Alkaline treatment of iron-containing MFI zeolites. Influence on mesoporosity development and iron speciation

The effects of alkaline treatment on the mesoporosity development and iron speciation in Fe-MFI zeolites have been investigated. To this end, a variety of samples derived from different synthetic routes and having distinct Si/Al ratios and Fe content were treated in NaOH solutions and characterized by N2 adsorption, SEM, TEM, UV/vis spectroscopy, and EPR. The alkaline treatment induces a significant intracrystalline mesoporosity development by framework silicon extraction and promotes disintegration of oligomeric iron species. Iron in framework positions has shown to provoke mesopore formation, whereas nonframework iron species suppresses silicon leaching and lowers the extent of extra porosity.     

In situ monitoring of desilication of MFI-type zeolites in alkaline medium

In situ pH and Attenuated Total Reflection (ATR) infrared techniques have been successfully applied in order to gain insights into the dissolution process connected to mesopore formation occurring upon alkaline treatment of ZSM-5 zeolites. Online pH measurements reveal a similar consumption of OH(-) ions in the initial stage of the reaction independent of the Si/Al ratio of the zeolite. In view of the greatly different mesoporosity development, the extraction of polymeric silica entities is anticipated, its structure depending on the framework Si/Al ratio. In agreement, ATR-IR experiments have confirmed dissolution of polymeric silicon-containing species that in the course of the alkaline treatment disintegrate into smaller entities. A direct relation between the type of porosity developed and the process of silicon extraction as measured in the liquid phase cannot be drawn.     

A Facile Top‐Down Protocol for Postsynthesis Modification of Hierarchical Aluminum‐Rich MFI Zeolites

High aluminum content constitutes a major hurdle for the postsynthesis modification of hierarchical zeolites. A facile protocol comprising fluorination and sequential alkaline treatment is presented for the postsynthesis modification of hierarchical Al‐rich MFI zeolites. By virtue of this protocol, uniform intracrystalline mesoporosity is introduced in an Al‐rich MFI zeolite (Si/Al=14.3). The obtained hierarchical zeolites exhibit a significant mesopore size distribution, centered around 6 nm, and show improved conversions in catalytic cracking of bulky aromatic molecules. The fundamental implications of the fluorination–alkaline treatment protocol are related to the formation of F‐bearing tetrahedral aluminum species in the antecedent fluorination step, which alleviates the resistance of Al sites to the alkaline medium and causes Al?F complexation for regulated hydrolysis of the Al species during the alkaline treatment process. This top‐down protocol and the derived mechanistic understandings are expected to be applied in the synthesis of hierarchical Al‐rich zeolites with other framework topologies.     

Desilication mechanism revisited: highly mesoporous all-silica zeolites enabled through pore-directing agents

The role of pore‐directing agents (PDAs) in the introduction of hierarchical porosity in silicalite‐1 in alkaline medium was investigated. By incorporation of various PDAs in aqueous NaOH, homogenously distributed mesopores were introduced in 2.5 μm silicalite‐1 crystals. It was proven for the first time that framework aluminum is not a prerequisite for the introduction of intracrystalline mesoporosity by desilication. The pore‐directing role is not directly exerted by framework trivalent cations metals, but by species on the external surface of the zeolite. The inclusion of metal complexes (Al(OH)₄^?, Ga(OH)₄^?) and tetraalkyl ammonium cations (tetramethyl ammonium (TMA⁺), tetrapropyl ammonium (TPA⁺)) in the alkaline solution led to distinct mesopore surface areas (up to 286 m² g^?1) and pore sizes centered in the range of 5–20 nm. In the case alkaline treatment was performed in the presence of Al(OH)₄^?, all aluminum partially integrated in the zeolite giving rise to both Lewis and Brønsted acidity. Apart from the concentration and location, the affinity of the PDA to the zeolite surface plays a crucial role in the pore formation process. If the PDA is attracted too strongly (e.g., TMA⁺), the dissolution is reduced dramatically. When the pore‐directing agent is not attracted to the zeolite’s external surface, excessive dissolution occurs (standard alkaline treatment). TPA⁺ proved to be the most effective PDA as its presence led to high mesopore surface areas (>200 m² g^?1) over a broad range of PDA concentrations (0.003–0.1 M ). Importantly, our results enable to extend the suitability of desilication for controlled mesopore formation to all‐silica zeolites.     

碱处理脱硅与提高Y型分子筛硅铝比——矛盾的对立与统一

考察了 "水热处理"以及"碱处理+水热处理"两种方法所制得的超稳 Y 分子筛的骨架硅铝比、孔结构特征以及酸量, 并探讨了"碱处理+水热处理"方法对起始 NaY 分子筛的适应性. 结果表明, 在水热处理前, 对 NaY 分子筛进行碱处理脱硅可在不改变最终样品的骨架超稳化水平和酸量的同时, 样品的介孔体积显著增加. 直接水热处理 NaY 分子筛所得样品介孔体积不超过 0.14 cm3/g, 而先碱处理后水热处理, 所得样品介孔体积可达 0.22 cm3/g. 该法适用于制备骨架硅铝比高的 NaY 分子筛. 起始原料的骨架硅铝比较低时, 所得样品的介孔体积增幅小, 而且微孔受损严重.     

Variation of Aluminium Distribution in Small-Sized ZSM-5 Crystals during Desilication

Hollow ZSM-5 zeolites of size below one micrometer can be produced by desilication of crystals with aluminium zoning. The parent crystals have a core–shell structure: the core part has nearly no aluminium, whereas the aluminium content in the shell increases when extending to exterior surface. Transmission electron microscopy confirmed the preservation of the crystalline shell after base leaching, but could not identify its subtle change. An increase of the Si/Al ratio of the surface was detected upon leaching the parent material to form the hollow zeolite by using ambient pressure X-ray photoelectron spectroscopy and infrared spectroscopy of substituted alkylpyridines. ²⁷Al MAS NMR showed that base leaching results in a reduced percentage of distorted tetrahedrally coordinated aluminium. The reprecipitation of dissolved species occurs and tetrahedrally coordinated tin atoms can thus be introduced to the shell framework. Overall, the formation of hollow ZSM-5 zeolites by desilication involves not only the removal of silicon-rich core, but also a reduced percentage of exterior aluminium-related acid sites, which should be considered while using hollow zeolites in acid-catalyzed reactions.     

How Water and Ion Mobility Affect the NMR Fingerprints of the Hydrated JBW Zeolite: A Combined Computational-Experimental Investigation

An important aspect within zeolite synthesis is to make fully tunable framework materials with controlled aluminium distribution. A major challenge in characterising these zeolites at operating conditions is the presence of water. In this work, we investigate the effect of hydration on the ²⁷Al NMR parameters of the ultracrystalline K,Na-compensated aluminosilicate JBW zeolite using experimental and computational techniques. The JBW framework, with Si/Al ratio of 1, is an ideal benchmark system as a stepping stone towards more complicated zeolites. The presence and mobility of water and extraframework species directly affect NMR fingerprints. Excellent agreement between theoretical and experimental spectra is obtained provided dynamic methods are employed with hydrated structural models. This work shows how NMR is instrumental in characterising aluminium distributions in zeolites at operating conditions.     

不同脱铝深度稀土超稳Y沸石的酸性质

用FT-IR和NH_2-TPD研究了稀土含量相同时脱铝程度不同的四种REUSY沸石的酸性质. 并对复杂羟基谱的归属进行了讨论. 同时将酸性质与骨架的Si、Al分布, 非骨架组份以及二次孔相关联, 提出浅、中度脱铝时主要脱除与超笼中HFOH相关联的Si(3Al)和Si(2Al)单元中的Al, 深度脱铝时则脱除与六方柱笼和方钠石笼中LFOH 羟基相关联的Si(1AI)中的铝和少量Si(2Al)中的铝. 另外发现, 非骨架组份使一部分HFOH羟基不能被吡啶分子接近. 而二次孔的形成使一部份LFOH 能被吡啶分子接近. 随着脱铝深度的加深, 总酸、B酸、L 酸量都减少. 但强酸和B酸的强度均相应增加.     

Influence of chemical dealumination on the properties of USY zeolite

Three series of dealuminated Y zeolites have been prepared by chemical extraction ofhydrothermally dealuminated Y zeolite(USY)with H_2Na_2EDTA,HCl and H_4EDTA.The unitcell constant,mesopore distribution,acidity and extraframework aluminum(EFAL)of thezeolites were studied with XRD,chemical analyses,adsorption,IR and NH_3-TPD techniques.It was shown that H_2Na_2EDTA only removed EFAL species deposited in the pores of USY,by contrast,HCl and H_4EDTA extract both extraframework and framework aluminum,andmake the zeolite framework further dealuminated.Adsorption tests gave evidence that a second-ary pore system exists in these dealuminated zeolites.H_2Na_2EDTA extraction increased bothmicropore and mesopore volumes,but after HCl and H_4EDTA treatments,new mesopores formedand the micropore volume was decreased.The pyridine-IR and NH_3-TPD measurements demon-strated that EFAL had no evident contribution to the zeolite acidity.     

Phase Transformations in Alkaline and Acid Leached Y Zeolites Dealuminated by Steaming

The steaming of zeolite Y (here at 873 K for 7 hours) leads to the formation of an amorphous aluminium aluminosilicate in addition to the dealuminated zeolite (DAY). An alkaline treatment of DAY causes the transformation of the non‐framework phase into an alkali aluminosilicate and the partial desilication of the DAY framework. The alkali aluminosilicate is decomposed by a moderate acid leaching under the formation of silica gel. The ²⁹Si MAS NMR and IR spectra of DAY and its chemically treated modifications are superimposed by the signals of the crystalline zeolite framework and the amorphous non‐framework materials whereas XRD measurements only characterize the current state of the framework.     

Creating mesopores in ZSM-48 zeolite by alkali treatment: Enhanced catalyst for hydroisomerization of hexadecane

ZSM-48 zeolites with various Si/Al ratios were hydrothermally synthesized in the H_2N(CH_2)_6NH_2(HDA)-containing media. The obtained samples were highly crystallized with minor mixed phases as evidenced by X-ray powder diffraction(XRD). The alkaline treated ZSM-48 zeolites maintained its structure under different concentrations of Na OH aqueous solution. Micropores remained unchanged while mesopores with wide pore size distribution formed after the alkaline treatment. The surface area increased from 228 to 288 m~2/g. The Br?nsted acid sites had little alteration while an obvious increase of Lewis acid sites was observed. The hydroisomerization of hexadecane was performed as the model reaction to test the effects of the alkali treatment. The conversion of hexadecane had almost no change, which was attributed to the preservation of the Br?nsted acid sites. While high selectivity to iso-hexadecane with an improved iso to normal ratio of alkanes was due to the mesopore formation and improved diffusivity.     

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.     

Mesopore Engineering for Well-Defined Mesoporosity in Al-Rich Aluminosilicate Zeolites

Desilication has been proven an effective approach for the construction of well-defined hierarchical porosities inside zeolites with an optimal framework Al content (Si/Al=25–50). However, for the Al-rich aluminosilicate zeolites, desilication is constrained by the excess and extensive shielding effects from high Al-contents. The developments in the desilication of siliceous zeolites convey a simplified principle of controlled dissolution of the microporous matrix for the construction of hierarchical porosities, which benefits the innovation of synthetic approaches for Al-rich zeolites. The perturbations to the environments of framework Al species may alleviate the excess shielding effects. This review highlights two corresponding protocols of sequential “fluorination–desilication” and “steaming–desilication” for the construction of hierarchical porosities inside Al-rich ZSM-5 zeolites. The success of these two protocols revitalizes the prevailing understanding of the interplay between dealumination and desilication, and implies the necessity of investigating the overlooked roles of extra-framework Al species. Despite the long history and significant achievements in the last decade, fundamental understandings at the molecule level are still limited for the desilication-based top-down approaches. In particular, the investigations on Al-rich zeolites just find their growing. The bridging of dealumination and desilication is essential for other industrially relevant Al-rich zeolites (e.g., faujasite zeolites). The complexities in the inherent characters (topology, spatial distribution, proximity, etc.) and apparent parameters (morphology, crystal/particle size, etc.) demand constructive synthetic toolboxes and further fundamental understanding.     

Bridging Dealumination and Desilication for the Synthesis of Hierarchical MFI Zeolites

The rational design of zeolite‐based catalysts calls for flexible tailoring of porosity and acidity beyond micropore dimension. To date, dealumination has been applied extensively as an industrial technology for the tailoring of zeolite in micropore dimension, whereas desilication has separately shown its potentials in the creation of mesoporosities. The free coupling of dealumination with desilication will bridge the tailoring at micro/mesopore dimensions; however, such coupling has been prevailingly confirmed as an impossible mission. In this work, a consecutive dealumination–desilication process enables the introduction of uniform intracrystalline mesopores (4–6 nm) into the microporous Al‐rich zeolites. The decisive impacts of steaming step have been firstly discovered. These findings revitalize the functions of dealumination in porosity tailoring, and stimulate the pursuit of new methods for the tailoring of industrially relevant Al‐rich zeolites.     

Mesopores in USY Zeolites II

Microporous zeolites Na‐Y and K‐Y were converted into the NaNH₄‐Y and KNH₄‐Y modifications by ion exchange being active in dealumination. Removal of framework aluminium and silicon is accompanied by formation of secondary mesopores. Internal mesopores are formed in the centre of zeolite crystals and external pores at their surface. Formation of mesopores changes the sorption behaviour.Residual alkali metal cations as Na⁺ or K⁺ stabilise, however, the framework ≡Si‐O‐Al≡ bonds. Because of inhomogeneous distribution of sodium ions, in NaNH₄‐Y less internal but more external mesopores are formed. Potassium ions of KNH₄‐Y are more homogeneous distributed over the framework why a more balanced formation of secondary pores takes place.     

Effect of aluminum on the nature of the iron species in Fe-SBA-15

We report the preparation of highly ordered mesoporous Fe-Al-SBA-15 with isolated extraframework Fe species under acidic conditions. The materials were characterized by means of UV resonance Raman spectroscopy, in conjunction with BET, XRD, TEM, UV-vis, H2-TPR, FT-IR, and 27Al MAS NMR spectroscopy. The addition of both Fe and Al to the synthesis gel of SBA-15 results in the formation of isolated extraframework Fe species located close to the framework Al ions and the Fe content an order of magnitude higher than that in Fe-SBA-15 synthesized without Al. The existence of anchored extraframework Fe species was confirmed by the presence of a strong absorption band at 270 nm, hydrogen reduction at relatively low temperature, and the presence of a resonance Raman band at 1140 cm(-1). The location of Fe in close proximity to framework Al nuclei is further supported by 27Al MAS NMR measurements. Two characteristic UV Raman bands at 510 cm(-1) and 1090 cm(-1) excited by 244-nm laser are assigned to Fe-O-Si symmetric and asymmetric stretching modes of isolated tetrahedral Fe ions in the silica framework for Fe-SBA-15. The resonance Raman band at 1140 cm(-1) excited by 325-nm laser is attributed to the asymmetric stretching mode of the isolated extraframework iron species in Fe-Al-SBA-15. The isolated Fe species close to framework Al species are stable in acidic HCl solution, whereas the majority of Fe species in Fe-SBA-15 can be easily removed.     

Behavior of extraframework Fe sites in MFI and MCM-22 zeolites upon interaction with N2O and NO

We report on the characterization of an isomorphously substituted Fe-MCM-22 sample containing both Fe and Al in framework positions (Si/Fe = 44, Si/Al = 25). XANES spectroscopy was used to study the evolution of Fe sites as a consequence of thermal activation at high temperature (1073 K) and subsequent oxidation with N2O. The results were compared to those obtained in the same conditions on a well-known Fe-silicalite sample (Si/Fe = 68, Si/Al = infinity). In both samples, thermal activation causes migration of a fraction of Fe ions from framework to extraframework positions, this migration being accompanied by a reduction of Fe3+ to Fe2+. Upon oxidation with N2O at 523 K, the two samples show a different behavior. While in Fe-silicalite practically all of the Fe2+ sites formed by thermal activation are reoxidized to Fe3+, in Fe-MCM-22 only a fraction of the extraframework iron sites is involved in the reoxidation process. The accessibility of the extraframework Fe sites was also investigated by using the NO molecule as a surface probe. Upon NO dosage on the sample, the modification of the pre-edge peak and of the edge position suggests an important charge release from the extraframework Fe2+ ions to the adsorbed molecules. This could be formalized with the formation of Fe3+(NO-) complexes, compatible (on the basis of the simple molecular orbital theory) with a bent NO geometry. The formation of a complex family of Fe2+ mono-, di-, and trinitrosyl complexes was also confirmed by FTIR spectroscopy. Similarly to what was observed in the oxidation experiments, the fraction of extraframework Fe sites able to interact with NO in Fe-MCM-22 sample is smaller than that in Fe-silicalite treated in the same conditions. This trend is explained with a major clustering of extraframework Fe sites in Fe-MCM-22 sample, as was also suggested by FTIR experiments. These results suggest that the dispersion of iron in zeolitic matrixes prepared by isomorphous substitution could also depend on the zeolitic structure.     

Controlled release of Ibuprofen from dealuminated faujasites

Four dealuminated faujasite samples have been employed as matrices for Ibuprofen adsorption and in vitro drug delivery with the aim of adapting the pore size to the size of the drug molecule and to study the influence of Al content upon the drug delivery. Ca. 15 wt% of Ibuprofen is adsorbed in the zeolite cavities. FTIR shows that the zeolite hydroxyl groups interact with Ibuprofen and, in addition, carboxylate species bonded to extraframework Al species are detected in the most dealuminated samples. Two stages are observed in the Ibuprofen delivery. In the first hours, the release is governed by a diffusion process, showing a similar delivery rate independently of the Al content. However, after this stage, the Al content is determinant in drug delivery, being the release faster when the framework Si/Al ratio increases up to 22, and then decreases for Si/Al=62. The behaviour of the highly dealuminated material is probably due to the predominance of Van der Waals interaction between the drug and the siliceous zeolite framework.     

Formation of Mesopores in USY Zeolites: A Case Revisited

Steaming of NH₄Y zeolite at 723 K and 873 K is accompanied by the formation of extra‐framework amorphous aluminosilicate and silica gel in addition to earlier observed extra‐framework aluminum species. Their occurrence is directly associated with the formation of mesopores. Bulk (intracrystalline) mesopores occur inside the crystallite nuclei and surface (intercrystalline) mesopores are located nearby the crystallite surface. Corrosion of the zeolite framework results in a loss of crystallinity and, consequently, decreased catalytic activity of the USY catalysts synthesized. Analysis of the reasons of mesopore formation may help to reduce these disadvantages.     

甘油脱水合成丙烯醛ZSM-5催化剂的孔结构和酸性调控

研究了ZSM-5 孔结构和表面酸性对甘油脱水合成丙烯醛反应性能的影响. 在碱浓度为0.2 mol·L^-1的NaOH溶液中, 分别在65和85 ℃条件下对ZSM-5进行化学刻蚀, 成功地制备了含微介孔的ZSM-5催化剂, 提高了催化剂的表面强酸密度. 碱处理后的ZSM-5催化剂在甘油脱水反应中的稳定性得到显著提高, 在ZSM-5-at85 (经85 ℃碱处理的ZSM-5)催化剂上甘油转化率在反应10 h 后仍可保持95%以上, 丙烯醛选择性达到78%. 采用N₂吸附-脱附等温线、X射线粉末衍射(XRD)、²⁷Al 固体核磁共振(²⁷Al MAS-NMR)和透射电子显微镜(TEM)等手段对ZSM-5 结构和表面性质进行了表征, 实验结果表明在碱处理过程中骨架中的硅发生了溶脱现象, 在分子筛表面上形成了大量介孔, 但是ZSM-5 的MFI 拓扑结构没有发生变化, 骨架中的大部分铝得到保持. X射线光电子能谱(XPS)、X射线荧光光谱(XRF)和氨气程序升温脱附(NH₃-TPD)证实了在碱处理后ZSM-5分子筛外表面的Si/Al 摩尔比低于其骨架中的比例, 由此表明脱硅现象主要发生在ZSM-5 的外表面, 在新产生的介孔区域由于Si/Al 摩尔比的降低使得强酸密度得到提高. 具有微介孔结构和较高酸密度的ZSM-5催化剂增强了反应物扩散性能和容碳能力, 这对于提高甘油脱水合成丙烯醛催化剂的活性和稳定性起到了关键作用.