Luca Maggiulli
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Maggiulli
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Luca
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03865 - Müller, Christoph R. / Müller, Christoph R.
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- Influence of the Acid Reactivity of Carbenium Ions in Zeolites on the Methanol‐to‐Olefins ProcessItem type: Journal Article
- Structure of Active Sites in the Methanol-to-Olefins ProcessItem type: Doctoral ThesisMaggiulli, Luca (2025)The increasing energy demand, along with the environmental issues associated with the use of fossil fuels, is directing scientific research toward sustainable and low-carbon-footprint alternatives. Renewable feedstocks such as lignocellulosic biomass and captured CO2, offer the possibility to obtain fuels and building block chemicals when first converted to intermediate (green) methanol. Methanol is specifically converted into olefins in the so-called methanol- to- olefins (MTO) process. Scientists at Mobil were the first to report in 1975 on the gas-phase reaction of methanol over zeolites to synthesize hydrocarbons. Since then, progress in scientific research has been made on both the fundamental mechanistic aspects of this reaction and its implementation at an industrial scale, culminating in industrialization over the past decade. In the acidic environment within the molecular-sized pores of zeolites, methanol molecules cannot directly couple to form multiple C-C bonds effectively. Instead, methanol reacts over a catalytic hydrocarbon pool (HCP) to yield olefins. The study of the catalytically active HCP over zeolites in the MTO process is the focus of this thesis. Chapter 1 provides the context of the thesis as well as its motivation. A brief overview of the production routes of petroleum fuels and olefins from fossil feedstocks opens the chapter. Next, the alternative new methanol synthesis routes from biomass, CO2, and hydrogen derived from water splitting are introduced. The description of the MTO process follows. A brief historical perspective is provided first, followed by a description of zeolite catalysts and the zeolite-catalyzed reaction of hydrocarbons and oxygenates within the context of the thesis. The stability and reactivity of olefinic and aromatic hydrocarbons in zeolites are discussed, after which the scope of the thesis is outlined. Chapter 2 covers the choice of zeolites and chemicals, as well as the methodological aspects of the work. The topologies of the zeolites ZSM-5 (MFI), SSZ-13 (CHA), and ferrierite (FER) used in this thesis are first described, followed by the description of the characterization methods. The experimental methodology, including the transient approach and feed-switching protocols, as well as the setups used to study the hydrocarbon pool over the zeolite samples, are then introduced. In brief, the zeolites and hydrocarbon species are analyzed under reactive conditions using operando infrared and UV-Vis spectroscopy, as well as powder X-ray diffraction, while the gas phase composition is analyzed by online gas chromatography (GC). Chapter 3 deals with the study of the correlation between adsorbed hydrocarbon species and product distribution during the reaction of methanol over ZSM-5, SSZ-13, and ferrierite zeolites. Using transient diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) coupled with online GC, the evolution of the adsorbed species along a sequence of methanol vapor pulses can be followed and quantitatively related to the parallel growth of product concentration. Polymethylbenzenium ions and alkyl-substituted cyclopentenyl cations are the main carbenium ions identified over ZSM-5 and SSZ-13. The polymethylbenzenium ions correlate with the formation of ethene, propene, and propane, which defines the catalytic role of the ions in short olefin formation. Alkyl-substituted cyclopentenyl cations, on the other hand, show a possible correlation with olefin concentrations only at the initial stage of the reaction and only over ZSM-5. Ferrierite exhibits a hydrocarbon pool of aliphatic oligomers that play a catalytic role in the evolution of olefins in the C2-C6 range, with high selectivity to C4 and C5, but concurrently causing premature deactivation. In Chapter 4, time-resolved powder X-ray diffraction (PXRD) is used to follow the changes in the unit cell dimensions of ZSM-5, SSZ-13, and ferrierite zeolites along the HCP formation. When methanol contacts the zeolite, it induces oscillations of the lattice parameters, i.e., expansion and/or contraction along the unit vectors of the three zeolites. The ZSM-5 lattice contracts when methanol is present, which is counteracted by its expansion once the methanol is flushed out. SSZ-13 expands along the c-axis and contracts along the a-axis with methanol pulsing, then contracts in both directions upon removal. Ferrierite shows unit cell expansion followed by contraction. Adsorbed hydrocarbon species lead to an irreversible expansion along the b- and c-axes and contraction along the a-axis over ZSM-5, which follows the growth of the product concentration. Mainly an expansion along the c-axis due to the accumulation of adsorbed hydrocarbon species is observed over SSZ-13, while the changes of the unit cell volume correlate with the product evolution. A continuous expansion along all three dimensions characterizes ferrierite but appears to be for the most part uncorrelated with product evolution. Chapter 5 introduces a novel operando transient CH3OH/CD3OD switching methodology to study the multiorigin of the olefins over ZSM-5 and SSZ-13. The pulsing methodology described in Chapter 3 is modified by introducing an intermediate sequence of CD3OD pulses (5 cycles) between two sequences of CH3OH pulses (5 cycles each). The introduction of the deuterated feed perturbs the kinetic steps of the mechanism though which olefins are formed. The product concentration undergoes a deviation from the expected values during the CD3OD phase. Over ZSM-5 ethene and C3+ hydrocarbons undergo respectively a negative and positive deviation. The opposite direction of the deviation is considered a signature of a separate mechanistic origin of ethene compared to higher hydrocarbons. Such trend was also observed after reaching maximum conversion at the same conditions. Over SSZ- 13 the concentrations of the C2-C4 olefins undergo a negative deviation instead. This suggests that the C2-C4 olefins (C2-C5 after reaching maximum conversion) share the same major synthetic pathway. The adsorbed polymethylbenzenium ions over ZSM-5 and SSZ-13 and the hydrocarbon products undergo deuterium exchange that can be followed by DRIFTS and GC and put in correlation. Ethene over ZSM-5 and C2-C4 olefins over SSZ-13 are mechanistically associated to the benzenium ion while C3+ hydrocarbons over ZSM-5 originate from a different source. The results reconcile with the existence of both olefin and aromatic cycles over ZSM-5, and an (almost) solely aromatic cycle over SSZ-13. Chapter 6 focuses on the acid reactivity of carbenium ions relevant to the MTO process. Polymethylbenzenium ions and alkyl-substituted cyclopentenyl cations were synthesized over two ZSM-5 with a different ratio between the Brønsted acid sites (BAS) and the Lewis acid sites (LAS) by reaction with methanol. The acid reactivity of the two ions was studied by in situ transmission FTIR during the reaction with two bases of different strengths. During the dosage of CD3CN over the ZSM-5 containing the carbenium ions, the selective deprotonation of the polymethylbenzenium ions was observed when adding an overstoichiometric amount of CD3CN. BAS form from the deprotonation of the ions and are complexed by the titrant. Furthermore, the deprotonation is reversible upon removing the excess CD3CN. Only the reaction with the stronger base pyridine leads to the deprotonation of the alkyl-substituted cyclopentenyl cations. The higher reactivity of the benzenium ions compared to the cyclopentenyl cations can have implications on the product selectivity in the MTO process when changing the relative amount of the ions. When polymethylbenzenium ions are favored, the C3+ hydrocarbons/ethene ratio is lower. Chapter 7 focuses on the possibility that HCP species require specific zeolite-related characteristics to express catalytic activity. The pretreatment of ZSM-5 (Si/Al ratios of 11.5 and 40) with C6–C7 olefins using a temperature program leads to the formation of adsorbed aliphatic oligomers, as well as cyclic and acyclic carbenium ions. These species, identified by DRIFT and DRUV-Vis spectroscopy, are distributed within the internal voids and experience different spatial environments. The as formed HCP species lead to the perturbation of the totality of the BAS available. The presence of HCP species before reaction with methanol shortens the induction time in products formation when the zeolites are tested in the MTO process with the pulsing method described in Chapter 2. However, the product selectivity of the untreated and pretreated zeolites does not differ significantly. Additionally, when the reaction is performed with CD3OD, only a fraction of the preformed HCP species can undergo the deuterium exchange. Therefore only hydrocarbons featuring precise zeolite-related characteristics are catalytically active. The shape selectivity imposed by the topology related to the reaction environment inside the internal voids can limit the catalytic function of certain hydrocarbons. Hence, not only is the nature of the HCP defined by the zeolite topology (Chapter 3), but its ability to react in a catalytic manner can also be governed by it. Chapter 8 summarizes and integrates the results from the previous chapters to outline the main conclusions of the thesis. Chapter 9 presents the outlook based on the experimental results of the thesis. The outlook includes the need for additional experimental or computational work to identify and disentangle the main effects when CD3OD is used in experiments such those of Chapter 5 and which, if satisfied, would provide a theoretical basis to quantitatively interpret the results of a CH3OH/CD3OD switching protocol. As a continuation of Chapter 7, it is desirable to investigate the spatial relationship between the type of HCP species (e.g., aliphatic oligomers or cyclic carbenium ions), the various Al T-sites in whose proximity the catalytic hydrocarbons may be found, and the internal volume available for reaction associated with them. Furthermore, a protocol to study the effect of the BAS/LAS ratio in ZSM-5 on the product selectivity is described. An experimental approach to reversibly modify the available BAS number over ZSM-5 is also proposed to stir on purpose the product selectivity of the reaction. Finally, an exploratory study is presented on the use of carbenium ions in zeolites for the activation of small molecules.
Publications 1 - 2 of 2