BEA Structure Research Articles

Correlation between Brønsted Acid Strength and Local Structure in Zeolites

As an index of acid strength, ammonia adsorption energies (Eads) were calculated with density functional theory on cluster models of Bronsted acid sites belonging to FAU, BEA, MFI, FER, MWW, and MOR structures, which were selected because of the availability of experimental data and industrial importance. The calculated Eads were reasonably consistent with experimental results from the ammonia IRMS-TPD (infrared mass spectroscopy−temperature-programmed desorption) method. The calculated value was slightly (10−20 kJ mol−1) lower than the observed value, and its change with varying structure was approximately in agreement with the experiments. A thorough study was carried out to find the geometric parameters of the zeolite clusters (in the H and NH4 forms) relevant to Eads and to discuss parameters controlling the acidic property. Hydrogen bonding interactions between ammonium cations and neighboring zeolitic oxygens were found to affect Eads observed in small cavities. When NH4+ was stabilized in relativel...

Cyclic diquaternary ammoniums for nanocrystalline BEA, MTW and MFI zeolites with intercrystalline mesoporosity

3,10-Diazoniabicyclo[10.2.2]hexadeca-12,14,15-triene-3,3,10,10-tetramethyl-dichloride is an effective structure-directing agent for BEA zeolite under a high concentration of Na+. The pseudomorphic crystallization process led to nanocrystalline BEA particles possessing a large volume of intercrystalline mesopores. The synthesis of nanocrystalline zeolite has been extended to other cyclic diammonium compounds that can generate BEA, MTW and MFI zeolite structures. Phase diagrams for these nanocrystalline zeolites have been constructed using sodium silicate as a silica source. The nanocrystalline morphologies and intercrystalline mesoporosity have been characterized by electron microscopy and N2 adsorption. The result indicates that the pseudomorphic crystallization process is limited to BEA, and not MTW or MFI despite their nanocrystalline morphologies.

Role of tetrahedral Co(II) sites of CoSiBEA zeolite in the selective catalytic reduction of NO: XRD, UV–vis, XAS and catalysis study

Abstract The CoSiBEA zeolites are prepared by a two-step postsynthesis method which allows to incorporate cobalt in BEA zeolite as isolated tetrahedral Co(II) species. The incorporation of Co ions in vacant T-sites and their reaction with silanol groups are evidenced by XRD and FTIR respectively. It does not generate acidic Bronsted sites as shown by FTIR of adsorbed pyridine. The state of cobalt species is characterized by DR UV–vis and XAS. These techniques allow to show that for low Co content (0.8 wt%) the cobalt is present as lattice tetrahedral Co(II) species. For much higher Co content (11 wt%) mainly extra-lattice octahedral Co(II) species and/or cobalt oxides are evidenced by DR UV–vis spectroscopy. Zeolite with isolated tetrahedral Co(II) species is active in SCR of NO by ethanol with selectivity towards N2 exceeding 85% for NO conversion from 30 to 70%. When extra-lattice octahedral Co(II) species and/or cobalt oxides appear, the full oxidation of ethanol and NO by dioxygen to CO2 and NO2 respectively, are the main reaction pathways. Possible way of the formation of lattice tetrahedral Co(II) species in the BEA structure is proposed.

Multicomponent sorption of hexane isomers in zeolite BETA

AbstractBreakthrough curves of single, binary, ternary, and quaternary mixtures of hexane (C6) isomers n‐hexane (nHEX), 3‐methylpentane (3MP), 2,3‐dimethylbutane (23DMB), and 2,2‐dimethylbutane (22DMB) were performed in commercial pellets of zeolite BETA (BEA structure), covering the temperature range between 423 and 523 K and partial pressures up to 30 kPa. From these data, single and multicomponent adsorption equilibrium isotherms were collected. A tri‐site Langmuir model (TSL) was developed to interpret the equilibrium data based on considerations about zeolite structure, and a dynamic adsorption model was tested predicting with a good accuracy the behavior of multicomponent fixed‐bed experiments. At the partial pressures studied, the sorption hierarchy in the zeolite BETA is nHEX >>> 3MP > 23DMB >> 22DMB. BEA structure demonstrates a significant selectivity between C6 isomers, especially at low coverage, giving a good perspective regarding their separation by adsorption processes. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Effect of the Preparation Method on the State of Nickel Ions in BEA Zeolites. A Study by Fourier Transform Infrared Spectroscopy of Adsorbed CO and NO, Temperature-Programmed Reduction, and X-Ray Diffraction

The state of nickel ions in two BEA zeolites was studied by X-ray diffraction (XRD), temperature-programmed reduction (TPR), and Fourier transform infrared (FTIR) spectroscopy using CO and NO as probe molecules. One of the samples, NiAlBEA (0.9 wt % of Ni), was prepared by conventional ion exchange, and the other, NiSiBEA (1.0 wt % Ni), was prepared by a two-step postsynthesis method involving dealuminated SiBEA zeolite. No structural changes are observed with AlBEA after ion exchange with nickel. In contrast, the incorporation of Ni into SiBEA leads to an increase of unit cell parameters of the BEA structure and to the consumption of silanol groups in vacant T-sites of the dealuminated zeolite. In the NiAlBEA sample Ni shows three TPR peaks in the region of 600−1100 K, whereas one sharp peak at 650 K with a shoulder at 750 K is observed with the NiSiBEA sample. Adsorption of CO at 100 K on NiAlBEA results in formation of Ni2+−CO species (2200 and 2214 cm-1). The latter are partially converted, at high coverages, to Ni2+(CO)2 dicarbonyls (2206−2204 cm-1). Reduction of the sample with CO leads to generation of Ni+ ions. The latter form, with CO, different carbonyl species that are in equilibrium: (i) Ni+−CO (2113 cm-1), (ii) Ni+(CO)2 (νs at 2138 cm-1 and νas at 2095 cm-1), and (iii) Ni+(CO)3 (2157, 2122, and 2113 cm-1). The polycarbonyl structures were proven by 12CO−13CO coadsorption. In line with these results, NO adsorption on NiAlBEA leads to formation of two kinds of nitrosyl species (1905 and 1897 cm-1). Adsorption of CO at 100 K on NiSiBEA leads to formation of several kinds of Ni2+−CO species observed in the 2196−2170 cm-1 region. The intensity of the bands was low, suggesting the majority of incorporated nickel ions are coordinatively saturated or inaccessible and thus not able to form carbonyls. Negligible amounts of Ni+ ions were present on the activated sample and formed various (poly)carbonyls. In this case the reduction hardly affected the state of the sample. Most of the nickel ions in NiSiBEA were not able to coordinate NO; the others formed nitrosyl complexes (1872−1845 cm-1) which were much less stable (up to 323 K) than those observed with NiAlBEA.

Mesoporous zeolite and zeotype single crystals synthesized in fluoride media

We report the synthesis and characterization of a series of new mesoporous zeolite and zeotype materials made available by combining new and improved procedures for directly introducing carbon into reaction mixtures with the fluoride route for conventional zeolite synthesis. The mesoporous materials were all prepared by hydrothermal crystallization of gels adsorbed on carbon matrices which were subsequently removed by combustion. The procedures presented here resulted in mesoporous zeolite and zeotypes materials with MFI, MEL, BEA, AFI and CHA framework structures. All samples were characterized by XRPD, SEM, TEM and N2 physisorption measurements. For the zeolite materials it was found that mesoporous MFI and MEL structured single crystals could indeed be crystallized from fluoride media using an improved carbon-templating approach. More importantly, it was found that mesoporous BEA-type single crystals could be crystallized from fluoride media by a newly developed procedure presented here. Thus, we here present the only known route to mesoporous BEA-type single crystals, since crystallization of this framework structure from basic media is known to give only nanosized crystals as opposed to mesoporous single crystals. For the zeotype materials it was found that highly crystalline mesoporous materials of AFI and CHA structure types could be synthesized using a newly developed procedure.

The synthesis and characterization of Al-Beta and all-silica Beta formed in fluoride and caustic media

The influence of various parameters upon the synthesis of the BEA structure in fluoride and caustic media is investigated and results are compared and related to some previous studies in the literature. High crystalline, low faulted BEA structure has been made in an excess of fluoride (F−/SiO2 ratio of 1.5) across the entire gel Si/Al ratio range of 20 to ∞ using a fumed silica source and in the absence of seed crystals or stirring. Although, for gel Si/Al ratios >20, no aluminium was found in the products. The scope for further investigation of the effects of H2O content and stirring vs. non-stirring upon crystallinity and aluminium uptake, especially in conditions of lower fluoride concentration, is shown. Results from the caustic media syntheses agree with those from previous studies and it is found that the uptake of aluminium into the zeolite during synthesis is required for the formation of the BEA structure. Product characterization of Al-Beta confirms the framework incorporation of aluminium through increased d-spacings, the presence of tetrahedral coordinate aluminium and lowering of the FT-IR external asymmetric stretch and double bend vibration relative to an all-silica sample.

Remarkable Effect of the Preparation Technique on the State of Cobalt Ions in BEA Zeolites Evidenced by FTIR Spectroscopy of Adsorbed CO and NO, TPR and XRD

The state of cobalt in two BEA zeolites was studied by XRD, TPR, and FTIR spectroscopy using CO and NO as probe molecules. One of the samples, CoAlBEA (0.4 wt % of Co), was prepared by conventional ion exchange and the other, CoSiBEA (0.7 wt % Co), by a two-step postsynthesis method involving dealuminated SiBEA zeolite. The introduction of Co into SiBEA leads to an increase of unit cell parameters of the BEA structure and to the consumption of silanol groups in vacant T-sites of the dealuminated zeolite. In contrast, no structural changes are observed after incorporation of cobalt into AlBEA by ion-exchange. The reduction temperature of cobalt in CoSiBEA zeolite (1130 K), is much higher than for CoAlBEA and indicates a strong interaction of cobalt ions with SiBEA. Low-temperature CO adsorption on CoAlBEA results in (i) H-bonded CO, (ii) Co(3+)-CO adducts (2,208 cm(-1)) and (iii) a small amount of Co(2+)-CO complexes (2,188 cm(-1)). In agreement with these results, NO adsorption leads to the appearance of (i) NO(+) (2,133 cm(-1), formed with the participation of the zeolite acidic hydroxyls), (ii) Co(3+)-NO (1932 cm(-1)), and (iii) a small amount of Co(2+)(NO)(2) dinitrosyls (nu(s) = 1,898 and nu(as) = 1,814 cm(-1)). Low-temperature CO adsorption on CoSiBEA leads to formation of two kinds of Co(2+)-CO adducts (2,185 and 2,178 cm(-1)). No Co(3+) cations are detected. In line with these results, adsorption of NO reveals the existence of two kinds of Co(2+)(NO)(2) dinitrosyls (nu(s) = 1,888 and nu(as) = 1,808 cm(-1) and nu(s) = 1,878 and nu(as) = 1,799 cm(-1), respectively).

Synthesis and characterization of hybrid organozeolites with high organic content

Abstract Different types of microporous hybrid organic–inorganic materials with ITQ-21, MFI and BEA structures have been synthesized from bridged organosiloxane precursors as silica sources. The methodology employed was similar to than for the synthesis of conventional zeolites. The organozeolites prepared contain the highest number of methylene and ethylene groups known up to now (∼9 wt.% of carbon), into their typical inorganic frameworks, while maintaining high porosity and free porous volume. This effect is very marked in the organozeolites with Beta topology. The presence of organic fragments in the framework is confirmed by 13 C and 29 Si NMR and IR spectroscopies. The organic–inorganic networks are thermally stable and remain after elimination of the structure directing agents (SDAs) by calcination. Characterization techniques show that the type of structure finally crystallized and the crystallinity achieved are key parameters to introduce high carbon content into the zeolitic frameworks. The hybrid organic–inorganic materials synthesized, with incipient structuration level, present the conventional morphology and textural properties of zeolitic materials.

Organic−Inorganic Hybrid Zeolites with Framework Organic Groups

A novel type of organic−inorganic hybrid zeolite has been successfully synthesized, in which lattice oxygen atoms are partially superseded by methylene groups. Various types of hybrid materials with the LTA, MFI, and *BEA structures are obtained from an organosilane in which a methylene group bridges two silicon atoms under synthesis conditions similar to those for conventional zeolites. The presence of a methylene group in the framework is demonstrated by 29Si and 13C MAS NMR and IR, although the Si−C bonds are partially cleaved during the hydrothermal synthesis to give rise to terminal methyl groups. The organic framework is thermally stable enough to remain after the combustion of the occluded organic structure directing agents (SDAs). The thermal stability of the organic framework is much higher than that in amorphous materials synthesized from the same silicon source and is dependent on the zeolite structures. The SDA-free materials not only show microporosity like ordinary zeolites but also exhibit ...

Highly Selective Formation of Cyclopentanol through Liquid-Phase Cyclopentene Hydration over MCM-22 Catalysts

Abstract MCM-22 catalyst exhibits an extremely high shape selectivity not observed on other zeolites of MFI, MOR and BEA structures, in that it effectively catalyzes the liquid-phase hydration of cyclopentene at a conversion up to 10% and a cyclopentanol selectivity as high as 99%.

Characterization of Chromium-Substituted Zeolite BEA

Abstract Chromium-incorporated BEA zeolite samples (CAB) are synthesized via a hydrothermal procedure, and their chemical compositions are analyzed by both the ICP-AES and EDX techniques. Synthesized samples are characterized for the local environment of chromium through FTIR, thermal analysis and cyclic voltammetry. An observation of a notable band in FTIR (at 955 cm-1) of calcined/acetic acid-washed samples of CAB revealed that at least a part of chromium is associated with the framework. In a thermal analysis, a high-temperature (643–773 K) desorption peak of TEA+ was noticed; the peak size increased along with an increase in the chromium content. Cyclic voltammograms of as-synthesized, calcined and acetic acid-treated samples gave ample evidence for the incorporation of chromium into the framework. Acid-washed CAB samples showed framework-stabilized chromium in a zeolite BEA structure, possibly under two different local environments, i.e. tetrahedral and square pyramidal sites.

Nanosized zeolite films for vapor-sensing applications

Colloidal zeolites LTA and BEA sized below 100 nm were synthesized as building blocks for the controlled growth of thin microporous films on piezoelectric sensor devices (quartz crystal microbalance, QCM). The zeolite films were prepared on pre-seeded gold substrates on QCM devices. Initially, a layer of colloidal particles was deposited on the support through chemical bonding with a silane coupling agent, followed by hydrothermal growth. BEA- and LTA-type zeolite films with thicknesses of 250 and 450 nm, respectively, were prepared by optimizing the synthesis conditions. The application of these zeolite films in microsensors for water and organic compounds is presented. The importance of the zeolite structure type with respect to the sensitivity towards different organic and water vapors at various concentrations is discussed. Both zeolites are thermally stable and show reproducible responses during long-term experiments. Based on these results, it was concluded that both zeolite films could be used effectively as humidity sensor materials for water vapor sensing purposes. High sensitivity, good reversibility and long life were demonstrated for this type of zeolite film at low water concentrations. In comparison to LTA, the BEA films show a higher sorption capacity towards water vapor and no rejection of pentane, hexane and cyclohexane, due to the larger pore size of the BEA structure.

Quantitative analysis of aluminium and iron in the framework of zeolites

Aluminosilicates of ZSM-5, mordenite, ferrierite and BEA structures, and ferrisilicates of ZSM-5 and -22 structures in NH4- and dehydrated/deammoniated H-form were investigated in order to determine the concentration of Al and Fe isomorphously substituted silicon in their frameworks. A comparative study has been made of the chemical composition of zeolites, the quantitative analysis of 27Al NMR spectra, the amount of released ammonia and the intensity of N–H IR vibration (at 1445 cm−1) in NH4-zeolites on the one hand, and the quantitative analysis of IR bands of Si–OH–M groups (M=Al or Fe) at 3605–3630 cm−1 of H-zeolites and the IR band at 2297 cm−1 related to the CN groups of adsorbed d3-acetonitrile interacting with the Brønsted sites of H-zeolites on the other hand. It has been shown that the intensity of N–H vibration at 1445 cm−1 of fully exchanged NH4-zeolites with the determined extinction coefficient, represents a quantitative measure of the concentration of NH4+ ions and, accordingly, the concentration of aluminium or iron in the framework positions of the hydrated zeolites. With dehydrated/deammoniated H-zeolites, the intensity of the IR band of the CN groups of adsorbed acetonitrile gives the concentration of the strong acidic bridging Si–OH–M groups. The characteristic band of adsorbed acetonitrile at 2297 cm−1 detects all the Si–OH–M groups, including those interacting via hydrogen bonding, while the IR bands at 3605–3630 cm−1 of Si–OH–M groups reflect only unperturbed OH groups.

Catalytic conversion of polyolefins into fuels over zeolite beta

The catalytic degradation of polypropylene and both low- and high-density polyethylene over different samples of zeolite beta has been investigated. Zeolite beta synthesized by the fluoride method has a low activity for the cracking of polyolefins, which is related to its large crystal size (12 μm) and poor aluminium incorporation. In contrast, zeolite beta synthesized from amorphous xerogels consists of small crystallites (⩽200 nm) and allows all the polyolefins to be degraded with high conversion (40–60%) and good selectivity towards C5–C12 hydrocarbons (60–70%), heavier products being hardly obtained (<6%). The incorporation of titanium into the BEA structure has been proved to enhance the catalytic activity, as denoted by the highest conversion per Al atom obtained with the Ti–Al-beta sample compared to Al-beta.

Acidity and m-Xylene Isomerization Activity of Large Pore, Zirconium-Containing Alumino-silicate with BEA Structure

Abstract A large pore, zirconium-containing alumino-silicate with BEA structure, Zr–Al-β (SiO 2 /ZrO 2 =100 and 50; SiO 2 /Al 2 O 3 =25), has been synthesized hydrothermally using TEAOH as template and characterized by XRD, FTIR, and diffuse reflectance UV–visible spectral techniques to show that Zr 4+ ions are isolated and are probably linked to the framework of the aluminosilicate of BEA structure. The FTIR studies of adsorbed pyridine, evacuated at different temperatures show an increase in Lewis as well as Bronsted acidity of Zr–Al-β samples in comparison to Zr-free Al-β and Zr-impregnated Al-β, having similar Al contents. The enhancement is probably the result of incorporation of Zr 4+ ions into the β structure as Si–O δ− ⋖Zr δ+ linkages. The catalytic activity of Zr–Al-β samples is demonstrated in the isomerization of m -xylene at atmospheric pressure in a fixed bed reactor. The additional acidity of Zr–Al-β catalyst has been correlated with a marginal increase in the isomerization activity as well as selectivity to the isomerization products rather than the disproportionation products, particularly at temperatures below 500 K.

Synthesis of ∗BEA-type molecular sieves using mesoporous materials as reagents

Abstract A new method for synthesizing ∗BEA-type molecular sieves that involves the use of mesoporous materials as reagents is presented. Si-Beta (pure-silica, ∗BEA-type molecular sieve) is synthesized by heating TEAOH-impregnated Si-MCM-41 to 150°C for several days. The Si-Beta obtained has a small number of Q3 sites that are mostly siloxy groups that balance the charge of TEA cations. TEA cations can easily be removed by contacting the Si-Beta with acetic acid solution, and as a result, a highly hydrophobic Si-Beta is formed. Ti-Beta is synthesized from Ti-containing mesoporous silica or Ti-impregnated Si-MCM-41 without adding Al3+, alkali-metal cations and seeds. The incorporation of Ti into the ∗BEA structure is confirmed by UV–vis data. ∗BEA-type materials that contain other species such as Al, B, V, Zr and Zn can also be prepared.

Crystallization of high silica BEA by dry gel conversion

Zeolite with BEA structure was prepared using a new crystallization method called ‘Dry Gel Conversion Technique’. This dry gel conversion technique is a convenient method to prepare zeolites and enables one to synthesize zeolite BEA with SiO 2 /Al 2 O 3 ratios from 30 to infinity using tetraethylammonium hydroxide (TEAOH) as a structure directing agent. Zeolite BEA with SiO 2 /Al 2 O 3 ratios from 30 to 730 were rapidly crystallized in 3 and 12 h, respectively. The 27 Al NMR spectrum for BEA with SiO 2 /Al 2 O 3 ratio of 30 confirmed the absence of octahedrally coordinated aluminum. The crystallization rates were higher for BEA with low SiO 2 /Al 2 O 3 ratios than for those with high SiO 2 /Al 2 O 3 ratios. BEA crystallized by this method had uniform particles of about 60 nm. Higher Na + ion concentrations in the gel promoted the formation of high silica BEA. Elimination of occluded TEA cations took place at lower temperatures (around 623 K) in high silica BEA than that in BEA with low SiO 2 /Al 2 O 3 ratios (around 900 K). This method also allowed to synthesize BEA in the form of self-bonded pellets.

Synthesis and characterization of large pore vanadium aluminosilicate beta molecular sieves

Microporous vanadium aluminosilicate with BEA structure was synthesized in a Na 2 O: SiO 2 : V 2 O 5 : Al 2 O 3 : N(C 2 H 5 ) 4 OH: H 2 O system. A kinetic study of the crystallization of V-Al-Beta reveals that there seem to exist optimum ranges of the Al 2 O 3 /SiO 2 and V 2 O 5 /SiO 2 ratio, out of which either the crystallinity is extremely low or other phases occur. The V incorporation in the framework is confirmed by i.r., e.s.r., and 51 V n.m.r. spectra, e.s.r. and n.m.r. spectra show that vanadium, both in the as-synthesized and calcined samples, is in an atomically dispersed state and that the clusters of vanadium as well as V 2 O 5 -like phases are absent. Part of the V species is easily oxidable, and the other part remains in the V 4+ states on calcining in air, possibly due to the effect of the strong Bronsted acid sites in the V-Al-Beta. In the hydroxylation of phenol with diluted H 2 O 2 aqueous, V-Al-Beta exhibits quite high catalytic activity and selectivity.

Dry-gel conversion technique for synthesis of zeolite BEA

A new method is developed to crystallize zeolites; for the first time, a zeolite with BEA structure is synthesized by a dry-gel conversion technique.