Brief introduction of the results
Upgrading ethanol (ETOH) to long-chain alcohols (LAS, C6+OH) offers an attractive and sustainable approach to carbon neutrality, but achieving efficient carbon chain extension is still a huge challenge, especially with the use of ***-free catalysts in the aqueous phase to produce LAS.
Based on this,Professor Xiong Yujie from the University of Science and Technology of China, Professor Wang Tiejun and Associate Professor Qiu Songbai from Guangdong University of Technology (co-corresponding author) and othersAn unconventional but effective strategy for catalyzing a highly efficient catalyst for the conversion of EToh to LAS is reported. The authors synthesized a highly efficient nickel catalyst encapsulated in graphitized carbon, modified with sulfur (ni@c-SX, X stands for S Nimolar ratio), and achieved by one-step carbonization of a nickel-organic complex gel containing a small amount of sulfur. Through this design, adding an appropriate amount of sulfur to the catalyst can not only accelerate the conversion of ETOH to LAS, but also break the limitation of the stepped growth model on carbon chain lengthening. After optimizing the sulfur content, the ni@c-S1 30 catalyst achieved a record high performance, reaching 742% LAS selectivity (C6OH 15.)2% and 59 of C8+OH0%) and 991% eToh conversion rate.
Combined with the in-situ characterization and density functional theory (DFT) calculations, the results show that the sulfur atoms forming strong Ni-S bonds can selectively block the active Ni sites on the surface through steric hindrance, while retaining most of the strong absorption sites of the aldehyde-rich intermediates. This study further demonstrates that surface sulfur modification is a versatile strategy that allows for a flexible selection of various sulfur precursors and addition methods.
Background:
Long-chain alcohols are an important and valuable component of the mass production of high-quality biofuels and fine chemicals such as plasticizers, thickeners, lubricants, detergents and so on in the modern chemical industry. At present, the industrial synthesis of long-chain alcohols is mainly through traditional oxygen synthesis methods, which require expensive organometallic catalysts, cumbersome synthesis processes, and use petroleum-derived olefins as raw materials. The sustainable production of LAS using biomass-derived ethanol (ETOH) has attracted attention, and usually the guerbet condensation of ethanol is carried out through a series of tandem reactions, including alcohol dehydrogenation to acetaldehyde and aldehyde reactions, followed by dehydration to form conjugated enal, and hydrogenation to generate n-butanol and LAS. Due to the lack of high-performance catalysts to break the limitations of the stepped growth model, the selectivity of LAS is still very low. Therefore, it is still challenging to design new catalysts to stabilize and accumulate more aldehyde intermediates at the catalytic active site and accelerate the coupling of ETOH to LAS. It was found that the preparation of a unique Ni surface by sulfur modification may provide a strong adsorption site for aldehydes, but is inert for side reactions. In addition, achieving controlled Ni site exposure and chalcogenide modification on the catalyst surface remains a challenge.
Figure 1Schematic diagram of ethanol upgrading to long-chain alcohols
**Explained
Catalytic performance
On a 180 No S ni@c-S0 catalyst, the LAS selectivity is 492% (C8+OH 26.)0%), and the ETOH conversion rate is 518%。In order to explore the promoting effect of surface sulfur modification, the authors systematically screened a series of ni@c-Sx catalysts with different S-Ni ratios, among which the peak value was 1 30 at the S-Ni ratio. On the ni@c-S1 30 catalyst, the ETOH conversion rate and LAS selectivity were significantly improved, reaching 76., respectively5% and 705% (C8+OH 43.)0%）。With the reaction time from 15 h to 12 h, the ETOH conversion rate increased from 376% gradually increased to 765% and remained largely unchanged for the next 12 h. In addition, the catalytic performance of the ni@c-S130 catalyst can be greatly improved at 99At a 1% ethyl dehydrogenation conversion rate, the LAS selectivity is 742% (15 for C6OH.)2%, C8+OH is 590%）。It should be noted that the prepared ni@c-S1 30 catalyst has the highest Etoh conversion and Las selectivity, especially for C8+OH. The proportion of LAS is as high as 96 out of all ETOH products4%, while the ETOH conversion rate and LAS selectivity were maintained well for 6 consecutive cycles.
Figure 2Catalytic performance of ni@c-SX catalysts
Mechanistic studies
Using density functional theory (DFT) calculations, the authors investigated the facilitation of surface sulfur during the conversion of water ETOH to LAS. Firstly, the authors constructed the optimized surface structure of Ni(111) with different sulfur coverage ratios, and studied the adsorption behavior of n-butyraldehyde. DFT calculations show that the n-butyraldehyde molecule is adsorbed on the bridge by oxygen atoms, and the n-butyraldehyde molecule can still be stably adsorbed when there are multiple sulfur atoms around. As the sulfur coverage increases from 0 to 20%, the adsorption energy of n-butyraldehyde is 121-1.34 EV range. During dehydrogenation, CH3CH2OH* and CH3CH2O* are adsorbed at the Ni site by oxygen atoms. The dehydroactivation energies at Ni-S0%, Ni-S14% and Ni-S25% were respectively. 70 and 080 ev。The dehydrogenation activation energy of Ni-S 14% was similar to that of Ni-S0%, indicating that the dehydrogenation process was not sensitive to low sulfur coverage.
For C-C bond cleavaged RLS, the activation barrier of CH3CO* to CH3* and CO* continues from 0 as the sulfur coverage increases from 0 to 25%.81 EV increased to 110 eV, indicating that C-C bond cleavage on the Ni surface with low sulfur coverage is effectively inhibited. The schematic diagram shows that the ETOH molecule is dehydrogenated into an acetaldehyde molecule and further upgraded to a n-butyraldehyde molecule. At the same time, the steric hindrance of sulfur on the Ni surface inhibited the cleavement of C-C bonds of EtoH, reduced the consumption of NaOH and the occupation of active sites, and promoted the aldehyde of acetaldehyde molecules. The strongly adsorbed acetaldehyde and n-butyraldehyde molecules are enriched on the Ni surface and finally converted to LAS through the direct growth pathway.
Figure 3Mechanism of ni@c-SX catalytic production of LAS
Figure 4DFT calculation results and reaction mechanism
Synthesis flexibility
During the reaction, dimethyl sulfoxide (DMSO) was directly introduced into the aqueous solution of ether to modify the ni@c-S0 catalyst in situ. With the increase of DMSO, the catalytic activity showed a volcanic trend, when the DMSO fraction increased to 0At 006%, the ETOH conversion rate and LAS selectivity are maximized. A catalyst with NiSO4 as a precursor and high selectivity for LAS (28. for C6OH).5%, C8+OH is 407%）。The results showed that the change of sulfur precursors could effectively improve the conversion rate and alcohol selectivity of EtoH, but there were some differences in the degree of toxicity of different sulfur precursors. The ni@c-S0 catalyst was compared with the ni@c-S1 30-Y (Y represents the sulfur precursor) and the best ni@c-DMSO0006 catalyst has a high conversion rate (> 800%), all of which showed ideal LAS selectivity (>58.).0%）。In conclusion, the flexibility of surface sulfur modification for the preparation of ni@c-SX catalysts was confirmed in terms of addition methods and sulfur precursors.
Figure 5Performance of catalysts prepared by different synthesis methods
Bibliographic information
enabling direct-growth route for highly efficient ethanol upgrading to long-chain alcohols in aqueous phase. nature communications,