Enhanced performance of Pd-[DBU][Cl]/AC mercury-free catalysts in acetylene hydrochlorination

doi:10.1016/S1872-2067(22)64204-9

Abstract

Abstract:

Compared with the rapid deactivation of the Pd/AC catalyst, an eco-friendly Pd-[DBU][Cl]/AC ([DBU][Cl], 1,8-diazabicyclo[5.4.0]undec-7-ene hydrochloride) catalyst with enhanced stability was synthesized and applied in the hydrochlorination of acetylene. The Pd-20[DBU][Cl]/AC catalyst exhibits 96.7% acetylene conversion and 98.9% selectivity of vinyl chloride within the 24 h evaluation under the conditions of T = 180 °C, GHSV(C₂H₂) = 360 h^-1 and V(HCl)/V(C₂H₂) = 1.2. The enhanced stability of the Pd-[DBU][Cl]/AC catalyst is attributed to the prevention of the reduction of Pd(II) to Pd(0), the inhibition of the formation of coke deposition on the catalyst surface, and the reduction of the loss of Pd species during the reaction. TPD characterization and DFT theoretical simulations confirmed that the PdCl₂-[DBU][Cl] complex has stronger adsorption of HCl and weaker adsorption of C₂H₂ and C₂H₃Cl compared to PdCl₂. Furthermore, the catalytic mechanism of acetylene hydrochlorination over the PdCl₂ and PdCl₂-[DBU][Cl] site was put forward based on theoretical simulations. The good activity and stability of Pd-20[DBU][Cl]/AC in the 60 h lifetime test indicates that it is likely to be a promising alternative to HgCl₂ catalysts.

Key words: Acetylene hydrochlorination, Mercury-free catalyst, Pd complex catalyst, Enhanced stabilityAdsorption energy

Xingzong Dong, Guangye Liu, Zhaoan Chen, Quan Zhang, Yunpeng Xu, Zhongmin Liu. Enhanced performance of Pd-[DBU][Cl]/AC mercury-free catalysts in acetylene hydrochlorination[J]. Chinese Journal of Catalysis, 2023, 46: 137-147.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(22)64204-9

https://www.cjcatal.com/EN/Y2023/V46/I3/137

Figures/Tables 12

Table 1 Texture parameters of catalysts.

Sample	S_BET^a (m² g^-1)		V_total^b (cm³ g^-1)
Sample	Fresh	Spent	Fresh	Spent
Pd/AC	1431	53	0.53	0.13
Pd-1[DBU][Cl]/AC	1238	29	0.42	0.09
Pd-5[DBU][Cl]/AC	1233	42	0.48	0.12
Pd-10[DBU][Cl]/AC	901	62	0.37	0.12
Pd-20[DBU][Cl]/AC	538	124	0.34	0.19
Pd-40[DBU][Cl]/AC	86	72	0.15	0.15

Fig. 1. Nitrogen adsorption-desorption isotherms (a), XRD pattern (b), XPS full spectrum (c), STEM image (d) of the Pd-20 [DBU][Cl]/AC catalyst.

Fig. 2. Acetylene conversion (a) and selectivity to VCM (b) over different samples. Reaction conditions: T = 180 °C, GHSV(C2H2) = 360 h-1 and V(HCl)/V(C2H2) = 1.2. (c) Acetylene conversion under different [DBU][Cl] loading. Reaction conditions: T = 180 °C, GHSV(C2H2) = 360 h-1 and V(HCl)/V(C2H2) = 1.2. (d) Acetylene conversion of Pd-20 [DBU][Cl]/AC at different reaction temperatures. Reaction conditions: GHSV(C2H2) = 360 h-1 and V(HCl)/V(C2H2) = 1.2.

Table 4 Pd content of catalysts.

Sample	Pd content^a (%)		Nominal loss rate (%)	Actual loss rate (%)
Sample	Fresh	Spent	Nominal loss rate (%)	Actual loss rate (%)
Pd/AC	0.219	0.136	38.1	20.3
Pd-1[DBU][Cl]/AC	0.226	0.131	42.1	28.2
Pd-5[DBU][Cl]/AC	0.214	0.144	32.7	17.6
Pd-10[DBU][Cl]/AC	0.208	0.143	31.2	22.3
Pd-20[DBU][Cl]/AC	0.177	0.145	18.1	11.4
Pd-40[DBU][Cl]/AC	0.142	0.137	3.38	1.85

Fig. 3. Pd 3d XPS spectra of Pd/AC (a) and Pd-20[DBU][Cl]/AC (b). TGA profiles of Pd/AC (c) and Pd-20[DBU][Cl]/AC (d) in air atmosphere.

Fig. 4. TPD profiles of C2H2 and HCl on the fresh catalysts. (a) C2H2-TPD; (b) HCl-TPD.

Fig. 5. The HOMO structures of PdCl2 (a) and [DBU][Cl] (c) and the LUMO structures of PdCl2 (b) and [DBU][Cl] (d).

Table 2 Relative contents of Pd species on the spent catalysts.

Sample	Relative content^a (area %)
Sample	Pd(0) (3d_5/2: 333.0eV)	Pd(II) (3d_5/2: 336.8eV)
Pd/AC-Spent	67	33
Pd-1[DBU][Cl]/AC-Spent	50	50
Pd-5[DBU][Cl]/AC-Spent	44	56
Pd-10[DBU][Cl]/AC-Spent	41	59
Pd-20[DBU][Cl]/AC-Spent	31	69
Pd-40[DBU][Cl]/AC-Spent	34	66

Table 3 Coke deposition of catalysts.

Sample	The amount of coke deposition^a (%)
Pd/AC	26.43
Pd-1[DBU][Cl]/AC	21.88
Pd-5[DBU][Cl]/AC	21.58
Pd-10[DBU][Cl]/AC	12.40
Pd-20[DBU][Cl]/AC	7.57
Pd-40[DBU][Cl]/AC	0.92

Fig. 6. (a) The adsorption energy of C2H2, HCl and C2H3Cl on PdCl2 and PdCl2-[DBU][Cl]. (b) Hirshfeld charge variations in the interaction of HCl, C2H2 and C2H3Cl with PdCl2 and PdCl2-[DBU][Cl]. Positive values represent catalysts as electron acceptors. (c) The long-term stability test of Pd-20[DBU][Cl]/AC. (d) TGA profiles of the fresh and spent Pd-20[DBU][Cl]/AC after evaluation.

Fig. 7. Reaction pathway diagram and geometric structures involved in acetylene hydrochlorination over the PdCl2-[DBU][Cl] site. H, C, N, Cl and Pd atoms are depicted in white, gray, slate-blue, green and cyan respectively. The unit of bond length and distance is ?.

Fig. 8. The catalytic cycle mechanism for acetylene hydrochlorination over PdCl2-[DBU][Cl] catalyst.

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