Reversibility of the Modification of HZSM-5 with Phosphate Anions

Article pubs.acs.org/JPCC

Reversibility of the Modification of HZSM‑5 with Phosphate Anions Miroslaw Derewinski,*,†,‡ Priit Sarv,§ Xianyong Sun,∥ Sebastian Müller,∥ Andre C. van Veen,∥ and Johannes A. Lercher*,∥ †

Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99354, United States J. Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Cracow, Poland § Institute of Chemical Physics and Biophysics, Estonian Academy of Sciences, Akadeemia tee 23, 12618 Tallinn, Estonia ∥ Department of Chemistry and Catalysis Research Center, Technische Universität München, Lichtenbergstr. 4, 85747 Garching, Germany ‡

S Supporting Information *

ABSTRACT: Quantitative NMR spectroscopic studies (27Al, 31 P, and 1H magic-angle spinning NMR) were combined with temperature-programmed desorption (TPD) to investigate the impacts of sequential treatment procedures, i.e., calcination and hot water washing, of phosphorus-modified ZSM-5 samples on their local structural changes for aluminum and phosphorus as well as on the acidities. Introduction of phosphorus resulted mainly in the transformation of tetrahedrally coordinated framework aluminum Altetra‑frame to distorted aluminum Altetra‑dist with a tetrahedral coordination. The lowered Altetra‑frame concentration accounted for the decrease in Brønsted acidity for the phosphorus-modified zeolites. Hightemperature calcination of P HZSM-5 resulted in formation of occluded condensed polyphosphates and Al−O−P complexes (Altetra‑dist−O−P and Alocta−O−P species where Al remained at the original framework positions), which were not present in the standard HZSM-5. Washing with hot water removed most of the introduced P and partly restored framework Altetra‑frame sites from these Al−O−P complexes, consequently increasing the corresponding Brønsted acidity. This could be explained by the partial reversibility of the modification process of HZSM-5 with phosphorus anions. Moreover, the treatments changed also the ratio of the framework Al sites, i.e., B1 and B2 sites. The larger extent of restoration of B2 compared to B1 acid sites increased the relative abundance of B2. The modified acidities were correlated to their catalytic performances in the methanol-to-olefins reaction.

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INTRODUCTION MFI type zeolites are extensively applied as catalysts in a wide variety of industrial processes. Their catalytic performance relies, to a large extent, on their acidity and shape-selective properties. Postsynthetic modification of zeolites with phosphorus is a frequently adopted method of tunig the acidity and consequently the catalytic properties in terms of activity, shapeselectivity, and hydrothermal stability.1−3 Various inorganic and organic phosphorus precursors were reported to have been successfully used. After wet impregnation and subsequent calcination in air at elevated temperatures, the modified zeolites usually showed adapted catalytic performance for catalyzed reactions including the methanol-to-olefins (MTO) process, toluene alkylation, fluid catalytic cracking, and C4 olefin cracking. Since the first reports by Butter and Kaeding,1−3 intense efforts have been made to understand the interaction of the MFI zeolite framework with phosphorus compounds and the species generated by this modification. Several models were proposed (Scheme 1). Phosphorus-induced optimization of the acidity of HZSM-5, however, requires a fundamental understanding of the © 2014 American Chemical Society

Scheme 1. Proposed Models on the Interaction of Phosphorus with the ZSM-5 Zeolite Framework Prepared by Impregnation with Orthophosphoric Acid and Calcination, As Proposed by (A) Kaeding et al.3 and later Védrine et al.,4 (B) Lercher et al.,5,6 (C) Xue et al.,7 and (D, E) Blasco et al.8

mechanism of phosphorus modification. Three intriguing impacts of phosphorus introduction were demonstrated, but controversy exists regarding the structure and the attribution. First, phosphorus-containing zeolites show enhanced stability against dealumination in the presence of steam.9 Several authors attributed this to the interaction between phosphorus Received: May 31, 2013 Revised: February 7, 2014 Published: February 10, 2014 6122

dx.doi.org/10.1021/jp4053677 | J. Phys. Chem. C 2014, 118, 6122−6131

The Journal of Physical Chemistry C

Article

ments were performed on a spinner with a 1/4 in. slit from 5− 50° 2θ (0.05° min−1). Scanning Electron Microscopy (SEM). SEM images were obtained on a JEOL 500 SEM (accelerating voltage 15 kV). Low-Temperature N2 Physisorption. BET specific surface areas, pore volumes, and pore size distribution were obtained using a PMI automatic BET-Sorptometer from N2 adsorption−desorption isotherms carried out at −196 °C. Before measurements, the samples were outgassed at 400 °C for 2 h. Brunauer, Emmet, and Teller (BET), Barrett, Joyner and Halenda (BJH; desorption branch), and t-plot methods were used for calculating specific surface area and meso- and micropore volume. NMR Spectroscopy. 1H, 27Al, 31P MAS NMR spectroscopy was used to characterize the P HZSM-5 zeolites under study. All NMR experiments were performed on Bruker Avance III 800 and with Bruker TriGamma probe equipped with 3.2 mm o.d. ZrO2 rotors. The single pulse MAS NMR experiment conditions are listed below. 27Al: spinning speed, 20 kHz; exciting pulse, 0.5 μs (10°); relaxation delay, 0.2 s; 10 000 scans. 31P: spinning speed, 20 kHz; exciting pulse, 1.5 μs (30°); relaxation delay, 5 s; 1768 scans. 1H: spinning speed, 18 kHz; exciting pulse, 3.7 μs (90°); relaxation delay, 40 s; 160 scans. In the case of 1H spectra the background signal was recorded separately and subtracted. The spectra were referenced as the following: 1H to adamantane (CS = 1.8 ppm); 27Al to alun (CS = 0 ppm); and 31P to Na3PO4·12H2O (CS = 6.0 ppm). Spectra were simulated with “dmfit program” by D. Massiot.14 Simulations are given in Supporting Information. Prior to 1H measurements, the samples were heated in air at 450 °C for 4 h and then under vacuum of 10−4 mbar for 30 min. Samples were cooled under vacuum and transported under vacuum in sealed vials to the glovebox, where the vials were opened in dry argon atmosphere (H2O < 1 ppm, O2 < 1 ppm) and samples were filled into rotors. Rotors were closed with vespel caps and a small quantity of Kel-F oil was spread between the cap and the rotor. All the handling of the rotors (filling, emptying) was done in the glovebox to avoid contamination with water. From the glovebox the rotors were transported to the spectrometer and spun with dry air. Temperature-Programmed Desorption. The change in intrinsic strength of the acidic sites of the used materials was investigated by temperature-programmed desorption (TPD) of adsorbed pyridine. For this, 50 mg of all samples were pressed into pellets with a specific particle size in the range of 0.5 and 0.71 mm. After activation in vacuum (p = 10−3 mbar) at 550 °C for 1 h (heating rate of 10 °C min−1), pyridine was adsorbed (p = 1 mbar) at 100 °C for 1 h. The physisorbed pyridine was removed by outgassing the materials for 2 h at 150 °C. The signal of desorbing pyridine was monitored from 150 °C to 800 °C (heating rate of 10 °C min−1) using a Pfeiffer QMS 200 mass spectrometer. Catalytic Testing. Catalytic testing was performed in a fixed bed quartz reactor with an internal diameter of 6 mm at 450 °C and a constant pressure of 1.07 bar. The proton-form of the catalysts was press-pelletized, crushed, and used in a sieve fraction ranging from 200 to 280 μm. The active zeolite was 1:20 (wt) diluted in 300−500 μm silicon carbide beads to ensure isothermal conditions. Catalyst activation prior to methanol conversion involved heating at 480 °C for 1 h under 50 mL min−1 N2 flow. Methanol was fed by passing 50 mL min−1 N2 through a methanol evaporator kept at 26 °C.

and the zeolite framework, stabilizing tetrahedrally coordinated framework aluminum.8,10 Second, introduction of phosphorus species significantly reduces the Brønsted acidity.2−6,10−12 Whereas most of the proposed models suggest the throughbonding interaction of phosphorus with bridging hydroxyl groups (Scheme 1), definitive experimental evidence, e.g., for Si−O−P bond formation, which would be the direct consequence of such direct interactions, is lacking. On the other hand, several authors attributed the acidity decrease to phosphorus-induced dealumination of tetrahedrally coordinated lattice aluminum together with the simultaneous formation of water insoluble amorphous extra-framework aluminum phosphates.10,13 Third, elution of phosphorus by washing of impregnated HZSM-5 samples with hot water prior to calcination could lead to complete restoration of Brønsted acidity and textural properties. Washing applied to calcined P HZSM-5 materials results in partial restoration of the acidic properties.10 The possibility of restoring Brønsted acid sites appears to contradict the model of extra-framework aluminum phosphates formation; the underlying mechanism for this reversibility was, however, hardly discussed. Thus, despite intensive investigations, fundamental questions concerning the chemical nature of the species formed during various treatments of P-modified HZSM-5 zeolites, as well as the mechanism of their formation, are at best hypothesized without being unequivocally established. In the present work, we investigate how steps during the preparation, i.e., impregnation with orthophosphoric acid and drying at 120 °C, calcination at 550 °C, and finally the hot water washing procedure (at 80 °C) influence the properties of the final catalysts. We aim to address (i) the reason for the decrease in the concentration of Brønsted acid sites in H3PO4 modified zeolites and (ii) the mechanism for the reversibility of modification of HZSM-5 zeolite with orthophosphoric acid. Quantitative NMR spectroscopy (27Al, 31P, and 1H MAS NMR) was combined with temperature-programmed desorption (TPD) to follow the changes in the coordination environment of aluminum and phosphorus and in the acidities of the samples after each treatment step. The various materials were explored with respect to the MTO reaction, which has been claimed to be critically influenced by the presence of phosphorus.

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EXPERIMENTAL SECTION

Sample Preparations. The ZSM-5 zeolite was synthesized under hydrothermal conditions using tetrapropylammonium ions (TPA+) as a template. The as-synthesized zeolite was calcined at 550 °C for 6 h and subsequently ion-exchanged at 80 °C using a 0.1 M NH4NO3 solution. xP HZSM-5 zeolite (Si/Al = 35) with different amounts of phosphorus (x = 1% and 2 wt %) have been obtained by impregnating HZSM-5 with an aqueous solution containing the respective amount of orthophosphoric acid (H3PO4). After being dried in a rotary vacuum evaporator at 50 °C, the sample was dried at 120 °C (5 h) then calcination at 550 °C (6 h) in flowing air. The subsequent washing with water at 80 °C (2 h) and thermal treatment (550 °C) was performed to study how physicochemical properties were altered by deposited P-containing species and if original properties can be restored. X-ray Diffraction (XRD). X-ray powder diffraction patterns were recorded on a Philips X′Pert Pro System operated with a Cu Kα1-radiation (0.154056 nm) at 40 kV/40 mA. Measure6123



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specific surface area (14 m2 g−1) was found for 2%P HZSM-5. This is tentatively attributed to removing oxide moieties in the pores by the washing procedure. TPD of pyridine was used to identify the concentration and to a lesser degree also the strength of acid sites in parent and Pmodified HZSM-5 samples. Desorption traces from the parent and the P-modified and subsequently calcined x%P HZSM-5 samples are compiled in Figure 2. Desorption curves show two

Weight hourly space times of 0.2 to 1.7 h gcat molMeOH−1 were adjusted by charging varying amounts of catalyst. The reactor effluent was online analyzed by a GC equipped with a HPPLOTQ capillary column (30 m, 0.32 mm i.d.) and a flame ionization detector. Because of the rapid interconversion, both oxygenates, i.e., methanol and dimethyl ether, were lumped as reactant. Conversion and yields were calculated on a carbon basis.

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RESULTS AND DISCUSSION ZSM-5 Impregnated with Orthophosphoric Acid and Subsequently Calcined. X-ray diffraction analysis was employed to confirm the high crystallinity of as-synthesized material, i.e., pure MFI (Si/Al = 35) crystalline phase without any contaminations (crystalline or amorphous) has been obtained. Introduction of P had no detectable effect on the crystallinity of the parent HZSM-5 zeolite and no new crystalline phase was found in the modified preparations after calcinations and washing procedures (XRD patterns not shown). SEM analysis revealed that the shape of the crystals is approximately that of a parallelepiped with dimensions c.a. 14 × 7 × 5 μm3 (Figure 1). In addition to the single crystals, the Figure 2. Temperature-programmed desorption of pyridine from parent HZSM-5 zeolite and P HZSM-5 samples of different phosphorus contents.

well-resolved peaks; the temperatures of the maxima are around 250 and 550 °C, respectively. These are attributed to desorption of pyridine from weak and strong acid sites, the latter being characteristic of bridging (SiOHAl) hydroxyl groups.6,15 The unmodified HZSM-5 showed a concentration of strong acid sites that was significantly higher than that of weak acid sites. Introduction of H3PO4 caused the concentration of strong Brønsted acid sites to decrease proportionally to the concentration of P deposited. Introduction of 1%P led the peak at approximately 550 °C to be considerably reduced. Introduction of 2%P eliminated these sites almost completely. In parallel to the introduction of 1% and 2%P, the concentration of weak acid sites increased. We attribute this increase to the formation of weak Lewis acid sites coordinatively adsorbing pyridine. These sites are located on small AlPO4 particles formed in the presence of phosphate ions with Al3+ being removed from the lattice via dealumination. For the sample with the higher phosphorus loading (2%P), the slight decrease of this peak after washing points to the participation of polyphosphate species in this adsorption mode.

Figure 1. SEM images of parent and 1%P impregnated HZSM-5 samples.

twinned crystals are present as well in a significant amount. Subsequent introduction of phosphorus into the zeolite led to little change in the morphology of the zeolite crystals. N2 physisorption results are presented in Table 1. The introduction of phosphorus led to a minor decrease in the BET surface area (20 m2 g−1 and 40 m2 g−1 for 1%P HZSM-5 and 2%P HZSM-5, respectively) and the micropore volume (0.095 mL g−1 and 0.090 mL g−1 for 1%P HZSM-5 and 2%P HZSM-5, respectively), proportional to the amount of P introduced. The washing procedure completely restored the specific surface area (BET) and the micropore volume for 1%P HZSM-5. A small increase in the micropore volume (0.08 mL g−1) and BET

Table 1. Specific Surface Area and Porosity of Parent and P-Modified HZSM-5 Samples sample HZSM-5 1%P HZSM-5 calcined 1%P HZSM-5 washed 2%P HZSM-5 calcined 2%P HZSM-5 washed

BET surface area (m2 g−1)

total pore volume (mL g−1)

micropore volume1 (mL g−1)

mesopore volume (mL g−1)

av mesop. diameter (nm)

365 345

0.214 0.205

0.098 0.095

0.116 0.11

1.68 1.69

364

0.206

0.097

0.11

1.69

325

0.181

0.090

0.09

1.73

379

0.243

0.107

0.11

1.73

1

Determined with the t-plot method. 6124



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A and B) with chemical shifts of 56.1 and 53.5 ppm, respectively, attributed to different framework aluminum species.18 Additionally, a narrow line of the extra-framework, octahedrally coordinated Al in a symmetrical environment Alocta‑classic (0.0 ppm) and a broad line of octahedrally coordinated Al (Alocta‑nonclassic) tentatively attributed to Al partially bound to the framework, i.e., Al(OSi)n(H2O)m (m + n = 6) at ∼4 ppm (23.0% of total Al), were observed. A very small concentration (2.5%) of distorted tetrahedrally coordinated Al at 41.0 ppm was also detected. Impregnation with orthophosphoric acid and subsequent drying at 120 °C resulted in the complete loss of the signal characteristic of Alocta‑classic and the decrease of the line attributed to Altetra‑frame (mainly line A at 53.5 ppm). These sites are converted to highly shielded octahedrally coordinated Al (Alocta) and to tetrahedrally coordinated distorted sites (Altetra‑dist) with chemical shift of −12.4 ppm and 40 − 48 ppm, respectively. However, the observed shifts were not very pronounced. The subsequent calcination decreased the concentration of Altetra‑frame sites considerably (reduction in the concentration from 14.6 × 1016 to 7.5 × 1016 atoms mg−1), converting them mainly to the Altetra‑dist (increase from 1.6 × 1016 to 8.3 × 1016 atoms mg−1) (Table 2) with the −12.4 ppm line becoming less shielded (shift of the resonance line to −10.4 ppm). Please note that the total Al concentration remained unchanged. On the basis of several references,8,10,13,19−22 we assigned the new Altetra‑dist and Alocta‑classic (at −10 ppm) to Al complexes with phosphorus. Impregnation with 2% of phosphorus resulted in significantly higher decrease of the number of Altetra‑frame sites (concentration almost three times lower than that recorded for 1%P HZSM-5 sample, i.e., 5.4 and 14.6 × 1016 atoms mg−1, respectively) which are converted to Alocta and Altetra‑dist (Table 2). Subsequent calcination at 550 °C led to further reduction in the concentration of Altetra‑frame followed by the increase of Altetra‑dist and Alocta. 31 P MAS NMR spectra of the impregnated (I) and calcined (II) 1%P HZSM-5 zeolite are shown in Figure 4. The assignment of the observed resonance lines in P HZSM-5 is mainly based on ref 21 related to the NMR data of phosphate and aluminophosphate species. The P-impregnated and dried sample yielded two main resonance lines at 0 and c.a. −8 ppm, which contribute more than 90% of the total intensity of the recorded spectrum (50% and 43%, respectively) (Figure 4). The first line is attributed to free monomeric orhophosphate (PO4) groups, the second one to “end group” containing two P atoms in linear pyrophosphates formed during heating the sample at 120 °C for several hours. The formation of Al−O−P bonds is manifested by weak resonances at −22 ppm (c.a. 3%) and −31.6 ppm (c.a. 2%) (Figure 4). According to Damaradan

Similar variations in the traces of desorbing pyridine have been reported in ref 16. 27 Al MAS NMR spectroscopy was used to characterize the nature and concentration of the aluminum species. Figure 3

Figure 3. 27Al MAS NMR spectra of the parent HZSM-5 zeolite (0) and impregnated (I) and calcined (II) 1%P HZSM-5 zeolite and difference spectra, i.e., Δplot(I−0) and Δplot(II−I).

shows the 27Al MAS NMR spectra of parent HZSM-5, of 1%P HZSM-5 after impregnation and drying at 120 °C, and 1%P HZSM-5 calcined at 550 °C. The 27Al MAS NMR spectra comprise three well-resolved 27Al resonance bands at around 55, 45, and −10 ppm, which were assigned to three groups of aluminum species: Altetra‑frame, Altetra‑dist, and Alocta, respectively, according to ref 17. Concentrations and relative intensities of the resonance bands attributed to these species are compiled in Table 2. In the spectrum of the parent HZSM-5 the main resonance line, attributed to tetrahedrally coordinated framework aluminum Altetra‑frame (74.5% of total Al), splits into two (lines

Table 2. Concentration and Distribution of Aluminum Species in the Parent and P-Modified HZSM-5 Samples Determined by 27 Al MAS NMR Al concentration (1016 atoms mg−1) sample HZSM-5 1%P HZSM-5 1%P HZSM-5 1%P HZSM-5 2%P HZSM-5 2%P HZSM-5 2%P HZSM-5

impregn calcined washed impregn calcined washed

Al distribution (%)

Altotal

Altetra‑frame

Altetra‑dist

Alocta

Altetra

Altetra‑dist

Alocta

23.8 23.8 24.0 22.5 25.5 23.2 22.9

17.8 14.6 7.5 9.4 5.4 3.3 4.7

0.6 1.6 8.3 6.7 7.2 9.0 9.3

5.5 7.6 8.2 6.4 12.9 10.9 8.9

74.5 61.3 31.2 41.8 21.2 14.2 20.5

2.5 6.7 34.6 29.8 28.2 38.8 40.5

23.0 31.9 34.2 28.4 50.6 46.9 38.9

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1016 atoms mg−1), which is speculated to be related to sublimation of phosphorus oxide formed during dehydration at elevated temperatures.13 Resonances, which can be attributed to phosphorus sites not associated with aluminum as well as resonances representing phosphorus species that have aluminum in their coordination sphere were observed in the spectrum. The first group consists of resonance lines at 0 ppm (residual free monomeric [PO4] orthophospate groups), −5.9 ppm (linear longer polyphosphate chains), −13.2 ppm (middle groups in condensed polyphosphates, i.e. PO[PO4]2), and −40 ppm (branching groups of OP[PO4]3 type in condensed polyphosphates). It has been additionally reported that species containing branched groups in condensed polyphosphates bound to aluminum (mainly Altetra‑dist) also contribute to −40 ppm resonance (see species 8, QP3 in ref 17). The resonance lines at −27.7 and −32.9 ppm are attributed to the presence of P−O−Al connectivity in the calcined 1%P and 2%P HZSM-5 zeolite. The resonance at −27.7 ppm represents species of Al3+(H2O)3(PO43−)3 type,21,23 whereas the resonance at −32.9 ppm is related mainly to Altetra‑dist bound to phosphates (aluminum phosphate). According to ref 21, the distortion of tetrahedral symmetry of Altetra‑frame is facilitated by acidic conditions during impregnation, leading to formation of (SiO)3Al(OH2)n type species. The presence of phosphorus in the vicinity of such species results in the formation of Altetra‑dist bound to phosphorus. It has to be stressed that middle chain groups in polyphosphate strongly deshielded by Al atoms can also contribute to this spectral region (−30 ± 2 ppm) (see species 7, i.e., QP21 in ref 21). A similar 31P MAS NMR spectrum was recorded for the sample impregnated with 2% of P and dried at 120 °C, as well as for the sample subsequently calcined at 550 °C (Table 3). The calcination procedure removed approximately 13% of the phosphorus, and the concentration of P, which remained in the 2%P HZSM-5 sample, is about 2.5 times higher than that found for the calcined 1%P HZSM-5 sample. Moreover, the observed higher concentration of Altetra‑dist and Alocta sites confirmed that considerably more Altetra‑frame sites are involved in generation of new Al−O−P species (58% and 82% for 1% and 2% Pmodified HZSM-5, respectively). The 1H MAS NMR spectra of the parent HZSM-5 sample and the 1%P HZSM-5 sample calcined at 550 °C and the difference spectrum are shown in Figure 5. The number of sites and their relative intensities are presented in Table 4. The simulated spectra can be found in the Supporting Information. Several resonance lines can be distinguished in the spectrum of the parent zeolite. The line at 1.9 ppm stems from terminal

Figure 4. 31P MAS NMR spectra of the impregnated (I) and calcined (II) 1%P HZSM-5 zeolite and difference spectrum Δplot(II−I).

et al17 the weak line at ca. −22 ppm is to be attributed to nonframework Al3+(H2O)6−n(PO43−)n species containing water, mostly in the coordination sphere of Alocta3+, and corresponds to the 27Al resonance line at −12.4 ppm in the spectrum of the impregnated sample (Figure 3). The weak resonance at −31.6 ppm is assigned to aluminum phosphate species formed as a result of phosphorus bonding to Altetra‑frame (53.5 ppm) species (resonance line at ca. 45−40 ppm in 27Al NMR spectrum). Because of very low intensities, a detailed description of the nature of species characterized by the resonances at −22 and −31.6 ppm must be considered with caution. Please note that the number of Altetra‑dist formed (1.0 × 1016 atoms mg−1) is equal to the number of P species described by the line at −33 ppm in 31P MAS NMR spectrum (0.9 × 1016 atoms mg−1) (Tables 2 and 3). The very weak line at ca. −5 ppm reflects the presence of free pyrophosphoric acid introduced as contaminant of phosphoric acid used for the impregnation. Because of their narrow line width, we conclude that these species are not coordinated to the framework. High-temperature (550 °C) treatment resulted in significant changes in the 31P MAS NMR spectrum (Figure 4). Calcination of the impregnated sample resulted in significant (ca. 27%) loss of phosphorus (Table 3) (decrease from 40.9 × 1016 to 29.8 ×

Table 3. Concentration and Contribution of Phosphorus Species in P-Modified Samples Determined by 31P MAS NMR P concentration (1016 atoms mg−1) resonance (ppm) sample

total

0

−6 ÷ −8

−13

−27 (−22)*

−33

−40

1%P HZSM-5 impregnated 1%P HZSM-5 calcined 1%P HZSM-5 washed P washed out (%) 2%P HZSM-5 impregnated 2%P HZSM-5 calcined 2%P HZSM-5 washed P washed out (%)

40.9 29.4 6.9 77 90.8 79.1 37.1 53

20.9 0.7 98 12.7 8.4 0.2 98

18.0 3.9 98 12.4 9.7 0.6 94

− 12.1 1.7 86 31.1 25.5 10.4 59

1.1* 5.8 2.6 55 5.2 12.9 10.6 18

0.9 2.3 1.1 52 7.1 15.9 9.5 40

− 4.6 1.4 70 2.1 5.9 5.2 12

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zeolite did not react with phosphorus. It has to be stressed that decrease of the concentration of Brønsted sites was mainly caused by reaction of B1 sites (drop from 4.8 × 1016 to 1.0 × 1016 atoms mg−1 for 1%P and 2%P HZSM-5, respectively), whereas the number of B2 sites did not change significantly (3.4 × 1016 and 3.1 × 1016 atoms mg−1 for 1%P and 2%P HZSM-5, respectively) (Table 4). Effect of Washing. The TPD traces of desorbing pyridine obtained for the samples before and after washing with hot water are compared in Figure 6. The applied washing procedure

Figure 5. 1H MAS NMR spectra of parent HZSM-5 (I) and calcined 1%P HZSM-5 (II) zeolite and difference spectrum (Δplot).

Si−OH groups, while that at 2.2 ppm is the result from internal SiOH groups (hydroxyl nests). The line at 2.7 ppm is attributed to nonframework Al−OH groups.24,25 The similarly intense line at 3.3 ppm is tentatively attributed to inaccessible OH groups on defects created during the removal of organic template.24 Two different Brønsted acid sites (Si−OH−Al groups) denoted as B1 and B2 shown by the resonance lines at 4.1 and 5.7 ppm, respectively, are noted. The one with the higher chemical shift is attributed to the more acidic site. The hydrogen atom of the bridging B2 site is able to move between the oxygen atoms surrounding the framework Al−O tetrahedron, thus giving rise to the third line at 4.8 ppm, which represents rapidly exchanging sites.24,26 The total concentration of Brønsted sites is about 2.0 × 1017 sites mg−1 with approximate B2:B1 ratio of 1:2, which is in good agreement with the previous HZSM-5 studies (Table 4).24,26 The total number of Brønsted sites correlates quite well with the concentration of framework Al (∼1.8 × 1017 atoms mg−1) (Table 2). After impregnation and subsequent calcination, the total concentration of Brønsted sites drops to 8.2 × 1016 sites mg−1, i.e., to less than half of the concentration observed for the parent zeolite. This is again consistent with the concentration of framework aluminum (7.5 × 1016 atoms mg−1) (Table 2). A new peak emerges at 6.5 ppm, which is assigned to complexation of B1 and B2 Brønsted sites by phosphate groups. The concentration of terminal and internal Si−OH and Alnonframe−OH decreased in a way that was much more pronounced than that of Brønsted acid sites because of the reactivity of the highly condensed polyphosphates in the 1%P HZSM-5 sample. A similar effect was observed for 2% HZSM-5 sample. However, after impregnation with 2% of P, only 20% of Brønsted sites (4.1 × 1016 atoms mg−1) present in parent

Figure 6. Temperature-programmed desorption of pyridine from calcined and washed 1%P HZSM-5 and 2%P HZSM-5 samples.

removes 77% and 53% of P initially introduced to 1%P and 2% P HZSM-5, respectively (Table 3). TPD of pyridine confirms that at concentration of 1−2 wt % P, washing partially recovers strong Brønsted acid sites. Additionally, the γ-peak (attributed to desorption of the base from strong Brønsted acid sites6) exhibit a sorption maximum at a temperature slightly higher than that of the nonwashed sample. The 27 Al MAS NMR spectra of the calcined and subsequently washed 1%P HZSM-5 samples as well as the difference spectrum are shown in Figure 7; the concentration of different Al species is compiled in Table 2. The analysis of the spectra shows that the washing procedure restored the concentration of tetrahedrally coordinated framework Al to more than 50% of its value in the parent zeolite (the

Table 4. Concentration and Distribution of Acid Sites in the Parent and P-Modified HZSM-5 Samples Determined by 1H MAS NMR H concentration (1016 atoms mg−1)

H distribution (%)

sample

Htotal

SiOHAltotal

B14.1ppm

B25.7ppm

B14.1ppm

B25.7ppm

HZSM-5 1%P HZSM-5 1%P HZSM-5 2%P HZSM-5 2%P HZSM-5

31.2 11.6 21.9 6.9 22.5

19.8 8.2 14.7 4.1 13.6

12.4 4.8 8.1 1.0 6.0

7.5 3.4 6.7 3.1 7.6

62 59 55 24 44

38 41 45 76 56

calcined washed calcined washed

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Figure 8. 31P MAS NMR spectra of calcined (I) and washed (II) 1%P HZSM-5 zeolite and difference spectrum (Δplot). Figure 7. 27Al MAS NMR spectra of calcined (I) and washed (II) 1%P HZSM-5 zeolite and difference spectrum (Δplot).

resonance lines of condensed polyphosphates overlapping with those of coordinated to Altetr‑dist, the Alocta−O−P links21,27,28 (species 3, 7, and 8, in ref 21) can also contribute to the observed decrease of resonances at −27.7, −32.2, and −40 ppm. The 27Al NMR data (Figure 7) clearly showed that washing of the calcined samples resulted in the recovery of Altetra‑frame line at 53.4 ppm and to a lesser extent of the 56.4 ppm line, indicating that the B2 sites were preferentially restored. This is accompanied by the decrease in the intensity of the lines attributed to Altetra‑dist, as well as those attributed to Aloct−O−P. All of which indicates that part of the Al−O−(HPO3)xH species formed mostly via partial covalent bonding of Al to the silica framework was transformed back into typical Altetra‑frame sites. Introduction of phosphorus loading of 1% and 2% leads to appearance of lines from the Al3+(H2O)3(PO43−)3 species in the 31P spectrum, paralleled by the increase of six-coordinated Al lines in the 27Al spectrum. Subsequent washing slightly decreases the concentration of Al3+(H2O)3(PO43−)3 species, and there is good correlation between the 27Al and 31P resonances (Tables 2 and 3). At the same time, phosphorus loading increases the signal intensity from the aluminum phosphate species (ca. −33 ppm) in the 31P NMR spectrum, paralleled by the signal increase from the distorted fourcoordinated Al species (Altetra‑dist) in 27Al NMR spectrum (Tables 2 and 3). But it is interesting to note how the difference between 1%P and 2%P loading affects the Altetra‑dist sites: 2% loading and subsequent calcination does not increase the intensity of Altetra‑dist lines proportionally to −33 ppm lines in 31P NMR spectrum, as if the number of potential aluminum sites for aluminum phosphate formation is limited. Therefore, it is reasonable to assign part of the −33 ppm 31P NMR intensity to soluble phosphorus species removed by washing and not solely to aluminum phosphate species. Moreover, after the washing, the absolute intensity of (Altetra‑dist) lines in the 27Al NMR spectrum of 2%P HZSM-5 is approximately the same as the absolute intensity of 31P NMR −33 ppm lines, assigned to aluminum phosphate sites (9.3 × 1016 atoms mg−1 versus 9.5 × 1016 atoms mg−1) (Tables 2 and 3). The 1H MAS NMR spectra of calcined and washed 1%P HZSM-5 and the difference between the two spectra are shown in Figure 9. Washing restored the concentration of Brønsted sites (14.7 × 1016 sites mg−1) to approximately 75% of that of the parent zeolite (Table 4). The concentration of SiOH groups was partially restored (terminal SiOH to a lesser

concentration of Altetra‑frame increased from 7.5 × 1016 sites mg−1 for the nonwashed sample to 9.4 × 1016 sites mg−1 for the washed sample). The difference spectrum clearly showed that both B1 and B2 sites were restored. Simultaneously, a decrease in the concentration of Altetra‑dist (line at 40 ppm) and Alocta‑classic (line at −10.4 ppm) was observed (from 8.3 × 1016 atoms mg−1 to 6.7 × 1016 atoms mg−1 and from 8.2 × 1016 atoms mg−1 to 6.4 × 1016 atoms mg−1, respectively). The relative contribution of these sites decreased from 69% to 58%. The higher concentration of Al removed from the Altetra‑dist and Alocta‑classic positions, in comparison to that restored in the framework Altetra‑frame positions (3.4 × 1016 atoms mg−1 and 1.9 × 1016 atoms mg−1, respectively), indicates slight irreversible dealumination of the zeolite. Washing of the 2%P HZSM-5 sample restores the concentration of Altetra‑frame to only ca. 26% of its value in the parent zeolite, i.e., the concentration of acid sites increased from 3.3 × 1016 atoms mg−1 for the nonwashed sample to 4.7 × 1016 atoms mg−1 for the washed sample (Table 2). This is accompanied by a decrease in concentration of Alocta−O−P sites, whereas the number of Altetra‑dist sites was almost unchanged (slight increase from 9.0 × 1016 to 9.3 × 1016 atoms mg−1 for calcined and washed samples, respectively). The 31P MAS NMR spectra of calcined and washed 1%P HZSM-5 zeolite as well as the difference spectrum are shown in Figure 8. The washing treatment eliminated more than 75% of P present in the calcined sample, i.e., the content of phosphorus decreased from 2.94 × 1017 to 6.9 × 1016 atoms mg−1 (Table 4, Figure 8). Similarly, washing of the 2%P HZSM-5 sample decreased the amount of P by 53%. The applied procedure removed polyphosphate species as shown by the almost complete disappearance of the resonances at ca. −6 and −13.6 ppm which are attributed to end and middle group in the condensed polyphosphates from the 31 P MAS NMR spectrum.21 Simultaneously, washing with hot water (80 °C) led to a decrease in the intensity of the resonance lines at −27.7, −32.9, and −40 ppm attributed to different phosphorus species bound to Al. Relatively high intensity of the resonance line at ca. −15.6 ppm in the spectrum of the washed 2%P HZSM-5 sample (Table 3) (Figure S13 of Supporting Information) was attributed to the presence of polyphosphates bound to octahedrally coordinated aluminum Alocta (species 6 and/or 9 in ref 21). It has to be noted that the removal of 6128



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modified and washed samples, although influenced by occluded P, leads to the formation of Brønsted acid sites. The concentration of phosphate correlates positively with the lower concentration of tetrahedrally coordinated Al3+ and the concentration of Brønsted acid sites, which is, however, 50% lower than the respective concentration in the washed materials. This suggests that the phosphate ions reacted with 50% of the Brønsted acid sites irrespective of their absolute concentration. The MTO reaction was used as a probe to correlate the modified acidity and modification procedures with the catalytic performance. The catalytic results of the methanol-to-olefins reaction are shown in Figure 11 for the parent, 1 wt % Pmodified (calcined), 1 wt % P-modified (calcined and hot water washed), and 2 wt % P-modified (calcined) HZSM-5 samples. Introduction of 1 wt % P by impregnation and subsequent calcination resulted in a significant loss of framework aluminum and, consequently, the Brønsted acidity, accounting for the lower catalytic activity of 1%P HZSM-5 compared to parent HZSM-5. The lower acid site concentration led to enhanced propene formation and suppressed hydrogen transfer (HT). This shift in selectivity is attributed to the decoupling of ethene formation and the formation of the other olefins as well as to the greater apparent dependence of the HT pathways on the acid site concentration. As a note in passing, we would like to stress that ethene formation showed a trend that was same as that of HT products, in line with the dual-cycle mechanism in which ethene is suggested to originate from the aromaticsbased catalytic cycle.29 Adding 2%P led to a further decrease in the activity, but not to a change in the selectivity, when comparing the catalysts at the same conversion. This shows that the increasing concentration of phosphorus does not lead to a change in the relative rates of the individual reactions. In particular, the extent of HT was identical to that of the 1%P sample. In passing it should also be noted that the relative distribution of the location of acid sites (B1 and B2) did not influence the kinetic results. Hot water washing of calcined 1%P HZSM-5 (or 2%P HZSM-5, not shown) resulted in a substantial restoration of framework aluminum and consequently the Brønsted acidity. Whereas the total Brønsted acid concentration was still lower for the washed material compared to the parent, the stronger B2 acid sites of the washed material were largely restored. Figure 11 shows similar catalytic activities and selectivities for the washed and the parent samples, which leads us to conclude that mainly the stronger B2 sites contribute to the methanol conversion rate. The C4 hydrogen transfer index (C4 HTI) was used to describe the hydrogen transfer ability of the working catalysts. Figure 12 shows the C4 HTI as a function of methanol conversion. The calcined 1%P HZSM-5 and 2%P HZSM-5 showed a C4 HTI that was remarkably lower than that of the parent sample. The washed material had a HTI that was comparable to that of the parent HZSM-5 at low methanol conversions, while at higher conversions the parent sample showed a C4 HTI that was higher than the washed material. This indicates that while B2 acid sites contribute more to the methanol conversion than do B1 acid sites, it is the total concentration of Brønsted acid sites (B1 + B2), rather than solely the stronger B2 sites, that contributed to hydrogen transfer. This would be in line with a model in which the acid sites act as a means to retain a higher hydrocarbon

Figure 9. 1H MAS NMR spectra of calcined (I) and washed (II) 1%P HZSM-5 zeolite and difference spectrum (Δplot).

degree) and the concentration of Al−OH groups (2.7 ppm) increased, probably due to some dealumination during the thermal activation prior to the 1H MAS NMR measurements. Again, the increase of Brønsted sites (from 8.2 × 1016 sites mg−1 for calcined sample to 14.7 × 1016 sites mg−1 for washed sample) correlates with the increase of framework Al, but there must be a considerable contribution of distorted and partially removed Al (Altetra‑dist). A quantitative analysis of the concentration of the strong Brønsted sites showed that the relative concentration of the B2 sites increased from 38% to 45% compared to the situation in the parent HZSM-5 (Table 4). Thus, impregnation and subsequent washing increased the fraction of B2 Si−OH−Al sites, which are more acidic compared to B1 type sites. The higher concentration of the more Brønsted acid sites in the total Brønsted acidity is concluded to be responsible for the slight shift toward higher temperatures of the maximum of the γ-peak in the TPD of pyridine desorbing from the washed P HZSM-5 samples. Figure 10 illustrates the effects of increasing concentrations of phosphate ions deposited. The total concentration of Br sites

Figure 10. Plot of total Brønsted site concentration versus 4coordinated aluminum concentration of parent and P-modified samples (calcined and washed).

(B1 + B2) correlates very well with the concentration of Altetra‑frame + Altetra‑dist. The concentration of B2 sites in samples after washing correlates with the Altetra‑dist sites, and the concentration of B1 sites in samples after washing correlates with the Alframe sites (Tables 2 and 4). This shows that the quality of changes induced is identical with increasing concentration of P, but its impact increases as the concentration of P increases. Washing of the sample restores most of the acid sites, but the procedure leads to slight dealumination (ca. 5%). It is remarkable that all tetrahedrally coordinated Al3+ in the 6129



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Figure 11. Impact of the P modification on the catalytic performance of HZSM-5 in the MTO reaction.

phosphorus-modified ZSM-5 samples. Introduction of phosphorus distorted the local coordination environment of tetrahedral framework aluminum but did not produce extraframework species. The reduction of Altetra‑frame concentration accounted for the decrease of Brønsted acidity of the phosphorus-modified zeolites. High-temperature calcination of P HZSM-5 resulted in the formation of occluded condensed polyphosphates and Al−O−P complexes (Altetra‑dist−O−P and Alocta−O−P species where Al remained at the original framework positions), which were not present in the parent HZSM-5. Washing with hot water removed most of the introduced phosphorus and restored framework Altetra‑frame sites from these Al−O−P complexes. In consequence, this increased the corresponding Brønsted acidity, demonstrating the partial reversibility of ZSM-5 of the modification process with phosphorus anions. The chemical and thermal treatments changed the ratio of the framework Al sites, i.e., B1 and B2 sites. The larger extent of restoration of B2 compared to B1 acid sites increased the overall average acid strength. The higher concentration of B2 Brønsted acid sites has a marked influence on the catalytic material properties for the MTO reaction. While the B2 type acid sites constitute the majority of the methanol conversion sites, it is the total concentration of Brønsted acid sites (B1+B2), rather than solely the stronger B2 sites, that contributes to hydrogen transfer.

Figure 12. Impact of the P modification on the C4 hydrogen transfer index observed in the MTO reaction on HZSM-5.

concentration in the pores, which is positive for the bimolecular hydride transfer reactions.

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CONCLUSIONS Quantitative NMR spectroscopic studies (27Al, 31P, and 1H MAS NMR) were combined with temperature-programmed desorption to explore the local structural changes for aluminum and phosphorus as well as Brønsted acid sites during sequential treatment procedures, i.e., calcination and hot water washing, of

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ASSOCIATED CONTENT

S Supporting Information *

Simulation of the NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org. 6130



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(18) Sarv, P.; Fernandez, C.; Amoureux, J.-P.; Keskinen, K. Distribution of Tetrahedral Aluminum Sites in ZSM-5 Type Zeolites: An 27Al (Multiquantum) Magic Angle Spinning NMR Study. J. Phys. Chem. 1996, 100, 19223−19226. (19) Zhuang, J.; Ma, D.; Yang, G.; Yan, Z.; Liu, X.; Liu, X.; Han, X.; Bao, X.; Xie, P.; Liu, Z. Solid-State MAS NMR Studies on the Hydrothermal Stability of the Zeolite Catalysts for Residual Oil Selective Catalytic Cracking. J. Catal. 2004, 228, 234−242. (20) Cabral de Menezes, S. M.; Lam, Y. L.; Damodaran, K.; Pruski, M. Modification of H-ZSM-5 Zeolites with Phosphorus. 1. Identification of Aluminum Species by 27Al Solid-State NMR and Characterization of Their Catalytic Properties. Microporous Mesoporous Mater. 2006, 95, 286−295. (21) Damodaran, K.; Wiench, J. W.; Cabral de Menezes, S. M.; Lam, Y. L.; Trebosc, J.; Amoureux, J.-P.; Pruski, M. Modification of H-ZSM5 Zeolites with Phosphorus. 2. Interaction Between Phosphorus and Aluminum Studied by Solid-State NMR Spectroscopy. Microporous Mesoporous Mater. 2006, 95, 296−305. (22) Goehlich, M.; Reschetilowski, W.; Paasch, S. Spectroscopic Study of Phosphorus Modified H-ZSM-5. Microporous Mesoporous Mater. 2011, 142, 178−183. (23) Montagne, L.; Pavalit, G.; Draoui, M. Mechanism of Polyphosphate Gel Formation in the Na2O−Al2O3−P2O5 System. J. Non-Cryst. Solids 1993, 155, 115−121. (24) Freude, D. Enhanced Resolution in the 1H NMR Spectra of Zeolite H-ZSM-5 by Heteronuclear Dipolar-Dephasing Spin-Echo MAS. Chem. Phys. Lett. 1995, 235, 69−75. (25) Freude, D.; Ernst, H.; Wolf, I. Solid-State Nuclear Magnetic Resonance Studies of Acid Sites in Zeolites. Solid State NMR 1994, 3, 271−286. (26) Beck, L. W.; White, J. L.; Haw, J. F. 1H{27Al} Double-Resonance Experiments in Solids: An Unexpected Observation in the 1H MAS Spectrum of Zeolite HZSM-5. J. Am. Chem. Soc. 1994, 116, 9657− 9661. (27) Lima, E. C. O.; Moita Neto, J.M. F.; Fujiwara, Y.; Galembeck, F. Aluminum Polyphosphate Thermoreversible Gels: A Study by 31P and 27 Al NMR Spectroscopy. J. Colloid Interface Sci. 1995, 176, 388−396. (28) Duncan, T. M. A Compilation of Chemical Shift Anisotropies; The Farragut Press: Chicago, 1990. (29) Bjørgen, M.; Svelle, S.; Joensen, F.; Nerlov, J.; Kolboe, S.; Bonino, F.; Palumbo, L.; Bordiga, S.; Olsbye, U. Conversion of Methanol to Hydrocarbons over Zeolite H-ZSM-5: On the Origin of the Olefinic Species. J. Catal. 2007, 249, 195−207.

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +1 509 375 3856. Fax: + 1 509 391 6498. E-mail: [email protected]. *Tel.: +49 89 28913540. Fax: + 49 89 28913544. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.S. acknowledges the support by grant SF0690034s09 from the Estonian Ministry of Education and Research. X.S. and S.M. acknowledge partial support for this work by Clariant in the framework of Municat.

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REFERENCES

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Reversibility of the Modification of HZSM-5 with Phosphate Anions

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