Method for gradient dehydrogenation of cyclohexanol and hydrogen recycling under catalysis of copper and silicon

Abstract

The invention relates to the technical field of chemical catalysis, in particular to a copper-silicon catalyzed cyclohexanol gradient dehydrogenation and hydrogen recycling method which comprises the following steps: step 1, preparing a gradient catalyst, namely carrying out surface hydroxyl regulation pretreatment on porous silica gel, and loading a copper salt solution by adopting a step-by-step impregnation method to form gradient distribution with gradually decreased copper loading capacity from outside to inside, carrying out segmented heating reduction in a reducing atmosphere containing 5vol% of H2 / N2; 2, gradient dehydrogenation reaction process control; and 3, hydrogen in-situ separation and cyclic utilization: separating hydrogen from the reaction outlet gas through a palladium alloy membrane, controlling the pressure difference of the two sides of the membrane to maintain the permeation flux, returning the separated hydrogen to the inlet of the reactor to adjust the hydrogen partial pressure, and storing the residual hydrogen. Through gradient catalyst preparation, accurate reaction process control and efficient hydrogen recovery technologies, the cyclohexanone production efficiency is improved, and long-term stability of the catalyst and efficient recycling of hydrogen are achieved.

Description

Technical Field

[0001] The invention relates to the technical field of chemical catalysis, and in particular to a method for gradient dehydrogenation of cyclohexanol catalyzed by copper silicon and hydrogen recycling. Background Art

[0002] Cyclohexanone is an important intermediate for synthetic fibers such as nylon-6 and nylon-66. Its industrial production mainly relies on the cyclohexanol dehydrogenation method. In the existing technology, copper-based catalysts have gradually replaced traditional zinc-iron-based catalysts due to their low reaction temperature and high selectivity, but the following key problems still exist: During long-term operation of copper-based catalysts, carbon deposits formed by condensation and dehydrogenation of organic matter on the surface will cause active sites to be covered. Studies have shown that the conversion rate of traditional copper-zinc catalysts drops by about 5% after 150 hours of operation, and the activity attenuation caused by carbon deposits is difficult to fully recover through regeneration.

[0003] Existing copper-silicon catalysts need to operate at 190-230°C. Although this is an improvement over zinc-based catalysts (≥350°C), external heat is still required to maintain the reaction temperature. For example, the energy consumption of the industrial comparison sample CDH-1 at 250°C is 30% higher than the theoretical value, and high temperature intensifies side reactions (such as deep dehydrogenation of cyclohexanone to produce phenol), resulting in a phenol content of more than 62ppm in the product.

[0004] The hydrogen produced as a byproduct of dehydrogenation is usually separated by condensation or pressure swing adsorption, which consumes a lot of energy and has a recovery rate of less than 70%. For example, although the purity of hydrogen produced as a byproduct of the chlor-alkali industry reaches 99.9%, the separation process requires multi-stage compression and membrane permeation, which increases the overall cost by 25%. In addition, the regulation of hydrogen partial pressure lacks a dynamic response mechanism, and traditional PID control is difficult to suppress the reverse reaction in real time, resulting in fluctuations in cyclohexanone selectivity (±2%).

[0005] Therefore, developing a gradient dehydrogenation technology that combines high activity, long life and efficient hydrogen recovery has become a bottleneck that needs to be broken through in the industrial production of cyclohexanone. Summary of the invention

[0006] Based on the above purpose, the present invention provides a method for gradient dehydrogenation of cyclohexanol and hydrogen recycling catalyzed by copper silicon, comprising the following steps: Step 1: Gradient catalyst preparation: After the porous silica gel was pretreated with surface hydroxyl regulation, the copper salt solution was loaded by a step-by-step impregnation method to form a gradient distribution with decreasing copper loading from the outside to the inside, and then the silica gel was reduced by step-by-step heating in a reducing atmosphere containing 5 vol% H2 / N2, where the heating rate was adjusted according to the distribution density of copper oxides. The gradient distribution of the decreasing copper loading is optimized by a dynamic adsorption experiment: a copper salt solution with increasing concentration is continuously injected into the silica gel column, and when the copper ion concentration of the effluent reaches 95% of the inlet concentration, the next concentration is switched; Step 2: Control of the gradient dehydrogenation reaction process: Introduce cyclohexanol vapor and an inert carrier gas into a fixed-bed reactor filled with the catalyst obtained in Step 1, which is divided into a low-temperature zone of 120 - 180 °C, a medium-temperature zone of 220 - 280 °C, and a high-temperature zone of 300 - 350 °C. Achieve gradient dehydrogenation by adsorbing cyclohexanol in the low-temperature zone, carrying out dehydrogenation reaction in the medium-temperature zone, and desorbing cyclohexanone in the high-temperature zone. At the same time, dynamically adjust the carrier gas flow rate by real-time monitoring of the hydrogen partial pressure to make the hydrogen partial pressure lower than the inhibition reverse reaction threshold of 0.05 - 0.15 MPa; The inhibition reverse reaction threshold of the hydrogen partial pressure is determined by a thermodynamic equilibrium experiment: Measure the cyclohexanone selectivity curve at different hydrogen partial pressures in a closed reactor, and take the hydrogen partial pressure corresponding to the inflection point of the curve as the inhibition reverse reaction threshold; Step 3: In-situ hydrogen separation and recycling: Separate hydrogen from the reaction outlet gas through a palladium alloy membrane, control the pressure difference across the membrane to maintain the permeation flux, return a part of the separated hydrogen to the reactor inlet to adjust the hydrogen partial pressure, and store the remaining hydrogen; The pressure difference across the membrane is determined by an experiment matching the hydrogen permeation flux and the mechanical strength of the membrane: Test the hydrogen permeation flux at different pressure differences, and combine scanning electron microscopy to observe the integrity of the membrane structure, and select the pressure difference range with a linear flux interval and no membrane damage.

[0007] Preferably, the specific method of the surface hydroxyl group regulation pretreatment in Step 1 is as follows: Immerse silica gel in a weakly alkaline solution, detect the intensity of the characteristic peak of surface hydroxyl groups on the silica gel by infrared spectroscopy, and select the pH value of the solution that makes the hydroxyl density reach the preset range; The preset range is determined by an adsorption isotherm experiment, which specifically includes loading copper salts on silica gel samples with different hydroxyl densities respectively, analyzing the copper dispersion by X-ray diffraction, and selecting the hydroxyl density range with the optimal copper dispersion; The pretreated silica gel is dried under vacuum to remove physically adsorbed water, and the drying temperature is determined by thermogravimetric analysis, with the end point being that the mass loss rate of the silica gel is lower than 5 wt%.

[0008] Preferably, the method for constructing the gradient distribution of the stepwise impregnation method in Step 1 is as follows: Immerse the pretreated silica gel in copper salt solutions with low, medium, and high concentrations in sequence, and carry out low-temperature drying and pre-calcination after each impregnation; The concentration gradient is optimized by a dynamic adsorption experiment, which specifically includes continuously injecting copper salt solutions with increasing concentrations into the silica gel, and real-time monitoring of the copper ion concentration in the effluent. When the copper ion concentration in the effluent reaches 95% of the inlet concentration, switch to the next concentration; The pre-calcination process is carried out under nitrogen protection, and the calcination temperature is determined by differential scanning calorimetry, with the end point being the end of the endothermic peak of copper salt decomposition.

[0009] Preferably, the specific operation of the stepwise temperature-raising reduction in Step 1 is as follows: Analyze the distribution density of copper oxide in the silica pores by X-ray photoelectron spectroscopy, and divide the catalyst into a high-density area and a low-density area; The high-density area adopts a stepped temperature-raising strategy. In the initial stage, it is heated to an intermediate temperature at a first heating rate and kept constant. After the hydrogen release rate decreases, it is heated to the final reduction temperature at a second heating rate; The low-density area is linearly heated to the final reduction temperature; the first heating rate, intermediate temperature, and second heating rate are optimized through a temperature-programmed reduction experiment, with the reduction degree of copper oxide and the particle size of copper particles as the judgment indexes.

[0010] Preferably, the method for determining the temperature ranges of the low-temperature zone, medium-temperature zone, and high-temperature zone in Step 2 is as follows: Determine the activation energy of the cyclohexanol dehydrogenation reaction by differential scanning calorimetry, and calculate the reaction rate constants of each temperature zone based on the Arrhenius equation; Set the temperature of the low-temperature zone so that the cyclohexanol adsorption rate is greater than the desorption rate, the temperature of the medium-temperature zone so that the dehydrogenation reaction rate is maximized, and the temperature of the high-temperature zone so that the cyclohexanone desorption rate is greater than the adsorption rate; The final temperature range is verified by on-line mass spectrometry monitoring of the gas composition at the reactor outlet to ensure the functional matching of each temperature zone.

[0011] Preferably, the method for determining the reverse reaction inhibition threshold of the hydrogen partial pressure in Step 2 is as follows: Introduce different ratios of hydrogen and cyclohexanol vapor into a closed reactor, and measure the equilibrium concentrations of cyclohexanone and cyclohexanol by gas chromatography; Plot the relationship curve between the hydrogen partial pressure and the cyclohexanone selectivity, and select the hydrogen partial pressure corresponding to the inflection point of the curve as the threshold; The carrier gas flow rate adjustment is achieved through a PID controller, whose input signal is the real-time data of the hydrogen partial pressure sensor, and the output signal is the opening degree of the carrier gas flow valve.

[0012] Preferably, the method for determining the pressure difference on both sides of the palladium alloy membrane in Step 3 is as follows: Test the influence of different pressure differences on the hydrogen permeation flux at a fixed temperature, and plot the pressure difference-flux curve; observe the membrane surface morphology by scanning electron microscopy to determine the maximum pressure difference that does not cause membrane layer rupture; the final operating pressure difference is selected within the linear range of the pressure difference-flux curve and lower than the maximum pressure difference value range.

[0013] Preferably, the method for optimizing the ratio of the hydrogen partially returned to the reactor in Step 3 is as follows: Establish a kinetic model including the dehydrogenation reaction rate, catalyst deactivation rate, and hydrogen recycling rate, and calculate the cyclohexanone yield and catalyst life at different circulation ratios through numerical simulation; Select the cycle ratio that maximizes the product of yield and lifespan; the model parameters are calibrated through laboratory bench-scale data, and the calibrated parameters include activation energy, mass transfer coefficient, and deactivation rate constant.

[0014] Preferably, the specific manner of partially returning hydrogen to the reactor is as follows: The permeated hydrogen is compressed and premixed with the raw material cyclohexanol vapor in a static mixer; The mixed gas enters the reactor through a porous distributor. The pore size gradient of the distributor is determined through a pressure drop experiment to ensure uniform distribution of the gas flow rate in each temperature zone; the compression pressure is dynamically adjusted according to the reactor inlet pressure to maintain the hydrogen partial pressure below the threshold.

[0015] Preferably, the method for adjusting the flow rate of the inert carrier gas is as follows: A hydrogen partial pressure sensor is arranged at the reactor outlet to collect data in real time. When the hydrogen partial pressure approaches the threshold, the opening degree of the carrier gas flow valve is increased to dilute the hydrogen concentration; The adjustment amount of the valve opening degree is calculated through a fuzzy control algorithm. Its input variables are the hydrogen partial pressure deviation and the deviation change rate, and the output variable is the opening degree increment.

[0016] Advantages of the present invention: 1. The gradient dehydrogenation and hydrogen recycling method proposed by the present invention avoids the problem of excessive carbon deposition on copper sites by precisely controlling the gradient distribution of copper loading. By adopting the stepwise impregnation method and dynamic adsorption experiments to optimize the copper salt solution concentration gradient, the uniformity and effectiveness of copper loading on the catalyst surface are ensured, thereby improving the stability of the catalyst, extending its service life, and effectively reducing the generation of carbon deposition. This measure significantly reduces the activity decay rate of the catalyst and improves the long-term stable operation ability of the catalyst.

[0017] 2. Through the design of gradient reaction temperature zones, the present invention divides the reactor into a low-temperature zone, a medium-temperature zone, and a high-temperature zone. Through precise control of the temperature intervals, high-efficiency adsorption of cyclohexanol is achieved in the low-temperature zone, dehydrogenation reaction is optimized in the medium-temperature zone, and desorption of cyclohexanone is completed in the high-temperature zone. This temperature zone distribution optimizes the heat effect and reaction rate of the reaction, not only reducing the energy consumption of the overall reaction, but also effectively suppressing side reactions (such as phenol generation), ensuring high selectivity and high yield of cyclohexanone. Therefore, the present invention not only improves the reaction efficiency, but also effectively reduces the energy consumption and the generation of by-products caused by too high temperature.

[0018] 3. The method of the present invention uses a palladium alloy membrane to separate hydrogen, which can achieve efficient hydrogen separation and dynamically control the hydrogen partial pressure to maintain it within the optimal range, thereby realizing the recovery of hydrogen and returning it to the reactor for reuse. By optimizing the pressure difference across the membrane, the efficiency of the hydrogen separation process and the integrity of the membrane are ensured, while avoiding high-energy-consuming multi-stage compression and membrane permeation operations. The energy consumption is significantly reduced, and the comprehensive cost in the separation process is decreased.

[0019] 4. By adopting the real-time hydrogen partial pressure monitoring and flow valve adjustment technology, the present invention enables the dynamic adjustment of the hydrogen partial pressure during the reaction process, avoiding the lag problem existing in the traditional system. This control mechanism can precisely control the hydrogen partial pressure to keep it within the reverse reaction inhibition threshold, maximizing the selectivity of cyclohexanone, avoiding side reactions caused by too high hydrogen partial pressure, and ensuring high purity and high selectivity of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only those of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0021] Figure 1 is the flowchart of the steps of the method of the present invention; Figure 2 is the flowchart of the specific operations of the stepwise temperature rise and reduction described in step 1 of the method of the present invention; Figure 3 is the flowchart of the method for determining the reverse reaction inhibition threshold of the hydrogen partial pressure described in step 2 of the method of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] The present invention will be described in detail below in combination with the drawings and specific embodiments. At the same time, it should be noted here that in order to make the embodiments more detailed, the following embodiments are the best and preferred embodiments. For some well-known technologies, those skilled in the art can also adopt other alternative methods for implementation; and the drawings are only for more specifically describing the embodiments, and are not intended to specifically limit the present invention.

[0023] Please refer to Figures 1-3, an embodiment of the present invention provides a method for gradient dehydrogenation of cyclohexanol and hydrogen recycling with a copper-silicon catalyst. In step 1, first, the surface of porous silica gel is pretreated by hydroxyl regulation to enhance its surface hydrophilicity and adsorption performance, laying a foundation for the loading of copper salts. Then, a stepwise impregnation method is used to load the copper salt solution on the silica gel, forming a gradient distribution with gradually decreasing copper loading from the outside to the inside. This gradient loading makes the copper sites inside the catalyst exhibit different activities under different temperature conditions, thereby optimizing the reaction performance of the catalyst. During the injection process of the increasing concentration of the copper salt solution, the change in copper ion concentration is monitored in real time by a dynamic adsorption experiment to ensure that the concentration of the copper salt solution gradually increases, forming an ideal copper gradient distribution, which helps to avoid the excessive aggregation of copper sites and improve the catalytic activity.

[0024] Subsequently, a stepwise temperature-raising reduction method is used for reduction in a reducing atmosphere containing 5 vol% H2 / N2. The heating rate is adjusted according to the difference in the distribution density of copper oxide, so as to ensure that copper oxide is completely reduced to metallic copper, optimize the active sites of the catalyst, and ensure the efficient progress of the reaction.

[0025] The gradient copper loading and stepwise temperature-raising reduction methods optimize the distribution of active sites of the catalyst, avoid the aggregation of copper sites and the generation of inactive regions caused by high temperature, and thus effectively improve the reaction efficiency and stability of the catalyst.

[0026] In step 2, cyclohexanol vapor and an inert carrier gas enter through a reactor. The catalyst in the reactor is divided into a low-temperature zone (120 - 180 °C), a medium-temperature zone (220 - 280 °C), and a high-temperature zone (300 - 350 °C) according to the temperature distribution. The function of the low-temperature zone is to adsorb cyclohexanol, the dehydrogenation reaction occurs in the medium-temperature zone, and the desorption of cyclohexanone is completed in the high-temperature zone. This gradient temperature control design realizes the efficient conversion of cyclohexanol and the high-selectivity production of cyclohexanone by optimizing the reaction temperature and reaction rate.

[0027] At the same time, by real-time monitoring of the hydrogen partial pressure and dynamically adjusting the carrier gas flow rate, it is ensured that the hydrogen partial pressure is maintained within the range of 0.05 - 0.15 MPa to inhibit the occurrence of the reverse reaction. The reverse reaction inhibition threshold of the hydrogen partial pressure is determined by a thermodynamic equilibrium experiment, and the cyclohexanone selectivity curve under different hydrogen partial pressures reflects the optimized range of the hydrogen partial pressure, thereby effectively avoiding the generation of by-products.

[0028] Through gradient temperature control and precise regulation of hydrogen partial pressure, the present invention can effectively improve the yield and selectivity of cyclohexanone, reduce the occurrence of side reactions, and improve the thermal efficiency of the reaction, significantly enhancing the overall performance of the catalytic process.

[0029] In step 3, the hydrogen gas during the reaction process is separated through a palladium alloy membrane. The palladium alloy membrane can effectively and selectively permeate hydrogen gas. Part of the separated hydrogen gas is returned to the reactor inlet to adjust the hydrogen partial pressure, and the remaining part is stored for other uses. The pressure difference across the membrane is determined through experiments that match the hydrogen permeation flux with the mechanical strength of the membrane. In the experiments, the hydrogen permeation flux at different pressure differences is tested, and the integrity of the membrane structure is observed through a scanning electron microscope. An appropriate pressure difference range is selected to ensure the permeation efficiency and structural stability of the membrane.

[0030] The hydrogen gas separation technology of the palladium alloy membrane not only improves the recovery rate of hydrogen gas, but also ensures the efficient reuse of hydrogen gas by precisely controlling the pressure difference across the membrane. At the same time, it reduces energy consumption and separation costs. The reuse of hydrogen gas improves the energy utilization rate of the reaction system and reduces the environmental impact and costs of overall production.

[0031] Through the preparation of gradient catalysts, precise reaction process control, and efficient hydrogen gas recovery technology, the present invention not only improves the production efficiency of cyclohexanone, but also achieves the long-term stability of the catalyst and the efficient reuse of hydrogen gas, showing significant prospects for industrial application.

[0032] In a possible implementation, first, the silica gel is immersed in a weakly alkaline solution. The weakly alkaline environment can promote the formation and regulation of hydroxyl groups on the surface of the silica gel, and solutions with different pH values will affect the quantity and distribution of surface hydroxyl groups. During this process, the characteristic peak intensity of the hydroxyl groups on the surface of the silica gel is detected through infrared spectroscopy analysis, and the pH value of the solution is adjusted to make the hydroxyl group density on the surface of the silica gel reach a preset range. In order to obtain the optimal surface hydroxyl group density, a pH range that can make the copper salt loading more uniform and stable is selected, thereby ensuring the efficiency and stability of subsequent catalytic reactions.

[0033] During this process, the preset hydroxyl group density range is determined through adsorption isotherm experiments. In the experiments, after adjusting silica gel samples with different hydroxyl group densities and loading copper salts, the dispersion of copper is analyzed using X-ray diffraction (XRD). The dispersion of copper affects the activity and stability of the catalyst. Selecting the hydroxyl group density range with the best copper dispersion can effectively improve the catalytic efficiency of the catalyst and reduce the aggregation phenomenon of copper.

[0034] After the pretreatment, the silica gel will be dried under vacuum to remove physically adsorbed moisture. During the drying process, thermogravimetric analysis (TGA) is used to monitor the change in drying temperature, and the optimal drying temperature is determined based on the mass loss rate of the silica gel. Ensure that the mass loss rate is lower than 5wt% as the drying end point to avoid damage to the silica gel structure or loss of active sites caused by over-drying.

[0035] Through this surface hydroxyl regulation pretreatment method, the copper salt loading capacity of silica gel and the dispersion of copper can be significantly improved, thereby enhancing the catalytic activity and reaction stability of the catalyst. The reasonable hydroxyl density and optimized drying process ensure the durability and activity of the catalyst at high temperatures, reducing the aggregation of copper ions and the deactivation of the catalyst. The application of this method effectively improves the performance of the copper-silica catalyst and can achieve efficient and durable catalytic effects in industrial applications.

[0036] In a possible implementation, first, the pretreated silica gel is successively impregnated in copper salt solutions with low, medium, and high concentrations. After each impregnation, the silica gel is dried at low temperature and then subjected to a pre-calcination treatment. The impregnation and drying processes can effectively ensure the uniform loading of copper salt on the silica gel surface and avoid the excessive aggregation of copper salt. The drying temperature and time after each impregnation are optimized to ensure that the copper salt can be evenly distributed on the silica gel surface and premature degradation of the copper salt does not occur.

[0037] During the optimization of the concentration gradient, the concentration gradient of the copper salt solution is precisely adjusted through a dynamic adsorption experiment. The specific method is to continuously inject copper salt solutions with gradually increasing concentrations into the silica gel and monitor the copper ion concentration in the effluent in real time. When the copper ion concentration in the effluent reaches 95% of the inlet concentration, it indicates that the silica gel surface has adsorbed sufficient copper ions, and at this time, the impregnation is switched to the copper salt solution of the next concentration. This dynamic adsorption process ensures the gradient distribution of copper salt on the silica gel surface, avoids the accumulation of copper salt at high concentrations, and at the same time ensures that the copper salt at low concentrations can be fully adsorbed.

[0038] After the copper salt impregnation is completed, the silica gel needs to be pre-calcined to remove excess solvents and unreacted substances. The pre-calcination process is carried out under nitrogen protection to ensure that the silica gel is not affected by oxidation during the calcination process. The calcination temperature is determined by differential scanning calorimetry (DSC), specifically by the end point of the endothermic peak when the copper salt decomposes to determine the calcination end temperature. This method ensures the complete decomposition of the copper salt and prevents the damage of the copper salt loading caused by too high temperature.

[0039] The gradient distribution constructed by this step-by-step impregnation method can significantly improve the loading uniformity of copper salt and the dispersion of copper, thereby enhancing the catalytic performance and stability of the catalyst. The copper salt with gradient distribution can better adapt to different reaction conditions, improve the catalytic activity and extend the service life of the catalyst. The dynamic adsorption experiment provides a means to precisely control the copper salt concentration, ensuring that the distribution of copper salt on the silica gel surface is more in line with the catalytic requirements. By controlling the calcination temperature through differential scanning calorimetry, the negative impact of high temperature on the catalyst performance is effectively avoided, enabling the copper-silica catalyst to exhibit the best performance in catalytic reactions.

[0040] In a possible implementation, first, the distribution density of copper oxide in the silica pores of the catalyst is analyzed by X-ray photoelectron spectroscopy (XPS). This analysis method can accurately identify the chemical state of copper oxide and determine its distribution on the catalyst surface and in the pores. According to the analysis results, the catalyst is divided into a high-density area and a low-density area, and different reduction treatments are carried out respectively.

[0041] In the high-density area, due to the high concentration of copper oxide, a stepwise temperature increase strategy is adopted for reduction. In the initial stage, the temperature is raised to an intermediate temperature at the first heating rate and kept constant. This temperature is usually relatively low, aiming to first reduce the more difficult-to-reduce copper oxide in the catalyst and ensure that the hydrogen release rate during the reduction process is in a relatively stable state. When the hydrogen release rate starts to decline, indicating that the relatively easy-to-reduce copper oxide has been completely reduced, the second stage begins. A higher heating rate is used to continue heating until the final reduction temperature is reached. In the second heating stage, the higher heating rate can accelerate the reduction process of copper oxide and ensure that the remaining copper oxide can be completely reduced within a shorter time.

[0042] In the low-density area, since the concentration of copper oxide is low, a linear temperature increase strategy is adopted to linearly raise the temperature from the initial temperature to the final reduction temperature. Linear heating can heat the catalyst more evenly and avoid uneven reduction during the heating process.

[0043] In the selection of parameters such as the first heating rate, intermediate temperature, and second heating rate, the reduction process is optimized through a temperature-programmed reduction experiment. The optimization process uses the reduction degree of copper oxide and the particle size of copper particles as judgment indicators. By measuring the reduction degree of copper oxide during the reduction process, it is possible to understand whether the reduction is complete; while the particle size of copper particles directly affects the activity and stability of the catalyst. Therefore, by controlling the particle size, it is possible to ensure that the catalyst has good catalytic performance.

[0044] The segmented temperature increase reduction strategy can effectively improve the reduction efficiency and catalytic performance of the copper-silica catalyst. The stepwise temperature increase can avoid the problem of copper particle aggregation during the rapid reduction process, helping to maintain the dispersion of copper particles, thereby improving the activity and stability of the catalyst. The linear temperature increase strategy in the low-density area can ensure uniform temperature distribution and avoid incomplete reduction caused by too high or too low temperature. By optimizing the parameters in the reduction process through a temperature-programmed reduction experiment, it is ensured that the catalyst obtains an ideal reduction degree of copper oxide and a suitable copper particle size under the best conditions, thereby effectively improving the catalytic performance and service life of the catalyst.

[0045] In a possible implementation, first, the activation energy of the cyclohexanol dehydrogenation reaction is determined by differential scanning calorimetry (DSC). Differential scanning calorimetry is a commonly used thermal analysis technique that can accurately measure the thermal effect during the reaction, thereby calculating the activation energy of the reaction. According to the Arrhenius equation, through the measured activation energy, the reaction rate constants corresponding to each temperature range can be calculated. The Arrhenius equation describes the relationship between temperature and the reaction rate constant, thus providing a theoretical basis for determining the temperature ranges of the low-temperature region, medium-temperature region, and high-temperature region.

[0046] The temperature setting in the low-temperature region is designed to make the adsorption rate of cyclohexanol greater than the desorption rate. At this time, the reactant (cyclohexanol) can be effectively adsorbed onto the catalyst surface, but the desorption rate of the reaction is slow, which is conducive to the accumulation of cyclohexanol and the progress of subsequent reactions. By this temperature control, a sufficient concentration of reactants can be maintained to promote the reaction.

[0047] The temperature setting in the medium-temperature region enables the dehydrogenation reaction rate to reach the maximum value. In this temperature range, cyclohexanol molecules can actively participate in the dehydrogenation reaction on the catalyst surface without desorbing or decomposing too quickly. This temperature range is the key area of the reaction, which can maximize the dehydrogenation reaction rate and the conversion efficiency of cyclohexanol to cyclohexanone.

[0048] The temperature setting in the high-temperature region makes the desorption rate of cyclohexanone greater than the adsorption rate. At this time, the generated cyclohexanone can quickly desorb from the catalyst surface, avoiding its reattachment to the catalyst surface or side reactions, and ensuring the smooth progress of the reaction process. The temperature setting in this temperature region helps to improve the overall conversion rate and selectivity of the reaction.

[0049] To ensure that the temperature settings in the low-temperature region, medium-temperature region, and high-temperature region are reasonable and effective, online mass spectrometry is used to monitor the composition of the gas at the reactor outlet for verification. By detecting the components of the gas during the reaction with an online mass spectrometer, the reaction process in each temperature region can be analyzed in real time to ensure that the functions of each temperature region achieve the expected goals. For example, the concentration changes of gases such as cyclohexanol, cyclohexanone, and hydrogen can be detected to determine whether the reaction conditions in each temperature region meet the predetermined reaction rate requirements. In this way, through mass spectrometry monitoring, the reaction conditions can be adjusted in a timely manner to ensure the functional matching of the temperature ranges during the reaction process and avoid side reactions or inefficient conversions caused by overheating or overcooling.

[0050] This method ensures the high efficiency and selectivity of the dehydrogenation reaction by precisely setting the temperature ranges of each temperature zone and combining the calculations of differential scanning calorimetry and the Arrhenius equation. The low-temperature zone can ensure the adsorption of cyclohexanol, while the medium-temperature zone maximizes the dehydrogenation reaction rate, thereby improving the efficiency of the conversion of cyclohexanol to cyclohexanone; the high-temperature zone helps to rapidly desorb the generated cyclohexanone and avoid side reactions. The effectiveness of the temperature setting is further verified by on-line mass spectrometry monitoring, ensuring the controllability and high efficiency of the reaction, and ultimately greatly enhancing the total conversion rate, selectivity of the catalytic reaction, and the stability of the catalyst.

[0051] In a possible implementation, first, different ratios of hydrogen and cyclohexanol vapor are introduced into a closed reactor to simulate the reaction conditions under different hydrogen partial pressures. Gas chromatography (GC) technology is used to measure the equilibrium concentrations of cyclohexanol and cyclohexanone. Gas chromatography can accurately separate and quantitatively determine each component in a gas sample. Therefore, during this process, the concentration changes of hydrogen, cyclohexanol, and cyclohexanone can be monitored in real time.

[0052] Based on the equilibrium concentration data measured by gas chromatography at different hydrogen partial pressures, the variation rules of the concentrations of cyclohexanol and cyclohexanone can be analyzed. In the experiment, with the change of the hydrogen partial pressure, the rate of the conversion of cyclohexanol to cyclohexanone and the occurrence of side reactions will change to varying degrees. Therefore, plotting the relationship curve between the hydrogen partial pressure and the selectivity of cyclohexanone is very important for determining the optimal hydrogen partial pressure.

[0053] By plotting the relationship curve between the hydrogen partial pressure and the selectivity of cyclohexanone, it can be observed that within a certain range of hydrogen partial pressures, the selectivity of cyclohexanone increases with the increase of the hydrogen partial pressure. However, after exceeding a certain specific hydrogen partial pressure, the reverse reaction of cyclohexanol begins to increase significantly, resulting in a decrease in the selectivity of cyclohexanone and even possibly causing unwanted side reactions to occur. The hydrogen partial pressure corresponding to the inflection point of this curve is the critical value for suppressing the reverse reaction. By selecting this point as the threshold, it can ensure that the reaction proceeds at the optimal hydrogen partial pressure and avoid the reverse reaction caused by excessive hydrogen.

[0054] To maintain the hydrogen partial pressure within the optimal range, a PID controller is used to adjust the carrier gas flow rate. The PID controller is a commonly used automatic control method, which adjusts the hydrogen flow rate through a feedback mechanism to maintain a constant hydrogen partial pressure during the reaction process. Specifically, the input signal of the PID controller is the detection data of a real-time hydrogen partial pressure sensor, and the sensor feeds back the real-time hydrogen partial pressure to the control system. The controller automatically adjusts the opening degree of the carrier gas flow valve according to the set target hydrogen partial pressure to keep the hydrogen flow rate within the preset range, thereby maintaining the hydrogen partial pressure in the reactor stable near the optimal value.

[0055] This precise method for adjusting hydrogen partial pressure can effectively avoid the occurrence of reverse reactions, thereby significantly improving the selectivity of cyclohexanone and the overall efficiency of the reaction. By measuring the equilibrium concentrations of cyclohexanol and cyclohexanone through gas chromatography and plotting the relationship curve between hydrogen partial pressure and cyclohexanone selectivity, it is possible to provide a scientific basis for optimizing the reaction conditions. The application of a PID controller in hydrogen partial pressure control enables real-time adjustment of the reaction process, ensuring that the hydrogen partial pressure always remains within the optimal range. The implementation of this process not only improves the conversion rate and selectivity of the catalytic reaction but also reduces side reactions and energy waste caused by inappropriate hydrogen partial pressure, ensuring the high efficiency and stability of the reaction process.

[0056] In a possible implementation, first, under constant temperature conditions (such as an operating temperature range of 300°C to 400°C), place the palladium alloy membrane in the hydrogen separation device. Gradually adjust the gas pressure difference across the membrane, for example, increasing it from 0.1 MPa to 1.0 MPa step by step, and measure the permeation flux of hydrogen through the membrane layer at different pressure differences. The flux can be quantitatively recorded through a gas flow meter and time integration, and converted into a standard flux value per unit area (such as mol / m²·s).

[0057] Plot the hydrogen permeation fluxes under different pressure differences as a curve to obtain a typical pressure difference - flux relationship graph. This curve usually shows the characteristics of first linear growth and then tending to a plateau. The linear region indicates that the hydrogen permeation performance of the membrane material is good, and there is no structural damage or flux saturation.

[0058] During the flux test, to prevent mechanical damage to the membrane layer, after conducting successive pressure application experiments on the membrane, observe its surface using a scanning electron microscope (SEM). Pay particular attention to whether there are structural changes such as cracks, holes, and deformations in the membrane layer. By comparing the surface morphology images of the membrane under different pressure differences, determine the maximum safe pressure difference that does not cause membrane layer rupture or fatigue damage. Usually, this pressure difference value is lower than the mechanical limit of the membrane material.

[0059] Based on a comprehensive analysis of the pressure difference - flux curve and the membrane structure stability data, while ensuring high - efficiency hydrogen permeation, select an operating pressure difference within the linear region of the pressure difference - flux curve and lower than the above - determined maximum safe pressure difference. This selection method ensures the best balance between permeation efficiency and the lifespan of the membrane material. For example, if the linear region is 0.2–0.6 MPa and the maximum safe pressure difference is 0.7 MPa, the finally recommended operating pressure difference is about 0.5 MPa.

[0060] By accurately selecting the operating pressure difference through this method, not only the long-term stable operation of the palladium alloy membrane under high-flux conditions is ensured, but also the physical damage of the membrane layer caused by overpressure is effectively prevented. In addition, this method provides theoretical support and practical basis for the high-selectivity recovery of hydrogen. The use of SEM morphology analysis improves the reliability of the judgment of the membrane layer safety, while the pressure difference-flux curve provides quantitative guidance for the engineering optimization of operating parameters, thereby improving the energy efficiency ratio and economy of the entire dehydrogenation and hydrogen recovery process, and having good industrial promotion value.

[0061] In a possible implementation, first, a kinetic model that comprehensively considers the dehydrogenation reaction rate, the catalyst deactivation rate, and the hydrogen recycling rate is established. The model includes: Dehydrogenation reaction rate: Describes the reaction rate of the cyclohexanol dehydrogenation reaction, which is usually affected by factors such as temperature, pressure, and catalyst activity.

[0062] Catalyst deactivation rate: As the reaction proceeds, the catalyst may be deactivated due to carbon deposition, poison adsorption, or other factors. The rate of deactivation and its impact on the reaction effect need to be considered in the model.

[0063] Hydrogen recycling rate: That is, the reflux ratio of hydrogen, which affects the hydrogen concentration in the reactor, thereby affecting the equilibrium and efficiency of the dehydrogenation reaction. The reasonable adjustment of this parameter can effectively control the balance between the reaction rate and the catalyst life.

[0064] Through numerical simulation, calculate the relationship between the yield of cyclohexanone and the catalyst life under different hydrogen recycling ratios. By changing the hydrogen recycling ratio, observe the accumulation of reaction products and the decline of catalyst activity. Specifically, under certain operating conditions, increasing the hydrogen recycling ratio can increase the hydrogen concentration in the reactor and promote the dehydrogenation reaction; however, too high a recycling ratio may lead to accelerated catalyst deactivation. Therefore, it is necessary to find a balance point through numerical simulation to maximize the product of the yield of cyclohexanone and the catalyst life.

[0065] To ensure the accuracy of the simulation results, it is necessary to calibrate the model parameters through laboratory bench-scale test data. These parameters include: Activation energy: Describes the sensitivity of the reaction rate to temperature changes, usually obtained through temperature-programmed reaction experiments.

[0066] Mass transfer coefficient: Describes the mass transfer efficiency between gas molecules on the catalyst surface and reactants, and is calibrated by measuring the mass transfer rate under different conditions through experiments.

[0067] Deactivation rate constant: The rate constant of catalyst deactivation, usually evaluated by long-term operation experiments to assess the stability of the catalyst, and this constant is obtained based on the decline of the catalyst.

[0068] Through the calculation and simulation of the above models and parameters, the hydrogen recycling ratio is selected to maximize the product of the cyclohexanone yield and the catalyst life. In actual operation, this means that while ensuring a high yield, the service life of the catalyst can be extended, thereby improving the economy and sustainability of the overall reaction process.

[0069] In one possible implementation, first, hydrogen is compressed by permeation and pre-mixed with the raw material cyclohexanol vapor in a static mixer. In this way, the cyclohexanol vapor and hydrogen can be evenly mixed to ensure that when entering the reactor, the distribution of the reactants and hydrogen is relatively uniform. The purpose of pre-mixing is to enable the dehydrogenation reaction of cyclohexanol and hydrogen in the optimal ratio when entering the reactor. This can avoid affecting the reaction selectivity and rate due to too high or too low hydrogen concentration.

[0070] After the mixed gas enters the reactor, the gas is distributed through a porous distributor. The pore size gradient of the distributor is precisely designed to ensure that the gas flow rate in each temperature zone of the reactor can be evenly distributed. To achieve this goal, a pressure drop experiment needs to be carried out to measure the gas distribution under different pore sizes and different gas flow rates. Through the analysis of experimental data, the optimal pore size gradient and distribution method are selected to ensure the flow uniformity of the gas in different temperature zones, thereby optimizing the reaction efficiency. Uniform gas distribution can avoid local overheating or insufficient reactants, reduce the burden on the catalyst, and improve the service life of the catalyst.

[0071] The compression pressure of hydrogen is dynamically adjusted according to the reactor inlet pressure, aiming to maintain the partial pressure of hydrogen below a specific threshold. In the dehydrogenation reaction, too high partial pressure of hydrogen may inhibit the dehydrogenation reaction and lead to a decrease in product selectivity. Therefore, the reactor inlet pressure and the partial pressure of hydrogen need to be precisely controlled to ensure that they are within the optimal range to achieve an ideal dehydrogenation effect. In actual operation, the dynamic adjustment of the compression pressure can be feedback-controlled based on real-time reaction data to maintain the partial pressure of hydrogen in the reactor at an appropriate level.

[0072] In one possible implementation, first, a hydrogen partial pressure sensor is installed at the reactor outlet to collect the partial pressure data of hydrogen in the reactor in real time. The role of the hydrogen partial pressure sensor is to precisely monitor the change in hydrogen concentration. When the hydrogen partial pressure in the reactor approaches the set threshold, the system will trigger adjustment measures. The monitoring of the hydrogen partial pressure can reflect the consumption rate of hydrogen and the progress of the reaction during the reaction process, helping to regulate the reaction conditions to ensure the stable progress of the reaction.

[0073] When the hydrogen partial pressure approaches the set threshold, to avoid excessive hydrogen concentration, the system will increase the flow rate of the inert carrier gas. The adjustment of the carrier gas flow rate is achieved by adjusting the opening degree of the carrier gas flow valve. Increasing the inert carrier gas flow rate can effectively dilute the hydrogen concentration and prevent the hydrogen partial pressure from being too high, which may affect the reaction selectivity and the performance of the catalyst.

[0074] The adjustment amount of the valve opening degree is calculated by a fuzzy control algorithm. The fuzzy control algorithm determines the increment of the valve opening degree to be adjusted based on the hydrogen partial pressure deviation (i.e., the difference between the current hydrogen partial pressure and the set threshold) and the deviation change rate (the change rate of the hydrogen partial pressure). Specifically, when the hydrogen partial pressure deviation is large, the system can quickly adjust the valve opening degree to increase the carrier gas flow rate; when the deviation changes relatively smoothly, the adjustment amplitude of the valve will be reduced accordingly. This control method based on deviation and change rate can more accurately control the hydrogen concentration, avoid too fast or too slow responses, and ensure the stability and efficiency of the reaction process.

[0075] This invention covers any substitutions, modifications, equivalent methods, and solutions made within the spirit and scope of this invention. To enable the public to have a thorough understanding of this invention, specific details are described in detail in the following preferred embodiments of this invention. However, those skilled in the art can fully understand this invention even without these detailed descriptions. Additionally, well-known methods, processes, procedures, components, and circuits are not described in detail to avoid unnecessary confusion to the essence of this invention.

[0076] The above description is only the preferred embodiment of this invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of this invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of this invention.

Claims

1. A method for gradient dehydrogenation of cyclohexanol and hydrogen reuse with a copper-silicon catalyst, characterized in that, It includes the following steps: Step 1: Preparation of gradient catalyst: After the porous silica gel is pretreated by surface hydroxyl regulation, it is loaded with a copper salt solution by the stepwise impregnation method to form a gradient distribution with a decreasing copper loading from the outside to the inside, and is reduced by stepwise heating in a reducing atmosphere containing 5 vol% H2 / N2, where the heating rate is differentially adjusted according to the distribution density of copper oxide; The gradient distribution with decreasing copper loading is optimized by a dynamic adsorption experiment: continuously inject copper salt solutions with increasing concentrations into the silica gel column, and switch to the next concentration when the copper ion concentration in the effluent reaches 95% of the inlet concentration; Step 2: Control of the gradient dehydrogenation reaction process: Inject cyclohexanol vapor and an inert carrier gas into a fixed-bed reactor filled with the catalyst obtained in Step 1, which is divided into a low-temperature zone of 120 - 180 °C, a medium-temperature zone of 220 - 280 °C, and a high-temperature zone of 300 - 350 °C. Gradient dehydrogenation is achieved by adsorbing cyclohexanol in the low-temperature zone, carrying out dehydrogenation reaction in the medium-temperature zone, and desorbing cyclohexanone in the high-temperature zone. At the same time, the carrier gas flow rate is dynamically adjusted by real-time monitoring of the hydrogen partial pressure to make the hydrogen partial pressure lower than the inhibition reverse reaction threshold of 0.05 - 0.15 MPa; The inhibition reverse reaction threshold of hydrogen partial pressure is determined by a thermodynamic equilibrium experiment: measure the cyclohexanone selectivity curve at different hydrogen partial pressures in a closed reactor, and take the hydrogen partial pressure corresponding to the inflection point of the curve as the inhibition reverse reaction threshold; Step 3: In-situ separation and recycling of hydrogen: The reaction outlet gas is separated from hydrogen through a palladium alloy membrane, and the pressure difference across the membrane is controlled to maintain the permeation flux. Part of the separated hydrogen is returned to the reactor inlet to adjust the hydrogen partial pressure, and the remaining hydrogen is stored; The pressure difference across the membrane is determined by an experiment on the matching of hydrogen permeation flux and membrane mechanical strength: measure the hydrogen permeation flux at different pressure differences, and combine scanning electron microscopy to observe the integrity of the membrane structure, and select the pressure difference range with a linear flux interval and no membrane damage; 2. The method for gradient dehydrogenation of cyclohexanol and hydrogen reuse using a copper-silicon catalyst according to claim 1, wherein The specific method of the surface hydroxyl regulation pretreatment described in Step 1 is: Immerse the silica gel in a weakly alkaline solution, detect the intensity of the characteristic peak of surface hydroxyl groups on the silica gel by infrared spectroscopy, and select the pH value of the solution that makes the hydroxyl density reach the preset range; The preset range is determined by an adsorption isotherm experiment, which specifically includes respectively loading copper salts on silica gel samples with different hydroxyl densities, analyzing the copper dispersion by X-ray diffraction, and selecting the hydroxyl density range with the best copper dispersion; The pretreated silica gel is dried in vacuum to remove physically adsorbed water, and the drying temperature is determined by thermogravimetric analysis, with the end point being that the mass loss rate of the silica gel is lower than 5 wt%; 3. A method for gradient dehydrogenation of cyclohexanol with hydrogen recycling using a copper-silicon catalyst according to claim 1, characterized in that, The method for constructing the gradient distribution of the stepwise impregnation method described in Step 1 is: Immerse the pretreated silica gel successively in copper salt solutions with low, medium, and high concentrations, and dry at low temperature and pre-calcine after each impregnation; The concentration gradient of the copper salt solutions with low, medium, and high concentrations is optimized by a dynamic adsorption experiment, which specifically includes continuously injecting copper salt solutions with increasing concentrations into the silica gel, and real-time monitoring the copper ion concentration in the effluent, and switching to the next concentration when the copper ion concentration in the effluent reaches 95% of the inlet concentration; The pre-calcination process is carried out under nitrogen protection, and the calcination temperature is determined by differential scanning calorimetry, with the end point being the end of the endothermic peak of copper salt decomposition; 4. A method for gradient dehydrogenation of cyclohexanol and hydrogen reuse with a copper-silicon catalyst according to claim 1, characterized in that, The specific operation of the stepwise heating reduction described in Step 1 is: The copper oxide distribution density in the silica pores was analyzed by X-ray photoelectron spectroscopy, and the catalyst was divided into a high-density region and a low-density region; In the high-density region, a stepped heating strategy was adopted. In the initial stage, it was heated to an intermediate temperature at a first heating rate and held at a constant temperature. After the hydrogen release rate decreased, it was heated to the final reduction temperature at a second heating rate; In the low-density region, it was linearly heated to the final reduction temperature; the first heating rate, intermediate temperature, and second heating rate were optimized through a temperature-programmed reduction experiment, with the reduction degree of copper oxide and the particle size of copper particles as the judgment indexes.

5. A method for gradient dehydrogenation of cyclohexanol and hydrogen reuse with a copper-silicon catalyst according to claim 1, characterized in that, The method for determining the temperature ranges of the low-temperature region, medium-temperature region, and high-temperature region in step 2 is as follows: The activation energy of the cyclohexanol dehydrogenation reaction was determined by differential scanning calorimetry, and the reaction rate constants of each temperature region were calculated based on the Arrhenius equation; The temperature of the low-temperature region was set so that the cyclohexanol adsorption rate was greater than the desorption rate, the temperature of the medium-temperature region was set so that the dehydrogenation reaction rate was maximized, and the temperature of the high-temperature region was set so that the cyclohexanone desorption rate was greater than the adsorption rate; The final temperature range was verified by on-line mass spectrometry monitoring of the gas composition at the reactor outlet to ensure the functional matching of each temperature region.

6. A method for gradient dehydrogenation of cyclohexanol and hydrogen reuse with a copper-silicon catalyst according to claim 1, characterized in that, The method for determining the inhibition reverse reaction threshold of the hydrogen partial pressure in step 2 is as follows: Different ratios of hydrogen and cyclohexanol vapor were introduced into a closed reactor, and the equilibrium concentrations of cyclohexanone and cyclohexanol were determined by gas chromatography; The relationship curve between the hydrogen partial pressure and the cyclohexanone selectivity was plotted, and the hydrogen partial pressure corresponding to the inflection point of the curve was selected as the threshold; The carrier gas flow rate adjustment was realized by a PID controller, whose input signal was the real-time data of the hydrogen partial pressure sensor, and the output signal was the opening degree of the carrier gas flow valve.

7. A method for gradient dehydrogenation of cyclohexanol and hydrogen reuse with a copper-silicon catalyst according to claim 1, characterized in that, The method for determining the pressure difference on both sides of the palladium alloy membrane in step 3 is as follows: The influence of different pressure differences on the hydrogen permeation flux was tested at a fixed temperature, and the pressure difference-flux curve was plotted; the surface morphology of the membrane was observed by scanning electron microscopy to determine the maximum pressure difference that does not cause membrane layer rupture; the final operating pressure difference was selected within the linear range of the pressure difference-flux curve and lower than the maximum pressure difference value range.

8. A method for gradient dehydrogenation of cyclohexanol and hydrogen reuse with a copper-silicon catalyst according to claim 1, characterized in that, The optimization method for the ratio of hydrogen partially returned to the reactor in step 3 is as follows: A kinetic model including the dehydrogenation reaction rate, catalyst deactivation rate, and hydrogen recycling rate was established, and the cyclohexanone yield and catalyst life under different circulation ratios were calculated by numerical simulation; The circulation ratio that maximizes the product of the yield and life was selected; the model parameters were calibrated by laboratory bench-scale test data, and the calibrated parameters included activation energy, mass transfer coefficient, and deactivation rate constant.

9. A method for gradient dehydrogenation of cyclohexanol and hydrogen recycling using a copper-silicon catalyst according to claim 1, characterized in that, The specific way of returning hydrogen partially to the reactor is as follows: The permeated hydrogen was compressed and premixed with the raw material cyclohexanol vapor in a static mixer; The mixed gas entered the reactor through a porous distributor, and the pore size gradient of the distributor was determined by a pressure drop experiment to ensure uniform distribution of the gas flow rate in each temperature region; the compression pressure was dynamically adjusted according to the reactor inlet pressure to maintain the hydrogen partial pressure below the threshold.

10. A method for gradient dehydrogenation of cyclohexanol and hydrogen reuse with a copper-silicon catalyst according to claim 1, characterized in that, The method for adjusting the flow rate of the inert carrier gas is as follows: A hydrogen partial pressure sensor was set at the reactor outlet to collect real-time data. When the hydrogen partial pressure approached the threshold, the opening degree of the carrier gas flow valve was increased to dilute the hydrogen concentration; The adjustment amount of the valve opening degree was calculated by a fuzzy control algorithm, whose input variables were the hydrogen partial pressure deviation and the deviation change rate, and the output variable was the opening degree increment.

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