Elsevier

Catalysis Today

Volume 192, Issue 1, 30 September 2012, Pages 189-196
Catalysis Today

Generation of Brønsted acid sites on Al2O3-supported Ta2O5 calcined at high temperatures

https://doi.org/10.1016/j.cattod.2012.02.035Get rights and content

Abstract

An alumina-supported tantalum oxide (Ta2O5/Al2O3) calcined at 1223 K promoted both Friedel–Crafts alkylation of anisole with benzyl alcohol and isomerization of α-pinene, and exhibited Brønsted acidity despite the high temperature calcination. Among the Ta2O5/Al2O3 catalysts tested, 33 wt% Ta2O5/Al2O3 calcined at 1223 K showed the highest activity. A monolayer of tantalum acid-like compound, which has distorted octahedral symmetry, was stabilized over 33 wt% Ta2O5/Al2O3 catalyst calcined at 1223 K. The two-dimensional Ta–O–Ta network of stabilized tantalum acid-like compound probably accounts for the generation of Brønsted acid.

Highlights

► Generation of acid sites on Ta2O5/Al2O3 calcined at high temperature. ► Ta2O5/Al2O3 showed catalytic activity for Friedel–Crafts alkylation and α-pinene isomerization. ► A monolayer of tantalum acid-like compound was stabilized. ► The two-dimensional Ta–O–Ta network probably accounts for the generation of Brønsted acid.

Introduction

Tantalum acid (Ta2O5·nH2O, hydrated tantalum oxide) shows mainly Lewis acid properties in the absence of water [1], [2], but Brønsted acid sites are generated by treatment with steam at 373 K. Compared to niobic acid (Nb2O5·nH2O, hydrated niobium oxide), which is known as a water-tolerant solid acid catalyst that exhibits strong Brønsted acidity and is effective for reactions in aqueous media such as esterification, olefin hydration, and alcohol dehydration [3], [4], [5], [6], [7], tantalum acid retains its strong acidity after calcination at higher temperatures. In the case of niobic acid, dehydration leads to lose of its acid property by heating above 773 K [8]. On the other hand, tantalum acid maintains its acidic property up to 1073 K. Generally, high temperature calcination usually causes solid acid catalysts to lose the acid property. Ordinary solid acid catalysts, such as aluminosilicate, zeolite and binary oxides, pretreated at >873 K lose their acid property [9], [10], [11], except in some cases like SiO2/Al2O3 prepared with CVD method [12], highly siliceous zeolites and layered mixed oxide [13], [14], [15]. High stability is one of the important factors on useful solid acid catalyst, because the life of solid acid catalyst strongly depends on the stability. If a solid acid could be prepared by calcination at high temperatures required for the formation of ceramics (>1173 K), it would be a useful acid-catalyst even under strict conditions because of its high stability.

We have reported that a solid acid prepared with high loading of Nb2O5 on an Al2O3 support and calcined at 1173 K exhibited Brønsted acid property despite the high temperature calcination. This strongly suggests that the alumina support stabilizes the Brønsted acid sites during calcination at high temperature [16], [17]. Amount of Brønsted acid site on Nb2O5/Al2O3 changed depending on both calcination temperature and loading amount of Nb2O5. This strongly suggests that the structural change around niobium is expected to have a major impact on Brønsted acid property. We have proposed that the stabilized niobic acid-like compound which has two-dimensional Nb–O–Nb network of probably account for the generation of Brønsted acid site.

Very recently, we reported that alumina-supported tantalum oxide (Ta2O5/Al2O3) calcined at high temperature such as 1223 K with high loading of Ta2O5 exhibited Brønsted acid property [18], suggesting that Ta2O5/Al2O3 would also act as a stable Brønsted acid catalyst. However, the relationship between the structural change around tantalum and generation of Brønsted acid site on Ta2O5/Al2O3 catalyst is still unclear. Here, we have prepared a series of Ta2O5/Al2O3 solid acids and the relationship between the local structure of tantalum species in Ta2O5/Al2O3 solid acids and acid property is investigated.

Section snippets

Preparation of catalysts

A series of Ta2O5/Al2O3 solid acids were prepared by impregnation of γ-alumina (JRC-ALO-8, Catalysis Society of Japan, 148 m2 g−1) with an ethanol solution of tantalum ethoxide (Ta(OC2H5)5), followed by calcination at various temperatures for 3 h in dry air. Ta2O5 loading amount was changed from 10 wt% to 50 wt%, and calcination temperature was changed from 773 K to 1423 K. Aluminum tantalate, AlTaO4, was synthesized from tantalum ethoxide and aluminum isopropoxide (Al(OC3H7)3) according to an

Catalytic activity

The catalytic activities of Ta2O5/Al2O3 catalysts have been investigated as a function of calcination temperature. Loading amount of tantalum oxide was 33 wt%. Fig. 1(A) shows the effect of the calcination temperature on the product yields in Friedel–Crafts alkylation of benzyl alcohol with anisole. The main products were ortho- and para-benzyl anisole isomers, which were presumably formed on the Brønsted acid sites, and by-product was dibenzyl ether produced mostly on the Lewis acid sites [14],

Conclusion

We found that the Ta2O5/Al2O3 catalysts calcined at high temperatures were effective for Friedel–Crafts alkylation and isomerization of α-pinene. Among the Ta2O5/Al2O3 catalysts tested, 33 wt% Ta2O5/Al2O3 calcined at 1223 K showed the highest activity in both Frieadel–Crafts Alkylation and isomerization of α-pinene. The Ta2O5/Al2O3 catalyst exhibits the Brønsted acidity despite calcination at such high temperatures. It is concluded that a monolayer of tantalum acid-like compound, which have

Acknowledgements

This work was supported in part by a Grant-in-Aid for Scientific Research (B) (Grant No. 23360355) and Grant-in-Aid for Young Scientists (B) (Grant No. 21760627) from the Ministry of Education, Culture, Sports, Science and Technology and by the Iwatani Naoji Foundation (Grant No. 09-011). The X-ray absorption experiments have been performed under the approval of the JASRI (Proposal Nos. 2009B1398 and 2010B1184).

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