~y~rother~a~ synthesis of zeolites Mark T Weller*
and Sandra E Dannl
Recent developments have included the synthesis of new, large pore zeolite structures, zeotypes and metal substituted zeolites, along with the in situ investigation of zeolite synthesis and control of morphoiogies. In terms of new zeolite structures there is increasing realisation of the aim to specifically design templates to control reolite structures, and the use of the fluoride ion as a mediating agent has been influential. In terms of zeotypes considerable effort has been directed at transition metal containing systems, particularly those of cobalt, with the potential to generate redox active catalytic centres. Growth of zeolite films on substrates has also been developed and progress is being made in studying directly, using diffraction and NMR techniques, the crystallisation of zeolites.
templates, sometimes synthesised specifically to induce a particular pore geometry. Often computer modelling and structural work define the interaction between the template and framework and the use of larger, more complex templates frequently leads to an open pore system with channels formed from 12- and 14-membered rings. Another notable influence has been the addition of anions, particularly fluoride ions, to reaction mixtures to facilitate and direct reactions, however, these anions are not often incorporated into the final product. Figure 1
Addresses *Department of Chemistry, University of Soffth~pto~, Highfield, Southampton SO1 7 1BJ, UK; e-mail: [email protected]
:Department of Chemistry, Loughborough University, Loughborough LEl 1 3TU, UK; e-mail: [email protected]
Current Opinion in Solid State & Materials Science 1998, 3:137-l
Electronic identifier: 1359-0286-003-00137 0 Current Chemistry ISSN 1359-0286
Introduction The purpose of this review is to highlight the advances achieved in the hydrothermal synthesis of zeolites in the past 1-2 years. Zeolites have been reviewed previously in this journal, most notably in volume 1 where articles on recent discoveries, future prospects and membranes were included. The material covered in this review is restricted to macroscopic crystalline materials and four salient areas are reviewed; new zeolite structures, known zeolite structures with different structuraf compositions (doped structures), zeotypes with zeolite morphoiogies and new methods of crystallisation/in situ crystallisation studies. As far as this review is concerned the term zeolite has been restricted to crystalline materials with nanoporous structures constructed mainly with aluminosilicate or silicate units; some nonaluminosilicate frameworks which have zeolite structures are discussed if their synthesis and structural chemistry are significant. Developments in the syntheses of mesoporous structures are not covered and hydrothermal synthesis of nonzeolitic frameworks (e.g titanosilicates and many noncobalt, transition metal phosphates), which is a rapidly developing area in its own right, also lies mainly outside the scope of this review.
in Solid State&Mater&
The structure of SSZ-4211104 shown as connected SiO, tetrahedra. The structure is viewed down the c direction and the undulating channel shown.
New zeolite structures
SSZ-42 ternplated using the N-benzyl-1,4-diazabicyclo [2.2.2] octane cation has a unique one-dimensional channel system as shown in Figure 1 [l”]. These channels consist of l&membered rings and have side walls similar to those found in zeolite p. C;omputer modelling indicates that the template molecule fits into the side pockets along the direction of the main channel. The adsorption properties of this material are exceptional for a one-dimensional material and are similar to those of a two-dimensional structure due to the presence of the large cage or pockets lying atong the channels.
The number of zeolite structures continues to develop, and recent work indicates that synthesis routes have become better directed with the use of carefully chosen
This large pore silica has also been described, as ITQ-1, in two artictes by Camblor and co-workers [2’,3’], The
Synthesis and reactivity of solids
synthesis described in this work is an example of the use of fluoride ions as mediators in zeolite synthesis; the template used in this case was the N-benylquinuclidinium cation. Furthermore structure solution was again achieved from X-ray powder data rather than from single crystal methods. The relatively thin walls in this material produce a large void volume of 0.21 cm3/g similar to that of the well-known zeolite B.
Previous attempts at the preparation of VPI-8 produced very small crystallites which were unsuitable for use in an ab initio structure determination due to the broadening of the X-ray reflections. Relatively large crystals of VPI-8 (4 pm), suitable for crystallographic work, have been produced using seed crystals [4”]. Single crystal electron diffraction experiments showed the crystals to be free of twinning and faulting and confirmed the tetragonal structure with a = 13.053 and c = 5.037 A. However, it was identification of the pinwheel building unit by HRTEM which provided the key to the full structure solution. The framework contains large l&membered rings running parallel to c separated by walls comprising of 5 and 6-membered rings. Two possible topologies were recognised although neither appeared to be preferred on stereochemical grounds. The biscyclopentadienyl cobalt (III) ion has recently been used to template a number of zeotypes which had previously required an amine to form, for example AlPO,-5. By using a large organometallic template, with methylatedcyclopentadienyl rings, a new high silica zeolite, UTD-1, with a 14-membered ring system was produced [5’,6]. The 14-membered elliptical pores represent one of the largest pore structures known for a zeolite. The reaction mixture consisting of a solution of 18% aqueous solution of (CpMe&Co(OH), NaOH and fumed SiO, were combined to form a gel of molar ratio 0.12.5:1:0.1:60H20 and aged for one hour. After transference to a teflon-lined autoclave the reactor was heated to 175°C under static conditions for two days. The mineral melinolite, K,Ca2[Al&,0,,].Z4H20 is a naturally occurring zeolite and exists in other cation forms, for example as a barium containing system; the material is structurally similar to phillipsite. Structural analysis of a synthetic zeolite of this type, termed zeolite W, has been carried out on powdered material grown from gels containing both potassium and sodium [7’]. The product, which contains only potassium as the nonframework cation, differs from the naturally occurring forms only in terms of the distribution of cations within the g-ring channels. The relationship between structure and ion type in this material would be worth studying further through detailed ionexchange and structural work. Another zeolite form of silica, SSZ-16, but with smaller pores has been reported [8’]. Again careful choice of template is required to maximise yield of the framework; however, many similar diquarternary ammonium cations
Current ODinion in Solid State & hktemls
The main cavity of the SSZ-16
direct its formation. Generally a tetramethylene system is optimum, though three and five bridge units have also been successfully employed. The structure directing agent provides an excellent fit for the large elongated cage in SSZ-16, (Figure ‘2). The structure is isomorphous with that of the aluminophosphate AlP04-52 with stacks of gmelinite cages. The largest windows to the main cages are only eight rings though there exists the potential for catalytic reactions involving small molecules such as NO,. Two zeolites currently of considerable importance as regards their ion-exchange properties are heulandite/clinoptilolite and zeolite P (gismondine type). The synthesis and structure of zeolite P and heulandite have been addressed by Khodabandeh and Davis  who have demonstrated a facile route to a heulandite via zeolite Pl. Zeolite Pl was formed by treatment of perlite glass with basic carbonate solutions and this was subsequently converted to heulandite by treatment with calcium ions. This conversion ofzeolite structures may be similar to the formation of zeolite A and then sodalite which occurs in high alumina/aluminosilicate/sodium hydroxide solutions.
Hydrothermal synthesis of zeolites Weller and Dann
zeolite) has not been previously fully characterised due to difficulties in producing a defect free material. An allsilica ZSM-11 has been prepared using the N,N-diethyl3,5-dimethylpiperidium cation as a template in a hydrothermal reaction at 150-170% [12’]. The use of this template is crucial in order to inhibit the formation of ZSM-5 (MFI). Structure investigation, as part of this work, using SEM, TEM and high resolution powder X-ray diffraction have confirmed that this material has a defect free high symmetry structure.
Current Opinion in Solid State & Materials Science
The large 14sring pore of the CIT-5 structure;
only the T atoms are
Zeolite P with a high aluminium content, Si:Al, 1:l (MAP, maximum aluminium P) and small uniform particle size may be obtained through seeding of vigorously stirred reaction mixtures [lo’]. The tight particle size distribution coupled with the micron sized crystallites and the unusual framework flexibility lead to unique, favourable ion-exchange and water sorption properties. These properties of MAP, resulting from the framework structure and the ability to maximise the aluminium content, clearly demonstrate the ability to tailor zeolite frameworks for specific applications. A further development of large pore systems, required for catalytic application on large molecules, has occurred with the synthesis and structural characterisation of CI’I5 [ll”] a high silica zeolite with 14-membered rings, (Figure 3). The templating agent used for this material was the complex cation N( 16)-methylsparteinium at 175°C in a high silica gel. A key feature of this synthesis was incorporation of lithium into the reaction mixture, however, this alkali metal is not incorporated into the zeolite, which is formally described as a pure silica. The structure is built from a zig-zag ladder of a four membered ring cross-linked by five ring units. Normally systems with such open structural features such as a 14-membered ring channel deliver poor thermal stability but remarkably the framework of CIT.5 seems to be stable to above 900°C. The structure shows a high degree of order which may confer this stability. Pentasil-based structures, particularly that of ZSM-5, have been studied due to their excellent catalytic properties. The pentasil-based structure of ZSM-11 (MEL type
Zeolite-p has commanded considerable interest in recent years due to its three-dimensional open pore structure and potential for forming a chiral framework. One problem with this material has been the presence of large numbers of defects associated with Si-O- or Si-OH units in those materials synthesised with typical Si:Al ratios of S-100. Recent work has reported the unseeded synthesis of pure silica zeolite-p using fluoride media [ 131. The fluoride ions seem to be occluded into the pores with the tetraethylammonium template negating the need for a framework charge associated with the connectivity defects. The structure remains a mixture of the polymorphs of this phase. Other publications on zeolites [14-221 have concerned synthesis using modified routes, for example using different templates such as crown ethers, or further structural characterisation of known zeolite frameworks. Examples of this structural work include changes in framework geometry as a function of silicon:aluminium ratio or determination of the template position.
Zeotypes While zeolites are strictly defined as aluminosilicate frameworks (though the aluminium level can be zero) other structurally related materials but without silicon and/or aluminum in the framework have been the focus of considerable attention. It is not the purpose of this review to discuss this work in detail but where it impinges on hydrothermal synthesis of true zeolites, for example through structural analogies, work of considerable note is discussed. STA-1, a microporous magnesium aluminophosphate was ternplated from aluminium hydroxide/magnesium acetate/phosphoric acid gels using diquinuclidinium ions [(C,H,,N-(CH,),(NC,Hr$]z+ n = 7-9 at 190°C for 48 h [‘23”]. STA-1 has the large pore volume and low framework density expected for such a large templating molecule. This work illustrates well how designing a particular organic template can lead to a designed framework. The structure consists of l&ring channels running in two orthogonal directions in the crystal which are also linked in the third direction (Figure 4). Introduction of various anions into gel mixtures can radically alter the product nature. For example a new zeotype, UiO-7 , was obtained from a fluoride modified
Synthesis and reactivity of solids
charge compensation framework following
Current ODinion in Solid State & Materials Science
The framework of STA-1 viewed down the b direction showing the 12sring openings. Only the connections between the T atoms are shown.
gel with the well known tetramethylammonium hydroxide template. The introduction of the.fluoride ion directs the reaction profile away from the normally formed sodalite type phase to a more open strutture. Reaction was carried out by crystallisation of a pseudoboehmite/phosphoric acid mixture with HF at 150°C. The structure solution of UiO-7 shows the developments taking place towards using several methods to obtain a structure where good single crystals are not available. In this case the structure was determined from a combination of synchrotron X-ray data, MASNMR data and through simulated annealing computational work. The structure determination in this case used the high quality diffraction data to obtain the unit cell parameters and possible space groups. From this, possible framework geometries consistent with structure symmetry were generated and tested against the diffraction data. In this instance one good fit was achieved and the basic structure then refined using the powder diffraction data. The structure contains an unusual two-dimensional eight-ring channel system, as in SAPO-40, but in this material the units are tilted producing a narrower pore system. The existence of UiO-6 has also recently been reported [25’]; this material is a l&ring ALP04 with one-dimensional channels.
Beryllophosphate and berylloarsenate analogues of the zeolite RHO were prepared by the hydrothermal reaction of beryllium nitrate, group (V) acid and alkali metal hydroxides in a gel . Transformation of the gel to the microcrystalline product was achieved by heating in vacuum at 70°C. Despite the low temperature required for synthesis and good ion-exchange properties, these materials have very poor thermal stability. Attempts to improve stability, by exchange reactions, were largely unsuccessful except for divalent calcium and barium where the higher charge imparts better stability by
for the highly dehydration.
The introduction of high levels of redox active metal sites in known zeolite structures remains a highly desirable goal of the synthetic chemist. A number of materials of this type are described in the later sections. An alternative route to materials containing transition metal centres in porous systems is the incorporation of large transition metal clusters of the anti-Keggin type  into a framework. Vanadium-based polyhedra of the general formula with -0.1
Modified structures Introduction of other transition metal species at reasonable levels into zeolite structures to enhance and modify catalytic activity remains the goal of the synthetic chemist. However, incorporation of transition metals other than cobalt into well-known zeolite frameworks is frequently difficult due to their preference for higher coordination environments than tetrahedral. Characterisation of such materials also needs to be undertaken rigorously to demonstrate that the transition metal dopant is actually in the framework. The synthesis of a zeolite having a silicon to iron ratio of 11:l is therefore of note [41’]. This iron silicate adopting the ferrierite (FER or ZSM-35) structure was obtained by addition of iron (III) sulphate to a silicate gel and crystallisation at 150°C for 1.5-20 days. Confirmation that the iron was incorporated into the zeolite framework was obtained from a significant increase in the unit cell parameters over those of ferrierite, adsorption characteristics and photoelectron spectroscopy. The replacement of iron by aluminum in this structure should alter its acid characteristics and, hopefully, reduce the effects of coking during isomerisation reactions The selectively and Lewis acidity of zeolitic materials can be modified by substitution of framework cations by transition metals. Trace amounts of nonframework iron, introduced by the addition of iron nitrate into the hydrothermal synthesis reaction mixture, have been shown to promote
Hydrothermal synthesis of reolites Weller and Dann
catalytic activity in the aromatisation of n-hexane 1421. Further chemistry involving substitution of framework aluminium has also been reported, though the levels of incorporation of the dopant ions are lower and evidence for their successful incorporation less robust [43-46].
Cobalt containing systems A range of cobalt-phosphate-based zeotype structures have been synthesised, as reported by Feng and colleagues . Framework analogues which have been prepared include, analcine, chabazite, thomsonite, sodalite, merlionite and pbi!Iipsite, with synthesis conditions typical of those used in zeolite formation. A variety of organic amines uere used as templates and the framework charge could be varied by changing factors such as the cobalt to phosphate ratio. Difficulties in synthesising zeolite analogues in this system result from the host-guest charge mismatch where the framework charge is too negative to be balanced by the positive charge of most protonated amines. This work followed the initial synthesis of a cobalt gallium phosphate of the gismondine type by Cowley and Chippindale . In comparison with the aluminosilicat~s, frameworks containing the BOh unit have been poorly studied despite enhancement of catalytic properties achieved by even small amounts of boron substitution, A new open framework cobalt borophosphate  has been synthesised using boric acid, phosphoric acid and cobalt trisethylenediamine trichloride. The channels are formed from polyhedra sharing corners in the sequence P04-CoQ,-P04-B04P04-B04-P04-Coo4 in a wavelike, oblong fashion. The short-axis of the opening is about 5.21 A and the long axis 7.6 A (O-O distances?. A novel cobalt-gallium phosphate with a microporous structure has been reported [SO] and cobalt has also been incorporated into the aluminophosphate ALPO-34 [Sl].
of uniform habit and size. Perchlorate appeared to have the greatest effect on the speed of crystallisation; in the case of zeolite Y it also appeared to improve selectivity in phase formation, where the normal zeolite P impurity remained undetected in the mother liquor two hours after complete crystallisation of zeolite Y. Other synthesis methods involving induction heating and crystallisation in a strong gravitational field have also been described [54,55].
Controlled reolite morphologies Several papers in the last eighteen months have dealt with the synthesis of zeolite films which have potential applications as membranes, in optoelectronic devices and in sensors. Three phases grown in this way are zeolite A [56’], ZSM-5 [.57’] and zeolite L [SS]. Of note here is the development of the technique of depositing nanocrystals of the zeolite from zeolite suspensions. Zeolite crystals are grown in solution until they reach a critical size and then deposited onto a substrate by dipping it into the nanosol; the nanounits are generally deposited in an ordered arrangement. The use of such membranes in the separation of alcoholether mixtures has also been described. The technique uses a pervaporation method employing an Na-Y membrane grown hydrothermally on a porous alumina support . This membrane consisted of randomly oriented crystallites but very high selectivities were obtained, particularly for alcohol ether combinations having large differences in molecular size.
In situ studies
ZSM-5, zeolite-fi and hexagonal Y (EMT) have been successfully synthesised by heating gels using microwaves [.52’]. The enhancement of single-phase crystallisation reduces reaction times -from days/hours to minutes; long reaction times at low temperature have previously been necessary to reduce the formation of impurity phases. The use of microwaves to reduce the cryscallisation time of useful zeolites may make the synthesis of such products more industrially viable.
The direct observation of zeolite crystallisation and formation remains a goal of the synthetic chemist. Because of the hydrothermal nature of these reactions, coupled with long nucleation times and subsequent rapid crystal growth, such studies are still problematical, though progress is being made. Such results provide a significant advance on previous studies of zeolite formation where the reaction mixture was separated or quenched. Solid state NMR methods, %i and 27Al have been used by Shi eta/. [60’] to observe the direct formation of zeolite A from aluminosilicate gels. These results currently throw little additional light on the nucleation/growth mechanism as they were obtained at a fairly low temperature, 65°C and the technique is therefore not really applicable to most conditions used in zeolite synthesis.
The use of Group VA and VIIA metal salts as promoters for selective crysrallisation of zeolites and their ability to increase the yield up to 4-6 times has been reported [53’]. Acid or Na/K salts of ions such as ClO,- or BrO,- are slowly added co the starting gel before it is transferred to teflon lined autoclaves before hydrothermal reaction. A range of zeolites with different pore sizes, for example, small (NU-1, FER), medium (ZSM-5) and large (beta and ZSM-12) have been synthesised using this methodology producing crystals
ln sitll diffraction experiments of zeolite growth are often hampered by the low levels of crystalline material and the time scales required to collect a reasonable diffraction profile. The development of a zeolitic phase, after nucleation, generally occurs too rapidly for all the stages to be observed using a laboratory source. These problems may be overcome by using high intensity synchrotron radiation as illustrated in the study of the synthesis of ULMJ [61’]. In this work a hydrothermal reaction occurring at 180°C
New synthesis methods
was studied by acquiring energy dispersive X-ray diffraction patterns at 60 s intervals. Results showed the formation of a short lived previously unobserved intermediate.
Conclusions The hydrothermal synthesis of zeolites and zeotypes remains a fertile area of study with the development of new framework structures through the use of tailored templating. Clearly this area has a great potential for future development as more complex templates are designed specifically for directing the formation of particular pore structures and features. The use of other influential structure-directing agents in hydrothermal reactions, such as fluoride, oxoanions and lithium, even though they are often not incorporated into the final material, is also an area where further understanding needs to be developed and employed.
Balkus KJ, Biscotto M, Gabrielov AG: The synthesis and characterization of UTD-1: the first lage pore zeolite based on a 14 membered ring system. Studies Surf Sci Catal A 1997, 105:415-421. A large pore high silica zeolite with one-dimensional channels constructed from 14-membered rings. Interestingly this material; was not templated with an amine but rather a cyclopentadienyl cobalt (Ill) derivative. The structure is faulted and involves intergrowths of various crankshaft chains. 6.
Bieniok A, Bornholdt K, Brendel U, Baur WH: Synthesis and crystal structure of zeolite-w, resembling the mineral merlinoite. J Mater Chem 1996,6:271-275. Synthesis and structure determination of a potassium form of the MER framework, natural phase merlionite.
Lobo RF, Zones SI, Medrud RC: Synthesis and Rletveld refinement of the small-pore zeolite SSZ-16. Chem Mater 1996,8:24092411. The use of various templating agents in the formation of this smaller pore zeolite, B-ring windows, is described. The structure contains two small cage types of the gmelinite and AFT type. .
Systems containing transition metals, either adopting zeolite structures or new framework geometries, have also recently seen rapid progress. While tetrahedral cobalt may be readily incorporated into frameworks other transition metals must also be targets if they can be stabilised in four -fold coordination at high levels within the structures. The final area where further rapid developments are likely to occur is in the in S&Uinvestigation of zeolite formation under hydrothermal conditions. High intensity X-ray and neutron beams, as well as the ability to carry out solid state NMR under high pressure conditions, promise significant advances of our understanding of zeolite formation and growth.
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Papers of particular interest, published within the annual period of review, have been highlighted as: l l
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Weller and Dann
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