The Silicon/Aluminium ratio is one of the most important structural parameters of zeolites because it controls the framework charge density, acidity, ion-exchange capacity, and hydrothermal stability. Solid-state NMR spectroscopy, particularly 29Si magic-angle spinning (MAS) NMR, provides a powerful method for determining the framework Si/Al ratio.
Under the Löwenstein rule, direct Al–O–Al linkages are avoided in aluminosilicate frameworks. Consequently, each framework aluminum atom is connected through oxygen bridges only to silicon atoms. This structural constraint allows the distribution of silicon environments observed in 29Si MAS NMR spectra to be related directly to the framework Si/Al ratio.
In 29Si MAS NMR, silicon atoms are commonly described as Si(nAl) or Q4(nAl), where n represents the number of neighboring aluminum atoms connected through Si–O–Al bridges. Distinct resonances are observed for Si(0Al), Si(1Al), Si(2Al), Si(3Al), and Si(4Al) environments. Increasing the number of neighboring Al atoms generally shifts the resonance by approximately +5 ppm per Al atom. This observation led to the Engelhardt correlation:
δ(29Si) ≈ δ0 + nΔ
where n is the number of neighboring Al atoms, Δ is typically 4–6 ppm per Al, and δ0 is the chemical shift of Si(0Al). Typical ranges are:
Si(0Al): −105 to −115 ppm
Si(1Al): −100 to −107 ppm
Si(2Al): −95 to −102 ppm
Si(3Al): −90 to −97 ppm
Si(4Al): −85 to −92 ppm
The original assignments were established using zeolites with known compositions. High-silica zeolites such as silicalite-1 and highly dealuminated ZSM-5 contain predominantly Si(0Al) environments and therefore provide reference chemical shifts for Si(0Al).
Aluminum-rich zeolites such as zeolite A and zeolite X contain large populations of Si(3Al) and Si(4Al) environments, allowing calibration of the full Si(nAl) sequence.
By deconvoluting the spectrum and integrating the intensity of each resonance, the average number of aluminum neighbors per silicon atom can be calculated as:
n̄ = Σ(nIn) / Σ(In)
where In is the integrated intensity of the Si(nAl) resonance.
Assuming all aluminum is tetrahedrally coordinated framework aluminum and that the Löwenstein rule is obeyed, the framework Si/Al ratio is obtained from:
Si/Al = 4Σ(In) / Σ(nIn)
This relationship arises because each framework aluminum atom is connected to four silicon atoms, while each Si(nAl) site contributes n Si–O–Al linkages.
How to determine the Si/Al ratio in JASON
There are a number of ways in which this ratio can be determined with JASON. One way would be to use BeautifulJASON in a similar way as shown in a previous post to study Styrene Butadiene Rubber. In this example I will use the table functionality of JASON, which has been enhanced recently to support custom equations and rules that preserve these equations. This is a faster way to set it up without the need to code, and it is equally fast to run, by simply loading the data we will automatically obtain an analysis like the following, with the last column of the peak table displaying the Si/Al ratio:
The first step is to load an example spectrum and process it as desired. In this example I will use line broadening to approximately match the signal decay.
Then we define the peaks of interest, and JASON will automatically deconvolve the peaks. We can then fit their models to the spectrum to refine the deconvolution. Note that we can manually readjust width, height or other parameters if desired at any point of this process by simply editing the values from the peak table.
We then create a peak table, once selected we can change its format. For example, we can hide columns with information we are not interested in. Then we will add two columns, one for our own reference to indicate which peak matches which silicon environment and another one to calculate the Si/Al ratio. The formula to be used resembles other spreadsheet software. We check in the Table Tools, under the columns tab the Letter associated with each column and each row is numbered as usual. For example, typically in a peak table the height column is associated with the letter C, so to add the height of the first two peaks we would calculate it in a cell as =C1+C2. So in this case, to calculate the Si/Al ratio based on the peak area (K column) we would need to type in a cell:
=4*(K1+K2+K3+K4+K5)/ (K1*4+K2*3+K3*2+K4*1+K5*0)
Once done, we can select the spectrum and create rules for the processing, analysis and layout, so next time we load a spectrum that meets the conditions of the rules it will be processed, analysed and displayed in the same way.
This procedure is demonstrated in the following video:
Beyond the Si/Al ratio
An alternative approach involves quantitative 29Si and 27Al MAS NMR measurements. In this method, the total silicon and aluminum signal intensities are measured under quantitative acquisition conditions, including sufficiently long recycle delays and appropriate pulse calibration. The Si/Al ratio is then obtained directly from the corrected integrated intensities. However, quantitative 27Al NMR can be complicated by quadrupolar broadening and excitation inefficiencies, and extra-framework aluminum species may contribute to the observed signal.
The distinction between framework and total Si/Al ratios is important. The Si(nAl)-based method yields the framework Si/Al ratio because it probes only tetrahedral framework aluminum connected to silicon atoms. In contrast, quantitative 27Al NMR or elemental analysis may include extra-framework aluminum species generated during dealumination.
The use of 29Si MAS NMR for determining framework Si/Al ratios was established in pioneering studies by Engelhardt and co-workers and remains a standard approach in zeolite characterization. Modern investigations often combine 29Si MAS NMR, 27Al MAS NMR, multidimensional correlation experiments, and density functional theory calculations to obtain a more detailed picture of aluminum distribution and framework ordering.
References
1. Engelhardt, G.; Lohse, U.; Patzelová, V.; Mägi, M. 29Si MAS NMR studies of zeolites and related aluminosilicates. Zeolites, 1983, 3, 233–238.
2. Engelhardt, G.; Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites. John Wiley & Sons, 1987.
3. Fyfe, C. A. Solid State NMR for Chemists. C.F.C. Press, 1983.
4. Kentgens, A. P. M. A Practical Guide to Solid-State NMR of Half-Integer Quadrupolar Nuclei with Some Applications to Disordered Systems. Geoderma, 1997, 80, 271–306.
5. Freude, D.; Hunger, M. NMR Spectroscopy in Zeolite Research. Catalysis Reviews, 1993, 35, 1–21.
6. Ashbrook, S. NMR of Microporous Materials http://www.solidstatenmr.org.uk/lectures_files/pdfs/Microporous.pdf