In Situ Solid-State ¹³C NMR Observation of Pore Mouth Catalysis in Etherification of β-Citronellene with Ethanol on Zeolite Beta

Abstract

The reaction mechanism of etherification of β-citronellene with ethanol in liquid phase over acid zeolite beta is revealed by in situ solid-state (13)C NMR spectroscopy. Comparison of (13)C Hahn-echo and (1)H-(13)C cross-polarization NMR characteristics is used to discriminate between molecules freely moving in liquid phase outside the zeolite and molecules adsorbed inside zeolite pores and in pore mouths. In the absence of ethanol, β-citronellene molecules enter zeolite pores and react to isomers. In the presence of ethanol, the concentration of β-citronellene inside zeolite pores is very low because of preferential adsorption of ethanol. The etherification reaction proceeds by adsorption of β-citronellene molecule from the external liquid phase in a pore opening where it reacts with ethanol from inside the pore. By competitive adsorption, ethanol prevents the undesired side reaction of β-citronellene isomerization inside zeolite pores. β-citronellene etherification on zeolite beta is suppressed by bulky base molecules (2,4,6-collidine and 2,6-ditertiarybutylpyridine) that do not enter the zeolite pores confirming the involvement of easily accessible acid sites in pore openings. The use of in situ solid-state NMR to probe the transition from intracrystalline catalysis to pore mouth catalysis depending on reaction conditions is demonstrated for the first time. The study further highlights the potential of this NMR approach for investigations of adsorption of multicomponent mixtures in general.

Ethanol conversion into 1,3-butadiene over a mixed Hf-Zn catalyst: A study of the reaction pathway and catalyst deactivation

“…17.6 eV (Fig. 11b) which is consistent with Hf 4+ species in HfO2 interacting with silica [56][57][58]. Unlike Zn, the Hf4f XP spectra for HfZnused and HfZnregen were very similar to that of HfZnfresh (Fig.…”

“…The nature of the carbon species present in the used sample was investigated by 1 H-to- 13 ppm are attributed, respectively, to the methyl carbon (CH3-) and to the carbon linked to the -OH group of ethanol [56], indicating that part of the carbon retained on the catalyst after the reaction is associated with the ethanol that remained adsorbed on the catalyst. On the other hand, the broad resonance with a maximum located at 127 ppm and the associated spinning side bands at 76 and 178 ppm are characteristic of aromatic carbons [57].…”

A Strategy for Probing the Evolution of Crystallization Processes by Low-Temperature Solid-State NMR and Dynamic Nuclear Polarization

“…5,6 In recent years, there has been increasing interest in the development and application of solid-state NMR strategies for in-situ monitoring of crystallization, particularly to establish the sequence of solid phases present as a function of time. [7][8][9][10][11][12][13][14] The time-resolution that can be achieved in in-situ solid-state NMR studies of crystallization depends on the time to record an individual spectrum of adequate quality to distinguish the different solid forms present at each stage of the process. However, even to record a simple one-dimensional solid-state NMR spectrum may require several tens of minutes (depending on isotopic abundance, the type of NMR measurement, relaxation times, magnetic field strength, etc), and the use of more sophisticated measurement techniques (e.g., two-dimensional correlation spectra, which yield more detailed structural insights) is generally not viable within the context of in-situ solid-state NMR studies of crystallization.…”

Monitoring Crystallization Processes in Confined Porous Materials by Dynamic Nuclear Polarization Solid-State Nuclear Magnetic Resonance

“…In recent years, in situ NMR spectroscopy − has been shown to be a promising strategy for investigating mechanistic aspects of crystallization processes from bulk solution, allowing the simultaneous evolution of both the liquid and solid phases to be probed as a function of time during crystallization. − The discriminating accuracy of solid-state NMR chemical shifts allows reliable identification of the solid form(s) (e.g., polymorphs) present at different stages during crystallization. − However, as transient solid forms are typically present in very low amounts, short-lived intermediate phases may be undetectable in in situ NMR studies due to the intrinsically low sensitivity of the NMR technique.…”