Application of self-thermal reforming of acetic acid to hydrogen production by scheelite-distorted gadolinium molybdate supported nickel catalyst

Abstract

This invention provides a nickel-based catalyst for the autothermal reforming of acetic acid to produce hydrogen. Addressing the technical problem of deactivation of existing catalysts during the autothermal reforming of acetic acid due to coking and sintering of active components, this invention introduces Gd and Mo elements via a hydrothermal method to construct a nickel-based catalyst with a Gd₂MoO₆ monoclinic structure derived from scheelite structural distortion as the support, its molar composition being (NiO). a (GdO 1.5 ) b (MoO3) c Where a ranges from 0.75 to 0.85, b from 1.18 to 1.28, and c from 0.56 to 0.66. This invention significantly improves the catalyst's resistance to coking and structural stability, and can effectively suppress the formation of byproducts such as methane and acetone.

Description

Technical Field

[0001] This invention relates to a nickel-supported scheelite-distorted gadolinium molybdate catalyst for hydrogen production from acetic acid via autothermal reforming, belonging to the field of hydrogen production from acetic acid via autothermal reforming. Background Technology

[0002] Hydrogen is a clean and efficient energy carrier, boasting advantages such as high energy density and pollution-free combustion products. Biomass is a sustainable resource that can be rapidly converted into biomass oil through pyrolysis. The aqueous phase of biomass oil contains up to 33 wt.% acetic acid; converting acetic acid into hydrogen via reforming is an important pathway for realizing the high-value utilization of biomass resources and has broad prospects for industrial applications.

[0003] The acetic acid autothermal reforming (ATR) hydrogen production technology integrates the core advantages of steam reforming (SR) and partial oxidation (POX) reactions. By precisely controlling the feed ratio of acetic acid, water and oxygen, it can achieve thermal neutrality of the reaction system and has the technical characteristics of rapid start-up and continuous stable operation.

[0004] In the autothermal reforming of acetic acid for hydrogen production, nickel-based catalysts are widely used due to their highly efficient activation of CH and C-C bonds. The transformation of acetic acid molecules (CH3COOH) on the surface of nickel-based catalysts follows an "adsorption-activation-stepwise cleavage" pathway: CH3COOH*→CH3COO*→CH3CO*→CH2CO*→CH2*+CO*→CH*, ultimately forming C* species through deep dehydrogenation of intermediates such as CH2* and CH*. However, nickel-based catalysts face two major challenges in practical applications: First, a large number of C* species easily aggregate and deposit to form coke, covering the active sites of the catalyst, leading to rapid catalytic activity decay; second, under high-temperature reaction conditions, nickel nanoparticles are prone to migration and aggregation (sintering), destroying the catalyst's microstructure and causing irreversible deactivation.

[0005] To address the challenges of coking and sintering faced by Ni-based catalysts in the autothermal reforming of acetic acid for hydrogen production, selecting a suitable support and constructing key active structures are crucial. This invention utilizes a hydrothermal method to prepare a gadolinium molybdate configuration derived from scheelite structural distortion as a support, loading nickel active components to form a multifunctional Ni-Gd-Mo-O catalytic system for the efficient catalytic reaction of acetic acid autothermal reforming for hydrogen production.

[0006] This invention is based on the ABO4 structure of scheelite. Gd is introduced at the A-site and Mo at the B-site, forming the GdMoO4 scheelite structure. Further introduction of Gd at the A-site results in the formation of a [GdO8] polyhedron, constructing a Gd2MoO6 gadolinium molybdate configuration derived from the scheelite structure distortion. This Gd2MoO6 configuration belongs to the monoclinic crystal system with space group C2 / c. Its structure contains three different coordinated Gd³⁺ sites, exhibiting either eight-coordinate or distorted body-centered cubic coordination geometry, while the Gd-O bond lengths are distributed over a wide range from 2.21 Å to 2.78 Å. This structure possesses the ability to flexibly store and release lattice oxygen, which is beneficial for providing active oxygen species in reactions, promoting the oxidation of intermediate products and eliminating coke deposits. Simultaneously, Mo in the structure... 6 The ⁺ structure, with its twisted triangular bipyramidal geometry and five O²⁻ coordinates, with Mo-O bond lengths ranging from 1.78 Å to 2.22 Å, further enhances the activity and mobility of lattice oxygen.

[0007] On the other hand, the structure contains six non-equivalent O²⁻ sites, many of which exhibit tri- or tetra-coordinate environments (such as [OGd3Mo] tetrahedra and [OGd4] tetrahedra). These unsaturated oxygen sites provide abundant interfacial active sites for anchoring nickel species, which is beneficial for the high dispersion and strong stability of nickel nanoparticles and effectively suppresses sintering at high temperatures. In addition, Gd2MoO6 forms an extended three-dimensional network structure through the shared corners and edges of [OGd3Mo] and [OGd4] tetrahedra, endowing the material with excellent thermal stability and structural integrity, and providing a durable and stable active interface for the autothermal reforming reaction.

[0008] The innovations in the carrier composition and structure of this invention enable the catalyst to exhibit high stability and excellent anti-sintering properties in the autothermal reforming reaction of acetic acid, significantly improving the conversion rate of acetic acid and the selectivity of hydrogen. Summary of the Invention

[0009] This invention utilizes a hydrothermal superimposed impregnation method to prepare a nickel-based catalyst supported on a Gd₂MoO₆ monoclinic structure derived from scheelite structural distortion. This solves the problems of poor thermal stability of existing catalysts and catalyst deactivation due to coking during the autothermal reforming of acetic acid to produce hydrogen. When the preferred catalyst of this invention is applied to the autothermal reforming of acetic acid to produce hydrogen, at a reaction temperature of 750°C, the conversion rate of acetic acid is close to 100%, and the hydrogen yield is consistently around 2.64 mol⁻¹H₂ / mol⁻¹HAc.

[0010] Technical solution of the present invention:

[0011] This invention addresses the characteristics of acetic acid autothermal reforming by employing a hydrothermal superimposition method to prepare a Gd₂MoO₆ supported nickel-based catalyst. The molar composition of this catalyst, based on oxides, is (NiO).a (GdO 1.5 ) b (MoO3) c The composition of nickel oxide by weight percentage is as follows: nickel oxide content is 14.6%-17.7%, gadolinium oxide content is 57.4%-62.9%, and molybdenum trioxide content is 21.4%-26.0%, and the sum of the weight percentages of each component is 100%.

[0012] The specific preparation and reaction steps are as follows:

[0013] 1) Weigh out appropriate amounts of gadolinium nitrate and ammonium molybdate tetrahydrate, add 50 mL of deionized water and 20 mL of ethylene glycol, and stir at 60°C for 30 minutes to obtain a suspension;

[0014] 2) The suspension was transferred to a high-pressure reactor lined with polytetrafluoroethylene, sealed, and placed in an oven for hydrothermal reaction at 180°C for 12 hours to obtain a precipitate mixture;

[0015] 3) The precipitate mixture was washed with deionized water and anhydrous ethanol three times and filtered three times. The washed precipitate was then dried in an oven at 105°C for 12 hours to obtain the carrier precursor.

[0016] 4) The precursor of the support was placed in a tube furnace and heated to 900°C at a heating rate of 10°C / min, and calcined at this temperature for 2 hours to obtain the Gd2MoO6 support.

[0017] 5) Weigh a certain amount of nickel nitrate and dissolve it in deionized water. After it is completely dissolved, add it dropwise to the above carrier and stir to mix it thoroughly. Let it stand at room temperature for 30 minutes. Then, evaporate it to dryness at 60°C and dry it in an oven at 60°C for 4 hours. Then grind the obtained material into a loose powder and dry it in an oven at 105°C overnight to obtain the catalyst precursor.

[0018] 6) The catalyst precursor was placed in a tube furnace and heated to 250°C at a rate of 3°C / min, then to 500°C at a rate of 5°C / min, and calcined at 500°C for 2 hours to finally obtain the nickel-based catalyst supported on Gd2MoO6. Its typical structure is shown in the X-ray diffraction pattern (attached). Figure 1 As shown in the attached diagram, a Gd₂MoO₆ monoclinic structure derived from scheelite structural distortion and a NiO phase were formed, resulting in Ni-Gd-Mo-O active centers and a mesoporous structure. Figure 2 As shown;

[0019] 7) The catalyst was loaded into the reactor and reduced in H2 at 700°C for 1 hour to carry out the autothermal reforming reaction of acetic acid. Nitrogen was used as the internal standard gas, and a mixed gas with a molar ratio of CH3COOH / H2O / O2 / N2=1.0 / (1.3-5.0) / (0.21-0.35) / (2.5-4.5) was introduced through the catalyst bed to carry out the reaction at a temperature of 600°C-800°C.

[0020] The beneficial effects of this invention are:

[0021] 1) This invention uses a hydrothermal method to construct a Gd2MoO6 support with a monoclinic C2 / c space group derived from the structural distortion of scheelite, and loads nickel-based active components to form a nickel-based catalyst supported by Gd2MoO6 with a unique coordination environment and interface structure. This structure is conducive to promoting the adsorption and activation of acetic acid, water and oxygen, and efficiently converting them into H2 and CO2.

[0022] 2) To address the issues of oxygen / water activation and acetic acid derivative conversion during the autothermal reforming of acetic acid, the Gd2MoO6 support prepared in this invention... It has three unequal Gd³⁺ sites. The first type of Gd³⁺ forms an octagonal coordination environment with eight O²⁻ atoms, with Gd–O bond lengths ranging from 2.21 to 2.69 Å. The second type of Gd³⁺ is in a highly distorted body-centered cubic (octagonal) configuration, with Gd–O bond lengths ranging from 2.26 to 2.78 Å. The third type of Gd³⁺ is also octagonal, with Gd–O bond lengths ranging from 2.24 to 2.52 Å. The three sites have different local charge environments, with the longer Gd-O bond reaching 2.78 Å, indicating that some bond energies are relatively weak and prone to polarization. This coordination structure is conducive to the chemisorption of oxygen (O2) molecules through termination or side connection, and the weakening and stretching of the OO bond through electron transfer, creating favorable conditions for subsequent dissociation to generate reactive oxygen species (O*). Mo in the structure 6The twisted triangular bipyramidal coordination configuration of ⁺ and its broad Mo-O bond length distribution (1.78–2.22 Å) together endow this site with a highly asymmetric and dynamically deformable coordination environment. This coordination flexibility caused by the structural characteristics of the Mo site allows the Gd₂MoO₆ lattice to undergo local, reversible structural relaxation under reaction conditions, thereby forming low-barrier lattice oxygen migration channels between the [MoO₅] polyhedra and the adjacent [GdO₈] polyhedra. In the oxygen-containing atmosphere of acetic acid autothermal reforming, active oxygen species (O) generated by surface adsorption and dissociation can be rapidly embedded into the lattice along these channels, transforming into interstitial oxygen or filling oxygen vacancies; simultaneously, once carbon species (C* or CHx*) are generated on the surface during the acetic acid autothermal reforming reaction, lattice oxygen can also rapidly migrate to the surface and directly oxidize them to CO / CO₂, thereby effectively inhibiting the formation and accumulation of carbon deposits. This mechanism enables efficient bidirectional transport between surface active oxygen and bulk lattice oxygen, providing a continuous oxygen supply and flexible redox regulation capability for the reaction.

[0023] 3) The multi-coordinated Gd³⁺ and O²⁻ on the Gd₂MoO₆ surface provide a multi-site anchoring effect for nickel species. By forming Gd-O-Ni interfacial bonds, nickel particles are confined to defects on the support surface, inhibiting their high-temperature migration and aggregation. Simultaneously, the reducible Mo in the structure... 6 ⁺ A reversible oxidation state change (Mo) occurs in the reaction atmosphere. 6 ⁺ ↔ Mo 5 ⁺ / Mo 4 (⁺) synergizes with the active center of nickel to dynamically adjust the electron density of nickel, maintaining its ability to break C-C bonds while enhancing its affinity for surface-adsorbed oxygen, thus maintaining excellent anti-sintering and anti-coking properties under high-temperature conditions.

[0024] 4) In the Gd2MoO6 support structure constructed in this invention, six non-equivalent O²⁻ sites are interconnected through four-coordinate [OGd3Mo] and four-coordinate [OGd4] polyhedra sharing vertices and edges, forming a stable three-dimensional coordination network. The four-coordinate [OGd4] units form a rigid supporting framework through strong covalent bonds, providing basic structural stability; while the four-coordinate [OGd3Mo] units, through their asymmetric coordination configuration and adjustable bond energy, form locally flexible structural domains that can buffer lattice stress caused by oxygen migration and valence state changes during the acetic acid autothermal reforming reaction. This structure not only maintains the integrity of the crystal structure at high temperatures (>600℃) but also provides a stable and tunable anchoring interface for the nickel active center, significantly improving the long-term operational stability of the catalyst in the acetic acid autothermal reforming reaction.

[0025] 5) The catalyst prepared in this invention is applied to the autothermal reforming of acetic acid to produce hydrogen. The catalyst exhibits advantages such as high nickel dispersion, stable structure, tunable redox properties, and strong resistance to coking. It can effectively promote the activation and conversion of reactants and achieve efficient and stable hydrogen production. Attached Figure Description

[0026] Figure 1 X-ray diffraction pattern of the catalyst of this invention

[0027] Figure 2 BJH pore size distribution diagram of the catalyst of this invention. Detailed Implementation

[0028] Reference example one

[0029] Weigh 9.961g of Gd(NO3)3·6H2O and dissolve it in 50 mL of deionized water, stirring until completely dissolved. Under continuous stirring, ammonia was slowly added dropwise until the solution pH reached 10, forming a homogeneous suspension. The suspension was transferred to a polytetrafluoroethylene-lined high-pressure reactor, sealed, and placed in an oven for hydrothermal reaction at 180°C for 12 hours to obtain a precipitate mixture. The precipitate mixture was washed three times each with deionized water and anhydrous ethanol, and filtered. The resulting precipitate was dried in an oven at 105°C for 12 hours to obtain the carrier precursor. The carrier precursor was placed in a tube furnace and heated to 900°C at a rate of 10°C / min, and calcined at this temperature for 2 hours to obtain the Gd2O3 carrier. 1.360 g of Ni(NO3)2·6H2O was dissolved in 10 mL of deionized water and added dropwise to 1.980 g of Gd2O3 carrier powder under stirring. The mixture was stirred until homogeneous and allowed to stand at room temperature for 30 minutes. Subsequently, it was rotary evaporated to dryness at 60°C and then dried in an oven at 60°C for 4 hours. h; The obtained solid was ground loosely and transferred to an oven at 105°C to dry overnight to obtain the catalyst precursor; The catalyst precursor was placed in a tube furnace and heated to 250°C at 3°C / min, then to 500°C at 5°C / min, and calcined at 500°C for 2 h to form Gd2O3 and NiO phases, thus obtaining the CDUT-NG catalyst. The molar composition of this catalyst, based on oxides, is (NiO). 0.80 (GdO 1.5 ) 1.88 The weight percentage composition based on oxides is as follows: nickel oxide content is 15.0%, and gadolinium oxide content is 85.0%.

[0030] The activity evaluation of the acetic acid autothermal reforming reaction was carried out in a continuous flow fixed-bed reactor. The catalyst was ground and compressed into tablets, then sieved into 20-40 mesh particles. 0.2 g of the tablets were weighed and loaded into the reactor, and reduced in H2 at 700°C for 1 h. Then, a mixed solution of acetic acid and water was injected into the vaporizer by a constant flow pump. After vaporization, oxygen was mixed in, and nitrogen was used as an internal standard gas to form a reaction feed gas with a molar composition of CH3COOH / H2O / O2 / N2 = 1.0 / (1.3-5.0) / (0.21-0.35) / (2.5-4.5). This feed gas was introduced into the reaction bed. The reaction conditions were 600-800°C, atmospheric pressure, and space velocity of 10000-35000 mL / (g-catalyst·h). The reaction tail gas was analyzed online by gas chromatography.

[0031] The activity of the CDUT-NG catalyst in the autothermal reforming of acetic acid was investigated. Under the reaction conditions of atmospheric pressure, space velocity of 25000 mL / (g-catalyst·h), reaction temperature of 750°C, and feed gas molar composition of acetic acid / water / oxygen / nitrogen of 1.0 / 4.0 / 0.28 / 3.9, the conversion rate of acetic acid was approximately 97.5% in the 10-h activity test, the hydrogen yield was approximately 2.38 mol-H2 / mol-HAc, the CO2 selectivity was approximately 61.3%, the CO selectivity was approximately 34.9%, and the byproduct CH4 selectivity was approximately 3.8%. These results indicate that the catalyst has low activity and produces a relatively large number of byproducts. Low-temperature nitrogen adsorption-desorption experiments were conducted on the CDUT-NG catalyst, and the characterization results showed a specific surface area of ​​3.316 m². 2 / g, pore volume is 0.028cm³ 3 / g, with an average pore size of 26.4nm.

[0032] Example 1

[0033] Weigh out 7.871g of Gd(NO3)3·6H2O and 1.539g of (NH4)6Mo7O. 24·4H2O was added to 50 mL of deionized water and 20 mL of ethylene glycol, and stirred at 60°C for 30 min to obtain a suspension. The suspension was transferred to a high-pressure reactor lined with polytetrafluoroethylene, sealed, and placed in an oven for hydrothermal reaction at 180°C for 12 hours to obtain a precipitate mixture. The precipitate mixture was washed three times with deionized water and three times with anhydrous ethanol, and filtered. The precipitate was dried in an oven at 105°C for 12 hours to obtain the support precursor. The support precursor was placed in a tube furnace and heated to 900°C at a rate of 10°C / min, and calcined at this temperature for 2 hours to obtain the Gd2MoO6 support. 1.360 g of Ni(NO3)2·6H2O was dissolved in 10 mL of deionized water and added dropwise to 1.820 g of Gd2MoO6 support powder under stirring. The mixture was stirred until homogeneous and allowed to stand at room temperature for 30 min. Subsequently, it was subjected to hydrothermal reaction at 60°C for 12 hours to obtain the precipitate. The solid was rotary evaporated to dryness at °C, and then dried in a 60°C oven for 4 h. The resulting solid was ground loosely and transferred to a 105°C oven to dry overnight to obtain the catalyst precursor. The catalyst precursor was placed in a tube furnace and heated to 250°C at 3°C / min, then to 500°C at 5°C / min, and calcined at 500°C for 2 h to obtain a catalyst with a NiO phase as the main body of the Gd2MoO6 monoclinic structure derived from the distortion of the scheelite structure, forming Ni-Gd-Mo-O active centers, thus obtaining the CDUT-NGM catalyst, the structure of which is shown in the attached figure. Figure 1 As shown. The molar composition of this catalyst is (NiO). 0.80 (GdO 1.5 ) 1.23 (MoO3) 0.61 The weight percentage composition based on oxides is as follows: nickel oxide content is 16.1%, gadolinium oxide content is 60.2%, and molybdenum trioxide content is 23.7%.

[0034] The CDUT-NGM catalyst was reduced in H2 at 700°C for 1 h, purged with nitrogen, and then a mixed gas with a molar ratio of acetic acid / water / oxygen / nitrogen of 1.0 / (1.3-5.0) / (0.21-0.35) / (2.5-4.5) was introduced. Autothermal reforming of acetic acid to hydrogen was conducted under atmospheric pressure, a space velocity of 25000 mL / (g-catalyst·h), and a temperature of 750°C. The acetic acid conversion remained at 100%, and the hydrogen yield was stable at 2.64 mol-H2 / mol-HAc. The CO2 selectivity was approximately 59.8%, the CO selectivity was approximately 40.0%, while the CH4 selectivity was only 0.20%, and no acetone byproduct was detected. During the 10-hour reaction, the catalyst activity remained stable without significant decrease. Low-temperature nitrogen adsorption-desorption characterization showed that the catalyst had a specific surface area of ​​10.8 m² / g, a pore volume of 0.067 cm³ / g, and an average pore size of 24.7 nm, exhibiting a typical mesoporous structure, as shown in the attached figure. Figure 2 As shown in the figure. The characterization results further indicate that the active structure of the catalyst did not undergo significant phase transformation or sintering during the reaction, and could effectively suppress the formation of byproducts such as methane and acetone, demonstrating high catalytic activity and stable hydrogen yield characteristics.

Claims

1. The application of a nickel-based catalyst supported on a Gd₂MoO₆ monoclinic structure derived from scheelite structural distortion in the autothermal reforming of acetic acid to produce hydrogen, characterized in that: 0.2 g of catalyst was reduced in H2 at 700°C for 1 h, purged with nitrogen, and then a mixed gas with a molar ratio of acetic acid / water / oxygen / nitrogen of 1.0 / (1.3-5.0) / (0.21-0.35) / (2.5-4.5) was introduced through the catalyst bed for reaction at 600-800°C. The catalyst was prepared by the following method: a certain amount of gadolinium nitrate and ammonium molybdate tetrahydrate were weighed, added to deionized water and ethylene glycol, and stirred at 60°C for 30 minutes to obtain a suspension; the suspension was transferred to a high-pressure reactor lined with polytetrafluoroethylene, sealed, and placed in an oven for hydrothermal reaction at 180°C for 12 hours to obtain a precipitate mixture; the precipitate mixture was washed with deionized water and anhydrous ethanol and filtered three times, then the washed precipitate was dried in an oven at 105°C for 12 hours, and then placed in a tube furnace at 10°C. The temperature was increased to 900°C at a rate of 3°C / min, and calcined at this temperature for 2 hours to obtain a Gd₂MoO₆ support. A certain amount of nickel nitrate was dissolved in deionized water, and after complete dissolution, it was added dropwise to the above support, stirred to ensure thorough mixing, and allowed to stand at room temperature for 30 minutes. Subsequently, it was rotary evaporated to dryness at 60°C, and then dried in an oven at 60°C for 4 hours. After removal, it was ground loosely and placed in an oven at 105°C overnight to obtain a catalyst precursor. The catalyst precursor was placed in a tube furnace, first heated to 250°C at a rate of 3°C / min, then heated to 500°C at a rate of 5°C / min, and calcined at 500°C for 2 hours to finally obtain a Gd₂MoO₆ monoclinic structure supported nickel-based catalyst derived from scheelite structural distortion; its molar composition is (NiO). a (GdO 1.5 ) b (MoO3) c The composition of nickel oxide by weight percentage is as follows: nickel oxide content is 14.6%-17.7%, gadolinium oxide content is 57.4%-62.9%, and molybdenum trioxide content is 21.4%-26.0%, and the sum of the weight percentages of each component is 100%.

2. The application of the nickel-based catalyst supported on a Gd₂MoO₆ monoclinic structure derived from scheelite structural distortion according to claim 1 in the autothermal reforming of acetic acid to produce hydrogen, characterized in that: The catalyst has the following composition by weight percentage based on oxides: 16.1% nickel oxide, 60.2% gadolinium oxide, and 23.7% molybdenum trioxide.

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