Hydrothermal synthesis of zeolite-like iron and zinc phosphates and its application in the methanol conversion

Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) © 2004 Elsevier B.V. All rights reserved.

1049

HYDROTHERMAL SYNTHESIS OF ZEOLITE-LIKE IRON AND ZINC PHOSPHATES AND ITS APPLICATION IN THE METHANOL CONVERSION Tagiyev, D.B., Aliyev, A.M., Mamedov, N.D. and FatuUayeva, S.S. Azerbaijan Medical University, Department of Biophysical and Bioorganic Chemistry, 23 Bakikhanov street, 1022, Baku, Azerbaijan. Fax: +99412903520. E-mail: dtagivev@,hotmail.com

ABSTRACT The reactions in R-ZnO(Fe203)-P205-H20 system (where R is an organic or an inorganic base) under the hydrothermal conditions have been investigated. The products of reactions have been characterized by X-ray diffraction, thermogravimetric and differential thermal analyses, IR-spectroscopy and elemental analysis. The influence of synthesis conditions (temperature, time, composition of initial mixture, nature and concentration of base) on the crystallization of zinc and iron phosphates has been studied. It has been established that zeolite-like KZnP04-1.5H20 with hexagonal unit cell parameters a=18.14 A and c=8.504 A, is formed in the presence of KOH. Three-dimensional framework consists of strictly alternating Zn04- H P04-tetrahedra. For compensation of negative charge of the framework the cations of potassium are located in the channels of it. In the presence of triethanolamine and low concentrations of alkaline cations the synthetic analogue of hopeite-mineral is crystallized either in the individual form or in the association with zinc phosphates of alkaline cations. A new three-dimensional microporous iron phosphate Fe2H3(P04)3-(EtOH)3N was obtained by hydrothermal synthesis in the presense of triethanolamine in the form of transparent elongated crystals of light green colour. The symmetry is orthorhombic with unit cell parameters a=9.61 A; b=8.64 A and c=14.19

A. The catalytic properties of zinc and iron phosphates have been investigated in methanol conversion. Iron phosphate is active in the formation of dimethyl ether, while zinc phosphates depending upon their composition leads to formation of dimethyl ether, lower olefins or formaldehyde. Keywords: Zinc, Iron, Phosphates, Hydrothermal Synthesis, Methanol Conversion. INTRODUCTION The crystallo-chemical possibility of phosphorus in the structure-formation permits to obtain crystal metal phosphates, structure of which to a large extent depends upon conditions of hydrothermal synthesis [1,2]. The possibility of obtaining microporous zinc phosphates having a similar structure with the known aluminosilicates has been shown in the work [3]. In particular, zinc phosphates with sodalite structure [4] are crystallized from sodium-containing gels in the presence of sodium, and with faujasite structure [5] in the presence of tetramethylammonium (TMA). More detailed study of the conditions of the hydrothermal synthesis (pH, Na/P ratio, temperature) on the formation of zinc phosphate in the presence of NaOH was carried out in [6]. In later investigations with use of various organic bases, for instance, 1,4-diaminobutane [7] and tetraethylenepentamine [8] zinc phosphates with laminar [7] and tomsonite [8] structure have been synthesized. In the abovementioned works the main attention was given to synthetic and structural aspects of zinc phosphates, while the catalytic properties of these materials have been studied insufficiently. At the same time zinc phosphates synthesized in the presence of sodium and tetramethylammonium are active catalysts of condensation of benzaldehyde with different esters [9]. The aim of the present investigation is the study of the influence of nature of organic and inorganic bases (NaOH, KOH, NH4OH and triethanolamine) on the crystallization of zinc and iron phosphates in RZnO(Fe203)-P205-H20 system, the obtaining of ion-exchanged forms of the synthesized zinc phosphates and the study of their catalytic properties in methanol conversion.

1050 EXPERIMENTAL Synthesis and characterization The hydrothermal synthesis of zinc and iron phosphates was carried out in the teflon-lined stainless steel autoclaves at 423-473K under autogenous pressure for 3-10 days. The freshly precipitated zinc or iron hydroxide and 85% orthophosphoric acid are used as a zinc (iron) and phosphorus sources. The cations of potassium, sodium and ammonium were introduced in the form of alkalis. The alkalinity of hydrogel has been varied within pH=8-10. Mole ratio of zinc (iron) oxides and phosphorus in the reaction mixture has been varied within 1:1-1:3. Triethanolamine was used as an organic base in some experiments. After cooling the autoclave, the solid samples werefiltered,washed and dried at 313-323K. Zinc phosphates were treated by aqueous solutions of Mg, Ca, Zn, Ni, Co, Cu, Fe, Cr and Al salts for obtaining the ion-exchanged forms of them. Four-fold exchange was carried out at 353-363K and every time the solution with the constant concentration of cation equal to 0.0 IN was used. X-ray diffraction, thermogravimetric and differential thermal analyses have been used for identification of the synthesized crystals. The primary diagnostics has been carried out on diffraction pattern taken on DRON-2 in CuKa-radiation. At the preliminary study of monocrystals by photo-method their quality, symmetry, unit cell parameters have been determined on Laue swing X-ray photograph with the subsequent more precise definition on the automatic diffractometer "Syntex R21"(^ MoKa graphite monochromator) by the least squares method on 15 reflections. The intensity of 911 independent zero reflections has been measured on the diffractometer (scanning, 20-60°) on the crystal by 0.2 x 0.3 x 0.4 mm^ size stretched out along the six-fold rotary axis z. Systematic extinctions have pointed to the belonging to R63 group in the framework of which the structure deciphering has been carried out. Catalytic activity The catalytic properties of zinc and iron phosphates and their ion-exchanged forms have been studied in methanol conversion. The reactions were carried out in fixed bed pyrex reactor at 573-773K, feed rate (WHSV) 2-6 h"\ Analysis of the reaction products were performed by chromatographic method. RESULTS AND DISCUSSION Synthesis and characterization of samples The product isostructural with a mineral-hopeite Zn3(P04)4-4H20 is formed at the hydrothermal crystallization of zinc phosphate hydrogel not-containing a base. The compound is crystallized in orthorhombic symmetry with unit cell parameters a=10.66 A; b=18.36 A and c=5.04 A. At the crystallization of hydrogel containing the organic base the product not containing an organic matrix and also isostructural with a hopeite has been obtained. The new unidentified phases have been obtained in the case of use of inorganic bases NaOH and NH4OH with a low concentration. Taking into account the results of chemical and thermal analyses the composition of these compounds can be expressed by formula NaZn4(P04)3 and NH4HZn2(P04)2-2H20. The compound KZnP04l.5H20 is the product of crystallization upon use of KOH solution as a base. Potassium zinc phosphate is crystallized with hexagonal unit cell parameters a=18.14 A H C=8.504 A. The three-dimensional framework is formed by the connection of Zn04- H P04-tetrahedra. For compensation of negative charge of framework the cations of potassium are located in the channels. As shown from the results, all three Na-, K-, NH4-forms of zinc phosphates are formed under the equal conditions. However, in spite of that and definite likeness of alkaline cations, all three phases are characterized by different structural features. In the absence of these cations in the reaction mass the synthetic analog of hopeite mineral is crystallized. This phase is formed both with and without participation of the organic base. Hopeite is formed at the low concentrations of alkaline cations in the association of zinc phosphates. With increasing concentration of alkaline cations this association is completely substituted by zinc phosphate. The investigation of ion-exchange properties of the synthesized zinc phosphates has shown that the cations of various metals can be introduced into the composition of potassium and ammonium zinc phosphates.

1051 The degree of ion exchange was measured by the X-ray photograph on which for cation-exchanged forms the strengthening of separate Hnes is observed corresponding to the cation location in the framework cavities. The X-ray photographs of initial ammonium zinc phosphate and its cation-exchanged forms are presented in Figure 1. At the comparison of X-ray photographs the changes of intensities of separate lines are seen distinctly while at 29=14.5; 39.0 and others new lines appear. The decrease of crystallinity is observed in the case of exchange of ammonium cation by cations of trivalent elements.

4^

I

10

I

20

I

30

•

•

I

40

50

60

2 6 , CiiK«

Figure 1. Diffractograms of NH4HZn2(P04)2-2H20 (a) and its Ni-(b), Co-(c) and Cr-substituted forms (d). The ion-exchanged forms of sodium zinc phosphates were not obtained, apparently because of the structure density of the obtained compound. The thermogravimetric and differential thermal analysis were used to investigate the dehydration process of the synthesized zinc phosphates and their thermal stability. The thermogravimetric analysis of the compound NH4HZn2(P04)2-21120 shows two weight losses centered at -655K and ~705K, respectively. These two losses should correspond to desorption of crystal water and to the compound decomposition with the ammonia release. Here the formation of pyrophosphate, Zn2P207has been established by X-ray analysis. The thermogravimetric analysis of the Ni-substituted form of ammonium zinc phosphate shows only a weight loss at -65OK, caused by the removal of adsorbed water. A weight loss corresponding to the removal of ammonia is absent. It testifies to exchange of NH4 cation by Ni cations. If the structure of the initial compound undergoes the changes after removal of ammonia, that cation-substituted form keeps its structure up to 880K. On the cation-substituted forms of ammonium zinc phosphate the dehydration process is reversible. This fact is an evidence of its zeoilte character. In the analysis of the KZnP04-1.5H20 sample two thermal events centered at ~405K and ~973K are observed. The first one can be assigned to the desorption of water, the other is essentially due to the transition of phase. The weight loss is 12 wt.%. In the case of Ni-substituted form three thermal events centered at ~400K, -995K and -HOOK are observed. The first one should correspond to desorptionb of water, whereas the two other are due to the transition of phase. Morever, in the case of cation-substituted form the total weight loss increased up to 14,7 % owing to the removal water. The thermogravimetric analysis of hopeite shows two well defined stages of nearly equivalent weight losses centered at ~385K and -605K, respectively. This two losses should correspond to desorption of water, as no organic species are present in the sample. Indeed, a total weight loss of 16,2 wt.% is obtained, which compared with the predicted value from the stoichiometry of hopeite Zn3(P04)2-4H20. DTA of the compound NaZn4(P04)3 indicated the absence of water molecules in the structure of the compound.

1052 At the hydrothermal synthesis of iron phosphate without the organic base the transparent crystals in the form of hexagonal prisms are formed reaching up to 0.2 x 0.4 x 0.3 mm^ sizes. The primary diagnostics has been carried out on diffraction pattern taken on DRON-2 in CuKa-radiation with the subsequent more precise definition on the diffractometer "Syntex RIVXX MoKa graphite monochromator). Iron phosphate is crystallized with hexagonal unit cell parameters a=9.143 A and b=l 6.777 A. The groupings-"dimers", including two Fe-octahedra and three P04-tetrahedra being the characteristic structural elements of the mixed frameworks and encountered in the crystalline silicates and phosphates structures are the architectural basis of structure [1]. The composition of one "dimer" is expressed by formula Fe206(P04)3. Beaded on the triple rotary axis "dimers" form uninterrupted columns along the axis z with alternation of filled octahedra and empty trigonal prisms. The crystal lattice can be examined as columns combined with octahedrons in the original three-dimensional consequency. The obtained motive consisted of Fe-octahedra and P04-tetrahedra has a negative charge for compensation of which the positive cations are demanded. According to crystallochemical analysis the protons converting the free end oxygen atoms of phosphate groups into OH ions may be such positive cations. Thus, taking into account water molecules observed by thermogravimetric analysis and statistically located in the above emptiness as well as hydrogen atoms positions of which were not managed to localize the structure is expressed by formula Fe3H3(P04)2-2H20. The water containing in it has a zeolite character, being desorbed from the cell at ~423K and again adsorbed by cooling. The composition of iron phosphate obtained in the presence of triethanolamine can be expressed as Fe2H3(P04)3(C2H40H)3N. According to thermogravimetric analysis the weight loss centered at ~423K characterizes the removal of the adsorbed water. This is followed by a d.t.g. peak centered at ~553K, which corresponds to the bum-out of the organic part, and a finally a weakly d.t.g. peaks centered at -85 8K and ~993K is observed. The crystal data of iron phosphates, obtained with participating of triethanolamine and without it are given in Table 1. Table 1. The crystal data of the two compounds.

Lattice parameters, A

Chemical formida

Fe2H3(P04)3-(EtOH)3N FeHs^OOrSHaO

9.61 9.143

8.64

14.19 16.777

V,(A)'

Crystal system

1178.2 1214.6

Rhombic Hexagonal

The reaction between oxides of iron (III) and phosphorus (V) leads to the formation of following compounds: hydrophosphate FeH3(P04)2-2H20, iron phosphate containing organic molecule in the structure Fe2H3(P04)3(C2H40H)3N and quartz-like phosphate - FeP04. The crystallization of the iron phosphates depends upon temperature of the process, for example, upon increasing temperature up to 473K at the constancy of all remaining parameters of synthesis the compound Fe2H3(P04)3(C2H40H)3N is associated with iron phosphate. The given association is crystallized also at 523K, however, above this temperature it is completely substituted by quartz-like phase. The analysis of kinetics of crystallization shows that iron phosphate obtained in the presence of the organic base is a stable phase at temperatures 423 and 473K. Upon increasing temperature up to 523K its content rises then with increasing the process duration the destruction occurs accompanied by formation of the more stable quartz-like phase. The kinetic analysis of crystallization process allows to suppose that quartz-like iron phosphate is not the product of Fe2H3(P04)3(C2H40H)3N destruction, i.e. its crystallization begins earlier than destruction. Catalytic activity The investigation of catalytic properties of zinc phosphates in methanol conversion has shown the absence of catalytic activity of sodium zinc phosphate. At the same time the ion-exchanged forms of potassium and ammonium zinc phosphates show an appreciable activity in methanol conversion. The results obtained with the different catalysts are presented in Table 2.

1053 Table 2. The catalytic properties of potassium and ammonium zinc phosphates in methanol conversion.

Catalyst

K-Zn-P-0 Ca(K)-Zn-P-0 Co(K:)-Zn-P-0 Cu(K)-Zn-P-0 Ni(K)-Zn-P-0 Fe(K)-Zn-P-0 NH4-Zn-P-0 Ca(NH4)-Zn-P-0 Co(NH4)-Zn-P-0 Cu(NH4)-Zn-P-0 Ni(NH4)-Zn-P-0 Fe(NH4)-Zn-P-0

Conversion of CN3OH, %

12.3 50.3 54.8 51.4 76.3 64.7 24.0 67.4 72.8 76.3 82.7 90.0

Yields of main reaction products, molVo C2-Q olefins

DME

1.6 14.1 4.4 18.4 5.3 30.4 6.0 29.8 28.8 48.8 50.1 62.3

8.4 32.7 39.4 28.4 64.2 31.0 16.4 27.4 20.3 13.6 21.3 14.4

CN^

6.3 0.5 1.3 14.8 18.1 10.4 8.1 3.7

Selectivity, mol^/o DME

C2-Q olefins

68.3 65.0 71.9 55.2 84.2 48.0 68.3 40.6 28.0 17.8 25.7 16.0

13.0 28.0 8.0 35.5 6.9 51.6 25.0 44.2 40.3 64.1 60.6 69.2

Temperature = 723K, WHSV= 4 h"^ As can be seen in the Table 2, the initial potassium and ammonium zinc phosphates are low-active in methanol conversion, however, upon K^- and NH4^-ions exchange by cations of bivalent metals an appreciable increase of catalyst activity takes place. From the series of catalysts obtained by potassium cations exchange in the KZnP04-1.5H20 Nisubstituted form is more active. Conversion of methanol is 76.3%. Dimethyl ether (DME) is the main product of reaction, selectivity of which reaches 84.2%. On Co-substituted form the yield of DME is decreased and formaldehyde is observed in the reaction products, the yield of which is 6.3%. Fe-form of potassium zinc phosphate possesses a higher activity in the formation of low olefins C2-C4. Thus, at methanol conversion 64.7% the yield of C2-C4-olefins is 30.4% at the selectivity 51.6%. On the cation-substituted forms of potassium zinc phosphate conversion of methanol occurs with the formation of DME as the main product, selectivity of which varies in the interval 65.0-84.2%. Only in the case of Fe-forms it is reduced down to 48.0% because of a higher yield of low olefins. The dependence of DME yield from temperature on these catalysts passes through maximum in the range of temperatures 710740K. At higher temperatures the yield of DME is reduced and the content of reaction products of methanol decomposition to CO and H2 is increased. The study of catalytic properties of cation-substituted forms NH4HZn2(P04)2-2H20 has shown that Fesubstituted form is the most active on which methanol conversion reaches 90,0% and selectivity on C2-C4olefins is 62.3%. On the other cation-substituted forms methanol conversion varies in the range 67.4-82.7%. A relatively high yield of formaldehyde (18.1%) is observed in the case of Co-form of ammonium zinc phosphate. The temperature dependence of yield of lower olefins on these types of catalysts is presented in Figure 2.

1054

o-Fe(NH4>2n-P-0 A -Ni(NH^-Zn-P-0 A-Ca(NH4)-Zn-P-0 •-NH*-Zn-P-0

mm

O

e

623

673

723

773

Temperature^ K Figure 2. The dependence of yield of lower olefins from temperature. Comparison of catalysts having the same substituting cations but distinctive by structure of zinc phosphate framework shows that the structure of the initial zinc phosphate influences the catalyst activity and the direction of methanol conversion. The catalysts on the basis of ammonium zinc phosphate are more active in the formation of C2-C4-olefins and formaldehyde but in the case of potassium zinc phosphate DME is formed more selectively. The obtained results can be explained in the following way: in the case of ion-exchanged forms of potassium zinc phosphate the acid centers are weak and mainly carry out dehydration of methanol into dimethyl ether. For further conversion of methanol or DME into hydrocarbons the strong acid centers absent over these catalysts are needed. Ammonia zinc phosphates and their ion-exchanged forms in the result of pretreatment of catalyst on air at 773-823K detach the ammonia molecule and at the same time formed protons play the role of the strong acid centers. Conversions of methanol and DME into hydrocabons occur with participating of the strong acid centers. Iron phosphate synthesized in the presence of the organic base after calcination at 823K possesses a very low activity in methanol conversion. Apparently the organic part being burnt-out blockades the active centers of catalyst by products of combustion. Therefore for the removal of organic part iron phosphate was placed in the teflon-lined autoclave, flooded by IM methanol solution in hydrochloric acid and kept up at 433K for 24 hours. After that it was washed by distilled water, dried on air and calcined in air flow at 873K for 8 hours. Then the catalytic properties of their were studied. The obtained results are presented in Table 3. Table 3. Conversion of methanol over iron phosphate synthesized in the presence of triethanolamine.

Temperature, K

673 723 WHSV = 2 h-^

Conversion, %

56.3 68.8

Yields of main products, moL%

Selectivity, mDL%

C2-Q olefins

DME

DME

C2-Q

6.5 18.1

34.3 43.7

61.0 63.5

11.6 26.3

1055 The cation-substituted ammonium zinc phosphates have been also used for preparation of multi-component catalysts of methanol conversion to hydrocarbons. It has been shown that the catalytic compositions on the basis of zinc phosphates, zeolites and Ni- or Zn-silicates have a high activity in methanol conversion into low olefins. At the same time it is interesting to note that Ni- and Zn-silicates show the activity only in the methanol decomposition to CO and H2. However, the introduction of the zeolite and Ni(NH4)-Zn-P-0 into the composition of Ni-silicate leads to the obtaining of a rather active catalyst on which nearly a complete conversion of methanol (98,7%) is observed at selectivity on C2-C4-olefins more than 85%. The sample displayed high selectivity to ethene (up to 52%) of the gas products). It indicates the possibility of use of zinc phosphates in the composition of multi-component catalysts for methanol conversion.

CONCLUSION At the hydrothermal synthesis in the R-ZnO-P205-H20 system although all three sodium-, potassium- and ammonium-zinc phosphates are formed under the equal conditions but they are characterized by the different structure features. In the case of NaOH and NH4OH the new phases NaZn4(P04)3 and NH4HZn2(P04)2-21120 have been obtained which were not identified. In the presence of KOH, KZnP04T.5H20 of zeolite-like structure is crystallized. Iron phosphate synthesized in the presence of triethanolamine shows an activity in methanol conversion after removal of the organic molecules from catalysts pores. The activity of zinc phosphates depends on their structure and composition. Sodium zinc phosphate practically is not active in methanol conversion. Potassium and ammonium zinc phosphates in the initial forms are low-active but their cation-substituted forms depending upon metal nature carry out the reaction in the direction of obtaining DME, lower olefins and formaldehyde. The multicomponent catalyst on the basis zinc phosphate, zeolite and Ni-silicate displayed high activity in the conversion of methanol to hydrocarbons. A nearly conversion of methanol (98,7%) is observed at selectivities to C2-C4-olefins more than 85,0% and 52,9% to ethene.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

Barrer R.M., Hydrothermal Chemistry of Zeolites, Akademic Press, London, New York, 1982. Flanigen E.M., Patton R.L., Wilson S.T., Study Surf Sci. Catal., 37 (1988) 13. Gier T.E., Stucky G.D., Nature, 349 (1991), 508. Nenoff T.M., Harrison W.T.A., Gier T.E., Stucky G.D., J. Am. Chem. Soc, 113 (1991), 378. Harrison W.T.A., Gier T.E., Moran K.L., Nicol J.M., Eckert H., Stucky G.D., Chem. Mater., 3 (1991), 27. Gier T.E., Harrison W.T.A., Nenoff T.M., Stucky G.D., Synthesis of Microporous Materials. Vol. 1 (1991), 407. Echavarria A., Saldarriaga C , Microporous and Mesoporous Materials, 42 (2001), 59. Neeraj S., Natarayan S., J. Phys. Chem. Solids, 62 (2001), 1499. Garcia-Serrano L.A., Blasco T., Perez-Pariente J., Sastre E., Stud. Surf Sci. Catal., 135 (2001), 317.