Understanding the formation of carbon deposits in zeolites is vital to developing new,s uperior materials for various applications,i ncluding oil and gas conversion processes.H erein, atom probe tomography (APT) has been used to spatially resolve the 3D compositional changes at the subnm length scale in as ingle zeolite ZSM-5 crystal, whichh as been partially deactivated by the methanol-to-hydrocarbons reaction using 13 C-labeled methanol. The results reveal the formation of coke in agglomerates that span length scales from tens of nanometers to atomic clusters with amedian size of 30-60 13 Ca toms.T hese clusters correlate with local increases in Brønsted acid site density,demonstrating that the formation of the first deactivating coke precursor molecules occurs in nanoscopic regions enriched in aluminum. This nanoscale correlation underscores the importance of carefully engineering materials to suppress detrimental coke formation.Zeolites are crystalline,m icroporous materials that exhibit robust hydrothermal stability,allowing them to be used under demanding process conditions,s uch as oil refinery operations [1,2] and automotive emissions treatments. [3,4] Commercially,one of the most important zeolites is ZSM-5 with MFI framework topology,w hich has become ubiquitous in petroleum refining and chemical manufacturing.[2] Thee normous quantities at which this material is utilized at the global scale continue to drive research targeting improved performance. Thed etrimental formation of coke is one of the factors limiting zeolite materials,p articularly ZSM-5, in highdemand catalytic processes,s uch as fluid catalytic cracking (FCC) and the methanol-to-hydrocarbons (MTH) reaction. ZSM-5 coking in the MTH reaction has long been studied, with ar ange of conclusions regarding the nature and mechanism of coke formation. [5,6] Despite ongoing investigations and debates,there is aconsensus that coking occurs due to the formation of alkylated mono-and polycyclic aromatics near internal channel intersections,followed by an increase in surface coke from polycyclica renes near pore openings, which finally form agraphitic layer and block pore access. [7][8][9][10][11][12][13] In order to more fully elucidate the material properties that promote the detrimental formation of coke during the MTH reaction on ZSM-5, it would be beneficial to study the carbon deposits on the sub-nm length scale.P revious coking studies on ZSM-5 in the MTH reaction have concentrated on the bulk, [7][8][9]13] or on micrometer length scales, [10,11,14,15] to gain some spatial insight, but none have been capable of delivering sub-nm resolution.Theo nly characterization method currently capable of spatially resolving 3D element distributions at the sub-nm scale is atom probe tomography (APT), which was first envisioned in the 1930s,b ut has recently experienced rapid growth due to improvements in instrumentation. [16][17][18][19][20] APT is able to create atom-by-atom 3D compositional reconstructions of materials within afabricated needle-...
Understanding the 3-D distribution and nature of active sites in heterogeneous catalysts is critical to developing structure–function relationships. However, this is difficult to achieve in microporous materials as there is little relative z-contrast between active and inactive framework elements (e.g., Al, O, P, and Si), making them difficult to differentiate with electron microscopies. We have applied atom probe tomography (APT), currently the only nanometer-scale 3-D microscopy to offer routine light element contrast, to the methanol-to-hydrocarbons (MTH) catalyst SAPO-34, with Si as the active site, which may be present in the framework as either isolated Si species or clusters (islands) of Si atoms. 29Si solid-state NMR data on isotopically enriched and natural abundance materials are consistent with the presence of Si islands, and the APT results have been complemented with simulations to show the smallest detectable cluster size as a function of instrument spatial resolution and detector efficiency. We have identified significant Si–Si affinity in the materials, as well as clustering of coke deposited by the MTH reaction (13CH3OH used) and an affinity between Brønsted acid sites and coke. A comparison with simulations shows that the ultimate spatial resolution that can be attained by APT applied to molecular sieves is 0.5–1 nm. Finally, the observed 13C clusters are consistent with hydrocarbon pool mechanism intermediates that are preferentially located in regions of increased Brønsted acidity.
Characterizing the structures of zeolites and their catalytic performance with high-spatial-resolution is vital to developing new solid catalysts. We demonstrate the application of photoinduced force microscopy (PiFM), with nanometer scale resolution across the infrared spectral range, for the study of zeolite ZSM-5 thin-films with various Si/Al ratios after the methanol-to-hydrocarbons reaction. This first-of-its kind nanometer scale infrared imaging of zeolite materials demonstrates the possibility of PiFM for the study of functional porous materials.
Hydrothermal treatment is a common method used to modify the physicochemical properties of zeolite‐based catalyst materials. It alters the number and type of acid sites through dealumination and increases molecular diffusion by mesopore formation. Steaming also reduces the structural integrity of zeolite frameworks. In this study, Raman microscopy has been used to map large zeolite ZSM‐5 crystals before and after steaming. 3D elemental maps of T−O (T: Al or Si) sites of the zeolite were obtained. The Raman active vibrational bands were determined, which are indicative of (non‐) framework Al, as well as of structural integrity. Zeolite steaming caused the introduction of additional heterogeneities within the zeolite framework. Al migration and the formation of extra‐framework Al species were observed. The described experiments demonstrate the capability of 3D Raman spectroscopy as a valuable tool to obtain information on the spatial distributions of framework elements as well as defects within a zeolite‐based material.
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