Effect of compositions of promoted VPO catalysts on the selective oxidation of n-butane to maleic anhydride

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A PT PA LL E IY DSS CA I A: GENERAL

ELSEVIER

Applied Catalysis A: General 147 (1996) 55-67

Effect of compositions of promoted VPO catalysts on the selective oxidation of n-butane to maleic anhydride Wu-Hsun Cheng Chemical Engineering Department, Chang Gung UniversiD,, Kweishan, Taoyuan, Taiwan Received 15 January 1996; revised 4 June 1996; accepted 5 June 1996

Abstract

A series of vanadium phosphate (VPO) catalysts with different P / V ratios and with or without indium and TEOS additives have been characterized by controlled-environment XRD, ICP, ESCA, TEM, SEM, BET and chemical titration and used for n-butane oxidation to maleic anhydride. The best catalyst contains slightly excess P over V with indium and TEOS promoters. It was found that excess P increases the resistance of catalyst precursors toward oxidation and results in VPO catalysts with a large exposed platelet face of a layer morphology. The promoters reduce the thickness of the platelet and facilitate the oxidation of the precursor which contains disordered VOHPO 4 • 0.5H20. The combination of the promoters and excess P results in a VPO catalyst with appropriate oxidizability and morphology and gives high yields of rnaleic anhydride. Keywords: Vanadium phosphate; Indium; TEOS; n-Butane; Maleic anhydride

1. Introduction Since Mitsubishi Chemical Industries first commercialized a maleic anhydride process based on the oxidation of C a hydrocarbons in 1971, the process has attracted considerable interest from both industrial and academic researchers. Recent studies have largely concentrated on using n-butane as a feedstock [ 1-4]. Vanadium phosphate (VPO) catalysts are unique in this 14 electron oxidation process. The effect of P / V ratio on the activity and selectivity of the catalyst has been studied [5-8] but the issue is still controversial. For example, it was believed that the good VPO catalyst has a P / V ratio of about 1 [5]. The maleic anhydride decomposition rate increases at P / V < 1 which results in reduced selectivity [6]. The activity and selectivity to maleic anhydride also decrease at P / V > 1.05 [6]. 0926-860X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 8 6 0 X ( 9 6 ) 0 0 2 13-X

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W.-H. Cheng /Applied Catalysis A: General 147 (1996) 55-67

Cavani et al. reported that the maleic anhydride selectivity decreased with increasing P / V ratio from 0.95 to 1.16 [8], while Hodnett et al. observed the opposite trend when the P / V ratio was increased from 0.95 to 1.07 [7]. However, most of the fundamental studies used the VPO catalysts without promoters. Patent literature, on the other hand, shows that the use of a slightly excess of phosphorus ( P / V = 1 to 1.2) with one or some of promoters such as In, U, Zn, Sb, Ta, Si, etc. [9] is advantageous. For example, VPO catalysts modified with a small amount of both indium and silica showed significantly improved maleic anhydride yield in n-butane oxidation [9]. Despite the extensive use of promoters in patent literature, fundamental studies on the effect of promoters have been very limited. The method of preparation of the promoted catalysts could lead to the formation of a higher surface area catalyst and thus enhance the catalyst activity [3,10]. Small amounts of some cations such as Ce, Co, Cr, Cu, Ni, Ti and Zr promote the n-butane oxidation reaction likely by the formation of defect sites because of the inclusion of these cations into the (VO)2P207 lattice [11]. The promotional effect of zirconium was also studied by Sant and Varma [12]. The primary role of Zr was attributed to the increase of the surface area or the change of the oxygen mobility. Zinc with Z n / V = 0.11 was shown to increase the conversion of the n-butane oxidation [13]. This was attributed to the increased catalyst oxidation rate (measured as mass increase in air at 500-600°C). The possible role of promoters was recently reviewed by Hutchings [14]. Two structural roles were suggested: (a) to form solid solutions which alter the catalytic activity, and (b) to decrease the formation of deleterious phases. Further detailed studies on the role of promoters are required as pointed out by Hutchings [10]. The XRD pattern of many VPO catalyst precursors corresponds to V O H P O 4 • 0 . 5 H 2 0 which transforms to (VO)2P207 via a topotactic transformation during the pretreatment [15,16]. V O - (HzPO4) 2 has also been identified as a phase in some precursors with a P / V ratio greater than 1 [17]. This phase transforms to VO(PO3) 2 on calcination [18,19]. Surface enrichment of phosphorus was found on the active vanadyl pyrophosphate [20,21]. This large amount of surface phosphorus is responsible for the stability of a bulk valence of four and for a high isolation of oxidation sites at the surfaces [5]. It is generally agreed that the oxidation of the VPO precursors containing excess P is slower than that without excess P during calcination in air. This paper shows that some phenomena in calcination, and catalyst morphology and disorder are significantly changed by the presence of catalyst promoters, indium and TEOS in VPO. Their promoting effect is discussed.

2. Experimental The catalyst synthesis procedure follows that already described [9]. In summary, the catalyst precursor was prepared by the reduction of V205 with organic

W.-H. Cheng/Applied Catalysis A: General 147 (1996) 55-67

57

Table 1 Catalyst designation used in this study Catalyst designation

Composition used in this study

IV/IP IV/I.2P 1V/lAP IV/1.2P/In

P / V (at. ratio)= 1.0 P / V = 1.2 P / V = 1.4 P / V = 1.2; 3 wt.-% indium and 1.5 wt.-% silica (retained in the precursor)

solvents. In a typical preparation, 600 g V20 s was mixed with 6 1 isobutanol and 0.6 1 benzyl alcohol. Indium acetate was added when indium was indicated as a component. The mixture was heated at reflux for 1 h for the reduction of V205. 700 g of tetraethylorthosilicate (TEOS) was then added to react with water produced at V205 reduction when silica was indicated as a component. The mixture was stirred for 2 more hours. A varied amount of 85% H3PO 4 was added. The mixture was heated at reflux overnight to give a light-blue slurry. After cooling, the solid was filtered off, air dried at l l0°C to give a catalyst precursor. A series of VPO catalyst precursors with different P / V ratios and with or without additives were prepared. Their designation is shown in Table 1. The XRD patterns of these precursors correspond to V O H P Q • 0.5H20 except for the I V / 1 . 4 P sample which shows an additional minor VO(H 2PO4)2 phase (Fig. 1). 2% sterotex was added into a precursor as a binder for pelletizing 1 / 8

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T W O THETA Fig. 1. X-ray diffraction patterns of VPO precursors, a: VOHPO 4 • 0.5H 2O; e: VO(H 2PO4)2.

32.0

58

W.-H. Cheng /Applied Catalysis A: General 147 (1996) 55-67

inch pellets before pretreatment. The standard catalyst pretreatment includes the calcination in air at about 400°C for 3 h in a belt furnace. For the reaction study, 60 g of a calcined catalyst pellet was loaded in a l inch OD reactor and was activated in 1.5% n-butane in air at 500 sccm at 450°C for 16 h before lowering the temperature to 360-410°C for the reaction study. The activity of the catalysts does not change significantly with time on stream after the activation. The above standard pretreatment and reaction conditions were used unless otherwise indicated in the paper. The controlled-environment XRD study was performed with a Rigaku 0-0 X-ray diffractometer equipped with a Rigaku controlled environment and high temperature chamber. A piece of beryllium metal was used as the X-ray window on the chamber. Cu K ce radiation was used as the X-ray source. The ESCA study was conducted with a DuPont 650 facility. Surface area was measured by the BET method. The bulk composition was determined with an ARL ICP instrument. Conventional titration was used to determine the oxidation state of vanadium [22]. The SEM study was conducted with an ETEC Autoscan instrument. The TEM analysis was taken on a Hitachi H-600 T E M / S T E M .

3. Results 3.1. Precursors

The crystalline phase, VOHPO4 • 0.5H20 of the precursors presented in Fig. 1 clearly indicates that the crystallite size of this phase is smaller or more disordered in 1 V / 1 . 2 P / I n than those without promoters. The 1 V / 1 . 2 P / I n sample shows much broadened XRD peaks. Preferred atomic ratios of P to V disclosed in the patent literature are mostly in the range of 1.0 to 1.2. However, these ratios reflect only compositions used in the catalyst synthesis. The actual catalyst compositions may be different from synthesis compositions. ICP analysis of the catalyst precursors reveals that excess phosphorus used in the synthesis is only partially retained in the VPO catalyst precursors (column 2 of Table 2).

Table 2 Bulk and surface P / V ratios of VPO precursors Catalyst precursors

Bulk P / V

Surface P / V

1V/IP 1V/1.2P 1V/I.4P 1V/1.2P/In

1.02 1.06 1.11 1.07

0.97 1.04 1.11 1.07

W.-H. Cheng / Applied Catalysis A: General 147 (1996) 55-67

59

Most of excess P was washed away during the synthesis. ESCA analysis further shows that the surface P / V ratio is similar to the bulk ratio (column 3 of Table 2). A V O P O 4 • 2 H 2 0 compound was used as a reference to convert the ESCA intensity ratio to the atomic ratio. The V O P O 4 • 2 H 2 0 reference was prepared from V205 and H3PO4 according to known practice [23]. No surface enrichment of phosphorus occurs in the precursors.

3.2. Catalyst pretreatment Phase transitions occur during catalyst pretreatment. Phase transitions of various precursors during air calcination at various temperatures were monitored by in-situ XRD as shown in Fig. 2. Calcination was conducted at each temperature for 1 h before XRD scans for about 30 min. Calcination at 400°C results in mostly amorphous VPO. A crystalline VOPO 4 phase, and minor ( V O ) z P z O 7 and 6-VOPO 4 phases were formed at 500°C calcination. However, the (VO)2P207 and 6-VOPO 4 phases were not observed in the 1 V / 1 . 2 P / I n catalysts at 500°C. The XRD pattern of the crystalline VOPO 4 phase characterized by 27.9 ° (20) and 21.0 ° (20) (measured at 500°C) does not the same as those of known V O P O 4 phases such as a-VOPO 4, c ~ I I - V O P O 4 , f l - V O P O 4,

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2e Fig. 2. XRD study on the effect of catalyst composition on phase transition of VPO during calcination in air. zx: VOHPO4.0.5H20; O: (VO)2P207; v : &VOPO4; 0 : o{-VOPO4 (see text). (A minor VO(H2PO4) 2 phase in 1V/1.4P at 25°C is omitted in the figure).

W.-H. Cheng / Applied Catalysis A: General 147 (1996) 55-67

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7-VOPO 4, and 8-VOPO 4 reported in the literature [20]. This VOPO 4 phase will be temporarily designated as the c e ' - V O P O 4 phase in this paper for the reason shown below. The hydration of the VPO catalysts containing a'-VOPO 4 results in a crystalline hydrate which can undergo the same phase evolution upon thermal dehydration as the a - V O P O 4 . 2 H 2 0 does [24]. The intensity of c~'V O P O 4 decreased upon hydration. Clearly, the formation of the oxidized a ' - W O P O 4 phase was suppressed when the P / V ratio increases. However, this suppression was not observed for the 1 V / I . 2 P / I n sample as evidenced by the high intensity of the a'-VOPO 4 phase. The vanadium valence of VPO calcined in a belt furnace for 3 h at various temperatures is shown in Fig. 3. The result is consistent with the XRD study and confirms that although the excess P suppresses the oxidation of VPO precursors, the precursor with both excess P and promoters does not reveal this suppression of oxidation during the calcination. Promoters even facilitate the oxidation of the precursor. The morphology of the samples after 400°C calcination was studied by SEM. The 1 V / 1 P sample shows a small-platelet morphology (Fig. 4a). The 1V/1.2P sample reveals clear platelet morphology with a larger exposed platelet face than the 1 V / 1 P sample (Fig. 4b). The thickness of the platelet is greatly reduced for the 1 V / 1 . 2 P / I n sample (Fig. 4c). The activation in 1.5% n-butane/air at 450°C for 16 h transforms amorphous calcined catalysts into working catalysts. The XRD analysis shows a crystalline

W.-H. Cheng /Applied Catalysis A: General 147 (1996) 55-67

1#

Fig. 4. SEM micrographs of calcined VPO catalysts. (a) IV/1P, (b) 1V/I.2P, (c) IV/I.2P/In.

61

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W.-H. Cheng /Applied Catalysis A: General 147 (1996) 55-67

a

Fig. 5. Bright field TEM images of (a) calcined and (19) activated IV/I.2P/In catalysts showing layer morphology.

(VO)2P207 phase. The crystallinity is reduced for the promoted VPO. The morphology of the VPO catalysts does not change during activation. Fig. 5 shows the TEM micrographs of the calcined and activated 1 V / 1 . 2 P / I n catalysts. Layered morphology of both samples was evidenced. Disorder between layers was detected by electron diffraction with the electron beam oriented parallel to the layers as shown in Fig. 6. The microdiffraction pattern from a 20 nm region near the edge of the particle showed streaks which indicated disorder. 3.3. Reaction

The oxidation of n-butane was conducted at 370-410°C. The temperature indicated is the highest temperature in the catalyst bed. The conversion of

W.-H. Cheng/Applied Catalysis A: General 147 (1996) 55 67

63

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Fig. 6. (a) Microdiffraction pattern showing streaking from the activated 1 V / 1 . 2 P / l n catalyst. (b) secondary electron image showing location where diffraction pattern in (a) was taken.

n-butane, selectivities and maleic anhydride yield (conversion times selectivity) over the I V / 1 . 2 P / I n catalyst are shown in Fig. 7. The carbon containing products include mostly maleic anhydride and some CO and CO 2. The maleic

Table 3 Oxidation of n-butane to maleic anhydride over VPO catalysts Catalyst sample

Surface area a (m 2 / g )

Relative MA yield b

Average vanadium valence c

1V/1P IV/1.2P IV/1.2P/In

10 9 11

0.52 0.81 1.00

4.3 4.1 4.15

a Surface area of fleshly calcined catalysts. b Maleic anhydride yield per gram of catalyst relative to that over I V / I . 2 P / I n with 1.5 n-butane/air in the feed. ~ Average vanadium valence in the used catalysts.

W.-H. Cheng /Applied Catalysis A: General 147 (1996) 55-67

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anhydride yield varies only within a small range with the reaction temperature used in the study. The maleic anhydride yield of other catalysts relative to that of 1 V / 1 . 2 P / I n at 400°C is given in Table 3. The 1 V / 1 . 2 P / I n catalyst has the highest yield indicating the promoting effect of the additives.

4. Discussion

4.1. Oxidizability of VPO precursors Promoters and excess amounts of P greatly influence the oxidizability of VPO precursors in calcination in air. Excess P in 1V/1.2P and 1V/1.4P samples retard the samples from oxidation. This is consistent with the effect of the P / V ratio on the oxidizability previously reported [5]. However, the promoted sample, 1 V / 1 . 2 P / I n , even with the excess P is easy to be oxidized in calcination as evidenced by both XRD (Fig. 2) and vanadium valence (Fig. 3) studies. Its vanadium valence is even higher than that in 1V/1P under the same calcination conditions. The VPO precursors have a layered morphology. The size of the platelet appears to increase with the P / V ratio and the thickness of the platelet decreases significantly in the 1 V / 1 . 2 P / I n sample (Fig. 4). It is

65

W.-H. Cheng/Applied Catalysis A: General 147 (1996) 55 67

known that the major phase of the VPO precursors, V O H P O 4 • 0 . 5 H 2 0 , has a layered structure [25]. Consistent with the SEM study, the XRD study also indicates a smaller crystallite size of the promoted sample compared to the unpromoted samples (Fig. 1). The thin-layer morphology of the promoted sample could facilitate its oxidation during the calcination. The (VO)zP207 phase in VPO catalysts is formed via the topotactic transformation from VOHPO4 • 0.5H20 [15,16]. The morphology is unchanged after the transformation. This is also consistent with the observation of XRD in this lab that (VO)2P207 peaks of the activated 1 V / 1 . 2 P / I n catalyst are broadened compared to the unpromoted catalysts. 4.2. Catalyst pe~. ormance

The effect of the P / V ratio in the unpromoted VPO catalysts on the reactivity in n-butane oxidation has been a subject of many studies. For example, Wenig and Schrader found that a slight excess of catalyst phosphorous ( P / V = 1.1) was beneficial to the activity and selectivity; used VPO catalysts without excess phosphorus contained the active but nonselective a - V O P O 4 phase [26]. The excess surface phosphate prevents the oxidation of the pyrophosphate to /3VOPO 4 which is active for the total oxidation to carbon oxides [21,27]. The active phase for the selective butane oxidation to maleic anhydride is still controversial. While some believe that the pyrophosphate is the active phase [21,26,27]. Others think the best catalyst consists of an oxidized surface(V 5+) in interaction with reduced matrix ((VO)2PzO 7) [20]. Evidence of dispersed V 5+ on the VPO matrix has been obtained by P NMR spin echo mapping of VPO catalysts [28]. Our study shows that the 1 V / 1 P sample is easy to be oxidized in calcination (Fig. 3) and in reaction (Table 3). The catalyst has a poor maleic anhydride yield (Table 3). It is also consistent with a study which concluded that highly oxidizable VPO catalysts are poor catalysts for maleic anhydride production [22]. Interestingly, the oxidation state of vanadium in the 1 V / 1 . 2 P / I n catalyst after calcination is high, but the promoted catalyst exhibits the best maleic anhydride yield. The thin-layer morphology and small crystallite size of the catalyst makes it quickly oxidized in calcination as discussed in the previous section. It could also be quickly reduced by butane in the reaction. The quick oxidation and reduction in the redox cycle would not make used 1 V / 1 . 2 P / I n catalyst overoxidized. The presence of excess P in the promoted catalyst also prevents the catalyst from overoxidation. The used 1 V / 1 . 2 P / I n catalyst shows lower oxidation state of vanadium compared to the 1 V / 1 P catalyst (Table 3). The slightly higher surface area of the promoted catalyst would contribute to a higher maleic anhydride yield than the 1V/1.2P catalyst. The promoted catalyst contains a small amount of silica and indium. In a separate study, a catalyst precursor (1V/1.2P/silica) was prepared with 1V/1.2P and just enough TEOS to react with water produced in the catalyst •

.

31

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66

W.-H. Cheng /Applied Catalysis A: General 147 (1996) 55-67

synthesis. It used about 50% of TEOS used in the I V / 1 . 2 P / I n sample. The performance of the 1V/1.2P/silica is similar to 1V/1.2P sample. Bither in his patent also reported that VPO catalysts with addition of indium alone or various amounts of TEOS alone did not enhance the production of maleic anhydride but the VPO catalysts prepared with both TEOS and indium give a significantly better maleic anhydride yield [9]. The promoting effect of the 1V/1.2P/In catalyst may represent the integrated effect of both indium and TEOS. The promoters alter the morphology, crystallite size and oxidizability of VPO precursors and working catalysts and increase the maleic anhydride yield.

5. Conclusions (1) VPO catalysts with slightly excess phosphorus and with indium and TEOS promoters give the best yield of maleic anhydride from n-butane. (2) Excess phosphorus increases the resistance of VPO toward oxidation in both calcination and reaction. The promoters greatly facilitate the oxidation of the VPO in calcination but not in the reaction environment. (3) The promoters drastically decrease the thickness of platelet in the layered morphology, and the crystallite sizes of the VOHPO4 • 0.5H20 a n d (VO)2P207 phases.

Acknowledgements The author thanks C. Lyman for assistance of TEM measurement, and DuPont company for the support of part of the program.

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