No abstract

of the 21st century. [2][3][4] Catalytic hydrogenation of CO 2 with renewable H 2 to value-added chemicals and clean fuels is considered to be one of the most promising ways to simultaneously relieve global warming and energy shortage. [5][6][7][8] In particular, CO 2 -to-methanol technology has been paid great attention because CH 3 OH not only is an alternative fuel but also can be used as a key intermediate for the production of other commodity chemicals, such as aromatics and olefins. [9][10][11] As a result, considering its practical application and economy efficiency, intense research efforts have been devoted to develop lowcost, efficient, and selective catalysts for CO 2 hydrogenation to methanol during the last few years. [12][13][14][15] The Cu-based catalysts, such as Cu/ ZrO 2 , Cu/CeO 2 /TiO x , and Cu/ZnO/Al 2 O 3 , have been extensively studied in methanol production, [16][17][18] but most of these catalysts still suffer from low selectivity and stability due to an intensive reverse watergas shift (RWGS) reaction and a waterinduced metal sintering. [15,19,20] In this regard, In 2 O 3 -based catalysts have emerged as promising candidates in methanol synthesis from the thermal hydrogenation of CO 2 . [21][22][23] Especially, when In 2 O 3 is supported on ZrO 2 , the methanol yield could further be dramatically increased because of the formation of oxygen vacancy or frustrated Lewis pairs at the In 2 O 3 /ZrO 2 interface. [22,[24][25][26][27][28] It is reported that ZrO 2 support can interact with In 2 O 3 and affect the adsorption and dissociation of CO 2 and H 2 , resulting in the change of the reaction pathways for the methanol formation. [21][22][23]26,29,30] For example, Javier et al. confirmed that the In 2 O 3 supported on monoclinic ZrO 2 (m-ZrO 2 ) can better activate CO 2 molecules because of its superior character of oxygen vacancies on In 2 O 3 in comparison with those on alumina and ceria. [27] Gong et al. also found that m-ZrO 2 can improve the electron density of In 2 O 3 , and the resulting electron-rich In 2 O 3 can promote H 2 dissociation and the conversion of formate (HCOO*) to methoxy (CH 3 O*). [29] Therefore, rational modulation of the electronic interactions, type, dimension, and nature of indium-zirconia interfaces is a very important strategy to improve their catalytic performance. [21,23,31]

Catalytic transfer hydrogenation (CTH) based on non-noble-metal catalysts has emerged as an environmentally friendly way for the utilization of biomass resources. However, the development of efficient and stable non-noble-metal catalysts is crucially challenging due to their inherent inactivity. Herein, a metal–organic framework (MOF)-transformed CoAl nanotube catalyst (CoAl NT160‑H) with unique confinement effect was developed via a “MOF transformation and reduction” strategy, which exhibited excellent catalytic activity for the CTH reaction of levulinic acid (LA) to γ-valerolactone (GVL) with isopropanol (2-PrOH) as the H donor. Comprehensive characterizations and experimental investigations uncovered that the confined effect of the ultrathin amorphous Al2O3 nanotubes could modulate the electronic structure and enhance the Lewis acidity of Co nanoparticles (NPs), thus contributing to the adsorption and activation of LA and 2-PrOH. The synergy between the electropositive Co NPs and Lewis acid–base sites of the CoAl NT160‑H catalyst facilitated the transfer of α-H in 2-PrOH to the C atom of carbonyl in LA during the CTH process via a Meerwein–Ponndorf–Verley mechanism. Moreover, the confined Co NPs embedded on am-Al2O3 nanotubes endowed the CoAl NT160‑H catalyst with superior stability and the catalytic activity was nearly unchanged for at least ten cycles, far surpassing that of the Co/am-Al2O3 catalyst prepared by the traditional impregnation method.