NMR Chemical Shift and Quadrupolar Interaction Parameters of Carbon‐Coordinated 27Al in Aluminium Carbide,Al4C3

Aluminium carbide, Al₄C₃, was characterised by ¹³C and ²⁷Al solid‐state NMR spectroscopy. The ¹³C NMR spectra display two resonances with an intensity ratio of 1:2, which is in agreement with the reported crystal structure. The ²⁷Al NMR spectra of Al₄C₃ under both static and MAS conditions were deconvoluted into two spectral components, belonging to the two aluminium species Al1 and Al2 in the crystal structure of Al₄C₃. The spectral fit allowed for determination of the relatively large quadrupolar coupling constants (χ ≈? 16 MHz) of both ²⁷Al species. One aluminium species displayed a tendency of having a χ of slightly smaller magnitude compared to the other. By carrying out DFT calculations of the EFG tensor at the ²⁷Al sites using the Wien2k software, we could tentatively assign the smaller χ site to be the crystallographic Al1 species. Also, the isotropic chemical shift for the carbon‐coordinated aluminium in Al₄C₃ could be determined, being in the range of 111 to 120 ppm. This is somewhat larger than those shift values observed for ²⁷Al in nitrogen coordination.     

Interactions of phosphorous molecules with the acid sites of H-Beta zeolite: Insights from solid-state NMR techniques and theoretical calculations

The local structures of various Brønsted and Lewis acid sites in H-Beta zeolite were resolved with the combined ³¹P MAS NMR, ³¹P–²⁷Al TRAPDOR NMR experiments and theoretical calculations at different levels. In addition, the interacting mechanisms of these acid sites with probe molecules such as trimethylphosphine (TMP) and trimethylphosphine oxide (TMPO) were clarified, which greatly aids the understanding of acid catalysis. Owing to the narrow chemical shift range and close Brønsted acid strengths, only an average resonance at −4.5 ppm was observed in TMP adsorbed H-Beta zeolite, consistent with the calculated data of acidities (substitution energies and proton affinities), geometries, adsorption energies as well as ³¹P chemical shifts. However, two types of Brønsted acids were distinguished by TMPO, and the HF/DZVP2 (MP2/DZVP2) chemical shifts were calculated at 68.1 (69.5) and 69.7–72.1 (71.7–74.9) ppm, respectively. Two types of Lewis acids were identified at −32.0 and −47.0 ppm with the latter exhibiting strong ³¹P–²⁷Al TRAPDOR effects. With theoretical calculations, these two peaks were attributed to the extra-lattice oxo-AlOH²⁺ species and the three-fold coordinated lattice-Al, extra-framework Al(OH)₃, oxo-AlO⁺ species, respectively. The peak at −60.0 ppm was conventionally assigned to the TMP physisorption, but the calculations indicated that the EFAL monovalent Al(OH)₂⁺ species coordinating with two lattice-O atoms near the framework Al atom can contribute to it as well.     

“NMR invisible” aluminum species in dealuminated mordenite zeolite: a1H/27 AI TRAPDOR NMR study

Various kinds of aluminum species in dealuminated mordenite were investigated in detail, and the quadrupole coupling constants (QCCs) for aluminum atoms associated with these species were obtained by means of the newly introduced¹H/²⁷ AI TRAPWR method as well as²⁷Al magic angle spinning (MAS) nuclear magnetic resonance (NMR). QCC values of 11.3, 15.3, 13.3 and (14.0± 0.6) MHz were determined from the TRAPDOR profiles for Lewis acid sites, Bronsted acid sites (SiOHAl) and two kinds of non-framework aluminum species Al(OH) _n , respectively. The source of the “invisible Al” is discussed on the basis of the NMR experimental results.     

Anomalous lattice parameter increase in alkali earth aluminium substituted tungsten defect pyrochlores

The structures of the defect pyrochlores AAl_0.33W_1.67O₆ where A=K, Rb or Cs have been investigated using X-ray and neutron powder diffraction methods as well as the ab initio modelling program VASP. The three cubic pyrochlores exhibit a non-linear increase in lattice parameter with respect to ionic radius of the A cation as a consequence of displacive disorder of the A-type cations. Solid state ²⁷Al MAS NMR studies of this pyrochlore system reveal shifts in the δ∼21-22 ppm range that are indicative of pseudo-5 coordinate Al environments and emanate from distorted Al octahedral with one abnormally long Al-O bond. Solid state ³⁹K, ⁸⁵Rb, ⁸⁷Rb and ¹³³Cs MAS and static NMR studies reflect the local cation disorder demonstrated in the structural studies.     

Effect of phenyl group on the structure and formation of transitional alumina from Al (OPh)<Subscript>3</Subscript>

The structure of freshly prepared Al(OPh)₃, its decomposition product, the hydrolyzed products and their structural evolution were investigated employing ²⁷Al MAS NMR spectroscopy, PXRD, TGA/DTA/DSC/FTIR techniques. In the ²⁷Al MAS NMR spectrum of the aluminium phenoxide, three signals with the chemical shift at 3.78, 21 and 45 ppm were observed. The chemical shift at 3.78 and 45 ppm revealed the presence of four and sixfold coordinated aluminum. The signal at 21 ppm corresponded to fivefold coordinated aluminium. When the aluminium phenoxide was directly decomposed in air at 600 °C, it resulted in amorphous product as evidenced from the PXRD pattern. The observed signals with chemical shifts at 10.1, 42, 73.6 ppm in the ²⁷Al MAS NMR spectrum indicated the presence of 6, 5 and 4 coordination for the aluminium atoms suggesting disordered transitional γ-alumina to be the product. The hydrolysis studies of Al(OPh)₃ with excess of water at 70 °C yielded bohemite (γ-AlOOH). The alumina obtained after dehydration at 600 °C was X-ray amorphous. The dehydrated product at 600 °C showed the presence of four and six coordinated aluminium atoms in the ²⁷Al MAS NMR spectrum confirming it to be ordered γ-Al₂O₃. Crystalline γ-Al₂O₃ was obtained on further heating at 800 °C.     

Preferential Siting of Aluminum Heteroatoms in the Zeolite Catalyst Al‐SSZ‐70

The adsorption and reaction properties of heterogeneous zeolite catalysts (e.g. for catalytic cracking of petroleum, partial oxidation of natural gas) depend strongly on the types and distributions of Al heteroatoms in the aluminosilicate frameworks. The origins of these properties have been challenging to discern, owing in part to the structural complexity of aluminosilicate zeolites. Herein, combined solid‐state NMR and synchrotron X‐ray powder diffraction analyses show the Al atoms locate preferentially in certain framework sites in the zeolite catalyst Al‐SSZ‐70. Through‐covalent‐bond 2D ²⁷Al{²⁹Si} J‐correlation NMR spectra allow distinct framework Al sites to be identified and their relative occupancies quantified. The analyses show that 94 % of the Al atoms are located at the surfaces of the large‐pore interlayer channels of Al‐SSZ‐70, while only 6 % are in the sub‐nm intralayer channels. The selective siting of Al atoms accounts for the reaction properties of catalysts derived from SSZ‐70.     

介孔硅铝酸盐的合成及其在傅克反应中的催化性能

采用溶胶凝胶法合成了一系列有序性好且酸性较强的介孔硅铝酸盐材料。利用X射线粉末衍射(XRD)、透射电镜(TEM)、27Al核磁共振(27Al NMR)、氨气程序升温脱附(HN3-TPD)及吡啶吸附红外光谱(Py-FT-IR)对制备的介孔硅铝酸盐材料的结构和性能进行表征,并考察了材料在苯甲醚和苯甲醇的傅克烷基化反应中的催化活性。实验结果表明:合成过程中,表面活性剂的用量、硅铝物质的量之比会影响材料结构的有序性,醋酸用量对材料结构有序性影响很小;进一步研究结果表明,nSi/nAl比会影响材料的酸催化活性,当nSi/nAl=10时材料的酸催化活性最高。氨气程序升温脱附和吡啶吸附红外光谱表明nSi/nAl=10的材料含有最多的B酸酸量。     

Reactivity of Extra‐framework Species of USY Zeolites in Alkaline Medium

Zeolites of type USY (ultra‐stable Y) were obtained by steaming of NH₄NaY modification. Samples were modified by subsequent alkaline treatment in KOH solution. USY and USY‐KOH were characterised by chemical element analysis, XRD, IR, ²⁹Al and ²⁹Si MAS NMR spectroscopic measurements. Correct silicon to aluminium ratios (Si/Al) were determined by XRD and IR (double ring vibration w_DR) data whereas values calculated according to data of ²⁹Si MAS NMR and IR spectroscopy (asymmetrical TOT valence vibration w_TOT) appeared to be too high., In the latter case, the signals of the zeolite framework were strongly superimposed by that of extra‐framework silica gel (EFSi) formed during steaming. It was found that alkaline leaching induces desilication of silicon‐rich area of the zeolite framework and partial dissolution of EFSi. Silicate ions of both react with likewise dissolved extra‐framework aluminium (EFAl) to form X‐ray amorphous aluminosilicate. Consequently, the superposition of the ²⁹Si MAS NMR signals of the zeolite framework by silica gel was reduced for Q⁴(0Al) but increased for Q⁴ (2Al) and Q⁴(3Al) structure units. A reinsertion of EFAl into the zeolite framework has not been observed.     

Solid state NMR characterization of formation of poly(ϵ‐caprolactone)/maghnite nanocomposites by in situ polymerization

Results of multinuclear MAS NMR spectroscopy are reported for poly (ε‐caprolactone)/maghnite nanocomposite formation, with ε‐caprolactone in situ polymerized in the presence of maghnite, a proton exchanged montmorillonite clay. Exfoliated and intercalated materials with different maghnite loading in the range 3–15 wt % were investigated. ¹H NMR evidences Brønsted acid hydroxyl groups in the silicate layers and shows that their broad signal at 7.6 ppm present in the parent clay disappears in the nanocomposite material. ²⁷Al MAS NMR results show that beside the hexacoordinated aluminum signal, two additional peaks corresponding to two different tetrahedral Al sites are present in the clay framework. The NMR signal intensity of only one of them was found to be affected in the nanocomposites compared with the parent maghnite, suggesting that these specific aluminum sites are the reactive ones at the initial stages of the polymerization. However almost no changes occurred in the ²⁹Si NMR spectra, confirming that the polymer grafting, as indicated earlier by atomic force microscopy, took place on the aluminum tetracoordinated sites rather than on the silicon sites. A mechanism of maghnite surface catalyzed polymerization of ε‐caprolactone was proposed, involving Brønsted and Lewis acid sites. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3060–3068, 2007     

Characterisation of Siliceous Extra‐Framework Species in DAY Zeolites by 29Si MAS NMR and IR Spectroscopic Measurements

Dealuminated Y zeolites (DAY) were obtained by steaming of NH₄NaY at temperatures between 450 °C and 700 °C. They were characterised by means of ²⁷Al and ²⁹Si MAS NMR, IR spectroscopic and XRD measurements. The Si/Al framework ratios of samples were calculated using the ²⁹Si MAS NMR signal intensities, the wave numbers of the double‐ring vibration band w_DR and the asymmetrical TOT valence vibration w_TOT of IR spectra as well as the XRD lattice constant a₀. In contrast to actual Si/Al ratio obtained from w_DR and a₀, the NMR spectroscopic and w_TOT values were determined to be too high because of the superposition of the signals coming from dealuminated zeolite framework and silica gel which forms in the zeolite as a result of steaming. The differently determined Si/Al ratios characterise the siliceous extra‐framework species.     

Synthesis of mesoporous zeolite single crystals with cheap porogens

Mesoporous zeolite (silicalite-1, ZSM-5, TS-1) single crystals have been successfully synthesized by adding soluble starch or sodium carboxymethyl cellulose (CMC) to a conventional zeolite synthesis system. The obtained samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen sorption analysis, ²⁷Al magic angle spinning nuclear magnetic resonance (²⁷Al MAS NMR), temperature-programmed desorption of ammonia (NH₃-TPD) and ultraviolet-visible spectroscopy (UV-vis). The SEM images clearly show that all zeolite crystals possess the similar morphology with particle size of about 300 nm, the TEM images reveal that irregular intracrystal pores are randomly distributed in the whole crystal. ²⁷Al MAS NMR spectra indicate that nearly all of the Al atoms are in tetrahedral co-ordination in ZSM-5, UV-vis spectra confirm that nearly all of titanium atoms are incorporated into the framework of TS-1. The catalytic activity of meso-ZSM-5 in acetalization of cyclohexanone and meso-TS-1 in hydroxylation of phenol was also studied. The synthesis method reported in this paper is cost-effective and environmental friendly, can be easily expended to prepare other hierarchical structured zeolites.     

Study of the geometry and location of the bridging OH groups in aluminosilicate and silicoaluminophosphate type zeolites using 1H MAS NMR sideband analysis and CP/MAS NMR

A numerical analysis of the ¹H MAS NMR sideband pattern of the bridging OH groups in various zeolites of the aluminosilicate type (LTA, faujasite, erionite, mordenite, pentasil) and of the silicoaluminophosphate type (SAPO-5, −17, −34, −37) yields H-Al distances of the hydroxyl protons to the adjacent framework aluminium nucleus which cover a range between 0.234 and 0.252 nm. From the results, a relation between the H-Al distance and the size of the oxygen rings (6−, 8−, 10− and 12-membered oxygen rings) could be derived. Using ²⁹Si CP/MAS NMR it was shown that the bridging OH groups in zeolites of the aluminosilicate type are located at the Si(nAl) sites with the largest possible number n. While the second coordination sphere of the T positions of the bridging OH groups in zeolites of the silicoaluminophosphate type (Si/(Al + P + Si) < 0.12) is occupied by the same number of phosphorus and aluminium atoms, the bridging OH groups in zeolites of the aluminosilicate type with medium framework aluminium densities are located at the Si(nAl) sites with n = 3, 2 and 1.     

Synthesis,Structure and Properties of an Extra-Large-Pore Aluminosilicate Zeolite NUD-6

Pure silica zeolites possessing uniform micropores, large surface area and high thermal and chemical stability have been widely studied and used in the fields of fine chemicals and oil industry. The incorporation of aluminium into the framework of silica zeolites changes their properties, making them more industrially useful as adsorbents and catalysts. Herein, we report the synthesis and characterization of an extra-large-pore aluminosilicate zeolite NUD-6 with a 16-membered-ring pore channel. Aluminium was directly incorporated into the zeolite NUD-6 framework, as confirmed by ²⁷Al MAS NMR studies and ammonia temperature-programmed desorption probes. Al-NUD-6 was not stable when heated at 550 °C to remove the organic templates. However, the organic templates in Al-NUD-6 could be removed by oxidation in nitric acid at room temperature. The obtained Al-NUD-6H retained the crystalline structure and possessed both micropores and mesopores despite the occurrence of severe structural distortions due to the presence of the corner-sharing Q³ Si₂O₇ units. The incorporation of aluminium resulted in both medium and strong acid sites in Al-NUD-6H, and could facilitate its use in adsorption and catalysis.     

Effect of small amounts of Al on the surface silanol structure and their correlation to the improved catalytic performances of WOx/SiO2–Al2O3 in the propene self-metathesis

Surface silanol structures, acid properties, and tungsten dispersion of the sol-gel-derived 7W/SiO₂–xAl₂O₃ (x = 0.2–23 wt%) were investigated by means of ²⁹Si, ²⁷Al, and ¹H MAS NMR, NH₃-TPD, in-situ NH₃-IR spectroscopy, XRD, and Raman spectroscopy. The surface silanol structure changed upon Al and tungsten loadings; however, loading of 1 wt% Al₂O₃ appeared to be the threshold for preserving the Si(OH)Al with isolated bridge after impregnation of 7 wt% W. The 7W/SiO₂–1Al₂O₃ (1 wt% Al₂O₃) was also found to exhibit the lowest ratio of Bronsted to Lewis acid with the highest amount of Lewis acid sites and the best catalyst performances in propene self-metathesis at 550 °C in terms of both propene conversion and ethylene/butene selectivity. Despite its low tungsten dispersion, the metathesis activity was correlated well with the higher amount of tungsten carbene species, which were formed on the catalysts containing higher isolated bridge silanol and the presence of higher Lewis acid sites.     

Synthesis of Mesoporous Silica Immobilized with 3-[(Mercapto or amino)propyl]trialkoxysilane by a Simple One-pot Reaction

Sodium silicate from rice husk ash (RHA) was transformed to functionalized silica with 3‐(mercaptopropyl)‐trimethoxysilane (MPTMS) or 3‐(aminopropyl)triethoxysilane (APTES) via a simple sol‐gel technique in a one‐pot synthesis to give RHAPrSH and RHAPrNH₂. The ²⁹Si MAS NMR of RHAPrSH and RHAPrNH₂ showed the presence of T¹, T², T³, Q², Q³ and Q⁴ silicon centers. The ¹³C MAS NMR showed that RHAPrSH had chemical shifts at δ 16.59, 32.73, consistent with two of the carbon atoms of the MPTMS moiety, while the ¹³C MAS NMR of RHAPrNH₂ had chemical shifts at δ 14.58, 26.13, 47.87, consistent with the three carbon atoms of the APTES moiety. The presence of carbon, silicon, sulfur and nitrogen in RHAPrSH and RHAPrNH₂ was determined by a combination of elemental analysis and EDX study.     

Crystal structure and MAS-NMR study of rehydrated UiO-7

The framework of rehydrated UiO-7 has the ZON type of topology. The crystal structure is described on the basis of high resolution, synchrotron powder X-ray diffraction data. The chemical composition of the unit cell is Al₃₂P₃₂O₁₂₈·62H₂O. Its dimensions are a=14.4930(2), b=14.8731(2) and c=17.5738(3) Å; space group Pbca. Of the four non-equivalent Al atoms, two have normal tetrahedral coordination, whereas one obtains five-coordination and one achieves octahedral coordination on additional bonding to three non-equivalent water molecules. Six more water molecules are located in the channels and are interconnected via hydrogen bonds. These features were confirmed by solid-state ²⁷Al MAS NMR data. Site assignment of ³¹P signals is discussed on the basis of POAl angles and Al-coordination.     

Topology and Equilibrium Analysis of the Monovalent Aluminum Compound Al4Cp*Ph4

The theoretical structure and thermochemistry of the tetrameric, low‐valence aluminum compound Al₄Cp*^Ph₄ (Cp*^Ph = C₅Me₄Ph) is discussed. The first synthesis of this compound was reported in 2005, but the compound failed to crystallize and experimental ²⁷Al NMR results were inconclusive in regard to the degree of association. Here density functional theory combined with a genetic algorithm is used to predict the expected structure and properties for Al₄Cp*^Ph₄. Synthesis efforts were repeated for this compound, resulting in a product with a ²⁷Al NMR chemical shift that differed from the previous report by nearly 20 ppm. However, calculated ²⁷Al NMR chemical shifts for the theoretically predicted structure are within one ppm of these new experimental results, strongly suggesting the tetrameric form has been synthesized. Previous work on five Al₄R₄ (R = C₅H₅, C₅Me₄H, C₅Me₅, C₅Me₄iPr, C₅Me₄Pr) compounds showed a general trend towards an increased likelihood of disassociation into monomeric species in solution as ligand bulk increased. Analysis of Al₄Cp*^Ph₄, the sixth and bulkiest compound in this series, indicates a departure from this trend. Bonding characteristics for monomer and tetramer forms in this series are examined in detail via topological analysis to understand this trend.     

gem-Diol-Type Intermediate in the Activation of a Ketone on Sn-β Zeolite as Studied by Solid-State NMR Spectroscopy

Lewis acid zeolites have found increasing application in the field of biomass conversion, in which the selective transformation of carbonyl-containing molecules is of particular importance due to their relevance in organic synthesis. Mechanistic insight into the activation of carbonyl groups on Lewis acid sites is challenging and critical for the understanding of the catalytic process, which requires the identification of reaction intermediates. Here we report the observation of a stable surface gem-diol-type species in the activation of acetone on Sn-β zeolite. ¹³C, ¹¹⁹Sn, and ¹³C–¹¹⁹Sn double-resonance NMR spectroscopic studies demonstrate that only the open Sn site ((SiO)₃Sn-OH) on Sn-β is responsible for the formation of the surface species. ¹³C MAS NMR experiments together with density functional theory calculations suggest that the gem-diol-type species exhibits high reactivity and can serve as an active intermediate in the Meerwein—Ponndorf–Verley–Oppenauer (MPVO) reaction of acetone with cyclohexanol. The gem-diol-type species offers an energy-preferable pathway for the direct carbon-to-carbon hydrogen transfer between ketone and alcohol. The results provide new insights into the transformation of carbonyl-containing molecules catalyzed by Lewis acid zeolites.     

Elucidating the Nature of the External Acid Sites of ZSM-5 Zeolites Using NMR Probe Molecules

The identification of acid and nonacid species at the external surface of zeolites remains a major challenge, in contrast to the extensively-studied internal acid sites. Here, it is shown that the synthesis of zeolite ZSM-5 samples with distinct particle sizes, combined with solid-state NMR and computational studies of trimethylphosphine oxide (TMPO) adsorption, provides insight into the chemical species on the external surface of the zeolite crystals. ¹H–³¹P HETCOR NMR spectra of TMPO-loaded zeolites exhibit a broad correlation peak at δ_P ∼35–55 ppm and δ_H ∼5–12 ppm assigned to external SiOH species. Pore-mouth Brønsted acid sites exhibit ³¹P and ¹H NMR resonances and adsorption energies close to those reported for internal acid sites interacting with TMPO. The presence of an external tricoordinate Al-Lewis site interacting strongly with TMPO is suggested, resulting in ³¹P resonances that overlap with the peaks usually ascribed to the interaction of TMPO with Brønsted sites.     

Insight into Three‐Coordinate Aluminum Species on Ethanol‐to‐Olefin Conversion over ZSM‐5 Zeolites

Commercial bioethanol can be readily converted into ethylene by a dehydration process using solid acids, such as Brønsted acidic H‐ZSM‐5 zeolites, and thus, it is an ideal candidate to replace petroleum and coal for the sustainable production of ethylene. Now, strong Lewis acidic extra‐framework three‐coordinate Al³⁺ species were introduced into H‐ZSM‐5 zeolites to improve their catalytic activity. Remarkably, Al³⁺ species working with Brønsted acid sites can accelerate ethanol dehydration at a much lower reaction temperature and shorten the unsteady‐state period within 1–2 h, compared to >9 h for those without Al³⁺ species, which can significantly enhance the ethanol dehydration efficiency and reduce the cost. The reaction mechanism, studied by solid‐state NMR, shows that strong Lewis acidic EFAl‐Al³⁺ species can collaborate with Brønsted acid sites and promote ethanol dehydration either directly or indirectly via an aromatics‐based cycle to produce ethylene.