Endogenous Growth Of Monolithic AlOOH/Al-Fiber And Its Applications In Development Of Structured Pd Nano-catalysts For Gas-phase CO Coupling To DMO

The gas-phase CO coupling to dimethyl oxalate(DMO)is attracting considerable attention due to the moderate reaction conditions,atom economy and non-oil route.Up to date,this process has been industrialized,but the commercial Pd/?-Al2O3 catalyst suffers from high Pd loading of 2-3 wt%(the state of the art)and poor mass/heat transfer.Recently,structured catalysts and reactors(SCRs)are a promising avenue to process intensification.For example,structured Al-substrates(e.g.,Al-fiber and-foam)have a great flexibility in geometric appearance for structured catalyst packing that is paving the way to the "ideal" reactor(ultralow pressure drop,mal-distribution free and remarkable decrease of hot/cold spot temperature).However,what to be noteworthy is that their practical applications in heterogeneous catalysis are severely restricted by the low surface area and inactive surfaces.Therefore,based on the "top-down" strategy,a green,versatile and general one-pot route to the morphology-controllable endogenous growth of nano-sheet AlOOH(ns-AlOOH)was developed on 3D Al-fiber through water-only hydrothermal oxidation between Al metal and H2O(2Al + 4H2O?2AlOOH + 3H2).A microfibrous-structured Pd catalyst for CO coupling to DMO has been prepared by employing the ns-AlOOH/Al-fiber composite as support.Such catalysts showed unique combination of high activity/selectivity and good stability with high permeability and high thermal conductivity.With the aid of various characterization techniques,including electron microscopic,spectroscopic,thermal and adsorption/desorption analysis,the catalytic performance and mechanism were investigated.The content of the thesis can be briefly summarized as follows.(1)Endogenous growth of AlOOH nanosheets on monolithic Al-fiber and its application for Pd-catalyzed CO coupling to DMOWe established a facile,green and generalized method of endogenous growth of ns-AlOOH on structured Al-fiber through steam-only oxidation process(2Al + 4H2O?2AlOOH +3H2).This core(Al-fiber)-shell(ns-AlOOH)composite had a honeycomb-like morphology and the ns-AlOOH content could be finely tuned by adjusting the steam-only oxidation time length.The proposed strategy opens new possibilities in fabrication of novel structured catalysts and reactors.As an example,microfibrous-structured Pd/ns-AlOOH/Al-fiber catalyst was prepared by directly employing the composite as support.Such catalyst delivered an intrinsic activity(TOF)three times higher than the traditional Pd/a-Al2O3 catalyst.(2)Monolithic Pd/ns-AlOOH/Al-fiber for CO coupling to DMO:Effect of morphology of AlOOH nanosheetThe ns-AlOOH/Al-fiber composite could also be fabricated in an autoclave through water-only hydrothermal oxidation process(2A1 + 42O?2AlOOH+3H2).Content and morphology of ns-AlOOH could be finely tuned by adjusting the hydrothermal oxidation time length and temperature.Interestingly,the Pd dispersion of Pd/ns-AlOOH/Al-fiber was very sensitive to the thickness of AlOOH nanosheet,and therefore the conversion showed strong AlOOH-nanosheet-thickness dependence whereas the intrinsic activity(TOF)was AlOOH-nanosheet-thickness independence.(3)Monolithic Pd/ns-AlOOH/Al-fiber for CO coupling to DMO:Synergistic interaction of hydroxyl and PdEffects of catalyst preparation and reaction conditions were studied over the Pd/ns-AlOOH/Al-fiber catalyst.The best catalyst with a low Pd-loading of only 0.25 wt%could deliver a high CO conversion of 67%and DMO selectivity of 96%for a feed of CH3ONO/CO/N2(1/1.4/7.6,vol)with a gas hourly space velocity of 3000 L kg-1 h-1 at 150?,and was maintained almost unchanged for at least 200 h.It was found that the intrinsic activity was intimately related to the hydroxyl amount on the catalyst surface,indicating the existence of Pd-hydroxyl synergistic interaction which was paramount to the enhanced catalytic performance.By nature,the presence of hydroxyls facilitated the adsorption of bridged CO on the Pd surface,which was the only reactive adsorbed CO species for the CO coupling reaction.(4)Monolithic Pd/ns-AlOOH/Al-fiber for CO coupling to DMO:In situ DRIFTS study of mechanismIn situ diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS)studies were performed to investigate the catalytic mechanism.Intermediate species consisting of stretching vibrations of C=O and C-O,and rocking vibration of CH3 was indeed captured,which could be assigned to either COCOOCH3*or COOCH3*(*,a surface site).To clarify the intermediate,methyl oxalyl chloride(CH3OCOCOCI)and methyl chloroformate(CH3OCOCI)were employed to form COCOOCH3*and COOCH3*species.Interestingly,the characteristic bands of the as-observed intermediate in the real reaction matched the obtained COCOOCH3*species from dissociative adsorption of CH3OCOCOCI.A double carbonylation reaction pathway was thus confirmed,i.e.,consecutive insertion of two CO*molecules into OCH3*to form COCOOCH3*followed by combining OCH3*to yield DMO.(5)Carbon-decoration monolithic Pd-C/ns-AlOOH/Al-fiber for CO coupling to DMO:Synergistic interaction of carbon and PdIn order to further improve the catalytic efficiency of Pd nanocatalysts,we attempted to modify the abovementioned mother Pd/ns-AlOOH/Al-fiber catalyst by carbon.Interestingly,the carbon-decorated Pd-C/ns-AlOOH/Al-fiber demonstrated 1.5 times higher TOF compared to the mother catalyst.It was found that TOF was intimately related to the form of carbon on the catalyst surface:the carbon of covering layer was unfavorable for TOF while the dissolved carbon could couple with Pd to form the new PdC species,which was paramount to the enhanced catalytic performance,by nature,as the result of carbon-promoted formation of the key intermediate species of COCOOCH3*.