Insights into the Correlation of Aluminum Distribution and BrÃ¶nsted

Letter pubs.acs.org/JPCL

Insights into the Correlation of Aluminum Distribution and Brönsted Acidity in H‑Beta Zeolites from Solid-State NMR Spectroscopy and DFT Calculations Rongrong Zhao, Zhenchao Zhao,* Shikun Li, and Weiping Zhang* State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China S Supporting Information *

ABSTRACT: Here we utilized 27Al MAS/MQMAS and 31P MAS NMR of quantitative adsorption of trimethylphosphine oxide (TMPO) and DFT calculations to elucidate the relationship between Al distribution and Brönsted acidity of series H-Beta zeolites derived from dealumination of Al-rich H-Beta zeolite. Three types of Brönsted acid strengths corresponding to different specific Al T-sites were demonstrated. The removal of one framework Al in 5MR2−-2Al and 6MR-2Al sites led to increasing the Brönsted acid strength of dealuminated H-Beta. Our findings on such exact correlation between specific Al distributions and corresponding Brönsted acid sites may guide the controlling Al distribution to get desired acid properties through zeolite synthesis or finely tuned dealumination, which has a great impact on the catalytic activity and selectivity of zeolite catalysts.

Z

adsorption of trimethylphosphine oxide (TMPO), 31P MAS NMR is also very sensitive to discriminate Brönsted/Lewis acid strength of zeolite catalysts and capable of covering the whole range from weak, medium, strong, to superacidity.18−20 Herein, 27Al MAS/MQMAS NMR, and 31P MAS NMR of quantitative TMPO adsorption as well as DFT calculations are utilized to elucidate the relationship between specific Al distributions and Brönsted acid properties in the widely used H-Beta zeolite catalysts. Fundamental insights into the variations of Al locations corresponding to the Brönsted acid strength have been demonstrated from Al-rich to high-silica HBeta zeolites through finely tuned dealumination. A series of H-Beta zeolites were obtained from Al-rich Beta zeolite synthesized from organotemplate-free approach after different degrees of dealumination.21 XRD patterns (Figure S1 in Supporting Information) show that there is no obvious change of the crystallinity after dealumination, but the diffraction peaks at 20−30° significantly shift to higher angles. This means the corresponding d spacings shrink as the T−T distance decreases when Si−O−Al changes into Si−O−Si due to dealumination.22 29Si MAS NMR spectra (Figure S2) demonstrate that the parent H-Beta (Si/Al = 7) has ∼4% Si(2Al) groupings, and the percentage of 2Al in the total Al can be estimated ranging from 29 to 58%. After dealumination the signal intensities of Si(1Al) groupings decrease significantly. 27 Al MAS NMR spectra of H-Beta zeolites with different degrees of dealumination are shown in Figure 1. The spectrum

eolites are essentially important microporous catalysts widely applied in petro- and fine-chemical industry, biomass conversion, and environmental catalysis.1−3 The catalytic performances of zeolites are strongly dependent on the acid properties and pore structures. With respect to the acid properties, the distribution, concentration, and strength of Brönsted acid sites have great impact on the activity, selectivity, and hydrothermal stability of zeolites.4−6 In fact, the properties of Brönsted acid sites are directly determined by the Al distributions in zeolites. The location of Al in the zeolite lattice impacts the confined environment of the acid site and varies the bond angles of the tetrahedral coordination of Al, leading to a subtle difference in Brö nsted acid strength.7 Therefore, elucidation of the Al distribution and the corresponding Brönsted acid properties is of great significance to make the acid properties engineering tunable both from zeolite synthesis and postmodification by controlling Al distributions. Many characterizations such as neutron diffraction, infrared spectroscopy (IR), extended X-ray absorption spectroscopy (EXAFS), X-ray standing wave (XSW), and solid-state NMR (ssNMR), combined with theoretical calculations, have been carried out to probe the Brönsted acidity or Al distributions in zeolites.8−15 However, most of them are focused solely on the Al or Brönsted acid distributions, and very few studies directly elucidate the relationship between the specific Al distribution and the corresponding distinct acid properties of Brönsted site. 1D and 2D solid-state NMR including 27Al MAS and MQ MAS NMR is a powerful tool to explore the coordination structures and specific T-sites in H-Beta zeolitic frameworks.16 Combined with DFT calculations, 10 distinct framework Al species were identified in ZSM-5 zeolites by 27Al MQ MAS NMR.17 After © XXXX American Chemical Society

Received: March 24, 2017 Accepted: May 10, 2017 Published: May 10, 2017 2323

DOI: 10.1021/acs.jpclett.7b00711 J. Phys. Chem. Lett. 2017, 8, 2323−2327

Letter

The Journal of Physical Chemistry Letters

Figure 1. 27Al MAS NMR spectra of H-Beta zeolites with different Si/Al ratios: (a) H-Beta-7, (b) H-Beta-22, and (c) H-Beta-36. (d) 27Al MQ MAS NMR spectrum of H-Beta-7.

for AlSiAl sites in as-prepared Na-type Al-rich Beta zeolite, but these Al sites are very susceptible to dealumination due to its low stability when exchanged into H-type.14 This prediction can be further confirmed when Na-type Al-rich Beta zeolite is exchanged into H-type; an appreciable amount of extraframework Al at 0 ppm appears in H-Beta (Si/Al = 7) (Figure 1a), which may result from the dealumination of Si(2Al) at 5MR1-T17 and 5MR2-T91 sites. 31 P MAS NMR spectra (Figure 2) of TMPO adsorbed in HBeta zeolites show significant changes in the line shape after

of H-Beta (Si/Al = 7) zeolite (Figure 1a) shows the main peak of four-coordinated framework and some six-coordinated extraframework Al at 59 and 0 ppm,23 respectively. After dealumination, the main peak shifts from 59 to 55 ppm (Figure 1b,c). 27Al MQ MAS NMR (Figure 1d) shows that there is a minor shoulder at 55 ppm in F1 dimension of H-Beta (Si/Al = 7), which means the broad peak at 59 ppm consists of at least two peaks. The isotropic chemical shifts (δiso) obtained from 27Al MQ MAS NMR are 60 and 57 ppm for the main peak and broad shoulder, respectively. 27Al MQ MAS NMR spectra of dealuminated high-silica H-Beta (Si/Al = 22, 36) (Figure S3a,b) clearly indicate that all of the zeolites contain these two peaks with isotropic chemical shift (δiso) at ca. 60 and 57 ppm. Therefore, 27 Al MAS NMR spectra can be deconvoluted by two peaks with δiso at 60 and 57 ppm, and the relative amounts are listed in Table S1. The results indicate that 78% framework Al locates at the T sites with 27Al δiso: 60 ppm for Al-rich Beta zeolite. After dealumination, 22% framework Al locates at δiso: 60 ppm for H-Beta (Si/Al = 22), while for H-Beta (Si/Al = 36) with more severe dealumination, only 10% total framework Al remains at δiso: 60 ppm. It means the selective dealumination of δiso: 60 ppm Al occurs in Al-rich H-Beta zeolite. The 27Al isotropic chemical shift (δiso) of each probable T site for both Al-rich and high-silica H-Beta zeolites was calculated using DFT cluster models, and the results are listed in Table 2 and Table S2, respectively. Even though nine inequivalent T sites exist in Beta zeolite (Figure S4), our theoretical calculations indicate that the most probable 1Al distributions in each unite cell are T1 (57 ppm), T9 (60 ppm), T5 (54 ppm), T6 (54 ppm), and T7 (62 ppm) sites. However, the amount of T7 site could be small because of its relatively higher Al substitution energy, as shown in our previous study.14 The most probable 2Al distributions are located in 5MR1-T92 (61, 56 ppm) and 5MR2-T15 (60, 58 ppm) for AlSiAl sites and 6MR1-T66 (54, 54 ppm) for AlSiSiAl sites.14 The DFTcalculated chemical shifts of these Al sites are near 55 and 60 ppm, which is in good agreement with the above 27Al MQ MAS NMR experimental results. It is worth noting that the 5MR1T17 and 5MR2-T91 sites are also very probable Al distributions

Figure 2. Deconvoluted 31P MAS NMR spectra of TMPO adsorbed in H-Beta zeolites with varied Si/Al ratios: (a) H-Beta-7, (b) H-Beta-22, and (c) H-Beta-36. The peak intensities are normalized.

dealumination. All spectra can be deconvoluted by the isotropic peaks at ca. 81−85, 67−71, 65−66, 55−64, 49−50, and 42 ppm, respectively, and the results are listed in Table 1. The 31P NMR chemical shifts ranging from 49 to 85 ppm are ascribed to chemically adsorbed TMPO complexes (TMPO molecules interact with Brönsted acid sites or Lewis acid sites). The signals at 42 ppm are attributed to the physisorbed TMPO.24 The peaks at 65−66 and 49−50 ppm are attributed to TMPO adsorbed on Lewis acid sites, while those at 81−85, 67−71, and 55−64 ppm are attributed to TMPO adsorbed on three types 2324


Letter


Table 1. Deconvoluted 31P Spectra with Corresponding Acid Contents of TMPO Adsorbed in H-Beta Zeolites with Varied Si/Al Ratios 31

samples H-Beta-7 H-Beta-22 H-Beta-36 a

a

81 (87) 84 (24) 85 (64)

P chemical shift/ppm (amount/μmol·g−1)

67 (710) 69 (270) 71 (130)

65 (70) 65 (122) 66 (79)

56 (159) 62 (110) 64 (85)

49 (40) 50 (63) 50 (27)

55 (44) 56 (10)

Outside the parentheses is 31P chemical shift; inside the parentheses is acid amount

of Brönsted acid sites.24,25 These discriminative assignments can be confirmed by the selective poissonation of Brönsted acid site by 2,6-dimethylquinoline (2,6-DMQ), whereas it will not affect the signal intensities of TMPO adsorbed on Lewis acid sites significantly (Figure S5).26 After dealumination, the chemical shifts of 31P peaks belonging to TMPO adsorbed on each type of Brönsted acid sites all move to the lower field. This means the acid strength of all Brönsted acid sites increases after dealumination.20 In general, 31P chemical shifts of adsorbed TMPO above 80 ppm are attributed to the relatively strong Brönsted acid sites or the confinement effect in 10-membered ring zeolite, which is not commonly observed in conventional H-Beta zeolite.20 To determine whether they are originated from the specific Al, H-Beta (Si/Al = 36) zeolite with significant amount of 85 ppm signals was treated with dilute hydrochloride (HCl) solution to eliminate the effect of extraframework Al. The results show that after acid treatment the extra-framework Al decreases significantly, and at the same time the signal at 85 ppm almost diminishes completely (Figure S6), while there is no change observed for the 64 and 71 ppm signals. Moreover, this signal also diminishes after 2,6-DMQ adsorption (Figure S5). Therefore, the signals above 80 ppm can be associated with both extra-framework Al (Lewis acid) and Brönsted acid sites and could be assigned to the strongest Brönsted acid sites due to the synergic effect between Lewis and Brönsted sites.27 The remaining 31P signals at 67−71 and 55−64 ppm are attributed to Brönsted sites with different acid strengths. The quantitative results from Table 1 demonstrate that the amount of Brönsted acid sites at 67−71 ppm decreases significantly from 710 to 270 μmol/g, while no obvious decrease from 159 to 154 μmol/g is observed at 55−62 ppm after first dealumination to Si/Al = 22. With more severe dealumination, the amount of Brönsted acid sites at both 67− 71 and 56−64 ppm decreases around half from 270 to 130 and 154 to 95 μmol/g, respectively. To get more detailed explanations of the acid strength changes associating with Al distributions during dealumination process, DFT calculations of 31P chemical shifts of adsorbed TMPO at specific acid sites were performed. The influence of Lewis acid sites was excluded, and only the Brönsted acid sites related to 55−64 and 67−71 ppm were considered. The optimized models for TMPO adsorbed on different T-sites and their structure parameters are shown in Figure 3 and Table S3, respectively. The calculated chemical shifts of 31P NMR are listed in Table 2. It is worth noting that only one TMPO molecule can be adsorbed on 6MR-2Al(T6,T6) due to the steric hindrance though two Brönsted acid sites (Figure 3c, Figure S7). The results clearly indicate that Al distributions at different T-sites show significant different 31P chemical shifts of adsorbed TMPO. Brönsted acid sites with 31P chemical shift of adsorbed TMPO at 55−64 ppm may result from 5MR1− 2Al(T9,T2), 6MR-2Al(T6,T6), and T6 sites. 5MR1-2Al(T9,T2) sites could be also excluded from having a significant amount due to the obvious deviation of 31P chemical shift at T9

Figure 3. Optimized models of TMPO adsorbed on different T sites: (a) T1, (b) T9, (c) 6MR-2Al(T6,T6), and (d) 5MR2-2Al(T1,T5). Al, pink; O, red; P, purple; C, black; H, white.

Table 2. Calculated Framework 27Al NMR Shielding Constants and Chemical Shifts and 31P NMR Shielding Constants and Chemical Shifts of TMPO Adsorbed on Potential T Sites

T site 5MR1-2Al 5MR2-2Al 6MR-2Al 1Al

T9 T2 T1 T5 T6 T6 T1 T5 T6 T9

27 Al shielding constants (ppm)

27 Al chemical shifts (ppm)

31 P shielding constants(ppm)

31 P chemical shifts (ppm)

499.2 503.7 500.1 502.2 506.0 506.0 502.8 506.2 506.6 500.1

61 56 60 58 54 54 57 54 54 60

255.9 234.9 233.9 229.8 251.0 248.9 231.7 234.5 238.3 233.9

46 68 69 72 52 54 71 68 64 69

site from the experimental value. Therefore, Al at 6MR(T6,T6) and T6 sites could contribute to the Brönsted acid sites adsorbed TMPO at 55−64 ppm. Although Al atoms at 6MR(T6,T6) site could be relatively stable, as demonstrated by our previous study14 and other groups,28 the dealumination process is rather complicated, many thermodynamic or kinetic factors will affect it significantly depending on the treating conditions. In our case, after first dealumination the amount of Brönsted acid sites at 55−62 ppm has nearly no change probed by one TMPO molecule on 6MR-2Al(T6,T6). This means one Al atom could be removed from 6MR-2Al(T6,T6) to form T6 site. Theoretically, the remaining T6 site will adsorb TMPO with 2325



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31

P chemical shift at 64 ppm (Table 2), which agrees with the experimental results that 31P chemical shifts of TMPO are 62− 64 ppm after dealumination (Figure 2). Therefore, Brönsted acid sites at 55−56 ppm should be associated with 6MR-2Al (T6,T6). As for the Brönsted acid sites at 67−71 ppm, they may be attributed to 5MR2-2Al(T1,T5), T1, T9, and T5 sites. The contribution of T5 site can be very small because in asprepared Na-Type Beta this T site has relatively high substitution energy.14 The pronounced decrease in 27Al δiso at 60 ppm is responsible for the decrease in these Brönsted acid sites, while Al at T1 site has δiso at 57 ppm in 27Al MAS NMR spectra and T9 site is relatively very stable. Therefore, the significant dealumination should occur at 5MR2-2Al(T1,T5) sites. In Al-rich H-Beta zeolite (Si/Al = 7), most Al at T5 site in 5MR2−2Al(T1,T5) will first be removed from the framework, and 5MR2-2Al(T1,T5) will be changed to T1 site with the computed 27Al chemical shift at 57 ppm well consisting of the observed 27Al δiso at 57 ppm. As expected from 29Si MAS NMR, the amount of 2Al can take up as much as 58% of the total framework Al in Al-rich H-Beta zeolite (Si/Al = 7). So, after first dealumination to Si/Al = 22, the corresponding Brönsted acid sites could be decreased to less than half from 710 to 270 μmol/g (Table 1). Considering that even if significant dealumination occurs the amount of residual Brönsted acid sites with adsorbed TMPO at 71 ppm (130 μmol/g) is still more than that at 64 ppm (85 μmol/g), then it is only reasonable that T1 site with 27Al δiso at 57 ppm has the major Al distribution in high-silica H-Beta. Although Al at T9 site (δiso at 60 ppm in 27Al NMR) is very stable, it is only about 10−22% of the total Al for the dealuminated H-Beta (Table S1) and may have little contribution to the Brönsted acid sites of Al-rich and high-silica H-Beta zeolites. The relatively small low-field shift of 31 P chemical shift of adsorbed TMPO after dealumination may be due to the relatively narrow chemical shift range (68−71 ppm) of different sites. The main contribution of Al at T1 site can also be confirmed by the very good agreement of computed 31 P chemical shift at 71 ppm with the experimental value at 71 ppm. In conclusion, the relationship between Al distributions and Brönsted acid sites of a series of H-Beta zeolites derived from dealumination of Al-rich Beta zeolite is elucidated by a combination of 27Al, 31P MAS NMR and theoretical calculation study. All of the zeolites contain three types of Brönsted acid sites: (1) the strongest Brönsted acid sites with 31P chemical shift of TMPO above 80 ppm arise from the synergic interaction between Lewis and Brö nsted sites; (2) the medium-strength Brönsted acid sites with 31P chemical shift of adsorbed TMPO at 67−71 ppm mainly result from Al at T1 site (δiso: 57 ppm in 27Al NMR) for dealuminated high-silica HBeta zeolite and from Al at T1 (δiso: 57 ppm in 27Al NMR), 5MR2-2Al(T1,T5) sites (δiso: 60 ppm in 27Al NMR) for Al-rich H-Beta zeolite; (3) the weakest Brönsted acid sites at 56−64 ppm originate from Al at 6MR-2Al(T6,T6) sites (δiso: 57 ppm in 27Al NMR) for Al-rich H-Beta and from Al at T6 site (δiso: 57 ppm in 27Al NMR) for dealuminated high-silica H-Beta zeolite. The removal of one framework Al in 5MR2-2Al and 6MR-2Al sites leads to increasing the Brönsted acid strength of dealuminated H-Beta. Our findings on the exact correlation between specific Al distributions and corresponding Brönsted acid sites may guide the control of Al distribution to get desired acid properties through zeolite synthesis or finely tuned dealumination.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b00711. Experimental details of the preparations of H-Beta zeolites with different Si/Al ratios. TMPO adsorption of H-Beta zeolites and theoretical calculations. Additional characterizations including XRD, 29Si MAS NMR, 27Al MAS/MQ MAS NMR, and structure parameters of 27Al and 31P chemical-shift calculation models. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.Z.). *E-mail: [email protected] (W.Z.). ORCID

Weiping Zhang: 0000-0003-1227-9769 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial supports from the National Natural Science Foundation of China (Nos. 21373035, 21673027, 21603022). We also thank Dr. Mathias Feyen and Dr. Ulrich Mueller of BASF SE, Germany for providing H-Beta zeolite samples.

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Insights into the Correlation of Aluminum Distribution and BrÃ¶nsted

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