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Structural transformation and chemical modifications of the unusual layered zeolite MWW form SSZ-70
Accepted Manuscript Title: Structural transformation and chemical modifications of the unusual layered zeolite MWW form SSZ-70 Authors: Justyna Grzybek, Martin Kubu, Wieslaw J. Roth, ˇ Barbara Gil, Jiˇri Cejka, Valeryia Kasneryk PII: DOI: Reference:
S0920-5861(18)31705-X https://doi.org/10.1016/j.cattod.2019.03.006 CATTOD 12028
To appear in:
Catalysis Today
Received date: Revised date: Accepted date:
27 November 2018 28 February 2019 4 March 2019
ˇ Please cite this article as: Grzybek J, Kubu M, Roth WJ, Gil B, Cejka J, Kasneryk V, Structural transformation and chemical modifications of the unusual layered zeolite MWW form SSZ-70, Catalysis Today (2019), https://doi.org/10.1016/j.cattod.2019.03.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural transformation and chemical modifications of the unusual layered zeolite MWW form SSZ-70
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Justyna Grzybek,a Martin Kubu,b Wieslaw J. Roth,a Barbara Gil,a Jiři Čejka,c , Valeryia Kasnerykb Faculty of Chemistry, Jagiellonian University in Kraków, Gronostajowa 2, 30-387 Kraków, Poland
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J. Heyrovský Institute of Physical Chemistry Academy of Sciences of Czech Republic, v.v.i., Dolejškova 2155/3, 182 23 Prague 8, Czech Republic c
Department of Physical and Macromolecular Chemistry, Faculty of Sciences, Charles University, Hlavova 8, 128 43 Prague 2, Czech Republic
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Dedication – this article is dedicated to Prof. M. Ziółek from UAM in Poznan on the occasion of her 70 th birthday to honor her contributions to catalysis sciences and her support and inspiration to several generations of scientists
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Graphical abstract
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Highlights:
Layered zeolites SSZ-70 with MWW topology were synthesized with boron and aluminum and investigated for post-synthesis modifications All show successful swelling, pillaring and stabilization to produce interlayer expanded zeolites (IEZ) except Al-SSZ-70, which did not show stabilization to IEZ Al insertion post-synthesis into B-SSZ-70 was less effective in producing active acid catalyst than direct synthesis with Al in the gel
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Pillared Al-SSZ-70 was more active than the parent zeolite in alkylation of mesitylene with benzyl alcohol proving benefits of creating more open structures.
Abstract
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SSZ-70 is one of several layered forms of zeolite MMW obtained by direct synthesis with consecutive layers shifted by approximately ±1/3 of a unit cell in the horizontal direction (⟨110⟩), according to the recent structure solution report. Zeolite MWW is a valuable industrial alkylation catalyst and SSZ-70 itself was called by the inventors 'a successful catalytic material'. SSZ-70 shows characteristic powder X-ray diffraction pattern (XRD), containing a unique broad band with a roughly triangular shape and maximum at 8.5-9.0° 2θ (Cu Kα radiation), which can be used to distinguish SSZ-70 from the other MWW materials with different layer arrangements. This work concerns post-synthesis treatments of SSZ-70 to investigate its ability to produce more expanded structures by silylation, swelling and pillaring. Both boron- and aluminum- containing SSZ-70 were investigated. They showed the ability for interlayer expansion by swelling with cationic surfactants and were pillared with silica to produce micro/mesoporous hybrids with increased BET area. Not too surprisingly, the original unique layer stacking of SSZ-70 seems to have become more random and not distinguishable from the swollen forms of the other swellable MWW materials MCM-22P and the monolayer MCM-56. Calcined SSZ-70 showed unusual peaks, assigned to (002) reflections, with d-spacing shorter than c-unit cell of the 3D MWW framework, which suggest collapse with formation of the so-called sub-zeolite form. Silylation resulted in different outcomes for B and Al SSZ-70 samples. The former produced Interlayer expanded (IEZ) forms of B-SSZ-70 but did not show significant increase in pore volume and BET surface area. Silylation of Al-SSZ-70 did not yield the stabilized (IEZ) form but had a positive effect as it prevented the collapse to sub-zeolite upon calcination. Catalytic testing was carried out for the Al-containing SSZ-70 obtained by direct synthesis and for B-SSZ-70 activated by Al insertion. The former showed much higher concentration of Brønsted acid sites and 5 times faster conversion in a model alkylation reaction between benzyl alcohol and mesitylene. Pillared Al-SSZ-70 showed highest activity despite lower Al concentration due to silica pillars. Both silylation and pillaring had a positive effect on the activity in alkylation reaction with bulky molecules. Keywords: MWW, SSZ-70, post-synthetic modification, alkylation
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1. Introduction
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Zeolites play a special role in heterogeneous catalysis as solid acids and oxidation catalysts with well-defined micropores of molecular dimensions. [1-7] There are 240 officially approved zeolite frameworks [8] but 5 have been particularly influential in the development of industrial applications and are unofficially recognized as the catalysis 'big five'. [9-13] They are zeolites X and Y (FAU), ZSM-5 (MFI), mordenite (MOR), ferrierite (FER) and beta (BEA). It can be justified to expand this group by the framework MWW containing 2 independent medium pore size channel systems and 0.7 nm wide supercages.[14] MWW shows exceptional catalytic activity and selectivity in aromatic alkylation [15, 16] and has been the leader in the development of layered, twodimensional (2D) zeolite materials.[17, 18] The latter represent a fundamental breakthrough concept [19] that has been spreading to other frameworks notably including also FER [20] and ZSM-5 [21]. It enabled formation of various layered structures by direct synthesis and post-synthesis modifications with potential for enhanced performances. [19, 22, 23] The framework MWW has been the model for the others by affording a great variety of different layered structures, about 15 so far.[22, 24] Some of them have expanded layer spacing and can be more accessible to larger molecules for catalytic reactions but also can allow functionalization with transition
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metals.[25-27] Another unsurpassed property of the MWW framework is its formation of different layered forms with high Al content, up to Si/Al = 10 [28]. This is a very important factor as high acidity is essential in demanding catalytic conversions of hydrocarbons into fuels, lubricants and other valuable products.[29] The majority of the other known layered zeolites are predominantly low Al materials, i.e. with small intrinsic activity.[30] Zeolite MWW stands out by giving 6 different types of layered materials and the 3D framework (MCM-49) by direct synthesis. Four of them, MCM-22P, EMM-10P, MCM-56 and SSZ-70, were obtained using conventional templates [31, 32] while the unilamellar MIT-1 and its multilayered equivalent designated by us UJM-1 and yet unpublished, were obtained with a bifunctional surfactant structure directing agents by analogy to layered MFI materials.[21, 33] The former 5 are distinguished based on XRD patterns, shown in Figure 1, containing unique combinations of sharp and broad peaks in the range 5-10° 2θ. They also show different catalytic characteristics and behavior in post-synthesis modifications, which include stabilization by treatment with silylating reagents to give the so-called interlayer expanded IEZ forms and swelling with surfactants.[34, 35] The top four XRDs in Fig. 1 can be viewed as resulting from combination of two sets of 2 basic types of visual features: one or two peaks around 6.5-7.5° 2θ, and 2 peaks or broad band at 8-10° 2θ. These features can be rationalized as due to a short interlayer distance, 2.5 nm (one peak at 6.5-7.5° 2θ), or expanded interlayer stacking, >2.6 nm (2 peaks), and vertically aligned (ordered, 2 peaks between 8-10° 2θ) or misaligned stacking (broad band). The expanded forms with two peaks at 6.5-7.5° 2θ collapse upon calcination to one peak with the 'normal', 2.5 nm, interlayer distance. The material SSZ-70 [36], which is the subject of this study is expanded showing 2 peaks at 6.5-7.5° 2θ but it is evidently not a vertically ordered MWW structure nor similar to the above disordered ones. It shows a unique band in the 8-10° 2θ range, namely a triangular broad maximum at 8.5-9° 2θ. The structure of SSZ-70 was solved and found to involve specific stacking faults with approximately ±1/3 shifts in the horizontal direction (⟨110⟩).[37] As a 'successful catalytic material', suggested by the inventors, SSZ-70 is of interest with regard to post-synthesis treatments with possible expansion and generation of open structures like those obtained earlier with the other layered MWW forms, especially MCM-22P and MCM-56. Previous reports about SSZ-70 transformation include delamination but it does not appear to be in the sense of obtaining single-sheet architectures like ‘house-of-cards'.[38] TEMs suggest mainly unaligned stacks with occasional single MWW layers. The primary transformation of layered zeolite forms include swelling with cationic surfactants, which tends to increase interlayer stacking disorder.[39] Thus, we wanted to determine, among others, if SSZ-70 would transform to these other types of MWW materials and lose its original specific layer offsets. The study of catalytic activity includes comparison of various layered derivatives of Al-containing SSZ-70.
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Expanded Unit cell c >2.6 nm 002 100 101 102
c ~2.5 nm
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1 peak 8-10 band
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Fig. 1. X-ray diffraction patterns in the range 5-10° 2θ for various as-synthesized MWW forms with layered structures rationalizing the observed features.
swelling detemplation stabilization
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Fig. 2. Principal transformations of the (multi)layered MWW materials as models for SSZ-70.
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2. Experimental 2.1. Structure directing agent (SDA)
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1,3-bis(cyclohexyl)imidazolium tetrafluoroborate was prepared according to the procedure described in [32]. 19.83 g of cyclohexylamine (Alfa-Aesar, 98%) and 200 mL of toluene (Sigma Aldrich) was placed in a room temperature water bath. Next, 6.32 g of paraformaldehyde (Fisher, 95%) was added with strong stirring. The solution was stirred at room temperature for 30 min and then ice was added to the water bath. After 1 h of solution cooling, another 19.83 g of cyclohexylamine was added dropwise via an addition funnel. 36.60 g of tetrafluoroboric acid (Alfa-Aesar 48 wt.% in water) was diluted to 30 wt.% with water and then added dropwise. The ice bath was removed, and the solution was allowed to warm for 30 min. Next, 28.98 g of glyoxal solution (Alfa-Aesar, 40 wt % in water) was added dropwise. The flask was heated at 40 oC overnight and then allowed to cool to room temperature. The solid precipitate was filtered off and washed with 150 mL of water and then 150 mL of diethyl ether and dried for 8 h under high vacuum. Recrystallization from 2:1 ethyl acetate/dichloromethane yielded 40.4 g of off-white solid after drying under high vacuum (82.4% yield). Final product was verified using 1H and 13C NMR. It was converted from the tetrafluoroborate to hydroxide form using hydroxide exchange resin (Ambersept 900 OH, Alfa Aesar) in water and then titrated with HCl solution to determine OH- concentration.
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2.2. Zeolite syntheses
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Borosilicate reactions were run at Si/B = 4 and 25 with gel compositions 1.0 SiO2:0.125 B2O3:0.25SDA+OH-:23 H2O designated B-SSZ-70(4) and 1.0 SiO2:0.02 B2O3:0.25SDA+OH-: 0.1 NaOH:30H2O designated B-SSZ-70(25), respectively, according to the procedures described in [40]. Al-SSZ-70 zeolite was obtained from the mixture composition: 1SiO2:0.2 OSDA:0.01 Al2O3:0.1 NaOH:30 H2O (Al-SSZ-70(50)) by the standard hydrothermal procedures. As the first step a weighted amount of the template was dissolved in water and converted to the hydroxide form by mixing with the ion exchange resin (Ambersept 900 OH, Alfa Aesar). The obtained aqueous solution of a template in hydroxide form was combined with boric acid (99.5%, Alfa Aesar) or aluminum hydroxide (Sigma Aldrich), NaOH (50%, Sigma Aldrich) and tetraethyl orthosilicate (98%, Sigma Aldrich). The gel was transferred to a Teflon liner, sealed in a pressure bomb and held at 433 K with rotation for 8 days for B-SSZ70 (25) and Al-SSZ-70 and 12 days for B-SSZ-70 (4). After crystallization the solids were recovered by filtration, washed with distilled H2O, and dried at 338 K overnight. Solids were calcined by heating on the air at 813 K for 6 h with 2K/min heating rate. MCM-22P zeolite were prepared according to the procedure described earlier [41]. 2.3. Post-synthetic modifications 2.3.1. Swelling and pillaring
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As-synthesized SSZ-70 samples were mixed with 25% cetyltrimethylammonium (CTMA) hydroxide solution (solid/liquid=0.05) obtained by ion exchange of the chloride with ion exchange resin as described previously.[28] The slurry was stirred overnight at room temperature. The solid was recovered by centrifugation, washed 3 times with distilled water and dried at room temperature. Pillaring was carried out by stirring of a swollen sample with tetraethyl orthosilicate (TEOS) (solid/liquid = 0.01, w/w) at room temperature. Solids were recovered by centrifugation, dried at room temperature and then calcined at 823 K for 6 h with 2K/min heating rate.
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2.3.2. Stabilization by silylation In a typical procedure, as-synthesized SSZ-70 was mixed with an aqueous solution of 2 M HNO3 (solid/liquid=0.5) and 2 mmol of dimethyldiethoxysilane Me2Si(OEt)2. The mixture was heated in a Teflon-lined autoclave at 423 K for 16 h. After silylation, the solid was collected by filtration, rinsed with distilled water and dried at 373 K. The product was calcined at 823 K for 6 h with 2K/min heating rate.
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2.3.3. Aluminum insertion into B-SSZ-70 The treatment was based on a literature procedure [42]. 0.40 g of calcined borosilicate zeolite and 0.10 g of Al(NO3)3·9H2O dissolved in 10 mL of H2O were placed in a Teflon-lined autoclave. The mixture was heated at 95°C for 5 days statically. The solids were washed twice with HCl solution (pH = 2) at room temperature, to remove weakly adsorbed Al salt before continuing to water wash. 2.3.4. Detemplatation of swollen SSZ-70 (deswelling)
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The swollen SSZ-70 zeolite was mixed with 0.625 M solution of NH4NO3 (Avantor Poland, p.p.a.) in ethanol for 1 h at room temperature (20 ml of solution per 0.5 g of zeolite), repeated two times, filtered, washed with anhydrous ethanol and dried at room temperature.
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2.4. Characterization
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The structure and crystallinity of obtained zeolites were determined by X-ray powder diffraction (XRD) with CuKα radiation (ʎ = 0.154 nm) using a Bruker AXS D8 Advance diffractometer equipped with a graphite monochromator and a position sensitive detector (Våntec-1) with Bragg-Brentano geometry and Rigaku MiniFlex diffractometer in a reflection mode. The XRD patterns were usually collected with steps of 0.02o.
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The samples were examined under scanning electron microscopy (SEM) using a JEOL JSM5500LV scanning electron microscope. For the measurements, crystals were covered with a thin platinum layer by sputtering in the vacuum chamber of a BAL-TEC SCD-050 sample sputter coater.
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Relative content of Al and Si was determined in the samples formulated into pellets, 20 mm in diameter, with the use of Energy-Dispersive XRF spectrometer (Thermo Scientific, ARL QUANT’X). The X-rays of 4-50 kV (1 kV step) with the beam size of 1 mm were generated with the Rh anode. The detector used was a 3.5 mm Si(Li) drifted crystal with a Peltier cooling (ca. 185 K). For quantitative analysis, calibration with a series of metallic standards and UniQuant software were used.
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Nitrogen adsorption isotherms were determined by the standard method at 77 K (liquid nitrogen temperature) using an ASAP 2020 (Micromeritics) static volumetric apparatus. Before adsorption the samples were outgassed at 623 K using turbomolecular pump to remove adsorbed water.
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The concentration of Lewis (LAS) and Brønsted (BAS) acid sites was determined using adsorption of pyridine (Py) followed by IR spectroscopy (Tensor 27 from Bruker, MTC detector, spectral resolution 2 cm -1). Zeolite samples were pressed into self-supporting wafers with a density of ca 8 mg/cm2 and activated in situ at 723 K for 1 hour at high vacuum (10-5 mBar). Excess of pyridine vapor was adsorbed at 440 K followed by desorption for 20 min at 440 K. Spectra were recalculated to a wafer mass equal 10 mg. Concentration of Lewis and Brønsted acid sites were evaluated from the intensities of bands at 1454 cm−1 (LAS) and at 1545 cm−1 (BAS) using absorption coefficients determined earlier in our laboratory [43] using external standards, ε (LAS) =0.165 cm2/μmol, and ε (BAS) =0.044 cm2/μmol, and the intensities of corresponding pyridine maxima after pyridine desorption at 440 K to ensure complete removal of weakly adsorbed species.
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2.5. Catalytic testing Calcined samples were ion exchanged into NH4+ form, by contacting with 1 M solution of NH4NO3 (Avantor Poland, p.p.a.) for 1 h at room temperature (20 ml of solution per 0.5 g of zeolite), repeated three times, filtered, washed with deionized water, dried, and activated at 723 K for 5 h.
conversion = k ∙
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The selected test reaction, liquid phase benzylation of mesitylene with benzyl alcohol (Scheme S1), was carried out in a three-necked round-bottom flask equipped with a reflux condenser with heating in a multi-experiment workstation StarFish (Radleys Discovery Technologies) under atmospheric pressure. The reaction temperature was 353 K. Typically, 160 mmol of mesitylene (22 ml) was combined with 50 mg of the catalyst and dodecane as an internal standard. The reaction mixture was maintained for 30 min at the required reaction temperature and then 2 mmol of benzyl alcohol was added. This was regarded as the starting reaction time. Liquid samples were withdrawn at regular intervals and analyzed by the gas chromatography in a Perkin Elmer chromatograph with an FID detector using a 30 m packed ELITE-1MS column. The conversion of alcohol was calculated as follows: Salcohol ∙ 100 Sstandard ∙ n0
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where S is the area of respective peak in the chromatogram, k is the calibration coefficient (mol), n0 is the starting amount of alcohol (mol).
Scheme 1. Alkylation of mesitylene with benzyl alcohol
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3. Results and discussion
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The study was carried out with three SSZ-70 samples: two boron-containing with Si/B in the gel equal to 4 and 25 and one with Si/Al =50, designated B-SSZ-70(4), B-SSZ-70(25) and Al-SSZ-70(50) . Increasing Al content can produce, depending on template and other conditions, other frameworks, e.g. we obtained zeolite beta with Si/Al = 25/1 (in the gel), and possibly some other layered MWW forms like the disordered EMM-10P. This highlights the critical aspects of this work, which is the proper identification of SSZ-70 and its differentiation from the other MWW layered structures. As already indicated in the introduction SSZ-70 is identified on the basis of the broad triangular peak with the maximum between 8.5 to 9° 2θ. This peak falls in between 2 reflections observed with the ordered MWW materials (MCM-22P and MCM-49) and in the middle of the broad band, with a more or less rectangular shape, exhibited by the disordered MWW materials (MCM-56 and EMM10P). The triangular SSZ-70 peak is clearly due to the particular arrangement of MWW layers, namely shifting by approximately ±1/3 in the horizontal direction (⟨110⟩). Changes in the appearance of this broad peak suggest transition to more random disorder. [37]
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3.1. Structure identification and characterization
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The as-synthesized SSZ-70 also shows 2 peaks in the range 6.5-7.5° 2θ, similar to (002) and (100) in MCM-22P and EMM-10P. They indicate expanded interlayer distance and usually merge into one peak or a close doublet slightly above 7° 2θ upon calcination. It should be also noted that XRD patterns of SSZ-70 frequently show very high intensities of the straight interlayer reflections (00l ,l=el), e.g. at 6.5° 2θ. This may indicate particularly severe 'preferred orientation' but other unknown effects can also be involved. These exaggerated intensities have no impact on the XRD interpretation since it involves only peak positions and shapes.
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The calcined SSZ-70 materials showed unexpected satellite peaks on the right of the (100) reflection at ca. 7.5° 2θ, i.e. at lower d-spacing. They are assigned to the interlayer reflection equivalent to (002) of MCM-22 (SSZ-70 unit cell has doubled unit cell z-dimension) that is apparently shifted to d-spacing shorter than in the 3D structure. It is a surprising but not unprecedented observation and was described before [38] as suggested evidence of delamination. The present case indicates that such collapse is possible without explicit delamination procedure but can occur through routine calcinations. This peak could be rationalized as due collapse of the layered MWW structures to below the crystallographic unit cell dimension of the 3D framework. Analogous collapse was previously postulated with some other layered precursors of NSI,[44] FER [45] and PCR [46], and the products were referred to as sub-zeolites.[44] Another possibility we considered initially was that SSZ-70, with unit cell c-dimension doubled in comparison to MCM-22, could show additional, superlattice, XRD peaks. For example, the interlayer reflections (101) of the larger unit cell could appear in this position. It was deemed less likely since such peak should be quite broad and most likely even invisible.
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The surface area and micropore volume of the calcined SSZ-70 materials are comparable to the typical values for MWW materials. In comparison, the above mentioned sub-zeolites of other frameworks, e.g. FER and NSI, were clearly less porous than the corresponding 3D frameworks. Layers in the latter have no internal porosity in contrast to MWW, which contains both interlayer channels and surface cavities and this may explain why calcined SSZ-70 shows normal BET areas, despite having the sub-zeolite form. The materials used in the reported structure solution studies [37] did not show the collapse of SSZ-70 upon calcination, suggesting formation of layer bridging like in MCM-22, of course with some unpaired free silanols. These differences in behavior with regard to calcination may be associated with the conditions of calcination itself and its effect on different samples, e.g. depending on template and other factors. Complete elucidation of this phenomenon does not seem crucial at this point due to the main focus on expanded materials.
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The structure of SSZ-70 and its behavior, e.g. upon calcination, can be linked to the nature of the bis(cyclohexyl)imidazolium template, which seems to induce the particular shift of MWW layer, i.e. by approximately ±1/3 in <110> direction. In the other known MWW zeolites synthesized directly the layers are either stacked more orderly or randomly arranged with hydrogen bonding involving surface silanols and template molecules. The particular layer stacking in SSZ-70 means that the density of interlayer bridges is likely smaller than in these other MWW zeolites and there are free silanols present. The objective of this work is to determine the properties and behavior of SSZ-70 in comparison to the other MWW layered materials.
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Fig. 3. X-Ray diffraction patterns of as synthesized (black) and calcined (red) SSZ-70 zeolites.
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The textural parameters of the studied SSZ-70 zeolites, namely BET area around 400 m2/g and pore volumes were typical for good quality MWW materials. It is evident that different chemical compositions of synthesis gel affects the morphology (SEM, Fig. 4) and is also reflected in the textural properties. The studied SSZ-70 zeolites form very thin plates but differ in size, shape and aggregation. The external surface areas are increasing in the order B-SSZ-70(4) < B-SSZ-70(25) < Al-SSZ-70(50), apparently as a reflection of crystal sizes observed in SEM, which are increasing in the opposite direction. The greater crystal sizes of B-MWW in comparison to Al-MWW are in line with previous observations, e.g. for MCM-22P [14]. Fig. 5 shows the change in overall and external surface areas upon pillaring and silylation/stabilization. The latter causes a small change while the former almost doubles the BET values.
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Fig. 4. SEM images of B-SSZ-70(4) (a), B-SSZ-70(25) (b) and Al-SSZ-70 (50) (c).
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Fig. 5.Textural parameters of the starting and modified SSZ-70 samples. 3.2 Swelling with surfactant
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Swelling of layered solids, i.e. expansion of interlayer space by intercalation with suitable organics like surfactant molecules has both a fundamental and practical side. The former represents validation of the true layered nature of materials in question,[39] which may need to be proven especially with zeolites. The practical side concerns the potential for producing expanded, more open structures like pillared and delaminated. The swelling of layered MWW materials with cetyltrimethylammonium cation (CTMA, hexadecyltrimethylammonium) is known to produce a low angle peak at ca. 5 nm d-spacing but also a new reflection, apparently the (003) one, located at around 5-5.5° 2θ, while the (002) peak of the original materials located at 6.5° 2θ must disappear.[47, 48] This is evidently the case for all SSZ-70 materials treated with CTMA-hydroxide as shown in Fig. 6, which confirms their highly efficient swelling. Another observation of interest is what happened with the characteristic triangular 'SSZ-70' peak with the maximum at around 8.9° 2θ. It seems to be broadened in all cases suggesting perturbation of the SSZ-70 layered structure. However, the change is not big enough to say conclusively that swelling changed SSZ-70 completely into a swollen MCM-22P-like material destroying the specific SSZ-70 layer arrangement. In the case of MCM-22P swelling, the original two peaks in the range 8-10° 2θ are changed into a 'rectangular' band, similar to EMM-10P and MCM-56, reflecting apparent lateral disorder (vertical misalignment). To learn more about these features the SSZ-70 swollen materials were calcined. All patterns show further shift towards the 'rectangular' shape of the 8-10° 2θ band suggesting transformation in the direction of EMM-10-like structures with the loss of the specific SSZ-70 features. This effect is quite pronounced for B-SSZ-70(25) where even some layer ordering can be detected, which is indicated by the dip near 9° 2θ, as it can be attributed to partial separation into distinct (101) and (102) peaks. The latter process became more pronounced with the dimethylformamide (DMF) added to the swelling mixture and calcination of the product. The corresponding XRD showed significantly developed (101) and (102) reflections of the MWW structure confirming lateral layer re-orientation with significant ordered 3D stacking. The calcined swollen AlSSZ-70(50) and high B-SSZ-70(4) show a distinct peak at ca 7.7° 2θ, which is assigned to sub-zeolite materials. Peak intensity is quite significant with B-SSZ-70(4) and its origin is not certain but it is considered to be the interlayer (002-like) reflection of a collapsed structure.
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Al-SSZ-70 swollen CTMA-OH calcined Al-SSZ-70-swollen CTMA-OH Al-SSZ-70 as synth.
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B-SSZ-70 (25) swollen CTMA-OH calcined B-SSZ-70 (25) swollen CTMA-OH B-SSZ-70 (25) as synth. B-SSZ-70 (4) swollen CTMA-OH calcined B-SSZ-70 (4) swollen CTMA-OH
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Fig. 6. X-Ray diffraction patterns of starting as-synthesized SSZ-70s and their modified forms obtained upon swelling with CTMA-OH and following calcination. Table 1. Textural parameters of examined zeolites calculated according to reference [49]. Sext m2/g
V micro cm3/g
V total cm3/g
B-SSZ-70 (4)
414
32
0.12
0.18
B-SSZ-70 (4)-IEZ
409
70
0.12
0.21
B-SSZ-70 (4) pillared
938
124
0.14
0.39
433
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0.13
0.20
434
78
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0.21
B-SSZ-70 (25) pillared
825
112
0.21
0.36
Al-SSZ-70
429
119
0.10
0.24
Al-SSZ-70-IEZ
492
129
0.12
0.27
Al-SSZ-70-pillared
764
357
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0.47
MCM-22
375
66
0.11
0.23
MCM-22-pillared
1017
180
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0.73
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Table 2. Acid properties from IR and Si/Al ratio (by XRF) of studied materials
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B-SSZ-70 (4) – Al reinsertion B-SSZ-70 (25) – Al reinsertion Al-SSZ-70 Al-SSZ-70-IEZ Al-SSZ-70 pillared MCM-22 MCM-22-pillared
Si/Al(IR) BAS+LAS 114 160 28 40
Si/Al BAS 155 232 31 52
BAS(IR) μmol/g 107 71 514 314
LAS(IR) μmol/g 38 32 65 95
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200
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Si/Al (XRF) 27 26 17 20 29
24 32
31 37
599 435
67 65
16 21
3.3. Pillaring and stabilization
SC RI PT
Zeolite
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Swollen SSZ-70 samples were pillared by treatment with TEOS according to the established procedure at room temperature [50] and then calcined. XRD patterns (Fig. A1) show successful pillaring indicated by low angle peaks at d-spacing in the range of 4.5 – 5.5 nm, which is further confirmed by significantly increased BET values to >750 m2/g. The question about preservation of the 'SSZ-70' structure cannot be answered decisively, as the band shape in the region 8-10° 2θ (Fig. A2) is not distinct enough to say how much the layer stacking arrangement shifted away from the SSZ-70 configuration.
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The treatment of multi-layered zeolite forms with silylating agents such as a monomeric silane source Si(OCH2CH3)2(CH3)2 was found to enable formation of permanent bridges between layers and prevention of structure contraction upon calcination.[51] This was referred to as stabilization because the interlayer (002) reflection (in MCM-22P) did not disappear/shift upon calcination. The products were referred to as interlayer expanded zeolites (IEZ).[52] In this process 10-ring interlayer pores become expanded by two SiO-ring units with formation of O-Si(OH)2- moieties instead of -O- as bridges between layers.[26] The silylated calcined B-SSZ-70s do show (Fig. A2) the interlayer 6.5° 2θ peaks at roughly the same position as they were in the as-synthesized material indicating that the structure was successfully stabilized. In the case of Al-SSZ-70 the interlayer peak is at 6.9° 2θ and shows significantly reduced intensity suggesting unsuccessful stabilization in terms of the -O-Si-Obridges between layers. This is not unusual since other layered MWW materials, e.g. MCM-22P, did show dependence of the stabilization capability on the amount of Al in the structure.[28, 53] This effect is not well understood but the stabilization potential decreases with increasing Al content. The present Al-SSZ-70 does not produce the IEZ form but its silylation does appear to have a positive effect by preventing collapse into subzeolite form. In the XRDs of the B-SSZ-70-IEZ the maximum of the triangular peak shifts to lower 2θ (from 8.9 to 8.4°) but not for silylated Al-SSZ-70. This seems consistent with changes to the structures but would require structural modeling for better understanding (like the structure of SSZ-70 itself).
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The BET area of stabilized B-SSZ-70-IEZ is not increased in comparison to the calcined-only samples. The apparently 'unstabilized' Al-SSZ-70 does show higher BET values than the untreated calcined product confirming positive impact of the silylation through prevention of the collapse to the sub-zeolite form. The differences in BET values may be within experimental errors or normal fluctuations of quantitative results but we believe they can be reasonably explained. In the case of MWW structures in general, the stabilization often does not result in significant increase of the BET values [51] despite the nominal pore size increases by two ring units and pore size enlargement by about 0.2 nm. As the bridging moieties contain geminal OH groups this may cause sufficient
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obstruction in the pore circumference to obliterate the nominal size gains. In the Al-SSZ-70, the silylation prevents collapse to sub-zeolite, which evidently resulted in higher BET area. 3.4. Acidity
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Catalytic potential of the obtained SSZ-70 zeolites was evaluated initially by determination of acid site concentration using FT-IR. The borosilicate forms of SSZ-70 were not expected to show strong acidity but were treated with aluminum nitrate to attempt conversion into active catalysts by insertion of aluminum. This was carried out by contacting calcined samples with aluminum nitrate solution for 5 days at 95°C according to the literature procedure [42]. The obtained aluminum modified borosilicates were compared to the Al-SSZ-70 and its derivatives pillared Al-SSZ-70 and Al-SSZ-70-IEZ. The concentrations of Brønsted and Lewis sites were determined by IR spectroscopy using the sorption of pyridine as a probe molecule (Table 2). The most important finding is that Al substitution into boron SSZ-70 by a routine method like applied here is much less efficient for activation than direct synthesis with Al in the gel. This is shown by the number of Brønsted acid sites in modified B-SSZ-70(4) and B-SSZ-70(25), 114 and 71 μmol/g respectively, which are approximately 5 times lower than that in directly synthesized Al-SSZ-70 (514 μmol/g). The IR spectra also showed significant retention of B in the structure, which may be because of pore restrictions in the condensed calcined structures. The concentrations of BAS in modified Al-SSZ-70 zeolites were lower for pillared and stabilized materials, 314 and 200 μmol/g respectively. These changes are consistent with the treatments, i.e. dilution due to silica pillars and partial Al extraction in acid medium during stabilization.
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3.5. Catalytic activity
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The framework MWW is known for exceptional performance in catalytic aromatic alkylation with small olefins but its expanded forms are viewed as promising catalysts for bulkier reactants.[54] The studies of Friedel-Crafts alkylation reported previously [33] used benzyl alcohol with benzene but we chose to replace the latter with mesitylene (Table 3). Since alkylation is an acid-catalyzed reaction the concentration of BAS in an important factor but accessibility is also crucial because mesitylene is a bulky molecule. The formation of relatively large products like 2-benzyl-1,3,5-trimethylobenzene (BTB) and dibenzyl ether (DBE) (Scheme 1) provides a measure of the accessibility of active sites.
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Al-SSZ-70 synthesized directly was found to be as active as the base case MCM-22 while samples obtained by Al substitution into B-SSZ-70 showed 5 times lower conversion of benzyl alcohol during the same reaction time (Fig.7). This confirms that Al insertion post-synthesis is a less effective approach, at least by the procedure applied here, than the direct synthesis. This outcome could have been influenced by the sub-zeolite character of the calcined B-SSZ-70 that were aluminated, by possibly not allowing better Al substitution for boron. The importance of the concentration of BAS for zeolites having similar textural properties is further evidenced by activity of the IEZ form. It has less Al because of treatment in acid environment and shows lower conversion than the parent with more Al. In the case of pillared Al-SSZ-70 the benefits of expanding the interlayer distance are clearly visible. It shows highly improved catalytic activity despite the decrease of BAS in comparison to the parent materials. This suggests impact of diffusion limitation in classical MWW zeolites in reaction with bulky molecules. This means that the reactants are not able to reach acid sites in micropores of 3-dimentional MWW and the reaction proceeds mostly on the external surfaces of the crystals. Pillared Al-SSZ-70 showed the highest conversion and selectivity percentage (100%), evidently due to the enhancement accessibility of acid sites located on the surface of the layers. Our study has shown that pillared Al-SSZ-70 and MCM-22 zeolites may be successfully used in the alkylation of the large molecules like mesitylene despite decreased number of acid sites in comparison to the parent zeolite.
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These results showed how important is the insight not only into acidity but also other properties like texture and porosity which are crucial to better selection of post-synthetic modifications for catalyst preparation. Table 3. Catalytic reaction data about various catalysts in the benzylation of mesitylene. BA conversion,%
Selectivity of BTB, %
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SSZ-70 (25) – Al reinsertion
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MCM-22
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MCM-22 pillared
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Time, min Fig. 7. Conversion of benzyl alcohol for SSZ-70 and MCM-22 catalysts in the benzlyation of mesitylene with benzyl alcohol.
4. Conclusions
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In this work SSZ-70 zeolites with Si/B ratios 4 and 25 and Si/Al ratio 50 in the gel have been synthesized by classical hydrothermal synthesis with the use of bis(cyclohexyl)imidazolium template. All three materials have been modified by post-synthetic treatments i.e. swelling, stabilization and pillaring. Our studies have shown that all tested zeolites easily transformed into expanded materials upon swelling which was confirmed by the 003 reflection located at around 5-5.5° 2θ and low angle peaks near 5 nm d-spacing. Calcination of SSZ-70 zeolites resulted in a collapse of the interlayer distances below that of the 3D framework, recognizable by the shift of 002 reflection towards higher values of 2θ deg. Pillaring of SSZ-70 materials resulted in a permanent layers separation (4.5 – 5.5 nm) confirmed by X-ray diffraction patterns as well as higher values of textural properties, e.g. over two-fold increase of surface areas. Due to the shift and blurring of the characteristic triangular reflection in the 8-10° 2θ range it cannot be unambiguously determined how much of the structure of SSZ-70 has been preserved. Silylation process to give the interlayer expanded zeolite form was successful only for Bcontaining zeolites. In the case of Al-SSZ-70 silylation prevented collapse to sub-zeolite upon calcination. Catalytic properties were investigated with directly synthesized Al-containing SSZ-70 and Al-substituted to boron
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SSZ-70. The former shows higher concentration of Brønsted acid sites and more promising catalytic activity in the reaction of mesitylene alkylation with benzyl alcohol. The highest activity and selectivity (100%) was showed by pillared Al-SSZ-70 apparently due to the enhancement accessibility of acid sites located on the surface of the layers. Catalytic results showed the advantage of direct synthesis of Al-SSZ-70 over the Al-inserted SSZ-70 samples as well as the positive effect of the modifications such as silylation and pillaring on the zeolite activity in alkylation reaction with bulky molecules.
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Acknowledgement This work was financed with the funds from the National Science Centre Poland, grant no 2016/21/B/ST5/00858. J.C. acknowledges OP VVV "Excellent Research Teams", project No. CZ.02.1.01/0.0/0.0/15_003/0000417 – CUCAM and the Czech Science Foundation (P106/12/G015). References
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[1] J. Čejka, G. Centi, J. Perez-Pariente, W.J. Roth, Zeolite-based materials for novel catalytic applications: Opportunities, perspectives and open problems, Catalysis Today, 179 (2012) 2-15. [2] E.G. Derouane, J.C. Védrine, R.R. Pinto, P.M. Borges, L. Costa, M.A.N.D.A. Lemos, F. Lemos, F.R. Ribeiro, The Acidity of Zeolites: Concepts, Measurements and Relation to Catalysis: A Review on Experimental and Theoretical Methods for the Study of Zeolite Acidity, Catalysis Reviews, 55 (2013) 454-515. [3] I. Fechete, Y. Wang, J.C. Védrine, The Past, Present and Future of Heterogeneous Catalysis, Catalysis Today, 189 (2012) 2-27. [4] A.F. Masters, T. Maschmeyer, Zeolites - From curiosity to cornerstone, Microporous and Mesoporous Materials, 142 (2011) 423-438. [5] D.P. Serrano, J.A. Melero, G. Morales, J. Iglesias, P. Pizarro, Progress in the design of zeolite catalysts for biomass conversion into biofuels and bio-based chemicals, Catalysis Reviews, 60 (2018) 1-70. [6] J. Přech, Catalytic performance of advanced titanosilicate selective oxidation catalysts – a review, Catalysis Reviews, 60 (2018) 71-131. [7] A. Burton, Recent trends in the synthesis of high-silica zeolites, Catalysis Reviews, 60 (2018) 132-175. [8] C. Baerlocher, L.B. McCusker, Database of Zeolite Structures, http://www.iza-structure.org/databases/, 2013. [9] T.F. Degnan, G.K. Chitnis, P.H. Schipper, History of ZSM-5 fluid catalytic cracking additive development at Mobil, Microporous and Mesoporous Materials, 35-36 (2000) 245-252. [10] J. Weitkamp, L. Puppe, Catalysis and Zeolites: Fundamentals and Applications, Springer Berlin Heidelberg2013. [11] T.F. Degnan, The implications of the fundamentals of shape selectivity for the development of catalysts for the petroleum and petrochemical industries, Journal of Catalysis, 216 (2003) 32-46. [12] M.W. Schoonover, M.J. Cohn, New materials discovery for industrial applications, Topics in Catalysis, 13 (2000) 367-372. [13] E.T.C. Vogt, G.T. Whiting, A.D. Chowdhury, B.M. Weckhuysen, Zeolites and Zeotypes for Oil and Gas Conversion, in: F.C. Jentoft (Ed.) Advances in Catalysis, Vol 582015, pp. 143-314. [14] M.E. Leonowicz, J.A. Lawton, S.L. Lawton, M.K. Rubin, MCM-22: A Molecular Sieve with Two Independent Multidimensional Channel Systems, Science, 264 (1994) 1910-1913. [15] C. Perego, P. Ingallina, Recent advances in the industrial alkylation of aromatics: new catalysts and new processes, Catalysis Today, 73 (2002) 3-22. [16] T.F. Degnan, Jr., C.M. Smith, C.R. Venkat, Alkylation of aromatics with ethylene and propylene: recent development in commercial processes, Applied Catalysis, A: General, 221 (2001) 283-294. [17] W.J. Roth, J. Čejka, Two-dimensional Zeolites: Dream or Reality?, Catalysis Science & Technology, 1 (2011) 43-53.
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[18] F.S.O. Ramos, M.K. de Pietre, H.O. Pastore, Lamellar zeolites: an oxymoron?, RSC Advances, 3 (2013) 20842111. [19] W.J. Roth, D.L. Dorset, Expanded view of zeolite structures and their variability based on layered nature of 3-D frameworks, Microporous and Mesoporous Materials, 142 (2011) 32-36. [20] L. Schreyeck, P. Caullet, J.C. Mougenel, J.L. Guth, B. Marler, PREFER: A new layered (alumino) silicate precursor of FER-type zeolite, Microporous Materials, 6 (1996) 259-271. [21] M. Choi, K. Na, J. Kim, Y. Sakamoto, O. Terasaki, R. Ryoo, Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts, Nature, 461 (2009) 246. [22] W.J. Roth, P. Nachtigall, R.E. Morris, J. Čejka, Two-dimensional zeolites: current status and perspectives, Chemical Reviews, 114 (2014) 4807–4837. [23] W.J. Roth, Chapter 7 - Synthesis of Delaminated and Pillared Zeolitic Materials, in: J. Čejka, H. van Bekkum, A. Corma, F. Schüth (Eds.) Catalysis by Microporous Materials, Elsevier2007, pp. 221-239. [24] L. Xu, P. Wu, Diversity of layered zeolites: from synthesis to structural modifications, New Journal of Chemistry, 40 (2016) 3968-3981. [25] P. Kaminski, I. Sobczak, P. Decyk, M. Ziolek, W.J. Roth, B. Campo, M. Daturi, Zeolite MCM-22 Modified with Au and Cu for Catalytic Total Oxidation of Methanol and Carbon Monoxide, Journal of Physical Chemistry C, 117 (2013) 2147-2159. [26] W.J. Roth, W. Makowski, B. Marszalek, P. Michorczyk, W. Skuza, B. Gil, Activity enhancement of zeolite MCM-22 by interlayer expansion enabling higher Ce loading and room temperature CO oxidation, Journal of Materials Chemistry A, 2 (2014) 15722-15725. [27] L.C. Liu, U. Diaz, R. Arenal, G. Agostini, P. Concepcion, A. Corma, Generation of subnanometric platinum with high stability during transformation of a 2D zeolite into 3D, Nature Materials, 16 (2017) 132-138. [28] W.J. Roth, J. Čejka, R. Millini, E. Montanari, B. Gil, M. Kubu, Swelling and interlayer chemistry of layered MWW zeolites MCM-22 and MCM-56 with high Al content, Chemistry of Materials, 27 (2015) 4620-4629. [29] J.C. Cheng, A.D. Fung, D.J. Klocke, S.L. Lawton, D.N. Lissy, W.J. Roth, C.M. Smith, D.E. Walsh, Process for preparing short chain alkyl aromatic compounds, US Patent 5,557,024, September 17 1995. [30] B. Marler, H. Gies, Hydrous Layer Silicates as Precursors for Zeolites Obtained through Topotactic Condensation: A Review, European Journal of Mineralogy, 24 (2012) 405-428. [31] W.J. Roth, D.L. Dorset, G.J. Kennedy, Discovery of New MWW Family Zeolite EMM-10: Identification of EMM-10P as the Missing MWW Precursor with Disordered Layers, Microporous and Mesoporous Materials, 142 (2011) 168-177. [32] R.H. Archer, J.R. Carpenter, S.-J. Hwang, A.W. Burton, C.-Y. Chen, S.I. Zones, M.E. Davis, Physicochemical Properties and Catalytic Behavior of the Molecular Sieve SSZ-70, Chemistry of Materials, 22 (2010) 2563-2572. [33] H.Y. Luo, V.K. Michaelis, S. Hodges, R.G. Griffin, Y. Román-Leshkov, One-pot synthesis of MWW zeolite nanosheets using a rationally designed organic structure-directing agent, Chemical Science, 6 (2015) 6320-6324. [34] W.J. Roth, B. Gil, B. Marszalek, Comprehensive system integrating 3D and 2D zeolite structures with recent new types of layered geometries, Catalysis Today, 227 (2014) 9-14. [35] W.J. Roth, B. Gil, W. Makowski, B. Marszalek, P. Eliášová, Layer like porous materials with hierarchical structure, Chemical Society Reviews, 45 (2016) 3400-3438. [36] S.I. Zones, A.W. Burton, Molecular sieve ssz-70 composition of matter and synthesis thereof, US Patent 7,108,843, September 19, 2006. [37] S. Smeets, Z.J. Berkson, D. Xie, S.I. Zones, W. Wan, X. Zou, M.-F. Hsieh, B.F. Chmelka, L.B. McCusker, C. Baerlocher, Well-Defined Silanols in the Structure of the Calcined High-Silica Zeolite SSZ-70: New Understanding of a Successful Catalytic Material, Journal of the American Chemical Society, 139 (2017) 16803-16812. [38] X. Ouyang, S.-J. Hwang, R.C. Runnebaum, D. Xie, Y.-J. Wanglee, T. Rea, S.I. Zones, A. Katz, Single-Step Delamination of a MWW Borosilicate Layered Zeolite Precursor under Mild Conditions without Surfactant and Sonication, Journal of the American Chemical Society, 136 (2014) 1449-1461.
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[39] W.J. Roth, C.T. Kresge, J.C. Vartuli, M.E. Leonowicz, A.S. Fung, S.B. McCullen, MCM-36: The First Pillared Molecular Sieve with Zeolite Properties, Catalysis by Microporous Materials, 94 (1995) 301-308. [40] R.H. Archer, S.I. Zones, M.E. Davis, Imidazolium structure directing agents in zeolite synthesis: Exploring guest/host relationships in the synthesis of SSZ-70, Microporous and Mesoporous Materials, 130 (2010) 255265. [41] B. Gil, W.J. Roth, J. Grzybek, A. Korzeniowska, Z. Olejniczak, M. Eliáš, M. Opanasenko, J. Čejka, The effect of hot liquid water treatment on the properties and catalytic activity of MWW zeolites with various layered structures, Catalysis Today, 304 (2018) 22-29. [42] S.I. Zones, A. Benin, S.-J. Hwang, D. Xie, S. Elomari, M.-F. Hsieh, Studies of Aluminum Reinsertion into Borosilicate Zeolites with Intersecting Channels of 10- and 12-Ring Channel Systems, Journal of the American Chemical Society, 136 (2014) 1462-1471. [43] B. Gil, B. Marszalek, A. Micek-Ilnicka, Z. Olejniczak, The Influence of Si/Al Ratio on the Distribution of OH Groups in Zeolites with MWW Topology, Topics in Catalysis, 53 (2010) 1340-1348. [44] W.J. Roth, C.T. Kresge, Intercalation chemistry of NU-6(1), the layered precursor to zeolite NSI, leading to the pillared zeolite MCM-39(Si), Microporous and Mesoporous Materials, 144 (2011) 158-161. [45] B.T. Yang, J.G. Jiang, H. Xu, P. Ji, P. Wu, Sub-zeolite of FER topology derived from an interlayer modification of PLS-3 lamellar precursor, Microporous and Mesoporous Materials, 203 (2015) 54-62. [46] W.J. Roth, O.V. Shvets, M. Shamzhy, P. Chlubna, M. Kubu, P. Nachtigall, J. Cejka, Postsynthesis Transformation of Three-Dimensional Framework into a Lamellar Zeolite with Modifiable Architecture, Journal of the American Chemical Society, 133 (2011) 6130-6133. [47] W.J. Roth, J.C. Vartuli, C.T. Kresge, Characterization of Mesoporous Molecular Sieves: Differences between M41S and Pillared Layered Zeolites, Catalysis by Microporous Materials, 129 (2000) 501-508. [48] S. Maheshwari, E. Jordan, S. Kumar, F.S. Bates, R.L. Penn, D.F. Shantz, M. Tsapatsis, Layer Structure Preservation during Swelling, Pillaring, and Exfoliation of a Zeolite Precursor, Journal of the American Chemical Society, 130 (2008) 1507-1516. [49] W. Makowski, K. Mlekodaj, B. Gil, W.J. Roth, B. Marszałek, M. Kubu, P. Hudec, A. Smiešková, M. Horňáček, Application of quasi-equilibrated thermodesorption of linear and di-branched paraffin molecules for detailed porosity characterization of the mono-layered zeolite MCM-56, in comparison with MCM-22 and ZSM-5, Dalton Trans., 43 (2014) 10574-10583. [50] M.E. Landis, B.A. Aufdembrink, P. Chu, I.D. Johnson, G.W. Kirker, M.K. Rubin, Preparation of molecular sieves from dense layered metal oxides, Journal of the American Chemical Society, 113 (1991) 3189-3190. [51] P. Wu, J.F. Ruan, L.L. Wang, L.L. Wu, Y. Wang, Y.M. Liu, W.B. Fan, M.Y. He, O. Terasaki, T. Tatsumi, Methodology for Synthesizing Crystalline Metallosilicates with Expanded Pore Windows through Molecular Alkoxysilylation of Zeolitic Lamellar Precursors, Journal of the American Chemical Society, 130 (2008) 8178-8187. [52] W.B. Fan, P. Wu, S. Namba, T. Tatsumi, A titanosilicate that is structurally analogous to an MWW-type lamellar precursor, Angewandte Chemie-International Edition, 43 (2004) 236-240. [53] T. Yokoi, S. Mizuno, H. Imai, T. Tatsumi, Synthesis and structural characterization of Al-containing interlayerexpanded-MWW zeolite with high catalytic performance, Dalton Transactions, 43 (2014) 10584-10592. [54] D. Liu, X. Zhang, A. Bhan, M. Tsapatsis, Activity and selectivity differences of external Brønsted acid sites of single-unit-cell thick and conventional MFI and MWW zeolites, Microporous and Mesoporous Materials, 200 (2014) 287-290.
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S0920-5861(18)31705-X https://doi.org/10.1016/j.cattod.2019.03.006 CATTOD 12028
To appear in:
Catalysis Today
Received date: Revised date: Accepted date:
27 November 2018 28 February 2019 4 March 2019
ˇ Please cite this article as: Grzybek J, Kubu M, Roth WJ, Gil B, Cejka J, Kasneryk V, Structural transformation and chemical modifications of the unusual layered zeolite MWW form SSZ-70, Catalysis Today (2019), https://doi.org/10.1016/j.cattod.2019.03.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural transformation and chemical modifications of the unusual layered zeolite MWW form SSZ-70
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Justyna Grzybek,a Martin Kubu,b Wieslaw J. Roth,a Barbara Gil,a Jiři Čejka,c , Valeryia Kasnerykb Faculty of Chemistry, Jagiellonian University in Kraków, Gronostajowa 2, 30-387 Kraków, Poland
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J. Heyrovský Institute of Physical Chemistry Academy of Sciences of Czech Republic, v.v.i., Dolejškova 2155/3, 182 23 Prague 8, Czech Republic c
Department of Physical and Macromolecular Chemistry, Faculty of Sciences, Charles University, Hlavova 8, 128 43 Prague 2, Czech Republic
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Dedication – this article is dedicated to Prof. M. Ziółek from UAM in Poznan on the occasion of her 70 th birthday to honor her contributions to catalysis sciences and her support and inspiration to several generations of scientists
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Graphical abstract
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Layered zeolites SSZ-70 with MWW topology were synthesized with boron and aluminum and investigated for post-synthesis modifications All show successful swelling, pillaring and stabilization to produce interlayer expanded zeolites (IEZ) except Al-SSZ-70, which did not show stabilization to IEZ Al insertion post-synthesis into B-SSZ-70 was less effective in producing active acid catalyst than direct synthesis with Al in the gel
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Pillared Al-SSZ-70 was more active than the parent zeolite in alkylation of mesitylene with benzyl alcohol proving benefits of creating more open structures.
Abstract
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SSZ-70 is one of several layered forms of zeolite MMW obtained by direct synthesis with consecutive layers shifted by approximately ±1/3 of a unit cell in the horizontal direction (⟨110⟩), according to the recent structure solution report. Zeolite MWW is a valuable industrial alkylation catalyst and SSZ-70 itself was called by the inventors 'a successful catalytic material'. SSZ-70 shows characteristic powder X-ray diffraction pattern (XRD), containing a unique broad band with a roughly triangular shape and maximum at 8.5-9.0° 2θ (Cu Kα radiation), which can be used to distinguish SSZ-70 from the other MWW materials with different layer arrangements. This work concerns post-synthesis treatments of SSZ-70 to investigate its ability to produce more expanded structures by silylation, swelling and pillaring. Both boron- and aluminum- containing SSZ-70 were investigated. They showed the ability for interlayer expansion by swelling with cationic surfactants and were pillared with silica to produce micro/mesoporous hybrids with increased BET area. Not too surprisingly, the original unique layer stacking of SSZ-70 seems to have become more random and not distinguishable from the swollen forms of the other swellable MWW materials MCM-22P and the monolayer MCM-56. Calcined SSZ-70 showed unusual peaks, assigned to (002) reflections, with d-spacing shorter than c-unit cell of the 3D MWW framework, which suggest collapse with formation of the so-called sub-zeolite form. Silylation resulted in different outcomes for B and Al SSZ-70 samples. The former produced Interlayer expanded (IEZ) forms of B-SSZ-70 but did not show significant increase in pore volume and BET surface area. Silylation of Al-SSZ-70 did not yield the stabilized (IEZ) form but had a positive effect as it prevented the collapse to sub-zeolite upon calcination. Catalytic testing was carried out for the Al-containing SSZ-70 obtained by direct synthesis and for B-SSZ-70 activated by Al insertion. The former showed much higher concentration of Brønsted acid sites and 5 times faster conversion in a model alkylation reaction between benzyl alcohol and mesitylene. Pillared Al-SSZ-70 showed highest activity despite lower Al concentration due to silica pillars. Both silylation and pillaring had a positive effect on the activity in alkylation reaction with bulky molecules. Keywords: MWW, SSZ-70, post-synthetic modification, alkylation
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1. Introduction
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Zeolites play a special role in heterogeneous catalysis as solid acids and oxidation catalysts with well-defined micropores of molecular dimensions. [1-7] There are 240 officially approved zeolite frameworks [8] but 5 have been particularly influential in the development of industrial applications and are unofficially recognized as the catalysis 'big five'. [9-13] They are zeolites X and Y (FAU), ZSM-5 (MFI), mordenite (MOR), ferrierite (FER) and beta (BEA). It can be justified to expand this group by the framework MWW containing 2 independent medium pore size channel systems and 0.7 nm wide supercages.[14] MWW shows exceptional catalytic activity and selectivity in aromatic alkylation [15, 16] and has been the leader in the development of layered, twodimensional (2D) zeolite materials.[17, 18] The latter represent a fundamental breakthrough concept [19] that has been spreading to other frameworks notably including also FER [20] and ZSM-5 [21]. It enabled formation of various layered structures by direct synthesis and post-synthesis modifications with potential for enhanced performances. [19, 22, 23] The framework MWW has been the model for the others by affording a great variety of different layered structures, about 15 so far.[22, 24] Some of them have expanded layer spacing and can be more accessible to larger molecules for catalytic reactions but also can allow functionalization with transition
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metals.[25-27] Another unsurpassed property of the MWW framework is its formation of different layered forms with high Al content, up to Si/Al = 10 [28]. This is a very important factor as high acidity is essential in demanding catalytic conversions of hydrocarbons into fuels, lubricants and other valuable products.[29] The majority of the other known layered zeolites are predominantly low Al materials, i.e. with small intrinsic activity.[30] Zeolite MWW stands out by giving 6 different types of layered materials and the 3D framework (MCM-49) by direct synthesis. Four of them, MCM-22P, EMM-10P, MCM-56 and SSZ-70, were obtained using conventional templates [31, 32] while the unilamellar MIT-1 and its multilayered equivalent designated by us UJM-1 and yet unpublished, were obtained with a bifunctional surfactant structure directing agents by analogy to layered MFI materials.[21, 33] The former 5 are distinguished based on XRD patterns, shown in Figure 1, containing unique combinations of sharp and broad peaks in the range 5-10° 2θ. They also show different catalytic characteristics and behavior in post-synthesis modifications, which include stabilization by treatment with silylating reagents to give the so-called interlayer expanded IEZ forms and swelling with surfactants.[34, 35] The top four XRDs in Fig. 1 can be viewed as resulting from combination of two sets of 2 basic types of visual features: one or two peaks around 6.5-7.5° 2θ, and 2 peaks or broad band at 8-10° 2θ. These features can be rationalized as due to a short interlayer distance, 2.5 nm (one peak at 6.5-7.5° 2θ), or expanded interlayer stacking, >2.6 nm (2 peaks), and vertically aligned (ordered, 2 peaks between 8-10° 2θ) or misaligned stacking (broad band). The expanded forms with two peaks at 6.5-7.5° 2θ collapse upon calcination to one peak with the 'normal', 2.5 nm, interlayer distance. The material SSZ-70 [36], which is the subject of this study is expanded showing 2 peaks at 6.5-7.5° 2θ but it is evidently not a vertically ordered MWW structure nor similar to the above disordered ones. It shows a unique band in the 8-10° 2θ range, namely a triangular broad maximum at 8.5-9° 2θ. The structure of SSZ-70 was solved and found to involve specific stacking faults with approximately ±1/3 shifts in the horizontal direction (⟨110⟩).[37] As a 'successful catalytic material', suggested by the inventors, SSZ-70 is of interest with regard to post-synthesis treatments with possible expansion and generation of open structures like those obtained earlier with the other layered MWW forms, especially MCM-22P and MCM-56. Previous reports about SSZ-70 transformation include delamination but it does not appear to be in the sense of obtaining single-sheet architectures like ‘house-of-cards'.[38] TEMs suggest mainly unaligned stacks with occasional single MWW layers. The primary transformation of layered zeolite forms include swelling with cationic surfactants, which tends to increase interlayer stacking disorder.[39] Thus, we wanted to determine, among others, if SSZ-70 would transform to these other types of MWW materials and lose its original specific layer offsets. The study of catalytic activity includes comparison of various layered derivatives of Al-containing SSZ-70.
3
Expanded Unit cell c >2.6 nm 002 100 101 102
c ~2.5 nm
5 6 7 8 9 10 11 12
A
N
2 peaks 8.5 band
SSZ-70
SC RI PT
1 peak 8-10 band
U
EMM-10P
5 6 7 8 9 10 11 12
5 6 7 8 9 10 11 12
MCM-56
5 6 7 8 9 10 11 12
2 peaks 8-10 band
3 peaks
MCM-49
MCM-22P
4 peaks
M
5 6 7 8 9 10 11 12
EP
TE
D
Fig. 1. X-ray diffraction patterns in the range 5-10° 2θ for various as-synthesized MWW forms with layered structures rationalizing the observed features.
swelling detemplation stabilization
A
CC
calcination
3D
detemplated
IEZ – interlayer expanded
swollen
silica pillared
delaminated
Ordered or disordered stacking
Fig. 2. Principal transformations of the (multi)layered MWW materials as models for SSZ-70.
4
2. Experimental 2.1. Structure directing agent (SDA)
N
U
SC RI PT
1,3-bis(cyclohexyl)imidazolium tetrafluoroborate was prepared according to the procedure described in [32]. 19.83 g of cyclohexylamine (Alfa-Aesar, 98%) and 200 mL of toluene (Sigma Aldrich) was placed in a room temperature water bath. Next, 6.32 g of paraformaldehyde (Fisher, 95%) was added with strong stirring. The solution was stirred at room temperature for 30 min and then ice was added to the water bath. After 1 h of solution cooling, another 19.83 g of cyclohexylamine was added dropwise via an addition funnel. 36.60 g of tetrafluoroboric acid (Alfa-Aesar 48 wt.% in water) was diluted to 30 wt.% with water and then added dropwise. The ice bath was removed, and the solution was allowed to warm for 30 min. Next, 28.98 g of glyoxal solution (Alfa-Aesar, 40 wt % in water) was added dropwise. The flask was heated at 40 oC overnight and then allowed to cool to room temperature. The solid precipitate was filtered off and washed with 150 mL of water and then 150 mL of diethyl ether and dried for 8 h under high vacuum. Recrystallization from 2:1 ethyl acetate/dichloromethane yielded 40.4 g of off-white solid after drying under high vacuum (82.4% yield). Final product was verified using 1H and 13C NMR. It was converted from the tetrafluoroborate to hydroxide form using hydroxide exchange resin (Ambersept 900 OH, Alfa Aesar) in water and then titrated with HCl solution to determine OH- concentration.
A
2.2. Zeolite syntheses
CC
EP
TE
D
M
Borosilicate reactions were run at Si/B = 4 and 25 with gel compositions 1.0 SiO2:0.125 B2O3:0.25SDA+OH-:23 H2O designated B-SSZ-70(4) and 1.0 SiO2:0.02 B2O3:0.25SDA+OH-: 0.1 NaOH:30H2O designated B-SSZ-70(25), respectively, according to the procedures described in [40]. Al-SSZ-70 zeolite was obtained from the mixture composition: 1SiO2:0.2 OSDA:0.01 Al2O3:0.1 NaOH:30 H2O (Al-SSZ-70(50)) by the standard hydrothermal procedures. As the first step a weighted amount of the template was dissolved in water and converted to the hydroxide form by mixing with the ion exchange resin (Ambersept 900 OH, Alfa Aesar). The obtained aqueous solution of a template in hydroxide form was combined with boric acid (99.5%, Alfa Aesar) or aluminum hydroxide (Sigma Aldrich), NaOH (50%, Sigma Aldrich) and tetraethyl orthosilicate (98%, Sigma Aldrich). The gel was transferred to a Teflon liner, sealed in a pressure bomb and held at 433 K with rotation for 8 days for B-SSZ70 (25) and Al-SSZ-70 and 12 days for B-SSZ-70 (4). After crystallization the solids were recovered by filtration, washed with distilled H2O, and dried at 338 K overnight. Solids were calcined by heating on the air at 813 K for 6 h with 2K/min heating rate. MCM-22P zeolite were prepared according to the procedure described earlier [41]. 2.3. Post-synthetic modifications 2.3.1. Swelling and pillaring
A
As-synthesized SSZ-70 samples were mixed with 25% cetyltrimethylammonium (CTMA) hydroxide solution (solid/liquid=0.05) obtained by ion exchange of the chloride with ion exchange resin as described previously.[28] The slurry was stirred overnight at room temperature. The solid was recovered by centrifugation, washed 3 times with distilled water and dried at room temperature. Pillaring was carried out by stirring of a swollen sample with tetraethyl orthosilicate (TEOS) (solid/liquid = 0.01, w/w) at room temperature. Solids were recovered by centrifugation, dried at room temperature and then calcined at 823 K for 6 h with 2K/min heating rate.
5
2.3.2. Stabilization by silylation In a typical procedure, as-synthesized SSZ-70 was mixed with an aqueous solution of 2 M HNO3 (solid/liquid=0.5) and 2 mmol of dimethyldiethoxysilane Me2Si(OEt)2. The mixture was heated in a Teflon-lined autoclave at 423 K for 16 h. After silylation, the solid was collected by filtration, rinsed with distilled water and dried at 373 K. The product was calcined at 823 K for 6 h with 2K/min heating rate.
SC RI PT
2.3.3. Aluminum insertion into B-SSZ-70 The treatment was based on a literature procedure [42]. 0.40 g of calcined borosilicate zeolite and 0.10 g of Al(NO3)3·9H2O dissolved in 10 mL of H2O were placed in a Teflon-lined autoclave. The mixture was heated at 95°C for 5 days statically. The solids were washed twice with HCl solution (pH = 2) at room temperature, to remove weakly adsorbed Al salt before continuing to water wash. 2.3.4. Detemplatation of swollen SSZ-70 (deswelling)
U
The swollen SSZ-70 zeolite was mixed with 0.625 M solution of NH4NO3 (Avantor Poland, p.p.a.) in ethanol for 1 h at room temperature (20 ml of solution per 0.5 g of zeolite), repeated two times, filtered, washed with anhydrous ethanol and dried at room temperature.
N
2.4. Characterization
M
A
The structure and crystallinity of obtained zeolites were determined by X-ray powder diffraction (XRD) with CuKα radiation (ʎ = 0.154 nm) using a Bruker AXS D8 Advance diffractometer equipped with a graphite monochromator and a position sensitive detector (Våntec-1) with Bragg-Brentano geometry and Rigaku MiniFlex diffractometer in a reflection mode. The XRD patterns were usually collected with steps of 0.02o.
D
The samples were examined under scanning electron microscopy (SEM) using a JEOL JSM5500LV scanning electron microscope. For the measurements, crystals were covered with a thin platinum layer by sputtering in the vacuum chamber of a BAL-TEC SCD-050 sample sputter coater.
EP
TE
Relative content of Al and Si was determined in the samples formulated into pellets, 20 mm in diameter, with the use of Energy-Dispersive XRF spectrometer (Thermo Scientific, ARL QUANT’X). The X-rays of 4-50 kV (1 kV step) with the beam size of 1 mm were generated with the Rh anode. The detector used was a 3.5 mm Si(Li) drifted crystal with a Peltier cooling (ca. 185 K). For quantitative analysis, calibration with a series of metallic standards and UniQuant software were used.
CC
Nitrogen adsorption isotherms were determined by the standard method at 77 K (liquid nitrogen temperature) using an ASAP 2020 (Micromeritics) static volumetric apparatus. Before adsorption the samples were outgassed at 623 K using turbomolecular pump to remove adsorbed water.
A
The concentration of Lewis (LAS) and Brønsted (BAS) acid sites was determined using adsorption of pyridine (Py) followed by IR spectroscopy (Tensor 27 from Bruker, MTC detector, spectral resolution 2 cm -1). Zeolite samples were pressed into self-supporting wafers with a density of ca 8 mg/cm2 and activated in situ at 723 K for 1 hour at high vacuum (10-5 mBar). Excess of pyridine vapor was adsorbed at 440 K followed by desorption for 20 min at 440 K. Spectra were recalculated to a wafer mass equal 10 mg. Concentration of Lewis and Brønsted acid sites were evaluated from the intensities of bands at 1454 cm−1 (LAS) and at 1545 cm−1 (BAS) using absorption coefficients determined earlier in our laboratory [43] using external standards, ε (LAS) =0.165 cm2/μmol, and ε (BAS) =0.044 cm2/μmol, and the intensities of corresponding pyridine maxima after pyridine desorption at 440 K to ensure complete removal of weakly adsorbed species.
6
2.5. Catalytic testing Calcined samples were ion exchanged into NH4+ form, by contacting with 1 M solution of NH4NO3 (Avantor Poland, p.p.a.) for 1 h at room temperature (20 ml of solution per 0.5 g of zeolite), repeated three times, filtered, washed with deionized water, dried, and activated at 723 K for 5 h.
conversion = k ∙
SC RI PT
The selected test reaction, liquid phase benzylation of mesitylene with benzyl alcohol (Scheme S1), was carried out in a three-necked round-bottom flask equipped with a reflux condenser with heating in a multi-experiment workstation StarFish (Radleys Discovery Technologies) under atmospheric pressure. The reaction temperature was 353 K. Typically, 160 mmol of mesitylene (22 ml) was combined with 50 mg of the catalyst and dodecane as an internal standard. The reaction mixture was maintained for 30 min at the required reaction temperature and then 2 mmol of benzyl alcohol was added. This was regarded as the starting reaction time. Liquid samples were withdrawn at regular intervals and analyzed by the gas chromatography in a Perkin Elmer chromatograph with an FID detector using a 30 m packed ELITE-1MS column. The conversion of alcohol was calculated as follows: Salcohol ∙ 100 Sstandard ∙ n0
TE
D
M
A
N
U
where S is the area of respective peak in the chromatogram, k is the calibration coefficient (mol), n0 is the starting amount of alcohol (mol).
Scheme 1. Alkylation of mesitylene with benzyl alcohol
EP
3. Results and discussion
A
CC
The study was carried out with three SSZ-70 samples: two boron-containing with Si/B in the gel equal to 4 and 25 and one with Si/Al =50, designated B-SSZ-70(4), B-SSZ-70(25) and Al-SSZ-70(50) . Increasing Al content can produce, depending on template and other conditions, other frameworks, e.g. we obtained zeolite beta with Si/Al = 25/1 (in the gel), and possibly some other layered MWW forms like the disordered EMM-10P. This highlights the critical aspects of this work, which is the proper identification of SSZ-70 and its differentiation from the other MWW layered structures. As already indicated in the introduction SSZ-70 is identified on the basis of the broad triangular peak with the maximum between 8.5 to 9° 2θ. This peak falls in between 2 reflections observed with the ordered MWW materials (MCM-22P and MCM-49) and in the middle of the broad band, with a more or less rectangular shape, exhibited by the disordered MWW materials (MCM-56 and EMM10P). The triangular SSZ-70 peak is clearly due to the particular arrangement of MWW layers, namely shifting by approximately ±1/3 in the horizontal direction (⟨110⟩). Changes in the appearance of this broad peak suggest transition to more random disorder. [37]
7
3.1. Structure identification and characterization
SC RI PT
The as-synthesized SSZ-70 also shows 2 peaks in the range 6.5-7.5° 2θ, similar to (002) and (100) in MCM-22P and EMM-10P. They indicate expanded interlayer distance and usually merge into one peak or a close doublet slightly above 7° 2θ upon calcination. It should be also noted that XRD patterns of SSZ-70 frequently show very high intensities of the straight interlayer reflections (00l ,l=el), e.g. at 6.5° 2θ. This may indicate particularly severe 'preferred orientation' but other unknown effects can also be involved. These exaggerated intensities have no impact on the XRD interpretation since it involves only peak positions and shapes.
A
N
U
The calcined SSZ-70 materials showed unexpected satellite peaks on the right of the (100) reflection at ca. 7.5° 2θ, i.e. at lower d-spacing. They are assigned to the interlayer reflection equivalent to (002) of MCM-22 (SSZ-70 unit cell has doubled unit cell z-dimension) that is apparently shifted to d-spacing shorter than in the 3D structure. It is a surprising but not unprecedented observation and was described before [38] as suggested evidence of delamination. The present case indicates that such collapse is possible without explicit delamination procedure but can occur through routine calcinations. This peak could be rationalized as due collapse of the layered MWW structures to below the crystallographic unit cell dimension of the 3D framework. Analogous collapse was previously postulated with some other layered precursors of NSI,[44] FER [45] and PCR [46], and the products were referred to as sub-zeolites.[44] Another possibility we considered initially was that SSZ-70, with unit cell c-dimension doubled in comparison to MCM-22, could show additional, superlattice, XRD peaks. For example, the interlayer reflections (101) of the larger unit cell could appear in this position. It was deemed less likely since such peak should be quite broad and most likely even invisible.
EP
TE
D
M
The surface area and micropore volume of the calcined SSZ-70 materials are comparable to the typical values for MWW materials. In comparison, the above mentioned sub-zeolites of other frameworks, e.g. FER and NSI, were clearly less porous than the corresponding 3D frameworks. Layers in the latter have no internal porosity in contrast to MWW, which contains both interlayer channels and surface cavities and this may explain why calcined SSZ-70 shows normal BET areas, despite having the sub-zeolite form. The materials used in the reported structure solution studies [37] did not show the collapse of SSZ-70 upon calcination, suggesting formation of layer bridging like in MCM-22, of course with some unpaired free silanols. These differences in behavior with regard to calcination may be associated with the conditions of calcination itself and its effect on different samples, e.g. depending on template and other factors. Complete elucidation of this phenomenon does not seem crucial at this point due to the main focus on expanded materials.
A
CC
The structure of SSZ-70 and its behavior, e.g. upon calcination, can be linked to the nature of the bis(cyclohexyl)imidazolium template, which seems to induce the particular shift of MWW layer, i.e. by approximately ±1/3 in <110> direction. In the other known MWW zeolites synthesized directly the layers are either stacked more orderly or randomly arranged with hydrogen bonding involving surface silanols and template molecules. The particular layer stacking in SSZ-70 means that the density of interlayer bridges is likely smaller than in these other MWW zeolites and there are free silanols present. The objective of this work is to determine the properties and behavior of SSZ-70 in comparison to the other MWW layered materials.
8
SC RI PT
Al-SSZ-70
B-SSZ-70 (25)
B-SSZ-70 (4) 6
8
10
12
14
16
2, deg.
U
Fig. 3. X-Ray diffraction patterns of as synthesized (black) and calcined (red) SSZ-70 zeolites.
CC
EP
TE
D
M
A
N
The textural parameters of the studied SSZ-70 zeolites, namely BET area around 400 m2/g and pore volumes were typical for good quality MWW materials. It is evident that different chemical compositions of synthesis gel affects the morphology (SEM, Fig. 4) and is also reflected in the textural properties. The studied SSZ-70 zeolites form very thin plates but differ in size, shape and aggregation. The external surface areas are increasing in the order B-SSZ-70(4) < B-SSZ-70(25) < Al-SSZ-70(50), apparently as a reflection of crystal sizes observed in SEM, which are increasing in the opposite direction. The greater crystal sizes of B-MWW in comparison to Al-MWW are in line with previous observations, e.g. for MCM-22P [14]. Fig. 5 shows the change in overall and external surface areas upon pillaring and silylation/stabilization. The latter causes a small change while the former almost doubles the BET values.
A
Fig. 4. SEM images of B-SSZ-70(4) (a), B-SSZ-70(25) (b) and Al-SSZ-70 (50) (c).
9
1000 900 800
B-SSZ-70 (4)
Specific surface area External surface area
B-SSZ-70 (25) Al-SSZ-70
S, m2g-1
700 600 500 400 300 200 100
B-SSZ-70 (25) B-SSZ-70 (4)
0
Calcined
Stabilized
Pillared
Fig. 5.Textural parameters of the starting and modified SSZ-70 samples. 3.2 Swelling with surfactant
SC RI PT
Al-SSZ-70
A
CC
EP
TE
D
M
A
N
U
Swelling of layered solids, i.e. expansion of interlayer space by intercalation with suitable organics like surfactant molecules has both a fundamental and practical side. The former represents validation of the true layered nature of materials in question,[39] which may need to be proven especially with zeolites. The practical side concerns the potential for producing expanded, more open structures like pillared and delaminated. The swelling of layered MWW materials with cetyltrimethylammonium cation (CTMA, hexadecyltrimethylammonium) is known to produce a low angle peak at ca. 5 nm d-spacing but also a new reflection, apparently the (003) one, located at around 5-5.5° 2θ, while the (002) peak of the original materials located at 6.5° 2θ must disappear.[47, 48] This is evidently the case for all SSZ-70 materials treated with CTMA-hydroxide as shown in Fig. 6, which confirms their highly efficient swelling. Another observation of interest is what happened with the characteristic triangular 'SSZ-70' peak with the maximum at around 8.9° 2θ. It seems to be broadened in all cases suggesting perturbation of the SSZ-70 layered structure. However, the change is not big enough to say conclusively that swelling changed SSZ-70 completely into a swollen MCM-22P-like material destroying the specific SSZ-70 layer arrangement. In the case of MCM-22P swelling, the original two peaks in the range 8-10° 2θ are changed into a 'rectangular' band, similar to EMM-10P and MCM-56, reflecting apparent lateral disorder (vertical misalignment). To learn more about these features the SSZ-70 swollen materials were calcined. All patterns show further shift towards the 'rectangular' shape of the 8-10° 2θ band suggesting transformation in the direction of EMM-10-like structures with the loss of the specific SSZ-70 features. This effect is quite pronounced for B-SSZ-70(25) where even some layer ordering can be detected, which is indicated by the dip near 9° 2θ, as it can be attributed to partial separation into distinct (101) and (102) peaks. The latter process became more pronounced with the dimethylformamide (DMF) added to the swelling mixture and calcination of the product. The corresponding XRD showed significantly developed (101) and (102) reflections of the MWW structure confirming lateral layer re-orientation with significant ordered 3D stacking. The calcined swollen AlSSZ-70(50) and high B-SSZ-70(4) show a distinct peak at ca 7.7° 2θ, which is assigned to sub-zeolite materials. Peak intensity is quite significant with B-SSZ-70(4) and its origin is not certain but it is considered to be the interlayer (002-like) reflection of a collapsed structure.
10
Al-SSZ-70 swollen CTMA-OH calcined Al-SSZ-70-swollen CTMA-OH Al-SSZ-70 as synth.
SC RI PT
B-SSZ-70 (25) swollen CTMA-OH calcined B-SSZ-70 (25) swollen CTMA-OH B-SSZ-70 (25) as synth. B-SSZ-70 (4) swollen CTMA-OH calcined B-SSZ-70 (4) swollen CTMA-OH
6
8
10
12
14
16
N
4
U
B-SSZ-70 (4) as synth.
A
2, deg.
M
Fig. 6. X-Ray diffraction patterns of starting as-synthesized SSZ-70s and their modified forms obtained upon swelling with CTMA-OH and following calcination. Table 1. Textural parameters of examined zeolites calculated according to reference [49]. Sext m2/g
V micro cm3/g
V total cm3/g
B-SSZ-70 (4)
414
32
0.12
0.18
B-SSZ-70 (4)-IEZ
409
70
0.12
0.21
B-SSZ-70 (4) pillared
938
124
0.14
0.39
433
55
0.13
0.20
434
78
0.13
0.21
B-SSZ-70 (25) pillared
825
112
0.21
0.36
Al-SSZ-70
429
119
0.10
0.24
Al-SSZ-70-IEZ
492
129
0.12
0.27
Al-SSZ-70-pillared
764
357
N/A
0.47
MCM-22
375
66
0.11
0.23
MCM-22-pillared
1017
180
N/A
0.73
A
CC
B-SSZ-70 (25-IEZ
EP
B-SSZ-70 (25)
D
SBET m2/g
TE
Zeolite
Table 2. Acid properties from IR and Si/Al ratio (by XRF) of studied materials
11
B-SSZ-70 (4) – Al reinsertion B-SSZ-70 (25) – Al reinsertion Al-SSZ-70 Al-SSZ-70-IEZ Al-SSZ-70 pillared MCM-22 MCM-22-pillared
Si/Al(IR) BAS+LAS 114 160 28 40
Si/Al BAS 155 232 31 52
BAS(IR) μmol/g 107 71 514 314
LAS(IR) μmol/g 38 32 65 95
63
82
200
59
Si/Al (XRF) 27 26 17 20 29
24 32
31 37
599 435
67 65
16 21
3.3. Pillaring and stabilization
SC RI PT
Zeolite
N
U
Swollen SSZ-70 samples were pillared by treatment with TEOS according to the established procedure at room temperature [50] and then calcined. XRD patterns (Fig. A1) show successful pillaring indicated by low angle peaks at d-spacing in the range of 4.5 – 5.5 nm, which is further confirmed by significantly increased BET values to >750 m2/g. The question about preservation of the 'SSZ-70' structure cannot be answered decisively, as the band shape in the region 8-10° 2θ (Fig. A2) is not distinct enough to say how much the layer stacking arrangement shifted away from the SSZ-70 configuration.
CC
EP
TE
D
M
A
The treatment of multi-layered zeolite forms with silylating agents such as a monomeric silane source Si(OCH2CH3)2(CH3)2 was found to enable formation of permanent bridges between layers and prevention of structure contraction upon calcination.[51] This was referred to as stabilization because the interlayer (002) reflection (in MCM-22P) did not disappear/shift upon calcination. The products were referred to as interlayer expanded zeolites (IEZ).[52] In this process 10-ring interlayer pores become expanded by two SiO-ring units with formation of O-Si(OH)2- moieties instead of -O- as bridges between layers.[26] The silylated calcined B-SSZ-70s do show (Fig. A2) the interlayer 6.5° 2θ peaks at roughly the same position as they were in the as-synthesized material indicating that the structure was successfully stabilized. In the case of Al-SSZ-70 the interlayer peak is at 6.9° 2θ and shows significantly reduced intensity suggesting unsuccessful stabilization in terms of the -O-Si-Obridges between layers. This is not unusual since other layered MWW materials, e.g. MCM-22P, did show dependence of the stabilization capability on the amount of Al in the structure.[28, 53] This effect is not well understood but the stabilization potential decreases with increasing Al content. The present Al-SSZ-70 does not produce the IEZ form but its silylation does appear to have a positive effect by preventing collapse into subzeolite form. In the XRDs of the B-SSZ-70-IEZ the maximum of the triangular peak shifts to lower 2θ (from 8.9 to 8.4°) but not for silylated Al-SSZ-70. This seems consistent with changes to the structures but would require structural modeling for better understanding (like the structure of SSZ-70 itself).
A
The BET area of stabilized B-SSZ-70-IEZ is not increased in comparison to the calcined-only samples. The apparently 'unstabilized' Al-SSZ-70 does show higher BET values than the untreated calcined product confirming positive impact of the silylation through prevention of the collapse to the sub-zeolite form. The differences in BET values may be within experimental errors or normal fluctuations of quantitative results but we believe they can be reasonably explained. In the case of MWW structures in general, the stabilization often does not result in significant increase of the BET values [51] despite the nominal pore size increases by two ring units and pore size enlargement by about 0.2 nm. As the bridging moieties contain geminal OH groups this may cause sufficient
12
obstruction in the pore circumference to obliterate the nominal size gains. In the Al-SSZ-70, the silylation prevents collapse to sub-zeolite, which evidently resulted in higher BET area. 3.4. Acidity
N
U
SC RI PT
Catalytic potential of the obtained SSZ-70 zeolites was evaluated initially by determination of acid site concentration using FT-IR. The borosilicate forms of SSZ-70 were not expected to show strong acidity but were treated with aluminum nitrate to attempt conversion into active catalysts by insertion of aluminum. This was carried out by contacting calcined samples with aluminum nitrate solution for 5 days at 95°C according to the literature procedure [42]. The obtained aluminum modified borosilicates were compared to the Al-SSZ-70 and its derivatives pillared Al-SSZ-70 and Al-SSZ-70-IEZ. The concentrations of Brønsted and Lewis sites were determined by IR spectroscopy using the sorption of pyridine as a probe molecule (Table 2). The most important finding is that Al substitution into boron SSZ-70 by a routine method like applied here is much less efficient for activation than direct synthesis with Al in the gel. This is shown by the number of Brønsted acid sites in modified B-SSZ-70(4) and B-SSZ-70(25), 114 and 71 μmol/g respectively, which are approximately 5 times lower than that in directly synthesized Al-SSZ-70 (514 μmol/g). The IR spectra also showed significant retention of B in the structure, which may be because of pore restrictions in the condensed calcined structures. The concentrations of BAS in modified Al-SSZ-70 zeolites were lower for pillared and stabilized materials, 314 and 200 μmol/g respectively. These changes are consistent with the treatments, i.e. dilution due to silica pillars and partial Al extraction in acid medium during stabilization.
A
3.5. Catalytic activity
TE
D
M
The framework MWW is known for exceptional performance in catalytic aromatic alkylation with small olefins but its expanded forms are viewed as promising catalysts for bulkier reactants.[54] The studies of Friedel-Crafts alkylation reported previously [33] used benzyl alcohol with benzene but we chose to replace the latter with mesitylene (Table 3). Since alkylation is an acid-catalyzed reaction the concentration of BAS in an important factor but accessibility is also crucial because mesitylene is a bulky molecule. The formation of relatively large products like 2-benzyl-1,3,5-trimethylobenzene (BTB) and dibenzyl ether (DBE) (Scheme 1) provides a measure of the accessibility of active sites.
A
CC
EP
Al-SSZ-70 synthesized directly was found to be as active as the base case MCM-22 while samples obtained by Al substitution into B-SSZ-70 showed 5 times lower conversion of benzyl alcohol during the same reaction time (Fig.7). This confirms that Al insertion post-synthesis is a less effective approach, at least by the procedure applied here, than the direct synthesis. This outcome could have been influenced by the sub-zeolite character of the calcined B-SSZ-70 that were aluminated, by possibly not allowing better Al substitution for boron. The importance of the concentration of BAS for zeolites having similar textural properties is further evidenced by activity of the IEZ form. It has less Al because of treatment in acid environment and shows lower conversion than the parent with more Al. In the case of pillared Al-SSZ-70 the benefits of expanding the interlayer distance are clearly visible. It shows highly improved catalytic activity despite the decrease of BAS in comparison to the parent materials. This suggests impact of diffusion limitation in classical MWW zeolites in reaction with bulky molecules. This means that the reactants are not able to reach acid sites in micropores of 3-dimentional MWW and the reaction proceeds mostly on the external surfaces of the crystals. Pillared Al-SSZ-70 showed the highest conversion and selectivity percentage (100%), evidently due to the enhancement accessibility of acid sites located on the surface of the layers. Our study has shown that pillared Al-SSZ-70 and MCM-22 zeolites may be successfully used in the alkylation of the large molecules like mesitylene despite decreased number of acid sites in comparison to the parent zeolite.
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These results showed how important is the insight not only into acidity but also other properties like texture and porosity which are crucial to better selection of post-synthetic modifications for catalyst preparation. Table 3. Catalytic reaction data about various catalysts in the benzylation of mesitylene. BA conversion,%
Selectivity of BTB, %
SSZ-70 (4) – Al reinsertion
13
87
SSZ-70 (25) – Al reinsertion
12
77
Al-SSZ-70
53
91
Al-SSZ-70-IEZ
38
92
Al-SSZ-70 pillared
100
100
MCM-22
55
87
MCM-22 pillared
94
98
U
Al-SSZ-70-pillared MCM-22-pillared
N
80 60
A
Al-SSZ-70 MCM-22
40
M
Al-SSZ-70-IEZ
20
B-SSZ-70 (4) - Al B- SSZ-70 (25) - Al
0 0
50
100
150
D
BzOH conversion, %
100
SC RI PT
Zeolite
200
250
300
EP
TE
Time, min Fig. 7. Conversion of benzyl alcohol for SSZ-70 and MCM-22 catalysts in the benzlyation of mesitylene with benzyl alcohol.
4. Conclusions
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In this work SSZ-70 zeolites with Si/B ratios 4 and 25 and Si/Al ratio 50 in the gel have been synthesized by classical hydrothermal synthesis with the use of bis(cyclohexyl)imidazolium template. All three materials have been modified by post-synthetic treatments i.e. swelling, stabilization and pillaring. Our studies have shown that all tested zeolites easily transformed into expanded materials upon swelling which was confirmed by the 003 reflection located at around 5-5.5° 2θ and low angle peaks near 5 nm d-spacing. Calcination of SSZ-70 zeolites resulted in a collapse of the interlayer distances below that of the 3D framework, recognizable by the shift of 002 reflection towards higher values of 2θ deg. Pillaring of SSZ-70 materials resulted in a permanent layers separation (4.5 – 5.5 nm) confirmed by X-ray diffraction patterns as well as higher values of textural properties, e.g. over two-fold increase of surface areas. Due to the shift and blurring of the characteristic triangular reflection in the 8-10° 2θ range it cannot be unambiguously determined how much of the structure of SSZ-70 has been preserved. Silylation process to give the interlayer expanded zeolite form was successful only for Bcontaining zeolites. In the case of Al-SSZ-70 silylation prevented collapse to sub-zeolite upon calcination. Catalytic properties were investigated with directly synthesized Al-containing SSZ-70 and Al-substituted to boron
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SSZ-70. The former shows higher concentration of Brønsted acid sites and more promising catalytic activity in the reaction of mesitylene alkylation with benzyl alcohol. The highest activity and selectivity (100%) was showed by pillared Al-SSZ-70 apparently due to the enhancement accessibility of acid sites located on the surface of the layers. Catalytic results showed the advantage of direct synthesis of Al-SSZ-70 over the Al-inserted SSZ-70 samples as well as the positive effect of the modifications such as silylation and pillaring on the zeolite activity in alkylation reaction with bulky molecules.
SC RI PT
Acknowledgement This work was financed with the funds from the National Science Centre Poland, grant no 2016/21/B/ST5/00858. J.C. acknowledges OP VVV "Excellent Research Teams", project No. CZ.02.1.01/0.0/0.0/15_003/0000417 – CUCAM and the Czech Science Foundation (P106/12/G015). References
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