CN116060020B - Calcium-chromium-based limonite-type nickel-based catalyst for hydrogen production by autothermal reforming of acetic acid - Google Patents

Abstract

The invention relates to a calcium-chromium-based limonite-type nickel-based catalyst for autothermal reforming of acetic acid to prepare hydrogen. The invention provides a novel catalyst with high activity, carbon deposit resistance and oxidation resistance aiming at the deactivation problem of the existing catalyst in the autothermal reforming process of acetic acid. The catalyst of the invention has the chemical composition of (NiO) _a (CaO) _b (CrO _1.5 ) _c Wherein a is 0.38-0.43, b is 0.88-1.33, and c is 1.27-1.61. The invention adopts the Pechni sol-gel method to prepare the Ca with Ni species loaded on the perovskite type Ca ₂ Cr ₂ O ₅ Novel catalysts supported on a carrier and forming Ni-Ca-Cr-O active centers. The catalyst of the invention effectively improves the reaction activity of hydrogen production by autothermal reforming of acetic acid.

Description

Calcium-chromium-based limonite-type nickel-based catalyst for hydrogen production by autothermal reforming of acetic acid

Technical field

The invention relates to a preparation method of a specific calcium-chromium-based limonite type oxide Ca ₂ Cr ₂ O ₅ supported Ni-based catalyst and its application in the process of hydrogen production by autothermal reforming of acetic acid, which belongs to the autothermal reforming of acetic acid. The technical field of hydrogen production.

Background technique

The extensive use of fossil energy has brought about environmental problems, such as the greenhouse effect, acid rain, ozone layer destruction, and particulate matter pollution. Hydrogen, as a clean fuel, has an energy density of up to 122MJ/Kg and is environmentally friendly. It is regarded as one of the most potential alternative energy sources. At present, common hydrogen production methods include hydrogen production from fossil fuels, hydrogen production from industrial by-products, and water electrolysis. Hydrogen production from fossil fuels has many by-products and gas impurities, as well as adverse effects on the environment. Hydrogen production methods such as hydrogen production from industrial by-products and hydrogen production by electrolysis have relatively strict requirements on technology and equipment, and the cost is high, and the hydrogen production is limited. Hydrogen production by reforming acetic acid derived from renewable biomass oil is a promising green hydrogen production method.

When acetic acid is reformed to produce hydrogen, according to the different ratios of acetic acid, water and oxygen as raw materials participating in the reaction, it can be divided into steam reforming (SR) and partial oxidation reforming (POX). ) and auto-thermal reforming (ATR). Hydrogen production by steam reforming is an endothermic reaction and requires external heat supply, which increases the cost of hydrogen production. The oxygen introduced by partial oxidation reforming to produce hydrogen can easily cause oxidative deactivation of the catalyst, and the hydrogen yield is not high; autothermal reforming Hydrogen production, by introducing an appropriate amount of oxygen, balances the heat of reaction and reduces the cost of hydrogen production.

In the process of hydrogen production by autothermal reforming of acetic acid, the catalyst plays a very critical role in the reaction. Reforming catalysts include noble metal catalysts and transition metal catalysts. Nickel-based catalysts are often used as Research object. However, the intermediate species such as CH ₃ CO* generated by the conversion of acetic acid on the Ni ⁰ active site on the catalyst will further decompose to generate CH ₃ * species, and the accumulation of its dehydrogenation product C* will form coke and deposit on the Ni ⁰ surface, causing the active sites of the catalyst to be covered, resulting in the deactivation of the catalyst. On the other hand, since the introduced oxygen is mainly consumed at the front end of the reaction, the temperature at the front end of the reaction bed rises sharply, reaching more than 1000°C. At this temperature, the active component Ni is prone to aggregation and sintering, and at the same time, it undergoes autothermal gravity The oxidizing atmosphere during the entire process can easily lead to the oxidation of the active metal Ni ⁰ , which causes the reaction front to continuously move backward, eventually causing the entire catalytic bed to sinter, oxidize and become deactivated. Therefore, during the hydrogen production reaction process of acetic acid autothermal reforming, it is necessary to improve the coking resistance, sintering resistance, and oxidation resistance of Ni-based catalysts, reduce the selectivity to by-products such as methane, acetone, etc., and improve hydrogen production. The rate is a key factor in the hydrogen production process of acetic acid autothermal reforming.

In order to solve the above problems, the present invention creatively introduces a Ca ₂ Cr ₂ O ₅ oxide-supported Ni-based catalyst with an A ₂ B ₂ O ₅ limonite structure.

First, the limonite structure Ca ₂ Cr ₂ O ₅ has abundant oxygen by introducing an ordered oxygen vacancy array and forming an alternating layer structure of octahedral and tetrahedral coordinated transition metal cations Ca ²⁺ /Cr ³⁺ . vacancies, and has strong lattice oxygen (O ^2- ) transfer ability, and the active component Ni also plays a positive role in the release of active oxygen, thereby promoting the formation of *CH _x (x= 0-3) and other intermediate species, improve the gasification of the coke precursor *C, and obtain the CO/CO ₂ product (C*+O*→CO,CO+O*→CO ₂ ); at the same time, this structure The components promote the forward progress of the WGS reaction (CO+H ₂ O→CO ₂ +H ₂ ), promote the generation of more hydrogen, and also greatly improve the catalyst's ability to resist coking.

Secondly, limonite-type Ca ₂ Cr ₂ O ₅ has good thermal stability and can provide a stable reaction interface during the conversion of acetic acid. Among them, the Ca species, as an alkaline earth metal, improves the overall alkalinity of the catalyst, helps to adsorb and activate water molecules, and forms intermediate species such as *OH and *O. Among them, the *OH species participates in the reforming reaction, and the *O species is responsible for the carbon deposition precursor. The gasification of the reactant is beneficial; and the Cr species has thermal stability and variable valence, among which the trivalent Cr ³⁺ species can effectively improve the adsorption and activation of the reactant H ₂ O, and can also promote the acetic acid in the autothermal reforming process. The possible oxidation of by-products such as formaldehyde, acetone, toluene and ethyl acetate can improve the selectivity and yield of hydrogen; at the same time, there is a synergistic effect between the active components Ni, Ca and Cr, forming Ni-Ca- The Cr-O active center improves the adsorption and activation of reactants acetic acid, water, and oxygen, promotes the formation and transformation of acetic acid-derived intermediate species such as CH ₃ COO*, CH ₃ CO*, and obtains target products such as H ₂ , improving Improve the catalyst activity and anti-coking ability.

Finally, during the preparation process, the Pechini sol-gel method was used to construct a new catalyst with a mesoporous structure, which promotes the reaction of reactant molecules CH ₃ COOH, H ₂ O, O ₂ and product molecules H ₂ , CO, CO ₂ , etc.; moreover, through the confinement effect of the mesoporous structure, the condensation effect of intermediate species such as CH ₂ CO* and CH ₃ OCH ₃ derived from acetic acid is effectively inhibited, the formation of coke is effectively inhibited, and the response to the reaction is improved. Product _H2 and CO/ _CO2 selectivity.

Contents of the invention

The problem to be solved by the present invention is that during the autothermal reforming process of acetic acid, the Ni-based catalyst is easily occupied by coke at high temperatures, affecting the conversion of acetic acid molecules and thus the production of hydrogen. At the same time, at high temperatures, the catalyst It is easy to be oxidized and sintered, which affects the activity of the catalyst. This provides a new type of catalyst that resists coke deposition, oxidation, and sintering.

The present invention uses Ni as an active component and prepares a calcium-chromium-based limonite-type Ca ₂ Cr ₂ O ₅ carrier-loaded nickel-based catalyst with a mesoporous structure through the Pechini sol-gel method; the catalyst of the present invention is used for acetic acid synthesis. In the thermal reforming hydrogen production reaction, when the reaction temperature is 650°C, the acetic acid conversion rate of the preferred catalyst is close to 100%, and the hydrogen production rate reaches about 2.20 mol-H ₂ /mol-HAc.

Technical solution of the present invention:

In view of the characteristics of acetic acid autothermal reforming, the present invention prepares a calcium-chromium-based limonite-type Ni/Ca ₂ Cr ₂ O ₅ catalyst by the Pechini sol-gel method. The molar composition of the catalyst of the present invention calculated as oxides is (NiO) _a (CaO) _b (CrO _1.5 ) _c , where a is 0.38-0.43, b is 0.88-1.33, and c is 1.27-1.61; calculated as oxides The weight percentage composition is: nickel oxide is 14.0%-16.1%, calcium oxide is 24.6%-37.7%, chromium oxide is 48.3%-61.3%, and the sum of the weight percentages of each component is 100%.

The specific preparation steps are as follows:

(1) According to the molar composition of the catalyst (NiO) _a (CaO) _b (CrO _1.5 ) _c , where a is 0.38-0.43, b is 0.88-1.33, and c is 1.27-1.61, weigh an appropriate amount of Ni(NO ₃ ) Dissolve ₂ ·6H ₂ O, Ca(NO ₃ ) ₂ ·4H ₂ O and Cr(NO ₃ ) ₃ ·9H ₂ O in deionized water and stir until all are dissolved to obtain 1# solution;

(2) According to the total number of moles of metal cations: citric acid: ethylene glycol ratio is 1:1:1, prepare a mixed solution 2# of citric acid and ethylene glycol, mix solution 2# and 1# solution, and then maintain the water bath temperature to 70°C, and continue stirring until a sol is formed;

(3) Place the obtained sol in an oven at 105°C for 24 hours, then raise it from room temperature to 750°C in a tube furnace at a rate of 5°C/min, and keep the calcination for 4 hours to obtain the calcium-chromium-based limonite type The nickel-based catalyst Ni/Ca ₂ Cr ₂ O ₅ forms a Ni-Ca-Cr-O active center and has a crystal phase structure in which NiO is supported on a calcium-chromium-based limonite-type Ca ₂ Cr ₂ O ₅ oxide. The typical structure is shown in Figure 1. At the same time, a mesoporous structure is constructed, and its typical BJH pore size distribution is shown in Figure 2;

(4) Catalyst activity test: Before use, the catalyst of the present invention is reduced in _H2 at a temperature of 500-800°C for 1 hour, and then the mixed solution of acetic acid and water is injected into the vaporizer with a constant flow pump. After vaporization, oxygen is mixed, and nitrogen is used as the Internal standard gas is used to form a reaction raw material gas with a molar composition of CH ₃ COOH/H ₂ O/O ₂ /N ₂ =1/(1.3-5.0)/(0.21-0.35)/(2.5-4.5), and this raw material The gas is introduced into the reaction bed, and the reaction temperature is 500-800°C.

Beneficial effects of the present invention:

(1) In the calcium-chromium-based limonite-type Ni/Ca ₂ Cr ₂ O ₅ catalyst, the Ni-Ca-Cr-O active center is formed, which improves the adsorption and activation of the reactants acetic acid, water, and oxygen, and promotes The formation and transformation of intermediate species such as CH ₃ COO* and CH ₃ CO* derived from acetic acid can obtain target products such as H ₂ and improve the activity and anti-coking ability of the catalyst.

(2) In the limonite Ca ₂ Cr ₂ O ₅ structure of the catalyst, an ordered oxygen vacancy array is formed and an alternating layer structure of octahedral and tetrahedral coordinated transition metal cations Ca ²⁺ /Cr ³⁺ is formed. , forming abundant oxygen vacancies and possessing strong lattice oxygen (O ^2- ) transfer ability, promoting the conversion of intermediate species such as *CH _x (x=0-3) formed during the conversion of acetic acid, improving By gasifying the coke precursor *C, CO/CO ₂ products (C*+O*→CO, CO+O*→CO ₂ ) are obtained; at the same time, this structure also promotes the WGS reaction (CO+H ₂ O→CO ₂ +H ₂ ) in the forward direction promotes the generation of more hydrogen and improves the catalyst's ability to resist coking.

(3) The limonite-type Ca ₂ Cr ₂ O ₅ oxide of the catalyst has good thermal stability, provides a stable reaction interface during the conversion of acetic acid, inhibits the sintering of the active component Ni, and improves the performance of the catalyst. stability. In this structure, Ca species, as an alkaline earth metal, improves the overall alkalinity of the catalyst and helps to adsorb and activate water molecules to form intermediate species such as *OH and *O. The *OH species participates in the reforming reaction, while the *O species The gasification of the coke precursor is beneficial; the Cr species has thermal stability and variable valence, among which the trivalent Cr ³⁺ species can effectively improve the adsorption and activation of the reactant H ₂ O, and also promotes the autothermal regeneration of acetic acid. The oxidation of by-products such as formaldehyde, acetone, toluene and ethyl acetate that may be produced during the whole process, thereby improving the selectivity and yield of hydrogen; there is a strong interaction between the active component Ni and the Ca ₂ Cr ₂ O ₅ carrier, It can effectively adsorb and activate acetic acid, and Ni also promotes the release of active oxygen, which effectively improves the activity of the catalyst and its ability to resist coking.

(4) The present invention constructs a new catalyst with a mesoporous structure. The mesoporous structure promotes the transmission and diffusion of reactants CH ₃ COOH, H ₂ O, O ₂ and product molecules H ₂ , CO, CO _2, etc.; moreover, through The confinement effect of the mesoporous structure effectively inhibits the polycondensation effect of acetic acid-derived intermediate species such as CH ₂ CO* and CH ₃ OCH ₃ , effectively inhibits the occurrence of coke deposition, and improves the selection of reaction products H ₂ and CO/CO ₂ sex.

(5) The catalyst of the present invention is used in the hydrogen production reaction process of acetic acid autothermal reforming. The results show that the catalyst of the present invention exhibits the advantages of anti-sintering, anti-coking, anti-oxidation, and high catalytic activity.

Description of the drawings

Figure 1: X-ray diffraction spectrum of the catalyst oxide of the present invention

Figure 2: BJH pore size distribution diagram of the catalyst of the present invention

Detailed ways

Reference example one

Weigh 12.497g Al(NO ₃ ) ₃ ·9H ₂ O and 1.175g Ni(NO ₃ ) ₂ ·6H ₂ O into a beaker, add a certain amount of deionized water to dissolve, and obtain solution 1#; then weigh Dissolve 7.849g of citric acid and 2.318g of ethylene glycol and mix to obtain solution 2#; then mix solution 2# with solution 1# and stir at 70°C until gel is formed; then Put the gel into an oven at 105°C, take it out after 24 hours, and finally heat it up to 750°C at a rate of 5°C/min for roasting in a tube furnace and keep it for 4 hours to obtain the CDUT-NA catalyst, which is supported on Al ₂ O Ni-based catalyst on ₃ ; the weight percentage composition of the catalyst in terms of oxides is: nickel oxide (NiO) is 15.1%, and aluminum oxide (AlO _1.5 ) is 84.9%.

The evaluation of acetic acid autothermal reforming reaction activity was carried out in a continuous flow fixed bed reactor. Grind the catalyst and press it into tablets, then sieve it into particles of 20-40 mesh, weigh 0.2g and put it into the reactor, reduce it in _H2 at a temperature of 500-800°C for 1 hour; then add the mixed solution of acetic acid and water with After the constant flow pump is injected into the vaporizer and vaporized, oxygen is mixed, and nitrogen is used as the internal standard gas to form a molar composition of CH ₃ COOH/H ₂ O/O ₂ /N ₂ =1/(1.3-5.0)/(0.21-0.35 )/(2.5-4.5) reaction raw gas, and introduce this raw gas into the reaction bed. The reaction conditions are 500-800°C, normal pressure, and space velocity 10000-35000ml/(g-catalyst·h). The reaction tail gas is Gas chromatograph online analysis.

The activity of catalyst CDUT-NA in acetic acid autothermal reforming was investigated. The reaction conditions were normal pressure, space velocity 25000ml/(g-catalyst·h), reaction temperature 650℃, and raw material gas acetic acid/water/oxygen=1/4.0/ At 0.28, the initial conversion rate of acetic acid is 99.6%, and as the reaction proceeds to 10 hours, the conversion rate of acetic acid decreases to 64.6%, and the hydrogen production rate gradually decreases to 0.65mol- _H2 /mol-HAc, the selection of _CO2 The selectivity is around 49.0%, the CO selectivity is around 38.5%, the CH ₄ selectivity is 5.6%, and the selectivity of the by-product acetone increases to around 35.7%. Characterization results such as XRD and BET show that the catalyst has poor stability during the autothermal reforming of acetic acid and has many by-products. The acetonation reaction is not effectively inhibited, sintering, coking and partial oxidation occur, and the activity is low.

Embodiment 1

Weigh 3.043g of Ca(NO ₃ ) ₂ ·4H ₂ O, 5.156g of Cr(NO ₃ ) ₃ ·9H ₂ O and 1.162g of Ni(NO ₃ ) ₂ ·6H ₂ O, pour them into the beaker, and add a certain amount Dissolve an amount of deionized water to obtain solution 1#; then weigh 6.254g of citric acid and 1.847g of ethylene glycol, and mix after dissolving to obtain solution 2#; then mix the 2# solution with the 1# solution, Place it at 70°C and stir until the gel forms; then put the gel into a 105°C oven, take it out after 24 hours, and finally heat it up to 750°C in a tube furnace at a rate of 5°C/min for roasting. In 4h, a calcium-chromium-based limonite-type nickel-based catalyst Ni/Ca ₂ Cr ₂ O ₅ , i.e., CDUT-NCC catalyst, was obtained. Its typical crystal structure is shown in Figure 1, appearing at 24.7°, 33.7° and 49.6°. There is an obvious diffraction peak of Ca ₂ Cr ₂ O ₅ , and the diffraction peaks of NiO appear at 37.5°, 43.5° and 63.1°, forming a crystal phase structure in which NiO is supported on calcium-chromium-based limonite-type Ca ₂ Cr ₂ O ₅ oxide. , and formed a Ni-Ca-Cr-O active center; the catalyst was tested by nitrogen low-temperature physical adsorption, and the pore diameter was concentrated at 4nm. The typical mesoporous structure is shown in Figure 2; the molar composition of the catalyst is (NiO) _0.40 (CaO) _1.29 (CrO _1.5 ) _1.29 , the weight percentage composition of the catalyst in terms of oxides is: nickel oxide (NiO) is 14.9%, calcium oxide (CaO) is 36.1%, and chromium oxide (CrO _1.5 ) is 49.0%.

The activity of catalyst CDUT-NCC in acetic acid autothermal reforming was investigated. The reaction conditions were normal pressure, space velocity 25000ml/(g-catalyst·h), reaction temperature 650℃, and raw material gas acetic acid/water/oxygen=1/4.0/ At 0.28, the catalyst's acetic acid conversion rate is about 100%, the hydrogen production rate is about 2.2mol-H ₂ /mol-HAc, the CO ₂ selectivity is about 45.7%, the CO selectivity is about 48.8%, and the CH ₄ selectivity At around 5.6%, there is almost no by-product acetone. The CDUT-NCC catalyst was characterized by low-temperature physical adsorption of nitrogen. The results showed that the specific surface area was 2.1m ² /g, the pore volume was 0.02cm ³ /g, and the pore size distribution was concentrated. The average pore diameter was 8.3nm, and the maximum pore diameter was 4.0nm. , which is a mesoporous material. Characterization results show that the catalyst has no sintering phenomenon and no significant carbon deposition, and the catalyst effectively suppresses the production of acetone as a by-product and has high catalytic activity.

Claims (2)

1. The application of calcium-chromium-based limonite-type nickel-based catalyst in the process of hydrogen production by autothermal reforming of acetic acid, which is characterized in that: the calcium-chromium-based limonite-type nickel-based catalyst is heated in H at a temperature of 500-800°C. Reduction in ₂ for 1 hour, then inject the mixed solution of acetic acid and water into the vaporizer with a constant flow pump. After vaporization, mix oxygen and use nitrogen as the internal standard gas to form a molar composition of CH ₃ COOH/H ₂ O/O ₂ /N ₂ =1/(1.3-5.0)/(0.21-0.35)/(2.5-4.5) reaction raw material gas, and this raw material gas is introduced into the reaction bed, the reaction temperature is 500-800°C; the catalyst is prepared by the following method Preparation: Weigh appropriate amounts of Ni(NO ₃ ) ₂ ·6H ₂ O, Ca(NO ₃ ) ₂ ·4H ₂ O and Cr(NO ₃ ) ₃ ·9H ₂ O and dissolve in deionized water and stir until all Dissolve to obtain solution 1#; according to the total number of moles of metal cations: citric acid: ethylene glycol ratio is 1:1:1, prepare a mixed solution 2# of citric acid and ethylene glycol, mix solution 2# and solution 1#, Then keep the water bath temperature at 70°C and continue stirring until the sol is formed; place the obtained sol in a 105°C oven for 24 hours, then raise it from room temperature to 750°C in a tube furnace at a rate of 5°C/min, and calcine to maintain After 4 hours, the calcium-chromium-based limonite-type nickel-based catalyst Ni/Ca ₂ Cr ₂ O ₅ was obtained, that is, the Ni-Ca-Cr-O active center was formed, and NiO was supported on the calcium-chromium-based limonite-type Ca ₂ Cr ₂ The crystal phase structure of O ₅ oxide; the molar composition of the catalyst in terms of oxide is (NiO) _a (CaO) _b (CrO _1.5 ) _c , where a is 0.38-0.43, b is 0.88-1.33, and c is 1.27- 1.61; The weight percentage composition based on oxides is: nickel oxide is 14.0%-16.1%, calcium oxide is 24.6%-37.7%, chromium oxide is 48.3%-61.3%, and the sum of the weight percentages of each component is 100 %. 2. Application of the calcium-chromium-based limonite-type nickel-based catalyst in the process of hydrogen production by autothermal reforming of acetic acid according to claim 1, characterized in that: the weight percentage of the catalyst in terms of oxide is: oxidation Nickel is 14.9%, calcium oxide is 36.1%, and chromium oxide is 49.0%.