Method for preparing propylene glycol through catalytic hydrogenolysis of glycerol

Abstract

The invention provides a method for preparing propylene glycol through glycerol hydrogenolysis. According to the method, a glycerol aqueous solution and hydrogen are subjected to countercurrent contact, and a hydrogenolysis reaction is carried out in a metal hydrogenolysis catalyst bed layer. Wherein the generated light component and a small amount of glycerol enter the cold trap along with hydrogen for gas-liquid separation, and the unreacted glycerol is discharged from the bottom of the reactor as a heavy component. In the invention, the upward gas phase flow increases the disturbance inside the reactor, thereby accelerating the diffusion of glycerol and propylene glycol inside and outside the catalyst and improving the mass transfer efficiency. Meanwhile, by utilizing the characteristic that the boiling point of the generated product is relatively low, the generated 1, 2-propylene glycol and 1, 3-propylene glycol can be rapidly taken out of a reaction system by hydrogen, and are prevented from being subjected to hydrogenolysis again, so that the overall selectivity of propylene glycol is effectively improved.

Description

Technical Field

[0001] The invention belongs to the field of glycerol processing and relates to a method for preparing propylene glycol from glycerol, and specifically relates to a method for preparing propylene glycol from glycerol by catalytic hydrogenolysis. Background Art

[0002] In recent years, the vigorous development of biomass diesel has brought about a large surplus of byproduct glycerol, and the global annual production has exceeded 4 million tons. How to convert glycerol into green and high-value has become an urgent problem to be solved. Among them, the process of preparing high-value-added 1,2-propylene glycol and 1,3-propylene glycol by catalytic hydrogenolysis of glycerol is relatively simple and pollution-free, and is one of the most promising routes. Among them, the product 1,2-propylene glycol is an important raw material for the production of unsaturated polyesters, polyurethane resins, surfactants and plasticizers, and can be widely used in food, medicine, toiletries and other industries; 1,3-propylene glycol is an important bulk chemical with a large application space in cosmetics, food, adhesives, lubricants, antifreeze and medicine. One of its most important application directions is the production of polypropylene terephthalate (commonly known as PTT) by polycondensation with terephthalic acid. PTT has excellent performance and combines the advantages of existing polyesters (polyester, nylon and acrylic). It has easy processing, resilience, stain resistance, and biodegradability. It is a hot spot for the development of synthetic fibers.

[0003] Although the use of bio-glycerol for catalytic hydrogenolysis to produce propylene glycol has broad prospects from an environmental and economic perspective, this process faces some challenges. The glycerol molecule contains two primary hydroxyl groups and one secondary hydroxyl group, of which the hydrogenolysis of the primary hydroxyl group mainly produces 1,2-propylene glycol, while the selective hydrogenolysis of the secondary hydroxyl group produces 1,3-propylene glycol. In addition, the generated propylene glycol will be further deeply hydrogenolyzed to form by-products such as n-propanol and isopropanol. These factors make the catalytic hydrogenolysis of glycerol technically difficult in terms of catalyst development and process design. The reaction pathway for the preparation of propylene glycol by catalytic hydrogenolysis of glycerol is as follows: Figure 1 shown.

[0004] At present, the preparation of propylene glycol from glycerol by catalytic hydrogenolysis usually uses two processes: intermittent reactor reaction and continuous trickle fixed bed reaction. Patent CN200810120727.8 discloses a method for preparing propylene glycol by catalytic hydrogenolysis of biodiesel-based crude glycerol. The method first injects a crude glycerol solution of a certain mass concentration into a stainless steel reactor with an inner liner, then adds a catalyst in a certain proportion, and reacts under set conditions to obtain propylene glycol as a product. The intermittent reactor process has certain advantages in the glycerol hydrogenolysis reaction. Its internal stirring function ensures that the catalyst can be evenly distributed in the reaction mixture, which helps to improve the reaction efficiency and speed. However, the process also has some disadvantages. Due to the small gas-liquid contact area, the slow diffusion rate of hydrogen in the liquid will affect the reaction efficiency. In addition, the product cannot be quickly removed from the reaction system, which easily leads to the deep hydrogenolysis of the product to produce by-product propanol, which greatly affects the selectivity of the reaction. Moreover, it is difficult to achieve continuous operation in the intermittent reactor, and the product separation caused by the solid-liquid mixing system is also relatively complicated. These factors limit its application in large-scale industrial production.

[0005] Patent CN200710305964.7 discloses a method for producing 1,2-propylene glycol using bio-based glycerol. The method is to fill a fixed bed reactor with CuO-CeO 2 -SiO 2 The catalyst is used to feed the glycerol solution and hydrogen into the reactor in the above manner, and a catalytic hydrogenolysis reaction is carried out under certain conditions to obtain the product. Although the continuous trickle bed process can significantly improve the mass transfer efficiency of hydrogen, thereby improving the conversion rate of the reaction, the fixed bed reactor has no stirring characteristics, which leads to a slow diffusion rate of glycerol and propylene glycol in the catalyst. Therefore, the 1,2-propylene glycol and 1,3-propylene glycol generated during the reaction will be hydrogenolyzed again in the catalyst bed to form a large amount of by-product propanol.

[0006] In summary, whether the intermittent reactor process or the continuous trickle bed process is used, the target product propylene glycol will be hydrogenolyzed to propanol again, making it difficult to achieve a high selectivity. At the same time, the secondary hydrogenolysis of the target product will also force the conversion rate of glycerol to decrease, and a large amount of unconverted glycerol needs to be separated and then reacted, which increases energy consumption. Summary of the invention

[0007] In order to solve the above problems existing in the prior art, the present invention provides a method for preparing propylene glycol by catalytic hydrogenolysis of glycerol. The method adopts a two-phase countercurrent process to effectively alleviate the slow diffusion problem of glycerol and propylene glycol inside and outside the catalyst, and can make the generated 1,2-propylene glycol and 1,3-propylene glycol quickly taken out of the reaction system by hydrogen, thereby reducing the possibility of further hydrogenolysis of the target product, so that the selectivity of propylene glycol reaches more than 76%. At the same time, due to the high boiling point of glycerol, most of the unreacted glycerol is discharged from the bottom of the reactor and directly recovered for reuse, thereby reducing the energy consumption caused by subsequent separation.

[0008] The method for preparing propylene glycol by catalytic hydrogenolysis of glycerol of the present invention comprises:

[0009] (1) hydrogen gas and glycerol aqueous solution are introduced into the reactor from the lower part and the upper part of the reactor respectively;

[0010] (2) the glycerol and hydrogen are contacted in the hydrogenation zone of the reactor to undergo a catalytic hydrogenolysis reaction to obtain a gas phase product and a liquid phase product;

[0011] (3) Separating the above gas phase products to obtain hydrogen, propylene glycol and glycerol.

[0012] In some embodiments, the hydrogen in step (3) is returned to step (1).

[0013] In some embodiments, the liquid product in step (2) is discharged from the bottom of the reactor and returned to step (1).

[0014] In some embodiments, the glycerol aqueous solution in step (1) is a 40-80 wt % glycerol aqueous solution, preferably a 50-70 wt % glycerol aqueous solution.

[0015] In some embodiments, the reactor in step (1) is a convection bed reactor.

[0016] In some embodiments, the molar ratio of the feed hydrogen to glycerol in step (1) is 10-100:1.

[0017] In some embodiments, the position where the glycerol enters the reactor in step (1) is always higher than the position where the hydrogen enters the reactor.

[0018] In some embodiments, the hydrogenation area in step (2) is a catalyst loading area.

[0019] In some further preferred embodiments, the catalyst loading area is based on the glycerol aqueous solution feed port, and the catalyst loading amount above the glycerol aqueous solution feed port is maintained below 30% of the total amount, preferably below 20%.

[0020] In some further preferred embodiments, the catalyst loading area is above the position where hydrogen enters the reactor.

[0021] In some further preferred embodiments, the catalyst is a hydrogenation catalyst commonly used in the art, preferably one of a copper-based catalyst, a platinum-based catalyst and an iridium-rhenium catalyst, and is prepared according to a preparation method recognized in the art.

[0022] In some further preferred embodiments, the particle size of the catalyst is 10-100 mesh, preferably 20-40 mesh.

[0023] The following is an example of the composition of platinum-based catalysts and their preparation methods. Platinum-based catalysts are mainly composed of three parts: hydrogenation component platinum, metal oxide promoter and carrier. The metal oxide promoter can be WO 3 , MgO, MnO or more; the carrier can be ZrO 2 、Al 2 O 3 、SiO 2 、TiO 2 The loading amount of platinum in the platinum-based catalyst is 1wt%-5wt% (relative to the carrier oxide); the loading amount of the oxide promoter is 5wt%-15wt% (relative to the carrier oxide).

[0024] The preparation method of platinum-based catalyst takes the impregnation method as an example. The catalyst is Pt-WO 3 / Al 2 O 3 For example, the preparation method is as follows: prepare a tungsten salt solution of a certain concentration and pour it into Al 2 O 3 The catalyst is impregnated on the carrier for 12-24 hours; after the impregnation, the carrier is dried in an oven at 80-120°C for 12-24 hours; then, the dried catalyst is calcined in a furnace at 350-700°C for 3-6 hours; a platinum salt solution of a certain concentration is prepared and poured into the calcined catalyst for 12-24 hours; after the impregnation, the catalyst is dried in an oven at 80-120°C for 12-24 hours; finally, the dried catalyst is calcined in a furnace at 350-550°C for 3-6 hours.

[0025] In some embodiments, the other parts of the reactor are filled with a carrier. Preferably, the carrier may be ZrO 2 、Al 2 O 3 、SiO 2 、TiO 2 More preferably, the other parts of the reactor are filled with carrier quartz sand. The particle size of the carrier is 10-100 mesh, preferably 20-40 mesh.

[0026] In some embodiments, the catalytic hydrogenation reaction conditions in step (2) are that the reaction space velocity of glycerol is 0.08h-0.5h -1 ; The reaction pressure is 2-8MPa, preferably 4-6MPa; the reaction temperature is 100-200℃, preferably 130-170℃.

[0027] In some embodiments, the gas phase product in step (2) is discharged from the top of the reactor.

[0028] In some embodiments, the gas phase product separation method in step (3) is cooling separation, specifically separation at a temperature of 30-70° C. to obtain hydrogen, propylene glycol and glycerol. The hydrogen can be returned to the hydrogenolysis reaction.

[0029] In some embodiments, the recycled hydrogen is mixed with the fresh hydrogen and then enters the reactor together to participate in the reaction.

[0030] In some embodiments, after the gas phase product is separated in step (3), by-products are obtained. Preferably, the by-products include isopropanol and n-propanol.

[0031] In some embodiments, the liquid product mainly contains glycerol and a small amount of propylene glycol, which can be formulated into a glycerol aqueous solution and re-enter the reactor to participate in the reaction.

[0032] The method for preparing propylene glycol by catalytic hydrogenolysis of glycerol of the present invention has the following beneficial effects:

[0033] (1) The method of the present invention utilizes two-phase countercurrent contact to carry out hydrogenolysis reaction. In this process, the upward gas phase can increase the disturbance inside the reactor, thereby accelerating the diffusion of glycerol and propylene glycol inside and outside the catalyst, and improving the mass transfer efficiency.

[0034] (2) The method of the present invention utilizes the relatively low boiling point of the generated product. The hydrogen can quickly remove the generated propylene glycol from the reaction system to avoid its re-hydrogenolysis, thereby effectively improving the overall selectivity of propylene glycol.

[0035] (3) The method of the present invention, by reasonably controlling the hydrogen flow rate, effectively avoids the situation where the reaction residence time is shortened due to excessive flow rate, and a large amount of unreacted glycerol is carried out, thereby reducing the reaction efficiency; and the situation where the propylene glycol cannot be carried out in time due to too small flow rate, resulting in secondary hydrogenolysis. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The accompanying drawings are a part of the specification and together with the detailed description, they provide further explanation of the present invention but are not intended to limit the present invention.

[0037] Figure 1 This is the reaction route of glycerol hydrogenolysis.

[0038] Figure 2 This is a schematic diagram of the process for preparing propylene glycol by catalytic hydrogenolysis of glycerol.

[0039] Attached Figure 2 The symbols are as follows: 1-aqueous glycerol solution, 2-hydrogen, 3-catalyst bed, 4-reactor, 5-cold trap, 6-hot trap, 7-light component, 8-heavy component. DETAILED DESCRIPTION

[0040] The specific embodiments of the present invention are described in detail below. The specific embodiments described here are only used to illustrate and explain the present invention, but are not used to limit the present invention.

[0041] The present invention is further described below by way of examples, but the present invention is not limited thereto.

[0042] Example

[0043] Unless otherwise specified, the reagents, materials and devices involved in the following examples are all commercially available in the art; the conventional operations involved in the following examples can be found in patents, patent applications and publications disclosed in the art (for example, He Yongde, ed., "Modern Coal Chemical Technology Handbook", Chemical Industry Press, 2003, but not limited thereto).

[0044] Example 1

[0045] Weigh 20-40 mesh Pt-WO 3 / Al 2 O 3 Catalyst (2wt% Pt, 10wt% WO 3 )20g, placed in the constant temperature section in the middle of the convection bed reactor, the catalyst bed is located between the gas phase and liquid phase feed ports. The rest of the reactor is filled with 20-40 mesh quartz sand. The catalyst needs to be activated and reduced first, and the temperature is set to 300°C. After the reduction is completed, the reaction temperature is set to 150°C, the reaction pressure is 5MPa, and the hydrogen flow rate is 100ml / min. At the same time, 60wt% glycerol solution is pumped into the reactor at a flow rate of 0.1ml / min, and sampling and analysis are carried out after the reaction continues for 24 hours. The reaction results are shown in Table 1.

[0046] Example 2

[0047] Weigh 20-40 mesh Pt-WO 3 / Al 2 O 3 Catalyst (2wt% Pt, 10wt% WO 3)20g, placed in the constant temperature section in the middle of the convection bed reactor, higher than the position where hydrogen enters the reactor, and based on the glycerol aqueous solution feed port, the catalyst loading above the feed port is 20% of the total catalyst amount. The rest of the reactor is filled with 20-40 mesh quartz sand. The catalyst needs to be activated and reduced first, and the temperature is set to 300°C. After the reduction is completed, the reaction temperature is set to 150°C, the reaction pressure is 5MPa, and the hydrogen flow rate is 100ml / min. At the same time, 60wt% glycerol solution is pumped into the reactor at a flow rate of 0.05ml / min, and sampling and analysis are performed after the reaction continues for 24 hours. The reaction results are shown in Table 1.

[0048] Example 3

[0049] Weigh 20-40 mesh Pt-WO 3 / Al 2 O 3 Catalyst (2wt% Pt, 10wt% WO 3 )20g, placed in the constant temperature section in the middle of the convection bed reactor, higher than the position where hydrogen enters the reactor, and based on the glycerol aqueous solution feed port, the catalyst loading above the feed port is 20% of the total catalyst amount. The rest of the reactor is filled with 20-40 mesh quartz sand. The catalyst needs to be activated and reduced first, and the temperature is set to 300°C. After the reduction is completed, the reaction temperature is set to 150°C, the reaction pressure is 5MPa, and the hydrogen flow rate is 200ml / min. At the same time, 60wt% glycerol solution is pumped into the reactor at a flow rate of 0.05ml / min, and sampling and analysis are performed after the reaction continues for 24 hours. The reaction results are shown in Table 1.

[0050] Example 4

[0051] Weigh 20-40 mesh Pt-WO 3 / Al 2 O 3 Catalyst (2wt% Pt, 10wt% WO 3 )20g, placed in the constant temperature section in the middle of the convection bed reactor, higher than the position where hydrogen enters the reactor, and based on the glycerol aqueous solution feed port, the catalyst loading above the feed port is 20% of the total catalyst amount. The rest of the reactor is filled with 20-40 mesh quartz sand. The catalyst needs to be activated and reduced first, and the temperature is set to 300°C. After the reduction is completed, the reaction temperature is set to 150°C, the reaction pressure is 5MPa, and the hydrogen flow rate is 300ml / min. At the same time, 60wt% glycerol solution is pumped into the reactor at a flow rate of 0.05ml / min, and sampling and analysis are performed after the reaction continues for 24 hours. The reaction results are shown in Table 1.

[0052] Example 5

[0053] Weigh 20-40 mesh Pt-WO 3 / Al 2 O 3 Catalyst (2wt% Pt, 10wt% WO 3 )20g, placed in the constant temperature section in the middle of the convection bed reactor, higher than the position where hydrogen enters the reactor, and based on the glycerol aqueous solution feed port, the catalyst loading above the feed port is 20% of the total catalyst amount. The rest of the reactor is filled with 20-40 mesh quartz sand. The catalyst needs to be activated and reduced first, and the temperature is set to 300°C. After the reduction is completed, the reaction temperature is set to 150°C, the reaction pressure is 5MPa, and the hydrogen flow rate is 600ml / min. At the same time, 60wt% glycerol solution is pumped into the reactor at a flow rate of 0.05ml / min, and sampling and analysis are performed after the reaction continues for 24 hours. The reaction results are shown in Table 1.

[0054] Comparative Example 1

[0055] Weigh 20-40 mesh Pt-WO 3 / Al 2 O 3 Catalyst (2wt% Pt, 10wt% WO 3 )10g, placed in the constant temperature section in the middle of the trickle bed reactor, and the rest of the reactor was filled with quartz sand of 20-40 mesh. The catalyst needs to be activated and reduced first, and the temperature is set to 300°C. After the reduction is completed, the reaction temperature is set to 150°C, the reaction pressure is 5MPa, and the hydrogen flow rate is 300ml / min. At the same time, a 60wt% glycerol aqueous solution is pumped into the reactor at a flow rate of 0.02ml / min, and the reaction is continued for 24 hours before sampling and analysis. The reaction results are shown in Table 1.

[0056] Table 1 Results of preparing propylene glycol by hydrogenolysis of glycerol

[0057]

[0058] According to the reaction results of Examples 1-5, the total selectivity of preparing propylene glycol by hydrogenolysis of glycerol in a convection bed reactor is relatively high. In Example 1, the catalyst bed is located between the gas phase and liquid phase feed ports, so the conversion rate of glycerol is relatively low. In Examples 2-5, the catalyst bed is appropriately moved upward to increase the conversion rate of glycerol, reaching a maximum of 57.6%. In Example 2, the amount of hydrogen is relatively small, resulting in an increase in the content of n-propanol and isopropanol. After gradually increasing the amount of hydrogen in Examples 3-4, the generated 1,2-propylene glycol and 1,3-propylene glycol are promptly taken out of the reaction system, thereby avoiding secondary hydrogenolysis reactions, and ultimately the total selectivity of propylene glycol reaches 76%. The reaction of excess hydrogen in Example 5 results in a significant decrease in the conversion rate.

[0059] The results of Comparative Example 1 show that in the trickle bed reaction, the product contains a large amount of secondary hydrogenolysis product n-propanol (32.8%), and the conversion efficiency of the reaction is relatively low.

[0060] The above is only a preferred embodiment of the present invention. It should be pointed out that ordinary technicians in this technical field can make several improvements and supplements without departing from the method of the present invention. These improvements and supplements should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing propylene glycol by catalytic hydrogenolysis of glycerol, comprising: (1) hydrogen gas and glycerol aqueous solution are introduced into the reactor from the lower part and the upper part of the reactor respectively; (2) the glycerol and hydrogen are contacted in the hydrogenation zone of the reactor to undergo a catalytic hydrogenolysis reaction to obtain a gas phase product and a liquid phase product; (3) Separating the above gas phase products to obtain hydrogen, propylene glycol and glycerol.

2. The method according to claim 1, characterized in that The hydrogen in step (3) is returned to step (1).

3. The method according to claim 1, characterized in that The liquid product in step (2) is discharged from the bottom of the reactor and returned to step (1).

4. The method according to claim 1, characterized in that: The glycerol aqueous solution in step (1) is a 40-80wt% glycerol aqueous solution, preferably a 50-70wt% glycerol aqueous solution.

5. The method according to claim 1, characterized in that: The reactor in step (1) is a convection bed reactor.

6. The method according to claim 1, characterized in that The molar ratio of the feed hydrogen to glycerol in step (1) is 10-100:

1.

7. The method according to claim 1, characterized in that The hydrogenation area in step (2) is a catalyst loading area; the catalyst loading area is based on the glycerol aqueous solution feed port, and the catalyst loading amount above the glycerol aqueous solution feed port is maintained below 30% of the total amount, preferably below 20%.

8. The method according to claim 1, characterized in that The catalyst loading area is above the point where hydrogen enters the reactor.

9. The method according to claim 1, characterized in that: The catalytic hydrogenation reaction conditions in step (2) are as follows: the reaction space velocity of glycerol is 0.08h-0.5h -1 ; The reaction pressure is 2-8MPa, preferably 4-6MPa; the reaction temperature is 100-200℃, preferably 130-170℃.

10. The method according to claim 1, characterized in that The gas phase product in step (2) is discharged from the top of the reactor.

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