Highly Efficient and Robust Au/MgCuCr 2 O 4 Catalyst for Gas-Phase Oxidation of Ethanol to Acetaldehyde

Transcription

1 Highly Efficient and Robust Au/MgCuCr O 4 Catalyst for Gas-Phase Oxidation of Ethanol to Acetaldehyde Peng Liu,*, and Emiel J. M. Hensen*, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 5, 5600 MB Eindhoven, The Netherlands. Experimental details () Catalyst preparation Supporting Information The various spinel supports (Mg 0.75 M Cr O 4 (M = Mg, Co, Ni, Cu), CuCr O 4, MgAl O 4, Mg 0.75 Cu Al O 4 ) were prepared by coprecipitation-calcination method using metal nitrates as precursors. Typically, for MgCuCr O 4 preparation, a mixed nitrate solution with stoichiometric amount of Mg(NO ) /Cu(NO ) /Cr(NO ) = //8 was adjusted to ph ~ by dropwise addition of M NaOH solution to ensure total coprecipitation. The precipitate was filtered, washed and dried at 0 o C overnight, then the solid was further calcined at 700 o C in air for 8 h to yield the spinel product. The Au/spinel catalysts (theoretical wt% Au loading) were prepared by the homogeneous deposition-precipitation (HDP) method using urea as the precipitation agent as developed by Louis et al. The catalysts were finally calcined in air at 50 o C for 5 h. A reference catalyst Au/SiO was prepared by using Au(en) Cl as precursor (theoretical Au loading wt%). Reference catalysts Au/TiO, Au/CuO-SiO were also prepared by the same HDP method with urea and the theoretical Au loading was wt%. Standard sample CuO/SiO (6. wt% Cu) was used as support for the Au/CuO-SiO catalyst. () Catalyst characterization XRD was performed on a Bruker Endeavour D4 with Cu K α radiation (40 kv and 0 ma). SEM micrographs were taken using a FEI Quanta 00F scanning electron microscope. TEM micrographs were acquired on a FEI Tecnai 0 electron microscope at an acceleration voltage of 00 kv with a LaB 6 filament. The average Au particle size was calculated by counting ~00 Au particles. The BET surface areas were recorded on a Tristar 000 automated gas adsorption system. The samples were degassed at 80 o C for h prior to analysis. The gold loading of the catalysts was determined by ICP-OES. After an aliquot of the sample was treated in a mixture of HCl/HNO (/), the residual solid was filtered and washed completely with distilled water until the filtrate reaching 00 ml. S

2 X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo Scientific K- Alpha spectrometer, equipped with a monochromatic small-spot X-ray source and a 80 o double focusing hemispherical analyzer with a 8-channel detector. Spectra were obtained using an aluminum anode (Al Kα = ev) operating at 7 W and a spot size of 400 μm. Survey scans were measured at a constant pass energy of 00 ev and region scans at 50 ev. The background pressure was 0-9 mbar and during measurement 0-7 mbar Argon because of the charge compensation dual beam source. Data analysis was performed using CasaXPS software. The binding energy was corrected for surface charging by taking the C s peak of contaminant carbon as a reference at 84.5 ev. Temperature programmed reduction (TPR) experiments were performed in a flow apparatus equipped with a fixed-bed reactor, a computer-controlled oven and a thermal conductivity detector. Typically, 0 mg catalyst was loaded in a tubular quartz reactor. Prior to TPR, the catalyst was oxidized by exposing to a flowing mixture of 4 vol.% O in He by heating to 00 o C. After cooling down to room temperature in flowing N, the sample was reduced in 4 vol.% H in N at a flow rate of 8 ml/min, whilst heating from room temperature up to 800 o C at a ramp rate of 0 o C/min. The H signal was calibrated using the CuO/SiO (6. wt% Cu) standard sample. Temperature programmed oxidation (TPO) of spent Au/MgCuCr O 4 catalyst after non-oxidative ethanol dehydrogenation was carried out as follows: 50 mg Au/MgCuCr O 4 catalyst was loaded in a tubular quartz reactor, then the catalyst was exposed to a flowing mixture of ethanol in He (by introducing 0 ml/min He stream through an ethanol saturator under room temperature) and heated to 00 o C for 0 h. It was cooled to 00 o C and swept with He (50 ml/min) overnight to remove physisorbed adsorbates. After the catalyst was cooled down to room temperature, it was oxidized in vol.% O in He at a flow rate of 50 ml/min, whilst heating to 600 o C at a ramp rate of 0 o C/min and the signals of acetaldehyde (M/e = 9), ethanol (M/e = ), O (M/e = ) and CO (M/e = 44) were simultaneously recorded by online mass spectrometry (quadrupole mass spectrometer, Balzers TPG-00). () Catalyst reaction Ethanol oxidation and dehydrogenation experiments were carried out in a fixed bed plug flow reactor system. Typically, 00 mg of catalyst (sieve fraction 5-50 μm) diluted with 00 mg of α-al O was loaded into a stainless steel reactor with an internal diameter of 6 mm. Prior to reaction, the catalyst was oxidized in a mixture 0 vol.% O in N at 00 o C for h. Two types of feed compositions were used. The volumetric composition of these feed mixtures were ethanol/o /He = //6 for oxidation, and ethanol/he = /66 for dehydrogenation. The reactant feed mixture was obtained by evaporating ethanol in a He stream in a controlled evaporator mixers (Bronkhorst). All tubings were kept above 00 C to avoid condensation of the reactants and products. The total gas flow rate was 67 ml/min with ethanol flow rate ~.5 ml/min (.5 S

3 vol.%) and the GHSV was 00,000 ml g - cat h - for all experiments. The reactor effluent was analyzed by online gas chromatography (Interscience GC-8000 Top, permanent gases on Shincarbon ST80/00 packed column connected to a TCD and hydrocarbons on a Rtx@-Wax column connected to a FID). In all cases, the carbon balances closed at 00 ± %. The ethanol conversion (X) and acetaldehyde selectivity (S) were calculated by: References () R. Zanella, S. Giorgio, C. R. Henry, C. Louis, J. Phys, Chem. B 00, 06, 764. () Y. Guan, E. J. M. Hensen, Appl. Catal. A 009, 6, S

4 Figure S. XRD patterns of various Au/spinel catalysts. For the Au/MgCr O 4, Au/MgCoCr O 4, Au/MgNiCr O 4, and Au/MgCuCr O 4 catalysts, the XRD patterns are consistent with the pure MgCr O 4 spinel phase (JCPDS 0-05). The XRD pattern of Au/CuCr O 4 is in line with pure CuCr O 4 spinel phase (JCPDS 4-044). The XRD patterns of Au/MgAl O 4 and Au/MgCuAl O 4 are in good agreement with the MgAl O 4 spinel phase (JCPDS -5). S4

5 Figure S. SEM images of (a) Au/MgCr O 4, (b) Au/MgCoCr O 4, (c) Au/MgNiCr O 4, (d) Au/MgCuCr O 4, (e) Au/MgCuCr O 4 after 500 h on stream, (f) Au/CuCr O 4, (g) Au/MgAl O 4, and (h) Au/MgCuAl O 4. S5

6 Au/MgCr O 4 d =. ± 0.9 nm Au/MgCoCr O 4 d =. ± 0.8 nm Au/MgCuCr O 4 d =. ± 0.8 nm Au/MgNiCr O 4 d =. ± 0.7 nm Au/CuCr O 4 d =.6 ± 0.9 nm Au/MgAl O 4 d =.4 ±.0 nm Au/MgCuAl O 4 d =.5 ± 0.6 nm Au/TiO d =. ± nm Au/SiO d = 6. ±. nm Au/CuO-SiO d =.8 ±. nm Au/MgCuCr O o C d =.5 ± 0.8 nm Au/MgCuCr O o C d = 4.5 ±. nm Figure S. TEM images and AuNP size distributions for various supported gold catalysts by calcination at 50 o C and two more Au/MgCuCr O 4 catalysts by calcination at 400 o C and 500 o C, respectively. S6

7 Figure S4. Effect of GHSV on the catalytic performance of Au/MgCuCr O 4 and Au/TiO catalysts for ethanol oxidation: A) ethanol conversion, B) acetaldehyde selectivity. Reaction conditions for low GHSV: g catalyst, vol% ethanol, ethanol/o /He = /0/9, GHSV = 6000 ml g cat - h -. Under GHSV = 6000 h -, Au/TiO converts 8% of the ethanol feed with an aldehyde selectivity of 9% at 5 o C and 96% of ethanol with 50% aldehyde selectivity at 50 o C. The Au/MgCuCr O 4 performs much better at an ethanol conversion of 5% with 99% aldehyde selectivity at 5 o C and about 90% conversion with 97% aldehyde selectivity at 75 o C. Evidently, at lower GHSV and temperature, our catalyst still outperforms Au/TiO. S7

8 Au/MgCuCr O 4 00 h used d =. ± 0.8 nm Au/MgCuCr O 4 00 h used d =. ± 0.8 nm Au/MgCuCr O h used d =.4 ± 0.9 nm Au/MgCr O 4 50 h used d =.8 ±.0 nm Au/CuCr O 4 50 h used d =.8 ± 0.9 nm Au/MgCuAl O 4 50 h used d =. ± 0.8 nm Au/SiO 50 h used d = 7. ±.5 nm Au/CuO-SiO 50 h used d =.4 ± 0.7 nm Figure S5. TEM images and AuNP size distributions for recovered gold catalysts after ethanol oxidation. For the reference Au/SiO, Au/CuO-SiO and Au/MgCuAl O 4 catalysts, the mean AuNP sizes increased from 6. to 7. nm, from.8 to.4 nm and from.5 to. nm, respectively. For Au/MgCr O 4 without Cu, the AuNP size increased from. to.8 nm. However for Au/CuCr O 4 without Mg, the AuNP size only slightly increased from.6 to.8 nm. More significantly, for the recovered Au/MgCuCr O 4 catalyst after 00, 00 and 500 h on-stream, the mean AuNP size only increased from. nm to.,. and.4 nm, respectively. Clearly, these results indicate that Cu-containing chromite spinel supports have beneficial effect on the stabilization of AuNPs, despite their relatively low surface area. A tentative reason for this stabilizating effect is the strong metal-support interactions between AuNP and the Cu-chromite spinel. S8

9 Figure S6. Effect of AuNP size on the catalytic performance of Au/MgCuCr O 4 catalyst for ethanol oxidation: A) ethanol conversion, B) acetaldehyde selectivity. Reaction conditions: 0. g catalyst,.5 vol% ethanol, ethanol/o /He = //6, GHSV = 00,000 ml g cat - h -. The ethanol conversion at 00 o C decreased from 68% (. nm) to 56% (.5 nm) and 8% (4.5 nm). The corresponding TOF values decreased with increasing AuNP size from 5 h - to 87 h - and 909 h -. Thus, increasing the size of the AuNP leads to lower catalytic activity of Au/MgCuCr O 4 catalyst.. S9

10 Figure S7. Cu p / (A) and Cu LMM (B) XP spectra of MgCuCr O 4 and Au/MgCuCr O 4 samples after the pretreatment in ethanol vapor or H at 00 o C. S0

11 Figure S8. Effect of pretreatment on the catalytic performance of Au/MgCuCr O 4 catalyst for ethanol oxidation: A) ethanol conversion, B) acetaldehyde selectivity. Pretreatment conditions: 0% H or O in N (00 ml/min), 00 o C, h. Reaction conditions: 0. g catalyst,.5 vol% ethanol, ethanol/o /He = //6, GHSV = 00,000 ml g - cat h -. S

12 Figure S9. H consumption traces (TPR) of (a) Au/MgCuCr O 4, (b) Au/CuCr O 4, and (c) Au/MgCuAl O 4 catalysts. The dashed line represents the trace of the corresponding spinel support. S

13 Figure S0. Cu p / (A) and Cu LMM (B) XP spectra of fresh and spent Au/MgCuAl O 4 and Au/CuO-SiO catalysts after ethanol oxidation. The Cu p / XPS results indicate that most Cu + species are reduced to Cu + and Cu 0 during ethanol oxidation. The Cu LMM XPS results show that all three Cu species (Cu +, Cu + and Cu 0 ) exist in the spent catalysts. For the spent Au/MgCuAl O 4 catalyst, the surface Cu + fraction decreased significantly from 54 to % after 50 h on-stream. Thus, these data show the facile reduction of Cu + /Cu + species to Cu 0 by ethanol in the Au/MgCuAl O 4 and Au/CuO-SiO catalysts, consistent with our hypothesis that this would preclude the formation of effective Au 0 -Cu + synergy explaining their inferior activity. S

14 Figure S. TPO profiles of spent Au/MgCuCr O 4 catalyst after non-oxidative ethanol dehydrogenation. S4

15 Scheme S. A proposed mechanism for Au/MgCuCr O 4 catalyzed aerobic oxidation of ethanol to acetaldehyde S5

16 Table S. Catalytic performance of various supported gold catalysts on aerobic oxidation of ethanol. Au/MgCuCr O 4 Au/MgCr O 4 Au/MgCoCr O 4 Au/MgNiCr O 4 Au/CuCr O 4 Au/MgAl O 4 Au/MgCuAl O 4 Au/TiO Au/SiO Au/CuO-SiO 00 o C o C o C S (ethylene) S (CO ) o C S (acetic acid) S (ethylene) S (CO ) o C S (acetic acid) S (ethylene) S (CO ) S6

17 Table S. XPS results (surface Cu + /Cu and Au/Cu ratios) of the sets of Cu-containing catalysts. Sample Surface Cu + /Cu atomic ratio (%) Surface Au/Cu atomic ratio (%) Au/MgCuCr O Au/CuCr O Au/MgCuAl O Au/CuO-SiO 0.9 Au/MgCuCr O 4 a Au/MgCuCr O 4 b a after 500 h on stream. b Pretreated in H at 00 o C for h. The surface Au/Cu atomic ratio of Au/MgCuCr O 4 is higher than Au/MgCuAl O 4 and Au/CuO-SiO catalysts, mainly due to the much higher surface area of the latter two supports, which is consistent with their higher gold dispersion. For Au/CuCr O 4 with similar surface area to Au/MgCuCr O 4, the lower surface Au/Cu ratio is due to the much higher Cu content. The decrease of the surface Au/Cu atomic ratio of Au/MgCuCr O 4 from 7.8 to 6.4% after reaction is consistent with the small increase of AuNP size from. to.4 nm S7

18 Table S. TPR results (H consumed, H /Cu ratios) of the sets of supports and catalysts. Sample Total Cu + (mmol/g cat ) a H uptake (mmol/g cat ) H /Cu + Cu + reduced below 00 o C (%) Au/MgCuCr O 4 ~ MgCuCr O 4 ~ Au/CuCr O 4 ~ CuCr O 4 ~ Au/MgCuAl O 4 ~ MgCuAl O 4 ~ ~00 Au/CuO-SiO ~ ~00 CuO/SiO ~ ~00 a Based on theoretic molecular formula. S8

19 Table S4. Catalytic performance comparison between aerobic oxidation of ethanol and non-oxidative ethanol dehydrogenation over MgCuCr O 4 spinel and Au/MgCuCr O 4 catalyst. Oxidation Dehydrogenation Au/MgCuCr O 4 MgCuCr O 4 Au/MgCuCr O 4 MgCuCr O 4 00 o C o C o C S (ethylene) S (CO ) o C S (acetic acid) S (ethylene) S (CO ) o C S (acetic acid) S (ethylene) S (CO ) o C S (acetic acid) S (ethylene) S (CO ) S9