Unexpected Destructive Dealumination of Zeolite Beta by Silylation

J. Phys. Chem. C 2010, 114, 8459–8468

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Unexpected Destructive Dealumination of Zeolite Beta by Silylation W. O. Parker, Jr.,*,† A. de Angelis,† C. Flego,† R. Millini,† C. Perego,‡ and S. Zanardi† eni S.p.A., Refining & Marketing DiVision, 20097 San Donato Milanese, Italy and eni S.p.A., Centro Ricerche per le Energie Non ConVenzionali, Istituto eni Donegani, Via G. Fauser 4, 28100 NoVara, Italy ReceiVed: February 4, 2010; ReVised Manuscript ReceiVed: March 24, 2010

Zeolite beta was silylated in a novel way, by chemical liquid deposition (CLD) of a cyclic organosiloxane, and characterized. Despite its large size, octamethyl-cyclotetrasiloxane (OMTS) did not provide a surface modification, as did the traditional tetra-alkoxysilanes (ethoxy, propoxy, and butoxy). OMTS treatment with minor Si deposition influenced the internal parts of the zeolite with destructive dealumination, as revealed by 27 Al MAS NMR, IR, and XRD studies. Three types of AlO6 sites were found, (a) 17 ppm 27Al shift, (b) 0 ppm shift, 3782 cm-1 IR frequency, and (c) -20 ppm shift, exhibiting no Brønsted and Lewis acidity, respectively, toward pyridine. Apparently, OMTS’s low reactivity toward condensation with the less acidic external hydroxyl groups promoted ring-opening at pore-mouth sites. This allowed the resulting linear siloxane tetramer to penetrate the pores and cause dealumination during calcination. Catalysts modified with OMTS had the lowest amounts of AlO4 sites, Lewis acidity, large void volume, and crystallinity. Introduction Large-pore zeolites, such as beta, are very useful catalysts for organic chemicals production. Postsynthesis surface modification of zeolites with silicon alkoxides (silylation) is commonly used to improve selectivity toward para-substituted isomers.1-7 Silica is deposited on the outer crystallite surface, by silylating agents too large to enter the pores, without blocking the access of small organic molecules. Product distribution is modified by (1) shutting-off catalytically “non-selective” external active sites (passivation) and (2) by augmenting sievingdiffusion effects through pore-mouth narrowing. Dealumination and compromised crystallinity are intentionally avoided. Recently, we reported silylation of H-beta with octamethylcyclotetrasiloxane (OMTS), an organosiloxane oligomer much larger than traditional agents.1a,b Unexpectedly, OMTS seemed to influence crystallinity and the “internal” parts of the zeolite. This unprecedented behavior drove an investigation of the structural-acidity changes associated, which must be understood to fully exploit silylation of H-beta as a selectivity promoter. Zeolite silylation methodology is largely based on chemical vapor deposition (CVD).2,8-10 But, chemical liquid deposition (CLD) is more amenable to large-scale industrial applications.11 H-beta is modified here with the most industrially appealing modality, CLD at 25 °C followed by calcination in air. CLD on zeolites with tetra-alkoxysilanes (usually TEOS) is far more practiced (e.g., refs 5-8 and 11-17) than CLD with cyclic organosiloxanes.18,19 Regardless of the silylating agent used, long-range structure (crystallinity) is always found, or considered, to be preserved. Silylation effects are usually gauged by analyzing adsorptive and catalytic properties. Aluminum coordination has seldom been addressed.7 Solid state 27Al NMR spectroscopy is a powerful structural tool routinely used to characterize zeolite beta, especially its dealuminated forms.20-34 But, studies of silylated betas11,12,16,35 are lacking NMR analysis. * To whom correspondence should be addressed. Fax number: 003902-52036347. E-mail: [email protected]. † eni S.p.A., via Maritano 26. ‡ eni S.p.A., Centro Ricerche per le Energie Non Convenzionali.

Thus, in this work, the effects of CLD silylation on H-beta are investigated for the first time using a combination of spectroscopic, diffractive, and adsorptive techniques. Emphasis is placed on aluminum coordination and acidity. On a previous occasion, hydroxyl IR absorption bands of H-beta were assigned to aluminum species with specified acidities.20 Here, associations between these IR bands (from dry species) and 27Al NMR signals (hydrated species) are explored. All the measurements confirm the contrast between alkoxysilane and organosiloxane silylating agents. Experimental Section Silylation. H-beta zeolite (Si/Al ) 11.5, BET surface area ) 730 m2/g), prepared from the NH4-form (purchased from PQ) by deep-bed calcination in air (550 °C, 5 h) was modified using fully dried materials, under dry nitrogen atmosphere within a glovebox. Five grams of zeolite, predried at 500 °C, was stirred in a 50 mL hexane solution of 5 wt % silylating agent (purchased from Aldrich) at 25 °C for 21 h. TEOS (tetra-ethoxysilane), TPOS (tetra-propoxysilane), TBOS (tetra-butoxysilane), and OMTS (octamethylcyclotetrasiloxane) were used to prepare samples A, B, C, and D, respectively (Table 1). After stirring, the suspension was dried under dynamic vacuum (5 mbar) at 25 °C overnight to remove hexane solvent (bp ) 68 °C), and partially remove unreacted silylating agents (boiling points ) 168 to 275 °C), followed by calcination at 550 °C under dry air flow. Successive silylation-calcination treatment of D with OMTS gave E (and likewise, F from E). Sample G was prepared as for D, except the reaction mixture was stirred at 60 °C. H was prepared by soaking the zeolite in 50 mL of pure OMTS for 21 h followed by drying and calcination as above. Elemental Analysis. Al2O3 content was analyzed, after acid digestion, with an inductively coupled plasma atomic emission spectrometer model Thermo “Intrepid”. SiO2 content was determined gravimetrically after removal by reaction with HF. Void Volume. The capacity to adsorb 1,3,5-trimethylbenzene (TMB) adsorption was made after evacuation under dynamic vacuum (10-4 mbar) at 200 °C for 1 h. Adsorption measurements were performed at 21 °C in a pyrex volumetric apparatus

10.1021/jp1010764  2010 American Chemical Society Published on Web 04/19/2010

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TABLE 1: Elemental Composition, Degree of Surface Silylation, and Large Void Volume for Silylated H-Beta Samplesa

H-beta A B C D E F G H c

silylating conditions

Si/Al (bulk)

deposited silylating agenta (% mole)

Si atomsb deposited per nm2

Si deposited per Al (mole ratio)

large void volumec (µL/g)

none TEOS (5%) TPOS (5%) TBOS (5%) OMTS (5%) OMTS (5%) 2nd treatment OMTS (5%) 3rd treatment OMTS (5%) 60 °C OMTS (pure)

11.5 14.6 13.3 13.2 14.3 14.9 16.0 13.9 14.4

9.3 7.4 8.5 3.0 1.8 1.5 2.6 0.2

0.18 0.11 0.11 0.17 0.20 0.24 0.15 0.17

0.21 0.12 0.11 0.19 0.22 0.30 0.16 0.19

100 nd 63 58 52 35 20 nd 53

a One gram of parent zeolite (Si/Al ) 11.5) contains 15.54 mmol of Si and 1.33 mmol of Al. b Using a constant surface area (730 m2/g). Microliters of liquid 1,3,5-trimethylbenzene adsorbed per gram of sample; nd ) not determined.

equipped with pressure and vacuum detectors. The volume of the adsorbed liquid TMB was calculated from the pressure difference in adsorbed gaseous TMB using a conversion factor of 0.139 mL/mmol (density ) 0.864 g/mL). The experimental error was 3%. NMR Spectroscopy. 27Al MAS NMR spectra were obtained at 78.2 MHz on a Bruker ASX-300 using hydrated (3d at 52% relative humidity, 25 °C) powders (ca. 80 mg) contained in 4 mm ZrO2 rotors spinning at 14 kHz. Two types of single-pulse experiments were made with rf field strengths of 83 kHz (νRF) for excitation. Quantification was made using spectra collected with an rf pulse duration of 0.5 µs (π/12 flip angle) to ensure quantitative excitation, 63 kHz spectral window, 1 s recycle delay and 20 000 scans. Isotropic chemical shifts were calculated from the median value for the inner spinning side-bands of the satellite transition (ST).36 Chemical shifts, reproducible within (0.7 ppm, were referenced externally to aq. AlCl3 at 0 ppm. Hydration provided complete detection within experimental accuracy ((10%) using a standard, Net4-beta (Si/Al ) 9.5), prepared previously.20 Quadrupole parameters were calculated from SATRAS (satellite transition) spectra,36,37 collected using an rf pulse duration of 1.0 µs (π/6 flip angle), spectral window of 1.25 MHz, 0.6 s recycle delay, 70 000 scans and backward linear prediction of the first 26 data points. Simulation of the full ST spinning sideband manifold was made with WINFIT “Q-1 mas all” software. The magic angle was finely adjusted using 2H NMR of NaDCO3 as previously.38 FT-IR Spectroscopy. Transmission FT-IR (Fourier Transform InfraRed) spectra were collected at 25 °C over the 4000-400 cm-1 region with a resolution of 1 cm-1 using a Perkin-Elmer model 2000 spectrometer, a self-supported pellet (ca. 10 mg/cm2) contained in a Pyrex cell, and KBr optical windows. The hydroxyl group distribution was analyzed after drying under dynamic vacuum (10-4 mbar) at 500 °C for 1 h. Signal maxima and relative areas (arbitrary units, estimated error 10%) were evaluated after normalization (i.e., for IR wafer thickness) over the 4000-400 cm-1 region. Acidity was measured adsorbing pyridine (13.3 mbar) at 200 °C on the activated sample (500 °C, 1 h, 10-3 mbar). After 1 h -3 of interaction, desorption (1 h, 10 mbar) was made at 200 °C followed by IR spectral collection. Acid site density was evaluated from the peak areas at 1455 (Lewis type) and 1545 cm-1 (Brønsted type), using the extinction coefficient calculated by Take et al.39 The error is estimated at ca. 10%. Diffraction. XRD patterns of nonhydrated samples were obtained with a Philips X’PERT diffractometer equipped with a peak-height analyzer and a monochromator on the diffracted beam. Data were collected stepwise over 3 e 2θ e 53°, with

a step size of 0.02° 2θ and 20 s/step accumulation time, using CuKR radiation (λ ) 1.54178 Å). Structural changes were monitored by analyzing three parameters (i.e., unit cell, width, and area of selected reflections) as detailed in the results sectioned. TEM. Samples for transmission electron microscopy (TEM) were prepared by mild grinding in an agate mortar, suspension in ethanol, and deposition on Cu grids covered with a porous carbon film. TEM observations were made with a field-emissiongun FEI Tecnai F20 super twin electron microscope, operating at 200 kV, equipped with a Gatan Slow Scan 794 CCD camera. Results Si Coverage. Silicon deposition was minor (Table 1). After calcination, only a small portion of the silylating agent remained: 7-9% mole for the alkoxysilanes (A-C), 2-3% for OMTS (D-G), and less than 1% for pure OMTS (H). The number of Si atoms deposited per nm2 of zeolite surface area was similar for the once-silylated samples (A-D): 0.11-0.17, corresponding to much less than one Si nucleus per Al. This minute coverage is comparable with CVD treatments,11 since the zeolite was predried. Large Void Volume. TMB was utilized to monitor changes in the large external void volume (e.g., pore mouths, voids between crystallites), accessible to a typical aminal1 molecule. With a van der Waals diameter (8.6 Å) larger than the crystallographic free diameters of the beta channels, 6.6 Å × 6.7 Å straight channel and 5.6 Å × 5.6 Å sinusoidal channel for polymorph A,40 TMB cannot enter the pores. In all cases, silylation reduced TMB adsorption, expressed as microliters of probe per gram of catalyst (Table 1). 27 Al MAS NMR. Figure 1 shows the 27Al MAS NMR spectra (central region) used to quantify aluminum types reported in Table 2. Five signals with isotropic chemical shifts (δiso) near 57, 41, 17, 1, and -20 ppm were distinguished. They were assigned to Al nuclei with different bonding coordinations: 57 ppm to four-coordinate (AlO4), 41 ppm to five-coordinate (AlO5) and those at 17, 1, and -20 ppm to six-coordinate (AlO6-a, -b and -c, respectively), consistent with the literature (Table 3).21,22,41 Framework AlO4 sites gave a signal with δiso ≈ 57 ppm (Table 2). The quadrupolar coupling constant (QCC) for AlO4 nuclei in the parent beta (Table 3) is less than 2.5 MHz, typical for framework sites in crystalline silico-aluminates.23,41 The apparent QCC calculated for AlO4 in heavily dealuminated samples was rather high (ca. 3.0 MHz, e.g., F in Table 3), suggesting a small contribution from nonframework AlO4 (QCC

Silylation of Zeolite Beta by CLD

Figure 1. 27Al MAS NMR spectra for N(C2H5)4-beta, the parent H-beta and silylated samples. Asterisk denotes spinning side-bands. Relative areas and shifts are given in Table 2.

ca. 6 MHz)21,23 or from AlO4 framework sites perturbed by the strongly polarizing effect of nearby cationic extra-framework aluminum.46 The AlO5 site with δiso ) 40.5 ppm (Figure 2), observed in samples E-G, was resolved using SATRAS spectroscopy.36,37 Signals making up the satellite transition (ST) spinning sidebands have narrower linewidths, compared to those of the central transition (CT), since second-order quadrupole effects are much smaller.47 The signal with isotropic shift near 17 ppm (AlO6-a) was observed in samples E-G. This shift is similar to that reported for AlO6 sites in aluminas, zeolite precursors, and dealuminated zeolites (Table 3). AlO6-a exhibited the same quadrupole parameters as those known for R-alumina. The QCC indicates an ordered phase (QCC < 3 MHz)48 or, perhaps, a contribution from motional averaging.49 AlO6-a was also observed for catalysts discharged from the reactor, after exposure to aniline during the aminal rearrangement.1 Dehydration caused its transformation to an “invisible” species (QCC > 10 MHz). AlO6-a did not exhibit Lewis acidity toward pyridine in dry form (due solely to AlO6-c sites, see below). Thus, the AlO6-a site is assigned to aluminum nuclei in small neutral extraframework aluminum oxide species. AlO6-a appears similar to the AlO6 species found in dealuminated beta23,24,50 and Y51,52 zeolites, but with a smaller QCC (see Table 3). The sharp signal near 0 ppm (AlO6-b) is due to partially hydrolyzed framework-associated Al. After evacuation, for IR measurements, this species gave the AlOOH IR band at 3782 cm-1,20 (Supporting Information Figure S1) with bland Brønsted acidity. Its QCC could not be determined due here to weak

J. Phys. Chem. C, Vol. 114, No. 18, 2010 8461 intensity or overlap with the intense AlO6-a signal (Figure 1). Other workers found the QCC to be ca. 1 MHz,41 suggesting species such as (H2O)5-nAl(OSi)nOH, where the number of framework bonds (n) are equal to 1,20,53 2,20,24 or 3.25,54 AlO6-b can be reconverted to a AlO4 site by exposure to NH3 gas, aqueous NH4+ exchange, or with heating.22,24,41,42 The broad signal near -20 ppm (AlO6-c) is attributed to extraframework Al nuclei with distorted octahedral symmetries experiencing strong quadrupolar broadening in amorphous polymeric oxides or oxide hydrates55 formed during dealumination.27 Before hydration, AlO6-c sites were Lewis acid sites,24 as confirmed by the NMR-acidity correlation in Supporting Information Figure S5. The QCC for this signal could not be measured here, but it is surely greater than 5 MHz. The Al nuclei in samples A, B, C, and H were little affected by CLD (Table 2, Figure 1). OMTS gave a large affect (D), influencing 25% of the framework AlO4 sites in H-beta with a deposition of only 0.2 Si atoms per Al (Table 1). This influence on internal Al sites became more drastic with repeated cycles (E,F in Table 2) and provided samples with large compositional changes useful for correlating analytical results. A curious IR-NMR contrast emerged for sample D. While IR reported a large loss in Lewis acid sites (compared to H-beta), NMR found a large increase in AlO6-c. Formation of extraframework AlO6-c sites usually increases the number of Lewis acid sites, since they are correlated. In fact, D is the furthest sample off this correlation (Supporting Information Figure S5). Ulterior silylation of D, to give E, caused two changes, (1) ca. 70% of the AlO6-c sites changed to AlO6-a and (2) ca. 25% of the AlO4 sites changed into AlO5. Silylation of E, to give F, caused no further NMR changes. A single OMTS treatment of H-beta at 60 °C (G) gave the same changes as the double treatment at 25 °C (E). F and G clearly have the most asymmetric AlO4 signals of the samples studied here. This indicates a contribution from perturbed or extra-framework AlO4 sites. Treatment with pure OMTS at 25 °C (H) caused a small conversion of AlO4 sites into extra-framework amorphous oxides (AlO6-c), accompanied by a strong reduction in the number of Brønsted acid sites and the SiOHAl IR band. The absence of internal modifications for sample H, as revealed by 27Al NMR (see Discussion), indicates that OMTS underwent polymerization (to polydimethylsiloxane) and remained outside. A high local concentration of OMTS, necessary for propagation of polymerization, was unavailable in the 5% wt solution used to make D. OH Distribution. The IR spectrum of the dehydrated parent beta exhibited 5 main bands in the hydroxyl region. Band assignments, made as previously,20 are given in Table 4. SiOHAl are bridging hydroxyl groups that furnish strong Brønsted acidity. AlOH is the absorption from OH groups bonded to extra-framework aluminum nuclei. The AlOOH species, giving the VHF (very high frequency) IR adsorption band at 3782 cm-1, is partially bonded to the framework. Relative Brønsted acid strengths are SiOHAl > AlOH > AlOOH.20 SiOHext are nonacidic silanols located on the external crystalline surface, which are spatially isolated from other silanols. SiOHint are internal silanols, located inside the pore system at structural defects. The degree of SiOHAl loss (17% A, 31% B, and 48% C) caused by the alkoxysilanes increased as the alkyl group size increased (ethyl > propyl > butyl) (Table 4). But, OMTS treatment (D) affected most the SiOHAl band (59%). Similar intensities of SiOHext and SiOHint bands reflected the small

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TABLE 2: Relative Amounts and Isotropic Chemical Shifts (δiso) for Aluminum Nucleia with Different Coordination Symmetries in Hydrated Samples Determined from 27Al MAS NMR Spectra % mole (δiso, ppm)

a

sample

AlO4

AlO5

AlO6-a

AlO6-b

AlO6-c

Net4-beta H-beta A B C D E F G H γ-Al2O3

99.0 ( 1.5 (56.8) 74.1 ( 2.5 (56.6) 71.8 ( 2.3 (56.6) 74.0 ( 2.5 (56.5) 70.7 ( 2.5 (56.5) 55.6 ( 3.0 (57.7) 35.9 ( 4.0 (59.1) 29.0 ( 2.8 (59.5) 31.0 ( 3.5 (58.0) 67.2 ( 3.0 (57.4) 25.0 ( 1.0 (77.4)

0 0 0 0 0 0 14.0 ( 2.0 (41.5) 18.0 ( 1.9 (40.5) 18.0 ( 2.0 (40.6) 0 0

0 0 0 0 0 0 37.6 ( 4.0 (17.3) 40.7 ( 4.5 (17.4) 31.2 ( 3.5 (17.3) 0 75.0 ( 2.5 (16.3)

1.0 ( 0.5 2.7 ( 0.3 (1.4) 1.5 ( 0.5 (1.4) 7.0 ( 1.0 1.8 ( 0.5 (1.6) 0.4 ( 0.2 0 0 0 0.5 ( 0.3 0

0 23.2 ( 2.0 26.7 ( 2.1 19.0 ( 2.0 27.5 ( 2.0 44.0 ( 3.0 12.5 ( 1.5 12.3 ( 1.5 19.8 ( 2.0 32.3 ( 2.5 0

δiso for some AlO6 species could not be determined due to insufficient signal intensity.

TABLE 3: 27Al NMR Quadrupole Parametersa for Aluminum Sites in Beta and in Comparative Samples from the Literature Determined by Satellite Transition (ST) or Multiple-Quantum (MQ) Methods sample H-beta (parent) E (OMTS silylated-beta) H-beta

H-beta (dealuminated, realuminated)

ASA (amorphous silica-alumina) zeolite A precursor (amorphous alumina) boehmite R-Al2O3 (corundum) Al2O3 (amorphous)

Al site

δ-iso (ppm)

QCC (MHz)

η

method

reference

AlO4 AlO4 AlO5 AlO6-a AlO4-1 AlO4-2 AlO4-3 AlO5 AlO6-1 AlO6-2 AlO4-1 AlO4-2 AlO4-3 AlO6-1 AlO6-2 AlO4-1 AlO4-2 AlO6 AlO6 AlO6 AlO6

56.6 59.1 40.5 17.4 61.5 57.5 61.5 33 2.7 1 57.1 53.4 57.4 0.2 10.6 61.3 55.0 3.8 16.5 12.6 16.0

1.8 ( 0.1 3.0 ( 0.4 3.2 ( 0.4 2.3 ( 0.1 2.5 1.9 6 6.5 0.25 5 1.6 1.5 5.2 1.5 5.6 2.9 4.6 3.8 4.6 1.8-2.8 2.4

0.6 ( 0.1 0.6 ( 0.2 0.6 ( 0.2 0.0

ST ST

this work this work

MQ

21

MQ

23

MQ

42 43 44 45

AlO4 AlO5 AlO6

71.5 38.0 11.5

5.0 2.7 4.5

0.3 0.3 0.3

MQ MQ ST MQ ST

0.5-1.0 0.0

38

a QCC (quadrupole coupling constant) ) e2qQ/h, where eQ is the nuclear quadrupole moment, eq is the electric field gradient. η (asymmetry parameter) ) (qxx - qyy)/qzz which ranges from 0 to 1.

crystal size (ca. 10 nm from TEM). All treatments decreased the SiOHext band (23 to 71%), consistent with condensation reactions on the surface. Acidity. The quantities of sites furnishing Lewis and Brønsted acidity toward pyridine are given in Table 4. Pyridine should give a useful measure of the number of acid sites available for reaction, since its diameter (6.5 Å) and pKb is similar to the aminal molecule. TBOS and OMTS-treatments caused the greatest reduction in Brønsted acidity measured by pyridine (Table 4). These agents could have caused an internal modification, since most (if not all) of the strongly acidic bridging hydroxyl groups (SiOHAl) are located inside the pores.15,56 Or instead, they could have blocked pyridine’s ingress and reaction with internal sites (SiOHAl). On the basis of other evidence (NMR, adsorption), we suggest that pore blockage was mainly operative for TBOS whereas internal modifications (dealumination) occurred with OMTS. Only OMTS largely reduced the Lewis acidity of H-beta. Multiple OMTS treatments gave F with Lewis acid sites

(shoulder at 1446 cm-1) not seen in the other samples. These peculiar sites were accompanied by (1) a band shoulder at 1596 cm-1, attributed to coordinatively unsaturated octahedral Al3+ cations57 and (2) an IR band at 1638 cm-1. Both indicate distorted coordinatively unsaturated AlO4 cations. Framework. Figure 3 shows that although the XRD pattern typical for beta is observable in all the samples, significant differences in intensities, widths, and positions of the reflections exist. Variations in long-range structure were evaluated in three ways (Table 5). (1) Crystallinity, relative to the parent beta, was measured by comparing the strong 302 reflection (2θ ≈ 22.4°) area, as suggested by Blasco et al.58 (2) Reflection width or the fwhm (full width at half-maximum) of two sharp reflections, 008 (2θ ≈ 27.1°) and 600 (2θ ≈ 43.6°) was used to detect changes in the average dimensions of the coherent scattering domains.27 (3) Unit cell parameters were determined by least-squares fit of the interplanar spacings associated with the sharp XRD reflections, indexed according to the tetragonal P4122 space group of the polymorph A of zeolite beta.


Figure 2. 27Al MAS NMR spectra of silylated beta F. The apparent shifts (δapp) of the central transition (CT) region are compared with the isotropic shifts (δiso) revealed by the second downfield spinning sideband of the satellite transition (ST). ST plot was displaced upfield by twice the spinning rate (28 000 Hz) to allow superimposition with CT region. Peak assignments in Table 2.

A large loss in crystallinity occurred for the OMTS-modified samples (D-G, Table 6). Decreased crystallinity along with increased reflection broadening (Supporting Information Figure S6) revealed structural breakdown in these samples. Unit cell shrinkage was paralleled by a loss in AlO4 sites (Supporting Information Figure S8). Long-range structural loss in OMTSsilylated catalysts was also evidenced by TEM observations comparing H-beta and sample E (Figure 4). Both samples had similar particle size (10 to 20 nm median dimensions); however, while the crystallites of the parent zeolite show a regular array of pores, those of E have amorphous character. The 29Si MAS NMR spectrum of H-beta was essentially unchanged by TEOS and TPOS (Supporting Information Table S1). TBOS treatment enhanced and broadened the -115 ppm signal, assigned to silicon nuclei making up the one fourmembered rings.59 OMTS treatment broadened all the signals and increased the relative area of the -104 ppm signal due (in part) to Q3 nuclei Si(OH)(OSi)3, suggesting an increase in structural defects or superficial area. Discussion Internal Modifications. NMR is entrusted here for quantifying changes to the internal portion of the zeolite, via the framework aluminum signal (AlO4). It is sustained that (most) all the AlO4 sites, with relatively high coordination symmetry detected by standard 27Al MAS NMR, are inside (and surrounded by) the framework. These centers provide the strongly acidic bridging hydroxyls (SiOHAl) which exist only inside the cavities.15,56 Results for the OMTS-treated samples (D-F) supported this contention. Since dealumination was destructive, the framework AlO4 content and crystallinity decreased to the same degree (Supporting Information Figure S8). Thus, the AlO4 NMR signal, of the hydrated samples, measures the framework

J. Phys. Chem. C, Vol. 114, No. 18, 2010 8463 aluminum (a substantial component making up the unit cell and crystalline long-range order), which is inside the crystallite. Aluminum sites with 4 bonds to the framework can exist on the outer surface. But these sites would be bonded to at least two Q3 Si sites (thus, not a strongly bridging hydroxyl) and would become AlO6 sites during hydration before 27Al NMR measurement. In principle, IR spectroscopy can estimate the number of framework AlO4 centers by detecting the SiOHAl band or by adsorption of bases at this strong Brønsted acid site. But pyridine, used here, is too large to interact with all the bridging hydroxyls, unlike ammonia.27 The intensity of the SiOHAl stretching band cannot be used either, due to uncertainties in the extinction coefficient60 and to the presence of nonproton cations (e.g., extra-framework Al)46 compensating the negatively charged framework aluminum nucleus. In fact, there was no clear relation between the number of AlO4 sites (NMR) and the SiOHAl band intensity (Supporting Information Figure S3). However, there was a trend between the number of Brønsted acid sites, measured by pyridine, and the SiOHAl band intensity (Supporting Information Figure S2). Thus, IR studies are not used here to quantify framework aluminum. Correlations. Spectroscopic relations between 27Al NMR and IR measurements are helpful because the link between hydroxyl IR adsorption bands and acidity has already been established for three aluminum species.20 The present sample series, with variable amounts of three AlO6 species, furnished an opportunity for linking NMR-IR-acidity measurements (see Supporting Information). Although the SiOHAl (IR) and AlO4 (NMR) signals arise from the same framework aluminum center, furnishing strong Brønsted acidity, they were not quantitatively related for the reasons just mentioned. Partially hydrolyzed framework-associated Al, with bland Brønsted acidity, gave the very high frequency AlOOH (IR, dry) band and the AlO6-b (NMR, hydrated) signal in good quantitative agreement (Supporting Information Figure S1). Extra-framework Al nuclei exhibiting Lewis acidity (in dry form) toward pyridine gave the AlO6-c (NMR) signal (Supporting Information Figure S5). Extraframework Al nuclei giving the AlOH (IR) band with moderate Brønsted acidity20 exhibited no parallel with NMR signals perhaps because the number of OH groups per extra-framework Al is variable. The extra-framework species responsible for the AlO6-a NMR signal (seen in E-G) could not be discerned from IR or acidity measurements. Alkoxysilanes. Reactions involving alkoxysilane silylation, that is, hydrolysis, condensation, and subsequent calcination, are well-known, for example, refs 61-64 and Supporting Information, much more than the reactions of organosiloxanes. However, very few structural studies were found regarding CLD of alkoxysilanes on dry zeolites. Since our parent H-beta was predried, only a small amount of the agent added was deposited (at 25 °C) through the action of acid sites involving aluminum (or trace water) on the external surface. This allowed partial condensation with the surface and/or self-oligomerization. Subsequently, during calcination, high-temperature condensation reactions form surface Si oxides with the elimination of any remaining nonhydrolyzed alkoxy groups and in situ production of smaller molecules (e.g., water, alcohol, ether). These small molecules are expected to slightly alter the framework Al (inside the cavities) since they are generated on the outer surface of the crystallite where the easily hydrolyzed agent reacted. TEOS is the most common silylating agent for zeolites. Citing some of the more recent reports, it was used to modify

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TABLE 4: Hydroxyl (Normalized area/g) and Acid Site Distributionsa (µmol/g) for the Parent and Silylated H-Beta (Dehydrated Samples) Determined by IR Spectroscopy

a

sample

SiOHAl

AlOH

AlOOH

SiOHext

SiOHint

Lewis

Brønsted

IR band (cm-1)

3610

3665

3782

3743

3730

1455

1545

H-beta A B C D E F G H

6.4 5.4 4.5 3.3 2.6 2.9 3.4 4.7 3.6

6.7 9.3 9.0 4.6 6.1 6.6 7.3 10.5 6.6

1.4 0.7 0.3 1.2 0.3 0.2 0.1 0 0.1

13.9 10.6 4.0 9.1 10.6 8.4 9.1 7.8 8.6

16.1 13.1 4.5 8.6 14.2 12.8 12.9 18.8 12.6

272 287 215 295 134 130 35b + 157 238 235

222 201 179 94 83 71 129 89 99

By chemisorption of pyridine at 200 °C. b From intensity of IR band at 1446 cm-1.

Figure 3. X-ray powder diffraction patterns for parent and silylated H-beta samples.

TABLE 5: Relative Crystallinity, fwhm of the 008 and 600 Reflections, and Unit Cell Parameters for Parent and Silylated H-Beta Samples (e.s.d.’s in parentheses) fwhm (deg)

unit cell parameters (Å)

sample

crystallinity (%)

008

600

a

c

V


100 80 80 80 62 44 40 63 72

0.332(4) 0.361(5) 0.361(1) 0.350(6) 0.349(6) 0.384(9) 0.371(3) 0.345(3) 0.340(1)

0.796(16) 0.814(16) 0.932(14) 0.847(17) 0.964(38) 1.19(9) 1.15(9) 0.884(54) 0.835(22)

12.432(3) 12.424(5) 12.441(2) 12.425(4) 12.372(2) 12.269(6) 12.255(3) 12.344(8) 12.367(5)

26.344(11) 26.352(19) 26.355(8) 26.344(15) 26.333(8) 26.233(22) 26.219(10) 26.343(29) 26.302(18)

4071.3(2.0) 4067.8(3.9) 4079.2(1.6) 4066.8(3.2) 4030.4(1.8) 3949.0(4.4) 3937.8(2.0) 4013.9(6.1) 4022.5(3.7)

ZSM-5,5-7,12-15 mordenite,7,11,12 beta,11,12,16,35 RHO, and ZK-5.17 In general, the calcined modified catalysts experienced a slight reduction in the pore-mouth size without affecting crystallinity or the interior of the zeolite. In line with expectations, CLD of TEOS on H-beta (sample A) had a minor effect on internal properties (AlO4 nuclei) and crystallinity (Table 6). TEOS was deposited at the weaker external acid sites without affecting the pore entrances. It caused a marginal increase in AlO6-c sites (15% mole), exhibiting Lewis acidity in dry form (Table 6). These results are similar to those

of Berger et al.7 who studied H-ZSM-5 (Si/Al ) 36) silylated by CLD of TEOS. CLD of TPOS and TBOS also had a minor influence on the internal portion of H-beta as measured by NMR and XRD (Table 6). The large decrease in Brønsted acidity for C (Table 4) indicates TBOS blocked pyridine’s ingress to the pore system. Organosiloxanes. One OMTS treatment caused large “internal” changes to H-beta, 25% loss in AlO4 sites (sample D). This is perhaps surprising, given the size of OMTS (kinetic diameter 8.6 Å). Since an alkyl group attached to Si makes


J. Phys. Chem. C, Vol. 114, No. 18, 2010 8465

TABLE 6: Changes in Aluminum Species, Acidity, Crystallinity, and Para-Selectivity of H-Beta Caused by Silylation % Al loss (NMR)a

% acidity loss (IR)b

sample

silylation conditions

AlO4

AlO6-c

Lewis

Brønsted

crystallinity loss (%)

para- selectivity factorc


TEOS (5%) TPOS (5%) TBOS (5%) OMTS (5%) OMTS (5%) 2 cycles OMTS (5%) 3 cycles OMTS (5%) 60 °C OMTS pure)

3 0 5 25 52 61 58 9

-15 18 -18 -90 46 47 15 -39

-5 21 -8 51 52 29 12 14

9 19 58 63 68 42 60 55

0 20 20 20 28 56 60 37 28

2.2 2.6 2.7 2.7 4.5 5.6 5.8 4.9 3.5

a Compared with parent H-beta having 74% AlO4 and 23% AlO6-c (Table 2). Negative values indicate % gain. b Compared with parent H-beta chemisorbing 272 and 222 µmoles of pyridine on Lewis and Brønsted sites, respectively at 200 °C (Table 5). c Evaluated from HPLC trace areas: ((p,p′)/((o,p′) + (o,o′)) measured at complete conversion (6 h, 150 °C) of preformed aminal in a batch reactor.1

Figure 4. TEM images of the parent H-beta (top) and after silylation twice with OMTS (sample E, bottom). The regular array of pores observed for H-beta was not detected in E, which experienced a large decrease in crystallinity.

nucleophilic attack on silicon more difficult,64 OMTS must react with the strong acid sites (available at the pore mouth) to drive hydrolysis and condensation. In the liquid hexane phase, OMTS could enter the pore system after undergoing ring-opening at the pore mouth. Strong acids catalyze ring-opening polymerization of OMTS at 25 °C.65 In

fact, high molecular weight poly dimethylsiloxane is industrially produced from OMTS. A plausible mechanism for acidolytic ring-opening by solids (zeolites, clays) has been suggested;65 see Scheme 1. Ring-opening does not require water, but detachment from the strong acid site does. Attached at the pore mouth or not, the ring-opened linear tetramer is narrow enough (ca. 6 Å) to fit inside the linear channel system of beta (6.6 Å × 6.7 Å straight channel, polymorph A)40 and long enough (ca. 12 Å) to reach some internal sites. However, given the large damage to D (40% long-range order loss) and the average crystallite size ca. 150 Å, ring-opened OMTS was likely able to detach and fully enter. Zeolite silylation with OMTS has been studied by CVD19 and CLD.18,66 But only external surface modifications were evidenced. Crystallinity loss was intentionally avoided to maintain para-selectivity.18 CVD of K-H-OFF (Si/Al ) 3.6, K/Al ) 0.25) with OMTS did not appreciably affect crystallinity or the SiOHAl IR band.19 Apparently, OMTS could not enter the cavities with pore openings (6.6 Å × 6.7 Å) similar to beta due to obstruction by the large potassium ion. A unilayer grafting of ring-opened OMTS was found on the external surface after treatment at 250 °C under vacuum.19 Each silicon of OMTS had lost one methyl group to give tetra-dentate attachment of the half-molecule to surface silanols: (O)2-Si(CH3)-O-Si(CH3)O2d]. CLD of OMTS on Na-Y did not affect the internal surface, because there were no strong acid sites (SiOHAl) to open the siloxane ring.66 CLD of low molecular weight polydimethylsiloxane (PDMS) on H-ZSM-5, under conditions similar to the present work, influenced only the external surface. Crystallinity and most of the framework sites (i.e., SiOHAl IR band) were unaffected.67 PDMS and its smaller oligomers were too large (ca. 7 Å) to enter the pores of the ZSM-5 channel system (5.1 × 5.5 Å2 and 5.3 × 5.6 Å2) and cause structural changes during subsequent calcination. Dealumination. Because of its catalytic importance, dealuminated beta has been well characterized, for example, refs 20-34. Zeolite beta is easily dealuminated and calcination in air always gives some extra-framework Al nuclei, mainly the completely detached species AlO6-c. Deep-bed calcination in air, as used here, causes greater dealumination than shallow-bed calcination.30 Calcination can also produce extra-framework21,23 or perturbed46 AlO4 sites with large QCC detectable by MQ-MAS (but not quantitatively). The Al coordination states of some T-sites in beta are flexible, permitting four- to six-coordinated transitions by interaction with water, adsorbed or generated in situ during calcination. Simply exposing calcined beta to water vapor at 25 °C can provoke

8466


Parker et al.

SCHEME 1: Hypothetical Ring-Opening Reaction of OMTS Initiated by a Strong Brønsted Acid Site

uncontrolled hydrolysis of framework aluminum68 to form framework-associated AlO6 species (i.e., AlO6-b). However, the conversion to AlO6 species observed here by NMR is interpreted as dealumination during calcination, rather than hydrolysis after calcination. This is supported by the large crystallinity loss evidenced by XRD for nonhydrated samples and also by NMR studies of nonhydrated samples. Large structural changes in zeolite silylations, as seen for OMTS silylated samples are exceptional. They are paramount to severe steaming treatments. Mild steaming of beta causes partial hydrolysis of framework Si-O-Al bonds to give AlO6b.22 This is supposedly the first step in the dealumination process. Severe steaming conditions (e.g., 650 °C) cause complete hydrolysis and creates extra-framework species (AlO6-c) with high QCCs that can migrate through the pores and enrich the Al content at the crystallite surface23,29 as an amorphous shell around zeolite grains.51 Repeated combustion of organic species trapped inside the pores of H-beta (in situ steaming) also causes structural collapse27 with AlO4 to AlO6-c conversion. Extensive dealumination in E-G caused the appearance of AlO6-a. In addition, AlO6-a was present in all the spectra of catalysts used in the aminal rearrangement reaction. Exposure to base caused the AlO6-c and AlO6-b signals to disappear as the AlO6-a signal grew. The hydrated species giving AlO6-a appears stable with respect to the other AlO6 forms. An AlO6 signal like AlO6-a, but broader (QCC ) ca. 5 MHz), was observed in the literature for partially hydrated H-beta samples. Realumination of dealuminated H-beta,23,50 and severe steaming of H-beta24 caused the 27Al signal to appear. Perhaps, the lower QCC found here for AlO6-a is due to motional averaging with water molecules (fully hydrated samples). In fact, dehydration caused the signal to disappear, indicating a large increase in QCC. Selectivity. Many factors can conspire to influence selectivity in zeolites. The specific molecular causes (e.g., structural-acidic properties) for the enhanced para-selectivity of OMTS-silylated catalysts, observed previously,1a are not the focus of this work. But some general considerations and recent related findings appear relevant. Selective production of the para MDA isomer (4,4′-methylenedianiline) from the preformed aminal (Scheme 2) is important for the synthesis of polyurethans. In solution, the para isomer is formed first, via acid catalyzed proto-dealkylation,69 which then isomerizes to the ortho isomers (2,4′- and 2,2′-MSA) within 2 h at 190 °C, giving a selectivity factor of 1.70 In acid zeolites with spaciousness indexes (SI) between 2.5 and 19, the factor is higher.1 Beta (SI ) 19) and ERB-1 (SI ) 8) gave factors above 2 with complete aminal conversion.1c Reduced crystallinity (framework integrity) and large void volume, caused by OMTS-silylation, gave para-selectivity

SCHEME 2: Reaction of Aniline and Formaldehyde to Give the Aminal, Catalytically Transformed into Methylenedianiline (MDA) Using H-Beta

factors above 4 (Table 6). Less large void volume suggests pore mouth narrowing, which can limit diffusion of the slightly bulkier ortho isomers (i.e., product shape-selectivity). Indeed, the pore mouth diameter of the beta structural model (6.6 Å × 6.7 Å straight channel) is similar to the van der Waals diameter of the para isomer (ca. 6.8 Å). But molecular mechanics (MM) calculations predicted no preference between ortho and para isomers inside the 12-membered ring systems of the *BEA structural model (polymorph A) or ZSM-12 (MTW) (Supporting Information Table S2). The difference in diffusional energy barriers between the p,p′ and o,p′ isomers is only ca. 9 kJ mol-1. In reality, the ring substituents of MDA are small. MM calculations did predict shape-selectivity effects for iso-propyl substituted rings in beta and MTW structures with a 42 kJ mol-1 energy barrier difference between meta and para isomers.71 Our MM calculations lacked free energy considerations, including entropy. Free energies of formation for adsorbed-phase molecules can explain shape selective.72 Adsorption entropy during the reaction is crucial because the pores are saturated giving conditions far from gas-phase equilibrium. Obviously, a reduction in large void volume, as observed here for OMTSsilylated catalysts, will impact adsorption entropy. Apart from size-shape considerations, acidity can also influence selectivity. The destruction of Lewis acid sites appears advantageous (Table 6), although their role in the overall reaction is still unclear. Dealumination can create, or make available, new active sites promoting para-selectivity. In fact, only the most selective catalysts (E-G) contained AlO5 sites (Table 2). Recently, other workers reported similar findings. The formation of AlO5 sites, caused by dealumination of H-ZSM-5, was associated with improved selectivity in the direct oxidation of benzene with N2O.73 Framework AlO4 sites with enhanced Brønsted acidity due to interaction with nearby extraframework Al were found in dealuminated H-Y zeolite.46 Extraframework aluminum clustering during steaming (400 °C) of

Silylation of Zeolite Beta by CLD dealuminated H-beta gave “cleaner” pores with more accessible active sites linked with enhanced para-selectivity in anisole acylation.21 Conclusions The unexpected generation of extra-framework aluminum in H-beta by OMTS silylation was confirmed by complementary methods. Dealumination was effective. Nearly every Si atom added caused AlO4 to AlO6 conversion, 0.2 Si atoms deposited per Al and ca. 25% of AlO4 converted. Dealumination was also destructive, since the loss in long-range framework structure was paralleled by the loss of framework AlO4 sites (Supporting Information Figure S4). OMTS with much more steric bulk than the traditional tetraalkoxysilanes was able to enter the crystallite. The low hydrolysis rate of OMTS limited its interaction to the stronger acid sites located at the pore mouths, causing ring-opening. In liquid phase, the narrow ring-opened oligomer was small enough to enter the pores and directly impact the interior during subsequent calcination. Multiple OMTS-silylated catalysts were exceptional since they contained AlO5 and AlO6-a sites. These catalysts had the lowest amounts of large voids (TMB adsorption, Table 1), crystallinity (XRD, Table 5), and extra-framework acid sites (AlO6-c, Table 6) exhibiting Lewis acidity. Lewis acidity was low since the extra-framework species were largely those giving the AlO6-a signal. 27Al NMR and acidity measurements identified the species giving AlO6-a as a fully hydrated nonacidic small aluminum oxide cluster. Aluminum species drawn out of the zeolite framework modifies acidity and the effective pore space. Usually, amorphous Al-rich debris caused by dealumination are removed by acid leaching to facilitate diffusion and catalysis.21,51 But here, for silylated beta, the amorphous phase is not detrimental for catalysis, since the most para-selective catalysts contained the largest amounts (Supporting Information Figure S9). Acknowledgment. Silylations were performed by O. Farias, N. Sommariva made elemental analysis, F. Frigerio estimated molecular diameters using the Insight computer program, and C. E. Barabino provided BET measurements. Supporting Information Available: Alkoxysilane reactions during silylation are summarized. Graphs compare the different observed quantities: Al species, acidity, crystallinity and unit cell parameters, 29Si MAS NMR data, details of molecular mechanics calculations, and simplified (homogeneous) mechanism for rearrangement of the preformed aminal into MDA isomers. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) de Angelis, A.; Ingallina, P.; Perego, C. Ind. Eng. Chem. Res. 2004, 43, 1169. (b) de Angelis, A.; Flego, C.; Farias, O.; Bosetti, A. WO patent 02-20458, 2002. (c) de Angelis, A.; Bellussi, G.; Carluccio, L. C.; Millini, R.; Perego, C. Stud. Surf. Sci. Catal. 2005, 158, 1367. (2) O’Connor, C. T.; Mol¨ler, K. P.; Manstein, H. J. Mol. Catal. A 2002, 181, 15. (3) Yoo, J. W.; Lee, C. W.; Park, S.; Ko, J. Appl. Catal., A 1999, 187, 225. (4) Halgeri, A. B.; Das, J. Catal. Today 2002, 73, 65. (5) Cˇejka, J.; Z´ilkova, N.; Wichterlova, B.; Eder-Mirth, G.; Lercher, J. Zeolites 1996, 17, 265. (6) Cˇejka, J.; Wichterlova, B. Catal. Lett. 1992, 16, 421. (7) Berger, C.; Raichle, A.; Rakoczy, R. A.; Traa, Y.; Weitkamp, J. Microporous Mesoporous Mater. 2003, 59, 1.

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JP1010764

Unexpected Destructive Dealumination of Zeolite Beta by Silylation

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