Hua Song

Catalysts for the selective catalytic reduction of NOx with NH3 at low temperature

Selective catalytic reduction of NOx with NH3 (NH3-SCR) at low temperature is a major challenge in environmental catalysis. In recent years, great efforts have been devoted to the development of low-temperature SCR catalysts for both stationary sources and diesel engines. Mn-based catalysts have attracted great attention due to their excellent low-temperature activity. However, vulnerability to SO2 and H2O poisoning and preference for N2O formation make these catalysts still far away from industrial application. V2O5 loaded on carbon materials has shown both high SCR activity and SO2 tolerance at low-temperature. This type of catalyst is very promising for applications in low-temperature SCR for stationary sources. Recently, Cu-containing small pore zeolites, such as Cu-SSZ-13 and Cu-SAPO-34 with a CHA structure and Cu-SSZ-39 and Cu-SAPO-18 with an AEI structure, were shown to have very high activity at low temperature and excellent hydrothermal stability at high temperature and thus received much attention for applications on diesel vehicles. In this review, we will focus on the recent studies on low-temperature NH3-SCR catalysts. In addition, the future directions of low-temperature SCR development will also be discussed.

Low-temperature and low-pressure non-oxidative activation of methane for upgrading heavy oil

It is highly desirable to upgrade viscous heavy oil, such as bitumen extracted from Canadian oil sand, to be transportable by pipeline. Conventionally, this is achieved by expensive catalytic hydrogenation under a hydrogen pressure of 15–20 MPa. In this study, it is reported that by using zinc and silver cation-modified HZSM-5 as the catalyst, methane can be activated at a low temperature of 380 °C and a pressure of 5 MPa to efficiently upgrade heavy oil, leading to the formation of partially upgraded crude oil which is more desirable for pipeline transportation and downstream refining. In addition, methane activation and its participation in the upgrading process were further evidenced by employing butylbenzene, styrene, and benzene as model compounds to represent heavy oil. This study opens a door for upgrading heavy oil with natural gas under fairly mild operation conditions instead of expensive hydrogen under rather stringent ones.

Converting solid wastes into liquid fuel using a novel methanolysis process

Biomass fast pyrolysis followed by hydrodeoxygenation upgrading is the most popular way to produce upgraded bio-oil from biomass. This process requires large quantities of expensive hydrogen and operates under high pressure condition (70-140 atm). Therefore, a novel methanolysis (i.e., biomass pyrolysis under methane environment) process is developed in this study, which is effective in upgraded bio-oil formation at atmospheric pressure and at about 400-600°C. Instead of using pure methane, simulated biogas (60% CH4+40% CO2) was used to test the feasibility of this novel methanolysis process for the conversion of different solid wastes. The bio-oil obtained from canola straw is slightly less than that from sawdust in term of quantity, but the oil quality from canola straw is better in terms of lower acidity, lower Bromine Number, higher H/C atomic ratio and lower O/C atomic ratio. The municipal solid waste and newspaper can also obtain relatively high oil yields, but the oil qualities of them are both lower than those from sawdust and canola straw. Compared with catalysts of 5%Zn/ZSM-5 and 1%Ag/ZSM-5, the 5%Zn-1%Ag/ZSM-5 catalyst performed much better in terms of upgraded bio-oil yield as well as oil quality. During the methanolysis process, the metal silver may be used to reduce the total acid number of the oil while the metal zinc might act to decrease the bromine number of the oil. The highly dispersed Zn and Ag species on/in the catalyst benefit the achievement of better upgrading performance and make it be a very promising catalyst for bio-oil upgrading by biogas.

Catalytic aromatization of acetone as a model compound for biomass-derived oil under a methane environment

The feasibility of upgrading acetone to fuels and valuable chemicals under a methane environment was investigated over various metal-modified zeolite catalysts at 400 °C and 3 MPa. Among these, HZSM-5 impregnated with Zn and Ga was found to produce the highest yield of BTEX in comparison to the other prepared catalysts. Catalyst characterization techniques, including XPS, NH3-TPD and TEM, showed that the excellent metal dispersion in the inner pores of the catalyst, a balance of weak, medium and strong acid sites as well as the stable chemical states of Zn and Ga under the methane environment favoured the selectivity towards BTEX. Additionally, the presence of methane could enhance the liquid yield by incorporation into the liquid products, as evidenced by NMR spectroscopy through an isotopic-labelling study. These findings lead to a better understanding of the interaction and reaction mechanism involved in the upgrading of biomass-derived ketones under a methane environment for production of fuels and valuable chemicals, which could potentially contribute to a cost-effective substitution of petroleum-derived chemicals and fuels for biomass-derived products.

Direct catalytic co-conversion of cellulose and methane to renewable petrochemicals

The catalytic co-conversion of cellulose and methane to aromatics was investigated over various Zn-containing zeolite catalysts. Higher aromatic yield (42.3% C) with BTEX selectivity of 70% and much lower solid residue yield (char: 9.45% C) are achieved over 3%Zn(II)–Znδ+/ZSM5 (0 < δ < 2) at 450 °C and 2.5 MPa. The loading of ZnO clusters or Zn2+ ions could be beneficial for the formation of aromatic products, and the introduction of Znδ+ species via CVD could improve BTEX selectivity and methane conversion. Co-feeding with methane inhibits the formation of coke or heavy substances and maximizes the carbon utilization efficiency for the formation of aromatics. Methane also enhances the oxygen removal efficiency to improve the quality of the liquid products. Methane participation in the formation of aromatic products is evidenced by liquid 1H, 2H and 13C NMR investigations, which reveal that methane tends to be incorporated into both the methyl group and the phenyl ring. The results of pyridine absorption and NH3-TPD indicate that the balanced distribution of Bronsted and Lewis acid sites and the appropriate ratio of weak to moderate acidic sites may benefit aromatic formation. XPS and XAS spectra of Zn species confirm the presence of Znδ+ species with oxygen vacancies, which show a higher selectivity for petrochemicals. The results reported in this work will give more insight into the catalytic chemistry of cellulose valorization under a methane environment and the design of rational catalysts for the cost-efficient utilization of biomass resources and natural gas.

Catalytic Biomass Valorization

Biomass is a significant non-conventional energy reserve, which has been considered as a promising alternative over other renewable sources such as solar, wind or hydroelectric storage due to its comparatively ample availability. A variety of biomass types can be converted into useful products via bioenergy technologies. The deep understanding and knowledge of these processes are necessary for optimization and advancement in a cost-effective way. A comprehensive comparison and discussion is conducted with respect to biochemical and thermochemical conversion technology such as microbic digestion and fermentation, pyrolysis, liquefaction and gasification. Pyrolysis is the process of converting biomass into bio oil, charcoal and gaseous factions by heating anaerobically to above 500°C. Liquefaction is a low temperature (LT) and high-pressure thermochemical process to produce marketable liquid over suitable catalysts under hydrogen or reductive environment. Gasification is the conversion of biomass into preferred combustible gas mixture (syngas) via the partial oxidation at high temperature, typically in the range of 800–900°C. The product gas is more versatile and can be burned in gas turbine for electricity production or synthesis of high-value chemicals. The parametric impact, mechanism, development status and future direction have been summarized for each of these technologies with the aim to pave the way for optimization of future investigation.