Structure and Catalytic Characterization of a Second Framework Al(IV) Site in Zeolite Catalysts Revealed by NMR at 35.2 T

Abstract

Ultra-high field 27 Al{ 1 H} 2D correlation NMR experiments demonstrate that at least two framework Al(IV) sites with hydroxyl groups can exist in acidic zeolite catalysts in their dehydrated and catalytically active states. In addition to the known Al(IV) at the framework bridging acid site (BAS), a new site created by a second tetrahedral Al atom and its hydroxyl group protons in zeolite HZSM-5 are clearly resolved at 35.2 T field strengths, enabled by recently developed series-connected hybrid (SCH) magnet technology. Coupled with computational modeling, extensive 27 Al MQMAS experiments at multiple field strengths, and 1 H MAS NMR experiments, these data indicate that this second tetrahedrally-coordinated Al site (denoted Al(IV)-2) experiences an increased chemical shift and unique quadrupolar parameters relative to the BAS in both dehydrated and hydrated states. These new experimental data, supported by computational and catalytic reaction work, indicates that the second site arises from partiallybonded framework (SiO) 4-n -Al(OH) n species that significantly increase catalyst reactivity in benzene hydride-transfer and n-hexane cracking reactions. Al(IV)-2 sites result either from framework crystallization defects or from incomplete post-synthetic hydrolysis of a framework Al, prior to the formation of extraframework Al. Populations of this second acidic proton site created by the Al(IV)-2 species are shown to be controlled via post-synthetic catalyst treatments, should be general to different catalyst structures, and significantly enhance catalyst reactivity in the cited probe reactions when they are present. The results herein communicate the highest magnetic field strength data on active zeolite catalyst structures to date and enable for the first time the detection of Al and H association on a dry HZSM-5 catalyst, i.e., under conditions representative of typical end-use processes.

Recent Advances of Solid‐State NMR Spectroscopy for Microporous Materials

“…Higher fields also improve the resolution, which is proportional to B 0 and B 0 2 for spectral dimensions corresponding to spin-1/2 and half-integer quadrupolar nuclei, respectively. Thanks to the availability of high-field (>18.8 T) NMR spectrometers, the NMR spectra with significantly enhanced spectral resolution were recently reported for halfinteger quadrupolar nuclei in microporous materials such as 27 Al in zeolite, [19] 25 Mg and 67 Zn in MOFs. [20] Further gain in sensitivity can be obtained using Carr-Purcell Meiboom-Gill (CPMG) detection or its QCPMG variants for quadrupolar nuclei.…”

A Practical Review of NMR Lineshapes for Spin-1/2 and Quadrupolar Nuclei in Disordered Materials

“…Figure 4 shows a few typical lineshapes in DQ–SQ correlation NMR spectra caused by disorder. Figure 4 a,c,e is the 1D MAS spectra of three different materials, namely, 31 P NMR for N , N -bis(diphenylphosphino)- N -((S)-R-methylbenzyl) amine [ 13 ], 13 C NMR for 10% carbon-13 labeled cellulose extracted from wood [ 13 ] and 1 H NMR for dehydrated zeolite catalyst HZSM-5 [ 8 ]. Figure 4 b,d,f are their associated 2D DQ–SQ spectra.…”

“…Disordered structures are found in a wide variety of solid-state materials, such as glasses [1,2], polymers [3,4], batteries [5,6], solid state catalysts [7,8], metal-organic frameworks [9,10], etc. Many of these materials have partial or complete crystallinity, and are therefore typically characterized using scattering and diffraction techniques.…”

“…Zeolite is usually known as a crystalline material with microporous framework structure, but the location of active sites and amorphous protonic contents can be considered as disordered on the environments to the nuclei under investigation. In addition, the existence of numerous distinct crystalline inequivalent sites, for example as many as 12 in the case of MFI zeolites [ 31 , 32 ], can have similar effects and consequences to 27 Al and 17 O NMR [ 8 , 33 , 34 ], when not resolved, as distributions of isotropic chemical shift and quadrupolar coupling parameters from disordered structures. DFT (Density Functional Theory) calculations have been used to directly relate NMR parameters to structural differences and variations, modeled either as many distinct crystalline sites or continuous distributions from disorder.…”

The accuracy challenge of the DFT-based molecular assignment of13C MAS NMR characterization of surface intermediates in zeolite catalysis

“…4 This technique is being actively used in combination with 27 Al 3Q MQMAS NMR spectroscopy for the investigation of Al atom distribution throughout the zeolite framework. 4,5 However, this has not yet been extensively applied for the mechanistic studies of hydrocarbon conversion on zeolites, raising, therefore, concerns regarding the accuracy and spectral resolution of the DFT-based assignments. There are a number of issues that could limit the accuracy of the calculated chemical shifts.…”