Alkylation Activity of Benzene with Syngas over Cu-based Catalysts

Zhao Xuebin; Zeng Feng; Zhao Bin; Gu Haohui

(SINOPEC Research Institute of Petroleum Processing, Beijing 10083)

Alkylation Activity of Benzene with Syngas over Cu-based Catalysts

Zhao Xuebin; Zeng Feng; Zhao Bin; Gu Haohui

(SINOPEC Research Institute of Petroleum Processing, Beijing 10083)

A series of Cu-based catalysts were developed for alkylation of benzene with syngas. The catalyst samples were prepared by the impregnation method, and were characterized by XRD, XRF, NH3-TPD, and TEM and evaluated in a fixed bed reactor. The optimized reaction temperature of Cu/Al2O3/ZSM-5 catalyst was 350 ℃, while higher contents of copper were conducive to alkylation of benzene with syngas. The new medium strength acid centers in the catalyst created by Cu were beneficial to alkylation. Hydrogenation reaction of CO was executed on the metal centers without dissociation, Dimethyl ether (DME) was the major intermediate over Cu-based catalysts. Higher selectivity of methylation and lower selectivity of heavy aromatics were confirmed after the second metal (Zn, Mn, or V) was added to the copper catalyst. Cu was partly covered by Zn in the Cu-Zn/Al2O3/ZSM-5 catalyst leading to low dispersion and low activity of copper. Cu-Mn/ Al2O3/ZSM-5 catalyst possessed the best yield of methylation product. Cu-Mn composite oxides were probably formed in fresh catalyst, which blocked the sintering of Cu in the reaction process. The loading of Cu decreased dramatically after the introduction of V, while causing an increase of the amount of medium strength acid centers at the same time. V prevented the sintering of copper particles during the reducing process and had a promoting effect on the activity of Cu.

copper; alkylation of benzene; syngas

1 Introduction

Benzene, toluene and xylene (BTX) are among those most important basic organic compounds, while para-xylene (PX) is the essential raw material for producing polymeric substances such as PET(polyethylene terephthalate), PBT(polybutylene terephthalate) and other chemical compounds[1-2]. Catalytic reforming and steam cracking of naphtha are the major sources of BTX aromatics industrially, and nevertheless the proportions of BTX in products from these processes are almost thermodynamically balanced[3]. So the studies on interconversion among BTX homologues are required. Alkylation, disproportionation, and trans-alkylation are the common methods for manufacturing the target aromatics[4].

There are several regular reactants for the alkylation of toluene, such as methanol, ethylene and alkyl halides. Because benzene has higher stability than toluene, the alkylation of benzene is more difficult. There are only few people focusing their attention on the alkylation of benzene with syngas[5-6], especially in China. The alkylation of benzene with syngas is a viable method for improving the yields of toluene and xylene. Using syngas, which is cheap and easily available, as the alkylation agent can reduce the production cost tremendously.

As shown below, the reaction of alkylation of benzene with syngas is highly exothermic. The Gibbs free energy of the reaction at 298.15 K is -99 kJ/mol, denoting that the reaction has a strong trend towards the target products.

The alkylation reaction needs a bifunctional catalyst to make the metal centers implement the hydrogenation activation of CO and the acidic centers execute the alkylation of benzene. Copper has been proved as an excellent metal center, while the HZSM-5 zeolite possesses a better catalytic activity for the alkylation of benzene[7]. In this paper, the catalytic activity of Cu-based catalysts during alkylation of benzene with syngas was systematically investigated.

2 Experimental

2.1 Preparation and characterization of catalyst

A series of catalysts with different contents of Cu were prepared by the excess impregnation method. The support was a mixture of γ-Al2O3and HZSM-5 zeolite. After being shaped and dried at 120 ℃ for 4 h and then calcined at 520 ℃ for 4 h, the support samples were impregnated with an aqueous solution of copper nitrate (analytically pure grade) for 6 h at room temperature, then the aqueous solution was poured out into another container. The catalyst was dried at 120 ℃ for 4 h and then calcined in air at 520 ℃ for 4 h. The catalyst was denoted as Cu/Al2O3/ ZSM-5. As for the Cu-M/Al2O3/ZSM-5 (M= Zn, or Mn, or V) catalysts, the atomic ratio of Cu to M in the aqueous nitrate solution was 7:3, with other treating steps being the same as mentioned above.

XRD was used to determine the crystalline phase and the average metal particle size of the catalyst samples by a Siemens D5005 diffractometer. The dispersion of the catalysts was viewed by a Tecnai G2 F20 transmission electron microscope. The composition of the catalyst was identified by a Rigaku’s 3013 X-ray fluorescence spectrometer.

The amount of acids in catalysts was estimated by means of the temperature-programmed desorption of NH3. The catalyst sample was placed in a quartz tube, which was at first heated to 500 ℃ in a N2atmosphere and kept for 30 min to remove the adsorbed molecules on the catalyst surface, and then was cooled down to 120 ℃. After the adsorption of NH3in a 10% NH3-He mixture at a flow rate of 25 cm3/min for 30 min, the NH3-TPD was carried out at a temperature increase rate of 10 ℃/min up to 500 ℃. The NH3-TPD curve was recorded by a TCD detector.

2.2 Evaluation of catalyst activity

The catalytic activity was examined in a fixed bed reactor after the catalyst samples were reduced at 220 ℃ for 16 h in a 3% H2- N2gas mixture under atmospheric pressure. The reactor was a 20 mm × 3 mm stainless steel pipe, 800 mm in length, heated in three sections by a vertical furnace. The feed oil was introduced by a piston pump, and the reaction gas was supplied to the catalyst sample through mass flow controllers. The molar ratio of H2/CO/ B (benzene) was 4:2:1 fed at a constant space velocity of 1.25 h-1with a catalyst loading of 12.0 g. After having run for 2 h and having discharged the unstable products, sampling was performed at the bottom of the gas-liquid separation tank after the reaction proceeded for 4 hours. The liquid products were analyzed by an Agilent gas chromatograph equipped with a flame ionization detector (FID). The conversion of benzene (CB), the selectivity of toluene (ST) and xylene (SX), the selectivity of para-xylene among xylene isomers (PX/X), the selectivity of heavy aromatics (SHA), the selectivity of methylation (SM) and the yield of methylation products (YM) were defined as follows:

Figure 1 Flow chart of experimental setup for gas–solid catalytic reaction

3 Results and Discussion

3.1 Alkylation of benzene with syngas over Cu/Al2O3/ ZSM-5 catalyst

The Cu/Al2O3/ZSM-5 catalyst samples containing 1.5%, 4.3% and 11.0% of Cu, respectively, were synthesized by the impregnation method and characterized by XRF, XRD, NH3-TPD and TEM. The catalytic activity of catalyst for alkylation of benzene with syngas under various conditions was evaluated.

Figure 2 The catalytic activity of catalyst samples with different copper contents at different temperatures▲—11.0%; ●—4.3%;■—1.5%

As shown in Figure 2, the conversion of benzene was improved with the increase of copper content in the catalyst and the 11.0%-Cu/Al2O3/ZSM-5 catalyst achieved the highest benzene conversion rate at every reaction temperature. As for the 11.0%-Cu/Al2O3/ZSM-5 catalyst, the benzene conversion soared at first and then declined dramatically as the reaction temperature increased beyond the optimal reaction temperature of 350 ℃. Table 1 summarizes the detailed results for evaluation of catalyst samples with different copper contents at 350 ℃ and 1.5 MPa. The selectivity of methylation and toluene increased at first and then declined with the increase of copper content, while on the other hand the selectivity of xylenes and p-xylene increased progressively, probably caused by the pore blocking of high content of copper species.

Figure 3 shows the NH3-TPD profiles of the support and Cu/Al2O3/ZSM-5 catalyst. Compared to the support which had typical strong acid centers and weak acid centers, the introduction of copper created new medium strength acid centers. The medium strength acid centers might come from the hydroxyl groups which were affected by the polarization effect of copper ions in the zeolite framework. With an increasing copper content, the amount of medium acid centers also increased, which might be probably one of the reasons leading to the improvement in benzene conversion. Meanwhile with respect to the 11.0%-Cu/Al2O3/ZSM-5 catalyst the increase in the amount of strong acid centers might cause the improvement in heavy aromatics yield.

The pyridine adsorption Fourier-transform infrared spectra at 350 ℃ illustrated the surface acidity of different copper species in Cu/Al2O3/ZSM-5 catalyst. The Br nsted acid sites, which were indicated at 1 540 cm-1, decreased after the loading of copper, while the Lewis acid sites, which were indicated at 1 450 cm-1, increased on contrary to the alkylation reaction outcome because alkylationreaction was mainly conducted at the Br nsted acid sites. The quantity of Br nsted acid sites also reduced with the increase of copper content in the catalyst. This occurred probably because the copper species had partly substituted for the protons of hydroxyl group.

Table 1 The results of copper catalysts evaluation at 350℃& 1.5 MPa

Figure 3 NH3-TPD curves of support and Cu/Al2O3/ZSM-5 catalyst with different Cu contents— —Support;— —1.5%;— —4.3%;— —11.0%

The XRD patterns of 11.0%-Cu/Al2O3/ZSM-5 catalyst showed that copper species existed in the form of copper oxide in the fresh catalyst, and in the form of zero-valency copper in the reduced catalyst and the discharged catalyst after reaction. The average particle size calculated by the Scherrer formula was 47.6 nm in the fresh catalyst, 94.9 nm in the reduced catalyst and 101.1 nm in the discharged catalyst after reaction. This outcome was probably related to the exothermic process during the reducing reaction and the alkylation reaction. The copper particles gathered together by heat especially during the reducing process and the limit of copper particle size was about 100 nm under this condition. It can be seen from the TEM images that in contrast to fresh catalyst there was an apparent sintering in the catalyst discharged after reaction.

3.2 The mechanism of alkylation of benzene with syngas over Cu/Al2O3/ZSM-5 catalyst

There was not an explicit explanation for the reaction mechanism in previous articles because of the complexity of the reaction. As regards the reaction mechanism, there may be two routes: (1) the dissociation mechanism of CO which deems that CHxmay be the reaction intermediates, and (2) the non-dissociation mechanism considering that CO is activated without removal of oxygen atoms. The main discrepancy between the two theories is the activation mode of CO, so the detection of reaction intermediates is the key to verify the reaction mechanism.

According to the first view, carbon monoxide must be strongly adsorbed on the catalyst surface, and the adsorbed C—O bonds are easily dissociated to form the adsorbed M—C and M—O bonds, which need a high CO adsorption energy on the catalyst. However being anexcellent methanol synthesis catalyst, copper cannot adsorb CO strongly enough. Therefore the non-dissociation mechanism might be the major CO activation pattern.

Figure 4 Py-IR spectra of Cu/Al2O3/ZSM-5 catalyst with different Cu contents at 350℃

Figure 5 XRD patterns of fresh, reduced and reacted 11.0% Cu/Al2O3/ZSM-5 catalyst▲—Cu; ■—CuO

Figure 6 The TEM images of fresh catalyst and the Cu/ Al2O3/ZSM-5 catalyst after reaction

In order to verify the non-dissociation mechanism, the two parts of bifunctional catalyst were prepared and evaluated respectively. Cu/Al2O3catalyst was prepared by the above-mentioned excess impregnation method. The Cu/Al2O3and HZSM-5 catalysts were placed in different reaction beds which were marked as Cu/Al2O3++ HZSM-5. The Cu/Al2O3+HZSM-5 catalyst represented a mechanical mixture of Cu/Al2O3and HZSM-5. Then the activity of Cu/Al2O3, HZSM-5, Cu/Al2O3++HZSM-5, Cu/ Al2O3+HZSM-5 and Cu/Al2O3/ZSM-5 catalysts under the same conditions was evaluated separately. Table 2 shows the main composition of the liquid products characterized by GC-MS.

Table 2 The content of key components of liquid products after 4 h of reaction at 350℃and 1.5 MPa

It can be seen from Table 2 that dimethyl ether (DME) was the key reaction intermediate. DME was first generated in the Cu/Al2O3metal centers, and then was shifted to the zeolite acid centers to take part in the alkylation reaction with benzene. Furthermore, the closer distance between the metal centers and the acid centers resulted in the higher activity of the catalyst. Therefore alkylation of benzene with syngas could be expressed as a combination of DME synthesis from syngas (reaction (1)) and alkylation of benzene with DME (reaction (2)):

By combining reactions (1) and (2), the overall alkylation reaction of benzene with syngas can be represented as follows:

It is well known that alkylation of benzene with DME and DME synthesis reaction proceed over the solid-acid catalyst and over the Cu/Al2O3catalyst, respectively, and the mechanisms of both reactions have been studied by many researchers[8-10]. On the basis of previous research the probable detailed mechanism of alkylation of benzene with syngas is proposed as follows:

3.3 Effect of a second metal added to Cu-based catalystIn order to improve the activity and stability of copper catalyst, a second metal, viz. Zn, or Mn, or V, was added to the copper catalyst system, respectively. The second metal had little effect on copper loading except V, which itself also had a lowest loading in the catalyst. Zinc had a higher loading than Mn to form the highest total metal loadings in the catalyst.

Figure 7 The probable mechanism of alkylation of benzene with syngas on Cu/Al2O3/ZSM-5 catalyst

Figure 8 The loadings of Cu-M/Al2O3/ZSM-5Catalyst(M=Zn, Mn, V)■—Cu; ■—M; ■—Total

Table 3 The average sizes of Cu particle calculated by XRDnm

Table 3 lists the average size of Cu particles calculated by XRD patterns. Compared to the Cu catalyst, Zn had no effect on the average size of Cu particles. The average size of Cu particles in the reduced Cu-Mn catalyst was smaller than that of reduced Cu catalyst and there were little changes in Cu particle size between the reduced catalyst and the Cu-Mn catalyst after reaction. Therefore Mn had the ability to deter the copper sintering, in particular in the course of reaction. As regards the Cu-V catalyst, vanadium could also block the sintering of Cu particles, especially in the course of reduction reaction.

Table 4 shows the results of catalyst evaluation at 350 ℃and 1.5 MPa. A conclusion can be drawn that the second element could improve the selectivity of methylation and reduce the selectivity of heavy aromatics. As for the Cu-Zn/Al2O3/ZSM-5 catalyst, the conversion of benzene was far lower than that of copper catalyst, but it achieved the highest PX/X ratio. The NH3- TPD curves indicated that the amount of total acid centers was less than that of copper catalyst, especially in terms of the weak acid centers and the strong acid centers. Zinc itself had a negative effect on the acid centers of the support, however it had no impact on the acid strength[7]. As a characteristic peak, the medium strength acid centers, which were created only by copper species, showed little changes. Therefore copper ions and zinc ions probably indicated no competitive adsorption on the catalyst support. The reduction of acid centers was caused by Zn species. The XRD spectra demonstrated that there was no zinc species detected in the fresh or reduced catalyst, and the catalyst discharged after reaction. ZnO itself was dispersed well on the support[7],

Table 4 The results for evaluation of Cu-M/Al2O3/ZSM-5 catalysts at 350℃and 1.5 MPa

Figure 9 The XRD patterns of fresh, reduced and recovered after reaction Cu-Zn/Al2O3/ZSM-5 and Cu-Mn/ Al2O3/ZSM-5 catalysts▲—Cu; ■—CuO

which was the major reason indicating that no zinc species were detected. It could be inferred that a part of copper species on catalyst surface were covered by zinc species which prevented the access of gas to copper species. On one hand, because of the low activity of Zn at 350 ℃, the covering of copper species by Zn species led to a low catalytic activity of the Cu-Zn/Al2O3/ZSM-5 catalyst. On the other hand, thanks to the high total metal loadings, excellent PX/X ratio was achieved by virtue of the poles blocking effect.

Mn had no impact on the loadings of copper and the conversion of benzene, however it improved the selectivity of methylation products and decreased the formation of heavy aromatics. As shown in the XRD patterns, copper oxide was not detected in the fresh Cu-Mn/ Al2O3/ZSM-5 catalyst and neither was manganese oxide detected. After having been reduced and having undergone the reaction, copper species were still detected. Mn also had a negative effect on the acidity of the support[7], and, nevertheless, NH3-TPD curves indicated that the addition of Mn caused an increase in the amount of medium strength acid centers and reinforced the acidity strength of acid centers. It can be presumed that Cu and Mn had an interaction in the catalyst, and the copper-manganese composite oxide would probably exist in the fresh catalyst. After the reduction process, copper species in the copper-manganese composite oxide were reduced leading to the change in catalyst acidity.

V metal itself had a low loading on the support and showed no obvious peaks in XRD patterns. It also had little influence on the strength and amount of acid centers[7]. Compared to the Cu/Al2O3/ZSM-5 catalyst, V species had a dramatic influence on the loadings of copper in the Cu-V/Al2O3/ZSM-5 catalyst. The conversion of benzene decreased correspondingly with a reducing copper loadings, but the yield of methylation product per Cu loading was the highest. The peak positions in XRD patterns were the same as the Cu/Al2O3/ZSM-5 catalyst and no vanadium species were detected. Despite the low copper contents, the NH3- TPD curve showed that the amountsof weak and medium acid centers were improved, and new strong acid centers at 520 ℃ were formed. It can be deduced that the interaction between V and Cu resulted in the variation of acid centers and V had a promoting effect on the activity of Cu.

Figure 10 The NH3-TPD curves of Cu/Al2O3/ZSM-5, Cu-Zn/Al2O3/ZSM-5 and Cu-Mn/Al2O3/ZSM-5 catalysts

Figure 11 The XRD patterns and NH3-TPD curves of Cu-V/Al2O3/ZSM-5 catalyst

4 Conclusions

The copper-based catalysts were synthesized and characterized in this study. The catalytic activity was evaluated by means of the alkylation of benzene with syngas and the reaction mechanism was verified. Conclusions drawn from this article are summarized as follows:

1) The optimal reaction temperature was 350 ℃ for the copper catalyst. The activity of copper catalyst improved with the increase of the loading of copper in the catalyst. The introduction of copper into the support could create new medium-strength acid centers which were beneficial to alkylation of benzene.

2) CO was activated in a non-dissociation environment in copper catalyst at 350 ℃ and DME was the key reaction intermediate.

3) Zn, or Mn, or V could improve the selectivity of methylation product and reduce the selectivity of heavy aromatics. Zn can cover a part of copper species on the surface resulting in a decrease of conversion of benzene. Mn probably existed in the form of copper-manganese composite oxide in the fresh catalyst. The reduced copper species in the composite oxide altered the acidity of catalyst. The Cu-Mn/Al2O3/ZSM-5 catalyst showed a best methylation yield. V may also have an interaction with copper which had a promoting effect on the activity of Cu species. It could prevent sintering of copper particles during the reduced process.

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Ethylene Carbonate Ionic Liquid Catalyst Successfully Developed by Liaoyang Petrochemical Company

The Liaoyang Petrochemical Company has successfully developed a novel ionic liquid catalyst for carbonylation of ethylene oxide with carbon dioxide to form ethylene carbonate (EC). This catalyst can achieve an 100 % conversion and a 98% selectivity at low temperature and under low pressure, featuring high catalytic activity, good stability, good adaptability to feedstocks and low production cost. In the course of project study the research team has achieved new breakthroughs in the area of heterogeneous load-type ionic liquid catalysts, and it is expected that a brand-new fixed-bed continuous production technique will be developed to command an internationally leading position, which can completely replace the traditional kettle-type reaction process to have a market competitive edge in terms of reaction efficiency and production cost.

date: 2014-11-13; Accepted date: 2015-01-09.

Zhao Xuebin, Telephone: +86-18810782811; E-mail: zhaoxuebin.ripp@sinopec.com.