Effects of ammonium exchange and Si/Al ratio on the conversion of methanol to propylene over a novel

Available online at www.sciencedirect.comJOUPINL.FScienceDirectNATURAL GASCHEMISTRYEL SEVIERJournal of Natural Gas Chemistry 20(201 1)261-265www.elsevier.comlocate/jngcEffects of ammonium exchange and Si/Al ratio on the conversion ofmethanol to propylene over a novel and large partical size ZSM-5Ruchao Wei, Chunyi Li*，Chaohe Yang，Honghong ShanState Key Laboratory of Heary Oil Processing, China University of Petroleumn, Qingdao 265555 Shandong, Chinal Manuseript reeved November 25, 2010; revised April 13, 2011 ]AbstractOne type of ZSM-5 zeolite with large partical size was prepared and characterized by XRD, SEM, N2 adsorption-desorption, XRF, Py-IRand NH3-TPD techniques. Effects of ammonium exchange and SiO2/AI2O3 molar ratios on the reaction of methanol to propylene (MTP)over Na-ZSM-5 and H-ZSM-5 zeolies have been studied in a fixed-bed flow reactor under the operating conditions ofT = 500°C,P= 1 atm,and WHSV =6h-l. Ammonium exchange led to a rapid decrease in Na content for Na-ZSM-5 zeolite. The reaction results indicated thatNa-ZSM-5 and H-ZSM-5 with diferent SiO2/Al2O3 molar ratios all exhibited high activity for methanol conversion. Ammonium exchangeand the decreased SiO2/Al2O3 molar ratio of ZSM-5 zeolite led to an increase both in strong acid sites and weak acid sites. Na ZSM-5 withhigh SiO2/Al2O3 molar ratio was favorable for the formaion of propylene. The highest propylene selectivity (45.9%) was obtained overNa-ZSM-5 zeolite catalyst with SiO2/Al2O3 molar ratio of 220.Key wordsZSM- 5; MTP; propylene; ammonium exchange; acidity1. Introductionpaper, a novel Na-ZSM-5 zeolite was prepared and effects ofammonium exchange and SiO2/Al2O3 molar ratio on the MTPPropylene is one of the most important intermediates inreaction are investigated.petrochemical industry. Nowadays, propylene is producedas a by-product in the steam cracking process for production2. Experimentalof ethylene, as well as in the FCC units. Due to the grow-2.1. Catalyst synthesising demand for propylene and the shortage of oil resourcein the future, new processes with high yield of propyleneare required. Methanol-to-olefins (MTO) and Methanol-to-Na-ZSM-5 catalysts with different SiO2/Al2O3 molar ra-propylene (MTP) processes are the promising alternative waystios were synthesized by hydrothermal crystallization method.for the production of propylene instead of petroleum route,N-butylamine (C4H1N) was used as the template and thesince methanol can be indutially produced from natural gassilica gel microspheres were used as the silicon source.and coal in large scale [1-8].The alkali source was NaOH and the aluminum source wasZSM-5 is a kind of high-silica zeolite discovered by Mo-NaAlO2. The molar composition of the synthesis mix-bil Oil Corporation in 1972. and this zeolite has been usedture with SiO2/Al2O3 molar ratio of 120 was 0.04Na2O :to prepare some unique catalysts for a variety of processes,SiO2 :xAl2O3 :0.11C4HjN :5.72H2O, where x =0.00833.such as conversions of methanol to olefins and gasoline, in- The SiO2/Al2O3 molar ratios were adjusted by changing thecreasing yield of propylene in FCC process, etc. Physicalratio of added raw materials. After being stired at 25 °Cand chemical properties of ZSM-5 zeolite (e.g. channel struc-for 2 h, the gel was transferred into an autoclave and crystal-ture, total acidity and crystal size) and operating conditions lized at 180°C for 6 h and 160。C for 36 h, respectively. Theaffect the selectity to propylene. Many works have beensynthesized products were filtered, washed, dried at 120°Cdone on the activity and selectivity of using H-ZSM-5 as theovermight and then calcined at 550°C for 5 h to remove thecatalyst in MTO&MTP processes [9- 18]. As far as we know,templa中国煤化工mples were labeled asNa-ZSM-5 zeolite with high activity and high propylene se-NZ-602:O3 molar ratio of thelectivity for the MTP reaction has not been reported. In thisstartinqY片C N M H Gily. Na-ZSM-5 zeolite* Corresponding author. Tel: 0532-86981862; Fax: 0532-86981787; E-mail: chyli@upc.edu.cnCopyrightO201 1. Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reseved.doi:10. 101/003-99310)60198-13262Ruchao Wei e al/Jourmal of Natural Gas Chemistry Vol. 20No. 32011was turned into the H-form by three consecutive ion ex-uously with a Thermal Conductivity Detector (TCD) to deter-changes in I M NH4NO3 solution with a solution/zeolite ratiomine the rate of ammonia desorption.of 10 cm'/g at 80 °C for 6 h, and resulting sample was driedat 120。C, and calcined again at 550。C for 2 h. The obtained3. Results and discussionH-ZSM-5 sample was labeled as HNZ. HNZ samples were de-noted in accordance with Na-ZSM-5, which means H-ZSM-5Figure 1 shows the XRD patterms of NaZSM-5 zeolitessarmple was labeled as HNZ-120 when the SiO2/Al2O3 molarwith different SiO2/Al2O3 molar ratios. It is evident that allratio of Na-ZSM-5 was 120. Moreover, to further investigatethe samples have a typical MFI structure.the effect of various ammonium exchange extents on the cat-alytic performance of MTP over ZSM-5 zeolite, HNZ sam-ples with SiO2/Al2O3 molar ratio of 180 by different NH4exchange times were prepared. HNZ 180-2, HNZ- 180-4 andHNZ-180-6 represented that the NZ samples were exchangedN2-220by 1 M NHNO3 by 1, 2 and 3 times, respectively, and theexchange time is 2 h for each time. Prior to the catalytic ac-NZ-180tivity tests, the samples were crushed and sieved in mesh sizeof 20 60 (corresponding to 250- 840 pum).JLNZ-1202.2. Reaction procedureNZ-60MTP reaction was conducted in a fixed-bed flow reac-tor at atmospheric pressure, 500。C, and methanol space ve-10203040locity (WHSV) of 6.0h-'. For each test, l.5g zeolite cat-20/09)alyst was loaded to the reactor and the feed was a 40 wt%Figure 1. XRD ptterns of dfferent NaZSM-5 zeoliesmethanol aqueous solution. The reactor outlet stream wascooled to 0 °C and then the gas and liquid products were sep-Figure 2 shows the SEM images of representativearated. The gas product was analyzed by a gas chromatog-NaZSM-5 zeolites. As shown, the samples are in rectangular-raphy (Varian CP, model 3800) which was equipped with aFID and two TCD. Aqucous and organic phases in the liq-uid product of the reactor were separated by a decanter. Asmall portion of the aqueous phase was sent to gas chromato-(a)graph (Agilent 6820) equipped with HP-INNOWAX capi-lary column (30 mx0.32 mmx0.25 um) and a FID. A smallportion of the organic phase was also analyzed with the gaschromatograph(CP- 3800GC), which was equipped with FIDand had a capillary column for separating Cs+ hydrocarbons.2.3. CharacteriationXRD patterns were recorded on an X'Pert PRO MPD(DANalytical Co,) diffractometer using Cu Ka radiation(45kV, 40 mA) with 2θ range of 50- -550.，The surfaceS4800 30KV7.2mm x1.OM SEMI 6/29/20105o Ounmorphology and crystallite size of the samples were deter-mined by scanning electron microscope (SEM) recorded on(b)an S-4800 microscope. Elemental analysis was carried out byXRF using an Axios X-ray flourescence spectrometer. Pyri-dine adsorption-IR was carried out on a Nexus Model In-frared Spectrophotometer (Termo Nicolet). Acid sites andacid type distribution of the catalysts were determined withinfrared spectroscopy (IR) of chemisorbed pyridine. Bronstedacidity was quantified according to the integrated areas of theabsorbance peaks at 1545 cm~ 1 . The NH3-TPD profile was中国煤化工measured with a conventional TPD apparatus. About 0.1 g ofsample was placed in a quartz reactor and saturated with am-0H.CNMHGmonia at 100 °C. TPD was carried out from 100°C to 600°CS480030kL72m w600 SEMT 6r28201035.......with a heating rate of 10 °C/min and with helium (30 mL/min)Figure 2. SEM micrographs of representative NaZSM-5 zeolites. (a) HZ-as the carrier gas. The effluent stream was monitored contin-120, (b) HZ-220Jourmal of Natural Gas Chenistry VoL 20No.3 2011263like shape and their particle sizes are in the range approxi-Table 2. The zeolite chemical composition and Bronstedmately from 30 pum to 40 μm.acidity of various zeolite catalystsFigure 3 ilustrates the N2 adsorption desorptionSampleSiO2/Al2O3 (mol) Na (w%)B acid (a.u.)isotherms of different NaZSM-5 samples at 77K. The N2NZ-6057.90.25791.7adsorption-desorption isothermns of NaZSM-5 samples areNZ-12097.90.156115.3of type I with a plateau at higher relative pressures and noNZ-180117.10.12152.1distinct hysteresis loop, which is typical for a microporousNZ-220136.50.11846.5material without significant mesoporosity. The textural prop-HNZ-6058.20.077239.0HNZ-12093.0 .0.074148.3erties of different NaZSM-5 zeolites are presented in Table 1.HNZ-180-2114.50.10688.2The BET surface areas of various NaZSM-5 with differentHNZ-180-4114.70.082110.2SiO2/Al2O3 molar ratios are above 370m2/g and the poreHNZ-180-6114.40.078112.4volumes are approximately 0.11cm3/g. The pore sizes ofHNZ-2200.07672.3different NaZSM-5 zeolites are 0.53 nm, in agreement withliteratures.a Obainecxd by XRF analysis; b Quanitatively calculated using thearea integral methodFigure 4 shows NH3-TPD profiles of different ZSM-5 ze-olites and their acidity data are listed in Table 3. Two des-orption peaks corresponding to the weak and strong acid sitesappear for ZSM-5 samples with different molar ratios. Theresult indicated that increasing the SiO2/Al2O3 molar ratioof ZSM-5 zeolites led to a decrease both in the number ofNZ-60。weak and strong acid sites for Na ZSM-5 and HZSM-5 sam-00048---0000081ples. And simutaneously the desorption peaks moved to-wards lower temperatures, indicating that the acid strengthalso decreased. Comparing NH3-TPD profles of Na ZSM-5with those of HZSM-5 samples with the same SiO2/Al2O3molar ratio, it can be found that the number of weak and strong.20.0.8.0acid sites both increased after NH4 exchange. There was al-Relative pessure (p/p,)most no change for desorption temperature corresponding toFigure 3. Nitrogen adsorption-desorption isotherms at 77 K for dfferentthe weak acid sites, while the desorption temperature corre-NaZSM-5 zeolitessponding to the strong acid sites moved towards higher tem-perature, suggesting that the acid strength of strong acid sitesTable 1. Specife surface area, pore volume and poresize of different NaZSM-5 zeolitesalso increased. The NH3-TPD data of different ZSM-5 ze0-Ser(m-/g) Pp:/(cm/g)D'/nmlites were in good agreement with the Py-IR results, i.e. for3790.120.53different ZSM-5 samples, with the increase of Brpnsted acid370.11sites, the strong acid sites also increased.N2-220 _37770.10■Calculated by t-plot method; b Calculated by H-K methodChemical compositions for different ZSM-5 zeolites arelisted in Table 2. SiO2/Al2O3 molar ratios of Na-ZSM-5 prod-NZ-220 .ucts decreased in different degrees compared with the initialcrystallization mixtures, while the difference increased withHNZ-180increasing the SiO2/Al2O3 molar ratio of samples. Na con-tent decreased with increasing the SiO2/Al2O3 molar ratio inNa-ZSM-5 zeolites. After exchanged to HZSM-5, the Na con-tent decreased significantly for all Na-ZSM-5 samples.Py-FTIR data for different ZSM-5 samples after pyridineadsorption at 200°C and subsequent evacuation at 200°C aregiven in Table 2. The results showed that the amount of中国煤化工Bronsted acid sites decreased with increasing the SiO2/AI2O3MHCNMHGmolar ratio for both Na-ZSM-5 and HZSM-5 samples. Com-1000600pared with Na-ZSM-5 zeolite, the amount of Bronsted acidTemperature (C)sites of the corresponding HZSM-5 sample with the sameFigure 4. NH3-TPD profiles of dffrent ZSM-5 zeolitesSiO2/AI2O3 molar ratio increased.264Ruchao Wei et al:/Jourmal of Natual Gas Chemistry Vol. 20 No.32011Table 3. NH3-TPD data of difTerent ZSM-5 zeolitessponding Na-ZSM-5 zeolites with the same SiO2/Al2O3 mo-Acidity (mmol NH3/g)lar ratio, while propylene and butylene selectivities decreased.Catalysttotalweak*strong°It can also be observed that as the NH2 exchange time ex-NZ-600.7290.5250.204tended for Na-ZSM-5 zeolite with the SiO2/Al2O3 molar ra-HNZ-600.7820.5310.251tio of 180, selectivities to propylene and butylene decreasedNZ-1200.4880.363gradually.HNZ-1200.550 .0.4120.138NZ-1800.3930.356 .0.037The conversion of methanol to olefins and hydrocarbonsHNZ-1800.4620.3650.097over acidic HZSM-5 zeolite is an intricate catalysis process.NZ-2200.3140.2890.025The overall reaction scheme consists of three categories of re-HNZ-2200.3790.3100.069actions: fast equilibrium of methanol (MeOH) with dimethyla From 100 °C to 400°C; b From 400°C to 600°Cether (DME) over Brosted acid sites; the conversion of theequilibrium mixture of MeOH and DME to the primary prod-Catalytic performances of different ZSM-5 zeolites foruct of ethylene and propylene; the subsequent conversion ofthe MTP reaction are shown in Table 4. Na-ZSM-5 sam-the primary products into a mixture of higher olefins, paraffinsples showed high activity toward the conversion of methanol.and aromatics by hydrogen transfer, alkylation and polycon-The conversion of methanol decreased slightly with increasingdention. There is general consensus that the intermediate inthe SiO2/Al2O3 molar ratios of Na-ZSM-5 zeolites. Increas-the dehydration of methanol to dimethyl ether over solid aciding the SiO2/Al2O3 molar ratio of Na-ZSM-5 zeolites alsocatalysts is a protonated surface methoxyl, which subjects toled to an increase in selectivities to propylene and butylene,a nucleophilic attack by methanol. However, the second stepwhile selectivities to other products including light alkanesi.e. the initial C-C bond formation from the C1 reactant, has(methane, ethane, propane and butane), ethylene and aromat-been the topic of an extensive discussion throughout the yearsics decreased. Similar trend was also observed for HZSM-5[2-3,19]. A large number of papers are available presentingzeolites with different SiO2/Al2O3 molar ratios.more than 20 possible mechanistic proposals for the formationHigher selectivities to light alkanes, ethylene and aromat-of the first c-C bond, indicating that a clear understanding ofics can be obtained on HZSM-5 zeolites compared with corre-the mechanism on the MTP reaction has not been achieved.Table 4. Catalytic performance of dfterent ZSM-5 zeolites for MTP reactionSelectivity (C-mol%)SampleConversion (%C1+C2C5CC3+C4Aromatics99.2.222.935.013.58.610.898.70.717.341.115.95.98.4.714.243.417.83.4.97.8.612.745.918.2.025432.3112.2.025.498.8).840.515.6:HNZ-180-216.641.816.34:HNZ-180-498.517.041.616.0HNZ.180-698.60.841.26.014.643.217.4自The other products are mainly Cs+ hydrocarbons excloding aromatics. Reaction conditions: T = 500 °C; catalyst amount, 1.5 g; WHSV (MeOH),6h-1; CHzOH: H2O=2:3; reaction time, 2hIt is generally acknowledged that the density of acidtion of propylene. Kimet al. [20] reported that Na ZSM-5 hassites and the acid strength decreases with the increasingonly weak acid sites. In their experiment, the total amount ofSiO2/Al2O3 molar ratios for ZSM-5 zeolites. The results ofacid sites of Na-ZSM-5 is much larger than that of HZSM-5,this paper are in good agreement with the rule. Ethylene andwhile Na-ZSM-5 showed a much lower activity than HZSM-5aromatics were formed over strong acid sites, so the selectiv-for conversion of glycerol to acrolein. Conversion of glycerolities to ethylene and aromatics decreased with increasing theto acrolein is also a dehydration reaction catalyzed by acidSiO2/Al2O3 molar ratio. Similarly, the same tendency was ob-catalysts like MTP reaction. The synthesized Na-ZSM-5 withserved for methane and ethane, as methane and ethane wereSiO2/Al2O3 molar ratio of 220 and high catalytic activity to-mainly formed by the dealkylation reaction of aromatics. Theward MTP reaction had a few strong acid sites. It is supposedreduction of the concentration of acid sites which are respon-that the strong acid sites rather than the acid amount are es-sible for the hydrogen transfer reaction, decreases the conver-senti:中国煤化工active catalyst towardsion of olefins to paraffins, thus selectivities to propane andMTPFhe used catalysts werebutane decreased with increasing the SiO2/AI2O3 molar ratiomostlf YHCNMHGstheNacontentofNa-for ZSM-5 zeolites.ZSM-5 produced industrially is mostly above 2% which leadsThe result ilustrated that a relatively weak acid strengthto a very low acidity, the conventional Na-ZSM-5 has to beand low concentration of acid sites are beneficial to the forma-exchanged to HZSM-5 before used as the catalyst. However,Journal of Natural Gias Chemistry Vol. 20 No. 32011265in this work the synthesized large size Na-ZSM-5 zeolite cat-creased SiO2/Al2Oz molar ratio of ZSM-5 zeolite led to an in-alysts exhibited high acidity and high activity toward MTP re- crease in both acidity and Bronsted acid sites. Na-ZSM-5 withaction, which should be atributed to the fact that silica gel mi-high SiO2/Al2O3 molar ratio favored the formation of propy-crospheres were used as the silicon source and the feature oflene in the MTP reaction. Among the tested Na-ZSM-5 andthe preparation process. The synthesized NaZSM-5 zeolites H ZSM-5 zeolites, the Na-ZSM-5 having a SiO2/Al2O3 ratiowith large particle size form as aggregates of small particles.of 220 exhibited the highest propylene selectivity of 45.9%.As the concentration of hydrothermal crysallization mixtureis rather high and the crytallization process is fast, some wa- Referencester molecules and hydroxyl groups on the surface of smallparticles were trapped within the large sized NaZSM-5. H+[1]ZhaoGL,TengJW,XiezK,linWQ,YangWM.ChenQL,within the hydroxyl groups which are chemically adsorbed onTang Y. J Catal, 2007, 248: 29the surface of silica gel can play as the charge balance ion dur-[2] MeiCS, WenPY,LiuZC,LiuHX, Wang Y D, Yang W M,ing the zeolite crytallization process. This led to a relativelyXie Z K, Hua W M, Gao Z. J Catal, 2008, 258: 243low Na content and high acidity of the synthesized large par-[3] Stocker M. Microporous Mesoporous Mater, 1999, 29: 3ticle size Na-ZSM- 5 zeolites.[4] Ivanova s, Lebrun C, Vanhaecke E, Pham-Huu C, Louis B. JIn NH3-TPD analysis, only Bronsted site-bound ammo-Catal, 2009, 265: 1nia can present at temperatures higher than 440 °C suggested[5] Travalloni L, Gomes A C L, Gaspar A B, da Silva M A P. CatalToday, 2008, 133-135: 406in Ref. [21], which means that the amount of Bronsted acid[6] Min H K, Park M B, Hong s B.J Catal, 2010, 271: 186sites should increase if there are more strong acid sites for[7] Chae HJ, Song Y H, Jeong K E, KimC U, Jeong s YJ PhysZSM-5 zeolites. In our cases, strong acid sites decreased withChem Solids, 2010,71: 600increasing the SiO2/Al2O3 molar ratio of ZSM-5 zeolite and[8] LiP, Zhang W P, Han x w, Bao x H. Catal Lett, 2010, 134: 124so did the Bronsted acid sites. After exchange with NHZ to[9] Firoozi M, Baghalha M, Asadi M. Catal Commun, 2009, 10:H-ZSM-5, the strong acid sites increased while the Brnsted1582acid sites also increased. As indicated in Refs. [2,10], a [10] Zhang s H, Zhang B L, Gao Z x, Han Y Z. Reac Kinet Mechcatalyst with a lower Brsted acidity favors the formation ofCat, 2010, 99: 447propylene in the MTP reaction. Our present resuts also in- (11] Park J w. SeoG. Appl CaralA, 2009, 356: 180dicated that a decreased amount of Bronsted acid sites was [12] Kaarsholm M.Joensen F, Nerlov J,Cenni R, ChaoukiJ,Patiencebeneficial to increasing the propylene selectivity. Moreover,G S. Chem Eng Sci, 2007, 62: 5527this work indicated that Na ZSM-5 could be used as a suit-[13] Zhao T s, Takemoto T, Yoneyama Y, Tsubaki N. Chem Lett,2005, 34: 970able catalyst for MTP process if the synthesized Na ZSM-5[14] Zhao T s, Takemoto T, Tsubaki N. Catal Commun, 2006, 7: 647zeolite has the appropriate physicochemical properties. The[15] Gujar A C, Guda V K, Nolan M, Yan Q G, Toghiani H, Whiteresults also suggested that ZSM-5 zeolite, which was used asM G. Appl CatalA.2009, 363: 115the catalyst for the MTP reaction, may be not ncessary to be[16] Moller K P, Bohnringer w, Schnitzler A E, van Steen E,fully exchanged with NHt to HZSM-5. Further studies alongO'Connor C T. Microporous Mesoporous Mater, 1999, 29: 127with this line are underway in our group.[17] Bjorgen M, Svelle s, Joensen F, Nerlov J, Kolboe S, Bonino F,Palumbo L, Bordiga s, OIsbye U.J Catal, 2007, 249: 1954. Conclusions[18]LiuJ,ZhangCX,ShenZH,HuaWM,TangY,ShenW,YueYH, Xu H L. Catal Commun, 2009, 10: 1506The conversion of methanol to propylene was carried[19] Dewaele O, Geers V L, Froment G F, Marin G B. Chem Eng Sci,1999, 54: 4385out using a novel large size ZSM-5 zeolite as catalyst.[20] Kim Y T, Jung K D, Park E D. Microporous Mesoporous Mater,The results indicated that Na-ZSM-5 and H-ZSM-5 with2010, 131: 28different SiO2/Al2O3 molar ratios all exhibited high activity [21] Zhang w M, Burckle E C Smimiotis P G. Microporous Meso-for methanol conversion. Ammonium exchange and the de-porous Mater, 1999, 33: 173中国煤化工MYHCNMHG