Method for determining number of Bronsted acid sites on outer surface of 5A zeolite molecular sieve

Abstract

The invention belongs to the technical field of zeolite molecular sieves, and particularly discloses a method for determining the number of Bronsted acid sites on the outer surface of a 5A zeolite molecular sieve. The method is used for solving the problem that the current method for measuring the acid sites of the zeolite molecular sieve cannot distinguish the components of the acid sites on the outer surface and the inner surface of the zeolite molecular sieve. The method comprises the following steps: firstly enabling the 5A zeolite molecular sieve to adsorb n-propylamine at room temperature, then purging with helium, then heating to 823-923K at a heating rate of 5-20K / min under the condition of introducing helium, and measuring the amount of generated propylene by using mass spectrum, thereby determining the number of Bronsted acidic sites on the outer surface of the 5A zeolite molecular sieve. The determination method is reliable and simple and convenient to operate, effectively realizes quantitative determination of the number of the Bronsted acid sites on the outer surface of the 5A zeolite molecular sieve, and has important significance in application of the 5A zeolite molecular sieve in the fields of catalysis and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of zeolite molecular sieves, and in particular relates to a method for determining the number of Bronsted acid sites on the outer surface of a 5A zeolite molecular sieve. Background Art

[0002] The basic structural unit of zeolite molecular sieve is silicon-oxygen tetrahedron (SiO 4 ) and aluminum oxide tetrahedron (AlO 4 ). In the aluminum-oxygen tetrahedron, since aluminum is +3 valence, the aluminum-oxygen tetrahedron carries a negative charge, which makes the zeolite molecular sieve framework carry a negative charge. Protons or cations must be used to neutralize it to achieve electrical neutrality. When these negative charges are replaced by hydrogen protons (H + ) to balance, the zeolite molecular sieve becomes a proton donor, that is, a Bronsted acid ( Acid, referred to as B acid). At the defect site of the zeolite molecular sieve framework or after loading metal ions, the unsaturated metal cations will enable the zeolite molecular sieve to accept electron pairs, which is Lewis acid (Lewis acid, referred to as L acid). The outer surface of the zeolite molecular sieve is hydroxylated and has Bronsted acidity, among which the acidity of aluminum hydroxyl (Al-OH) is stronger than that of silicon hydroxyl (Si-OH). Bronsted acid sites have important applications in catalytic reactions, such as catalytic heavy oil cracking, fuel oil hydrodesulfurization, gasoline and diesel reforming in the oil refining industry, catalytic addition reactions, substitution reactions, esterification reactions in organic synthesis, etc.; they also have important applications in the field of adsorption and separation, such as material surface grafting modification, carbon dioxide capture, and adsorption separation of propylene and propane. The Bronsted acidity of zeolite molecular sieves can be improved by reducing the silicon-aluminum ratio of zeolite molecular sieves (increasing the aluminum content) and introducing acidic additives (such as phosphoric acid and sulfuric acid). The Bronsted acid sites on the outer and inner surfaces of zeolite molecular sieves behave differently in catalytic reactions and material surface modification. Therefore, in addition to quantitatively determining the total amount of Bronsted acid sites on the outer and inner surfaces of zeolite molecular sieves, distinguishing the Bronsted acid site components on the outer and inner surfaces is of great significance to the application of zeolite molecular sieves.

[0003] The more mature methods for determining the type, quantity and strength of acid sites in zeolite molecular sieves are: (1) ammonia programmed temperature desorption (NH 3-TPD method): After the zeolite molecular sieve adsorbs ammonia, it is subjected to programmed temperature desorption, and the amount of ammonia desorbed at different temperatures is recorded to evaluate the concentration and strength of the acid sites on the surface of the zeolite molecular sieve (acid site distribution). The desorption peak at a relatively low temperature corresponds to the weak acid site, and the desorption peak at a relatively high temperature corresponds to the strong acid site. (2) Pyridine adsorption infrared spectroscopy (Py-FTIR method): Using pyridine molecules as probes, when pyridine molecules are adsorbed on the acid sites on the surface of the zeolite molecular sieve, the vibration frequency changes of the pyridine molecules are analyzed by Fourier transform infrared spectroscopy. Different types of acid sites will lead to different frequency changes when pyridine molecules are adsorbed, so that the type and relative strength of the acid sites in the zeolite molecular sieve can be determined. (3) Nuclear magnetic resonance method (NMR method): The acid sites in the zeolite molecular sieve are studied by solid-state nuclear magnetic resonance. By analyzing the proton spectrum or silicon spectrum in the sample, the different types of acid sites in the zeolite molecular sieve and their relative abundance can be determined. This method is very helpful for understanding the internal structure of the zeolite molecular sieve and the distribution and properties of various acid sites. (4) Thermogravimetric analysis-mass spectrometry (TGA-MS): Thermogravimetric analysis combined with mass spectrometry can be used to evaluate the gas released by zeolite molecular sieves during heating. By detecting the type and amount of released gas, the number and activity of acid sites in the zeolite molecular sieve can be inferred. This method is particularly suitable for analyzing the gas release characteristics of zeolite molecular sieves at different temperatures, thereby indirectly reflecting the characteristics of its acid sites.

[0004] The above method can only determine the total amount of Bronsted acid sites on the surface of the zeolite molecular sieve, but cannot distinguish the amount of Bronsted acid sites on the outer surface and the inner surface of the zeolite molecular sieve. Summary of the invention

[0005] The purpose of the present invention is to provide a method for determining the number of Bronsted acid sites on the outer surface of a 5A zeolite molecular sieve, which effectively solves the problem that the current method for determining the acid sites of zeolite molecular sieves cannot distinguish the components of the acid sites on the outer surface and the inner surface of the zeolite molecular sieve.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for determining the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve, comprising the following steps:

[0007] S1. Load 5A zeolite molecular sieve into a fixed bed, introduce helium at a flow rate of 50-150 mL / min, heat treat at 473-573 K for 1-3 h, cool to room temperature, and set aside.

[0008] S2. Introduce helium at a flow rate of 2.5 to 10 mL / min into a bubbler containing n-propylamine liquid to bring out the n-propylamine vapor from the bubbler, and then dilute it with helium at a flow rate of 50 to 200 mL / min. Allow the 5A zeolite molecular sieve heat-treated in step S1 to in-situ adsorb n-propylamine for 0.5 to 1.5 h at 298 K to obtain sample I.

[0009] S3. Purge sample I in situ with helium at a flow rate of 50-150 mL / min for 6-16 h, and then heat it to 823-923 K at a heating rate of 5-20 K / min under helium. Analyze the exhausted gas with a mass spectrometer during the heating period, and record the changes in mass spectrometry signal intensity of mass-to-charge ratio 41 and mass-to-charge ratio 30 with heating time.

[0010] S4, multiplying the mass spectrum signal intensity of the mass-to-charge ratio 30 obtained in step S3 by 0.05 to obtain the mass spectrum signal intensity of the mass-to-charge ratio 41 generated by n-propylamine, and then subtracting the mass spectrum signal intensity of the mass-to-charge ratio 41 generated by n-propylamine at the same time point (same temperature) obtained above from the mass spectrum signal intensity of the mass-to-charge ratio 41 obtained at any time point in step S3, to obtain a curve of the mass spectrum signal intensity of the mass-to-charge ratio 41 of propylene generated by the chemical reaction versus heating time, and further obtain a curve of the change of propylene yield versus heating time.

[0011] S5, integrating the curve of propylene yield versus heating time obtained in step S4 over the entire heating time range to obtain the number of Bronsted acid sites (μmol / g, number / nm) on the outer surface of 5A zeolite molecular sieve. 2 ).

[0012] Furthermore, the measuring device used includes a helium cylinder, a bubbler, a fixed bed, a mass spectrometer and a temperature controller for controlling the heating temperature of the fixed bed, the outlet pipeline of the helium cylinder is divided into a first pipeline and a second pipeline, wherein the first pipeline is connected to the inlet of the bubbler, the outlet pipeline of the bubbler is connected to the inlet of the fixed bed after intersecting with the second pipeline, mass flow meters are provided on the first pipeline and the second pipeline, an injection port is provided on the pipeline after the outlet pipeline of the bubbler intersects with the second pipeline, and the outlet of the fixed bed is connected to the mass spectrometer.

[0013] Furthermore, when the fixed bed is not loaded with 5A zeolite molecular sieve, at this time, at 298K, helium is introduced into the fixed bed at a flow rate of 100mL / min, and 1mL of propylene gas is injected through the injection port. The mass spectrometer is used to record the change in the mass spectrum signal intensity of the mass-to-charge ratio of 41 over time until the propylene gas is completely blown out by the helium. The mass spectrum signal intensity change curve is integrated over the entire time range to obtain the integrated intensity of the mass-to-charge ratio of 41 for 1mL of propylene gas.

[0014] The mass spectrum signal intensity of the mass-to-charge ratio 41 of the propylene generated by the chemical reaction obtained in step S4 is divided by the integrated intensity of the mass-to-charge ratio 41 of 1 mL of propylene gas to obtain a curve of the change of propylene yield with heating time.

[0015] Furthermore, in step S1, the flow rate of the introduced helium gas is 100 mL / min, the heat treatment temperature is 523 K, and the heat treatment time is 2 h.

[0016] Furthermore, in step S2, the flow rate of helium used to bring the n-propylamine vapor out of the bubbler is 5 mL / min, and the flow rate of helium used for dilution is 100 mL / min; the time for adsorbing n-propylamine is 1 hour.

[0017] Further, in step S3, the flow rate of helium gas used to purge sample I is 100 mL / min, and the purge time is 12 h.

[0018] Furthermore, in step S3, the heating rate of sample I is 10K / min, and the final heating temperature is 873K.

[0019] Compared with the prior art, the beneficial technical effects of the present invention are:

[0020] (1) The probe molecule used in the present invention to determine the Bronsted acid sites on the outer surface of 5A zeolite molecular sieve is n-propylamine. The desorption of n-propylamine occurs in a relatively low temperature range, while the chemical reaction to generate propylene occurs in a relatively high temperature range. The two have little overlap, which is conducive to the accurate quantitative determination of propylene yield by mass spectrometry, and further determination of the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve.

[0021] (2) The adsorbent used in the present invention is low-concentration n-propylamine vapor (partial pressure of about 0.04 atm) with helium as carrier gas, and the temperature is room temperature, so n-propylamine liquid will not condense in the test system to affect the operation of the system; and the adsorption of n-propylamine on the 5A zeolite molecular sieve is also carried out at room temperature. At room temperature, n-propylamine is only adsorbed on the outer surface of the 5A zeolite molecular sieve and does not enter the interior of the 5A zeolite molecular sieve. The 5A zeolite molecular sieve does not need to be treated to block the pores or shield the inner surface, and the number of Bronsted acid sites on the outer surface can be directly measured.

[0022] (3) The present invention is used to measure the low concentration of n-propylamine vapor at the Bronsted acid sites on the outer surface of 5A zeolite molecular sieve. The vapor is generated in real time by volatilizing the n-propylamine liquid in a bubbler at room temperature using helium as a carrier gas, without the need for heating or using a high-pressure gas cylinder.

[0023] (4) The present invention is safe and reliable, easy to operate, and can effectively realize the quantitative determination of the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve, which is of great significance to the application of 5A zeolite molecular sieve in the field of catalysis. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the structure of the measuring device of the present invention.

[0025] Figure 2 It is a scanning electron microscope photograph of the 5A zeolite molecular sieve used in Embodiment 1, wherein the positions marked with Spectrum 1 and Spectrum 2 are sampling points for measuring elemental composition with an energy spectrometer.

[0026] Figure 3 It is the nitrogen adsorption-desorption isotherm of 5A zeolite molecular sieve at liquid nitrogen temperature.

[0027] Figure 4 This is the pore size distribution diagram of 5A zeolite molecular sieve.

[0028] Figure 5 This is a schematic diagram of the adsorption structure of n-propylamine molecules on the outer surface of 5A zeolite molecular sieve.

[0029] Figure 6 This is a graph showing the change in n-propylamine yield with temperature during programmed temperature increase for the unmodified, n-hexylphosphonic acid (HPA) modified and 3-aminopropane-1-phosphoric acid (APPA) modified 5A zeolite molecular sieves in Example 1, Example 6 and Example 7.

[0030] Figure 7 It is a graph showing the change in propylene yield with temperature during programmed temperature increase of the unmodified, n-hexylphosphonic acid (HPA) modified and 3-aminopropane-1-phosphoric acid (APPA) modified 5A zeolite molecular sieves in Examples 1, 6 and 7. DETAILED DESCRIPTION

[0031] Embodiment 1: A method for determining the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve, using a measuring device such as Figure 1As shown, it includes a helium cylinder 1, a bubbler 2 (volume 300mL, containing 100mL of n-propylamine liquid, room temperature), a fixed bed 3, a mass spectrometer 4 and a temperature controller 5. The outlet pipeline of the helium cylinder 1 is divided into a first pipeline and a second pipeline, wherein the first pipeline is connected to the inlet of the bubbler 2, and the outlet pipeline of the bubbler 2 is connected to the inlet of the fixed bed 3 after intersecting with the second pipeline. The first pipeline and the second pipeline are both provided with a mass flow meter 6, and the total pipeline after the outlet pipeline of the bubbler 2 intersects with the second pipeline is provided with an injection port 7 and a third valve 8. A first valve 9 is provided on the first pipeline, a second valve 10 is provided on the outlet pipeline of the bubbler 2, a fourth valve 11 is provided on the outlet pipeline of the fixed bed 3, and the outlet of the fourth valve 11 is connected to the discharge port and the mass spectrometer 4, the bypass of the fixed bed 3 is controlled by the fifth valve 12 (when the fixed bed 3 is loaded with a sample, the fifth valve 12 is opened, and the third valve 8 and the fourth valve 11 are closed at the same time), and the heating of the fixed bed 3 is controlled by the temperature controller 5.

[0032] The following steps are included: (1) Figure 1 In the device shown, no solid sample (5A zeolite molecular sieve) is loaded into the fixed bed. At 298K, helium is introduced at a flow rate of 100mL / min, and 1mL of propylene gas is injected through the injection port. The mass spectrometer is used to record the change in the mass spectrum signal intensity of a mass-to-charge ratio of 41 over time until the injected propylene gas is completely blown out by the helium. The mass spectrum signal intensity change curve is integrated over the entire time range to obtain the integrated intensity of the mass-to-charge ratio of 41 for 1mL of propylene gas. The measurement is repeated 5 times and the average value is taken.

[0033] (2) 0.5 g of 5A zeolite molecular sieve was loaded into a fixed bed, and helium gas was introduced at a flow rate of 50 to 150 mL / min, and heat-treated at 473 to 573 K for 1 to 3 h, and then cooled to room temperature.

[0034] (3) Helium is introduced into the bubbler at a flow rate of 2.5 to 10 mL / min to bring the n-propylamine vapor out of the bubbler, and then diluted with helium at a flow rate of 50 to 200 mL / min to allow the 5A zeolite molecular sieve heat-treated in step (2) to in-situ adsorb n-propylamine for 0.5 to 1.5 h at 298 K.

[0035] (4) The 5A zeolite molecular sieve adsorbing n-propylamine in step (3) is purged in situ with helium at a flow rate of 50 to 150 mL / min for 6 to 16 h, and then heated to 823 to 923 K at a heating rate of 5 to 20 K / min under the flow of helium. During the heating period, the discharged gas is analyzed by a mass spectrometer, and the changes in the mass spectrometry signal intensity of the mass-to-charge ratio of 41 and the mass-to-charge ratio of 30 with the heating time are recorded.

[0036] (5) The mass spectrometer signal intensity of the mass-to-charge ratio 30 obtained in step (4) is multiplied by 0.05 to convert the mass spectrometer signal intensity of the mass-to-charge ratio 41 generated by n-propylamine.

[0037] (6) The intensity of the mass-to-charge ratio 41 obtained at any time point in step (4) is subtracted from the mass spectrum signal intensity of the mass-to-charge ratio 41 generated by n-propylamine at the same time point (same temperature) obtained in step (5) to obtain a curve of the mass spectrum signal intensity of the mass-to-charge ratio 41 of propylene generated by the chemical reaction as a function of the heating time.

[0038] (7) The intensity of the mass-to-charge ratio 41 of propylene generated by the chemical reaction obtained in step (6) is divided by the integrated intensity of the mass-to-charge ratio 41 of 1 mL of propylene gas obtained in step (1) to obtain a curve of the change of propylene yield with heating time.

[0039] (8) The curve of the change of propylene yield with heating time obtained in step (7) is integrated over the entire heating time range to obtain the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve (μmol / g, number / nm 2 ).

[0040] The morphology of the 5A zeolite molecular sieve (Sigma-Aldrich 233676) used in this embodiment is as follows: Figure 2 Scanning electron microscope photos (taken by Hitachi SU3500 VP scanning electron microscope) show that the large particles are about 4μm cubes, and there are a large number of nano-sized particles. The elemental composition measured by energy dispersive spectrometer is shown in Table 1, and the silicon / aluminum atomic ratio is about 1, and the calcium / sodium atomic ratio is about 2.

[0041] Table 1 Elemental composition of 5A zeolite molecular sieve determined by energy spectrometer

[0042] element Content (wt%) Si 17.4 Al 17.0 O 46.4 Ca 8.5 Na 4.1

[0043] The adsorption-desorption isotherm of nitrogen (purity 99.999%) of the 5A zeolite molecular sieve used in this embodiment at liquid nitrogen temperature (tested by 3Flex gas adsorption instrument of Micromeritics Instruments, USA) is as follows: Figure 3 As shown. According to the relative pressure p / p 0 In 10 -7 The specific surface area of ​​the 5A zeolite molecular sieve used in this embodiment is 680 m 2 / g; according to relative pressure p / p 0 The adsorption data in the range of 0.20 to 0.40 were used to calculate the external specific surface area (pore size>2.5 nm) using the t-Plot method. The external specific surface area of ​​the 5A zeolite molecular sieve used in this embodiment is 30 m 2 / g; The pore size distribution is calculated by the Horvath-Kawazoe method. The pore size distribution of the 5A zeolite molecular sieve used in this embodiment is as follows: Figure 4 As shown, the average pore size is

[0044] The principle of the present invention is that when n-propylamine in helium carrier gas contacts 5A zeolite molecular sieve at room temperature, n-propylamine molecules can be adsorbed on the outer surface of 5A zeolite molecular sieve, such as Figure 5 As shown, it cannot diffuse into the interior of 5A zeolite molecular sieve (due to the limitation of the smaller pores of 5A zeolite molecular sieve and the obstruction of n-propylamine molecules adsorbed at the pores). The adsorption sites on the outer surface of 5A zeolite molecular sieve include Bronsted acid sites (surface hydroxyl groups) and other adsorption sites. The n-propylamine molecules adsorbed on the Bronsted acid sites undergo Hofmann elimination reaction at high temperature (>600K) under the catalysis of Bronsted acid: CH 3 CH 2 CH 2 NH 2 +Z-OH→CH 3 CH 2 CH 2 NH 3 + +ZO - →C 3 H 6 +NH 3 +Z-OH, propylene and ammonia are generated, and the number of Bronsted acid sites is equal to the number of molecules of propylene or ammonia generated (1:1 relationship); while the n-propylamine molecules adsorbed on other sites do not undergo chemical reaction and desorb during the heating process. The yield of desorbed n-propylamine in Example 1, Example 6 and Example 7 varies with temperature as shown in Figure 6 The yield of propylene during the programmed temperature rise was analyzed by mass spectrometry. The yield of propylene in Example 1, Example 6 and Example 7 changed with temperature as shown in FIG. Figure 7 As shown, the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve can be quantitatively determined. The amount of generated ammonia is not used to quantify the number of Bronsted acid sites because the mass spectrometry signal of ammonia with a mass-to-charge ratio of 17 is interfered by the water precipitated by the zeolite molecular sieve (water also produces a mass spectrometry signal with a mass-to-charge ratio of 17).

[0045] The present invention is further described in detail below with reference to specific embodiments.

[0046] Embodiment 1: (1) in Figure 1In the device shown, no solid sample is loaded into the fixed bed. At 298K, the third valve and the fourth valve are opened and the first valve, the second valve and the fifth valve are closed. Helium (purity 99.999%, the same below) is introduced at a flow rate of 100mL / min (the flow rate is controlled by MFC1), 1mL of propylene gas is injected from the injection port with a syringe, and the change of the mass spectrum signal intensity of the mass-to-charge ratio (m / z) of 41 with time is recorded by a mass spectrometer until the injected propylene gas is completely blown out by the helium (the mass spectrum signal intensity of the mass-to-charge ratio of 41 returns to 0), and the mass spectrum signal intensity curve is integrated over the entire time range to obtain the integrated intensity of the mass-to-charge ratio of 41 for 1mL of propylene gas. The measurement is repeated 5 times and the average value is taken.

[0047] (2) 0.5 g of 5A zeolite molecular sieve solid sample (<10 μm, Sigma-Aldrich 233676, the same below) was loaded into the fixed bed, the third valve and the fourth valve were opened and the first valve, the second valve and the fifth valve were closed, and helium was introduced at a flow rate of 100 mL / min (the flow rate was controlled by MFC1), and heat treated at 523 K for 2 h (heating was controlled by a temperature controller), and then cooled to room temperature.

[0048] (3) Open the first valve, the second valve, the third valve and the fourth valve and close the fifth valve, introduce helium at a flow rate of 5 mL / min into the bubbler (containing n-propylamine liquid, CAS No. 107-10-8, purity 99%, room temperature, the same below) (the flow rate is controlled by MFC2), use helium as a carrier gas to bring the n-propylamine vapor out of the bubbler, and then dilute it with helium at a flow rate of 100 mL / min (the flow rate is controlled by MFC1, at this time the partial pressure of n-propylamine in helium is 0.038 atm), and allow the solid sample heat-treated in step (2) to adsorb n-propylamine in situ at 298 K for 1 h.

[0049] (4) The third valve and the fourth valve were opened, and the first valve, the second valve and the fifth valve were closed. The solid sample treated in step (3) was purged in situ with helium at a flow rate of 100 mL / min (the flow rate was controlled by MFC1) for 12 h. Then, the solid sample was heated to 873 K at a heating rate of 10 K / min under the flow of helium. During this period, the discharged gas was analyzed by a mass spectrometer (MALDI-TOF type, Bruker Company), and the changes in the mass spectrometry signal intensity of mass-to-charge ratio 41 and mass-to-charge ratio 30 with heating time were recorded.

[0050] (5) The mass spectrometer signal intensity of the mass-to-charge ratio 30 obtained in step (4) is multiplied by 0.05 (the ratio of the intensity of the mass-to-charge ratio 41 of n-propylamine to the intensity of the mass-to-charge ratio 30 is 0.05) to convert the mass spectrometer signal intensity of the mass-to-charge ratio 41 generated by n-propylamine.

[0051] (6) The intensity of the mass-to-charge ratio 41 obtained at any time point in step (4) is subtracted from the mass spectrum signal intensity of the mass-to-charge ratio 41 generated by n-propylamine at the same time point (same temperature) obtained in step (5) to obtain a curve of the mass spectrum signal intensity of the mass-to-charge ratio 41 of propylene generated by the chemical reaction as a function of the heating time.

[0052] (7) The intensity of the mass-to-charge ratio 41 of propylene generated by the chemical reaction obtained in step (6) is divided by the integrated intensity of the mass-to-charge ratio 41 of 1 mL of propylene gas obtained in step (1), and then multiplied by 1 mL to obtain the propylene gas yield (mL / min) of the chemical reaction at any time point during the heating process after 0.5 g of 5A zeolite molecular sieve adsorbs n-propylamine, and then the mL in the unit of the propylene gas yield is converted to μmol, and then divided by the mass of the solid sample 0.5 g to obtain a curve of the change of propylene yield (μmol / g·min) with heating time.

[0053] (8) The curve of propylene yield versus heating time obtained in step (7) is integrated over the entire heating time range to obtain the number of Bronsted acid sites (μmol / g) on ​​the outer surface of the 5A zeolite molecular sieve. The number of propylene yield per nm can be calculated based on the outer surface area of ​​the 5A zeolite molecular sieve. 2 The number of Brønsted acid sites.

[0054] The curve of propylene yield changing with temperature in this example is as follows Figure 7 As shown in the "unmodified 5A" curve, the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve was determined to be 132±21μmol / g (three replicates), i.e. 2.6±0.4 / nm 2 (Based on external specific surface area 30m 2 / g calculation).

[0055] Example 2: The difference between this example and Example 1 is that: in step (2), the flow rate of helium introduced is 50 mL / min, the heat treatment temperature is 473 K, and the heat treatment time is 3 h; in step (3), the flow rate of helium introduced into the bubbler is 10 mL / min, the gas containing n-propylamine coming out of the bubbler is diluted with helium at a flow rate of 200 mL / min, and the solid sample adsorbs n-propylamine for 0.5 h; in step (4), the flow rate of helium used for purging is 150 mL / min, and the purging time is 6 h; in step (4), the heating rate is 5 K / min, and the final heating temperature is 823 K.

[0056] The number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve measured in this example is 121 μmol / g, i.e., 2.4 / nm 2 .

[0057] Example 3: The difference between this example and Example 1 is that: in step (2), the flow rate of helium introduced is 150 mL / min, the heat treatment temperature is 573 K, and the heat treatment time is 1 h; in step (3), the flow rate of helium introduced into the bubbler is 2.5 mL / min, the gas containing n-propylamine coming out of the bubbler is diluted with helium at a flow rate of 50 mL / min, and the solid sample adsorbs n-propylamine for 1.5 h; in step (4), the flow rate of helium used for purging is 50 mL / min, and the purging time is 16 h; in step (4), the heating rate is 20 K / min, and the final heating temperature is 923 K.

[0058] The number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve measured in this example is 118 μmol / g, i.e., 2.4 / nm 2 .

[0059] Example 4: The difference between this example and Example 1 is that: in step (2), the flow rate of helium introduced is 75 mL / min, the heat treatment temperature is 498 K, and the heat treatment time is 1.5 h; in step (3), the flow rate of helium introduced into the bubbler is 7.5 mL / min, the gas containing n-propylamine coming out of the bubbler is diluted with helium at a flow rate of 150 mL / min, and the solid sample adsorbs n-propylamine for 0.8 h; in step (4), the flow rate of helium used for purging is 75 mL / min, and the purging time is 10 h; in step (4), the heating rate is 15 K / min, and the final heating temperature is 848 K.

[0060] The number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve measured in this example is 129 μmol / g, i.e., 2.6 / nm 2 .

[0061] Example 5: The difference between this example and Example 1 is that: the heat treatment time in step (2) is 1.5 h; the adsorption time of n-propylamine on the solid sample in step (3) is 0.8 h; the helium purge time in step (4) is 10 h, the heating rate is 15 K / min, and the final heating temperature is 823 K.

[0062] The number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve measured in this example is 115 μmol / g, i.e., 2.3 / nm 2 .

[0063] Example 6: 5A zeolite molecular sieve modified with n-hexylphosphonic acid was used as a sample for testing the Bronsted acid sites on the external surface.

[0064] (1) Figure 1In the device shown, no solid sample is loaded into the fixed bed. At 298K, the third valve and the fourth valve are opened and the first valve, the second valve and the fifth valve are closed. Helium is introduced at a flow rate of 100mL / min, and 1mL of propylene gas is injected from the injection port with a syringe. The mass spectrometer is used to record the change in the mass spectrum signal intensity of a mass-to-charge ratio of 41 over time until the injected propylene gas is completely blown out by the helium. The mass spectrum signal intensity curve is integrated over the entire time range to obtain the integrated intensity of the mass-to-charge ratio of 41 for 1mL of propylene gas. The measurement is repeated 5 times and the average value is taken.

[0065] (2) 0.5 g of a solid sample of 5A zeolite molecular sieve modified with n-hexylphosphonic acid was loaded into a fixed bed, the third valve and the fourth valve were opened and the first valve, the second valve and the fifth valve were closed, helium was introduced at a flow rate of 100 mL / min, and heat-treated at 523 K for 2 h, and then cooled to room temperature.

[0066] The preparation method of 5A zeolite molecular sieve modified with n-hexylphosphonic acid is as follows: 0.0331 g n-hexylphosphonic acid (HPA, CAS No. 4721-24-8, (HO) 2 PO(CH 2 ) 5 CH 3 , purity 99%) solid powder and 1g 5A zeolite molecular sieve were added with 250mL tetrahydrofuran solvent (CAS No. 109-99-9, purity 99%), stirred for 24h, then centrifuged to remove the supernatant, then heat treated at 130°C for 7h, and finally washed 4 times with tetrahydrofuran solvent.

[0067] (3) Open the first valve, the second valve, the third valve, and the fourth valve, and close the fifth valve, and introduce helium into the bubbler at a flow rate of 5 mL / min, use helium as a carrier gas to carry the n-propylamine vapor out of the bubbler, and then dilute it with helium at a flow rate of 100 mL / min, and allow the solid sample heat-treated in step (2) to adsorb n-propylamine in situ at 298 K for 1 hour.

[0068] (4) Open the third valve and the fourth valve, and close the first valve, the second valve and the fifth valve, and purge the solid sample treated in step (3) with helium at a flow rate of 100 mL / min for 12 h, and then heat it to 873 K at a heating rate of 10 K / min under the flow of helium. During the heating period, analyze the discharged gas with a mass spectrometer, and record the changes in the mass spectrum signal intensity of the mass-to-charge ratio of 41 and the mass-to-charge ratio of 30 as a inverse of the heating time.

[0069] (5) The mass spectrometer signal intensity of the mass-to-charge ratio 30 obtained in step (4) is multiplied by 0.05 to convert the mass spectrometer signal intensity of the mass-to-charge ratio 41 generated by n-propylamine.

[0070] (6) The fixed bed is reloaded with a solid sample of 5A zeolite molecular sieve modified with n-hexylphosphonic acid, and heat treated according to step (2), then step (3) is skipped, and the change in the mass spectrometry signal intensity of the mass-to-charge ratio of 41 during the heating process as a function of the heating time is measured according to step (4). This step can be skipped if the measurement requirements are not high.

[0071] (7) The intensity of the mass-to-charge ratio 41 obtained at any heating time point in step (4) is subtracted from the mass spectrum signal intensity of the mass-to-charge ratio 41 generated by n-propylamine at the same heating time point (same temperature) obtained in step (5), and then the mass spectrum signal intensity of the mass-to-charge ratio 41 generated by n-hexylphosphonic acid on the outer surface of the 5A zeolite molecular sieve at the same heating time point obtained in step (6) is subtracted to obtain a curve of the mass spectrum signal intensity of the mass-to-charge ratio 41 of propylene generated by the chemical reaction as a function of the heating time.

[0072] (8) The intensity of the mass-to-charge ratio 41 of propylene generated by the chemical reaction obtained in step (7) is divided by the integrated intensity of the mass-to-charge ratio 41 of 1 mL of propylene gas obtained in step (1), and then multiplied by 1 mL to obtain the propylene gas yield (mL / min) of the chemical reaction at any time point during the heating process after 0.5 g of 5A zeolite molecular sieve adsorbs n-propylamine, and then the propylene yield (μmol / g·min) versus heating time curve is obtained.

[0073] (9) The curve of the change of propylene yield with heating time obtained in step (8) is integrated over the entire heating time range to obtain the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve (μmol / g, number / nm 2 ).

[0074] The propylene yield of this example changes with temperature as shown in Figure 7 As shown in the 5A-HPA curve, the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve is determined to be 24 μmol / g, i.e. 0.48 / nm 2 , while the number of Bronsted acid sites on the outer surface of the unmodified 5A zeolite molecular sieve is >100 μmol / g, indicating that most of the Bronsted acid sites on the outer surface of the 5A zeolite molecular sieve are occupied by n-hexylphosphonic acid.

[0075] Example 7: 5A zeolite molecular sieve modified with 3-aminopropane-1-phosphoric acid was used as a sample for testing the Bronsted acid sites on the external surface.

[0076] The preparation method of 5A zeolite molecular sieve modified with 3-aminopropane-1-phosphoric acid is as follows: 0.0277 g of 3-aminopropane-1-phosphoric acid (APPA, CAS No. 13138-33-5, (HO) 2 PO(CH 2 ) 3 NH2 , purity 99%) solid powder and 1g 5A zeolite molecular sieve were added with 250mL tetrahydrofuran solvent (CAS No. 109-99-9, purity 99%), stirred for 24h, then centrifuged to remove the supernatant, then heat treated at 130°C for 7h, and finally washed 4 times with tetrahydrofuran solvent.

[0077] The same test method as in Example 6 was used to measure the change of propylene yield with temperature. Figure 7 As shown in the 5A-APPA curve, the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve is determined to be 1 μmol / g, i.e. 0.02 / nm 2 , indicating that the Brønsted acid sites on the outer surface of 5A zeolite molecular sieve are basically covered by 3-aminopropane-1-phosphoric acid.

[0078] Example 8: The difference between this example and Example 1 is that the 5A zeolite molecular sieve in Example 1 is first ground in an agate mortar for 30 minutes to reduce its particle size and increase its external specific surface area to 32 m 2 / g (nitrogen adsorption isotherm was measured at liquid nitrogen temperature, and the external specific surface area was calculated by t-Plot method). Using the same test method as in Example 1, the number of Bronsted acid sites on the external surface of the 5A zeolite molecular sieve in this example was measured to be 143 μmol / g, i.e., 2.7 / nm 2 The number of Bronsted acid sites per unit mass of 5A zeolite molecular sieve is increased compared with that before grinding, and the number of sites per unit area is similar to that before grinding.

[0079] Example 9: The difference between this example and Example 1 is that the 5A zeolite molecular sieve in Example 1 is first ground in an agate mortar for 60 minutes to reduce its particle size and increase its external specific surface area (nitrogen adsorption isotherm measured at liquid nitrogen temperature, external specific surface area calculated by t-Plot method) to 34 m 2 The same test method as in Example 1 was used to measure the number of Bronsted acid sites on the outer surface of the 5A zeolite molecular sieve of this example to be 153 μmol / g, i.e., 2.7 sites / nm. 2 The number of Bronsted acid sites per unit mass of 5A zeolite molecular sieve is significantly increased compared with that before grinding, and the number of sites per unit area is similar to that before grinding.

[0080] Example 10: The difference between this example and Example 1 is that the 5A zeolite molecular sieve in Example 1 is first ball milled (ND8-1L ball mill, Nanjing Nanda Tianzun Electronics Co., Ltd., zirconium dioxide grinding balls, the ratio of grinding balls to material mass is 45:1, and the ball milling speed is 540r / min), the ball milling time is 40min, and the external specific surface area after ball milling is increased from 30m 2 / g increased to 36m 2The same test method as in Example 1 was used to measure the number of Bronsted acid sites on the outer surface of the 5A zeolite molecular sieve of this example to be 192 μmol / g, i.e., 3.2 sites / nm. 2 The number of Bronsted acid sites per unit mass of 5A zeolite molecular sieve is significantly increased compared with that before ball milling, and the number of sites per unit area is significantly increased compared with that before ball milling.

[0081] Example 11: The difference between this example and Example 1 is that the 5A zeolite molecular sieve in Example 1 is first ball milled (ND8-1L ball mill, Nanjing Nanda Tianzun Electronics Co., Ltd., zirconium dioxide grinding balls, the ratio of grinding balls to material mass is 45:1, and the ball milling speed is 540r / min), the ball milling time is 80min, and the external specific surface area after ball milling is reduced from 30m 2 / g increased to 41m 2 The same test method as in Example 1 was used to measure the number of Bronsted acid sites on the outer surface of the 5A zeolite molecular sieve of this example to be 228 μmol / g, i.e., 3.3 sites / nm. 2 The number of Bronsted acid sites per unit mass of 5A zeolite molecular sieve is significantly increased compared with that before ball milling, and the number of sites per unit area is significantly increased compared with that before ball milling.

[0082] Example 12: This example is different from Example 1 in that hydrogen-type 5A zeolite molecular sieve is used as a sample for testing the Bronsted acid sites on the outer surface.

[0083] The preparation method of hydrogen type 5A zeolite molecular sieve is:

[0084] 2 g of 5A zeolite molecular sieve was placed in 400 mL of a 0.1 mol / L hydrochloric acid solution, stirred at room temperature, and ion exchange was performed for 24 h; then, the supernatant was removed, 400 mL of a 0.2 mol / L hydrochloric acid solution was added, stirred at room temperature, and ion exchange was performed for 24 h; then, centrifugation was performed, the supernatant was removed, and the mixture was washed with deionized water for 5 times, each time with 100 mL of deionized water; finally, the solid powder was placed in a vacuum drying oven and dried at room temperature and vacuum (1 Pa) for 24 h to obtain a hydrogen-type 5A zeolite molecular sieve.

[0085] The same test method as in Example 1 was used to measure the Bronsted acid sites on the outer surface of the hydrogen-type 5A zeolite molecular sieve to be 130 μmol / g, i.e., 2.6 sites / nm. 2 , indicating that H + After ion exchange, although the number of Bronsted acid sites on the inner surface of 5A zeolite molecular sieve increases (becomes hydrogen type), the number of Bronsted acid sites on the outer surface remains basically unchanged.

[0086] It can be seen from Examples 1 to 12 that the method of adsorbing n-propylamine at room temperature and analyzing propylene yield by mass spectrometry at high temperature effectively realizes the quantitative determination of the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve.

[0087] The preferred operating conditions for determining the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve are as follows: the flow rate of helium introduced during heat treatment of 5A zeolite molecular sieve is 100 mL / min, the heat treatment temperature is 523 K, and the heat treatment time is 2 h; the flow rate of helium introduced into the bubbler when generating n-propylamine vapor is 5 mL / min, the flow rate of helium used to dilute the n-propylamine gas coming out of the bubbler is 100 mL / min, and the time for 5A zeolite molecular sieve to adsorb n-propylamine is 1.0 h; the flow rate of helium for purging after 5A zeolite molecular sieve adsorbs n-propylamine is 100 mL / min, the purging time is 12 h, the heating rate during heating is 10 K / min, and the final heating temperature is 873 K.

[0088] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for determining the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve, characterized in that: The following steps are involved: S1. Load 5A zeolite molecular sieve into a fixed bed, introduce helium at a flow rate of 50-150 mL / min, heat treat at 473-573 K for 1-3 h, cool to room temperature, and set aside; S2, introducing helium at a flow rate of 2.5-10 mL / min into a bubbler containing n-propylamine liquid, taking out the n-propylamine vapor from the bubbler, and then diluting it with helium at a flow rate of 50-200 mL / min, and allowing the 5A zeolite molecular sieve heat-treated in step S1 to in-situ adsorb n-propylamine for 0.5-1.5 h at 298 K to obtain sample I; S3, purge sample I in situ with helium at a flow rate of 50-150 mL / min for 6-16 h, and then heat to 823-923 K at a heating rate of 5-20 K / min under helium, analyze the exhausted gas with a mass spectrometer during the heating period, and record the changes in mass spectrometry signal intensity of mass-to-charge ratio 41 and mass-to-charge ratio 30 with heating time; S4, multiplying the mass spectrum signal intensity of the mass-to-charge ratio 30 obtained in step S3 by 0.05 to obtain the mass spectrum signal intensity of the mass-to-charge ratio 41 generated by n-propylamine, and then deducting the mass spectrum signal intensity of the mass-to-charge ratio 41 generated by n-propylamine at the same time point (same temperature) from the mass spectrum signal intensity of the mass-to-charge ratio 41 at any time point obtained in step S3, to obtain a curve of the mass spectrum signal intensity of the mass-to-charge ratio 41 of the propylene generated by the chemical reaction versus the heating time, and further deriving a curve of the propylene yield versus the heating time; S5. Integrate the curve of propylene yield versus heating time obtained in step S4 over the entire heating time range to obtain the number of Bronsted acid sites on the outer surface of the 5A zeolite molecular sieve.

2. The method for determining the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve according to claim 1, characterized in that: The measuring device used includes a helium cylinder, a bubbler, a fixed bed, a mass spectrometer and a temperature controller for controlling the heating temperature of the fixed bed. The outlet pipeline of the helium cylinder is divided into a first pipeline and a second pipeline, wherein the first pipeline is connected to the inlet of the bubbler, and the outlet pipeline of the bubbler is connected to the inlet of the fixed bed after intersecting with the second pipeline. Mass flow meters are provided on the first pipeline and the second pipeline. An injection port is provided on the pipeline after the outlet pipeline of the bubbler intersects with the second pipeline, and the outlet of the fixed bed is connected to the mass spectrometer.

3. The method for determining the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve according to claim 2, characterized in that: When the fixed bed is not loaded with 5A zeolite molecular sieve, at this time, at 298K, helium is introduced into the fixed bed at a flow rate of 100mL / min, and 1mL of propylene gas is injected through the injection port, and the change of the mass spectrum signal intensity of the mass-to-charge ratio of 41 with time is recorded by a mass spectrometer until the propylene gas is completely blown out by the helium. The mass spectrum signal intensity change curve is integrated over the entire time range to obtain the integrated intensity of the mass-to-charge ratio of 41 for 1mL of propylene gas; The mass spectrum signal intensity of the mass-to-charge ratio 41 of the propylene generated by the chemical reaction obtained in step S4 is divided by the integrated intensity of the mass-to-charge ratio 41 of 1 mL of propylene gas to obtain a curve of the change of propylene yield with heating time.

4. The method for determining the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve according to claim 3, characterized in that: In step S1, the flow rate of helium gas is 100 mL / min, the heat treatment temperature is 523 K, and the heat treatment time is 2 h.

5. The method for determining the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve according to claim 4, characterized in that: In step S2, the flow rate of helium used to bring the n-propylamine vapor out of the bubbler is 5 mL / min, and the flow rate of helium used for dilution is 100 mL / min; the time for adsorbing n-propylamine is 1 hour.

6. The method for determining the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve according to claim 5, characterized in that: In step S3, the flow rate of helium gas used to purge sample I is 100 mL / min, and the purge time is 12 h.

7. The method for determining the number of Bronsted acid sites on the outer surface of 5A zeolite molecular sieve according to claim 6, characterized in that: In step S3, the heating rate of sample I is 10K / min, and the final heating temperature is 873K.

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