In recent years- is this a prepositional or adjective phrase

"In recent years" is a prepositional phrase. At the minimum, a prepositional phrase will begin with a preposition and end with a noun, pronoun, gerund, or clause, the "object" of the preposition.
Another example: She is on the computer.

An adjective phrase or an adjectival phrase is a more complicated issue. This term can refer to a phrase built upon an adjective, which functions as the head of that phrase. For example, the phrase "much quicker than I" is based on the adjective 'quick', and the phrase "fond of animals" is based on the adjective 'fond'.

A different use of the term "adjectival phrase" is for a phrase that modifies a noun as an adjective would, even if it does not contain or is not based on an adjective. These may be more precisely distinguished as phrasal noun modifiers. For example, in "Mr Clinton is a man of wealth," the prepositional phrase "of wealth" modifies a man the way an adjective would, and it could be reworded with an adjective as "Mr Clinton is a wealthy man."