Study the hydro-isomerization of light paraffin over bifunctional catalysts at elevated pressures

Noble metal based catalysts being 0.8 wt.%Pd and 0.35 wt.%Pt supported on HZSM-5 zeolite were subjected to the investigation. Physico-chemical characteristics of catalysts were determined by the methods of nitrogen physi-sorption, SEM, XRD, TEM, NH3-TPD, TPR, and Hydrogen Pulse Chemi-sorption (HPC). Activity of catalysts in the isomerization of n-C6 and n-C5 + n-C6 mixture was studied in a micro-flow reactor in the temperature range of 225 − 325 oC at pressure of 0.1 MPa and 0.7 MPa; the molar ratio H2/ hydrocarbon: 5.92, concentration of n-C5 or n-C6: 4.6 mol.%, GHSV 2698 h-1. The stability of catalysts was also determined. The obtained catalysts expressed high acid density, good reducing property, and high metal dispersion with the cluster size in the range of nanometer. They were an excellent contact for isomerization of n-hexane or n-C5 + n-C6 mixture with high conversion, selectivity, and yield. At optimal temperature, n-paraffin conversions were as high as 57 – 63 % and selectivity was 90 %. Pressure had the positive effect to the catalyst conversion, selectivity, and stability. At 0.7 MPa, both catalysts produced 6669 RON liquid product containing friendly environmental iso-paraffins which is superior blending stock for green gasoline.


INTRODUCTION
The production of gasoline for automobiles has been becoming challenged in the aspect of environmental concerns.Being toxic tetraethyl lead, MTBE, benzene, and its derivatives are banded or gradually eliminated from the gasoline pool.In order to meet the strict gasoline specifications refineries have been forced to find out new processes and new catalysts to produce good blending stock for green gasoline.
Isomerate is rising as one of the excellent replacements for the required characteristics.
The skeletal hydro-isomerization of npentane + n-hexane mixture is an essential process to produce iso-pentane and iso-hexane for low aromatics gasoline.Bi-functional catalysts consisting of noble metals supported on micro-porous materials have been widely used

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for this reaction.Among the available supports, HZSM-5 showed to be superior [1].However, it has been found that isomerization over bifunctional catalyst is influenced by the H2 spillover phenomenon [1].The H2 migrating or spilling over from the noble metal sites onto the acidic support is responsible for the promoting effect of H2.However, this effect has been observed on limited numbers of catalysts.Therefore, finding out new catalyst with superior H2 spillover effect is evaluable for isomerization reaction.Metals like Pd and Pt have been found to be good active components for hydrogendehydrogenation functions as well the H2 spillover effect.The easy dissociation of hydrogen on their surfaces is responsible for those abilities.This work dealt with Pd and Pt catalysts supported on HZSM-5 for hydroisomerization of the mixture of n-pentane and nhexane.Several experiments had been conducted in order to find out the active catalysts.
Most of the studies on isomerization are carried out at pressures higher than atmospheric [2], [3] in order to be correlated with the industrial operation (20-30 kg cm -2 ) [4].Neither a systematic study nor the selection of this parameter can be found in the literature although there are papers indicated that hydrogen is necessary for the isomerization reaction [5].Allain [2] proposed the reaction order with respect to hydrogen is -0.7 and that can be explained by a kinetic bi-functional monomolecular model while Iglesia et al. [6] reported n-heptane isomerization rate showed positive hydrogen kinetic order.This study was conducted in order to study the actual effect of pressure to the catalyst activity and stability.
From those points of view two zeolite supported catalysts based on Pt and Pd were subjected to the investigation for the isomerization of n-pentane and n-hexane mixture at elevated pressures.
Physico-chemical properties of the catalysts were characterized by methods of BET-N2, SEM, TEM, XRD, TPR, H2 pulse chemi-sorption (HPC), and TPD.Activity of the studied catalysts was tested in a micro-flow reactor at the temperature range of 225 ÷ 325 o C; mol ratio of H2/hydrocarbons was 5.92; concentration of n-C5 or n-C6 was 4.6 mol.%;GHSV 2381 h -1 .The mixture of reaction products was analyzed on the GC Agilent Technologies 6890 Plus with a FID detector, and DB 624 column with 30 m of length and 0.32 mm of outer diameter.

RESULTS
SEM image of Pd/HZSM-5 catalyst presented rectangular cubic crystallites of zeolites with dimensions 90 -263 nm for catalysts Pd/HZSM-5.Values of HZSM-5 crystallite size (dzeol) and zeolite particle dimension (d) varied in the ranges of 29.6 -31.5 nm and 37.7 -44.7 nm, respectively (Tab.1).Loading metal onto HZSM-5 more or less reduced the specific area of zeolite may be because of the pore blockage of the support by added metal species [9].

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Catalyst Pt/HZSM-5 expressed higher metal dispersity and smaller metal crystallite size in comparison with Pd/HZSM-5.Table 1 showed that Pt has the dispersity of 76.99 % and crystallite size of 1.5 nm while those of Pd are 23.3 % and 5.0 nm.Pd cluster size (dPd) calculated by HPC and measured by TEM are relatively closed; on Pd/HZSM-5, dPd is 5 nm by HPC and 7.36 nm by TEM (Fig. 1).Occhiuzzi found that on catalysts 0.88 wt.%Pd/ZrO2-WO3 the values 29.3 nm for quantity dPd and 3.8 % for Pd dispersity (γPd), were observed [10].Another work on 2.0 wt.%Pd/HZSM-5 by Thomson specified the Pd crystallite size in the range of 12.5 nm with metal dispersion of 9.4% [11].The differences between metal dispersion in our work and those in the others happened because the fact that, surface area of our catalyst (298 m 2 /g) is much higher than that of ZrO2-WO3 (36 m 2 /g) and the metal content of our catalyst (0.8 wt.%) is lower than that in catalyst in Thomson's study (2.0 wt.%).Relatively small metal crystallite sizes in this work are good for isomerization reaction [12].
EDS image of Pd/HZSM-5 (not shown) shows that Pd is distributed on catalyst surface fairly evenly and the value of Si/Al on the surface is fairly high about 18.
XRD patterns of catalysts were presented in These results also indicated that Pd or Pt may interact with defect sites stabilizing the crystalline structure of HZSM-5 and making framework structure to be more ordered.The absence of characteristic peaks of Pd and Pt may be due to very small amounts of those species.
The negative peak at 75 o C in TPR diagram of Pd/HZSM-5 catalyst (Fig. 3, line 1) characterized the decomposition of palladium hydride formed through H-diffusion within the Pd crystallites [13].While Pt supported on HZSM-5 is characterized by 2 reduction peaks at 202 o C and 402 o C (Fig. 3, line 2).Maora H. Jorda˜o also showed two reduction peaks at 270 o C and 400 o C for 1.0 wt.% Pt/HUSY calcined at 500 o C [14].Those peaks characterized the reduction of Pt 4+ to Pt 2+ and Pt 2+ to Pt o .
Loading metal onto HZSM-5 resulted in the depression of acidity (Tab.1).This result was also announced by Ho Si Thoang [12] and Villegas [15].This may be because of the replacement of proton H + of silanol group of HZSM-5 by the metal [16], [17].Pd/HZSM-5 possesses relatively higher total acidity and acid strength compared to Pt/HZSM-5.
In the reaction of pure feed (Tab.2) or mixed feed (Tab.3) the activity of Pd/HZSM-5 and Pt/HZSM-5 are comparable but the lifetime of Pd/HZSM-5 is a little shorter than Pt/HZSM-5.This may come from the fact that the higher acidity of Pd/HZSM-5 leading to the stronger cracking reaction and the shorter lifetime of Pd/HZSM-5 compared to Pt/HZSM-5.Trang 56 The higher selectivity in the reaction of mixture compared to pure C6 one leading to the catalyst stability is much improved.The lifetime of Pd/HZSM-5 and Pt/HZSM-5 catalysts increased from 1 h and 4 h in case of the reaction of pure C6 to 7 h and > 30 h in case of a mixture, respectively.This fact can be explained by the lower concentration of C6 in feed [18] and the apparent energies of activation of cracking decreased with increasing chain length of the hydrocarbon [19].
Increasing pressure of the reaction from 0.1 MPa up to 0.7 MPa helped to increase both catalyst conversion, selectivity, and, as a result, is the lifetime of catalyst (Tab.2 and Tab.3).The lifetime of Pd/HZSM-5 catalyst increased from 1 h (pure feed) and 7 h (mixed feed) to over 30 h when the pressure was brought up from 0.1 MPa to 0.7 MPa.And those of Pt/HZSM-5 catalyst increased from 4 h (pure feed) to over 30 h.
It seems that high pressure has a good impact to the catalyst conversion, selectivity, and stability.Iglesia and co-workers [6] found that isomerization rate has positive hydrogen kinetic order.They proposed that the reaction takes place via hydride transfer step which terminates the isomerized carbocation by hydride transferred from the neutral species.
They called those by the bi-functional bimolecular mechanism which sounds in contrast with the conventional mono-molecule mechanism.By proposing this kind of mechanism they also specified the rate-limiting step is the desorption of the intermediate.The same observation was made by Manoli and coworkers [20].They found the maximal activity of Pt/SZ in n-hexane isomerization with increasing hydrogen partial pressure.At high hydrogen partial pressure, hydride ions are provided for rate-limiting desorption step leading to the improvement of catalyst activity.At low hydrogen partial pressure, there are less hydride transfer phenomena leading to the longer lifetime of surface intermediate.In this situation, cracking and oligomerization will dominate at short residence time; polymerization and coke formation will dominate at long residence time leading to the depression of catalyst lifetime.

CONCLUSIONS
Catalyst Pt/HZSM-5 expressed higher metal dispersity, smaller metal crystallite size, and lower acidity in comparison to Pd/HZSM-5.The advantage of Pt based catalyst over Pd based catalyst is that its lifetime is little higher than that of Pd catalyst at atmospheric pressure.
The presence of n-pentane in the mixture elevated reaction conversion, reaction selectivity, and catalyst lifetime.
Pressure had a positive effect to the reaction conversion, reaction selectivity, and catalyst lifetime.At 0.7 MPa, Pd and Pt supported catalysts produced 66-69 RON liquid product containing friendly environmental iso-paraffins which are superior blending stock for green gasoline.

Fig. 2 .
Fig. 2. The peaks in the range of 2Ɵ = 7 -10 o and 22 -25 o were identified as peak reflections of HZSM-5 zeolite [1].The impregnation of Pd or Pt onto HZSM-5 did not change the peak position but slightly increased the intensity of the peaks and the crystallinity of HZSM-5 due to the