Catalytic cracking of 1-butene to propylene by Ag modified HZSM-5☆

Rongrong Zhang,Zhengbao Wang*

College of Chemical and Biological Engineering,Zhejiang University,Hangzhou 310027,China

Keywords:1-Butene Propylene Cracking HZSM-5 Zeolite Silver

ABSTRACT Silver modified HZSM-5(AgHZ)zeolite catalysts were prepared by ion exchange method and their catalytic properties in the 1-butene cracking reaction were measured.The catalysts were characterized by infrared spectroscopy with pyridine adsorption(Py-IR),N2 adsorption and X-ray diffraction(XRD).The effects of Ag loading and steaming treatment on catalytic performances were studied.It is found that the activity of HZSM-5(HZ)catalyst significantly decreases with the steaming time,whereas AgHZ catalysts shows table activity in the steaming time of24–48 h and their activities increase with the Ag loading.When the steaming time is 24–48 h,the yield of propylene over HZ catalyst significantly decreases,whereas it is stable over AgHZ catalysts.The AgHZ catalysts with Ag loadings of 0.28%–0.43%(by mass)show similar propylene yields(～30%),which are higher than that over the AgHZ catalyst with a Ag loading of 0.55%(by mass).These results indicate that the steam-treated AgHZ catalysts with optimum Ag loadings have higher yield of propylene and are more stable than the steam treated HZ catalyst.The regeneration stability measurement in butene cracking also shows that the AgHZ catalyst steam-treated under a suitable condition has better stability than the HZ catalyst.

1.Introduction

Propylene is one of the most important fundamental chemical intermediates and is widely used in chemical fields.Recently,the demand for propylene derivatives is increasing steeply.The traditional methods for the production of propylene cannot satisfy the increasing demand.It is mainly produced by steam cracking of petroleum with low yield under harsh operation conditions(higher than 800°C),while the yield of propylene is also limited in the fluid catalytic cracking(FCC)process[1].Other techniques require high investment and their economic viability is low,such as propane dehydrogenation[2]and 2-butene disproportionation process[3].On the other hand,lots of C4olefin resources are produced as by-products in the catalytic cracking and steam cracking processes.Near 200 million C4ole fins are produced every year while mostly are used as fuels.The catalytic cracking of C4resources appears to be very attractive for researchers since the waste can be put to good use and valuable propylene can be obtained.

Much work has been done on C4catalytic cracking techniques.Many zeolite catalysts such as ZSM-23,MCM-22,MCM-49,SAPO-34 and ZSM-5[4–12]have been studied in the C4catalytic cracking process.ZSM-5 zeolite has been mostly investigated due to its relatively high activity and the unique pore structure.Acidity is a crucial factor for obtaining high conversion and propylene yield.To obtain a better propylene yield,various metal elements such as alkaline earths,Cr,Fe and rare earths have been used to modify the acidity of HZSM-5[9–13].Besides the activity and selectivity,the hydrothermal stability is another important factor for commercial catalysts.Up to now,many efforts have been made on the improvement of the hydrothermal stability of zeolite catalysts,especially HZSM-5.Phosphorus modified HZSM-5(P/HZSM-5)zeolite has shown attractive hydrothermal stability[14–19].The effect of phosphorus has been commonly accepted by researchers for years although the mechanism is still controversial.It is found that in C4catalytic cracking process using P modified HZSM-5 zeolites as catalysts,not only the hydrothermal stability of catalysts can be improved,but also the selectivity of propylene is increased[15].But phosphorus species tend to lose in steaming treatment.It is fatal for catalysts in the long term regeneration process.

Tsutsumi et al.[20]reported that Ag+cations were reduced to Ag atoms and proton cation sites were created under reducing atmosphere for Ag modified zeolites prepared by ion exchange method.Shibata et al.[21]had the same conclusion about AgY zeolite.In other words,Ag modification does not change the acidity of zeolite.Based on our previous study[22],silver species can greatly increase the hydrothermal stability of ZSM-5 zeolite because dealumination of zeolite can be improved by protons in cation sites and Ag species can protect framework aluminum species by occupying the cation sites under steaming treatment in airatmosphere.Up to now,there is no report about Ag modified HZSM-5 as a catalyst in C4catalytic cracking process.In this article,the catalytic properties of Ag modified HZSM-5 catalysts prepared by ion exchange method were investigated in the 1-butene catalytic cracking reaction.By adjusting the Ag loading and steaming treatment condition,the acidity of zeolite catalysts was tuned properly,and catalysts with high selectivity of propylene and high stability were obtained.

2.Experimental

2.1.Catalyst preparation

HZSM-5(Si/Al2mass ratio=280),designated as “HZ”,was purchased from Zeolyst Company.To obtain different Ag loadings,Ag was introduced to ZSM-5 zeolites by ion exchange of HZ with AgNO3solution at room temperature for different times.Ag modified HZSM-5 zeolite catalysts(designated as AgHZ)were dried at 60°C overnight and calcined at 550°C for 4 h.The ion-exchange conditions are listed in Table 1.Catalysts with different Ag loadings are designated as“x AgHZ”.For example,0.28AgHZ means Ag modified HZSM-5 zeolite with an Ag loading of 0.28%(in mass).

Table 1 Preparation conditions of Ag modified ZSM-5 catalysts

Steaming treatment:After pretreatment at550°C in nitrogen gas for 1 h,the steaming treatment of the catalysts was carried out at600°C in a stream of air(12.5 ml·min-1)and deionized water(6 g·h-1)for different periods.“-S”was added after the catalyst name for zeolite catalysts steam-treated for 24 h.Catalysts were regenerated at 550°C for 24 h with a steam partial pressure of 0.028 MPa.

2.2.Characterization of catalysts

X-ray powder diffraction(XRD)analyses(Dmax-RA)were performed using Cu Kαradiation to identify the productphase and to calculate crystallinity.Nitrogen adsorption/desorption isotherms were recorded on a Micrometrics ASAP 2020 instrument.The total surface area was calculated according to the Brunner–Emmet–Teller(BET)method,and the micropore volume,the micropore surface area and external surface were evaluated by t-plot method.The Ag loading was measured by the inductively coupled plasma mass spectroscopy(ICPMS)method.The acidic properties(Br?nsted and Lewis acid sites)of the samples were investigated by IR of pyridine adsorption.The samples were pretreated at 450°C for 1 h.Pyridine was introduced to the IR cell at 150°C.The excess pyridine was desorbed by evacuation for 1 h,and then the spectrum was recorded on a Bruker TENSOR 27 spectrometer.

2.3.1-Butene catalytic cracking tests

1-Butene(＞99%purity)catalytic cracking was performed in a fixed bed reactor with a quartz tube of 8 mm inner diameter at atmospheric pressure.The catalysts were pressed to pellets and crushed to particles of 20–40 mesh for catalytic cracking,and 0.15 g catalyst was loaded for all tests.Catalysts were pretreated at 550 °C for 1 h in 30 ml· min–1of N2.Then pure 1-butene flowed through the reactor and the weight hourly space velocity(WHSV)was 20 h-1for the most tests.The reactor effluent products were analyzed by on-line gas chromatograph(SHIMAZU GC 2010,Japan)using a HP-AL/S capillary column with a FID detector.For simplicity,all types of butene were grouped as the overall feed.The butene conversion(X)and selectivity(S)of product component were defined as(where all percentages are by mass):

3.Results and Discussion

3.1.Influence of Ag loading

3.1.1.Characterization of AgHZ catalysts

Pyridine was used as probe molecule to characterize the acidic properties of Ag modified HZ catalysts.To ensure that Ag+ions were reduced and protons were created,AgHZ catalysts were on-line reduced in the IR cell prior to the pyridine adsorption.IR spectra of pyridine adsorption are shown in Fig.1.The band at 1546 cm-1is assigned to pyridine ions adsorbed on Br?nsted acid sites and the band at 1450 cm-1is assigned to pyridine ionsadsorbed on Lewis acid sites.With the increase of Ag loading,Lewis acid sites increase,which are mainly related to pyridine adsorbed on silver species,as we reported previously[22].The Br?nsted acidities of HZ and AgHZ catalysts are nearly the same.It has been proved that silver ions in the cation sites of AgHZ catalysts can be easily reduced to Ag atoms,and the proton cations produced simultaneously[20–22].This is the reason that AgHZ catalysts with different Ag loadings have the same Br?nsted acidic sites.

Fig.1.IR spectra of pyridine adsorption on HZ and AgHZ catalysts with different Ag loadings,which were in situ reduced prior to pyridine adsorption.

The textural properties of catalysts are listed in Table 2.Compared with the unmodified HZ catalyst,the AgHZ catalysts all have a slight decline on surface area and micropore volume.With the increase of Ag loading,the BET surface area decreases from 358 to 320 m2·g-1and the micropore volume from 0.18 to 0.15 cm3·g-1,indicating that silver modification has a slight effect on the zeolite structure.

3.1.2.Catalytic cracking performance of AgHZ catalysts

The influence of Ag loadings on 1-butene catalytic cracking performance was studied.As shown in Fig.2(a),conversions of 1-butene over untreated AgHZ catalysts with different Ag loadings are almost the same as that of untreated HZ catalyst,and the product distributions over these catalysts have no significant difference(Table 3).The selectivity and yield of propylene are almost the same for all untreated zeolite catalysts[Fig.2(b)].That is,silver species have no significant influence on the 1-butene catalytic cracking performance of AgHZ catalysts.Hydrogen is produced in dehydrogenation reactions during 1-butene cracking.As described above,AgHZ catalysts with different Ag loadings theoretically have the same Br?nsted acidic sites with HZSM-5.This means that the working proton acid sites of AgHZSM-5 are theoretically the same with HZSM-5 in the cracking reaction.This is the reason that the AgHZ catalysts with different Ag loadings have the similar catalytic cracking performance with the HZ catalyst.

Table 2 Textural properties of HZ and AgHZSM-5 catalysts

Fig.2.Effects of steaming time on(a)conversion of butene and(b)propylene selectivity and yield in 1-butene cracking over HZ and AgHZ catalysts with different Ag loadings(reaction conditions:T=550°C,WHSV=20 h-1,time of steaming(TOS)=5 min).

Table 3 Catalytic performance of 1-butene cracking at initial reaction time over AgHZ catalysts with different Ag loadings(steaming condition:600 °C,24 h,reaction conditions:T=550 °C,WHSV=20 h-1,TOS=5 min)

3.2.Influence of steaming treatment

Steam will be produced during the regeneration process of cokeburning and sometimes steam is introduced into the coke-burning stream to prevent from temperature runaway.On the other hand,the acid zeolite catalysts tend to dealuminate under steam.To study their hydrothermal stability and the influence of acidities on the catalytic performance,HZSM-5 catalyst and AgHZSM-5 catalysts with different Ag loadings were steam-treated under 600°C for 6 to 48 h.

3.2.1.Characterization of steam-treated AgHZ catalysts

Fig.3.IR spectra of pyridine adsorption on steam-treated HZ and AgHZ catalysts with different Ag loadings.

IR spectra of HZ and AgHZ catalysts steam-treated for 24 h are shown in Fig.3.The change of Lewis acid sites near 1450 cm-1is related to silver species and non-framework Al species.Increasing the Ag loading,pyridine absorbed on silver species increases,while pyridine absorbed on the non-framework Al species decreases.With the increase of Ag loading,the Br?nsted acid sites increase.It has been proposed previously that silver species can prevent their correlated Al sites in zeolite catalysts from dealumination because of the replacement of protonic cations with Ag cations under steaming in air[22].Therefore,AgHZ catalysts with higher Ag loadings will have more proton acid sites and less non-framework Al species after steaming in air.

The textural properties of steam-treated catalysts are listed in Table 2.Compared with the non-treated catalysts,the surface areas and micropore volumes of AgHZ-S and HZ-S catalysts all decline slightly.Compared with HZ-S catalyst,the decline for AgHZ-S catalysts is lower.The crystallinity of HZ-S catalyst clearly decreases,while the loss for AgHZ-S catalysts is less(Fig.4).It is certain that the destruction is caused by steaming treatment.There are no characteristic diffraction peaks of silver species at 38°and 44°,indicating that silver species are dispersed uniformly on the zeolite.

Fig.4.XRD patterns of HZ catalyst,and steam-treated HZ and AgHZ catalysts.

3.2.2.Catalytic cracking performance of steam-treated AgHZ catalysts

The production of propylene with high selectivity in catalytic cracking of butene depends on the acidity of ZSM-5 zeolite.Xu et al.[23]have investigated the cracking of C4alkenes on zeolites with different pore structure and Si/Al2ratios.It was reported that higher acidities will enhance the formation of secondary reactions of alkenes(mainly oligomerization,aromatization and hydrogen transfer reactions).Thus,suppressing the formation of by-products in butene cracking is very important.1-Butene molecules first dimerize to C8intermediates and then the latter crack to small molecules.Propylene selectivity is mainly balanced by both C8and C5+cracking and C3H6hydrogen transfer reaction.For that,catalysts should have optimum acidities.

The catalytic performance of steam-treated catalysts in 1-butene cracking is shown in Fig.2 and Table 3.The activity of HZ catalyst is significantly decreased along with the steaming time.For example,the conversion decreases from about 80.0%to 70.0%after 6 h steaming treatment and to 51.4%after 48 h steaming treatment.This is because the acidity of HZ catalyst is significantly decreased after 48 h steaming(data not shown),indicating that HZ catalyst is not stable under steaming.As for AgHZ catalysts,the activity is slightly decreased after 6 h steaming treatment,whereas the decrease of the activity becomes very slow after 24 h steaming treatment,indicating that the dealumination of Ag modified HZSM-5 catalysts steam-treated after 24 h is slight.Moreover,the more the Ag loading,the less decrease of the activity for the same steaming time[Fig.2(a)].For example,after 24 h steaming treatment,the conversion over 0.55AgHZ-S is 77.4%(80.0%over 0.55AgHZ),while it is 68.5%over 0.28AgHZ-S(80.2%over 0.28AgHZ).That is,the steamtreated AgHZ catalyst with higher Ag loading has higher cracking activity(Table 3),which is consistent with the result for the cracking of hexane in reference[22].Table 3 also lists the product distribution of 1-butene cracking over the steam-treated catalysts.The selectivity of ethylene and by-products(C5+excepted)all increases with increasing Ag loading,whereas the selectivity of propylene and C5+decreases.As can be seen from Fig.2(b),the selectivity of propylene over steamtreated 0.55AgHZ is always lower than other steam-treated AgHZ catalysts under different steaming times.The selectivity of propylene over HZ catalyst steam-treated for 48 h is the lowest.After＞24 h steaming,the yield of propylene over HZ catalyst is significantly decreased,whereas itis stable over AgHZ catalysts.The steam-treated AgHZ catalysts with Ag loadings of 0.28%–0.43%(by mass)show similar propylene yield(～30%),which is higher than that over the steam-treated 0.55AgHZ catalyst.This is because steam-treated 0.28AgHZ,0.38AgHZ and 0.43AgHZ catalysts have optimum acidic sites,whereas steam-treated 0.55AgHZ has too more acidic sites.

3.3.Influence of reaction conditions

The cracking reaction is an endothermic process and the temperature plays an important role.By comprehensively considering the activity and propylene selectivity,catalyst 0.43AgHZ-S was chosen for further study.The effects of reaction temperature on butene conversion and product distribution over 0.43AgHZ-S catalyst are shown in Fig.5.No significant difference occurs to the conversion from 450°C to 600°C,while an obvious increase(from 73.6%to 82.0%)happens from 600 °C to 650 °C.The change is determined by the cracking mechanism.Butene molecules dimerize to C8intermediates,and the latter crack to smaller olefins.The former reaction is exothermic.Raising the temperature from 450 °C to 550 °C,the cracking reaction is suppressed while the dimerization reaction is accelerated,therefore,the change on conversion is little[6].Further increasing the temperature,the pyrolytic cracking reaction is enhanced.As a result,the conversion of 1-butene increases.

The propylene selectivity first increased from 25.4%to 43.3%when the temperature changes from 450 °C to 600 °C,and then it decreases to 33.9%when the temperature rises to 650°C[Fig.5(a)].The ethylene selectivity increases steadily with temperature,and the selectivity at 650°C is as high as 16.5%due to the increase of the pyrolytic cracking.The total selectivity of propylene and ethylene reaches the maximum value of 57.8%at 600°C.1-Butene cracking is an endothermic reaction,while hydrogen transfer reaction is exothermal.Therefore,higher temperature is beneficial for the production of propylene and ethylene,but it is also favorable to the formation of aromatics produced by oligomerization,dehydrogenation and cyclization reaction of low olefins[24].Therefore,if the temperature is too high(e.g.650°C),the selectivity of low olefins(e.g.propylene)will decrease.

As shown in Fig.5(b),the selectivity of ethane increases slowly with increasing temperature,while the selectivities of propane,butane and C5+decrease.As the temperature rises above 550°C,the selectivity of methane and aromatics increases dramatically.At 650°C,the selectivities of aromatics,methane,propane and ethane are 25%,11.3%,1.7%and 1.3%,respectively.Methane,ethane,propane,and aromatics are considered as main secondary products in 1-butene cracking.Higher temperature promotes aromatics dealkylation and hydrocarbons protolytic cracking reactions,therefore,the selectivity of methane and ethane increases.Meanwhile,the selectivity of propane and butane decreases due to inhibition of hydrogen transfer reaction.According to the product distribution at different temperatures,it can be concluded that the reaction temperature of 550°C is preferred.

Fig.5.Effects of reaction temperature on(a)the conversion of 1-butene,selectivities and yields of ethylene and propylene and(b)selectivities of by-products(reaction conditions:WHSV=20 h-1,TOS=5 min).

The effect of space velocity(WHSV)on 1-butene conversion and product selectivity is shown in Fig.6.With the increase of WHSV,1-butene conversion and the selectivity of ethylene decrease,however,the selectivity of propylene increases.From 5 to 20 h-1,the conversion goes down steadily from 83.0%to 72.1%,and then decreases slowly to 70.0%at 30 h-1.The effect of WHSV on by-products is more obvious than on main products.When the WHSV is as low as 5 h-1,the selectivity of propane,butane and aromatics is as high as 10.4%,9.1%and 13.5%,respectively.Raising the WHSV to 20 h-1,they fell to 3.3%,7.1%and 4.3%,respectively.As described above,these three products are mainly produced by hydrogen transfer reaction.The high WHSV is unfavorable for hydrogen transfer reaction due to shorter contact time.As the primary products of the cracking process,C5+hydrocarbons increase with WHSV.In general,20 h-1is a better choice for obtaining a high yield of propylene in 1-butene cracking.

Fig.6.Effects of the space velocity on(a)the conversion of 1-butene,selectivity and yields of ethylene and propylene and(b)selectivity of by-products(reaction conditions:T=550°C,TOS=5 min).

3.4.Regeneration performance of AgHZ catalyst

Fig.7.The regeneration performance of butene cracking over(a)HZ steam-treated for 6 h(b)0.43AgHZ-S catalysts;0,1,2,3,4 represented fresh catalyst and catalyst regenerated for one to four times,respectively(reaction conditions:T=550°C,WHSV=20 h-1).

To explore the regeneration stability of the AgHZ catalyst,the catalytic performance of the 0.43AgHZ-S catalyst after four cycles of reaction-regeneration was evaluated,and the results are shown in Fig.7.Catalysts were regenerated in a mild condition of the humid air with a steam partial pressure of 0.028 MPa at 550°C for 24 h[25].For comparison,considering the similar conversion and product distribution,the catalytic performance of HZ catalyst steam-treated for 6 h was also investigated.In the four reaction–regeneration cycles,the conversion over HZ catalyst decreases gradually,while the regenerated 0.43AgHZ-S catalyst still maintains the initial activity.Although the yield of propylene over steam-treated HZ catalyst has no significant decrease within four recycles,it is estimated that the yield will decrease after more recycles.It is implied that the stability of 0.43AgHZ-S catalyst is better than steam-treated HZ catalyst.

4.Conclusions

AgHZ catalysts with different Ag loadings showed similar performance in butene cracking with HZ catalyst before steaming treatment.Steaming treatment was a good method to tune the acidities of zeolite.After the steaming treatment,the remaining Br?nsted acid sites increased with the Ag loading.In other words,the addition of Ag species made the dealumination of HZSM-5 zeolite under control.The activity of HZ catalyst significantly decreased with increasing steaming time,whereas AgHZ catalysts showed stable activity after steaming of＞24 h and their activities increased with increasing Ag loading.After＞24 h steaming,the yield of propylene over HZ catalyst significantly decreased,whereas it was stable over AgHZ catalysts.The AgHZ catalysts with Ag loadings of 0.28%–0.43%(by mass)showed similar propylene yield(～30%),which was higher than that over the AgHZ catalyst with the Ag loading of 0.55%(by mass).The results indicated that the steam-treated AgHZ catalyst with optimum Ag loading had higher yield of propylene and was more stable than the steam-treated HZ catalyst.From the reaction results in reaction-regeneration cycles,it was clear that the regeneration stability over the steam-treated AgHZ catalyst was better than that over the steam-treated HZ catalyst with good initial catalytic performance.