Effect of support redox character on catalytic performance in the gas phase hydrogenation of benzaldehyde and nitrobenzene over supported gold

The authors are grateful to Dr. N. Perret for her involvement in this work. EPSRC support for free access to the TEM facility at the University of St. Andrews and financial support to Dr. M. Li and Dr. X. Wang through the Overseas Research Students Award Scheme (ORSAS) are also acknowledged.Peer reviewedPostprin

Zn-Promoted Selective Gas-Phase Hydrogenation of Tertiary and Secondary C4 Alkynols over Supported Pd

We have investigated the gas-phase (P = 1 atm; T = 373 K) hydrogenation of (tertiary alkynol) 2-methyl-3-butyn-2-ol (MBY) and (secondary) 3-butyn-2-ol (BY) over a series of carbon (C), non-reducible (Al2O3 and MgO), and reducible (CeO2 and ZnO) supported monometallic [Pd (0.6–1.2% wt) and Zn (1% wt)] and bimetallic Pd–Zn (Pd:Zn mol ratio = 95:5, 70:30, and 30:70) catalysts synthesized by deposition–precipitation and colloidal deposition. The catalysts have been characterized by H2 chemisorption, hydrogen temperature-programmed desorption (H2-TPD), specific surface area (SSA), X-ray photoelectron spectroscopy (XPS), and transmission (TEM) and scanning transmission electron microscopy (STEM) analyses. Reaction over these catalysts generated the target alkenol [2-methyl-3-buten-2-ol (MBE) and 3-buten-2-ol (BE)] through partial hydrogenation and alkanol [2-methyl-butan-2-ol (MBA) and 2-butanol (BA)]/ketone [2-butanone (BONE)] as a result of full hydrogenation and double-bond migration. The catalysts exhibit a similar Pd nanoparticle size (2.7 ± 0.3 nm) but a modified electronic character (based on XPS). Hydrogenation activity is linked to surface hydrogen (from H2 chemisorption and H2-TPD). An increase in H2:alkynol (from 1 → 10) results in enhanced alkynol consumption with a greater rate in the transformation of MBY (vs BY); H2:alkynol had negligible effect on product distribution. Reaction selectivity is insensitive to the Pd site electron density with a similar response (SMBE = 65 ± 9% and SBE = 70 ± 8%) over Pdδ− (on Al2O3 and MgO) and Pdδ+ (on C and CeO2). A Pd/ZnO catalyst delivered enhanced alkenol selectivity (SMBE = 90% and SBE = 96%) attributed to PdZn alloy phase formation (proved by XRD and XPS) but low activity, ascribed to metal encapsulation. A two-fold increase in the consumption rate was recorded for Pd–Zn/Al2O3 (30:70) versus Pd/ZnO with a similar alloy content (32 ± 4% from XPS), ascribed to a contribution due to spillover hydrogen (from H2-TPD) where high alkenol selectivity was maintained.This research was funded by the Engineering and Physical Sciences Research Council (EPRSC; grant number EP/L016419/1; Ph.D. studentship to A.G.-F., CRITICAT program), the Spanish Ministerio de Ciencia Innovación y Universidades, Generalitat Valenciana and FEDER (RTI2018-095291-B-I00, MAT2017-87579-R MINECO/FEDER and PROMETEO/2018/076)

Effect of Crystallographic Phase (β vs. γ) and Surface Area on Gas Phase Nitroarene Hydrogenation Over Mo2N and Au/Mo2N

The catalytic action of Mo2N and Au/Mo2N has been assessed in the selective gas phase hydrogenation of p-chloronitrobenzene (p-CNB) to p-chloroaniline (p-CAN). The nitrides were synthesised via temperature programmed treatment of MoO3 in H2+N2 and Au introduced by deposition-precipitation with urea. We have examined the influence of nitride crystallographic phase (tetragonal β-Mo2N vs. cubic γ-Mo2N) and surface area (7-66m2g−1) on the catalytic response. Catalyst activation by temperature programmed reduction has been monitored and the reduced catalysts characterised in terms of BET area/pore volume, H2 chemisorption/temperature programmed desorption (TPD), powder X-ray diffraction (XRD), elemental analysis, scanning (SEM) and transmission (TEM) electron microscopy and X-ray photoelectron spectroscopy (XPS) measurements. The formation of β- and γ-Mo2N was confirmed by XRD and TEM. γ-Mo2N exhibits a platelet morphology whereas β-Mo2N is characterised by an aggregation of small crystallites. Hydrogen chemisorption and TPD analysis have established a greater hydrogen uptake capacity (per unit area) for β-Mo2N relative to γ-Mo2N, which is associated with surface nitrogen deficiency, i.e. higher surface Mo/N for β-Mo2N. Incorporation of Au on both nitrides resulted in an increase in surface hydrogen. The Au phase takes the form of nano-scale particles with a mean size of 7 and 4nm on β-Mo2N and γ-Mo2N, respectively. Both β-Mo2N and γ-Mo2N promoted the exclusive hydrogenation of p-CNB to p-CAN where the β-form delivered a higher specific (per m2) rate; the specific rate for γ-Mo2N was independent of surface area. The inclusion of Au on both nitrides served to enhance p-CAN productio

Gas phase chemoselective hydrogenation of p-nitrobenzonitrile over gold:effect of metal particle size, support and the metal-support interface

We report the catalytic gas phase hydrogenation of p-nitrobenzonitrile (p-NBN) to p-aminobenzonitrile (p-ABN) over a series of oxide (CeO2, Fe2O3, Fe3O4, TiO2, ZrO2 and Al2O3) supported (1 mol%) Au catalysts (mean size 3–8 nm from electron microscopy). Hydrogenation rate was structure sensitive with lower turnover frequencies (TOF) over larger Au nanoparticles in the 4–8 nm interval and a decrease in TOF for Au ≤ 3 nm. This size dependence also applies to H2 chemisorption under reaction conditions. Rate normalised with respect to H2 uptake showed a dependence on support redox potential where the formation of oxygen vacancies (from O2 titration) served to stabilise the −NO2 group, lowering reactivity. Reaction over Au/TiO2 with modified electronic character (from XPS) deviated from this trend and delivered the highest specific hydrogenation rate, which is attributed to –NO2 activation at the Au-TiO2 interface; this effect extends to TiO2 supported Ag and Pd

Gas phase chemoselective hydrogenation of p

We report the catalytic gas phase hydrogenation of p-nitrobenzonitrile (p-NBN) to p-aminobenzonitrile (p-ABN) over a series of oxide (CeO2, Fe2O3, Fe3O4, TiO2, ZrO2 and Al2O3) supported (1 mol%) Au catalysts (mean size 3–8 nm from electron microscopy). Hydrogenation rate was structure sensitive with lower turnover frequencies (TOF) over larger Au nanoparticles in the 4–8 nm interval and a decrease in TOF for Au ≤ 3 nm. This size dependence also applies to H2 chemisorption under reaction conditions. Rate normalised with respect to H2 uptake showed a dependence on support redox potential where the formation of oxygen vacancies (from O2 titration) served to stabilise the −NO2 group, lowering reactivity. Reaction over Au/TiO2 with modified electronic character (from XPS) deviated from this trend and delivered the highest specific hydrogenation rate, which is attributed to –NO2 activation at the Au-TiO2 interface; this effect extends to TiO2 supported Ag and Pd

Gas Phase Hydrogenation of Furaldehydes via Coupling with Alcohol Dehydrogenation over Ceria Supported Au-Cu

We have investigated the synthesis and application of Au-Cu/CeO2 (Cu: Au = 2) in the continuous gas phase (P = 1 atm; T = 498 K) coupled hydrogenation of 5-hydroxymethyl-2-furaldehyde (HMF) with 2-butanol dehydrogenation. STEM-EDX analysis revealed a close surface proximity of both metals in Au-Cu/CeO2 post-TPR. XPS measurements suggest (support → metal) charge transfer to form Auδ− and strong metal-support interactions to generate Cu0 and Cu+. Au-Cu/CeO2 promoted the sole formation of 2,5-dihydroxymethylfuran (DHMF) and 2-butanone in the HMF/2-butanol coupling with full hydrogen utilisation. Under the same reaction conditions, Au/CeO2 was fully selective to DHMF in standard HMF hydrogenation (using an external hydrogen supply), but delivered a lower production rate and utilised less than 0.2% of the hydrogen supplied. Exclusive -C=O hydrogenation and -OH dehydrogenation is also demonstrated for the coupling of a series of m-substituted (-CH3, -CH2CH3, -CH2OH, -CF3, -N(CH3)2, -H) furaldehydes with alcohol (1-propanol, 1-butanol, 2-propanol, 2-butanol, cyclohexanol) dehydrogenation over Au-Cu/CeO2, consistent with a nucleophilic mechanism. In each case, we observed a greater hydrogenation rate and hydrogen utilisation efficiency with a 3⁻15 times lower E-factor in the coupling process relative to standard hydrogenation. Our results demonstrate the feasibility of using hydrogen generated in situ through alcohol dehydrogenation for the selective hydrogenation of m-furaldehydes with important industrial applications