Spatial activity profiling along a fixed bed of powder catalyst during selective oxidation of propylene to acrolein

Spatially resolved activity profiling along a fixed bed of powder catalyst during selective oxidation of propylene to acrolein revealed gradients in the gas phase composition and temperature, and thus the reaction network.


Raw data during profile acquisition
For acquisition of the profiles along the fixed-bed catalyst at an oven temperature of 400 °C (setpoint), the position with respect to the sampling orifices (via linear stage), the gas phase temperature at the sampling orifice (via inserted type-K thermocouple), the local gas phase concentration (via mass spectrometer (MS)) and integral catalytic performance (via gas chromatograph (GC)) were recorded simultaneously. Corresponding data is shown in Figure   S1. Figure S1: Obtained raw data during profile acquisition. Position along the catalyst bed (a), gas phase temperature at the sampling orifice (b), MS data representing local gas phase composition (c), and integral GC data (d).

Position and time averaged data
For the depiction of the gas phase concentration and temperature profiles, the last five minutes of temperature and ion current acquisition at each position were averaged. In the case of temperature this corresponded to the averaging of 60 individual points, resulting in the profile shown in Figure S2a. In the case of mass spectrometric data, 5-6 individual points for each m/z ratio were averaged, resulting in the profiles shown in Figure S2b. Furthermore, bypass and individual GC measurements during profile acquisition were averaged (see Figure S2c) to correlate MS with GC data for the quantification of the MS data along the catalyst bed. The averaged performance of the catalyst during profile acquisition is shown in Figure S2d.

Species fragmentation during mass spectrometry
The fragmentation patterns 1 of the observed species by gas chromatography (GC) are shown in Figure S3. For the correlation of quantified GC data with local mass spectrometry (MS) data, preferentially unique fragments for each species were used (e.g., m/z = 22 for CO2). More information about selected m/z ratios can be found in Table S1. Furthermore, the ratios of different m/z signals (e.g., main fragment for allyl alcohol m/z = 58) were checked to resolve the formation of intermediates not observed by GC. However, no clear evidence for their formation was found as the corresponding ratios were almost constant along the catalyst bed.  (1)

Concentration profiles
As the amount of water was not quantified by the GC, it was calculated according to the stoichiometry of the reactions leading to the observed main-and side-products (c.f. equations (2)-(5)).

Reaction progress along the catalyst bed
The propylene (C3H6) conversion along the catalyst bed at position x was calculated according to equation (6). The selectivity towards acrolein (C3H4O), acrylic acid (C3H4O2), carbon monoxide (CO), carbon dioxide (CO2), and the sum of carbon oxides (COx) at point x was calculated according to equations (7)-(11), respectively.

Local reaction rates
First, the total volume flow was converted to total molar flow assuming ideal gas conditions. Subsequently, the obtained value was multiplied with the individual concentration profiles (i.e., points) shown in Figure S4. Thus, molar flow rates along the catalyst bed were obtained.
Finally, the first derivative of these spatially resolved molar flow rates was calculated, representing the local reaction rate. Figure