Predicting the DNP-SENS efficiency in reactive heterogeneous catalysts from hydrophilicity

Abstract

Identification of surfaces at the molecular level has benefited from progress in dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS). However, the technique is limited when using highly sensitive heterogeneous catalysts due to secondary reaction of surface organometallic fragments (SOMFs) with stable radical polarization agents. Here, we observe that in non-porous silica nanoparticles (NPs) (d_particle = 15 nm) some DNP enhanced NMR or SENS characterizations are possible, depending on the metal-loading of the SOMF and the type of SOMF substituents (methyl, isobutyl, neopentyl). This unexpected observation suggests that aggregation of the nanoparticles occurs in non-polar solvents (such as ortho-dichlorobenzene) leading to (partial) protection of the SOMF inside the interparticle space, thereby preventing reaction with bulky polarization agents. We discover that the DNP SENS efficiency is correlated with the hydrophilicity of the SOMF/support, which depends on the carbon and SOMF concentration. Nitrogen sorption measurements to determine the BET constant (C_BET) were performed. This constant allows us to predict the aggregation of silica nanoparticles and consequently the efficiency of DNP SENS. Under optimal conditions, C_BET > 60, we found signal enhancement factors of up to 30.

Fig. 1. Free energy as a function of distance (r) between centers of masses of two prototypical silica particles M-0, M-Me or M-CH₂^tBu illustrates the trend of aggregation between these two particles in o-DCB solution. M-0, M-Me, and M-CH₂^tBu correspond to [SiO_1.5(OH)]₈, [SiO_1.5(O^tBu)_0.5(OH)_0.5]₈, and [SiO_1.5(OC(CH₂^tBu)₃)_0.5(OH)_0.5]₈, respectively. Details of the free energy calculation are summarized in the ESI.

Fig. 2. (a) Correlation of C_BET and c_carbon of SOMFs 1–6 at various concentrations; (b) correlation of C_BET with ¹H enhancement factors ε_H (o-DCB) and (c) correlation of C_BET and c_SOMF of SOMFs 1–6 at different concentrations. The various regions (yellow, blue, red and green) indicate where DNP SENS measurements are possible and where they are not. S/N_(SOMF) corresponds to the signal to noise ratio of the SOMF.

Fig. 3. (a) DNP SENS ¹H MAS, (b) ¹³C CP MAS and (c) ²⁹Si CP MAS (100 K, 400 MHz/263 GHz gyrotron) of 3-med in 16 mM TEKPol o-DCB solution obtained at 8 kHz MAS. The red line represents the spectra without microwave irradiation (μwave off), and the black line represents experiments with microwave irradiation (μwave on). The stars indicate the spinning side band.