Liquid phase selective hydrogenation of furfural on Raney nickel modified by impregnation of salts of heteropolyacids
Introduction
There are two ways of producing furfuryl alcohol through hydrogenation of furfural, gas phase hydrogenation and liquid phase hydrogenation. Most companies adopt gas phase hydrogenation, while in some countries liquid phase hydrogenation is usually employed. In both cases, the catalyst used is a Cu–Cr one. In liquid phase hydrogenation, high temperature and pressure are required. The greatest drawback of a Cu–Cr catalyst is its high toxicity, which causes severe environmental pollution. A report by Thomas et al. [1]indicated that Raney nickel with (NH4)6Mo7O24 deposited on its surface was able to catalyze liquid phase hydrogenation of furfural giving a high selectivity for furfuryl alcohol under mild conditions. Socol'skii et al. [2]added 20–30% of Gd in the process of making a Ni–Al alloy which was used to make Gd-containing Raney nickel. The activity and the selectivity of the resulting catalyst in the selective hydrogenation of furfural was greatly enhanced. Raney cobalt was found to be an effective catalyst for the reaction 3, 4. In order to enhance the activity and selectivity of Raney cobalt catalyst in hydrogenation of furfural, 0.5–15% of various transition metals were added to Co–Al alloy. Fifteen percent Re-promoted Raney cobalt showed the best selectivity. Plyusnin [5]reported that Fe–Cr or Fe–Ti modified Raney copper catalyst was much more improved in catalytic activity and selectivity in selective hydrogenation of furfural than unmodified Raney copper. Heteropolyacids (HPAs) and their salts possess acid and oxidation–reduction properties and can act as molecular catalysts. Among HPAs, those of the Keggin type are structurally stable and are often used in industrial practice [6]. Recently, it was reported that as modifiers of transition metal catalysts, HPAs have remarkable effects in homogeneous catalytic hydrogenation reactions. For example, the catalytic effect of palladium-sulfate–molybdate in the catalytic hydrogenation of propanol to propenol is improved from 50% to 96% when being modified by 12-molybdophosphoric acid [7]. Furthermore, with Willkinson catalyst, [RhCl(PPh3)3], after being modified by Li4SiW12O40, relative rates of catalytic hydrogenation of substituted monoalkenes to that of unsubstituted monoalkenes are noticeably reduced [8]. So far, no work concerning Raney nickel promoted by salts of HPAs has been reported.
This paper reports Raney nickel catalysts modified by salts of HPAs and effects of various factors on the activity and the selectivity in selective hydrogenation of furfural to produce furfuryl alcohol.
Section snippets
Preparation of catalyst
Ni–Al alloy powder (Ni/Al=50/50, m/m), a commercial product supplied by Dalian Oil and Fat Chemical Factory having a particle fineness of 200–300 mesh, was slowly added to a 20% aqueous solution of NaOH at about 50°C. The temperature was kept constant until all the alloy powder had been added, then raised to 90°C, which was then maintained for 2 h. The resulting alkali leached powder was filtered, then washed with deionized water until the washings became neutral, and finally kept in deionized
Results and discussion
The reaction of furfural hydrogenation can be represented by Scheme. 1.
Products of hydrogenation comprise furfuryl alcohol, tetrahydrofurfural and tetrahydrofurfuryl alcohol. The formation of furfuryl alcohol and tetrahydrofurfural are parallel reactions, which are competitive, while tetrahydrofurfuryl alcohol is the final product of further hydrogenation of both furfuryl alcohol and tetrahydrofurfural. In the molecule of furfural, there is a conjugated system of double bonds on the ring, and a
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