Biosynthesized Ag/α-Al2O3 catalyst for ethylene epoxidation: the influence of silver precursors

Biosynthesized Ag/α-Al2O3 catalysts toward ethylene epoxidation were prepared with Cinnamomum camphoratrees (CC) extract using AgNO3, silver–ammonia complex ([Ag (NH3)2]+) and silver–ethylenediamine complex ([Ag(en)2]+) as the silver precursors. The catalyst from [Ag(en)2]+ demonstrated better activity compared to the catalysts from the other two precursors, 1.41% EO concentration with EO selectivity of 79.1% and 12.0% ethylene conversion were achieved at 250 °C. To investigate the influence of silver precursors on the catalytic performance, three catalysts were characterized by XRD, UV-Vis, XPS, SEM and O2-TPD techniques. The results indicated that [Ag(en)2]+ precursors could be reduced more effectively by CC extract, and Ag particles were successfully immobilized onto the α-Al2O3 support under mild conditions. Moreover, a silver defects surface on the Ag/α-Al2O3 catalyst from [Ag(en)2]+ precursors had the best oxygen activation ability, playing an important role in the generation of electrophilic oxygen species which were responsible for the epoxidation reaction of CC to EO.


Introduction
With the increasing environmental problems due to emissions of pollutants from chemical industry, development of green synthesis of metal nanoparticles with various biological organisms in a more economical and eco-friendly mode has received more and more attentions.Biosynthesis with the plant extract as both the reductant and stabilizing agents can successfully prepare and control the morphology of the metal nanoparticles in a simply way under mild conditions. 1,2  series of plant extracts have been used to successfully synthesize Au 3,4 , Ag [4][5][6] , Pd 7 , Au-Ag 8,9 and Au-Pd 10 nanoparticles.For example, Au-Pd bimetallic nanoparticles were prepared based on simultaneous bioreduction of Au (III) and Pd(II) precursors with Cacumen platycladi (CP) leaf extract 5 .To date, the as-synthesized biogenic metal nanoparticles have been reported in many fields including optics 11 , antibacterial or antimicrobial agent 2 , biological control 12 and so on.Of special interest is that metal nanoparticles from plant extracts could also be used as catalyst which exhibited comparative or more excellent performance comparing with those from the conventional methods 3,[13][14][15][16][17][18] .Reddy et al. reported that by using Sapindus mukorossi Gaertn fruit pericarp, gold nanoparticles from HAuCl 4 were obtained with good catalytic activity for the chemical reduction of p-nitroaniline 3 . Vlchis-Nestor et al. synthesized Au and AgAu nanostructures supported on SiO 2 -Al 2 O 3 by C. sinensis extract, the obtained catalysts possessed high catalytic performance and stability in oxidation and hydrogenation of CO 13 .Recent years, our group has also made great efforts on the preparation of catalysts with plant extracts [14][15][16][17][18] .It is found that bioreduction methods are easy to incorporate metal nanoparticles into supports under mild conditions. Besides with plant biomass playing as both reductant and stabilizer, bioreduction could prepare metal particles with a narrower size distribution and a desired diameter, which is very important for the catalytic activity.Highly stable and active Au nanocatalyst toward propylene 50 epoxidation was prepared through immobilizing biosynthesized Au nanoparticles onto TS-1 support and achieved excellent catalytic performance 14,15 , demonstrating the advantage of plant extract in fabrication of supported metal catalysts.
Ethylene oxide (EO) is vital chemical intermediate with diverse 55 applications, the main method for producing EO is by the direct oxidation of ethylene with air or oxygen over a silver-based α-alumina catalyst [19][20][21] .Great interests have been focused on developing and optimizing the catalyst for more effective selectivity of EO due to both economic and technical reasons 22 .

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Conventionally, the Ag catalyst for ethylene epoxidation is prepared by impregnation through thermal decomposition process, in which silver nitrate 20 was the most common Ag precursors.Besides, Ag 2 O 23 , silver lactate 24 and silver oxalate/ ethylenediamine complex 25 were also employed.The catalysts 65 prepared from different precursors showed varied properties closely related to their activities such as the oxygen adsorption and activation ability, Ag particle size and distribution on the support etc.
Very recently, we reported that biomass in Cinnamomum 70 camphoratrees (CC) extract could reduce sintering during the thermal decomposition process. 18In the present work, the catalytic performance of silver catalysts was obviously enhanced by replacing silver nitrate with other silver precursors.screened with a 20-mesh sieve.15 g powder was added to 100 mL deionized water and the mixture was shaken at 30 °C for 12 h with a rotation rate of 150 rpm.After that, the solution was filtered and proper amount of deionized water was added to keep the volume of the filtrate at 100 mL.The concentration of the CC 30 extract was denoted as 15 g/L.Preparation of Ag/α-Al 2 O 3 catalyst: The catalyst was prepared by an impregnation-bioreduction process with three different Ag precursors, namely, AgNO 3 , silver-ammonia complex and silver-ethylenediamine complex (concentration of Ag in different 35 precursors is 1.36 M).Firstly, the industrial α-Al 2 O 3 was crushed, size of 20-40 mesh was collected and calcined at 600 °C for 6 h.The pretreated α-Al 2 O 3 support (1.0 g) was impregnated with silver precursors (1.2 mL), and dried at 50 o C for 12 h in vacuum.Then the obtained solid was dipping with CC extract in an iso

Catalytic activity measurements
Catalytic reactions were carried out in a vertical fixed-bed 75 stainless-steel reactor at 2 MPa pressure.The feed gas was comprised of 15 vol.% C 2 H 4 , 7 vol.%O 2 and 5 vol.%CO 2 balanced with N 2 .0.5 mL catalyst was used at a space velocity of 7000 mL•h -1 •mL cat -1 .The gas leaving the reactor was heated at about 115 o C and analyzed online by a gas chromatograph.The 80 chromatograph was equipped with a TCD, using a Porapak Q packed column (2 mm × 3 m), and a flame ionization detector (FID), using a ß-ß-oxydipropionitrile packed column (2 mm × 1.5 m).Generally, more attentions are focused on controlling the EO selectivity rather than the ethylene conversion when the industrial requirement of EO concentration (1.30~1.50%) is 110 reached.In order to compare the selectivity of different catalysts under the similar EO concentration in a more intuitive way, the EO selectivity against EO concentration over Ag/α-Al 2 O 3 with different silver precursors is shown in Fig. 2. In the range of reaction temperature, the EO concentration of Ag/α-Al 2 O 3 -n (0.80~1%) did not achieve the industrial requirement of EO concentration, when increasing the reaction temperature, the EO selectivity declined rapidly from 84.43 to 64.40%.As to the Ag/α-Al 2 O 3 -a and Ag/α-Al 2 O 3 -en, both catalysts could reach 5 the industrial requirement of EO concentration.Ag/α-Al 2 O 3 -en achieved 1.41% EO concentration at 250 °C with 79.11% EO selectivity, while for Ag/α-Al 2 O 3 -a catalyst, at 260 °C the comparable EO concentration could be reached with a lower selectivity of 71.04%.Hence, it could be concluded that the 10 catalyst prepared from silver-ethylenediamine complex exhibited the best catalytic activity with 1.41% EO concentration, 79.1% EO selectivity and space time yield of 195.63 g h -1 L cat -1 at 250 °C.SEM was used to observe the morphology of the supported Ag nanoparticles on α-Al 2 O 3 .Al 2 O 3 is large flake-shape, while Ag particles (see arrows), much smaller and spherical are supported on it.A small quantity of Ag particles was found on Ag/α-Al 2 O 3 -n sample (Fig. 4A).Though more Ag were observed on Ag/α-Al 2 O 3 -a (Fig. 4B), Ag particles aggregated obviously and distributed unevenly as in Fig. 4A.With regard to the catalyst 15 prepared from [Ag(en) 2 ] + , large amount of Ag nanoparticles were well dispersed on the support (Fig. 4C) with a statistic diameter of 93.2±41.2nm ( Fig. 4D).

UV-Vis DRS characterization
UV-Vis DRS was used to identify Ag oxidation state of the three catalysts.As shown in Fig. 5, obvious absorption band at ~200 nm aroused by the electron transition of 4d 10 -4d 9 5s 1 from the highly dispersed Ag + on the support 26 , emerged on both Ag/α-Al 2 O 3 -n and Ag/α-Al 2 O 3 -a catalysts, indicating that AgNO 3 and [Ag(NH 3 ) 2 ] + precursors were not reduced completely.Absorption band at 250 ~ 260 nm, attributed to the electron transition of 4d 10 5s 1 -4d 9 5s 1 5p 1 , 4d 10 5s 1 -4d 9 5s 1 6p 1 or 5s 1 -5p 1 from metallic Ag [27][28][29] , was observed on the three catalyst, indicating the existence of Ag 0 on them.Ag/α-Al 2 O 3 -en catalyst (Fig. 5c) had the strongest band intensity of ~250 nm compared with the other two samples, suggestting that more metallic Ag existed on this sample.Bands at the 296 nm and 350 nm assigned to the Ag n δ+ clusters, which were formed by the interaction between metallic silver and the support, could promote the reaction of ethylene epoxidation [30][31][32][33][34] .Comparing with the other two, band at 296 nm is stronger of Ag/α-Al 2 O 3 -en which exhibiting the best catalytic activity of ethylene epoxidation.

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XPS experiment was performed in order to get more information about the electronic states of Ag on the three catalysts prepared from different precursors.As shown in Fig. 6, two bands of Ag 3d 5/2 were observed at 367.2 and 368.1~368.7 eV.The former band (367.2 eV) is assigned to the metallic Ag 0 particles 19,35 and 40 the latter (368.1~368.7 eV) is associated with the presence of high oxidation valence state Ag δ+ species [43][44][45] .Generally the binding energy of metallic Ag3d 5/2 is at 367.9 eV 36,37 .Fig. 6 shows that the peak shifts obviously to lower binding energy, which was attributed to the differential charging effect aroused by 45 the interaction between Ag and the support 35 .XPS spectra in Fig. 6    O 2 -TPD analysis Oxygen activation on the catalyst surface plays an important role in ethylene epoxidation reaction, and O 2 -TPD is one of the most effective techniques to study the oxygen adsorption and activation ability of the catalysts 38,39 .Molecularly adsorbed 5 O 2 species desorbs from Ag at a relatively low temperature 40 while desorption of lattice oxygen occurs at above 750 °C41 , and it is generally believed that the atomic oxygen which desorbs at 200~500 °C is the reactive species on silver catalysts 38 .Fig. 7 shows the O 2 -TPD characterizations of the three catalysts.Based 323 °C for the Ag/α-Al 2 O 3 -en catalyst (Fig. 7c), the peak at 248 °C could be attributed to the nucleophilic oxygen species desorbing from regular surface and the peak at 323 °C to electrophilic oxygen species desorbing from imperfect or defect regions of the silver surface [42][43][44] .The desorption peak at 403 o C 20 was referred to oxygen species desorbing from the subsurface of silver 45 .And it was believed that nucleophilic oxygen species located on the regular surface of silver was responsible for promoting the adsorption of C 2 H 4 on Ag surface 21 , and the electrophilic oxygen species adsorbed on the silver defects

40 volumetric
photoelectron spectra (XPS) of the catalysts were obtained on a Quantum-2000 ESCA Microprobe spectrometer (PHI, USA) with

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Ethylene epoxidation over the Ag/α-Al 2 O 3 catalystsThe catalytic performance for ethylene epoxidation over the Ag/α-Al 2 O 3 catalysts prepared from different silver precursors is shown in Fig.1.It can be seen that the reaction activities of the three catalysts are very different from one another.For the 90 ethylene epoxidation over the Ag/α-Al 2 O 3 -en catalyst, considerable EO concentration (0.82%) was achieved at the initial evaluation temperature 230 °C and with the increasing temperature, EO concentration increased obviously.EO concentration of 1.41 % with EO selectivity of 79.1% and 12.0 % 95 ethylene conversion were achieved at 250 °C.The EO concentration further increased to 2.22% at 260 °C while the EO selectivity decreased to 62.03%.The performance of the catalysts prepared from AgNO 3 and silver-ammonia complex (Ag/α-Al 2 O 3 -n and Ag/α-Al 2 O 3 -a) was relatively lower than that 100 of Ag/α-Al 2 O 3 -en at the same temperature.For the Ag/α-Al 2 O 3 -a catalyst, EO concentration increased from 0.67 to 1.50% and the selectivity decreased from 76.88 to 57.68% with the temperature increasing from 250 to 270 °C.For the ethylene epoxidation over Ag/α-Al 2 O 3 -n catalyst, lower EO concentration (≤ 1%) and 105 ethylene conversion (6.33~10.43%)was shown between 260~280 °C.
process with CC extract, silver oxidation state from different precursors on the same support was different.Though CC extract could reduce the three precursors to Ag 0 , obvious Ag + (~ 200 nm) was detected on the catalyst surface for Ag/α-Al 2 O 3 -n and Ag/α-Al 2 O 3 -a, while for Ag/α-Al 2 O 3 -en, no obvious band of Ag + emerged.It could be concluded that CC completely reduced the [Ag (en) 2 ] + precursor.
also exhibited the three catalysts had different ratios of Ag 0 and Ag δ+ : Ag species on Ag/α-Al 2 O 3 -n catalyst had a high concentration of Ag δ+ while on Ag/α-Al 2 O 3 -en, there was mainly Ag 0 species.The valence distributions of Ag of different 50 Ag/α-Al 2 O 3 catalysts are presented in Tab. 1.It could be clearly seen that Ag precursors had significant effects on the electronic state of Ag on the catalysts.The percentage of metallic Ag were 78.64, 48.16 and 27.21 % on the catalysts from AgNO 3 , [Ag(NH 3 ) 2 ] + and [Ag(en) 2 ] + , respectively.And it should be noted 55 that, based on the results of XRD (Fig. 3) and UV-Vis DRS (Fig. 5), Ag δ+ on Ag/α-Al 2 O 3 -n and Ag/α-Al 2 O 3 -a catalysts originated from the incompletely reduced precursors and the Ag n δ+ clusters formed by the interaction between metallic silver and the support, while for Ag/α-Al 2 O 3 -en sample, Ag δ+ were mainly the Ag n δ+ 60 clusters.

10 on
the corresponding area of the peaks between 200~500 °C, the ratio of desorbed O 2 was 1(Ag/α-Al 2 O 3 -n):1.5(Ag/α-Al 2 O 3 -a) : 7.21 (Ag/α-Al 2 O 3 -en), indicating that Ag/α-Al 2 O 3 -en catalyst had a stronger oxygen adsorption ability.Note that two shoulder peaks emerged at 248 and 403 °C in addition to the main one at 15

25 surface
could attack the carbon-carbon double bond of ethylene to produce EO42 .Thus, O 2 -TPD results also clarified the catalyst prepared from Ag[(en) 2 ] + with CC extract had the best reaction activity due to the excellent oxygen activation ability.