The Methanol-to-Hydrocarbons (MTH) process is currently of great interest as methanol, that can be produced from conventional as well as renewable sources, such as natural gas, biomass and waste, can be transformed into valuable commodity chemicals, such as ethylene or propylene.1 The MTH reaction can be catalyzed by zeolites, and small-pore zeolites are suitable catalysts when light olefins are the desired products.2 A key component of the catalytic process is the formation of reactive aromatic intermediate species, which serve as reaction centers to produce the desired olefins, though these intermediates can deactivate the material if they become too large and block the active sites and hinder diffusion.3 Zeolites are notoriously difficult to characterize at the nanoscale due to instability under electron beams, e.g. from transmission electron microscopy (TEM).4 Atom probe tomography (APT) is uniquely positioned among all tomographic techniques as it alone can currently provide 3D chemical location and identity with sub-nm resolution, and is especially for studying elements that offer no significant z-contrast differences, such as the primary zeotype elements Al, Si and P, as it is a Time of Flight (ToF) technique.
A sample for APT is fabricated to a needle shape specimen using conventional Focused Ion Beam milling in a Scanning Electron Microscope (FIB-SEM). After being loaded into the APT instrument, an electric field is applied between the local electrode and the needle. Figure 1 illustrates a schematic representation of the working mechanism of the technique. This electric field causes the ions to evaporate one-by-one. A laser is used to trigger this field evaporation of the ions. The laser pulse is the beginning of the ToF and the detection event, located at the position sensitive detector, is the end of the ToF.4 For the APT samples the MTH reaction will be performed by using 13C labelled methanol so that any occluded 13C can be unambiguously assigned as coke (precursor) deposits. Additionally, using 13C labelled methanol enhances the distinguishability of C and e.g. Mg (mass of 24 Da) as Mg2+ overlaps with 12C+, and Mg+ with 12C2+.
Figure 1: Schematic representation of the APT technique, with the corresponding sizes indicated
Our group has extensive experience in innovatively applying this technique for zeolite research in collaboration with Oak Ridge National Lab (Tennessee, USA). Previous works have shown that for modified ZSM-5 and aged SAPO-34, 3D material reconstructions and spatial and quantitative information can be derived from this technique. Importantly, characteristic behaviors of the MTH mechanism and deactivation were correlated to the 3D elemental reconstructions and data obtained from the APT measurements. In Figure 2, the different 3D framework systems of ZSM-5 and SAPO-34 are illustrated, with a wrap of the accessible pore space. APT has determined that the ZSM-5 framework contains coke depleted regions on length scales of tens of nanometers, while in SAPO-34 there were no coke depleted regions. ZSM-5 contained more 13C clusters than SAPO-34, since the smaller pore size of SAPO-34 prevents the formation of numerous polycyclic aromatic species. APT was used to obtain information about the 3D spatial composition and showed that different frameworks and compositions resulted in different coking behavior under similar reaction conditions.5,6 Additionally, it was proven that, with APT, affinities between template molecules, carbon and acid sites can be determined.5
Figure 2: A comparison of significant differences in coking behavior identified by APT in ZSM-5 and SAPO-34.4,5,7 Carbon is depicted in black.
Incorporation of alkaline earth metals by either ion-exchange or impregnation shows an increased selectivity to light olefins and an enhanced life time of the catalyst compared to the parent material.8,9 However, the exact mechanism of this influence on the reactivity is poorly understood besides some modification of the acidity.8,9,10,11 Correlations between the activity, selectivity and deactivation rates of the zeolites with and without incorporation of alkaline earth metals and the atomic composition and distribution of the metals, silica, aluminum and coke could provide insights in the actual reaction mechanism within the pore structures of the zeolites. For this, we are studying zeolite H-SSZ-13 and its analogues with introduced alkaline earth metals, as this catalyst seems suitable for APT research. This work, and the comparison between different MTH active zeolite materials, will be presented in this lecture. The relation between the position and distribution of the coke, silica, aluminum and metals obtained with APT provides detailed information about coke (precursor) formation mechanism during the MTH process on sub-nm scale as so far no other technique is capable of.
Reference list:
(1) Ji, Y.; Deimund, M. A.; Bhawe, Y.; Davis, M. E. Organic-Free Synthesis of CHA-Type Zeolite Catalysts for the Methanol-to-Olefins Reaction. ACS Catal. 2015, 5, 4456–4465.
(2) Goetze, J.; Meirer, F.; Yarulina, I.; Gascon, J.; Kapteijn, F.; Ruiz-Martínez, J.; Weckhuysen, B. M. Insights into the Activity and Deactivation of the Methanol-to-Olefins Process over Different Small-Pore Zeolites As Studied with Operando UV-Vis Spectroscopy. ACS Catal. 2017, 7, 4033–4046.
(3) Dai, W.; Wu, G.; Li, L.; Guan, N.; Hunger, M. Mechanisms of the Deactivation of SAPO-34 Materials with Different Crystal Sizes Applied as MTO Catalysts. ACS Catal. 2013, 3, 588–596.
(4) Schmidt, J. E.; Peng, L.; Poplawsky, J. D.; Weckhuysen, B. M. Nanoscale Chemical Imaging of Zeolites Using Atom Probe Tomography. Angew. Chem., Int. Ed.. 2018, 57, 10422–10435.
(5) Schmidt, J. E.; Peng, L.; Paioni, A. L.; Ehren, H. L.; Guo, W.; Mazumder, B.; Matthijs De Winter, D. A.; Attila, Ö.; Fu, D.; Chowdhury, A. D.; et al. Isolating Clusters of Light Elements in Molecular Sieves with Atom Probe Tomography. J. Am. Chem. Soc. 2018, 140, 9154–9158.
(6) Schmidt, J. E.; Poplawsky, J. D.; Mazumder, B.; Attila, Ö.; Fu, D.; de Winter, D. A. M.; Meirer, F.; Bare, S. R.; Weckhuysen, B. M. Coke Formation in a Zeolite Crystal During the Methanol-to-Hydrocarbons Reaction as Studied with Atom Probe Tomography. Angew. Chem., Int. Ed.. 2016, 55, 11173–11177.
(7) Baerlocher, C.; McCusker, L. B. Database of Zeolite Structures, <http://www.iza-structure.org/databases/>. Accessed March 26, 2018.
(8) Goetze, J.; Weckhuysen, B. M. Spatiotemporal Coke Formation over Zeolite ZSM-5 during the Methanol-to-Olefins Process as Studied with Operando UV-Vis Spectroscopy: A Comparison between H-ZSM-5 and Mg-ZSM-5. Catal. Sci. Technol. 2018, 8, 1632–1644.
(9) Yarulina, I.; De Wispelaere, K.; Bailleul, S.; Goetze, J.; Radersma, M.; Abou-Hamad, E.; Vollmer, I.; Goesten, M.; Mezari, B.; Hensen, E. J. M.; et al. Structure–performance Descriptors and the Role of Lewis Acidity in the Methanol-to-Propylene Process. Nat. Chem. 2018, 10, 804–812.
(10) Ji, Y.; Birmingham, J.; Deimund, M. A.; Brand, S. K.; Davis, M. E. Steam-Dealuminated, OSDA-Free RHO and KFI-Type Zeolites as Catalysts for the Methanol-to-Olefins Reaction. Microporous Mesoporous Mater. 2016, 232, 126–137.
(11) Mentzel, U. V.; Højholt, K. T.; Holm, M. S.; Fehrmann, R.; Beato, P. Conversion of Methanol to Hydrocarbons over Conventional and Mesoporous H-ZSM-5 and H-Ga-MFI: Major Differences in Deactivation Behavior. Appl. Catal. A Gen. 2012, 417–418, 290–297.