The methanol-to-hydrocarbons (MTH) reaction is a crucial process that converts methanol into essential chemical products such as light olefins, aromatics and gasoline fractions. Methanol can be derived from non-petroleum routes including coal, natural gas, biomass and CO 2 conversion. Therefore, the MTH reaction not only bridges coal, natural gas and petroleum chemical industries, but is also regarded as an important technology for building a diversified resource utilization system.
At first glance, the reaction may sound simple: methanol enters a catalyst and useful hydrocarbons are produced. In reality, this process is much more complex. Methanol molecules first enter the zeolite pores and are activated at acid sites, subsequently forming a series of reactive intermediates. These intermediates continuously undergo methylation, cracking, hydrogen transfer, cyclization and aromatization reactions, driving the formation of light olefins and aromatics. Meanwhile, some reactive species gradually evolve into coke species, which block pores, cover acid sites and ultimately lead to catalyst deactivation.
Zeolite-catalyzed MTH reactions inherently exhibit multiscale heterogeneity. In other words, the reaction microenvironments are not identical in different regions of the same zeolite crystal. Even within the same particle, there may be significant differences in temperature, diffusion, acid-site distribution and coke deposition. This spatially nonuniform and temporally evolving behavior defines the multiscale spatiotemporal heterogeneity of MTH reactions.
This review systematically summarizes this heterogeneity across four scales. Firstly, at the molecular-level, the diffusion capabilities of different guest molecules within the zeolite pores are not the same. Molecular size, pore topology and the distribution of acid sites all influence the diffusion behavior of species such as methanol, dimethyl ether, olefins and aromatics, thereby leading to differences in reaction pathways and product selectivity.
Secondly, at the crystal-scale, a zeolite crystal is far from a completely uniform “ideal crystal”. The distribution of acid sites, Si and Al atoms, pore connectivity and crystal surface barriers can all influence the formation of reaction zones in the MTH reaction. Meanwhile, coke formation proceeds in a highly localized and framework-sensitive manner, generating distinct coke gradients that decrease from the surface to the core.
Thirdly, at the particle-scale, industrial catalyst particles are usually not composed of a single zeolite component. They also contain various additional components, such as promoters, binders, and pore-forming agents. The introduction of these components can further amplify heterogeneity in molecular diffusion, pore connectivity among different components and temperature distribution within the particles. Therefore, catalyst preparation methods and binder types can directly affect the MTH reaction process.
Finally, at the reactor-scale, along the fixed-bed reactor, intense exothermicity and mass-transfer limitations generate pronounced axial and radial temperature gradients, with hot spots and thermal fronts that migrate as the catalyst deactivates. In parallel, coke deposition forms highly structured axial “cigar-burn” fronts and radial core–shell patterns, whose propagation rate and direction are controlled by contact time, reactor hydrodynamics and zeolite topology.
In summary, this review systematically summaries the spatiotemporal heterogeneity of the MTH reaction across the molecular diffusion (micro), crystal, particle and reactor scales (macro). This review highlights that such spatiotemporal heterogeneity is not an accidental phenomenon in MTH reactions, but a general characteristic of zeolite-catalyzed reactions. Therefore, understanding the multiscale spatiotemporal heterogeneity in MTH reactions will contribute to the rational design of next-generation high-performance zeolite catalysts and may also provide insights into other zeolite-catalyzed processes, such as fluid catalytic cracking, aromatic cracking, CO 2 hydrogenation and syngas conversion.
National Science Review