Zeolite rehydration

Rehydration of the samples occurs even after a 900°C-pretreatment and gives rise to IR OH bands having the usual frequencies. It had been reported that rehydration of lanthanum zeolites heated at 700° C regenerated the catalytic activity (5). Therefore, the thermal stability of activity seems to be related to the rehydration. This rehydration of the lanthanum zeolites pretreated at 900° C leads to the formation of a significant number of acid sites of the same strength in regard to Py as those present on the 550°C-pretreated Na-8.7 zeolite rehydrated in the same conditions. 

Zeolite Dehydrated zeolite Rehydrated zeolite (0 = 1) [Al(acac)3]... 

However, when the sample is exposed to moist air and the zeolite rehydrated this band shifts back to 2044 cm". Above 200C... 

Solid state H and 27Al MAS NMR spectra were collected using a Broker DSX-500 spectrometer (11.7 T) and a Broker 4 mm CPMAS probe. For dehydration studies, the calcined and rehydrated zeolites were packed in a 4mm Zr02 NMR rotor, evacuated (10" 3 Torr) while heated and then held at desired temperature for 2 h. 

Pyridine sorption studies have shown the presence of both Bronsted and Lewis acid sites in USY zeolites, although to a lesser extent than in the corresponding HY zeolite (51,53). Acidity is maintained even after strong dehydroxylation of USY-B at 820°C. Rehydration of the calcined material did not regenerate significantly Bronsted acid sites, due to irreversible changes in the zeolite framework (51). 

In the spectrum of zeolite KL, at the temperature of evacuation (100°-500°C), stronger water-bound bands (3665, 3685, 3700 cm 1 and 1602, 1630, and 1650-1660 cm 1) are found. Absorbance of these bands decreases above 400°C. A parallelism in the absorbance decrease is shown for the 1602 and 3700 cm 1 bands (preserved to 600°C) at increased dehydration temperature. These bands are not recovered on rehydration, at either low or at high temperatures. Water molecules are, evidently, localized in the secondary system of channels (8). Perhaps water is localized in cancrinite cells. 

Tphe excellent catalytic activity of lanthanum exchanged faujasite zeo-A lites in reactions involving carbonium ions has been reported previously (1—10). Studies deal with isomerization (o-xylene (1), 1-methy 1-2-ethylbenzene (2)), alkylation (ethylene-benzene (3) propylene-benzene (4), propylene-toluene (5)), and cracking reactions (n-butane (5), n-hexane, n-heptane, ethylbenzene (6), cumene (7, 8, 10)). The catalytic activity of LaY zeolites is equivalent to that of HY zeolites (5 7). The stability of activity for LaY was studied after thermal treatment up to 750° C. However, discrepancies arise in the determination of the optimal temperatures of pretreatment. For the same kind of reaction (alkylation), the activity increases (4), remains constant (5), or decreases (3) with increasing temperatures. These results may be attributed to experimental conditions (5) and to differences in the nature of the active sites involved. Other factors, such as the introduction of cations (11) and rehydration treatments (6), may influence the catalytic activity. Water vapor effects are easily... 

Figure 8. Bronsted acidity ( ) and Lewis acidity (O) of La zeolites pretreated at 900° C and rehydrated, against cation contents/unit celt. Bronsted aridity (c) and Lewis aridity ( ) of the Na-8.7 zeolite pretreated at 800°C and rehydrated are also plotted...

Table II. Bronsted and Lewis Acidities (in Arbitrary Units) of the La-7 Zeolite Pretreated at Indicated Temperatures and Rehydrated...

The La zeolites calcined above 700°C (I, 5, 13) do not have hydroxyl groups. Since the samples we used for catalytic and acidic measurements are rehydrated after calcination, the question is raised as to whether this rehydration causes OH groups to reappear. For this purpose, a 900° C-pretreated La-7 sample was rehydrated and then evacuated at 450° C. Its spectrum presents weak OH bands around 3740, 3680, 3640, and 3535 cm-1. 

Influence of Temperature. Data concerning the thermal stability of the catalytic activity are given in Figure 2 and Table I. The thermal stability of the starting materials Na-8.7 and D.Na-5.4 is discussed first. The limit of stability of the Na-8.7 sample appears to be higher than for the NaHY zeolites studied previously (3, 6, 27, 28). Nevertheless, this sample cannot be considered ultrastable since neither its structural data nor the thermal stability of its OH groups are characteristic of ultrastable zeolites (17). This increase in the stability may be explained by dry air heating and subsequent rehydration. 

The relationship of Lewis and Brpnsted acid site concentrations on H—Y zeolite was explored further in a study by Ward (156) of the effect of added water. At low calcination temperatures (<500°C) only a small increase in the Brpnsted acid site concentration occurred upon addition of water to the sample. Rehydration of samples dehydroxylated by calcination above 600°C resulted in a threefold increase in the amount of Brpnsted-bound pyridine. However, no discreet hydroxyl bands were present in the infrared spectrum after rehydration. Thus, the hydroxyl groups reformed upon hydration must be in locations different from those present in the original H—Y zeolite, which gave rise to discreet OH bands at 3650 and 3550 cm-1. 

These results strongly pointed toward the involvement of the acidic hydroxyl groups in the catalytic reaction as suggested by Benesi (157), since the maximum activity was obtained when the zeolite was completely deammoniated. In addition, catalysts which had been dehydroxylated by high-temperature calcination demonstrated low activity. Thus, Benesi proposed that the Brtfnsted acid sites rather than the Lewis acids were the seat of activity for toluene disproportionation. This conclusion was supported by the enhancement in toluene disproportionation activity observed when the dehydroxylated (Lewis acid) Y zeolite was exposed to small quantities of water. As discussed previously, Ward s IR studies (156) indicated a substantial increase in Brdnsted acidity upon rehydration of dehydroxylated Y sieve. 

X-ray and aluminium MAS NMR measurements were carried out on samples rehydrated in a desiccator over an aqueous NH,C1 solution. A portion of the zeolites synthesized with organic templates was heated for 5 h at 600 °C to remove organic compounds. The Na /H ion exchange was carried out at room temperature with an aqueous solution of 0.5 N HC1. The preparation of 1H MAS NMR samples was performed under shallow bed activation conditions in a glass tube of 5.5 mm inner diameter and 10 mm height of the zeolite layer. The temperature was increased at a rate of 10 K/hr. After maintaining the samples at the final activation temperature of 400 °C under a pressure below 10 L Pa for 24 hrs., they were cooled and sealed. 

Zeolite Hydrated Partially Hydrated Dehydrated Rehydrated... 

The results in CsyNas-A and (NH )i2 A are basically similar to those for Kj -A. In freshly prepared, hydrated CsyNaj-A only Cux is observed (30). Partial dehydration at 50-100° C gives a weak signal of Cuxx and mainly Cuq. Complete dehydration to give Cuq is readily achieved at 400° C and subsequent rehydration at room temperature reforms Cux as expected. Note the contrast here with the anomalous rehydration result in Kj -A. In hydrated (Nlfy)i2 A Cux is predominant with sometimes a little Cuxx. Partial dehydration at 50° C converts Cux to Cujj As found in Ki2 A and rehydration at room temperature restores Cux- Dehydration at higher temperatures than 100° C cannot be done because it results in the irreversible decomposition of this zeolite. 

Most zeolites have three-dimensional structures with cavities connected to charmels that exhibit ion sieve properties. If the pore volume of the cavity is large enough, the hydration stripped cations exchanged can be rehydrated in the cavity to simulate their hydration in the solution. The large cations (Rb, Cs, organic cations) cannot enter the channels or windows of the cavities of zeolites with small pores. Table 3 lists the pore openings of some hydrated zeolites [133]. 

C. Naccache Figure 1 shows that rehydration results in a large decrease of spin concentration. For example, as can be determined in this figure (dotted line corresponds to sample activated at 450° and then rehydrated), LaY zeolite dehydrated at 450° gives about 1.5 X 10 positive radical ions, while after rehydration the number of radical ions is only 1.1 X 10 . The decrease in electron-acceptor sites resulting from rehydration of the zeolite is demonstrated. 

Figure 5.14. Al triple-quantum MAS NMR spectra of commercial zeolite mordenite in the hydrogen form, unheated and heated to various temperatures followed by hydration. Spinning side bands are marked by asterisks. The final spectrum is of a sample heated in two steps, of 650°C and 500°C with an intermediate rehydration step. After Chen et al. (2000), by permission of copyright owner.

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