(349d) Weak Zeotype Solid-Acids As Promising Materials for Selective Dehydration of Oxygenates

Significant research in the last decade has been devoted to harness the supply of renewable carbon in lignocellulosic biomass to derive bulk chemicals. Biomass-derived intermediates are replete with multi-functional oxygen moieties which notoriously lead to coking. Consequently, the use of aluminosilicate zeolites in catalytic chemistries dealing with oxygenate dehydration frequently suffers from poor stability and the need for energy-intensive regeneration cycles. Furthermore, activating these feeds selectively on such ‘strongly acidic’ materials is often challenging, and designing new solid-acid catalysts may be required to efficiently process these alternative feedstock.

The industrial relevance of aluminosilicates for hydrocarbon catalysis has led to rich understanding of their active sites. Using two distinct classes of solid acid zeotype materials with weak Brønsted acidity (namely, borosilicates, and phosphorous-modified zeosils), we argue that attaining a deeper understanding of weak solid acids would afford further advancements in efficient processing of oxygenate feeds. Taking the example of a Brønsted acid catalyzed chemistry with relative C-O to C-C bond scission rates as a selectivity descriptor, we first show that both set of weakly acidic materials highly prefer C-O scission events over C-C, leading to significant improvements in the selectivity descriptor (Figure 1A). Using the gained insights, we then demonstrate the limited applicability of traditional Brønsted acid site counting methods to milder solid acids (Figure 1B), and in doing so, shed light on the implications of weak surface binding in dehydration catalysis. Indeed, while isopropanol dehydration mechanistics remain unaltered (Figure 1C), the reaction performance (propene selectivities, and site time yields) are drastically different on the phosphorus-modified zeosils viz-a-viz their aluminosilicate counterparts due to lower adsorption enthalpies. Importantly, the nature of catalytic sites in these weak solid acids remains unaltered in a host of micro-, and mesoporous supports (Figure 1D), possibly opening up avenues for shape-selective oxygenate conversion catalysis on these materials.