CN109954510A - A kind of chromium-based ammonia synthesis and ammonia decomposition catalyst and application - Google Patents

Abstract

The present invention relates to a chromium-based catalyst for ammonia synthesis and ammonia decomposition, which comprises chromium element and related supports and additives. As a novel catalytic material, the present invention exhibits good catalytic activity in ammonia synthesis and ammonia decomposition reactions.

Description

A kind of chromium-based ammonia synthesis and ammonia decomposition catalyst and application

technical field

The invention relates to catalyst technology, and particularly provides a chromium-based catalyst for ammonia synthesis and ammonia decomposition and its application in ammonia synthesis and ammonia decomposition reactions.

Background technique

Ammonia is the basic raw material for the production of important chemical products such as fertilizers, nitric acid, plastics, and medicine, and it is also a hydrogen source carrier with potential application prospects. Therefore, the synthesis and decomposition of ammonia is of great significance in industry. The catalytic conversion of nitrogen and hydrogen on transition metals is the main way of industrial ammonia synthesis, and the main industrial process currently used is the Haber-Bosch process. The reaction conditions for ammonia synthesis in this process are harsh (iron-based catalyst: 350-525°C, 100-300atm), and the equipment requirements are very high, so the energy consumption is very high, and the annual energy consumption is 1% of the world's total annual energy consumption. At present, the catalysts widely used in the industry for ammonia synthesis and ammonia decomposition are transition metal catalysts such as iron-based catalysts, ruthenium-based catalysts and nickel-based catalysts. High temperature and high pressure are required. For the ammonia decomposition reaction, nickel-based catalysts are inexpensive but low in activity. Therefore, the development of new low-temperature, low-pressure and high-efficiency catalyst systems for ammonia synthesis and ammonia decomposition is still a very important research topic. It is an effective strategy to develop a new generation of ammonia synthesis and ammonia decomposition catalysts by jumping out of the limitations of iron-based and ruthenium-based catalysts and using existing basic theories and technologies to develop catalysts with other components.

There are very few studies on non-iron (ruthenium)-based catalysts for ammonia synthesis and decomposition, especially chromium-based catalysts. Early results showed that chromium-based catalysts have low ammonia synthesis and decomposition activities [A.Mittasch, Adv. Catal., 1950, 2, 81-104; CR Lotz, F. Sebba, Trans. Faraday Soc., 1957, 53, 1246-1252.], so it has not attracted the attention of researchers for a long time. In 2009, Zhu et al. reported that chromium oxide (Cr ₂ O ₃ ) can exhibit a certain catalytic activity for ammonia decomposition at higher temperatures (>500°C), such as at 600°C and WHSV=60000ml _NH3 g _cat ^-1 h. Under the reaction conditions of ^-1 , the conversion rate of ammonia gas can reach 43.4% [L.Li,ZHZhu,SBWang,XDYao,ZFYan,J.Mol.Catal.A,2009,304,71-76.].

In summary, there are few studies on chromium-based ammonia synthesis and ammonia decomposition catalysts, and the mechanism of its ammonia decomposition and synthesis is still unclear. How to improve the activity and stability of the catalyst and reduce the cost of the catalyst by modulating the composition and structure of the catalyst needs further research.

SUMMARY OF THE INVENTION

Alkaline (earth) metal amide compounds, such as lithium amide, potassium amide, magnesium amide, etc., are a class of hydrogen storage materials with important application prospects. When we studied the properties of this type of material, we found that when the transition metal chromium element was added to it, its thermal decomposition properties changed significantly. For example, after adding Cr to lithium amide (LiNH ₂ ), when the reaction temperature was higher than 200 degrees, a large amount of nitrogen and hydrogen were detected in the gas products, as shown in Fig. 1 . Since the decomposition product of _LiNH2 is mainly ammonia gas below 500 degrees, the presence of Cr may change the decomposition path of lithium amide and may be used as a catalyst for ammonia decomposition reaction.

To prove this hypothesis, we synthesized a CrN/ _LiNH2 composite catalyst and investigated its catalytic performance for ammonia decomposition. As shown in Figure 2, nitrogen and hydrogen can be generated at 300 degrees, and the ammonia conversion rate increases gradually with the increase of temperature.

Further studies show that Cr has similar catalytic activity to nitrogen-containing compounds of main group elements such as Na, K, Cs, Be, Mg, Ca, Sr, Ba, and Al, respectively. Nitrogen- or/and hydrogen-containing compounds include mono- or multi-component nitrides, amino compounds, imino compounds, nitrogen oxides, nitride-hydrides and hydrides or mixtures of two or more thereof. Its basic composition is M _x N _y H _3y-nx , wherein M is one or more of the above-mentioned IA, IIA, IIIA group elements, n is the chemical valence state of M, x=1～4, y = 0 to 3.

The nitrogen- or/and hydrogen-containing compounds of these main group elements can be supported on a certain carrier. The carrier can be oxides of main group elements such as Li ₂ O, Na ₂ O, K ₂ O, MgO, CaO, SrO, BaO, SiO ₂ , Al ₂ O ₃ etc. or its nitrides such as BN, Si ₃ N ₄ , One or more combinations of Mg ₃ N ₂ , Ca ₃ N ₂ , AlN and molecular sieves, carbon materials, and metal organic framework materials (MOFs). The mass ratio of catalyst to support can be from 1000:1 to 1:500; after optimization, it can be from 200:1 to 1:100.

Other metal nitrides can be added to the Cr element. The metal of the metal nitride may be one or two or more of the elements of Group IVB, VB, VIB, VIIB or VIIIB. The mass ratio of catalyst to metal nitride can be from 1000:1 to 1:500; after optimization, it can be from 200:1 to 1:100.

Metal alloys can be added to the Cr element. Metal alloys are: binary or multi-element alloys formed between IVB, VB, VIB, VIIB or VIIIB group elements and C, B or N. The mass ratio of catalyst to metal alloy can be from 1000:1 to 1:500; after optimization, it can be from 200:1 to 1:100.

In the ammonia decomposition reaction, the catalyst provided by the present invention can achieve a relatively ideal effect: the CrN/LiNH ₂ (molar ratio is 1:0.8) catalyst is obviously improved compared with CrN. In the ammonia synthesis reaction, the CrN/LiH (molar ratio is 1:5) catalyst in a nitrogen-hydrogen mixture (N ₂ : H ₂ =1:3), 10 atm, under the reaction conditions of 350 ℃, the ammonia synthesis reaction rate can be Up to 8627umol g _cat ^-1 h ^-1 .

Description of drawings

Figure 1. Ar-TPD spectrum of CrN/LiNH2 (molar ratio ₁ :0.8).

Figure _2. Activity of CrN and CrN/LiNH2 (molar ratio 1:0.8) in 5% _NH3 /Ar. Figure 3. Activity of CrN/ _NaNH2 (molar ratio 1:0.4) in 5% _NH3 /Ar.

Figure 4. Activity of CrN and CrN/LiNH2 ( ₁ :0.8 molar ratio) in pure ammonia.

Figure 5. Activity of CrN and CrN/ _NaNH2 (1:0.4 molar ratio) in pure ammonia.

Figure 6. Ammonia synthesis reactivity of CrN/LiH (molar ratio 1:5) in _N2 / _H2 (1/3).

Figure 7. Ammonia synthesis reactivity of CrN/CaH2 (molar ratio ₁ :1.3) in _N2 / _H2 (1/3).

Detailed ways

To further illustrate the present invention, the following specific examples are given, but they do not limit the scope of the invention defined by the appended claims.

Example 1:

In an argon glove box, 1.0000 g of chromium nitride (CrN) and 0.2788 g of lithium amide (LiNH ₂ ) were accurately weighed and placed in a self-made stainless steel ball mill jar. After sealing the ball mill jar, it was loaded into a planetary ball mill (Fischt PM400), and the ball milling condition was 150 rpm for 3 hours. The sample CrN/LiNH ₂ (molar ratio 1:0.8) was obtained.

In an argon glove box, 0.0300 g of CrN/LiNH ₂ (molar ratio 1:0.8) was accurately weighed and placed in a fixed-bed stainless steel reactor. The sample was heated to the desired temperature in the reaction atmosphere (5% NH ₃ /Ar mixed gas), the flow rate of the reaction gas was controlled at 1.8 L/h and 3.6 L/h, and the samples were taken for analysis after 30 minutes. The test results are shown in Figure 2. CrN only began to show a small amount of ammonia decomposition activity when the temperature was higher than 475 degrees, and the ammonia conversion rate was only 3% at 500 degrees. However, CrN/LiNH ₂ showed a certain ammonia decomposition activity at 300 degrees (ammonia conversion rate was about 7%), and the ammonia conversion rate gradually increased with the increase of temperature; at 500 degrees, The ammonia conversion rate can reach 90%.

Example 2:

In an argon glove box, 1.0000 g of chromium nitride (CrN) and 0.2364 g of sodium amide (NaNH ₂ ) were accurately weighed and placed in a self-made stainless steel ball mill jar. After sealing the ball mill jar, it was loaded into a planetary ball mill (Fischt PM400), and the ball milling condition was 150 rpm for 3 hours. The sample CrN/LiNH ₂ (molar ratio 1:0.4) was obtained.

In an argon glove box, 0.0300 g of CrN/NaNH ₂ (molar ratio 1:0.4) was accurately weighed and placed in a fixed-bed stainless steel reactor. The sample was heated to the desired temperature in the reaction atmosphere (5% NH ₃ /Ar mixed gas), the flow rate of the reaction gas was controlled at 1.8 L/h and 3.6 L/h, and the samples were taken for analysis after 30 minutes. The test results are shown in Figure 3. The ammonia conversion rate increases gradually with the increase of temperature. At 300 degrees, the ammonia conversion rate on CrN/NaNH ₂ is about 11%, which is slightly higher than that on CrN/LiNH ₂ ; at 500 degrees , the ammonia conversion rate can reach 82%.

Example 3:

In an argon glove box, 1.0000 g of chromium nitride (CrN) and 0.2788 g of lithium amide (LiNH ₂ ) were accurately weighed and placed in a self-made stainless steel ball mill jar. After sealing the ball mill jar, it was loaded into a planetary ball mill (Fischt PM400), and the ball milling condition was 150 rpm for 3 hours. The sample CrN/LiNH ₂ (molar ratio 1:0.8) was obtained.

In an argon glove box, 0.0300 g of CrN/LiNH ₂ (molar ratio 1:0.8) was accurately weighed and placed in a fixed-bed stainless steel reactor. The sample was heated to the desired reaction temperature in a pure ammonia atmosphere, the pressure was 1 atm, the flow rate of the reaction gas was controlled at 2.4 L/h, and the samples were taken for analysis after 30 minutes. The test results are shown in Figure 4. CrN only began to show a small amount of ammonia decomposition activity when the temperature was higher than 500 degrees, and the ammonia conversion rate was only 2.4% at 550 degrees. However, CrN/LiNH ₂ showed a certain ammonia decomposition activity at 300 degrees (the conversion rate of ammonia gas was about 2%), and the conversion rate of ammonia gas gradually increased with the increase of temperature; at 550 degrees, the The ammonia conversion rate can reach 38%.

Example 4:

In an argon glove box, 1.0000 g of chromium nitride (CrN) and 0.2364 g of sodium amide (NaNH ₂ ) were accurately weighed and placed in a self-made stainless steel ball mill jar. After sealing the ball mill jar, it was loaded into a planetary ball mill (Fischt PM400), and the ball milling condition was 150 rpm for 3 hours. The sample CrN/LiNH ₂ (molar ratio 1:0.4) was obtained.

In an argon glove box, 0.0300 g of CrN/NaNH ₂ (molar ratio 1:0.4) was accurately weighed and placed in a fixed-bed stainless steel reactor. The sample was heated to the desired reaction temperature in a pure ammonia atmosphere, the pressure was 1 atm, the flow rate of the reaction gas was controlled at 2.4 L/h, and the samples were taken for analysis after 30 minutes. The test results are shown in Figure 5. CrN/NaNH ₂ showed a certain ammonia decomposition activity at 300 degrees (ammonia conversion rate was about 1.4%), and the ammonia conversion rate gradually increased with the increase of temperature; but at 300-550 degrees In the temperature range, the activity is lower than that of CrN/LiNH ₂ ; at 550 degrees, the ammonia conversion rate on CrN/NaNH ₂ can reach 22%.

Example 5:

In an argon glove box, 1.0000 g of chromium nitride (CrN) and 0.6060 g of lithium hydride (LiH) were accurately weighed and placed in a self-made stainless steel ball mill jar. After sealing the ball mill jar, it was loaded into a planetary ball mill (Fischt PM400), and the ball milling condition was 150 rpm for 3 hours. The sample CrN/LiH (molar ratio 1:5) was obtained.

In an argon glove box, accurately weigh 0.0300 g of CrN/LiH (molar ratio 1:5) and place it in a fixed-bed stainless steel reactor. The sample was heated to the desired reaction temperature in a N ₂ /H ₂ (1/3) atmosphere, the pressure was 10 atm, the flow rate of the reaction gas was controlled at 1.8 L/h, and the sample was taken for analysis after at least 1 hour. The test results are shown in Figure 6. CrN/LiH has shown a certain ammonia synthesis activity at 225 degrees (ammonia generation rate is about 437umol g _cat ^-1 h ^-1 ), and the ammonia generation rate increases gradually with the increase of temperature; at 350 degrees , the rate of ammonia formation on CrN/LiH can reach 8627umol g _cat ^-1 h ^-1 .

Example 6:

In an argon glove box, 1.0000 g of chromium nitride (CrN) and 0.8273 g of calcium hydride (CaH ₂ ) were accurately weighed and placed in a self-made stainless steel ball mill jar. After sealing the ball mill jar, it was loaded into a planetary ball mill (Fischt PM400), and the ball milling condition was 150 rpm for 3 hours. The sample CrN/CaH ₂ (molar ratio 1:1.3) was obtained.

In an argon glove box, 0.0300 g of CrN/CaH ₂ (molar ratio 1:1.3) was accurately weighed and placed in a fixed-bed stainless steel reactor. The sample was heated to the desired reaction temperature in a N ₂ /H ₂ (1/3) atmosphere, the pressure was 10 atm, the flow rate of the reaction gas was controlled at 1.8 L/h, and the sample was taken for analysis after at least 1 hour. The test results are shown in Figure 7. CrN/CaH ₂ shows a certain ammonia synthesis activity only when the temperature is above 275 degrees, and the ammonia generation rate increases gradually with the increase of temperature; at 350 degrees, the ammonia generation rate on CrN/CaH ₂ can reach 3472umol g _cat ^-1 h ^-1 , which is lower than the rate of ammonia formation on CrN/LiH.

Claims (8)

1. a kind of for ammonia synthesis and the chromium-based catalysts of ammonolysis craft, it is characterised in that: the catalyst includes main body and addition Agent, main body are one of crome metal, chromium nitride or its metal evanohm or two kinds or more, and additive includes containing for major element One of nitrogen compound or/and hydrogen-containing compound or two kinds or more and carrier；

The range that the mass ratio of the catalyst body and additive is is 1000:1 to 1:500.

2. catalyst as described in claim 1, it is characterised in that:

Nitrogenous or/and hydrogen-containing compound the molecular formula of the major element are as follows: M_xN_yH_m(3y-nx), wherein M is I A, II A, III A One of race's element or two kinds or more, the chemical valence state that n (can be 1,2,3) is M, m can be 1, -1) chemical valence state that is H, when When m=1, molecular formula M_xN_yH_3y-nx, x=1~3, y=1~3；As m=-1, molecular formula M_xN_yH_nx-3y, x=1~4, y= 0~1.

3. catalyst as claimed in claim 1 or 2, it is characterised in that: the major element be Li, Na, K, Rb, Cs, Mg, The mixture of one of Ca, Sr, Ba, Al or two kinds or more.

4. catalyst as described in claim 1, it is characterised in that: the carrier is Li₂O、MgO、CaO、SrO、BaO、 Al₂O₃、SiO₂、TiO₂、ZrO₂、CeO₂、BN、Si₃N₄、Mg₃N₂、Ca₃N₂, AlN, molecular sieve, carbon material, metal-organic framework materials One of (MOFs) or two kinds or more of combination, the quality of carrier is 20-99wt% in catalyst.

5. catalyst as described in claim 1, it is characterised in that: the chromium nitride is CrN, Cr₂One in the nitride such as N Kind or two kinds or more of combination.

6. catalyst as described in claim 1, it is characterised in that: the metal evanohm are as follows: IV B, V B, VI B, VII B or More than one of VIII B race element or two kinds or more of the binary or ternary being combined into Cr metal alloy or IV B, V B, VI B, VII B or VIII B race element, preferably such as Ti, Zr, V, Nb, Ta, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt One of or two kinds or more of the binary or ternary being combined into Cr more than metal alloy；Or IV B, V B, VI B, VII B or Alloy more than one of VIII B race element or two kinds or more of the binary or ternary formed between Cr and C and/or N, preferably As one of Ti, Zr, V, Nb, Ta, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt or two kinds or more and Cr And the alloy more than binary or ternary formed between C and/or N.

7. catalyst as described in claim 1, it is characterised in that: the mass ratio of the catalyst body and additive it is more excellent Range is 200:1 to 1:100.

8. a kind of application of any catalyst of claim 1-7, it is characterised in that:

The catalyst is used for operating condition when ammonia synthesis: sample is in nitrogen and hydrogen mixture (volume ratio N₂: H₂=1:3) in heating To 200-500 (preferably 300 degrees centigrade), gross pressure 1-30atm, reaction gas flow velocity is 1.8-2.4L/h, with conventional conductance The generating rate of rate method detection ammonia；

The catalyst is used for operating condition when ammonolysis craft: sample is in reaction atmosphere (volume content 1-100% (preferably 5%) NH₃/ Ar or pure ammonia) in rise to 300-600 degrees Celsius of reaction temperature, product composition is divided online using gas-chromatography Analysis.