Mg2vo4 inverse spinel type nickel-based catalyst for hydrogen production by acetic acid autothermal reforming

Abstract

The present application relates to a kind of Mg2VO4 anti-spinel type nickel-based catalyst for hydrogen production of acetic acid autothermal reforming.The present application is directed to the problems such as catalyst easy deactivation, poor stability in the process of acetic acid autothermal reforming, provides a new type of catalyst with high activity, anti-coking, anti-sintering. The present application utilizes sol-gel method to introduce V and Mg to form a nickel-based catalyst with Mg2VO4 anti-spinel structure, with Ni-Mg-V-O as active center. The present application catalyst effectively promotes acetic acid adsorption conversion, inhibits the production of by-products such as acetone and methane, and improves the hydrogen yield and reaction stability. The chemical composition of the catalyst of the present application is (NiO) a (MgO) b (VO 2.5 ) c , wherein a is 0.75-0.86, b is 3.54-4.37, and c is 1.84-2.21.

Description

TECHNICAL FIELD

[0001] The application relates to a Mg2VO4 inverse spinel type nickel-based catalyst for hydrogen production by self-thermal reforming of acetic acid and belongs to the field of hydrogen production by self-thermal reforming of acetic acid. BACKGROUND

[0002] The global main energy supply relies on fossil fuels, and the consumption of fossil fuels produces carbon dioxide emissions, causing a series of environmental problems. Hydrogen energy, regarded as a substitute for fossil fuels, has a wide application prospect.

[0003] In the hydrogen production method, the conversion of fossil raw materials such as natural gas and coal to produce hydrogen will still bring carbon emissions, and the cost of water electrolysis to produce hydrogen is high; therefore, biomass, which has the advantages of abundant reserves, wide sources and low price, is considered as a good source for hydrogen production; acetic acid, which is the main component of the aqueous phase of biomass oil obtained by pyrolysis (about one third of the aqueous phase components), is a good hydrogen production raw material.

[0004] Acetic acid hydrogen production includes a steam reforming process, which is an endothermic process that requires continuous external energy supply, limiting its application; the partial oxidation reforming process of acetic acid and oxygen is an exothermic process, but the hydrogen production rate is low; the self-thermal reforming of acetic acid to produce hydrogen combines the exothermicity of partial oxidation reforming and the high H2 production rate of steam reforming, and the heat balance of the overall reaction system is achieved by controlling the amount of oxygen introduced, that is, CH3COOH+xO2+yH2O→aCO+bCO2+cH2(ΔH=0kJ / mol), which significantly reduces the dependence on external heat sources, improves the hydrogen production rate, and has good application prospects.

[0005] In the process of self-thermal reforming of acetic acid to produce hydrogen, nickel-based catalysts exhibit high initial activity in the activation of C-C, C-H and O-H in oxygen-containing compounds; however, the introduction of oxygen forms a high oxidation atmosphere at the front end of the catalyst bed, which can reach a local high temperature of 1000℃, so the catalyst is easy to agglomerate and sinter, which is one of the important reasons for the deactivation of the catalyst; the oxygen atmosphere in the self-thermal reforming process also easily oxidizes the active component Ni in the nickel-based catalyst and deactivates it; at the same time, after the adsorption and activation of acetic acid on the active center, a series of processes such as dehydrogenation, dehydroxylation and decarboxylation (CH3COOH*→CH3COO*→CH3CO*→CH2CO*→CH2*→CH*) and ketone reaction (CH3CO*+CH3*→CH3COCH3) are generated in the reaction process, a large amount of intermediate species such as CH3CO*, CH x *(x=1-3) and C* are generated, which further condense to form coke and cover the active sites of the nickel-based catalyst, reducing the active sites for the conversion of reactant molecules and reducing the reaction activity, thereby deactivating the nickel-based catalyst. 0 x

[0006] To address the sintering, oxidation, and coking problems of nickel-based catalysts in the autothermal reforming of acetic acid, this invention creatively constructs a nickel-based catalyst with a stable anti-spinel phase Mg2VO4 supported by a sol-gel method, which is applied to the autothermal reforming of acetic acid to produce hydrogen, and has the following innovative features.

[0007] First, regarding the problem of controlling the conversion of acetic acid using nickel-based catalysts in the autothermal reforming process of acetic acid, based on the characteristic that the NiMgO2 solid solution formed in the Mg-Ni-O composite oxide structure is difficult to reduce, an inverse spinel Mg2VO4 structure was creatively prepared: (1) This inverse spinel structure belongs to the cubic phase. Space group, Mg 2 + occupies the octahedral position in the anti-spinel structure, V 3 Occupying octahedral and tetrahedral positions in the inverse spinel structure, the Mg2VO4 inverse spinel structure exhibits good thermal stability and high electron mobility. By rearranging the electron cloud density, the spatial density of the Ni electron cloud increases, and the modulation of the Ni electron environment allows Ni in the NiMgO2 solid solution to... 0 (1) It is easier to be reduced; (2) The V species in the nickel-based catalyst supported by the Mg2VO4 anti-spinel structure interact with the Mg species to form V-Mg-O bonds, thereby weakening the interaction between Mg and Ni and further promoting the reduction of the active component Ni; (3) The Ni-Mg-VO catalyst with a permeable nanoporous structure can inhibit the agglomeration and sintering of the active component Ni and promote the entry of reactant molecules into the porous channels.

[0008] Furthermore, the introduction of the alkaline earth metal Mg, which has good thermal stability, effectively increases the basicity of the catalyst support, especially enhancing the medium-basicity sites. This is beneficial for the adsorption of CH3COOH* at the Ni-Mg-VO active center, thereby improving the conversion of the feedstock acetic acid and the generation of the product hydrogen. Simultaneously, the increased catalyst basicity effectively promotes the adsorption of acidic CO2 gas, converting it into carbonate species, thus enhancing the forward progress of the water-gas conversion reaction (WGSR) (CO + H2O → CO2 + H2) and inhibiting the methanation reaction (CO2 + 4H2 → CH4 + 2H2O or CO + 3H2 → CH4 + H2O). The enhanced interaction between the metal and the support, through the formation of the NiMgO2 solid solution, reduces the migration rate of Ni, inhibiting the sintering of the active component nickel. However, under a reducing atmosphere, Ni in the NiMgO2 solid solution is difficult to reduce, leading to a reduction in active sites, which is detrimental to the breaking of C-C bonds in acetic acid, thus deactivating the catalyst.

[0009] Species V can be linked by redox pairs (V 5 + / V 3) transition, promotes the formation of oxygen vacancies, and improves the oxygen mobility. It shows strong adsorption capacity and oxidation capacity for carbon-containing species derived from acetic acid, such as small molecule species CH3*, CO*, CO2* and the like, thereby promoting the carbon oxidation reaction (C*+O*→CO*+O*→CO / CO2), gasifying the carbon deposit, and inhibiting the formation of carbon deposit on the catalyst surface, further improving the carbon deposit resistance of the catalyst in the process of acetic acid autothermal reforming; the redox properties of V species can promote electron transfer and increase the electron cloud density around the active Ni, so that the Ni is reduced from the NiMgO2 solid solution, providing active sites for the breaking of C-H and C-C bonds in CH3COOH, and improving the dissociation performance of acetic acid.

[0010] Therefore, the innovation of the catalyst in the catalytic reaction pathway and structure improves the thermal stability, carbon deposit resistance and sintering resistance of the catalyst in the process of acetic acid autothermal reforming. The activity test results of the catalyst applied in the process of acetic acid autothermal reforming also show that the catalyst has excellent activity, selectivity and stability. SUMMARY

[0011] The present application creatively prepares a nickel-based catalyst loaded with Mg2VO4 inverse spinel structure by a sol-gel method, and solves the problems of poor stability, easy carbon deposit, easy sintering and difficult reduction of active components of the catalyst in the process of acetic acid autothermal reforming. When the catalyst is used in the process of acetic acid autothermal reforming to produce hydrogen, the acetic acid conversion rate is close to 100% and the hydrogen production rate can be stabilized at about 2.65 mol-H2 / mol-HAc under the condition of a reaction temperature of 700℃. The activity test and phase analysis results confirm that the catalyst has the characteristics of carbon deposit resistance, sintering resistance and stable catalytic activity.

[0012] Technical scheme of the present application:

[0013] The present application prepares a Ni / Mg2VO4 catalyst by a sol-gel method according to the characteristics of acetic acid autothermal reforming. The chemical composition of the catalyst of the present application is (NiO) a (MgO) b (VO 2.5 ) c , wherein a is 0.75-0.86, b is 3.54-4.37, and c is 1.84-2.21; the weight percentage composition according to the oxide is as follows: nickel oxide is 14.0%-16.0%, magnesium oxide is 35.7%-44.1%, and vanadium pentoxide is 41.8%-48.3%, and the sum of the weight percentages of the components is 100%.

[0014] The specific preparation method comprises the following steps:

[0015] 1) According to the component ratio of the chemical composition of the catalyst (NiO)a (MgO) b (VO 2.5 ) c Wherein a is 0.75-0.86, b is 3.54-4.37, c is 1.84-2.21, a certain amount of nickel nitrate, magnesium nitrate and ammonium metavanadate are dissolved in deionized water, and continuously stirred until completely dissolved to obtain a mixed solution #1;

[0016] 2) According to the mole ratio of citric acid: ethylene glycol: metal nitrate is 1:1:1, a mixed solution #2 of citric acid and ethylene glycol is prepared; solution #1 and solution #2 are fully mixed at 65℃, continuously stirred until a gel appears, and the gel is placed in an oven at 105℃ for 12h, and the sample is foamed and expanded;

[0017] 3) The sample obtained in step 2) is crushed and placed in a tube furnace and calcined at a temperature rising rate of 10℃ / min from room temperature to 600-800℃, and reduced for 1h at 600-800℃ in a H2 flow of 30mL / min, to obtain a composite oxide with mesoporous structure of Mg2VO4 inverse spinel structure and NiMgO2 solid solution, and the typical crystal structure is shown in the XRD pattern of the attached Figure 1 , the solid solution in the structure indicates that there is a strong interaction between Ni and Mg, and the Ni grains are highly dispersed on the Mg2VO4 inverse spinel structure carrier, that is, the Ni / Mg2VO4 phase is obtained, forming a stable Ni-Mg-V-O active center, and the typical BJH pore size distribution is shown in the attached Figure 2 ;

[0018] 4) After the catalyst is reduced at 600-800℃ in a H2 atmosphere, it is purged with nitrogen atmosphere, and a mixed gas with a mole ratio of acetic acid / water / oxygen=1 / (1.3-5.0) / (0.21-0.35) is introduced, and the self-thermal reforming reaction is carried out through the catalyst bed, and the reaction temperature is 500-800℃, atmospheric pressure, and the space velocity is 20000-60000mL / (g-catalyst·h).

[0019] The beneficial effects of the present application are:

[0020] 1) For the problem of controlling the conversion of acetic acid by nickel-based catalyst in the process of acetic acid self-thermal reforming, based on the characteristics of the difficulty of reduction of NiMgO2 solid solution in Mg-Ni-O composite oxide structure, ① the present application uses sol-gel method to construct a Ni-Mg-V-O reaction interface formed by highly dispersed Ni metal in stable inverse spinel structure Mg2VO4 and NiMgO2 solid solution; Mg 2+ occupies the octahedral position in the inverse spinel structure, V 3+The occupation in the octahedral and tetrahedral positions of the inverse spinel structure makes the structure have good thermal stability and high electron mobility, rearranges the electron cloud density, adjusts the chemical environment of the active metal Ni, and thus promotes the reduction of Ni from the NiMgO2 solid solution; ② The V species in the structure can interact with the Mg species to form a vanadium-metal-oxygen (V-M-O) structure, thereby weakening the interaction between Mg and Ni, and further reducing part of the Ni in the NiMgO2 solid solution, increasing the content of the active component Ni 0 .

[0021] 2) The unique redox cycle performance (V 5+ / V 3+ ) of the variable-valence transition metal V species in the inverse spinel structure promotes electron transfer and increases the electron cloud density around the active Ni, promoting the dissociation of the reactant acetic acid molecules at the Ni-Mg-V-O active center; at the same time, the redox performance of the V species causes the catalyst to produce oxygen vacancy defects, which is beneficial to the migration of lattice oxygen in the MgO6 octahedron and the VO4 tetrahedron in the inverse spinel to the surface, improves the oxygen mobility, and also promotes the adsorption and dissociation of the reactants H2O and O2 at the oxygen vacancy defects into O* and OH* active oxygen species, thereby promoting the gasification of the carbon (C*+O*→CO*+O*→CO / CO2) and inhibiting the condensation of C*, CH x *(x=1-3) and other carbon-containing intermediates, thereby improving the carbon deposition resistance of the catalyst.

[0022] 3) On the Ni-Mg-V-O active center, the alkaline earth metal Mg species neutralizes the Lewis acid sites of the V species, increases the basicity of the catalyst, is beneficial to the adsorption and activation of the acidic reactant molecules CH3COOH at the Ni-Mg-V-O active center, and promotes the conversion and generation of the intermediate products CH3COO* and CH2CO* in the reaction process, thereby increasing the conversion rate of acetic acid; at the same time, it is beneficial to the adsorption of CO2 gas and the generation of carbonate, thereby promoting the forward progress of the water-gas shift reaction and producing more H2, while also inhibiting the generation of by-product methane. In addition, the interaction between Ni and the carrier reduces the mobility of Ni through electron transfer, inhibits the aggregation and growth of Ni crystal grains, and in a catalyst with a larger specific surface area, Ni 0 shows greater dispersion, which can more effectively inhibit the sintering of the active component Ni 0 .

[0023] 4) The catalyst of the present application has high acetic acid conversion rate and high hydrogen production rate in the acetic acid conversion process, effectively inhibits carbon deposition, sintering and oxidation of the active component Ni 0 . BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1X-ray diffraction spectrum of the catalyst of the present application

[0025] Figure 2 BJH pore size distribution graph of the catalyst of the present application DETAILED DESCRIPTION

[0026] Reference Example 1

[0027] Take 0.876g of Ni(NO3)2·6H2O and 8.110g of Mg(NO3)2·6H2O, add an appropriate amount of deionized water to prepare solution #1; take 7.279g of C6H8O7·H2O and 2.151g of (CH2OH)2, add an appropriate amount of deionized water to prepare solution #2; mix solution #1 and solution #2, continuously stir in a constant temperature water bath at about 65°C until a gel is formed; dry the gel in a 105°C vacuum drying oven for 12h; crush the dried and foamed sample, heat to 700°C at a heating rate of 10°C / min, calcine for 4h, then reduce in H2 atmosphere for 1h to obtain CDUT-NM catalyst, which has a NiMgO2 solid solution structure; the weight percentage composition of the catalyst is: the content of nickel oxide is 14.9%, and the content of magnesium oxide is 85.1%.

[0028] 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 tabletized, crushed, and sieved to obtain particles of 20-40 mesh, which were loaded into the reactor, reduced in H2 atmosphere at 700°C for 1h, then the mixed solution of acetic acid and water was injected into the vaporizer for vaporization, mixed with oxygen after vaporization, and nitrogen was used as the internal standard gas to form a reaction feed gas with a molar composition of CH3COOH / H2O / O2=1 / (1.3-5.0) / (0.21-0.35), which was then introduced into the reaction bed for acetic acid autothermal reforming reaction. The reaction conditions were 500-800°C, atmospheric pressure, space velocity 20000-60000mL / (g-catalyst·h), and the reaction was analyzed online by gas chromatography.

[0029] The CDUT-NM catalyst was subjected to activity evaluation of acetic acid autothermal reforming, and under the reaction conditions of atmospheric pressure, space velocity 30000mL / (g-catalyst·h), reaction temperature 700°C, feed gas acetic acid / water / oxygen=1 / 4.0 / 0.28, and reaction time 10h, the hydrogen yield was 1.98ml-H2 / mol-HAc. The catalyst composition was Mg-Ni-O solid solution, the reduction degree of the active component nickel was low, and partial sintering and carbon deposition occurred, resulting in low activity.

[0030] Example 1

[0031] Take 1.179 g of Ni(NO3)2·6H2O and 5.073 g of Mg(NO3)2·6H2O and 1.157 g of NH4VO3, add an appropriate amount of deionized water to prepare solution #1; take 7.088 g of C6H8O7·H2O and 2.037 g of (CH2OH)2, add an appropriate amount of deionized water to prepare solution #2; mix solution #1 with solution #2, continuously stir in a constant temperature water bath at about 65°C until a gel is formed; dry the gel in a 105°C vacuum drying oven for 12 h; crush the dried and foamed sample, heat to 700°C at a heating rate of 10°C / min, calcine for 4 h, then reduce in H2 atmosphere for 1 h to obtain a CDUT-NMV catalyst, which has a Mg2VO4 structure with a reverse spinel phase and a NiMgO2 solid solution phase, as shown in the X-ray diffraction spectrum of Figure 1 Fig. 1, a Ni / Mg2VO4 structure with Ni-Mg-V-O active center is formed; the molar composition of the catalyst is (NiO) 0.82 (MgO) 3.94 (VO 2.5 ) 1.98 The weight percentage composition of the catalyst in terms of oxides is: 15.0% nickel oxide, 39.8% magnesium oxide, and 45.2% vanadium pentoxide.

[0032] The CDUT-NMV catalyst was subjected to activity investigation in the acetic acid autothermal reforming reaction under the reaction conditions of atmospheric pressure, space velocity of 30000 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 100%, the hydrogen production rate was about 2.65 mol-H2 / mol-HAc, the CO2 selectivity was about 58.1%, the CO selectivity was about 41.9%, and there was almost no by-product such as methane and acetone. The catalyst was characterized by low-temperature nitrogen adsorption, and the specific surface area was 8.1 m 2 / g, the pore volume was 0.045 cm 3 / g, and the average pore size was 10.3 nm. The characterization results show that the synergistic effect between Ni, Mg and V in the catalyst improves the ability of the catalyst to resist sintering, resist carbon deposition and inhibit oxidation of active metals, effectively improves the conversion rate of acetic acid and the hydrogen production rate by controlling the adsorption and conversion of reactant molecules.

Claims

1. Use of Mg2VO4 inverse spinel type nickel-based catalyst in the process of hydrogen production by autothermal reforming of acetic acid, characterized in that: After the catalyst is reduced at 600-800℃ for 1h in H2 atmosphere, a mixed gas with a molar ratio of CH3COOH / H2O / O2=1 / (1.3-5.0) / (0.21-0.35) is introduced, and the self-thermal reforming reaction of acetic acid is carried out by passing through the catalyst bed, with a reaction temperature of 500-800℃; the catalyst is prepared by the following method: according to the chemical composition, a certain amount of nickel nitrate, magnesium nitrate and ammonium metavanadate are dissolved in deionized water to prepare a mixed solution #1 with different mass ratios; a mixed solution #2 of citric acid and ethylene glycol is prepared according to the total molar ratio of citric acid:ethylene glycol:metal nitrate being 1:1:1; solution #1 and solution #2 are mixed at 65℃, and stirred constantly until a gel appears; the gel is dried in an oven at 105℃ for 12h; after crushing the dried and foamed sample, it is heated to 700℃ at a heating rate of 10℃ / min, calcined for 4h, and reduced at 600-800℃ for 1h in H2 atmosphere, to form a Ni 0 The particles are dispersed in the nickel-based catalyst Ni / Mg2VO4 with Mg2VO4 inverse spinel structure, and form Ni-Mg-V-O active centers; the proportion of each component in the catalyst (NiO) a (MgO) b (VO 2.5 ) c , wherein a is 0.75-0.86, b is 3.54-4.37, and c is 1.84-2.21; the weight percentage composition according to the oxide is: nickel oxide is 14.0%-16.0%, magnesium oxide is 35.7%-44.1%, vanadium pentoxide is 41.8%-48.3%, and the sum of the weight percentages of each component is 100%.

2. Use of Mg2V04inverse spinel type nickel-based catalyst according to claim 1 in the process of hydrogen production by autothermal reforming of acetic acid, characterized in that: The catalyst has the following composition in terms of weight percentage of oxides: 15.0% of nickel oxide, 39.8% of magnesium oxide and 45.2% of vanadium pentoxide.

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