催化丁烯裂解高选择性生成丙烯的SAPO-18的合成

胡云峰， 吕忠媛， 苏寻明， 李兆飞， 阎立军

(1.东北石油大学 化学化工学院， 黑龙江 大庆 163318；2.中国石油 石油化工研究院， 北京 100195)



催化丁烯裂解高选择性生成丙烯的SAPO-18的合成

胡云峰1， 吕忠媛1， 苏寻明1， 李兆飞2， 阎立军2

(1.东北石油大学 化学化工学院， 黑龙江 大庆 163318；2.中国石油 石油化工研究院， 北京 100195)

合成了不同n(Si)/n(Si+Al+P)的SAPO-18分子筛，并将其应用于催化丁烯裂解反应中；同时采用XRD、SEM、XRF、TG和NH3-TPD技术对其进行表征，以考察如何通过改变凝胶混合物中的Si含量来提高其在催化丁烯裂解反应中的丙烯选择性。结果表明，与ZSM-5分子筛相比，n(Si)/n(Si+Al+P)为0.036的SAPO-18能获得较高的低碳烯烃选择性。在反应初期 (TOS=3 min)，丁烯转化率达到71.6%，丙烯选择性可以达到54.2%。这主要归因于SAPO-18具有独特的八元环结构以及适宜的酸性。

SAPO-18； 1-丁烯裂解； 丙烯； Si含量

Light olefins are important petrochemical raw materials. It is reported that average annual growth of propylene production reached 9%, while the average annual growth of propylene consumption reached 10% in domestic since 2008[1]. Approximately, 68% of propylene was derived from ethylene steam cracker plant byproduct, 29% of propylene was obtained from FCC cogeneration, and the remaining 3% of propylene was derived from the olefins conversion, propane dehydrogenation, methanol to olefins and other crafts. What deserves the attention to 1-butene cracking to light olefins, particularly to propylene, is that not only there is full advantage of the low value-added C4fractions of refinery, but also an effective way is provided to alleviate the phenomenon of the propylene shortages. Due to the increasing demand for propylene nowadays, it is significant to hunt for a new and more effective catalyst for 1-butene cracking.

ZSM-5 was earlier used as the catalyst for 1-butene cracking, by which a fraction of alkanes and a plenty of C5+hydrocarbons were produced from hydrocarbon transformations and aromatization side reaction, except light olefins[2-4]. Thus, ZSM-5 needs to be modified to produce enough propylene to meet the growing market demand[5]. In recent years, more attention is paid to SAPO-34 for its exceptionally high selectivity to propylene[6]. SAPO-18/SAPO-34 intergrown molecular sieve was applied to catalyze the 1-butene cracking and it is found that molecular sieve pore structure played a great role on the product distribution of 1-butene cracking[7]. It is worth noting that SAPO-18 and SAPO-34 are the silico-aluminophosphate molecular sieves with eight-membered ring and small-pore, which are the effective catalysts for the methanol-to-olefins (MTO) reaction[8-10]. Due to the fact that SAPO-18 exhibited better catalytic performance in MTO reaction, SAPO-18 may be used for the 1-butene cracking reaction. In this work, the different impact of the port size on 1-butene cracking was further explored by comparing ZSM-5 and SAPO-18, and the influence of acidity on 1-butene cracking product distribution was also studied.

1 Experimental

1.1 Materials and reagents

Phosphoric acid (w(H3PO4)=85.00%), Yaohua chemical product.N,N-Diisopropylethylamine (w(C8H19N)=25.00%) and Fumed silica (w(SiO2)=25.00%), Aladdin industries product. Pseudo boehmite (w(Al2O3)=30.00%), Tongjie chemical Co. Ltd. Product. Butene(≥99.9%), Bright special gas chemical research institute product.

1.2 Catalyst preparation

Samples of SAPO-18 were prepared according to the way of Robson[11]. The final gel with then(SiO2)∶n(Al2O3)∶n(P2O5)∶n(H2O)∶n(R)=a∶1∶0.90∶50∶1.60, in whicharange is 0-0.33 and R is the template ofN,N-diisopropylethylamine, was sealed in a Taflon-lined stainless steel autoclave and crystallized at 160℃ under autogenous pressure for 8 d without agitation. The product was recovered by centrifugation and washed with distilled water, and then dried over night at 50℃ in air, finally, calcined for 5 h at 550℃. The obtained samples withaof 0, 0.07, 0.13, 0.27, 0.33, respectively, were denoted as SAPO-18-00, SAPO-18-07, SAPO-18-13, SAPO-18-27 and SAPO-18-33, corresponding ton(Si)/n(Si+Al+P) of 0, 0.04, 0.06, 0.13, 0.15, respectively, tested by XRF.

1.3 Catalyst characterization

XRD patterns of synthesized samples were acquired on a XRD-6000 diffractometer with Cu-Kαradiation, operating at 40 kV and 30 mA. XRD data were collected for 2θbetween 5° and 60°. The crystal morphology and crystal size of samples were observed by a JSM-6380 scanning electron microscope (SEM). By means of NH3temperature programmed desorption (NH3-TPD), carried out in the Auto Chem 3000 system with a Micromeritics ASAP 2020, the acid properties could be calculated. The total remaining carbon was performed by a NETZSCH TG 209 F3 analyzer with the temperature-programmed rate of 20℃/min from 30℃ to 850℃ under oxygen flow. Si, P and Al contents of SAPO-18 were measured by a XRF-1800 instrument with the powder-pressing method to prepare the sample.

1.4 Catalytic activity evaluation

The ZSM-5 provided by Petrochemical Research Institute of CNPC and he prepared SAPO-18 were pressed and then ground and sieved for the fraction of 20-40 mesh. Generally, 1.0 g sample was filled in a stainless steel fixed bed reactor placing in a furnace. The catalyst was heated to 550℃ under flowing N2at the desire MHSV (3.5 h-1). Thereafter, the 1-butene (≥99%) was charged into the reactor. The product distribution was analyzed by an online gas chromatography (GC-14) with a FID at the reaction of 1, 3, 10 and 30 min, respectively.

2 Results and discussion

2.1 Physical and chemical properties of SAPO-18 with different Si contents

2.1.1 XRD Analysis

The XRD patterns of prepared SAPO-18 with different Si contents in the gel and the SEM image of SAPO-18-07 were showed in Fig.1. From Fig.1(a) it can be seen that there existed the XRD peaks at 15.9°, 20.3°, 21.3°, 25.7°, 27.8°, indicating the existence of the AEI structure, which was similar to those reported previously[12], and also that the intrinsic AEI structure of SAPO-18 was preserved and the relative crystallinity changed slightly, meaning that there is no impact of Sicontent change in the gel on the crystal structure of SAPO-18.

Fig.1 XRD patterns of SAPO-18 with different Si contents and SEM image of SAPO-18-07(a) XRD; (b) SEM

2.1.2 SEM Analysis

The SEM images of SAPO-18 with different Si content were obtained, one of which was shown in Fig.1(b). The SEM results indicated that SAPO-18 exhibited uniform and regular cubic crystals with an average particle size of 0.8 μm and high crystallinity, being consistent with those obtained from XRD[13].

2.1.3 NH3-TPD Analysis

Changing the Si content in the gel is a known method used to modify the Si content in SAPO-18 molecular sieves framework. Si can substitute for both Al and P atoms in the framework via mechanisms I (Si→P) and II (2Si→Al+P), respectively[8,14]. As Si content influences acidic sites, it affects the catalytic performance of the catalysts. The profile of NH3-TPD was illustrated in Fig.2. Except SAPO-18-00 presented only one NH3desorption peak at approximately 220℃ corresponding to weak acid sites, all the samples exhibited two prominent NH3desorption peaks at approximately 200 and 400℃, corresponding to weak and strong acid sites, respectively. The acid amount of SAPO-18 first increased and then stabilized with the increase of Si content, existing a threshold level, after which the number of acid sites remained nearly constant. This result was consistent with XRF data and similar to the result obtained by Izadbakhsh et al[15]. The threshold level of acid sites in SAPO-18 could be determined by Si capacity of AEI framework structure and formation of Si islands rather than the Si content of the synthesis gels mixture. Fortunately, SAPO-18 with suitable acid sites for 1-butene cracking can be effectively synthesized by adjusting Si content in the gel.

Fig.2 NH3-TPD curves of SAPO-18 with different Si contents

2.2 The product distribution of 1-butene cracking catalyzed by SAPO-18

Table 1 The catalytic performance comparison of SAPO-18 and ZSM-5 in 1-butene catalytic cracking

m(Catalyst)=1.0 g;T=500℃;p=0.1 MPa; MHSV=3.5 h-1; TOS—Time on stream

1) Determined by TG at TOS of 30 min.

Although the catalytic activity of SAPO-18 is very high in the initial reaction period, it cannot be ignored that SAPO-18 is incomparable with ZSM-5 in terms of activity stability. 1-Butene conversion over SAPO-18 decreased to 57% after 10 min cracking, and the catalyst was nearly completely deactivated after 30 min cracking. However, the activity of ZSM-5 remained stable at a TOS of 30 min. In fact, with the use of a fluidized bed to compensate for rapid deactivation, SAPO-18 would be a potential industrial catalyst with significant research value for 1-butene cracking.

Fig.3 Three reaction pathways of dimerization cracking of 1-butene over SAPO-18 P/E—m(Propylene)/m(Ethylene)

2.3 The influence of the SAPO-18 structure on its catalytic performance

With regard to activity stability, the small window size of SAPO-18 limits the large hydrocarbon molecules to diffuse outside the cages, leading to rapid deactivation, which primarily causes poor activity stability. SAPO-34 with similar structure to SAPO-18 has similar catalytic deactivation performance in the MTO reaction[19,21]. Coke is not easy to form because of the three-dimensional channel, resulting in the excellent activity stability of ZSM-5. The total amounts of carbon deposition on the two catalysts after the stream of 30 min were also listed in Table 1. The mass fractions of coke in SAPO-18 and ZSM-5 were 2.60% and 0.24%, respectively, which further proved that the poor activity stability of SAPO-18 was due to its relatively high carbon deposition comparing to ZSM-5.

2.4 Catalytic performance of SAPO-18 with different Si contents

In 1-butene cracking, besides the by-products of C5+, propane is also an important by-product, which is mainly generated by hydrogen-transfer reactions. The effect of Si content on 1-butene catalytic cracking over SAPO-18 at a TOS of 3 min was studied and shown in Fig.4. It can be seen in Fig.4 that the SAPO-18-00 with the least acid sites presented the lowest catalyst activity. With the decrease of Si mole fraction in gel of SAPO-18 from 0.33 to 0.07, the propylene yield of 1-butene cracking increased from 31.1% to 38.8%, the propane yield decreased from 12.7% to 7.2%, and feed conversion slightly changed, which illustrated that the higher the Si content in SAPO-18 catalyst, the lower the selectivity of light olefins is. The hydrogen transfer side reaction during 1-butene cracking was suppressed to some extent when the Si content in SAPO-18 catalyst decreased and the production of propane from propylene was decreased, but an appropriate Si content is necessary for catalytic activity. Thus, in addition to the structure, suitable acidity of catalyst also played a very important role in controlling the hydrogen transfer reactions effectively for optimal catalytic performance.

Fig.4 Conversion(x), yield (y) and selectivity (s) of 1-butene catalytic cracking over SAPO-18m(Catalyst)=1.0 g; T=500℃; p=0.1 MPa; MHSV=3.5 h-1; TOS=3 min x; s (Propylene-Ethylene); y (Propylene); y (Propane); y (C5+)

As shown in Table 1, when then(SiO2)/n(Al2O3) of ZSM-5 were increased from 60 to 300, the yield of propylene increased from 10.2% to 18.6% and the yield of propane decreased from 27.1% to 13.0% at a TOS of 3 min. Xu et al.[22]and Lin et al.[16]observed that with the increase ofn(SiO2)/n(Al2O3), the total amount of acid sites of ZSM-5 decreased and the selectivity to propylene was enhanced. These results were in agreement with the results about the effect of acidity of SAPO-18 on 1-butene cracking reaction. However, in order to improve the light olefins selectivity over ZSM-5, a lot of modified work should be done.

3 Conclusion

The acidity of SAPO-18 by changing the Si content in the gel was successfully changed. SAPO-18 (n(Si)/n(Si+Al+P)=0.036) with large cages, small windows and suitable acidity compared to ZSM-5 presents excellent selectivity towards propylene at the initial reaction period of 1-butene cracking. The acidity and the framework topology of molecular sieves are key factors to affect the catalytic performance of zeolites in the 1-butene cracking reaction.

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Synthesis of SAPO-18 for High Propylene Selectivity in 1-Butene Catalytic Cracking

HU Yunfeng1, LÜ Zhongyuan1, SU Xunming1， LI Zhaofei2, YAN Lijun2

(1.DepartmentofChemistry&ChemicalEngineering,NortheastPetroleumUniversity,Daqing163318,China;2.PetrochemicalResearchInstitute,CNPC,Beijing100195,China)

SAPO-18 samples with differentn(Si)/n(Si+Al+P) were synthesized and characterized by XRD, SEM, XRF, TG and NH3-TPD techniques and used to catalyze 1-butene cracking in order to explore how to maximize propylene selectivity by changing the Si content in the gel mixture. The selectivity to light olefins obtained in 1-butene cracking catalyzed by SAPO-18 withn(Si)/n(Si+Al+P) of 0.036 was higher than that catalyzed by ZSM-5. In the initial reaction (TOS=3 min), the butene conversion rate could reach 71.6% and the propylene selectivity could reach 54.2%. The catalytic effect of SAPO-18 was mainly due to unique eight-membered ring structure and appropriate acidity.

SAPO-18; 1-butene cracking; propylene; Si content

2016-04-14

中国石油科技创新基金研究项目(2012D-5006-0403)和中国石油天然气集团公司科技开发项目(2014A-2610)资助

胡云峰，男,教授,博士,从事分子筛催化和Cu基催化剂方面的研究；Tel:0459-6503731；E-mail:huyfdqlh@aliyun.com

1001-8719(2017)01-0150-07

O643

A

10.3969/j.issn.1001-8719.2017.01.021