Rapid Hydrogenation of Toluene in a One-Pass Reactor at Ambient Temperature and Pressure on a Novel Nanostructured Super-Alloy Catalysts without Containing Noble Metals

Novel Ni-Re super-alloy catalyst system is found for rapid hydrogenation of toluene to methylcyclohexane (MCH) in a one-pass reactor under mild reaction conditions. The novel bimetallic Ni-Re alloy catalysts exhibit the remarkably enhanced activity and stability for hydrogenation of toluene by comparison with monometallic Ni catalyst. The catalytic performance of novel Ni-Re catalyst is compared with that of Pt catalysts for hydrogenation of toluene. The better activity maintenance of the Ni-Re catalysts than the Pt-based catalysts are found. The alloying effect of Ni with Re is identified by XRD and EXAFS measurements.

Keywords: Liquid Organic Hydride; Hydrogenation of Toluene; Ni-Re Super-Alloy Catalysts

The hydrogenation of aromatics is an important reaction for the production of cyclohexane – an important precursor in the manufacturing of nylon-6,6 [1-3]. Hydrogen is a clean energy carrier and it can be generated from renewable energy including solar energy, wind energy, etc. The toluene/methylcyclohexane couple is a promising cyclic hydrocarbon combination for the safe and feasible storage of hydrogen [4]. In this method, hydrogen is fixed to toluene (C₇H₈) with the hydrogenation reaction and converted into MCH (C₇H₁₄) that is able to be transported with a chemical tanker as same as toluene. In the demand site, hydrogen is generated from MCH by dehydrogenation reaction, and toluene is recovered for the recycle use. Liquid organic hydrogen carrier (LOHC) is suitable for large-scale storage and long distance transportation due to the ambient condition with the low potential risk. Not only is the toluene/methylcyclohexane couple reversible and highly selective, but it is free from carcinogenic products. Furthermore, it has a relatively high hydrogen storage capacity (6.2 wt.%, 46.5 kg-H2/m³), and the volatility range of the constituents makes the couple compatible with existing transportation infrastructure, such as storage tanks and refueling stations, giving it an advantage over other solid hydrocarbons for hydrogen storage and transport [4-7]. However, the hydrogenation of aromatics, including toluene, is traditionally performed using elevated temperatures and pressures, which often exceed 100 °C and 50 atm H₂ [8]. Thus, effecting the reduction under mild conditions is an important challenge in terms of energy conservation and associated environmental considerations. This is of particular concern when examining the suitability of the toluene/methylcyclohexane couple for the reversible storage of hydrogen, i.e. the system's usability in a decentralised transport infrastructure. Despite much research on the hydrogenation of toluene at mild temperature, elevated pressures are still required. Additionally, the catalysts are often with low activity and/or low stability [2]. A further approach to overcome the problems of stability has been to immobilise the nanoparticles on a solid support, such as silica, boehmite nanofibers, multi-walled carbon nanotubes, titanium dioxide, and others [1,2,9-11]. Recently, Özkar et al. reported the hydrogenation of aromatics at room temperature under 3 atm H₂ using colloidal nanozeolite framework-stabilised ruthenium (0) nanoclusters, and obtained a turnover-frequency (TOF) of 1800 h−1 for the hydrogenation of toluene. Although the hydrogenation of toluene and tetralin at ambient temperature and pressure in a batch reactor has been recently reported and 79% of toluene conversion was obtained, noble metal catalysts are necessary for the reaction [12]. Ni-Re based superalloy catalysts have been successfully applied to the steam reforming of gasoline to generate hydrogen [13-18]. In the present presentation, rapid hydrogenation of toluene in a one-pass reactor at ambient temperature and pressure on a novel super-alloy catalysts without containing noble metals are studied and compared with Pt-based catalysts.

Ni/Al₂O₃ and Pt/Al₂O₃ catalysts were prepared by impregnating Al₂O₃ with aqueous solution of Ni(NO₃)₂ and Pt(NH3)4(NO3)2 respectively at room temperature, followed by drying at 333 K for 4h and 393 K for 4h, calcination at 773 K for 4 h. Ni-Re/Al₂O₃ was prepared by impregnating Ni/Al₂O₃ with aqueous solution of NH₄ReO₄ at room temperature, followed by drying at 333K for 4h and 393K for 4h, calcination at 773 K for 4 h. The catalytic tests for hydrogenation of toluene to MCH was conducted in a demonstration equipment with two reactors. The volume of each reactor is about 200 ml. The catalytic tests were also conducted in a fixed-bed one-pass flow smaller reactor. The low rates of toluene, MCH and hydrogen are controlled by mass-flow controllers. The products including MCH and toluene are condensed and collected in condenser at room temperature. All the collected products in the condenser were analyzed by gas chromatographs with a flame ionization detector (FID).

XRD patterns were recorded by using a Rigaku Ultima III diffractometer with Cu Kα radiation (λ = 0.154178) at 40 kV and 40 mA. The scanning angle (2θ) ranged from 5 to 50◦ at a scanning rate of 5◦ min−1.

Figure 1 shows conversions with tome on stream for hydrogenation of toluene on Ni(5wt%)/Al₂O₃ and Ni(5wt%)-Re/Al₂O₃ with different Re loadings at 393K and toluene WSV of 3.4h^-1 and H₂/Toluene/N₂ mole ratio of =3.8/1/0.6.

We can see from Figure 1 that the initial conversion for toluene hydrogenation on Ni(5wt%)/Al₂O₃ is about 10% and the conversion is declined to zero after about 20 hours of time on stream. The initial toluene conversion on Ni(5wt%)Re(0.1wt%)/Al₂O₃ is about 88% and it is declined to about 80% after about 20 hours of time on stream, which is about 8 time higher than that on Ni(5wt%)/Al2O3 catalyst. The initial toluene conversions on Ni(5wt%)Re(0.25wt%)/Al₂O₃ and Ni(5wt%)Re(0.5wt%)/Al₂O₃ catalysts are about 90% and they are stable during about 20 hours of time on stream. The initial maximum toluene conversion on Ni(5wt%)Re(1wt%)/Al₂O₃ is about 98% and it is declined to about 93% after about 20 hours of time on stream. Therefore the activity of Ni(5wt%)/Al₂O₃ is remarkably enhanced by addition of about only 0.1wt% Re. The activity of Ni(5wt%)-Re/Al₂O₃ is increased further with increasing Re loadings from 0.1wt% to 1wt% for toluene hydrogenation under the reaction conditions. Catalytic activities of Ni(5wt%)-Re/Al₂O₃ catalysts with different Re loadings of from 0.1wt% to 2wt% are compared further for hydrogenation of toluene at 393 K and a higher toluene WSV of 5.9 h^-1 and H₂/Toluene/N₂ mole ratio of =3.8/1/0.6 as shown in Figure 2.

In the higher toluene WSV of 5.9 h-1, the activity variation of catalysts with different Re loadings from 0.1wt% to 2wt% are more evident than that at a lower WSV of 3.4 h^-1. The catalytic activity order for toluene hydrogenation of the Ni(5wt%)-Re/Al₂O₃catalysts with different Re loadings from 0.1wt% to 2wt% is as follows, Ni(5wt%)Re(0.1wt%)/Al2O3 Ni(5wt%)Re(0.25wt%) /Al₂O₃ Ni(5wt%)-Re(0.5wt%)/Al₂O₃> Ni(5wt%)-Re(1wt%)/Al₂O₃> Ni(5wt%)-Re(2wt%)/Al₂O₃

The toluene conversion on Ni(5wt%)-Re/Al₂O₃ catalysts is increased with increasing Re loadings from 0.1wt% to 0.5wt% and toluene conversion on Ni(5wt%)-Re/Al₂O₃ catalysts is decreased with increasing Re loadings from 0.5wt% to 2wt% as shown in Figure 2.

Toluene conversions with time on stream on Ni(5wt%)-Re(1wt%)/Al₂O₃ , Ni(10wt%)-Re(1wt%)/Al₂O₃ and Ni(15wt%)-Re(1wt%)/Al₂O₃catalysts with different Re-loadings for hydrogenation of toluene at 393K and toluene WSV of 5.9h^-1 and H2/Toluene/N2 mole ratio of =3.8/1/0.6 are shown in Figure 3.

We can see from Figure 3 that the toluene conversions with time on stream on Ni(5wt%)-Re(1wt%)/Al₂O₃ and Ni(10wt%)-Re(1wt%)/Al₂O₃ are similar and they are about 85% under the reaction conditions. The toluene conversion on Ni(15wt%)-Re(1wt%)/Al₂O₃ is 88%, which is about 3% higher than that on Ni(5wt%)-Re(1wt%)/Al₂O₃ and Ni(10wt%)-Re(1wt%)/Al₂O₃. These indicate that the catalyst activity almost has no increase by increasing Ni-loading from 5 wt% to 10 wt% and it has a little increase by increasing Ni-loading from 10 wt% to 15 wt% as shown in Figure 3. Therefore the activity of the Ni/Al₂O₃ for hydrogenation of toluene is remarkably enhanced by alloying of Ni with Re to form the new alloy active sites. The higher Ni-Loadings or higher Re-loadings are not necessary for the high activities of the catalysts. The suitable Ni-loading is about 5wt% and the suitable Re loading is about 0.25-0.5 wt% for obtaining the best catalytic performance of Ni-Re/Al₂O₃ catalysts for hydrogenation of toluene.

The dependence of conversion and selectivity on the composition of Ni-Re/Al₂O₃is further studied in Table 1. We can see from Table 1 that a high toluene conversion of about 99.8% with a high selectivity of about 99.9% attained on Ni-Re/Al₂O₃ catalyst. In order to collect the relatively higher purity liquid product by condensation in one step, the high conversion of up to 100% is highly desired for hydrogen storage by one pass flow hydrogenation of toluene.

It has been well known that Pt has been the best element as the active component of the catalysts for hydrogenation of unsaturated hydrocarbons and dehydrogenation of saturated hydrocarbons. However the Pt is high cost noble element. It is highly desirable if we could found the other catalyst materials to instead of Pt catalysts for hydrogenation or dehydrogenation reactions. Figure 4 shows the toluene conversions with time on stream on Pt(1wt%)Al2O3 and Ni(5wt%)-Re(0.5wt%)/Al₂O₃ catalysts for hydrogenation of toluene at 393K and toluene WSV of 5.9 h-1 and H2/toluene/N₂ mole ratio of 3.8/1/0.6. The initial toluene conversion on Ni(5wt%)-Re(0.5wt%)/Al₂O₃ is about 91% and it is quite stable with time on stream, which is quite similar to the toluene conversion on Pt(1wt%)Al₂O₃ catalyst for toluene hydrogenation at 393K and toluene WSV of 5.9 h^-1 and H₂/ toluene/N₂ mole ratio of 3.8/1/0.6 as shown in Figure 4.

Figure 5 shows the toluene conversions with time on stream on Pt(1wt%)Al₂O₃ and Ni(5wt%)-Re(0.5wt%)/Al₂O₃ catalysts for hydrogenation of toluene at 393 K and a lower toluene WSV of 3.4 h ^-1 and H₂/Toluene/N₂ mole ratio of =3.8/1/0.6 during a longer time of about 20 hours on stream.

A high initial toluene conversion of over 99% is obtained on both Pt(1wt%)Al₂O₃ and Ni(5wt%)-Re(0.5wt%)/Al₂O₃ catalysts for hydrogenation of toluene at 393 K and a lower toluene WSV of 3.4 h^-1 and H₂/toluene/N₂ mole ratio of =3.8/1/0.6. The conversion of toluene hydrogenation is declined from over 99% to 20-30% after about 20 hours of time on stream on Pt(1wt%)Al₂O₃ catalyst. The conversion of toluene hydrogenation is still maintained at over 92% after about 20 hours of time on stream on the Ni(5wt%)-Re(0.5wt%)/Al₂O₃ catalyst for hydrogenation of toluene at 393 K and a lower toluene WSV of 3.4 h^-1 and H₂/toluene/N₂ mole ratio of =3.8/1/0.6.as shown in Figure 5. Therefore the Ni(5wt%)-Re(0.5wt%)/Al₂O₃ catalyst exhibit the much better activity maintenance at the high toluene conversion than the Pt(1wt%)Al₂O₃ catalyst for hydrogenation of toluene at 393K and toluene WSV of 3.4 h^-1 and H₂/Toluene/N₂ mole ratio of =3.8/1/0.6. It is of significance that our developed novel Ni-Re alloy catalyst exhibits the similar activity to that of the Pt catalyst and the activity maintenance Ni-Re catalyst is much better than that of Pt catalyst for hydrogenation of toluene at high toluene conversion. Figure 6 shows comparison of Ni(5wt%)Re(1wt%)/Al₂O₃ catalyst with other alloys such as Ni(5wt%)Zn(1wt%)/Al₂O₃ and Ni(5wt%)V(1wt%)/Al₂O₃ catalysts for hydrogenation of toluene at 393 K and toluene WSV of 3.4 h^-1 and H₂/toluene/N₂ mole ratio of =3.8/1/0.6.

We can see from Figure 6 that the Ni(5wt%)Zn(1wt%)/Al2O3 and Ni(5wt%)V(1wt%)/Al₂O₃ alloy catalysts exhibit the much lower activity than the Ni(5wt%)Re(1wt%)/Al₂O₃ catalyst for hydrogenation of toluene at 393K and toluene WSV of 3.4h^-1 and H₂/toluene/N₂ mole ratio of =3.8/1/0.6 as shown in Figure 6. The alloying effect of Ni with Re on the structure of Ni nanoparticles is characterized by EXAFS and obtained XRD and the EXAFS and XRD results are shown in Figures 7 and 8. Figure 8 compared the EXAFS profiles of Ni(5wt%)/Al₂O₃ and Ni(5wt%)-Re(1wt%)/Al₂O₃ samples after reaction of toluene hydrogenation. We can see from Figure 7 that the Ni-Ni bond length on Ni(5wt%)-Re(1wt%)/Al₂O₃ is about 0.258 nanometer and the Ni-Ni bond length on Ni(5wt%)/Al₂O₃ sample is about 0.248 nanometer. The Ni-Ni bond length on Ni(5wt%)-Re(1wt%)/Al₂O₃ sample is about 0.01nanometer longer than that on Ni(5wt%)/Al₂O₃ sample. The alloying of Ni with Re lead to the expanding of the Ni-Ni bonds in the Ni-Re alloy nanoparticles. Expanding of Ni-Ni bond length in the Ni-Re alloy nanoparticles is the reason why the bimetallic Ni-Re alloy nanoparticles are much more active than the monometallic Ni nanoparticles for catalyzing the hydrogenation of toluene.

The alloying effect of Ni with Re on the structure Ni-Re/Al₂O₃ catalyst is also characterized by XRD. The XRD patterns of Al₂O₃, Ni(5wt%)/Al₂O₃, Ni(5wt%)-Re(0.1wt%)/Al₂O₃, Ni(5wt%)-Re(0.25wt%)/Al₂O₃, Ni(5wt%)-Re(0.5wt%)/Al2O3, Ni(5wt%)-Re(1wt%)/Al₂O₃ and Ni(5wt%)-Re(2wt%)/Al₂O₃ samples after reduction by flowing hydrogen at 703K for 1h are shown in Figure 8.

Since the Ni(111) and Ni(200) XRD peaks of Ni(5wt%)/Al₂O₃, Ni(5wt%)-Re(0.1wt%)/Al₂O₃, Ni(5wt%)-Re(0.25wt%)/Al₂O₃, Ni(5wt%)-Re(0.5wt%)/Al₂O₃, Ni(5wt%)-Re(1wt%)/Al₂O₃ and Ni(5wt%)-Re(2wt%)/Al₂O₃ samples are overlayed with the XRD peaks of Al2O3 substrate (support), the Ni(220) XRD peaks of Ni(5wt%)/Al₂O₃, Ni(5wt%)-Re(0.1wt%)/Al₂O₃, Ni(5wt%)-Re(0.25wt%)/Al₂O₃, Ni(5wt%)-Re(0.5wt%)/Al₂O₃, Ni(5wt%)-Re(1wt%)/Al₂O₃ and Ni(5wt%)-Re(2wt%)/Al₂O₃ samples are compared in the 2θrange of 71-83degree although the Ni(220) peak is relatively more weak than the Ni(111) peak or Ni(200) peak. We can see from Fig 8 that the 2 theta angel of Ni (220) peak in the Ni(5wt%)-Re(0.1wt%)/Al₂O₃, Ni(5wt%)-Re(0.25wt%)/Al₂O₃, Ni(5wt%)-Re(0.5wt%)/Al₂O₃, Ni(5wt%)-Re(1wt%)/Al₂O₃ and Ni(5wt%)-Re(2wt%)/Al₂O₃ samples are shifted to the higher value by comparison with that of Ni(5wt%)/Al₂O₃ sample because of the alloying effect of Ni with Re.

In order to observed the alloying effect of Ni with Re more clearly by XRD, Ni(5wt%)/Ni-foil, Ni(5wt%)-Re(0.5wt%)/Ni-foil, Ni(5wt%)-Re(1wt%)/Ni-foil, Ni(5wt%)-Re(2wt%)/Ni-foil, samples are prepared by using Ni-foil as substrate instead of Al₂O₃. The XRD patterns of the Ni(5wt%)/Ni-foil, Ni(5wt%)-Re(0.5wt%)/Ni-foil, Ni(5wt%)-Re(1wt%)/Ni-foil and Ni(5wt%)-Re(2wt%)/Ni-foil samples after reduction by following hydrogen at 553K for 1h are compared in Figure 9. I can clearly see that the Ni(111) peak of Ni-Re samples is shifted to the lower 2 theta angel values and Ni(220) peak shifted to the higher 2theta values by comparison with the Ni(111) and Ni(220) pears of Ni sample. The XRD results in Figure 9 confidently prove the alloying effect of Ni with Re in the Ni-Re catalysts.

A novel bimetallic Ni-Re alloy nanoparticles catalyst system is found for highly efficient hydrogenation of toluene to methylcyclohexane. The alloying effect of Ni with Re lead to the 2theta angel of Ni(220) XRD peak being shifted to the higher value and a new XRD peak at 2thea angel of about 44.3 degree is appeared near the Ni(111) peak at the 2theta angel of about 44.5 degree. Bond length expanding of Ni-Ni resulted from the alloying effect of Ni with Re is identified by EXAFS measurements.