Gold is a well-recognised catalyst of selective alcohol oxidation. Combining with other metals may improve its properties. Earlier we reported synergetic effect of AuAg/SiO2 of Au/Ag>3/1 atomic ratio in aerobic selective benzyl alcohol (BzOH) oxidation in toluene in base free conditions1. In this work alumina supported analogues were studied and the effect of base addition to the reaction. Bimetallic Au-Ag (Au/Ag=4/1; 1/1 atomic ratios, designated by Au4Ag1 and Au1Ag1, respectively)) and corresponding monometallic aqueous sols were formed1, and then adsorbed on alumina or silica. Alumina supported samples were prepared also by solvated metal atom deposition (SMAD) technique. For structural characterization of the samples UV-vis spectroscopy, HR-TEM, XPS, temperature programmed reduction (TPR), CO adsorption followed by DRIFT and its temperature programmed desorption by QMS was applied. The samples were tested in BzOH oxidation in toluene solution (30 mL 0.1 M BzOH, 15-30 mg catalyst, 80°C, O2 bubbling, 1 atm) without or with addition of equimolar K2CO3 after calcination (400°C/air/1h) and following reduction treatment (350°C/H2/30 min). The initial reaction rates were compared. The mean particle sizes of Au-Ag and monometallic Au particles in the different catalysts varied between 2-5 nm, which did not change much in the pretreatments. Ag particles in the monometallic Ag samples were much larger and polydisperse (5-17 nm). The formation of bimetallic particles with alloyed phase, and especially in Au1Ag1 systems and in SMAD samples, also silver segregated on the surface of bimetallic particles or separated as well was suggested by SPR bands in UV-vis and indirectly by XPS and TPR and CO adsorption measurements. In BzOH oxidation only benzyl-benzoate was detected beside the main product benzaldehyde (>90% selectivity). In base free reaction all the catalysts deactivated quickly, because of poisoning by trace amount of benzoic acid. By added K2CO3 this could be avoided, the reaction rates increased, the complete conversion of BzOH could be reached. The monometallic Ag and the bimetallic Au1Ag1 samples had no or negligible activity in the reaction conditions applied. The alumina supported Au4Ag1=4/1 and Au catalysts' activity was similar in calcined and reduced state, in case of the silica supported samples the reduced form was more active. The alumina supported samples were much more active than the silica supported ones. The synergetic effect of the sol derived bimetallic Au4Ag1/SiO2 significantly decreased on base addition, and on alumina support almost disappeared. The SMAD derived Ag4Ag1/Al2O3 was much less active, than Au/Al2O3. Strong influence of the support, preparation method, Au/Ag molar ratio, the surface composition of bimetallic particles and K2CO3 addition to the reaction was demonstrated in evolution of synergetic Au-Ag bimetallic effect in benzyl alcohol oxidation.

Sol derived alumina and silica supported Au-Ag bimetallic catalysts: structure and activity in aerobic selective oxidation of benzyl alcohol

Claudio Evangelisti
2017

Abstract

Gold is a well-recognised catalyst of selective alcohol oxidation. Combining with other metals may improve its properties. Earlier we reported synergetic effect of AuAg/SiO2 of Au/Ag>3/1 atomic ratio in aerobic selective benzyl alcohol (BzOH) oxidation in toluene in base free conditions1. In this work alumina supported analogues were studied and the effect of base addition to the reaction. Bimetallic Au-Ag (Au/Ag=4/1; 1/1 atomic ratios, designated by Au4Ag1 and Au1Ag1, respectively)) and corresponding monometallic aqueous sols were formed1, and then adsorbed on alumina or silica. Alumina supported samples were prepared also by solvated metal atom deposition (SMAD) technique. For structural characterization of the samples UV-vis spectroscopy, HR-TEM, XPS, temperature programmed reduction (TPR), CO adsorption followed by DRIFT and its temperature programmed desorption by QMS was applied. The samples were tested in BzOH oxidation in toluene solution (30 mL 0.1 M BzOH, 15-30 mg catalyst, 80°C, O2 bubbling, 1 atm) without or with addition of equimolar K2CO3 after calcination (400°C/air/1h) and following reduction treatment (350°C/H2/30 min). The initial reaction rates were compared. The mean particle sizes of Au-Ag and monometallic Au particles in the different catalysts varied between 2-5 nm, which did not change much in the pretreatments. Ag particles in the monometallic Ag samples were much larger and polydisperse (5-17 nm). The formation of bimetallic particles with alloyed phase, and especially in Au1Ag1 systems and in SMAD samples, also silver segregated on the surface of bimetallic particles or separated as well was suggested by SPR bands in UV-vis and indirectly by XPS and TPR and CO adsorption measurements. In BzOH oxidation only benzyl-benzoate was detected beside the main product benzaldehyde (>90% selectivity). In base free reaction all the catalysts deactivated quickly, because of poisoning by trace amount of benzoic acid. By added K2CO3 this could be avoided, the reaction rates increased, the complete conversion of BzOH could be reached. The monometallic Ag and the bimetallic Au1Ag1 samples had no or negligible activity in the reaction conditions applied. The alumina supported Au4Ag1=4/1 and Au catalysts' activity was similar in calcined and reduced state, in case of the silica supported samples the reduced form was more active. The alumina supported samples were much more active than the silica supported ones. The synergetic effect of the sol derived bimetallic Au4Ag1/SiO2 significantly decreased on base addition, and on alumina support almost disappeared. The SMAD derived Ag4Ag1/Al2O3 was much less active, than Au/Al2O3. Strong influence of the support, preparation method, Au/Ag molar ratio, the surface composition of bimetallic particles and K2CO3 addition to the reaction was demonstrated in evolution of synergetic Au-Ag bimetallic effect in benzyl alcohol oxidation.
2017
Istituto di Scienze e Tecnologie Molecolari - ISTM - Sede Milano
Au-Ag bimetallic catalysts
selective oxidation
benzyl alcohol
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.14243/333765
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