CN104128187A - Ni/La2O3 catalyst for steam reforming of LPG with low water-to-carbon ratio and preparation method thereof - Google Patents

Abstract

The invention relates to a Ni/ _La2O3 catalyst used for steam reforming of LPG with a low water-to-carbon ratio and a preparation method _thereof , belonging to the technical field of catalyst preparation technology. The present invention is an LPG steam reforming catalyst supported by nickel on lanthanum oxide obtained by reducing perovskite composite oxides of nickel and lanthanum. It is characterized in that the atomic molar ratio of lanthanum to nickel is 1:1, and its preparation method is a precipitation-combustion method. Use 2mol/L ammonium carbonate solution to slowly titrate the mixed solution of lanthanum nitrate, nickel nitrate and citric acid to form a suspension. The suspension is dried and roasted to obtain LaNiO ₃ perovskite composite oxide, which is reduced by hydrogen at 600℃ Afterwards, a Ni/La ₂ O ₃ catalyst was obtained. The catalyst prepared by the invention has the advantages of high catalytic activity, wide service temperature, good catalytic stability, high mechanical strength, low production cost and the like. The life test results show that the catalyst is a potential low temperature, low water to carbon ratio, high carbon alkane steam reforming catalyst.

Description

Ni/La2O3 catalyst for steam reforming of LPG with low water-to-carbon ratio and preparation method thereof

technical field

The invention discloses a Ni/La ₂ O ₃ catalyst for steam reforming of LPG with a low water-to-carbon ratio and a preparation method thereof, belonging to the technical field of catalyst preparation technology.

Background technique

LPG has the advantages of high combustion calorific value, high energy density, stable market source, convenient storage and transportation, and a mature commercial distribution network. It is used for the development of on-board hydrogen production for fuel cell vehicles and has huge potential commercial value. Hydrogen production by catalytic reforming of hydrocarbon steam is currently the most widely used method for hydrogen production. Coke deposition on the catalyst has adverse effects on its activity, life, and mechanical strength. To prevent carbon deposition, excessive water vapor (S /C = 3~5). Increasing the amount of steam means increased energy consumption, large equipment, large floor space, and expensive investment and operation costs. An effective solution is to use a pre-conversion process. It is to add an adiabatic reactor in front of the reformer, so that the hydrocarbons in the raw material can be converted into small molecular gases such as CH ₄ and H ₂ under the conditions of low temperature, low water-to-carbon ratio, and high space velocity. Then CH ₄ is reformed to generate H ₂ , CO and CO ₂ at a high temperature above 700°C.

The process of pre-reforming hydrocarbons to produce methane-enriched gas is generally carried out in the range of 400-550 °C. High-activity nickel catalysts are used to ensure that high-carbon hydrocarbons are cracked, steam-reformed, hydrogenated and shifted at lower temperatures, and finally methane gas is obtained. Oxygenation and CO transformation balance. Endothermic reactions such as pyrolysis and conversion are coupled with exothermic reactions such as methanation and transformation to supply heat to each other and reduce heat exchange with the outside world. The difference between the pre-reforming reaction and the traditional reaction is that the traditional conversion is mainly based on hydrocarbon and steam reforming to produce hydrogen, which needs to be carried out at a higher temperature; while the pre-reforming reaction requires cracking and hydrogenation reactions to be carried out at the same time. Through the adjustment of process parameters, the overall The thermal effect is small, and it is carried out in an adiabatic reactor. The pre-reforming catalyst is different from the reforming catalyst, and should have the following characteristics under the condition of low water-to-carbon ratio: (1) high low-temperature activity; (2) high resistance to carbon deposition; (3) good thermal stability. Therefore, it is necessary not only to ensure that the catalyst has a higher activity, but also to prolong the service life and the activity and stability of the catalyst after repeated use.

Ni/Al ₂ O ₃ catalyst is considered as a promising catalyst for steam reforming of hydrocarbons due to its high activity and low price (Appl. Catal. A. 314(2006): 9, 81; Int. J . Hydrogen Energy. 33(2008):7427; Chin. J. Catal. 30(2009):690). However, active metallic Ni species are easily oxidized to nickel oxides under water vapor and oxygen atmosphere, which completely deactivates the nickel-based catalysts.

LaNiO ₃ perovskite-type composite oxide has high thermal stability, and the Ni/La ₂ O ₃ catalyst formed after reduction can make the particle size of the active component nickel small and uniformly disperse in La ₂ O ₃ , so that The catalyst has good anti-coking ability and is widely used in the catalytic reforming reaction of alkanes. Catalysis Today 107–108 (2005) 474–480 and Catalysis Today 212 (2013) 98– 107 Catalysis Today 212 (2013) 98– 107 Preparation of LaNiO ₃ perovskite-type composite oxides for carbon dioxide and methane reforming by combustion method using glycine and nitrate as raw materials . In Energy Fuels 2009, 23, 4883–4886, glycine and nitrate were used as raw materials to prepare LaNiO ₃ perovskite composite oxides by combustion method and used for dry reforming of methane. Catalysis Today 171 (2011) 24–35 Using citric acid and lanthanum and nickel nitrates as raw materials, LaNiO ₃ perovskite-type composite oxides were prepared by sol-gel method for catalytic decomposition of methane. Applied Catalysis A: General 450 (2013) 73–79 Using glycine and nitrate as raw materials, LaNiO ₃ perovskite-type composite oxides were prepared by combustion method for catalytic ethanol to hydrogen and carbon nanotubes. The present invention prepares LaNiO ₃ perovskite type oxide by precipitation-combustion method, and the lanthanum oxide-loaded nickel LPG steam reforming reaction catalyst prepared after reduction has quite high activity and long-term stability. Under low water-to-carbon ratio conditions, multiple cycles were successfully carried out, and the catalytic activity did not decrease during daily start-up and shutdown operations.

Contents of the invention

The object of the present invention is to provide a Ni/La ₂ O ₃ low water-to-carbon ratio LPG steam reforming catalyst with high catalytic activity, wide service temperature, good catalytic stability, high mechanical strength and low production cost.

The novel nickel-based LPG steam reforming catalyst of the present invention is characterized in that it is formed by reduction of perovskite-type composite oxides of lanthanum and nickel, wherein the atomic molar ratio of nickel and lanthanum is 1:1, and the preparation of the catalyst of the present invention The process adopts the inorganic salt precipitation-combustion method, which mainly includes the following steps:

a) Prepare a certain amount of mixed aqueous solution of Ni(NO ₃ ) ₂ 6H ₂ O, La(NO ₃ ) ₃ 9H ₂ O and citric acid according to the atomic ratio of lanthanum and nickel in the catalyst as 1:1, so that the metal The molar concentration of ions is 0.2 mol/L, and the concentration of citric acid is 0.4 mol/L;

b) Move the prepared mixed solution to a constant temperature water bath at 40°C, slowly drop in 1 mol/L ammonium carbonate solution at a rate of 1 mL/min, and keep stirring; the stirring speed is 200 rpm, Titrate until the pH is 6-7, a precipitate is formed, and the solution becomes a suspension;

c) Move the obtained suspension to an oven at 100 °C for 24 h to dry. The dried powder is calcined in a muffle furnace from room temperature to 400 °C for 5 h, and then gradually raised to 600 °C for 5 h. h, the heating rate is 1 ℃/min. The calcined powder is extruded in a mold under a pressure of 25-35 kN; then crushed and sieved to obtain particles with a particle size of 35-45 mesh, that is, to obtain LaNiO ₃ perovskite composite oxide;

d) After completely reducing the LaNiO ₃ perovskite composite oxide at 600 ℃ and H ₂ atmosphere for 3-4 h, the Ni/La ₂ O ₃ catalyst is obtained.

Description of drawings

Fig. 1 is the XRD pattern and the reduced XRD pattern of the LaNiO ₃ perovskite composite oxide prepared after roasting in Example 1 of the present invention.

Fig. 2 is the catalytic evaluation result of the catalyst prepared in Example 1.

Fig. 3 is the comparison of the catalytic performance evaluation results of Example 1 and Comparative Example 1.

Fig. 4 is the maximum space velocity and the corresponding equilibrium value (indicated by dotted lines) of complete conversion of LPG at different reaction temperatures of the embodiment

Fig. 5 is the experimental result of the catalyst circulation operation of the embodiment.

Detailed ways

The present invention is further illustrated by the following examples.

Example 1

Weigh 0.04 mol each of nickel nitrate and lanthanum nitrate, add deionized water to 0.08 mol of citric acid, and stir until dissolved. Control the concentration of nickel nitrate and lanthanum nitrate to 0.2 mol/L, and the concentration of citric acid to 0.4 mol/L. Move the solution to a water bath at 40°C, keep stirring, and titrate the prepared mixed solution with the prepared 1 mol/L ammonium carbonate solution with a peristaltic pump at 1 mL/min until the pH is 6-7 to form a suspension. Stirring was continued for 2 h, the obtained suspension was evaporated to dryness at 80 °C, transferred to an oven, dried at 120 °C for 12 h, then transferred to a muffle furnace for roasting, and the temperature was raised from room temperature to 400 °C at a rate of 1 °C/min Roast for 5 h, then gradually increase the temperature to 600 °C for 5 h. The calcined powder is extruded in a steel mold under a pressure of 25-35 kN; then crushed and sieved to obtain particles with a particle size of 35-45 meshes to obtain a perovskite composite oxide LaNiO ₃ . Then the temperature was raised from room temperature to 600 ℃ at a rate of 10 ^o C/min, and the Ni/La ₂ O ₃ catalyst was obtained after reduction with 15 vol% H ₂ /N ₂ for 3 h.

Comparative example 1

Lanthanum oxide-supported nickel catalyst was prepared by traditional impregnation method, and lanthanum trioxide was impregnated with 0.2 mol/L nickel nitrate; ₂ O ₃ agree.

evaluation experiment

LPG was purchased from Shanghai ENN Jiuhuan Automotive Energy Co., Ltd. Its composition is: C ₂ H ₆ 3.1 vol%, C ₃ H ₈ 84.0 vol%; C ₄ H ₁₀ 12.9 vol%, and was used directly without purification treatment.

The activity evaluation of the catalyst was carried out in a traditional atmospheric pressure fixed-bed reactor. The quartz tube reactor was 900 mm long and 10 mm in diameter. After mixing 0.1 g of catalyst and 0.9 g of quartz sand, they were placed in the middle of the quartz tube. In the middle of the layer of quartz wool, the actual temperature of the catalyst is detected by a thermocouple placed in the catalyst bed. The reaction gas is controlled by a mass flow meter, and the water is controlled by a micro pump to enter a preheater with a constant temperature of 300 ℃ and equipped with quartz balls to mix evenly. The space velocity of the control reaction is 11000 mL/g _cat h, and the water-carbon ratio S/C=1. Before evaluating the reaction, the catalyst should be reduced for 4 h in a hydrogen gas mixture, i.e. 15 vol.% H ₂ /N ₂ (the flow rate is 30 Ml min ^-1 ) at a temperature of 600 °C; to avoid initial carbon deposition, the reducing gas was switched to After 5 minutes into nitrogen and the required amount of water vapor, turn off the nitrogen and replace it with LPG to enter the reaction system. The gas discharged from the catalytic reaction is cooled by the condenser at room temperature, and all the water is removed by anhydrous calcium sulfate. Finally, dry gas hydrocarbon products were analyzed using an online GC-FID gas chromatograph with a CP–SIL 5 CB column, and CH4, CO, _CO2 and _H2 were analyzed by another GC-TCD gas chromatograph. The gas outlet flow rate is measured by a soap film flowmeter, and the outlet gas has no other gases except H ₂ , CO, CO ₂ and CH ₄ . Then the LPG conversion rate was measured between 400 and 550 °C. The LPG conversion rate is calculated by the following formulas.

                                                 

In the formula, X _LPG is the conversion rate of LPG, S _i and S _H2 are the selectivity of products respectively, N is the molar flow rate, and i is CO, CO ₂ , CH ₄ .

In the daily start-up and shutdown operations, the lanthanum nickelate perovskite composite oxide is first reduced at 600 °C, first reacted at 450 °C, and the space velocity is 11000 mL/g _cat h for liquefied petroleum gas steam reforming reaction, The water/carbon molar ratio was 1, and the reaction system and catalyst were completely stabilized after running for 13 h. After the first round of reaction, the LPG was switched to an equal volume of _N2 and steam purge. The used catalyst was cooled in situ from 450 °C to 200 °C within 2 h. After purging with N ₂ and water vapor for 4 h, the catalyst temperature was raised to 450 °C again at a rate of 8 °C/min. After 5.5 h, N ₂ and water vapor were completely converted to the same conditions as the first round of reaction to start the second round circular reaction. Subsequent purging by water vapor followed by the third and fourth cycles of the reaction was carried out in the same mode of operation.

(a) in Figure 1 is the XRD pattern of the preparation after calcination at 600 ^oC , and the corresponding LaNiO ₃ perovskite PDF card number is (JCPDS 33-0710). (b) is the XRD pattern after hydrogen reduction at 600 ^o C, corresponding to nickel and lanthanum oxide, indicating that after LaNiO ₃ is reduced by hydrogen gas, LaNiO ₃ with a perovskite structure transforms into a lanthanum oxide-supported nickel catalyst.

Figure 2 is the result of LPG steam reforming reaction. The reaction conditions are as follows: the space velocity is 11000 mL/g _cat h, the water-to-carbon ratio is 1, and the reaction temperature is 450 °C. It can be seen from the figure that the Ni/La ₂ O ₃ catalyst prepared by the precipitation-combustion method has high activity and can completely convert LPG into CH ₄ , CO, CO ₂ and H ₂ . During the long-term operation of 380 h, The conversion rate of LPG did not drop significantly, showing better stability, and no carbon deposition was found in the catalyst after the reaction.

Fig. 3 is the catalytic activity comparison of _{embodiment 1 and comparative example 1, as shown in the figure, in the reaction of 13 h, the Ni/La2O3 prepared} by precipitation-combustion method passes through 5 h of reaction, and the conversion rate of LPG is constant Stabilized at 95%, while the Ni/La prepared by the impregnation method in the comparative example. _Catalyst , after the catalytic reaction _was carried out for 4 h, after the conversion of LPG reached the maximum conversion rate of 87%, the conversion rate of LPG gradually decreased, and the reaction reached After 12 h, the conversion rate of LPG dropped to 21%. It can be seen from the comparative test results that the catalyst prepared by the precipitation-combustion method of the present invention has higher catalytic activity and stability than the catalyst prepared by the traditional impregnation method.

Figure 4 shows the maximum space velocities for complete conversion of LPG to CH ₄ , CO, CO ₂ and H ₂ when the water/carbon ratio is 1 and the temperature ranges from 425 ^o C to 550 ^o C. As shown in the figure, the maximum space velocity at different temperatures increases almost linearly with the increase of temperature. At 425 ^o C, the maximum space velocity of LPG complete conversion is 4100 mL/g _cat h, and at 550 ^o C, the maximum space velocity is 16500 mL/g _cat h. There is no carbon deposition and thermal cracking of hydrocarbons, and the selectivity of products is close to the equilibrium value calculated by thermodynamics, indicating that the methanation of carbon oxides and hydrogen, and the water-gas shift reaction of the reforming system have reached a chemical equilibrium. This shows that the Ni/La ₂ O ₃ catalyst prepared by the precipitation self-combustion method has the characteristics of high activity and high stability.

Fig. 5 is a diagram of the catalytic activity of the Ni/La ₂ O ₃ catalyst during the LPG pre-reforming shutdown operation. Reaction conditions: 450 ^o C reaction, water-to-carbon ratio = 1, space velocity = 11000 mL/g _cat h, treat with nitrogen and water vapor when the reaction is interrupted. The first round of cyclic reaction was carried out for 13 h to make the reaction system completely stable. The second round of reaction was carried out for 5.5 h, and the subsequent rounds of reaction followed the same pattern. When passing LPG into the reaction system, the initial conversion rate exceeds 90%. Within 1.5 h, it quickly rose to about 95%, reaching the same catalytic activity. It can be seen from Figure 5 that after three rounds of cyclic operation, the catalytic activity of the catalyst remains unchanged and has high stability. It is a very potential low-temperature low-water-carbon ratio high-carbon alkane steam reforming catalyst.

Claims (3)

1. the Ni/La for LPG low steam carbon ratio steam reforming ₂o ₃catalyst, is characterized in that, in this lanthana supported nickel catalyst, the atomic molar of nickel, lanthanum is than being 1:1.

2. the Ni/La for LPG low steam carbon ratio steam reforming ₂o ₃the preparation method of catalyst, is characterized in that having following process and step:

A) prepare a certain amount of Ni (NO ₃) ₂6H ₂o, La (NO ₃) ₃9H ₂the mixed aqueous solution of O and citric acid, making metal ion molar concentration is 0.2 mol/L, and the atomic molar of nickel, lanthanum is than being 1:1, and the concentration of citric acid is 0.4mol/L;

B) mixed solution preparing is moved to 40 ℃ of thermostat water baths, with the sal volatile of 2 mol/L, with 1 mL/min speed, slowly splash into, and ceaselessly stir, mixing speed is 200 revs/min, and being titrated to PH is 6-7, forms suspension;

C) suspension obtaining is moved to dry 24 h in the convection oven of 100 ℃, the roasting in Muffle furnace of dried powder; From room temperature, be raised to 400 ℃ of roasting 5 h, be more progressively warmed up to 600 ℃ of roasting 5 h, programming rate is 2 ℃/min; By the extrusion modling in punching block under the pressure of 25 ~ 35 kN of the powder after roasting; With by broken, sieve, obtaining particle diameter is 35～45 object particles, obtains LaNiO ₃perovskite composite oxide, then in nitrogen atmosphere, temperature is at 600 ℃, to reduce 3～4 h, finally obtains lanthana supported nickel catalyst.

3. the Ni/La for LPG low steam carbon ratio steam reforming as claimed in claim 2 ₂o ₃the preparation method of catalyst, the nickel nitrate described in it is characterized in that and lanthanum nitrate can use nickel acetate, lanthanum acetate to substitute; Described ammonium carbonate can substitute with ammonium oxalate.