Mechanisms of Quick Zeolite Beta Crystallization - American Chemical

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Mechanisms of Quick Zeolite Beta Crystallization Nathan Hould,*,† Mohamed Haouas,*,‡ Vladimiros Nikolakis,§ Francis Taulelle,‡ and Raul Lobo†,§ †

Center for Catalytic Science and Technology, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States ‡ Tectospin, Institut Lavoisier de Versailles, University of Versailles Saint Quentin en Yvelines, France § Catalysis Center for Energy Innovation, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: Silicate self-assembly and zeolite beta crystallization in the presence of 4,4′-trimethylenebis(N-methyl, N-benzyl piperidinium) (TMP2+) structure directing agent were studied using a combination of 29Si and 27Al nuclear magnetic resonance (NMR), small angle scattering (SAS), and attenuated total reflectance Fourier transform infrared (ATR/FTIR) spectroscopy. Crystallization of nanosized siliceous TMP2+ zeolite beta proceeds along three populations of particles (i.e., primary, secondary, and tertiary), differing in size and internal structure similar to crystallization of aluminum containing zeolite beta with tetraethylammonium (TEA+) as structure directing agent. Aluminosilicate TMP2+ zeolite beta, however, crystallizes along a very unique pathway, wherein secondary particles are final partially crystalline colloidally stable zeolite beta. TMP2+ zeolite beta crystallizes quickly when compared to TEA+ zeolite beta, which takes nearly a hundred hours to crystallize under similar synthesis conditions. KEYWORDS: zeolite beta, small angle x-ray scattering, small angle neutron scattering, nucleation, nuclear magnetic resonance, colloid

1. INTRODUCTION Zeolite crystallization remains an interesting and viable topic of research to understand mechanisms ranging from aluminosilicate concrete hardening1 to biomimetic growth of diatomaceous earth.2 Silicalite-1 has served for a long time as a model system for studying silicate self-assembly and structuring because of its ease of preparation.3−18 A great deal of attention has been devoted to tetraethylorthosilicate (TEOS) hydrolysis and silicate assembly in tetrapropylammonium hydroxide (TPAOH) solutions10,19,20 and recently on aluminosilicate assembly in trimethyladamantammonium hydroxide (TMAdaOH) solutions.21,22 These reports show that when starting from monomeric precursors, silicates and aluminosilicates assemble into nanoparticles above a critical aggregation concentration (CAC)23−26 (i.e., CSiOx/COH− ∼ 1) independent of the structure directing agent (SDA; e.g., TMAda+ and TPA+). We have investigated previously the mechanism of zeolite beta crystallization using tetraethylammonium (TEA+) as the SDA.27−29 Although TEA+ is the most economical SDA to produce zeolite beta, in part because of its use in phase transfer catalysis,30 other SDAs can be used to produce zeolite beta compositions of matter unattainable using TEA+. Among these SDAs is a group having carbon to nitrogen ratios greater than 11 that can be used to prepare hydrophobic siliceous zeolite beta. One cyclic diquaternary compound from this group has been important in the synthesis of an unique type of hierarchical zeolite material from diatoms.31 This SDA induces transformation of the amorphous diatom into a crystalline zeolite while preserving the mesoporous character of the diatom. Other diquaternary SDAs have been used in the synthesis of siliceous © 2012 American Chemical Society

zeolite beta having isomorphous transition metal substitutions. Notably, Saxton’s32 4,4′-trimethylenebis(N-methyl, N-benzyl piperidinium) (TMP2+) in Scheme 1 has been used Scheme 1. Structure of TMP2+ SDA used to Crystallize Zeolite Beta

by Davis et al.33,34 and our group to prepare titanosilicate35 and niobosilicate36 TMP2+ zeolite beta, respectively. Recently, especially interesting synthesis pathways to niobosilicate,37 stannosilicate,38,39 and titanosilicate40 zeolite beta have also been revealed. Here, we report mechanisms of crystallization of siliceous and aluminosilicate forms of TMP2+ zeolite beta.

2. EXPERIMENTAL SECTION 2.1. Synthesis of 4,4′-Trimethylenebis(N-methyl, N-benzylpiperidinium) Dihydroxide. 4,4′-trimethylenebis(N-methyl, N-benzylpiperidinium) dibromide (TMP(Br)2) was prepared from two solutions. The first solution was 50 g of 4,4′-trimethylenebis(Nmethylpiperidine) (>98%, Aldrich) in 200 mL of acetone. The second solution was 143 g of benzyl bromide (98%, Aldrich) in 1000 mL of acetone. The first solution was added dropwise to the second solution in Received: July 6, 2012 Revised: August 22, 2012 Published: August 27, 2012 3621

dx.doi.org/10.1021/cm3020995 | Chem. Mater. 2012, 24, 3621−3632

Chemistry of Materials

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a 2000 mL 3-neck round-bottom flask while stirring. The resulting solution was stirred for 12 h. Afterward, the mother liquor was removed from the white solid using a 1500 mL Buchner funnel. The solid was then washed with 500 mL of ether before being dissolved in 2000 mL of ethanol at reflux conditions. The clear yellow solution was then supersaturated by removing 1000 mL of ethanol using a rotary evaporator. This solution was placed in a refrigerator at 278 K for 48 h. The resulting crystal product was separated from the mother liquor using a Buchner funnel and then washed with 500 mL ether. The crystal product was shown to be pure TMP2+ using 1H and 13C nuclear magnetic resonance (NMR) and D2O as solvent. A 70% yield of product was obtained after one recrystallization. TMP(OH)2 was obtained by ion exchange using an Ambersep 900, OH− ion-exchange resin (0.8 mmol/mL). The exchange was completed in a static batch at room temperature in 72 h using a 2-fold excess of ionexchange resin and water solvent. TMP(OD)2 was obtained by using a 1.5-fold excess of Ag2O in D2O solvent. The solution was shaken for 144 h. Conversions were near 85% of TMP(Br)2 to TMP(OH)2 and TMP(OD)2 was determined by titration of the product solution using phenolphthalein. Afterward, the solution was concentrated to 25% w/w using the dihydroxide or dideuteroxide basis by removing water or D2O at 343 K by rotary evaporation. 2.2. Preparation of Synthesis Solutions and Sols. Here, when the silicate concentration is above the CAC, the term sol is applied (to indicate a colloidal dispersion) in place of the term solution, which, for the purpose of this report, solely contains silicate monomers and oligomers and is free of nanoparticles. Zeolite beta synthesis sols were prepared with the following molar composition 1 Si(OCH2CH3)4/30 (H2O + D2O)/0.25 (TMP(OH)2 + TMP(OD)2)/L Al(OCH2CH3)3 where L was 0 or 0.01. The above sols were made in two steps. In the first step, the silicate source, water, and SDA were mixed and stirred for 2 h. To the resulting clear sol, the heteroatom sources were added. The sol was then stirred for 24 h. The final clear sol was divided into several equal parts. Each was loaded into Parr acid digestion autoclaves that were subsequently heated at 393 K for up to 192 h. After the desired heating time, the synthesis sols were removed from the oven and quenched to room temperature. Silicate precursor assembly was studied in solutions and sols prepared as described above (no heating), with the following molar composition X Si(OCH2CH3)4/30 H2O/0.25 TMP(OH)2/L Al(OCH2CH3)3 where L was 0 or 0.01 and X was 0.125, 0.375, 0.625, or 1.000. The materials used in the syntheses were tetraethyl orthosilicate (TEOS), supplied by Sigma Aldrich at 98% w/w purity, 25% w/w TMP(OH)2 in water, prepared as described above, and aluminum triethoxide, supplied by Gelest or Strem Chemicals with a purity of 99% w/w. 2.3. NMR Methods. The one-dimensional NMR experiments were carried out on a Bruker Avance 500 spectrometer, operating at 99.353 MHz for 29Si and 130.326 MHz for 27Al. In a modified background-free probe, 10 mm quartz tubes were used with relatively short recycle delays between pulses to avoid the background signal of quartz. The 29Si spectra were recorded with single-pulse acquisition at room temperature (300 K) using a pulse of 3.6 microseconds (π/4), a recycle delay of 5 s, and an acquisition time of 1.6 s and accumulating 4096 scans. To account for the longer relaxation times of Q3 and Q4 silicates in nanoparticles, correction factors were applied to the spectra.41 The correction factors were determined by comparing two spectra of selected samples acquired with short (5 s) and long (90 s) recycle delays using PTFE NMR tubes (to prevent quartz contribution to the signals). The 27Al NMR spectra were obtained by applying π/12 pulses (2.2 microseconds pulse duration) and a recycle delay of 0.5 s, sufficient to equilibrate magnetization. The 29Si and 27Al chemical shifts were referenced to tetramethylsilane and octahedral [Al(H2O)6]3+, respectively. Simulation of the narrow lines was conducted with Lorenzian shape, while the broad bands were conducted with Lorenzo−Gaussian shape to better simulate the chemical shift distribution using the NMRnotebook software program. 2.4. Small Angle Scattering. Small angle neutron scattering (SANS) patterns were collected using 1 mm suprasil quartz sample cells at beamline NG-3 at the National Center for Neutron Research, National Institute for Standards and Technology (NIST).42,43 The incident neutron wavelength used was 0.6 nm, with a wavelength spread (Δλ/λ) of 0.15. The beam was collimated using source and sample apertures with diameters of 50 and 12.7 mm, respectively. The scattering

data were collected on a two-dimensional detector44 over a q-range from 0.038 to 4.121 nm−1. The raw data set was corrected for detector background, sensitivity, and scattering from the empty cell.45 All twodimensional scattering patterns collected were azimuthally symmetric and averaged. Siliceous and aluminosilicate sols used for SANS measurements were diluted 1 to 1 by weight with 50% w/w deuterated methanol in D2O and pure D2O, respectively. Small angle X-ray scattering (SAXS) patterns were collected on a SAXSess instrument (Anton-Paar) using line collimated Cu Kα radiation (1.542 Å). The sample holder was a TCS120 (Anton-Paar) fitted for liquid samples in a quartz capillary (1 mm diameter and 10 μm thickness). The sample to detector distance was 264.5 mm. X-ray scattering patterns were measured between 0.07 nm−1 and 26 nm−1 using a phosphor image plate for 5 to 40 min and scaled to I[q = 0] = 1 to correct for time and absorption effects. 2.5. ATR/FTIR Spectroscopy. Attenuated total reflectance Fourier transform infrared (ATR/FTIR) spectra46 were measured using a Nicolet 8700 FTIR spectrometer equipped with a single reflection diamond attenuated total reflectance attachment (Golden Gate) and TlBrxI1−x lenses (KRS-5) having a usable energy domain from 400 to 4000 cm−1. The infrared beam was incident π/4 to the surface of internal reflection and a solution of molar composition 30 H2O/0.25 TMP(OH)2/4 CH2CH3OH was used as a background. All ATR/FTIR spectra are the average of 32 scans collected using 4 cm−1 resolution. They were also corrected for penetration depth9 using a refractive index of 1.54.

3. RESULTS AND DISCUSSION 3.1. Silicate Connectivity in Zeolite Beta Crystallization. 29Si NMR is a noninvasive technique used here to determine silicate connectivity and relative amounts of 29Si in oligomers (and monomers), nanoparticles, and ‘silent’ 29Si silicates. Silicate oligomers give rise to sharp peaks in NMR spectra that can be assigned to specific silicates such as single 3- and 4-rings and double 3-, 4-, and 5-rings (i.e., 3R and 4R and D3R, D4R, and D5R, respectively) as well as lower symmetry oligomers.47−50 These can be separated into different regions of the spectra for Q0, Q1, Q2, and Q3 silicates at −72, −78 to −81, −81 to −90, and −90 to −105 ppm, respectively.51,52 As the oligomers condense into nanoparticles, the peaks broaden due to two primary reasons. First, 29Si nuclei in nanoparticles average their conformations more slowly (i.e., due to enhanced framework rigidity), producing a distribution of chemical shift for identical environments. Second, the nanoparticles have a longer rotational correlation time because of the larger size leading to a shorter T2 relaxation time, broadening each line for each environment.41 Therefore, in nanoparticles, the Q1, Q2, Q3, and Q4 silicates sites distributions are detected as broad peaks centered at about −80, −90, −98, and −106 ppm, respectively. As crystallization takes place, 29Si in nanoparticles becomes NMR ‘silent’ due to the drastic peak broadening from further framework rigidity (reducing fluctuations of the chemical shift anisotropy) and aggregation of the nanoparticles that increase rotational correlation times (and decrease the T2 relaxation time). On top of these observations, a nuclei balance can be used to analyze 29Si NMR spectra to determine relative amounts of the three silicate populations. 3.1.1. Assembly of Silicate Precursors to Zeolite Beta Prior to Heating. To characterize silicate self-assembly taking place during the preparation of the initial zeolite synthesis precursor, two sols and two solutions in the presence and absence of aluminum were prepared and analyzed by 29Si NMR. The spectra and quantitative decomposition (see Figures S.1−S.3 in the Supporting Information) allowed the determination of silicate distribution inside the oligomers and nanoparticles, as presented in Figure 1. At low silicate concentrations, mostly Q0 oligomers were in solution as partially deprotonated silicic acid. As TEOS is easily hydrolyzed, 3622



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illustrated by broadening of NMR lines from quadrupolar interactions and accelerated exchange from rapid hydrolysis and oxolation of Si−O and Al−O bonds60,61 (see Figure S.4 in the Supporting Information showing the Q0 silicate signal line width as a representative example). The average silicate connectivity (i.e., in Q) in Figure 2 provides insight into the aggregation process which occurs above

Figure 1. Distribution of 29Si inside of (top) oligomers and (bottom) nanoparticles in siliceous synthesis solutions and sols prior to heating as a function of total silicate content.

Figure 2. Average connectivity of oligomeric silicates and silicate nanoparticles in synthesis solutions and sols prior to heating as a function of total silica content.

k1

Si(OCH 2CH3)4 + 4H 2O HooI Si(OH)4 + 4CH3CH 2OH

a CAC. was calculated for each of the populations individually by averaging over silicates belonging exclusively to the oligomer or nanoparticle populations. At low silicate concentrations, the oligomers had a low , and as the silicate concentration increased, plateaued near 2.1. The difference between nanoparticle and oligomer connectivity suggests that the nanoparticles are formed by a combination of aggregation and interoligomer oxolation. Interestingly, again, aluminum did not have an impact on the silicate connectivity in the nanoparticles or the oligomers, in agreement with the results from the CAC analysis. The connectivity of aluminum was measured by 27Al NMR spectroscopy (see Figure S.5 in the Supporting Information). Unlike 29Si NMR spectra, 27Al NMR spectra did not show any narrow features specific to oligomers because the quadrupolar interaction of 27Al broadens their NMR signal. However, broad, resolved peaks in the spectra correspond to q0/q1, q2, q3, and q4 oligomeric aluminosilicates, which are centered near 75, 70, 65, and 60 ppm, respectively.62 The 27Al q4 peak shifts to near 54 ppm when aluminum is inside of the nanoparticles.21 The 27Al connectivity was observed to be higher than the 29Si connectivity expected along thermochemistry (i.e., energetically favorable Al−O compared to Si−O bonds)63 effecting the aluminosilicate distribution. Above the CAC, when CSiOx/COH− = 2, all of the aluminum was in a 4-connected configuration as observed in Figure 3 and apparently also located inside of the nanoparticles. Actually, as the aluminum to silicon ratio decreases, the interconversion between silicate oligomers becomes slower, as indicated by the narrowing of 29Si linewidths (see Figure S.4 in the Supporting Information). 3.1.2. Siliceous TMP2+ Zeolite Beta Crystallization upon Heating. At early times in crystallization, a sol was measured by 29 Si NMR (see Supporting Information S.2). As shown in Figure 4a and b, the concentration of Q0, Q1, and Q2 silicates were low relative to the Q3 oligomers. The total concentration of oligomeric silicates was constant during a 12 h induction time followed by a quick increase between 12 and 48 h of heating mirroring the Q3 population and, to a lesser extent, Q2 silicates.

k −1

(1)

and silicic acid is easily deprotonated, k2

Si(OH)4 HooI Si(OH)3 O− + H+ k −2

(2)

in basic synthesis solutions. As more silicate was added into the synthesis solution, the silicates change from having the majority of 29Si in Q0 units to more connected oligomers by oxolative condensation53,54 (i.e., oxolation) reaction: SixO−4xz− y H4x − 2y − z + SidO4−dh− f H4d − 2f − h k1, x , y , z , d , f , h , m

HooooooooooooI Six + dO−4((xz++dh))− (y + f + m)H4(x + d) − 2(y + f + m) − (z + h) k −1, x , y , z , d , f , h , m

+ m H 2O

(3)

As complex as reaction 3 appears for simple cases such as k1,1,0,0,1,0,0,1 (i.e., neutral silicic acid monomers condensing to create a dimer), activation energy has been measured to be 93(6) kJ/mol.55 At silicate concentration just below the CAC, D3R silicates (i.e., prismatic hexamer) were the major cage-structure units in solution, as shown by the peak at −88.8 ppm.56,57 When nanoparticles began to be formed above X = 0.50, a peak appeared near −98.756−59 showing D4R and D5R units (i.e., cubic octomer and pentonic decamer) were formed in amounts comparable to that of the prismatic hexamer. At the maximum silicate concentration, D4R and D5R became the most prominent oligomeric silicates in the sol. The silicate distribution in the nanoparticles also changed with silicate concentration from a loosely connected nanoparticle having very little Q4 silicate to one that was more connected at higher silicate concentrations. Interestingly, aluminum did not affect the CAC position (at the resolution of the experiment) or the silicate distribution in the nanoparticles (see Table S.1 in the Supporting Information). Aluminum was associated with the silicates, even in the synthesis solutions least concentrated in silicates, as 3623



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leading to an average connectivity (i.e., ) approximately limited to 3 (Figure 5). This is a consequence of the difference in activation energy for the forward reaction along

Figure 5. Average connectivity of oligomers and nanoparticles at selected heating times during zeolite beta crystallization.

1 TMP2 + 2 k4 1 HooI [≡Si−O−Si≡]A + TMP2 + + OH− 2 k −4

[≡SiOH]A + [≡SiO]−A + Figure 3. (top) Distribution of 27Al and (bottom) average connectivity of 29Si and 27Al in aluminosilicate synthesis solutions and sols prior to heating.

(4)

and 2[≡SiO]−A + TMP2 + + H 2O k5

The siliceous nanoparticle population (contributed to by both siliceous primary and secondary particles) exhibits concentration trends different from the siliceous oligomer population. Unlike the oligomer population, which increased in concentration between 12 and 48 h, the nanoparticle population decreased in concentration. The majority of changes occurred in the fraction of nanoparticle Q3 silicates. The signal from nanoparticle Q1, Q2, and Q4 silicates remained low and nearly constant during heating

HooI [≡Si−O−Si≡]A + TMP2 + + 2OH− k −5

(5)

where the square brackets denote a chemically bonded silicate framework and the subscript A represents an amorphous moiety. The last oxolation reaction has a higher activation energy and proceeds with a lower rate constant.64 The primary nanoparticles are effectively nanoaggregates of oligomers having greater

Figure 4. 29Si NMR Qn populations of (a and c) oligomers and (b and d) nanoparticles in siliceous TMP2+ (a and b) and aluminosilicate TMP2+ (c and d) synthesis sols at selected heating times during zeolite beta crystallization. 3624



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zeolite beta as detected by 29Si NMR spectra (Figures S.6 and S.7 in the Supporting Information). Here, the sols were heated for 96 h instead of up to 192 h because aluminum accelerates zeolite beta crystallization. The oligomer population was observed to have an initial silicate distribution similar to that obtained in siliceous synthesis sols, as shown in Figure 4c. When heat was applied to the synthesis sols, the oligomer population formed more connected silicates immediately, unlike the siliceous oligomers, which exhibited a 12 h induction time before changes occurred. The total oligomer concentration passed a maximum at 12 h of heating in the aluminosilicate synthesis sol, suggesting that intermediate steps are important in the solubility of the silicates during the early stages of crystallization. The presence of a pronounced peak at the maximum agrees with the lower solubility of zeolite beta crystals (see the ATR/FTIR in Section 3.3) compared to amorphous nanoparticle, which have a higher chemical potential. The aluminosilicate nanoparticles’ connectivity changed quickly within 12 h of heating (Figure 4d), unlike the siliceous system, which passed a 12 h induction time before changing slowly up to 48 h of heating. At this stage, the concentration of Q3 silicates inside of the nanoparticles decreased while the Q4 silicate concentration increased slightly. The Q1 and Q2 silicate concentration remained essentially steady state during crystallization. This translated into 29Si in aluminosilicate nanoparticles having lower , becoming ‘silent’ and leaving behind 29Si in nanoparticles in more highly connected configurations to be detected by 29Si NMR (see Figure 5). This suggests 29Si in Q3 silicates such as siloxy groups in IPs change to become ‘silent’ crystalline 29Si in zeolite beta along reaction 7 and

than 1.75 (i.e., of the oligomers below the CAC). The thermal energy activates oxolation to expel water and TMP2+ at the interface of the oligomers that have aggregated into nanoparticles. In other cases, the nanoaggregates may occlude {2[SiO]− TMP2+}A ion pairs (IPs) along [≡Si−O−Si≡]A + TMP2 + + 2OH− k6

HooI {2[≡SiO]− TMP2 +}A + H 2O k −6

(6)

where the curly brackets denote covalently and ionically bonded framework. Reaction 6 shows that upon occluding TMP2+ into a silicate framework IPs are formed while framework connectivity changes to retain charge neutrality. IPs in a siliceous nanoparticle core associate with at least 2 Q3 created along elementary steps: first, adding a water molecule to hydrolyze a [Si−O−Si] moiety to create two silanol groups, second, adding two hydroxide anions to deprotonate two silanol groups while eliminating two water molecules, and third, adding a TMP2+ to create an IP. These three elementary steps are combined into a single reaction 6. IPs may associate with more Q3 units because T-atom vacancies and intergrowth defects can be formed at siliceous−organic IPs in zeolite beta. 65,66 Heating can lead to oxolation inside of the nanoparticles to increase , eliminating progressively the hydroxide and TMP2+ along reverse reaction 6, wherein water is consumed as TMP2+ and OH− are released, while the network connectivity increases (still being amorphous). The siliceous nanoparticles do not exhibit increases in either the Q3 or Q4 signals, suggesting that reverse reaction 6 occurs infrequently inside nanoparticles. This leads us to conclude that as primary particles aggregate into secondary particles TMP2+ are already occluded (see Section 3.4). It is very interesting that siliceous nanoparticles immediately reach essentially steady state , which suggests charge density is essentially steady state as well. The nanoparticle Q3 concentration decreases between 12 and 48 h as the fraction of NMR ‘silent’ 29Si quickly increases. This happens as already well-connected silicates transform into crystalline zeolite beta following the reaction

{[≡SiO]− [AlO2/4 ]− TMP2 +}A k9

HooI {[≡SiO]− [AlO2/4 ]− TMP2 +}C k −9

Aluminosilicate nanoparticles also had a higher connectivity than siliceous nanoparticles, as expected, because of the higher stability of [Al−O−Si] compared to [Si−O−Si] moieties.63,67,68 Thermochemistry combined with synthesis sol chemical composition leads to exchange of {[SiO]− 1 /2TMP2+} with {[AlO2/4]− 1/2TMP2+} IPs and a reduction of silanol groups and a fourth pathway to crystalline moieties:

k7

{2[≡SiO]− TMP2 +}A HooI {2[≡SiO]− TMP2 +}C k −7

(9)

(7)

k10

{2[AlO2/4 ]− TMP2 +}A HoooI {2[AlO2/4 ]− TMP2 +}C

and

k −10

k8

[≡Si−O−Si≡]A HooI [≡Si−O−Si≡]C k −8

(10)

These reactions are analogous to chemistry in monoquaternary systems such as high silica TEA+ zeolite beta and other siliceous zeolites, which contain a high density of internal Q3 silicates associated with {[SiO]− SDA+} IPs.28,65,69−71 Silicate nanoparticles occlude aluminum at early stages of crystallization before heating as was determined from the 27Al NMR peak broadness (see Figure S.8 in the Supporting Information). 27Al was 4-connected at room temperature when the CSiOx to COH− ratio was 2 (see Section 3.1.1). Upon heating, the synthesis sol 27Al NMR peak shifted from 54 to 53 ppm due to changes in environment as aluminum remained 4-connected to the framework (see Figure S.8 in the Supporting Information). A similar resonance at 53 ppm was detected in 29Si MAS NMR spectra from nanosized assemblies and colloidal powders of zeolite beta.72 Lower aluminum connectivity leads to peaks at lower field21 not observed in the spectra of these synthesis sols. Aluminum increases the connectivity of the silicate nanoparticles

(8)

where the subscript C represents a crystalline moiety. Implicit in reactions 7 and 8 are hydrolysis and oxolation steps illustrated in reaction 3 (unenclosed by brackets and unpaired) that lead to a thermochemically stabilized structure. In coupling forward reaction 6 with forward reaction 7, an unbound TMP2+ becomes bound to a siloxy group to create an ion pair (IP) unit while changing local silicate structure. These reactions are intended to illustrate important moieties observed using NMR. Below, moiety changes from amorphous to local structure propagating to global structure are better understood by incorporating ATR/FTIR and small angle scattering (SAS) analysis. 3.1.3. Aluminosilicate TMP2+ Zeolite Beta Crystallization upon Heating. As we will show, crystallization of aluminosilicate TMP2+ zeolite beta proceeds similar to crystallization of siliceous 3625



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(i.e., ) agreeing with 27Al having higher average connectivity than 29Si, which exists largely in Q3 connectivity. When aluminum is 4-connected in a silicate framework, a negative charge is compensated for in a {[AlO2/4]− 1/2TMP2+} IP. This type of aluminous IP is, intrinsically, associated with a more connected framework than siliceous IPs.28,65 This second effect leads to enhanced connectivity of aluminosilicate nanoparticles compared to siliceous nanoparticles. Within error, 27Al and 29Si in nanoparticle signal disappearance correlates nearly 1 to 1, suggesting an aluminum rich ‘silent’ phase (see Figure 6). We

Figure 6. Distribution of aluminosilicates in the synthesis sol at selected heating times during zeolite beta crystallization.

have shown 4-connected aluminum accelerates zeolite beta crystallization and increases final framework structure connectivity. 3.2. Nanoparticle Aggregation and Zeolite Beta Crystallization. Growth of TEA+ zeolite beta crystals proceeds along three populations of nanoparticles (primary, secondary, and tertiary) as observed by SAS.27−29 Crystallization of aluminosilicate and siliceous TMP2+ zeolite beta proceeded along two and three populations of nanoparticles, respectively. In the latter crystallization pathway, amorphous primary particles aggregated into secondary particles, which began as amorphous and finally became colloidally stable TMP2+ zeolite beta nanocrystals. In siliceous synthesis sols, TMP2+ zeolite beta crystallization proceeded along primary, secondary, and tertiary nanoparticles similar to aluminosilicate TEA+ zeolite beta. It is important to remember that nanoparticle populations observed by SAS are not the same as those detected by 29Si NMR: first, the silicate oligomers are not observed in our SAS measurements (possibly because they are too small), and second, the primary and secondary particle populations observed using SAS are indistinguishable in 29Si NMR spectra. The NMR ‘silent’ 29Si nuclei are in structured secondary and tertiary particles and in large amorphous silicate particles. 3.2.1. Primary Particles before Heating. At room temperature, primary particles assemble above the CAC and contribute to the nanoparticle signal observed in the 29Si NMR spectra. The SAXS patterns from sols containing aluminosilicate primary particles show higher scattering intensity at low-q values than those from the siliceous system (Figure 7a). The pair distance distribution functions (PDDF) obtained by indirect Fourier transform (IFT)73 of the SAXS patterns show that the aluminosilicate primary particles are more polydisperse than the silicate primary particles (Figure 7b). The effective Rg of the particles was 1.89 and 0.79 nm in the aluminosilicate and siliceous systems, respectively. The PDDF from the aluminosilicate

Figure 7. (a) SAXS patterns and (b) PDDFs of silicates in synthesis sols prior to heating.

primary particles extends to 10 nm, the value of the largest dimension of the primary particles in sol. The low intensity at low-q in the siliceous system suggests interparticle repulsion forces between siliceous primary particles. In siliceous TEA+ systems, interparticle repulsive forces between primary particles were enhanced with respect to the aluminosilicate system as well.73 Silicates join in two steps. In the first step smaller silicates aggregate into larger silicates (e.g., primary particles aggregate into secondary particles). In the second step, oxolation of surface silanols groups on adjacent silicates particles already aggregated into a secondary particle leads to a covalently bonded framework (see reaction 3 wherein two steps are combined into one reaction). Interparticle repulsive forces may provide an activation barrier and induction time to be passed before aggregation and oxolation happen. 3.2.2. Primary Particles after Heating. Upon heating aluminosilicate primary particles to 393 K, they aggregate quickly into secondary particles. In siliceous sols, a 6 h induction time passed before primary particles aggregate into secondary particles (depicted in Figure 8). This is similar to what was measured using 29Si NMR spectroscopy, which shows an induction time to changes in the oligomer and nanoparticle concentrations. We assign the difference between aggregation of aluminosilicate and siliceous TMP2+ primary particles to differences in interparticle repulsion observed in SAXS patterns collected prior to heating. See Figure 7, where the depressed SAS intensity for the siliceous primary particles at low-q indicates enhanced repulsive interactions between the siliceous primary particles compared to the 3626



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Figure 8. SAS patterns from (a) siliceous and (b) aluminosilicate synthesis sols at selected heating times (t).

Figure 9. SAXS patterns showing a rise in the first (broad) diffraction peak of zeolite beta centered near 5 nm−1 for (a) siliceous and (b) aluminosilicate TMP2+ zeolite beta at selected heating times (t).

aluminosilicate primary particles. This observation is similar to the TEA+ system where aluminosilicate (and borosilicate) primary particles’ attraction was enhanced compared to siliceous TEA+ primary particles.27,29 Aluminum enhances secondary particle colloidal stability in contrast to primary particles wherein aluminum induces aggregation. In siliceous sols secondary particles formed prior to 6 h of heating time. The siliceous secondary particles aggregated into tertiary particles during the first 48 h of heating time, as is shown by the power law behavior in Figure 8 and disappearance of the knee near 0.2 nm−1 after 24 h of heating time. During aggregation, the silicates’ structure was changing, as shown by the peak at 5 nm−1 in the SAXS patterns, becoming more pronounced (see Figure 9). Similarly, upon heating, aluminosilicate TEA+ secondary particles change to become more organized while simultaneously they become less colloidally stable and aggregate into tertiary crystals.27 Our results agree well with the mechanism of titanosilicate TEA+ zeolite beta crystallization reported by Kaučič et al.40 The aluminosilicate TMP2+ secondary particles, on the other hand, are stable in solution for at least 480 h. Compared to the siliceous TMP2+ system, the aluminosilicate system changed more quickly to global structure, as shown by the SAXS patterns, becoming more pronounced in Figure 9. The aluminosilicate TMP2+ secondary particles had some zeolite beta structure at synthesis times as early as 6 h of heating, showing that secondary particles can be crystalline zeolite beta, agreeing with high resolution transmission electron micrographs (HRTEM) of titanosilicate secondary particles.40 Because the secondary particles do

not aggregate in our aluminosilicate solution, we conclude that synthesis conditions can be selected to stabilize zeolite beta precursor nanoparticles (in the colloidal sense) while they change to be crystals. To characterize the secondary particles in solution, the SAS patterns were analyzed using an IFT.73 The secondary particle PDDFs showed that the Rg of the aluminosilicate secondary particles was initially about 4 nm and then increased to nearly 8 nm after 96 h of heating. The secondary particles were observed to be slightly larger by SANS measurements, perhaps because larger length scales are sampled with neutrons. The siliceous secondary particles heated for 12 h had an Rg of 16.7 nm, nearly 4 times larger than aluminosilicate secondary particles. This is interesting in light of the nanoparticles’ average connectivity measured by NMR. We have observed that aluminosilicate nanoparticles have a higher connectivity than siliceous nanoparticles, which combined with their significantly higher surface area per unit volume (i.e., smaller radius of gyration) suggests that the difference in the nanoparticles’ internal silanol density is higher than that calculated from the Q3 to Q4 ratio measured by 29Si NMR. At later times in the siliceous system when secondary particles aggregated into tertiary particles, a power law model was used to characterize the structures. The power −n was measured to increase from 1.6 after 18 h of heating time to a plateau near 2.4 after 24 h of heating time. The values suggest a mass fractal network comprising secondary and tertiary particles in onedimensional pseudochain configuration which changes to become a denser pseudogel network upon futher heating.74 3627



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Figure 10. ATR/FTIR spectra measured from (a) siliceous and (b) aluminosilicate TMP2+ sols at different heating times (t). The top spectra marked with solid squares were measured from aluminosilicate TEA+ zeolite beta powder. Spectra are marked with dotted lines at 567, 1073, and 1220 cm−1. * indicates an artifact from background subtraction near 875 cm−1. Absorbance changes at four frequencies relative to spectra prior to heating are shown for (c) siliceous and (d) aluminosilicate TMP2+ sols.

and aluminosilicate TMP2+ synthesis sols after 18 and 6 h of heating, respectively. Relative absorbance at selected wavenumbers from spectra of siliceous and aluminosilicate TMP2+ sols is shown in Figures 10c and d, respectively. Wavenumbers were selected to be near absorption band maxima. In the spectra from the siliceous TMP2+ sols the absorbance near 1022 cm−1 remains nearly constant for up to 18 h of heating time and then decreases at later stages. The absorbance near 1041 cm−1 increases with time from the onset of heating, while the absorbance bands near 1073 cm−1 and 1220 cm−1 start to increase after 12 h of heating time. All three of them pass maxima near 18 h of heating time before reaching essentially steady state near 48 h of heating time. In the aluminosilicate TMP2+ sol, similar observations were made, with the most striking difference being the absence of an induction time. The graphs shown in Figure 10c and d have several similarities with the graphs shown in Figure 4. The induction time observed for the absorption near 1022 cm−1 of the siliceous sols coincides with that observed for the oligomeric units in Figure 4a. This observation supports this bands assignment to amorphous oligomeric siliceous units. In both sols, there was no induction time to changes of the absorbance near 1041 cm−1. By considering the changes of the nanoparticle connectivity shown in Figure 4, we argue that the absorbance band near 1041 cm−1 be assigned structured silicates. We link decreases in nanoparticle 29Si fraction with time measured by NMR (Figures 4b and d) to decreases in amorphous silicate band near 1022 cm−1. While simultaneously crystals were formed, an absorbance near 1041 cm−1 increased leading to its being

3.3. Local Connectivity in Zeolite Beta Crystallization. To understand silicate local connectivity in TMP2+ zeolite beta crystallization, ATR/FTIR spectra of siliceous and aluminosilicate TMP2+ sols are shown after selected heating times in Figure 10a and b, respectively. In a similar fashion to siliceous TPA+ sols,9 the spectra of both fresh samples prior to heating have four absorption bands appearing near 1022 cm−1, 1073 cm−1, 1121 cm−1, and 1150 cm−1. The band at 1022 cm−1 can be attributed to the presence of linear or cyclic oligomeric units, while the other three can be attributed to symmetric Si−O−Si stretching of amorphous nanoparticles formed above the CAC.9,75 Characteristic FTIR bands of zeolite beta structure appearing near 567 cm−1 and 1220 cm−1 are missing prior to heating. The band near 1220 cm−1 is characteristic of zeolites which contain 5-member ring moieties;76,77 thus, it indicated zeolite beta structure. The band near 567 cm−1 can be attributed to the vibration of double 5-ring moieties76 present in zeolite beta. As zeolite beta was formed, the band near 1022 cm−1 shifted to be near 1045 cm−1 (as shown in the aluminosilicate TMP2+ sols), and bands near 1220 and near 1073 cm−1 appeared and became more pronounced. The band near 1073 cm−1 can be assigned either to asymmetric stretching of Si−O in zeolite beta or to cyclic 4-mers to 6-mers of oligomeric units.9,75,78−80 We argue for relation between asymmetric stretching and zeolite beta structure because it becomes pronounced at the same time as structure is formed (its absence in aluminosilicate TEA+ zeolite beta powder is subject to more detailed analysis). The band at 1220 cm−1 indicates zeolite beta structure27 and was observed in the siliceous 3628



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assigned to structure silicates being formed on the zeolite beta crystallization pathway. In aluminosilicate TEA+ synthesis sols, 240 h of heating were required to organize local silicate structure such that a band near 1220 cm−1 appears. In the aluminosilicate TMP2+ sols, signals associated with local structure (the bands near 1220 cm−1 and 567 cm−1) were observed at heating times as early as 6 h and as diffraction was observed in SAXS patterns above. In both siliceous and alumosilicate systems, appearance of bands assigned to structure correlated well with the ‘silent’ portion of 29Si NMR spectra. In siliceous systems, the induction time to structure changes as measured by ATR/FTIR are linked to changes the oligomer connectivity as measured using 29Si NMR. The band near 1076 cm−1 increased after 12 h of heating time simultaneously along with initial changes in oligomer connectivity. After 18 h of heating, bands near 1220 and 567 cm−1 become more pronounced as the oligomer Q3 silicate concentration increased. In the siliceous system, a 24 h passed before global structure was measured by SAXS (see the diffraction peak at 5 nm−1 in Figure 9a). Local structure was measured at earlier stages by ATR/FTIR (see the bands near 1220 cm−1 and 567 cm−1 in Figure 10a) showing global structure (having a 24 h induction time) proceeds upon passing local structure (having an 18 h induction time). 29Si NMR spectra in Figure 4b show the induction time to silicate oxolation corresponds to local structuring. It is reasonable because structure that diffracts would be formed after tetrahedra become locally structured by T−O bond breaking−reformation, as illustrated by reversible reaction 3 for the silicate case. 3.4. Two-Step Nucleation Model. The two-step nucleation model, which characterizes the aluminosilicate TEA+ zeolite beta nucleation pathway,27,28 could fit our measured TMP2+ zeolite beta nucleation. In the two-step nucleation model, as applied to zeolite beta nucleation, secondary particles that are formed by aggregation of primary particles are nucleation centers. After aggregation, secondary particles have a composition near that of final zeolite beta, but they are still amorphous. At this stage, the connectivity of the soluble oligomers is still changing by combination of reactions 3−8. After further heating, the secondary particles change from amorphous to zeolite beta by reorganizing local structure, illustrated in a single step in reactions 7, 8, 9, and 10, until global structure is attained; see the stoichiometry marked 11. In aluminosilicate TEA+ zeolite beta nucleation, the second step is rate-limiting. The first step finished 48 h after heating is applied, whereas the majority of structural changes happen up to 480 h of heating, as measured by the Scherrer equation.27 In the TMP2+ system measured here, composition and structure changes happen more quickly than in the similar TEA+ system.27 To determine whether zeolite beta nucleates in accordance with the two-step model, independent techniques were used to investigate secondary particle composition and structure (measured by SAXS and ATR/FTIR, respectively). The composition (density) was measured by neutron scattering length density (SLD) matching by varying the background SLD using mixtures of water and deuterium oxide (D2O). Values were measured at four background SLDs, and the secondary particles’ SLD (i.e., the match point) was determined by interpolation (Figure 11). The siliceous secondary particles’ SLD was measured to be 2.52 × 10−4 nm−2 after 12 h of heating, while they were amorphous by SAXS and ATR/FTIR measurements. The SLD of the aluminosilicate TMP2+ secondary particles heated for 96 h was 2.47 × 10−4 nm−2. The slightly lower value

Figure 11. Neutron SLD matching measured on siliceous secondary particles after 12 h of heating and aluminosilicate secondary particles after 96 h of heating.

compared to siliceous secondary particles may be due to increased T-atom density in the aluminosilicate TMP2+ secondary particles. In contrast to siliceous TMP2+ secondary particles, aluminosilicate TMP2+ secondary particles had local zeolite beta structure by ATR/FTIR spectroscopy. To progress, we assigned a SLD of 2.47 × 10−4 nm−2 to TMP2+ zeolite beta and determined that siliceous TMP2+ secondary particles have the composition of zeolite beta, as well. Our assignment is justifiable when compared to the significantly higher SLD of 3.54(1) × 10−4 nm−2 for more silica dense Ludox nanoparticles.81 The SLDs for TMP2+ secondary particles were slightly lower than 2.75(4) × 10−4 and 2.72(8) × 10−4 nm−2, as obtained for aluminosilicate TEA+ secondary particles after 48 and 96 h of heating time, respectively.27 We conclude that the composition of the siliceous TMP2+ secondary particles is already near zeolite beta before local structure is formed. To check the secondary nanoparticles’ composition, SLD calculations were performed on a model of TMP2+ zeolite beta having the following composition: |TMP2N+I + NII|[Si 64 − 2NI − 2NIIAl 2NIIO128 − 8NI − 2NII × (OH)6NI O−2(NI + NII)]

(11) −

2+

where NI is the number of {2[SiO] TMP } IPs coupled with T-atom vacancies and NII is the number of {2[AlO2/4]− TMP2+} IPs free of defects both on an unit cell (uc) basis. This simple model does not account for mixed aluminosilicate (as illustrated in reaction 9) and {2[SiO]− TMP2+} IPs that may be associated with hydrolyzed [Si−O−Si] moieties and intergrowth defects in the framework65,66,70 but does qualitatively account for differences in 29Si Q3 composition between siliceous and aluminosilicate TMP2+ nanoparticles. Siliceous TMP2+ zeolite beta was already measured to have 0.27 w/w combustible materials (by thermal gravimetric analysis (TGA)) and a microporous volume of 0.27 cm3/g (by N2 adsorption). The combustible materials were assigned to TMP2+ to calculate that there were 3.24 {2[SiO]− TMP2+}/uc in siliceous TMP2+ zeolite beta. The SLD was calculated using the scattering lengths of the elements making up uc composition marked (11) and 4.2329 nm3 as the Vuc for zeolite beta polymorph A (i.e., BEA*).82,83 This procedure yields a SLD of 2.89 × 10−4 nm−2, slightly higher than for siliceous TMP2+ secondary particles. 3629



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Scheme 2. Mechanism of Siliceous TMP2+ Zeolite Beta Crystallization Wherein Time Evolution of Particle Size and Structure Distributions Are Qualitatively Illustrated

zeolite beta, which required a small amount of aluminum to crystallize.27,29 In both systems, primary particles comprising silicate monomers and oligomers self-assemble at room temperature when prepared at silicate compositions above a CAC. Here, we have shown that the assembly of TMP2+ primary particles proceeds by formation of monomers and oligomers at low silicate concentrations. At high silicate concentrations above the CAC, well connected oligomers aggregate into primary particles, leaving behind a low concentration of soluble oligomers and monomers. Aluminum addition into the synthesis sol has a negligible effect on the CAC, while aluminum is 4-connected inside of the nanoparticles and not left behind in solubilized oligomeric and monomeric forms. Nanoparticles formed above the CAC have an effective Rg of 1.89 and 0.79 nm in the TMP2+ aluminosilicate and siliceous compositions, respectively. Aluminosilicate primary particles appear to be more polydisperse (or elongated) as shown by their broad PDDFs. The silicate selfassembly behavior is similar to what has been observed in silicate sols containing TMAda+,21 TPA+,24 and TEA+ SDAs.29 Upon heating siliceous TMP2+ primary particles, an induction time prior to aggregation into secondary particles was measured. On the other hand, the aluminosilicate TMP2+ primary particles aggregated immediately into aluminosilicate TMP2+ secondary nanoparticles. A considerable difference in secondary particle size was observed. Siliceous secondary particles had a Rg of 16.7 nm compared to smaller aluminosilicate TMP2+ secondary particles that had Rg values near 5 nm during synthesis. The aluminosilicate TMP2+ secondary nanoparticles containing {2[AlO2/4]− TMP2+} IPs had higher framework connectivity than siliceous TMP2+ secondary nanoparticles, even while primary particles had similar connectivity. We conclude that charge compensation occurs after heat is applied, perhaps during aggregation of the primary particles. Association of organicsiliceous IPs with a high density of silanol groups is in agreement with previous N2 adsorption and TGA measurements in TEA+ zeolite beta.28,65 ATR/FTIR and SLD analyses were used to show that siliceous TMP2+ zeolite beta nucleates according to two step nucleation model. During nucleation of aluminosilicate TMP2+ zeolite beta, partially crystalline secondary particles remained colloidally stable in sol, ultimately leading to colloidally stable aluminosilicate zeolite beta crystals having a final dimension near 10 nm. This mechanism is unlike aluminosilicate TEA+ secondary particles, which become less colloidally stable as they proceed along the nucleation reaction coordinate to aggregate into tertiary particles, and similar to siliceous TEA+ secondary particles, which remain stable in the form of amorphous secondary particles. Siliceous TMP2+ secondary particles, on the other hand, become less stable and aggregate into tertiary particles, similar to the aluminosilicate TEA+ secondary particles and titanosilicate TEA+ zeolite beta reported by Kaučič et al.,40 which initially have local structure (by ATR/FTIR) but not yet global structure (by SAXS). Our observations are similar to those made on aggregates of siliceous TPA+ nanoparticles using HRTEM and cryogenic transmission electron microscopy (cryo-TEM) revealing amorphous nanoparticles at early stages, which change to become crystalline.18 All of these results together build a case for structuring inside of silicates having a composition near the final crystal product’s (obtained after completing step 1 of the twostep nucleation model) along stepwise hydrolysis and oxolation reactions, which initially rearrange silicate local structure (mediated by framework-SDA electrostatic and van der Waals

This enhancement could be explained by the secondary nanoparticles having a 17% increase in volume relative to BEA*. In conclusion, siliceous TMP2+ zeolite beta fits our two-step nucleation model, as proven by measuring amorphous secondary particles having a composition away from dense amorphous silica (i.e., 2.2 g cm−3) and more resembling zeolite beta which eventually change to have both the composition and structure of zeolite beta. Key to the nanoparticles’ composition is the formation of IPs. We have shown how siliceous nanoparticles containing only {2[SiO]− TMP2+} IPs have a higher silanol group density than those containing {2[AlO2/4]− TMP2+} IPs. These IPs affect the nanoparticles’ free energy, composition, and local structure. This report, combined with our earlier measured induction times to nucleation, enables a proposal of an empirical correlation between the free energy of IPs in precursor nanoparticles and the reciprocal of induction time to nucleation. The free energy of TEA+ occlusion into zeolite beta frameworks is exergonic as measured by calorimetry (−32 ± 15 kJ/mol TEA+ in siliceous zeolite beta).84 The free energy of the TMP2+ IPs (−181 ± 21 kJ/ mol SDA) is lower, and they do nucleate zeolite beta more quickly than TEA+. Also, {2[SiO]− TMP2+} IPs are higher in energy than {2[AlO2/4]− TMP2+} IPs, in part, because four Al− O−Si moieties are lower in energy than Si−O−Si, Si−O−H, and Si−O− moieties formed at a siliceous IP.63,70 This leads to a tentative free energy ranking as follows: {2[AlO2/4]− TMP2+} < {2[SiO]− TMP2+]} ∼ 2{[AlO2/4]− TEA+} < 2{[SiO ]− TEA+}, which is positively correlated with the induction time to nucleation, that is, 6 < 36 < 288 h, and an unknown heating time, respectively. This correlation should be valuable to model zeolite nucleation to advance the classical work of Zhdanov et al.85 and Warzywoda et al.86 We add that similarity in free energy of 2{[AlO2/4]− TEA+} and {2[SiO]− TMP2+]} IPs along with measured differences in nucleation induction time leads us to understand that we must bring together synthesis, structure, and thermochemistry to model zeolite nucleation well.

4. CONCLUSIONS Very interesting mechanisms of TMP2+ zeolite beta crystallization were determined (as depicted in Scheme 2 and Scheme 1 in the Supporting Information) and compared with that of TEA+ 3630



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interactions) and finally propagate silicate structure to attain globally ordered zeolite beta powder.

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ASSOCIATED CONTENT

S Supporting Information *

Section S.1: Monitoring TEOS hydrolysis in aqueous TMP(OH)2 solutions and sols with NMR. Figure S.1: 29Si NMR spectra in the system X Si(OCH2CH3)4/30 H2O/0.25 TMP(OH)2. X from bottom to top is 0.125, 0.375, 0.625, and 1.000. Figure S.2: 29Si NMR spectra in the system X Si(OCH2CH3)4/30 H2O/0.25 TMP(OH)2/0.01 Al(OCH2CH3)3. X from bottom to top is 0.125, 0.375, 0.625, and 1.000. Figure S.3: Example of 29Si NMR spectral decomposition: black line (experimental), blue line (calculated), broad red lines (nanoparticles components), narrow red lines (oligomers components), green line (difference). Figure S.4: 29Si NMR line width of monomer species in the system X Si(OCH2CH3)4/30 H2O/0.25 TMP(OH)2/L Al(OCH2CH3)3 here L was 0 or 0.01. Table S.1: The distribution of silicates inside of the oligomers and naoparticles in system X Si(OCH2CH3)4/30 H2O/0.25 TMP(OH)2/L Al(OCH2CH3)3 where L was 0 or 0.01 and X was 0.125, 0.375, 0.625, or 1.000. Figure S.5: 27Al NMR spectra in the system X Si(OCH2CH3)4/ 30 H2O/0.25 TMP(OH)2/0.01 Al(OCH2CH3)3. X from bottom to top is 0.125, 0.375, 0.625, and 1.000. Section.S.2) Monitoring zeolite crystallization with NMR. Figure S.6: 29Si NMR spectra in the system 1 Si(OCH2CH3)4/30 H2O/0.25 TMP(OH)2 heated at 393 K for from bottom to top 0, 3, 6, 12, 18, 24, 48, 96, and 192 h, (F.S.7: 29Si NMR spectra in the system 1 Si(OCH2CH3)4/30 H2O/0.25 TMP(OH)2/0.01 Al(OCH2CH3)3 at 393 K for from bottom to top 0, 3, 6, 12, 18, 24, 48, and 96 h. Figure S.8) 27Al NMR spectra in the system 1 Si(OCH2CH3)4/30 H2O/0.25 TMP(OH)2/0.01 Al(OCH2CH3)3 heated at 393 K for from bottom to top 0, 3, 6, 12, 18, 24, 48, and 96 h. Scheme S.1: Mechanism of aluminosilicate TMP2+ zeolite beta crystallization wherein time evolution of particle size and structure distributions are qualitatively illustrated. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +1.302.831.2061. Fax: +1.302.831.1048. E-mail: nhould@ udel.edu. Tel.: +33.1.39.25.4.54. Fax: +33.1.39.25.44.76. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.H. acknowledges A.F. and B.P. for measuring thermal gravimetric analysis isotherms and SAS patterns. V.N. acknowledges financial support from the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001004. This manuscript was prepared under cooperative agreement 70NANB7H6178 from NIST, U.S. Department of Commerce. The statements, findings, conclusions and recommendations are those of the author(s) and do not necessarily reflect the view of NIST or the U.S. Department of Commerce.

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REFERENCES

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