Preparation Of Hydrotalcite-Like Compounds Containing Pd, Cu, Ni, Zr And Their Catalytic Performances

Methyl isobutyl ketone (MIBK) is a good origanic slovent and the advanced process for producing MIBK is one-step synthesis by gas-phase condensation and hydrogenation of acetone. In this process, the key technology is to develop the multi-fuctional catalyst, which has both the ability of hydrogenateon and condensation, as well as suitable acid-base sites. The catalysts made from noble or transition metals by impregenation or coprecipatation have bad thermal stability and low acetone conversion or MIBK selectivity. Hydrotalcite-like compounds (HTLcs) are layered compounds. The composition of the metals in the sheets can be adjusted and the anions between the layers can be exchanged. In this paper, based on these special properties of HTLcs, HTLcs containing palladium, copper or nickel are synthesized in order to introduce them into the sheets of MgAl hydrotalcite, then the prepared HTLcs are calcined to give their corresponing composite oxides. It is expected that the resulted catalysts have better catalytic activity in the synthesis of MIBK from acetone gas phase condensation and hydrogenateon by adjusting metal molar contents and M²⁺/M³⁺ molar ratios in the sheets of HTLcs precursors. ZrO₂ is both acid and base catalyst and has the ability to adjust acid-base strength of catalyst, thus it is introduced into the above catalysts by calcining HTLcs containing zirconium in order to further adjust the dispersion and the acid-base strength of the above catalysts. Because of the exchangable property of anions between the sheets of HTLcs, PdCl₄^2- is intercalated into the space of sheets so as to prepare Pd-based catalyst via this PdCl₄-intercalated HTLcs.In this paper, PdMgAl, CuMgAl and NiMgAl HTLcs with different metal molar contents and M²⁺/M³⁺ molar ratios are synthesized at pH=9-10, aging temperature of 60℃and aging time of 12 hours by coprecipatation using NaOH and Na₂CO₃ as the precipitator, then M/Mg(Al)O(M=Pd, Cu, Ni) catalysts are prepared by calcining the resulted HTLcs. The resulted samples are characterized by the titrition with EDTA, XRD, IR, SEM, SEM-EDS, DTA, CO₂-TPD, TPR and BET special surface area mesurement. Some effects such as the metal molar contents, M²⁺/M³⁺ molar ratios on the structure of HTLcs and the phase change during the thermal treatment, metal dispersion and the reducibility, and base strength of the derived mixedcomposite oxides are discussed. Then the catalytic acitivities of acetone gas-phase condensation and hydrogenation over these catalysts are tested. PdMgAlZr, CuMgAlZr and NiMgAlZr HTLcs are prepared by the same method. The effects of Zr molar contents on the structure of PdMgAlZr, CuMgAlZr and NiMgAlZr HTLcs and base strength, metal dispersion and reducibility, and catalytic activities of the derived composite oxides are investigated. It is showed that PdMgAl, CuMgAl and NiMgAl HTLcs with single hydrotalcite phase and perfect layered structure can be prepared at M²⁺/M³⁺ molar ratios in the range of 2 and 4 along with Pd molar contents of 0.1-1mol%, Cu or Ni molar contents of 1-10mol%. Highly-dispersed metal-based catalysts with MgO phase and weak base strength are then obtained by calcining the resulted HTLcs and they have better catalytic acitivities than those prepared by impregenation and coprecipatation in the synthesis of MIBK from acetone gas-phase condensation and hydrogenation. PdMgAlZr, CuMgAlZr and NiMgAlZr HTLcs with single hydrotalcite phase and better layered structure can be prepared at Zr molar contents not more than 20mol%. The introduction of Zr into the catalyst precursors improves the dispersions of metals on the resulted catalysts and the base strength and then the catalytic activities of the catalysts can be improved with suitable Zr molar contents.MgAl-PdCl₄ HTLcs are prepared by exchanging NO₃^- in the spaces of MgAl-NO₃ HTLcs using PdCl₄^2-. Then the catalytic activities of the catalysts derived from MgAl-PdCl₄ HTLcs are studied. It is showed that MIBK selectivity is in the range of 23% and 43%, DIBK selectivity is about 30%. The resulted catalysts are suitable for combining production of MIBK and DIBK.