Swelling and Interlayer Chemistry of Layered MWW Zeolites MCM-22

Article pubs.acs.org/cm

Swelling and Interlayer Chemistry of Layered MWW Zeolites MCM-22 and MCM-56 with High Al Content Wieslaw J. Roth,*,†,‡ Jiri Č ejka,‡,§ Roberto Millini,∥ Erica Montanari,⊥ Barbara Gil,† and Martin Kubu‡ †

Faculty of Chemistry, Jagiellonian University in Kraków, ul. Ingardena 3, 30-060 Kraków, Poland J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of Czech Republic, Dolejškova 2155/3, 182 23 Prague 8, Czech Republic § King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia ∥ Renewable Energy & Environmental Laboratories, Eni S.p.A., Via Fauser 4, I-28100 Novara, Italy ⊥ Physical Chemistry Dept., Downstream Laboratories, Eni S.p.A., Via F. Maritano 26, I-20097 San Donato Milanese, Italy ‡

S Supporting Information *

ABSTRACT: Swelling of layered zeolite precursors such as MCM-22P with cationic surfactants at high pH is the key step in their subsequent conversion into expanded lamellar materials by pillaring and delamination. Increasing Al content in the precursors can yield more active catalysts but affects their swelling efficiency especially at lower temperature, which was reported as favorable for layer structure preservation with more siliceous MCM-22P. The latter, a (multi)layered precursor, was investigated in this work and showed inadequate swelling of its high-Al representatives with organic hydroxide/surfactant mixtures and especially when NaOH is the source of high pH. In contrast, the unilamellar MCM-56 was found to swell readily at room temperature with various hydroxide sources, and notably with NaOH, in combination with the surfactant. The observed differences between MCM-56 and MCM-22P, especially with regard to swelling with NaOH, are attributed to the fundamentally different nature of their layer surface and interlayer linking. The former has surface terminated with AlOH−Na+ moieties, producing weak connections, instead of the pyramidal SiOHs populating the MCM-22P surface and forming interlayer H-bonding. The methods used for validating the swelling and product characterization included XRD, nitrogen sorption, IR spectroscopy and TEM imaging. The microscopy confirmed, by direct visualization, the extensive but not complete swelling of MCM-56, which can be enhanced by treatment at higher temperature.

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INTRODUCTION Zeolites are well-known and widely used in commercial applications as exceptional heterogeneous catalysts1,2 and selective sorbents3 with extraordinary properties arising from their framework structures with uniform micropores. They had been known exclusively as networks extended continuously in 3D until framework MWW revealed formation via a layered precursor, denoted MCM-22P4,5 and also ERB-1.6 Subsequently other frameworks such as ferrierite,7 sodalite,8 and recently MFI,9 showed formation of layered forms as well. There are now approximately 15 frameworks, out of over 200 known, recognized to produce layered precursors and other lamellar forms10 consisting of monolayers with thickness of approximately 1−3 nm.4,6,7 This was a significant fundamental development but it also has very beneficial practical side. In contrast to virtually immutable 3D zeolite structures, their 2D lamellar derivatives can be modified11,12 into new architectures and compositions like the well-established 2-dimensional solids.13−15 In particular, they can be converted into more © XXXX American Chemical Society

open materials with enhanced accessibility to zeolitic active centers,16−19 which is often quite limited in the conventional 3D frameworks. A more general concept of zeolite structures, integrating the 3D and 2D forms as a two-dimensional representation, has been proposed.10,20 The starting layered zeolites have been obtained by three different pathways: bottom-up direct syntheses using organic templates21 that are usually discovered by accident but recently also by design9,22 and the novel top-down process from other frameworks conducive to selective degradation.23−26 The principal transformations of layered zeolite precursors are illustrated in Figure 1 based on the framework MWW, and in principle they are applicable to all others. The intercalation of large organic molecules such as surfactants, which is called swelling,27 is the first step toward generation of expanded Received: March 19, 2015 Revised: June 9, 2015

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DOI: 10.1021/acs.chemmater.5b01030 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

was proposed as an alternative allowing better layer preservation and avoidance of degradation.35 It was developed with precursors having relatively low Al content. Subsequent catalytic evaluation postulated that layer preservation (through swelling at RT) resulted in a better catalytic activity of the MCM-36 obtained after pillaring of swollen precursors.36 Although the best MCM-36 performance was observed with MCM-22P swollen at RT it was also the most Al-rich precursor used in this study. Direct comparison of MCM-36 samples prepared from the same precursor swollen at RT and higher temperatures indicated very similar catalytic performance. This observation of minimal activity difference proves that the choice of lower (RT) vs higher temperature has secondary influence on catalytic activity, provided that the swelling efficiency is comparable.36 At the same time it was evident that the initial higher Al content in the starting MCM-22P was the primary source of higher catalytic activity in its derivatives. The increasing activity with Al content is a general trend for zeolites37 and therefore it is important to understand the behavior of representatives with maximized Al content, although it does not ensure the best ultimate performance. The framework MWW is so far unique by offering several modifiable layered forms with high Al content down to Si/Al around 10/1.38 They are also available via convenient straightforward syntheses amenable to a scale-up. As the essentially only example of a layered precursor that can achieve high Al content, MCM-22P shows that the swelling efficiency is strongly influenced by Si/Al ratio of the layers.39 This was also observed40 in the preparation of ITQ-2, which exhibited lower BET area and pore volume with increasing Al content of the precursor. In addition, mild swelling conditions were found to be insufficient to enable ITQ-2 formation by delamination.35 This work concerns swelling and characterization of high-Al MWW materials MCM-22P and MCM-56. The former shows sensitivity to the type of hydroxide added to provide high pH in the surfactant solution. In particular, NaOH, which would be the most convenient reagent to use, is found to be very ineffective,29 while some of the organic quaternary hydroxides allow only partial swelling. In contrast, MCM-56, which has intrinsic high Al content,41 shows easy swelling under mild

Figure 1. Principal transformations reported for MCM-22P and MCM-56 with representative interlayer d001 spacing distances from XRD. Filled pores indicate presence of SDA molecules. The idealized representation of MCM-22P as ordered stacks of MWW layers is explained in the first section below.

structures with permanently separated layers by processes like pillaring and delamination.28 It is typically the most difficult and crucial stage that in the case of 2D zeolites requires unprecedented severity, namely, high pH of 12.5 and above.29 This calls for a particular attention because, in addition to the possibility of incomplete swelling, other undesirable effects like partial layer dissolution with accompanying degradation and even formation of mesophases like the M41S materials30,31 may occur.32 The difference between swelling and the undesired reactions may be difficult to detect. There are in fact quite a few literature reports that erroneously claim zeolite swelling and successful pillaring or delamination. The breaking of interlayer hydrogen bonding between the layers is considered the main reason for the needed high severity, i.e., pH, of the swelling conditions.33,34 Swelling was initially developed as a high temperature process carried out at near 100 °C (using MCM-22P with a high Al content).27 Later on, room temperature swelling (RT)

Table 1. Summary of Preparation Conditions, Positions of Low Angle XRD Maxima (Cu Kα radiation, λ = 0.154178 nm) and Textural Properties of the Studied Materialsa XRDlow angle peak maximum swollen material MCM-22P + calc MCM-22P + swell

MCM-56 + calc MCM-56 + swell

a

pillared

alkalinity, MOH (mol/dm3)

estd. swell (%)

2θ (deg)

d (nm)

2θ (deg)

NaOH TMA-OH HD-OH/Cl TPA-OH TPA-OH 95C

0.54 0.39 0.31 0.40 0.40

0 80 100 10 50

6.5 6.0 1.8 1.7 1.8 1.8

1.36 1.47 4.90 5.20 4.90 4.90

N/A 1.9 1.8 N/A N/A

TPAOH/calc NaOH NaOH HD-OH/Cl HD-OH/Cl 99C HD-OH/Cl 99C, hydr

0.51 0.52 0.66 0.16 0.16 0.16

100 100 100 100 100 100

1.75 1.6 1.6 1.55 1.55 1.55

5.04 5.52 5.52 5.69 5.69 5.69

N/A N/A 1.8 1.7 1.75 1.8

swelling, MOH

d (nm)

4.65 4.90

4.90 5.20 5.04 4.90

textural properties BET (m2/g)

pore vol. (cm3/g)

520

0.34

593 735

0.51 0.55

419 499

0.53 0.60

697 676 970 851

0.59 0.85 0.68 0.57

HD = hexadecyltrimethylammonium. B


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Adsorption isotherms of nitrogen were determined at −196 °C using an ASAP 2020 (Micromeritics) volumetric apparatus as described earlier.43 The detailed procedure is provided in the Supporting Information. The concentration of Lewis (LAS) and Brønsted (BAS) acid sites was determined using adsorption of pyridine (Py, Sigma-Aldrich, analytical grade) followed by IR spectroscopy (Tensor 27 from Bruker, MTC detector, spectral resolution 2 cm−1).43 Full details are available in Supporting Information. Concentrations of Lewis and Brønsted acid sites were evaluated from the intensities of bands at 1454 cm−1 (LAS) and at 1545 cm−1 (BAS) after desorption at 250 °C based on absorption coefficients determined in the laboratory using external standards,46 ε(LAS) = 0.165 cm2/μmol and ε(BAS) = 0.044 cm2/ μmol. 27 Al NMR spectra were recorded as described earlier, using APOLLO console (Tecmag) at the magnetic field of 7.05 T (Magnex).43 Additional details are in Supporting Information. TEM investigations were carried out using a JEOL JEM 3010 instrument operating at 200 kV accelerating voltage, equipped with an energy dispersive spectrometer (EDS) by Oxford (Isis). The powders were embedded in epoxy resin, and 35−40 nm thick sections were obtained with a Reichert-Jung ULTRACUT ultramicrotome equipped with a diamond knife.

conditions even with the use of NaOH/surfactant. MCM-56 is considered to be a unilamellar zeolite with MWW topology comprising layers stacked disorderly without interlayer connectivity, except for possible incidental silanol condensations.28,41,42 It crystallizes as a distinct intermediate during preparation of the three-dimensional MCM-49 (high-Al MWW) and must be intercepted at the right time in the course of synthesis.41 If quenched prematurely or too late the MCM-56 monolayers end up contaminated with either unreacted amorphous solid or partially condensed (MCM49), respectively. For example, a recent article34 quotes MCM56 as consisting of 70% “submerged layers”, i.e., disordered or, in our view, monolayers, and 30% condensed, i.e., equivalent to MCM-49. This result can be attributed to overcrystallized MCM-56 (synthesis too long), while the sample we use here is judged to be close to the idealized MCM-56. The present results show MCM-56 swelling readily without interference from other cations, which contrasts the behavior of MCM-22P. This supports its proposed unilamellar character and provides new insights into the nature of MCM-56 and its formation, which have been quite elusive so far.42,43 A study concerning preparation and testing of pillared MCM-36 materials obtained from MCM-22P and MCM-56 but with different Al content has been reported recently.44 The MCM-36 obtained from the former had much lower BET area, which is indicative of inferior sample quality.

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RESULTS AND DISCUSSION Since evaluation of the swelling efficiency of different high-Al MWW materials under various conditions is the main experimental objective in this work the criteria used for this are briefly reviewed first.20 Table 1 summarizes details of the performed syntheses and treatments with physical characteristics of the starting MWW materials and the products after calcinations. Criteria for Estimation of Swelling Efficiency and the Problem of MCM-22P Disorder. The evaluation of the swelling efficiency of MCM-22P is based on XRD and its four specific features shown in Figure 2. In practice one can focus on

EXPERIMENTAL SECTION

Preparation of High-Al MCM-22. The preparation was described earlier.39,43 It is similar to the MCM-56 synthesis below except for the Si/Al of the gel equal to 15/1. Preparation of MCM-56. MCM-56 was prepared according to the published procedure.41,45 The synthesis mixture comprised water, 50% NaOH solution, sodium aluminate (Riedel-de-Haen, 40−45% Na2O, 50−56% Al2O3), hexamethylenimine (HMI), and Ultrasil VN3 in the following molar ratios: Si/Al = 11.5, OH−/Si = 0.21, HMI/Si = 0.35, water/Si = 19.2. The hydrothermal synthesis was carried out with agitation in a Teflon lined Parr reactor at 143 °C. The crystallization was interrupted after 29 h, sampled for crystallinity, and continued for an additional 4 h. The product was isolated by filtration, washed with water, and dried at 110 °C. Swelling of MCM-22P and MCM-56. The swelling solutions were prepared using 25% aqueous solution of hexadecyltrimethylammonium chloride (HDTMA-Cl) either original or partially converted to hydroxide (vide infra) with additions of the hydroxides of Na, tetramethylammonium (TMA), or tetrapropylammonium (TPA) and water. The calculated alkalinities are given in Table 1. Swelling was typically carried out overnight at room temperature and in some cases near boiling as indicated. Solids were isolated by centrifugation, washing with water and decantation, two times, and dried at temperatures no higher than 65 °C. The HDTMA−hydroxide solution was obtained as previously reported45 by contacting 100 g of the 25% solution of HDTMA-Cl with 80 mL of anion exchange resin AG 1-X8. The slurry was slowly stirred overnight and filtered. The [OH−] concentration was approximately 0.4 M. Pillaring. Pillaring was carried out with 0.3−0.4 g of the dried swollen solid in 20 mL of TEOS. The mixture was stirred and heated under reflux at 95 °C for 24 h. The solid was isolated by centrifugation, dried at room temperature, hydrolyzed in water (20 mL), and dried at 100 °C. In one case no hydrolysis was applied. The final product was obtained upon calcination at 540 °C for 4−8 h. Characterization. The characterization by X-ray powder diffraction was carried out using a Bruker AXS D8 Advance diffractometer in the ranges 1−10 and 3−50° 2θ with Cu Kα (λ = 0.154178 nm) radiation.

Figure 2. XRD patterns of MCM-22P treated with various swelling mixtures (hexadecyltrimethylammonium chloride (HDTMA-Cl) + MOH) and nitric acid with features for diagnosing the extent of MCM-22P conversion.

just one: the disappearing separation between the original (101) and (102) reflections at 8 and 10° 2θ, resulting in a broad band without a dip/valley in the center. The other three, i.e., new peaks at ∼5 and 1.7 nm ((001) and (003) reflections) and disappearance of the initial (002) reflection at 6.5° 2θ, are known from experience to follow suit when the valley is absent. For MCM-56 the broad band at 8−10° 2θ is its defining C


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Chemistry of Materials feature,41 which leaves only the (001) and (003) peaks as the criteria for swelling evaluation. They provide only visual semiquantitative measure and must be supplemented by a more reliable test, which involves pillaring and BET surface area determination. The BET area value indicative of satisfactory swelling/pillaring is approximately 700 m2/g and higher. It is the typical minimal value obtained when the XRD criteria, especially the no-valley condition for MCM-22P swelling, are fulfilled. These criteria for “complete” or satisfactory swelling are not synonymous with separating all layers in the sample. This is a complex problem, which is further illustrated and discussed based on TEM microscopy. MCM-22P has been often described38 and is represented in Figure 1 as having ordered stacking of its MWW layers with ca. 2.7 nm repeat. This apparent 3D order was postulated because of the XRD pattern containing discrete interlayer reflections with nonzero h and/or k indices (i.e., hkl, with l and h or k ≠ 0),47 which implies periodic repeat in the stacking direction. Other observations6 and techniques, like electron diffraction showing streaking along c*,48 and theoretical simulations,34 suggest limited order or even stacking faults. This is difficult to quantify but is evidently quite limited in comparison to MCM56 and the other multilayered MWW materials EMM-10P, which are distinguished by not having distinct (101) and (102) reflections like in MCM-22P. Because of this contrast in XRD we will consider and represent MCM-22P as relatively ordered. It will allow us to clearly differentiate from the situation when swelling of MCM-22P produces obliteration of any lateral order/3D periodicity evidenced by the absence of (101) and (102) reflections in the properly swollen product. Interestingly, as-made ERB-1 precursor can produce a 3D ordered structure by intercalating [2,2,2]-azabicyclooctane.6 Swelling of High-Al MCM-22P. MCM-22P precursor investigated in this work was prepared according to the most common formulation resulting in Al content at or near the possible maximum corresponding to Si/Al ∼11/149 (if more Al is added the transition to MCM-56 occurs). As shown in the literature it could be swollen with high efficiency with surfactant hydroxide at the temperature near 100 °C.27 It is evident from the XRD patterns (Figure 2) and the data in Table 1 that highAl MCM-22P is completely swollen by the surfactant hydroxide solution at room temperature but in general its swelling efficiency varies depending on the type of cation-hydroxide used to generate high pH in the swelling mixture. Even increasing temperature may be only partially effective as illustrated by the TPA-OH case. This contrasts the apparent ease of swelling of the more siliceous MCM-22P reported earlier35,36 and the MCM-56 described here. Especially notable is the fact that the multilayered MCM-22P shows hardly any swelling with the NaOH/surfactant solution. It is particularly consequential as behavior contrasting that of MCM-56 and is analyzed further in comparison to it in the next section. The high Al content in MCM-22P affects its ability toward the other transformations as exemplified by the already mentioned delamination to ITQ-2.40 It is seen here to also extend to the reaction with acid (XRD shown in Figure 2). This treatment was reported previously with more siliceous samples to cause collapse of the interlayer space (c-cell d-spacing change from 2.7 to 2.5 nm) accompanied by considerable disorder, which was concluded from the XRD pattern resembling MCM56. The product is called MCM-56 analogue,50,51 but it is still unknown if it is the same as MCM-56; the present authors

believe the answer is negative since the analogue may retain its original interlayer H-bonding and possibly show swelling behavior like MCM-22P rather than MCM-56.16 The XRD shown in Figure 2 indicates that treatment of the studied here high-Al MCM-22P with acid produces only partial “collapse” and disorder, i.e., incomplete conversion into the MCM-56 analogue. This is evidenced by the remaining (002) reflection at 6.5° 2θ and largely unchanged valley around 9° 2θ, which suggest that some parts of MCM-22P resisted the collapse. The third transformation of MCM-22P shown in Figure 1 affording the Interlayer Expanded Zeolite (IEZ) form,52 is also known to diminish in efficiency as the Al content increases toward lowest Si/Al.53 The observed more difficult and incomplete conversion of Al-rich MCM-22P, demonstrated in various conversions, is hard to justify with specific explanation at this point. It may result from different inherent chemistries during treatments and transformations when more Al is present in the framework and apparently at the surface. Alternatively, having more Al in the MCM-22P synthesis mixture may favor formation of sites or regions with regular T−O−T bridges, i.e., 3-D framework fragments (MCM-49), instead of the hydrogen bonded silanol pairs shown in Figure 3.33,34 This would preclude local swelling,

Figure 3. Difference between Si and Al centers on the MWW layer leading to MCM-22P34 and MCM-56, respectively. There are two “pyramidal” sites per unit cell (72 T atoms) on each side. (MWW layer rendition was adopted from IZA Web site).

IEZ-stabilization, and disordering that are possible for the layered MCM-22P structure but not the condensed framework. The corresponding materials would be effectively MCM-22P/ 49 mixtures, and they may go undetected as such by XRD unless sufficiently extensive47 or specifically looked for. We also observed time aging effects upon storage of high-Al precursors demonstrated by some dry MCM-22P samples examined 2 years after preparation. They manifested much lower efficiency of swelling than the original fresh samples. This suggests the possibility of spontaneous condensation of the interlayer hydrogen bonded silanol bridges into T−O−T moieties occurring upon prolonged storage. Swelling of MCM-56Comparison to MCM-22P. In contrast to MCM-22P, the swelling of MCM-56 having slightly greater Al content occurs with high efficiency irrespective of the type of hydroxide used as was already seen in the previous studies that included TMA-OH.45 It is confirmed and significantly extended here by the novel outcome of efficient swelling occurring when NaOH is used in combination with the surfactant to generate basic pH. It is notable and a bit surprising because MCM-22P shows little swelling with this medium. D


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3D framework MCM-49 with Al content higher than in MCM22P, as the synthesis progresses. The observed switch from MCM-22P formation to MCM-56 can be attributed to Al incorporation on the layer in the T6 site (initially denoted T1) because the Al amount is greater than can be accommodated in the layer. The first five to six Al atoms per unit cell of MWW (72 T atoms; Si/Al ∼12/1), if present, are accommodated “inside” the layer judging from the amount of hexamethylenimine in the intralayer 10R channels and the surface pockets.47,56 The unique “pyramidal” T6 are populated predominantly or entirely with silica atoms as surface Si OH groups (see Figure 3). These silanols connect opposite layers via H-bonding producing relatively rigid and stable multilayered structure MCM-22P. The formation of MCM-56 occurs as the effective content of Al increases above 5−6 and the “extra” Al atoms cannot be accommodated in the layer but must populate the surface, i.e., the pyramidal T6-sites. This has a chemical consequence since the replacement of SiOH on the surface with AlOH generates a negative charge (Al3+ replacing Si4+) and requires presence of a cation for neutrality producing AlOH−Na+ moieties. In agreement with this is the fact that solid MCM-56 (and MCM-49) shows higher content of Na and lower of the organic SDA, hexamethylenimine (HMI), than MCM-22P. The latter contains more HMI than acid sites (molar HMI > Al). This “excess” is accommodated between layers as neutral molecules forming the interlayer H-bonding with silanols. MCM-56 contains more Al than HMI, which requires additional compensating cations, like Na+. The observed contrast between swelling of MCM-56 and MCM-22P and their different surface populations of Al OH−Na+ and SiOH, respectively, suggests that the former moieties form a rather weak interlayer bond and, during MCM56 synthesis, thwart attachment of additional layers causing disorder and monolayer nature. What remains to be explained is what triggers the formation of MCM-49, i.e., onset of the multilayered structure, which occurs when nearly all gel is converted into MCM-56. Usually there is a sudden pH jump near the end of templated zeolite syntheses, which may activate the OH attached to Al and initiate the second layer (MCM-49 begins). The presence of AlOH−Na+ moieties has further consequences with regard acid site generation, as is evident if one carries out exchange and calcination, which can be expressed formally by the following reactions:

The difference between NaOH/surfactant treatment of MCM-22P and that of MCM-56 is shown in Figure 4. High-

Figure 4. XRD of MCM-56 and MCM-22P treated with various swelling mixtures comprising the surfactant (HDTMA-Cl) plus a hydroxide NaOH or HDTMA-OH; the corresponding pillared derivatives are shown in blue.

Al MCM-22P is seen as practically unswollen because of the intense maximum at 6° 2θ, assigned to the (002) reflection and especially due to the almost unchanged valley at 9° 2θ. The presumed shift of the (002) reflection to 6° 2θ from 6.5° 2θ indicates slight expansion of MCM-22P from the original d002spacing 1.36 to 1.47 nm. This expansion is attributed to intercalation of surfactant molecules in lateral rather than vertical orientation.54 It may be envisioned when interlayer hydrogen bonds remain unbroken and continue holding the layers together allowing intercalation but preventing larger expansion, i.e., true swelling. The low angle region shows increased broad scattering around 2° 2θ, which may indicate some swelling, but overall it is considered quite incomplete. The rationale for the lack of MCM-22P swelling with surfactants when NaOH or smaller organic bases were used to generate high pH was proposed before.33 It presumed that small cations diffused faster or more effectively than surfactants into the interlayer space, thus interfering in some way with the swelling. Some new observations came to light since this original proposal, especially that swelling may occur in the presence of tetramethylammonium hydroxide TMA-OH (vide supra).45 Overall, there is no simple explanation to account for the observed behavior of MCM-22P toward swelling reagents. MCM-56 shows facile swelling judging from XRD and pore characteristics of its pillared products. It provides a convenient alternative route to high-Al expanded MWW materials. Rationalizing Differences between MCM-56 and MCM-22P. MCM-22P is more difficult to swell than MCM56 because its layers are strongly interconnected by hydrogen bonds between surface silanols. These bonds must be severed by high pH environment, but there is also dependence on the cations present in the swelling mixture. MCM-56 comprises disorganized monolayers with much weaker interlayer interactions indicated by its much easier swelling. In both materials the layers are assumed to have the same structure so there must be a particular reason for their different disposition and bonding. An explanation is provided by the fact that MCM-56 is formed from the “MCM-22P” synthesis mixture when its effective Al content becomes increased (directly or through increased alkalinity) near Si/Al ∼ 11/1.49,55 Furthermore, MCM-56 is an intermediate that converts into fully condensed

AlOH−Na + + NH4NO3 → AlOH−NH4 + → Al + ↑NH3 + H 2O

This shows that Al sites on the surface produce trigonal, pyramidal Al Lewis not Brønsted acid sites. A similar case was reported recently and attributed to defects.57 MCM-56 may contain similar centers but as the consequence of its structure. Additional discussion about detectability of this Al site is carried out in the NMR section right before conclusions. The MCM-22P crystal growth shows simultaneous propagation in three directions: very fast growth of layers and their more or less ordered stacking. This is evidenced by XRD patterns, which show early on the separate (101) and (102) maxima (valley in the region 8−10° 2θ) and are indicative of multilayered nature when crystallinity is low and the amorphous component still evident. The XRDs during MCM-56 crystallization show profiles without the valley. It E


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Chemistry of Materials begins to show up only when the amorphous solid is almost exhausted, marking the onset of MCM-49. MCM-56 Swelling with Different NaOH Amounts. The corresponding XRD patterns are shown in Figure 5. The

Figure 5. XRD of MCM-56 treated with swelling mixtures comprising HDTMA-Cl and various amounts of NaOH in mol/kg of the swelling solution (25% surfactant salt). Materials considered fully swollen are shown in red.

experiment started with MCM-56 suspension in a neutral surfactant chloride. Portions of NaOH were added, the slurry stirred overnight, and a sample withdrawn for XRD; more hydroxide was added, and the cycle was repeated several times. There is a gradual increase in the swollen portion judging from the low angle peaks at ca. 2 and 5.5° 2θ. Complete swelling is observed with ≥0.31 mol of NaOH per kg of the swelling solution. Lower NaOH concentrations produce incomplete partial or negligibleswelling. TEM Evaluation of Swelling and Pillaring. The images of selected swollen and pillared samples, presented in Figure 6, confirm extensive separation of the MWW layers consistent with the analysis based on the XRD patterns. The layers can be identified unequivocally in the edge-on view because of distinct light lines in the middle corresponding to the unique intralayer sinusoidal pores. Although swelling was judged to be quite efficient based on XRD and BET, the images reveal the presence of some unseparated layers, i.e., formally MCM-22 domains. This suggests that the extent of swelling and pillaring can be still improved. The observed gaps between layers in the pillared calcined sample indicate the presence of thermally stable pillars. They are not distinct, which proves their nonperiodic distribution and/or amorphous character. The crystals are maximum 100 nm wide and sometimes appear disordered or bent; the layers are often arranged almost parallel to each other with a relative distance that varies but are in general not less than about 1.5 nm (except for the unswollen parts). The average distances are estimated from positions of the low angle line in XRD: between about 2.5 and 2 nm for the swollen and pillared samples. TEM image of the pillared sample also reveals the presence of an amorphous phase, which is most likely originating from the TEOS pillaring treatment. It is considered to be a contamination and together with the mentioned presence of unseparated layers should be minimized. The possibility for better swelling was examined by treatment at higher temperature, namely, 99 °C, for overnight using the surfactant hydroxide/chloride solution 1:1. The analogous room temperature swelling was reported earlier.43 Subsequent pillaring of

Figure 6. TEM micrographs evaluating different swelling efficiencies of MCM-56: swollen with NaOH as base (a,b), pillared (c,d) compared with MCM-56 swollen with HDTMA-OH/Cl at 99 °C, and pillared without TEOS hydrolysis before calcination (e,f).

the sample treated at elevated temperature was carried out in two ways: with and without hydrolysis of the TEOS treated swollen MCM-56 before final calcination.54 As can be seen in Table 1, the high temperature swelling resulted in enhanced BET area of the pillared derivatives approaching 1000 m2/g. Calcination without hydrolysis after TEOS pillaring gave higher BET than with hydrolysis. According to TEM observations, the sample of MCM-56 swollen at 99 °C and pillared without TEOS hydrolysis contains a majority of MWW zeolite layers separated as single unit cell lamellae. This confirms the potential for improved swelling efficiency by increasing temperature of the treatment. It also appears that crystals are wider/longer in comparison to those treated with the NaOH/surfactant mixture. This suggests that swelling with NaOH in solution may cause more crystal breakage. Evaluation of Pillared MCM-56 by Nitrogen Isotherms. Nitrogen isotherms obtained for the pillared MCM56 derivatives show the expected significant increase of porosity compared to the starting MCM-56 (lowest profile). The observed hysteresis loops are typical for layered solids like MCM-56/36.58 The notable feature is the difference between MCM-56 swollen with NaOH/HDTMA-Cl (red curve) and HDTMA-OH/Cl at room temperature (green one) in the F


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Chemistry of Materials region above p/p0. Both materials show almost identical overlapping profiles below p/p0 = 0.5. At higher pressure the former is horizontal with a flat hysteresis loop, while the latter shows continuous increase with an upward incline. The isotherms in Figure 7 also confirm that swelling at elevated temperature can lead to enhanced porosity of the

MCM-56 analogue with disorganized layer stacking. It is possible that acid deswelling did not remove all of the surfactant present between layers and its residual amount remained in horizontal orientation producing in effect an intercalated MCM-22P with slight expansion and layer ordering. Acid deswelling of Al rich swollen zeolite precursors has a drawback as it may result in dealumination by leaching of the framework Al. An alternative treatment with salts instead of acid, such as ammonium nitrate in alcoholic solution, which was applied here, provides a milder approach. An aqueous environment is less conducive to surfactant extraction, producing little deswelling. Salts of organic cations can be also used for deswelling, in which case they would afford organic intercalates of layered zeolites. Acidity Evaluation by FT-IR Studies; 27Al NMR. Table 2 summarizes acid site concentration values for the studied MCM-22 and MCM-56 materials including those deswollen by exchange with ammonium nitrate in alcohol. Two types of zeolite activation were applied: air/vacuum involving calcination in air at 550 °C, ammonium exchange and heating in vacuum to 450 °C in situ in the IR cell; and vacuum activation−as-synthesized material heated in situ in vacuum to 550 °C with elimination of organic and other degradable/ volatile compounds. The latter did not entail cation exchange, and the final numbers could be affected by the presence of Na+ as indeed is observed with the starting MCM-56. Both starting MCM-22 and MCM-56 demonstrate high acid site concentration (>900 μmol/g) consistent with their high Al content. MCM-56 was reported earlier to be sensitive to postsynthesis treatments and activation, which could result in dealumination and structural deformation producing low acid site concentration and poor catalytic activity.42 More recent studies have shown that MCM-56 can show high acid site concentrations comparable to MCM-22 and MCM-49,43,46 which may be consistent with its demonstrated high catalytic activity in some processes like ethylbenzene synthesis.59 Swelling produces a decrease in the Brønsted acid site concentration (BAS) by about 1/3 to around 600 μmol/g while the Lewis sites (LAS) remain about the same. Pillaring, which introduces amorphous silica, produces further decrease of BAS to around 400−500 μmol/g.43 Swelling at elevated temperature, which produces more extensive layer separation observed in TEM and enhanced porosity in comparison to the RT treatment, makes hardly any

Figure 7. Nitrogen isotherms of the original MCM-56 and its pillared derivatives swollen with HDTMA-Cl plus NaOH (red) and with HDTMA-OH (50/50) at RT (green) and 99 °C (blue without hydrolysis, purple with hydrolysis prior to calcinations). The isotherms are not shifted, represent actual values, and indicate gradual increase in porosity.

pillared MCM-56. This may be due to both increased amount of separated layers, as seen in TEM, and desilication causing some layer fragmentation and additional porosity. Surfactant Extraction from Swollen MWW. Swelling is typically carried out in order to produce permanently expanded or delaminated solids, and so, the reverse process, which has been dubbed deswelling, appears to have little practical significance. On the other hand, it may be quite valuable for appraising the state of the treated precursor layers by freeing it from the swelling agent. The other alternative is calcination at over 500 °C, which by comparison is quite severe and may cause additional damage especially due to heat because of the enormous amount of organic that is burned off. Tsapatsis et al. described deswelling of MCM-22P swollen at room temperature by acidification.35 The recovered product was identified as the 3D ordered MCM-22P precursor. This is an unexpected outcome because acid treatment of assynthesized MCM-22P50 induces disorder and collapse to the

Table 2. Concentration of Brønsted (BAS) and Lewis (LAS) Acid Sites in MCM-22P and MCM-56 Samples after Various Treatmentsa sample type

heat activation

BAS, μmol/g

LAS, μmol/g

Si/Al BAS + LAS

Si/Al BAS

MCM-22 as synthesized MCM-22 calc, NH4NO3 exchange MCM-56, as synth, with original Na+ MCM-56 calc, NH4NO3 exchange MCM-56 sw HDTMA-OH/Cl MCM-56-sw HDTMA-OH/Cl calc, NH4NO3 MCM-56 sw NaOH, deswell MCM-56 sw NaOH, deswell MCM-56 sw HDTAMA-OH/Cl 99C, deswell MCM-56 sw HDTAMA-OH/Cl 99C, deswell

Vac Air/Vac Vac Air/Vac Vac Air/Vac Vac Air/Vac Vac Air/Vac

1092 828 585 942 N/A 635 1577 564 1227 649

156 167 55 139

12 16 29 15

14 19 33 17

179 251 154 162 154

19 8 22 11 20

25 10 29 13 25

ref 41

41

Samples were activated by heating under vacuum at 550 °C (Vac) and air (550 °C) followed by cation exchange and heating under vacuum, at 450 °C (Air/Vac).

a

G


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forms show high acid site concentrations. This can result in high catalytic activity in processes like benzene alkylation with small olefins, in which MWW zeolites play a prominent commercial role.

difference for the BAS values. The MCM-56 swollen with HDTMA-OH/Cl at RT and 99 °C have BAS equal to 635 and 649 μmol/g, respectively. A significant difference is seen by IR in the silanol region (Figure 8). The intensities of maxima due

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ASSOCIATED CONTENT

S Supporting Information *

Detailed description of characterization by adsorption, FTIR and 27Al NMR NMR spectra, compilation of MCM-22 catalytic data from ref 36. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.chemmater.5b01030.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +48-126632016. E-mail: [email protected].

Figure 8. FTIR of MCM-56 pillared at different temperatures in the OH stretching region.

Notes

The authors declare no competing financial interest.

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to terminal silanol and bridging hydroxide are quite similar. However, for the high temperature treatments the background is elevated substantially, which can be attributed to the presence of various (additional) silanol groups interacting with each other. This is consistent with much more extensive desilication than at RT, which is believed to occur upon swelling at higher temperature. The activation in vacuum can produce initially much higher BAS values than the standard air/vacuum procedure (see Table 2). This potentially attractive enhancement of acidity is especially evident in the cases of mild deswelling with ammonium nitrate in alcohol. These high concentrations revert to typical values when followed by regular calcination. 27 Al NMR spectra recorded for selected samples showed preservation of tetrahedral Al coordination until final calcinations. There was very little octahedral Al seen in the swollen and calcined MCM-56. Deswelling followed by calcinations produced a significant amount of octahedral Al especially upon calcinations in air. Typically for MWW materials the signals were broad, preventing detection of different structural environments in the framework. Swollen MCM-56 shows the expected less intense signal than the parent zeolite. This signal does not regain all of the original intensity after surfactant removal by calcinations or chemical deswelling, suggesting formation of undetectable Al centers. The notable issue with the proposed Al site on the surface is the problem of its detectability. As observed here and in ref 57, the 27Al NMR spectrum of the Na forms does not show signals that could be assigned to it. It may be possible to obtain evidence through advanced NMR methods like 27Al{1H} REDOR (3Q) combined with computational methods.57 The complexity of this issue is quite obvious, and most likely extensive investigation will be required to settle outstanding problems.

ACKNOWLEDGMENTS ̌ J.C. acknowledges the Czech Science Foundation for the project of the Centre of Excellence (P106/12/0189). The research leading to these results has received also a partial funding for J.Č . from the European Union Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 604307. This work was financed with the funds from the Narodowe Centrum Nauki provided on the basis of decision number DEC-2011/03/B/ST5/01551. The IR measurements were carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (Contract No. POIG.02.01.00-12-023/08).

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REFERENCES

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CONCLUSIONS The efficiency of swelling and other transformations of MCM22P, a multilayered precursor to zeolite MCM-22, are affected by the increasing Al content in the framework. In contrast, the unilamellar analogue MCM-56 can be readily swollen under mild conditions and notably with NaOH as the source of the required high pH. Swelling at higher temperature increases the amount of separated layers and results in the pillared product with increased porosity. MCM-22, MCM-56, and their various H


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Swelling and Interlayer Chemistry of Layered MWW Zeolites MCM-22

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