CN114486635A - Method and system for measuring molecular diffusion coefficient in porous material - Google Patents
Method and system for measuring molecular diffusion coefficient in porous material Download PDFInfo
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- 238000009792 diffusion process Methods 0.000 title claims abstract description 57
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- 238000002347 injection Methods 0.000 claims abstract description 136
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- 238000010926 purge Methods 0.000 claims description 18
- IMNFDUFMRHMDMM-UHFFFAOYSA-N N-Heptane Chemical compound CCCCCCC IMNFDUFMRHMDMM-UHFFFAOYSA-N 0.000 claims description 16
- XDTMQSROBMDMFD-UHFFFAOYSA-N Cyclohexane Chemical compound C1CCCCC1 XDTMQSROBMDMFD-UHFFFAOYSA-N 0.000 claims description 11
- TVMXDCGIABBOFY-UHFFFAOYSA-N octane Chemical compound CCCCCCCC TVMXDCGIABBOFY-UHFFFAOYSA-N 0.000 claims description 11
- VLKZOEOYAKHREP-UHFFFAOYSA-N n-Hexane Chemical compound CCCCCC VLKZOEOYAKHREP-UHFFFAOYSA-N 0.000 claims description 9
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N13/00—Investigating surface or boundary effects, e.g. wetting power; Investigating diffusion effects; Analysing materials by determining surface, boundary, or diffusion effects
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N13/00—Investigating surface or boundary effects, e.g. wetting power; Investigating diffusion effects; Analysing materials by determining surface, boundary, or diffusion effects
- G01N2013/003—Diffusion; diffusivity between liquids
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Abstract
The present disclosure relates to a method and system for measuring the diffusion coefficient of molecules within a porous material. The method disclosed by the invention has the advantages that the liquid-phase probe molecules enter the gasification chamber for gasification at a constant flow rate through the control of the sample injection needle and the automatic injection pump, then are mixed with the carrier gas at a constant flow rate and enter the sample cell to be contacted with the porous material, the concentration signal of the probe molecules in the mixed gas after contact is detected, and the molecular diffusion coefficient of the liquid-phase probe molecules in the porous material can be directly obtained through fitting calculation according to the detected concentration signal of the probe molecules.
Description
Technical Field
The disclosure relates to the technical field of measurement and characterization of porous materials, in particular to a method and a system for measuring a molecular diffusion coefficient in a porous material.
Background
The pore channels in the material are effective channels for molecular diffusion, and for heterogeneous reactions on the surface of a solid-phase medium, the diffusion capacity of reactant and product molecules in the pore channels of the porous material has a significant influence on the chemical reaction rate, selectivity and separation and adsorption capacity of the porous material. Conventional N2The adsorption-desorption method and other means can represent the ratio of the interior of the materialThe parameters such as surface area, pore volume and pore size distribution have certain correlation with the diffusion capacity of molecules in the pore channels, but the diffusion rate of the molecules cannot be directly reflected. The diffusion coefficient of molecules in the porous material is a quantitative parameter for representing the diffusion rate of the molecules, is an important index for directly researching the diffusion performance of the material, and the development of the method capable of directly measuring the diffusion coefficient of the molecules in the porous material has great significance for the research of the porous material.
The diffusion coefficient can be measured by various means such as nuclear magnetic resonance, molecular simulation, gravimetric method, and zero-length column method. Among them, the Zero Length Column (ZLC) method is an effective means for studying adsorption and diffusion kinetics by introducing a very thin (short) adsorbent layer (column) and controlling experimental conditions, so as to measure the diffusion coefficient of probe molecules in the adsorbent crystal regardless of the interference of factors such as heat transfer and mass transfer resistance, and has been continuously focused and improved by researchers. However, the devices and operation flows used by the existing zero-length column method technology are very complex, especially when the adsorbent which is liquid phase at normal temperature is used, the sample injection system needs complex temperature control devices, multi-path airflow flow control equipment, four-way valves and other equipment, the price is high, and the rapid and accurate control of the concentration of the probe molecules is difficult to realize.
Disclosure of Invention
The present disclosure provides a method and system for measuring the diffusion coefficient of molecules in a porous material, in order to simply and efficiently measure the diffusion coefficient of liquid phase probe molecules in the porous material.
To achieve the above object, the present disclosure provides a method of measuring a diffusion coefficient of molecules in a porous material, the method comprising:
s1: injecting liquid-phase probe molecules in the sample injection device into the gasification chamber at a first flow rate for gasification; enabling carrier gas with a second flow rate to enter the gasification chamber and mixing the carrier gas with the gasified probe molecules to form mixed gas;
s2: allowing the mixed gas to enter a sample cell and contact with a porous material in the sample cell to form a contact gas;
s3: detecting and recording the contact gasThe probe molecule concentration signal in the body, when the detected probe molecule concentration signal in the contact gas stops rising, the sample introduction device stops introducing the sample, and the moment of stopping introducing the sample is t0；
S4: continuously purging the carrier gas with the second flow rate into the sample cell through the gasification chamber until the detected probe molecule concentration signal in the contact gas is unchanged;
s5: in t pairs
Performing linear fitting to obtain a fitting relation shown in a formula (1); calculating the molecular diffusion coefficient D of the porous material according to the formula (2),
c0concentration of the probe molecules at the time when the increase in the concentration of the probe molecules ceases, ctIs from t0The concentration of probe molecules in the contact gas at time t, R being the particle size of the porous material, b and k being independently selected from any constant;
the sample introduction device comprises a sample introduction needle and an automatic injection pump, the needle head of the sample introduction needle is detachably communicated with the inlet of the automatic injection pump, and the outlet of the automatic injection pump is communicated with the probe molecule inlet of the gasification chamber;
the sample cell has a volume of 0.1 to 1000. mu.L, and is formed in a cylindrical shape having an aspect ratio in the range of 0.01 to 25, the axis of the cylindrical shape extending in the gas flow direction.
Optionally, the volume of the sample injection needle is 1-1000 μ L, preferably 2-200 μ L;
the sample cell has a volume of 0.1 to 50 μ L, the sample cell is formed in a cylindrical shape having an aspect ratio in the range of 0.1 to 20, and an axis of the cylindrical shape extends in a gas flow direction;
preferably, the sample cell has a length of 5 to 100mm and an internal diameter of 0.5 to 12 mm.
Optionally, the probe molecule has a boiling point of 30 to 160 ℃;
preferably, the probe molecule is selected from one or more of cyclohexane, n-hexane, n-heptane and n-octane.
Optionally, in step S1, the first flow rate is 1 μ L/h to 10mL/h, preferably 10 μ L/h to 1 mL/h;
the second flow rate is 1-100mL/min, preferably 5-60 mL/min.
Optionally, in step S2, the temperature of the gasification chamber is 80-200 ℃, preferably 100-180 ℃;
the temperature of the sample pool is 120-260 ℃; the particle size of the porous material is 0.01-0.5 times of the diameter of the sample cell.
Optionally, the method further comprises: before step S1, a gas path purge is performed using the carrier gas until the detected probe molecule concentration signal in the contact gas is unchanged.
Optionally, the porous material is selected from one or more of ZSM-5 type molecular sieve, Y type molecular sieve and porous alumina;
the carrier gas is one or more selected from helium, nitrogen, hydrogen and argon.
A second aspect of the present disclosure provides a system for measuring a diffusion coefficient of a molecule in a porous material using the method according to the first aspect of the present disclosure, the system comprising a carrier gas feed line, a sample introduction device, a gasification chamber, a sample cell, and a detector;
the carrier gas feeding pipeline is communicated with a carrier gas inlet of the gasification chamber, a mixed gas outlet of the gasification chamber is communicated with an inlet of the sample cell, and an outlet pipeline of the sample cell is connected with the detector; the sample cell is used for filling a porous material to be tested, and an inlet of the sample cell is in fluid communication with an outlet of the sample cell.
Optionally, the volume of the sample injection needle is 1-1000 μ L, preferably 2-200 μ L;
the volume of the sample pool is 0.1-50 mu L, and the length-diameter ratio of the sample pool is 0.1-20; preferably, the sample cell has a length of 5-100mm and an inner diameter of 0.5-12 mm;
the detector is selected from one of a flame ionization detector, a thermal conductivity cell detector, and a catalytic combustion detector.
Optionally, the system further comprises a carrier gas control device disposed on the carrier gas feed line;
the carrier gas control equipment comprises carrier gas flow control equipment and/or carrier gas pressure control equipment, the carrier gas flow control equipment is selected from one or more of a mass flow meter, an electronic flow controller and a rotor flow meter, and the carrier gas pressure control equipment is selected from a pressure reducing valve and/or an electronic pressure controller.
The method disclosed by the invention has the advantages that the liquid-phase probe molecules enter the gasification chamber for gasification at a constant flow rate through the control of the sample injection needle and the automatic injection pump, then are mixed with the carrier gas at a constant flow rate and enter the sample cell to be contacted with the porous material, the concentration signal of the probe molecules in the mixed gas after contact is detected, and the molecular diffusion coefficient of the liquid-phase probe molecules in the porous material can be directly obtained through fitting calculation according to the detected concentration signal of the probe molecules.
Additional features and advantages of the disclosure will be set forth in the detailed description which follows.
Drawings
The accompanying drawings, which are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosure and together with the description serve to explain the disclosure without limiting the disclosure. In the drawings:
FIG. 1 is a schematic illustration of a system for measuring the diffusion coefficient of molecules within a porous material in one embodiment of the present disclosure.
Description of the reference numerals
1. Carrier gas 2, carrier gas control equipment 3, automatic injection pump
4. Sample injection needle 5, gasification chamber 6, thermostat
7. Sample cell 8, detector 9, computer
Detailed Description
The following detailed description of specific embodiments of the present disclosure is provided in connection with the accompanying drawings. It should be understood that the detailed description and specific examples, while indicating the present disclosure, are given by way of illustration and explanation only, not limitation.
In the present disclosure, unless otherwise stated, the use of directional words such as "up" and "down" generally refers to the up and down of the device in normal use, and specifically refers to the orientation of the drawing in fig. 1. "inner and outer" are meant to refer to the profile of the device itself.
As shown in fig. 1, a first aspect of the present disclosure provides a method of measuring a diffusion coefficient of molecules within a porous material, the method comprising: s1: injecting liquid-phase probe molecules in the sample injection device into the gasification chamber 5 at a first flow rate for gasification; enabling carrier gas 1 with a second flow rate to enter the gasification chamber 5, and mixing the carrier gas with the gasified probe molecules to form mixed gas; s2: the mixed gas enters a sample cell 7 and contacts with the porous material in the sample cell 7 to form contact gas; s3: detecting and recording the concentration signal of the probe molecules in the contact gas, and stopping the sample introduction of the sample introduction device when the concentration signal of the probe molecules in the contact gas stops rising, wherein the time of stopping the sample introduction is t0(ii) a S4: continuously purging the carrier gas 1 with the second flow rate into the sample cell 7 through the gasification chamber 5 until the detected probe molecule concentration signal in the contact gas is unchanged; s5: in t pairs
Performing linear fitting to obtain a fitting relation shown in a formula (1); calculating the molecular diffusion coefficient D of the porous material according to the formula (2),
c0concentration of the probe molecules at the time when the increase in the concentration of the probe molecules ceases, ctIs from t0The time of the start of time t(ii) the concentration of probe molecules in the contact gas, R being the particle size of the porous material, b and k being independently selected from any constant; the sample introduction device comprises a sample introduction needle 4 and an automatic injection pump 3, the needle head of the sample introduction needle is detachably communicated with the inlet of the automatic injection pump, and the outlet of the automatic injection pump is communicated with the probe molecule inlet of the gasification chamber; the volume of the sample cell 7 is 0.1 to 1000. mu.L, and the sample cell 7 is formed in a cylindrical shape having an aspect ratio in the range of 0.01 to 25, the axis of the cylindrical shape extending in the gas flow direction.
The method disclosed by the invention has the advantages that the liquid-phase probe molecules enter the gasification chamber for gasification at a constant flow rate through the control of the sample injection needle and the automatic injection pump, then are mixed with the carrier gas at the constant flow rate and enter the sample cell to be contacted with the porous material, the concentration signal of the probe molecules in the mixed gas after contact is detected, and the molecular diffusion coefficient of the liquid-phase probe molecules in the porous material can be directly obtained through fitting calculation according to the detected concentration signal of the probe molecules.
As shown in FIG. 1, the volume of the needle 4 in the present disclosure is not limited, and may be selected according to the physicochemical properties and the content of the porous material in the sample cell 7, and in one embodiment, the volume of the needle 4 may be 1 to 1000. mu.L, and preferably may be 2 to 200. mu.L.
According to the present disclosure, the accuracy of the measurement result of the molecular diffusion coefficient can be improved by reducing the length of the adsorbent layer, increasing the flow rate of the carrier gas in the sample cell 7 and the porosity of the adsorbent, and in one embodiment, the sample cell 7 can be loaded with a smaller volume and aspect ratio, for example, the volume of the sample cell 7 can be 0.1 to 50 μ L, and preferably can be 1 to 40 μ L, and the sample cell 7 is formed in a cylindrical shape whose aspect ratio can be in the range of 0.1 to 20, and the axis of the cylindrical shape extends along the gas flow direction. In a preferred embodiment, the sample cell 7 may have a length of 5 to 100mm, preferably 20 to 60mm, and an inner diameter of 1.5 to 8 mm.
According to the present disclosure, the porous material may be selected from one or more of ZSM-5 type molecular sieve, Y type molecular sieve and porous alumina; the carrier gas 1 may be selected from one or more of helium, nitrogen, hydrogen and argon.
In a preferred embodiment according to the present disclosure, the probe molecule may be selected from one or more of cyclohexane, n-hexane, n-heptane and n-octane, the boiling point of the probe molecule may be 30 to 160 ℃, preferably 50 to 150 ℃, the above preferred probe molecule is close to the property of the reaction raw material, and the diffusion coefficient determined by using the probe molecules can be used as a basis for screening a suitable porous material as a catalyst system.
In order to allow the probe molecules and the carrier in the liquid phase to enter the vaporizing chamber 5 at a constant flow rate to maintain a constant concentration of the probe molecules in the mixed gas, in an embodiment according to the present disclosure, in step S1, the first flow rate may be 1 μ L/h to 10mL/h, and preferably may be 10 μ L/h to 1 mL/h; the second flow rate may be 1 to 100mL/min, and preferably may be 5 to 60 mL/min.
In order to gasify the liquid phase probe molecules, the temperature of the gasification chamber 5 of the present disclosure may be 80-200 ℃, and preferably may be 100-; further, in order to keep the probe molecules in the mixed gas in the sample cell 7 in a gaseous state, in one embodiment, the temperature of the sample cell 7 may be 120-.
In order to fill the sample cell 7 with porous material as much as possible, so as to make the contact between the probe molecules and the porous material more uniform and avoid the probe molecules not in contact with the porous material from directly entering the detector 8, in one embodiment according to the present disclosure, the particle size of the porous material is 0.01 to 0.5 times, preferably 0.05 to 0.4 times, the diameter of the sample cell 7 in step S2. Wherein, the diameter of the sample cell 7 refers to the inner diameter of the cylindrical sample cell 7.
According to the present disclosure, in order to make the experimental result more accurate, a gas path purge may be performed using the carrier gas 1 before step S1 until the detected probe molecule concentration signal in the contact gas does not change any more.
As shown in fig. 1, the second aspect of the present disclosure provides a system for measuring the diffusion coefficient of molecules in a porous material by using the method of the first aspect of the present disclosure, the system comprising a carrier gas feed line, a sample introduction device, a gasification chamber 5, a sample cell 7 and a detector 8; the carrier gas feeding pipeline is communicated with a carrier gas inlet of the gasification chamber, a mixed gas outlet of the gasification chamber is communicated with an inlet of the sample cell, and an outlet pipeline of the sample cell is connected with the detector 8; the sample cell 7 is filled with a porous material, and the inlet of the sample cell is in fluid communication with the outlet of the sample cell.
The system disclosed by the invention has the advantages that the liquid-phase probe molecules enter the gasification chamber for gasification at a constant flow rate through the control of the sample injection needle and the automatic injection pump, then are mixed with the carrier gas at the constant flow rate, the mixed gas enters the sample cell to be contacted with the porous material, the concentration signal of the probe molecules in the contacted gas is detected and recorded by the detector 8, and the molecular diffusion coefficient of the liquid-phase probe molecules in the porous material can be directly obtained through fitting calculation according to the concentration signal of the probe molecules.
The volume of the injection needle 4 is not limited in the present disclosure, and may be selected according to the physicochemical properties and the content of the porous material in the sample cell 7, and in one embodiment, the volume of the injection needle 4 may be 1 to 1000 μ L, and preferably may be 2 to 200 μ L.
According to the present disclosure, in order to improve the accuracy of the measurement result of the molecular diffusion coefficient by reducing the length of the adsorbent layer, increasing the flow rate of the carrier gas in the sample cell 7 and the porosity of the adsorbent, in one embodiment, the sample cell 7 may be loaded with a smaller volume and aspect ratio, for example, the volume of the sample cell 7 may be 0.1 to 50 μ L, preferably 1 to 40 μ L, and the sample cell 7 is formed in a cylindrical shape having an aspect ratio that may be in the range of 0.1 to 20, the axis of the cylindrical shape extending along the gas flow direction. In a preferred embodiment, the sample cell 7 may have a length of 5 to 100mm, preferably 20 to 60mm, and an inner diameter of 1.5 to 8 mm.
According to the present disclosure, the detector 8 is selected from one of a flame ionization detector, a thermal conductivity cell detector, and a catalytic combustion detector.
In order to enable the carrier to enter the gasification chamber 5 at a constant flow rate and pressure according to the present disclosure, in one embodiment according to the present disclosure, the system may further comprise a carrier gas control device disposed on the carrier gas feed line; preferably, the carrier gas control device may include a carrier gas flow control device and/or a carrier gas pressure control device, the carrier gas flow control device may be selected from one or more of a mass flow meter, an electronic flow controller and a rotameter, and the carrier gas pressure control device may be selected from a pressure reducing valve and/or an electronic pressure controller.
According to the present disclosure, in order to maintain the sample cell 7 at a constant temperature to ensure the accuracy of the detection result, the exterior of the sample cell 7 may be provided with an incubator 6, as shown in fig. 1, the temperature of the incubator 6 may be 120-.
The present disclosure is further illustrated by the following examples, but is not to be construed as being limited thereby.
In the following examples and comparative examples, a Legato 110 model autosampler from KD Scientific and a 10. mu.L injection needle from Agilent Technologies were used, and an injection port, a vaporizing chamber, a carrier gas control device, a gas circuit, an incubator, a flame ion detector 8, and a control program therefor were used in a gas chromatograph model 7890B from Agilent Technologies.
The specific surface area, the total pore volume, the micropore volume and the mesopore volume of the material are measured by a nitrogen low-temperature static adsorption method and by an ASAP 2020 of micromeritics, and the standard for calculating the specific surface area is GB/T-5816-.
The following examples all adopt the system shown in fig. 1 to measure the diffusion coefficient of molecules in the porous material, wherein, a carrier gas feed line is communicated with a carrier gas inlet of a gasification chamber 5, a mixed gas outlet of the gasification chamber 5 is communicated with an inlet of a sample cell 7, an outlet line of the sample cell 7 is connected with a detector 8, a computer 9 is communicated with the detector 8, and a carrier gas control device 2 is arranged on the carrier gas feed line; an inlet of the sample cell 7 is in fluid communication with an outlet of the sample cell 7, a porous material to be measured is filled in the sample cell 7, and a constant temperature box 6 is arranged outside the sample cell 7; the inlet of the automatic injection pump 3 is detachably communicated with the needle head of the sample injection needle 4, and the outlet of the automatic injection pump 3 is communicated with the probe molecule inlet of the gasification chamber 5.
Example 1
A passivated column with a length of 23mm and an internal diameter of 1.4mm was used as the sample cell 7. Filling one end of a chromatographic column with quartz wool, then filling 50mg of ZSM-5 molecular sieve particles (the particle size is 0.18-0.38 mm), and sealing the other end with the quartz wool after tapping. The sample cell 7 is placed in a thermostat 6, connected to the gas circuit and airtight. The temperature of the vaporizer 5 was set to 160 ℃ and the temperature of the incubator 6 was set to 200 ℃. Helium was used as the carrier gas, and the flow rate of carrier gas 1 was set to 10 mL/min. And introducing carrier gas 1 to perform gas path purging until all parameters are stabilized at the set value and the signal of the detector 8 is stable.
The detector 8 reading begins to be recorded. The volume of the sample injection needle 4 is set to be 10 mu L on the control interface of the automatic injection pump 3, and the sample injection flow is set to be 0.5 mu L/min. 10 mu L of cyclohexane is extracted by a sample injection needle 4, the sample injection needle 4 is inserted into a sample injection port of an automatic injection pump 3, and the sample injection port of the automatic injection pump 3 is fixed on a probe molecule inlet of a gasification chamber. And starting the automatic injection pump 3 to perform constant-speed sample injection, and monitoring and recording the reading of the detector 8 in real time. After about 9 minutes, the concentration of the probe molecule no longer increased and equilibrium of adsorption was reached, and this concentration was taken as c0. After 5 minutes, the automatic injection pump 3 is closed to stop automatic sample injection, and the sample injection needle 4 is pulled out, and the moment is taken as t0. The detector 8 reading is measured and recorded and the purge is continued until the detector 8 signal stabilizes.
From t in the desorption process0The concentration of the probe molecule at time t from the time point ctUsing ln (c)t/c0) Plot t. From t in the figure0Linear segments are obtained within about 850s after the start. Linear fitting is carried out on the linear segment by using a least square method, and the decision coefficient r of the fitting2Is 0.9977. And (3) calculating the diffusion coefficient D of the cyclohexane in the molecular sieve according to the formula (1) by the fitted slope and the particle size of the molecular sieve. Molecular sievesThe diffusion coefficient and the channel parameters are shown in table 1.
TABLE 1
| Test items | Test results |
| Specific surface area (m)2/g) | 356.9 |
| Total pore volume (cm)3/g) | 0.196 |
| Micropore volume (cm)3/g) | 0.144 |
| Mesopore volume (cm)3/g) | 0.052 |
| Coefficient of diffusion (m)2/s) | 4.6×10-14 |
Example 2
A passivated column with a length of 23mm and an internal diameter of 1.4mm was used as the sample cell 7. Filling one end of a chromatographic column with quartz cotton, then filling 50mg of ZSM-5 molecular sieve particles (the particle size is 0.18-0.38 mm) subjected to NaOH hole expansion treatment for 6 hours, and sealing the other end with quartz cotton after tapping. The sample cell 7 is placed in a thermostat 6, connected to the gas circuit and airtight. The temperature of the vaporizer 5 was set to 160 ℃ and the temperature of the incubator 6 was set to 200 ℃. Helium was used as the carrier gas, and the flow rate of carrier gas 1 was set to 10 mL/min. And introducing carrier gas 1 to perform gas path purging until all parameters are stabilized at the set value and the signal of the detector 8 is stable.
The detector 8 reading begins to be recorded. The volume of the sample injection needle 4 is set to be 10 mu L on the control interface of the automatic injection pump 3, and the sample injection flow is set to be 0.5 mu L/min. 10 mu L of cyclohexane is extracted by a sample injection needle 4, the sample injection needle 4 is inserted into a sample injection port of an automatic injection pump 3, and the sample injection port of the automatic injection pump 3 is fixed on a probe molecule inlet of a gasification chamber. And starting the automatic injection pump 3 to perform constant-speed sample injection, and monitoring and recording the reading of the detector 8 in real time. After about 9 minutes, the concentration of the probe molecule no longer increased and equilibrium of adsorption was reached, and this concentration was taken as c0. After 5 minutes, the automatic injection pump 3 is closed to stop automatic sample injection, and the sample injection needle 4 is pulled out, and the moment is taken as t0. The detector 8 reading is measured and recorded and the purge is continued until the detector 8 signal stabilizes.
From t in the desorption process0The concentration of the probe molecule at time t from the time point ctUsing ln (c)t/c0) Plot t. From t in the figure0Within about 800s after the start is a linear segment. Linear fitting is carried out on the linear segment by using a least square method, and the decision coefficient r of the fitting2Is 0.9963. And (3) calculating the diffusion coefficient D of the cyclohexane in the molecular sieve according to the formula (1) by the fitted slope and the particle size of the molecular sieve. The diffusion coefficients and pore parameters of the molecular sieves are shown in table 2.
TABLE 2
| Test items | Test results |
| Specific surface area (m)2/g) | 390.0 |
| Total pore volume (cm)3/g) | 0.288 |
| Micropore volume (cm)3/g) | 0.113 |
| Mesopore volume (cm)3/g) | 0.175 |
| Coefficient of diffusion (m)2/s) | 6.3×10-14 |
According to tables 1 and 2, as can be seen from the comparison of the specific surface areas and the pore distribution data of the molecular sieve particles in examples 1 and 2, in the case that the specific surface area of the ZSM-5 molecular sieve particles subjected to the pore-expanding treatment with NaOH in example 2 is not significantly changed, the mesoporous volume is significantly increased, and the diffusion performance of the molecular sieve particles should be significantly improved; according to the comparison of the diffusion coefficients of the embodiment 1 and the embodiment 2 obtained by the measurement method disclosed by the disclosure, the diffusion coefficient of the ZSM-5 molecular sieve particles subjected to pore-expanding treatment by NaOH in the embodiment 2 is obviously higher than that of the embodiment 1, and coincides with the prediction of the trend of improving the diffusion performance of the molecular sieve particles subjected to pore-expanding treatment, so that the method disclosed by the disclosure has high accuracy and reliability in measuring the diffusion coefficient of the molecules in the porous material.
Example 3
A passivated column with a length of 23mm and an internal diameter of 1.4mm was used as the sample cell 7. Filling one end of a chromatographic column with quartz wool, then filling 40mg of Y-type molecular sieve particles (the particle size is 0.18-0.38 mm), and sealing the other end with quartz wool after tapping. The sample cell 7 is placed in a thermostat 6, connected to the gas circuit and airtight. The temperature of the vaporizer 5 was set to 160 ℃ and the temperature of the incubator 6 was set to 200 ℃. Helium was used as the carrier gas, and the flow rate of carrier gas 1 was set to 10 mL/min. And introducing carrier gas 1 to perform gas path purging until all parameters are stabilized at the set value and the signal of the detector 8 is stable.
Start recording detectorAnd 8 reading. The volume of the sample injection needle 4 is set to be 10 mu L on the control interface of the automatic injection pump 3, and the sample injection flow is set to be 0.5 mu L/min. 10 mu L of cyclohexane is extracted by a sample injection needle 4, the sample injection needle 4 is inserted into a sample injection port of an automatic injection pump 3, and the sample injection port of the automatic injection pump 3 is fixed on a probe molecule inlet of a gasification chamber. And starting the automatic injection pump 3 to perform constant-speed sample injection, and monitoring and recording the reading of the detector 8 in real time. After about 8 minutes, the concentration of the probe molecule no longer increased and equilibrium of adsorption was reached, and this concentration was taken as c0. After 5 minutes, the automatic injection pump 3 is closed to stop automatic sample injection, and the sample injection needle 4 is pulled out, and the moment is taken as t0. The detector 8 reading is measured and recorded and the purge is continued until the detector 8 signal stabilizes.
From t in the desorption process0The concentration of the probe molecule at time t from the time point ctUsing ln (c)t/c0) Plot t. From t in the figure0Linear segments within about 300s after the start. Linear fitting is carried out on the linear segment by using a least square method, and the decision coefficient r of the fitting2Is 0.9954. And (3) calculating the diffusion coefficient D of the cyclohexane in the molecular sieve according to the formula (1) by the fitted slope and the particle size of the molecular sieve. The diffusion coefficients and pore parameters of the molecular sieves are shown in table 3.
TABLE 3
| Test items | Test results |
| Specific surface area (m)2/g) | 737.2 |
| Total pore volume (cm)3/g) | 0.369 |
| Micropore volume (cm)3/g) | 0.275 |
| Mesopore volume (cm)3/g) | 0.094 |
| Coefficient of diffusion (m)2/s) | 1.9×10-13 |
Example 4
A passivated column with a length of 23mm and an internal diameter of 1.4mm was used as the sample cell 7. One end of the column was packed with quartz wool, then 40mg of the Y-type molecular sieve particles of example 3 were loaded, and after tapping, the other end was sealed with quartz wool. The sample cell 7 is placed in a thermostat 6, connected to the gas circuit and airtight. The temperature of the vaporizer 5 was set to 160 ℃ and the temperature of the incubator 6 was set to 200 ℃. Helium was used as a carrier gas, and the flow rate of carrier gas 1 was set to 10 mL/min. And introducing carrier gas 1 to perform gas path purging until all parameters are stabilized at the set value and the signal of the detector 8 is stable.
The detector 8 reading begins to be recorded. The volume of the sample injection needle 4 is set to be 10 mu L on the control interface of the automatic injection pump 3, and the sample injection flow is set to be 0.5 mu L/min. Extracting 10 μ L n-hexane with a sample injection needle 4, inserting the sample injection needle 4 into the sample injection port of the automatic injection pump 3, and fixing the sample injection port of the automatic injection pump 3 on the probe molecule inlet of the gasification chamber. And starting the automatic injection pump 3 to perform constant-speed sample injection, and monitoring and recording the reading of the detector 8 in real time. After about 8 minutes, the concentration of the probe molecules does not rise any more, the adsorption balance is achieved, after 5 minutes, the automatic injection pump 3 is closed to stop automatic sample injection, the sample injection needle 4 is pulled out, the reading of the detector 8 is measured and recorded, and the purging is continued until the signal of the detector 8 is stable.
The remaining liquid in the needle 4 was discharged, and n-heptane was sucked in until the space in the needle 4 was filled. This step is repeated three times, and the injection needle 4 is cleaned. The detector 8 reading begins to be recorded. The volume of the sample injection needle 4 is set to be 10 mu L on the control interface of the automatic injection pump 3, and the sample injection rate is 0.5 mu L/min. 10 mu L of n-heptane is extracted by a sample injection needle 4, the sample injection needle 4 is inserted into a sample injection port of the automatic injection pump 3, and the sample injection port of the automatic injection pump 3 is fixed on a probe molecule inlet of the gasification chamber. And starting the automatic injection pump 3 to perform constant-speed sample injection, and monitoring and recording the reading of the detector 8 in real time. After about 10 minutes, the concentration of the probe molecules does not rise any more, the adsorption balance is achieved, after 5 minutes, the automatic injection pump 3 is closed to stop automatic sample injection, the sample injection needle 4 is pulled out, the reading of the detector 8 is measured and recorded, and the purging is continued until the signal of the detector 8 is stable.
The residual liquid in the injection needle 4 is discharged, and n-octane is sucked in until the space in the injection needle 4 is filled. This step is repeated three times, and the injection needle 4 is cleaned. The detector 8 reading begins to be recorded. The volume of the sample injection needle 4 is set to be 10 mu L on the control interface of the automatic injection pump 3, and the sample injection rate is 0.5 mu L/min. 10 mu L of n-octane is extracted by a sample injection needle 4, the sample injection needle 4 is inserted into a sample injection port of the automatic injection pump 3, and the sample injection port of the automatic injection pump 3 is fixed on a probe molecule inlet of the gasification chamber. And starting the automatic injection pump 3 to perform constant-speed sample injection, and monitoring and recording the reading of the detector 8 in real time. After about 18 minutes, the probe molecule concentration no longer rises and adsorption equilibrium is reached. After 2 minutes, the automatic injection pump 3 is closed to stop automatic sample injection, the sample injection needle 4 is pulled out, the reading of the detector 8 is measured and recorded, and the purging is continuously carried out until the signal of the detector 8 is stable.
The results of the three experiments were each linearly fitted using the least squares method, and the coefficients of determination of the fitting and the diffusion coefficients of the different probe molecules in the Y-type molecular sieve are shown in table 4.
TABLE 4
| Probe molecule | r2 | D(m2/s) |
| N-hexane | 0.9941 | 1.2×10-13 |
| N-heptane | 0.9946 | 4.8×10-14 |
| N-octane | 0.9988 | 1.6×10-14 |
According to table 4, as can be seen from the comparison of the results of the diffusion coefficient measurement of the same porous material by using three probe molecules in example 4, the diffusion coefficient of the material decreases with the increase of the size of the probe molecule, and conforms to the theoretical diffusion rule, which indicates that the method disclosed by the present disclosure has feasibility in testing the dynamic diffusion coefficient of the porous material, and the test results have accuracy.
Example 5
A passivated column with a length of 23mm and an internal diameter of 1.4mm was used as the sample cell 7. One end of the chromatographic column was filled with quartz wool, followed by loading 30mg of porous alumina particles (particle size 0.18-0.38 mm), and sealing the other end with quartz wool after tapping. The sample cell 7 is placed in a thermostat 6, connected to the gas circuit and airtight. The temperature of the vaporizer 5 was set to 160 ℃ and the temperature of the incubator 6 was set to 200 ℃. Helium was used as the carrier gas, and the flow rate of carrier gas 1 was set to 10 mL/min. And introducing carrier gas 1 to perform gas path purging until all parameters are stabilized at the set value and the signal of the detector 8 is stable.
The detector 8 reading begins to be recorded. The volume of the sample injection needle 4 is set to be 10 mu L on the control interface of the automatic injection pump 3, and the sample injection rate is 0.5 mu L/min. Extracting 10 μ L cyclohexane with injection needle 4, inserting the injection needle 4 into the injection port of the automatic injection pump 3, fixing the injection port of the automatic injection pump 3 at the probe molecule inlet of the gasification chamberThe above. And starting the automatic injection pump 3 to perform constant-speed sample injection, and monitoring and recording the reading of the detector 8 in real time. After about 3 minutes, the concentration of the probe molecule does not rise any more and the adsorption equilibrium is reached, and this concentration is taken as c0. After 10 minutes, the automatic injection pump 3 is closed to stop automatic sample injection, and the sample injection needle 4 is pulled out, and the moment is taken as t0. The detector 8 reading is measured and recorded and the purge is continued until the detector 8 signal stabilizes.
From t in the desorption process0The concentration of the probe molecule at time t from the time point ctUsing ln (c)t/c0) Plot t. From t in the figure0Within about 50s after the start is a linear segment. Linear fitting is carried out on the linear segment by using a least square method, and the decision coefficient r of the fitting2Is 0.9968. And (3) calculating the diffusion coefficient D of the cyclohexane in the alumina according to the formula (1) by the fitted slope and the particle size of the alumina. The diffusion coefficients and channel parameters of the alumina are shown in table 5.
TABLE 5
| Test items | Test results |
| Specific surface area (m)2/g) | 343.7 |
| Total pore volume (cm)3/g) | 0.713 |
| Coefficient of diffusion (m)2/s) | 9.6×10-13 |
The preferred embodiments of the present disclosure are described in detail with reference to the accompanying drawings, however, the present disclosure is not limited to the specific details of the above embodiments, and various simple modifications may be made to the technical solution of the present disclosure within the technical idea of the present disclosure, and these simple modifications all belong to the protection scope of the present disclosure.
It should be noted that, in the foregoing embodiments, various features described in the above embodiments may be combined in any suitable manner, and in order to avoid unnecessary repetition, various combinations that are possible in the present disclosure are not described again.
In addition, any combination of various embodiments of the present disclosure may be made, and the same should be considered as the disclosure of the present disclosure, as long as it does not depart from the spirit of the present disclosure.
Claims (10)
1. A method of measuring the diffusion coefficient of molecules within a porous material, the method comprising:
s1: injecting liquid-phase probe molecules in the sample injection device into the gasification chamber at a first flow rate for gasification; enabling carrier gas with a second flow rate to enter the gasification chamber and mixing the carrier gas with the gasified probe molecules to form mixed gas;
s2: allowing the mixed gas to enter a sample cell and contact with a porous material in the sample cell to form a contact gas;
s3: detecting and recording the concentration signal of the probe molecules in the contact gas, and stopping the sample introduction of the sample introduction device when the concentration signal of the probe molecules in the contact gas stops rising, wherein the time of stopping the sample introduction is t0；
S4: continuously purging the carrier gas with the second flow rate into the sample cell through the gasification chamber until the detected probe molecule concentration signal in the contact gas is unchanged;
s5: in t pairs
Performing linear fitting to obtain a fitting relation shown in formula (1)Is represented by the formula; calculating the molecular diffusion coefficient D of the porous material according to the formula (2),
c0concentration of the probe molecules at the time when the increase in the concentration of the probe molecules ceases, ctIs from t0The concentration of probe molecules in the contact gas at time t, R being the particle size of the porous material, b and k being independently selected from any constant;
the sample introduction device comprises a sample introduction needle and an automatic injection pump, the needle head of the sample introduction needle is detachably communicated with the inlet of the automatic injection pump, and the outlet of the automatic injection pump is communicated with the probe molecule inlet of the gasification chamber;
the sample cell has a volume of 0.1 to 1000. mu.L, and is formed in a cylindrical shape having an aspect ratio in the range of 0.01 to 25, the axis of the cylindrical shape extending in the gas flow direction.
2. The method according to claim 1, wherein the volume of the injection needle is 1-1000 μ L, preferably 2-200 μ L;
the sample cell has a volume of 0.1 to 50 μ L, the sample cell is formed in a cylindrical shape having an aspect ratio in the range of 0.1 to 20, and an axis of the cylindrical shape extends in a gas flow direction;
preferably, the sample cell has a length of 5 to 100mm and an internal diameter of 0.5 to 12 mm.
3. The method of claim 1, wherein the probe molecule has a boiling point of 30 to 160 ℃;
preferably, the probe molecule is selected from one or more of cyclohexane, n-hexane, n-heptane and n-octane.
4. The method according to claim 1, wherein in step S1, the first flow rate is 1 μ L/h to 10mL/h, preferably 10 μ L/h to 1 mL/h;
the second flow rate is 1-100mL/min, preferably 5-60 mL/min.
5. The method as claimed in claim 1, wherein the temperature of the gasification chamber is 80-200 ℃, preferably 100-180 ℃ in step S2;
the temperature of the sample pool is 120-260 ℃; the particle size of the porous material is 0.01-0.5 times of the diameter of the sample cell.
6. The method of claim 1, wherein the method further comprises: before step S1, a gas path purge is performed using the carrier gas until the detected probe molecule concentration signal in the contact gas is unchanged.
7. The method according to any one of claims 1 to 6, wherein the porous material is selected from one or more of ZSM-5 type molecular sieve, Y type molecular sieve and porous alumina;
the carrier gas is one or more selected from helium, nitrogen, hydrogen and argon.
8. A system for measuring the diffusion coefficient of molecules in a porous material by using the method of any one of claims 1 to 7, wherein the system comprises a carrier gas feed line, a sample introduction device, a gasification chamber, a sample cell and a detector;
the carrier gas feeding pipeline is communicated with a carrier gas inlet of the gasification chamber, a mixed gas outlet of the gasification chamber is communicated with an inlet of the sample cell, and an outlet pipeline of the sample cell is connected with the detector; the sample cell is used for filling a porous material to be tested, and an inlet of the sample cell is in fluid communication with an outlet of the sample cell.
9. The system according to claim 8, wherein the volume of the injection needle is 1-1000 μ L, preferably 2-200 μ L;
the volume of the sample pool is 0.1-50 mu L, and the length-diameter ratio of the sample pool is 0.1-20; preferably, the sample cell has a length of 5-100mm and an inner diameter of 0.5-12 mm;
the detector is selected from one of a flame ionization detector, a thermal conductivity cell detector, and a catalytic combustion detector.
10. The system of claim 8, further comprising a carrier gas control device disposed on the carrier gas feed line;
the carrier gas control equipment comprises carrier gas flow control equipment and/or carrier gas pressure control equipment, the carrier gas flow control equipment is selected from one or more of a mass flow meter, an electronic flow controller and a rotor flow meter, and the carrier gas pressure control equipment is selected from a pressure reducing valve and/or an electronic pressure controller.
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