Influence of ZSM-5 Crystal Size on Methanol-to-Olefin (MTO) vs. Ethanol-to-Aromatics (ETA) Conversion

Crystal size is a key parameter of zeolites applied as catalysts. Herein, ZSM-5 crystals with similar physicochemical and acid properties, few defects, and aluminum exclusively in tetrahedral coordination are synthesized and the influence of the crystal size on the MTO and ETA conversion is investigated. Short olefins are the main products of the MTO conversion, whereas larger olefins and aromatics dominate the products after ETA conversion. In the case of both feeds, an increased crystal size decreases the catalyst's lifetime. The MTO conversion over larger ZSM-5 altered the product distribution, which was not the case for the ETA conversion. The reason is that the instantly available aromatics during ETA conversion lead to fast coking and zeolite crystals only active in the outer layers. Thus, the different reactivity of different-sized ZSM-5 is direct proof of a different conversion mechanism for both alcohols.

Recent advances in solid-state NMR of zeolite catalysts

Zeolites are important inorganic crystalline microporous materials with a broad range of applications in the areas of catalysis, ion exchange, and adsorption/separations. Solid-state nuclear magnetic resonance (NMR) spectroscopy has proven to be a powerful tool in the study of zeolites and relevant catalytic reactions because of its advantage in providing atomic-level insights into molecular structure and dynamic behavior. In this review, we provide a brief discussion on the recent progress in exploring framework structures, catalytically active sites and intermolecular interactions in zeolites and metal-containing ones by using various solid-state NMR methods. Advances in the mechanistic understanding of zeolite-catalysed reactions including methanol and ethanol conversions are presented as selected examples. Finally, we discuss the prospect of the solid-state NMR technique for its application in zeolites.

Carbocation chemistry confined in zeolites: spectroscopic and theoretical characterizations

Acid-catalyzed reactions inside zeolites are one type of broadly applied industrial reactions, where carbocations are the most common intermediates of these reaction processes, including methanol to olefins, alkene/aromatic alkylation, and hydrocarbon cracking/isomerization. The fundamental research on these acid-catalyzed reactions is focused on the stability, evolution, and lifetime of carbocations under the zeolite confinement effect, which greatly affects the efficiency, selectivity and deactivation of zeolite catalysts. Therefore, a profound understanding of the carbocations confined in zeolites is not only beneficial to explain the reaction mechanism but also drive the design of new zeolite catalysts with ideal acidity and cages/channels. In this review, we provide both an in-depth understanding of the stabilization of carbocations by the pore confinement effect and summary of the advanced characterization methods to capture carbocations in zeolites, including UV-vis spectroscopy, solid-state NMR, fluorescence microscopy, IR spectroscopy and Raman spectroscopy. Also, we clarify the relationship between the activity and stability of carbocations in zeolite-catalyzed reactions, and further highlight the role of carbocations in various hydrocarbon conversion reactions inside zeolites with diverse frameworks and varying acidic properties.

Potentials of bio-butanol conversion to valuable products

In the last decade, there was observed a growing demand for both n-butanol as a potential fuel or fuel additive, and propylene as the only raw material for production of alcohol and other more bulky propylene chemical derivatives with faster growing outputs (polymers, propylene oxide, and acrylic acid). The predictable oilfield depletion and the European Green Deal adoption stimulated interest in alternative processes for n-butanol production, especially those involving bio-based materials. Their commercialization will promote additional market penetration of n-butanol for its application as a basic chemical. We analyze briefly the current status of two most advanced bio-based processes, i.e. ethanol–to-n-butanol and acetone–butanol–ethanol (ABE) fermentation. In the second part of the review, studies of n-butanol and ABE conversion to valuable products are considered with an emphasis on the most perspective catalytic systems and variants of the future processes realization.

Theoretical investigation of the olefin cycle in H-SSZ-13 for the ethanol-to-olefins process using ab initio calculations and kinetic modeling

The formation of the hydrocarbon pool (HCP) in the ethanol-to-olefins (ETO) process catalyzed by H-SSZ-13 is studied in a kinetic model with ab initio computed reaction barriers.

In Situ Observation of Non-Classical 2-Norbornyl Cation in Confined Zeolites at Ambient Temperature

Carbonium ions are an important class of reaction intermediates, but their dynamic evolution is difficult to be monitored by in situ techniques under experimental conditions because of their extremely short lifetime. Probably the most famous case is 2-norbornyl cation (2NB⁺ ): its existing form (classical or non-classical) had been debated for decades, until the concrete proof of non-classical geometry was achieved by X-ray crystallographic characterization at ultra-low temperature (40 K) and super acidic environment. However, we lack the understanding about 2NB⁺ at ambient conditions. Herein, by taking advantage of the confinement effect and delocalized acidic environment of zeolites, we successfully stabilized 2NB⁺ and unequivocally confirmed its "non-classical" structure inside the ZSM-5 zeolite by ab initio molecular dynamics simulations and ¹³ C solid-state nuclear magnetic resonance experiments. It is the first time to in situ observe the non-classical 2NB⁺ without the super acidic environment at ambient temperature, which provides a new strategy to expand the carbocation chemistry.

Dehydration of butanol towards butenes over MFI, FAU and MOR: influence of zeolite topology

The effect of zeolite topology on the kinetics, selectivity and catalyst stability for the dehydration of butanol is studied experimentally and through microkinetic modeling.

Ethanol to Aromatics on Modified H-ZSM-5 Part II: An Unexpected Low Coking

In this study a commercial H‐ZSM‐5 zeolite (Si/Al=11) was post‐synthetically modified by a combined dealumination procedure to adjust its catalytic properties for the selective formation of aromatics from ethanol. The solid‐state properties of original and modified zeolites are determined by structural, textural and acidity analysis. The formation of aromatics and durability of the zeolites were investigated depending on space velocity or contact time in the catalyst bed. In particular, the formation rate and desorption of aromatics from solid‐state surface as well as their tendency to form coke precursors by consecutive build‐up reactions determine the formation of coke. Therefore, the rate of build‐up and finished aromatization by hydride transfer (pre‐determined by the kind, location and geometric arrangement of surface acid sites) and the statistical number of reaction events until final desorption at the specific contact time have to be harmonized to increase aromatics yield and to decrease catalyst decay by coke simultaneously.

Ethanol-to-butadiene: the reaction and its catalysts

Catalytic conversion of ethanol is a promising technology for producing sustainable butadiene. This paper reviews the reaction and its catalysts, and discusses the challenges their development faces.

Observation of an oxonium ion intermediate in ethanol dehydration to ethene on zeolite

Zeolite-catalyzed dehydration of ethanol offers promising perspectives for the sustainable production of ethene. Complex parallel-consecutive pathways are proposed to be involved in the reaction network of ethanol dehydration on zeolites, where the initial step of ethanol dehydration is still unclear particularly for the favorable production of ethene at lower temperature. Here we report the observation of a triethyloxonium ion (TEO) in the dehydration of ethanol on zeolite H-ZSM-5 by using ex situ and in situ solid-state NMR spectroscopy. TEO is identified as a stable surface species on the working catalyst, which shows high reactivity during reaction. Ethylation of the zeolite by TEO occurs at lower temperature, leading to the formation of surface ethoxy species and then ethene. The TEO-ethoxide pathway is found to be energetically preferable for the dehydration of ethanol to ethene in the initial stage, which is also supported by theoretical calculations.

Unraveling the Homologation Reaction Sequence of the Zeolite-Catalyzed Ethanol-to-Hydrocarbons Process

Although industrialized, the mechanism for catalytic upgrading of bioethanol over solid-acid catalysts (that is, the ethanol-to-hydrocarbons (ETH) reaction) has not yet been fully resolved. Moreover, mechanistic understanding of the ETH reaction relies heavily on its well-known "sister-reaction" the methanol-to-hydrocarbons (MTH) process. However, the MTH process possesses a C1 -entity reactant and cannot, therefore, shed any light on the homologation reaction sequence. The reaction and deactivation mechanism of the zeolite H-ZSM-5-catalyzed ETH process was elucidated using a combination of complementary solid-state NMR and operando UV/Vis diffuse reflectance spectroscopy, coupled with on-line mass spectrometry. This approach establishes the existence of a homologation reaction sequence through analysis of the pattern of the identified reactive and deactivated species. Furthermore, and in contrast to the MTH process, the deficiency of any olefinic-hydrocarbon pool species (that is, the olefin cycle) during the ETH process is also noted.

Methanol, ethanol, propanol, and butanol adsorption on H-ZSM-5 zeolite: an ONIOM study

The search for renewable raw materials less harmful to the environment, such as methanol, ethanol, 1-propanol, and 1-butanol has become attractive. These products are obtained more rapidly and efficiently by specific solid catalysts, mainly the zeolites. The Brønsted acid sites distributed over the sinusoidal and the straight channels are important for the alcohol dehydration reaction that produces widely used chemicals. Therefore, the ONIOM method was used to study methanol, ethanol, propanol, and butanol adsorption in H-ZSM-5 zeolite. PM6 and DFT levels were used for the high layer ONIOM, while the low layer was calculated using the UFF force field. DFT was calculated using the B3LYP global hybrid GGA, M06-2X hybrid meta-GGA, and the hybrid range separated ωB97X-D functionals at 6-31+G(d) basis set. The high layer ONIOM was completely relaxed. The binding energy shows dependence on the relaxed tetrahedra and position of acid site. The Si/Al ratio was also studied. Graphical Abstract HOMO orbital of adsorbed alcohols showing the main contribution of zeolite for small alcohols.

Mechanistic insights into the formation of butene isomers from 1-butanol in H-ZSM-5: DFT based microkinetic modelling

First principles microkinetic modelling provides in-depth mechanistic insights into the competing reaction pathways for zeolite-catalyzed conversion of 1-butanol to butene isomers.

Effect of zeolite confinement on the conversion of 1-butanol to butene isomers: mechanistic insights from DFT based microkinetic modelling

First principles microkinetic modelling shows that, unlike in H-ZSM-5 and H-ZSM-22, trans-2-butene formation in H-FER occurs via direct dehydration of 1-butanol.