Technical field of the invention

This invention relates to a Bi-metal doped ZSM-5 Catalyst for the oligomerisation of light olefins. The invention also relates to the oligomerisation of olefins and a method for manufacturing a catalyst.

Background to the invention

The inventor is aware of the use of metal promoted ZSM-5 catalysts for the heterogeneous oligomerisation of light olefins.

The MOGD (Mobil Olefins to Gasoline and Distillate) and COD (Conversion of Olefins to Distillate) are the most popular commercially developed technologies used for conversion of light olefins to distillates and gasoline using HZSM-5 type of zeolite [1 ,3-6,9-1 1 ]. HZSM-5 zeolite is one of the best solid acid catalyst and have been widely applied in petrochemical industry, due to their unique structures, thermal stability, acidity and shape selectivity [10, 12, 13].

Many researchers studied oligomerization process over HZSM-5 modified by Fe, Mo, Co, Ni, Cr, Zn, Ga, Ti, Zr, Mn, Mg etc. , with the goal of reducing coke formation, which leads to rapid catalyst deactivation and poor stability [14, 15]. The results of these investigations suggest a need for the development of a more active catalyst for oligomerization of 1 -hexene and finding ways to prolong the catalyst lifetime still remains a major problem [15, 16].

It is an object of the invention to provide an improved catalyst for the selective oligomerisation of a 1 -hexene containing feed stream to obtain distillates in the gasoline (C5-C10) and diesel (C10-C20) range, and to provide a catalyst which can be manipulated to express selectivity towards gasoline with a relatively low Ci to C5 selectivity. It is a further object to manufacture clean high quality fuels. General description of the invention

According to the invention there is provided a catalyst for the heterogeneous oligomerisation of a 1 -hexene containing feed stream to produce distillates in the gasoline and/ or diesel range, which catalyst includes:

a HZSM-5 catalyst modified with between 4 and 6 weight percent Fe and Mo in a ratio range of 1 :5 to 5:1 .

The catalyst may preferably modified with between 2.4:2.6 and 3.1 :2.9 weight percent Fe and Mo to provide a catalyst with high gasoline selectivity and low Ci to C5 selectivity.

Oligomerisation may carried out at about 350°C for about 6 hours on-stream at atmospheric pressure using a fixed-bed reactor. The reactor may be a quartz tube reactor.

The method may include selecting a specific ratio of Fe and Mo to manipulate the selectivity towards gasoline or diesel during oligomerisation.

The invention also extends a method to produce distillates in the gasoline and/ or diesel range by contacting a 1 -hexene containing feed stream with a HZSM-5 oligomerisation catalyst modified with between 4 and 6 weight percent Fe and Mo in a ratio range of 1 :5 to 5: 1 .

The invention also extends to the use of the heterogeneous oligomerisation catalyst to produce distillates in the gasoline and/ or diesel range as described.

The invention also extends to a method for the manufacture of an oligomerisation catalyst, wherein calcined NH ₄ -ZSM-5 zeolite (S1O2/AI2O3 ~ 30) was impregnated by incipient wetness impregnation method firstly with a solution of ammonium heptamolybdate (ΝΗ ₄ )6Μθ ₇ θ ₂ -4Η2θ and secondly with a solution of iron(lll) nitrate Fe(N0 ₃ ) ₃ -9H ₂ 0. Detailed description of the invention

The invention is now described by way of example with reference to the accompanying Figures.

In the Figures:

Figure 1 shows N2-adsorption and desorption isotherms (a); BJH pore size distribution (b) of the HZSM-5 and xFeyMo-ZSM-5 samples.

Figure 2 shows HRSEM images of the HZSM-5 and xFeyMo-ZSM-5 samples with fixed loading and variable Mo/Fe ratio (A - HZSM-5, B - 5Mo, C - 1 Fe4Mo, D - 1.25Fe3.75Mo, E - 1.43Fe3.57Mo, F - 1 .67Fe3.33Mo, G - 2Fe3Mo, H - 2.5Fe2.5Mo, I - 3Fe2Mo, J - 4Fe1 Mo, K - 5Fe).

Figure 3 shows Element mapping of the HZSM-5 (A), 5Fe-ZSM-5 (B), 5Mo- ZSM-5 (C), 2.5Fe2.5Mo-ZSM-5 (D).

Figure 4 shows HRTEM images of the xFeyMo-ZSM-5 catalysts with fixed loading and variable Mo/Fe ratio (A - HZSM-5, B - 5Mo, C - 1 Fe4Mo, D - 1.25Fe3.75Mo, E - 1.43Fe3.57Mo, F - 1 .67Fe3.33Mo, G - 2Fe3Mo, H - 2.5Fe2.5Mo, I - 3Fe2Mo, J - 4Fe1 Mo, K - 5Fe).

Figure 5 shows EDX spectra of HZSM-5 and xFeyMo-ZSM-5 samples.

Figure 6 shows FT-IR spectra of the HZSM-5 and xFeyMo-ZSM-5 catalysts.

Figure 7 shows XRD patterns of the HZSM-5 and xFeyMo-ZSM-5 catalysts.

Figure 8 shows XRD patterns of the HZSM-5 and xFeyMo-ZSM-5 samples.

Figure 9 shows the Conversion of 1 -hexene for the experiments with 1.0 g catalyst and time-on-stream of 6 h.

Figure 10 shows the Effect of Mo/Fe ratio with TOS = 5 hours on the selectivity of C1-C5, Ce-Cg, C10+ hydrocarbons.

Figure 1 1 shows Selectivity C1 -C5, C6-C9, Cg and C10+ hydrocarbons over HZSM-5, 2.5Fe2.5Mo-ZSM-5 and 3Fe2Mo-ZSM-5 samples.

Figure 12 shows Product distribution of fuel range hydrocarbons over HZSM- 5, 2.5Fe2.5Mo-ZSM-5 and 3Fe2Mo-ZSM-5 samples (TOS= 6 hours, T=350°C, FR = 0.098 ml/min, WHSV = 2 h ^"1 ). Experimental

Chemicals used

Reagents that have been used in the preparation of catalyst and the catalytic reaction: H ₄ - ZSM-5 zeolite (Zeolyst Int., CBV 3024E), Iron(III) nitrare (Sigma-Aldrich, 98+%), Ammonium molybdate tetrahydrate (Sigma-Aldrich, 99.98%), 1-Hexene (Sigma-Aldrich,

97%).

Catalyst synthesis

The preparation of the xFeyMo-ZSM-5 catalysts was involved the impregnation of a calcined commercial H4-ZSM-5 zeolite (S1O ₂ /AI2O3 ~ 30) using the incipient wetness impregnation method with solutions of ammonium heptamolybdate (ΝΗ ₄ ) ₆ Μθ7θ24 4H ₂ 0 and iron(III) nitrate Fe(N0 ₃ ) ₃ - 9H ₂ 0. The NH ₄ -ZSM-5 powder was calcined at 550 °C in air for 6 h to convert the powder from the ammonium form to its protonated form. Samples containing both molybdenum and iron were prepared by a two-step impregnation procedure, in which the molybdenum phase was introduced first. Then samples were dried overnight at 120 °C and calcined at 500 °C for 6 hours. Similarly, 5wt%Fe-ZSM-5 and 5wt%Mo-ZSM-5 catalysts were prepared by impregnation method.

Catalyst characterization

FTIR spectroscopy analysis

The FTIR framework spectra were obtained by using Perkin Elmer FTIR spectrometer. The sample were then analysed 400 to 4000 cm ^"1 at room temperature. The spectral data were collected on the computer equipped copyright 2012 Perkin Elmer, Inc. version 10.03.07.

Surface area and micropore analysis (BET)

BET surface area was determined using a Micrometrics 3300, TriStar surface area and porosity analyser. About 0.3 g of sample was degassed at 400 °C for 4 hours. After degassing process the samples were then loaded on the analysis station for determination of the isotherms at -196 °C. The pore size distributions were calculated using the Barrett-Joyner- Halenda (BJH) model applied to the desorption branch of the isotherm, assuming cylindrical pore geometry. SEM analysis

Scanning electron microscopy (SEM) micrographs were obtained using a high resolution SEM EHT 5.00 kV. All samples were carbon coated before imaging. The HRSEM (AURIGA) was also equipped with an EDS spectrometer with the INCA EDS system by Oxford Instruments for elemental analysis of zeolites.

TEM analysis

Transmission electron microscopy images were obtained using aid of HRTEM techniques using a FEI Tecnai TF20 (20()kV) equipped with a STEM: unit, high-angle annular dark-field (HAADF) detector and X-Twin lenses.

Powder X-ray diffraction analysis

Powder X-ray diffraction data were collected using a Brucker AXS D8 diffractometer equipped with a primary beam Gobel mirror, a radial Soller slit, a VAntec- 1 detector and using Cu-Κα radiation (40 kV, 40 mA, λΚαΙ = 1.5406 A). Data were collected in the 2Θ range 5 to 70° and 90° in 0.021° steps, using a scan speed resulting in an equivalent counting time of 14.7 s per step.

Catalytic evaluation

The performances of the zeolite catalysts were tested in the conversion of 1 -hexene in a fixed bed quartz tube reactor at a weight hourly space velocity (WHSV) of 4 h ^"1 and the reaction temperature was kept at 350 °C under atmospheric pressure. The reactor tube was charged with 1 g of a catalyst then heated from room up to 350 °C. 1 -hexene was introduced by syringe pump at a flow rate of 0.098 ml/min with the syringe of 29 mm diameter continuously for 6 hours. All products were analysed on an offline Bruker 450 GC equipped with a BR-Alumina/Na2SC>4 column ( _\ -Ce), BR- 1 column (C ₇ -C ₁₇ ) and a flame ionization (FID) detector.

Catalyst characterization

Surface area and micropore analysis (BET)

Table 1 summarizes the nominal composition of samples with different Mo/Fe ratio and fixed loading (5 wt%). The BET surface area (SBET), microporous surface area (Smicro), external surface area (S _ex trer), microporous volume (V _m i _cr o), total pore volume (V _to tai) and average pore diameters of the studied catalysts are listed in Table 2 [17].

Correspondingly, the micropore volume of HZSM-5 zeolite was reduced from 0.105 to 0.092 cmVg, as revealed in Table 2. After loading 5wt% of Mo on HZSM-5, the BET surface area decreased from 377 to 313 cm ² /g due to strong Mo interaction with HZSM-5 and the high dispersion of Mo into the zeolite channels [17-19]. Adding 5wt% Fe to HZSM-5 catalyst has shown in a decrease of SBET, Smicro, S _ex trer, Vmicro, V _to tai indicating that Fe mostly deposited on the external surfaces and could have blocked some of the micropores of HZSM- 5 [17].

For the xFeyMo-ZSM-5 samples is noticed a different behavior with variable Mo Fe ratio. The xFeyMo-ZSM-5 samples exhibit a greater surface area than 5wt%Mo-ZSM-5 suggesting a better dispersion when both metals are present [17, 19].

Table 1

Nominal composition for prepared samples of xFeyMo-ZSM-5 catalysts

Table 2

Surface properties of promoted HZSM-5 catalysts

Figure. 1 (a) provides the N ₂ ad sorption-de sorption isotherms for samples with different Mo Fe ratio. The characteristic of the xFeyMo-ZSM-5 samples tends to agglomerate into microsized agglomerates is also featured by its N ₂ adsorption and desorption isotherms, which could also be supported by the results of XRD and SEM results [18] . As shown in Figure. 1 all catalysts exhibit typical type I isotherms with a small hysteresis loop in the range of p/p° = 0.5-0.9, which indicate that the HZSM-5 and xFeyMo-ZSM-5 samples have high microporosity and inter-crystal mesoporosity possibly related to the aggregation of the crystals [ 16, 18,20-23] .

Compared with the isotherm of the HZSM-5, for the xFeyMo-ZSM-5 samples the amount of N ₂ adsorption decreased. It was observed little difference in the shape of N ₂ adsorption/desorption isotherms for xFeyMo-ZSM-5 samples, but presence of both Fe and Mo on the external surfaces of zeolite crystals has little influence on their agglomeration behaviors [ 18] .

Figure. 1. N ₂ -adsorption and desorption isotherms (a); BJH pore size distribution (b) of the HZSM-5 and xFeyMo-ZSM-5 samples.

The pore size distribution was obtained by applying Barrett-Joyner-Halenda (BJH) method from the desorption branches of nitrogen isotherms [24] . Figure. 1 (b) shows the meso- and macropores with sizes in a wide range of 3-100 nm [16] . The majority of these pores concentrated at about 12-15 nm. Thus, impregnation does not change the texture of catalysts which agrees with XRD, FTIR, SEM and TEM results [25].

Compared with HZSM-5 catalyst, a notable increase of the pore size distribution for 4wt%Felwt%Mo-ZSM-5 (12.5 nm), 3wt%Fe2wt%Mo-ZSM-5 (14.5 nm), 2.5wt%Fe2.5wt%Mo-ZSM-5 (14.5 nm) and 5wt%Fe-ZSM-5 (21.0 nm) samples, exhibiting that the incorporation of Fe ₂ (MoC> ₄ ) ₃ , M0O ₃ and Fe ₂ C> ₃ in the framework made the pore diameter larger.

3.1.2. Electron microscopy and X-ray microanalysis

The SEM images of the HZSM-5 and xFeyMo-ZSM-5 zeolite catalysts are shown in Figure. 2.

Figure. 2. HRSEM images of the HZSM-5 and xFeyMo-ZSM-5 samples with fixed loading and variable Mo/Fe ratio (A - HZSM-5, B - 5Mo, C - lFe4Mo, D - 1.25Fe3.75Mo, E - 1.43Fe3.57Mo, F - 1.67Fe3.33Mo, G - 2Fe3Mo, H - 2.5Fe2.5Mo, I - 3Fe2Mo, J - 4FelMo, K - 5Fe).

The SEM images showed aggregates of particles. No major morphological differences were observed between zeolite catalysts. Figure. 2 shows two typical SEM images obtained. It is clear from the images that the promotion zeolite catalysts have a cube or elongated prismatic shapes [17, 18,20,21].

The element mapping of the HZSM-5, 5wt%Fe-ZSM-5, 5wt%Mo-ZSM-5 and 2.5wt%Fe2.5wt%Mo-ZSM-5 samples by SEM is showed in Figure. 3.

Figure. 3. Element mapping of the HZSM-5 (A), 5Fe-ZSM-5 (B), 5Mo-ZSM-5 (C), 2.5Fe2.5Mo-ZSM-5 (D).

From Figure. 3 it can be seen that the contribution of the element Fe and Mo the same as Si and Al is homogeneous. The element analysis shows that the 0-, A1-, Si- Fe-K, Fe-L, Mo K and Mo L signals display uniformly. In other words, the Fe ₂ 0 ₃ , M0O ₃ and Fe2(MoC>4)3 crystallites are well dispersed on the external surface of the HZSM-5 crystals [26]. Table 3 lists the elementary compositions of the xFeyMo-ZSM-5 samples on the SEM images and analysed by EDX technique. Table 3.

Energy Dispersive X-ray Spectroscopy (SEM)

3.1.3. The high resolution transmission electron microscopy (HRTEM)

HRTEM and Energy Dispersive Spectroscopy (EDS) were performed for the HZSM- 5 and xFeyMo-ZSM-5 samples to confirm the presence of Fe, Mo and Fe-Mo in FIZSM-5 and study the composition of the metallic phase in the catalysts. Figure. 4 shows of brightfield TEM images and Table 6 lists the elementary compositions of spots on the TEM images. The presence of Fe-Mo particles in these objects is also evidenced by EDS analysis, as it is shown in Figure. 5, where the peaks corresponding to 0-, Al- Si-, Fe-K, Fe-L, Mo- K and Mo-L emissions are clearly outlined together with other peaks that arise from the carbon and the copper grid [27,28]. As it is seen from EDS spot area the Fe and Mo were always detected together and involved in the formation of Fe2C>3, M0O3 and Fe2(MoC>4)3 in the HZSM-5 zeolite catalyst [18].

Figure. 4. HRTEM images of the xFeyMo-ZSM-5 catalysts with fixed loading and variable Mo/Fe ratio (A - HZSM-5, B - 5Mo, C - lFe4Mo, D - 1.25Fe3.75Mo, E - 1.43Fe3.57Mo, F - 1.67Fe3.33Mo, G - 2Fe3Mo, H - 2.5Fe2.5Mo, I - 3Fe2Mo, J - 4FelMo, K - 5Fe).

Figure. 5. EDX spectra of HZSM-5 and xFeyMo-ZSM-5 samples.

Table 6

Energy Dispersive X-ray Spectroscopy (TEM)

3.1.4. FT-IR spectra for promoted HZSM-5 catalysts

The FT-IR spectra of the HZSM-5 and xFeyMo-ZSM-5 samples were recorded in the range of 400-4000 cm and shown in Figure. 6. The absorption bands at 1220, 1075, 797, 542, 433 cm ^"1 are considered as the characteristic signals for the framework vibration of the HZSM-5 zeolite catalyst [29]. It was found that the band 433 cm ^"1 belongs to the T-0 bending vibration of internal tetrahedral, 542 cm ^"1 (double ring), 797 cm ^"1 (external symmetric stretch), 1075 cm ^"1 (internal asymmetric stretch) and 1220 cm ^"1 (external asymmetric stretch), respectively [25,29-31].

According to the IR spectra recorded from the xFeyMo-ZSM-5 samples, all the structure sensitive bands are similar to those of HZSM-5, but for 5wt%Mo-ZSM-5, lwt%Fe4wt%Mo-ZSM-5 and 2wt%Fe3wt%Mo-ZSM-5 catalysts it was found new bands.

Figures. 6. FT-IR spectra of the HZSM-5 and xFeyMo-ZSM-5 catalysts. The spectrum of xFeyMo-ZSM-5 catalyst shows major changes in the region of 750-1000 cm ^"1 , weakening of strong absorption of band at 797 cm ^"1 . It was found new broad at 901 cm ^"1 , which corresponds to the overlap between Mo-O-Mo bond vibrations and Mo-O-Fe bond vibrations in Fe2(MoC>4)3. The band at 1075 cm ^"1 is sensitive to the ratio of framework Si/Al. By the loading of FeMo species, the band at 1075 cm ^"1 shifted to 1081 cm ^"1 (Figures. 6).

3.1.5. X-ray diffraction (XRD) analysis Figures. 7-8 show the XRD patterns of samples with different Fe Mo ratios. The similarity between the XRD patterns of the HZSM-5 and xFeyMo-ZSM-5 samples indicates that the HZSM-5 framework was preserved after impregnation [32]. It can be seen that there are very weak diffraction peak at 2Θ = 27.35° assigned to M0O ₃ in 5wt%Mo-ZSM-5 [17,26,33,34]. In contrast, the sample impregnated with pure iron (5wt%Fe-ZSM-5) does not exhibit any reflections related to Fe2C>3 or Fe ₃ C>4, indicating a very small iron oxide particles [29]. For the samples of xFeyMo-ZSM-5 the XRD patterns exhibited reflections corresponding to molybdate species: peak at 2Θ = 27.45° is an evidence of iron molybdate phase (Fe2(MoC>4)3) [Error! Reference source not found.]. This indicates that there is an interaction between M0O3 and Fe20 in xFeyMo- ZSM-5 samples As shown in Figures. 7-8 the Fe-Mo additives causes a decrease on the HZSM-5 crystallinity, since a reduction of the HZSM-5 characteristic peaks is observed.

Figure. 7. XRD patterns of the HZSM-5 and xFeyMo-ZSM-5 catalysts.

Figure. 8. XRD patterns of the HZSM-5 and xFeyMo-ZSM-5 samples.

Once all the samples compared with HZSM-5 exhibited a decrease in the peaks between 2Θ = 7.6° and 9.2°, it is reasonable to suppose that the dispersion of molybdenum trioxide, iron oxides and iron(III) molybdates takes place on HZSM-5 surface as well as inside the channels. 3.2. Catalyst evaluation

The effect of Fe/Mo ratio on the catalytic conversion of 1-hexene was studied at reaction temperature of 350°C for 6 hours on-stream (Figure. 9). The reaction was performed at atmospheric pressure. The best conversion of 1-hexene (TOS 6 hours) was observed 99.9% for 2.5wt%Fe2.5wt%Mo-ZSM-5, 3wt%Fe2wt%Mo-ZSM-5 and HZSM- 5.

Figure. 9. Conversion of 1-hexene for the experiments with 1.0 g catalyst and time-on-stream of 6 h.

Figures. 10-11 show that addition of Fe-Mo to HZSM-5 led to decrease in the selectivity of C1-C5 fragments. An average selectivity of between 23-37% was obtained for the 2.5wt%Fe2.5wt%Mo-ZSM-5 and 3wt%Fe2wt%Mo-ZSM-5 zeolite catalysts. The average selectivity for the parent FIZSM-5 was between 27-43%.

A high gasoline selectivity (>50%) was observed for the 2.5wt%Fe2.5wt%Mo- ZSM-5, 3wt%Fe2wt%Mo-ZSM-5, 1.43wt%Fe3.57wt%Mo-ZSM-5 and parent HZSM-5 zeolite catalysts, with other catalysts being kept at 40% and less. The best selectivity for C ₆ -C ₉ was obtained over 3wt%Fe2wt%Mo-ZSM-5 catalyst, but C1 ₀ + (>25%) over 2.5wt%Fe2.5wt%Mo-ZSM-5 and being higher (9 wt%) than that of the parent HZSM-5 catalyst after TOS=6 hours. The 2.5wt%Fe2.5wt%Mo-ZSM-5 and 3wt%Fe2wt%Mo- ZSM-5 catalytic systems being higher (6-11 wt%) than that of the parent HZSM-5 catalyst in the selectivity of C9 fragments. Figure. 10. Effect of Mo/Fe ratio with TOS = 5 hours on the selectivity of C ₁ -C5, C ₆ -C ₉ , Ci ₀₊ hydrocarbons. Figure. 11. Selectivity C ₁ -C5, C ₆ -C _¾ C ₉ and Ci ₀₊ hydrocarbons over HZSM-5, 2.5Fe2.5Mo-ZSM-5 and 3Fe2Mo-ZSM-5 samples.

Figure. 12. Product distribution of fuel range hydrocarbons over HZSM-5, 2 5Fe2.5Mo-ZSM-5 and 3Fe2Mo-ZSM-5 samples (TOS= 6 hours, T=350°C, FR = 0.098 ml/min, WHSV = 2 h ^"1 ).

Figure. 12 shows the product distribution of grouped fractions of C 1-C17+ from the liquid product for 2.5wt%Fe2.5wt%Mo-ZSM-5, 3wt%Fe2wt%Mo-ZSM-5 and HZSM-5. The product distribution of C ₈ , C9 and C ₁₀ were 15%, 19% and 14% respectively for the HZSM-5 catalyst. Upon addition of FeMo the product distribution of C ₈ , C9, C ₁₀ and Cn increase to 33%, 33%, 23% and 15% respectively for the 2.5wt%Fe2.5wt%Mo-ZSM-5 and 3wt%Fe2wt%Mo-ZSM-5. Summary

Adding of the Fe-Mo to HZSM-5 catalysts has been shown to be positive for the stability of gasoline and distillate production both at standard experimental conditions, i.e. using atmospheric pressure. The 2.5wt%Fe2.5wt%Mo-ZSM-5 and 3wt%Fe2wt%Mo-ZSM-5 catalysts show higher conversion and selectivity in gasoline and distillate range than parent HZSM-5 catalyst. It shall be understood that the examples are provided for illustrating the invention further and to assist a person skilled in the art with understanding the invention and are not meant to be construed as unduly limiting the reasonable scope of the invention. REFERENCES

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