CN115974646A - Method for preparing ethanol by catalyzing carbohydrate with monatomic catalyst - Google Patents
Method for preparing ethanol by catalyzing carbohydrate with monatomic catalyst Download PDFInfo
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E50/00—Technologies for the production of fuel of non-fossil origin
- Y02E50/10—Biofuels, e.g. bio-diesel
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- Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
Abstract
The invention provides a method for preparing bioethanol by efficiently catalyzing carbohydrate conversion with a monatomic catalyst. The assistant and the active metal in the catalyst are in a monoatomic dispersion state. The method is suitable for a kettle type reactor, the cellulose conversion rate is 100 percent on the monatomic catalyst with low load, and the ethanol space-time yield is as high as 30.4g ethanol ·g metal ‑1 ·h ‑1 Much higher than the results reported in other literatures and patents. The catalyst is simple and convenient to prepare, low in cost, easy to recover, easy to separate products, green and environment-friendly in reaction process, and has excellent ethanol yield and excellent stability. The method for preparing the bioethanol provided by the invention is not only suitable for cellulose, but also suitable for carbohydrates such as glucose, starch and the like and corncobsPrimary lignocelluloses such as miscanthus and pine. The technical scheme of the invention provides a new way for preparing bioethanol from renewable resources which are abundant in reserves, cheap and easy to obtain, and has excellent application prospects.
Description
Technical Field
The invention belongs to the technical field of biomass carbohydrate catalytic conversion, and relates to a method for preparing bioethanol by efficiently catalyzing cellulose hydrogenolysis with a monatomic catalyst.
Background
Cellulosic ethanol is an ideal gasoline additive and it is predicted that by 2030, up to 30% of transportation fuels will come from biomass energy sources, with cellulosic ethanol being the most important liquid fuel. The current mature commercial production of bioethanol is the fermentation of carbohydrates in sugar-or starch-containing raw materials (e.g. sugar cane, sugar beet, maize or wheat, etc.), also known as first generation bioethanol (j.chem.technol.biotechnol.2016, 91 (2), 304-317). However, corn and wheat are important food sources, and therefore, the second generation of bioethanol has come to be produced by taking lignocellulose abundant in nature as raw material, and has the advantages of easily available materials and no competition for food (bioresourc.technol.2008, 99 (13), 5270-95). Second generation bioethanol is mainly prepared by biofermentation processes, first hydrolysis of pretreated lignocellulose, followed by post-fermentation and distillation using enzymes or acids to break down the cellulose into simple sugars (e.g. glucose) (chinese patent: CN101319196A, CN104073524A, CN109468345 a). However, enzymatic processes in biofermentations are limited by technical bottlenecks and economic costs, such as severe pretreatment processes, high cellulase costs, etc. (Green chem.2019,21 (9), 2234-2239). In addition, CO is released when pyruvate is generated in the glucose fermentation process 2 Thus, the theoretical maximum molar yield of ethanol is 67% (chem.commun.2019, 55 (30), 4303-4306). Compared with the biological fermentation process, the direct catalysis of the cellulose conversion by the chemical method can adjust the cellulose conversion path by reasonably designing the catalyst, is beneficial to improving the ethanol production efficiency, reducing the cost and avoiding generating CO 2 To improve the atom economy (Chinese patent CN111217672A, CN 109382104A). However, the existing field of ethanol preparation by one-step direct conversion of cellulose by chemical method has the problems of low product selectivity, poor catalyst stability, low ethanol space-time yield and the like (Joule, 3 (2019) 1937-1948 ethanol space-time yield of 12.6g ethanol ·g metal -1 ·h -1 ；Green Chem.2019,21 (9), 2234-2239 ethanol space time yield 0.3g ethanol ·g metal -1 ·h -1 (ii) a chem.Commun.2019,55 (30), 4303-4306 ethanol space-time yield of 4.5g ethanol ·g metal -1 ·h -1 (ii) a Fuel 2021,292,120311-120318 ethanol space-time yield 8.2g ethanol ·g metal -1 ·h -1 ). Therefore, the development of a more stable and efficient catalyst for preparing ethanol from cellulose is of great significance.
Disclosure of Invention
The invention provides a method for preparing bioethanol by efficiently catalyzing carbohydrates, which can realize the efficient breaking of C-C bonds and C-O bonds of carbohydrates such as cellulose, cellobiose, straw sugar solution, starch, glucose and the like and native biomass, and finally realize the high-selectivity preparation of bioethanol by a 'one-pot method'. The prepared single-atom catalyst not only greatly reduces the consumption of noble metal and the cost, and the atom utilization rate reaches 100 percent, but also has excellent stability. The method provides a way for preparing bioethanol in a large scale from non-fossil sources, has good application prospect, and is favorable for realizing the strategic targets of carbon peak reaching and carbon neutralization.
A method for preparing bioethanol by efficiently catalyzing cellulose with a monatomic catalyst comprises the following steps:
1) Firstly, preparing a WOx carrier with rich defect sites by an ethanol solvothermal method, wherein the specific preparation process comprises the following steps: 5-10g WCl 6 Dissolving in 200mL ethanol at room temperature, transferring the yellow liquid into a hydrothermal kettle, heating in an oven at 120-200 deg.C for 24-96h, cooling, washing, and drying to obtain carrier WOx (2.65)<x<2.83, preferably x = 2.72).
2) The catalyst is prepared by a step-by-step isometric impregnation method, wherein the first step impregnation method comprises the following steps: the precursor solution of the active component M is evenly dipped on a WOx carrier of 2-5g in equal volume, and the loading capacity of the active metal is 0.05-2wt%, preferably 0.05-1wt%. Drying the prepared catalyst at 120 ℃ for 4h, then reducing the catalyst at 200-500 ℃ for 0.5-3h in a hydrogen atmosphere, preferably at 300-400 ℃ for 1-2h, and finally obtaining the catalyst M/WOx with dispersed monoatomic atoms.
3) Then, a reducible metal oxide auxiliary agent N is further impregnated on the M/WOx catalyst in equal volume, and the specific steps are as follows: adding an auxiliary component N (N is Nb) 2 O 5 And Ta 2 O 5 Isoreducible metal oxide) precursor salt solution (niobium chloride, tantalum chloride) is uniformly dispersed on the M/WOx catalyst by an isovolumetric impregnation method, and the loading amount of the auxiliary N is 0.05-0.5wt%, preferably 0.05-0.2wt% of the mass of the catalyst. And then drying the obtained catalyst at 120 ℃ for 4h, roasting the catalyst in a muffle furnace at 200-500 ℃ for 1-3h in the air atmosphere, and reducing the catalyst in a tubular furnace at 200-500 ℃ for 0.5-3h to obtain the N/M/WOx monatomic catalyst, wherein the roasting temperature and the roasting time are preferably 300-400 ℃ for 2-3h respectively, and the reducing temperature and the reducing time are preferably 300-400 ℃ for 1-2h respectively. The molar ratio of the auxiliary N to the active metal Pt is between 0.05 and 3, and preferably between 0.05 and 1.0.
The performance evaluation of the original biomass or the bioethanol prepared by the hydrogenolysis of the carbohydrate derived from the biomass is carried out in a batch autoclave reactor, and the substrates comprise cellulose, cellobiose, straw sugar solution, starch, glucose and other carbohydrates and pine, miscanthus sinensis, corncob and other original lignocellulose. The operating process conditions are as follows: the concentration of the substrate is 1-30wt.%, the reaction temperature is 200-260 ℃, the hydrogen pressure is 2-8MPa, the reaction time is 0.5-8h, and water is used as a solvent.
The advantages of the invention are as follows:
(1) The auxiliary agent N and the active component M in the multifunctional catalyst prepared by the invention are highly dispersed and exist in a monoatomic form, the atom utilization rate reaches 100%, and the auxiliary agent N is tightly combined with the active metal M to form a unique N-M-WOx interface, so that the step of selectively breaking C-C bonds and C-O bonds in the conversion process of carbohydrates such as cellulose is greatly promoted. The catalyst is simple and convenient to prepare, easy to recover, easy to separate products, high in ethanol selectivity and environment-friendly in reaction process.
(2) Compared with the M/WOx catalyst, after the auxiliary agent N is impregnated, the selectivity of ethanol is obviously improved, and rich oxygen vacancies on the surface of WOx are beneficial to anchoring the active metal M and the auxiliary agent N, thereby laying a good foundation for improving the stability of the catalyst.
(3) And the literature reported at presentCompared with the patent, the metal loading capacity of the catalyst is greatly reduced, the preparation cost of the catalyst is obviously reduced, the higher ethanol yield is kept, and the ethanol yield per unit mass of Pt per unit time is far higher than the currently reported result (30.4 g) ethanol ·g metal -1 ·h -1 ) Has excellent industrial application prospect.
Drawings
FIG. 1 is a TEM image (a-d) of the 0.1Nb/0.5Pt/WOx monatomic catalyst prepared in example 3, the corresponding elemental plane scans (e-i) and the EDS spectroscopy results (j);
FIG. 2 is a TEM image and a line scan image of the 0.1Nb/4Pt/WOx catalyst prepared in comparative example 10 (upper yellow line: pt, lower purple line: nb)
FIG. 3 is a stability test of monatomic 0.1Nb/0.5Pt/WOx catalyzed cellulose hydrogenolysis to bioethanol: reaction conditions are as follows: 0.3g of cellulose, 20g of water, 0.2g of 0.1Nb/0.5Pt/WOx, the reaction temperature of 240 ℃, the reaction pressure of 4MPa, the reaction time of 4h and the rotation speed of 800rpm.
Detailed Description
The following detailed description of the embodiments of the invention is provided in connection with the accompanying drawings.
Example 1: preparation of WOx vector
5g of WCl 6 Dissolving in 200mL of absolute ethyl alcohol, stirring to dissolve fully, then transferring to a hydrothermal kettle, sealing, and heating at 160 ℃ for 36h. And after the temperature is reduced, filtering, washing and freeze-drying to obtain a blue solid, namely WOx, wherein x =2.72.
Example 2: preparation of Metal catalyst M/WOx
Preparation of 0.5wt% of a monatomic catalyst Pt/WOx (abbreviated as 0.5 Pt/WOx): adding appropriate amount of H 2 PtCl 6 The solution was impregnated in an equal volume on the WOx support obtained in example 1, and the loading of the active component Pt in the catalyst was 0.5wt%. Standing at room temperature for 12h, drying at 120 deg.C for 4h, reducing at 300 deg.C for 1h in hydrogen atmosphere, cooling, and adding 1%O 2 /N 2 (volume fraction) was deactivated for 4h and the resulting catalyst was reported as 0.5Pt/WOx.
Example 3: preparation of multifunctional catalyst N/M/WOx
Taking the preparation method of 0.1wt% Nb/0.5Pt/WOx of the single atom catalyst as an example (abbreviated as 0.1Nb/0.5 Pt/WOx): adding an appropriate amount of NbCl 5 The precursor solution was impregnated with an equal volume onto the 0.5Pt/WOx catalyst of example 2 to obtain a final catalyst with Nb loading of 0.1wt%. It was then left to stand at room temperature for 12h and dried at 120 ℃ for 4h. Then roasting in a muffle furnace at 400 ℃ for 3h in the air atmosphere, reducing the temperature in a hydrogen atmosphere at 300 ℃ for 1h, and recording the obtained catalyst as 0.1Nb/0.5Pt/WOx (the values in the front of the metal in the following examples represent the same meanings). It can be seen from the spherical aberration transmission electron microscope with atomic resolution in fig. 1 that the additive Nb and the active metal Pt respectively exhibit monoatomic dispersion on the WOx surface of the support, and do not aggregate. The surface scanning analysis (TEM mapping) of the catalyst element further proves that Nb and Pt are monodisperse, 1Nb atom is attached to 1 Pt atom to form a 'diatomic' (Nb atom and Pt atom) catalytic active center, the rest Pt is still dispersed on the surface of the carrier in a monoatomic form (at the moment, the Nb/Pt molar ratio is 0.4), and the energy spectrum result proves that Nb and Pt elements exist on the surface of the catalyst, and the Nb and Pt elements are highly dispersed and are not agglomerated.
Examples 4-7 in Table 1 differ from example 3 in the volume of NbCl in the impregnating solution equal to the volume 5 The catalyst preparation conditions were the same as in example 3 except for the content. The spherical aberration transmission electron microscope of atomic resolution confirmed that Pt and Nb appeared monoatomic and closely spaced on the prepared catalyst. Comparative examples 2 to 5 the single atom catalyst was prepared by the same method as in example 3 except that the kind of the precursor of the selected promoter was different from that of the precursor of the selected promoter. The prepared monatomic catalyst is used for comparing the influence of different auxiliary agent types and contents on the catalytic conversion performance of cellulose. The conditions for catalyzing the cellulose conversion reaction by the single atom catalyst in the examples 4-7 and the comparative examples 2-5 are consistent, taking the example 5 as an example: 0.2g of monatomic 0.05Nb/0.5Pt/WOx catalyst, 0.3g of cellulose and 20g of water, wherein the reaction temperature is 240 ℃, the hydrogen pressure is 4MPa, and the reaction time is 4h. Upon completion of the reaction, the liquid product was centrifuged and analyzed using high performance liquid chromatography. The liquid product comprises: ethanol, ethylene glycol, propylene glycol, n-propanol, n-butanol, sorbitol andisosorbide.
From the table 1, it can be seen that the N/M/WOx catalyst with a unique structure designed and prepared in the present invention can realize the high efficiency conversion of cellulose into ethanol. In example 5, the addition of Nb as a co-agent in an amount of 0.05wt.% alone significantly improves ethanol yield, which is mainly benefited by the formation of the Nb-Pt-WOx interface. In example 3, the yield of 0.1Nb/0.5Pt/WOx catalyzing the conversion of the cellulose into the ethanol is as high as 57.7 percent. Furthermore, as the Nb content of the adjuvant increased from 0.05wt% to 0.5wt%, the ethanol yield increased from 46.7% to 57.7% and then decreased to 45.8%, exhibiting an increasing and decreasing "volcano-type" curve. When the content of Pt is 0.5wt%, the molar ratio of Nb atoms to Pt atoms is increased (Nb/Pt: 0.2-2.1) with the increase of the content of the auxiliary Nb, the coverage of the auxiliary Nb for the monoatomic Pt is increased, so that the exposed active sites are reduced, and therefore, the yield of ethanol is reduced, and a proper amount of the auxiliary is beneficial to accurately constructing an active interface, so that the hydrogenolysis of C-O bonds in the reaction process is obviously promoted. In addition, for example 7, when the Nb/Pt molar ratio reached 2.1, each Nb atom was in close proximity to a Pt atom, and the remaining Nb atoms were monoatomic and dispersed on the support WOx. Although Nb and Pt still exhibited monoatomic dispersion at this time, the degree of coverage of the Pt surface was slightly increased, and a part of Nb was uniformly dispersed on the support surface. Although the ethanol yield at this time slightly decreased, it was still close to 46%, showing unusual results. In summary, the catalysts produced showed better results with ethanol yields of greater than 45% over the selected promoter range than the catalyst without promoter addition, and with ethanol yields of greater than 54% over the promoter preferred range.
EXAMPLE 8A monatomic catalyst of 0.1Ta/0.5Pt/WOx was prepared similarly to example 3, except that the promoter precursor selected was TaCl 5 The transmission electron microscopy corrected for spherical aberration of atomic resolution confirmed that Ta and Pt on the surface of the catalyst appeared monoatomic and close to one another (atomic ratio Ta/Pt = 0.2), with the remainder of Pt appearing monoatomic on the support. The monatomic catalyst prepared in example 8 also showed good performance in the process of preparing ethanol by selective hydrogenolysis of cellulose, and the ethanol yield reaches 50%, meaning that the auxiliary agents Nb and Ta can effectively promote the fiberAnd (4) performing hydrogenolysis on the cellulose.
TABLE 1.N/0.5Pt/WOx catalytic cellulose conversion Performance
Note: substrate conversion was 100% in all examples; the concentration of the cellulose is 1.5wt%,20g of water and 0.2g of catalyst, the reaction temperature is 240 ℃, the reaction pressure is 4MPa, and the reaction time is 4h; other products mainly included propylene glycol, n-propanol, n-butanol, sorbitol, etc. (0.1 Nb-0.5Pt/WOx in comparative example 1 represents a co-impregnated catalyst, and the mass of the two-component catalysts in comparative examples 6 and 7 is 0.1g each, and they were added together to the reaction system before the reaction, for example, in comparative example 6, 0.1g of 0.5Pt/WOx (the loading amount of Pt as an active component in the catalyst is 0.5 wt%) and 0.1g of Nb 2 O 5 Added into a reaction kettle together for reaction).
The catalyst 0.1Nb-0.5Pt/WOx in comparative example 1 is prepared by co-impregnation method, i.e. WOx carrier is prepared by ethanol solvothermal method (same as example 1), and then certain amount of chloroplatinic acid and NbCl are added simultaneously 5 The solution was impregnated on WOx support in equal volume, dried and calcined at 400 ℃ for 1h, and reduced with hydrogen at 300 ℃ for 1h. The electron microscope result proves that the Pt and Nb on the catalyst prepared by the co-impregnation method are not completely attached, but are randomly dispersed, and partial areas are aggregated (the Pt grain size is 0.5-1.5 nm). Compared with the catalyst prepared by the step-by-step impregnation method (example 3), the ethanol yield is remarkably reduced, which shows that the unique catalyst structure constructed by the step-by-step impregnation method is beneficial to promoting the breakage of C-C bonds and C-O bonds. The catalysts modified with the aids Fe, zn, cu and W of comparative examples 2-5 (which were prepared in the same manner as in example 3 except that the metal cations of the aids were different) exhibited much lower performance than those of examples 3 and 8, although they could improve the ethanol yield in the ethanol production from cellulose. Comparative examples 6-7 two-phase catalysts (0.5 Pt/WOx and 0.5 Pt/Nb) 2 O 5 The preparation method differs from that of example 2 in that the selected carrier is Nb 2 O 5 And WOx, the rest being the same, the electron microscope results confirmed that 0.5Pt/WOx and 0.5Pt/Nb 2 O 5 Gold (II)Pt is in monoatomic dispersion. The hydrogenolysis reaction result of the catalytic cellulose proves that the N/M/WOx multifunctional catalyst prepared by the step-by-step impregnation method can obviously promote the generation of ethanol.
The catalyst in comparative example 8 was prepared in a similar manner to example 2, except that chloroplatinic acid was impregnated on the WOx support and calcined directly at 400 ℃ for 1h and reduced with hydrogen at 300 ℃ for 1h. From comparative example 8, it can be seen that the ethanol yield was only 22.7%, which is lower than the result obtained in example 4, mainly because the direct firing process after impregnation of the Pt precursor resulted in partial Pt aggregation and uneven dispersion. Comparative example 9 where the support was Nb-doped WOx, i.e., a quantity of WCl was first dissolved in 200mL of ethanol 6 And NbCl 5 Followed by heating at 160 ℃ for 36 hours, impregnation with 0.5wt% of Pt (loading amount of active component Pt in the catalyst was 0.5wt%, nb was 0.1 wt%) after filtration, washing, freeze-drying, and then hydrogen reduction at 300 ℃ for 1 hour. As can be seen from comparative example 9, the ethanol yield was only 19.3%. Comparative examples 8 and 9 further demonstrate the unique advantage of the monatomic 0.1Nb/0.5Pt/WOx catalyst prepared by a step-and-volume impregnation method for the hydrogenolysis of cellulose to ethanol.
It can be seen from Table 1 that Nb is calculated when the loading of auxiliary is 0.1wt% 2 O 5 When the yield of ethanol is optimum, examples 9 to 14 and comparative example 10 for this purpose were further examined as to the content of the auxiliary agent of 0.1wt% Nb 2 O 5 In this case, the effect of catalysts of different active metal Pt contents on catalyzing cellulose conversion is shown in table 2.
TABLE 2 catalytic cellulose conversion Performance of catalysts with different Pt metal contents
Note: substrate conversion was 100% in all examples; the cellulose concentration is 1.5wt%,20g of water and 0.2g of catalyst, the reaction temperature is 240 ℃, the reaction pressure is 4MPa, and the reaction time is 4h; other products mainly include propylene glycol, n-propanol, n-butanol, sorbitol, etc.
The catalyst of example 9 was prepared as follows: adding a certain amount of NbCl 5 The auxiliary agent precursor solution was impregnated on the WOx carrier prepared in example 1, dried, calcined at 400 ℃ for 1 hour in an air atmosphere, and then reduced at 300 ℃ for 1 hour, and the electron microscope results confirmed that Nb is monoatomic dispersed on WOx. Examples 10 to 14 and comparative example 10 were prepared in a similar manner to examples 2 and 3, except that the content of the metal precursor chloroplatinic acid solution was different. The results of the spherical aberration electron microscope confirmed that the active metal Pt and the auxiliary Nb in examples 10 to 14 are monoatomic and mostly close. The reaction is as an example 10: 0.2g of monatomic 0.1Nb/0.05Pt/WOx catalyst, 0.3g of cellulose and 20g of water, wherein the reaction temperature is 240 ℃, the hydrogen pressure is 4MPa, and the reaction time is 4 hours. Upon completion of the reaction, the liquid product was centrifuged and analyzed using high performance liquid chromatography. The liquid product comprises: ethanol, ethylene glycol, propylene glycol, n-propanol, n-butanol, sorbitol, and isosorbide. As can be seen from examples 10-14 in Table 2, the ethanol yield gradually increased as the metal Pt loading was gradually increased from 0.05wt% to 2wt%. When the loading amount of the metal Pt is within 0.5-2wt%, the ethanol yield is kept stable. From example 9, it can be found that the ethanol yield is extremely low in the absence of active metal Pt, meaning the indispensable property of active metal Pt. In order to highlight the advantages of the monatomic catalyst, a Pt nanoparticle catalyst of 0.1Nb/4Pt/WOx was further prepared by a step-by-step impregnation method and used for the catalytic cellulose hydrogenolysis performance evaluation (comparative example 10). FIG. 2 is a spherical aberration corrected transmission electron microscope image of 0.1Nb/4 Pt/WOx. It can be seen from the figure that the Pt particle size is about 4nm, which is a typical Pt nanoparticle. The electron microscope line scanning result proves that the auxiliary agent Nb is dispersed on the surface of Pt particles instead of the WOx carrier, which means that the stepwise impregnation method is beneficial to constructing an auxiliary agent-active metal-carrier catalyst structure. As can be seen from table 2, the nanoparticle catalyst formed after increasing the Pt loading of the active metal did not significantly improve the ethanol yield, and when the Pt loading was increased to 4wt%, the ethanol space-time yield (space-time yield = (ethanol content (mol) × yield × 46.07)/(reaction time × catalyst mass × Pt loading)) decreased to 3.5g ethanol ·g metal -1 ·h -1 And the time-space yield of the ethanol as the cellulose product catalyzed by the monatomic catalyst 0.1Nb/0.5Pt/WOx is as high as 30.4g ethanol ·g metal -1 ·h -1 . This means that the additive Nb and the active metal Pt are dispersed and attached to each other in a single atom manner, and the 0.1Nb/0.5Pt/WOx has the best catalytic activity, thereby being beneficial to reducing the consumption of noble metals and the preparation cost of the catalyst, and showing the remarkable advantages of the single-atom catalyst.
TABLE 3 Effect of different Nb/Pt molar ratios on catalytic cellulose conversion Performance
Note: substrate conversion was 100% in all examples; the concentration of the cellulose is 1.5wt%,20g of water and 0.2g of catalyst, the reaction temperature is 240 ℃, the reaction pressure is 4MPa, and the reaction time is 4h; other products mainly include propylene glycol, n-propanol, n-butanol, sorbitol, etc.
From the foregoing, it can be seen that a catalyst with an appropriate Nb/Pt molar ratio can effectively promote the performance of cellulose hydrogenolysis to ethanol within a certain range of the content of the auxiliary agent and the active metal, and therefore the influence of different Nb/Pt molar ratios on the conversion of cellulose was examined, as shown in table 3. The catalysts of examples 15-20 in Table 3 were prepared in a similar manner to that of examples 3 and 2, except that the amounts of the promoter and the active metal precursor were different, and the electron microscope results confirmed that the promoter and the active metal in the prepared catalysts exhibited monoatomic dispersion. Taking example 15 as an example: 0.2g of monatomic 0.5Nb/0.05Pt/WOx catalyst, 0.3g of cellulose and 20g of water, wherein the reaction temperature is 240 ℃, the hydrogen pressure is 4MPa, and the reaction time is 4 hours. Upon completion of the reaction, the liquid product was centrifuged and analyzed using high performance liquid chromatography. The liquid product comprises: ethanol, ethylene glycol, propylene glycol, n-propanol, n-butanol, sorbitol and isosorbide.
In example 15, the atomic ratio of Nb/Pt was 21, and although Nb and Pt still showed monoatomic dispersion, pt at an extremely low content was heavily covered with Nb, and most of Nb was monodisperse on the support surface except for a small portion of Nb closely attached to Pt, resulting in a serious deficiency of Nb-Pt-WOx interface sites, and thus a low ethanol yield. In example 16, the Nb/Pt atomic ratio was 1.1, and at this time 90% or more of Nb atoms were respectively adhered to one Pt atom, and the ethanol yield was remarkably improved to 46.7%. When the Nb/Pt atomic ratio was further decreased to 0.55 (example 17), the ethanol yield was slightly decreased. The Nb content of the fixing assistant in examples 18 to 20 was 0.05wt%, and as the Pt content was increased from 0.05wt% to 2wt%, that is, as the Nb/Pt atomic ratio was decreased from 2.1 to 0.05, the ethanol yield was slightly decreased after increasing. As the Nb/Pt atomic ratio decreases from 2.1 to 0.05, the remaining Nb atoms are distributed in a monoatomic form on the support WOx (example 18, nb/Pt = 2.1), and the remaining Pt atoms are distributed in a monoatomic form on the support WOx (example 20, nb/Pt = 0.05). For example 18, insufficient levels of promoter and active metal Pt in the catalyst resulted in an ineffective promotion of the activated hydrogen and C — O bond cleavage step in the cellulose hydrogenolysis process. For example 20, the ethanol yield still reached 50.2%, but there was no significant advantage over example 3 and the amount of precious metal was increased. In summary, a suitable Nb/Pt atomic ratio catalyst is effective in promoting hydrogenolysis of cellulose to ethanol over the selected range of promoter and active metal content, i.e., nb/Pt atomic ratio between 0.05 and 3, preferably between 0.05 and 1.
TABLE 4.0.1Nb/M/WOx Performance for ethanol conversion from cellulose
Note: substrate conversion was 100% in all examples; the concentration of the cellulose is 1.5wt%,20g of water and 0.2g of catalyst, the reaction temperature is 240 ℃, the reaction pressure is 4MPa, and the reaction time is 4h; other products mainly include propylene glycol, n-propanol, n-butanol, sorbitol, etc. ( Note: comparative examples 14-16, in which the molar ratio of the two active metals is 1:1, the total mass fraction is 0.5wt% )
Comparative examples 11-16 in Table 4 examine the effect of different active metals on the performance of catalyzing the conversion of cellulose to ethanol. Comparative examples 11-16 catalysts were prepared similarly to examples 2 and 3, except that the active metal precursor was of a different type, and the remainder was the same, and electron microscopy results confirmed that comparative examples 11-16 were monatomic catalysts. Taking comparative example 11 as an example: 0.2g of monoatomic 0.1Nb/0.5Pd/WOx,0.3g of cellulose and 20g of water, the reaction temperature is 240 ℃, the hydrogen pressure is 4MPa, and the reaction time is 4h. Upon completion of the reaction, the liquid product was centrifuged and analyzed using high performance liquid chromatography. It can be seen from the table that the active metal Pt has the best catalytic performance, and in addition the metal Ru also shows unusual results.
TABLE 5 Effect of support on cellulose conversion Performance
Note: substrate conversion was 100% in all examples; the cellulose concentration is 1.5wt%,20g of water and 0.2g of catalyst, the reaction temperature is 240 ℃, the reaction pressure is 4MPa, and the reaction time is 4h; other products mainly include propylene glycol, n-propanol, n-butanol, sorbitol, etc.
Comparative examples 17-24 in Table 5 examine the effect of vectors on cellulose conversion and were prepared similarly to examples 3 and 2, except that the choice of vector was not consistent. Taking comparative example 17 as an example: 0.2g0.1Nb/0.5Pt/Al 2 O 3 0.3g of cellulose and 20g of water as catalysts, the reaction temperature is 240 ℃, the hydrogen pressure is 4MPa, and the reaction time is 4h. Upon completion of the reaction, the liquid product was centrifuged and analyzed using high performance liquid chromatography. As seen from example 3 and comparative examples 18-20 in Table 4, the carrier WOx has unique advantages for C-C bond breakage in the cellulose degradation process, and the efficiency of preparing bioethanol by cellulose conversion can be improved by taking WOx as the carrier. In addition, the WOx surface of the carrier has abundant defect sites, which is beneficial to anchoring active metal and auxiliary agent, so that the monatomic catalyst is prepared, and the atom utilization rate is 100%. When alumina, silica, zirconia, titania and ceria are used as carriers, the yield of ethanol is not ideal, because side reactions occur in the hydrogenolysis process of cellulose, the cracking of C-C bonds in the cellulose cannot be effectively promoted, and thus the content of intermediate products, namely erythrose and ethylene glycol, is reduced, and further the yield of ethanol is reduced.
TABLE 6 catalytic conversion Performance of different carbohydrate substrates
Note: in all the examples, the concentration of the substrate is 1.5wt%,20g of water, 0.2g of 0.1Nb/0.5Pt/WOx, the reaction temperature is 240 ℃, the reaction pressure is 4MPa, and the reaction time is 4h; other products mainly include propylene glycol, n-propanol, n-butanol, sorbitol, etc.
The monatomic 0.1Nb/0.5Pt/WOx catalyst prepared in the present invention gave excellent results during cellulose conversion, for which the performance of the catalyst in other carbohydrate conversions was further examined, as shown in examples 21-27 of Table 6. The catalysts used in examples 21-27 were all the 0.1Nb/0.5Pt/WOx monatomic catalysts prepared in example 3. Using example 25 as an example: 0.3g of corncob, 20g of water, 0.3g of monatomic 0.1Nb/0.5Pt/WOx catalyst, the reaction temperature of 240 ℃, the reaction pressure of 4MPa and the reaction time of 4 hours. After the reaction was completed and cooled to room temperature, the liquid product was centrifuged and analyzed by high performance liquid chromatography.
As can be seen from table 6, both small molecular carbohydrates such as cellobiose and glucose and native lignocellulose (lignin has been extracted in advance) can be efficiently converted to prepare bioethanol on the monatomic catalyst designed by the present invention, and the present invention has a wide application range.
TABLE 7 Effect of reaction temperature on cellulose conversion Performance
Note: in all the examples, the cellulose concentration is 1.5wt%,20g of water, 0.2g of 0.1Nb/0.5Pt/WOx, the reaction pressure is 4MPa, and the reaction time is 4h; other products mainly include propylene glycol, n-propanol, n-butanol, sorbitol, etc.
In Table 7, examples 28 to 30 and comparative examples 25 to 26 were examined the effect of different reaction temperatures on the conversion of cellulose by using the monatomic catalyst 0.1Nb/0.5Pt/WOx prepared in example 3. Taking example 28 as an example: 0.3g of cellulose, 2g of water, 0.2g of monatomic 0.1Nb/0.5Pt/WOx catalyst, the reaction temperature of 200 ℃, the reaction pressure of 4MPa and the reaction time of 4h. After the reaction was completed and cooled to room temperature, the liquid product was centrifuged and analyzed by high performance liquid chromatography.
As can be seen from Table 7, the reaction temperature has a significant effect on the performance of the conversion of cellulose to bioethanol. As can be seen from comparative examples 25-26, the ethanol yield was lower than 20% at reaction temperatures below 200 ℃. With further increase of the reaction temperature, the ethanol yield increased significantly. When the reaction temperature is higher than 240 ℃, ethanol is easy to generate ethane by transitional hydrogenolysis, so that the yield of ethanol is slightly reduced, but still higher than 49 percent, and good results are shown. The proper reaction temperature is beneficial to H in water + The transmission of the method is also beneficial to destroying rigid hydrogen bonds in the cellulose structure, promotes the depolymerization of the cellulose and lays a foundation for the efficient production of ethanol.
Examples 31-33 in Table 8 are comparisons of catalytic cellulose conversion performance of monatomic 0.1Nb/0.5Pt/WOx catalysts (prepared in example 3) under different hydrogen pressures. Using example 31 as an example: 0.3g of cellulose, 20g of water, 0.2g of monatomic 0.1Nb/0.5Pt/WOx catalyst, the reaction temperature of 240 ℃, the reaction time of 4 hours and the hydrogen pressure of 2MPa. After the reaction was completed and cooled to room temperature, the liquid product was centrifuged and analyzed by high performance liquid chromatography. It can be seen from Table 8 that the ethanol yield is maintained at a high level and the ethanol yield is higher than 52% in the hydrogen pressure range of 4-6 MPa.
TABLE 8 influence of reaction Hydrogen pressure on cellulose conversion Performance
Note: in all the examples, the cellulose concentration is 1.5wt%,20g of water, 0.2g of 0.1Nb/0.5Pt/WOx, the reaction temperature is 240 ℃, and the reaction time is 4h; other products mainly include propylene glycol, n-propanol, n-butanol, sorbitol, etc.
Examples 34-38 in Table 9 are designed to produce 0.1Nb/0.5Pt/WOx monatomic catalysts for conversion performance in varying concentrations of cellulose. Taking example 36 as an example: 1.6g of cellulose, 20g of water and 0.8g of a monoatomic 0.1Nb/0.5Pt/WOx catalyst (50 percent of the mass of the cellulose), wherein the reaction temperature is 240 ℃, the reaction pressure is 4MPa, and the reaction time is 4 hours. After the reaction was completed and cooled to room temperature, the liquid product was centrifuged and analyzed by high performance liquid chromatography. From the table 9, it can be seen that in the selected cellulose concentration range, the 0.1Nb/0.5Pt/WOx catalyst still shows excellent performance, realizes the efficient hydrogenolysis of cellulose to prepare ethanol, and has excellent application prospects.
TABLE 9 influence of cellulose concentration on catalytic performance
Note: in all the examples, the catalyst is 0.1Nb/0.5Pt/WOx,20g of water, the reaction temperature is 240 ℃, the reaction pressure is 4MPa, and the reaction time is 4h; other products mainly include propylene glycol, n-propanol, n-butanol, sorbitol, etc.
TABLE 10 Effect of calcination temperature on the conversion Performance of 0.1Nb/0.5Pt/WOx catalyzed cellulose
Note: substrate conversion was 100% in all examples; the cellulose concentration is 1.5wt%,20g of water and 0.2g of catalyst, the reaction temperature is 240 ℃, the reaction pressure is 4MPa, and the reaction time is 4h; other products mainly include propylene glycol, n-propanol, n-butanol, sorbitol, etc.
Examples 39-41 in Table 10 examined the effect of different catalyst calcination temperatures on the selective hydrogenolysis of cellulose to ethanol using a monoatomic 0.1Nb/0.5Pt/WOx catalyst (300 ℃ hydrogen reduction 1h after calcination). Taking example 39 as an example, 0.2g of a monatomic 0.1Nb/0.5Pt/WOx catalyst after calcination at 200 ℃ and reduction at 300 ℃,0.3g of cellulose, 20g of water, a reaction temperature of 240 ℃, a hydrogen pressure of 4MPa, and a reaction time of 4 hours. Upon completion of the reaction, the liquid product was centrifuged and analyzed using high performance liquid chromatography. The liquid product comprises: ethanol, ethylene glycol, propylene glycol, n-propanol, n-butanol, sorbitol, and isosorbide. As can be seen from examples 39-41 and example 3 in the table, the ethanol yield is gradually increased with the gradual increase of the calcination temperature of the carrier, which means that the appropriate increase of the calcination temperature is beneficial to the construction of the monatomic assistant-active metal-WOx active interface, and within the appropriate calcination temperature range, the ethanol yield is more than 42%, and particularly within the preferred calcination range, the ethanol yield is more than 52%.
Catalyst reduction temperature is also one of the important factors affecting catalyst performance, and Table 11 examples 42-44 are the effect of different reduction temperatures on the conversion of 0.1Nb/0.5Pt/WOx catalyzed cellulose by a monatomic catalyst (reduction after calcination at 400 ℃ for 1 h). Taking example 42 as an example: 0.3g of cellulose, 0.20 g of water, 0.2g of 0.1Nb/0.5Pt/WOx catalyst calcined at 400 ℃ and reduced for 1h at 200 ℃, the reaction temperature is 240 ℃, and the reaction time is 4h. After the reaction was completed and cooled to room temperature, the liquid product was centrifuged and analyzed by high performance liquid chromatography.
As can be seen from Table 11, the ethanol yield remained high in the reduction temperature range of 200-500 deg.C, and in the preferred reduction temperature range, the ethanol yield exceeded 53%. Wherein the yield of ethanol is highest after reduction at 300 ℃, and the Nb-Pt-WOx interface sites constructed at the moment are most abundant, which is beneficial to promoting the cellulose to be efficiently converted into bioethanol.
TABLE 11 influence of catalyst reduction temperature on catalytic performance
Note: in all the examples, the cellulose concentration is 1.5wt%,20g of water, 0.2g of 0.1Nb/0.5Pt/WOx, the reaction temperature is 240 ℃, the reaction pressure is 4MPa, and the reaction time is 4h; other products mainly include propylene glycol, n-propanol, n-butanol, sorbitol, etc.
Example 45 is a stability test of a monatomic catalyst 0.1Nb/0.5Pt/WOx catalyzed cellulose hydrogenolysis to bioethanol, with the following specific operating steps: 0.3g of cellulose, 20g of water, 0.2g of 0.1Nb/0.5Pt/WOx monatomic catalyst, the reaction temperature of 240 ℃, the reaction pressure of 4MPa and the reaction time of 4h. After each reaction, the catalyst is centrifugally separated, washed by ultrapure water for 2 times and then added into a reaction kettle, 0.3g of cellulose and 20mL of water are added, 4MPa of hydrogen is filled, and the temperature is raised to 240 ℃ for the next cycle reaction. The liquid product after each reaction was analyzed by high performance liquid chromatography after centrifugal separation, and the reaction results are shown in fig. 3. From the figure, the ethanol yield of the 0.1Nb/0.5Pt/WOx catalyst prepared by the invention is basically kept unchanged after 10 cycles, which shows that the catalyst has excellent stability and good industrial application prospect.
Claims (7)
1. A method for preparing ethanol by catalyzing carbohydrate with a monatomic catalyst is characterized by comprising the following steps: the catalyst is abbreviated as N/M/S, wherein the auxiliary N and the active metal M are respectively dispersed on the surface of the carrier S in a monoatomic dispersion (not aggregated) state, and the auxiliary N is Nb 2 O 5 、Ta 2 O 5 Isoreducible metal oxides, preferably Nb 2 O 5 The content of the auxiliary agent is 0.05 to 0.5 weight percent, preferably 0.05 to 0.2 weight percent; m is a noble metal Pt, and the content of the active metal Pt is 0.05-2wt%, preferably 0.05-1wt% of the sum of the mass of Pt and the mass of the carrier S; within the range of the chosen content of promoter and active metal, the Nb/Pt atomic ratio is between 0.05 and 3, preferably between 0.05 and 1; s is catalyst support WOx (2.63 < x < 2.82, preferably x = 2.72).
2. The method of claim 1, wherein: the preparation method of the required catalyst is a step impregnation method, and the prepared catalyst is abbreviated as N/M/S:
firstly, dipping a precursor solution of an active component M (preferably in the same volume) on a carrier WOx, drying the obtained catalyst, and reducing the dried catalyst for 0.5 to 3 hours at the temperature of between 200 and 500 ℃ by hydrogen, preferably reducing the catalyst for 1 to 2 hours at the temperature of between 300 and 400 ℃ to obtain M/WOx; then further dipping (preferably equal-volume dipping) precursor solution of the auxiliary agent N on M/WOx, drying at 120 ℃, air-roasting at 200-500 ℃ for 1-3h, and then reducing at 200-500 ℃ for 0.5-3h to obtain the monatomic N/M/WOx catalyst, wherein the roasting temperature and time are preferably 300-400 ℃ and roasting for 2-3h respectively, and the reducing temperature and time are preferably 300-400 ℃ and reducing for 1-2h respectively.
3. The method according to claim 1 or 2, characterized in that:
vector WOx through WCl 6 The ethanol is prepared by a solvothermal method, and the specific process is as follows: 5-10g WCl 6 Dissolving in 200mL of ethanol under stirring at room temperature, transferring the yellow liquid into a hydrothermal kettle, heating at 120-200 ℃ for 24-96h, preferably at 140-180 ℃ for 48-72h, cooling, washing and drying to obtain the carrier WOx.
4. The method of claim 2, wherein:
the selected M precursor is one or two of chloride and nitrate corresponding to the active metal M; the precursor of the selected auxiliary agent N is one or two of chloride and oxalate corresponding to the auxiliary agent N.
5. The method of claim 1, wherein:
the reaction is carried out in a batch autoclave reactor, and the operation process conditions are as follows: the carbohydrate concentration of a substrate is 1-30wt%, the dosage of a catalyst is 1-50wt% of the mass of the substrate, the reaction temperature is between 200 and 260 ℃, the hydrogen pressure is 2-8MPa, the reaction time is 0.5-8h, and water is used as a solvent.
6. The method according to claim 1 or 5, characterized in that:
the carbohydrate is one or more of glucose, cellulose, starch, straw sugar solution, cellobiose, corn cob, miscanthus and pine.
7. The method of claim 1, wherein:
when the atomic ratio of N/M is between 0.05 and 1, each Nb atom or Ta atom on the prepared catalyst is tightly attached to one Pt atom; pt atoms not forming a "diatomic" active centre with Nb (or Ta) atoms are dispersed on the support WOx in monoatomic form;
when the atomic ratio of N/M is between 1<N/M ≦ 3, each Pt atom on the prepared catalyst is tightly attached to one Nb atom or Ta atom; the Nb (or Ta) atoms that do not form a "diatomic" active center with the Pt atoms are dispersed on the support WOx in monoatomic form.
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