Quantitative 3D Fluorescence Imaging of Single Catalytic Turnovers Reveals Spatiotemporal Gradients in Reactivity of Zeolite H-ZSM-5 Crystals upon Steaming

Abstract

Optimizing the number, distribution, and accessibility of Brønsted acid sites in zeolite-based catalysts is of a paramount importance to further improve their catalytic performance. However, it remains challenging to measure real-time changes in reactivity of single zeolite catalyst particles by ensemble-averaging characterization methods. In this work, a detailed 3D single molecule, single turnover sensitive fluorescence microscopy study is presented to quantify the reactivity of Brønsted acid sites in zeolite H-ZSM-5 crystals upon steaming. This approach, in combination with the oligomerization of furfuryl alcohol as a probe reaction, allowed the stochastic behavior of single catalytic turnovers and temporally resolved turnover frequencies of zeolite domains smaller than the diffraction limited resolution to be investigated with great precision. It was found that the single turnover kinetics of the parent zeolite crystal proceeds with significant spatial differences in turnover frequencies on the nanoscale and noncorrelated temporal fluctuations. Mild steaming of zeolite H-ZSM-5 crystals at 500 °C led to an enhanced surface reactivity, with up to 4 times higher local turnover rates than those of the parent H-ZSM-5 crystals, and revealed remarkable heterogeneities in surface reactivity. In strong contrast, severe steaming at 700 °C significantly dealuminated the zeolite H-ZSM-5 material, leading to a 460 times lower turnover rate. The differences in measured turnover activities are explained by changes in the 3D aluminum distribution due to migration of extraframework Al-species and their subsequent effect on pore accessibility, as corroborated by time-of-flight secondary ion mass spectrometry (TOF-SIMS) sputter depth profiling data.

Figure 1

Schematic of the single molecule fluorescence approach used to map in 3D the reactivity of a single H-ZSM-5 crystal. (a) Intergrowth structure of a zeolite H-ZSM-5 crystal indicating the direction of straight and sinusoidal pores in different subunits (color coded). (b) Accumulated image of individual fluorescent products depicted with respect to the size of the zeolite crystal. (c) Formation of fluorescent products (red) upon protonation of FA (black) on a Brønsted acid site. (d) Estimate of the analyzed crystalline volume depicting the 3D distribution of fluorescent molecules (red). Note that the localization precision in the Z-direction is estimated to be ∼500 nm.

Figure 2

NASCA localization approach. (a) Three isolated catalytic events (bursts), as identified by the 2D Gaussian localization algorithm. (b) A single burst, as detected by the EMCCD camera, and (c) subsequent localization of a fluorescent event appearing in 10 consecutive frames. The red circle denotes a diameter of 20 nm, indicating the lateral spatial precision of the method. (d) High-resolution map of fluorescence activity based on 100 consecutive frames. (e) Typical fluorescence trajectories of the events shown in (d). (f) Histogram of the photobleaching times for 370 single molecules.

Figure 3

Single turnover stochastics of the oligomerization reaction monitored at the surface of the H-ZSM-5-P single crystal for 4 h in a 5.75 mM solution of FA. (a) Total number of detected turnovers per frame as a function of time. Each time interval is 100 s. Note the large time gaps between the measurements. (b) Poisson distributions of the corresponding color-coded trajectories presented in (a), including the Poisson parameter λ.

Figure 4

Single molecule reactivity maps for H-ZSM-5-P, H-ZSM-5-MT, and H-ZSM-5-ST crystals recorded at three different focal depths (Z = 0 (surface), 2, and 4 μm). Reactivity is accumulated for 1000 frames after 3 h of reaction in a 5.75 mM solution of FA. Yellow arrows indicate the regions with lower reactivity due to a different crystallographic orientation of the subunits. Color bar: turnovers per 200 × 200 nm².

Figure 5

Models of the H-ZSM-5 intergrowth structure: red (top/bottom subunits), straight pores run parallel to the image plain; blue (side subunits), straight pores run perpendicular to the image plain. The overlaid single molecule maps indicate differences in reactivity for planes that are 2 and 4 μm below the surface, after 2.5 h in a 5.75 mM solution of FA. Color bar: turnovers per 800 × 800 nm².

Figure 6

Normalized turnover activities of zeolites H-ZSM-5-P, H-ZSM-5-MT, and H-ZSM-5-ST in a 5.75 mM solution of FA plotted as a function of time and focal depth Z. The turnover rates are calculated and normalized for the top subunit (see Figure 5) in order to eliminate the polarization effect and higher background scattering from the side subunits. The first two experimental points for the H-ZSM-5-MT crystal (after 5 and 42 min) were recorded from two different crystals. The color bars indicate turnover rates, as plotted in the 3D graphs. Note the logarithmic axis for H-ZSM-5-ST. The black dots in the 3D graphs indicate the experimental values.

Figure 7

Aluminum TOF-SIMS sputter depth profiles for single zeolite crystals: H-ZSM-5-P (blue), H-ZSM-5-MT (green), and H-ZSM-5-ST (red). The approximate number of Al atoms is calculated on the basis of the TOF-SIMS response of the Si⁺/Al⁺ signal with respect to 96 T atoms per unit cell of zeolite H-ZSM-5.

Figure 8

High-resolution imaging of accessible acid sites. (a, b) High-resolution images of surface reactivity based on movies (2000 frames) for (a) a H-ZSM-5-P crystal in a 23 mM solution of FA and (b) a H-ZSM-5 MT crystal in a 11.5 mM solution of FA. The color bar denotes the number of detected turnovers per 48 × 48 nm². Insets marked with arrows indicate high-resolution images of 384 × 384 nm² domains, used for calculating the histograms displayed in (c) and (d). The yellow square in (a) indicates a region of interest used to construct the scatter plot in Figure 9a. (c, d) Corresponding histograms of turnover activity, calculated from 384 × 384 nm² binned regions in (a) and (b) and normalized to the molar concentration of FA, for (c) H-ZSM-5-P and (d) H-ZSM-5-MT.

Figure 9

(a, b) Scatter plots of reactivity reconstructed for (a) the H-ZSM-5-P crystal and the region of interest indicated in Figure 8a (yellow square) and (b) a simulated, random scatter plot. Each dot in the scatter plots represents one catalytic turnover. (c, d) Histograms of the number of nearest-neighbors (NN) detected within a radius of 100 nm. (c) Comparison of H-ZSM-5-P and the simulated pattern, calculated from (a) and (b). (d) Comparison of H-ZSM-5-MT and the corresponding simulated pattern (see Supporting Information S5 for the corresponding scatter plots).

Figure 10

(a) Single turnover trajectory recorded for a 384 × 384 nm² zeolite domain. Inset: definition of the waiting time as the time between two subsequent catalytic turnovers. (b) Waiting time trajectory reconstructed from (a). (c) Evolution of turnover numbers for five exemplified zeolite domains: the black line is derived from (b), and the red lines in the background represent all 784 trajectories. (d) Mean distribution of waiting times calculated for all 784 surface domains (dark blue). The blue and red lines denote the fitted exponential decays of the waiting time histograms for the blue and red trajectories in (c), respectively. (e) 2D conditional histogram of consecutive waiting times recorded at t_n and t_n+1. The color bar indicates the occurrence of pairs of waiting times.