Nature of Catalytically Active Sites in the Supported WO₃/ZrO₂ Solid Acid System: A Current Perspective

Abstract

Tungstated zirconia (WO3/ZrO2) is one of the most well-studied solid acid catalyst systems and continues to attract the attention of both academia and industry. Understanding and controlling the properties of WO3/ZrO2 catalysts has been a topic of considerable interest over almost the past three decades, with a particular focus on discovering the relationship between catalytic activity and the molecular structure of the surface acid site. Amorphous tungsten oxide (WOx) species on ZrO2 surfaces were previously proposed to be very active for diff erent acidic reactions such as alcohol dehydration and alkane isomerization. Recent developments in electron optical characterization and in situ spectroscopy techniques have allowed researchers to isolate the size, structure, and composition of the most active catalytic species, which are shown to be three-dimensional distorted Zr-WOx clusters (0.8−1.0 nm). Complemen-tary theoretical calculations of the Brønsted acidity of these Zr-WOx clusters have confirmed that they possess the lowest deprotonation energy values. This new insight provides a foundation for the future characterization and theory of acidic supported metal oxide catalytic materials that will, hopefully, lead to the design of more active and selective catalysts. This perspective presents an up-to-date, comprehensive summary of the leading models of WO3/ ZrO2 solid acid catalysts.

Alkane isomerization over sulfated zirconia solid acid system

“…For CFB isomerization technology, the stability of active sulfate species during regeneration process needs to be further improved by optimizing the catalyst composition and operating conditions. It is worth noting that several alternatives to SZ have been well developed, such as WO x /ZrO 2 and MoO x /ZrO 2 , which are also considered as solid superacids and show similar catalytic performance in alkane isomerization with SZ catalysts. In conclusion, the SZ and its modified versions are much more promising in n ‐alkane isomerization and are expected to draw great interest in the coming years.…”

“…Amorphous zirconium hydroxide and crystalline zirconia were often used in the preparation of zirconia‐based catalysts . For n ‐alkane isomerization, SZ catalyst prepared by sulfating the crystalline zirconia exhibited higher surface area, but lower acidity and reactivity compared with the catalysts obtained from the amorphous zirconia .…”

Major Advances and Challenges in Heterogeneous Catalysis for Environmental Applications: A Review

“…As it was mentioned in the review paper of [57], there is a need to project and prepare heterogeneous catalysts with sophisticated surface sites (e.g. [58]) in order to selectively promote reactions. Selectivity is the next important issue in heterogeneous catalysis [57].…”

Assessing carbon or tungstates coverage of ZrO2 nanoparticles supported on MWCNT via NOx-TPD

“…The aforementioned ZrO 2 /MWCNT composite has been further functionalized by adding W to make tungsten-promoted ZrO 2 nanoparticles supported on MWCNT (WO 3 -ZrO 2 /MWCNT). Due to their intrinsic acid properties, tungstated zirconias have attracted the attention of the catalysis community for about three decades [29]. Several studies have shown that the W surface density (W/nm 2 ), also reflecting the coverage of ZrO 2 by tungstates [30,31], is the critical parameter when determining performance of these catalysts in various catalytic reactions [25,29].…”

“…Due to their intrinsic acid properties, tungstated zirconias have attracted the attention of the catalysis community for about three decades [29]. Several studies have shown that the W surface density (W/nm 2 ), also reflecting the coverage of ZrO 2 by tungstates [30,31], is the critical parameter when determining performance of these catalysts in various catalytic reactions [25,29]. Consequently, much work has been devoted to determining the W surface density for which monolayer coverage of WO x on a ZrO 2 support is achieved.…”