Heterogeneous base catalysts

Kaneda and coworkers found that HTacted as a heterogeneous base catalyst for the epoxidation of various a, 3-unsaturated ketones with 30% aqueous hydrogen peroxide as the oxidant [41]. Notably, oxidation systems using HT give epoxides exclusively,... 

Kumbhar, P. S., Sanchez-Valente, J. and Figueras, F. (1998). Modified Mg-Al hydrotalcite a highly active heterogeneous base catalyst for cyanoethylation of alcohols../ Chem. Soc., Chem. Commun., 1091. 

Synthesis of 2-aminothiophenes 564 by using calcined Mg-Al (Mg Al = 4 1) hydrotalcite (HT) as a heterogeneous base catalyst has been reported (Equation 26) <2001SC3113>. 

Mg/Al hydrotalcite as a heterogeneous base catalyst has been used by Samant and co-workers [170] in combination with microwave irradiation for the S3mthesis of 2-aminochromenes 118 via a multicomponent condensation of an aromatic aldehyde, malononitiile and 1-naphthol. The hydrotalcite is a heterogeneous basic catalyst and could easily be separated from the reaction mixture by simple filtration. The recovered catalyst was used for successive runs to investigate its reusability,... 

Bass JD, Anderson SL, Katz A (2003) The effect of outer-sphere acidity on chemical reactivity in a synthetic heterogeneous base catalyst. Angew Chem Int Ed 42 5219... 

Synthesis of heterogeneous base catalysts nitrogen containing carbon nanotubes... 

Valizadeha, H. and Azimib, A.A. 2011. ZnO/MgO containing ZnO nanoparticles as a highly effective heterogeneous base catalyst for the synthesis of 47f-pyrans and coumarins in [bmim]BF4. J. Iran. Chem. Soc. 8 123-130. 

H. Sheibani, M. Seifi, A. Bazgir, Synth. Commun. 2009, 39, 1055-1064. Three-component synthesis of pyrimidine and pyrimidinone derivatives in the presence of high-surface-area MgO, a highly effective heterogeneous base catalyst. 

D Cruz, A., Kulkarni, M.G., Meher, L.C. and Dalai, A.K. 2007. Synthesis of biodiesel from canola oil using heterogeneous base catalyst. J. Amer. Oil Chem. Soc. 84(10) 937-943. 

R. Rajagopal, T.M. Jyothi, T. Daniel, K.V. Srinivasan, B.S. Rao, Calcined Mg-Al hydrotalcite as a heterogeneous base catalyst for gewaldami-nothiophene synthesis, Synth. Commun. 31 (2001) 3113-3117. 

S.B. KhaliH, A.R. Sardarian, KF/AI2O3 an efficient solid heterogeneous base catalyst in one-pot synthesis of benzimidazoles and bis-benzimidazoles, Monatsh. Chem. 143 (2012) 841-846. 

MgO was found as a highly effective heterogeneous base catalyst for the three-component reaction of aryl aldehydes, malononitrile, and 4-hydroxy-6-methylpyrone (11) leading to the formation of funchonalized 2-amino-7-methyl-5-oxo-4-aryl-4,5-dihydropyrano[4,3-l7]p3n-an-3-carbonitriles (12) under reflux in aqueous ethanol (Scheme 24) [125]. 

M. Seifi, H. Sheibani, High surface area MgO as a highly effective heterogeneous base catalyst for three-component s)fnthesis of tetrahydro-benzopyran and 3,4-dihydropyrano[c]chromene derivatives in aqueous media, Catal. Lett. 126 (2008) 275-279. 

The reported order of activity of alkaline earth oxide catalysts was observed to be BaO>SrO>CaO>MgO. Among them, MgO was more active than CaO in the transesterifieation of rapeseed oil, but was also responsible for soap formation. In the transesterifieation of Camelina sativa oil, BaO and SrO were found to be effective catalysts for the synthesis of biodiesel at 100° C compared to CaO and MgO [20]. The authors suggested that SrO can be preferred over BaO due to the noxious nature of BaO. Similar results were also observed for the transesterifieation of palm oil using CaO, SrO and BaO as heterogeneous base catalyst imder ultra sonic assisted preparation of biodiesel. BaO and SrO were found to be efficient catalysts than CaO for production biodiesel with a yield of 95%. With BaO, there was a drop in catalytic activity due to the dissolution of BaO catalyst in methanol [21]. 

TABLE 12.1 Single metal oxides as heterogeneous base catalysts. 

K/KOH/Al Oj as heterogeneous base catalyst was employed for the trans-esteriflcation of rapeseed oil with methanol. Under optimized conditions of 1 9 of oil to methanol ratio at 60° C with 4% catalyst produced 84.5% yield of biodiesel in 1 h [49]. Though the eatalyst was recovered by filtration, the reusability studies were not encouraging as the biodiesel yield with recycled eatalyst decreased to 37.6%. The deerease in yield was observed due to the leaching of active species of catalyst into methanol. KOH loaded on Al Oj and NaY zeolite support as heterogeneous was used for the transesterification of palm oil to biodiesel. With 25 wt% KOH/Al Oj and 10% KOH/NaY, a biodiesel yield of 91.07% was achieved at 70° C in 3 h at 1 15 molar ratios of palm oil to methanol with 3-6% of catalyst. However, leaching of potassium species with both catalysts was observed, which decreases the biodiesel yield [50]. 

Mg-Al hydrotalcites (HTC) as heterogeneous base catalyst was studied for transesterification of soybean oil with methanol. It was found that oil to methanol ratio of 1 12 at a temperature of 60-65° C with 2.5% of catalyst and reaction time of 4 h were the optimized conditions. The characterization of the catalyst by (X-ray dilTraction) XRD pattern showed that MgO was the main substance present in the structure [59], Mg-Al hydrotalcite derived catalyst... 

Hydrotalcite catalysts or layered double hydroxides as catalysts are reported to give high conversion and yields in transesterification of triglycerides. Development of hydrotalcite materials for immobilization of biocatalysts for transesterification can overcome the need for higher temperature for hydro-talcite-catalyzed transesterification, which usually requires higher molar ratio of methanol. Table 12.5 summarizes the applications of hydrotalcite and hydrotalcite like compoimds as heterogeneous base catalysts for the production of biodiesel. 

In another study, neodymium oxide (Nd Oj) loaded with potassium hydroxide (30 wt%) was used as heterogeneous base catalyst for transesterrlica-tion of soybean oil to produce biodiesel. With oil to MeOH ratio of 1 14 at 60° C with 6% catalyst, biodiesel yield of 92.41% was achieved in 1.5 h. It was showed that the catalyst was envirorunentally friendly and noncorrosive with longer lifetime and was used for five times without losing its activity [81]. 

KOH supported on SBA 15 was used as a heterogeneous base catalyst for the transesterification of palm oil with methanol. It was found that 1 11.6 ratio of oil to MeOH and a temperature of 70° C for 5 h with a catalyst amount of 3.91% resulted in 83.7% of biodiesel [89]. Table 12.6 gives brief information of base catalysts, which are not derived fix)m metal oxides and hydrotalcites. 

South African Class F fly ash derived Na-X zeolite was used as heterogeneous base catalyst for the transesterification of sunflower oil to produce biodiesel. It was shown that a biodiesel yield of 83.53% was achieved with reaction conditions of oil to methanol ration of 1 6, catalyst amoimt of 3% and at a temperature of 65° C for a period of 8 h [92]. The study showed that the process offers value addition to the large amount of fly ash generated every year. 

Cinder supported CaO/KF was used as a heterogeneous base eatalyst for the transesterification of raw soybean oil for producing biodiesel. The reaction conditions of oil to methanol ratio of 1 12, catalyst amount of 2.1% at a temperature of 65° C for 20 min gave 99% conversion. The study showed that the catalyst can be reused for several times with good conversions and it was reported that 96.6% conversion was observed in the fourth cycle [96]. Table 12.7 describes the use of natural materials as supports for heterogeneous base catalyst preparation for biodiesel preparation. 

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