CdS Nanorods Anchored with Crystalline FeP Nanoparticles for Efficient Photocatalytic Formic Acid Dehydrogenation

Selective Transformation of Biomass and the Derivatives for Aryl Compounds and Hydrogen via Visible-Light-Induced Radicals

ConspectusFor sustainable development, exploring renewable resources is an urgent priority. Nonfood biomass, one of the largest renewable resources on Earth, primarily comprises three key components: lignin (ca. 15-30%), cellulose (ca. 35-50%), and hemicellulose (ca. 20-30%). Theoretically, nonfood biomass can be converted into green chemicals and energy. However, most studies have focused on the generation of chemicals and carbon-based energy under harsh conditions, often resulting in lower selectivities. Therefore, further efforts to explore efficient and selective methods for producing chemicals and hydrogen (H2) are essential to promoting the practical applications of renewable biomass. In this Account, we summarize our contributions to the efficient and selective transformation of biomass and its derivatives into aryl compounds and H2. These transformations were achieved using visible-light-induced photocatalytic systems that generate active radicals to selectively cleave C-C, C-O, C-H, and O-H bonds under mild conditions, without using noble metals. First, aryl compound production was achieved by chemoselective cleavage of C-C and C-O bonds using aryl carboxyl radicals and aryl ether radical cations. Specifically, the aryl carboxyl radical in the charge-transfer complex induced the chemoselective cleavage of C-C bonds of aryl carboxylic acids, which are platform molecules derived from lignin oxidation; the aryl carboxyl radical in free form facilitated the chemoselective cleavage of C-O bonds in the model of the 4-O-5 lignin linkage. Moreover, arenols and aryl alcohols were obtained via cooperation of the aryl ether radical cation and the vanadate-induced chemoselective cleavage of the C-O bonds of the models of various lignin linkages. Second, we developed a streamlined strategy for H2 production from biomass using a one-pot, two-step route with formic acid (HCO2H) as an intermediate for H2 storage by thermocatalysis. Using this strategy by photoredox catalysis, HCO2H was initially obtained via the alkoxy radical-induced gradual cleavage of C-C bonds in cellulose, hemicellulose, glucose, and their derivatives. Subsequently, efficient H2 production from biomass-based HCO2H was realized via hydroxyl radical (·OH)-induced C-H and the following cleavage of the O-H bonds, with cooperation of the nickel catalysis. Third, the highest H2 production capability from biomass was achieved via efficient water reforming. This process utilized alkoxy radicals followed by generated carbon cations via electrocatalysis, inducing a well-organized cleavage of C-C, O-H, and C-H bonds. We anticipate that these insights will inspire the development of more efficient, stable, and cost-effective catalytic systems, accelerating the utilization of biomass as a renewable resource and driving other related significant transformations.

State-of-the-Art Light-Driven Hydrogen Generation from Formic Acid and Utilization in Enzymatic Hydrogenations

A concept of combining photocatalytically generated hydrogen with green enzymatic reductions is demonstrated. The developed photocatalytic formic acid (FA) dehydrogenation setup based on Pt(x)@TiO2 shows stable hydrogen generation activity, which is two orders of magnitude higher than reported values of state-of-the-art systems. Mechanistic studies confirm that hydrogen generation proceeds via a photocatalytic pathway, which is entirely different from purely thermal reaction mechanisms previously reported. The viability of the presented approach is demonstrated by the synthesis of value-added compounds 3-phenylpropanal and (2R, 5S)-dihydrocarvone at ambient pressure and room temperature, which should be applicable for many other hydrogenation processes, e. g., for the preparation of flavours and fragrance compounds, as well as pharmaceuticals.

Mixed Crystalline Covalent Heptazine Frameworks with Built-in Heterojunction Structures towards Efficient Photocatalytic Formic Acid Dehydrogenation

Covalent heptazine frameworks (CHFs) are widely utilized in the recent years as potential photocatalysts. However, their limited conjugated structures, low crystallinity and small surface areas have restricted the practical photocatalysis performance. Along this line, we report herein the synthesis of a kind of mixed crystalline CHF (m-CHF-1) with built-in heterojunction structure, which can efficiently catalyze the formic acid dehydrogenation by visible light driven photocatalysis. The m-CHF-1 is synthesized from 2,5,8-triamino-heptazine and dicyanobenzene (DCB) in the molten salts, in which DCB plays as organic molten co-solvent to promote the rapid and ordered polymerization of 2,5,8-triamino-heptazine. The m-CHF-1 is formed by embedding phenyl-linked heptazine (CHF-Ph) units in the poly(heptazine imide) (PHI) network similar to doping. The CHF-Ph combined with PHI form an effective type II heterojunction structure, which promote the directional transfer of charge carriers. And the integration of CHF-Ph makes m-CHF-1 have smaller exciton binding energy than pure PHI, the charge carriers are more easily dissociated to form free electrons, resulting in higher utilization efficiency of the carriers. The largest hydrogen evolution rate reaches a value of 42.86 mmol h-1 g-1 with a high apparent quantum yield of 24.6 % at 420 nm, which surpasses the majority of other organic photocatalysts.

Solar-Driven Biomass Reforming for Hydrogen Generation: Principles, Advances, and Challenges

Hydrogen (H2) has emerged as a clean and versatile energy carrier to power a carbon-neutral economy for the post-fossil era. Hydrogen generation from low-cost and renewable biomass by virtually inexhaustible solar energy presents an innovative strategy to process organic solid waste, combat the energy crisis, and achieve carbon neutrality. Herein, the progress and breakthroughs in solar-powered H2 production from biomass are reviewed. The basic principles of solar-driven H2 generation from biomass are first introduced for a better understanding of the reaction mechanism. Next, the merits and shortcomings of various semiconductors and cocatalysts are summarized, and the strategies for addressing the related issues are also elaborated. Then, various bio-based feedstocks for solar-driven H2 production are reviewed with an emphasis on the effect of photocatalysts and catalytic systems on performance. Of note, the concurrent generation of value-added chemicals from biomass reforming is emphasized as well. Meanwhile, the emerging photo-thermal coupling strategy that shows a grand prospect for maximally utilizing the entire solar energy spectrum is also discussed. Further, the direct utilization of hydrogen from biomass as a green reductant for producing value-added chemicals via organic reactions is also highlighted. Finally, the challenges and perspectives of photoreforming biomass toward hydrogen are envisioned.

Boosting photocatalytic conversion of formic acid to CO over P-doped CdS

In this work, NaH2PO2, Na2S2O3 and CdCl2 were used to synthesize P-doped CdS samples for the photocatalytic decomposition of formic acid to CO reaction. The CO production rates and selectivity of P-doped CdS are as high as 24.5 mmol g-1 h-1 and 92.4%, in which the rate is 7 times higher than that of the pure CdS. Multiple characterizations show that the P-doping increases the specific surface area, widens the band gap and shifts the energy band position of CdS, resulting in enhanced photocatalytic activity.

Hydrogen Production from Formic Acid by In Situ Generated Ni/CdS Photocatalytic System under Visible Light Irradiation

Simple and practical noble-metal-free catalyzed hydrogen production from sustainable resources, such as renewable formic acid, is highly desirable. Herein, the development of an efficient photocatalytic hydrogen production from aqueous solution of formic acid using in situ generated Ni/CdS photocatalytic system was described. CdS-Cys (Cys=l-cysteine) quantum dots (QDs) acting as photocatalyst with Ni(OAc)₂ as H₂ production catalyst precursor, a 94 % yield was obtained within 5 h under visible light irradiation at 50 °C. The average rate of H₂ production reached up to 282 μmol mg^-1  h^-1 with 99.8 % H₂ selectivity. Mechanistic studies indicate cooperation of dynamic quenching and static quenching of CdS-Cys QDs by Ni(OAc)₂ . Especially, Ni⁰ , generated in the dynamic quenching, accelerated the electron transfer by acting as an electron outlet and enhancing the stability of CdS to slow down the photocorrosion distinctly, delivering efficient H₂ production with high selectivity. Our study will inspire exploration of various efficient non-noble-metal catalysts for practical H₂ production from bio-based formic acid.

Liquid Sunshine: Formic Acid

"Liquid sunshine" is the conceptual green liquid fuel that is produced by a combination of solar energy, CO₂, and H₂O. Alcohols are commonly regarded as the preferred candidates for liquid sunshine because of their advantages of high energy density and extensive industrial applications. However, both the alcohol synthesis and H₂ release processes require harsh reaction conditions, resulting in large external energy input. Unlike alcohols, the synthesis and dehydrogenation of formic acid (FA)/formate can be performed under mild conditions. Herein, we propose liquid sunshine FA/formate as a promising supplement to alcohol. First, we outline the vision of using FA/formate as liquid sunshine and discuss its feasibility. Then, we concentrate on the application of FA/formate as liquid organic hydrogen carrier and summarize the recent developments of CO₂ hydrogenation to FA/formate and FA/formate dehydrogenation under mild conditions. Finally, we discuss the current applications, challenges, and opportunities surrounding the use of FA/formate as liquid sunshine.

Selective Photocatalytic Dehydrogenation of Formic Acid by an In Situ-Restructured Copper-Postmetalated Metal-Organic Framework under Visible Light

Formic acid is considered as one of the most promising liquid organic hydrogen carriers. Its catalytic dehydrogenation process generally suffers from low activity, low reaction selectivity, low stability of the catalysts, and/or the use of noble-metal-based catalysts. Herein we report a highly selective, efficient, and noble-metal-free photocatalyst for the dehydrogenation of formic acid. This catalyst, UiO-66(COOH)₂-Cu, is built by postmetalation of a carboxylic-functionalized Zr-MOF with copper. The visible-light-driven photocatalytic dehydrogenation process through the release of hydrogen and carbon dioxide has been monitored in real-time viaoperando Fourier transform infrared spectroscopy, which revealed almost 100% selectivity with high stability (over 3 days) and a conversion yield exceeding 60% (around 5 mmol·g_cat^–1·h^–1) under ambient conditions. These performance indicators make UiO-66(COOH)₂-Cu among the top photocatalysts for formic acid dehydrogenation. Interestingly, the as-prepared UiO-66(COOH)₂-Cu hetero-nanostructure was found to be moderately active under solar irradiation during an induction phase, whereupon it undergoes an in-situ restructuring process through intraframework cross-linking with the formation of the anhydride analogue structure UiO-66(COO)₂-Cu and nanoclustering of highly active and stable copper sites, as evidenced by the operando studies coupled with steady-state isotopic transient kinetic experiments, transmission electron microscopy and X-ray photoelectron spectroscopy analyses, and Density Functional Theory calculations. Beyond revealing outstanding catalytic performance for UiO-66(COO)₂-Cu, this work delivers an in-depth understanding of the photocatalytic reaction mechanism, which involves evolutive behavior of the postmetalated copper as well as the MOF framework over the reaction. These key findings pave the way toward the engineering of new and efficient catalysts for photocatalytic dehydrogenation of formic acid.

An in situ derived MOF@In2S3 heterojunction stabilizes Co(II)-salicylaldimine for efficient photocatalytic formic acid dehydrogenation

We report here the hierarchical construction of a molecular Co(II)-salicylaldimine catalyst and an in situ derived In₂S₃ semiconductor in a MOF@In₂S₃ heterojunction through sequentially controllable in situ etching and post-synthetic modification for photocatalytic hydrogen production from formic acid. The enhanced catalyst stability and facilitated charge carrier mobility between the In₂S₃ photosensitizers and Co catalyst realize a superior H₂ production rate of 18 746 μmol g^-1 h^-1 (selectivity > 99.9%) with a turnover number (TON) of up to 6146 in 24 h (apparent quantum efficiency of 3.8% at 420 nm), indicating a 165-fold enhancement over that of the pristine MOF. This work highlights a powerful strategy for synergistic Earth-abundant metal-based MOF photocatalysis in promoting H₂ production from FA.

A metal-organic framework with rich accessible nitrogen sites for rapid dye adsorption and highly efficient dehydrogenation of formic acid

MOFs with adequate free nitrogen sites have potential applications in dye adsorption and formic acid dehydrogenation. Here, we successfully synthesized a novel 3-D MOF 1 ([(CH₃)₂NH₂][Cd(L)DMA]·0.5DMA·1.5H₂O) with a special two-fold interpenetrating framework through a simple solvothermal reaction between CdCl₂·1.5H₂O and a nitrogen-rich triangular tricarboxylate-based linker (H₃L, 4,4',4''-s-triazine-2,4,6-tribenzoic acid). After removing the guest molecules of dimethylacetamide (DMA) and H₂O, including the coordinated DMA from 1 by vacuum activation at 423 K, a compound named 1' with a formula of [(CH₃)₂NH₂][Cd(L)] and a similar interpenetrating framework structure was obtained. In comparison with compound 1, the total void volume of 1' is nearly doubled, and thus may provide higher potential for the adsorption of other guest molecules. Notably, the pyridine N atoms located in the middle of the triangular tricarboxylate-based linker are not involved in the coordination with Cd²⁺, and are all uniformly dispersed throughout the whole framework of the 3-D MOFs. Due to its unique structural features, the 3-D MOF 1' could effectively adsorb the cationic dye MB⁺ for recycling purposes. The rapid adsorption rate (0.7 × 10^-2 g mg^-1 min^-1) and the relatively high capacity (900 mg g^-1) for MB⁺ demonstrate the potential of 1' in dye adsorption. In addition, 1' may also be used as an effective support to immobilize PdAu NPs via the double-solvent method. The resultant catalyst Pd0.8Au0.2/1' exhibits decent catalytic activity for the dehydrogenation of formic acid with a TOF value of 1854 h^-1 at 333 K. The existence of a large void volume and accessible pyridine N atoms provide a suitable environment for achieving a high dispersion of PdAu NPs, thereby leading to the formation of a catalytically active and stable supported noble-metal NP catalyst for H₂ generation from formic acid decomposition.

Au nanorods modified PCN-222(Cu) for H2 evolution from HCOOH dehydrogenation by photothermal enhanced photocatalysis

Formic acid that can be produced by CO2 reduction are considered as liquid organic hydrogen carriers. Herein, Au nanorods loaded PCN-222(Cu) prepared by a seed-induced growth route exhibits highly selective...