Synthesis of IM-5 Zeolite by Template Pre-Reaction

Sun Min

(State Key Laboratory of Catalytic Materials and Reaction Engineering, Research Institute of Petroleum Processing, SINOPEC, Beijing 100083)

Abstract: IM-5 zeolite was synthesized by hydrothermal crystallization method with 1,5-bis (N-methylpyrrolidinium)pentane bromide using the precursors N-methylpyrrolidine and 1,5-dibromopentane as raw materials of template after pre-reaction, and then aluminum, alkali, water and silicon sources were added into the reaction system. The effects of the proportion of precursors and other materials and the reaction conditions on the crystallinity, crystal morphology and pore structure of the synthesized zeolites were systematically investigated, which provided the basic data for industrial production. The physical properties of the synthesized samples were analyzed by XRD, SEM, and N2 adsorptiondesorption techniques, and the catalytic performance of the samples was evaluated. The results show that IM-5 zeolite can be synthesized effectively by using the template pre-reaction method, and its physical properties and catalytic activity in catalytic alkylation of benzene and methanol are comparable to those of industrial samples synthesized by traditional method.

Key words: IM-5 zeolite; template; pre-reaction; catalytic performance

1 Introduction

IM-5 is a new type of silica-alumina zeolite. It has a distinctive pore structure, which is composed of unique two-dimensional 10-membered rings channel system and some three-dimensional characteristic cavities. The twodimensional pore structure retains its diffusion-limiting effect in catalytic reactions, and the presence of threedimensional characteristic cavities of IM-5 can increase the contact probability between reactants and active centers to play a positive role in promoting the diffusion of products and reducing the carbon deposits. In addition,IM-5 has better thermal and hydrothermal stability than ZSM-5[1-5]. The IM-5 zeolite catalyst exhibits excellent activity, shape selectivity and stability in the catalytic cracking of hydrocarbons, light aromatics alkylation,catalytic reduction of nitrogen oxides and other reactions,and has shown excellent application prospects in the petrochemical field[6-15].

IM-5 was first successfully synthesized using 1,5-bis(methylpyrrolidinium) pentane bromide (MPPB)as a template by Benazzi in 1997[16]. However, the highcost method for synthesizing IM-5 restricts its wide application. Therefore, there is an urgent need to develop a low-cost method to solve the problem in order to expand its application. In the cost composition of IM-5,the existing method of synthesizing template MPPB is expensive, which is one of the main reasons for the high cost of IM-5. Its synthesis generally requires the reaction ofN-methylpyrrolidine (NMP) and 1,5-dibromopentane(DBP), the precursors for the synthesis of template, in a certain proportion with a suitable solvent, and then after undergoing tedious separation and purification operations,the pure MPPB is finally obtained[9,17]. Synthesis of IM-5 by this method will bring on a lot of energy and material consumption, and MPPB is easy to absorb moisture and not easy to store. Therefore, the related improvement studies were carried out to solve these problems. Yang Weiya, et al.[18]introduced ultrasonic dispersion method into the reaction system for synthesizing the template agent, which could reduce the amount of solvent required for the reaction. Ji Xiangfei, et al.[19]directly mixed the raw materials for synthesis of template agent with other raw materials in a certain proportion to synthesize zeolite, without the need of synthesizing template agent,and synthesized IM-5 by an in-situ template method.However, the in-situ template method has some problems,such as low utilization rate of NMP and DBP and the need to recycle the unreacted template materials. It can be seen that this method is difficult to reduce the amount of NMP and DBP, and the need for a separate recovery device will increase the synthesis cost. Up to now, it is impossible to synthesize high quality IM-5 zeolite with high efficiency and low cost. According to the above enlightenment, in order to improve the effective utilization rate of template precursors in the preparation process, we would try to adopt the template pre-reaction method which directly uses NMP, DBP and reaction medium at an appropriate ratio for a certain prereaction time as the template requires for the synthesis of IM-5. However, up to now, the synthesis of IM-5 by this template pre-reaction method has not been reported yet. For the first time, an 1.0-L PARR crystallization reactor is used to simulate the feeding method used in the industrial production of zeolite. First, NMP, DBP and the reaction medium are added into the reactor, heated to a certain temperature under stirring, and are subjected to pre-reaction for a certain time in the sealed reactor. Then the aluminum source, inorganic alkali, deionized water and silicon source are directly added into the reactor for crystallization reaction to synthesize IM-5. Herein, the effects of the raw material ratio and the crystallization reaction conditions on the physical properties of IM-5 zeolite are systematically investigated, and the catalytic activity of IM-5 zeolite in the alkylation reaction of benzene and methanol is evaluated, which can provide the basic data support for industrial production.

2 Experimental

2.1 Materials

Industrial grade raw materials such as solid silica gel, NaAlO2, IM-5 zeolite (RZM-2) were purchased from the Hunan Jianchang Petrochemical Co., Ltd.N-methylpyrrolidine and 1,5-dibromopentane were purchased from the Tokyo Chemical Industry Co., Ltd.Benzene, methanol, and sodium hydroxide were purchased from the Sinopharm Chemical Reagent Co., Ltd.

2.2 Synthesis of IM-5 by template pre-reaction

IM-5 zeolite was synthesized according to the following molar ratio of the materials, viz.:n(SiO2) :n(Al2O3) :n(Na2O) :n(DBP) :n(NMP) :n(H2O)=100 : (0.5—10) :(15—30) : (6—16) : (10—45) : (500—3000). To be specific, a certain amount of NMP, DBP and solvent were first added into the crystallization reactor, and the reaction was conducted under autogenous pressure at 40—80 ℃for 6—48 h under continuous stirring. Then a certain amount of NaAlO2, NaOH, deionized water, and solid silica gel was added, and the autogenously-pressurized hydrothermal crystallization was carried out for 4—7 days at 130—180 ℃ under stirring. After crystallization, the reactor was cooled down to room temperature, the solid products were recovered by filtration, washed repeatedly with deionized water, and then dried overnight at 110 ℃.As-synthesized zeolites were calcined at 550 ℃ for 5 h to remove the template occluded. The calcined samples were then refluxed twice in NH4NO3solution for 4 h followed by calcination at 550 ℃ for 8 h to ensure that they were completely in their proton form.

2.3 Characterization of IM-5

The crystal structure of zeolite was determined by EMPYREAN X-ray powder diffractometry (XRD, made by the Palytical Company in Holland). The diffractometer was provided with Cu target, Kα radiation, and Ni filter,operating under conditions covering a tube voltage of 40 kV, a tube current of 40 mA, and a scanning range of 2θangle of 5° — 50°. The pore properties of the zeolite were determined by an ASAP 2010 static nitrogen adsorption instrument from Micromeritics, USA. The specific surface area was calculated by the BET method and the pore volume was calculated by the t-plot method. The morphology characteristics of the zeolite were characterized by a Quanta 200F scanning electron microscope (SEM) produced by FEI Company, USA. The test conditions were as follows:the acceleration voltage of the SEM was 20 kV, and the magnification was 1 000―30 000.

2.4 Catalytic performance evaluation of IM-5

The catalytic performance of the synthesized samples was evaluated by alkylation of benzene and methanol, and was carried out in a pure hydrocarbon high-pressure miniature reactor. The evaluation conditions covered: an(benzene)/n(methanol) ratio of 1:1, a reaction temperature of 440 ℃, a reaction pressure of 0.28 MPa, a mass hourly space velocity of 4 h-1, and an(nitrogen)/n(benzene and methanol) ratio of 10 with nitrogen serving as the carrier gas, and the composition of the product was analyzed by online gas chromatography.

3 Results and Discussion

3.1 Effect of template precursors ratio on the synthesis of IM-5 zeolite

In the synthesis of template MPPB, its precursors DBP and excessive NMP firstly enter into reaction under appropriate conditions, and then pure MPPB is obtained through separation and purification operation[17]. In this study, the template pre-reaction method was used to synthesize IM-5 zeolite. Because the products after the pre-reaction of precursors do not go through separation and purification operation and directly participate in the crystallization system of IM-5 zeolite, the unreacted precursors or side reaction products under specific reaction conditions may have an impact on the synthesis of IM-5. The effects of the proportions of precursors and other materials in the crystallization system on the zeolite crystallinity were investigated, with the results shown in Table 1. It can be seen that the relative crystallinity of zeolite changes with the change of the NMP/DBP ratio and the DBP/silica gel ratio. Compared with the sample D-2, the crystallinity of the sample D-1 increased with the increase of NMP feeding amount when the DBP feeding amount was the same. Compared with the sample D-2,the crystallinity of the sample D-3 increased with the increase of DBP feeding amount under the same NMP feeding amount. In the experimental range, increasing the amount of any precursor may increase the amount of the generated template, resulting in an increase in the crystallinity of the zeolite.

Table 1 Effect of material ratio on zeolite crystallinity

The XRD patterns of the products are shown in Figure 1.It can be seen from Figure 1 that the sample D-2 is a pure phase IM-5 zeolite. The characteristic peaks of mordenite at 2θof 6.5°, 8.5°, 9.7°, 13.5°, and 26.5° were found in the sample D-1. The characteristic peaks of α-quartz at 2θof 20.86° and 26.65° were observed in the samples D-3.This indicates that pure IM-5 zeolite can be synthesized whenn(NMP)/n(DBP) ratio is 2.5, while both D-1 having a higher value and D-3 having a lower value can produce hetero-crystals. Compared with samples D-2 and D-1,although the crystallinity of the zeolite increased due to the increased generation of template agent, the amount of unreacted NMP also increased at the same time. NMP,an organic amine, may play the role of template agent in the crystallization system and can lead to the generation of other zeolites. According to the XRD pattern of D-1, a small amount of mordenite zeolite might have generated.Similarly, the unreacted DBP could lead to the formation of α-quartz as compared to D-2 and D-3. Therefore,the proper ratio of DBP to NMP and other materials in the crystallization system is an important factor for the synthesis of pure phase IM-5.

Figure 1 XRD patterns of samples synthesized with different proportions of template precursors

3.2 Effect of 1,5-dibromopentane on the synthesis of IM-5

In the conventional synthesis of MPPB, excess NMP reacts upon DBP with the hope that the DBP will react completely, and the excess NMP is easy to be separated in the subsequent separation and purification operation.However, if the template pre-reaction method still adopts the traditional method for synthesis of MPPB with excessive NMP, the price of which is about 2 times that of DBP, the direct introduction of excessive NMP into the zeolite synthesis system will inevitably lead to the rise of production cost. At the same time, excessive NMP will lead to the generation of hetero-crystals such as mordenite zeolite, as well as the sample D-1. Being different from the traditional method, this study focuses on improving the NMP conversion rate and avoiding the formation of mordenite hetero-crystals by increasing the DBP dosage under the condition of using fixed amount of NMP and other materials. The effect of template precursor DBP on the synthesis of zeolite is shown in Table 2. It can be seen from Table 2 that the relative crystallinity of the product improves with an increasingn(DBP)/n(SiO2) ratio. This is true, because with the increase ofn(DBP)/n(SiO2) ratio, the NMP and DBP pre-reaction can produce more MPPB, and the effective template dose in the reaction system increased,thus leading to the increase of crystallinity of the product.The XRD patterns of the products are shown in Figure 2. It can be seen from Figure 2 that the sample e has a highest crystallinity, with the characteristic peaks of α-quartz identified at 2θof 20.86° and 26.65°. Other samples are pure phase IM-5 zeolite. This indicates that the relative crystallinity of the zeolite can be improved by increasing the dosage of DBP featuring a low cost. It also indicates that there is a proper amount of DBP in the crystallization reaction system, which will not produce hetero-crystals,and the pure phase IM-5 can be synthesized. However,when the amount of NMP is too large, it is easy to lead to the formation of α-quartz, and the appropriate DBP amount is about 0.15. It can be concluded that appropriately increasing the dosage of DBP with low cost in the template precursor reaction can increase the effective utilization rate of NMP with high cost, improve the relative crystallinity of zeolite, and reduce the synthesis cost of zeolite. Compared with the industrial IM-5 zeolite (RZM-2) with a relative crystallinity of 95%, the relative crystallinity of sample d is 93%, which is close to that of industrial sample.

Table 2 Effect of 1,5-dibromopentane on synthesis of IM-5

The SEM morphology of RZM-2 and the sample d is shown in Figure 3. It can be seen from Figure 3 that both the sample d and RZM-2 have good crystal morphology and regularity.The specific surface area and pore volume data of the sample d and the industrial sample RZM-2 are shown in Table 3.It can be seen from Table 3 that the specific surface area and pore volume data of the two samples are at the same level. Specifically, the specific surface area of the sample d is slightly lower than that of industrial sample. Pore volume of the sample d is slightly higher than the industrial sample,and the specific surface area and micropore volume of the sample d are similar to those of the industrial sample. In conclusion, the specific surface area, pore volume, and crystal morphology of the synthesized IM-5 zeolite have reached the level of industrial samples.

Figure 2 XRD patterns of the products

Figure 3 SEM morphology of the sample d and RZM-2

Table 3 Pore structure data of sample d and RZM-2

3.3 Effect of water-silicon ratio on the synthesis of IM-5

The molar ratio of water to silicon is an important technical index for industrial production of zeolites.The lower the water/silicon molar ratio, the higher the yield of single reactor would be. Reducing the water/silicon molar ratio is an important means to improve the production efficiency. During the actual production of zeolite, the ratio of reaction materials should be adjusted when the molar ratio of water to silicon is reduced. The influence of water/silicon molar ratio on the synthesis of IM-5 zeolite is shown in Table 4. The XRD spectra of the corresponding products are shown in Figure 4. It can be seen from Table 4 and Figure 4 that pure phase IM-5 zeolite with high crystallinity can be synthesized at a water/silicon molar ratio range of 10—20, and the product has the lowest crystallinity at a lowest water/silicon molar ratio of 7, which is accompanied by the formation of heterocrystalline phases of ZSM-12 and α-quartz. It can also be seen from Table 4 and Figure 4 that the crystallinity of IM-5 decreases with a decreasing water/silicon molar ratio and an increasingn(DBP)/n(H2O) ratio. The molar ratio of DBP to H2O represents the concentration of template to some extent,and the crystallinity decreases with the increase of this concentration. On the one hand, the decrease of the water/silicon molar ratio may cause the crystallization system to become sticky, which would affect the mass transfer of materials. On the other hand, it may be caused by the accelerated decomposition of the template in the crystallization system under conditions of high temperature and high alkalinity.

Table 4 Effect of water/silicon molar ratio on the synthesis of IM-5

3.4 Effect of crystallization time on the synthesis of IM-5

Crystallization time is another important technicalindicator for industrial production of zeolites. In order to improve the production efficiency, in the course of zeolite production, a higher temperature and a shorter time crystallization scheme are generally adopted. In order to meet the needs of industrial production, the effect of crystallization time on the synthesis of zeolites was investigated under a high crystallization temperature of 175 ℃. Figure 5 shows the XRD patterns of zeolites synthesized at different crystallization times. In Figure 5 the crystallinity of samples obtained at a crystallization time of 84 h, 96 h, 120 h, and 144 h is 62%, 94%, 97%,and 97%, respectively. Figure 5 shows the effect of reaction time on crystallinity: the sample obtained after 72 h of crystallization time has no obvious IM-5 zeolite diffraction peak, and the sample obtained at a reaction time of 84 h shows the IM-5 zeolite diffraction peak.This indicates that the synthesis induction period of IM-5 zeolite is longer. With the increase of crystallization time,the crystallinity of IM-5 zeolite increased gradually. The results showed that IM-5 zeolite underwent a process of gradual formation and growth with an increasing reaction time. The crystallinity of the sample after 120 h of crystallization reached 97%, and then the crystallinity did not increase with a further increase of reaction time. The optimum crystallization time is 120 — 144 h.

Figure 4 XRD patterns of the products

Figure 5 XRD patterns of IM-5 synthesized at different crystallization time

3.5 Evaluation of catalytic performance of IM-5

The catalytic performance of IM-5 zeolite synthesized by the template pre-reaction method and the industrial sample (RZM-2) assessed in benzene and methanol alkylation evaluation reaction is shown in Figure 6. It can be seen from Figure 6 that when the physical properties of the zeolites are similar,the benzene conversion over the zeolite obtained by the template pre-reaction method is similar to that of the industrial sample zeolite, indicating that the IM-5 zeolite synthesized by the template pre-reaction method has the same catalytic performance as the industrial sample zeolite. On the other hand, RZM-2 was synthesized from pure MPPB template agent, and the physical and chemical properties of IM-5 synthesized by the template pre-reaction method were similar to those of IM-5 zeolite synthesized by pure template agent,indicating that the template pre-reaction method could retain the physical and chemical properties of IM-5 while reducing the material and energy consumption in the synthesis process.Thus, the synthesis cost of IM-5 is reduced.

Figure 6 Catalytic properties of RZM-2 and IM-5 synthesized by template pre-reaction are evaluated in the alkylation of benzene and methanol

4 Conclusions

(1) The IM-5 zeolite was synthesized by the template prereaction method, which effectively solved the problem of high material consumption and energy consumption of template agent in the conventional method for synthesizing zeolite, and reduced the production cost.

(2) The quality of IM-5 zeolite synthesized by the template pre-reaction method is satisfactory, and its physical properties are similar to those of industrial IM-5 synthesized by the conventional method.

(3) The IM-5 zeolite synthesized by the template pre-reaction method showed high catalytic activity, and its catalytic performance was comparable to that of industrial IM-5 evaluated in the alkylation reaction of benzene and methanol.

Acknowledgements:This study was completed under the guidance and help of Professor Shu Xingtian, Member of Chinese Academy of Engineering, and Professor Mu Xuhong,Professor Luo Yibin, Dr. Shi Chunfeng, et al., from the SINOPEC Research Institute of Petroleum Processing. I would like to express my thanks.