2.1. Cu-MOFs (Cu3BTC2) and ZIF-9(Co) preparation
In a typical synthesis process, 0.01 mmol Cu (NO3)2.3H2O and 0.03 mmol 1,3,5 pyromellitic acid were firstly weighed and dissolved in 50 mL ethanol,then the mixture was stirred for 1 h and transferred to an oven at 140 °C for 24 h. After being washed and dried,blue Cu-MOFs powder was obtained. The synthesis of pure ZIF-9 was carried out in aqueous solution at room temperature. 0.23 g benzimidazole and 0.28 g Co (NO3)3.6H2O were weighand dissolved in 40 ml of water.After the mixture was stirred for 15 min, it was added 5 mL of ammonia and then stirred for 45 min. Finally, it was kept in the dark for 8 h and dried by centrifuge to obtain the product.
2.2 Composite sample preparation
In the process of synthesizing ZIF-9, a certain amount of pre-synthesized Cu-MOFs was added during the aging process under dark conditions. Cu-Co-x(x = 1, 2, 3, 4. x refers to the changed Cu-MOFs mass) was obtained by adjusting the precursor of ZIF-9 synthesized by benzimidazole and Co(NO)3.6H2O. Cu-Co-xP-2(x = 1, 2, 3, 4; x is the mass ratio of precursor to sodium hypophosphite) composite catalyst was obtained by phosphating different Cu-Co-2 (Cu-MOFs@ZIF-9) with a 0.125 g Cu-MOFs. Phosphating experiments of the catalyst were carried out under the conditions of nitrogen protection by calcination at 300 °C for 2 h. In addition, Cu-Co-x (x = 1, 2, 3, 4.) was subjected to phosphatization experiments of the same degree (sodium hypophosphite has twice the mass of Cu-Co-x) to obtain Cu-Co-2P-x (x = 1, 2, 3, 4.) composite catalyst. The phosphorylated products of pure Cu-MOFs and ZIF-9 are referred to as Cu-P and Co-P. The specific catalyst synthesis diagram is shown in Figure 1.