Ga-promoted tungstated zirconia catalyst for n-butane
isomerization
Xiao-Rong Chena, Chang-Lin Chena;*, Nan-Ping Xua, Song Hana, and Chung-Yuan Moub
aCollege of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China
bDepartment of Chemistry and Center of Condensed Matter, National Taiwan University, 1 Roosevelt Road, Section 4, Taipei, Taiwan
Received 15 August 2002; accepted 29 October 2002
Ga-promoted tungstated zirconia (GWZ) was prepared by a slurry impregnation method. The textural properties as well as the acidities of the Ga-promoted catalysts were characterized by X-ray powder diffraction (XRD), N2 adsorption, NH3 temperature-programmed
desorption (NH3TPD), microcalorimetry and H2temperature-programmed reduction (H2TPR). The catalytic behavior of GWZ for
n-butane isomerization was studied in the presence of hydrogen. In comparison to tungstated zirconia (WZ), the catalytic activity of the Ga-promoted catalyst was greatly improved. The reason proposed for the higher activity of the Ga-promoted catalysts was that Ga enhances the oxidizing ability of the catalysts.
KEY WORDS: tungstated zirconia; gallium; n-butane isomerization.
1. Introduction
Strong solid acids as catalysts for hydrocarbon isomerization reactions are sought because they have the potential to offer key advantages over liquid acids such as much easier handling and storage, less corrosion and more friendly to the environment [1]. For example, there is a worldwide need for removing aromatics in gasoline because of their toxicity. The loss of octane number as a result of the removal of aromatics could be mitigated by the addition of other high-octane com-pounds such as multi-branched hydrocarbons [2]. Since the discovery of its strong solid acidity by Hino and Arata [3,4], tungstated zirconia (WZ) has attracted much attention as a potential petrochemical process catalyst. The strong acidity and high activity of WZ made it attractive as a catalyst in the isomerization of hydrocarbons. WZ was demonstrated by several authors to be active in the isomerization of normal alkanes such as n-butane [5], n-pentane [6,7] and n-hexane [8,9].
Similar environmental concerns also arise regarding the use of sulfated zirconia (SZ), the well-known solid super-acid first discovered by Tanabe and Hattori [10]. Although high catalytic activity and product selectivity can be achieved in skeletal isomerization reactions, serious con-cerns remain about the long-term stability of zirconia-supported sulfate species in reducing and oxidizing environments that are typical of hydrocarbon reactions and catalyst regeneration. This may lead to the generation of hazardous SOx and H2S [11,12]. This situation raises many doubts about their possible industrial application.
Although WZ is less active than SZ and the catalytic activity of WZ for n-butane isomerization is low, tung-stated zirconia (WZ) has become increasingly important as an alternative to SZ. The stable and inorganic nature of these types of mixed metal oxide catalysts (WO3/ ZrO2) ensures that they are environmentally friendly in both oxidizing and reducing environments, even at high reaction temperature.
Recently, Al and Ga introduced to SZ have shown good activity and selectivity for n-butane isomerization [13–15]. Moreno and Poncelet [14] reported that Ga shows better catalytic performances than Al. Hua and Sommer [16] reported alumina-doped Pt/WOx/ZrO2 catalysts for n-heptane isomerization.
In this paper we are interested in the study of the promotion of WZ with Ga and its potential as a catalyst for the isomerization of butane. Isomerization of n-butane on a series of Ga-promoted tungstated zirconia catalysts was studied in a fixed-bed flow reactor. A large increase in the catalytic activity can be observed on the Ga-promoted WZ catalysts. Pt addition improved further the stability, activity and the selectivity of Ga-promoted WZ in the isomerization of n-butane. The reasons for the improvement in activity and stability of the Ga-promoted catalysts are discussed.
2. Experimental
2.1. Catalyst preparation
Zr(OH)4 was prepared from zirconia nitrate solution by adding dropwise ammonium hydroxide solution up to pH9–10. The precipitated hydrogel was filtered and
1011-372X/03/0200-0177/0 # 2003 Plenum Publishing Corporation * To whom correspondence should be addressed.
washed repeatedly until the filtrate liquid showed a pHof 7. The gel was dried at 105 8C. The dried particles were
impregnated with aqueous ammonium tungstate
(H8N2O4W, Acoros) in order to obtain a W content of 10 wt% in the final catalyst. In the synthesis of Ga-promoted WZ catalyst, the appropriate amount of Ga(NO)3 was added to a WZ slurry. The resultant suspension was refluxed overnight at 120 8C, dried at 110 8C, and then calcined at final temperature in static air for 3 h.
Pt-impregnated GWZ catalyst was prepared by impregnating 1.0GWZ850 (the first digits indicate the Ga weight percent in the final catalyst, the final digits the calcination temperature in 8C) with hydrogen hexa-chloroplatinate (Acoros, 40% Pt) solution overnight. The concentration of the solution was adjusted in order to obtain 0.3% Pt in the final catalyst. Then it was dried at 110 8C and calcined at 450 8C in an air stream for 3 h.
2.2. Catalyst characterization
XRD patterns of the samples were obtained using a Bruker D8 Advance instrument with CuK radiation at 40 kV and 30 mA. BET surface areas of the samples were acquired using a Micromeritics ASAP 2010 auto-matic adsorption instrument using N2as the adsorbent. The temperature-programmed desorption of ammonia (NH3TPD) was carried out using a Micromeritics Auto-Chem 2910 instrument. An amount of 150 mg of sample was pretreated at 500 8C under pure He (50 ml/min) for 1 h and then cooled to 120 8C. After introducing 3% NH3 in He at 120 8C for 0.5 h, the sample was flushed with He for 1.5 h. The TPD profile of NH3was obtained from 120 to 800 8C at a heating rate of 10 8C/min. The desorption process was monitored by a quadruple mass spectrometer (Thermo ONIX, ProLab) connected on-line through a heated capillary interface. The mass number 16 was followed to obtain TPD profiles of NH3 because the mass intensity is relatively strong and the interference from H2O is negligible. Micro-calorimetric studies of the adsorption of NH3 were carried out at 150 8C using a self-assembled differential calorimeter. H2 temperature-programmed reduction (H2 TPR) of samples was performed using a self-assembled instrument equipped with a thermal conduc-tivity detector (TCD). Samples were pretreated in a flow of air at 400 8C for 1 h, cooled to room temperature in N2, and then heated to 800 8C at a rate of 10 8C/min in a gas stream of 5% H2in N2.
2.3. Catalytic experiments
The isomerization of n-butane was performed in a fixed-bed flow reactor. Prior to reaction, the catalyst was pretreated at 450 8C for 3 h under air conditions, and then contacted with flowing hydrogen at the reaction
temperature for 1 h. The following reaction conditions were used: reaction temperature, 300 8C; pressure, 101.3 kPa; catalyst mass, 1.0 g; feed flow rate (at NTP), 3 ml/min of n-butane mixed with 12 ml/min of H2 at n-butane WHSV of 0.47/h. An on-line SP6800A gas chromatograph equipped with a FID was used to analyze the reaction products.
3. Results and discussion 3.1. X-ray diffraction
The XRD patterns of WZ and GWZ were recorded for investigating the effect of calcination temperature on the zirconia crystal structure. The results are presented in figure 1. All samples calcined at 750 8C, 800 8C contained zirconia primarily in the tetragonal phase and no WO3 crystallite was observed for these samples. The emergence of WO3 crystallite (2¼ 23– 258) on the zirconia surface was found with an increase of the calcination temperature. Two small peaks of monoclinic zirconia (2¼ 28.58, 31.58) were observed from WZ850 while WZ900 showed a mixture of a monoclinic phase and a tetragonal phase. When Ga is incorporated, it suppresses the monoclinic zirconia. No monoclinic zirconia was present on the XRD patterns of 1.0GWZ850 and 1.0GWZ900. Huang et al. [17] reported that the tetragonal structure was essential in highly active acid catalysts and a monoclinic phase would result in a material of distinctly lower acidity. This suggested that Ga influenced catalytic activity through the crystallization behavior. It favored the formation of the tetragonal phase and stabilized tetra-gonal zirconia crystallites even at higher calcination temperature. Previously, we have also found that Al suppressed the formation of monoclinic zirconia in sulfated zirconia [18].
3.2. BET surface area
The BET surface areas of all promoted and non-promoted WZ catalysts calcined at 850 8C are around 50–60 m2/g, shown in table 1. Ga addition to WZ had less effect on the surface area of WZ. The surface area of the 1.0 wt% Ga-doped WZ catalysts decreased with increasing calcination temperature.
3.3. Acidity measurements
We used the NH3 TPD technique to compare the acidic characteristic of the same set of WZ850 and 1.0GWZ850 catalysts. Figure 2 shows there is no obvious difference in the NH3 TPD profiles between WZ850 and 1.0GWZ850.
The surface acidities of WZ850 and 1.0GWZ850 were measured by microcalorimetry using ammonia as a basic
probe molecule. The results are shown in figure 3. The differential heat of adsorption decreases with NH3 coverage, indicating a distribution of acid site strengths in the catalysts. The calorimetric data provided parallel results which give the initial adsorption heat of WZ850 and 1.0GWZ850 to be 136 and 135 kJ/mol. The total acid sites of WZ850 and 1.0GWZ850 are 195 and 182 mol/g, respectively.
3.4. H2 TPR of WZ and GWZ
To understand the promotion effect of Ga, we used the H2 TPR technique to compare the characteristics of WZ850 and 1.0GWZ850. As shown in figure 4, the H2TPR profile of WZ850 presents a peak of hydrogen consumption at 505 8C while 1.0GWZ850 shows a peak at 440 8C. Addition of Ga to WZ decreased the
reduction temperature of WZ. The redox properties of WZ had been observed and related to the activation of n-alkane [19,20]. Kuba et al. [19] reported that the redox properties of W6þ in WOx/ZrO2 played a crucial role as a redox initiator in the activation of alkanes. It was suggested that Ga addition improved the redox properties of W6þ. We can infer that GWZ might be bifunctional, exhibiting both acidic and redox proper-ties, and that alkane isomerization might be initiated by oxidation of the alkane. The decreased reduction temperature of GWZ may make n-butane isomerization easier.
3.5. Catalytic activity in n-butane isomerization
Rossi et al. [21] reported that the reaction temperature influences the activity and selectivity of n-butane isomer-ization and reaction at 300 8C shows good performances. The isomerization reaction of n-butane was carried out at 300 8C. The performances of WZ catalysts with different Ga content and GWZ calcined at differential temperature are listed in table 2. We can see that the n-butane initial activity increased with Ga content up to 1.0 wt%, and decreased again as the Ga content was further increased. For 1.0GWZ calcined at different
temperatures, a maximum catalytic performance
occurred of about 37.3% at 850 8C. The catalysts calcined at 750 and 800 8C showed low initial activities. On increasing the calcination temperature to 900 8C, the initial activity decreased. Except for the low selectivity to i-C4of WZ850 catalyst, not much difference was observed in selectivity to i-C4 for Ga-promoted Figure 1. XRD patterns of samples: (1) WZ750; (2) WZ800; (3) WZ850; (4) WZ900; (5) 1.0GWZ750; (6) 1.0GWZ800; (7) 1.0GWZ850; (8) 1.0GWZ900.
T, tetragonal zirconia; M, monoclinic zirconia; W, tetragonal WO3.
Table 1
Surface area of various catalysts Catalyst Calcination temperature (8C) Ga content (wt%) Surface area (m2/g) GWZ 750 1.0 77.6 800 1.0 58.6 850 1.0 58.1 900 1.0 31.0 850 0.5 53.4 850 1.5 57.6 850 2.0 53.0 WZ 850 0 53.3
catalysts. It is concluded that the best conversion is achieved when the catalyst is promoted by 1.0 wt% Ga addition and calcination at 850 8C.
The Ga-promotion effect of n-butane conversion versus time-on-stream (TOS) in the flow reactor is shown in figure 5. The influence of platinum addition was also considered. The conversion over WZ850 was much lower and attained a stable conversion of 3.9% after 180 min; its selectivity was less than 80%. The initial activity of 1.0GWZ850 showed a tremendous
improvement compared with WZ850, reaching 37.3%, slightly decreased with TOS and reached a stable conversion of 23.7% while the initial selectivity of 1.0GWZ850 was less than 80% and attained a stable selectivity of 85.5%. In contrast, Pt/1.0GWZ850 immediately attained a stable conversion of 42% and
selectivity of 94%. Although deactivation of
1.0GWZ850 in the initial stage was observed, the deactivated catalyst 1.0GWZ850 can be completely regenerated in air at 450 8C.
Figure 2. NH3TPD profiles: (1) WZ850; (2) 1.0GWZ850.
4. Conclusions
From the results of this study it can be concluded that the best Ga loading in these catalysts is around 1.0 wt% and calcination should be at 850 8C. The high isomeriza-tion activity of the catalysts promoted by the addiisomeriza-tion of Ga in the presence of H2 can be explained by assuming that Ga suppresses the monoclinic zirconia, and favors and stabilizes tetragonal zirconia. In contrast to WZ, Ga-promoted WZ showed less difference in acidity characteristics, but had lower reduction temperature, which improved the redox properties of W6þ. As a redox initiator, W6þ plays an important role in the oxidation of the alkane. The decreased reduction temperature of GWZ made n-butane isomerization easier.
Acknowledgments
We thank Professor Jianyi Shen from Nanjing University for help in the microcalorimetric study. Financial support from the Educational Department of Jiangsu Province (Project 00KJB530001 to CLC), the Science and Technology Department of Jiangsu Province (Project GB2002017 to CLC), and Key Laboratory of Chemical Engineering and Technology of Jiangsu Province is gratefully acknowledged.
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Table 2
Catalytic performances over various catalysts
Catalyst n-C4conversion (%) Selectivity to i-C4(%)
10 min 60 min 300 min 10 min 60 min 300 min
WZ850 5.1 4.1 3.9 78.9 78.6 78.7 1.0GWZ750 11.9 8.7 7.5 81.8 81.3 81.5 1.0GWZ800 26.2 21.3 16.3 81.5 83.9 85.9 1.0GWZ850 37.3 31.1 23.7 79.2 82.8 85.5 1.0GWZ900 29.4 24.2 18.5 81.1 83.9 85.1 0.5GWZ850 12.8 11.7 10.4 83.9 84.1 84.1 1.5GWZ850 29.1 25.2 20.1 82.4 84.1 84.9 2.0GWZ850 19.5 16.5 13.9 84.5 85.8 85.1
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