Effects of P content in a P/HZSM-5 catalyst on the conversion of ethanol to hydrocarbons

Availableonlineatwww.sciencedirect.comJoumal ofScience DirectNatural GasChemistryELSEVIERJournal of Natural Gas Chemistry 20(2011)162-166Effects of P content in a p/hzsm-5 catalyst on theconversion of ethanol to hydrocarbonsLirsity, Urumgi 830046, Xinjiang, ChineAbstractA series of P/HZSM-5 catalysts prepared by impregnation method were used for ethanol conversion to lower olefins. The catalysts werecharacterized by X-ray diffraction(XRD), NH3-temperature-programmed desorption(NH3-TPD)and N2 adsorption-desorption measurementsIt was found that the P/HZSM-5 catalysts showed high activity and selectivity toward light olefins. The selectivities of propylene and butylenecan be improved with the introduction of phosphorus P). When the content of reached 3.0 wt%, more thanropylene in the gaseousproducts was obtained over the P/HZSM-5 catalyst 4t 450.Ctroductid of P modified the strong Bronsted acid sites of the originalHZSM-5 catalysts and P/HZSM-5 catalysts could resist coke forgood stabilityKey wordsethanol; P/HZSM-5; hydrocarbons; modific1. Introductionofifork al atoftment [8]. Several studies reported that P-modifiedLight olefins, such as ethylene, propylene and bulylene HZSM-5 catalysts were active for ethanol conversion to ethyare important raw materials in the petrochemical industrylene 19-11]. Tynjala et al. [9] found that ethylene was theTraditional crude oil (mainly naphtha)cracking can not meet final product over P-modified HZSM-5 catalysts containingthe great demand due to oil supply crisis. It has driven much 2.2 wt% and 2.7 wt% P, and no higher hydrocarbons wereeffort to seek alternative and renewable sources. Ethanol observed at high reaction temperature(643 K). Zhang et alis particularly attractive since it is a renewable agricultural supported in their work that no higher hydrocarbons werresource and it can be obtained easily by fermentation of formed during ethanol dehydration, but not all the P-modifiddnAHZSM-5 catalysts favored dehydration of ethanol to ethylIt has been reported that as solid acid catalysts, HZSM-5 at high reaction temperature [10]. Ramesh et al. [11] investizeolites are widely used for conversion of ethanol to light gated the influence of H PO4 loading on the catalytic perforolefins [1-7. These studies revealed that the distribution mance of modified HZSM-5 catalysts, and found that higherof various products depends on the concentration of Bronsted hydrocarbons were produced to a significant extent duringacid sites on the catalyst. The acidity of HzsM-5 with lowerethanol conversion However no further studies were carriedSiO2/Al2O3 ratio suppressed the formation of C2, C3 and C4, out on ethanol conversion to C2-C4 olefins over P/HZSM-5but favored more C6+ aromatics as well as Cl-5 paraffins. catalysts. In this paper, we investigated the effects of Pcon-The selectivities of propylene and ethylene were improved tent on the selectivity of light olefins derived from ethanolprominently over metal-exchanged HZSM-5over P/HZSM-5 catalysts prepared by impregnation methodhe introduction of phosphorus on H-MFI zeoliFurthermore, the correlation between the production of C2-4stabilize the mfi zeolite structure by preventing theolefins and the acidity of the catalysts was also studiedCorrespondingauthor.Tel:+86-991-8581012-808:E-mail:yinglu@xju.edu.cnFor This project was supported by the Doctor Fund of Science Research of Xinjiang University(Gr中国煤化工 lal Natural Scienceundation of China( Grant No. 20963010)CNMHGCopyright(2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reservedoi:10.1016/S1003-9953(1060163-6Journal of Natural Gas Chemistry VoL. 20 No. 2 201I2. Experimentalthen cooled down to room temperature. The adsorption ofNH3 was performed at room temperature for 30 min. PhysiCatalyst preparationcally adsorbed NH3 was removed at 120C. NH3-TPD curveswere recorded under helium flow with heating the samplesCommercial HZSM-5 zeolite (ratio of Sio2/Al2 O3=25from 120C to 900C at the rate of 10C/minNankai University, China) was used. (NH4)3 PO4 modifiedHZSM-5 catalysts were prepared by impregnation method: 2.3. Catalyst activity testthe HZSM-5 zeolite was impregnated in the aqueous solutioncontaining(NH4)3 PO4. The amounts of P were 0, 1.0 wt%Ethanol conversion was performed in a continuous flow2.0 wt%, 3.0 wt%, 4.0 wt% and 5.0 wt%, respectively. Ater fixed bed quartz tubular reactor with an inner diameter ofImpregnation, the wet catalysts were dried at 120'C for 4 h, 6 mm. About 0.3 g catalyst was put into the reactor. Previ-followed by a calcination at 550C in air flow for 5 h. The ous activation in situ was done in nitrogen flow(40 mL/min)P-modified samples were marked as HZ, IPZ, 2PZ, 3PZ, 4Pz for 30 min at 470 C with a heating rate of 2C/min. Pureand 5PZ based on the corresponding P concentrations. The ethanol (>99.7%), preheated to 120C by heating taps beforecoked catalyst was marked as UZ. Finally, the calcined cat- entering the reactor, was charged by a metering pump(Agialyst was pelletized, crushed and sieved, and the grains with lent P200lI series) into a nitrogen stream. The total pressure0.5-1.0 mm size were takenwas kept at O I MPa. The catalyst bed was heated electricallyA thermocouple, which reached the catalyst bed, was used2.2. Catalyst characterizationto measure the temperature during the reaction. The gaseousproducts were analyzed on-line using a gas chromatographThe phase identification of the original and modified cat-(Agilent 3420)equipped with a PEG-20M capillary columnalysts were carried out by an X-ray diffractometer( Mac Sci- and FID detector; and the atomic products were analyzed usence Ml8XHF22-SRA) withradiation operated at ing a HPI capillary column35 kV and 25 mA. XRD patterns were collected over 20 val-ues in the range of3°-70° at the rate of2%min3. Results and discussionAfter alkali fusion, the Si content of the samples was measured by gravimetric method, and then the contents of P andAl were tested with phospho-molybdenum blue method and 3. CharacterizatioEDTA volume method, respectivelyThe nitrogen adsorption-desorption measurements wereNitrogen adsorption results, the real P content of the prearried out at -196C in a MicromeritiCs ASAP 2010pared samples and the relative crystallinity are listed in Tastrument(Micromeritics Instrument Corporation, USA). The ble 1. It can be noted that the introduction of P caused a clearsamples were pretreated under a stable vacuum of 0.67 Padecrease in the BEt surface area, microporous volume and200° C for 1I.crystallinity. The crystallinity of the used catalyst was notNH3 temperature-programmed desorption (NH3-TPD) changed prominently, but there is a distinct decrease in surfaceexperiments were performed in an Autosorb-TP-5080 appa- area. The results indicate that the deactivation of catalysts canratus(Xianquan, China). In the NH3-TPD experiment, about be attributed to the coke deposition on the surface of catalysts,100 mg catalyst was placed in the quartz tube. First, the sam- which causes the covering of active centers or the blocking ofples were pretreated at 400C in helium flow for 0.5 h, and zeolite pore channelsTable 1. Physical properties of parent HZSM and P modified HZSM-5Sample Ratio of Si/Al2 P content(wt%) Ratio of P/Al XRD(%)BET surface area(m-g) Micro pore volume(cm/g)1002,1692,23.810.95213.121.04.861.3172.6191.6ePZ1.67UZ0.93The relative crystallinity; The coked 3PZ catalystFigure 1 shows XRD patterns of original HZSM-5 and the increase of PP-modified HZSM-5 catalysts. All samples exhibited typi- dealumination ofrVL中国煤化omcal HZSM-5 zeolite structure. After introduction of P, the can replace frameCN MH Gtra amorphousintensity of the XRd patterns was weakened gradually with framework aluminum phosphates [12-14. This effect is even164Jiangyin Lu et al./ Journal of Natural Gas Chemistry Vol, 20 No. 2 2011more evident in the 5PZ catalyst. Higher content of P on 4PZ). While for the case of 5Pz catalyst, the NH3 desorptionHZSM-5 support may block the pore in the modified cata- peak disappeared completely in the strong acid sites. Thislysts and influence the H-MFI Zeolite structure [15]. After may be attributed to the dealuminzation of tetrahedral frame10 h reaction the intensity of XRd pattern is scarcely changed work aluminum (TFAL) in the HZsM-5 zeolite framework,(Figure 2)which primarily contributes to the strong acidity of the catalysts [11, 17]. The deactivation of the catalysts seems to berelated to the type I acid sites, which has already proposed bysome authors 83PZIPZ4PZ入Figure 1. XRD patterns of HZSM-5 and P-modified HZSM-5 catalysts200300Figure 3. NH3-TPD profiles of parent and P-modified HZSM-5 catalysts3. 2. Effects of P content on catalytic performance of theHZSM-5 supportCoked catalystConsidering the amount of the Bronsted acid sites of thecatalysts, one should investigate the influence of the relativeactivities over P/HZSM-5 catalyst for ethanol conversion tolight olefins. Indeed, the strong Bronsted acidity plays an im-portant role in the selectivity of lower olefins. The conversionFresh catalystof ethanol and the selectivity of each product are shown inTable 2. In most cases. ethanol conversion was almost 100%obut the product selectivity of each catalyst was different. It canbe clearly seen that when P species was added into the origiFigure 2. XRD patterns of different P/HZSM-5 catalyst samplenal HZSM-5, the catalytic activity was increased significantlyHZSM-5 support favored the formation of aromatics and C1-4NH3-TPD profiles of zeolite support and P-modified cat- paraffins but suppressed the formation of C3-4 olefins. Thealysts in the temperature range of 50-750C are shown in selectivity of paraffin was decreased gradually with the in-Figure 3. Two peaks of HZsM-5 zeolite can be clearly seen crease of P content. As P content was increasing, the selecat 225C(type I acid sites)and 450C(type II acid sites) tivity of propylene increased first, passing through a maximalder experimental condition, suggesting that there are two value over 3 wt% P/HZSM-5 catalyst(3PA), then decreasedtypes of distinct acidic strength in the catalyst. They are as-ylene incrsigned to weak acid sites and strong acid sites, respectively. nificantly when the content of p was over 3 wt%. When theThe NH3-TPD experiments showed that the peaks of the weak content of P was above 4.0 wt %, the amount of ethylene wasacid shifted to lower temperature with P contents increasing, more than 95%. Further increase of the P loading brought aindicating the strength decrease of those acid sites. And the slight increase in the selectivity of ethylene. Of all the catpeak areas of the strong acid sites decrease slowly, indicating alysts, the 3PZ catalyst presents an excellent performance inthat the strong acid sites were replaced by new acidic cen- propylene selecti中国煤化工 lyst is more seters caused by the introduction of P [16]. With P content lective for C3+CNMHincreasing, the decrease in acidity looked more pronouncedwiul nguIu uviling point, par-ly one peak was observ240 the weak acid 3 Ligure 3, tic 0y aromatics, can 5 @o a relatively fa6@ctivation of 70Journal of Natural Gas Chemistry VoL. 20 No. 2 201Ithe catalyst. The higher catalytic activity of HZSM-5 towards of strong acid sites varied with the reaction temperature, re-aromatics may be attributed to the higher surface acidity as sulting in differnet products and product distributionOconpared with that of P-modified HZSM-5. The polymer-It can be summarized that ethylene is the main product ofzation reaction of ethylene to higher hydrocarbons(C5+ and the gaseous fraction during ethanol conversion to light olefinsaromatics) is difficult to take place on P/HZSM-5 catalysts. on P modified HZSM-5 catalysts below 300C. As the reA9+含 OKe fomation once teamipens or wa s nd rcasearardm lv increased and theyield of C2 dramatically decreased. This indicates that higherTable 2. InfluenP会temperature favors the formation of C3 species. As the temperature was further increased, Ca was further polymerized tohigher olefins, some of which were cracked into C3 species98.50.427.114.213.617827.3while the others cyclized and aromatized [20]30.816.512.216.3The composition and distribution of liquid hydrocarbonsre presented in Figure 4. The main components of liquidproducts are aromatics, mostly toluene and xylene. The fraction of xylene in liquid hydrocarbons is far more pronouncedReaction condition: SiO2/Al2O3=25(molar ratio), 450C,0.3 gover the P/HZSM-5 catalyst than that of HZSM-5catalyst, pure ethanol, WHSV=3. 16h 0.1 MPa3.3. Catalytic activity test with temperatureLZZ HZSM-50.3区3PZAs mentioned above. ethanolipletelyto hydrocarbons over P-modified HZSM-5 catalysts undersuch experimental conditions. When the P content exceeded3 wt%o. thetivity for each product changed with differentreaction temperature. Therefore, the performance of the catE0.2alysts as a function of temperature is investigated. Table3 3shows the catalytic performance of 3Pz catalyst as a functionof temperature. At 270C, the main product was ethylenee reaction temperature in001Ecreasing, the yield of ethylene dropped markedly to 77.7% at300C, passing through a minimum of 22. 6% at 420C, thenincreased to 3 1.3% at 470C. However, the yield of propy-lene is only 0.4% at 270C, increased slightly with the rise of Figs catalysts4. Organic fraction distribution after 4 h of reaction over HZSM-5temperature, reaching 18.9% at 450C. Similarly, the seletivity of butylenes increased first, passing through a maximalvalue of 15.9%o at 390C, and then slowly dropped to 10.9% at470C. Why a wide range of hydrocarbons rather than ethy- 4. Conclusionslene are produced over P/HZsM-5 catalyst at high tempera-Otute? The reason may be as follows. NH3-TPD results showIn the reaction of ethanol conversion to hydrocarbonsthat the original HZSM-5 catalyst has the highest number of (NH4)3PO4 modified HZSM-5 catalysts are in favor of protrong acid sites whicha key role in the transforma-ducing large amounts of light olefins. The optimum phospho-tion of ethanol into hydrocarbons [11, 17-19]. The amountrus content is close to 3.0 wt %o. The introduction of P reducesTable 3. EfTect of temperature on product distribution in thethe percentage of strong Bronsted acid sites in the catalystsethanol conversion to hydrocarbons over 3Pz catalysthe selectivity of lower olefins is also closeTemperaturelectivity(%)C1-C4 C6-Cg aroma树lount of Bronsted acid sites and the reactperaturephosphorus introduction has a great impthe stabil.3.6f hzsM-5 zeol11amework al atom:阴巧talysts she Nigh9994.39.69,211.8for the conversion of ethanol to light olefins and strong resis48.510.49.612.1tance to coke formation28917.515914.61505b/A八Acknowledgement中国煤化工This project waCNMHGd of Science Re-Reaction csearch of XinjiangtOran INO. DsvovlOl) and the Na-ol.0.1tional Natural Science Foundation of China( Grant No. 20963010)166Jiangyin Lu et al./ Journal of Natural Gas Chemistry Vol, 20 No. 2 2011The authors thank the Physics and Chemistry Detect Center of theCosta A F, Cerqueira H s Appl Catal A, 2006, 314: 160Xinjiang University for XRD analyses and BET experiments[9] Tynjala P, Pakkanen T T, Mustamaki S J Phys Chem B, 1998102(27):5280References[10] Zhang D S, Wang R J, Yang XX. Catal Lett, 2008, 124(3-4)[1] Inaba M, Murata K, Saito M, Takahara L. Green Chem, 2007. [11] Ramesh K, Hui L M, Han Y F, Borgna A. Catal Commun, 2009,10(5):5679(6):638[12] Lischke G, Eckelt R, Jerschkewitz H G, Parlitz B, Schreier E,[2 Machado NR C F, Calsavara V, Astrath NGC, Matsuda U K,Storek W, Zibrowius B, Ohlmann G. J Catal, 1991, 132(1): 229Paesano A, Baesso M L. 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