Mechanistic studies of zeolite catalysis via in situ solid-state nuclear magnetic resonance spectroscopy: progress and prospects

Abstract

Zeolites, with their exquisite microporous frameworks and tailorable acidities, serve as ubiquitous catalysts across a diverse spectrum of industrial applications, ranging from petroleum and coal processing to sustainable chemistry and environmental remediation. Optimizing their performance hinges on a thorough understanding of the structure-performance relationship. In situ solid-state nuclear magnetic resonance spectroscopy has emerged as a critical tool, providing unparalleled atomic-level insights into both structure and dynamic aspects of zeolite-catalyzed reactions. Herein, we review recent progress in the development and application of the in situ solid-state nuclear magnetic resonance technique to zeolite catalysis. We first review the in situ nuclear magnetic resonance techniques used in zeolite-catalyzed reaction, including batch-like and continuous-flow reaction modes. The conditions and limitations for these techniques are thoroughly summarized. Subsequently, we review the applications of in situ nuclear magnetic resonance techniques in zeolite-catalyzed reaction, focusing on some important catalytic reactions like methanol-to-hydrocarbons, ethanol dehydration, alkane activation, and beyond. Emphasis is placed on the strategies of specific in situ nuclear magnetic resonance methodologies to tackle critical challenges encountered in these fields, such as probing intermediates and unraveling reaction mechanisms. Additionally, we discuss the burgeoning opportunities and prospective challenges associated with in situ nuclear magnetic resonance studies of zeolite-catalyzed processes.

Graphical abstract

Keywords

heterogeneous catalysis / solid-state NMR / reaction mechanism / zeolites / characterization

1 Introduction

Acidic zeolites are crystalline aluminosilicate solids that serve as porous solid acid catalysts with important roles in the petroleum and chemical industries. In early applications, zeolites were commonly utilized for fluid catalytic cracking of crude oil into higher value hydrocarbon products [1]. Beyond this, zeolites have found widespread use in alkane transformations such as propane dehydrogenation [2−6], methanol-to-hydrocarbons (MTH) reaction [7−17], toluene disproportionation [18,19].

The catalytic activity of zeolites depends on the presence of both Brønsted and Lewis acid sites. The Brønsted acid sites are generated by substituting Si⁴⁺ with trivalent cations (e.g., Al³⁺, Fe³⁺, Ga³⁺, B³⁺) in the SiO₄ tetrahedral units (T-sites) [20−23]. The Lewis acid sites are formed by introducing heteroatoms, such as Ti⁴⁺, Sn⁴⁺, or Zr⁴⁺, into the framework [24−28], or by creating extra-framework Al species on the zeolite surface [29−31]. Moreover, zeolites can act as effective supports to load metal cations or encapsulate noble metals, which can be tuned to catalyze specific reactions [32,33]. The acidity and porosity of zeolites influence the formation and reactivity of intermediates or transition states, resulting in different catalytic activities and selectivities. By modifying the zeolite structure or active site composition, zeolites and zeolite-based composite materials can be designed to catalyze some reactions that are challenging for conventional catalysts [34−38]. Current applications of zeolites have expanded to fine chemicals production and sustainable processes, based on zeolites’ tunable acid-base functionality within constrained nanospaces [39−41].

Gaining molecular-level insights into heterogeneous reaction mechanisms can provide critical clues for designing high-performance catalysts and optimizing catalytic processes. Under working conditions, obtaining structural information of catalysts, capturing key intermediates, and monitoring dynamic reaction processes over catalysts are challenging but important for understanding mechanisms. Currently, various techniques such as in situ infrared (IR) [42,43], Raman [44−46], ultraviolet-visible (UV-Vis) [47,48], X-ray absorption spectroscopy [49,50], and electron paramagnetic resonance [51,52] are widely employed in heterogeneous catalysis research. Nuclear magnetic resonance (NMR) technique is a powerful tool to obtain the structural and dynamic information at molecular level in many fields [53−55], such as chemistry, pharmacy, and biology. Solid-state NMR (ssNMR) is particularly suitable for probing the short-range ordering of solids and determining the structure and geometry of solid materials by versatile one-dimensional (1D) and two-dimensional (2D) spectroscopy [56−62]. It has been commonly used to investigate the framework structure and the active site of zeolites by observing the signals of nuclei of interest [63−67], such as ¹H, ²⁷Al, ³¹P, ²⁹Si, ¹⁷O. Advances in ssNMR methods have enabled the observation of transition metal sites in zeolites containing metal nuclei with low gamma, low natural abundance, and low loading [59,68−74].

In situ ssNMR technique has great advantages in determining the structures of surface species or intermediates over catalysts in heterogeneous catalysis reactions [58,69,75−80]. In the past 20 years, significant progress has been made in the development of in situ ssNMR technique [60,78,81−83]. Various in situ reaction techniques have been developed for the ssNMR investigation of catalytic reactions under conditions of high temperature, high pressure, moisture, etc. The ¹H and ¹³C magic angle spinning (MAS) NMR spectroscopy can provide the qualitative and quantitative information of adsorbed organic species over catalysts. Furthermore, the ²⁹Si, ²⁷Al, ¹¹B, etc., MAS NMR spectroscopy can be utilized to monitor the catalyst structure during catalytic reactions. Therefore, in situ ssNMR techniques offer invaluable insights into the relationship between catalyst structure and performance. Although NMR offers numerous advantages, its primary limitation is its relatively lower sensitivity compared to other spectroscopic techniques such as IR, UV-Vis, and Raman. This sensitivity issue presents a considerable challenge for advancing in situ ssNMR techniques. Consequently, developing a high-sensitivity in situ ssNMR method is essential for its continued evolution and broader application.

This review summarizes the recent advances of In situ ssNMR techniques and their application in zeolite-based heterogeneous catalysis. We mainly focus on the literature published in the past 20 years, especially the recent 10 years. In situ ssNMR methods are first addressed. Typical operating conditions and characteristics of these techniques are summarized. The application of the in situ ssNMR techniques in various heterogeneous catalysis reactions over zeolites is then illustrated with some representative examples. Through these cases, we demonstrate the roles of in situ ssNMR techniques in the studies of zeolite reaction systems, such as identifying intermediates, measuring reaction kinetics, and probing host-guest interactions between adsorbates and zeolites. The unique advantages of in situ ssNMR techniques in elucidating the heterogeneous catalysis reaction mechanisms and understanding the structure-performance relationships of zeolite catalysts are highlighted.

2 In situ ssNMR techniques

In situ investigation of heterogeneous catalysis reactions by ssNMR requires both high-speed rotation (MAS) and high reaction temperature, which poses great challenges for the experimental setup. Therefore, some auxiliary tools or methods are necessary. So far, the in situ ssNMR technique has been developed into several branches that operate under batch-like or continuous-flow (CF) conditions in general. Each of the in situ ssNMR techniques has its own merits and limitations, depending on the specific system being investigated. Tab.1 provides an overview comparing the key characteristics and operational parameters of the different in situ ssNMR techniques. While each method strives to monitor catalytic processes more realistically, their applicability is also restricted by their inherent capabilities and design.

Tab.1 Summary of in situ ssNMR techniques

Reaction mode	Experimental protocol	Reaction condition	Detection temperature	Advantage	Limitation
Batch-like	Glass ampoule [84]	Pressure: < 70 barTemperature: < melting point of ampoule material	From ambient to high temperature	Suitable for reactants that are difficult to adsorb or undergo transformation;provides sufficient contact time for reactants with catalysts;capable of reheating after quenching the reaction; capable of quantifying transformation of observed species	Difficult to differentiate reactants, intermediates and products; not amenable to repeated/renewed usage; challenging to secure the tube assembly and facilitate NMR rotor rotation; difficult to maintain stable/unvarying pressure levels
	Cryogenic adsorption vessel enabling rotor nestling (CAVERN) [85,86]	Pressure: atmosphere pressureTemperature: low (i.e., liquid N2 temperature) temperature to 300 °C	From cryogenic to high temperature	Capable of reactant adsorption at low temperatures;the reaction can be rapidly quenched after a very short time; can be heated inside a MAS NMR probe; the rotor can be reloaded with the reactant	Challenging to differentiate the individual intermediate, reactant and product species; unable to withstand/tolerate elevated reaction pressures
	HTHP (high-temperature and high-pressure) MAS NMR rotor [87−91]	Pressure: < 400 barTemperature: < 250 °C	< 250 °C	Can operate at high temperatures and pressures; able to maintain a constant pressure; can be used multiple times; can operate in liquid-solid, gas-liquid-solid, or other multi-phase systems	Specialized rotors are required
Continuous flow	CF MAS NMR [83,92]	Pressure: atmosphere pressureTemperature: < 400 °C	< 400 °C	Mimicking a real flow reaction in fixed-bed reactor; capable to combine with other operando spectroscopy or online chromatography/mass spectrometer	A lower MAS rate results in a reduced signal-to-noise ratio; sensitivity loss at high temperatures; hard to conduct at high pressure
	Pulse-quench technique [93,94]	Pressure: atmosphere to high pressureTemperature: reaction temperature	Ambient temperature	Reaction can be quickly quenched; active species can be “frozen” on catalyst surface; facile to connect with online gas chromatography/mass spectrometer (GC/MS)	Analysis is performed on the used catalyst

Under batch-like conditions, the catalysts and the adsorbed species are sealed in an NMR rotor for measurement. Diverse reaction techniques, including glass ampoules, CAVERN, and high-pressure operando NMR rotors, are suitable for sample preparation. The ampoule method was established by Anderson et al. [84], in which the gaseous or liquid reactants can be introduced, reacted and analyzed by NMR. The CAVERN technique was developed by Xu’s group [85,86]. This setup permits meticulous control over both catalyst activation and reactant adsorption. The key advantage of CAVERN lies in its ability to analyze catalytic reactions in real-time across a wide temperature range (from –196 to 300 °C) within the NMR probe [85,86]. This enables researchers to observe not only the final products but also the intermediate involved in the reaction, which can offer invaluable insights into the heterogeneous catalysis.

Recently, much attention has been paid on the HTHP MAS NMR rotors, which can be used to investigate the reactions performed at high-temperature, high-pressure or solution conditions (Fig.1) [87−91]. In a spinning rotor, the pressures mainly originate from both the internal fluid pressure and the centrifugal force of samples. As shown in Fig.1(a), the rotor cylinder, fabricated from zirconia using diamond grinding tools, exhibits a range of sizes (9.5–3.2 mm) and incorporates a unique internal thread pattern (sharp-V or square) for enhanced sealing efficacy. This design culminates in a threaded screw zirconia cap equipped with dual high-temperature O-rings, enabling the rotor to withstand pressures exceeding 100 bar and temperatures surpassing 250 °C. After introducing liquid or solid sample into the rotor, the charge or recharge of high-pressure gas can be conducted in a custom loading chamber. More recently, a Walter, Hoyt, Mehta, and Sears (WHiMS) rotor was created that can withstand pressures over 400 bar (for a 5 mm rotor) and achieve temperatures above 250 °C [90]. As illustrated in Fig.1(b), the WHiMS rotor also employs a 5 or 7.5 mm cavern-style zirconia sleeve sealed on one side by a rigid ceramic end. It is secured using a polymeric bushing containing one or two O-rings (Fig.1(b) C). The bushing features a flexible slit allowing the locking screw to flex inward during insertion until its locking ridge aligns with a matching groove machined into the rotor. After inserting the screw cap into the bushing, the bushing expands so the ridge engages while also blocking the centrally located pinhole. Pressurization of the rotor occurs through a one-way valve built into the bushing ((E) of Fig.1(b)) and an attached loading chamber. Depressurization happens via the pinhole after loosening the locking screw. Compared to other designs, the WHiMS rotor is easier to manufacture and more practical for sampling. By using the high-temperature and high pressure resistant NMR rotor techniques, the reactions under extremely condition such as supercritical phenomena [95], zeolite crystallization [96,97], and aqueous phase heterogeneous catalysis [98,99] can be investigated.

Fig.1 Diagram of (a) HTHP MAS NMR rotors developed by Jaegers et al. Reprinted with permission from Ref. [88], copyright 2020, American Chemical Society. (b) The WHiMS rotors. Reprinted with permission from Ref. [90], copyright 2018, American Chemical Society.

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Given that many heterogeneous catalysis reactions occur under continuous flow conditions, there was incentive to develop NMR methods to monitor such reacting systems. Hunger and Horvath [83,92] created the first CF MAS NMR technique that combined reagent flow with sample spinning. Their development merged these important aspects of capturing kinetics under realistic flow reactions while preserving spectral resolution through rotational NMR methods. This innovation helped open new opportunities for using NMR to study catalysis under conditions more representative of industrial applications vs. prior batch-like approaches. Moreover, by adding a quartz window and a glass fiber, the UV-Vis spectroscopy can simultaneously detect the adsorbed species in conjunction with in situ NMR [100]. Combined with a stopped-flow protocol, the intermediate species from various adsorbed surface species can be distinguished [70,101−104]. In addition, there are some other in situ NMR probes, developed around the same time as Hunger’s or later, such as Isbester and Goguen’s CF-MAS NMR probe [105,106] and Hu’s 9.5 mm large-sample-volume CF-MAS probe [107]. Another important flow in situ NMR technique is the pulse-quench reactor technique, which was developed by Haw et al. [93,94]. This technique allows the reaction to be performed under continuous flow conditions and at a wide range of temperature. After reacting for a desired time, the reaction can be quickly quenched by pulsing liquid N₂ onto catalysts within 200 ms. Meanwhile, the adsorbed species and potential active intermediates can be frozen and detected by NMR after transferring the reacted catalysts into rotors. The adsorbed species over catalysts can be examined at various spinning speeds and for sufficient time, in order to achieve NMR spectra with high resolution and high signal-to-noise ratio. The volatile products that leave the reactor can be detected by GC or GC-MS as well. Combined investigation of adsorbed intermediates and effluents helps clarify possible reaction pathways [93,94].

3 In situ NMR investigation of heterogeneous catalytic reactions over zeolites

3.1 Conversion of alcohols over zeolite

Alcohols derived from coal, biomass and natural gas are often regarded as platform molecules for the production of light olefins, gasoline and aromatics, which can serve as alternatives to conventional petroleum-based routes [15,69,108−110]. The conversion of alcohols over zeolites involves a complex network of sub-reactions, which poses significant challenges for the mechanistic understanding of such process. Various in situ spectroscopic techniques, such as NMR, IR, UV-Vis, Raman, have been employed to monitor the reaction process and identify the surface intermediate species [111−116]. Among them, in situ ssNMR has the advantage of being able to observe and differentiate the surface species adsorbed on zeolites. By using in situ NMR, alkoxy species and carbocations have been frequently detected as the key intermediates in the conversion of alcohols to hydrocarbons [71,93,104,117−119]. Generally, sample preparation under continuous flow conditions is preferable, as most intermediates are highly sensitive to water produced during alcohol dehydration. Therefore, pulse-quench or CF-MAS NMR probes are ideal for isolating and characterizing unstable surface species formed during these complex catalytic reactions.

3.1.1 MTH reaction

Methanol can be transformed over acidic zeolite catalysts into various hydrocarbon products. Depending on the catalyst and reaction conditions, it includes methanol-to-olefins (MTO), methanol-to-propylene (MTP), methanol-to-gasoline (MTG), and methanol-to-aromatics (MTA) reactions [7,120−123]. The MTH reaction is a highly complex process, involving multiple reaction pathways such as methylation, alkylation, oligomerization, cyclization. Despite the extensive research on the MTH reaction in the past four decades, the reaction mechanisms are still under debate and many different proposals have been made [9,75,124−129].

The MTH reaction is an autocatalytic process that consists of four main stages: methanol dehydration, induction period, steady-state period and zeolite deactivation. Significant research has focused on the formation of C–C bond products in this reaction [130−133]. The C–C bond formation can occur through both direct and indirect mechanisms (Fig.2). The direct mechanism is more prevalent in the early stage of the MTH reaction when the zeolite surface is saturated with methanol or dimethyl ether (DME) molecules (Fig.2(a)). Over 20 direct mechanisms have been proposed to explain the initial C–C bond formation [9,69,126], generally involving the C–C coupling of methanol with an active C₁ species like radical, CO or formaldehyde derived from another methanol molecule [133−137]. Methyl acetate, acetaldehyde, and ethoxy species formed in this way are considered the first C–C bonded intermediates. After the first C–C bond species are formed, the indirect mechanism, also known as the hydrocarbon pool (HCP) mechanism, takes over. The HCP species are composed of some active C–C bond hydrocarbons, such as alkenes, aromatics and carbenium ions, which can undergo methylation by methanol/DME and subsequent cleavage of light olefins [8,11,138−143]. Based on the HCP species, a well-established dual-cycle mechanism has been proposed, in which the reactants can be catalyzed by either alkenes or aromatics independently [8,11]. Alternatively, another cycle based on cyclopentadienes/cyclopentenyl cations has been proposed, which involves cyclopentenyl cations as intermediates [143,144]. It is widely accepted that the HCP mechanism is more favorable than the direct mechanism under the operating conditions, due to the higher stability of the intermediates and the lower activation energy.

Fig.2 Proposed reaction mechanism of MTH. (a) Direct reaction mechanism (Zeo represents zeolite); (b) HCP mechanism.

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3.1.2 Identification of key intermediates in the first C–C bond formation

The formation of the first C–C bond species is a precursor step to the HCP mechanism, which occurs preferentially in the early stage of the MTH reaction or at the entrance of the catalyst beds. The initial reaction in this stage is the dehydration of methanol, which produces some intermediates such as surface methoxy species and trimethyloxonium ions [71,104,145]. Wang et al. [71,104] systematically studied the formation and reactivity of the surface methoxy species on zeolite, which were found to be highly reactive with other organic molecules such as methanol, DME, aromatics, CO in the MTH reaction.

The methoxy species exhibit carbocation-like properties (strong electrophilicity) to some extent on zeolite surface, owing to the electron withdrawing effects of the zeolite framework oxygen atoms bonded to them [134,146,147]. They can attack some electron-rich entities, such as the O atoms of DME, to form trimethyloxonium ions [133,148]. In addition, the surface methoxy species can attack methanol or DME and initiate a hydrogen transfer reaction between them, leading to the formation of formaldehyde, methoxymethyl cation and methane [149,150]. Based on these intermediates, methane-formaldehyde [151] and methoxymethyl cation [150] reaction mechanisms have been proposed. Moreover, formaldehyde can be further dehydrogenated to CO over zeolites. The study by Liu et al. found that CO undergoes a carbonylation reaction with methanol over ZSM-5, resulting in the generation of the initial C–C bond compound [135]. The rate-limiting step is the carbonylation of surface methoxyl species with a moderate activation energy of 80 kJ·mol^–1. Subsequent investigations by Chowdhury et al. identified intermediates like surface acetate species and methyl acetate generated during MTO reactions over SAPO-34 [136,152]. As shown in Fig.3, the surface acetate species and methyl acetate were characterized by 2D ¹³C-¹³C proton-drive spin-diffusion (PDSD) NMR spectra and ¹³C-¹H heteronuclear chemical shift correlation experiments (HETCOR) NMR spectra, which exhibited distinct ¹³C NMR signals of carbonyl groups at 180.5 and 178.5 ppm, respectively. Both the 2D ¹³C-¹³C PDSD and ¹³C-¹H HETCOR NMR experiments rely on the spatial dipole-dipole interaction, enabling the acquisition of information on the spatial proximity among adjacent nuclei. This information is invaluable for attributing the NMR signals of surface species over zeolites. In their subsequent work, the authors suggested that the C₂–C₄ alkenes could be formed from the alkyl chain growth and decarbonylation of ketene intermediates, which were derived from the deprotonation of surface acetate species [136,153].

Fig.3 Combination of 13C-13C PDSD and 13C-1H HETCOR MAS NMR spectroscopy to identify the surface-acetate species and methyl acetate. (a) Zooms from 2D 13C-13C (blue) and 13C-1H (red) MAS ssNMR spectra, respectively, indicating surface acetate and methyl acetate resonances. (b) ssNMR signals of surface-bound formate in the 13C-1H spectra (light blue). (c) Zoom of aromatic signals from 2D 13C-13C (blue) and 13C-1H (light blue) MAS NMR spectra, respectively. The sample was prepared from 30 min 13C methanol reaction over H-SAPO-34 at 400 °C. Reprinted with permission from Ref. [152], copyright 2016, Wiley-VCH.

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The Lewis acid sites on catalyst also play a role in the formation of the first C–C bond species. Comas-Vives et al. [154] reported that the Lewis acid sites on γ-Al₂O₃ could activate DME to produce C–C bond species, involving a transient oxonium ion intermediate. Zeolites also have various types of Lewis acid sites, which can influence the MTH reaction, including the first C–C bond formation [29]. Wang and coworkers [133] showed that the induction period of the MTH reaction was significantly reduced over dealuminated ZSM-5, suggesting that the Lewis acid sites created by dealumination facilitated the formation of initial C–C bond species. By using ¹³C cross polarization (CP)/MAS NMR, they observed a new ¹³C signal at 52.4 ppm over dealuminated ZSM-5 (Fig.4). This signal was assigned to surface methoxy species on extra-framework aluminum (SMS-EFAL), which were further characterized by measuring the ¹³C-²⁷Al distance by ¹³C-²⁷Al symmetry-based rotational-echo saturation-pulse double-resonance (S-RESPDOR) NMR method and theoretical calculations. Moreover, after introducing formaldehyde with methanol at 250–350 °C, some C–C bond species such as surface ethoxy species, acetaldehyde, methyl acetate and surface acetate species were formed. Interestingly, the ethoxy species, which are intermediates of ethene, were also detected in the co-reaction. Based on the experimental results, the authors proposed that the SMS-EFAL could react with formaldehyde to form acetaldehyde as the first C–C bond species, with a reasonable activation energy of 28.3 kcal·mol^–1. The subsequent hydrogen transfer between acetaldehyde and methanol led to the formation of ethanol or surface ethoxy species as the precursors to ethene. This study highlights that formaldehyde or its analogs serve as reactive intermediates in the formation of initial C–C bond species during the MTH reaction. Additionally, beyond their interaction with methane in the methane-formaldehyde mechanism, they are also capable of reacting with methanol or surface methoxy species to facilitate the formation of C–C bond species.

Fig.4 Identification of the formation of SMS-EFAL and its reactivity in MTH reaction. (a) 13C NMR spectra of trapped products obtained from reaction of 13C-methanol for 1 min, followed by co-feeding 13C-methanol and 13C-formaldehyde for another 1 min over dealuminated H-ZSM-5 at 250–350 °C. (b) 13C-{27Al} RESPDOR NMR spectra and 13C-27Al internuclear distance measurement, confirming the formation of SMS-EFAL. Theoretically optimized model of SMS-EFAL is also shown. (c) Reaction mechanism for the formation of the first C–C bond in MTH reaction. The theoretically calculated activation energy (Eact) and reaction energy (Ereact) values are given in kcal·mol–1. Reprinted with permission from Ref. [133], copyright 2018, Wiley-VCH.

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3.1.3 Identification of HCP species

HCP species formed during the MTH reaction over zeolites can generally be classified into three main categories: aromatics, alkenes, and carbenium ions. However, directly identifying the specific active species within the zeolite pores is challenging due to the complex adsorbed intermediates present. in situ ssNMR spectroscopy provides unique advantages for characterizing such complex reaction systems through its ability to obtain advanced 1D and 2D NMR spectra. In earlier studies, Goguen et al. [155] applied the pulse-quench method and detected the 1,3-dimethylcyclopentenyl cations at a preset time in the MTG reaction over ZSM-5. They later also detected pentamethylbenzenium ion intermediates through co-pulsing of methanol and aromatic compounds over ZSM-5 [156]. Based on these observed carbenium ions, a plausible pairing reaction mechanism was proposed to explain the formation of butene as a reaction product. This mechanism was further supported through complementary density functional theory (DFT) calculations [139]. While the pairing mechanism provided insights, it remained incomplete as the pentamethylbenzenium ions were only indirectly observed through co-reacting aromatics and methanol. More direct evidence was still needed to fully validate the proposed reaction routes. In addition to butene formation, isotopic labeling experiments indicated ethene and propene could also be produced via pairing mechanisms over ZSM-5 [111,157,158]. Therefore, unambiguous identification of reactive intermediates is critical for establishing a comprehensive MTH reaction mechanism. In more recent work, Wang and colleagues [141] applied advanced high-field ssNMR combined with solution NMR to further characterize adsorbed species under MTO conditions over ZSM-5. Utilizing these advanced high-resolution NMR techniques, a greater variety of HCP species were successfully identified. As shown in Fig.5, in addition to the well-established 1,3-dimethylcyclopentenyl cation, 1,2,3-trimethylcyclopentenyl cation and pentamethylbenzenium ion, the study directly observed 1,3,4-trimethylcyclopentenyl cation and various ethyl-substituted cyclopentenyl cations. Due to strong deshielding effects of the carbocation centers, these species exhibited characteristic chemical shift ranges of 240–260 ppm for cyclopentenyl cations and 180–210 ppm for benzenium ions. These carbenium ions were identified as very reactive HCP species, responsible for the formation of light olefins. For instance, based on 1,3,4-trimethylcyclopentenyl cation and pentamethylbenzenium ion, propene could be formed via the ring contraction and C–C cracking process [111]. Moreover, the ethyl group could be cleaved from ethyl substituted cyclopentenyl cations, leading to the production of ethene. Meanwhile, ring contraction of methylbenzenium ions generates ethyl-substituted cyclopentenyl cations [140,141]. Thus, pairing routes for selective propene and ethene production were fully constructed based on these identified intermediates. However, it is important to note that the current pairing routes for ethene formation typically encounter a significant activation barrier. Establishing a feasible reaction mechanism is crucial, drawing on the combination of in situ spectroscopic techniques and theoretical calculations.

Fig.5 Combination of solid-state 13C MAS NMR and solution 13C NMR to analyze the HCP from methanol conversion over H-ZSM-5. (a) The trapped products obtained from reaction of 13C methanol over H-ZSM-5 at 350 °C for 30 min. The 13C chemical shifts of both solid-state and liquid-state NMR are indicated for the observed carbocations (those from liquid-state NMR are in the parentheses). Asterisks denote spinning sidebands. For liquid-state NMR, the carbocations were regenerated and stabilized by adding concentrated sulfuric acid (98%) into the extract solution of the reacted H-ZSM-5 at room temperature. The confirmed cyclopentenyl cations are shown in the top. Reprinted with permission from Ref. [141], copyright 2015, Wiley-VCH. (b) The reaction mechanism for the formation of ethene and propene in methanol conversion over H-ZSM-5. Reprinted with permission from Ref. [140], copyright 2015, Wiley-VCH, and Ref. [141], copyright 2015, Elsevier.

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The carbenium ions have also been detected over other zeolites besides ZSM-5 in the MTO reaction [118,159−162]. For example, Xu et al. [118] simultaneously observed pentamethylcyclopentenyl cations and heptamethylbenzenium ions during continuous flow MTH reactions over CHA zeolites using in situ ssNMR. Based on the experimental results, the authors proposed a feasible propene formation mechanism from the aromatic-based cycle, either in the pairing or the side-chain routes. For CHA zeolites, the side-chain route is preferred for the formation of propene compared to the pairing mechanism, which contrasts with the behavior observed in MFI zeolites. Subsequent work by Xiao et al. utilizing advanced J-coupling based 2D ¹³C-¹³C correlation NMR experiments further characterized additional carbenium ions over ZSM-5. This included identification of butyl-substituted cyclopentenyl cations that provided new insights into reaction intermediates and mechanisms [163]. As shown in Fig.6, the refocused Incredible Natural Abundance DoublE AUAntum Transfer Experiment (INADEQUATE) NMR method was used to confirm the HCP species in the MTO reaction over H-ZSM-5. This method can explore the through-bond correlations, which can provide straightforward pathway for ¹³C signal assignments. Besides the reported carbenium ions such as methyl-substituted cyclopentenyl cations (I, II) and pentamethylbenzenium ion (III), the 1,5-dimethyl-3-sec-butyl cyclopentenyl cation (IV) is observed. The experimental identification of butyl-substituted cyclopentenyl cation provides direct evidence for the paring route to form butene in aromatic-based cycle [139].

Fig.6 (a) 2D 13C-13C refocused INADEQUATE spectrum of 13C enriched MTO activated H-ZSM-5. Signals corresponding to carbenium ions (black) and to other neutral carbon species (blue) are highlighted to distinguish them. The assignments of the different carbenium species are given in different colors. Asterisks (*) denote spinning sidebands. (b) Molecular structures of the carbenium ions are identified, color-coded according to their assignments. (c) Extracted horizontal traces of carbenium ion I with arrows in dashed lines indicating their positions in the 2D map. The corresponding double quantum frequency δDQ of each slice is also given in the figure. The chemical shifts of different 13C sites are given in parenthesis. Unlabelled peaks are from other carbenium ions or aromatic species. Reprinted with permission from Ref. [163], copyright 2017, the Royal Society of Chemistry.

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3.2 Ethanol-to-hydrocarbon reaction (ETH)

The ETH reaction has attracted much attention in recent years, as it can produce commodity chemicals without relying on the petroleum route [110,164−166]. The ETH reaction consists of two consecutive stages (Fig.7) [167]. In the first stage, ethanol is dehydrated to ethene, involving either monomolecular or bimolecular routes [168]. In the second stage, C₃₊ products are formed from the ethene produced in the first stage. In this stage, the active adsorbed hydrocarbon species can either be converted into products such as light olefins or aromatics or be alkylated by ethene to form larger hydrocarbons. The nature and function of these active adsorbed intermediates in the ETH reaction mechanism bears strong similarities to the HCP species discussed previously for MTH conversions over zeolites (Section 3.1.3).

Fig.7 Proposed reaction mechanism for the conversion of ethanol to hydrocarbons over ZSM-5 (EtOH: ethanol, DEE: diethyl ether, C2H4: ethene, C3H6: propene, C4H8: butene, C5+: olefinic hydrocarbons containing more than five carbon atoms, aromatics: hydrocarbons containing one or more aromatic rings, C2H4*: ethene surface species, C4H8*: butene surface species, C*ali: aliphatic surface species, C*aro: aromatic surface species. Route I (violet): dimerization of ethene to butene, Route II (green): formation of propene and butene via aliphatic surface intermediates, Route III (blue): formation of propene via aromatic surface intermediates). Reprinted with permission from Ref. [167], copyright 2016, Wiley-VCH.

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Like the MTH reaction, the ETH reaction undergoes a rather complex reaction network over zeolites. in situ ssNMR excels in monitoring the dynamic ETH reaction process and identifying the key intermediates over zeolites. The reaction mechanism of the ETH reaction can be elucidated through the application of in situ ssNMR techniques, serving as a significant adjunct to understanding the ETH reaction mechanism. For example, Wang et al. [117] used a stopped-flow protocol to study the ethanol dehydration process over Y zeolites by combining in situ MAS NMR-UV/Vis spectroscopy. They ambiguously identified the surface ethoxy species as the key intermediate for the dehydration of ethanol and the subsequent conversion to higher hydrocarbons. More recently, Zhou et al. [169] investigated the ethanol dehydration process over ZSM-5 under CF conditions by using in situ ssNMR. As shown in Fig.8(a), a narrow ¹³C NMR signal at 85 ppm appeared when ¹³C ethanol flowed over ZSM-5 at 220 °C. With the help of J-coupling based 2D ¹³C-¹³C INADEQUATE NMR spectrum, this signal was unambiguously assigned to triethyloxonium ion (TEO). The TEO species was formed from the dehydration of three ethanol molecules over zeolites, similar to the formation of trimethyloxonium ion in the MTH reaction [145]. The TEO species showed very high reactivity. As shown in Fig.8(a), the TEO species were very unstable and decomposed above 250 °C, while the hydrocarbon species were also formed, as indicated by the appearance of high field ¹³C signals at 8.7–32.6 ppm. Further ¹³C NMR experiments demonstrated that the TEO was also a key intermediate in the dehydration of ethanol. Based on the experimental results and DFT calculations, the authors proposed a plausible ethanol dehydration mechanism (Fig.8(b)). The ethanol dehydration involved a complex reaction network. According to the DFT calculations, the TEO species could be easily transformed into ethoxy species and then produce ethene (steps 1–8). Compared to the other routes in the intricate reaction network, this route was kinetically favorable for the dehydration of ethanol to ethene. Aided by the in situ ssNMR technique, the TEO mechanism is demonstrated to be a possible route for the ethanol dehydration over zeolites. It is worth noting that the TEO and surface ethoxy species exhibit a high sensitivity to water molecules. Therefore, during the capture of these intermediates, it is essential to remove water vapor, for instance, by employing a high-flow carrier gas.

Fig.8 Investigation of ethanol transformation over zeolites: (a) in situ flow 13C MAS NMR spectra of 13CH313CH2OH conversion over H-ZSM-5 with time on stream and at elevating temperatures; (b) proposed ethanol dehydration routes. Reprinted with permission from Ref. [169], copyright 2019, Springer Nature.

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The homologation reaction of ethanol from dehydration to deactivation process was studied by Chowdhury et al. by combining ssNMR and UV-Vis diffuse reflectance spectroscopy [170]. They identified the complex adsorbed species over ZSM-5 after the ETH reaction by using various 1D and 2D ssNMR experiments. As shown in Fig.9(a), in addition to adsorbed ethanol and surface ethoxy species, carbonyl-containing compounds and ethyl-substituted benzenes were identified from 2D ¹³C-¹³C PDSD NMR spectra. The ethyl-substituted benzenes were the active HCP species in the ETH reaction, which could be either decomposed into olefins or further converted into coking species. Based on these characterizations, they proposed a homologation reaction mechanism for the ETH process. As in the previous case, ethanol dehydration to ethene was the initial step. Then, the homologation of ethanol with ethene led to the formation of adsorbed butylene species. The repeated homologation reaction eventually produced the ethylated aromatics as the final products. At the same time, the non-homologated products such as propene with odd carbon number could also be formed from the cracking of homologated products.

Fig.9 13C MAS NMR investigation of ethanol transformation over ZSM-5: (a) 2D 13C-13C PDSDNMR spectra of adsorbed species over ZSM-5 after 13C ethanol reaction; (b) proposed mechanism for the homologation-reaction of ethanol in ETH process. Reprinted with permission from Ref. [170], copyright 2019, Wiley-VCH.

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In addition to ethanol dehydration and homologation chemistry, the ETH reaction also involves complex olefin transformation reactions occurring within the zeolite framework. Cyclization and aromatization of olefinic intermediates represent conventional processes analogous to those observed in MTH conversions. Accordingly, carbocationic species such as cyclopentenyl cations can form as important reaction intermediates. By using J-coupling based 2D ¹³C-¹³C INADEQUATE NMR spectrum, Zeng et al. [171] directly identified cyclopentenyl cations generated during ETH reactions catalyzed by ZSM-5 zeolites (Fig.10(a)). Both the methyl and ethyl-substituted cyclopentenyl cations are formed in the ETH reaction. Combined with ¹²C/¹³C isotopic switching experiments, the cyclopentenyl cations were identified as the active intermediates for the ethanol conversion. Based on the experimental results, an HCP reaction network was theoretically proposed for the ETH process over ZSM-5 (Fig.10(b)). Alkenes, aromatics and cyclopentenyl cations were claimed to independently contribute to the formation of ethene and propene. In the alkenes-based cycle, surface alkoxide species are produced from the oligomerization of ethene, which can be transformed into ethene and propene via the β-elimination; in the cyclopentadienes-based cycle, cyclopentenyl cations are initially deprotonated to neutral cyclopentadienes whose ring or side-chain can be ethylated to ethyl- or propyl-substituted cyclopentenyl cations followed by splitting ethene or propene; in the aromatics-based cycle, ethene and propene are formed via the side-chain route, in which ethylation of aromatics produces benzenium ions as the reaction intermediates.

Fig.10 (a) Advanced 2D 13C-13C INADEQUATE NMR experiment probing trapped carbocations of 13CH313CH2OH conversion. The assignments of the different carbenium ions are highlighted in different colors. (b) Ethene and propene formation via triple-cycle routes with the participation of different intermediate species in ETH over H-ZSM-5. Calculated free energy barriers at 250 °C are given in kcal·mol–1. (R the alkyl groups). Reprinted with permission from Ref. [172], copyright 2022, Elsevier.

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3.3 Conversion of light alkanes over zeolite

Light alkanes (C₁–C₄) are abundant in natural gas, biogas and co-produced from petroleum and coal chemical industries [32,33,173−175]. However, light alkanes are hard to activate due to the inertness of C–C and C–H bonds. Moreover, the reactivity of alkanes decreases as the hydrocarbon chain length decreases. Methane, in particular, is the least reactive hydrocarbon molecule, with a high C–H bond energy (104 kcal·mol^–1) [2]. Zeolites play a dual role by supplying active sites for alkane activation and microporous confinement environments that impact adsorption, diffusion and stability of intermediates along reaction pathways. This allows overcoming inertness limitations and enables targeted conversion of light alkanes. The active sites for the activation of alkanes are either Brønsted acid sites or metal sites [32,176−179]. In the activation process over Brønsted acid sites, the alkanes are attacked by either C–H or C–C bond, forming penta-coordinated carbonium ions as the initial intermediates [32,174,180]. The subsequent decomposition of carbonium ion produces H₂ and a carbenium ion with the same size as the alkane in the C–H activation, or a shorter carbenium ion and alkane in the C–C activation. The final deprotonation of carbenium ions leads to the formation of alkenes.

The stability of the carbocation intermediates is crucial for the alkane activation. Methane and ethane form primary carbocations that are very unstable, which makes the activation of these alkanes very challenging. For instance, methane is almost inert over pure zeolites even at high temperature [181]. A possible way to activate alkanes is to introduce some metals in the extra-framework or framework positions of zeolites, such as Cu, Fe, Mo, Zn, Ga, Pt, Pd. These bifunctional zeolites can effectively transform light alkanes under oxidative or non-oxidative conditions [182−196].

In addition to acid-catalyzed protolysis pathways, a carbenium ion chain mechanism has also been proposed to explain alkane activation over zeolites, particularly for C₂₊ alkanes [174,197,198]. In this mechanism, the alkane molecules are activated by carbenium ions or alkoxyl species that act as reaction initiators. The initiators attract hydride to the carbenium ions, forming new carbenium ions or alkoxyls, which then decompose into alkenes or crack by β-scission reaction. Potential precursors for the initiating species could come from alkene impurities, alkane activation at Lewis acid sites, or protolytic reactions over Brønsted acids [199]. The carbenium ion chain mechanism is favored thermodynamically in large-pore zeolites due to reduced steric hindrance. Kinetically, it also benefits from a lower activation energy compared to the initial C–H bond cleavage in protolysis. Therefore, this bimolecular route tends to predominate at lower temperatures and high alkane conversions that facilitate chain propagation.

In situ ssNMR has been demonstrated to be an effective tool for unraveling the intricate mechanisms of light alkane activation over zeolites. Its ability to directly probe reaction intermediates through their unique NMR signatures and monitor isotopic exchange processes provides invaluable insights into the catalytic cycle at the atomic level. However, the inherently low reactivity of light alkanes presents a significant hurdle for in situ ssNMR studies. The majority of experimental work to date has relied on batch-like sampling methods to accumulate sufficient quantities of intermediates for spectral analysis. Approaches, such as sealed glass ampoules and the CAVERN technique, have dominated existing literature due to their practicality, despite limitations in dynamic and temporal resolution.

3.3.1 Identification of surface species in light alkane activation over zeolites

3.3.1.1 Methane

Methane is more inert and harder to transform than other higher alkanes, even at high temperature. Transition metals like Ga, Zn, Mo, Cu deposited on zeolites effectively lower the activation energy via either metallic site dehydrogenation or synergistic effects with Brønsted acidity [176,178,200−205]. Theoretical calculations [206−208] and H/D exchange experiments [209,210] have shown that the C–H bond of methane can be polarized by Zn species on zeolite at low temperatures, even at room temperature. In this process, some intermediates, such as zinc-methyl species (Zn-CH₃), are formed from the heterolytic cleavage of methane. Luzgin et al. [211,212] reported that the heterolytic cleavage of the C–H bond of methane molecules by ZnO clusters on Zn/H-BEA produced both zinc-methyl species and methoxy species at 523 K. The methoxy species could be partly oxidized to surface formate, which could couple with zinc-methyl species to form acetaldehyde as a C–C bond species. Similarly, Kolyagin et al. [188] detected zinc-methyl formation from methane dissociation over Zn/H-ZSM-5 at room temperature. In addition to the heterolytic cleavage, Xu et al. [213] found that homolytic cleavage of the C–H bond of methane could also take place over Zn/H-ZSM-5 based on in situ ¹³C NMR experiments. The homolytic cleavage of methane molecule by a dizinc cluster generated methyl radical, which could be further converted into surface methoxy species trapped by zeolite. Moreover, the heterolytic cleavage of methane could also occur over Ga or Zn modified ZSM-5, resulting in the formation of Ga/Zn-CH₃ [187,214]. Zhao et al. [215,216] identified the active sites and the formation process of Ga-CH₃ and Zn-CH₃ by using ssNMR spectra. The metal methyl behaved similarly to organozinc compounds, which can react with some nucleophilic species such as CO₂ [217] or can be oxidized into methoxy species under oxidizing conditions [214].

Methane dehydroaromatization (MDA) to produce high-value hydrocarbons from methane has attracted significant research interest. Modified zeolites containing Mo, W, Fe, V, Cr, etc. are commonly used as MDA catalysts [218−220]. However, elucidating the reaction mechanism remains challenging and debated due to its complexity [221−223]. Recent studies found an induction period during MDA, similar to MTH, where active Mo species are formed via methane reduction and an HCP is built up containing polyaromatics [224,225]. Çağlayan et al. [226] detected some intermediates during the early stages of the MDA reaction over Mo/H-ZSM-5 by using 2D ssNMR spectra. As shown in Fig.11, the mobile and rigid organics in the zeolite pores were characterized by J-coupling based and dipolar-coupling based 2D ¹³C-¹H correlation experiments, respectively.

Fig.11 Identification of surface species over Mo/H-ZSM-5 in methane transformation by ssNMR. (a) J-coupling based 2D MAS ssNMR 13C-1H correlations experiment to confirm the mobile molecules. (b) Dipolar-coupling based 2D MAS ssNMR 13C-1H correlations experiment to confirm the rigid molecules. The spectra were obtained after (13C-) MDA reaction over Mo/ZSM-5 at 725 °C for 50 min (in blue) and 2 h (in red). (c) Reaction mechanism for the formation of C–C bond from methane. Reprinted with permission from Ref. [226], copyright 2020, Wiley-VCH.

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Besides the alkylated unsaturated hydrocarbon (olefins/aromatics), acetylene (81 ppm (¹³C), 2 ppm (¹H)), acetal (104 ppm (¹³C), 6.5 ppm (¹H)), surface-formate (174 ppm (¹³C), 10 ppm (¹H)) and Mo=CH₂ (42–45 ppm (¹³C), 5–6 ppm (¹H)) were also identified. Based on the spectroscopic evidence, they proposed two plausible C–C coupling pathways. One involves C–H bond activation of methane by MoO_(3–x) to form a carbene-like CH₂=MoO_(3–x) intermediate transformed to acetylene, which aromatizes over Mo-carbide. The other was similar to the first C–C bond formation process in the MTH reaction. The consecutive oxidation of methane by MoO_(3–x) generated CO, which could react with methoxy species like the carbonylation process for the C–C bond formation in the MTH reaction [135,152]. Finally, the HCP species such as alkenes and aromatics were formed and accelerated the MDA reaction.

In a recent work by Gao et al. [74], the host-guest interaction between the HCP species (aromatics and olefins) and active sites over Mo/ZSM-5 were investigated by ¹H-{⁹⁵Mo} double-resonance NMR (Fig.12). In the early MDA period, the olefins (6.4 ppm) were found to strongly interact with the nearby acidic proton-Mo site, reflected by the strong ¹H-⁹⁵Mo dipolar (ΔS) between the two species. The strong interaction between olefins and Mo sites results in the formation of aromatics (8.1 ppm). Furthermore, the studies found that catalyst deactivation correlated with a reduction in the proximity of these active sites along with stronger interactions developing between aromatics/derivatives and Brønsted sites within the zeolite over time. Afterwards, using 1D ⁹⁵Mo NMR and 2D ¹H-⁹⁵Mo HETCOR ssNMR spectroscopy, they identified the active sites responsible for the methane MDA over Mo/ZSM-5 [72]. Two kinds of MoO_xC_y sites were determined, but exhibit different activity in the MDA reaction. The one with more carbidic properties exhibits higher activity for methane activation and benzene formation.

Fig.12 1H{95Mo} S-RESPDOR NMR spectra of (a) fresh Mo/ZSM-5, (b) Mo/ZSM-5 reacted for 30 min and (c) for 120 min of MDA reaction at 973 K. Normalized ΔS of (d) Brønsted acid, (e) olefins, and (f) aromatics vs. MDA reaction time (ΔS = S0 – S). Reprinted with permission from Ref. [74], copyright 2021, Wiley-VCH.

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3.3.1.2 C₂–C₄ alkanes

The transformation of C₂–C₄ alkanes over zeolites typically occurs through two major processes: initial alkene formation followed by secondary reactions. Brønsted acidic sites on the zeolite facilitate C–H or C–C bond cleavage of C₂–C₄ alkanes, generating carbocationic intermediates such as carbonium ions. Ivanova et al. [227,228] detected a cycloproponium ion as a key intermediate in propane activation over H-ZSM-5 by using ¹³C MAS NMR spectra. The metal species on zeolite can act either as active sites or as synergistic enhancers of Brønsted acid. Similar to methane, the C–H bonds of ethane and propane can also be polarized by metal sites, resulting in the formation of metal alkyl species at low temperature [229,230]. Zn-ethyl and Zn-propyl have been observed after activating ethane and propane over Zn/ZSM-5 at room temperature [196]. Wang et al. [227,228] also identified Zn-ethyl species as a key intermediate in the carbonylation of ethane by CO using in situ ssNMR spectra [231]. Propene and hydrogen could form from Zn-propyl dehydrogenation as well [194]. Synergistic mechanisms involving metal and Brønsted sites have also been proposed [178,232−234]. Schreiber et al. [176] found that the Lewis-Brønsted acid pairs over Ga/ZSM-5 were more active than pure Brønsted acid pairs by forming the active [Ga-H]²⁺ species.

The secondary reaction of alkenes and alkanes, which are formed from the activation of C₂–C₄ alkanes, further transforms them into long chain hydrocarbons or aromatics. The introduction of metal Lewis acid such as Mo, Zn, Ga can enhance the dehydrogenation process of hydrocarbon, favoring the formation of aromatics [33,74,192,218,235,236]. Consequently, direct alkane aromatization for valuable aromatic chemicals becomes feasible over metal-modified zeolites. Gabrienko et al. [237] investigated full n-butane transformation pathways over Zn/Beta using ¹³C ssNMR. As shown in Fig.13, at room temperature, besides a major ¹³C NMR signal of n-butane (13.5 ppm for methyl and 27.1 ppm for methylene with natural abundance), two new signals at 8 and 21 ppm were detected with rigid properties, which were assigned to α and δ C-atoms of n-butylzinc, respectively. When the sample was heated at 453 K, the ¹³C signals of methyl groups of n-but-2-ene with cis- and trans-isomer (13 and 18 ppm) appeared, indicating the dehydrogenation of n-butane. When the reaction temperature was increased to 523 K, the olefin oligomerization was further enhanced, as shown by the strong ¹³C signals of olefin oligomers at 10–40 ppm and the specific signals at 93, 117, and 141 ppm. Aromatic formation initiated at 573–623 K as evidenced by the 130/150 ppm peaks. Moreover, hydrogenolysis also occurred based on methane (–7.9 ppm), ethane (5.9 ppm), and propane (17.0 ppm) signals. In this study, the in situ ssNMR technique demonstrates its proficiency in elucidating the dynamic evolution of light alkanes over metal-modified zeolites. The dehydrogenation of light alkanes at the metal active sites has been unequivocally confirmed.

Fig.13 13C CP/MAS NMR spectra of n-butane-1-13C adsorbed on Zn2+/H-BEA zeolite. The spectrum before sample heating at elevated temperatures is shown in (a). The sample was heated (b) at 453 K for 100 min, (c) at 523 K for 60 min, (d) at 573 K for 60 min, and (e) at 623 K for 60 min (All spectra were recorded at room temperature (about 298 K). The spectrum region from –30 to 50 ppm is highlighted in the frame for better observation of the detected signals. Asterisks denote spinning side bands). Reprinted with permission from Ref. [237], copyright 2020, Elsevier.

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Identifying key intermediates and reaction pathways for alkane transformation poses significant challenges due to the formation of numerous organic species and the complex sub-reactions involved. Additionally, harsh reaction conditions like high temperature and pressure required further complicate in situ investigations. Wang et al. [238] used a batch-like reactor to investigate the propane aromatization and successfully observed the cyclopentenyl cations as the key intermediates by ¹³C MAS NMR. Using the batch-like reactor (glass ampoule), the propane molecules could contact well with the active sites on zeolite and transform at a mild temperature. As shown in Fig.14(a), after ¹³C-propane reacted over Ga/ZSM-5 for 10 min, the aromatics were formed, as indicated by the specific ¹³C signals at 130 ppm. At the same time, some weak but well-resolved ¹³C signals at 240–250, 147–156, 48 ppm appeared, which were assigned to cyclopentenyl cations. Cyclopentenyl cations promote aromatization over zeolites via ring expansion and dehydrogenation reactions as established in MTH chemistry. Their preferential formation over Ga/ZSM-5 vs. H-ZSM-5, as evidenced by the relatively higher ¹³C peak intensities, indicated that Ga incorporation accelerates aromatization by facilitating cyclopentenyl cation generation. Furthermore, an autocatalytic effect was observed where initial propene formed promotes further propane conversion to cyclopentenyl cations (Fig.14(b)), which then dehydrogenate additional propane molecules (Fig.14(c)). This provided direct experimental validation of carbenium ion mechanisms and revealed cyclopentenyl cations as key reactive intermediates enabling Ga/ZSM-5 to overcome kinetic barriers for alkane conversion under milder reaction conditions.

Fig.14 In situ solid state 13C NMR to investigate the propane aromatization over Ga/ZSM-5: (a) 13C CP/MAS NMR of the adsorbed hydrocarbon species on 1% Ga-ZSM-5 during propane aromatization reaction at 350 °C for 0–320 s; (b) propane conversion with reaction time over H-ZSM-5 and Ga/ZSM-5; (c) proposed propane direct and cyclopentenyl cations mediated aromatization routes. Reprinted with permission from Ref. [238], copyright 2021, Wiley-VCH.

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3.4 Zeolite catalyzed reactions in liquid phase

In addition to conventional gas-solid catalytic reactions, liquid-solid and gas-liquid-solid phase systems are also widely employed, especially for reactants that are difficult to vaporize or thermodynamically unstable. Numerous biomass conversion transformations proceed in aqueous or organic solvent environments, such as reactions involving glucose, lignin derivatives, and others [99,239,240]. However, the solvent in a high-temperature reaction environment can result in a significantly high-pressure system, posing significant challenges for investigating reaction mechanisms using traditional in situ spectroscopic techniques.

Recently, developments in HTHP MAS NMR technique have enabled in situ investigation of these liquid-phase heterogeneous catalytic systems through measurement of kinetics, identification of reactive intermediates, and characterization of reactant adsorption onto active sites [240]. For example, the phenol alkylation process in liquid-solid phase has been investigated by in situ ssNMR spectroscopy [98,99]. The phenol derived from lignin can be alkylated by both olefins and alcohols to adjust the carbon number and improve the carbon retention in liquid products, which are widely used as additives in gasoline, lubricants and other products. Four mechanistic possibilities have been proposed based on vapor and liquid-phase studies: (1) electrophilic attack of phenol by alcohol-derived carbenium ions, (2) cyclohexyl phenyl ether intramolecular rearrangement, (3) phenol alkylation by olefin-derived carbenium ions re-adsorbed on acid sites, and (4) concerted direct dehydration of phenol and alcohol. Zhao et al. [98] investigated the specific phenol alkylation by cyclohexanol over H-BEA zeolite in the apolar solvent decalin by using in situ¹³C MAS NMR spectra. As shown in Fig.15(a) and Fig.15(b), phenol alkylation (156 ppm) initiation coincided with near-complete cyclohexanol conversion (400 min), indicating direct reaction was not occurring. Instead, carbenium ions formed from cyclohexene (dehydration product of cyclohexanol) re-adsorbed on Brønsted acid sites appeared. Weaker cyclohexene adsorption vs. cyclohexanol explained the induction period. Co-reacting phenol-cyclohexene eliminated this delay (Fig.15(a)). Besides the C-alkylation products, O-alkylation product cyclohexyl phenyl ether (158.8 ppm) was also formed and gradually decreased after reaching the maximum concentration. This implied that the O-alkylation of phenol was kinetically favored but not stable, which could be further transformed by intramolecular rearrangement. Subsequent work revealed alcohol monomers, not just olefins, could directly form electrophilic carbenium ions (Fig.15(c)). The cyclohexanol-cyclohexanol dimers initially hindered the formation of carbenium ions. As cyclohexanol levels decreased, the ability of cyclohexanol monomers to form reactive carbenium ions became increasingly pronounced, thereby accelerating the alkylation of phenol. Once cyclohexanol was completely consumed, the re-adsorption of cyclohexene (the dehydration product) to form alkylating carbenium ions through interaction with acid sites became the predominant pathway for phenol alkylation. This work demonstrated the efficiency of in situ ssNMR technique on the investigation of solution-solid phase reaction. Aided by this technique, the mechanism of cyclohexanol dehydration over zeolites is well depicted.

Fig.15 HTHP MAS NMR to investigate the cyclohexanol dehydration over beta zeolite. (a) Concentration-time profiles of reactants and products in the alkylation of phenol by cyclohexanol and cyclohexene. The concentration was determined by 13C MAS NMR spectra. (b) Stacked plot of in situ 13C MAS NMR spectra of 1-13C-phenol alkylation with 1-13C-cyclohexanol at 127 °C. Reprinted with permission from Ref. [98], copyright 2017, American Chemical Society. (c) Reaction pathways proposed on the basis of in situ 13C NMR measurements of cyclohexanol dehydration. Reprinted with permission from Ref. [99], copyright 2018, Springer Nature.

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4 Conclusions and perspectives

This review has illustrated the powerful capabilities of in situ ssNMR techniques for investigating heterogeneous catalytic reactions proceeding over zeolites under both gas-solid and liquid-solid phase conditions. Key organic intermediates and reaction kinetics have been elucidated for important processes through application of ¹³C and ¹H NMR spectroscopy under working reaction environments.

Most of the studies using in situ ssNMR focused on the observation or monitoring of the organic adsorbed species such as the active intermediates on zeolites. However, zeolite frameworks may also undergo changes during the reaction, which would impact catalytic performance. Future studies should therefore expand in situ NMR investigations to explore the dynamic roles of inorganic zeolite frameworks in their interactions with adsorbed organic molecules during catalytic reactions. Exploring the host-guest interaction between the zeolite framework and surface adsorbed organic molecules is crucial for understanding elementary reaction steps. However, directly characterizing these interactions at the molecular level remains challenging due to the complexity of surface-adsorbed organics and the non-covalent nature of these interactions. To overcome these limitations, high-resolution ssNMR techniques, such as ultra-fast MAS, need further development to improve the resolution of NMR spectra. Additionally, NMR methods specifically designed to probe non-covalent interactions, like double resonance or multidimensional (2D and 3D) NMR spectroscopy, are essential for investigating the spatial proximity between the zeolite framework and surface organic species. While liquid-phase systems can now be examined, adsorbed species and possible intermediates at solid-liquid interfaces remain poorly understood for developing accurate mechanistic models. Liquids can introduce complex non-covalent molecular networks within reaction systems, significantly hindering the characterization of liquid-solid phase zeolite-catalyzed reactions. For instance, water, a common component in zeolite catalysis, can form hydrogen-bond networks within zeolite pores. These interactions include hydrogen bonds between water molecules themselves, between water and the zeolite framework, and between water and reactants or intermediates. Therefore, developing advanced NMR methodologies to characterize host-guest interactions and the molecular environment of adsorbates under liquid-phase conditions is another critical challenge.

Conventional MAS NMR suffers from intrinsic low sensitivity, limiting acquisition of spectra with high chemical and temporal resolution. To some extent, these limitations impede the development of in situ ssNMR techniques for investigating heterogeneous reactions over zeolites. For instance, in certain alkane transformations over zeolites, intermediate species are extremely scarce, making them challenging to detect by using in situ ssNMR techniques. Techniques aimed at enhancing NMR sensitivity, such as dynamic nuclear polarization (DNP) [241,242] and parahydrogen-induced polarization (PHIP) [243−247], which are capable of improving sensitivity by two to four orders of magnitude, hold promise for enhancing the capabilities of in situ NMR experiments by facilitating acquisition of high-resolution spectra with improved temporal information. DNP-enhanced NMR occurs through the transfer of electron spin polarization to surrounding nuclei via microwave irradiation. Both radicals and paramagnetic ions can act as polarization agents [248]. When reactions take place over paramagnetic metal ion-modified zeolites, such as in alkane dehydrogenation, there is significant potential for enhanced NMR signal sensitivity through DNP. Moreover, radicals can be generated in specific zeolite-catalyzed reactions, such as the MTH transformation [137]. These radicals can serve as polarization agents, thereby facilitating DNP-enhanced NMR. Combining DNP techniques with in situ NMR experiments under batch conditions offers a promising approach for studying zeolite catalysis with greater sensitivity and resolution. PHIP combined with in situ ¹H MAS NMR under flow conditions has been used in the study of propene hydrogenation in metal-modified zeolites [249−251]. Theoretically, the PHIP technique can be adapted for use in more zeolite-catalyzed reactions. For instance, alkene reactants could be initially polarized through PHIP, leading to the formation of alkanes that contain polarized nuclei like ¹H, ¹³C, or ¹⁵N. This polarization transfer allows these alkanes to act as reactants in zeolite-catalyzed reactions. Such an approach not only facilitates the monitoring of molecular fate with enhanced NMR sensitivity but also aids in uncovering the underlying reaction mechanisms. Additionally, there are still limitations in loading samples and conducting in situ flow experiments at elevated temperatures under controlled gas atmospheres within current probe designs. Designing specialized flow probes capable of withstanding high temperatures is a promising avenue for expanding the scope of reactions amenable to investigation. Addressing these technical issues will further strengthen ssNMR as an indispensable tool for characterizing dynamic heterogeneous catalysis under working conditions.

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Competing interests

The authours declare that they have no competing interests.

Acknowledgements

This work was supported by the National Key R&D Program of China (Grant Nos. 2023YFB4103600 and 2023YFB4103700), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB0540000), the National Natural Science Foundation of China (Grant Nos. 22072165, 22127801, 22225205, 22320102002, and 22161132028), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No. 2021329), the Natural Science Foundation of Hubei Province (Grant No. 2021CFA021), the Hubei International Scientific and Technological Cooperation Program (Grant No. 2022EHB021), and the International Science & Technology Cooperation Base for Sustainable the Catalysis and Magnetic Resonance (Grant No. SH2303), and the Young Top-notch Talent Cultivation Program of Hubei Province.

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