Zirconium pyrovanadate nickel-based composite catalyst for acetic acid autothermal reforming hydrogen production

Abstract

The invention relates to a zirconium pyrovanadate nickel-based composite catalyst for preparing hydrogen by autothermal reforming of acetic acid. Aiming at the problem that the existing catalyst is inactivated in the autothermal reforming reaction of acetic acid, the invention provides a novel catalyst with stable activity, carbon deposition resistance and sintering resistance. The chemical composition of the catalyst is (NiO) a (ZrO2) b (VO2.5) c, a ranges from 0.76 to 0.86, b ranges from 1.01 to 1.19, and c ranges from 2.09 to 2.36. According to the invention, nickel, zirconium and vanadium species are introduced through a sol-gel method, and the composite catalyst of ZrV2O7 zirconium pyrovanadate supported Ni2V2O7 nickel vanadate is prepared. The catalyst provided by the invention improves the yield and stability of hydrogen, and effectively inhibits the generation of by-products such as methane, acetone and the like.

Description

Technical Field

[0001] The invention relates to a zirconium pyrovanadate nickel-based composite catalyst for producing hydrogen through autothermal reforming of acetic acid, belonging to the field of producing hydrogen through autothermal reforming of acetic acid. Background Art

[0002] As a clean and efficient energy carrier, hydrogen has important application value in the fields of fuel cells and chemical synthesis. Among the traditional hydrogen production methods, the cost of water electrolysis is relatively high; petroleum cracking and hydrogen production, and natural gas catalytic reforming will produce carbon dioxide emissions. In order to overcome the defects of traditional hydrogen production technology, biomass hydrogen production technology came into being. Biomass is a renewable resource with a wide range of sources; biomass can generate biomass oil containing hydrocarbons through pyrolysis, gasification and other processes; among them, acetic acid is one of the main components of biomass oil, becoming a highly potential raw material in the process of biomass hydrogen production.

[0003] The autothermal reforming of acetic acid to produce hydrogen is to add a small amount of oxygen or air to the steam reforming feedstock, combining the advantages of steam reforming and partial oxidation reforming. By changing the oxygen-carbon ratio to adjust the degree of reaction of partial oxidation reforming, the thermal balance of the overall reaction system can be achieved, that is, CH3COOH+xO2+yH2O→aCO+bCO2+cH2(ΔH=0kJ / mol). While maintaining a high hydrogen yield, it significantly reduces dependence on external heat sources and has significant advantages in hydrogen production technology.

[0004] In the process of hydrogen production by autothermal reforming of acetic acid, the catalysts used are mainly noble metals such as Pt, Ru and Rh in Group VIII and non-noble metals such as Fe, Co and Ni. Transition metal Ni-based catalysts can promote the activation of CC bonds and CH bonds in acetic acid molecules, which is beneficial to improve hydrogen yield and selectivity.

[0005] In the process of acetic acid autothermal reforming, due to the poor thermal stability of the catalyst itself or the weak interaction between the active components and the carrier, and under high-temperature reforming reaction conditions, the active Ni species tend to migrate and aggregate, eventually leading to the gradual deactivation of the catalyst due to sintering and agglomeration. In addition, the presence of O2 in the raw materials of the autothermal reforming process will lead to the oxidation of the active metal Ni, reducing the active sites on the catalyst surface. As for the reactants, the acetic acid molecules are adsorbed on the surface of the Ni-based catalyst. After activation on the catalyst surface, they undergo dehydrogenation, deoxygenation, demethylation and other reaction steps to produce intermediate products such as CH3COO*, CH3CH2OH*, CH3CO*, and CH3*. CH3CO* will undergo a condensation reaction to form by-products such as acetone, and produce intermediate species such as C*, forming carbon deposits that are deposited on the catalyst surface, isolating the catalyst and the reactants, resulting in a decrease in catalyst activity.

[0006] In order to solve the above problems of easy sintering, carbon deposition, oxidation, etc. in the autothermal reforming process, the present invention introduces nickel, zirconium and vanadium species through the sol-gel method, and creatively prepares a composite catalyst with ZrV2O7 zirconium pyrovanadate supported Ni2V2O7 nickel vanadate. The structure of ZrV2O7 zirconium pyrovanadate belongs to the cubic Pa3 space group, in which Zr 4 ⁺Formed a [ZrO6] octahedron that shares corners with 6 equal-valued [VO4] tetrahedra; V 5+ A [VO4] tetrahedron is formed that shares a corner with three equivalent [ZrO6] octahedra, of which two are not equivalent to O 2- The linear geometry and two equivalent V 5 ⁺Atomically bonded, bonded to a Zr in linear geometry 4+ and a V 5 ⁺ Atomically, vacancy defects and interstitial atomic defects of Zr and V atoms are formed during the high-temperature preparation process; the Ni2V2O7 nickel vanadate structure (P21 / c space group) belongs to a p-type semiconductor oxide, and the composite structure constructed with zirconium pyrovanadate (ZrV2O7) is beneficial to increase the transferable oxygen species O in the carrier during the conversion of acetic acid molecules. - 2. O2 2- and O - The amount of vanadium (V 3+ 、V 4+ ) and zirconium (Zr 4+ 、Zr 5+ ) species and induce redox reactions, thus having higher catalytic performance; at the same time, the defects and reaction interfaces of the constructed ZrV2O7-Ni2V2O7 composite catalyst promote the adsorption and activation of reactant molecules CH3COOH, O2 and H2O, thereby promoting the dissociation of CH3COOH* at the Ni-Zr-VO active center, as well as the further dissociation of intermediate products such as CH3COO*, CH3CO*, and CH3*.

[0007] The ZrV2O7-Ni2V2O7 composite catalyst has a mesoporous structure, and its spatial confinement effect anchors the nickel active component in a specific area, reducing its migration and aggregation during the reaction, while being beneficial to the diffusion and transfer of reactant molecules and product molecules; at the same time, it helps to maintain the dispersion of active metals and prevent the reduction of active sites due to the aggregation of metal particles; since carbon deposits are easily formed around large-particle Ni clusters, the high dispersion of Ni species further inhibits the formation of carbon deposits.

[0008] Therefore, the innovation of the composite catalyst constructed by the present invention in the catalytic reaction pathway and structure improves the thermal stability, anti-coking and anti-sintering capabilities of the catalyst in the acetic acid autothermal reforming reaction. The activity test results of the catalyst applied to the acetic acid autothermal reforming reaction also show that the catalyst of the invention has excellent activity, selectivity and stability. Summary of the invention

[0009] The technical problem to be solved by the present invention is to provide a novel catalyst with stable activity, resistance to carbon deposition and resistance to sintering, in view of the problem that the existing catalyst has low activity, poor stability, easy carbon deposition and easy sintering in the autothermal reforming reaction of acetic acid, thereby causing the deactivation of the catalyst. The present invention uses nickel as an active component, introduces zirconium and vanadium components, and adopts a sol-gel method to create a Ni2V2O7-ZrV2O7 composite catalyst; the catalyst of the present invention is used in the autothermal reforming reaction of acetic acid to produce hydrogen. When the reaction temperature is 700°C, the acetic acid conversion rate is close to 100%, and the hydrogen yield is stable at about 2.68 mol-H2 / mol-HAc.

[0010] Aiming at the characteristics of acetic acid autothermal reforming, the present invention prepares Ni2V2O7-ZrV2O7 composite catalyst by sol-gel method. The chemical composition of the catalyst of the present invention is (NiO) a (ZrO2) b (VO 2.5 ) c , wherein a is 0.76-0.86, b is 1.01-1.19, and c is 2.09-2.36; the weight percentage composition is: nickel oxide is 14.1%-16.0%, zirconium oxide is 31.2%-36.7%, and vanadium pentoxide is 47.6%-53.7%; the weight percentage composition of the preferred catalyst is: nickel oxide is 15.0%, zirconium oxide is 34.3%, and vanadium pentoxide is 50.7%.

[0011] The specific preparation method steps are as follows: (1) According to the proportion of each component in the catalyst (NiO) a (ZrO2) b (VO 2.5 ) c , wherein a is 0.76-0.86, b is 1.01-1.19, and c is 2.09-2.36, nickel nitrate, zirconium nitrate and ammonium metavanadate are dissolved in deionized water to prepare a mixed solution #1; (2) Prepare a mixed solution of citric acid and ethylene glycol #2 according to the total molar ratio of citric acid:ethylene glycol:metal ion of 1:1:1; mix solution #1 and solution #2 and stir, keep heating in a water bath at 65°C, continue stirring until gel appears, move the gel into an oven at 105°C for 12 hours, and allow the sample to foam and expand; (3) The sample obtained in step (2) was crushed and placed in a tube furnace for calcination. The temperature was raised to 600-800°C at a rate of 10°C / min and the calcination was maintained for 4 hours to obtain the Ni2V2O7-ZrV2O7 composite catalyst of the present invention. The typical crystal structure is shown in the attached figure. Figure 1 As shown in the XRD pattern; (4) Before use, the catalyst is reduced in a H2 flow at a flow rate of 30 mL / min at a temperature of 600-800°C for 1 h, then purged with nitrogen, and a mixed gas with a molar ratio of acetic acid / water / oxygen = 1 / (1.3-5.0) / (0.21-0.35) is introduced through the catalyst bed to carry out autothermal reforming reaction. The reaction temperature is 500-800°C, atmospheric pressure, and the space velocity is 20000-60000 mL / (g-catalyst.h).

[0012] Beneficial effects of the present invention: (1) The present invention constructs a composite catalyst of ZrV2O7 zirconium pyrovanadate supported by Ni2V2O7 nickel vanadate in a cubic Pa3 space group unit cell. The high temperature structure of ZrV2O7 (>102°C) is a three-dimensional framework composed of corner-sharing [ZrO6] octahedra and [VO4] tetrahedra; Zr is located at the 4b Wyckoff site, V is located at the 8c site, and there are two different O 2- sites, i.e. one site connects two [VO4] tetrahedra (forming VOV), and the other site connects one [VO4] tetrahedron and one [ZrO6] octahedron (forming VO-Zr); due to the bending of the VOV bond, a 3×3×3 superstructure is formed below 77°C, and at high temperatures (above 102°C) it shows isotropic negative thermal expansion properties (negative thermal expansion coefficient -191×10 -6 K -1 ), and there is a displacement phase transition from a 3×3×3 superstructure to a 1×1×1 structure; Ni2V2O7 belongs to the monoclinic system of the P21 / c space group, the Ni atom is surrounded by six O atoms, these O atoms come from different [VO4] tetrahedrons and [V2O7] groups, some V atoms coordinate with four O atoms to form [VO4] tetrahedrons, and some V atoms coordinate with six O atoms to form distorted octahedral structures; therefore, the catalyst obtained by the present invention is a composite structure composed of ZrV2O7 supporting Ni2V2O7 as a whole; the composite structure has excellent thermal stability, provides a stable interface for the acetic acid autothermal reforming reaction, is conducive to the formation of Ni-Zr-VO active centers, and is conducive to the adsorption and dissociation of reactant molecules and the formation of intermediate species (CH3COOH→CH3COOH*→CH3COO*+H*, H2O→OH*+O*, O2→2O*).

[0013] (2) The multivalent transition metals in the ZrV2O7-Ni2V2O7 composite structure of the present invention are V 4+ +Zr 4+ ↔V 3+ +Zr 5+ Charge circulation, ① Due to the different electronegativity differences between Zr, V and O, electrons shift from Zr or V atoms with relatively weak electronegativity to O atoms with stronger electronegativity. During the charge circulation process, the conduction of electrons generates a local electric field. Under the action of the electric field, oxygen atoms migrate from their original lattice positions to the catalyst surface to form oxygen vacancies, which is conducive to the activation and dissociation of oxygen-containing molecules such as H2O, CO2, and O2 into active oxygen species (O*) at oxygen vacancies, and promotes the migration of oxygen in the carrier phase to the surface oxygen vacancies, thereby increasing the oxygen mobility, which is conducive to the oxidation and gasification of C* formed by further dehydrogenation of carbon-containing intermediates (acetone, ethylene ketone, CHx*, etc.) in the acetic acid autothermal reaction to CO2 / CO (C*+O*→CO, CO*+O*→CO2), and promotes the water gas shift reaction to proceed in the forward direction (CO+H2O→CO2+H2), increasing the transferable oxygen species in the carrier, namely superoxide O2 - 、Peroxide O2 2- and oxygen ions O 2- , effectively improving the selectivity of CO2 and the hydrogen yield, while improving the deactivation problem of the catalyst caused by carbon deposition; ② During the charge transfer process, due to the transfer and circulation of electrons in the Zr-OV electron bridge, the electron distribution on the catalyst surface is changed, causing the electrons to be redistributed on the catalyst surface, and more electrons to gather around Ni atoms. The electrons are transferred and rearranged between V, Zr and Ni, forming a synergistic effect, which promotes the aggregation of electrons around Ni atoms and increases the electron cloud density around Ni, which is beneficial to the active component Ni. 0 Therefore, the V and Zr modification reduces the reduction temperature of the catalyst, improves the reducibility of the catalyst, and makes the surface have more active sites, which is conducive to the breaking of CC, CH, CO, HO and other bonds of CO2*, CH3COOH*, H2O* and other molecules on the Ni-Zr-VO active sites, which is conducive to the occurrence of further dehydrogenation reactions; ③ In the composite structure of the catalyst of the present invention, the electron transfer between V and Ni and Zr makes Ni 0 A metal-support interaction is formed between the catalyst and the support, which promotes the anchoring of Ni on the support and inhibits the agglomeration of Ni to form larger Ni clusters, showing a higher dispersion. More active sites promote the further dissociation of CH3COOH* (CH3COOH*→CH3COO*+H*), thereby improving the acetic acid conversion rate and hydrogen yield.

[0014] (3) In the Ni-Zr-VO active center, the outer electron structure of the Zr atom gives it a certain electron donating ability, which can interact with the empty V[Ar]3d orbital 3 4s 2 The combination of Zr and V produces an inductive effect, which causes the electron cloud of the surrounding atoms or groups to shift, and the electrons are partially transferred from the Zr atom to the V atom, resulting in the weakening of the Lewis acidity of the V atom. At the same time, a certain electron-rich region is formed around it; the formation of the electron-rich region increases the region's ability to attract electron pairs, thereby enhancing the basicity of some originally weak or medium Lewis basic sites, thereby increasing the adsorption of acidic gases CO2 and CH3COOH, which is beneficial to the formation of bidentate carbonates of CO2 at the surface basic sites, inhibiting the ketonization (2CH3COOH→(CH3)2CO+H2O+CO2) caused by the formation of excessive Lewis acid by V species to generate acetone as a carbon deposit precursor, and is beneficial to the further dissociation of CH3COOH*.

[0015] (4) The formation of Ni-Zr-VO active sites provides a stable reaction interface for the autothermal reforming process of acetic acid, effectively solving the deactivation problem caused by sintering of active components and carbon deposition, ultimately resulting in high acetic acid conversion rate and high hydrogen yield. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 : X-ray diffraction spectrum of the catalyst of the present invention after calcination.

[0017] Figure 2 : BJH pore size distribution diagram of the catalyst of the present invention. DETAILED DESCRIPTION

[0018] Weigh 1.175g of Ni(NO3)26H2O and 12.497g of Al(NO3)39H2O, pour into a beaker, add 12mL of deionized water, stir thoroughly until dissolved, and obtain solution 1#; then weigh 7.849g of citric acid, dissolve in 12mL of deionized water, stir evenly to obtain solution 2#; weigh 2.318g of ethylene glycol to obtain solution 3#; slowly drop solution 2# into solution 3#, and then heat the mixed solution in a 70°C water bath. Slowly add it dropwise to the 1# solution, stir until it becomes a gel, and then place it in an oven at 105°C and dry it for 24 hours to obtain a catalyst precursor; put the precursor sample into a tube furnace, heat it to 750°C at a heating rate of 5°C / min, and calcine it at this temperature for 4 hours to obtain a CDUT-NA catalyst, forming a Ni-based catalyst supported on Al2O3; the weight percentage of the catalyst calculated by oxide is: nickel oxide (NiO) is 15.1%, aluminum oxide (AlO 1.5 ) is 84.9%.

[0019] The activity evaluation of acetic acid autothermal reforming reaction is carried out in a continuous flow fixed bed reactor. The catalyst is ground and tableted, and then sieved into 20-40 mesh particles, and 0.1-0.2g of the tableted catalyst is weighed and mixed with quartz sand and loaded into the reactor, and reduced in H2 at a temperature of 600℃-800℃ for 1h; then the acetic acid-water mixed solution is injected into the vaporizer by a constant flow pump for vaporization, and then mixed with nitrogen, and nitrogen is used as the internal standard gas to form a reaction raw gas with a molar ratio of CH3COOH / H2O / O2=1 / (3.0-5.0) / (0.2-0.5), and this raw gas is passed into the reaction bed, and the reaction conditions are 600-800°C, normal pressure, and a space velocity of 20000-60000mL / (g-catalyst.h), and the reaction tail gas is analyzed online by a gas chromatograph.

[0020] The activity of the CDUT-NA catalyst was investigated by the autothermal reforming reaction of acetic acid. Under the reaction conditions of space velocity of 25000mL / (g-catalyst.h), reaction temperature of 650°C, and feed molar ratio of CH3COOH / H2O / O2=1 / 4.0 / 0.28, the initial conversion rate of the catalyst to acetic acid was 99.6%. After 10 hours of reforming reaction, the acetic acid conversion rate dropped to 64.6%, the H2 yield gradually decreased to 0.65mol-H2 / mol-HAc, CO2 selectivity fluctuated around 49.0%, CO selectivity fluctuated around 38.5%, by-product methane selectivity fluctuated around 5.6%, and the selectivity for by-product acetone increased to around 35.7%. XRD and other characterization results show that the catalyst has low activity and more by-products in the autothermal reforming process of acetic acid, sintering and carbon deposition occurred during the reaction, and the overall stability was poor. Example

[0021] Weigh 2.334g of Ni(NO3)26H2O, 2.577g of ZrO(NO3)22H2O, and 2.608g of NH4VO3, add 10mL of deionized water, and stir to obtain solution 1#; then weigh 8.713g of citric acid, dissolve it in 10mL of deionized water, and stir to obtain solution 2#; weigh 2.574g of ethylene glycol to obtain solution 3#; slowly drop solution 2# into solution 3#, and slowly drop the mixed solution into solution 1# under a 65°C water bath, stir until it becomes a gel, and then place it in an oven at 105°C and dry it for 12h to obtain a catalyst precursor; put the sample into a tubular furnace, heat it to 800°C at a heating rate of 10°C / min, and calcine it at this temperature for 4 hours to obtain the catalyst CDUT-NZV of the present invention, whose typical crystal structure is shown in the attached figure. Figure 1As shown in the figure, significant diffraction peaks of ZrV2O7 are formed at 20.3°, 22.7° and 24.9°, while diffraction peaks of Ni2V2O7 appear at 28.82°, 31.04° and 35.36°, and a composite catalyst of ZrV2O7 zirconium pyrovanadate supported Ni2V2O7 nickel vanadate structure is obtained, forming a Ni2V2O7-ZrV2O7 composite catalyst structure with Ni-Zr-VO active center; the results of nitrogen low-temperature physical adsorption test show that the pore size of the catalyst is concentrated at 3.1nm, and the typical mesoporous structure is as shown in the attached Figure 2 As shown; the molar composition of the catalyst is (NiO) 0.81 (ZrO2) 1.02 (VO 2.5 ) 2.24 The weight percentage composition of the oxides is: 15.0% nickel oxide, 34.3% zirconium oxide, and 50.7% vanadium pentoxide.

[0022] The activity of the CDUT-NZV catalyst was investigated by the autothermal reforming reaction of acetic acid. Under the reaction conditions of normal pressure, space velocity of 50000 mL / (g-catalyst.h), reaction temperature of 700°C, feed ratio of CH3COOH / H2O / O2=1 / 4.0 / 0.28, and reaction time of 10 h, the acetic acid conversion rate of the catalyst was stable at about 99.3%, the hydrogen yield was 2.68 mol-H2 / mol-HAc, the CO2 selectivity was about 65.1%, the CO selectivity was about 34.9%, the CH4 selectivity was about 0.1%, and the selectivity of the byproduct acetone tended to 0. As the reaction proceeded, the catalyst activity remained stable. The NZV catalyst was characterized by low-temperature nitrogen adsorption as shown in the attached figure. Figure 2 The result is: the specific surface area is 6.05m 2 / g, pore volume is 0.03cm 3 / g, and the average pore diameter is 8.4nm.

[0023] According to the activity test results, the catalyst of the present invention exhibits good catalytic performance in the autothermal reforming reaction of acetic acid. Due to the strong synergistic effect between Ni, Zr and V in the catalyst, the catalyst's ability to resist carbon deposition, sintering and thermal stability is improved, which ultimately manifests as a high acetic acid conversion rate and a high hydrogen yield.

Claims

1. Application of zirconium pyrovanadate nickel-based composite catalyst in the process of hydrogen production by autothermal reforming of acetic acid, characterized in that: Before use, the catalyst is reduced in H2 at a flow rate of 30 mL / min at a temperature of 600-800° C. for 1 h, then purged with nitrogen, and a mixed gas with a molar ratio of acetic acid / water / oxygen=1 / (1.3-5.0) / (0.21-0.35) is introduced, and an autothermal reforming reaction is carried out through the catalyst bed, and the reaction temperature is 500-800° C., normal pressure, and the space velocity is 20000-60000 mL / (g-catalyst.h); the catalyst is prepared by the following method: preparing a mixed solution #1 of nickel nitrate, zirconium nitrate and ammonium metavanadate; according to citric acid: The total molar number of ethylene glycol: metal ions is 1:1:1, and a mixed solution of citric acid and ethylene glycol #2 is prepared; solution #1 and solution #2 are mixed and stirred, and the water bath is kept heated at 65°C, and stirring is continued until gel appears, and the gel is moved to an oven, and the temperature is 105°C, and the time is 12 hours, and the sample foams and expands; the obtained sample is crushed and placed in a tubular furnace for roasting, and the temperature is raised from room temperature to 600-800°C at a heating rate of 10°C / min, and the roasting is maintained for 4 hours to form a composite catalyst of ZrV2O7 zirconium pyrovanadate supported Ni2V2O7 nickel vanadate; its chemical composition is (NiO) a (ZrO2) b (VO 2.5 ) c , wherein a is 0.76-0.86, b is 1.01-1.19, and c is 2.09-2.36; the composition by weight percentage is: nickel oxide is 14.1%-16.0%, zirconium oxide is 31.2%-36.7%, vanadium pentoxide is 47.6%-53.7%, and the sum of the weight percentages of each component is 100%.

2. The use of the zirconium pyrovanadate nickel-based composite catalyst according to claim 1 in the process of hydrogen production by autothermal reforming of acetic acid, characterized in that: The catalyst is composed of 15.0% nickel oxide, 34.3% zirconium oxide and 50.7% vanadium pentoxide in terms of weight percentage of oxides.

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