Crystalline phase transition in as-synthesized pure silica zeolite RTH containing tetra-alkyl phosphonium as organic structure directing agent

The choice of structure directing agents (SDAs) in zeolite synthesis significantly impacts the arrangement of active sites, thereby influencing the stabilization of reaction intermediates with profound implications for catalytic applications....

. The 1 H-13 C cross polarization (CP) MAS NMR spectra, ν 0 ( 13 C) = 100.6MHz and ν 0 ( 1 H) = 400.1 MHz, were carried out in a 4 mm probe spinning the samples at 10 kHz, using 1 H π/2 pulse length of 2.3 μs, 2 ms contact time, SPINAL to decouple 1 H nuclei and recycle delays of 3 s.The 31 P spectra, ν 0 ( 31 P) = 161.9MHz, were recorded using a 4 mm probe spinning the samples at 10 kHz, using 20 s as recycle delay, 31 P π/2 pulse length of 4.0 μs and SPINAL for 1 H decoupling.   Recording the 19 F NMR spectra at low spinning rates do not average out the chemical shift anisotropy (CSA) giving rise to spinning side bands (SSB) with a contour shape that reflects the local symmetry of the fluoride anions.Figure S6a compares the 19 F MAS-NMR spectra of the RTH-9 and RTH-30 zeolites, recorded at 3.5 and 3 kHz spinning rates, respectively.The spectrum of the RTH-30 zeolite was simulated by using a unique component with δ 19 F iso = -68.1 ppm and that of the RTH-9 zeolite with two components at δ 19 F iso = -72.6 ppm and δ 19 F iso = -72.0ppm ignoring the tiny contribution at δ 19 F iso = -67.2ppm.The CSA parameters obtained in the simulation of the spinning side bands (SSB) patterns are shown in Figure S6b.The η ≈ 0.2 and κ ≈ 0.7 values are characteristic of an axial symmetry [1][2][3] and the span, Ω = 77.0-86.0 ppm, higher than 70.0 ppm is consistent with occurrence of Si-F bonding (O 4 Si-F species). 4The Ω values for a mobile F  atoms in as-prepared zeolites ranges between 45 -65 ppm as reported previously for MFI and CHA zeolites.According to the IZA database 5 the calcined pure silica RTH has four inequivalent T sites, namely T1, T2, T3 and T4, each with multiplicity 8.Because the calcined RTH unit cell contains 32 Si atoms there are 496 possible ways of arranging the two fluoride anions (r) in the 32 Si sites (n).
However, two F  bonded to two different T sites will have different chemical environments and consequently different chemical shifts, which would not agree with the 19 F NMR results.We have considered only combinations of two F  in equivalent crystallographic sites.That yields a total of 112 combinations, 28 for each of the four sites (equation 1 with n = 8 and r = 2).For every combination exchanging the two F  produces the same chemical structure, that is why we have calculated m combinations of r objects from a set of n objects (equation 1) because the order does not matter.In summary, for each crystallographic site (T1, T2, T3, T4), the 8 Si atoms have been labelled A, B, C, D, E, F, G, H and pairs of F  have been located in 28 combinations without repetition, namely, AB, AC, AD, AE, … GH. Figure S8 shows the labels for every T crystallographic site.Each set with 28 structures is called a T group.

Figure S9.
Relative energy (blue) in kcal/mol with respect to the most stable structure (CD of T2) and absolute difference (Δσ iso ) between the calculated shieldings of the two F  of every structure.Arrows mark the 16 structures with equivalent F  anions (Δσ iso ≈ 0).

Figure S9
shows the relative energy of the 112 models with respect to the most stable (CD model of the T2 group) and the difference of the absolute shielding between both fluorides (Δσ iso ) for each model.In general, when Δσ iso >> 1 the corresponding structure is relatively unstable.The opposite is not necessarily true.Some models with Δσ iso ≈ 0 are also relatively unstable (e.g AF of T2 and T4).The Δσ iso is zero for the models AB, CD, EF, GH (marked with an arrow) of the four T groups, meaning that both F  are chemically equivalent, according to the observed results by 19 F NMR.Moreover, in these 16 models the 32 Si atoms can be grouped in 16 equivalent pairs (see Tables S2, S3, S4, S5), that is, there are 16 different calculated 29 Si σ iso which agrees with the 16 signals observed in the 29 Si NMR results.These is not true for the rest of the 96 models.A close inspection of these 16 models (Tables S2 to S5) reveals that the two equivalent F  are related by the following symmetry operation: x+½, y+½, z ( left).The same holds for the 16 Si pairs.These 16 particular configurations are among the most stable because the distance between both fluoride anions is maximized ( right).Analysis of the symmetry shows that there is no space group with the symmetry operations (x, y, z; x+½, y+½, z) found in the best 16 models and consequently the unit cell must be redefined.Table S1.Experimental  iso 19 F (ppm) and theoretical  iso 19 F (ppm) values calculated with the modified Becke-Johnson exchange potential (TB-mBJ). 6The linear regression curve is shown in Figure S2.S8.Calculated absolute isotropic 29 Si shielding (σ iso , ppm) of the RTH optimized models with a triclinic unit cell and the fluoride anion located in T1 and T2 sites.The σ iso values of the model with the F  anion in position 6 (in italics) were used to make the plot shown in Figure 8.  S10.Calculated absolute isotropic 29 Si shieldings (σ iso , ppm) of the RTH optimized models with a triclinic unit cell and the fluoride anion located in T4 sites.The σ iso values of the model with the F atom in position 16-I (in italics) were used to make the plot shown in Figure 9.

Figure
Figure S3.a) 1 H-13 C CP MAS NMR spectra of the RTH samples and the assignment to the different groups of the P-OSDA + cation; the inset of the figure shows the amplification of signal of CH 3 and b) 31 P MAS NMR spectra of the RTH samples.

Figure
Figure S3a display the13  C MAS NMR spectra of the RTH-x zeolites obtained at different synthesis time.The splitting of the PCH 3 signal is due to the scalar J-coupling with the 31 P (I =1/2) J C-P = 49 Hz.The  13 C of the signal assigned to the isolated CH 3 group of the P-OSDA + (inset of the FigureS3a) is slightly different for the RTH samples, suggesting a little variation in their orientation inside the cavities.The 31 P MAS NMR spectra of the RTH-x samples FigureS3bconsist of a main signal at δ 31 P = 43.2ppm and another one much weaker at δ 31 P = 44.6 ppm suggesting that some phosphorous are in a slightly different environment.

Figure S4 .
Figure S4.Simulation of the 29 Si MAS NMR spectra of the zeolites a) RTH-30 and b) RTH-9.A total of 16 Lorentzian signals of equal intensity, 15 of them in the Q 4 region and one of penta-coordinated silicon have been used.Note that the scale in both spectra is different

Figure S6 .
Figure S6.a) 19 F MAS NMR spectra of the zeolite RTH-30 (blue) and RTH-9 (purple) recorded at the spinning rates indicated in the figure.b) CSA parameters obtained from the simulation of the spectra shown in a).

Figure S7 .Figure S8 .
Figure S7. 19F MAS NMR spectrum of the RTH zeolite synthesized with PSODA + by heating the gel at 150 ºC for 15 days (RTH(150)-15 sample).The 19 F NMR spectrum the RTH(150)-15 sample synthesized following the procedure depicted in the experimental part but heating at 150 ºC (instead of 175 ºC) during 15 days contains two signals at δ 19 F = 71.8and δ 19 F = 67.3ppm with practically equal intensity indicating the presence of similar amounts of RTH-A and RTH-B phases

Figure S10 .
Figure S10.Representation of two equivalent fluoride anions (left panel).Si, O, P, C, H, F are depicted in orange, red, yellow, gray, light gray, blue respectively.Unit cell vectors a, b and c are depicted in red, green and blue respectively.The right panel shows the relative energy as a function of the shortest F-F distance.Circles corresponding to models with equivalent pairs of fluorides have a black border.

Figure S11 .
Figure S11.Correlation of the calculated σ iso 29 Si of the models 5 (a), 6 (b), 7 (c) and 8 (d) with the experimental δ iso 29 Si of samples RTH-9 and RTH-30.In the four models, the fluoride anion is located in the T2 sites.Notice how the distinctive 29 Si signal at -103.70 ppm is not predicted properly with these models.

Figure S12 .
Figure S12.Correlation of the calculated σ iso 29 Si of the models 9-II (a), 10-I (b), 11-II (c) and 12-II (d) with the experimental δ iso 29 Si of samples RTH-9 and RTH-30.In the four models, the fluoride anion is located in T3 sites.

Figure S13 .
Figure S13.Correlation of the calculated σ iso 29 Si of the models 13-I (a), 14-II (b), 15-II (c) and 16-I (d) with the experimental δ iso 29 Si of samples RTH-9 and RTH-30.In the four models, the fluoride anion is located in T4 sites.Notice how the distinctive 29 Si signal at -103.70 ppm well predicted with these models.They correlation is better with the signals of sample RTH-9.

Figure S14 .
Figure S14.Plot of the heat flow against the temperature of the RTH-9 (red lines) and RTH-30 (blue line) samples upon heating from 25 C to 150 C (left graphical) and subsequent cooling down to 25 C.

Table S2 .
Starting coordinates for the OSDA in RTH-9 calculated from the PXRD data assuming a monoclinic unit cell.

Table S4 .
Calculated absolute shielding (σ iso 29 Si, ppm) of the 16 Si atoms (multiplicity of two) of the optimized monoclinic RTH model (32 Si atoms) with combinations of two F  anions in T1 sites.

Table S5 .
Calculated absolute shielding (σ iso 29 Si, ppm) of the 16 Si atoms (multiplicity of two) of the optimized monoclinic RTH model (32 Si atoms) with combinations of two F  anions in T2 sites.

Table S6 .
Calculated absolute shielding (σ iso 29 Si, ppm) of the 16 Si atoms (multiplicity of two) of the optimized monoclinic RTH model (32 Si atoms) with combinations of two F  anions in T3 sites.

Table S7 .
Calculated absolute shielding (σ iso 29 Si, ppm) of the 16 Si atoms (multiplicity of two) of the optimized monoclinic RTH model (32 Si atoms) with combinations of two of two F  anions in T4 sites.

Table S9 .
Calculated absolute isotropic29Si shielding (σ iso , ppm) of the RTH optimized models with a triclinic unit cell and the fluoride anion located in T3 sites.

Table S11 .
Cell parameters and volume of the triclinic unit cell of models 6 and 16-I and those obtained experimentally from the XRD data for the RTH-A, RTH-B and RTH-C phases.