The 31P MAS NMR spectra of phosphorus-modified chabazite (P-CHA) zeolites have been observed during the hydrothermal treatment to probe the structural changes of phosphorus species in zeolites. Characteristic changes of the spectra were observed in the range of 27 ~ 42 ppm, which correlates to the hydrothermal structure changes in P-CHA zeolite
Microporous and Mesoporous Materials 294 (2020) 109908 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso Theoretical study on CHA zeolites 31 P NMR chemical shifts of phosphorus-modified Pei Zhao a, Bundet Boekfa b, Toshiki Nishitoba c, Nao Tsunoji d, Tsuneji Sano d, Toshiyuki Yokoi c, Masaru Ogura e, Masahiro Ehara a, f, * a Research Center for Computational Science, Institute for Molecular Science, Okazaki, 444-8585, Japan Department of Chemistry, Faculty of Liberal Arts and Science, Kasetsart University, Kamphaengsaen Campus, Nakhonpathom, 73140, Thailand Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa, 226-8503, Japan d Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8527, Japan e Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo, 153-8505, Japan f Element Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto, 615-8245, Japan b c A R T I C L E I N F O A B S T R A C T Keywords: P-CHA zeolite 31 P NMR chemical shift Density functional theory calculation The 31P MAS NMR spectra of phosphorus-modified chabazite (P-CHA) zeolites have been observed during the hydrothermal treatment to probe the structural changes of phosphorus species in zeolites Characteristic changes of the spectra were observed in the range of À 27 ~ À 42 ppm, which correlates to the hydrothermal structure changes in P-CHA zeolites Theoretical calculations on the 31P and 27Al NMR chemical shifts have been sys tematically performed to disclose the possible phosphorus species of intra- and extra-framework and the struc tural changes during the hydrothermal treatment The shift of the 31P resonances toward higher field were identified for condensed phosphates and aluminophosphates Based on the calculated 31P NMR chemical shifts, the peak with increased intensity at À 42 ppm in the initial stage of the hydrothermal treatment is mainly assigned to the formation of siliconoaluminophosphate (SAPO) species in the 6-membered ring of the zeolite framework, while the peak with increased intensity at À 29 ppm in the later stage of treatment is ascribed to the accumulation of the extra-framework condensed phosphate or aluminophosphate species after the partial framework decomposition The dominant peak at À 33 ppm in all 31P NMR spectra is assigned to the phosphates in the framework Introduction Zeolites are inorganic microporous crystalline materials composed by SiO4 and AlO4 tetrahedra that link to form channels and cavities of molecular dimensions [1], which are important heterogeneous catalysts widely used in the modern chemical and petrochemical industries [2–4] The substitution of tetrahedrally coordinated framework Si atoms with the trivalent aluminum introduces bridging hydroxyl groups in the or dered structure, which significantly affect the catalytic activity The modification of zeolites with phosphorus is a widely adopted method to tune the acidity and consequently the catalytic properties in terms of activity, shape selectivity, and hydrothermal stability [5,6] Two intriguing impacts of phosphorus introduction in zeolites have been demonstrated (1) The interaction between phosphorus and the zeolite framework can effectively stabilize tetrahedrally coordinated framework aluminum, presenting enhanced stability against deal umination in the presence of steam [7,8] (2) The Brønsted acidity is reduced upon the introduction of phosphorus species [9], which can be ascribed to the bonding interaction of phosphorous with bridging hy droxyl groups and phosphorus-induced dealumination of tetrahedrally coordinated lattice aluminum together with the simultaneous formation of water insoluble amorphous extra-framework aluminum phosphates [8,10] In particular, regarding selective catalytic reduction (SCR) of NOx with NH3, it was found that the phosphorus-modified CHA (P-CHA) zeolites show high hydrothermal stability The systematic examinations of wide range of P-CHA zeolites have been performed [11–13] P-CHA could exhibit NO purification ability in the SCR process even after severe hydrothermal treatment at 900 � C As in general, the role of phosphorus species in zeolites has been intensively analyzed by various * Corresponding author Research Center for Computational Science, Institute for Molecular Science, Okazaki, 444-8585, Japan E-mail address: ehara@ims.ac.jp (M Ehara) https://doi.org/10.1016/j.micromeso.2019.109908 Received 24 September 2019; Received in revised form 29 October 2019; Accepted 18 November 2019 Available online 21 November 2019 1387-1811/© 2019 The Authors Published by Elsevier Inc This is an open access (http://creativecommons.org/licenses/by-nc-nd/4.0/) article under the CC BY-NC-ND license P Zhao et al Microporous and Mesoporous Materials 294 (2020) 109908 spectroscopic measurement including 31P Magic Angle Spinning (MAS) NMR However, the origin of the above advantages has still not been clear partly because the fundamental theoretical characterization or identification of the 31P NMR chemical shift of P-CHA zeolites has not been satisfactory Up to now, the phosphorus modification has been extensively stud ied on H-ZSM-5 stemmed from a well-ordered MFI type framework [9] Based on various spectroscopy information especially 27Al, 29Si, and 31P MAS NMR, several models have been proposed to understand the interaction of the ZSM-5 zeolite framework with phosphorus compounds and the species generated by this modification [7,14–16] However, the preferred chemical models during the severe hydrothermal treatment are still elusive due to the absence of compelling evidences Meanwhile, the difference in experimental conditions during and after the intro duction of phosphorus also makes the interpretation of data and spectra ambiguous Thus, theoretical studies play a significant role in disclosing the feasible species However, systematic studies of the 31P NMR spectra regarding the phosphorus-modified zeolites have not been performed so far Note that zeolitic materials based on the chabazite topology have high symmetry and only one symmetry-distinct tetrahedral site in the framework [17], which is an advantage of theoretical calculations Herein, the 31P MAS NMR spectra of P-CHA zeolites under the hy drothermal treatment have been observed The 31P MAS NMR is sensi tive to phosphorus species involved and therefore, is an effective method to simulate or probe its structural change To give the identification of the phosphorus species and to disclose the structural changes of the PCHA zeolites during hydrothermal treatment, we performed systematic theoretical calculations on the 31P and 27Al NMR chemical shifts Different phosphates and aluminophosphates were first considered to evaluate the 31P NMR chemical shifts of different types of phosphorus species The 31P and 27Al NMR chemical shifts of possible chemical structures for the phosphorus-zeolite interaction system were studied to uncover the preferred species during the hydrothermal treatment of the P-CHA zeolite NMR chemical shifts were referenced to 1.6 ppm, ammonium dihy drogen phosphate, respectively, and samples were spun at 15 kHz by using a mm ZrO2 rotor For 31P MAS NMR spectra, which were recorded by using a single pulse, the pulse width was set at 4.25 μs and 1000 scans were accumulated at a sample spinning rate of 15 kHz A s relaxation delay was determined so as to be long enough to permit quantitative analysis of zeolite samples Computational details For the identification of 31P NMR spectrum of P-CHA zeolites under the hydrothermal treatment, 31P NMR chemical shifts of both extra- and intra-framework phosphorus species as well as 27Al NMR chemical shifts have been systematically studied by the density functional theory (DFT) calculations For the extra-framework phosphorus species, phosphates and aluminophosphates were examined Regarding the phosphateframework interacting systems, both monophosphates and di phosphates bound to P-CHA were considered The 42T quantum cluster where T stands for the tetrahedral Si or Al atoms was used to represent the H-CHA zeolite The quantum cluster was adopted from the crystallographic structure of CHA zeolite [18] The chemical formula of the 42T cluster can be expressed as (Al2Si40O66H2)H36 (H2: Brønsted protons; H36: terminal hydrogen atoms) The model was carefully treated to represent the effect of the Brønsted acid at the six-membered ring (6-MR) and the framework effect inside the cavity where the phosphorus species dominate Two Si atoms at the 6-MR are replaced by two Al atoms to generate the proton Brønsted acid site by adding a proton to the O atom which is adjacent to €wenstein’s rule [19], i.e., the each Al atom According to the Lo –Al–O–Al– bond formation is forbidden, two different positions at the 6-MR were considered to place two Al atoms: 3NN (third nearest neighbor Al site, Al–O–Si–O–Si–O–Al) and 2NN (second nearest neighbor Al site, Al–O–Si–O–Al) [20] Meanwhile, two Brønsted protons were attached to the bridging O atom of 6-MR and 8-MR to locate their favorable position (Table S4) At the boundary, the terminal Si–H bonds were treated at the bond length of 1.47 Å with the same direction of Si–O bond from the crystallographic data Only the terminal hydrogen atoms were kept fixed with X-ray structure while other atoms were allowed to relax The adsorption or reaction energy (ΔEr) was calculated as the energy difference between the considered chemical structure (Ephosphorus-zeolite) and the sum of the H3PO4 (Ephosphorus) and water (Ewater) molecules as well as single zeolite (Ezeolite), Er ẳ Ephosphorus-zeolite - (Ephosphorus ỵ Ewater ỵ Ezeolite) Note that the amounts of the H3PO4 and water mole cules depend on the chemical structure under consideration The M06-L functional [21] with the basis set of 6-31G(d,p) was utilized in the geometry optimizations, and vibrational frequency ana lyses were also conducted at the same level of theory to confirm the considered structures to be local minima The series of M06 functional can well describe the electrostatic and van der Waals interaction [22] Particularly, the M06-L functional has improved performance for calculating NMR chemical shielding constants [23] NMR chemical shielding values were evaluated employing the gauge-invariant atomic orbital (GIAO) method [24] at the optimized geometry The calculated 31 P and 27Al chemical shifts in the considered structures are referenced to H3PO4 and Al(NO3)3, respectively, which are often used as reference molecules [16,25,26] The following formula is used to obtain the calculated NMR chemical shift (δ): Experimental details Phosphorus-modified CHA zeolite was prepared according to the previous report [13] Sodium hydroxide, dealuminated FAU zeolite, N, N,N-trimethyl-1-adamantammonium hydroxide (TMAda), tetraethyl phosphonium hydroxide (TEP), and distilled water were mixed to obtain a gel with the molar composition 1:0.0625:0.25–0.03:0.05–0.27:0.1:7.5 of Si/Al/TMAda/TEP/NaOH/H2O The resulting gel was transferred into a 100 cm3 Teflon-lined stainless-steel autoclave (stirring-type hy drothermal synthesis reactor, R-100, Hiro Company, Japan) and heated at 150 � C for seven days with tumbling at 10 rpm After crystallization, the solid product was collected by centrifugation, washed thoroughly with distilled water until it was almost neutral, and then dried overnight at 70 � C To remove organic molecules, the obtained as-synthesized P-CHA zeolite was calcined in air at 600 � C for 10 h Phosphorus-modification degree (P/Al) was controlled by tuning the amount of TMAda and TEP in the synthesis gel The obtained Na-form P-CHA sample was converted to NH4-form by ion-exchange method; g of Na-form P-CHA sample was stirred in 100 ml of 1.0 M NH4NO3 aqueous solution at 60 � C for h The treatment was repeated three times Then, Copper ions were introduced into the NH4-form P-CHA by ion-exchanged method; g of the NH4-form P-CHA was stirred in 100 mL of 0.012 M Cu(CH3COO)2 aqueous solution at room temperature for 24 h The sample was filtered, washed with distilled water, dried over night at 100 � C Thus, obtained Cu ion exchanged P modified CHA-type zeolite was denoted as “Cu/P-CHA” The Si/Al, P/Al and Cu/Al atomic ratios of Cu/P-CHA were found to be 11, 0.20 and 0.25, respectively, which were estimated by inductively coupled plasma-atomic emission spectrometer (ICP-AES, Shimadzu ICPE-9000) The high-resolution 31P MAS NMR spectra were obtained on a JEOL ECA-600 spectrometer (14.1 T) equipped with an additional kW power amplifier The 31P δ ¼ σref - σ where σref is the calculated chemical shielding of the P or Al atom in H3PO4 or Al(NO3)3, and σ is the calculated chemical shielding of the atom under consideration The functional dependence was also examined using the B3LYP functional [27] with the basis set of 6-31G(d,p) All DFT calculations P Zhao et al Microporous and Mesoporous Materials 294 (2020) 109908 of the framework of CHA is decomposed after the 15-h treatment (Fig S1) These spectroscopic signatures provide valuable information for the interactions between phosphorus species and the zeolite framework Subsequently, in this work, theoretical studies on 31P NMR chemical shifts have been carried out to disclose the possible formed species for the P-CHA zeolites The phosphorus sites are denoted as QPnm , where n and m represent the number of P–O–P and P–O–Al connectivity, respectively, with given tetrahedrally coordinated phosphorus 4.2 Phosphate species Previous studies on the 31P NMR chemical shifts of orthophosphates, short chain polyphosphates and other condensed phosphates suggested that QP00 resonances are observed at � ppm, QP10 at À � 10 ppm, QP20 at À 17 � 10 ppm and QP30 at À 40 � ppm [29–32], presenting considerable overlaps in chemical shifts caused by different types of phosphorous In order to improve spectral assignments, various phos phates including linear and branched polyphosphates as well as cyclic phosphates as shown in Fig were first considered to evaluate their 31P NMR chemical shifts As shown in Table 1, the calculated 31P NMR chemical shift of the end groups (QP10 ) in pyrophosphate H4P2O7 are around À ppm, while the chemical shift of the middle group (QP20 ) in H5P3O10 is À 34 ppm Three configurations were considered for the polyphosphate H6P4O13: the linear one has the lowest energy and the relative energies of two Fig 31P MAS NMR spectra of the Cu ion exchanged P modified CHAtype zeolites were conducted using Gaussian 09 suite of programs version E.01 [28] Results 4.1 31P MAS NMR spectra of P-CHA zeolites under hydrothermal treatment Fig presents the 31P MAS NMR spectra of the Cu/P-CHA zeolites before and after the hydrothermal treatment, the introduction of P into the zeolite leads to several characteristic peaks around À 27, À 33, and À 40 ppm in the fresh state, accompanied by the weak shoulders of the peaks centered at À 14 and À 22 ppm Apparently, the peak around À 33 pm is dominant, presenting the highest intensity in the considered range of chemical shift Meanwhile, the peaks around À 27 and À 40 ppm also exhibit considerable intensities After the hydrothermal treatment at 900 � C, changes in the position and intensity of 31P NMR peaks can be observed, indicating the structural changes of the formed species during the hydrothermal treatment The peaks around À 33 ppm are always dominant during the 1–15 h treatment It should be noted that the peak intensity around À 42 ppm drastically increases after the 1-h treatment, which is still strong after 7-h but decreases after the 9–15 h treatment Meanwhile, the intensity of peaks around À 29 ppm increases after the 7–9 h treatment The XRD and 29Si MAS NMR spectra reveal that a part Table The 31P NMR chemical shifts (in ppm) and Mulliken charges of phosphorus in phosphates Phosphate P type NMR Charge H3PO4 QP00 0.0 1.216 2.4 1.286 QP20 a À 34.0 1.343 32.4 27.3 30.4 16.2 36.6 24.4 1.355 1.340 1.353 1.381 1.378 1.337 H4P2O7 H5P3O10 H3P3O9 QP10 QP20 L-H6P4O13 QP20 A-H6P4O13 QP30 B–H6P4O13 P4O10 QP30 QP30 a b c b c a À À À À À À a À 37.6 1.405 À 52.9 1.387 Fig Optimized structures of different phosphate species The lowercase letters (a–d) label distinct phosphorus atoms The values in parentheses represent relative energies for H6P4O13 with different configurations (in kcal⋅molÀ 1) Color code: O red; H white; P orange (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) P Zhao et al Microporous and Mesoporous Materials 294 (2020) 109908 Fig Optimized structures of monodentate aluminophosphates (six coordinate) The lowercase letters label (a–d) distinct phosphorus atoms, and the distances are in Å The values in parentheses represent relative energies in three H6P4O13–AlO3H3(H2O)2 configurations (in kcal⋅molÀ 1) Color code: O red; H white; Al blue; P orange (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) branched configurations are very close Two middle groups (QP20 ) in LH6P4O13 resonate at À 16.2 and À 36.6 ppm for b and c, respectively AH6P4O13 and B–H6P4O13 exhibit different 31P NMR chemical shifts for QP30 , that is, À 24.4 and À 37.6 ppm, indicating the chemical shift is sensitive to the difference in configuration, which may originate from the discrepancy in charge distribution Chemical shifts of the end groups in H5P3O10 and H6P4O13 fall into the range of À 2.1~-10.1 ppm (Table S1) As for the species after the intense dehydration, the QP20 atoms resonate at À 32.4, À 30.4, and À 27.3 ppm in the cyclic phosphate H3P3O9, and four QP30 atoms in P4O10 resonate at around À 53 ppm In addition, water and chloroform were taken into account to evaluate the solvent effect on chemical shifts, which was found to be negligible within the polarizable continuum model (PCM) (Table S1) Overall, the 31 P resonances are shifted towards higher field as the number of P–O–P increases, which exhibits the similar trend to previously experimental studies [26,33] However, except for one QP20 (b) in L-H6P4O13 and QP30 in P4O10, other QP20 and QP30 atoms possess rather close chemical shifts in the range of À 24 ~ À 37 ppm Based on these results, the dominant peak observed at around À 33 ppm in Fig may have some contributions from QP20 or QP30 groups, in particular after the partial decomposition of the zeolite framework in the present work Additionally, as for the considered phosphates, the P atoms with higher positive charge tend to exhibit higher-field 31P NMR chemical shifts (Table 1), indicating the charge distribution may affect the 31P chemical shift The similar trends can also be found at the B3LYP/6-31G(d,p) level of theory (Table S1) Table The 31P and 27Al chemical shifts (in ppm) and Mulliken charges of P and Al in monodentate aluminophosphates Aluminophosphate P type H3PO4–AlO3H3(H2O)2 QP01 H4P2O7–AlO3H3(H2O)2 H5P3O10–AlO3H3(H2O)2 A-H6P4O13–AlO3H3(H2O)2 B–H6P4O13–AlO3H3(H2O)2 L-H6P4O13–AlO3H3(H2O)2 31 P NMR Charge 27 Al NMR Charge 9.2 1.332 5.5 1.028 QP11 a À 1.9 1.378 2.0 1.101 QP21 a À 20.0 1.389 5.6 1.096 QP31 a À 25.8 1.432 1.5 1.090 QP31 a À 26.2 1.490 4.9 1.094 QP11 a À 2.5 1.493 8.7 1.057 QP20 b c À 16.9 À 32.1 1.380 1.340 atoms tend to migrate to the hydroxyl connected to aluminum, which leads to the formation of hydrogen bonds As shown in Table S2, except for the Al atom bound to H5P3O10 and H4P2O7, the 31P resonance dif ference between the tetrahedral and octahedral coordinate aluminum is small in other structures, while the Al atoms exhibit typical resonances for the tetrahedral or octahedral coordinate structure [7,16] This work focuses on the six-coordinate octahedral aluminum phosphates As shown in Table 2, the 31P NMR of aluminophosphates tend to shift to lower field compared to the corresponding phosphates The P atom of QP01 in H3PO4–Al(OH)2(H2O)2 resonates at 9.2 ppm The QP21 , and QP31 (B–H6P4O13–AlO3H3) species are also shifted to lower field, i.e., À 20.0 and À 26.2 ppm, respectively The QP11 in H4P2O7–AlO3H3(H2O)2, and QP31 in A-H6P4O13–AlO3H3(H2O)2 have similar chemical shifts (À 1.9 and À 25.8 ppm) to the corresponding phosphoric acid (À 1.8 and À 24.4 ppm) The QP10 chemical shifts fall into the range of 14.2 to ỵ1.7 ppm (Table S2) It is found that the linear structure L-H6P4O13–AlO3H3(H2O)2 is more unstable (22.4 kcal molÀ 1) than the branched one Note that the QP11 (-2.5 ppm) and QP20 (-32.1 ppm) chemical shifts in LH6P4O13–AlO3H3(H2O)2 are also shifted to lower field after attaching the Al atom When the hydroxyl group (P–OH) of the phosphate is connected to the Al atom, the attachment of two hydroxyl groups on the aluminum atom results in the bidentate aluminophosphate species, as shown in Fig and Fig S3 Note that the attachment of three hydroxyl groups 4.3 Aluminophosphate species Aluminophosphate species considered in the previous study [26] were assessed since the amorphous extra-framework aluminophosphate species play a significant role during framework dealumination Two types of aluminophosphates were studied via the interaction of – O) or the hydroxyl group (P–OH) aluminum with the double bond (P– Fig shows the structures with the octahedral coordinate aluminum – O bond, which results in the in the first coordination sphere via the P– monodentate aluminum phosphates Note that the corresponding structures with the tetrahedral coordinate aluminum were also consid ered (Fig S2) These optimized structures revealed that some hydrogen P Zhao et al Microporous and Mesoporous Materials 294 (2020) 109908 Fig Optimized structures of bidentate aluminophosphates The lowercase letter label (a–d) distinct phosphorus atoms Color code: O red; H white; Al blue; P orange (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) aluminophosphate species are slightly shifted to lower field compared to the corresponding monodentate structures, as shown in Table Meanwhile, it is found these species possess low-field 27Al-NMR chem ical shifts compared to other aluminumphosphate species As for bidentate aluminophosphate species containing a hexagon, the aluminum atom is connected to a hydroxyl group of one phosphate and a double bond of another phosphate The QP11 chemical shift exhibits a high-field value of À 20.1 ppm, and the QP21 resonance in the condensed phosphate is higher-field (À 27.4 ppm) Additionally, the formed hexagon-structure is much more stable than the tetragon-structure, which may play an important role in aluminophosphate species Table The 31P and 27Al chemical shifts (in ppm) and Mulliken charges of P and Al in bidentate aluminophosphates Aluminophosphate P type H2PO4–AlO2H2 (H2O)2 QP02 A-H3P2O7–AlO2H2(H2O)2 H4P3O10–AlO2H2(H2O)2 B–H3P2O7–AlO2H2(H2O)2 H5P4O13–AlO2H2(H2O)2 31 P NMR Charge 27 Al NMR Charge 13.4 1.497 14.7 0.958 QP12 a 3.0 1.517 17.5 0.986 a À 14.0 1.538 18.1 0.976 QP11 a b a b À À À À 20.1 2.3 27.4 18.2 1.392 1.339 1.448 1.405 3.5 1.042 3.2 1.069 QP22 QP21 4.4 Geometry structure of the CHA zeolites As for the CHA framework, the 3NN structure with two protons located on 6-MR and 8-MR has the lowest relative energy (Table S4), which is considered in the following calculations of P-CHA zeolite As shown in Fig 5, as for the Al/Si–O bonds without the Brønsted proton on the O atom, the distances of Al–O bonds fall into the range of 1.69–1.76 Å, while those of Si–O bonds are around 1.59–1.63 Å, which are consistent with the experimental data (1.70–1.73 Å and 1.58–1.64 Å for Al–O and Si–O bond distances, respectively) [34] Nevertheless, the Si–O and Al–O bonds in the vicinity of Brønsted acid sites are much longer, that is, 1.895 and 1.926 Å for the Al–O bonds; 1.706 and 1.721 Å for the Si–O bonds The O–H bond distances are around 0.97 Å Exper imentally, the 27Al resonance of the CHA zeolite is detected at 58 ppm [35] However, the calculated 27Al chemical shifts are 33.3 and 26.0 ppm for α and β positions, respectively, whose Mulliken charges are 1.056 e (α) and 1.109 e (β) The confinement of the finite computational model as well as the methodology errors should contribute to the observed difference between theoretical and experimental values Consequently, a constant shift of ỵ24.7 ppm is applied to the following 27 Al chemical shifts, which is a simple way to improve the calculated values Therefore, the corrected 27Al chemical shifts in the pristine framework are 58.0 and 50.7 ppm for the α and β Al atoms, respectively Fig The optimized structure (left) of the most stable structure (3NN-6R8R) and the geometrical parameters in the 6-MR (right, distances are in Å) Color code: O red; Si tan; H white; Al blue (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 4.5 Interaction of phosphorus with the Brønsted acid sites of the CHA framework leads to the open-shelled electronic structure of aluminophosphates, which is not considered in the present calculations The six-coordinate phosphates attached with two extra H2O molecules are shown in Fig It is found that the 31P NMR chemical shifts in bidentate Previous studies have proposed various structures for the in teractions between phosphates and the Brønsted acid site as illustrated P Zhao et al Microporous and Mesoporous Materials 294 (2020) 109908 influence the catalytic activity [9] The hydrothermal stability, on the other hand, is enhanced by the intra-framework phosphates [36,37] In M0 structure, the phosphate approaches the Brønsted acid site with the distance of 1.707 Å and the bridging O–H bond being slightly lengthened, indicating the formation of a hydrogen bond The calculated interaction energy (À 29.0 kcal molÀ 1) also suggests the formation of M0 is favorable The 31P NMR chemical shift is À 6.0 ppm, which is in Table Adsorption or reaction energies (ΔEr, in kcal⋅molÀ 1), 31P and 27Al chemical shifts (in ppm) and Mulliken charges in structures shown in Fig Charge 27 À 6.0 1.338 α QP00 4.6 1.359 α 1.2 QP01 À 5.7 1.445 α M2 13.8 QP01 À 17.2 1.423 α M3 38.7 QP00 Si2 À 23.5 1.512 α M4 À 36.6 QP00 7.3 1.430 α M5 À 8.8 QP01 À 31.9 1.590 α Model ΔEr M0 Scheme Chemical structures (M0-M5) of the interaction models of phos phorus with the Brønsted acid sites proposed by (M0) Abubakar et al [36], (M1) Kaeding et al [6], (M2) Lercher et al [15], (M3) Xue et al [38], (M4, 5) Blasco et al [7] in Scheme 1, and their corresponding optimized structures are shown in Fig The 31P NMR chemical shifts of these structures were investigated for the identifications of the present hydrothermal treatment and also for the reference data of the future related works The calculated reac tion energies, Mulliken charges, and NMR chemical shifts of the P and Al atoms are listed in Table It has been recognized that these formed species are essential for decreasing the zeolite acid strength and 31 P NMR À 29.0 QP00 M0-b À 35.7 M1 a β β β β β β β Al NMRa 57.4 51.9 58.1 50.6 54.1 49.4 57.0 57.0 50.4 49.6 60.7 50.2 53.2 49.2 Charge 1.073 1.103 1.055 1.107 1.073 1.104 1.086 1.116 1.103 1.110 1.056 1.098 1.082 1.107 Values corrected by 24.7 ppm Fig Optimized structures of the interaction models of phosphorus with the Brønsted acid sites The α and β represent two Al atoms Color code: O red; Si tan; H white; Al blue; P orange The terminal H atoms are omitted for clarity (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) P Zhao et al Microporous and Mesoporous Materials 294 (2020) 109908 Scheme Chemical structures (M6-M10) of the interaction models of diphosphates with the Al sites in the CHA zeolite higher-field than free H3PO4 due to the increased charge The adsorption – O group in energy is more negative (À 35.0 kcal molÀ 1) when the P– teracts the Brønsted acid site (M0-b) via a hydrogen bond, as shown in Fig S4 However, the M0-b structure exhibits a low-field 31P resonance of 4.6 ppm The chemical shifts of two Al atoms in these two structures are almost the same as those in the pristine zeolite The phosphorous can bound to framework via the formation of tetư rahydroxy phosphonium cation P(OH)4ỵ [7], M4 and M5, having different H3PO4 configurations and coordination numbers of the P atom after protonation M4 leads to the most stable structure (À 36.6 kcal molÀ 1) A recent study proposed that M4 is an effective phosphorous species for improving the framework stability in P-modified ZSM-5 ze olites [37] Meanwhile, the calculated 31P NMR chemical shift is 7.3 ppm, which suggests that M4 is not the dominant species observed here For M5, the adsorption of the protonated orthophosphoric acid results in a smaller adsorption energy of À 8.8 kcal molÀ compared to M0 and M4 It is found that the 31P resonance in M5 is significantly shifted to high field, i.e., À 31.9 ppm, which suggests this structure may have a signif icant contribution to the 31P NMR MAS spectra observed here and therefore, is a possible candidate in the initial stage of P-CHA zeolite Compared to the pristine framework, the 27Al resonances (α) in M4 and M5 are shifted to lower and higher field, respectively In M1 structure, the interaction of orthophosphate with the bridging hydroxyl leads to the dehydration of one water molecule and generates a slightly less stable structure The calculated 31P NMR chemical shift is À 5.7 ppm, which is similar to that in M0 even though the P atom in M1 is more positively charged M2 structure has the Si–O–P–O–Al structure by breaking an Al–O bond, resulting in a positive reaction energy of 13.8 kcal molÀ Note that the addition to the Brønsted acid site in 8-MR was also considered, but the geometry optimization leads to the more stable structure like M1, indicating the steric repulsion between the phosphate and the 4-MR is an advantage of the formation of M2 It is found that M2 shows high-field 31P resonance of À 17.2 ppm than M1 In M3, the P atom is connected with two oxygen atoms by breaking two Al–O bonds, and the dehydration caused by the orthophosphate and two Brønsted acid hydrogen atoms leads to a less stable structure with the energy of 38.7 kcal molÀ The corresponding 31P resonance of À 23.5 ppm is shifted higher field compared to M2, which is also a promising species in P-CHA in terms of the chemical shift Overall, the high-field 31P NMR chemical shift was found in M5 (À 31.9 ppm), followed by M3 (À 23.5 ppm) and M2 (À 17.2 ppm) among the considered models M0, M4, and M5 described the interactions of phosphate with the framework via hydrogen bonds, which result in negative adsorption energies that may be indicative of the formation upon the impregnation of phosphoric acid In contrast, M1, M2, and M3 were unstable because of the dehydration or breakage of the Si–O–Al bond, which should be difficult to revert by single cationic exchange or hot-water washing [7] Compared to the pristine framework, the small changes in the 27Al chemical shift also reveal that the interaction be tween phosphate groups and framework has a minor effect on the 27Al resonances To further disclose the 31P NMR chemical shifts of condensed phos phates in the zeolite framework, the pyrophosphoric acid was also entrained in the mentioned models (Fig S5 and Table S5) When the pyrophosphoric acid is attached, all the structures M00 -M50 except for M30 exhibit more favorable reaction energies than orthophosphoric acid, especially for M5’ (À 20.3 kcal molÀ 1), indicating there is a high possi bility to trap pyrophosphate in the P-modified zeolites As for M30 , the reaction energy (39.1 kcal molÀ 1) is comparable to that for orthophos phate Importantly, the 31P NMR chemical shifts are shifted to higher field, and the shifted values depend on the models The chemical shift in M50 is shifted to À 54.0 ppm from À 31.9 ppm, showing the largest change M30 exhibits the smallest change of 4.6 ppm The shifted values for other models fall into the range of 7.1–13.5 ppm The 31P NMR chemical shifts labeled as a are À 19.5, À 54.0, À 26.7, and À 28.1 ppm for M00 , M50 , M20 , and M30 , respectively The end group (b) exhibits the relatively high-field resonance in M4’ (À 19.2 ppm), M1’ (À 16.3 ppm), and M2’ (À 13.6 ppm) 4.6 Interaction of phosphorus with the aluminum atom in the CHA framework Previous studies proposed that the presence of phosphorous around the tetrahedral Al atom in the framework would lead to the distorted P Zhao et al Microporous and Mesoporous Materials 294 (2020) 109908 Fig Optimized structures of the interaction models of diphosphates with the Al sites The α and β represent two Al atoms Color code: O red; Si tan; H white; Al blue; P orange The terminal H atoms are omitted for clarity (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) phosphates were considered to construct the intra-framework structures bound to framework Al atom (Scheme and Fig 7) [26] As shown in Table 5, the structures with phosphates bound to the framework tetrahedral aluminum have large reaction energies in the range of -32.8 ~ À 68.5 kcal molÀ 1, indicating that these intraframework structures of M6-M10 are thermodynamically favorable Note that M7 leads to the largest reaction energy of À 68.5 kcal molÀ 1, followed by M8 with a reaction energy of À 58.7 kcal molÀ The results also further prove that the P-CHA zeolite frameworks prefer to be sta bilized by expelling the framework Al – O bond of diphosphate and two monophosphates at When the P– tacks the Al atom in the framework, respective M6 and M7 structures are generated The shortest Al–O distances between phosphate and the Al atom are 1.946 and 1.904 Å for M6 and M7, respectively Note that the other H3PO4 molecule in M7 has a longer distance with the framework tetrahedral aluminum (3.550 Å), which should be ascribed to the steric repulsion It is found that the QP11 atom in M6 and the QP01 atom in M7 have higher-field chemical shifts (À 11.5 and À 2.1 ppm) compared to free H4P2O7–AlO3H3 (7.2 ppm) and H3PO4–AlO3H3 (8.1 ppm) M9 and M10 represent two possible structures that the framework aluminum is attacked by the hydroxyl group of phosphates Reaction energies of M9 and M10 are À 37.7 and À 32.8 kcal molÀ 1, respectively, indicating the former one is slightly stable In the case of M10, there is a – O bond and the other bridging hydroxyl hydrogen bond between the P– The QP2 chemical shift in M9 is 11.8 ppm, which is comparable with the isolated one (10.1 ppm) The QP11 and QP10 chemical shifts in M10 are in Table Absorption or reaction energies (ΔEr, in kcal⋅molÀ 1), 31P and 27Al NMR chemical shifts (in ppm) and Mulliken charges in structures shown in Fig Model ΔEr M6 À 46.9 M7 M8 M9 M10 a À 68.5 À 58.7 À 37.7 À 32.8 31 P NMR Charge 27 Al NMRa Charge a À 11.5 1.363 α 57.9 1.076 QP10 b À 5.8 1.470 β 44.1 1.084 a À 2.1 1.512 α 55.7 1.079 QP00 b 6.8 1.306 β 42.5 1.103 a À 16.9 1.455 α 57.4 1.063 QP11 b À 7.5 1.444 β 36.2 1.191 a 11.8 1.530 α 56.0 1.095 QP10 b À 7.2 1.345 β 45.8 1.078 a À 18.8 1.420 α 55.9 1.067 QP10 b À 16.3 1.405 β 52.6 1.157 QP11 QP01 QP11 QP12 QP11 Values corrected by 24.7 ppm O–Al–O structures, which was caused by the hydrolysis of the tetrahe dral Al atoms that can be facilitated by acidic conditions [10,26] Meanwhile, it was pointed out that the formation of Al–O–P bonds may take place within the framework in a manner similar to the onset of dealumination process during steaming, leading to the formation of the local SAPO interfaces [25,26] Eventually, the process can result in the formation of amorphous extra-framework aluminophosphate species discussed above [26] In this work, monodentate and bidentate of P Zhao et al Microporous and Mesoporous Materials 294 (2020) 109908 Fig Optimized structures of phosphorus-framework interaction based on M8 (A) dealumination caused by diphosphates; (B) phosphates containing three P atoms; (C) phosphates containing four P atoms Color code: O red; Si tan; H white; Al blue; P orange The terminal H atoms are omitted for clarity (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table Reaction energies (ΔEr, in kcal⋅molÀ 1), 31P and 27Al NMR chemical shifts (in ppm) and Mulliken charges in structures shown in Fig Structure ΔEr A À 59.3 B C a À 64.9 À 65.7 31 P NMR Charge 27 Al NMRa Charge a À 21.8 1.454 α 56.5 1.071 QP11 b À 5.8 1.427 β 64.0 1.135 a À 27.8 1.489 α 57.8 1.059 QP11 b À 12.0 1.453 β 36.6 1.181 QP10 c À 15.2 1.389 QP21 a À 33.0 1.492 α 55.8 1.063 QP11 b À 3.9 1.443 QP20 c À 34.8 1.462 β 35.3 1.204 QP10 d À 4.2 1.336 QP11 QP21 Values corrected by 24.7 ppm slightly higher-field than the isolated structure (À 14.4 and À 13.9 ppm, Fig S6) – O group simultaneously attack the Al One hydroxy group and one P– atom, resulting in the formation of a hexagon as shown in M8 Compared to resonances of the isolated structure, B–H3P2O7–AlO2H2 (À 22.1 and 3.1 ppm for a and b), one QP11 (a: -16.9 ppm) is slightly low-field, while the other QP11 (b: -7.5 ppm) is high-field Fig 8(A) shows the structure of M8 after expelling the framework Al atom, in which the QP11 (a, À 21.8 ppm) is close to the corresponding value in the isolated structure, and the Q P11 (b, À 5.8 ppm) is close to the corresponding value in M8 Due to the high-field 31P NMR chemical shift in M8 as well as the large reaction energy, the condensed phosphates in the framework based on M8 are also studied as shown in Fig As summarized in Table 6, the chemical shift of the P atom (a) is further shifted to the lower chemical shifts of À 27.8 ppm (B) and À 33.0 ppm (C) In addition, it is found that the 27Al-NMR chemical shifts (β) in M6-9 are shifted to higher field due to the distorted tetrahedral aluminum in the framework [39] Particularly, M8 exhibits the smallest 27Al reso nance (36.2 ppm), and chemical shifts of other models are in the order of M7 (42.5 ppm) < M6 (44.1 ppm) < M9 (45.8 ppm) Interestingly, the corresponding 27Al NMR chemical shift in M10 is slightly shifted to lower field (52.6 ppm), which may suggest a small distortion for the Al atom Fig Optimized structures of the SAPO species based on the 42 T structure Color code: O red; Si tan; H white; Al blue; P orange The terminal H atoms are omitted for clarity (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 4.7 Silicoaluminophosphate (SAPO) species in the CHA framework In the present preparation procedure of P-CHA [11,13] and consid ering the contents of aluminum and phosphorus in the sample (Si/Al ¼ 11 and P/Al ¼ 0.2), it is assumed that only one-phosphorus species exist in one-unit cell of zeolite cage To simulate this P-CHA cage, the possible SAPO species are constructed in the 6-MR of the framework, in which one framework Si atom is replaced by one P atom and one or two Al atoms are placed in the 6-MR, as shown in Fig Note that one proton is P Zhao et al Microporous and Mesoporous Materials 294 (2020) 109908 sites (M0-M5), M3 and M5 generate significant 31P resonances at À 23.5 and À 31.9 ppm, respectively, while other models exhibit relatively lowfield resonances, as shown in Fig 10b When one more H3PO4 molecule is condensed on these models by dehydration, 31P resonances are shifted to higher field, for example, the QP11 resonance in M2 (À 26.7 ppm) and M5 (À 54.0 ppm); the QP10 Si2 resonance in M3 (À 28.1 ppm); the QP10 resonances in M0 (À 19.5 ppm), M4 (À 19.2 ppm), and M1 (À 16.3 ppm) In the case of dealuminated structures caused by phosphates (M6M10), the QP11 resonances in M8 (À 16.9 ppm) and M10 (À 18.8 ppm) may play a significant role, while other models exhibit relatively lowfield 31P resonances, as shown in Fig 10b As for the condensed phos phates in M8, higher-field resonances were found for QP21 (-27.8 and À 33.0 ppm for B and C in Fig 8) The SAPO species in the framework may be generated by substituting the Si atom with the P atom in the hydrothermal treatment The for mation of SAPO species in the 6-MR zeolite framework results in sig nificant resonances at the range of À 35.8 ~ À 48.8 ppm Based on the theoretical results and the contents of Si, Al, and P atoms in the sample, the possible assignments of the 31P MAS NMR spectra of P-CHA zeolites before and after the hydrothermal treatment are considered The SAPO species generated in the framework can be candidates for the peaks with increased intensity at À 42 ppm after 1-h treatment, and the intensity-decrease at À 42 ppm after the long-time treatment should be caused by the partial framework decomposition with the SAPO species Meanwhile, the highly condensed phosphates may also contribute to the peak at À 42 ppm Considering the decrease in the crystallinity of zeolite by the hydrothermal treatment with long time (Fig S1), the peaks with increased intensity at around À 29 ppm after treatment for over 7–9 h should be due to the accumulation of amor phous extra-framework phosphate and aluminophosphate species caused by the framework decomposition, such as the QP20 resonances in H3P3O9, the QP31 resonances in monodentate, and the QP11 and QP21 resonances in bidentate with a hexagon As for the dominant peak at À 33 ppm in all 31P NMR spectra, the phosphates in the framework like M5 and M8 may contribute to the peak Table Relative energies (ΔE, in kcal⋅molÀ 1), 31P and 27Al chemical shifts (in ppm) and Mulliken charge in structures shown in Fig model ΔEa 31 A1 A2 B1 0.0 0.3 0.0 À 48.8 À 40.1 À 40.5 B2 5.5 P NMR À 35.8 Charge 27 1.505 1.518 1.558 54.2 50.2 1.537 Al NMRb α β α β a b 57.0 51.9 61.0 51.1 Charge 1.088 1.099 1.059 1.121 1.064 1.114 Relative energy with respect to the most stable species Values corrected by 24.7 ppm added to compensate the negative charge in the model with two Al atoms Based on the L€ owenstein’s rule [19], the possible positions for the Al atom are considered to determine the stable structures (Fig S7), and the structures with the Al atom next to the P atom are much more stable As listed in Table 7, the 31P resonances of these stable SAPO species locate at the much higher field as À 40.1 ppm (A2), À 48.8 ppm (A1), À 40.9 ppm (B1), and À 35.8 ppm (B2), species of which should contribute to the observed 31P NMR chemical shifts at À 42 ppm in the initial stage of the hydrothermal treatment The chemical shifts of the framework tetrahedral aluminum are slightly smaller than the pristine framework Discussion Based on the present theoretical results, the possible structures are qualitatively clarified for the observed 31P resonances in the range of À 27 ~ À 42 ppm Fig 10 summarizes the 31P NMR chemical shifts of the species examined in this work For the phosphates without aluminum, QP20 and QP30 mainly fall into the range of À 24 ~ À 37 ppm, as shown in Fig 10a After attaching the aluminum atom to phosphates, some 31P resonances are shifted to the lower field One QP21 and two QP31 reso nances are found at À 20.0, À 25.8 and À 26.2 ppm for monodentate aluminophosphate species Note that bidentate aluminophosphate spe cies caused by attaching the Al to the hydroxyl group of the phosphate are slightly shifted to lower field compared to the corresponding mon odentate The bidentate aluminophosphate species forming a hexagon exhibit a significant resonance at À 20.1 ppm for QP11 , which possesses higher-field chemical shifts for condensed species (À 27.4 ppm for QP21 ) For the possible interactions of phosphorus with the Brønsted acid Conclusions The 31P MAS NMR spectra of P-CHA zeolites were observed under the hydrothermal treatment Characteristic changes of the spectra were identified in the range of À 27~-42 ppm It was observed that the Fig 10 The 31P NMR chemical shifts of different types of phosphorus species considered in this work (a) Phosphates (orange) and aluminophosphates (blue: monodentate; magenta: bidentate); (b) P-CHA models (purple diamond: structures shown in Figs 6, and 9; green triangle: structures shown in Fig S5; circle: structures A (dark cyan), B (pink), and C (wine) shown in Fig (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 10 P Zhao et al Microporous and Mesoporous Materials 294 (2020) 109908 resonance at À 42 ppm significantly increases after 1-h treatment, while the resonance at À 29 ppm increases after further treatment for 7-h The 31 P NMR chemical shifts of a wide range of phosphates have been theoretically studied by the DFT calculations to propose the identifica tion of phosphate species generated by the hydrothermal treatment of PCHA zeolite, that may correlate to the structural changes of P-CHA zeolite The results revealed that the 31P resonances in phosphates and alu minophosphates are shifted to higher field in the condensed species Interestingly, the bidentate aluminophosphate species forming a hexa gon exhibit a rather high-field resonance at À 20.1 ppm for QP11 Note that the 31P resonances in aluminophosphates are slightly shifted to lower field compared to the corresponding phosphates without aluminum, particularly, the shifted values in bidentate structures are larger than those in monodentate structures Six possible models have been considered to study the interactions between phosphorus with the Brønsted acid sites M3 and M5 show the 31 P resonances at À 23.5 and À 31.9 ppm and therefore, these structures are one of the candidates of the P-CHA prepared here Five models are also considered for the interactions of phosphorus with the framework aluminum, in which the QP11 resonances in M8 (À 16.9 ppm) and M10 (À 18.8 ppm) may play a significant role, while other models exhibit relatively low-field 31P resonances The increased intensity at À 42 ppm in the initial stage of the hy drothermal treatment is mainly attributed to the SAPO species in the framework, while the increased intensity at À 29 ppm in the later stage of treatment is ascribed to the accumulation of the extra-framework condensed phosphate and aluminophosphate species caused by the partial framework 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... Fig 31P MAS NMR spectra of the Cu ion exchanged P modified CHAtype zeolites were conducted using Gaussian 09 suite of programs version E.01 [28] Results 4.1 31P MAS NMR spectra of P -CHA zeolites. .. theoretical calculations on the 31P and 27Al NMR chemical shifts Different phosphates and aluminophosphates were first considered to evaluate the 31P NMR chemical shifts of different types of phosphorus... higher-field chemical shifts for condensed species (À 27.4 ppm for QP21 ) For the possible interactions of phosphorus with the Brønsted acid Conclusions The 31P MAS NMR spectra of P -CHA zeolites