Silicalite-1 Crystals Etched with Hydrofluoric Acid Dissolved in Water

J. Phys. Chem. C 2010, 114, 13685–13694

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Silicalite-1 Crystals Etched with Hydrofluoric Acid Dissolved in Water or Acetone Libor Brabec* and Milan Kocirik J. HeyroVsky Institute of Physical Chemistry of the ASCR, V.V.i., DolejsˇkoVa 3, 182 23 Prague 8, Czech Republic ReceiVed: March 25, 2010; ReVised Manuscript ReceiVed: May 23, 2010

Highly siliceous (Si/Al ≈ 350) MFI-type crystals of various sizes and morphologies were etched with HF dissolved either in water or in acetone. The highest concentration of HF used was 4 wt % in water and 5.5 wt % in acetone with etching time shorter than 1 h. After rapid etching, the lateral faces of coffin-shaped crystals exhibited differently resistant triangular areas. Slow etching lasting weeks or months was performed in 100× diluted solutions and provided oriented rectangular pits corresponding to the point-group symmetry of MFI structure. Our coffin-shaped crystals were found to be twins of lateral faces as {100} consisting of two pyramidal segments ingrown to the crystal bed. Inner resistant triangle skins were found in calcined crystals indicating preferential deposition of organic residues on the segment interface. Such skins also appeared in small flat monocrystals at the top of the calcined polycrystalline layer. They seem to be analogous to those found in coffin-shaped crystal twins. Etching of calcined crystals with HF-acetone solution led to dissolution of inner crystal bulk contrary to the impregnated outer shell. The etching patterns both on as-synthesized coffin-shaped crystals (surface triangles) and on calcined ones (inner triangles) resemble the well-known optical hourglass effect. Introduction A dispute about orientation of individual crystal segments in silicalite-1 crystals and its development in the course of crystal growth continues. The relevance of this problem increases because of a raising need to grow polycrystalline layers of MFI crystals with desired crystal orientation. Individual crystals represent the building units of such layers. Thus, this effort is directed not only toward acquisition of fundamental knowledge about growth of the zeolitic phase but also toward development of zeolitic membranes, films to cover sensors, and organized layers of zeolitic catalysts in microfluidic reactors. Dispute also continues about the physical nature of crystal parts and their boundaries in particular about (1) crystallographic orientation of the parts, (2) occurrence of various defects and channel system mismatch at the boundaries (diffusion barriers), (3) permeabilities for species along the boundaries, and (4) separability of the crystal parts along the boundaries. Depending on the conditions of their synthesis, silicalite-1 crystals differ in size and morphology. Models of internal crystal morphology taken from the literature1-3 are illustrated in Figure 1. Very often, MFI-type crystals are twins. This means that they consist of segments of different crystallographic orientation. A crystal bed and two 90° intergrown pyramidal parts (Figure 1A) were assumed in many articles.3-11 Models composed of four lateral pyramids and two wedges (Figure 1C and D) were also considered1,2,12-15 where at least one pair of pyramids and both wedges are of the same crystallographic orientation. Thin, tabular monocrystals were reported by Geus et al.15 These crystals were 260 µm in length with a high ratio of their width to thickness (120 µm:25 µm). Starting from the optical hourglass effect, well visible after calcining the crystals, the authors suggested that the crystal consisted of three segment pairs (Figure 1D without the crystal core). * To whom correspondence should be addressed. Phone: (+420) 26605 3775. Fax: (+420) 28658 2307. E-mail: [email protected].

Agger et al.1 abided by monocrystals with pyramidal domains (Figure 1D) differing in the types of defects and in the number of OH groups in directions [100] and [010]. They suggested switching between MFI and MEL structures in the growth direction [100] and supposed a higher concentration of silanol groups in the [010] one. Different heights of terraces on adjacent lateral crystal faces constitute a serious argument in favor of such an inhomogeneous monocrystallinity. The hourglass effect has been explained by different refractive indices of individual zones identical to pyramidal segments of crystal. The existence of coffin-shaped ZSM-5 monocrystals 80 × 15 × 10 µm3 was also confirmed by Roeffaers et al.13 They used 4-(4-diethylaminostyryl)-1-methylpyridinium iodide (DAMPI) as a fluorescence agent preferentially adsorbed on (010) face with straight channel mouths. Altogether, large MFI monocrystals (80-300 µm in length) were only referred to in the aforementioned three articles.1,13,15 All these authors synthesized the crystals from alkali-free batches according to Mu¨ller and Unger16 and assumed that the crystal consisted of at least six parts (Figure 1C and D). On the other hand, Weidenthaler et al.4 referred to the same synthesis work16 and concluded from optical methods that crystals consisted of a crystal bed and two 90° intergrown parts (Figure 1A). In addition, their crystals were not of the typical coffin shape with end roofs but exhibited unusual facets perpendicular to the c-axis. The following two works reported on twinned large crystals. Karwacki et al.14 accepted the model shown in Figure 1C. This idea was deduced from the hourglass effect visible during template removal and from a dumbbell-shaped crystal stage typical of silicoaluminophosphate (SAPO) and aluminophosphate (AlPO) materials. Schmidt et al.2 used the same morphological model in their article but concluded that the central wedges should not be considered separately from those two pyramids which are of identical crystallographic orientation as the wedges. Consequently, they support the model in Figure 1A. A remarkable finding was published recently on the basis

10.1021/jp1027228  2010 American Chemical Society Published on Web 07/26/2010

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Figure 1. Models of internal crystal morphology according to literature. (A) A crystal bed with two pyramids, (B) a virtually homogeneous monocrystal, (C) two wedges on the c-axis and two pairs of lateral pyramids around, and (D) segments of a monocrystal basically similar to those of crystal twins.

of analysis of MFI crystals exhibiting a broad range of length/ width and Si/Al ratios. The authors reported on the occurrence of additional crystal subunits comprising four narrow wedges (subunits β).17 The growth of twinning crystal components was always observed on parent (010) faces. With regard to their orientation, these components are usually considered to be turned by 90° around the c-axis toward the parent crystal. However, this statement does not apply absolutely: in the case of twinning, that is, reflection on (110) plane, there must be a deviation from 90° because the unit cell parameters a and b are not equal (a/b ≈ 1.008, deviation 0.45°). This fact was emphasized by Rieder et al.18 A similar difference between parameters a and b was found earlier also by Price et al.19 for silicalite-1 containing Fanions and by Olson et al.20 for ZSM-5 (MFI structure

containing Al). The adjacent lateral faces were examined by microhardness measurement, but no significant difference was found.21 This result supports the equivalence of all lateral faces, that is, the crystal twinning. HF acid was expected to be a suitable etching agent to reveal boundaries of higher permeability and separability in silica and zeolitic materials. However, except for our previous papers,22-24 only two reports were found about zeolites treated with HF acid: Hay et al.25 used HF to remove ramps on (100) faces of ZSM-5 crystals, and Wloch26 washed the surface of ZSM-5 crystals with HF acid diluted with acetone. His assynthesized crystals did not exhibit any pits or cracks in contrast to calcined crystals, which dissolved easily. These crystals probably contained a very low number of defects and were completely filled with template.

Crystals Etched with Hydrofluoric Acid

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TABLE 1: Typical Crystal Dimensions, Synthesis Temperature, and Synthesis Time sample

dimensions [µm]

T [°C]

time

S1 S2 S3 SL

220 × 50 × 50 180 × 45 × 45 2.5 × 1.5 × 0.5 25 × 10 × 5

160 185 95 160

17 days 7 days 7 days 16 h

Schmidt et al.2 attained corrosion of crystals by H2O2/NaOH treatment in a microwave oven followed by 1 h exposition to ultrasound in water. The present work reports on comparative etching results for highly silicious MFI crystals of various sizes and morphologies, as-synthesized and calcined, and etched under different conditions. The aim of the study is to reveal the orientation, morphology, and separability of internal crystal segments. The effect of template content and that of organic residues on etching process constitute important questions to be answered. Another aim of the study is a search for conditions (in particular the etching bath composition) that favor the formation of characteristic etching patterns on the crystal faces. Such patterns would represent negative crystals revealing the elements of point-group symmetry of the etched crystal. The theory and the experience of etch pattern formation on numerous nonporous crystals have been described in detail in the monograph by Heimann.27 To the best of our knowledge, this phenomenon has not yet been observed in zeolites. Experimental Section Three samples (S1-S3) of highly silicious (Si/Al ≈ 350) MFI-type crystals used in this study were synthesized in our laboratory by modification of the procedure proposed by Kornatowski.28 Table 1 provides the dimensions of crystals and two synthesis parameters. The longest dimension is along the c-axis, and the shortest is along the b-axis of the crystal bed. In the case of the S3 sample, the latter dimension represents the thickness of the parent crystals, that is, the thickness without the outlying twinned segments (cf. Figure 7). Samples S1 and S2 are coffin-shaped crystals roughly 200 µm long. The molar composition of the synthesis batch was 100 SiO2:4.4 Na2O:5.6 TPABr:2832 H2O (TPABr ) tetrapropylammonium bromide). Colloidal solution Tosil (30 wt % SiO2, pH ) 9.0, manufactured by KOMA s.r.o., Czech Republic) was used as the source of silica. NaHCO3 (Solvay) served as the source of sodium. Crystals were grown in Teflon-lined stainless steel autoclaves. The small crystals S3 (cf. Figure 7) grew from the mixture 96 SiO2:11.5 TPAOH:0.77 NaOH:5760 H2O:384 EtOH (TPAOH ) tetrapropylammonium hydroxide, tetraorthosilicate (TEOS) was used as the source of silica). Before heating, the mixture was shaken at room temperature for 24 h. After centrifuging and washing by distilled water in an ultrasound bath, the crystals were calcined. Calcination was mostly performed by heating the crystals in a dry-air stream to 550 °C at the rate of 0.5 °C/min. The high temperature was held for 24 h, and the sample was then cooled at 2 °C/min. Sample S1 was heated either at the rate of 5 °C/ min (S1-h5) or at the rate of 0.5 °C/min (S1-h05). Self-supporting silicalite-1 layers with 25 µm crystals SL on their surface were synthesized23 from the molar mixture 10 SiO2: TPABr:NaOH:800 H2O and were calcined at 400 °C for 24 h (standard heating rate of 0.5 °C/min). Both as-synthesized and calcined large crystals S1 and S2 were etched. Small crystals S3 and SL were etched after their calcination. Etching proceeded with diluted HF acid obtained

from 40 wt % water solution supplied by Lach-Ner, s.r.o., Czech Republic. The basic concentrations used were 4 wt % in water and 5.5 wt % in acetone. A few milligrams of crystals were etched in 1.5 mL of one of these solutions. For the least resistant sample S1, initial etching patterns could be already observed after a few minutes both in water and acetone HF solutions. In the most diluted solutions (0.02-0.05 wt % HF), crystals were kept for 3 weeks or 3 months to obtain as clear and distinct as reasonably possible characteristic etching patterns on the crystal faces. Micrographs were made using a JEOL JSM-5500LV scanning electron microscope after covering the samples with a Pt layer ca. 15 nm thick to prevent accumulation of surface charge. Results and Discussion Coffin-Shaped Crystals As-Synthesized (with TPA+ in Zeolitic Channels). In a previous paper,18 we reported on twinning of S1 crystals, which means the equivalence of their lateral faces as {100}. We concluded that for these crystals the model in Figure 1A according to Caro et al.3 and the references contained therein adequately describes the number and arrangement of their crystal segments. The same holds true for S2 crystals. All their lateral faces exhibit a pair of triangle figures formed by an array of etch pits (Figure 2A and B). Complementary triangular areas on the faces are very rarely pitted. When acetone was used instead of water, crystals S2 were etched somewhat slower. After etching with 5.5% HF for 15 and 45 min, less resistant triangular areas became discernible and well visible, respectively (Figure 2E and F). The phenomenon can be understood if one takes into account the permeability of HF into the crystal along the boundaries of the pyramids. A trace of the etchant is apparent on transversal cuts of S1 crystals etched with 4% HF solution in water for 3 min as demonstrated in Figure 2C and D. This result suggests that formation of triangle figures on the lateral faces of the crystal bed can be explained by a combined HF attack from outside and from the inner triangle interface (prolate sides of pyramids). The weakly pitted triangular areas are much less accessible for inner attack. As can be seen from Figure 1A, the strongly pitted triangles (dotted area) represent the projection of ingrown pyramids on lateral faces of the crystal bed. The basal faces of pyramids exhibit similar triangle etch patterns although a lower accessibility of weakly pitted triangles for the inner attack is not so obvious. The interface between the transverse sides of pyramids and the crystal bed seems to be tight enough (further indicia will be mentioned in the section Coffin-Shaped Crystals Calcined). Also, a less probable possibility of some chemical or physical differences in the triangle regions should be considered for completeness’ sake. The loose intergrowth of pyramids with the crystal bed in coffin-shaped crystals can be also demonstrated by visible intrusion of iodine along those boundaries of pyramids as shown in Figure 3. However, iodine did not intrude into as-synthesized crystals whereas HF obviously did. On the other hand, impregnation of this interface resulting from calcination represents a barrier for HF (see below), but iodine in toluene or other organic solvents can penetrate.5 Coffin-Shaped Crystals Calcined (after TPA+ Removal). The calcined MFI crystals were found to behave during HF etching in a very different way compared with as-synthesized ones. This can be clearly seen in Figure 4 which shows crystals S1 etched with 4% HF solution in water for 10 min. A rapid and uniform recession of crystal surface took place throughout the sample. Neither pitting nor visualization of

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Figure 2. As-synthesized crystals S1 and S2 after etching. (A-D) Crystals etched with 4% HF in water: A, S2 crystals etched for 30 min; B, S2 crystal magnified; C, transversal cut through the middle of S1 crystal; D, transversal cut in one-fourth of S1 crystal length. (Cuts were etched for 3 min. The rind around the crystal is a glue.) (E, F) Crystals S2 etched with 5.5% HF in acetone: E, 15 min; F, 45 min.

boundaries as in the case of as-synthesized crystals (Figure 2) was observed. At the same time the crystal edges became rounded, crystals took a cigar shape, and the terminal roofs dissolved more rapidly than the middle of the crystal body. This finding also contrasts with noncalcined crystals where the terminal roofs are the most resistant crystal parts (cf. Figure 2A). The rapid and uniform surface dissolution can be explained taking into account that the channel system in calcined crystals is free for transport of the etchant molecules. On the other hand, in our previous work,29 we gave evidence that products of template decomposition (propylene etc.) tend to associate on crystal surfaces and in confined spaces of appropriate dimensions. Thus, the interface space along the segment boundaries was found to be filled with such associates, which are expected to be resistant to HF solutions in water owing to their chemical nature. A distinct difference in the etching process appeared when water was replaced by acetone. Figure 5 shows calcined crystals S1 and S2 after etching with 5.5% HF in acetone.

Figure 3. Silicalite-1 crystal twin colored with iodine. After a standard calcination, the crystal was filled with toluene. Coloring proceeded for 8 h in saturated solution of iodine in toluene.



Figure 4. Calcined crystals S1 etched with 4% HF in water for 10 min.

Figure 5. Calcined crystals S1 and S2 etched with 5.5% HF in acetone. (A, B) Crystals S1-h5 etched for 10 min. (C) Crystal S1-h5 etched for 20 min. (D) Crystal S2 etched for 45 min. (E, F) Crystals S1-h05 etched for 40 min. A, resistant crystal siding; B, mouth spatula-like body inside the crystal bed; C, incrustation over bed edges; D, hourglass etching pattern; E, resistant inner interface; F, incrustation surviving over the dissolving crystal bed.

The etching time was between 10 and 45 min. New interesting features of internal morphology were revealed. An example of crystals S1-h5 etched for 10 min is shown in Figure 5A and B

(in this case, the heating rate in the calcination program was 10 times higher than in the remaining instances). Some crystals were completely dissolved inside while the circumferential walls

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Figure 6. Etched fracture areas of calcined coffin-shaped crystals similar to S1. (A) Crystal fragment etched with 3% HF in water for 10 min. (B) Crystal fragment etched with 5.5% HF in acetone for 30 min.

Figure 7. Calcined crystals S3 etched with 5.5% HF in acetone. (A) 15 min; (B) 30 min; (C, D) 60 min.

as well as the end roofs survived. This “rampart” indicates that the walls were impregnated by deposited organic residues protecting them against etching. It is not clear why the impregnated walls are more resistant than the crystal interior in the case of HF-acetone solution while the opposite is true with HF in water. The etching patterns in Figure 5B indicate that crystal beds do not represent monolithic blocks. They contain embedded bodies in the form of mouth spatula separable from the bed remainder by a boundary permeable to HF. In comparison with the S1-h5 crystals, slowly heated crystals S1-h05 and S2 were more resistant to etching (Figure 5C-F). The low heating rate caused a high residence time of TPA+ decomposition products in the interfacial spaces where they associate and are deposited. A weaker effect of microcracking on segment boundaries might represent another reason for the higher resistivity of slowly heated crystals. Etching revealed a thin incrustation formed along the prolate triangle interface between the crystal bed and pyramids (Figure

5C-F). The incrustation extended to the lateral sides of the crystal bed via bed edges (Figure 5C) and was more resistant to etching than the remaining crystal bed (Figure 5F). The incrustation visible only on those prolate triangular areas can be explained as follows: on these areas inside the crystal twin, both types of channels (straight and sinusoidal) are open while on the transverse triangle-area interface only straight channels lead from the crystal bed and only sinusoidal ones from the pyramid. Thus, the flow of organic compounds from pores during calcination should be slower on the transverse interface. Moreover, with regard to the spatula-like imprints (Figure 5B and E) and to the resistant outer triangular areas in the case of as-synthesized crystals (Figure 2A), the pyramids seem to be intergrown with the crystal bed more tightly along their transverse sides than along the prolate ones. Consequently, the interfacial space along the transverse triangle sides would be too narrow for a pronounced accumulation of organic residues. Another finding apparent from Figure 5D is that incrustation regions assume an hourglass shape. In contrast to the optical



Figure 8. Layer crystals SL calcined at 400 °C, etched with 0.4% HF in water for 30 s. (A) Layer top side before etching; (B) etched layer with many “love letters”, i.e., exposed crystal insides with pairs of triangular skins; (C, D) dissolving crystal bulks with skins covering the lateral faces completely (before lost of outer skins); (E) triangular skins start to appear under fading top skin; (F) triangular skins in detail.

hourglass effect observed via light microscopy, here a tangible hourglass-shaped solid figure was formed. We summarize our findings concerning a contingent separability of four pyramids around the c-axis illustrated in Figure 1C. To the best of our knowledge, these four pyramids have never been obtained or indicated by any procedure. Their separability should be indicated by an etching fissure around an isolated square facet (i.e., profile of the prospective central wedge, cf. Figure 1C) in the middle of a transversal crystal cut. However, etching patterns on transversal section areas of our as-synthesized crystals24 showed clearly the presence of only two pyramidal segments separable from the crystal bed (Figure 2C and D). In the case of some calcined crystals prepared in a manner similar to that used for sample S1, we observed rectangular or square figures on fracture areas (Figure 6). We do not ascribe these figures to the presence of a core segment or central wedges with four pyramids around. The explanation of this phenomenon observed on profiles of calcined crystals remains open.

Figure 9. A hypothetical scheme of an SL monocrystal from the calcined polycrystalline layer. Triangular skins occur below a thin pyramidal crystal segment. The outer skin (gray rectangle area) covers the b-face completely. On the right-hand side, the skins are depicted in c-axis view.

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Figure 10. Pits on lateral sides of as-synthesized crystals S1 (4% HF in water for 10 min). (A) View to (100) face; (B) individual pits in detail.

Another reflection belongs to a possible occurrence of further crystal subunits mentioned by Karwacki et al.17 in their recent work. These authors presented convincing evidence of the existence of additional wedges in MFI crystals. These wedges exhibited a slightly different orientation toward the parent crystal structure and were found on crystals with a substantial Al amount and with a low length/width ratio (L/W ∼ 2) whereas oblong crystal prisms (L/W > 3.8) did not exhibit this attribute. We did not observe these additional wedges in our crystals using our etching procedure. For samples S1 and S2 (W ) T (thickness) < L and L/W ∼ 4), the above result of our observation is consistent with that in ref 17. In the case of crystals S3 and SL, the dimension criterion L/W in ref 17 may not be applicable because these crystals were tabular (W > T). Moreover, Al content in all our samples was at least 1 order of magnitude lower than that in aforementioned crystals with additional wedges. Small Twinned Crystals Calcined. The results of etching small calcined crystals S3 are presented in Figure 7. The crystals consisted of a boat-shaped platelet and a pair of younger crystal individuals ingrown into the platelet on its basal faces. Under the given etching conditions, a prevailing portion of twins was decomposed into these three parts. A dovetail is noticeable on the younger segments interlocking the complemental hollow in the platelet. The platelets themselves are assumed to be monocrystals. After etching with 4% HF in water or in 5.5% HF in acetone for 15 min, the crystal surface appeared to be unaffected (Figure 7A). Further etching was rather more effective in the HF-acetone solution. After 30 min etching, the crystal interior was partially dissolved (Figure 7B). After 1 h etching, many crystal bulks were dissolved completely while the impregnated outer shells survived (Figure 7C and D). It may be concluded that the surface of small crystals is resistant to the etchant because of association of template decomposition products. Calcined Polycrystalline Layers. The aforementioned, rather surprising finding on organic deposits in the crystal surface region is expected to have serious consequences in particular for zeolitic membranes. Etching patterns in our previous studies on self-standing polycrystalline silicalite-1 layers have already indicated this effect.22,23 Plugging of crystal surface region by various obstacles (1) decreases the crystal surface permeability to various penetrants and (2) changes the adsorption energy of penetrants at the crystal surface. Their combined effect has been predicted using a model given in a theoretical study.30 Apart from an open question on crystal surface permeability in zeolitic membranes because of the presence of template residues, there is another open question: what is the effect of some stable

template residues on mass transport in nonzeolitic (intercrystalline) pores of the membrane. The discovered etching procedure may thus become a diagnostic method to check the template residue distribution or its complete removal. In view of the present strong requirements on diminishing calcination temperature of zeolitic membranes down to or below 400 °C, we examined the presence of organic deposits by etching the crystals grown on the top side of self-standing silicalite-1 layer calcined at 400 °C. Figure 8A shows their appearance before etching. The other pictures shown in Figure 8 represent patterns after etching with 0.4% HF in water for 30 s. Figure 8B demonstrates the occurrence of crystals with triangle-area skins on their top (010) face analogic to those in the aforementioned coffin-shaped twins after removing the pyramidal domains. Very probably, these skins represent an interface (resistant to etching because of organic deposits) between the basic crystal body and a segment (likely a thin pyramid) which dissolved. This result might comply with the well-founded model of a monocrystal with pyramidal domains proposed by Agger et al.1 (Figure 1D). For greater clarity, the skins are depicted schematically in Figure 9. Protracted Etching of As-Synthesized Coffin-Shaped Crystals: Characteristic Pits. Our further effort in etching methodology was directed to development and analysis of characteristic pits on crystal faces. Such phenomena have been reported for nonporous crystals together with theoretical principles of etching pattern analysis and relation between pit shapes and crystal structure.27 Pits of this kind have never been observed on zeolite crystals and, accordingly, the first task was to screen a broad range of etching conditions and to examine very slow processes lasting weeks or even months. Our investigation has shown that formation of characteristic pits occurs on {100} faces and that the pit shape depends on HF concentration and etching time. The first series of observations relates to etching with HF solutions in water (Figures 10 and 11). During short, strong etching, shallow square pits with round corners formed (Figure 10; the touch of snow in Figure 10B is an etch product settled from the liquid phase). Slow etching led to deep and rather rectangular pits with sharp corners (Figure 11). Other observations focused on etching with HF solutions in acetone (Figure 12). It is apparent from protracted experiments with noncalcined crystals (etching for weeks and months) that there is no significant difference between shapes of pits after etching with HF in water and with HF in acetone. All observed shapes of separate etch pits (i.e., rectangles or rounded squares) on various MFI crystals meet the symmetry conditions (three 2-fold rotation axes and three reflection planes perpendicular to each other) given by the point group mmm



Figure 11. As-synthesized crystals S1 after etching with 0.05% HF in water for 3 weeks. (A) General view; (B) view approximately in the parent [101] direction; (C) a 90°-intertwisted crystal with equally shaped pits on both wings; (D) detail of picture B.

Figure 12. As-synthesized crystals S1 etched with 0.04% HF in acetone for 3 months. A - general view; B - half of a crystal with a well developed pit; C - a crystal dissolved in the thinnest part of the crystal bed; D - view to fracture of a crystal.

(otherwise 2/m 2/m 2/m or D2h). This point group corresponds to the space group Pnma assigned to the MFI framework type. Other important findings from protracted etching (Figures 11 and 12) are as follows:

(1) Etched faces (010) both of the crystal bed and both pyramids became visible after dissolution of the lateral crystal edges. The {010} faces of segments diminished continuously without any discrete deep pits appearing on them. Obviously,

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the etching rate normal to (010) face is slower than the tangential rate along this face. (2) Rectangular pits appeared exclusively on {100} faces. Main axes of these rectangles are parallel to b-axes of individual segments, that is, to MFI straight channels. Etching rate on the surface along the c-axis is lower than that along the b-one probably because the transport along the c-axis is possible only via a tortuous path through sinusoidal and straight channels. (3) A unique distorted crystal was found among etched ones (Figure 11C; note the different orientation of terminal roofs). The image provides a direct comparison of the pyramidal base and the lateral side of the crystal bed. These areas appear to be equivalent as already mentioned; the etch pits of rectangular shape at the left and right crystal halves are identical. Conclusion Silicalite-1 coffin-shaped crystals ca. 200 µm long, prepared by a procedure described above, were found to be twins consisting of a crystal bed and one pair of lateral pyramids. These crystal parts were visualized by etching since HF penetrated preferentially along their boundaries. Each lateral face {100} of the as-synthesized coffin-shaped crystal exhibited a pair of rapidly etched triangular areas. HF-water dissolves the calcined coffin-shaped crystals preferentially from the outside, and HF-acetone dissolves them from the inside. Calcined crystals contained organic residues accumulated along the prolate pyramid boundaries and also on the lateral faces of the crystal. These areas were more resistant than the crystal bulk in particular in the case of HF solution in acetone. After removal of the pyramids from calcined coffin-shaped crystals, hourglass-like boundaries appeared in the crystal bed. They bear the character of tangible objects. Etched small boat-shaped crystals (very probably monocrystals) present on the upper side of calcined polycrystalline layers exhibited the same triangle-area boundaries inside as the coffinshaped crystal twins. Thus, diffusion barriers may be present in these crystals. Protracted etching both with HF-water and HF-acetone solutions resulted in well-developed rectangular pits on {100} faces. The shape and orientation of these pits conform to the symmetry elements of the MFI structure. To the best of our knowledge, this is the first time that an etching technique applied to zeolite crystals provided pits bearing information on the respective point-group symmetry. Acknowledgment. The financial support by Grant Agency of Academy of Sciences of the Czech Republic via grant IAA 400400909 is gratefully acknowledged. The authors are obliged to Prof. M. Rieder for a stimulating discussion.

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JP1027228

Silicalite-1 Crystals Etched with Hydrofluoric Acid Dissolved in Water

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