Effect of Metal Dispersion on the Hydrogenation of 2-Amyl Anthraquinone over Pd/Al₂O₃Catalyst

Abstract

A series of highly dispersed Pd/Al 2 O 3 catalysts were prepared via the polyol method. The catalysts were characterized by nitrogen adsorption, X-ray diffraction (XRD), UV-Vis spectrophotometry, temperature programmed reduction (TPR) and transmission electron microscopy (TEM). The influence of Pd particle size on the hydrogenation of 2-amyl anthraquinone (AAQ) was investigated in a trickle-bed reactor. The turnover frequency (TOF) showed antipathetic size dependence while the space time yield (STY) peaked at 4 nm. Also, the selectivity and deactivation rate were affected by the size of palladium particles. The structure-sensitivity relations for the catalysts may be ascribed to the necessities of specific Pd cluster structure for the activation of π-bond. Keywords: anthraquinone hydrogenation, H 2 O 2 , dispersion IntroductionHydrogen peroxide (H 2 O 2 ) is widely used in the chemical industry and environmental protection as an environmentally friendly oxidant. [1][2][3] Today, hydrogen peroxide is manufactured almost exclusively by the autoxidation of 2-alkyl anthrahydroquinone (2-alkyl AQH 2 ) to the corresponding 2-alkyl anthraquinone (2-alkyl AQ) in the so-called AQ process. In the cyclic process, AQ is hydrogenated to yield anthrahydroquinone, then oxidation of the latter produces hydrogen peroxide and regenerates the starting AQ (Scheme 1). 4 Major producers commonly use either the 2-ethyl or the 2-amyl derivative of AQ. In recent years, along with the production growth of downstream products such as propylene oxide and caprolactam, the market demand for H 2 O 2 solution at high concentration has been increasing substantially. Given its high solubility, 2-amylanthraquinone is considered as a good replacement for the widely used 2-ethylanthraquinone to increase productivity. 5 The network of the reactions taking place during the hydrogenation of 2-amyl anthraquinone (AAQ) is quite complicated. The desired products considered as active anthraquinones are 2-amyl anthrahydroquinone (AAQH 2 ) and 2-amyl tetrahydroanthrahydroquinone (H 4 AAQH 2 ), which could produce H 2 O 2 after oxidation. However, other degradations such as 2-amyl octahydroanthrahydroquinone (H 8 AAQH 2 ) and anthrone (AN) are useless to the H 2 O 2 production.6 Previous literature about the hydrogenation of anthraquinone used the 2-ethyl-anthraquinone (EAQ) system rather than the 2-amylanthraquinone system. In addition, the research focused on the topics such as the enhancement of mass transfer and the properties of support. For example, Santacesaria et al. 7 reported that the hydrogenation of 2-ethyl-anthraquinone over Pd catalysts was a fast reaction and the mass transfer of EAQ was the rate limiting step. Based on this, Luo and co-workers 8 designed an egg shell Pd/glass catalyst which can achieve a high instant yield of H 2 O 2 of 11.2 g L -1. Besides, the acid-base property of the support was a key point to influence the catalyst activity and selectivity, as the acid sites of the support were considered as the adsorption sites of EAQ...

Synthesis of Tailored Egg-Shell Pd@Al2O3 Catalyst for Catalytic Hydrogenation of 2-Alkylanthraquinone

“…Similar structural information on the samples (109.8–112.4 m 2 ·g –1 , 0.51–0.56) were determined by nitrogen adsorption–desorption isotherms, indicating that the texture properties of the support substance were pretty much nothing affected by the hydrophobic microemulsions. The powder XRD patterns of the researched catalysts were collected, and the results are shown in Figure (a), in which peaks locating at approximately 2θ = 31.4°, 32.8°, 36.6°, 39.5°, 45.8°, and 67° are attributed to the (400), (002), (111), (222), (400), and (440) crystal faces of mixed γ-Al 2 O 3 and θ-Al 2 O 3 (JCPDS Card No.10-0425 and No.1086-1410, respectively) . In addition, there are three peaks at 2θ = 40.1°, 46.5°, and 51.9°, attributed to the (111), (200), and (422) crystal faces of metallic palladium (JPDS Card No.46-1043 and 011310, respectively) .…”

“…For catalytic performance comparison of the hydrophobic microemulsion, a similar alumina substance was employed as the support with the same intended palladium concentration of 0. 28 In addition, there are three peaks at 2θ = 40.1°, 46.5°, and 51.9°, attributed to the (111), (200), and (422) crystal faces of metallic palladium (JPDS Card No.46-1043 and 011310, respectively). 29 Similar with the results of the texture properties, pretty much nothing of influence has been observed from the powder XRD patterns of alumina substance, yet crystal faces of the formed palladium particles differ from the nature of the hydrophobic microemulsion systems.…”