Comparative Study Beetween Zn – Cu- HZSM-5 and Zn-HZSM-5 ...

http://www.revistadechimie.ro REV.CHIM.(Bucharest)♦ 67♦ No. 4 ♦ 2016734

Comparative Study Beetween Zn – Cu- HZSM-5 and Zn-HZSM-5(Acetate) Catalysts in Conversion of C4 – C4

= Technical Fraction

IULIEAN VASILE ASAFTEI1, KAMEL EARAR2*, NECULAI CATALIN LUNGU1, LUCIAN-MIHAIL BIRSA1, MARIA IGNAT1,VALENTIN PLESU3*, IOAN GABRIEL SANDU4,5*1 Alexandru Ioan Cuza University of Iasi, Faculty of Chemistry, Laboratory of Materials Chemistry and Laboratory of OrganicChemistry, 11 Carol I Blvd, 700506, Iasi, Romania2 Dunarea de Jos University of Galati, Romania, Faculty of Medicine and Farmacy, 35. Al. I. Cuza Str., 800010, Galati, Romania3 Politehnica University of Bucharest,Centre for Technology Transfer for the Process Industries, Department of Chemical andBiochemica Engineering, 1 Gh. Polizu Str., Bilding A, 011061, Bucharest, Romania4 Gheorghe Asachi Technical University, Faculty of Materials Science and Engineering, 64 Mangeron Blvd., 700050, Iasi, Romania5 Romanian Inventors Forum, 3 Sf. Petru Movila Str., 700089, Iasi, RomaniaAlexandru Ioan Cuza University of Iasi, ARHEOINVEST Interdisciplinary Platform, 22 Carol I Blvd, 700506, Iasi, Romania

The catalytic activity and product distribution over Zn-Cu-HZM-5, Zn-HZSM-5 (acetate) and HZSM-5 catalystsduring the C4 – C4

= conversion, have been studied. The experimental results show that the conversion ofhydrocarbons and the selectivity to aromatics BTX on the Zn-Cu-HZSM-5 catalyst are higher than that on Zn-HZSM-5 (acetate) and on HZSM-5. On HZSM-5, protons (Bronsted acid sites), on Zn-HZSM-5 (acetate), andon Zn-Cu-HZSM-5, both Zn2+and Zn2+/ Cu2+ cations and protons intervene in alkanes dehydrogenation andfurther in dehydrocyclooligomerization reactions that form C6-C8 aromatics; the aromatic products (principallytoluene, xylenes and benzene) concentration of liquid products varied with the time on stream of theprocess. Product distributions at 450oC indicated that Zn-Cu-HZSM-5 and Zn-HZSM-5(acetate) exhibitedhigher aromatization activities compared with oligoaromatization activity of HZSM-5. Meanwhile the timeon stream increases from 24h on HZSM-5 to over 70h on bifunctional catalysts and the resulted gasescontain hydrogen.

Keywords: aromatization, Zn-HZSM-5 (acetate), Zn-Cu-HZSM-5, C4 – C4= technical fraction.

Among the known aluminosilicate zeolite networks thatthe MFI (ZSM-5) type has the greatest interest due to itsunique two-dimensional 10 ring pore structure relativelyslow deactivation by cocking and to appreciable thermalstability [1]. Much attention has been drawn to thetransformations of lower alkanes into aromatichydrocarbons from both industrial and academic points ofview. The aromatic hydrocarbons can be utilized as abooster for high octane number gasoline and arefundamental raw for chemicals into petroleum industry

The aromatization of light alkanes contained in non-associated natural gas, in associated gas (as petroleumcasing-head gas) and from petroleum refining processes(as liquefied petroleum gas, LPG) represent a newattractive way of producing BTX aromatics. Already a fewcommercial processes have been announced based on:HZSM-5 (M2 Forming Process – Mobil Oil [2, 3] and M-Forming Process – Mobil) [4]; Ga/HZSM-5 (Cyclar-BP/UOP[5, 6] and Z-Forming from Mitsubishi Oil and Chiyoda [7];Zn/HZSM-5 (Alpha process of Asahi Chemical andPetrochemical) [8]; Pt/K(Ba)L (AromaxTM process –Chevron – Phillips Chemical Co. [9]; RZ- Platformingprocess – UOP [10, 11]; Aroforming from IFP; Salutecbased on metal oxides-HZSM-5 [12, 13].

Csicsery has described the dehydrocyclodimerizationof lower alkanes over bifunctional catalysts such asplatinum on alumina and Cr2O3 on alumina [14].

Activity is mostly determined by the zeolite Bronstedacid sites and by the active metal-phase supported by zeoliteand selectivity is due to the zeolite microspores and/orcavities size and shape. The structure of ZSM-5 zeolite ischaracterized by two dimensional types of intersecting

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channels (2-D pore system) with 10-member ring (MR)openings: one type is sinusoidal (zigzag) with near-circularopenings (0.53 x 0.56 nm) and the other one is straightwith elliptical openings (0.51x0.55 nm) [15]. Due to theshape-selective properties of the ZSM-5 framework(determined by product shape selectivity or/and transitionstate selectivity), mainly small aromatics (BTX) are formedand activation on account of coke deposition is relativelyslow [16].

Many studies have focused on the ability of themonofunctional acid catalyst (HZSM-5) to convert lighthydrocarbons to BTX [17-32]. HZSM-5 catalysts are notthe best dehydrogenating catalysts because the hydrogenrejection from catalyst occurs by hydrogen transfer toolefins which limits the aromatics selectivity. Aromatizationover HZSM-5 is accompanied by substantial cracking of C–C bond of alkanes with a production of 3 moles of smallalkanes per one mole of aromatics.

Modification of the proton forms of ZSM- (MFI) zeolitesby metals such as Ga, or Zn, increased the selectivitytowards aromatic hydrocarbons. Ga-HZSM-5 [17-39] andZn-HZSM-5 [17-23, 26, 34-57] zeolite catalysts have beenstudied extensively for the conversion of lights alkanes intoaromatic hydrocarbons. The Zn-HZSM-5, have on essentialinstability because of easily evaporation of zinc at around500-520oC. Introduction of Cu led to increase thehydrogenation /dehydrogenation activity at hightemperature [57].

In connection with this work, in present papers we reportthe catalytic activity and selectivity of the Zn-Cu-HZSM-5,Zn-HZSM-5 (acetate) in comparison with HZSM-5 in theconversion of technical fraction C4

= - C4.

REV.CHIM.(Bucharest)♦ 67♦ No. 4 ♦ 2016 http://www.revistadechimie.ro 735

Experimental partNaZSM-5 zeolite synthesis

NaZSM-5 zeolite (Si/Al = 36.02; 2.65 wt.% Na2O), wassynthesized in our laboratory by hydrothermalcrystallization from alkaline media containing sodiumsilicate (29.63 wt.% SiO2, 9.55 wt.% Na2O, 60.82 wt.% H2O,pycnometric density 1.443 kg·dm-3), aluminum sulphate,Al2(SO4)3·18H2O (15 wt.% Al2O3), sulphuric acid (96 wt.%,1.835 kg·dm-3), ethylene glycol (1.1132 kg·dm-3) as a gelmodifier and as a void filler, deionizer water and ammoniumhydroxide (25 wt.% NH3) to control the pH of the gel (11.0– 11.5). All chemicals are Romanian technical productsand were used as received [58]. The gel was allowed tocrystallize in stainless steel autoclave at180 ± 5oC for 24hwith stirring. The product was than filtered, washed, anddried at 110oC for 6h and calcined at 550oC in air for 6h. Thepurity and crystallinity of Na-ZSM5 was checked by X- raydiffraction.

Catalysts preparationThe HZSM-5 form was obtained by triple ion – exchange

with 1M NH4NO3 at 80ºC for 6h and by calcinations in air at550ºC for 6h. Zeolite HZSM-5 was converted to Zn-HZSM-5(acetate) by process of ion exchanges two times with 0.1MZn(CH3COO)2 aqueous solution (solid : solution = 1g : 5mL) under stirring at 80oC for 6h each time. The Zn-HZSM-5 (acetate) sample was filtered, washed, dried at 110oCfor 6h and calcined at 450oC in air for 6h. The zinc containedin the sample was 0.77 wt% at ZnO. The Zn-Cu-HZM-5zeolite (1.73 ZnO wt% and 0.64 wt % CuO) was preparedby ion – exchange of the HZSM-5 two times with 0.1MZn(NO3)2 aqueous solution. The Zn-HZSM-5 sample waswashed, dried at 110o C for 6h and calcined in air at 450oCfor 6h after Zn-HZSM-5 (1.73 ZnO wt%) sample was ion-exchange using 0.1 M Cu(NO3)2 solutions under stirring at80oC for 6h. The product was filtered, washed, dried at110oC for 6h and calcined in air at 550oC for 6h.

The HZSM-5, Zn-Cu-HZSM-5 and Zn-HZSM-5 (acetate)powders with 20 wt%, γ - Al2O3 as binder was extrudedand then cut into short cylinders, dried at 110oC for 6h andcalcined at 475oC in air for 6 h.

CharacterizationThe structure type, phase purity and degree of

crystallinity were determined by X-ray powder diffractionpattern which were obtained on a Philips PW 1830diffractometer using Ni filtered Cu Kα radiation at ascanning speed of 0.02o·s-1 in the range of 6–45 o, 2θ. XRDpowder pattern of the Na-ZSM-5 sample exhibit onlydiffraction lines proper to MFI structure high crystallinity(fig. 1). The pattern confirms that the synthesized zeolitehas the structure identical to MFI-type zeolite [15]. Themorphology and size of the individual crystals wereobtained by scanning electron microscopy (SEM) with aMicrospec WDX-2A using a 25 kV accelerating potential.

The SEM image of parent NaZSM-5 is presented in figure 2.It reveals the well-defined morphology of crystals indicatinghighly crystalline material.

The acidity and strength distribution on HZSM-5, Zn-HZSM-5 (acetate) and Zn-Cu-HZSM-5 catalysts weremeasured using Temperature Programmed Desorption(TPD) technique using ammonia. A known weight of thesample was activated in a dry N2 at 500oC for 4h then cooledto 80oC when ammonia was admitted. The amount ofammonia desorbed from 100 to 800oC (at a heating rate of10oC/min) was quantitatively monitored by absorption in1M HCl. The ammonia desorbed represents the total acidity(weak and strong) of the sample. The TPD ammoniadesorption presents two peaks, one at low temperature(LT) and one at high temperature (HT)(table 1).Temperature peak correspond to higher acid strength andis done to ammonia bound to strong structural Brönstedsites (Si – O – Al bridging OH), and possible to strong Lewissites (≡ Al and ≡Si+). Low temperature peak correspondto less acidic sites (terminal OH groups, cationic sites Mn+,AlO+). The temperature and the amount of desorbedammonia give information about strength and number ofthe acid sites.

The BET specific surface area applying the BET equationwas determined using a Carlo – Erba Sorptomatic Series1800 instrument at -196oC and at sub-atmospheric pressurewith nitrogen as the analysis gas.

The values of the BET specific surface area and acidityof the HZSM-5 Zn – Cu – HZSM-5 and Zn-HZSM-5 catalystsare presented in table 1.

Fig. 1. X-ray diffraction pattern of NaZSM-5 zeolite

Fig. 2. SEM image of parent NaZSM-5 zeolite

Table 1PHYSICO-CHEMICAL

CHARACTERISTICS OF THE STUDIEDCATALYSTS


The incorporation of the Zn2+ and Cu2+ ions into cationicpositions affects the acidity of the HZSM-5 zeolite,decreasing the number of Brönsted strong acid sites andincreasing the Lewis acid sites.

Catalytic performanceThe catalytic activity of HZSM-5 and Zn-Cu-HZSM-5 for

C4 – C4= technical fraction conversion at 450oC and 8 atm.

(HZSM-5), under atmospheric pressure (Zn-Cu-HZSM-5),and at 4 atm. (over Zn-HZSM-5 (acetate) with WHSV 1h-1,to aromatics BTX, in a fixed-bed continuous flow stainless– steel reactor (a commercial Twin Reactor System NakyMetrimpex, Hungary) was studied. The catalysts were pre-treated with N2 for 6 h at 450oC to remove the adsorbedimpurities and the moisture.

The reaction products were separated into liquid andgas fractions through an ice – trap. Composition of productswas obtained with two gas chromatographs (GC Carlo Erba,model C and Vega) using a fused silica capillary column(25 m length and 0.32 mm i.d.) with SE-52 stationary phaseand flame ionization detector (FID) for liquid phase and acolumn (6m length) with squalane and dimethylsulpholaneand a thermal conductivity detector (TCD) for gaseousphase, respectively.

Results and discussionsCatalytic performance of HZSM-5 catalyst in C4- C4

=

hydrocarbons aromatizationFive catalytic tests at 450oC, and 8 atm. total pressures,

using the butanes – butenes technical fraction wereperformed on HZSM-5 with regeneration of catalyst aftereach test: 475oC for 6h in nitrogen flow with 2% oxygen.

Conversion of mixed butanes-butenes to aromatics overHZSM-5 takes place with low selectivity to BTX, thereactants forming predominantly the cracking products.The operating conditions (temperature 450oC, WHSV 1 h-1

and pressure 8 atm.) were in advance selected to obtainthe high yield of liquid product during the catalytic tests.

The change of the liquid yield, conversion of butenes inraw materials and BTX concentration in liquid productsover HZSM-5 with time on-stream (TOS) in the conversionof butanes and butenes are presented in figure 3.

aromatics BTX over HZSM-5 monofunctional catalyst donot go beyond 35 wt% in the liquid phase (expectedlycatalytic test number3) and the formation of xylenes andtoluene are of preference (fig. 4).

It is well known that the acidity of the surface of thecatalyst has decisive effect on the activity of the catalyst.The acidity is influenced by the type of the acid sites, suchas the Bronsted (protic acid sites) and Lewis (aprotic acidsites) sites as well as the number and the strength of theacid sites.

Monofunctional HZSM-5 catalyst exhibits preferentiallyhigh cracking, isomerization and β-scission reactivity thatlead to loss of carbon atoms to undesirable products.Hydrogen rejection from surface occurs by hydrogentransfer to alkanes which limits to aromatic yield that canbe obtained on HZSM-5. Formation of aromatichydrocarbons on HZSM-5 catalyst involving Bronsted-acidcenters as active sites. Aromatic molecules were formedfrom alkenes oligomers by successive deprotonation andhydride transfer to carbenium ions. With this mechanism,the formation of one molecule of aromatic hydrocarbonsinevitably accompanies the formation of three moleculesof alkanes.

Catalytic performance of Zn-Cu-HZSM-5 catalyst in C4- C4=

hydrocarbons aromatizationFive catalytic tests at 450oC, using the butanes – butenes

technical fraction were performed on Zn-Cu-HZSM-5 withregeneration of catalyst after each test: 475oC for 6h innitrogen flow with 2% oxygen.

The average yields in liquid product in the five tests wereover 40 wt %. The liquid product contained over 75 wt %aromatic hydrocarbons.

The operating conditions (temperature 450oC, WHSV1 h-1 and atmospheric pressure) were in advance selectedto obtain the high yield of liquid product during the catalytictests.

The dependence of liquid yield, BTX concentration andbutenes conversion on TOS in the on Zn-Cu-HZSM-5 duringC4 – C4

= conversion is presented in figure 5. Butenes areconsumed almost totally during the whole test, while thebutanes are consumed during the first 4 h of reaction, afterwhich their concentration increases continuously. The yieldin liquid product is over 40%wt.

The average output of aromatics BTX over Zn-Cu-HZSM-5 bifunctional catalyst is over 75 wt% in the liquid phase(fig. 6)

In the liquid product resulted on Zn-Cu-HZSM-5 aromatichydrocarbons are predominant together with the “oligo”fraction which contains mostly alkanes and alkenes in theiso- series.

Butenes are consumed almost totally during in the firstly16h of reaction (conversion of C4

= is over 95%), after whichtheir concentration increases continuously.

The molecular hydrogen was not detected in thegaseous phase. The liquid yield not exceeds 27% duringthe whole test

The catalytic activity and selectivity to aromatichydrocarbons decreased continuously after 8 h of reaction(at ~50 wt.% to 20% after 24h). Aliphatic hydrocarbons C5– C10 are progressively formed. The average output of

Fig. 3. Variation of liquid yield, BTX concentration and butenesconversion over HZSM-5 catalyst (Test No. 3)

Fig. 4.The aromatics and C10+ fraction average outputover HZSM-5 catalyst (450oC, 8 atm., WHSV = 1h-1)


The corresponding liquid product contains over 75 wt.%mononuclear aromatic hydrocarbons, distributes in theorder:

toluene > xylenes and ethyl benzene» benzene.

Zinc cooper dehydrogenation function acts in both steps,the dehydrogenation of i-butane (especially), the dienesformation and then dehydrogenation of naphtenes toaromatic hydrocarbons [57].

Catalytic activity and selectivity of Zn-HZSM-5(acetate)during C4-C4

= hydrocarbons aromatizationAromatization of a mixture containing butanes and

butenes over Zn-HZSM-5 (acetate) takes place with goodselectivity to aromatics BTX and with production ofmolecular hydrogen.

The operating conditions (temperature 450oC, WHSV1 h-1 and 4 atm. pressures) were in advance selected toobtain the high yield of liquid product during the catalytictests.

The same catalyst Zn-HZSM-5 (acetate) has been usedin five tests with regeneration after each test: 475oC for 6hin nitrogen flow with 2% oxygen.

The changes of the liquid yield, butenes conversion andaromatics hydrocarbon BTX concentration over Zn-HZSM-5 (acetate) with time on-stream (from four to four hours),are shown in figure 7.

The butenes are consumed in more quantity in firstly40h of reaction, but in small quantity comparative over Zn-Cu-HZSM-5 catalyst. The liquid yield not exceed of 37 wt.%during the catalytic test and aromatic hydrocarbons BTXconcentration is over 60 wt.% in firstly 8 h time of run afterthat is continuously decreased at ~43 wt.% after 96h.

The concentration of butanes (n + i) decreased after 8h of reaction, after that is continuously increasing without

to rise above the initial concentration. The main gaseoushydrocarbon over Zn/HZSM-5 (acetate) is propane (~30vol.%), less than over HZSM-5 (about 60 vol.%) [56].

The average output of aromatics BTX over Zn-HZSM-5(acetate) bifunctional catalyst does not go beyond 60 wt%in the liquid phase (fig. 8)

Aromatic hydrocarbons were mainly toluene (~25 wt%)and xylenes (~25 wt%), benzene being about 5 wt%. Thealiphatic hydrocarbons C5-C10 content is increasing from~20 wt% after 4h of reaction to ~40 wt% after 96h ofreaction and is based on C9 and C10 hydrocarbons. Theformation of aliphatic hydrocarbons with more than 10carbon atoms (> C10) is limited to about ~7wt%. Zn-HZSM-5 (acetate) is less active and selective that Zn-Cu-HZSM-5[56].

Aromatization of a mixture containing butanes andbutenes over Zn-Cu-HZSM-5 and Zn-HZSM-5 (acetate)takes place with high selectivity to aromatics BTX and withproduction of molecular hydrogen.

In contrast to the conversion over HZSM-5, the yields oflower alkanes were very small over metal ion containingHZSM-5, indicating that the incorporation of zinc (andcooper) species does not generate acidic centers capableof catalyzing hydride transfer reactions [56, 57].

Since the dehydrogenation path with metal cations doesnot result in the simultaneous formation of alkanes, thereis essentially no limit to higher yield of aromatics. Thisexplains the higher yields of aromatic hydrocarbons overZn-Cu-HZSM-5, and Zn-HZSM-5 (acetate) catalysts.Alkenes formed by cracking of alkanes can be effectivelytransformed to aromatic hydrocarbons by the dehydro-genation activity of these cations.

Fig. 6.The aromatics and C10+ fraction average outputover Zn-Cu-HZSM-5 catalyst (450oC, atmospheric pressure and

WHSV = 1h-1)

Fig. 5. Variation of liquid yield, BTX concentration andbutenes conversion, vs. time on stream over Zn-Cu-HZSM-5

catalyst (Test No.3)

Fig. 7. Variation of liquid yield, BTX concentration andbutenes conversion, vs time on stream over Zn-HZSM-5 (acetate)

catalyst (Test No.3)

Fig. 8. The aromatics and C10+ fraction average outputover Zn-HZSM-5 (acetate) catalyst (450oC, 4 atm. pressure and

WHSV = 1h-1)


The difference in the product distribution between HZSM-5 and Zn-Cu-HZSM-5 and Zn-HZSM-5 (acetate) catalysts isbrought about by the differences in the acid strength ofthese catalysts. The zinc (and cooper) cations werereduced by hydrogen which was produced during thedehydrogenation of hydrocarbons with resulting in theformation of acidic OH groups [52, 53, 55-60].

For Zn-HZSM-5, the formation of Brönsted acid sites wasconfirmed by the presence of a band at 1548 cm-1

(pyridinium ion) a band at 1454 cm-1, was observed inFT-IR spectra of pyridine adsorbed on HZMS-5 and Zn-HZSM-5, which was plausibly due to pyridine moleculeinteracting with Zn cations [60].

Butanes-butylenes mixtures conversion over HZSM-5,Zn-Cu-HZSM-5 and Zn-HZSM-5 (acetate) catalysts occurs

via a complex sequence of cracking, dehydrogenation,oligomerization, isomerization, cyclization, β-scission andH transfer (scheme 1) [56].

Zn-Cu-HZSM-5 catalyst contains medium acidity thatminimizes the occurrences of cracking reactions. Zn andCu incorporated in HZSM-5 zeolite through ion exchange isvery well dispersed and is stable in isolated cationic (Zn2+,Cu2+) positions with tetrahedral symmetry. In the case ofZn-Cu-HZSM-5 and Zn-HZSM-5 (acetate) catalysts thealkanes dehydrocyclodimerization proceed via bifunctionalpathways involving exchanged cations for dehydro-genation of alkanes and dehydrocyclization of alkenicoligomers and acidic OH groups for alkenes inter-conversion and aromatic formation. Zn2+ (and Cu2+) cationsas Lewis acid sites promote the alkanes dehydrogenation(heterolytic cleavage of the C – H bond) to alkenes(dehydrogenation function), oligomers dehydrogenation tooligomers with one double bond, decrease b-scission rates,exert strong hydrogen attracting action and promote

Scheme 1

Table 2THE MON AND LIQUID PRODUCTS

DENSITY RESULTS ON HZSM.5,Zn-HZSM-5 AND Zn-Cu-HZSM-5

CATALYSTS HZSM-5


removal of hydrogen atoms adsorbed as hydrogenmolecular, prevents hydrogenation of alkenes required incyclization and consequently, enhances aromatization.

Alkanes are activated by a monomolecular or abimolecular mechanism depending on the reactionconditions (temperature, surface concentration ofreactants and conversion) [59, 60].

The zinc and Zn/Cu species are efficient in the increaseof aromatization selectivity since they activate the C-Hbonds and promote the dehydrogenation as well as themigration of hydrogen atom by hydrogen transfer.

At the same time alkenes C4= react much faster than

the alkanes C4 in presence of protonic sites of zeoliteyielding the carbenium ions which may add to anothermolecule of alkenes to yield a new carbenium ion.

Consequently, it is clear that there exist two types ofactivation of reactant alkanes molecules over Zn-Cu-HZSM-5 and Zn-HZSM-5 (acetate) catalysts, dehydrogenation bythe action of Zn or Zn/Cu cations and cracking on Brönstedacid sites. Thus, relative contribution of the two types ofactivation seems to depend on both the nature of reactingalkanes and the primary reactions over acid sites.

The average output of the MON (Motor Octane Number)and liquid products density of liquid fractions results onHZSM-5, Zn-Cu-HZSM-5 and Zn-HZSM-5 (acetate) catalystsin conversion of C4/C4

= technical fraction is presented intable 2.

The liquid fraction results on the on HZSM-5, Zn-Cu-HZSM-5 and Zn-HZSM-5 (acetate) catalysts can be used as ablending mixture for the octane number enhancing ofgasoline because the MON is over 90, or as raw materialsfor production of petrochemicals and chemicalsintermediates.

ConclusionsMonofunctional acid catalyst HZSM-5 exhibits a low

selectivity to aromatics BTX in the catalytic aromatizationof butanes-butenes mixture, due to preferentially cracking,isomerization, and â-scission reactivity. The averageoutputs of aromatics BTX do not go beyond 30 wt% in theliquid phase and the formation of xylenes and toluene is ofpreference.

Bifunctional zeolite Zn-Cu-HZSM-5 is very active andselective catalyst in the aromatization of C4 – C4

= technicalfraction. The incorporation of Zn (II) and Cu (II) into cationicpositions promotes a decrease of the number of Brönstedacid sites and an increase of Lewis acid sites that are ableto abstract hydride from the adsorbed hydrocarbonsmolecules and catalyze the formation of hydrogen gas.Zn/Cu species is active in dehydrogenation /hydrogenationreactions responsible for formation of hydrogen gas andaromatics BTX.

There is a general acceptance that the major role of themetal in the catalyst is to accelerate the combination ofsurface hydrogen, which formed from the dehydrogenationand dehydrocyclization processes involved as key steps inthe conversion of light hydrocarbons to aromatics viaalkenic intermediates. The Zn/Cu may have a more markedtendency to accelerate the combination of surfacehydrogen that on Zn-HZSM-5 (acetate) catalyst, so thearomatics selectivity of the Zn-Cu-HZSM-5 is higher thanthat of Zn-HZSM-5 (acetate) and HZSM-5 catalysts.

The HZSM-5 catalyst deactivates fast and Zn-Cu-HZSM-5 and Zn-HZSM-5 (acetate) catalysts are able to sustainactivity and selectivity for a longer period.

The presence of butenes in the butanes feed exercisethe activation of butanes: it is thought that butenes are

protonate to carbenium ions from a Bronsted acid site andthen activate butanes through hydride abstraction.

As described above, the bifunctional nature of thecatalysts is important for aromatization of alkenes andalkanes. The acidic sites are responsible for oligomerizationof olefins and the metal cations are responsible fordehydrogenation of alkenic intermediates or oligomerizedproducts. Acidic sites are also responsible for cracking ofoligomers and hydrogen transfer reactions.

The catalytic aromatization reactions over Zn-Cu-HZSM-5 and Zn-HZSM-5 (acetate) catalysts can upgrade the low-value light hydrocarbon byproduct streams from refineryand cracker operations, producing aromatics BTX andhydrogen as co-product.

Small selectivity for BTX aromatics of Zn-HZSM-5(acetate) comparative with Zn-Cu-HZSM-5 catalyst inconversion of butanes-butenes technical mixtures is veryprobably because as of the organic macromolecularspecies that results after calcinations of Zn-HZSM-5(acetate) fresh catalyst (and/or Zn content). Thismacromolecular species blocked (partially) access of thereactants molecules on the active catalytic centers(Brönsted and Lewis acids centers and Zn centers).

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Comparative Study Beetween Zn – Cu- HZSM-5 and Zn-HZSM-5 ...

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