Communication pubs.acs.org/cm
Interconversion of the CDO Layered Precursor ZSM-55 between FER and CDO Frameworks by Controlled Deswelling and Reassembly Wieslaw J. Roth,*,† Barbara Gil,† Wacław Makowski,† Andrzej Sławek,† Justyna Grzybek,† Martin Kubu,‡ and Jiri Č ejka‡ †
Faculty of Chemistry, Jagiellonian University in Kraków, Ingardena 3, 30-060 Kraków, Poland J. Heyrovský Institute of Physical Chemistry Academy of Sciences of Czech Republic, v.v.i., Dolejškova 2155/3, 182 23 Prague 8, Czech Republic
‡
S Supporting Information *
Z
eolites are very useful and valuable crystalline materials1 with exceptional catalytic and selective sorption properties2,3 that are a direct consequence of their microporous framework structures.4,5 Zeolite structures are usually extended in three dimensions (3D)6 as crystals with a submicrometer and greater size. An increasing number of frameworks, starting with the MWW,7,8 have demonstrated formation of layers with one dimension equal to 1−3 nm corresponding to about one or a fraction of a unit cell.9 They form analogues of layered solids, such as clays and layered oxides,10,11 and have been named 2D zeolites.12 The typical primary representatives of 2D zeolites, i.e., those that can be further modified into various other layered structures and forms, are recognized as layered zeolite precursors. They can produce the conventional 3D frameworks by topotactic condensation,13 which represents the second zeolite synthesis pathway complementing the conventional direct assembly in 3D.14 The area of zeolites has been undergoing enormous qualitative and quantitative expansion due to these 2D forms, which have revealed many novel materials and phenomena15,16 with possible applications in catalysis17 and a new platform for drug delivery. Approximately 15 zeolites out of 230 approved by IZA have been found to produce precursors,18 and in the future many more can be found to afford a layered form. One of the unusual features found with zeolite precursors has been that some layers can produce by topotactic condensation two or more different complete frameworks due to peculiarities of their structure. The examples are FER/CDO,19 CAS/NSI,20 HEU/RRO,21 and PCR/IPC-922,23 frameworks. The corresponding layers are denoted fer, cas, heu,13 and pcr (our designation adopted here). The pcr layer can produce four different frameworks, and two have not been made yet.24,25 Different structures can be obtained with the same layer by stacking according to different symmetry elements like translation or mirror reflection, provided it does not produce the identical outcome,26 like when a layer has the lateral mirror plane. The fer and pcr layers show additional twist because the corresponding framework can also result from a lateral shift of the layers. This is a consequence of the propagating symmetry operation (translation or reflection) combined with their internal symmetry (perpendicular mirror). It enabled conversion of the precursor of CDO to the FER framework27 and the PCR precursor to IPC-9, the latter being an “unfeasible” structure, so designated because of its energetic disadvantage.22 The other examples of zeolite pairs produced by the same layer that were mentioned © XXXX American Chemical Society
above require separate synthesis of the precursors as their interconversion would involve layer rotation. The first example of the precursor interconversion by layer shift was reported with RUB-36, the precursor of zeolite CDO, which by appropriate treatment involving layer separation by swelling and calcination, produced FER.27 The same CDO precursor also produced FER upon various alkaline treatments.28 In general, the fer layers can adopt after calcination many different arrangements: not only periodic frameworks FER and CDO but often afford disordered solids. The composites with organic molecules incorporated as templates or by intercalation also exhibit various structures.19 One of the reasons for the preferred transformation into FER structure after complete layer separation may be that it represents a more energetically favorable pathway. Thus, it was interesting to investigate if the alternative is also possible, namely to induce a shift to the CDO structure after separating the fer layers. It is addressed in this article, which describes formation of both zeolites, FER and CDO from a CDO precursor, ZSM-55, by intercalation of different organic compounds as bases or salts after the swelling. A related process has been carried out with the pcr layers, which produce two structures with FER and two with CDO projections.29 The shift was between the two with the “FER” projection, PCR and IPC-9. 22 The latter is strongly disadvantaged energetically. The transformations of ZSM-55 and its deswelling with diethydimetylammonium (dedma) cation are shown in Figure 1. The borosilicate ZSM-5530 and its Al-containing analogue ZSM-5231 were obtained hydrothermally with choline chloride as the structure directing agent (SDA). The structure of ZSM55 as a CDO precursor32 with C-centered orthorhombic unit cell (0. 78, 2.2, 1.4 nm) similar to MCM-4719 was confirmed by electron diffraction.32 ZSM-55 is one of several layered CDO precursors.13,19,32 It shows d-spacing equal to 1.1 nm that contracts upon calcination to 0.9 nm producing zeolite CDO. As-synthesized ZSM-55 was converted to the organic-free form, H-ZSM-55, by treatment with 3 M HCl in methanol. It also reduced B content from 0.2 to 0.012% (Si/B ∼ 103, 11B NMR shown in Figure 1S). The interlayer d-spacing of the air-dried detemplated HReceived: April 1, 2016 Revised: May 16, 2016
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DOI: 10.1021/acs.chemmater.6b01302 Chem. Mater. XXXX, XXX, XXX−XXX
Communication
Chemistry of Materials
Figure 1. Transformations of ZSM-55 with framework assignments based on powder X-ray diffraction.
ZSM-55 was surprisingly unchanged vs the original precursor with the SDA, but the higher angle diffraction lines were clearly altered and included broadening. The expected contraction by 0.2 nm did occur upon drying at 65 or 100 °C (see Figure 2). Figure 3. XRD patterns of ZSM-55 and the calcined derivatives obtained after swelling with surfactant and deswelling with dedma−Cl and −OH in ethanol in comparison to the FER and CDO patterns from the IZA Structure Web site.
as-synthesized ZSM-55 to produce its interlayer expanded form CDO-IEZ, which is included in Table 1. The apparent preference of the separated fer layers to produce FER-dominant structure, even when derived from a CDO precursor, was changed in favor of the CDO. It was achieved by deswelling of the swollen CDO precursor with dedma−OH in alcohol and calcination. Deswelling with dedma chloride afforded FER. The results are summarized in Figure 3 and in Table 1. Deswelling with dedma produced unambiguous direction into either CDO or FER-dominant zeolites. Deswelling with TEA+ was different: chloride gave FER while hydroxide gave intercalated solid that became amorphous upon calcination (see Figure 2S in Supporting Information). The detemplation with dedma−OH was complete within 1 h (Figure 6S), ruling out the alternative conversion pathway: dissolution and recrystallization into FER or CDO due to high pH and organic compounds acting as SDAs. This alternative is highly unlikely due to low temperature and short duration of treatments, which are insufficient to crystallize fer layers (e.g., syntheses of ZSM-55 and FER with CDO seeds33 take >90 h at 150 °C). Additionally, SEM images show preservation of the original crystal habit and size (Figure 5S). The XRD results present a relatively simple view of the interconversions between CDO and FER as well as disordered structures. The real situation may be much more complex as already revealed by Marler et al.,19 who distinguished four different layered arrangements with SDAs included before calcination. In our case this complexity is indicated by the textural parameters data shown in Table 1. All of the products obtained by transformation of the detemplated H-ZSM-55, including the CDO zeolite, show BET surface areas above 300 m2/g and micropore volume in the range 0.11−0.15 cm3/g. For the CDO zeolite obtained by calcination of as-synthesized ZSM-55 these values were
