The layered transition-metal oxide HNbMoO(6) is demonstrated to exhibit remarkable catalytic performance for the hydrolysis of saccharides such as sucrose, cellobiose, starch, and cellulose, attributable to water tolerance and the facile accessibility of saccharides into the strong acidic interlayer gallery of the solid.

Solid acid catalysts offer the opportunity to reduce environmental impact owing to such advantages as ease of product separation and recyclability of the catalyst, which contribute considerably to green chemistry. Nanosheets, crystalline two-dimensional metal oxide sheets prepared from cationexchangeable layered metal oxides through exfoliation and aggregation, are a novel class of potential solid acid catalysts for replacing liquid acids, such as sulfuric acid. This article reviews the acid strength and acid catalysis of several types of nanosheets, which are strongly dependent on novel strong Brønsted acid sites attributed to bridged OH groups formed only on nanosheets. An efficient acid catalysis of layered protonated niobium molybdate in Friedel-Crafts alkylation, esterification and hydrolysis, owing to its unique intercalation availability, is also discussed. ''green technology'' dictate that production should have a minimal adverse effect on the environment and human health, 5,6 stimulating the replacement of these acid catalysts with recyclable, nontoxic solid acids with strong acid sites. From the 1940s onwards, liquid acids have been replaced by solid acids for the production of fuels and bulk chemicals in the fields of oil refining and petrochemicals on an enormous scale. 4 Most of these reactions are carried out in the gas phase, and zeolites are widely used because of their excellent thermal and chemical stability, acid properties and shape selectivity. [1][2][3][4][5][6][7] At present, more than 180 industrial processes, such as alkylation, isomerization, dehydration, condensation, cracking and etherification, employ solid acid catalysts, including zeolites, clays, metal oxides and ion-exchange resins. 4 However, in liquid phase reactions, most processes are still catalyzed by homogeneous acids, such as H 2 SO 4 , HF and AlCl 3 . This disagreeable situation is due to the lack of suitable highly active solid acid catalysts. Thus, the development of solid acids as heterogeneous catalysts is desirable from the viewpoint of environmentally sustainable chemistry. One of the most attractive classes of acid-catalyzed reactions is chemical synthesis in water. Organic synthesis using water as a solvent has several advantages, such as non-flammability, non-toxicity and safety, which make it a highly enviromentally-benign process. These reactions require the replacement of liquid acids with water-tolerant solid

Layered HNbMoO6 was found to function as a strong solid acid catalyst, exceeding the activity of zeolites and ion-exchange resins for Friedel-Crafts alkylation. HNbMoO6 also exhibited high catalytic activity for esterification of hydrocarboxylic acid and hydration. The catalytic performance of layered HNbMoO6 is attributed to the intercalation of reactants into the interlayer and the development of strong acidity.

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