An NMR method for characterizing the evolution of acid pairs in solid catalysts and its application

Abstract

The invention discloses a kind of NMR method and application of characterizing solid catalyst acid pair evolution, including by selecting double-headed 1,2-bis(dimethylphosphine)ethane (DMPE) as probe molecule, taking solid catalyst molecular sieve as example, carry out a series of advanced one-dimensional and two-dimensional homonuclear / heteronuclear related NMR detections;By carrying out systematic analysis to experimental result, obtain the spatial interaction information between different acid centers, obtain direct evidence of acid pair evolution, mainly solve the problem that previous experimental means are difficult to accurately characterize the acid pair evolution in solid catalyst, can deepen the understanding and recognition of acid pair synergistic catalytic reaction mechanism in solid catalyst.The inventive method is simple to operate, and result is accurate, and is applicable to the identification of acid pairs in various solid catalysts such as molecular sieve, oxide, heteropoly acid.

Description

Technical Field

[0001] The present invention relates to a solid NMR experimental method, in particular to an NMR method for characterizing the evolution of solid catalyst acid pairs assisted by a double-headed phosphine-containing probe molecule and its application. Background Art

[0002] As a type of heterogeneous catalyst with regular pore structure and unique acidity, molecular sieves are widely used in various key reaction processes in fine chemical and petrochemical industries, such as catalytic cracking, alkylation, disproportionation, isomerization, etc. Generally, the most typical active sites of molecular sieves are derived from the skeleton bridged hydroxyl groups. Acid center. Dealumination is a common strategy to improve the thermal stability and acidity of molecular sieves. Dealumination will cause some of the tetracoordinated aluminum to be removed from the framework, forming extra-framework aluminum (EFAL) species with Lewis acidity. Various spatially adjacent and Lewis acid centers (acid pairs) significantly enhance the catalytic activity of dealuminated molecular sieves. The spatial interaction mechanism between the Lewis acid centers and the acid pair during the dealumination process is still unclear, which greatly hinders people's understanding of the complete dealumination mechanism of zeolites and the cooperative catalytic reaction mechanism of the acid pair.

[0003] Solid-state NMR technology can characterize the active site structure and catalytic performance of solid catalysts at the atomic scale. It should be noted that the combination of solid-state NMR and probe molecule technology has been proven to be able to accurately and reliably characterize the local fine structure and acid properties of various acid sites. However, traditional single-head probe molecules, such as NH3, deuterated acetonitrile (CD3CN), deuterated pyridine (pyridine-d5), 2- 13 C-acetone, trimethylphosphine (TMP) and trialkylphosphine oxide (TRPO) usually contain only one basic group, which means that one probe molecule can only detect one active site. In order to further obtain the spatial proximity and interaction information between adjacent active sites, more advanced two-dimensional solid-state NMR technology is used. For example, using 1 The fine structure and structure of EFAL species in dealuminated HY zeolite were revealed by H DQ MAS NMR technique. / Lewis acid synergistic mechanism; using 27 Al DQ MAS NMR constructs the interaction network between the framework tetracoordinated aluminum and EFAL species in dealuminated molecular sieves. However, these methods cannot accurately describe the interaction between the framework tetracoordinated aluminum and EFAL species in dealuminated molecular sieves. By combining two-dimensional NMR technology with traditional probe molecules, although some useful spatial correlation information can be provided to a certain extent, it is still impossible to distinguish the adjacent Whether the Lewis acid center (acid pair) is located in the same pore of the molecular sieve or in two adjacent pores. After all, only the acid pair in the same pore has real synergistic catalytic significance. Summary of the Invention

[0004] The purpose of the present invention is to provide an NMR method and application for characterizing the evolution of acid pairs in solid catalysts, which mainly solves the current problem of lack of accurate experimental characterization of acid pairs in solid catalysts, thereby providing important information for in-depth understanding of the evolution of acid pairs in solid catalysts and the mechanism of acid pair synergistic catalytic reactions.

[0005] In order to achieve the above-mentioned object of the present invention, the present invention provides the following technical solutions:

[0006] An NMR method for characterizing the evolution of solid catalyst acid pairs comprises the following steps:

[0007] ① Dealumination treatment of molecular sieve samples: calcination and dealumination treatment are performed on different molecular sieve samples to obtain dealumination molecular sieve samples;

[0008] ② Dehydration treatment: heating and dehydrating the molecular sieve samples that have not been dealuminated and those that have been dealuminated in step ① to obtain dehydrated molecular sieve samples;

[0009] ③ Probe molecule adsorption treatment: adsorbing a sufficient amount of double-ended probe 1,2-bis(dimethylphosphine)ethane molecules into the molecular sieve sample after dehydration and activation in step ②;

[0010] ④Solid-state NMR experiment:

[0011] Combined with one dimension 1 H MAS NMR and one-dimensional 27 Al MAS NMR method to determine the formation of non-framework aluminum species in dealuminated molecular sieves;

[0012] use 31 The P CP / MAS NMR method was used to characterize the acid properties of different acid centers in the adsorbed 1,2-bis(dimethylphosphino)ethane probe molecule sample, combined with two-dimensional 1 H- 31 P HETCOR heteronuclear correlation NMR spectrum clearly attributes each signal;

[0013] Combined with two-dimensional 31 P- 31 P DARR MAS NMR and two-dimensional 31 P- 31 PDQ MAS NMR experiments were used to obtain spatial information about the different acid centers in dealuminated molecular sieves and to determine the evolution of acid pairs.

[0014] Optionally, in step ①, the molecular sieve sample includes NaY with Si / Al≈2.8 and HUSY molecular sieve with Si / Al≈3.5;

[0015] The NaY molecular sieve is converted into NH4Y molecular sieve by ion exchange. The ion exchange includes:

[0016] NaY molecular sieves were added to 1 mol / L NH4NO3 solution at a ratio of 1 g / 100 mL and stirred at 353 K for 10 h.

[0017] Filter while hot, then wash and filter repeatedly with ultrapure water until there is no NO3 in the sample - ion;

[0018] The above ion exchange process was repeated 4 times;

[0019] It was then dried in an oven at 383 K overnight to obtain NH4Y molecular sieve.

[0020] Optionally, in step ①, the dealumination treatment includes:

[0021] Place a small amount of NH4Y or HUSY molecular sieve in a clean porcelain boat and place it in the center of the tube furnace;

[0022] In a dry air atmosphere, gradually heat to 723 K at a heating rate of 1 K / min and maintain at this temperature for 3.5 h;

[0023] Then it was allowed to cool naturally to room temperature;

[0024] After the above treatment, a dealuminated molecular sieve is obtained.

[0025] Optionally, in step ②, the dehydration treatment includes:

[0026] The molecular sieve samples were placed in glass tubes, tightly connected to a vacuum system, and subjected to heating and dehydration treatment;

[0027] The temperature was gradually increased from room temperature to the target temperature of 673 K at a heating rate of 1 K / min;

[0028] At a temperature of 673K ​​and a pressure of <10 -3 Pa conditions, the dehydration was continued for 10 h, and then the sample was naturally cooled to room temperature to obtain a dehydrated sample.

[0029] Optionally, in step ③, the adsorption process includes:

[0030] Open the valve above the sample tube to adsorb sufficient volatile 1,2-bis(dimethylphosphino)ethane molecules onto the dehydrated activated molecular sieve sample, and freeze the sample tube with liquid nitrogen to accelerate the adsorption process;

[0031] After the adsorption is completed, close the valve at the upper end of the sample tube to seal it, and let it stand for 1 hour to ensure that 1,2-bis(dimethylphosphino)ethane reaches adsorption equilibrium;

[0032] Open the valve above the sample tube and desorb at room temperature for 1 h to remove excess adsorbed 1,2-bis(dimethylphosphino)ethane molecules;

[0033] Subsequently, the sample tube was sealed using a flame gun;

[0034] The sealed sample tube was placed in an oven at 373 K and heated for 3 h;

[0035] Long-term heat treatment can ensure sufficient diffusion of 1,2-bis(dimethylphosphino)ethane molecules in the molecular sieve pores and uniform adsorption on the acid sites.

[0036] Optionally, in step ④, combining one-dimensional 1 H MAS NMR and one-dimensional 27 Al MAS NMR method to determine the formation of non-framework aluminum species in dealuminated molecular sieves, including:

[0037] Place the rotor loaded with dehydrated samples in the probe and gradually increase the speed to 12 kHz for tuning;

[0038] Will 1 The π / 2 pulse width of the H nucleus is set to the optimized result, the pulse delay time is set to 5s, and the number of sampling is set according to the actual signal strength of the sample, preferably 64;

[0039] 1 After the H MAS NMR spectrum acquisition is completed, reduce the speed to 0 and remove the rotor;

[0040] because 27 Al is a quadrupole nucleus, and the sample is dehydrated. 27 The NMR signal of Al nucleus is weak, so the one-dimensional 27 Al MAS NMR experiments were performed using undehydrated samples;

[0041] Place the rotor containing the undehydrated sample in the probe and gradually increase the speed to 12 kHz for tuning;

[0042] Development 27 Before the Al MAS NMR experiment, the π / 2 pulse width of the solid sample was calculated based on the optimization results of the standard Al(NO3)3 solution;

[0043] One-dimensional 27The Al MAS NMR experiment used the small-flip-angle technique, with a pulse width of π / 12 and a pulse delay of 1 s. The sampling times were set according to the actual signal intensity of the sample.

[0044] Optionally, in step ④, 31 The P CP / MAS NMR method was used to characterize the acid properties of different acid centers in the adsorbed DMPE probe molecule sample, combined with two-dimensional 1 H- 31 PHETCOR heteronuclear correlation NMR spectra clearly attribute each signal, including:

[0045] The rotor containing the adsorbed probe molecule sample was placed in the probe, and the speed was gradually increased to 12 kHz. The cross-polarization matching power and decoupling power were set according to the standard sample optimization results. The contact time was 4 ms, the pulse delay time was set to 3 s, and the one-dimensional 31 The number of sampling times of P spectrum;

[0046] Keep the rotation speed constant and continue to collect the two-dimensional 1 H- 31 PHETCORNMR spectra, heteronuclear correlation spectra can be correlated 1 H nucleus and 31 The spatial information between P nuclei can be obtained by comparing the heteronuclear correlation spectra at different contact times, thereby assisting 31 Assignment of P chemical shifts;

[0047] At the time of sampling 1 The H channels are decoupled simultaneously, with contact times of 0.1ms and 4.0ms, respectively.

[0048] Optionally, in step ④, the different 31 The chemical shift assignment results of P were obtained by two-dimensional 31 P- 31 P DARR MAS NMR experiments observed spatial correlations between acid centers with different chemical shifts, including with Lewis acid centers;

[0049] Mixing time t m is 100ms.

[0050] Furthermore, the present invention also claims the use of the aforementioned NMR method for characterizing the evolution of acid pairs in solid catalysts in characterizing the spatial correlation between different acid centers and the evolution of acid pairs.

[0051] Furthermore, the present invention also claims the use of the aforementioned NMR method for characterizing the evolution of acid pairs of solid catalysts in revealing the detailed acidic characteristics of catalytic materials.

[0052] From the above, the present invention is based on the original single-headed phosphine-containing probe molecule (such as trimethylphosphine) combined with the solid NMR experimental method, and innovatively proposes a double-headed phosphine-containing probe molecule 1,2-di(dimethylphosphine)ethane (DMPE), and introduces a variety of two-dimensional solid NMR experimental techniques. While ensuring the accurate characterization of the local fine structure and acid properties of various acid centers, it further obtains the spatial proximity information between different acid centers, providing direct experimental evidence for the evolution of acid pairs during the calcination and dealumination of molecular sieves. The present invention innovatively proposes a double-headed DMPE probe molecule, the structural formula of which is (CH3)2P-(CH2)2-P(CH3)2, which contains two basic sites, and the distance between PP atoms is relatively fixed, about It can be anchored to two adjacent acid centers (acid pairs) simultaneously, so it is very suitable for characterizing the evolution of molecular sieve acid pairs during calcination and dealumination. 1 H MAS NMR and one-dimensional 27 Al MAS NMR method confirmed that calcination at 723K can dealuminate Y molecular sieve and form EFAL species. On this basis, DMPE probe molecules were adsorbed and the 31 P CP / MAS NMR combined with two-dimensional spectroscopy with different contact times 1 H- 31 PHETCORR NMR experiments, supplemented by density functional theory (DFT) calculations, clarified the signal attribution and characterized the detailed acid properties of the dealuminated molecular sieve, including the type and intensity of the acid center. 31 P- 31 P DARR MAS NMR and two-dimensional 31 P- 31 PDQ MAS NMR, directly obtain various The spatial correlation between the two Lewis acid centers is studied, and direct experimental evidence of the evolution of the acid pairs during the calcination process is obtained. The research objects of this invention are HY / HUSY before dealumination and HY-d450 and HUSY-d450 molecular sieves after dealumination (both topological structures are FAU). Such molecular sieves have important applications in catalytic cracking, hydrocracking and isomerization reactions in the fields of petroleum refining and chemical industry. Dealumination is a common strategy to improve its thermal stability and acidity. Dealumination can cause the molecular sieve to form Lewis acidic EFAL species, and various spatially adjacent The presence of Lewis acid centers (acid pairs) significantly enhances the catalytic activity of dealuminated molecular sieves. Therefore, a clear understanding of the local fine structure, properties, and potential evolution of acid pairs in dealuminated molecular sieves is the theoretical basis for developing new, highly efficient industrial catalysts. The analytical method of this invention is universally applicable. This multi-probe molecule combined with NMR technology can be used to explore the spatial interactions between different acid centers in solid catalysts such as molecular sieves, metal oxides, and heteropolyacids, potentially opening up new avenues for revealing the local fine structure of catalytic active centers.

[0053] Compared with the prior art, the present invention has the following advantages and positive effects:

[0054] 1. It can characterize the acid type, acid strength, acid content and other information of different acid centers of solid catalysts;

[0055] 2. It can detect the host-guest interaction between the acid center of the solid catalyst and the probe molecule;

[0056] 3. It can obtain the spatial proximity information between different acid centers of solid catalysts;

[0057] 4. You can get close or the local fine structure properties of Lewis acid centers (acid pairs);

[0058] 5. By studying the molecular sieve samples with different dealumination degrees, we can get the various Spatial interaction network with Lewis acid centers and evolution of acid pairs. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 Is DMPE in various possible dealuminized molecular sieves Schematic diagram of adsorption on and Lewis acid centers.

[0060] Figure 2 It is the HY and HUSY molecular sieves before and after dealumination 1 H MAS NMR spectrum.

[0061] Figure 3 It is the HY and HUSY molecular sieves before and after dealumination 27 Al MAS NMR spectrum.

[0062] Figure 4 It is the adsorption of DMPE by HY and HUSY molecular sieves before and after dealumination. 31 P CP / MAS NMR spectrum.

[0063] (a) HY molecular sieve;

[0064] (b) HY-d450 molecular sieve;

[0065] (c) HUSY molecular sieve;

[0066] (d) HUSY-d450 molecular sieve.

[0067] Figure 5 It is a two-dimensional molecular sieve adsorbing DMPE. 1 H- 31 PHETCORNMR spectrum.

[0068] (a) Contact time is 0.1ms;

[0069] (b) The contact time is 4.0 ms.

[0070] Figure 6 The two-dimensional adsorption of DMPE on HY and HUSY molecular sieves before and after dealumination 31 P- 31 PDARR MAS NMR spectrum.

[0071] (a) HY molecular sieve;

[0072] (b) HY-d450 molecular sieve;

[0073] (c) HUSY molecular sieve;

[0074] (d) HUSY-d450 molecular sieve.

[0075] Figure 7 It is a two-dimensional molecular sieve adsorbing DMPE. 31 P- 31 PDQ MAS NMR spectrum.

[0076] (a) HY molecular sieve;

[0077] (b) HY-d450 molecular sieve;

[0078] (c) Schematic diagram of DMPE molecules adsorbed on spatially adjacent acid centers.

[0079] Figure 8 It’s the operating procedure. DETAILED DESCRIPTION

[0080] The outstanding features of the present invention are further illustrated by the following examples, which are only intended to illustrate the present invention but in no way limit the present invention.

[0081] Description of experimental methods

[0082] 1. One Dimension 1 H MAS NMR experiments

[0083] Experimental conditions: one-dimensional1 The samples used in H MAS NMR experiments need to be dehydrated at high temperature in advance. 1 The H nucleus resonant frequency was 500.57 MHz. A 4 mm probe was used, along with a 4 mm ZrO2 rotor sealed with a Kel-F cap. The magic angle spinning rate was 12 kHz. The π / 2 pulse width was 4.0 μs, the pulse delay was 5 s, and the number of accumulated samples was 64.

[0084] 2. One-dimensional 27 Al MAS NMR experiments

[0085] Experimental conditions: one-dimensional 27 The samples used in the Al MAS NMR experiment were non-dehydrated molecular sieve samples. 27 The resonant frequency of the Al nucleus is 130.44 MHz. A 4 mm probe was used, along with a 4 mm ZrO2 rotor sealed with a Kel-F cap. The magic angle spinning rate was 12 kHz. The experiment employed a small-flip-angle technique, a pulse width of 0.26 μs (π / 12), and a pulse delay of 1 s.

[0086] 3. One Dimension 31 P cross-polarization (CP) / MAS NMR experiments

[0087] Experimental conditions: one-dimensional 31 The samples used in PCP / MAS NMR experiments need to be dehydrated at high temperature and adsorbed with an appropriate amount of DMPE probe molecules. 31 The resonant frequency of the P nucleus was 202.63 MHz. A 4 mm probe was used, along with a 4 mm ZrO2 rotor sealed with a Kel-F cap. The magic angle spinning rate was 12 kHz. The contact time was 4 ms, and the pulse delay was 3 s.

[0088] 4. 2D 1 H- 31 PHETCORNMR experiment

[0089] Experimental conditions: Sampling was performed at different CP contact times (0.1–4 ms) and a pulse delay time of 1.5 s.

[0090] 5. Two-dimensional 31 P- 31 PDARR MAS NMR experiments

[0091] Experimental conditions: mixing time is 100ms.

[0092] 6. Two-dimensional 31 P- 31 PDQ MAS NMR experiments

[0093] Experimental conditions: POST-C7 pulse sequence was used.

[0094] Example 1

[0095] like Figure 8 As shown, an NMR method for characterizing the evolution of solid catalyst acid pairs includes ion exchange 1, dealumination treatment 2, dehydration treatment 3, adsorption of probe molecules 4, loading the sample into a rotor 5, and solid NMR experiment 6. The specific steps are as follows:

[0096] ① Ion exchange and dealumination treatment of molecular sieve samples:

[0097] NaY (Si / Al≈2.8) and HUSY (Si / Al≈3.5) molecular sieves were purchased from the Nankai University Catalyst Plant. First, an ion exchange operation was performed to convert the NaY molecular sieve into NH4Y molecular sieve. Then, portions of the NH4Y and HUSY molecular sieves were calcined and dealuminated to obtain dealuminated molecular sieve samples.

[0098] The specific steps are as follows:

[0099] Ion exchange: Add NaY molecular sieve to 1 mol / L NH4NO3 solution at a ratio of 1g / 100mL and stir continuously for 10h at 353K; then filter while hot and wash repeatedly with ultrapure water until there is no NO3 in the sample. - The above ion exchange process was repeated 4 times and then dried in an oven at 383K overnight to obtain NH4Y molecular sieve.

[0100] Dealumination treatment: A small amount of NH4Y (or HUSY) molecular sieve is spread flat on a clean porcelain boat and placed in the center of a tube furnace. In a dry air atmosphere, the boat is gradually heated to 723K (450°C) at a heating rate of 1K / min and maintained at this temperature for 3.5 hours. The boat is then allowed to cool naturally to room temperature. The dealumination molecular sieves obtained after this treatment are named HY-d450 and HUSY-d450 (d450 indicates a dealumination temperature of 450°C).

[0101] ②Dehydration treatment:

[0102] Four molecular sieves, NH4Y, HY-d450, HUSY and HUSY-d450, were placed in self-made glass tubes, tightly connected to the vacuum system, and then heated and dehydrated. The upper end of the self-made glass tube was ground to ensure a tight connection between the glass tube and the vacuum system; the middle part was tightened so that after the sample was dehydrated, it could be burned off with a flame gun to ensure the sample was sealed. The temperature was gradually increased from room temperature to the target temperature of 673K ​​at a heating rate of 1K / min; at a temperature of 673K ​​and a pressure of <10 -3 Under Pa conditions, dehydration was continued for 10 h, and then naturally cooled to room temperature to obtain dehydrated HY, HY-d450, HUSY and HUSY-d450 samples. The valves above all sample tubes were closed; one of each of the four dehydrated molecular sieve samples was taken and sealed with a flame gun for use in 1 HMAS NMR detection; other dehydrated samples were subjected to probe molecule adsorption treatment.

[0103] ③Probe molecule adsorption treatment:

[0104] After the high-temperature dehydration pretreatment in step ②, the molecular sieve sample can be adsorbed with the double-ended probe molecule 1,2-di(dimethylphosphino)ethane (DMPE). The valve at the top of the sample tube is opened to allow sufficient volatile DMPE probe molecules to adsorb onto the dehydrated and activated molecular sieve sample. The sample tube is then chilled with liquid nitrogen to accelerate the adsorption process. After adsorption is complete, the valve at the top of the sample tube is closed to seal it, and the sample tube is allowed to stand for 1 hour to ensure that DMPE reaches adsorption equilibrium. The valve at the top of the sample tube is opened, and desorption is carried out at room temperature for 1 hour to remove excess adsorbed DMPE molecules. The sample tube is then sealed with a flame gun. The sealed sample tube is placed in an oven at 373K and heated for 3 hours. This prolonged heat treatment ensures sufficient diffusion of DMPE molecules within the molecular sieve pores and uniform adsorption on acid sites.

[0105] ④Solid-state NMR experiment preparation part:

[0106] Before solid-state NMR experiments, sealed samples were transferred to ZrO2 rotors in a glove bag filled with dry nitrogen and sealed with Kel-F caps.

[0107] Solid-state NMR experiments are performed on a high-field (400 MHz and above) solid-state nuclear magnetic resonance spectrometer. Before the experiment, select a suitable probe (e.g., 4 mm MAS probe) and correctly install it into the magnet. Open the corresponding software on the computer, read the probe information, and confirm that the preamplifier and power amplifier units are connected correctly. Select a suitable standard sample, including 1 H. 31 P and 27Al standard samples were adamantane (1.91 ppm), (NH₄)₂HPO₄ (1.0 ppm), and a 1 mol / L Al(NO₃)₃ solution (0 ppm). Each standard sample was placed in the probe, and the rotation speed was gradually increased from 5 kHz to 12 kHz (the Al(NO₃)₃ solution was a liquid, sampled under static conditions). The tuning interface was opened, and the absorption line was adjusted to the optimal position by adjusting the Match and Tuning buttons. Key parameters such as the π / 2 pulse width, cross-polarization (CP) matching power, and decoupling power were optimized, and the corresponding nuclei were calibrated.

[0108] ⑤ Combined with one dimension 1 H MAS NMR and one-dimensional 27 The Al MAS NMR method is used to determine the formation of non-framework aluminum species in dealuminated molecular sieves. The specific steps are:

[0109] First, place the rotor loaded with the dehydrated sample in the probe, gradually increase the speed to 12kHz, and tune it; set the π / 2 pulse width to the optimized result, the pulse delay time to 5s, and set the number of sampling according to the actual signal strength of the sample, usually 64. 1 After the H MAS NMR spectrum acquisition is completed, reduce the speed to 0 and remove the rotor. 27 Al is a quadrupole nucleus, and the sample is dehydrated. 27 The NMR signal of Al nucleus is weak, so the one-dimensional 27 The Al MAS NMR experiment was conducted using undehydrated samples. The rotor containing the undehydrated sample was placed in the probe and the speed was gradually increased to 12 kHz for tuning. 27 Before the Al MASNMR experiment, the π / 2 pulse width of the solid sample was calculated based on the optimization results of the standard Al(NO3)3 solution. 27 The Al MAS NMR experiment uses the small-flip-angle technique. The pulse width is set to π / 12 during sampling, the pulse delay time is 1 s, and the number of sampling times is set according to the actual signal intensity of the sample, usually 1024 times.

[0110] ⑥ Adoption 31 The P CP / MAS NMR method was used to characterize the acid properties of different acid centers in the adsorbed DMPE probe molecule sample, combined with two-dimensional 1 H- 31 The P HETCOR heteronuclear correlation NMR spectrum clearly attributes each signal. First, the rotor containing the adsorbed probe molecule sample is placed in the probe, and the speed is gradually increased to 12 kHz. The cross-polarization matching power and decoupling power are set according to the standard sample optimization results, the contact time is 4 ms, the pulse delay time is set to 3 s, and the one-dimensional31 The sampling times of PCP / MAS NMR spectrum are usually 256. Then, keep the speed constant and continue to collect the two-dimensional 1 H- 31 P HETCORNMR spectra, heteronuclear correlation spectra can be associated 1 H nucleus and 31 The spatial information between P nuclei can be obtained by comparing the heteronuclear correlation spectra at different contact times, thereby assisting 31 Attribution of P chemical shift; at the time of sampling 1 The H channels are decoupled simultaneously, with contact times of 0.1ms and 4.0ms, respectively.

[0111] ⑦ Combined with two-dimensional 31 P- 31 PDARR MAS NMR and 2D 31 P- 31 P DQ MAS NMR experiments were used to obtain spatial information between different acid centers in the dealuminated molecular sieve and determine the evolution of acid pairs. 31 The chemical shift assignment results of P were obtained by two-dimensional 31 P- 31 PDARR MAS NMR experiments observe spatial correlations between acid centers with different chemical shifts (e.g., with Lewis acid center); mixing time t m 100ms. 2D 31 P- 31 PDQ NMR experiments can not only obtain spatial correlation information between acid centers with different chemical shifts, but also establish spatial correlation between different acid centers with the same chemical shift (e.g. and and Lewis, Lewis and Lewis acid centers); the commonly used DQ double quantum recoupling pulse sequence is POST-C7. Generally speaking, only when the internuclear distance is less than Only when the double quantum correlation signal of the same nucleus is detected can it be detected.

[0112] Figure 1 Various possible solutions for DMPE in dealuminated molecular sieves and schematic diagram of the adsorption configuration on the Lewis acid center.

[0113] Single pulse 1 H MAS NMR experiments are a common method used to characterize various hydroxyl species in molecular sieves. Figure 2 The one-dimensional structure of the four molecular sieves HY, HUSY before dealumination and HY-d450 and HUSY-d450 after dealumination at 450℃ 1H MAS NMR spectra. Comparing the spectra before and after dealumination, the Al-OH signals of the EFAL species (2.7 and 0.8 ppm) in the HY-d450 and HUSY-d450 samples were significantly enhanced, indicating that high-temperature calcination can effectively dealuminate the molecular sieve and generate Lewis-acidic EFAL species, laying the foundation for subsequent experiments.

[0114] One-dimensional 27 Al MAS NMR experiments can accurately provide information on the content and coordination state of aluminum atoms, and effectively distinguish between framework aluminum and EFAL species. Figure 3 As shown, before dealumination, the HY and HUSY molecular sieves are mainly composed of four-coordinated framework aluminum. 27 The Al chemical shift is 60 ppm; a significantly increased -1 ppm resonance peak is observed after dealumination, indicating the presence of EFAL species. Furthermore, a broad peak at approximately 40 ppm is observed in HUSY and HUSY-d450, likely originating from pentacoordinated EFAL or distorted tetracoordinated aluminum species. Therefore, after dealumination, abundant EFAL species are present in HY-d450 and HUSY-d450 molecular sieves.

[0115] like Figure 4 As shown in the figure, after adsorption of DMPE probe molecules, the non-dealuminized HY powder 31 There are two resonance peaks in the P CP / MAS NMR spectrum, with chemical shifts of 2 and -37 ppm respectively. 31 The PCP / MAS NMR spectrum shows four main resonance peaks at 2, -25, -37 and -44 ppm. Similar phenomena can also be observed on HUSY and HUSY-d450 molecular sieves. Combined with the DFT theoretical calculation results, the 2 ppm 31 The P resonance peak can be attributed to the adsorption of DMPE on Protonated DMPEH generated from the acid center + , and the -37ppm signal may originate from the unprotonated P atom at the other end of the single-head protonated DMPE molecule. In addition, the -44ppm 31 The P resonance peak may be derived from the adsorption of DMPE on a series of weakly acidic Lewis acid centers containing hydroxyl groups (e.g., Al(OH)3, Al(OH)2 + and AlOH 2+ ), while -25ppm is attributed to the signal of DMPE adsorbed on the strong Lewis acidic Al 3+ Species signaling.

[0116] Comparison of the two-dimensional 1 H- 31PHETCORNMR spectrum can further reveal the molecular structure of DMPE. The adsorption configuration on the Lewis acid center is clearly assigned. Figure 5 As shown, when the contact time τ c = 0.1ms, the HUSY-d450 molecular sieve adsorbing DMPE showed an obvious correlation peak at (2,7.5) ppm. This signal corresponds to 31 P resonance (F2 dimension: δ 31 P = 2ppm) and 1 H resonance (F1 dimension: δ 1 H=7.5ppm), which can be attributed to DMPE adsorption Protonated DMPEH formed by acid center + When the contact time increases to 4.0ms, in addition to (2,7.5)ppm, new related peaks appear, namely (2,2.0), (-25,2.0) and (-37~-44,1.8)ppm, which correspond to DMPE adsorption to Acid center (δ 31 P=2ppm) and Lewis acid center (δ 31 P = -25 and -44 ppm) and the unprotonated P atom at the other end of the single-head protonated DMPE molecule (δ 31 P=-37ppm) and its methyl (or methylene) protons (δ 1 Due to the long contact time, the spectrum also showed related signals at (-37 to -44, 7.5) ppm, which further confirmed that one end was an unprotonated P atom (δ 31 P=-37ppm) of single-head protonated DMPE (δ 1 H=7.5ppm), also indicates the presence of a weak Lewis acid center (δ 31 P = -44ppm) and Acid center (δ 1 H=7.5ppm) space is close. Acid (δ 1 H=7.5ppm) and strong Lewis acid center (δ 31 There is no spatial correlation between the

[0117] 2D 31 P- 31 P DARR MAS NMR experiments can be used to investigate the steric interactions between different acid sites in HY, HUSY, and dealuminated HY-d450 and HUSY-d450 molecular sieves. Figure 6 As shown, in HY molecular sieve ( Figure 6a), only the diagonal peak autocorrelation signals at (2,2) and (-37,-37) ppm were observed, and the off-diagonal correlation peaks at (2,-37) and (-37,2) ppm were not observed. This may be due to the unprotonated P atom at the other end of the single-head protonated DMPE molecule (δ 31 P=-37ppm) has high mobility, which is not conducive to the generation of NMR related signals. After dealumination, non-diagonal related peak signals of (2,-44) and (-44,2) ppm appeared in HY-d450 molecular sieve ( Figure 6 b), confirmed the weak Lewis acid center (δ 31 P = -44ppm) and also with Acid center (δ 31 P=2ppm) spatial correlation. Figure 6 c) and dealuminated HUSY-d450 ( Figure 6 d) Similar phenomena also exist in the sample, especially for the HUSY-d450 molecular sieve with a high content of EFAL species, where strong non-diagonal correlation peaks at (2,-44) and (-44,2) ppm, as well as two groups of weak non-diagonal correlation peak signals at (2,-37) and (-37,2), (-25,-44) and (-44,-25) ppm can be clearly observed. 31 P- 31 P DARR MAS NMR experiments also show that the strong Lewis acid Al 3+ Species are relatively isolated.

[0118] Two-dimensional adsorption of DMPE molecules 31 P- 31 PDARR MAS NMR spectra can only provide relevant information between P atoms in different chemical environments, and cannot identify the spatial correlation between P atoms with the same chemical environment. 31 P- 31 P DQMAS NMR can complement it. Figure 7 As shown in a, the autocorrelation peak signal of (2,4) ppm indicates that the HY molecular sieve The acid center is close to the space, that is, there Acid pair; In addition, Figure 7 In b, non-diagonal correlation peak signals (2, -42) and (-44, -42) ppm were also observed in the dealuminated HY-d450 sample, corresponding to (δ 31 P=2ppm) and weak Lewis acid center (δ 31 P=-44ppm) between the spatial correlation, that is, there is -Lewis acid pair; while the diagonal correlation signal at (-44, -88) ppm indicates that there is also a Lewis-Lewis acid pair in the sample, which reveals the presence of Acid to -Lewis acid pairs, and then to Lewis-Lewis acid pairs. Figure 7 In c, the DMPE molecules are listed respectively. Acid pair, -Lewis acid pairs and adsorption states on Lewis-Lewis acid pairs.

[0119] Example 2

[0120] Solid-state NMR experiments combined with probe molecule technology are a method for accurately and reliably characterizing the local fine structure and acid properties of various acid centers in solid catalysts. However, traditional probe molecule-assisted solid-state NMR methods cannot accurately describe the structural characteristics of acid pairs. The present method uses HY and HUSY molecular sieves before and after dealumination as examples and innovatively proposes a new double-headed probe molecule 1,2-di(dimethylphosphine)ethane (DMPE), which has a spacing of approximately The P-atom dual probe can simultaneously anchor to two adjacent acid sites (acid pairs), making it ideal for characterizing the evolution of acid pairs in solid catalysts. Furthermore, this probe molecule, combined with solid-state NMR technology, can be used to explore the spatial interactions between different acid sites in various solid catalysts, such as molecular sieves, metal oxides, and heteropolyacids, potentially opening up new avenues for revealing the localized fine structure of solid catalyst active sites.

[0121] The specific applications are as follows:

[0122] 1. Obtain detailed acid characteristics of the active center of the solid catalyst, such as acid type, acid content, and acid strength;

[0123] 2. Obtain the spatial interaction network of each acid center in the solid catalyst and the acid pair evolution information;

[0124] 3. Expand the multi-probe molecule-assisted solid-state NMR method to reveal the acidic characteristics of various catalytic materials.

[0125] The above description is only a specific embodiment of the present invention, and the protection scope of the present invention is not limited thereto. Any person familiar with the technology can understand and think of any changes or replacements within the technical scope disclosed by the present invention, which should be included in the scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. An NMR method for characterizing the evolution of solid catalyst acid pairs, characterized in that: The following steps are involved: ① Dealumination treatment of molecular sieve samples: calcination and dealumination treatment are performed on different molecular sieve samples to obtain dealumination molecular sieve samples; ② Dehydration treatment: heating and dehydrating the molecular sieve samples that have not been dealuminated and those that have been dealuminated in step ① to obtain dehydrated molecular sieve samples; ③ Probe molecule adsorption treatment: adsorbing an appropriate amount of double-ended probe 1,2-bis(dimethylphosphine)ethane molecules onto the molecular sieve sample activated by the dehydration treatment in step ②; ④Solid-state NMR experiment: Combined with one dimension 1 H MAS NMR and one-dimensional 27 Al MAS NMR method to determine the formation of non-framework aluminum species in dealuminated molecular sieves; use 31 The P CP / MAS NMR method was used to characterize the acid properties of different acid centers in the adsorbed 1,2-bis(dimethylphosphino)ethane probe molecule sample, combined with two-dimensional 1 H- 31 P HETCOR heteronuclear correlation NMR spectrum clearly attributes each signal; Combined with two-dimensional 31 P- 31 P DARR MAS NMR and two-dimensional 31 P- 31 PDQ MAS NMR experiments were used to obtain spatial information about the different acid centers in dealuminated molecular sieves and to determine the evolution of acid pairs.

2. The NMR method for characterizing the evolution of solid catalyst acid pairs according to claim 1, wherein In the step ①, the molecular sieve samples include NaY with Si / Al ≈ 2.8 and HUSY molecular sieve with Si / Al ≈ 3.5; The NaY molecular sieve is converted into NH4Y molecular sieve by ion exchange. The ion exchange includes: NaY molecular sieves were added to 1 mol / L NH4NO3 solution at a ratio of 1 g / 100 mL and stirred at 353 K for 10 h. Filter while hot, then wash and filter repeatedly with ultrapure water until there is no NO3 in the sample - ion; The above ion exchange process was repeated 4 times; It was then dried in an oven at 383 K overnight to obtain NH4Y molecular sieve.

3. The NMR method for characterizing the evolution of solid catalyst acid pairs according to claim 1, wherein: In the step ①, the dealumination treatment includes: Place a small amount of NH4Y or HUSY molecular sieve in a clean porcelain boat and place it in the center of the tube furnace; In a dry air atmosphere, the sample was gradually heated to 723 K at a heating rate of 1 K / min and maintained at 723 K for 3.5 h. Then it was allowed to cool naturally to room temperature; After the above treatment, a dealuminated molecular sieve is obtained.

4. The NMR method for characterizing the evolution of solid catalyst acid pairs according to claim 1, wherein: In the step ②, the dehydration treatment includes: The molecular sieve samples were placed in glass tubes, tightly connected to a vacuum system, and subjected to heating and dehydration treatment; The temperature was gradually increased from room temperature to the target temperature of 673 K at a heating rate of 1 K / min; At a temperature of 673 K and a pressure of < 10 -3 Pa conditions, the dehydration was continued for 10 h, and then the sample was naturally cooled to room temperature to obtain the dehydrated sample.

5. The NMR method for characterizing the evolution of solid catalyst acid pairs according to claim 1, wherein: In the step ③, the adsorption process includes: Open the valve above the sample tube to adsorb sufficient volatile 1,2-bis(dimethylphosphino)ethane molecules onto the dehydrated activated molecular sieve sample, and freeze the sample tube with liquid nitrogen to accelerate the adsorption process; After the adsorption is completed, close the valve at the upper end of the sample tube to seal it and let it stand for 1 h to ensure that 1,2-bis(dimethylphosphino)ethane reaches adsorption equilibrium; Open the valve above the sample tube and desorb at room temperature for 1 h to remove excess adsorbed 1,2-bis(dimethylphosphino)ethane molecules; Subsequently, the sample tube was sealed using a flame gun; The sealed sample tube was placed in an oven at 373 K and heated for 3 h; Long-term heat treatment can ensure sufficient diffusion of 1,2-bis(dimethylphosphino)ethane molecules in the molecular sieve pores and uniform adsorption on the acid sites.

6. The NMR method for characterizing the evolution of solid catalyst acid pairs according to claim 1, characterized in that: In step ④, combining one-dimensional 1 H MAS NMR and one-dimensional 27 Al MAS NMR method to determine the formation of non-framework aluminum species in dealuminated molecular sieves, including: Place the rotor loaded with dehydrated samples in the probe and gradually increase the speed to 12 kHz for tuning; Will 1 The π / 2 pulse width of the H nucleus was set to the optimized result, the pulse delay time was set to 5 s, and the number of samplings was set according to the actual signal intensity of the sample, with a total of 64 samplings; 1 After the H MAS NMR spectrum acquisition is completed, reduce the speed to 0 and remove the rotor; because 27 Al is a quadrupole nucleus, and the sample is dehydrated. 27 The NMR signal of Al nucleus is weak, so the one-dimensional 27 Al MAS NMR experiments were performed using undehydrated samples; Place the rotor containing the undehydrated sample in the probe and gradually increase the speed to 12 kHz for tuning; Development 27 Before the Al MAS NMR experiment, the π / 2 pulse width of the solid sample was calculated based on the optimization results of the standard Al(NO3)3 solution; One-dimensional 27 The Al MAS NMR experiment used the small-flip-angle technique. The pulse width was set to π / 12 during sampling, the pulse delay time was 1 s, and the sampling times were set according to the actual signal intensity of the sample.

7. The NMR method for characterizing the evolution of solid catalyst acid pairs according to claim 1, characterized in that: In the step ④, 31 The P CP / MAS NMR method was used to characterize the acid properties of different acid centers in the adsorbed DMPE probe molecule sample, combined with two-dimensional 1 H- 31 PHETCOOR heteronuclear correlation NMR spectra clearly attribute each signal, including: The rotor containing the adsorbed probe molecule sample was placed in the probe, and the speed was gradually increased to 12 kHz. The cross-polarization matching power and decoupling power were set according to the standard sample optimization results. The contact time was 4 ms, the pulse delay time was set to 3 s, and the one-dimensional 31 P is the sampling number of CP / MAS NMR spectrum; Keep the rotation speed constant and continue to collect the two-dimensional 1 H- 31 P HETCOR NMR spectrum, heteronuclear correlation spectrum 1 H nucleus and 31 The spatial information between P nuclei is obtained by comparing the heteronuclear correlation spectra at different contact times to obtain the spatial distance information between different nuclei, thereby assisting 31 Assignment of P chemical shifts; At the time of sampling 1 The H channels were decoupled simultaneously, with contact times of 0.1 ms and 4.0 ms, respectively.

8. The NMR method for characterizing the evolution of solid catalyst acid pairs according to claim 1, characterized in that: In the step ④, the different 31 The chemical shift assignment results of P were obtained by two-dimensional 31 P- 31 P DARR MAS NMR experiments observed spatial correlations between acid centers with different chemical shifts, including Brønsted and Lewis acid centers; Mixing time t m is 100 ms.

9. Use of the NMR method for characterizing the evolution of acid pairs in solid catalysts according to any one of claims 1 to 8 in characterizing the spatial correlation between different acid centers and the evolution of acid pairs.

10. Use of the NMR method for characterizing the evolution of acid pairs of solid catalysts according to any one of claims 1 to 8 in revealing the detailed acidic characteristics of catalytic materials.

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