Advanced Solid-State NMR Approaches and Their Application in Catalyst Development

Subramanian Prasad

BASF

Transition metal containing zeolites such as Cu and Fe exchanged CHA are of crucial importance for the remediation of environmentally hazardous engine exhaust gases [1]. Direct measurement of the active species of exchanged transition metals is generally challenging due to a number of effects, particularly their paramagnetic nature [2]. We have observed through numerous studies that these paramagnetic ions, when in close proximity with silicon-29 nuclei within a zeolite framework, cause drastic yet consistent reductions in the spin-lattice relaxation time (T1) of those nuclei. This reduction can be correlated with distribution of active sites and overall activity in Selective Catalytic Reduction (SCR). As a relatively simple exchange ion in CHA, Cu2+ demonstrates that T1 decreases asymptotically as Cu/Al ratio [3] is increased until the exchange sites are saturated reaching an effective maximum activity. The mobility of these ions in poorly exchanged CHA is demonstrated with reduced T1 and improved activity. The much more complex system of Fe ion exchange shows a parallel improvement in ion evolution after hydrothermal treatment in CHA. A broader study of efficacy of ion exchange vs pore size shows that other zeolites MFI and BEA which have larger pores (10 and 12 Member Rings respectively, vs 8MR for CHA) display lower silicon-29 T1 values as pore size increases. This trend is reflected in the catalytic activity, further showing the power of using silicon-29 T1 analysis as a nondestructive probe for final SCR functionality [4].

Reaction properties of these catalysts, including hydrothermal stability and product selectivity, depend on the distributions of Al heteroatoms in the zeolite framework, particularly “paired” Al heteroatoms separated by one or two O-Si-O linkages [5]. The local Al distributions in zeolites can be varied by zeolite synthesis approaches or post-synthetic treatments. However, atomic-level evidence for these paired framework Al sites has been challenging to obtain, along with their effects on cation distributions and catalytic reaction properties. Through a combination of two-dimensional (2D) 27Al{29Si} and 29Si{29Si} through-bond NMR correlation experiments, direct evidence of “paired” framework Al atoms in aluminosilicate chabazite zeolite catalysts is obtained. In particular, the 2D NMR analyses enable second- and third-nearest-neighbor tetrahedral sites to be detected and resolved, which are challenging to distinguish by other methods [6]. In situ 13C NMR measurements, furthermore, enable the distributions of adsorbed reactant species on chabazite catalysts to be resolved, quantified, and correlated with macroscopic catalytic reaction properties for methanol dehydration. The results yield new atomic-level insights on how different distributions of framework heteroatoms, such as aluminum, affect the activity and selectivity of zeolite catalysts.

References

  1. Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts, I. Nova, E. Tronconi (Eds.) Springer New York, New York, NY, 2014.
  2. A.J. Pell, G. Pintacuda, C.P. Grey, Paramagnetic NMR in solution and the solid state, Prog. Nucl. Magn. Reson. Spec., 111 (2019) 1-271.
  3. J. Palamara, K. Seidel, A. Moini, S. Prasad, “Ion distribution in copper exchanged zeolites by using Si-29 spin lattice relaxation analysis” J. Magn. Reson. 267 (2016) 9–14.
  4. J. Palamara, J. Yang, A. Moini, S. Prasad, “Exploiting inherent paramagnetic effects in transition metal exchanged zeolites as predictors of catalytic activity“ 61st Experimental NMR Conference, Baltimore, MD, March 3-8, 2020.
  5. J. Di Iorio, C. Nimlos, R. Gounder, “Introducing catalytic diversity into single-site chabazite zeolites of fixed composition via synthetic control of active site proximity” ACS Catal., 7 ( 2017) 6663 and references cited therein.
  6. M. Schmithorst, B. Chmelka, A. Moini, S. Prasad, “Distributions of Al atoms in chabazite zeolite frameworks and their effects on adsorption and catalytic reaction properties“ 20th International Zeolite Conference, Valencia July 3-8, 2022.