Unlocking the Potential of Photocatalysts in Biomass Refinery
2020; Elsevier BV; Volume: 6; Issue: 11 Linguagem: Inglês
10.1016/j.chempr.2020.10.012
ISSN2451-9308
Autores Tópico(s)Nanomaterials for catalytic reactions
ResumoIn this issue of Chem, Wang and co-workers report the fine-tuning of product selectivity over facet-engineered TiO2. They evaluate three morphology-controlled TiO2 nanocrystals in the photocatalytic upgrading of biomass-derived platform compounds in methanol. Hydrogenation and reductive coupling products are produced in high yields over oxygen-vacancy-rich and -vacancy-free TiO2 surfaces, respectively. In this issue of Chem, Wang and co-workers report the fine-tuning of product selectivity over facet-engineered TiO2. They evaluate three morphology-controlled TiO2 nanocrystals in the photocatalytic upgrading of biomass-derived platform compounds in methanol. Hydrogenation and reductive coupling products are produced in high yields over oxygen-vacancy-rich and -vacancy-free TiO2 surfaces, respectively. The photocatalytic activity of certain metal oxides was discovered even before the term “photocatalyst” came into existence. In 1929, Keidel reported the rapid decomposition of dyes in contact with TiO2 under photoirradiation.1Keidel E. The fading of aniline dyes in the presence of titanium white.Farben-Zeitung. 1929; 34: 1242-1243Google Scholar By then, TiO2 and other photocatalysts were called “photosensitizers,” referring to “substances which absorb light and cause a photochemical change, but appear more or less unchanged at the end of the reaction.”2Goodeve C.F. Kitchener J.A. The mechanism of photosensitisation by solids.Trans. Faraday Soc. 1938; 34: 902-908Crossref Google Scholar It turns out that Keidel’s work was but a prelude to thousands of further studies on TiO2 in photocatalysis in the decades to come. At present, TiO2-based photocatalysts have found wide applications covering environmental remediation, energy generation, and chemical synthesis. Exciting new catalytic applications associated with improved fundamental insights continue to pop up even today. Along this line, Wang and co-workers at Xiamen University reported the fine-tuning of product selectivity over facet-engineered TiO2, representing an exciting advance in the photocatalytic valorization of bio-feedstock. Although several lignocellulosic-biomass-derived platform compounds, such as furfural3Chen S. Wojcieszak R. Dumeignil F. Marceau E. Royer S. How catalysts and experimental conditions determine the selective hydroconversion of furfural and 5-hydroxymethylfurfural.Chem. Rev. 2018; 118: 11023-11117Crossref PubMed Scopus (237) Google Scholar and vanillin,4Wong S.S. Shu R. Zhang J. Liu H. Yan N. Downstream processing of lignin derived feedstock into end products.Chem. Soc. Rev. 2020; 49: 5510-5560Crossref PubMed Google Scholar are already commercially available, creating specific reaction pathways to direct these chemicals into a particular end product instead of a mixture of products remains a challenge. In this issue of Chem,5Wu X. Li J. Xie S. Duan P. Zhang H. Feng J. Zhang Q. Cheng J. Wang Y. Selectivity control in photocatalytic valorization of biomass-derived platform compounds by surface engineering of titanium oxide.Chem. 2020; 6: 3038-3053Abstract Full Text Full Text PDF Scopus (14) Google Scholar these authors evaluated three morphology-controlled TiO2 nanocrystals—a bipyramid-shaped anatase exposing {101} facets, an anatase nanosheet dominated by {001} facets on the surface, and a rod-shaped rutile enclosed mainly by {110} facets—in the photocatalytic upgrading of furfural in methanol and observed astounding differences in product selectivity. Whereas anatase nanosheets favored the hydrogenation of the aldehyde group, offering furfural alcohol in about 90% selectivity, rutile nanorods exclusively led to the formation of furoin and hydrofuroin via C–C coupling of two furfural molecules. Bipyramid-shaped anatase provided a mixture of hydrogenation and C–C coupling products. Similar selectivity patterns were observed for the transformation of cellulose-derived 5-methyl furfural and lignin-derived vanillin. Electron paramagnetic resonance, X-ray photoelectron, and UV-visible diffuse reflectance spectroscopies revealed that the generation of oxygen vacancies on TiO2 under irradiation in methanol is strongly dependent on the exposed facet. The density of oxygen vacancies, as measured by thionine titration, is two orders of magnitude higher on anatase nanosheets than on rod-shaped rutile, hinting that the surface-oxygen vacancies are pivotal in regulating the reaction pathway. Further investigations based on infrared spectroscopy, radical scavenger tests, and density functional theory calculations led the authors to conclude that the oxygen-vacancy-rich surface interacts strongly with furfural, leading to the preferential formation of a CH–O⋅ intermediate followed by hydrogenation to alcohol, whereas a ⋅C–OH intermediate is formed on the oxygen-vacancy-free surface, enabling C–C coupling (Scheme 1). Furfural alcohol is a renewable fine chemical, whereas furoin and hydrofuroin are precursors for green jet fuels. Their synthesis from furfural via thermal catalytic pathways has been extensively studied. Very different catalytic systems and reaction conditions are needed: the formation of furfural alcohol relies on metallic catalysts in the presence of H2 or a hydrogen donor, whereas the production of furoin and hydrofuroin is often conducted over strong base catalysts. Consequently, these two types of reactions are commonly studied and optimized separately. In sharp contrast, the Wang group’s work indicates that in similar catalytic systems under identical reaction conditions, the reaction channel can be rationally switched toward either C=O reduction or C–C coupling simply through engineering of the exposed facets of TiO2. As such, this study not only provides cheap, easily available, and environmentally benign photocatalytic systems to upgrade bio-feedstock with high selectivity and activity under mild conditions but also inspires the development of a new generation of catalytic systems to upgrade furfural and relevant platform chemicals in the future. From a fundamental perspective, the structure-selectivity correlation is rationalized by the morphology-dependent surface property of TiO2. Coincidently, a strong morphology-dependent activity of cadmium sulfide (CdS) in promoting the conversion of bio-based α-hydroxyl acids into amino acids under visible-light irradiation was recently reported.6Song S. Qu J. Han P. Hülsey M.J. Zhang G. Wang Y. Wang S. Chen D. Lu J. Yan N. Visible-light-driven amino acids production from biomass-based feedstocks over ultrathin CdS nanosheets.Nat. Commun. 2020; 11: 4899Crossref PubMed Scopus (15) Google Scholar Nanosheet-shaped CdS was identified as the most active form of CdS for amination reaction as a result of the preferential formation of oxygen-centered radical intermediates. In contrast, its H2 evolution activity was the poorest among all tested CdS materials. Corroborating with each other, these two works strongly suggest that morphology control is a viable and general approach to further unlocking the catalytic potential of various photocatalysts in the transformation of multifunctional chemicals. In a bigger context, this study is another milestone testifying to the effectiveness of photocatalysis in biomass refinery. Whereas existing research is by far dominated by thermo-driven catalytic approaches,3Chen S. Wojcieszak R. Dumeignil F. Marceau E. Royer S. How catalysts and experimental conditions determine the selective hydroconversion of furfural and 5-hydroxymethylfurfural.Chem. Rev. 2018; 118: 11023-11117Crossref PubMed Scopus (237) Google Scholar,4Wong S.S. Shu R. Zhang J. Liu H. Yan N. Downstream processing of lignin derived feedstock into end products.Chem. Soc. Rev. 2020; 49: 5510-5560Crossref PubMed Google Scholar,7Zang H. Wang K. Zhang M. Xie R. Wang L. Chen E.Y.X. Catalytic coupling of biomass-derived aldehydes into intermediates for biofuels and materials.Catal. Sci. Technol. 2018; 8: 1777-1798Crossref Google Scholar the high temperature and high pressure generally applied in these systems increase energy consumption and often lead to undesired side reactions. In contrast, photocatalysis is usually carried out at ambient temperature and atmospheric pressure, and by involving the unique mechanism of photogenerated electrons and holes, photocatalysis has the potential to accomplish reactions that are otherwise difficult to be achieved. From the same group in Xiamen, a redox-neutral photocatalytic system was established for the cleavage of β–O–4 linkage under conditions much more benign than thermocatalytic processes.8Wu X. Fan X. Xie S. Lin J. Cheng J. Zhang Q. Chen L. Wang Y. Solar energy-driven lignin-first approach to full utilization of lignocellulosic biomass under mild conditions.Nat. Catal. 2018; 1: 772-780Crossref Scopus (173) Google Scholar In another study, Feng Wang and colleagues reported the biomass-derived conversion of furanic molecules into diesel fuel precursors and H2 via photocatalytic dehydrocoupling on Ru-doped ZnIn2S4, which has not been realized in thermal catalytic processes.9Luo N. Montini T. Zhang J. Fornasiero P. Fonda E. Hou T. Nie W. Lu J. Liu J. Heggen M. et al.Visible-light-driven coproduction of diesel precursors and hydrogen from lignocellulose-derived methylfurans.Nat. Energy. 2019; 4: 575-584Crossref Scopus (84) Google Scholar As a result of the outstanding contributions from many teams in the last decade, photocatalysis has indeed emerged as an attractive route to upgrading biomass and its derivatives.10Wu X. Luo N. Xie S. Zhang H. Zhang Q. Wang F. Wang Y. Photocatalytic transformations of lignocellulosic biomass into chemicals.Chem. Soc. Rev. 2020; 49: 6198-6223Crossref PubMed Google Scholar Despite these encouraging findings, there is still a long way ahead. Biomass refinery—in which hydrogenation, hydrodeoxygenation, (de)carbonylation, decarboxylation, C–C coupling, and/or C–C/C–O breakage reactions can occur in parallel and/or in tandem—is abnormally complex. Establishing the surface-structure-selectivity correlations of common photocatalysts with these reactions is a daunting task. Meanwhile, although the activity of photocatalysts (measured by unit mol of product formed per unit mass of catalyst per unit time) might compare favorably with catalysts developed in thermal processes, the conversion seldom reaches close to 100%. This will complicate product purification and increase overall cost, adding extra hurdles to commercializing the process. Another challenge is to make chiral compounds from bio-feedstock in photocatalytic processes. Cellulose and hemicellulose have multiple chiral centers that are often lost during processing. It is important to recover some of the original stereo-structure to increase the product value. Finally, continued effort in band-gap engineering is still needed for photocatalysts that mainly absorb UV light to better utilize visible light. In nature, plants use sunlight to transform and assemble H2O and CO2 in a complex but precise manner into biomass via photosynthesis. With improved understanding and control of photocatalysts, we could, one day, partially reverse the process by rationally dismantling biomass and re-assembling the pieces into useful products in the sunshine. Selectivity Control in Photocatalytic Valorization of Biomass-Derived Platform Compounds by Surface Engineering of Titanium OxideWu et al.ChemSeptember 25, 2020In BriefWang and colleagues found that the selectivity for photocatalytic transformations of lignocellulose-derived platform chemicals including furfural, methyl furfural, and vanillin depended strongly on the exposed facet of TiO2. They demonstrated that the facet-dependent density of oxygen vacancies governs the charge distribution and adsorption strength of surface species, and thus determines product selectivity. Hydrogenation products such as fine chemicals and coupling products as biofuel precursors can be produced in high yields over oxygen-vacancy-rich and oxygen-vacancy-free TiO2 surfaces, respectively. Full-Text PDF
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