Understanding and Tuning the Effects of H2O on Catalytic CO and CO2 Hydrogenation.

Catalytic COx (CO and CO2) hydrogenation to valued chemicals is one of the promising approaches to address challenges in energy, environment, and climate change. H2O is an inevitable side product in these reactions, where its existence and effect are often ignored. In fact, H2O significantly influences the catalytic active centers, reaction mechanism, and catalytic performance, preventing us from a definitive and deep understanding on the structure-performance relationship of the authentic catalysts. It is necessary, although challenging, to clarify its effect and provide practical strategies to tune the concentration and distribution of H2O to optimize its influence. In this review, we focus on how H2O in COx hydrogenation induces the structural evolution of catalysts and assists in the catalytic processes, as well as efforts to understand the underlying mechanism. We summarize and discuss some representative tuning strategies for realizing the rapid removal or local enrichment of H2O around the catalysts, along with brief techno-economic analysis and life cycle assessment. These fundamental understandings and strategies are further extended to the reactions of CO and CO2 reduction under an external field (light, electricity, and plasma). We also present suggestions and prospects for deciphering and controlling the effect of H2O in practical applications.

Functionalisation of MUF-15 enhances CO2/CH4 selectivity in mixed-matrix membranes.

MUF-15 (MUF = Massey University Framework) is a metal-organic framework with pores that can be tuned by ligand functionalisation. Crystallites of MUF-15 and derivatives were blended with the organic polymer 6FDA-DAM to produce mixed-matrix membranes (MMMs). At a loading of 30 wt%, membranes with MUF-15-F, MUF-15 with an appended fluoro group, exhibited a CO2 permeability of 1300 Barrer and CO2/CH4 selectivity of 37.1. These values surpass membranes with the parent MUF-15 and exceed the Robeson upper bound.

Heterogeneous Catalytic Systems for Carbon Dioxide Hydrogenation to Value-Added Chemicals.

Utilizing renewable energy to hydrogenate carbon dioxide into fuels eliminates massive CO2 emissions from the atmosphere and diminishes our need for using fossil fuels. This review presents the most recent developments for designing heterogeneous catalysts for the hydrogenation of CO2 to formate, methanol, and C2+ hydrocarbons. Thermodynamic challenges and mechanistic insights are discussed, providing a strong foundation to propose a suitable catalyst. The main body of this review focuses on nanostructured catalysts for constructing efficient heterogeneous systems. The most important factors affecting catalytic performance are highlighted, including active metals, supports and promoters that can potentially be used. The summary of the results and the outlook are presented in the final section. During the past few decades, heterogeneous CO2 hydrogenation has gained much attention and made tremendous progress. Thus, many highly efficient catalysts have been studied to discover their active sites and provide mechanistic insights. This paper summarizes recent advances in CO2 hydrogenation and its conversion into various hydrocarbons such as formate, methanol, and C2+ products. As for formate production, Au and Ru nanocatalysts show superior activity. However, considering the catalyst cost, Cu-based catalysts have an excellent prospect for methanol production, among other catalysts. Ultra-small nanoparticles and nanoclusters appear promising to provide highly active cost-effective catalysts. A growing number of researchers are investigating the possibility of directly synthesizing C2+ products through CO2 hydrogenation. The major challenge in producing heavy hydrocarbons is breaking the ASF limitations, which have been achieved over bifunctional catalysts using zeolites. Using suitable support and promoter can lead to a superior activity, ascribed to structural, electronic, and chemical promotional effects.

Rational Design of MXene Hollow Fiber Membranes for Gas Separations.

One scalable and facile dip-coating approach was utilized to construct a thin CO2-selection layer of Pebax/PEGDA-MXene on a hollow fiber PVDF substrate. An interlayer spacing of 3.59 Å was rationally designed and precisely controlled for the MXene stacks in the coated layer, allowing efficient separation of the CO2 (3.3 Å) from N2 (3.6 Å) and CH4 (3.8 Å). In addition, CO2-philic nanodomains in the separation layer were constructed by grafting PEGDA into MXene interlayers, which enhanced the CO2 affinity through the MXene interlayers, while non-CO2-philic nanodomains could promote CO2 transport due to the low resistance. The membrane could exhibit optimal separation performance with a CO2 permeance of 765.5 GPU, a CO2/N2 selectivity of 54.5, and a CO2/CH4 selectivity of 66.2, overcoming the 2008 Robeson upper bounds limitation. Overall, this facile approach endows a precise controlled molecular sieving MXene membrane for superior CO2 separation, which could be applied for interlayer spacing control of other 2D materials during membrane construction.