Process Formation Investigation Of NiAL-LDHs And Its Catalytic Performances Evaluation

Despite the tremendous research efforts on the synthesis of LDHs with increasingly novel compositions and structures, the researchers are still far more acquainted with the formation process of LDHs, and even still have some differences. To gain the resulting LDHs material with specific applications, a detailed understandment of the formation process of the LDHs is very important.It is well known that abundant hydroxyl of LDHs surface can offer a number of basic sites, but as acid-base bifunctional catalyst, the source of acid sites is still need to explore for its strongly influenced by the preparation process. In this paper, we reported a systematic investigation of the formation process of Ni Al-LDHs by HMT hydrolysis method from nickel and aluminum precursor salt solution to explore the source of acid sites. Being based on the detailed characterizations, the structures and catalytic performances of resultant products were subsequently investigated and evaluated. The primary contents and results are displayed as follows.1. The formation process of Ni Al-LDHs was studied in detail under different synthetic environment: for c(Ni²⁺) being 0.5mol/L, 0.2 mol/L, 0.1mol/L, while altering the ratios of n(Ni²⁺)/n(Al³⁺) and n（NO-3）/n（HMT） in orderto ensure that n(Ni²⁺)/n(Al³⁺) are 1:2, 1:1, 3:1 and n（NO-3）/n（HMT） are 8:1, 3:1,1:1 in solution, respectively. XRD, FT-IR, BET, XPS, SEM were carried out to reveal that Ni Al-LDHs was transformed by Al OOH with similar layered structure. Concrete as follows:（1） For c(Ni²⁺) being 0.5mol/L and hydrothermal time being 12 h, when n（NO-3）/n（HMT） was equal to 8:1, if n(Ni²⁺)/n(Al³⁺) was 1:2, only pure Al OOH could be acquired and if the n(Ni²⁺)/n(Al³⁺) was modulated to 1:1, the samples were mixtures of Al OOH and Ni Al-NO₃-LDH. The totally transition from Al OOH to Ni Al-NO₃-LDH was observed only when n(Ni²⁺)/n(Al³⁺) was 3:1.When n（NO-3）/n（HMT） was set to 3:1 and 1:1, if n(Ni²⁺)/n(Al³⁺) were tuned to1:2 and 1:1, Al OOH and Ni Al-CO₃-LDH were achieved simultaneously, while n(Ni²⁺)/n(Al³⁺) was 3:1, Al OOH could be transformed to Ni Al-CO₃-LDH completely. It is obvious that basic surroundings is conducive to the formation of Ni Al-LDHs. When c(Ni²⁺) decreased to 0.2mol/L and 0.1 mol/L respectively,although the initial concentration of Ni²⁺varies greatly, it has not much effect on the transformation process, which had similar phenomenon of c(Ni²⁺) being0.5mol/L.（2） When n(Ni²⁺)/n(Al³⁺) was set to 3:1, Ni Al-LDHs was transformed from Al OOH gradually with altering hydrothermal time. We can find that the whole forming process of Ni Al-LDHs can be divided into the following stages: HMT hydrolysis, the formation of Al OOH, the continuous incorporation of surrounding Ni²⁺into Al OOH, the intercalation of anions into the interlayergalleries, the stack of main layers to build a three-dimensional network, the formation of plate-like structure of Ni Al-LDHs. Throughout the above-mentioned processes, the incorporation of Ni²⁺into the sheet of the lamellar Al OOH play a primary role for the formation of Ni Al-LDHs crystallites. Furthermore, we selected Ni Al-NO₃-LDH and Ni Al-CO₃-LDH obtained at c(Ni²⁺) being 0.5mol/L as the research object and found that Ni Al-CO₃-LDH had larger specific surface area and more stable structure than Ni Al-NO₃-LDH by comparing the layer spacing, specific surface area, element composition, pyrolysis behavior, surface morphology and other aspects.2. The synthesis process of Al OOH via HMT hydrolysis was studied in detail with seriously monitoring the ratio of n（NO-3）/n（HMT） during the preparation process. Samples obtained at various n（NO-3）/n（HMT） were characterized by XRD, FT-IR, TG-DTG, SEM and TEM. As the result showed that Al OOH obtained at different n（NO-3）/n（HMT） had different structures and morphologies.3. The obtained Al OOH, Ni Al-NO₃-LDH and Ni Al-CO₃-LDH were introduced to the one-pot synthesis procedure of benzoin ethyl ether from benzaldehyde and ethanol for catalyst evaluation purposes. To differentiate the catalytic behaviours of these three kinds of catalysts explicitly, probe the microstructure and reveal the possible origins of effective catalytic activities for the superior catalyst, instrumental experiments of XPS, EXAFS, ESR were carried out. The results showed Ni Al-NO₃-LDH and Al OOH prepared by HMThydrolysis presented vividly positive catalytic activities, but Ni Al-NO₃-LDH exhibited a higher catalytic activity than single hydroxide Al OOH, indicating that Ni and Al should have obvious metal ion synergistic effect. Especially,samples of Ni Al-NO₃-LDH with c(Ni²⁺) was 0.5mol/L behaved best with benzaldehyde conversion of 79.8% and selectivity for benzoin ethyl ether of100%, and it could be reuse many times with a stable structure. Oppositely, the benzaldehyde conversion and selectivity for benzoin ethyl ether was zero for cases with Ni Al-CO₃-LDH as catalysts. It seems that interlayer anions was closely related with the catalytic performances of Ni Al-NO₃-LDH and Ni Al-CO₃-LDH, but elemental analysis and FT-IR denied this speculation.Further experiment gave evidence to the conclusion that the appearance of proper Lewis acidic sites arising from low coordination number of Ni²⁺in Ni Al-NO₃-LDH was responsible for the excellent catalytic performances.