit strong poisoning effects on all kinetic rates. Their data can be fitted by equations of the form ... 

Autocatalysts, based on monoliths, are probably the most extensively used catalytic reactors around a hundred million have been installed and are performing well in car exhaust systems [10-12]. Reduction of volatile organic carbon (VOC) emissions [13] and removal of NOj, from stationary sources [14, 15] are also... 

Several uncertainties in this periodic process have not been resolved. Pressure drop is too high at SV = 10,000 h 1 when packed beds of carbon are used. Study of carbon-coated structured packing or of monoliths with activated carbon washcoats is needed to see if lower pressure drops at 95% SO2 removal can be achieved. Stack gas from coal or heavy oil combustion contains parts-per-million or -per-billion quantities of toxic elements and compounds. Their removal in the periodically operated trickle bed must be examined, as well as the effect of these elements on acid quality. So far, laboratory experiments have been done to just 80°C use of acid for flushing the carbon bed should permit operation at temperatures up to 150°C. Performance of periodic flow interruption at such temperatures needs to be determined. The heat exchange requirements for the RTI-Waterloo process shown in Fig. 26 depend on the temperature of S02 scrubbing. If operation at 150°C is possible, gas leaving the trickle bed can be passed directly to the deNO, step without reheating. 

Lenz and Aicher reported the experimental results obtained with an autothermal reformer fed with desulfurized kerosene employing a metallic monolith coated with alumina washcoat supporting precious metal catalysts (Pt and Rh) [78]. The experiments were performed at steam-to-carbon ratios S/C = 1.5-2.5 and... 

The use of a monolithic stirred reactor for carrying out enzyme-catalyzed reactions is presented. Enzyme-loaded monoliths were employed as stirrer blades. The ceramic monoliths were functionalized with conventional carrier materials carbon, chitosan, and polyethylenimine (PEI). The different nature of the carriers with respect to porosity and surface chemistry allows tuning of the support for different enzymes and for use under specific conditions. The model reactions performed in this study demonstrate the benefits of tuning the carrier material to both enzyme and reaction conditions. This is a must to successfully intensify biocatalytic processes. The results show that the monolithic stirrer reactor can be effectively employed in both mass transfer limited and kinetically limited regimes. 

Similar results were achieved over a Rh/alumina monolith catalyst " using catalytic POX for the reforming of a simulated JP-8 military feed containing 500 ppm of sulfur (as benzothiophene or dibenzothiophene). Stable performance for over 500 h with complete conversion of the hydrocarbons to syngas at 1,050°C, 0.5 s contact time, and LHSV of about 0.5 h was reported. At this high temperature, carbon formation was not reported and the sulfur exited as hydrogen sulfide. 

Germani et al. [82] compared the performance of their catalyst coating developed for water-gas shift in a micro structured reactor with that of the same catalyst coated on a cordierite monolith under identical reaction conditions. Higher conversion was achieved in the micro channels at same modified residence time under all experimental conditions applied. Figure 2.88 shows the CO conversion vs. a modified residence time (catalyst weight/flow of carbon monoxide) measured at various reaction temperatures. 

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