Adding Zn to the ZSM-5 zeolite effectively increases the aromatic selectivity in the methanol-to-aromatics (MTA) process. The formation of metal-derived Lewis acid sites promotes the dehydrogenation but at the cost of a rapid deactivation of the catalyst by coke, due to the increased aromatic formation. In this work, we impregnated a Zn-modified catalyst (2 wt%) with variable contents of Ca (0.02 and 0.5 wt%) and evaluated their kinetic behavior in the MTA and ethane dehydrogenation reactions. The results proved the superior performance of the Zn(2)Ca(0.02) catalyst due to a synergistic effect between the two metals. The Ca ions limit coke formation from excessive aromatization, increasing catalyst stability and removing Zn clusters, resulting in a recovery of Brønsted acid sites (BAS) active for the formation of light aromatics. Combining these effects results in a more efficient and viable catalyst for aromatic production from methanol.@en

Monitoring complex catalytic pathways under industrially-relevant conditions is one of the key challenges in catalysis chemistry and technology. Herewith we describe a direct technique called ‘fast scanning-pulse analysis’ (FASPA) that allows the direct characterization and detailed kinetic analysis of intimately interweaved catalytic pathways. The power and potential of the FASPA approach are demonstrated with an industrially relevant methanol-to-hydrocarbons (MTH) process over H-ZSM-5 zeolite. This reaction proceeds via a hydrocarbon pool (HCP) mechanism producing olefins and aromatics. The HCP is built-up upon exposure to methanol during the induction period, followed by a transition regime to a quasi steady-state MTH operation. This FASPA technique allows (sub-)second resolution of the full temporal products response upon a methanol pulse providing direct and quantitative insights into the MTH reactions. Globally, two consecutive pathways can be discerned: a very fast primary product formation in the presence of methanol in a narrow active MTH reaction zone, followed by a slower formation of light aromatics, which is closely related to the decomposition and release of HCP species and secondary reactions in absence of methanol in the downstream part of the catalyst bed. The time delay between the appearance of inert tracer and primary products represents the time needed to build-up the HCP in the induction period, where methane is observed prior to other products. The primary products (alkanes, olefins, and light aromatics) are nearly instantaneously formed from the pulsed methanol. These results demonstrate the highly dynamic character of the HCP in the MTH process over H-ZSM-5.

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The nature of hydrocarbon pool (HCP) intermediates in the methanol-to-hydrocarbons (MTH) process has been thoroughly investigated, especially for BEA- and CHA-type zeolite catalysts like H-β and H-SAPO-34. Herein, we further reveal the dynamic mechanistic details of the MTH process over the H-ZSM-5 catalyst at 400 °C, based on the dual-cycle mechanism and HCP in this medium-pore zeolite. Application of switching sequences of ¹³C-labeled and unlabeled methanol pulses over a model H-ZSM-5 catalyst combined with on-line MS analysis and a recently reported technique called “fast scanning-pulse GC analysis” provides a direct and quantitative insight into the MTH reactions under quasi-steady-state conditions. The transient product responses showed the almost instant formation of hydrocarbons upon a small pulse of methanol, followed by secondary formation of light aromatics via HCP decomposition and olefin alkylation-dealkylation, especially in a long catalyst bed when methanol is quickly consumed in the initial reaction zone in the catalyst bed. The isotopic analysis of typical aliphatic C₃₊ product responses after switching ¹³C-methanol pulses to the unlabeled methanol pulses showed a fast isotope scrambling in the formation of C₃₊ species. MS analysis of the light aromatics indicates a complete consecutive but slower isotope incorporation process of ¹²C into ¹³C-aromatics. Results provide direct experimental confirmation of the kinetically preferred olefin-based cycle over the aromatic-based cycle. The sequential isotopic incorporation strongly suggests that the paring reaction pathway through aromatic ring contraction and re-expansion steps is operative. In the appearance of aromatics upon pulsing methanol over larger catalyst beds, four processes are directly discerned, involving the displacement of adsorbed species by formed water, isotope incorporation yielding directly labeled and unlabeled products through the paring mechanism and direct aromatization, and HCP conversion through secondary reactions.

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The MTH process is recognized as one of the key ingredients of the methanol (MeOH) economy concept enabling the production of crucial hydrocarbon building blocks, such as light olefins and aromatics, starting from CO2 as the primary carbon source. Besides the driving force of reducing reliance on fossil fuels, the highly tunable nature of this process in response to rapidly changing market demands attracts continuous process development by industry. In academia, proposing comprehensive structure-performance relationships relying on a deeper understanding of the complex reaction pathways makes MTH a perfect model reaction system for new porous material synthesis and catalyst design...@en

Identification of the catalyst characteristics correlating with the key performance parameters including selectivity and stability is key to the rational catalyst design. Herein we focused on the identification of property-performance relationships in the methanol-to-olefin (MTO) process by studying in detail the catalytic behaviour of MFI, MEL and their respective intergrowth zeolites. The detailed material characterization reveals that both the high production of propylene and butylenes and the large MeOH conversion capacity correlate with the enrichment of lattice Al sites in the channels of the pentasil structure as identified by ²⁷Al MAS NMR and 3-methylpentane cracking results. The lack of correlation between MTO performance and other catalyst characteristics, such as crystal size, presence of external Brønsted acid sites and Al pairing suggests their less pronounced role in defining the propylene selectivity. Our analysis reveals that catalyst deactivation is rather complex and is strongly affected by the enrichment of lattice Al in the intersections, the overall Al-content, and crystal size. The intergrowth of MFI and MEL phases accelerates the catalyst deactivation rate.

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The catalytic conversion of glycerol to aromatics (GTA, e.g., benzene, toluene, and xylenes, BTX) over a shaped H-ZSM-5/Al₂O₃ (60/40 wt%) catalyst was investigated in a continuous fixed-bed reactor to study the addition of the Al₂O₃ binder in the catalyst formulation on catalyst performance. The experiments were performed under N₂ at 550 °C, a WHSV of glycerol (pure) of 1 h⁻¹, and atmospheric pressure. The spent H-ZSM-5/Al₂O₃ catalysts were reused after an oxidative regeneration at 680 °C and in total 5 reaction-regeneration cycles were performed. Catalyst characterization studies show that the addition of the Al₂O₃ binder does not affect the surface area and crystallinity of the formulation, but increases the total pore volume (mesopores in particular) and total acidity (Lewis acidity in particular). The H-ZSM-5/Al₂O₃ (60/40 wt%) catalyst shows a considerably prolonged catalyst life-time (8.5 vs. 6.5 h for H-ZSM-5), resulting in a significant increase in the total BTX productivity (710 vs. 556 mg g⁻¹ H-ZSM-5). Besides, the addition of the Al₂O₃ binder retards irreversible deactivation. For instance, after 3 regenerations, catalyst performance is comparable to the fresh one. However, after 4 regenerations, some irreversible catalyst deactivation occurs, associated with a reduction in total pore volume, crystallinity, and acidity (Brønsted acidity in particular), and meso-porosity of the Al₂O₃ binder. This study shows that both the stability and reusability of H-ZSM-5-based catalysts for GTA are remarkably enhanced when using a suitable binder.

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The production of valuable aromatics and the rapid catalyst deactivation due to coking are intimately related in the zeolite-catalyzed aromatization reactions. Here, we demonstrate that these two processes can be decoupled by promoting the Ga/HZSM-5 aromatization catalyst with Ca. The resulting bimetallic catalysts combine high selectivity to light aromatics with extended catalyst lifetime in the methanol-to-aromatics process. Evaluation of the catalytic performance combined with detailed catalyst characterization suggests that the added Ca interacts with the Ga-LAS, with a strong effect on the aromatization processes. A genetic algorithm approach complemented by ab initio thermodynamic analysis is used to elucidate the possible structures of bimetallic extraframework species formed under reaction conditions. The promotion effect of minute amounts of Ca is attributed to the stabilization of the intra-zeolite extraframework gallium oxide clusters with moderated dehydrogenation activity.

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