One-step synthesis and characterization of niobia-phosphate aerogels

ELSEVIER

Catalysis Today 28 (1996) 41-47

One-step synthesis and characterization of niobia-phosphate aerogels Akshay Waghray a, Edmond I. Ko b,* a Depurtmenr of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, USA b Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890,

USA

Abstract Niobia-phosphate aerogels, containing 5 and 10 mol-% phosphate, were prepared by a one-step sol-gel synthesis followed by supercritical drying with carbon dioxide. After calcination at 773 and 1073 K for two hours, these samples were characterized by nitrogen adsorption, X-ray diffraction, infrared study of adsorbed pyridine and I-butene isomerization. The effect of calcination temperature on their acidic and catalytic properties, along with similar data for a sample prepared by immersing a niobia aerogel in phosphoric acid, allowed us to identify a surface phosphate. with near P=O double bonds to be the acidity-enhancing species. Keywords: Niobia-phosphate aerogels; One-step synthesis

1. Introduction Niobic acid (a hydrated form of niobia) is well known for its high acid strength and its ability to catalyze a wide variety of reactions [1,2]. However, niobic acid is almost neutral after a calcination temperature of about 773 K and the loss of acidity is accompanied by the crystallization of niobia. In a series of papers, we have explored the structure-acidity relationship of niobia by trying to understand the origin of acidity in terms of molecular building blocks. Our approach was to stabilize niobia from crystallization by (i) supporting it on another oxide [3], (ii) mixing it with another oxide [3-51 and (iii) preparing it as an aerogel [6,7]. Our results confirmed the earlier findings of Tanabe and * Corresponding author.

co-workers [ 1,2] in that it is desirable to maintain X-ray amorphous niobia in order to preserve its high acid strength. Specifically, we believe that tetrahedral or octahedral niobia units containing Nb=O bonds are responsible for strong Lewis acid sites. These molecular building blocks are less prevalent, or no longer exist, when niobia crystallites become detectable by X-ray diffraction. Another way to prevent niobia from crystallization is to dope it with an anion. Okazaki and co-workers [8,9] have demonstrated the effectiveness of such an approach by treating niobic acid with phosphoric acid. However, using either an oxide or an anion to stabilize niobia from crystallization introduces a complicating factor in that the dopant may be acidic itself or introduce new acid sites via interactions with niobia. Thus, in characterizing these doped sys-

0920-5861/%/$32.00 0 1996 Elsevier Science B.V. All rights resewed SSDI 0920-5861(95)00226-X

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A. Waghray, E.I. Ko / Cutalysis Toduy 28 (1996) 41-47

terns, it is important to compare the results with un-doped samples and to understand the molecular origin of acid sites. In their work, Okazaki and Kurosaki [8] established that treating niobic acid with phosphoric acid leads to enhanced catalytic activities for the dehydration of cyclohexanol and the alkylation of benzene. They further showed that the active species is likely to be an amorphous niobium phosphate [8,9]. Recently, we have used a one-step synthesis for zirconia-sulfate aerogels to study solid superacidity [lo]. By characterizing the acidic properties of these samples as a function of activation temperature, we were able to arrive at a better understanding of the role of surface sulfate groups. We believe that this strategy, i.e., using different preparative methods to study catalytic systems, is of general utility. Thus, we undertook a study of niobia-phosphate aerogels to demonstrate the applicability of our approach in general and to shed light on the acidic properties of niobia-containing materials in particular.

2. Experimental 2.1. Sample preparation A niobia aerogel, denoted as A-Nb,O,, was prepared by following a procedure previously described [6]. Briefly, an alkoxide/alcohol solution was prepared by dissolving 15 mmol of niobium(V) ethoxide &rem) in 15 ml of s-butyl alcohol (99 + % anhydrous, Aldrich). This solution was quickly added to a well-mixed alcohol/water/acid solution which contained 15 ml of s-butyl alcohol, 150 mmol of doubly deionized water, and 12 mmol of nitric acid (70%, Fisher). The combined solution was stirred with a magnetic stir bar and turned into an alcogel in about 10 s. Our one-step synthesis of niobia-phosphate aerogels involved adding the phosphate precursor, ammonium dihydrogen phosphate (Fisher), directly into the sol-gel preparation. Specifically, the appropriate amount of ammonium

phosphate was added to the alcohol/water/acid solution with the acid content kept constant. Two alcogels, containing 5 and 10 mol-% phosphate, were prepared by this method. These samples were denoted as A-NbPO,-95 and ANbPO,-90, respectively. Each alcogel was covered and aged for 1 h at room temperature. The alcohol was then removed by supercritical drying with carbon dioxide [6]. The resultant aerogel was grounded into a powder of < 100 mesh. Further heat treatment consisted of heating under flowing nitrogen (18 l/h) at 673 K for 2 h and then in flowing oxygen ( 18 l/h) at 773 K for 2 h. Each sample was heated at a rate of 10 K/min from room temperature to the final treatment temperature. Samples calcined at 873, 1073 and 1173 K were previously calcined at 773 K. For comparison a phosphate-promoted sample was prepared by immersing a niobia aerogel that had been calcined as previously described at 773 K in a 25 mM solution of ortho-phosphoric acid (85%, Fisher) at room temperature for 24 h. This suspension was evaporated to dryness by heating in air at 383 K for 3 h, followed by a vacuum treatment at 383 K for 3 h. The dried sample was then calcined at 773 K for 2 h in flowing oxygen. The calcined sample was denoted as 5% HsPO,/Nb,O, to reflect its nominal loading of 5 mol-% phosphate. 2.2. Physical and chemical characterization Textural characterization was performed on an Autosorb-l gas sorption system (Quantachrome Corp.). Prior to analysis, samples were outgassed at 383 K for 3 h under vacuum. Forty-point desorption isotherms were obtained, from which BET surface area (taken at P/P, at about 0.3), total pore volume (at P/P, close to unity) and pore size distribution (BJH method [ 111) were calculated. Crystal structures were determined by X-ray diffraction (XRD) using a Rigaku D/Max diffractometer with Cu K, radiation. Isomerization of 1-butene was performed in a

A. Wughray.

E.I. Ko / Catalysis

differential, downward flow, fixed bed catalytic reactor. Approximately 100 mg of catalyst was pretreated under 50 seem (3 l/h) of He flow (Matheson HP) at 473 K for 1 h. The sample was then cooled to 423 K and exposed to a feed stream mixture of 5 seem (0.3 l/h) 1-butene (Matheson, research grade) and 95 seem (5.7 l/h) helium. A Gow-Mac 55OP gas chromatograph with a thermal conductivity detector was used to analyze the composition of 1-butene, cis- and trans-2-butene. A Mattson Galaxy 5020 spectrometer with a Harrick diffuse reflection attachment (DRA-2) was used to collect diffuse reflectance infrared Fourier transform (DRIFT) spectra. For in situ experiments samples were diluted in KBr to 2 wt.-% and placed into an in situ DRIFT Harrick Reaction Chamber (HVC-DR2). They were then pretreated under conditions that were similar to those used in conditioning the catalysts for lbutene isomerization. Pyridine exposure was carried out at 423 K by diverting a stream of helium through a liquid saturator containing pyridine at room temperature for 15 min. The samples were kept at 423 K under helium flow for another hour before DRIFT spectra were collected. The ratio of Bronsted to Lewis acid sites was determined by a method previously described [lo].

3. Results and discussion 3.1. Stabilizing

effect of phosphate promotion

After calcination at 773 K for 2 h, the two niobia-phosphate aerogels, A-NbPO,-95 and A-NbPO,-90, have BET surface areas of about 220 and 240 m*/g, respectively. These values are 15%-25% higher than that of a niobia aerogel after the same heat treatment [6], suggesting that phosphate has inhibited the sintering process. This stabilization of surface area is even more apparent at higher calcination temperatures. Fig. la shows the fractional surface area, defined as the surface area at a given

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1 Y

E 0.0 i;i t ;i 0.6

E 0.4

a $ r” a h

0.2

0 7do

0CiO

960

1060

1100

1

Temperature(K) 1.4 1.2

a

n’

1

E = 0.8 E 5 0.6 >

i

0.4

: 0.2 0 10 Temperature(K)

Fig. I. Effect of treatment on (a) surface area and (b) pore volume of niobia areogels with and without phosphate promotion. 0, A-Nb,O,; ?? , A-NbPO,-95; 0, A-NbPO,-90; 0, 5% H,PO, /Nb,O,.

calcination temperature divided by that at 773 K, of our samples as a function of calcination temperature. The niobia-phosphate aerogels clearly retain a larger fraction of their surface areas at 873 and 1073 K compared to a niobia aerogel, even though the differences disappear at 1173 K. For comparison, we show the corresponding data for the 5% H3P04/Nb20, sample which retains an even larger fraction of its surface area than the two niobia-phosphate aerogels. This observation may be explained in terms of the location of phosphate in these samples, a point that we will address below. Our phosphoric acid-treated niobia aerogel behaves similarly to the phosphoric acid-treated niobic acid reported by Okazaki and Kurosaki

Bl.

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Fig. lb shows the variation of pore volume with calcination temperature for the same set of samples. Again, the presence of phosphate clearly retains the pore volume better. Pore size distribution measurements show that all samples are mesoporous after calcination at 773 K for 2 h. The pore size distributions are quite broad with maxima in the range of lo-30 nm (pore radius). At 1073 K, the significant drop in the pore volume of niobia aerogel is accompanied by a disappearance of mesopores. In contrast, at the same calcination temperature, all the phosphate-containing samples maintain a large pore volume with very little change in their pore size distributions. The effect of phosphate on the crystallization of niobia is more subtle than its effect on textural properties. As shown in Table 1, after calcination at 773 and 873 K, all the samples are X-ray amorphous and crystallize into the TTphase of niobia, respectively. The effect of phosphate promotion does manifest itself at 1073 K, in that only pure niobia crystallizes into the T-phase, whereas the phosphate-containing samples remain in the n-phase. This observation is not surprising because TT, a less well crystallized structure of T, is stabilized by impurities [ 121. The stabilizing effect of phosphate does not show up at lower calcination temperatures because the niobia aerogel itself is capable of resisting crystallization [6]. In fact, one key difference between this work and previous studies of Okazaki and co-workers [8,9] is that whereas their niobic acid crystallizes and loses acidity after calcination at 773 K, our niobia aerogel does not [ 121. Our focus of phosphate

28 (19%) 41-47

-

A-NW,

-

A-NbPO,-95

A-IWO,-90 S%H,F’O,IA-WO, A-NbPO,-95 (1073) A-NbPO&W (1073)

--@+ +

0.0:.

0

1.1

20

1

40

60

1

60

(773.2) (873.2) (1073.2)

Fig. 2. lsomerization of I-butene over niobia aerogels with and without phosphate promotion. All samples were calcined at 773 K for 2 h, unless specified otherwise.

promotion is, thus, on enhancing rather than retaining acidity. 3.2. Catalytic and acidic properties Fig. 2 shows the time-on-stream data of lbutene isomerization over our samples, reported on a per-surface-area basis. All samples deactivated with time on stream, reaching steady-state activities in about 100 min. The selectivity at steady state, defined as the ratio of c&2butene/truns-2-butene, is between 1.3 and 1.5 which suggests that the reaction is catalyzed by Bronsted acid sites [13]. Consistent with earlier results [6], the niobia aerogel which is X-ray amorphous is active in isomerizing 1-butene. In fact, adding phosphate

Sample A-Nb,O, g Tb

b

A-NbPO,-95

A-NbPO,,-90

5% H,PO,/Nb,O,

A

A

A

l-r

l-r

T-r

TT

Tr

l-r

a A = amorphous. b ‘IT, T = two low-temperature modifications of niobia. See 1121.

1

120

lime on stream (mins)

Table 1 X-Ray diffraction results of niobia aerogels with and without phosphate promotion as a function of heat treatment Heat treatment (temperature in K, time in h)

1

100

A. Wqhruy,

E.I. Ko/Curulysis

5%H,POdA-Nb,O, ’ P

A-NbPOi90

E 8

A-NbPO,-95

? ?1073K

A-Nb,O, 0

10

20

30

, 40

% 6 Fig. 3. Fraction of Brvnsted acid sites on niobia aerogels with and without phosphate promotion. Data for the two niobia-phosphate aerogels are shown at two activation temperatures (773 and 1073

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45

event, Fig. 3 shows that this significant increase in the specific activity for 1-butene isomerization is accompanied (and explained) by an increase in the fraction of Brprnsted acid sites in these two samples upon activation at 1073 K. Since the specific isomerization activities for pure niobia aerogels are the same at calcination temperatures of 773 and 1073 K 161, the observed increase in niobia-phosphate aerogels must be due to the activation of phosphate.

K).

3.3. Location and nature of the phosphate species

directly in the sol-gel step only has a small effect (about 50% increase) on the isomerization activities of the two niobia-phosphate aerogels. In contrast, the phosphoric acid-immersed niobia aerogel is a factor of three more active (in terms of specific activity) than the niobia aerogel. Fig. 3 shows the fraction of Bronsted acid sites on these samples as determined by pyridine adsorption. There is a qualitative agreement between catalytic and acidic properties in that (i) the niobia-phosphate aerogels and niobia aerogel have similar fractions of Brprnsted acid sites and (ii) the phosphoric acid-immersed sample has the largest fraction of Bronsted acid sites in this set of samples. However, we could not make a quantitative comparison between the results in Figs. 2 and 3 because we did not determine the total number of acid sites in these samples. In order to understand the effect of different methods of phosphate introduction (i.e., co-gelling versus immersion), we activated our two niobia-phosphate aerogels at a higher calcination temperature of 1073 K. As shown in Fig. 2, these two samples, denoted as A-NbPO,90( 1073) and A-NbPO,-95( 1073), have specific activities that are very close to the phosphoric acid-immersed niobia aerogel calcined at 773 K. But they are less active on a per-mass basis because of the loss in surface area at the higher calcination temperature (see Fig. la). In any

Our catalytic and acidic results point to two key observations: (i) the niobia-phosphate aerogels are very different at calcination temperatures of 773 and 1073 K and (ii> the sample calcined at 1073 K is similar to the phosphoric acid-immersed sample calcined at 773 K. We believe these observations can be explained by the location of the phosphate species in these samples. In our study of zirconia-sulfate aerogels, we used a one-step synthesis that is similar to the one used in this work [lo]. By adding sulfuric acid directly in the sol-gel step, we found that sulfate is initially trapped in the bulk of the resultant aerogel. As the sample is activated at higher temperatures, sulfate is expelled onto the surface and transforms into an active surface species that promotes strong Bronsted acidity. At the same time, the sintering of the sample leads to a loss in surface area and the crystallization of zirconia. Apparently the same sequence of events takes place in our niobiaphosphate aerogels. Phosphate that is introduced in the gelling step remains in the bulk of the samples calcined at 773 K. It does affect the stability of niobia somewhat, as shown by the slightly higher surface area, but does not lead to enhanced activity significantly. In fact, the increase in isomerization activity could be due to a better-stabilized niobia itself. When the niobia-phosphate aerogels are calcined at 1073 K, phosphate species are expelled onto the surface

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A. Wughray, E.I. Ko/ Catalysis Today 28 (1996) 41-47

1350

1250

1150

1050

950

5 10

Wavenumbers (cm”) Fig. 4. DRIFT spectra of niobia-phosphate aerogels (A-NbPO,-95) showing the effect of activation temperature.

of niobia as the sample sinters and loses surface area. These samples thus resemble the phosphoric acid-immersed niobia aerogel, for which the phosphate species is on the surface of niobia to start. DRIFT data of the A-NbPO,-95 sample calcined at 773 and 1073 K confirm this movement of phosphate with increasing activation temperature. Fig. 4 shows that, from 773 to 1073 K, the main peak shifts from about 1100 to 1200 cm-‘. According to the literature assignments on phosvibrations [ 14,151, these phorous-oxygen wavenumbers and the observed shift is consistent with a PO:- species changing into a PO, species or more generally, a species with predominantly P-O single bonds changing into one with near P=O double bonds. We summarize our results by proposing in Fig. 5 two models of our niobia-phosphate aerogels. The drawings are meant to be schematic and do not represent the correct structure. The important point is that phosphate species move from the bulk onto the surface at a higher calcination temperature. The surface phosphate species, with its P=O double bonds, is responsible for enhanced Bronsted acidity, perhaps in the same way that S=O double bonds act in surface sulfate species 1161.In our recent work on the preparation and characteriza-

tion of zirconia-phosphate aerogels [17], we proposed models that are similar to those in Fig. 5. Unlike the niobia aerogel, a zirconia aerogel does not contain Bronsted acid sites. For this reason, one difference between niobia-phosphate and zirconia-phosphate is that the latter sample calcined at 773 K contains no Brransted acid sites that are detectable by the adsorption of pyridine at 423 K. For zirconia-phosphate, the appearance of Brprnsted acidity as phosphate is expelled onto the surface at a higher calcination temperature is thus even more convincing. As mentioned earlier, phosphate promotion affects the stabilization of surface area differently depending on the method of introduction. Now we know the difference is due to the location of the phosphate species. Data in Fig. la suggest that the surface phosphate species is more effective in preventing a loss in surface area. Okazaki and co-workers proposed that amorphous niobium phosphate is responsible for enhanced acidic properties of phosphoric acid-

:

I

/““\ /““\/““\ /““\

(b)

Fig. 5. Schematic diagrams of niobia-phosphate aerogels vation temperatures of (a) 773 K and (b) 1073 K.

at acti-

A. Waghray, E.I. Ko/Cak~lysis

treated niobic acid [8,9]. This suggestion is not inconsistent with our proposed model because before niobium phosphate crystallizes fully, it would be possible to find in the sample regions that contain phosphorous-oxygen bonds with a double bond character. Along this line further infrared study of the phosphorous-oxygen stretching region of niobium phosphate samples at different stages of crystallization might provide useful insight. The effect of phosphate loading, a parameter that was not quantitatively studied in our work, also deserves further attention.

4. Summary We have demonstrated two approaches to prepare phosphate-promoted niobia aerogels that are more active in 1-butene isomerization than niobia aerogels. Since the large surface area and pore volume are lost at the temperature that is necessary to activate niobia-phosphate aerogels, the simpler, one-step approach offers no catalytic advantage over the phosphoric acidimmersed aerogel when activities are compared on a per-mass basis. Still, the real advantage of having an alternate synthetic approach in this case is that comparing samples prepared by different methods provides an effective way to understand the activation behavior of our sam-

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47

ples and the catalytically active species itself. Specifically, our results establish that the Bronsted acidity of niobia aerogels can be enhanced by phosphate promotion and that the responsible surface phosphate species has near P=O double bonds.

References [l] T. Iizuka, K. Ogasawara and K. Tanabe, Bull. Chem. Sot. Jpn., 56 (1983) 2927. [2] K. Ogasawara, T. lizuka and K. Tanabe, Chem. Lett., (1984) 645. [3] P.A. Burke and E.I. Ko, J. Catal., 129 (1991) 38. [4] SM. Maurer, D. Ng and E.I. Ko, Catal. Today, 16 (1993) 319. [5] SM. Maurer and E.I. Ko, Bull. Chem. Sot. Jpn., 66 (1993) 645. [6] S.M. Maurer and E.I. Ko, J. Catal., 135 (1992) 125. [7] S.M. Maurer and E.I. Ko, Catal. Lett., 12 (1992) 231. [S] S. Okazaki and A. Kurosaki, Catal. Today, 8 (1990) 113. [9] S. Okazaki and N. Wada, Catal. Today, 16 (1993) 349. [lo] D.A. Ward and E.I. Ko, J. Catal., 150 (1994) 18. [l l] E.P. Barrett, LG. Joyner and P.P. Halenda, J. Am. Chem. sot., 73 (1951) 373. [ 121 E.I. Ko and J.G. Weissman, Catal. Today, 8 (1990) 27. [13] J. Goldwasser, J. EngeIhardt and W.K. Hall, J. Catal., 71 (1981) 381. [14] K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, 2nd ed., Wiley, New York, 1970, pp. 116-7. [ 151 M. Grayson and E.J. Griffith (Editors), Topics in Phosphorus Chemistry, Vol. 6, Wiley, New York, 1969. [16] T. Jin, T. Yamaguchi and K. Tanabe, J. Phys. Chem., 90 ( 1986) 4794. [17] R. Boyse and E.I. Ko, Catal. Lett., in press.