Coke Formation in a Zeolite Crystal During the Methanol‐to‐Hydrocarbons Reaction as Studied with Atom Probe Tomography

Abstract

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-...

Discerning the Location and Nature of Coke Deposition from Surface to Bulk of Spent Zeolite Catalysts

“…In another recent work by J. E. Schmidt et al . C 13 clusters were noted to be present in low Si/Al ratio regions (or high Al concentration regions) after 90 minutes of methanol-hydrocarbon conversion reaction35. Based on our APT results, we also observed a non-uniform distribution of Al in as-prepared HZSM-5 as revealed by the 2D ion concentration map in Fig.…”

“…More recently work by Schmidt et al . analyzed spatial correlation of C 13 clusters formed in ZSM-5 catalyst particles after 90 minute C 13 isotopically labeled methanol to hydrocarbon conversion reaction35. In their work, based on APT analysis, authors indicated formation of early C 13 clusters which appeared to correlate with regions of high Al content in ZSM particles.…”

Probing the Location and Speciation of Elements in Zeolites with Correlated Atom Probe Tomography and Scanning Transmission X‐Ray Microscopy

“…After STXM measurements, a needle shaped specimen was carefully removed from the cross section using Ga FIB milling and then studied with APT using the LEAP 4000XR local electrode atom probe equipped with laser pulsing capabilities and an energy compensating reflectron lens and data analysis was performed using CAMECA's IVAS software. We have previously reported both in detail ,,,. The APT experiment was performed on a tungsten wire instead of a Si micropost array, which is the more historical method of conducting APT experiments …”

Recent trends and fundamental insights in the methanol-to-hydrocarbons process

“…Recently, the first use of atom probe tomography (APT) to investigate the 3-D distribution of elements in zeolites at the subnm scale has been reported. 108,109 In 2016 Schmidt et al used APT to study coke formation and its relation to inhomogeneous Al distribution in large ZSM-5 crystals after MTH (using 13 C-labelled methanol to distinguish the formed coke species from contaminations). 109 APT samples were taken from the (Al rich) surface and the (Al poor) core of the ZSM-5 crystal showing a clear correlation of coke formation and Brønsted acid site density.…”