Diamond Annual Review 2021/22

90 91 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 Upgrading zeolite catalysts to improve propene production Related publication: Lin, L., Fan, M., Sheveleva, A. M., Han, B., X., Tang, C. C., Carter, J. H., da Silva, I., Parlett, C. M. A., Tuna, F., McInnes, E. J. L., Sastre, G., Rudić, S., Cavaye, H., Parker, S. F., Cheng, Y., Daemen, L. L., Ramirez-Cuesta, A. J., Attfield, M. P., Liu, Y., Tang, C. C., Han, B., Yang, S. Control of zeolitemicroenvironment for propene synthesis frommethanol. Nature Communications , 12 822 (2021). DOI: 10.1038/s41467-021-21062-1 Publication keywords: Methanol-to-propene; Hetero-atomic zeolite; Microenvironment; First carbon-carbon bond P ropene is a key compound used in the production of a wide range of polymers and chemicals. With rapidly growing demand for propene, the world faces a shortage of this crucial ingredient unless more efficient productionmethods can be developed. Commercial methanol-to-propene (MTP) plants use the ZSM-5 and SAPO-34 zeolite catalysts to produce propene from methanol. However, thisprocess ispoorlyunderstoodandproducesa rangeof products. Fundamental investigationsof theprocessandthedevelopment of more selective and stable catalysts are both challenging goals for MTP research. A team from the University of Manchester synthesised an MFI-zeolite within tantalum(V) and aluminium(III) centres in the framework. Using Diamond Light Source's Energy Dispersive EXAFS beamline (I20-EDE), they studied the interaction of methanol with the Tantalum(V) centre during the MTP reaction. They also used complementary measurements on the High-Resolution Powder Diffraction beamline (I11) to examine the configurations of adsorbedmethanol molecules in zeolite pores. Their results show that the new zeolite offers remarkable propene selectivity (51%), propene/ethene ratio (8.3) and catalytic stability (>50 hours) at fullmethanol conversion. Combining insitu synchrotronX-raypowder diffractionandX-ray absorption spectroscopy at Diamondand inelastic neutron scattering at ISIS with DFT calculations, the teamwas able to uncover key details of the process to add to our fundamental understanding. This work will help develop sustainable manufacturing techniques for propene and other light olefins, using renewable resources such as biomass and CO 2 . Propene is a versatile building block for many important polymers, intermediates and fine chemicals 1 . It is predicted that the rapidly growing demand for propene will lead to a global propene shortage 2 . Methanol- to-propene (MTP) processes is a promising route to alleviate this short fall. This process has been commercialised by using ZSM-5 or SAPO-34 zeolites as catalysts 3 , but many by-products such as alkanes, aromatics, and coke, form via complex reaction pathways, resulting in low propene selectivity and rapid deactivation of the catalysts owing to the coke formation. Thus, the development of new catalysts with the merit of the balance in “propene selectivity-propene/ethene ratio-catalytic stability” attracts much attention from academia and industry. Moreover, in academia, more than 20 different, sometimes controversial, formation mechanisms of the first carbon-carbon bond have been postulated to date 4 . Thus, unravelling the explicit mechanism and developing efficient catalysts remain unsolved problems 5 . Recently, a new hetero-atomic zeolite (denoted as TaAlS-1) was designed and synthesised by direct hydrothermal method to catalyse the MTP process. The Ta(V) and Al(III) centres were readily incorporated into the framework by addingTa(V) and Al(III) sources in themixture during the synthesis, resulting in anewzeolitematerialwithdual-metal-decoratedpores. ElectronParamagnetic Resonance (EPR) spectroscopy has confirmed the successful incorporation of Ta(V) and Al(III) into framework sites. The pyridine-adsorption infrared spectroscopy and ammonia temperature-programmed desorption show that the introduction of Ta(V) and Al(III) centres alters the acidity of the resultant zeolite, and thus it provides a unique microenvironment for the conversion of methanol. A remarkable propene selectivity (51%), propene/ethene ratio (8.3) and catalytic stability (>50 hours) at full methanol conversion were achieved simultaneously over the TaAlS-1 zeolite. Its catalytic performance compares favourably with that of all state-of-the-art catalysts for MTP reactions (Fig. 1). The binding domains of methanol molecules within TaAlS-1 zeolite were determined by in situ Synchrotron X-ray Powder Diffraction (SXPD). For comparison, the same investigations on HZSM-5 and TaS-1 (the isostructural analogue without Al sites) zeolites were also performed. Adsorbed methanol molecules show three distinct spatial orientations within the framework of HZSM-5, TaAlS-1 and TaS-1 (Fig. 2a-c), indicating the presence of varying host-guest interactions. MeOH II and MeOH IV in TaAlS-1 interact with bridging O(H)-centres through its C-OH groups via hydrogen bonds (Fig. 2d). MeOH I interacts with MeOH II and MeOH IV to assemble a {MeOH} 3 trimer that favours the formation of trimethyloxonium (TMO) as the reaction intermediate. The TMO-type configuration is also observed in HZSM-5 (Fig. 2e), but not in TaS-1 due to lack of Brønsted acid sites (Fig. 2f ). MeOH III is located close to MeOH IV in both TaAlS-1 (Fig. 2g) and HZSM-5 (Fig. 2h), and it interacts with Ta(V) sites in TaAlS-1 (Fig. 2g), which is potentially crucial for the activation of the C-O bond and thus to the formation of the first C-C bond. To understand the effect of active Ta(V) sites, their local environment was interrogated through Ta L3-edge X-ray Absorption Spectroscopy (XAS). Temperature programmed operando XAS revealed that the degree of methanol adsorption decreased with increasing reaction temperature (Fig. 3a,b). At the reaction temperature, adsorbed methanol molecules were revealed at the Ta(V) sites. This indicates that the adsorbed methanol molecule is activated by Ta(V) sites and then released for reaction to form products, resulting in “open” Ta(V) sites for next reaction cycle. Thus, the framework Ta(V) sites are ideal active sites for adsorption, activation and desorption of methanol substrates. The detailed reaction mechanism was investigated by combined Inelastic Neutron Scattering (INS) and DFT calculations. Adsorbed methanol on Brønsted acid sites reacts with one weakly-adsorbed methanol molecule to form dimethyl ether (DME). Then DME reacts with a third methanol molecule to form TMO as a key intermediate which was clearly observed by INS (Fig. 3c, 365 cm -1 ). Further, a fourth methanol molecule adsorbed on Ta(V) sites attacks the TMO intermediate to form the first carbon-carbon bond, resulting in the formation of olefins (Fig. 3d). In conclusion, the new hetero-atomic zeolite with Ta(V) and Al(III) centres in the framework shows unique microenvironment that is ideally suited for the conversion of methanol to propene. Ta(V) sites in zeolite framework play a vital role in adsorbing and activating methanol molecules to form the first carbon-carbon bond. The formation of the first carbon-carbon bond between TMO and activated methanol molecule was evidently revealed. This will inform the design of future catalytic systems in these challenging industrial processes. References: 1. Torres Galvis, H. et al. Supported iron nanoparticles as catalysts for sustainable production of lower olefins. Science , 335 835–838 (2012). DOI: 10.1126/science.1215614 2. Grant, J. T. et al. Selective oxidative dehydrogenation of propane to propene using boron nitride catalysts. Science , 354 1570–1573 (2016). DOI: 10.1126/science.aaf7885 3. Tian, P. et al. Methanol to olefins (MTO): from fundamentals to commercialization. ACS Catalysis , 5 1922–1938 (2015). DOI: 10.1021/ acscatal.5b00007 4. Yarulina, I. et al. Recent trends and fundamental insights in the methanol-to-hydrocarbons process. Nature Catalysis , 1 398–411 (2018). DOI: 10.1038/s41929-018-0078-5 5. Olsbye, U. et al. Conversion of methanol to hydrocarbons: how zeolite cavity and pore size controls product selectivity. Angewandte Chemie International Edition , 51 5810–5831 (2012). DOI: 10.1002/ anie.201103657 Funding acknowledgement: We thank EPSRC (EP/P011632/1), the Royal Society, National Natural Science Foundation of China (21733011, 21890761, 21673076), and the University of Manchester for funding. We thank EPSRC for funding the EPSRC National Service for EPR Spectroscopy at Manchester. Corresponding author: Dr. Sihai Yang, University of Manchester, Sihai.Yang@manchester.ac.uk . Spectroscopy Group Beamline I20-EDE (and Crystallography Group Beamline I11 ) Figure 3: XAS and INS spectra for TaAlS-1 zeolite and proposed reaction mechanism for MTP; (a) Operando XAS spectra at the Ta L3 edge of TaAlS-1 zeolite during the conversion of methanol, only selected spectra shown; (b) Deconvolution of XAS via linear combination fitting to activated and adsorbed species. (c) INS spectra for TaAlS-1 on the adsorption and catalytic conversion of methanol; (d) Proposed reaction mechanism for the conversion of methanol in the induction period over TaAlS-1. Figure 1: Comparison of propene selectivity-propene/ethene ratio-catalytic stability of selected MTP catalysts. Figure 2: Views of crystal structures of MeOH-loaded TaAlS-1, HZSM-5 and TaS-1. Four distinct binding sites (I to IV) for MeOH have been observed in the channels. (a) TaAlS-1·MeOH; (b) HZSM-5·MeOH; (c) TaS-1·MeOH; Only three binding sites were observed in TaS-1. Detailed views of MeOH I , II , IV (d,e,f ) and MeOH III (g,h,i) in TaAlS-1, HZSM-5 and TaS-1, respectively. Ta/ Al/Si, violet; C, grey; O, orange; H, white.