I performed the CO2 adsorption on PEI impregnated SBA-15 and the adsorption capacity as followed:
30 wt% = 2.0 mmol co2/g
40wt% = 2.5 mmol CO2/g
50wt% = 3.0 mmolCO2/g
60wt% = 3.5 mmol CO2/g
please suggest me, how to calculate the N efficiency (molar ratio (CO2/N))?
As shown in our groups paper below, amine efficiency is calculated as the moles of CO2 per moles of Nitrogen as you suggest. The best way to calculate this for PEI is to make an assumption about the amount of available moles of N for carbon capture. As I'm sure you are aware primary amines are seemingly more effective than secondary, which are more effective than tertiary, and on a branched PEI you will have some amount of all three amine types. However, for ease of calculation we will take all amines to be as effective as others for efficiency calculation (this gives a slight overestimate of the real case as you can imagine, but barring some truly detailed knowledge of your PEI polymer it is difficult to make assumptions otherwise). So on PEI you have one effective N mole per 43.04 grams of repeat unit. This calculates to 0.325 mass fraction of Nitrogen to PEI repeating monomer. This can be calculated to moles per gram as well, so 1 mole of N per 43.04 grams of monomer. You can then do capacity in (moles CO2/grams polymer)*(gram polymer/moles Nitrogen) and obtain amine efficiency. Obviously for PEI this efficiency factor (the gram polymer/moles Nitrogen) is seemingly high, but in reality very low due to the presence of tertiary and secondary amine groups in hyperbranched PEI at least.
Also as an aside, an amine efficiency seemingly higher than the maximum (0.5 moles Co2/mole N for dry conditions) either denotes humidity in the capture stream (which would bring maximum efficiency up to 1) OR a mixed physisorption/chemisorption process where the effective capacity is being overstated by the simple calculation of amine efficiency. Please let me know if you have any other questions!
Amine efficiency is nothing but the mole of CO2 adsorbed per mole of Nitrogen(in the form of primary/sec/tertiary amine). As Christopher Cogswell explained, you need to know the moles of nitrogen loaded. The ratio of moles of CO2 to moles of N will give you the amine efficiency. I suggest you to carry out the CHN-elemental analysis for the accurate quantification of amine loading.
You can refer to the attached file. I have clearly stated the equation for amine efficiency calculation. Also, refer table 2 for the calculations.
XPS is more sensitive than EDS. The most sensitive technique is PIXE. see ref. J. Radioanal. Nucl. Chem., v. 240, n. 3. pp. 893-896.
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