Fig. 1. The reaction categories of the MTH conversion over solid acidic catalysts.

Fig. 2. A simplified scheme of the MTH process consisting of the three stages. Reprinted with permission from Ref. [15]. Copyright 2022, Wiley-VCH GmbH.

Fig. 3. (a) The development of the direct mechanism in the MTH conversion in recent decades. (b) Several recent direct reaction mechanisms for the MTH conversion over solid acidic catalysts. Reprinted with permission from Ref. [15]. Copyright 2022, Wiley-VCH GmbH.

Fig. 4. (a) In situ solid-state 13C MAS NMR spectra recorded during 13C-methanol conversion over H-ZSM-5 at 573 K. Reprinted with permission from Ref. [34]. Copyright 2017, Wiley-VCH GmbH. (b) Plausible reaction pathways for the formation of the earliest-detected hydrocarbons during the initial stage of the MTO reaction. Reprinted with permission from Ref. [54]. Copyright 2018, American Chemical Society. (c) In situ solid-state 13C MAS NMR spectra of HSSZ-13 with continuous-flow (CF) 13C-methanol conversion at 220 °C. (d) Plausible reaction pathways for C-C bond formation. Catalytic cycles and the related activation barriers for the initial C-C bond formation with SMS-(left)- and TMO-(right)-mediated methanol (upper part) and DME (lower part) conversion on HSSZ-13 zeolite predicted by AIMD simulations. Reprinted with permission from Ref. [56]. Copyright 2021, Elsevier.

Fig. 5. (a) 13C CP/MAS NMR spectra of trapped products obtained from the reaction of 13C-methanol for 1 min, followed by co-feeding 13C-methanol and 13C-FA for another 1 min over H-ZSM-5-De at 250-350 °C (a-e). (b) Proposed reaction routes for the formation of C-C bond-containing species. The calculated activation energy (ΔEact) and reaction energy (ΔEreact) are indicated in kcal mol-1 for the elementary steps. Reprinted with permission from Ref. [35]. Copyright 2018, Wiley-VCH GmbH.

Fig. 6. (a) First C-C bond formation in MTH through coupling between nucleophilic and electrophilic carbon atoms. Reprinted with permission from Ref. [30]. Copyright 2016, Wiley-VCH GmbH. (b) Solid-state NMR spectra of acetate species in H-SAPO-34 after the MTH reaction for 30 min at 673 K. Zooms from 2D 13C?13C (blue) and 13C?1H (red) MAS ssNMR spectra with long mixing (150 ms) or CP contact time (500 ms), respectively, indicating surface acetate and MA resonances. (c) ssNMR signals of surface-bound formate in the 13C?1H spectra (light blue) with a short CP contact time (50 ms). (d) Zoom of aromatic signals from 2D 13C?13C (blue) and 13C?1H (light blue) MAS NMR spectra with long mixing (150 ms) or short CP contact time (50 ms), respectively. Reprinted with permission from Ref. [31]. Copyright 2016, Wiley-VCH GmbH.

Fig. 7. (a) Online MS monitoring of acetaldehyde, CO and H2 during the co-feeding of CO in methanol conversion at 523 K. (b) 13C CP MAS NMR spectra of selected samples: (i) methylated H-ZSM-5 zeolite, (ii) after loading of 50 mbar 13CO at 473 K for 5 min, (iii) of 50 mbar 13CO and 50 mbar H2 at 473 K for 5 min, (iv) zeolite H-ZSM-5 after methanol conversion at 523 K for 5 min, and (v) zeolite H-ZSM-5 after loading 50 mbar CH313CHO at room temperature for 5 min. (c) Energy diagram at 0 K and the key transition state structures of the formation of ketene from CO and MA (CH3COOCH3). (d) The key transition state structures of the evolution and formation of acetaldehyde related to the induction period of the MTH conversion in H-ZSM-5. Reprinted with permission from Ref. [57]. Copyright 2019, America Chemical Society.

Fig. 8. Overview of the most relevant initiation mechanisms of the MTO reaction. Al-OH and Al-OMe are used to abbreviate bridging hydroxyl and methoxy groups (for example Al-OH-Si). Mechanisms are grouped into boxes according to the oxidation state of carbon. Formation of the surface methoxy group is shown explicitly only for the reaction 1a→1b and is omitted for clarity in all subsequent reactions since it is not rate-limiting. In the reaction of 3f to 3h, the intermediate formation of Al-OEt is omitted. Reprinted with permission from Ref. [32]. Copyright 2017, America Chemical Society.

Fig. 9. ms-TPES of m/z = 42 (a) and m/z = 56 (b) in MA conversion over HZSM-5. (c) Proposed formation routes for (top) the first product with a C-C bond, (middle) first ethylene, and (bottom) first propylene based on SA on acidic zeolites. Molecules in blue ellipses are the detected precursors for ethylene. Molecules in the red rectangle are predicted precursors for propylene. Reprinted with permission from Ref. [58]. Copyright 2022, Wiley-VCH GmbH.

Fig. 10. Reaction pathways of catalytic C1 coupling. Proposed reaction network of the oxygenate-driven reaction in the MTH process (a) and of the direct CH3? radical-addition pathway in the MTH and MCTH reactions (b) to yield C5 intermediates in the micropores of H-ZSM-5. The insets on top of the molecular schemes show the ms-TPE (open squares) and reference spectra (solid lines) of the identified species for MTH in a and MCTH in b. Color code: C (dark gray), O (red), H (light gray), X (X = OH or Cl; violet). (c) Reaction pathway for the chain-propagation reaction of C5H8 to generate benzene and toluene in the MTH process. The ms-TPE (open squares) and reference spectra (solid lines) of the identified isomeric products are also shown. Reprinted with permission from Ref. [59]. Copyright 2022, Springer Nature.

Fig. 11. (a) Proposed hydrocarbon pool mechanism. Reprinted with permission from Ref. [67] Copyright 1996, Elsevier. (b) Representation of the paring and side-chain reaction concepts in MTH catalysis. Reprinted with permission from Ref. [93]. Copyright 2009, Wiley-VCH.

Fig. 12. Proposed dual-cycle mode for the conversion of methanol to olefins. Reprinted with permission from Ref. [5]. Copyright 2012, Wiley-VCH.

Fig. 13. (a) Proposed reaction pathway containing the observed intermediates on H-SAPO-34 for the early stages of MTO conversion. (b) Proposed route for the role of the polymethylcyclopentenyl and polymethylcyclohexenyl cations in the dual-cycle mechanism. Reproduced with permission from Ref. [104]. Copyright 2015, America Chemical Society. (c) Traditional dual cycles (alkene-based and aromatic-based cycles) and the cyclopentadiene-based cycle. Reproduced with permission from Ref. [119]. Copyright 2019, America Chemical Society. (d) Proposed route for the formation of aromatics from cyclohexene over H-ZSM-5 with detailed steps in the ring contraction process. Reproduced with permission from Ref. [120]. Copyright 2020, America Chemical Society. (e) Three stages of the whole process of MTO conversion. Reproduced with permission from Ref. [121]. Copyright 2020, America Chemical Society.

Fig. 14. (a) Schematic reaction network for methanol-mediated alkanes and aromatics formation involving alkanes from methanol-induced hydrogen transfer at LASs and aromatics from formaldehyde-based reactions at BASs. Reproduced with permission from Ref. [122]. Copyright 2016, America Chemical Society. (b) Potential reaction pathways that qualitatively agree with observed trend in site-loss yield and site-loss selectivity during the MTH conversion with co-feeds. Reproduced with permission from Ref. [128]. Copyright 2021, America Chemical Society.

Fig. 15. Domino cascade of MTO conversion roadmap. The autocatalytic sets, operating by a hypercyclic network embedded in the large interlinked network, are interlinked by three autocatalytic entities (olefin, MCP, and aromatic species), driving autocatalytic turnover. Reproduced with permission from Ref. [130]. Copyright 2021, America Chemical Society.

Fig. 16. (a) 2D 13C-13C MAS solid-state NMR spectra of rigid molecules (290 K, 12 kHz, 700 MHz). Polarization of 13C atoms was achieved through cross-polarization (CP, purple) or direct excitation (DE, blue), and a 120 ms PARIS mixing period was used. Gray filled regions indicate spinning side bands. (b) Expansion of the aromatic/olefinic signals from the 2D 13C-13C CP MAS solid-state NMR spectrum. (c) Expansion from the 2D 13C-13C CP MAS solid-state NMR spectrum, indicating acetic acid and MA resonances. (d) Identified generic molecular keys/structures. (e) Proposed reaction pathways of the methyl-acetate-to-hydrocarbon (MATH) conversion catalyzed by zeolite (denoted as Z-H). Reproduced with permission from Ref. [131]. Copyright 2018, Wiley-VCH.

Fig. 17. (a) Overview of the initiation and autocatalytic part (olefin cycle only) of the MTO process. The initiation part is further broken down into a part responsible for CO formation and CO-mediated C-C coupling. O(n) is an abbreviation for any isomer of olefin CnH2n. Reproduced with permission from Ref. [132]. Copyright 2019, America Chemical Society. (b) Proposed roadmap of the MTH conversion over H-ZSM-5 (denoted as HZ). Reproduced with permission from Ref. [57]. Copyright 2019, America Chemical Society.

Fig. 18. Proposed roadmap of the MTH conversion over H-ZSM-5 (denoted HZ) with the participation of both Br?nsted (denoted B) and Lewis acid sites (denoted L). Reproduced with permission from Ref. [133]. Copyright 2022, Elsevier.

Fig. 19. Mechanism of syngas conversion for ZnAlOx/H-ZSM-5. Syngas participates in the overall catalytic cycle, apart from simply producing C1 intermediates. The oxygenate-based cycle is vigorously involved in the syngas conversion reaction network, further consuming CO and H2, and contributes to final products such as olefins (bottom left) and aromatics (bottom right). The hydrogenation reaction is destructive in the formation of unsaturated products by conventional HCP dual cycles (top and middle). Reproduced with permission from Ref. [134]. Copyright 2022, Springer 5. Improvement of the MTH performance based on the reaction mechanism

Fig. 20. (a) Ellipsoidal model for the cage-defining ring and the cage-defining ring size. (b) Selection of the cage-defining ring from a CHA cage. (c,d) Correlation of cage-defining ring size by cage structures and light olefin product distribution categories. Reproduced with permission from Ref. [138]. Copyright 2019, America Chemical Society.

Fig. 21. Proposed mechanism for aromatics formation on CMTA over H-ZSM-5. Reproduced with permission from Ref. [144]. Copyright 2018, Wiley-VCH.

Fig. 22. (a) Schematic illustrations of the selective transformation of coke into specific intermediates. (b) Selective transformation of coke into a specific naphthalenic species-rich catalyst and improvement of MTO performance and atom economy implemented in the circulating fluidized bed reactor-regenerator configuration. Evolution of the detailed distribution of carbonaceous intermediates with molecular masses smaller than 200 Da during nitrogen and steam cracking at 680 °C, which was analyzed by GC-MS. (c) Evolution of carbonaceous species with molecular masses larger than 200 Da during nitrogen and steam cracking at 680 °C, which was analyzed by MALDI FT-ICR MS. (d) Operando UV-Raman spectra of ZEOS-Coked samples treated by steam at 650 °C for different time. (e) Evolution of the textural property and coke quantity of ZEOS-Coked samples during steam cracking at 680 °C. Reproduced with permission from Ref. [149]. Copyright 2021, Springer Nature.

Fig. 23. (a) Schematic diagrams of the directional construction of active hydrocarbon pool species within SAPO-34 crystals. (b) Time-dependent methanol conversion over commercial SA-34-II catalyst before and after treatments (pre-coking of 5 min and subsequent steaming of 5 h) during the MTO conversion at 698 K with a WHSV of 1.0 h?1 and, and the corresponding selectivity of ethylene and propylene recorded at TOS = 100 min. Reproduced with permission from Ref. [151]. Copyright 2022, America Chemical Society.

Fig. 24. (a) Proposed reaction pathway containing the dual-cycle mechanism of MTA conversion before and after n-butanol cofeeding. (b) Methanol conversion and hydrocarbon product distribution over 4%Ga-HZ-5 catalysts at 650 °C and with a TOS of 40 h using methanol or a mixture of methanol and n-butanol as reagents. (c) The mechanistic interpretation for the improvement of the MTA conversion after n-butanol cofeeding. Reproduced with permission from Ref. [152]. Copyright 2018, America Chemical Society.

Fig. 25. (a) Relationship between the amounts of BASs in SAPO-34 and the dual-cycle mechanism as well as the catalyst lifetime. Reproduced with permission from Ref. [110]. Copyright 2017, the Royal Society of Chemistry. (b) The selectivity to C4-C8 alkenes and aromatics over different plate-like H[Al]MFI and H[Ga]MFI zeolites in MTP conversion at 748 K with a TOS of 20 h, plotted as a function of the BASs density. (c) Propylene/ethylene ratios and catalyst lifetime over different catalysts during the MTP conversion at 475 °C with a TOS of 20 h. Reproduced with permission from Ref. [155]. Copyright 2022, Elsevier.

Fig. 26. (a) Relationship between the β/(α+γ) and Al(54)/Al(56) ratios; β/(α+γ) and Al(54)/Al(56) ratios are determined from the deconvolution results of UV-vis-DRS spectra and 27Al MAS NMR spectra, respectively. (b) Relationship between Al locations in the ZSM-5 framework and the dual-cycle mechanism. Product distribution and methanol conversion as a function of time on stream for MTO over S-HZ-40 (c) and T-HZ-40 (d) at 450 °C, with a methanol WHSV of 4.23 h?1. Reproduced with permission from Ref. [158]. Copyright 2016, American Chemical Society.

Fig. 27. (a) The modification with Ca (AE3) and Mg (AE7) leads to significant prolongation of catalyst lifetime (black squares), increases yield of propylene (black stars) and decreases yield of ethylene (black triangles). (b) Methanol throughput and lifetimes of the catalysts under study. Reaction conditions: T = 500 °C, WHSV = 8 g MeOH gcatalyst?1 h?1. (c?e) Descriptors of ZSM-5 acidity in MTO. Z (black), pre-synthetically modified ZSM-5 with different Si/Al ratios; M (green), post-synthetically demetallated ZSM-5; AE (orange), zeolites modified by post-synthetic incorporation of alkaline-earth metals. The acidity-performance relationship clearly shows that the density of BASs (CBAS) determines propylene (c) and ethylene (d) selectivity, whereas the ratio of LAS and BASs (CBAS/CLAS) governs catalyst lifetime (e). Reproduced with permission from Ref. [165]. Copyright 2022, Springer Nature.

Fig. 28. (a) Schematic illustration of the hydrolysis process of framework Si?O?Al bonds for fresh and spent SAPO-34 catalysts after MTO conversion. (b) Recycling studies of SAPO-34 catalysts applied in MTO conversion at 673 K with and without pre-coking. (c) XRD patterns and 1H MAS NMR spectra of the regenerated SAPO-34 catalysts after 30 cycles in the MTO conversion. (d) 27Al and 31P MAS NMR spectra of the regenerated SAPO-34 catalysts after 30 cycles in the MTO conversion and after hydration at room temperature for 24 h. (e) Methanol conversion before and after water co-feeding (20 vol%) over SA-34-F and SA-34-P catalysts at 400 °C (left). 1H MAS NMR spectra of the regenerated SAPO-34 catalysts after 20 cycles in the MTO conversion with water co-feeding (right). Reproduced with permission from Ref. [175]. Copyright 2021, Springer Nature.