NMR (nuclear magnetic resonance) method for characterizing acid pair evolution of solid catalyst and application

Abstract

The invention discloses an NMR (nuclear magnetic resonance) method for characterizing solid catalyst acid pair evolution and application, which comprises the following steps: selecting double-ended 1, 2-bis (dimethyl phosphine) ethane (DMPE) as a probe molecule, taking a solid catalyst molecular sieve as an example, and carrying out a series of advanced one-dimensional and two-dimensional homonuclear / heteronuclear related NMR detection; through systematic analysis of experimental results, spatial interaction information among different acid centers is obtained, direct evidence of acid pair evolution is obtained, the problem that the evolution of acid pairs in the solid catalyst is difficult to accurately characterize by the conventional experimental means is mainly solved, and understanding and understanding of a concerted catalytic reaction mechanism of the acid pairs in the solid catalyst can be deepened. The method is easy to operate, accurate in result and suitable for identifying acid pairs in various solid catalysts such as molecular sieves, oxides and heteropoly acid.

Description

Technical Field

[0001] The 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 reaction, etc. Generally, the most typical active site of molecular sieves is originated from the skeleton bridge hydroxyl group. 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 non-framework aluminum (EFAL) species with Lewis acidity. Various spatially adjacent and Lewis acid centers (acid pairs) significantly improve the catalytic activity of dealuminated molecular sieves. The spatial interaction mechanism between the Lewis acid center and the acid pair during the dealumination process is still unclear, which greatly hinders people's understanding of the complete dealumination mechanism of molecular sieves 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-headed probe molecules, such as NH 3 , deuterated acetonitrile (CD 3 CN), deuterated pyridine 5 ),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 NMR technology is used. For example, using 1 HDQ MAS NMR technique reveals the fine structure and / Lewis acid synergistic mechanism; using 27 Al DQ MAS NMR constructs the interaction network between framework tetracoordinated aluminum and EFAL species in dealuminated molecular sieves. However, these methods cannot accurately describe the interaction between various acid strengths. 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, mainly to solve 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 reaction.

[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 the molecular sieve samples 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-headed probe 1,2-di(dimethylphosphine)ethane molecules into the molecular sieve sample after dehydration and activation in step ②;

[0010] ④Solid 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-di(dimethylphosphine)ethane probe molecule sample, combined with two-dimensional 1 H- 31 P HETCOR heteronuclear correlation NMR spectra clearly attribute each signal;

[0013] Combined with two-dimensional 31 P- 31 P DARR MAS NMR and 2D 31 P-31 P DQ MAS NMR experiments were used to obtain spatial information about the different acid centers in the dealuminated molecular sieves and determine the evolution of the 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] NaY molecular sieve was converted into NH 4 Y molecular sieve, ion exchange includes:

[0016] According to the ratio of 1g / 100mL, NaY molecular sieve was added to 1mol / L NH 4 NO 3 The solution was stirred continuously at 353 K for 10 h;

[0017] Filter while hot, then wash and filter repeatedly with ultrapure water until there is no NO in the sample. 3 - ion;

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

[0019] It was then dried in an oven at 383K overnight to obtain NH 4 Y molecular sieve.

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

[0021] A small amount of NH 4 Y or HUSY molecular sieves are laid flat in a clean porcelain boat and placed 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 let it 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 respectively, which were tightly connected to the vacuum system for 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-di(dimethylphosphine)ethane molecules into 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(dimethylphosphine)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(dimethylphosphine)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 the sufficient diffusion of 1,2-bis(dimethylphosphine)ethane molecules in the molecular sieve pores and the uniform adsorption on the acid sites.

[0036] Optionally, in step ④, 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 with the dehydrated sample in the probe, gradually increase the speed to 12kHz, and perform tuning;

[0038] Will 1 The π / 2 pulse width of the H core 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, the speed is reduced to 0 and the rotor is removed;

[0040] because 27 Al is a quadrupole nucleus, and the sample is dehydrated. 27 The NMR signal of Al nuclei 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 27Before the Al MAS NMR experiment, the standard Al(NO 3 ) 3 The solution optimization results were used to calculate the π / 2 pulse width of the solid sample;

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

[0044] Optionally, in step ④, using 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 associated 1 H nucleus and 31 Spatial information between P nuclei can be obtained by comparing heteronuclear correlation spectra with different contact times, thus assisting 31 Assignment of P chemical shifts;

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

[0048] Optionally, in step ④, the 31 The chemical shift attribution 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 of 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] As described above, the present invention innovatively proposes a double-headed phosphine-containing probe molecule 1,2-di(dimethylphosphine)ethane (DMPE) based on the original single-headed phosphine-containing probe molecule (such as trimethylphosphine) combined with the solid NMR experimental method, and introduces a variety of two-dimensional solid NMR experimental techniques to ensure accurate characterization of the local fine structure and acid properties of various acid centers, while further obtaining 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 (CH 3 ) 2 P-(CH 2 ) 2 -P(CH 3 ) 2 , 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 The Al MAS NMR method confirmed that calcination at 723K can dealuminate the Y molecular sieve and form EFAL species. On this basis, the DMPE probe molecule was adsorbed and the 31 P CP / MAS NMR combined with two-dimensional spectroscopy with different contact times 1 H- 31 P HETCOR NMR experiments, supplemented by density functional theory (DFT) calculations, clarified the signal attribution and characterized the detailed acid properties of the dealuminated molecular sieves, such as the type and strength of the acid centers. 31 P- 31 P DARR MAS NMR and 2D 31 P- 31 PDQ MAS NMR, directly obtain various The spatial correlation between the Lewis acid centers and the Lewis acid centers is used to obtain direct experimental evidence of the evolution of the acid pairs during the calcination process. The research objects of the present 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 Lewis acid center (acid pair) significantly improves the catalytic reaction activity of the dealuminated molecular sieve. Therefore, a clear description of the local fine structure and performance of the acid pair in the dealuminated molecular sieve and its possible evolution is the theoretical basis for the development of new and efficient industrial catalysts. The analytical method in the present invention is universal. This method combining multi-headed probe molecules 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 heteropoly acids, and is expected to open up a new path for revealing the local fine structure of the catalyst active center.

[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 nearby 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 It 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 3It 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) The contact time is 0.1 ms;

[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 8It’s the operating procedure. DETAILED DESCRIPTION

[0080] The outstanding features of the present invention are further illustrated by the following examples, which are intended only to illustrate the present invention but not to limit the present invention.

[0081] Description of experimental methods

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

[0083] Experimental conditions: one-dimensional 1 The samples used in H MAS NMR experiments need to be dehydrated at high temperature in advance. On the BrukerAVANCE-III 500MHz spectrometer, 1 The resonance frequency of the H nucleus is 500.57 MHz. A 4 mm probe is used and a 4 mm ZrO 2 The rotor is sealed with a Kel-F cap. The magic angle rotation rate is 12kHz. The π / 2 pulse width is 4.0μs, the pulse delay time is 5s, and the cumulative sampling number is 64 times.

[0084] 2. One Dimension 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 is used and a 4 mm ZrO 2 The rotor is sealed with a Kel-F cap, and the magic angle rotation rate is 12kHz. The experiment uses small-flip-angle technology, with a pulse width of 0.26μs (π / 12) and a pulse delay time of 1s.

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

[0087] Experimental conditions: one-dimensional 31 The samples used in the PCP / MAS NMR experiment need to be dehydrated at high temperature and adsorbed with an appropriate amount of DMPE probe molecules. On a BrukerAVANCE-III 500MHz spectrometer, 31 The resonance frequency of the P core is 202.63MHz. A 4mm probe is used and a 4mm ZrO 2 The rotor was sealed with a Kel-F cap. The magic angle spinning rate was 12 kHz. The contact time was 4 ms and the pulse delay time was 3 s.

[0088] 4. Two-dimensional 1 H- 31 PHETCORNMR experiments

[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 catalyst plant of Nankai University. First, an ion exchange operation was performed to convert the NaY molecular sieve into NH 4 Y molecular sieve; then take part of NH 4 Y and HUSY molecular sieves were subjected to calcination and dealumination treatment respectively to obtain dealumination molecular sieve samples.

[0098] The specific steps are as follows:

[0099] Ion exchange: NaY molecular sieve was added with 1 mol / L NH 4 NO 3 The solution was stirred continuously at 353K for 10 h; then filtered while hot and washed repeatedly with ultrapure water until there was no NO in the sample. 3 - ions; the above ion exchange process was repeated 4 times. Then it was dried in a 383K oven overnight to obtain NH 4 Y molecular sieve.

[0100] Dealumination treatment: a small amount of NH4 The Y (or HUSY) molecular sieve was laid flat in a clean porcelain boat and placed in the center of a tube furnace. In a dry air atmosphere, it was gradually heated to 723K (450℃) at a heating rate of 1K / min and maintained at this temperature for 3.5h. It was then allowed to cool naturally to room temperature. The dealuminated molecular sieves obtained after the above treatment were named HY-d450 and HUSY-d450 (d450 means the dealuminated temperature is 450℃).

[0101] ②Dehydration treatment:

[0102] NH 4 The four molecular sieves Y, HY-d450, HUSY and HUSY-d450 were placed in self-made glass tubes, which were tightly connected to the vacuum system for heating and dehydration. 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, and after the sample was dehydrated, it was convenient to burn it 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 Pa conditions, continue dehydration for 10 hours, then cool naturally to room temperature to obtain dehydrated HY, HY-d450, HUSY and HUSY-d450 samples. Close the valves above all sample tubes; take one of each of the four dehydrated molecular sieve samples and seal them with a flame gun to 1 HMAS NMR detection; other dehydrated samples were subjected to probe molecule adsorption treatment.

[0103] ③Probe molecule adsorption treatment:

[0104] After the molecular sieve sample is pretreated with high-temperature dehydration in step ②, it can adsorb the double-headed probe molecule 1,2-di(dimethylphosphine)ethane (DMPE). Open the valve above the sample tube, adsorb a sufficient amount of volatile DMPE probe molecules into the dehydrated and 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 hour to ensure that DMPE reaches adsorption equilibrium; open the valve above the sample tube, desorb at room temperature for 1 hour, and remove the excessively adsorbed DMPE molecules. Subsequently, seal the sample tube with a flame gun. Place the sealed sample tube in an oven at 373K and heat for 3 hours; long-term heat treatment can ensure the sufficient diffusion of DMPE molecules in the molecular sieve pores and uniform adsorption on the acid sites.

[0105] ④Solid NMR experiment preparation part:

[0106] Prior to solid-state NMR experiments, the sealed samples were transferred to a ZrO 2 The rotor is sealed with a Kel-F cap.

[0107] Solid-state NMR experiments are conducted on a high-field (400MHz and above) solid-state nuclear magnetic resonance spectrometer. Before the experiment, select a suitable probe (e.g., 4mm MAS probe) and install it correctly in the magnet. Open the corresponding software on the computer, read the probe information, and confirm that the preamplifier and power amplifier are connected correctly. Select a suitable standard sample, including 1 H. 31 P and 27 The standard samples of Al are adamantane (1.91 ppm), (NH 4 ) 2 HPO 4 (1.0ppm) and 1mol / LAl(NO 3 ) 3 The standard samples were placed in the probes and the speed was gradually increased from 5kHz to 12kHz (Al(NO 3 ) 3 The solution is liquid and is sampled under static conditions), open the tuning interface, adjust the absorption line to the optimal position by adjusting Match and Tuning, then optimize key parameters such as π / 2 pulse width, cross-polarization (CP) matching power, decoupling power, and calibrate the corresponding core.

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

[0109] First, place the rotor 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, the speed is reduced to 0 and the rotor is removed. 27 Al is a quadrupole nucleus, and the sample is dehydrated. 27 The NMR signal of Al nuclei 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 standard Al(NO 3 ) 3 The solution optimization results calculated the π / 2 pulse width of the solid sample, and the other one-dimensional 27The 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 sampling times are set according to the actual signal strength 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 12kHz. The cross-polarization matching power and decoupling power are set according to the standard sample optimization results, the contact time is 4ms, the pulse delay time is set to 3s, and the one-dimensional 31 The sampling frequency of the P CP / MAS NMR spectrum is usually 256. Then, keep the speed constant and continue to collect the two-dimensional 1 H- 31 P HETCOR NMR spectra, heteronuclear correlation spectra can be associated 1 H nucleus and 31 Spatial information between P nuclei can be obtained by comparing heteronuclear correlation spectra with different contact times, thus assisting 31 Attribution of P chemical shift; at the time of sampling 1 The H channels are decoupled simultaneously, and the contact times are 0.1ms and 4.0ms respectively.

[0111] ⑦Combined with two-dimensional 31 P- 31 PDARR MAS NMR and 2D 31 P- 31 P DQ MAS NMR experiment, to obtain the spatial information between different acid centers in the dealuminated molecular sieve, to determine the evolution of acid pairs. 31 The chemical shift attribution 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. and 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 distance between the nuclei is less than Only when the same nuclear double quantum correlation signal can be detected.

[0112] Figure 1 For DMPE in the dealuminated molecular sieve various possible 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 the various hydroxyl species in molecular sieves. Figure 2 The one-dimensional graph of the four molecular sieves HY, HUSY before dealumination and HY-d450 and HUSY-d450 after dealumination at 450℃ 1 H MAS NMR spectrum. Comparing the changes in the spectra before and after dealumination, the Al-OH signals (2.7 and 0.8 ppm) of the EFAL species in the HY-d450 and HUSY-d450 samples were significantly enhanced, indicating that high-temperature calcination treatment can effectively dealuminate the molecular sieve and generate Lewis-acidic EFAL species, laying the foundation for subsequent experiments.

[0114] One Dimension 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 tetracoordinated framework aluminum. 27 The Al chemical shift is 60ppm; the significantly increased -1ppm resonance peak observed after dealumination indicates the presence of EFAL species. In addition, a broad peak was also observed at about 40ppm in HUSY and HUSY-d450, which may be derived from pentacoordinated EFAL or distorted tetracoordinated aluminum species. Therefore, after dealumination treatment, abundant EFAL species exist in HY-d450 and HUSY-d450 molecular sieves.

[0115] like Figure 4 As shown in the figure, after adsorbing DMPE probe molecules, the non-dealuminized HY original powder 31 Two resonance peaks appeared 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 31The P resonance peak can be attributed to DMPE adsorbed 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. 31 The P resonance peak may originate 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 signal.

[0116] Comparison of the two-dimensional 1 H- 31 PHETCORNMR spectra can further reveal the molecular The adsorption configuration on the Lewis acid center can be 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 adsorbed on Protonated DMPEH formed by acid center + When the contact time increases to 4.0 ms, in addition to (2,7.5) ppm, new related peaks appear, namely (2,2.0), (-25,2.0) and (-37~-44,1.8) ppm, corresponding to DMPE adsorbed 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 (-37~-44,7.5) ppm related signals, which further confirmed that one end was an unprotonated P atom (δ 31 P=-37ppm) of single-head protonated DMPE (δ1 H=7.5ppm), also indicates 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 P = -25ppm) with no spatial correlation.

[0117] 2D 31 P- 31 P DARR MAS NMR experiments can be used to investigate the spatial 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 6 a), 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) is highly mobile, 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), confirming 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 samples, 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+ The species is relatively isolated.

[0118] Two-dimensional adsorption of DMPE molecules 31 P- 31PDARR MAS NMR spectra can only provide relevant information between P atoms in different chemical environments, but 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 (2,4) ppm autocorrelation peak signal indicates that the HY molecular sieve The acid center is close in space, that is, there is 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 centers (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 that during the calcination dealumination process 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 an accurate and reliable method for characterizing the local fine structure and acid properties of various acid centers in solid catalysts. However, the traditional probe molecule-assisted solid-state NMR method cannot accurately describe the structural characteristics of acid pairs. The method of the present invention takes HY and HUSY molecular sieves before and after dealuminization as examples, and innovatively proposes a new double-headed probe molecule 1,2-di(dimethylphosphine)ethane (DMPE), which has a spacing of about The P-atom dual probe can be simultaneously anchored to two adjacent acid centers (acid pairs), and is therefore very suitable for characterizing the evolution of acid pairs in solid catalysts. Moreover, this method of combining probe molecules with solid NMR technology can be used to explore the spatial interactions between different acid centers in different solid catalysts such as molecular sieves, metal oxides, and heteropoly acids, and is expected to open up a new path for revealing the local fine structure of the active center of solid catalysts.

[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 implementation 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 substitutions 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 the molecular sieve samples that have been dealuminated in step ① to obtain dehydrated molecular sieve samples; ③ Probe molecule adsorption treatment: adsorbing an appropriate amount of double-headed probe 1,2-di(dimethylphosphine)ethane molecules into the molecular sieve sample after dehydration and activation in step ②; ④Solid 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-di(dimethylphosphine)ethane probe molecule sample, combined with two-dimensional 1 H- 31 P HETCOR heteronuclear correlation NMR spectra clearly attribute each signal; Combined with two-dimensional 31 P- 31 P DARR MAS NMR and 2D 31 P- 31 P DQ MAS NMR experiments were used to obtain spatial information about the different acid centers in the dealuminated molecular sieves and determine the evolution of the acid pairs.

2. The NMR method for characterizing the evolution of solid catalyst acid pairs according to claim 1, characterized in that: In the step ①, the molecular sieve samples include NaY with Si / Al≈2.8 and HUSY molecular sieves with Si / Al≈3.5; The NaY molecular sieve is converted into NH4Y molecular sieve by ion exchange, and the ion exchange includes: NaY molecular sieve was added into 1 mol / L NH4NO3 solution at a ratio of 1 g / 100 mL and stirred continuously 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 383K overnight to obtain NH4Y molecular sieve.

3. The NMR method for characterizing the evolution of solid catalyst acid pairs according to claim 1, characterized in that: In the step ①, the dealumination treatment comprises: Spread 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, gradually heat to 723 K at a heating rate of 1 K / min and maintain at this temperature for 3.5 h; Then let it 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, characterized in that: In the step ②, the dehydration treatment comprises: The molecular sieve samples were placed in glass tubes respectively, which were tightly connected to the vacuum system for 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 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.

5. The NMR method for characterizing the evolution of solid catalyst acid pairs according to claim 1, characterized in that: In the step ③, the adsorption process includes: Open the valve above the sample tube to adsorb sufficient volatile 1,2-di(dimethylphosphine)ethane molecules into 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 hour to ensure that 1,2-bis(dimethylphosphine)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(dimethylphosphine)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 the sufficient diffusion of 1,2-bis(dimethylphosphine)ethane molecules in the molecular sieve pores and the 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 with the dehydrated sample in the probe, gradually increase the speed to 12kHz, and perform tuning; Will 1 The π / 2 pulse width of the H core 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; 1 After the H MAS NMR spectrum acquisition is completed, the speed is reduced to 0 and the rotor is removed; because 27 Al is a quadrupole nucleus, and the sample is dehydrated. 27 The NMR signal of Al nuclei 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 Dimension 27 The Al MAS NMR experiment used the small-flip-angle technique, with a pulse width of π / 12 and a pulse delay time of 1 s set during sampling. 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 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: 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 spectra; Keep the rotation speed constant and continue to collect the two-dimensional 1 H- 31 PHETCORNMR spectra, heteronuclear correlation spectra can be associated 1 H nucleus and 31 Spatial information between P nuclei can be obtained by comparing heteronuclear correlation spectra with different contact times, thus assisting 31 Assignment of P chemical shifts; At sampling time 1 The H channels are decoupled simultaneously, and the contact times are 0.1ms and 4.0ms respectively.

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

9. 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 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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