Tunable Preparation Investigation And Catalytic Performances Evaluation Of Ni-based Layered Double Hydroxides Compounds

Based on the special structural traits, Layered double hydroxides (simplified asLDHs) confer themselves the favorable acid-base catalytic performances. Despite thetremendous research efforts on the synthesis and application of LDHs with increasinglynovel compositions and structures, the researchers are still far more acquainted withLDHs. As for the recent research context, catalytic behaviors investigations of LDHsmainly focus on the basic activities toward LDHs precursors as well as their subsequentcompound metal oxides after calcination. However, LDHs are seldom hired as acidcatalysts. The limited employment of LDHs as acid catalysts is presumably caused by theabsence of suitable acid sites. The appearance of feasible acid-base activities is closelyrelated with the composition, acquiring route and synthesis circumstance. Of all theinfluential factors in the synthesis of LDHs materials, the pH value is reckoned as anessential one and a basic pH is usually confirmed as necessity for the successfulpreparation. It is really worthy attention to make clear whether LDHs can be achieved under acidic circumstance, and if feasible, whether the obtained samples will possessacid-base sites which are different from conventional ones obtained under basic pH value.In this paper, NiAl-LDHs, NiFe-LDHs and Ni(HCO3)2were correspondingly fabricatedvia urea hydrolysis method by varying the pH value during preparation process. Thestructures and catalytic performances of resultant products were subsequentlyinvestigated and evaluated. The primary contents and results are displayed as follows.1. The synthesis process of NiAl-LDHs and NiFe-LDHs via urea hydrolysis was studiedin detail with seriously monitoring pH value during the preparation process. As the resultspresents, when with proper n(NO-3)/n(urea) ratio and hydrothermal treatment, pH valuecould deeply influence the morphology and constituents of resulting samples. Thespecific synthesis points are concluded here.(1) For preparation process of NiFe-LDHs via urea hydrolysis method, for c(NO-3)being0.5mol/L and hydrothermal treatment at150℃for24h, pH value was tuned byaltering ratio of n(NO-3)/n(urea). When final pH value was below6, no NiFe-LDHsproducts could be acquired; when pH in scope of6～7, NiFe-NO3-LDH were achieved;For pH being7～9, the samples were mixtures of NiFe-NO3-LDH and NiFe-CO3-LDH;as to condition of beyond9, pure NiFe-CO3-LDH were obtained. The results suggest thefinal products underwent a gradual transition from NiFe-NO3-LDH to NiFe-CO3-LDHwith elevating pH value, along with the emerging and vanishing of Ni(HCO3)2.(2) For preparation process of NiAl-LDHs using urea hydrolysis method, settingc(NO-3) of0.5mol/L and hydrothermal treatment of150℃and24h, ratio of n(NO-3)/n(urea)was modulated to result in the final pH value in intervals of3~6,6~7and above7; thecorrespondingly outcomes were NiAl-NO3-LDH, mixture of NiAl-NO3-LDH and NiAl-CO3-LDH, NiAl-CO3-LDH respectively. Also, the transition from NiAl-NO3-LDHto NiAl-CO3-LDH was observed. However, no Ni(HCO3)2was detected in the resultingsamples.2. The synthesis procedure of Ni(HCO3)2via urea hydrolysis and its resultant nickeloxides after calcination was systematically studied and the obtained samples werecomprehensively characterized by various instruments. As the results supports, the initialconcentration ratio of nickel nitrate to urea in the aqueous solution held a vital place indetermining the final product to be Ni(HCO3)2, α-Ni(OH)2or β-Ni(OH)2. Whenconcentration of Ni2+was equal or greater than0.25mol/L, the ratio of n(NO-3) to n(urea)being1:3and hydrothermal treatment at150℃for12h, pure Ni(HCO3)2with finecrystalline was obtained. In what follows, the Ni(HCO3)2was calcined at600℃for2hand NiO spheres with rough surfaces, loose structures and specific surface area of748.25m2/g were acquired.3. For catalyst evaluation purposes, the obtained NiAl-LDHs, NiFe-LDHs andNi(HCO3)2were introduced to the one-pot synthesis procedure of benzoin ethyl etherfrom benzaldehyde and ethanol. To explicitly differentiate the catalytic behaviours ofthese three kind catalysts and probe the physicochemical properties, instrumentalexperiments of XRD, FT-IR, TG-DTG, BET, XANES, NMR and SEM-EDX were carriedout. The results contends, NiAl-NO3-LDH, NiFe-NO3-LDH and Ni(HCO3)2prepared byurea hydrolysis presented vividly positive catalytic activities. Especially, samples ofNiAl-NO3-LDH with final pH value being3.36behaved best with benzaldehydeconversion of78%and selectivity for benzoin ethyl ether of100%. Oppositely, thebenzaldehyde conversion and selectivity for benzoin ethyl ether was zero for cases with NiAl-CO3-LDH and NiFe-CO3-LDH as catalysts. Further characterization gave credenceto the conclusion that the catalytic performances kept closely related with coordinationsaturation between the metal ions and hydroxides on the sheets. Both4-coordination-M(M=Al or Ni) and6-coordination-M(M=Al or Ni) in NiAl-NO3-LDHsynthesized under acid pH rather than the unique6-coordination-M(M=Al or Ni) inNiAl-CO3-LDH under basic pH were detected.4. For theoretically verify the validity of one-pot synthesis process of benzoin ethyl ether,taking Ni(HCO3)2as the catalyst, based on the available kinetics data from experiment,the possible reaction routes, thermodynamic functions for transient state, apparentactivation energy (Ea) value and plausible catalytic mechanism were established with theaid of quantum chemical calculation. These basic findings addressed a first-order reactionfor synthesis of benzoin ethyl ether from benzaldehyde and ethanol catalyzed byNi(HCO3)2and42.72kJ/mol for apparent activation energy. Quantum chemicalcalculation suggested that benzoin ethyl ether was formed via a series of states i.e. acomplex, transition states and intermediates. The apparent activation energy fromcis-C6H14NiO8…2+&&6H5to cis-C4H8NiO7…&9H12O2was about27.34kJ/mol and thelatter was succedently converted to C9H12O2with the help of alcohol. Noticeably, the roleof C9H12O2as an intermediate from cis-C4H8NiO7…&9H12O2dissociation is significant.The Ea value for conversion of R-C9H12O2…&6H5HCO to R-C16H16O2…+2O was41.44kJ/mol and this step was evidenced to be the rate determining step. The Ea valuefrom quantum chemical calculation (41.44kJ/mol) kept consistent with that fromexperimental kinetics (42.72kJ/mol).