Patent Application
20190275505
The present invention belongs to preparation of light olefins using syngas, and particularly relates to a catalyst and a method for preparing light olefins from direct conversion of syngas.
Light olefins refer to alkenes with the number of carbon atoms less than or equal to 4. Light olefins represented by ethylene and propylene are very important basic organic chemical raw materials. With the fast growth of economy in China, the market of the light olefins is in short supply for a long time. At present, the light olefins are produced mainly through a petrochemical route of cracking of light hydrocarbons (ethane, naphtha and light diesel fuel). Due to the increasing shortage of global petroleum resources and the long-term high price of crude oil, the development of the light olefins industry relying only on a tubular cracking furnace technology based on petroleum light hydrocarbons as raw material will encounter more and more difficulties in raw material. The technology and the raw material for producing the light olefins must be diversified. The source of the raw material can be widen by technologies based on syngas for the production of light olefins, which can be derived from crude oil, natural gas, coal and renewable materials, thus provide an alternative solution for the steam cracking technology based on the high-cost raw materials like naphtha. One-step direct preparation of the light olefins using the syngas is a process of directly preparing the light olefins with the number of carbon atoms less than or equal to 4 through Fischer-Tropsch synthesis reaction of carbon monoxide and hydrogen under the action of the catalyst. This process simplifies the process flow and greatly reduces the investment unlike an indirect method that further prepares the alkene from the syngas through the methanol or dimethyl ether.
Direct preparation of light olefins using syngas through Fischer-Tropsch synthesis has become one of research hotspots in the development of catalysts for Fischer-Tropsch synthesis. In patent CN1083415A disclosed by Dalian Institute of Chemical Physics, Chinese Academy of Sciences, high activity (CO conversion rate: 90%) and selectivity (light olefins selectivity: 66%) can be obtained under reaction pressure of 1.0 to 5.0 MPa and reaction temperature of 300 to 400° C., catalyzed by an iron-manganese catalyst system, with IIA alkali metal oxide such as MgO or silica rich zeolite (or phosphorous-aluminum zeolite) as the support, and alkali K or Cs ion as the auxiliary. In patent ZL03109585.2 declared by Beijing University of Chemical Technology, Fe/activated carbon catalyst with manganese, copper, zinc, silicon or potassium as auxiliaries is prepared by a vacuum impregnation method for the reaction of preparation of the light olefins from the syngas. Under this catalyst, the CO conversion rate is 96%, and the selectivity of light olefins in hydrocarbons is 68%, with no feedstock gas circulation. Recently, Professor de Jong's team at Utrecht University in Netherlands made good progress by using Fe catalyst modified byFe, Naor S, and supported on SiC, carbon nanofiber or other inert carriers, obtained 61% of selectivity of light olefins. However, the selectivity of light olefins will reduce when the syngas conversion increases. In direct preparation of alkenes from syngas, cryogenic separation is generally needed due to the gaseous raw material and low-boiling ethylene product. If C3-C4 alkenes, i.e., propylene and butylene, can be obtained with high selectivity, cryogenic separation will not be needed, thereby greatly reducing energy consumption and cost for product separation and bringing great application value. In the above reports, metal iron or iron carbide was applied as the active component of the catalyst, and the reactions followed the carbon chain growth mechanism on metal surfaces. The selectivity of the product light olefins is low, while the selectivity of C3-C4 alkene is lower.
Recently, a bifunctional catalyst with the composite of ZnCr2O4 oxide and hierarchical pore SAPO-34 zeolite has been reported by Dalian Institute of Chemical Physics, Chinese Academy of Sciences (Jiao et al., Science 351 (2016) 1065-1068), which realized 80% of the selectivity of light olefins when the CO conversion was 17%. The selectivity of light paraffins was 14% so the olefin/paraffin ratio of light hydrocarbons (o/p) was 5.7. When CO conversion increased to 35%, the selectivity of light olefins was 69%, the selectivity of light paraffins was 20%, making an o/p of 3.5, and propylene and butylene selectivity was 40-50%.
In view of the above problems, the present invention provides a catalyst and a method for preparing light olefins by direct conversion of syngas.
The technical solution of the present invention is as follows:
A catalyst, characterized in that, the catalyst is a composite catalyst A+B and is formed by compounding catalyst A and catalyst B in a mechanical mixing mode; the active ingredient of the catalyst A is active metal oxide; the catalyst B is a zeolite of CHA and/or AEI topology; and the active metal oxide is one or more than one of MnO, MnCr2O4, MnAl2O4, MnZrO4, ZnO, ZnCr2O4, ZnAl2O4, CoAl2O4 and FeAl2O4.
A spacing between geometric centers of the active metal oxide of the catalyst A and the particle of the catalyst B is 5 μm-40 mm. A spacing between axes of the particles is preferably 100 μm-5 mm, and more preferably 200 μm-4 mm.
A weight ratio of the active ingredients in the catalyst A and the catalyst B is within a range of 0.1-20 times, and preferably 0.3-5.
The active metal oxide is composed of grains with a size of 5-30 nm, and a large amount of oxygen vacancies exist within a distance range of 0.3 nm from the surfaces of the grains to the internal direction of the grains, i.e., the molar weight of oxygen atoms occupies a value less than 80% compared with the theoretical stoichiometric ratio; and preferably, the molar weight of oxygen atoms occupies a value of 80%-10% of the oxygen molar content in theoretical stoichiometric ratio, more preferably 60%-10% and most preferably 50%-10%. The surface oxygen vacancies are defined as: (1−the molar weight of oxygen atoms in theoretical stoichiometric ratio of oxygen molar weight); and corresponding oxygen vacancy concentration is preferably 20%-90%, more preferably 40%-90% and most preferably 50%-90%.
A dispersing agent is also added to the catalyst A; the dispersing agent is one or more than one of Al2O3, SiO2, Cr2O3, ZrO2 and TiO2; the active metal oxide is dispersed in the dispersing agent; and the content of the dispersing agent in the catalyst A is 0.05-90 wt %, and the balance is the active metal oxide.
The catalyst component B is a zeolite of CHA and/or AEI topology. The CHA and/or AEI zeolite has eight-membered ring orifices and a three-dimensional porous channel and comprises cha cage.
The skeleton element composition of the zeolite of CHA and AEI topologies may be one or more than one of Si—O, Si—Al—O, Si—Al—P—O, Al—P—O, Ga—P—O, Ga—Si—Al—O, Zn—Al—P—O, Mg—Al—P—O and Co—Al—P—O.
H may be connected or not connected to the O element of the zeolite skeleton. The H may be entirely or partially replaced by one or more than one of Na, Ca, K, Mg, Ge, Zr, Zn, Cr, Ga. Sn, Fe, Co, Mo and Mn by ion exchange; and the total molar ratio of the substitute metal to oxygen is 0.0002-0.001.
A molar ratio of (Si+Zn+Mg+Co) to (Al+Ga) in the zeolite composition Si—O of the CHA topology and in the skeleton element composition outside is less than 0.6.
A molar ratio of (Si+Zn+Mg+Co) to (Al+Ga) in the zeolite composition Si—O of the AEI topology and in the skeleton element composition outside is less than 0.6.
The zeolite has the amount of medium-strength acidic sites of 0-0.3 mol/kg, preferably 0.003-0.2 mol/kg, and more preferably 0.003-0.06 mol/kg, wherein the peak temperature range corresponding to the desorption peak of NH3-TPD for mediate strong acid is 275-500° C., and preferably 275-370° C.
The acid strength is defined by the peak temperature of NH3-TPD, including three kinds of acid:weak acid, medium-strength acid and strong acid.
The NH3-TPD is according to the position of a desorption peak of NH3; the position of the desorption peak means that under standard test conditions that a ratio of sample mass w and carrier gas flow rate f (w/f) is 100 g·h/L and a heating rate is 10° C./min, a TCD records a thermal conductivity signal of desorption of NH3 and draws a desorption curve; according to the peak temperatures of the NH3 desoption curve, the acid strength of inorganic solid is divided into three kinds; the weak acid is an acid site where the deposition temperature of NH3 is less than 275° C.; the medium-strength acid is an acid site where the deposition temperature of NH3 is between 275° C. and 500° C.; and the strong acid is an acid site where the deposition temperature of NH3 is greater than 500° C.
The mechanical mixing can adopt one or more than one of mechanical agitation, ball milling, rocking bed mixing and mechanical grinding for composition.
A method for preparing light olefins using direct conversion of syngas, wherein syngas is used as reaction raw material; a conversion reaction is conducted on a fixed bed or a moving bed; and the adopted catalyst is the catalyst of any one of claims 1-8.
The pressure of the syngas is 0.5-10 MPa; reaction temperature is 300-600° C.; and space velocity is 300-10000 h−1.
The ratio of syngas H2/CO for reaction is 0.2-3.5, and preferably 0.3-2.5.
The dual-function composite catalyst is used for preparing lower alkene using one-step direct conversion of syngas, wherein the selectivity for propylene and butylene is 40-75%, and preferably 50-75%, while the selectivity for methane side product is lower than 15%, and preferably less than 10%.
The present invention has the following beneficial effects that:
Different from the traditional technology for preparing the light olefins through methanol (MTO for short), this technology realizes the preparation of light olefins through one-step direct conversion of syngas.
Propylene and butylene selectivity is as high as 40-75%. The products can be separated without deep cooling, thereby greatly reducing separation energy consumption and cost.
The composite catalyst in the patent is simple in preparation process and mild in conditions. The creaction process has an extremely high product yield and selectivity, with the selectivity for C2-C4 light olefins reaching 50-90% and especially high selectivity for C3-C4 alkenes. Meanwhile, the selectivity for methane side product is low (<15%), and the catalyst has a long lifetime greater than 700 hours. The present invention has excellent application prospect.
The present invention is further illustrated below by embodiments, but the scope of claims of the present invention is not limited by the embodiments. Meanwhile, the embodiments only give some conditions for achieving the purpose, but it doesn't mean that the conditions must be satisfied to achieve the purpose.
I. Preparation of Catalyst A
(I) Synthesizing ZnO Material with Polar Surface Through an Etching Method:
(1) weighing 0.446 g (1.5 mmol) of Zn(NO3)2.6H2O; weighing 0.480 g (12 mmol) of NaOH and adding to the above container; weighing 30 ml of deionized water and adding to the container; stirring for a time greater than 0.5 h to uniformly mix a solution; increasing the temperature to 160° C. with the reaction time of 20 h; decomposing precipitate into zinc oxide; naturally cooling to room temperature; centrifugally separating reaction liquid to collect the centrifugally separated precipitate; and washing with deionized water twice to obtain ZnO oxide;
(2) ultrasonically mixing an etching agent with ZnO oxide uniformly under normal temperature; immersing the ZnO oxide in the solution of the etching agent; and generating a complexing or direct reduction reaction by the etching agent and the zinc oxide; and
heating the above suspended matter; then taking out the suspended matter for washing and filtering the suspended matter to obtain active nano ZnO material having a large amount of surface oxygen holes.
In Table 1: the mass ratio of the catalyst to the etching agent is 1:3. The mass ratio of the oleic acid to the hexamethylenetetramine is 1:1, without solvent. The mass ratio of the oleic acid to the hydrazine hydrate is 95:5, without solvent. Specific treatment conditions include temperature, treatment time and atmosphere types as shown in Table 1 below.
(3) Drying or Drying and Reducing:
after centrifuging or filtering the above obtained products and washing the products with deionized water, drying or drying and reducing the products in an atmosphere which is inert atmosphere gas or a gas mixture of inert atmosphere gas and a reducing atmosphere, wherein the inert atmosphere gas is one or more than one of N2, He and Ar, the reducing atmosphere is one or both of H2 and CO, a volume ratio of the inert atmosphere gas to the reducing gas in the drying and reducing gas mixture is 100/10-0/100, the temperature of drying and reducing is 350° C., and time is 4 h. ZnO material with abundant oxygen vacancies on the surface is obtained. Specific samples and preparation conditions thereof are shown in Table 1 below. The oxygen vacancies on the surface are: 100%-percent of the molar weight of oxygen atoms in theoretical stoichiometric ratio of oxygen molar weight.
The surface oxygen vacancies are the percent of the molar weight of oxygen atoms in theoretical stoichiometric ratio of oxygen molar content within a distance range of 0.3 nm from the surfaces of the grains to the internal direction of the grains. The surface oxygen vacancies are defined as: 100%-percent of the molar weight of oxygen atoms in theoretical stoichiometric ratio of oxygen molar weight.
As a reference example, ZnO 9 which is not etched in step (2) and has no oxygen vacancy on the surface; and metal Zn 10 by completely reducing Zn. (II) Synthesizing MnO material with polar surface through an etching method: the preparation process is the same as that of the above ZnO. The difference is that, the precursor of Zn is changed for the corresponding precursor of Mn, which is one of manganous nitrate, manganese chloride and manganese acetate (manganous nitrate herein).
The etching process is the same as step (2) in above (I), and the process of drying or drying and reducing is the same as the preparation processes of products ZnO 3, ZnO 5 and ZnO 8 in step (3) in above (I). The catalyst having a great number of surface oxygen vacancies is synthesized. The surface oxygen vacancies are 67%, 29% and 27%.
Corresponding products are defined as MnO 1-3.
(III) Synthesizing Nano ZnCr2O4, ZnAl2O4, MnCr2O4, MnAl2O4 and MnZrO4 Spinel with High Specific Surface Area and High Surface Energy:
selecting corresponding nitrate, zinc nitrate, aluminum nitrate, chromic nitrate and manganous nitrate as precursors according to chemical composition of the spinel, and mixing the precursors with urea at room temperature in water; aging the above mixed liquid; then taking out the mixed liquid for washing, filtering and drying the obtained precipitants; and calcining the obtained solid under an air atmosphere to obtain spinel oxide which grows along the (110) crystal plane direction. The sample is also treated by the etching method to synthesize the catalyst with a great number of surface oxygen vacancies. The etching process and aftertreatment process are the same as step (2) and step (3) in above (I). The sample has large specific surface area and many surface defects, and can be applied to catalyzing the conversion of syngas.
Specific samples and preparation conditions thereof are shown in Table 2 below. Similarly, the surface oxygen vacancies are defined as: 100%-percent of the molar weight of oxygen atoms in theoretical stoichiometric ratio of oxygen molar weight.
(IV) Cr2O3, Al2O3 or ZrO2 Dispersed Active Metal Oxide
Preparing Cr2O3, Al2O3 or ZrO2 dispersed active metal oxide through a precipitate deposition method by taking Cr2O3, Al2O3 or ZrO2 as carriers. Taking the preparation of dispersed ZnO as an example, commercial Cr2O3, Al2O3 or ZrO2 carrier is dispersed in a base solution in advance, and then one or more than one of zinc acetate, zinc nitrate, zinc sulfate and other Zn precursors are taken as Zn raw material, mixed with one or more than one of sodium hydroxide, ammonium bicarbonate, ammonium carbonate and sodium bicarbonate, and precipitated at room temperature. Herein, taking zinc nitrate and sodium hydroxide as an example, the molar concentration of Zn2+ in the reaction liquid is 0.067M; the ratio of molar fractions of Zn2+ and precipitant may be 1:8; and then aging is conducted at 160° C. for 24 hours to obtain carrier Cr2O3, Al2O3 or ZrO2 dispersed ZnO oxide, and the contents of the dispersing agents in catalyst A are 0.1 wt %, 10 wt % and 90 wt %.
The etching process is the same as the preparation processes of products ZnO 3, ZnO 5 and ZnO 8 in step (2) in above (I). The catalyst having a great number of surface oxygen vacancies is synthesized. The surface oxygen vacancies are 65%, 30% and 25%. The aftertreatment process is the same as step (3) in above (I).
Corresponding products from top to bottom are defined as dispersed oxides 1-3.
The same method is used to obtain carrier Cr2O3, Al2O3 or ZrO2 dispersed MnO oxide, wherein the contents of the dispersing agents in catalyst A are 5 wt %, 30 wt % and 60 wt %. The surface oxygen vacancies are 62%, 27% and 28%. Corresponding products from top to bottom are defined as dispersed oxides 4-6.
II. Preparation of Catalyst B (Zeolite of CHA and AEI Topologies):
The CHA and/or AEI topology has eight-membered ring orifices and a three-dimensional porous channel and comprises cha cage.
1) The Specific Preparation Process is as Follows:
The raw materials of 30% silica sol (mass concentration), AlOOH, phosphoric acid, TEA (R) and deionized water are weighed according to the oxide SiO2:Al2O3:H3PO4:R:H2O=1.6:16:32:55:150 (mass ratio); after mixing at room temperature, auxiliary HF is added with a molar weight of 0.5 time of the template agent; the mixture is stirred and aged at 30° C. for 2 h, and transferred into a hydrothermal reactor and crystallized at 200° C. for 24 h. The autoclave is quenched by water bath to room temperature. Centrifugal washing is conducted repeatedly until the pH of the supernatant is 7 at the end of washing. After the precipitate is dried at 110° C. for 17 h, the precipitate is calcined in air at 600° C. for 3 h to obtain the silicon-phosphorus-aluminum inorganic solid acid with hierarchical pore structure.
The skeleton element composition of the zeolite of CHA and AEI topologies may be one or more than one of Si—O, Si—Al—O, Si—Al—P—O, Al—P—O, Ga—P—O, Ga—Si—Al—O, Zn—Al—P—O, Mg—Al—P—O and Co—Al—P—O.
O element of part of the skeleton is connected with H, and corresponding products are successively defined as zeolites 1-7.
2) The H connected to the O element of skeletons of the above products 1-7 is partly replaced by the following metal ions: Na, Ca, K, Mg, Ge, Zr, Zn, Cr, Ga. Sn, Fe, Co, Mo and Mn by ion exchange; and the preparation process is:
SiO2:Al2O3:H3PO4:R:H2O=1.1:16:32:55:150 (molar ratio), wherein R is the template agent.
The aluminum sulphate is mixed with the sodium hydroxide solution, and then silica sol, phosphoric acid, TEA(R) and deionized water are added and stirred for 1 h to obtain initial gel with uniform phase. Then, the mixture is transferred into a synthesis autoclave, is statically crystallized at 165° C. for 80 h, and then is quenched, washed and dried to obtain a zeolite sample. The above samples are then mixed with 0.5 mol/L of metal ion nitrate solution to be exchanged with the solid-liquid mass ratio of 1:30. The mixture is stirred at 80° C. for 6 h, washed and dried. The exchange procedure is conducted twice continuously, and the as-prepared powder is calcined at 550° C. for 3 h to obtain CHA or AEI zeolite after metal ion exchange.
Corresponding products are successively defined as zeolites 9-22.
III. Catalyst Preparation
The catalyst A and the catalyst B in the required ratio are added to the container to achieve the purposes of separation, crushing, uniform mixing and the like, through one or more than one of extrusion force, impact force, shear force and friction force generated by high-speed motion of the material and/or the container, and realize conversion among mechanical energy, thermal energy and chemical energy by regulating the temperature and the atmosphere of carrier gas, thereby further enhancing the interaction between different components.
In the mechanical mixing process, the mixing temperature can be set as 20-100° C., and the mechanical mixing process can be conducted in an atmosphere or directly in the air. The atmosphere is one or more of: a) nitrogen and/or inert gas; b) mixed gas of hydrogen, nitrogen and/or inert gas, with the volume ratio of hydrogen in the mixed gas being 5-50%; c) mixed gas of carbon monoxide, nitrogen and/or inert gas, with the volume ratio of carbon monoxide in the mixed gas being 5-20%; and d) mixed gas of oxygen, nitrogen and/or inert gas, with the volume ratio of oxygen in the mixed gas being 5-20%. The inert gas is one or more of helium, argon and neon.
Mechanical stirring: mixing the catalyst A and the catalyst B with a stirring rod in a stirring tank; and regulating the mixing degree and the relative distance of the catalyst A and the catalyst B by controlling stirring time (5 min-120 min) and rate (30-300 r/min).
Ball milling: Rolling the abrasive and the catalysts at a high speed in a grinding tank thus producing strong impact and milling on the catalysts to achieve the effects of dispersing and mixing the catalyst A and the catalyst B. The ratio of the abrasive (which is stainless steel, agate and quartz; and the size range is 5 mm-15 mm) to the catalysts (the mass ratio scope is 20-100:1) is controlled to regulate the particle size and the relative distance of the catalysts.
Shaking table mixing: premixing the catalyst A and the catalyst B and placing the catalysts into the container; realizing the mixing of the catalyst A and the catalyst B by controlling the reciprocating oscillation or circumferential oscillation of the shaking table; and realizing uniform mixing and regulating the relative distance by regulating oscillation speed (range: 1-70 r/min) and time (range: 5 min-120 min).
Mechanical grinding: premixing the catalyst A and the catalyst B and placing the catalysts into a container; and under a certain pressure (range: 5 kgf/cm2-20 kgf/cm2), making the ground and the mixed catalysts do relative motion (speed range: 30-300 r/min) to achieve the effects of regulating the particle size and the relative distance of the catalysts and realizing uniform mixing.
Specific catalyst preparation and parameter features are shown in Table 6.
Example of Catalytic Reactions
A fixed bed reaction is taken as an example, but the catalyst is also applicable to a fluidized bed reactor. The apparatus is equipped with gas mass flow meters and online product analysis chromatography (the tail gas of the reactor is directly connected with the metering valve of chromatography, and thus periodic and real-time sampling and analysis will be achieved).
2 g of the above catalyst in the present invention is placed in a fixed bed reactor. The air in the reactor is replaced with Ar; and then the temperature is raised to 300° C. in the H2 atmosphere, and then the inlet gas is switched to the syngas (H2/CO molar ratio=0.2-3.5). The pressure of the syngas is 0.5-10 MPa. The temperature is raised to reaction temperature of 300-600° C., and the space velocity of the reaction raw gas is regulated to 500-8000 ml/g/h. On-line chromatography is used to detect and analyze the product.
The reaction performance can be changed by changing the temperature, pressure, space velocity and H2/CO molar ratio in the syngas. The sum selectivity of propylene and butylene is 30-75%. The selectivity of light olefins (the sum of ethylene, propylene and butylene) is 50-90%. Due to the low hydrogenation activity of the surface of the metal composite of the catalyst, a large amount of methane will be avoided and the selectivity of methane is low. Table 7 lists specific application and effect data of the catalysts.
In reference example 1, the catalyst component A is ZnO 9, and component B is Zeolite 1.
In reference example 2, the catalyst component A is Zn 10, and component B is Zeolite 1.
The component A in the catalyst adopted in reference example 3 is metal ZnCo+Zeolite 1. The molar ratio of Zn to Co is 1:1. The mass ratio of ZnCo to Zeolite 1 is 1:1. Other parameters and the mixing process are the same as those of catalyst A.
The catalyst adopted in reference example 4 is TiO2 without surface oxygen vacancy+Zeolite 1. Other parameters and the mixing process are the same as those of catalyst A.
The zeolite in the catalyst adopted in reference example 5 is a commodity SAPO-34 purchased from Nankai University Catalyst Factory, wherein the temperature of desorption peak of medium-strength acid on NH3-TPD is 390° C.
The zeolite in the catalyst adopted in reference example 6 is a commodity ZSM-5 purchased from Nankai University Catalyst Factory, wherein the zeolite is of a full microporous structure and the Si/Al ratio is 30.
Reaction results of reference examples 5 and 6 show that, the topological structure and acid strength of CHA or AEI are crucial to the modulation of the selectivity of products.
The distance between the metal oxide and the zeolite in the catalyst adopted in reference example 7 is 10 mm. Other parameters and the mixing process are the same as those of catalyst A.
The metal oxide in the catalyst adopted in reference example 8 is located in porous channels of the zeolite and is in close contact with the zeolite. Other parameters and the like are the same as those of catalyst A.
Results of reference examples 7 and 8 show that, the distance between component A and component B is also crucial to product selectivity.
In the reference technology of the document (Jiao et al., Science 351 (2016) 1065-1068), the acid amount of the adopted SAPO-34 zeolite was large. The amount of the medium-strength acid reached 0.32 mol/kg according to the NH3-TPD test. Therefore, when the conversion increased to 35%, the selectivity of light olefins was 69%, the selectivity of light paraffins was 20%, o/p decreased to 3.5 and the selectivity of propylene and butylene was 40-50%.
It is observed from the above table that, the structure of the zeolite including the topologies, acid strength and acid amount of CHA&AEI, and the matching of the distance between the metal oxide and the zeolite are crucial and directly affect the conversion of carbon monoxide and the selectivity of propylene and butylene.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201611101851.0 | Dec 2016 | CN | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2017/114446 | 12/4/2017 | WO | 00 |
