Propane dehydrogenation membrane reactor based on W-MFI molecular sieve membrane

Abstract

The invention relates to the technical field of chemical catalysis and separation, in particular to a tungsten (W) doped MFI type molecular sieve membrane (W-MFI) and catalyst integrated membrane reactor which is used for efficiently catalyzing propane dehydrogenation to prepare propylene. The W-doped MFI molecular sieve membrane (W-MFI) is used as a key membrane material of the membrane reactor, so that the overall performance and stability of the membrane reactor are remarkably improved. The introduction of W not only enhances the thermal stability and chemical stability of the molecular sieve membrane and effectively prevents the collapse risk of the membrane structure at high temperature, but also enables the membrane reactor to maintain stable performance output in a 24-hour operation process. And in the aspect of catalytic performance, the prepared membrane reactor has the propane conversion rate of 44% and the propylene selectivity of more than 98% under the conditions that the temperature is 550 DEG C, the mass space velocity is 3.6 h <-1 >, the ratio of C3H8 to N2 is 1: 3 and argon purging is performed (30 mL / min), and compared with a membrane reactor of a pure silicon MFI molecular sieve membrane, the performance is greatly improved.

Description

Technical Field

[0001] The present invention relates to the field of chemical catalysis and separation technology, and in particular to a membrane reactor integrating a tungsten (W)-doped MFI molecular sieve membrane (W-MFI) and a catalyst, for efficient catalytic dehydrogenation of propane to produce propylene. Background Art

[0002] Propylene is a very important basic chemical raw material, widely used in high-value-added chemical products. In recent years, many processes for producing propylene have been developed, including methanol to ethylene (MTO), Fischer-Tropsch (FT), and propane dehydrogenation (PDH). Among them, PDH technology, as a process that specifically produces propylene rather than mixed hydrocarbon products, is considered one of the most promising methods for producing propylene. Furthermore, with the continuous improvement of fracturing technology, large-scale extraction of shale gas condensate with high propane content has become feasible, making the raw material cheaper and more abundant. Therefore, propane dehydrogenation to propylene is considered an important propylene production route, and related research has received widespread attention both domestically and internationally.

[0003] Conventional propane dehydrogenation (PDH) processes are limited by thermodynamic equilibrium, resulting in low conversion rates (<40%). The accumulation of H₂ byproducts in fixed-bed reactors leads to catalyst carbon deposition and deactivation. PDH membrane reactors, with their ability to separate hydrogen at high temperatures, can remove hydrogen promptly during the reaction, accelerating the reaction and mitigating catalyst deactivation. They are considered a promising approach to addressing the low efficiency of conventional PDH. Existing palladium-based hydrogen separation membranes are costly and have poor high-temperature stability (susceptibility to hydrogen embrittlement above 500°C). Molecular sieve membranes such as SAPO-34 are susceptible to clogging by coke precursors in hydrocarbon environments. The acidity (B- and L-acidity) of SAPO-34 is significantly affected by synthesis conditions (such as the Si / Al ratio and template), but precise control of acid site density and strength is difficult and reproducible, potentially leading to fluctuations in catalytic activity. In PDH, excessive propylene reaction can lead to significant carbon deposition, reducing propylene selectivity. Furthermore, carbon deposition can coat the SAPO-34 molecular sieve membrane surface, degrading membrane performance.

[0004] The MFI zeolite membrane features three-dimensional intersecting pores, including straight cylindrical channels (along the b-axis, with a pore diameter of approximately 0.53 nm × 0.56 nm) and sinusoidal channels (along the a-axis, with a pore diameter of approximately 0.51 nm × 0.55 nm), along with an all-silicon backbone. This enables it to achieve exceptional performance in separating small molecule gases such as H2 under harsh conditions. The inventors attempted to construct a membrane reactor by encapsulating a Pt catalyst within a pure silicon MFI zeolite membrane, which significantly improved propane conversion compared to a packed-bed reactor. However, overall, this membrane reactor still has significant room for improvement in propane conversion and propylene selectivity, and the zeolite membrane structure is at risk of collapse at temperatures exceeding 500°C.

[0005] Therefore, how to optimize the above membrane reactor is an urgent problem to be solved. Summary of the Invention

[0006] To address the above problems, the present invention adopts W-doped MFI molecular sieve membrane as the key membrane material of the membrane reactor, and uses Silicalite-1 molecular sieve with the same configuration as the molecular sieve membrane as the carrier of the propane dehydrogenation catalyst to form a new propane dehydrogenation membrane reactor, which can significantly improve the propane conversion rate, propylene selectivity and 24-h operation stability.

[0007] In order to achieve the technical purpose of the present invention, the present invention adopts the following technical solutions: A propane dehydrogenation membrane reactor based on a W-MFI molecular sieve membrane, the membrane reactor comprising a W-MFI molecular sieve membrane arranged at the center of the membrane reactor, a propane dehydrogenation catalyst being filled in the cavity between the outer surface of the W-MFI molecular sieve membrane and the inner side of a membrane reactor shell, the W-MFI molecular sieve membrane comprising a hollow fiber support and a W-MFI molecular sieve membrane layer supported on the outer surface of the support, and the carrier of the propane dehydrogenation catalyst being a Silicalite-1 molecular sieve.

[0008] Preferably, the W-MFI molecular sieve membrane uses SiO2 and W as skeleton sources, and the molar ratio of SiO2:W in the casting solution is 1:0.02-0.05.

[0009] Preferably, the W-MFI molecular sieve membrane has a H2 permeability of ≥1.5×10⁻ at 550°C. 7 mol / (m²·s·Pa), H2 / C3H8 selectivity ≥4.0.

[0010] Preferably, the hollow fiber support is a single-channel hollow fiber or a multi-channel hollow fiber.

[0011] Preferably, the cavity between the outer surface of the W-MFI molecular sieve membrane and the inner shell of the membrane reactor is filled with solid particles mixed with a propane dehydrogenation catalyst and a filler.

[0012] Preferably, the support of the propane dehydrogenation catalyst is a hierarchical pore Silicalite-1 molecular sieve synthesized with the assistance of lysine, and the active component of the propane dehydrogenation catalyst is one or more of Pt, Sn, Cr, Co, and Zn.

[0013] Preferably, the active components of the propane dehydrogenation catalyst are Pt and Sn, wherein the Pt loading is 0.5-1.0 wt%, and the Sn / Pt molar ratio is 0.3-0.5.

[0014] The present invention also provides a propane dehydrogenation method based on W-MFI molecular sieve membrane, in which propane gas generates propylene and hydrogen under the catalytic action of a propane dehydrogenation catalyst. The hydrogen permeates the W-MFI molecular sieve membrane into the hollow fiber cavity and is removed by a purge gas.

[0015] Preferably, the operating temperature of the membrane reactor is 500-600 °C, the pressure is atmospheric pressure, WHSV=1.8-4.8 h⁻¹, and C3H8:N2=1:1-1:4.

[0016] Preferably, the flow rate of the sweep gas is 10-40 mL / min.

[0017] Compared with the prior art, the present invention has the following advantages: First, the present invention uses W-doped MFI molecular sieve membrane (W-MFI) as the key membrane material of the membrane reactor, which significantly improves the overall performance and stability of the membrane reactor. The introduction of W not only enhances the thermal and chemical stability of the molecular sieve membrane, effectively prevents the risk of collapse of the membrane structure at high temperatures, but also enables the membrane reactor to maintain stable performance output during operation for up to 24 hours. In terms of catalytic performance, the membrane reactor prepared by the present invention has a high catalytic performance at 550 ° C and a mass space velocity of 3.6 h -1 Under the conditions of C3H8:N2=1:3 and argon purge (30 mL / min), a propane conversion rate of 44% and a propylene selectivity of >98% were achieved, which achieved a significant performance improvement compared with the membrane reactor of pure silicon MFI molecular sieve membrane.

[0018] Secondly, in the propane dehydrogenation reaction, the W-MFI molecular sieve membrane and the Silicalite-1 molecular sieve used as the catalyst support adopt the same aluminum-free MFI configuration. This design significantly promotes the efficient transfer of reactants and products between the membrane layer and the catalyst layer. The hydrogen generated by propane decomposition can pass through the membrane and catalyst layers more smoothly, reducing diffusion resistance and improving reaction efficiency. Both contain no aluminum, thus avoiding the side effects of aluminum on propane dehydrogenation.

[0019] Furthermore, the use of lysine-assisted hierarchical silicalite-1 as a catalyst support not only provides a larger surface area, exposing more active sites, but also facilitates the rapid diffusion of reactants and products. The combination of the W-MFI molecular sieve membrane and the hierarchical silicalite-1 catalyst support of the same configuration provides an efficient and stable solution for propane dehydrogenation. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1Schematic diagram of the membrane reactor structure prepared in Example 2 of the present invention, wherein 1-reactor shell, 2-molecular sieve membrane, 3-catalyst, 4-quartz sand, 5-graphite ring, 6-quartz wool, 7-analytical chromatogram; Figure 2 is the XRD spectrum of the W-MFI film prepared in Example 1; Figure 3 SEM images of the W-MFI film prepared in Example 1 and the Si-MFI film prepared in Comparative Example 1; Figure 4 UV-vis graph of the W-MFI film prepared in Example 1; Figure 5 The effect of temperature on the H2 / C3H8 separation performance of the W-MFI molecular sieve membrane prepared in Example 1; Figure 6 Long-term stability (550 °C) of the W-MFI molecular sieve membrane H2 / C3H8 binary system prepared in Example 1; Figure 7 The effect of temperature on the H2 / C3H8 separation performance of the Si-MFI molecular sieve membrane prepared in Comparative Example 1; Figure 8 SEM characterization of the PtSn / P-1 catalyst prepared in Example 2; Figure 9 XRD characterization of the PtSn / P-1 catalyst prepared in Example 2; Figure 10 Comparison of long-term reaction stability between the membrane reactor and the fixed-bed reactor constructed in Example 3; Figure 11 TG curves of the PtSn / P-1 catalyst prepared in Example 2 and the catalyst after reaction in the membrane reactor and fixed bed reactor constructed in Example 3; Figure 12 Comparison of regeneration performance of fixed bed and membrane reactors constructed for propane dehydrogenation according to Example 3. DETAILED DESCRIPTION

[0021] Example 1 The present invention will be further described below with reference to the accompanying drawings.

[0022] The preparation method of W-MFI molecular sieve membrane is as follows: (1) Seed preparation: Tetraethyl orthosilicate, tetrapropylammonium hydroxide, ethanol, and deionized water were mixed in a molar ratio of 1TEOS:0.3TPAOH:4EtOH:24H2O. Stir at room temperature for 6 hours and react at 150°C for 48 hours. Wash by centrifugation and dry for 12 hours. Calcinate at 550°C for 8 hours to remove the organic template.

[0023] (2) The seed crystals prepared in step (1) are uniformly coated on the Al2O3 carrier by the immersion pulling method, specifically: the molecular sieve prepared in the above step (1) is made into a 0.5 wt% seed crystal suspension, the carrier is immersed in the seed crystal suspension for 30 seconds, and then dried at 60 ° C for 6 hours.

[0024] (3) Preparation of W-MFI molecular sieve membrane: A synthesis solution was prepared with a molar ratio of 1TEOS: 0.12TPAOH: 180H2O: 4EtOH: 0.04Na2WO4·2H2O, and the mixture was stirred and aged at room temperature for 6 h. The carrier coated with the seed layer prepared in step (2) was fixed and placed vertically in a reactor, and reacted with the membrane synthesis solution at 140°C for 12 h. The membrane after the reaction was washed with deionized water, soaked in clean water for 12 h, dried at 60°C for 12 h, and then calcined in an ozone atmosphere to remove the template agent. The calcination time was 96 h, the calcination temperature was 230°C, and the heating rate was 0.5°C / min.

[0025] Example 2 Preparation of hierarchical pore PtSn@P-1 catalyst: (1) Preparation of catalyst carrier: 1SiO2 in molar ratio Tetraethyl orthosilicate, tetrapropylammonium hydroxide, and deionized water were mixed and stirred with a mixture of 0.46 TPAOH, 0.36 L-lysine, and 9H2O. After the silicon source was completely hydrolyzed, the lysine component was added to the solution and stirring continued. The homogenized mixed solution was then transferred to a vacuum rotary evaporator to remove excess water, yielding an initial gel. Crystallization was performed at 90°C for 12 hours, followed by centrifugal washing and freeze-drying. The resulting catalyst support is labeled P-1.

[0026] (2) Preparation of hierarchical PtSn@MFI catalyst by impregnation: 630 μL of H2PtCl6·H2O solution (1 g H2PtCl6·H2O dissolved in 50 mL ethanol) and 0.006 g SnCl2 were added to 100 μL of 3% HCl solution, corresponding to 0.025 mM platinum ions and 0.031 mM tin ions. The mixture was shaken to form a purple-red metal precursor. 0.50 g of P-1 was added to a preheated mortar (80 °C), followed by the metal precursor, and ground until the water evaporated. The resulting catalyst was labeled PtSn / P-1.

[0027] Example 3 Membrane reactor system construction: like Figure 1As shown, the molecular sieve membrane 2 (prepared in Example 1) is encapsulated in a customized membrane reactor shell 1 (effective cavity size: length 5 cm, inner diameter 0.4 cm), and the two ends of the membrane are squeezed and sealed by a graphite ring 5. In the remaining space outside the effective area of ​​the reactor assembly, the filling medium is composed of a catalyst 3 (PtSn / P-1 prepared in Example 2) and a quartz sand 4 mixed in a specific ratio (1:2). The gas inlet and outlet of the reactor are blocked by quartz wool 6 to prevent the filling medium from scattering, and are connected to the raw gas and analytical chromatograph 7 respectively. High-purity argon gas is connected to the permeate side of the reactor for purging, and all gases are controlled by a mass flow controller. Membrane reactor operating parameters: reaction temperature is 550 ° C, mass space velocity (WSHV) is 3.6 h -1 , C3H8 : N2 = 1 : 3, purge gas flow rate is 30 mL / min Comparative Example 1 Step (1) and step (2) are the same as in Example 1.

[0028] The preparation process of step (3) is basically the same as step (3) of Example 1, except that the molar ratio of each substance in the gel of the final synthetic molecular sieve membrane is 1TEOS: 0.12TPAOH: 180H2O:, and the prepared molecular sieve membrane is marked as Si-MFI.

[0029] Characterization The gas separation performance of the membrane is expressed by two parameters: gas permeation rate P and separation coefficient α. The gas permeation rate P represents the total amount of gas passing through the membrane per unit area under unit time and unit pressure. P=N / (A×△P) is expressed in mol / (m 2 ·s·Pa); the separation coefficient α is used to evaluate the membrane separation efficiency, α =PA / PB.

[0030] The reaction was analyzed online using a gas chromatograph (GC 9790 plus, Zhejiang Fuli) equipped with dual detectors: a hydrogen ion flame detector (FID) equipped with a 50 m × 0.32 nm × 10 μm capillary column and a thermal conductivity detector (TCD) equipped with a GDX-502 (2 m × 4 mm) packed column.

[0031] The relevant calculation formula is as follows: Propane conversion (C3H8 Conversion) calculation: Calculation of propylene selectivity (C3H6 Selectivity): Calculation of propylene yield (C3H6 Yield): Where Fi represents the flow rate.

[0032] Airspeed calculation (WHSV): in is the volume flow rate of propane gas passing through the fixed bed reactor per unit time, is the density of the propane gas introduced, is the mass of the catalyst.

[0033] The XRD pattern of the molecular sieve membrane prepared in Example 1 is as follows: Figure 2 The surface and cross-sectional SEM images of the W-MFI molecular sieve membrane prepared in Example 1 are shown in FIG. Figure 3 As shown in the figure, it can be seen that the crystals on the surface of the film are all regular rectangular parallelepiped morphology, and the crystals grow alternately to form a continuous and dense film layer without obvious intercrystalline defects. UV-vis ( Figure 4 ) The results show that the DR-UV / vis spectrum in the range of 200-500 nm confirms the incorporation state of W. The absorption band at 207 nm is attributed to the charge transfer from the ligand to the metal, originating from the isolated [WO 4 ] 2- The absorption band at 260 nm corresponds to O 2- → W 6+ The charge transfer is related to the octahedral polytungstate species, which strongly proves that W has successfully entered the framework structure of the MFI molecular sieve.

[0034] The H2 / C3H8 gas separation performance of the W-MFI and Si-MFI molecular sieve membranes in Example 1 and Comparative Example 1 at different temperatures and the long-term stability of the H2 / C3H8 gas separation of the W-MFI molecular sieve membrane at 550°C were investigated. The experimental results are shown in Figure 5. For H2 / C3H8 separation, the hydrogen permeability was stably maintained at 1.5×10 -7 mol / (m 2 ·s·Pa); As shown in Figure 6, the W-MFI zeolite membrane exhibited excellent separation stability during the 24-hour continuous test. The selectivity of hydrogen to propane remained at around 4.1 throughout the 24-hour continuous operation. The Si-MFI membrane (Figure 7) had a selectivity of 1.8 at 550°C and a hydrogen permeability of 1.1×10 -7 mol / (m 2 ·s·Pa), which were lower than those of W-MFI membrane.

[0035] The XRD pattern of the molecular sieve prepared in Example 2 is as follows: Figure 8 The SEM images of the molecular sieves prepared in Example 2 are shown as follows. Figure 9 As shown. Figure 8It can be seen that PtSn / P-1 is an amorphous structure. This is because under low temperature (90 °C), small-sized amorphous nanoparticles are preferentially formed. At this time, lysine acts as a crystal phase regulator to inhibit the growth of nanoparticles, so the sample morphology shows small-sized spheres, which is consistent with the Figure 9 consistent.

[0036] For the membrane reactor constructed in Example 3, the effects of feed composition, space velocity, and temperature on product distribution were investigated. The results are shown in Tables 1, 2, and 3, respectively. The membrane reactor (PBMR) exhibited significant performance in propane dehydrogenation compared to a conventional fixed-bed reactor (PBR). Under identical operating conditions (550°C, mass space velocity of 3.6 h⁻¹, and C₃H₈:N₂ = 1:3), the PBMR achieved a propylene selectivity of 98.45 mol%, a 0.67 percentage point increase over the PBR (97.78 mol%). The PBMR also exhibited significantly enhanced high-temperature adaptability—at 600°C, the PBMR propylene selectivity (94.59 mol%) increased by 9.21 percentage points over the PBR (85.38 mol%). Furthermore, the PBMR effectively suppressed cracking side reactions, generally reducing the production of byproducts such as methane, ethylene, and ethane. A typical example was a 45.1% reduction in ethane production (0.28 mol%) at 550°C compared to the PBR (0.51 mol%). Furthermore, the PBMR maintains a propylene selectivity of 99.39 mol% under the demanding conditions of high space velocity (WHSV = 4.8 h⁻¹) and high dilution ratio (C₃H₈:N₂ = 1:4), consistently outperforming the PBR. This advantage stems from the in-situ hydrogen separation capability of the W-MFI zeolite membrane within the PBMR: real-time H₂ removal overcomes reaction equilibrium constraints, inhibiting side reactions and carbon deposition deactivation. Combined with the isomorphous design of the membrane and Silicalite-1 catalyst support, this optimizes the mass transfer pathway, achieving breakthrough performance under conditions of high temperature (500–600°C), high space velocity (1.8–4.8 h⁻¹), and a wide range of feed compositions (C₃H₈:N₂ = 1:1–1:4).

[0037] Table 1 Effect of feed composition on product distribution Table 2 Effect of space velocity on product distribution Table 3 Effect of temperature on product distribution Secondly, at a reaction temperature of 550 °C and a mass space velocity of 3.6 h -1The membrane reactor was subjected to a 24-hour stability test using a feed gas ratio of C₃H₃:N₂ = 1:3 and a purge flow rate of 30 mL / min. The results were compared with a conventional fixed-bed system. As shown in Figure 10, the membrane reactor demonstrated superior propane conversion and propylene yield over the 24-hour continuous operation period. While both reactors exhibited performance degradation with extended reaction time, the membrane reactor demonstrated significantly enhanced reaction stability. The membrane reactor system maintained a stable propane conversion of approximately 44%, while the fixed-bed reactor experienced a decline in propylene selectivity over the same test period.

[0038] At a reaction temperature of 550 °C and a mass space velocity of 3.6 h -1 The spent catalysts were analyzed by TG after 24 hours of reaction in fixed bed and membrane reaction with a feed gas ratio of C3H8:N2=1:3 and a purge gas flow rate of 30 mL / min. The results are shown in Figure 11. The reaction conditions were 550 °C and WHSV of 3.6 h -1 The feed gas ratio was C₃H₈₈:N₂=1:3. For the membrane reactor system, a continuous argon purge at a flow rate of 30 mL / min was used. Analysis results showed that the amount of carbon deposited on the catalyst surface treated in the membrane reactor was lower than that of the catalyst used in a traditional fixed-bed reactor.

[0039] Finally, at a reaction temperature of 550 °C and a mass space velocity of 3.6 h -1 The membrane reactor was regenerated using a feed gas ratio of C3H8:N2=1:3 and a purge gas flow rate of 30 mL / min. Following the initial catalytic run, air (20% O2 / 80% N2, 60 mL / min) was introduced at 550°C to remove carbon deposits. H2 was then introduced at a flow rate of 60 mL / min to reduce the catalyst for 60 minutes. Propane (25% C3H8 / 75% N2) was then introduced under the same reaction conditions as the initial catalytic run, resulting in three regeneration runs. As shown in Figure 12, the membrane reactor after redox regeneration exhibited the same performance as during initial operation, demonstrating that the PtSn / P-1-based hollow fiber W-MFI molecular sieve membrane reactor can be stably regenerated under propane dehydrogenation reaction conditions.

Claims

1. A propane dehydrogenation membrane reactor based on W-MFI molecular sieve membrane, characterized in that: The membrane reactor includes a W-MFI molecular sieve membrane arranged at the center of the membrane reactor, the cavity between the outer surface of the W-MFI molecular sieve membrane and the inner side of the membrane reactor shell is filled with a propane dehydrogenation catalyst, the W-MFI molecular sieve membrane includes a hollow fiber support and a W-MFI molecular sieve membrane layer supported on the outer surface of the support, and the carrier of the propane dehydrogenation catalyst is Silicalite-1 molecular sieve.

2. The propane dehydrogenation membrane reactor according to claim 1, characterized in that The W-MFI molecular sieve membrane uses SiO2 and W as skeleton sources, and the molar ratio of SiO2:W in the casting solution is 1:0.02-0.

05.

3. The propane dehydrogenation membrane reactor according to claim 1, characterized in that The W-MFI molecular sieve membrane has a H2 permeability of ≥1.5×10⁻ at 550°C 7 mol / (m²·s·Pa), H2 / C3H8 selectivity ≥4.

0.

4. The propane dehydrogenation membrane reactor according to claim 1, characterized in that: The hollow fiber support is a single-channel hollow fiber or a multi-channel hollow fiber.

5. The propane dehydrogenation membrane reactor according to claim 1, characterized in that: The cavity between the outer surface of the W-MFI molecular sieve membrane and the inner shell of the membrane reactor is filled with solid particles mixed with a propane dehydrogenation catalyst and fillers.

6. The propane dehydrogenation membrane reactor according to claim 1, characterized in that: The carrier of the propane dehydrogenation catalyst is a hierarchical pore Silicalite-1 molecular sieve synthesized with the assistance of lysine, and the active component of the propane dehydrogenation catalyst is one or more of Pt, Sn, Cr, Co, and Zn.

7. The propane dehydrogenation membrane reactor according to claim 5, characterized in that: The active components of the propane dehydrogenation catalyst are Pt and Sn, wherein the Pt loading is 0.5-1.0 wt%, and the Sn / Pt molar ratio is 0.3-0.

5.

8. A propane dehydrogenation method using a propane dehydrogenation membrane reactor based on a W-MFI molecular sieve membrane according to claim 1, characterized in that: Propane gas generates propylene and hydrogen under the catalytic action of the propane dehydrogenation catalyst. The hydrogen permeates the W-MFI molecular sieve membrane into the inner cavity of the hollow fiber and is removed by the purge gas.

9. The propane dehydrogenation method according to claim 8, characterized in that The operating temperature of the membrane reactor is 500-600 °C, the pressure is atmospheric pressure, WHSV=1.8-4.8 h⁻¹, and C3H8:N2=1:1-1:

4.

10. The propane dehydrogenation method according to claim 8, characterized in that The flow rate of the purge gas is 10-40 mL / min.

Images