Comparative Study of Pd–Ni Bimetallic Catalysts Supported on UiO-66 and UiO-66-NH2 in Selective 1,3-Butadiene Hydrogenation

Selective hydrogenation of 1,3-butadiene (BD) is regarded as the most promising route for removing BD from butene streams. Bimetallic Pd–Ni catalysts with changed Pd/Ni molar ratios and monometallic Pd catalysts were synthesized using two differently structured metal-organic framework supports: UiO-66 and UiO-66-NH2. The effects of the structure of support and the molar ratio of Pd/Ni on the catalytic property of selective BD hydrogenation were studied. The Pd–Ni bimetallic supported catalysts, PdNi/UiO-66 (1:1) and PdNi/UiO-66-NH2 (1:1), exhibited fine catalytic property at low temperature. Compared with UiO-66, UiO-66-NH2 with a certain number of alkaline sites could reduce the catalytic activity for the BD hydrogenation reaction. However, the alkaline environment of UiO-66-NH2 is helpful to improve the butene selectivity. PdNi/UiO-66-NH2 (1:1) catalyst presented better stability than PdNi/UiO-66 (1:1) under the reaction conditions, caused by the strong interaction between the –NH2 groups of UiO-66-NH2 and PdNi NPs. Moreover, the PdNi/UiO-66-NH2 (1:1) catalyst presented good reproducibility in the hydrogenation of BD. These findings afford a beneficial guidance for the design and preparation of efficient catalysts for selective BD hydrogenation.

The support has a considerable effect on the catalytic performance for the selective BD hydrogenation reaction [20][21][22][23][24][25]. The interactions among metal NPs and supports, charging transfers, and support acidity play important roles in the catalytic activity and and support. Furthermore, the Pd/Ni molar ratio of bimetallic Pd-Ni catalysts had a significant effect on the activity and product selectivity.

Synthesis of UiO-66-NH 2 and UiO-66
The UiO-66-NH 2 crystal was prepared via the solvothermal method [52]. ZrCl 4 (2.40 mmol, 0.56 g) was melted into N,N'-dimethylformamide (DMF, 9 mL) under intense stirring. The 2-aminoterephthalic acid (NH 2 -BDC, 5.10 mmol, 0.92 g) and benzoic acid (72.06 mmol, 8.80 g) were melted into 20 mL DMF. Afterwards, the DMF solution of ZrCl 4 was added into the solution of NH 2 -BDC and benzoic acid. After stirring for 1 h, the homogenized solution was heated at 180 • C in a 100 mL Teflon autoclave for 24 h in a drying oven. The light-yellow powder was collected by centrifugation at 3800 rpm for 20 min. The powder was washed four times with DMF and methanol, respectively. Finally, it was dried for 24 h in a vacuum oven at 60 • C. UiO-66 was also prepared via the solvothermal method by using the same procedure, except using terephthalic acid (BDC) instead of NH 2 -BDC.

Catalytic Activity Measurement
The catalytic property of Pd-Ni NP and Pd NP supported catalysts was assessed using a continuous flowing system with a 6 mm inner diameter of quartz fixed-bed reactor under atmospheric pressure. Initially, 5 mg of fine powder catalyst was diluted with 0.5 g quartz sand (25-40 mesh), then placed at the center of the quartz reactor. The reactants, H 2 (6.5 mL/min) and BD/N 2 (1.0 vol.%, 20 mL/min), passed through the fixed catalyst bed with a space velocity of 318,000 mL/(h·g cat ) at 40-110 • C. The high space velocity can eliminate significant mass-transfer limitation during the hydrogenation of BD. The reactor effluent was measured online using the Al 2 O 3 capillary column.
Pd/UiO-66 and Pd/UiO-66-NH2 presented a different catalytic property compared with Pd-Ni bimetallic catalysts ( Figure S5). For Pd/UiO-66 catalyst, the conversion of BD To evaluate the reproducibility of the catalyst, two batches of PdNi/UiO-66-NH 2 (1:1) catalysts were synthesized by the impregnation method. The catalytic activity and selectivity of the two batches of catalysts for the hydrogenation of BD were investigated under the same conditions ( Figure S6). Two batches of PdNi/UiO-66-NH 2 (1:1) displayed similar BD conversions and product selectivities for the BD hydrogenation at 55 • C ( Figure S6). PdNi/UiO-66-NH 2 (1:1) catalyst displayed good reproducibility.
Pd/UiO-66 and Pd/UiO-66-NH 2 presented a different catalytic property compared with Pd-Ni bimetallic catalysts ( Figure S5). For Pd/UiO-66 catalyst, the conversion of BD was 90.2% at the initial stage of hydrogenation, and then quickly deactivated within 0.5 h. The BD conversion decreased to 13.9% at 50 • C for 12 h. However, there is a rise in selectivity to total butenes with increasing reaction time. The selectivity toward total butenes raised from 82.0% to 99.7% when the reaction time increased from 0.25 h to 8 h. Pd/UiO-66-NH 2 catalyst presented complete conversion at the initial stage of hydrogenation. The BD conversions slightly decreased in the next 4 h, and then rapidly decreased in the next 8 h, while the selectivity to total butenes steadily increased with the increase of the reaction time. The BD conversion and butene selectivity were 52.4% and 95% at 50 • C for 12 h, respectively. Obviously, the Pd NPs supported on UiO-66-NH 2 (100%) presented higher BD conversion than that supported on UiO-66 (90.2%). This result is contrary to those of Pd-Ni bimetallic catalysts. This may be explained by a previous study, which displayed that the catalysts with the smallest NP show higher BD conversion [31]. The TEM characterization shows that the Pd NP mean particle sizes were 14.6 nm and 7.7 nm for Pd/UiO-66 and Pd/UiO-66-NH 2, respectively . The catalytic activity for the hydrogenation of BD was dependent on both MOF support structure and particle size. Although the PdNi/UiO-66-NH 2 (1:1) bimetallic catalyst (42.8% BD conversion at 50 • C)

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.