Metal Foam-/Fiber-Structured Ni-Based Catalysts For Partial Oxidation Of Methane And CO₂ Reforming Of Methane To Syngas

To prompt the harmonious development of economy and environment,it is urgently to transform the traditional coal-based energy consumption structure in our country.Accordingly,the conversion technologies of methane have been attracting more and more attentions.Among them,the methane to syngas process,as the prerequisite operation in the indirect methane conversion is strongly exo-or endo-thermic,which induces severe mass/heat transfer limitations in the traditional catalyst bed and then leads to high energy consumption and low efficiency in the practical application.Monolithic metal-structured catalysts and reactors,as an effective chemical process intensification technology,are giving an opportunity for the development of novel monolithic catalysts for the methane reforming to syngas due to improved hydrodynamics in combination with enhanced heat/mass transfer.Hence,the development of novel methods for effectively and efficiently embedding catalytic components onto the monolithic metallic substrates and the application of as-prepared structured catalysts for converting methane to syngas are highly desirable from academic and industrial standpoints.Monolithic metallic foam-/fiber-structured Ni-based catalysts were triumphantly prepared by the novel catalytic functionalization methods,such as wet chemical etching,interface-assisted growth,and coupling agent-assisted self-organization.Based on the unique synergistic coupling of the flow and heat/mass transfer in the reactor and surface catalysis,these structured catalysts were qualified for partial oxidation of methane?POM?and CO₂ reforming of methane?also called dry reforming of methane,DRM?to syngas.The content of the thesis can be briefly summarized as follows.?1?Ni-foam-structured Ni-based catalyst and its catalytic performance for partial oxidation of methane to syngasA series of monolithic Ni-foam-structured Ni-based catalysts for the POM reaction have been developed via wet chemical etching method,in situ growth or sol-assisted impregnation method.The results are as follows:?i?The NiO-Al₂O₃/Ni-foam catalyst prepared by wet chemical etching method exhibits the satisfying catalytic activity and syngas selectivity.But the CH₄ conversion and H₂/CO selectivity are declined during the 85 h test originated from the serious carbon deposition.?ii?A Ni-foam-structured NiO-MgO-Al₂O₃ nanocomposite catalyst was developed via thermal decomposition of NiMgAl layered double hydroxides?LDHs?in situ hydrothermally grown onto the Ni-foam.Originated from the uniform distribution of NiO-MgO-Al₂O₃ in the nanocomposites,the strong interaction between Ni O and MgO-Al₂O₃,and the enhanced basicity,CH₄ conversion of 86.5%and H₂/CO selectivity of 91.8%/88.0%can be achieved with a stability for at least 200 h over this catalyst at 700 ^oC,a gas hourly space velocity?GHSV?of 100 L g^-1 h^-1 and a feed gas with CH₄/O₂ molar ratio of 1.8/1.0.?iii?The Ni-foam-structured Ni-CeAlO₃-Al₂O₃ nanocomposite catalyst was obtainable via direct growth of NiAl-LDHs nanosheets and subsequent impregnation assisted by boehmite sol and thermal treatment.Such catalyst delivers enhanced carbon resistance due to the introduction of CeAlO₃.We speculate that carbon formation rate is dramatically reduced thanks to a feasible Ce³⁺/Ce⁴⁺chemical cycling.Specific verification experiments will be conducted in part?3?.The above results demonstrate the feasibility of the monolithic metal-structured catalysts for the POM reaction.Moreover,the nanocomposites derived from LDHs deliver the homogeneous component-distribution and strong interaction between NiO and MgO-Al₂O₃,which can improve the sintering-/carbon-resistance of the catalyst.Furthermore,the introduction of MgO and the formation of Ce³⁺/Ce⁴⁺chemical cycling markedly improve the carbon resistance of the catalyst.What to be noted is that the Ni-foam is powdered in the long time reaction process,which will deteriorate the catalytic performance.?2?Fe CrAl-fiber-structured LDHs-derived nanocomposite catalyst and its catalytic performance for partial oxidation of methane to syngasA thin-felt microfibrous-structured LDHs-derived NiO-MgO-Al₂O₃/FeCrAl-fiber catalyst was fabricated by employing FeCrAl-fiber with high permeability,enhanced heat/mass transfer and excellent temperature-/oxidation-resistance as support.A crystallization-driven self-assembly process was used to achieve the spontaneous growth of Al?OH?₃ on the FeCrAl-fiber with the aid of precipitation of Al?OH?₃ from a sodium aluminate solution,which can be transformed into Al₂O₃ layer by calcination treatment.Via the Al₂O₃/water interface-assisted strategy,LDHs were embedded onto monolithic substrates and NiO-MgO-Al₂O₃/FeCrAl-fiber catalyst was fabricated after a thermal treatment which transformed LDHs to nanocomposites.The thin-felt NiO-MgO-Al₂O₃/FeCrAl-fiber catalyst achieves the combination of the homogeneous distribution of catalytic components,enhanced heat/mass transfer and high permeability,thus offering high activity and stability for the high throughput and exothermic POM reaction.At 700 ^oC,a GHSV of 72 L g^-1 h^-1 and a feed gas with CH₄/O₂ molar ratio of1.8/1.0,CH₄ conversion and syngas selectivity maintain almost constant within 300 h running.Moreover,we anticipate the feasibility and generality of the above Al₂O₃/water interface-assisted strategy to be a new point which might stimulate exploitation of the new-generation structured catalyst.?3?FeCrAl-fiber-structured Ni-CeAlO3-Al2O3 nanocomposite catalyst and its catalytic performance for partial oxidation of methane to syngasA thin-felt microfibrous-structured NiAl-LDHs-derived NiO-Al₂O₃/FeCrAl-fiber support was fabricated via the Al₂O₃/water interface-assisted strategy and calcination treatment.Then the Ce?NO₃?₃ precursor was loaded on the support via the impregnation method and the Ni-CeAlO₃-Al₂O₃/FeCrAl-fiber catalyst was obtained after the calcination and reduction.At 700 ^oC,a GHSV of 72 L g^-1 h^-1 and a feed gas with CH₄/O₂molar ratio of 2.0/1.0,CH₄ conversion and syngas selectivity maintain almost constant within 325 h running.The catalyst shows a significantly reduced carbon formation rate compared to the Ni-Al₂O₃/FeCrAl-fiber catalyst under the same reaction conditions:0.15 mg_car g^-1_cat h^-1 for the Ni-CeAlO₃-Al₂O₃/FeCrAl-fiber catalyst vs.1.40 mg_car g^-1_catat h^-1 for the Ni-Al₂O₃/FeCrAl-fiber catalyst.Based on the analysis results,it is believed that CeAlO₃ can capture O₂/CO₂ to form Ce⁴⁺and surface active oxygen due to its reducibility,so that the surface carbon from CH₄ fragments can be timely oxidized.Meanwhile,Ce⁴⁺can convert to Ce³⁺under CH₄/H₂ atmosphere and Ce³⁺will participate in the new round of Ce³⁺/Ce⁴⁺chemical cycling.Namely,carbon formation rate is reduced to a great extent due to the feasible Ce³⁺/Ce⁴⁺chemical cycling.?4?Fe CrAl-fiber-structured LDHs-derived nanocomposite catalyst and its catalytic performance for CO2 reforming of methane to syngasFor the DRM reaction which is conducted at a low GHSV,the excess of active component of Ni and acidic site of Al₂O₃ always facilitates the formation of carbon.Therefore,the optimized catalyst for the POM reaction in part?2?is not suitable for the DRM reaction.And it is difficult to obtain Al₂O₃/FeCrAl-fiber support with low content of Al₂O₃ via the preparation strategy in part?2?.The continuous and uniform shell of AlOOH nanosheets with low content was grown onto the FeCrAl-fiber surface via a simple hydrothermal treatment in a solution of sodium aluminate and urea.And the subsequent calcination treatment turned the AlOOH into Al₂O₃.FeCrAl-fiber-structured NiO-MgO-Al₂O₃ nanocomposite catalyst was developed via thermally decomposing NiMgAl-LDHs that can be grown onto the FeCrAl-fiber through the Al₂O₃/water interface-assisted method.By taking advantages of homogeneous component-distribution in the LDHs-derived NiO-MgO-Al₂O₃ nanocomposites and enhanced heat/mass transfer,this catalyst delivers satisfying performance for the DRM reaction.At 800 ^oC,a GHSV of 5 L g^-1 h^-1 and a feed gas with CH₄/CO₂ molar ratio of1.0/1.1,CH₄/CO₂ conversion maintains almost constant within the initial 90 h and then slides in a smooth downturn within another 180 h running.Despite a slow coking rate,gradually blocking of the Ni particle surface is inevitable by the carbon deposit along with prolonged time on stream,which is the main cause for the catalyst deactivation.?5?FeCrAl-fiber-structured Ni@SiO2/Al2O3 catalyst and its catalytic performance for CO2 reforming of methane to syngasA thin-felt microfibrous-structured Ni@SiO₂/Al₂O₃/FeCrAl-fiber catalyst was fabricated by one-step top-down“macro-micro-nano”organization with the aid of the cross-linking molecules?H₂NCH₂CH₂CH₂Si?OC₂H₅?₃,APTES?followed by the calcination and reduction treatment.APTES acted as the bidirectional bridging:the first briding occurred between-NH₂ of APTES and Ni²⁺by the preferential chelation;and the second was the silanisation reation between the ethoxy groupts of APTES and surfae OH groups on AlOOH nanoarrays to form Al-O-Si bonds.Due to the confinement effect of core-shell-like nanostructure,strong interaction between Ni core and SiO₂shell and satisfying heat/mass transfer of FeCrAl-fiber,CH₄/CO₂ conversion maintains almost constant at 90.8%/89.9%within 500 h running at 800 ^oC,a GHSV of 5 L g^-1 h^-1and a feed gas with CH₄/CO₂ molar ratio of 1.0/1.1.Not surprisingly,the core-shell-like Ni@SiO₂ nanostructures are well preserved and the Ni size is increased a little from7-9 to 10-16 nm.Better yet,carbon deposition is not detectable on the 500 h tested Ni@SiO₂/Al₂O₃/FeCrAl-fiber catalyst.