CN1132664A - Composite catalyst for one-step process preparing dimethyl ether from synthetic gas and its prepn - Google Patents

Abstract

The invention relates to a composite catalyst for preparing dimethyl ether by a one-step method of synthesis gas and a preparation method thereof, belonging to the technical field of chemical catalyst preparation. The invention proposes to prepare the mixed metal oxide catalyst by co-precipitation impregnation method, saves the alumina which only plays an isolation role in the methanol synthesis catalyst, and introduces co-catalyst boron. The molar content of each metal is: copper 20-40%, zinc 10-30%, aluminum 40-60%, boron 0.5-2%. The catalyst of the invention has the advantages of mild reaction conditions, high yield of dimethyl ether and the like.

Description

Composite catalyst for one-step preparation of dimethyl ether from synthesis gas and preparation method thereof

The invention relates to a composite catalyst for preparing dimethyl ether by a one-step method from synthesis gas and a preparation method thereof, belonging to the technical field of chemical catalyst preparation.

The one-step preparation of DME from synthesis gas (a mixture of carbon monoxide and hydrogen) was developed as part of the synthesis gas to gasoline process. The process of producing gasoline from syngas (MTG), originally developed by Mobil Corporation, consists of three steps to achieve the following three chemical reactions:

(1)

(2)

(3)

Among them, reaction (1) uses traditional methanol synthesis catalysts, such as copper/zinc/aluminum, copper/zinc/chromium, etc.; reaction (2) uses solid acid catalysts, such as γ-alumina, aluminum silicate, molecular sieves, etc.

Later, the TIGAS process developed by Topsoe Company completed reaction (1) and reaction (2) in one reactor in one step, and the catalyst used was a composite catalyst composed of methanol synthesis catalyst and methanol dehydration catalyst (US Patent 4520216). The main advantage of this is that the methanol generated in reaction (1) is continuously converted into dimethyl ether through reaction (2), which overcomes the thermodynamic equilibrium limitation of reaction (1), and the single-pass conversion rate of carbon monoxide and hydrogen can be greatly improved.

Many patents disclose composite catalysts for the one-step preparation of dimethyl ether from syngas, such as US patents 3894102, 4417000, 4520216; Japanese patents 60-179494, 63-254188; European patents 0148126A2, 0164156A1; British patent 2093365A, etc. The composite catalysts used in these patents are mechanical mixtures of methanol synthesis catalysts and methanol dehydration catalysts. This composite catalyst has two disadvantages: one is that the two catalyst particles cannot be in close contact, and the other is that the carrier (alumina, chromium oxide) that plays an isolation role in the methanol catalyst has lost its effect in the composite catalyst, and its existence is in vain. Increased catalyst volume. Both of the aforementioned disadvantages lead to a reduction in catalyst activity. For example: the composite catalyst announced by U.S. Patent No. 4,520,216 is a methanol synthesis catalyst (the copper/zinc/chromium mol ratio prepared by the general co-precipitation method is a mixed oxide catalyst of 30～40/10～60/50～18) particles and A mixture of methanol dehydration catalyst (γ-alumina) particles or particles obtained by mechanical mixing of two catalysts. Another example is the preparation method of the composite catalyst announced by European patent 164156: a) prepare a mixed solution of copper, zinc, aluminum and or chromium, wherein each metal molar ratio is: copper/zinc＜10, (chromium+aluminum)/(copper + zinc) < 2, copper/(copper + zinc + chromium + aluminum) > 0.1; b) adding alkali at a temperature lower than 70°C for co-precipitation; c) drying and calcining the resulting precipitate; d) combining the obtained product with a dehydrating Catalysts (γ-alumina, etc.) are mixed.

The purpose of the present invention is to overcome the above two disadvantages, so that the composite catalyst activity can be improved.

The present invention will now be outlined.

The present invention is a composite catalyst for preparing dimethyl ether by one-step synthesis gas, which is a mixed metal oxide, wherein the molar percentages of each metal are:

Copper 20～40

Zinc 10～30

Aluminum 40-60 is characterized in that it is prepared by coprecipitation impregnation. A small amount of boron can also be added to the composite catalyst as a cocatalyst, the molar content of which is 0.5-2%.

The present invention is also a preparation method of the above-mentioned composite catalyst, which is characterized in that the nitrate of copper and zinc is prepared into a mixed nitrate solution with a concentration of about 1 mole per liter with deionized water according to the ratio of copper and zinc in the above-mentioned catalyst ; This solution and 1 mole of sodium carbonate solution per liter are added dropwise to a beaker filled with γ-alumina for co-precipitation and impregnation. The amount of γ-alumina meets the content of aluminum in the catalyst. Immersion, co-precipitation impregnation is carried out under constant stirring, the temperature is 50-90°C, the pH value is between 6-8, and the relative flow rate of the two solutions is controlled; after stopping the feeding, continue to stir the obtained precipitate and keep it at a constant temperature for aging 15-60 minutes; filter the aged precipitate and wash it with deionized water; dry the washed precipitate in an oven at 80-120°C for 8-16 hours; put it in a muffle furnace at 350-450°C Calcining for 8-16 hours; molding the obtained solid with a tablet press.

In the above preparation method, it is also possible to add boric acid so that the molar content of boron in the catalyst is 0.5-2% in the mixed nitrate solution of copper and zinc.

Example:

Comparative Example 1: The methanol synthesis catalyst was first prepared by the traditional co-precipitation method, the method being: 14.5 grams of Cu(NO ₃ ) ₂ 3H ₂ O (copper nitrate trihydrate), 8.9 grams of Zn(NO ₃ ) ₂ 6H ₂ O (zinc nitrate hexahydrate) and 3.8 g of Al(NO ₃ ) ₃ ·9H ₂ O (aluminum nitrate nonahydrate) were dissolved in 100 ml of deionized water. This mixed nitrate solution and 1 mole per liter of sodium carbonate aqueous solution (dissolving 10.6 grams of anhydrous sodium carbonate in 100 milliliters of deionized water) were simultaneously added dropwise to a beaker filled with a small amount of deionized water for co-precipitation. Stir continuously during the co-precipitation process, keep the temperature at 70°C, and keep the pH value at about 7. After the co-precipitation is completed, it is aged for 30 minutes. The precipitate was filtered and washed with deionized water. Dry the washed precipitate in an oven at 100°C for 12 hours, and then calcinate it in a muffle furnace at 400°C for 12 hours to obtain an unshaped catalyst for methanol synthesis. Wherein the copper/zinc/aluminum molar ratio is 6:3:1. Gained unformed methanol synthesis catalyst is mixed with 5.1 grams of gamma-alumina powder, ground to 200 orders, and then pressed into tablets to obtain a composite catalyst prepared by mechanical mixing, wherein the copper/zinc/aluminum mol ratio is 6: 3: 11.

Example 1: 14.5 grams of Cu(NO ₃ ) ₂ 3H ₂ O (copper nitrate trihydrate) and 8.9 grams of Zn(NO ₃ ) ₂ 6H ₂ O (zinc nitrate hexahydrate) were dissolved in 100 ml of deionized A mixed nitrate solution is made in water. This solution and 1 mole per liter of sodium carbonate solution were simultaneously added dropwise to a beaker containing 5.1 g of γ-alumina for co-precipitation impregnation. The gamma-alumina is soaked with a small amount of deionized water before impregnation. The co-precipitation impregnation is carried out under constant stirring, the temperature is 70°C, and the pH value is kept at about 7. After stopping the feeding, continue to stir the obtained precipitate and keep it at a constant temperature for aging for 30 minutes; filter the aged precipitate and wash it with deionized water; Put the washed precipitate into an oven and dry at 100°C for 12 hours, then put it into a muffle furnace and calcinate at 350°C for 6 hours, and shape the obtained solid with a tablet machine to obtain the boron-free compound of the present invention. Catalyst, wherein the copper/zinc/aluminum molar ratio is 6:3:10.

Embodiment 2: method, step and condition are the same as embodiment 1, just add 2.1 grams of boric acid again in copper, zinc mixed nitrate solution. The molar ratio of copper/zinc/aluminum/boron in the obtained catalyst is 6:3:10:0.2. The catalyst is a boron-containing composite catalyst of the present invention.

Comparative example 2: with comparative example 1, just the copper/zinc/aluminum mol ratio is 5: 7: 2 in the methanol synthesis catalyst, and the copper/zinc/aluminum mol ratio is 5: 7: 14 in the composite catalyst prepared by mechanical mixing method.

Embodiment 3: with embodiment 1, just copper/zinc/aluminum mol ratio is 5: 7: 12 in the composite catalyst.

Embodiment 4: same as embodiment 2, except that the copper/zinc/aluminum/boron molar ratio is 5:7:12:0.3.

Catalyst activity evaluation method:

The prepared catalyst is crushed and sieved, and 20-40 mesh particles are taken and loaded into a tubular reactor with an inner diameter of 6 mm. First use 1 to 10% hydrogen (the rest is nitrogen) to reduce under normal pressure according to a certain temperature rise program (increase from room temperature to 250°C at a rate of 0.5°C per minute, keep the temperature at 250°C for 4 hours), and then carry out reaction. Feed gas composition is hydrogen/carbon monoxide=2: 1 (molar ratio), reaction pressure=3 MPa, space velocity=1000 per hour, measure catalyst activity under three kinds of temperatures of 230,250,270 ℃, product gas uses gas chromatography for analysis. The results of the measurements expressed in terms of dimethyl ether yield are listed in Table 1.

Table 1 Catalyst activity evaluation results

example DME production rate (moles per liter per hour) 230℃ 250℃ 270°C Comparative Example 1 Embodiment 1 Embodiment 2 Comparative Example 2 Embodiment 3 Embodiment 4 0.871.843.720.821.713.45 1.974.255.921.794.015.70 3.725.726.143.545.406.03

It can be seen from the table that the catalytic activity of preparing dimethyl ether from synthesis gas in one step by using the catalyst prepared by the method of the present invention is greatly improved. The reasons are: 1. During the co-precipitation and impregnation process, the copper and zinc basic salt precipitates formed in the γ-Al ₂ O ₃ channels are limited by the pores, and the particles are small. After calcining and decomposition, they can form highly dispersed and closely contacted particles The mixed metal oxide strengthens the synergistic effect between the components, and the methanol generated on the copper-zinc oxide can also be converted into dimethyl ether on the γ-alumina in time. 2. Infrared spectroscopy experiments show that the presence of boron strengthens the adsorption of carbon monoxide on copper, thereby promoting the methanol synthesis reaction.

Claims (4)

1. A composite catalyst for preparing dimethyl ether by synthesis gas one-step method, it is a kind of mixed metal oxide, wherein each metal mole percentage is: Copper 20～40 Zinc 10～30 The characteristic of aluminum 40-60 is that it is prepared by co-precipitation impregnation method. 2. According to the said composite catalyst of claim 1, it is characterized in that a small amount of boron is added as a cocatalyst, and its molar content is 0.5～2%. 3. a kind of preparation method of the said composite catalyst of claim 1 is characterized in that the nitrate of copper, zinc is made into concentration by the ratio of copper, zinc in the above-mentioned catalyzer with deionized water and is the mixed mixture of about 1 mole per liter. Nitrate solution, drop this solution and 1 mole per liter of sodium carbonate solution into a beaker filled with γ-alumina for co-precipitation and impregnation. The amount of γ-alumina is in line with the aluminum content in the catalyst. Deionized water infiltration, co-precipitation impregnation is carried out under constant stirring, the temperature is 50-90 ° C, the pH value is between 6-8, and the relative flow rate of the two solutions is controlled; after the feeding is stopped, continue to stir the obtained precipitate and Keep constant temperature for aging for 15-60 minutes; filter the aged precipitate and wash it with deionized water; put the washed precipitate in an oven and dry at 80-120°C for 8-16 hours; then put it in a muffle furnace at 350 Calcination at ~450°C for 8-16 hours; shape the obtained solid with a tablet press. 4. according to the said preparation method of claim 3, it is characterized in that adding the boric acid that makes the molar content of boron in the catalyst be 0.5～2% again in said copper, zinc mixed nitrate solution.