acidity and acid strength distribution of the crystalline microporous mapo-36 molecular sieve

Acidity and acid strength distribution of the crystalline microporous MAPO-36 molecular sieve

Deepak B. Akolekar Department of Chemical Engineering, University of Laval, Ste-Foy, Quebec, Canada

/n situ i.r. acidity measurements of MAPO-36 show the presence of Br6nsted and Lewis acid sites. The temperature-programmed desorption spectra of pyridine, obtained by the mass spectrometric technique, over MAPO-36, AIPO4-5, and SAPO-5 reveal the presence of the two types of acid sites. At a comparable concentration of the substituted element (Si or Mg) in the aluminophosphates of types 5 and 36, the concentration of Br6nsted acid sites is significantly higher in MAPO-36. The results of temperature-programmed desorption and stepwise thermal desorption of pyridine (using g.c. techniques) indicates that MAPO-36 possesses a very high number of strong acid sites (i.e., 6, 7.5, and 16 times higher than that of MAPO-5, SAPO-5, and AIPO4-5 respectively) above 673 K and fewer weaker acid sites than the other aluminophosphates of type 5. The generation of negative framework charge, hydroxyl structure, BrSnsted acidity, and the chemisorption of pyridine above 673 K indicate the substitution of Mg 2+ for some of the AI 3+ in the aluminophosphate framework of type 36.

Keywords: Acidity; acid strength distribution; MAPO-36; AIPO4-5; SAPO-5; MAPO-5; Br6nsted and Lewis acidity

I N T R O D U C T I O N

Microporous aluminophosphates are a new generation molecular sieves 1-6 with potential importance as adsorbents and catalysts. The crystal structure of MAPO-36 (magnesium-containing aluminophosphate) reported by Smith et al. 7,8 has a unique three-dimensional structure with monoclinic sym- metry and cell constant: a = 13.148/~, b = 21.577 ~, c = 5.164/~,, and [3 = 91.84 °. MAPO-36 has the 4.6.12 two-dimensional net similar with annular side pockets (unlike cancrinite). The channels are elliptical (10.1 × 9.2/~ across in projection) and parallel to the c-axis and have a pore opening of 7.4 x 6.5 /~. In the MAPO-36 framework, the hypothetical AI 3+ site is substituted by Mg 2+, which develops a negative framework charge and generates acidity. 5'9

MAPO-36 shows relatively higher catalytic activity for alkanes, i0,i i aromatics, and alcoholsl i than that of the other aluminophosphates. Because of its typical structure and higher catalytic activity, the acidity and acid strength distribution of MAPO-36 were investi- gated and compared with the type 5 aluminophosphates having the same concentration of the substituted element.

Address reprint requests to Dr. Akolekar at his present address: Dept. of Physical Chemistry, The University of New South Wales, P.O. Box 1, Kensington, NSW 2033, Australia. Received 4 January 1993; revised 28 June 1993; accepted 10 July 1993

EXPERIMENTAL

MAPO-36 was obtained from Pr3N-MAPO-36 by removing the organic template from the latter by heating it in air at 825 K (heating rate: 5 K min -1) for 12 h. The Pr3-N-MAPO-36 was synthesized according to the method outlined by Wilson and Flanigen 1° with certain modifications. 9 The gel of composition 1.8 Pr3N'0.17 MgO.0.92 A1203"1.0 P205-40 H20"33 HOAc was hydrothermally treated in a Teflon-coated autoclave at autogenous pressure, initially at 378 K for 50 h and finally at 423 K for 24 h. The details of synthesis, processing, and characterization (XRD, SEM, elemental analysis, X.p.s.t.g./d.t.g./d.t.a., N2- sorption capacity, etc.) of MAPO-36 have been reported elsewhere. 9

A1PO4-5, SAPO-5, and MAPO-5 were prepared according to the procedure described by Wilson et al. x The preparation and characterization of these materials are reported elsewhere. 12-14

Infrared-mass spectrometric studies were per- formed on self-supported wafers [15 mg, thickness 10.0 mg cm-2]. All the precalcined materials were first activated in situ in the presence of He and 02 (20%) at 825 K for 5 h and further in vacuum (5 x 10 -7 mbar) at the same temperature for 5 h. Freshly distilled pyridine was adsorbed at 473 K for 2 h contacting the vapor of the liquid at 273 K (0.12 mbar) and, subsequently, pyridine was desorbed at 473 K for 1 h under 10-Ombar. Further, pyridine was desorbed stepwise for 2 h as a function of evacuation temperature from 473 to 823 K and the

© 1994 Butterworth-Heinemann ZEOLITES, 1994, Vol 14, January 53

Acidity measurements of MAPO-36: D.B. Akolekar

Table 1 Characteristics of MAPO-36, ALP04-5, SAPO-5, and MAPO-5 molecular sieves

N2-sorption Molecular capacity sieve Elemental composition (mmol g- l )

Framework Strong acid sites, charge qi(673 K) p.mol g-1

MAPO-36 (0.04 Mg.0.46 AI.0.50 P)O2 5.6 AIPO4-5 (0.50 AI.0.50 P)O2 4.9 SAPO-5 (0.042 Si-0.05 A1.0.458 P)O2 4.8 MAPO-5 (0.039 Mg.0.46 AI.0.50 P)O2 4.9

-0 .040 40.0 - 2.5

-0.042 5.4 -0.039 6.5

i.r. spectrum was recorded at each step. Br6nsted and Lewis acidities were determined on the basis of the absorbances of the PyB band near 1543 cm -l and the PyL band near 1447 cm -I, respectively. Br6nsted and Lewis acid sites concentrations were calculated from the maximum absorbances of the respective bands and taking the same sample weight into considera- tion.

The temperature-programmed desorption (t.p.d.) of pyridine over MAPO-36, A1PO4-5, and SAPO-5 were carried out in an ultrahigh vacuum apparatus using the i.r.-m.s, technique. The t.p.d, was carried out from 473 to 823 K at linear heating rate of 20 K min- 1. While recording the t.p.d, spectra of pyridine using the m.s. instrument, masses originating from the parent pyridine molecule, e.g., m/e = 79, and from fragmentation, m/e = 52 ( C 4 H 4 from C5H5N), were monitored.

The quadrapole mass spectrometer and the i.r. spectrometer equipped with a Data Station used were a model QMG 311, Balzers, Germany, and i.r. 580B, Perkin-Elmer, USA, respectively. Details of the apparatus and procedure are reported elsewhere. 15,16

The number of strong acid sites and the acid strength distribution on MAPO-36, A1PO4-5, SAPO- 5, and MAPO-5 were measured by studying the stepwise thermal desorption (s.t.d.) of pyridine and also by determining the chemisorption of pyridine at different temperatures, using g.c. techniques. Iv'is The procedures for the measurement of chemisorption and s.t.d, of pyridine and also for evaluating the chemisorption data from s.t.d, have been given earlier. 17-20

RESULTS A N D DISCUSSION

Characteristics of MAPO-36, AIPO4-5, SAPO-5, and MAPO-5 are presented in Table I. The characteriza-

-- 9 12 14 tion of these materials ' - by XRD, SEM, t.g./d.t.g./ d.t.a, and N2 and H20-sorption capacity measurements indicated their high crystalline nature. The elemental composition of these materials obtained by AAS and chemical analysis shows that the materials do not possess any extraneous A1 or P or the substituted element. On basis of the elemental analysis and from the metal substitution mechanism proposed by Flanigen et al., 6 it follows that in MAPO-36 and MAPO-5, Mg is exclusively incorporated into the hypothetical A1 site, and in SAPO-5, Si is substituted into the hypothetical P site. The MAPO-36 possessed higher N2- and H20-sorption capacity than that of

A1PO4-5, SAPO-5, and MAPO-5. This may be attri- buted to the presence of annular side pockets in the MAPO-36 crystal structure, s

Figure 1A shows the hydroxyl region infrared spectrum of MAPO-36 after vacuum activation. The bands at 3754 cm- 1 are characteristic of terminal OH or impurities, whereas the bands at 3665 and 3632 cm -~ are assigned to the lattice-terminating P-OH group and the acidic hydroxyl group,6'2]-2~respec - tively. The incorporation of Mg into the aluminophosphate framework generates the acidic hydroxyls. These acidic hydroxyls are generated after the re- moval of organic template at 823 K. The anionic framework charge generated by the Mg substitution leads to Br6nsted acidity.

The infrared spectrum after the desorption of pyridine at 473 K from MAPO-36 pretreated at 798 K in vacuum is shown in Figure lB. The bands at 1447 and 1543 cm-l ascribed to Lewis and Br6nsted acid sites, respectively, are close to their usual values. 24 The decrease of all the hydroxyl groups of the MAPO-36 was observed upon the pyridine adsorp- tion, which is in agreement with earlier studies. 25 Infrared investigations of adsorbed pyridine con- firms the presence of Br6nsted and Lewis acid sites on the MAPO-36. The generation of Br6nsted acidity indicates substitution of Mg 2+ for the AI 3+ into the aluminophosphate framework.

The results of the changes in the absorbance (1447 and 1543 cm- 1 bands)(peak height/mg) of pyridine as a function of evacuation temperature for the MAPO- 36 (Figure 2) indicate that there exists distribution of Lewis and Br6nsted acid sites on MAPO-36 and the concentration of Lewis acid sites is higher than that of Br6nsted acid sites. With the increase in the evacua-

~ A

3 ~ 0 0

WAVE NUMBER, Cb~ =

Figure 1 (A) Hydroxyl region infrared spectrum of MAPO-36 after 775 K vacuum activation. (B) Infrared spectrum of chemisorbed pyridine on MAPO-36 molecular sieve (pyridine chemisorbed at 473 K).

54 ZEOLITES, 1994, Vol 14, January

0 . 2 ~ . - ?

<

0"2C

o - 1 " .

z o~ o, Ic

0 - 0 !

• LEWIS Ao0 5~TES AT 1447 =~1

~o ~o 6~o 6~o ,~o 7;0 8;0 TEMPERATURE,(K)

8~o

Figure 2 Changes in the absorbance of pyridine for the MAPO- 36 as a function of evacuation temperature.

tion temperature, a decrease in the concentration of Br6nsted and Lewis acid sites is observed.

The ratio of Br6nsted and Lewis acid sites at 473 K (Table 2) is highest for MAPO-36 as compared with the type 5 aluminophosphates. The order of the ratio of BrSnsted and Lewis acid sites at 473 K is as follows: MAPO-36 > MAPO-5 > SAPO-5 > A1PO4-5. The order of concentration of Br6nsted acid sites at 473, 573, and 673 K evacuation temperature is as follows: A1PO4-5 < SAPO-5 < MAPO-5 < MAPO-36. At 473 and 573 K evacuation temperatures, Lewis acidity is highest for the MAPO-36 and lowest for the SAPO-5 while it is the same for A1PO4-5 and MAPO-36 at 673 K. The B/L ratio at 473 K of the A1PO4-5 is much lower than that of MAPO-36, which indicates the incorporation of Mg into some A1 sites in the A1POa- f r a m e w o r k , g e n e r a t i n g a negat ive ly cha rged framework hydroxyl structure and Br6nsted acidity.

The t.p.d, spectra of pyridine on MAPO-36, AIPO4-5 and SAPO-5 obtained by the m.s. technique are given in Figure 3. The t.p.d, spectra show a large difference in the extent of pyridine desorption from A1PO4-5 to that of the MAPO-36 and SAPO-5. The total number of acid sites on the MAPO-36 is higher than that on the A1PO4-5 and SAPO-5. The t.p.d. spectra show two peaks, indicating the kind of acid site present. The temperature maximum for these peaks for MAPO-36, A1PO4-5, and SAPO-5 are 630 and 755 K, 543 and 673 K, and 648 and 673 K,


10

"3 8

o ~

o tJJ

&73 57] 67] 773

TEMPERATURE (K}

Figure 3 T.p.d. spectra of pyridine desorption from MAPO-36, AIPO4-5, and SAPO-5 molecular sieves [obtained using the m.s. technique; amount of catalyst: 15 mg; heating rate: 20 K min-1; m/e = 79 (pyridine)].

respectively. The higher peak maxima temperature for MAPO-36 indicate that MAPO-36 possesses stronger acid sites than those of AIPO4-5 and SAPO- 5.

The temperature dependence of the chemisorption of pyridine and the acid strength distribution obtained by the stepwise thermal desorption (s.t.d.) of pyridine on the aluminophosphates are shown in Figures 4 and 5. The strong acid sites (measured in terms of the amount of pyridine chemisorbed at 673 K) on the aluminophosphates are compared in Table 1.

The decrease in the chemisorption of pyridine with increase in the temperature for all the aluminophosphates (Figure 4) reveals that the pyridine chemisorption sites (which are expected to be mostly the acid sites) on all the aluminophosphates are not of equal s t rength, thus indicating the presence of acid strength (site energy) distribution on the aluminophosphates. The acid-strength distribution on the aluminophosphates is clearly shown in Figure 5. The columns in the figure show the distribution of the acid sites involved in the chemisorption/desorption of pyridine. Each column of the acid-strength distribution represents the number of sites measured in

Table 2 Data on BrSnsted and Lewis acid sites of MAPO-36, AIPO4-5, SAPO-5, and MAPO-5 molecular sieves

Absorbance of pyridine [a.u.]-102

Br6nsted acid sites Lewis acid sites

Evacuation temp. (K)

Sample B/L a 473 573 673 773 473 573 673 773

MAPO-36 0.29 6.5 3.3 1.7 0.3 22.1 8.9 4.3 0.7 AIPO4-5 0.07 0.9 0.5 0.1 - 12.7 6.6 4.3 1.4 SAPO-5 0.22 2.0 1.1 0.4 0° 1 8.9 4.5 2.8 1.2 MAPO-5 0.24 3.0 1.3 0.5 0.1 12.5 5.0 2.6 1.3

"Ratio of the absorbance of the bands at 1543 cm -1 (B) and 1447 cm -1 (L) at 473 K

ZEOLITES, 1994, Vol 14, January 55


terms of pyridine desorbed in corresponding temperature step. The strength of these sites is expressed in terms of the desorption temperature of pyridine, Ta, which lies in the range in which chemisorbed pyridine is desorbed.

A comparison of the chemisorption vs. temperature curves and the acid-strength distribution for the aluminophosphates shows that A1PO4-5, SAPO-5, and MAPO-5 differ markedly from MAPO-36 in their acidity and acidity distribution. The interesting results obtained from the t.p.d. (Figure 4) and s.t.d. (Figure 5) of pyridine (using the g.c. technique) over MAPO-36 show the existence of the site-energy distribution; the distribution is less broad and very few weak acid sites are present as compared to those of A1PO4-5, SAPO-5, and MAPO-5. The amount of pyridine chemisorbed above 673 K (Table 1) over MAPO-36 is extremely higher than over A1PO4-5 (16-fold), SAPO-5 (7.5-fold), and MAPO-5 (6-fold). This shows that the MAPO-36 possesses very high concentration of strong acid sites, which is, in turn, reflected by its higher catalytic activity. ~

C O N C L U S I O N S

The following conclusions were drawn from the present study:

Infrared acidity measurements of MAPO-36 show the presence of Br6nsted and Lewis acid sites. At a comparable concentration of the substituted element (Si in the SAPO-5, Mg in the MAPO-5, and Mg in the MAPO-36) in the aluminophosphate framework, the concentration of Br6nsted acid sites is significantly higher in the case of MAPO-36. The order of the ratio of Br6nsted and Lewis acid sites at 473 K is as follows: MAPO-36 > MAPO-5 > SAPO-5 > AIPO4-5. The B/L ratio at 473 of the A1PO4-5 is much lower than that of MAPO-36, which indicates the incorporation of Mg into some A1 sites in A1PO4-5 framework generating negatively charged framework, hydroxyl structure, and Br6nsted acidity.

The t.p.d, spectra of pyridine (obtained by the m.s. technique) on MAPO-36, AIPO4-5, and SAPO-5 show

.--.

I=l

¢r

0.8

0.6

0.4

0.2

0.0 273

°• o - - o ALP04-5 o - - Q S A P O - 5

o A - - A M A P 0 - 5

A - - A ~ L ~ P 0 - - 3 6

A o

3"/3 473 5'73 673

~,E~at'ttmg. T (x)

Figure 4 The temperature dependence of the chemisorption of pyridine on MAPO-36, AIPO4-5, SAPO-5, and MAPO-5.

I 0.25

-6 E

.EE 0.20

i~ ~ 0,15

== (no 0.10

0.05

0.00 1 2

[XS] ALP04-5

r ~ l S A P O - 5

M A P O - 5

M A P O - 3 6

3 4 5 6 7

1) 323 ( [d < 373 K 2) 373 < Td ( 423 K 3) 423 ( T d ( 473 K 4) 473 ( T d ( 523 K

5) 523 ( T d ( 573 K 6) 573 ( T d ( 673 K 7) 673 ( T d ( T d *

Figure 5 Acid strength distribution on MAPO-36, AIPO4-5, SAPO-5, and MAPO-5.

two peaks indicating the two types of acid sites present and a large difference in the extent of pyridine desorpt ion. The higher peak maxima temperature for MAPO-36 indicate that the MAPO- 36 possesses stronger acid sites than those of the AIPO4-5 and SAPO-5.

A comparison of the t.p.d, and s.t.d. (using g.c. techniques) of pyridine over MAPO-36, AIPO4-5, SAPO-5, and MAPO-5 showed a remarkable difference in their acidity and acid strength distribution. The strong acid sites present on MAPO-36 is extremely higher than the A1PO4-5 (16-fold), SAPO-5 (7.B-fold), and MAPO-5 (6-fold).

The typical crystal structure, high sorption capacity, higher acidity and catalytic acidity of the MAPO- 36 reflects its potential as a useful catalyst and adsor- bent.

A C K N O W L E D G E M E N T S

The author is sincerely thankful to Dr. H.G. Karge, Fritz Haber Institute of the Max Planck Society, Berlin, Germany, for providing the experimental facilities and helpful discussions. The author is grate- ful to the Alexander von Humboldt Foundation, Bonn, Germany, for an award of an international research fellowship.

R E F E R E N C E S

1 Wilson, S.T., Lok, B.M., Messina, C.A., Cannon, T.R. and Flanigen, E.M.J. Am. Chem. Soc. 1982, 104, 1146

2 Wilson, S.T., Lok, B.M. and Flanigen, E.M. US Pat. 4 310 440 (1982)

3 Flanigen, E.M., Lok, B.M., Patton, R.L. and Wilson, S.T. Pure Appl. Chem. 1986, 58, 135

4 Flanigen, E.M., Lok, B.M., Patton, R.L. and Wilson, S.T. Stud. Surf. ScL Catal. 1986, 28, 109

5 Flanigen, E.M., Lok, B.M., Patton, R.L. and Wilson, S.T., in Proceedings of 7th International Zeolite Conference, Kodansha-Elsevier, Tokyo-Amsterdam, 1986, p.103

6 Flanigen, E.M., Patton, R.L. and Wilson, S.T. Stud. Surf. ScL Catal. 1988, 37, 13

7 Smith, J.V., Pluth, J.J. and Andries, K.J., in Atlas of Zeolite Structure Types, 3rd revised ed. (Eds. W.M. Meir and D.H.

56 ZEOLITES, 1994, Vol 14, January

Olson) Butterworth-Heinemann, London, UK, 1992, p.50 8 Smith, J.V., Pluth, J.J. and Andries, K.J. Zeo/ites 1993, 13(3),

166 9 Akolekar, D.B., J. Catal. 1993, 143, 227

10 Wilson, S.T° and Flanigen, E.M., ACS Symp. Sero 398, Am. Chem. Soc., Washington, DC, 1989, p. 329

11 Akolekar, D.B., J° Catal. 1993, 144, 148 12 Akolekar, D.B. PhD Thesis, University of Poona, Poona, 1987 13 Choudhary, VoR. and Akolekar, D.B.J. Catal. 1987, 103, 115 14 Akolekar, D.B. submitted 15 Karge, H.G.Z. Phys. Chem. Neue Folge 1971, 76, 133 16 Karge, H.G., Hatada, K., Zhang, Y. and Fiedorow, R. Zeo/ites

1983, 3, 13


17 Choudhary, V.R.J. Chromatogr. 1983, 268, 207 18 Choudhary, V.R. and Nayak, V.S. AppI. Catal. 1982, 4, 32 19 Akolekar, D.B. and Choundhary, VoR. J. Catal. 1987, 105, 416 20 Nayak, V.S. and Choudhary, V.R.J. Catal. 1983, 81, 26 21 Dzwigaj, D.S., Briend, M., Shikholeslami, A., Peltre, M°J. and

Barthomeuf, D. Zeolites 1990, 10, 157 22 Arlidge, E.Z. and Farmer, V.C.J. Appl. Chem. 1963, 13, 17 23 Tapp, N.J. and Milestone, N.B. Zeolites 1990, 10, 105 24 Ward, J.W., in Zeolite Chemistry and Catalysis (Ed. J.A.

Rabo) ACS Monograph 171, Am. Chem. Soc., Washington, DC, 1976, p. 118

25 Matens, J.A., Mertens, M. and Jacobs, P.A. Stud. Surf. ScL Catal. 1988, 37, 97

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acidity and acid strength distribution of the crystalline microporous mapo-36 molecular sieve

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