Liangshu Zhong

Cobalt carbide nanoprisms for direct production of lower olefins from syngas

Lower olefins—generally referring to ethylene, propylene and butylene—are basic carbon-based building blocks that are widely used in the chemical industry, and are traditionally produced through thermal or catalytic cracking of a range of hydrocarbon feedstocks, such as naphtha, gas oil, condensates and light alkanes. With the rapid depletion of the limited petroleum reserves that serve as the source of these hydrocarbons, there is an urgent need for processes that can produce lower olefins from alternative feedstocks. The ‘Fischer–Tropsch to olefins’ (FTO) process has long offered a way of producing lower olefins directly from syngas—a mixture of hydrogen and carbon monoxide that is readily derived from coal, biomass and natural gas. But the hydrocarbons obtained with the FTO process typically follow the so-called Anderson–Schulz–Flory distribution, which is characterized by a maximum C2–C4 hydrocarbon fraction of about 56.7 per cent and an undesired methane fraction of about 29.2 per cent (refs 1, 10, 11, 12). Here we show that, under mild reaction conditions, cobalt carbide quadrangular nanoprisms catalyse the FTO conversion of syngas with high selectivity for the production of lower olefins (constituting around 60.8 per cent of the carbon products), while generating little methane (about 5.0 per cent), with the ratio of desired unsaturated hydrocarbons to less valuable saturated hydrocarbons amongst the C2–C4 products being as high as 30. Detailed catalyst characterization during the initial reaction stage and theoretical calculations indicate that preferentially exposed {101} and {020} facets play a pivotal role during syngas conversion, in that they favour olefin production and inhibit methane formation, and thereby render cobalt carbide nanoprisms a promising new catalyst system for directly converting syngas into lower olefins.

Direct conversion of CO2 into liquid fuels with high selectivity over a bifunctional catalyst

Although considerable progress has been made in carbon dioxide (CO2) hydrogenation to various C1 chemicals, it is still a great challenge to synthesize value-added products with two or more carbons, such as gasoline, directly from CO2 because of the extreme inertness of CO2 and a high C–C coupling barrier. Here we present a bifunctional catalyst composed of reducible indium oxides (In2O3) and zeolites that yields a high selectivity to gasoline-range hydrocarbons (78.6%) with a very low methane selectivity (1%). The oxygen vacancies on the In2O3 surfaces activate CO2 and hydrogen to form methanol, and C−C coupling subsequently occurs inside zeolite pores to produce gasoline-range hydrocarbons with a high octane number. The proximity of these two components plays a crucial role in suppressing the undesired reverse water gas shift reaction and giving a high selectivity for gasoline-range hydrocarbons. Moreover, the pellet catalyst exhibits a much better performance during an industry-relevant test, which suggests promising prospects for industrial applications. It is still a great challenge to synthesize value-added products with two or more carbons directly from CO2. Now, a bifunctional catalyst composed of reducible metal oxides (In2O3) and zeolites (HZSM-5) is prepared and yields high selectivity to gasoline-range hydrocarbons (78.6%) with a high octane number directly from CO2 hydrogenation.

CuFe, CuCo and CuNi nanoparticles as catalysts for higher alcohol synthesis from syngas: a comparative study

Higher alcohol synthesis (HAS) from syngas has attracted much attention and Cu-modified Fischer–Tropsch (FT) catalysts exhibited promising catalytic performance for HAS. In this paper, three model modified FT catalysts, CuFe, CuCo and CuNi nanoparticles, were synthesized by co-reduction method for the comparison of their performance in HAS. XRD, TEM and EDS characterizations for spent samples indicate that severe phase separation of Cu and Fe took place for CuFe, and Cu@Co core–shell structure formed with co-existence of Cu–Co alloy nanoparticles for CuCo, but only Cu–Ni alloys were observed for CuNi. Such structural change led to different performance in higher alcohol synthesis. As a result, CuFe mainly kept the original FT property of Fe, CuCo showed different performance from Co, and CuNi performed as methanol catalyst.

Preparation and activity of Cu/Zn/Al/Zr catalysts via hydrotalcite-containing precursors for methanol synthesis from CO2 hydrogenation

A series of Cu/Zn/Al/Zr catalysts were synthesized by calcination of hydrotalcite-containing precursors with different Cu2+/Zn2+ atomic ratios (n). Two other catalysts (n = 2) were also prepared via phase-pure hydrotalcite-like and conventional rosasite precursors for comparison. XRD and UV-Vis-NIR DRS characterizations demonstrate that most Cu2+ of hydrotalcite-containing materials did not enter the layer structure. The Cu dispersion of the catalysts decreases with the increase of Cu content, while both the exposed Cu surface area and the Cu+ and Cu0 content on the reduced surface reach a maximum when n is 2. The catalytic performance for the methanol synthesis from CO2 hydrogenation was also tested. The catalytic activity and selectivity of the catalysts (n = 0.5–4) via hydrotalcite-containing precursors rise first and then decrease with increasing Cu2+/Zn2+ ratios, and the optimum performance is obtained over the catalyst with Cu2+/Zn2+ = 2. Moreover, the Cu/Zn/Al/Zr catalyst (n = 2) via hydrotalcite-containing precursor exhibits the best catalytic performance, which is mainly due to the maximum content of active species compared with another two catalysts derived from different precursors.

Advances in bifunctional catalysis for higher alcohol synthesis from syngas

Abstract Bifunctional catalysis on dual sites plays an important role in higher alcohol synthesis from syngas. It makes use of two types of active sites of which one type dissociates CO and forms surface alkyl species and the other type catalyzes non-dissociative CO adsorption for CO insertion and alcohol formation. To improve catalytic activity for higher alcohol synthesis, it is necessary to design dual sites on the atomic scale to give them high stability. The recent advances in higher alcohol synthesis using bifunctional catalysts are reviewed. The design of the dual sites, the structure of the dual sites on several typical catalyst systems, and the structural evolution of the dual sites during reaction are discussed using our latest research results.

Higher alcohol synthesis over Cu-Fe composite oxides with high selectivity to C2+OH

Abstract Cu-Fe composite oxides were prepared by co-precipitation method and tested for higher alcohol synthesis from syngas. The selectivity to C 2+ OH and C 6+ OH in alcohol distribution was very high while the methane product fraction in hydrocarbon distribution was rather low, demonstrating a promising potential in higher alcohols synthesis from syngas. The distribution of alcohols and hydrocarbons approximately obeyed Anderson-Schulz-Flory distribution with similar chain growth probability, indicating alcohols and hydrocarbons derived from the same intermediates. The effects of Cu/Fe molar ratio, reaction temperature and gas hourly space velocity (GHSV) on catalytic performance were studied in detail. The sample with a Cu/Fe molar ratio of 10/1 exhibited the best catalytic performance. Higher reaction temperature accelerated water-gas-shift reaction and led to lower total alcohols selectivity. GHSV showed great effect on catalytic performance and higher GHSV increased the total alcohol selectivity, indicating there existed visible dehydration reaction of alcohol into hydrocarbon.

Yttrium oxide modified Cu/ZnO/Al2O3 catalysts via hydrotalcite-like precursors for CO2 hydrogenation to methanol

Y2O3-modified Cu/ZnO/Al2O3 catalysts (Cu2+ : Zn2+ : (Al3+ + Y3+) = 2 : 1 : 1) via hydrotalcite-like precursors were synthesized with Y3+ : (Al3+ + Y3+) atomic ratios between 0 and 0.5. With the introduction of certain amounts of Y2O3, the surface area and dispersion of Cu for the Cu/ZnO/Al2O3 catalysts increased significantly. However, excess Y2O3 content would decrease the dispersion of both Cu and ZnO as a result of the reduced amount of hydrotalcite-like phases in the precursors. It was suggested that Y2O3 and the hydrotalcite-like structure could prevent the aggregation of Cu nanoparticles during reduction and reaction and improve the reducibility of Cu2+ species. As Cu0 species were the predominant active sites for methanol synthesis from CO2 hydrogenation, addition of suitable amounts of Y2O3 to the Cu/ZnO/Al2O3 catalyst enhanced the catalytic activity for CO2 hydrogenation remarkably. Nevertheless, CO2 conversion decreased significantly when Y3+ : (Al3+ + Y3+) > 0.1 due to the lower surface area of Cu and a relatively weaker interaction between Cu and ZnO. The Cu/ZnO/Al2O3/Y2O3 catalyst with Y3+ : (Al3+ + Y3+) = 0.1 derived from hydrotalcite-like compounds exhibited the best catalytic performance with high stability.

Elucidation of reaction network of higher alcohol synthesis over modified FT catalysts by probe molecule experiments

The reaction network of higher alcohol synthesis over a CuFe/ZrO2 modified FT catalyst was investigated by the probe molecule technique, and the influence of various probe molecules (alkane, alcohol, aldehyde and olefin) on the catalytic performance was evaluated in detail. Alcohol selectivity and CO conversion remained almost constant when alkane was used as the probe molecule. However, the products shifted to higher carbon number significantly due to the alleviation of the retention of heavy products by the extraction of the added alkane. When alcohol was introduced into the reaction system, a considerable amount of esters with unique structures was formed with the formula RCO2R′, where R′ is the alkyl group from the added alcohol, and such esters followed ASF distribution with a similar chain propagation factor to that of the corresponding higher alcohols. With the addition of aldehyde as the probe molecule, most of the added aldehyde was directly hydrogenated to the corresponding alcohol, but a considerable amount of esters was also detected in the reaction products, which could be attributed to the reaction between the surface acyls adsorbed from the introduced aldehyde molecules and the alcohols synthesized by syngas. The added 1-olefins were directly hydrogenated to the corresponding paraffins, incorporated into the growing carbon chain to form higher products or transformed into alcohols with one more carbon number by CO insertion reaction. Based on these results, the catalytic mechanism and reaction network of higher alcohol synthesis over a modified FT catalyst were discussed.

A review of the catalytic hydrogenation of carbon dioxide into value-added hydrocarbons

Chemical utilization of CO2 to chemicals and fuels is very attractive because it can not only alleviate global warming caused by increasing atmospheric CO2 concentration but also offer a solution to replace dwindling fossil fuels. Hydrogen is a high-energy material and can be used as the reagent for CO2 transformation. Moreover, when hydrogen originates directly from renewable energy, CO2 hydrogenation can also provide an important approach for dealing with the intermittence of renewable sources by storing energy in chemicals and fuels. Therefore, much attention has been paid to CO2 hydrogenation to various value-added hydrocarbons, such as lower olefins, liquefied petroleum gas, gasoline, aromatics and so on. The focus of this perspective article is on the indirect and direct routes for production of hydrocarbons from CO2 hydrogenation and recent developments in catalyst design, catalytic performance and reaction mechanism. In addition, a brief overview on CO2 hydrogenation to methanol is given, which is a critical process in the indirect route involving conversion of CO2 into methanol and subsequent transformation into hydrocarbons. We also provide an overview of the challenges in and opportunities for future research associated with CO2 hydrogenation to value-added hydrocarbons.

Effects of acid pretreatment on Fe-ZSM-5 and Fe-beta catalysts for N2O decomposition

Two series of ZSM-5 and beta zeolites were pretreated in 1.0 mol/L HNO 3 solution at room temperature for various time periods. The catalytic performances of their Fe-exchanged products in N 2 O decomposition were evaluated. The Fe-zeolite catalysts were characterized using N 2 adsorption-desorption, inductively coupled plasma optical emission spectroscopy, X-ray diffraction, ultraviolet-visible spectroscopy, temperature-programmed desorption of NH 3 , and scanning and transmission electron microscopies. For the ZSM-5 zeolite, acid leaching primarily takes place on the crystal surface and the particle size is reduced, therefore the pore channels are shortened. However, because of the good stability of MFI zeolites, the acid does not greatly penetrate the pore channels and new mesopores are not created. For the beta zeolite, because the amorphous material is inclined to dissolve (deagglomerate), some of the micropores are slightly dilated. The improved catalytic activities can be explained by the increased active Fe loading as a result of structural changes.© 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.