2.1. Tetrahydrofurfuryl alcohol (THFA)
Tetrahydrofurfuryl alcohol with a molecular formula of C5H10O2 is a transparent, mobile, high-boiling liquid with mild odor, and completely miscible with water. THFA is regarded as ―green solvent‖ because of its relatively benign nature and very low toxicity.[16] The industrial production of THFA is commercially manufactured by Koatsu Chemical Industries in Japan with annual production volume of ~30 t. Up till now, the potential applications of THFA are only limited for agricultural solvent, printing inks, industrial and electronics cleaners.[16]
First published article that could be traced on Web of Science about production of THFA was in 1989, performed by Antoine Gaset’s group. They attempted to synthesize THFA according to two procedures involving either furfural or furfuryl alcohol. Copper-supported catalysts were efficient for transforming furfural into furfuryl alcohol, but couldn’t obtain THFA. On the other hand, catalysts based upon metals from group VIII (Ni, Ru, Rh, Pd, Pt) could lead to the formation of THFA. Ni-supported catalysts was commonly used because its cheap, but the reaction was not selective. In their study, they compared activity, selectivity, and lifetime of different catalysts in hydrogenation of furfural and furfuryl alcohol.[17]
An interesting report was showed by Folami T. Ladipo’s group. This group presumed that the nature of heterogeneous catalysts in selectivity for desired products by concentrating on composition of catalysts and operating conditions of furfural hydrogenation appeared somehow limited. There is a current need for development of soluble catalysts that display high activity and product selectivity for furfural hydrogenation or hydrogenolysis. They synthesized ruthenium(II) bis(diimine)
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complexes with different ligands using data conducted by Gadi Rothenberg’s group[18]
as standpoint. They could achieve both 99% yield of furfuryl alcohol from furfural and 99% yield of THFA from furfuryl alcohol by changing the ligands.[19]
Liquid-phase chemoselective hydrogenation of furfural was studied by Virginia Vetere’s group, employing Pt, Rh, and Ni in bimetallic systems. Bimetallic systems, containing amounts of tin, were obtained by means of controlled surface reactions between a monometallic catalyst and Sn(C4H9)4. Relationship between two metals employed activity and selectivity. All systems allowed obtaining furfuryl alcohol from furfural with high selectivity (99, 97, and 76% were achieved with Pt, Rh, and Ni catalysts, respectively). These bimetallic catalysts only achieved ~4% selectivity to THFA in hydrogenation of furfural.[20]
More detail information regarding hydrogenation of furfuryl alcohol to THFA was performed by Xiaochun Chen’s group. They performed liquid phase hydrogenation on a series of new special supported Ni catalyst QD3 using continous stirred autoclave.
Operating condition studied in the experiments was opted as the following range : temperature within 433-453 K, pressure within 3.0-4.0 MPa, catalyst loading 20g/L and stirring rate at 1000 rpm. They concluded temperature is most important factor to furfuryl alcohol conversion, catalyst loading affects THFA selectivity, hydrogen pressure has a relatively weak impact on furfuryl alcohol conversion and THFA selectivity. They claimed that under optimized condition, conversion above 99.9%, and selectivity of THFA higher than 98.3% could be achieved within 3.5h.[21]
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Selective low-temperature hydrogenation of furfuryl alcohol to THFA could be achieved by hectorite-supported ruthenium nanoparticles. Metallic ruthenium nanoparticles intercalated in hectorite (particle size ~4 nm) under mild conditions, 40°C, 20 bar of H2 pressure could achieve 100% conversion, selectivity >99%).[22]
2.2 Sodium borohydride (NaBH
4) as hydrogen-generator
[12]Sodium borohydride can be classified as both a complex metal hydride and chemical hydride as it can release hydrogen by two methods : thermolysis, where the stored hydrogen is released by heating, and hydrolysis, where the stored hydrogen is released by reaction with water. The former is not attractive for portable applications since sodium borohydride is stable up to 400°C. The latter is particularly attractive for three major reasons. Firstly, hydrolysis of sodium borohydride is a spontaneous, exothermic (-210 kJ mol-1) process that can be easily accelerated by simple addition of metal catalyst. Secondly, as can be seen from equation 1, half of the hydrogen comes from the water, giving sodium borohydride a relatively high theoretical hydrogen storage capacity of 10.8 wt%. Finally, the hydrolysis reaction can produce pure hydrogen at temperature as low as 298K.
However, hydrogen generation by hydrolysis of sodium borohydride is not without problems. A major issue is the volume of water required. Equation 1 shows the stoichiometric chemical reaction, but in reality at least 4 molar equivalents of water are required for each mole of sodium borohydride in the reaction. This is for two reasons.
Firstly, as shown in Equation 2, sodium metaborate (NaBO2) is rapidly hydrated.
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Secondly, the solubility of sodium borohydride in water is relatively low (55 g per 100 g at 25°C), requiring more water than that required by stoichiometry to ensure the sodium borohydride remains in solution (although sodium borohydride does have a considerably higher solubility than ammonia borane (33.6 g per 100 g at 25°C), and other hydrolysis materials such as aluminium and silicon which are insoluble). This is further compounded by the even lower solubility of sodium metaborate (28 g per 100 g of water at 25°C), which means that the concentration of sodium borohydride must be kept below 16 g per 100 g of water to ensure that sodium metaborate does not precipitate from the reaction mixture.
Sodium borohydride undergoes self-hydrolysis upon the addition of water, and thus typically stabilized by the addition of sodium hydroxide (the self-hydrolysis reaction rate drops to negligible above pH 13). The mechanism of self-hydrolysis has been described as follows :
Step 1 : NaBH4(s) Na+(aq) + BH4-(aq) Step 2 : BH4-(aq) + H+(aq) BH3(aq) + H2(g) Step 3 : BH3(aq) + 3H2O(l) B(OH)3(aq) + 3H2(g) Step 4 : B(OH)3(aq) + H2O(l) B(OH)4-(aq) + H+(aq) Step 5 : 4B(OH)4-(aq) + 2H+(aq) B4O72-(aq) + 9H2O(l)
The decrease in the amount of protons in basic media results in Step 2 of the self-hydrolysis being disfavored and the hydrogen generation process thus slowed. By increasing the amount of protons and thus accelerating Step 2, the addition of homogenous acid catalysts to aqueous sodium borohydride solutions results in an increase the rate of hydrolysis.
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2.3 Hydrogenation using NaBH
4as hydrogen-generator
Catalyst Brown’s P-2 Ni, Caubere’s nickel complex reducing agent (NiCRA) and Ni2B-BER, are well known in hydrogenation of alkenes using nickel compounds.
For other systems, also NaBH4-CoCl2, FeCRA, LiH-VCl, and LaNi5H6 have been reported to be good hydrogenation systems. Takashi’s group have found system (sodium borohydride (NaBH4)/moist alumina in hexane) is excellent for the good selective reduction of aliphatic and aromatic carbonyl compounds to corresponding alcohols in high yield. They tested using different types of support, which styrene as test substrate and could achieve 90% of ethyl benzene. [23]
Ioannis’s group investigated that mesoporous titania-supported gold nanoparticles assemblies (Au/MTA) catalyze the activation of NaBH4, which act as transfer hydrogenation agents for the reduction of nitroarenes to the corresponding anilines in moderate high yields. Nitroalkanes are reduced to the corresponding diazo and hydrazo compounds. [24]
Stephen P. Thomas’s group performed simple and environmentally benign formal hydrogenation using highly abundant iron (III) salts and an inexpensive, bench stable, stoichiometric reductant, NaBH4, in ethanol, under ambient conditions. Also this reaction has been applied to the reduction of terminal alkenes up to 95% yield, and nitro-groups up to 95% yield. [25]
Poul Nielson’s group directly used the relatively inexpensive reagent RuCl3.xH2O as catalyst for the reduction of olefins in the presence of water.
Combination of cheap and readily available sodium borohydride and a catalytic amount of RuCl3.xH2O selectively reduces mono- and disubstituted olefins, whereas trisubstituted olefins, unless activated, and benzyl ethers remain inert. [26]
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