Tetrahedral Framework Aluminum Research Articles

Dynamic Evolution of Aluminum Coordination Environments in Mordenite Zeolite and Their Role in the Dimethyl Ether (DME) Carbonylation Reaction

Part of tetrahedral framework aluminum in a protonic mordenite (HMOR) will convert geometry to distorted tetrahedral and octahedral coordination. High-field 27 Al NMR data show that more framework Al atoms at T3 and T4 sites change geometry to nonframework structures than others. These nonframework Al species preferentially reside in the side pockets, which will decrease the accessibility of acid sites in the 8-membered ring (MR) channel, impairing the dimethyl ether (DME) carbonylation reaction. The arisen octahedrally coordinated Al species are framework-associated, which can be reverted into the zeolite framework. Herein, we find that a facile treatment with pyridine could force the octahedral coordination Al back into a tetrahedral environment, which could increase the number of available active sites and enhance the diffusion of DME, thus improving the reactivity (4 times) of the DME carbonylation reaction and prolonging the lifetime of catalysts.

Dynamic Evolution of Aluminum Coordination Environments in Mordenite Zeolite and Their Role in the Dimethyl Ether (DME) Carbonylation Reaction

AbstractPart of tetrahedral framework aluminum in a protonic mordenite (HMOR) will convert geometry to distorted tetrahedral and octahedral coordination. High‐field 27Al NMR data show that more framework Al atoms at T3 and T4 sites change geometry to nonframework structures than others. These nonframework Al species preferentially reside in the side pockets, which will decrease the accessibility of acid sites in the 8‐membered ring (MR) channel, impairing the dimethyl ether (DME) carbonylation reaction. The arisen octahedrally coordinated Al species are framework‐associated, which can be reverted into the zeolite framework. Herein, we find that a facile treatment with pyridine could force the octahedral coordination Al back into a tetrahedral environment, which could increase the number of available active sites and enhance the diffusion of DME, thus improving the reactivity (4 times) of the DME carbonylation reaction and prolonging the lifetime of catalysts.

Acidity and Stability of Brønsted Acid Sites in Green Clinoptilolite Catalysts and Catalytic Performance in the Etherification of Glycerol

Natural zeolite clinoptilolite CLIN with a framework ratio of Si/Al ≥ 4 containing mainly potassium and calcium ions in its internal channel system was used as a starting material. The acidic HCLIN catalysts were prepared under soft conditions avoiding the use of environmental less-benign mineral acids. The starting material was ion exchanged using a 0.2 M aqueous ammonium nitrate solution at a temperature 80 °C for 2 h. The obtained NH4CLIN was converted into the acid HCLIN catalyst by calcination at 300–600 °C. The obtained samples were characterized by XRD, FTIR, SEM/TEM, AAS, and EDX element mapping. The state of aluminium and silicon was studied by 27Al- and 29SiMAS NMR spectroscopy. The textural properties of the catalysts were investigated by nitrogen adsorption and desorption measurements. The Brønsted acidity of the HCLIN catalysts was studied by temperature-programmed decomposition of the exchanged ammonium ions releasing ammonia as well as 1H MAS NMR, {1H–27Al} Trapdor, and {1H–27Al} Redor experiments. The strongly agglomerated samples were crystalline and thermally stable up to >500 °C. Although a part of the clinoptilolite framework is maintained up to 600 °C, a loss of crystallinity is already observed starting from 450 °C. The specific surface areas of the starting CLIN and ammonium exchanged NH4CLIN are low with ca. 26 m2/g. The pores are nearly blocked by the exchangeable cations located in the zeolite pores. The thermal decomposition of the ammonium ions by calcination at 400 °C causes an opening of the pore entrances and a markable increase in the specific micropore area and micropore volume to ca. 163 m2/g and 0.07 cm3/g, respectively. It decreases with further rising calcination temperature indicating some structural loss. The catalysts show a broad distribution of Brønsted acid sites (BS) ranging from weak to strong sites as indicated the thermal decomposition of exchanged ammonium ions (TPDA). The ammonium ion decomposition leaving BS, i.e., H+ located at Al–O–Si framework bridges, starts at ≥250 °C. A part of the Brønsted sites is lost after calcination specifically at 500 °C. It is related to the formation of penta-coordinated aluminium at the expense of tetrahedral framework aluminium. The Brønsted sites are partially recreated after repeated ammonium ion exchange. The catalytic performance of the acidic HCLIN catalysts was tested in the etherification of glycerol as a green renewable resource with different C1-C4 alcohols. The catalysts are highly active in the etherification of glycerol, especially with alcohols containing the branched, tertiary alkyl groups. Highest activity is observed with the soft activated catalyst HCLIN300 (300 °C, temperature holding time: 1 min). A total of 78% conversion of glycerol to mono and di ether were achieved with tert-butanol at 140 °C after 4 h of reaction. The mono- and di-ether selectivity were 75% and 25%, respectively. The catalyst can be reused.

Role of Al in Na-ZSM-5 zeolite structure on catalyst stability in butene cracking reaction

The Na-ZSM-5 catalysts (SiO2/Al2O3 molar ratio = 20, 35, and 50) were prepared by rapid crystallization method to investigate their performance in butene cracking reaction. The XRD, XRF, NH3-TPD, FT-IR, TPO, UV–Vis, and 1H, 27Al, 29Si MAS NMR techniques were used to identify the physical and chemical properties of Na-ZSM-5 catalysts. The silanol group (Si–OH) was the main acid site of Na-ZSM-5, and it was proposed to be the active site for the butene cracking reaction. The butene conversion and coke formation were associated with the abundance of silanol groups over the Na-ZSM-5 catalyst. The dealumination, resulting in the deformation of tetrahedral framework aluminum species was a key factor for Na-ZSM-5 catalyst deactivation, because of the Si–O–Al bond breaking and formation of Si–O–Si bond. The stability of the Si–O–Al bond was linked to the molar number of sodium since the Na atom interacts with the Si–O–Al bond to form Si–ONa–Al structure, which enhances the stability of the silanol group. Therefore, the Si–ONa–Al in zeolite framework was an essential structure to retain the catalyst stability during the reaction. The Na-ZSM-5 with the lowest SiO2/Al2O3 molar ratio showed the best performance in this study resulting the highest propylene yield and catalyst stability.

Understanding structure-function relationships in HZSM-5 zeolite catalysts for photocatalytic oxidation of isopropyl alcohol

The HZSM-5 molecular sieves are a kind of special photocatalyst with photocatalytic activity, while their relationship between structure and function has been a matter of debate. Here, we report that for HZSM-5, the distinguished transformation of coordination environment derived from the removal of tetrahedral framework aluminium by thermal treatment will result in a considerable change in photocatalytic behaviours. The established NMR analysis combined with the theoretical calculations demonstrates the formation of extra-framework aluminium (EFAL) upon calcinations. This leads to a changed reaction path of photocatalytic oxidation of isopropyl alcohol in gas-phase over calcined HZSM-5 catalysts. DFT calculation results show the formed of EFAL species can increase the interaction energy between reactant molecules and zeolites which affords an outstanding photocatalytic reactivity and a controllable photocatalytic selectivity in the oxidation of isopropyl alcohol. This clearly indicates that the framework aluminium (FAL) species are responsible for acetone formation, while the EFAL species are apt to promote the deep mineralization of isopropyl alcohol to inorganic CO2. This study provides new understanding in the issue of photocatalysis for zeolite molecular sieves.

Selectively convert fructose to furfural or hydroxymethylfurfural on Beta zeolite: The manipulation of solvent effects

The selective synthesis of furfural or hydroxymethylfurfural (HMF) from fructose on single catalyst (Beta zeolite, Hβ) is challenging task. However, in this study, selectivity of Hβ zeolite was discovered easily to tune by solvent effects. Strong solvent effects on the selectivity of fructose conversion were observed in different manners depending on the solvent used. It was shown that the coordinated state of framework aluminum, induced by solvent effects, has a major impact on the selectivity. The solvents with amide group were discovered to induce the reversible tetrahedral-octahedral framework aluminum transformation, but the configuration of aluminum was no influenced by other solvents such as γ-butyrolactone (GBL). Compared with other solvents, the GBL was not able to enhance turnover frequency (TOF) value of reaction but also suppress the degradation of furfural. Interestingly, a considerable yield of furfural (50.25%) was obtained combined with tetrahedral and octahedral framework aluminum active sites in GBL, while high selectivity of HMF (83.3%) was achieved in presence of single tetrahedral framework aluminum over Hβ in NMP.

Influence of Levulinic Acid Hydrogenation on Aluminum Coordination in Zeolite-Supported Ruthenium Catalysts: A 27 Al 3QMAS Nuclear Magnetic Resonance Study.

The influence of a highly oxygenated, polar protic reaction medium, that is, levulinic acid in 2‐ethylhexanoic acid, on the dealumination of two zeolite‐supported ruthenium catalysts, namely Ru/H‐β and Ru/H‐ZSM‐5, has been investigated by 27Al triple‐quantum magic‐angle spinning nuclear magnetic resonance spectroscopy (3QMAS NMR). Upon use of these catalysts in the hydrogenation of levulinic acid, the heterogeneity in aluminum speciation is found to increase for both Ru/H‐ZSM‐5 and Ru/H‐β. For Ru/H‐ZSM‐5, the symmetric, tetrahedral framework aluminum species (FAL) were found to be mainly converted into distorted tetrahedral FAL species, with limited loss of aluminum to the solution by leaching. A severe loss of both FAL and extra‐framework aluminum (EFAL) species into the liquid phase was observed for Ru/H‐β instead. The large decrease in tetrahedral FAL species, in particular, results in a significant decrease in strong acid sites, as corroborated by Fourier transform infrared spectroscopy (FT‐IR). This decrease in acidity, evidence of the inferior stability of the strongly acidic sites in Ru/H‐β relative to Ru/H‐ZSM‐5 under the applied conditions, is considered as the main reason for differences seen in catalyst performance.

Phosphorus modified hierarchically structured ZSM-5 zeolites for enhanced hydrothermal stability and intensified propylene production from 1-butene cracking

Hierarchically structured ZSM-5 zeolites (HSZ) were synthesized by a mesoporogen-free procedure and subsequently modified with varied amounts of phosphorus (1–3 wt%) through impregnation of phosphoric acid solution. Materials characterization using various techniques showed that the hierarchical structures of HSZ were well preserved after phosphorus modification, and more interestingly, their hydrothermal stability were improved significantly and the main textural properties kept almost unchanged even after hydrothermally treated at 750 °C for 4 h in 100% steam. The strong acid sites of HSZ were found to be gradually eliminated by the phosphorus induced dealumination of tetrahedral framework aluminum (TFAL), however, weak acid sites remained almost intact. In the 1-butene cracking reactions, benefitting from the auxiliary mesopores and phosphorus modification, P-modified HSZ showed remarkably improved selectivity (∼52%) and yield (∼43%) of propylene as well as superior anti-deactivation ability. All these properties of P-modified HSZ made it a promising catalyst for industrial application.

Effect of nickel on phosphorus modified HZSM-5 in catalytic cracking of butene and pentene

Nickel was introduced into phosphorus modified HZSM-5 (P/HZSM-5) with different loadings by an impregnating method. The effect of nickel loading on the structure and acidity of catalysts was investigated by XRD, N2 adsorption, XPS, NMR, and FTIR of adsorbed pyridine. Nickel interacted with pre-introduced phosphorus, and the interaction promoted the migration of some phosphorus species from channels to external surface and broke part of the AlOP bonds. This increased the content of tetrahedral framework aluminum, and restored some Brønsted acid sites which were neutralized by phosphorus modification. In the cracking of butene and pentene, the performance of catalysts was strongly dependent on nickel content. For the cracking of butene, with increasing nickel content, butene conversion and ethylene selectivity increased, while propylene selectivity first increased, passed through a maximum and then decreased with further increment of nickel content; for the cracking of pentene, with increasing nickel content, pentene conversion, ethylene selectivity and propylene selectivity increased.

Property, active species and reaction mechanism of NO and NH3 over mesoporous Fe-Al-SBA-15 via microwave assisted synthesis for NH3-SCR

A series of bimetallic Fe-Al-SBA-15 DeNOx catalysts were synthesized by microwave (MW) and conventional heating (CH) methods with various levels of iron and aluminum contents, all of which was used to identify and study the roles of aluminum and different iron species in SCR. Synergistic effect between iron and aluminum was achieved showing a satisfactory NO conversation of approximately 95% over Fe-Al-SBA-15-MW at temperature as low as 350°C. The catalysts were characterized by XRD, TEM, N2 desorption/adsorption, UV–vis DRS, 27Al MAS NMR, in situ DRIFTS and EPR. Through UV–vis DRS and 27Al MAS NMR, we found that MW method tended to produce more oligomeric FexOy clusters and tetrahedral framework aluminum than CH method. We also did some research about the reaction mechanism of NH3-SCR. Specifically, oligomeric FexOy clusters played an important role in NO oxidation. NH3 were mainly adsorpted to the tetrahedral framework aluminum and existed as NH4+ ions. Although the reaction between adsorbed NH3 and O2 occurred, the reaction rate was much slower than that between NH3 and NO+O2. Moreover, in situ EPR indicated that Fe3+ species, which did not react with pure NH3, could be easily reduced to Fe2+ by NH3+NO. When O2 was introduced along with NH3 and NO, the non-paramagnetic Fe2+ species would be reoxidezd to Fe3+, which meant that O2 participated in the reaction cycle and had a great function on reoxidization of Fe2+. Besides, the active complex NH4+(NO2) which attributed to g′∼3.3 in EPR might be the key intermediates in the SCR reaction.

Effect of Impregnating Fe into P-Modified HZSM-5 in the Coupling Cracking of Butene and Pentene

Iron-containing phosphorus-modified HZSM-5 catalysts were prepared and used for the coupling cracking of butene and pentene (mole ratio of butene to pentene = 4:1). The impact of iron content on the structure and acidity of catalysts was studied by XRD, N2 adsorption, UV–vis spectroscopy, NMR, and FTIR of adsorbed pyridine. The results revealed that iron interacted with phosphorus and increased the content of tetrahedral framework aluminum after impregnating iron into P/HZSM-5. Such interaction recovered partial Brønsted acid sites which were previously neutralized by phosphorus modification, and introducing iron created some Lewis acid sites, significantly promoting the concentration and strength of acid sites. In the coupling cracking of butene and pentene, the addition of iron into phosphorus-modified HZSM-5 significantly increased the conversions of butene and pentene. As iron loading increased from 0 to 2 wt %, butene conversion and pentene conversion increased by 26.8% and 45.8% at 550 °C, weight hourly space velocity = 6 h–1, steam ratio = 0.36 g/g, and time-on-stream = 4 h. With increasing iron loading in phosphorus modified HZSM-5, ethylene selectivity increased while propylene selectivity decreased. The catalytic performance of catalyst was strongly influenced by reaction conditions in the cracking. Proper reaction conditions could inhibit the secondary reactions and enhance propylene selectivity.

Methane Aromatization over Cobalt and Gallium -Impregnated HZSM-5 Catalysts

The influence of the catalyst acidity, the ratio of cobalt in the catalyst on the conversion of methane and the stability were evaluated using a fixed-bed microreactor at atmospheric pressure and at a flow rate of 1500 mL/g h (GHSV 600 h−1). The reaction was conducted at 973 K and 1023 K over gallium and cobalt -impregnated HZSM-5 catalysts. The 2%Ga–2% Co/HZSM-5 catalyst exhibited remarkable stability with no significant deactivation for 100 h on stream, and yielded a maximum conversion of methane to benzene equal to 9.9%. These catalysts were thoroughly characterized using XRD, N2 adsorption measurements, TPD of NH3 and FT-IR. The acidity changes severely affected aromatization, and resulted in drastic modifications in product distribution. From this work, we found that only a small fraction of tetrahedral framework aluminum, which corresponds to the Bronsted acid sites, is sufficient to accomplish the aromatization of the intermediates in methane aromatization reaction, while the superfluous strong Bronsted acid sites, which can be decreased by adding Ga and Co, are shown to be related with the aromatic carbonaceous deposits on the catalysts. After adding Ga and Co the strength of Lewis acid sites of the catalyst increased. But the total amount of the acidity on the catalyst decreased.

Spectral studies on the interaction of acetylacetone with aluminum-containing MCM-41 mesoporous materials

Diffuse reflectance spectroscopy (DRS) was used to study the interaction of acetylacetone (acac) with the mesoporous aluminum-containing MCM-41 materials. A room temperature synthesis method was used for preparation of purely siliceous MCM-41 and for aluminum-containing MCM-41 materials. Samples with Si/Al ratios of 50, 20, 10 and 5 were synthesized. The synthesized mesoporous materials possess highly ordered structure and high surface area as evidenced from X-ray diffraction and nitrogen physisorption measurements, respectively. The treatment of the as-synthesized aluminum-containing MCM-41 samples with acac shows a distinct band at ≈290 nm. This band is assigned to six coordinated aluminum atoms in the structure which is produced by diffusion of acac molecules through surfactant micelles and their interaction with aluminum atoms. The 290-nm band disappears upon several successive washing of the sample with ethanol. The treatment of the calcined aluminum-containing MCM-41 sample with acac produces the same 290-nm band where its intensity increases with the aluminum content of the sample. The intensity of this band is reduced upon successive ethanol washing, but remains nearly constant after three times washing. This irremovable aluminum species can be assigned to framework aluminum. The measured acidity for our aluminum-containing MCM-41 samples correlates linearly with the intensity of 290-nm band for the ethanol treated samples. This supports the idea that the Brønsted acidity in aluminum-modified MCM-41 samples is a function of the amount of tetrahedral framework aluminum in the structure.

Aromatization of methane by using propane as co-reactant over cobalt and zinc-impregnated HZSM-5 catalysts

The activities of the cobalt and zinc-impregnated HZSM-5 catalysts to the non-oxidative conversion of propane (C3) and methane (C1) into aromatic hydrocarbons were evaluated using a fixed-bed microreactor. C1 conversion reached 36.7% and the selectivity of aromatic products reached above 88.7% at atmospheric pressure, weight (hourly) space velocity (WHSV) 1.6 g h −1/(g cat) −1 and 873 K. The influence of the acidity and the ratio of cobalt in the catalyst on the conversion of methane and propane was evaluated. C1 incorporation was conclusively confirmed by the mass spectral analyses of aromatic products produced in a run with 13CH 4 which shows a significant 13C enrichment in the C 6H 6 +, C 7H 8 + and C 8H 10 + fragments. The methane activation could result from its hydrogen-transfer reaction with alkenes. These catalysts were thoroughly characterized using XRD, N 2 adsorption measurements, TPD of NH 3, and FT-IR. The results showed that the activation of methane in low temperature was due to existence of propane. The acidic changes and micropore area of the catalyst severely affected aromatization, and resulted in drastic modifications in product distribution. From this work, we found that only a small fraction of tetrahedral framework aluminum, which corresponds to the Bronsted acid sites, is sufficient to accomplish the aromatization of the intermediates in methane and propane aromatic reaction, while the superfluous strong Bronsted acid sites, which can be decreased by adding Co and Zn, are showed to be related with the aromatic carbonaceous deposits on the catalysts. The density of acidic site and the strength of strong acid decreased when the concentration of Co and Zn in the catalyst increased. Therefore, a much higher benzene yield and a longer durability of the catalysts are obtained when compared with the conventional HZSM-5 catalysts.

Effect of phosphorus on HZSM-5 catalyst for C 4-olefin cracking reactions to produce propylene

A series of HZSM-5 catalysts with various phosphorus (P) loadings (0–2.1 wt%) were prepared by impregnation of phosphoric acid. The effect of phosphorus on the structure and acidity of HZSM-5 catalyst and its catalytic performance for C 4-olefin cracking reactions were studied by XRD, N 2 adsorption, NMR, NH 3-TPD, FTIR spectra of adsorbed pyridine, TGA, and catalytic tests. The results revealed that the structure of HZSM-5 samples was scarcely damaged with increasing P loadings, whereas strong acidity of them was weakened mainly due to the dealumination of tetrahedral framework aluminum (TFAL) after P modification. The results of C 4-olefin cracking reactions showed a great increment of the propylene selectivity for the P-modified HZSM-5 with the optimal propylene yield at a P-loading of 1.5 wt%. It was found that both coke deposition and dealumination were the crucial factors in the deactivation of HZSM-5. Due to the dealumination and thus elimination of part of the strong acid sites, P-modified HZSM-5 showed excellent anticoking ability. TFAL of P-modified HZSM-5 changed mainly to distorted tetrahedral aluminum (DTAL) or pentacoordinated aluminum (PAL) species in the process of C 4-olefin cracking reaction, which can still act as stable active centers for C 4-olefin cracking to produce propylene. Thus, P-modified HZSM-5 shows excellent stability. A possible reaction pathway for C 4-olefin cracking reactions has been proposed based on the reaction results.

Modification of H-ZSM-5 zeolites with phosphorus. 1. Identification of aluminum species by 27Al solid-state NMR and characterization of their catalytic properties

Aluminum species in P-ZSM-5 zeolites were identified and quantified using 27Al MAS and MQMAS NMR methods. Samples containing between 0% and 15% P 2O 5 were studied after calcination or calcination followed by steaming. In addition to the tetrahedral framework aluminum, Al tet-f, and the octahedral aluminum, Al oct, we also observed significant quantities of highly distorted aluminum with tetrahedral coordination, Al tet-dis, and octahedrally coordinated aluminum in aluminum phosphate, Al oct-O-P. Furthermore, small quantities of the pentahedrally coordinated species Al pent-O-P were seen in samples with high phosphorus content. There was a good correlation between the amount of framework aluminum and acidity measured by TPD of n-propylamine. In contrast, the aluminum phosphate was shown to be catalytically inactive both by acidity measurements and catalytic tests.

Spectroscopic and XRD characterisation of zeolite catalysts active for the oxidative methylation of benzene with methane

The benzene methylation with methane over zeolite catalysts was previously shown in our laboratory to require the presence of oxygen. Thus, a two-step mechanism involving the intermediate formation of methanol by partial oxidation of methane followed by the methylation of benzene with methanol in the second step, was postulated. This paper now reports the results of the characterisation of the zeolite catalysts used for the oxidative benzene methylation reaction in order to provide some information about their composition, structure, properties and their behaviour before and after the reaction. The catalysts were characterised by X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray fluorescence (XRF), FT-IR and solid state NMR. XRD results indicate that the crystalline structures of all the ZSM-5 and H-beta catalysts remained unchanged after batch reaction of benzene with methane over the catalysts in agreement with the observation that the catalysts recovered from the reactor could be reused without loss of activity. Elemental analyses and FT-IR data show that as the level of metal ion exchange increases, the Brönsted acid concentration decreases but this metal ion exchange does not totally remove Brönsted acidity. FT-IR results further show that only a small amount of acid sites is actually necessary for a catalyst to be active since used catalysts containing highly reduced Brönsted acidity are found to be reusable without any loss of their activity. 29 Si and 27 Al magic angle spinning (MAS) NMR together with FT-IR spectra also show that all the active zeolites catalysts contain some extra-framework octahedral aluminium in addition to the normal tetrahedral framework aluminium. The presence of this extra-lattice aluminium does not, however, have any adverse effect on the crystallinity of the catalysts both before and after oxidative benzene methylation reaction. There appears also to be no significant dealumination of the zeolite catalysts during reaction since their catalytic performance was retained after use.

Remarkable Improvement on the Methane Aromatization Reaction: A Highly Selective and Coking-Resistant Catalyst

An effective way (without addition of any oxidative gases to the reactant) to suppress coke formation (by modification of the Mo/HZSM-5 catalyst) and thus to significantly improve the aromatic selectivity in the methane aromatization reaction is presented in this study. A greater yield of benzene and an enhanced durability of the catalyst when compared with the conventional Mo/HZSM-5 catalysts are observed. Multinuclear solid-state NMR, TPD, TPO, TG, and UV-Raman methods are applied to characterize and rationalize the structure-property relationship of the improved catalytic performance of the modified catalysts. From this work, we found that only a small fraction of tetrahedral framework aluminum, which corresponds to the Bronsted acid sites, is sufficient to accomplish the aromatization of the intermediates in methane aromatization reaction, while the superfluous strong Bronsted acid sites, which can be removed upon steaming treatment, are shown to be related with the aromatic carbonaceous deposits on the catalysts.

Stable and Selective Dehydrocondensation of Methane Towards Benzene on Modified Mo/HMCM-22 Catalyst by the Dealumination Treatment

A modified Mo/HMCM-22 catalyst by the dealumination treatment (Mo/HMCM-22-D) exhibited remarkable performance for the catalytic dehydrocondensation of methane with a higher selectivity of benzene and a lower selectivity of coke, in comparison with the same Mo catalyst supported on parent HMCM-22 (Mo/HMCM-22). Excellent catalytic stability as well as a high benzene formation rate of 1500 nmol/(g-cat·s) was obtained on a 6%Mo/HMCM-22-D catalyst at 1023 K, 3 atm and 2700 ml/(g·h) owing to the efficient suppression of coke formation. Dealumination of the HMCM-22 zeolite was characterized by XRD, 27Al and 1H MAS NMR and NH3-TPD techniques. It was found that the dealumination treatment of HMCM-22 zeolite resulted in an effective suppression of acid sites, particularly the Bronsted acid sites (proton form in Al--O--Si) owing to the removal of tetrahedral framework aluminum, while the microporous structure and the zeolite framework remained unchanged. It was suggested that the stable and selective dehydrocondensation of methane towards benzene is based on the suppression of coke formation owing to the effective decrease of strong Bronsted acid sites by the dealumination treatment of the HMCM-22 zeolite.

MAS NMR Studies on the Dealumination of Zeolite MCM-22

The dealumination of Zeolite MCM-22 by calcination or hydrothermal treatment was investigated by conventional multinuclear solid-state NMR and checked by an ultrahigh-field NMR experiments (19.6 T) with a very fast spin rate (19.1 kHz). The presence or variation of different species (e.g. silanol nest, 4-coordinated framework, 6-coordinated and 5-coordinated extraframework aluminum) during or after dealumination was detected, and their changes were followed. Four tetrahedral framework aluminum sites were identified upon the 27Al MAS NMR experiments showing chemical shifts at 61.5, 57.0, 53.5, and 50.0 ppm, respectively. After hydrothermal treatment, aluminum at 57.0 ppm which corresponded to T3, T4, T7, T10, and T12 sites in the orthorhombic model is preferentially expelled from the zeolitic framework. Moreover, a dehydroxylation process without the release of four-coordinated framework Al also happened, resulting in the presence of a tricoordinated framework aluminum species, which, together with the formation of tetrahedral framework aluminum species, may serve as the Lewis acid site of the zeolite.